Recent advances and perspective on heterogeneous catalysis using metals and oxide nanocrystals

Abstract

Inorganic nanomaterials are widely used in heterogenous catalysis. With the development of nanoscience and nanotechnology, chemists can precisely control the size, morphology, and exposed facets of nanocrystals (NCs) in an elegant manner, further tailoring their catalytic performance towards diverse reactions. In this review, we focus on the design principles of high-performance catalysts using metal NCs and oxide NCs. Special attention is paid to the shape control and surface engineering of NCs. Some practically important heterogeneous catalytic reactions over nanocatalysts will be discussed. Finally, an outlook and perspective in nano-catalysis will be provided. The goal of this review is to discuss recent advances and perspective on heterogeneous catalysis using inorganic nanomaterials and motivate researchers to design efficient catalysts with high activity, selectivity and stability.

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Yong Xu

Yong Xu is currently a Professor at the School of Materials and Energy, Guangdong University of Technology. He obtained his PhD from the Department of Chemical Physics of University of Science and Technology of China in 2013. After being a Postdoctoral Fellow in Soochow University and Lawrence Berkeley National Laboratory, he joined Soochow University as an Associate Professor in 2017. His current research interests focus on the selective activation of hydrocarbons at the surface and interface of functional catalysts.

Muhan Cao

Muhan Cao obtained her PhD degree in Environmental Science and Technology from Hohai University in 2015. Currently, she is an Assistant Research Fellow at Soochow University. Her current research interests focus on the design of metal and semiconductor nanocrystals and their applications in the optoelectronic field.

Qiao Zhang

Qiao Zhang is a Professor in the Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, China. He received his PhD from the University of California Riverside (2012). After being a Postdoctoral Fellow in the Department of Chemistry, University of California Berkeley (2012–2014), he joined Soochow University in 2014. His main research field is molecular level studies of surface and heterogeneous catalysis, controllable synthesis of all-inorganic perovskites with tunable optical properties and their applications.

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1. Introduction

Heterogeneous catalysis plays a vital role in industry for the production of transportation fuels, lubricants, drugs, polymers, fibers, and other important high value-added chemicals. In heterogeneous catalysis, catalysts not only greatly accelerate the kinetics of reactions by reducing the activation energy, but also selectively form the desired products through the control of reaction pathways. It has been well accepted that the earliest report in catalysis can be traced back to the 18th century. Accordingly, with over 250 years of development, more than 90% of industrial chemicals are produced with the addition of catalysts. Generally, the majority of catalysts in industry are solid powders in which the active sites are dispersed in the form of very small nanoparticles and clusters.¹ To obtain high-performance catalysts, the active elements need to be loaded on porous supports (e.g., Al₂O₃ and SiO₂) to stabilize the active sites and add some promoters to avoid their deactivation.

It is well-known that nanostructures are more likely to exhibit much better catalytic performances due to the “nano-effect” originating from their small size.² Due to the large ratio of surface atoms, these small nanoparticles can expose much more active sites than the bulk, leading to a significant increase in catalytic activity. Furthermore, small nanoparticles and clusters can bring new structural and electronic properties to enhance the catalytic performance. In addition, nanostructures provide an ideal platform for the fundamental investigation of the catalytic mechanism through precise surface engineering.³ With the development of nanoscience and nanotechnology, chemists have gained the ability to synthesize nanostructures with tunable morphologies and sizes.^4–8 Over the past decades, various metal (Pt, Pd, Rh, Ru, etc.) and oxide nanostructures (NiO, FeO_x, CoO_x, CeO₂, etc.) have been successfully synthesized as catalysts for diverse reactions including CO₂ reduction,^9–25 selective hydrogenation,^26–33 CH₄ activation,^34–38 CO oxidation.^39–44 that the catalytic performance of NCs is strongly dependent on their size, shape, composition and interfacial interactions. Recently, it has been gradually realized that compositing a single-component with another can significantly enhance the catalytic performance since the synergistic effects between the two components may bring additional properties to the composite catalyst. Consequently, tremendous effort has been devoted to the modification of NCs, leading to significant progress in nano-catalysis.

In this review, we discuss recent advances and perspective on heterogeneous catalysis using inorganic nanomaterials, with a specific focus on the design principle of high-performance catalysts using metal NCs and oxide NCs. Attention is paid to the shape control and surface engineering of NCs for achieving enhanced catalytic performances in some important heterogeneous catalytic reactions including CO₂ reduction, selective hydrogenation, CH₄ activation, CO oxidation, and oxygen reduction reaction (ORR). Finally, the outlook and perspective in nano-catalysis will be provided. The goal of this review is to motivate researchers to search for more efficient catalysts with high activity, selectivity and stability.

2. Metal NCs

Noble metals (Pt, Pd, Rh, Ru, Au, Ag, etc.) have been used as catalysts for diverse reactions due to their superior activities. Compared to non-noble catalysts, noble metal-based catalysts generally exhibit higher activity because their strong affinity to reactants. Thus, to achieve decent activity at a low cost, noble metals are generally dispersed on porous supports with a large surface area via the well-established wet-impregnation method. Indeed, catalyst preparation via the wet-impregnation method has a long history in industry, and it is still regarded as one of the most popular strategies nowadays. Scientists have empirically realized that the catalytic performance from supported catalysts is strongly dependent on the synthetic parameters, pre-treatment conditions, reduction temperature, and the support. With the progress in science, especially in characterization techniques, scientists have found that these noble metals exist as small nanoparticles, clusters and even isolated atoms. Thus, a variation in the synthesis, pre-treatment and reduction conditions will result in different sizes, morphologies, and geometry and coordination environment of the active sites. Many reports indicate that low-coordinated atoms exhibit much higher activity than that of full-coordinated atoms. For example, NCs with exposed high-index facets generally exhibit higher activity than that with low-index facets (e.g., (100) and (111) facets) because the surface atoms on high-index facets, especially at the edges and corners, are generally low-coordinated. However, NCs with high-index facets are unstable due to their high surface energy. Thus, the stabilization of these high-index facets in the synthetic process and catalytic process is critical to achieve superior catalytic performance in nano-catalysis. Therefore, tremendous effort has been devoted to the stabilization of NCs via surface engineering. In this section, we summarize the recent advances on the shape control and surface engineering of some typical noble metal-based NCs.

2.1 One-dimensional (1-D) metal NCs

Over the past decades, great progress has been achieved in the synthesis of 1-D metal NCs, including nanowires, nanorods and nanotubes. To date, chemists have empirically realized that the preferential protection of some specific facets can enable the anisotropic growth of metal nanoparticles. To obtain 1-D NCs, the anisotropic growth of NCs needs to be induced along some specific facets with the assistance of some specific capping agents. Although the detailed mechanism for the shape control of nanostructures is not completely understood to date, many strategies have been successfully developed for the preparation of 1-D metal NCs. In this section, we focus on this type of metal NCs which have been widely used as catalysts, such as Pt, Pd, and Rh.

2.1.1 Synthesis of 1-D Pt-based NCs.
Synthesis of 1-D Pt NCs. Xia and co-workers synthesized Pt nanowires with a mean length and diameter of 100 nm and 5 nm, respectively, by reducing H₂PtCl₆ with ethylene glycol (EG) in the presence of poly(vinyl pyrrolidone) (PVP).⁴⁵ A trace amount of Fe³⁺ or Fe²⁺ ions exhibited dual functions in the growth of the Pt nanowires as follows: (1) Fe³⁺ or Fe²⁺ ions can induce the aggregation of Pt nanoparticles into larger structures, which serve as nucleation sites and (2) Fe³⁺ or Fe²⁺ ions can greatly decrease the reaction rate and supersaturation level and further induce anisotropic growth. Subsequently, they described the synthesis of Pt single-crystal nanowires using the same precursor, reductant and capping agent on functional solid supports including Pt or W gauze,^46,47 TiO₂,⁴⁸ SiO₂,⁴⁹ Ni(OH)₂,⁵⁰ Ti_0.7Ru_0.3O₂ composite oxide,⁵¹ and sulfur-doped graphene.⁵² Sun et al. demonstrated a wet-chemical method for the growth of Pt single-crystalline nanowires on carbon nanospheres using H₂PtCl₆ and HCOOH as the precursor and reductant, respectively.⁵³ They claimed that the anisotropic growth of the Pt nanowires along the (111) direction was strongly related to the slow reduction rate. Similarly, nitrogen-doped CNTs could also be used as a support for growing Pt nanowires. The obtained Pt nanowires had a mean diameter of 2.5 nm with a length of up to 100 nm.⁵⁴ In addition to the support-induced synthetic method, atomic layer deposition (ALD) is a straightforward method to produce Pt nanowires, in which the obtained NCs are shaped by the support. For instance, Bent et al. prepared Pt nanowires on highly ordered pyrolytic graphite (HOPG) through ALD.⁵⁵ It was shown that Pt was preferentially deposited only at the step edges of HOPG, leading to the formation of laterally aligned Pt nanowires. Shui and Li prepared ultralong Pt nanowires on the centimeter scale with a few nanometers in diameter via electrospinning. It was found that the parameters such as PVP concentration, water/ethanol ratio, feeding rate and electric field strength are critical for the formation of uniform Pt nanowires (Fig. 1).⁵⁶ Ponrouch and co-workers obtained highly porous and preferentially oriented (100) Pt nanowires via electrodeposition. They highlighted that H₂ plays a critical role in the growth of Pt nanowires because the surface free energy of the (100) facet in the presence of H₂ is lower than that of the (111) facet, leading to the growth of predominantly exposed Pt(100) facets.⁵⁷ Besides the success in the synthesis of Pt nanowires, the support also plays a critical role in the anisotropic growth of Pt nanowires, whose functions are not understood to date. On the other hand, the above-mentioned method cannot precisely tune the length and diameter of Pt nanowires, which has seriously impeded their practical application. Therefore, other synthetic methods have been developed to synthesize pure Pt nanowires in solution. For example, Teng and co-workers reported the synthesis of ultrathin Pt nanowires with a width of less than 2.5 nm and a length of over 30 nm via a phase-transfer method, in which inorganic Pt chloride was transferred to a mixture of octadecylamine and toluene in the presence of n-dodecyltrimethylammonium bromide (DTAB). After the addition of sodium tetrahydroborate (NaBH₄) to the aqueous solution, the Pt cations were rapidly reduced and formed thermodynamically unstable elongated primary nanostructures, followed by an anisotropic growth along the (111) direction to form thread-like nanowires.⁵⁸ Subsequently, the organic phase has been widely employed to produce Pt nanowires.^59,60 In addition, the template-synthesis method has been reported for synthesizing Pt nanowires. Choi and co-workers prepared well-defined Pt nanowires with a diameter of 47 ± 9.8 nm and a length of 6 μm via a template-synthesis method by the electrodeposition of Pt within the pores of a track-etched polycarbonate membrane (Fig. 2a).⁶¹ Zhang et al. synthesized ordered porous Pt nanowires with controlled large mesopores (15–45 nm) using dual templates of porous anodic aluminum oxide (AAO) membranes and silica nanospheres self-assembled in the channels (Fig. 2b).⁶² Zhang and co-workers produced highly uniform single-crystal Pt nanowires with a diameter of about 1.8 nm and a superhigh aspect ratio of >104 using insulin amyloid fibrils as sacrificial templates (Fig. 2c and d). It was shown that these fibrils were critical for obtaining the ultralong Pt nanowires with preferential exposure of the low-energy crystal facets.⁶³

Fig. 1 Morphology of electrospun Pt nanowires at different ratios of PVP/H₂PtCl₆: (a) PVP/H₂PtCl₆ = 33.1/10.6 mg mL⁻¹ and (b) PVP/H₂PtCl₆ = 33.1/2.6 mg mL⁻¹. Morphology of the nanowires with H₂O/C₂H₅OH ratios of (c) 0.036 and (d) 0.075. Morphology of the electrospun nanowires at the feeding speeds of (e) 0.1 mL h⁻¹ and (f) 0.2 mL h⁻¹. Reprinted with permission from ref. 56.

Fig. 2 (a) Transmission electron microscopy (TEM) images of the Pt nanowires prepared from the track-etched polycarbonate membrane. Reprinted with permission from ref. 61. (b) Scanning electron microscopy (SEM) image of the mesoporous Pt nanowires and inset: high-magnification SEM image. Reprinted with permission from ref. 62. (c) Low-magnification TEM image of Pt nanowires and (d) high-resolution TEM (HRTEM) image of a single Pt nanowire. Reprinted with permission from ref. 63.

Synthesis of 1-D Pt/noble metal nanocomposites. Despite the on successful synthesis of 1-D Pt nanowires in the above-mentioned reports, several disadvantages need to be overcome before their practical application: (1) a reduction in the amount of Pt in the catalysts due to its high cost; (2) pure Pt catalysts exhibit high activity, but suffer from low selectivity; and (3) pure Pt tends to rapidly deactivate due to sintering and poisoning. Therefore, it is necessary to modify pure 1-D Pt NCs prior to their utilization. A general strategy for modification is to composite Pt with other elements or compounds to form alloys, core@shell nanostructures and heterojunctions. Over the past decades, many strategies have been successfully developed to modify 1-D nanowires, which greatly promote their activity, selectivity and stability in heterogeneous catalysis. Teng and co-workers synthesized Au₂₅Pt₇₅ and Au₄₈Pt₅₂ ultrathin alloyed nanowires with an average width of less than 3 nm via a wet chemistry approach at room temperature.⁶⁴ In these alloyed nanowires, the charge transfer between Au and Pt, especially d-charge depletion at the Au site and d-charge gain at the Pt site were observed, suggesting a strong interaction between Au and Pt. Hong et al. demonstrated that the synthesis of dendritic Au/Pt nanowires could be achieved through an epitaxial growth strategy.⁶⁵ Zhong's group reported the synthesis of ultrathin Pt–Au alloy nanowires featuring a composition-tunable and (111) facet-dominant surface via a modified hydrothermal method (Fig. 3a–d).⁶⁶ The authors emphasized that it was a surfactant-free method; however, the detailed mechanism for shape control was not discussed. Ag has been widely used to alloy with Pt to form Pt–Au nanowires. Cao and co-workers prepared uniform Pt–Ag nanowires via the co-reduction of AgNO₃ and H₂PtCl₆ in a Teflon-lined autoclave at 220 °C for 20 h.⁶⁷ Subsequently, they investigated the mechanism of the growth of Pt–Ag nanowires during the hydrothermal treatment. It was shown that the morphologies of the Pt–Ag NCs were strongly dependent on the amount of AgNO₃ in the growth solution. Specifically, the morphology evolved from nanowires to nanotrees, and then nanochains with a decrease in the amount of AgNO₃. By altering the concentration of AgNO₃, uniform bimetallic Pt–Ag nanowires were obtained (Fig. 3e).⁶⁸ Peng et al. demonstrated that Pt–Ag nanowires could be synthesized in oleylamine and oleic acid through oriented attachment (Fig. 3f). Mechanism studies indicated that the formation of nanowires through oriented attachment is driven by both thermodynamics (surface adsorption energy and potential energy) and kinetics (surface reconstruction). Consequently, the shape control of Pt–Ag NCs is plausible by altering the parameters to create synthetic conditions that meet both the thermodynamic and kinetic requirements.⁶⁹ In addition, it has been reported that hollow Pt–Ag nanowires can be obtained by adding Ag nanowires to a solution of Pt⁴⁺. Because of the difference in the redox potentials, Pt⁴⁺ ions can be reduced by Ag nanowires via a galvanic reaction, leading to the formation of hollow Pt–Ag nanowires.⁷⁰ The galvanic reaction can also be used to synthesize Pt–Au nanowires, in which Pt nanowires can reduce AuCl₃ in the presence of DTAB.⁷¹ Other noble metals including Rh and Ru have also been used to modify 1-D Pt nanowires.^72–77 For instance, Huang's group reported branched Rh–Pt bimetallic ultrathin nanowires through seed displacement and epitaxial growth in aqueous solution containing Rh nanocubes as seeds, K₂PtCl₆ and PVP.⁷⁸ Another work by the same group showed that Rh–Pt nanowires could be obtained using Pt(acac)₂ and Rh(acac)₃ in the presence of oleic acid and oleylamine.⁷⁹

Fig. 3 HRTEM images of (a–c) Pt₇₂Au₂₈ nanowires, where the inserts are the magnified views of areas for the determination of the lattice fringes, and (d) model illustrating the helix exhibited by an individual nanowire. Reprinted with permission from ref. 66. (e) TEM image of the as-prepared Pt_0.8Ag_0.2 nanowires. Reprinted with permission from ref. 68. (f) TEM images of Pt₅₃Ag₄₇ nanowires. Reprinted with permission from ref. 69.

Synthesis of 1-D Pt–Ni NCs. In addition to noble metals, non-noble metals have attracted great attention in the modification of 1-D Pt nanowires. As one of the most important metals, Ni has been widely used to composite with Pt to improve its catalytic performance. Ma and co-workers reported a generic gas–solid method for the preparation of surfactant-free ultrathin Pt–Ni nanowires, in which H₂ was essential for the formation of a unique 1-D morphology.⁸⁰ Yang et al. fabricated multisegmented Pt–Ni nanowires via pulsed electrodeposition using porous AAO membranes as templates (Fig. 4a).⁸¹ It was shown that highly uniform wavy Pt–NiO hollow nanopeapods could be obtained by regulating the oxidation of Ni in the multisegmented Pt–Ni nanowires (Fig. 4b). Bu and co-workers demonstrated a protocol for the synthesis of twisted Pt–Ni nanowires using Pt(acac)₂ and Ni(acac)₂ as the precursors in the presence of hexadecyltrimethylammonium chloride (CTAC), glucose and oleylamine (Fig. 4c and d), in which three steps were involved: (1) formation of ultrathin Pt nanowires, (2) reduction of Ni species on the preformed nanowires, and (3) Pt diffusion into the Ni-rich 1-D nanowires.⁸² Another work from the same group studied the surface engineering of Pt–Ni nanowires through a post-treatment, in which the ratio of Ni and Pt significantly varied after treatment in air or H₂ (Fig. 4e and f).⁸³ Recently, it has been shown that the introduction of non-metallic elements into Pt–Ni nanowires can greatly enhance their catalytic performance. Wang et al. reported a two-step method for synthesizing Pt–Ni/NiS composite nanowires, in which segregated Pt–Ni nanowires were prepared in the first step and then reacted with sulfur in oleylamine (Fig. 5).⁸⁴ Liu et al. achieved sulfur modification in aqueous solution with the introduction of sulfite (e.g., SO₃²⁻ or HSO₃⁻) in an acidic medium.⁸⁵ Moreover, Xie and co-workers modified Pt–Ni nanowires by nitrogen through a high temperature treatment of as-obtained Pt–Ni nanowires (250 °C) in NH₃. The final composite exhibited a structure of Pt–Ni/Ni₄N nanowires after surface modification. Zhang and co-workers coated Pt–Ni nanowires with a metal–organic framework (MOF) to form a Pt–Ni@MOF composite, which could be used as an efficient catalyst for the chemoselective hydrogenation of cinnamaldehyde.{Zhang, 2018 #93}. It should be noted that modifying 1-D nanowires with non-metallic elements (e.g., N, S and C) may provide a novel and efficient strategy to promote their catalytic performance. However, the doping amounts should be carefully controlled since an excess amount may result in strong coordination between the metal and non-metallic elements (e.g., S), leading to the poisoning of the catalysts.

Fig. 4 (a) SEM images of electrodeposited multisegmented Pt–Ni nanowires and (b) SEM images of the multisegmented Pt–Ni nanowires after annealing at 350 °C for 1 h in air. Reprinted with permission from ref. 81. (c) Scanning transmission electron microscopy (STEM) image and (d) TEM image of the 1-D Pt–Ni nanowires. Reprinted with permission from ref. 82. High angle circular dark field (HAADF)-STEM images of the treated Pt₃Ni₃ nanowires/C in (e) air and (f) H₂. Reprinted with permission from ref. 83.

Fig. 5 Representative (a, d, g and j) high-magnification TEM images, (b, e, h and k) HAADF-STEM images and (c, f, i and l) HAADF-STEM images, and corresponding energy dispersive X-ray spectrometry (EDS) elemental mappings (Ni in red, S in green and Pt in blue) of (a–c) Pt₃Ni₁ NWs–S, (d–f) Pt₃Ni₂ NWs–S, (g–i) Pt₃Ni₃ NWs–S and (j–l) Pt₃Ni₄ NWs–S. Scale bars: 20 nm. Reprinted with permission from ref. 84.

Synthesis of 1-D Pt–Fe NCs. Introducing Fe into 1-D nanowires has been regarded another efficient way to enhance the catalytic performance of Pt NCs.^86–89 Wang and co-workers synthesized Pt–Fe nanowires with diameters in the range of 2–3 nm in the presence of oleylamine and octadecene.⁹⁰ By controlling the reduction of Pt(acac)₂ and decomposition of Fe(CO)₅, the length of the Pt–Fe nanowires could be tailored from 20 to 200 nm (Fig. 6a). Zhu et al. used a seed-induced method to synthesize Pt–Fe nanowires in the presence of oleylamine, oleic acid and 1-octadecene.⁴³ It was reported that the diameter of the Pt–Fe nanowires could be systematically tuned through a secondary seeded growth step, in which the as-prepared Pt–Fe nanowires were used as seeds and Fe(CO)₅ was used as the precursor.⁹¹ Kou's group prepared ultrafine Pt–Fe nanowires using Fe(Ac)₂ as the precursor in the presence of W(CO)₆, glucose, CTAC, oleylamine and 1-octadecene (Fig. 6b).⁹² Interestingly, when glucose in the growth solution was replaced by phloroglucinol (anhydrous), the obtained Pt–Fe nanowires exhibited a wicker-like morphology (Fig. 6c).⁹³ They claimed that the smooth nanowires connected with each other and gradually grew into uneven interfacial nanowires with branch-rich exteriors. Moreover, Luo and co-workers used Fe(acac)₃ as the precursor of Fe to synthesize zigzag-like Pt–Fe nanowires in the presence of Pt(acac)₂, glucose, CTAC and oleylamine (Fig. 6d–f).⁹⁴ Similarly, Bai et al. prepared zigzag-like Pt–Fe nanowires in organic solution using a different Fe precursor (e.g., Fe₂(CO)₉) and reductant (e.g., benzoin) (Fig. 6g–j).⁶⁰ Thus, based on the above reports, it can be concluded that the changes in the types of precursor, reductants and surfactants in colloidal synthetic methods can systematically tune the final structures and morphologies.

Fig. 6 (a) TEM images of Fe₅₅Pt₄₅ nanowires with a length of 200 nm. Reprinted with permission from ref. 90. (b) TEM image of ultrafine Pt₃Fe nanowires. Reprinted with permission from ref. 92. (c) TEM image of Pt₃Fe nanowires. Reprinted with permission from ref. 93. (d) Low- and (e) high-magnification HAADF-STEM images and (f) STEM-EDS elemental mapping of Pt₃Fe zigzag-like nanowires. Reprinted with permission from ref. 94. (g) HAADF-STEM image with low magnification and high magnification (inset), (h) HAADF-STEM image and elemental mappings, (i) TEM image and (j) HRTEM image of Pt–Fe nanowires. Reprinted with permission from ref. 60.

Synthesis of 1-D Pt–Co NCs. Co, which is located in the same group in the periodic table, has also been widely used to composite with 1-D Pt NCs.^95–100 It has been shown that the synthetic method for Pt–Co nanowires is similar to that of Pt–Ni nanowires by replacing the Ni precursor (e.g., NiCl₂) with a Co precursor (e.g., CoCl₂).^80,85 Generally, CoCl·6H₂O and Co(acac)₃ are used as Co precursors, and H₂PtCl₆·6H₂O and Pt(acac)₂ are correspondingly used as Pt precursors for the preparation of Pt–Co nanowires. Based on the physicochemical properties of these precursors, Pt–Co nanowires can be obtained in both aqueous and organic solution. Owing to the low boiling point of water, the aqueous-phase synthesis of Pt–Co nanowires is performed in a pressed Teflon-lined stainless-steel autoclave to reach the desired temperature. By contrast, organic solvents, such as oleylamine, oleic acid and 1-octadecene, have a much higher boiling point than water. Therefore, the organic-phase synthesis of Pt–Co nanowires is performed in a glass reactor. Lu and co-workers synthesized Pt–Co nanowires via a hydrothermal method using H₂PtCl₆·6H₂O and CoCl·6H₂O as precursors in the presence of KOH, EG and dimethyl formamide (DMF) (Fig. 7a–e).¹⁰¹ The mechanism studies showed that small nanoparticles, elongated nanorods, and some nanochains were formed in the initial stage, and further assembled into a wire-like structure to reduce the surface energy under the solvothermal conditions, which is consistent with the typical oriented attachment mechanism. Bu et al. demonstrated a protocol for the synthesis of Pt–Co nanowires in the organic phase, in which Pt(acac)₂ and Co(acac)₃ were used as precursors, oleylamine as the solvent and surfactant, CTAC as the structure-directing agent, and glucose as a reducing agent (Fig. 7f–h).{Bu, 2016 #111} Guo et al. used a similar organic method to prepare Pt₃Co nanowires for the electro-reduction of oxygen (Fig. 7i and j).¹⁰² Moreover, Bai and co-workers synthesized zigzag Pt₄Co nanowires using Pt(acac)₂ and Co(acac)₃ as precursors in the presence of glucose, CTAC and oleylamine (Fig. 7k). It was shown that the zigzag Pt₄Co nanowires with abundant steps/edges and Pt-rich surface could be used as highly efficient catalysts for the hydrogenation of CO₂.¹⁰³

Fig. 7 (a) HAADF-STEM images and (b–e) elemental mapping images of Pt₉₅Co₅ nanowires. Reprinted with permission from ref. 101. (f) STEM, (g) TEM and (h) STEM-ADF images and EDS elemental mappings of the hierarchical Pt₃Co nanowires. Inset in (f) is an enlarged STEM image. The scale bars in (f), inset of (f), (g) and (h) are 200, 20, 10 and 10 nm, respectively. Reprinted with permission from ref. 104. (i) TEM image and (j) STEM-EDS elemental mapping of as-synthesized Pt₃Co nanowires.¹⁰² (k) HAADF-STEM image of Pt₄Co nanowires. Inset in (k) is an enlarged HAADF-STEM image. The scale bars in (k), inset of (a) are 100 and 20 nm, respectively. Reprinted with permission from ref. 103.

Synthesis of other 1-D Pt-based nanocomposites. Yu et al. fabricated Pt–Cu nanowires using Pt(acac)₂ and Cu(acac)₂ in the presence of tetrabutylammonium bromide and oleylamine via a well-established oriented attachment process.¹⁰⁵ Zhang and co-workers prepared screw thread-like Pt–Cu nanowires with high-index facets and controllable compositions in oleylamine via a colloidal synthesis method, in which Pt(acac)₂ and CuCl₂·2H₂O were used as metal precursors, respectively (Fig. 8).¹⁰⁶ Furthermore, Cao et al. synthesized Pt–Cu nanowires in aqueous solution using Na₂PtCl₆·6H₂O and CuCl₂·6H₂O as the precursors via the classic hydrothermal method. The proposed mechanism can be briefly described as follows: (1) reduction of precursors by NaBH₄ solution; (2) Pt and Cu atoms emerge, aggregate and fuse to form Pt–Cu nuclei, accompanied by the generation of abundant hydrogen bubbles from the hydrolysis and oxidation of NaBH₄; and (3) Pt–Cu crystal nuclei anchor and then grow on the surfaces and interspaces of hydrogen the bubbles via self-assembly to form Pt–Cu nanowires.¹⁰⁷ Similarly, Liao and co-workers obtained ultrathin (ca. 1 nm) and long Pt–Cu nanowires up to the micrometer scale using EG as the reductant (Fig. 9a and b).¹⁰⁸ The above two cases indicate that the reduction kinetics may strongly influence the obtained morphologies of Pt–Cu nanowires. Compared to NaBH₄, which can rapidly reduce the precursors, EG is a much weaker reductant and it can slow down the reduction rate, leading to the formation of long nanowires. In addition, the galvanic reaction can also be used for the synthesis of Pt–Cu nanowires due to the different redox potentials of Pt⁴⁺/Pt and Cu²⁺/Cu. Alia et al. synthesized Pt-coated Cu nanowires via the partial galvanic displacement of Cu nanowires. The obtained Pt–Cu nanowires possessed a mean diameter and length of 100 nm and 25–40 μm, respectively.¹⁰⁹ The compositions of Pt–Cu nanowires can be tuned by controlling the degree of galvanic displacement, where complete displacement will lead to pure Pt nanowires.¹¹⁰ In addition, Xu and co-workers synthesized zigzag-like Pt–Zn alloy nanowires with high-index facets. In this synthetic protocol, Pt(acac)₂ and Zn(acac)₂ were used as precursors, glucose as a reductant, and CTAC/oleylamine as surfactants.¹¹¹ However, compared to Pt–Co and Pt–Fe, reports on Pt–Zn nanowires are still limited, which may probably due to their moderate performance.

Fig. 8 (a) Low-magnification TEM image, (b and e) high-magnification TEM images, (c) HAADF-STEM image, and (d) HAADF-STEM image and mapping images of PtCu_1.8 nanowires. (f) HRTEM and (g) TEM image and corresponding selected-area-electron diffraction (SAED) (inset) of a single PtCu_1.8 nanowire. Reprinted with permission from ref. 106.

Fig. 9 (a) Field emission SEM (FESEM) image and (b) TEM image of Pt₃₂Cu₆₈ nanowires. Reprinted with permission from ref. 108. (c) TEM image, (d) HAADF-STEM image, (e) aberration-corrected atomic-resolution STEM image and (f) STEM-EDS elemental mapping of as-prepared zigzag Pt–Zn nanowires. Reprinted with permission from ref. 111.

2.1.2 Synthesis of 1-D Pd-based NCs.
Synthesis of 1-D Pd NCs. The solution-phase approach is regarded as a powerful and efficient protocol for the synthesis of Pd NCs. In general, the final morphologies of Pd NCs may be tuned by manipulating the thermodynamics or kinetics during their growth. In the thermodynamic mode, the obtained NCs tend to expose the facets with the lowest surface energy, in which surface capping agents play a vital role in shape control. In the kinetical mode, the shapes of Pd NCs can be tuned by adjusting the reduction kinetics.^112,113 Template-assisted methods were used to synthesize Pd nanowires in earlier stages since the size and shape of NCs can be easily directed by templates. Lee et al. reported the preparation of Pd nanowires with a diameter of less than 10 nm using mesoporous silicate materials with different pore sizes and SBA-15 as templates.¹¹⁴ During the synthetic process, the Pd precursor was loaded into the template matrix via chemical vapor infiltration, followed by mild thermal decomposition to generate Pd nanowires inside the template. Finally, freestanding Pd nanowires were obtained by removing the silicate templates. Koenigsmann et al. reported the synthesis of Pd nanowires using polycarbonate or AAO membranes as a template.¹¹⁵ In addition to the hard template, some surfactants can act as soft templates for synthesizing Pd nanowires. Yu and co-workers used didodecyldimethylammonium chloride (DODAC) as a soft template to shape the morphology.¹¹⁶ It was shown that the double hydrophobic carbon chains of DODAC can make the mesophase more stable owing to the stronger hydrophobic interaction and effectively confine the crystalline growth of Pd nanostructures among the cylinders of a hexagonal micellar structure. Despite the great success in the template-assisted method for the synthesis of Pd nanowires, some disadvantages are still remaining. The key limitation of template-assisted methods is that they do not readily afford rational control over the diameter and length of the resulting nanowires. Therefore, other methods have been developed for the synthesis of Pd nanowires. Liang et al. developed a new template-assisted chemical process for the controllable synthesis of uniform Pd nanowires/nanotubes with an ultrahigh aspect ratio of 10 000, in which ultrathin Te nanowires were used as both a reducing agent and sacrificial template.¹¹⁷ To obtain uniform Pd nanowires, the reduction kinetics needs to be precisely controlled. Wang et al. systematically studied the relationship between the reaction kinetics and the obtained morphologies of Pd NCs. It was shown that two major parameters were critical for obtaining Pd nanowires, i.e., fast reaction kinetics during the nucleation stage and low surface charge of the small Pd particles generated in the early stage.¹¹⁸ Teng et al. developed a phase-transfer method for the synthesis of Pd nanowires, in which palladium nitride was transferred to a mixture of octadecylamine and toluene in the presence of DTAB, a phase-transfer agent.⁵⁸ Huang and Zheng prepared 5-fold twinned Pd nanowires using PVP as the reductant in the presence of sodium iodide (Fig. 10a–f).¹¹⁹ It was shown that both PVP and I⁻ were critical for the formation of the 5-fold twinned Pd nanowires. As one of the most famous shape-directing agents, PVP can also be used as a reductant during synthesis. I⁻ can preferentially bind to the (100) facet, leading to the anisotropic growth of NCs. The above conclusion was further verified by Huang and co-workers, who synthesized penta-twinned Pd nanowires with diameters of less than 8 nm and aspect ratios of up to 100 in the presence of PVP and NaI (Fig. 10g–j). In this case, I⁻ not only can coordinate with Pd (PdI₄²⁻) to slow the reaction rate, but also preferentially bind to the (100) facet, leading to the formation of penta-twinned nanowires.¹²⁰ Moreover, Chen and co-workers used a seed-mediated synthesis process to prepare uniform Pd nanorods with average lengths of ∼200 and 300 nm with the addition of copper acetate. It was shown that copper acetate is critical for the formation of Pd nanorods since Cu has a similar but lower reduction potential than that of Pd, which results in the periodic deposition and reoxidation of the Cu atoms on the Pd nanorods and faceted particles, leading to the formation of nanorods and other shaped NCs.¹²¹ In addition to chemical methods, several techniques have also been employed for the preparation of Pd nanowires. Bhuvana and Kulkarni prepared Pd nanowires via direct-write electron beam lithography.¹²² On the other hand, Yoo et al. reported the growth of twin-free single-crystalline Pd nanowire arrays on an SrTiO₃(110) substrate in a very high density. In particular, Pd powder was vaporized at 1100–1250 °C and transported to the lower temperature region by a carrier gas. Consequently, twin-free epitaxial Pd nanowires were formed on the substrate.¹²³

Fig. 10 (a–d) TEM images of as-prepared Pd nanowires. (e and f) HRTEM image and SAED patterns of individual nanowires. Reprinted with permission from ref. 119. (g and h) TEM images at low and high magnifications, respectively, and (i) HRTEM image of as-prepared Pd nanowire. The red arrow and dotted line indicate the twin boundary along the long axis of the nanowire. (j) Fourier transform pattern derived from the lattice fringes in panel (i). Reprinted with permission from ref. 120.

Synthesis of 1-D Pd/noble metal nanocomposites. Many physical and chemical methods, including the electrodeposition method,^124,125 template-assisted synthesis,^126–131 polyol-assisted synthesis,^132,133 galvanic replacement reaction,^134–136 hydrothermal and solvothermal treatment,¹³⁷ seed-mediated synthesis,¹³⁸ and chemical reduction,^139–141 have been employed for the modification of Pd nanowires over the past decades. Jang et al. reported the synthetic of hollow Pd–Ag alloy nanowires via the electrodeposition of lithographically patterned Ag nanowires, followed by galvanic replacement reaction to form Pd. The relative atomic ratio between Pd and Ag could be controlled by controlling the reaction time of aligned Ag nanowires and Pd²⁺.¹²⁵ Yang and co-workers prepared superlattice-structured Pd–Cu nanowires with random-gapped, screw-threaded and spiral shapes via electrodeposition on a template of AAO.¹²⁵ Koenigsmann et al. developed a template-based technique for the preparation of a series of bimetallic Pd–Au and Pd–Pt nanowires with control over their composition and size without surfactants.¹²⁹ Zhu et al. developed a facile method for the synthesis of Pd–Pt and Pd–Au nanowires, in which Te nanowires were employed as a sacrificial template and reducing agent (Fig. 11a).¹³⁵ Moreover, Li and co-workers reported a scalable template-assisted method for the preparation of ultrathin and uniform Pd@Pt core@shell nanowires with a controllable composition and shell thickness, high aspect ratio, and smooth surface, in which Te NWs were employed as the sacrificial template and reductant (Fig. 11b–i).¹³⁴ In another work from the same group, tri-metallic PdPtTe nanowires were produced with a similar method, in which the composition of the Pd–Pt–Te nanowires was controlled by the amount of Pt and Pd precursors in the growth solution.¹²⁶ In addition to Te nanowires, Cu nanowires can also be used as a sacrificial template. For instance, Li and co-workers fabricated trimetallic Pt–Pd–Cu and bimetallic Pt–Cu nanotubes in dimethyl sulfoxide using Cu nanowires as templates.¹²⁷ Teng and co-workers reported the synthesis of Pd–Au nanowires through the galvanic replacement reaction between Pd ultrathin nanowires and Au precursor (e.g., AuCl₃), in which toluene, DTAB and octadecylamine were used as the solvent, phase transfer agent and capping agent, respectively.¹³⁶ To obtain well-alloyed bimetallic Pd nanowires, the reduction rate of the different precursors in the growth solution needs to be precisely controlled. NaBH₄ is one of the most common reductants in the colloidal synthesis of NCs, which can rapidly reduce noble metal ions in solution.¹⁴² Zhu et al. used NaBH₄ to reduce Na₂PdCl₄ and HAuCl₄ for the synthesis of twisted Pd–Au nanowires without any templates.¹⁴³ It should be noted that the morphologies are determined by many other parameters in chemical reduction methods, such as solvent type, surfactant type, halide ion, and reaction temperature.¹⁴²

Fig. 11 (a) TEM image of the obtained Pd₈₀Pt₂₀ nanowires. Reprinted with permission from ref. 135. TEM images of the Pd@Pt nanowires with Pt atomic ratios of (b) 6.0%, (c) 13.5%, (d) 21.2% and (e) 28.5%, (f) HRTEM image of Pd@Pt nanowires with a Pt atomic ratio of 21.2%. (g–i) EDS mapping analysis showing the elements in the Pd@Pt nanowires with a Pt atomic ratio of 21.2%. Reprinted with permission from ref. 134.

Synthesis of 1-D Pd/non-noble metal nanocomposites. The modification of 1-D Pd NCs with non-noble metals has attracted great attention because it can greatly decrease the usage of noble metals and enhance the catalytic performance. As one of the most common non-noble metals, Fe has been widely used to composite with 1-D Pd NCs. Shui and co-workers synthesized Pd–Fe nanowires by electrospinning. Although the composition of the Pd–Fe nanowires was not uniform, the electrospinning method provides a simple strategy to obtain long nanowires up to millimeters in length (Fig. 12a).¹⁴⁴ Wang et al. synthesized trimetallic PdPtFe nanowires using Pd(acac)₂, Pt(acac)₂ and Fe(CO)₅ in the presence of oleylamine, 1-octadecene and sodium oleate (Fig. 12b). This method combined template-assisted synthesis and galvanic replacement reaction. Fe–Pt nanowires were pre-prepared as a template, and further reacted with a Pd precursor to form Pd–Pt–Fe nanowires through a galvanic replacement reaction (Fe + Pd²⁺ → Fe²⁺ + Pd).⁸⁶ Oleylamine plays a vital role in the synthesis, which self-organizes into an elongated reverse-micelle-like structures to direct the growth of the Pt–Fe nanowires.⁹⁰ Du and co-workers demonstrated the synthesis of Pd–Co nanowires with different Co contents via electrodeposition on AAO.¹²⁸ As a template, AAO provides nanochannels for the electrodeposition of Pd and Co precursors, which can be removed to obtain uniform Pd–Co nanowires (Fig. 12c–k). On the other hand, Imura et al. used a template-assisted method for the synthesis of Pd–Ni nanowires in the presence of a long-chain amidoamine derivative. Different from hard templates, soft templates, such as some specific surfactants, can form micelles to direct the anisotropic growth of nanowires.¹⁴⁵ Soft template-assisted methods have been widely used in the synthesis of 1-D Au NCs.^146–150 In addition, other non-noble elements, such as Ce¹³² and Cu,¹²⁵ were reported to composite with Pd nanowires.

Fig. 12 (a) SEM image of the obtained PdFe₅ nanowires. Reprinted with permission from ref. 144. (b) TEM image of Pd–Pt–Fe nanowires. Reprinted with permission from ref. 86. (c) Schematic of the synthesis and integration of Pd–Co nanowires including electrodeposition, removal of AAO and integration. (d) Top-view and (e) side-view SEM images of Pd–Co nanowire arrays. (f) TEM image of Pd–Co NWs, and overlapped (g) and separated elemental mappings of (h) Pd, (i) Co and (j) O. (k) HRTEM image with the inset SAED pattern taken from the dashed rectangle and circle in (d), respectively. The scale bars in inset (k) are 1/(10 nm). Reprinted with permission from ref. 128.

2.1.3 Synthesis of other 1-D metal-based NCs.
Synthesis of 1-D Au NCs. The synthesis of Au nanorods can be dated back to more than 30 years ago. Many review papers have been published with respect to the synthesis of Au nanorods.^{148,151–153} Thanks to the pioneering work from Murphy et al., seeded growth has been widely used for the synthesis of Au nanorods, in which CTAB and silver ions are used as the surfactant and directing agent, respectively.^154,155 Three popular mechanisms were proposed for explaining the effects of CTAB and silver during the synthesis of Au nanorods: (1) silver deposits on the longitudinal faces of Au nanorods and enables the anisotropic growth of Au nanorods; (2) CTAB reacts with silver to form a complex of CTA–Br–Ag⁺, which can be used as a facet-specific capping agent for the growth of Au nanorods; and (3) the existence of silver can alter the shape of CTAB micelles from spherical to cylindrical shapes, which can be used as a soft template for the growth of Au nanorods.¹⁵¹ Ye et al. used a binary system including CTAB and sodium oleate for the synthesis of Au nanorods. It was found that the presence of sodium oleate could significantly improve the morphological yield of Au nanorods (>98%).¹⁵⁶ Later, we used CTAC and sodium oleate as binary surfactants for revealing the mechanism for the synthesis of Au nanorods. In this work, it was found that sodium oleate could regulate the kinetics via the pre-reduction of Au³⁺ to Au⁺. On the other hand, the CTAC could react with Br⁻ and Ag⁺ to form CTA–Br–Ag⁺, a key complex for the formation of Au nanorods (Fig. 13).¹⁴⁹

Fig. 13 (a) Powder X-ray diffraction (XRD) patterns of different compounds: (from top to bottom) AgBr, CTAC, CTAB, CTA–Br–Ag–Br, and CTA–Br–Ag–Cl. (b) UV-vis spectra of Au nanorods prepared using CTA–Br–Ag–Cl (black curve) and CTA–Br–Ag–Br (red curve) as the ligands. (c and d) TEM images of Au nanorods prepared by using (c) CTA–Br–Ag–Cl and (d) CTA–Br–Ag–Br as the ligand. TEM images of Au nanorods prepared by dissolving AgBr crystals in (e) CTAC and (f) CTAB solution. Reprinted with permission from ref. 149.

Synthesis of 1-D Ag NCs. The seeded growth method has also been used for the synthesis of Ag nanorods. Jana and co-workers reported a seed-mediated growth approach for the synthesis of Ag nanorods with various aspect ratios, in which nearly 4 nm spherical Ag nanoparticles were used as seeds.¹⁵⁷ Similarly, Pietrobon et al. synthesized monodisperse size-controlled faceted Ag nanorods via the thermal regrowth of decahedral silver nanoparticles in the presence of citrate at 95 °C.¹⁵⁸ It was found that the rod length could be varied precisely during the regrowth, while the width was determined by the choice of decahedra seeds since rod growth occurred exclusively along the five-fold axis. Rekha et al. prepared silver nanorods using a modified seed-mediated method. It was found that the aspect ratios of these nanorods could be regulated by controlling the amount of seed solution during the synthesis. Ni et al. studied the mechanism of the seeded growth method for the synthesis of Ag nanorods via a seed-mediated method.¹⁵⁹ The wormlike micelles of cetyltrimethylammonium tosylate (CTAT) were characterized by HRTEM. Ag nanorods grew from multiply twinned decahedral seeds, and the blockage of circumferential growth led to an increase in the aspect ratio of the Ag nanorods. Mirkin's group demonstrated that Ag nanorods could be obtained via the plasmon excitation of Ag seed particles in the presence of Ag⁺ and trisodium citrate (Fig. 14). It was found that a red shift of the excitation wavelength of the Ag spherical seed particles reduced the rate of Ag⁺ reduction and led to the deposition of silver onto the tips of the growing nanorods compared to their sides, resulting in the generation of higher aspect ratio rods.¹⁶⁰ On the other hand, Hu et al. reported a template-less and non-seed process for the synthesis of Ag nanorods and nanowires, in which silver nitrate was reduced by tri-sodium citrate in the presence of sodium dodecylsulfonate. Furthermore, the concentration of trisodium citrate was crucial for the synthesis of Ag nanocrystals, and sodium dodecylsulfonate acted an assistant in controlling the diameters and aspect ratios of the products.¹⁶¹

Fig. 14 (a) TEM image of the silver seed nanoparticles. (b) SEM and (c) TEM images of silver nanorods synthesized with a bandpass filter centered at 600 ± 20 nm. (d) Selective-area electron diffraction (SAED) pattern of a single silver nanorod, showing the interpenetrating [100] (red) and [112] (blue) zone patterns. (Scale bars: 100 nm). Reprinted with permission from ref. 160.

In addition to above-mentioned cases, other 1-D metal-based NCs have been synthesized as catalysts. Fu and co-workers synthesized ultrathin wavy Rh nanowires using Na₃RhCl₆ and sodium ascorbate in the presence of PVP.¹⁶² Although the detailed mechanism was not mentioned, the halide ions (KI) played a critical role in the formation of the Rh nanowires. It was shown that NaI can also be used as an I⁻ source for Rh nanowires, while the other parameters are similar to that in Fu's work.¹⁶³ Mohamed and co-workers prepared Rh nanowires using a DNA template-assisted method. In this work, RhCl₃ was used as a precursor, which could be reduced by an electrochemical method and NaBH₄.¹⁶⁴ Schoiswohl et al. modified Rh nanowires by decorating the steps of vicinal Rh(111) surfaces with Ni. After modification, the Rh–Ni nanowires exhibited much higher chemical reactivity towards oxygen than pure Rh nanowires.¹⁶⁵ Liu and co-workers prepared single-crystalline RuO₂ nanowires via a thermal evaporation method.¹⁶⁶ It was shown that the diameter and length of the RuO₂ nanowires could be tailored by altering the growth time.¹⁶⁶ In addition, other 1-D metal NCs, including Fe, Co, and Ni, were synthesized.^167–169 Compared to noble metal-based NCs, there are only a few reports on the synthesis of non-noble metal NCs, which may be attributed to the fact that noble metals exhibit much higher catalytic activity than non-noble metals.

2.2 Two-dimensional metal NCs

2.2.1 Synthesis of 2-D Pt-based nanocomposites.
Synthesis of 2-D Pt NCs. Template-assisted methods have been widely used for the synthesis of 2-D Pt nanosheets or ¹⁴⁹dendritic Pt nanosheets using multilamellar 1,2-dioctadecanoyl-sn-glycero-3-phosphocholine vesicles as templates in ascorbic acid solution. Under electron-beam irradiation, the dendritic nanosheets gradually evolved to metastable “holey” nanosheets, leading to an improved durability in electrocatalytic reactions due to their remarkable ripening resistance.¹⁷⁰ Kijima and co-workers synthesized nanohole-structured single-crystalline Pt nanosheets via the reduction of a Pt precursor (e.g., Na₂PtCl₆) confined to the lyotropic liquid crystals of polyoxyethylene sorbitan monooleate.¹⁷¹ Furthermore, 2-D Pt nanosheets or nanoplates can be obtained by partially sacrificing the templates in bimetallic Pt-based nanoplates. For example, Chhetri et al. reported the synthesis of free-standing Pt nanosheets by the inducing mechanochemical transformation of Te template nanorods via galvanic displacement.¹⁷² Because Te is highly dissolvable in acid, it can be easily etched by chemical reaction. Feng et al. reported a protocol for the synthesis of porous Pt nanosheets, in which the Te in PtTe₂ nanowires was removed by acetic acid.¹⁷³ Liu and co-workers prepared ultrathin Pt nanoplates (2 nm) by etching the Ag templates from Ag@Pt core@shell nanoplates with concentrated nitric acid (Fig. 15).¹⁷⁴ Moreover, Funatsu et al. reported the synthesis of monolayer Pt nanosheets with a homogeneous thickness via the exfoliation of layered Pt precursor materials with the addition of sodium dodecyl sulfate (SDS) during the electrochemical reduction.¹⁷⁵

Fig. 15 (a) Low-magnification TEM image of ultrathin Pt nanoplates. (b) TEM image of the ultrathin Pt nanoplates. The arrow indicates a defect. (c) HRTEM image of an individual ultrathin Pt nanoplate imaged with an electron beam perpendicular to the basal (111) facet. Inset: Typical SAED pattern. Black and white arrows indicate the (220) and the formally forbidden 1/3(422) diffractions, respectively. (d) HRTEM image of a vertically aligned ultrathin Pt nanoplate. Lines indicate the alignment of the atoms along the [111] direction. Inset: A low-magnification TEM image. Reprinted with permission from ref. 174.

Synthesis of 2-D Pt-based NCs. Bi has been widely used to composite with Pt to form Pt–Bi nanoplates. Qin et al. synthesized Pt–Bi nanoplates using Pt(acac)₂ and Bi(Ac)₃ as precursors in the presence of NH₄Br, 1-octadecene and oleylamine.¹⁷⁶ Wang and co-workers obtained hexagonal Pt–Bi nanoplates in a mixture of PVP, CH₃COOH and DMF, while Pt(acac)₂ and Bi(Ac)₃ were used as precursors.¹⁷⁷ Zhang's group recently synthesized Pt–Bi nanoplates using Pt(acac)₂ and bismuth neodecanoate as precursors in the presence of DEG and PVP. The detailed mechanism studies showed that Pt²⁺ ions were preferentially reduced in the initial stage, which acted as the seeds for the latter growth process.¹⁷⁸ To achieve the anisotropic growth of Pt–Bi nanoplates, both precise control of the reduction kinetics and suppression of the growth along the (100) facet are necessary. Generally, the thickness of Pt–Bi nanoplates is related to the capping ability of the surfactant. Feng et al. revealed that the introduction of CO could act as both a reducing agent and capping agent during the synthesis of PtBi nanoplates.¹⁷⁹ However, the effects of CO has not been discussed to date, which may be attributed to the strong adsorption of CO on the Pt surface. Similarly, Pt–Pb nanoplates were obtained when the Bi precursor was replaced by a Pb precursor.¹⁸⁰ Sun et al. reported a synthetic protocol for Pt–Pb nanoplates by adding Pt(acac)₂ and Pb(acac)₂ to a mixture of L-ascorbic acid, oleylamine and 1-octadecene.¹⁸¹ Bu and co-workers synthesized hexagonal Pt–Pb@Pt core@shell nanoplates using Pt(acac)₂ and Pb(acac)₂ in the presence of L-ascorbic acid, oleylamine and 1-octadecene (Fig. 16). It was found that the edge-Pt and top (bottom)-Pt(110) facets underwent large tensile strain, which helped optimize the Pt–O bond strength, leading to a superior catalytic performance towards the ORR.¹⁸² Luo and co-workers fabricated intermetallic Pt₄₅Sn₂₅Bi₃₀ nanoplates via a wet-chemistry method through the thermal decomposition of Pt(acac)₂, SnCl₂ and bismuth acetate [Bi(act)₃] in a mixture of oleylamine and octadecene.¹⁸³ The detailed characterizations revealed that the structure of Pt₄₅Sn₂₅Bi₃₀ nanoplates consisted of the intermetallic (100) plane of Pt–Sn and Pt–Bi (Fig. 17). In addition, other metals, such as Co,¹⁸⁴ Cu¹⁸⁵ and Ag,¹⁸⁶ were reported to composite with 2-D Pt nanoplates.

Fig. 16 Morphology and structure characterization of Pt–Pb hexagonal nanoplates. Representative (a) HAADF-STEM image, (b) TEM image, (c) EDS, and (d) X-ray diffraction (XRD) pattern of Pt–Pb hexagonal nanoplates. (e) SAED and (f) HRTEM of a single hexagonal nanoplate. Insets of (f) are the fast Fourier transform (FFT) patterns from the white squares at the edge of and inside the nanoplate, respectively. (g) Elemental mapping of Pt–Pb hexagonal nanoplates: HAADF-STEM image, Pt mapping in green, Pb mapping in red, and integrated mapping of Pt and Pb. The compositional ratio between Pt/Pb is 55.9/44.1. Reprinted with permission from ref. 182.

Fig. 17 (a) TEM image, (b) EDS spectrum, and (c) XRD pattern of Pt₄₅Sn₂₅Bi₃₀ nanoplates. The inset in (c) shows the unit cell of Pt₅₀Sn₂₅Bi₂₅ intermetallic, in which green, purple, and yellow spheres represent Pt, Sn, and Bi atoms, respectively. (d) Aberration-corrected HAADF-STEM image of a typical hexagonal nanoplate. (e) High-resolution HAADF-STEM image and corresponding EDS mapping of the representative area in the nanoplate. (f) Schematic illustration of the atomic arrangement in the nanoplate. Reprinted with permission from ref. 183.

2.2.2 Synthesis of 2-D Pd-based NCs.
Synthesis of 2-D Pd NCs. In 2009, Siril et al. reported a protocol for the production of ultrathin Pd nanosheets in emulsions formed by a ternary system (water/toluene containing Pd complexes/cetyltrimethylammonium bromide (CTAB)) and in quaternary hexagonal mesophases formed by water/toluene containing Pd complexes and CTAB/surfactant.¹⁸⁷ They highlighted the critical effects of CO on the formation of Pd nanosheets. Subsequently, Huang and wo-workers modified this method by replacing the surfactant and solvent by PVP/CTAB and DMF, respectively, while CO was still used as a surface-confining agent.¹⁸⁸ The freestanding hexagonal Pd nanosheets were as thin as 10 atomic layers, which exhibited a well-defined but tunable surface plasmon resonance peak in the near-infrared region (Fig. 18). The effects of CO on the formation of ultrathin Pd nanosheets were also highlighted in other reports.^189–195 Li et al. demonstrated the shape-controlled synthesis of ultrathin Pd nanosheets by simply mixing a Pd carbonyl complex [Pd₂(μ-CO)₂Cl₄]²⁻ with H₂O in the absence of any organic capping agents. The mechanism studies showed that the CO ligands in the complex serve as both a reductant and capping agent for the formation of Pd nanosheets.¹⁹⁶ In another work from the same group, the authors demonstrated the face-to-face assembly of two-dimensional Pd nanosheets into one-dimensional Pd superlattice nanowires in the presence of trace amounts of Fe³⁺ and Al³⁺, in which the surfactant-free feature plays a critical role in the assembly of the Pd nanosheets into superlattice nanowires.¹⁹⁷

Fig. 18 (a) TEM image of Pd nanosheets. Inset: Photograph of an ethanol dispersion of the as-prepared Pd nanosheets in a cuvette. (b) HRTEM image of a Pd nanosheet flat lying on the TEM grid. (c) SAED pattern of a single Pd nanosheet (shown in the inset). (d) TEM image of an assembly of Pd nanosheets perpendicular to the TEM grid. Inset: Thickness distribution of the Pd nanosheets. Reprinted with permission from ref. 188.

Synthesis of 2-D Pd nanocomposites. 2-D composite Pd-based nanosheets or nanoplates are generally synthesized via the direct co-reduction of precursors and epitaxial growth of other metals on Pd nanoplates. Yang and co-workers synthesized Pd–Cu nanosheets via the co-reduction of Pd(acac)₂ and Cu(acac)₂ in the presence of Mo(CO)₆.¹⁹⁸ Li et al. used Na₂PdCl₄ and CuCl₂·2H₂O as precursors to produce Pd–Cu nanoplates in the presence of W(CO)₆, acetic acid, and DMF.¹⁹⁹ Furthermore, the precursors of Na₂PdCl₄ and CuCl₂ could be reduced by CO to form Pd–Cu nanoplates.²⁰⁰ As discussed before, CO not only serves as a reducing agent, but also can strongly confine the formation of ultrathin nanoplates or nanosheets. For example, Li et al. synthesized surfactant-free PdFe ultrathin nanosheets via a hydrothermal method. It was shown that CO played a vital role in the anisotropic growth of the nanosheets, which acted as a capping and reducing agent.²⁰¹ Moreover, Cheong and co-workers obtained free-standing Pd–Ni alloy wavy nanosheets in a Teflon-lined high-pressure vessel by charging with 4 atm of CO.²⁰² Other structure-directing agents, such as NH₄Br, have been reported to direct the anisotropic growth 2-D nanoplates. Tang et al. synthesized Pd₃Pb/Pd nanoplates in an organic solution of oleylamine/1-octadecene, in which Pd(acac)₂ and Pb(HCOO)₂ were used as the metal precursors, and glucose and NH₄Br as the reducing agent and structure-directing agent, respectively (Fig. 19).²⁰³ Other Pd-based nanoplates (PdM, M = Ga, In, Sn, Pb, and Bi) have also been reported.²⁰⁴ In addition to non-noble metals, noble metals including Ag and Pt were also reported to alloy with Pd.^205,206 Hong et al. reported the synthesis of Pd–Pt–Ag alloy nanosheets using K₂PdCl₄, K₂PtCl₄ and AgNO₃ as precursors, in which the solution of precursors and ascorbic acid was sequentially injected into a CO-saturated aqueous solution of CTAC. It was shown that CO plays a vital role in the formation of ultrathin Pd–Pt–Ag ternary-alloy nanosheets with a thickness of approximately 3 nm.²⁰⁷ On the other hand, Pd nanoplates can be used as seeds or templates for the synthesis of bimetallic nanoplates via epitaxial growth and deposition. Yan and co-workers used Pd nanoplates as seeds for the synthesis of Pd@Pt nanoplates and multimetallic Pd@PtM (M = Ni, Rh, and Ru) nanoplates (Fig. 20).²⁰⁸ Chen et al. synthesized hexagonal Pd nanosheets, which were used as seeds for the formation of bimetallic Pd@Au nanoplates by chemically reducing AuPPh₃Cl with hydrazine hydrate in DMF.²⁰⁹ Furthermore, Lim and co-workers prepared Pd@Pt core@shell nanoplates using a similar overgrowth method, in which hexagonal and triangular Pd nanoplates were employed as seeds.²¹⁰

Fig. 19 (a) Model, (b) TEM image from out-of-plate view, (c) SAED pattern, and (d) HRTEM image of a single nanosheet. (e and f) HRTEM images from the selected area in (d). Simulated HRTEM images as well as the atomic models in (e and f) are superimposed on the experimental images. (g) Atom models of the nanosheet showing the top interface [(100)Pd//(100)Pd₃Pb] and the side interface [(001)Pd//(001)Pd₃Pb]. Reprinted with permission from ref. 203.

Fig. 20 TEM and HAADF-STEM-EDS mapping images for (a and d) Pd@Pt nanoplates, (b and e) Pd@PtRh nanoplates, and (c and f) Pd@PtRu nanoplates, respectively, prepared using the standard procure, except for the different metal precursors. The scale bar in top, left part in panel d is 1 nm. Reprinted with permission from ref. 208.

2.2.3 Synthesis of 2-D Rh-based NCs.
Synthesis of 2-D Rh NCs. Solvothermal treatment has been widely used for the synthesis of Rh nanoplates.^211,212 Duan and co-workers obtained ultrathin Rh nanosheets in the presence of PVP via a solvothermal synthetic approach.²¹³ The as-obtained Rh nanosheets were semi-transparent with an edge length of ca. 500–600 nm (Fig. 21a and b), which showed a superior catalytic performance towards the hydrogenation of phenol and hydroformylation of 1-octene.²¹³ Using a similar solvothermal synthetic approach, Zhang et al. produced hierarchically-coiled ultrathin nanosheets by mixing RhCl₃·3H₂O, PVP and HCOONH₄ in benzyl alcohol.²¹⁴ Zhang et al. mixed RhCl₃ with 1-octadecylamine in a Teflon-lined stainless-steel autoclave. Single-crystal hyperbranched Rh nanoplates were obtained after solvothermal treatment at 200 °C for 10 h.²¹⁵ Zheng's group demonstrated a CO-confined growth strategy to prepare single-crystalline Rh nanosheets with atomic thickness and controllable size, in which CO was critical for the formation of nanosheets but did not vary the ratio of long edge to short edge of the different-sized Rh nanosheets (Fig. 21c). Jang and co-workers synthesized Rh nanoplates by dissolving [Rh(CO)₂Cl]₂ in oleylamine at 50 °C for 10 days (Fig. 21d). They emphasized that the solution must be left undisturbed.²¹⁶ The mechanism studies showed that the coordinated oleylamine was located up and down the 1-D chains. The chains interact with each other via van der Waals interaction between alkyl groups to form a lamellar structure. The gradual reduction of Rh(I) to metallic Rh formed 2-D plates with shape evolution to triangle or quadrangle shapes.²¹⁶

Fig. 21 (a) Low-magnification TEM image of the PVP-capped Rh nanosheets. Scale bar, 1 μm. (b) High-magnification TEM image of the PVP-capped Rh nanosheets. Scale bar, 100 nm. Reprinted with permission from ref. 213. (c) TEM images of Rh nanosheets with a tunable size obtained under different CO pressures. Reprinted with permission from ref. 224. (d) TEM image of the top view of the prepared nanoplates. Reprinted with permission from ref. 216.

Synthesis of 2-D Rh nanocomposites. Although there are many studies on Rh-based composite NCs,^217–220 2-D Rh-based composite NCs have rarely been reported over the past decades. Köhler and co-workers reported the synthesis of bimetallic Bi–Rh nanoplates via a microwave-assisted polyol process. In this process, bismuth acetate and rhodium acetate dimer were used as metal precursors, and EG as the reducing agent. The average diameter and thickness of the Bi–Rh nanoplates were 60 nm and 20 nm, respectively.²²¹ Compared to the ultrathin Rh nanosheets, these Bi–Rh nanoplates were much thicker, indicating the significance of the capping agents in the synthesis of 2-D NCs. Zhao et al. reported a cyanogel-reduction method for the synthesis of ultrathin Rh–Co alloy nanosheets, in which a (K₃[Co(CN)₆])–RhCl₃ cyanogel was initially prepared and then reduced by HCHO at 240 °C for 6 h.²²² Kang composited Rh nanosheets with reduced graphene oxide for achieving an enhanced methanol oxidation reaction performance.²²³

2.2.4 Synthesis of 2-D Au NCs. The chemical reduction approach is a general method for the synthesis of 2-D Au NCs, which generally involves the reduction of an auric solution to form nanoplates in the presence of appropriate stabilizers (e.g., CTAB and PVP).²²⁵ The stabilizers are critical during the growth of the Au nanoplates since they can suppress the aggregation of nanoplates and control the growth process. Hence, the size of the prepared Au nanoplates can be tuned by varying in parameters involved in the synthesis, such as reagent concentration, solution temperature and reaction duration. In chemical reduction, the reductants are the key parameters for the synthesis of Au nanoplates. Sun's group reported the synthesis of morphology-controlled Au nanoplates in high yield and purity with a narrow size distribution via the chemical reduction of tetrachloroauric acid using trisodium citrate, which kinetically controlled the reaction pathway.²²⁶ The obtained Au nanoplates with two hexagonal shapes were highly [111] oriented with a flat top normal to the electron beam. The size of the nanoplates was determined by the relative amount of nuclei that were initially formed by the reduction by sodium citrate, leading to an increase in size with a decrease in the concentration of sodium citrate. Furthermore, they confirmed that the rate of the reaction can be controlled by sodium citrate, which proceeds in a two-step manner with self-seeding, resulting in the formation of planar nanocrystals or nanoplates with high yield and purity.

Chu and co-workers synthesized Au nanoplates via the reduction of HAuCl₄ by trisodium citrate, and they claimed that the preheating of the HAuCl₄ solution was quite important to control the size distribution of the nanoplates.^227,228L-Ascorbic acid has been widely used for reducing tetrachloroauric acid in the seeded growth of Au nanorods.^{146,156,229–232} Yamamoto et al. reported a facile approach for the synthesis of star-shaped Au nanoplates via the reduction of HAuCl₄ by L-ascorbic acid at room temperature in the presence of PVP.²³³ The obtained nanoplates were single-crystal nanoplates lying flat on their [111] planes with six symmetric hones extending to the [112] directions, but not the stacked two triangular plates or twin. Our group demonstrated a high morphology yield of gold nanoplates synthesized in the presence of CTAC and iodide ions.²³⁴ The kinetics of the growth could be controlled by fine-tuning the pH value using NaOH, resulting in the rapid growth of gold nanoplates in less than 10 min. Using the method, a high morphology yield (>90%) of triangular nanoplates with tunable edge lengths from about 45 nm to 120 nm were obtained (Fig. 22). Detailed investigations showed that the formation of the nanoplates was mainly determined by the selective binding of iodide ions, typically, the tri-iodide ions could facilitate the formation of nanoplates through selective binding on the Au(111) facets, and also formed I₃⁻ to selectively remove other shaped impurities in the early stage through chemical etching (Fig. 22g). This method was the first report on the rapid synthesis of small gold nanoplates with a high morphology yield of over 90% without any further purification steps, providing new insight to achieve cost-effective synthesis for practical applications.

Fig. 22 TEM images of Au nanoplates with tunable edge length. Average edge length of each sample is (a) 45.22 ± 3.38 nm; (b) 65.48 ± 2.92 nm; (c) 70.26 ± 3.07 nm; (d) 78.25 ± 3.16 nm; (e) 107.34 ± 8.01 nm; and (f) 117.29 ± 5.78 nm. The scale bars are 100 nm. (g) Proposed growth pathway of gold nanoplates through oxidative etching (gold nuclei with various crystal structures are produced in the early stage of nucleation. In the presence of oxygen and iodide, dominant gold nuclei with a planar twined structure are left and grow into nanoplates, while the other shaped nuclei are oxidized by tri-iodide ions). Reproduced with permission from ref. 234.

The seed-mediated approach is an effective procedure to obtain nanoparticles, which was originally developed for the synthesis of Au nanorods in 2001 by Murphy's group, in which CTAB and ascorbic acid are used as the shape-directing agent and reductant, respectively. Generally, seeds with a size in the range of 2–3 nm are prepared via the rapid reduction of auric solution by NaBH₄.^155,235 Owing to the high production yields, seeded growth methods have been widely used for generating nanoparticles including nanorods,^{146,230,236,237} bipyramids,^238–242 and nanoplates.^243–245 In 2004, Murphy's group reported that the shape of the Au nanoparticles could be controlled by systematically varying the experimental parameters in seeded growth. The growth solution contained CTAB, HAuCl₄, L-ascorbic acid, and in some cases a small quantity of AgNO₃, and the seed solution consisted of CTAB, HAuCl₄ and NaBH₄. The obtained shapes of the Au nanoparticles consisted of rod-like, hexagonal, cubic and triangular plates.²⁴⁶ Afterwards, Mirkin's group reported a procedure for generating Au nanoplates based on Murphy's seeded growth methods in 2005.²⁴⁷ All the obtained nanoplates with uniform edge lengths and thicknesses were triangular, while spherical nanoparticles were the major by-products (almost 50%). The obtained nanoplates possessed an average edge length and thickness of 144 and 7.8 nm, respectively, which were single crystalline with a [111] faceted flat top and bottom face according to the hexagonal electron diffraction pattern. To further tune the size of Au nanoplates, the subsequent work by Mirkin et al. reported an over-growth method for systematically controlling the edge length of Au nanoplates, in which seeds (Au nanoplates) were added into the growth solution containing a mixture of CTAB, HAuCl₄, ascorbic acid and NaOH. They found that the edge length of the nanoplates could further grow from 100 nm to about 300 nm while preserving their original shape, crystal properties and thickness.²⁴⁸ The highly preferential growth of the side facets of the nanoplates over the triangular faces required the step-by-step injection of Au ions to avoid the formation of pseudospherical particles, which was attributed to the Gibbs-Thomson effect, indicating that a convex surface had a higher surface energy than a flat surface of the same material in the same phase.

Furthermore, Kim's group studied the tuning of both the edge length and thickness of triangular Au nanoplates using a modified seeded growth method.²⁴⁹ They claimed that multiple growth steps and the interval between multiple steps were critical for the preferential expansion of Au nanoplates. The edge length increased with an increase in the volume ratio between the isotropic growth solution and triangular Au nanoplates, ranging from 140 nm to about 260 nm, and the thickness correspondingly increased from 10 nm to 83 nm.²⁴⁹ Although the detailed mechanism was not mentioned, the procedure was similar to that of the conventional seeded growth methods, except for the addition of a certain amount of iodide ions, indicating that the iodide ions may play an important role in the formation of triangular nanoplates. Later, another work from Mirkin's group claimed that the morphologies of Au nanoparticles were strongly dependent on iodide ions.²⁴⁴ It was found that a high concentration of iodide ions (50 μM) favoured the formation of nanoplates in high yield (65% before purification), while a mixture of nanoparticle morphologies including nanorod, nanoprism, and nanosphere morphologies was observed at a lower iodide ion concentration of 10 μM. They claimed that the iodide ions were the dominant shape-directing agents, even more than the seed particles since nanorods, nanoprisms and nanospheres could be synthesized using the prepared seeds. This phenomenon was further confirmed by Sastry and co-workers, where they investigated the role of iodole ions on the morphology of gold nanotriangles in detail.²⁵⁰ The proposed mechanism could be understood based on the preferential adsorption of iodide on the [111] crystal facet of Au, which leads to a lack of preferential growth on the [111] facet.^248,251,252

Besides, the seeded growth of Au nanoplates can also be performed on the surface. Akrajas and Munetaka reported the synthesis of two-dimensional Au nanoplates on the surface of indium tin oxide (ITO) through liquid-phase reduction from Au seed particles attached to the ITO surface in the presence of PVP. The obtained nanoplates were single crystalline in nature with [111] facets and the edge length of up to about 2 μm, which grew parallel to the surface of ITO. The concentration of PVP was the key factor to reduce the spherical or irregular-shaped nanoparticles and promote the growth of Au nanoplates on the surface of ITO.²⁵³ Zamborini and co-workers reported the growth of Au nanoplates directly on surfaces of glass and Si/SiO_xvia a chemical seed-mediated growth method, in which involved purification by tape or sonication to improve the yield of Au nanoplates by up to 90%.²⁵⁴ A photochemical approach was reported by Jin et al. for generating Ag nanoplates in 2001.²⁵⁵ Zhu and co-workers reported the synthesis of Au plates via a photochemical approach using the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) as the reaction medium, template, and capping agent.²⁵⁶ The proposed mechanism involved radical reactions photoexcited by UV light to reduce Au³⁺. As a template, the effects of [BMIM][BF₄] were summarized as follows. Specifically, Au³⁺ ions were reduced to Au clusters by photoinduction, and the preferential adsorption of [BMIM][BF₄] on the planes of Au crystals led to the formation of Au nanoplates. Ding's group developed a continuous method for generating Au nanoplates under UV irradiation in the presence of citric acid and PVP at room temperature. They claimed that the morphologies of the Au nanoparticles were strongly dependent on the concentration of PVP and the flow rate of the solution.²⁵⁷ Furthermore, Soejima and Kimizuka claimed that two processes were involved in the photo-assisted synthesis of Au nanoplates: (1) the growth of 2-D Au nanoplates via the photoreduction of Au(OH)₄ complexes, and (2) the oxidative etching of Au nanoplates by dissolved molecular oxygen.²⁵⁸ NaBr plays multiple roles in terms of forming stable adlayers on Au surfaces and promoting the growth of 2-D nanoplates, and converting the oxidized Au(I) species to AuBr₂⁻ ions for the oxidative etching of gold nanocrystals. In addition, Pereira et al. developed a photocatalytic approach for the synthesis of triangular Au nanoplates in aqueous solution over a tin(IV) porphyrin photocatalyst, in which CTAB was used as stabilizer and triethanolamine as the final electron donor.²⁵⁹ The size of the Au nanoplates could be systematically tuned by varying the concentration of photocatalyst and CTAB. Specifically, tin(IV) porphyrin was photoexcited by light to form an intermediate with the electrons from triethanolamine, accompanied by the catalytic reduction of Au³⁺ over the intermediate to form Au nanoplates.

Synthesis of 2-D Au-based nanoplates. Tsuji and co-workers investigated the shape-dependent evolution of Au@Ag core–shell nanocrystals using PVP-assisted N,N-dimethylformamide reduction. The final shapes of the Au@Ag core–shell nanostructures were determined by the shapes of the Au nanoplate seeds, and the edge length of the silver shells increased with an increase in the molar ratio of [AgNO₃]/[HAuCl₄].²⁶⁰ Besides, Park et al. demonstrated that the final shapes of the Ag shell on Au nanoplates could be tuned by the presence of iodide ions.^261,262 It was found that the Ag shell was formed homogeneously over the entire surface of the flat Au nanoplates without iodide ions, whereas the presence of iodide ions induced the selective coating of the Ag shell in the direction perpendicular to the basal plane of the nanoplates, leading to the formation of Au@Ag core–shell hollow nanostructures via galvanic replacement reactions.

Mirkin et al.²⁶³ reported the plasmon-driven synthesis of triangular core–shell nanoplates from gold seeds, in which colloidal Au and Ag nanoparticles with diameters of 11 nm and 5 nm, respectively, were added to the aqueous solutions, followed by light irradiation with 550 nm light for 5 h. It was found that the edge length of the core–shell nanoplates was determined by the excitation wavelength, and long excitation wavelengths (600 nm) led to an increase in the average edge length (80 ± 7 nm). Zhang and co-workers synthesized hexagonal Au@Ag core–shell nanoplates on the (0002) face of ZnO nanorods via a two-step deposition-annealing method,²⁶⁴ in which high-quality ZnO nanorods with a flat (0002) face were grown via the chemical vapor deposition method. Au was firstly deposited by e-beam evaporation, and subsequently annealed at 700 °C for 60 s to enable the formation of a first layer of well-defined Au nanoplates, and then the prepared Au nanoplates underwent e-beam evaporation again for 1 nm Ag capping.

2.2.5 Synthesis of 2-D Ag NCs. Jin and co-workers reported a photochemical approach for the preparation of 2-D Ag nanoplates in the presence of citrate and bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt (BSPP).²⁵⁵ They revealed that this phenomenon disappeared in the dark even when the reaction time was over a two-month period, and the triangular shape silver nanoplates grew bigger with an increase in the radiation intensity. Thus, the synthesis of silver nanoplates could be simply controlled by turning on/off the light. The potential mechanism was that under the effect of BSPP, silver seeds were converted into small clusters through the photoinduced fragmentation of silver nanoparticles, and then to small nanoplates (edge lengths of 5–10 nm). The small nanoplates acted as seeds and grew at the expense of the small nanoparticles. Xia et al. demonstrated that citrate ions could promote the formation of nanoplates from spheres through a combination of chemical and photochemical methods.²⁶⁵ Another work presented by Maillard indicated that the silver nanoplates were formed in a solution of seed colloid and citrate, which grew through the reduction of silver ions induced by the incident beam.²⁶⁶

Mirkin et al. synthesized Ag nanoplates via dual-beam illumination (termed as the primary beam and secondary beam).²⁶⁷ It was shown that the excitation of a silver nanoparticle colloid with a single beam resulted in nanoplates with two size distributions, named Type 1 (edge length 70 ± 12 nm) and Type 2 (edge length 150 ± 16 nm). Although the size of the Type 2 nanoplates was almost twice that of the Type 1 nanoplates, their thickness was essentially the same, which may imply a mechanism that the triangular Type 2 nanoplates were assembled by three pieces of triangular Type 1 nanoplates. By contrast, only Type 1 nanoplates were formed when the solution was simultaneously excited via dual-beam with the wavelengths of 550 nm (dipole SPR excitation) and 450 nm (or 340 nm, quadrupole SPR excitation) light, and the edge lengths ranged from 40 to 120 nm with a variation in the length of the primary beam from 400 to 750 nm, which were also vividly shown in the TEM images (Fig. 23). The above results indicated that the fusion of the smaller nanoplates was inhibited by quadrupole or high energy excitation, indicating that it was feasible to control the edge length of the nanoplates.²⁶⁸ Another report by Markin's group demonstrated the mechanism of photo-mediated growth.²⁶⁹ They proposed that the growth of Ag nanoplates from nanoparticles may undergo several steps, i.e. plasmon excitation-initiated photo-reaction, shape transformation of spherical silver nanoparticles to small nanoplates, and growth from small nanoplate seeds to large nanoplates. They assumed that dipole and quadrupole plasmon excitation may play different roles in the shape control process. Dipole plasmon excitation favoured the growth of sharp corner nanoplates, while in-plane quadrupole excitation favoured the growth of truncated nanoplates.

Fig. 23 Unimodal growth of nanoplates. (a) Schematic diagram of dual-beam excitation. (b) Optical spectra (normalized) for six different-sized nanoplates (1–6 edge length: 38 ± 7 nm, 50 ± 7 nm, 62 ± 9 nm, 72 ± 8 nm, 95 ± 11 nm and 120 ± 14 nm) prepared by varying the primary excitation wavelength (central wavelength at 450, 490, 520, 550, 650 and 750 nm, respectively; width, 40 nm) coupled with a secondary wavelength (340 nm; width, 10 nm). (c) Edge length as a function of the primary excitation wavelength. (d–f) TEM images of Ag nanoplates with average edge lengths of (d) 38 ± 7 nm, (e) 72 ± 8 nm and (f) 120 ± 14 nm. Scale bar applies to panels d–f. Reprinted with permission from ref. 267.

In addition, the effects of incident light on the control of the morphology were further verified in other studies. Callegari and co-workers demonstrated that the shape could be controlled by choosing the wavelength of light used to drive the photochemical growth. Specifically, the edge length of the obtained triangular Ag nanoplates increased with an increase in the wavelength of the incident light.²⁷⁰ Liz-Marzán et al. reported that the later dimensions of the silver nanoplates could be controlled by illuminating pre-formed silver seeds under different illumination conditions.²⁷¹ It was noted that although the later size of silver nanoplates increased with the wavelength of the incident light, the thickness was almost constant. Besides, the morphology of silver nanoplates could be tuned by altering other parameters (e.g., citrate, pH, and BSSP).²⁷² Xue and Mirkin demonstrated a straightforward route for controlling the bimodal growth of silver nanoplates by simply adjusting the pH value of the solution based on the hypothesis that a variation in pH value can adjust the process of nanoplate fusion, thus affecting the growth pathways.²⁷³ It was shown that highly basic conditions were adverse for the formation of silver NCs since a passivating layer of AgOH likely formed on the surface of silver NCs and inhibited the photoreaction. However, a lower pH value may lead to the formation of nanoparticles with irregular shapes, and even further reduce the interplate repulsion. A subsequent work by Mirkin et al. investigated the effects of pH value and BSSP on the morphology and size of Ag NCs.²⁷⁴ It was found that a high pH value favoured the formation of bipyramids, whereas a low pH value favoured the formation of nanoplates, and the aspect ratio increased with an increase in the ratio of [BSPP]/[Ag⁺]. The proposed mechanism was that a fast reaction rate achieved by a high pH (10 or 11) and a relatively low [BSPP]/[Ag⁺] ratio (close to 1) favoured the growth of (100)-faceted right triangular bipyramids via the preferential deposition of Ag on the (111) facets compared to the (100) facets. By contrast, a slower reaction rate achieved by low pH value and high [BSPP]/[Ag⁺] ratio favoured the preferential deposition of Ag on the (100) facets, resulting in the formation of Ag NCs with a higher percentage of (111) facets (e.g., nanoplates). Zhang et al. reported that the morphology of the Ag nanoplates changed from thin triangular plates to thick round plates with the assistance of PVP through a UV-light-induced reconstruction process.²⁷⁵

In addition, the chemical reduction method, which is also named the thermal synthesis method, has been widely used for the synthesis of Ag nanoplates.²⁷⁶ As one of the most popular approaches for the synthesis of nanoplates, the chemical reduction method involves the gradual conversion of colloidal Ag nanoparticles into nanoplates.^{265,277–279} In principle, chemical reduction methods simply involve the reduction of metal ions by a specific chemical reducing agent with the help of a capping agent to form small nanoparticles, then grow into larger structures at an appropriate pH value and temperature.²⁶⁸ Zhang et al. demonstrated that various Ag sources could be directly converted into Ag nanoplates in H₂O₂ with the assistance of an appropriate capping ligand.²⁸⁰ Sun and Lei reported the conversion of Ag nanoplates into Au–Ag alloy nanoplates for enhancing their stability in catalytic applications, in which the Au–Ag nanoplates were formed by immersing a sample of Ag nanoplates into an aqueous solution of NaAuCl₄ at room temperature.²⁸¹ Sun and co-workers reported the synthesis of highly stable Ag–Au triangular nanoplates via galvanic reaction based on face-selective etching.²⁸² The obtained alloy nanoplates exhibited tunable localized surface plasmon resonance and efficient two photon luminescence as well as high stability under the strong laser and tough ion-contained environment owing to the close match of the lattice spacing between Au and Ag. Moreover, galvanic displacement reactions have been applied for the synthesis of Ag–Pd nanoplates. Generally, Ag nanoplates are used as templates, and Ag–Pd nanoplates are obtained by sacrificing the Ag templates for the reduction of Pd²⁺ into Pd⁰via a galvanic displacement reaction. The properties of the obtained Ag–Pd alloy nanoplates were determined by the concentration of Pd²⁺ and the Ag templates. It has been reported that the size of the obtained alloy nanoplates could be tuned by altering the size of the Ag templates²⁸³ and the concentration of the Pd precursor.^284–286 Moreover, this galvanic displacement method can be used to synthesize other Ag-based nanoplates, such as Ag–Pt nanoplates.²⁸⁷

2.2.6 Synthesis of other 2-D metal-based NCs. Leng et al. reported the synthesis of single crystalline Ni nanosheets via the decomposition of Ni(COOH)₂ with the aid of iron species.²⁸⁸ The edge lengths of the Ni nanosheets ranged from several tens to hundreds of nanometers, with the shapes of triangle and hexagon.²⁸⁸ In another work from the same group, they synthesized monodisperse fcc Ni nanoplates (Fig. 24) with a triangular shape and uniform sizes via a thermolysis method, in which oleic acid and oleylamine were used as the stabilizers. A plausible mechanism was proposed as follows: (1) small triangular nanoplates acted as the initial nuclei, and continued to grow within the (111) planes along the 〈110〉 direction by Ni atomic attachment, forming large triangular nanoplates and (2) some of the small nanoplates with the same size will be connected along the (110) lateral planes, leading to the formation of large triangular and hexagonal nanoplates.²⁸⁹ Another typical case for 2-D non-noble metal nanostructures is Cu nanoplates. Sun reported the synthesis of size- and shape-controlled planar Cu nanoplates in high yield via the reduction of a Cu(II)–tartrate complex using the mild reductant NaH₂PO₂ in the presence of PVP. The size of the Cu nanoplates could be systematically tuned from the nano-scale to submicron-scale by altering the type of complex agent or by adjusting the amount of tartrate complex.²⁹⁰ Pastoriza-Santos et al. synthesized single-crystalline Cu nanoplates with prominent surface plasmon bands through the reduction of copper salts (i.e., Cu(OAc)₂) with hydrazine, using PVP as a stabilizer and without the need for an inert atmosphere.²⁹¹ Moreover, Guo and co-workers reported the synthesis of hexagonal and triangular Ni–Cu alloy nanoplates with a well-defined twin structure via a one-pot non-aqueous synthetic approach in an Ar atmosphere. More importantly, the morphology of the Ni–Cu alloy nanoplates evolved from hexagon to triangle when dibenzyl ether was added to the growth solution. The mechanism was summarized as follows: (1) Ni(II) is reduced Ni(0) atoms, which then combined into crystal embryos; (2) Cu(II) will partially etch the surface Ni(0) into Ni(II) via galvanic replacement reactions in the presence of Cl⁻; and (3) the deposited Cu(0) on the surface of the Ni embryos will further catalyze the reduction of Ni(II) to form alloy nuclei. The reiteration of this oxidative etching and catalytic growth process enabled the alloy nuclei to grow into spherical seeds, which finally developed into alloy nanoplates.²⁹² Bai and co-workers recently synthesized lichen-like Ru–Cu nanosheets via a solvothermal method.²⁹³ These lichen-like nanostructures with a thickness of about 3 nm exhibited a superior catalytic performance towards the direct oxidation of CH₄. Compared to the noble metal-based NCs, non-noble metals, such as Ni, Cu, and Co, are less popular, mainly because they are prone to fast oxidation.²⁹¹

Fig. 24 TEM images of the standing Ni nanoplates by tilting to confirm the self-assembled plate structure. Tilted TEM images obtained at (a) −10°, (b) 0°, (c) +10°, and (d) +25° tilt. The inset in (b) is the enlarged image of the standing edges of several triangular Ni nanoplates. Reprinted with permission from ref. 289.

2.3 Three-dimensional metal NCs

2.3.1 Synthesis of 3-D Pt-based NCs.
Synthesis of 3-D Pt NCs. Based on previous reports, 3-D Pt NCs are generally enclosed by (100) and/or (111) facets. The first example is Pt nanocubes, which are enclosed by six (100) facets. Generally, Pt(acac)₂ is used as a Pt source, and oleic acid and oleylamine are used as capping agents. In addition, some additives are needed to act as a shape-directing agent. Sun demonstrated the synthesis of Pt nanocubes by reducing Pt(acac)₂ in the presence of oleic acid and oleylamine, in which Fe(CO)₅ was employed as the shape-directing agent.²⁹⁴ Liu et al. reported that W(CO)₆ could also be used as a shape-directing agent for the synthesis of Pt nanocubes.²⁹⁵ Loukrakpam and co-workers prepared Pt nanocubes via a chromium-assisted synthetic method in the presence of Cr(CO)₆. The plausible mechanism suggests that Cr species can preferentially adsorb on Pt(111), leading to the effective blocking of the Pt(111) facet.²⁹⁶ In addition to metal hexacarbonyls (e.g., Fe(CO)₅, W(CO)₆, and Cr(CO)₆), CO has been confirmed to direct the formation of Pt nanocubes.^297,298 For Pt NCs enclosed by (111) facets, Kang et al. demonstrated that Mn₂(CO)₁₀ enabled the formation of Pt octahedrons (Fig. 25).²⁹⁹ Furthermore, they claimed that the obtained morphologies of the Pt NCs varied strongly by altering the ratio of Mn₂(CO)₁₀ and Pt(acac)₂, as presented in Fig. 25. Liao et al. revealed the growth process in a liquid cell using TEM with high spatial and temporal resolution. It was shown that the growth rates of all the low index facets are similar until the (100) facets stop growing, while the continuous growth of the other facets leads to the formation of nanocubes.³⁰⁰ Discussing the effects of shape-directing agents on the morphologies of Pt NCs is critical for studying their mechanism; however, the mechanism is not completely understood to date. One popular explanation proposed by Kang et al. suggests that Mn from the decomposition of Mn₂(CO)₁₀ may selectively deposit on Pt(100) and then reduce Pt²⁺ to Pt, which takes the position occupied by the Mn atom.²⁹⁹ To determine the detailed mechanism, a more sensitive tool is necessary to unambiguously identify the efforts of shape-directing agents on the morphology control.

Fig. 25 (a) TEM and (b) HRTEM images of Pt octahedra. Scale bars: (a) 50 nm and (b) 2 nm. TEM images of (c) Pt cuboctahedra, (d) Pt spheres, (e) Pt tetrapods, (f) Pt truncated cubes, (g) Pt star (octapods), and (h) Pt multipods. Scale bars: (c and d) 20 nm, (e) 100 nm, (f–h) 50 nm, insets of (c, d and f) 5 nm, insets of (e and h) 20 nm, and inset of (g) 10 nm. Reprinted with permission from ref. 299.

Pt NCs with concave surfaces are interesting in catalysis since their corners and edges generally exhibit higher catalytic activity than that of conventional Pt NCs. On the other hand, Pt NCs enclosed by high-index facets are much more active than NCs enclosed by low-index facets (e.g., (111) and (100) facets).³⁰¹ However, Pt NCs with high-index facets generally have high surface energy, leading to instability during their synthesis and catalytic application.³⁰² In principle, two strategies can be used for the synthesis of concave Pt NCs: (1) site-specific dissolution through etching and galvanic replacement and (2) directionally controlled overgrowth by facet-selective capping, kinetic control, and template-directed epitaxy.³⁰³ Zhang et al. summarized the most popular and common concave Pt NCs in previous works.³⁰³ Thus, here, we simply introduce some classic examples of the synthesis of Pt NCs enclosed by high-index facets. Zhang et al. presented the synthesis of Pt concave NCs with high-index facets in an aqueous solution via the reduction of H₂PtCl₆ in the presence of PVP and glycine, in which PVP acted as a surfactant, stabilizing agent and reductant, and glycine as a surface controller and co-reductant.³⁰⁴ Huang and co-workers synthesized concave Pt NCs enclosed by both high-index (411) and (100) facets via a solvothermal method (Fig. 26).³⁰⁵ It was shown that the amine played a vital role in the formation of the concave NCs. Zhang et al. reported the preparation of high-index facets of (211) and (411) via a solvothermal method using Pt(acac)₂ as the Pt source, 1-octylamine as the solvent and capping agent, and formaldehyde as an additional surface structure regulator. The plausible mechanism suggests that amine can stabilize the monoatomic step edges, leading to the formation of the (211) facet. Formaldehyde decomposes into CO, and further leads to the formation of (411) surfaces by the additional adsorption of the CO on the (100) terraces.³⁰⁶ Moreover, an electrochemical method was employed to synthesize Pt NCs enclosed by high-index facets. Wei et al. reported the electrochemical synthesis of tetrahexahedral (THH) Pt NCs in deep eutectic solvents for the first time. The as-prepared THH Pt NCs were bound with (910) and vicinal high-index facets, which exhibited superior catalytic activity and stability for the electrooxidation of ethanol.³⁰⁷ In another work from the same group, the synthesis of a complex shape of Pt micro/nano-sized crystals was demonstrated, where a convex hexoctahedra enclosed with 48 (15 5 3) high-index facets were obtained via the electrochemical square-wave-potential method. It was realized that the shape of the Pt NCs with high-index facets evolved from THH to hexoctahedron (HOH), and then to trapezohedron (TPH) with an increase in the upper (E_U) or lower potential (E_L) (Fig. 27).³⁰⁸

Fig. 26 (a, d and g) TEM images, (b, e, and h) SAED patterns, and (c, f, and i) geometric models of individual concave Pt NCs oriented along the (a–c) [100], (d–f) [111], and (g–i) [110] directions. (j) HRTEM image of the region indicated by the box in (g). (k) Atomic model corresponding to the region indicated by the box in (j). Reprinted with permission from ref. 305.

Fig. 27 Shape evolution of Pt NCs from THH to TPH via HOH by increasing the E_U or E_L of the square-wave-potential. Increasing E_U will increase the etching rate, and increasing E_L will decrease growth rate of Pt NCs. Both approaches result in the same trend of shape evolution. Reprinted with permission from ref. 308.

Synthesis of 3-D Pt nanocomposites. Among the composite Pt NCs,^309–314 Pt₃Ni nanostructures are some of the most popular catalysts in electrocatalysis over the past two decades.^315–328 Zhang et al. synthesized monodisperse Pt₃Ni nanoctahedra and nanocubes enclosed by (111) and (100) facets, respectively, via a high-temperature organic solution chemistry approach under an inert atmosphere. In this process, the precursors of Pt(acac)₂ and Ni(acac)₂ were dissolved in a mixture of oleylamine and oleic acid and then heated to 130 °C under an Ar gas flow. W(CO)₆ was added as a shape-reducing agent, followed by heating to 230 °C for 40 min under vigorous agitation.³¹⁷ Wu and co-workers presented a strategy for the synthesis of truncated-octahedral Pt₃Ni catalysts with (111) facets. They claimed that short-alkane-chain amines appeared to favor the formation of (111) facets. When dodecylamine was used as the capping agent, only octahedra and truncated octahedra were obtained, while a certain of amount of cubes were observed when octadecylamine and hexadecylamine were used.³¹⁶ In addition to the capping agent, the reduction kinetics may influence the final morphology of Pt₃Ni NCs.³¹⁸ Zhang et al. obtained Pt₃Ni NCs with Pt skin and Ni-rich Pt₃Ni NCs under thermal treatment.³²⁴ The different thermal stability of Pt and Ni may pull the Ni atoms to surface to decrease the surface energy. Shen and co-workers demonstrated the combinational use of multiple cutting-edge in situ techniques to study the growth process of octahedral Pt₃Ni NCs. It was shown that Pt nuclei are formed in the initial stage, which further catalyze the reduction of Ni to form an alloy with moderate elemental segregation (Fig. 28a–e).³²⁹ To significantly expose more Pt atoms on the surface of Pt₃Ni NCs, Huang et al. synthesized highly porous Pt₃Ni NCs in the presence of PVP, DMF and phenol. They claimed that the selective use of phenol was critical for the formation of porous Pt₃Ni NCs. PVP acted as a surfactant to prevent the aggregation of the NCs.³²⁰ Furthermore, the superstructure of octahedral Pt₃Ni NCs, which was formed during the controlled evaporation of the solvents, was proven as an efficient strategy to reduce the surface ligands.^319,323 As discussed above, it has been widely reported that the high-index facets of NCs may exhibit higher catalytic activity than that with low-index facets. Wang and co-workers demonstrated the synthesis of high-indexed Pt₃Ni alloy THH nanoframes through preferential CO etching. In this process, PtNi₄ THH NCs were prepared via the co-reduction of PtCl₄ and NiCl₂·6H₂O in the presence of oleylamine, which were further subjected to sequential cleaning, carbon loading, CO-etching and annealing treatment to obtain the Pt₃Ni alloy THH nanoframes (Fig. 28f–k).³²⁵ Huang et al. demonstrated the surface doping of Mo on Pt₃Ni octahedrons on commercial carbon black without the use of any bulky capping agents, in which Mo(CO)₆, Pt(acac)₂ and Ni(acac)₂ were added to a suspension of Pt₃Ni/C in DMF and left to react at 170 °C for 48 h.³²²

Fig. 28 Shape evolution of Pt₃Ni nanoparticle. (a) In situ STEM images of a growing Pt₃Ni nanoparticle (upper row) and corresponding 2-D projection model (lower row), scale bar: 1 nm. (b) Representative atomic model (not to scale) of the nanoparticle during its growth. (c) Number of grown layers of the (001) and (111) facets and (d) their surface exposure ratio as function of experimental time. (e) 3-D model of the Pt₃Ni growth process. The early stage model 1 was constructed to show a possible dynamical cluster structure. Reprinted with permission from ref. 329. (f) HAADF-STEM image of PtNi₄ THH NCs (the inset is a THH model along the [001] zone). (g) HRTEM image of a single PtNi₄ THH NC recorded along the [001] zone axis with the measured angles between surfaces. (h) HRTEM image zoomed from the boxed area in (g), displaying the surface atomic steps marked as (210) and (310) and XRD pattern of the obtained THH NCs, showing the alloy phase with the fcc structure. (i) HAADF-STEM image of THH nanoframes on carbon evolved by CO thermal annealing. (j) HAADF-STEM image of THH nanoframes on carbon obtained by annealing in the absence of air. (k) TEM image of generated nanoframes by annealing in air after 5000 electrochemical potential cycles from 0.6 to 1.0 V versus RHE at 0.5 V s⁻¹ in 0.1 M HClO₄, showing that the THH profile was perfectly preserved. Reprinted with permission from ref. 325.

2.3.2 Synthesis of 3-D Pd-based NCs.
Synthesis of 3-D Pd NCs. Various Pd NCs have been synthesized over the past two decades. Zhang et al. summarized several typical shapes of Pd NCs in 2013.¹¹² It was shown that most of the Pd NCs exposed their low-index facets including (100), (110) and (111) facets (Fig. 29). It is well known that the final shape of Pd NCs is determined by many parameters, such as capping agent, reduction kinetics and structure of the seeds. Xia's group reported that citrate ions could selectively stabilize the (111) facets, and thus favor the formation of Pd NCs enclosed by (111) facets.³³⁰ In contrast, halide ions (e.g., Br⁻) could selectively adsorb onto the (100) facets of Pd NCs, and thus stabilize these facets during growth, leading to the formation of Pd nanocubes enclosed by (100) facets.²⁹⁴ Further investigations revealed that halide ions (e.g., Br⁻ and Cl⁻) may coordinate with Pd²⁺ to retard the reduction kinetics, leading to variable sizes of Pd nanocubes.^40,331 Jin et al. demonstrated the synthesis of Pd NCs with controlled shapes and different proportions of (100) and (111) facets via the seeded growth method. By varying the ratio of precursor to seed, the shape of the Pd NCs evolved from cubes into truncated cubes, cuboctahedrons, truncated octahedrons, and octahedrons due to the faster growth along the 〈100〉 directions relative to the 〈111〉 directions (Fig. 30a–e).³³² On the other hand, the seeded growth method was also used to prepare Pd concave nanocubes enclosed by 24 high-index facets through preferential overgrowth on Pd cubic seeds. In this process, Br⁻ ions were added to reduce the reduction kinetics and stabilize the (100) facets.³³³ Huang and co-workers demonstrated the preparation of polyhedral concave Pd NCs with a tunable ratio of (111) and (110) facets in the presence of formaldehyde. They claimed that the aldehyde group (e.g., formaldehyde and benzaldehyde) was critical for the formation of concave Pd NCs, and the percentage of (110) of the NCs could be tuned by altering the amount of formaldehyde and the reaction temperature (Fig. 30f–i).³³⁴

Fig. 29 Typical Pd NCs with different shapes reported in the literature. Reprinted with permission from ref. 112.

Fig. 30 (a) Schematic illustration of how continuous growth on the (100) planes eventually leads to the transformation of a Pd cube bound by (100) facets into an octahedron enclosed by (111) facets, and (b–e) SEM images of Pd polyhedrons obtained by controlling the amount of Na₂PdCl₄ added to the reaction solution: (b) 5.8 mg, (c) 8.7 mg, (d) 17.4 mg, and (e) 29.0 mg. TEM images of the Pd polyhedrons are given in the insets (scale bars: 10 nm). The scale bars in (b–e) are 50 nm. Reprinted with permission from ref. 332. (f) Large-area and (g) enlarged TEM, (h) HAADF-STEM, and (i) SEM images of as-synthesized concave Pd NCs. Reprinted with permission from ref. 334.

Synthesis of 3-D Pd nanocomposites. Pd–Au bimetallic NCs have recently attracted significant attention in heterogeneous catalysis due to their excellent catalytic performances. Accordingly, tremendous effort has been devoted to the controllable synthesis of Pd–Au bimetallic NCs with tailored shapes and sizes over the past decades. Generally, three major methods have been reported for the synthesis of Pd–Au bimetallic NCs, including seeded growth, template-assisted synthesis and one-pot synthesis.³³⁵ The seeded growth method has been regarded as an efficient and flexible strategy for the preparation of Pd–Au bimetallic NCs, in which the effect of the complicated nucleation process does not need to be considered. The final morphology of the NCs is determined by thermodynamic and kinetic parameters. Seeded growth methods have been widely used for the synthesis of different-shaped Pd–Au bimetallic NCs since Au seeds can be obtained easily. Li et al. reported that when spherical Au seeds were used, a number of morphologies including octahedrons, rectangular bars, cubes, and dendrites could be obtained.³³⁶ It has been reported that the final shapes of Pd–Au NCs are strongly related to the morphologies of the Au seeds. For example, Yang and co-workers prepared Au@Pd octahedrons, truncated octahedrons, cuboctahedrons and truncated cubes using octahedral Au seeds (Fig. 31a–e).³³⁷ Subsequently, chemists realized that kinetic control enabled the formation of NCs enclosed by high-index facets. Generally, the reduction kinetics can be controlled by changing the precursor concentration, reaction temperature, reducing agents and capping agents. Lee and co-workers demonstrated the synthesis of trioctahedral (TOH), hexoctahedral (HOH) and THH Au@Pd NCs using TOH Au seeds, in which the growth kinetics was tuned by altering the Pd/Au atomic ratio or the amount of introduced NaBr (Fig. 31f–i).³³⁸ In the growth solution with a low Pd/Au atomic ratio, Pd prefers to form a TOH shell via a layer-by-layer mode due to the template effect of Au seeds. With an increase in the Pd/Au ratio, the reduction kinetics becomes fast, leading to a fast growth rate in the 〈110〉 direction to form TOH NCs. Moreover, THH NCs will be obtained at much faster reduction kinetics.

Fig. 31 SEM images of the synthesized Au@Pd core@shell NCs with (a) octahedral, (b) truncated octahedral, (c) cuboctahedral, (d) truncated cubic, and (e) concave cubic shapes. All scale bars are 100 nm. Reprinted with permission from ref. 337. (f) SEM images of TOH Au@Pd NCs, (g) HOH Au@Pd NCs, and (h) THH Au@Pd NCs. (i) Schematic illustration of the reaction regions that form Au@Pd NCs with different polyhedral shapes and different high-index facets. Reprinted with permission from ref. 338.

The template-assisted method involves the epitaxial growth process. Generally, Au NCs with different shapes are used as a template, on which Pd atoms gradually grow to form Pd–Au NCs. Wang and co-workers prepared two types of Au@Pd NCs (e.g., TOH and THH) with high-index facets exposed through template synthesis, as shown in Fig. 32.³³⁹ Comparing the abovementioned two methods, one-pot synthesis is much more complicated since it needs precise control of the co-reduction of two different precursors. Lee et al. demonstrated the synthesis of Au@Pd octahedrons through the co-reduction of HAuCl₄ and H₂PdCl₄ in the presence of CTAC.³⁴⁰ It was shown that CTAC was a weak reducing agent, and the final shape was controlled by thermodynamics. Consequently, the reaction was finished within 48 h, and Au@Pd octahedrons with low energy (111) facets were formed. In addition, Au@Pd NCs with high-index facets were prepared via a one-pot synthesis, such as truncated HOH Au@Pd NCs.³⁴¹ Br⁻ ions were added to the growth solution to form AuBr₄⁻ to control the reduction kinetics. It should be noted that the shape control and evolution of Pd–Au NCs have been well studied; however, it is still challenging to completely understand the detailed mechanism of this process.³³⁵

Fig. 32 (a) Schematic showing the growth of THH Au@Pd core@shell NCs from THH Au NCs, (b) SEM image, (c) TEM image of a single core@shell NC viewed along the [00−1] axis. Inset: SAED pattern recorded on the NC, (d) schematic showing the growth of TOH Au@Pd core@shell NCs from TOH Au NCs, (e) SEM image, and (f) TEM image of a single Au@Pd core@shell NC viewed along the [−110] axis. Inset: SAED pattern recorded on the NC. Reprinted with permission from ref. 339.

Other metals have also been reported to composite with Pd NCs. Wang et al. demonstrated the synthesis of bimetallic Pd–Ni hollow nanoparticles via a modified galvanic replacement method using Ni nanoparticles as sacrificial templates in an aqueous medium. The Ni nanoparticles were synthesized via the re-reduction of NiCl₂·6H₂O by NaBH₄ in the presence of sodium citrate. Afterwards, H₂PdCl₄ was added dropwise to the Ni particle/colloid for another 2 h to obtain Pd–Ni bimetallic nanoparticles (Fig. 33a).³⁴² Fu and co-workers synthesized bimetallic Pd–Ag alloy networks on a large scale using a rapid and aqueous solution method at room temperature. Briefly, Pd(NO₃)₂·xH₂O and AgNO₃ were used as precursors, and NaBH₄ solution was used the reducing agent.³⁴³ Luo et al. prepared concave Pd–Ag bimetallic NCs by introducing a Pd precursor (e.g., Na₂PdCl₄) in an aqueous suspension of Ag nanocubes in the presence of CTAC (Fig. 33b and c).³⁴⁴ A plausible mechanism was proposed. As illustrated in Fig. 33d, the Cl⁻ ions from CTAC could preferentially bind to the (100) facets on the Ag nanocubes, making them more susceptible to oxidation and etching. On the other hand, Pd atoms grew on the Pd cubes through the galvanic replacement reaction to form concave NCs. Zhang et al. reported the fabrication of echinus-like Pd–Cu NCs via a one-step and template-free method. Experimentally, KBr and Pluronic F127 were first dissolved in distilled water. Subsequently, a certain amount of Na₂PdCl₄, CuCl₂, HCl, and AA was added successively to the above solution at 90 °C for 2 h.³⁴⁵ Zhang et al. synthesized concave Pd–Cu NCs via a one-pot method, in which an Na₂PdCl₄ and CuCl₂·2H₂O solution was added to a mixture of PVP, decylamine, glucose and DMF (Fig. 33e).³⁴⁶ Li and co-workers synthesized Pd–Cu₂O hybrid nanoconcaves via a seeded growth method. In the first step, octadecahedral Cu₂O seeds were prepared and dispersed in hexane, which was added to a solution of Pd(acac)₂ in 1-octadecene and oleylamine (Fig. 33f). Afterwards, the mixture was heated at 90 °C under agitation to evaporate the hexane, followed by a growth process in a capped 25 mL vial at 120 °C for 24 h.³⁴⁷ Xie et al. demonstrated the synthesis of Pd–Rh concave nanocubes via the site-specific overgrowth of Rh on Pd cubic seeds, as presented in Fig. 33g. It was shown that Pd could be etched in an acidic aqueous solution to generate Rh nanoframes.³⁴⁸

Fig. 33 (a) TEM image of the hollow Pd–Ni NCs. Reprinted with permission from ref. 342. TEM images of concave NCs prepared by reacting Ag nanocubes with different volumes of Pd(II) precursor solution: (b) 0.3 mL and (c) 0.4 mL in the presence of CTAC. (d) Schematic illustration of the proposed mechanism for the oxidation of Ag on the (100) facets of a CTAC-capped Ag nanocube, the reduction of Pd(II) by Ag to generate Pd atoms for their deposition on the (110) facets, and the diffusion of the Pd atoms across the surface. (bottom) Illustration of the transformation of an Ag nanocube into an Ag–Pd nanocube and two subsequent concave NCs with different concavities on the side faces of the nanocubes with different volumes of Pd(II) precursor in the presence of CTAC. Reprinted with permission from ref. 344. (e) TEM image of the concave Pd–Cu NCs. Reprinted with permission from ref. 346. (f) SEM image of Pd–Cu₂O nanoconcaves. Reprinted with permission from ref. 347. (g) TEM image of the Pd@Rh core@frame concave nanocubes. Reprinted with permission from ref. 348.

2.3.3 Synthesis of 3-D Rh-based NCs.
Synthesis of 3-D Rh NCs. Rh has been widely used as a catalyst for a variety of chemical transformation processes. Thus, the shape control of Rh NCs offers an effective way to investigate the correlation between their catalytic performance and surface atomic structure.³⁴⁹ It is well known that the shape of Rh NCs is strongly related to the capping agents, reduction kinetics, etc.³⁴⁹ With the development of nanoscience, several protocols have been developed for the synthesis of Rh NCs. Zhang and co-workers synthesized Rh nanocubes with a mean size of 10 nm via a seedless polyol method. Their mechanism studies showed that the Br⁻ ions from trimethyl(tetradecyl)ammonium bromide (TTAB) could selectively adsorb on the (100) faces, resulting in the formation of Rh nanocubes with (100) facets.³⁵⁰ Kim et al. demonstrated the synthesis of different-shaped Rh nanostructures including nanotetrahedrons, nanotetrapods, and hierarchical dendrites via a so-called assembly strategy.³⁵¹ In the typical synthesis of Rh nanotetrahedrons, a solution of Rh(acac)₃, 1,2-hexadecanediol, and stearic acid in 1-octadecene was placed in a flask under vacuum for 25 min at 100 °C under stirring, followed by heating to 245 °C at a rate of 5 °C min⁻¹ in an O₂/Ar flow. They claimed that O₂ could oxidize the unstable side products (e.g., Rh nanoplates) into Rh ions (Fig. 34a–h). Chen et al. prepared Rh NCs with high-index facets via an oxidative etching process (Fig. 34i–l). HCl could etch the Rh NCs at 200 °C under hydrothermal conditions through two half-reactions: anode dissolution of Rh and cathode reduction of H⁺ ions. With an increase in the HCl concentration, the hydrogen electrode potential became more positive, leading to an increase in the dissolved corrosion rate of Rh simultaneously.³⁵² Xie and co-workers etched Pd from Pd–Rh nanocubes with an Fe³⁺/Br⁻ pair in the presence of HCl.²⁸⁷ The addition of HCl aimed to avoid the formation of insoluble Fe(OH)₂. Consequently, cubic Rh nanoframes with a unique open structure were obtained after Pd etching. In addition to the chemical etching method, the solvothermal method has also been employed to synthesize Rh NCs with tunable sizes and shapes.²⁹² Zhang et al. recently demonstrated the synthesis of different-shaped Rh NCs in the presence of oleylamine and 1-octadecene. It was shown that the morphology evolved from tetrahedron to concave tetrahedron, and to nanosheet when the volume ratios of oleylamine to 1-octadecene were changed (Fig. 34m–p).³⁵³

Fig. 34 TEM images and schematic diagram of hierarchical Rh dendritic nanostructure (a–c) grown from a nanopyramid core and (e–g) grown from a nanoframe core. (d and h) Expanded schematic view of the nanodendrite with pertinent structural parameters. Reprinted with permission from ref. 351. (i) TEM image of Rh concave nanocubes. Inset: SAED pattern. (j) Size distribution of Rh concave nanocubes. (k) HRTEM image of an individual Rh concave nanocube projected along the [100] zone axis. The angles (α) between the concave facets and the (100) facets are marked. (l) Model of concave nanocube projected along the [100] zone axis and a table showing the calculated values for the angle α when the concave nanocube is bound by different high-index crystallographic facets. Reprinted with permission from ref. 352. Morphological and structural characterizations for (m and n) Rh tetrahedrons to (o and p) concave tetrahedrons. Reprinted with permission from ref. 353.

Synthesis of 3-D Rh-based nanocomposites. Sneed and co-workers reported the synthesis of Rh–Pd nanoboxes through the control of metal migration in core@shell nanoparticles (Fig. 35a–h). They proposed that Pd atoms or ions could migrate from the particle cores via the small gaps between the Rh islands in the shell, which was further accelerated by Br⁻ from CTAB in the growth solution.³⁵⁴ Similar Rh–Pd nanostructures were obtained by Ye et al. via the selective removal of the Pd cores from different Pd–Rh nanocubes. It was shown that HCHO played a vital role during the synthesis since it could alter the reaction kinetics and the growth behavior of Pd and Rh, leading to the formation of different nanocubes, which determined the following formation of hollow nanostructures, nanoframes or nanoboxes.³⁵⁵ Wang et al. demonstrated the one-pot shape-selective synthesis of Pd–Rh NCs with tunable compositions and morphologies, including hollow nanocubes, nanoicosahedrons, and truncated octahedrons via a facile hydrothermal approach (Fig. 35j–m). In this process, PVP was used as both the reductant and capping agent and halide anions (Br⁻/I⁻) as the shape control agents.³⁵⁶ Zhang et al. reported the synthesis of Ag@Rh core@frame nanocubes via the galvanic replacement reaction between Rh(III) and Ag nanocubes.³⁵⁷ Initially, the Rh atoms tended to nucleate and deposit at the edges of the Ag nanocubes when the amount of Rh(III) precursor was relatively low. With an increase in the amount of Rh(III) precursor, the galvanic replacement reaction between Rh(III) and the Ag nanocubes occurred to form Ag–Rh core-frame nanocubes (Fig. 35n–q). In addition to the abovementioned nanostructures, Rh–Pt nanocubes have also been prepared to improve the catalytic performance of Rh NCs.^217,358,359

Fig. 35 TEM images of (a and e) Pd nanocubes, (b and f) Pd@Rh core@island shell nanocubes, (c and g) Pd–Rh nanoboxes, and (d and h) Rh cubic nanoframeworks. Scheme in (i) shows models for the structural evolution from Pd nanocube to Rh cubic NCs. Reprinted with permission from ref. 354. (j) TEM image and (k–m) HRTEM images (insets are corresponding FFT patterns and geometrical models with 2-, 3-, and 5-fold symmetries, respectively). Reprinted with permission from ref. 356. (n) TEM and (o) SEM images of Ag–Rh hollow nanocubes. TEM images of (p) Ag–Rh nanoboxes and (q) Rh nanoboxes with highly porous walls. The Ag–Rh hollow nanocubes were prepared via the titration of 0.3 mL of Na₃RhCl₆ solution. The Ag–Rh and Rh nanoboxes were obtained by etching the Ag–Rh hollow nanocubes with H₂O₂ and Fe(NO₃)₃, respectively. Reprinted with permission from ref. 357.

2.3.4 Synthesis of other 3-D metal NCs. Lee et al. synthesized concave rhombic dodecahedron Au NCs via a seeded growth method in the presence of 4-aminothiophenol (4-ATP). The obtained 3-D Au NCs were enclosed by stabilized facets of (331), (221), and (553). The mechanism studies revealed that the formation of Au–S between 4-ATP and Au NCs and the aromatic geometry of 4-ATP resulted in the excellent stability of the concave Au NCs even after rigorous electrochemical reactions.³⁶⁰ Moreover, the galvanic replacement reaction has also been used for the synthesis of Au NCs, in which Ag was used as the sacrificial seeds. Xia and co-workers described a method for the synthesis of Au nanocubes via the galvanic replacement reaction. In the first step, Ag nanocubes were prepared via the reduction of Ag(I) by ethylene glycol in the presence of PVP and a trace amount of Na₂S, which served as sacrificial templates for the preparation of Au nanocages.³⁶¹

To improve the stability and the catalytic performance of Au NCs, they are generally composited with other metals to form alloys or intermetallic compounds. Ma et al. reported the synthesis of Au–Ag core–shell nanocubes with edge lengths in the range of 13.4–50 nm via a seeded method, in which spherical single-crystal Au nanocrystals (11 nm), ascorbic acid, and CTAC were used as the seeds, reductant, and capping agent, respectively.³⁶² Lu et al. developed a method for the synthesis of Au–Pd core–shell heterostructures with an unusual tetrahexahedral (THH) morphology enclosed by (730) facets using Au nanocubes as the structure-directing cores (Fig. 36).³⁶³ It was shown that the lattice mismatch between Au and Pd, chloride, oxygen, CTAC, and reaction temperature were the key factors for the formation of the THH core–shell nanocrystals. Consequently, the composite NCs exhibited a significantly enhanced catalytic performance toward the reduction of CO₂. More information on the controllable synthesis of Au–Pd NCs was well summarized by Zhang et al.³³⁵ Other elements, such as Ni³⁶⁴ and Cu,³⁶⁵ have also been used to composite with Au to improve its stability.

Fig. 36 (a) SEM images of the cubic Au nanocrystals used as the cores. (b and c) SEM and TEM images of the THH Au–Pd core–shell nanocrystals. (d) HAADF-STEM image of the THH Au–Pd core–shell nanocrystals. (e) EDS line scan and elemental mapping image of a THH Au–Pd core–shell nanocrystal. (f) Schematic drawing of a THH nanocrystal viewed from different angles. The axes projecting along the [100], [110], [111], and [730] directions are also shown. Reprinted with permission from ref. 363.

2.4 Shape evolution of metal NCs

The final morphology of metal NCs is determined by many factors, such as the capping agent, reducing agent, reaction temperature, and kinetics. Therefore, changes in these parameters during the growth of metal NCs will lead to significant changes in the morphology of NCs. Specifically, the evolution of the morphology of metal NCs is determined by thermodynamics and kinetics in the steps of nucleation and growth. Langille et al. studied the roles of Ag⁺ and halides (Cl⁻, Br⁻, and I⁻) in the seed-mediated synthesis of Au NCs.³⁶⁶ They claimed that halides could adjust the reduction potential of Au³⁺ in solution and passivated the surface of Au NCs, leading to the control of growth kinetics, and thus the formation of Au NCs with different shapes. Moreover, Ag⁺ could be used as an underpotential deposition (UPD) agent to access a different set of particle shapes by controlling the growth of the resulting gold nanoparticles through surface passivation (Fig. 37). Xia and co-workers summarized the reaction pathway for the synthesis of metal NCs.^367,368 During the nucleation process, the seeds may take a single-crystal, singly twinned, multiply twinned structure, or plate form. The inherent structures of seeds may lead to the growth of different seeds (Fig. 38). Generally, from single-crystal seeds, growth along the (111) and (100) will lead to the production of cubes, octahedrons, and cuboctahedrons,³⁶⁹ while the uniaxial growth of cuboctahedral and cubic seeds will result in the formation of octagonal rods and rectangular bars, respectively.³⁶⁸ Moreover, right bipyramids and nanobeams will be produced from singly twinned seeds,^370,371 and icosahedrons, decahedrons, and pentagonal nanorods can be produced from multiply twinned seeds.^229,372,373 Additionally, plates will be obtained when the seeds contain stacking faults.³⁶⁸

Fig. 37 Scheme illustrating how halides and silver ions can be used to direct the growth of gold seeds in different growth pathways to yield different shaped products: (a) kinetically controlled products in the absence of silver ions; (b) Ag underpotential deposition-controlled products, where the interactions of silver with the particle surface dictate the product shape; and (c) effect of varying the stability of the Ag UPD layer with high concentrations of chloride, bromide, or iodide in the growth solution, yielding concave cubes, tetrahexahedra, and stellated particles, respectively. Reprinted with permission from ref. 366.

Fig. 38 Schematic illustration of the reaction pathways that lead to Pd nanostructures with different shapes. Briefly, a palladium precursor is reduced to produce Pd atoms, which subsequently aggregate to form nuclei. Once the nuclei have grown past a certain size, they become seeds with a single-crystal, single-twinned, or multiple-twinned structure. If stacking faults are involved, the seeds will grow into plate-like nano-structures. The green, orange, and purple colors represent the (100), (111), and (110) facets, respectively. Twin planes are delineated in the figure with red lines. The parameter R is defined as the ratio between the growth rates along the 〈100〉 and 〈111〉 axes. Reprinted with permission from ref. 368.

3. Metal oxide NCs

Metal oxide NCs have attracted significant attention because of their outstanding structural flexibility combined with excellent properties and wide applications in diverse fields including piezoelectricity, chemical sensing, photodetection and catalysis. Compared to their bulk materials, metal oxide NCs may possess other unique properties associated with their highly anisotropic geometry and size confinement. Specifically, the “nano-effects” of NCs will lead to a variety of surface oxidation states, coordination numbers, symmetry, crystal-field stabilization, density, stoichiometry and acid–base surface properties, resulting in the significant electronic properties and crystal structures of metal oxides.³⁷⁴ Over the past decades, tremendous effort has been devoted to the development of metal oxide NCs. It has been well realized that metal oxide NCs exhibit significantly enhanced catalytic performances. For example, Zhou et al. reported that CeO₂ nanorods exhibited much higher activity than their nanoparticle counterparts in the CO oxidation reaction.³⁷⁵ Xie and co-workers also demonstrated that Co₃O₄ nanorods with (110) planes favored the CO oxidation reaction at low temperature.³⁷⁶ Considering the wide variety of applications of metal oxide NCs, here, we focus on the design of metal oxide NCs for promoting their catalytic performance.

3.1 One-dimensional (1-D) oxide NCs

3.1.1 Synthesis of 1-D titanium oxide NCs. There are four main polymorphs of TiO₂ in nature, including anatase (tetragonal, space group I4₁/amd), rutile (tetragonal, space group P4₂/mnm), brookite (orthorhombic, space group Pbca), and TiO₂(B) (monoclinic, space group C2/m).³⁷⁷ As an n-type semiconductor, TiO₂ has been widely used in photocatalysis.³⁷⁸ The sol–gel method has also been widely used to prepare 1-D TiO₂ NCs.^379–381 Yun and co-workers reported the synthesis of TiO₂ nanorods via the sol–gel method. Titanium tetraisopropoxide was added to an aqueous solution of triethanolamine. Afterwards, the resulting turbid white solution gradually turned transparent light yellow, followed by the addition of oleic acid, diethylamine, and diethylenetriamine aqueous solution to synthesize spherical and ellipsoidal TiO₂ nanoparticles, respectively. Finally, the mixtures were added to Pyrex bottles and heated at 100 °C for 24 h, and then transferred to a Teflon-lined autoclave and heated at 250 °C for 72 h.³⁸² Solvothermal methods are the most common and important methods for the synthesis of 1-D TiO₂ NCs.^383–388 Nian demonstrated the formation of single-crystalline anatase nanorods with a specific crystal-elongation direction through the hydrothermal treatment of titanate nanotube suspensions in an acidic environment. Before the hydrothermal treatment, the nanotube suspensions were prepared via treatment of TiO₂ in NaOH, followed by mixing with HNO₃ to achieve different pH values.³⁸⁹ Tahir et al. prepared monocrystalline rutile TiO₂ nanorods via hydrothermal treatment at 150 °C using an aqueous TiCl₄ solution under acidic conditions in the presence of dopamine.³⁹⁰ Yu et al. demonstrated the large-scale synthesis of single-crystalline anatase TiO₂ nanorods via the hydrothermal treatment of H-titanate fibers. In this process, the pH value played an important role in the control of the shape and crystal structures.³⁹¹ Liu reported a facile hydrothermal method for the growth of oriented single-crystalline rutile TiO₂ nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates (Fig. 39).³⁹² It was shown that the epitaxial relation between the FTO substrate and rutile TiO₂ with a small lattice mismatch played a key role in driving the nucleation and growth of the rutile TiO₂ nanorods on FTO, and the sizes of the nanorods could be tuned by altering the parameters including growth time, temperature, reactant concentration, acidity, and additives. Bryan et al. demonstrated the preparation of Co²⁺- and Cr³⁺-doped TiO₂ nanorods and NCs via a low-temperature hydrolytic route, which employed cobalt(II) acetate tetrahydrate and titanium(IV) tetraisopropoxide in the presence of trimethylamine N-oxide dehydrate and oleic acid.³⁹³ It has reported that AAO can be used as a hard template for the preparation of TiO₂ nanorods, and the diameter of the TiO₂ nanorods can be well controlled by tuning the pore size of AAO.^394–396 In general, AAO is immersed into the sol solution of precursors, followed by high temperature treatment. TiO₂ nanorods are obtained when AAO is removed in H₃PO₄ aqueous solution. The crystalline structure of the as-prepared TiO₂ nanorods can be controlled by altering the calcination temperature. For example, a low temperature will lead to the formation of anatase TiO₂ nanorods, while a high temperature favors the formation of rutile nanorods.

Fig. 39 FESEM images of oriented rutile TiO₂ nanorod film grown on FTO substrate. (a) Top view, (b) cross-sectional view, and (c and d) tilted cross-sectional views. (e) HRTEM image and SAED pattern of a single TiO₂ nanorod shown in the inset of (f). Reprinted with permission from ref. 392.

TiO₂ nanotubes are also important 1-D NCs with unique electronic properties including high electron mobility, high specific surface area, and high mechanical strength.^397–401 Tian and co-workers reported the synthesis of large arrays of oriented TiO₂-based nanotubes and continuous films via a one-pot protocol. Initially, a TiO₂ suspension was prepared by dispersing P25 powder in water. Secondly, a titanium foil with TiO₂ nanoparticles, which obtained through dip coating from the TiO₂ suspension, reacted with an alkaline solution (NaOH) in a sealed Teflon reactor. Thirdly, the Ti foil was removed, and the products were soaked and washed with H₂O, and dried in air.⁴⁰² Anodization is widely employed for the preparation of regular TiO₂ nanotubes.^403–406 For instance, Albu et al. demonstrated the formation of free-standing TiO₂ nanotubes via a three-step process: (1) preparation of a TiO₂ nanotubular layer on Ti foil with a high aspect ratio; (2) selective dissolution of the metallic substrate; and (3) opening of the closed tube bottom by selective chemical etching. The TiO₂ array consisted of regular tubes with a diameter of 160 ± 30 nm and a wall thickness of 20 ± 5 nm (Fig. 40a and b).⁴⁰⁷ Similarly, Jennings synthesized TiO₂ nanotubes via anodization on 2.5 cm × 1.5 cm × 125 μm pieces of titanium foil (99.6% purity, Advent) in a nonaqueous fluoride-containing electrolyte (Fig. 40c and d).⁴⁰⁸ Macak et al. reported that a highly viscous glycerol electrolyte could suppress local concentration fluctuations and changes in pH during the anodization of titanium, leading to the formation of layers of smooth TiO₂ nanotubes up to 7 μm thick with an average pore diameter of 40 nm (Fig. 40e).⁴⁰⁹ In another work by Macak et al., they synthesized TiO₂ nanotubes by directly immersing a Cu clip into an electrolyte, which consisted of H₂SO₄/HF, Na₂SO₄/NaF and 1,2,3-propanetriol/NH₄F. The size of the TiO₂ nanotubes ranged from 500 nm to 4.5 μm.⁴¹⁰ Kuang et al. synthesized highly ordered TiO₂ nanotube arrays with a length of 5–14 μm via the electrochemical anodization of Ti foil.⁴⁰⁴ Sun and co-workers synthesized highly ordered TiO₂ nanotube films via anodic oxidation in an NH₄F organic electrolyte (Fig. 40f).⁴¹¹ Prior to anodization, the Ti foils were cleaned with ethanol and deionized water, followed by immersing the cleaned Ti foils into a 0.05% NH₄F solution (in EG), and then they were subjected to a constant 60 V anodic potential for 20 h at room temperature in a two-electrode electrochemical cell connected to a DC power supply. After rinsing with deionized water and drying under N₂, the obtained TiO₂ nanotube arrays were annealed at 450 °C for 2 h with heating and cooling rates of 10 °C min⁻¹ in O₂. In addition, Perera et al. reported the synthesis of TiO₂ nanotube/reduced graphene oxide composites via an alkaline hydrothermal process. They added TiO₂ (P90) powder to a graphene oxide suspension under stirring for 1 h to ensure complete mixing. Subsequently, NaOH was added to the above system, and the mixture was transferred to a Teflon-lined autoclave. The hydrothermal process was performed at 120 °C for 24 h. The as-obtained TiO₂ nanotubes with a small diameter of 9 nm exhibited promising activity towards the photocatalytic degradation of malachite green.⁴¹²

Fig. 40 SEM images of the TiO₂ nanotube layer obtained via anodization, (a) top view and (b) side view. Scale bar: 1 μm. Reprinted with permission from ref. 407. (c and d) Cross-section and oblique SEM images of TiO₂ nanotube arrays used to fabricate dye-sensitized cells. Reprinted with permission from ref. 408. (e) Smooth TiO₂ nanotubes 7 μm long produced in glycerol electrolyte with 0.5 wt% NH₄F. The inset shows the walls of the nanotubes. Reprinted with permission from ref. 409. (f) Typical top view SEM image of the TiO₂ nanotube array film. Reprinted with permission from ref. 411.

3.1.2 Synthesis of 1-D cerium oxide NCs. CeO₂ is one of the most important catalysts, which has recently attracted great attention in heterogeneous catalysis due to its high oxygen storage capacity (CeO₂ → CeO_2−x + 1/2O₂). Numerous studies have shown that the oxygen vacancies in CeO₂ can significantly promote the catalytic performance. Consequently, CeO₂ has been widely used as a support, promoter and co-catalyst in industrial catalytic processes. A variety of shaped CeO₂ NCs, such as CeO₂ nanorods, CeO₂ nanowires, CeO₂ nanotubes, and CeO₂ nanocubes have been synthesized, which have greatly prompted their application in catalysis.

1-D CeO₂ nanorods are generally prepared via the hydrothermal method since this surfactant-free method can provide CeO₂ nanorods with a clean surface for investigating their catalytic activity and mechanism.^{375,413–418} NaOH can also be used to synthesize CeO₂ nanorods in the hydrothermal method. Liyanage and co-workers synthesized Y-doped CeO₂ nanorods via the hydrothermal method, in which NaOH was used as a precipitant.⁴¹⁸ Liu et al. mixed a CeCl₃ aqueous solution with an NaOH aqueous solution under vigorous stirring. Subsequently, the mixed solution was transferred to a Teflon-lined stainless autoclave at 130 °C for 18 h.⁴¹⁹ Mai and co-workers also used NaOH as a precipitant for the synthesis of CeO₂ nanorods via the hydrothermal method.⁴²⁰ Yan et al. demonstrated the synthesis of uniform single-crystalline CeO₂ nanorods via a hydrothermal process using Ce(NO₃)₃·6H₂O and Na₃PO₄·6H₂O as the cerium source and mineralizer, respectively. By altering the hydrothermal treatment time, the morphology of the CeO₂ NCs evolved from nano-octahedrons to nanorods. Different from the reported cases using a strong base as the precipitant, such as NaOH, KOH and other organic alkalis, the use of Na₃PO₄ does not leave impurities in the hydrothermal reaction system and makes this process very simple to obtain and separate CeO₂ nanorods.⁴²¹ Ji and co-workers used a similar hydrothermal method for the synthesis of CeO₂ nanorods. An aqueous solution of CeCl₃·7H₂O was vigorously mixed with Na₃PO₄ solution and then transferred to a Teflon-lined stainless-steel autoclave. Moreover, the as-prepared CeO₂ nanorods were used as seeds to enable the growth of nanorods in length, but had a negligible influence on their diameter (Fig. 41).⁴²² Bai and co-workers recently demonstrated the synthesis of CeO₂ nanowires via a simple hydrothermal reaction by adding cerium chloride, sodium oleate, deionized water, and n-butylamine to a stainless reactor.⁴²³ The obtained CeO₂ nanowires with a mean diameter of 6 nm and length of 300 nm can be used as an excellent support for Rh single atoms (SAs) due to the abundant vacancies on their surface.

Fig. 41 TEM images of CeO₂ nanorods obtained from (a–c) primary and (d–f) secondary synthesis. When CeO₂ nanorods obtained from primary synthesis using (a) 0.1 M, (b) 0.2 M, and (c) 0.4 M CeCl₃ were used as seed crystals in a secondary hydrothermal treatment, CeO₂ (d) nanowires and (e and f) nanorods were obtained, respectively. Both the primary and secondary syntheses were conducted at 220 °C. Reprinted with permission from ref. 422.

Soft template-assisted methods have also been reported for the synthesis of CeO₂ nanorods. Vantomme and co-workers used CTAB as a surfactant for the preparation of CeO₂ nanorods.⁴²⁴ Typically, this surfactant and CeCl₃·7H₂O with a molar ratio of 0.33 were dissolved in H₂SO₄ aqueous solution (pH = 2) at 40 °C under stirring for 1 h. Afterwards, 35 mL of ammonia solution (25 wt%) was added dropwise to the mixed solution to obtain a purple precipitation, which was transferring to a Teflon-lined autoclave. The CeO₂ nanorods were obtained after hydrothermal treatment at 80 °C for 4 days under static conditions. Sun et al. demonstrated the synthesis of CeO₂ nanorods via a solvothermal method using hydrated cerium(III) chloride as the cerium source, octadecylamine as the surfactant, and ethanol/water and ethylenediamine as the co-solvent. By altering the concentration of the precursors, the diameter and length could be tuned in the range of 40–50 nm and 0.3–2 μm, respectively. It was shown that ethylenediamine played a vital role because it acted both as a complexing agent to influence the ions competing in the complex reaction and a basic agent to influence the viscosity and adjust the pH value of the solution.⁴²⁵ In addition, Zhang and co-workers prepared polycrystalline CeO₂ nanorods via an ultrasonication method, in which polyethylene glycol (PEG) was used as a structure-directing agent at room temperature. The obtained nanorods possessed a diameter and length in the range of 5–10 nm and 50–150 nm, respectively.⁴²⁶ In a typical synthesis, PEG600 was mixed with Ce(NO₃)₃ solution (PEG600: 1%) under ultrasonication at room temperature, followed by the addition of a certain amount of NaOH solution with vigorous stirring during ultrasonication for about 1 h. Afterwards, the precipitate was further sonicated for an additional 1 h to obtain the final products. It was shown that ultrasonication was critical for the formation of nanorods, where irregular nanoparticles were obtained without ultrasonication.

Han and co-workers reported the synthesis of well-crystalline CeO_2−x nanotubes via a mild hydrothermal reaction route using cerium nitrate and ammonium hydroxide as reactants. More importantly, this hydrothermal method was conducted without high pressure and high temperature.⁴²⁷ Two successive steps were involved in the synthesis. Firstly, Ce(NO₃)₃·6H₂O was added to deionized water and heated to 100 °C, followed by the addition of a certain amount of 5% ammonia hydroxide solution. Secondly, once the yellowish precipitates were formed, the products were aged for 3 min, and quickly cooled at 0 °C.⁴²⁷ Furthermore, Tang et al. produced CeO₂ nanotubes through the thermal treatment of Ce(OH)₃ nanotubes.⁴²⁸ Firstly, Ce(OH)₃ nanotubes were prepared via a standard alkali thermal treatment process in a glove box under oxygen-free conditions as a hard template. In a typical process, anhydrate CeCl₃ was added to an aqueous NaOH solution and placed in a Teflon-lined stainless-steel autoclave. Then, the autoclave was sealed and held at 120 °C for 72 h. Secondly, the as-synthesized Ce(OH)₃ nanotubes were placed in a quartz tube after evacuation (10⁻² Torr). Then, a gas flow of Ar/NH₃ (20 : 1) was introduced at a flow rate of 500 sccm and gas pressure of 500 Torr. Finally, the samples were heated to 450 °C for 2 h at a heating rate of 50 °C h⁻¹. Zhou and co-workers reported the formation CeO₂ nanotubes with extended cavities via an oxidation–coordination-assisted dissolution process of partially oxidized 1-D Ce(OH)₃ as follows:⁴²⁹ (1) 1-D Ce(OH)₃ was exposed to air at room temperature for 24 h to partially oxidize Ce(OH)₃; and (2) the partially oxidized Ce(OH)₃ was dispersed in distilled water and H₂O₂, and treated under ultrasonication for 2 h. The mechanism studies showed that the partial oxidation of Ce(OH)₃ and the addition of H₂O₂ are essential for the formation of CeO₂ nanotubes (e.g., Ce³⁺ + OH⁻ → Ce(OH)₃; Ce(OH)₃ + OH⁻ → CeO₂ + H₂O).

3.1.3 Synthesis of 1-D copper oxide NCs. Copper oxide has two typical compounds, cuprous oxide (Cu₂O) and cupric oxide (CuO). Both of them are p-type semiconductors and used as promising candidate materials for the conversion of solar energy to electrical or chemical energy.³⁷⁴ Compared to Cu₂O, CuO has a much a narrow band gap (1.2 eV vs. 2.17 eV), and thus has been widely used as a photocatalyst in many reactions.

Several methods have been reported for the synthesis of 1-D CuO NCs. Chen and co-workers demonstrated the synthesis of CuO nanorods with different surface areas. They simply mixed Cu(NO₃)₂·3H₂O and NaOH in ethanol under stirring in a flask to obtain CuO nanorods. The surface area of the CuO nanorods could be systematically tuned upon further hydrothermal treatment in NaOH solution by altering the temperature in the range of 60–160 °C.⁴³⁰ Liu et al. used another Cu precursor, such as cupric dodecyl sulfate, to react with NaOH at 80 °C for the preparation of CuO nanorods. It was shown that the morphological yield of CuO nanorods was over 95%.⁴³¹ Chang and co-worked systematically investigated the effects of the reaction parameters on the morphology and size of the CuO NCs. It was shown that single-crystalline CuO nanorods with a selected width of 5–15 nm could be obtained by simply altering the concentration of Cu(NO₃)₂ at low temperatures and atmospheric pressure. Long CuO nanorods/nanoribbons of up to 1 μm in length were produced via a two-step overgrowth process. A plausible mechanism for the overgrowth was proposed, where short nanorods in the growth solution can self-assemble into a two-dimensional netted structure with (001), (100) and (110) crystal planes.⁴³² Shrestha et al. further verified the effects of the reaction parameters on the size and morphology by comparing three chemical combinations for the hydrothermal synthesis of CuO nanorods, including (1) copper nitrate, lactic acid/sodium hydroxide, (2) copper sulfate/sodium lactate/sodium hydroxide, and (3) copper nitrate/sodium hydroxide.⁴³³ Alammar and co-workers reported the ultrasound-assisted synthesis of CuO nanorods in a neat room-temperature ionic liquid (e.g., 1-butyl-3-ethylimidazoliumbis(trifluoromethylsulfonyl)imide).⁴³⁴ Gusain et al. further confirmed that the additional sonication of the as-prepared CuO nanorods with two different ionic liquids (e.g., 1-methylimidazole and tetrabutylammonium acetate) could guide the morphological changes of CuO nanorods with a high aspect ratio.⁴³⁵

Jia et al. demonstrated the direct synthesis of ultrathin CuO nanorods with high-index facets using oleylamine as both the solvent and the surface controller (Fig. 42a–c).⁴³⁶ It was shown that the growth of the CuO nanorods follows a classic oriented attachment mechanism. PEG can also be used as growth-directing agent for the synthesis of CuO nanorods. Chen and co-workers synthesized ultrathin CuO nanorods by adding PEG to a mixture of NaOH and CuCl₂ solution (Fig. 42d). The control experiments showed that the length of the CuO nanorods decreased with an increase in the molecular weight of PEG.⁴³⁷ Wang et al. revealed the effects of PEG during the synthesis of CuO nanorods. As a nonionic surfactant, PEG monomer (CH₂–CH₂–O) can easily form chain structures in aqueous solution, which acts as a soft template for the growth of CuO nanorods.⁴³⁸ In addition to the soft template-assisted method, hard templates have also been employed for the synthesis of CuO nanorods. For instance, Wang et al. synthesized Cu(OH)₂ from CuCl₂ and NaOH, which was further subjected to calcination at 200–400 °C to obtain CuO nanorods.⁴³⁹ Porous CuO nanorods could be obtained by controlling the calcination temperature. Zhong et al. prepared CuO nanorods from a sacrificial Cu foil via naturally generated electrochemical corrosion. In particular, a wetted Cu foil was partially immersed in KOH solution (1 M) for 7 days. The CuO nanorods grew at the gas–liquid interfaces by sacrificing the Cu foil.⁴⁴⁰ Zhou and co-workers reported a similar method. Typically, a piece of Cu foil was immersed in a mixed solution of NaOH and (NH₄)₂S₂O₈ at 3 °C for serval min. Afterwards, the Cu(OH)₂ product was cleaned with water, dried at 100 °C, and finally calcined at 400 °C in an N₂ atmosphere to obtain Cu₂O nanorods.⁴⁴¹ Furthermore, Xia's group reported the synthesis of CuO nanorods through the chemical oxidation of Cu foil, in which the cleaned Cu foil was calcined in air at different temperatures (400–600 °C).⁴⁴² The results showed that the lateral dimension of the CuO nanowires decreased from ∼100 to ∼30 nm when the calcination temperature was increased from 400 °C to 600 °C (Fig. 42e–g). A similar calcination process has been reported to prepare CuO nanorods.⁴⁴³

Fig. 42 (a) TEM image and (b) HAADF-STEM image of ultrathin CuO nanorods. The inset in (a) shows a photograph of the as-prepared CuO nanorods dispersed in cyclohexane. (c) Typical HRTEM image of the individual nanorods projected from the [010] direction. The white dashed lines highlight the orientation between two parallel (201) facets. The blue arrows indicate the necking sites. The insets show the FFT patterns and the corresponding simulated two-dimensional shape of a nanorod. Small particles as “building blocks” attach with each other via the (201) facet to form a rod-like structure. Reprinted with permission from ref. 436. (d) TEM image of the CuO nanorods. Reprinted with permission from ref. 437. TEM images of CuO nanowires prepared by heating copper grids in air for 4 h at (e) 400 °C, (f) 500 °C and (g) 600 °C. Reprinted with permission from ref. 442.

Other methods such as seeded-mediated growth, solution combustion method, and directional aggregation method have been reported for the preparation of 1-D CuO NCs. Zhang and co-workers demonstrated the growth of a large-scale epitaxial array of CuO nanowires on the surface of a copper nanostructure via a seeded-mediated growth process.⁴⁴⁴ Yao et al. demonstrated the synthesis of uniform and monodisperse CuO nanorods through the directional aggregation and crystallization of tiny CuO nanoparticles. In this process, two steps are involved, i.e. the synthesis of CuO nanoparticles and aging of CuO nanoparticles to form CuO nanorods. The mechanism studies showed that these Cu nanoclusters were oxidized into CuO primary nanoparticles, which experienced a further self-aggregation and Ostwald ripening process to form CuO nanorods.⁴⁴⁵ Christy and co-workers reported a solution combustion method using citric acid as fuel for the preparation of CuO nanorods.⁴⁴⁶ In a typical synthetic process, a stoichiometric amount of citric acid was added to a Cu(NO₃)₂ aqueous solution. Then, the solution was kept in a furnace at 300 °C. CuO nanorods were obtained via the following chemical equation: Cu(NO₃)₂ + 0.611C₆H₈O₇ → CuO + N₂ + 3.66CO₂ + 2.44H₂O.

3.1.4 Synthesis of 1-D manganese oxide NCs. MnO₂ has several crystalline forms, such as α-, β-, γ- and δ-type MnO₂.^447–449 It is an important oxide and has been widely used in the magnetic field, electrochemistry, and catalysis.^450–454 Based on the redox reactions of MnO₄⁻ and/or Mn²⁺, several methods have been developed for the synthesis of MnO₂ NCs including thermal treatment,⁴⁵⁵ refluxing,^456–458 hydrothermal,⁴⁵⁹ and sol–gel method.^460–465

Li and co-workers synthesized single-crystal MnOOH nanorods with high aspect ratios via a simple hydrothermal using KMnO₄ as the Mn precursor in the presence of PVP.⁴⁶⁶ The dimensions could be tuned by altering the hydrothermal temperature, reaction time and ratio of PVP/KMnO₄. Further calcination of the as-prepared MnOOH nanorods led to the formation of MnO₂ and Mn₂O₃ nanorods with high aspect ratios. Similarly, Zhang et al. prepared β-MnO₂ nanorods through the thermal decomposition of a template precursor of MnOOH, which was obtained via the hydrothermal treatment of KMnO₄ in an aqueous solution.⁴⁶⁷ Zhang et al. demonstrated a facile route for the synthesis of γ-MnO₂ nanorods through the hydrothermal treatment of Mn₃O₄ nanoparticles, which were prepared by mixing an aqueous solution of MnSO₄ and Na₂CO₃ (Fig. 43a and b).⁴⁶⁸

Fig. 43 (a) TEM and (b) HRTEM images of the as-prepared γ-MnO₂ nanorods. Reprinted with permission from ref. 468. Low magnification TEM images of a free standing (c) α-MnO₂ nanorod and (d) β-MnO₂ nanorod. Lattice-resolved HRTEM images of (e) α-MnO₂ nanorod and (f) β-MnO₂ nanorod. Left bottom inset images in (e and f) are the corresponding FFT patterns. The middle insets in (e and f) simulate the [111] projection of the tetragonal α-MnO₂ crystal structure and the [110] projection of the tetragonal β-MnO₂ crystal structure, respectively (Mn and O marked as purple and red in color, respectively). Reprinted with permission from ref. 471.

Liu and co-workers demonstrated the preparation of β-MnO₂ nanorods via a refluxing route using MnSO₄·H₂O, Na₂S₂O₈ and NaOH as precursors.⁴⁶⁹ In this process, MnSO₄·H₂O and Na₂S₂O₈ were dissolved in deionized water, followed by the addition of a certain volume of NaOH aqueous solution under stirring for 30 min until a suspended liquid was formed. Afterwards, the suspended liquid was refluxed at 50 °C for 18 h, then refluxed at 80 °C for 5 h, and finally at 100 °C for 3 h. Wang et al. reported the formation of single-crystalline α-MnO₂ nanorods with a diameter of 30–70 nm and length of 2 μm via the hydrothermal reaction of KMnO₄ under acidic conditions.⁴⁷⁰ A plausible mechanism based on Ostwald ripening process was proposed. Firstly, KMnO₄ decomposes into MnO₂ and O₂ in acidic conditions, and hydrothermal treatment may further enhance its decomposition. Secondly, the produced MnO₂ colloids tend to form some larger agglomerates, and then small nanorods. When the KMnO₄ is consumed completely, the whole system tends to be thermodynamically stable. Thirdly, the amorphous and poorly crystalline components in the agglomerates dissolve again and grow into nanorods through a dissolution–recrystallization process. Su and co-workers prepared α-MnO₂ and β-MnO₂ nanorods via the hydrothermal method (Fig. 43c–f).⁴⁷¹ For the synthesis of α-MnO₂ nanorods, 1.5 mmol KMnO₄ and 5 mmol concentrated HCl were added to 15 mL deionized water with vigorous stirring until a transparent purple solution was formed, which was transferred to a Teflon-lined stainless steel autoclave and heated to 140 °C for 12 h. For the synthesis of β-MnO₂ nanorods, 1 mmol (NH₄)₂S₂O₈ and MnSO₄·H₂O were dissolved in 20 mL deionized water. 0.2 g CTAB as a soft template was added into the solution and heated to 140 °C in a Teflon-lined autoclave for 12 h. Toufiq et al. investigated the effects of the hydrothermal dwell time on the dimensions of MnO₂ nanorods.⁴⁷² It was shown that the diameter of the MnO₂ nanorods decreased from 460 nm to 250 nm with an increase in the hydrothermal reaction time from 5 h to 15 h. A plausible mechanism in the hydrothermal method was proposed, which involved two critical processes of oriented attachment and rolling-cum-phase.⁴⁷³ Tang et al. demonstrated a low-temperature sol–gel process associated with different surfactants (e.g., CTAB and PVP) in ethanol solvent for the preparation ultrafine MnO₂ nanowires and nanorods. In a typical synthesis, 1.0 g surfactant was dissolved in 80 mL ethanol to form a transparent solution at room temperature, followed by the addition of 5 mL concentrated nitric acid and Mn(OAc)₂·4H₂O. After the mixture was sealed, vigorous stirring was performed for a few hours until (CH₃COO)₂Mn was completely dissolved. Finally, the mixed solution was dried at 80 °C to evaporate the solvent for 48 h, and the products were washed with water 5 times, and then dried in an oven at 80 °C overnight.

3.1.5 Synthesis of 1-D vanadium oxide NCs. Vanadium oxides are well known as active catalysts for many hydrocarbon oxidation reactions. The research on vanadium oxides as catalysts can be dated back to the last century. It has been shown that the surface unsaturated coordinated V species can greatly promote their catalytic activity.⁴⁷⁴ Generally, vanadium oxides have several different valences, such as VO, V₂O₃, VO₂, and V₂O₅. It should be noted that phase equilibria in the vanadium-oxygen system are rather complicated, which makes vanadium oxides display complex structural chemistry and structure-dependent catalytic performances.³⁷⁴ Among these oxides, the rutile phase VO₂(R) is the most stable structure, and thus has recently attracted much attention.

Over the past decades, several methods have been developed for the synthesis of vanadium oxide NCs. Among them, hydrothermal treatment is one of the most important and common strategies for the preparation of 1-D VO₂ nanorods.^475–483 For instance, Gui and co-workers reported the synthesis of single-crystal VO₂ hydrate nanorods via the hydrothermal reaction of KOH, V₂O₅, and hydrazine in a sealed autoclave at low temperature. They claimed that hydrazine played a key role in the control of the 1-D morphology.⁴⁸⁴ Chen et al. fabricated VO₂ nanorods by reducing V₂O₅ with oxalic acid during the hydrothermal process.⁴⁸⁵ It was found that monoclinic phase VO₂(B) was firstly formed and then transformed into the tetragonal VO₂(A) phase with an increase in the system temperature or extended reaction time (Fig. 44). A plausible mechanism was proposed. Vanadium pentoxide was firstly dissolved and hydrated to form VO₂⁺ and V₁₀O₂₆(OH)₂₄⁻, which were further reduced by H₂C₂O₄·2H₂O to VO₂⁺ under hydrothermal treatment. When the temperature was increased to 180 °C, VO₂(B) nanobelts were formed from the VO₂ nuclei through aggregation. Finally, the short VO₂(B) nanobelts assembled into VO₂(A) nanorods through crystallographic adjustment with an increase in the temperature from 180 °C to 230 °C. Ji et al. synthesized pure phase VO₂(R) nanorods via the reduction of V₂O₅ by oxalic acid during a one-step hydrothermal treatment.⁴⁷⁶ It was shown that a lower temperature favored the formation of the VO₂(B) phase, which gradually changed to the VO₂(R) phase with an increase in temperature. They claimed that H₂SO₄ could affect the zeta potential and the crystallization-dissolution properties of VO₂(R) in the solution as a result of the morphology control of the collected products.

Fig. 44 SEM images of VO₂ nanorods formed at different temperatures for 24 h with a filling ratio of 0.4: (a) 180 °C, (b) 210 °C, (c) 230 °C, and (d) 250 °C. Reprinted with permission from ref. 485.

V₂O₅ has been considered as one of the most attractive candidates for cathodes due to its fascinating merits such as abundance, low cost, and extraordinarily high theoretical capacity.^486–488 Pinna and co-workers reported the preparation γ-V₂O₅ nanorods and nanowires via a reverse micelle technique. In a mixture of sodium bis(ethyl-2-hexyl)sulfosuccinate Na(AOT)/isooctane/H₂O, the ratio of [H₂O]/[AOT] was kept constant at 10. Reverse micelles were formed under these conditions. A solution of alkoxide VO(OR)₃ (5 × 10⁻³ M) with RCH(CH₃)₂ in isooctane was then rapidly added to the micellar solution. When the reaction time was extended from 24 h to 100 days, the short γ-V₂O₅ nanorods evolved into long nanowires, as shown in Fig. 45a and b.⁴⁸⁹ Ostermann et al. demonstrated the synthesis of single-crystal V₂O₅ nanorods on electrospun rutile nanofibers by carefully calcining composite nanofibers consisting of amorphous V₂O₅, amorphous TiO₂, and PVP (Fig. 45c and d).⁴⁹⁰ Detailed experiments indicated that the shape and size of the obtained V₂O₅ nanorods could be controlled by the content of TiO₂ and calcination conditions. In the initial stage, the TiO₂ networks in the composite fibers favored the formation of small nuclei at low temperature, resulting in the formation of thin and long nanorods. On the other hand, the presence of TiO₂ may influence the crystallization and diffusion rates of V₂O₅ and further promote the growth of V₂O₅ nanorods. Glushenkov and co-workers demonstrated the preparation of V₂O₅ nanorods via a solid-state, mass-quantity transformation from V₂O₅ powder.⁴⁹¹ Two steps were involved in the synthetic process, i.e., controlled nanoscale growth from the nanocrystals and ball milling treatment along the [010] direction.

Fig. 45 TEM images of V₂O₅ nanorods synthesized in reverse micelles observed after (a) 24 h and (b) 100 days. Reprinted with permission from ref. 489 SEM images of V₂O₅–TiO₂ nanofibers prepared by electrospinning from a solution of 2-propanol containing different ratios (r) of VO(OiPr)₃ to Ti(OiPr)₄ and then calcination at various temperatures (T): (c) r = 1 : 0, T = 475 °C, (d) r = 4 : 1, T = 475 °C, (e) r = 1 : 1, T = 425 °C, and (f) r = 1 : 1, T = 525 °C. Reprinted with permission from ref. 490.

3.1.6 Synthesis of 1-D gallium oxide NCs. Ga₂O₃ has recently attracted significant attention due to its promising catalytic activity towards alkane dehydrogenation.⁴⁷⁴ The calcination of gallium oxide hydrate (GaOOH) nanorods is a straightforward strategy for the synthesis of Ga₂O₃ nanorods. Zhang and co-workers synthesized GaOOH via a facile large-scale hydrothermal process from a mixed solution of GaCl₃, H₂O and NaOH under suitable pH conditions. After calcination at 900 °C, GaOOH could be transformed into β-Ga₂O₃ crystals with a similar morphology. In our recent work, GaOOH nanorods were prepared via a hydrothermal process from Ga(NO₃)₃ and NaOH without any surfactant, which could be further converted into Ga₂O₃ nanorods after calcination at 600 °C for 2 h.⁴⁹² It was shown that the length of the GaOOH rods was strongly related to the concentration of NaOH. More specifically, a high concentration of NaOH led to the formation of GaOOH rods with a short length. Kumar et al. synthesized β-Ga₂O₃ nanorods through the ultrasonic irradiation of molten gallium in warm water to form α-GaOOH. β-Ga₂O₃ was formed after the calcination of α-GaOOH in air at 600 °C for 3 h.⁴⁹³

Li et al. demonstrated the production of β-Ga₂O₃ nanorods with diameters in the range of 10–60 nm through a simple gas reaction method.⁴⁹⁴ In a typical process, a substrate of single-crystal LaAlO₃ (8 mm × 8 mm × 0.5 mm) was quickly dipped into a Co(NO₃)₂ ethanol solution (0.01 M). Then, 2 g of Ga metal and substrate were put in the center of a horizontal quartz tube with a distance of about 6 mm after drying. Afterwards, the quartz tube was evacuated for 30 min, and then heated to 780 °C. Finally, and the quartz tube was ventilated by air at a rate of 26 cm³ min⁻¹. The final Ga₂O₃ nanorods were collected on the substrate after cooling to room temperature. Shi et al. prepared Ga₂O₃ nanorods on an Si(111) substrate via magnetron sputtering through ammoniating Ga₂O₃ thin films.⁴⁹⁵ It was shown the ammoniating time affected the crystallinity of the Ga₂O₃ nanorods. Girija and co-workers demonstrated the preparation of pure monoclinic phase of single crystalline β-Ga₂O₃ nanorods through a facile reflux condensation process.⁴⁹⁶ In a typical synthetic process, a stoichiometric amount of CTAB was added to Ga(NO₃)₃ solution under magnetic stirring for 30 min. The solution was heated and refluxed with continuous stirring at 90 °C for 12 h in a two-neck refluxing pot. Afterwards, the reaction system was cooled to room temperature, and the collected white precipitate was washed with ethanol and distilled water. Finally, the product was dried at 100 °C and calcined at 900 °C for 3 h to obtain pure β-Ga₂O₃ nanorods.

In addition to the above-mentioned 1-D oxide NCs, other 1-D NCs, including ZnO nanorods,^497–501 In₂O₃ nanorods,^502–507 SnO₂ nanorods,^508–517 WO₃ nanorods and nanowires,^518–523 and ZrO₂ nanorods,^524–526 have also been synthesized in the literature. However, herein, we are not going to give a detailed review of them due to the following two reasons: (1) these oxides are generally not used as active components but as promoters in catalysis and (2) their synthetic protocols are similar to that of the oxides mentioned in Section 3.1. Thus, readers may refer the corresponding literature for more information.

3.2 Two-dimensional (2-D) oxide NCs

3.2.1 Synthesis of 2-D titanium oxide NCs. Sofianou and co-workers synthesized anatase TiO₂ nanoplates with exposed (001) facets via a solvothermal process.⁵²⁷ In a typical protocol, Ti(C₃H₇O)₄ was dissolved in ethanol under vigorous stirring at room temperature. Subsequently, hydrofluoric acid (HF 40%) was added to the above solution as a capping agent, which was further transferred to a Teflon-lined autoclave. The reaction was operated in an oven at 180 °C for 24 h. The as-synthesized product was collected via centrifugation. Zhang et al. demonstrated a similar hydrothermal method for the synthesis of black hydrogenated/N-doped anatase TiO₂ nanoplates with a flower-like hierarchical architecture, which involved two steps of hydrothermal synthesis and hydrogenation.⁵²⁸ In the hydrothermal step, a Ti(OC₄H₉)₄ solution was added dropwise to methanol with vigorous stirring for 30 min. Subsequently, tripolycyanamide was added to the above solution with vigorous stirring for 2 h and subsequent ultrasonication for 20 min. Then, the suspension was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated to 180 °C for 12 h. In the hydrogenation step, after drying at 60 °C for 3 h, this dry gel was then calcined at 600 °C for 4 h to obtain flower-like hierarchical N-doped TiO₂ anatase, which was further subjected to an additional hydrogenation process at 600 °C in a continuous H₂ flow (100 mL min⁻¹) for 3 h to obtain the final nanoplates. Sun and co-workers demonstrated a fluorine-free solvothermal synthetic method for the preparation of rhombic TiO₂ nanoplates with (010) facets in non-aqueous solvent.⁵²⁹ In this process, tetrabutyl titanate was used as a precursor, and acetic acid and DMF were employed as capping agents. They claimed that carboxylic acids tended to bind on the anatase (001) facets, while amide preferred the (101) facets. Consequently, the selective binding on different facets of TiO₂ restricted the crystal growth in the corresponding [001] and [101] direction, leading to the formation of rhombic-shaped TiO₂ nanoplates.

Furthermore, Katoch and co-workers demonstrated the fabrication of polyaniline/TiO₂ hybrid nanoplates via a sol–gel chemical method by adding TiO₂ NCs with a size of 3–5 nm in the process of the chemical polymerization of aniline. It was shown that the morphology evolved from a plate-like structure to an aggregate structure with an increase in the amount of TiO₂.⁵³⁰ Song et al. reported the synthesis of 2-D anatase TiO₂ hollow nanoplates via a templates-assisted route using α-Fe₂O₃ nanoplates.⁵³¹ α-Fe₂O₃ nanosheets were dispersed in ethanol with sonication for 6 h, followed by the addition of tetrabutyl titanate to the suspension under stirring. After the addition of ammonia solution (28 wt%), the mixture was transferred to an ultrasound bath for 4 h to obtain α-Fe₂O₃@amorphous TiO₂ nanoplates. After washing and drying, these Fe₂O₃@amorphous TiO₂ nanoplates were calcined at different temperatures (350 °C, 450 °C, and 600 °C) in air for 2 h at a heating rate of 1 °C min⁻¹ to promote the crystallization of the TiO₂ nanoplates. Hu and co-workers prepared NH₄TiOF₃ nanoplates with a thickness of 100 nm via a solvent-free low-heating solid-state chemical method in the absence of a template. Moreover, the NH₄TiOF₃ nanoplates could be converted into TiO₂ nanoplates with exposed (001) facets after calcination in the temperature range of 200–800 °C.⁵³² Specifically, when the calcination temperature was below 800 °C, smooth TiO₂ nanoplates with a thickness of 50 nm were obtained, as shown in Fig. 46. However, when the calcination temperature was over than 800 °C, the nanoplates disappeared under high temperature calcination, with the presence of big particles.

Fig. 46 TiO₂ nanoplates calcined at 800 °C: (a–c) SEM images, (d) TEM image, (e) SAED pattern and (f and g) HRTEM images. Reprinted with permission from ref. 532.

3.2.2 Synthesis of 2-D cerium oxide NCs. Pan et al. fabricated CeO₂ nanoplates via the hydrothermal treatment of Ce(NO₃)₃·6H₂O, CTAB, and NH₃·H₂O at 100–160 °C. It was shown that the controlled conversion of nanoplates into nanotubes and nanorods could be achieved by altering the reaction time, temperature and CTAB/Ce³⁺ ratio. They claimed that CTAB could strongly protect the growth along (111) facet. When the concentration of CTAB is low, the insufficient protection may lead to the formation of nanorods. With an increase in the concentration of CTAB, the capping ability of CTAB increased significantly, leading to a significant restriction in the formation of nanoplates.⁵³³ Lin and co-workers demonstrated the synthesis of CeO₂ nanoplates via the thermal decomposition of a mixture of cerium acetate, oleic acid, oleylamine and 1-octadecene.⁵³⁴ They claimed that when oleylamine was added to the mixed solution together with oleic acid in a one-step process, the final product was CeO₂ nanoplates. By contrast, only CeO₂ nanorods could be obtained when oleylamine was added in a separate step (Fig. 47a and b). Furthermore, Zhang and co-workers presented a hydrothermal approach for preparing single-crystal CeO₂ nanoplates with new hexagonal unit cells via a disproportionation–reduction–oxidation process of Ce(III)–Ce(IV)–Ce(III) species (Fig. 47c).⁵³⁵ A plausible mechanism was proposed for revealing the formation process of the CeO₂ nanoplates as follows: (1) the formation of CeO₂ small nanocubes from polyhedra; (2) the subsequent self-assembly of nanocubes into hexagonal stacks; and (3) the formation of CeO₂ nanoplates from CeO₂ nanocubes and rearrangement of the CeO₂ lattices via oriented attachment. On the other hand, Hu and co-workers prepared CuO–CeO₂ binary oxide nanoplates via a solvothermal method (Fig. 47d and e).⁵³⁶ In a typical procedure, an aqueous solution of Cu(NO₃)₂·3H₂O, Ce(NO₃)₃·6H₂O and PEG were prepared, followed by the dropwise addition of NaOH solution under magnetic stirring at room temperature. After aging for 2 h, the products were collected by filtering and washing with deionized water and ethanol. Subsequently, the as-obtained gel was dispersed in 500 mL ethanol by sonication and then transferred to an autoclave. After reaction at 260 °C for 2 h, the resultant powder was cooled to room temperature under a continuous Ar flow. Finally, the powder was calcined in air at 400 °C for 4 h.

Fig. 47 TEM images of the as-prepared CeO₂ (a) nanorods and (b) nanoplates. The insets are the corresponding SAED patterns. Reprinted with permission from ref. 534. (c) XRD pattern recorded from the hexagonal CeO₂ nanoplates. The left and right insets show the SEM image and SAED pattern of the CeO₂ nanoplates, respectively. Reprinted with permission from ref. 535. (d) TEM and (e) HRTEM images of the as-prepared CuO–CeO₂ nanoplates. Reprinted with permission from ref. 536.

3.2.3 Synthesis of other 2-D oxide NCs. Other 2-D oxide nanoplates have also been synthesized, such as NiO, CuO, MnO₂, ZnO, and WO₃ nanoplates. Behm and co-workers synthesized NiO nanoplates through the calcination of Ni(OH)₂ nanoplates, which were prepared by mixing NiCl₂·6H₂O, ammonium hydroxide, and hexadecyltrimethylammonium bromide in H₂O (Fig. 48a).⁵³⁷ Cao et al. demonstrated the synthesis of CuO nanoplates via a hydrothermal reaction from Cu(NO₃)·3H₂O (Fig. 48b).⁵³⁸ Ai and co-workers synthesized MnO₂ nanoplates via a microwave irradiation method in aqueous solution in the absence of any templates, catalysts or organic reagents (Fig. 48c).⁵³⁹ Also, Jin et al. used a microwave-assisted method for the production of ZnO nanoplates (Fig. 48d).⁵⁴⁰ Zheng and co-workers fabricated WO₃ nanoplates via the hydrothermal method, in which L-lactic acid was used as a capping agent in the hydrothermal process for the preparation of rectangular WO₃ nanoplates (Fig. 48e).⁵⁴¹

Fig. 48 (a) TEM image of NiO nanoplates. Reprinted with permission from ref. 537. (b) SEM image of CuO nanoplates. Reprinted with permission from ref. 538. (c) SEM image of MnO₂ nanoplates. Reprinted with permission from ref. 539. (d) SEM image of ZnO nanoplates. Reprinted with permission from ref. 540. (e) SEM image of WO₃ nanoplates. Reprinted with permission from ref. 541.

3.3 Three-dimensional (3-D) oxide NCs

3.3.1 Synthesis of 3-D titanium oxide NCs. Many studies have focused on the preparation of 3-D TiO₂ structures. Although the overall size of these 3-D TiO₂ structures are on the micrometer scale, all of them consist of nano units. For example, Zhou and co-workers synthesized 3-D TiO₂ nanostructures with various microstructures via hydrothermal methods.⁵⁴² It was shown that the solvent used in hydrothermal treatment strongly influenced the microstructures of the TiO₂ nanostructures. Typically, 3-D dandelion-like TiO₂ structures self-assembled from nanorods were obtained in a non-polar solvent based on the water–non-polar solvent interface. By contrast, 3-D microspheres were obtained when n-hexane was used as the solvent. Zhu et al. demonstrated the fabrication of flower-like anatase TiO₂ nanostructures via a one-step template-free hydrothermal method using titanocene dichloride as the precursor and ethylenediamine as the chelating agent in aqueous solution.⁵⁴³ Liu and co-workers reported the synthesis of flower-like TiO₂ nanostructures with exposed (001) facets via a low-temperature hydrothermal process from Ti powder (Fig. 49a and b). It was shown that HF acted as a critical shape-directing agent, which could react with Ti under hydrothermal conditions. On the other hand, F⁻ could reduce the surface energy of (001) facets, leading to the formation of flower-like TiO₂ nanostructures with exposed (001) facets.⁵⁴⁴ Sun and co-workers demonstrated a hydrothermal method for the preparation of 3-D TiO₂ nanostructures with different morphologies by adjusting the precursor hydrolysis rate and the surfactant aggregation (Fig. 49c–k).⁵⁴⁵ Detailed experimental data showed that the final morphologies are strongly determined by the hydrolysis rate. Specifically, a rapid hydrolysis rate cause the nano-units surrounding the TiO₂ nuclei to grow in the form of nanorods. By contrast, the nano-units tend to become nanoribbons when EG is introduced since EG can retard the hydrolysis rate in this reaction system.

Fig. 49 (a and b) SEM images of the as-prepared flower-like TiO₂ nanostructures at low and high magnifications, respectively. Reprinted with permission from ref. 544. (c–k) Typical SEM images of 3-D dendritic TiO₂ nanostructures with different morphologies. Reprinted with permission from ref. 545.

3.3.2 Synthesis of 3-D cerium oxide NCs. CeO₂ nanocubes enclosed by six (200) planes were prepared through a facile one-pot method with the assistance of solvothermal treatment. Typically, 15 mL toluene, 1.5 mL oleic acid and 0.15 mL tert-butylamine were added to a solution of cerium(III) nitrate aqueous in a Teflon-lined stainless-steel autoclave without stirring (Fig. 50a and b). After the autoclave was capped, it was put in an oven and heated to 180 °C for 24 h. It was shown that the shape and size of the CeO₂ NCs were related to the concentration of the reactants, the amount of stabilizing agents, and the water/toluene ratio in the reaction system.⁵⁴⁶ Dang and co-workers reported a similar method for the preparation of CeO₂ nanocubes using toluene, oleic acid, tert-butylamine and cerium(III) nitrate. They revealed the shape evolution from polyhedral particles to nanocubes with a prolonged reaction time (Fig. 50c and d). Detailed studies showed that the growth of the [100] direction of CeO₂ is dominant because the (111) face has a lower surface energy, leading to the formation of octahedron or truncated octahedron in the aqueous solution. In contrast, OLA prefers to bind to the (100) face with a high surface energy and limits the growth of the [100] direction in the oil phase, resulting in the formation of CeO₂ nanocubes.⁵⁴⁷ The synthesis of CeO₂ nanocubes at the liquid–liquid interface has also been reported.⁵⁴⁸ On the other hand, it has been reported that CeO₂ nanocubes can be produced via the hydrothermal reaction of Ce(NO₃)₃ and NaOH. For instance, Zhang et al. demonstrated the synthesis of CeO₂ nanocubes by mixing 0.1 M Ce(NO₃)₃ solution and 0.3 M NaOH (Fig. 50e and f).⁵⁴⁹ After stirring for 6 h, the precursor was centrifuged and washed several times with distilled water. Afterwards, 2.5 mL of the above precursor was transferred to a stainless-steel autoclave, followed by the addition of an appropriate amount of decanoic acid. After the autoclave was capped, the reaction was performed in the reactor at 400 °C for 10 min and terminated by submerging the reactor in a water bath at room temperature. Wu et al. discussed two different precursors in detail, including cerium(III) chloride and cerium(III) nitrate, for the synthesis of CeO₂ nanocubes. Moreover, the CeO₂ nanorods could be converted into nanocubes with the assistance of NO₃⁻ ions. Other ions such as Br⁻, I⁻, Cl⁻ and SO₄²⁻ can lead to the formation of CeO₂ nanorods.⁵⁵⁰ Also, Qian and co-workers demonstrated the synthesis of cubic-like CeO₂ NCs via a solvothermal process using hexadecylamine as a capping agent and CeCl₃ as the precursor at 180 °C for 24 h. It was found that both the shape and size of the CeO₂ NCs can be systematically tuned by altering the amount of water, the concentration of ligand and different types of aliphatic amine.⁵⁵¹ In addition to nanocubes, flower-like CeO₂ NCs were synthesized.^552–556

Fig. 50 (a) TEM image and (b) XRD pattern of the as-prepared CeO₂ nanocubes. Insets of (b) HRTEM images. Reprinted with permission from ref. 546. TEM images of (c) CeO₂ polyhedral particles and (d) CeO₂ nanocubes. Reprinted with permission from ref. 547. (e) TEM and (f) HRTEM images of the as-prepared CeO₂ nanocubes. Reprinted with permission from ref. 549.

3.3.3 Synthesis of 3-D copper oxide NCs. Huang and co-workers demonstrated the synthesis of Cu₂O NCs by mixing an aqueous solution of CuCl₂, SDS, NaOH, and NH₂OH·HCl reductant.⁵⁵⁷ It was shown that the shapes could systematically evolve from cubic to face-raised cubic, edge- and corner-truncated octahedral, all-corner-truncated rhombic dodecahedral, (100)-truncated rhombic dodecahedral, and rhombic dodecahedral structures with an increase in the NH₂OH·HCl concentration (Fig. 51). In another work from the same group, Kuo and co-workers synthesized monodisperse Cu₂O truncated cubic, cuboctahedral, truncated octahedral, and octahedral NCs using CuCl₂, SDS, hydroxylamine and NaOH, respectively.⁵⁵⁸ It was shown that the morphologies of the Cu₂O NCs were strongly related to SDS, while their sizes were controlled by the amount of NaOH. Yao et al. synthesized Cu₂O nanocubes and dodecahedra using 1-hexadecylamine and copper(II) acetate monohydrate. For the synthesis of Cu₂O nanocubes, copper(II) acetate monohydrate and warm 1-hexadecylamine were added to a 100 mL flask. For the synthesis of Cu₂O dodecahedra, copper(II) acetate monohydrate, warm HDA and undecane were added to a 100 mL flask. The above mixtures were preheated to 90 °C and kept for 10 min under vigorous stirring, followed by a rapid heating to 160–220 °C for 15–90 min.⁵⁵⁹ Susman and co-workers described the fabrication of Cu₂O NCs on substrates through chemical deposition. In the absence of additives, the morphologies could be well controlled. Through the chemical deposition of Cu₂O NCs on Au-seeded substrates in citrate-based solutions, the morphologies of Cu₂O NCs could be precisely controlled from cubes to octahedra.⁵⁶⁰

Fig. 51 SEM images and the corresponding schematic drawings of the Cu₂O NCs synthesized with morphology evolution from cubes to rhombic dodecahedra upon increasing the amount of NH₂OH·HCl. Two orientations are shown for each particle shape. Reprinted with permission from ref. 557.

The synthesis of hollow Cu₂O NCs was also reported using the hydrothermal method. Kuo and co-workers demonstrated the fabrication of Cu₂O nanocages and nanoframes possessing an unusual truncated rhombic dodecahedral structure.⁵⁶¹ The growth solution consisted of CuCl₂, SDS, NH₂OH·HCl, HCl, and NaOH. This protocol was quite similar to that for the synthesis of Cu₂O NCs in Fig. 52, except for the addition of HCl. In a typical synthetic process, 0.087 g SDS was added to 0.1 mL CuCl₂ solution (0.1 M) under vigorous stirring. Subsequently, 0.45 mL NH₂OH·HCl (0.2 M) and 0.09 mL of HCl (2 M) were added with slight shaking by hand, followed by the rapid injection of 0.25 mL 1 M NaOH with additional shaking for 10 s. Type-I nanoframes and nanocages were obtained after aging for 45 min and 2 h, respectively, while ethanol was added under sonication for the formation of type-II nanoframes (Fig. 52).

Fig. 52 SEM and TEM images of the truncated rhombic dodecahedral Cu₂O nanoparticles: (a–d) type-I nanoframes, (e–h) nanocages, and (i–l) type-II nanoframes. The magnified images clearly show the hollow structures of the nanoparticles. (m) Schematic illustration of the procedure for the synthesis of Cu₂O nanocages and nanoframes. Reprinted with permission from ref. 561.

3.3.4 Synthesis of other 3-D oxide NCs. Wang and co-workers prepared 3-D hierarchical WO₃ nanostructures with a novel nanosheet-assembled morphology through the acidification of Na₂WO₄·2H₂O and HCl.⁵⁶² After mixing Na₂WO₄·2H₂O with HCl, a yellow precipitate was formed, which was washed with deionized water and ethanol, and then dried at 80 °C for 10 h. Finally, the precipitates were sintered at 500 °C to obtain the WO₃ sample. A work from Huang et al. claimed that the Ostwald ripening process was involved the formation of 3-D WO₃ nanostructure.⁵⁶³ Qiu et al. prepared flower-like WO₃ nanostructures via a solvothermal method using H₂WO₄ as the precursor. An additional treatment of H₂WO₄ with H₂O₂ was highlighted (Fig. 53a).⁵⁶⁴ In a typical process, H₂O₂ (50 wt%) was added to an aqueous solution of H₂WO₄ under stirring on a hot plate (95 °C). Subsequently, a mixture of the treated H₂WO₄ solution, oxalic acid, urea, HClO₄ and acetonitrile in a certain ratio was sealed in a Teflon-lined stainless steel autoclave and maintained at 180 °C for 10 h. Wang and co-workers demonstrated the synthesis of flower-like WO₃ structures via the simple calcination of W₁₈O₄₉ nanowires. Initially, W₁₈O₄₉ nanowires were prepared via a microwave-assisted solvothermal method using WCl₆ as a precursor. Different from the reported solvothermal systems, this system was heated by microwave at 200 °C for 2 h.⁵⁶⁵ Zhang and co-workers demonstrated that BF₄⁻ could be added to the autoclave as a guiding agent, which led to the formation of orthorhombic WO₃ NCs with a tunable percentage of (001) facets.⁵⁶⁶ Ma et al. prepared uniform flower-like hierarchical Co₃O₄ nanostructures via a surfactant-free hydrothermal process at low temperature using CoCl₂, NaCl and C₆H₁₂N₄ (Fig. 53b).⁵⁶⁷ Also, the hydrothermal method has been used for the synthesis of other flower-like nanostructures, such as MoO₃–In₂O₃ (Fig. 53c)⁵⁶⁸ and SnO₂ (Fig. 53d).⁵⁶⁹

Fig. 53 (a) SEM image of flower-like WO₃ nanostructures. Inset: High magnification SEM image. Reprinted with permission from ref. 564. (b) SEM image of flower-like Co₃O₄ nanostructures. Reprinted with permission from ref. 567. (c) SEM image of flower-like MoO₃–In₂O₃ nanostructures. Reprinted with permission from ref. 568. (d) SEM image of flower-like SnO₂ nanostructures. Reprinted with permission from ref. 569.

4. Catalytic applications

4.1 CH₄ oxidation

CH₄ is among the most important and attractive feedstocks in the chemical industry due to its large reserves in the Earth.⁴²³ Thus, the production of high value-added products from CH₄ is highly important, not only because it can alleviate the oil crisis, but also reduce the emission of greenhouse gas.^570–572 Indeed, the oxidation of CH₄ has been widely investigated over the past decades for the production of methanol, olefins and aromatics.^573–582 However, these existing processes suffer from the disadvantages of high operating temperature and/or high pressure, leading to high cost. Generally, NCs with specific shapes, sizes and structures cannot bear these harsh conditions. Consequently, chemists have attempted to disperse metals on a support to prevent them from sintering under high temperature. Recently, the selective oxidation of CH₄ to produce methanol has attracted significant attention in the chemical industry, which has been regarded as a dream reaction. Despite the huge challenges arise from the strong C–H bonds in CH₄, a low operating temperature is beneficial for nanocatalysts with tailored morphologies and sizes. Over the past decade, great effort has been devoted to the direct oxidation of CH₄ to CH₃OH; however several critical points need to be overcome as follows: (1) the existing catalytic systems use H₂O₂ as an oxidant or a co-oxidant, and the high cost of H₂O₂ seriously impedes the practical application of this type of catalytic process and (2) the balance of CH₄ conversion and methanol selectivity still requires further investigation. Therefore, recent reports aimed to design efficient catalysts for reducing the concentration of H₂O₂ and/or replacing H₂O₂ by O₂ for the direct oxidation of CH₄ to CH₃OH.

Rahim and co-workers reported that H₂O₂ could be used as an efficient oxidant for CH₄ conversion to CH₃OH under mild conditions.⁵⁸³ With the assistance of electron paramagnetic resonance (EPR), ˙CH₃ and ˙OH were observed in the reaction system, indicating that the oxidation of CH₄ was triggered by radicals. Moreover, they compared the performance of TiO₂-supported Au–Pd nanoparticles and Au–Pd/TiO₂ prepared via the conventional impregnation method. It was shown that Au–Pd/TiO₂ exhibited a much lower yield than TiO₂-supported Au–Pd nanoparticles. Detailed experiments showed that Au–Pd/TiO₂ prepared via a conventional impregnation method tended to decompose H₂O₂ to form H₂O besides ˙OH, while Au–Pd nanoparticles could suppress the decomposition of H₂O₂, leading to a significant increase in CH₃OH yield. Subsequently, another work from the same group demonstrated the production of CH₃OH through the direct oxidation of CH₄ with H₂O₂ and O₂ at 50 °C on colloidal Au–Pd nanoparticles.³⁷ They claimed that the presence of H₂O₂ can trigger the oxidation of CH₄via a radical process. These PVP-stabilized Au–Pd nanoparticles can strongly suppress the degradation of H₂O₂ to form H₂O, leading to a significant increase of the CH₃OH yield. The yield of CH₃OH and CH₃OOH reached about 53.6 mol kg_cat.⁻¹ h⁻¹ at a total selectivity of 88% H₂O₂ (Table 1). Furthermore, to reduce the usage of H₂O₂, O₂ was added as a co-oxidant. Detailed experiments showed that the PVP-stabilized Au–Pd nanoparticles not only promoted the formation of radicals and CH₄ activation, but also suppressed the degradation of H₂O₂. Isotope experiments were performed to further reveal the mechanism using H₂¹⁶O₂ and ¹⁸O₂. The appearance of ¹⁶O in the products suggests that ˙CH₃ may react with ˙¹⁶O¹⁶OH or ¹⁶O₂ (from H₂¹⁶O₂). When 10 mmol of H₂O₂ and 5 bar of ¹⁸O₂ pressure were employed, the primary products contained 70% ¹⁸O and 30% ¹⁶O. Consequently, a plausible mechanism was proposed, where H₂O₂ can produce ˙OH, which can activate CH₄ to form ˙CH₃. Moreover, ˙CH₃ will further react with ˙OOH or O₂ to form CH₃OOH.

Table 1 Comparison of the catalytic activity of supported and unsupported colloidal Au-only, Pd-only, and Au–Pd catalysts for the liquid phase oxidation of CH₄ using H₂O₂. Test conditions: reaction time = 0.5 h; stirring speed = 1500 rpm; CH₄ pressure = 30 bar; reaction temperature = 50 °C; and stirred heating ramp rate = 2.25 °C min⁻¹. Dash indicates not applicable. Reprinted with permission from ref. 37

Entry	Catalyst	H2O2 amount (μmol)	O2 pressure (bar)	Amount of product (μmol)				Primary oxygenate selectivity (%)	Oxygenate productivity (mol kgcatalyst^−1 hour^−1)	H2O2 consumed (%)	Gain factor
				CH3OOH	CH3OH	HCOOH	CO2
a Entry 1: sol-immobilized solid catalyst, 100 mg (6.6 μmol of metal in 10 mL of water). b Entries 2 to 7: colloidal catalysts (10 mL, 6.6 μmol of metal). c Entries 8 and 9: homogeneous metal precursor solutions (6.6 μmol of metal in 10 mL of water).
1^a	1%Au–Pd/TiO2	1000	0	0.0	0.4	0.00	1.2	26	0.03	73	2 × 10^−3
2^b	Au–Pd colloid	1000	0	11.8	3.3	0.6	1.1	90	29.4	38	3 × 10^−2
3^b	Au–Pd colloid	1000	5	17.4	7.6	1.8	1.5	88	53.6	27	9 × 10^−2
4^b	Au–Pd colloid	0	5	0	0	0	0.2	0	0	0	—
5^b	Au–Pd colloid	50	5	15.7	2.8	1.2	0.3	92	39.4	44	1.2
6^b	Pd colloid	50	5	0	0	0	0.7	0	0	22	0
7^b	Au colloid	50	5	0	0	0	0.1	0	0	12	0
8^c	PdCl2	50	5	0	0	0	0.3	0	0	5	0
9^c	HAuCl4	50	5	0	0	0	0.2	0	0	10	0

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Based on the above two cases, two critical points should be highlighted as follows: (1) only one C–H bond should be activated in CH₄ to increase the selectivity of CH₃OH. Sometimes, highly dispersed metal atoms may over-activate C–H to form the products of CO_x and (2) H₂O₂ is a very important chemical, which is much more expensive than CH₃OH, and thus this dream process of CH₄ oxidation with H₂O₂ may be seriously impeded in industry. Thus, to overcome these problems, recent studies focused on the surface modification of metals to balance their activity and selectivity, and the reduction of the amount of H₂O₂ used in reaction. For example, Bai and co-workers modified Pd nanoparticles with Cu to boost the selective oxidation of CH₄ to CH₃OH and CH₃OOH.⁵⁸⁴ It was shown that the interface of CuPdO₂/CuO, which was formed by calcining Pd–Cu nanoparticles in air, could efficiently activate and convert CH₄ to CH₃OH using H₂O₂ or O₂ as an oxidant under mild conditions. By altering the amount of Pd and Cu in these nanoparticles, different ratios of CuPdO₂ and CuO were observed, leading to a variable oxygenate yield. The optimized catalyst gave a yield of oxygenates of 4076.5 μmol g⁻¹ h⁻¹ and a selectivity of 93.9% (Fig. 54). Bai et al. further tuned the Ru⁴⁺ ratio at the interface of Ru–Cu lichen-like nanoparticles.²⁹³ It was shown that the composition-dependent synergistic effects between Ru and Cu in these nanoparticles, which is related to the ratio of Ru⁴⁺ on the surface of the catalysts, can significantly promote the selective oxidation of CH₄ to form CH₃OH. To further demonstrate the role of Ru⁴⁺ species, they calcinated these lichen-like nanoparticles at different temperatures. High temperature calcination led to a high ratio of Ru⁴⁺ as a result of the stronger synergy at the interface and higher oxygenate yield. Although high temperature calcination will result in the sintering of Ru–Cu nanoparticles, surface engineering of composite catalysts indeed affects their catalytic performance strongly. They proposed a radical trigger mechanism for the selective oxidation of CH₄ by detecting ˙CH₃ and ˙OH radicals in the reaction system. This work highlighted the significance of interface modification on the selective oxidation of CH₄. More importantly, the amount of H₂O₂ was as low as 10 μmol, while the oxygenate yield was 3241.6 μmol g⁻¹ h⁻¹ with a total selectivity of 93.8%.

Fig. 54 Product yields and oxygenate selectivity of (a) Cu/C, Pd/C, and Pd_0.3Cu_0.7/C, (b) CuO/C, PdO/C, and Pd_0.3Cu_0.7O/C, (c) Pd_xCu_1−xO/C and (d) Pd_0.3Cu_0.7O/C, CuPdO₂/C, and CuPdO₂/C + CuO/C. Reaction conditions: 10 mg of catalyst, 0.5 MPa CH₄, 20 mL of 1 mol L⁻¹ H₂O₂, and 50 °C, 1 h. (e) HAADF-STEM image of Pd_0.3Cu_0.7 nanoparticles. (f) HRTEM image and (g) atomic model of Pd_0.3Cu_0.7O/C. Reprinted with permission from ref. 584.

To achieve the selective activation of C–H bonds in CH₄, Bai et al. recently demonstrated that the cooperation of CeO₂ nanowires and Rh SAs can efficiently catalyze the selective oxidation of CH₄ to CH₃OH.⁴²³ It was shown that the abundant vacancies on CeO₂ nanowires can well anchor isolated Pt atoms. A plausible mechanism was proposed, where H₂O₂ can be decomposed into ˙OH on two neighboring Ce(III) atoms with vacancies and into ˙OOH on Ce(IV) atoms. CH₄ will be adsorbed and activated on Rh SAs to form ˙CH₃ with the assistance of radicals. ˙CH₃ further reacts with ˙OH and ˙OOH to form CH₃OH and CH₃OOH. The over-oxidation of ˙CH₃ is suppressed on Rh SAs based on density functional theory (DFT) calculations. By contrast, CH₄ activation with H₂O₂ on Rh clusters is different from that on Rh SAs. ˙CH₃ and CH₃OOH can be oxidized into *CH₂ and then to HCOOH by ˙OH on Rh clusters. Finally, HCOOH will react with ˙OH to form CO and CO₂. The vacancies can be compensated by H₂O₂via a revisable process of Ce(III) ↔ Ce(IV). Different from the existing systems, CeO₂ is involved in the selective oxidation of CH₄ by promoting the formation of ˙OH, leading to a significant increase in the oxygenate yield in comparison to Rh clusters on CeO₂ nanowires (Fig. 55).

Fig. 55 (a) TEM image of the SAs Rh–CeO₂ nanowires. (b) Aberration-corrected (AC)-HAADF-STEM image in temperature color of the SA Rh–CeO₂ nanowires. The isolated Rh atoms are marked with black circles. (c) Intensity profile recorded from the line in (b). (d) AC-HAADF-STEM image in temperature color of the Rh/CeO₂ NWs. The Rh cluster is marked with blue line. (e) Intensity profile recorded from the line in panel (d). (f) Yield and selectivity of oxygenates from Rh/CeO₂-com, Rh/CeO₂ nanowires, and SAs Rh–CeO₂ nanowires. Reaction conditions: P_CH4: 0.5 MPa, H₂O₂: 20 mL (1 M), T: 50 °C, reaction time: 1 h, and catalyst weight: 10 mg. (g) Catalytic performance at different temperatures over SA Rh–CeO₂ nanowires. Reaction conditions: P_CH4: 0.5 MPa, H₂O₂: 20 mL (1 M), T: 30–90 °C, reaction time: 1 h, and catalyst weight: 10 mg. (h) Catalytic performance with different H₂O₂ concentrations over SA Rh–CeO₂ nanowires. Reaction conditions: P_CH4: 0.5 MPa, H₂O₂: 20 mL (0.1–1.5 M), T: 50 °C, reaction time: 1 h, and catalyst weight: 10 mg. (i) Catalytic performance at different CH₄ partial pressures over SA Rh–CeO₂ nanowires. Reaction conditions: P_CH4: 0.5–3 MPa, H₂O₂: 20 mL (1 M), T: 50 °C, reaction time: 1 h, and catalyst weight: 10 mg. The error bars represent the standard deviation of three experiments. Reprinted with permission from ref. 423.

Despite the promising activity of CH₄ conversion in the presence of H₂O₂, the high cost of H₂O₂ is the main problem for the practicability of this process. Accordingly, because H₂O₂ can be directly produced from H₂ and O₂, a feasible process by converting CH₄ with H₂ and O₂ to CH₃OH was proposed. To achieve this process, one should well balance the formation of H₂O₂ and the activation of CH₄ because H₂O₂ production from H₂ and O₂ is generally operated at a low temperature (e.g., 0 °C),^585,586 while CH₄ activation requires a high temperature. Another challenge is that H₂O₂ tends to rapidly decompose to H₂O and O₂, leading to a very low amount of H₂O₂ and low CH₄ conversion. Thus, to solve these problems, Jin and co-workers recently reported a strategy for the in situ formation of peroxide in the CH₄ oxidation process to CH₃OH.³⁵ They loaded Au–Pd alloy nanoparticles in aluminosilicate zeolite crystals and modified the external surface of the zeolite with organosilanes. The silanes allowed the diffusion of H₂, O₂ and CH₄ molecules in the Au–Pd nanoparticle, but suppressed the desorption of formed H₂O₂, leading to a great enhancement in the reaction activity. The optimized catalyst gave a methane conversion of 17.3% at a methanol selectivity of 92%, as shown in Fig. 56. The concept of a molecular fence was further verified by testing the catalysts in the presence of H₂O₂. Compared to the high CH₃OH yield from the conventional ZSM-5-supported Au–Pd catalyst, the capped Au–Pd nanoparticles exhibited much lower activity, indicating that silanes can inhibit the diffusion of H₂O₂ from the solution to the catalyst surface. Similarly, the silanes will also prohibit the desorption of H₂O₂ in the solution as a result of the high CH₄ conversion.

Fig. 56 (a) Oxidation of CH₄ with H₂ and O₂ over various catalysts. Reaction conditions: 10 mL water, 30 min, 70 °C, 27 mg of catalyst, and feed gas at 3.0 MPa with 3.3% H₂/6.6% O₂/1.6% CH₄/61.7% Ar/26.8% He, and 1200 revolutions per minute (rpm). (b and c) Dependence of the methane conversion (Conv.), methanol selectivity (Sel.), methanol productivity (Prod.), and H₂O₂ concentration in water solution on the reaction time over (b) Au–Pd@ZSM-5-C16 and (c) Au–Pd@ZSM-5 catalysts. Reaction conditions: 10 mL of water, 70 °C, 27 mg of catalyst, feed gas at 3.0 MPa with 3.3% H₂/6.6% O₂/1.6% CH₄/61.7% Ar/26.8% He, and 1200 rpm. Each reaction was tested eight times to obtain the error bars. Reprinted with permission from ref. 35.

4.2 CO₂ reduction

CO₂ reduction is another hot topic in catalysis, which has attracted great attention in chemical industry. Generally, different types of catalytic processes have been reported for the reduction of CO₂, such as thermal catalysis, electrocatalytic process, and photocatalysis. For the conventional thermal catalytic process, CO₂ can be converted into high value-added products, such as methanol, ethanol and olefins, through hydrogenation. For an electrochemical process, CO₂ can be converted into CO, CH₄, and alcohols. For a photocatalytic process, CO₂ can be reduced into CO, CH₄ and other chemicals. The thermal catalytic CO₂ reduction has been studied for several decades; however, it still requires further investigation to increase its conversion and selectivity. Compared to CO oxidation, thermal catalytic CO₂ reduction is much more difficult due to the chemical inertness of CO₂. Thus, a common strategy for improving the CO₂ reduction is to increase the reaction temperature. However, a high temperature will lead to an increase in the energy consumption and bring great difficulty in the control of selectivity. On the other hand, a high temperature may result in the aggregation of nanocrystals. Thus based on these issues, electrochemical and photocatalytic CO₂ reduction under mild conditions have become a hot topic in the chemical industry. Although the progress in nanomaterials has significantly improved their catalytic performance, the electrochemical and photocatalytic reduction of CO₂ still suffer from the disadvantages of poor activity and low selectivity. To achieve high-efficiency electrochemical CO₂ reduction, one needs to design efficient and robust nanocatalysts that can balance the activation of CO₂ and coupling of C-C bonds to form value-added products. The main challenges for photocatalytic CO₂ reduction are the low activity and poor selectivity for the product, especially poor selectivity for C₂₊ products. Therefore, the design of efficient catalysts is highly necessary for the efficient reduction of CO₂.

4.2.1 Thermal catalytic CO₂ reduction. Zhang and co-workers reported that the d-band center and negative charge density of Rh are dependent on the dimensions and structures of NCs.⁵⁸⁷ They alloyed Rh with W to form RhW nanosheets and nanoparticles for the hydrogenation of CO₂ to methanol. The experimental results showed that the activity of the Rh₇₅W₂₅ nanosheets is 5.9, 4.0, and 1.7 times higher than that that of Rh nanoparticles, Rh nanosheets, and Rh₇₃W₂₇ nanoparticles, as shown in Fig. 57a and b. Two factors were considered to explain this enhancement in activity. The addition of W into Rh nanosheets can promote the charge transfer from W to Rh, leading to a high d-band center. On the other hand, the quantum confinement on the nanosheets will also strengthen the d-band center of Rh in comparison to nanoparticles, resulting in the stronger adsorption of CO₂ on Rh. A high density of electrons on active sites can promote CO₂ adsorption. In addition to alloying with another element to increase the electron density via charge transfer, some specific morphologies may favor the accumulation of electrons. Khan et al. synthesized Pt₃Co octapods, Pt₃Co nanocubes, Pt octapods, and Pt nanocubes for the hydrogenation of CO₂ to methanol.⁵⁸⁸ It was shown that the vertices in the Pt₃Co octapods are more negatively charged than that in the other three NCs due to the sharp-tip and alloy effects. Consequently, the CO₂ conversion was 2.2, 6.1, and 6.6 times higher than that of Pt₃Co nanocubes, Pt octapods, and Pt nanocubes (Fig. 57c and d). Bai et al. demonstrated that ordered PdCu nanoparticles can promote *CO hydrogenation to *HCO, the rate-determining step for the hydrogenation of CO₂ to ethanol, leading to a TOF value of 359.0 h⁻¹ at a high ethanol selectivity of 92.0% (Fig. 57e–h).⁵⁸⁹ X-ray photoelectron spectroscopy (XPS) measurements also confirmed that the electrons transfer from Cu to Pd, leading to a significant increase in ethanol selectivity.

Fig. 57 (a) Products of CO₂ hydrogenation over Rh nanoparticles, Rh nanosheets, Rh₇₃W₂₇ nanoparticles, and Rh₇₅W₂₅ nanosheets at 150 °C after 5 h. (b) Comparison of TOF_metals and TOF_Rh for different catalysts. Reprinted with permission from ref. 587. (c) Product yields achieved on Pt nanocubes, Pt octapods, Pt₃Co nanocubes, and Pt₃Co octapods (20 mg, 5% mass loading) in the hydrogenation of CO₂ at 150 °C for 5 h. (d) Comparison of the TOF_Metal and TOF_Pt numbers of different catalysts. Reprinted with permission from ref. 588. Achieved product yields of CH₃OH and C₂H₅OH (ROH) and selectivity of C₂H₅OH of Pd₂Cu nanoparticles with different loadings on (e) P25 and (f) over different supports in the hydrogenation of CO₂ at 200 °C for 5 h. (g) Achieved product yields of ROHs and selectivity of C₂H₅OH and (h) comparison of the TOF_Pd values of Pd–Cu NPs/P25. Error bars correspond to deviations from three independent experiments. Reprinted with permission from ref. 589.

Other non-noble metals have been widely used for CO₂ hydrogenation. With the development of nanoscience, chemists can synthesize non-noble NCs with desired shapes to investigate the mechanism of catalytic performance on different-shaped NCs. Liao and co-workers presented the shape effect of ZnO NCs on their interaction with Cu in the synthesis of methanol from the hydrogenation of CO₂. It was shown that the interface of Cu–ZnO facilitated the reduction of ZnO via surface Cu oxides by spilled hydrogen at a much lower temperature.⁵⁹⁰ Rungtaweevoranit et al. composited Cu nanoparticles with a Zr-based metal–organic framework (MOF) (UIO-66) for the hydrogenation of CO₂.⁵⁹¹ It was shown that the strong interaction between the Cu nanoparticles and Zr oxide [Zr₆O₄(OH)₄(–CO₂)₁₂] secondary building units of the MOF could strongly promote the catalytic performance. The performance of optimized catalyst exceeded the benchmark Cu/ZnO/Al₂O₃ catalyst and gives a steady 8-fold enhanced yield and 100% selectivity for methanol.

4.2.2 Electrocatalytic CO₂ reduction. The electrocatalytic reduction of CO₂ is a promising process to convert CO₂ into useful products. Compared with other conversion methods, the electrocatalytic reduction of CO₂ does not require H₂ as a feedstock since it is intrinsically coupled with the electrocatalytic splitting of H₂O.⁵⁹² However, the electrocatalytic reduction of CO₂ suffers from the disadvantages of high overpotential (about −1.0 V) and low selectivity for desirable products.⁵⁹³ It has been reported that both the activity and selectivity depend on the sizes and shapes of Cu NCs. Loiudice and co-workers demonstrated the non-monotonic size dependence of the selectivity in cubic Cu NCs in the electrocatalytic reduction of CO₂.⁵⁹⁴ They prepared Cu nanocubes with sizes of 24 nm, 44 nm and 63 nm. It was shown that the cubes with a 44 nm edge length gave the highest selectivity towards the electrocatalytic reduction of CO₂ and faradaic efficiency for ethylene (Fig. 58a). Manthiram et al. demonstrated that Cu nanoparticles supported on glassy carbon (n-Cu/C) exhibited 4 times greater methanation current densities compared to high-purity copper foil electrodes in the electrocatalytic reduction of CO₂ (Fig. 58b and c).⁵⁹⁵ Moreover, Baturina and co-workers reported that Cu nanoparticles exhibited much higher C₂H₄ selectivity than smooth Cu films (Fig. 58d).⁵⁹⁶ The selectivity of C₂H₄ increased with a decrease in the size of the Cu nanoparticles, which may be attributed to the increase of under-coordinated sites, such as corners, edges, and defects in Cu nanoparticles with small sizes. A similar correlation between particle size and catalytic performance was reported on other metals, such as Au,^597,598 Ag⁵⁹⁹ and Pd nanoparticles.⁶⁰⁰ For example, it was shown that the electrocatalytic activity significantly increased when the size of Au nanoparticles was lower than 5 nm, which may be attributed to the significant increase in low-coordinated Au atoms on the surface of small nanoparticles.⁶⁰¹

Fig. 58 (a) Bar graph reporting the faradaic efficiencies for each product in Cu nanocubes with different sizes and Cu foil at −1.1 V vs. RHE. The glassy carbon signal was subtracted. Reprinted with permission from ref. 594. (b) Total current density, demonstrating that n-Cu/C has greater overall reduction activity than copper foil. (c) Methanation current density, in which the combined effect of the improved current density and faradaic efficiency on n-Cu/C is apparent. Reprinted with permission from ref. 595. (d) Faradaic efficiency vs. time for the generation of H₂ (solid squares) and CO (open squares) on different thin films held at −1.2 V. CO₂-Purged 0.1 M KHCO₃ solutions at pH = 6.8, 1600 rpm, and 20 μg Cu cm⁻² Cu loading. Reprinted with permission from ref. 596.

Furthermore, it has been found that the electrocatalytic performance of Cu NCs is strongly related to their shape. Wang and co-worked discussed the electrocatalytic performance for the reduction of CO₂ on different facets of Cu NCs. They used Cu nanocubes with (100) facets as a starting model to create high energy surface of (110) via a chemical etching process. It was shown that the etched NCs exhibited much higher catalytic activities in comparison to Cu nanocubes.⁶⁰² Roberts and co-workers compared the catalytic performance on different facets of Cu NCs, including Cu(100), Cu(211) and Cu(111).⁶⁰³ It was shown that Cu(100) exhibited the highest selectivity for the production of ethylene, while Cu(111) gave the worst ethylene selectivity (Fig. 59). They claimed that the single-crystal studies failed to explain the formation of methane. Despite the highest ethylene selectivity on Cu (100), the (100) facet generates a significant amount of methane. On the other hand, compared to bulk foil, the local pH value of Cu NCs is much higher, leading to an improvement in the selectivity for ethylene.

Fig. 59 (a) Comparison of cyclic voltammograms (CVs) in 0.1 M KHCO₃. Methane and ethylene formation on (b) Cu(211), (c) Cu(100), and (d) Cu(111). Reprinted with permission from ref. 603.

4.2.3 Photocatalytic CO₂ reduction. The photocatalytic reduction of CO₂ has attracted increasing attention recently since it is regarded as a clean and renewable process for simultaneously capturing this major greenhouse gas and solving the shortage of sustainable energy. Thus, many reviews regarding the photocatalytic reduction of CO₂ have been published over the past decade. They discussed the fundamental concept, catalyst design and modification and mechanism about photocatalysis in detail.^604–617 Hence, here, we focus on the shape-dependent photocatalytic performance.

Yu and co-workers demonstrated the correlation of photocatalytic CO₂ reduction activity and exposed surface of anatase TiO₂.⁶¹⁸ It was shown that the yield of CH₄ was strongly dependent on the ratio of (001) and (101) facets. The optimized catalyst has a (001) ratio of 58% (Fig. 60a). A plausible mechanism was proposed. Most of electrons and holes are mainly accumulated on the (101) facets due to the low percentage of (001) facets. The charge carriers easily recombine with holes and only a small fraction of electrons and holes are separated, leading to a poor photocatalytic performance. Once an optimal ratio of (001) facet and (101) facet was achieved, the electron and hole pairs could be separated, since the electrons would migrate to (101) facet, while holes would migrate to (001) facet. Consequently, an improved photocatalytic performance was achieved. Moreover, Liu and co-workers compared the photocatalytic performance on different phases of TiO₂ NCs.⁶¹⁹ It was shown that the densities of surface oxygen vacancies were different on the TiO₂ NCs after He pre-treatment. Consequently, Ti³⁺ sites were observed on anatase and brookite but not on rutile. These oxygen vacancies can significantly promote the formation of CO and CH₄ from the photocatalytic reduction of CO₂ (Fig. 60b). Liu et al. also claimed that the co-existence of (001) and (101) facets could greatly promote the formation of oxygen vacancies, leading to a significant enhancement in photocatalytic efficiency (Fig. 60c and d).⁶²⁰ The synergistic effects associated with the co-existence of (001) and (101) facets enabled a 4-fold enhancement in CO₂ reduction in comparison to TiO₂ with a single (001) plane or (101) plane and P25. Truong and co-workers demonstrated that branched TiO₂ NCs with (331) facets exhibited much higher activity than rods with exposed (110) facets in the photocatalytic reduction of CO₂.⁶²¹ It was shown this high-index facet could not only provide more active sites such as edge and corner sites, but also promote the separation of electron–hole pairs. The presence of defects, such as oxygen vacancies and uncoordinated Ti atoms, can greatly enhance the photocatalytic performance for the reduction of CO₂. Kong and co-workers demonstrated that both surface and bulk defects in TiO₂ NCs could strongly influence the performance for the photocatalytic reduction of CO₂.⁶²² It was shown that a decrease in the relative concentration ratio of bulk defects to surface defects could significantly promote the separation of electron and hole pairs, leading to a sharp increase in activity.

Fig. 60 (a) Comparison of the photocatalytic CH₄-production activity of P25 and TiO₂ samples prepared by varying the HF content. Reprinted with permission from ref. 618. (b) Production of CO and CH₄ on the three unpretreated and He pre-treated TiO₂ polymorphs under 6 h illumination. Reprinted with permission from ref. 619. CO production over TiO₂ and TiO_2–x NCs with different exposed facets under (c) UV-vis light (200–700 nm) and (d) visible light (400–700 nm) irradiation. Reprinted with permission from ref. 620.

4.3 Selective hydrogenation

Selective hydrogenation is one of the most important processes in the chemical industry, which requires catalysts that can selectively catalyze the desired reaction paths but suppress side reactions.^623–629 To achieve selective hydrogenation, one needs to precisely control the surface properties of catalysts to promote the formation of desired products, but suppress the generation of by-products. Conventional strategies such as increasing the dispersity of metals and exposing more active sites can significantly increase the activity. However, selective hydrogenation generally requires the suppression of the activity to some extent since a high dispersity of active sites may favor the formation of by-products. For example, the selective hydrogenation of light alkynes (e.g., acetylene and propyne) to relative alkenes (e.g. ethylene and propylene) is a critical process to remove trace amounts of alkynes the alkenes before polymerization since the presence of alkynes can poison the catalyst. To remove trace amount alkynes, one needs to selectively convert alkynes to alkenes rather than alkanes. Noble metal-based catalysts (e.g., Pd) are active for hydrogenation; however, they favor over-hydrogenation. Another important case is the compounds with co-existing C=C and C=O bonds. The selective hydrogenation of C=O is a very important process in the chemical industry. However, the C=C bond is much more active than the C=O bond, which is generally preferentially converted. Specifically, selective hydrogenation requires activity to be partially sacrificed for selectivity. With the development of nanoscience and nanotechnology, chemists achieved control of the morphology and composition of nanocrystals, and thus can regulate the surface properties of catalysts for selectively adsorbing/desorbing the reactant/intermediate/product.

Chung and co-workers demonstrated that Pd nanocubes enclosed by (100) facets exhibited much higher alkene selectivity (>90%) than a commercial Pd/C catalyst in the selective hydrogenation of 5-decyne, 2-butyne-1,4-diol, and phenylacetylene.⁶³⁰ They claimed that the enhancement in selectivity was caused by the large adsorption energy of the carbon–carbon triple bond on the (100) facets of the Pd nanocubes. However, the detailed mechanism should be further studied since the strong adsorption of the carbon–carbon triple bond on (100) facet is insufficient for explaining the high selectivity of alkenes. Crespo-Quesada et al. investigated the catalytic performance of the selective hydrogenation of acetylene on Pd NCs enclosed by (100) and (111) facets. They considered Pd–C as the active sites for the selective hydrogenation of acetylene. Pd cubes with (100) facets reacted with acetylene to form a PdC_0.13 phase at a rate roughly 6-fold higher than that of Pd octahedrons with (111) facets. Consequently, the Pd nanocubes exhibited higher activity than Pd octahedrons towards the selective hydrogenation of acetylene.⁶³¹ Niu and co-workers claimed that poisoning the Pd surface with Pb could significantly promote the selective hydrogenation of alkyne.⁶³² They found that the addition of Pb influenced the morphologies of Pd–Pb NCs. However, only octahedrons exhibited high selectivity for the hydrogenation of phenylacetylene (Fig. 61). The poisoning effects of Pb have also been reported by Zhang and co-workers.⁶³³ They indicated that Pd–Pb NCs exhibited much higher styrene selectivity than that of Pd-C catalysts. Moreover, they found that the concave shape of Pd–Pb nanocubes gave much higher activity at a comparable styrene selectivity (Fig. 62).

Fig. 61 Hydrogenation of phenylacetylene on Pd–Pb NCs with different shapes and Pb surface compositions: (a) cubes, 0%; (b) cuboctahedra, 0.2%; (c) truncated octahedra, 2.1%; (d) slightly truncated octahedra, 13.8%; (e) octahedra, 27.0%; and (f) octahedra, 0%. Reprinted with permission from ref. 632.

Fig. 62 (a) Hydrogenation of phenylacetylene over different concave degree Pd–Pb catalysts and a benchmark catalyst: (b) Pd₃Pb nanocubes, (c) Pd₃Pb concave NCs, (d) Pd₃Pb concave NCs and (e) Pd/C. Reaction conditions: phenylacetylene (0.1 mmol), ethanol (10 mL), H₂ (2 atm), 50 °C, and catalyst (5 mg; 10% Pd/C, 5 mg). Reprinted with permission from ref. 633.

Our recent work indicated that Pd octahedrons enclosed by (111) facets could be used as highly active, selective and stable catalysts for the selective hydrogenation of propyne.⁶³⁴ By contrast, Pd nanocubes enclosed by (100) facets favored the over-hydrogenation of propyne. Different from these existing reports that claimed Pd carbides are the active sites for the selective hydrogenation of alkyne, we found that both the (111) and (100) facets tended to over-hydrogenate propyne; however, the apexes of cubes and octahedrons are the active sites since propyne prefers to adsorb at the apexes. Theoretical calculations further confirmed that propyne molecules can be easily converted into propylene. However, over-hydrogenation is kinetically suppressed because the formed propylene tends to desorb from the apexes other than over-hydrogenate into propane. To further confirm the theoretical calculations, more control experiments were performed. For example, FeCl₃ solution was used to etch the apexes of Pd octahedrons. It was found that the apexes were gradually etched with an increase in time, which completely evolved into spherical Pd nanoparticles. The selectivity of propylene correspondingly decreased from about 96% to 50% after the apexes were etched (Fig. 63).

Fig. 63 TEM images of etched Pd octahedrons (∼19 nm) and their catalytic performance for the hydrogenation of propyne. TEM images of Pd octahedrons and SIRAL40-supported Pd nanoparticles after etching by FeCl₃ solution for different times: (a and e) 0 min; (b and f) 10 min; (c and g) 30 min; and (d and h) 60 min. (i) Propyne conversion, propylene selectivity, and propane selectivity of SIRAL40-supported Pd nanoparticles etched by FeCl₃ for propyne hydrogenation: purple, propyne conversion; red, propylene selectivity; and green, propane selectivity. (j) Propylene yield of SIRAL40-supported Pd nanoparticles etched by FeCl₃ for propyne hydrogenation. Conditions for etching: 1 mL FeCl₃ solution (1 M) was added to 5 mL Pd octahedron solution with moderate stirring (600 rpm) at 45 °C. Conditions for hydrogenation: T = 35 °C, P_total = 0.1 MPa, C₃H₄/H₂/N₂ = 6/12/32 mL min⁻¹, weight of supported catalyst 0.2 g. Reprinted with permission from ref. 634.

The selective hydrogenation of α,β-unsaturated ketones and aldehydes to unsaturated alcohols is a very important process in the petrochemical, biological and chemical industries.^635–647 As mentioned above, α,β-unsaturated ketones and aldehydes have an unsaturated C=C and C=O bond; however, the C=C bond is thermodynamically preferred during hydrogenation. Thus, highly efficient catalysts with high selectivity for the hydrogenation of the C=O bond need to be designed.

Zhu and co-workers reported that ultrasmall Au₂₅(SR)₁₈ (25 and 18 represent the number of Au atoms and ligands, respectively) nanoparticles could achieve 100% chemoselectivity for the unsaturated alcohol in the selective hydrogenation of α,β-unsaturated ketones (e.g. benzalacetone).⁶⁴⁸ A plausible mechanism was proposed that Au₂₅ possessed low-coordinated Au atoms, which could be used as active sites for C=O activation. To further reveal the mechanism, Ouyang and Jiang simulated the catalytic process using first-principles DFT calculations. It was shown that that H₂ and benzalacetone were co-adsorbed on Au₂₅(SR)₁₈ clusters. When H₂ was cleaved into two H atoms on the surface of Au, one of them diffused from the Au atom to the adsorbed benzalacetone. The energy barrier for the hydrogenation of C=O and C=C is 0.99 and 1.12 eV, respectively, indicating that the hydrogenation of C=O is preferred. Moreover, ethanol was found to further stabilize the partially hydrogenated intermediate in the hydrogenation of C=O via a hydrogen bond, leading to a smaller H₂ cleavage energy (0.90 eV).⁶⁴⁹ Wu et al. demonstrated that alkylamine-capped Pt₃Co alloy NCs could be used as highly selective catalysts for the chemoselective hydrogenation of cinnamaldehyde and citral.⁶⁵⁰ It was shown that the selectivity for unsaturated alcohols gradually deceased with the shortening of the amine chain length, which was coated on the Pt₃Co NCs, as presented in Fig. 64a and b. The plausible mechanism suggests that an amine with a long chain length can retard the adsorption of the reactants on Pt₃Co alloy NCs, leading to a slow reaction rate, but a high selectivity for unsaturated alcohol. Serrano-Ruiz and co-workers studied the surface structure and shape of Pt NCs in the selective hydrogenation of crotonaldehyde and cinnamaldehyde.⁶⁵¹ It was reported that the Pt(111) facet showed higher selectivity for unsaturated alcohol than the Pt(100) facet but a much slower reaction rate. It seems that sacrificing activity is a general strategy to increase the selectivity of unsaturated alcohols in many cases. This is consistent with the conclusion that the adsorption of H₂ or α,β-unsaturated ketones/aldehydes should be weakened on the surface of catalysts to further tune the selectivity via kinetic control. This contradiction may be solved by compositing active sites with other active elements, such as Fe. The synergistic effects between different active sites can significantly increase the selectivity of unsaturated alcohols. For example, Bai et al. used PtFe zigzag nanowires as selective catalysts for the selective hydrogenation of cinnamaldehyde.⁶⁰ Compared to the pure Pt zigzag nanowires, the PtFe zigzag nanowires exhibited both higher activity and selectivity for cinnamyl alcohol (Fig. 64c and d). They claimed that the high selectivity can be attributed to the lower electron density of Pt owing to the electronic interaction between Fe and Pt.

Fig. 64 Histograms of (a) selectivity for cinnamyl alcohol (white), hydrocinnamaldehyde (cross-hatch), and hydrocinnamyl alcohol (gray) at nearly 100% conversion in the hydrogenation of cinnamaldehyde with different amine-capped Pt₃Co catalysts. (b) Rates of hydrocinnamyl alcohol production after complete conversion of cinnamaldehyde with different amine-capped Pt₃Co nanocatalysts. Conditions: Pt₃Co nanocatalysts (11 mg), n-butanol (10 mL), cinnamaldehyde (3 mmol), nonane (1 mmol), H₂ (1.5 atm), and 25 °C. Reprinted with permission from ref. 650. Cinnamaldehyde hydrogenation performances of (c) Pt ZNWs and (d) PtFe ZNWs. Reprinted with permission from ref. 60.

4.4 CO oxidation

The low-temperature oxidation of CO is one of the most extensively studied reactions in heterogeneous catalysis, which has attracted increasing attention in the context of cleaning the air and lowering automotive emissions.^{39,41,376,652–661} This reaction is also fundamentally important for investigating the mechanism of catalysts as a model reaction. An ideal catalyst should be capable increasing the activity and reducing the reaction temperature. The advances in precise surface/interface engineering have enabled chemists to carefully investigate the correlation between structure and activity. It has been reported that the catalytic performance for the oxidation of CO is dependent on the size and structure of metal NCs. For example, Jin and co-workers demonstrated that the activity for the oxidation of CO increased by a factor of 10 when the size of Pd nanoparticles decreased from 18 nm to 6 nm.⁴⁰ The decrease in size can largely increase the content of surface atoms, leading to a significant enhancement in activity. However, something must be overlooked since the increased activity is not exactly linear with an increase in the content of atoms. Specifically, the activity on every single exposed atom is different, which may strongly be related to their coordination environment. For example, the atom arrangements differ on different facets, which means that the electronic properties are different, leading to different activity. On the other hand, alloying with another metal or depositing on an oxide can also change the coordination environment of atoms, resulting in different catalytic performances.⁶⁶² For example, Zhang and co-workers synthesized different Rh NCs with (100) and (111) facets. It was shown that the Rh(111) facet exhibited higher activity than Rh(100) facet towards the oxidation of CO. Choi et al. reported that octahedral and cubic Pd@Rh NCs gave higher activity compared with the Rh/C catalyst towards the oxidation of CO.⁴⁴ However, they claimed that only slight differences were observed on octahedral and cubic Pd@Rh NCs.

Najafishirtari and co-workers demonstrated that the synergistic effects between different metals could largely boost the oxidation of CO.⁴² When Au–Cu nanoparticles were treated at high temperature (e.g., 350 °C), the occurrence of segregation between Au and Cu led to a weak interaction and low activity. Moreover, when the sample was further reduced at 350 °C, Au and Cu re-alloyed to form an fcc phase NC. Furthermore, George and co-workers reported the interaction between metal-based nanoparticles and an oxide support could significantly promote their activity towards the oxidation of CO.⁶⁶³ They compared the CO oxidation performance of two different nanostructures, i.e. Au_0.80Pd_0.20–Fe_xO_y dumbbell NCs and Au_0.75Pd_0.25 NCs, on Al₂O₃. It was shown that the Au_0.80Pd_0.20–Fe_xO_y dumbbell NCs exhibited higher activity, indicating that the strong interaction between the metal alloy and oxide could highly enhance the CO oxidation.

Strong interaction between metals and oxides can be achieved via the formation of a metal-oxide interface on an oxide with abundant O vacancies, such as CeO₂. It should be noted that the vacancies in CeO₂ can greatly promote the activity of metals. Carrettin et al. demonstrated that depositing Au on nanocrystalline particles of CeO₂ resulted in an enhancement in activity by two orders of magnitude in comparison to that of the Au catalyst deposited on a regular CeO₂ support.⁶⁶⁴ They highlighted that the vacancies, which could be produced by H₂ treatment, greatly promoted the oxidation of CO. Guzman et al. provided spectroscopic evidence of the enhancement from the active O vacancies in a CeO₂-supported Au catalyst.⁶⁵⁶ Kim and co-workers further confirmed the effects of vacancies in CeO₂ on the oxidation of CO through theoretical calculations.⁶⁶⁵ Other oxides, such as FeO_x,³⁹ MgO,⁶⁶⁶ and Co₃O₄,³⁷⁶ can also been used to promote the activity of metal atoms through strong M–O–M interaction. For instance, Xie and co-workers demonstrated that Co₃O₄ nanorods exhibited superior catalytic performance towards the oxidation of CO at temperatures as low as −77 °C.³⁷⁶ Detailed characterization indicated that more active Co³⁺ species were observed on the (111) planes of the Co₃O₄ nanorods, leading to a significantly higher reaction rate (Fig. 65a). Kinetics studies showed that the reaction order of CO and O₂ over the Co₃O₄ nanorods was 0.12 and 0.28, respectively. At low temperature (e.g. −77 °C), the adsorbed CO and O₂ on Co₃O₄ nanorods can usually exceed the rate of surface reaction, leading to the occurrence of CO oxidation on the Co³⁺ sites (Fig. 65b). On the other hand, the similar apparent activation energy of CO oxidation on the Co₃O₄ nanorods (22 kJ mol⁻¹) and Co₃O₄ nanoparticles (21 kJ mol⁻¹) indicated that Co³⁺ could be assigned as the active sites (Fig. 65c).

Fig. 65 (a) Effects of moisture content, regeneration and temperature on the oxidation of CO over Co₃O₄ nanorods. CO oxidation with a feed gas of 1.0 vol% CO/2.5 vol% O₂/He under normal (moisture 3–10 ppm, blue symbols) and dry (moisture <1 ppm, red symbols) conditions at −77 °C. The used sample was regenerated with a 20 vol% O₂/He mixture at 450 °C for 30 min and then tested for CO oxidation under normal conditions (green symbols) at −77 °C. CO oxidation at 25 °C (black symbols) was tested with the normal feed gas. Reaction kinetics of CO oxidation. (b) Reaction rates (r_CO) as a function of CO or O₂ concentration over Co₃O₄ nanorods: (triangle symbols) CO and (circle symbols) O₂. The reaction rates were measured at −77 °C and the concentrations of CO and O₂ were in the range of 0.5–1.2 vol% and 1.0–5.1 vol%, respectively. P_CO and P_O2, partial pressures. (c) Arrhenius plots for the reaction rate constants (activation energies, E_a) on Co₃O₄ nanorods (triangle symbols) and nanoparticles (circle symbols) in the temperature (T) range of −86 °C to −56 °C with a feed gas of 1.0 vol% CO/2.5 vol% O₂/He. Reprinted with permission from ref. 376.

4.5 Oxygen reduction

The ORR is of utmost importance in advanced electrochemical energy conversion technologies. However, the ORR suffers from the disadvantages of sluggish kinetics.^667–674 Pt has been regarded as promising catalyst for the ORR; however, the high cost and poor activity of Pt seriously impede its practical application.⁵⁹⁹ Over the past decades, many strategies have been developed for designing efficient Pt-based catalysts by compositing with non-noble metals, such as Ni and Co. On the other hand, many shaped Pt-based NCs with high-index facets and/or low coordinated atoms were synthesized for achieving high ORR activity. The use of a non-noble component in catalysts can significantly increase their ORR activity and reduce their cost; however, maintaining the durability of catalysts during such long-term harsh working conditions is another key point.

Pt–Ni alloys have been widely reported as promising ORR catalyst over the past decade. Wu and co-workers synthesized Pt₃Ni NCs for the ORR.²⁵⁴ The carbon-supported Pt₃Ni catalysts exhibited an area-specific activity of 850 μA cm_Pt⁻² at 0.9 V, which is ∼4 times higher than that of the commercial Pt/C catalyst (∼0.2 mA cm_Pt⁻² at 0.9 V). Huang et al. prepared highly porous Pt₃Ni NCs for the ORR. It was shown that the porous Pt₃Ni NCs exhibited a much better ORR performance that commercial Pt black and commercial Pt/C catalyst.³²⁰ The current density and mass activity of Pt₃Ni NCs were 5 times and 6 times higher than that of the commercial Pt catalyst. Moreover, the porous Pt₃Ni NCs exhibited better stability in 6000 potential sweeps. Zhang and co-workers demonstrated that the composition evolution of the Pt₃Ni NCs influenced their ORR performance. For example, it was shown that Pt₃Ni NCs with an Ni-rich surface gave a 300% increase in mass activity towards to HCOOH oxidation compared with the commercial Pt/C catalyst, while Pt₃Ni NCs with a Pt-rich surface showed a 300% increase in mass activity towards the ORR in comparison to a commercial ORR catalyst.³²⁴ Gong and co-workers synthesized 1 nm-thick Pt₃Ni bimetallic alloy nanowires for the ORR.²⁶⁷ It was shown that the Pt₃Ni nanowires exhibited a significant enhancement in mass activity in comparison to the commercial Pt/C catalyst (0.546 vs. 0.135 A mg_Pt⁻¹). Also, Pt₃Ni nanowires exhibited better durability than Pt nanowires and the commercial Pt/C catalyst, as shown in Fig. 66. Huang and co-workers demonstrated that the addition of a transition metal can significantly enhance the ORR performance of Pt₃Ni octahedra.³²² Among the transition metals, Mo resulted in the highest increase in ORR activity. The Mo-doped Pt₃Ni/C exhibited the best ORR performance with a specific activity of 10.3 mA cm⁻² and mass activity of 6.98 A mg_Pt⁻¹, which are 81- and 73-fold enhancements compared with the commercial Pt/C catalyst (0.127 mA cm⁻² and 0.096 A mg_Pt⁻¹), respectively (Fig. 67).

Fig. 66 Electrocatalytic durability measurements: (a) CVs curves and (b) positive-going linear sweep voltammetry (LSV) curves of Pt₃Ni nanowires during accelerated durability test. (c) Mass activity and (d) electrochemical surface area (ECSA) comparison of Pt and Pt nanowires, and Pt₃Ni nanowires at different cycles. Reprinted with permission from ref. 267.

Fig. 67 Electrochemical durability of the high-performance octahedral Mo–Pt₃NiCo/C catalyst and octahedral Pt₃Ni/C catalyst. (a and b) ORR polarization curves and (inset) corresponding CVs of (a) octahedral Mo–Pt₃Ni/C catalyst and (b) octahedral Pt₃Ni/C catalyst before and after 4000, and after 8000 potential cycles between 0.6 and 1.1 V versus RHE. (c) Changes in the ECSA (left), specific activity (middle), and mass activity (right) of the octahedral Mo–Pt₃Ni/C catalyst and octahedral Pt₃Ni/C catalyst before and after 4000, and after 8000 potential cycles. The durability tests were carried out at room temperature in O₂-saturated 0.1 M HClO₄ at a scan rate of 50 mV s⁻¹. Reprinted with permission from ref. 322.

5. Summary and outlook

In this review, we discussed the controllable synthesis of metal and oxide NCs with tailored shapes, sizes and compositions. Special attention was paid to the fabrication, stabilization and modification of the typical metals and oxides that are regarded as promising catalysts. Finally, we summarized some typical and popular processes, such as CH₄ selective oxidation, CO₂ reduction, selective hydrogenation, CO oxidation and ORR, to elucidate the importance of shape, size and composition on catalytic performance. This review aimed to summarize the correlation of structure–activity of metal and oxide NCs.

Despite the tremendous progress in the controllable synthesis of metals and oxides and their catalytic application, some challenges remain. With the development of nanoscience and nanotechnology over the past decades, chemists have gained the ability to fabricate metal NCs. Many methods including seeded growth, polyol method, hydrothermal method, solvothermal method, and templated-assisted method have been developed for the preparation of nanostructures. The successful experience has been applied for the synthesis of oxide NCs, such as CeO₂, Co₃O₄, and TiO₂ nanostructures, which significantly promoted their promising application in both thermal-catalysis and photocatalysis. However, the detailed mechanism for their growth has not been completely understood to date. Some principles have been proposed for interpreting the morphology control. For example, thermodynamics shows that the obtained NCs will expose the most stable facets (lowest surface energy). To obtain high-index facets with high surface energy, precise control of the growth kinetics is necessary. During the control of the kinetics, some capping agents are required, such as PVP, oleylamine, and oleic acid, to selectively protect the facets. In some cases, halide ions are used to tune the shapes of NCs via selective etching in the presence of capping agents. However, the intrinsic mechanism of these capping agents is unclear, leading to the unrepeatable synthesis of NCs. Thus, some advanced technologies should be developed for revealing the growth process of nanocrystals. For instance, the advances on liquid TEM measurement allow chemists to observe the growth process of nanocrystals and monitor the growth rate of different positions of the seeds. Some other surface technologies are essential for investigating the effects of surface ligands on the growth of nanocrystals.

Another challenge is the stability of shaped NCs. As discussed before, thermodynamics principles suggest that NCs tend to aggregate or expose low-index facets to decrease their surface energy. Some capping agents are used for obtaining metal and oxide NCs with tailored morphologies and sizes. However, the presence of capping agents will retard the adsorption of reactant molecules on the metal surface, leading to a decrease in catalytic performance. The removal of capping agents from NCs can favor an increase in exposed active sites on their surface; however, these NCs will suffer from aggregation and/or shape evolution without the protection of a capping agent. It is widely accepted that the apex, edge, terrace and low-coordinated atoms are the most active. Thus, it is still challenging to apply NCs with high-index facets in industry unless this contradiction is solved. One probable strategy for improving the stability is the surface modification of nanocrystals with other elements. (1) Surface modification with metals to form alloys or intermetallics will lead to significantly enhanced stability and activity. (2) Surface modification with non-metal elements, such as N and C, can also promote the activity and improve the stability of catalysts. Metal-free catalysts have recently become a hot topic in catalysis, such as C₃N₄. These metal-free catalysts are generally produced under harsh conditions, which may exhibit better stability than metal-based catalysts under the same conditions.

Thirdly, catalysis is a systematic and complicated process. The catalytic performance is strongly related to the surface properties of catalysts. However, is impossible for the surface of a catalyst to be invariable. Specifically, the chemical environment of surface atoms strongly varies after the adsorption of reactant molecules. This variation in the chemical environment of the surface atoms will further influence their electronic properties, and thus their catalytic performance. Specifically, one probable situation is that the highlighted active sites from high-index facets or terraces may not actually exist during the catalytic reaction. With the development of science and technology, more advanced in situ techniques may greatly help scientists to reveal the real mechanism on the surface of catalysts.

Finally, heterogeneous catalysis requires a good balance between activity and selectivity. For most catalytic reactions in the chemical industry, there are at least two different pathways, and only one of them is preferred. Sometimes, selectivity is more important than activity in industry because unreacted raw materials can be recycled into other reactants for further conversion. To achieve actual industrial application, catalysts should exhibit high selectivity, high activity, excellent stability and low cost. Accordingly, it is still a long way for chemists to pursue more efficient and economical catalytic processes. On the other hand, more attention should be devoted to renewable and eco-friendly processes, such as photocatalysis.

In summary, we attempted to review the recent advances on thermal catalysis, photocatalysis and electrocatalysis using metal and oxide nanostructures. We hope that this review can help readers better understand the correlation of structure–activity, and inspire researchers to design highly active, selective and stable catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported National Natural Science Foundation of China (21673150, 51802206, 21703146, 51922073) and Natural Science Foundation of Jiangsu Province (BK20180846, BK20180097). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

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Footnote

† Yong Xu and Muhan Cao contributed equally.

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