Sonochemical catalysis as a unique strategy for the fabrication of nano-/micro-structured inorganics

Abstract

Ultrasound-assisted approaches, as an important trend in material synthesis, have emerged for designing and creating nano-/micro-structures. This review simply presents the basic principles of ultrasound irradiation including acoustic cavitation, sonochemical effects, physical and/or mechanical effects, and on the basis of the latest progress, it newly summarizes sonochemical catalysis for the fabrication of nano-structured or micro-structured inorganic materials such as metals, alloys, metal compounds, non-metal materials, and inorganic composites, where the theories or mechanisms of catalytic synthetic routes, and the morphologies, structures, sizes, properties and applications of products are described in detail. In the review, a few technological potentials and probable challenges of sonochemical catalysis are also highlighted for the future advance of synthesis methods. Therefore, sonochemical catalysis or ultrasound-assisted synthesis will serve as a unique strategy to reveal its great significance in material fabrication.

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Zhanfeng Li

Dr Zhanfeng Li is an associate professor at the College of Chemistry and Chemical Engineering of Qingdao University. He received his PhD degree in applied chemistry from Jilin University in 2016, under the supervision of Prof. Xuejun Cui. After joining Qingdao University, he worked as a Postdoctoral Research Fellow at the College of Materials Science and Engineering (Qingdao University), under the supervision of Prof. Zonghua Wang. His research interests focus on sonochemical synthesis, the design of new micro-/nano-scale materials, and sonochemical applications in catalysis and biomedicine.

Huiqi Wang

Dr Huiqi Wang is a lecturer at the College of Chemistry and Chemical Engineering of Qingdao University. She received her PhD degree in analytical chemistry from Jilin University in 2018, under the supervision of Prof. Qiong Jia. After graduation, she joined Qingdao University and worked as a Postdoctoral Research Fellow at the College of Materials Science and Engineering (Qingdao University), under the supervision of Prof. Zonghua Wang. Her research interests focus on the design of new nano-materials, fabrication of functional adsorbing materials, and applications in biological detection and analysis.

Xuejun Cui

Dr Xuejun Cui is a professor in the Department of Chemistry at the University of Jilin. He graduated in applied chemistry in 2003 and completed his PhD at Jilin University in 2008, under the supervision of Prof. Hongyan Wang. After that, he joined the faculty of the Chemistry Department in Jilin University. During the employment, he spent time as a junior research fellow at the Max Planck Institute of Colloids and Interfaces and Nanyang Technological University. His research interests focus on acoustic chemistry, and the design and preparation of biomedical materials for drug delivery.

Zonghua Wang

Prof. Zonghua Wang is a Group Leader at the Department of Chemistry, and a Director of Instrumental Analysis Center, Qingdao University. She obtained her M.S. degree in analytical chemistry from China University of Geosciences in 1987, and obtained her PhD in analytical chemistry from Tsinghua University in 2004, under the supervision of Prof. Guoan Luo. Her current research interests include the functionalization and application of nano-materials, nano-analytical chemistry, electroanalytical chemistry, and biofunctional interfaces and materials.

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1. Introduction

Sonochemistry, coined in 1953, is a research area in which powerful ultrasound irradiation can assist to induce some chemical reactions (synthesis, degradation, catalysis, etc.),¹ or it is defined as a nonclassical mechanical method by which one or more chemical processes can be induced or strengthened or even weakened when a reacting system is irradiated with ultrasound. Since sonochemistry or the chemical effects from ultrasound irradiation were reported firstly in the 1920s or earlier, many important sonochemistry-based applications have been found in chemical synthesis, material processing, industrial manufacturing, and so on.² For instance, ultrasound irradiation is a very efficient method to clean the debris from solid surfaces, which has been widely used in research and industry;³ ultrasound irradiation is often utilized to purify wastewater by degrading contaminants (e.g., textile dyes);^4,5 and in polymer chemistry, ultrasound irradiation can be used as a driving force to cleave polymer chains,^6,7 or act as a simple but effective way to initiate radical-mediated polymerizations or to carry out emulsion polymerizations by emulsifying oil and water phases.^8–10

Nano-structured or micro-structured materials are interesting and versatile in many applications such as catalysis, biomedicine, energy storage, hydrogen storage, etc.,^11–13 so numerous synthesis methods have been developed to prepare well-designed functional materials, where sonochemistry serving as a unique catalytic technology is often involved. This review will overview the latest developments of ultrasound-assisted effects or reactions to prepare nano-scale and micro-scale inorganic structures, give a fundamental understanding of the basic principles of sonochemistry, and cover the powerful and unique aspects in the synthesis of products (i.e., metals, alloys, metal compounds, and non-metal materials) as a greater emphasis, e.g., the morphologies, structures, sizes, properties as well as applications. As well, the fabrication of multimodal or multifunctional inorganic composites (e.g., metal/metal compounds, metal/non-metal materials, metal compound/metal compound, metal compound/non-metal material) will be a focus. Meanwhile, the review will point out the prospective challenges and trends in the programming of ultrasound-assisted preparation, and conclude with the significance of sonochemical catalysis in material synthesis.

2. Ultrasound

Ultrasound refers to a concept or terminology in which a sound wave has a frequency higher than 20.0 kHz exceeding the upper limit of human hearing, and its industrial production can be often achieved by the electrical energy from a magneto-strictive or piezo-electric transducer.^14–16 A piezo-electric transducer is able to create ultrasound irradiation across the whole frequency range, but the element is more fragile and destroyed by heat. In comparison, the ultrasonic frequency produced by a magneto-strictive transducer, for practical reasons, is restricted to be 20.0–100.0 kHz.¹⁷ For ultrasound, the frequency that is governed by the transducer is a key factor for many applications. Low-frequency ultrasound (20.0–100.0 kHz) is usually called power ultrasound, and the majority of sonochemical processes as well as ultrasound-assisted surface treatments occurs in this frequency range.^3,18 For example, cleaning or decontamination is the most common use of low-frequency ultrasound in a general chemistry laboratory. High-frequency ultrasound (0.5–10.0 MHz) is typically used in medical applications for diagnosis and therapy.^19–21 For example, high-intensity focused ultrasound (HIFU) can be used in surgery to break down blood clots or destroy tumors, and the ultrasound irradiation at a frequency of around 1.0 MHz has aided the delivery of some nanodrugs. Fig. 1 shows the general applications of ultrasound, and many of them are briefly explained in terms of chemical technology.

Fig. 1 General applications of ultrasound in chemical, material and manufacturing processes.

2.1. Acoustic cavitation

When sound spreads in a liquid, its typical velocity and wavelength are both much larger than the molecular scale, so theoretically no direct interaction can occur at a molecular level between ultrasound irradiation and chemical species.^22,23 However, ultrasonic irradiation, different from heat, light or other energy sources, can offer some extreme transient conditions (e.g., localized high pressure and temperature) and physical and/or mechanical effects to complete some chemistry-correlative applications, and the whole processes are basically induced by acoustic cavitation created from either low-frequency or high-frequency ultrasound.²⁴

Acoustic cavitation is an instantaneous process spanning the formation, growth and violent collapse of micro-sized bubbles which are caused by ultrasound irradiating a liquid.²⁵ As we know, ultrasound is a longitudinal pressure wave, and when the sound moves and propagates through a medium, the acoustic pressure cycles will cause the medium containing gas or particles to experience an alternate compression and rarefaction phase. The cavitation bubbles are usually nucleated during the rarefaction phase, because some liquid molecules may be pulled apart at “weak spot” sites containing some gaseous impurity by overcoming intermolecular interactions.²⁶ Following this, the bubbles will grow in every acoustic pressure cycle along with a gas inflow, where the average bubble size during the rarefaction phase is larger than that during the compression phase. During the oscillating process, the bubbles accumulate ultrasonic energy effectively causing an enormous energy concentration within their small volume. The bubbles will be no longer stable after growing to a critical size range of typically tens of micrometers, and they go through a rapid inertial overgrowth by strongly coupling to the acoustic field until a catastrophic collapse occurs at some point. Once collapsing, the concentrated energy stored in the bubbles will be released within a very short time at a heating and cooling rate higher than a billion K s⁻¹, and moreover, such implosive collapse is a nearly adiabatic process, resulting in the formation of localized micro-scale “hotspots” with high temperature (≈5000 K) and high pressure (≈1000 bar).^27,28Fig. 2 shows the formation of a “hotspot” under acoustic cavitation.

Fig. 2 Schematic illustration of “hotspot” formation under acoustic cavitation, and the reaction zones of sonochemistry.

2.2. Sonochemical effects

Since acoustic cavitation is able to account for the extreme characteristic conditions to induce some possible chemistry effects, the hotspot theory has been postulated to explain sonochemical events, though many phenomena reported by several researchers still contradict it.² In a liquid system, there is no direct chemical reaction except the sonolysis of liquid, but the sonolysis-inducing radicals may diffuse into the liquid to create potential chemical reactions, which involve primary or secondary sonochemical reactions (Fig. 2). Once the cavitation bubbles collapse, the conditions in the bubble center or at the hotspots are very harsh, e.g., large temperature gradients, pressure and shear are experienced. The volatiles and/or gases inside the bubbles (Zone 1) and/or in the interfacial region (Zone 2) between the bubble center (Zone 1) and the bulk liquid (Zone 3) will lead to the formation of many radical species, namely primary sonochemical reactions. The majority of radical production is thought to occur in the interior vapor phase of bubbles, because the liquid layer surrounding the cavitation bubbles will immediately inject into the bubbles upon cavitation collapse. When the initially produced species migrate into the surrounding liquid, a variety of reactions with solutes may create secondary sonochemical reactions.²⁹ What is more, the sonochemical effects involve different types of cavitation bubbles. In the system, the transient bubbles, which often grow and collapse quickly after one acoustic cycle, may account for the majority of chemical activity. Differently, the stable bubbles which can persist for more acoustic cycles will create a large shear gradient when collapsing, and the shear gradient plays an important role in the sonochemical system. For example, ultrasound irradiation is able to contribute to the efficient formation of emulsions in a two-phase liquid system.³⁰

Like other chemical reactions or processes, sonochemistry also involves the dissociation and formation of chemical bonds, since the compressive heat created by collapsing bubbles generates high enough temperatures locally. In order to confirm this proposal, Pflieger et al. carried out the sonolysis of water at different ultrasound frequencies and photographically captured the generated highly reactive hydroxyl radicals (˙OH) (reaction (1)), and the addition of luminol could highlight the phenomenon owing to the blue sonochemiluminescence generated near the cavitation bubbles (Fig. S1†).³¹ Upon diffusing out, the primary H˙ and ˙OH, if taking no account of the recombination to their original form (reaction (2)), can further combine to produce H₂ and/or H₂O₂ (reactions (3) and (4)), and can also generate some secondary radicals, e.g., R˙ by reacting with organic additives (reaction (5)), or by a combination of H˙ with dissolved oxygen (O₂) (reaction (6)). Either strong oxidants or reductants are capable of initiating various sonochemical reactions in aqueous or nonaqueous solution including reduction, oxidation, hydroxylation, degradation, polymerization, etc.

H₂O → H˙ + ˙OH (1)

H˙ + ˙OH → H₂O (2)

2H˙ → H₂ (3)

2˙OH → H₂O₂ (4)

RH + ˙OH (or H˙) → R˙ + H₂O (or H₂) (5)

2.3. Physical and/or mechanical effects

Accompanied by the sonochemical consequences, the acoustic cavitation in a liquid often gives rise to numerous physical and/or mechanical effects including simple heating, shock waves and microjets (Fig. 3), which can also affect the efficiency of sonochemical reactions.³² Heating during sonication is readily understood because tens or more of watts of acoustic energy will be delivered into the liquid, so ultrasound can be useful for activating reactions. A microjet in a liquid is often created in any heterogeneous system, since the asymmetrical collapse of highly energetic bubbles usually induces the surroundings to form a doughnut-like shape.²⁹ Experimentally, the microjets have high speeds, and those greater than 100 m s⁻¹ can cause surface modification (pitting and erosion) or the generation of nanostructures.³³ If a surface-unperturbed bubble rebounds rapidly from its minimum spherical radius causing the surrounding liquid to compress and then to propagate outward, a shock wave will occur.^34,35 Likewise, the shock waves can produce several different mechanical and physical consequences, such as increasing the mass transport, accelerating the suspension of solid particles, and inducing changes in size distributions, morphologies and surface compositions. All in all, many ultrasound-assisted processes or technologies, e.g., the fragmentation for brittle materials, the emulsification for immiscible liquids, the agglomeration for malleable materials, and the exfoliation of layered materials into 2D layers, can be explained by these physical and/or mechanical effects.^36,37

Fig. 3 Physical and/or mechanical effects from acoustic cavitation in a liquid.

3. Sonochemical catalysis to synthesize nano-/micro-structures

Nano-structured and micro-structured materials have been opening up new opportunities for many applications in electronics, catalysis, energy, chemistry, medicine and biology, because nano-/micro-sized regimes often exhibit a lot of properties distinct from those of their bulk counterparts. With a rapid advance of nanoscience and nanotechnology, a variety of synthetic methods (e.g., gas phase techniques, liquid phase methods, and mixed phase approaches) have been developed to design and prepare nano-/micro-structures. Sometimes, many chemical, physical and/or mechanical properties which often prescribe the applications of nano-/micro-structured devices are heavily dependent upon the synthetic routes of materials, so that an appropriate technology will be a driving force for the new synthesis methodologies.^38,39 Among versatile methods, the utilization of ultrasound has been extensively examined for material synthesis over the years, and it is demonstrating its powerful and unique aspects in the fabrication of nano- and micro-structured materials including inorganics, polymers, and hybrid materials (Fig. 4). The specialty that ultrasound irradiation provides a facile and fast route to yield a variety of nano-/micro-materials from inorganic or organic precursors is mainly dependent on the primary and/or secondary sonochemistry, or more specifically, is decided by primary and/or secondary radicals from bubble collapse and post-produced chemical consequences in the liquid phase. In addition, physical/mechanical effects also play an important role in such formation. As an indispensable emphasis, the common types of nano/micro inorganic materials will be methodically presented in the following sections.

Fig. 4 Nano-/micro-structured inorganic, polymeric, and hybrid materials produced by sonochemical catalysis.

4. Metals and alloys

Metal nanomaterials have a useful superiority in many fields such as catalysis, sensing, imaging, biomedicine, and so on. Under ultrasound irradiation, volatile organometallic compounds such as Fe(CO)₅, Cr(CO)₆, Ni(CO)₄ and Co(CO)₃NO easily release individual metal atoms by dissociation of the organic groups as well as the multiple metal–ligand bonds, forming metal nanostructures in a nonvolatile solvent (e.g., silicone oil, long-chain hydrocarbons). In the synthesis, the atoms of organometallic compounds can be thermally excited to the point of emitting visible light analogous to the emission from flame excitation, and the nonvolatile solvents are very significant for absorbing the available energy in the collapsing bubble, so that the achieved conditions are much less extreme.

Suslick et al. carried out the sonication of Fe(CO)₅ in alkane solvent or ionic liquid with a low vapor pressure under argon gas, which efficiently yielded a black iron powder at the freezing point.^40,41 Ultrasound irradiation generated a very high temperature within the short lifetime of a cavitation event, enabling a rapid quenching process to rapidly cool the metal atoms and prevent crystallization, so the obtained iron powder appeared to be an agglomeration of 20 nm amorphous nanoparticles. Later on, Enomoto et al. examined sonochemically derived amorphous metallic iron from Fe(CO)₅, and found that there was a certain amount (about 15%) of volatile component. By heating in an inert atmosphere, the volatile component could be removed, but the as-synthesized fine iron, if exposed to air, would be oxidized showing a weight gain. The agglomerated appearance of products would not be favorable especially for catalytic application, so oleic acid or other similar surfactants often could be present in the solvent to stabilize the metal atoms (Fig. S2-I†).⁴² Interestingly, it was concluded that the surfactant addition should be done after the sonochemical synthesis rather than before the synthesis. This might be because the unfavorable sonochemical interaction between the surfactant and the carbonyl would result in large-sized product. When oleic acid (1 mass% in decane) was added after the synthesis, a fine dispersion of iron nanoparticles could be easily achieved (Fig. S2-II†).⁴² Other precursor compounds also have produced porous amorphous metal nanoparticles, even alloy nanoparticles by varying the composition in the solution.^43,44

Besides volatile organometallic compounds, a few nonvolatile precursors also can create nanostructured metals when subjected to ultrasound irradiation in a volatile solvent. Different from the metal synthesis from volatile organometallic compounds, the process of nonvolatile precursors forming metal nanostructures is mainly based on the mechanism of secondary sonochemistry, because the sonolysis of solvent vapor can produce some strong debris but does not need additional catalysts,⁴⁵ for example, H˙ and ˙OH radicals for water, R˙ radicals for volatile organic vapor, and so on. The reactive debris are responsible for producing nano-/micro-structured materials via some redox reactions. Moreover, the products from nonvolatile compounds are usually well crystallized, which is obviously different from the amorphous structures created by volatile precursors.

As a useful alternative to the traditional approaches (e.g., photochemical reduction, controlled chemical reduction, radiolytic reduction, and solvothermal synthesis), the sonochemical preparation of nanostructured noble metals (e.g., Au, Ag, Pt, Pd, etc.) has been carried out in many studies. Grieser and coworkers published a systematic study on the sonochemical synthesis of gold nanospheres.⁴⁶ When HAuCl₄ was sonicated in aqueous solution containing alcohols or other similar organic additives, spherical gold nanoparticles would be formed by Au(III) reduced to Au(0), and the nanoparticle size was related to the ultrasound frequency. At around 213 kHz in the presence of 1-propanol, the size of gold nanoparticles was smallest owing to the maximum rate of Au(III) reduction (Fig. 5-I). Yasuda et al. synthesized size-controlled gold nanoparticles by using the ultrasound irradiation of HAuCl₄ aqueous solutions with the aid of ultrafine bubbles (UFBs) rather than any capping and reducing agents.⁴⁷ The addition of air-UFBs largely decreased the diameter of the spherical gold nanoparticles, which was attributed to the sonochemical reduction of Au(III) ions being accelerated by the air-UFBs. And because gold nanospheres electrostatically adsorbed onto those UFBs with a long lifetime in water, the product was very stably dispersed in an aqueous solution for more than two months (Fig. 5-II). Additionally, pulsed ultrasound delivered with the same time-averaged power as continuous irradiation could further decrease the nanoparticle diameter.

Fig. 5 (I) Left: average size of Au nanoparticles; middle: rate of Au(III) reduction as a function of ultrasound frequency (0.2 mM Au(III), 20 mM 1-propanol, Ar atmosphere, 0.1 W mL⁻¹ ultrasonic power), where each error bar corresponds to the standard deviation of the size of Au nanoparticles; right: TEM micrograph (a) and size distribution histogram (b) of Au nanoparticles synthesized after 120 min irradiation with 213 kHz ultrasound; the scale bar is 50 nm in (a). Reproduced with permission from ref. 46. Copyright 2005 American Chemical Society. (II) Left: electron micrographs of spherical Au nanoparticles (AuNPs) synthesized with and without air-UFBs, where the scale bars are 100 nm; right: photographs of AuNPs colloids prepared with and without air-UFBs, and stabilization mechanism of AuNPs colloids containing UFBs. Reproduced with permission from ref. 47. Copyright 2020 Elsevier.

Nonspherical metal nanoparticles such as nanorods, nanobelts, nanocones, nanoplates, nanowires, and nanorings can also be prepared by sonochemical reduction. For example, Au nanorods have been formed from HAuCl₄ in the presence of cetyltrimethylammonium bromide and AgNO₃,⁴⁸ when the formation of monodispersed Au nano-decahedra was realized with poly(vinylpyrrolidone) as a stabilizing polymer.⁴⁹ Ashokkumar et al. reported the sonochemical synthesis of different gold nanoparticles by using HIFU at 463 kHz, and the shapes and size distributions of products were dependent on the ultrasound condition.⁵⁰ In this case, the radical-driven reduction process played an important role in nuclei formation and growth, and thus the size of gold nanoparticles decreased with acoustic power increasing. Experimentally, irregular shapes appeared at 30 W, and primarily icosahedral occurred at 50 W with a significant amount of nanorods at 70 W. Significantly, some nonvolatile precursors under high-intensity ultrasound irradiation were capable of creating extremely small nanoclusters (e.g., Ag nanoclusters, Au nanoclusters and Cu nanoclusters) when using polymer molecules as a capping agent.^51–55 To avoid the resource-/time-consuming synthesis processes of nanoclusters, Xin et al. utilized a rapid sonochemical route to prepare fluorescent Au nanoclusters (about 2 nm) in a large quantity within about 40 min (Fig. S3-I†).⁵⁶ The Au nanoclusters had good dispersion in aqueous solution due to a protecting agent glutathione, and showed a strong orange-red photoluminescence (E_m = 598 nm) with high photo-, storage-, metal- and pH-stability (Fig. S3-II†), which was used as a sensing probe for highly sensitive and selective label-free detection of Cu²⁺.⁵⁶ Recently, Aminabhavi et al. reported a green synthesis of three-dimensional hexagonal-like zero-valent Cu materials via the sonolysis of copper(II) acetate in a medium consisting of ethylene glycol-ethanol (1 : 1),⁵⁷ and the resulting nanostructures with high purity (>99%) demonstrated a high efficiency for the reduction of nitrate from polluted waters.

Also, bimetal or alloy nanoparticles have been obtained by an ultrasound-assisted Ostwald ripening process. In the electrocatalysis field, alloy nanoparticles containing Pt have attracted enormous interest not only for their low material cost by reducing the Pt amount, but also for their enhanced catalytic activity, durability and selectivity. By sonicating metal salts in an argon-purged non-aqueous solution containing a strong reducing agent, Gümeci et al. prepared bimetallic nanoparticles consistent with Pt₃Ni stoichiometry by adjusting the mole ratio of Pt⁴⁺ and Ni²⁺ ions.⁵⁸ According to both specific activity and mass activity, the activated Pt₃Ni sono-nanoparticles demonstrated a 2–3 times higher oxygen reduction activity than a commercial Pt/C catalyst. To obtain another oxygen reduction electrocatalyst, they later described the synthesis of homogeneous Pt/Cu nanoparticles with the aid of high-intensity ultrasound.⁵⁹ Such amorphous, uniformly alloyed nanomaterial showed particle diameters of 2–3 nm, and had a composition consistent with a PtCu₃ stoichiometry. Compared with commercial Pt catalysts, the de-alloyed sample exhibited a 3- to 6-fold enhancement in oxygen reduction reaction activity. Wang et al. successfully synthesized Au@Pt nanocolloids with nanostructured dendritic Pt shells by chemically reducing both H₂PtCl₆ and HAuCl₄ species in a low-concentration surfactant solution,⁶⁰ and an ultrasonic treatment dramatically decreased the particle size with a narrower size distribution. As a result of a better reduction potential, Au ions were preferentially reduced over a short time to form Au seeds, followed by the overgrowth of Pt dendritic nanowires on the seeds. By controlling the Pt ions/Au ions molar ratios in the precursor solutions, the thicknesses of Pt shells on Au cores could be tuned easily.

To make toxic mercury (Hg) serve as a safe material in electrochemistry and catalysis fields, Gedanken et al. successfully prepared bimetallic Hg/Pd alloys (HgPd 1 : 1, HgPd 2.5 : 1, and HgPd 4 : 1) by sonicating an aqueous Pd(II) nitrate solution with liquid mercury.⁶¹ Hg/Pd catalyst exhibited superior stability and reusability to other noble metal-based catalysts. Later on, they also achieved pure crystalline Hg–Ag amalgam microspheres with uniform morphology by ultrasonically reacting liquid mercury with an AgNO₃ aqueous solution (Fig. 6).⁶² In the study of Kwon and coworkers,⁶³ another kind of bimetallic alloy, namely Pd_nM (n = 1 for M = Mn, Fe, and Co; n = 1, 2, and 3 for M = Ni) nanoparticles, was synthesized on carbon supports by sonochemical catalytic reactions of Pd(acetylacetonate)₂ with Ni(acetylacetonate)₂, Co(acetylacetonate)₂, Mn(acetylacetonate)₂ or Fe(acetylacetonate)₃ in ethylene glycol. Similarly, trimetallic nanoparticles ((Pd,Co)@Pt) with different elemental compositions have been obtained by the ultrasound-assisted polyol synthesis of Co(acetylacetonate)₂, Pd(acetylacetonate)₂ and Pt(acetylacetonate)₂, where the nanoparticles had a Pt-enriched surface with Pd and Co forming the core.⁶⁴ Other sonchemical syntheses of bimetal (e.g., Au/Ag, Au/Pd, Pt/Ru) or even multi-metal nanomaterials (e.g., NiCoPd, Pt/PdNiMo) have been reported as well.^65–68

Fig. 6 SEM images of Hg–Ag amalgam with a 1.5 : 1 molar ratio prepared by sonication (a) and stirring methods (b). HRSEM image and elemental mapping (c) of composite Hg : Ag (1.5 : 1). The scale bars are 20 μm in (a) and (b), and 6 μm in (c). Reproduced with permission from ref. 62. Copyright 2018 Elsevier.

5. Metal compounds

Both volatile organometallic compounds and nonvolatile metal precursors are not only limited to the synthesis of nanostructured metals or alloys by sonochemical catalysis or ultrasound-assisted reduction, and various metal compounds including metal oxides, metal hydroxides, metal chalcogenides, metal carbides, etc., can also be prepared from the relevant precursors. Certainly, apart from ultrasound irradiation and reacting precursors, the synthesis processes sometimes involve templates or other technologies.

5.1. Template-free approach

Upon the collapse of bubbles, the high temperature in the localized hotspots can make the reactive metal atoms decompose from metal precursors and react with dissolved gas, other solutes or even solvent molecules, producing metal compounds without any assistance including templates and other structure-directing agents. For instance, Mo(CO)₆ in hexadecane by sonochemical heating under CH₄/H₂ gas could cause the formation of nanostructured Mo₂C porous aggregates,⁶⁹ and iron nitride nanoparticles have been produced when the ultrasound treatment of iron carbonyl compounds occurred under a reductive NH₃/H₂ gas mixture.⁷⁰ Recently, Argirusis and coworkers sonochemically prepared metal oxide nanoparticles on anode materials from organic solvents containing W(CO)₆ or Mo(CO)₆ in 80 °C ambient air.⁷¹ They found that ultrasound intensity and solvents were responsible for the size distribution and morphology of resultant nanoparticles when the ultrasound intensity/duration as well as the ratio of precursor/substrate compounds affected the decoration loading extent of nanoparticles on substrates.

For metal oxides or hydroxides, the sonication of metal salts in an aqueous solution is a typical synthesis method in the presence of oxygen sources (e.g., air, oxygen gas or hydroxides). Using sonication, Price et al. produced copper oxide nanoparticles from a copper acetate and hydrogen peroxide solution, or obtained zinc oxide nanoparticles from a zinc acetate and NaOH solution.⁷² Uzunov et al. utilized different methods (i.e., precipitation, mechanochemical treatment and sonochemical process) to prepare nano-sized zinc oxides, and three products had various morphologies with a crystallite size below 20 nm.⁷³ Interestingly, the ZnO nanorods from the sonochemistry-assisted process exhibited the highest photocatalytic activity for the degradation of malachite green under UV irradiation, and the polycrystalline zinc oxide from the precipitation-assisted process had a better photocatalytic efficiency under visible irradiation, but on the contrary, the nano-sized ZnO from the mechanochemistry-assisted process had lower photocatalytic activity because of the inhomogeneous size distribution.

Spinel ferrites are promising magnetic materials and widely used in various fields, so lately, the sonochemical syntheses of novel spinel ferrite nanostructures have become an active research area.^74–78 For example, Almessiere, Slimani and coworkers produced a series of high-purity spinel ferrite compositions via ultrasonic irradiation, such as Mn_0.5Zn_0.5Eu_xDy_xFe_1.8−2xO₄ (x ≤ 0.1), Mn_0.5Zn_0.5Fe_2−2x(Dy_xY_x)O₄ (0.0 ≤ x ≤ 0.05), Ni_0.3Cu_0.3Zn_0.4Tm_xFe_2−xO₄ (0.0 ≤ x ≤ 0.10), CoTb_xFe_2−xO₄ (0.00 ≤ x ≤ 0.10), CoTm_xFe_2−xO₄ (0.0 ≤ x ≤ 0.08), Co_0.7Zn_0.3Tm_xFe_2−xO₄ (0.0 ≤ x ≤ 0.04), etc.^79–93 They examined the structural properties, morphological properties, and physical properties (e.g., magnetic traits, optical traits, and electrical traits) of the products, and even evaluated their biological characteristics for potential anti-cancer and anti-bacterial activities. Additionally, they also utilized ultrasonic-assisted approaches to prepare many ferromagnetic M-type hexaferrites including BaTm_xTb_xFe_12−2xO₁₉ (x = 0.00–0.05) hexaferrites, Ba_0.5Sr_0.5Nd_xEu_xFe_12−2xO₁₉ (x = 0.00–0.05) hexaferrites, Sr_0.5Ba_0.5Tm_xSm_xFe_12−2xO₁₉ (x = 0.00–0.05) hexaferrites, Sr_0.5Ba_0.5Tm_xTb_xFe_12−2xO₁₉ (x = 0.00–0.05) hexaferrites, and so on.^94–98

Metal chalcogenides can be obtained by the sonication of metal salts and chalcogen sources (e.g., thiourea for sulfur source, selenourea for selenium source, or others). Suslick et al. ultrasonically irradiated Mo(CO)₆ and elemental sulfur in 1,2,3,5-tetramethylbenzene under argon gas, producing clustered MoS₂ nanoparticles with a high edge surface area, and the product exhibited substantially high catalytic activity for the hydrodesulfurization of thiophene.⁹⁹ To develop a viable non-enzymatic electrochemical sensor for H₂O₂ detection, Chen et al. prepared a smooth grass-like vanadium disulfide from VCl₃ and Na₂S precursors through a simple sonochemical method without surfactants or templates.¹⁰⁰ The grass-like structure was comprised of a large number of randomly oriented leaf-like flakes which had an average diameter of 250 nm and a thickness of 60 nm (Fig. S4†), and there were no significant impurities but vanadium (V) and sulfur (S) elements with a 1 : 2 atomic ratio.

In a recent study by Park et al., a substantially green procedure was designed to yield Cu(In_0.7Ga_0.3)Se₂ nanoparticles on the basis of a slightly modified sonochemical method.¹⁰¹ The reaction was carried out at room temperature with ethanol as the only solvent, NaBH₄ as a reducing agent, and a 5 h ultrasound irradiation. In the reaction pathways, ultrasound irradiation played an important role in decomposing the intermediate Cu₂Se, so prolonging the sonification time would result in the formation of Cu(In_xGa_1−x)Se₂ compounds with much better crystallinity, and wherein a 5 h sonication yielded tetragonal Cu(In_0.7Ga_0.3)Se₂ nanocrystals without any Cu₂Se crystals. In order to develop an electrocatalyst for the effective sensing of diphenylamine, Chen et al. prepared ytterbium-doped molybdenum selenide (YbMoSe₂) by using a one-step ultrasonic method.¹⁰² YbMoSe₂ displayed a densely packed hierarchical sheet-like or flower-like structure with uniform shape and size (Fig. 7), and there was a mixing and disordered arrangement in the atomic crystal lattice due to the substitution of Yb with MoSe₂. In the hexagonal lattice structure of YbMoSe₂, the lattice spaces of Yb and MoSe₂ were 0.6 nm assigned to the (002) plane, and lattice distortion/defects on the MoSe₂ crystal lattice occurred due to the exposed lattice plane of Yb into the atomic lattice of MoSe₂.

Fig. 7 TEM images ((A) scale bar = 0.5 μm; (B) scale bar = 0.2 μm), HRTEM image ((C) scale bar = 5 nm), and selected area electron diffraction pattern ((D) scale bar = 1 nm) of layered YbMoSe₂. Fast Fourier transform pattern of YbMoSe₂ ((E) scale bar = 1 nm), and selected area electron diffraction pattern in the case of basal plane of YbMoSe₂ (F). Schematic diagram of YbMoSe₂ (G). Reproduced with permission from ref. 102. Copyright 2019 Elsevier.

Other metallic compounds can also serve as element or ligand sources for the ultrasound-aided synthesis of metal compounds. Kipcak et al. used magnesium chloride hexahydrate and different boron sources (borax, tincalconite, boric acid, and boron oxide) as the precursor materials to synthesize magnesium borate by sonochemistry-assisted routes, and all the outcomes were MgO(B₂O₃)₃·7H₂O with high crystallinity and higher than 80% yields.¹⁰³ Pinkas and coworkers utilized a sonochemical precipitation method to form insoluble amorphous uranium phosphates from UO₂²⁺ precursors in trialkyl phosphate solutions.¹⁰⁴ In this sonolysis experiment, the high-boiling trialkyl phosphates served as a phosphate source to create some active species by the decomposition of ester molecules and then reacted with uranium precursors in the solution, followed by the obtained amorphous precipitates changing into crystalline uranium diphosphate (UP₂O₇) when heated to 1000 °C. The efficiency of uranium removal was higher than 30% by sonochemical precipitation, which might become a potential remote separation technology for uranium in waste streams.

Mastai et al. described a surfactant-free sonochemical method to synthesize CoFe₂O₄ nanoparticles for the catalytic degradation of elastomer-based polymers, where Co(CH₃COO)₂ and Fe(C₅H₇O₂)₃ were sonicated in a slightly basic solution (pH = 8) at 75 °C.¹⁰⁵ After calcination at 500 °C, such cobalt ferrite nanoparticles demonstrated an average particle size of around 27 ± 6 nm. Salavati-Niasari et al. synthesized pristine uniform DyVO₄ nanoparticles by a sonochemical method (60 W, 18 kHz) so as to create a highly efficient photocatalyst for water treatment.¹⁰⁶ Since the selective adsorption of surfactants on the crystal facets could control the growth rates along the different crystal directions due to the anisotropy in adsorption stability, the researchers conducted a size and uniformity repair of nanomaterials through using different types of surfactants such as anionic surfactants, cationic surfactants and nonionic surfactants. They found that cetyltrimethylammonium bromide could be used to prepare optimum DyVO₄ nanoparticles with suitable uniformity and small sizes. Later in the presence of PEG-6000, they synthesized pure Zn_0.35Fe_2.65O₄ nanostructure as a ferrite catalyst material by sonicating Fe(NO₃)₃ and Zn(OAc)₂ in a basic aqueous solution, aiming to purify waste water containing different pollutants.¹⁰⁷ In the report of Ashiri et al.,¹⁰⁸ the sonochemical catalysis approach was used for the synthesis of BaTiO₃ nano-powders from titanium tetrachloride, barium(II) chloride, and sodium hydroxide in an aqueous solution. The results were contrasted with those of a mechanochemical method, demonstrating that the sonochemical synthesis allowed the preparation of highly pure tetragonal BaTiO₃ at a much lower temperature and a shorter time span, because the reactant ions in the solution mixture would homogenously enter into the perovskite structure in order.

5.2. Template-directing approach

In the sonication process, an appropriate structure-directing agent or removable template can serve to create different nanostructured materials. To make one-dimensional ZnO nanorods and two-dimensional ZnO nanoflakes, Vabbina et al. deposited vertically aligned nanostructures on Au-coated Si substrates by sonicating a zinc salt, in which hexamethylenetetramine was used as a shape-directing agent to attach to the nonpolar facets of ZnO and allow the growth only in the 〈0001〉 direction.¹⁰⁹ Under an ammonia condition, Zhu et al. sonochemically reacted CdCl₂ with NaSeSO₃ producing hollow spherical assemblies composed of CdSe nanoparticles,¹¹⁰ where amorphous Cd(OH)₂ from the hydrolysis acted as an in situ template to direct the growth of CdSe nanoparticles and the formation of spherical assemblies. Chu et al. fabricated coupled twin-shaped hollow hemispherical calcium molybdate microstructures via a facile ultrasound-assisted approach at room temperature,¹¹¹ where ethylenediaminetetraacetic acid as a structure-directing agent played pivotal roles in the formation of unique architecture on the basis of the oriented attachment and subsequent Ostwald ripening mechanism (Fig. S5†). The coupled twin-shaped hollow hemispheres had high specific surface area, hollow interior, mesoporous structure and low density.

Expectedly, SiO₂ materials or carbon nanoparticles usually give an aid to directly depositing produced free metal atoms onto a support, forming various micro-/nano-structures. Under air, Suslick et al. carried out the ultrasound irradiation of Mo(CO)₆ and silica nanoparticles in 1,2,3,5-tetramethylbenzene, and the MoO₃-coated silica nanoparticles as the pre-product would form hollow MoO₃ when using HF to etch the silica component (Fig. 8-I).¹¹² Similarly, hollow MoS₂ could be yielded by replacing air with Ar gas and adding sulfur into the system.⁵¹ In another study, Suslick et al. prepared nanosized hollow hematite from the precursor Fe(CO)₅ when carbon nanoparticles created from polypyrrole carbonization were employed as a sacrificial template (Fig. 8-II).¹¹³ Also, Yang et al. obtained porous Co₃O₄ from the sonochemical product of Co₄(CO)₁₂ with carbon nanotubes as a template which spontaneously removed itself via combustion attributed to the rapid oxidation of elemental Fe upon air exposure (Fig. 8-III).¹¹⁴ During the sonochemical synthesis of nanostructured PbWO₄, Zhu and coworkers realized the control of unique morphologies in the presence of soft templates.¹¹⁵ Sonicating a mixed solution containing Pb(CH₃COO)₂, NaWO₄ and P123 (Pluronic block copolymer) would result in the formation of hollow metal oxide spindles (Fig. 8-IV), but only solid particles were obtained in the absence of ultrasound. This could probably be attributed to the proposal that ultrasound irradiation changed P123 micelles into micelle aggregates with a hollow spindle structure.

Fig. 8 (I) TEM micrographs of sonochemically prepared hollow MoO₃ nanospheres from HF etching of MoO₃/SiO₂ before thermal annealing (left: scale bar = 1 μm) and after thermal annealing at 350 °C (right: scale bar = 500 nm), where insets are at higher magnification (left: scale bar = 200 nm; right: scale bar = 100 nm). Reproduced with permission from ref. 112. Copyright 2005 American Chemical Society. (II) Bright-field TEM image ((a) scale bar = 20 nm) and dark-field TEM image ((b) scale bar = 20 nm) of nanosized hollow iron oxide (inset: SAED pattern). Reproduced with permission from ref. 113. Copyright 2007 American Chemical Society. (III) TEM images (left: scale bar = 200 nm; right: scale bar = 30 nm) of CNTs–CoO_x nanocables prepared by sonication and Co₃O₄ nanotubes after the calcination of CNTs–CoO_x nanocables, where the upper-left inset corresponds to the HRTEM image (scale bar = 5 nm). Reproduced with permission from ref. 114. Copyright 2007 Wiley. (IV) Typical SEM image ((a) scale bar = 200 nm) of as-prepared PbWO₄ sample, (b) SEM images of several broken hollow spindles viewed from different angles, and typical TEM image ((c) scale bar = 300 nm), SAED pattern ((d) scale bar = 150 nm), and HRTEM images recorded on the tip ((e) scale bar = 5 nm) and the shell ((f) scale bar = 5 nm) of a hollow spindle. Reproduced with permission from ref. 115. Copyright 2006 American Chemical Society.

5.3. Technology-cooperating approach

In cooperation with other approaches, ultrasound irradiation is capable of enabling the preparation of more metal compound nano-/micro-structures. To decrease the sintering temperature in the preparation of strontium hexaferrite (SrFe₁₂O₁₉) nanoparticles, Jesús and Miró described a synthesis route of sonochemistry and annealing.^116,117 The sonochemical process yielded an amorphous phase containing Fe³⁺, Fe²⁺ and Sr²⁺ ions from a complexed polyol solution of metallic acetates, and the annealing of the amorphous phase caused the generation of hexagonal SrFe₁₂O₁₉ by the creation and phase conversion of intermediates. Moreover, high output power and long time (up to 3 h) of sonication could promote the crystallinity of amorphous phases, decrease in crystal size, and modify the morphology of particles. Similarly, Cavalcante and coworkers synthesized triclinic-structured tungstate (CuWO₄) crystals by a sonochemical approach and a subsequent heat treatment in a conventional furnace.¹¹⁸ Within the sol–gel preparation of nanostructured metal compounds, ultrasound irradiation can accelerate the hydrolysis process, improve the phase purity, and create products with a narrower size distribution and a higher surface area. Yu et al., Mansour et al. and Haridas et al. separately produced TiO₂ nanoparticles by the sonochemical hydrolysis of titanium tetra-isopropoxide precursor.^119–121 In these synthesis cases, the accelerated hydrolysis in sonication improved the crystallinity of TiO₂, showing the significant effects of ultrasound irradiation on the microstructure and crystallization kinetic mechanism of amorphous TiO₂. In the report of Chen et al., differently, rod-like ZnO nanocrystallites were synthesized from zinc acetate via an ultrasound-assisted non-hydrolytic sol–gel process,¹²² in which the reactant diethylene glycol acting as a capping agent limited particle growth and prevented agglomeration. The final ZnO nanorods had a width of about 30 nm and length of 50–70 nm. To obtain thin ZrO₂ layers on structured reactors, Jodłowski developed an efficient ultrasound-assisted sol–gel method through the synergistic combination of three processes:¹²³ sonochemical synthesis of Zr sol–gel from a Zr(IV) precursor solution, addition of ethylene glycol and glycerol as stabilizing agents, and deposition of ZrO₂ on FeCr alloy supports. The resultant ZrO₂ films on the metallic structure were dense and uniform without any impurities.

Furthermore, many technologies such as sono-fragmentation and ultrasonic spray pyrolysis have been developed to produce nanostructured materials on account of the physical or mechanical effects from the ultrasound.¹²⁴ In sonication, few-layered materials such as MoSe₂, MoS₂, WS₂ and Bi₂Te₃ can lead to single-layered nanosheets by direct liquid-phase exfoliation, which is actually a response from sono-fragmentation.¹²⁵ Suslick et al. have prepared MnO₂ microspheres via an ultrasonic spray pyrolysis process, where a KMnO₄ and HCl mixed solution was nebulized into micro-sized droplets serving as a microreactor and produced microspheres upon heating.¹²⁶ By tuning the precursor concentrations and furnace temperatures, the microsphere morphology could be controlled. For instance, the synthesis yielded amorphous MnO₂ microspheres at 150 °C but crystalline α-MnO₂ at 500 °C. In the study of Skrabalak et al.,¹²⁷ ultrasonic spray pyrolysis coupled with a molten salt synthesis successfully produced single-crystalline hexagonal NaInS₂ nanoplates with an average diameter of 572 ± 175 nm and a width of 51 ± 16 nm, and the photoanode films fabricated from the NaInS₂ nanoplates provided nearly 25 times greater photocurrent upon illumination than films made with traditional NaInS₂. Later in order to develop a facile and general synthetic strategy for particles with size, shape and compositional control, they described a simple reactor for ultrasonic spray synthesis,¹²⁸ where the particle formation occurred within spatially and temporally confined droplets. By such a step-by-step procedure, AgSbO₃ visible-light photocatalysts were synthesized when single-crystalline NaSbO₃ nanoplates prepared by salt-assisted aerosol combustion were used as a topotactic template.¹²⁹Fig. 9 shows ultrasonic spray pyrolysis creating some nano- and micro-materials.

Fig. 9 (I) Simple reactor schematic for ultrasonic spray synthesis (USP). Reproduced with permission from ref. 128. Copyright 2017 American Chemical Society. (II) SEM image ((a) scale bar = 250 nm) and TEM image ((b) scale bar = 50 nm) of NaSbO₃ at 900 °C, where the inset in (b) displays the corresponding SAED pattern, and for comparison, SEM images of NaSbO₃ product obtained from an aerosol-assisted synthesis without CsNO₃ flux ((c) scale bar = 250 nm) and NaSbO₃ prepared from conventional solid-state synthesis ((d) scale bar = 2 μm). Reproduced with permission from ref. 129. Copyright 2015 American Chemical Society. (III) SEM image ((A) scale bar = 1 μm) and TEM image ((B) scale bar = 100 nm) showing the hexagonal shape of NaInS₂ nanoplates produced via USP (the inset electron diffraction pattern in (B) demonstrates the single-crystalline nature); SEM image ((C) scale bar = 10 μm) of NaInS₂ prepared through a non-USP method for comparison, and (D) X-ray diffraction patterns of the USP and non-USP materials. Reproduced with permission from ref. 127. Copyright 2012 Wiley-VCH. (IV) Morphology of MnO₂ microspheres produced via USP at 150 °C (a–e) and 500 °C (f–j) ((b, c, g and h) scale bars = 100 nm; (d) scale bars = 20 nm; (e and j) scale bars = 2 nm; (i) scale bars = 5 nm); TEM images (c, d, h and i) where increasing the reaction temperature caused the formation of larger crystals; SEM images (b and g); and electron diffraction patterns (e and j). Reproduced with permission from ref. 126. Copyright 2015 Wiley-VCH.

6. Non-metal materials

Apart from metals, alloys, and metal compounds, nonmetal nanomaterials can be also facilely produced by sonochemical effects or ultrasound-assisted technologies, mainly covering carbon materials and non-carbon materials.

6.1. Carbon materials

Based on the physical/chemical effects of ultrasound, single- and few-layered materials are often prepared by breaking 3D layer structures down to 2D planar structures, where the attractive forces between individual layers are completely overcome. As a typical case, the synthesis of graphene oxide (GO) using Hummers' method is widely employed and admirable: oxidization of pristine graphite increases the interlayer distance and consequently weakens the van der Waals force relative to graphite, so that a bath or horn sonication facilitates the exfoliation of single- or few-layered GO from the bulk material. Sagadevan et al. synthesized perfectly crystalline GO by a modified Hummers' method utilizing a sonochemical technique,¹³⁰ and the sample was wanted specifically for the design of smart opto-electronic devices. However, the presence of oxides (i.e. epoxy bridges, hydroxyl groups, and carboxyl groups) would disrupt the band structure and completely degrade the electronic properties, though the oxides could be removed by a reduction step. In another study, Lee et al. reported the synthesis of high-purity reduced GO (rGO) from pristine graphite via a one-step ultrasonic reduction method,¹³¹ where the processes avoided the harsh reaction conditions including high temperature and highly toxic hydrazine. The as-synthesized rGO exhibited a very thin wrinkled paper-like structure with sheet folding, where minimal layers were about 4 layers with a layer spacing of about 1 nm.

Sonication-assisted liquid-phase exfoliation methods can allow the extraction of low-defect graphene materials (Fig. 10-I).¹³² Coleman et al. successfully utilized the direct exfoliation of graphite in N-methylpyrrolidone by sonication to produce single- and few-layered graphene (a monolayer yield of about 1 wt%), where the surface energy between solvent and graphite had to match (about 40 mJ m⁻²) to obtain high-yield exfoliated graphene without defects or oxides.¹³³ Romero-Salguero et al. reported another ultrasound-assisted liquid-phase exfoliation method to synthesize graphene nanosheets from microcrystalline graphite powder in o-dichlorobenzene,¹³⁴ and the resulting nanostructures were demonstrated to be unfunctionalized, non-oxidized and isolated. Trusova's group developed the synthesis of oxygen-free graphene sheets by the sonochemical exfoliation of graphite powder in N,N-dimethyloctylamine-aqua medium (pH was equal to 3),¹³⁵ followed by the fixation of graphene sheets at the organic-aqua interface (Fig. 10-II). In order to avoid the commonly used organic exfoliating solvents, Yusoh and coworkers used a commercial coffee as a green medium to facilely create few-layer graphene flakes in sonication (Fig. 10-III).¹³⁶ However, there was a large presence of hydroxyl groups in the graphene, which was different from the other ultrasound-assisted liquid-phase exfoliations, suggesting that the graphene was successfully functionalized by the active chlorogenic acid in coffee.

Fig. 10 (I) Schematic representation of the liquid-phase exfoliation process of graphite in the absence of surfactant molecules. Reproduced with permission from ref. 132. Copyright 2014 Royal Society of Chemistry. (II) TEM micrographs (bright field: scale bar = 1 μm; dark field: scale bar = 200 nm) of graphene films obtained from the decanted liquid substance after graphene-containing colloid sedimentation during 12 h. Reproduced with permission from ref. 135. Copyright 2018 Hindawi. (III) TEM images of GC (left: scale bar = 200 nm), and at higher magnification with the presence of bilayer and trilayer graphene (right: scale bar = 40 nm) with very similar orientations to each other. Reproduced with permission from ref. 136. Copyright 2019 Elsevier.

Other nonmetal materials, such as carbon nanotubes, graphene quantum dots, carbon nanodots, and black phosphorus quantum dots, can be prepared from corresponding precursors by ultrasonic treatment as well. Ha and Jeong reported a sonochemical route to prepare crystalline multi-walled carbon nanotubes,¹³⁷ in which ultrasound was used to irradiate a p-xylene solution containing silica powder and ferrocene. The p-xylene acting as a carbon source was pyrolyzed to carbon atoms or moieties via the catalysis of Fe clusters that originated from the sonochemically decomposed ferrocene, and finally created high-purity multi-walled carbon nanotubes. The nanotube wall thickness was in the range of 25–40 nm and consisted of 10–30 graphitic walls. Kumar et al. made carbon nanodots from poly(ethylene glycol) in sonication, forming a coating layer of carbon nanodots on polyethylene.¹³⁸ The samples had an average diameter of 2–9 nm depending on the preparation conditions, and showed good fluorescence properties because the highest quantum yield of emission was about 16%. Similarly, many other carbon sources can also be subjected to ultrasonic treatments to prepare carbon dots or doped nanodots. Li et al. used alkali- or acid-assisted ultrasonic treatment of glucose to obtain carbon dots, while Wei et al. developed a facile sonochemical method to make nitrogen-doped carbon dots by using the pyrolysis of carbon precursors (citric acid and ethylene diamine).^139–141 Recently by sonicating dopamine in dimethylformamide, Zhou et al. fabricated nitrogen-doped carbon quantum dots with temperature-dependent fluorescence.¹⁴² The product had superior dispersibility and stability in water, strong and stable fluorescence against various pH and ionic strengths, low cytotoxicity, and high photostability, as well as exhibiting a sensitive quenching phenomenon towards Fe²⁺ ions.

6.2. Non-carbon materials

In recent years, many non-carbon materials have been also produced by sonochemical catalysis technologies. In the absence of any templates or surfactants, Xia et al. employed the sonication of amorphous selenium colloids in alcohol to thermodynamically form trigonal selenium (t-Se) nanowires via a solid-solution-solid transformation mechanism,¹⁴³ where selenium nanocrystals were sonochemically nucleated in a trigonal structure acting as the seeds of the nanowires. Different from Xia's work, Li et al. used a longer ultrasonication time (40 kHz, 100 W cm⁻²) to synthesize single-crystalline t-Se nanotubes and nanowires.¹⁴⁴ When the sonication period was prolonged to 30–60 min, Se nanotubes were initially formed from the spherical-like t-Se seeds and gradually grew within the aging time. With the aging time being prolonged to 24 h, the t-Se nanotubes grew into a well-organized shape with diameters of less than 200 nm, whereas the tubular structure tended to collapse and change into t-Se nanorods and nanowires with diameters of 20–50 nm after further aging.¹⁴⁴ The two stages could not be separated obviously. Salavati-Niasari et al. sonochemically synthesized Se nanostructures by using SeCl₄ as a new precursor in a reducing environment.¹⁴⁵ When the ultrasonic power was set at about 60 W with a 30–40 min period, the sufficient amounts of energy produced Se nanosheets with an average diameter of 200 nm; if the stronger reducing agent potassium borohydride instead of hydrazine was used in the synthesis, it did not create Se nanosheets but Se nanoparticles with an average particle size of 20 nm. Besides, solvent, acid or base environment, and surfactant also played a key role in controlling the morphology and particle size of resultant Se nanostructures. Similarly, they obtained rod-like Se nanostructures by a sonochemical route using thioglycolic acid as the reducing agent.

In the study by Bedini et al., isolated silicon nanoparticles were sonochemically synthesized by Si₃H₈ solution undergoing a sonication process (26 kHz, 10–40 °C) in an inert N₂ atmosphere.¹⁴⁶ The whole formation was mainly attributable to the interplay between the extreme conditions during the bubble collapse and the high density of active species (e.g., silicon radicals) created at such spatially localized points (Fig. S6†). Therein, the incomplete diffusion during the short-lived collapse caused residual bonded hydrogen existing in the particles, and the silicon matrix did not have an enough interaction time to reconstruct into a crystalline state, so the as-synthesized silicon nanoparticles were discrete, hydrogenated, and amorphous. The size of silicon samples was tunable within a 1.5–50 nm range by varying the ultrasound amplitude or the trisilane concentration, where low-amplitude ultrasound or low-concentration Si₃H₈ favored the creation of discrete and monodisperse silicon nanoparticles (size distribution of 2.96 ± 0.82 nm) probably attributed to the smaller microbubbles and/or less trisilane in-diffusion during bubble growth.¹³³

Silica (SiO₂) nanoparticles occupy a significant position in scientific research due to their potential applications (e.g., pharmaceuticals, catalysis, pigments, stabilizers, polishing, and sensors), and sonochemical catalysis has extended to the synthesis of various SiO₂ nanostructures. Via an ultrasound-assisted sol–gel method, Kim et al. successfully prepared monodisperse and size-controlled silica particles with a uniformly spherical shape from tetraethyl orthosilicate in the presence of ammonia.¹⁴⁷ The average diameters of the silica particles were distributed in the range from 40 to 400 nm, and could be controlled by the reactants' molar ratio which determined the nucleation and growth rates of particles. Salavati-Niasari et al. also synthesized spherical SiO₂ nanoparticles by the sonochemical method,¹⁴⁸ and, dissimilarly, Schiff-bases (i.e., bis(acetylacetonato)propylene-1,3-diimine and bis(acetylacetonato)buthylene-1,4-diimine) acted as capping agents for the hydrolysis of tetraethyl orthosilicate in aqueous alcohol solution. The molar concentration of Schiff-base ligand could be effective in control of particle size, but the conclusions for bis(acetylacetonato)propylene-1,3-diimine and bis(acetylacetonato)buthylene-1,4-diimine were inconsistent, which was not further explained. Sankar et al. reported the synthesis of amorphous silica nanoparticles from brown rice husk via the sonochemical process,¹⁴⁹ where the sonication time was varied so as to modulate the product's properties. By increasing the sonication time (0–50 min), the mean particle sizes were facilely controlled from 5 to 40 nm while the pore volumes were changed from 176.33 m² g⁻¹ to 226.56 m² g⁻¹, and moreover, the band-gap energy was decreased from 5.77 eV to 5.68 eV. In the study of Jiang et al.,¹⁵⁰ a facile ultrasonic-assisted approach was presented to prepare hollow mesoporous silica. Prior to the hollow mesoporous structure, a whole solid was obtained through CTAB-directed sol–gel processes, and then a one-step ultrasonic treatment was conducted acting as a selective etching. Fig. 11 exhibits the conversion process of the whole solid to hollow structure for different ultrasound durations.

Fig. 11 Conceptual demonstrations of the preparation process of hollow mesoporous silica spheres, where TEM images (scale bars = 200 nm) show the silica spheres prepared at 20 kHz for (A) 2 h, (B) 6 h, and (C) 48 h. Reproduced with permission from ref. 150. Copyright 2019 Elsevier.

7. Inorganic composites

Sometimes, the attributes of a single material cannot satisfy some specific demands in a proposal, or it is not able to serve as a qualified candidate for some harsh processes or reactions. Hence, the merging or synergy between elements or materials is a superior choice on the basis of various properties including low cost, high activity, good stability, and so on. So far, nano/micro inorganics have passively formed thousands of combinations for actual requirements, and here, as a highlight, are only provided the hybrid incorporations from metals, metal compounds, and non-metal materials via sonication. Sonochemical catalysis represents a feasible, facile and short-time approach for the synthesis of hybrid inorganic materials.

7.1. Metal/metal compounds

On the basis of surface plasmon resonance, a series of hybrid noble metal/metal compounds can be developed to decompose various organic pollutants as plasmonic catalysts. Cho et al. produced Ag/AgCl nanocubes with Ag nanoparticles embedded into the AgCl cubic matrices when sonicating sodium chloride, silver nitrate and poly(vinyl pyrrolidone) in ethylene glycol (Fig. 12-I).¹⁵¹ Such Ag/AgCl nanocrystals had a 115 nm edge length, and their sizes and size distribution could be well controlled by changing the Ag⁺ and poly(vinyl pyrrolidone) concentrations in the precursor solutions. Under ambient conditions, the as-obtained Ag/AgCl hybrids displayed an enhanced photocatalytic performance under visible light. Later, Bao et al. synthesized Ag/AgX (X = Cl, Br) plasmonic photocatalysts via a facile one-pot ultrasonic spray pyrolysis method,¹⁵² wherein AgNO₃ and KCl (or KBr) separately worked as Ag source and Cl (or Br) source with no additional conditions (e.g., high pressure), surfactants and reducing agents. Through the thermal decomposition of residual AgNO₃ solution, metallic Ag particles uniformly nucleated and covered the surface of AgX grains, becoming an up- and down-structure like a sequence of small mounds (Fig. 12-II). Due to the surface plasmon resonance of small-sized Ag nanoparticles with uniform distribution, Ag/AgX photocatalysts had excellent catalytic activity for the visible light-driven photodegradation of methyl orange and methylene blue pollutants.

Fig. 12 (I) SEM images (scale bars = 500 nm) of Ag/AgCl nanocrystals at various stages with sonication durations of (a) 10 min, (b) 15 min, (c) 20 min, (d) 25 min, (e) 30 min, and (f) 35 min. (g) Plot of the particle size of AgCl nanocrystals as a function of sonication time (poly(vinyl pyrrolidone) = 20 g L⁻¹, [Ag⁺] = 0.05 mol L⁻¹). (h) Schematic growth process of AgCl nanocrystals under sonication conditions. Reproduced with permission from ref. 151. Copyright 2012 Wiley. (II) Schematic diagram of experimental setup, illustrating the preparation process of Ag/AgX (X = Cl, Br) photocatalyst, and SEM images of (a) AgCl, (b) Ag/AgCl, (c) AgBr, and (d) Ag/AgBr ((a, b and d) scale bars = 1.00 μm; (c) scale bar = 3.00 μm). Reproduced with permission from ref. 152. Copyright 2018 Elsevier.

In the study of Chen et al., a two-step sonochemical approach was described to prepare core/satellite Cd(OH)₂/Ag nanorods (Fig. S7-I†).¹⁵³ Firstly, Cd(OH)₂ nanorods were yielded from cadmium chloride in a base medium by a single ultrasound-aided reaction, and then changed into highly crystalline CdO nanorods after a thermal annealing process; afterwards, the CdO nanorods in the presence of Ag precursor underwent a second ultrasonication producing a Cd(OH)₂/Ag core/satellite complex, where a phase transformation occurred from the cubic structure of CdO to the monoclinic crystalline structure of Cd(OH)₂ followed by Ag nanodots being deposited on the Cd(OH)₂ surface. Recently, they employed a surfactant-free sonochemical approach to synthesize unique Fe₂O₃/ZrO₂ nanostructures with nanocube morphology,¹⁵⁴ and then achieved Au nanodots decorated on the Fe₂O₃/ZrO₂ composites by a one-step sonochemical method (Fig. S7-II†). Apart from the conclusion that the Fe/Zr molar ratio played an important role in catalytic activity for the production of liquid fuel or organic chemicals from syngas (CO and H₂), the dopant Au nanodots also served as a promoter to contribute to the enhanced catalysis of Fe₂O₃/ZrO₂ nanocubes, wherein 2.5 wt% Au significantly increased the CO conversion ratio. However, higher Au concentration (e.g., 5 wt%) in the catalyst would in turn cause a deactivation effect due to the aggregation and poisoning of particles.

Via promoting the diffusion of metal ions or particles, the physical/mechanical effects of ultrasound can participate in doping and/or modification processes creating hybrid metal/metal compounds. By ultrasound irradiation, Belova et al. produced Au nanoparticle-intercalated mesoporous TiO₂ structures with an enhanced photocatalysis from a slurry containing Au colloids and TiO₂ particles, while Xiong et al. made Zn(OAc)₂ and Mg(OAc)₂ into photoluminescence-tunable Mg²⁺-doped ZnO nanoparticles without MgO phase.^155,156 Recently, Xu and coworkers described a simple and facile ionic liquid-assisted sonochemical method to synthesize hexagonal and tetragonal LnPO₄:Eu³⁺ nanostructures (Ln = La, Gd, Y),¹⁵⁷ where the acceleration effect of ionic liquid combined with ultrasound irradiation led to a rapid reaction rate. Perhaps related to the period of the sonication process, the as-synthesized LnPO₄:Eu³⁺ displayed four different morphologies: nanowires, nanorods, nanowire bundles, and nanoparticles. By a modified sonochemical method using aloe vera gel as a surfactant, Nagabhushana et al. carried out the synthesis of Eu³⁺-doped Y₂O₃ nanophosphors.¹⁵⁸ The sonication period had an important effect on the morphology of Eu³⁺-doped Y₂O₃. If the sonication period was very short, the structure of nanophosphors easily agglomerated, and when the sonication period was prolonged but less than 6 h, the structure gradually became uniformly spherical with the size slightly reducing, and the agglomerated structure occurred again after more than 6 h because uniform structures were again increasing.

With 1-butyl-3-methylimidazolium tetrafluoroborate as fluoride source and EuBr₂ or EuSO₄ as europium source, Wickleder et al. designed another novel ionic liquid-assisted strategy to fabricate Eu²⁺-doped BaFCl nanoparticles by applying sonochemical approaches.¹⁵⁹ Such Eu²⁺-doped nanoparticles in 1-butyl-3-methylimidazolium tetrafluoroborate presented a relatively narrow size distribution ranging from 20 to 90 nm with 80% being smaller than 40 nm. In the visible range, BaFCl nanoparticles doped with Eu²⁺ had a high reflectance value similar to that of undoped BaFCl nanoparticles owing to nearly no efficient absorption. In the UV spectral range, Eu²⁺-doped BaFCl particles exhibited a decreased reflectance efficiently emitting bluish white light, even though the undoped particles also displayed weak dark blue emission probably attributed to the C-dot present in the ionic liquid. By the same synthesis route, Ln(NO₃)₃ and 1-hexyl-3-methylimidazoliumdihydric phosphate led to Eu³⁺-doped lanthanide nanophosphates as well. Akbari et al. tried to use the functionalized surface oxide of liquid metals to build up advanced optoelectronic devices.¹⁶⁰ By employing the strong shear forces of ultrasound waves, liquid Galinstan could be simply fractured and separated in an aqueous solution, where ultra-thin Ga₂O₃ nanosheets could be achieved on the alloying nanoparticle surface with the nanoparticle core remaining liquid (Fig. 13). When Ag ions were present in the reacting system, the alloying nanoparticles acted as a sacrificial template where Ga with low standard reduction potential would be feasibly replaced by Ag ions with high standard reduction potential. The decoration of Ag facilitated the visible light responsivity of Ga₂O₃ nanosheets.

Fig. 13 (a) Schematic of ultrasonic-assisted synthesis of Galinstan nanoparticles in different ionic solutions. Left: formation of irregular shaped nanoparticles after ultrasonic-assisted synthesis of Galinstan nanoparticles (scale bar = 500 nm). The presence of fractured ultra-thin skin oxide on the nanoparticle surface is evidently seen. It seemed that during the size reduction of nanoparticles in the ultrasonic process, brittle skin oxides broke and separated from host liquid alloy. Right: schematic showing the sequential dispersion of liquid alloy under ultrasonic waves. The core–shell structure of Galinstan nanoparticles consists of a liquid InGaSn core covered with ultra-thin surface gallium oxide. (b) Spherical morphology (scale bar = 500 nm) of the nanoparticles and distribution of alloying elements on the surface of the nanoparticles synthesized in silver ion-containing solutions. Reproduced with permission from ref. 160. Copyright 2019 Royal Society of Chemistry.

7.2. Metal/non-metal materials

There are many approaches to combine metals with non-metals forming hybrid materials, for example, doping, intercalation, coating, decoration, etc. Gedanken et al. produced carbon dots doped with Ga atoms (Ga@C-dots) when sonicating molten Ga and polyethylene glycol in a reaction medium for 30–120 min.¹⁶¹ The Ga@C-dots were monodispersed and spherical in shape, and had an average size of about 5 nm with a range of 3 to 8 nm, while the pristine C-dots produced from polyethylene glycol had an average size of about 6 nm (Fig. 14-I). The dopant Ga induced the crystallinity of C-dots and enhanced the formation of more C-dots with uniform size, so Ga@C-dots in selected area electron diffraction (SAED) revealed a ring pattern corresponding to a hexagonal carbon structure. With respect to C-dots, the Ga@C-dots had high photosensitization producing more singlet oxygen, which was significant for biomedical applications. Later, Kumar et al. investigated the interaction between molten gallium and the hydrocarbon medium induced by ultrasonic energy, so as to explore the possibility that the product was gallium carbide or another gallium–carbon complex.¹⁶² When the sonication of molten gallium immersed in decane, dodecane and hexadecane was carried out, the partial decomposition of the hydrocarbon medium would occur and form a carbon film surrounding a gallium core prior to the interaction between carbon and gallium (Fig. 14-II). Thus, it was not a kinetic barrier but a thermodynamic barrier against the formation of gallium carbide. Based on the antibacterial activity of Ga@C-dots, Gedanken et al. further fabricated Ga@C-dots@Ga nanoparticles by Ga-doped C-dots decorated on Ga nanoparticles,¹⁶³ when 2.5 h of ultrasound irradiation with 70% amplitude caused the dispersion of molten Ga in the form of particles (Fig. 14-III). Once the Ga@C-dots@Ga nanoparticles were coated on the cell incubation substrate, the cell growth exhibited a 97% increase in the number of branches for the neurites deriving from the soma (Fig. 14-IV).

Fig. 14 (I) TEM images (scale bars = 20 nm) of C-dots: (A) PEG sonicated without gallium and (B) with gallium presented with a size distribution plot; (C) HRTEM image (scale bar = 20 nm) of Ga@C-dots synthesized by PEG in the presence of molten Ga (inset: SAED pattern marked by plane); (D) HRTEM images (scale bars = 5 nm) of Ga@C-dots (inset: lattice fringes of Ga@C-dots). Reproduced with permission from ref. 161. Copyright 2015 Royal Society of Chemistry. (II) Proposed structure for (A) Ga-dodecane sample and (B) Ga-decane sample from current characterization data (red: gallium, violet: oxygen, gray: carbon). Reproduced with permission from ref. 162. Copyright 2017 American Ceramic Society. (III) SEM micrograph ((a) scale bar = 100 nm), TEM image ((b) scale bar = 50 nm), magnified view ((c) scale bar = 50 nm) of single particles, and (d) EDS spectrum of Ga@C-dots@Ga nanoparticles. (IV) Light microscopy images for the growth rate of SH-SY5Y cells: cells growing on a glass substrate after (a) 2 days and (c) 7 days, and cells growing on the Ga@C-dots@Ga substrate after 2 days (b) and 7 days (d). Reproduced with permission from ref. 163. Copyright 2017 Royal Society of Chemistry.

Ultrasound irradiation is a facile method to dramatically enhance the intercalation of guest substances into layered materials. In the study of Walter et al.,¹⁶⁴ H₂PtCl₆ was intercalated into natural graphite under ultrasound, and after exposure to a H₂ flow, Pt-intercalated graphite could be formed by the reduction of Pt ions to metal nanoparticles in the graphite layers. Jones and coworkers prepared K-intercalated compounds (KC₈) by sonicating graphite with potassium in toluene,¹⁶⁵ but the formation was difficult to achieve in either THF or hexane. This could be responded by the possibility that an aromatic solvent was necessary for interplanar expansion or intercalation prior to the addition of potassium to the lattice. In fact, the highly efficient exfoliation for graphene using sonication can also be carried out in the presence of inorganic ions (e.g., SO₄²⁻, NO₃⁻, Cl⁻, Li⁺, Na⁺, etc.), and has attracted much interest. Such successful exfoliation for few-layer nanosheets is often the result of ion intercalation with the aid of the physical effects of ultrasound irradiation. To realize the eco-friendly production of graphene, Cheng et al. designed an ultrasonic-assisted Li⁺/Na⁺ co-intercalated exfoliation route for few-layer graphene without organic solvent,¹⁶⁶ wherein mixed Li⁺ and Na⁺ served as the intercalated agent of natural flake graphite to prepare the as-exfoliated graphene before the exfoliation of graphene nanosheets. The variation of Li⁺/Na⁺ ratio affected the structure and morphology of graphene nanosheets, and the thicknesses of products were about 2.38–2.56 nm (about 7–8 layers) at the optimal Li⁺/Na⁺ ratio (namely Li_0.7/Na_0.3).

Coating or decoration is pervasive and efficient for the combination of heterogeneous materials. Lacroix-Desmazes et al. used a three-step process (i.e., latex synthesis, sonochemistry, and sol–gel) to produce a silica-supported Pt catalyst in aqueous medium, and enhanced the distribution of noble metal Pt at low loadings by controlling the hierarchical porosity of silica material.¹⁶⁷ Zhang et al. prepared metal/non-metal hybrids by a ternary NiCoPd nanocatalyst dispersed on multiwalled carbon nanotubes (CNTs),¹⁶⁸ where the NiCo nanoparticles from Co²⁺ and Ni²⁺ were mixed with the CNT solution under ultrasonication followed by the reducing and loading of the third ion Pd²⁺ on the NiCo/CNTs in the presence of NaBH₄. The NiCoPd/CNT hybrids exhibited an enhanced electrocatalytic activity for the methanol oxidation reaction, which likely resulted from the synergistic function of doped Ni and Co metals as well as the size and distribution of ternary-metal nanoparticles. In the fabrication of platinum-based electrocatalysts, Okoli et al. developed carbon-supported palladium–nickel–molybdenum materials by a sonochemical method using ionic liquid as a dispersion medium.¹⁶⁹ After depositing a platinum monolayer on the ternary PdNiMo nanoparticle cores, the carbon-supported electrocatalysts could improve cathode activity and durability during the oxygen reduction reaction of a fuel cell.

Yang et al. reported, for the first time, a facile and powerful method to integrate hollow Pt–M (M = Ni, Co) nanoparticles into graphene cellular monoliths (GCMs) by using sonochemical-assisted reduction together with gelatinization reaction (Fig. S8-I†).¹⁷⁰ Firstly, the Pt²⁺ and M²⁺ precursors were ultrasonically reduced to three-dimensional hollow Pt–M nanoparticles in the presence of sodium borohydride and distributed on GO sheets. Then the transformation of GO into graphene was successfully achieved via an ascorbic acid reduction, and the as-synthesized Pt–M/graphene hybrid materials were assembled into Pt–M/GCM hydrogels. The Pt–M/GCM had a well-defined macroscopic porous structure on which hollow Pt–Ni or Pt–Co nanoparticles with a size of 10–17 nm were uniformly distributed (II and III in Fig. S8†).¹⁷⁰ Obviously, Pt–M nanoparticles promoted the formation of hierarchical porous structures and caused high specific surface areas and large total pore volumes. Compared with commercial Pt/C, the Pt–M/GCM had better electrocatalytic activity and higher durability toward oxygen reduction reaction. Abbas's group synthesized morphology-controlled Cu nanoparticles wrapped on reduced graphene oxide (Cu/RGO) when Cu(NO₃)₂ and GO as the precursor were subjected to an in situ ultrasonication reaction prior to the reduction of pre-product CuO/RGO in H₂ gas.¹⁷¹ The morphology and catalytic performance of Cu/RGO were affected by ultrasound, NH₄OH, and Cu loading. For the pre-product CuO/RGO, the absence of NH₄OH resulted in polydisperse CuO particles with irregular shapes or amorphous-like structure, and without the aid of ultrasound, the produced Cu catalyst had an elongated-like shape with thick RGO layers owing to aggregation, whereas ultrasound alone was also not enough to completely reduce the GO into RGO. The Cu/RGO catalysts with a 10 wt% or 25 wt% Cu loading had an excellent dispersion and without any aggregation (Fig. S8-IV†),¹⁷¹ but on the contrary, the morphology with a 45 wt% Cu loading appeared to be a leaf-like shape. In the catalytic hydrogenation reaction of dimethyl oxalate, Cu/RGO catalysts with 25 wt% Cu loading displayed the highest selectivity to methyl glycolate, while the catalysts with 45 wt% Cu loading favored the selectivity to ethanol.

7.3. Metal compounds/metal compounds

Among metal compounds, the all-out cooperation has attracted much attention for the preparation, properties and applications. In the study of Zavala-Rivera et al.,¹⁷² iron carbide@iron oxide nanoparticles with core–shell architecture (around 6.38 nm in diameter) were obtained when the sonochemical reaction of Fe(CO)₅ in octanol was carried out under an inert air condition or pure oxygen condition. According to the composition of iron carbide core and magnetite shell, a supposed synthesis mechanism was described as follows. With the aid of oxygen, the decarbonization of Fe(CO)₅ produced the nucleation points of iron carbide (Fe₃C) due to the hotspot formation by cavitation. Afterwards, an iron oxide (γ-Fe₂O₃) layer (thickness of around 2.5 nm) was gradually formed via the secondary interaction of iron carbide and oxygen. Such iron carbide@iron oxide nanoparticles presented a ferromagnetic behavior at 4 K, suggesting a potential application in theranostics. Jiang et al. reported a facile two-step route to fabricate a ZnO/CdS hierarchical heterostructure (average size of approximately 2 μm) with enhanced photocatalytic efficiency under natural sunlight.¹⁷³ The hierarchical ZnO microspheres with ultrathin nanosheets (thickness of about 20 nm) as secondary structures were firstly synthesized through a hydrothermal method, following which hexagonal wurtzite CdS nanoparticles with diameters between 50 and 100 nm were produced by sonicating cadmium chloride and thiourea and then incorporated on the wurtzite ZnO sheets. Under natural sunlight, the ZnO/CdS heterostructures exhibited higher photocatalytic activity or degradation efficiency than pure ZnO sample, and the CdS loading amount was crucial for the photocatalytic performance, because the visible-light-absorbing capability of the hybrid material was enhanced by the coupling effects of the two semiconductors.

Adhyapak et al. attempted to merge hetero-valent tin oxide (Sn₃O₄) with ZnO, since ZnO with a wide band gap (3.2 eV) could be a good alternative for band coupling with Sn₃O₄ to fulfil the effective separation and transfer of photo-generated holes and electrons.¹⁷⁴ By a combined hydrothermal and sonochemical method, they synthesized a ZnO@Sn₃O₄ nano-heterostructure composite, where a nanosheet-like Sn₃O₄ structure with a thickness of 20 nm was decorated with hexagonal rod-like ZnO of 50–180 nm in width and 200–400 nm in length (Fig. 15-I). Compared to pristine ZnO and Sn₃O₄, ZnO@Sn₃O₄ showed a significantly superior photocatalytic activity towards dye degradation as well as hydrogen production via water splitting, and the complete degradation of methylene blue was achieved within 40 min under natural sunlight (Fig. 15-II).¹⁷⁴ Aiming at thiophene photocatalytic oxidative desulfurization, Mousavi-Kamazani and coworkers utilized a facile and single-step sonochemical method to prepare composite Cu₂O–CeO₂ nanostructures from Ce(NO₃)₃ and Cu(NO₃)₂ (Fig. 15-III).¹⁷⁵ Interestingly in the absence of ultrasound irradiation, the main product was not Cu₂O–CeO₂ but CuO–CeO₂, implying that ultrasound irradiation facilitated the reduction of Cu²⁺ to Cu⁺. Besides the composition, ultrasound irradiation had a direct effect on the product morphology, and Cu₂O–CeO₂ composite nanoparticles were smaller in size (about 10 nm) and higher in uniformity unlike agglomerated CuO–CeO₂ structures. Compared with CuO–CeO₂, Cu₂O–CeO₂ exhibited much higher photocatalytic efficiency for thiophene oxidative desulfurization under visible light because hexagonal-like Cu₂O nanostructures in the heterostructure not only increased the amount of light absorption but also had an electron-synergistic effect to promote electron–hole separation.

Fig. 15 (I) TEM images ((a) scale bar = 50 nm; (b) scale bar = 20 nm), SAED pattern image ((c) scale bar = 5 nm), and HRTEM image ((d) scale bar = 5 nm) of ZnO@Sn₃O₄. (II) (a) Plot of change in absorbance vs. irradiation time in the presence of ZnO, Sn₃O₄, P25 and ZnO@Sn₃O₄, (b) spectral changes for the degradation of methylene blue in the presence of ZnO@Sn₃O₄, (c) pseudo-first-order kinetic plots of ln(C₀/C_t) vs. irradiation time for the degradation of methylene blue using ZnO, Sn₃O₄, P25 and ZnO@Sn₃O₄, and (d) hydrogen production as a function of irradiation time in the presence of ZnO, Sn₃O₄ and ZnO@Sn₃O₄. Reproduced with permission from ref. 174. Copyright 2019 Royal Society of Chemistry. (III) Schematic mechanism for the photocatalytic oxidation of thiophene on Cu₂O–CeO₂ nanocomposite: (a) photocatalytic oxidation of thiophene under visible light irradiation by CuO–CeO₂, Cu₂O–CeO₂, CeO₂ and Cu₂O, and (b) repeatability of as-synthesized Cu₂O–CeO₂ nanocomposite. Reproduced with permission from ref. 175. Copyright 2020 Elsevier.

TiO₂ is a known semiconductor material in light science, and has an excellent photocatalytic activity under UV light. Nevertheless, the separation of photoinduced electron–hole pairs restricts the photocatalysis of TiO₂. Therefore, combining TiO₂ with other transition metal semiconductors is a usual and efficient approach to overcome this issue. By sonochemical catalysis with a 480 W ultrasound irradiation of 40 kHz, Majumder et al. fabricated crystalline hexagonal disk-shaped anatase TiO₂ within manganese oxide (Mn₂O₃) nanorods forming TiO₂–Mn₂O₃ composites.¹⁷⁶ The synthesized TiO₂–Mn₂O₃ had unique morphology including nanorods and nanodisks, which could provide higher surface area to enhance the adsorption efficiency and promote the electron–hole separation. In comparison to the composites obtained by a sol–gel method, TiO₂–Mn₂O₃ showed a comparable photocatalytic decolorization efficiency when the decomposition of malachite green oxalate was carried out under ultraviolet irradiation, and with an aid of ultrasound-functionalized H₂O₂, the photoactivity of such a catalyst was found to be higher than that of all bare samples. To extend the absorption of TiO₂ to the visible region, Fallah et al. employed an ultrasonic method in combination with microemulsion to prepare core–shell CdS/TiO₂ nanoparticles (i.e., CdS core and TiO₂ shell) and control their particle size.¹⁷⁷ By optimizing the influential parameters such as temperature, synthesis retention time, TiO₂/CdS ratio and ultrasound power, nanocomposites with an average size of up to 10 nm were successfully synthesized, namely a nano-sized TiO₂ layer (about 1 nm) fully enclosed the CdS core (about 9 nm), as shown in Fig. S9.†¹⁷⁷ Owing to the participation of CdS in the core–shell composites, the absorption spectra of TiO₂ occurred with a red shift, suggesting the optical absorption spectrum of nanocrystals had extended to the visible region.

In the report from Cheng et al.,¹⁷⁸ TiO₂@ZIF-8 composite materials made up of hollow TiO₂ nanospheres (TiO₂ HNPs) and zeolitic imidazolate framework-8 (ZIF-8) were fabricated via a facile sonochemical or sonocrystallization route. Spherical polystyrene with a uniform size was selected as the template in the preparation of polystyrene@TiO₂ core–shell particles, and after annealing to remove the polystyrene, the obtained TiO₂ hollow nanospheres (shell thickness of 60 nm) were carboxylated and externally decorated with ZIF-8 that was produced by a self-nucleus growth under ultrasonic treatment, forming double-shell γ-TiO₂@ZIF-8 hybrid materials (Fig. 16-II).¹⁷⁸Via experimental sequences different from the overall sonocrystallization, two other kinds of semiconductor–metal organic framework samples were also produced, namely α- and β-TiO₂@ZIF-8, whereas some failures occurred in ZIF-8 coverage on the surface of TiO₂ HNPs due to the over-independence of two components. The large surface area and high porosity of hollow TiO₂ and ZIF-8 would expose more active sites in photocatalysis, and the composite heterostructures provided efficient charge separation with electron injection from ZIF-8 to hollow TiO₂ (Fig. 16-I).¹⁷⁸ Under artificial solar light, the hybrid double-shell γ-TiO₂@ZIF-8 exhibited an enhanced hydrogen evolution rate, 3.5 times higher than that of the bare TiO₂ HNPs, with up to 50.89% apparent quantum efficiency at 380 nm (Fig. 16-III). Moreover, the hydrogen evolution rate remained nearly the same after several consecutive photocatalytic reactions, demonstrating the exceptional photocatalytic stability of γ-TiO₂@ZIF-8.¹⁷⁸

Fig. 16 (I) Illustration of hydrogen evolution mechanism over TiO₂@ZIF-8 under artificial solar light irradiation. (II) SEM images ((a) scale bar = 2 μm; (b) scale bar = 1 μm) and TEM images ((c and d) scale bar = 200 nm) of γ-TiO₂@ZIF-8. (III) (a) Photocatalytic hydrogen evolution performance of TiO₂ HNPs, ZIF-8 and α-/β-/γ-TiO₂@ZIF-8, (b) H₂ evolution rates, (c) recyclability of H₂ evolution behavior of γ-TiO₂@ZIF-8, and (d) PXRD patterns of γ-TiO₂@ZIF-8 before and after recycling 4 times. Reproduced with permission from ref. 178. Copyright 2019 Elsevier.

7.4. Metal compounds/non-metal materials

The coupling of metal compounds and non-metal materials catches the eye as a consequence of their superior complementation in particular domains. Carbon materials are popularly selected for many composites not only for their incomparable chemical, physical and/or mechanical properties but also for their variety in structure. Nikitenko et al. carried out the sonolysis of W(CO)₆ in diphenylmethane under 20 kHz power ultrasound in argon at 80 °C,¹⁷⁹ and the pre-product WC_x nanoparticles were embedded in amorphous sonopolymer followed by carbon-supported WC_x nanoparticles (W₂C/C) being created after annealing in an inert atmosphere at 600 °C. The W₂C/C nanocomposites were stable and consisted of amorphous W₂C with average particle size ranging from 4 to 7 nm and hexagonal carbon particles with average size of 30–40 nm. Later, Yamada et al. prepared an amorphous iron/carbon mixture by the sonolysis of ferrocene in diphenylmethane.¹⁸⁰ Interestingly, the Fe and C atoms migrated to consistently form a spherical Fe–C phase with diameters of about 50 nm when the pre-produced amorphous sample was subjected to heating at 900 °C, but the product would be accompanied by carbon nanotubes if heated at 1200 °C, which was caused by the catalysis of iron or iron carbide nanoparticles. In order to develop an efficient and durable catalyst for the oxygen evolution reaction, Xu et al. reported the synthesis of amorphous nickel-iron oxides/carbon nanohybrids (a-NiFeO_x/C) with tunable compositions via a sonochemical method.¹⁸¹ As an electrocatalyst, the compositions of mixed-metal oxides conveniently affected the activities of the oxygen evolution reaction, wherein the optimal a-Ni₅₀Fe₅₀O_x/C exhibited a low overpotential, outperforming the benchmark RuO₂ catalyst. Sonochemistry, as another assisting factor, provided the nickel-iron oxides/carbon nanohybrids with highly stable amorphism remaining even after prolonged cycling, and led to a simultaneously formed carbon scaffold and internal Ni(0), which was important to enhance the stability and activity for the oxygen evolution reaction.

Metal compound-based composites with graphene or GO as a support are diverse and abundant. Zhu et al. succeeded in a controlled incorporation of TiO₂ nanoparticles on graphene layers homogeneously by ultrasonically irradiating graphene and TiCl₄ in ethanol.¹⁸² In terms of photocatalytic activity, the graphene–TiO₂ composite containing 25 wt% TiO₂ was better than commercial pure TiO₂. This was due to the graphene–TiO₂ composite structure consisting of a homogeneous dispersion of crystalline TiO₂ nanoparticles with extremely small size (around 4–5 nm) on the graphene sheets, and the good contact between the two materials enhanced the photo-electron conversion of TiO₂ through reducing the recombination of photo-generated electron–hole pairs. Prakash et al. reported the sonochemical synthesis of nanoporous MnCo₂O₄/graphene as a hybrid cathode material in non-aqueous Li–O₂ batteries (Fig. 17-I).¹⁸³ The product offered a strong synergistic coupling between MnCo₂O₄ nanospheres and graphene sheets, and a porous nanosphere architecture with internal oxygen diffusion pathways and peripheral conductive graphene sheet extensions fulfilled the requirements of a robust cathode and overcame the harsh LiO₂ battery conditions (Fig. 17-II),¹⁸³ so when catalyzing oxygen reduction, MnCo₂O₄/graphene exhibited better rate capability than the widely used Vulcan carbon and benchmark Pt/C catalysts, as well as excellent cycling stability and low overpotential. Zhang et al. sonochemically synthesized SnO₂/graphene nanocomposites from SnCl₂ and GO precursors,¹⁸⁴ and SnO₂ nanoparticles were in situ created, and fully and well adhered on the two sides of graphene nanosheets by the reduction of GO under continuous ultrasonication after Sn²⁺ was homogeneously dispersed on the GO surface. The resulting SnO₂/graphene structure had high specific surface area, a stable framework, and remarkable electron/ion transport, and hence, compared to bare SnO₂, the SnO₂/graphene nanocomposites serving as a lithium ion battery anode exhibited significantly improved specific capacity, better cycling stability, and competitive rate performance. In the study of Chen et al.,¹⁸⁵ (ZrO₂-Al₂O₃)/GO nanocomposites with special 3D network were successfully prepared by a sonochemical catalysis route, where the coexisting Zr and Al sources quickly produced metal oxide nanoparticles aggregated on the GO surface under ultrasound irradiation. Owing to a good fluoride adsorption capacity, the nanocomposites exhibited exceptionally high defluoridation performance for drinking water. Similarly, ZnO is also often deposited on carbon materials using ultrasound-assisted approaches. For example, Cobianu et al. prepared 3D nanostructured ZnO–graphene hybrids for gas sensing applications, while Khairy et al. employed an ultrasonic/hydrothermal process to fabricate ZnO nanosphere-carbon nanotube nanocomposites and ZnO nanosphere-rGO nanocomposites for the heterogeneous photocatalytic degradation of 4-nitrophenol.^186,187

Fig. 17 (I) Microscopic images of MnCo₂O₄: FESEM images of MnCo₂O₄ ((a and b) scale bar = 200 nm) and MnCo₂O₄/graphene composite ((c) scale bar = 2 μm; (d) scale bar = 1 μm); TEM images of MnCo₂O₄ ((e) scale bar = 100 nm; (f) scale bar = 50 nm) and MnCo₂O₄/graphene composite ((g) scale bar = 50 nm; (h) scale bar = 100 nm), as well as SAED pattern (inset); (i) EDX mapping images of Mn, Co, O, and C elements. (II) Schematic representation of the formation-decomposition mechanism of discharged products on MnCo₂O₄/graphene hybrid cathodes. Reproduced with permission from ref. 183. Copyright 2017 Wiley.

So far, many efforts have been also made to prepare metal compound/silica materials. Santoyo-Salazar et al. obtained core–shell Fe₃O₄@SiO₂via the combination of Stöber and sonochemical methods using low-power ultrasound irradiation, where magnetite nanoparticles were coated with silica.¹⁸⁸ With an increase of tetraethyl orthosilicate volume, the composites had a controlled SiO₂ layer encapsulating crystalline Fe₃O₄ distribution. In order to validate the mechanism of sonoluminescence in SiO₂ suspensions, Sharipov et al. produced porous SiO₂ nanoparticles containing Ru₃(CO)₁₂ with a size range of 10–30 nm and an average pore size of 5.8 nm via ultrasonic dispersal,¹⁸⁹ ahead of which the pores on the precursor micro-mesoporous silica powder (particle size of 100–300 μm, and average pore size of 5.8 nm) were saturated with ruthenium dodecacarbonyl by solvent evaporation. In some studies, metal compound-based SiO₂ composites have been developed to form nanocatalysts. Kim et al. employed a short-time sonochemical method for the synthesis of PdO-doped silica nanocomposites,¹⁹⁰ in which PdO nanoparticles were deposited on the silica surface without aggregation. Compared with unsupported PdO nanoparticles, the PdO-doped composites had higher catalytic activity and selectivity for the aerobic alcohol oxidation reaction due to the synergetic effect of PdO with the silica support. In the presence of a basic agent, Salavati-Niasari et al. fabricated Nd₂O₃–SiO₂ nanocomposites using ultrasonic waves for decomposition.¹⁹¹ By adjusting the mole ratio of Si and Nd, type of basic agent and ultrasonic power, Nd₂O₃–SiO₂ nanocomposites were optimized in terms of shape, size and photocatalytic activity. Under ultraviolet illumination, the decomposition efficiency of the as-formed nanocatalyst toward methyl violet contaminant was remarkably increased, suggesting the introduction of SiO₂ to Nd₂O₃ had an important effect on the catalytic performance of Nd₂O₃.

Also, other metal compound/inorganic materials have been developed for on-demand applications. Akbari et al. reported the decoration of Se nanostructure on the surface of Ga₂O₃ nanosheets to facilitate the visible light responsivity of Ga₂O₃, which was similar to the proposal that they modulated the surface properties of Ga₂O₃ nanosheets with Ag nanostructure.¹⁶⁰ Chen et al. fabricated CuFe₂O₄/g-C₃N₄ composite nanostructures by a two-step sonochemical route at a frequency of 20 kHz and power of 70 W,¹⁹² where sphere-like CuFe₂O₄ nanoparticles were synthesized using sonochemical and hydrothermal techniques in basic medium and then fixed on graphitic carbon nitride (g-C₃N₄) nanosheets by a second ultrasonication producing network-like magnetic nanocomposites (Fig. 18-I). Such CuFe₂O₄/g-C₃N₄ exhibited better electrochemical activity towards the oxidation of xanthine to uric acid with higher anodic current, larger linear range and lower detection limit (Fig. 18-II).¹⁹² Nowee and coworkers utilized ultrasound-assisted precipitation to synthesize Cu(OH)₂/g-C₃N₄ photocatalysts for hydrogen production from water splitting under visible light.¹⁹³ The product demonstrated a higher performance (about 31 times) than pure C₃N₄ because the stable Cu(OH)₂ cocatalyst acted as electron traps and active sites. Certainly, ultrasound irradiation provided an intimate cocatalyst-semiconductor interfacial contact, higher surface area and better dispersion of cocatalysts, which facilitated the efficient transfer of electrons to cocatalyst and decreased the recombination of charge carriers. In the study of Lv et al.,¹⁹⁴ g-C₃N₄ loaded with MoS_x nanodots (3–20 nm) was successfully prepared by ultrasonic treatment. MoS_x/g-C₃N₄ had better photocatalytic H₂ evolution performance than Pt/g-C₃N₄, but its activity decreased with time probably ascribed to the photocorrosion of MoS_x. To overcome this, Ni was deposited on the surface of MoS_x nanodots by in situ photo-deposition, producing more stable Ni–MoS_x/g-C₃N₄ cocatalysts under visible light. The resultant composites inhibited the recombination of electron–hole pairs excited in g-C₃N₄ and provided more active sites for hydrogen evolution, causing a much better photocatalytic performance.

Fig. 18 (I) TEM analysis of CuFe₂O₄/g-C₃N₄ ((A) scale bar = 0.5 μm; (B) scale bar = 100 nm), and electrochemical oxidation mechanism of xanthine based on CuFe₂O₄/g-C₃N₄/SPCE (C). (II) (A) Electrochemical oxidation of xanthine using (a) bare SPCE, (b) g-C₃N₄/SPCE and (c) CuFe₂O₄/g-C₃N₄/SPCE; (B) electrochemical detection of xanthine with different concentrations (10–100 μM); (C) anodic current vs. different concentrations. Reproduced with permission from ref. 192. Copyright 2020 Elsevier.

7.5. Other composites

There are numerous and varied inorganic composites that do not belong to the defining categories in the above description. For instance, Badhulika et al. prepared red-black phosphorus from low-cost red phosphorus via a simple scalable sonication process,¹⁹⁵ and then combined the hybrid phosphorus with sulfonated porous carbon in an ultrasound-assisted carbonization process, yielding red-black phosphorus/sulfonated porous carbon composites. The synergistic combination of red-black phosphorus and sulfonated porous carbon provided shorter pathways for the ion diffusion and rapid electron transfer, and the composites exhibited a high structural stability attributed to the carbon network with phosphorus bonding and heteroatom doping. Hence the red-black phosphorus/sulfonated porous carbon was able to serve as a high-performance supercapacitor electrode to deliver superior capacitive properties towards energy storage devices. To improve the efficiency of dye-sensitized solar cells, Salavati-Niasari et al. prepared two kinds of GQDs/Sb₂S₃/TiO₂ ternary hybrids using an ultrasonic probe with 60 W cm⁻² intensity and 18 kHz frequency.¹⁹⁶ When analyzing the electronic structure of GQDs/Sb₂S₃/TiO₂, the nanomaterials with Sb₂S₃ inserted between GQDs and TiO₂ were more stable than those with Sb₂S₃ placed on GQDs/TiO₂. The band gap of GQDs/Sb₂S₃/TiO₂ was less than that of TiO₂ and GQDs/TiO₂ so that the electronic transition from the valence band to the conduction band was easier and the photocatalytic performance was stronger. In the reports of Mortazavi-Derazkola et al. and Orooji et al.,^197,198 sonochemical synthesis produced nanocomposite-based platforms for drug delivery and photocatalytic degradation, respectively. Wherein, mesoporous Fe₃O₄@SiO₂–hydroxyapatite showed low toxicity, high drug loading, slow release and biodegradation,¹⁹⁷ while Fe₃O₄/SiO₂/ZnO–Pr₆O₁₁ nanocomposites displayed remarkably enhanced photocatalytic activity towards rhodamine B and Congo red degradation under UV irradiation.¹⁹⁸

Hybrid structures simultaneously consisting of metal, metal compounds and nonmetal compounds have been emerging in large numbers in the field of sonochemical synthesis. Kipp et al. developed an ultrasound-assisted technique to create in situ gold nanocrystals on the surface of silica-coated CdSe-dot/CdS-rod core/shell nanoparticles (Fig. 19-I),¹⁹⁹ where polyethylene glycol acted as the solvent and reducing agent for Au³⁺ ions. Due to the high viscosity of polyethylene glycol, the mobility of gold precursors was efficiently decreased to prevent the penetration of Au³⁺ into the silica shell whose surface promoted the heterogeneous nucleation of Au nanocrystals. Besides, the collective effect from polyethylene glycol and ultrasound irradiation ensured the formation of Au nanocrystals with a relatively narrow size distribution ranging from 2 to 7 nm (Fig. 19-II).¹⁹⁹ With the aid of the hybrid structures, the strong plasmon-exciton interaction between the CdSe-dot/CdS-rod and Au nanocrystals decreased the photoluminescence decay rate and suppressed fluorescence blinking behavior. Wang et al. reported the fabrication of magnetic-based flower-like silver microflowers through a sonochemistry-assisted strategy,²⁰⁰ where the product was comprised of a 200 nm Fe₃O₄ core, a 50 nm SiO₂ inner shell and a highly branched Ag petal outer shell (Fig. 19-III). The size and morphology of Ag petals on the Fe₃O₄@SiO₂@Ag microflowers could be well controlled and tuned by varying the concentrations of Ag precursor. Owing to the large effective surface area of the branched Ag petals, Fe₃O₄@SiO₂@Ag microflowers were capable of forming sufficient plasmonic hotspots and capturing target molecules. The Fe₃O₄ core with good magnetic responsiveness allowed enrichment of the targeted analytes and creation of abundant interparticle hotspots via magnetism-induced aggregation, and made the versatile microflowers easily recycled or reproducible as a surface-enhanced Raman scattering substrate. As for non-adsorbed R6G molecules or adsorbed pesticide thiram, Fe₃O₄@SiO₂@Ag had corresponding detection limits as low as 1 × 10⁻¹⁴ M or 1 × 10⁻¹¹ M (Fig. 19-IV),²⁰⁰ and it could easily capture S. aureus in tap water when functionalized by aptamers, and significantly enhanced the surface-enhanced Raman scattering signal. Fe₃O₄/SiO₂/Pr, graphene–Ce–TiO₂ and graphene–Fe–TiO₂ nanocomposites also have been fabricated in the presence of ultrasound irradiation, with the aim of being employed for the photocatalytic degradation of organic dyes.^201,202

Fig. 19 (I) (a) Evolution of the absorption spectra of CdSe-dot/CdS-rod/SiO₂-HAuCl₄-PEG mixture under ultrasound irradiation as a function of the reaction time, and (b) TEM images of the samples after different reaction times, where the frame color of each image (50 nm × 50 nm in scale) corresponds to that of the absorbance plot as displayed in (a). (II) TEM image ((a) scale bar = 100 nm) of an aggregation of CdSe-dot/CdS-rod/SiO₂-Au hybrid nanoparticles after 4 h reaction time, (b) corresponding Au particle size distribution histogram, and TEM images ((c–e) scale bars = 20 nm) of individual CdSe-dot/CdS-rod/SiO₂-Au hybrid nanoparticles. Reproduced with permission from ref. 199. Copyright 2019 American Chemical Society. (III) Schematic of the synthetic process of Fe₃O₄@SiO₂@Ag microflowers (a), and TEM images (scale bars = 100 nm) of (b) Fe₃O₄, (c) Fe₃O₄@SiO₂, (d) Fe₃O₄@SiO₂-Ag seed and (e) Fe₃O₄@SiO₂@Ag microflowers. (IV) Surface-enhanced Raman scattering spectra of (a) R6G and (b) thiram at different concentrations on the Fe₃O₄@SiO₂@Ag microflowers, where the inset shows a magnified view of spectrum E in (a). Reproduced with permission from ref. 200. Copyright 2016 Royal Society of Chemistry.

Furthermore, rare earth ions often participate in the sonochemical synthesis of inorganic composites. Muniswamy et al. reported excellent assembly of BaTiO₃:Eu³⁺@SiO₂ superstructures using CTAB as a surfactant.²⁰³ The nanocomposites had a broomlike structure because CTAB kinetically controlled the growth rates of various faces by interacting with the faces through adsorption and desorption when chemically adsorbed onto the surface of nucleated BaTiO₃:Eu³⁺ nanoparticles. Thus, the CTAB concentration was an influential parameter to set in favor of nucleation and growth of crystals. Certainly, the pH of reactants and ultrasound irradiation time were also involved in the growth mechanism of the broomlike architectures leading to core–shell entities. Compared to commercially available toxic dusting powders, dormant fingerprints could be visualized by BaTiO₃:Eu³⁺@SiO₂ (5 mol%) as an efficient phosphor exhibiting high efficiency and high sensitivity. In the study of Kokila et al., SiO₂-coated SrTiO₃:Dy³⁺ (5 mol%) nanopowders were obtained on the basis of a low-temperature sonochemical route in the presence of bio-surfactant aloe vera gel extract. As compared to uncoated SrTiO₃:Dy³⁺ (5 mol%),²⁰⁴ SiO₂@SrTiO₃:Dy³⁺ exhibited an increasing enhancement in the photoluminescence intensity with the numbers of SiO₂ coating layers probably owing to the passivation of surface states and the reduced surface defects of SrTiO₃:Dy³⁺. As a labelling agent for the visualization of latent fingerprints and dosimetry, SiO₂@SrTiO₃:Dy³⁺ exhibited high clarity, high sensitivity, low fading (20%), and good reproducibility (up to four cycles).

8. Future outlook of sonochemical catalysis in synthesis

Sonochemical catalysis has been developed for the synthesis of numerous materials owing to the simplicity and effectiveness of ultrasound irradiation. Ultrasound irradiation is also a useful tool to catalyze the reactions among other synthesis methods, where the shear force and intense shock waves allow a faster mass transfer to cause a better contact of reactants. Furthermore, ultrasonic treatment is usually able to increase the catalytic activity by changing the crystallinity of product materials, giving catalytic enhancement for reactions such as reduction, oxidation, degradation, polymerization, and so on.²⁰⁵

In the future, there are still some probable challenges to overcome in both the design of ultrasound-assisted approaches and the controllability of product properties. For instance, the indeterminacy of sonochemical catalysis may bring two-sidedness to the synthesis platforms as a result of the deficient mechanistic and experimental understanding of the cavitation phenomenon; sonochemical catalysis should have a chance to assemble pre-defined materials for purpose-specific or site-specific modification; the synergy of ultrasound irradiation with other synthesis methods or technologies should be highly demanding to control morphologies, structures, sizes and compositions; the resultant nano/micro devices still have to emphasize the activity, stability, durability, selectivity, low cost, long shelf life, or other requirements; and regarding the synthesis of inorganic materials via sonochemical catalysis, further studies also need to involve more mechanisms and product varieties for in-depth development. Certainly, sonochemical catalysis at the nano- and microscale levels will be explored according to on-site conditions or stimuli.

9. Conclusion

Ultrasound can act as a nonclassical mechanical method to directly or indirectly induce or strengthen some chemical syntheses. In this review, the basic principles of acoustic cavitation have been overviewed as well as the chemical effects and physical and/or mechanical effects from acoustic cavitation, which are responsible for sonochemical catalysis. According to the latest developments, sonochemical catalysis has been described with emphasis on the synthesis of nano/micro inorganic structures including metals, metal compounds, non-metal materials, and inorganic composites, and many aspects are covered in detail with morphologies, structures, sizes, properties and applications of well-designed products. Also, the review points out the prospective challenges and trends in ultrasound-assisted synthesis, highlighting the significance of sonochemical catalysis. The ultrasound-assisted methods or sonochemical catalysis are versatile in material preparation or modification as a unique strategy, and will be an available future direction to broaden applications in catalysis, biomedicine or other fields.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to thank Natural Science Foundation of Shandong (2018GGX102030), Taishan Scholar Program of Shandong Province (ts201511027), China Postdoctoral Science Foundation (2017M622125), and Research Project of Postdoctoral Application in Qingdao. The authors thank Royal Society of Chemistry, Elsevier, American Chemical Society, Springer and Wiley-VCH for copyright permissions of reproduction, adaptation, and redrawing of figures.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0na00753f

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