Precursor chemistry of metal nitride nanocrystals

Abstract

Metal nitride nanocrystals are a versatile class of nanomaterials. Depending on their chemical composition, the optical properties vary from those of traditional semiconductor nanocrystals (called quantum dots) to more metallic character (featuring a plasmon resonance). However, the synthesis of colloidal metal nitride nanocrystals is challenging since the underlying precursor chemistry is much less developed compared to the chemistry of metal, metal chalcogenide or metal phosphide nanocrystals. Here, we review chemical approaches that lead (or could lead) to the formation of colloidally stable metal nitride nanocrystals. By systematically comparing different synthetic approaches, we uncover trends and gain insight into the chemistry of these challenging materials. We also discuss and critically evaluate the plausibility of certain suggested mechanisms. This review is meant as a guide for the further development of colloidal nitride nanocrystals.

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Mahsa Parvizian

Mahsa Parvizian completed her Bachelor and Master in chemistry at the University of Paris. In 2020 she started her PhD under the supervision of Prof. De Roo at the University of Basel. Her research focuses on the synthesis of metal nitride nanocrystals and their reaction mechanism.

Jonathan De Roo

Dr Jonathan De Roo studied chemistry at Ghent University and obtained his PhD on the surface chemistry of metal oxide nanocrystals in 2016. During his PhD, he was also three months at ETH Zurich to study the surface of CsPbBr3 nanocrystals. He completed a postdoc at Columbia University, working on crystallization mechanisms and the design of new ligands. In 2019, he joined the University of Basel as Tenure Track Assistant Professor. His research team focuses on mechanistic aspects in the synthesis of ceramic nanocrystals and atomically precise oxo-clusters. Both precursor conversion and crystallization mechanism are studied.

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1. Introduction

Various nanomaterials show size-dependent properties, e.g., magnetic, electrical, and optical properties that cannot be achieved in the bulk material. Therefore, the synthesis of uniform nanocrystals (NC) with controlled size and shape has been attracting a lot of attention.¹ While atomic precision is still more an aspiration than a reality in nanocrystal science,² it is imperative to produce colloids with narrow size distributions (low polydispersity) to allow the study of size dependent properties.³ One of the most well-known phenomena is the tunable luminescence of quantum dots (QDs), covering the entire UV-VIS-NIR spectrum depending on the QD size and chemical composition.⁴

Ligand-assisted, wet chemical synthesis has provided the necessary control over nanocrystal size and shape.⁵ However, it is a complex process, combining metal–organic chemistry, inorganic crystallization and surface chemistry to yield an organic/inorganic hybrid object that is colloidally stable. The precursor conversion mechanism of many metals, metal chalcogenide, metal phosphide and metal halide NCs has been elucidated.^6–9 and also their surface chemistry is increasingly better understood.^10–16 The crystallization mechanism (i.e., nucleation and growth) causes more discussion in the literature.^17,18 Approaches based on classical nucleation theory are at odds with the Finke–Watzky based mechanisms.^19,20 Recent evidence shows that many NCs do not nucleate via a burst nucleation event, but rather that nucleation is a continuous process.^21–23 Continuous nucleation does not necessarily lead to polydisperse ensembles because small NCs grow faster than larger NCs, allowing the newly formed NCs to catch up with the growing NC population.^22–25 Even without a comprehensive framework for NC crystallization, chemists have been able to adjust the final NCs size by controlling the precursor conversion rate.²⁶ The three aspects of colloidal synthesis (precursor conversion, crystallization and surface chemistry) are interdependent. An additional, often neglected parameter is the solvent. During high temperature nanocrystal synthesis, certain popular solvents decompose or polymerize, forming products that compromise the purity of the final NCs.^27,28 Moreover, the purity of the ligand can considerably influence the solubility of the starting precursors and provide a better capping of the final NCs.²⁹

While metal, metal chalcogenide and metal halide NCs are well studied, the metal pnictides have received less attention, with the exception of metal phosphides.^9,30–33 Metal nitrides in particular are a versatile class of nanomaterials but their synthesis is not well developed. One difficulty is the higher thermodynamic stability of the oxides, compared to the nitrides (see Table 1). One can rationalize the lower standard formation free energy for nitrides because of the high bond enthalpy of dinitrogen (941 kJ mol⁻¹) versus dioxygen (500 kJ mol⁻¹).^34,35 Another factor is the lower ionic character of metal nitrides since the enthalpy of formation of N³⁻ is much higher than for O²⁻ (2300 kJ mol⁻¹versus 700 kJ mol⁻¹).³⁶ Only AlN and alkali(ne earth) metal nitrides are regarded as saline nitrides and they react quickly with water, releasing ammonia.³⁷ On the other hand, boron and silicon nitride are almost purely covalent compounds with high stability.³⁷ Transition metals like titanium form interstitial nitrides with extremely high chemical, thermal and mechanical stability.³⁷ TiN has the fcc structure of metallic titanium with interstitial nitrogen atoms in the octahedral holes (Fig. 1). Given the above considerations, it is no surprise that metal nitride synthesis is generally carried out at a high temperature and with nitrogen precursors that are more reactive than molecular nitrogen.

Fig. 1 (a) Crystal structure of group 4 metal nitride. The metal is represented in purple and the nitrogen atom in grey. Structures plotted using VESTA.⁴⁸ Cif file obtained from ref. 49–51 (b) Absorption spectra of aqueous dispersions of TiN, ZrN, and HfN NCs. The inset shows a photograph of the NCs dispersed in water. Reprinted from ref. 40 with permission from John Wiley and Sons, Copyright 2019.

Table 1 Standard free energies of formation (kJ mol⁻¹) for selected oxides and nitrides at room temperature³⁸

Oxides		Nitrides
TiO2	−891.2	TiN	−309.2
ZrO2	−1042	ZrN	−336.8
MnO2	−464.4	Mn3N2	−146.4
Fe2O3	−744.8	Fe2N	+12.6
CoO	−213.8	Co3N	+8.4
CuO	−128.4	Cu3N	+74.5
ZnO2	−318.4	Zn3N2	−20.9

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Here, we review the existing synthetic strategies that lead to metal nitride NCs, with a focus on the chemical conversion reactions. A broader review on nitride nanomaterials, including porous and 2D materials can be found elsewhere.³⁹ We target predominantly wet chemical syntheses and colloidally stable nanocrystal products. Other methods such as gas phase or solid state synthesis are only included when informative. We do not discuss nitride NCs of the alkali metals or alkaline earth metals since there are no synthetic procedures for nanocrystals available and they are extremely sensitive towards humidity, rendering them less technologically interesting. We will go systematically through the periodic system and discuss the nitrides per group, starting with group 4 and ending with group 13.

2. Group 4 metal nitrides

Group 4 transition metal nitrides have a high melting point, a high chemical stability and excellent corrosion resistance. TiN, ZrN and HfN have a cubic crystal structure (Fm3̄m space group) with a lattice constant of 4.239 Å, 4.585 Å, and 4.510 Å respectively, see Fig. 1. Their NCs exhibit a localized surface plasmon resonance (LSPR) in the visible region (HfN and ZrN) or in the near infrared regime (TiN), see Fig. 1.⁴⁰ The plasmonic response translates in a highly efficient conversion of sunlight into heat, an effect that has been used for water evaporation and desalination.^41–44 The position of the LSPR of TiN in the infrared region makes it suitable for photothermal therapies or plasmon-induced photocatalysis.⁴⁵ Given the optical properties of the group 4 metal nitrides, they are an attractive alternative for gold NCs. Furthermore, they exhibit interesting catalytic properties. ZrN NCs possess excellent activity and long-term stability (superior to platinum) as catalysts for the electrochemical oxygen reduction reaction (ORR).⁴⁶ Also HfN NPs are catalytically active for the oxygen evolution reaction (OER).⁴⁷

In 1988, TiN films were prepared using atomic layer epitaxy at 500 °C; TiCl₄ precursor was pulsed into a reactor with gaseous ammonia.⁵² The colour depends on the stoichiometry of the film. For 40% N a golden yellow color is observed while for 50% a dark violet is obtained. Alternatively, Ti(NMe₂)₄ is reacted with ammonia. Similar strategies apply to ZrN and HfN. An extensive account of customized precursors for Chemical Vapour Deposition (CVD) of the group 4 nitrides is reviewed elsewhere.⁵³

2.1. Non-thermal plasma synthesis

In 2017, powders of TiN were synthesized in a non-thermal plasma reactor (Fig. 2) from gaseous TiCl₄ and ammonia.⁵⁴

6TiCl₄ + 8NH₃ → 6TiN + N₂ + 24HCl (1)

Fig. 2 Diagram of the nonthermal plasma apparatus for TiN NCs synthesis. Reprinted with permission from ref. 55. Copyright 2018 American Chemical Society.

The particle size is tuned from 3 to 8 nm by decreasing the flow rate of ammonia. The smaller particles are more oxidized than the bigger particles, but all show a plasmon resonance between 800 and 1000 nm. One can obtain ZrN nanocrystals in a similar fashion using ZrCl₄ and NH₃ as the precursors.⁵⁶ After synthesis, the powders need to be heated at 200 °C in an Ar atmosphere to remove the ammonium salt by-products. To avoid this last step, TiCl₄ was later replaced with tetrakis(dimethylamido) titanium (Ti(NMe₂)₄).⁵⁵

6Ti(NMe₂)₄ + 8NH₃ → 6TiN + N₂ + 24HNMe₂ (2)

Also here, the particles are 6–8 nm in size (Fig. 3) and have a well-defined plasmon resonance (around 800 nm). However, the particles are not monocrystalline (consisting of 2–3 nm crystallites) and contain a considerable amount of carbon.

Fig. 3 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the obtained TiN NCs via non-thermal plasma synthesis. Scale bars are (a) 20 nm and (b) 5 nm. Reprinted with permission from ref. 55. Copyright 2018 American Chemical Society.

2.2. Solution chemistry of titanium amido and imido complexes

In solution, TiCl₄ reacts with tert-butylamine to the mixed chloro amido complex 1, see Fig. 4.⁵⁷1 has been used as a single source precursor for the CVD deposition of TiN. When 1 is reacted with triphenylphosphine oxide, 2 is formed (Fig. 4), which does not convert to TiN during CVD. Complex 1 is unstable at room temperature and slowly looses tert-butylamine, probably forming an imido species. This is supported by the formation of the monomeric tert-butylimido complex 3 by the addition of tert-butylpyridine as neutral Lewis base (Fig. 4).⁵⁸

Fig. 4 Reactions of titanium chloride with tertbutylamine and thiphenylphosphine oxide or tertbutylpyridine.

When Ti(NMe₂)₄ is treated with primary amines such as n-propylamine, n-octylamine or tert-butylamine, transamination occurs.^59–61 Subsequently, the primary amido group self-condenses and thus forms alkylimido bridges, see Fig. 5. The rate of transamination and condensation is clearly dependent on the steric hindrance between the alkyl groups of the amine since it proceeds swiftly at room temperature for n-propylamine while tert-butylamine only forms the condensed product at 100 °C.⁶¹ When using an excess of primary amine, a polymeric, insoluble product is obtained.⁶⁰ However, sols can be formed in tetrahydrofurane (THF) by controlling the amine to titanium stoichiometry. These sols were then used to coat silica via dip-coating and Ti(C,N) and TiN were obtained upon pyrolysis at high temperature (minimum 800 °C) under nitrogen or ammonia atmosphere respectively.⁶²

Fig. 5 Transamination followed by self-condensation of primary metal amide group.

Upon addition of one equivalent of primary amine to Ti(NMe₂)₄, the dimer 4 is formed, which can rearrange to a monomeric imido species by coordination of a Lewis base. Addition of two equivalents of trimethylsilylchloride to the latter complex yields the dichloroimido complex.^61,63 The same product can be made by the addition of a primary amine to Ti(NMe₂)₂Cl₂.⁶⁴ Although most of the above titanium complexes have not been used so far to synthesize TiN NCs, they serve as valuable inspiration for future synthetic development and grant insight in the precursors chemistry of titanium and the rest of the group 4.

Recently, titanium chloride was reacted with potassium amide in pyridine at 300 °C.⁶⁵ While the TEM image showed an isolated TiN nanocrystal of 5 nm, the XRD pattern was too broad to be consistent with a large ensemble of 5 nm crystals. Indeed, another TEM image clearly shows aggregated particles in an amorphous matrix.

2.3. Synthesis of group 4 nitride powders from the group 4 oxides

TiN NCs were obtained by Fischer et al. by using a mesoporous graphitic carbon nitride (mpg-C₃N₄) as both the confinement and nitrogen source.⁶⁶ In this strategy, amorphous TiO₂ (confined in the pores) is first prepared via a sol–gel procedure starting from TiCl₄ in ethanol. In a second step, this composite product is heated to 800 °C under inert atmosphere or in sealed quartz ampoules. The confined metal oxide is transformed into pure TiN while the mpg-C₃N₄ is decomposed, see Fig. 6. The size of the final TiN NCs can be slightly adjusted (5.6 nm to 7.1 nm) by controlling either the pores of the starting mpg-C₃N₄ or changing the concentration of the initial TiCl₄ precursor (filling rate). However, the resulting NCs are strongly aggregated.⁶⁶

Fig. 6 Temperature-dependent powder XRD analysis of the TiO₂ precursor/mpg-C₃N₄ composites. (+) represents mpg-C₃N₄; (o), TiO₂; and (*), TiN. Reprinted from ref. 66 with permission from John Wiley and Sons, Copyright 2007.

In 2007, Buha et al. investigated the thermal transformation of pre-synthesized anatase TiO₂ NCs into nanocrystalline TiN by using cyanamide or urea as a nitrogen source at 800 °C under nitrogen atmosphere.⁶⁷ Complete transformation is achieved when starting from 5 nm titania while with 10 and 20 nm titania, anatase impurities are still present. This reflects the solid state nature of the approach. When using a high excess of cyanamide or urea, a large amount of amorphous carbon is also produced (20–25 wt%). This can be minimized to 2 wt% by reducing the cyanamide or urea amount. The resulting TiN NCs have an average crystallite size of 3 nm and are aggregated to 50 nm agglomerates, embedded in amorphous carbon. Similarly, ZrN nanocrystals were synthesized by reacting 10–15 nm ZrO₂ nanocrystals with urea at 1100 °C under an ammonia flow.⁶⁸ In 2009, Antonietti and coworkers presented the “urea glass route” which is a combination of sol–gel chemistry and the above strategy of Buha et al.⁶⁹ First, TiCl₄ is dissolved in ethanol, forming titanium ethoxide and releasing HCl as side product. Then a varying amount of solid urea is added to the solution and stirred until complete dissolution and the solvent is allowed to evaporate. The metal center is mostly coordinated to urea via the carbonyl oxygen as shown by FTIR data. The gel was then heated under N₂ flow at 800 °C for 3 hours at a very slow rate of 3 °C min⁻¹. A mechanism has been proposed based on thermal gravimetric analysis (TGA) and powder X-ray diffraction (XRD analysis. First, a glassy intermediate with nanoscale anatase and rutile titania is formed at 400 °C. Second, at 600 °C, loss of oxygen and metal reduction/nitridation is observed. Higher temperatures (800 °C) are required to obtain a complete recrystallization, see Fig. 7. A key parameter R has been defined as the ratio of urea and titanium. For R = 3, a mixture of TiN and TiO₂ is obtained. Pure TiN is retrieved for R = 4, while Ti(C,N) was formed for R = 10. The higher R, the higher the carbon contamination so the optimal ratio for TiN is four. The authors expanded the procedure to many other metal nitrides (VN, NbN, GaN, CrN, Mo₂N), but each metal has another optimal value for R.⁶⁹ Later, the method was also applied to produce Sn₃N₄,⁷⁰ HfN,⁴⁷ and ZrN,⁴⁶ as well as other nanocomposites such as graphene g-C₃N₄ doped with TiN.^71,72 The nitride NCs obtained via this process are typically aggregated powders. Finally, other nitrogen sources were explored such as guanidine carbonate and barbituric acid. However, both features a particularly low solubility and it was not possible to achieve R > 1.⁶⁹

Fig. 7 XRD pattern of titanium–urea gels (R = 10) treated at different temperatures. The * symbol refers to crystalline TiO₂. Reprinted with permission from ref. 69. Copyright 2009 American Chemical Society.

In 2019, Karaballi and coworkers also synthesized TiN, ZrN and HfN NCs from their parent oxide NPs.⁴⁰ Their strategy consists of a high temperature (1000 °C) solid-state metathesis reaction of magnesium nitride with commercial TiO₂ (17 nm), ZrO₂ (19 nm), and HfO₂ (43 nm).

The final product is treated with 1 M HCl to remove MgO and unreacted Mg₃N₂, and could be dispersed in water.⁴⁰ All compounds have the expected cubic crystal structure and the nanocrystal size is 12, 15 and 11 nm for TiN, ZrN and HfN respectively. The NCs seem aggregated on transmission electron microscopy (TEM) but it is unclear whether this occurred during the sample preparation or already in solution. The reaction could be easily scaled to 1 g of metal nitride. Efforts to significantly increase the particle size by using larger oxide NCs were unsuccessful, indicating new nucleation events of the nitride, rather than a simply anion exchange reaction. The localized surface plasmon resonance of the NCs is blue-shifted and more narrow in the series: TiN, ZrN and HfN (Fig. 1). Finally, all particles were slightly oxidized at the surface and this effect was worse for ZrN and HfN, compared to TiN. This scalable strategy of Karaballi et al. appears to produce high quality group 4 nitrides. While they are highly crystalline, their size dispersion could be improved. The method has already been adopted by other groups to produce, e.g., plasmonic HfN nanocrystals.⁷³

In 2017, Kan et al. explored molten salts (here: a eutectic mixture of MgCl₂ and NaCl) as a reaction medium for the synthesis of TiN from TiO₂ and Mg powder under N₂ atmosphere. The use of a higher Mg equivalent versus TiO₂ (up to 4 equivalent), higher heating temperature (up to 1100 °C), or longer reaction time (up to 7 h) leads to phase pure TiN with a better crystallinity as judged by the well-defined diffraction peaks obtained by XRD. The authors obtained 5–30 nm aggregated TiN NPs by using 2.5 equivalent of Mg heated at 1100 °C for 3 h. The authors described the mechanism as being a dissolution–precipitation type based on the increase of the surface area from the starting TiO₂ (9.83 m² g⁻¹) to the obtained TiN (93.09 m² g⁻¹).⁷⁴

3. Group 5 metal nitrides

Group 5 metal nitrides (VN, NbN and TaN) have the same crystal structure as group 4 metal nitrides; a cubic crystal structure with a Fm3̄m space group with a lattice constant of 4.13 Å,⁷⁵ 4.394 Å,⁴⁹ and 4.331 Å (ref. 76) respectively. These group 5 nitrides exist as well in other oxidation states with less common crystal structures (orthorhombic, tetragonal and hexagonal), e.g., Ta₃N₅, Ta₄N₅ or Ta₅N₆. Vanadium nitride has attracted interest as an active material for supercapacitors (SC) due to its high specific capacitance as well as high electrical conductivity.⁷⁷ Niobium nitrides (Nb₂N, Nb₄N₃, NbN, etc.) are mainly investigated as superconductors. Furthermore, nitrogen-rich niobium nitrides are good candidates for electrochemical capacitors.^78,79

VN and NbN nanopowders were synthesized via the urea glass route (see above).⁶⁹ Agglomerated VN NCs with a crystallite size of 6 nm have been synthesized by Choi et al. via a two-step method. First, ammonolysis of VCl₄ forms amorphous V(NH₂)₃Cl. Second, the amorphous precursor is crystallized at 400 °C in the presence of NH₄Cl under an ammonia atmosphere.⁸⁰ This method is one of the main chemical methods to form VN NCs,⁸¹ as well as NbN NCs.⁸²

In 1972, Ta₃N₅ powder and thin films were obtained at 1100 K starting from ammonia and Ta₂O₅ powder or Ta thin films respectively.⁸³ This strategy was recently pursued to prepare Ta₃N₅ nanotubes.⁸⁴ In 1995, TaN films were made using the CVD method with tert-butylimidotris(diethylamido) tantalum as precursor at temperatures ranging from 450 °C to 650 °C.⁸⁵ Nanocrystalline TaN with a cubic phase has been synthesized via a solid combustion synthesis using K₂TaF₇, NaN₃, and NH₄F precursors under high temperatures up to 850 to 950 °C.⁸⁶

In 2011, Ho et al. have reported three methods for the synthesis of colloidal Ta₃N₅ NCs.⁸⁷ In their first method TaCl₅ is dispersed in octadecene (ODE). At 310 °C, tris(trimethylsilyl)amine ((TMS)₃N, dissolved in trioctylphosphine) is injected and the particles are left to crystallize for 20 min at 300 °C.

The resulting NCs (23 nm, see Fig. 8A) could not be redispersed in any solvent after precipitation due to a lack of strongly binding ligands. Smaller (10 nm) NCs were formed at a reaction temperature of 200 °C. The authors report that the synthesis is sensitive to the purity of TaCl₅; at least 99.99% purity is required, and even then the yield was very low (which was attributed to the low solubility of TaCl₅ in ODE). In their second method, tantalum chloride was dissolved in a mixture of oleylamine (OLAm) and trioctylphosphine oxide (TOPO). Li₃N was added at 300 °C.

Fig. 8 TEM images of Ta₃N₅ obtained by Ho et al.⁸⁷ (a) NPs obtained with TaCl₅ and (TMS)₃N (scale 20 nm), (b) with Ta(NMe₂)₅ and (TMS)₃N (scale 20 nm) (c) with TaCl₅ and Li₃N (scale 10 nm). (d) Absorption spectrum of the obtained 10 nm Ta₃N₅. The inset showing the two limiting regimes. Adapted with permission from ref. 87. Copyright 2010 American Chemical Society.

The resulting NCs (2–8 nm after 20 min reaction, see Fig. 8B) could be redispersed in hexane after precipitation with methanol. When the reaction is left to proceed for 1 hour, 10 nm NCs are obtained. Their absorption spectrum has two absorption onsets (467 nm and 515 nm) which is explained by the absorption of Ta₃N₅ (515 nm) and that of a higher bandgap material, likely an oxidized shell. XPS data indeed confirm the presence of oxidic tantalum. To counter the low yields in the previous methods, the authors devised a third method. Pentakis(dimethylamido)tantalum and (TMS)₃N were dissolved in ODE and injected in pure ODE at 300 °C (Fig. 8C). However, the formed particles were amorphous and highly sensitive to air.⁸⁷ For all three methods, the crystallinity of the resulting particles was solely assessed by TEM and an ensemble measurement (e.g., XRD) is missing.

In 2013, a sol–gel pathway was demonstrated with Ta(NMe₂)₅ and n-propylamine.⁸⁸ Transamination occurs readily but the condensation appears to be slower than for titanium (see section on titanium nitride for details on transamination and condensation). Upon solvent evaporation a gel was formed. Solvent removal by vacuum resulted in a xerogel that converted in an ammonia atmosphere at 800 °C into Ta₃N₅ powder with a crystal size of 20 nm. Thin films were obtained by coating the a silica slides with the sol. Firing at 600 °C leads to TaN while heating to 800 °C yields Ta₃N₅.⁸⁸

It is thus clear from the above that nanocrystals of the group 5 nitrides have not been convincingly obtained in colloidal form. While this is the first challenge for the future, the second challenge is to control the oxidation state and the polymorphism of these materials.

4. Nickel nitride

Nickel nitride Ni₃N has a hexagonal epsilon-Fe₃N-type structure with a P6(3)22 space group, see Fig. 9. The two lattice constants are 4.622 Å and 4.305 Å.⁸⁹ Nickel nitride is interesting for various applications such as lithium-ion batteries,⁹⁰ magnetic NPs,^91,92 catalysis and electrocatalysis,^93,94 including hydrogenation, hydrogen, and oxygen evolution reactions (OER).^95,96 The early chemical synthetic methods to obtain Ni₃N were based on the decomposition of nickel complexes at high temperatures and in presence of ammonia or hydrazine.^90,92

Fig. 9 Crystal structure of Ni₃N. The nickel atoms are represented in dark red and the nitrogen atoms in cyan and white. Structures plotted using VESTA.⁴⁸ Cif file obtained from ref. 89.

Recently a one-step synthesis of colloidally stable Ni₃N NCs has been reported by Shanker et al.⁹⁷ Nickel acetate is used as precursor and OLAm is chosen as ligand. Different nanocrystal sizes are obtained (5 to 80 nm) by varying the time and temperature in a narrow window, see Fig. 10. The NCs are reasonably monodisperse and colloidally stable. The authors detect ammonia during the reaction and thus infer that ammonia is the nitrogen source. However, it is unclear how ammonia is being generated since the most logical reaction would be the formation of oleylacetamide and NiO. Furthermore, the synthesis requires a specific reagent purity and supplier.

Fig. 10 TEM images of the different Ni₃N NCs obtained. (a) 5.6 ± 0.9 nm, (b) 10.6 ± 1.0 nm, (c) 36.6 ± 0.8 nm, (d) 56.4 ± 6.5 nm, and (e) 76.4 ± 3.8 nm. (f) High resolution transmission electron microscopy (HRTEM) imageof a Ni₃N nanocrystal with a size of 56.4 ± 6.5 nm. Reprinted with permission from ref. 97. Copyright 2021 American Chemical Society.

5. Copper nitride

Copper nitride, Cu₃N, has an anti-ReO₃ crystal structure with a Pm3m space group, and a lattice constant of 3.817 Å, see Fig. 11.^98,99 Copper nitride is a semiconductor with a reported band-gap of 1.7 eV which could be potentially interesting for solar cells.¹⁰⁰ Thin films of copper nitride have been used in optical storage devices,¹⁰¹ and resistive random access memory chips.¹⁰² Cu₃N NCs were applied as catalyst in the reduction of CO₂ to ethylene,¹⁰³ alkaline water electrolysis,¹⁰⁴ and the oxygen reduction reaction (ORR).¹⁰⁵ Cu₃N can be doped with other metals such as Pd to form Cu₃Pd_xN with 0.02 < x < 1 which shows metallic behaviour.^106–108 The crystal structure of the latter is similar to the Cu₃N structure, with a palladium atom in the center of the unit cell (with a larger lattice constant of 3.854 Å), see Fig. 11. Cu₃N forms at relatively low temperature but decomposes at higher temperature to metallic copper. Under inert atmosphere, copper nitride films decompose at 475 °C,¹⁰¹ and particles decompose at 300 °C.¹⁰⁹ When heated in air, Cu₃N decomposes to CuO.¹⁰⁹ We calculated an exciton Bohr radius of 3.12 nm for copper nitride based on the effective masses of electrons and holes, as reported by Birkett et al.¹¹⁰

Fig. 11 Crystal structure of Cu₃N and the doped Cu₃PdN. The copper is represented in dark red, the palladium in grey and the nitrogen atom in cyan. Structures plotted using VESTA.⁴⁸ Cif file obtained from ref. 111 and 112.

In 1939, Juza et al. synthesized Cu₃N powders by reacting CuF₂ with ammonia gas (aminolyis reaction).¹¹³ Phase pure copper nitride was obtained at reaction temperatures between 250 °C and 325 °C (6 hours reaction time).¹¹⁴ At higher temperatures, Cu(0) is formed. Alternatively, Zachweija and Jacobs synthesized Cu₃N by thermal decomposition of [Cu(NH₃)_x](NO₃).¹¹¹ In 2005, Choi et al. combined copper(II) chloride and sodium azide in superheated toluene or THF.¹⁰⁹ The intermediate copper azide decomposes to Cu₃N powder at 185 °C.

The reaction temperature is remarkably lower than previously reported for the similar synthesis of other metal nitrides such as TiN and GaN. Unfortunately, the reaction time is very long (about three days) and metal azides are thermally unstable and shock sensitive, which limits the reaction to small scales. Finally, the obtained Cu₃N is not phase-pure and some NaCl impurities remain present in the sample.

5.1. Cu₃N nanocrystal synthesis from copper nitrate and primary amine

In 2011, Wang et al. obtained colloidally stable Cu₃N NCs by reacting Cu(NO₃)₂·3H₂O in octadecylamine (ODA) at 220–280 °C.¹¹⁵ Octadecylamine acts here both as solvent and as ligand.

The authors hypothesize that NO₂ and O₂ are formed as side products but did not provide experimental evidence. This hypothesis was based on the solid state decomposition reaction of Cu(NO₃)₂·3H₂O into CuO, NO₂ and O₂ at around 180 °C.¹¹⁶ The size of the NCs is tuned by using different reaction temperatures (220–280 °C) and reaction times (5–20 min). The conditions and their outcomes are also summarized in Table 2. Pure Cu₃N NCs are only obtained with 32 equivalents of ODA. Cu₂O is obtained with 8 equivalents, while 80 equivalents lead to Cu(0) NCs. Forty and thirteen equivalents lead to a mixture of Cu/Cu₃N or Cu₃N/Cu₂O mixture respectively. Please note that the copper concentration also changes in these experiments since ODA is the solvent. Wang et al. argue that nitrate (NO₃⁻) is the nitrogen source for the nitride (N³⁻), even though this would involve the transfer of eight electrons to N(V) in nitrate. They based their argument on the observation that copper nitrate is the only precursor that delivers copper nitride. Copper chloride does not yield any products while copper acetate yields a Cu(0)/Cu₂O mixture and copper sulfate delivers copper sulfide NCs. However, no other hypotheses were tested or disproved, thereby leaving the option open that the nitride is provided by ODA, as is the case for certain syntheses of InN NCs (see below).

Table 2 Synthetic conditions for colloidal copper nitride NCs

Precursor^a	[Cu] (mM)	Solvent^b/ligand^c	Ligand/Cu ratio	Temperature (°C)	Time (min)	Morphology	Phase	Size (nm)	Ref.
a HMT = hexamethylenetetraamine, PPC = pyrrole-2-carbaldpropyliminato Cu(II). b ODE = 1-ODE. c ODA = octadecylamine, OLAm = oleylamine, HDA = hexadecylamine.
Cu(NO3)2·3H2O	101	ODA	32	240	10	Cubic	Cu3N	15	115
Cu(NO3)2·3H2O	101	ODA	32	280	5	Spherical	Cu3N	10	115
Cu(NO3)2·3H2O	101	ODA	32	260 + 240	5 + 5	Spherical	Cu3N	15	115
Cu(NO3)2·3H2O	101	ODA	32	220 + 240	5 + 5	Spherical	Cu3N	15	115
Cu(NO3)2·3H2O	101	ODA	32	220	20	Irregular	Cu3N	50	115
Cu(NO3)2·3H2O		ODA	40	240	10		Cu + Cu3N		115
Cu(NO3)2·3H2O		ODA	13	240	10		CuO + Cu3N		115
Cu(NO3)2·3H2O	24	ODE/ODA	67	250	30	Cubic	Cu3N	26	117
Cu(NO3)2·3H2O	24	ODE/HDA	69	250	30	Cubic	Cu3N	19	117
Cu(NO3)2·3H2O	24	ODE/OLAm	63	250	30	Cubic	Cu3N	11	117
Cu(NO3)2·3H2O	24	OLAm		250	30	Irregular	Cu3N		117
Cu(NO3)2·3H2O	25	ODE/OLAm	31	240	15	Cubic	Cu3N		118
Cu(NO3)2·3H2O + Pd(acac)2	25	ODE/OLAm	31	240	15	Cubic	Cu3PdN	16	118
Cu(NO3)2·3H2O + Pd(acac)2	25	ODE/OLAm	31	190	30	Cubic	Cu3PdN		119
Cu(NO3)2·3H2O	25	ODE/OLAm	31	210	30	Cubic	Cu3N		119
Cu(NO3)2·3H2O	75	ODA/OLAm	45	240	20	Cubic	Cu3N	25	105
Cu(NO3)2·3H2O	88	ODA/OLAm	41	240	30	Cubic	Cu3N	25	103
Cu(NO3)2·3H2O	88	ODA/OLAm	41	250	30	Cubic	Cu3N	20	103
Cu(NO3)2·3H2O	88	ODA/OLAm	41	260	30	Cubic	Cu3N	10	103
Cu(NO3)2·3H2O	88	ODA/OLAm	41	230	30	Spherical	Cu3N	20	103
Cu(NO3)2·5H2O + HMT	50	Hexanol	—	200	60	Spherical	Cu3N	80	122
Cu(acetate)2·H2O + NH3	10	Nonanol	—	190	60	Spherical	Cu3N	200	123
PPC	50	ODA	74	260	5	Spherical	Cu3N	3	124
Cu(NO3)2·3H2O	50	ODA	74	260	5	Cubic	Cu3N	19	124
Cu(OMe)2	92	BnNH2	117	140	15	Spherical	Cu3N	2	125
CuI + KNH2 + NH3	21	Pyridine	—	−35 + 130	10 + 30	Spherical	Cu3N	4	126

---

Also in 2011, Wu and co-workers described the formation of Cu₃N from Cu(NO₃)₂·3H₂O in the presence of both a primary amine (surfactant) and octadecene (ODE) (solvent), see eqn (11).¹¹⁷ By changing the primary amine (ODA, hexadecylamine or OLAm), the size of the final Cu₃N varies from 26 ± 5.6 nm to 18 ± 2.3 nm and 10 ± 2.7 nm respectively (Fig. 12). All NCs have a cubic morphology and are phase-pure copper nitride.

Fig. 12 TEM images and the corresponding size histograms of Cu₃N NCs synthesized from copper nitrate and various primary amines in ODE solvent, according to ref. 117. (a, b) Octadecylamine (c, d) hexadecylamine, and (e, f) oleylamine. Reprinted with permission from ref. 117. Copyright 2011 American Chemical Society. Note that the authors labelled the axis diameter. Given the cubic morphology of the nanocrystals, this is confusing and the cube edge length is a more appropriate measure for the size.

In a first stage, the reaction is heated to 150 °C with a concomitant color change from dark blue to green and then yellow, indicating the reduction of Cu(II) to Cu(I).¹¹⁵ Subsequently, the solution is heated to 250 °C for 30 min and the solution turns brown, consistent with the formation of copper nitride. According to Wu et al. ODE is necessary as solvent to obtain well-defined nanocubes since irregular and polydisperse Cu₃N NCs have been obtained using only OLAm. Most likely, other solvents would be more suitable since ODE polymerizes during nanocrystal synthesis.²⁷ Nonetheless, this method was adopted by other groups and Vaughn II et al. used it to produce Cu₃N and Cu₃PdN, albeit with half the number of amine equivalents, see also Table 2.¹¹⁸ The reaction mixture is degassed for 10 min at 120 °C and then heated to 240 °C under argon for 15 min. To produce Cu₃Pd_xN, Pd(acac)₂ is also added at the start. The average diameter of the quasicubic Cu₃PdN NCs is 16 ± 2 nm. Cu₃N and Cu₃PdN can also be grown on Pt and Au nanoparticle seeds to form nanoscale heterostructures at 190 °C see Fig. 13.¹¹⁹

Fig. 13 Time-dependent growth studies of Cu₃PdN on Pt nanocubes at 170 °C for (a–f) 5 min, (g–l) 10 min, and (m–r) 15 min. The data for each time point include (a, g, and m) TEM images, (b, h, and n) HAADF-STEM images, (c–e, i–k, and o–q) energy dispersive X-ray spectroscopy (STEM-EDS) element maps for Pt (red), Cu (blue), and Pd (green), and (f, l, and r) HRTEM images of individual particles. Reprinted with permission from ref. 119, Copyright 2019 American Chemical Society.

In 2019, Braman et al. synthesized Au decorated Cu₃N NCs. First, Cu₃N NCs (10 ± 5 nm) were obtained via a similar approach to the one of Vaughn et al. but with a higher amine equivalent and a lower heating temperature (210 °C). Subsequently, Au NPs were grown on top of the Cu₃N cubes. The resulting heterostructure demonstrated a better catalytic activity than pure Cu₃N.¹²⁰

In 2014, Xi et al. performed the reaction in a mixture of ODA and OLAm.¹⁰⁵ First the mixture is heated to 110 °C and than quickly heated to 240 °C for 10 min. They obtained uniform nanocubes of Cu₃N of around 20 nm. The growth mechanism was investigated by analyzing reaction aliquots at 2, 5 and 10 min. From TEM measurements (Fig. 14), Xi et al. conclude that first small NPs are formed, which subsequently ripen into bigger nanocubes. This growth mechanism was later confirmed under slightly different conditions.¹²¹ In addition, long reaction times (at 260 °C) lead to the decomposition of the formed Cu₃N to Cu(0) particles. Xi et al. also studied the magnetic properties of the Cu₃N NCs and observed paramagnetic and ferromagnetic contributions. Such properties are not expected for d⁰ materials and are assumed to originate from surface defects.¹⁰⁵

Fig. 14 (A) TEM of the Cu₃N NCs synthesized according to ref. 105, after 2, 5 and 10 min. (B) The proposed growth procedure of Cu₃N nanocubes. Reproduced from ref. 105 with permission from the Royal Society of Chemistry.

In 2019, Yin et al. used the same strategy of Xi et al. By tuning the heating temperature, the size and morphology of the final Cu₃N is changed, see Table 2.¹⁰³ Above 240 °C cubic NCs are formed while below 240 °C spherical NCs are formed. The size of the final Cu₃N decreases from 25 nm to 10 nm by increasing the reaction temperature from 240 °C to 260 °C. The Cu₃N NCs are stable even after 2 weeks of air exposure and no degradation is observed in the crystal structure and morphology.

5.2. Cu₃N nanocrystal synthesis from Cu(OMe)₂ and benzylamine

Deshmukh et al. reacted Cu(OMe)₂ in benzylamine (BnNH₂) and obtained very small, spherical Cu₃N NCs (2.2 ± 0.5 nm).^125,127

The standard synthesis is performed at 140 °C under inert atmosphere, in a pressure tube. Over the course of the reaction, the color changes from dark blue to red. The average crystal size (determined from XRD) is quite insensitive to the reaction conditions (80 °C–180 °C, 5–60 min) and varies from 1.8 to 2.8 nm. The authors further suggested a precursor conversion mechanism by analyzing the reaction by-products with gas chromatography–mass spectrometry (GC-MS), see Fig. 15. In the first step, copper(II) methoxide oxidizes BnNH₂ to benzenemethanimine and is reduced to Cu(I). This proposal is based on the detection of methanol in GC-MS. This reactivity is special for Cu(OMe)₂ since other Cu(II) salts such as Cu(II) chloride make stable complexes with amines and fail to produce Cu₃N. The benzenemethanimine intermediate reacts with benzylamine, yielding N-benzylidenebenzylamine (detected by GC-MS) and ammonia. The in situ formed ammonia reacts with Cu(OMe) to form Cu₃N. Traces of benzonitrile are also observed at the GC, which is justified by further oxidation of benzenemethanimine in presence of Cu(II). This synthetic strategy was adopted by Liu and co-workers, but also requires a specific reagent purity and supplier.¹²⁸

Fig. 15 Proposed reaction mechanism of Cu(OMe)₂ with BnNH₂ that leads to the formation of Cu₃N.¹²⁵ Molecules in red are the one detected by GC-MS.

5.3. Cu₃N nanocrystal synthesis via sol–gel chemistry

In aqueous sol–gel chemistry, dissolved metal precursors are hydrolyzed with water and further condense in metal oxide particles and gels.¹²⁹ Similarly, researchers have synthesized copper nitride NCs by aminolysis (reaction with ammonia) and condensation. In 2018, Egeberg et al. reacted CuI with ammonia under basic conditions to form Cu(NH)₂.¹²⁶

By condensation of the copper amide under reflux conditions, small copper nitride NCs are obtained (4.2 ± 0.7 nm), see Fig. 16. Only pyridine is observed at the surface of the NCs according to FT-IR and elemental analysis, which could explain why these NCs are air and moisture sensitive.

Fig. 16 TEM images of Cu₃N NCs synthesized by sol–gel chemistry according to ref. 126. Reproduced from ref. 126 with permission from the Royal Society of Chemistry.

Alternatively, Nakamura et al. have formed Cu₃N from copper(II) acetate in nonanol by bubbling ammonia through the mixture at 190 °C for 1 h.¹²³ The final NCs are not colloidally stable and form aggregates of about 200 nm with crystallite sizes of 20–30 nm. The same colour changes are observed; from blue to yellow to dark brown. Single phase CuO is formed by using shorter chain alcohols such as pentanol, hexanol and heptanol at 130 °C. With increasing temperature, an increasing amount of copper nitride is obtained and from 170 °C pure copper nitride is formed. The authors propose the following mechanism. First, ammonia forms a complex with Cu(II), evidenced by a color change from green to blue. Second, the authors hypothesize Cu(II) is reduced by nonanol. The color change to yellow is consistent with a Cu(I) species but the authors do not detect the direct product, nonyl aldehyde. They do detect 9-ODEamide, of which it is unclear how it is exactly formed. Finally, the Cu(I) species reacts with ammonia to form copper nitride. In the background, there is also the esterification of acetate with nonanol to nonyl acetate. According to the mechanism, a lot of protons are released but the authors did not try to uncover the fate of these protons. Nor were alternative hypotheses considered such as the reduction of Cu(II) by ammonia itself.

5.4. Alternative precursors for Cu₃N nanocrystal synthesis

In 2018, Mondal and co-workers obtained Cu₃N by reacting hexamethylenetetraamine (HMT) and Cu(NO₃)₂·5H₂O in n-hexanol at 200 °C for one hour under an inert atmosphere.¹²² HMT undergoes hydrolysis and forms formaldehyde and NH₃. The latter is the source of nitrogen as described in Fig. 17. Copper nitrate is in this case simply the source of water and Cu and not the nitrogen source. Cu(II) is assumed to be reduced by the formaldehyde intermediate, but the authors do not show evidence for the formation of formic acid. The final NCs are around 80 nm in size, which is most likely due to the absence of a strongly coordinating ligand.

Fig. 17 Proposed reaction mechanism for Cu₃N formation from hexamethylenetetraamine and Cu(NO₃)₂·5H₂O.¹²²

Also in 2018, Sithole et al. synthesized a novel copper precursor: pyrrole-2-carbaldpropyliminato Cu(II) (PPC), see Fig. 18.¹²⁴ PPC is synthesized from pyrrole-2-carboxaldehyde, copper acetate and propylamine in deionized water, filtered and recrystallized. In a second step, PPC is heated with ODA to form Cu₃N (Fig. 18). The resulting spherical NCs are about 1.5–4 nm in diameter, in contrast to the 18 nm cubes that are obtained with copper nitrate under identical conditions. The photoluminescence of the smaller Cu₃N NCs is slightly blue-shifted to the emission of the larger NCs, which could possibly be ascribed to quantum confinement. Indeed, we estimated from the calculated dielectric constant and the effective hole and electron masses, an exciton Bohr radius of about 3 nm. However, a direct relation between size and optical properties is currently missing.

Fig. 18 UV-Vis absorption and photoluminescence spectra of Cu₃N obtained via (a) PPC and (b) Cu(NO₃)₂·3H₂O method. Reprinted from ref. 124 with permission from the Royal Society of Chemistry. (c) Reaction mechanism proposed for Cu₃N formation from the reaction of pyrrole-2-carbaldpropyliminato Cu(II) (PPC) and ODA shown below.¹²⁴

6. Zinc nitride

Zn₃N₂ has a cubic crystal structure (Ia3̄ space group) with a lattice constant of 9.7691 Å (Fig. 19).¹³⁰ It is a nontoxic, earth-abundant, and low-cost semiconductor,¹³¹ and is therefore promising for various applications. However, it has been less exploited than other nitrides due to the difficulties in preparation and its high reactivity towards water.¹³² As a II–V semiconductor, Zn₃N₂ has a high electron mobility and carrier concentration. The reported band gap for Zn₃N₂ varies from 1.0 to 3.3 eV depending on the preparation methods.^133–135

Fig. 19 (a) Crystal structure of Zn₃N₂ metal nitride. The metal is represented in purple and the nitrogen atom in grey. Structures plotted using VESTA.⁴⁸ Cif file obtained from ref. 130. (b) Toluene solutions of OLAm capped Zn₃N₂ NCs sampled at different reaction times and excited with a 365 nm lamp. (c) Absorption (thin lines) and emission (thick lines) spectra of Zn₃N₂ NCs with mean diameters of 2.1 ± 0.4 nm (solid lines), 4.6 ± 2.3 nm (dashed lines) and 7.1 ± 2.0 nm (dotted lines). Reproduced from ref. 136 with permission from the Royal Society of Chemistry.

Various physical methods or nitridation approaches have been employed to synthesize Zn₃N₂ as thin films,¹³⁷ powders,¹³⁸ hollow balls,¹³⁹ or nanowires.¹³⁴ One chemical approach was reported by Taylor et al. where diethylzinc was reacted with an ammonia flow in ODE, with OLAm as the capping ligand.

The size is tuned by the stepwise addition of the diethylzinc precursor. Thus, NCs emitting in the range 500–1100 nm were obtained,¹⁴⁰ with a photoluminescence quantum yield greater than 50%. The authors further investigated the effect of the ratio between the diethylzinc to ammonia on the growth. A lower ammonia flow rate or higher diethylzinc injection promotes growth and large particles are obtained. Higher ammonia flow rates lead to smaller NCs.¹³⁶

7. Group 13 nitrides

Among the group 13 nitrides, GaN has acquired most fame due to its use in blue LEDs, a feat that has been awarded with the 2014 nobel prize in Physics.¹⁴¹ The most stable phase of the group 13 nitrides has the wurtzite crystal structure. The bandgap of AlN, GaN and InN is 6.2 eV, 3.4 eV and 0.7 eV respectively. As discussed before AlN is the least interesting nitride of group 13 since it is considered a saline nitride and quickly reacts with water. However, aluminum nitride (AlN) has a high thermal conductivity (up to 320 W m⁻¹ K⁻¹), high acoustic wave velocity, and electrical resistance.¹⁴² These properties raise the potential of AlN for applications such as thermal interfaces for electronic laser, brackets for LED technology, resonators, sensors, actuators, etc.^143–145 The different synthetic approaches for cubic AlN formation have been presented elsewhere.¹⁴⁶

GaN and InN nanocrystals are considered the possible next generation of QDs. While the exciton Bohr radius of gallium nitride is 3.2 nm, the exciton Bohr radius of indium nitride is 8 nm. Together with the formation of solid solutions (In_xGa_1−xN), this provides ample opportunity to tune the optical properties using either quantum confinement or compositional control. GaN is being investigated for high frequency and high power transistor design.^147,148 InN is a promising compound for optoelectronic applications due to its low bandgap, small effective mass and efficient electron transport. InN is also considered for low-cost, high-efficiency solar cells, photomasks, laser diodes, sensors, and biological imaging.^149–153 InN has a complex plasmonic behavior due to a nonparabolic dispersion of its conduction band.^154,155

7.1. Synthesis of group 13 nitride powders

In 2004, InN was prepared by the conversion of In₂O₃ under ammonia flow in a temperature range of 600 °C to 730 °C. Depending on the temperature, either spherical NCs or hollow microtubes are formed. This is due to the decomposition of InN at a temperature around 650 °C into metallic In and nitrogen¹⁵⁶

In₂O₃ + 2NH₃ → 2InN + 3H₂O

GaN NCs in a silica matrix were synthesized by first forming an oxide gel of gallium and silicon. The gel is then annealed at 900 °C for 5 h under NH₃ atmosphere.¹⁵⁷ Variations on this strategy exist where pure GaN nanocrystalline powders are formed by annealing Ga₂O₃, GaO(OH), or NH₄[Ga(OH)₂CO₃] under an NH₃ atmosphere at 900–950 °C.^158–161 Similar to the case of TiN, the solid state reaction between Ga₂O₃ (or γ-Al₂O₃) and cyanamide CN₂H₂ yields GaN (or AlN) NCs.^67,162 Finally, the urea glass route was also successful in synthesizing GaN nanopowders.

7.2. Synthesis strategies based on azide or amide precursors

Colloidally stable, but poorly crystalline GaN NCs are obtained by refluxing [Et₂Ga(N₃)]₃, (N₃)₂Ga[(CH₂)₃NMe₂], or (Et₃N)Ga(N₃)₃ in triglyme at 220 °C for 5 hours.¹⁶³ Also, GaCl₃ and NaN₃ react to form gallium azide, which forms amorphous GaN at 220 °C or 260 °C in THF or toluene respectively. Annealing and NaCl removal are necessary in order to form crystalline powders.¹⁶⁴ Finally, crystalline GaN NCs were formed from NaN₃ and metal Ga by heating to 700 °C under nitrogen in a high-pressure apparatus.¹⁶⁵

Janik et al. formed GaN NCs via pyrolysis at 450–500 °C of a gallium imide [Ga(NH)_3/2]_n precursor (obtained by reaction of [Ga(NMe₂)₃]2 and NH₃).¹⁶⁶ In 1999, this method was improved by crystallizing the [Ga(NH)_3/2]_n precursor in hot trioctylamine (360 °C for 24 h) under a flow of ammonia.¹⁶⁷ During cooldown of the reaction mixture, a solution of hexadecylamine in trioctylamine was added, which improved the colloidal stability of the final GaN NCs. These NCs have a diameter of 3 nm and feature a first exitonic emission at 3.65 eV, higher than the bulk bandgap of GaN.

β-GaN NPs of 3–8 nm in size were obtained via microwave heating of Ga(NH₂)₃ in ionic liquid at 300 °C for 1 h. The colorless suspension of Ga(NH₂)₃ was obtained by dissolving GaCl₃ in liquid NH₃ at −35 °C and adding KNH₂. The obtained NCs show a bandgap around 3.9 eV which is 0.6 eV above the β-GaN bulk bandgap. The authors attributed this blue shift to quantum confinement. The fluorescence properties of the as-synthesized NCs are shown in Fig. 20.¹⁶⁸

Fig. 20 (a) Emission spectrum at 274 nm and excitation spectrum at 523 nm of the synthesized GaN. (b) Photographs of the as synthesized GaN NCs in daylight and under excitation at 254 nm. Reprinted from ref. 168 with permission from the Royal Society of Chemistry.

In 2006 Dingman et al. reported the synthesis of InN nanofibres at low temperatures (203 °C), catalyzed by nanometer sized droplets of In metal (also called a solution–liquid–solid (SLS) mechanism).¹⁶⁹ The authors prepared two different indium precursors (ⁱPr₂InN₃ and ^tBu₂InN₃) by reacting trialkylindium first with methanol and subsequently with trimethylsilylazide. Crystalline InN was then formed by reacting the as mentioned precursors with 1,1-dimethylhydrazine (H₂NNMe₂) at 203 °C for 20 h. The role of 1,1-dimethylhydrazine is to reduce part of the indium precursors to In droplets, which then catalyze the InN crystallization.

In 2006 Choi et al. synthesized InN NCs from indium bromide and sodium azide under solvothermal conditions.¹⁷⁰ The indium azide intermediate decomposes into InN and N₂.

The solvent used during the solvothermal treatment determines the final properties of the NCs. While toluene delivered 10 nm aggregated wurtzite InN, hexadecane yielded poorly crystalline InN. The reaction typically takes 3–5 days during which the reaction temperature was increased step by step to avoid explosive azide decomposition. Also In_xGa_1−xN alloyed particles were synthesized via this strategy.¹⁷⁰

The reaction of potassium amide (KNH₂) and indium iodide in liquid ammonia yields an insoluble indium amide product. The latter is probably highly condensed, featuring imido bridges. Decomposition of this insoluble product at 400 °C under nitrogen atmosphere delivers InN, In, KNH₂, and NH₃.¹⁷¹ Alternatively, InI₃ and NaNH₂ were reacted in anhydrous benzene in a Teflon-lined pressure bomb.¹⁷² The crystallite size of the resulting InN NCs varies from 9.9 ± 5 nm to 35.1 ± 10 nm by increasing the reaction time or temperature. At ambient pressure, InN NCs (6.2 ± 2 nm diameter) have been synthesized using InBr₃ and NaNH₂ in hexadecane under an ammonia atmosphere.

A post-synthetic acid treatment (to remove In(0)) and functionalization with OLAm yielded colloidally stable particles, see Fig. 21.¹⁷³ This method has been further studied by Palomaki et al. where they use In₂S₃ instead of InBr₃ and ODE as their solvent.¹⁷⁴

Fig. 21 TEM images of the colloidally stable InN obtained by Hsieh et al. after acid treatment. (a) Low magnification image, (b) high resolution image and (c) diffraction pattern obtained via fast Fourier transform (FFT). Reprinted with permission from ref. 173 with permission from the Royal Society of Chemistry.

To improve the size dispersion, size-selective precipitation was performed by the dropwise addition of acetonitrile to OLAm-capped InN NCs, and centrifugation. The final nanocrystal size was tuned from 4.5 to 9.9 nm by varying the reaction time from 15 min to 12 h at 220 °C. The NCs size also increases from 4.1 to 8.0 nm by increasing the reaction temperature from 175 °C to 200 °C (at a fixed reaction time of three hours). Below 175 °C no InN was formed.

When InCl₃·4H₂O, NaNH₂, and sulfur were reacted in a pressure bomb at 160 °C for 30 h, wurtzite InN crystals (50–200 nm) were obtained.¹⁷⁵ According to XRD, the reaction proceeds through a In₂S₃ intermediate. Even larger particles (500 nm) were obtained using In(NO₃)₃·5H₂O as precursor.

7.3. Colloidal InN NCs from lithium oleylamide

Karan and coworked reported a surfactant-assisted procedure where all precursors are soluble.¹⁷⁶ InBr₃ is first dissolved in a mixture of OLAm and ODE solvent (or hexadecane). At 220 °C the authors inject lithium oleylamide, which was synthesized from OLAm, n-butyllithium (n-BuLi) and tetramethylethylenediamine (TMEDA). After 10 minutes, the reaction is stopped and the product is purified. TEM analyis shows that the obtained particles consist of a 30 nm In(0) core and InN outgrowths (Fig. 22). The mixed composition of In and InN is confirmed by XRD with sharp reflections for In and broader reflections for InN, consistent with their size difference (Fig. 22). Using the same strategy as mentioned before, In is removed by an nitric acid treatment and the InN NCs are stabilized by functionalization with OLAm.¹⁷⁶

Fig. 22 TEM images of the NPs. (a) Overview of the as prepared sample, showing the InN branches stemming out of larger In(0) NPs. (b) HRTEM image of one of the InN branches in (a) at the surface of the In(0) nanoparticle. XRD pattern for the (c) as-prepared sample and (d) acid treated sample. The vertical lines show the XRD patterns of bulk zinc blende-InN (red) and tetragonal In(0) (blue). Reprinted with permission from ref. 176. Copyright 2016 American Chemical Society.

Intrigued by the high dissociation energy of C–N bonds (330 kJ mol⁻¹), Beaulac et al. investigated the mechanism of this reaction by analyzing the by-products via¹H NMR, ¹³C NMR, 2D COSY NMR, and mass spectrometry.¹⁷⁷ The authors propose that oleylamide is oxidized to oleylimine, thereby reducing In(III) to In(0), see Fig. 23. A second equivalent of oleylamide performs a nucleophilic attack on the imine, yielding a secondary aldimine and releasing NH₂⁻. The latter goes on to react with In(III) to form InN. The proposed mechanism is based on the observation of secondary aldimine, ammonia, hydrogen, InN and In as reaction products. Oleylimine is too unstable to be isolated. In(0) is always formed in larger yields than InN, meaning that not all reactive NH₂⁻ are converted to InN.

Fig. 23 Overall reaction. Reaction 1a, alkylamide precursor oxidation; reaction 1b, formation of the secondary imine leading to NH₂⁻ production; reaction 2, nitride formation; R1, R2, R3, three hypothetical reduction processes concomitant with reaction 1a; S1, detrimental hypothetical side reaction. Orange, reactants; blue, intermediates (not isolated); red, identified products. Uncoordinated forms depicted here for simplicity. Reprinted with permission from ref. 177. Copyright 2018 American Chemical Society.

7.4. Other surfactant based strategies for colloidal nitrides

AlCl₃, GaCl₃, and InCl₃ react with urea to form the crystalline complexes: [Al(H₂NCONH₂)₆]Cl₃, [Ga(H₂NCONH₂)₆]Cl₃, and [In(H₂NCONH₂)₃Cl₃].¹⁷⁸ In the Al and Ga complexes, six urea molecules are coordinated to the metal and the chloride is simply the counter ion of the positively charged complex. In the In complex, three urea molecules and 3 chlorides build up the first coordination sphere. Pyrolysis of the complexes under nitrogen yields AlN, GaN or a mixture of InN and In₂O₃. Refluxing [Ga(H₂NCONH₂)₆]Cl₃ in trioctylamine (365 °C) for 24 hours yields colloidal GaN NCs (2–3 nm). These particles show photoluminescence (PL) emission bands at 340 and 380 nm. Refluxing the Al and In complex yields AlN and InN (with a minor indium oxide impurity) respectively. Later, the authors showed that the obtained GaN NCs were amorphous and further annealing in ammonia at 450 °C for 24 h was necessary to improve their crystallinity.¹⁷⁹

In 2019, Chang Choi et al. designed a hot-injection strategy to obtain GaN QDs.¹⁸⁰ Gallium chloride and stearic acid were dissolved in ODE and heated to 280 °C under argon. Subsequently, a solution of lithium bis(trimethylsilyl)amide in hexane was rapidly injected.

The nanocrystal size was hard to determine from TEM but surely below 5 nm, which is also confirmed by the very broad nature of the XRD reflections. The PL emission of the QDs could be tuned from 315–355 nm, while the bulk PL is at 365 nm. Successful doping with zinc red-shifted the PL to 330–460 nm.¹⁸⁰

8. Summary and outlook

We have focused on precursor chemistry that allows synthesizing colloidal metal nitrides varying from plasmonic nitrides (Group 4) to semiconductors (Group 13). Approaches that lead to nanocrystal formation are described based on a systematic comparison between synthetic strategies. To date, the late transition metal and group 13 metal nitrides have been more extensively studied. While the reaction mechanism (precursor conversion) is most studied for the group 13 nitrides, the most uniform nanocrystals are obtained for Cu₃N. At the same time, we were forced to conclude that there does not exist any convincing reports of a colloidal synthesis of group 4 and group 5 nitrides. Only nanopowders were succesfully obtained, of which some could be dispersed in water, albeit in aggregated form. It is striking that the ease of nitride formation seems to correlate with the valence of the elements. Whereas Cu⁺ and Ni⁺ easily crystallize as nitrides, Ga³⁺ and In³⁺ are more difficult to crystallize and the most successful approaches are based on crystallization, catalyzed by liquid nanodroplets of metal. An additional hurdle for the group 4 and 5 nitrides is the required reduction during the synthesis. For example, most common precursors for TiN and TaN are based on Ti(IV) or Ta(V), while the final nitrides contain the metal in the +III oxidation state. To avoid the reduction step, future precursors of group 4 and 5 nitrides could start from the metal in the correct oxidation state. When designing new precursors, it is also useful to keep in mind the lessons from the metal chalcogenide field. The consensus seems to be that for optimal nanocrystal formation (and control over size), the conversion of precursor (P) to monomer (M) needs to be slow, relative to the crystallization (nucleation and growth) of the monomer into the final nanocrystals.

Since group 4 and 5 nitrides typically require high temperatures for crystallization, the logical conclusion is that their crystallization rate is low. Therefore, one should design nitride precursors that decompose even slower, or to apply strategies that enhance the rate of crystallization by lowering the barrier for bond forming and bond breaking. To move the field forward in a rational way, we need to understand the mechanism of the colloidal syntheses that do provide satisfactory results. While the precursor conversion has been studied for InN, there are many systems for which the mechanism is left unexplored. Even more problematic, the crystallization part of the mechanism has not been studied for any of the nitrides discussed here. However, the crystallization is likely the most difficult step in the whole process and thus the most critical to understand to make progress. Finally, it will be equally important that researchers treat colloidal synthesis as a true chemical reaction that has a chemical equation (which needs to be written down) and for which an accurate yield needs to be reported. By putting more rigor into writing the Experimental section, we can enhance the reproducibility of the field. Given the use of early transition metal nitrides for many applications such as catalysis, photothermal therapy, superconductors, and electrochemical capacitors, synthetic efforts should focus on providing these NCs as size-tunable colloidal particles. The field of colloidal metals nitrides is clearly challenging and still young, but adding colloidal nitrides (with synthetic control) to the portfolio of colloidal materials would open up many exciting possibilities for the future of materials science and nanotechnology.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the University of Basel and the NCCR Molecular Systems Engineering (project number: 182895) for financial support. We thank Ajmal Roshan for help with the Table of Content graphic.

References

1. S. G. Kwon and T. Hyeon, Acc. Chem. Res., 2008, 41, 1696–1709.
2. Z. Hens and J. De Roo, J. Am. Chem. Soc., 2020, 142, 15627–15637.
3. E. Matijevic, Chem. Mater., 1993, 5, 412–426.
4. Z. Hens, Science, 2015, 348, 1211–1212.
5. J. Park, J. Joo, S. Kwon, Y. Jang and T. Hyeon, Angew. Chem., Int. Ed., 2007, 46, 4630–4660.
6. I. Ojea-Jiménez, F. M. Romero, N. G. Bastús and V. Puntes, J. Phys. Chem. C, 2010, 114, 1800–1804.
7. R. Garcia-Rodriguez, M. P. Hendricks, B. M. Cossairt, H. T. Liu and J. S. Owen, Chem. Mater., 2013, 25, 1233–1249.
8. M. Niederberger and G. Garnweitner, Chem. – Eur. J., 2006, 12, 7282–7302.
9. M. D. Tessier, K. De Nolf, D. Dupont, D. Sinnaeve, J. De Roo and Z. Hens, J. Am. Chem. Soc., 2016, 138, 5923–5929.
10. M. A. Boles, D. Ling, T. Hyeon and D. V. Talapin, Nat. Mater., 2016, 15, 141–153.
11. N. C. Anderson, M. P. Hendricks, J. J. Choi and J. S. Owen, J. Am. Chem. Soc., 2013, 135, 18536–18548.
12. J. De Roo, F. Van den Broeck, K. De Keukeleere, J. C. Martins, I. Van Driessche and Z. Hens, J. Am. Chem. Soc., 2014, 136, 9650–9657.
13. F. W. Eagle, N. Park, M. Cash and B. M. Cossairt, ACS Energy Lett., 2021, 977–984.
14. J. De Roo, K. De Keukeleere, Z. Hens and I. Van Driessche, Dalton Trans., 2016, 45, 13277–13283.
15. Y. Yang, H. Qin and X. Peng, Nano Lett., 2016, 16, 2127–2132.
16. T. Kister, D. Monego, P. Mulvaney, A. Widmer-Cooper and T. Kraus, ACS Nano, 2018, 12, 5969–5977.
17. J. D. Martin, Chem. Mater., 2020, 32, 3651–3656.
18. R. G. Finke, M. A. Watzky and C. B. Whitehead, Chem. Mater., 2020, 32, 3657–3672.
19. V. K. Lamer and R. H. Dinegar, J. Am. Chem. Soc., 1950, 72, 4847–4854.
20. S. Özkar and R. G. Finke, J. Am. Chem. Soc., 2017, 139, 5444–5457.
21. S. Mozaffari, W. Li, C. Thompson, S. Ivanov, S. Seifert, B. Lee, L. Kovarik and A. M. Karim, Nanoscale, 2017, 9, 13772–13785.
22. B. M. McMurtry, K. Qian, J. K. Teglasi, A. K. Swarnakar, J. De Roo and J. S. Owen, Chem. Mater., 2020, 32, 4358–4368.
23. P. T. Prins, F. Montanarella, K. Dümbgen, Y. Justo, J. C. van der Bok, S. O. M. Hinterding, J. J. Geuchies, J. Maes, K. De Nolf, S. Deelen, H. Meijer, T. Zinn, A. V. Petukhov, F. T. Rabouw, C. De Mello Donega, D. Vanmaekelbergh and Z. Hens, Nano Lett., 2021, 21, 2487–2496.
24. S. Mozaffari, W. Li, M. Dixit, S. Seifert, B. Lee, L. Kovarik, G. Mpourmpakis and A. M. Karim, Nanoscale Adv., 2019, 1, 4052–4066.
25. D. R. Handwerk, P. D. Shipman, C. B. Whitehead, S. Özkar and R. G. Finke, J. Am. Chem. Soc., 2019, 141, 15827–15839.
26. M. P. Hendricks, M. P. Campos, G. T. Cleveland, I. J.-L. Plante and J. S. Owen, Science, 2015, 348, 1226–1230.
27. E. Dhaene, J. Billet, E. Bennett, I. Van Driessche and J. De Roo, Nano Lett., 2019, 19, 7411–7417.
28. K. De Keukeleere, S. Coucke, E. De Canck, P. Van Der Voort, F. Delpech, Y. Coppel, Z. Hens, I. Van Driessche, J. S. Owen and J. De Roo, Chem. Mater., 2017, 29, 10233–10242.
29. D. Baranov, M. J. Lynch, A. C. Curtis, A. R. Carollo, C. R. Douglass, A. M. Mateo-Tejada and D. M. Jonas, Chem. Mater., 2019, 31, 1223–1230.
30. J. J. Calvin, J. K. Swabeck, A. B. Sedlak, Y. Kim, E. Jang and A. P. Alivisatos, J. Am. Chem. Soc., 2020, 142, 18897–18906.
31. M. R. Friedfeld, J. L. Stein, A. Ritchhart and B. M. Cossairt, Acc. Chem. Res., 2018, 51, 2803–2810.
32. M. E. Mundy, D. Ung, N. L. Lai, E. P. Jahrman, G. T. Seidler and B. M. Cossairt, Chem. Mater., 2018, 30, 5373–5379.
33. A. G. Rachkov and A. M. Schimpf, Chem. Mater., 2021, 33, 1394–1406.
34. D. R. Sadoway, JOM, 1991, 43, 15–19.
35. A. MIURA, J. Ceram. Soc. Jpn., 2017, 125, 552–558.
36. K. Fung and G. Mantov, J. Electroanal. Chem. Interfacial Electrochem., 1972, 35, 27–34.
37. C. Housecroft, Inorganic Chemistry, Pearson, 2018, vol. 72, pp. 650–651.
38. S. H. Elder, F. J. DiSalvo, L. Topor and A. Navrotsky, Chem. Mater., 1993, 5, 1545–1553.
39. Y. Ma, L. Xiong, Y. Lu, W. Zhu, H. Zhao, Y. Yang, L. Mao and L. Yang, Front. Chem., 2021, 9, 390.
40. R. A. Karaballi, G. Humagain, B. R. A. Fleischman and M. Dasog, Angew. Chem., Int. Ed., 2019, 58, 3147–3150.
41. S. Ishii, R. P. Sugavaneshwar and T. Nagao, J. Phys. Chem. C, 2016, 120, 2343–2348.
42. E. Traver, R. A. Karaballi, Y. E. Monfared, H. Daurie, G. A. Gagnon and M. Dasog, ACS Appl. Nano Mater., 2020, 3, 2787–2794.
43. R. A. Karaballi, Y. E. Monfared and M. Dasog, Chemistry, 2020, 26, 8499–8505.
44. R. A. Karaballi, Y. Esfahani Monfared and M. Dasog, Langmuir, 2020, 36, 5058–5064.
45. U. Guler, S. Suslov, A. V. Kildishev, A. Boltasseva and V. M. Shalaev, Nanophotonics, 2015, 4, 269–276.
46. Y. Yuan, J. Wang, S. Adimi, H. Shen, T. Thomas, R. Ma, J. P. Attfield and M. Yang, Nat. Mater., 2019, 19, 282–286.
47. C. Defilippi, D. V. Shinde, Z. Dang, L. Manna, C. Hardacre, A. J. Greer, C. D’Agostino and C. Giordano, Angew. Chem., Int. Ed., 2019, 58, 15464–15470.
48. K. Momma and F. Izumi, J. Appl. Crystallogr., 2011, 44, 1272–1276.
49. A. N. Christensen, V. Romano, R. Hesse, A. F. Andresen and P. Fischer, Acta Chem. Scand., Ser. A, 1975, 29, 563–564.
50. F. W. Glaser, D. Moskowitz and B. Post, JOM, 1953, 5, 1119–1120.
51. K. Becker and F. Ebert, Z. Phys., 1925, 31, 268–272.
52. L. Hiltunen, M. Leskela, M. Makela, L. Niinisto, E. Nykanen and P. Soininen, Thin Solid Films, 1988, 166, 149–154.
53. A. Kafizas, C. J. Carmalt and I. P. Parkin, Coord. Chem. Rev., 2013, 257, 2073–2119.
54. A. Alvarez Barragan, N. V. Ilawe, L. Zhong, B. M. Wong and L. Mangolini, J. Phys. Chem. C, 2017, 121, 2316–2322.
55. K. S. Schramke, Y. X. Qin, J. T. Held, K. A. Mkhoyan and U. R. Kortshagen, ACS Appl. Nano Mater., 2018, 1, 2869–2876.
56. S. Exarhos, A. Alvarez-Barragan, E. Aytan, A. A. Balandin and L. Mangolini, ACS Energy Lett., 2018, 3, 2349–2356.
57. C. H. Winter, P. H. Sheridan, T. S. Lewkebandara, M. J. Heeg and J. W. Proscia, J. Am. Chem. Soc., 1992, 114, 1095–1097.
58. S. C. Dunn, A. S. Batsanov and P. Mountford, J. Chem. Soc., Chem. Commun., 1994, 2007–2008.
59. D. C. Bradley and I. M. Thomas, J. Chem. Soc., 1960, 3857–3861.
60. D. C. Bradley and E. G. Torrible, Can. J. Chem., 1963, 41, 134–138.
61. C. Lorber and L. Vendier, Dalton Trans., 2013, 42, 12203–12219.
62. A. W. Jackson and A. L. Hector, J. Mater. Chem., 2007, 17, 1016–1022.
63. C. Lorber, R. Choukroun and L. Vendier, Eur. J. Inorg. Chem., 2006, 4503–4518.
64. N. Adams, H. R. Bigmore, T. L. Blundell, C. L. Boyd, S. R. Dubberley, A. J. Sealey, A. R. Cowley, M. E. G. Skinner and P. Mountford, Inorg. Chem., 2005, 44, 2882–2894.
65. A. Egeberg, O. Wenzel, R. Popescu, D. Gerthsen and C. Feldmann, ChemistryOpen, 2020, 10, 334–339.
66. A. Fischer, M. Antonietti and A. Thomas, Adv. Mater., 2007, 19, 264–267.
67. J. Buha, I. Djerdj, M. Antonietti and M. Niederberger, Chem. Mater., 2007, 19, 3499–3505.
68. Y. Liu, X. Zhang, L. Lu, J. Ye, J. Wang, X. Li, X. Bai and W. Wang, Chin. Chem. Lett., 2021 DOI:10.1016/j.cclet.2021.07.054.
69. C. Giordano, C. Erpen, W. T. Yao, B. Milke and M. Antonietti, Chem. Mater., 2009, 21, 5136–5144.
70. F. D. Qu, Y. Yuan and M. H. Yang, Chem. Mater., 2017, 29, 969–974.
71. G. S. Shanker, R. Bhosale, S. Ogale and A. Nag, Adv. Mater. Interfaces, 2018, 5, 1801488.
72. G. S. Shanker, G. B. Markad, M. Jagadeeswararao, U. Bansode and A. Nag, ACS Energy Lett., 2017, 2, 2251–2256.
73. D. B. O'Neill, S. K. Frehan, K. Zhu, E. Zoethout, G. Mul, E. C. Garnett, A. Huijser and S. H. C. Askes, Adv. Opt. Mater., 2021, 2100510.
74. X. Kan, J. Ding, H. Zhu, C. Deng and C. Yu, Powder Technol., 2017, 315, 81–86.
75. S. Hosoya, T. Yamagishi and M. Tokonami, J. Phys. Soc. Jpn., 1968, 24, 363–367.
76. J. Gatterer, G. Dufek, P. Ettmayer and R. Kieffer, Monatsh. Chem., 1975, 106, 1137–1147.
77. Y. Liu, Q. Wu, L. Liu, P. Manasa, L. Kang and F. Ran, J. Mater. Chem. A, 2020, 8, 8218–8233.
78. H. Cui, G. Zhu, X. Liu, F. Liu, Y. Xie, C. Yang, T. Lin, H. Gu and F. Huang, Adv. Sci., 2015, 2, 1500126.
79. Y. Li, J. Zhang, X. Qian, Y. Zhang, Y. Wang, R. Hu, C. Yao and J. Zhu, Appl. Surf. Sci., 2018, 427, 884–889.
80. D. Choi, G. Blomgren and P. Kumta, Adv. Mater., 2006, 18, 1178–1182.
81. T. Huang, S. Mao, G. Zhou, Z. Wen, X. Huang, S. Ci and J. Chen, Nanoscale, 2014, 6, 9608.
82. D. Choi and P. N. Kumta, J. Am. Ceram. Soc., 2011, 94, 2371–2378.
83. J. H. Swisher and M. H. Read, Metall. Trans., 1972, 3, 489.
84. L. Wang, X. Zhou, N. T. Nguyen, I. Hwang and P. Schmuki, Adv. Mater., 2016, 28, 2432–2438.
85. M. H. Tsai, S. C. Sun, H. T. Chiu, C. E. Tsai and S. H. Chuang, Appl. Phys. Lett., 1995, 67, 1128–1130.
86. Y.-J. Lee, D.-Y. Kim, K.-H. Lee, M.-H. Han, K.-S. Kang, K.-K. Bae and J.-H. Lee, Nanoscale Res. Lett., 2013, 8, 128.
87. C.-T. Ho, K.-B. Low, R. F. Klie, K. Maeda, K. Domen, R. J. Meyer and P. T. Snee, J. Phys. Chem. C, 2010, 115, 647–652.
88. C. F. Mallinson, B. M. Gray, A. L. Hector, M. A. McLachlan and J. R. Owen, Inorg. Chem., 2013, 52, 9994–9999.
89. A. Leineweber, H. Jacobs and S. Hull, Inorg. Chem., 2001, 40, 5818–5822.
90. F. Gillot, J. Oró-Solé and M. R. Palacín, J. Mater. Chem., 2011, 21, 9997.
91. D. C. Dunand and P. Müllner, Adv. Mater., 2010, 23, 216–232.
92. N. Gajbhiye, R. Ningthoujam and J. Weissmller, Phys. Status Solidi A, 2002, 189, 691–695.
93. C. Bartholomew, J. Catal., 1976, 45, 41–53.
94. M. Shalom, V. Molinari, D. Esposito, G. Clavel, D. Ressnig, C. Giordano and M. Antonietti, Adv. Mater., 2013, 26, 1272–1276.
95. K. Xu, P. Chen, X. Li, Y. Tong, H. Ding, X. Wu, W. Chu, Z. Peng, C. Wu and Y. Xie, J. Am. Chem. Soc., 2015, 137, 4119–4125.
96. D. Gao, J. Zhang, T. Wang, W. Xiao, K. Tao, D. Xue and J. Ding, J. Mater. Chem. A, 2016, 4, 17363–17369.
97. G. S. Shanker and S. Ogale, ACS Appl. Energy Mater., 2021, 4, 2165–2173.
98. M. J. Winiarski, J. Solid State Chem., 2018, 266, 161–165.
99. D. Bocharov, A. Anspoks, J. Timoshenko, A. Kalinko, M. Krack and A. Kuzmin, Radiat. Phys. Chem., 2020, 175, 108100.
100. A. Zakutayev, C. M. Caskey, A. N. Fioretti, D. S. Ginley, J. Vidal, V. Stevanovic, E. Tea and S. Lany, J. Phys. Chem. Lett., 2014, 5, 1117–1125.
101. T. Maruyama and T. Morishita, Appl. Phys. Lett., 1996, 69, 890–891.
102. W. Zhu, X. Zhang, X. Fu, Y. Zhou, S. Luo and X. Wu, Phys. Status Solidi A, 2012, 209, 1996–2001.
103. Z. Yin, C. Yu, Z. Zhao, X. Guo, M. Shen, N. Li, M. Muzzio, J. Li, H. Liu, H. Lin, J. Yin, G. Lu, D. Su and S. Sun, Nano Lett., 2019, 19, 8658–8663.
104. C. Panda, P. W. Menezes, M. Zheng, S. Orthmann and M. Driess, ACS Energy Lett., 2019, 4, 747–754.
105. P. X. Xi, Z. H. Xu, D. Q. Gao, F. J. Chen, D. S. Xue, C. L. Tao and Z. N. Chen, RSC Adv., 2014, 4, 14206–14209.
106. U. Hahn and W. Weber, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 53, 12684–12693.
107. F. Gulo, A. Simon, J. Kohler and R. K. Kremer, Angew. Chem., Int. Ed., 2004, 43, 2032–2034.
108. X. Y. Cui, A. Soon, A. E. Phillips, R. K. Zheng, Z. W. Liu, B. Delley, S. P. Ringer and C. Stampfl, J. Magn. Magn. Mater., 2012, 324, 3138–3143.
109. J. Choi and E. G. Gillan, Inorg. Chem., 2005, 44, 7385–7393.
110. M. Birkett, C. N. Savory, A. N. Fioretti, P. Thompson, C. A. Muryn, A. D. Weerakkody, I. Z. Mitrovic, S. Hall, R. Treharne, V. R. Dhanak, D. O. Scanlon, A. Zakutayev and T. D. Veal, Phys. Rev. B, 2017, 95, 115201.
111. U. Zachwieja and H. Jacobs, J. Less-Common Met., 1990, 161, 175–184.
112. H. Jacobs and U. Zachwieja, J. Less-Common Met., 1991, 170, 185–190.
113. R. Juza and H. Hahn, Z. Anorg. Allg. Chem., 1939, 241, 172–178.
114. G. Paniconi, Z. Stoeva, H. Doberstein, R. I. Smith, B. L. Gallagher and D. H. Gregory, Solid State Sci., 2007, 9, 907–913.
115. D. Wang and Y. Li, Chem. Commun., 2011, 47, 3604–3606.
116. D. Flaxbart, J. Am. Chem. Soc., 1999, 121, 2339–2339.
117. H. Wu and W. Chen, J. Am. Chem. Soc., 2011, 133, 15236–15239.
118. D. D. Vaughn Ii, J. Araujo, P. Meduri, J. F. Callejas, M. A. Hickner and R. E. Schaak, Chem. Mater., 2014, 26, 6226–6232.
119. R. W. Lord, C. F. Holder, J. L. Fenton and R. E. Schaak, Chem. Mater., 2019, 31, 4605–4613.
120. D. Barman, S. Paul, S. Ghosh and S. K. De, ACS Appl. Nano Mater., 2019, 2, 5009–5019.
121. R. Kadzutu-Sithole, L. F. E. Machogo-Phao, T. Kolokoto, M. Zimuwandeyi, S. S. Gqoba, K. P. Mubiayi, M. J. Moloto, J. V. Wyk and N. Moloto, RSC Adv., 2020, 10, 34231–34246.
122. S. Mondal and C. R. Raj, J. Phys. Chem. C, 2018, 122, 18468–18475.
123. T. Nakamura, H. Hayashi, T. A. Hanaoka and T. Ebina, Inorg. Chem., 2014, 53, 710–715.
124. R. K. Sithole, L. F. E. Machogo, M. A. Airo, S. S. Gqoba, M. J. Moloto, P. Shumbula, J. Van Wyk and N. Moloto, New J. Chem., 2018, 42, 3042–3049.
125. R. Deshmukh, G. B. Zeng, E. Tervoort, M. Staniuk, D. Wood and M. Niederberger, Chem. Mater., 2015, 27, 8282–8288.
126. A. Egeberg, L. Warmuth, S. Riegsinger, D. Gerthsen and C. Feldmann, Chem. Commun., 2018, 54, 9957–9960.
127. R. Deshmukh, E. Tervoort, J. Kach, F. Rechberger and M. Niederberger, Dalton Trans., 2016, 45, 11616–11619.
128. Y. Liu, D. Lin, P. Y. Yuen, K. Liu, J. Xie, R. H. Dauskardt and Y. Cui, Adv. Mater., 2017, 29, 1605531.
129. J. Livage, C. Sanchez, M. Henry and S. Doeuff, Solid State Ionics, 1989, 32–33, 633–638.
130. D. Partin, D. Williams and M. O'Keeffe, J. Solid State Chem., 1997, 132, 56–59.
131. N. C. Coronel, Ph.D. thesis, 2016.
132. T. Yang, Z. Zhang, Y. Li, M. Lv, S. Song, Z. Wu, J. Yan and S. Han, Appl. Surf. Sci., 2009, 255, 3544–3547.
133. K. Kuriyama, Y. Takahashi and F. Sunohara, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 48, 2781–2782.
134. F. Zong, H. Ma, J. Ma, W. Du, X. Zhang, H. Xiao, F. Ji and C. Xue, Appl. Phys. Lett., 2005, 87, 233104.
135. S. Prabha, D. Durgalakshmi, K. Subramani, P. Aruna and S. Ganesan, ACS Appl. Mater. Interfaces, 2020, 12, 19245–19257.
136. P. N. Taylor, M. A. Schreuder, T. M. Smeeton, A. J. D. Grundy, J. A. R. Dimmock, S. E. Hooper, J. Heffernan and M. Kauer, J. Mater. Chem. C, 2014, 2, 4379–4382.
137. A. H. Jayatissa, T. Wen and M. Gautam, J. Phys. D: Appl. Phys., 2012, 45, 045402.
138. G. Paniconi, Z. Stoeva, R. I. Smith, P. C. Dippo, B. L. Gallagher and D. H. Gregory, J. Solid State Chem., 2008, 181, 158–165.
139. F. Zong, H. Ma, C. Xue, W. Du, X. Zhang, H. Xiao, J. Ma and F. Ji, Mater. Lett., 2006, 60, 905–908.
140. R. Ahumada-Lazo, S. M. Fairclough, S. J. O. Hardman, P. N. Taylor, M. Green, S. J. Haigh, R. Saran, R. J. Curry and D. J. Binks, ACS Appl. Nano Mater., 2019, 2, 7214–7219.
141. H. Amano, N. Sawaki, I. Akasaki and Y. Toyoda, Appl. Phys. Lett., 1986, 48, 353–355.
142. M. Kaur, K. Singh, P. Singh, A. Kaur, R. Meena, G. P. Singh, H. Barabadi, M. Saravanan and A. Kumar, Nanomed. J., 2020, 7, 194–198.
143. Y. Bian, M. Liu, G. Ke, Y. Chen, J. DiBattista, E. Chan and Y. Yang, Surf. Coat. Technol., 2015, 267, 65–69.
144. C. Li, X. Liu, L. Shu and Y. Li, Mater. Express, 2015, 5, 367–370.
145. C. Li, X.-Z. Liu, B. Peng, L. Shu and Y.-R. Li, Rare Met., 2016, 35, 408–411.
146. V. Kudyakova, R. Shishkin, A. Elagin, M. Baranov and A. Beketov, J. Eur. Ceram. Soc., 2017, 37, 1143–1156.
147. G. G. Arizaga, M. J. Oviedo and O. E. Lopez, Colloids Surf., B, 2012, 98, 63–71.
148. P. Antoniammal and D. Arivuoli, J. Nanomater., 2012, 2012, 1–11.
149. S. Nakamura and G. Fasol, The Blue Laser Diode, Springer Berlin Heidelberg, 1997, pp. 313–315.
150. E. Starikov, P. Shiktorov, V. Gružinskis, L. Reggiani, L. Varani, J. Vaissière and J. H. Zhao, Phys. B, 2002, 314, 171–175.
151. H. Neff, O. Semchinova, A. Lima, A. Filimonov and G. Holzhueter, Sol. Energy Mater. Sol. Cells, 2006, 90, 982–997.
152. H. P. T. Nguyen, Y.-L. Chang, I. Shih and Z. Mi, IEEE J. Sel. Top. Quantum Electron., 2011, 17, 1062–1069.
153. J. Wu, J. Appl. Phys., 2009, 106, 011101.
154. Z. H. Liu and R. Beaulac, Chem. Mater., 2017, 29, 7507–7514.
155. Z. Liu, L. M. Janes, M. Saniepay and R. Beaulac, Chem. Mater., 2018, 30, 5435–5443.
156. B. Schwenzer, L. Loeffler, R. Seshadri, S. Keller, F. F. Lange, S. P. DenBaars and U. K. Mishra, J. Mater. Chem., 2004, 14, 637.
157. G. Sinha, S. K. Panda, P. Mishra, D. Ganguli and S. Chaudhuri, J. Phys.: Condens. Matter, 2007, 19, 346209.
158. Y. Chen, N. Jyoti and J. Kim, Appl. Phys. A, 2010, 102, 517–519.
159. Y. Chen, M. Maniruzzaman and J. Kim, Int. J. Precis. Eng. Manuf., 2011, 12, 573–576.
160. H. Xiao, H. Pei, J. Liu, J. Cui, B. Jiang, Q. Hou and W. Hu, Mater. Lett., 2012, 71, 145–147.
161. H. Yan, K. Liu, R. Luo, S. Chen and H. Cao, Mater. Lett., 2010, 64, 2109–2111.
162. P. G. Li, M. Lei, Y. X. Du, X. Guo and W. H. Tang, Appl. Surf. Sci., 2009, 255, 3843–3847.
163. A. Manz, A. Birkner, M. Kolbe and R. A. Fischer, Adv. Mater., 2000, 12, 569–573.
164. L. Grocholl, J. J. Wang and E. G. Gillan, Chem. Mater., 2001, 13, 4290–4296.
165. F. Kawamura, K. Watanabe, T. Takeda and T. Taniguchi, J. Cryst. Growth, 2011, 321, 100–105.
166. J. F. Janik and R. L. Wells, Chem. Mater., 1996, 8, 2708–2711.
167. O. I. Micic, S. P. Ahrenkiel, D. Bertram and A. J. Nozik, Appl. Phys. Lett., 1999, 75, 478–480.
168. H. F. Gaiser, R. Popescu, D. Gerthsen and C. Feldmann, Chem. Commun., 2020, 56, 2312–2315.
169. S. D. Dingman, N. P. Rath, P. D. Markowitz, P. C. Gibbons and W. E. Buhro, Angew. Chem., Int. Ed., 2000, 39, 1470–1472.
170. J. Choi and E. G. Gillan, J. Mater. Chem., 2006, 16, 3774–3784.
171. A. P. Purdy, Inorg. Chem., 1994, 33, 282–286.
172. C. Wu, T. Li, L. Lei, S. Hu, Y. Liu and Y. Xie, New J. Chem., 2005, 29, 1610.
173. J. C. Hsieh, D. S. Yun, E. Hu and A. M. Belcher, J. Mater. Chem., 2010, 20, 1435.
174. P. K. Palomaki, E. M. Miller and N. R. Neale, J. Am. Chem. Soc., 2013, 135, 14142–14150.
175. L. Wang, Q. Shen, D. Zhao, J. Lu, W. Liu, J. Zhang, K. Bao and Q. Zhou, J. Cryst. Growth, 2017, 471, 62–65.
176. N. S. Karan, Y. Chen, Z. Liu and R. Beaulac, Chem. Mater., 2016, 28, 5601–5605.
177. Y. Chen, N. T. Landes, D. J. Little and R. Beaulac, J. Am. Chem. Soc., 2018, 140, 10421–10424.
178. K. Sardar, M. Dan, B. Schwenzer and C. N. R. Rao, J. Mater. Chem., 2005, 15, 2175–2177.
179. B. Chitara, D. J. Late, S. B. Krupanidhi and C. N. R. Rao, Solid State Commun., 2010, 150, 2053–2056.
180. Y. C. Choi, H. Kim, C. Lee, J. Son, H. Baik, S. Park, J. Kim and K. S. Jeong, Chem. Mater., 2019, 31, 5370–5375.

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