Preparation of ultrafine InN powder by the nitridation of In₂O₃ or In(OH)₃ and its thermal stability

Abstract

Crystalline InN powder has been prepared by the nitridation of In₂O₃ and In(OH)₃ with NH₃ gas for the first time, and ultrafine InN powder in the size range 50–300 nm and with a specific surface area of 8 m² g⁻¹ has been obtained using In₂O₃ nanoparticles as the starting material. The resulting powders were characterized by XRD, FE-SEM, TEM, TG-DSC and BET surface area techniques. It was found that nanosized In₂O₃ was completely converted into InN at 600 °C within 8 h. The thermal stability of the ultrafine InN powder in air and under a nitrogen atmosphere was also investigated. It was observed that InN began to be oxidized into In₂O₃ at 389 °C and decomposed in the narrow temperature range of 595–696 °C in flows of air and N₂, respectively.

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Introduction

The group III nitride compounds have recently attracted much attention because of their applications in the blue and ultraviolet spectral range. Indium nitride (InN), one of III–V compound semiconductors with wurtzite crystal structure, has a direct band gap of about 1.9 eV.¹ It has potential applications in optoelectronics² and is a promising material for color display, high efficiency solar cells of low cost and high mobility devices because of the steep gradient near Γ in its band gap structure.³ Moreover, light-emitting devices in the blue–orange range light have been expected by varying the ratio of gallium to indium to adjust the energy gap between 1.9 and 3.5 eV.⁴ InN has also been found to show electrochromic behavior.⁵

Due to the thermodynamic properties of the In + N₂ system, InN is extremely difficult to synthesize. The high bonding energy of the N₂ molecule and the relatively low bonding energy of InN make direct growth from the constituents (In and N₂) extremely difficult.⁶ Reactive sputtering, reactive magnetron sputtering, nitrogen microwave plasma, pulse discharge, and molecular beam epitaxy have all been used to grow InN films.^7–11 Methods for the growth of InN films have recently been reviewed.^12,13 InN in powder form is very important for understanding the physical and chemical properties of this semiconductor. However, there is little in the literature that focuses on the preparation of powdered InN.^14–16 Zhang et al.¹⁷ reported the preparation of InN nanowires using a mixture of metallic In and In₂O₃ as starting materials at 700 °C, some metallic In remained in their nitrided products. García et al.¹⁸ prepared In_xGa_{1 − x}N and InN powders by nitridation of (NH₄)₃In_xGa_{1 − x}F₆ in a flow of NH₃ gas, and obtained InN particles of 2 µm in size. Here, we describe a novel method for the synthesis of InN powder by the direct nitridation of In(OH)₃ and In₂O₃ powders. Ultrafine InN powder with a particle size ranging from 50 to 300 nm was obtained by using home-synthesized In₂O₃ nanoparticles as the starting material. This simple and facile route for the preparation of InN powder makes a thorough investigation of the thermodynamic, optical and thermal properties of InN possible.

Experimental

(a) Synthesis of nanosized In(OH)₃ and In₂O₃

Indium nitrate (99.5% In(NO₃)₃·4.5H₂O) was used as the main starting material without any further purification, the desired amount of indium nitrate was dissolved in distilled water and the concentration of indium adjusted to 0.3 mol l⁻¹. This aqueous solution was then neutralized with 1.0 mol l⁻¹ ammonia solution at room temperature until a pH value higher than 8 was obtained. The mixture was stirred at high speed while the required ammonia solution was added dropwise. Subsequently, the precipitate was separated from the solution by filtration, and repeatedly washed with distilled water. Then, the white cake was dried at 110 °C for 24 h. XRD analysis verified the dried cake was In(OH)₃. After calcination at 450 °C for 2 h, crystalline In₂O₃ nanoparticles were obtained.

The well washed cake of In(OH)₃ was peptized with 0.5 mol l⁻¹ HNO₃ at 60 °C for 5 h, and a transparent sol containing of 0.3 mol l⁻¹ In(OH)₃ was obtained. Then, the sol was mixed with 0.3 mol l⁻¹ silica sol isovolumetrically and gelled after ageing at 60 °C for 48 h, thus, a white gel with an In ∶ Si ratio of 1 ∶ 1 was obtained. The gel was dried at 110 °C for 24 h and was then ground and sieved with a 200 mesh screen. Before nitridation, the xerogel was calcined at 600 °C for 2 h to remove any undesired water.

(b) Preparation of InN powder

Commercially available In(OH)₃ and In₂O₃ powders (Shanghai Jinchao High-Purity Electronic Materials Co.) with a minimum purity of 99.99% as well as home-made In₂O₃ and In(OH)₃ nanoparticles were used as the starting materials. 1 g of indium oxide or hydroxide was placed in a high-purity quartz boat and set in a quartz tube furnace (inner diameter of 82 mm) with air-tight end gaskets. The reactor was flushed with argon to eliminate oxygen in the system during the heat-up period. As the temperature reached 500 °C, NH₃ (99.9% purity) was introduced from one end of the reactor at a flow rate of 0.5–2.0 l min⁻¹. The furnace was heated to the experimental reaction temperature (500–700 °C) at a rate of 10 °C min⁻¹. The temperature was subsequently kept constant for 1–8 h, then the sample was cooled to room temperature at approximately 6 °C min⁻¹ in an ammonia atmosphere.

(c) Characterization

The crystallite size was calculated from the broadening of the (100) peak of InN by the Scherrer formula:

D = Kλ/(βcosθ) (1)

Where D is the crystallite size, λ is the wavelength of the X-ray radiation (Cu Kα = 1.5406 Å), K is usually taken as 0.89, and β is the line width at half-maximum height, after subtraction of equipment broadening.

XRD patterns were obtained at room temperature with a diffractometer D/max 2550V using Cu Kα radiation at wavelength 1.5406 Å. The lattice parameters (a and c) were calculated by the relation: sin²θ = A(h² + hk +k²) + Cl², where A = λ²/3a² and C = λ²/4c² using the (100) and (101) peaks. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) observations were carried out using a JEOL-200CX and JSM-6700F electron microscope, respectively. The Brunauer–Emmett–Teller (BET) surface area was determined using a Micromeritics ASAP 2010 nitrogen adsorption apparatus. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed on a NETZSCH STA 449C at a rate of 5 °C min⁻¹ in air or nitrogen flow. CHN (carbon, hydrogen and nitrogen) element analysis was carried out on an Elementar Vario EL element analysis instrument, and pure oxygen was introduced to assure the nitride solid was combusted completely.

Results and discussion

(a) The influence of reaction temperature on nitridation

Fig. 1 shows the powder XRD patterns of the nitrided powders. All the XRD peaks of the powder obtained by the nitridation of commercial In₂O₃ powder at 600 °C for 8 h are attributed to the wurtzite phase of InN (Fig. 1d). The estimated lattice parameters of InN obtained by nitridation of commercial In₂O₃ at 550 °C and 600 °C, as well as the synthesized In₂O₃ nanoparticles at 600 °C for 8 h are presented in Table 1. However, for the partially nitrided product, the cell parameter a is 3.541 Å, which is slightly higher than that of the completely converted products. The discrepancy in the value of a may result from oxygen contamination in the mixture of In₂O₃ and InN. The fact that both lattice parameters a and c of completely converted InN are in good agreement with those values reported by García et al.¹⁸ and JCPDS 79-2498 card, respectively, suggests the high crystallinity and purity of InN derived from nitridation of indium oxide or hydroxide. Energy dispersive spectra were used to measure the amount of oxygen in the completely converted products; no oxygen was detected, which indicates that possible impurities are within the detection limits of the instrument. The nitrogen content of In₂O₃ nanoparticles nitrided at 600 °C for 8 h is 10.82 wt.% with a relative error of ±0.5%, which is consistent with the theoretical value of 10.86 wt.%. Both the hydrogen and carbon contents are below the detection limits of this instrument (0.3%).

Fig. 1 XRD patterns of the nitrided powders: (a) 550 °C for 1 h, (b) 550 °C for 5 h, (c) 550 °C for 8 h and (d) 600 °C for 8 h using commercial In₂O₃ as the starting material; (e) 600 °C for 8 h using the synthesized In₂O₃ nanoparticles as the starting material. ■, In₂O₃ in cubic phase; and ○, InN in hexagonal phase.

Table 1 Lattice parameters of InN powders

Parameter	InN^a	InN^b	InN^c	InN^d	InN^e
a InN in partially nitrided sample. b InN was prepared by the nitridation of a commercial In2O3 at 600 °C for 8 h. c InN was prepared by the nitridation of In2O3 nanoparticles at 600 °C for 8 h. d Reported in ref. 18. e JCPDS 79-2498 card.
Crystal structure	Wurtzite	Wurtzite	Wurtzite	Wurtzite	Wurtzite
Lattice constant, a/Å	3.541	3.536	3.536	3.536	3.536
Lattice constant, c/Å	5.704	5.702	5.702	5.700	5.709

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The conversion of In₂O₃ to InN is very sensitive to the nitridation temperature and reaction time, and pure InN powder can be prepared only in a relatively narrow temperature range (580–620 °C). When the nitridation temperature was higher than 650 °C, black droplets of indium metal could be seen in the resultant powder by the naked eye. This temperature was close to the reported value (660 °C) for the dynamic decomposition of InN.⁶ On the other hand, when the nitridation temperature was as low as 550 °C for 8 h, the In₂O₃ powder partially converted into InN powder (Fig. 1c).

(b) The influence of raw materials on the morphology of resulting InN powder

When commercial In₂O₃ and In(OH)₃ were used as the raw materials, the particle morphology of the resulting InN is predominately polyhedral and spherical in the range 100 nm to 2.4 µm, which is shown by SEM in Fig. 2a. However, the relatively broad distribution of the particle size may limit its applications. In our previous work, we demonstrated that nanosized oxide powder was beneficial to the gas–solid reaction, and much finer nitride powder was obtained by using nanocrystalline oxide as the starting material for the synthesis of TiN and CrN.^19,20 So, we therefore attempted to prepare ultrafine InN powder by using In₂O₃ nanoparticles instead of commercial In₂O₃ or In(OH)₃ powders as the starting material. Fig. 2b shows the SEM micrograph of the resulting InN obtained by the nitridation of nanosized In₂O₃ (specific surface area of 92 m² g⁻¹), a much finer InN powder was successfully prepared by using this fine oxide powder as the starting material.

Fig. 2 SEM images of InN powders obtained from: (a) commercial In₂O₃ powder; and (b) synthesized In₂O₃ nanoparticles by nitriding the oxide powders at 600 °C for 8 h in NH₃.

Fig. 3 shows the TEM micrographs of the In₂O₃ nanoparticles, the resulting InN powders and metallic In well-dispersed in silica matrix. In₂O₃ nanoparticles were obtained by calcination of the In(OH)₃ precipitate at 450 °C for 2 h, it can be seen that the particles of In₂O₃ are uniform with a crystallite size of 10 nm (Fig. 3a). InN was obtained by the nitridation of In₂O₃ nanoparticles at 600 °C for 8 h in a flow of NH₃. However, the InN particle size, which falls in the range 50–300 nm as shown in Fig. 3b, is much larger than that of the precursor oxide. The calculated value for the particle size, based on the broadening of the XRD peak, of this InN powder is 25.6 nm, but with a relatively low surface area 8.0 m² g⁻¹. Assuming all particles are spherical, the value of the surface area corresponds to a particle size of 103 nm for InN powder. The crystallite size of InN estimated by Scherrer's equation (eqn. (1)) is much finer than that from TEM observation and BET surface area. The accuracy for determining the crystallite size based on XRD broadening is affected significantly by the uniformity of the crystallite size, the crystallinity and the size effect of the samples. As shown in Fig. 2b and Fig. 3b, the particle size is relatively widely distributed, which may be mainly responsible for the broadening of XRD peaks and the roughness of the estimation of the crystallite size of InN.

Fig. 3 TEM micrographs of: (a) In₂O₃ nanoparticles; (b) InN powder obtained by nitridation of In₂O₃ nanoparticles at 600 °C for 8 h in NH₃; and (c) metallic In well-dispersed in a silica matrix obtained by nitridation of In₂O₃/SiO₂ at 600 °C for 8 h in NH₃.

By comparison, the synthesized In(OH)₃ powder after calcination at 600 °C also for 8 h (as for the nitridation reaction) in air had a surface area as high as 68.3 m² g⁻¹, corresponding to a crystallite size of 13 nm for In₂O₃. The unusually dynamic growth of the InN powder may be interpreted according to the mechanism presented in the next section.

(c) Mechanism of the nitridation reaction

The whole reaction can be expressed as:

In₂O₃ + 2NH₃ = 2InN + 3H₂O (2)

When In(OH)₃ was used as the precursor, it decomposed into In₂O₃ and H₂O at a temperature of about 300 °C and the resulting water vapor was removed by the flowing NH₃. So, indium hydroxide was also applied successfully to synthesize InN powder as described in eqn. (2). When the In₂O₃ nanoparticles were calcined at 600 °C for 8 h in air, they still had a specific surface area as high as 68.3 m² g⁻¹, which was much higher than that of the resulting InN. However, the crystallite size of the InN powder prepared by the nitridation of the In₂O₃ nanoparticles at 600 °C for 8 h in the stream of NH₃, is widely distributed and not as uniform as the In₂O₃ nanoparticles. Indium oxide may be reduced by the H₂ derived from the decomposition of NH₃ at this temperature, the liquid metallic indium then aggregates to form large droplets before complete reaction with N₂. A possible formation mechanism for the InN particles is:

In₂O₃ + 3H₂ = 2In + 3H₂O (3)

2In + N₂ = 2InN (4)

Besides metallic indium, other low-valence oxides of indium might also occur in the reduction reaction of In₂O₃ with H₂. Low-valence oxides of indium could be nitrided with NH₃, similar to reaction (2). The fact that no In, In₂O and InO could be detected in the partially nitrided In₂O₃ powder suggests reaction (3) might be the controlling step for the formation of InN. In addition, fresh metallic indium with high reactivity on the indium oxide surface also contributed to the nitridation reaction. Eqn. (4) is in conflict with the fact that metallic In did not react with the N₂ molecules.⁶ As we know, metallic indium forms a bulk droplet above its melting point (156.61 °C) and there is only a very limited surface available for the gas–liquid reaction. Although aggregation of the liquid In nanoparticles was not avoided completely by using nanosized indium oxide or hydroxide as the precursor, much more active and much finer sites were formed during the reduction of the In₂O₃ nanoparticles. In order to verify eqn. (3), we used a confined reaction system of In₂O₃/SiO₂ to carry out the nitridation reaction. As expected, well-dispersed metallic indium rather than InN was obtained in the silica matrix. Fig. 3c shows a TEM micrograph of the indium dispersed in the silica, which was prepared by nitridation of SiO₂/In₂O₃ derived from a sol–gel process at 600 °C for 8 h. Nanosized In particles are observed clearly in high contrast due to the electron density of indium being higher than that of SiO₂. When In₂O₃/SiO₂ was used as the raw material, the reaction that proceeds from the surface to the core could be limited by the formation of a dense layer blocking the surface and hindering the diffusion of gases (N₂, NH₃) inside the large particles. The smaller molecules of H₂ can diffuse to the surface of the In₂O₃ in binary oxides easily, therefore metallic indium rather than InN was obtained in those cases. The network of silica also separated the indium particles from each other and only the particles on the surface, not those sub-surface, merged. As we know, the melting point of In is 156.61 °C, it is impossible to obtain nanosized indium particles by the reduction of In₂O₃ at a relatively high temperature without the network preventing liquid indium from aggregating. Metallic indium well-dispersed in silica has potential for application as a gentle and mild reducing agent as well as for a catalyst in some organic syntheses.²¹Fig. 4 shows the XRD patterns of the In/SiO₂ powders prepared by the nitridation of In₂O₃/SiO₂ at 600 °C for 8 h and 24 h, respectively.

Fig. 4 X-Ray diffraction patterns of well-dispersed In in a silica matrix prepared by nitridation of In₂O₃/SiO₂ at (a) 600 °C for 8 h and (b) 600 °C for 24 h. ■, In in tetragonal phase; and ○, In₂O₃ in cubic phase.

(d) Thermal stability

TG and DSC were used to investigate the thermal stability of the resulting InN powder. Fig. 5 shows the TG–DSC curves of InN powder prepared by nitridation of nanosized In₂O₃ nanoparticles at 600 °C for 8 h. InN powder was oxidized into In₂O₃ in the temperature range 389–695 °C in air with a weight increase of 8.1 wt.%, combined with a distinct exothermal peak at 475.7 °C. On the other hand, InN was decomposed into In and N₂ in a flow of N₂. The TG curve shows a marked weight loss (11.24 wt.%) in the range 595–696 °C, which is associated with the decomposition of the InN in the stream of N₂. A long tail rather than a distinct endothermal peak occurred in the DSC curve and was interpreted as being due to its relatively low binding energy. The value of weight loss from the TG analysis is slightly higher than the theoretical content of pure InN (10.86 wt.%). As mentioned in the first section, the nitrogen content of the InN powder is 10.82 wt.% with an error of ±0.5% (detected by CHN elemental analysis), which is consistent with the theoretical value of 10.86 wt.%. So, the small difference of nitrogen content determined by TG may result from an instrumental error. It is noticeable that fine InN powder starts decomposing at temperature as low as 595 °C. The result suggests that the best temperature for the preparation of crystalline InN may be around this temperature since the formation of metallic indium is avoided.

Fig. 5 TG–DSC curves of the InN powder prepared by nitridation of nanosized In₂O₃ at 600 °C for 8 h in a flow of: (a) air; and (b) nitrogen.

Conclusions

In conclusion, a novel route for the preparation of ultrafine InN powder based on the nitridation of indium oxide and hydroxide is presented. The crystalline InN powder was of 50–300 nm in size with a specific surface area of 8.0 m² g⁻¹. The quality of the resulting powder was very sensitive to the preparation parameters especially the nitridation temperature. The formation of intermediate metallic In and the aggregation of the liquid In nanoparticles were responsible for the unusually dynamic growth of the InN powders. InN commenced being oxidized into In₂O₃ at 389 °C and decomposed in the narrow temperature range 595–696 °C in flows of air and N₂, respectively. This strategy can also be applied to the synthesis of ternary group III-nitrides as well as InN in film form.

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