Synthesis of In₂O₃ nanoparticles via a green and solvent-free method

1 Introduction

Indium oxide (In₂O₃) is one kind of important n-type semiconductors with a wide direct band gap of about 3.6 eV [1], [2], [3], which has been applied in solar cells [4], lithium-ion batteries [5], gas sensors [6], [7], [8], [9], optoelectronic devices [10] and photocatalysis [11]. Up to now, In₂O₃ nanoparticles have been synthesized by several techniques, including thermal evaporation [12], sol-gel method [13], hot injection technique [14], solvothermal [15], hydrothermal route [16], thermolysis [17] and chemical vapor deposition (CVD) [18], [19]. Although the above-mentioned methods have been successfully used to synthesize In₂O₃ nanoparticles, the simple and cost-effective routes to prepare pure In₂O₃ nanoparticles by using cheap, nontoxic and environmentally starting materials are still the key issues.

In this study, we presented a green method for the facile synthesis of In₂O₃ nanoparticles. By mixing and heating the indium nitrate and ammonium bicarbonate, In₂O₃ nanoparticles were obtained. The heating drove off several additional components such as gases, leaving behind pure In₂O₃ nanoparticles. In contrast to the wet chemical method, this solvent-free synthesis can be scaled up easily, which is important for large-scale industrial production. Furthermore, the influences of ratios of reactants and annealing atmosphere on the structure, optical properties and morphology of In₂O₃ nanoparticles were investigated.

2 Materials and methods

Figure 1 shows the schematic diagram of the preparation of In₂O₃ nanoparticles by a solvent-free mechanochemical method (15 mmol, Aladdin Industrial Corporation). The process started with mixing indium nitrate hydrate (15 mmol, Sinopharm Chemical Reagent Co., Ltd) and ammonium bicarbonate (15 mmol, 22.5 mmol or 30 mmol). The molar ratios of indium nitrate to ammonium bicarbonate are determined to be 1:1, 1:1.5 and 1:2. In a typical synthesis, the required amount of nitrates and ammonium bicarbonate were mixed thoroughly and ground with a mortar and pestle for 10 min. The mixture was then baked at 400°C for 3 h under air, Ar gas flow or vacuum atmosphere to obtain In₂O₃ powder. After calcination, the color of the powder turned from white to light yellow, indicating that the resultant product is In₂O₃.

Figure 1:

Illustrative diagram of In₂O₃ nanoparticle preparation processes.

The thermal properties of the calcined grinded mixture were investigated by simultaneous thermogravimetric analysis [differential thermal analysis (DTA) and thermogravimetric analysis (TG)], which use a Rigaku 8101D MTS-9000 thermal balance in the flowing air condition (50 ml/min). The temperature range was 25–500°C, and the heating rate was 10°C/min. The crystal structure of the powders was identified by the XRD method (Rigaku, D/max-rA, Japan). The Raman measurements were performed at room temperature using a LABRAM-HR micro-Raman system in the back scatting configuration with a laser source of 514 nm. The optical absorption spectrum was recorded on a UV-Vis-365-type spectrophotometer. The nanoparticles’ morphology was investigated by transmission electron microscopy (TEM). Additionally, high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were taken in order to determine the nanocrystal structure.

3 Results and discussion

3.1 Solvent-free synthesized mechanism of In₂O₃

Most In₂O₃ nanoparticles were prepared by a wet chemical method (e.g. thermal hydrolysis [20], solvothermal calcination [21]). In these approaches, precipitate nanoparticles formed from ions in liquid and water molecules which could trap inside causing agglomeration [22]. Moreover, the drying of the precipitate also led to a partial re-agglomeration of particles; such an irreversible agglomeration cannot be thoroughly eliminated although using mechanical grinding. So a solvent-free mechanochemical method was employed to solve the above problems. This process will minimize the agglomeration. The In₂O₃ nanoparticles were prepared via a solvent-free solid-state reaction as follows:

6NH4HCO3+2In(NO3)3⋅4.5H2O→Grind 10 min 2In(OH)3+ 6NH4NO3+6CO2(gas)+9H2O2In(OH)3→Bake at 380∘CIn2O3+3H2O NH4NO3→Bake at 380∘CNH3+NO2(gas)+N2O(gas)+H2O

It is known that a conventional solid-state reaction under ambient conditions is unfavorable, because the ionic diffusion in solid state is difficult. Therefore, the grinding is used to energize the reactant to make the reaction possible. On the other hand, the hydration of nitrate plays an important role in the synthetic process, where it acts as a reaction zone for the formation of oxide nanocrystalline. As surface nitrates are dissolved in the hydration, they have high mass-transfer rates. As a result, the solid-state reaction is accelerated, which controls the nucleation and growth of nanoparticles to improve particle size uniformity. The non-agglomerated In₂O₃ nanoparticles are obtained due to the reaction carried out in a water-free system. Mixing indium nitrate and ammonium bicarbonate produced ammonium nitrate (see XRD data of Figure 2A and B) and indium hydroxide (see XRD data of Figure 2C), with excess carbon and oxygen bubbling off as CO₂ (see the reaction equation above). Finally, the mixture was baked. The heating drove off several additional components such as gases, leaving behind pure oxide nanoparticles. XRD patterns of the resulting nanoparticles are shown in Figure 2D. All the X-ray peaks were indexed, which confirmed the formation of In₂O₃ nanoparticles [23].

Figure 2:

XRD patterns of the (A) mixture after grinding 5 min, (B) mixture after grinding 10 min, (C) In(OH)₃ after removing NH₄NO₃ and (D) mixture baked at 400°C.

TG/DTA analysis has been taken to study the phase transformation of the grinded mixture. Figure 3 shows TG/DTA of the mixture recorded in static air atmosphere from ambient temperature to 500°C. Five endothermic peaks were observed in the DTA curve. The weak endothermic peak at 52°C is due to the decomposition of the excess ammonium bicarbonate. Two strong endothermic peaks were noticed at 126 and 163°C, which can be assigned to the decomposition of ammonium nitrate. An abrupt weight loss was observed from 219 to 277°C, which is due to the dehydration of indium hydroxide to indium oxide. The DTA curve showed two endothermic reactions that can be attributed to the transformation of In(OH)₃ to InOOH and InOOH to In₂O₃, respectively [24]. These results are consistent with the results reported by Ho and Yen [25]. The dehydration process is as follows:

Figure 3:

TGA and DTA of the diagram of the grinded mixture.

In(OH)3→In OOH+H2O(gas)2In OOH→In2O3+H2O(gas)

3.2 The influences of ratios of reactants and annealing atmosphere on the crystallite sizes, optical properties and morphological of In₂O₃

Figure 4 shows the XRD patterns of In₂O₃ prepared by annealing in air with the different molar ratios of indium nitrate to ammonium bicarbonate: (a) 1:1, (b) 1:1.5, (c) 1:2 and annealing in (d) Ar and (e) vacuum. All the X-ray peaks were indexed, which confirmed the formation of In₂O₃ nanoparticles despite using different ratios of reactants and annealing atmosphere. The full width at half maximum (FWHM) of the In₂O₃ (222) XRD peak was enclosed, as shown in Figure 4. The FWHM of the In₂O₃ (222) XRD peak decreased and its intensity increased, while the ratios of nitrate to ammonium decreased, as shown in Figure 4. This implies an increase in the crystallite sizes as the excess ammonium is used. It may be due to the fact that the growth rate of nanocrystalline will be accelerated when the more ammonium is used. On the other hand, the nitrate and ammonium cannot mix at molecular level by mechanical grind. The solvent-free solid-state reaction will be partly undone when the nitrate reacts with ammonium. So the ammonium in excess is necessary for the full reaction. But the crystallite size increased slowly when the molar ratios of nitrate to ammonium exceeded 1:1.5 (Figure 4B and C). Therefore, the molar ratios of nitrate to ammonium were fixed at 1:1.5. Meanwhile, the crystallite size was almost invariable when the annealing atmosphere was replaced with Ar or vacuum atmosphere (Figure 4D and E).

Figure 4:

XRD patterns of In₂O₃ prepared by annealing in air with the different ratios of nitrate to ammonium bicarbonate: (A) 1:1, (B) 1:1.5, (C) 1:2 and annealing in (D) Ar and (E) vacuum.

Room temperature Raman spectra of In₂O₃ nanoparticles prepared by different processing parameters are shown in Figure 5. All the Raman spectra showed a very low frequency peak (around 131 cm^-1), two low frequency peaks (around 307 and 369 cm^-1) and two high frequency peaks (around 496 and 628 cm^-1), corresponding to the Raman modes of vibration of a bcc In₂O₃. The observed results are in good agreement with the reported values of bcc In₂O₃ [26], [27], [28].

Figure 5:

Raman spectrum of In₂O₃ prepared by annealing in air with the different ratios of nitrate to ammonium bicarbonate: (A) 1:1, (B) 1:1.5, (C) 1:2 and annealing in (D) Ar and (E) vacuum.

Figure 6 shows the optical absorption spectra of In₂O₃ nanoparticles prepared by different processing parameters. It can be seen that the absorption edge shifts toward the longer wavelengths with the increase of ammonium, due to the increase of particle size. It may attribute to the quantum confinement effect. It is well known that the semiconductor nanoparticle energy gap increases with decreasing grain size, which leads to a blue shift of the optical absorption edge and has been observed in a system of many semiconductor nanoparticles [29]. On the other hand, the red shift of the optical absorption spectra edge can be observed, induced by annealing under different atmospheres. In our work, In₂O₃ nanoparticles were formed by thermolysis of In(OH)₃. There are oxygen vacancies at the surface of the In₂O₃ nanoparticles during the annealing process. It has been reported that the concentration of oxygen vacancy seriously depends on the annealing atmosphere [30]. When the air annealing is replaced by the Ar annealing, the concentration of the oxygen vacancy increases. This can be explained by the fact that when oxygen is absent in the annealing atmosphere, some of the lattice oxygen may be released in the form of O₂ gas, thus leaving oxygen vacancies. Meanwhile, the oxygen concentration in the vacuum system is lower than that in the Ar system, due to the high pumping speed. This induces the higher concentration of oxygen vacancies in the In₂O₃ nanoparticles annealed in vacuum. It is known that oxygen vacancies induce the formation of new energy levels in the band gap and as a result the red shift may be formed [31], [32], [33]. Therefore, with decreasing oxygen concentration of annealing atmosphere (air > Ar > vacuum), the concentration of the oxygen vacancy in In₂O₃ nanoparticles increases, which results in a larger red shift of the optical band gap.

Figure 6:

UV-Vis spectrum of In₂O₃ prepared by annealing in air with the different ratios of nitrate to ammonium bicarbonate: (A) 1:1, (B) 1:1.5, (C) 1:2 and annealing in (D) Ar and (E) vacuum.

The morphology and structure of the In₂O₃ nanoparticles were investigated by TEM. Figure 7 shows the TEM of In₂O₃ nanoparticles prepared by annealing under air with the different ratios of nitrate to ammonium bicarbonate: (a) 1:1, (b) 1:1.5, (c) 1:2. With regard to the morphology, the powders prepared by the stoichiometric reaction are random spherical (Figure 7A). On the contrary, the powders prepared with excess ammonium are irregular in shape (Figure 7B and C). It can be clearly seen that in addition to spherical particles, nanorods are also observed. The changing morphology of In₂O₃ particles can be attributed to different reaction rates. Initially, nucleation results in the formation of small spherical nanoparticles. As the particles grow larger, with the excess ammonium used, some particle is in adhesion of neighboring nanoparticles, which finally leads to the formation of larger crystalline nanorods [34].

Figure 7:

TEM of In₂O₃ nanoparticles prepared by annealing in air with the different ratios of nitrate to ammonium bicarbonate: (A) 1:1, (B) 1:1.5, (C) 1:2.

Figure 8 shows the TEM (i), HRTEM (ii) micrographs and SAED pattern (iii) of In₂O₃ nanoparticles prepared by annealing under (a) Ar and (b) vacuum. Unlike the In₂O₃ nanoparticles prepared under air, the In₂O₃ nanoparticles prepared under both Ar and vacuum have well-defined spheroidal shapes (Figure 8a(i) and b(i)). Caruntu et al. [35] revealed that the morphological features of In₂O₃ nanoparticles can be tuned upon changing the reaction atmosphere, which is attributed to the different growth kinetics. The HRTEM image of In₂O₃ nanoparticles presented in Figure 8a(ii) and b(ii) reveals that the particles are monocrystalline in nature. The nanocrystal exhibits lattice fringes with interplanar distances of around 0.29 nm, corresponding to the (222) lattice plane of cubic In₂O₃. Figure 8a(iii) and b(iii) shows a typical SAED pattern taken from In₂O₃ nanoparticles prepared under Ar and vacuum, respectively. The pattern reveals five diffraction rings with a spotted appearance, which is indicative of the high crystallinity of the as-prepared In₂O₃ nanoparticles. The SAED of the nanoparticles is consistent with the cubic structure of In₂O₃ featuring strong ring patterns assigned to the (211), (222), (400), (440) and (622) planes (Figure 8iii) [36]. The interplanar spacings (d_hkl) calculated from the electron diffraction pattern are in good agreement with the X-ray diffraction data (JPCDS file no. 006-0416).

Figure 8:

TEM (i), HRTEM (ii) micrographs and SAED pattern (iii) of In₂O₃ nanoparticles prepared by annealing in (a) Ar and (b) vacuum.

4 Conclusions

In summary, we have successfully demonstrated the synthesis of In₂O₃ nanoparticles by a novel and easy solvent-free reaction. Nanoparticles are formed after simple grind and bake of the mixture of indium nitrate and ammonium bicarbonate. The influences of ratios of reactants and annealing atmosphere on the structure, optical properties and morphology of In₂O₃ nanoparticles were investigated carefully. It was found that the optical absorption band edge of In₂O₃ nanoparticles exhibits a blue shift with the decrease of ammonium bicarbonate, due to the decrease of particle size. On the other hand, with the decrease of the oxygen concentration of annealing atmosphere (air > Ar > vacuum), the concentration of the oxygen vacancy increases, which results in a larger red shift of the optical band gap. This is attributed to the oxygen vacancy in In₂O₃ nanoparticles.