CRYSTAL GROWTH & DESIGN
One-Step Aqueous Solvothermal Synthesis of In2O3 Nanocrystals
2008 VOL. 8, NO. 2 695–699
Jun Yang, Chunxia Li, Zewei Quan, Deyan Kong, Xiaoming Zhang, Piaoping Yang, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China ReceiVed April 6, 2007; ReVised Manuscript ReceiVed October 9, 2007
ABSTRACT: Highly crystalline and nearly monodisperse In2O3 nanocrystals with both cube and flower shapes were successfully synthesized in one step through a facile aqueous solvothermal method for the first time, free of any surfactant or template. X-ray diffraction (XRD), transmission electron microscopy (TEM), selective area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the samples. In our work, the use of diethylene glycol (DEG) is a crucial factor for the formation of the In2O3 phase. CO(NH2)2 and a defined amount of water play important roles in the formation of the In2O3 with different nanostructures. Detailed formation mechanisms for the In2O3 phase have been proposed on the hydrolysis of indium alkoxide complexes at elevated temperature in DEG solutions, and the general growth mechanisms for the In2O3 nanostructures have been discussed. Compared with nonaqueous solvothermal synthesis of In2O3, we present a simple aqueous synthesis method by combining a hydrothermal process with a polyol method.
1. Introduction Semiconductor nanostructures have been attracting increasing attention due to their exceptional properties, which differ from those of their bulk counterparts, and their potential applications in optoelectronic devices. Among them, indium oxide (In2O3) has been investigated extensively for its semiconducting properties. Indium oxide is a very important wide band gap (direct band gap around 3.6 eV) n-type transparent semiconductor (TCO) and has been widely used in microelectronic areas including window heaters, solar cells, liquid crystal displays,1 and ultrasensitive gas sensors for detection of O3,2 CO2,3 H2,4,5 NO2,5 and Cl2.6 So, many methods were developed to prepare In2O3 nanocrystals. Traditional synthesis methods of In2O3 nanostructures include physical methods, such as the sputtering method,7 chemical vapor phase deposition (CVD),8 and soft chemical methods, such as the sol–gel method,9 the homogeneous precipitation method,10 the template method,11 and the microemulsion method.12 Recently, highly monodisperse In2O3 nanocrystals were synthesized by Fang and co-workers in a hightemperature organic solution.13 Among the above methods to prepare In2O3, the vapor phase process has a high facility request for material synthesis with lower quality of samples per batch, and most liquid phase processes for In2O3 include multiple steps with complicated flows. So a facile direct process to prepare highly crystalline and well separated In2O3 nanocrystals is desired. In recent years, the hydrothermal/solvothermal process has been widely used to prepare nano- to micrometer particles with uniform size and shape.14 It is an important technique for inorganic materials synthesis because of unity of ideal crystal growth and perfect crystallization with few or no disfigurements. So far, hydrothermal synthesis has not yielded In2O3 directly, and In2O3 can only be obtained through calcination of corresponding hydrothermal precursors, such as In(OH)315,16 and InOOH.17 One-step solvothermal synthesis of In2O3 has been reported only in nonaqueous systems (O2 and H2O < 0.1 ppm) as far as we know.18–20 At the same time, the polyol method, which is based on direct precipitation in a multivalent, high * Corresponding author. E-mail address:
[email protected].
Table 1. Summary of the Experimental Conditions and the Corresponding Denotations for the Final Samples sample
temp (°C)
time (h)
In(NO3)3 · 4.5H2O (mmol)
DEG (mL)
H 2O (mL)
CO(NH2)2 (g)
S1 S2 S3 S4
200 200 200 200
24 24 24 24
2 2 2 2
35 35 37 0
2 2 0 37
2 0 2 2
boiling alcohol (e.g., diethylene glycol) and a defined amount of water, has been successfully used to prepare a large variety of oxide materials including Cu2O, TiO2, Nb2O5, Cr2O3, ZnO, Fe3O4, etc.21–23 But there is no report about synthesis of In2O3 using this polyol-mediated process. In this paper, we combine the hydrothermal process with the polyol method and present an alternative synthesis approach to In2O3 nanoparticles based on the hydrolysis of indium alkoxide complexes at elevated temperature in solutions of the parent chelating alcohols (DEG). Furthermore, we present a study on the influences of solvent and additive on the formation of In2O3 phase and its shape and size. Despite the moderate reaction temperature of 200 °C, the as-synthesized In2O3 is highly crystalline.
2. Experimental Section Preparation. All the chemicals used in our experiment were of analytical reagent (AR) grade, purchased from Beijing Fine Chemical Company, China. In a typical synthesis, 2 mmol of In(NO3)3 · 4.5H2O and 2 g of CO(NH2)2 were added to the mixture of 35 mL of diethylene glycol (DEG) and 2 mL of H2O. The solution was stirred for 3 h. Then the transparent feedstock was charged into a 45 mL Teflon-lined stainless autoclave and heated at 200 °C for 24 h. After cooling to room temperature naturally, the products were separated by centrifugation, washed with ethanol and distilled water for several times, and dryed in atmosphere at 60 °C for 4 h. The detailed experimental parameters are listed in Table 1, and the products are denoted as S1-S4. Characterization. The phase purity and crystallinity of the samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ ) 0.15405 nm). The morphology and structure of the samples were inspected using transmission electron microscopy (TEM). Samples for TEM were prepared by depositing a drop of samples dispersed in ethanol onto a
10.1021/cg070340x CCC: $40.75 2008 American Chemical Society Published on Web 11/27/2007
696 Crystal Growth & Design, Vol. 8, No. 2, 2008
Yang et al.
Figure 1. XRD patterns of the as-formed solvothermal products of S1, S2, and S3. The standard data for In2O3 (JCPDS card 71-2194) is also presented in the figure for comparison. carbon grid. The excess liquid was wicked away with filter paper, and the grid was dried in air. Low-resolution transmission electron microscopy (TEM) and selective area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. High-resolution transmission electron microscopy (HRTEM) was performed using FEI Tecnai G2 S-Twinwere with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. All the measurements were performed at room temperature (RT).
3. Results and Discussion Phase. Figure 1 shows the XRD patterns of the as-formed solvothermal products of S1, S2, and S3 (from top to bottom). All diffraction peaks for as-formed samples can be readily indexed to a cubic lattice [space group: Ia3 (No. 206)] of pure In2O3 according to the Joint Committee on Powder Diffraction Standards (JCPDS) data card No. 71-2194. No additional peaks of other phases have been found. The calculated lattice constants using Jade 5.0, a ) 10.1319 ( 0.0015 Å for S1, a ) 10.0630 ( 0.0057 Å for S2, and a ) 10.0779 ( 0.0075 Å for S3, are well compatible with the literature value of a ) 10.1170 Å (JCPDS 71-2194). It is worth noting that in the XRD patterns of S2 and S3 the width of the peaks is much broader than that of S1, implying that the crystallite size of S2 and S3 is smaller than that of S1, which can be proven by the results of the estimated average crystallite sizes and TEM images in a later section. The crystallite size of the samples (S1, S2, and S3) can be estimated from the Scherrer equation, D ) Kλ/(β cos θ), where D is the average grain size, K is a constant (0.90), λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full-width at half-maximum (fwhm, in radian) of an observed peak, respectively.24 The estimated average crystallite sizes are about 29 nm for S1, 13 nm for S2, and 11 nm for S3. It should be mentioned that for the present samples (S1, S2 and S3), no obvious shift is observed for the diffraction peaks with respect to those of the bulk In2O3 (Figure 1), which is due to the similar lattice constants for them. Therefore, it can be said that the particle shape or particle size has little impact on the lattice dimension. Morphology. TEM was used to characterize the morphology and crystal structure of the samples. Figure 2A provides a representative TEM overview image of S1. More than 95% of the particles of the as-obtained In2O3 in S1 were shaped with a regular cube structure with a mean edge length of 20–30 nm from 100 measured particles, which is basically in the same size range with those estimated by XRD. The high-magnification image (Figure 2B) shows the nanocubes more distinctly. The
Figure 2. (A) Low- and (B) high-magnification TEM images of S1, (C) HRTEM image of a part of an In2O3 nanocube, and (D) SAED pattern. The inset in panel B shows a simulated cube enclosed with crystal faces of {001}.
HRTEM image (Figure 2C) of a part of one nanocube shows several lattice planes with perfect crystallinity. The lattice plane spacing of (200) is 0.506 nm due to the In2O3 structure for a nanocube oriented along the [001] direction.18 The TEM image of an individual square nanocube (Figure 2B) showed that the nanocube was heavily truncated and simply enclosed with crystal faces of {001} (inset in Figure 2B).15 The electron diffraction pattern (SAED) of the nanocubes is consistent with cubic In2O3 with strong ring patterns due to (222), (400), (440), and (622) planes, respectively (Figure 2D). Diethylene glycol (DEG) is a versatile, widely used, cheap, and safe chelating organic molecule for uniform nanoparticles besides being used as a solvent.21–23 In our work, CO(NH2)2, a defined amount of water, and DEG play critical roles in the formation of the In2O3 phase and nanostructures. We did a series of experiments to verify this viewpoint (Table 1). When no CO(NH2)2 was added to the solution under the same conditions, we obtained S2. Figure 3A,B shows typical TEM images of S2, which reveals that In2O3 in S2 consists of flower-like agglomerates with diameter of 55–85 nm. Clearly, the individual In2O3 flower-like agglomerate seems to be also composed of smaller nanoparticles (basically shaped as cubes) with size ranging from 6 to 8 nm (inset in Figure 3B). The HRTEM image (Figure 3C) of a part of one nanoparticle of flower-like agglomerate shows several lattice planes with high crystallinity. The lattice plane spacing of (400) is 0.253 nm. The electron diffraction pattern (SAED) of the flower-like agglomerates is consistent with cubic In2O3 with strong ring patterns due to (222), (400), (440), and (622) planes, respectively (Figure 3D). S3 was obtained without additional water. Figure 4A shows a TEM image of the corresponding In2O3 nanoparticles (halfbaked cube) in S3 with an average length of 7 nm. The HRTEM image and SAED are presented in Figure 4, panels B and C, respectively. Enlightened by the above results of S1, S2, and S3, we can control the size and shape of In2O3 through adding water and CO(NH2)2. Additionally, when no DEG (only water) was used, no In2O3 product was obtained (S4). Figure 5A shows the XRD patterns of S4. The XRD peaks of S4 can be indexed to a cubic lattice [space group Im3 (No. 204)] of pure In(OH)3 according to the JCPDS file no. 73-1810. The TEM image
Solvothermal Synthesis of In2O3 Nanocrystals
Crystal Growth & Design, Vol. 8, No. 2, 2008 697
Figure 3. (A) Low- and (B) high-magnification TEM images of S2, (C) HRTEM image of a part of an In2O3 nanoparticle, and (D) SAED pattern. The inset in panel B shows a part of a flower-like agglomerate.
Figure 5. (A) XRD patterns of S4 with the standard data for In(OH)3 (JCPDS card 73-1810) also presented in the figure for comparison and (B) TEM image of S4. The inset in panel B shows the HRTEM image of a part of an In(OH)3 nanoparticle.
Figure 4. (A) TEM image of S3, (B) HRTEM image of a part of an In2O3 nanoparticle, and (C) SAED pattern.
(Figure 5B) indicates that In(OH)3 in S4 mainly consists of cubic particles with size ranging from 150 to 230 nm. The HRTEM
image (inset in Figure 5B) of a part of one particle shows several lattice planes with high crystallinity. Therefore, the presence of DEG is a prerequisite for the formation of the In2O3 phase. The Formation Mechanism for the In2O3 Phase and Nanostructures. In previous research, hydrothermal synthesis has not yielded In2O3 directly.15–17,25,26 Yu and co-workers17 reported novel hexagonal InOOH nanofibers prepared by a controlled hydrolysis solvothermal reaction of In3+ in ether with 1 mL of deionized H2O added. Other researchers obtained different In(OH)3 nanostructures via a hydrothermal process at 200 °C,15,25,26 which was confirmed by another work of ours.16 It is well-known that In3+ may be entirety hydrolyzed to In(OH)3 because of the existence of water. In a hydrothermal system, In(OH)3 can be kept in line (In3+ + 3H2O f In(OH)3 + 3H+), while in a solvent system with a little water, In(OH)3 can be further dehydrated to form InOOH (In(OH)3 f InOOH + H2O),17 but in these aqueous systems, In2O3 cannot be directly obtained and can only be obtained after calcination of corresponding hydrothermal precursors. However, in the present work, In2O3 crystals were directly obtained in DEG with a defined amount of water under solvothermal conditions (S1, S2, and S3 in Figure 1); on the other hand, when no DEG (only water) was used, In(OH)3 product (S4 in Figure 5A) was
698 Crystal Growth & Design, Vol. 8, No. 2, 2008 Scheme 1. Proposed mechanism for indium chelated complex formation (I), hydrolysis of complexes (II), and dehydration of hydrolysates, which form indium oxide (III) in DEG solutions
obtained under the same conditions. Comparing the above results, we can know that the use of DEG is a crucial factor for the formation of the In2O3 phase. There must be a new reaction occurring in DEG with a little water instead of simple hydrolysis reaction of In3+ in water under the same conditions. We know that diethylene glycol (DEG) is a convenient reaction medium27 due to its useful physical and chemical properties.28 It can function as a complexing agent and a solvent for performing the synthesis. It remains liquid in a wide range of temperatures (-10 to 245 °C) and has a high dielectric constant ( ) 32) that enables it to dissolve polar and ionic substances. In addition, the structure of its molecule is ideal for forming chelating complexes with metals, either neutral or anionic, when its molecules are deprotonated. Application of DEG as a solvent was reported for synthesis of Fe3O4 nanocrystals by hydrolysis of chelating iron alkoxide complexes at elevated temperature,23 in which the detailed formation mechanism for oxides was discussed. Enlightened by our experimental results and the previous reports,23,27,29 we explain the formation process for the In2O3 phase (Scheme 1) as follows. Under the present experimental conditions, first, anionic chelating indium alkoxide complexes are formed (reaction I of Scheme 1); second, the hydrolysis of complexes occurs (reaction II); finally, dehydration of hydrolysates yields indium oxide (reaction III). In our work, In2O3 was directly obtained in an aqueous solvothermal synthesis route due to a new reaction mechanism occurring as proposed above compared with conventional reaction through hydrothermal treatment, and the process flow of In2O3 synthesis was shortened. In principle, crystal growth and crystal morphology are governed by the degree of supersaturation, the diffusion of the reactant species to the surface of the crystals, the surface and interfacial energy, and the structure of the crystals; that is, extrinsic and intrinsic factors, the crystal structure, and the growth surroundings are accounted for in the final morphology.15 The geometric shape of a crystal is determined directly by the external expression of a selected set of symmetry-related faces. Although the unit cell symmetry governs the spatial relations between the faces, their selection is mechanistically determined by the relative rates of growth along different crystallographic directions. In general, faces perpendicular to the fast directions of growth have smaller surface areas, and slow growing faces therefore dominate the final morphology. For example, a needleshaped crystal therefore corresponds to fast growth along one specific axis, whereas preferential growth along two directions produces a platelike morphology.30 As we know, those crystal-
Yang et al. Scheme 2. Illustration for the formation of the In2O3 with different nanostructures
lographic faces with low diffraction index are always kept in the final products.15 As the final solvothermal product, In2O3 was simply enclosed with {001} faces (S1 in Figure 2) because these faces hold the slowest growth rate and lowest surface energy. The cubical shape is consistent with the cubic crystal structure of In2O3. In the In2O3 cubic structure, the {001} family planes contain the three equivalent planes, (100), (010), and (001), which are perpendicular to the three directions [100], [010], and [001], respectively. The as-prepared In2O3 nanocrystallites grow along the three directions at an equal speed.31 Consequently, the cubic morphology of the product enclosed with crystal faces of {001} is obtained (S1 in Figure 2A,B). The preferential growth along the 〈001〉 direction can be observed by the lattice plane of (200) in Figure 2C. We performed a series of synthetic experiments by varying CO(NH2)2 and water compared with that of S1 and found some significant differences in the nanocrystal shapes and sizes, which may be explained by the proposed reaction mechanisms in Scheme 1. It is well-known that OH– can be generated in aqueous solution by hydrolysis of CO(NH2)2.32 When no CO(NH2)2 was used (S2), [OH–] in the reaction system is much lower than when CO(NH2)2 was used, so the production rate of indium alkoxide complexes is slow and the concentration of complexes is low (reaction I); as a result, the production rate of In2O3 is slow too (reactions II and III). Under these conditions, the collision rate between In2O3 crystallites is much quicker than the growth rate of a single In2O3 crystal particle in solution, and then flower-like agglomerates composed of smaller nanoparticles (S2 in Figure 3A,B) are obtained through random aggregation for minimizing the surface energy. When no additional water used (S3), there is much less water in reaction system, which comes from the raw materials In(NO3)3 · 4.5H2O and DEG. Only a small part of the anionic chelating indium alkoxide complexes have hydrolyzed due to absence of water in reaction system (reaction II). Certainly, the formation and growth of In2O3 crystal particles stop (reaction III) and S3 is realized (Figure 4A). The size of S3 is smaller than that of S1 because of insufficient hydrolysis of complexes and incomplete growth of single In2O3 crystal particle. In addition, less output of S3 also sustains our supposition. Abovementioned analyses can be simply shown in Scheme 2.
4. Conclusions In summary, In2O3 nanocrystals with both cube and flower shapes were successfully synthesized in one step via a templateand surfactant-free aqueous solvothermal reaction route for the first time, based on the hydrolysis of indium alkoxide complexes at elevated temperature in solutions of DEG. The results suggest that DEG, CO(NH2)2, and a defined amount of water play critical
Solvothermal Synthesis of In2O3 Nanocrystals
roles in the formation of the In2O3 phase and nanostructures. It is noted interestingly that only In(OH)3 cubic particles were obtained when no DEG was used. It is believed that the present aqueous solvothermal synthesis approach (combining a hydrothermal process with a polyol method) can be used to prepare some other oxides that cannot be obtained from the polyol method. The complete absence of surfactants in our reaction system is a crucial point for potential applications of In2O3 nanoparticles in ultrasensitive gas sensors, where a well-defined and easily accessible crystal surface is required. Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua”of Chinese Academy of Sciences, the MOST of China (Grant Nos. 2003CB314707 and 2007CB935502), and the National Natural Science Foundation of China (Grants 50572103, 20431030, and 00610227).
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