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In(OH)3 and In2O3 Nanorod Bundles and Spheres Subscriber access provided by SHAANXI NORMAL UNIV Inorganic Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Article In(OH)3 and In2O3 Nanorod Bundles and Spheres:  Microemulsion-Mediated Hydrotherma...

In(OH)3 and In2O3 Nanorod Bundles and Spheres
Subscriber access provided by SHAANXI NORMAL UNIV Inorganic Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Article In(OH)3 and In2O3 Nanorod Bundles and Spheres:  Microemulsion-Mediated Hydrothermal Synthesis and Luminescence Properties Jun Yang, Cuikun Lin, Zhenling Wang, and Jun Lin Inorg. Chem., 2006, 45 (22), 8973-8979• DOI: 10.1021/ic060934+ • Publication Date (Web): 05 October 2006 Downloaded from http://pubs.acs.org on April 21, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: • Supporting Information • Links to the 29 articles that cite this article, as of the time of this article download • Access to high resolution figures • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article In(OH)3 and In2O3 Nanorod Bundles and Spheres: Microemulsion-Mediated Hydrothermal Synthesis and Luminescence Properties Jun Yang, Cuikun Lin, Zhenling Wang, and Jun Lin* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China Received May 29, 2006 Indium hydroxide, In(OH)3, nano-microstructures with two kinds of morphology, nanorod bundles (around 500 nm in length and 200 nm in diameter) and caddice spherelike agglomerates (around 750−1000 nm in diameter), were successfully prepared by the cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol microemulsion- mediated hydrothermal process. Calcination of the In(OH)3 crystals with different morphologies (nanorod bundles and spheres) at 600 °C in air yielded In2O3 crystals with the same morphology. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. The pH values of microemulsion play an important role in the morphological control of the as-formed In(OH)3 nano-microstructures from the hydrothermal process. The formation mechanisms for the In(OH)3 nano-microstructures have been proposed on an aggregation mechanism. In2O3 nanorod bundles and spheres show a similar blue emission peaking around 416 and 439 nm under the 383-nm UV excitation, which is mainly attributed to the oxygen vacancies in the In2O3 nano-microstructures. 1. Introduction Semiconductor nanostructures have been attracting in- creasing attention because of 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. In2O3 is a very important wide- band-gap (direct band gap around 3.6 eV), n-type transparent semiconductor 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 H2,3,4 NO2,4 and Cl2.5 Inorganic particles often show unique size- and shape-dependent properties. For instance, the gas-sensing ability of In2O3 has been shown to increase significantly by decreasing its particle size.6 It is imaginable that size- and shape-controllable growth of In2O3 nanopar- ticles might pave the way to further elevate its performance. Up to now, various morphologies of In2O3 have been synthesized via different methods. For example, In2O3 nanobelts through thermal evaporation,7 In2O3 nanowries by using the vapor-liquid-solid technique,8 In2O3 nanoparticles prepared from thermal decomposition,9 tin-doped In2O3 whiskers formed by the electron shower physical vapor deposition process,10 In2O3 nanowire arrays,11 and nanorods12 induced by template-assisted growth have been reported. However, there are relatively few reports about the hydro- thermal synthesis of In2O3 nanostructures.13,14 In recent years, soft templates such as reverse micelles or microemulsions have been widely used as an ideal media to prepare inorganic nanoparticles.15 As the nanosized water * To whom correspondence should be addressed. E-mail: jlin@ciac.jl.cn. (1) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, 123. (2) Gagaoudakis, E.; Bender, M.; Douloufakis, E.; Kataarakis, N.; Natsakou, N.; Cimalla, V.; Kiriakids, G. Sens. Actuator, B 2001, 80, 155. (3) Chung, W. Y.; Sakai, G.; Shimanoe, K.; Miura, N.; Lee, D. D.; Yamazoe, N. Sens, Actuator, B 1998, 46, 139. (4) Liess, M. Thin Solid Films 2002, 410, 183. (5) Tamaki, J.; Naruo, C.; Yamanoto, Y.; Matsuoka, M. Sens. Actuator, B 2002, 83, 190. (6) Gurlo, A.; Ivanovskaya, M.; Barsan, N.; Schweizer-Berberich, M.; Weimar, U.; Gopel, W.; Dieguez, A. Sens. Actuators, B 1997, 44, 327. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Li, C.; Zhang, D.; Han, S.; Liu, X.; Tang, T.; Zhou, C. AdV. Mater. 2003, 15, 143. (9) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795. (10) Yumoto, H.; Sako, T.; Gotoh, Y.; Nishiyama, K.; Kaneko, T. J. Cryst. Growth 1999, 203, 136. (11) Zheng, M. J.; Zhang, L. D.; Li, G. H. Appl. Phys. Lett. 2001, 79, 839. (12) Kuo, C. Y.; Lu, S. Y.; Wei, T. Y. J. Cryst. Growth 2005, 285, 400. Inorg. Chem. 2006, 45, 8973−8979 10.1021/ic060934+ CCC: $33.50 © 2006 American Chemical Society Inorganic Chemistry, Vol. 45, No. 22, 2006 8973 Published on Web 10/05/2006 pools, they have been widely used as spatially constrained nanoreactors for controlled synthesis of nanoparticles with a desired narrow size distribution such as Ag nanodisks,16 CdS nanotriangles,17 V2O5 nanowires/nanorods,18 CaCO3 nanowires,19 and BaSO4 filaments cones.20 At the same time, the combination of microemulsion techniques with a hydro- thermal/solvothermal process has also been explored for the preparation of nanocrystals such as ZnS nanocrystallites,21 CdS nanorods,22 SbSO4 nanorods,23 SrCO3 nanostructures,24 Ca10(PO4)6(OH)2 nanofibers,25 ZnO nanowires,26 TiO2 nanoc- rystallites,27 SnO2 nanorods,28 BaF2 whiskers,29 and molecular sieve fibers.30 Enlightened by the above reports, we believe that it is reasonable to prepare In(OH)3 or In2O3 nanostructures via microemulsion-mediated hydrothermal synthesis. In this paper, we first report the synthesis of In(OH)3 nano- microstructures with different morphologies (nanorod bundles and spheres) in a cetyltrimethylammonium bromide (CTAB)/ water/cyclohexane /n-pentanol microemulsion system under hydrothermal conditions by carefully controlling fundamental experimental parameters such as the pH values of the microemulsion. The obtained In(OH)3 nanorod bundles and spheres, both consisting of individual nanoparticles with size ranging from 30 to 55 nm, can be efficiently achieved at a hydrothermal temperature as low as 140 °C. To the best of our knowledge, this is the lowest temperature reported by far for the fabrication of In(OH)3 crystals through the hydrothermal process. Calcination of the In(OH)3 crystals yielded In2O3 crystals with the same morphology. Finally, the optical properties of the resulting In2O3 nano-microstruc- tures were investigated. 2. Experimental Section Preparation. All of the chemicals used in our experiment were of analytical-reagent (A.R.) grade, purchased from Beijing Fine Chemical Co., Beijing, China. A quaternary microemulsion system consisting of cetyltrimethylammonium bromide (CTAB)/water/ cyclohexane/n-pentanol was selected for this study. The micro- emulsion was prepared by dissolving CTAB (1.0 g) in 30 mL of cyclohexane and 1.5 mL of n-pentanol. The mixing solution was stirred for 30 min. Then a certain volume of a 0.5 M In(NO3)3 aqueous solution was added to the solution. The molar ratio of water to CTAB was set at 5, and the pH value of the microemulsion was changed from 5 to 3 through the addition of drops of concentrated nitric acid. The solution was stirred for another 60 min. Then the transparent feedstock was charged into a 45-mL Teflon-lined stainless steel autoclave and heated at 140 °C for 24 h. After the autoclave was cooled to room temperature naturally, the precursors were separated by centrifugation, washing with ethanol and distilled water several times, and drying in an air atmosphere at 60 °C for 4 h. The final products were retrieved through a heat treatment at 600 °C for 2 h in air. To investigate the growing process for In- (OH)3 crystals with different morphologies, the hydrothermal reactions were stopped after designed times of 1.5, 5.5, 16, and 24 h (for nanorod bundles) or 4, 7, 16, and 24 h (for nanorod spheres), respectively. Then the products were isolated and analyzed. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (ì ) 0.154 05 nm). The morphology of the samples was inspected using a field emission scanning electron microscope (Philips XL 30) equipped with an energy-dispersive spectrometer and a transmission electron micro- scope (JEOL-2010, 200 kV). Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spec- trophotometer equipped with a 150-W xenon lamp as the excitation source at room temperature. The luminescence decay curves were obtained using a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) and a 383-nm UV laser (pulse width ) 4 ns) as the excitation source. 3. Results and Discussion 3.1. Phase Formation. Figure 1 shows the XRD patterns of the as-formed products through the hydrothermal process (a) and those after a heat treatment at 600 °C (b). The XRD peaks for as-formed samples can be indexed to a cubic lattice [space group: Im3h(204)] of pure indium hydroxide, In(OH)3, as shown in Figure 1a. The calculated lattice constant, a ) 0.7977 nm, is in good agreement with that (a ) 0.7979 nm) from the standard card (JCPDS No. 85-1338). After anneal- (13) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 147. (14) Yu, D.; Yu, S. H.; Zhang, S.; Zuo, J.; Wang, D.; Qian, Y. T. AdV. Funct. Mater. 2003, 13, 497. (15) (a) Schwuger, M.; Stickdom, K.; Schomacker, R. Chem. ReV. 1995, 95, 849. (b) Gan, L. M.; Liu, B.; Chew, C. H.; Xu, S. J.; Chua, S. J.; Loy, G. L.; Xu, G. Q. Langmuir 1997, 13, 6427. (c) Feng, P.; Bu, X.; Stucky, G. D.; Pine, D. J. J. Am. Chem. Soc. 2000, 122, 994. (16) Maillard, M.; Giorgio, S.; Pileni, M. P. AdV. Mater. 2002, 14, 1084. (17) (a) Pinna, N.; Weiss, K.; Kongehl, H. S.; Vogel, W.; Urban, J.; Pileni, M. P. Langmuir 2001, 17, 7982. (b) Pinna, N.; Weiss, K.; Urban, J.; Pilent, M. P. AdV. Mater. 2001, 13, 261. (18) Pinna, N.; Willinger, M.; Weiss, K.; Urban, J.; Schlogl, R. Nano Lett. 2003, 3, 1131. (19) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Ou, H. D.; Liu, H. Q. J. Cryst. Growth 2002, 244, 379. (20) (a) Hopwood, J. D.; Stephen, M. Chem. Mater. 1997, 9, 1819. (b) Li, M.; Stephen, M. Langmuir 2000, 16, 7088. (21) Xu, S. J.; Chua, S. J. Appl. Phys. Lett. 1998, 73, 478. (22) Zhang, P.; Gao, L. Langmuir 2003, 19, 208. (23) Xiang, J. H.; Yu, S. H.; Geng, X.; Liu, B. H.; Xu, Y. Cryst. Growth Des. 2005, 5, 1157. (24) Cao, M. H.; Wu, X. L.; He, X. Y. Langmuir 2005, 21, 6093. (25) Cao, M. H.; Wang, Y. H.; Guo, C. X. Langmuir 2004, 20, 4784. (26) Zhang, J.; Sun, L. D.; Pan, H. Y. New J. Chem. 2002, 26, 33. (27) (a) Wu, M. M.; Long, J. B.; Huang, A. H.; Luo, Y. J. Langmuir 1999, 15, 8822. (b) Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist, A. J. Phys. Chem. B 2002, 106, 10674. (28) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. AdV. Mater. 2003, 15, 1022. (29) Cao, M. H.; Hu, C. W.; Wang, E. B. J. Am. Chem. Soc. 2003, 125, 11196. (30) Lin, J. C.; Dipre, J. T.; Yates, M. Z. Chem. Mater. 2003, 15, 2764. Figure 1. XRD patterns of the as-formed sample (a) and that after annealing at 600 °C (b). The standard data for In(OH)3 (JCPDS No. 85- 1338) and In2O3 (JCPDS No. 71-2194) are also presented in the figure for comparison. Yang et al. 8974 Inorganic Chemistry, Vol. 45, No. 22, 2006 ing at 600 °C in air, the XRD pattern of the resulting sample exhibits multiple intense peaks, all of which can be perfectly indexed to the cubic phase [space group: Ia3h(206)] of In2O3, as shown in Figure 1b. The calculated lattice parameter (a ) 1.0125 nm) for In2O3, is in good agreement with the known lattice parameter for crystalline In2O3 (a ) 1.0117 nm; JCPDS No. 71-2194). The crystallite size of the samples can be estimated from the Scherrer equation, D ) 0.941ì/â cos ı, where D is the average grain size, ì is the X-ray wavelength (0.154 05 nm), and ı and â are the diffraction angle and full-width at half-maximum (fwhm) of an observed peak, respectively.31 The strongest peaks (200) at 2ı ) 22.30° and (222) at 2ı ) 30.62° were used to calculate for the average crystallite size (D) of the In(OH)3 and In2O3 particles, respectively. The estimated average crystallite sizes are about 60 nm for In(OH)3 particles and 25 nm for In2O3 particles. The synthesis reactions can be simply expressed as Under the hydrothermal process, In(OH)3 nano-micro- structures are prepared by the hydrolysis reaction of In3+ at 140 °C, as described in reaction (1). The dehydration of the In(OH)3 precursors at 600 °C results in the formation of the final product In2O3 nano-microstructures, as described in reaction (2). 3.2. Morphology Control. The pH values of the micro- emulsion have great effects on the morphology of the hydrothermal products. Figure 2a shows the SEM images of the as-formed In(OH)3 samples prepared from the micro- emulsion system with pH ) 5. It can be seen that the In- (OH)3 samples are composed of uniform nanorod bundles (around 500 nm in length and 200 nm in diameter) with an aspect ratio of about 2.5:1, and the nanorod bundles consist of individual parallel nanorods with diameters ranging from 30 to 45 nm. Every nanorod is further composed of the smaller nanosized particles with size ranging from 30 to 45 nm, basically in the same size range as those estimated by XRD. The inset in Figure 2a is the magnified image of In- (OH)3 nanorod bundles. After calcination at 600 °C, the obtained In2O3 samples retain the morphology and size of In(OH)3, as shown in Figure 2b. The chemical composition of the In2O3 nanorod bundles was investigated with energy- dispersive spectrometry (EDS; Figure 2c), which indicates an atomic ratio of In:O ) 2:2.94. The deviation from In:O ) 2:3 may be due to the experimental error of EDS experiments. The EDS result gives further support for the XRD analysis above. To further study the fine structure of the above nanorod bundles, transmission electron microscopy (TEM) was performed. Representative TEM micrographs for the In(OH)3 and In2O3 nanorod bundles are shown in parts a-d of Figure 3. As can be seen from Figure 3a, In(OH)3 nanorod bundles with a width of about 210 nm and a length of about 500 nm are obtained, which is composed of parallel nanorods with a mean diameter of 40 nm. High-resolution TEM and selected-area electron diffraction (SAED) were performed in the region of nanorod bundles as labeled in Figure 3a, and the micrographs are shown in parts b and c of Figure 3, respectively. The lattice fringes of crystalline In(OH)3 can be seen clearly (Figure 3b); the SAED images, which combine partial ring and dot patterns, indicate that the nanorod is of polycrystalline nature. Figure 3d is the TEM image of the In2O3 nanorod bundles, which is in agreement with the SEM results (Figure 2b). The average diameter of the nanorod (the subunit of nanorod bundles) of In2O3 (31) Zhang, Y. W.; Jin, S.; Tian, S. J.; Li, G. B.; Jia, T.; Liao, C. S.; Yan, C. H. Chem. Mater. 2001, 13, 372. In3+ + 3H2O f In(OH)3 + 3H+ (1) 2In(OH)3 f In2O3 + 3H2O (2) Figure 2. Field emission scanning electron micrsocopy (FE-SEM) images of the as-formed hydrothermal products of In(OH)3 (a) and the corresponding In2O3 (b) obtained from the microemulsion with pH ) 5. The inset in part a is the magnified image of an individual In(OH)3 nanorod bundle. EDS of the In2O3 nanorod bundles (c) found In and O from the sample with an atom ratio of In:O ) 2:2.94 (Au from the Au coating and Si from the Si substrate were used for measurement). In(OH)3 and In2O3 Nanorod Bundles and Spheres Inorganic Chemistry, Vol. 45, No. 22, 2006 8975 reduces to 20-30 nm compared with that of In(OH)3 in Figure 3a (mean diameter of 40 nm), indicating that the decomposition of In(OH)3 resulted in a reduction of the particle size in In2O3. There are some dark parts on the TEM image because those parts may be too thick for the electrons to penetrate. The SAED of In2O3 nanorod bundles is given in the inset of Figure 3d, whose strong diffraction rings can be indexed as (222), (400), and (440) planes of cubic In2O3. When the pH value of the microemulsion was decreased to 3, the hydrothermal product of In(OH)3 consisted of caddice spherelike agglomerates (instead of the nanorod bundles for pH ) 5), as shown in the SEM image of Figure 4a. The diameters of the spherical agglomerates of In(OH)3 are in the range of 0.75-1 ím. Clearly, the individual In- (OH)3 sphere seems to be also composed of crooked nanorods with diameters ranging from 30 to 55 nm, and each nanorod contains smaller nanosized particles with sizes ranging from 30 to 55 nm. The initial shape and size of the products are basically kept in the phase transformation from In(OH)3 to In2O3 after calcination at 600 °C (Figure 4b). Figure 4c is the TEM image of the In2O3 spheres. In agreement with the SEM results (Figure 4b), from Figure 4c we can distinctly see that there are many rods grown on the surface of the black spheres, which sustains the fact that the In2O3 spheres are composed of nanorods. These spheres are well crystallized, which can be corroborated by the SAED pattern of the sample (inset of Figure 4c). The SAED of the spheres is consistent with cubic In2O3 with strong ring patterns due to (222), (400), (440), and (622) planes. 3.3. Formation Mechanism for the In(OH)3 Nano- microstructures. In principle, crystal growth and crystal morphology are governed by the degree of supersaturation, the diffusion of the reaction, the species to the surface of the crystals, the surface and interfacial energies, 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.13 From the above experimental results, it can be seen that the pH value of the microemulsion has a significant effect on the morphologies and sizes of the final products. Furthermore, it was found that the sizes of the crystals obtained by this microemulsion-mediated hydrothermal Figure 3. TEM images of the as-formed hydrothermal products of In- (OH)3 (a) with its HRTEM (b) and SAED pattern (c) as well as the TEM image of the In2O3 nanorod bundles (d) with the SAED pattern (inset). Figure 4. FE-SEM images of the as-formed hydrothermal products of In(OH)3 (a) and the corresponding In2O3 (b) with its TEM image (the SAED pattern in the inset) of the In2O3 spheres (c) obtained from the microemulsion with pH ) 3. The inset in part a is the magnified image showing an individual In(OH)3 sphere. Yang et al. 8976 Inorganic Chemistry, Vol. 45, No. 22, 2006 process are much larger than the typical dimensions for individual microemulsion droplets (around 5-100 nm) under appropriate reaction conditions.24 Thus, it can be inferred that both the pH value of the microemulsion and aggregation and coalescence of individual nanoparticles (formed from the microemulsion droplets) are responsible for the formation of products with different morphologies mentioned in the last section. The growing process of In(
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