Lotus-Root-Like In2O3 Nanostructures: Fabrication, Characterization

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J. Phys. Chem. C 2007, 111, 13398-13403

Lotus-Root-Like In2O3 Nanostructures: Fabrication, Characterization, and Photoluminescence Properties Cuiqing Wang, Dairong Chen,* Xiuling Jiao,* and Changlong Chen School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, P.R. China ReceiVed: April 25, 2007; In Final Form: July 9, 2007

Novel lotus-root-like In2O3 nanostructures with a diameter of ca. 300 nm and a length of 1.5-4.0 µm have been prepared by annealing In(OH)3 nanostructures with the same morphology derived from a mild solution reaction. The hierarchical nanostructures are composed of several segments aggregated orderly from In2O3 nanorods with the length of 50-90 nm and diameter of 15-40 nm. The as-prepared products were characterized by X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectra (XPS) in detail. Furthermore, the effects of reaction parameters on the formation of nanostructures were also discussed, and the hydrolysis and oriented-aggregation mechanism was proposed. Room-temperature photoluminescence (PL) spectra of the In2O3 nanostructure showed the peculiar visible emission from the blue to orange region, with the strongest peak centered at 468 nm.

1. Introduction It is well-known that the inorganic nanomaterials exhibit sizeand shape-dependent properties, and much effort has been made to control the synthesis of nanomaterials on shape, size, dimensionality, and structures.1 Particularly, the synthesis of three-dimensional (3D) micro-/nanostructures with hierarchical or complex structures has been an exciting field because of their peculiar properties and extensive applications. The hierarchical nanostructures built from nanocells, such as nanoparticles,2 nanorods/wires/belts,3 and nanoplates/disks/sheets,4 and which exhibit unique physical and chemical properties different from those of nanocells, have been widely investigated. Many hierarchical nanostructures including metal,5 oxides,6 sulfides and ternary chalcogenides,7 hydrates,8 etc.9 have been successfully fabricated. However, to find and investigate the novel nanoarchitectures or hierarchical nanostructures for some functional compounds is still an interesting task not only in answering basic research questions but also in technological applications. As an important n-type III-VI semiconductor with a wide band gap of ca. 3.6 eV, indium oxide (In2O3) exhibits novel electronic and optical properties. Therefore, it has extensive applications in electrooptic modulators, low emissive windows, solar cells, electrochromic windows in dissipating static electricity from the windows on xerographic copiers, optoelectronic devices, gas sensors, flat panel display materials, and lightemitting diodes.10 Many approaches have been developed to prepare In2O3 with different morphologies, such as nanocrystals and nanodots,11 nanofilms,12 nanoribbons and nanobelts,13 nanowires/fibers,14 nanotubes,15 nanocrystal chains,16 and nanocubes/pyramids.17 In the 1990s, Matijevic et al. prepared the spindly In2O3 microparticles by calcining the hydrolysis product of indium nitrate.18 Recently, Peng’s group fabricated the In2O3 nanocrystals to form three-dimensional (3D) nanoflowers using indium carboxylates as the precursors with or * Corresponding author. Telephone: +86-0531-88364280. Fax: +860531-88364281. E-mail: [email protected].

Figure 1. XRD patterns of In(OH)3 precursors after (a) and before (b) calcinations.

without ethanol as the activating reagent in a hydrocarbon solvent under elevated temperatures,19a and Lin and co-workers prepared In(OH)3 nanorod bundles and caddice sphere-like agglomerates by use of the cetyltrimethyl-ammonium bromide (CTAB)/water/cyclohexane/ n-pentanol microemulsion-mediated hydrothermal process and calcined them to obtain In2O3 nanocrystals with the same morphologies.19b Herein, fabrication of a novel lotus-root-like In2O3 hierarchical nanostructures through a simple solution route followed by a calcining process was introduced. First, the lotus-root In(OH)3 nanostructures were synthesized by a simple hydrolyzation and precipitation process of InCl3 in the mixture of deionized water, ethanol, and formide at low temperature. Second, the In(OH)3 nanostructures were calcined in air to transform into In2O3 powders with the same morphology. The products were characterized by XRD, SEM, TEM, FT-IR, TG, and XPS techniques, and the PL spectrum of In2O3 nanostructures was measured. Furthermore, the effects of reaction parameters on the formation of nanostructures and their formation mechanism were also discussed. One object of this research is to find a new hierarchical nanostructure and investigate its formation process and mechanism. The other object is to understand what effects the structures and morphologies have on the physical and chemical properties of the functional compounds.

10.1021/jp073177p CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

Lotus-Root-Like In2O3 Nanostructures

Figure 2. FE-SEM (a) and TEM (b) images of In2O3 nanostructures. (Inset, a) High magnification FE-SEM image. (Inset, b) Enlarged TEM image.

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13399 a copper grid for further analysis. TG analysis was employed to evaluate the weight loss of the sample under an air flow of 20.0 mL·min-1 and a heating rate of 20.0 °C·min-1 using a thermal analyzer (TGA/SDTA, 851e METTLER). FT-IR spectra were recorded on a Nicolet 5DX-FTIR spectrometer using KBr pellet method in the range of 400-4000 cm-1. XPS spectra were recorded on a PHI-5300 ESCA spectrometer (PerkinElmer) with its energy analyzer working in the pass energy mode at 35.75 eV, and the Al KR line was used as the excitation source. The binding energy reference was taken at 284.7 eV for the C1s peak arising from surface hydrocarbons. 2.3. PL Measurements. The PL spectrum of In2O3 nanostructures was measured by a Hitachi M-850 fluorescence spectrophotometer at room temperature under ambient atmosphere. The In2O3 powders were pressed into the thin slices, and the excitation wavelength was 370 nm. 3. Results and Discussion

Figure 3. HR-TEM image (a), and SAED pattern (b) of In(OH)3 nanostructures.

2. Experimental Section 2.1. Synthesis. All reagents were analytical grade and were used without further purification. In a typical synthesis, 0.4 g (1.8 mmol) of InCl3 was dissolved into 10.0 mL of deionized water, forming a colorless solution A. 2.0 mL (34.0 mmol) of formide, 2.0 mL (50.0 mmol) of ethanol and 6.0 mL of deionized water were mixed to form another colorless solution B. The solutions A and B were mixed rapidly to form a clear solution, and this solution was aged at 80 °C for 1.0 h to produce the white colloid precipitate. The precipitate was collected by centrifugation, washed with deionized water and ethanol for several times, and then calcined in a boat crucible at 450 °C in air for 2.0 h to form the lightyellow In2O3 powders. 2.2. Characterization. The crystal structure of the product was determined from the X-ray diffractometer (Rigaku D/Max 2200PC) with a graphite monochromator and Cu KR radiation (λ ) 0.15418 nm) in the range of 20-70° at room temperature while the tube voltage and electric current were held at 40 kV and 20 mA. The morphology and microstructure of the products were determined by field emission SEM (FE-SEM, JSM-6700F), TEM (JEM-100CXII) with an accelerating voltage of 80 kV, and high-resolution TEM (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV. For HR-TEM observations, the samples were sliced off with a diamond knife and attached to

Figure 4. XPS of In2O3 nanostructures.

3.1. Morphology and Structure of the Products. The XRD pattern of the product (Figure 1a) shows its nature of cubic In2O3 (JCPDS no. 06-0416), and the peaks indicate its good crystallinity. The main peaks on the XRD pattern of the product before calcination (Figure 1b) match with those of In(OH)3 (JCPDS no. 85-1338), showing that most of the precursors are cubic In(OH)3 phase. A few other peaks indicate that there is a little impurity in the In(OH)3 precursor, which can be indexed to the indium hydroxide chloride. The FE-SEM image shows that the In(OH)3 precursor exhibits lotus-root-like morphology, and the corresponding TEM image clearly demonstrates that the nanostructures have a diameter of ca. 300 nm and a length of 1.5-4.0 µm, and one lotus root is composed of several segments (Supporting Information, Figure S1). After calcination, the lotus-root morphology still remains, and the length and diameter do not exhibit obvious changes when compared with those of In(OH)3 precursor (Figure 2). For the In2O3 nanostructures, one segment is built from a large number of In2O3 nanorods with length of 50-90 nm and diameter of 15-40 nm by regular aggregation (the inset in Figure 2a). The enlarged TEM image (the inset in Figure 2b) further shows that these nanostructures are not surface-smooth and some nanorods are conglutinated on their surface. The experiments demonstrate that the In2O3 nanostructures are sufficiently stable and cannot be destroyed even after lengthy ultrasonication. The HR-TEM image confirms that the In(OH)3 and nanostructures are crystalline. The lattice-fringe image of In(OH)3 reveals the calculated planar space of 0.291 nm of the {220} plane in cubic In(OH)3 (Figure 3a). It can also be seen from Figure 3a that the segments of the lotus-root-like In(OH)3 nanostructure are composed of nanoparticles with the size of ca. 10 nm and a small misorientation exists in the interface (denoted by the arrows) although the planar fringes are ordered

13400 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Figure 5. In situ XRD patterns of In(OH)3 nanostructures calcined at different temperatures.

in the particles, which implies an oriented aggregation growth mechanism.9e In addition, the corresponding selected area electron diffraction (SAED) patterns (Figure 3b) indicate that the lotus-root nanostrutures grow along the [110] direction of cubic In(OH)3 (denoted by the arrow) and the elongated diffraction spots result from the small misorientation deviating from perfect alignment between nanocrystals.6h The XPS analysis of In2O3 nanostructures exhibits the characteristic spin-orbit splits of In3d5/2 and In3d3/2 signals (Figure 4a), and the O1s peak (Figure 4b). The observed O1s peak at 529.9 eV can be assigned to the lattice oxygen in crystalline In2O3. Moreover, with the calcination temperature raising, the concentration of O atoms in the sample gradually decreases, and that of In atoms increases (Supporting Information, Table S1), indicating the transformation from In(OH)3 to In2O3. The TG curve of In(OH)3 precursor (Supporting Information, Figure S2) shows a weight-loss of ca. 17.63%. This matches with the theoretical weight-loss (16.27%) of transformation from In(OH)3 to In2O3 within the error. The IR spectrum (Supporting Information, Figure S3) demonstrates the adsorption of In-O vibration at 500 cm-1 and the stretching vibration absorption of hydroxyls at 3200 cm-1. There are no bands between 3000 cm-1 and 2500 cm-1, indicating no organics on the particle surface. When the In(OH)3 precursor is calcined at 450 °C, only the adsorption of In-O vibration in the range of 500-600 cm-1 exists. The adsorption around 3450 cm-1 should be assigned to a small amount of adsorbed water. In order to determine the temperature of phase transformation from In(OH)3 to In2O3, the in situ XRD patterns of In(OH)3 precursor at different temperatures were measured. As shown in Figure 5, the In(OH)3 nanostructures at 250 °C are still at the In(OH)3 phase, but the peaks of indium hydroxide chloride are obviously decreased. It can be concluded that a temperature higher than 280 °C is necessary for complete phase transformation from In(OH)3 to In2O3. 3.2. Fabrication of In2O3 Nanostructures. SEM and TEM observations show that the lotus-root-like nanostructures remain during the phase transformation from In(OH)3 to In2O3. Thus, the formation process of In(OH)3 hierarchical nanostructures is studied to understand its fabrication mechanism. First, the effects of additives and reaction parameters on the fabrication of In(OH)3 nanostructures were investigated. It was

Wang et al. found that as the reaction conditions were the same as those in the typical experiment save that formide was absent, the reaction could not occur to produce In(OH)3, even with heating for 1.0 h. Moreover, by using ethylenediamine or tri-ethylamine as the additive, only nanoflakes were found (Supporting Information, Figure S4). When ammonia was used as additive, the precipitate was nanorod aggregate, whereas urea resulted in the formation of rectangular aggregates. These results indicated that the formide should be responsible for the fabrication of lotus-rootlike In(OH)3 nanostructures. As a comparison, the morphology and structures can be controlled by adjusting the volume ratio of ethanol to water in solution B while keeping other reaction parameters constant. The product was 1.5 µm spindle-like aggregates in absence of ethanol (Figure 6a), and each spindle was also made of nanosized subunits. With the volume ratio of 1/3-3/1, the lotus-root-like nanostructures could be obtained (Figure 6b,c). Further experiments showed that longer particles with similar morphology could be formed (Figure 6d) even in the absence of water. Thus, the presence of a suitable quantity of ethanol in solution B is crucial for the fabrication of lotusroot nanostructures. When water was replaced by ethanol in solution A, only irregular In(OH)3 particles were obtained (Figure 6e). For the formation of the In(OH)3 precursor, it is necessary to heat the reaction solution up to a designed temperature. The experiments showed that as the reaction temperature was 50 °C and other parameters were the same as those in the typical experiment, no precipitate was produced even when the reaction time was as long as 1.0 h. However, when the temperature was up to 60 °C, the In(OH)3 product was formed after heating for 30 min. With the temperature increasing, the required time for producing the In(OH)3 precipitate would decrease. For example, as the reaction temperature was 100 °C, 5 min was long enough to form the In(OH)3 precipitate. The TEM images show that the new precipitate comprises the irregular aggregates, and the corresponding XRD pattern indicates (Supporting Information, Figure S5) the cubic In(OH)3 nature with a little impurity. To track the fabrication process of lotus-root-like In(OH)3 nanostructures, a detailed time course was studied of the typical synthesis. After the reaction proceeded for a designed time, the solution was rapidly cooled to room temperature, and the precipitate was collected by centrifugation and washed with deionized water and ethanol. As shown in Figure 7, after the solution heated for 4 min, only a weak peak at 22.2° appeared on the XRD pattern, corresponding to the (200) plane of cubic In(OH)3, whose intensity increased, and other peaks also appeared gradually with the time prolonging. When the reaction was conducted for 1.0 h, the well-crystalline In(OH)3 was formed. In fact, as the reaction proceeded for 2 min, the particles would be produce although the solution was still transparent, but the quantity of the product was too small to record the XRD pattern. The collected product was composed mainly of the irregular particles with the size of ca. 10 nm accompanied by a few elliptical particles with the size of ca. 20 nm (Figure 8a). After 4 min, the small elliptical particles formed (Figure 8b),

Figure 6. TEM images of products prepared under different volume ratio of ethanol to water, (a) 0, (b) 1/3, (c) 3/1, and (d) no water in solution B, (e) no water in solution A.

Lotus-Root-Like In2O3 Nanostructures

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13401 reaction systems were sealed or opened completely, the OHconcentration of solution was too large or small to form the hierarchical nanostructures.

Figure 7. XRD patterns of the products as the reaction conducted at 80 °C for different times, (a) 4 min, (b) 8 min, (c) 12 min, (d) 30 min, and (e) 1.0 h.

which were aggregated from the irregular particles in a very short time. With the reaction proceeding, the elliptical particles kept on growing at the cost of In3+ cations residing in the solution and grew to the large ones with a size of 500 nm in the short direction 8 min later (Figure 8c). However, as the reaction proceeded, the aspect ratio of the particles gradually increased, and only spindles with the diameter of 500 nm were found with the time over 16 min, and some of them began to join together (Figure 8d). Finally, the lotus-root-like nanostructures were formed during the following 30 min, while the size of spindles composing the lotus-root nanostructures hardly changed (Figure 8e). On the basis of the above experimental results, the formation process of In(OH)3 nanostructures can be illustrated as in Figure 9. In the initial stages, when the solution was heated, the In3+ cations hydrolyzed to form the indium hydroxide colloid particles due to the decomposition of formide at high temperature, and the as-formed amorphous nanoparticles subsequently aggregated to small elliptical particles to minimize their surface energy, which can be revealed by the HRTEM observation (Figure 3a). At the same time, the transformation from the amorphous indium hydroxide to cubic In(OH)3 occurred. With the reaction proceeding, grain growth was carried out with the consumption of the reactants in the solution, the particles became larger and larger, and the spindles with the size of 500 nm were formed after the reaction was conducted for 30 min. At this stage, the grain growth exhibited anisotropy due to the anisotropy of the crystallographic structure. When the reactants were consumed completely, the grain growth almost stopped. Then the oriented aggregation along [110] direction started due to the large surface energy of (110) planes, as a result, the lotusroot-like In(OH)3 nanostructures were formed. However, it was also found that if the experiments in a completely sealed reactor or open to the air were made with other reaction parameters kept constant, larger irregular spindles in stead of lotus-root nanostructures were obtained (Supporting Information, Figure S6). It was proposed that formide would decompose to produce OH- anions, and the concentration of OH- anions was crucial for the formation of lotus-root-like structures. When the reaction system was partially exposed to the environment, the OHconcentration could be controlled at a moderate value, and the lotus-root In(OH)3 nanostructures were formed. But as the

3.3. PL Spectrum. The PL spectrum of In2O3 nanostructures at room temperature exhibits three peaks centered at 468, 551, 632 nm and a shoulder at 445 nm in the visible light region (Figure 10). The strongest peak centered at 468 nm locates in the blue-green region, and the weak peak centered at 632 nm locates in the orange region. To further identify the peaks on the PL spectrum, PL property of the In2O3 nanostructure was determined with another excitation wavelength of 244 nm (Supporting Information, Figure S7). As a result, the peaks at 468 and 551 nm also existed with the same emission wavelengths, although their intensities decreased, which indicates these two peaks are PL peaks but not due to Raman scattering. The PL properties of In2O3 nanostructures with different morphologies given in Table 1 indicate that the bulk of In2O3 exhibit no PL emission.20a Nanocrystalline In2O3 shows PL emission at different wavelengths, indicating that the PL property of In2O3 might be affected by its dimensionality, shape, and size. As shown in Table 1, the PL emission spectra of In2O3 nanoparticles are different due to the various parameters and additives.11b,d However, In2O3 nanowires by the vapor-solid method14e and those by thermal vapor oxidation14b have the same emission spectra although the synthetic parameters and reagents are different. Comparing with the reports, the lotus-root nanostructures exhibit the characteristic PL emission spectrum. As an n-type semiconductor, the cubic In2O3 has a oxygen-deficient fluorite structure with twice the unit-cell edge of the corresponding fluorite cell and with 1/4 of the anions missing in an ordered way.12a,b,14b The blue PL emission of In2O3 nanostructures is mainly attributed to the existence of oxygen vacancies. The oxygen vacancies related to the preparing process can act as donors and induce the formation of new energy levels in the band gap, which results in PL emission under photoexcitation. As a result, the emission can be attributed to the radioactive recombination of a photoexcited hole with an electron occupying the oxygen vacancies.17a It is reported that the point defects, dislocations, and slabs of structurally distinct material can be created as a consequence of oriented aggregation growth mechanism.21 Here, the aggregation of the indium hydroxide colloid particles and the oriented aggregation of the nanorods as well as the calcination process17a,19b might induce the formation of different oxygen vacancies; as a result different PL emission peaks are determined. However, the PL emission spectrum of In2O3 nanostructures also shows a peak at 632 nm in an orange region, which shifts to 619 nm with the excitation wavelength changing to 244 nm and is only found in the PL spectrum of In2O3 films.20c Although more information including theories and experiments are needed to understand the PL spectra of nanocrystalline In2O3, the peculiar visible PL emission from the blue to orange region of the lotus-root In2O3 nanostructures must be due to its novel hierarchical nanostructures.

Figure 8. TEM images of the products as the reaction conducted at 80 °C for different times, (a) 2 min, (b) 4 min, (c) 8 min, (d) 16 min, and (e) 1.0 h.

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Wang et al. Acknowledgment. This work was supported by Program for New Century Excellent Talents in University, P. R. China. Supporting Information Available: FE-SEM image and TGA curve of In(OH)3, IR spectra of In(OH)3 after calcinations at different temperatures, TEM images of In(OH)3 prepared using different additives, XRD patterns and TEM image of the product after reaction at 100 °C for 5 min, the PL spectrum of the In2O3 nanostructure with the excitation wavelength of 244 nm. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Proposed formation process of the In(OH)3 nanostructures.

References and Notes

Figure 10. PL spectrum of lotus-root In2O3 nanostructures. Excitation wavelength: 370 nm.

TABLE 1: PL Properties of In2O3 Nanostructures with Different Morphologies

samples nanoparticles11b nanoparticles11d nanofibers14b nanowires14e nanowires14f nanowires embedded20b films20c nanocubes17a bulk20a

preparative methods thermal decomposition thermal decomposition thermal vapor oxidation method vapor-solid method electro-depositionoxidizing alumina template technology deposition and thermal oxidation hydrothermal treatment -

excitation wavelength (nm)

emission wavelength (nm)

234 275

423, 392 (shoulder) 325, 330, 332

250

470

382

470

250 274

425, 429, 442, 460 398

364

637

380

450

-

no

4. Conclusions Lotus-root-like In(OH)3 hierarchical nanostructures have been successfully fabricated by a mild solution method. The reaction parameters including the ratio of water to ethanol, reactant concentration, and temperature show the significant effects on the formation of lotus-root-like nanostructures. The formide is crucial in the assembly of lotus-root-like structures. On the basis of the experiments, a hydrolysis and oriented-aggregation mechanism for the fabrication of lotus-root In(OH)3 nanostructures is proposed. After calcination, the In(OH)3 nanostructures with diameter of ca. 300 nm and length of 1.5-4.0 µm can be transformed into In2O3 nanostructures without changing of the morphology. The lotus-root-like In2O3 nanostructures are composed of several smaller segments which are fabricated by the oriented aggregation of nanorods with length of 50-90 nm and diameter of 15-40 nm. The PL emission spectrum of lotusroot-like In2O3 exhibits the peculiar visible emission centered at 445, 468, 551, and 632 nm.

(1) (a) Service, R. F. Science 2005, 309, 95. (b) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (2) (a) Ewers, T. D.; Sra, A. K.; Norris, B. C.; Cable, R. E.; Cheng, C. H.; Shantz, D. F.; Schaak, R. E. Chem. Mater. 2005, 17, 514. (b) Yang, H.; Zeng, H. Angew. Chem., Int. Ed. 2004, 43, 5930. (3) (a) Chen, X.; Wang, X.; Wang, Z.; Yang, X.; Qian, Y. Cryst. Growth Des. 2005, 5, 347. (b) Mao, Y.; Kanungo, M.; Hemraj-Benny, T.; Wong, S. S. J. Phys. Chem. B 2006, 110, 702. (c) Cao, A.; Hu, J.; Liang, H.; Wan, L. Angew. Chem., Int. Ed. 2005, 44, 4391. (4) (a) Mo, M.; Yu, J.; Zhang, L.; Li, S.-K. A. AdV. Mater. 2005, 17, 756. (b) Cheng, Y.; Wang, Y.; Zheng, Y.; Qin, Y. J. Phys. Chem. B 2005, 109, 11548. (c) Zhong, L.; Hu, J.; Liang, H.; Cao, A.; Song, W.; Wan, L. AdV. Mater. 2006, 18, 2426. (5) (a) Duan, G.; Cai, W.; Luo, Y.; Li, Z.; Lei, Y. J. Phys. Chem. B 2006, 110, 15729. (b) Qu, L.; Dai, L. J. Phys. Chem. B 2005, 109, 13985. (6) (a) Lou, X.; Zeng, H. J. Am. Chem. Soc. 2003, 125, 2697. (b) Li, Z.; Ding, Y.; Xiong, Y.; Yang, Q.; Xie, Y. Chem. Commun.2005, 918. (c) Liang, J.; Liu, J.; Xie, Q.; Bai, S.; Yu, W.; Qian, Y. J. Phys. Chem. B 2005, 109, 9463. (d) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. Angew. Chem., Int. Ed. 2005, 44, 4197. (e) Fang, X.; Ye, C.; Zhang, L.; Zhang, J.; Zhao, J.; Yan, P. Small 2005, 1, 422. (f) Li, Z.; Ding, Y.; Xiong, Y.; Xie, Y. Cryst. Growth Des. 2005, 5, 1953. (g) Xu, Y.; Chen, D.; Jiao, X. J. Phys. Chem. B 2005, 109, 13561. (h) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. AdV. Mater. 2005, 17, 42. (7) (a) Gao, F.; Lu, Q.; Xie, S.; Zhao, D. AdV. Mater. 2002, 14, 1537. (b) Huang, Y.; Duan, X.; Lieber, C. M. Small 2005, 1, 142. (c) Khomenkova, L.; Fernandez, P.; Piqueras, J. Cryst. Growth Des. 2007, 7, 836. (d) Zhao, N.; Qi, L. AdV. Mater. 2006, 18, 359. (e) He, Z.; Yu, S.; Zhou, X.; Li, X.; Qu, J. AdV. Funct. Mater. 2006, 16, 1105. (f) Gou, X.; Cheng, F.; Shi, Y.; Zhang, L.; Peng, S.; Chen, J.; Shen, P. J. Am. Chem. Soc. 2006, 128, 7222. (8) (a) Zhang, Z.; Shao, X.; Yu, H.; Wang, Y.; Han, M. Chem. Mater. 2005, 17, 332. (b) Zhang, J.; Liu, S.; Lin, J.; Song, H.; Luo, J.; Elssfah, E. M.; Ammar, E.; Huang, Y.; Ding, X.; Gao, J.; Qi, S.; Tang, C. J. Phys. Chem. B 2006, 110, 14249. (9) (a) Chen, D.; Tang, K.; Li, F.; Zheng, H. Cryst. Growth Des. 2006, 6, 247. (b) Gong, Q.; Qian, X.; Ma, X.; Zhu, Z. Cryst. Growth Des. 2006, 6, 1821. (c) Shi, H.; Qi, L.; Ma, J.; Cheng, H.; Zhu, B. AdV. Mater. 2003, 15, 1647. (d) Shi, H.; Qi, L.; Ma, J.; Wu, N. AdV. Funct. Mater. 2005, 15, 442. (e) Cheng, Y.; Wang, Y.; Chen, D.; Bao, F. J. Phys. Chem. B 2005, 109, 794. (10) (a) Gopchandran, K. G.; Joseph, B.; Abaham, J. T.; Koshy, P.; Vaidyan, V. K. Vacuum 1997, 48, 547. (b) Cui, J.; Wang, A.; Edleman, N. L.; Ni, J.; Lee, P.; Armstrong, N. R.; Marks, T. J. AdV. Mater. 2001, 13, 1476. (c) Bogdanov, P.; Ivanovskaya, M.; Comini, E.; Faglia, G.; Sberveglieri, G. Sens. Actuators B 1999, 57, 153. (d) Wu, C.; Wu, C.; Sturm, J. C.; Kahn, A. Appl. Phys. Lett. 1997, 70, 1348. (11) (a) Zhao, Y.; Zhang, Z.; Wu, Z.; Dang, H. Langmuir 2004, 20, 27. (b) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem. Soc. 2005, 127, 5276. (c) Soulantica, K.; Erades, L.; Sauvan, M.; Senocq, F.; Maisonnat, A. AdV. Funct. Mater. 2003, 13, 553. (d) Seo, W. S.; Jo, H. H.; Lee, K.; Park, J. T. AdV. Mater. 2003, 15, 795. (e) Murali, A.; Barve, A.; Leppert, V. J.; Risbud, S. H. Nano Lett. 2001, 1, 287. (12) (a) Tamaki, J.; Naruo, C.; Yamamoto, Y.; Matsuoka, M. Sens. Actuators B 2002, 83, 190. (b) Liess, M. Thin Solid Films 2002, 410, 183. (13) (a) Pan, Z.; Dai, Z.; Wang, Z. Science 2001, 291, 1947. (b) Jeong, J. S.; Lee, J. Y.; Lee, C. J.; An, S. J.; Yi, G. C. Chem. Phys. Lett. 2004, 384, 246. (14) (a) Peng, X.; Meng, G.; Wang, X.; Wang, Y.; Zhang, J.; Liu, X.; Zhang, L. Chem. Mater. 2002, 14, 4490. (b) Liang, C.; Meng, G.; Lei, Y.; Phillipp, F.; Zhang, L. AdV. Mater. 2001, 13, 1330. (c) Li, C.; Zhang, D.; Han, S.; Liu, X.; Tang, T.; Zhou, C. AdV. Mater. 2003, 15, 143. (d) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2004, 4, 651. (e) Peng, X.; Meng, G.; Zhang, J.; Wang, X.; Wang, Y.; Wang, C.; Zhang, L. J. Mater. Chem. 2002, 12, 1602. (f) Zheng,

Lotus-Root-Like In2O3 Nanostructures M.; Zhang, L.; Li, G.; Zhang, X.; Wang, X. Appl. Phys. Lett. 2001, 79, 839. (g) Zhang, D.; Li, C.; Han, S.; Liu, X.; Tang, T.; Jin, W.; Zhou, C. Appl. Phys. Lett. 2003, 82, 112. (15) (a) Cheng, B.; Samulski, E. T. J. Mater. Chem. 2001, 11, 2901. (b) Li, Y.; Bando, Y.; Golberg, D. AdV. Mater. 2003, 15, 581. (c) Chen, C.; Chen, D.; Jiao, X.; Wang, C. Chem. Commun. 2006, 4632. (16) Lao, J.; Huang, J.; Wang, D.; Ren, Z. AdV. Mater. 2004, 16, 65. (17) (a) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y. Cryst. Growth Des. 2005, 5, 147. (b) Guha, P.; Kar, S.; Chaudhuria, S. Appl. Phys. Lett. 2004, 85, 3851. (c) Jia, H.; Zhang, Y.; Chen, X.; Shu, J.; Luo, X.; Zhang, Z.; Yu, D. Appl. Phys. Lett. 2003, 82, 4146.

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13403 (18) Yura, K.; Fredrikson, K. C.; Matijevic, E. Colloids Surf. 1990, 50, 281. (19) (a) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Kim, M.; Peng, X. J. Am. Chem. Soc. 2006, 128, 10310. (b) Yang, J.; Lin, C.; Wang, Z.; Lin, J. Inorg. Chem. 2006, 45, 8973. (20) (a) Ohhata, Y.; Shinoki, F.; Yoshida, S. Thin Solid Films 1979, 59, 255. (b) Cao, H.; Qiu, X.; Liang, Y.; Zhu, Q. Appl. Phys. Lett. 2003, 83, 761. (c) Lee, M. S.; Choi, W. C.; Kim, E. K.; Kim, C. K.; Min, S. D. K. Thin Solid Films 1996, 279, 1. (21) (a) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (b) Penn, R. L. Science 1998,281, 969.