In2O3 Hollow Microspheres - American Chemical Society

Sep 27, 2006 - formed under hydrothermal conditions and NH3 from the hydrolysis of formamide acted as the OH- provider. Morphological and structural ...
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Langmuir 2006, 22, 9380-9385

In2O3 Hollow Microspheres: Synthesis from Designed In(OH)3 Precursors and Applications in Gas Sensors and Photocatalysis Benxia Li,† Yi Xie,*,† Meng Jing,† Guoxin Rong,† Yecang Tang,‡ and Guangzhao Zhang‡ Department of Nanomaterials and Nanochemistry and Department of Low-Dimensional Physics and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed June 27, 2006. In Final Form: August 23, 2006 In this work, well-shaped In(OH)3 hollow microspheres have been successfully prepared via a novel surfactant-free vesicle-template-interface route in the “formamide-resorcinol-water” system, in which spontaneous vesicles were formed under hydrothermal conditions and NH3 from the hydrolysis of formamide acted as the OH- provider. Morphological and structural characterizations indicate that the shells of as-prepared In(OH)3 hollow microspheres were constructed by numerous nanocubes about 80 nm in size. As desired, In2O3 hollow microspheres were obtained from annealing the designed In(OH)3 precursors, and the as-obtained In2O3 hollow microspheres performed well as a gas-sensing material in response to both ethanol and formaldehyde gases and as a photocatalyst for photocatalytic degradation of rhodamine B. The facile preparation method and the improved properties derived from special microstructures are significant in the synthesis and future applications of functional nanomaterials.

Introduction In addition to current investigations on zero- and onedimensional nanomaterials,1 controlled organization of primary building units into curved structures represents another challenge for material self-assembly. Such a capability is attractive not only in understanding the concept of self-assembly with building blocks but also because of the importance of its potential application.2 Hollow spheres, as typical three-dimensional curved structures with inner cavities, are of much importance because of their unique properties of low density, high specific surface area, and good permeation, and their potential applications in various areas.3 So far, hollow spheres of many materials have been fabricated through various methods, including templateassisted synthesis,4 direct evacuations with Ostwald ripening, and the Kirkendall effect.5 Template-assisted synthesis is usually adopted for the preparation of hollow spheres. The templates are basically classified as hard templates6 and soft templates7 (mainly supermolecular assemblies, such as vesicles, emulsions, droplets, and large molecule aggregates). Among them, vesicle templates * To whom correspondence should be addressed. Address: Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. Tel: 86-551-3603987. Fax: 86-5513603987. E-mail: [email protected]. † Department of Nanomaterials and Nanochemistry. ‡ Department of Low-Dimensional Physics and Chemistry. (1) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (c) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.; Yang, P. Nature 2003, 422, 599. (d) Yang, R.; Ding, Y.; Wang, Z. L. Nano Lett. 2004, 4, 1309. (2) (a) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G., Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (b) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (c) Cao, A. M.; Hu, J. S.; Liang, H. P.; Wan, L. J. Angew. Chem., Int. Ed. 2005, 44, 4391. (3) (a) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 19, 2028. (b) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (c) Meier, W. Chem. Soc. ReV. 2000, 29, 295. (d) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. AdV. Mater. 2002, 14, 1499. (4) (a) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 19. (b) Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (c) Yuan, Z. Y.; Ren, T. Z.; Su, B. L. AdV. Mater. 2003, 15, 1462. (5) (a) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (b) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711.

are simple and inexpensive, and can be easily removed from the final hollow particles without any loss of structure. Meso- and nanoscopic hollow spheres have been synthesized from the selfassembly of building blocks via the as-formed vesicles.8 Our group has synthesized some hollow inorganic micro- or nanospheres by in situ vesicle-template-interface reactions with the help of surfactants.9 In this work, we designed a novel surfactant-free vesicle-template route for the preparation of inorganic hydroxide or oxide hollow spheres. This route was inspired from a common technique related to formamidephenolic emulsions, which are usually used for DNA reassociation in biological studies.10 Can such emulsion systems be applied in the preparation of interesting nanostructures? Combined with the consideration of the basic characteristics of the hydrolysis of formamide,11 a “formamide-resorcinol-water” system was preliminarily designed here for possible application in the preparation of metal hydroxide hollow spheres. As expected, the well-defined vesicles formed spontaneously in the present system were detected by laser light scattering (LLS) and observed clearly by transmission electron microscopy (TEM). (6) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Sun, Y. G.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 481. (c) Yang, Z. Z.; Niu, Z. W.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (d) Liang, Z. J.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (7) (a) Hsieh, C. C.; Lin, K. F.; Chien, A. T. Macromolecules 2006, 39, 3043. (b) Hentze, H. P.; Raghavan, S. R.; Mckelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069. (c) Wu, M.; Wang, G.; Xu, H.; Long, J.; Shek, F. L. Y.; Lo, S. M. F.; Williams, I. D.; Feng, S.; Xu, R. Langmuir 2003, 19, 1362. (d) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Langmuir 2003, 19, 4040. (e) Huang, J. X.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. AdV. Mater. 2000, 12, 808. (f) Gao, X.; Zhang, J.; Zhang, L. AdV. Mater. 2002, 14, 290. (8) (a) Hubert, D. H. W.; Jung, M.; German, A. L. AdV. Mater. 2000, 12, 1291. (b) Schmidt, H. T.; Ostafin, A. E. AdV. Mater. 2002, 14, 532. (c) He, T.; Chen, D. R.; Jiao, X. L.; Xu, Y. Y.; Gu, Y. X. Langmuir 2004, 20, 8404. (d) Wang, S. F.; Gu, F.; Lu, M. K. Langmuir 2006, 22, 398. (9) (a) Xiong, Y. J.; Xie, Y.; Yang, J.; Zhang, R.; Wu, C. Z.; Du, G. A. J. Mater. Chem. 2002, 12, 3712. (b) Zheng, X. W.; Xie, Y.; Zhu, L. Y.; Jiang, X. C.; Yan, A. H. Ultrason. Sonochem. 2002, 9, 311. (c) Zhu, L. Y.; Zheng, X. W.; Liu, X. M.; Zhang, X.; Xie, Y. J. Colloid Interface Sci. 2004, 273, 155. (10) (a) Gulick, P. J.; Dvorak, J. Gene 1990, 95, 173. (b) Nishi, Y.; Akiyama, K.; Kopf, B. R. Mamm. Genome 1992, 2, 11. (c) Miller, R. D.; Riblet, R. Nucleic Acids Res. 1995, 23, 2339. (d) Laman, A. G.; Kurjukov, S. G.; Bulgakova, E. V.; Anikeeva, N. N.; Brovko, F. A. J. Biochem. Biophys. Methods 2001, 50, 43. (11) Cheng, N. L. Handbook of SolVents, 2nd ed.; Chemical Industry Press: Beijing, 1994; p 747.

10.1021/la061844k CCC: $33.50 © 2006 American Chemical Society Published on Web 09/27/2006

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Indium oxide, an important n-type semiconductor with a direct band gap of 3.55-3.75 eV, was chosen as the present studied object. In2O3 nanostructures hold promising applications in optoelectronic devices, such as UV lasers, solar cells, and gas sensors.12 According to a previous report,13 In2O3 materials can be prepared by thermal-treating their oxyhydroxides with desired morphology. For example, In2O3 nanocubes and nanofibers have been prepared by annealing their In(OH)3 or InOOH counterparts.14 In situ release of H2O from oxyhydroxide materials during the decomposing process would retain the morphology of their oxyhydroxide precursors. Thus, In(OH)3 hollow microspheres, as an example, were synthesized via the designed surfactant-free vesicle-template-interface route, here the “formamide-resorcinol-water” system. The shells of as-obtained In(OH)3 hollow microspheres are constructed by numerous nanocubes with sizes around 80 nm, and such a morphology has been rarely reported previously. As expected, the desired In2O3 hollow spheres from annealing as-prepared In(OH)3 precursors exhibited photocatalysis for the degradation of rhodamine B (RhB) and good sensitivity in response to ethanol and formaldehyde gases. Such a synthetic route is interesting in understanding self-assembly with building blocks and helps to prepare functional hollow microspheres. Furthermore, the facile preparation method and investigated properties reported here are of much significance in the synthesis and future applications of nanomaterials. Experimental Section All chemical reagents in this work were purchased from the Shanghai Chemical Company, China. They were of analytical grade and used without further purification. In(OH)3 Hollow Spheres. The details of a typical experiment for the synthesis of In(OH)3 hollow spheres are as follows. Under stirring, 1.0 mmol of InCl3‚4H2O and 5 mmol of resorcinol were dissolved in 10 mL of mixed solvent (5 mL of distilled water and 5 mL of formamide). The final mixture was transferred into a Teflon-lined stainless autoclave (15 mL capacity), which was kept at 160 °C for 12 h and then cooled to room temperature naturally. The white precipitate was washed with distilled water and absolute alcohol several times to remove the possible residues and then dried at 50 °C under vacuum for 6 h. For detecting the as-formed vesicles, a parallel experiment without InCl3‚4H2O under the same conditions was carried out. The final solution was collected for subsequent characterization. In2O3 Hollow Spheres. In the procedures, 0.1 g of the as-prepared sample of In(OH)3 hollow spheres was put into a corundum crucible with a capacity of 30 mL, kept at 400 °C for 3 h, and then cooled to room temperature naturally. The final powder was collected for subsequent characterization. Sample Characterization. The average hydrodynamic radius and hydrodynamic radius distribution of the vesicles formed in the reaction system was detected by LLS on an ALV-5000E with a He-Ne laser (λo ) 632 nm) as the light source at 297 K. The vesicle solution was prepared by diluting the as-collected solution to about 3.0 × 10-4 g/mL and was filtered through a 0.8 µm Millipore MillexLCP filter to remove dust before the LLS measurement. The hydrodynamic radius (Rh) of the vesicles was determined by dynamic LLS. Fourier transform infrared (FTIR) spectroscopic study was carried out with a MAGNA-IR 750 (Nicolet Instrument Co.) FTIR spectrometer. The as-prepared vesicle-containing In(OH)3 sample (12) (a) Granqvist, C. G. Appl. Phys. A: Solids Surf. 1993, 57, 19. (b) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Jin, W.; Zhou, C. Appl. Phys. Lett. 2003, 82, 112. (c) Gurlo, A.; Ivanovskaya, M.; Barsan, N.; Schweizer-Berberich, M.; Weimar, U.; Gopel, W.; Dieguez, A. Sens. Actuators 1997, B44, 327. (13) Roy, R.; Shafer, M. W. J. Phys. Chem. 1954, 58, 372. (14) (a) Tang, Q.; Zhou, W. J.; Zhang, W.; Ou, S. M.; Jiang, K.; Yu, W. C.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 147. (b) Yu, D.; Yu, S. H.; Zhang, S.; Zuo, J.; Wang, D.; Qian, Y. AdV. Funct. Mater. 2003, 13, 497. (c) Sorescu, M.; Diamandescu, L.; Tarabasanu-Mihaila, D.; Teodorescu, V. S. J. Mater. Sci. 2004, 39, 675.

Langmuir, Vol. 22, No. 22, 2006 9381 was directly dried in a vacuum and then a thick film was prepared using a mixture of KBr and the sample. X-ray powder diffraction (XRD) measurements were carried out with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphite monochromatized high-intensity Cu-KR radiation (λ ) 1.54178 Å). The morphologies and dimensions of the products were observed by field emission scanning electron microscopy (FE-SEM) taken on a JEOL JSM-6700FSEM. The TEM images and electronic diffraction (ED) patterns were taken on a Hitachi Model H-800 instrument with a tungsten filament, using an accelerating voltage of 200 kV. For SEM and TEM observation, the samples were dispersed in ethanol or water by ultrasonic treatment and dropped on copper foils or carbon-copper grids. Thermogravimetric differential thermal analysis (TG/DTA) of the as-synthesized In(OH)3 sample was carried out on a Shimadzu TA-50 thermal analyzer at a heating rate of 10 K min-1 from room temperature to 700 °C in air. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an excitation source of Mg KR )1253.6 eV. BrunauerEmmett-Teller (BET) nitrogen adsorption-desorption was measured using a Micromeritics ASAP 2000 system. The photocatalytic activity was investigated using an RhB aqueous solution as a probe and a Pyrex beaker (250 mL) as the photoreactor vessel. The reaction system containing 100 mL of RhB solution with an initial concentration of 1.0 × 10-5 M and 25 mg of the as-prepared In2O3 samples was magnetically stirred in the dark for 15 min to reach uniform dispersity and the adsorption equilibrium. The solution was then exposed to UV irradiation from a 200-W high-pressure Hg lamp at room temperature. Samples were collected every 10 min to measure the RhB degradation by UV-vis spectra (Shimadzu UV2550). Measurements on gas sensitivity were performed with a WS-30A system (Weisheng Instruments Co., Zhengzhou, China). The In2O3 sensors were fabricated by dip-coating the as-prepared In2O3 alcohol colloids on the ceramic tube of the sensor body without an additional annealing process except for drying at 50 °C.

Results and Discussion To understand preliminarily what happened to the formamideresorcinol-water system under the present hydrothermal conditions, TEM and LLS were used to study the solution from the system after reaction. Figure 1a,b shows TEM images for the diluted solution of the formamide-resorcinol-water system after reaction, indicating that there are many as-formed vesicles with a size of about 500 nm. In addition, the hydrodynamic radius distribution of the vesicles was measured at room temperature (297 K) with a scan angle of θ ) 45°. Figure 1c shows the average hydrodynamic radius of the vesicles, ) 275 nm, and the hydrodynamic radius distribution f(Rh), which can be calculated using the Stokes-Einstein equation:

Rh ) (kBT/6πη)D-1 where kB, T, and η are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. The above analysis demonstrated that well-defined vesicles have formed in the present system. As for the formation of the vesicles in the reaction system, we deemed that it came from the polymerization between formamide and resorcinol, in which the detailed reactions are presently unclear and still under investigation. The FTIR spectra (Figure S1, Supporting Information) provide preliminary proof for the intermolecular bonding. It was presumed that the as-formed oligomers with hydrophilic and hydrophobic segments could self-organize into vesicle-like aggregates, utilizing the intermolecular interactions and the difference in solubility of different fragments in the selective solvents. Although the exact mechanism for the formation of vesicles in the formamide-resorcinol-water system is still under

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Figure 3. Image of In(OH)3 hollow microspheres obtained in the formamide-resorcinol-water system: (a) panoramic SEM image, (b) magnified SEM image of an open hollow sphere, (c,d) TEM images, and (e) SAED pattern taken on the shell of the hollow sphere.

Figure 1. (a,b) TEM images of the vesicles formed in the reaction system. (c) Hydrodynamic radius distribution of the as-formed vesicles in water. The concentration is about 3.0 × 10-4 g/mL.

Figure 2. XRD pattern of the as-prepared In(OH)3 hollow microspheres.

investigation, such a system is undoubtedly interesting and usable for the preparation of some inorganic hydroxide or oxide hollow spheres, with the as-formed vesicles acting as templates for an interface reaction and as-released NH3 providing OH- ions. As an example, In(OH)3 hollow spheres have been successfully created in a high morphological yield in the present reaction system. Figure 2 shows a typical XRD pattern of an as-prepared In(OH)3 sample, which could be readily indexed to the pure bodycentered cubic phase of In(OH)3 (JCPDS No. 85-1338), indicating high purity and good crystallinity of the sample. The morphologies and microstructures of the In(OH)3 sample were illuminated by SEM and TEM images, as shown in Figure 3. A panoramic SEM image in Figure 3a indicates that the sample consists of a large scale of hollow microspheres with diameters of 0.5-1.0 µm. Many broken spheres present in the image and their apparent cavities can be observed, suggesting that the shells of the spheres are rather thin and could be fractured by the post-sonication for

SEM observation. A magnified SEM image of an open hollow sphere with a diameter of about 1.0 µm is shown in Figure 3b, which reveals the interesting fact that the In(OH)3 hollow sphere is assembled by nanocubes with sizes around 80 nm. Numerous nanocubes arrange compactly and spherically to form a hollow sphere. In addition, the In(OH)3 product was further investigated by TEM, and the TEM images are shown in Figure 3c,d. The obvious contrast between the dark edge and the relatively bright center further confirms their hollow nature (Figure 3c). From the detailed observation in Figure 3d, the hollow interior occupies 70-90% volume in a sphere, and the thickness of a shell is about 85 nm, which is very close to the sizes of the nanocubes. Meanwhile, the ED pattern (Figure 3e) taken on the shell of the hollow microsphere can be attributed to the [001] zone axis diffraction of body-centered cubic (bcc) In(OH)3, indicating the good crystallinity of the nanocubes. The presence of some unordered spots in the ED pattern is attributed to the interference from the diffractions of other In(OH)3 nanocubes. To better understand the formation of In(OH)3 hollow microspheres, a series of controlled experiments were carried out (Figure S2, Supporting Information). The absence of resorcinol in the reaction solution with the other experimental conditions unchanged resulted in dispersive In(OH)3 nanocubes with sizes about 100 nm (Figure S3, Supporting Information). However, using distilled water or formamide alone as the solvent and in the presence of resorcinol, no In(OH)3 precipitate was obtained. All the observations indicate that all of three memberssresorcinol, formamide, and watersare indispensable for the formation of hollow In(OH)3 spheres. A rational mechanism called a vesicletemplate-interface reaction for the formation of In(OH)3 hollow microspheres is described in Scheme 1. Initially, the vesicles formed in the formamide-resorcinol-water system. The hydrophilic hydroxyls on the surface of the vesicles attracted In3+ ions in the solution and formed the In3+-covered vesicles. Because of the hydrolysis of In3+ and NH3, In(OH)3 single-crystalline nanocubes formed around the In3+-covered vesicles as a result of the growth characteristic of bcc In(OH)3.14a With the reaction proceeding, a relatively compact layer of In(OH)3 nanocubes was shaped around the vesicle template. As a result, In(OH)3 hollow microspheres assembled by nanocubes were formed, in which the vesicles can be easily rinsed by a washing process without destroying the spherical hollow structures. In addition,

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Scheme 1. Schematic Illustration of the Formation Process of In(OH)3 Hollow Microspheresa

a Resorcinol resolved in the mixed solvents of formamide and water f Vesicles were formed by polymerization and self-assembly f In3+ ions were adsorbed on the surfaces of the vesicles f In(OH)3 nanocubes formed on the surfaces of the vesicles f In(OH)3 hollow microspheres constructed by nanocubes were obtained as the final product after removing the vesicles by washing.

the as-obtained In(OH)3 hollow microspheres have good dispersibility and can be well-dispersed in some solvents by simple sonication. For the formation of the present hollow microspheres with good dispersibility, formamide has played an important role. First, because of the hydrolysis of formamide, the rapid production of supersaturated In(OH)3 single-crystalline nanocubes in the mixed solvent and their regular spherical assembly around the as-formed vesicles lead to the good dispersibility of In(OH)3 hollow microspheres. Second, formamide could act as an electrostatic stabilizer in the present system to prevent the agglomeration of the microspheres. Last, the as-formed oligomers from the polymerization between formamide and resorcinol could act as a surfactant to reduce the aggregation of the hollow microspheres. Thus, In(OH)3 hollow microspheres with good dispersibility were obtained in our preparing system. The conversion process of the In(OH)3 sample during calcination in air was studied via TG/DTA, as shown in Figure 4a. The TGA curve can be mainly divided into three weight-loss steps. The first step between 35 and 250 °C was attributed to physical water evaporation from the In(OH)3 sample, and the DTA curve indicates an endothermic process during this weightloss step. The second step between 250 and 300 °C, corresponding to an endothermic reaction, was ascribed to chemical dehydration (2In(OH)3 f 2InOOH + 2H2O), and the weight loss of this step is about 10%. The last step at 300-500 °C shows an ∼5% weight loss and an exothermic reaction around 305 °C, which was considered as the reaction 2InOOH f In2O3 + H2O. The results of the thermal analysis were in good agreement with theoretical values and the literature report.15 As expected, In2O3 hollow microspheres have been prepared by calcination of the as-obtained In(OH)3 precursors at 400 °C in air. Figure 4b shows the XRD pattern of the product obtained by annealing as-prepared In(OH)3. All of the peaks can be indexed to the pure cubic phase of In2O3 (JCPDS No. 76-0152), indicating that the pure phase of In2O3 can be obtained by calcining In(OH)3. The XPS analysis (Figure S4) on the as-obtained In2O3 sample provides some preliminary information about the surface atoms and electronic structure, and further confirms its high purity. SEM and TEM images of the In2O3 hollow microspheres were shown in Figure 5. The panoramic observation (Figure 5a) indicates that the as-obtained In2O3 sample inherits well the morphology of the In(OH)3 precursor and consists of well-shaped hollow microspheres with diameters of 0.5-1.0 µm. As seen from the magnified SEM image in Figure 5b, the shapes of nanocubes in the hollow microspheres were mainly retained, but many small particles were observed on their surfaces, indicating that, after the heat treatment, the pristine single-crystalline In(OH)3 nanocubes were transformed into polycrystalline In2O3 nanocubes consisting of numerous In2O3 nanocrystallites. The TEM image in Figure 5c also reveals the hollow trait of as(15) Ho, W. H.; Yen, S. K. Thin Solid Films 2006, 498, 80.

Figure 4. (a) TG/DTA curves of In(OH)3 hollow microspheres. (b) XRD pattern of the In2O3 product from annealing In(OH)3 hollow microspheres.

Figure 5. (a,b) SEM and (c,d) TEM images of In2O3 hollow spheres from annealing In(OH)3 hollow spheres. (e) SAED pattern taken from an In2O3 hollow microsphere.

obtained In2O3 samples, and the magnified TEM image (Figure 5d) confirms that In2O3 hollow microspheres are assembled by nanocubes. The ED pattern (Figure 5e) taken from the shell of In2O3 hollow spheres exhibits a polycrystalline characteristic indexed to be the cubic phase of In2O3, which coincides with the observation in Figure 5b. From the observation of the FE-SEM

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and TEM images, there is no obvious change in the aggregation state of the hollow microspheres after heat treatment, indicating that the heat treatment has little influence on the aggregation state as well as the morphology of the as-obtained hollow microspheres. One of the most important applications of In2O3 material is used in gas sensors.16 More and more strict regulations on the release of toxic gases require fast and accurate detection at low concentrations. More practical and wider applications of gas sensors require their good performance at lower temperatures, especially room temperature.17 It has been demonstrated that a decrease in the size of the crystallites in the sensing layer can result in a considerable increase in sensitivity.18 Thus, nanomaterials with desirable morphology and small size may help to improve their own sensitivity at room temperature. To demonstrate the performance of an as-obtained In2O3-hollow-microsphere sample as a sensing material at room temperature, its sensitivity in response to ethanol (C2H5OH) and formaldehyde (HCHO) gas was investigated. In addition, to testify the effect of the sample’s morphology on its sensitivity in a gas sensor, the sample of In2O3 dispersive nanocubes (Figure S5, Supporting Information) from annealing the as-obtained In(OH)3 dispersive nanocubes (Figure S3, Supporting Information) was also studied under the same conditions. The sensitivity-gas concentration plots are shown in Figure 6, in which the sensitivity was defined as Rair/ Rgas, where Rair and Rgas are the electrical resistances for sensors in air and in gas, respectively.17 Panels a and b of Figure 6 display the plots of sensitivity versus gas-concentration when In2O3 hollow microspheres and dispersive nanocubes were exposed to ethanol (C2H5OH) and formaldehyde (HCHO) gases, respectively. It was obvious that, under the same gas concentration, the sensitivity of In2O3 hollow microspheres is greater than that of the In2O3 dispersive nanocubes. When exposed to C2H5OH gas, the sensitivity of In2O3 hollow microspheres is nearly twice that of the In2O3 dispersive nanocubes. From these results, it is evident that as-prepared In2O3 hollow microspheres exhibited better sensitivity than did In2O3 dispersive nanocubes. In addition, both of the sensors made of as-prepared In2O3 hollow microspheres and In2O3 dispersive nanocubes are more sensitive to C2H5OH. Upon explaining these results, it refers to the theory for such metal-oxide sensors involving absorption/desorption phenomena and reactions at the surface of the metal oxide.19 The In2O3 sensors can adsorb oxygen from the atmosphere; then the adsorbed oxygen and the surface lattice oxygen (O2-) of the In2O3 take part in the oxidation of detected gases. Once the oxidation reaction occurs, electrons will enter into the sensors, resulting in their decreased resistance. BET measurements revealed that the specific surface area of In2O3 hollow microspheres is 98 m2/g, whereas that of In2O3 nanocubes is 77 m2/g. One important reason for the above results is that the sample of In2O3 hollow microspheres has a larger surface area and much more capacious interspaces than the In2O3-dispersive-nanocube sample, which can provide sufficient space and more active sites for the interaction between adsorbed O2 species and detected gases, resulting in the better sensitivity of the In2O3-hollow-

sphere sensor. In comparison with other gas-sensing materials,20 the present In2O3 microspheres have exhibited fine sensitivity and are promising for future application. In addition, as an important wide band gap metal oxide, In2O3 has been applied to improve the photocatalytic efficiency of some other semiconductors.21 Yet, there is little information on the photocatalytic activity of indium oxide alone.22 To demonstrate the photocatalysis of the present In2O3-hollow-microsphere sample for the degradation of organic pollutants, we have carried out the experiments of the photocatalytic degradation of RhB with the as-prepared In2O3 samples as photocatalysts. The characteristic absorption of RhB at 553 nm was chosen as the parameter to be monitored for the photocatalytic degradation process. Figure 7a shows the UV-vis absorption spectrum of an aqueous solution of RhB (initial concentration: 1.0 × 10-5 M, 100 mL) in the presence of an In2O3-hollow-microsphere sample (25 mg) under exposure to UV light for various durations. The absorption peaks corresponding to RhB, diminished gradually as the exposure time was extended. During this process, the intense pink color of the starting RhB solution gradually faded with increasingly longer exposure time. To demonstrate the influence of the sample’s morphology on its photocatalytic

(16) (a) Neri, G.; Bonavita, A.; Micali, G.; Rizzo, G.; Galvagno, S. Chem. Commun. 2005, 6032. (b) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345. (17) (a) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. H. AdV. Mater. 2005, 17, 582. (b) Zhao, Q. R.; Gao, Y.; Bai, X.; Wu, C. Z.; Xie, Y. Eur. J. Inorg. Chem. 2006, 8, 1643. (18) Yamazoe, N. Sens. Actuators, B 1991, 5, 7. (19) (a) Gurlo, A.; Barsan, N.; Weimar, U.; Ivanovskaya, M.; Taurino, A.; Siciliano, P. Chem. Mater. 2003, 15, 4377. (b) Sun, Z. P.; Liu, L.; Zhang, L.; Jia, D. Z. Nanotechnology 2006, 17, 2266.

(20) (a) Li, W. Y.; Xu, L. N.; Chen, J. AdV. Funct. Mater. 2005, 15, 851. (b) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. AdV. Mater. 2005, 17, 582. (c) Feng, P.; Wan, Q.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 213111. (d) Zhang, J.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2006, 18, 867. (21) (a) Shchukin, D.; Poznyak, S.; Kulak, A.; Pichat, P. J. Photochem. Photobiol., A 2004, 162, 423. (b) Poznyak, S. K.; Talapin, D. V.; Kulak, A. I. J. Phys. Chem. B 2001, 105, 4816. (c) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 2287. (22) Poznyak, S. K.; Golubev, A. N.; Kulak, A. I. Surf. Sci. 2000, 454-456, 396.

Figure 6. Room-temperature sensitivity of the sensors made of as-prepared In2O3 samples to (a) ethanol (C2H5OH) and (b) formaldehyde (HCHO).

In2O3 Hollow Microspheres

Langmuir, Vol. 22, No. 22, 2006 9385

tance spectra (DRS) (Figure S6, Supporting Information) of In2O3 hollow spheres and In2O3 dispersive nanocubes are quite similar and have the same absorption band gap, indicating that the light absorption property of the two samples has little influence on the evaluation of their photocatalytic activity. The relatively higher photocatalytic activity of In2O3 hollow spheres relative to that of In2O3 nanocubes can be explained by their larger surface area and more capacious interspaces, which provide more active sites for the photocatalytic degradation of RhB molecules. In addition, the photocatalysis of the present In2O3 samples toward other organic molecules such as eosin Y has also been observed. Because of the drawback of In2O3 material on photocatalysis, the photocatalytic activity of the present In2O3 hollow microspheres is somewhat poorer than that of Degussa P25 (a commercial photocatalyst with the best photocatalytic activity). While we know that both the shapes and the sizes of functional materials have a distinct effect on their properties, the present hollow microspheres with larger specific surface area will offer a good subject for future improvement of their performance in applications.

Conclusions

Figure 7. (a) Absorption spectrum of the RhB solution (5.0 × 10-5 M, 100 mL) in the presence of 25 mg of In2O3 hollow spheres under exposure to UV light. (b) Photodegradation of RhB (5.0 × 10-5 M, 100 mL) under different conditions: (1) with 25 mg of In2O3 hollow spheres and without UV light, (2) without catalyst and under UV light, (3) with 25 mg of In2O3 dispersive nanocubes and under UV light, and (4) with 25 mg of In2O3 hollow spheres and under UV light.

efficiency, we studied the degradation process of RhB with the In2O3 dispersive nanocubes as photocatalyst as well. Figure 7b displays the results of the RhB degradation in a series of experimental conditions. The concentration of RhB hardly changed under both of the conditions with catalyst in the dark (curve 1) and exposure to UV light without catalyst (curve 2). However, as indicated in curves 3 and 4, the concentration of RhB obviously decreased with each of the as-prepared In2O3 samples as a catalyst under exposure to UV light, revealing the obvious photocatalytic ability of In2O3. Moreover, the In2O3hollow-sphere sample exhibits superior photocatalysis over the In2O3-dispersive-nanocube sample. The UV-vis diffuse reflec-

In summary, In(OH)3 hollow microspheres constructed by uniform nanocubes have been successfully synthesized in a high morphological yield via a novel surfactant-free vesicletemplate-interface route in the formamide-resorcinol-water system. The as-formed vesicles in the present system were detected by LLS and observed by TEM. Such a synthetic route is interesting in understanding self-assembly with building blocks and helps to prepare functional hollow microspheres. As desired, well-shaped In2O3 hollow microspheres have been obtained by annealing as-prepared In(OH)3 precursors. The as-obtained In2O3 hollow spheres exhibited more effective photocatalysis and better sensitivity in gas sensors compared to that of the as-prepared In2O3 dispersive nanocubes, which is possibly due to the sample of In2O3 hollow microspheres having a larger surface area, providing sufficient space and more active sites for interactions. The facile preparation method and the improved properties derived from hollow microstructures reported here are of much significance in the synthesis and application of nanomaterials. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20321101) and the state key project of fundamental research for nanomaterials and nanostructures (2005CB623601). The authors sincerely thank Caiyuan Pan and Huarong Liu for their helpful discussions. Supporting Information Available: FTIR spectra for resorcinol, formamide, and the as-prepared In(OH)3 sample (S1); Details of a series of controlled experiments with different conditions (S2); TEM image of In(OH)3 nanocubes obtained without resorcinol (S3); XPS spectra of the In2O3 sample (S4); TEM image and ED pattern of In2O3 nanocubes prepared from In(OH)3 nanocubes (S5); and UV-vis DRS of In2O3 hollow spheres and dispersive nanocubes (S6). This material is available free of charge via the Internet at http://pubs.acs.org. LA061844K