Hydrothermal Synthesis of Mesoporous InVO4 Hierarchical

Nov 27, 2008 - spectra, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and selected...
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Langmuir 2009, 25, 1705-1712

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Hydrothermal Synthesis of Mesoporous InVO4 Hierarchical Microspheres and Their Photoluminescence Properties Yao Li,† Minhua Cao,*,†,‡ and Liyun Feng§ Department of Chemistry, Northeast Normal UniVersity, Changchun, 130024, P. R. China, Department of Chemistry, Beijing Institute of Technology, Beijing, 100081, P. R. China, and Center of Analysis and Testing, Beihua UniVersity, Jilin, 132013, China ReceiVed September 11, 2008. ReVised Manuscript ReceiVed NoVember 27, 2008 In this work, uniform InVO4 hierarchical microspheres with the assistance of sodium dodecyl benzene sulfonate (SDBS) have been successfully synthesized by a facile hydrothermal method. X-ray diffraction, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction were used to characterize the as-synthesized samples. The results indicate that such 3D hierarchical architecture InVO4 microspheres are polycrystalline and assembled by numerous nanocrystals. Nitrogen adsorption-desorption measurement and the pore size distribution curve suggest that mesopores exist in these hierarchical microarchitectures. The formation mechanism has been proposed based on Oswald ripening and nonoriented attachment. It is further found that SDBS plays a key role in the formation process, and when the amount of SDBS is adjusted, the average diameter of such InVO4 microspheres could be controlled from 5 to 1 µm. UV-vis diffuse reflectance and PL spectra are employed to estimate the photoluminescence property of as-prepared InVO4 samples.

1. Introduction The systematic control over size and morphology of inorganic materials at micro- and nanoscale levels represents a great challenge in the modern synthetic field. This is because there is a close relationship between the morphology and the property of these materials.1 Recently, scientists have been paying more and more attention to the organization of complex microarchitectures, especially three-dimensional (3D) hierarchical architectures which are assembled by nanostructured building blocks such as nanoplates, nanoparticles, nanoribbons, nanorods, and so forth.2 Such complex architectures combining the features of micrometer- and nanometer-scaled building blocks could show unique properties different from those of the monomorphological structures.3 To date, a wide variety of inorganic materials, including metal oxide, sulfide, hydrate, and other minerals, have been successfully prepared with hierarchical structures,2,4 in which those hierarchically structured materials with porous architectures received special attention because they provide opportunities for exploiting novel properties due to their high surface-to-volume ratio and permeability. Therefore, the synthesis of mesoporous materials represents a remarkable level in modern materials chemistry.5 Mesoporous materials are generally synthesized through a soft or hard template formation process.4b,6 However, this method often suffers from the removal of templates. At present, there have always been emerging attempts to directly * To whom correspondence should be addressed. E-mail: caomh043@ nenu.edu.cn. † Northeast Normal University. ‡ Beijing Institute of Technology. § Beihua University. (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (2) (a) Li, Y. Y.; Liu, J. P.; Huang, X. T.; Li, G. Y. Cryst. Growth Des. 2007, 7, 1350. (b) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. AdV. Mater. 2005, 17, 42. (c) Gu, Z. J.; Zhai, T. Y.; Gao, B. F.; Sheng, X. H.; Wang, Y. B.; Fu, H. B.; Ma, Y.; Yao, J. N. J. Phys. Chem. B 2006, 110, 23829. (d) Yang, J.; Lin, C. K.; Wang, Z. L.; Lin, J. Inorg. Chem. 2006, 45, 8973. (3) Gao, X. F.; Jiang, L. Nature 2004, 432, 36. (4) (a) Zhao, Q.; Xie, Y.; Zhang, Z.; Bai, X. Cryst. Growth Des. 2007, 7, 153. (b) Di, Y.; Meng, X.; Wang, L.; Li, S.; Xiao, F. S. Langmuir 2006, 22, 3068.

grow inorganic materials with porous architectures without the assistance of any templates.7,8 In comparison to the template method, the direct synthesis method is considerably simpler. Although great advances have been made, the fabrication of mesoporous hierarchical 3D architecture is still a significant challenge.2a Indium vanadate (InVO4) belongs to the family of orthovanadates oxide. As an important fundamental material, it has received great interest because of its potential applications in various fields,16 such as photocatalyst for degradation of organic pollutants,9a,16a air purification9b and water splitting,10,15 gas sensor for monitoring ethanol,11 and anode materials for lithium secondary batteries.12,16b Over the past few years, synthesis of InVO4 has been accomplished by traditional solid(5) (a) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W. J. Am. Chem. Soc. 1992, 114, 10834. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (6) (a) Deng, D.; Tang, R.; Liao, X.; Shi, B. Langmuir 2008, 24, 368. (b) Ren, T. Z.; Yuan, Z. Y.; Su, B. L. Langmuir 2004, 20, 1531. (c) Tan, Y.; Srinivasan, S.; Choi, K. S. J. Am. Chem. Soc. 2005, 127, 3596. (d) Tian, B.; Liu, X.; Solovyov, L. A.; Liu, Z.; Yang, H.; Zhang, Z.; Xie, S.; Zhang, F.; Tu, B.; Yu, C.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2004, 126, 865. (e) Jiao, F.; Harrison, A.; Jumas, J. C.; Chadwick, A. V.; Kockelmann, W.; Bruce, P. G. J. Am. Chem. Soc. 2006, 128, 5468. (7) (a) Yu, J. C.; Xu, A.; Zhang, L.; Song, R.; Wu, L. J. Phys. Chem. B 2004, 108, 64. (b) Yuan, J.; Laubernds, K.; Zhang, Q.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966. (c) Ba, J.; Polleux, J.; Antonietti, M.; Niederberger, M. AdV. Mater. 2005, 17, 2509. (8) (a) Choi, W. S.; Koo, H. Y.; Zhongbin, Z.; Li, Y.; Kim, D. Y. AdV. Funct. Mater. 2007, 17, 1743. (b) Zhu, Y.; Zhang, L.; Schappacher, F. M.; Po¨ttgen, R.; Shi, J.; Kaskel, S. J. Phys. Chem. C 2008, 112, 8623. (c) Zhu, H.; Wang, X.; Qian, L.; Yang, F.; Yang, X. J. Phys. Chem. C 2008, 112, 4486. (d) Yang, D.; Sun, S.; Meng, H.; Dodelet, J. P.; Sacher, E. Chem. Mater. 2008, 20, 4677. (9) (a) Zhang, L. W.; Fu, H. B.; Zhang, C. J. Solid State Chem. 2006, 179, 804. (b) Xiao, G. C.; Wang, X. C.; Li, D. Z.; Fu, X. Z. J. Photochem. Photobiol. A: Chem. 2008, 193, 213. (10) (a) Ye, J.; Zou, Z.; Oshikiri, M.; Matsushita, A.; Shimoda, M.; Imai, M.; Shishido, T. Chem. Phys. Lett. 2002, 356, 221. (b) Ye, J.; Zou, Z.; Arakawa, H.; Oshikiri, M.; Shimoda, M.; Matsushita, A.; Shishido, T. J. Photochem. Photobiol. A: Chem. 2002, 148, 79. (c) Xu, L. X.; Sang, L. X.; Ma, C. F.; Lu, Y. W.; Wang, F.; Li, Q. W.; Dai, H. X.; He, H.; Sun, J. H. Chin. J. Catal. 2006, 27, 100. (11) Chen, L.; Liu, Y.; Lu, Z.; Zeng, D. J. Colloid Interface Sci. 2006, 295, 440. (12) (a) Wang, Y.; Cao, G. J. Mater. Chem. 2007, 17, 894. (b) Denis, S.; Baudrin, E.; Touboul, M.; Tarascon, J. M. J. Electrochem. Soc. 1997, 144, 4099.

10.1021/la803682d CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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Figure 1. XRD patterns of the as-prepared microspheres.

state reaction between In2O3 and V2O5,10a,b,13 and by pyrolysis of amorphous complex precursor,17 which often needs higher temperature and long reaction time, and by sol-gel method,12a,14,16a which is limited by the stability of its precursor system. To our knowledge, different material preparation methods may have some important effects on material microstructures and physical properties. Therefore, developing different synthesis methods is very important for exploring their novel properties. Very recently, there has been a strong trend toward the application of solution routes for direct synthesis of advanced materials at low temperature.18 Hydrothermal methods, as one of the most promising solution chemical methods, have been widely used to produce various inorganic materials due to its mild synthetic conditions, low cost, and mass production.19 Such soft chemical method demonstrated efficient synthesis and morphological control of InVO4-based materials. For example, nanosized InVO4 powders have been prepared from NaVO3 and InCl3 basic solution under hydrothermal conditions at 150 °C.20 The InVO4 sol contains orthorhombic InVO4 nanocrystals which were obtained by hydrothermal treatment.21 And InVO4 powders with the rodlike, cubiclike, irregular, and bricklike shapes have been successfully synthesized by a hydrothermal method in the presence of different organic additives.11 Nevertheless, to the best of our knowledge, the synthesis of mesoporous InVO4 with hierarchical architecture has never been explored due to the lack of synthetic capability. In this paper, we for the first time report the synthesis of mesoporous InVO4 3D uniform hierarchical architecture microspheres by a hydrothermal process. The as-prepared microspheres are self-assembled by InVO4 nanocrystals in the presence of sodium dodecyl benzene sulfonate (SDBS) at a low temperature (150 °C). Such microarchitecture is new in the family of InVO4 micro-/nanostructures. The cooperation mechanism of Oswald ripening and nonoriented attachment (13) Touboul, M.; Toledano, P. Acta Crystallogr. B. 1980, 36, 240. (14) (a) Orel, B.; Sˇurca Vuk, A.; Opara Krasˇovec, U.; Drazˇicˇ, G. Electrochim. Acta 2001, 46, 2059. (b) Cimino, N.; Artuso, F.; Decker, F.; Orecl, B.; Sˇurca Vuk, A.; Zanoni, R. Solid State Ionics 2003, 165, 89. (15) Lin, H. Y.; Chen, Y. F.; Chen, Y. W. Int. J. Hydrogen Energy 2007, 32, 86. (16) (a) Ge, L.; Xu, M. X.; Fang, H. B. Mater. Lett. 2007, 61, 63. (b) Sˇurca Vuk, A.; Krasˇovec, U. O.; Orel, B.; Colomban, P. J. Electrochem. Soc. 2001, 148, H49. (c) Touboul, M.; Melghit, K.; Ben´ard, P. Eur. J. Solid State Inorg. Chem. 1994, 31, 151. (17) Zhang, S. C.; Zhang, C.; Yang, H. P.; Zhu, Y. F. J. Solid State Chem. 2006, 179, 873. (18) (a) Henderson, N. L.; Schaak, R. E. Chem. Mater. 2008, 20, 3212. (b) Zhang, J.; Sun, K.; Kumbhar, A.; Fang, J. J. Phys. Chem. C 2008, 112, 5454. (c) Lee, J. Y.; Connor, S. T.; Cui, Y.; Peumans, P. Nano Lett. 2008, 8, 689. (d) Fulmer, M.; Brown, P. W. J. Am. Ceram. Soc. 1992, 75, 3401. (19) (a) Lou, X. W.; Zeng, H. C. Chem. Mater. 2002, 14, 4880. (b) Gu, Z. J.; Zhai, T. Y.; Gao, B. F.; Ke, D. M.; Ma, Y.; Yao, J. N. Cryst. Growth Des. 2008, 8, 750. (c) Eckert, J. O., Jr.; Hung-Houston, C. C.; Gersten, B. L.; Lencka, M. M.; Riman, R. E. J. Am. Ceram. Soc. 1996, 79, 2929. (d) Morey, G. W. J. Am. Ceram. Soc. 1953, 36, 279.

Figure 2. X-ray photoelectron spectra (XPS) of InVO4.

has been proposed based on observation from time-dependent InVO4 morphologies evolvement. It is found that the different amounts of SDBS played a key role in controlling the size of as-synthesized microspheres. Pore-size distribution analysis results indicate that there are mesopores in these 3D spherical microstructures. As far as we know, previous photoluminescence research studies on semiconductors mostly concentrated on metal-sulfurets22 and simple metal oxides.23InVO4, as a multicomponent metal oxide compound, photoluminescence behavior has rarely been a concern until now. Herein, we first investigated the room temperature photoluminescence property of this newly structured InVO4 sample. These InVO4 microspheres show an intense green emission peak around 521 nm under the 314 nm UV excitation, which is mainly attributed to the oxygen vacancies in the InVO4 microstructures. This work not only gives new insight into the hierarchical growth of complex InVO4 microarchitectures by cooperation of two mechanisms but also sheds some light on their photoluminescence property, which may provide promising applications in optical fields.

2. Experimental Section Synthesis. All of the chemicals are analytical grade and were used as received without further purification. The typical procedure is as follows: First, SDBS was dissolved into 30.0 mL of deionized water to form a transparent solution. Second, 0.25 mmol of InCl3 · 4H2O was dissolved in 5.0 mL of 1.0 M HNO3 solution and (20) Xiao, G.; Li, D.; Fu, X.; Wang, X.; Liu, P.; Chin, J. Inorg. Chem. 2004, 20, 195. (21) Fang, H. B.; Xu, M. X.; Ge, L.; He, Z. Y. Trans. Nonferrous Met. Soc. China 2006, 16, s373. (22) (a) Wang, Y.; Meng, G.; Zhang, L.; Liang, C.; Zhang, J. Chem. Mater. 2002, 14, 1773. (b) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 3298. (c) Xia, B.; Lenggoro, I. W.; Okuyama, K. Chem. Mater. 2002, 14, 4969.

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Figure 3. (a) Low-magnification, (b) enlarged, (c) high-magnification SEM images of the as-prepared microspheres; (d) SEM of a single microsphere.

0.25 mmol of NH4VO3 was dissolved in 10.0 mL of 1.0 M NaOH solution. Then the nitric acid solution of InCl3 · 4H2O and sodium hydroxide solution of NH4VO3 were added to the SDBS aqueous solution, under continuous stirring, and yellow precipitates formed gradually. The pH value of the final system was adjusted to 4 with 1.0 M aqueous nitric acid and the volume was fixed to 50 mL at room temperature under constant magnetic stirring. Finally, the resulting slurry was transferred to a Teflon-lined stainless steel autoclave of 80.0 mL capacity, maintained at 150 °C for 24 h. After reaction under autogenous pressure and cooling to room temperature naturally, the yellowish white precipitate was collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried at room temperature in air. The chemical reactions involved in the entire synthesis of InVO4 can be formulated, as shown in eqs 1-3:11,24

In3+ + 3OH- f In(OH)3

(1)

VO33- + OH- f VO43- + H+

(2)

In(OH)3 + VO34- f InVO4 + 3OH-

(3)

Characterization. The crystalline structure of the as-prepared product was analyzed by X-ray diffraction (XRD) using a Rigaku/ Dmax2000 diffraction meter with Cu KR radiation (λ ) 1.5418 Å). The step scan covered the angular range 10-70° in steps of 0.01°. Three-dimensional observations of the morphology of products (23) (a) Liang, J.; Deng, Z.; Jiang, X.; Li, F.; Li, Y. Inorg. Chem. 2002, 41, 3602. (b) Yang, H.; Yao, X.; Wang, X.; Xie, S.; Fang, Y.; Liu, S.; Gu, X. J. Phys. Chem. B 2003, 107, 13319. (c) Lin, Y. R.; Yang, S. S.; Tsai, S. Y.; Hsu, H. C.; Wu, S. T.; Chen, I. C. Cryst. Growth Des. 2006, 6, 1951. (d) Dutta, D. P.; Sudarsan, V.; Srinivasu, P.; Vinu, A.; Tyagi, A. K. J. Phys. Chem. C 2008, 112, 6781. (e) Lu, H. B.; Liao, L.; Li, H.; Tian, Y.; Li, J. C.; Wang, D. F.; Zhu, B. P. J. Phys. Chem. C 2007, 111, 10273. (24) (a) Sun, L. D.; Zhang, Y. X.; Zhang, J.; Yan, C. H.; Liao, C. S.; Lu, Y. Q. Solid State Commun. 2002, 124, 35. (b) Wang, H.; Meng, Y. Q.; Yan, H. Inorg. Chem. Commun. 2004, 7, 553.

were conducted on a field emission scanning electron microscope (EESEM, JEOL, JSM-6700F) operated at an acceleration voltage of 5.0 kV. Transmission electron microscopy (TEM) observation was carried out on a JEOL, JEM-2010 instrument in bright field. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) models were carried out on a JEOL, JEM-2100F (operated at 200 kV). The X-ray photoelectron spectra (XPS) analysis was measured on a PHI 5300 ESCA instrument using Al KR line as the excitation source. The pass energy of the analyzer was set at 35.75 eV. The binding energy was calibrated with respect to the C(1s) peak arising from surface hydrocarbons taken at 284.7 eV. The nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) methods were analyzed on a Micromeritics ASAP 2010 analyzer. Room temperature photoluminescence (PL) excitation/emission spectra were recorded on an Hitachi F-4500 spectrophotometer. The FTIR pattern was acquired from a Magna 560 FT-IR spectrometer (American Nicolet). Raman spectrum was recorded on an Invia Raman microscope (Renishaw Company).

3. Results and Discussion 3.1. Morphology and Structure. The synthesis of hierarchical InVO4 microspheres with porous architecture has been achieved in aqueous solution containing InCl3 · 4H2O and NH4VO3 by thermal treatment in the presence of 0.5 g of SDBS for 24 h. Figure 1 shows the typical X-ray diffraction (XRD) pattern that all of the diffraction peaks can be well-indexed as an orthorhombic (Cmcm) phase of InVO4 and match very well with the standard card (JCPDS 48-0898, a ) 5.753 Å, b ) 8.520 Å, c ) 6.587 Å). No impurity peaks are observed, indicating that the product is pure. The peaks are sharp and narrow, indicating the high crystallization of the product. The Raman spectrum and the FTIR spectrum of the as-prepared InVO4 sample further confirm its orthorhombic phase (see

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Figure 4. (a) TEM image and SAED result of a single InVO4 microsphere; (b) enlarged TEM image of a single microsphere; (c) and (d) HRTEM image of the microsphere.

Figures S1 and S2, Supporting Information) where the structure is composed of chains of InO6 octahedral linked together by the VO4 tetrahedral.10a,14,25 The X-ray photoelectron spectroscopy (XPS) analysis was carried out to determine the surface chemical composition of the as-prepared sample and the valence states of various species. The spectra of the sample exhibit characteristic spin-orbit split of In(3d5/2) and In(3d3/2) signals (Figure 2a) and V(2p3/2) and V(2p1/2) (Figure 2b). The analysis of the In(3d) and V(2p) core lines clearly indicates the presence of In3+ oxidation state (444.3 and 451.7 eV for In(3d))9b,26 and V5+ oxidation state (516.79 and 524.7 eV for V(2p)).9b,26 Figure 3a is a typical low-magnification scanning electron microscopy (SEM) image of InVO4 sample obtained with the addition of SDBS of 0.5 g at 150 °C for 24 h, from which a number of uniform microspheres with average diameter of 2 µm can be clearly observed. No other morphologies are detected, indicating a high yield of these microspheres. High-magnification SEM images in Figure 3b-d clearly show the rough surface of the microspheres and some detailed structural information of the sample. As can be seen in Figure 3c, the spheres are constructed of numerous nanoparticles. These nanoparticles are densely selfassembled and form 3D hierarchical structures, which will be discussed in a later section. With close views of an individual microsphere, we find out that the structure of as-synthesized microsphere is porous (marked by arrowheads in Figure 3d). Such self-assembled micrometer-scale sphere structure was further investigated by transmission electron microscopy (TEM) (25) Touboul, M.; Tole´dano, P. Acta Crystallogr., Sect B: Struct. Sci. 1980, 36, 240. (26) Briggs, D.; Seah, M. P. Practical Surface Analysis Auger and X-ray Photoelectron Spectroscopy, Vol. 1, (second ed); Wiley: New York, 1990.

to reveal the detail of its organization. Figure 4a presents a typical TEM image of an individual InVO4 microsphere with obvious rough surface, in good accordance with the SEM images. Crystallinity of these microspheres was confirmed by select area electron diffraction (SAED) (inset in Figure 4a), revealing diffraction rings typical for its polycrystalline feature. Figure 4b is a magnified TEM image taken from the fringe of a microsphere marked by a red rectangle in Figure 4a, from which nanoparticles can be clearly seen, manifesting the 3D hierarchical structures consisting of subunits of nanoparticles. The nanoparticles are about 20-30 nm in average diameter, as disclosed in Figure 4c. The microstructure of the nanoparticle subunits of the 3D hierarchical architecture was investigated by high-resolution transmission electron microscopy (HRTEM). A HRTEM image (Figure 4d) of a single nanoparticle (marked in Figure 4c) shows clear lattice fringes with the lattice interplanar spacing of 0.425 nm, giving additional evidence that the nanoparticles are highly crystalline. The stability of these microspheres is extraordinarily good at ultrasonic conditions, though they are composed of nanoparticles. These microspheres could keep their morphology intact when the ultrasonic time was prolonged from 5 to 30 min (see Figure S3). Recently, many reports demonstrated that functional surfactants on the surface of nanoparticles have crucial influence on their self-assembly behavior.2a,27 The adsorption of organic surfactants on crystalloids also has been an effective way to (27) (a) Cao, H.; Qian, X.; Wang, C.; Ma, X.; Yin, J.; Zhu, Z. J. Am. Chem. Soc. 2005, 127, 16024. (b) Jia, B.; Gao, L. Cryst. Growth Des. 2008, 8, 1372. (c) Liu, J.; Huang, X.; Sulieman, K. M.; Sun, F.; He, X. J. Phys. Chem. B 2006, 110, 10612. (28) Liang, J.; Bai, S.; Zhang, Y.; Li, M.; Yu, W.; Qian, Y. J. Phys. Chem. C 2007, 111, 1113.

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Figure 5. SEM images of InVO4 product obtained without SDBS: (a) low magnification; (b) high magnification.

Figure 6. TEM images of InVO4 samples obtained using different amounts of SDBS: (a) without SDBS; the molar ratio of SDBS-to-InCl3 of (b) 3.5, (c) 8, and (d) 10.

control the size and shape of nanocrystals.28 In our experiment, to highlight the influence of SDBS as a capping agent on the assembly behavior of InVO4, a series of experiments were conducted. Figure 5 displays SEM investigations of the InVO4 sample obtained without the use of any surfactant at 150 °C. An overview image is given in Figure 5a and shows that the particles exhibit a spherical shape with a nonuniform size ranging from 2 to 4 µm in diameter. an SEM image of a single microsphere at high magnification shows (Figure 5b) that the ill-defined microsphere with incompact structure was aggregated by irregular

particles, which is different from that case with the use of SDBS. The reason may be that when the particles were not restricted by the functional surfactant SDBS, they would grow anisotropically and aggregate in a random way to form spheres. TEM image (Figure 6a) further confirms its shape, in good agreement with that from above SEM images. Figure 6b-d gives a comparison of representative TEM images of microstructures obtained with a constant temperature of 150 °C dependent on the SDBS-to-InCl3 molar ratio. When the SDBS-to-InCl3 molar ratio was 3.5 (0.3 g of SDBS), microspheres with an average

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Figure 7. Typical N2 gas adsorption-desorption isotherm of InVO4 microspheres. The inset is the corresponding pore-size distribution.

diameter of 3 µm could be observed, although the dominant structure remained a little loose and the subunits still contained some irregular particles (Figure 6b). Further increase in the molar ratio of SDBS-to-InCl3 to 6 (0.5 g of SDBS) led to the generation of relatively regular microspheres with diameter of 2 µm, as shown in Figure 2. When the molar ratio of SDBS-to-InCl3 was increased to 8 (0.7 g of SDBS), we got InVO4 microspheres of 1 µm in diameter with compact structure (Figure 6c). These results clearly confirmed that the amount of SDBS used here could affect the size of microspheres. It can be seen that SDBS not only strongly adsorbed on the surface of the nanoparticles, controlling them to grow into uniform nanocrystals serving as building blocks, but also profoundly influenced the assembly of these building blocks by interaction force. However, when the molar ratio of SDBS-to-InCl3 was increased to 10 (0.9 g of SDBS),

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well-dispersed microspheres were obtained but still with diameter size of 1 µm (Figure 6d), which illuminated that the structure had achieved a relatively stable state. Therefore, it was proposed that the interactions between adsorbed SDBS were not only attractive but also repulsive, which functioned together and were beneficial for the system to reach the desired balance between the crystalline growth and assembly.4a In addition, the XRD results of the samples obtained by using different SDBS-toInCl3 molar ratios are summarized in Figure S4, from which the pure orthorhombic phase of InVO4 can be identified. To understand the porous structure of the 3D hierarchical InVO4 architecture, full nitrogen sorption isotherms were measured to gain the information about the nature of porosity. As can be seen in Figure 7, a molecular monolayer is first formed in the low-pressure region (below the pressure as marked by the arrow). Further increase of the pressure leads to multilayer adsorption.29 When the pressure is quite high, the adsorption volume increases sharply but the desoption hysteresis occurs (in the reduced pressure range from 0.6 to 0.98), indicating the condensation of nitrogen in the pores within this pressure range. The BET nitrogen sorption isotherms are of reverse “S” shape, which is identified as type IV and characteristic of mesoporous materials with pores in the size ranging from 2 to 50 nm.2a,29 The corresponding BJH analyses exhibit bimodal mesoporous distribution: the peak values are 2 and 15 nm, respectively, as presented in the inset of Figure 7. Most of the pores fall into the size range from 6 to 17 nm, which is in agreement with the results of the FESEM analysis (see Figure 3d). These pores presumably arise from the spaces among the small nanocrystallites within an InVO4 microsphere. And the bimodal mesopores will endow the as-prepared hierarchical microspheres with novel application potentials.7a,30 In addition, the surface area and pore volume of these microspheres are 18.4 m2/g and 0.057 cm3/g

Figure 8. SEM images of InVO4 products attained after (a) 8 h, (b) 16 h, (c) 20 h, and (d) 24 h.

Mesoporous InVO4 Hierarchical Microspheres

Figure 9. Schematic illustration of the formation process of InVO4 hierarchical microspheres.

Figure 10. (a) UV-vis diffuse reflectance spectrum of as-prepared InVO4 powders. (b) PL excitation (red) and emission spectra of InVO4 microspheres.

from BJH desorption summary, respectively. The apparently special low surface area for mesoporous material results from the high density of InVO4 because the metal oxides, especially multicomponent metal oxides, always have a high density.31 3.2. Growth Mechanism. To reveal the growth process of InVO4 hierarchical microspheres, time-dependent experiments were also carried out. In this experiment, only reaction time was changed, keeping other reaction parameters constant. As shown in Figure 8a, when a short reaction time of 8 h was used, irregular particles were formed without a discernible morphology. When the reaction time was prolonged to 16 h, these irregular particles started to grow into comparatively regular nanoparticles (Figure 8b). In addition, it can be observed that some nanoparticles tend

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to organize into micrometer-scale spheres (marked by circularity). With further extention of the reaction time to 20 h, well-defined microspheres were almost formed though some nanoparticles still existed around them, as shown in Figure 8c. After 24 h of reaction time, nanoparticles disappeared, and perfect 3D hierarchical InVO4 microspheres with average diameter size of 2 µm were obtained (Figure 8d). On the basis of time-dependent morphology evolution and SDBS-to-InCl3 molar ratio influence observation, along with crystal structure analysis and the SEM and TEM investigations above, the formation process of the 3D hierarchical InVO4 microspheres can be proposed to include Oswald ripening and self-assembly aggregation. The whole process could be schematically illustrated in Figure 9. First, large numbers of tiny InVO4 crystalline nuclei are generated through conventional nucleation after the ions of In3+ and VO43- meet in aqueous solution in the presence of SDBS (step 1). During the second stage, the growth of crystals results in the formation of irregular InVO4 nanoparticles through the typical Oswald ripening process (step 2) in which smaller particles form initially but slowly disappear due to their high solubility and crystallize to larger particles according to the Gibbs-Thomson law.32,33 Then the bigger particles further grow at the cost of the smaller ones. At the same time, these as-formed nanoparticles have a higher surface energy. Driven by the minimization of interfacial energy, the InVO4 microspheres are formed through the self-assembly aggregation of the nanoparticles (step 3). In the following step, Oswald ripening and self-assembly aggregation continues (step 4). This aggregation process of nanocrystals gives rise to textural mesopores of the product. Ultimately, the well-dispersed polycrystalline microspheres are formed through the cooperation of Oswald ripening and self-assembly. In this work reported here, the two basic processes to some extent assisted each other in the formation of final stable InVO4 microspheres. 3.3. Photoluminescence Property. The room-temperature UV-vis diffuse reflectance and photoluminescence spectra (excitation and emission) of the mesoporous InVO4 hierarchical microspheres are shown in Figure 10. For the UV-vis diffuse reflectance measurement, the as-synthesized InVO4 microspheres exhibited a strong peak in the UV region, as shown in Figure 10a, which is in accordance with the value of literature reported.9a,10a The excitation spectrum consists of a broad band below 350 nm with a maximum at 314 nm (Figure 10b, red curve), which is almost in good agreement with the result from the UV-vis diffuse reflectance spectrum. Upon such maximum excitation, the obtained PL spectrum (Figure 10b, black curve) shows that the InVO4 microspheres display an intensive visible emission which is mainly located in the green region with its maximum intensity centered at 521 nm. Additionally, this green emission is beneficial for its application in optic devices.34 Generally, the PL spectrum in the range of visible light for metal oxides is mainly attributed to the effect of impurities and structural defects, such as oxygen vacancies.35,36 The mesoporous InVO4 microspheres should favor the existence of large quantities of (29) Lu, F.; Cai, W. P.; Zhang, Y. G. AdV. Funct. Mater. 2008, 18, 1047. (30) (a) Rolison, D. R. Science 2003, 299, 1698. (b) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Bieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (c) Wong, S. T.; Lin, H. P.; Mou, C. Y. Appl. Catal., A 2000, 198, 103. (d) Zhang, L. Z.; Yu, J. C. Chem. Commun. 2003, 16, 2078. (31) (a) Ba, J. H.; Polleux, J.; Antonietti, M.; Niederberger, M. AdV. Mater. 2005, 17, 2509. (b) Nguyen, P.; Ng, H. T.; Kong, J.; Cassell, A. M.; Quinn, R.; Li, J.; Han, J.; McNeil, M.; Meyyappan, M. Nano Lett. 2003, 3, 925. (32) (a) Oswald, W. Z. Phys. Chem. 1897, 22, 289. (b) Oswald, W. Z. Phys. Chem. 1900, 34, 495. (33) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemaan: Oxford, U.K., 1997. (34) Tang, Q.; Zhou, W.; Zhang, W.; Ou, S.; Jiang, K.; Yu, W.; Qian, Y. T. Cryst. Growth Des. 2005, 5, 148.

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oxygen vacancies. The emission from InVO4 microspheres may result from the radioactive recombination of electron occupying oxygen vacancies with a photoexcited hole, which is analogous to the PL mechanism of ZnO, and In2O3 semiconductors.2d,28,36 The green emission process in InVO4 samples can be described as follows: under the excitation of 314 nm irradiation, the electrons are excited from the valence band (VB) to the conduction band (CB). The electrons move freely in the CB and finally relax to the oxygen vacancies. Similar to the oxide semiconductors, the oxygen vacancies would induce the formation of new energy levels in the band gap.2d,36 The recombination of an electron occupying oxygen vacancies with a photoexcited hole yields the green emission with a maximum wavelength at 521 nm.

4. Conclusion In summary, hierarchical mesoporous InVO4 3D microsphere structures constructed with numerous uniform nanoparticles were successfully synthesized by a hydrothermal process with the assistance of SDBS. Nitrogen absorption-desorption measurement indicated that there are mesopores existing in these InVO4 (35) Fleischauer, P. D.; Fleischauer, P. Chem. ReV. 1970, 70, 199. (36) (a) Chen, S. J.; Liu, Y. C.; Shao, C. L.; Mu, R.; Lu, Y. M.; Zhang, J. Y.; Shen, D. Z.; Fan, X. W. AdV. Mater. 2005, 17, 586. (b) Liang, C. H.; Meng, G. W.; Lei, Y. AdV. Mater. 2001, 13, 1330.

Li et al.

microspheres. The shape evolution process was investigated by time-dependent experiment investigations. The cooperation of Oswald ripening and nonoriented attachment is responsible for the formation of such structure. SDBS played an important role in the formation of such well-defined microsphere structures. The as-formed InVO4 shows a strong green emission due to the oxygen vacancies at room temperature. We believe that the photoluminescence property of the InVO4 microspheres may have some promising applications in the future. Acknowledgment. The authors thank the National Natural Science Foundation of China (NSFC, No. 20771022) and the Huo Yingdong Foundation for financial support. This work also was supported by Analysis and Testing Foundation of Northeast Normal University and Jilin Province Science and Technology Development Planning. Supporting Information Available: Raman spectrum of the as-prepared 3D hierarchical InVO4 microspheres; infrared spectra of the as-prepared 3D hierarchical InVO4 microspheres; TEM images of InVO4 samples at different ultrasonic times; XRD results of the InVO4 products prepared by using different amounts of SDBS. This material is available free of charge via the Internet at http://pubs.acs.org. LA803682D