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Facile Synthesis and Assemblies of Flowerlike SnS2 and In3+-Doped SnS2: Hierarchical Structures and Their Enhanced Photocatalytic Property Yongqian Lei,†,‡ Shuyan Song,†,‡ Weiqiang Fan,†,‡ Yan Xing,†,‡ and Hongjie Zhang*,† State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: NoVember 16, 2008
Novel flowerlike SnS2 and In3+-doped SnS2 hierarchical structures have been successfully synthesized by a simple hydrothermal route using biomolecular L-cysteine-assisted methods. The L-cysteine plays an important role both as assistant and as sulfur source. Experiments with various parameters indicate that the pH values have a strong effect on the morphology of the assembly. Based on the experiments, a growth mechanical process was proposed. The synthetic samples were characterized by XRD, SEM, TEM (HRTEM), BET measurement, TGA, and XPS in detail. Further investigation of the photocatalytic degradation of three different dyes, methylene blue, methylene green, and ethyl violet, indicate that both samples have high photocatalytic activity, and the doped In3+ has enhanced the photoactivity of SnS2. Introduction In the past decades, ordered nanostructures, assemblies using semiconductor nanoparticles, nanorods, nanobelts, and nanosheets, as building blocks, have attracted great interest because of their collected physics and chemistry properties.1 It is an important process for the fabrication of functional electronic and photonic nanodevices using these building blocks. In this field, remarkable progress has been made for the controllable synthesis of complex inorganic materials.2 However, it remains a big challenge to freely operate the material at the micro/nanolevel using a facile method. Among all the semiconductors, the metal chalcogenides have driven extensive research interest when they were prepared at the micro/nanolevel.3 Metal sulfides have narrow band gaps and band at relatively negative potentials compared with corresponding oxides, which can be good candidates for photovoltaic materials, photocatalysts, and water treatment especially when they have hierarchical structures.4 Nowadays diverse morphologies such as fullerene-like nanoparticles, nanobelts, nanotubes, and nanoflakes have been synthesized by various methods including chemical vapor deposition, electrochemical deposition, molecular beam epitaxy, template-assisted solvothermal, hydrothermal, and solvothermal treatments.5 However, there are still very few reports on a general facile, low-energy spent route. Of all the methods, biomolecule-assisted synthesis methods have became a new and promising focus in the preparation of various micro/nanomaterials. Biomolecules play an important role in morphology control as shown in our previous work and are also environmently friendly ways to practice green chemistry.6 Recently, Xie et al. synthesized Bi2S3 and PbS hierarchical architectures by biomolecular L-cysteineassisted methods.7 Inspired by their work, it is interesting to investigate the preparation and assembly of other sulfide nanomaterials by L-cysteine-assisted biological approaches. It * To whom correspondence should be addressed. Telephone: +86-43185262127. Fax: +86-431-85698041. E-mail:
[email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
is also necessary to fabricate facile, rapid synthesis methods of micro/nano hierarchical materials to meet the potential application fields. As one of the important IV-VI semiconductors, SnS2 is known for its strong anisotropy of optical properties and potential applications in efficient solar cell materials as well as electrical switching.8 In our previous work, SnS2 nanosheets were synthesized by the surfactant-assisted hydrothermal route, which represented excellent gas sensitivity.9 In this work, we reported a facile one-pot hydrothermal route synthesis of flowerlike SnS2 and In3+-doped SnS2 hierarchical structures and investigated the structure and morphology with comprehensive techniques including electron microscopy (SEM/TEM), X-ray diffraction (XRD), XPS, TGA, and nitrogen adsorption characteriztion. By monitoring the structure evolution with various experimental parameters during the reaction process with electron microscopy, we proposed a formation mechanism of the hierarchical flowerlike structure assembly process. Furthermore, it is known that composition affected their properties. Therefore, another aim of this work is to investigate the evolution of photocatalytic activity because the serious environmental issues associated with industrial pollution drive us to search for new catalytic materials for contamination treatment. It is also a hot topic for the direct solar-to-chemical conversion/ degradation of organic pollutants by semiconductor-based catalysts in environmental science.10 Experimental Section All the reagents were of analytical grade and were purchased and used as received without further purification. In a typical experiment, 0.35 g (1 mmol) SnCl4 · 5H2O and 0.25 g (about 2.1 mmol) L-cysteine were dissolved in 30 mL of deionized water, and then the mixture underwent ultrasonic treatment for a few minutes to become a transparent solution. The above solution was transferred to a 50 mL Teflon-lined autoclave and heated under an electronic oven at 160 °C for about 24 h. The In3+-doped SnS2 sample was obtained when 0.4 mL of 0.05 M InCl3 (2%) solution was added before hydrothermal treatment. The samples were washed several times using deionized water
10.1021/jp8079974 CCC: $40.75 2009 American Chemical Society Published on Web 01/06/2009
Flowerlike SnS2 and In3+-Doped SnS2
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and ethanol, respectively, and dried in a vacuum oven at 60 °C overnight for further characterization. Characterization X-ray Diffraction and Electron Microscopy Characterization. The crystalline and phase purity of the products were examined by powder XRD. Measurements were performed on a Rigaku X-ray diffractometer with Cu KR radiation; the accelerating voltage and applied current were 40 kV and 40 mA, respectively. The size, general morphology, and structure of the as-synthesized samples were characterized using fieldemission scanning electron microscopy (FEI XL30 EFSEM) at an accelerating voltage of 15 kV and a Hitachi 8100 transmission electron microscope (TEM) at an operating voltage of 200 kV. Surface Properties and TGA Characterization. A Thermo ESCALAB 250 X-ray photoelectron spectroscope (XPS) equipped with a standard and monochromatic source (Al KR hν ) 1486.6 eV) was employed for surface analysis. TGA measurements were performed on a Perkin–Elmer Thermal Analysis Pyris Diamond TG/DTA. The samples were heated under a N2 gas stream from 40 to 700 °C at a heat rate of 10 ° C/min. Nitrogen Adsorption and Desorption. The specific areas of the powders were determined by using a Micromeritics ASAP 2020 specific area and porosity analyzer using the method of Brunauer–Emmett–Teller (BET). All the samples were degassed at 373 K overnight before BET measurements. Photocatalytic Activity Test. The photocatalytic measurements were carried out in an aqueous solution at ambient temperature and performed on a three-necked column container, which was equipped with a glass pipe inset. Briefly, a 10 mg sample was suspended in a 300 mL aqueous solution of 0.025 mM dye. The cylindrical quartz pipe was surrounded by a circulating water jacket to cool the system. Prior to irradiation, the suspension was magnetically stirred in the dark for 10 min to establish an adsorption/desorption equilibrium. The samples were evaluated by degradation of dyes in an aqueous solution under a 300 W mercury lamp. The cooling water was placed around the quartz lamp and pipe wall to take the heat away. The concentration of the dyes during the degradation was monitored by collecting solutions in step time using a UV-vis spectrometer (TU-1901). Simultaneously, the collected solution was filtered to remove the residue before UV-vis characterization.
Figure 1. XRD patterns of the samples (a) SnS2 and (b) In3+-doped SnS2.
Results and Discussion The phase purity of the prepared samples was characterized by powder X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the products, and the sharp pattern indicates that the products were well crystallized. All the peaks in the XRD pattern can be readily indexed to a pure hexagonal phase of SnS2 with lattice constants a ) 3.649 Å and c ) 5.899 Å, which are in good agreement with the reported values (JCPDS card No. 22-0951). Interestingly, the (001) diffraction peak showed the strongest intensity in the pattern. These observations may indicate that their (001) planes tend to be preferentially oriented parallel to the surface of the supporting substrate. Compared with the XRD patterns of the two samples, the introduction of 2% In3+ did not destroy the hexagonal phase structure. The typical overall SEM image shows that the samples are uniform flowerlike spheres with an average diameter of 1.5 µm (Figure 2a). A magnified view shows that these individual flower-shaped spheres consist of large number of uniform nanoplate architectures as building blocks. These quantities of
Figure 2. SEM images of the synthesized SnS2: (a) overview, (b) enlarged and (c) TEM image of the samples, and (d) an individual nanoplate. SEM image of the synthesized In3+-doped SnS2: (e) overview, (f) enlarged and (g) TEM image of the samples, and (h) an individual nanoplate.
nanoplates with interpenetrating growth order are connected to each other to build flowerlike architectures. With long-time ultrasonic treatment, a part of the individual flowerlike sphere is observed. The deep black TEM image indicates that the nanoplates are closely packed across the flower center. The individual flowerlike structure is further characterized by high-resolution TEM (HRTEM). As shown in Figure 3, the distortion of the
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Figure 3. (a) TEM images of the individual SnS2 nanoflower. (b) HRTEM image and (c) corresponding FFT pattern.
Figure 4. XPS spectra of the doped SnS2 sample: (a-c) high-resolution spectra of S 2p, Sn 3d, and In 3d of the sample without surface treatment (black line) and with surface treatment (red line). (d) Survey of the XPS spectrum.
nanoplate is clearly observed. The magnified image shows the clear lattice stripe (Figure 3b) of the nanoplate. This image reveals that the interplanar distance of the lattice fringes is 0.295 nm, which is consistent with the (002) planes of a hexagonal phase SnS2. The fast Fourier transform (FFT) pattern displays a bright spot of the lattice fringes in Figure 3c, which shows a SnS2 hexagonal phase diffraction pattern. Interestingly, when 2% In3+ was introduced in the synthetic SnS2 sample, the color of the dried sample changed from buff to reseda. Simultaneously, the assembled building block of the nanoplates seems to be frizzy, and their surface tends to be smooth (Figure 2e). Compared with the pure sample, the flowerlike spheres tend to connect to each other (see enlarged Figure 2f). The difference between the two samples can also be seen in the TEM images (Figure 2g and 2h). Unlike the closepacked SnS2 hierarchical structure, the clear contrast of the dark center and bright edge indicate that the crimped nanoplates grew outward. A close view of the individual flower also shows the inner configuration. The building blocks of the nanoplates were observably thin and crimped. The surface electronic states and the chemical composition of the doped sample were further confirmed by XPS analysis. Figure 4a shows the high-resolution S 2p spectra. The weak shoulder peak may be due to the residual organic sulfur reactant. The two strong peaks of Figure 4b can be indexed to the Sn 3d3/2 and Sn 3d5/2, respectively. To precisely investigate the In electronic state, the sample was examined in situ with electron
Lei et al.
Figure 5. TGA curve of the samples: (a) In3+-doped SnS2 and (b) SnS2.
Figure 6. Nitrogen adsorption-desorption isotherm of the prepared SnS2 sample and pore volume distribution curve (inset).
beam treatment for 6 min to remove the surface area. It can be seen that the In 3d peaks are remarkably enhanced (red line). The maximum peak of In 3d5/2 at 444.5 eV can be attributed to the In-S binding energy (BE). The small offset of the peaks at the surface (black line) may be attributed to the partial oxidation. Figure 4d shows the overall spectrum of the doped sample; the weak C1s and O1s peaks come from adsorbed reactant and gaseous molecules in the atmosphere. The normalized TGA analysis curves (Figure 5) indicate that the two samples are decomposed at about 430 °C. The slow decline of the curve before 400 °C may be attributed to the adsorbed water in the atmosphere and the residual organic molecules on their large surface area. The clearly different weight percentages of the final products are ascribed to the decreased percentage weight loss of the doped In atom. The samples at different pH values were also tested (see Figure S4). It can be seen that the curves of samples at pH 3 and 7 are similar. The remarkable decrease before 400 °C of the sample at pH 10 may be attributed to hydroxyl at basic conditions. All the samples show maximum weight loss at about 430 °C, which indicates their stability toward the pH value. Nitrogen gas adsorption-desorption isotherms and BarrettJoyner-Halenda (BJH) methods characterization further verified the adsorption ability of the hierarchical flowerlike SnS2 (Figure 6) and In3+-doped SnS2 (Figure S1). The type-IV isotherm with a hysteresis loop in the range of 0.4-1.0 P/P0 is in accordance with the assembled platelike structure11 and similar to the flowerlike In2S3 hierarchical nanostructure.12 The quantitative calculation shows that the BET surface area of as-prepared SnS2
Flowerlike SnS2 and In3+-Doped SnS2
Figure 7. SEM images of the samples at different pH values: SnS2 (a) pH 7.6 with detailed image (inset); (b) pH 10, In3+-doped SnS2; (c) pH 7.3 with detailed image (inset); (d) pH 9.7.
and In3+-doped SnS2 flowerlike architectures are 95 and 106 m2/g, respectively. This result is higher than the reported flowerlike Fe2O3 (40 m2/g),13 In2S3 (78 m2/g),12 MnO2 (60.4 m2/ g),14 CeO2 (34 m2/g),15 and Ni(OH)2 (68.03 m2/g)16 with similar morphology. The increased surface area may be attributed to the small assembled hierarchical building blocks. This type of hierarchical 3D architecture with high surface area is beneficial for the applications as catalyst or for water treatment. The Influence of pH Value. We further investigated the influence of experimental parameter variation on the assembled morphology. It is well-known that amino acids have hydrolysis equilibrium in solution. The cysteine solution presents acidity owing to the stronger dissociation of the carboxyl group than the amino group (-NH3+).17 The pH value is about 3 when the reactants are mixed naturally. While the solution was adjusted to pH ) 7 by ammonia solution, the obtained sample turned to gray. Furthermore, only deep gray colloid was obtained under the same conditions when the pH value was 10. The detailed corresponding morphologies of the samples are shown in Figure 7. When the pH value was adjusted to 7, the obtained products were near uniform spheres 1-2 µm in diameter (Figure 7a). The detailed individual image (inset) indicated that these microspheres were composed of small crimp nanosheets with close-packed assembly. When the pH value was changed to 10, only crimped nanosheets were obtained at the same reaction conditions. This morphology variation tendency was also found in the synthesis of In-3+ doped SnS2. The products were crimped rose flowers (as shown in Figure 7c) when the pH value was set at 7. The detailed flower (Figure 7c inset) indicated that the nanosheet across the axis formed the flower architecture. When the pH increased higher, only crimped colloid was obtained. The Influence of Various S Sources. During the metal sulfur synthesis, various S sources were used such as S powder, thiourea Na2S, and thioacetamide. In our reaction system, these S sources were also used to study the morphology of the assembly. As shown on the Figure 8, four different morphologies were obtained with Na2S, thiourea (Tu), thiosemicarbazide, and thioacetamide (TTA) instead of L-cysteine. When Na2S was added to the SnCl4 solution, yellow precipitate generated rapidly owing to the high concentration of S2- quickly supplied, and the final morphology was nanoparticles (Figure 8a). With thiourea and TTA, the morphology changed to irregular nanoplates. Compared with those S sources, L-cysteine as an ordinary amino acid biomolecule has three functional groups, -NH2, -COOH, and -SH, which have a strong tendency to coordinate with inorganic cations and metals.9 This precursor complex
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Figure 8. SEM images of SnS2 samples with different S sources: (a) Na2S, (b) thiourea (Tu), (c) thiosemicarbazide, and (d) thioacetamide (TTA).
SCHEME 1: Possible Growth Mechanical Scheme of the Assembly Flowerlike Structure
decomposed to form the SnS2 nuclei in a supersaturated medium at the initial stage. Followed by a general crystal growth kinetic at the expense of the small crystals, the hexagonal plate formed in advance with the inner hexagonal phase. The presence of these groups has made cysteine a commonly used self-assembly reagent in the preparation of 3D hierarchical structure. It acts both as S source and template inducer agent in the solution during the hydrothermal route. It is believed that the physical and chemical properties of the solvent can influence the solubility, reactivity, and diffusion behavior of the reagents and the intermediate.18 In our reaction system, pH-dependent experiments indicate that the morphology of the samples is sensitive to the pH value. It seems that high basicity is beneficial for the formation of crimped nanosheets. Although the exact assembly process is unclear, a possible growth mechanical process can be illustrated in Scheme 1 based on above experiments. Here, we present evidence for the formation mechanism of the assembled objects from nanoplates to two types of the flower-shaped morphologies directly captured by electron microscopy. The two major morphologies can be assumed in which there are two steps in the generation process of the flowerlike structures: First, flat and smooth 2D nanosheets are formed based on the traditional nuclei formation growth theory.19 Second, with the random connection there are two types of connected nanosheets: one is edge to edge; the other is angle to angle. Under basic conditions, the nanosheet growth tends
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Figure 9. UV-vis absorption spectra of photocatalytic decomposition of methylene blue dye at step time (left) and curves of concentration decay for different samples: (a) blank, (b) commerce P25, (c) SnS2, and (d) In3+-doped SnS2.
to be the former. With acidic conditions, the growth tends to be the latter. Interestingly, in the former type, two kinds of helixes we called left-hand and right-hand are observed during the assembly process. This phenomenon may be due to the different growth direction of the border. Finally, the two kinds of helical nanosheets continue to grow across the axis and radial direction and form the flowerlike architecture. In the other type, the angle to angle nanosheets grow under two directions evenly matched. With the continued growing, the cross-linked nanosheets are eventually formed. This type of assembly was also observed on the Co(OH)2 and fullerene structure.20 Photocatalytic Property. We further compared the photocatalytic activity of the two samples by analyzing the photodegradation of three different dyes under UV-vis light irradiation. Methylene blue (MB), methylene green (MG), and ethyl violet (EV) were used as probes of the prepared samples. The initial dye concentration is 0.025 mM. The photocatalytic performance of the two samples was estimated from the variation of the color in the reaction system by its visible light absorption intensity. Total concentrations of all dyes were simply determined by the maximum peaks of the MB, MG, and EV, which were located at 664, 653, and 595 nm, respectively. Figure 9 shows the typical photodegradation MB curve of the prepared SnS2 sample. The intensity of major absorption peaks at 664 nm decreases step by step. In order to compare the photocatalytic performance, the Degussa TiO2 photocatalyst commerce P25 was used as the reference. As shown on the concentration charge curve, the blank sample, without any catalyst, shows liner decrease (Figure 9a). With both our samples (Figure 9c and 9d) and P25 (Figure 9b), the dye concentration decreases remarkably in the initial 10 min. The concentration decays to half of the initial for about 20 min. Only after 1 h of irradiation, the conversion is close to 90% for both samples and 80% for P25, which suggests the excellent photocatalytic activity of the prepared samples. This typical concentration curve obeys the pseudo-first-order kinetics law according to the reported photodegradation MB experiment.21 Obviously, the photocatalytic performance of In3+-doped SnS2 is enhanced compared with the pure SnS2 sample. Other dyes were also tested under the same conditions, and similar results were obtained. The corresponding adsorption and concentration decay curve (Figure S2-S3) shows the same enhanced photocatalytic performance. Based on these results, it can be seen that the dopant can improve the photocatalytic ability of SnS2. We also did the photocatalytic experiments under visible light irradiation. However, these photocatalysts are inactive in the visible region. The absorption spectrum of the doped sample (Figure S5) shows a weak blue shift, and little enhancement of absorption in the UV region. However, the detailed mechanism is still unclear, and there are no related reports so far. For the photocatalysts, even though the principle and activity-controlling
Lei et al. factors of the photomineralization process in semiconductor based photocatalysts have been discussed previously,9,22 many aspects of the function of inorganic photocatalysts are still unclear, such as the detailed mechananism reduction and oxidation on the semiconductor surface, electrons and holes transferring in and out of the catalyst, and the effect of variable material preparations and surface impurities on the catalytic activity of semiconductors.23 Considering our prepared samples, the enhancement may be attributed to the introduced dopant ion result in the generation of holes which efficiently suppress the photocorrosion and improve the photocatalytic activity of sulfide photocatalysts.24 On the other hand, the large value of surface to volume ratio can increase the number of active surface sites where the photogenerated electron-hole pairs are able to induce more hydroxyl and superoxide radicals to react with absorbed molecules.25 As for our prepared two samples, SnS2 (BET 95 m2/g) and In3+-doped SnS2 (BET 106 m2/g), the hierarchical structure is beneficial to quicken the rate of interfacial charge transfer and inhibit high rate of charge carrier recombination.26 Conclusion In summary, 3D flowerlike SnS2 and In3+-doped SnS2 hierarchical structures were successfully synthesized through a simple one-pot hydrothermal route. Electronic microscopy, XRD, and XPS characterization confirmed their structure. TGA and BET characterization indicated that the samples have good thermal stability and large surface area, which ensured their potential application. A systematic approach was applied to the fundamental factors’ (various S sources, pH values) effect on the synthesis, assembly, and morphology. During our experiments, L-cysteine played an important role both as S source and template agent. Interestingly, there are two types of assembly when the pH value changed. These two kinds of assembly finally lead to two types of flowerlike structures, and a possible growth mechanical was discussed. The experiments of photocatalytic degradation of three dyes indicated that both of our prepared samples have high photocatalytic activity, and the doped In3+ can improve the photocatalytic activity. This facile preparation method and assembly reported here are expected to be utilized in fabrications of unique hierarchical structures of various semiconductor functional materials with advanced properties. Acknowledgment. The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grant Nos. 20631040 and 20771099) and the MOST of China (Grant Nos. 2006CB601103 and 2006DFA42610).. SupportingInformationAvailable: N2 adsorption-desorption isotherms of the doped sample, photocatalytic decomposed curves of MG and EV, TGA curves of the samples at different pH values, and absorption spectra of the samples. These materials are free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (b) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (c) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (2) (a) Alivisatas, A. P. Science 1996, 271, 933. (b) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (c) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (d) Zeng, H. C. J. Mater. Chem. 2006, 16, 649. (3) (a) Pedro, U. J.; Camargo, H. C.; Lee, Y. H.; Xia, Y. N. J. Mater. Chem. 2006, 16, 3893. (b) Mitzi, D. B.; Kosbar, L. L.; Murray, C. E.; Copel, M.; Afzali, A. Nature 2004, 428, 299. (c) Kumar, S.; Nann, T. Small 2006, 2, 316.
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