Microtubes with Square Cross-Sections: Microstructure, Growth

interior structure, small wall thickness, large surface areas, and possible ... synthetic techniques and processes, it is of great importance to study...
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J. Phys. Chem. C 2007, 111, 13659-13664

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ARTICLES Single-Crystalline BiVO4 Microtubes with Square Cross-Sections: Microstructure, Growth Mechanism, and Photocatalytic Property Lin Zhou, Wenzhong Wang,* Lisha Zhang, Haolan Xu, and Wei Zhu State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China ReceiVed: August 10, 2006; In Final Form: May 29, 2007

Single-crystalline bismuth vanadate (BiVO4) microtubes, with novel square cross-sections and flower-like morphology constructed by several tubes radiating from the center, were synthesized by a facile reflux method at 80 °C. No surfactants or templates were involved in the shaping process. The microtubes are of monoclinic structure with a [010] growth direction with a side length of ca. 800 nm and a wall thickness of ca.100 nm. A series of morphology evolutions of BiVO4 from nanoparticles, to microrods, and then to microtubes have been arrested. The growth mechanism for the BiVO4 microtubes is proposed to be a dissolutionrecrystallization-induced concentration depletion mechanism, which is different from other micro/nanotubes formed by layered structure materials reported previously. The presence of NaHCO3 is crucial in forming the tubular structure by the equilibrium of the formation and dissociation of carbonic acid. Optical absorption experiments revealed the BiVO4 microtubes had strong absorption in the visible light region in addition to the UV light region, and the energy of the band gap was estimated to be 2.36 eV. The as-synthesized microtubes exhibited higher photocatalytic activity under visible light irradiation (λ > 400 nm) than that of the reference sample prepared by solid-state reaction, which may be ascribed to the special single-crystalline tubular structure and/or flower-like morphology.

Introduction Since the discovery of carbon nanotubes in 1991,1 onedimensional (1D) micro/nanoscale tubular materials with hollow interior structure, small wall thickness, large surface areas, and possible quantum-confinement effects have attracted extraordinary attention owing to their unique properties and potential applications as sensors, catalysts, electric transportation, optics, or templates for construction of other functional materials.2 On the other hand, recently there has been an increasing number of excellent studies on novel 1D micro/nanomaterials with various cross-section configurations which are found to have a “shape effect” on their properties.3 Many 1D materials with distinct cross-section morphologies have been achieved. Wang et al. synthesized semiconducting nanobelts or nanoribbons (ZnO, SnO2, In2O3, CdO, and Ga2O3) with rectangular-like cross-sections by a simple thermal evaporation method.4 Using a vapor transport and condensation technique, Yang et al. reported epitaxial growth of hexagonal cross-section ZnO nanowires that grow perpendicular to sapphire substrates.5 Most recently, Hu et al. reported a simple catalyzed vapor-liquidsolid (VLS) growth resulting in silica fibers with triangular cross-sections.6 These results indicated that the cross-section configuration could be tailored by controlling the growth conditions. However, the cross-sections of micro/nanotubes generally tend to be circular due to the relatively low surface * To whom correspondence should be addressed. Phone: +86-21-52415295. Fax: +86-21-5241-3122. E-mail: [email protected].

energy.7 Although macroscale tubes of square cross-sections are frequently used as energy absorbing structural elements,8 few kinds of rectangular or square inorganic micro/nanotubes have been reported so far.9 To the best of our knowledge, the synthesis of single-crystalline square-shaped tubular structure via a mild template-free solution strategy has not yet been reported. Bismuth vanadate (BiVO4) has long been recognized as an important semiconductor due to its unique properties such as ferroelasticity,10 acousto-optical,11 ionic conductivity, etc.12 It has been used for a wide range of applications including gas sensors, posistors, solid-state electrolytes, positive electrode materials for lithium rechargeable batteries, and nontoxic yellow pigment for high-performance lead-free paints13 and recently proved to be a good photocatalyst for water splitting and pollutant decomposing under visible light irradiation.14 BiVO4, a layered structured compound, exists in three phases: monoclinic scheelite, tetragonal zircon, and tetragonal scheelite.15 In the early literature, monoclinic BiVO4 is usually synthesized by solid-state or melting reactions at high temperature while tetragonal zircon is prepared by the low-temperature method in aqueous solution.16 Recently, some new wet chemical methods have been developed for preparation of BiVO4, such as chemical bath deposition, hydrothermal treatment, metalorganic decomposition, sonochemical method, and mild solution process17 For example, Kudo et al.18 selectively synthesized monoclinic and tetragonal zircon BiVO4 at room temperature in aqueous solution by reactions of layered potassium vanadates

10.1021/jp065155t CCC: $37.00 © 2007 American Chemical Society Published on Web 08/28/2007

13660 J. Phys. Chem. C, Vol. 111, No. 37, 2007 with Bi(NO3)3. Later, they successfully obtained BiVO4 of a high-temperature form with tetragonal scheelite phase at room temperature by hydrolyzing a nitric acid solution of Bi(NO3)3and NaVO3.19 Monoclinic BiVO4 was also fabricated via a hydrothermal treatment (140-200 °C) of an aqueous solution of bismuth nitrate and two different vanadium sources (V2O5 and NaVO3).17b More recently, Devillers et al.20 reported the synthesis of monoclinic BiVO4 from hybrid materials made from an organic polymer and inorganic salts. The properties of BiVO4 are strongly dependent on its morphology and crystal phase. It is commonly understood that the performance of an inorganic pigment depends on its morphological characteristics, such as shape and particle size distribution.21 Zhang et al.22 reported sheet-like BiVO4 nanostructures that have outstanding spectral selectivity and improved color properties compared with the bulk material. In addition, BiVO4 with a monoclinic phase exhibited much higher photocatalytic activity than that of the other two phases.18,19 Considering the properties and applications are closely related to the microstructure and corresponding synthetic techniques and processes, it is of great importance to study the controllable synthesis of BiVO4 with the desired crystalline phase and morphology. Herein, we reported the preparation of single-crystalline monoclinic BiVO4 microtubes with novel square cross-sections by a simple reflux method; no surfactants or templates were involved in our synthetic system. To study the growth process and corresponding mechanism, reactions between Bi(NO3)3‚ 5H2O and NH4VO3 were carried out for different reaction times to arrest the BiVO4 morphologies at different growth stages. The phase-transition behavior, growth mechanism, and specific roles of experimental parameters were investigated. The photocatalytic activity of the microtubes was also evaluated, which is much higher than that of the corresponding sample prepared by solid-state reaction. Experimental Section The BiVO4 microtubes were synthesized in an aqueous media under ambient pressure. All reagents used in our experiments were of analytical purity and used as received from Shanghai Chemical Co. In a typical process, Bi(NO3)3‚5H2O (2 mmol) and NH4VO3 (2 mmol) were dissolved in concentrated HNO3. Then 30 mL of deionized water was added into these two solutions, which gave a final concentration of 2 mol/L HNO3. After these nitric acid solutions of Bi(NO3)3 and NH4VO3 were mixed, NaHCO3 was added to adjust the pH to 6-7. Yellow precipitates formed immediately. The suspension was heated at 80 °C for different times in a reflux system being stirred magnetically. The final products were centrifuged, washed with deionized water and absolute ethanol several times, and dried at 80 °C for 10 h in air. For comparison, BiVO4 was also prepared by the traditional solid-state reaction (SSR-BiVO4) according to ref 23. The powder X-ray diffraction (XRD) patterns of the assynthesized samples were measured on a D/MAX 2250V diffractometer (Rigaku, Japan) using monochromatized Cu KR (λ ) 0.15418 nm) radiation under 40 kV and 100 mA and scanning with the 2θ ranging from 10° to 70°. The morphologies and microstructures of as-prepared samples were examined with transmission electron microscopy (TEM, JEOL JEM-2100F; accelerating voltage 200 kV) and scanning electron microscopy (SEM, JSM-6700F). High-resolution transmission electron microscopy (HRTEM) images were obtained by a JEM 2100F field emission transmission electron microscope operated at an accelerating voltage of 200 kV. The specimens used for TEM

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Figure 1. XRD patterns of (a) precursor and the samples after reflux treatment at 80 °C for (b) 2, (c) 6, and (d) 9 h. Inset: the widened part of XRD pattern near 19°, showing the characteristic split of the monoclinic scheelite phase.

studies were dispersed in absolute ethanol by ultrasonic treatment. The sample was then dropped onto a copper grid coated with a holey carbon film and dried in air. UV-visible diffuse reflectance spectra of the samples were obtained on an UVvis spectrophotometer (Hitachi U-3010) using BaSO4 as reference. Photocatalytic activities of the BiVO4 samples were evaluated by photocatalytic decolorization of rhodamine-B (RhB) under visible light. A 500 W Xe lamp was used as a light source with a 400 nm cutoff filter to provide visible light irradiation. The experiments were performed at room temperature as follows: 0.5 mmol of photocatalyst was added into 100 mL of rhodamine-B solution (10-5 mol/L). Before illumination, photocatalyst was dispersed in RhB solution by ultrasonic bath for 5 min. Then the solution was stirred and exposed to visible light irradiation. The concentrations of the RhB were monitored by checking the absorbance at 553 nm during the photodegradation process using a Hitachi U-3010 UV-vis spectrophotometer. Results and Discussion The phase and composition of the products obtained after different reaction times were investigated using XRD measurement. Figure 1 shows the XRD patterns of time series samples. It can be seen that the starting precipitate obtained at the first stage was almost amorphous before reflux treatment (Figure 1a). After 2 h, as shown in Figure 1b, tetragonal BiVO4 formed (JCPDS No. 48-0744). As the time increased to 6 h, all the diffraction peaks can be indexed to be a pure monoclinic phase BiVO4 (Figure 1c). The cell constants calculated by the leastsquares refinement method are as follows: a ) 5.18 Å, b ) 11.73 Å, and c ) 5.11 Å, which are in agreement with the reported values (JCPDS No. 14-0688). Compared with the tetragonal phase, the monoclinic one has a different crystal structure in which the Bi-O polyhedron is more distorted by a 6s2 lone pair of Bi3+ and the angle of the monoclinic crystal is 90.4°, whereas that of the tetragonal crystal is 90°. As shown in Figure 1c and inset, the peaks at 2θ ) 18.5°, 35°, 46°, and 59° are obviously split, which are evidence to differentiate between monoclinic and tetragonal phase,18,24 indicating that a phase transformation from the tetragonal to monoclinic phase of BiVO4 had taken place during the reaction. As the time increased to 9 h, the XRD patterns of the products are nearly the same as that of the 6 h one except for a little intensity increasing. All these results demonstrate that pure monoclinic BiVO4 was successfully synthesized by a mild reflux method at a relatively low temperature (80 °C) and under ambient pressure.

Single-Crystalline BiVO4 Microtubes

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Figure 2. BiVO4 microtubes synthesized at 80 °C for 6 h: (a) lowmagnification SEM image of the product and high-magnification SEM image (inset) for a single BiVO4 microtube, showing the hollow structure and wall thickness; (b) TEM and SEM image (inset) of an individual BiVO4 microflower; (c) the corresponding SAED pattern taken from the rectangular part of the microtube in Figure 2b; (d) HRTEM image near the nozzle of a single microtube (inset).

The morphology and microstructure of BiVO4 microtubes were revealed by SEM and TEM. Shown in Figure 2a and b are the SEM and TEM images of BiVO4 microtubes after refluxing for 6 h. The panoramic view in Figure 2a clearly demonstrates that the as-synthesized products are almost entirely microtubes with a length of 2-5 µm. As shown in Figure 2a inset, the individual tube has well-defined square cross-sections with a side length of ca. 800 nm and a wall thickness of ca.100 nm. Close observation of the samples revealed by highmagnification SEM image (Figure 2b inset) shows that a large quantity of the products is flower-like. Several microtubes construct a microflower by radiating from the center. The flower-like morphology and hollow interior structure of the products were also confirmed by a high-magnification TEM image (Figure 2b). The corresponding selected area electron diffraction (SAED) pattern (Figure 2c) taken from the rectangular part of the microtube in Figure 2b reveals that the microtube is single crystalline. Figure 2d shows a typical HRTEM image of a single BiVO4 microtube, and the inset of this figure shows the low-resolution TEM image. The clear lattice fringe indicates the high-crystallinity and single-crystalline nature of the microtubes. It can be measured that the d spacings are 0.581 and 0.468 nm, which agree well with the lattice spacings of (020) and (011) of monoclinic BiVO4. Both the HRTEM and SAED pattern reveal that the microtubes grew along the [010] direction. To investigate the morphological evolution process of the BiVO4 microtubes, time-dependent experiments were carried out by extracting products at different reflux reaction stages, while the molar ratio of Bi(NO3)3‚5H2O and NH4VO3 was fixed at 1:1 and the reaction temperature was kept at 80 °C. Figure 3a-f shows a series of SEM and TEM images of the precursor and products by varying the reflux time from 3 to 6 h and 9 and 15 h. The detailed morphological evolution process of the microtubes can be described as follows. The precursor contains

Figure 3. Morphology evolution of the BiVO4 microtubes with the reaction time: SEM images of (a) the precursor and the products obtained at 80 °C for (b) 3, (c) 6, (d) 9, (e) and (f) 15 h.

nanoparticles with a size of ca. 100 nm (Figure 3a). The corresponding XRD pattern (Figure 1a) confirms that these nanoparticles are amorphous. When the reaction time is 3 h, some flower-like square-shaped microtubes appeared in addition to the nanoparticles (Figure 3b). Furthermore, as shown in Figure 3b inset, a few smaller microflowers formed by several squareshaped microrods with a side length of ca. 500 nm and length of ca. 1 µm. It implies that the tubular structures grow on the microrods. When the time increases to 6 h, the product consists of almost wholly square-shaped microtube flowers; no particles can be found, as shown in Figure 2a. In addition, Figure 3c shows that a small quantity of intermediary microtubes exists, which clearly revealed the different growth stages from microrods to microtubes gradually (indicated by the arrow). After refluxing for 9 h, the microtubes begin to grow longer (Figure 3d). Finally, as the time was extended to 15 h, BiVO4 microtubes with a size of ∼1 µm and length up to 10 µm were formed (Figure 3e and f). The hollow interior structure of microtubes remained, as shown in the inset of Figure 3f. Shown in Figure 4 is the schematic crystal structure of monoclinic phase BiVO4. It is a layered structure composed of VO4 tetrahedron layers and Bi layers. Recently, the rolling mechanism25 and surfactant-directed growth mechanism26 have been reported to account for formation of the tubular structure, especially for those layered structure materials such as graphite, Bi, BN, WS2, NiCl, and VOx. However, these two mechanisms cannot elucidate the formation of BiVO4 microtubes since neither surfactant was added nor was a rolling phenomenon found, though it is a layered structured compound. On the basis

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Figure 4. Schematic crystal structures of monoclinic phase BiVO4.

Figure 7. UV-visible diffuse reflectance spectra of BiVO4 microtubes.

Figure 5. Schematic representation of the tube-formation process.

by Xi et al. when they synthesized tellurium nanotubes via a hydrothermal reduction process.29 Thus, formation of BiVO4 microtubes may represent a new interesting growth phenomenon for inorganic materials with layered structure. It is noticeable that NaHCO3 is crucial in forming the monoclinic BiVO4 microtubes. First, adding NaHCO3 resulted in generation of tetragonal phase BiVO4 at the first stage and the following transformation to a monoclinic one via a dissolution-recrystallization process. Though the monoclinic phase is thermodynamically more stable, the tetragonal phase BiVO4 is kinetically preferable during a sudden increase of pH by adding NaHCO3.19 Second, NaHCO3, as a buffer, served as the ratedetermining factor in controlling the BiVO4 concentration within the bulk solution to mediate the nucleation and growth of BiVO4. On the basis of our experiment, relevant chemical reactions can be formulated as follows

Figure 6. (a) Low-magnification SEM image and (b) high-magnification SEM image of products obtained at 80 °C for 6 h when NaHCO3 was replaced by NaOH.

of the experimental observations, growth of BiVO4 microtubes can be proposed as a dissolution-recrystallization-induced concentration depletion mechanism. The whole process is illustrated in Figure 5. Initially, the amorphous fine particles were obtained as the precursor. At the beginning of the reaction these amorphous particles aggregated and crystallized in the form of tetragonal BiVO4. Under constant reflux reaction at 80 °C the nanoparticles gradually dissolved and recrystallized. Phase transportation from the tetragonal phase to the monoclinic one took place during the recrystallization process.18 Thus, the monoclinic phase BiVO4 nuclei were generated and the energetically favorable directions will preferentially grow to form the microrod flowers. As the reflux reaction proceeded, further feeding BiVO4 to the seeds (microrod flowers) surface will result in the continued growth along the 1D direction. Because of the different rate between the dissolution and recrystallization process under the mild conditions in solution at 80 °C, simultaneous formation of many monoclinic phase BiVO4 seeds (microrod flowers) will greatly reduce the concentration of BiVO4 in the bulk solution. Analogous concentration depletion has been reported by Whitesides et al. when they grew calcite crystals on the gold substrates.27 Therefore, it could not provide enough BiVO4 for growth of the growing microrod. Mass transportation to the growing regions would lead to undersaturation in the central part of the growing faces of each seeds; thus, microtubes with a well-defined hollow interior structure finally formed. A similar mechanism was reported by Xia and co-workers in the case of fabricating tellurium nanotubes with well-controlled structures.28 The phenomenon was also observed

H+ + HCO3- H H2CO3 H H2O + CO2

(a)

Bi(NO3)3 + H2O H 2HNO3 + BiONO3

(b)

BiONO3 + VO3- H BiVO4 + NO3-

(c)

Initially, the reactants were dissolved in concentrated HNO3; no precipitate could be found. During the addition of NaHCO3 to the solution H+ was consumed and the pH increased (Formula a). Then, as the pH slowly increased greater than or equal to 1-2, Bi(NO3)3‚5H2O gradually reacted with water to form slightly soluble BiONO3, as shown in Formula b. Third, BiONO3 can react with VO3- and a yellow BiVO4 precipitate was obtained (Formula c). As a parallel experiment, when NaHCO3 was replaced by NaOH, keeping other conditions unchanged, a large quantity of particles with a size of several hundreds of nanometers were observed; no tubes or rods were found after 6 h of reaction (Figure 6). When the time increased, the particles aggregated and grew bigger. Thus, we believe that NaHCO3 plays an important role in formation of microtubes by the equilibrium due to formation and dissociation of carbonic acid (Formula a).30 It is well known that the optical absorption property and migration of the light-induced electrons and holes of a semiconductor, which are relevant to the electronic structure feature, are recognized as the key factors in determining its photocatalytic activity.31 The UV-vis diffuse reflectance spectrum of BiVO4 microtubes is shown in Figure 7. The BiVO4 microtubes show strong absorption in the visible light region until ∼525 nm in addition to that in the UV light region. The steep shape of the spectrum indicates that the visible light adsorption is not due to the transition from the impurity level but to the bandgap transition.32 The energy of the band gap of BiVO4

Single-Crystalline BiVO4 Microtubes

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13663 structures might be useful to be aligned as building blocks or as small containers for many functional devices. Acknowledgment. We acknowledge the financial support from the Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) and the National Natural Science Foundation of China (no. 50672117). References and Notes

Figure 8. Changes of UV-visible spectra of BiVO4 microtubes suspended RhB solution as a function of irradiation time. Inset: RhB concentration changes over SSR-BiVO4 and BiVO4 microtubes, λ > 400 nm.

microtubes estimated from the main absorption edges of the UV-vis diffuse reflectance spectrum is 2.36 eV, which is comparable with literature reports.18 To study the photocatalytic activities of BiVO4 microtubes, RhB with a major absorption band at 553 nm is chosen as a model organic pollutant. As shown in Figure 8, absorption of RhB/BiVO4 microtubes suspension gradually decreased during the photodegradation under visible light irradiation. In addition, the major absorption band shifts from 553 to 496 nm step by step, indicating removal of ethyl groups one by one, which is in good accordance with that in the literature.32,33 At the same time, the color of the suspension changes gradually, demonstrating that the chromophoric structure of the dye is destroyed. The corresponding plot for the concentration changes of RhB is shown in the inset of Figure 8. The photodegradation rate by BiVO4 microtubes is up to 96% in 180 min under visible light irradiation, which is much higher than that of the corresponding sample prepared by solid-state reaction (ca. 16%), indicating a prominent improvement of the photocatalytic activity. The higher photocatalytic activities of these microtubes can be ascribed to the novel square tubular structure and/or the flowerlike morphology. Conclusions In summary, single-crystalline BiVO4 microtubes constructing a flower-like morphology have been synthesized by a facile reflux process in open air without any surfactants or templates. The cross-section of individual microtubes appears to be a novel square shape. Unlike other micro/nanotubes formed by layered structure materials reported previously, the formation mechanism of the square-shaped microtubes has been proposed to be a dissolution-recrystallization-induced concentration depletion process based on an investigation of morphology evolutions. The presence of NaHCO3 is found to be a crucial factor in forming the tubular structure by controlling the BiVO4 concentration within the bulk solution to mediate the nucleation and growth of BiVO4. The as-synthesized microtubes showed strong absorption in the visible light region and exhibited prominent improvement of photocatalytic activity, which could be attributed to the distinctive morphology. These results not only enrich the tubular structures of inorganic compounds but also provide a new mild template-free solution strategy to synthesize single-crystalline tubular structures, especially for those layered structure materials. Furthermore, these square-shaped tubular

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