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Nanosheet-Based Bi2MoxW1-xO6 Solid Solutions with Adjustable Band Gaps and Enhanced Visible-Light-Driven Photocatalytic Activities Liang Zhou,† Minmin Yu,† Jie Yang,† Yunhua Wang,† and Chengzhong Yu*,†,‡ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China, and ARC Centre of Excellence for Functional Nanomaterials and Australian Institute for Bioengineering and Nanotechnology, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: September 29, 2010
Bi2MoxW1-xO6 solid solutions with various compositions (x ) 0, 0.25, 0.50, 0.75, and 1.00) have been synthesized by a facile hydrothermal crystallization method. All the as-synthesized products are composed of nanosheets with similar orthorhombic Aurivillius layered structures. With decreasing Mo content, the thickness of the nanosheets decreases. The nanosheets begin to intergrow at x ) 0.75 and further assemble into hierarchical superstructures through “oriented attachment” with even less Mo content. For the Bi2MoxW1-xO6 solid solutions with x ) 0.25, 0.50, and 0.75, the valence band is widened and the conduction band is elevated when compared with the most intensively studied Bi2WO6 photocatalyst. As a result, the Bi2MoxW1-xO6 solid solutions (x ) 0.25, 0.50, and 0.75) show narrowed band gaps of ∼2.69 eV when compared with Bi2WO6 (2.94 eV) and Bi2MoO6 (2.72 eV). The photocatalytic activities of the as-prepared products have been evaluated using the photodecomposition of methylene blue under visible light irradiation as a model reaction. The sample, Bi2Mo0.25W0.75O6, exhibits the highest photocatalytic activity. The intrinsic layered structure, nanosheet morphology, manipulated band structure and band gap, and the W content play important roles in the photocatalytic activity. Our approach provides a facile technique to tune both the nanostructure and the band gap of photocatalysts by simply adjusting the composition of the solid solutions, leading to photocatalysts with enhanced visible light photocatalytic activity. This method may be further extended to the designed synthesis of novel and highly efficient visible-light-driven semiconductors for environmental remediation. 1. Introduction Heterogeneous semiconductor photocatalysts have sparked worldwide interest for converting solar energy to chemical energy and for decomposing organic contaminants, thus addressing the increasing global concerns of clean energy production and environmental remediation.1-6 TiO2-based materials are currently the most popular photocatalysts due to their high photocatalytic activity, long-term stability against photocorrosion, abundance in nature, low cost, and low toxicity.7-13 A major handicap of TiO2 is its rather large optical band gap (3.2 eV for the anatase phase and 3.0 eV for the rutile phase), which means it can only be activated by ultraviolet (UV) light (λ < 400 nm). According to the solar spectrum, UV light accounts for only a small fraction (4%) of the incoming solar energy, whereas visible light makes up as large as 43%. From the viewpoint of solar energy utilization, it is highly desirable to develop visible-light-responsive photocatalysts. Consequently, either cation doping with transition metals14 or anion doping with C,15,16 N,17-21 S,22,23 and B24-26 has been widely used to functionalize TiO2-based materials into visible-light-driven photocatalysts. However, it is difficult for doped TiO2 to absorb a sufficient quantity of visible light. Moreover, the photocatalytic activity is still rather low mainly due to massive charge carrier recombination. In addition to TiO2-based materials, many researchers have diverted their attention to exploit new photocatalysts for visible * To whom correspondence should be addressed. E-mail:
[email protected]. † Fudan University. ‡ The University of Queensland.
light harvesting. The pioneering study done by Zou et al.27 showed that In1-xNixTaO4 displayed water-splitting capability to produce H2 and O2 in stoichiometric amounts under visible light irradiation. Following this work, many binary, ternary, and multicomponent metal oxide semiconductor photocatalysts with intense visible light absorption, such as WO3,28,29 Bi2WO6,30,31 Bi2MoO6,32 BiVO4,33-35 CaBi2O4,36 and InVO4,37 have been developed. Among the above-mentioned candidates, Bi2WO6 with a typical Aurivillius layered structure is particularly interesting. Recently, Kudo et al. demonstrated that Bi2WO6 had photocatalytic activity for O2 evolution in AgNO3 solution.35 Tang et al. revealed the mineralization of CHCl3 and CH3CHO contaminants under visible light irradiation over Bi2WO6.31 More recently; Amano et al. found that the apparent quantum efficiency for the photocatalytic oxidative decomposition of gaseous CH3CHO over polycrystalline Bi2WO6 can reach as high as 8%.38 Enlightened by these contributions, great attention has been paid to the photocatalytic activity of Bi2WO6.39-53 Bi2MoO6 has a similar Aurivillius layered structure to that of Bi2WO6. Compared with Bi2WO6, Bi2MoO6 with a narrower band gap (for example, 2.77 eV for Bi2WO6 and 2.63 eV for Bi2MoO6 in Zhou et al’s report54) has the ability to harness more sunlight. However, its photocatalytic activity is not as high as that of Bi2WO6; thus, Bi2MoO6 has attracted less attention. A question arises naturally: is it possible to integrate the advantages of both Bi2WO6 (high photocatalytic activity) and Bi2MoO6 (narrower band gap) into one compound and how do you achieve this? Because the W component can endow the material with high photocatalytic activity, whereas the Mo constituent contributes to the narrow band gap, it is reasonable to postulate
10.1021/jp107061p 2010 American Chemical Society Published on Web 10/20/2010
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that the Bi2MoxW1-xO6 solid solutions may provide a suitable option. Although Bi2MoxW1-xO6 can be prepared by either classical solid reaction or mechanochemical activation,55,56 homogeneous Bi2MoxW1-xO6 solid solutions can only be obtained in the range of 0 e x e 0.5 by these methods. Moreover, to the best of our knowledge, there has been only one report concerning the photocatalytic properties of Bi2MoxW1-xO6 until now.57 In the present work, we report the synthesis of homogeneous Bi2MoxW1-xO6 (x ) 0, 0.25, 0.50, 0.75, and 1.00) solid solutions in the entire range of 0 e x e 1.0 by a facile hydrothermal treatment method. Their visible light (λ > 400 nm) photocatalytic activities have been systematically investigated. It is shown that the introduction of Mo into the Bi2WO6 lattices affects not only the nanostructured morphology but also the band-gap configuration. As a result, Bi2Mo0.25W0.75O6 exhibits the highest photocatalytic activity for methylene blue (MB) photodecomposition and even outperforms the most intensively studied Bi2WO6. Our contribution provides insight into the design of new visible-light-driven semiconductor photocatalysts with enhanced activity. 2. Experimental Section 2.1. Synthesis of the Bi2WxMo1-xO6 Solid Solutions. “y” mmol (y ) 0, 2.5, 5.0, 7.5, and 10.0) of W powder and 10 - y mmol of Mo powder were dissolved in 40 mL of 15 wt % H2O2 under stirring at 293 K. The mixture was kept at 293 K for 12 h under stirring. Afterward, 40 mmol of urea (CO(NH2)2) and 20 mmol of bismuth nitrate pentahydrate [Bi(NO3)3 · 5H2O] were added to the solution and stirred for another 10 min. The solution as well as the precipitates was transferred into a Teflonlined autoclave, sealed, and hydrothermally treated at 473 K for 24 h. The products were collected by filtration, washed with water, and dried at room temperature. The resultant products with y ) 0, 2.5, 5.0, 7.5, and 10.0 were designated as Bi2MoO6, Bi2W0.25Mo0.75O6, Bi2W0.50Mo0.50O6, Bi2W0.75Mo0.25O6, and Bi2WO6, respectively. The Mo content in the precursor solution was defined as x, and x ) Mo/(Mo + W). 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded on a German Bruker D8 Advanced X-ray diffractometer with Ni-filtered Cu KR radiation at a voltage of 40 kV and a current of 40 mA. The crystallite size of the products was determined from the full width at half-maximum (fwhm) of the (020*) and (131*) diffraction peaks by using the Scherrer equation, D ) Kλ/β cos θ, where K is the shape factor with a typical value of 0.89, λ is the X-ray wavelength (0.154 nm in our case), β is the line broadening at fwhm in radians, and θ is the Bragg angle. Scanning electron microscopy (SEM) images were obtained on a Philips XL30 microscope operated at 20 kV. Transmission electron microscope (TEM) experiments were conducted on JEOL 2100 and JEOL 2011 microscopes with an accelerating voltage of 200 kV. The samples for TEM measurements were dispersed in ethanol by sonication and then supported onto a holey carbon film on a copper grid. UV-vis diffusive reflectance spectra (DRS) were obtained using a JASCO V-550 UV-vis spectrometer using BaSO4 as a reference. The N2 adsorption-desorption isotherms were measured at 77 K on a nitrogen adsorption apparatus (Quadrasorb SI, Quantachrome) after degassing the samples at 453 K for 6 h. The Brunauer-Emmett-Teller (BET) surface areas were determined from the adsorption branch of the isotherm in a relative pressure range from 0.05 to 0.30. Valence band spectra were recorded on a Kratos Axis ULTRA X-ray photoelectron spectrometer using a monochromatic Al KR (1486.6 eV) X-ray
Figure 1. XRD patterns of (a) Bi2WO6, (b) Bi2Mo0.25W0.75O6, (c) Bi2Mo0.50W0.50O6, (d) Bi2Mo0.75W0.25O6, and (e) Bi2MoO6.
source and a 165 mm hemispherical electron energy analyzer. All binding energies were calibrated using contaminant carbon (C 1s ) 284.6 eV) as a reference. 2.3. Photocatalytic Test. The photocatalytic activities of the Bi2MoxW1-xO6 solid solutions were evaluated by photodecomposition of MB under visible light at 20 °C. A 300 W Xe lamp was used as the light source with a 400 nm cutoff filter to provide visible light irradiation. In each experiment, 0.1 g of photocatalyst was dispersed into 100 mL of aqueous MB solution (10 ppm). Before illumination, the dispersion was stirred for 2 h in the dark to ensure the adsorption/desorption equilibrium. The dispersion was then exposed to visible light irradiation under magnetic stirring. At given time intervals, 5 mL of the suspension was sampled and centrifugated to remove the precipitates. The concentrations of the MB in the supernatant were monitored by checking the absorbance at 664 nm using an Agilent HP8453 spectrophotometer. 3. Results 3.1. Structure and Morphology Characterization. The Bi2MoxW1-xO6 solid solutions were prepared by a facile hydrothermal treatment method using Bi(NO3)3 · 5H2O as the Bi source, peroxo-molybdic acid as the Mo source, peroxopolytungstic acid as the W source, and urea to adjust the pH value of the solution. The crystalline phase and crystallinity of the as-synthesized samples were well characterized by XRD. As shown in Figure 1, pattern a, without Mo doping (x ) 0), the product can be indexed to orthorhombic Bi2WO6 with lattice parameters of a ) 0.5437 nm, b ) 1.643 nm, and c ) 0.5458 nm and a space group of Pca21 (Joint Committee on Powder Diffraction Standards, JCPDS Card No. 79-2381). No other peaks can be detected, indicating the high purity of the sample. The relatively broad diffraction peaks demonstrate the nanocrystalline nature of the product. For x ) 1.0 (Figure 1, pattern e), high-purity orthorhombic Bi2MoO6, the so-called γ(L)Bi2MoO6 phase, with lattice parameters of a ) 0.5482 nm, b ) 1.619 nm, and c ) 0.5509 nm and a space group of Pca21 (JCPDS Card No. 77-1246) can be obtained. For x ) 0.25, 0.50, and 0.75 (Figure 1, patterns b-d), the XRD patterns generally
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TABLE 1: Physicochemical Properties of the Bi2MoxW1-xO6 Solid Solutions samples
D020/nma
D131/nmb
SBET/m2/gc
Eg/eVd
Ra/%e
k′/min-1 f
Bi2MoO6 Bi2Mo0.75W0.25O6 Bi2Mo0.50W0.50O6 Bi2Mo0.25W0.75O6 Bi2WO6
36.6 28.6 20.5 17.5
41.6 36.0 24.1 22.3 21.2
8.8 10.9 13.3 12.9 15.2
2.72 2.68 2.70 2.69 2.94
21 28 44 39 50
0.0073 0.0093 0.0086 0.0129 0.0107
a The crystallite size determined from the (020*) diffraction peak by using the Scherrer equation. b The crystallite size determined from the (131*) diffraction peak by using the Scherrer equation. c BET surface area. d Band gap. e Extent of MB adsorption. f The apparent reaction rate constant.
Figure 2. Representative SEM images of (a) Bi2MoO6, (b) Bi2Mo0.75W0.25O6, (c, d) Bi2Mo0.50W0.50O6, (e, f) Bi2Mo0.25W0.75O6, and (g, h) Bi2WO6.
Figure 3. TEM images (a, b), ED pattern (c), and HRTEM image (d) of Bi2MoO6.
resemble that of Bi2WO6 and/or γ(L)-Bi2MoO6; thus, these materials can be referred to as Bi2MoxW1-xO6 solid solutions. Taking a close look at Figure 1, one can also find that, with the increase of the Mo content, (1) the relative intensity of the (020*) diffraction peak versus that of the most intensive (131*) peak increases, (2) the diffraction peaks originally located at 2θ ) 32.87, 47.16, and 55.93° split gradually, and (3) the diffraction peaks sharpen sequentially (see the 131* diffraction), indicating that the more the incorporated Mo content, the larger the crystallite size. Take the crystallite size determined from the fwhm of the most intensive (131*) diffraction peak by using the Scherrer equation, for example (Table 1); it increases dramatically from ca. 21.2 nm for Bi2WO6 to 41.6 nm for Bi2MoO6. The high crystallinity of Mo-rich compositions may be attributed to the lower crystallization temperature of γ(L)Bi2MoO6 compared with that of Bi2WO6. It is noteworthy that homogeneous Bi2MoxW1-xO6 solid solutions can be obtained in the whole range of 0 e x e 1 by our hydrothermal approach, whereas either classical solid reaction or mechanochemical activation employed in previous reports leads to mixtures of Bi2WO6 and γ(H)-Bi2MoO6 phases in the range of 0.5 e x e 1.55,56 The different synthesis methods are responsible for this difference. The relatively low-temperature hydrothermal treatment at 473 K induces the formation of homogeneous Bi2MoxW1-xO6 solid solutions in the whole range of 0 e x e 1 by preventing the formation of the γ(H)Bi2MoO6 phase, which begins to form at temperatures higher than 943 K and remains unaltered on cooling.55,56 Figure 2 represents the SEM images of the as-synthesized Bi2MoxW1-xO6 solid solutions. For Bi2MoO6, large quantities of irregular nanosheets can be observed (Figure 2a). These nanosheets randomly aggregate into larger particles with sizes
up to tens of micrometers (Figure S1, Supporting Information). Such irregular nanosheets are also predominant in Bi2Mo0.75W0.25O6; however, a small fraction (less than 1%) of nanosheets begins to intergrow, as indicated by the white arrow in Figure 2b. The intergrowth of nanosheets into hierarchical structures becomes prominent when only 50% of Mo is introduced into the Bi2WO6 lattices (Figure 2c,d). With even less Mo doping (x ) 0.25), large quantities of hierarchical flower-like superstructures with diameters of 5-15 µm are constructed from 2-dimensional nanosheets by “oriented attachment” and intergrowth (Figure 2e,f). For Bi2WO6, its panoramic morphology is similar to that of Bi2Mo0.25W0.75O6 with a uniform particle size of 5-10 µm (Figure 2g,h). To understand the detailed morphological and structural characteristics of the Bi2MoxW1-xO6 solid solutions, TEM was employed. Figure 3a shows an overall morphology of the Bi2MoO6 nanosheets with lengths of 100-800 nm and widths of 100-300 nm. Most of the nanosheets lie flat on the copper grid, whereas a small fraction of them stand on their side surfaces (indicated by black arrows), from which the thickness of the nanosheet can be determined to be ∼32 nm. Figure 3b displays a TEM image of a typical Bi2MoO6 nanosheet with its corresponding selected area electron diffraction (SAED) pattern and high-resolution TEM (HRTEM) image displayed in Figure 3c,d, respectively. The set of diffraction spots can be indexed to orthorhombic Bi2MoO6 on the [010] projection, indicating the single-crystalline nature of the nanosheet. It should be mentioned that, besides the strong allowed diffraction spots, such as 200* and 002*, some weak diffraction spots can also be found at those forbidden sites (such as 100* and 102*), as indicated by white arrows in Figure 3c. These weak diffraction spots can be assigned to the high-order Laue zone caused by the combined
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Figure 4. TEM images (a-c) and ED pattern (d) of Bi2Mo0.50W0.50O6.
Figure 5. TEM (a, b), HRTEM image (c), and ED pattern (d) of Bi2WO6.
effects of elongation of the diffraction spots along the normal of the nanosheet and narrowed Laue zone along the [010] direction (the ultralarge {010} spacing, 1.619 nm). Similar phenomena have also been found in our previous studies on WO3 · 0.33H2O nanosheets.58 The single-crystalline characteristic of the Bi2MoO6 nanosheet can be further confirmed by the HRTEM image (Figure 3d). Two sets of atomic spacings (0.272 and 0.273 nm) can be distinguished unambiguously from Figure 3d, which correspond to the {200} and {002} lattice fringes of orthorhombic Bi2MoO6, respectively. Considering that the basal plane of the Bi2MoO6 nanosheets is the (010) plane, the thickness of the nanosheet should correspond to the crystallite size along the [010] direction. When the thickness of the nanosheets measured from the TEM image is compared to the D(020) value determined from the XRD pattern (see Table 1), it is found that these two values agree fairly well (32 nm vs 36.6 nm). An individual Bi2Mo0.50W0.50O6 hierarchical architecture with a diameter of ∼1.6 µm is shown in Figure 4a. From an enlarged TEM image of the superstructure (Figure 4b), one can find that the superstructure is assembled from densely packed rectangular nanosheets in a parallel manner (“oriented attachment”). A nanosheet peeled from the superstructure by ultrasonication is shown in Figure 4c. Instead of being perfectly flat, the nanosheet shows intrinsic out-of-plane wrinkles, which indicate the high crystallinity and ultrathin nature of the nanosheet. By detailed analysis of the contrast (the darker contrast in the center), one can deduce that this nanosheet is actually composed of two nanosheets sharing part of their top and/or bottom surfaces. The SAED pattern (Figure 4d) taken from a corner of the nanosheet shows a diffraction pattern that can be assigned to the [010] zone axis. Again, some weak diffraction spots caused by the high-order Laue zone are found at those forbidden sites (indicated by white arrows in Figure 4d). An individual Bi2WO6 hierarchical superstructure with a diameter of ∼2.2 µm is shown in Figure 5a. Figure 5b is the enlarged TEM image of the superstructure showing the primary 2-D nanosheet building blocks clearly. Figure 5c,d shows the HRTEM image and the corresponding SAED pattern of a nanosheet on the superstructure. From Figure 5c, the (200) and/ or (002) lattice fringes (0.27 nm) can be clearly identified. The corresponding SAED pattern (Figure 5d) can be indexed to the orthorhombic Bi2WO6 viewed from the [010] direction. By
combining Figure 5c,d, it is unambiguously confirmed that the top and bottom surfaces of the nanosheets are the {020} planes, whereas the side surfaces are enclosed by the {202} facets. Strikingly, the Bi2WO6 nanosheets are not stable under electronbeam exposure (Figure S2, Supporting Information). The originally smooth surfaces and explicit edges of the nanosheet became rough gradually after tens of seconds of electron bombardment at 200 kV. In addition, tens of small nanoparticles with diameters of 5-10 nm oozed out of the surfaces and distributed homogeneously along the edges. A similar phenomenon can also be observed in the case of Bi2Mo0.50W0.50O6 nanosheets, as shown in Figure 4c, and it has also been reported by Zhang and Zhou et al.39,54 Unlike the Bi2WO6 and Bi2MoxW1-xO6 solid solutions (x ) 0.25, 0.50 and 0.75), the Bi2MoO6 nanosheets are stable under 200 kV electron exposure; no detectable damages can be observed. By carefully checking different Bi2MoxW1-xO6 nanosheets, it is found that all of the nanosheets, no matter what the composition is, have a same normal of [010], which hints that the crystallite size along the [010] direction should correspond to the thickness of the nanosheets. As we mentioned in the XRD results, the (020*) diffraction peak sharpens and the relative intensity increases sequentially with increasing Mo fraction. By combining the TEM results and the XRD results together, it is safe to draw the conclusion that the lower the Mo content, the thinner the Bi2MoxW1-xO6 nanosheets. This conclusion is strongly supported by the N2 adsorption-desorption results (see Table 1). Generally, the BET surface area of the Bi2MoxW1-xO6 nanosheets increases with decreasing Mo content, although the density of the materials shows the opposite trend. Because it is the bottom and top surfaces that contribute to a lion’s share of the total surface area of the Bi2MoxW1-xO6 nanosheets, rather than the side surfaces, it can be deduced that the increase in the BET surface area with decreasing Mo content is caused by the decreased thickness of the nanosheets. 3.2. Electronic Structure. UV-vis DRS is a useful tool for characterizing the electronic states of semiconductor materials. Figure 6 displays the diffuse reflectance spectra of the Bi2MoxW1-xO6 solid solutions, which have intensive absorption bands with steep edges in the visible light region. The steep absorption edges and the almost parallel characteristics of the absorption edges strongly indicate the nature of band-to-band transition in the Bi2MoxW1-xO6 solid solutions. Bi2WO6 is able
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Figure 6. UV-vis diffusive reflectance spectra and digital photograph of the Bi2MoxW1-xO6 solid solutions.
to absorb visible light up to 442 nm. A great red shift (∼50 nm) of the absorbance threshold can be observed when 25% of Mo is introduced into the lattice, indicating a narrowing of the band gap. However, a further increase of the Mo content does not cause further significant red shifts of the absorbance threshold. The band gaps of the Bi2MoxW1-xO6 solid solutions are determined from the plot of the Kubelka-Munk function versus the energy of light absorbed (Figure S3, Supporting Information) and are listed in Table 1. For Bi2WO6, the band gap is ∼2.94 eV, for Bi2MoxW1-xO6 (x ) 0.25, 0.50, 0.75) solid solutions, the band gaps are in the range of 2.68-2.70, and for Bi2MoO6, the band gap is ∼2.72 eV. In agreement with the UV-vis spectra, Bi2WO6 has a yellowish color, whereas all the other samples have a yellow color (Figure 6). The relative energy levels of the valence band (VB) maximum and conduction band (CB) minimum play key roles in determining the redox power of photoinduced carriers in photocatalytic reactions.59,60 To determine the effect of Mo content on the relative positions of the CB and VB edges and VB width of the as-prepared Bi2MoxW1-xO6 solid solutions, the total densities of states of VBs have been measured (Figure 7). As can be seen from Figure 7a, the Bi2MoxW1-xO6 solid solutions (x ) 0.25, 0.50, and 0.75) show identical VB maxima and widths. Considering the similar band gaps (2.68-2.70 eV) of these three materials, they should show very similar electronic structures. When compared with Bi2WO6, Bi2Mo0.25W0.75O6 shows a VB maximum upshift of ∼0.37 eV as well as a widened VB (Figure 7b). This upshift is larger than the band-gap narrowing value determined by the UV-vis data (∼0.25 eV), which indicates that the CB minimum of Bi2Mo0.25W0.75O6 is elevated by ∼0.12 eV when compared with that of Bi2WO6. Although their VB maxima are almost at the same position, the Bi2Mo0.25W0.75O6 shows a wider VB than BiMoO6 (Figure 7b).
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Figure 7. Valence band spectra of the Bi2MoxW1-xO6 solid solutions (red line for Bi2Mo0.25W0.75O6, blue line for Bi2Mo0.50W0.50O6, orange line for Bi2Mo0.75W0.25O6, black line for Bi2WO6, and green line for Bi2MoO6).
Figure 8. Schematic band structures of Bi2WO6, Bi2MoxW1-xO6 (x ) 0.25, 0.50, and 0.75), and Bi2MoO6.
On the basis of the VB spectra and the band gaps determined from UV-vis data, the schematic electronic structures for Bi2MoxW1-xO6 solid solutions can be proposed (Figure 8). As shown from Figure 8, the Bi2MoxW1-xO6 solid solutions (x ) 0.25, 0.50, and 0.75) have a narrower band gap than that of Bi2WO6 because of their widened and up-shifted VB. However, when compared to Bi2MoO6, the band gap narrowing value is small (∼0.02 eV), and it is hard to deduce whether this small difference is caused by the upward shift of the VB maximum or the downward shift of the CB minimum. 3.3. Photocatalytic Activities of the Bi2MoxW1-xO6 Solid Solutions. The photocatalytic activities of the Bi2MoxW1-xO6 solid solutions have been evaluated by the photodecomposition of MB under visible light irradiation (λ > 400 nm). The MB, which is a thiazine dye, is a popular probe in the heterogeneous photocatalysis community. The temporal evolution of the spectra during MB adsorption and photodecomposition over Bi2Mo0.25W0.75O6 is shown in Figure 9a. MB solution shows a major absorption band at 664 nm. In the presence of Bi2Mo0.25W0.75O6, the absorbance decreased by ca. ∼40%
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J. Phys. Chem. C, Vol. 114, No. 44, 2010 18817 determined from the slope of the fitted lines, which is 0.0107, 0.0129, 0.0086, 0.0093, and 0.0073 min-1 for x ) 0, 0.25, 0.50, 0.75, and 1.00, respectively. From the kinetic data, it is concluded that all the W-containing samples outperformed Bi2MoO6 in MB photodecomposition. Moreover, although Bi2WO6 adsorbs the highest amount of MB due to its largest surface area (see Table 1), Bi2Mo0.25W0.75O6 shows the fastest MB decomposition behavior among all materials under study. Such observations will be discussed in detail in the following section. 4. Discussion
Figure 9. Absorption changes of the MB solution in aqueous Bi2Mo0.25W0.75O6 dispersions under visible light irradiation (a). Firstorder plots for the photodecomposition of MB over the Bi2MoxW1-xO6 solid solutions (b).
before irradiation, reflecting the dark adsorption ability of Bi2Mo0.25W0.75O6 for MB. During the following photodecomposition process, a quick decrease of the MB absorption at 664 nm is observed, indicating the rapid decomposition of MB under visible light. After 200 min of irradiation, more than 95% of the MB has been decomposed, demonstrating the excellent photocatalytic activity of Bi2Mo0.25W0.75O6 for MB decomposition under visible light. Because dark adsorption is a prerequisite for good photocatalytic activity, the dark adsorption ability of the Bi2MoxW1-xO6 solid solutions in terms of the extent of MB adsorption (Ra) is listed in Table 1. Interestingly, the extent of MB adsorption is consistent with the BET surface areas determined by N2 adsorption-desorption (see Table 1), showing the advantage of nanostructured photocatalysts over conventional bulk materials with relatively small surface areas. The temporal evolution of the MB concentration under visible light irradiation in the presence of Bi2MoxW1-xO6 is displayed in Figure 9b. A blank experiment with no photocatalyst added was also conducted, in which the MB decomposition is very slow (data not shown). All Bi2MoxW1-xO6 samples are able to degrade MB, and the composition of the catalysts has a significant effect on the degradation rate. The ln(C/C0) decreases linearly with increasing time (where C0 is the initial concentration of MB, in our case, 10 ppm, and C is the concentration of MB after the irradiation), which means that the photodecomposition of MB in the presence of Bi2MoxW1-xO6 can be described as a (pseudo)first-order reaction. Thus, the kinetic data, as shown in Figure 9b, can be expressed by the first-order reaction equation, ln(C/C0) ) -k′t (t represents the irradiation time in min). The apparent reaction rate constant, k′, can be
It is of interest to investigate the inherent correlations between the crystal structure and the morphology of the Bi2MoxW1-xO6 solid solutions. Bi2WO6 has a typical Aurivillius layered structure constructed by corner-shared [WO6] octahedral layers sandwiched between [Bi2O2]∞layers (Figure S4, Supporting Information). Because of the lanthanide contraction, Mo has a rather similar atomic radius with W (134.2 vs 137 pm). As a result, a large fraction or even all of the W atoms in the Bi2WO6 lattices can be substituted by Mo atoms without causing obvious changes to the original orthorhombic structure. Evidence supporting this deduction include that (1) Bi2MoO6 has a similar Aurivillius layered structure to that of Bi2WO6 and (2) the XRD patterns of the Bi2MoxW1-xO6 solid solutions with x ) 0.25, 0.50, 0.75, and 1.00 generally resemble that of Bi2WO6. Thus, the schematic structure of Bi2WO6, as shown in Figure S4 (Supporting Information), can also be used to represent the structures of the Bi2MoxW1-xO6 solid solutions (x ) 0.25, 0.50, and 0.75) and Bi2MoO6. Because of the intrinsic layered structure of the Bi2MoxW1-xO6 solid solutions, all samples crystallize into nanosheets with a thin thickness along the [010] direction, which is also the stacking direction of the alternating [Bi2O2]∞ layers and corner-shared [WO6] octahedral layers. A number of factors play crucial roles in the photocatalysis: (1) The intrinsic layered structure. It has been reported that a series of layer-structured compounds show high activity for water splitting and decomposition of organic pollutants under UV or visible light irradiation.6 In our case, the intrinsic Aurivillius-type layered structure, especially the corner-shared [Mo/WO6] octahedral layers, may promote the generation and separation of charge carriers.32,43 (2) The nanosheet morphology. The nanosized characteristic of the materials allows for more efficient transfer of the photogenerated electrons and holes from inside of the particles to the outside surfaces, thus diminishing the bulk electron-hole recombination, which is detrimental to the photocatalytic process. Moreover, the high surface area of the nanosheets not only ensures a large adsorption quantity of MB but also supplies more active sizes for organic contaminant decomposition. (3) The band structure. It has been reported that the VB of Bi2WO6 is composed of Bi 6s and O 2p hybrid orbitals.40 The hybridization of Bi 6s and O 2p orbitals makes the VB largely dispersed, which favors the mobility of the photogenerated holes in the VB.36,40 In our case, the introduction of 25-75% Mo further widens the VB (Figures 7 and 8), which is beneficial to the photooxidation reaction. In addition, the CB is also elevated by ∼0.12 eV (Figure 8), which is beneficial to the photoreduction process. (4) The band gap. The replacement of 25-100% W in Bi2WO6 lattices by Mo reduces the band gap by 0.22-0.25 eV; thus, Bi2MoxW1-xO6 (x ) 0.25, 0.50, 0.75, and 1.0) harnesses more visible light than Bi2WO6.
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(5) The W content. Zhou et al.54 found that Bi2WO6 nanoplates exhibited higher photocatalytic activities than Bi2MoO6 nanoparticles, and Li et al.57 demonstrated that the W-containing Bi2MoxW1-xO6 samples showed better photocatalytic activities than Bi2MoO6. In our case, the W content is also one of the key factors that affect the photocatalytic activities of the solid solutions. When Bi2Mo0.25W0.75O6 is compared with the most intensively studied Bi2WO6 photocatalyst, the effects of VB widening, CB elevation, and band gap narrowing surpasses other factors (such as the surface area and the W content), leading to the superior photocatalytic activity of Bi2Mo0.25W0.75O6. 5. Conclusions Nanosheet-based homogeneous Bi2MoxW1-xO6 solid solutions with various compositions (x ) 0, 0.25, 0.50, 0.75, and 1.00) have been successfully synthesized by a hydrothermal crystallization method. For Bi2MoxW1-xO6 solid solutions with x ) 0.25, 0.50, and 0.75, the VB is widened and the CB is elevated when compared with the most intensively studied Bi2WO6 photocatalyst, leading to Bi2MoxW1-xO6 materials (x ) 0.25, 0.50, 0.75) with a narrowed band gap and enhanced visiblelight-harvesting ability. Bi2Mo0.25W0.75O6 with a relatively high W content shows the highest photocatalytic activity for MB photodecomposition under visible light irradiation. Our present study has shown that both the nanostructures and the band gaps of photocatalysts can be manipulated to obtain enhanced activity by simply adjusting the composition of the solid solutions. This strategy is expected to shed light on band-gap engineering and design of novel and highly efficient visible-light-driven semiconductor photocatalysts for environmental remediation. Acknowledgment. This work was supported by the 973 Program (2010CB226901), STC of Shanghai Municipality (0852nm01500), and the Australian Research Council. Supporting Information Available: SEM image, TEM images, UV-vis data, and schematic structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Rajeshwar, K.; de Tacconi, N. R. Chem. Soc. ReV. 2009, 38, 1984– 1998. (3) Kudo, A.; Miseki, Y. Chem. Soc. ReV. 2009, 38, 253–278. (4) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851–7861. (5) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (6) Osterloh, F. E. Chem. Mater. 2008, 20, 35–54. (7) Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428–4453. (8) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891–2959. (9) Liu, G.; Wang, L. Z.; Yang, H. G.; Cheng, H. M.; Lu, G. Q. J. Mater. Chem. 2010, 20, 831–843. (10) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. J. Am. Chem. Soc. 2009, 131, 4078–4083. (11) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638–641. (12) Li, H. X.; Bian, Z. F.; Zhu, J.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 4538–4539. (13) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406–8407. (14) Litter, M. I. Appl. Catal., B 1999, 23, 89–114. (15) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243–2245. (16) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908– 4911. (17) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (18) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y. B.; Chen, X. B. J. Phys. Chem. B 2004, 108, 1230–1240. (19) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483–5486.
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