MoxW1−xO3·0.33H2O Solid Solutions with Tunable Band Gaps - The

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MoxW1-xO3 · 0.33H2O Solid Solutions with Tunable Band Gaps Liang Zhou,† Jie Zhu,† Meihua Yu,‡ Xiaodan Huang,† Zhen Li,‡ 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, ARC Centre of Excellence for Functional Nanomaterials and Australian Institute for Bioengineering and Nanotechnology, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed: May 20, 2010; ReVised Manuscript ReceiVed: October 24, 2010

A series of MoxW1-xO3 · 0.33H2O (x ) 0, 0.25, 0.50, 0.75) micro/nanostructures and R-MoO3 nanobelts have been prepared by a facile hydrothermal treatment method starting from aqueous peroxo-polytungstic acid and peroxo-molybdic acid solutions. The WO3 · 0.33H2O lattice can be substituted with up to 75% Mo without structural alterations of the orthorhombic host structure. With the increase of the Mo content (x) from 0 to 0.75, the band gap of the as-prepared MoxW1-xO3 · 0.33H2O micro/nanostructures is narrowed from 3.25 to 2.77 eV. The increased M5+ (M ) Mo and W) fraction and thus enhanced intervalency-transition are responsible for the narrowing of the band gap. 1. Introduction Redox-active transition metal oxides and hydrates such as WO3 · nH2O and MoO3 · nH2O (n ) 0, 0.33, 1 or 2) have attracted significant attention due to their rich polymorphs, intriguing physical/chemical properties, and widespread applications in chromogenic (electro-, photo-, thermochromic) devices,1-8 gas sensors,9,10 photocatalysts,11-17 field-emission devices,18-20 rechargeable batteries,21-25 etc. Compared with unary metal oxide constituents (WO3 and MoO3), binary molybdenum-tungsten oxide (MoxW1-xO3) materials are more promising due to the additional opportunity for components control, structural characteristics tailoring, physical/chemical properties modulation, as well as improved performance in the above-mentioned applications due to the expected “synergistic effect” in the composites. Thus, tremendous effort has been dedicated to the preparation,26-28 formation mechanism study,29-31 and property investigation32-46 of MoxW1-xO3 over the past years. MoxW1-xO3 showed improved electrochromic,32-35 gas sensing,36-40 catalytic,41 lithium ion transport,42 and photocatalytic properties43 when compared with their unary oxide counterparts WO3 and MoO3. In addition, the band gaps of the MoxW1-xO3 materials can be modulated by varying the Mo/W ratio.44,45 However, the diversity of MoxW1-xO3 nanostructures is relatively limited although various WO3 and MoO3 nanostructures (e.g., nanoparticulates,21,47 single crystals,16,48 nanowires,9,25 nanobelts,22,24 nanotubes,12,49 porous materials,15,17,50-53 hollow spheres,10,11 and hierarchical structures11,54,55) have been synthesized. To investigate the composition-property relationship, it is essential to prepare MoxW1-xO3 materials with tunable Mo/W ratios with the same or similar crystal structure. However, to the best of our knowledge, it remains a great challenge to prepare pure phase MoxW1-xO3 materials over a wide composition range.29,30,42,43,45 For example, the resulting structures from the solvothermal reaction of MoO3 · 2H2O and (NH4)6H2W12O41 · H2O were rather complicated (including MoO3, hexagonal Mo bronze, MoxW1-xO3 · 0.33H2O, and (NH4)zMoxW1-xO3).29,30 In * To whom correspondence should be addressed. E-mail: [email protected]. † Fudan University. ‡ The University of Queensland.

another example, the structure evolution of MoxW1-xO3 materials prepared by electrodeposition from aqueous peroxo-polymolybdotungstate solutions generally follows a trend from triclinic to monoclinic and then to orthorhombic phase via triclinic/ monoclinic and monoclinic/orthorhombic mixed phases with increasing Mo fraction.42,45 The difficulty in preparing MoxW1-xO3 materials with the same crystal structure over a wide composition range makes it difficult to perform in-depth studies to directly discern the composition-optical/electronic-property relationships without ruling out the effects of crystal phase difference, thus hard to design optimized MoxW1-xO3 materials for certain applications. Fortunately, it has been reported that the MoxW1-xO3 · 0.33H2O solid solutions over the full compositional range of 0 e x e 1 can be obtained by the hydrothermal treatment of an aqueous suspension of tungstic acid gel and molybdic acid solution at 110 °C.27 Thus, the MoxW1-xO3 · 0.33H2O solid solutions are desirable candidates for the composition-property study. In our previous work, we found that hexagonal shaped WO3 · 0.33H2O nanodisks composed of nanosheets can be prepared by hydrothermal treating of an aqueous peroxopolytungstic acid solution.48 In this work, we further extended this method to prepare a series of MoxW1-xO3 · 0.33H2O micro/ nanostructures with controlled stoichiometry (x ) 0, 0.25, 0.50, 0.75) via hydrothermal treating of aqueous peroxo-polytungstic acid and peroxo-molybdic acid solutions, and investigated their band gaps as a function of the Mo content. 2. Experimental Section Chemicals. Hydrogen peroxide (H2O2, 30 wt %) was purchased from Yixing City No. 2 Chemical Reagent plant. Molybdenum (batch no. F20090305) and tungsten powder (F20071101) were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received without purification. Synthesis. In a typical synthesis, y mmol (y ) 0, 2.5, 5.0, 7.5, and 10.0) of Mo powder and (10 - y) mmol of W powder were dissolved in a mixture of 20 mL of 30 wt % H2O2 and 20 mL of H2O under stirring at 293 K. The mixture was kept at

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293 K for 12 h under stirring. The solution was then transferred into a Teflon-lined stainless-steel autoclave, sealed, and hydrothermally treated at 473 K for 24 h. The products were collected by centrifugation, washed by water 3 times, and dried at room temperature. The resultant products with y ) 0, 2.5, 5.0, 7.5, and 10.0 were designated as WO3 · 0.33H2O, Mo0.25W0.75O3 · 0.33H2O, Mo0.50W0.50O3 · 0.33H2O, Mo0.75W0.25O3 · 0.33H2O, and R-MoO3, respectively. The Mo content in molar ratio in the precursor solution was defined as x, and x ) Mo/(Mo + W). Characterizations. X-ray diffraction (XRD) patterns were recorded on a German Bruker D4 X-ray Diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54056 Å) at a voltage of 40 kV and a current of 40 mA. Raman spectroscopy was performed on a Horiba Jobin Yvon HR800 micro-Raman spectrometer at room temperature. The standard He-Ne laser with a wavelength of 632.8 nm and a liquid-N2 cooled CCD was chosen. Scanning electron microscopy (SEM) images were obtained on a Philips XL30 microscope operated at 20 kV. An energy-dispersive X-ray spectroscopy (EDS) facility attached to the SEM was employed to analyze the chemical composition. 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 deposited onto a holey carbon film on a copper grid. UV-vis diffusive reflectance spectra (DRS) were obtained on a JASCO V-550 UV-vis spectrometer with BaSO4 as reference. The chemical composition of the products was analyzed by inductively coupled plasma (ICP, IRIS Itrepid, Thermo Element Company) after dissolving the samples in H2O2 or NaOH solution. The Brunauer-EmmettTeller (BET) surface areas were measured on a nitrogen adsorption apparatus (Quadrasorb SI, Quantachrome) at 77 K. The samples were degassed at 453 K for 6 h before the measurement. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis ULTRA X-ray photoelectron spectrometer, using a monochromatic Al KR (1486.6 eV) X-ray source and a 165 mm hemispherical electron energy analyzer. Survey spectra in the range of 0 to 1200 eV were recorded with use of a pass energy of 160 eV, dwell time of 100 ms, and step size of 1 eV. Narrow high-resolution scans were run with a pass energy of 20 eV, dwell time of 250 ms, and step size of 0.05 eV. All binding energies were calibrated by using contaminent carbon (C1s ) 284.6 eV) as a reference. 3. Results The mixture of peroxo-polytungstic acid and peroxo-molybdic acid solutions was prepared by dissolving Mo and W powders in aqueous H2O2 solution. When pure W powder was used as the reactant, a relatively cloudy peroxo-polytungstic acid solution was obtained (Figure S1a, Supporting Information). When Mo was introduced into the system, the color of the solution changed from light blue to yellow (Figure S1b-e, Supporting Information). As the Mo content increased, the solution became clearer gradually. The hydrothermal treatment at 473 K leads to the decomposition and condensation of peroxopolytungstic acid and peroxo-molybdic acid. Figure 1 shows the XRD patterns of the as-synthesized products as a function of Mo fraction in the initial mixed solution. For x ) 0 (Figure 1, pattern a), all diffraction peaks can be exclusively indexed as orthorhombic WO3 · 0.33H2O (Joint Committee on Powder Diffraction Standards, JCPDS No. 35-0270, space group Fmm2, lattice parameters a ) 0.7359 nm, b ) 1.251 nm, c ) 0.7704 nm), suggesting the high purity of the product. For x ) 0.25, 0.50, and 0.75 (Figure 1, patterns

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Figure 1. XRD patterns of (a) WO3 · 0.33H2O (x ) 0), (b) Mo0.25W0.75O3 · 0.33H2O (x ) 0.25), (c) Mo0.50W0.50O3 · 0.33H2O (x ) 0.50), (d) Mo0.75W0.25O3 · 0.33H2O (x ) 0.75), and (e) R-MoO3 (x ) 1.00).

b-d), the XRD patterns all resemble that of WO3 · 0.33H2O except for minor differences, indicating that as high as 75% W in WO3 · 0.33H2O (see Table 1) can be substituted by Mo while keeping the original orthorhombic host structure generally unaltered. With the increase of the Mo content x, a new diffraction peak centered at 2θ ) 29.8° appears, and the peak originally located at 2θ ) 37.74° splits into two peaks gradually. According to the references,27,28,56 the appearance of new diffraction peaks in this case is due to the reduced symmetry from an F-centered orthorhombic cell for WO3 · 0.33H2O to a C-centered orthorhombic cell for Mo0.75W0.25O3 · 0.33H2O caused by increased distortions induced by Mo substitution. Taking a close look at the XRD patterns of the MoxW1-xO3 · 0.33H2O solid solutions, two trends can be found: (1) generally, the diffraction peaks of the products sharpen gradually with x increasing from 0 to 0.75 and (2) the position of the diffraction peaks for these four samples does not shift with the increase of x due to the similar atomic radius of Mo and W (134.2 pm for Mo and 137 pm for W). For x ) 1.00 (Figure 1, pattern e), the XRD pattern can be assigned to orthorhombic MoO3, the so-called R-MoO3 phase (JCPDS No. 35-0609, space group Pbnm, lattice parameters a ) 0.3963 nm, b ) 1.3856 nm, c ) 0.3697 nm). The strong intensities of 020*, 040*, and 060* diffraction peaks indicate a preferred orientation of the product. Characteristic Raman spectra of the as-synthesized samples are shown in Figure 2. WO3 · 0.33H2O (x ) 0, Figure 2, pattern a) exhibits two strong peaks located at 683 and 804 cm-1, two intermediate bands centered at 337 and 949 cm-1, and many weak bands located below 300 cm-1. The two bands at 683 and 804 cm-1 with high intensity can be assigned to stretching vibrations ν(W-O-W) of the bridging oxygen atoms, the band at 337 cm-1 is characteristic of δ(O-W-O) deformation mode, while the band at 949 cm-1 is attributed to the stretching mode of the terminal WdO double bond.57 Generally, the Raman spectra of the samples with x ) 0.25, 0.50, and 0.75 (Figure 2, patterns b-d) are similar to that of WO3 · 0.33H2O (x ) 0), further demonstrating the structural similarity of these samples. However, three minor differences caused by increased distortions induced by Mo substitution

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TABLE 1: Theoretical and Experimental Mo Content, Mo5+ Fraction, M5+ Fraction, BET Surface Area, and Band Gap sample

xtheoreticala (%)

xEDSb (%)

xICPc (%)

xXPSd (%)

xMo(V)e (%)

xM(V)f (%)

SBETg (m2/g)

Egh (eV)

WO3 · 0.33H2O Mo0.25W0.75O3 · 0.33H2O Mo0.50W0.50O3 · 0.33H2O Mo0.75W0.25O3 · 0.33H2O R-MoO3

0 25 50 75 100

0 21 44 69 100

0 33 52 75 100

0 20 46 68 100

0 7.54 4.70 3.25 1.66

0 1.51 2.16 2.21 1.66

10.8 9.8 9.3 8.7 12.3

3.25 2.90 2.85 2.77 2.87

a Theoretical Mo content. b Mo content determined from EDS. c Mo content determined from ICP. d Mo content measured from XPS. e Mo5+ fraction ([Mo5+ ]/[Mo]) measured from XPS. f M5+ fraction {([Mo5+ ] + [W5+ ]) + ([Mo]/[W])} measured from XPS. g BET surface area. h Band gap.

Figure 2. Raman spectra of (a) WO3 · 0.33H2O (x ) 0), (b) Mo0.25W0.75O3 · 0.33H2O (x ) 0.25), (c) Mo0.50W0.50O3 · 0.33H2O (x ) 0.50), (d) Mo0.75W0.25O3 · 0.33H2O (x ) 0.75), and (e) R-MoO3 (x ) 1.00).

can also be found. First, the intensities of the Raman bands increase with the Mo content x; as a result, after intensity normalization the weak bands for Mo-rich samples become well-resolved. Second, the positions of some Raman bands shift with the increase of x from 0 to 0.75. For example, the ν(W-O-W) stretching vibration (originally located at 683 cm-1) shifts to higher wavenumbers continuously (702 cm-1 for Mo0.75W0.25O3 · 0.33H2O); the stretching mode of the terminal WdO double bond originally centered at 949 cm-1 shifts to low wavenumbers gradually, 929 cm-1 for Mo0.75W0.25O3 · 0.33H2O, while the position change of the ν(W-O-W) stretching vibration originally located at 804 cm-1 is irregular. Third, the relative intensity of the 683-702 cm-1 band to the 804-815 cm-1 band increases gradually with the increase of x. The differences in the positions and intensities of the Raman bands are most probably caused by the replacement of W by Mo. For x ) 1.0, the Raman spectrum (Figure 2, pattern e) agrees well with that of R-MoO3.42,58 The rather narrow band at 995 cm-1, the strongest peak at 820 cm-1, and the intermediate band at 665 cm-1 are coming from the antisymmetric ν(ModOterminal) stretching along the b-axis direction, the symmetric ν(MoOcorner-shared-Mo) stretching with the bonding aligning along the a axis direction, and the antisymmetric ν(Mo3-Oedge-shared) stretching, respectively. Intermediate and weak Raman shifts due to deformation and lattice bands can also be found at low wavenumbers of 379, 338, 292, 246, and 159 cm-1.

Figure 3. SEM images of (a) WO3 · 0.33H2O (x ) 0), (b) Mo0.25W0.75O3 · 0.33H2O (x ) 0.25), (c) Mo0.50W0.50O3 · 0.33H2O (x ) 0.50), (d) Mo0.75W0.25O3 · 0.33H2O (x ) 0.75), and (e, f) R-MoO3 (x ) 1.00).

Figure 3 represents the SEM images of the products. For WO3 · 0.33H2O (Figure 3a) and Mo0.25W0.75O3 · 0.33H2O (Figure 3b), the products are mainly composed of micrometer sized spheres (1-2 µm in size) aggregated by nanoparticles. Increase of x to 0.50 induces uniform and smooth submicrometer-sized hexagonal-shaped slabs with diameters of 300-600 nm and thicknesses of 100-200 nm (Figure 3c). A mixture of snowflakelike microparticles and highly branched microspheres with diameters ∼4 µm (Figure 3d) can be obtained for x ) 0.75. A very small fraction of microbelts can also be observed (Figure S2, Supporting Information). In good agreement with previous literature, hydrothermal treatment of peroxo-molybdic acid solution (x ) 1.0) leads to the formation of uniform R-MoO3 nanobelts with widths of 200-500 nm and lengths of tens of micrometers (Figure 3e,f).59-62 TEM was employed to investigate the structural information of the products. Figure 4a shows a typical well-developed hexagonal-shaped Mo0.50W0.50O3 · 0.33H2O slab with a diameter of ∼350 nm. From the contrast of an edge of a slab (Figure 4b), it is clear that each slab is composed of several thinner nanosheets (at least 5 layers in this case). Parts c and d of Figure 4 show the corresponding selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) image, respectively. The set of diffraction spots can be indexed as the [001] zone axis of single-crystalline orthorhombic Mo0.50W0.50O3 ·

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Figure 5. TEM images (a, b, d) and ED pattern (c) of Mo0.75W0.25O3 · 0.33H2O (x ) 0.75).

Figure 4. TEM images (a, b, e), HRTEM image (d), and ED patterns (c, f) of Mo0.50W0.50O3 · 0.33H2O (x ) 0.50).

0.33H2O. Interestingly, weak diffraction spots caused by high order Law zone can also be found at some forbidden sites as indicated by white arrows in Figure 4c.48 The comparison of parts b and c of Figure 4 confirmed that the top and bottom surfaces are of the {001} planes and the six side surfaces are surrounded by the {130} and {100} planes. Two sets of atomic spacings, 0.37 and 0.63 nm, which correspond to the {200} and {020} lattice fringes, respectively, can be distinguished clearly from Figure 4d. A side-standing slab with a thickness of 120 nm is shown in Figure 4e. The corresponding SAED pattern (Figure 4f) can be indexed as the [100] zone axis of orthorhombic Mo0.50W0.50O3 · 0.33H2O. A representative snowflake-like Mo0.75W0.25O3 · 0.33H2O microparticle with a diameter of ∼3.5 µm is shown in Figure 5a. It has six dendritic trunks with lengths and widths of ∼1.75 and 0.75-1.0 µm, respectively. Some thin slabs attached to the trunks can also be found. Notably, some of the trunks show brighter contrast at the center and darker contrast at the edges, suggesting these trunks may have hollow interiors at the center. We carefully checked several snowflake-like microparticles, and found this phenomenon is common (Figure S3, Supporting Information). The magnified TEM image of a hollow trunk and the corresponding SAED pattern are shown in parts b and c of Figure 5, respectively. The SAED pattern can be indexed to the [001] zone axis. By combining parts b and c of Figure 5, it is confirmed that the top and bottom surfaces are of the {001} planes and the other surfaces are covered by {010} and {110} planes. A typical highly branched microsphere is shown in Figure 5d, where no hollow interiors can be found. In agreement

Figure 6. TEM images (a, b), ED pattern (c), and HRTEM image (d) of R-MoO3 (x ) 1.0).

with the SEM results, a small fraction of microbelts can be observed. By SAED, we demonstrate that the microbelts are actually R-MoO3 (Figure S4, Supporting Information). Figure 6a is a low-magnification TEM image showing the overall morphology of the as-prepared R-MoO3 nanobelts. Numerous R-MoO3 nanobelts with widths of 200-500 nm and lengths of tens of micrometers are observed. A typical R-MoO3 nanobelt with a width of 230 nm lain flat on the copper grid is shown in Figure 6b with its corresponding SAED pattern and HRTEM image shown in Figure 6, parts c and d, respectively. The SAED pattern can be indexed to the [010] zone axis of single-crystalline R-MoO3, confirming that the R-MoO3 nanobelt in Figure 6b lies on its (010) plane and grows preferentially along the [001] direction. Interestingly, besides the strong allowed diffraction spots such as 200* and 002*, some weak diffractions can also be observed at those forbidden sites (such

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Figure 7. XPS spectra of the W4f core level region for (a) WO3 · 0.33H2O (x ) 0), (b) Mo0.25W0.75O3 · 0.33H2O (x ) 0.25), (c) Mo0.50W0.50O3 · 0.33H2O (x ) 0.50), and (d) Mo0.75W0.25O3 · 0.33H2O (x ) 0.75).

as 100* and 001* as indicated by white arrows in Figure 6c). In this case, the weak 100* and 001* diffractions are due to double diffraction caused by the dynamic scattering of the strong electron beam rather than the “high-order Laue zone” as observed in Mo0.50W0.50 · 0.33H2O. Consistent with the SAED pattern, the HRTEM image exhibits clear lattice fringes with d spacings of 0.40 and 0.37 nm, corresponding to the {200} and {002} atomic spacings, respectively. The BET surface areas of the products have been measured by N2 adsorption in the relative pressure range of 0.05-0.30 and listed in Table 1. There are two major factors that influence the BET surface areas of the products, that is, the particle size and the Mo content. The R-MoO3 nanobelts show the largest BET surface area of 12.3 m2/g due to the nanobelt morphology and the highest Mo content. For the MoxW1-xO3 · 0.33H2O solid solutions, the particles size (the size of the primary particles) play a more important role. With the increase of the Mo content from 0 to 0.75, the BET surface area of the products decreases from 10.8 to 8.7 m2/g, which can be attributed to the increase of the primary particle size deduced from the sharpening of the diffraction peaks shown in Figure 1, also in accordance with the SEM observations. XPS was used to determine the surface chemical composition and valence states of the products. As shown in Table 1, the surface Mo content determined by XPS data is quite similar to the bulk chemical composition measured by EDS (over more than 10 positions) and ICP, indicating a homogeneous distribution of Mo and W in the products. Figures 7 and 8 show the evolution of W4f ad Mo3d core level regions as a function of Mo content, respectively. The W4f core level spectra (Figure 7) of the products are dominated by a spin-orbit doublet with peaks at binding energies of 35.75 ( 0.10 (W4f7/2) and 37.90 ( 0.10 eV (W4f5/2) (see also Table 2). The spin-orbit separation

between the two peaks is ∆E ) 2.14 eV and the peak area ratio is 0.75. This doublet is associated with the W6+ oxidation state.63 The W5+ and W4+ species with lower binding energies are not detected in the spectra. As expected, the intensity of the doublet decreases when the Mo content increases. Different from the W4f spectra, the Mo3d spectra (Figure 8) show the 5/2-3/2 spin-orbit doublet of two oxidation states. The most intense doublet at binding energies of 232.95 ( 0.15 (Mo3d5/2) and 236.10 ( 0.15 eV (Mo3d3/2) (Table 2) corresponds to Mo6+, whereas the weak doublet at lower binding energies of 231.65 ( 0.15 and 234.8 ( 0.15 eV corresponds to Mo5+.64 In the same doublet, the spin-orbit separation between the two peaks is ∆E ) 3.13 eV and the peak area ratio is 0.667. With increasing Mo content from 0.25 to 1, the fraction of Mo5+ ([Mo5+ ]/[Mo]) decreases from 7.54% to 1.66% (Table 1). However, when the fraction of reduced W species (∼0) is considered, the fraction of M5+ {([Mo5+ ] + [W5+ ])/([Mo] + [W])} increases from 0 to 2.21% with increasing Mo content from 0 to 0.75 (Table 1). The optical absorption properties of the as-synthesized materials are key factors in determining their photocatalytic activities and also highly relevant to their photochromic and electrochromic properties. UV-vis diffuse reflectance spectroscopy is a useful tool for characterizing the electronic states of semiconductor materials. Figure 9 displays the UV-vis DRS of the as-synthesized materials. The absorption edge (λg) can be estimated from the x-axis intercept of the tangent at the steep rising part of the spectra (as shown in Figure 9, the dotted lines). Then, the band gaps (Eg) of the as-prepared materials can be determined by the equation Eg ) 1240/λg. WO3 · 0.33H2O displays an absorption edge of ∼381.8 nm, corresponding to a band gap of 3.25 eV. A minor tail extending from 380 to 450 nm can also be observed. The overall profiles of the Mo-doped

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Figure 8. XPS spectra of the Mo3d core level region for (a) Mo0.25W0.75O3 · 0.33H2O (x ) 0.25), (b) Mo0.50W0.50O3 · 0.33H2O (x ) 0.50), (c) Mo0.75W0.25O3 · 0.33H2O (x ) 0.75), and (d) R-MoO3 (x ) 1.00).

TABLE 2: Binding Energies for Mo3d and W4d Core Lines of the Products binding energy (eV)

W4f7/2 (VI)

W4f5/2 (VI)

WO3 · 0.33H2O Mo0.25W0.75O3 · 0.33H2O Mo0.50W0.50O3 · 0.33H2O Mo0.75W0.25O3 · 0.33H2O R-MoO3

35.77 35.74 35.82 35.66

37.92 37.89 37.97 37.81

samples are rather similar to that of WO3 · 0.33H2O except that they show no tails in the UV-vis range. With the introduction of 25% of Mo, a significant red shift can be observed, indicating a narrowing of the band gap; with even more Mo fraction, relatively minor red shifts can be observed. For x ) 0.25, 0.50,

Mo3d5/2 (VI)

Mo3d3/2 (VI)

Mo3d5/2 (V)

Mo3d3/2 (V)

232.90 232.99 232.87 233.10

236.03 236.12 236.00 236.23

231.77 231.75 231.52 231.62

234.90 234.88 231.65 234.75

and 0.75, the band gaps of the solid solutions are 2.90, 2.85, and 2.77 eV, respectively. The band gap of R-MoO3, which is 2.87 eV, does not follow the trend of the MoxW1-xO3 · 0.33H2O solid solutions due to its structural difference. The variation of band gap as a function of Mo content is shown in Figure S5 (Supporting Information). The DRS results clearly demonstrate the importance of structure (crystalline phase) in studying the composition-property relationship of the binary MoxW1-xO3 materials. 4. Discussions

Figure 9. DRS of MoxW1-xO3 · 0.33H2O (x ) 0, 0.25, 0.50, 0.75) and R-MoO3 (x ) 1.00).

The structure of WO3 · 0.33H2O (Figure 10) can be described as follows.28,65 All the tungsten atoms are bonded to six oxygen atoms by WsO single bond, WdO double bond, or WsOH2 coordination bond in a slightly distorted octahedral coordination. The [WO6] and [WO5(OH2)] octahedra share their corners and form six-membered rings in the (001) plane. As a result, the basic structural element, that is, an infinite layer is formed. The stacking of such layers along the [001] direction with a shift of a/2 between adjacent layers leads to the formation of the complete WO3 · 0.33H2O structure. Both Mo and W are group 6 transition metals. Furthermore, Mo and W have quite approximate atomic radii (134.2 pm vs

MoxW1-xO3 · 0.33H2O Solid Solutions

J. Phys. Chem. C, Vol. 114, No. 49, 2010 20953 the trend of MoxW1-xO3 · 0.33H2O solid solutions due to its structural difference, demonstrating the importance of keeping a similar structure when comparing a property (band gap here). 5. Conclusion

Figure 10. Schematic structure of MoxW1-xO3 · 0.33H2O. The large blue spheres represent the Mo or W atoms, and the small red spheres represent the oxygen atoms or H2O molecules. The pink and yellow squares represents the [Mo/WO6] or [Mo/WO5(H2O)] octahedra from adjacent layers.

137 pm) because of the lanthanum contraction. Thus they have quite similar chemical properties, and numerous tungsten trioxides and hydrates have their isostructural analogues for Mo. The isostructural analogue for WO3 · 0.33H2O is MoO3 · 0.33H2O. These two hydrates have not only similar structure but also approximate lattice constants, which can also be reflected by the great similarity of the XRD patterns. Due to the similarity of the structure and the approximation of the lattice constants, a large fraction (up to 75%) of the W atoms can be substituted by Mo atoms, while keeping the original orthorhombic structure of WO3 · 0.33H2O almost unchanged. The only difference is that the substitution of W by Mo induces more distortion in the lattice and the symmetry reduced gradually and slightly from an F-orthorhombic cell for WO3 · 0.33H2O to a C-orthorhombic cell for Mo0.75W0.25O3 · 0.33H2O.25,45 In spite of this lowering of symmetry, this series of hydrates (MoxW1-xO3 · 0.33H2O, x ) 0, 0.25, 0.50, and 0.75) are almost isostructural. Thus, the schematic structure of WO3 · 0.33H2O shown in Figure 8 can also represent the structures of MoxW1-xO3 · 0.33H2O solid solutions (x ) 0.25, 0.50, and 0.75). As mentioned above, each W6+ is octahedrally surrounded by six O in WO3 · 0.33H2O. As a consequence of this arrangement, the W5d orbitals split into t2g and eg levels. For defectfree tungsten oxide and hydrates, the band gap (from O2p to Wt2 g) is wide enough to render it transparent. When some oxygen defects are created or some electrons are added, the excess electrons trapped by W6+ ions must occupy the t2g bands and lead to the transformation from transparency to a colored state because of the d-d transition (t2g to eg),66 that is, the reduced W species acts as the color center. When Mo is introduced into the lattices of WO3 · 0.33H2O, the Mo6+ ions act as preferential localized trapping sites for electrons than W6+ and lead to reduced Mo species (in our case, Mo5+).32 This is also supported by our XPS study, the reduced W species (W5+ and W4+) are out of the detection limit of XPS while 3.25-7.54% of Mo6+ have been reduced to Mo5+ in the MoxW1-xO3 · 0.33H2O solid solutions. With increasing Mo content, the fraction of M5+ (M ) Mo and W, {([Mo5+ ] + [W5+ ])/([Mo] + [W])} in MoxW1-xO3 · 0.33H2O also increases. The increased M5+ fraction and thus enhanced intervalencytransition,32 that is electron transition between metal ions with different valencies (e.g., Mo5+ to Mo6+ and Mo5+ to W6+), are responsible for the red shift in DRS (the narrowing of the band gaps). However, for the band gap of R-MoO3, it does not follow

In conclusion, we have successfully prepared a series of MoxW1-xO3 · 0.33H2O micro/nanostructures with controlled stoichiometry (x ) 0, 0.25, 0.50, 0.75). The WO3 · 0.33H2O lattice can be substituted with up to 75% Mo without structural alteration of the orthorhombic host structure. With the increase of the Mo content, the band gap of MoxW1-xO3 · 0.33H2O narrowed from 3.25 to 2.77 eV. The increased M5+ fraction and thus enhanced intervalency-transition are responsible for the narrowing of the band gap. Our work may provide an insight into optimization of molybdenum-tungsten binary oxides and hydrates for photocatalytic and chromogenic applications. Acknowledgment. The authors thank Prof. Zhanghai Chen and Mr. Qijun Ren for the Raman measurements. This work is financially supported by the State Key Research Program of China (2010CB226901, 2006CB932302), the NSF of China (20721063), Science & Technology Commission of Shanghai Municipality (08DZ2270500, 0852 nm01500), and the Australia Research Council. Supporting Information Available: Digital photographs, SEM images, TEM image, and ED pattern. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yao, J. N.; Hashimoto, K.; Fujishima, A. Nature 1992, 355, 624– 626. (2) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201–262. (3) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1529– 1598. (4) Gesheva, K.; Szekeres, A.; Ivanova, T. Sol. Energy Mater. Sol. Cells 2003, 76, 563–576. (5) He, T.; Yao, J. N. J. Mater. Chem. 2007, 17, 4547–4557. (6) Niklasson, G. A.; Granqvist, C. G. J. Mater. Chem. 2007, 17, 127– 156. (7) Wu, J. J.; Hsieh, M. D.; Liao, W. P.; Wu, W. T.; Chen, J. S. ACS Nano 2009, 3, 2297–2303. (8) Wang, S. T.; Feng, X. J.; Yao, J. N.; Jiang, L. Angew. Chem., Int. Ed. 2006, 45, 1264–1267. (9) Polleux, J.; Gurlo, A.; Barsan, N.; Weimar, U.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2006, 45, 261–265. (10) Li, X. L.; Lou, T. J.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2004, 43, 5442–5449. (11) Chen, D.; Ye, J. H. AdV. Funct. Mater. 2008, 18, 1922–1928. (12) Zhao, Z. G.; Miyauchi, M. Angew. Chem., Int. Ed. 2008, 47, 7051– 7055. (13) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. J. Am. Chem. Soc. 2008, 130, 7780–7781. (14) Morales, W.; Cason, M.; Aina, O.; de Tacconi, N. R.; Rajeshwar, K. J. Am. Chem. Soc. 2008, 130, 6318–6319. (15) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Ueda, W.; Abe, R. Chem. Commun. 2008, 6552–6554. (16) Zhao, Z. G.; Liu, Z. F.; Miyauchi, M. Chem. Commun. 2010, 46, 3321–3323. (17) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Abe, R.; Ueda, W. J. Mater. Chem. 2010, 20, 1811–1818. (18) Zhou, J.; Xu, N. S.; Deng, S. Z.; Chen, J.; She, J. C.; Wang, Z. L. AdV. Mater. 2003, 15, 1835–1840. (19) Fang, X.; Bando, Y.; Gautam, U. K.; Ye, C.; Golberg, D. J. Mater. Chem. 2008, 18, 509–522. (20) Khademi, A.; Azimirad, R.; Zavarian, A. A.; Moshfegh, A. Z. J. Phys. Chem. C 2009, 113, 19298–19304. (21) Lee, S. H.; Kim, Y. H.; Deshpande, R.; Parilla, P. A.; Whitney, E.; Gillaspie, D. T.; Jones, K. M.; Mahan, A. H.; Zhang, S. B.; Dillon, A. C. AdV. Mater. 2008, 20, 3627–3632. (22) Mai, L. Q.; Hu, B.; Chen, W.; Qi, Y. Y.; Lao, C. S.; Yang, R. S.; Dai, Y.; Wang, Z. L. AdV. Mater. 2007, 19, 3712–3716. (23) Li, W. Y.; Cheng, F. Y.; Tao, Z. L.; Chen, J. J. Phys. Chem. B 2006, 110, 119–124.

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