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Band Gap Tailored Zn(Nb1-xVx)2O6 Solid Solutions as Visible Light Photocatalysts Sang Min Ji,† Sun Hee Choi,‡ Jum Suk Jang,† Eun Sun Kim,† and Jae Sung Lee*,† Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang 790-784, Korea, and Beamline Research DiVision, Pohang Accelerator Laboratory, San 31, Hyoja-dong, Pohang 790-784, Korea ReceiVed: March 1, 2009; ReVised Manuscript ReceiVed: August 14, 2009
Zn(Nb1-xVx)2O6 solid solutions (0.01 e x e 0.1) were synthesized by incorporation of V into the lattice of ZnNb2O6 by solid-state reaction. The wide-band-gap semiconductor ZnNb2O6 (3.98 eV) became visiblelight-active with the band gap energy shifted down to ca. 2.5 eV. Physical characterization with XRD, XPS, and XANES revealed that homogeneous solid solutions were synthesized, which showed systematic variation of electronic structure with concentration of incorporated vanadium. Theoretical band structure calculation indicated that d orbitals of vanadium were responsible for the band gap reduction by forming the bottom part of the conduction band of the solid solution. All V-incorporated solid solutions loaded with RuO2 as a cocatalyst showed photocatalytic activities for H2 or O2 production from aqueous solutions of sacrificial reagents under visible light irradiation (λ g 420 nm) and RuO2(3 wt %)-loaded Zn(Nb0.94V0.06)2O6 showed the highest activity. Introduction Photocatalytic water splitting is an ideal method for utilizing solar energy to produce hydrogen, which is considered a key energy carrier and fuel of the future. For the efficient hydrogen production via solar energy conversion, photocatalytic materials that respond to visible light photons are essential. In spite of extensive research in the past, efficient visible light photocatalysts with the suitable band gap energy and band edge positions are still rare.1,2 A fruitful method to acquire visible light photocatalysts is to control the band structure by modification of the well-known UV-active phtocatalysts with wide band gap energy. This approach, sometimes termed band engineering,3 employs techniques such as transition-metal doping,4-6 valenceband controlling,7,8 and formation of solid solutions.9,10 In a solid solution between two semiconductors of wide and narrow band gaps, the band gap energy and band edge positions can be controlled by varying the composition. Being a single homogeneous phase, the solid solution material usually exhibits a strong, single absorption edge, which is highly desirable for a high efficiency photocatalyst. In contrast, cation or anion doping produces materials that show two absorption edges; main edge in UV range and a small shoulder due to dopants in the visible light range. Thus, visible light absorption is limited. In this work, we selected orthorhombic ZnNb2O611 with columbite structure as UV light-active photocatalyst with wide band gap (ca. 4 eV) and ZnV2O612 as a material with a narrow band gap. Both materials have the same crystal structure and difference between atomic radii of Nb and V is less than 15%. Therefore, it is expected that these materials form readily a homogeneous solid solution.13 Indeed, the prepared solid solution, i.e., Zn(Nb1-xVx)2O6 (x ) mole fraction of V, 0-0.1) loaded with RuO2 as a co-catalyst was found to be an active photocatalyst that could produce hydrogen or oxygen from aqueous solutions of sacrificial reagents under visible light * To whom correspondence should be addressed. E-mail: jlee@postech. ac.kr. Tel: +82-54-279-2266. Fax: +82-54-279-5528. † Pohang University of Science and Technology. ‡ Pohang Accelerator Laboratory.
irradiation (λ g 420 nm). The synthesis, structure, and electronic properties of the Zn(Nb1-xVx)2O6 solid solutions are discussed for various x values based on the results of physicochemical characterization and theoretical band structure calculation. Finally, these properties are correlated with photocatalytic activity. Experimental Section Preparation of Materials. A series of Zn(Nb1-xVx)2O6 solid solutions (x ) 0.01-0.1) and ZnNb2O6 were synthesized by a conventional solid-state reaction method at a high temperature. A stoichiometric mixture of ZnO (Alfa, 4 N), Nb2O5 (Aldrich, 4 N), and V2O5 (Aldrich, 2 N) was ground in a agate mortar in the presence of ethanol and dried at 373 K for 1 h in air. The obtained mixture powders were calcined at 973 K for 2 h, and then the palletized powders were sintered at 1148 K for 2 h in static air under ambient pressure. The resulting pellet was ground in an agate mortar in the presence of ethanol to obtain fine powder. The colors of as-prepared powders were changed from white to yellow with an increase in the amount of vanadium. In order to enhance photocatalytic activity, RuO2 nanoparticles were loaded as co-catalyst, which would function as an electron trap or a reaction site on the surface of prepared photocatalyst. RuO2 nanoparticle was loaded by impregnation with ruthenium carbonyl complex, Ru3(CO)12, dissolved in tetrahydrofuran at 333 K for 5 h, and then evaporated to dryness and finally oxidized at 623 K for 1 h in air.14 Characterization of Materials. High-resolution X-ray Powder diffraction (HRPD) patterns were obtained by using synchrotron radiation on beamline 8C2-HRPD of Pohang Accelerator Laboratory (PAL), Pohang, Korea. The incident X-rays were vertically collimated by a mirror, and monochromatized to the wavelength of 1.54960 Å by a double-crystal Si(111) monochromator. A data set was collected in the range of 10° e 2θ e 152° with a step size of 0.01° (2θ angle). From diffraction patterns, the value of d-spacing (the distance between adjacent planes in the set (hkl)) was calculated by the Bragg’s Law and the lattice parameters and volume of unit cell were
10.1021/jp901882q CCC: $40.75 2009 American Chemical Society Published on Web 09/21/2009
Zn(Nb1-xVx)2O6 Solid Solution Photocatalyst calculated by the least-squares method applying the orthorhombic columbite structure.15 The diffuse reflectance spectrum was obtained by a UV-vis recording spectrophotometer (UV-2401PC, Shimadzu Co.) with an integrating sphere (ISR-240A, Shimadzu Co.) at room temperature and converted to the absorption spectrum by the Kubelka-Munk function, F(R).16 The optical band gap energy could be estimated from the absorption spectra extrapolating (F(R) · hV)2 versus hV plot.17 The electronic state (oxidation state) and qualitative composition were analyzed by X-ray photoelectron spectra (XPS) acquired with a VG-Scientific ESCALAB 220 iXL spectrometer equipped with a hemispherical electron analyzer and an Mg KR (hν ) 1253.6 eV) X-ray source. The binding-energy calibration was performed using the C 1s peak at 284.6 eV as the reference energy. HR FE-SEM images were obtained on a JEOL JSM 7401F instrument operated at 8 kV. X-ray absorption near-edge structure (XANES) experiments were carried out on wiggler beamline 5A of PAL, Korea. The incident beam was monochromatized using a Si(111) doublecrystal monochromator. The spectra for the K-edge of Nb (Eo ) 18986 eV) were taken at room temperature in transmission mode with a He-filled IC Spec ionization chamber for incident beam and with a N2-filled chamber for transmitted beam. The energy scan was performed in four regions for good energy resolution in a steep absorption; 5 eV step in the region of 18786-18936 eV, 1 eV step in 18936-18976 eV, 0.2 eV step in 18976-19026 eV, and 2.0 eV step in 19026-19586 eV. The XANES regions of obtained spectra were processed in the usual way to obtain the absorbance and analyzed with ATHENA in the IFEFFIT suite of software programs.18 Background absorption including any instrumental absorption and absorption from other edges was removed by using a simple linear fit in the pre-edge region. The edge energy E0 was identified as the energy of the maximum derivative of absorption. Then, the pre-edge background-removed absorption spectra were normalized by using the edge step ∆µ0 (E0) where µ0 (E) was the smooth bare atom background function, calculated with the cubic spline in the post-edge region. For the theoretical electronic band-structure calculation of solid solutions, partial LDOS (angular momentum projected density of states) was calculated with FEEF 8.2 code.19 The calculations are based on a relativistic, self-consistent real space Green’s function formalism. A Hedin-Lundqvisit exchange potential was applied to the cluster of 71 atoms. As an electron-hole interaction has a considerable influence on the density of states of semiconductors, the core-hole effect was taken into account by putting a core hole in the Nb LIII level.20,21 Crystallographic parameters including lattice parameters and atomic positions were adopted from the literature for this calculation.22,23 Photocatalytic Reactions. The photocatalytic reactions were carried out at room temperature in an upper-irradiation-type Pyrex reaction vessel hooked up to a closed gas circulation system. Two half reactions, i.e., photocatalytic reduction of H+ to H2 or oxidation of H2O to O2 were examined under visible light irradiation in the presence of an electron donor (methanol) or acceptor (Ag+) as sacrificial agents. H2 evolution was examined in an aqueous solution (distilled water 80 mL and methanol 20 mL) while stirring with 0.2 g of the catalyst loaded with RuO2 as co-catalyst. For the O2 evolution, the reaction was examined in an aqueous AgNO3 solution (0.01 M, 100 mL) while stirring with 0.2 g of the catalyst loaded with RuO2. Photocatalytic reactions were performed under the visible light irradiation with 450 W Hg-arc lamp equipped with cutoff filter
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Figure 1. HRPD patterns of (a) ZnNb2O6 and (b-j) Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0.01-0.1) and the variation of diffraction peak of (311) plane of samples.
(λ g 420 nm) and liquid filter to remove IR, after nitrogenpurging for 1 h to deaerate. The evolved amounts of H2/O2 were analyzed by gas chromatography (HP6890) equipped with a thermal conductivity detector (TCD) and molecular sieve fivecolumn with Ar carrier gas. The quantum efficiency (QE) was calculated using the following equation: QE ) 2 × number of H2 or 4 × number of O2 generated per number of incident photon. The number of incident photons were determined by an optical power meter (1815-C, Newport) with the light sensor (Si photodiode, 818-UV, Newport) attached to the photocatalytic reactor. Results and Discussion Crystal Structure. Figure 1 shows the HRPD patterns obtained with synchrotron X-ray radiation for ZnNb2O6 and a series of Zn(Nb1-xVx)2O6 solid solutions (x ) 0.01-0.1) prepared at 1423 and 1148 K, respectively. The different temperatures reflect the low melting temperature of precursor V2O5, which consequently lowers the optimum crystallization temperature for Zn(Nb1-xVx)2O6 solid solutions. The HRPD pattern of ZnNb2O6 prepared is consistent with that of columbitestructured ZnNb2O6 with an orthorhombic symmetry (JCPDS 76-1827). It belongs to the space group of D142h-Pbcn with lattice parameters of a ) 14.208, b ) 5.726, and c ) 5.04.20 All prepared solid solutions exhibited a single phase with columbite structure without apparent impurity phase. All diffraction peaks shifted to higher angles with increasing value of x in Zn(Nb1-xVx)2O6. This successive shift of diffraction peaks indicates that the prepared samples were not a mixture of ZnNb2O6 and ZnV2O6, but a single phase of Zn(Nb1-xVx)2O6 solid solutions. In addition, the lattice parameter, a and unit cell volumes, calculated from d-spacing of measured diffraction patterns, deceased linearly with increasing amounts of V+5 incorporated into Nb5+ site, as shown in Figure 2. Such variation of lattice parameters should be expected due to the different ionic radii of Nb5+ (1.429 Å) and V+5 (1.321 Å). Thus, it was confirmed that solid solutions with columbite structure were successfully synthesized for all samples. Optical Properties. The optical properties were probed by UV-vis diffuse reflectance (UV-DR) spectroscopy for solid
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Figure 2. Calculated lattice parameter, a and unit cell volume of Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0.01-0.1); These values were calculated based on d-spacings obtained from HRPDs.
Figure 3. UV-vis diffuse reflection spectra of ZnNb2O6 and Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0.01-0.1).
solutions of different compositions. The measured reflectance was converted to absorbance using the Kubelka-Munk function as shown in Figure 3. In case of ZnNb2O6, absorption edge appeared near 300 nm corresponding to 3.98 eV, consistent with the literature.10 However, in case of solid solutions of low V contents (x e 0.03), two absorption edges appeared. The main absorption edge should originate from the band-to-band transition in ZnNb2O6 while the weak absorption shoulder in the region of longer wavelengths might originate from level-to-band transition of some impurity energy level. As mentioned, such weak absorption shoulder is often observed for transition metaldoped semiconductors.24 For samples with high V contents (x e 0.04), the weak absorption shoulder was not observed and the main absorption edge became steeper, suggesting that asprepared samples have a direct band gap and absorb visible light due to the band-to-band transition. In addition, absorption edge was red-shifted slightly to longer wavelength as increasing amounts of V were incorporated, as shown in inset of Figure 3. In order to determine band gap energy of samples from the absorption spectra, a plot of (F(R) · hV)2 vs hV was introduced, as shown in inset of Figure 4. Figure 4 shows the variation of band gap energies determined by extrapolating the linear part of (F(R) · hV)2 plot to the energy axis. As V atoms were incorporated, the band gap energy decreased rapidly initially, and then slowly approached ca. 2.5 eV. The energy did not vary even when the concentration of V increased further (x g 0.06). This band gap energy is highly desirable for water splitting
Ji et al.
Figure 4. Band gap energies of ZnNb2O6 and Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0.01-0.1). Inset: plot of (F(R) · hν)2 vs hν for samples with x ) 0.03-0.1.
photocatalysts because it is larger than the Gibbs free energy change for water dissociation (237 kJ/mol or 1.23 eV per electron) and still small enough to absorb a significant part of solar spectrum.1,2 The detailed examination for the change in band gap energy is followed. Electronic Structures. The optical properties just discussed are strongly related to the electronic structure of the material, and the electronic state was determined by XPS and XANES. In Figure 5a, XPS spectra of Nb 3d core level for solid solution samples revealed a characteristic Nb 3d3/2-Nb 3d5/2 spin-orbital splitting, indicating the octahedral structure of Nb coordinated with six oxygen atoms. The measured binding energies of Nb 3d3/2 and Nb 3d5/2 were ca. 209.5 and ca. 206.9 eV, respectively, which were in accordance with the literature.25 This indicates that Nb of all samples are in a pentavalent Nb(V) oxidation state independent of the concentration of V incorporated. However, the intensity of these peaks decreased with the increase of V concentration in the solid solution. Figure 5b shows XPS V 2p3/2 core level spectra of solid solutions. The measured binding energy of V 2p was ca. 517.2 eV in accordance with literature for pentavalent V(V).23 Further, the intensity of V 2p peaks increased with the increasing V concentration. These XPS results reveal that there is no change of the formal oxidation states of Nb and V in solid solutions. XANES may be more sensitive to minute variation in electronic state of the solid solution. Figure 6a shows Nb K-edge XANES of Nb foil (standard reference), NbO2 (tetravalent), Nb2O5 (pentavalent), and ZnNb2O6. Two niobium oxides used as reference materials were commercial products. Binding energies were calibrated with that of standard Nb foil. The main absorption peaks in Figure 6 denotes the electronic transition from 1s to 5p orbitals. In addition, a weak pre-edge shoulder was observed in spectra of Nb2O5 and ZnNb2O6 containing Nb(V) atoms. This characteristic pre-edge is due to the dipoleforbidden electronic transition from 1s to 4d orbitals.26 XANES spectra of Zn(Nb1-xVx)2O6 solid solutions (x ) 0, 0.02, 0.04, 0.06, 0.08, 0.1) are shown in Figure 6b. All samples showed the characteristic pre-edges, indicating the oxidation state of Nb(V) in solid solutions. However, there was some fine variation in the position and intensity of pre-edges in spectra of solid solutions, which reflects the different electronic states of the 4d orbital of Nb in the material. Because the weak preedge shoulder was originated from the s-d transition, the reduction of intensity of pre-edge shoulder reflects a decrease in the density of empty d-states as V content increased. Furthermore, red-shift the pre-edge shoulder is interpreted as
Zn(Nb1-xVx)2O6 Solid Solution Photocatalyst
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Figure 6. Nb K-edge XANES spectra of (a) Nb foil, NbO2, Nb2O5, ZnNb2O6 prepared and (b) Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0-0.1). Figure 5. X-ray photoelectron spectra of (a) Nb 3d and (b) V 2p core level of ZnNb2O6 and Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0.02-0.1).
the decrease in the attractive potential of the nucleus on the 1s electron due to the partial occupancy of the outer 4d orbital of Nb5+ ion.27 Thus, the s-d transition would become easier in the solid solutions relative to ZnNb2O6. To interpret the fine structure of the pre-edge shoulder in XANES spectra in more detail, the normalized absorption spectra were converted to the second derivative ones in Figure 7 to see better the variation in pre-edge. Thus, the positions of pre-edges were determined from Figure 7 and their intensities from the deconvolution of XANES spectra in Figure 6. The results are summarized in Figure 8. The pre-edge of ZnNb2O6 was located at 18994.3 eV with an intensity of 0.0589. However, the position and intensity of pre-edge of solid solutions decreased to 18992.8 eV and 0.0351, respectively, as the concentration of V increased to 6% (x ) 0.06). This means that the empty 4d orbital of Nb in solid solution is partially occupied by the incorporation of V. It might be suggested that such partial occupation of Nb 4d orbital is originated from the hybridization between Nb 4d and V 3d orbitals. However, for higher concentrations of V, the position and intensity of preedge became the same as those of ZnNb2O6. It might be due to the weak hybridization or dehybridization between those two d orbitals. In addition, at x ) 0.06, the binding energy was the lowest. Thus, the s-d transition can occur most readily in Zn(Nb1-xVx)2O6 (x ) 0.06).
Figure 7. Second derivatives of Nb K-edges XANES spectra of Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0-0.1).
Antonio et al. observed the similar phenomena between Nb and Ti and suggested that the variation of Nb 4d orbital is originated from the random interaction between Nb and Ti at low Nb concentrations.27 The variation of the Nb 4d orbital in the present work could be explained as depicted in Figure 9. At low V concentrations, the V atoms are incorporated randomly into Nb sites and some V 3d orbitals hybridized with Nb 4d orbitals, resulting in the partial occupation of Nb 4d orbital by
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Figure 10. Valence band spectra of Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0-0.1). Figure 8. Variation of (a) intensity and (b) energy position of preedges in Nb K-edges XANES spectra of Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions (x ) 0-0.1)
Figure 9. Proposed band structures of ZnNb2O6 and Zn(Nb1-xVx)2O6 solid solutions prepared with different compositions.
electrons from V 3d orbital. However, as the V concentration increased, V atoms begin to interact with each other and form localized V 3d orbital, suggesting the formation of new conduction band edge made of only V 3d orbitals at a lower energy level than Nb 4d orbitals. The theoretical calculation in the following section supports the formation of the additional conduction band consisting of V 3d orbitals. With regard to the band positions, the bottom of conduction band should be located at a more negative position than 0 V and the top of valence band at a more positive position than + 1.23 V, because Zn(Nb1-xVx)2O6 (x > 0) could produce both H2 and O2 photocatalytically (Figure 12). Recalling that the band gap of Zn(Nb1-xVx)2O6 (x ) 0.06) is ca. 2.5 eV, the valence band would be located approximately at 2.0-2.5 eV and the conduction band at -0.5-0 eV. To investigate valence band of solid solutions, XPS measurement was performed especially for the valence band region (0-14 eV), as shown in Figure 10. All solid solutions show two peaks; one is a broad peak of O 2p orbital ranging from 3
to 8 eV and the other is a narrow peak of Zn 3d orbital. It reveals that valence bands of all solid solutions consist mainly of O 2p orbitals hybridized with Zn 3d orbitals and there is no contribution of incorporated V in forming the valence band. Band Structure Calculation. To investigate the band gap tuning effect by incorporated V and to confirm the proposed band structure of the solid solution on the basis of experimental results, partial LDOS (angular momentum projected density of states) for band structure region was calculated with FEEF 8.2 code.19 The ab initio real space Green’s function code FEFF8 permits simultaneous, self-consistent calculations of both XAS (X-ray absorption fine structure) and electronic structure.28 In Figure 11, the conduction band of solid solutions consists of the combination of Nb 4d orbital and V 3d orbital, while that of ZnNb2O6 consists of only Nb 4d orbitals. The incorporated V 3d orbitals were located at a lower energy level than Nb 4d orbital, contributing to the formation of new bottom of conduction band. It can also be deduced from the different crystal field splittings (10Dq) of Nd 4d and V 3d orbitals. The greater the splitting of energy levels, the more hybridization with O 2p orbitals. Therefore, the antibondings associated with Nd 4d orbitals places at a higher energy level than those with V 3d orbitals. In low V concentrations (x ) 0.06), V 3d orbital overlapped with Nb 4d orbital, forming a hybrid orbital. However, in high V concentration (x ) 0.44), V 3d orbitals are separated from Nb 4d orbital and form a new conduction band at a lower energy level, resulting in reduction of band gap energy. The formation of the additional conduction band has been reported in the energy bands for SrTiO3 where the additional conduction band associated with the metal s-p and Sr d orbitals lies several eV above the Fermi level.29 These computed band structure are consistent with the proposed band structure from XANES results discussed above. Furthermore, it was revealed that the valence band of all materials consists of the hybrid orbitals of Zn 3d and O 2p orbitals. In particular, the top of valence band has a dominant contribution from O 2p orbitals, which is also consistent with valence band XPS in
Zn(Nb1-xVx)2O6 Solid Solution Photocatalyst
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Figure 12. Photocatalytic (a) hydrogen and (b) oxygen evolution rate over RuO2(5 wt %)-Zn(Nb1-xVx)2O6 solid solutions (x ) 0-0.1) in the presence of methanol (20 vol %) and AgNO3 (0.01M), respectively, under visible light irradiation (λ g 420 nm). Figure 11. Theoretically computed partial LDOS for ZnNb2O6 (a) and Zn(Nb1-xVx)2O6 solid solutions with x ) 0.06 (b) and 0.44 (c). LDOS were calculated with FEEF 8.2 code.
Figure 10. The contribution of V 3d orbital to the valence band of the solid solution is rather concentrated toward bottom side of the band. Photocatalytic H2/O2 Production from Aqueous CH3OH or AgNO3 Solutions. Photocatalytic activities of Zn(Nb1-xVx)2O6 solid solutions (x ) 0-0.1) loaded with RuO2 as a co-catalyst were determined by hydrogen or oxygen production from water in the presence of an electron donor (methanol) or an electron acceptor (AgNO3) as sacrificial reagents, respectively, under visible light irradiation (λ g 420 nm). As mentioned, ZnNb2O6 (promoted with NiOx) showed modest activity for water splitting only under UV irradiation.11 Figure 12 shows photocatalytic H2 or O2 production rates of RuO2 (5 wt %)-loaded Zn(Nb1-xVx)2O6 solid solutions. All V-incorporated solid solution (with RuO2) samples produced H2 or O2 in the presence of methanol or AgNO3 under visible light irradiation, while RuO2-ZnNb2O6 with a wide band gap (ca. 3.98 eV) was not active at all. In both H2 and O2 production, RuO2-Zn(Nb1-xVx)2O6 (x ) 0.06) showed the maximum activities. Note that at this composition, the hybridization between Nb 4d and V 3d is maximized in the formation of the bottom part of the conduction band of the solid solution. This represents a nice correlation between electronic structure and photocatalytic activity of solid solution photocatalysts. Zn(Nb0.94V0.06)2O6 loaded with RuO2 as a co-catalyst showed the maximum photocatalytic activity, i.e., 14.1 µmol/h of H2 and 89.3 µmol/h of O2. Considering the stoichiometry of water decomposition, the activity of H2 production represents ca. 1/13 of O2 production activity. Thus, the material is an excellent O2
Figure 13. Photocatalytic hydrogen evolution rate over Zn(Nb1-xVx)2O6 solid solution (x ) 0.06) with different loadings of RuO2 in the presence of methanol (20 vol %) under visible light irradiation (λ g 420 nm).
producing photocatalyst but a mediocre H2 producing photocatalyst. For all solid solutions, H2 or O2 evolved continuously for 300 min without an apparent deactivation. Overall water splitting, i.e., simultaneous H2 and O2 production from pure water was tried for these solid solutions without success. There should be some kinetic barrier for the reaction. In any case, a new visible light-active photocatalyst has been discovered for H2 or O2 production from water by the band gap tuning of RuO2-Zn(Nb1-xVx)2O6. The dependence of photocatalytic hydrogen evolution on the loaded amount of RuO2 on Zn(Nb1-xVx)2O6 (x ) 0.06) is shown in Figure 13. Little hydrogen was produced without RuO2 loading. Hydrogen evolution increased with RuO2 loading up to 16.1 µmol/h (QE ) 0.12% at 420 nm) at 3 wt % RuO2 and then decreased. Also, the maximum oxygen evolution rate was
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Figure 14. SEM images of Zn(Nb1-xVx)2O6 solid solution (x ) 0.06) with different loading of RuO2; (a) 1, (b) 3, and (c) 6 wt %. Scale bar ) 100 nm.
102.3 µmol/h (QE ) 1.5% at 420 nm) at 3 wt % RuO2. Figure 14 shows the SEM images of Zn(Nb1-xVx)2O6 (x ) 0.06) with various loadings of RuO2 nanoparticles. In all samples, it was observed that RuO2 nanoparticles of less than 40 nm were loaded on the surface of sample. At low RuO2 loadings (1 and 3 wt %), the surface of Zn(Nb1-xVx)2O6 is sparsely covered with RuO2 particles, the photocatalytic activity increases with RuO2 loading. At higher loadings, excessive coverage by RuO2 reduces the activity probably by blocking the light absorption by Zn(Nb1-xVx)2O6. Thus, it appears that with 3 wt % RuO2 loading, an optimum surface coverage is achieved that gives the maximum photocatalytic activity.30 Conclusion The wide-band-gap semiconductor ZnNb2O6 (3.98 eV) became visible-light-active with band gap energy shifted down to ca. 2.5 eV by forming solid solution Zn(Nb1-xVx)2O6 (x ) 0-0.1). In this band gap tuning, d orbitals of V were responsible for the band gap reduction by forming the bottom part of the conduction band of the solid solution. All V-incorporated solid solutions loaded with RuO2 as a co-catalyst showed photocatalytic activities for H2 or O2 production from aqueous solutions of sacrificial reagents under visible light irradiation (λ g 420 nm) and RuO2(3 wt %)-doped Zn(Nb0.94V0.06)2O6 showed the highest activity. Acknowledgment. This work was supported by the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, National Research Laboratory, General Motors R&D Center, and the Brain Korea 21 program.
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