J. Phys. Chem. C 2007, 111, 439-444
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Overall Splitting of Water by RuO2-Loaded PbWO4 Photocatalyst with d10s2-d0 Configuration Haruhiko Kadowaki,† Nobuo Saito,† Hiroshi Nishiyama,† Hisayoshi Kobayashi,‡ Yoshiki Shimodaira,§ and Yasunobu Inoue*,† Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan, Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan, and Department of Applied Chemistry, Faculty of Science, Science UniVersity of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: August 31, 2006; In Final Form: October 13, 2006
Tetragonal lead tungstate, PbWO4, consisting of a WO4 tetrahedron showed high and stable photocatalytic activity for the overall splitting of water, producing a stoichiometric quantity of H2 and O2 under UV irradiation when loading RuO2 on the metal oxide. The activity was found to strongly depend on the preparation temperature of PbWO4. In the temperature range 773-1273 K, a maximum appeared at around 1023 and 973 K when prepared in air and under a N2 atmosphere, respectively. X-ray diffraction analysis and SEM observation showed that high activity was achieved with a combination of crystallized PbWO4 with a high dispersion of RuO2 particles. The DFT calculation for PbWO4 with the d10s2-d0 electronic configuration showed that the top of the valence band (HOMO) was composed of hybridized O 2p + Pb 6s orbitals, while the bottom of the conduction band (LUMO) consisted of hybridized W 5d + O 2p + Pb 6p orbitals. Considerable dispersion was observed for both the conduction and valence bands, which differed from the small dispersion observed for the photocatalytically inactive CaWO4 with a similar crystal structure. Photoexcited electrons and holes with high mobility were considered to be responsible for the high photocatalytic performance of PbWO4. The useful role of a Pb2+ ion with the d10s2 electronic configuration in photocatalysis has been elucidated.
Introduction Recently, a series of p-block metal oxides consisting of Ga3+, Ge4+, Sn4+, and Sb5+ with the d10 electronic configuration has been determined to become photocatalytically active for the decomposition of water under UV irradiation during RuO2 loading.1-9 Typical metal oxides consist of indates (MIn2O4 (M ) Ca,Sr), NaInO2, LaInO3), zinc gallate (ZnGa2O4), zinc germanate (Zn2GeO4), strontium stannate (Sr2SnO4), and various antimonates (M2Sb2O7 (M ) Ca,Sr), CaSb2O6, NaSbO3). On the other hand, conventional transition metal oxides recognized as active photocatalysts for water splitting over the last three decades are metal oxides consisting of the following metal ions: Ti4+, Zr4+, Nb5+, and Ta5+, which have the d0 electronic configuration. The representative transition metal oxides are SrTiO3,10 A2Ti6O13 (A ) Na, K, Rb),11 BaTi4O9,12-14 A2La2Ti3O10 (A ) K, Rb, Cs),15-16 Na2Ti3O7,17 K2Ti4O9,18 ZrO2,19 A4Nb6O17 (A ) K, Rb),20 Sr2Nb2O7,21 ATaO3 (A ) Na, K),22-23 MTa2O6 (M ) Ca, Sr, Ba),24-25 Sr2Ta2O7,15 ACa2Ta3O10 (A ) H, Na, Ca),26 A2SrTa2O7‚nH2O (A ) H, K, Rb),27 and Ba5Ta4O15.28 For both d10 and d0 metal oxides, either NiO or RuO2 was used in most cases as a promoter.8 In an attempt to find a new photocatalytst, it is needed to examine transition metal oxides with the d0 configuration that differs from the metal ions described above. Recently, we reported that lead tungstate, PbWO4, bearing a scheelite structure, formed a stable photocatalyst for the overall splitting of H2O into H2 and O2 when In3+,
* Address correspondence to this author. † Nagaoka University of Technology. ‡ Kyoto Institute of Technology. § Science University of Tokyo.
combined with RuO2.29 PbWO4 was the first example of a tungsten oxide capable of photocatalytically splitting H2O into H2 and O2. Since most of the tungsten metal oxides were found to be photocatalytically inactive for the splitting of water, this finding requires further detailed investigation to understand its photocatalytic properties, particularly from the viewpoint of electronic structures. In the present work, optimal experimental conditions to achieve high photocatalytic performance were investigated by changing the preparation conditions of PbWO4. Density function theory (DFT) calculations were employed to determine its band structures. Furthermore, CaWO4 was taken as a model for the other tungstates, and their photocatalytic properties and band structures were compared. The d10s2-d0 electronic configuration of PbWO4 yielded high dispersions in not only the conduction band, but also the valence band, which was demonstrated to be responsible for the photocatalytic activity of PbWO4 for the splitting of water. Furthermore, the role of a d10s2 metal ion (Pb2+) in photocatalysis by metal oxides is demonstrated. Experimental Section Lead tungstate, PbWO4, was synthesized by a solid-state reaction under different conditions. An equimolar mixture of PbO (Nakarai tesque, GR) and WO3 (Nakarai tesque, for analytical use) was calcined in air in the temperature range of 873-1273 K for 16 h. Another attempt to prepare PbWO4 under different conditions involved subjecting it to heat treatment under a N2 atmosphere. For the latter, a mixture of the starting oxides was heated under a N2 flow in the temperature range of 773-1173 K for 16 h (denoted here as (N)PbWO4). CaWO4
10.1021/jp065655m CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006
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Figure 1. Overall water splitting by 1 wt % RuO2-loaded PbWO4 (b; H2, O; O2) and 1 wt % RuO2-loaded CaWO4 (9; H2).
was prepared by calcination in air at 1073 K for 16 h. To load RuO2 as the promoter, prepared PbWO4 was impregnated up to incipient wetness, using a ruthenium carbonyl complex, Ru3(CO)12, in THF. After drying at 353 K, it was oxidized in air at 673 K for 4 h to convert the loaded ruthenium carbonyl complex to RuO2 particles. The photocatalytic reaction was carried out in a closed gas circulation reaction system using a quartz reaction cell. Details of the procedure for photocatalytic decomposition of water have been reported elsewhere.6,7 Briefly, RuO2-loaded PbWO4 powder (250 mg) was placed in distilled and ion-exchanged water (ca. 30 mL) in a quartz reaction cell. The photocatalyst was dispersed in water by continuous bubbling with Ar gas (13.3 kPa) during the photocatalytic reaction under illumination with an outer 200 W Hg-Xe lamp (Hamamatsu L566-02). The amounts of H2 and O2 produced in the gas phase were analyzed by an on-line gas chromatograph. The X-ray diffraction patterns of PbWO4 were recorded on an X-ray diffractometer (Rigaku RAD III). UV diffuse reflectance spectra were obtained with a UV-vis spectrometer (JASCO V-570). Scanning electron microscopy (SEM) images were obtained with a Shimazu EPMA 1600. The surface area was measured by a BET system (Yuasa Chem BET3000). The band calculation of PbWO4 was carried out with use of the plane-wave DFT program package Castep.30 According to the ultrasoft core potentials scheme,31 the following configurations were used in the valence atomic configurations of PbWO4 and CaWO4: 5d106s26p2 for Pb atoms, 3s23p64s2 for Ca atoms, 5d46s2 for W atoms, and 2s22p4 for O atoms. The unit cell compositions are [PbWO4]4 and [CaWO4]4, and the numbers of occupied orbitals are 88 and 80 for PbWO4 and CaWO4, respectively. The kinetic energy cutoff was set to 330 eV. Results Figure 1 shows the water splitting reaction on 1.0 wt % RuO2dispersed PbWO4 under Hg-Xe lamp irradiation. Both H2 and O2 were produced starting at the onset of the reaction in proportion nearly equal to irradiation time. In repeated runs, the production of H2 and O2 was constant, and the ratio of H2 to O2 was 2.1 in the fourth run. The stable production of both H2 and O2 with nearly a stoichiometric ratio indicates that the production proceeds photocatalytically. To compare the photocatalytic properties, CaWO4 with a similar crystal structure was employed. RuO2-loaded CaWO4 produced a small amount of H2 without evolution of O2, indicative of negligible photocatalytic activity. Figure 2 shows the photocatalytic activity of 1.0 wt % RuO2dispersed PbWO4 as a function of the temperature at calcination used during PbWO4 preparation. The activity of RuO2-loaded PbWO4 was quite small for PbWO4 prepared by calcination at 873 K. However, with increasing calcination temperature, the activity increased markedly in the range of 923-973 K, passed
Figure 2. Photocatalytic activity of RuO2-dispersed PbWO4 and (N)PbWO4 for H2 (b) and O2 (O) production as a function of temperature used for the preparation.
Figure 3. XRD patterns of PbWO4 calcined at 873 (a), 923 (b), 1023 (c), 1123 (d), and 1223 K (e), and (N)PbWO4 heated at 773 (f), 873 (g), 973 (h), and 1073 K (i).
through a maximum at 1023 K, and gradually decreased. RuO2loaded (N)PbWO4 also exhibited photocatalytic activity, producing both H2 and O2. As the temperature used for preparation increased, the activity reached a maximum at 973 K, beyond which it decreased markedly. The photocatalytic activity of RuO2-loaded PbWO4 was comparable with that of previously reported RuO2-loaded BaTi4O9 whose quantum yield was evaluated to be approximately 10% under similar conditions.13 Figure 3 shows the X-ray diffraction patterns of PbWO4 and (N)PbWO4 prepared at various temperatures. In PbWO4 prepared in air at 873 K, in addition to major peaks due to PbWO4, weak peaks attributed to unreacted WO3 and PbO were observed. Upon calcination at 923 K, the extra peaks disappeared, and all the major diffraction peaks were consistent with those of tetragonal PbWO4 reported previously.32 In the calcination temperature range of 1023-1223 K, the diffraction patterns were exactly the same, indicative of a single phase formation
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Figure 5. UV diffuse reflectance spectra of PbWO4 calcined at 923 (a), 1023 (b), 1123 (c), and 1223 K (d), and (N)PbWO4 heated at 773 (e), 873 (f), and 973 K (g).
Figure 4. SEM images of PbWO4 calcined at 923 (a), 1023 (b), 1123 (c), and 1223 K (d), and (N)PbWO4 heated at 773 (e), 873 (f), 973 (g), and 1073 K (h).
of PbWO4. The peak width of the major diffraction peaks became narrower with increasing temperature. In (N)PbWO4 preparation, a small peak due to WO3 was observed. This peak was also observed for the preparation at 773 K. The WO3 peak disappeared upon heat treatment at 873 K. Heat treatment in the temperature range of 873-1073 K yielded diffraction patterns due entirely to the single-phase PbWO4. Upon increasing the temperature from 873 to 1073 K, the major diffraction peaks narrowed, and their intensity increased. The relative intensity of each diffraction peak was similar for PbWO4 and (N)PbWO4 over the entire range of preparation temperatures. Neither high temperatures nor the atmosphere during preparation caused preferential growth of a specific crystal plane. Figure 4 shows SEM micrographs of PbWO4 and (N)PbWO4 prepared at different temperatures. Calcination of PbWO4 at 923 K produced extremely fine particles. At 1023 K, considerable growth occurred while maintaining the morphology. At 1123 K, the particles were markedly enlarged as a result of agglomeration. Heat treatment of (N)PbWO4 at 773 K yielded irregularly shaped particles with moderate sizes. The particles grew considerably at 873 K and significantly at 973 K. A marked growth was observed at around 1073 K, and the particles exhibited clear crystal structures with edges and corners. At 1273 K, the surface of the grown particles became smooth. Figure 5 shows UV diffuse reflectance spectra of PbWO4 and (N)PbWO4 prepared at different temperatures. For PbWO4 calcined at 923 K, gradual absorption occurred at around 450
Figure 6. Band dispersion and density of states (DOS) for PbWO4. The valence and conduction band region of part a is enlarged and is given as part b.
nm, followed by an absorption between 440 and 330 nm. Absorption increased sharply at 330 nm and leveled off at 310 nm. For PbWO4 calcined at 1023 K, the general absorption trend was similar; however, the absorption at around 440-330 nm disappeared. The absorption reached a maximum at 310 nm. For PbWO4 calcined in the temperature range of 1123-1223 K, nearly the same spectrum with a maximum absorption at 310 nm was obtained. The spectrum of (N)PbWO4 treated at 773 K showed a wide absorption in the wavelength range of 480-330 nm and a sharp absorption at 330 nm. Heat treatment of (N)PbWO4 at 873 K changed it to a moderately sharp absorption at around 340 nm, and the absorption leveled off at 310 nm. Nearly the same spectrum was obtained for (N)PbWO4 heated at 973 K. Figure 6 shows the band structure and density of states (DOS) of PbWO4 in which the energy at the top of valence band is taken as zero. The DOS breaks down into the angular momentum of atomic orbitals (AOs) represented by different line
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Figure 7. AO partial DOS for Pb atom, W atom, and O atom of PbWO4 and the total.
shapes. The occupied bands listed in order of increasing energy are O 2s (#1-#16), Pb 5d (#17-#36), Pb 6s (#37-#40), and O 2p + Pb 6s (+W 5d) (#41-#88), where the highest is the valence band. The bottom of the conduction band is composed of the W 5d band hybridized with O 2p and Pb 6p orbitals. In an effort to obtain more exact information on the atom-specific character of each band, the DOS was further decomposed into the AO projected DOS (PDOS) in terms of atomic and angular momentum contributions. Figure 7 shows AO PDOS for Pb, W, and O atoms in the higher energy region (-8 to 0 eV) of the occupied bands and the lower energy region (0 to 8 eV) of unoccupied bands. The density spread over 0 eV is an artifact due to the band shape smearing technique, and has no physical meaning. The band gap is estimated to be 3.24 eV. The narrow band gap for PbWO4 was ascribed to the hybridization between the O 2p and Pb 6s orbitals. The Pb 6s band already appeared in the lower energy region, #37-#40, but the Pb 6s orbitals mix with the O2 p orbitals at the top of the valence band. This type of hybridization is called “split-off state”. The electron density contour maps of the HOMO and LUMO levels related to photoexcitation are shown in Figure 8. In the electron density contour of the HOMO, the contribution of not only O 2p, but also the Pb 6s orbitals was observed. Discussion The dependence of the photocatalytic activity on increasing temperature in the preparation of PbWO4 and (N)PbWO4 caused a significant enhancement in the activity, a maximum, and considerable decrease. The correlation was analogous, irrespective of the conditions under which it was prepared (air or N2 atmosphere), but the temperature for maximum activity was lower by about 100 K for (N)PbWO4 than for PbWO4. This was attributed to the dependence of the reactivity of solid phases on the atmosphere. Compared with air, a N2 atmosphere is not an oxidative atmosphere, which is thought to increase the reactivity of PbO with WO3. A correlation between activity and preparation temperature was also observed for the other metal oxides such as MIn2O4 (M ) Ca, Sr),2,6 NaInO2,5 ZnGa2O4,4 and Zn2GeO4.8 In the low-temperature region, the activity increases, the X-ray diffraction peaks of both PbWO4 and (N)PbWO4 narrow, and the particle size increases significantly with increasing preparation temperature. The crystallization of PbWO4 and (N)PbWO4 particles proceeded significantly enough to eliminate impurities and structural imperfections that frequently work as traps for charge recombination. Thus, it is the
Figure 8. Electron density contour map for the top (#88) of the valence band (HOMO) (a) and for the bottom (#89) of the conduction band (LUMO) (b) of PbWO4. Pb: black, W: blue, O: red.
crystals that are thought to be responsible for activity enhancement. In the high-temperature region at which the activity decreases, the SEM images showed an exaggerated growth of PbWO4 and (N)PbWO4 particles. This resulted in a marked decrease in the surface area of PbWO4 and (N)PbWO4. Previous investigations of the relationship between the amount of RuO2 loading and the photocatalytic activity showed that a remarkable decrease in the surface area of active metal oxides caused agglomeration of RuO2 particles deposited and reduced the density of active sites associated with the RuO2 particles, thus leading to a decrease in the photocatalytic activity. It is therefore evident that high photocatalytic activity was achieved for wellcrystallized PbWO4 and (N)PbWO4 combined with dispersed RuO2 particles, which is consistent with previous conclusions.2,4 There are two crystal structures for PbWO4: one is tetragonal and the other is monoclinic. As shown in Figure 3, all of the major peaks in the X-ray diffraction patterns for PbWO4 were assigned to those of the former structure. Tetragonal PbWO4 is a scheelite-type species with a lattice parameter of a ) 0.5456(2) nm, c ) 1.2020(2) nm, and R ) β ) γ ) 90°.32 An interesting structural feature is that photocatalytically active PbWO4 is formed by a network of WO4 tetrahedron. Although RuO2-loaded Zn2GeO4 consisting of a GeO4 tetrahedron was reported to be photocatalytically active in d10 metal oxides,8 almost all conventional d0 transition metal oxides reported thus far to be photocatalytically active for water splitting consisted of octahedral MO6 units (M ) transition metal ion). On the other hand, tungsten oxides, Na2W4O13, Bi2W2O9, and Bi2WO6, were reported to have the photocatalytic ability to produce H2 from CH3OH and O2 from an aqueous solution of AgNO3, but not of decomposing H2O.33-34 These oxides possess WO6 octahedra as the fundamental unit. PbWO4 is the first example of a d0 metal oxide that consists of a metal-oxygen tetrahedron and is photocatalytically active for splitting water. Thus, the
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Figure 10. AO partial DOS for Ca atom, W atom, and O atom of CaWO4 and the total. Figure 9. Band dispersion and density of states (DOS) for CaWO4. The valence and conduction band region of part a is enlarged and is given as part b.
availability of a WO4 tetrahedron unit as a photocatalytic site for a water splitting reaction could be proposed. Calcium tungstate, CaWO4, which belongs to the scheelite group, has a tetragonal structure with a unit cell of a ) b ) 5.230 nm, c ) 11.384 nm, and R ) β ) γ ) 90° and consists of a WO4 tetrahedron.35 Its crystal structure is analogous to that of PbWO4. However, RuO2-loaded CaWO4 produced a small amount of H2, and was determined to be photocatalytically inactive for water splitting. This suggests that the photocatalytic activity of PbWO4 is related to its unique electronic structure. As shown in the DFT calculation (Figure 6), the O 2s and Pb 5d bands had strong single peaks with little mixing with other AOs. The Pb 6s band appeared weakly as a single peak just below the O 2p valence band. As shown in Figure 7, although the O 2p AOs mainly form the valence band, there was a considerable amount of mixing from the W 5d AOs in the lower region of the valence band. The small hybridization of the Pb 6s and 6p AOs occurs over the entire energy region. At the top of the valence band, the Pb 6s AOs mix more strongly with the O 2p AOs. This strong mixing is shown in the contour map for the HOMO in Figure 8, which is a characteristic feature for several (not all) lead oxide materials. It should be noted that the dispersion of the valence band is relatively small (flat), and is large only at the top (Figure 6). This is evidently due to the contribution from Pb 6s AOs. For comparison, Figure 9 shows the band dispersion and the DOS for CaWO4, and Figure 10 shows its AO partial DOS for Ca, W, and O atoms. The valence band consisted of O 2p orbitals without a contribution from Ca atoms, and its dispersion was very small. Therefore, the considerably large dispersion in the valence band for PbWO4 had a unique band structure, which evidently induced the large mobility of generated holes. The recent findings that typical d10 metal oxides become strong photocatalysts for water decomposition are explained in terms of the advantages of sp orbitals with a large dispersion in the conduction bands. However, d0 transition metal oxide photocatalysts have a conduction band consisting of narrow d orbitals with small dispersions. In photocatalytically inactive CaWO4, the conduction band exhibits a small dispersion, characteristic of typical d-orbitals. Small band dispersion indicates low mobility of photoexcited electrons. In contrast, as shown in the contour map for the LUMO of PbWO4 (Figure
Figure 11. Electron density contour map for the top (#80) of the valence band (HOMO) (a) and for the bottom (#81) of the conduction band (LUMO) (b) of CaWO4. Ca: green, W: blue, O: red.
8), the bottom of the conduction band of PbWO4 is composed of the W 5d AOs with a considerable amount of mixing with the Pb 6p AOs, and shows moderate band dispersion. In conclusion, the electronic structures for the HOMO and LUMO regions of PbWO4, which are related to the photoexcitation, are formed by the O 2p + Pb 6s AOs and the W 5d + Pb 6p AOs, respectively. The photocatalytic performance of RuO2-loaded PbWO4 is due to large dispersions in both the valence and conduction bands, generating photoexcited holes and electrons with large mobility. On the basis of the present findings, the d10s2 (Pb2+)-d0 electronic configuration is expected to be useful for the design of efficient photocatalysts for the overall splitting of water. Acknowledgment. This work was supported by the Core Research for Evolutional Science and Technology (CREST) and Solution Oriented Research for Science and Technology (SORST) programs of the Japan Science and Technology Corporation (JST) and by a Grant-in-Aid for Scientific Research in Priority
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