17420
J. Phys. Chem. C 2007, 111, 17420-17426
Photocatalytic O2 Evolution of Rhodium and Antimony-Codoped Rutile-Type TiO2 under Visible Light Irradiation Ryo Niishiro,† Ryoko Konta,† Hideki Kato,† Wang-Jae Chun,‡ Kiyotaka Asakura,‡ and Akihiko Kudo*,†,§ Department of Applied Chemistry, Faculty of Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan, Catalysis Research Center, Hokkaido UniVersity, 11 Kita 10 Nishi, Kita-ku, Sapporo 060-0811, Japan, and Core Research for EVolutional Science and Technology, Japan Science and Technology Agency (CREST, JST), 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan ReceiVed: June 18, 2007; In Final Form: August 22, 2007
The effects upon the photophysical and photocatalytic properties of the codoping of rhodium and antimony in rutile-type TiO2 were investigated. TiO2 codoped with rhodium and antimony (TiO2:Rh/Sb) had an absorption band in the visible light region. Its energy gap was 2.1 eV. The TiO2:Rh/Sb powder showed photocatalytic activity for the oxidation of water to form O2 in the presence of Ag+ ions as an electron acceptor under visible light irradiation (λ > 440 nm), whereas TiO2 doped with only rhodium showed no activity. The photocatalytic activity strongly depended on the amount and ratio of the Sb/Rh dopant. Electron spin resonance, diffuse reflection spectroscopy, and X-ray absorption near-edge structure analyses revealed that the doped rhodium exists in a trivalent oxidation state in the active photocatalyst. The photocatalytic activity was decreased remarkably even by a small amount of Rh4+ ions. Antimony played an important role in suppressing the formation of Rh4+ ions by charge compensation. Thus, it has been clarified that Rh3+ is the species responsible for the photocatalytic activity under visible light irradiation in the TiO2:Rh/Sb photocatalyst.
1. Introduction Photocatalytic water splitting has been studied intensively as one of the potential approaches for the usage of solar energy because it is such an attractive chemical reaction for the conversion of solar energy into chemical energy. Many metal oxide1-4 and some metal nitride5,6 photocatalysts have been found that can split water into H2 and O2 efficiently in a stoichiometric ratio under UV irradiation. However, it is inevitably necessary to develop visible-light-driven photocatalysts in order to use solar energy efficiently. There are a few photocatalyst systems that can split water into H2 and O2 in a stoichiometric ratio under visible light irradiation.7-11 Alternatively, it has been reported that metal oxide, sulfide, and (oxy)nitride materials show photocatalytic activities for H2 or O2 evolution from aqueous solutions containing sacrificial reagents under visible light irradiation.1,4,12-33 In particular, some sulfide photocatalysts show high activities for H2 evolution under visible light irradiation, although they cannot oxidize water to form O2 because of photocorrosion.14,17,18,29 In contrast, metal oxides are suitable materials for the photocatalytic reaction in water because of their stabilities toward photocorrosion. It is still an important topic to design new photocatalyst materials, especially with visible-light response. Constructing the database of photocatalyst materials will clarify the factors affecting the photocatalytic activities. It is also important to design photocatalysts that are active in either H2 or O2 evolution because they can be applied to a Z-scheme photocatalyst system for overall water splitting under visible light irradiation.7-9 * Corresponding author. Tel: +81-35228-8267. Fax: +81-35261-4631. E-mail:
[email protected]. † Tokyo University of Science. ‡ Hokkaido University. § Japan Science and Technology Agency (CREST, JST).
There are several strategies to develop visible-light-driven photocatalysts.4,11 Doping of transition metal ions in wide band gap photocatalysts is a well-known method for the design of visible-light-driven photocatalysts.15-19,31-33 The authors have reported previously that ZnS,17,18 TiO2,19,33 and SrTiO319,31-33 doped with transition metal ions are active for photocatalytic H2 or O2 evolution from aqueous solutions containing sacrificial reagents under visible light irradiation. These studies by the present authors indicated that control of the oxidation number of the doped transition metal ions to maintain the charge balance in the crystal structure is important for the photocatalytic properties. For example, when Cr3+ and Ni2+ ions are partly substituted for Ti4+ sites in the crystal lattice of TiO2 and SrTiO3 hosts, the charge compensation by codoping of high-valent ions such as Sb5+, Ta5+, and Nb5+ improves their photocatalytic activities under visible light irradiation.19,31,33 The authors have also reported that platinum-loaded SrTiO3 doped with rhodium is a highly active visible-light-driven photocatalyst for H2 production in the presence of methanol as an electron donor.32 The visible-light response of rhodium-doped SrTiO3 is due to the electron transition from the electron donor levels formed by doped Rh3+ ions to the conduction band of SrTiO3. These transition-metal-doped SrTiO3 photocatalysts play important roles in the construction of Z-schemes for water splitting under visible light irradiation.7,8 Alternatively, it has been reported that a rhodium-doped TiO2 electrode showed photocurrent under visible light irradiation.34 Therefore, it is expected that the doping of rhodium will be effective in enhancing the visiblelight response of other wide band gap oxide photocatalysts such as TiO2. In the present study, the photocatalytic activities of rutiletype TiO2 doped with rhodium and codopants under visible light irradiation were investigated in order to develop new visible-
10.1021/jp074707k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007
Photocatalytic O2 Evolution
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17421
light response photocatalysts and extend the codoping effect. The role of codoping is discussed based on characterization by X-ray diffraction, diffuse reflection spectra, electron spin resonance, X-ray photoelectron spectra, and X-ray absorption near-edge structure analyses. 2. Experimental Section TiO2 doped with rhodium and either antimony, tantalum, or niobium, which are denoted as TiO2:Rh and TiO2:Rh/M (M ) Sb, Ta, and Nb), were prepared from TiO2 (Soekawa Chemical; 99.9%), Rh2O3 (Wako Pure Chemical; 99.5%), Sb2O3 (Nacalai Tesque; 98%), Ta2O5 (Rare Metallic; 99.99%), and Nb2O5 (Kanto Chemical; 99.95%) by solid-state reactions. The starting materials were mixed well using a mortar in the ratio corresponding to the composition of Ti1-X-YRhXMYO2. The mixture was calcined at 1423 K for 10 h in air using an alumina crucible. Part of the antimony might volatilize during the calcination. The non-doped TiO2 powder as a reference was also treated at 1423 K for 10 h in air. Crystal phases of the powders obtained were confirmed by X-ray diffraction (Rigaku; Miniflex, Cu KR). A hydrogen treatment at 573 K for 2 h was carried out for some samples to reduce Rh4+ ions to the Rh3+ state because part of the rhodium ions doped in the TiO2 host can be oxidized to Rh4+ during the high-temperature calcination. Diffuse reflection spectra (DRS) were obtained using a UVvisible-NIR spectrometer (Jasco; UbestV-570) and were converted from reflectance to absorbance by the Kubelka-Munk method. X-band electron spin resonance measurements (JEOL; JES-FA200) were carried out at 143 K in vacuo. X-ray photoelectron spectra were obtained using an X-ray photoelectron spectrometer (KRATOS; ESCA-3400). A Ti 2p peak (458.7 eV) of rutile-type TiO2 was used for the correction of the binding energies. Photocatalyst powders were observed using a scanning electron microscope (JEOL; JSM-6700F). X-ray absorption near-edge structure analyses of rhodium-doped TiO2 at the Rh K-edge were carried out at the BL10B facility of the Photon Factory at the Institute of Materials Structure Science, HighEnergy Accelerator Research Organization (KEK-IMSS-PF) in Tsukuba. The electron energy and currents for the storage ring were 2.5 GeV and 300-450 mA, respectively. X-rays emitted from the storage ring were monochromatizesd using a Si(311) channel-cut single-crystal monochromator. A portion of the rhodium-doped TiO2 photocatalyst (1.4 g) was mixed with 1.4 g of boron nitride (Soekawa Chemical; 99.5%), and then the mixture was pressed into a 20-mm-diameter disk. X-ray absorption spectra of all samples, including a Rh foil and Rh2O3 powder as reference samples, were obtained in the transmittance mode. The incident and transmitted X-rays beams were monitored by ionization chambers filled with Ar and Kr, respectively. The energies were corrected by using the Rh K-edge (23219.8 eV) of the Rh-foil. The photocatalytic reaction for O2 evolution from an aqueous silver nitrate solution (0.05 mol L-1) was carried out in a gasclosed circulation system. Argon gas (20 Torr) was introduced into the system after deaeration. The photocatalyst powder (0.3 g) was dispersed in the reactant solution (150 mL) by a magnetic stirrer in a cell with a top window made of Pyrex. A 300-W Xe lamp (Perkin-Elmer; CERMAX-PE300F) with attached cutoff filters (HOYA; Y44 and L42) was employed for visible light irradiation. The amount of evolved O2 was determined by online gas chromatography (Shimazu; GC-8A, MS-5A column, TCD, Ar carrier).
Figure 1. Diffuse reflection spectra of (a) non-doped TiO2, (b) TiO2: Rh(1%), (c) H2-reduced TiO2:Rh(1%), (d) TiO2:Rh(1%)/Sb(1%), (e) TiO2:Rh(1%)/Nb(1%), (f) TiO2:Rh(1%)/Ta(1%), (g) TiO2:Rh(1%)/ Nb(2%), and (h) TiO2:Rh(1%)/Ta(2%).
3. Results and Discussion 3.1. Characterization of Rhodium Species in TiO2:Rh and TiO2:Rh/M (M ) Sb, Ta, and Nb). XRD analyses indicated that all samples had a single phase of rutile-type TiO2. Rhodium and codopants were incorporated into the TiO2 host. The XRD diffraction peaks of TiO2:Rh and TiO2:Rh/M shifted to lower angles compared to those of non-doped TiO2. This is reasonable because Rh3+ ions (0.805 Å) are larger than Ti4+ ions (0.745 Å).35 These results revealed the fact that rhodium and the codopant were substituted for Ti4+ sites. Figure 1 shows diffuse reflection spectra of TiO2:Rh(1%) and TiO2:Rh(1%)/M(1 and 2%) (M ) Sb, Ta, and Nb). All of the doped samples showed new absorption bands in the visible light region. TiO2:Rh, which was a dark-brown powder, showed significant absorption bands around 650 and 1200 nm in addition to a visible light absorption band with an onset around 600 nm. However, TiO2:Rh treated with H2 at 573 K for 2 h showed a simple absorption profile with an onset around 620 nm although there was a tailing. The absorption bands observed around 650 and 1200 nm in TiO2:Rh disappeared as a result of H2 reduction treatment. This suggests that the disappeared absorption bands were due to Rh4+ ions formed in the high-temperature calcination process. The absorption profile of H2-reduced TiO2:Rh was similar to the action spectrum of the TiO2:Rh electrode reported.34 Alternatively, TiO2:Rh(1%)/Sb(1%) showed a simple absorption profile with onset at 600 nm in spite of the fact that no H2 reduction treatment was used. The color was clear orange. When 1% of tantalum and niobium was codoped, the peak intensities of 650 and 1200 nm that were observed for TiO2: Rh(1%) decreased drastically although these bands were still observed, in contrast to 1% of antimony codoping. The absorption band around 650 nm seemed to become a tailing absorption band. These absorption bands disappeared with 2% of tantalum and niobium codoping. Figure 2 shows ESR spectra of TiO2:Rh(1%) and TiO2: Rh(1%)/M(1 and 2%) (M ) Sb, Ta, and Nb). TiO2:Rh showed ESR signals assigned to Rh4+. One percent of tantalum and niobium codoping also gave similar signals. However, it should be noted that the intensity of the signals decreased significantly. The signals disappeared when 2% of tantalum and niobium was codoped. The signal was hardly observed even if 1% of antimony was codoped. XANES spectra showed that the absorption of TiO2:Rh shifted to higher energy in comparison with Rh2O3, whereas TiO2:Rh(2%)/Sb(4%) showed almost the same absorption as Rh2O3 (Figure 3), suggesting that Rh4+ became Rh3+ with the codoping of antimony. These DRS, ESR, and XANES measurements revealed that codoping of antimony, tantalum, and niobium suppressed the
17422 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Niishiro et al. TABLE 1: Photocatalytic O2 Evolution from an Aqueous Silver Nitrate Solution over Rutile-Type TiO2:Rh/Ma dopant
energy gap/ incident light/ rate of O2 evolutionb/ µmol h-1 eV nm
none Rh(1.0%)
3.0
Rh(1.0%)c
2.0
Rh(1.0%)/Sb(1.0%) Rh(1.0%)/Ta(1.0%) Rh(1.0%)/Nb(1.0%) Rh(1.0%)/Sb(2.0%)
2.2
Rh(1.0%)/Ta(2.0%) Rh(1.0%)/Nb(2.0%)
2.2 2.2
2.2
λ > 300 λ > 300 λ > 440 λ > 300 λ > 440 λ > 440 λ > 440 λ > 440 λ > 300 λ > 440 λ > 440 λ > 440
101 0 0 3.8 1.1 2.7 0 0 19.5 11.1 4.7 0.9
a Catalyst, 0.3 g; reactant solution, 150 mL of 0.05 mol L-1 aqueous silver nitrate solution; cell, top-irradiation type; light source, 300-W Xe lamp. b Initial rate. c H2-reduced.
Figure 2. ESR spectra measured at 143 K in vacuo of (a) TiO2: Rh(1%), (b) TiO2:Rh(1%)/Nb(1%), (c) TiO2:Rh(1%)/Ta(1%), (d) TiO2: Rh(1%)/Sb(1%), (e) TiO2:Rh(1%)/Nb(2%), (f) TiO2:Rh(1%)/Ta(2%), and (g) TiO2:Rh(1%)/Sb(2%).
Figure 3. X-ray absorption spectra of (a) TiO2:Rh(2%); (b) TiO2: Rh(2%)/Sb(4%); (c) Rh2O3; and (d) Rh foil at the rhodium K-edge.
formation of some impurity levels due to Rh4+ species in the band structure. One percent of antimony codoping was enough to eliminate the impurity levels, whereas 1% of niobium and tantalum codoping was not sufficient. It is concluded that antimony is the most effective codopant to suppress the Rh4+ formation. The oxidation state of the doped rhodium ion was controlled by a codopant as mentioned above. This effect of codopant is reasonable if the charge balance is considered as follows. When Rh3+ ions (starting material, RhIII2O3) are partly substituted for Ti4+ ions in the crystal lattice of TiO2, Rh4+ ions and/or oxygen defects should form in order to maintain the charge balance. The composition is expressed as Ti1-2X-YRhIII2XRhIVYO2-X. High-valent Rh4+ ions formed by charge compensation in TiO2: Rh are reduced to Rh3+ by H2 treatment, resulting in the disappearance of the absorption bands around 650 and 1200 nm. However, the tailing to 900 nm in the absorption was observed for the H2-reduced TiO2:Rh(1%), suggesting the existence of some defects. Alternatively, only Rh3+ ions are doped in TiO2:Rh/M because high-valent Sb5+, Ta5+, and Nb5+ compensate for the charge according to the composition Ti1-X-YRhIIIXMVYO2. It is considered that the absorption bands
responsible for the 2.2-eV energy gap observed for TiO2:Rh/M are due to electronic transitions from the electron donor levels formed by doped Rh3+ ions to the conduction band, as observed for SrTiO3:Rh.32 3.2. Photocatalytic Activities of TiO2:Rh and TiO2:Rh/M (M ) Sb, Ta, and Nb). Table 1 shows photocatalytic O2 evolution from an aqueous silver nitrate solution over TiO2: Rh(1%) and TiO2:Rh(1%)/M(1 and 2%) (M ) Sb, Ta, and Nb). TiO2:Rh showed no photocatalytic activity under full-arc (λ > 300 nm) and visible light (λ > 440 nm) irradiation, whereas H2-reduced TiO2:Rh showed low activity. The difference in the photocatalytic activity between non-treated and H2-reduced TiO2:Rh was due to Rh4+ ions, which work as effective recombination centers between photogenerated electrons and holes. The significant amount of Rh4+ species was detected in the non-treated TiO2:Rh by DRS, ESR, and XANES analyses as shown in Figures 1-3. This result indicated that the photocatalytic activity of H2-reduced TiO2:Rh under visible light irradiation was due to the electronic transition concerned with Rh3+ ions. When 1% of antimony was codoped with 1% of rhodium, the photocatalytic activity was observed. In contrast, TiO2: Rh(1%)/Ta(1%) and TiO2:Rh(1%)/Nb(1%) did not show the activity. Small amounts of Rh4+ species that work as recombination centers between photogenerated electrons and holes were detected by DRS and ESR for TiO2:Rh(1%)/Ta(1%) and TiO2:Rh(1%)/Nb(1%) as shown in Figures 1 and 2, but not for TiO2:Rh(1%)/Sb(1%). When 2% of tantalum and niobium was codoped with 1% of rhodium, the activities were obtained as well as TiO2:Rh(1%)/Sb(2%). However, the activity of TiO2: Rh(1%)/Sb(2%) was much higher than those of TiO2:Rh(1%)/ Ta(2%) and TiO2:Rh(1%)/Nb(2%). Empty Ta 5d and Nb 4d orbitals may form some levels near the bottom of the conduction band consisting of Ti 3d orbitals. It disturbs the conduction band of TiO2, resulting in low activities of TiO2:Rh(1%)/Ta(2%) and TiO2:Rh(1%)/Nb(2%). The photocatalytic activity of TiO2:Rh/Sb was also much higher than that of H2-reduced TiO2:Rh. This difference is explained by the existence of oxygen defects. Rhodium ions were doped as only Rh3+ in both the H2-reduced TiO2:Rh and TiO2:Rh/M. However, when Rh4+ ions in TiO2:Rh were reduced to Rh3+ ions by H2 reduction treatment, oxygen defects that work as recombination centers should form. The formation of the oxygen defects was suggested by the tailing in DRS as shown in Figure 1. In contrast, in the cases of TiO2:Rh/M, Rh3+ ions were doped by maintaining the charge balance via the
Photocatalytic O2 Evolution
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17423 TABLE 2: Dependence of Energy Gap and Photocatalytic O2 Evolution from a Silver Nitrate Solution over TiO2:Rh(X%)/Sb(2X%) on the Amount of Doped Rhodium Ionsa
Figure 4. Photocatalytic O2 evolution from an aqueous silver nitrate solution over TiO2:Rh(1.0%)/Sb(2.0%) under visible light irradiation. Catalyst, 0.3 g; reactant solution, 150 mL of 0.05 mol L-1 aqueous silver nitrate solution; light source, 300-W Xe lamp; incident light, λ > 440 nm; and cell, top-irradiation type.
Figure 5. Dependence of photocatalytic activity for O2 evolution from an aqueous silver nitrate solution over TiO2:Rh(1.3%)/Sb(2.6%) upon cutoff wavelength in the incident light. Catalyst, 0.3 g; reactant solution, 150 mL of 0.05 mol L-1 aqueous silver nitrate solution; light source, 300-W Xe lamp; and cell, top-irradiation type.
codoping of high-valent ions, resulting in the suppression of the oxygen defect formation. A typical time course of photocatalytic O2 evolution from an aqueous silver nitrate solution over TiO2:Rh(1%)/Sb(2%) under visible light irradiation is shown in Figure 4. The photocatalytic activity of TiO2:Rh(1%)/Sb(2%) was decreased gradually as usual because a metallic Ag shielding incident photon and partly blocking the surface active sites for photocatalytic reaction was photodeposited on the TiO2:Rh/Sb surface. An induction period was observed for H2 evolution using a SrTiO3:Rh photocatalyst,32 whereas that for O2 evolution on the TiO2:Rh/Sb photocatalyst was negligible. The induction period was due to the reduction of high-valent Rh species such as Rh4+ to Rh3+. The amount of the high-valent Rh species in the chargecompensated TiO2:Rh/Sb was smaller than that in SrTiO3:Rh. It resulted in the induction period being negligible for TiO2: Rh/Sb. Figure 5 shows the dependence of photocatalytic O2 evolution from an aqueous silver nitrate solution over TiO2:Rh(1.3%)/ Sb(2.6%) upon the minimum wavelength of the incident light. The wavelength at which the photocatalytic activity appeared agreed well with the onset of DRS. This photocatalyst responded to 600 nm, which is quite a long wavelength for O2 evolution photocatalysts. It was concluded that this O2 evolution reaction proceeded photocatalytically with electronic transition in the new visible light absorption band formed by doped Rh3+ ions. As revealed in this section, antimony was the most effective codopant for the TiO2:Rh/M photocatalyst. Therefore, the effects of the doping amount and ratio in TiO2:Rh/Sb on the photophysical and photocatalytic properties were further investigated
X/ %
energy gap/ eV
rate of O2 evolutionb/ µmol h-1
0.1 0.5 1.0 1.3 2.0 3.0 5.0
2.26 2.16 2.15 2.13 2.06 1.99 1.91
9.6 9.9 11.1 16.9 10.2 6.0 1.3
a Catalyst, 0.3 g; reactant solution, 150 mL of 0.05 mol L-1 aqueous silver nitrate solution; cell, top-irradiation type; light source, 300-W Xe lamp; incident light, λ > 440 nm. The ratio of codoping of antimony to rhodium was fixed at two. b Initial rate.
Figure 6. Diffuse reflection spectra of TiO2:Rh(X%)/Sb(2X%): X ) (a) 0.1; (b) 0.5; (c) 1.0; (d) 1.3; (e) 2.0; (f) 3.0; and (g) 5.0.
in order to clarify the role of antimony codoping and optimize the TiO2:Rh/Sb photocatalyst. 3.3. Effect on Photocatalytic Activity, and Band Structure, of the Doping Amount in TiO2:Rh/Sb. The photocatalytic activity for O2 evolution from an aqueous silver nitrate solution depended on the amount of rhodium dopant, as shown in Table 2. Here, the ratio Sb/Rh was maintained at two. The increase in the doping amount gives two effects on the photocatalytic performance. One is a positive effect, that is, an increase in the number of absorbed photons. Another is a negative effect, that is, an increase in the number of recombination centers. Therefore, the optimum amount of dopant existed. TiO2: Rh(1.3%)/Sb(2.6%) showed the highest activity. When the amounts of rhodium were larger than 2%, the negative effect became predominant, resulting in the decrease in photocatalytic activity. Diffuse reflection spectra of TiO2:Rh/Sb with different amounts of dopants, in which the ratio Sb/Rh was maintained at two, are shown in Figure 6. No impurity phases were observed by XRD for all samples. When a small amount of Rh3+ ions was doped, a visible light absorption band was observed as a shoulder in addition to the intrinsic band gap absorption of the TiO2 host. This is a typical profile for doped materials. As the amount of doped Rh3+ ions increased, the absorption band in the visible light region became more intense and, at the same time, the absorption edges red-shifted. In particular, the absorption profiles for TiO2:Rh/Sb samples with higher doping levels resembled a band gap transition, even though the shapes of the absorption edges were not so steep. This result indicated that the doped Rh3+ ions interacted with each other at the high doping level, resulting in the formation of a sub-band with 4d occupied orbitals of Rh3+. As the amount of doped Rh3+ ions increased further, the interaction between Rh3+ ions became strong. The stronger the interaction is, the thicker the sub-band
17424 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Niishiro et al.
Figure 7. Scheme of photocatalytic O2 evolution on TiO2:Rh(1.3%)/ Sb(2.6%). Figure 9. Diffuse reflection spectra of TiO2:Rh(1.3%)/Sb(Y%) with Sb/Rh ratios (a) 0; (b) 0.5; (c) 1; (d) 2; and (e) 3.
Figure 8. SEM images of TiO2:Rh(1.3%)/Sb(Y%) with Sb/Rh ratios (a) 0; (b) 0.5; (c) 1; and (d) 2.
is, resulting in the red-shift of visible light absorption band and energy gap narrowing as shown in Table 2. The scheme of photocatalytic O2 evolution over TiO2: Rh(1.3%)/Sb(2.6%), which showed the highest photocatalytic activity under visible light irradiation, is shown in Figure 7. The photocatalytic O2 evolution proceeds by electronic transition from an electron donor sub-band consisting of Rh 4d orbitals to a conduction band consisting of Ti 3d orbitals. The scheme is similar to those for TiO2:Cr/Sb19 and TiO2:Ni/(Ta, Nb).33 TiO2:Rh/M was active only for photocatalytic O2 evolution from an aqueous silver nitrate solution under visible light irradiation, whereas SrTiO3:Rh was only active for photocatalytic H2 evolution from an aqueous methanol solution under visible light irradiation.32 Transition-metal-doped rutile-type TiO2 usually shows activity for O2 evolution.19,33 In the case of rhodium doping, the TiO2:Rh/M photocatalyst possessed O2 evolution sites for four-electron oxidation of water, in contrast to a SrTiO3: Rh photocatalyst. This is one of the major differences between TiO2:Rh/M and SrTiO3:Rh photocatalysts. It is interesting that holes photogenerated in the sub-band formed by Rh3+ ions can oxidize water to form O2. 3.4. Effect on Morphology, Photoabsorption Property, Oxidation States of Rhodium and Antimony, and Photocatalytic Property of the Doping Ratio in TiO2:Rh/Sb. Figure 8 shows SEM images of TiO2:Rh(1.3%)/Sb(Y%) with different Sb/Rh ratios. Here, the doping amount of rhodium was fixed at 1.3%. The Sb/Rh ratio strongly affected the particle size and morphology. The particle sizes of TiO2:Rh/Sb with Sb/Rh ratios of 0 and 0.5 were larger than 5 µm due to sintering during the high-temperature calcinations, while such sintering was remarkably suppressed in other samples, that is, those with ratios Sb/ Rh g 1. The morphology of TiO2:Rh/Sb samples with Sb/Rh ratios of 0 and 0.5 was rough. As the amount of codoped
Figure 10. X-ray photoelectron spectra for Sb 4d of (a) Sb2O3, (b) Sb2O5, and TiO2:Rh(1.3%)/Sb(Y%) with Sb/Rh ratios (c) 0.5; (d) 1; (e) 2; (f) 2.5; and (g) 3.
antimony increased, the particles became smooth. These TiO2: Rh(1.3%)/Sb(Y%) samples were prepared at 1423 K by a solidstate reaction. Particles usually sinter under such conditions. Antimony doping suppressed the sintering probably by the difference in size between Ti4+ (0.745 Å) and Sb3+ (0.90 Å).35 It resulted in the formation of fine and crystalline particles. The sintering was also suppressed when only antimony was doped in TiO2. Figure 9 shows the diffuse reflection spectra of TiO2: Rh(1.3%)/Sb(Y%). As the amount of codoped antimony increased, the absorption band around 650 and 1200 nm decreased as observed in Figure 1. This absorption band eventually disappeared when the Sb/Rh ratio was equal to or larger than unity, giving a simple absorption band with an onset at 600 nm. The color of the photocatalysts changed from dark brown for Sb/Rh ) 0 and 0.5 to orange for Sb/Rh g 1, corresponding to the changes in the DRS profiles. Figure 10 shows the XPS spectra for Sb 4d in TiO2:Rh/Sb with different Sb/Rh ratios. The peaks for Sb5+ and Sb3+ were detected at 36.6 and 34.4 eV, respectively, in all TiO2:Rh/Sb samples. The intensity of Sb3+ was low when the Sb/Rh ratios were 0.5 and 1. The detection of Sb3+ in the case of Sb/Rh ) 1 means that the ratio of Sb5+/Rh3+ was not unity. In addition, a part of the antimony in the starting mixture would be lost due to volatilization during the high-temperature calcination
Photocatalytic O2 Evolution
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17425 ments revealed that rhodium was completely doped as Rh3+ in the highly active TiO2:Rh/Sb photocatalyst, while a quite small amount of Rh4+ remained in the lowly active TiO2:Rh(1.3%)/ Sb(1.3%) photocatalyst. Alternatively, the excess antimony was spontaneously converted to Sb5+ and Sb3+ to maintain the charge balance, as shown by XPS, resulting in no remarkable decrease in photocatalytic activity for Sb/Rh ratios g2, as observed for Cr3+ and Ni2+-doped TiO2 photocatalysts.19 However, the photocatalytic activity decreased slightly at the Sb/Rh ratios 2.5 and 3. The excess doping of antimony would cause lattice distortion, which disturbed the migration of photogenerated carriers and work as recombination centers. 4. Conclusions
Figure 11. ESR spectra measured at 143 K in vacuo for Rh4+ of TiO2: Rh(1.3%)/Sb(Y%) with Sb/Rh ratios (a) 0; (b) 0.5; (c) 1; and (d) 2.
Figure 12. Dependence of photocatalytic activity of TiO2:Rh(1.3%)/ Sb(Y%) on the ratio of doped antimony to rhodium. Catalyst, 0.3 g; reactant solution, 150 mL of 0.05 mol L-1 aqueous silver nitrate solution; light source, 300-W Xe lamp; incident light, λ > 440 nm; and cell, top-irradiation type.
process.19
Sb3+
The presence of and volatilization of antimony suggests that the charge compensation was not completed and/ or some defects formed by codoping for the Sb/Rh )1 sample. Figure 11 shows the ESR spectra for Rh4+ in TiO2:Rh(1.3%)/ Sb(Y%). The intense signals due to the Rh4+ species were observed for samples of Sb/Rh ) 0 and 0.5. This observation is consistent with the results of the diffuse reflection spectra in which some absorption bands were observed in the visible light region. The ESR intensity of the sample with Sb/Rh ) 1 was considerably weak compared with those with Sb/Rh ) 0 and 0.5. No signals were observed for the Sb/Rh ) 2 sample. Figure 12 shows the dependence of the photocatalytic activity of TiO2:Rh(1.3%)/Sb(Y%) upon the Sb/Rh ratio. The codoping of antimony significantly affected the photocatalytic activity, as observed for TiO2:Cr/Sb.19 TiO2:Rh/Sb samples with Sb/Rh ratios of 0 and 0.5 showed no photocatalytic activity. In contrast, all photocatalysts with the Sb/Rh ratios g1 showed activity for O2 evolution under visible light irradiation. An optimum ratio of Sb/Rh was two. The particle size and morphology changed remarkably, and the color of the photocatalyst turned from dark brown to orange for Sb/Rh ratios g1. However, the photocatalytic activities for TiO2:Rh/Sb samples with Sb/Rh ratios g2 were remarkably higher than that for Sb/Rh ) 1, even though the unity is the stoichiometric ratio from the viewpoint of charge compensation of Rh3+ with Sb5+ at Ti4+ sites. ESR measure-
Rutile-type TiO2 codoped with rhodium and antimony, tantalum, or niobium was developed as a new photocatalyst for O2 evolution from an aqueous silver nitrate solution under visible light irradiation. TiO2:Rh(1.3%)/Sb(2.6%) showed the highest activity and responded to 600 nm, corresponding to 2.1 eV of the energy gap. Thus, rhodium was an effective dopant in the visible-light response for O2 evolution on the TiO2 photocatalyst, in contrast to the SrTiO3:Rh photocatalyst, which was active for H2 evolution. The visible-light response was due to the transition from the electron donor sub-band formed by Rh3+ to the conduction band. The relationship between the existence of Rh4+ and photocatalytic activity was clarified by characterizations using ESR, DRS, and XANES. It was important to suppress the formation of Rh4+ ions and oxygen defects, which work as recombination centers, by the codoping of high-valent Sb5+, Ta5+, or Nb5+ ions. The codoping of Rh3+ with either Sb5+ or Ta5+ was effective for the improvement of the photocatalytic activities of transition-metal-ion-doped TiO2 under visible light irradiation, as well as that of Cr3+ and Ni2+, with Sb5+, Ta5+, and Nb5+. Among them, antimony is the most effective codopant. It was confirmed by the present study that codoping is a suitable strategy to develop visible-light-responsive photocatalysts by transition-metal doping. Acknowledgment. This work has been supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology (JST) Agency, a GrantAid (No. 14050090) for Priority Area Research (No. 417) from the Ministry of Education, Culture, Science and Technology, and the Nissan Science Foundation. X-ray absorption measurements were carried out under the approval of the Photon Factory Advisory Committee (Proposal No. 2004G308). References and Notes (1) Domen, K.; Kondo, J. N.; Hara, M.; Takata, T. Bull. Chem. Soc. Jpn. 2000, 73, 1307. (2) Sato, J.; Saito, N.; Nishiyama, H.; Inoue, Y. J. Phys. Chem. B 2003, 107, 7965. (3) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (4) Kudo, A.; Kato, H.; Tsuji, I. Chem. Lett. 2004, 33, 1534. (5) Sato, J.; Saito, N.; Yamada, Y.; Maeda, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K.; Inoue, Y. J. Am. Chem. Soc. 2005, 127, 4150. (6) Arai, N.; Saito, N.; Nishiyama, H.; Inoue, Y.; Domen, K.; Sato, K. Chem. Lett. 2006, 35, 796. (7) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 2416. (8) Kato, H.; Hori, M.; Konta, R.; Shimodaira, Y.; Kudo, A. Chem. Lett. 2004, 33, 1384. (9) Abe, R.; Takata, T.; Sugihara, H.; Domen, K. Chem. Commun. 2005, 3829. (10) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295.
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