Solar Water Splitting Using Powdered Photocatalysts Driven by Z

Seiji Kawasaki , Kazuto Akagi , Kan Nakatsuji , Susumu Yamamoto , Iwao Matsuda , Yoshihisa Harada , Jun Yoshinobu , Fumio Komori , Ryota Takahashi , M...
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J. Phys. Chem. C 2009, 113, 17536–17542

Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator Yasuyoshi Sasaki, Hiroaki Nemoto, Kenji Saito, and Akihiko Kudo* Department of Applied Chemistry, Faculty of Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: July 27, 2009; ReVised Manuscript ReceiVed: August 20, 2009

Ru/SrTiO3:Rh photocatalyst powder for H2 evolution and varied photocatalyst powders for O2 evolution such as BiVO4 and WO3 were suspended in acidified aqueous solutions, resulting in showing activities for water splitting into H2 and O2 in a stoichiometric ratio without an electron mediator under visible light irradiation. The photocatalytic activities were dependent on pH. The highest activity was obtained at pH 3.5. An optical microscope observation of the aqueous suspension containing Ru/SrTiO3:Rh and BiVO4 powders at pH 3.5 revealed that these powders aggregated with suitable contact. The condition of Rh doped in SrTiO3 also affected strongly the photocatalytic activity and quenching of the photoluminescence of BiVO4. The high photocatalytic activity was obtained and the luminescence was remarkably quenched, when SrTiO3:Rh containing Rh species with reversible redox properties was used and mixed. These results indicated that the photocatalytic water splitting and quenching of the photoluminescence occurred through interparticle electron transfer from the conduction band of BiVO4 to impurity level consisting of the reversible Rh species doped in SrTiO3. Thus, we succeeded in constructing unique and simple Z-scheme photocatalysis systems driven by interparticle electron transfer under visible light irradiation. In addition, the (Ru/SrTiO3:Rh)-(BiVO4) system split water under simulated sunlight (AM-1.5). 1. Introduction Solar H2 production from water is one of the most important themes because of issues of the depletion of fossil fuels and CO2 emission. Semiconductor electrodes1-5 and photocatalysts4-19 will be useful for solar water splitting. Solar water splitting using a powdered photocatalyst makes an excellent candidate for a practical large-scale use for an H2 production system because of its simplicity of fabrication, i.e., the process requires only a water pool containing a photocatalyst powder under sunlight irradiation. For achieving solar water splitting, it is important to develop photocatalysts that respond to visible light occupying a large part of sunlight. Visible-light-driven photocatalysts such as (Ga1-XZnX)(N1-XOX) solid solution driven by one-step photoexcitation and Z-scheme photocatalysis systems (two-step photoexcitation) mimicking photosynthesis in a green plant have so far been reported.8,10-19 However, the number of visiblelight-driven photocatalysts for overall water splitting is limited. Moreover, there are few reports that indicate solar energy conversion efficiencies of powdered photocatalysts.7,19 There are many reports about visible-light-driven photocatalysts that show the activities for H2 or O2 evolution from water containing sacrificial reagents (designated as H2- and O2photocatalyst, respectively),8,10,20-33 for example, WO3,21 BiVO4,22 TiO2:Cr,Sb,23 AgNbO3,24 SrTiO3:Cr,Ta,26 SrTiO3:Rh,27 CdS,20 Sm2Ti2S2O5,25 Ta3N5,8 and TaON.28 These photocatalysts are inactive for overall water splitting into H2 and O2 in the absence of sacrificial reagents. However, overall water splitting has been attained by constructing a Z-scheme photocatalysis system composed of H2- and O2-photocatalysts and a suitable electron mediator.13-19 The electron mediator plays a role in shuttling the photogenerated carriers between the H2- and O2* To whom correspondence should be addressed. E-mail: a-kudo@ rs.kagu.tus.ac.jp. Phone: +81-35228-8267. Fax: +81-35261-4631.

photocatalysts. Sayama and Abe et al. have constructed Zscheme systems composed of Pt/SrTiO3:Cr,Ta and Pt/TaON as a H2-photocatalyst, Pt/WO3 as an O2-photocatalyst, and an IO3-/Ielectron mediator to split water under visible light irradiation.13,15 Domen et al. have also reported Z-scheme systems using Pt/ MTaO2N (M ) Ca, Ba)28 as a H2-photocatalyst.18 SrTiO3:Rh is a unique photocatalyst in terms of the high activity for H2 evolution in spite of a doping-type oxidephotocatalyst.27 Moreover, the SrTiO3:Rh photocatalyst possesses a novel character that is the reversible oxidation state (+3, +4, etc.) of the Rh ion doped in SrTiO3. The Rh ions have +3 and +4 of the oxidation state in SrTiO3:Rh as prepared. The SrTiO3:Rh photocatalyst is activated by reduction of Rh(IV) to Rh(III) during H2 evolution in an aqueous methanol solution under visible light irradiation. The absorption spectrum of the SrTiO3:Rh photocatalyst changes by accompanied with the change in the oxidation state. This photocatalyst responds to 520 nm for the H2 evolution. We have also succeeded in overall water splitting using this SrTiO3:Rh photocatalyst for constructing visible-light-driven Z-scheme systems with WO3, BiVO4,22 and Bi2MoO631 as an O2-photocatalyst and an Fe3+/Fe2+ electron mediator.14,16,19 The Fe3+/Fe2+ electron mediator is indispensable for overall water splitting using the system employing Pt/SrTiO3: Rh because the iron ion not only shuttles the carriers but also suppresses a back reaction to form water from evolved H2 and O2 on the Pt cocatalyst by covering the Pt surface with iron species.14,16 In contrast to the Pt cocatalyst, a Ru cocatalyst is an effective cocatalyst that does not enhance the back reaction.19 Z-scheme combined systems that are active for overall water splitting under visible light irradiation are expected to be constructed using varied visible-light-driven H2- and O2photocatalysts that have been reported. In general, an electron mediator is required for the construction of the Z-scheme photocatalysis system. However, an affinity between photocata-

10.1021/jp907128k CCC: $40.75  2009 American Chemical Society Published on Web 09/14/2009

Solar Water Splitting Using Powdered Photocatalysts lysts and the electron mediators actually limits such a combination to several systems. Moreover, the electron mediators may give undesirable effects such as backward reactions to form water from H2 and O2 evolved, or their intermediates, and a shielding effect of irradiated incident light. Some photocatalytic reactions, such as H2 and O2 evolution from aqueous solutions containing sacrificial reagents and oxidation of organic compounds under UV or visible light irradiation, are induced by the electron transfer from the conduction band of one photocatalyst to the conduction band or the valence band of another photocatalyst in two photocatalyst-combined systems prepared by physical mixing,34-36 CVD,37 and thermal treatment.38 We have tried to fabricate an overall water splitting system driven by the interparticle electron transfer between H2- and O2-photocatalysts without an electron mediator because the undesirable reactions and negative effects by an electron mediator can be excluded. In the present work, visible-light-driven Z-scheme photocatalysis systems for overall water splitting were investigated only by suspending the H2- and O2-photocatalyst powders in one reactor in the absence of an electron mediator. Solar water splitting was also demonstrated using the present photocatalyst system. 2. Experimental Section 2.1. Preparation and Characterization of Photocatalysts. SrTiO3,27 SrTiO3:Rh(1%),27 SrTiO3:Cr(3%),Ta(3%),26 TiO2: Rh(1.3%),Sb(2.6%),32 TiO2:Cr(2.3%),Sb(3.45%),24 and AgNbO323 powders were prepared by a solid-state reaction as previously reported. In the case of SrTiO3:Rh, the starting materials SrCO3 (Kanto Chemical; 99.9%), TiO2 (Soekawa Chemical; 99.9%), and Rh2O3 (Wako Pure Chemical; 95%) were mixed in a mortar according to the ratio Sr/Ti/Rh ) 1.07:0.99: 0.01. The mixture was calcined at 1273 K for 10 h in air using an alumina crucible. BiVO4 powder was prepared by a liquid-solid reaction.22 A mixture of Bi(NO3)3 · 5H2O (Kanto Chemical; 99.9%) and V2O5 (Wako Pure Chemical; 99.0%) in the ratio Bi/V ) 1:1 was added to 0.5 mol L-1 aqueous nitric acid solution. The suspension was stirred at room temperature for 3 days. The obtained BiVO4 powder was washed with water and dried. Bi2MoO631 and (CuAg)0.22In0.5ZnS30 powders were prepared by reflux and coprecipitation methods, respectively. Commercial anatase (Merck; 99%) and rutile (Soekawa Chemical; 99.9%)-phase TiO2, and WO3 (Nacalai Tesque; 99.5%) were used as received. Ru (1 wt %) and Pt (0.3 wt %) cocatalysts working as active sites for H2 evolution were loaded on photocatalysts by photodepotion from aqueous methanol solutions (10 vol%) containing RuCl3 · nH2O (Wako Pure Chemical; 99.9%) and H2PtCl6 (Tanaka Kikinzoku; 37.55% as Pt).19 The cocatalyst-loaded photocatalysts were collected by filtration and washed with water. Diffuse reflectance spectra of photocatalysts were obtained using a UV-vis-NIR spectrometer (JASCO; UbestV-570) and were converted from reflection to absorbance by the KubelkaMunk method. Photoluminescence was measured in vacuo at 25 K using a fluorospectrometer (HORIBA JOBIN YVON; SPEX Fluolog-3). Z-potentials of photocatalysts were measured by the electrokinetic sonic amplitude method (Colloidal Dynamics; AcoustSizer IIs). Photocatalysts were observed using an optical microscope (KEYENCE; VHX-200) and a scanning electron microscope (JEOL; JSM-7400F). 2.2. Photocatalytic Reactions. Photocatalytic reactions were mainly conducted in a gas-closed-circulation system. A reaction cell was made of Pyrex glass. Photocatalyst powders (0.05-0.4

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17537 g) were suspended in water (120 mL) at proper pH (adjusted with H2SO4 and NaOH) by a magnetic stirrer. The reactant solution was kept at 293 K. Argon gas (40 Torr) was introduced into the system after deaeration. The photocatalysts were irradiated using a 300-W Xe-arc lamp (Parkin Elmer; CermaxPE300BF). Visible light (λ > 420 nm) was controlled by a cutoff filter (HOYA; L42). The amounts of evolved H2 and O2 were determined using an online gas chromatograph (Shimadzu; GC8A, MS-5A column, TCD, Ar carrier). Apparent quantum yields were defined by the following equation because of a 4-electron process for H2 production in the Z-scheme system. Apparent quantum yield (%) ) 100 × [The number of reacted electrons]/ [The number of incident photons] ) 100 × [The number of evolved H2 molecules × 4]/ [The number of incident photons]

(1)

The monochromatic light was obtained with band-pass filters attached with a 300-W Xe-arc lamp (Asahi Spectra; MAX-301). The photon flux of the monochromatic light was measured by a silicon photodiode (OPHIRA; PD300-UV of a head and NOVA of a power monitor). Photocatalytic water splitting was also carried out in an Ar flow system using a solar simulator with an air-mass 1.5 filter (Yamashita Denso; YSS-80QA, 100 mW cm-2). In this case, reactions were conducted in a reaction cell with 33 cm2 of an irradiation area under a 15 mL min-1 Ar stream. A solar energy conversion efficiency was defined by the following equation. Solar energy conversion efficiency (%) ) 100 × [Output energy as H2]/([Energy density of incident solar light] × [Irradiated area]) ) 100 × o [Standard Gibbs free energy of water, ∆G298 ]× [Rate of H2evolution]/([Energy density of incident solar light] × [Irradiated area]) (2)

Here, the energy density of irradiated solar light was 100 mW cm-2. 3. Results and Discussion 3.1. Photocatalytic Water Splitting. An electron mediator has been considered to be indispensable for overall water splitting using the Z-scheme photocatalysis system composed of powdered H2- and O2-photocatalysts. However, in the present study, overall water splitting into H2 and O2 in a stoichiometric ratio under visible light irradiation proceeded by suspending the H2-photocatalyst of Rh-doped SrTiO3 loaded with Ru cocatalyst (designated as Ru/SrTiO3:Rh) and various O2photocatalysts in an aqueous solution without an electron mediator, as shown in Table 1 (runs 1, 7-16). Thus, various kind of Z-scheme systems have been developed for water splitting. Among them, the combination of Ru/SrTiO3:Rh and BiVO4 gave the highest activity under visible light irradiation (run 1). The overall water splitting proceeded steadily for a long time as shown in Figure 1. The turnover numbers of reacted electrons to the total numbers of Rh and Bi (or V) in the employed SrTiO3:Rh and BiVO4 photocatalysts were estimated to be 309 and 5.4, respectively, indicating that overall water splitting photocatalytically proceeded. The activity depended on the ratio of Ru/SrTiO3:Rh and BiVO4 in weight. The highest

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TABLE 1: Overall Water Splitting Using Z-Scheme Photocatalysis Systems Composed of Various Powdered H2and O2-Photocatalysts without Electron Mediatorsa activity/ µmol h-1

run

H2-photocatalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Ru/SrTiO3:Rh Pt/SrTiO3:Rh Ru/SrTiO3:Cr,Ta Ru/(CuAg)0.22In0.5ZnS Ru/SrTiO3 Ru/TiO2 (anatase) Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh Ru/SrTiO3:Rh -

weight incident O2-photocatalyst ratiob light/nm H2 BiVO4 BiVO4 BiVO4 BiVO4 BiVO4 BiVO4 AgNbO3 Bi2MoO6 TiO2:Cr,Sb TiO2:Rh,Sb WO3 SrTiO3 TiO2 (rutile) BiVO4 BiVO4 BiVO4 BiVO4

0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.1/0.1 0.04/0.1 0.2/0.1 0.4/0.1 -

>420 >420 >420 >420 >300 >300 >420 >420 >420 >420 >420 >300 >300 >420 >420 >420 >420 >420

40 1.3 0.3 4.4c 0.05 Trace 1.9 12 6.7 5.1 5.7 19 67 13 22 14 0.6 0

O2 19 0 0.1 0c 0 0 0.7 5.2 3.3 2.2 2.4 8.2 31 6.0 9.6 5.8 0 0

Figure 2. Overall water splitting under visible light irradiation by (a) mixed and (b) separated systems of Ru/SrTiO3:Rh and BiVO4 powder under visible light irradiation. Reaction conditions: catalyst; 0.1 g each, reactant solution; aqueous H2SO4 solution, 120 mL, pH 3.5, light source; 300-W Xe-arc lamp (λ > 420 nm), cell; top-irradiation cell with a Pyrex glass window.

a

Reaction conditions: reactant solution; aqueous H2SO4 solution, pH 3.5, 120 mL, light source; 300-W Xe-arc lamp, cell; top-irradiation cell with a Pyrex glass window. b Weight ratio of H2-photocatalyst/O2-photocatalyst (g/g). c pH 7.

Figure 1. Overall water splitting under visible light irradiation by the (Ru/SrTiO3:Rh)-(BiVO4) system. Catalyst: 0.1 g each. Reactant solution: aqueous H2SO4 solution, pH 3.5, 120 mL. Light source: 300-W Xe-arc lamp with a cutoff filter (λ > 420 nm).

activity was obtained when Ru/SrTiO3:Rh/BiVO4 was 0.1/0.1 (runs 1, 14-16). This dependence was probably due to the balance of the number of contact between Ru/SrTiO3:Rh and BiVO4 particles and the quantity of photon flux absorbed by each photocatalyst. When Pt cocatalyst was loaded on the SrTiO3:Rh photocatalyst, overall water splitting did not proceed because of enhanced backward reaction of water formation from evolved H2 and O2, or their intermediates (run 2). An electron mediator of Fe3+/Fe2+ was indispensable for the Z-scheme photocatalysis system employing the Pt-loaded SrTiO3:Rh photocatalyst.14,16 Moreover, other H2-photocatalysts except Ru/ SrTiO3:Rh could not be employed for overall water splitting (runs 3-6), indicating that doped Rh significantly participated in not only formation of the impurity level in the forbidden band for visible light response but also the electron transfer between photocatalyst particles. Figure 2 shows overall water splitting using the present photocatalysis systems with/without contact between Ru/SrTiO3: Rh and BiVO4. This experiment was carried out using the two top-opened containers sunk in one reaction cell without stirring. Overall water splitting proceeded in the mixed system, while it was not so in the separated system. This result indicated that

Figure 3. Photoresponses of the Z-scheme photocatalysis systems. (a) Action spectra of overall water splitting using the Z-scheme photocatalysis systems. (Ru/SrTiO3:Rh)-(BiVO4) system: blue circles, (Ru/ SrTiO3:Rh)-(TiO2-rutile) system: red triangles. Catalyst: 0.1 g each. Reactant solution: aqueous H2SO4 solution, pH 3.5, 120 mL. Light source: 300-W Xe-arc lamp. Monochromatic light was obtained using band-pass filters and a Xe-arc lamp. The number of photons at 420 nm was 0.38 mmol h-1. Error bars show regions of the incident light. (b) Diffuse reflection spectra of reduced SrTiO3:Rh, BiVO4, and TiO2rutile.

the physical contact was necessary for overall water splitting. Moreover, the metallic or ionic species that would work as an electron mediator were not detected in the aqueous solution before and after the photocatalytic water splitting. These results indicated that the present Z-scheme-type water splitting proceeded via electron transfer between the Ru/SrTiO3:Rh and BiVO4 particles. Figure 3a shows action spectra for overall water splitting using the Z-scheme photocatalysis systems of (Ru/SrTiO3:Rh)(BiVO4) and (Ru/SrTiO3:Rh)-(TiO2-rutile). The (Ru/SrTiO3: Rh)-(BiVO4) and (Ru/SrTiO3:Rh)-(TiO2-rutile) systems re-

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Figure 4. Solar water splitting into H2 and O2 using Z-scheme (Ru/ SrTiO3:Rh)-(BiVO4) system. Catalyst: 0.1 g each. Reactant solution: aqueous H2SO4 solution, pH 3.5, 180 mL. Light source: a solar simulator with an AM-1.5 filter (100 mW cm-1). Reactor: Ar flow system. Irradiated area: 33 cm2.

Figure 5. Dependence of the photocatalytic activity of the (Ru/SrTiO3: Rh)-(BiVO4) system on pH adjusted with H2SO4 and NaOH. Catalyst: 0.1 g each. Light source: 300-W Xe-arc lamp with a cutoff filter (λ > 420 nm).

sponded to 520 and 420 nm, respectively (the transmission width of band-pass filters: 20 nm). The onsets of absorption of SrTiO3: Rh, BiVO4, and TiO2-rutile were estimated to be 520, 512, and 411 nm from the diffuse reflectance spectra as shown in Figure 3b. Hence, the photoresponses for these Z-scheme photocatalysis systems were limited by the photocatalysts having wider band gap. Taking account of the fact that overall water splitting did not proceed on Ru/SrTiO3:Rh or BiVO4 as shown in Table 1 (runs 17 and 18), the photoexcitation of both H2- and O2photocatalysts is necessary for overall water splitting. Moreover, electrons photogenerated in a conduction band of BiVO4 have to be consumed by the reaction with holes photogenerated at an impurity level of SrTiO3:Rh if the water splitting proceeds photocatalytically. These results indicated that the present photocatalysis systems work through a Z-scheme mechanism. The apparent quantum yield of the (Ru/SrTiO3:Rh)-(BiVO4) system was 1.7% at 420 nm. This value is quite high compared with those of heterogeneous visible-light-driven photocatalysts for overall water splitting.10 The visible-light-driven O2-photocatalysts employed in this system have been reported to be active only for O2 evolution in the presence of sacrificial reagents such as a silver cation.10 However, it was first revealed that the Ru/ SrTiO3:Rh photocatalyst enabled us to achieve overall water splitting under visible light irradiation using various O2photocatalysts, extending the diversity of photocatalyst systems for this reaction. This is due to release from limitation of suitable affinity of the electron mediator with photocatalysts and suppression of the inhibition reactions.17 The Z-scheme photocatalysis system developed in this study requires no electron mediator, enabling us to use the varied O2-photocatalysts for overall water splitting. The (Ru/SrTiO3:Rh)-(BiVO4) system was experimentally confirmed to split water under sunlight irradiation (AM-1.5, 100 mW cm-1) as shown in Figure 4. When 1 m2 of the irradiated area was supposed, the rates of H2 and O2 production were 390 and 194 mL h-1, respectively. The solar energy conversion efficiency of this system was determined to be 0.12%. Although this value has hardly been reported for powdered photocatalysts, it is higher than that (0.03%) of powdered UV-light-driven NiOX/ TiO2 photocatalyst.7 3.2. Effects of pH on Photocatalytic Activities and Suspensibilities of Photocatalysts. The photocatalytic activity for overall water splitting using the (Ru/SrTiO3:Rh)-(BiVO4) system depended on pH of an aqueous solution as shown in Figure 5. The activity remarkably increased in acidic conditions below pH 4.0, and the highest activity was obtained at pH 3.5. Figure 6 shows the optical microscope images of the Ru/SrTiO3:

Rh and BiVO4 powders suspended in water adjusted to pH 7.0, 4.0, 3.5, and 2.5. Photocatalyst powders that were partly sintered in the size of 300-800 nm (see Supporting Information, Figure S1) were suspended well at pH 7.0, while the sintering particles were aggregated in the size of ca. 10 µm under acidic conditions (pH 4.0, 3.5, and 2.5). The largest aggregate formed at pH 3.5 that was the optimal condition for overall water splitting. When these photocatalyst particles did not contact each other, overall water splitting and interparticle electron transfer did not proceed as shown in Figure 2. Therefore, the observed aggregations in acidic solutions must be the mixture of Ru/SrTiO3:Rh and BiVO4 particles. The correlation between the photocatalytic activity and the degree of the aggregation is also supported by the results from Figures 5 and 6. Because the isoelectric point of Ru/SrTiO3:Rh was ∼4 (see Supporting Information, Figure S2), negligible electrostatic repulsion gave the aggregation under acidic conditions. As the control experiment, suspension of Ru/ SrTiO3:Rh and BiVO4 were also observed by an optical microscopy (see Supporting Information, Figure S3). BiVO4 showed no pH dependence of the aggregation, whereas the strong aggregation occurred by acidifying the suspended solution of Ru/SrTiO3:Rh. Moreover, the dependence of the photocatalytic activity of the BiVO4-combined system on pH seen in Figure 5 was similar to that of TiO2-rutile or WO3-combined system. The maximum activities of these systems were obtained around pH 3.5 being independent of O2-photocatalysts. These results indicated that the degree of the aggregation which affected the photocatalytic activity was mainly dominated by the property of the H2-photocatalyst of Ru/SrTiO3:Rh. On the other hand, the zeta potential of BiVO4 was negative at pH 2-9 (see Supporting Information, Figure S2). Electrostatic interaction between Ru/SrTiO3:Rh and BiVO4 particles can also contribute to the aggregations around pH 3.5. 3.3. Relationship between the Photocatalytic Activity and Rh Species Doped in SrTiO3. Reversibility of an oxidation state of Rh species doped in SrTiO3 is important for H2 evolution ability of the SrTiO3:Rh photocatalyst.27 Therefore, the effect of H2-reduction of SrTiO3:Rh on the activities of the present Z-scheme photocatalysis system was investigated as shown in Figure 7. The photocatalytic activities of SrTiO3:Rh reduced at temperature lower than or equal to 473 K were almost as high as that of the nontreated photocatalyst, whereas those reduced at 573 and 673 K were negligible. Diffuse reflectance spectra of SrTiO3:Rh treated with different conditions were measured to reveal the effect of the oxidation state of doped Rh species on overall water splitting as shown in Figure 8. The absorption band in the visible light region up to 520 nm is attributed to

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Figure 6. Optical microscope images of pH-adjusted aqueous suspension containing Ru/SrTiO3:Rh and BiVO4 at pH (a) 7.0, (b) 4.0, (c) 3.5, and (d) 3.0.

Figure 7. Dependence of the activity of the (Ru/SrTiO3:Rh)-(BiVO4) system on reduction temperature for SrTiO3:Rh. Catalyst: 0.1 g each. Reactant solution: aqueous H2SO4 solution, pH 3.5, 120 mL. Light source: 300-W Xe-arc lamp with a cutoff filter (λ > 420 nm).

Figure 8. Diffuse reflectance spectra of SrTiO3:Rh. The dotted, broken, and solid lines indicate spectra obtained immediately, at 1 day, and at 5 days later after H2 reduction, respectively.

the electronic transition from the donor level formed by the Rh3+ species to the conduction band, while that around 580 nm is due to the electronic transition from the valence band to the acceptor level formed by the Rh4+ species.27 The absorption band in visible light region up to 520 nm contributed to photocatalytic H2 evolution, whereas the absorption around 580 nm was inactive. H2-reduction resulted in the decrease in the absorption around 580 nm attributed to Rh4+ with the rise of absorption due to the existence of Rh3+ around 420 nm. In the case of SrTiO3:Rh reduced at 473 K, Rh3+ was gradually oxidized to Rh4+ by air-exposure. In contrast, Rh3+ in SrTiO3:

Figure 9. Photoluminescence of BiVO4 mixed with SrTiO3:Rh reduced at various temperatures (ratio in weight; BiVO4:SrTiO3:Rh ) 9:1), excitation: 449 nm at 25 K.

Rh reduced at 573 K was not oxidized even if it was exposed in air for a few days. Thus, SrTiO3:Rh reduced at temperature lower than or equal to 473 K maintained the reversibility of the oxidation state of Rh doped in SrTiO3 that was required for producing H2, while the higher temperature H2-reduction led to the loss of the reversibility. Figure 9 shows photoluminescence spectra of the BiVO4 powder mixed with the SrTiO3:Rh powder reduced at different temperatures. The broad emission band at 715 nm which was caused by a band gap excitation of BiVO4 at 25 K was quenched by the mixed SrTiO3:Rh powder. The emission intensity of BiVO4 decreased to reach a constant value by mixing with SrTiO3:Rh reduced at 473 K or nonreduced SrTiO3:Rh. The emission spectrum of BiVO4 overlapped with only the absorption band around 580 nm but not the absorption band up to 520 nm that responds to photocatalytic reaction. However, the degree of the quenching using SrTiO3:Rh reduced at 473 K was equal to that using nonreduced SrTiO3:Rh, even if the absorption around 580 nm of SrTiO3:Rh reduced at 473 K was much weaker than that of nonreduced SrTiO3:Rh. This result suggested that the quenching was due to not energy transfer from excited BiVO4 to the absorption mode around 580 nm but electron transfer. On the other hand, the degree of the quenching was small when the SrTiO3:Rh powder reduced at 573 K was mixed. These results suggested that the quenching of the emission from BiVO4 was due to the electron transfer from the BiVO4 to

Solar Water Splitting Using Powdered Photocatalysts

Figure 10. Mechanism of water splitting using the Z-scheme photocatalysis system driven by electron transfer between H2- and O2photocatalysts. (a) Suspension of Ru/SrTiO3:Rh and BiVO4 at neutral and acidic conditions. (b) Scheme of photocatalytic water splitting.

SrTiO3:Rh particles, and SrTiO3:Rh containing surface Rh species with the reversibility of the oxidation state significantly contributed to the electron transfer process to afford the higher photocatalytic activities for overall water splitting. Although the experimental condition of the quenching of luminescence is different from that of photocatalytic water splitting, these results support the electron transfer mechanism for the Z-scheme-type water splitting. 3.4. Mechanism of Overall Water Splitting. Figure 10 shows a proposed mechanism for the present Z-scheme photocatalysis system. This mechanism is mainly supported by effects of mixing (Figure 2) and aggregation (Figures 5 and 6) on the activity and the relationship between the activity and photoluminescence quenching (Figures 7-9). The Ru/SrTiO3: Rh particles aggregate incorporating the BiVO4 particles by acidifying the aqueous suspension, providing effective contact for the interparticle electron transfer. The simple and certain contact between the H2- and O2-photocatalyst particles by pH adjustment was the characteristic point of the present system. When the suspension is irradiated with visible light, photogenerated holes and electrons form in the impurity level of SrTiO3: Rh and in the valence band of BiVO4, and the conduction bands of SrTiO3:Rh and BiVO4, respectively. The electron transfer occurs from the conduction band of BiVO4 to the impurity level of SrTiO3:Rh. Then, the excited electrons in SrTiO3:Rh reduce water to form H2 on Ru cocatalyst, and the holes in BiVO4 oxidize water to form O2 to accomplish overall water splitting. As shown in Figures 7-9, the interparticle electron transfer process requires Rh species with the reversibility of the oxidation state in SrTiO3:Rh, indicating that the reversible Rh species at the surface of photocatalyst plays a pivotal role for the electron transfer between particles. 4. Conclusion Acidifying the aqueous solution caused the aggregation of Ru/SrTiO3:Rh and BiVO4 particles to provide effective contact, so that photocatalytic water splitting efficiently proceeded by the Z-schematic interparticle electron transfer without an electron mediator under visible light irradiation. In addition, this unique powdered photocatalysis system was active for solar water splitting. Only SrTiO3:Rh containing Rh species with revers-

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17541 ibility of oxidation number was effective as a H2-photocatalyst for the Z-scheme photocatalysis systems driven by the direct electron transfer between photocatalyst particles. It is the remarkable character for the SrTiO3:Rh photocatalyst. The present Z-scheme system that can employ varied photocatalysts has actually extended the visible-light-driven photocatalyst system for overall water splitting. In the conventional Z-scheme systems, an electron mediator not only shuttles the photogenerated carrier but also causes the inhibition reaction and a filter effect due to absorption by itself. These negative effects for attaining overall water splitting can be excluded by the use of the electron mediator-free Z-scheme system. Although the contact control between photocatalyst particles only by acidifying a solution was very simple and interesting, it would not be sufficient still for electron transfer. Therefore, the electron transfer process between photocatalyst particles is probably a rate-determining step on the photocatalytic reaction in the present Z-scheme system. The efficiency of the present system will be improved if the connection between Ru/SrTiO3:Rh with BiVO4 particles becomes more effective. For example, preparation of fine photocatalyst particles and suitable post-treatment of the mixture will lead to constructing an efficient solar H2 production system from water using this simple system. Acknowledgment. This work was supported by a Grant-inAid (No. 20037061) for Chemistry of Concerto Catalysis (No. 460) from the MEXT Japan and the ENEOS Hydrogen Trust Fund. Supporting Information Available: SEM images, zeta potentials of suspension, and optical micro images of suspension of Ru/SrTiO3:Rh and BiVO4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Khaselev, O.; Turner, J. A. Science 1998, 280, 425–427. (3) Gra¨tzel, M. Nature 2001, 414, 338–344. (4) Licht, S. J. Phys. Chem. B 2001, 105, 6281–6294. (5) Kudo, A. Int. J. Hydrogen Energy 2007, 32, 2673–2678. (6) Domen, K.; Hara, M.; Kondo, J. N.; Takata, T. Bull. Chem. Soc. Jpn. 2000, 73, 1307–1331. (7) Arakawa, H.; Sayama, K. Catal. SurVeys Jpn. 2000, 4, 75–80. (8) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851–7861. (9) Osterloh, F. E. Chem. Mater. 2008, 20, 35–54. (10) Kudo, A.; Miseki, Y. Chem. Soc. ReV. 2009, 38, 253–278. (11) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (12) Liu, H.; Yuan, J.; Shangguan, W.; Teraoka, Y. J. Phys. Chem. C 2008, 112, 8521–8523. (13) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 2416–2417. (14) Kato, H.; Hori, M.; Konta, R.; Shimodaira, Y.; Kudo, A. Chem. Lett. 2004, 33, 1348–1389. (15) Abe, R.; Takata, T.; Sugihara, H.; Domen, K. Chem. Commun. 2005, 3829–3831. (16) Kato, H.; Sasaki, Y.; Iwase, A.; Kudo, A. Bull. Chem. Soc. Jpn. 2007, 12, 2457–2464. (17) Higashi, M.; Abe, R.; Ishikawa, A.; Takata, T.; Ohtani, B.; Domen, K. Chem. Lett. 2008, 37, 138–139. (18) Higashi, M.; Abe, R.; Teramura, K.; Takata, T.; Ohtani, B.; Domen, K. Chem. Phys. Lett. 2008, 452, 120–123. (19) Sasaki, Y.; Iwase, A.; Kato, H.; Kudo, A. J. Catal. 2008, 259, 133– 137. (20) Mau, A. W.; Huang, C.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, A. E. J. Am. Chem. Soc. 1984, 106, 6537–6542. (21) Darwent, J. R.; Mills, A. J. Chem. Soc., Faraday Trans. 2 1982, 78, 359–367. (22) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459– 11467. (23) Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 12441–12447.

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