Recent advances in visible-light-responsive ...

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PERSPECTIVE

Cite this: Phys. Chem. Chem. Phys., 2013, 15, 13243

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Recent advances in visible-light-responsive photocatalysts for hydrogen production and solar energy conversion – from semiconducting TiO2 to MOF/PCP photocatalysts Yu Horiuchi, Takashi Toyao, Masato Takeuchi, Masaya Matsuoka* and Masakazu Anpo* The present perspective describes recent advances in visible-light-responsive photocatalysts intended to develop novel and efficient solar energy conversion technologies, including water splitting and photofuel cells. Water splitting is recognized as one of the most promising techniques to convert solar energy as a clean and abundant energy resource into chemical energy in the form of hydrogen. In recent years, increasing concern is directed to not only the development of new photocatalytic materials but also the importance of technologies to produce hydrogen and oxygen separately.

Received 4th April 2013, Accepted 20th May 2013

Photofuel cells can convert solar energy into electrical energy by decomposing bio-related compounds

DOI: 10.1039/c3cp51427g

technologies have been going on since the discovery of semiconducting titanium dioxide materials and

and livestock waste as fuels. The advances of photocatalysts enabling these solar energy conversion have extended to organic–inorganic hybrid materials, such as metal–organic frameworks and porous

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coordination polymers (MOF/PCP).

1. Introduction Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. E-mail: [email protected], [email protected]; Fax: +81 72-254-9910; Tel: +81 72-254-9288

Yu Horiuchi received his PhD from Osaka University in 2011. From 2010 to 2011, he worked in Professor Hiromi Yamashita’s laboratory at Osaka University as a JSPS research fellow. In 2011 he joined Osaka Prefecture University as an Assistant Professor in Chemistry. His research interests include the development of visible-lightresponsive photocatalysts for water splitting and solar energy Yu Horiuchi conversion, and the rational design and synthesis of metal–organic frameworks as heterogeneous catalysts and photocatalysts.

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the Owner Societies 2013

In recent years, the depletion of fossil fuels that have been formed from organic remains of dead plants and animals over hundreds of millions of years and the environmental problems caused by their combustion have stimulated research on the

Takashi Toyao

Takashi Toyao was born in 1988 in Shimane, Japan. He completed BS degree in 2011 and MS degree in 2013 at Osaka Prefecture University. He is currently a PhD student and a JSPS research fellow in the department of applied chemistry at Osaka Prefecture University. His research interests include the development of visible-light-responsive photocatalysts utilizing TiO2 thin films and metal–organic frameworks.

Phys. Chem. Chem. Phys., 2013, 15, 13243--13253

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development of new renewable energy production technologies. So far, several approaches have been proposed in order to satisfy the requirements. Among those explored, the system combining photocatalysts and solar energy as a clean and abundant energy resource is recognized to be of great promise. Pioneering work on solar energy conversion using photocatalysts was conducted by Honda and Fujishima in 1971– 1972.1 They discovered a photosensitization effect of the TiO2 electrode for the electrolysis of water into H2 and O2 upon applying an external bias, which is the so-called Honda– Fujishima effect. The irradiation of UV light with energy greater than the band gap of TiO2 leads to the generation of electrons and holes in the conduction band and valence band, respectively, with TiO2 as an anode electrode. The photo-formed electrons move to the counter electrode, the Pt black electrode, by applying an anodic potential through an external circuit, and

Masato Takeuchi

Masato Takeuchi is currently an associate professor in the graduate school of engineering at Osaka Prefecture University (OPU). He received his PhD degree at OPU in 2002. Prior to working in OPU, he was a JSPS postdoctoral fellow in OPU (Prof. Anpo) and Torino University (Profs Collucia and Martra) in 2002–2004. His major interests include the NIR investigations on the hydrogen bond networks in water, alcohol, ammonia clusters adsorbed on catalyst surfaces.

Masaya Matsuoka obtained his doctoral degree in applied chemistry from Osaka Prefecture University in 1997. During 1997 he worked as a postdoctoral fellow at the Universite´ Pierre et Marie Curie, Paris. In 1998 he again joined the Osaka Prefecture University as a research associate and is presently a professor at the same university. His current research interests include the development of the visible light Masaya Matsuoka responsive photocatalysts and their applications for the environmental purifications as well as the H2 production from pure water.

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reduce H+ into H2 there. On the other hand, photo-formed holes oxidize H2O to produce O2 on the TiO2 surface. These reactions occurring at the cathode and anode and the overall reaction are described as follows: Cathode:

2H+ + 2e - H2

(1)

Anode:

H2O + 2h+ - 12O2 + 2H+

(2)

Overall:

H2O - H2 + 12O2

(3)

The Gibbs free energy change of the water splitting reaction described by eqn (3) (two-electron process) is 238 kJ mol 1 (2.47 eV), showing an endothermic reaction. Hence, photoelectrochemical solar water splitting under an applied potential of less than 1.23 eV enables the storage of solar energy as chemical energy. In the powdered system, Pt-loaded TiO2 has emerged as water splitting photocatalysts.2 Pt nanoparticles act as cocatalysts and help to reduce overpotential of hydrogen evolution reaction and to promote charge separation. Currently, the field of photocatalyst research is rapidly expanding, and various environmentally-benign applications as well as water splitting are being investigated. However, TiO2 photocatalysts whose bandgap energies are ca. 3.2 eV are active only under the irradiation of UV light, which accounts for only 3–5% of the solar spectrum. Therefore, highly desired are the development of visible-light-responsive photocatalysts and the effective utilization of solar light by using them. In this perspective, we highlight the recent advances in the development of visible-light-responsive photocatalysts and their applications to various environmentally-benign photocatalytic reaction systems including water splitting, hydrogen production from aqueous media (half reaction of water splitting) and photofuel cells. The first part describes the history of the development of powdered photocatalysts for water splitting

Masakazu Anpo is presently the Vice President and Executive Director of Osaka Prefecture University. He is a pioneer in the study of photochemical reactions on solid surfaces including catalysts and has published more than 100 books with Elsevier, Wiley, Springer, Nova Sciences, Asakura-shoten, etc. He has received many awards from Japan Photochemical Society, Catalysis Society of Japan, Masakazu Anpo Chemical Society of Japan, and Ministry of Education of Japan as well as other honors such as a member of Academia Europaea and Science Council of Japan. He has published over 485 original papers and is the Editor-in-Chief of the international journal, ‘‘Research on Chemical Intermediates’’ (Springer, USA). His dream is the establishment of solar chemistry as a new environmentally-friendly science and technology.

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induced by the irradiation of solar light and visible light. The second part deals with the details of the design concept and performances of an H-type reactor to realize the separate evolution of hydrogen and oxygen from water. In the third part, the development of photofuel cells, particularly separate-type photofuel cells, which can convert solar energy into electrical energy by decomposing the bio-related compounds and livestock waste as fuels is described. The final part focuses on recent applications of metal–organic frameworks and porous coordination polymers (MOF/PCP) to visible-light-responsive photocatalysts.

2. Recent advances in visible-lightresponsive photocatalysts 2.1.

Solar water splitting induced by powdered photocatalysts

Considerable efforts have hitherto been made to design visiblelight-responsive photocatalysts by many researchers. Since most semiconducting materials typified by TiO2 have wide bandgap energies, bandgap engineering, such as cation doping and anion doping, is a promising way to develop photocatalysts with visible-light responsivity. For example, Asahi et al. reported the visible-light-promoted photocatalysis of N-doped TiO2, in which p states of the N atom contribute to the bandgap narrowing by mixing with O2p states.3 Moreover, visible-lightpromoted photocatalytic decomposition of organic compounds was attained by Abe et al. by employing a Pt-loaded WO3 photocatalyst, whose bandgap energy is small enough to absorb visible light (Fig. 1).4 Although the conduction band level of WO3 is more positive than the reduction potential of O2, the efficient decomposition reaction occurs because Pt nanoparticles as cocatalysts can promote multielectron O2 reduction into H2O2. On the other hand, the combination of light harvesting antennas with semiconducting photocatalysts is also an effective way to design visible-light-responsive photocatalysts. Dye-sensitized photocatalysts5 and plasmonic photocatalysts6 have been proven to act as photocatalysts under visible-light irradiation through the light absorption of light harvesting antennas and subsequent

Fig. 1 (a) Time course of acetaldehyde (1000 ppm, ca. 15 mmol) decomposition over Pt–WO3 (0.1 wt% Pt), bare WO3, Pt–N–TiO2 (0.5 wt% Pt), and bare N–TiO2 under visible light irradiation (400 o l o 500 nm). (b) Time course of CO2 generation from isopropyl alcohol (1200 ppm, ca. 17 mmol) over the photocatalysts under full-arc or visible light irradiation. Reprinted with permission from ref. 4. Copyright (2008) American Chemical Society.

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electron transfer to the conduction band of semiconducting photocatalysts. However, it is difficult to design rational band structures and realize overall water splitting into H2 and O2 under solar light irradiation by employing a semiconducting photocatalyst alone. Along these lines, some researchers focus on two-step photoexcitation processes that mimic photosynthesis of green plants, the so-called Z-scheme, in order to realize overall water splitting. The conceptual diagram is shown in Fig. 2. This photocatalytic system consists of two visible-light-responsive semiconducting photocatalysts specialized for respective hydrogen evolution reaction and water oxidation reaction and a redox mediator. The photocatalyst for hydrogen evolution reaction (photocatalyst A) is excited by the irradiation of visible light, and then, photo-formed electrons reduce H+ into H2 and photo-formed holes oxidize the redox mediator. Simultaneously, in the photocatalyst for water oxidation reaction (photocatalyst B), photo-formed holes oxidize H2O to produce O2 and photo-formed electrons reduce the redox mediator under visible-light irradiation. Totally, water splitting into H2 and O2 is attained photocatalytically. Bard described research studies that were the origin of Z-scheme photocatalysts in 1979.7 In 1997, an alternative evolution of H2 and O2 using a RuO2–WO3 photocatalyst and a Fe3+/Fe2+ redox system shown in Fig. 3 was reported by Sayama et al.8 In this system, only H2 is evolved in the absence of the photocatalyst under the irradiation of the full-range of UV light through quartz glass. On the other hand, when the light is irradiated through Pyrex glass in the presence of the photocatalyst, only O2 is evolved. This unique system allows for the evolution of H2 and O2 without mixing the two gases, which is not attained by using conventional semiconducting photocatalyst systems. In 2001, Abe et al. realized photocatalytic water splitting into H2 and O2 induced by a two-step photoexcitation system composed of an IO3 /I shuttle redox mediator and two different TiO2 photocatalysts under the irradiation of light at a wavelength of l > 300 nm.9 One is Pt-loaded anatase TiO2 for H2 evolution reaction, and the other is rutile TiO2 for O2 evolution reaction. Although the water splitting reaction proceeds efficiently in this reaction system, the utilization of visible light had not yet been achieved. Subsequently, the same research group reported in the same year that the Z-scheme photocatalytic system

Fig. 2

Conceptual diagram of a Z-scheme photocatalytic system.


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Fig. 3 (A) Presumed reaction mechanism of the photocatalytic decomposition of water using the Fe3+/Fe2+ redox system. (B) Alternative evolution of H2 and O2 gases using a RuO2–WO3 catalyst and a Fe3+/Fe2+ redox system (H2:K, O2: J). Run 1: Mixture of FeSO4 (1 mmol) and H2SO4 (10 mmol) and distilled water (350 mL) was irradiated through a quartz glass filter (>200 nm). Run 2: RuO2 (1 wt%)–WO3 catalyst (1 g) was added to the solution after run 1 and light was irradiated through a Pyrex glass filter (>300 nm). Run 3: Catalyst powder was filtered from the solution (or sedimented by stop stirring) after run 2 and then the solution was irradiated again through a quartz glass filter (>200 nm). Run 4: same as run 2. Reprinted with permission from ref. 8. Copyright (1997) Elsevier.

consisting of platinized Cr–Ta-doped SrTiO3 and platinized WO3 photocatalysts and an IO3 /I shuttle redox mediator for water splitting effectively works under visible-light irradiation (l > 420 nm), as shown in Fig. 4.10 These pioneering studies inspire interest in the development of novel Z-scheme photocatalytic systems, and various combinations of photocatalysts and shuttle redox mediators have hitherto been reported, such as (Pt/TaON)– (Pt–WO3)–(IO3 /I ) in 2005,11 (Pt–TaON)–(RuO2/TaON)–(IO3 /I ) in 2008,12 (Pt–ATaO2N; A = Ca, Ba)–(Pt/WO3)–(IO3 /I ) in 2008 (ref. 13) and (Pt/ZrO2/TaON)–(Pt/WO3)–(IO3 /I ) in 2010.14 Kudo and co-workers have studied redox mediators strenuously. In 2004, the Fe3+/Fe2+ redox mediator was revealed to be effective for the Z-scheme photocatalytic system consisting of Ru/SrTiO3:Rh

and BiVO4 photocatalysts.15 Moreover, in 2009, solar water splitting was attained in the absence of redox mediators by using (Ru/SrTiO3:Rh)–(BiVO4) systems at adequate pH values of acidified aqueous solutions (Fig. 5A and B).16 These powdered photocatalysts aggregate with suitable contact at an adequate pH value, and the reaction proceeds through interparticle electron transfer. The photoreduced graphene oxide (PRGO) was found to act as a solid electron mediator, which can improve the transfer of photo-formed electrons from BiVO4 to Ru/SrTiO3:Rh under visible-light irradiation (Fig. 5C and D).17 On the other hand, it is noteworthy that visible-light-driven water splitting using one powdered photocatalyst loaded with appropriate cocatalysts has been realized by Maeda et al. A solid solution of gallium and zinc nitrogen oxide, (Ga1 XZnX)(N1 XOX), modified with nanoparticles of a mixed oxide of rhodium and chromium behaves as an overall water splitting photocatalyst, in which the Cr2O3 shell provides an H2 evolution site at the external surface while preventing water formation from H2 and O2 on Rh (Fig. 6).18 More recently, the same research group achieved the successful overall water splitting under visible-light irradiation using an IrO2/Cr2O3/RuOX/ZrO2/TaON photocatalyst.19 2.2.

Fig. 4 (A) Speculated reaction mechanism for the water splitting using a mixture of Pt–SrTiO3, Pt–WO3 and NaI aqueous solution. (B) Reaction time course of the photocatalytic splitting of water into H2 and O2 under visible light (l > 420 nm, 3.1 W, window area: 16 cm2). (a) 0.2 g of Pt (0.3 wt%)–SrTiO3 doped with Cr and Ta (both 1 mol% of Ti) was suspended in the NaI aqueous reaction solution (100 mmol L 1, 250 mL, pH 7.0). (b) 0.2 g of Pt (1 wt%)–WO3 were suspended in the NaI aqueous reaction solution (100 mmol L 1, 250 mL, pH 7.0). The magnetic stirring was continued during the light irradiation and in the dark. Reproduced from ref. 10 with permission from The Royal Society of Chemistry.

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Separate evolution of hydrogen and oxygen

Solar water splitting has been recognized as one of the most promising chemical processes to convert solar energy into chemical energy in the form of hydrogen. Many studies have been devoted to the realization of solar water splitting by using various types of powdered photocatalysts. However, in these powdered systems, hydrogen and oxygen are produced as a mixture, and thus, the development of efficient separation processes is inevitable toward practical usage of hydrogen as fuel. The utilization of visible-light-responsive thin film photocatalysts offers one solution to this issue. As shown in Fig. 7, a photoelectrochemical cell composed of a visible-light-responsive thin film photocatalyst as a photoanode, a platinum cathode and electrolytes separated by a proton exchange membrane allows for the production of hydrogen at the cathode and oxygen at the anode. In addition, a unique H-type reactor for the separate evolution of hydrogen and oxygen shown in Fig. 8 has been developed by Anpo and co-workers.20 A unique visible-light-responsive titanium dioxide

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Fig. 5 (A) Mechanism of water splitting using the Z-scheme photocatalysis system driven by electron transfer between H2- and O2-photocatalysts. (a) Suspension of Ru/SrTiO3:Rh and BiVO4 under neutral and acidic conditions. (b) Scheme of photocatalytic water splitting. (B) Solar water splitting into H2 and O2 using the 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 2). Reactor: Ar flow system. Irradiated area: 33 cm2. Reprinted with permission from ref. 16. Copyright (2009) American Chemical Society. (C) (a) Schematic image of a suspension of Ru/SrTiO3 and PRGO/BiVO4 in water at pH 3.5. (b) Mechanism of water splitting in a Z-scheme photocatalysis system consisting of Ru/SrTiO3:Rh and PRGO/BiVO4 under visible-light irradiation. (D) Overall water splitting under visible-light irradiation by the (Ru/SrTiO3:Rh)– (PRGO/BiVO4) system. Conditions: catalysts (0.03 g each) in H2SO4(aq.) (pH 3.5, 120 mL); light source, 300 W Xe lamp with a 420 nm cutoff filter; top-irradiation cell with a Pyrex glass window. Reprinted with permission from ref. 17. Copyright (2011) American Chemical Society.

thin film photocatalyst (Vis-TiO2) prepared by a radio frequency magnetron sputtering deposition (RF-MS) method is deposited on the Ti metal substrate for oxygen evolution, and Pt nanoparticles are also deposited on the other side of the substrate for hydrogen evolution (Vis-TiO2/Ti/Pt). This thin film device is sandwiched by two compartments, and thus, the complete separate evolution of hydrogen and oxygen is realized. In this section, the details of the unique preparation of Vis-TiO2 by using an RF-MS method and the performances of the H-type reactor based on Vis-TiO2 under solar light irradiation are described. Vis-TiO2 prepared by using an RF-MS method at high substrate temperatures (TS) above 673 K has visible light absorption characteristics.21 As shown in Fig. 9A, UV-vis transmission spectra of TiO2 thin films prepared at TS below 473 K (UV-TiO2) show absorption bands only in the UV-wavelength region, while observable absorption bands in the visible wavelength region are seen in spectra of Vis-TiO2 prepared at TS above 673 K. It is also found from photographic images that a yellow colored TiO2 thin film is formed when a TS of 873 K is employed for the preparation (Fig. 9A inset). This visible light absorption

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Fig. 6 (A) A schematic reaction mechanism of overall water splitting on Rh/Cr2O3loaded (Ga1 XZnX)(N1 XOX) and the corresponding processes on supported Rh NPs and Cr2O3 NPs. (B) Time courses of overall water splitting using Rh/Cr2O3-loaded (Ga1 XZnX)(N1 XOX) under visible-light irradiation (l > 400 nm). (a) 0.15 g of catalyst, (b) a mixture of 0.15 g of catalyst with 0.15 g of a sample loaded with Rh-NPs. Reactions were performed in pure water (370 mL) upon illumination using a high-pressure mercury lamp (450 W) through an aqueous 2 M NaNO2 solution filter. Reprinted with permission from ref. 18b. Copyright (2006) Wiley-VCH.

Fig. 7 Schematic illustration of a photoelectrochemical cell based on a visiblelight-responsive photocatalyst for water splitting under an applied bias.

characteristic is attributable to the unique composition of the synthesized TiO2 thin film. Fig. 9B shows secondary ion mass spectrometry (SIMS) depth profiles of TiO2 prepared at TS of 473 and 873 K. The secondary ion intensity derived from 18O for Vis-TiO2 prepared at TS of 873 K gradually decreases from the top surface (O/Ti = 2.00 0.01) to the inside bulk (O/Ti = 1.93 0.01), whereas the stoichiometric O/Ti ratio of 2.00 0.01 observed for TiO2 prepared at TS of 473 K does not change over the entire range. The gradually-reduced bulk composition


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Fig. 8 Schematic illustration of an H-type reactor for the separate evolution of H2 and O2 under solar light irradiation using a sunlight concentrating system.

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Fig. 10 (A) Time course of separate evolution of H2 and O2 under solar light irradiation by using H-type systems based on Vis-TiO2/Ti/Pt or UV-TiO2/Ti/Pt. The TiO2 side is suffused with 1.0 M NaOH aq. and the Pt side is suffused with 0.5 M H2SO4 aq. to gain a chemical bias of ca. 0.83 V to assist electron transfers from TiO2 to Pt. The device is irradiated using a sunlight concentrating system from the TiO2 side. (B) The effect of HF treatment of Vis-TiO2 on photocatalytic performances of H-type systems for the separate evolution of H2 and O2 under visiblelight irradiation (l > 450 nm). The reactions were performed under the same conditions as (A) except for using a different light source.

exhibited enhanced performance for water splitting reaction, compared to that employing Vis-TiO2/Ti/Pt. Moreover, the solar light conversion efficiency of the system employing HF(60)-VisTiO2/Ti/Pt and the sunlight concentrating system was determined to be 0.38% based on the H2 evolution rate. 2.3.

Fig. 9 (A) UV-vis transmission spectra of TiO2 thin films prepared by RF-MS methods at different substrate temperatures (TS = 373–973 K). (B) SIMS depth profiles of TiO2 thin films prepared by RF-MS methods at different substrate temperatures (TS = 473 or 873 K).

of Vis-TiO2 was thus considered to lead to a perturbation in the electronic structure of the TiO2 thin film, resulting in the absorption of visible light. It is noteworthy that only the RF-MS method conducted at a high substrate temperature enables to synthesize such visible-light-responsive TiO2 thin films. As shown in Fig 10A, the H-type system employing Vis-TiO2/ Ti/Pt shows efficient photocatalytic activities for water splitting reaction under concentrated sunlight irradiation and enables to produce stoichiometric H2 and O2 separately. For further improvement of photocatalytic activities, chemical etching of Vis-TiO2 by HF solution was attempted.22 The photocurrent of Vis-TiO2 was increased dramatically by HF treatment under both UV- and visible-light irradiation, showing that the efficiency of the photooxidation of water increased with HF treatment. The highest photocurrent was observed for Vis-TiO2 etched for 60 min (HF(60)-Vis-TiO2). The IPCE (incident photon-to-current conversion efficiency) values of HF(60)-Vis-TiO2 (+0.1 V vs. SCE) under UV and visible light were determined to be 66% and 9.4%, respectively. This is due to the increase in the surface roughness and surface area of Vis-TiO2 after the HF treatment. Fig. 10B shows the results of the separate evolution of H2 and O2 from water using HF(60)-Vis-TiO2/Ti/Pt under visible-light irradiation (l > 450 nm). The system employing HF(60)-Vis-TiO2/Ti/Pt

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Separate type photofuel cell

The utilization of biomass and its derivatives as carbon-neutral feedstocks alongside solar energy has attracted much attention for the development of new energy production technologies from clean and renewable resources. Photofuel cells (PFCs) developed by Kaneko et al. enable the production of electricity using various biomass and bio-related compounds as fuels.23 This device comprises a nanoporous TiO2 thin film photoanode, a Pt cathode for O2 reduction and an electrolyte containing various biomass and bio-related compounds as fuels and operates under light irradiation, as shown in Fig. 11. The photo-formed holes oxidize biomass and bio-related compounds at the photoanode, and the photo-formed electrons transfer to the cathode and reduce O2 to form H2O. For example, when ammonia is used as the fuel, the following reactions occur at the anode and the cathode. Anode: Cathode:

2NH3 - N2 + 6H+ + 6e 3 2O2

+ 6H+ 6e - 3H2O

(4) (5)

Thus, simultaneous utilization of solar light and biomass will be realized by using PFC systems. However, several issues related to the efficient utilization of solar light and the improvement of the conversion efficiency need to be addressed for the practical application of PFCs. Nishikiori et al. have made improvements to photoanodes by using sol–gel processes.24 The hybridization of a clay mineral, allophane, with a TiO2 electrode was revealed to enhance cell performances of the PFC, in which allophane effectively adsorbed fuels and then transported them to photocatalytically active TiO2 (Fig. 12).24b As such, although activities of photoanodes in PFCs continue to improve, strongly desired are the development of visible-light-responsive

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Fig. 11 Energy diagram of ammonia direct PFC (at pH 12) using a nanoporous TiO2 film photoanode and a Pt cathode soaked in an aqueous ammonia solution. Reprinted with permission from ref. 23b. Copyright (2006) Elsevier.

photoanodes and the enhancement of the reduction reaction rate at the cathode. We demonstrate here a novel separate-type photofuel cell (SPFC) shown in Fig. 13.25 The separation of the PFC into two compartments, comprised of the anode and cathode, enables the use of two different electrolytes. Various biomass derivatives are incorporated as fuels in the anode electrolyte and an I3 /I redox solution, as is typically used in dye-sensitized solar cells, are incorporated in the cathode electrolyte for the enhancement of the reduction reaction rate at the cathode. In addition, the above-mentioned Vis-TiO2 with visible-light-responsivity is employed for the photoanode for the efficient utilization of solar light. This device design meets requirements for the advances of PFCs toward practical applications. The photovoltaic performances of various SPFCs observed under simulated solar light irradiation from a PEC-L 11 solar simulator at AM 1.5 (100 mW cm 2) are summarized in Fig. 14. Fig. 14A shows the effect of the addition of iodine redox couples into the cathode electrolyte on photovoltaic performances of SPFCs in the presence of methanol in the anode electrolyte solution as fuel. The addition of iodine redox couples can enhance their photovoltaic performances significantly.

Fig. 12 I–V curves of the (1) titania electrode and the allophane–titania composite electrodes containing (2) 0.10% and (3) 1.0% allophane observed using the electrolyte solutions with and without glucose during UV irradiation. Reprinted with permission from ref. 24b. Copyright (2010) The Chemical Society of Japan.

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Fig. 13 Schematic illustration of a separate-type photofuel cell (SPFC) based on a Vis-TiO2 photoanode and a carbon cathode.

This finding indicates that oxygen reduction at the cathode (eqn (6)) is replaced by the reduction reaction of I3 ions to I ions (eqn (7)), resulting in the promotion of the reduction reaction at the cathode. O2 + 4H+ + 4e - 2H2O

(6)

I3 + 2e - 3I

(7)

It is noteworthy that the device using a carbon cathode shows the same performance as the one using a Pt cathode in the presence of I3 /I redox couples, proving that a cost-efficient carbon can be used as an alternative to Pt for the cathode. Moreover, as shown in Fig. 14B and Table 1, their photovoltaic performances can be enhanced by the addition of biomass derivatives, suggesting that various biomass derivatives are available as fuels in this device. From a standpoint of the efficient utilization of solar light, the use of Vis-TiO2 with visible-light-responsivity is expected to be effective. In fact, the SPFC employing a UV-TiO2, which is a UV-light active TiO2 thin film prepared by an RF-MS method, as the photoanode exhibited lower photovoltaic performance than one employing Vis-TiO2. This performance difference is due to a good visible light responsivity of Vis-TiO2. The results indicate that the use of Vis-TiO2 for the anode of the SPFC enables the efficient utilization of visible light present in solar light. For further improvement of cell performances of SPFCs, studies are proceeding with the amelioration of the Vis-TiO2 photoanode.26 The loading of Rh3+ ions onto Vis-TiO2 was found to enhance photovoltaic performances of SPFCs. This is due to the improvement of the visible light absorption as well as the increase in the electron donor density of Vis-TiO2. Furthermore, the enhancement of energy conversion efficiency was achieved by an HF treatment of Vis-TiO2. In addition to the increase in surface area and surface roughness after HF treatment, the enhanced conductivity was responsible for the improvement of cell performances of the SPFC.


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Fig. 14 I–V curves of SPFCs under simulated solar light irradiation at AM 1.5 observed under various conditions. Effects of the addition of (A) iodine redox couples and (B) biomass derivatives on photovoltaic performances of SPFCs.

Table 1 Photovoltaic performances of SPFCs with or without biomass derivatives under simulated solar light irradiation at AM 1.5

Biomass derivatives

VOC/V

JSC/mA cm

None Methanol (10 vol%) Ethanol (10 vol%) Ethylene glycol (10 vol%) Glycerin (10 vol%) Glucose (9 wt%)

0.42 0.66 0.65 0.68 0.72 0.68

0.10 0.39 0.33 0.52 0.57 0.41

2

FF

Z/%

0.22 0.24 0.20 0.28 0.27 0.26

0.009 0.062 0.043 0.099 0.111 0.073

2.4. Visible-light-responsive metal–organic framework photocatalysts The hybridization of organic and inorganic materials opens up a new field in visible-light-responsive photocatalysts by the integration of useful organic and inorganic characteristics within a single composite. Dye-sensitized photocatalysts combine efficient lightharvesting properties of dye molecules and excellent photocatalysis of semiconducting materials and realize visible-light-promoted photocatalytic reactions.5a,b In this line, efficient hydrogen evolution from water containing a sacrificial electron donor under visiblelight irradiation has been achieved by the adsorption of dye sensitizers on Pt-loaded TiO2 (e.g., Fig. 15).5b,c Hydrogen evolution using visible-light-responsive photocatalysts is an essential half reaction in water splitting, which converts solar energy into chemical potential of hydrogen molecules. Metal–organic frameworks (MOFs) and porous coordination polymers (PCPs) are organic–inorganic hybrid materials consisting of organic linkers and metal-oxo clusters, and their applications to adsorbents, separation materials, ion conductive materials

Fig. 15 Dye-sensitized photocatalyst for the H2 evolution system in two-steps water splitting. Reprinted with permission from ref. 5c. Copyright (2000) Elsevier.

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and catalysts are currently active areas of research.27 In addition to the 3-dimensional porous network structures of MOFs, their designability enabling accurate material design based on the diversity of combinations of organic linkers and metal-oxo clusters provides an opportunity to develop new functional materials. Herein, we introduce unique visible-light-responsive MOF photocatalysts whose organic linkers act as light-harvesting units. Lin and co-workers reported that Pt-loaded Zr-based MOFs consisting of an organic linker containing a light harvesting Ir complex are effective for hydrogen evolution reaction under visible-light irradiation.28 In this photocatalytic system, the reaction proceeds through the light absorption by the MOF framework, Ir complex, and subsequent electron injection into Pt nanoparticles as shown in Fig. 16. Similarly, Rosseinsky and co-workers employed a porphyrin MOF (Al-PMOF) as a light harvesting unit for the hydrogen evolution reaction (Fig. 17).29 Mori and co-workers demonstrated the hydrogen evolution from water in the multicomponent system consisting of various MOFs, [Ru(bpy)3]2+, methyl viologen and EDTA-2Na.30 On the other hand, in recent years, there has been intensive interest in semiconducting properties of MOF.31 Garcıá et al. and Majima et al. reported independently that photocatalysis and photoluminescence phenomena have been observed in a Zn-based MOF (MOF-5).31a,b Garcıá et al. proposed in their article that the photophysical processes in MOF-5 occur through a

Fig. 16 Scheme showing the synergistic photocatalytic hydrogen evolution process via photoinjection of electrons from the light harvesting MOF frameworks into the Pt NPs. The red balls represent Zr6(O)4(OH)4(carboxylate)12 cores, while the green balls represent the Ir–phosphor ligand of the MOF. Reprinted with permission from ref. 28. Copyright (2012) American Chemical Society.

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Fig. 17 The photocatalytic reaction using Zn0.986(12)TCPP-[AlOH]2 (2). (i) Reaction involving 2, methyl viologen, colloidal platinum, and sacrificial EDTA. (ii) Reaction involving 2, colloidal platinum, and sacrificial EDTA. Reprinted with permission from ref. 29. Copyright (2012) Wiley-VCH.

Fig. 19 Schematic illustrations of (A) the structure of Ti–MOF–NH2 and (B) the reaction mechanism for hydrogen evolution over Ti–MOF–NH2 induced by visiblelight irradiation. (C) Action spectrum for hydrogen evolution from water containing TEOA as a sacrificial electron donor over Ti–MOF–NH2. Inset shows the photograph of Ti–MOF–NH2.

Fig. 18 Photophysical processes that occur after the irradiation of the MOF-5 solid material and an aqueous solution containing the terephthalate unit and Zn2+. Reprinted with permission from ref. 31a. Copyright (2007) Wiley-VCH.

linker-to-cluster charge-transfer (LCCT) process shown in Fig. 18. The combination of visible-light-harvesting organic linkers and metal-oxo clusters with photofunctional characteristics will enable the development of dye-sensitized type MOF photocatalysts operating under visible-light illumination. In this context, Garcıá and co-workers achieved photocatalytic hydrogen evolution via the LCCT mechanism under the irradiation with 370 nm light.32 More recently, Li and co-workers described the successful tuning of the absorption of a Tibased MOF by using a visible-light-active organic linker, 2-aminoterephthalic acid and its application to a photocatalyst for CO2 reduction reaction to form the formate anion, HCOO , under visible-light irradiation.33 Subsequently, Matsuoka and co-workers reported that the same MOF consisting of titaniumoxo clusters and 2-aminoterephthalic acid organic linkers (Ti–MOF–NH2) behaves as an effective photocatalyst for visiblelight-promoted hydrogen evolution reaction (Fig. 19).34 Matsuoka et al. elucidated in their article the reaction mechanism of the visible-light-promoted photocatalytic reaction over Ti–MOF–NH2 by using ESR measurements. In the measurements, the suspension of Ti–MOF–NH2 and an aqueous triethanolamine solution

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was evacuated at 77 K and then irradiated with visible light (l > 420 nm) for 3 h at room temperature. After irradiation, the ESR spectrum was recorded at 77 K. The obtained ESR spectrum shows typical signals corresponding to paramagnetic Ti3+ centers in a distorted rhombic oxygen ligand field, which are not observed in the spectrum before visible-light irradiation (Fig. 20). These results indicate that photo-formed electrons transfer from the excited 2-aminoterephthalic acid organic linker to the conduction band of the titanium-oxo cluster under visible-light irradiation and reduce Ti4+ species to form Ti3+ species, that is, the visible-light-promoted hydrogen evolution reaction occurs over Ti–MOF–NH2 on the basis of the LCCT mechanism. Thus, the accurate combination of metal-oxo clusters and organic linkers is of extreme importance for the development of more efficient MOF photocatalysts. Since excited state hydrogen bonding structures formed between molecular chromophores and

Fig. 20 ESR spectra observed at 77 K for (A) Ti–MOF–NH2 and (B) Ti–MOF, immersed in aqueous 0.01 M TEOA solution, before (dotted line) and after (solid line) visible-light irradiation (l > 420 nm). The suspension was degassed under vacuum at 77 K and irradiated with visible light for 3 h at room temperature, followed by spectrum acquisition at 77 K.


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Perspective solvents as typified by alcohols are known to have strong influence on photophysical and photochemical processes, such as intermolecular or intramolecular electronic transition, fluorescence and phosphorescence,35 it is also important to design novel photofunctional materials in consideration of these kinds of interactions.


3. Summary and outlook Solar energy conversion technologies using photocatalysts have been expected to be a means of solving various environmental problems and have rapidly advanced as described above. Solar water splitting is being realized by the development of novel visible-light-responsive photocatalysts, such as a solid solution of gallium and zinc nitrogen oxide, (Ga1 XZnX)(N1 XOX), and an IrO2–Cr2O3–RuOX–ZrO2–TaON composite, and also by the design of Z-scheme photocatalytic systems mimicking photosynthesis of green plants. Moreover, separate evolution of hydrogen and oxygen from water with a special emphasis on realistic applications has been extensively studied. An H-type reactor for the water splitting enables us to produce completelyseparated hydrogen and oxygen. The development of other solar energy conversion systems has also attracted much attention. In particular, photofuel cells that can convert solar energy into electrical energy by decomposing bio-related compounds and livestock waste as fuels have been of interest from a viewpoint of reducing the environmental load. In the light of photocatalyst design, MOF/PCP have recently emerged as materials with great potential for realization of visiblelight-driven hydrogen evolution owing to their structural designability. Several visible-light-responsive MOF photocatalysts for hydrogen evolution have been hitherto developed by employing rationally-designed organic linkers as light-harvesting units. In future researches on photocatalytic solar energy conversion, largely desired are not only the development of efficient and stable visible-light-responsive photocatalysts but also the system design to produce hydrogen and oxygen separately with high conversion efficiency. In particular, electrode-type photocatalysts are recognized as a suitable form for industrial applications. Furthermore, the findings until now let us reaffirm the importance of cocatalysts. Thus, the development of accurately-designed cocatalysts and the elucidation of their roles in water splitting will help to progress the research on photocatalytic solar energy conversion. Energy production using photocatalysts and solar light would be realized in industry in the near future through these considerations.

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