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To solve resource, energy, and environmental issues, development of sustainable clean energy system is strongly required. In recent years, hydrogen has been paid much attention to as a clean energy. Solar hydrogen production by water splitting using a photocatalyst as artificial photosynthesis is a promising method to solve these issues. Efficient utilization of visible light comprised of solar light is essential for practical use. Three strategies, i.e., doping, control of valence band, and formation of solid solution are often utilized as the useful methods to develop visible light responsive photocatalysts. This minireview introduces the recent work on visible-light-driven photocatalysts developed by substitution with metal cations of those strategies.

Na0.5Bi0.5TiO3 (BG 3.1 eV) with a valence band formed by Bi(III) was found as a new photocatalyst for solar water splitting. The water splitting activity of highly crystalline Na0.5Bi0.5TiO3 synthesized by a flux method was much higher than that of the samples synthesized by a solid-state reaction. The optimized RhCr2Ox(0.1 mol%)/Na0.5Bi0.5TiO3/CoOOH(0.02 mol%) gave a 5.1% apparent quantum yield at 350 nm and split water even under simulated sunlight irradiation with a 0.05% solar to hydrogen energy conversion efficiency. We have successfully achieved solar water splitting using a one-step photoexcitation type photocatalyst valence-band-controlled with Bi(III).

We successfully improved a (CuGa)0.5ZnS2 metal sulfide photocatalyst by preparation in a molten LiCl–CsCl flux for sacrificial H2 evolution compared to the counterpart prepared by a solid-state rea...

The activity of many water‐splitting photocatalysts could be improved by the use of RhIII–CrIII mixed oxide (Rh2−xCrxO3) particles as cocatalysts. Although further improvement of water‐splitting activity could be achieved if the size of the Rh2−xCrxO3 particles was decreased further, it is difficult to load ultrafine (, Rh2−x Cr x O3 particles with a size of approximately 1.3 nm were loaded onto a BaLa4Ti4O15 photocatalyst. The photocatalyst exhibited the highest quantum yield reported for BaLa4Ti4O15. The developed preparation technique of Rh2−xCrxO3 particles is also expected to improve the activity of other advanced water‐splitting photocatalysts. SG=glutathionate.

Improvement of water splitting performance of AgTaO3 (BG 3.4 eV) of a valence-band-controlled photocatalyst was examined. Survey of cocatalysts revealed that a Rh0.5Cr1.5O3 cocatalyst was much more effective than Cr2O3, RuO2, NiO and Pt for water splitting into H2 and O2 in a stoichiometric amount. The optimum loading amount of the Rh0.5Cr1.5O3 cocatalyst was 0.2 wt%. The apparent quantum yield (AQY) at 340 nm of the optimized Rh0.5Cr1.5O3(0.2 wt%)/AgTaO3 photocatalyst reached to about 40%. Rh0.5Cr1.5O3(0.2 wt%)/AgTaO3 gave a solar to hydrogen conversion efficiency (STH) of 0.13% for photocatalytic water splitting under simulated sunlight irradiation. Bubbles of gasses evolved by the solar water splitting were visually observed under atmospheric pressure at room temperature.

Linear conjugated polymers have potential as photocatalysts for hydrogen production from water but so far, most studies have involved non-scalable sacrificial reagents. Z-schemes comprising more than one semiconductor are a potential solution, but it is challenging to design these systems because multiple components must work together synergistically. Here, we show that a conjugated polymer photocatalyst for proton reduction can be coupled in a Z-scheme with an inorganic water oxidation photocatalyst to promote overall water splitting without any sacrificial reagents. First, a promising combination of an organic catalyst, an inorganic catalyst, and a redox mediator was identified by using high-throughput screening of a library of components. A Z-scheme system composed of P10 (homopolymer of dibenzo[b,d]thiophene sulfone)–Fe2+/Fe3+–BiVO4 was then constructed for overall water splitting under visible light irradiation. Transient absorption spectroscopy was used to assign timescales to the various steps in the photocatalytic process. While the overall solar-to-hydrogen efficiency of this first example is low, it provides proof of concept for other hybrid organic–inorganic Z-scheme architectures in the future.

Artificial photosynthesis, which splits water into H2/O2 or reduces CO2 and N2 by visible light, catalyzed by molecular catalysts (MCs) with/without being coupled with a solar cell should serve as ...

Microspherical particles of Rh-doped SrTiO3 (SrTiO3:Rh) having size of 1–2 μm were synthesized by a spray drying (SD) method employing a water-soluble ammonium trilactatotitanate. When the calcination temperature increased up to 1200 °C, primary particles grew significantly while the microspherical morphology of the secondary particles was kept. Dynamic light scattering analysis proved well-dispersive nature of the SD sample. Photocatalytic activities of SrTiO3:Rh synthesized by the SD method, solid state reaction (SSR), and a polymerizable complex (PC) method were evaluated for H2 evolution using an electron donor such as methanol, Fe2+, or [Co(bpy)3]2+ and Z-scheme water splitting combined with BiVO4 of an O2-evolving photocatalyst and either Fe3+/2+, [Co(bpy)3]3+/2+, or reduced graphene oxide as an electron mediator. SrTiO3:Rh prepared by the SD method showed much higher activities for all Z-scheme water splitting than the samples synthesized by the SSR method. Activities of the SD sample for Z-scheme water splitting using electron shuttles, Fe3+/2+ and [Co(bpy)3]3+/2+, were slightly higher or comparable to those of the PC sample whereas the SD sample achieved higher activity in Z-scheme water splitting based on interparticle electron transfer. In addition, even when the mass of SrTiO3:Rh used for the Z-scheme system with Fe3+/2+ was decreased from 50 mg to 30 mg, the SD sample still showed high activity while the activity decreased for the SSR and PC samples. Thus, usefulness of the SD method to obtain highly active SrTiO3:Rh with well-dispersive nature has been demonstrated.

Various studies on functionalization of water-splitting photocatalysts have been performed toward their practical usage. Control of the cocatalyst has been investigated, and recently, in addition to particle-size control, alloying has been extensively used to achieve this goal. It is essential to investigate photocatalysts with precisely controlled cocatalysts to obtain a detailed understanding of the effect of heteroatom doping of the cocatalyst on the photocatalytic activity and thereby establish clear design guidelines for functionalization. However, previous studies have investigated photocatalysts with a variety of particle sizes and doping ratios (chemical compositions). In this study, we succeeded in loading precisely controlled Au24Pd and Au24Pt clusters on BaLa4Ti4O15, which is one of the most advanced photocatalysts, using precisely synthesized alloy clusters as the precursor. Experiments with the photocatalysts loaded with precisely controlled cocatalysts revealed the following three features o...

Solid solutions with of Cu3 VS4 with either Cu3 NbS4 or Cu3 TaS4 (Cu3 Nb1-x Vx S4 or Cu3 Ta1-x Vx S4 ) were prepared by a solid-state reaction and adopted a sulvanite structure. Their band gaps were 1.6-1.7 eV corresponding to the absorption of a wide range of visible light. Ru cocatalyst-loaded Cu3 MS4 (M=V, Nb, Ta), Cu3 Nb1-x Vx S4 , and Cu3 Ta1-x Vx S4 showed photocatalytic activities for sacrificial H2 evolution under visible-light irradiation. Most solid solutions showed better activities than the single-component Cu3 MS4 (M=V, Nb, Ta). Cu3 MS4 (M=V, Nb), Cu3 Nb1-x Vx S4 , and Cu3 Ta1-x Vx S4 also functioned as photoelectrodes and gave cathodic photocurrents under visible-light irradiation, indicating a p-type semiconductor character. Cu3 Nb0.9 V0.1 S4 showed the best photocatalytic and photoelectrochemical performances. When Ru-loaded Cu3 Nb0.9 V0.1 S4 was used as a photocathode with a CoOx -loaded BiVO4 photoanode, photoelectrochemical water splitting proceeded under simulated sunlight irradiation without an external bias.

Various types of Z-scheme systems for water splitting under visible light irradiation were successfully developed by employing Rh- and Ir-doped metal oxide powdered materials with relatively narrow energy gaps (EG): BaTa2O6:Ir,La (EG: 1.9-2.0 eV), NaTaO3:Ir,La (EG: 2.1-2.3 eV), SrTiO3:Ir (EG: 1.6-1.8 eV), NaNbO3:Rh,Ba (EG: 2.5 eV) and TiO2:Rh,Sb (EG: 2.1 eV), with conventional SrTiO3:Rh (an H2-evolving photocatalyst) or BiVO4 (an O2-evolving photocatalyst), and suitable electron mediators. The Z-scheme systems were classified into three groups depending on the combination of H2- and O2-evolving photocatalysts and electron mediator. The Z-scheme systems combining BaTa2O6:Ir,La with BiVO4, and NaTaO3:Ir,La with BiVO4 were active when a [Co(bpy)3]3+/2+ redox couple was used rather than an Fe3+/2+ one. The combination of SrTiO3:Ir with SrTiO3:Rh gave an activity when the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couple ionic mediators were used. The Z-scheme systems combining NaNbO3:Rh,Ba and TiO2:Rh,Sb with SrTiO3:Rh showed activities by using the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couples and also via interparticle electron transfer by just contact with/without reduced graphene oxide (RGO). These suitable combinations can be explained based on the impurity levels of doped Rh3+ and Ir3+ toward the redox potentials of the ionic mediators for the Z-scheme systems employing ionic mediators, and p-/n-type and onset potentials of the photocurrent in the photoelectrochemical properties of those photocatalyst materials for the Z-scheme systems working via interparticle electron transfer.

In the synthesis method of a BiVO4 photoanode via BiOI flakes, a BiOI film is formed by electrochemical deposition in Step 1, and a vanadium (V) source solution is placed by drop-casting on the BiOI film in Step 2. Following this, BiVO4 particles are converted from the BiOI–(V species) precursors by annealing. However, it is challenging to evenly distribute vanadium species among the BiOI flakes. As a result, the conversion reaction to form BiVO4 does not proceed simultaneously and uniformly. To address this limitation, in Step 2, we developed a new electrochemical deposition method that allowed the even distribution of V2O5 among Bi–O–I flakes to enhance the conversion reaction uniformly. Furthermore, when lactic acid was added to the electrodeposition bath solution, BiVO4 crystals with an increased (040) peak intensity of the X-ray diffractometer (XRD) pattern were obtained. The photocurrent of the BiVO4 photoanode was 2.2 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) under solar simulated light of 100 mW/cm2 illumination. The Faradaic efficiency of oxygen evolution was close to 100%. In addition, overall water splitting was performed using a Ru/SrTiO3:Rh–BiVO4 photocatalyst sheet prepared by the BiVO4 synthesis method. The corresponding hydrogen and oxygen were produced in a 2:1 stoichiometric ratio under visible light irradiation.

Ba2In2O5 (BG 3.0 eV) with a brownmillerite structure has arisen as a novel photocatalyst for water splitting under UV irradiation. Ba2−xLaxIn2O5+0.5x (x = 0.04) with a brownmillerite structure had ...

Nitrogen/fluorine-codoped rutile TiO2 (R-TiO2:N,F) was newly synthesized, and its photocatalytic activity for water oxidation was evaluated. R-TiO2:N,F could be prepared by nitridation of the rutile TiO2 (R-TiO2) and (NH4)2TiF6 mixture at 773 K. The prepared samples produced O2 from aqueous AgNO3 solution under visible light irradiation, while R-TiO2 nitrided at the same temperature without any fluorine source showed negligible activity. The highest activity was obtained with the sample prepared at the (NH4)2TiF6/R-TiO2 ratio of 15/85, exhibiting water oxidation activity even in the presence of a reversible electron acceptor such as IO3− or Fe3+ with the aid of a RuO2 cocatalyst. Stoichiometric water splitting into H2 and O2 was achieved using a mixture of Ru/SrTiO3:Rh and RuO2/TiO2:N,F in the presence of [Co(bpy)3]3+/2+ (bpy = 2,2′-bipyridine) as a shuttle redox mediator without noticeable degradation of activity under visible light and even under AM1.5G simulated sunlight. Transient absorption spectroscopy revealed that appropriate nitrogen/fluorine codoping reduces the density of mid-gap states working as deep traps of photogenerated electrons, and increases the number of free electrons compared to only nitrogen-doped R-TiO2. Experimental results highlighted that the photocatalytic activity of R-TiO2:N,F could be enhanced by improving visible-light absorption capability through N/F codoping while suppressing the density of deep trap sites.

We report here a new class of co-catalysts for hydrogen evolution on TiO2 particles upon photo-irradiation. TiO2 particles prepared by sol/gel method was allowed to graft metal ion by a simple treatment with metal salts (Pt (IV), Rh (III), Cu (II), Fe(III), Al (III) ions) in water. The metal ion surface-grafted particles of TiO2(sol/gel) exhibit excellent photochemical hydrogen evolution compared to Pt (0) cluster loaded particles of TiO2(sol/gel) under the UV light irradiation of λ > 290 nm. XPS analysis shows that the valence states of the metal ions grafted on the TiO2 particles are the same with the parent metal ion salts and no metal (0) clusters were detected. DFT calculation of the simple model of metal ion/TiO2 indicated that an injected electron mostly localized on the metal ion sites, but not on Ti sites, which rationalizes the metal ion serve as the hydrogen evolution sites. Earth abundant metal ions, Cu (II), Fe (III), and Al (III) thus can provide new class of hydrogen evolution sites for artificial photosynthetic systems. To explore the possibility of utilizing semiconductor as an electron relay in the future system of molecular-catalyst-sensitized water splitting into hydrogen and hydrogen peroxide, the stability of hydrogen peroxide on various TiO2 surfaces was examined. Hydrogen peroxide was found to be pretty stable on Rh (III), Fe (III), and Al (III) ions surface-grafted TiO2, while Pt (IV) and Cu (II) ion grafted one similarly caused decomposition of hydrogen peroxide as Pt (0) cluster loaded TiO2 (Pt(0)/TiO2).

We report herein remarkable improvement of activity and stability of an Au25-loaded BaLa4Ti4O15 water-splitting photocatalyst. We first examined the influence of refining the gold cocatalyst on the individual reactions over the BaLa4Ti4O15 photocatalyst in this water-splitting system. The results revealed that refining the gold cocatalyst accelerates not only the hydrogen generation reaction, but also oxygen photoreduction reaction, which suppresses the H2 generation via photoreduction of protons. This finding suggests that photocatalytic activity will be enhanced if the O2 photoreduction reaction can be selectively suppressed by covering Au25 with a Cr2O3 shell which is impermeable to O2 but permeable to H+. Then, we developed new method for the formation of the Cr2O3 shell onto Au25. Our method utilizes the strong metal–support interaction between them. Water-splitting photoactivity of Au25–BaLa4Ti4O15 was improved by 19 times under an optimized coverage of the Cr2O3 shell. The Cr2O3 shell also elongate...

Photoelectrochemical water splitting has attracted much attention in recent years as an alternative energy source. However, there have been several significant issues such as low efficiency, high cost and less scalability for its practical application. Here, we show that (CuGa1−yIny)1−xZn2xS2 (CGIZS)-based photocathodes fabricated with a particle transfer method exhibited a photocurrent of 4.5 mA cm−2 at 0.6 V vs. RHE along with a largely positive onset potential of 1.0 V vs. RHE under simulated sunlight (AM 1.5G) and an initial solar-to-hydrogen energy conversion efficiency of 1.1% was obtained with photoelectrochemical water splitting using a two-electrode cell composed of the CGIZS-based photocathode and a BiVO4-based photoanode. CGIZS used for the particle transfer method was obtained in a powder state with high crystallinity by a flux method using a molten-salt of LiCl–KCl, and also formed solid solutions with a chalcopyrite single phase structure in a wide range of Ga/In ratios and Zn contents, in which the absorption edges of photocatalysts were tunable up to 880 nm.

Water is a promising source of clean hydrogen. Besides water electrolysis, the direct photoelectrochemical dissociation of water that combines light absorption and electrochemical water splitting into a single device is actively investigated by the scientific community. We report results on the p-type rhodium-doped strontium titanate (Rh:SrTiO 3 ) surface-modified by adsorption of a 3d -transition metal complex which is known to be a good catalyst of the hydrogen evolution reaction (HER) from water. The tris-dioximate cobalt hexachloroclathrochelate with an encapsulated cobalt(II) ion was selected as a surface HER co-catalyst for this study. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The kinetics of the photoelectrochemical water dissociation was studied using the open circuit photovoltage decay (OCPD) and photoelectrochemical impedance spectroscopy (PEIS) methods. Under open circuit conditions, the cobalt cage complex did not substantially affect the HER kinetics compared to the bare doped Rh:SrTiO 3 semiconductor. But when a reverse bias was applied, a significant difference caused by presence of the clathrochelate was observed. The rate constants of the charge transfer and the recombination processes were both affected, leading to the conclusion that the macrobicyclic cobalt-encapsulated species not only increased the rate of charge transfer, but also played a role as recombination centers for photoexcited minority carriers. The space charge capacitance was determined under the inversion conditions. Only under strong reverse bias it was found that, whereas bulk Rh:SrTiO 3 has a p- type conductivity, the surface of both bare Rh:SrTiO 3 and the cobalt clathrochelate-covered Rh:SrTiO 3 show the properties of a n -type semiconductor.

CuGaS2, (AgInS2)x–(ZnS)2−2x, Ag2ZnGeS4, Ni- or Pb-doped ZnS, (ZnS)0.9–(CuCl)0.1, and ZnGa0.5In1.5S4 showed activities for CO2 reduction to form CO and/or HCOOH in an aqueous solution containing K2SO3 and Na2S as electron donors under visible light irradiation. Among them, CuGaS2 and Ni-doped ZnS photocatalysts showed relatively high activities for CO and HCOOH formation, respectively. CuGaS2 was applied in a powdered Z-scheme system combining with reduced graphene oxide (RGO)-incorporated TiO2 as an O2-evolving photocatalyst. The powdered Z-scheme system produced CO from CO2 in addition to H2 and O2 due to water splitting. Oxygen evolution with an almost stoichiometric amount indicates that water was consumed as an electron donor in the Z-schematic CO2 reduction. Thus, we successfully demonstrated CO2 reduction of artificial photosynthesis using a simple Z-scheme system in which two kinds of photocatalyst powders (CuGaS2 and an RGO–TiO2 composite) were only dispersed in water under 1 atm of CO2.

Ir and La-codoped BaTa2O6 was developed as a novel photocatalyst for H2 evolution utilizing visible light up to 640 nm. The Ir and La-codoped BaTa2O6 with a Ru cocatalyst functioned as an H2-evolving photocatalyst for Z-schematic water splitting under visible light irradiation upon combination with a BiVO4 O2-evolving photocatalyst and a [Co(bpy)3]3+/2+ electron mediator.

We have successfully demonstrated solar water splitting using a newly fabricated photoelectrochemical system with a Pt-loaded SiC photocathode, a CoOx -loaded BiVO4 photoanode, and a perovskite solar cell. Detection of the evolved H2 and O2 with a 100 % Faradaic efficiency indicates that the observed photocurrent was used for water splitting. The solar-to-hydrogen (STH) efficiency was 0.55 % under no additional bias conditions.

Water splitting using semiconductor photocatalysts has been attracting growing interest as a means of solar energy based conversion of water to hydrogen, a clean and renewable fuel. Z-scheme photocatalytic water splitting based on the two-step excitation of an oxygen evolution photocatalyst (OEP) and a hydrogen evolution photocatalyst (HEP) is a promising approach toward the utilisation of visible light. In particular, a photocatalyst sheet system consisting of HEP and OEP particles embedded in a conductive layer has been recently proposed as a new means of obtaining efficient and scalable redox mediator-free Z-scheme solar water splitting. In this paper, we discuss the advantages and disadvantages of the photocatalyst sheet approach compared to conventional photocatalyst powder suspension and photoelectrochemical systems through an examination of the water splitting activity of Z-scheme systems based on SrTiO3:La,Rh as the HEP and BiVO4:Mo as the OEP. This photocatalyst sheet was found to split pure water much more efficiently than the powder suspension and photoelectrochemical systems, because the underlying metal layer efficiently transfers electrons from the OEP to the HEP. The photocatalyst sheet also outperformed a photoelectrochemical parallel cell during pure water splitting. The effects of H+/OH− concentration overpotentials and of the IR drop are reduced in the case of the photocatalyst sheet compared to photoelectrochemical systems, because the HEP and OEP are situated in close proximity to one another. Therefore, the photocatalyst sheet design is well-suited to efficient large-scale applications. Nevertheless, it is also noted that the photocatalytic activity of these sheets drops markedly with increasing background pressure because of reverse reactions involving molecular oxygen under illumination as well as delays in gas bubble desorption. It is shown that appropriate surface modifications allow the photocatalyst sheet to maintain its water splitting activity at elevated pressure. Accordingly, we conclude that the photocatalyst sheet system is a viable option for the realisation of efficient solar fuel production.

Z-Scheme water splitting by photocatalyst sheets is a promising approach for efficient and scalable H2 production. One of the most important challenges in developing such photocatalyst sheets is the application of photocatalysts with long absorption edges. In this study, RhCrOx-loaded LaMg1/3Ta2/3O2N (RhCrOx/LaMg1/3Ta2/3O2N) and Mo-doped BiVO4 (BiVO4:Mo), with absorption edges at 600 and 540 nm, are investigated as the hydrogen evolution photocatalyst and oxygen evolution photocatalyst in photocatalyst sheets, respectively. The (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet exhibits water splitting activity that is ∼5 times higher than that of corresponding powder suspensions. Stable water splitting is achieved under visible light using the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet coated with an amorphous TiO2 protective layer. However, the results show the low H2 evolution activity of the RhCrOx/LaMg1/3Ta2/3O2N imposes a ceiling on the activity of the photocatalyst sheet. The surface modification of LaMg1/...

Photocatalyst sheets consisting of H2 evolution photocatalyst (HEP) and O2 evolution photocatalyst (OEP) particles applied to an underlying conductive layer show promise with regard to promoting efficient and scalable water splitting. One of the most important challenges in enhancing the performance of such systems is establishing efficient charge transfer between photocatalyst particles that are often thickly stacked on the conductive layer. In this study, reduced graphene oxide (RGO) is investigated as an additional solid mediator to the conductive layer to bridge particulate photocatalysts and thus ensure effective charge transfer. Photocatalyst sheets made of RhCrOx/LaMg1/3Ta2/3O2N as the HEP and BiVO4:Mo as the OEP are applied to an Au layer together with RGO. The activity of this system is 3.5 times greater following the incorporation of the RGO. Charaterization analyses reveal that RhCrOx/LaMg1/3Ta2/3O2N particles tens of nanometers in size are fixed on larger, micrometer-sized, BiVO4:Mo particles by RGO photoreduced from GO in situ. The RGO facilitates charge transfer between particles that are distant from the underlying Au layer and thus involves more photocatalyst particles in the water splitting reaction. It is concluded that the incorporation of conductive materials into the photocatalyst particle layer can effectively enhance the water splitting activity of photocatalyst sheets.

Metal sulfides are highly active photocatalysts for water reduction to form H2 under visible light irradiation, whereas they are unfavorable for water oxidation to form O2 because of severe self-photooxidation (i.e., photocorrosion). Construction of a Z-scheme system is a useful strategy to split water into H2 and O2 using such photocorrosive metal sulfides because the photogenerated holes in metal sulfides are efficiently transported away. Here, we demonstrate powdered Z-schematic water splitting under visible light and simulated sunlight irradiation by combining metal sulfides as an H2-evolving photocatalyst, reduced graphene oxide (RGO) as an electron mediator, and a visible-light-driven BiVO4 as an O2-evolving photocatalyst. This Z-schematic photocatalyst composite is also active in CO2 reduction using water as the sole electron donor under visible light.

CuLi1/3Ti2/3O2 powders with hexagonal- and trigonal-delafossite structures were prepared by treating cubic-Li2TiO3 and monoclinic-Li2TiO3 with molten CuCl, respectively. It was confirmed that Li+ at the interlayer of Li2TiO3 was replaced with Cu(I). Both hexagonal- and trigonal-CuLi1/3Ti2/3O2 showed photocatalytic activities for sacrificial H2 evolution using visible light up to 600 nm which corresponds to their absorption edges (BG 2.1 eV), when Ru- and Pt-cocatalysts were loaded. The activity of the hexagonal-CuLi1/3Ti2/3O2 was higher than that of the trigonal-CuLi1/3Ti2/3O2. The apparent quantum yield of the sacrificial H2 evolution from an aqueous K2SO3 + Na2S solution over the hexagonal-CuLi1/3Ti2/3O2 loaded with Ru-cocatalyst was 3.3% at 420 nm. Moreover, Z-schematic water splitting was achieved when hexagonal- and trigonal-CuLi1/3Ti2/3O2 with a Pt-cocatalyst as H2-evolving photocatalysts were combined with a reduced graphene oxide (RGO)-TiO2 composite as an O2-evolving photocatalyst.

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion. Mechanically robust photoelectrodes are formed by embedding self-assembled metal nanopillars in a semiconductor thin film, forming tubular Schottky junctions around each pillar. The photocarrier transport efficiency is strongly enhanced in the Schottky space charge regions while the pillars provide an efficient charge extraction path. Ir-doped SrTiO3 with embedded iridium metal nanopillars shows good operational stability in a water oxidation reaction and achieves over 80% utilization of photogenerated carriers under visible light in the 400- to 600-nm wavelength range., Nanoscale designs are known to increase the energy conversion efficiency of photoelectrochemical water splitting. Here, the authors report a self-organized nanocomposite formed by embedding self-assembled metal nanopillars in a semiconductor thin film, for enhanced photocarrier separation efficiency.

K4Nb6O17 (BG: 3.67 eV) and Na2W4O13 (BG: 3.12 eV) layered oxide photocatalysts with wide band gaps were treated with a molten AgNO3 to substitute K+ and Na+ with Ag+, resulting in red-shifts of absorption edges in diffuse reflectance spectra. A part of Na+ ions in the interlayer of Na2W4O13 was substituted with Ag+ ions by the molten AgNO3 treatment with keeping the layered structure. Both Ag(I)-substituted K4Nb6O17 and Na2W4O13 showed photocatalytic activities for O2 evolution from aqueous solutions containing a sacrificial reagent utilizing the absorption bands newly formed by the Ag(I)-substitution. Notably, the Ag(I)-substituted Na2W4O13 produced O2 under visible light irradiation. When ball-milled Na2W4O13 was treated with a molten AgNO3, the Ag(I)-substitution rate increased. The Ag(I)-substituted Na2W4O13 with ball-milling showed higher photocatalytic activity for O2 evolution than that without ball-milling. Z-schematic water splitting proceeded under visible light irradiation by combining the Ag(I)-substituted Na2W4O13 of an O2-evolving photocatalyst with Ru-loaded SrTiO3 doped with Rh of a H2-evolving photocatalyst.

We have recently succeeded in loading extremely small monodisperse gold clusters (1.2 ± 0.3 nm) as cocatalysts on a water-splitting BaLa4Ti4O15 photocatalyst using a glutathione-protected Au25 cluster (Au25(SG)18) as a precursor; an improved photocatalytic activity of 2.6-fold was obtained when compared with that of BaLa4Ti4O15 onto which large gold nanoparticles (10–30 nm) were loaded. In the current study, the controlled loading of a series of ultrasmall Aun clusters onto BaLa4Ti4O15 using various Aun(SG)m clusters (n = 10, 15, 18, 22, 25, 29, 33, 39) was examined. The results revealed that the use of a highly stable cluster as a precursor is essential for achieving control over the loading of the gold clusters. Additionally, the origin of the improved photocatalytic activity owing to the ultra miniaturization of the cocatalyst was reconsidered herein based on the photocatalytic activities of the obtained photocatalysts. The results strongly suggested that the activity per gold atom on the surface decre...

Z-schematic water splitting was successfully demonstrated using metal sulfide photocatalysts that were usually unsuitable for water splitting as single particulate photocatalysts due to photocorrosion. When metal sulfide photocatalysts with a p-type semiconductor character as a H2-evolving photocatalyst were combined with reduced graphene oxide-TiO2 composite as an O2-evolving photocatalyst, water splitting into H2 and O2 in a stoichiometric amount proceeded. In this system, photogenerated electrons in the TiO2 with an n-type semiconductor character transferred to the metal sulfide through a reduced graphene oxide to achieve water splitting. Moreover, this system was active for solar water splitting.

A powdered Z-scheme system for the photocatalytic water splitting reaction under visible light irradiation was successfully demonstrated utilizing a combination of a metal-complex catalyst, reduced graphene oxide (RGO), and semiconductor photocatalysts. A Ru-complex electrocatalyst, [Ru (2,2′-bipyridine)(4,4′-diphosphonate-2,2′-bipyridine)(CO)2]2+, linked with SrTiO3:Rh (Rh: 4 at%) as a H+ reduction photocatalyst was coupled with BiVO4 as a water oxidation photocatalyst and RGO as a solid-state electron mediator. H2 and O2 evolved stoichiometrically under visible light irradiation (λ > 420 nm) and the turnover number for the Ru-complex was calculated to be 450 after 8.5 h of irradiation time, which indicates that this Z-scheme system splits water photocatalytically and the Ru-complex functions as an efficient cocatalyst for hydrogen evolution by water splitting.

Mo-doped BiVO4 powder was successfully prepared by a facile liquid-solid-state reaction and was applied to a photoanode for water splitting. Pulverization of the Mo-doped BiVO4 particles by ball-mi...

Development of sunlight-driven water splitting systems with high efficiency, scalability, and cost-competitiveness is a central issue for mass production of solar hydrogen as a renewable and storable energy carrier. Photocatalyst sheets comprising a particulate hydrogen evolution photocatalyst (HEP) and an oxygen evolution photocatalyst (OEP) embedded in a conductive thin film can realize efficient and scalable solar hydrogen production using Z-scheme water splitting. However, the use of expensive precious metal thin films that also promote reverse reactions is a major obstacle to developing a cost-effective process at ambient pressure. In this study, we present a standalone particulate photocatalyst sheet based on an earth-abundant, relatively inert, and conductive carbon film for efficient Z-scheme water splitting at ambient pressure. A SrTiO3:La,Rh/C/BiVO4:Mo sheet is shown to achieve unassisted pure-water (pH 6.8) splitting with a solar-to-hydrogen energy conversion efficiency (STH) of 1.2% at 331 K a...

The effect of iridium valence in Ir:SrTiO3 on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO3. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir4+ or Ir3+ dopants in SrTiO3. The measurements show that Ir3+:SrTiO3 forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir4+:SrTiO3 under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir4+:SrTiO3 contains an in-gap state close to the top of the SrTiO3 valence band. The performance of Ir4+:SrTiO3 in electrochemical reactions is compared with cathodic Rh3+:SrTiO3, clearly illustrating the...

Cocatalysts are usually needed to improve photoconversion efficiency in water splitting with heterogeneous photocatalysts. Here, we show that NiOx (0 < x < 1) nanoparticles loaded on a layered perovskite, BaLa4Ti4O15 (BLT), serve as an effective electron sink. We have measured the time profiles of transient absorption (TA) of BLT with and without loading NiOx cocatalyst in air and in water in a wide wavelength range from 400 nm to 4 μm upon excitation with a femtosecond pulse at 266 nm. TA at 4 μm indicates that photogenerated electrons are rapidly transferred to cocatalyst within 1 ps. The time profile of TA at 402 nm from sub-μs to 10 ms contributed by surface photoholes is affected by loading of cocatalyst and redox reactions with water. Fittings of the decay profiles of TA at 402 nm with a trap–detrap kinetic model indicate that the oxidation of water appreciably starts at around 1 ms after the pump pulse, while the reduction of water takes place prior to oxidation in much early time domains. This imp...

The codoping effect of antimony on the photocatalytic activity of visible-light-driven SrTiO 3 doped with rhodium (SrTiO 3 :Rh) was investigated. SrTiO 3 doped with rhodium and antimony (SrTiO 3 :Rh/Sb) prepared by a hydrothermal method was found to be active for photocatalytic H 2 evolution from an aqueous methanol solution and O 2 evolution from an aqueous silver nitrate solution under visible light irradiation, although SrTiO 3 doped with rhodium and no antimony was active only for the H 2 evolution. Photocatalytic activities of SrTiO 3 :Rh/Sb were strongly dependent on the ratio of codopant to dopant (Sb/Rh). Diffuse reflection spectroscopy (DRS), electron spin resonance (ESR), Raman, and action spectrum analyses revealed the contribution of rhodium and antimony to visible-light response of SrTiO 3 :Rh/Sb. Unstable and reversible Rh 3+ ions in oxidation state were the superior species for the H 2 evolution. On the other hand, Rh 3+ ions stabilized by codoping of antimony without the formation of Rh 4+ ions and oxygen defects which would work as undesirable recombination sites between photogenerated electrons and holes played an important role in the O 2 evolution. Moreover, when an IrO X cocatalyst was loaded on the surface of the SrTiO 3 :Rh/Sb photocatalyst, the photocatalytic activity of the O 2 evolution drastically increased. The apparent quantum yield for the H 2 evolution over Pt(0.3 wt%)/SrTiO 3 :Rh(1%)/Sb(1%) and the O 2 evolution over IrO X (3.0 wt%)/SrTiO 3 :Rh(1%)/Sb(1%) at 420 nm were 0.8% and 4.5%, respectively. The Z -scheme system composed of Ru(1.0 wt%)/SrTiO 3 :Rh(2%) as a H 2 -evolving photocatalyst, IrO X (3.0 wt%)/SrTiO 3 :Rh(1%)/Sb(1%) as an O 2 -evolving photocatalyst, and an Fe 3+ /Fe 2+ redox couple as an electron mediator showed photocatalytic activity for overall water splitting under visible light irradiation.

KCaSrTa5O15 with tungsten bronze structure and a band gap of 4.1 eV showed activity for water splitting without cocatalysts. The activity was improved by loading the NiO cocatalyst. The apparent quantum yield of optimized NiO-loaded KCaSrTa5O15 was 2.3% at 254 nm for water splitting. When CO2 gas was bubbled into the reactant aqueous solution, Ag cocatalyst-loaded KCaSrTa5O15 produced CO and H2 as reduction products of CO2 and H2O, respectively, and O2 as an oxidation product of H2O. The carbon source of CO was confirmed to be CO2 molecules by using (13)CO2. The ratio of the number of electrons to that of holes calculated from the amounts of products (CO, H2 and O2) was almost unity. Additionally, the ratio of the turnover number of electrons consumed for CO production to the total number of an Ag atom of the cocatalyst that was the active site for CO2 reduction was 8.6 at 20 h. These results indicate that water was consumed as an electron donor for this photocatalytic CO2 reduction in an aqueous medium. Thus, KCaSrTa5O15 with tungsten bronze structure has arisen as a new photocatalyst that is active for water splitting and CO2 reduction utilizing water as an electron donor.

SnNb2O6 is one of the promising photoanode materials for photoelectrochemical water splitting. We prepared the SnNb2O6 thin-film photoanode with various Sn/Nb ratios by a pulsed-laser deposition method and investigated impacts of lattice defects in the SnNb2O6 photoanode on the water oxidation property. Photoelectrochemical measurements and Raman spectroscopy analyses for the SnNb2O6 photoanodes presented that the water oxidation property can be improved by suppressing the formation of the acceptor site caused by the substitution of Sn4+ for the Nb5+ site in the crystal lattice of SnNb2O6. This study provides material’s design strategy based on defect chemistry to realize an efficient SnNb2O6 photoanode used for the photoelectrochemical cell, which can operate without an external bias.

Tantalum nitride (Ta3N5) modified with various O2-evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ420 nm) in an attempt to construct a redox-mediator-free Z-scheme water-splitting system. Ta3N5 was prepared by nitriding Ta2O5 powder under a flow of NH3 at 1023-1223 K. The activity of Ta3N5 for water oxidation from an aqueous AgNO3 solution as an electron acceptor without cocatalyst was dependent on the generation of a well-crystallized Ta3N5 phase with a low density of anionic defects. Modification of Ta3N5 with nanoparticulate metal oxides as cocatalysts for O2 evolution improved water-oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO(x)) was found to be the most effective, improving the water-oxidation efficiency of Ta3N5 by six to seven times. Further modification of CoO(x)/Ta3N5 with metallic Ir as an electron sink allowed one to achieve Z-scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru-loaded SrTiO3 doped with Rh as a H2-evolution photocatalyst.

[Co(bpy)3](3+/2+) and [Co(phen)3](3+/2+) redox couples were revealed to play as electron mediators for Z-scheme photocatalyst systems composed of Ru/SrTiO3:Rh and BiVO4 powders for overall water splitting under visible light irradiation. These electron mediators were effective for only the combination of SrTiO3:Rh with BiVO4. They did not work when nondoped SrTiO3 and TiO2 of H2-evolving photocatalysts and WO3 of O2-evolving photocatalysts were employed. These results indicated that the affinity between photocatalysts and the Co-complex electron mediators was important. The photocatalytic activity depended on pH. Neutral pH conditions gave the highest activity for overall water splitting. Overall water splitting by the present system steadily proceeded for a long time. The Z-scheme photocatalyst system was also confirmed to split water under sunlight irradiation at the rates depending on weather. Moreover, overall water splitting by the Z-scheme photocatalyst system with the Co-complex electron mediator using a reaction cell in which the Ru/SrTiO3:Rh suspension was divided from BiVO4 suspension by a membrane filter resulted in H2 evolution separated from that of O2.

Photoelectrochemical (PEC) water splitting offers a means for distributed solar hydrogen production. However, the lack of stable and cost-effective photoanodes remains a bottleneck that hampers their practical applications. Here we show that particulate Mo-doped BiVO4 water oxidation photoanodes, without costly and complex surface modifications, can possess comparable stability to that of solar cells. The photoanode exhibits enhanced intrinsic photocorrosion inhibition and self-generation and regeneration of oxygen evolution catalysts, which allows stable oxygen evolution for >1,000 h at potentials as low as 0.4 V versus the reversible hydrogen electrode. The significantly improved photocorrosion resistance and charge separation are attributed to the unusual high-temperature treatment. In situ catalyst regeneration is found to be a site-specific and oxygen evolution rate change-induced process. Our findings indicate the potential of PEC water splitting to compete with other solar hydrogen production solutions, and should open new opportunities for the development of feasible PEC water splitting systems. Using photoelectrodes to split water is a promising approach to convert solar energy to fuel, but photoanode stability is often an issue. Now, a Mo-doped BiVO4 photoanode is shown to stably evolve oxygen for 1,000 h due to in situ regeneration of the catalyst, and inhibition of photocorrosion.

UTokyo Research掲載「太陽光を用いて水を分解できる粉末光触媒シート」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/solar-water-splitting-using-particulate-photocatalyst-sheets.html, UTokyo Research "Solar water splitting using particulate photocatalyst sheets" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/solar-water-splitting-using-particulate-photocatalyst-sheets.html

Water splitting to produce H2 using sunlight is a form of artificial photosynthesis in that light energy is converted to chemical energy. As such, water splitting using powdered photocatalysts has attracted attention in the framework of energy and environmental issues. This article reviews z-scheme photocatalyst systems for water splitting under visible light irradiation, especially focused on the systems consisting of SrTiO3:Rh of a H2-evolving photocatalyst, and O2-evolving photocatalysts with and without electron mediators. These photocatalyst systems showed activities for water splitting into H2 and O2 in a stoichiometric amount under visible light irradiation and even under sunlight irradiation. The photocatalytic activity was sensitive to pH. The optimum pH was 2.4 when iron ions were used as electron mediators. Co-catalysts also affected the activity. The photodeposited Ru co-catalyst gave an excellent performance. The best performance achieved by the pH adjustment and the selection of a co-catalyst was obtained mainly by suppression of back reactions to form H2O from evolved H2 and O2.

CaTa4O11, Ca1−xSrxTa4O11 (0 ≤ x ≤ 0.3), and LaZrTa3O11 with laminated structure consisting of MO6 and MO7 (M = Ta and/or Zr) layers were prepared by a polymerized complex method. These tantalates s...

Indium borate (InBO 3 ) with a calcite structure that was prepared by a solid-state reaction and a two-step precipitation method was employed as a photocatalyst for water splitting under UV light irradiation. The InBO 3 photocatalyst prepared by the precipitation method showed higher activity than that prepared by the solid-state reaction when a NiO cocatalyst was loaded on the photocatalyst. In the case of the precipitation method, the photocatalytic activity over NiO/InBO 3 strongly depended on the calcination temperature and an excess amount of boric acid in the preparation. The correlation among preparation conditions, physical properties, and photocatalytic activities for water splitting has revealed that crystallinity and particle size mainly contributed to the increase in the photocatalytic activity.

Bismuth vanadate (BiVO4) is incorporated with reduced graphene oxide (RGO) using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 10-fold enhancement in photoelectrochemical water splitting reaction is observed on BiVO4−RGO composite compared with pure BiVO4 under visible illumination. This improvement is attributed to the longer electron lifetime of excited BiVO4 as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination. Improved contact between BiVO4 particles with transparent conducting electrode using RGO scaffold also contributes to this photoresponse enhancement.