Enhancing Photocathodic Performances of Particulate-CuGaS2-Based Photoelectrodes via Conjugation with Conductive Organic Polymers for Efficient Solar-Driven Hydrogen Production and CO2 Reduction.

Modification with conductive organic polymers consisting of a thiophane- or pyrrole-based backbone improved the cathodic photocurrent of a particulate-CuGaS2-based photoelectrode under simulated solar light. Among these polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) was the most effective in the improvements, providing a photocurrent 670 times as high as that of the bare photocathode. An incident-photon-to-current efficiency (IPCE) for water reduction to form H2 under monochromatic light irradiation (450 nm at 0 V vs RHE) was ca. 11%. The most important point is that modification of the conductive organic polymers does not involve any vacuum processes. This importance lies in the use of an electrochemically oxidative polymerization, not in a physical process such as vapor deposition of metal conductors. This is expected to be advantageous in the large-scale application of photocathodes consisting of particulate photocatalyst materials toward industrial solar-hydrogen production using photoelectrochemical-cell-based devices. Artificial photosynthesis of water splitting and CO2 reduction under simulated solar light was demonstrated by combining the PEDOT-modified CuGaS2 photocathode with a CoOx-loaded BiVO4 photoanode. Furthermore, how the cathodic photocurrent of the particulate-CuGaS2-based photocathode was drastically improved by the modification was clarified based on various characterizations and control experiments as follows: (1) selectively filling cavities between the particulate CuGaS2 photocatalysts and a conductive substrate (FTO; fluorine-doped tin oxide) with the polymers and (2) using a large driving force for carrier transportation governed by the polymers' redox potentials adjusted by functional groups.

Modification of Visible-Light-Responsive Pb2Ti2O5.4F1.2 with Metal Oxide Cocatalysts to Improve Photocatalytic O2 Evolution toward Z-Scheme Overall Water Splitting.

The development of a highly active photocatalyst for visible-light water splitting requires a high-quality semiconductor material and a cocatalyst, which promote both the migration of photogenerated charge carriers and surface redox reactions. In this work, a cocatalyst was loaded onto an oxyfluoride photocatalyst, Pb2Ti2O5.4F1.2, to improve the water oxidation activity. Among the metal oxides examined as cocatalysts, RuO2 was found to be the most suitable, and the O2 evolution activity depended on the preparation conditions for Ru/Pb2Ti2O5.4F1.2. The highest activity was obtained with RuCl3-impregnated Pb2Ti2O5.4F1.2 heated under a flow of H2 at 523 K. The H2-treated Ru/Pb2Ti2O5.4F1.2 showed an O2 evolution rate an order of magnitude higher than those for the analogues without the H2 treatment (e. g., RuO2/Pb2Ti2O5.4F1.2). Physicochemical analyses by X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and time-resolved microwave conductivity measurements indicated that the optimized photocatalyst contained partially reduced RuO2 species with a particle size of ~5 nm. These partially reduced species effectively trapped the photogenerated charge carriers and promoted the oxidation of water into O2. The optimized Ru/Pb2Ti2O5.4F1.2 could function as an O2-evolving photocatalyst in Z-scheme overall water splitting, in combination with an Ru-loaded, Rh-doped SrTiO3 photocatalyst.

Enhanced Z-schematic water splitting using a (CuGa)0.5ZnS2 H2-evolving photocatalyst with treatment in aqueous solutions.

The effectiveness of the treatment of a (CuGa)0.5ZnS2 H2-evolving photocatalyst in an aqueous Na2S solution for Z-schematic water splitting under visible light irradiation is demonstrated. The treatment suppresses undesired consumption of photogenerated holes, including photocorrosion of (CuGa)0.5ZnS2.

Water splitting and CO2 reduction over an AgSr2Ta5O15 photocatalyst developed by a valence band control strategy.

Ag+ substitution was applied to a tungsten-bronze-type metal oxide. An AgSr2Ta5O15 photocatalyst has emerged for water splitting and CO2 reduction. DFT calculation and diffuse reflection spectra revealed that the Ag d-orbital formed a new valence band, leading to a narrow band gap (3.91 eV) compared to that of NaSr2Ta5O15 (4.11 eV).

The development of an Ir and Sr-codoped KNbO3 photocatalyst as an O2-evolving photocatalyst for Z-schematic water splitting under visible light irradiation.

Ir and Sr-codoped KNbO3 has been found to be a novel photocatalyst for sacrificial O2 evolution under visible light irradiation. The Ir and Sr-codoped KNbO3 developed worked as an O2-evolving photocatalyst for Z-schematic water splitting under visible light irradiation when Ru-loaded SrTiO3 doped with Rh was employed as a H2-evolving photocatalyst.

CO2 Reduction Using Water as an Electron Donor over Heterogeneous Photocatalysts Aiming at Artificial Photosynthesis.

Photocatalytic and photoelectrochemical CO2 reduction of artificial photosynthesis is a promising chemical process to solve resource, energy, and environmental problems. An advantage of artificial photosynthesis is that solar energy is converted to chemical products using abundant water as electron and proton sources. It can be operated under ambient temperature and pressure. Especially, photocatalytic CO2 reduction employing a powdered material would be a low-cost and scalable system for practical use because of simplicity of the total system and simple mass-production of a photocatalyst material.In this Account, single particulate photocatalysts, Z-scheme photocatalysts, and photoelectrodes are introduced for artificial photosynthetic CO2 reduction. It is indispensable to use water as an electron donor (i.e., reasonable O2 evolution) but not to use a sacrificial reagent of a strong electron donor, for achievement of the artificial photosynthetic CO2 reduction accompanied by ΔG > 0. Confirmations of O2 evolution, a ratio of reacted e- to h+ estimated from obtained products, a turnover number, and a carbon source of a CO2 reduction product are discussed as the key points for evaluation of photocatalytic and photoelectrochemical CO2 reduction.Various metal oxide photocatalysts with wide band gaps have been developed for water splitting under UV light irradiation. However, these bare metal oxide photocatalysts without a cocatalyst do not show high photocatalytic CO2 reduction activity in an aqueous solution. The issue comes from lack of a reaction site for CO2 reduction and competitive reaction between water and CO2 reduction. This raises a key issue to find a cocatalyst and optimize reaction conditions defining this research field. Loading a Ag cocatalyst as a CO2 reduction site and NaHCO3 addition for a smooth supply of hydrated CO2 molecules as reactant are beneficial for efficient photocatalytic CO2 reduction. Ag/BaLa4Ti4O15 and Ag/NaTaO3:Ba reduce CO2 to CO as a main reduction reaction using water as an electron donor even in just water and an aqueous NaHCO3 solution. A Rh-Ru cocatalyst on NaTaO3:Sr gives CH4 with 10% selectivity (Faradaic efficiency) based on the number of reacted electrons in the photocatalytic CO2 reduction accompanied by O2 evolution by water oxidation.Visible-light-responsive photocatalyst systems are indispensable for efficient sunlight utilization. Z-scheme systems using CuGaS2, (CuGa)1-xZn2xS2, CuGa1-xInxS2, and SrTiO3:Rh as CO2-reducing photocatalyst, BiVO4 as O2-evolving photocatalyst, and reduced graphene oxide (RGO) and Co-complex as electron mediator or without an electron mediator are active for CO2 reduction using water as an electron donor under visible light irradiation. These metal sulfide photocatalysts have the potential to take part in Z-scheme systems for artificial photosynthetic CO2 reduction, even though their ability to extract electrons from water is insufficient.A photoelectrochemical system using a photocathode is also attractive for CO2 reduction under visible light irradiation. For example, p-type CuGaS2, (CuGa)1-xZn2xS2, Cu1-xAgxGaS2, and SrTiO3:Rh function as photocathodes for CO2 reduction under visible light irradiation. Moreover, introducing a conducting polymer as a hole transporter and surface modification with Ag and ZnS improve photoelectrochemical performance.

Photocatalytic CO2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa)0.5ZnS2 in the Presence of Basic Additives.

We demonstrated photocatalytic CO2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 prepared by a flux method as a CO2-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoOx/BiVO4) as an O2-evolving photocatalyst produced CO of a CO2 reduction product accompanied by H2 and O2 in a simple suspension system without any additives under visible light irradiation and 1 atm of CO2. When a basic salt (i.e., NaHCO3, NaOH, etc.) was added into the reactant solution (H2O + CO2), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO2 reduction using SO32- as an electron donor over the bare (CuGa)0.5ZnS2 photocatalyst. The selectivity for the CO formation of the Z-schematic CO2 reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa)0.5ZnS2 metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O2 evolution, indicating that water was an electron donor for the CO2 reduction. Moreover, the present Z-scheme system also showed activity for solar CO2 reduction using water as an electron donor. The bare (CuGa)0.5ZnS2 powder loaded on an FTO glass was also used as a photocathode for CO2 reduction under visible light irradiation. CO and H2 were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.

Development of visible-light-responsive Ir and La-codoped KTaO3 photocatalysts for water splitting.

Ir and La-codoped KTaO3 has arisen as a novel powdered photocatalyst responding to visible light up to a wavelength of 600 nm. RhCrO3-loaded KTaO3 codoped with Ir and La splits water into H2 and O2 under visible light irradiation.

Solar water splitting over Rh0.5Cr1.5O3-loaded AgTaO3 of a valence-band-controlled metal oxide photocatalyst.

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.

Activation of Water-Splitting Photocatalysts by Loading with Ultrafine Rh-Cr Mixed-Oxide Cocatalyst Nanoparticles.

The activity of many water-splitting photocatalysts could be improved by the use of RhIII -CrIII mixed oxide (Rh2-x Crx O3 ) particles as cocatalysts. Although further improvement of water-splitting activity could be achieved if the size of the Rh2-x Crx O3 particles was decreased further, it is difficult to load ultrafine (<2 nm) Rh2-x Crx O3 particles onto a photocatalyst by using conventional loading methods. In this study, a new loading method was successfully established and was used to load Rh2-x Crx O3 particles with a size of approximately 1.3 nm and a narrow size distribution onto a BaLa4 Ti4 O15 photocatalyst. The obtained photocatalyst exhibited an apparent quantum yield of 16 %, which is the highest achieved for BaLa4 Ti4 O15 to date. Thus, the developed loading technique of Rh2-x Crx O3 particles is extremely effective at improving the activity of the water-splitting photocatalyst BaLa4 Ti4 O15 . This method is expected to be extended to other advanced water-splitting photocatalysts to achieve higher quantum yields.

The Importance of the Interfacial Contact: Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)Catalytic Water Oxidation?

Optimizing interfacial contact between graphene and a semiconductor has often been proposed as essential for improving their charge interactions. Herein, we fabricated bismuth vanadate-reduced graphene oxide (BiVO4/rGO) composites with tailored interfacial contact extents and revealed their disparate behavior in photoelectrochemical (PEC) and powder suspension (PS) water oxidation systems. BiVO4/rGO with a high rGO coverage on the BiVO4 surface (BiVO4/rGO HC) exhibited an 8-fold enhancement in the PEC photocurrent density with respect to neat BiVO4 at 0 V versus Ag/AgCl, while BiVO4/rGO with a low rGO coverage (BiVO4/rGO LC) gave a lesser 3-fold enhancement. In contrast, BiVO4/rGO HC delivered a detrimental effect, while BiVO4/rGO LC exhibited an enhanced performance for oxygen evolution in the PS system. The phenomenon is attributed to changes in the hydrophobicity of the BiVO4/rGO composite in conjunction with the interfacial contact configuration. A better BiVO4/rGO interfacial contact was found to improve the charge separation efficiency and charge transfer ability of the composite material, explaining the superior PEC performance of BiVO4/rGO HC. Additionally, optimization of the interfacial contact extent was revealed to further improve the energetics of the composite material, as evidenced by a Fermi level shift to a more negative potential. However, the high hydrophobicity of BiVO4/rGO HC arising from the higher rGO reduction extent triggered poor water miscibility, reducing the surface wettability and therefore hampering the photocatalytic O2 evolution activity of the sample. The study underlines water miscibility as a governing issue in the PS system.

Z-scheme photocatalyst systems employing Rh- and Ir-doped metal oxide materials for water splitting under visible light irradiation.

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.

Cu3 MS4 (M=V, Nb, Ta) and its Solid Solutions with Sulvanite Structure for Photocatalytic and Photoelectrochemical H2 Evolution under Visible-Light Irradiation.

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.

Enhanced H2 evolution over an Ir-doped SrTiO3 photocatalyst by loading of an Ir cocatalyst using visible light up to 800 nm.

Ir cocatalyst-loaded SrTiO3:Ir has arisen as a promising photocatalyst for H2 evolution with a response to the whole range of visible light up to 800 nm. The key factor for the high activity lies in H2-reduction after loading of an Ir cocatalyst to obtain metallic and adhesive Ir-cocatalysts.

Z-Schematic and visible-light-driven CO2 reduction using H2O as an electron donor by a particulate mixture of a Ru-complex/(CuGa)1-xZn2xS2 hybrid catalyst, BiVO4 and an electron mediator.

Visible-light-driven Z-schematic CO2 reduction using H2O as an electron donor was achieved using a simple mixture of a metal-sulfide/molecular hybrid photocatalyst for CO2 reduction, a water oxidation photocatalyst and a redox-shuttle electron mediator. This is the first demonstration of a highly selective particulate CO2 reduction system accompanying O2 generation utilizing a semiconductor/molecular hybrid photocatalyst.

Decomposition of an aqueous ammonia solution as a photon energy conversion reaction using a Ru-loaded ZnS photocatalyst.

Metal sulfides are an excellent material group for highly efficient water reduction to H2. We demonstrate the uphill reaction using a single-particulate metal sulfide photocatalyst. A Ru cocatalyst-loaded ZnS photocatalyst showed an activity for the decomposition of an aqueous solution into H2 and N2 under simulated sunlight irradiation.

Solar Water Splitting Utilizing a SiC Photocathode, a BiVO4 Photoanode, and a Perovskite Solar Cell.

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.

Capturing local structure modulations of photoexcited BiVO4 by ultrafast transient XAFS.

Ultrafast excitation of photocatalytically active BiVO4 was characterized by femto- and picosecond transient X-ray absorption fine structure spectroscopy. An initial photoexcited state (≪500 fs) changed to a metastable state accompanied by a structural change with a time constant of ∼14 ps. The structural change might stabilize holes on oxygen atoms since the interaction between Bi and O increases.

Development of Ir and La-codoped BaTa2O6 photocatalysts using visible light up to 640 nm as an H2-evolving photocatalyst for Z-schematic water splitting.

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.