Shunya Yoshino

We present a novel synthesis method for single-phase LaHfO2N via the ammonolysis of amorphous oxide-based precursors in the presence of active metal powders such as Mg and MgH2. Hf-containing oxynitrides and nitrides are difficult to synthesize by conventional ammonolysis because oxygen removal from the thermodynamically stable Hf–O framework and subsequent nitridation require strongly reducing conditions. The present active-metal-assisted ammonolysis process provides an effective synthetic strategy for such Hf-containing compounds, as demonstrated by the successful formation of phase-pure LaHfO2N. The addition of Mg or MgH2 significantly enhanced the nitridation process; increasing the Mg or MgH2 content led to a higher proportion of the perovskite LaHfO2N phase and a corresponding decrease in the residual oxide-based precursor phase, La2Hf2O7. In contrast, excessive addition of active metals induced over-reduction, resulting in colored impurity phases and enhanced optical absorption in the visible and near-infrared regions. Therefore, careful control of the active-metal content was essential for obtaining high-purity LaHfO2N powders. Optimal phase purity was achieved at precursor-to-Mg or MgH2 mixing ratios of approximately 1.0:2.0. The optical absorption of the resulting LaHfO2N powders decreased when oxide-based precursors preheated at lower temperatures were used, indicating that amorphous oxide-based precursors are effective for suppressing undesired coloration. After treatment with a dilute HNO3 solution, the perovskite phase remained intact, with no detectable impurities such as HfNx and no significant Mg substitution at the Hf sites. The photocatalytic activity of the LaHfO2N samples was evaluated under ultraviolet irradiation. The powders exhibited water-splitting capability, predominantly producing oxygen with only trace amounts of hydrogen.

Fe2O3 is a widely studied material as a photoanode and O2 evolution photocatalyst responsive to visible light. Although Z-scheme systems employing Fe2O3-based photocatalysts have been reported, the kind of combined H2-evolving photocatalyst is limited to C3N4. In this study, we demonstrated water splitting by the Z-scheme systems utilizing Ti-doped Fe2O3 (Fe2O3:Ti) and Rh-doped SrTiO3 (SrTiO3:Rh). Z-scheme water splitting (Z-WS) was achieved by the interparticle electron transfer from Fe2O3:Ti to SrTiO3:Rh. Ti doping was essential to achieve Z-WS. Doping Ti into Fe2O3 increased the carrier density, causing improvement of electronic conductivity and enhancement of charge separation through band bending. These factors remarkably improved the photoelectrochemical property, resulting in achievement of Z-WS. The Ru/SrTiO3:Rh-Fe2O3:Ti system was active for water splitting even without cocatalyst modification of Fe2O3:Ti. The activity was improved by the IrOx deposition, while modification with MnOx, FeOx, CoOx, and RhOx decreased activity. The impregnation method involving heat treatment at 500–600 °C and the adsorption method followed by heat treatment at 500 °C were found to be an appropriate condition for IrOx deposition through systematic characterization. The external quantum yield of the present system was determined to be 0.23% at 430 nm.

Hybrid photocathodes combining metal sulfide powders with poly-3,4-ethylenedioxythiophene (PEDOT) as an effective hole transporting polymer were prepared through a facile drop-casting method and electrochemically oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT). The PEDOT modification remarkably enhanced a photocurrent density of a powder-based (CuGa)_0.5ZnS₂ photocathode (bandgap (BG): 2.3 eV) for hydrogen evolution under visible light irradiation at 0 V vs RHE, 60-times higher compared with an unmodified photocathode. The improvement was advantageously effective when using small metal sulfide particles prepared by a flux method compared to large particles synthesized by a solid-state reaction due to improved necking and interfacial contact facilitated by PEDOT in the fine physical structure. The optimized PEDOT/(CuGa)_0.5ZnS₂-composited photocathode (PEDOT-(CuGa)_0.5ZnS₂) achieved an incident photon-to-current conversion efficiency (IPCE) of 30% at 420 nm and 0 V vs RHE for the H₂ evolution. PEDOT modification was also effective for Cu₂ZnSnS₄ (BG: 1.4 eV) and Cu₃VS₄ (BG: 1.5 eV) black metal sulfide photocathodes. Furthermore, solar water splitting proceeded in a two-electrode photoelectrochemical cell consisting of a PEDOT-(CuGa)_0.5ZnS₂ photocathode and a BiVO₄-based photoanode, even without applying an external bias. The photoelectrochemical cell gave 0.08% for nonbias and 0.21% for 0.4 V of an external bias of a solar-to-hydrogen energy conversion efficiency (STH), even though both photoelectrodes were fabricated without any vacuum process and using just two ingredients. Our findings will offer a viable strategy for developing photocathodes that unite simplicity and high performance by using powder materials for the scalable application of photoelectrochemical green H₂ production.

Highly efficient water splitting under visible light was achieved using Ir, Sb, and Al-codoped SrTiO ₃ of a single particulate photocatalyst by a solid-state reaction followed by flux treatment using SrCl ₂ and loading of a RhCrO _x cocatalyst.

Synthesis of N-doped MgTiO₃ with an ilmenite structure was examined as a new ilmenite-type oxynitride material. N-doped MgTiO₃ with Sc-substitution was successfully obtained by flux-assisted nitridation using a LiCl-KCl eutectic under pure NH₃ or 5% NH₃. The oxynitrides gave visible light absorption which MgTiO₃ did not show, and were active for sacrificial O₂ evolution using AgNO₃ under visible light irradiation.

Nanoporous graphene (NPG) materials have the pronounced electrochemical stability of the seamless graphene structures developed over the 3D space. We revisited the Raman spectra of nanoporous carbons (NPCs) synthesized using θ-/γ-Al₂O₃ templates and NPGs converted from NPCs by annealing at 1800 °C to identify the type and density of defects. We found that both the NPCs and NPGs mostly consist of single-layered graphene with a few single vacancies and Stone–Wales defects. The density of vacancy defect per hexagon in the graphene sheet is estimated to be 10⁻² for NPCs, while the annealing reduced the value to 10⁻³–10⁻⁴ for NPGs. This supports the outstanding chemical and electrochemical stability of the novel porous carbon materials.

CO₂ reduction as an artificial photosynthetic system is a promising technology to produce green energies and chemicals because it uses light energy to convert H₂O and CO₂ into valuable products such as CO, HCOOH, CH₃OH, CH₄, and preferably higher hydrocarbons. In photocatalytic reduction, water should be used as hydrogen and electron sources for CO₂ reduction. Moreover, CH₄ formation is an attractive and challenging topic because of the eight-electron-reducing product of CO₂. Herein, we report the development of a new Rh-Ru cocatalyst decorated on an alkaline earth-doped NaTaO₃ surface for the photocatalytic CO₂ reduction to form CH₄ using water as an electron donor. CH₄ was obtained by a photocatalytic "uphill" reaction of CO₂ reduction using Rh-Ru cocatalyst-loaded NaTaO₃:Sr, water, and CO₂ in an aqueous suspension system. About 10% of a selectivity (electronic efficiency) was obtained for CH₄ formation under ambient conditions accompanied with O₂ evolution of the oxidation product of H₂O.