Akinobu Nakada

Photocatalytic CO2 reduction to formic acid using a Ru(II)-Re(I) supramolecular complex in an aqueous solution.

In an aqueous solution, photophysical, photochemical, and photocatalytic abilities of a Ru(II)-Re(I) binuclear complex (RuReCl), of which Ru(II) photosensitizer and Re(I) catalyst units were connected with a bridging ligand, have been investigated in details. RuReCl could photocatalyze CO2 reduction using ascorbate as an electron donor, even in an aqueous solution. The main product of the photocatalytic reaction was formic acid in the aqueous solution; this is very different in product distribution from that in a dimethylformamide (DMF) and triethanolamine (TEOA) mixed solution in which the main product was CO. A (13)CO2 labeling experiment clearly showed that formic acid was produced from CO2. The turnover number and selectivity of the formic acid production were 25 and 83%, respectively. The quantum yield of the formic acid formation was 0.2%, which was much lower, compared to that in the DMF-TEOA mixed solution. Detail studies of the photochemical electron-transfer process showed back-electron transfer from the one-electron-reduced species (OERS) of the photosensitizer unit to an oxidized ascorbate efficiently proceeded, and this should be one of the main reasons why the photocatalytic efficiency was lower in the aqueous solution. In the aqueous solution, ligand substitution of the Ru(II) photosensitizer unit proceeded during the photocatalytic reaction, which was a main deactivation process of the photocatalytic reaction. The product of the ligand substitution was a Ru(II) bisdiimine complex or complexes with ascorbate as a ligand or ligands.

Highly efficient visible-light-driven CO2 reduction to CO using a Ru(II)–Re(I) supramolecular photocatalyst in an aqueous solution

In an aqueous solution, [Ru(dmb)2–(BL)–Re(CO)3Cl]2+ (BL = bridging ligand) most efficiently photocatalyzed the reduction of CO2 to CO under visible-light irradiation using 2-(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)benzoic acid (BI(CO2H)H) as a water-soluble sacrificial reductant (ΦCO = 13%, TON = 130). Since BI(CO2H)H could efficiently produce one-electron-reduced species of [Ru(diimine)3]2+-type complexes under visible-light irradiation even in an aqueous solution, that is one of the main reasons why the photocatalytic system induced the highly efficient CO2 reduction. This result strongly indicates that BI(CO2H)H should be a useful reductant for evaluating the real abilities of various photocatalytic systems in water as well.

Solar-driven Z-scheme water splitting using tantalum/nitrogen co-doped rutile titania nanorod as an oxygen evolution photocatalyst

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2:Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O2 in the presence of reversible electron acceptors (IO3− or Fe3+), while TiO2:N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H2 and O2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2:Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (λ > 420 nm) and under AM 1.5G simulated sunlight.

Improved water oxidation under visible light on oxyhalide Bi4MO8X (M = Nb, Ta; X = Cl, Br) photocatalysts prepared using excess halogen precursors

We herein report the synthesis of a series of Bi-based oxyhalides, Bi4MO8X (M = Nb, Ta; X = Cl, Br), via solid-state reaction at different calcination temperatures and using various precursor ratios to improve their photocatalytic activity for water oxidation under visible light. The calcination temperature employed had an influence on the crystallite sizes, morphologies, specific surface areas, and surface compositions. Interestingly, when stoichiometric precursors were heated at higher temperatures, the crystallite sizes of Bi4MO8X decreased with cleavage occurring along the in-plane direction. Elemental analysis indicated that the unusual trend in Bi4MO8X can be explained by the volatile nature of halogen at higher temperatures. The volatilization of the halogen species was minimized when an excess of the halogen precursor BiOX was employed. The Bi4MO8X samples prepared with excess BiOX exhibited considerably higher photocatalytic activities for water oxidation under visible light, due to the suppressed halogen defects at which recombination of photogenerated carriers can occur. These findings offer a useful insight into the preparation of active oxyhalide photocatalysts.

Two-step synthesis of Sillén–Aurivillius type oxychlorides to enhance their photocatalytic activity for visible-light-induced water splitting

A two-step synthesis via the polymerized complex method (2PC) was developed to improve the photocatalytic activity of a Sillen–Aurivillius oxychloride Bi4TaO8Cl and related oxychlorides for O2 evolution (i.e., water oxidation) in Z-scheme water splitting under visible light. This method uses the polymerized complex reaction to prepare a precursor oxide (e.g., Bi3TaO7), which is subsequently calcined with BiOCl to yield a pure Bi4TaO8Cl phase with smaller particle sizes than those obtained via a conventional single-step solid-state reaction (1SSR). Furthermore, time-resolved microwave conductivity (TRMC) measurements revealed that the Bi4TaO8Cl sample prepared by the 2PC method at 973 K (2PC_973) achieved more than five times longer-lived charge separation than that by the 1SSR at 973 K (1SSR_973), which probably arises from lower numbers of charge-recombination centers produced in the 2PC synthesis. Thus, the synthesized Bi4TaO8Cl samples exhibited a higher rate of O2 evolution (e.g., 20 μmol h−1 for 2PC_973 vs. 4 μmol h−1 for 1SSR_973). Overall water splitting into stoichiometric H2 and O2 was demonstrated by constructing a Z-scheme photocatalytic system consisting of 2PC_973, Ru-modified SrTiO3:Rh, and an Fe3+/Fe2+ shuttle redox mediator, with an external quantum efficiency of 0.9% at 420 nm, which was much higher than that using the sample derived from the optimized 1SSR method at 1173 K (0.4%). The 2PC synthesis was successfully extended to other Sillen–Aurivillius type oxychlorides, Bi4NbO8Cl, Bi6NbWO14Cl and Sr2Bi3Ta2O11Cl, all of which exhibited superior water splitting activity compared to those prepared through the 1SSR.

Flux Synthesis of Layered Oxyhalide Bi4NbO8Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light

An oxyhalide photocatalyst Bi4NbO8Cl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi4NbO8Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi4NbO8Cl at as low as 650 °C. Thus, synthesized Bi4NbO8Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higherxa0O2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO2 or Pt cocatalyst on Bi4NbO8Cl further enhanced the activity for O2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H2-evolving photocatalyst Ru/SrTiO3:Rh along with an Fe3+/Fe2+ redox mediator, the RuO2/Bi4NbO8Cl is an excellent O2-evolving photocatalyst, exhibiting highly effective water splitting into H2 and O2 via the Z-scheme.