(AgIn)(x)Zn(2(1-x))S(2) solid solutions between ZnS photocatalyst with a wide band gap and AgInS(2) with a narrow band gap showed photocatalytic activities for H(2) evolution from aqueous solutions containing sacrificial reagents, SO(3)(2)(-) and S(2)(-), under visible-light irradiation (lambda >or= 420 nm) even without Pt cocatalysts. Loading of the Pt cocatalysts improved the photocatalytic activity. Pt (3 wt %)-loaded (AgIn)(0.22)Zn(1.56)S(2) with a 2.3 eV band gap showed the highest activity for H(2) evolution, and the apparent quantum yield at 420 nm amounted to 20%. H(2) gas evolved at a rate of 3.3 L m(-2) x h(-1) under irradiation using a solar simulator (AM 1.5). The diffuse reflection and the photoluminescence spectra of the solid solutions shifted monotonically to a long wavelength side as the ratio of AgInS(2) to ZnS increased in the solid solutions. The photocatalytic H(2) evolution depended on the compositions as well as the photophysical properties. The dependence of the photophysical and photocatalytic properties upon the composition was mainly due to the change in the band position caused by the contribution of the Ag 4d and In 5s5p orbitals to the valence and conduction bands, respectively. It was found from SEM and TEM observations that the solid solutions partially had nanostep structures on their surfaces. The Pt cocatalysts were selectively photodeposited on the edge of the surface nanosteps. It was suggested that the specific surface nanostructure was effective for the suppression of recombination between photogenerated electrons and holes and for the separation of H(2) evolution sites from oxidation reaction sites.
Photocatalysts for water splitting developed by the present authors are reviewed. A NiO (0.2 wt %)/NaTaO3:La (2%) photocatalyst with a 4.1-eV band gap showed high activity for water splitting into H2 and O2 with an apparent quantum yield of 56% at 270 nm. Many visible-light-driven photocatalysts have also been developed through band engineering by doping of metal cations, forming new valence bands with Bi6s, Sn5s, and Ag4d orbitals, and by making solid solutions between ZnS with wide band gap and narrow band gap semiconductors. Overall water splitting under visible light irradiation has been achieved by construction of a Z-scheme photocatalysis system employing the visible-light-driven photocatalysts for H2 and O2 evolution, and the Fe3+/Fe2+ redox couple as an electron relay.
The photocatalytic splitting of water to generate H 2 and O 2 has attracted attention as a clean energy system. An important feature of this system is that it does not require complicated devices: the photocatalysts are simply placed in water, irradiated with sunlight, and then hydrogen is produced by a photocatalytic reaction. This water-splitting reaction does not depend on fossil fuels, and is therefore an ideal method to produce clean hydrogen fuel. The reaction is also attractive from the viewpoint of achieving an artificial
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