Photocatalytic H2 Evolution Reaction from Aqueous Solutions over Band Structure-Controlled (AgIn)xZn2(1-x)S2 Solid Solution Photocatalysts with Visible-Light Response and Their Surface Nanostructures

Abstract

(AgIn)_xZn_2(1-x)S₂ solid solutions between ZnS photocatalyst with a wide band gap and AgInS₂ with a narrow band gap showed photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃^2- and S^2-, under visible-light irradiation (λ ≥ 420 nm) even without Pt cocatalysts. Loading of the Pt cocatalysts improved the photocatalytic activity. Pt (3 wt %)-loaded (AgIn)_0.22Zn_1.56S₂ with a 2.3 eV band gap showed the highest activity for H₂ evolution, and the apparent quantum yield at 420 nm amounted to 20%. H₂ gas evolved at a rate of 3.3 L m^-2·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 AgInS₂ to ZnS increased in the solid solutions. The photocatalytic H₂ 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 H₂ evolution sites from oxidation reaction sites.