Novel stannite-type complex sulfide photocatalysts A₂ ^I -Zn-A^IV-S₄ (A^I = Cu and Ag; A ^IV = Sn and Ge) for hydrogen evolution under visible-light irradiation

Photophysical properties and photocatalytic activities for H₂ evolution of the series of stannitetype complex sulfides A ₂ ^I-Zn-A^IV-S₄ (A^I = Cu and Ag; A ^IV = Sn and Ge) were studied. Diffuse reflection spectra and the plane-wave-based density functional theory (DFT) calculation suggested that the conduction bands were made up of Ge4s4p or Sn5s5p with S3p orbitals, while the valence bands consisted of Cu3d and Ag4d with S3p orbitals. The constituting elements of A^I and A^IV affected the photophysical and photocatalytic properties. Ru cocatalyst-loaded Cu₂ZnGeS₄, Ag₂ZnGeS₄, and Ag2ZnSnS4 showed high activities for photocatalyticH₂ evolution from an aqueous Na₂SK ₂SO₃ solution under visible-light irradiation. Combination of Cu and Ag made it possible that the CuAgZnSnS₄ photocatalyst with a narrow band gap (1.4 eV) utilized a wide range of visible light for the H₂ evolution.