Electronic Band Structures and Photochemical Properties of La−Ga-based Oxysulfides

The oxysulfides LaGaS₂O and La₃GaS₅O are prepared and characterized through measurements of photo-electrochemical properties and photocatalytic activity. The band gap energies of LaGaS₂O and La₃GaS₅O are estimated from diffuse reflectance spectra to be 3.0 and 2.3 eV, respectively. The oxide LaGaO₃ exhibits a band gap of 4.4 eV. The band gap thus appears to narrow with increasing sulfur content. La₃GaS₅O prepared on a transparent conducting electrode is shown to function as an n-type semiconductor in the presence of 0.01 M Na₂S−Na₂SO₃ and 0.1 M Na₂SO₄ under visible irradiation (λ ≥ 420 nm), whereas the LaGaS₂O electrode only produces anodic photocurrent under ultraviolet light (λ ≥ 290 nm). LaGaS₂O and La₃GaS₅O have similar conduction band bottoms, although La₃GaS₅O exhibits a lower valence band top. These oxysulfides are demonstrated to evolve H₂ or O₂ from water containing hole scavengers (Na₂S−Na₂SO₃) or an electron scavenger (Ag⁺), in reasonable agreement with the estimated band gap position.