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.