Factors affecting photocatalytic oxygen evolution over SrTiO3

Abstract

For efficient photocatalytic water splitting, it is needed to accelerate O₂ evolution process which is a rate-limiting step. We studied systematically to elucidate key factors for enhancing the O₂ evolution of Na⁺-doped SrTiO₃. By using a commercially available SrTiO₃ (STO(C)), the optimal synthetic condition was determined. As substrates for the doping, we synthesized STO by hydrothermal methods (HT) under various temperatures. All particles of HT(180°C) were near-spherical but many cubic particles were obtained for HT(200 °C). The optimal Na⁺-doping amount for both HT samples was 4.0 atom% whereas 2.0 atom% for STO(C). Their O₂ evolution rates were compared in relation to their physical properties such as the crystallite size, the specific surface area, and the chemical composition of the surface. In X-ray photoelectron spectroscopy (XPS), the ratio of lattice oxygen to adsorbed oxygen on the surface is varied depending on the substrates and the Na⁺-doping amounts but is obtained to be 7: 3 for Na(2.0)-STO(C), Na(4.0)-HT(180 °C), and Na(4.0)-HT(200 °C). The O₂ evolution rate of Na(2.0)-STO(C) is higher than that of Na(4.0)-HT(200 °C) or Na(4.0)-HT(180°C) by a factor of 1.8 or 3.2, respectively. The highest O₂ evolution rate observed with Na(2.0)-STO(C) is attributable to the largest crystalline size (75.6 nm). Furthermore, it is likely that more oxygen vacancy is formed on Na(2.0)-STO(C) because the binding energies of the Ti2p and O_lattice peaks in XPS shifted significantly to lower energies.