Photocatalytic hydrogen production from water on Pt-free SrTiO₃ in alkali hydroxide solutions

Abstract

Semiconductor surfaces can act as photosensitizers for small molecules which on their own do not absorb solar photons. Such sensitizers can be used for the photoassisted decomposition of water to hydrogen and oxygen. Most previous work on this process has concentrated on photoelectrochemical ceils using oxide semiconductor photoanodes for oxygen evolution and platinum cathodes for the evolution of hydrogen^1–3. We report here the sustained photogeneration of hydrogen on SrTiO₃ single crystal surfaces by a different mechanism, when no platinum coating or platinum counterelectrode is present. Hydrogen yields far exceed the monolayer amounts (∼10¹⁵ molecules cm⁻²) expected from a surface stoichiometric reaction. The reaction takes place on illumination of the crystal with band gap radiation (hv ≧ 3.2eV) in aqueous alkaline solution. The rate of hydrogen evolution increased with increasing hydroxide concentration in the solution. We also report the use of water vapour as a reactant through saturation of a layer of a basic deliquescent compound, such as NaOH, which coated the crystal. The photocatalytic generation of hydrogen on the illuminated surface of metal-free SrTiO₃ crystals shows that strongly reductive as well as oxidative reactions can be carried out and sustained on illuminated oxide semiconductors. A wide range of photocatalytic reactions may possibly be carried out in conditions not amenable to the operation of photoelectrochemical cells by using this new mechanism.