Electronic structure basis for enhanced overall water splitting photocatalysis with aluminum doped SrTiO3 in natural sunlight

Abstract

Overall water splitting with photocatalyst particles presents a potentially cost-effective pathway to hydrogen fuel, however, photocatalysts that can compete with the energy conversion efficiency of photovoltaic and photoelectrochemical cells are still lacking. Recently, Goto et al. reported (Joule, 2018, 2(3), 509–520) that Al-doped SrTiO₃ microparticles, followed by modification with Rh_2−yCr_yO₃ support overall water splitting with 0.4% solar to hydrogen efficiency and with 56% apparent quantum yield at 365 nm. Earlier, based on transient IR spectroscopy results, the improved activity of Al:SrTiO₃ had been attributed to the removal of Ti³⁺ deep recombination sites by the Al³⁺ ions. Here we use X-ray photoelectron spectroscopy to show that Al³⁺ incorporation not only reduces the Ti³⁺ concentration but also diminishes the n-type character of SrTiO₃ and shifts the Fermi level to more oxidizing potentials. According to DFT, the electronic structure of Al-doped SrTiO₃ depends sensitively on the relative locations of Al³⁺ and oxygen vacancies sites, with Al³⁺ ions next to the oxygen vacancies being most effective at suppressing the sub-band gap states. Reduced hole and electron trapping resulting from the elimination of Ti³⁺ states is confirmed by surface photovoltage spectroscopy and electrochemical scans. These findings not only provide an experimental basis for the superior water splitting activity of Al-doped SrTiO₃, under ultraviolet and solar irradiation, but they also suggest that aliovalent doping may be a general method to improve the solar energy conversion properties of metal oxides. Additionally, overall water splitting with a type 1 single bed particle suspension ‘baggie’ reactor under direct sunlight illumination with 0.11% solar to hydrogen efficiency is also demonstrated for the first time. This provides a proof of concept for one of the models in the 2009 US Department of Energy Technoeconomic analysis for photoelectrochemical hydrogen production.