Electronic structure basis for enhanced overall water splitting photocatalysis with aluminum doped SrTiO3 in natural sunlightElectronic supplementary information (ESI) available: SEM images and powder X-ray diffraction patterns, irradiance data and gas evolution data, optical spectra, XPS spectra, DFT calculated band structures, and a movie showing gas evolution from a catalyst suspension in direct sunlight. See DOI: 10.1039/c9ee00310j

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 SrTiO 3 microparticles, followed by modification with Rh 2− y Cr y O 3 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:SrTiO 3 had been attributed to the removal of Ti 3+ deep recombination sites by the Al 3+ ions. Here we use X-ray photoelectron spectroscopy to show that Al 3+ incorporation not only reduces the Ti 3+ concentration but also diminishes the n-type character of SrTiO 3 and shifts the Fermi level to more oxidizing potentials. According to DFT, the electronic structure of Al-doped SrTiO 3 depends sensitively on the relative locations of Al 3+ and oxygen vacancies sites, with Al 3+ 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 Ti 3+ 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 SrTiO 3 , 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. Aliovalently Al 3+ doped strontium titanate enables overall water splitting in type 1 baggie particle suspension reactors in direct sunlight.