UC Davis

Aliovalently Al ³⁺ doped strontium titanate enables overall water splitting in type 1 baggie particle suspension reactors in direct sunlight.

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.