Linking in-situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen evolving photocatalysts

Recently, record solar to hydrogen efficiencies have been demonstrated using La,Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in-situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 VRHE) and a Rh oxidation state dependent reorganisation of the electronic structure, involving the loss of a vacant Rh 4d mid gap state. This reorganisation enables Rh:SrTiO3 to be reduced by co-doping without compromising p-type character. In-situ time resolved spectroscopies show the electronic structure reorganisation induced by Rh reduction controls electron lifetime in photocatalyst sheets. In Rh:SrTiO3 , enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, resulting in long-lived photogenerated electrons even at very positive potentials (+1 VRHE), where both components of the complete device operate effectively. This understanding of role of co-dopants provides new insight into the design principles for water splitting devices based on bandgap engineered metal oxides.