Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes

High-efficiency photoelectrochemical water-splitting devices require the integration of electrocatalysts (ECs) with light-absorbing semiconductors (SCs), but the energetics and charge-transfer processes at SC/EC interfaces are poorly understood. We fabricate model EC-coated single-crystal TiO 2 electrodes and directly probe SC/EC interfaces in situ using two working electrodes to independently monitor and control the potential and current at both the SC and the EC. We discover that redox-active ion-permeable ECs such as Ni(OH) 2 or NiOOH yield ‘adaptive’ SC/EC junctions where the effective Schottky barrier height changes in situ with the oxidation level of the EC. In contrast, dense, ion-impermeable IrO x ECs yield constant-barrier-height ‘buried’ junctions. Conversion of dense, thermally deposited NiO x on TiO 2 into ion-permeable Ni(OH) 2 or NiOOH correlated with increased apparent photovoltage and fill factor. These results provide new insight into the dynamic behaviour of SC/EC interfaces to guide the design of efficient SC/EC devices. They also illustrate a new class of adaptive semiconductor junctions. Photoelectrochemical water-splitting devices require integrating electrocatalysts with light-absorbing semiconductors, but understanding charge-transfer processes at interfaces has proved difficult. Ion-permeable electrocatalysts deposited onto TiO 2 photoelectrodes now result in adaptive semiconductor/electrocatalyst junctions where both the effective interface barrier height and the photovoltage output change depending on the oxidation state of the electrocatalyst.