High-efficiency photoelectrochemical water-splitting devices require integrating electrocatalysts (ECs) onto light-absorbing semiconductors (SCs), but the energetics and charge-transfer processes at SC|EC interfaces are poorly understood. In order to study ECs on photoanodes, we fabricate model EC-coated single-crystal TiO electrodes and directly probe SC|EC interfaces in situ using a new dual-electrode photoelectrochemistry technique to independently monitor and control the potential/current at both the SC and the EC.
We discover that redox-active ion-permeable ECs such as Ni(OH)/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 ECs yield constant-barrier-height “buried” junctions. Conversion of dense, thermally deposited NiO on TiO into ion-permeable Ni(OH)/NiOOH correlated with increased apparent photovoltage and fill-factor. A new theory of adaptive EC|SC junctions is proposed and applied via numerical simulation to understand this behavior. The theory can also be used to understand catalyst-modified hydrogen-evolving photocathodes as well as catalyst-modified visible-light-absorbing oxides such as BiVO, and experiments are underway to directly test the predictions. These results provide new insight into the dynamic behavior of SC|EC interfaces that help guide the design of efficient SC|EC devices. They also illustrate a new class of adaptive semiconductor junctions.
(1) Lin, F.; Boettcher, S. W. Adaptive semiconductor-electrocatalyst junctions in water splitting photoanodes. Nat. Mater. 2013.
(2) Mills, T. J.; Boettcher, S. W. Theory and simulations of electrocatalyst-coated semiconductor electrodes for solar water splitting. . 2013.