Semiconductor structures (for example, films, wires, particles) used in photoelectrochemical devices are often decorated with nanoparticles that catalyse fuel-forming reactions, including water oxidation, hydrogen evolution or carbon-dioxide reduction. For high performance, the catalyst nanoparticles must form charge-carrier-selective contacts with the underlying light-absorbing semiconductor, facilitating either hole or electron transfer while inhibiting collection of the opposite carrier. Despite the key role played by such selective contacts in photoelectrochemical energy conversion and storage, the underlying nanoscale interfaces are poorly understood because direct measurement of their properties is challenging, especially under operating conditions. Using an n-Si/Ni photoanode model system and potential-sensing atomic force microscopy, we measure interfacial electron-transfer processes and map the photovoltage generated during photoelectrochemical oxygen evolution at nanoscopic semiconductor/catalyst interfaces. We discover interfaces where the selectivity of low-Schottky-barrier n-Si/Ni contacts for holes is enhanced via a nanoscale size-dependent pinch-off effect produced when surrounding high-barrier regions develop during device operation. These results thus demonstrate (1) the ability to make nanoscale operando measurements of contact properties under practical photoelectrochemical conditions and (2) a design principle to control the flow of electrons and holes across semiconductor/catalyst junctions that is broadly relevant to different photoelectrochemical devices.
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The data that support the findings of this study are available from the corresponding author upon reasonable request.
Code used for the pinch-off simulations can be downloaded as a Supplementary Information file.
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This work was funded by the Department of Energy, Basic Energy Sciences (award no. DE-SC0014279). F.A.L.L. acknowledges support from a NSF graduate research fellowship (no. 1309047). S.Z.O. acknowledges support from a research fellowship of the German Research Foundation (Deutsche Forschungsgemeinschaft, under project no. 408246589 (OE 710/1-1)). The atomic force microscope was purchased using funds provided by the NSF Major Research Instrumentation Program (grant no. DMR-1532225). We acknowledge use of shared instrumentation in the Center for Advanced Materials Characterization in Oregon and Rapid Materials Prototyping facilities, which are supported by grants from the M.J. Murdock Charitable Trust, the W.M. Keck Foundation, Oregon Nanoscience and Microtechnologies Institute and the National Science Foundation.
The authors declare no competing interests.
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