Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
Recent advances in photoelectrochemical water splitting have led to record solar-to-hydrogen conversion efficiencies. Those studies were able to leverage the insights from the photovoltaics community regarding efficient light absorption and charge carrier extraction and the electrocatalysis community regarding the right choice of oxygen and hydrogen evolution catalysts. However, the before mentioned research fields are unable to answer the most important remaining scientific challenge in the field: long-term stability of a photoelectode in a corrosive electrolyte environment.
To prevent corrosion, semiconductor surfaces are often coated conformably with a thermodynamically stable oxide film. This approach allows for relative simple fabrication, however it requires the film to not only provide chemical protection but also electrical transport between semiconductor and electrocatalyst. This creates a fundamental trade-off: thick or non-conductive films can prevent corrosion but such films do not sufficiently conduct current. The requirement for electrical transport through a protective film furthermore increases the chances of corrosion in films which are thermodynamically predicted to be stable (according to their Pourbaix diagram), but show a deviating behaviour at finite currents, e.g. due to kinetically active defect states.
Here we investigate the use of nanoscale point contacts in water-splitting devices on the nanoscale by using electrical and electrochemical atomic force microscopy techniques. We use micro- and nanofabrication techniques to surround the contacts by non-conductive oxide layers for robust chemical protection while simultaneously guaranteeing sufficient electrical transport, a combination of properties that seems out of reach for conventional geometries. Furthermore, we show that nanoscale contacts are able to generate large photovoltages due to the “pinch-off”/ surface-gate effect, which allows the control over the charge carrier-selectivity by utilizing adjacent materials with varying work functions/ redox potentials.
This work is being funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Projektnummer 408246589, and the US Department of Energy, Basic Energy Sciences, award number DE-SC0014279.
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