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Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes

Abstract

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 TiO2 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 IrOx ECs yield constant-barrier-height ‘buried’ junctions. Conversion of dense, thermally deposited NiOx on TiO2 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.

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Figure 1: Conventional single-working-electrode JV data for TiO2/EC samples and schematics of the DWE experiment.
Figure 2: Cyclic voltammograms of DWE samples and steady-state Vcat as a function of Vsem under illumination.
Figure 3: The measured steady-state open-circuit SC potential Vsem and calculated Voc across the SC/ECjunction as a function of the EC potential.
Figure 4: Current–voltage curves collected across the SC/ECinterface by sweeping Vsem and holding Vcat fixed.
Figure 5: Voltammetry of the Ni(OH)2 EC (through the porous Au WE2) while Vsem was held at 0 V versus in the dark (blue) and under 1 sun illumination (black).

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Acknowledgements

This work was supported by the DOE Basic Energy Sciences, Grant DE-FG02-12ER16323. S.W.B. acknowledges support from the DuPont Young Professor Program. The authors thank T. J. Mills for helpful discussions and acknowledge use of equipment in the CAMCOR and SuNRISE Laboratories.

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F.L. and S.W.B. designed the study, analysed the data and wrote the manuscript. F.L. performed the experiments.

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Correspondence to Shannon W. Boettcher.

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Lin, F., Boettcher, S. Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes. Nature Mater 13, 81–86 (2014). https://doi.org/10.1038/nmat3811

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