Tuning Hole Accumulation of Metal Oxides Promotes the Oxygen Evolution Rate
Although much is known about the increased rate of a catalyst required for the oxygen evolution reaction (OER), the hole accumulation behaviors of the catalyst on both photo- and dark electrodes remain uncovered. We present a rational tuning of the hole-accumulation and hole-transfer ability of a catalyst by in situ deposition of IrOx on NiOx islands on model Si electrodes and we demonstrate a distinct hole accumulation behavior during the OER through electrochemical methods, in situ UV–vis and Raman spectroelectrochemistry. We find that the NiOx islands with <10% coverage on an n++-Si dark electrode demonstrate a 6-fold higher capacity of hole accumulation relative to that on an n-Si photoanode under illumination. A further in situ targeted deposition of soluble hexahydroxyiridate on the NiOx leads to the formation of NiOx/IrOx junctions, where the IrOx further depletes the extracted holes by NiOx from the n-Si photoanode decreasing the overpotential by ∼260 mV at 20 mA cm–2, yet the IrOx promotes the hole accumulation at the NiOx/IrOx on the dark electrodes presenting a similar decrease of the overpotential. In contrast to the dark-holes, an extremely lower charging level of photogenerated holes of the same catalyst at the identical current density reasoned a seemingly higher reactivity to drive the OER, and reasonably demonstrated a stronger oxidation/corrosion to the Si electrode.
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