Balancing photovoltage generation and charge-transfer enhancement for catalyst-decorated photoelectrochemical water splitting: A case study of the hematite/MnOx combination

To improve semiconductor-based water-splitting performance, one popular approach is to modify the electrode surface with catalysts. The strategy is to increase charge-transfer kinetics and, hence, to reduce overpotential requirements. Relatively underwhelming attention has been paid to how such surface treatments influence the nature of the semiconductor/solution interface so as to reduce photovoltage generated by the photoelectrode. Using atomic layer deposition-grown (ALD) MnOx on hematite (α-Fe2O3) as an example, here, we show that increased charge-transfer kinetics does not necessarily lead to improved overall performance. Compared to bare hematite, MnOx-decorated photoelectrode exhibits significant anodic on-set potential shift. The phenomenon is understood as a substantial reduction in photovoltage generation by hematite, and the origin is identified as Fermi-level pinning effect due to MnOx introduction. This work sheds light on the importance of maintaining band-edge pinning for semiconductor-based photoelectrochemical reactions.