The role of ultra-thin MnOx co-catalysts on the photoelectrochemical properties of BiVO4 photoanodes

Abstract

Metal oxide semiconductors are promising as photoanodes for solar water splitting, but they typically suffer from poor charge transfer properties due to the slow surface reaction kinetics for oxygen evolution. To overcome this, their surfaces are usually modified by depositing earth-abundant, efficient, and inexpensive water oxidation co-catalysts. While this effort has been successful in enhancing the photoelectrochemical performance, a true understanding of the nature of the improvement is still under discussion. This is due to the fact that the co-catalyst can have multiple functionalities, e.g., accelerating charge transfer, passivating surface states, or modifying band bending. Disentangling these factors is challenging, but necessary to obtain a full understanding of the enhancement mechanism and better design the semiconductor/co-catalyst interface. In this study, we investigate the role of atomic layer deposited (ALD) MnO_x co-catalysts and their thickness in the photoelectrochemical performance of BiVO₄ photoanodes. Modified MnO_x/BiVO₄ samples with an optimum thickness of ∼4 nm show higher photocurrent (a factor of >3) as well as lower onset potential (by ∼100 mV) compared to the bare BiVO₄. We combine spectroscopic and photoelectrochemical measurements to unravel the different roles of MnO_x and explain the photocurrent trend as a function of the thickness of MnO_x. X-ray photoelectron spectroscopy (XPS) studies reveal that the surface band bending of BiVO₄ is modified after the addition of MnO_x, therefore reducing surface recombination. At the same time, increasing the thickness of MnO_x beyond the optimal 4 nm provides shunting pathways, as shown by energy dispersive X-ray scanning transmission electron microscopy (EDX-STEM) and redox electrochemistry. This cancels out the band bending effect, which explains the observed photocurrent trend. Therewith, this study provides additional insights into the understanding of the charge transfer processes occurring at the semiconductor–catalyst interface.