Photoelectrocatalytic Water Splitting by Conformal Copper-Oxide on Hematite Nanostructures: Dependence on Surface-States

Understanding the pivotal role of surface co-catalysts is paramount in the strategic design of forthcoming photoelectrodes. However, the nuanced impacts of co-catalysts remain elusive, particularly in promoting the water oxidation reaction on hematite, especially in connection to surface states denoted as S1 (higher energy) and S2 (lower energy). For this purpose, we tailored two isomorphous hematite nanoarrays with a thin layer of amorphous copper oxide (CuOx), composed of a blend of Cu(I) and Cu(II) species, via a soft electrodeposition technique. Remarkably, we discovered that in pristine hematite (α-Fe₂O₃), the S2 state played a pivotal role in activating the CuOx ad-layer for water oxidation. At lower external biases (approximately 0.9–1.1 V_RHE), CuOx served as charge reservoir in equilibrium with the S2 state. Notably, beyond 1.1 V_RHE, where the high-energy holes of the S1 state became available, CuOx was activated indirectly through the equilibrium with the S2 state, and a pronounced enhancement in photocurrent was observed. Conversely, in the case of Ti-doped hematite (Ti : α-Fe₂O₃) devoid of the S2 state, the presence of CuOx resulted in a decline in charge transfer efficiency. Instead of facilitating water oxidation, CuOx adversely affected the S1 surface sites and reduced the charge carrier density in Ti : α-Fe₂O₃.