Consequences of Annealing on Metal–Insulator–Semiconductor Water Splitting Photoelectrocatalysts

Metal–insulator–semiconductor (MIS) systems, combining light-harvesting semiconductors with electrocatalytic metals, are promising photoelectrocatalysts for efficient and stable photoelectrochemical water splitting. Despite considerable advancements, MIS systems fall significantly below the theoretical photovoltage limits. Important hurdles are the (1) presence of interfacial defects that serve as charge recombination centers and (2) insufficient charge carrier selectivity of the insulators. In this study, we investigate thermal annealing to overcome these obstacles. The impact of annealing is demonstrated by comparing the oxygen evolution reaction (OER) performance of two MIS systems that employ common insulators (Al₂O₃ vs HfO₂) in Ir/Si photoelectrocatalysts. Experimental and modeling results reveal an elaborate interplay between underlying mechanisms at different annealing conditions. We demonstrate how to quantify these mechanisms, showing that annealing can significantly improve performance by passivating interfacial defects but can also impair the insulators’ charge tunneling characteristics. Insights from this study offer direction toward approaching the maximum photovoltage in MIS photoelectrocatalysts.