Evolution of Surface Oxidation on Ta₃N₅ as Probed by a Photoelectrochemical Method

In this work, we present an in situ method to probe the evolution of photoelectrochemically driven surface oxidation on photoanodes during active operation in aqueous solutions. A standard solution of K₄Fe(CN)₆-KPi was utilized to benchmark the photocurrent and assess progressive surface oxidation on Ta₃N₅ in various oxidizing solutions. In this manner, a proportional increase in the surface oxygen concentration was detected with respect to oxidation time and further correlated with a continuous decline in the photocurrent. To discern how surface oxidation alters the photocurrent, we experimentally and theoretically explored its impact on the surface carrier recombination and the interfacial hole transfer rates. Our results indicate that the sluggish photocurrent demonstrated by oxidized Ta₃N₅ arises because of changes in both rates. In particular, the results suggest that the N-O replacement present on the Ta₃N₅ surface primarily increases the carrier recombination rate near the surface and to a lesser degree reduces the interfacial hole transfer rate. More generally, this methodology is expected to further our understanding of surface oxidation atop other nonoxide semiconductor photoelectrodes and its impact on their operation.