Speaker
Description
Photo-electrocatalytic water splitting is a promising approach for meeting the global demand of sustainable hydrogen production, but its efficiency is often limited by charge carrier dynamics in the photoanode materials. Bismuth vanadate (BiVO$_4$) is a leading candidate for this application, yet its performance is strongly affected by the trapping of photogenerated charges, which can hinder transport. While trapping of charges in separate polarons has been extensively studied, the role of self-trapped excitons (STEs) as an alternative, potentially dominant trapping pathway remains far less understood. While it has been shown experimentally and computationally that STEs can form in BiVO$_4$, it is not clear whether they remain stable or dissociate under operating conditions. This gap limits our ability to rationally design materials with improved charge transport.
In our study, we use hybrid density functional theory with the nudged elastic band (NEB) method to quantify the stability of STEs, as well as the activation barriers for their hopping, dissociation and transformation in BiVO$_4$, enabling estimates of the relevant kinetic timescales. Our results reveal distinct behaviours on timescales ranging from sub-picoseconds to nanoseconds for two STE types: a separated, more mobile state and a compact, more stable one with higher barriers. We also study an alternative charge trapping mechanism involving binding pairs of holes within O – O dimers, resulting in formation timescales on the order of microseconds. These findings provide fundamental insights into the kinetic stability and mobility of trapped charge states in BiVO$_4$, providing a basis for interpreting charge trapping dynamics under operating conditions. We also quantify activation barriers for O – O dimer formation and for electron detachment from a dimer, providing an alternative multi-polaron binding pathway with distinct kinetics.