Unraveling Solvent-Dependent Carrier Dynamics in WO3 Nanoparticles for Photocatalytic Water Oxidation

Understanding how the solvent environment influences charge-carrier dynamics is essential for elucidating the photocatalytic behavior of semiconductor nanoparticles. Here, we report the solvent-dependent carrier relaxation and trapping dynamics in monoclinic WO3 nanoparticles, a well-known oxygen evolution reaction (OER) photocatalyst, using femtosecond and nanosecond transient absorption spectroscopy. We show that photogenerated hot holes rapidly relax to the band edge and subsequently populate an oxygen-vacancy-associated midgap state in both water and acetonitrile. However, the subsequent evolution of these midgap states differs between water and acetonitrile. In water, molecules interact with oxygen vacancy sites and facilitate interfacial electron transfer, partially replenishing the depleted midgap states. In contrast, such an electron-transfer pathway is suppressed in acetonitrile. This difference is consistent with the observation that WO3 is active for water oxidation in water but is inactive in acetonitrile. These results highlight the important role of solvent-defect interactions in governing charge-carrier dynamics and photocatalytic activity in WO3 nanoparticles.