Inhibition of Oxidative Activity by Photo-induced Surface Hydroxyl Groups in Bismuth Titanium Oxide Photocatalysts Detected by Single-Molecule Fluorescence Imaging

The surface structure of semiconductor photocatalysts plays a critical role in the extraction of charge carriers during photocatalytic reactions. However, the surface structures of many photocatalysts are dynamic under catalytic conditions, which makes identifying photo-induced changes and their effect on photocatalytic activity challenging. Herein, we use super-resolution, single-molecule fluorescence imaging to map the spatial distribution of catalytically active regions and quantify the changes in activity of those regions for photocatalytic oxidation in bismuth titanium oxide (Bi4Ti3O12) nanoplates. Individual nanoplates varied widely in both activity and stability, reflecting significant heterogeneity among a single synthesis batch of photocatalyst particles. We observed that while the nanoplates are more active around the perimeter of the basal facet, those perimeter regions decrease in activity at the same relative rate as the rest of the basal facet, indicating a spatially uniform degradation mechanism. Through ensemble structural characterization of the nanoplates after irradiation, we identified surface hydroxyl groups as the photo-induced feature. Our results indicate that surface hydroxyl groups introduce hole trap states that increase charge recombination at the surface of the nanoplates, leading to the observed decrease in activity. Surprisingly, we identified nanoscale features that do not begin with high activity but exhibit a slower rate of activity loss, leading to regions with greater activity relative to the rest of the basal facet at the end of imaging. Our findings highlight the importance of not only understanding how different nanoscale features influence catalytic activity but also how these features evolve over time, which dictates catalyst stability.