Single-Molecule Colocalization of Redox Reactions on Semiconductor Photocatalysts Connects Surface Heterogeneity and Charge-Carrier Separation in Bismuth Oxybromide

The surface structure of semiconductor photocatalysts controls the efficiency of charge-carrier extraction during photocatalytic reactions. However, understanding the connection between surface heterogeneity and the locations where photogenerated charge carriers are preferentially extracted is challenging. Herein we use single-molecule fluorescence imaging to map the spatial distribution of active regions and quantify the activity for both photocatalytic oxidation and reduction reactions on individual bismuth oxybromide (BiOBr) nanoplates. Through a coordinate-based colocalization analysis, we quantify the spatial correlation between the locations where fluorogenic probe molecules are oxidized and reduced on the surface of individual nanoplates. Surprisingly, we observed two distinct photochemical behaviors for BiOBr particles prepared within the same batch, which exhibit either predominantly uncorrelated activity where electrons and holes are extracted from different sites or colocalized activity in which oxidation and reduction take place within the same nanoscale regions. By analyzing the emissive properties of the fluorogenic probes, we propose that electrons and holes colocalize at defect-deficient regions, while defects promote the selective extraction of one carrier type by trapping either electrons or holes. Although previous work has used defect engineering to enhance the activity of bismuth oxyhalides and other semiconductor photocatalysts for useful reductive half-reactions (e.g., CO2 or N-2 reduction), our results show that defect-free regions are needed to promote both oxidation and reduction in fuel-generating photocatalysts that do not rely on sacrificial reagents.

Automated summary

The study investigates the photocatalytic oxidation and reduction reactions of bismuth oxybromide nanoplates using single-molecule fluorescence imaging, revealing two distinct photochemical behaviors within the same batch of samples. The results suggest that defect-free regions promote the colocalization of electrons and holes, while defects selectively extract one carrier type by trapping either electrons or holes.