Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis in which photoexcited charge carriers are used to perform charge-transfer reactions. However, the fate of photoexcited charges once they reach the surface, i.e., whether they recombine or are extracted to initiate useful redox reactions, is highly sensitive to the structure of the surface. Our research group is investigating how oxygen vacancies, a common type of surface defect in metal oxide semiconductors, control their photocatalytic activity. Using single-molecule, super-resolution fluorescence microscopy, we image both spatial and temporal variations in the photocatalytic activity of individual semiconductor nanocrystals such as bismuth oxybromide and tungsten oxide. To understand the chemical origins of these variations in activity we apply a combination of ensemble structural characterization, electronic-structure calculations, and the quantitative spatial correlation of multiple fluorogenic probes. Our results show that both photoexcitation and chemical modifications to the surface of semiconductor nanocrystals can be used to tune the concentration and distribution of oxygen vacancies in these materials, which has a significant impact on their resulting catalytic activity. In one system, bismuth oxybromide, a high concentration of oxygen vacancies decreases activity, while in tungsten oxide, it increases activity. Thus, to achieve high performance for a desired catalytic transformation, our results demonstrate it is necessary to tune the concentration and type of defects for the specific photocatalyst. Ultimately, photocatalysts containing a stable, intermediate concentration of oxygen vacancies may prove to be optimal for balancing the activity of both reductive and oxidative transformations in a system that generates chemical fuels from sunlight.
Bio: Bryce Sadtler graduated from Purdue University in 2002 with a B.S. degree in Chemistry. He conducted his graduate studies at the University of California, Berkeley under the guidance of Paul Alivisatos and received a Ph.D. in Physical Chemistry in 2009. He was then a Beckman Institute Postdoctoral Fellow at the California Institute of Technology, where he worked with Nathan Lewis and Harry Atwater. Bryce joined the Department of Chemistry at Washington University in St. Louis in 2014. His research interests include solid-state chemistry and light–matter interactions in nanoscale materials for applications in solar energy conversion and catalysis. He has received an NSF Career award (2018), an ACS PRF Doctoral New Investigator Award (2017), and was named an Emerging Investigator by the Journal of Materials Chemistry (2017). Bryce was promoted to an Associate Professor at Washington University in July 2022.