Hematite is an earth abundant semiconductor showing promise as a photocatalyst for water oxidation. Unfortunately, the efficiency of this material is hindered by small polaron formation, which is known to limit charge transport via ultrafast carrier self-trapping. While much work has been done to study polaron formation and hopping dynamics in bulk hematite, a complete description of surface dynamics in this material is still lacking. Theoretical predictions indicate that the kinetics and thermodynamics of polaron formation are significantly different at a surface compared to bulk. However, testing these predictions requires the ability to probe ultrafast electron dynamics at surfaces with chemical state resolution. Toward this goal, we have recently developed extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. This method extends the benefits of x-ray absorption spectroscopy, including element, oxidation, and spin state specificity, with surface sensitivity and ultrafast time resolution, having a probe depth less than 3 nm and a time resolution faster than 100 fs. Using XUV-RA spectroscopy we provide systematic comparison of surface and bulk polaron formation in hematite. We find that the rate of surface polaron formation (250 ± 40 fs) is approximately three times slower than formation in bulk (90 ± 5 fs). These results are interpreted in the framework of a Marcus model for polaron formation and transport. Additionally, we demonstrate that these dynamics can be systematically tuned by surface molecular functionalization offering the possibility for design of photocatalytic interfaces with enhanced carrier transport.

Controlling polaron formation at hematite surfaces probed by XUV reflection-absorption spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.