Time-Resolved X-Ray Absorption Spectroscopy: Visualizing the Time Evolution of Photophysics and Photochemistry in Photocatalytic Solar Energy Conversion

The time-resolved X-ray absorption technique with femto-microsecond timescale has had a huge impact on the mechanistic understanding of photochemical reactions due to the powerful ability to probe, in real time, the electronic and geometric structures within homogeneous and heterogeneous photocatalytic systems. The time-resolved X-ray absorption technique can "snapshot" the charge transfer and dynamic electronic and geometric structures similarly to a camera, showing the whole process of solar energy conversion clearly in the form of a "molecular movie." Herein, the aim is to provide a systematic overview of the time-resolved X-ray absorption technique and its applications in the study of photocatalytic systems. First, the dynamic charge kinetics and structural changes for the excited state of light-harvesting units are specifically summarized. Then the charge transfer behavior between light-harvesting units and catalytic sites is interpreted. After that, the geometric and electronic changes of catalytic units during the photochemical process, as well as the complete reaction path and key information for rate-limiting steps, are elaborated. Capturing the dynamic electronic and geometric changes in the photophysical and photochemical process on a time scale gives progressive guidance for designing advanced systems for solar energy conversion.