Electronic Structure and Ultrafast Electron Dynamics in CuO Photocatalysts Probed by Surface Sensitive Femtosecond X-ray Absorption Near-Edge Structure Spectroscopy.

CuO is often employed as a photocathode for H evolution and CO reduction, but observed efficiency is still far below the theoretical limit. To bridge the gap requires understanding the CuO electronic structure; however, computational efforts lack consensus on the orbital character of the photoexcited electron. In this study, we measure the femtosecond XANES spectra of CuO at the Cu M and O L edges to track the element-specific dynamics of electrons and holes. Results show that photoexcitation represents an O 2p to Cu 4s charge transfer state indicating the conduction band electron has primarily Cu 4s character. We also observe ultrafast mixing of Cu 3d and 4s conduction band states mediated by coherent phonons, with Cu 3d character of the photoelectron reaching a maximum of 16%. This is the first observation of the photoexcited redox state in CuO, and results provide a benchmark for theory where electronic structure modeling still relies heavily on model-dependent parametrization.