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

Abstract

CuO is often employed as a photocathode for H2 evolution and CO2 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 M2,3 and O L1 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.

Effect of Surface Electron Trapping and Small Polaron Formation on the Photocatalytic Efficiency of Copper(I) and Copper(II) Oxides

“…Figures D, E, and F show XPS spectra of the three samples and provide information on the oxidation states of the Cu oxide samples. While UV–vis absorption and Raman spectroscopy probe the bulk films, XPS is a surface sensitive technique that primarily samples only the top ∼2 nm of the Cu oxide samples. , This value is comparable with the probe depth of XUV-RA measurements . Hence XPS measurements are helpful to understand the nature of the surface and characterize the defect, which influences the ultrafast surface dynamics observed by XUV-RA spectroscopy.…”

“…XUV-RA instrumentation and methodology have been previously described in detail in recent publications and a concise description is given in section 1 of Supporting Information. , To briefly explain, there are two types of XUV-RA experiments that are conducted, namely, static (i.e., ground state) and transient. Static Cu M 2,3 -edge spectra are measured based on the reflected XUV intensity from Cu oxide samples, referenced to TiO 2 , which has high XUV reflectivity and no resonant absorption features overlapping with Cu M 2,3 -edge.…”

“…In this study, we utilize extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy to study ultrafast electron and hole thermalization and trapping in Cu 2 O, CuO, and Cu 2 O/CuO with well characterized defect states and phase composition. XUV-RA spectroscopy based on tabletop high harmonic generation (HHG) combines ultrafast time resolution (<100 fs) with element and oxidation state-specific measurements similar to X-ray absorption by probing the Cu M 2,3 -edge resonance between 70 and 80 eV. − Additionally, XUV-RA measured at near-grazing angle represents a surface sensitive measurement having a probe depth of 2–3 nm. , Consequently, this method provides a detailed picture of how specific surface defect states mediate carrier relaxation in Cu oxide photocatalysts. As described below, these measurements are combined with photoelectrochemical studies of the same Cu 2 O, CuO, and Cu 2 O/CuO systems, which show a strong correlation between ultrafast electron trapping at Cu vacancy defect states and the loss of photocatalytic efficiency.…”

Band-Edge Electronic Structure on Photo(electro)catalytic Performance of ABO2 (A = Cu, Ag; B = Al, Ga, In): Elucidating the Role of Valence Electron States

“…18 In a similar vein, Bandaranayake et al conducted femtosecond XANES spectroscopy on CuO, allowing them to monitor the element-specific dynamics of electrons and holes. 19 Xiong et al undertook a systematic screening of thermoelectric semiconductors, discerning their conduction types based on the energy positions of their band edges. 20 Additionally, some newly discovered band-edge electronic states, such as Urbach-edge, 21 have received widespread attention.…”

“…Slavney et al observed that defect-induced band-edge reconstruction enables the bismuth-halide double perovskite Cs 2 AgBiBr 6 to effectively absorb visible light . In a similar vein, Bandaranayake et al conducted femtosecond XANES spectroscopy on CuO, allowing them to monitor the element-specific dynamics of electrons and holes . Xiong et al undertook a systematic screening of thermoelectric semiconductors, discerning their conduction types based on the energy positions of their band edges .…”