Investigation of Cu-Oxidation and Its Roles in Unassisted CO2 Reduction and Electrochemical Oxygen Evolution

Sunlight-driven photocatalytic CO 2 reduction to CO under unassisted (unbiased) conditions was recently demonstrated using heterostructured catalysts that combine p-type GaN with plasmonic Au nanoparticles and Cu nanoparticle co-catalysts (p-GaN/Al 2 O 3 /Au/Cu) [1]. However, the exact oxidation state of Cu and the hole transfer directionality between Au and Cu remain under debate. Here, we combine computational and experimental efforts to investigate the different oxidation states of Cu under unassisted photocatalytic operating conditions. Experimentally we found that the Cu nanoparticles are primarily composed of Cu 2 O and CuO, CuCO 3 .Cu(OH) 2 species, with no detectable metallic Cu present. The calculated bulk thermodynamics confirmed the high stability of CuCO 3 .Cu(OH) 2 which is also validated by its largest contribution in the Pourbaix diagram, consistent with experimental observations. To gain insights into the origin of charge transfer, we simulate material interfaces namely, Au/CuO, Au/Cu 2 O, and Au/CuCO 3 .Cu(OH) 2 . Our theoretical analysis supports experimentally observed light-driven hole transfer from Au to Cu. More recently significant progress has been made in understanding copper oxide (CuO) behavior at oxidizing electrochemical conditions (> 1.5 V) [2]. In our study, we show CuO transforms into the OER active phase CuOOH, as evident in the bulk Pourbaix diagram presented here. Our calculations show a novel square planar, non-magnetic CuOOH phase. This discovery is supported by calculated Raman spectroscopic analysis, revealing distinct features corresponding to Cu 3+ species at 567 and 585 cm -1 , closely matching experimental peaks at 587 cm -1 . These findings provide insights into the electrochemical properties of copper oxide phases and their relevance towards electrocatalytic OER. Acknowledgments : This material is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266. Reference : [1] H. A. Atwater et al. ACS Energy Lett, 6, 1849 (2021) [2] M J. Janik et al. ACS Appl. Mater. Interfaces, 15, 27878 (2023)