(62i) Understanding the Role of Cu Oxidation State in Unassited CO2 Reduction and Electrochemical Oxygen Evolution

Sunlight-driven photocatalytic CO₂ 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/Al2O3/Au/Cu) [1]. However, the exact oxidation state of Cu and the direction of hole transfer between Au and Cu remain under debate. Here, we combined computational and experimental efforts to investigate the different oxidation states of Cu under unassisted photocatalytic operating conditions. Our experimental collaborators found that the Cu particles in the system are primarily composed of Cu₂O and CuO, CuCO₃.Cu(OH)₂ species, with no detectable metallic Cu present. The calculated bulk Pourbaix thermodynamics confirmed the high stability of CuCO₃.Cu(OH)₂ and validates its largest contribution consistent with the experimental observations. Further, we simulate materials interfaces Au/CuO, Au/Cu₂O, and Au/CuCO₃.Cu(OH)₂ to gain insights into the atomistic properties, such as charge transfer and band bending. Our theoretical calculations suggest light-driven hole transfer from Au to Cu, consistent with experimental results.

In addition to photocatalysis, significant progress has been made in understanding copper oxide (CuO) behavior at positive potentials, approximately 1.7 V, under electrochemical conditions [2]. We found that CuO transforms into the OER active phase CuOOH at this potential, as suggested by the updated bulk Pourbaix diagram. Our calculations show a square planar, non-magnetic CuOOH phase. This discovery is supported by simulated Raman spectroscopy, revealing distinct features corresponding to Cu³⁺ species at 567 and 585 cm^-1, closely matching experimental data at 587 cm^-1. These findings deepen our insight into the electrochemical properties of copper oxide phases and their relevance in electrocatalysis.

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)