Defect‐Assisted Electron Tunneling for Photoelectrochemical CO2 Reduction to Ethanol at Low Overpotentials

Abstract A Si/ZnO/Cu 2 O p‐n‐p heterojunction potential well with electron tunnels is fabricated for selective photoelectrochemical CO 2 reduction to ethanol. This heterojunction is formed by growing n‐type ZnO nanosheets between defect‐rich p‐type Cu 2 O nanoparticles and nanoporous p‐type Si. Due to the existence of this potential well, the photogenerated electrons are trapped and accumulate inside n‐ZnO at low biases with the assistance of a ≈ 0.6 V built‐in potential, and escape into the Cu 2 O defect band. Under simulated sunlight, the Si/ZnO/Cu 2 O photocathode exhibits an onset potential of 0.2 V versus reversible hydrogen electrode (RHE) for aqueous photoelectrochemical CO 2 reduction. Due to the confined electron energy in tunneling, the product selectivity is substantially tuned from CO or formate to ethanol, with an excellent Faradaic efficiency of ethanol over 60% at 0 V versus RHE.