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

A Si/ZnO/Cu2O p‐n‐p heterojunction potential well with electron tunnels is fabricated for selective photoelectrochemical CO2 reduction to ethanol. This heterojunction is formed by growing n‐type ZnO nanosheets between defect‐rich p‐type Cu2O 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 Cu2O defect band. Under simulated sunlight, the Si/ZnO/Cu2O photocathode exhibits an onset potential of 0.2 V versus reversible hydrogen electrode (RHE) for aqueous photoelectrochemical CO2 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. A defect‐rich Si/ZnO/Cu2O heterojunction with a p‐n‐p potential well is developed for photoelectrochemical CO2 reduction. The photogenerated electrons are accumulated under a built‐in electric field, and can transfer to the oxygen‐vacancy‐abundant Cu2O through tunneling. This Si/ZnO/Cu2O photocathode enables an efficient photoelectrochemical CO2 reduction to ethanol, with an excellent Faradaic efficiency of over 60%.