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 approximate to 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.