Role of TiO2coating layer on the performance of Cu2O photocathode in photoelectrochemical CO2reduction.

TiO2is usually employed as a protective layer for Cu2O in photoelectrocatalytic CO2reduction. However, the role of TiO2layer on CO2reduction activity and selectivity is still elusive. In this work, a systematic investigation is carried out to probe the impact of the deposition parameters of TiO2overlayer, including the temperature and thickness, on CO2reduction performance. Compositional and (photo-)electrochemical analysis is performed to explore the property of TiO2overlayers. Carrier behavior, including donor density and electron energy, and stability of TiO2are demonstrated to be influenced by atomic layer deposition conditions and thus play a role in controlling CO2reduction reaction. Specifically, as the thickness of the TiO2layer increases from 2 to 50 nm, the electron energy tends to be lowered accompanying the electron transfer mode from tunneling for TiO2thin layers to type II for thick TiO2, leading to a decrease in CO2reduction selectivity. With an increase of the TiO2deposition temperature, the stability increases with a loss of conductivity. Cu2O coated with 2 nm TiO2at 150 °C is proven to be the optimized candidate in this work for photoelectrochemical reduction of CO2to CO, HCOOH and CH3COOH under an applied bias of -0.4 versus RHE.