Photocatalytic Reduction of CO2 to CO on TiO2-Passivated InP at High Underpotentials in Ionic Liquids

Wednesday, October 14, 2015: 11:30
104-B (Phoenix Convention Center)
G. Zeng (University of Southern California) and S. B. Cronin (University of Southern California)
Lowering the overpotential required to drive the photocatalytic reduction of CO2 to useful products is a very important challenge. In this article, we use an ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) co-catalyst to improve the selectivity and efficiency of CO2 reduction to CO on TiO2-passivated InP. Here, the InP surface is passivated using a thin film of TiO2 deposited by atomic layer deposition (ALD), which improves the photoconversion efficiency by as much as 18X compared to bare InP (at -1.57V vs. NHE). We use a non-aqueous ionic liquid solution, which enables CO2 reduction with a Faradaic efficiency of 89% at an overpotential of 0.02V. More importantly, we observe CO2 reduction to CO at an underpotential of 0.68V with a Faradaic efficiency of 39%. We believe there are three mechanisms of enhancement in this photocatalytic system. Firstly, the TiO2 deposited by ALD is n-type due to oxygen vacancies and forms an np-junction with the underlying p-type InP photocathode. Secondly, the catalytically active sites on the TiO2 lower the reduction reaction energy barrier. Thirdly, the [EMIM]+ ions in solution further lowering the energy of CO2 intermediate, most likely by complexation