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

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