Highly Selective Photocatalytic CO$_{2}$ Reduction on TiO$_{2}$-Passivated InP in Ionic Liquids

Lowering the overpotential required to drive the photocatalytic reduction of CO$_{2}$ to useful products is a very important challenge. In this article, we use an ionic liquid [EMIM]BF$_{4}$ co-catalyst to improve the selectivity and efficiency of CO$_{2}$ reduction to CO on TiO$_{2}$-passivated InP. Here, the InP surface is passivated using a thin film of TiO$_{2}$ deposited by atomic layer deposition (ALD), which improves the photoconversion efficiency by as much as 17X compared to bare InP. We believe there are three mechanisms of enhancement in this photocatalytic system. Firstly, the TiO$_{2}$ deposited by ALD is $n$-type due to oxygen vacancies, and forms a \textit{pn}-junction with the underlying $p$-type InP photocathode, resulting in a built-in potential which reduces electron-hole recombination through charge separation. Secondly, the Ti$^{3+}$ active sites formed on the TiO$_{2}$ surface lower the energy of the CO$_{2}^{-}$ intermediate through the formation of an intermediate complex. Thirdly, the [EMIM]$^{+}$ ions in solution also stabilize the CO$_{2}^{-}$ intermediate, further lowering the energy barrier of this reaction. Here, we use a non-aqueous ionic liquid solution, which prohibits hydrogen formation and enables highly selective CO$_{2}$ reduction with a Faradaic efficiency of 46{\%}. This general approach of passivating narrower band gap semiconductors with TiO$_{2}$ and utilizing a non-aqueous ionic liquid solution allows a wide range of materials to be considered for photocatalysis, enabling more efficient photocatalysts to be developed.