Photoelectrochemical Proton-Coupled Electron Transfer of TiO₂ Thin Films on Silicon

TiO₂ thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO₂ thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO₂ reduction, N₂ reduction, and H₂ evolution. On both n-type Si and irradiated p-type Si, TiO₂ reduction is proton-coupled with a 1e^-:1H⁺ stoichiometry, as demonstrated by the Nernstian dependence of the Ti^4+/3+ E_1/2 on the buffer pK_a. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO₂/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO₂ films after reduction reveals restructuring with the formation of islands, rendering TiO₂ films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO₂ films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.