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

Abstract

TiO 2 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 TiO 2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO 2 reduction, N 2 reduction, and H 2 evolution. On both n-type Si and irradiated p-type Si, TiO 2 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. TiO 2 /Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO 2 films after reduction reveals restructuring with the formation of islands, rendering TiO 2 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 TiO 2 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 protoncoupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive chargecarrying mediators.

Hydrogen Atom Binding Energy of Structurally Well-Defined Cerium Oxide Nodes at the Metal–Organic Framework–Liquid Interfaces

“…The half-wave potential ( E 1/2 ) derived from the CVs scaled linearly with respect to the electrolyte pH with a slope of 65 ± 9 mV/pH. The close-to-Nernstian slope of 59 mV/pH suggests a 1H + /1e – stoichiometry (Figure B). ,,,− E values measured at the same pH with different buffers were mostly within ±50 mV difference, suggesting that the Ce 4+/3+ redox is independent of the proton source. Combining these results with the previous thermochemical measurements of bulk ceria, ,, we ascribe the Ce 4+/3+ redox reaction to a 1H + /1e – PCET reaction (eq ).…”

Lithium-coupled electron transfer reactions of nano-confined WOx within Zr-based metal–organic framework

“…To this day, reports on LCET reaction thermodynamics remain rare in the literature, particularly when compared to those related to PCET reactions. Free energies of PCET reactions have long been examined both experimentally and computationally on molecular species, metals, and even binary and ternary material surfaces ( Nørskov et al, 2005 ; Strmcnik et al, 2008 ; Seh et al, 2017 ; Wise et al, 2020 ; Agarwal et al, 2021 ; Fertig et al, 2021 ; Noh and Mayer, 2022 ; Fortunato et al, 2023 ; Nedzbala et al, 2024 ). A few reports related to LCET of cobalt, nickel, vanadium, tungsten, and many other oxides suggest that the reaction free energy (ΔG LCET ) is dependent on synthesis, structure, morphologies, chemical history, and many other parameters ( Okubo et al, 2007 ; Kerisit et al, 2009 ; Pan et al, 2010 ; Wang et al, 2016 ; Ng et al, 2020 ).…”