A femtosecond time resolved view of vibrationally assisted electron transfer across the metal/aqueous interface

TL;DR

This study uses femtosecond spectroscopy to show that vibrational excitation of specific solvent molecules can directly influence charge transfer processes at the metal/aqueous interface, revealing solvent's role in electrocatalysis.

Contribution

It provides experimental evidence linking solvent vibrational modes to charge transfer, highlighting the importance of specific solvent motions in electrochemical reactions.

Findings

Charge transfer can be induced by vibrational excitation of ferrocene aromatic CH.

The energy of the vibrational mode is comparable to solvent interaction energies.

Solvent motions may significantly influence reduction/oxidation rates in electrocatalysis.

Abstract

Understanding heterogeneous charge transfer is crucial if we are to build the best electrolyzers, fuel cells and photoelectrochemical water splitting devices that chemistry allows. Because the elementary processes involved have timescales ranging from femto- to milliseconds, direct simulation is not generally possible. Model Hamiltonian approaches thus have a crucial role in gaining mechanistic insight. Current generations of such theories describe a reactant(s) or product(s) that interacts with electrolyte via a single effective interaction. Such approaches thus obscure the extent to which particular solvent fluctuations influence charge transfer. Here we demonstrate experimentally that for a prototypical system, a ferrocene terminated alkane thiol self-assembled monolayer (SAM) on gold in contact with aqueous electrolyte, charge transfer from the Au to the ferrocene can be induced by vibrational excitation of the ferrocene aromatic CH. Intriguingly the energy of the aromatic CH vibration, 0.38 eV, is a large fraction of the effective solvent interaction strength inferred for the ferrocene/ferrocenium system in prior electrochemical studies: 0.85 eV. Our results thus demonstrate the coupling of charge transfer to a specific solvent motion and more generally imply that solvent may affect reduction/oxidation rates in electrocatalysis by coupling to a few distinct solvent motions. Identifying these motions is crucial in rationalizing trends in reactivity with change in electrolyte and thus in pursuing electrolyte engineering from first principles.