Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Charge transfer across solid/water interfaces is ubiquitous in such diverse fields as (photo)electrochemistry, microbial ecology and geophysics. Because gold interacts relatively weakly with many solutes, the planar gold/water system is a particularly attractive system for elucidation of the elementary processes underlying metal/water electron transfer. While water electrolysis on gold electrodes was first demonstrated more than 225 years ago, the mechanism of electron transfer from gold to water is still unclear. From an experimental point of view part of the challenge has been a lack of methods that allow characterization of the transient intermediates with femtosecond-picosecond time resolution. Here we report two novel experimental approachs that overcome this challenge. In both we initiate electron transfer from gold to water (i.e. the hydrogen evolution reaction) using an intense, femtosecond, UV pulse to promote an electron from gold's fermi level into water's conduction band. In one experiment we probe the fate of this electron by its perturbation of the gold electrode’s open circuit potential following a second fs laser pulse at wavelengths ranging from 0.65 - 4 μm. In the second we probe the fate of the interfacial solvated electron optically, using the surface specific, laser-based technique sum frequency spectroscopy. The results of this combined approach suggest that (i) at the gold/water interface the delocalized, conduction band, solvated electron has a lifetime of 150-250 fs depending on surface potential (ii) conduction band electrons relax to form localized solvated electrons the great majority of which have a potential dependent lifetime of 1-18 ps while a small fraction live for much longer, > 40 ps, and presumably drive chemistry. These results furnish a potential-dependent, mechanistic picture of the transfer of electrons from gold into liquid water, the first step in hydrogen evolution, and its dependence on pH.
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