Distinguishing Plasmonic Photoinduced Electron Transfer and Photothermal Enhancement Mechanisms for Photoelectrocatalytic Ethanol Oxidation on Au Nanoparticle-Decorated Photoelectrodes

Plasmonic Au nanoparticle photoelectrodes were fabricated and characterized with ultrafast transient absorption (TA) and photomodulated cyclic voltammetry (CV) to determine whether the plasmonic photoenhancement mechanism for electrooxidation of ethanol is driven by hot electron transfer or by photothermal heating. The different timescales of the ultrafast TA and CV experiments enable the characterization of the photocatalytic event from start to finish. From TA experiments, visible-light photoexcitation of plasmonic Au nanoparticles is followed by rapid picosceconds excited-state relaxation, whereas photomodulated CV experiments exhibited slow photophysics, with the ethanol electrooxidation photocurrent growing in and decaying over 100s of milliseconds. These outcomes are only consistent with a photothermal heating mechanism for ethanol electrooxidation. The slow timing of the photocurrent decay after illumination has ceased is incompatible with a photoinduced electron-transfer mechanism given the rapidity of excited-state relaxation and carrier recombination. Systematic analysis of the photomodulated CV further corroborates the dominance of the photothermal mechanism in that it shows the photocurrent response quantitatively follows the temperature response in photothermal heating experiments, with no contribution from photoinduced electron transfer being detected whatsoever. Beyond elucidating the photoenhancement mechanism for ethanol electrooxidation, the methodology presented here provides a logical framework for the photomechanistic analysis of other reactions and systems driven by light absorption of plasmonic nanoparticles.