Mechanistic Understanding of Plasmon-Enhanced Electrochemistry

Noble metals are prime candidates for electrocatalysts for electrochemical energy conversions. In a separate realm, nanostructured noble metals are found to exhibit photocatalytic activity under plasmonic excitation with no electrical bias. Here, we explore the convergence of the electrocatalytic and photocatalytic activity of nanostructured noble metals in electrochemical reactions performed under plasmonic excitation. We use as a model the hydrogen evolution reaction (HER) on Au nanoparticle (NP) electrocatalysts. We observe that plasmonic excitation of the Au NP electrocatalyst enhances the HER activity. We trace the mechanistic origin of this plasmonic enhancement by using a combination of electrochemical techniques. Specifically, we resolve the contribution of hot carriers from that of heat generated by plasmonic excitation in the observed enhancement. Although photothermal heating plays a minimal role, hot electrons generated by plasmon excitation result in cathodic photocharging of the Au NP electrode, which enhances interfacial electron-transfer kinetics and manifests in a reduced overpotential. The higher the light intensity, the greater is the decrease in the overpotential, that is, the activation barrier. The photopotential resulting from plasmonic excitation exclusively enhances the Faradaic reaction without an increase in the double-layer charging. In summary, this article provides a unified model of the influence of plasmonic excitation on an electrochemical reaction.