Hot Carriers versus Thermal Effects: Resolving the Enhancement Mechanisms for Plasmon-Mediated Photoelectrochemical Reactions

Nonradiative decay of localized surface plasmons results in the production of hot charge carriers and the generation of heat, both of which can affect the efficiency of plasmon-mediated photoelectrochemical processes. Unfortunately, decoupling the impact of each effect on measured photocurrents is extremely challenging because the relative contribution of the two plasmon decay pathways cannot be controlled or easily measured. Here, we present a methodology for exploring the roles of hot carriers and heat generation on plasmon-mediated photoelectrochemical processes using scanning electrochemical microscopy (SECM). Light is used to drive a redox reaction at a plasmonic substrate, while an ultra-microelectrode tip is positioned close to the substrate to read out both the reaction products and the mass transfer rate of the redox species. By controlling the potential at the tip and substrate electrodes, the roles of photoinduced reactions at the substrate and enhanced mass transport to the tip due to local heating can be isolated and investigated independently. We observe enhanced photo-oxidation at the substrate that is due to both plasmon-generated hot holes as well as a thermal-induced change in the equilibrium potential of the redox molecules. The concentration of the reaction products changes as a function of excitation intensity, showing a linear dependence on hot carrier effects and an exponential dependence for thermal effects, and allowing us to quantify the relative contributions of the two plasmon decay pathways to enhanced photo-oxidation. This SECM approach is suitable for probing a variety of photoactive structures used in photovoltaic and photocatalytic devices.