Surface plasmons on a Au waveguide electrode open new redox channels associated with the transfer of energetic carriers

Abstract: Plasmonic catalysis holds significant promise for opening new reaction pathways inaccessible thermally, or for improving the efficiency of chemical processes. However, challenges persist in distinguishing photothermal effects from effects due to energetic electrons and/or holes excited in the metal comprising a plasmonic structure. Here we use a Au stripe waveguide along which surface plasmon polaritons (SPPs) propagate, operating simultaneously as a working electrode in a 3-electrode electrochemical cell. Cyclic voltammograms were obtained under SPP excitation as a function of incident optical power and wavelength ~ 1350 nm, enabling processes involving energetic holes to be separated from processes involving energetic electrons by investigating oxidation and reduction reactions separately. Redox current densities increase by 10x under SPP excitation. The oxidation, reduction and equilibrium potentials drop by as much as 2x and split beyond a clear threshold with SPP power in correlation with the photon energy. The temperature of the working electrode under SPP excitation is monitored in situ and independent control experiments isolate thermal effects. Chronoamperometry measurements with on-off modulated SPPs at 600 Hz, yield a diffusion-limited rapid current response modulated at the same frequency, ruling out thermally-enhanced mass transport. Our observations are attributed to the opening of optically controlled non-equilibrium redox channels associated with energetic carrier transfer to the redox species.