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

Abstract

Plasmonic catalysis holds promise for opening new reaction pathways inaccessible thermally or for improving the efficiency of chemical processes. We report a gold stripe waveguide along which infrared (λ 0 ~ 1350 nanometers) surface plasmon polaritons (SPPs) propagate, operating simultaneously as an electrochemical working electrode. Cyclic voltammograms obtained under SPP excitation enable oxidative processes involving energetic holes to be investigated separately from reductive processes involving energetic electrons. Under SPP excitation, redox currents increase by 10×, redox potentials decrease by ~2× and split in correlation with photon energy, and the charge transfer resistance drops by ~2× as measured using electrochemical impedance spectroscopy. The temperature of the working electrode was monitored in situ, ruling out thermal effects. Chronoamperometry measurements with SPPs modulated at 600 hertz yield a commensurately modulated current response, ruling out thermally enhanced mass transport. Our observations indicate opening of optically controlled nonequilibrium redox channels associated with energetic carrier transfer to the redox species.

Real-Time Convolutional Voltammetry Enhanced by Energetic (Hot) Electrons and Holes on a Surface Plasmon Waveguide Electrode

“…The threshold at P inc ≈ 1 mW indicates the opening of these redox channels as the current densities associated with energetic carrier transfer overcome noise and the equilibrium redox current densities. (Extensive and in situ thermal control experiments performed in previous work on a similar setup rule out thermal effects as the cause of the current increase …”

“…The current flowing through the WE into the redox species is the electrical measurand. Advantages of this system include a compact arrangement, a WE that is defined lithographically and is well-controlled structurally, SPPs that propagate over the entire length of the WE with complete overlap, and a thin WE which enhances the escape probability of energetic carriers created therein via SPP absorption …”

“…Advantages of this system include a compact arrangement, a WE that is defined lithographically and is well-controlled structurally, SPPs that propagate over the entire length of the WE with complete overlap, and a thin WE which enhances the escape probability of energetic carriers created therein via SPP absorption. 23 The expression that relates the output optical power (P out ) to the incident optical power (P inc ) is written as…”

“…Advantageously, by separating oxidation and reduction reactions electrochemically, the role of energetic holes can be distinguished from that of energetic electrons. Energetic carriers are created in a WE via SPP absorption therein, but absorption also raises the temperature of the metal and heat diffuses into the surrounding reaction volume. − It is not trivial to separate the effects of temperature from the effects of energetic carriers, given that electrochemical reactions and fluid dynamics are temperature dependent, but it is possible to isolate effects through careful experimental design and control experiments. − …”

“…18−23 In recent work, infrared SPPs propagating along a Au WE shaped as a stripe waveguide induced significant increases in redox currents (10×) along with significant decreases in redox potentials, attributed to the creation of energetic carriers (electrons and holes) by SPP absorption in the Au stripe. 23 Here, we investigate convolutional voltammetry in this system by monitoring the electrochemical current and the output optical power in real time versus the applied potential, as chronoamperometry and cyclic voltammetry (CV) measurements are performed. Output optical power voltammograms indicate that changes in optical signal are caused mainly by changes in the refractive index near the WE, following changes in concentration of the reduced and oxidized forms of the redox species.…”