Plasmon-Resonant Vibrational Sum Frequency Generation of Electrochemical Interfaces: Direct Observation of Carbon Dioxide Electroreduction on Gold.

Here we present plasmon-resonant vibrational sum frequency generation spectroscopy for use in electrochemical measurements. Using surface plasmon resonance we couple light through a CaF2 prism to Au films of >50 nm in order to reach the buried Au/electrolyte interface. The approach enables us to use bulk electrolyte, and high current densities (>1 mA/cm2), and therefore is suitable to probe active intermediates under relevant electrochemical reaction conditions. Fresnel factor modeling of the plasmon resonance for a three layer system (CaF2/Au/electrolyte) shows good agreement with experimental data. Off-angle momentum-matching to the surface plasmon resonance allows us to measure functional groups (-CH, -CD, -CN, -NO2) across a wide range of infrared frequencies by simply scanning the infrared wavelength without any angular realignment. Additionally we report a detection limit <1% of a monolayer for the Au/electrolyte interface. Using this method we observe an active intermediate during CO2 reduction on Au at catalytic currents. Consequently, we believe that this method will provide mechanistic understanding of electrochemical reactions.