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.