As a second order nonlinear technique, sum-frequency generation (SFG) spectroscopy is forbidden, under the electric-dipole approximation, in the bulk of a medium with inversion symmetry and then generate only at the surface of such media where the symmetry is broken and appear to be highly surface-specific for interfaces between the centrosymmetric media. So far, many studies of electrochemical processes at various interfaces have been performed by using SFG, and molecular level insight like the molecular adsorption, orientation, packing, and dynamics has been obtained.

We have developed broadband sum-frequency generation (BB-SFG) spectroscopy. It enables us to obtain information on in situ electrochemical systems. One of the electrochemical processes is the CO₂ reduction on a polycrystalline copper and polycrystalline gold electrode. The antisymmetric C−H stretching modes of methyl group and antisymmetric C−H stretching modes of methylene group were observed on copper electrode surface, no adsorbed CO intermediate was observed. As for the process on gold electrode surface, the adsorbed CO was the main intermediate detected, and CO was observed at a relative high potential (-0.5V vs SCE). In addition, we present some other examples that investigate the electrochemical interface with SFG techniques, such as the potential-dependent nonresonant (NR) SFG.

Figure 1 (a)(b) Potential-dependent BB-SFG spectra of 0.1M CO₂-saturated NaHCO₃ solution at poly-Cu electrode (C-H region) and poly-Au electrode (CO region). (c) Variations of the wavenumbers (black) and intensities (red) of the CO peak in (b) from the fitting data.

Acknowledgement: This work was financially supported by NSFC (21327901 and 21621091).