Probing the Gold/Water Interface with Surface-Specific Spectroscopy

Water is an integral component in electrochemistry, inthe generationof the electric double layer, and in the propagation of the interfacialelectric fields into the solution; however, probing the molecular-levelstructure of interfacial water near functioning electrode surfacesremains challenging. Due to the surface-specificity, sum-frequency-generation(SFG) spectroscopy offers an opportunity to investigate the structureof water near working electrochemical interfaces but probing the hydrogen-bondedstructure of water at this buried electrode-electrolyte interfacewas thought to be impossible. Propagating the laser beams throughthe solvent leads to a large attenuation of the infrared light dueto the absorption of water, and interrogating the interface by sendingthe laser beams through the electrode normally obscures the SFG spectradue to the large nonlinear response of conduction band electrons.Here, we show that the latter limitation is removed when the goldlayer is thin. To demonstrate this, we prepared Au gradient filmson CaF2 with a thickness between 0 and 8 nm. SFG spectraof the Au gradient films in contact with H2O and D2O demonstrate that resonant water SFG spectra can be obtainedusing Au films with a thickness of similar to 2 nm or less. The measuredspectra are distinctively different from the frequency-dependent Fresnelfactors of the interface, suggesting that the features we observein the OH stretching region indeed do not arise from the nonresonantresponse of the Au films. With the newfound ability to probe interfacialsolvent structure at electrode/aqueous interfaces, we hope to provideinsights into more efficient electrolyte composition and electrodedesign.

Automated summary

Water is essential in electrochemistry and plays a crucial role in the electric double layer and interfacial electric fields. Investigating the molecular-level structure of interfacial water near working electrode surfaces has been challenging. However, using sum-frequency-generation (SFG) spectroscopy, it is now possible to probe the structure of water at buried electrode-electrolyte interfaces. Thin gold layers were found to effectively remove the limitation of obscuring the SFG spectra. This newfound ability to probe interfacial solvent structure may contribute to more efficient electrolyte composition and electrode design.