Water is integral in electrochemistry, in the generation of the electric double layer, and in propagating interfacial electric fields; yet our molecular-level understanding of interfacial water near functioning electrode surfaces is limited. Sum-frequency-generation (SFG) spectroscopy offers an opportunity to investigate the structure and dynamics of the solvent near working electrochemical interfaces, as the method is inherently surface-specific. Previous efforts that propagated infrared (IR) beams through the solvent suffered due to the IR absorption of water. Furthermore, sending visible and IR beams through standard electrode films, obscures the SFG spectra, due to the large nonlinear response of conduction band electrons. To overcome this limitation, we prepared and investigated thin, yet conductive gradient sputtered Au films on CaF2 using SFG, FTIR, AFM, and UV-Vis spectroscopies to explore the ideal thickness of an Au electrode layer for SFG experiments. SFG spectra of dry and wet 8.0-0.4 nm thick Au films suggest that unobscured interfacial water spectra can be obtained using Au films with a thickness of ~2 nm or less. Experimental spectra are compared with the frequency dependent Fresnel Factors generated from each interface, which suggest that features we observe in the OH stretching region do not arise from the nonresonant response of our gold films. With the newfound ability to probe interfacial solvent structure at electrode/aqueous interfaces, we hope to provide insights into more efficient electrolyte composition and electrode design.

Probing the gold/water interface with surface specific spectroscopy

ACS Fall 2021

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