Plasmonic Nano-gratings For in situ Nonlinear Optical Spectroelectrochemistry

Understanding electrochemical (EC) reactions at the molecular-level is crucial for fundamental science and applications, yet it is highly challenging to probe the buried interfaces between electrodes and electrolyte. Previously by exploiting the surface plasmon (SP) enhancement, we successfully monitored molecular signatures of EC reactions in situ with the surface-specific sum-frequency vibration spectroscopy (SFVS). However, the SP suffers from severe attenuation toward absorption bands of electrolytes. Here we show that metal electrodes engraved with plasmonic nano-gratings can provide a general solution for SFVS of EC interfaces. With optimal composition and geometry, the nano-gratings can efficiently compensate for the electrolyte attenuation, and lead to strong near-field enhancement highly specific to electrode/electrolyte interfaces. Using a gold nano-gratings electrode, we demonstrate the in situ SFVS in the midst of strong absorption bands of the water electrolyte, and unveiled sequential changes of the hydrogen bonding network of interfacial water molecules during redox cycling of gold, showing that our approach offers a powerful tool toward the microscopic and mechanistic understanding of EC interfaces.