In Situ Raman Monitoring of Potential-Dependent Adlayer Structures on the Au(111)/Ionic Liquid Interface

Probing the adlayer structures on an electrode/electrolyte interface is one of the most important tasks in modern electrochemistry for clarifying the electrochemical processes. Herein, we have combined cyclic voltammetry and electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy techniques to explore the potential-dependent adlayer structures on Au(111) in a room-temperature ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) without or with pyridine (Py). It is clearly found that the BMI⁺ cations strongly adsorb on the negatively charged surface with a flat-lying orientation, leaving a little space for Py adsorption. Upon increasing the potentials of the electrode, the variations of Raman band intensities and frequencies reveal that the interaction between the BMI⁺ cations and the Au surface becomes weak; meanwhile, the Py adsorption becomes strong, and its geometry turns from flat, tilted to vertical. Finally, BMI⁺ cations desorb and leave plenty of surface sites for Py adsorption in bulk solution, and a N-bonded compact Py adlayer is formed on the very positively charged surface. This causes obvious anodic peaks in cyclic voltammograms, and the peak currents increase with the square root of the scanning rate. The present work provides a fair molecular-level understanding of electrochemical interfaces and molecular adsorption of Py in ionic liquids.