Probing the Effect of pH Value and Voltage on the Near-Surface Proton Concentration at the Electrochemical Interface by In Situ Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS)

Understanding the variation of proton concentration near the surface of the electrochemical interface is of great significance for understanding the mechanism of electrochemical reactions. In this work, 4-Mpy molecules that are protonated and deprotonated depending on the surrounding pH value adsorb on the Au nanoparticle film electrode with high SERS activity, and by virtue of the highly interfacial-sensitive EC-SERS technique, we systematically studied the effects of electrolyte pH value and external voltage on the protonation and deprotonation of 4-Mpy at the interface between Au-NP film electrode and phosphate buffer, to analyze the changes of near-surface proton concentration at the electrochemical interface. It is found that the pH value of the electrolyte plays a decisive role in the protonation process of 4-Mpy at the electrode interface at low reduction voltage (< −0.1 V). In acidic and neutral solution, 4-Mpy exists mainly in protonated form on the electrode surface. However, in alkaline solutions, 4-Mpy exists mainly on the electrode surface in the form of deprotonation. At high reduction voltage (≥ −0.1 V), the protonation and deprotonation of 4-Mpy on the electrode surface are mainly determined by the adsorption structure of 4-Mpy on the electrode surface. At the same time, we conducted a comparative study of 2-Mpy and 4-Mpy molecules and found that the adsorption modes were different depending on the position of the N atom. 2-Mpy is inclined adsorbed on the surface of the Au-NP film electrode, and 4-Mpy is vertically adsorbed on the surface of the Au-NP film electrode.