Surface enhanced Raman spectroscopy measurement of surface pH at the electrode during Ni electrodeposition reaction

In this work, we developed a precise approach to analyze local proton concentration at the solid/liquid interface of electrodes, i.e. “surface pH”, during electrochemical reactions. For this, surface enhanced Raman spectroscopy (SERS) was applied to analyze pH-dependent structural changes of the –COOH group of p-mercaptobenzoic acid (p-MBA) modified onto Au nanoparticles (NPs) on the substrate close to a working electrode. Measurements using this system identified deprotonation of –COOH of p-MBA. Since preliminary experiments and density functional theory calculations suggest that the pKa of p-MBA attached to Au NPs is close to that in bulk solution, the SERS results indicate pH increase due to proton consumption by the cathodic overpotential of the working electrode. As an example, we applied this system to surface pH monitoring in electrodeposition process of Ni in an acidic bath, which indicated the validity of our method for precise detection of pH changes at electrode interfaces in situ.