The Role of Chloride in Raman Signal Enhancement by Electrochemical Silver Oxidation Revealed by Dark Field Microscopy

Raman spectroscopy is a widely used technique in several contexts, including chemical analysis, materials characterization, and catalysis. However, to exploit the high capacities of this technique, signal enhancement is needed. For this purpose, several methodologies can be used, and those known as surface enhanced Raman scattering (SERS), or resonance Raman (RR) have been widely used. However, there are some new strategies, such as electrochemical surface oxidation enhanced Raman scattering (EC-SOERS), that require further understanding for optimum exploitation in diverse analytical contexts. In EC-SOERS, the enhancement of the Raman signal is observed during the electrochemical oxidation of silver in the presence of a precipitating agent, but only for specific concentrations of this agent. In this work, we use electrochemical dark-field microscopy (DFM) to explore and reveal the origin of this concentration dependency by monitoring the oxidative formation of EC-SOERS substrates in solutions of different chloride concentrations. These operando studies provide a complete picture of the processes taking place on the electrode surface and at the solution adjacent to it with a high time resolution, showing that the formation of the EC-SOERS substrate requires sufficient Cl^- to generate AgCl nanocrystals without blocking the surface and allowing the release of Ag⁺ cations. Thanks to the gained mechanistic insights, the selection of a suitable precipitation agent concentration can move from a trial and error selection process to a knowledge-based selection, allowing the rational design of different SOERS substrates that will facilitate the efficient application of SOERS in different research contexts.