SERS Chemical Enhancement of Water Molecules from Halide Ion Coadsorption and Photoinduced Charge Transfer on Silver Electrodes

The surface-enhanced Raman scattering (SERS) signal of water is hard to be measured due to its very small Raman scattering cross section and weak adsorption on coinage metals, only electrochemical SERS spectra of water have been observed in electrode/electrolyte interfaces so far. Our present work focuses on the chemical enhancement from halide ions on SERS signals of water adsorbed on silver electrodes, by combining the metallic cluster model and hybrid density functional theory (DFT) calculations. The interfacial structures, binding interactions and the anion effect from different halides including chloride, bromide, and iodide ions have been analyzed and compared with experimental measurements in literatures. Then the excited states of halide ions modified active sites on roughened silver electrodes have been discussed. In particular, our time-dependent DFT (TD-DFT) calculations predicted that halide ions can form low-lying excited states of surface complexes, like the photon-induced charge transfer (PICT) states, and finally contribute to the chemical enhancement of SERS signals of water. Furthermore, we proposed that the halide effect on the relative SERS intensities of water is a good example for understanding the chemical enhancement of SERS active sites modified by halide ions in electrochemical systems. This is different from the chemical enhancement of SERS of water at metal cathodes.