Microphotoelectrochemical Surface-Enhanced Raman Spectroscopy: Toward Bridging Hot-Electron Transfer with a Molecular Reaction

The rational design and applications of plasmon-mediated chemical reactions (PMCRs) are fundamentally determined by an understanding of photon-electron-molecule interactions. However, the current understanding of the PMCR of plasmon-decayed hot electron-mediated reactions remains implicit, since there has not been a single measurement of both hot-electron transfer and molecular transformation following photon excitation. Therefore, we invented a method called microphotoelectrochemical surface-enhanced Raman spectroscopy (μPEC-SERS) that uses an ultramicroelectrode (UME) whose dimensions match those of the focused laser spot. This system can simultaneously record the photocurrent (∼picoamps) of hot-electron transfer with a high signal-to-noise ratio and the SERS spectra of a molecular reaction in the same electrode area. The responses of the photocurrent and SERS spectra to laser illumination can correlate the surface reaction activated by hot electrons with the SERS spectral changes. A typical PMCR of p-aminothiophenol (PATP) on a Ag UME was used to illustrate that the correlation of the photocurrent with the spectral changes is capable of revealing the reaction mechanism in terms of the formation of activated oxygenated species. The laser power-, laser wavelength-, and surface roughness-dependent photocurrents link the formation of activated oxygenated species to the hot-electron transfer. Further comparisons of the photocurrent with the conventional electrochemical current of the oxygen reduction reaction indicate that the activated oxygenated species are oxidative in transforming PATP to p,p'-dimercaptoazobenzene, which is supported by a density functional theory (DFT) calculation. Therefore, μPEC-SERS could be a powerful tool for investigating PMCRs and other systems involving photon-electron-molecule interactions.