Catalytic Hot-Electron SERS Analytical Substrates and a Case Study on Graphene Nanocomposite Inspection

Hot-electron transfer between an electrode surface and an adsorbate is potentially useful in numerous electrochemical technologies, especially those operating at room temperature. Although challenging, estimation of the parameters controlling such a transfer process is fundamentally important to electrode design and utilization. Herein, nanothin Au/SiO₂/Si electrodes are fabricated and analyzed for the source, generation, energetic characteristics, and interfacial transfer process of photogenerated surface hot electrons using excitation wavelength- and electron density-dependent Raman spectroscopy with methylene blue (MB) as the target molecule. Although Raman signal intensity exponentially increases with Si doping concentration when illuminated by a green laser, the signal intensification appears constant for a red laser. In both cases, Raman enhancement factors follow catalytic degradation with increasing measurement cycles. In conjunction with the calculated density of state distribution of the MB/Au interface, the observed Raman signals are ascribed to the charge-transfer (CT) surface-enhanced Raman scattering (SERS) mechanism, induced by the indirect and the resonant surface plasmon-enhanced direct interfacial transfers of hot electrons originating from Si and Au when excited by green and red lasers, respectively. Applying the electrodes as a CT SERS platform based on the above information to inspect a reduced graphene oxide/silica (rGO/SiO₂) nanocomposite, conducting rGO core/insulating SiO₂ shell structure, which implies plasmonic-heating property, is proposed and correlated with its photothermal efficiency. The findings shed light on the applicability of the hot-electron injector not only for molecular analysis but also for relevant technologies such as photoelectrocatalysis.