Time-resolved surface-enhanced resonance Raman scattering study of the reduction process of heptylviologen monocation radical film on silver electrode surfaces

Abstract

Abstract Time-resolved SERRS spectroscopy was applied to elucidate the mechanism of the reduction process of a heptylviologen monocation radical film to a neutral species on Ag electrode surfaces under various conditions. The film deposited on Ag electrodes at −0.65 V (vs. Ag/AgCl) consists of dimers, (HV +. ) 2 . On application of a step potential from −0.65 to −1.2 V, the radical dimer is converted to a neutral species, HV 0 . The time-resolved spectra measured as a function of time after application of the step potential indicates that on the electrode immersed in KBr solutions (0.3 and 3 mol l −1 ) the radical dimer is at first converted to an intermediate state, which is a surface complex of a monocation radical monomer and a Br − ion (the radical monomer in a type B state), and then reduced to the neutral species. The time-resolved spectra proved also the existence of a disproportionation reaction, i.e. 2HV +. (type B) → HV 2+ + HV 0 . The increase in the KBr concentration (0.3 → 3 mol l −1 ) stabilizes the intermediate surface complex, causing an appreciable decrease in the reduction rate from (HV +. ) 2 to HV 0 . The reduction process on a silver electrode in 0.3 mol l −1 Na 2 SO 4 consists of two reaction paths; one is a direct conversion from (HV + ) 2 to HV 0 and another is a path through a radical monomer, which gives SERRS features appreciably different from those of type B. The first process proceeds much faster than that on the electrode in the KBr solutions.