Time-dependent Raman spectra from two types of adsorption sites at Ag electrodes

Time-dependent surface-enhanced Raman scattering (SERS) form thiourea (TU) and ClO4− coadsorbed at Ag electrodes was investigated using a potential step. The intensity–time profiles for the different vibrational modes can be classified into two categories according to the response rate of the band intensity to the potential step, i.e. a fast response within 5 s and a slow response up to 5 min. More interestingly, all band intensities show the fast response characteristics after the electrode was made to undertake a negative excursion to the potential of −2.0 V. This phenomenon is explained in terms of co-existence of two types of adsorption sites (SERS active sites), i.e., neutral Ag atom(s) and partial charged Ag atom(s). It is shown that time-dependent SERS combined the potential step method may be helpful not only for gaining a deeper insight into the SERS mechanism(s), but also for revealing the nature and structural dynamics of adsorption sites formed by strong chemical interaction.