SERS and DFT investigation of the adsorption behavior of 4-mercaptobenzoic acid on silver colloids

4-mercaptobenzoic acid (4-MBA) adsorbed on Ag colloids has been investigated by surface-enhanced Raman scattering (SERS) spectroscopy. From the intensity variation of the symmetric stretching COO− vibration band corresponding to different pH values, it shows that 4-MBA molecules are bonded to the Ag surface only through the S atoms at acidic conditions, but through the S atoms and the COO− groups at neutral or alkaline conditions. The 4-MBA bonding to Ag shows SERS activity with multilayer adsorption (Freundlich isotherm model). Raman spectra calculated by density functional theory (DFT) with emphasis on long-range charge-transfer effect were compared with experimental observation to interpret the adsorption behavior of 4-MBA on the Ag surface. From both DFT simulations and experimental observation, we have verified that the contribution of electromagnetic and chemical enhancements in the SERS spectra of 4-MBA adsorbed on Ag are equally important.