[Adsorption of methylene blue on colloidal silver--a surface-enhanced Raman spectroscopy study combined with density...

Surface-enhanced Raman spectra of methylene blue (MB) at different concentrations in silver colloid were obtained. The results indicate that the physical adsorption is dominant at high concentration while the chemical adsorption is the main fashion at relatively low concentration; there are different adsorption orientations at different concentration: MB+ molecule is perpendicular to the surface of silver nanoparticle at high concentration and adopts a parallel orientation on the surface of nanoparticle at low concentration. The effect of adsorbing time of MB molecule in Ag colloid was investigated and the adsorption dynamics study shows that the parallel orientation at low concentration does not change with the adsorbing time increasing. Density functional theory (DFT) calculations at the level of B3LYP/6-311+G * (for C, S, N, H)/LANL2DZ (for Ag) were employed to optimize the structures and predict Raman frequencies of MB+ and various MB+ -Ag complexes. The results of experiments and calculations suggest that the silver atom prefers to be bound to N and S atoms in the aromatic ring, and thus two different complexes are formed, i.e., conformer N-Ag and conformer S-Ag. Moreover, the Mulliken charge population analysis indicates that N atom in the aromatic ring prefers to interact with Ag than S atom does. Finally, the Raman frequencies observed in the experiments and their vibrational modes were tentatively assigned and discussed.