Interactions between Keggin-type lacunary polyoxometalates and Ag nanoparticles: A surface-enhanced Raman scattering spectroscopic investigation

Interactions between two Keggin-type lacunary polyoxometalates (POMs), PW11O397- and SiW11O398-, and pre-formed citrate-protected colloidal Ag nano-particles (NPs) were studied by surface-enhanced Raman scattering (SERS) and UV-Vis spectroscopies. Clear SERS evidence of displacing the originally surface-bound citrate ions by the subsequently added POMs indicates a stronger metal-ligand interaction between the POMs and the Ag NPs. Accompanied red-shifts in the surface plasma peak position of the Ag NPs were also observed for both POMs. The simultaneous presence of a number of SERS vibrational bands which can be assigned to vibrational modes of the POM molecule suggests that its overall structure is intact upon adsorption. Based on the SERS observations, a bonding between the penta-dentate aperture of the POMs and the Ag NP surface was proposed.