Charge transfer between triphenyl phosphine and colloidal silver: A SERS study combined with DFT calculations

The adsorption of triphenyl phosphine (PPh3) on colloidal silver has been investigated by means of surface-enhanced Raman spectroscopy (SERS). On the basis of surface selection rule, it is deduced from SERS results that PPh3 adsorbs on silver surface via its P atom with three phenyl rings tilted with respect to the silver surface. The electron-donor effect of PPh3 can be sensitively probed by the coadsorbed SCN-. The Raman frequency of nu(CN) of the adsorbed SCN- shifts to lower frequencies when PPh3 is coadsorbed with SCN-, and the red shift of CN stretching frequency is found to increase with increasing the surface coverage of PPh3. This could be explained in terms of the electron-donor effect of PPh3: PPh3 adsorbed on silver surface donates its lone pair of electrons to silver surface, and then the negative charge of silver surface transfers to the pi* orbital of CN bond via S-Ag bond. Consequently, the CN bond is weakened, and the frequency of nu(CN) shifts to lower frequencies. Density functional theory (DFT) calculations further confirm the experimental results that the charge transfer is from PPh3 to silver surface rather than reversely. The information obtained from the adsorption of PPh3 on silver by SERS may be helpful to understand the mechanism of heterogeneous catalysis involving phosphine ligands coordinated on transition-metal surfaces.