Surface-enhanced Raman scattering study of the redox adsorption of p-phenylenediamine on gold or copper surfaces.

The adsorption of the p-phenylenediamine (PPD(+)) radical cation on gold or copper nanoparticle (NP) surfaces was studied through surface-enhanced Raman scattering (SERS) spectroscopy, excited at 1064 nm. The SERS spectra were obtained from gold or copper NPs after exposure to non-oxidized p-phenylenediamine (PPD) aqueous solution, in millimolar concentration. The gold NPs were synthesized as nanoshells involving silica cores (SiO(2)@Au) and the copper NPs were obtained in aqueous medium, undergoing surface oxidation with the formation of Cu(II) oxide nanoshell (Cu@CuO). In the latter, the oxidative adsorption of PPD(+) led to the reduction of the copper oxide, present on NP surface, allowing obtaining the PPD(+) SERS spectrum. The vibrational assignments of the SERS spectra of the adsorbate were performed using the results of Density Functional Theory calculations of the Raman frequencies, which together with the SERS surface selection rules, allowed to infer the adsorption geometry of PPD(+) radical cation on both metallic surfaces. This work stress the investigation of redox processes involved in the molecular adsorption is imperative for the interpretation of the SERS results, which is even more important when copper surfaces are studied.