Charge-Transfer Interaction of Poly(vinylpyrrolidone) with Platinum and Rhodium Nanoparticles

The vibrational spectra of platinum and rhodium nanoparticles (2.4−7 nm) capped with poly(vinylpyrrolidone) (PVP) were investigated by deep UV−Raman and Fourier transform infrared (FTIR) spectroscopy. Raman spectra of PVP/Pt and PVP/Rh showed selective enhancement of CO, C−N, and CH2 vibrational modes of the pyrrolidone ring as a result of donor−acceptor interactions between polymer functional groups and surface metal atoms. This was observed in the UV−Raman spectra of PVP-capped metal nanoparticles by in-situ measurements in both reduced and oxidized states. Charge-transfer interactions between the polymer donor groups and surface Pt atoms in the first layer of the PVP/Pt system changed reversibly as a function of metal oxidation state (Pt(II) and Pt(0)), induced by heating under a flow of H2 or O2. In contrast, the UV−Raman spectra of PVP/Rh were not reversible upon heating at 150 °C in an atmosphere of O2, H2/N2, or N2; in this case, fragments of thermal decomposition of PVP on Rh were detected. Furthermore, it was demonstrated that UV−Raman spectroscopy (244 nm) is a highly sensitive tool to study the effect of surface oxide layers on chemically enhanced surface-enhanced Raman spectroscopy (SERS). In the case of PVP/Al, a nonreducible oxide layer of Al2O3 blocked the involvement of conducting electrons from making bonding interactions with pyrrolidone rings.