The vibrational spectra of platinum nanoparticles (2.4-9 nm) capped with poly(N-vinylpyrrolidone) (PVP) were investigated by deep UV-Raman and FTIR spectroscopy and compared with those of pure PVP. Raman spectra of PVP/Pt show selective enhancement of C=O, C-N, and CH2 vibrational modes attributed to the pyrrolidone ring. Selective enhancement of ring vibrations is attributed both to the resonance Raman effect and SERS chemical enhancement. A red shift of the PVP carbonyl frequency on the order of 60 cm-1 indicates the formation of strong >C=O-Pt bonds. It is concluded that PVP adheres to the nanoparticles through a charge-transfer interaction between the pyrrolidone rings and surface Pt atoms. Heating the Pt nanoparticles under reducing conditions initiates the decomposition of the capping agent, PVP, at a temperature 100 degrees C below that of pure PVP. Under oxidizing conditions, both PVP/Pt and PVP degrade to form amorphous carbon.

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 CdO, C-N, and CH 2 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 H 2 or O 2 . In contrast, the UV-Raman spectra of PVP/Rh were not reversible upon heating at 150°C in an atmosphere of O 2 , H 2 /N 2 , or N 2 ; 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 Al 2 O 3 blocked the involvement of conducting electrons from making bonding interactions with pyrrolidone rings. IntroductionPoly(vinylpyrrolidone) (PVP) has been used extensively as a macroligand for stabilizing metallic nanocrystals with different shapes 1 and as flexible membrane around metallic colloids. In some instances, the PVP has been shown to influence the selectivity of catalytic reactions. 2 The application of PVP/Pt and PVP/Rh nanoparticles in low-temperature catalytic reactions has recently offered promising new opportunities to create tailored catalytic systems. However, very little is known about the structure and nature of the chemical interaction between PVP and metallic nanoparticles at the molecular level. Several experimental techniques have been used to explore the PVP/Pt and PVP/Rh interfaces. 3 Recently, we have shown that deep UV-Raman spectroscopy can be advantageous for probing the interaction between PVP and Pt nanoparticles because of substantial intensity enhancement for certain vibrational modes as a result of resonance Raman and surface chemical enhancement effects. 4 By combining Fourier transform infrared (FTIR) and Raman methods, it is possible to differentiate between vibrational modes arising from unbound pyrrolidone rings and those with direct interactions with surface atoms of the metal nanoparticles.In this paper, we present results that demonstrate a correlation between the extent of interaction of PVP donor groups with

Poly(N-vinylpyrrolidone (PVP) capped platinum and rhodium nanoparticles (7 -12 nm) have been studied with UV-VIS, FTIR and Raman spectroscopy. The absorption bands in the region 190 -900 nm are shown to be sensitive to the electronic structure of surface Rh and Pt atoms as well as to the aggregation of the nanoparticles. The behavior of surface Rh and Pt atoms with ligated CO and amide groups of pyrrolidones resemble that of surface coordination compounds.