ABSTRACT
The vibrational spectra of platinum nanoparticles (12 nm) capped with tetradecyltrimethylammonium bromide, C(14)TAB, were investigated by Fourier transform infrared (FTIR) spectroscopy. We have shown that the thermal decay of Pt-C(14)TAB nanoparticles in N(2), H(2), and O(2) atmospheres leads to the release of the hydrocarbon chain of the surfactant and the formation of a strongly bonded layer of ammonium cations on the platinum surface. The platinum atoms accessible to CO chemisorption were not reducible by hydrogen in the temperature range from 30 to 200 degrees C. A FTIR spectrum of C(14)TAB adsorbed on Pt nanoparticles was dramatically perturbed as compared with pure C(14)TAB. New intense and broad bands centered at 1450 cm(-1) and 760 cm(-1) are making their appearance in Pt-C(14)TAB. It may be speculated that new bands are the result of coupling between conducting electrons of Pt and molecular vibrations of adsorbed C(14)TAB, and as a consequence, specific vibrational modes of ammonium cation are transformed into electron-vibrational modes.
Subject(s)
Metal Nanoparticles , Trimethyl Ammonium Compounds/chemistry , Microscopy, Electron , Molecular Structure , Spectroscopy, Fourier Transform Infrared , TemperatureABSTRACT
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.
Subject(s)
Nanostructures/chemistry , Platinum/chemistry , Polyvinyls/chemistry , Pyrrolidinones/chemistry , Carbon/chemistry , Ethylenes/chemistry , Molecular Structure , Nitrogen/chemistry , Oxidation-Reduction , Oxygen/chemistry , Spectrophotometry, Ultraviolet/methods , Spectroscopy, Fourier Transform Infrared/methods , Spectrum Analysis, Raman/methods , Temperature , Tissue AdhesionsABSTRACT
The vibrational properties of mesoporous silica (SBA-15) were investigated by deep ultraviolet (UV) Raman and infrared spectroscopies with and without the presence of platinum nanoparticles in the mesopores that were incorporated by sonication. Raman and IR spectral line assignments were made by comparison to amorphous silicas. This procedure permitted identification of vibrations of longitudinal (LO) and transverse (TO) optical lattice modes, the presence of Si-OH, and vibrational modes associated with the presence of three-, four-, and six-membered siloxane rings. Hydraulic pressing of the mesoporous silica with pressure in the range 3-7 tons cm(-2) destroys the X-ray diffraction pattern and strongly decreases the Raman peak (D2) associated with three-membered rings at the surface. In the presence of platinum nanoparticles in the silica mesopores, a peak attributed to a Pt-O stretching vibration appears at between 530 and 580 cm(-1) in the UV-Raman spectrum, which can be used to monitor the presence of the platinum particles and their interaction with the support. The D2 feature in the UV-Raman spectra also decreases with increasing Pt loading, which is attributed to interactions of the Pt nanoparticles with the silica surface.
