IR Optical Properties of Pt Nanoparticles and Their Agglomerates Investigated by in Situ FTIRS Using CO as the Probe Molecule

Abstract

Pt nanoparticles (Ptn) and their agglomerates (Ptnag) were synthesized by chemical reduction methods. The mean sizes of Ptn and Ptnag were determined to be respectively 3.4 and 400 nm using transmission electron microscopy (TEM). CO adsorption on electrodes made of the two kinds Pt nanomaterials was used as the probe reaction. Studies of cyclic voltammetry revealed that the current peak potential of COad oxidation on electrodes of Ptnag supported on glassy carbon (Ptnag/GC) has been shifted negatively by about 80 mV in comparison with that on a bulk Pt electrode. In situ electrochemical FTIRS results illustrated that Ptn/GC and Ptnag/GC exhibit different IR properties for CO adsorption. In comparison with CO adsorbed on bulk Pt, a phenomenon of enhanced IR absorption (EIRA) was observed on dispersed Pt nanoparticles (Ptn). IR absorption of COad on Ptn/GC is enhanced 31 times and the full width at half-maximum (FWHM) of the COL band is increased to 40 cm-1, which is 25 cm-1 larger than the value (15 cm-1) measured on a bulk Pt electrode. In contrast to the EIRA of CO adsorbed on dispersed Pt nanoparticles, the abnormal infrared effects (AIREs) were encountered for CO adsorbed on agglomerates of Pt nanoparticles, in which a strong interaction is presented between Ptn. The IR features of CO adsorbed on Ptnag/GC illustrated that, except for the IR absorption of COad being enhanced 9 times and the FWHM of the COL band being increased to 25 cm-1, the direction of the COad bands is inverted from absorption to antiabsorption. The results of the present paper demonstrated that the EIRA and the AIREs are closely related to the agglomerate states of Pt nanoparticles, and throw a light on the origin of the particular IR properties of nanomaterials.