X-ray spectroscopy studies on the surface structural characteristics and electronic properties of platinum nanoparticles.

The surface structural characteristics and electronic behavior of three platinum nanoparticle (NP) samples prepared with tertiary amine (Pt-TA), primary amine (Pt-PA), and thiol (Pt-SR) molecules were studied using Pt 4f, 5d, and S 2p x-ray photoelectron spectroscopy (XPS), Pt L(3)-edge x-ray absorption spectroscopy (XAS), and theoretical projected local density of states (l-DOS) calculations. Transmission electron microscopy and XPS composition analysis indicated that the three NPs were all very small (1-2 nm), the NP size decreasing in the order of Pt-TA>Pt-PA approximately Pt-SR. All the three samples showed a positive Pt 4f binding energy (BE) shift relative to that of the bulk, in the order of bulk<Pt-TA<Pt-PA<Pt-SR. The origin of the BE shift was elucidated by XAS and deconvolution of the Pt 4f XPS peak, indicating that the observed BE shifts were largely associated with the initial state effect (i.e., nanosize and surface structure). The surface and size effects on the electronic behavior of Pt were further studied by valence band XPS and the results were interpreted with calculated d-DOS of three Pt(55) model clusters with varied surface structures. Finally, the implication of these results on tuning the electronic properties of Pt NPs with size, surface, and alloying effects was discussed.