XAFS Studies On The Structure And Formation Mechanism Of Noble Metal Nanoparticles

Noble metal nanoparticles are extremely interesting because of their large applications in catalysis, fuel cell, gas sensing, et al. Since nanopartles properties strongly depend on partle size, morphology and electronic structure, and the initial nucleation mechanism is critical to determine the final partile size and morphology, it is mandatory characterizing the atomic distribution, the electronic structure and the formation mechanism of nanoparticle for its controllable synthesis and control of performance. In this dissertation, we will discuss the geometrical structure of Pt(IV) in H2PtCl6ammonia solution, the formation mechanism of Pt nanoparticles in the reduction of H2PtCl6to Pt nanoparticles by methanol, and the electronic and atomic structure of Cu-Pt nanoparticles synthesized by a polyol liquid phase method. To this purpose we used a combination of X-ray absorption fine structure spectroscopy (XAFS), transmission electron microscopy (TEM), X-ray diffraction (XRD) patterns, UV-Vis, liquid chromatography mass spectrometry (LCMS), multiple scattering theory and First-principle calculation method. Besides, we also studied the change of the inversion parameter of ZnCr2O4nanoparticles synthesized by a co-precipitation and postcalcined method vs the annealing temperatue. This dissertation includes:1. We investigated the geometry structure of Pt(IV) in H2PtCl6ammonia solution. By means of extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) calculations. Data show that C1ligands around Pt with a Pt-Cl bond length of2.32A are completely replaced by NH3ligands with a Pt-N distance of2.05A. Data point out both the Pt-Cl and a Pt-N orbital hybridization will cause a post-edge feature that indicates the number of neighboring atoms.2. We investigated the formation mechanism of Pt nanoparticles and the effect of H2O on the reduction of H2PtCl6to Pt nanoparticles by methanol. The transformation route is:[PtCl6]2-to [PtCl5(CH3O)]2-to [PtCl4]2-to [PtCl3(CH3O)]2-to [PtCl2]2-to Pt30and finally to Pt nanoparticles in a pure CH3OH solution. With10vol%water added into the CH3OH solution, a new chemical reduction pathway from [PtCl2]2-to [PtCl5(CH3O)(H2O)]2-to [PtCl4]2-to [PtCl3(CH3O)(H2O)]2-to [PtCl2]2-to Pt30and finally to Pt nanoparticles was detected. In the latter case, H2O not only participated but also greatly accelerated the reaction acting as a "catalyst".3. We investigated the electronic and atomic structure of Cu-Pt nanoparticles with different Cu/Pt molar ratio synthesized by a glycol-water reduction method. Results show that majority of Cu atoms segregated to the surface to form a CuO-like shell around a Pt-rich Cu/Pt core. The analysis of the Pt-L2，3white line (WL) intensities points out a novel change compared to the corresponding bulk Cu-Pt alloy. Actually, forming Cu-Pt alloy nanoparticles Pt atoms lose d electrons while an increase of Pt d-electron is observed when the Cu atoms on the surface are gradually oxidized. Results suggest that the electronic properties of Cu-Pt nanoparticles can be tuned by manipulating surface and alloying effects.4. ZnCr2O4nanocrystals with different sizes were synthesized by a co-precipitation and post-calcined method. XRD and TEM showed that the nanocrystals annealed at different temperature have the same crystal structure and their size increases with the annealing temperature. Cr and Zn K-edge XANES and EXAFS has been used to investigate the change of the inversion parameter vs. the annealing temperature. The inversion parameter is found to decrease with increasing annealing temperature. In particular, samples annealed at750℃has a normal spinel structure, like the bulk ZnCr3O4. On the contrary, the inversion parameter of the sample annealed at300℃can get0.24. This is the first study that shows in the ZnCr2O4a variation of the inversion degree tuned by the annealing temperature, and provides also an original method to obtain ZnCr2O4nanoparticles with different inversion parameters.