High-index faceted Pt-based alloy nanocrystals: Shape-controlled synthesis, formation mechanisms and their structure-dependent catalytic properties.

摘要

The motivation for tailoring the shape, size and composition of nanocrystals (NCs) lies in the fact that the properties of nanocrystalline solids are strongly correlated to overall structures and exposed crystallographic facets. In the past two decades, there has been widespread research centered on the Pt-based NCs with well-defined shapes to meet the demand of high performances in various catalytic reactions. The major objective of this dissertation is shape-controlled synthesis of Pt-M (M=Ni, Co and Fe) bimetallic NCs. As an extensive investigation, the shape evolution mechanism and structure-property dependence in catalytic aspect are also involved.;Pt3Co and Pt3Fe concave nanocubes bounded by high-index facets were innovatively synthesized via developed co-reduction approach in organic solution. It was further revealed that the particle size and concavity could be controlled by precisely tuning a number of parameters such as ratio between oleylamine and oleic acid, capability of metal carbonyl, metal valence in the precursor and ratio of metal precursors. In addition, the growth progress was extensively surveyed and proposed to be highly dependent on an appropriate nucleation process with a successive anisotropic overgrowth and a preservation of the resultant high-index planes by control-binding of capping agents. Moreover, hydrogenation of styrene and reduction of 4-nitrophenol were selected as model reactions to evaluate the catalytic properties of as-synthesized Pt3 Co and Pt3Fe concave nanocubes. These concave nanocubes as a new class of nanocatalysts with more opened structure and active atomic sites located on their high-index crystallographic planes exhibit an enhanced catalytic activity in comparison with low-indexed surface terminated Pt 3Co nanocubes in similar size. Meanwhile, the intentionally introduced transition metals (Co and Fe) not only reduce the overall preciousness but also endow them with superior catalytic performances due to their modified surface electronic structures, when comparing with pure Pt counterpart.;Besides, tetrahexahedral (THH) Pt-Ni NCs were synthesized in a similar system with surface coverage of {730} high-index facets and extraordinary spatial distribution of Pt and Ni atoms. Specifically, the edges of THH are abundant in Pt forming a compositionally segregated structure, whereas the Ni component behaves as paddings. An innovative solid-state annealing was implemented as a supplement of the solution-phase processing, unexpectedly, induced the relocation of Ni moieties through a channel from surface to interior and eventually resulted in the generation of hollow alloy nanostructures. The overall morphologies, surface details and compositional distribution were well resolved and carefully examined by TEM, HRTEM, XRD and HAADF-EDS. Coordinatively unsaturated atoms on the retained THH rims and open architecture allowing more accessibility to reactants merit the Pt-Ni THH nanoframes pronounced enhancement in catalytic activity and distinctive reaction pathway for the formic acid oxidation.;It is as though this dissertation work resides on a fundamental level, the insights attained in the study could be further extended to a more broad range of nanomaterials. The size-, shape- as well compositional distribution-controlled NCs would be the most promising building units applied in a variety of technologically important fields such as bio-imaging, electronic packaging and clean energy production.