Structural and Architectural Evaluation of Bimetallic Nanoparticles: A Case Study of Pt−Ru Core−Shell and Alloy Nanoparticles

Abstract

A comprehensive structural/architectural evaluation of the PtRu (1:1) alloy and Ru@Pt core-shell nanoparticles (NPs) provides spatially resolved structural information on sub-5 nm NPs. A combination of extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), pair distribution function (PDF) analyses, Debye function simulations of X-ray diffraction (XRD), and field emission transmission electron microscopy/energy dispersive spectroscopy (FE-TEM/EDS) analyses provides complementary information used to construct a detailed picture of the core/shell and alloy nanostructures. The 4.4 nm PtRu (1:1) alloys are crystalline homogeneous random alloys with little twinning in a typical face-centered cubic (fcc) cell. The Pt atoms are predominantly metallic, whereas the Ru atoms are partially oxidized and are presumably located on the NP surface. The 4.0 nm Ru@Pt NPs have highly distorted hcp Ru cores that are primarily in the metallic state but show little order beyond 8 A. In contrast, the 1-2 monolayer thick Pt shells are relatively crystalline but are slightly distorted (compressed) relative to bulk fcc Pt. The homo- and heterometallic coordination numbers and bond lengths are equal to those predicted by the model cluster structure, showing that the Ru and Pt metals remain phase-separated in the core and shell components and that the interface between the core and shell is quite normal.

Ordered bilayer ruthenium–platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts

“…Ru(hcp) and Pt(fcc), both consist of closely packed planes of atoms, differing in their stacking sequence: The hcp layers cycle between two shifted positions, expressed as ABAB, whereas the fcc layers cycle between all three equivalent shifted positions, that is, ABCABC. Their distinctive XRD features were used in previous studies to distinguish the Ru@Pt core-shell NPs, synthesized by using a sequential polyol process at 200°C, from the Ru-Pt alloy NPs 21,25 . We obtained a similar XRD profile for the Ru@Pt sample prepared with the as-synthesized Ru cores (Fig.…”

“…It differs from that for pure Pt NPs by the notable intensity between the fcc Pt (111) and (200) peaks, originating from the hcp-Ru (002) and (101) diffractions. Because of the relative weakness of the hcp diffractions, the Ru cores were considered to be highly disordered 25 .…”

“…These profound thickness-dependent effects suggest the need and the opportunity for advancing the HOR catalysts' CO tolerance and corrosion resistance via precisely controlling the thickness of well-defined Pt shells. However, it was questioned whether a sharp Ru-Pt core-shell interface is possible at the atomic level, because Ru cores were considered disordered in previously synthesized Ru@Pt NPs based upon the commonly observed weak Ru intensities in their X-ray diffraction (XRD) profiles 21,25 .Here we develop an economically viable method for reliably producing well-ordered Ru@Pt NPs. With in-depth characterizations and analyses of the hcp to fcc structural phase transition at the Ru-Pt core-shell interface, we find that defect-induced partial alloying is the cause of Ru disordering, and so succeed in fabricating single crystalline Ru@Pt NPs with well-defined, energetically favourable hcp to fcc structural transition at the core-shell interface.…”

“…These profound thickness-dependent effects suggest the need and the opportunity for advancing the HOR catalysts' CO tolerance and corrosion resistance via precisely controlling the thickness of well-defined Pt shells. However, it was questioned whether a sharp Ru-Pt core-shell interface is possible at the atomic level, because Ru cores were considered disordered in previously synthesized Ru@Pt NPs based upon the commonly observed weak Ru intensities in their X-ray diffraction (XRD) profiles 21,25 .…”

CO-Tolerant PtMo/C Fuel Cell Catalyst for H2Oxidation

“…A powerful aspect of the analysis of X-ray absorption near edge structure (XANES) lies in its capability of providing useful information on changes in the electronic structure of Pt-based alloy catalysts. [33][34][35][36] Typically, the Pt d-band vacancy is derived from the analysis of Pt L III edge, which is created by the excitation of 2p 3/2 electrons to empty states in the vicinity of Fermi level. The change in the Pt L III peak is normally interpreted as the variation in Pt d-band vacancy: the intensity of Pt L III peak varies proportionally with the increase in Pt d-band vacancy.…”