Pd‐Catalyzed Growth of Pt Nanoparticles or Nanowires as Dense Coatings on Polymeric and Ceramic Particulate Supports

Abstract

This Communication describes an approach that can control the growth of Pt into shells that consist of nanoparticles or nanowires on colloidal spheres. Core/shell particles have been extensively studied largely because of their attractive properties (optical, mechanical, magnetic, or catalytic) that are often different from their bulk counterparts. As a result of their unique features, core/shell particles hold promise in potential applications such as controlled delivery, catalysis, magnetic information storage, optical sensing, and confinement of reactions. Many research efforts have been directed towards the development of new techniques for “engineering” such materials with well-controlled properties. In general, the properties of core/shell particles can be tailored precisely by varying the composition, dimension, and structure of the cores or shells. One-dimensional (1D) nanostructures, especially nanowires, have attracted much attention due to their potential use as interconnects in fabricating electronic devices. However, with respect to the procedure and cost effectiveness, producing nanowires is far from being trivial. It remains a grand challenge to develop a plausible method for generating large quantities of nanowires from various metals. Here we present a versatile approach capable of generating core/ shell particles, with the cores being polymer or silica beads and the shells being dense, uniform coatings of Pt nanoparticles or quasiradial Pt nanowires. We recently discovered that the introduction of a trace amount of iron species (Fe or Fe) to the polyol process could induce the formation of Pt nanowires or multipods by significantly reducing the net reduction rate of the salt precursor. We have also shown that these nanowires could be catalytically activated to grow from the surface of micrometersized aggregates consisting of Pt nanoparticles. Herein, we demonstrate a more affordable and practical method for growing Pt nanowires by using Pd-coated colloidal spheres to imitate the catalytic property of the Pt aggregates. Moreover, this procedure can be modified to grow thick, uniform shells composed of Pt nanoparticles. The key to the success of these syntheses are Pd nanoparticles (2–4 nm in size) that can be readily generated in situ as sub-monolayers on colloidal spheres terminated by an amino functional group by reducing a palladium precursor with ethanol under sonication. No growth of Pt nanoparticles or nanowires was observed when there were no Pd nanoparticles on the surface of the colloidal spheres. We note that such Pd nanoparticles have been widely used as a catalyst in the electroless deposition of thin films of metals such as Ni, Cu, and Ag on various substrates. In the present work, Pt nanowires with aspect ratios of up to 30 could be controllably grown through an iron-mediated polyol reduction when there was a catalytic metal exposed on the substrate surface. Furthermore, shells of Pt nanoparticles with thicknesses of up to 100 nm could be formed by reducing the amount of iron species added to the reaction solution. Figure 1 illustrates the multistep protocol designed to grow Pt nanoparticles or nanowires as thick coatings on colloidal spheres. In the first step, amino-functionalized melamine beads were sonicated in a solution of [PdCl4] 2– and ethanol for 1 h, resulting in the direct attachment of Pd nanoparticles to the polymer surface as a sub-monolayer (product A in Fig. 1). Here, ethanol acts as a reducing agent to produce Pd from Pd. The amino-terminated coupling agents on the substrate surface might also act as a primer to attract the Pd nanoparticles, which nucleated in the solution phase. Dokoutchaev et al. have reported that the surface of amino-derivatized polymer beads showed a large affinity for Pd nanoparticles. The resulting beads were recovered from the reaction solution by centrifugation and washed several times with ethanol and water. In the next step, the Pd-coated beads were dispersed in ethylene glycol (EG) and heated to 110 °C. After heating for 1 h to activate the immobilized Pd nanoparticles and to decompose some EG to aldehyde, specific amounts of H2PtCl6/EG and poly(vinyl pyrrolidone)/EG (PVP/EG) were added dropwise to the reaction solution. At this stage, H2PtCl6 was reduced by the aldehyde to form a Pt II intermediate, which could remain in this state at room temperature for more than one month without being reduced further to Pt. The solution was continuously heated for another 2 h (or until the solution turned yellow-green) to ensure C O M M U N IC A IO N