Surfactant‐Free Platinum‐on‐Gold Nanodendrites with Enhanced Catalytic Performance for Oxygen Reduction

Platinum and its alloys play important roles in many industrial applications, such as CO/NOx oxidation in catalytic converters, synthesis of nitric acid, oil cracking, and fuel cells. In particular, platinum has been the most effective catalyst in proton-exchange membrane fuel cells (PEMFCs) owing to its outstanding electrocatalytic characteristics, which facilitate both hydrogen oxidation and oxygen reduction. Although carbon-supported platinum nanoparticles are currently used as cathode catalysts in fuel-cell technology, the commercialization of this technology for automotive applications still requires more economical and effective catalytic materials that can operate with a much smaller amount of expensive platinum. Over the last decade, pioneering researchers reported that the electrocatalytic performance of platinumbased materials can be improved by controlling the morphology of platinum nanocrystals or alloying platinum with other transition metals. In particular, the recently developed platinum nanocrystals with dendritic structures displaying a large number of edges and corner atoms exhibit dramatically enhanced catalytic activity in the oxygen reduction reaction (ORR), the slow kinetics of which is a major problem limiting the efficiency of PEMFCs. Although there has been considerable progress in the preparation of platinum nanodendrites through metal seed or block copolymer mediated processes, the lack of a facile synthetic route has limited the practical applications of nanodendrites. In particular, there is strong demand for a new synthetic method for the mass production of nanodendrites. In this context, the study described herein examines the possibility of large-scale synthesis of platinum nanodendrites in a controllable manner using the recently developed hollow nanoreactor, consisting of a porous silica nanoshell and an entrapped Au nanocrystal, which can confine the growth of metal species inside the silica cavity. The hollow silica nanoreactors provide a consistent and well-isolated environment for the growth of nanocrystals, which enables the morphologycontrolled synthesis of Pt nanodendrites even from a highly concentrated reaction suspension. Moreover, the porous silica shell can be removed readily under basic conditions, leaving a Pt nanodendrite in a surfactant-free form. The surfactant-free Pt dendrites were ready for catalytic applications without additional surfactant-removing processes under harsh conditions, which frequently cause the deformation of the nanocrystals and the decline of catalytic activities. In addition, the surfactant-free nature of the nanocrystals could make it possible to lend the surface various properties and functions through simply coordination of functional ligand molecules. Herein we report the novel synthesis of Pt nanodendrites by Au-seed-mediated growth inside hollow silica nanospheres. The proposed synthetic protocol based on isolated nanoreactors is quite remarkable in terms of the product yield per unit reaction volume compared to traditional cappingagent-based synthesis, which produces only a few milligrams of Pt nanodendrites per milliliter of reaction suspension. The nanoreactor-based synthetic route enabled the synthesis of as much as 1.5 g of uniform Pt nanodendrites from a single reaction in 40 mL aqueous suspension. The prepared ligandfree Pt nanodendrites exhibited greater ORR activity than commercial Pt black catalysts. We also report the extendable utility of the current method to prepare Pt nanodendrite colloid with tunable dispersity and hybrid nanocrystals of various metals. Scheme 1 summarizes the general synthetic routes for various Pt-based nanoparticles based on seed-mediated growth inside porous hollow silica nanospheres. The hollow nanoreactor (Au@h-SiO2), which consists of a porous silica nanoshell with inner and outer diameters of (14 2) and (28 2) nm, respectively, and a (4.0 0.6) nm Au nanocrystal captured inside the cavity, was prepared by selective etching of Fe3O4 from a silica nanosphere encapsulating a Fe3O4/Au hybrid nanocrystal. When an aliquot of Na2PtCl4 was added to an aqueous suspension containing Au@h-SiO2 nanospheres