Platinum Concave Nanocubes with High‐Index Facets and Their Enhanced Activity for Oxygen Reduction Reaction

Abstract

Platinum nanoparticles are widely used as the primary catalysts in a myriad of industrial processes such as CO/NOx oxidation in catalytic converters, nitric acid production, petroleum cracking, as well as hydrogen (or alcohol) oxidation and oxygen reduction reactions in fuel-cell technology. For most catalytic reactions, it has been shown that high-index planes, which are associated with large numbers of atomic steps, edges, and kinks hold the key to the enhancement of catalytic performance in terms of activity and/or selectivity. A number of protocols have been demonstrated for generating Pt nanoparticles enclosed by high-index facets, including those based on electrochemical reduction and heat treatment. For example, Sun and co-workers have reported the synthesis of tetrahexahedral (THH) Pt nanocrystals with high-index facets, such as {730}, {210}, and {520}, by applying a square-wave potential to polycrystalline Pt microspheres supported on a glassy carbon electrode. Although these Pt nanocrystals have been shown to have high catalytic activity, their sizes are still relatively too large and the method of preparation is rather limited in terms of production volume. It still remains a challenge to produce Pt nanocrystals with high-index facets by using a simple, scalable route based on wet chemical reduction. Over the past several years, kinetic control has been demonstrated as a simple and versatile approach to the shapecontrolled synthesis of noble-metal nanocrystals in the solution phase. In general, kinetic control can be achieved by: 1) substantially slowing down the formation rate of atoms, 2) using a weak reducing agent, 3) introducing an oxidation process, and 4) taking advantage of Ostwald ripening. When the concentration of metal atoms in the solution is low, the atoms tend to add to the edges and corners of a seed rather than the entire surface, thus leading to the formation of nanocrystals with thermodynamically unfavorable morphologies, including rods, plates, multipods, and dendritic structures. In recent years, nanocrystals with concave rather than flat faces have attracted attention because of their high-index facets. To this end, Zheng and co-workers have demonstrated the synthesis of concave Pd polyhedral nanocrystals with high electrocatalytic activity for formic acid oxidation. Mirkin and co-workers have also reported the synthesis of concave cubic Au nanocrystals, and demonstrated higher chemical activity compared to octahedra enclosed by lowindex {111} facets. Herein we report the first synthesis of Pt concave nanocubes enclosed by high-index facets including {510}, {720}, and {830} by slowly adding an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 into deionized water by using two syringe pumps. In this synthesis, the formation of a Pt pyrophosphato complex (that is formed by mixing K2PtCl4 and Na2H2P2O7) and the slow addition of this precursor by a syringe pump are believed to play a key role in the formation of Pt concave nanocubes. In this case, the seeds selectively overgrow from corners and edges, and the Br ion serves as a capping agent to block the growth of the h100i axis. The Pt concave nanocubes exhibited substantially enhanced specific activity (per unit surface area) relative to those of Pt nanocubes, cuboctahedra, and commercial Pt/C catalysts that are bounded by low-index facets such as {100} and {111} toward the oxygen reduction reaction (ORR), which is the ratedetermining step in a proton-exchangemembrane (PCM) fuel cell. In a typical synthesis, an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 were prepared separately and then injected simultaneously at an injection rate of 67 mLmin 1 by using two syringe pumps into deionized water maintained at 95 8C. The color of the solution immediately turned from light pink to black upon the addition of the reactant solutions, thus indicating rapid reduction of PtCl4 2 into elemental Pt by NaBH4. Figure 1a shows a typical transmission electron microscopy (TEM) image of the product that contains Pt nanocubes with a concave structure. [*] Dr. T. Yu, D. Y. Kim, Prof. H. Zhang, Prof. Y. Xia Department of Biomedical Engineering Washington University Saint Louis, MO 63130 (USA) E-mail: xia@biomed.wustl.edu D. Y. Kim Department of Chemical and Biomolecular Engineering (BK21 graduate program) Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahangro, Yuseong-gu, Daejeon 305-701 (Korea)