Synthesis of Pt-Cu nanodendrites through controlled reduction kinetics for enhanced methanol electro-oxidation.

Metal nanoparticles with dendritic structures are of particular interest because of their different chemical and physical properties compared to polyhedral nanoparticles. Owing to the large surface area and the rich step-surface atoms, dendritic nanostructures have been demonstrated to be more catalytically active than conventional polyhedra for many reactions, such as O2 reduction, [1–3] methanol oxidation, formic acid oxidation, ethanol oxidation, and other small molecule reactions. 9] A small number of synthetic strategies have previously been developed for the synthesis of bimetallic nanodendrites, for example a seed-mediated method has been reported for synthesizing a dense array of Pt branches on a Pd core, with a truncated-octahedral shape, by directly reducing the precursor on the core or by attaching the reduced clusters to the core. Seeded growth has also been demonstrated in nonaqueous systems by using oleylamine to reduce Pt-monolayer particles on Pd cores. In addition, the seeded-growth method has been confined within silica templates to generate Pt-on-Au nanodendrites. Another approach is a block-copolymer-mediated method, in which Pd and Pt precursors were simultaneously reduced by using ascorbic acid, yielding Pt-onPd nanodendrites. In this case, the triblock copolymer assisted the phase segregation to form a core–shell dendritic structure. Although most strategies have focused on core–shell nanodendrites, synthetic methods for alloyed nanodendrites are largely lacking. Recently, alloyed Pt–Ni nanodendrites were obtained through a solvothermal approach, which facilitated the effective diffusion to form alloys in a autoclave device; however, control over the composition remains a challenge. Herein, we report a co-reduction method for the synthesis of alloyed nanodendrites that involves a kinetically controlled reduction, in which ethylene glycol couples to oxidative etchants before fast reduction with ascorbic acid. This approach is initially demonstrated by using a Pt–Cu system. Pt–Cu nanostructures have received considerable interest because of their bifunctionality that synergistically improves their catalytic activity towards electrochemical reactions associated with fuel-cell applications, for example methanol oxidation. For this reaction, Pt is the best-known catalyst for the oxidation of methanol to CO-related intermediate species, whereas Cu is active for CO oxidation with water. Together, Pt–Cu alloys activate both methanol and water in the electrooxidation of methanol, which can effectively remove adsorbed intermediates from the catalyst surface. A number of Pt–Cu nanostructures that further enhance the electrocatalytic activity of methanol have previously been reported, including nanocubes, nanocages, nanorods, and concave structures. Because the catalytic activity improves with an increased number of active sites, nanodendrites, having a large surface area and high density of step-surface atoms, are promising catalyst candidates for heterogeneous catalysis. In this work, Pt–Cu nanodendrites were synthesized for the first time by using a kinetically controlled co-reduction method. The composition and complexity of the nanodendrites were readily manipulated by varying the reaction time of the rate-limiting step. These dendritic nanostructures show superior catalytic activity in the electro-oxidation of methanol. As an initial demonstration, Pt3Cu nanodendrites were synthesized by using a two-step co-reduction of Pt and Cu precursors in a 1:1 molar ratio. The first step was modified from the previously reported polyol synthesis of branched Pt nanoparticles by introducing Cu precursors, and the second step was to accelerate the reduction process by using a stronger reducing agent (i.e. , ascorbic acid). At the initial stage, the precursors were reduced by using ethylene glycol in the air, with a small amount of additives (i.e. , FeCl3 and HCl). These additives and O2 in the air served as oxidative etchants, which could react with the reduced species to return it to the oxidized form. The color of the reaction mixture turned from orange–yellow to green–yellow within 1 h, and the mixture retained the green–yellow color for a long period of time (i.e. , 24 h) before the addition of ascorbic acid, indicating that the reduction of the precursor was very slow. As soon as the ascorbic acid was added to the reaction, the solution turned from green–yellow to dark-brown within 15 min, yielding the nanodendrites with a diameter of approximately 30 nm (Figure 1A). The lattice constant (a) of the nanodendrites was obtained through analysis of the X-ray diffraction (XRD) pattern to be 0.384 nm (Figure 1B). Assuming the linear-mixing rule, the composition of the nanodendrites was estimated as Pt3Cu, based on Vegard’s Law (aPt=0.392 nm and aCu=0.361 nm). [23] [a] E. Taylor, Dr. S. Chen, Prof. J. Chen Department of Chemistry and Biochemistry University of Arkansas, Fayetteville, AR 72701 (USA) Fax: (+1)479-575-6203 E-mail : chenj@uark.edu [b] Dr. J. Tao, Dr. L. Wu, Dr. Y. Zhu Condensed Matter Physics and Materials Science Department Brookhaven National Laboratory, Upton, NY 11973 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300527. Part of a Special Issue on “Shaping Nanostructures for Applications in Energy Conversion and Storage“. To view the complete issue, visit : http://onlinelibrary.wiley.com/doi/10.1002/cssc.v6.10/issuetoc