Controllable polyol synthesis of uniform palladium icosahedra: effect of twinned structure on deformation of crystalline lattices.

Palladium plays a key role in technologies used for hydrogen storage, hydrogen purification, water treatment, and fuel cells. Palladium is also widely used as the primary catalyst for low-temperature reduction of automobile pollutants, organic reactions, hydrogenation, and petroleum cracking. These numerous applications result in palladium drawing considerable interest. Most applications of palladium are related to its remarkable hydrogen-adsorption capacity. A recent study indicated that icosahedral palladium nanoparticles (Pd NPs) can absorb a larger quantity of hydrogen than their cuboctahedral analogues. Moreover, the catalytic activity of a metal NP is commonly enhanced by surface atoms located on the corners and edges. Accordingly, icosahedral Pd NPs, with a high density of twins and corners on their surfaces, are expected to be the most active catalysts, and this has led to an explosion of interest in their synthesis. Much effort has been devoted to synthesizing various metal NPs with specific shapes in aqueous or nonhydrolytic media. Among these strategies, the polyol process is a convenient, versatile, and low-cost route for the synthesis of metal NPs. In recent years, this technique has been further modified through the introduction of polymers, foreign ions, and seeds, as well as careful regulation of reaction temperature, to yield metal NPs with well-defined sizes and geometric shapes. 8,12, 13] Initial nucleation is known to be one of the determining factors for the shape of final products. The metal nuclei can adopt single-crystal, singly twinned, or multiply twinned structures. Compared with gold, multiply twinned Pd nuclei are highly susceptible to oxidation under the reaction conditions. Multiply twinned Pd NPs are depleted in favor of stable single-crystalline NPs during the growth process. Consequently, the shape of the final Pd products of a solution-phase synthesis is restricted to spherical NPs, single-crystalline plates, bars, rods, cubes, and cuboctahedra owing to highly oxidative etching, poorer protection of the twinned structures, and fast reduction and growth rate. 16] In comparison, under slow reaction conditions, nucleation of metal atoms and growth of nuclei can be kinetically controlled through polymers or foreign ions. The icosahedral structure is favored over both decahedral and cuboctahedral structures for Pd at small sizes (number of Pd atoms N< 309). Therefore, it is possible to selectively synthesize high-quality Pd icosahedra by carefully manipulating the growth process of icosahedral seeds generated by slow reactions. Thus, Pd icosahedra were synthesized in 80% yield by a water-based synthetic strategy. To the best of our knowledge, it remains a challenge to control the synthesis of uniform Pd icosahedra in high yield. Here we present a facile and effective polyol route for controllable synthesis of icosahedral Pd NPs with uniform size in ethylene glycol (EG) solution. A high yield of icosahedral Pd NPs was obtained in a one-pot reaction. Furthermore, the dimensions of the icosahedral NPs can be readily tailored from 15 to 42 nm by tuning the experimental parameters. A unique powder X-ray diffraction (PXRD) pattern of Pd owing to the multiply twinned structure was observed for the first time for Pd icosahedra. The as-synthesized Pd icosahedra are stable in air for months. Compared with spherical Pd NPs, Pd icosahedra can maintain their high catalytic activity even after several cycles. For a typical synthesis of Pd icosahedra, an EG solution containing a given amount of sodium chloride (NaCl), polyvinylpyrrolidone (PVP, Mw = 360 000), and sodium tetrachloropalladate (Na2PdCl4) was vigorously stirred and heated in air at an appropriate temperature (see the Supporting Information). Transmission electron microscopy (TEM) images demonstrate that the Pd NPs synthesized in an EG solution containing 5 mm Na2PdCl4, 10 mm NaCl, and 200 mm PVP have a hexagonal projection with a size of 31 2 nm (Figure 1a and b). Energy-dispersive X-ray spectroscopy (EDS, Figure S1, Supporting Information) and field-emission scanning electron microscopy (FESEM, Figure 1c) analyses showed that these Pd NPs consist of only Pd and have an icosahedral shape. Figure 1 d shows a high-resolution TEM [*] Dr. C. Li , R. Sato, Dr. M. Kanehara, Prof. T. Teranishi Department of Chemistry Graduate School of Pure and Applied Sciences University of Tsukuba Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan) Fax: (+ 81)29-853-4011 E-mail: teranisi@chem.tsukuba.ac.jp Homepage: http://www.chem.tsukuba.ac.jp/teranisi/index_E.html