Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction.

Synthesis of platinum (Pt) nanoparticles with controlled sizes and shapes is one of the most attractive goals in developing highly active Pt catalysts for fine chemical synthesis.1 Well-dispersed Pt nanoparticles are also an important catalyst for fuel cell reactions: they catalyze hydrogen (or alcohol) oxidation at an anode and oxygen reduction at a cathode.2 Pt nanoparticles are commonly prepared by reducing a Pt salt with hydrogen,3 an alcohol,4 sodium borohydride,5 or hydrazine.6 The syntheses have also achieved partial control of nanoparticle shape in cube,3,4b,5b multipod,4c,7 and one-dimensional nanostructure,8 among which Pt nanocubes are of particular interest in catalysis due to the presence of (100) dominant faces on the particle surface. Studies on oxygen reduction in acid electrolytes show that Pt (100) planes are more active than the (111) ones.9 Using Pt nanocubes as catalyst, one would expect that less Pt is needed in the fuel cell reactionsa goal that has long been sought for commercialization of the fuel cell technology.10 Despite the synthetic progresses made in the past, a reliable process for producing monodisperse Pt nanocubes with controlled sizes is still not available to date. Here we report a simple high-temperature organic phase synthesis of monodisperse Pt nanocubes and study their catalysis for oxygen reduction in a fuel cell reaction condition. We recently reported the synthesis of monodisperse FePt nanocubes by simultaneous decomposition of iron pentacarbonyl, Fe(CO)5, and reduction of platinum acetylacetonate, Pt(acac)2, in the presence of oleic acid and oleylamine.11 Oleic acid and oleylamine were used not only for nanoparticle stabilization but also for nanoparticle shape control. Sequential addition of oleic acid and oleylamine facilitated the formation of FePt nanocubes. Our further synthesis revealed that, regardless of the addition sequence of oleic acid and oleylamine, nearly monodisperse Pt nanocubes could be separated as long as a trace amount of Fe(CO)5 was present during the reaction. Selfassembly of these nanocubes gave a (100) textured array, indicating a very narrow shape distribution in cubic Pt nanoparticles. Electrochemical studies showed that the Pt nanocubes catalyzed oxygen reduction in a 1.5 M H2SO4 aqueous solution with specific activity over 2-fold higher than that from the commercial Pt catalyst. Monodisperse 8 nm Pt nanocubes were synthesized by dissolving Pt(acac)2, oleic acid, and oleylamine in 1-octadecene (ODE) at room temperature and heating the solution at 120 and 200 °C in the presence of Fe(CO)5 solution in ODE.12 Figure 1A shows the typical transmission electron microscopic (TEM) image of the assynthesized 8 nm nanocubes (the lower-resolution TEM image is also given in Figure S1). They are nearly 100% in cubic (or cubelike) shape. High-resolution TEM (HRTEM) image of a single Pt nanocube (Figure 1B) shows the Pt lattice fringes with the interfringe distance measured to be 0.19 nm, close to the lattice spacing of the {100} planes at 0.196 nm in the face-centered cubic (fcc) Pt crystal. Energy dispersive spectroscopic (EDS) analysis on the nanocube assembly shows no evidence of Fe in the nanocubes (Figure S2), indicating that the trace amount of Fe in the synthesis does not have measurable effect on the particle composition. The crystal structure of the Pt nanocubes was characterized by X-ray diffraction (XRD). Figure 1C shows the XRD pattern of the fcc structured 8 nm Pt nanocubes. Slow evaporation of the hexane dispersion (30 min) of the 8 nm Pt nanocubes on a silicon substrate led to a textured assembly, as shown in Figure 1D. The strong (200) peak in the diffraction pattern indicates that the Pt nanocubes align flat on the substrate. This further proves that the nanocubes have a very narrow shape distribution. The Pt nanocubes were synthesized by high-temperature (200 °C) reduction of Pt(acac)2. The presence of the trace amount of Fe(CO)5 was the key to achieve this shape control. Without Fe(CO)5, Pt nanoparticles were still prepared, but the shape of the particles was much less controlled (Figure S3). In the presence of Fe(CO)5, the cubes were obtained by controlling the heating rate at 3-5 °C per minute to 200 °C and heating at this temperature for 30 min.12 TEM image of the reaction intermediate separated from the mixture 15 min after the addition of Fe(CO)5 shows the presence of ∼3 nm nanoparticles with polyhedral shapes (Figure S4). This suggests that Pt nuclei produced in the presence of trace amount of Fe(CO)5 have polyhedral shapes, and nanocubes evolve from the growth of these polyhedral Pt nuclei. The shape of the Pt particles was also controlled by the presence of oleylamine and † Brown University. ‡ Hitachi Maxell, Ltd. Figure 1. (A) TEM image of the 8 nm Pt nanocubes; (B) HRTEM image of a single Pt nanocube; (C) XRD pattern of the 8 nm Pt nanocube array; and (D) XRD pattern of the self-assembled 8 nm Pt nanocubes showing (100) texture. Published on Web 05/15/2007