Formation of Hollow Nanocrystals Through the Nanoscale Kirkendall Effect

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Platinum acetylacetonate was reduced with a long-chain polyol to form uniform platinum nanoparticles in the presence of surfactants such as oleic acid oleylamine and trioctylphosphine. The size of the platinum particles was tuned from 1 to 10 nm depending on the concentration of surfactants. Co 2 (CO) 8 was then injected into the hot solution and decomposed to form a conformal coating on platinum nanocrystals. Oxidation of the Pt@Co nanocrystals was performed a few minutes after we introduced the cobalt carbonyl by blowing a stream of O 2 /Ar mixture (1:4 in volume ratio 120 ml/min) into the colloidal solution at 455 K. The system was kept under stirring for 3 hours. A black stable colloidal dispersion in o -dichlorobenzene was obtained. Finally the Pt@CoO particles were precipitated by methanol washed with toluene and methanol three times and dried under vacuum. Typical nitrogen adsorption/desorption measurement on the powder at 77 K showed a type IV isotherm with type H2 hysteresis with a Brunauer-Emmet-Teller surface area of 65 m 2 /g and a total pore volume of 0.0676 cm 3 /g.

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Rates were measured on a per-gram basis. They were normalized per mole of surface platinum species (Pt s ) to obtain a turnover frequency (molecule Pt s –1 s –1 ). Moles of Pt s was determined by D = 1.13/ d where D is the platinum dispersion [the ratio of Pt s to the total platinum content (Pt t )] and d is the particle size in nm. The platinum particle size was determined from number average TEM measurements.

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We thank J. Fréchet for the valuable discussions. Supported by the Air Force Office of Scientific Research under award no. F49620-01-1-0033; by the Director Office of Energy Research Office of Science Division of Materials Sciences of the U.S. Department of Energy under contract no. DE-AC03-76SF00098; and by the Ford Motor Company and the Berkeley Catalysis Center (R.M.R.).