Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy PtxY is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model PtxY nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mgPt(-1) at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of PtxY over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core.

A matter of size: The particle size effect on the activity of the oxygen reduction reaction of size‐selected platinum clusters was studied. The ORR activity decreased with decreasing Pt nanoparticle size, corresponding to a decrease in the fraction of terraces on the surfaces of the Pt nanoparticles (jk=kinetic current density, see picture).

We report on a real-time in situ TEM study of the coalescence of individual pairs of decahedral gold nanoparticles, which have been synthesized in solution. We observe the rate of growth of the neck that joins two particles during coalescence and compare this to classical continuum theory and to atomistic kinetic Monte Carlo simulations. We find good agreement between the observations and the simulations but not with the classical continuum model. This disagreement is attributed to the faceted nature of the particles.

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Eine Frage der Größe: Der Einfluss der Partikelgröße auf die Aktivität größenselektierter Platin‐Cluster in der Sauerstoffreduktionsreaktion (ORR) wurde untersucht. Die Aktivität der Pt‐Nanopartikel sank mit kleiner werdender Größe der Nanopartikel, entsprechend einer Abnahme des Anteils an Terrassen auf der Oberfläche der Pt‐Nanopartikel (jk=kinetische Stromdichte, siehe Bild).

We use kinetic Monte Carlo simulations to investigate the coalescence of fcc nanoparticles via lattice-based diffusion of surface atoms. The radius of the neck region connecting the two nanoparticles is found to develop with characteristic power laws r ϳ t a with a ϳ 1 3 and a ϳ 1 6 for the early and intermediate stages of coalescence, respectively. For late coalescence stages, when the nucleation of new atomic layers on nanoparticle facets is required for further coalescence, the nanoparticle size, temperature, and nanoparticle orientation all influence the development of the neck. In contrast, classical theory predicts an approximately constant value of a͑ϳ 1 6 ͒. We also examine the temperature dependence of the equilibration times for relaxing nanoparticles and distinguish the limiting processes to be nucleation of new germs on a facet and/or the detachment of atoms from atomic layers.

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Dynamical response of surface metallic states in single crystalline ultrathin Bi(001) films on Si(111) 7 Â 7 surface was investigated at a spectral range of 0.1-12 THz by broadband terahertz time-domain spectroscopy. The observed transmittance increased with a decrease in the thickness, without showing a gap structure. The measured complex dielectric dispersion was analyzed using a Drude model, and the plasma frequency (x p) and damping constant (c) were found to be inversely proportional to the thickness. The results strongly indicate the existence of surface metallic states, whose carrier density and damping constant are estimated to be 3.08 Â 10 19 cm À3 and 4.83 Â 10 2 THz, respectively. V