Herein we report the synthesis and properties of Fe(55)Pt(45) nanoparticles, both monodisperse and self-assembled into hexagonal close-packed and cubic arrays of 4.0 +/- 0.2 nm size in an L1(0) structure, obtained by a modified polyol process. The new synthetic route improved the control over the particle composition, thereby reducing the temperature required to convert from face-centered cubic (fcc) to face-centered tetragonal (fct) phase by some 30-50 degrees C without additives. Annealing at 550 degrees C for 30 min converts the self-assembled nanoparticles into ferromagnetic nanocrystals with large coercivity, H(C) = 11.1 kOe. Reducing the fcc-to-fct (L1(0)) ordering temperature avoided particle coalescence and decreased the loss in particle positional order without compromising the magnetic properties, as is generally observed when additives are used.

In this work, carbon-supported Pt 70 Co 30 nanoparticles were prepared by a polyol process using a long-chain diol as reducer (hexadecanediol) and oleic acid and oleylamine as stabilizers. Depending on the synthesis conditions, Pt 70 Co 30 /C nanocatalysts with very small particle size (1.9 ( 0.2 nm) and narrow distribution homogeneously dispersed on the carbon support and having a high degree of alloying without the need of thermal treatments were obtained. The as-prepared catalyst presents an excellent performance as proton exchange membrane fuel cells (PEMFC) cathode material. In terms of mass activity (MA), the Pt 70 Co 30 /C electrocatalysts produced single fuel cell polarization response superior to that of commercial catalyst. To analyze alloying effects, the result of thermal treatment at low temperatures (200-400 °C) was also evaluated and an increase of average crystallite size and a lower degree of alloying, probably associated to cobalt oxidation, were evidenced.

A phase diagram of the pseudo-ternary Aerosol OT (AOT)+n-butanol/n-heptane/water system, at a mass ratio of AOT/n-butanol=2, is presented. Conductivity measurements showed that within the vast one-phase microemulsion region observed, the structural transition from water-in-oil to oil-in-water microemulsion occurs continuously without phase separation. This pseudo-ternary system was applied to the synthesis of carbon-supported Pt70Fe30 nanoparticles, and it was found that nanoparticles prepared in microemulsions containing n-butanol have more Fe than those prepared in ternary microemulsions of AOT/n-heptane/water under similar conditions. It was verified that introducing n-butanol as a cosurfactant into the AOT/n-heptane/water system lead to complete reduction of the Fe ions that allowed obtaining alloyed PtFe nanoparticles with the desired composition, without the need of preparing functionalized surfactants and/or the use of inert atmosphere.

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