Electrocatalysis of Pt-Fe alloys produced by ion beam mixing

Ion beam mixing using 120 keV Fe+ ions at doses varying from 3.0 to 20 ⊠ 1015 cm2 was carried out on Pt–Fe bilayer samples over a temperature range from 298 to 523 K to produce alloys of various compositions. The mixing was characterized using Rutherford backscattering with 2 MeV 4He+ ions and transmission electron microscopy. Ion beam mixing and thermal anneals led to grain growth of the polycrystalline films and to formation of Pt3Fe as an alloy phase. At T ≤ 373 K, mixing was athermal and took place by ballistic collisional processes. At T ≥ 473 K, iron migrated rapidly into platinum; the observed activation energy of 0.3 eV suggested that diffusion was of the short-circuit type controlled by lattice vacancy and grain boundary transport. The surface Pt concentration in the mixed films remained high at ∼90 at. %. This resulted in a reduction of ∼25% in the cathodic overpotential compared to pure Fe electrodes for H2 evolution in 30 wt. % KOH solution. An ion beam mixed Pt–Fe surface layer was more stable than Fe coated with evaporated Pt; this is attributed to improved Pt–Fe adhesion as a result of the ion beam mixing process.