Electrocatalysis of Pt-Fe alloys produced by ion beam mixing
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Ion beam mixing using 120 keV Fe+ ions at doses varying from 3.0 to 20 x 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 =S 373 K, mixing was athermal and took place by ballistic collisional processes. At T 3= 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 H 2 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.
I. INTRODUCTION A potentially attractive application of surface modification techniques is the formation of electrodes for use in electrocatalysis. For such applications, a high surface concentration of a catalytically active component is required. Such materials are usually the expensive noble metals. Hence, a desirable alternative is to use a less expensive base metal electrode with its surface modified in such a way to be as catalytically active as the noble metal catalyst. Thermal annealing processes are usually not useful in making electrodes, as grain boundary diffusion can lead to a nonuniform surface which can be easily corroded in the electrolyte. Both ion implantation1 and ion beam mixing2'3 have been used to modify the electrochemical properties of surfaces. In this investigation, ion beam mixing of a Pt-Fe bilayer was carried out using 120 keV Fe+ ions over a temperature range of 298-523 K. The ion beam mixed surface layer compositions were determined using Rutherford backscattering (RBS) analysis. Additional microstructural characterizations were carried out using transmission electron microscopy (TEM). II. EXPERIMENTAL
In order to study the phenomenon of ion beam mixing between an alloying component and a substrate material it is imperative to ensure that the interface between the materials is free of contamination. This is particularly important for mixing into substrate materials such as iron where a natural oxide, 20 to 40 A thick, is always formed after exposure to air. Consequently, for the current experiments, in which the alloying ele98
J. Mater. Res., Vol. 5, No. 1; Jan 1990
ment, Pt, was to be mixed into an Fe substrate, careful sample preparation techniques were adopted. Two "sets of samples were prepared, one for measuring the extent of mixing and the other for electrochemical measurements. The samples prepared for the mixing studies consisted of about 700 A
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