Compositional variations in the near surface layers, an atom-probe study of cosegregation of sulfur in Pt–Rh and Pt–Ir alloys

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Abstract
Equilibrium composition depth profiles with true single atomic layer depth resolution have been obtained for the (001) plane of a Pt–Ir and five Pt–Rh alloys using the atom-probe field ion microscope. Pt segregates to the top two layers in Pt–Ir and Rh segregates to the top layer in Pt–Rh alloys after annealing at 700 °C for 5 min. In addition, an overlayer of sulfur is found to cosegregate to the surface of these alloys even though the bulk impurity content is less than 100 ppm. The coverage of sulfur overlayer for Pt–Rh alloys varies between 0.22 to 0.52 of a monolayer, and it increases with increasing surface Rh concentration. The depth dependence of the composition depth profiles has been found to be monotonic for the Pt–Ir alloy and nonmonotonic, or possibly oscillatory, for the Pt–Rh alloys. Pending the uncertainty of the effect of the sulfur overlayer, Pt segregation in Pt–Ir is consistent with most theoretical models, and Rh segregation in Pt–Rh is consistent with the bond-breaking model and the atomic model, but is inconsistent with a surface energy model and an electronic model. Rh segregation may also be an effect of chemisorption enhanced surface segregation. While the experimental result on the top surface layer can be explained by various theoretical models, the compositional variation in the near surface layers may shed some light on the atomic interactions in the surface region. For the Pt–Ir alloy, we have also observed a diffusion kinetic effect in a composition depth profile.