Temperature-dependent segregation on Pt25Rh75(111) and (100)

Surface segregation is studied on Pt25Rh75(111) and Pt25Rh75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80 at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt25Rh75(111) until 1000 degrees C, whereas it reaches a maximum enrichment around 500 degrees C on Pt25Rh75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt25Rh75(100) surface by the interchange of Pt and Ph atoms within the near-surface layers, and on the Pt25Rh75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100 degrees C. The presence of carbon contamination on the Pt25Rh75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED. (C) 1999 Published by Elsevier Science B.V. All rights reserved.