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

E. Platzgummer, M. Sporn, R. Koller, S. Forsthuber, M. Schmid, W. Hofer, and P. Varga

Surf. Sci. 419 (1999) 236-248

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°C, whereas it reaches a maximum enrichment around 500°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 learn about 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 interchange of Pt and Rh 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°C. The presence of carbon contamination on the Pt₂₅Rh₇₅(100) surface causes a significant reduction of the Pt segregation. There is an excellent agreement between the top layer concentrations derived by LEIS and quantitative LEED.

Corresponding author: P. Varga (varga).

Users with online access to Surface Science can load the article from the publisher.