Segregation processes in PdCu(110) and the effects of sulphur impurity on surface composition and microstructure from annealing

摘要

Various surface compositions (in ～ 4 uppermost layers) are prepared on a PdCu(110) single-crystal alloy. Low energy electron diffraction, Auger electron spectroscopy and thermal desorption spectroscopy of CO are used for surface characterisation. Rejection of foreign atoms (sulphur) from the matrix with increasing temperature or in CO at higher pressure (～ 10 ↑(-3) mbar) is accompanied by drastic changes of the Pd-Cu surface microstructure. Sulphur segregation is accompanied by Cu co-segregation and reverse segregation, surface clustering, an increasing amount of defects on the surface and a decrease of surface roughening temperature. The (1 × 2) structure in the Cu-rich and the Pd-rich surfaces changes by annealing into a Pd-rich (1 × 1) structure, and sulphur segregates from the bulk to the surface. Sulphur atoms statistically bind on Pd atoms in the top layer at atomic concentration ratios S/Pd ≤ 0.2. For higher coverages sulphur develops the commensurate c(2 × 2) overlayer and enriches in grain boundaries of substrate domains. For CO chemisorption there is a correspondence between the electronegativity of the impurity atom and the inhibition of surface adsorption and change of the binding energy. Besides surface microstructure, long-range electronic effects also play an important role in the PdCu single-crystal alloy surface. # 1997 Elsevier Science Ltd. All rights reserved