Surface analysis of the PdCu(110) single crystal alloy at different segregation rates

Abstract

The PdCu(110) plane, with Pd: Cu = 1:1 in the bulk, was prepared to have various surface compositions by applying cycles of argon ion sputtering (ion energy of ⩽ 750 eV with ion current densities ⩽ 1 μA/ cm 2) and annealing temperatures ( T AN) of 420 < T < 820 K. With this preparation method different compositions of the top surface layers (TL) were obtained: (i)exclusively Pd atoms; (ii) composition of various Cu Pd ratios, including the bulk ratio; and (iii) exclusively Cu atoms. The surface region (SR) of ~ 4 layers depth analysed by AES, and the CO TDS used for TL-characterization, allowed the average ratio of Cu/Pd in the next three subsurface layers (3SSL) to be evaluated. LEED enabled the determination of the surface structure after each step. The high temperature treatment of the surface followed by low temperature sputtering ( TSP) and TAN below that of Cu segregation ( TSEG) ~ 550 K, produced a smooth Pd-rich surface. On the other hand, a large number of sputtering and annealing cycles without prior high TAN produced a rough surface, enabling the Cu atoms lying beneath the TL to be accessible for surface reactions. A diagram describing LEED structures can be drawn from the results which present the large variety of surface structures for the clean PdCu(110) single crystal surface depending on the T SP and T AN . In addition to this diagram the SR Cu Pd ratio for various TSP and TANis given for slow and fast annealing modes at long and short pretreatment periods. Three phase diagrams comparing experimental surface properties for each of the SR, TL and 3SSL with bulk composition are presented.