STM STUDIES OF OXYGEN-INDUCED RECONSTRUCTION ON A PT-RH(100) ALLOY SURFACE

Oxygen-induced reconstruction of a Pt-Rh(100) alloy surface was studie d by scanning tunneling microscopy (STM). When a clean Pt-enriched Pt- Rh(100) surface was exposed to O-2 at more than 600 K, oxygen-induced reconstruction took place with segregation of Rh atoms to the surface. Depending on the coverage of oxygen, two types of reconstruction on t he Pt-Rh(100) alloy surface are induced. A p(3 x 1)-O structure appear ed at relatively low oxygen coverage, and the TPD spectrum of the p(3 x 1)-O surface gave a desorption peak of O-2 at ca. 930 K. The p(3 x 1 )-O structure showed several different STM images depending on the tip condition, from which we could distinguish the two component metals, Rh-O rows and Pt rows. Based on this, a model structure was deduced. A t a high oxygen coverage, more Rh atoms were segregated to the surface and the STM image suggests a c(2 x 20)-O structure. The c(2 x 20)-O s urface changed into the p(3 x 1)-O by heating with the concomitant des orption of O-2 at ca. 830 K. The STM images for this c(2 x 20)-O surfa ce is analogous to the arrangement of Ph atoms on the top layer of a R h(lll) hexagonal lattice. Due to the lattice mismatch between the quas i-Rh(lll) overlayer and Pt-Rh(100) substrate, a moire pattern with 20 times periodicity along the [011] directions was observed. Both the p( 3 x 1)-O and c(2 x 20)-O surfaces are easily reduced by H-2 even at ro om temperature.