STM studies of oxygen-induced reconstruction on a PtRh(100) alloy surface

Oxygen-induced reconstruction of a PtRh(100) alloy surface was studied by scanning tunneling microscopy (STM). When a clean Pt-enriched PtRh(100) surface was exposed to O2 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 the PtRh(100) alloy surface are induced. A p(3 × 1)-O structure appeared at relatively low oxygen coverage, and the TPD spectrum of the p(3 × 1)-O surface gave a desorption peak of O2 at ca. 930 K. The p(3 × 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. At a high oxygen coverage, more Rh atoms were segregated to the surface and the STM image suggests a c(2 × 20)-O structure. The c(2 × 20)-O surface changed into the p(3 × 1)-O by heating with the concomitant desorption of O2 at ca. 830 K. The STM images for this c(2 × 20)-O surface is analogous to the arrangement of Rh atoms on the top layer of a Rh(111) hexagonal lattice. Due to the lattice mismatch between the quasi-Rh(111) overlayer and PtRh(100) substrate, a moire pattern with 20 times periodicity along the 〈111〉 directions was observed. Both the p(3 × 1)-O and c(2 × 20)-O surfaces are easily reduced by H2 even at room temperature.