Surface Science 409 (1998) 252–257 Morphological changes of Rh crystallites in the presence of adsorbed oxygen C. Voss, N. Kruse * Chemical Physics of Surfaces and Heterogeneous Catalysis, Free University of Brussels, CP243, 1050 Brussels, Belgium Received 19 December 1997; accepted for publication 2 March 1998 Abstract We studied the oxygen-induced reconstruction of Rh tips by means of field ion microscopy. The overall 3D morphology of the clean tip was transformed from nearly hemispherical to polyhedral in the presence of adsorbed oxygen at temperatures between 500 and 550 K. Shape transformation is associated with a strong coarsening of the crystal and the appearance of facets with {111} and {001} orientation. The only high-index planes present in the polyhedral form are of {137} symmetry. Local reconstruction forms of the missing-row type were identified in {011} and {113} planes of the tip surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Field ion microscopy; Oxygen adsorption; Reconstruction; Rhodium; Subsurface diffusion 1. Introduction reconstruction forms may appear along with over- all changes in the particle morphology. From the viewpoint of phenomenological thermodynamics, Metallic surfaces frequently undergo structural the concept of the minimization of surface free modifications in the presence of adsorbates. Such energy of individual planes during reconstruction modifications may range from small relaxations in must be replaced by the concept of the minimiza- interatomic distances to drastic reconstructions tion of the total surface free energy. and facetting depending on the surface electronic An excellent example of the rich variety of properties in the presence of adsorbed species. reconstruction forms on extended 2D surfaces is Since the invention of the scanning tunnelling the oxygen/Rh(110) system. Using STM, Leibsle microscope (STM ), a variety of reconstruction et al. [1] and Murray et al. [2] demonstrated the forms have been made visible on extended 2D occurrence of (1×2) missing row reconstructions surfaces of metal single crystals. Much less is for this system. Interestingly, not every second row known about the behaviour of particle surfaces of atoms along the [11 : 0] direction is missing on which simultaneously expose a large number of that surface plane but, depending on the oxygen small planes of different crystallography and size. coverage, only every third, fourth or fifth row is Rather than interacting independently these planes absent. This gives rise to (2×2)p2mg, c(2×6), may communicate so that entirely different local c(2×8) and c(2×10) phases, respectively. A con- siderable mobility of surface atoms or compounds containing oxygen is necessary to bring about these * Corresponding author. Fax: ( +32 ) 2 6505708; e-mail: nkruse@ulb.ac.be reconstructive changes. 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0039-6028(98)00210-6