par Visart de Bocarmé, Thierry ;Bar, Thomas ;Kruse, Norbert
Référence International Field Emission Symposium (IFES) (46: 2000-07-10->14: Pittsburgh, PA, USA)
Publication Non publié, 2000-07-10
Communication à un colloque
Résumé : The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals was studied using video-FIM (Field Ion Microscopy) at temperatures between 350 and 550 K and up to 2x10-2 Pa total pressure. Following on previous works (1,2) a polyhedral Rh crystal was prepared by reaction with oxygen. Subsequent imaging at 500 K in a hydrogen rich gas mixture (H2/O2 = 9) revealed considerable morphological changes of the (001)-oriented field emitter tip, i.e. the extension of low index planes and an enlargement of the {137} planes detrimental to {012}. The faceting of planes lying along the <110> zone lines was also observed. In further experiments the hydrogen partial pressure was slowly decreased while keeping the oxygen pressure constant and monitoring time dependencies by means of a video-recorder. {110} planes were first observed to transform from Oad/Had into Oad-covered. The Oad-covered region then extended anisotropically towards the central (001) pole. A cross-like pattern appeared and divided the apex into four equivalent quadrants with {111} planes in their centers. With a further decrease of the hydrogen partial pressure these quadrants also changed independently from Oad/Had to Oad-covered. The transformation was reversible upon increasing the hydrogen pressure back to its initial value. However, a hysteresis behavior was observed, i.e. a larger hydrogen partial pressure was found to be necessary to establish the initial patterns of a reactive Oad/Had layer. We associate this behavior with the reduction of small clusters of RhxOy formed during adsorption and reaction of pure oxygen with Rh in the presence of an electric field of ~13V/nm. By varying the temperature from 350 to 550 K a kinetic phase diagram was established for the Oad/Had system.

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