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Chemisorption studies on the Rh(100) and Zr(0001) surfaces

University of British Columbia

The work in this thesis includes diffracted beam intensity measurements with low-energy electron diffraction (LEED) for surfaces formed by the chemisorption of phosphorus and sulphur on to the (100) surface of rhodium, and by the chemisorption of oxygen and sulphur on to the (0001) surface of zirconium. The actual surfaces considered are designated Rh(100)-c(2X2)-P, Rh(1OO)-p(2X2)-S, Rh(100)-c(2X2)-S, Zr(0001 )-(2X2)-O, Zr(0001)-(1X1 )-O (low coverage ), Zr(0001)-(1X1)-O ( high coverage ) and Zr(000l)-(3X3)-S. In each case intensity-versus-energy I(E) curves were measured with a video LEED analyzer (VLA) for a set of independent diffracted beams. For particular surfaces the intensities measured have been analyzed with multiple scattering calculations for various structural models, the search for best correspondence between measured and calculated I(E) curves being undertaken with LEED R factor methods. In other cases the data measured here provide a resource for subsequent multiple scattering analyses. The multiple scattering analysis for the Rh(100)-c(2x2)-P surface structure including normal and off normal incidence measurements considered regular overlayer models; this study indicates that the P atoms adsorb on the “expected” 4-fold coordination adsorption sites with the P-Rh interlayer spacing close to 1.13 A, and the nearest neighbor P to Rh bond distance equal to 2.13 A. A high level of correspondence has been reached between my measured 1(E) curves for the Rh(100)-c(2X2)-S and Rh(100)-p(2X2)-S surfaces and those calculated with inclusion of relaxations for both the metallic and overlayer structures. Briefly, the S atoms are indicated to shift by approximately 0.28 A from the 4-fold coordinate site towards the bridge site; the two pairs of S-Rh bond distance are found to be close to 2.15 and 2.46 A. An initial multiple scattering analysis for the Zr(0001 )-(2X2)-O surface indicates that the most probable structure has oxygen occupying octahedral holes between the first and second layers of hcp zirconium. This analysis does not support a fcc-type reconstruction for the metallic structure reported earlier. The zirconium to oxygen interlayer spacing is indicated to equal 1.30 A, and the nearest neighbour Zr-O bond length is 2.25 A. Because of its importance in CANDU technology there is a need to study the surface chemistry of Zr02 single crystals; however, conventional preparation procedures have not given samples useful for surface science study. Therefore we have initiated a new project to explore the preparation of very thin films developed on appropriate substrates. We have found that the deposition on to a Au(111) surface of a few monolayers of zirconium in an oxygen atmosphere of 10 Torr can yield, after a brief anneal at around 1025 K, thin crystalline films of cubic zirconium oxide grown parallel to the (111) plane. With heating to 675 K during the deposition process, layer-by-layer growth occurs, whereas in the absence of such additional heating island formation provides the better growth model. In the latter case facets can be formed, apparently from the ordering of atomic steps. With prolonged heating of the Zr02 film at 1025 K, or above, there is a decrease in the Auger signals of both Zr and 0, and an increase for Au, but further work is needed to distinguish between the mechanisms of overlayer agglomeration and intermetallic mixing.

Thesis/Dissertation

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2008-12-17

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http://hdl.handle.net/2429/3022

Chemistry

University of British Columbia

UBCV

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