UBC Theses and Dissertations
Studies of the adsorption of C₂H₄, CO and PH₃ on the (OOO1) surface of zirconium Lau, Ji-Ren
The work in this thesis includes crystallographic investigations with low-energy electron diffraction (LEED) for two surface structures designated Zr(0001)-(1x1)-C (one of lower C coverage and the other of higher C coverage), as well as investigations of the adsorption of CO and PH₃ on the (0001) surface of zirconium by using Auger electron spectroscopy (AES) and LEED. For the lower-coverage Zr(0001)-(1x1)-C surface, the intensity-versus-energy (I(E)) curves for a set of diffracted beams were measured with a video LEED analyzer, and then compared with the results of multiple scattering calculations made for various structural models. Levels of correspondence between experimental and calculated I(E) curves were assessed with the reliability index proposed by Pendry, and surface geometries determined by the conditions for the best correspondence between experiment and calculation. The LEED intensity analysis suggests that under the lower C coverage, the C atoms in Zr(0001)-(1x1)-C appear to just occupy octahedral holes between the first and second layers of hep zirconium; this is similar to the situation reported by Wong et al for the N adsorption structure formed on Zr(0001). The LEED-determined Zr-C bond distance is 2.30 Å, which is in reasonable agreement with the value determined by X-ray crystallography for bulk ZrC (2.34 Å). Only a very preliminary analysis could be attempted for the higher-C-coverage Zr(0001)-(1x1)-C structure; there the C atoms may occupy all tetrahedral holes between the first and second layers of hep zirconium. Some I(E) curves were measured for CO adsorption on the Zr(0001) surface; a (2x2) LEED pattern formed with the initial CO exposure while a (1x1) pattern occurs with greater exposures. A comparison was made with CO adsorption on the Ti(0001) surface, and with oxygen adsorption on Zr(0001). The PH₃ adsorption on Zr(0001) can give a (3x3) LEED pattern which is unstable and only measurable in a short energy range. Unlike H₂S adsorption on Zr(0001) surface, ordering to form the (3x3)-PH₃ surface is inhibited by the loss of phosphorus during heating. An attempt was made to find the optimal recipe to get a sharp (3x3)-PH₃ pattern; it appears that the R[sub p] value (a ratio of Auger peak heights to indicate coverage) should be between 1.3 to 1.6, and that the ordering temperature should be higher than 560°C.
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