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Title:X-ray diffraction study on CU(111)/electrolyte interfaces
Author(s):Chu, Yong Song
Doctoral Committee Chair(s):Robinson, Ian K.
Department / Program:Physics
Subject(s):X-ray diffraction
CU(111)/electrolyte interfaces
Pb monolayer
free-electron gas model
Abstract:An experimental study of the interfacial structures at the Cu(111) surface under various electrochemical environments is presented. The interfacial structures have been examined with the synchrotron x-ray diffraction. Three different Cu(111)/electrolyte systems are discussed. In the first experiment, the properties of the underpotentially deposited (UPD) Ph monolayer on the Cu(111) surface were studied. The Pb UPD monolayer formed an incommensurate hexagonal closed-packed structure, aligned with the Cu(111) substrate. Both in-plane and surface-normal structures ofPb/Cu(111) were studied as a function of the electrochemical potential. While the surface-normal structure exhibited no potential dependence, the in-plane structure of the Pb monolayer responded sensitively to the potential. The compression behavior the Ph monolayer showed excellent agreement with the free-electron gas model. Experiments on of the bare Cu(111) surface in 0.1 M HCl04 (pH :::::1) revealed that there were two distinct interfacial structures. For a wide potential range negative of the Nernst potential, the Cu(111) surface was, in fact, not bare but adsorbed with oxygen or hydroxide. The bare Cu(lll) surface was observed only at the onset of the hydrogen evolution. CTR (crystal truncation rods) analysis showed that the oxygen atoms are adsorbed at 3-fold hollow sites of the Cu(lll) substrate. The observed bond length was 111 consistent with the known Cu-0 bond length. The presence of the oxygen adlayer in the pH regions (pH <3. 5 ), in which the bulk oxide is forbidden, has significant thermodynamic implications, and the adsorbed oxygen monolayer can be interpreted as the precursor to the bulk oxide. In a less acidic electrolyte (pH=4.5), the growth of aqueous oxide was observed. The aqueous oxide exhibited an epitaxial cubic Cu20 (cuprite) structure. Two distinct epitaxial structures were found: the "aligned' cuprite with its ( 1 T 0) axis parallel with the substrate ( 1 T 0) axis and the "reversed" cuprite with its ( 11 0) axis anti-parallel with the substrate ( 1 T 0) axis. While the "reversed" cuprite was found to grow into a thick film, the "aligned" cuprite remained as a thin two-dimensional film. The growth of the aqueous oxide showed a distinct growth behavior, which was different from the that of the previously reported "dry" oxide grown under vacuum.
Issue Date:1997
Genre:Dissertation / Thesis
Other Identifier(s):4048121
Rights Information:©1997 Chu
Date Available in IDEALS:2012-04-26

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