Full text
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| Author(s): |
Alexandre Pancotti
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin |
| Defense date: | 2009-09-12 |
| Examining board members: |
Richard Landers;
George Gershon Kleiman;
Fernando Alvarez;
Edmar Avellar Soares;
Jonder Morais
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| Advisor: | Richard Landers; Abner de Siervo |
| Abstract | |
This thesis presents a study of the atomic structure of the surface of two materials with potential as supports for metallic nanoparticles in model catalysts. The first is Chromium Oxide that is used as a support for commercial catalysts, and the second is SrTiO3(100) (STO), which has the very interesting characteristic of inducing the growth of nanoparticles of some metals with different shapes depending on the type of heat treatment of the support. The Chromium Oxide used in this study was in the form of ordered films grown epitaxially on a Pd(111) crystal, while the SrTiO3 consisted of a bulk crystal doped with Nb cleaved along the (100) face. The epitaxial Chromium Oxide films were grown in situ in a surface analysis chamber by evaporating metallic Cr under 1.0x10-6 mBar of O2 pressure on to the substrate heated to 623K. The films as grown showed clear LEED (Low Energy Electron Diffraction) patterns and constant stoichiometry. Two different reconstructions were studied: p(2x2), that was present for thin films of about 3.5 Å, and (V3xV3)R30o, which is characteristic of thicker films (above 12.0 Å). The composition and detailed surface structures were determined by XPS (X-Ray Photo electron Spectroscopy) and XPD (X-Ray Photoelectron Diffraction). SrTiO3 is an insulator, but doping with Nb makes it sufficiently conducting so that it is possible to use LEED, XPS and XPD without charging problems inherent to insulating samples. Two types of radiation were used for exciting the samples: synchrotron radiation (700eV) from the SGM beam line at the Brazilian National Synchrotron Radiation Light Laboratory and conventional X-rays from Al and Mg anodes. To simulate the surface structures the MSCD package [1] was used. To accelerate the optimization of the structural parameters a genetic algorithm [2] was used in conjunction with the MSCD package. The thinner Chromium Oxide film was shown to consist of CrO with a p(2x2) reconstruction, having as first and second interlayer distances 0,16 Å and 1,92 Å respectively. The thicker Chromium Oxide film was studied as grown and after annealing at 973K. In both cases the structure was determined to be a-Cr2O3(0001), but with different surface termination. XPD revealed that the film as grown was terminated by an O monolayer, with the first interlayer distance expanded by 9.5% relative to bulk values. After annealing the film was shown to terminate in two atomic layers of Cr, who¿s interlayer distance was reduced by 68% relative to the bulk. Results for the surface of STO showed regions covered by SrO and TiO2. In both cases the cation layers relaxed inwards and the atoms of the second layer outwards, resulting in corrugated surfaces. These results compare very well with results obtained by LEED. By using the genetic algorithm [2], it was possible to show that 30% of the surface was covered by SrO(100) islands (AU) | |