Papers

AVS2001 Session SS-TuA: Metal Oxides: Structure and Photocatalysis

Tuesday, October 30, 2001 2:00 PM in Room 121

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS-TuA-1 Scanning Tunneling Microscopy Study of the Reduced Fe₃O₄(111) Terminated Selvedge on a Natural Single Crystal α-Fe₂O₃(0001) Surface: Termination and Surface Reaction with Carbon Tetrachloride K.T. Rim, J.P. Fitts, T. Muller, K. Adib, N. Camillone, R.M. Osgood (Columbia University); S.A. Joyce (Pacific Northwest National Laboratory); G.W. Flynn (Columbia University) Scanning Tunneling Microscopy(STM) and Low Energy Electron Diffraction(LEED) have been used to study the degradation of CCl₄ on the reduced surface of a natural α-Fe₂O₃(0001) surface. STM and LEED results indicate that repeated cycles of Ar⁺ sputtering at 2kV and annealing in vacuum and in 10^-6Torr of O₂ partial pressure at 850K reduces the α-Fe₂O₃(0001) surface. Based on STM images approximately 90 % of the reduced surface is terminated by Fe₃O₄(111), while the remaining 10 % is terminated by FeO(111) and an unidentified phase. CCl₄ vapor was dosed on the reduced surface at room temperature. STM images were obtained at room temperature before and after the surface was exposed to CCl₄. STM images before dosing CCl₄ reveal a lateral distance of 6.0Â±0.5Å between atoms and a step height of 4.8Â±0.5Å between terraces. A related Temperature Programmed Desorption(TPD) study on the same surface concludes that CC l₄ is adsorbed dissociatively on the Fe₃O₄(111) terminated surface and that only Cl atoms remain on the surface when the dosed surface is flashed up to 590K. STM images after dosing and flashing up to 590K show bright features on the Fe ₃O₄(111) terminated surface that are identified as individual Cl atoms. These atoms are bound atop Fe atoms, which occupy tetrahedral Fe sites on the Fe₃O₄(111) surface, located in three-fold hollow sites of the underlying close-pa cked O plane. STM images before dosing and the chemical reactivity of a Fe₃O₄(111) terminated surface with Cl atoms suggest that this Fe₃O₄(111) surface is iron-terminated. In contrast, no changes indicating reactivity of the FeO(111) terminated surface were observed. To further clarify the nature of bright features, STM studies are performed where the surface is exposed to Cl₂ at room temperature.
2:20 PM		SS-TuA-2 Photo-catalytic Reactions of Organic Molecules Over TiO₂(001) Single Crystal - Effect of Surface Structure H. Idriss, J. Wilson (The University of Auckland, New Zealand) No work has addressed so far the effect of surface reconstruction on the photochemical reactions of wide band-gap oxide semiconductors. Rutile, TiO₂(001) single crystal a n-type semiconductor, band-gap 3.0 eV, is unique in that it possess two stable surface structures that can be obtained by thermal treatment. They are the (011)-faceted (750 K) and the (114)-faceted (> 950 K) surface. We have investigated the steady state reaction of acetic acid (as a prototype organic molecule) under photo-excitation (365 nm) over the two thermodynamically stable surfaces under ultra high vacuum conditions. Acetic acid was readily decomposed at room temperature into CO₂ and CO. Moreover the formation of ethane, by the so-called "Photo-Kolbe" mechanism was clearly observed. On the (011)-faceted and at an acetic pressure of 2.4 x 10^-7 Torr the CO₂, CO and, ethane production was 1.02 x 10^-8, 1.7 x 10^-8, and 4.51 x 10^-9 Torr, respectively. The product distribution on the (114)-faceted surface was found similar. However, it appears that this latter surface, the one containing Ti⁴⁺ cations in 4, 5 and 6-fold coordination to oxygen anions is less active than the (011)faceted surface (all Ti⁴⁺ cations are 5-fold coordinated to oxygen anions), under similar conditions.
2:40 PM		SS-TuA-3 Semiconductor Photocatalysis over Titanium Dioxide D.F. Ollis (North Carolina State University) Near-UV illuminated titania surfaces allow for total oxidation of trace organic contaminants in water or air, at room temperatures and with molecular oxygen as the ultimate oxygen source. We examine the origins, progress, and prospects for photocatalysis in four key areas: air purification and remediation, air sterilization, self-cleaning surfaces, and photo-initiated polymeric coatings.
3:20 PM		SS-TuA-5 Valence Band Photoemission from Pure and Sr Diffused Single Crystal Anatase TiO₂(001) Surfaces S. Thevuthasan, V. Shutthanandan, M.A. Henderson, G.S. Herman, S.A. Chambers, Y. Liang, C.H.F. Peden (Pacific Northwest National Laboratory); S. Mun, N.M. Hamdan, D. Shuh, C.S. Fadley (Lawrence Berkeley National Laboratory) Single crystal rutile titanium dioxide has been extensively studied as a model oxide surface because of its commercial availability and its wide use in a variety of technological applications. In contrast, investigations of high-quality anatase titanium dioxide are limited in the literature due primarily to the lack of availability of single crystals. Recently, we have successfully grown epitaxial single crystal anatase TiO₂(001) thin films with high quality surfaces on SrTiO₃(001) substrates at the Environmental Molecular Sciences Laboratory's (EMSL) MBE facility.¹ We have carried out valence band photoemission experiments on pure anatase TiO₂(001) thin films, and those with outdiffused Sr at the Advanced Light Source. During these experiments, we investigated the valence band region that includes the band gap and O 2p band as a function of photon energy ranging from 35 eV to 80 eV. The results were compared with those for rutile TiO₂ surfaces. Valence band spectra from sputtered and annealed (in vacuum and oxygen environments) anatase show differences compared to the valence band spectra from rutile surfaces. However, outdiffused Sr results in no visible changes to the valence band spectra. This result suggests that Sr may occupy the cation sites in a strontium titanate-like structure that exhibiting the band gap similar to the band gap of anatase titanium oxide. These results will be discussed along with the ion channeling measurements of Sr outdiffusion and substitution for cation sites. foontote 1G.S. Herman, M.R. Sievers, Y. Gao, Phys. Rev. Lett. 84, 3354 (2000). Work supported by the U.S. Department of Energy, Offices of Basic Energy Sciences and Biological and Environmental Research, the Environmental Management Science Program, and the laboratory directed research and development (LDRD) program.
3:40 PM		SS-TuA-6 Surface Investigations of TiO₂ Anatase (101) N. Ruzycki, D.L. Ederer (Tulane University); G.S. Herman, M.A. Henderson (Pacific Northwest National Laboratory); U. Diebold (Tulane University) Titanium dioxide exists in three cyrstallographic structures, rutile, anatase, and brookite. The surfaces of rutile, especially the most stable (110) face, are very well investigated, but far less is know about the technologically much more relevant anatase phase. We report a study of the lowest-energy face, the (101) surface of an anatase mineral sample. After sputtering and annealing to a temperature of 650°C, a sharp (1x1)LEED pattern is observed. In STM the surface morphology is dominated by small triangularly-shaped terraces separated by monoatomic steps.¹ The step directions are consistent with the ones expected from simple bond-counting rules. Surprisingly, very few point defects are observed with atomic-resolution STM images and HREELS. This is consistent with the low surface free energy predicted in first-principles calculations.² It is in contrast to rutile (110), however, where a similar surface treatment creates a few percent of oxygen vacancies. These surface defects play a major role in the surface chemistry of rutile (110), for example, they promote the dissociation of water.² Adsorption of water is used to probe the surface reactivity of anatase (101). Predominantly molecular adsorption is found, consistent with the picture that po int defects do not play a major role. ¹W. Hebenstreit, N. Ruzycki, G. S. Herman, and U. Diebold, Phys. Rev. B 64 (24) (2000) R16334. ²A. Vittadini, A. Selloni, and M. Gratzel, Phys.Rev.Lett.. 81 (14) 2954. 3. M. A. Henderson, Surf. Sci. 335 (1996) 151.
4:00 PM		SS-TuA-7 Characterization of Surface Defects on Flat and Porous MgO Surfaces Z. Dohnálek, D. McCready, J.S. Young, A. Dohnálková, G.A. Kimmel, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory) We use the physisorption of gases such as N₂, Ar, and CO and standard surface analytical techniques to characterize the surface defects on thin films of dense and porous MgO. Different types of defects are prepared on the MgO films by careful adjustment of the deposition conditions. Significant changes are observed on the surfaces of films grown on the Mo(100) substrate at various substrate temperatures, O₂ pressures, deposition rates, and film thicknesses. Temperature programmed desorption (TPD) of physisorbed molecules reveals distinct desorption features associated with different binding sites on the defective films. The analysis of TPD spectra yields the binding energies and concentrations of these defect sites. The information from flat MgO(100) surfaces is utilized in the characterization of porous, crystalline MgO films. Details of the experimental techniques, results, and implications of the results will be presented. Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.
4:20 PM		SS-TuA-8 A Study of the Surface Reconstruction of Fe₃O₄ (100) Using Antiferromagnetic Tips G. Mariotto, S. Murphy, I. Shvets (Trinity College Dublin, Ireland) We have studied the surface structure and chemical composition of an artificially grown single crystal of Fe₃O₄ (100) using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The crystal has been also characterized by electrical resistivity measurements as a function of temperature. Two different preparation procedures are discussed. The first one consists of mechanically polishing the sample with diamond paste and then annealing it in situ at a temperature of 990 ± 50 K. It has been found that this procedure gives rise to a surface characterized by the presence of square terraces with the edges aligned along the [110] and [1-10] directions. A p(1x1) LEED pattern has been associated with this type of images and sample preparation. The steps separating adjacent terraces are 2 ± 0.2 Å high, which corresponds to the distance between like sites of iron atoms. A longer annealing induced the onset of a different type of structure: the square terraces split into rows that become sharper and narrower as the annealing time is increased. The separation between these rows ranges from 24 to 40 Å. This type of surface structures manifests itself in the LEED pattern with the presence of satellite spots around the primary spots. Surface contamination by impurities segregating from the bulk are discussed. The second preparation procedure consists of argon ion etching followed by annealing. This method produces a c(2x2) reconstruction that is clearly visible in the LEED pattern. AES spectra indicate a clean surface of magnetite. Terraces separated by 2 ± 0.2 Å are routinely observed. Atomically resolved pictures have been obtained on a sample prepared according to this procedure using an antiferromagnetic MnNi tip. The differences between the two preparation procedures are discussed.
4:40 PM		SS-TuA-9 Characterization of Copper Oxides Formed by Thermal and Plasma Oxidation Using Linear Sweep Voltammetry, Galvanostatic Reduction and XPS M.A. Hossain (Lamar University); J.R. Parga (Instituto Tec. de Saltillo); H. McWhinney (Prairie View A&M University); R. Schennach (Technical University of Graz); D.L. Cocke (Lamar University) The growing importance of copper in the semiconductor industry has led to a renewed interest in the properties and growth modes of copper oxides under a variety of conditions. While thermal oxidation of copper has been studied extensively over the last decades, recent surface studies seem to ignore the possible formation of Cu₃O₂. It has been shown that thermal oxidation of copper leads to multilayer structures, which consist of Cu_xO, Cu₂O, Cu₃O₂ and CuO, depending on the oxidation conditions. These oxides have been analyzed by electrochemical methods, which probe the buried interfaces in an electrical potential controlled order. XPS combined with depth profiling has been used to obtain information which applied with controlled growth complements the electrochemical methods. Linear sweep voltammetry (LSV) and galvanostatic reduction (GR) have been used to characterize the oxides formed by thermal and plasma oxidation. LSV and GR show that plasma oxidation at room temperature leads to the formation of a pure CuO film, which cannot be formed using thermal oxidation and provides insight into the formation of the other oxides and about their subsurface interfaces.
5:00 PM		SS-TuA-10 The Surface Kinetics of the Initial Stages of Metal Oxidation Visualized by In situ UHV-TEM J.C. Yang, G.W. Zhou, M.D. Bharadwaj, D. Evans (University of Pittsburgh) Copper has played a significant role in the development of oxidation theories, ranging from the classic oxidation studies of the epitaxial growth of the thermodynamically stable oxide scale to the surface science investigations of the dynamics of oxygen interaction with the bare metal surface. We are visualizing the initial oxidation stage of Cu thin films by in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM) where we have varied temperature, oxidation pressures and oxidizing environment. In situ UHV-TEM provides unique information, such as real-time structural changes and information on buried interfaces, where extremely clean surface conditions are obtained inside the microscope. Based on our data, we have developed a semi-quantitative model of the initial oxidation stage where the dominant mechanism for transport, nucleation and growth of oxide islands is oxygen surface diffusion. We are presently examining other metallic systems, such as Aluminum.