Papers

AVS1997 Session SS2-MoA: Metal Oxide Surface Science II

Monday, October 20, 1997 2:00 PM in Room A1/2-A

Monday Afternoon

Start	Invited?	Item
2:00 PM		SS2-MoA-1 Effect of Sulfur on Oxide Adhesion on Fe(111) J.A. Kelber, J.S. Lin, B.M. Ekstrom, H. Cabibil (University of North Texas) The impurity-induced debonding of protective oxide scale are not well understood. We report an observation of topography-related iron oxide scale de-wetting on S pre-covered Fe(111). Prior to oxidation, a (1x1)-S phase was formed by annealing the crystal to 920- 970 K. Upon 1700 L exposure of O ₂ at room temperature, a diffuse (1x1) LEED pattern is observed, indicating a disordered surface. Flash annealing the oxide surface to 720 K results in the removal of most of the oxide. This is accompanied by a transformation of the LEED pattern to a (2 √3 x1)R30 ⁰. Previous results indicate that this LEED pattern is associated with a S coverage higher than that of the (1x1)-S phase, suggesting an increase in the S coverage on the surface . STM measurements reveal the presence of scattered oxide islands with exposed underlayer substrate, exhibiting triangular pitting and pyramid formation. This triangular pitting and pyramid formation may be responsible for the oxide instability at relatively low temperature for S pre-covered iron surface.
2:20 PM		SS2-MoA-2 The Interaction of Water with the (012) Surface of α-Fe₂O₃: Experiment and Theory M.A. Henderson, S.A. Joyce, J.R. Rustad, E. Wasserman (Pacific Northwest National Laboratory) The interaction of water with α-Fe₂O₃(012) was examined experimentally with TPD, SSIMS and LEED, and theoretically with Ewald summation methods. Both the 1x1 and 2x1 structures of the (012) surface were examined experimentally, while only the former was considered theoretically. Water desorbs from the 1x1 surface in TPD states at 240 and 350 K. The shape and coverage dependence of the 350 K state are first order. By monitoring, the H₃O⁺, FeO(OH)⁺ and FeO(H₂O)⁺ ions, SSIMS results also suggest that both states are molecular. However, when the surface is 'labelled' with ¹⁸O, scrambling between oxygens in the surface and the adlayer occurs in the 350 K state, but not the 240 K state, suggesting the latter is molecular and the former is from hydroxyl recombination. In agreement with the isotope experiments, energy minimization calculations for the 1x1 surface show that full hydroxylation is favored by 1.03 J/m2 over full hydration, while 50% hydroxylation is favored by 1.16 J/m2. Of the 45 hydroxylated structures considered, the lowest energy configurations were 75% hydroxylated, with the lowest of these favored by 1.29 J/m2. HREELS experiments will be conducted in the near future to assist in resolving this conflict. In contrast, water TPD from the 2x1 surface gives states at 220-250 and 410 K, the latter exhibiting complex coverage dependence. SSIMS and isotope results suggest that the 220-250 K state is molecular and the 410 K state is from hydroxyl recombination. Water TPD experiments conducted during stepwise transformation of the 1x1 to 2x1 surface (accomplished by heating in 50 K increments from 550 to 950 K) suggest that the reconstruction process occurs at specific nucleation sites and extends over large regions instead of occurring on a random site by site basis. LEED results concur with this conclusion. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
2:40 PM		SS2-MoA-3 Growth and Surface Structure of FeO(001) Epitaxial Thin Films Y. Gao, Y.J. Kim, S.A. Chambers (Pacific Northwest National Laboratory) Well-ordered, pure-phase epitaxial films of FeO(001) were prepared on MgO(001) and on Fe₃O₄(001)/MgO(001) by oxygen-plasma-assisted MBE. FeO(001) growth on Fe₃O₄(001)/MgO(001) appears in a step-flow fashion, whereas the growth of FeO(001) on MgO(001) occurs initially by island formation, and then island coalescence. FeO(001) epitaxial film surface exhibits a (2x2) reconstruction, as evidenced by both RHEED and LEED. High- resolution Fe 2p core-level XPS reveals that the reconstruction is accompanied by a significant amount of Fe³⁺ in the surface region, suggesting that the (2x2) surface structure is attributed to the non-stoichiometric defects. The (2x2) surface structure disappears and the concentration of Fe³⁺ species in the surface region decreases after thermal annealing in ultra-high vacuum. A defect model based on the 4:1 clusters is proposed to interpret the (2x2) surface periodicity. The model shows that the (2x2) FeO(001) surface structure consists of an ordered array of tetrahedral Fe³⁺ interstitials.
3:00 PM		SS2-MoA-4 Photoexcited Fe₂O₃ Surfaces: Properties and Chemisorption D.S. Toledano, V.E. Henrich (Yale University) We are using α-Fe₂O₃ in model catalyst studies of the interaction of atmospheric gases with photoexcited transition-metal-oxide surfaces. Transition-metal oxides comprise a small but extremely important component of atmospheric aerosol particles because their charge-transfer bandgaps are generally smaller than the 4.3 eV cut-off of solar radiation in the troposphere. Solar radiation is thus able to change the valence state of surface transition-metal cations via electron-hole pair creation: in α-Fe₂O₃, some Fe³⁺ cations become Fe²⁺, which exhibit very different surface chemistry than do Fe³⁺, and offer the potential to catalyze heterogeneous photochemical reactions for which α-Fe₂O₃ would otherwise be inert. Reduced and stoichiometric single-crystal α-Fe₂O₃ (0001) surfaces were prepared and studied in UHV by UPS, XPS, Auger and LEED; tropospheric solar radiation was simulated with a broad-spectrum UV lamp. SO₂ was the first gas to be examined as it is a prevalent pollutant whose oxidation leads to acid rain production. We find enhanced adsorption of SO₂ on the photoexcited surface compared with adsorption of SO₂ in the dark, and distinctly different changes in valence electronic structure for dark versus illuminated adsorption. SO₂ adsorption on illuminated surfaces is found to be similar to that on Fe₂O₃ that has been reduced by ion bombardment; XPS spectra indicate that SO₂ adsorbs molecularly on both surfaces. Illumination following SO₂ exposure in the dark has interesting effects that differ from either illuminated adsorption or photon-stimulated desorption of SO₂. Adsorption of CO and water vapor are currently under study.
3:20 PM		SS2-MoA-5 Core-level X-ray Photoelectron Spectra and X-ray Photoelectron Diffraction of RuO₂(110) Grown by Molecular Beam Epitaxy on TiO₂(110) S.A. Chambers, Y.J. Kim, Y. Gao (Pacific Northwest National Laboratory) RuO₂ is a conductive metallic oxide which is of interest in a variety of technologies. Accurate determination of the chemical states present on RuO₂ surfaces by x-ray photoelectron spectroscopy (XPS) is essential for an accurate understanding of surface chemical activity and electronic properties. The origin of the fine structure in Ru 3d core-level XPS spectra has been the source of some controversy. Part of the problem stems from the use of ill-defined surfaces. In order to address this controversy, we have grown well-characterized RuO₂(110) films on TiO₂(110) by oxygen- plasma-assisted molecular beam epitaxy. We then measured Ru 3d, 4s, 4p and O 1s high-resolution core-level x-ray photoelectron spectra, along with Ru 3d and O 1s scanned-angle x-ray photoelectron diffraction (XPD) angular distributions. XPS spectral interpretation and the nature of the XPD scans strongly suggest that the complex line shapes are due to final-state screening effects, rather than the presence of Ru in oxidation states other than +4.
3:40 PM		SS2-MoA-6 Studies of the Defect Chemistry on (110) and (100) TiO₂ Surfaces L.-Q. Wang, K.F. Ferris, D.R. Baer, M.H. Engelhard (Pacific Northwest National Laboratory) Defects play an important role in chemical reactivity of oxide surfaces. This paper relates new and existing XPS and UPS results of molecule interactions with ideal and defected rutile surfaces to recent ab initio electronic structure calculations. The interactions of water, formic acid and methanol have been investigated. Electronic structure calculations corroborate the observed increase in adsorption as Ti sites are exposed (by electron-beam exposure or Ar+ bombardment). The calculation also shows a strong adsorptive interaction for formate onto both stoichiometric and defective TiO₂ surfaces. In contrast to water absorption, modeling calculations on defective TiO₂ have found that formate is located in an asymmetric position with respect to the Ti3+ sites. HCOOH at saturation exposure apparently binds to all available Ti sites including Ti3+ sites, preventing further O₂ healing of defects. The excess adsorption promoted by surface defects quantitatively corresponds to the number of newly exposed sites created by electron-beam exposure. Surface structural influences on defect reactivity have also been addressed by examining the interaction of water with both defective TiO₂ (110) and (100) surfaces. It was found that defect intensities for (100) surfaces are reduced more readily than for (110) surfaces after the same water exposure. The electronic structure calculations show that water dissociates more readily on TiO₂(100) surface than on (110) surfaces. For TiO₂(110) surface, methanol can reduce defect intensities more readily than water for a given exposure. In contrast, formic acid produces little healing at saturation exposure.
4:00 PM		SS2-MoA-7 Synthesis and Structure of Metal Clusters Supported on TiO₂(110): A Scanning Tunneling Microscopy Study X. Lai, C. Xu, D.W. Goodman (Texas A&M University) Pd and Al clusters supported on TiO₂(110) have been investigated using scanning tunneling microscopy (STM). For Pd, homogeneous hemispherical Pd clusters were preferentially nucleated near atomic step edges. At very low Pd coverages dimeric and tetrameric Pd clusters were observed. The atomically-resolved STM images suggest that 3-D Pd clusters are formed via the growth of 2-D Pd layers, although no evidence for any chemical interfacial reaction between Pd and the TiO₂(110) substrate is apparent. A metal to nonmetal transition has been found for Pd clusters consisting of some 300-400 atoms, with clusters smaller than this size having a bandgap from <1V to several volts. Al binds to the surface more strongly than Pd, consistent with the smaller Al clusters being more highly dispersed. Highly oxidized Al clusters were found to disorder the TiO₂ substrate in the low coverage regime illustrating that Al exhibits a strong chemical interaction with the oxide substrate. Changes in the TiO₂(110) substrate from a "row" structure to a "net" structure due to Al interaction with oxygen in the top-most layer of the substrate will also be discussed.
4:20 PM		SS2-MoA-8 Oxygen-Induced Reconstruction of a TiO₂(110) Surface: A Scanning Tunneling Microscopy Study L. Gross, M. Kuhn, U. Diebold (Tulane University) The large-scale and atomic-scale effects of various annealing temperatures, in ultra-high vacuum (UHV), on a TiO₂(110) surface has been systematically studied by scanning tunneling microscopy (STM). After preparing the surface in oxygen at 750 K, the sample was annealed in UHV to varying temperatures ranging from 540 K to 920 K. With increasing annealing temperatures, the overall surface morphology changes from one that shows rounded step edges and rounded patches of a typical width of 5 nm as the top most layer to one that shows large flat elongated terraces (typical width is 10 nm in (001) direction) with step edges preferably along the (001), (1-1-1) and (1-11) directions. First the edges line along these directions and then the patches, now terminated in straight lines along the preferred directions, disappear and the surface flattens out. At high temperatures the appearance of point defects (single bridging-oxygen vacancies) is noted. For annealing temperatures below 700 K, atomic scale features appear on the surface. These were found to be induced by the presence of oxygen. They form rows along the (1-10) direction. Rows along (001) direction, that show similar dimensions, have been previously observed at higher annealing temperatures. Small-scale images with atomic resolution were taken to determine the structure of these features. Single and double strand models of a Ti₂O₃ surface species are proposed to account for these features. Upon annealing to high temperature, this oxygen-induced surface phase decomposes into the expected TiO₂(110) phase. The implications for a low-temperature oxygen-induced surface phase of Ti₂O₃ is discussed.
4:40 PM		SS2-MoA-9 Atomic Resolution UHV STM of Insulating Oxides: NiO and UO₂ M. Castell, C. Muggelberg, S. Dudarev (University of Oxford, United Kingdom); D. Goddard (BNFL, United Kingdom); A.P. Sutton, G.A.D. Briggs (University of Oxford, United Kingdom) Oxides that are insulating at room temperature such as NiO and UO₂ can be imaged with atomic resolution using an STM at elevated temperature. In NiO(001) surfaces the empty-states images reveal nickel atoms while the filled states images reveal oxygen atoms. This shows that NiO is not a true Mott-Hubbard insulator, but should rather be classified as a charge-transfer insulator. Images of defects and step edges show that it is possible to see directly the nature of the covalent bonding between Ni-3d states and O-2p states. Images of UO₂(111) reveal the presence of interstitial oxygen atoms, but in a different arrangement from their bulk positions, and images of UO₂(110) address puzzles about this surface that have been around for over thirty years. Like many oxides, NiO and UO₂ have strongly correlated electrons, and so adequate interpretation of all these results required the development of a new method of density functional theory incorporating both spin and a Hubbard U term. In this way we are able to account quantitatively both for the structures seen and for the voltage-dependent contrast in the images.
5:00 PM		SS2-MoA-10 Characterization of Defective Oxide Surfaces by Surface-Sensitive and Titration Techniques H. Öfner, F. Zaera (University of California, Riverside) Thin NiO films, grown by oxidation of Ni(110) surfaces, were characterized under ultrahigh vacuum by low-energy electron diffraction (LEED), x-ray photoelectron (XPS), Auger (AES) and ion scattering (ISS) spectroscopies as well as by CO temperature-programmed desorption (TPD) titrations. The effect of introducing defects via Ar ion irradiation at room temperature was compared with the results from partially oxidized ordered Ni(110) surfaces. CO TPD proved to be a useful local probe for the investigation of defective NiO surfaces, and revealed that Ar ion irradiation of thin NiO films leads to the formation of Ni-O phases similar to those found during the early oxidation stages of this surface.