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Epitaxial growth of ultrathin Ni films on NiO(100) and TiO2(110)
AI01 Surface Science 279 (1992) 1-12 North-Holland
Structure of the Ni{ l10}-p(2 x 1)-OH surface from time-of-flight scattering and recoiling spectrometry C.D. Roux, H. Bu and J.W. Rabalais Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA Received 24 May 1992; accepted for publication 13 August 1992 The structure of the Ni{ll0}-p(2 × I).OH surface resulting from exposure of the 0 2 pre-dosed Ni(ll0).p(3 x 1).O surface to H 2° vapor at ~ 350 K has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS). Variations in the scattered neon and recoiled hydrogen, oxygen, and nickel neutral plus ion fluxes are monitored as a function of 4 keV Ne + or Ar + beam incidence angle and crystal azimuthal angle. Analysis of shadowing effects in these scattering and recoiling events, with calibrated shadow cones, is used in the structure determination. The results show that the reaction between H 2 0 and pre-adsorbed O removes the Ni substrate (3 x 1) reconstruction induced by 02, producing an unreconstructed (1 X 1) Ni substrate. The resulting OH ovedayer is arranged in a (2 x 1) rectangular lattice of length 3.52 A along (001) and 4.98 A along (110) corresponding to a 0.5 ML coverage. Hydroxyl is bonded to Ni through the O atom whose site is more than 1.1 ,~ above the surface. The OH bond is inclined 45* + 4* from the surface normal and aligned along the (001) azimuth. The short bridge site along the (110) Ni rows is proposed as the OH adsorption site.
Surface Science 279 (1992) 13-22 North-Holland
A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen J.G. Chen, D.A. Fischer 1, J.H. Hardenbergh and R.B. Hall Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, USA Received 14 May 1992; accepted for publication 17 August 1992 The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K,. indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EEI_~ and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50.4 NiO(100) crystalline clusters and c(2 × 2)-O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H 2 in the temperature range of 600-800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H 2 pressure. The reaction of NiO/Ni(100) with H 2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 x 10 ~3 molecules cm -2 s-1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial H - H bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.
Surface Science 279 (1992) 23-32 North-Holland
Epitaxial growth of ultrathin Ni films on NIO(100) and TIO2(110) Ming-Cheng Wu 1 and Preben J. M~ller Laboratory of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen O, Denmark Received 16 April 1992; accepted for publication 5 August 1992
A102 Epitaxial growth of ultrathin Ni films on (1 × 1) NiO(100) and (1 × 1) TiO2(ll0) has been examined using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It is found that the epitaxial orientational relationship between Ni overlayers and NiO(100) can be described using site-coincidence growth theory. However, the growth of Ni on TiO2(110) exhibits a complex behaviour. Two types of Ni islands are formed concurrently on TiO2(l10). The first is a hexagonal structure which is oriented parallel to the substrate, and the second a hexagonal structure whose direction of growth is inclined with reference to the substrate plane. The growth modes of ultrathin Ni films on NiO(100) and TiO2(l10) have also been examined using AES. These three-dimensional epitaxial structures of Ni overlayers can be well described by the Stranski-Krastanov growth mechanism using a quasi-isotropic growth model.
Surface Science 279 (1992) 33-48 North-Holland
Dissociative adsorption of methane on Pd(679) surface Yarw-Nan Wang, Richard G. Herman and Kamil Klier Zettlemoyer Center for Surface Studies and Department of Chemistry, 7 Lehigh University, Beth&hem, PA 18015, USA Received 8 June 1992; accepted for publication 13 August 1992 The reaction of methane with a Pd(679), Pd(S)-[7(111) × (310)], single crystal was studied using a high-pressure cell and Auger electron spectroscopy, low energy electron diffraction, and temperature programmed desorption techniques. Methane was dissociatively chemisorbed at 1 Torr and at surface temperatures of > 400 K with the formation of surface carbon and hydrogen species. The surface hydrogen also diffused into the bulk of the crystal. The initial dissociative sticking probability of methane on Pd(679) was 10-9-10 -7 with an activation energy of ~ 10.7 kcal/mol. The extent of CH 4 decomposition was strongly temperature dependent in that the surface carbon formed fractional monolayers (ML) at 400-500 K but produced multilayer coverages, e~g. 10 ML, at 600 K. The multilayer carboneous layers were disordered irrespective of the surface coverage. Oxidative titration cycles that included post-saturation of the carbon- and hydrogen-covered surfaces with oxygen followed by temperature programmed oxidation (TPO) led to the desorption of CO, CO 2 and H 2. No H20 formation was observed in the TPO cycles, and the desorbed H 2 was stoichiometrically equivalent to carbon originating from the methane decomposition. The desorption data, along with CO chemisorption experiments, indicated that carbon species resulting from CH 4 decomposition on Pd(679) formed clusters that were located primarily in the vicinity of stepped and kinked edges and left the majority of the Pd surface bare as further adsorption sites. The overall reaction, albeit stepwise executed, amounted to partial oxidation reactions CH 4 + ½02 ~ CO + 2H 2 and CH 4 + O 2 CO2 + 2H2. These reaction stoichiometries indicate that Pd metal possesses the basic chemical properties for low temperature catalytic oxidative reforming of CH 4 to CO, CO2, and H 2.
Surface Science 279 (1992) 49-67 North-Holland
Nucleation, growth, and structure of futlerene films on A u ( l l l ) Eric I. Altman and Richard J. Colton Chemistry Division - Code 6177, Naval Research Laboratory, Washington, DC 20375-5000, USA Received 12 June 1992; accepted for publication 6 August 1992 The growth of fullerene films by vapor deposition on A u ( l l l ) was studied using UHV-STM, LEED, AES, and TOF-SIMS. The fullerenes were found to grow in a layer-by-layer manner. The molecules initially adsorb at intersections of multiple steps and edges of monatomic steps on narrow terraces. As the coverage is increased, periodic arrays of short chains form at steps separating wider terraces with the periodicity determined by the (23 × v~)-Au(lll) reconstruction. At higher coverages, hexagonal layers with a lattice constant of 1.0 nm grow out from the steps on both the upper and lower terraces. The adsorption of the fullerenes lifts the reconstruction indicating a strong interaction between the fullerenes and Au(lll). STM indicates that two ordered commensurate structures predominate on the A u ( l l l ) surface: (1) a layer with a periodicity of approximately 38 Au spacings; and (2) a layer with a (2vC3 × 2v~)R30 ° unit cell. Second layer molecules were found to display no affinity for step edges. On Au(lll), the second layer desorbs at 300°C, while the first layer is strongly bound and does not begin to desorb until 500"C. After annealing to 500°C the remaining C60 is largely disordered. Intramolecular structure was observed within molecules at the step edges and in the hexagonal arrays.
Structure of the Ni{ l10}-p(2 x 1)-OH surface from time-of-flight scattering and recoiling spectrometry C.D. Roux, H. Bu and J.W. Rabalais Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA Received 24 May 1992; accepted for publication 13 August 1992 The structure of the Ni{ll0}-p(2 × I).OH surface resulting from exposure of the 0 2 pre-dosed Ni(ll0).p(3 x 1).O surface to H 2° vapor at ~ 350 K has been investigated by time-of-flight scattering and recoiling spectrometry (TOF-SARS). Variations in the scattered neon and recoiled hydrogen, oxygen, and nickel neutral plus ion fluxes are monitored as a function of 4 keV Ne + or Ar + beam incidence angle and crystal azimuthal angle. Analysis of shadowing effects in these scattering and recoiling events, with calibrated shadow cones, is used in the structure determination. The results show that the reaction between H 2 0 and pre-adsorbed O removes the Ni substrate (3 x 1) reconstruction induced by 02, producing an unreconstructed (1 X 1) Ni substrate. The resulting OH ovedayer is arranged in a (2 x 1) rectangular lattice of length 3.52 A along (001) and 4.98 A along (110) corresponding to a 0.5 ML coverage. Hydroxyl is bonded to Ni through the O atom whose site is more than 1.1 ,~ above the surface. The OH bond is inclined 45* + 4* from the surface normal and aligned along the (001) azimuth. The short bridge site along the (110) Ni rows is proposed as the OH adsorption site.
Surface Science 279 (1992) 13-22 North-Holland
A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen J.G. Chen, D.A. Fischer 1, J.H. Hardenbergh and R.B. Hall Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, USA Received 14 May 1992; accepted for publication 17 August 1992 The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K,. indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EEI_~ and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50.4 NiO(100) crystalline clusters and c(2 × 2)-O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H 2 in the temperature range of 600-800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H 2 pressure. The reaction of NiO/Ni(100) with H 2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 x 10 ~3 molecules cm -2 s-1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial H - H bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.
Surface Science 279 (1992) 23-32 North-Holland
Epitaxial growth of ultrathin Ni films on NIO(100) and TIO2(110) Ming-Cheng Wu 1 and Preben J. M~ller Laboratory of Physical Chemistry, H.C. Orsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen O, Denmark Received 16 April 1992; accepted for publication 5 August 1992
A102 Epitaxial growth of ultrathin Ni films on (1 × 1) NiO(100) and (1 × 1) TiO2(ll0) has been examined using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It is found that the epitaxial orientational relationship between Ni overlayers and NiO(100) can be described using site-coincidence growth theory. However, the growth of Ni on TiO2(110) exhibits a complex behaviour. Two types of Ni islands are formed concurrently on TiO2(l10). The first is a hexagonal structure which is oriented parallel to the substrate, and the second a hexagonal structure whose direction of growth is inclined with reference to the substrate plane. The growth modes of ultrathin Ni films on NiO(100) and TiO2(l10) have also been examined using AES. These three-dimensional epitaxial structures of Ni overlayers can be well described by the Stranski-Krastanov growth mechanism using a quasi-isotropic growth model.
Surface Science 279 (1992) 33-48 North-Holland
Dissociative adsorption of methane on Pd(679) surface Yarw-Nan Wang, Richard G. Herman and Kamil Klier Zettlemoyer Center for Surface Studies and Department of Chemistry, 7 Lehigh University, Beth&hem, PA 18015, USA Received 8 June 1992; accepted for publication 13 August 1992 The reaction of methane with a Pd(679), Pd(S)-[7(111) × (310)], single crystal was studied using a high-pressure cell and Auger electron spectroscopy, low energy electron diffraction, and temperature programmed desorption techniques. Methane was dissociatively chemisorbed at 1 Torr and at surface temperatures of > 400 K with the formation of surface carbon and hydrogen species. The surface hydrogen also diffused into the bulk of the crystal. The initial dissociative sticking probability of methane on Pd(679) was 10-9-10 -7 with an activation energy of ~ 10.7 kcal/mol. The extent of CH 4 decomposition was strongly temperature dependent in that the surface carbon formed fractional monolayers (ML) at 400-500 K but produced multilayer coverages, e~g. 10 ML, at 600 K. The multilayer carboneous layers were disordered irrespective of the surface coverage. Oxidative titration cycles that included post-saturation of the carbon- and hydrogen-covered surfaces with oxygen followed by temperature programmed oxidation (TPO) led to the desorption of CO, CO 2 and H 2. No H20 formation was observed in the TPO cycles, and the desorbed H 2 was stoichiometrically equivalent to carbon originating from the methane decomposition. The desorption data, along with CO chemisorption experiments, indicated that carbon species resulting from CH 4 decomposition on Pd(679) formed clusters that were located primarily in the vicinity of stepped and kinked edges and left the majority of the Pd surface bare as further adsorption sites. The overall reaction, albeit stepwise executed, amounted to partial oxidation reactions CH 4 + ½02 ~ CO + 2H 2 and CH 4 + O 2 CO2 + 2H2. These reaction stoichiometries indicate that Pd metal possesses the basic chemical properties for low temperature catalytic oxidative reforming of CH 4 to CO, CO2, and H 2.
Surface Science 279 (1992) 49-67 North-Holland
Nucleation, growth, and structure of futlerene films on A u ( l l l ) Eric I. Altman and Richard J. Colton Chemistry Division - Code 6177, Naval Research Laboratory, Washington, DC 20375-5000, USA Received 12 June 1992; accepted for publication 6 August 1992 The growth of fullerene films by vapor deposition on A u ( l l l ) was studied using UHV-STM, LEED, AES, and TOF-SIMS. The fullerenes were found to grow in a layer-by-layer manner. The molecules initially adsorb at intersections of multiple steps and edges of monatomic steps on narrow terraces. As the coverage is increased, periodic arrays of short chains form at steps separating wider terraces with the periodicity determined by the (23 × v~)-Au(lll) reconstruction. At higher coverages, hexagonal layers with a lattice constant of 1.0 nm grow out from the steps on both the upper and lower terraces. The adsorption of the fullerenes lifts the reconstruction indicating a strong interaction between the fullerenes and Au(lll). STM indicates that two ordered commensurate structures predominate on the A u ( l l l ) surface: (1) a layer with a periodicity of approximately 38 Au spacings; and (2) a layer with a (2vC3 × 2v~)R30 ° unit cell. Second layer molecules were found to display no affinity for step edges. On Au(lll), the second layer desorbs at 300°C, while the first layer is strongly bound and does not begin to desorb until 500"C. After annealing to 500°C the remaining C60 is largely disordered. Intramolecular structure was observed within molecules at the step edges and in the hexagonal arrays.