Exposure of a Ru(0001 ) surface covered by submonolayer quantities of Cs to O2 leads to the emiss... more Exposure of a Ru(0001 ) surface covered by submonolayer quantities of Cs to O2 leads to the emission (with very low probabilities) of O-ions. The yield of O-reaches a maximum at Cs coverages around 0.3-0.4 monolayers (1 ML is equivalent to an absolute coverage 0c,=0.33 ), and in this range is much higher for low (~260 K) than for high sample temperatures. These effects are traced back to the varying degree of "metallization" of the Cs overlayers as substantiated by metastable deexcitation spectroscopy (MDS) experiments.
The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface ar... more The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d 5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d 5/2 spectra with oxygen exposure at 475 K and the corresponding O 2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO 2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d 5/2 images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.
Atomic oxygen adsorbed on oxide domains terminating an oxidized Ru(0 0 0 1) surface is marked by ... more Atomic oxygen adsorbed on oxide domains terminating an oxidized Ru(0 0 0 1) surface is marked by its unusually high chemical reactivity as compared to other known oxygen species. We applied thermal desorption spectroscopy employing isotopic labeling to characterize the thermally driven incorporation of these oxygen atoms into the subsurface region. This O ad 3 O sub conversion takes place in temperature region below 600 K where the pre-adsorbed oxygen becomes removed via the associative oxygen desorption. The eciency of the process depends linearly upon the occupation of the available adsorption sites. The conversion probability reaches a value of 0.25 per adatom. The rather low activation energy of 0.22 eV for this oxide mediated oxygen incorporation promises a high eciency of this reaction pathway in the high temperature regime. We propose a model based on a thermal transformation of Ru x O y domains as the decisive step for the oxygen conversion.
The carbon monoxide (CO) oxidation reaction, taking place over oxygen-rich Ru(0001) surfaces, has... more The carbon monoxide (CO) oxidation reaction, taking place over oxygen-rich Ru(0001) surfaces, has been used here as a simple probe of the reactivity of oxygen species formed by high temperature oxidation procedures. For surfaces oxidized at temperatures lower than 950 K a high reaction probability, within a range of 10-2, has been found. The kinetics of CO 2 emission exhibit a fast and a long term component which reflects the contributions of the topmost and the subsurface oxygen phases, respectively. Both kinetic components are drastically reduced when working with surfaces resulting from the application of the same oxidation procedure but at temperatures above 950 K. For preparation temperatures above 1100 K the reaction probability is below the detection limit. In this temperature range, the oxygen phases show valence region ultraviolet photoelectron spectroscopy features characteristic of a regular RuO 2 solid phase. Moreover, these non-reactive surfaces are marked by intense emission of RuO 3 , RuO 4 and Ru 2 O molecular fragments as appearing in thermal desorption spectra in a temperature range 1000-1300 K.
Reactive scattering of CO molecules at oxygen-rich Ru(0001) surfaces with concentrations equivale... more Reactive scattering of CO molecules at oxygen-rich Ru(0001) surfaces with concentrations equivalent up to 16 monolayers and sample temperatures between 300 and 700 K led to the identification of two distinct reaction channels in the transient CO 2 rate. The first reaction channel is related to the recombination of CO molecules with oxygen atoms already located on the surface. The second reaction channel, which can be observed at sample temperatures above about 400 K, is controlled by the diffusion of oxygen atoms from the near-surface region toward the surface.
The formation of CO 2 , by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated a... more The formation of CO 2 , by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated as a function of the oxygen coverage for sample temperatures up to 900 K. It turned out that the reaction probability per incident CO molecule is below 5 × 10 -4 for O coverages up to 3 monolayers (ML); oxygen in excess of 1 ML is located in the subsurface region. The reaction probability for the (1 × 1)-1O phase is in agreement with the data derived from high-pressure experiments by Peden and Goodman [J. Phys. Chem. 1986, 90, 1360]. Even for CO molecules with a translational energy of 1.2 eV (supersonic molecular beam experiments), the reaction probability is less than 5 × 10 -2 . This value is consistent with the activation barrier derived from DFT calculations for a reaction by direct collision from the gas phase (Eley-Rideal mechanism). Beyond an oxygen load of 3 ML, however, the reaction probability increases by 2 orders of magnitude. It is suggested that this enhancement is due to a further destabilization of the surface oxygen by the onset of oxide formation.
Electron emission during oxidation of thin Cs films is confined to the stage Cs{sub 2}O{sub 2}{r ... more Electron emission during oxidation of thin Cs films is confined to the stage Cs{sub 2}O{sub 2}{r arrow}CsO{sub 2} and is caused by Auger deexcitation accompanying the (formal) reaction O{sub 2}{sup 2{minus}}({ital s})+O{sub 2}{r arrow}2O{sub 2}{sup {minus}}({ital s}). This nonadiabatic process becomes possible because resonance ionization of the affinity level of the impinging O{sub 2} molecule upon crossing the Fermi level {ital E}{sub {ital F}} is efficiently suppressed due to the absence of occupied states near {ital E}{sub {ital F}} at the surface.
An atomic oxygen species adsorbed on Ru͑0001͒ containing subsurface oxygen has been found and is ... more An atomic oxygen species adsorbed on Ru͑0001͒ containing subsurface oxygen has been found and is characterized by the means of thermal description spectroscopy ͑TDS͒, ultraviolet photoemission spectroscopy, and reactive CO scattering. The surplus oxygen can be adsorbed on an oxygen-rich ruthenium crystal at room temperature when more than two monolayers of oxygen are deposited in the subsurface region (1 ML ϭ1.58ϫ10 15 cm Ϫ2 ). The maximum number of oxygen atoms occupying this state depends on the oxygen content in the subsurface region. It saturates for oxygen contents higher than about 10 ML at a level of 0.25 ML. The initial binding energy of an adsorbed oxygen atom, as derived from TDS data, is lower than 0.75 eV per atom. It continuously decreases with increasing lateral adatom density. The work function varies with the increasing population of this oxygen state. For saturating oxygen coverage it reaches a maximum that can be by 1.3 eV higher than the value known for a clean metallic Ru surface. The CO oxidation was performed in a beam scattering experiment and it reveals a high reactivity of this weakly bound state. At room temperature the CO/CO 2 -conversion probability reaches a value of 0.18, which is nearly by one order of magnitude higher than the one found for subsurface oxygen at much higher temperatures. ͓S0163-1829͑99͒12243-4͔
The non-IPR fulleride compound Cs<i><sub>x</sub></i>C<sub>68</su... more The non-IPR fulleride compound Cs<i><sub>x</sub></i>C<sub>68</sub> was investigated in analogy to previous experiments on Cs<i><sub>x</sub></i>C<sub>58</sub>. While the vibrational properties as well as the electronic structure in the valence band range appear to be pretty similar to Cs<i><sub>x</sub></i>C<sub>58</sub>, a striking difference could be observed by XPS and Photoemission Microscopy analysis: apparently, C<sub>68</sub> forms different phases within a thick film including islands in which it is able to store more than 70 Cs atoms per fullerene cage and to keep these amounts even after annealing over 1100 K. This means that Cs<i><sub>x</sub></i>C<sub>68</sub> constitutes the highest alkali metal doping degree of any Fullerene species known so far. Spatially resolved photoemission analysis also revealed that the Cs-rich ...
The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces un... more The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces under single-collision conditions has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Methyl radicals were generated by thermal decomposition of azomethane N 2 (CH 3) 2. Significant carbon deposition under elevated surface temperature conditions was observed if substrates were used which had been decorated by nanometer sized defects prior to methyl radical exposure. Graphene layers as well as protrusions were observed to be formed depending on the defect. No carbon deposition was observed for surface temperature below 800°C. Largest sticking probabilities of up to 10 À6 were observed for HOPG (highly oriented pyrolytic graphite) surfaces prestructured with hexagonal nanometer sized etch pits. Here, the initially resulting mono-atomic layers are pinned by the hole periphery and exhibit a densely packed hexagonal atomic structure corresponding to the graphite basal plane. For surfaces held at 1000°C, the lateral growth rate of a graphene layer around a single hole can exceed 230 A A 2 /s at a CH Å 3 flux of 3 • 10 17 molecules/cm 2 s. The deposition kinetics switches from 2D to 3D growth prior to completion of the first graphene layer. A growth mechanism based on CH Å 3 decomposition and hydrogen desorption is proposed.
The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface ar... more The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d 5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d 5/2 spectra with oxygen exposure at 475 K and the corresponding O 2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO 2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d 5/2 images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.
Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diff... more Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction ͑LEED͒, and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru͑0001͒. Emission of RuO x (xϽ4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (T p Ͻ850 K͒ a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular Ru x O y phase takes place within the temperature region of 900-1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO 2 coating layer manifests itself by LEED patterns characteristic for a particular RuO 2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO 2 .
... Recently, it has been convincingly demonstrated that the CO oxidation performed at an oxygen-... more ... Recently, it has been convincingly demonstrated that the CO oxidation performed at an oxygen-rich Ru 0001 surface, becomes more efficient by two orders of magnitude when the sample has contained subsurface oxygen.13 Such a pronounced reactivity jump obviously ...
ABSTRACT The capacity of intentionally created defects to act as possible channels for oxygen inc... more ABSTRACT The capacity of intentionally created defects to act as possible channels for oxygen incorporation into the subsurface region has been studied by means of thermal desorption spectroscopy and ultraviolet photoelectron spectroscopy (21.2 eV). Thermal-energy atom scattering has been applied to determine the overall surface roughness as achieved by exposing the surface to a flux of low-energy Ar+ ions. Within a wide range of experimental conditions the low-temperature (T less than or equal to 600 K) and high-pressure (p approximate to 1 bar) oxidation regime applied here does not lead to a significant formation of bulk oxides. Instead two competing channels for oxygen incorporation into the subsurface region have been identified. The first path proceeds via the penetration of the oxygen monolayer adsorbed on top of the defect-free surface areas. The second path is defect-mediated and exhibits a maximum capacity of about 10 oxygen atoms per defect. This reaction channel is thermally activated with an apparent activation energy of 0.15 eV, which is nearly 3 times lower than the activation energy corresponding to the growth of oxide domains.
Exposure of a Ru(0001 ) surface covered by submonolayer quantities of Cs to O2 leads to the emiss... more Exposure of a Ru(0001 ) surface covered by submonolayer quantities of Cs to O2 leads to the emission (with very low probabilities) of O-ions. The yield of O-reaches a maximum at Cs coverages around 0.3-0.4 monolayers (1 ML is equivalent to an absolute coverage 0c,=0.33 ), and in this range is much higher for low (~260 K) than for high sample temperatures. These effects are traced back to the varying degree of "metallization" of the Cs overlayers as substantiated by metastable deexcitation spectroscopy (MDS) experiments.
The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface ar... more The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d 5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d 5/2 spectra with oxygen exposure at 475 K and the corresponding O 2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO 2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d 5/2 images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.
Atomic oxygen adsorbed on oxide domains terminating an oxidized Ru(0 0 0 1) surface is marked by ... more Atomic oxygen adsorbed on oxide domains terminating an oxidized Ru(0 0 0 1) surface is marked by its unusually high chemical reactivity as compared to other known oxygen species. We applied thermal desorption spectroscopy employing isotopic labeling to characterize the thermally driven incorporation of these oxygen atoms into the subsurface region. This O ad 3 O sub conversion takes place in temperature region below 600 K where the pre-adsorbed oxygen becomes removed via the associative oxygen desorption. The eciency of the process depends linearly upon the occupation of the available adsorption sites. The conversion probability reaches a value of 0.25 per adatom. The rather low activation energy of 0.22 eV for this oxide mediated oxygen incorporation promises a high eciency of this reaction pathway in the high temperature regime. We propose a model based on a thermal transformation of Ru x O y domains as the decisive step for the oxygen conversion.
The carbon monoxide (CO) oxidation reaction, taking place over oxygen-rich Ru(0001) surfaces, has... more The carbon monoxide (CO) oxidation reaction, taking place over oxygen-rich Ru(0001) surfaces, has been used here as a simple probe of the reactivity of oxygen species formed by high temperature oxidation procedures. For surfaces oxidized at temperatures lower than 950 K a high reaction probability, within a range of 10-2, has been found. The kinetics of CO 2 emission exhibit a fast and a long term component which reflects the contributions of the topmost and the subsurface oxygen phases, respectively. Both kinetic components are drastically reduced when working with surfaces resulting from the application of the same oxidation procedure but at temperatures above 950 K. For preparation temperatures above 1100 K the reaction probability is below the detection limit. In this temperature range, the oxygen phases show valence region ultraviolet photoelectron spectroscopy features characteristic of a regular RuO 2 solid phase. Moreover, these non-reactive surfaces are marked by intense emission of RuO 3 , RuO 4 and Ru 2 O molecular fragments as appearing in thermal desorption spectra in a temperature range 1000-1300 K.
Reactive scattering of CO molecules at oxygen-rich Ru(0001) surfaces with concentrations equivale... more Reactive scattering of CO molecules at oxygen-rich Ru(0001) surfaces with concentrations equivalent up to 16 monolayers and sample temperatures between 300 and 700 K led to the identification of two distinct reaction channels in the transient CO 2 rate. The first reaction channel is related to the recombination of CO molecules with oxygen atoms already located on the surface. The second reaction channel, which can be observed at sample temperatures above about 400 K, is controlled by the diffusion of oxygen atoms from the near-surface region toward the surface.
The formation of CO 2 , by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated a... more The formation of CO 2 , by exposing oxygen precovered Ru(0001) surfaces to CO, was investigated as a function of the oxygen coverage for sample temperatures up to 900 K. It turned out that the reaction probability per incident CO molecule is below 5 × 10 -4 for O coverages up to 3 monolayers (ML); oxygen in excess of 1 ML is located in the subsurface region. The reaction probability for the (1 × 1)-1O phase is in agreement with the data derived from high-pressure experiments by Peden and Goodman [J. Phys. Chem. 1986, 90, 1360]. Even for CO molecules with a translational energy of 1.2 eV (supersonic molecular beam experiments), the reaction probability is less than 5 × 10 -2 . This value is consistent with the activation barrier derived from DFT calculations for a reaction by direct collision from the gas phase (Eley-Rideal mechanism). Beyond an oxygen load of 3 ML, however, the reaction probability increases by 2 orders of magnitude. It is suggested that this enhancement is due to a further destabilization of the surface oxygen by the onset of oxide formation.
Electron emission during oxidation of thin Cs films is confined to the stage Cs{sub 2}O{sub 2}{r ... more Electron emission during oxidation of thin Cs films is confined to the stage Cs{sub 2}O{sub 2}{r arrow}CsO{sub 2} and is caused by Auger deexcitation accompanying the (formal) reaction O{sub 2}{sup 2{minus}}({ital s})+O{sub 2}{r arrow}2O{sub 2}{sup {minus}}({ital s}). This nonadiabatic process becomes possible because resonance ionization of the affinity level of the impinging O{sub 2} molecule upon crossing the Fermi level {ital E}{sub {ital F}} is efficiently suppressed due to the absence of occupied states near {ital E}{sub {ital F}} at the surface.
An atomic oxygen species adsorbed on Ru͑0001͒ containing subsurface oxygen has been found and is ... more An atomic oxygen species adsorbed on Ru͑0001͒ containing subsurface oxygen has been found and is characterized by the means of thermal description spectroscopy ͑TDS͒, ultraviolet photoemission spectroscopy, and reactive CO scattering. The surplus oxygen can be adsorbed on an oxygen-rich ruthenium crystal at room temperature when more than two monolayers of oxygen are deposited in the subsurface region (1 ML ϭ1.58ϫ10 15 cm Ϫ2 ). The maximum number of oxygen atoms occupying this state depends on the oxygen content in the subsurface region. It saturates for oxygen contents higher than about 10 ML at a level of 0.25 ML. The initial binding energy of an adsorbed oxygen atom, as derived from TDS data, is lower than 0.75 eV per atom. It continuously decreases with increasing lateral adatom density. The work function varies with the increasing population of this oxygen state. For saturating oxygen coverage it reaches a maximum that can be by 1.3 eV higher than the value known for a clean metallic Ru surface. The CO oxidation was performed in a beam scattering experiment and it reveals a high reactivity of this weakly bound state. At room temperature the CO/CO 2 -conversion probability reaches a value of 0.18, which is nearly by one order of magnitude higher than the one found for subsurface oxygen at much higher temperatures. ͓S0163-1829͑99͒12243-4͔
The non-IPR fulleride compound Cs<i><sub>x</sub></i>C<sub>68</su... more The non-IPR fulleride compound Cs<i><sub>x</sub></i>C<sub>68</sub> was investigated in analogy to previous experiments on Cs<i><sub>x</sub></i>C<sub>58</sub>. While the vibrational properties as well as the electronic structure in the valence band range appear to be pretty similar to Cs<i><sub>x</sub></i>C<sub>58</sub>, a striking difference could be observed by XPS and Photoemission Microscopy analysis: apparently, C<sub>68</sub> forms different phases within a thick film including islands in which it is able to store more than 70 Cs atoms per fullerene cage and to keep these amounts even after annealing over 1100 K. This means that Cs<i><sub>x</sub></i>C<sub>68</sub> constitutes the highest alkali metal doping degree of any Fullerene species known so far. Spatially resolved photoemission analysis also revealed that the Cs-rich ...
The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces un... more The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces under single-collision conditions has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Methyl radicals were generated by thermal decomposition of azomethane N 2 (CH 3) 2. Significant carbon deposition under elevated surface temperature conditions was observed if substrates were used which had been decorated by nanometer sized defects prior to methyl radical exposure. Graphene layers as well as protrusions were observed to be formed depending on the defect. No carbon deposition was observed for surface temperature below 800°C. Largest sticking probabilities of up to 10 À6 were observed for HOPG (highly oriented pyrolytic graphite) surfaces prestructured with hexagonal nanometer sized etch pits. Here, the initially resulting mono-atomic layers are pinned by the hole periphery and exhibit a densely packed hexagonal atomic structure corresponding to the graphite basal plane. For surfaces held at 1000°C, the lateral growth rate of a graphene layer around a single hole can exceed 230 A A 2 /s at a CH Å 3 flux of 3 • 10 17 molecules/cm 2 s. The deposition kinetics switches from 2D to 3D growth prior to completion of the first graphene layer. A growth mechanism based on CH Å 3 decomposition and hydrogen desorption is proposed.
The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface ar... more The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d 5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d 5/2 spectra with oxygen exposure at 475 K and the corresponding O 2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO 2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d 5/2 images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.
Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diff... more Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction ͑LEED͒, and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru͑0001͒. Emission of RuO x (xϽ4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (T p Ͻ850 K͒ a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular Ru x O y phase takes place within the temperature region of 900-1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO 2 coating layer manifests itself by LEED patterns characteristic for a particular RuO 2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO 2 .
... Recently, it has been convincingly demonstrated that the CO oxidation performed at an oxygen-... more ... Recently, it has been convincingly demonstrated that the CO oxidation performed at an oxygen-rich Ru 0001 surface, becomes more efficient by two orders of magnitude when the sample has contained subsurface oxygen.13 Such a pronounced reactivity jump obviously ...
ABSTRACT The capacity of intentionally created defects to act as possible channels for oxygen inc... more ABSTRACT The capacity of intentionally created defects to act as possible channels for oxygen incorporation into the subsurface region has been studied by means of thermal desorption spectroscopy and ultraviolet photoelectron spectroscopy (21.2 eV). Thermal-energy atom scattering has been applied to determine the overall surface roughness as achieved by exposing the surface to a flux of low-energy Ar+ ions. Within a wide range of experimental conditions the low-temperature (T less than or equal to 600 K) and high-pressure (p approximate to 1 bar) oxidation regime applied here does not lead to a significant formation of bulk oxides. Instead two competing channels for oxygen incorporation into the subsurface region have been identified. The first path proceeds via the penetration of the oxygen monolayer adsorbed on top of the defect-free surface areas. The second path is defect-mediated and exhibits a maximum capacity of about 10 oxygen atoms per defect. This reaction channel is thermally activated with an apparent activation energy of 0.15 eV, which is nearly 3 times lower than the activation energy corresponding to the growth of oxide domains.
Uploads
Papers by A. Böttcher