1. |
- Elgammal, Karim, et al.
(author)
-
Density functional calculations of graphene-based humidity and carbon dioxide sensors : effect of silica and sapphire substrates
- 2017
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 663, s. 23-30
-
Journal article (peer-reviewed)abstract
- We present dispersion-corrected density functional calculations of water and carbon dioxide molecules adsorption on graphene residing on silica and sapphire substrates. The equilibrium positions and bonding distances for the molecules are determined. Water is found to prefer the hollow site in the center of the graphene hexagon, whereas carbon dioxide prefers sites bridging carbon-carbon bonds as well as sites directly on top of carbon atoms. The energy differences between different sites are however minute - typically just a few tenths of a millielectronvolt. Overall, the molecule-graphene bonding distances are found to be in the range 3.1-3.3 (A) over circle. The carbon dioxide binding energy to graphene is found to be almost twice that of the water binding energy (around 0.17 eV compared to around 0.09 eV). The present results compare well with previous calculations, where available. Using charge density differences, we also qualitatively illustrate the effect of the different substrates and molecules on the electronic structure of the graphene sheet.
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2. |
- Erdtman, Edvin, 1981-, et al.
(author)
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Simulations of the thermodynamics and kinetics of NH3 at the RuO2 (110) surface
- 2017
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 656, s. 77-85
-
Journal article (peer-reviewed)abstract
- Ruthenium(IV)oxide (RuO2) is a material used for various purposes. It acts as a catalytic agent in several reactions, for example oxidation of carbon monoxide. Furthermore, it is used as gate material in gas sensors. In this work theoretical and computational studies were made on adsorbed molecules on RuO2 (110) surface, in order to follow the chemistry on the molecular level. Density functional theory calculations of the reactions on the surface have been performed. The calculated reaction and activation energies have been used as input for thermodynamic and kinetics calculations. A surface phase diagram was calculated, presenting the equilibrium composition of the surface at different temperature and gas compositions. The kinetics results are in line with the experimental studies of gas sensors, where water has been produced on the surface, and hydrogen is found at the surface which is responsible for the sensor response.
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3. |
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4. |
- Ghadami Yazdi, Milad, et al.
(author)
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Structure dependent effect of silicon on the oxidation of Al(111) and Al(100)
- 2019
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 684, s. 1-11
-
Journal article (peer-reviewed)abstract
- The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy. Upon O 2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al 2 O 3 , and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied. Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness.
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5. |
- Gladh, Jörgen, et al.
(author)
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Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 633, s. 77-81
-
Journal article (peer-reviewed)abstract
- We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.
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6. |
- Hosseinpour, Saman, et al.
(author)
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Atmospheric corrosion of Cu, Zn, and Cu-Zn alloys protected by self-assembled monolayers of alkanethiols
- 2016
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 648, s. 170-176
-
Journal article (peer-reviewed)abstract
- In this article results from earlier studies have been compiled in order to compare the protection efficiency of self-assembled monolayers (SAM) of alkanethiols for copper, zinc, and copper-zinc alloys exposed to accelerated indoor atmospheric corrosion conditions. The results are based on a combination of surface spectroscopy and microscopy techniques. The protection efficiency of investigated SAMs increases with chain length which is attributed to transport hindrance of the corrosion stimulators in the atmospheric environment, water, oxygen and formic acid, towards the copper surface. The transport hindrance is selective and results in different corrosion products on bare and on protected copper. Initially the molecular structure of SAMs on copper is well ordered, but the ordering is reduced with exposure time. Octadecanethiol (ODT), the longest alkanethiol investigated, protects copper significantly better than zinc, which may be attributed to the higher bond strength of Cu-S than of Zn-S. Despite these differences, the corrosion protection efficiency of ODT for the single phase Cu20Zn brass alloy is equally efficient as for copper, but significantly less for the heterogeneous double phase Cu40Zn brass alloy.
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7. |
- Langhammer, David, 1991-, et al.
(author)
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SO2 adsorption on rutile TiO2(110) : An infrared reflection-absorption spectroscopy and density functional theory study
- 2018
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 677, s. 46-51
-
Journal article (peer-reviewed)abstract
- The adsorption of SO2 on single crystalline TiO2(110) has been investigated by means of polarized infrared reflection-absorption spectroscopy (IRRAS) experiments and density functional theory (DFT) calculations. IR absorption bands were detected at 1324 cm-1 and 985 cm-1 with p-polarized light incident along both the [110] and [001] crystallographic directions at 123 K. When the temperature was increased to 153 K, the peak at 1324 cm-1 disappears, while a new, weak band appears at 995 cm-1. Simultaneously, a band at 995 cm-1 also emerges with s-polarized light along the [110] direction. Based on the symmetry properties of the IRRAS spectra and accompanying ab initio simulations of the spectra employing a three layer model (vacuum-adsorbate-substrate), it is shown that the low temperature absorption IRRAS bands can be attributed to an SO3-like adsorbate structure. This is also the most stable adsorption structure (Ead = - 0.58 eV) on the stoichiometric surface. The combined IRRAS and DFT results show that the band appearing at 995 cm-1 is associated with a surface sulfite specie which is stabilized by residual surface water. The DFT calculations also revealed that a stable adsorption structure exists on a reduced TiO2 surface, where SO2 binds strongly to an oxygen vacancy site. It is suggested that this is an intermediate that form surface sulfate upon further reactions with water, although it was not observed on the stoichiometric surface studied in this work.
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8. |
- Lee, J. -Y, et al.
(author)
-
The surface energy and stress of metals
- 2018
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 674, s. 51-68
-
Journal article (peer-reviewed)abstract
- We investigated surface properties of metals by performing first-principles calculations. A systematic database was established for the surface relaxation, surface energy (γ), and surface stress (τ) for metallic elements in the periodic table. The surfaces were modeled by multi-layered slab structures along the direction of low-index surfaces. The surface energy γ of simple metals decreases as the atomic number increases in a given group, while the surface stress τ has its minimum in the middle. The transition metal series show parabolic trends for both γ and τ with a dip in the middle. The dip occurs at half-band filling due to a long-range Friedel oscillation of the surface charge density, which induces a strong stability to the Peierls-like transition. In addition, due to magnetic effects, the dips in the 3d metal series are shallower and deeper for γ and τ, respectively, than those of the 4d and 5d metals. The surface stress of the transition metals is typically positive, only Cr and Mn have a negative τ for the (100) surface facet, indicating that they are under compression. The light actinides have an increasing γ trend according to the atomic number. The present work provides a useful and consistent database for the theoretical modelling of surface phenomena.
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9. |
- Li, Cui, et al.
(author)
-
Theoretical study of para-nitro-aniline adsorption on the Au(111) surface
- 2016
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 649, s. 124-132
-
Journal article (peer-reviewed)abstract
- The electronic structure, bonding properties and dynamics of para-nitro-aniline (PNA) adsorbed on the Au(111) surface for a sub-monolayer coverge have been investigated by density-functional theory (DFT) static calculations and quantum molecular dynamics simulations. Four main adsorption geometries have been identified by DFT energy optimization with the gradient corrected PBE functional and accounting for the role of the van del Waals (vdW) interaction. Quantum dynamics calculations starting from the four different structures have been performed at room temperature to estimate the relative stability of the adsorbates and the presence of barriers for their interconversion. Quantum simulations suggest that the most stable adsorption geometry at room temperature is that of PNA with a slightly distorted molecular plane almost parallel to the Au(111) surface. In a second less populated configuration the PNA molecule interacts with the substrate by its NO2 group while the molecular plane is orthogonal to the surface. The N 1s electron photoemission spectrum has been simulated for the identified adsorbate geometries and a measurable variation of the absolute and relative chemical shift for the two nitrogen atoms in comparison with the known values for PNA in gas phase is predicted.
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10. |
- Lousada, Cláudio M., et al.
(author)
-
Molecular and dissociative adsorption of water at a defective Cu(110) surface
- 2017
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 658, s. 1-8
-
Journal article (peer-reviewed)abstract
- We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of water at the perfect Cu(110) and at a defective Cu surface that provides a broad spectrum of adsorption sites in terms of coordination of Cu atoms, with the aim of understanding the role of surface defects in the dissociation of water molecules. The molecular adsorption of water is spontaneous at both surfaces but at the defective surface we found two stable molecular adsorption structures that differ slightly in the disposition of the O-atoms of H2O on the surface plane but differ considerably on the orientation of their H-atoms. Additionally we studied the dissociative adsorption of water accompanied with formation of H2(g). At the defective surface, starting from 1 ML of molecularly adsorbed H2O, the dissociation of 0.22 ML of H2O leading to 0.22 ML of HO, 0.78 ML H2O and H2(g) is exergonic but the dissociation of 0.44 ML or more H2O molecules is endergonic. These findings are discussed in terms of the two main factors that affect the adsorption energies: the existence of exposed adatoms provides an environment that facilitates the interaction with small adsorbates leading to stronger bonds between the surface and such adsorbates; and at the same time, the limited polarizability of the defect sites causes high coverages of adsorbates that drag electron density from the surface to be unfavorable. The overall effect is that at the defective Cu(110) formation of low coverages of HO groups is more favorable than at the perfect Cu(110) while forming coverages higher than 0.44 ML of HO is less favorable than at the perfect Cu(110). These effects have their origin in the extent of the polarization of the Cu–O bonds.
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11. |
- Maleshlijski, S., et al.
(author)
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Imaging SPR combined with stereoscopic 3D tracking to study barnacle cyprid-surface interactions
- 2016
-
In: Surface Science. - : ELSEVIER SCIENCE BV. - 0039-6028 .- 1879-2758. ; 643, s. 172-177
-
Journal article (peer-reviewed)abstract
- Barnacle larvae (cyprids) explore surfaces to identify suitable settlement sites. This process is selective, and cyprids respond to numerous surface cues. To better understand the settlement process, it is desirable to simultaneously monitor both the surface exploration behavior and any close interactions with the surface. Stereoscopic 3D tracking of the cyprids provides quantitative access to surface exploration and pre-settlement rituals. Imaging surface plasmon resonance (SPR) reveals any interactions with the surfaces, such as surface inspection during bipedal walking and deposition of temporary adhesives. We report on a combination of both techniques to bring together information on swimming behavior in the vicinity of the interface and physical interactions of the cyprid with the surface. The technical requirements are described, and we applied the setup to cyprids of Balanus amphitrite. Initial data shows the applicability of the combined instrument to correlate exploration and touchdown events on surfaces with different chemical termination. (C) 2015 Published by Elsevier B.V.
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12. |
- Mattsson, Andreas, 1976-, et al.
(author)
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Co-adsorption of oxygen and formic acid on rutile TiO2 (110) studied by infrared reflection-absorption spectroscopy
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 663, s. 47-55
-
Journal article (peer-reviewed)abstract
- Adsorption of formic acid and co-adsorption with oxygen have been investigated on the rutile TiO2(110) surface using p- and s-polarized infrared reflection-absorption spectroscopy (IRRAS) at O2 exposures between 45 L to 8100 L and at temperatures between 273 K and 343 K. On the clean surface formic acid dissociates into a formate ion (formate) and a proton. Formate binds to two five-fold coordinated Ti atoms in the troughs along the [001] direction, and the proton binds to neighboring bridging O atoms. Exposure of adsorbed formate to O2 leads to a decrease in the asymmetric νas(OCO) band at 1532 cm−1 and to the concomitant formation of a new vibration band at 1516 cm−1. From the s-and p-polarized IRRAS measurements performed at different O2 exposures, surface pre-treatments and substrate temperatures, and by comparisons with previous reports, we conclude that the new species is a bidentate surface hydrogen carbonate, which is formed by reaction between formate and oxygen adatoms on the surface. The σv reflection plane of the surface hydrogen carbonate molecule is oriented along the [001] direction, i.e. the same direction as the adsorbed formate molecule. On the clean TiO2(110) surface exposed to O2 prior to formic acid adsorption, similar results are obtained. The reaction rate to form surface hydrogen carbonate from formate is found to follow first-order kinetics, with an apparent activation energy of Er=0.25 eV.
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13. |
- Migas, D. B., et al.
(author)
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Quasi-2D silicon structures based on ultrathin Me2Si (Me = Mg, Ca, Sr, Ba) films
- 2018
-
In: Surface Science. - : Elsevier B.V.. - 0039-6028 .- 1879-2758. ; 670, s. 51-57
-
Journal article (peer-reviewed)abstract
- By means of ab initio calculations with hybrid functionals we show a possibility for quasi-2D silicon structures originated from semiconducting Mg2Si, Ca2Si, Sr2Si and Ba2Si silicides to exist. Such a 2D structure is similar to the one of transition metal chalcogenides where silicon atoms form a layer in between of metal atoms aligned in surface layers. These metal surface atoms act as pseudo passivation species stabilizing crystal structure and providing semiconducting properties. Considered 2D Mg2Si, Ca2Si, Sr2Si and Ba2Si have band gaps of 1.14 eV, 0.69 eV, 0.33 eV and 0.19 eV, respectively, while the former one is also characterized by a direct transition with appreciable oscillator strength. Electronic states of the surface atoms are found to suppress an influence of the quantum confinement on the band gaps. Additionally, we report Sr2Si bulk in the cubic structure to have a direct band gap of 0.85 eV as well as sizable oscillator strength of the first direct transition.
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14. |
- Mohsenzadeh, Abas, et al.
(author)
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DFT study of the water gas shift reaction on Ni (111), Ni (100) and Ni (110) surfaces
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 644, s. 53-63
-
Journal article (peer-reviewed)abstract
- Density functional theory (DFT) calculations were used to study the water gas shift (WGS) reaction on Ni(111), Ni(100) and Ni(110) surfaces. The adsorption energy for ten species involved in thereaction together with activation barriers and reaction energies for the nine most important elementary steps were determined using the same model and DFT methods. The results reveal that these energies are sensitive to the surface structure. In spite of this, the WGS reaction occurs mainly via the direct (also referred to as redox) pathway with the CO + O → CO2 reaction as the rate determining step on all three surfaces. The activation barrier obtained for this rate limiting step decreases in the order Ni(110) > Ni(111) > Ni(100). Therefore, if O species are present on the surfaces then the WGSreaction is fastest on the Ni(100) surface. However, the barrier for desorption of H2O (which is the source of the O species) is lower than its dissociation reaction on the Ni(111) and Ni(100) surfaces, but not on the Ni(110) surface. Hence, at low H2O(g) pressures, the direct pathway on the Ni(110) surface will dominate and will be the rate limiting step. The calculations also show that the reason that the WGS reaction does not primarily occur via the formate pathway is that this species is a stable intermediate on all surfaces. The reactions studied here support the Brønsted-Evans-Polanyi (BEP) principles with an R2 value of 0.99. © 2015 Elsevier B.V. All rights reserved.
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15. |
- Muhammad Sohail, Hafiz, et al.
(author)
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Umklapp induced surface band structure of Ag/Ge(111)6 x 6
- 2015
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 635, s. 55-60
-
Journal article (peer-reviewed)abstract
- This study focuses on the electronic structure of a 6 x 6 surface which is formed by 0.2 monolayer of Ag on top of the Ag/Ge(111) root 3 x root 3 surface. The 6 x 6 periodicity was verified by low energy electron diffraction. Angle resolved photoelectron spectroscopy was employed to study the electronic structure along the (Gamma) over bar-(M) over bar-(Gamma) over bar and (Gamma) over bar-(K) over bar-(M) over bar high symmetry lines of the 6 x 6 surface Brillouin zone. There are six surface bands in total. Out of these, three were found to be related to the 6 x 6 phase. The surface band structure of the 6 x 6 phase is significantly more complex than that of the,root 3 x root 3 surface. This is particularly the case for the uppermost surface band structure which is a combination of a surface band originating from the underlying root 3 x root 3 surface and umklapp scattered branches of this band. Branches centered at neighboring 6 x 6 SBZs cross each other at an energy slightly below the Fermi level. An energy gap opens up at this point which contains the Fermi level. The complex pattern of constant energy contours has been used to identify the origins of various branches of the surface state dispersions. (C) 2014 Elsevier B.V. All rights reserved.
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16. |
- Pasti, Igor A., et al.
(author)
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Adsorption of nonmetallic elements on defect-free MgO(001) surface - DFT study
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 632, s. 39-49
-
Journal article (peer-reviewed)abstract
- Adsorption of 11 non-metals (H, B, C, N, O, F, Si, P, S, Cl and Br) on defect-free MgO(001) surface was investigate using DFT approach. Adsorption energies were found to be between -0.56 eV (hydrogen adsorption) and -2.63 eV (carbon adsorption). Charge transfer from substrate to adsorbate was observed to follow the periodicity in the Periodic Table of Elements, as increases from left to right and decreases from top to bottom. All investigated adsorbates prefer oxygen sites on MgO surface. The analysis of adsorbate-MgO(001) electronic structure suggested that the electronic structure of the O adsorption center and adsorbate atom is molecule-like and there is no strong interaction with MgO electronic bands. Based on the obtained dataset for adsorption energies of selected non-metallic adsorbates (X) the reactivity of MgO towards the bond cleavage in the cases of X-X, H-X and HO-X bonds was discussed. Obtained results point to weak reactivity of MgO(001) towards atomic adsorption and low activity for bond cleavage. However, these results can be used as a starting point for the functionalization of MgO, particularly in the cases where bond cleavage activity and surface-mediated stabilization of dissociation products are desired.
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17. |
- Posada Borbon, Alvaro, 1990, et al.
(author)
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Initial oxidation of Cu(100) studied by X-ray photo-electron spectroscopy and density functional theory calculations
- 2018
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 675, s. 64-69
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Journal article (peer-reviewed)abstract
- Density functional theory calculations and ambient pressure X-ray photoelectron spectroscopy are used to investigate initial oxidation of Cu(100). Surface stability with respect to oxygen coverage is calculated together with O 1s core level shifts. Oxidation of Cu(100) is found to occur via the formation of a p(2 × 2) overlayer (0.25 ML) followed by a reconstructed r2×r2 R45° -O missing-row (MR) structure (0.50 ML). A c(4 × 6) structure with a 0.3 ML coverage is close in stability for intermediate oxygen chemical potentials. The relative stability is found to be weakly dependent on the applied exchange-correlation functional. The calculated shifts in the O 1s binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O 1s binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The O 1s core-level shifts here obtained with CO2 as oxidant, are similar to previous measurements of Cu(100) oxidation with O2.
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18. |
- Ragazzon, Davide, et al.
(author)
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Growth of TiO2(B)(001) on Au(111) by Chemical Vapor Deposition
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 633, s. 102-108
-
Journal article (peer-reviewed)abstract
- This study presents how a TiO2(B) film exposing the (001) face can be grown on Au(111) by chemical vapor deposition. Identification and characterization of the TiO2(B)(001) layer are carried out with low-energy electron diffraction (LEED), synchrotron radiation photoelectron spectroscopy (PES), scanning tunneling microscopy (STM) and X-ray absorption spectroscopy (XAS). Formation of the TiO2(B) film requires a two-step preparation procedure: deposition at 280 °C followed by annealing to 500 °C. This suggests that the interaction between a substrate and an overlayer stabilizes the TiO2(B) film, preventing the formation of thermodynamically more stable rutile islands. The study thus gives insight into how the morphology and the atomic structure of the titania overlayer can be controlled.
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19. |
- Sandell, Anders, et al.
(author)
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Adsorption and photolysis of trimethyl acetate on TiO2(B)(001) studied with synchrotron radiation core level photoelectron spectroscopy
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 666, s. 104-112
-
Journal article (peer-reviewed)abstract
- We present a synchrotron radiation photoelectron spectroscopy study of the adsorption and photooxidation of trimethyl acetate (TMA) on TiO2(B)(001). The TiO2(B)(001) substrate was realized in the form of 2nm thick film on Au(111). The TMA species adopt the bidentate bonding configuration, as expected for carboxylic acids on TiO2, but cannot coordinate to all surface Ti ions due to steric hindrance. The proposed arrangement of the TMA species thus allows for the formation of an overlayer with a (2 x 1) periodicity. The thermal stability is found to be comparable to that on rutile (110) although the results indicate differences in the threshold for the TMA+H -> TMAA reaction. Photolysis using both ultraviolet (UV) light and soft x-ray synchrotron radiation (SR) was studied and compared to the reaction on the reduced ruffle (110) surface. A kinetic analysis suggests that the photoreaction rate for TMA on the TiO2(B) thin film is initially two times faster than that on the reduced rutile TiO2(110) surface. The higher activity of the TiO2(B) film is assigned to a reduced influence from surplus electrons associated with reduced Ti species, thereby decreasing the probability for hole-annihilation at high TMA coverage.
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20. |
- Sangiovanni, Davide, et al.
(author)
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N and Ti adatom dynamics on stoichiometric polar TiN(111) surfaces
- 2016
-
In: Surface Science. - : ELSEVIER SCIENCE BV. - 0039-6028 .- 1879-2758. ; 649, s. 72-79
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Journal article (peer-reviewed)abstract
- We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Ti-ad and N-ad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Ti-ad and N-ad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that N-ad species are considerably more mobile than Ti-ad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, E-a = 1.03 for Ti-ad and 0.61 eV for N-ad, compared to 0 K values E-aOK = 1.55 (Ti-ad) and 0.79 eV (N-ad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111). (C) 2016 Elsevier B.V. All rights reserved.
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21. |
- Schaefer, Andreas, 1981, et al.
(author)
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First layer water phases on anatase TiO2(101)
- 2018
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 674, s. 25-31
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Journal article (peer-reviewed)abstract
- The anatase TiO2(101) surface and its interaction with water is an important topic in oxide surface chemistry. Firstly, it benchmarks the properties of the majority facet of TiO2 nanoparticles and, secondly, there is a controversy as to whether the water molecule adsorbs intact or deprotonates. We have addressed the adsorption of water on anatase TiO2(101) by synchrotron radiation photoelectron spectroscopy. Three two-dimensional water structures are found during growth at different temperatures: at 100 K, a metastable structure forms with no hydrogen bonding between the water molecules. In accord with prior literature, we assign this phase to chains of disordered molecules. Growth 160 K results in a metastable structure with expressed hydrogen bonding between the water molecules. At 190 K, the water molecules become disordered as the thermal energy is too high and hence the hydrogen bonds break. The result is a structure with isolated monomers. Partial dissociation is observed for all three growths, with the molecular state only slightly favored in energy (20–40 meV) over the dissociated state. Heating of a thick film leads to more dissociation compared to a bilayer, when formed at 100 K. Thus, extending the water network facilitates proton transport and hence dissociation. The results reconcile apparent conflicting experimental results previously obtained by scanning tunneling microscopy (STM) and core level photoelectron spectroscopy.
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22. |
- Sohail, Hafiz Muhammad, et al.
(author)
-
Electronic and atomic structures of a Sn induced 3√3x3√3 superstructure on the Ag/Ge(111) √3x√3 surface
- 2016
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 644, s. 29-33
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Journal article (peer-reviewed)abstract
- We have investigated sub-monolayer coverages of Sn on the Ag/Ge(111) surface. It was found that ≈0.45 monolayer (ML) resulted in a new, well-defined, reconstruction with a periodicity. The periodic structure of the surface atoms was verified by low energy electron diffraction and scanning tunneling microscopy. The electronic structure was studied in detail using angle resolved photoelectron spectroscopy and core level spectroscopy at a temperature of 100 K. Several surface bands were identified and their dispersions are presented along the –– and –– high symmetry lines of the surface Brillouin zone (SBZ). The surface has a metallic character since there is a strong surface band crossing the Fermi level near -points coinciding with -points of the 1×1 SBZ. The Fermi contour of the metallic band showed a hexagonal shape in contrast to the circular shaped Fermi contour of the initial surface. Both empty and filled state STM images showed a hexagonal arrangement of protrusions which show a local periodicity and a superimposed modulation of the apparent heights with a periodicity.
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23. |
- Sohail, Hafiz Muhammad, et al.
(author)
-
Experimental studies of an In/Pb binary surface alloy on Ge(111)
- 2016
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 649, s. 146-151
-
Journal article (peer-reviewed)abstract
- In this study, we present a binary In/Pb surface alloy on Ge(111) formed by evaporating0.85 monolayer (ML) of In on the Pb/Ge(111) surface with 1.33 ML of Pb. A welldefined3×3 periodicity is formed after annealing at a temperature of ≈200 °C, as verified by bothlow energy electron diffraction (LEED) and scanning tunneling microscopy (STM). OverviewSTM images, obtained at 50 K, show a clear 3×3 periodicity. Detailed STM images reveal thatthe protrusions consist of atomic sized features with a local hexagonal arrangement. Each 3×3unit cell contains nine such features indicating a structure with 9 atoms per 3×3 cell. Based onangle resolved photoelectron spectroscopy (ARPES) data, we have identified five surface bandswithin the bulk band gap. Four of them cross the Fermi level leading to a metallic character of thesurface. The dispersions of these bands have been mapped in detail along the high symmetrydirections of the 3×3 surface Brillouin zone. Fermi contours, mapped in 2D k-space, showinteresting features. In particular, the occurrence of two differently rotated hexagon like contoursis discussed.
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24. |
- Soldemo, Markus, et al.
(author)
-
A well-ordered surface oxide on Fe(110)
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 639, s. 13-19
-
Journal article (peer-reviewed)abstract
- A well-ordered surface oxide grown on Fe(110) has been studied using scanning tunneling microscopy (STM), low energy electron diffraction, low energy electron microscopy, and core level photoelectron spectroscopy. The iron oxide film exhibits wide terraces and is formed after exposure to 100-1000 L at 1 x 10(-6) mbar O-2 and 400 degrees C. Two domains, mirror symmetric in the Fe(110)-lattice mirror symmetry planes but otherwise equal, are observed. The surface oxide forms a relatively large coincidence surface unit cell (16.1 angstrom x 26.5 angstrom). Imaging by STM reveals a strong bias dependence in the apparent height within the surface unit cell. The oxygen terminating atomic layer has a hexagonal atomic structure, FeO(111)-like, with the atomic sparing of 3.2 angstrom, that is expanded by similar to 63% relative to bulk FeO(111). (C) 2015 Elsevier B.V. All rights reserved.
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25. |
- Soldemo, Markus, et al.
(author)
-
Oxidation of Fe(110) in oxygen gas at 400 °c
- 2016
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 644, s. 172-179
-
Journal article (peer-reviewed)abstract
- The initial oxidation of Fe(110) in oxygen gas at 400 °C beyond initial adsorbate structures has been studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM). Formation of several ordered phases of surface oxides is observed at oxygen coverages between approximately 2.3 and 3.5 oxygen atoms/Fe(110) surface atom. Initially, a FeO(111)-like film is formed with a parallelogram-shaped moiré pattern. It has two mirror domains that are formed symmetrically around the growth direction of a zigzag-shaped adsorbate structure. With increased local oxygen coverage, the moiré structure transforms into a ball-shaped form. Both these moiré structures have equal atomic stacking at the surface and equal apparent height in STM, suggesting oxygen ions diffusing into the film upon oxidation and that the oxide growth takes place at the iron-iron oxide interface. The FeO(111)-like film turns into a Fe3O4(111)-like film with a triangular bistable surface termination as the oxidation proceeds further. The FeO(111)-like film growth proceeds according to the Frank-van der Merwe mechanism while the Fe3O4(111)-like film grows according to the Stranski-Krastanov mechanism.
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26. |
- Starfelt, Samuel, et al.
(author)
-
STM/STS and photoemission study of Ag thin films on Ga/Si(111)(√3 × √3)R30°
- 2019
-
In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 682, s. 25-32
-
Journal article (peer-reviewed)abstract
- Silver thin films have been formed by room temperature deposition of Ag on a Ga/Si(111) (√3 × √3)R30° surface. Scanning tunneling microscopy and spectroscopy (STM/STS) have been used to study both the clean Ga/Si(111) (√3 × √3)R30° surface and Ag films with different coverages. For the film formation, Ag first grows into 2D islands on Ga/Si(111) (√3 × √3)R30°. The first layer of the islands forms atomic lines with three-fold symmetry in the 1 × 1 direction of the underlying Si(111) substrate. From a 2 ML coverage, the growth becomes layer-by-layer mode. STS measurements have been performed on Ag films with different coverages inorder to investigate their electronic structures. Between 2–4 ML, the STS spectra reveal 2 and 3 peaks below the Fermi level. These peaks move towards the Fermi level with increased film thicknesses and thus share the same behavior as those of quantum well states (QWSs). The energy positions of the peaks have been compared with valence band photoelectron spectra in order to assign them to various parts of the QWSs. In addition, the photoemission results also confirm the existence of QWSs for a 2 ML Ag film on Ga/Si(111) (√3 × √3)R30°.
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27. |
- Öberg, Henrik, et al.
(author)
-
Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser : DFT prediction and X-ray confirmation of a precursor state
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 640, s. 80-88
-
Journal article (peer-reviewed)abstract
- We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate.
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28. |
- Arman, Alif, et al.
(author)
-
Adsorption of hydrogen on stable and metastable Ir(100) surfaces
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 656, s. 66-76
-
Journal article (other academic/artistic)abstract
- Using the combination of high resolution core level spectroscopy and density functional theory we present adetailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complementedby an investigation of the hydrogen desorption process from various phases using temperature programmeddesorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core levelshifts match very well with those predicted by density functional theory based on established structural models.In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core levelshift of surface Ir atoms is altered by +0.17 eV for each Ir–H bond formed. In the submonolayer regime we findevidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identifyfour different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hexphase also four surface components are identified, which undergo a rather rigid core level shift of +0.15 eV uponhydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms.Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogenindependent of the total coverage. We conclude that the surface always separates into patches of fully covereddeconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with thedesorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts intothe reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption forall phases to be θmax = 1.0 ML1×1−H , θ = 0.8 ML max5×1−H , and θ ≥ 1.0 ML max5×1−hex−H .
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29. |
- Arman, Alif, et al.
(author)
-
Co3O4(100) films grown on Ag(100): Structure and chemical properties
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 657, s. 90-95
-
Journal article (peer-reviewed)abstract
- Spinel type Co3O4(100) is successfully grown on Ag(100) at ultrahigh vacuum conditions and its structure, electronic and chemical properties are compared with those of Co3O4(111) grown on Ir(100). We find that the Co3O4(100) is unreconstructed. In contrast to the defect free Co3O4(111) surface the Co3O4(100) surface contains a high concentration of defects that we assign to subsurface cation vacancies analogous to those observed for Fe3O4(100). Our photoemission and absorption spectroscopy experiments reveal a very similar electronic structure of the Co3O4(111) and Co3O4(100) surfaces. The similar electronic structure of the two surfaces is reflected in the CO adsorption properties at low temperatures, as we observe adsorption of molecular CO as well as the formation of carbonate (CO3) species on both surfaces upon CO exposure at 85 K.
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30. |
- Borck, O., et al.
(author)
-
Methylbenzenes on graphene
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 664, s. 162-167
-
Journal article (peer-reviewed)abstract
- We present a theory study of the physisorption of the series of methylbenzenes (toluene, xylene and mesitylene), as well as benzene, on graphene. The aim is two fold: we provide data that will be used as input to larger-scale methods like molecular-dynamics simulations, and at the same time we enhance the basic understanding of graphene used as a material for sensors and as an idealized model for the carbon in active carbon filters. The molecules are studied in a number of positions and orientations relative to graphene, using density functional theory with the van der Waals functional vdW-DF. The molecules are adsorbed fractional coverage. We focus on the vdW-DF1 and vdW-DF-cx functionals, and find that the binding energy of the molecules on graphene grows linearly with the number of methyl groups, at the rate of 0.09 eV (vdW-DF1) to 0.11 eV (vdW-DF-cx) per added methyl group. We further find that the orientation of the methyl groups of the molecules relative to graphene is at least as important as the lateral position of the whole molecule on graphene. © 2017 Elsevier B.V.
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31. |
- Gibson, Andrew J., et al.
(author)
-
Resonant core spectroscopies of the charge transfer interactions between C60 and the surfaces of Au(111), Ag(111), Cu(111) and Pt(111)
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 657, s. 69-78
-
Journal article (peer-reviewed)abstract
- Charge transfer interactions between C60 and the metal surfaces of Ag(111), Cu(111), Au(111) and Pt(111) have been studied using synchrotron-based photoemission, resonant photoemission and X-ray absorption spectroscopies. By placing the X-ray absorption and valence band spectra on a common binding energy scale, the energetic overlap of the unoccupied molecular orbitals with the density of states of the underlying metal surface have been assessed in the context of possible charge transfer pathways. Resonant photoemission and resonant Auger data, measuring the valence region as a function of photon energy for C60 adsorbed on Au(111) reveals three constant high kinetic energy features associated with Auger-like core-hole decay involving an electron transferred from the surface to the LUMO of the molecule and electrons from the three highest occupied molecular orbitals, respectively and in the presence of ultra-fast charge transfer of the originally photoexcited molecule to the surface. Data for the C60/Ag(111) surface reveals an additional Auger-like feature arising from a core-hole decay process involving more than one electron transferred from the surface into the LUMO. An analysis of the relative abundance of these core-hole decay channels estimates that on average 2.4 ± 0.3 electrons are transferred from the Ag(111) surface into the LUMO. A core-hole clock analysis has also been applied to assess the charge transfer coupling in the other direction, from the molecule to the Au(111) and Ag(111) surfaces. Resonant photoemission and resonant Auger data for C60 molecules adsorbed on the Pt(111) and Cu(111) surfaces are shown to exhibit no super-Auger features, which is attributed to the strong modification of the unoccupied molecular orbitals arising from stronger chemical coupling of the molecule to the surface.
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32. |
- Grånäs, Elin, et al.
(author)
-
Hydrogen intercalation under graphene on Ir(111)
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 651, s. 57-61
-
Journal article (peer-reviewed)abstract
- Using high resolution X-ray photoelectron spectroscopy and scanning tunneling microscopy we study the intercalation of hydrogen under graphene/Ir(111). The hydrogen intercalated graphene is characterized by a component in C 1s that is shifted −0.10 to −0.18 eV with respect to pristine graphene and a component in Ir 4f at 60.54 eV. The position of this Ir 4f component is identical to that of the Ir(111) surface layer with hydrogen atoms adsorbed, indicating that the atomic hydrogen adsorption site on bare Ir(111) and beneath graphene is the same. Based on co-existence of fully- and non-intercalated graphene, and the inability to intercalate a closed graphene film covering the entire Ir(111) surface, we conclude that hydrogen dissociatively adsorbs at bare Ir(111) patches, and subsequently diffuses rapidly under graphene. A likely entry point for the intercalating hydrogen atoms is identified to be where graphene crosses an underlying Ir(111) step.
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33. |
- Hellman, Anders, 1974, et al.
(author)
-
Electro-oxidation of water on hematite: Effects of surface termination and oxygen vacancies investigated by first-principles
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 640, s. 45-49
-
Journal article (peer-reviewed)abstract
- The oxygen evolution reaction on hydroxyl- and oxygen-terminated hematite was investigated using first-principle calculations within a theoretical electrochemical framework. Both pristine hematite and hematite containing oxygen vacancies were considered. The onset potential was determined to be 1.79 V and 2.09 V vs. the reversible hydrogen electrode (RHE) for the pristine hydroxyl- and oxygen-terminated hematite, respectively. The presence of oxygen vacancies in the hematite surface resulted in pronounced shifts of the onset potential to 3.09 V and 1.83 V. respectively. Electrochemical oxidation measurements conducted on thin-film hematite anodes, resulted in a measured onset potential of 1.66 V vs. RHE. Furthermore, the threshold potential between the hydroxyl- and oxygen-terminated hematite was determined as a function of pH. The results indicate that electrochemical water oxidation on hematite occurs on the oxygen-terminated hematite, containing oxygen vacancies.
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34. |
- Jhang, Jin-Hao, et al.
(author)
-
CO and D2O chemistry on continuous and discontinuous samaria thin films on Pt(111) : The surface science of heterogeneous catalysis: In honor of Robert J. Madix
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 650, s. 221-229
-
Journal article (peer-reviewed)abstract
- Abstract The chemistry of CO and D2O, individually adsorbed or co-adsorbed, on epitaxial thin films of samaria on Pt(111) was studied by temperature-programmed desorption spectroscopy (TPD). Continuous thin films as well as discontinuous films composed of samaria islands on bare Pt(111) were prepared. Their comparative study indicates that Sm2O3 islands provide lattice oxygen at their perimeter for CO oxidation on adjacent exposed Pt area where CO adsorption takes place. CO2 production was observed only on as-prepared discontinuous films. While, in particular on thermally reduced samaria islands, TPD after D2O adsorption revealed D2 production which indicates a pathway for D2O dissociation, no evidence for the water gas shift reaction of CO and residual OD species on the surface was found after co-adsorption of CO and D2O. Instead, interaction between CO and OD species at the perimeter of islands on reduced discontinuous SmOx thin films obviously promotes D2 formation without yielding CO2 as desorbing product.
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35. |
- Knudsen, Jan, et al.
(author)
-
A versatile instrument for ambient pressure x-ray photoelectron spectroscopy : The Lund cell approach
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 646, s. 160-169
-
Journal article (peer-reviewed)abstract
- During the past one and a half decades ambient pressure x-ray photoelectron spectroscopy (APXPS) has grown to become a mature technique for the real-time investigation of both solid and liquid surfaces in the presence of a gas or vapour phase. APXPS has been or is being implemented at most major synchrotron radiation facilities and in quite a large number of home laboratories. While most APXPS instruments operate using a standard vacuum chamber as the sample environment, more recently new instruments have been developed which focus on the possibility of custom-designed sample environments with exchangeable ambient pressure cells (AP cells). A particular kind of AP cell solution has been driven by the development of the APXPS instrument for the SPECIES beamline of the MAX IV Laboratory: the solution makes use of a moveable AP cell which for APXPS measurements is docked to the electron energy analyser inside the ultrahigh vacuum instrument. Only the inner volume of the AP cell is filled with gas, while the surrounding vacuum chamber remains under vacuum conditions. The design enables the direct connection of UHV experiments to APXPS experiments, and the swift exchange of AP cells allows different custom-designed sample environments. Moreover, the AP cell design allows the gas-filled inner volume to remain small, which is highly beneficial for experiments in which fast gas exchange is required. Here we report on the design of several AP cells and use a number of cases to exemplify the utility of our approach.
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36. |
- Murdoch, A., et al.
(author)
-
The influence of CO adsorption on the surface composition of cobalt/palladium alloys
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 646, s. 31-36
-
Journal article (peer-reviewed)abstract
- Segregation induced by the adsorption of gas phase species can strongly influence the composition of bimetallic surfaces and can therefore play an important role in influencing heterogeneous catalytic reactions. The addition of palladium to cobalt catalysts has been shown to promote Fischer Tropsch catalysis. We investigate the adsorption of CO onto bimetallic CoPd surfaces on Pd{111} using a combination of reflection absorption infrared spectroscopy and medium energy ion scattering. The vibrational frequency of adsorbed CO provides crucial information on the adsorption sites adopted by CO and medium energy ion scattering probes the surface composition before and after CO exposure. We show that cobalt segregation is induced by CO adsorption and rationalise these observations in terms of the strength of adsorption of CO in various surface adsorption sites.
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37. |
- Nilsson, Viktor, 1985, et al.
(author)
-
Trends in adsorbate induced core level shifts
- 2015
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 640, s. 59-64
-
Journal article (peer-reviewed)abstract
- Photoelectron core level spectroscopy is commonly used to monitor atomic and molecular adsorption on metal surfaces. As changes in the electron binding energies are convoluted measures with different origins, calculations are often used to facilitate the decoding of experimental signatures. The interpretation could in this sense benefit from knowledge on trends in surface core level shifts for different metals and adsorbates. Here, density functional theory calculations have been used to systematically evaluate core level shifts for (111) and (100) surfaces of 3d, 4d, and 5d transition metals upon CO, H, O and S adsorption. The results reveal trends and several non-intuitive cases. Moreover, the difficulties correlating core level shifts with charging arid d-band shifts are underlined. (C) 2015 Elsevier B.V. All rights reserved.
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38. |
- Shipilin, Mikhail, et al.
(author)
-
The influence of incommensurability on the long-range periodicity of the Pd(100)-(root 5 x root 5)R27 degrees-PdO(101)
- 2017
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 660, s. 1-8
-
Journal article (peer-reviewed)abstract
- The structural model of the (root 5 x root 5)R27 degrees PdO(101) surface oxide grown on Pd(100) has been proposed and refined by a number of authors over more than a decade. In the current contribution we discuss the long-range periodicity of this structure arising along one of the crystallographic directions due to its incommensurability with the substrate. Analyzing the results of surface sensitive diffraction studies, we determined a slight distortion of the previously reported perfect (root 5 x root 5)R27 degrees surface oxide unit cell. Considering it, we were able to achieve both qualitatively and quantitatively better fit to the experimental diffraction data than it was possible for the perfect structure. Further, taking into account the experimentally obtained scanning tunneling microscopy data and closely examining high-resolution patterns recorded by means of high-energy surface X-ray diffraction, we developed a qualitative structural model based on a larger non-orthogonal surface unit cell to shed more light on the long-range order of the PdO(101) surface oxide. The model comprises a shift of the atoms of the PdO perpendicularly to the direction of the incommensurability to correct for it. This structural model reproduces the fine details of the high-resolution diffraction patterns and qualitatively explains the periodic stripes of structural distortion observed in the images recorded by a scanning tunneling microscope.
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39. |
- Simonsen, S. B., et al.
(author)
-
Coarsening of Pd nanoparticles in an oxidizing atmoshere studied by in situ TEM
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 648, s. 278-283
-
Journal article (peer-reviewed)abstract
- The coarsening of supported palladiumnanoparticles in an oxidizing atmospherewas studied in situ by means of transmission electron microscopy (TEM). Specifically, the Pd nanoparticles were dispersed on a planar and amorphousAl2O3 support and were observed during the exposure to 10 mbar technical air at 650 °C. Time-resolved TEM image series reveal that the Pd nanoparticles were immobile and that a few percent of the nanoparticles grew or shrank, indicating a coarsening process mediated by the Ostwald ripening mechanism. The TEM image contrast suggests that the largest nanoparticles tended to wet the Al2O3 support to a higher degree than the smaller nanoparticles and that the distribution of projected particle sizes consequently broadens by the appearance of an asymmetric tail toward the larger particle sizes. A comparison with computer simulations based on a simple mean-fieldmodel for the Ostwald ripening process indicates that the observed change in the particle size distribution can be accounted for by wetting of the Al2O3 support by the larger Pd nanoparticles.
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40. |
- Soldemo, Markus, et al.
(author)
-
Oxidation of Fe(110) in oxygen gas at 400 degrees C
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 644, s. 172-179
-
Journal article (peer-reviewed)abstract
- The initial oxidation of Fe(110) in oxygen gas at 400 degrees C beyond initial adsorbate structures has been studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM). Formation of several ordered phases of surface oxides is observed at oxygen coverages between approximately 2.3 and 3.5 oxygen atoms/Fe(110) surface atom. Initially, a FeO(111)-like film is formed with a parallelogram-shaped moire pattern. It has two mirror domains that are formed symmetrically around the growth direction of a zigzag-shaped adsorbate structure. With increased local oxygen coverage, the moire structure transforms into a ball-shaped form. Both these moire structures have equal atomic stacking at the surface and equal apparent height in STM, suggesting oxygen ions diffusing into the film upon oxidation and that the oxide growth takes place at the iron-iron oxide interface. The FeO(111)-like film turns into a Fe3O4(111)-like film with a triangular bistable surface termination as the oxidation proceeds further. The FeO(111)-like film growth proceeds according to the Frank-van der Merwe mechanism while the Fe3O4(111)-like film grows according to the Stranski-Krastanov mechanism. (C) 2015 Elsevier B.V. All rights reserved.
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41. |
- Zhdanov, Vladimir, 1952
(author)
-
Kinetic model of oxidation of metal nanoparticles: Cabrera-Mott and Kirkendall effects
- 2019
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 684, s. 24-27
-
Journal article (peer-reviewed)abstract
- The conventional Cabrera-Mott model for the formation of a thin (nm-sized) flat oxide film on the surface of a macroscopic metal crystal implies that the process is limited by the field-facilitated activated jumps of metal ions at the metal-oxide interface. Earlier, it was modified in order to describe oxidation of metal nanoparticles with the formation of the oxide shell around the close-packed metallic core. In reality, oxidation of metal nano particles is often accompanied by the appearance of a hollow in the metallic core (Kirkendall effect). Herein, the analytical treatment takes this effect into account. The numerical results obtained by integration of the corresponding equation show the specifics of the oxidation kinetics in this case.
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42. |
- Zhdanov, Vladimir, 1952
(author)
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Two-dimensional Ostwald ripening on a patterned support and in a mixed overlayer
- 2016
-
In: Surface Science. - : Elsevier BV. - 0039-6028. ; 644, s. 191-198
-
Journal article (peer-reviewed)abstract
- The author presents lattice Monte Carlo simulations of Ostwald ripening of islands formed of monomers with attractive nearest-neighbor interaction in two situations. The first one implies that monomers of one kind are located on a heterogeneous support which itself includes embedded frozen patterns of the Ostwald type. This feature is demonstrated to be able to terminate the island ripening. In the second case, the support is considered to be uniform while the overlayer is assumed to contain monomers of two kinds. This complicating factor is found to nearly not modify the ripening kinetics or make it somewhat slower despite appreciable correlations in the arrangement of different monomers.
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