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Sökning: WFRF:(Borg A) > Sandell Anders

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1.
  • Schaefer, A., et al. (författare)
  • Controlled modification of nanoporous gold : Chemical vapor deposition of TiO2 in ultrahigh vacuum
  • 2013
  • Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332 .- 1873-5584. ; 282, s. 439-443
  • Tidskriftsartikel (refereegranskat)abstract
    • TiO2 has been deposited in the first 400 nm of a nanoporous gold (NPG) structure using metal organic chemical vapor deposition with titanium-tetraisopropoxide as single source precursor in ultra high vacuum. The NPG has been pretreated by ozone to clean and stabilize the structure for deposition. The deposited oxide stabilizes the porous structure, otherwise prone to coarsening at elevated temperatures, up to 300 degrees C. The study combines the controlled sample preparation with a functional test of the prepared catalyst under real conditions in a continuous gas flow reactor. The catalytic activity of the loaded NPG at 60 degrees C for CO oxidation is found to be superior to unloaded as-prepared NPG.
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2.
  • Farstad, M. H., et al. (författare)
  • Water Adsorption on TiOx Thin Films Grown on Au(111)
  • 2015
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:12, s. 6660-6669
  • Tidskriftsartikel (refereegranskat)abstract
    • High resolution photoelectron spectroscopy has been used to investigate water adsorption on four different TiOx ultrathin film structures, grown on Au(111) by chemical vapor deposition. Two of the structures are reduced TiOx single layer phases, forming a honeycomb (HC) and a pinwheel (PW) structure, respectively. The other two phases have TiO2 stoichiometry, one in the form of islands and one in the form of a TiO2(B)(001) extended layer. Partial water dissociation is observed for all phases but the HC phase, and the dissociation propensity and adsorbate thermal stability structure result from interplay between the atomic structure of the particular TiOx phase and defects formed in the preparation. The dissociation on the TiO2(B) film is mainly related to different types of defect sites. The TiO2 islands, interpreted as surface reconstructed rutile TiO2(100), generate the highest amount of hydroxyls with a behavior consistent with reconstruction into a mixed (100) and (110) termination. Water dissociation on the PW layer can be assigned to particular sites of the structure and it stands out by leading to oxidation of Ti species.
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3.
  • Ragazzon, Davide, et al. (författare)
  • Chemical vapor deposition of ordered TiOx nanostructures on Au(111)
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 617, s. 211-217
  • Tidskriftsartikel (refereegranskat)abstract
    • The deposition of TiOx (x <= 2) structures on Au(111) by chemical vapor deposition (CVD) in ultrahigh vacuum (UHV) has been investigated with high-resolution core level photoelectron spectroscopy (PES), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Using titanium tetra-isopropoxide as single source precursor it is possible to form different TiOx phases on the surface after deposition: at low coverages, we observe large two-dimensional (2D) honeycomb-lattice Ti2O3 islands with a (2 x 2) registry with the substrate. Higher coverages are dominated by the formation of three-dimensional (3D) TiO2 structures. The TiO2 structures are atomically well ordered provided that the deposition temperature is high enough (500 degrees C). The ordered structure exhibits a LEED pattern characteristic for a rectangular surface unit cell. By performing the deposition at different temperatures it is possible to tune the balance between the 2D and 3D phases: Growth at 500 degrees C significantly favors the formation of 3D TiO2 islands as compared to growth at 200 degrees C and 300 degrees C. (C) 2013 Elsevier B.V. All rights reserved.
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4.
  • Sandell, Anders, et al. (författare)
  • Adsorption and photolysis of trimethyl acetate on TiO2(B)(001) studied with synchrotron radiation core level photoelectron spectroscopy
  • 2017
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 666, s. 104-112
  • Tidskriftsartikel (refereegranskat)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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5.
  • Sandell, Anders, et al. (författare)
  • Photochemistry of Carboxylate on TiO2(110) Studied with Synchrotron Radiation Photoelectron Spectroscopy
  • 2016
  • Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 32:44, s. 11456-11464
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a dedicated synchrotron radiation photoelectron spectroscopy (SR-PES) study of a photochemical reaction on the surface of rutile TiO2(110). The photoreaction kinetics of carboxylate species (trimethyl acetate, TMA) upon irradiation by UV and soft X-rays were monitored, and we show that it is possible to control the reaction rates from UV light and soft X-rays independently. We directly observe Ti4+ -> Ti3+ conversion upon irradiation, attributed to electron trapping at Ti sites close to surface OH groups formed by deprotonation of the parent molecule, trimethylacetic acid (TMAA). TMA photolysis on two surface preparations with different oxygen vacancy densities shows that the vacancy-related charge quenches the amount of charge that can be trapped at hydroxyls upon irradiation. During the initial stages of reaction the correlation between the amount of photodepleted TMA and the amount of charge trapped in the Ti 3d band gap state is nearly 1:1. A first-order kinetics analysis reveals that the reaction rate decreases with decreasing TMA coverage. There is also a coverage-dependent difference in the electronic structure of TMA moieties, primarily involving the carboxyl anchor group. These changes are consistent with a decreased hole affinity of the adsorbed TMA and hence a decreased reaction rate. This discovery adds to the previously presented picture of a reactivity that is inversely proportional to the number of surface hydroxyls, suggesting that the balance between the amounts of TMA, OH, and trapped charge needs to be considered.
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6.
  • Schaefer, Andreas, et al. (författare)
  • Toward Controlled Modification of Nanoporous Gold : A Detailed Surface Science Study on Cleaning and Oxidation
  • 2012
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:7, s. 4564-4571
  • Tidskriftsartikel (refereegranskat)abstract
    • Nanostructured metals and especially unsupported nanoporous gold (np-Au) have attracted considerable attention in a variety of fields because of their special surface chemical properties. For applications in catalysis and sensorics, the oxidation, of the metal and the availability of oxygen at the very surface are crucial and also are capable of altering structural properties. In this article, we will discuss the state of the np-Au surface after annealing in vacuum. We shed light on the nature of Au-oxide obtained after cleaning the surface from carbon impurities with atomic oxygen provided by ozone decomposition, and we consider the effect of this procedure on silver residues. The results provide new insight into possible oxide species at the np-Au surface and represent a vital step toward controlled modification of the np-Au surface in the future.
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7.
  • Amft, Martin, et al. (författare)
  • A Molecular Mechanism for the Water-Hydroxyl Balance during Wetting of TiO2
  • 2013
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:33, s. 17078-17083
  • Tidskriftsartikel (refereegranskat)abstract
    • We show that the formation of the wetting layer and the experimentally observed continuous shift of the H2O-OH balance toward molecular water at increasing coverage on a TiO2(110) surface can be rationalized on a molecular level. The mechanism is based on the initial formation of stable hydroxyl pairs, a repulsive interaction between these pairs, and an attractive interaction with respect to water molecules. The experimental data are obtained by synchrotron radiation photoelectron spectroscopy and interpreted with the aid of density functional theory calculations and Monte Carlo simulations.
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8.
  • Farstad, M. H., et al. (författare)
  • Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100)
  • 2018
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:2, s. 688-694
  • Tidskriftsartikel (refereegranskat)abstract
    • Thin films of TiOx on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO2 films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO2/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.
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9.
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10.
  • Johansson, E. M. J., et al. (författare)
  • Comparing Surface Binding of the Maleic Anhydride Anchor Group on Single Crystalline Anatase TiO2 (101), (100), and (001) Surfaces
  • 2010
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:35, s. 15015-15020
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the surface binding of the maleic anhydride (C4H2O3, MA) on the TiO2 anatase (101), (100), and (001) single crystal surfaces. The MA anchor group has recently been used for dye adsorption in solar cells based on nanostructured anatase, and the results reported here are partly discussed with respect to such systems. MA was deposited simultaneously onto the (101), (100), and (001) TiO2 single crystal surfaces in UHV, and the surface binding was investigated with electron spectroscopy. The O1s and C1s core-level spectra were compared to a multilayer of MA to investigate the differences in bonding to the anatase surfaces. The results suggest a surface chemistry where the molecule reacts and the MA ring opens when adsorbed at the (101) and (100) surfaces. The molecule anchors via four oxygen atoms, similar to bonding with two carboxylic groups on TiO2. For the (001) surface, the spectra indicated a different adsorption geometry. A small amount of electronic states in the bandgap of the TiO2 surfaces was observed both before and after MA was deposited onto the surfaces, and on the (101) and (100) surfaces, the intensity of these surface states was slightly enhanced after deposition of MA.
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