Ronnie T. Vang

Aarhus University, Aarhus, Central Jutland, Denmark

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Publications (35)212.74 Total impact

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    ABSTRACT: We report on a new, one-step electrochemical oxidation method for the rapid synthesis of mixed phase, polycrystalline TiO2 porous films with oriented rutile within a few minutes. The orientation as well as the surface chemical composition of rutile nanocrystallites can readily be tuned by adjusting the additive concentrations of HCl or HF in the electrolyte during synthesis. All TiO2 films show similar large specific surface area, which is ideal for the application of photocatalysis and photoelectrocatalysis. Compared to the random-oriented TiO2 film, films with an increasing portion of exposed rutile (101) facets were found to be characterized by enhanced photocatalytic oxidation and photoelectrochemical performances. We also observed a synergistic promotion effect of the orientation and surface F impurity. Most interesting, our tailor-oriented porous TiO2 films prepared using HF as additive show an impressive photocurrent generation at zero bias, which is similar to 50 times higher compared to that of the random-oriented TiO2 film.
    No preview · Article · Dec 2013 · The Journal of Physical Chemistry C
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    ABSTRACT: We report on high-quality polycrystalline Fe-doped TiO2 (Fe–TiO2) porous films synthesized via one-step electrochemical oxidation. We demonstrate that delicate properties such as the impurity concentration and the microstructure that strongly influence the performance of the material for photovoltaic and photocatalysis applications can be controlled by adjusting the electrolyte composition. Compared to Fe-doped TiO2 films prepared with traditional phosphate- or silicate-based electrolytes, our newly synthesised Fe–TiO2 films contain solely Fe dopants, which results in excellent photocatalytic and photovoltaic performance under visible light irradiation.
    No preview · Article · Aug 2012 · Journal of Materials Chemistry
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    ABSTRACT: In 1991, Bickley et al. proposed a synergetic effect between anatase and rutile in Degussa P25. Since then, there has been an intensive debate about the correctness of this proposal, the origin of the synergism, and the right polymorph composition. However, a comparison of pure titanium dioxide samples with various anatase-to-rutile ratios, but otherwise identical properties, is missing. In this paper, we report about a series of utterly pure, highly porous titanium dioxide films with identical grain sizes, surface areas, and crystallinity, but varying polymorph compositions. Photocatalytic oxidation of methylene blue was utilized to investigate the influence of the anatase-to-rutile ratio on the photoreactivity. We clearly observe the synergetic effect within a well-defined range of anatase-to-rutile ratios. A film with 60% anatase and 40% rutile exhibits optimal performance at a 50% improved activity compared with pure anatase.
    No preview · Article · Nov 2011 · The Journal of Physical Chemistry C
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    ABSTRACT: Through an interplay between density functional theory and scanning tunneling microscopy, we investigated the mechanism of low-temperature CO oxidation on Ni(111) and NiO(111) surfaces. We systematically examined CO oxidation on different possible active sites. We find that sub- and full monolayers of O chemisorbed on Ni(111) surfaces play no significant role in low-temperature CO oxidation. We further show that CO oxidation at the perimeter of O islands on Ni(111) cannot occur at low temperatures. In contrast, we suggest that oxidized Ni(111) surfaces, i.e., NiO(111), can catalyze low-temperature CO oxidation when NiO(111) is saturated by O2. Our findings can rationalize low-temperature CO oxidation on Ni(111) surfaces that have been predosed with large amounts of oxygen, as observed in recent experiments.
    No preview · Article · Nov 2010 · The Journal of Physical Chemistry C
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    ABSTRACT: From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O(2)-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO(2) production.
    No preview · Article · Aug 2010 · ACS Nano
  • R. T. Vang · S. Wendt · F. Besenbacher
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Jul 2010 · ChemInform
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    ABSTRACT: The adsorption of 2,6-naphthalenedicarboxylic acid (NDCA) molecules on the Ag(110), Cu(110), and Ag(111) surfaces at room temperature has been studied by means of scanning tunnelling microscopy (STM). Further supporting results were obtained using X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS). On the Ag(110) support, which had an average terrace width of only 15 nm, the NDCA molecules form extended one-dimensional (1-D) assemblies, which are oriented perpendicular to the step edges and have lengths of several hundred nanometres. This shows that the assemblies have a large tolerance to monatomic surface steps on the Ag(110) surface. The observed behaviour is explained in terms of strong intermolecular hydrogen bonding and a strong surface-mediated directionality, assisted by a sufficient degree of molecular backbone flexibility. In contrast, the same kind of step-edge crossing is not observed when the molecules are adsorbed on the isotropic Ag(111) or more reactive Cu(110) surfaces. On Ag(111), similar 1-D assemblies are formed to those on Ag(110), but they are oriented along the step edges. On Cu(110), the carboxylic groups of NDCA are deprotonated and form covalent bonds to the surface, a situation which is also achieved on Ag(110) by annealing to 200 °C. These results show that the formation of particular self-assembled molecular nanostructures depends significantly on a subtle balance between the adsorbate-adsorbate and adsorbate-substrate interactions and that kinetic factors play an important role. KeywordsMolecular self-assembly-hydrogen bonding-scanning tunnelling microscopy-X-ray photoelectron spectroscopy
    Full-text · Article · Jul 2010 · Nano Research
  • Stefan Wendt · Ronnie T. Vang · Flemming Besenbacher
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    ABSTRACT: Scanning Tunnelling Microscopy, Photoelectron Spectroscopy and Density Functional Theory simulations are used to identify reaction pathways, active sites and the electronic defects on rutile TiO2(110). Examples from our laboratory show how fundamental work performed under ultra-high vacuum conditions helps to improve our understanding of important aspects of catalysis and photocatalysis.
    No preview · Article · Mar 2010
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    ABSTRACT: Using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations we have studied the reduction of ultra-thin films of FeO(1 1 1) grown on Pt(1 1 1) after exposure to atomic hydrogen at room temperature. A number of new ordered, partly-reduced FeOx structures are identified and as a general trend we reveal that all the reduced FeOx structures incorporate 2-fold coordinated Fe atoms as opposed to the original 3-fold coordinated Fe atoms in the FeO film. We find that when all the Fe atoms are 2-fold O-coordinated the FeOx surface structure is resistant to further reduction at room temperature. We observe that water easily dissociates on the most heavily reduced FeOx, structure in contrast to the initially inert FeO film, and reveal that it is possible to partially re-oxidize the FeOx film by heating the surface slightly in the presence of water.
    No preview · Article · Jan 2010 · Surface Science
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    ABSTRACT: A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH-LiCl as a function of temperature and time. Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data reveals that Cl readily substitutes for BH in the structure of LiBH. Prolonged heating a sample of LiBH-LiCl (1:1 molar ratio) above the phase transition temperature and below the melting point (108 < T < 275 C) can produce highly chloride substituted hexagonal lithium borohydride, h-Li(BH){sub 1-x}Cl, e.g., x 0.42, after heating from room temperature (RT) to 224 C at 2.5 C/min. LiCl has a higher solubility in h-LiBH as compared to orthorhombic lithium borohydride, o-LiBH, which is illustrated by a LiBH-LiCl (1:1) sample equilibrated at 245 C for 24 days and left at RT for another 13 months. Rietveld refinement reveals that this sample contains o-Li(BH){sub 0.91}Cl{sub 0.09} and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH as compared to a slower segregation of LiCl from o-LiBH, which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH){sub 0.91}Cl{sub 0.09} sample. The substitution of the smaller Cl for the larger BH ion is clearly observed as a reduction in the unit cell volume as a function of time and temperature. A significant stabilization of h-LiBH is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) Li and ¹¹B NMR experiments on pure LiBH show an increase in full width at half maximum (fwhm) when approaching the phase transition from o- to h-LiBH, which indicates an increase of the relaxation rate (i.e., T decreases). A less pronounced effect is observed for ion-substituted Li(BH){sub 1-x}Cl, 0.09 < x < 0.42. The MAS NMR experiments also demonstrate a higher degree of motion in the hexagonal phase, i.e., fwhm is reduced by more than a factor of 10 at the o- to h-LiBH phase transition.
    Preview · Article · Dec 2009 · Chemistry of Materials
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    ABSTRACT: A surprisingly high degree of structural and compositional dynamics is observed in the system LiBH4−LiCl as a function of temperature and time. Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data reveals that Cl− readily substitutes for BH4− in the structure of LiBH4. Prolonged heating a sample of LiBH4−LiCl (1:1 molar ratio) above the phase transition temperature and below the melting point (108 < T < 275 °C) can produce highly chloride substituted hexagonal lithium borohydride, h-Li(BH4)1−xClx, e.g., x 0.42, after heating from room temperature (RT) to 224 °C at 2.5 °C/min. LiCl has a higher solubility in h-LiBH4 as compared to orthorhombic lithium borohydride, o-LiBH4, which is illustrated by a LiBH4−LiCl (1:1) sample equilibrated at 245 °C for 24 days and left at RT for another 13 months. Rietveld refinement reveals that this sample contains o-Li(BH4)0.91Cl0.09 and LiCl. This illustrates a significantly faster dissolution of LiCl in h-LiBH4 as compared to a slower segregation of LiCl from o-LiBH4, which is also demonstrated by in situ SR-PXD from three cycles of heating and cooling of the same Li(BH4)0.91Cl0.09 sample. The substitution of the smaller Cl− for the larger BH4− ion is clearly observed as a reduction in the unit cell volume as a function of time and temperature. A significant stabilization of h-LiBH4 is found to depend on the degree of anion substitution. Variable temperature solid-state magic-angle spinning (MAS) 7Li and 11B NMR experiments on pure LiBH4 show an increase in full width at half maximum (fwhm) when approaching the phase transition from o- to h-LiBH4, which indicates an increase of the relaxation rate (i.e., T2 decreases). A less pronounced effect is observed for ion-substituted Li(BH4)1−xClx, 0.09 < x < 0.42. The MAS NMR experiments also demonstrate a higher degree of motion in the hexagonal phase, i.e., fwhm is reduced by more than a factor of 10 at the o- to h-LiBH4 phase transition.
    Full-text · Article · Nov 2009
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    ABSTRACT: Following the development of the scanning tunneling microscope (STM), the technique has become a very powerful and important tool for the field of surface science, since it provides direct real-space imaging of single atoms, molecules and adsorbate structures on surfaces. From a fundamental perspective, the STM has changed many basic conceptions about surfaces, and paved the way for a markedly better understanding of atomic-scale phenomena on surfaces, in particular in elucidating the importance of local bonding geometries, defects and resolving non-periodic structures and complex co-existing phases. The so-called “surface science approach”, where a complex system is reduced to its basic components and studied under well-controlled conditions, has been used successfully in combination with STM to study various fundamental phenomena relevant to the properties of surfaces in technological applications such as heterogeneous catalysis, tribology, sensors or medical implants. In this tribute edition to Gerhard Ertl, we highlight a few examples from the STM group at the University of Aarhus, where STM studies have revealed the unique role of surface defects for the stability and dispersion of Au nanoclusters on TiO2, the nature of the catalytically active edge sites on MoS2 nanoclusters and the catalytic properties of Au/Ni or Ag/Ni surfaces. Finally, we briefly review how reaction between complex organic molecules can be used to device new methods for self-organisation of molecular surface structures joined by comparatively strong covalent bonds.
    No preview · Article · Jun 2009 · Surface Science
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    ABSTRACT: The oxidized Ag(111) surface has been studied by a combination of experimental and theoretical methods, scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory. A large variety of different surface structures is found, depending on the detailed preparation conditions. The observed structures fall into four classes: (a) individually chemisorbed atomic oxygen atoms, (b) three different oxygen overlayer structures, including the well-known p(4×4) phase, formed from the same Ag6 and Ag10 building blocks, (c) a c(4×8) structure not previously observed, and (d) at higher oxygen coverages structures characterized by stripes along the high-symmetry directions of the Ag(111) substrate. Our analysis provides a detailed explanation of the atomic-scale geometry of the Ag6/Ag10 building block structures and the c(4×8) and stripe structures are discussed in detail. The observation of many different and co-existing structures implies that the O/Ag(111) system is characterized by a significantly larger degree of complexity than previously anticipated, and this will impact our understanding of oxidation catalysis processes on Ag catalysts.
    Full-text · Article · Apr 2009 · Physical review. B, Condensed matter
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    ABSTRACT: By chemically modifying the FeO(111) thin film on Pt(111), we show that it is possible to unambiguously correlate its STM morphology with its underlying structure without recourse to STM simulations. Partial reduction of the oxide surface leads to the formation of triangularly-shaped oxygen vacancy dislocation loops at specific sites in the moiré structure of the film. Their presence allows unambiguous identification of the high-symmetry domains of the moiré structure, whose differing chemical properties govern the templating effect on adsorbed metal atoms, clusters and molecules.
    No preview · Article · Jan 2009 · Surface Science
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    No preview · Article · Dec 2008 · ChemInform
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    ABSTRACT: The mechanisms of CO oxidation on Ni(111) surfaces pre-dosed with Oxygen are systematically investigated using a combination of UHV experimental (XPS, TPD, STM) and first-principles (DFT) methods. Different CO oxidation mechanisms on Ni(111) surfaces with local O-islands, O chemisorbed on Ni(111) as a function of surface coverage up to 1 ML, and on highly oxidized Ni(111) surfaces, i.e., NiO(111) surfaces, are discussed. These different CO oxidation mechanisms can be used to rationalize experimental results on CO oxidation on Ni(111) surfaces under different experimental conditions. Insights derived should be applicable to CO oxidation on other transitional metal surfaces.
    No preview · Conference Paper · Nov 2008
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    ABSTRACT: The surface science approach to catalysis, pioneered by 2007 Nobel Laureate in chemistry Gerhard Ertl, has helped revolutionize our understanding of heterogeneous catalysis at the atomic level. In this tutorial review we show how the scanning tunnelling microscope (STM), in combination with this surface science approach, is a very important tool for the study of catalytically relevant model systems. We illustrate how the high spatial and temporal resolution of the STM can be used to obtain quantitative information on elementary processes involved in surface catalyzed reactions. Furthermore, we show that the STM is an outstanding surface science tool to bridge the materials gap and the pressure gap between surface science experiments and real catalysis. Finally, we show that we are approaching an era where the atomic-scale insight gained from fundamental STM surface science studies can be used for the rational design of new catalysts from first principles.
    No preview · Article · Nov 2008 · Chemical Society Reviews

  • No preview · Article · May 2008 · Physical Review Letters
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    ABSTRACT: The sections in this article areIntroductionScanning Probe TechniquesHigh-Resolution STM StudiesDynamic ImagingHigh-Pressure STM StudiesImaging of Oxide SurfacesCatalyst Design Based on Model System StudiesConclusion AcknowledgementsKeywords:solid catalysts;scanning probe microscopy
    No preview · Chapter · Mar 2008
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    ABSTRACT: 2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag110 surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.
    Full-text · Article · Mar 2008 · Physical Review Letters