G. Thornton

University College London, Londinium, England, United Kingdom

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Publications (311)966.35 Total impact

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    ABSTRACT: The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge transfer processes involved, partly because the excited states involved are difficult to study. Here, we present evidence for the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ~3.1 eV. The onset is shifted to lower energy by the co-adsorption of molecular water, which suggests a means of tuning the energy of the excited state.
    Journal of Physical Chemistry Letters 08/2015; DOI:10.1021/acs.jpclett.5b01508 · 7.46 Impact Factor
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    ABSTRACT: Supported metal nanoparticles form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that adsorbates bond in an identical fashion as on a semiinfinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past 40 years to be used to model heterogeneous catalysts. Using a surface science approach to CO adsorption on supported Pd nanoparticles, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single-crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step edges. This strain also weakens the CO-metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation.
    Proceedings of the National Academy of Sciences 06/2015; 112(26). DOI:10.1073/pnas.1506939112 · 9.81 Impact Factor
  • Materials Science and Technology 06/2015; DOI:10.1179/1743284715Y.0000000086 · 0.80 Impact Factor
  • Chi Ming Yim · Chi Lun Pang · Geoff Thornton
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    ABSTRACT: The electronic structure of cross-linked TiO2(110)-(1 × 2) has been investigated using scanning tunneling spectroscopy (STS) and by monitoring changes in ultraviolet photoelectron spectroscopy (UPS) following exposure of the surface to O2. STS reveals two states located in the bandgap, at 0.7 and 1.5 eV below the Fermi level. The population of these two states varies over different parts of the (1 × 2)-reconstructed surface. An addition state at 1.1 eV above the Fermi level is observed at the double link part of the structure. All of the bandgap states are attenuated following exposure to O2, while the workfunction is increased. We attribute this to an electron transfer from the surface to the adsorbed oxygen.
    Surface Science 05/2015; DOI:10.1016/j.susc.2015.04.022 · 1.87 Impact Factor
  • Physical Review B 01/2015; 91(3). DOI:10.1103/PhysRevB.91.039910 · 3.74 Impact Factor
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    David C Grinter · Thomas Woolcot · Chi-Lun Pang · Geoff Thornton
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    ABSTRACT: As models for probing the interactions between TiO2 surfaces and the dye molecules employed in dye-sensitized solar cells, carboxylic acids are an important class of molecules. In this work, we present a scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study of three small carboxylic acids (formic, acetic, and benzoic) that were reacted with the TiO2(110) surface via a dipping procedure. The three molecules display quite different adsorption behavior, illustrating the different interadsorbate interactions that can occur. After exposure to a 10 mM solution, formic acid forms a rather disordered formate overlayer with two distinct binding geometries. Acetic acid forms a well-ordered (2 × 1) acetate overlayer similar to that observed following deposition from vapor. Benzoic acid forms a (2 × 2) overlayer, which is stabilized by intermolecular interactions between the phenyl groups.
    Journal of Physical Chemistry Letters 12/2014; 5(24):4265-4269. DOI:10.1021/jz502249j · 7.46 Impact Factor
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    ABSTRACT: We have used noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM) to study the rutile TiO2(011) surface. A series of (2n × 1) reconstructions were observed, including two types of (4 × 1) reconstruction. High-resolution NC-AFM and STM images indicate that the (4 × 1)-α phase has the same structural elements as the more widely reported (2 × 1) reconstruction. An array of analogous higher-order (2n × 1) reconstructions were also observed where n = 3-5. On the other hand, the (4 × 1)-β reconstruction seems to be a unique structure without higher-order analogues. A model is proposed for this structure that is also based on the (2 × 1) reconstruction but with additional microfacets of {111} character.
    The Journal of Physical Chemistry C 10/2014; 118(40):23168-23174. DOI:10.1021/jp507422s · 4.77 Impact Factor
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    ABSTRACT: CeO2-x(111) ultrathin films consisting of small, discrete islands decorating a Pt(111) substrate have been studied using a combination of Scanning Tunnelling Microscopy, Low-Energy Electron Microscopy, and Low-Energy Electron Diffraction. Significantly, the chemical nature of the ceria film has also been probed using X-Ray Absorption Spectroscopy (XAS) combined with X-ray PhotoEmission Electron Microscopy (XPEEM) in the same ultrahigh vacuum system. XAS spectra over the Ce M5 absorption edge demonstrated that the ceria islands contained ∼50% Ce4+and ∼50% Ce3+, leading to an overall stoichiometry of CeO1.75, which was uniform across the film. The unique advantage of this experimental setup is the application of multiple techniques on the same sample: high-resolution STM to monitor the morphology, XPEEM to probe the stoichiometry, and LEEM to act as a bridge between the two.
    Journal of Electron Spectroscopy and Related Phenomena 08/2014; 195. DOI:10.1016/j.elspec.2014.03.014 · 1.55 Impact Factor
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    ABSTRACT: Nanometer-sized gold particles supported on ceria are an important catalyst for the low-temperature water−gas shift reaction. In this work, we prepared a model system of epitaxial, ultrathin (1−2 nm thick) CeO 2−x (111) crystallites on a Rh(111) substrate. Low-energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM) were employed to characterize the in situ growth and morphology of these films, employing Ce 4f resonant photoemission to probe the oxidation state of the ceria. The deposition of submonolayer amounts of gold at room temperature was studied with scanning tunneling microscopy (STM) and XPEEM. Spatially resolved, energy-selected XPEEM at the Au 4f core level after gold adsorption indicated small shifts to higher binding energy for the nanoparticles, with the magnitude of the shift inversely related to the particle size. Slight reduction of the ceria support was also observed upon increasing Au coverage. The initial oxidation state of the ceria film was shown to influence the Au 4f binding energy; more heavily reduced ceria promoted a larger shift to higher binding energy. Understanding the redox behavior of the gold/ceria system is an important step in elucidating the mechanisms behind its catalytic activity.
    The Journal of Physical Chemistry C 07/2014; 118(33). DOI:10.1021/jp5055205 · 4.77 Impact Factor
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    ABSTRACT: Surface x-ray diffraction has been employed to elucidate the structure of the interface between a well characterized (001) surface of 0.1 wt% Nb-SrTiO3 and liquid H2O. Results are reported for the clean surface, the surface in contact with a drop of liquid water and the surface after the water droplet has been removed with a flow of nitrogen. The investigation revealed the clean surface, prepared via annealing in 1 x 10-2 mbar O2 partial pressure, is unreconstructed and rough on a short length scale. The surface is covered with large terraces, the topmost layer of which is either TiO2 or SrO with an area ratio of about 7/3. For the surface in contact with water our results reveal that associative H2O adsorption is favoured for the TiO2 terminated terrace whereas adsorption is dissociative for the SrO terminated terrace, which validates recent first principles calculations. After removal of the water droplet, the surface largely resembles the water-covered surface but now with a disordered overlayer of water present on the surface.
    The Journal of Physical Chemistry C 04/2014; 118(20). DOI:10.1021/jp5034118 · 4.77 Impact Factor
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    Chi L Pang · David C Grinter · Jai Matharu · Geoff Thornton
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    ABSTRACT: Ultrathin films of rutile TiO 2 (110) have been grown on a W(100)-O(2 × 1) surface and characterized with a combination of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). LEED shows the presence of two orthogonal rotational domains of rutile TiO 2 (110). In line with this, STM images reveal that the rutile TiO 2 grows as discrete islands and can be aligned in either of the principal directions of the underlying substrate W(100)-O(2 × 1). High-resolution STM images reveal atomic-scale rows and the presence of point defects on the rutile islands that are characteristic of the native rutile TiO 2 (110)-(1 × 1).
    The Journal of Physical Chemistry C 12/2013; 117(48):25622. DOI:10.1021/jp409948u · 4.77 Impact Factor
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    ABSTRACT: We report a new method for introducing metal atoms into silicon wafers, using negligible thermal budget. Molecular thin films are irradiated with ultra-violet light releasing metal species into the semiconductor substrate. Secondary ion mass spectrometry and x-ray absorption spectroscopy show that Mn is incorporated into Si as an interstitial dopant. We propose that our method can form the basis of a generic low-cost, low-temperature technology that could lead to the creation of ordered dopant arrays.
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    J. Matharu · G. Cabailh · G. Thornton
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    ABSTRACT: We present a study of the growth and reactivity of ultra-thin films of TiO2 grown on W(100). Three approaches to film growth are investigated, each resulting in films that show order in low-energy diffraction (LEED) and a low level of non-stoichiometry in X-ray photoelectron spectroscopy (XPS). H2O is used as a probe of the reactivity of the films, with changes in the Ti 2p and O 1s core levels being monitored by XPS. Evidence for the dissociation of H2O on the TiO2(110) ultra-thin film surface is adduced. These results are discussed with reference to related studies on native TiO2(110).
    Surface Science 10/2013; 616:198-205. DOI:10.1016/j.susc.2013.05.020 · 1.87 Impact Factor
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    ABSTRACT: Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO2(110)-(1 × 1) in ultrahigh vacuum. The wires have a width down to ∼20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO)5. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring.
    The Journal of Physical Chemistry C 08/2013; 117(34):17674-17679. DOI:10.1021/jp405640a · 4.77 Impact Factor
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    ABSTRACT: High resolution X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy (LEEM) have been used to investigate the growth of ultrathin CeOx(111) on Re(0001), a model catalyst system. Rotational domains of CeOx(111) are identified with microprobe low energy electron diffraction (LEED) and dark-field LEEM. In the regions not covered by the ceria islands, a surface rhenium-oxide layer has been observed using energy-filtered XPEEM imaging and spectroscopy. The oxidation state of the ceria is key to its catalytic activity. For this reason we have employed resonant photoelectron spectroscopy of the Ce 4f contributions to the valence band to monitor the relative Ce3+ and Ce4+ concentrations. The overall stoichiometry of the moderately reduced film was CeO1.63. Resonant energy-filtered XPEEM imaging of the Ce oxidation state allowed us to confirm the uniformity of this stoichiometry across the ceria islands that constituted the film.
    The Journal of Physical Chemistry C 07/2013; 117(32):16509. DOI:10.1021/jp405887h · 4.77 Impact Factor
  • Chi Lun Pang · Robert Lindsay · Geoff Thornton
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    ABSTRACT: TiO2 is exploited in a number of applications, including solar power, catalysis, gas sensing, and corrosion protection, where surface properties are central to operation. This commercial importance drives cutting-edge research into this material; activity extends from development of novel titania-based structures exhibiting enhanced surface functionality to fundamental work aiming to gain mechanistic insight into surface processes. TiO2 is exploited in a number of applications, including solar power, catalysis, gas sensing, and corrosion protection, where surface properties are central to operation. This commercial importance drives cutting-edge research into this material; activity extends from development of novel titania-based structures exhibiting enhanced surface functionality to fundamental work aiming to gain mechanistic insight into surface processes.
    Chemical Reviews 05/2013; 113(6). DOI:10.1021/cr300409r · 45.66 Impact Factor
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    ABSTRACT: We have prepared a model catalytic system by depositing Pd onto a TiO2(110) surface held at 720 K. Scanning tunneling microscopy (STM) reveals well-defined Pd nanocrystals consisting of (111) top facets with {111} and {100} side facets. The Pd nanocrystals go down to about 10 nm in width and 1.3 nm in height. Top facets can be imaged with atomic resolution, indicating the absence of TiOx encapsulation. The model catalyst was probed by exposure to CO and O2. By varying the CO exposure, different CO overlayers were formed on the (111) top facets, with coverages ranging from 0.33 to 0.75 of a monolayer. Near edge X-ray absorption fine structure (NEXAFS) measurements at 300 K reveal that at around 0.5 ML coverage, CO is oriented with the molecular axis more or less normal to TiO2(110). Dosing small amounts of O2 separately on a Pd/TiO2(110) surface led to an overlayer of p(2 × 2)-O formed on the (111) top facet of the Pd nanocrystals at 190 K.
    Faraday Discussions 04/2013; 162:-. DOI:10.1039/C2FD20137B · 4.61 Impact Factor

Publication Stats

6k Citations
966.35 Total Impact Points

Institutions

  • 2004–2014
    • University College London
      • • London Centre for Nanotechnology
      • • Department of Chemistry
      Londinium, England, United Kingdom
  • 1979–2013
    • University of California, Berkeley
      • • Department of Materials Science and Engineering
      • • Department of Chemistry
      Berkeley, California, United States
    • CSU Mentor
      • Department of Chemistry
      Long Beach, California, United States
  • 2010–2012
    • London Centre for Nanotechnology
      Londinium, England, United Kingdom
  • 1983–2011
    • The University of Manchester
      • School of Chemistry
      Manchester, England, United Kingdom
    • Lawrence Berkeley National Laboratory
      Berkeley, California, United States
  • 2009
    • Fritz Haber Institute of the Max Planck Society
      Berlín, Berlin, Germany
  • 2007
    • Graz University of Technology
      • Institute for Technical Informatics
      Gratz, Styria, Austria
  • 2001
    • University of Washington Seattle
      • Department of Chemistry
      Seattle, Washington, United States
  • 1992–1999
    • University of Liverpool
      • Surface Science Research Centre
      Liverpool, England, United Kingdom
  • 1998
    • The University of York
      • Department of Physics
      York, ENG, United Kingdom
  • 1989
    • Imperial College London
      Londinium, England, United Kingdom
  • 1988
    • Trinity College
      Hartford, Connecticut, United States
  • 1986
    • Trinity College Dublin
      Dublin, Leinster, Ireland