Geoff Thornton

Chemical reactions on rutile TiO2(110)

Understanding the surface chemistry of TiO2 is key to the development and optimisation of many technologies, such as solar power, catalysis, gas sensing, medical implantation, and corrosion protection. In order to address this, considerable research effort has been directed at model single crystal surfaces of TiO2. Particular attention has been given to the rutile TiO2(110) surface because it is the most stable face of TiO2. In this critical review, we discuss the chemical reactivity of TiO2(110), focusing in detail on four molecules/classes of molecules. The selected molecules are water, oxygen, carboxylic acids, and alcohols-all of which have importance not only to industry but also in nature (173 references).

Electron traps and their effect on the surface chemistry of TiO2(110)

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO2(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

Size-Dependent Dissociation of Carbon Monoxide on Cobalt Nanoparticles

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Surface relaxation of SrTiO3(001)

Surface X-ray diffraction has been used to examine the 300 K structure of SrTiO3(001)1×1 with a termination of 78% TiO and 22% SrO. The data indicate that a lateral ferroelectric distortion is absent on both terminations, consistent with a recent theoretical calculation. Although the negligible Ti atom relaxation found on the TiO termination is consistent with medium-energy ion scattering data, it differs from recent density functional theory calculations by 0.13 and 0.16 A. In contrast, the Sr displacement towards the SrO surface of 0.22±0.07 A is in good agreement with theory.

Final-state effects in the 3d and 4d X-Ray photoelectron spectra of CeO2

The complex 3d and 4d X-ray photoelectron spectra of CeO2 arc re-interpreted with the aid of multiple scattering Xα and intermediate coupling calculations. Features in the 3d spectrum are identified with Ce 3d94f0, 3d94f1VB−1, and 3d95p5np final-state configurations. Multiplet coupling in the 3d94f1 system is found to be significant.

C60 adsorption on the quasicrystalline surface of Al70Pd21Mn9

Abstract Room temperature adsorption of C 60 on the flat quasicrystalline surface of Al 70 Pd 21 Mn 9 has been investigated using scanning tunnelling microscopy. A dispersed overlayer is formed at low coverage, with avoidance of step-edges. There is no evidence of island formation or clustering. As the coverage is increased, a higher density layer is formed with no evidence of the formation of hexagonal ordered adsorbate structures seen on other substrates. This is followed by the onset of second layer formation. A range of bonding sites for C 60 molecules is implied from measurements of apparent molecular heights and from thermal effects. Detailed analysis of the surface at a low coverage (∼0.065 ML) provides evidence of adsorbate local order, with Fibonacci ( τ -scaling) relationships between the C 60 molecules. Where this occurs, the preferred adsorption site is tentatively identified as the pentagonal hollow. These local correlations however are not found to extend over larger regions of the surface.

Beam line 4: A dedicated surface science facility at Daresbury Laboratory

We describe a beam line currently under construction at the Daresbury Laboratory which forms part of a surface science research facility for the Interdisciplinary Research Centre in Surface Science. The beam line has three branches, two of which are described here. The first branch covers the high‐energy range 640 eV≤hν≤10 keV, being equipped with a double‐crystal monochromator and a novel multicoated premirror system. The second branch line is optimized for the energy range 15≤hν≤250 eV, using cylindrical focusing mirrors, a spherical diffraction grating and an ellipsoidal refocusing mirror to achieve high resolution with a small spot size.

The atomic and electronic structure of the (001) surface of monoclinic pyrrhotite (Fe7S8) as studied using STM, LEED and quantum mechanical calculations

Abstract The properties of monoclinic Fe 7 S 8 (001) surfaces have been examined using LEED and STM following sputtering and annealing of the surface at ∼300°C. A phase transition is observed in LEED patterns taken at elevated temperatures. At temperatures above ∼300°C, the LEED patterns show only the periodicity of the roughly hexagonal closest-packed S atoms, whereas at lower temperatures, additional LEED spots appear which reflect the ordering of Fe vacancies. This transition is reversible. STM images taken at negative bias voltages exhibit triangular terraces separated by steps. The measured step heights are integer multiples of 2.9 A, which is one quarter of the 4C Fe 7 S 8 unit cell size in the c direction, and corresponds to the distance between two consecutive Fe or S layers. Although the STM images of single terraces appear to have an atomic arrangement corresponding to the ordering within those Fe layers which contain vacancies, bright spots in the images are most likely to represent S atoms, with a vacancy ordering which is induced by the Fe vacancies. This conclusion is supported by experimental STM images, which show a reversed orientation of the surface geometry on successive terraces when separated by steps of 2.9 A, and by quantum mechanical calculations of STM images which show S 3p-like states at the top of the valence band. Pyrrhotite (001) surfaces contain triangular etch pits with dimensions ranging from the atomic scale to more than 100 A across. These are formed by the successive removal of Fe 3 S 3 units from the surface. Images taken following exposure of the surface to 6000 L O 2 did not alter the flat terraces, but the formation of adsorptive structures near steps, especially at corners, was observed.

Thin film TiO2 on nickel(110): an STM study

Titanium oxide films grown on the surface of a Ni(110) single crystal have been investigated using STM, LEED and AES for Ti coverages ranging from 1 to 10 ml [1 ml of Ti is defined here as equivalent to the number of top layer Ni atoms of Ni(110)]. The oxide overlayers were prepared by vapour phase deposition of Ti followed by oxidation in 1×10−7 mbar O2 at 800 K. Oxidation of Ti coverages between 1 and 10 ML results in STM images indicating the presence of two terminations coexisting on the surface. One termination consists of islands of epitaxial rutile TiO2(110), the second having cell parameters of 2.98±0.1×3.15±0.2 A. The latter unit cell is consistent with TiO(001) (2.99×2.99 A2). On oxidation of higher Ti coverages (10 ml), only epitaxial rutile TiO2(110) islands are observed.