Harrison, George Timothy; (2017) An STM and Photocatalysis Study of Single Crystal and Ultrathin Films of Rutile and Anatase TiO2. Doctoral thesis , UCL (University College London).

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Abstract

Synthesis of two ultrathin TiO2 films: model rutile / anatase TiO2(101) and rutile TiO2 / lepidocrocite-like TiO2 / W(100) interfaces: Scanning tunneling microscopy (STM) tip-pulse modification of a well characterised e-beamed single crystal anatase TiO2(101) surface composed of a high step edge density, resulted in the formation of a new crystalline ordered rutile structure, characterised in real space by STM. This region was embedded in a ~400 Å diameter, ~50 Å tall mound in a surface hole surrounded by a ca. pristine (0.035 ML water) outer area of anatase TiO2(101)-(1 × 1). One area of the modified surface had an atomically resolved unit mesh of (3.1 ± 0.2 Å × 14.2 ± 0.5 Å) and saw-tooth structure consistent with the surface structure of (1 × 3) reconstructed rutile TiO2(100) surface. In-situ adsorption of acetic acid resulted in a monolayer coverage (2 adsorbates per (1 × 3)) of adsorbates resulting in a (2 × 3) (6.1 ± 0.4 Å × 14.7 ± 0.4 Å) unit mesh. Analysis of the STM images allowed this to be precisely assigned as the reduced rutile TiO2(100)-(1 × 3)-β surface structure [Phys. Rev. Lett. Vol. 82, 26, 5265-5268]. This presents a novel methodology to form a system composed of two single crystal variants of anatase and rutile in close enough contact ~30 Å to study the interfaces influence on the electronic properties of rutile and anatase with scanning tunnelling microscopy (STS) and photochemical properties with a photo-active molecule. Ultrathin films of TiO2 were grown on a W(100)-O-(2 × 1) substrate and characterised using a combination of STM and low energy electron diffraction (LEED). In addition to islands of rutile TiO2(110) with (1 × 1) termination that have been reported previously, rutile TiO2(110) islands with a (1 × 2) film termination were observed. A lepidocrocite-like TiO2 nanosheet, of 5 Å height was observed; closely related structurally to an anatase (001) bi-layer, was also observed on the W(100) surface in combination with rutile islands. High-resolution STM images show that the nanosheet grows in the principal orthogonal directions of the W(100) substrate and forms a commensurate (1 × 7) coincident cell. This two phase titania ultrathin film system of rutile TiO2(110)/lepidocrocite-like TiO2 provides a suitable model system to study a model interface between an anatase derived and rutile TiO2. Here macroscopic bulk averaging techniques can be utilised to study the interface. Photooxidation study on single crystal anatase TiO2(101): As little mechanistic information of a photooxidation reaction has yet to be derived
 from single-crystal studies on anatase TiO2(101) surface with photo-active probe molecules a study is required prior to the investigation of a tip modified rutile / anatase TiO2(101) surface. The photooxidation of ethanol (as a
prototype hole-scavenger organic molecule) adsorbed on the anatase TiO2(101) surface was studied by STM 
and online mass spectrometry to determine the adsorbate species in the dark and under UV
 illumination in the presence of O2 and to extract kinetic reaction parameters under photo-excitation. Here, the reaction rate for the photooxidation of ethanol to acetaldehyde was found to depend on the O2
 partial pressure and surface coverage. An order of the reaction with respect to O2 was found to be close to
0.15. Carbon−carbon bond dissociation leading to the formation of CH3 radicals in the gas phase
was found to be a minor pathway, which is contrary, to the case of the rutile TiO2(110) single-crystal. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that 
can be attributed to its molecular and dissociative modes. A mixed adsorption is also supported by
DFT calculations performed by SABIC, in which they determined similar energies of adsorption (Eads) for the
molecular (1.11 eV) and dissociative (0.93 eV) modes. Upon UV exposure at (and above) 3 × 10-8 mbar O2, a third species was identified on the surface as a reaction product that can be tentatively attributed to acetate/formate species on the basis of C 1s XPS results. The kinetics of the initial oxidation steps were evaluated using the STM and mass spectrometry data.

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