Morphology and Photocatalytic Activity of Highly Oriented Mixed Phase Titanium Dioxide Thin Films

Thin TiO2 films on quartz substrates were prepared by spin coating of undoped and metal-ion-doped Sol-Gel precursors. These films were characterised by Scanning Electron Microscopy, Laser Raman Microspectroscopy, X-ray Diffraction and UV-Vis Transmission. The photocatalytic performances of the films were assessed by the photo-degradation of methylene-blue in aqueous solution under UV irradiation. Films exhibited a high degree of orientation and a thermal stabilization of the anatase phase as a result of substrate effects. In the absence of dopants, the rutile phase formed as parallel bands in the anatase which broadened as the transformation progressed. TiO2 films doped or co-doped with transition metals exhibited the formation of rutile in segregated clusters at temperatures under ~800°C as a result of increased levels of oxygen vacancies. Photocatalytic activity of the films synthesised in this work was low as likely result of poor TiO2 surface contact with dye molecules in solution. The presence of transition metal dopants appears detrimental to photocatalytic activity while the performance of mixed phase films was not observed to differ significantly from single phase material

Ab initio study of phase stability in doped TiO2

Ab initio density functional theory calculations of the relative stability of the anatase and rutile polymorphs of TiO2 were carried out using all-electron atomic orbitals methods with local density approximation. The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative to the anatase phase in pristine material. From computational analysis of the formation energies of Si, Al, Fe and F dopants of various charge states across different Fermi level energies in anatase and in rutile, it was found that the cationic dopants are most stable in Ti substitutional lattice positions while formation energy is minimised for F− doping in interstitial positions. All dopants were found to considerably stabilise anatase relative to the rutile phase, suggesting the anatase to rutile phase transformation is inhibited in such systems with the dopants ranked F > Si > Fe > Al in order of anatase stabilisation strength. Al and Fe dopants were found to act as shallow acceptors with charge compensation achieved through the formation of mobile carriers rather than the formation of anion vacancies

XPS studies of high-Tc superconducting Bi2Sr2Ca(Cu1-yFey)2Ox single crystals

Fe-substituted Bi2Sr2Ca(Cu1-[gamma]Fe[gamma])2Ox single crystals have been studied by X-ray photoelectron spectroscopy (XPS) to determine the changes of oxidation states of their constitutes as well as the electronic structure of the valence band of the system. The core level lines of Bi exhibit two main components, which indicates either inequivalent sites or the presence of different oxidation states. The second component shows a systematic increase with Fe-substitution. The Sr lines also exhibit two components in which the lower binding energy component decreases with the dopant content. No significant changes in CuO2 bilayers and Ca-O layer were observed. The results suggest a dramatic charge transfer between the Bi-O and Sr-O layers which compensates the increase of positive charge near the CuO2 planes induced by Fe-substitution for Cu, and the impurity scattering of the dopants in the conduction band is one of the key factors for the Tc suppression

Powder melting process for YBa2Cu3O7-y superconductor by eutectic formation

YBa2Cu3O7-y with high grain alignment has been successfully fabricated by a modified powder melting process at a temperature of ~930oC which is near the eutectic point of the starting materials Y2BaCuO5, BaCuO2, and CuO. In terms of the levitation force and YBa2Cu3O7-y grain alignment, the best result was shown in a sample having a Y2BaCuO5:BaCuO2:CuO molecular ratio of 1:3:5. In the present work, the Cu-surplus eutectic liquid was completely absorbed by use of a Y2BaCuO5 substrate, by which the final composition was driven to stoichiometric YBa2Cu3O7-y. The results were comparable to those obtained by the conventional melt-texture-growth process. A new invariant point apparently occurred at ~930-940oC owing to an interfacial reaction between YBa2Cu3O7-y matrix and Y2BaCuO5 substrate