Using an organometallic precursor, TiO$_2$ coatings were fabricated on surfaces of quartz, zircon and rutile sands. X ray Diffraction, X ray Fluorescence, UV Vis spectroscopy and surface area measurement were used to characterise support materials. T
he phase composition and morphology of the coatings were characterised by laser Raman spectroscopy and SEM respectively. A packed bed reactor was used to study the inactivation of Escherichia coli in recirculating water by the supported photocatalysts. It was found that the sand grains were well coated with a homogenous layer of TiO2 and coatings were well adhered, exhibiting a mixed anatase rutile composition after firing at 850C. Photocatalytic activity was highest in coatings applied to quartz sand, although sterilisation of the recirculating water was not achieved with any of the materials investigated. The advantages of quartz as a TiO$_2$ photocatalyst support material are likely a result of this materials higher purity and optical transmittance. Potential enhancement through Si doping cannot be ruled out.
Ab-initio density functional theory (DFT) calculations of the relative stability of anatase and rutile polymorphs of TiO2 were carried using all-electron atomic orbitals methods with local density approximation (LDA). The rutile phase exhibited a mod
erate 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.
