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

Retention of the anatase phase in TiO2 through the use of bauxite and zircon dopants

Preserving the anatase phase of TiO2 is of interest due to the photocatalytic activity of this phase, which is generally considered superior to rutile Ti02 due to a higher surface area. The retardation of the anatase to rutile transition through the use of low cost dopants, zircon and bauxite, was investigated. Pressed powder compacts of anatase TiO2 doped with varying levels of bauxite and zircon powders were fired at 1025°C for durations of 2, 3 and 4 hours. X-ray diffraction analysis, Scanning electron microscopy, Energy dispersive X-ray spectroscopy and Raman spectroscopy were used to evaluate the effect these dopants have on the transition to rutile.It was found that both zircon and bauxite retard the transition to rutile in the solid state. Increasing the level of bauxite dopant consistently increases the preservation of the anatase phase. It was found that at levels of bauxite doping over 5 wt%, over 90% of the anatase phase is retained through 2 hour firing cycles.Using EDS, zircon particles were observed to nucleate elongated prismatic rutile grains. Whilst XRD analysis showed that the addition of zircon has a marked effect on retarding the overall transition of anatase to rutile, increasing levels of this dopant over 2.5% brings about a reduction in preserved anatase, probably due to increased rutile nucleation. Findings showed Samples doped with 2.5% ZrSi04 retained approximately 95% anatase through 2 hour firing cycles

Fabrication of single- and mixed-phase titanium dioxide photocatalysts

Multifaceted research was conducted into the fabrication of single- and mixed-phase titanium dioxide photocatalysts using organometallic precursors and commercially available powders. Three photocatalyst immobilisation techniques involving the use of electrophoretic deposition, spin coating of sol-gels and the deposition of coatings on sand grains were explored. Associated enabling work involved the development of novel methods for the enhancement of dispersion and aqueous electrophoretic deposition of oxides on large-scale surfaces. Similar work was done on the coating of small-scale surfaces in the form of TiO2 coatings on sand grains, which are a potential low-cost support for water purification photocatalysts.The preceding studies were interpreted in terms of the anatase to rutile and brookite to rutile phase transformation behaviour was investigated through the thermal treatment of supported and powdered TiO2 materials. Various compositional and morphological factors were found to have significant effects on the anatase to rutile phase transformation and the temperatures at which mixed-phase TiO2 materials were obtained. These effects were discussed in light of an interpretive and predictive review of the phase transformations.The performance of these single- and mixed-phase supported titanium dioxide materials were investigated in pilot-scale water purification studies through the inactivation of bacteria and through the photo-oxidation of methylene blue. The similarity in the results was such that it was not possible to determine if these data resulted from the impact of insufficient sensitivity of the testing and analytical procedures on the ability to differentiate between the competing effects of compositional and morphological aspects of the phase assemblage

Single and mixed phase TiO2 powders prepared by excess hydrolysis of titanium alkoxide

To investigate the excess hydrolysis of titanium alkoxides, TiO2 powders were fabricated from titanium tetraisopropoxide using 6 : 1 and 100 : 1 H2O/Ti (r) ratios. The powders were dried and fired at a range of temperatures ((800uC). Hydroxylation and organic content in powders werecharacterised using attenuated total reflectance Fourier transform infrared spectroscopy (FTIR), laser Raman microspectroscopy and elemental microanalysis; surface area and pore size distribution were evaluated using N2 gas adsorption; phase composition was analysed using Xraydiffraction (XRD) and laser Raman microspectroscopy; and crystallite size was evaluated by XRD, TEM and SEM. Results showed near complete hydrolysis in a predominantly aqueous medium (r5100), resulting in precipitated crystalline powders exhibiting brookite and anatase, which begin to transform to rutile below 500uC. The powders precipitated in a predominantly organic medium (r56) underwent partial hydrolysis, were highly porous and exhibited an amorphous structure, with the crystallisation of anatase occurring at y300uC and the transformation to rutile beginning at 500–600uC

The Effects of Carboxylic Acids on the Aqueous Dispersion and Electrophoretic Deposition of ZrO2

The agglomeration, electrokinetic properties and electrophoretic deposition behaviour of aqueous suspensions of ZrO2 with carboxylic acid additives were studied in comparison with conventional pH adjustment. It was found that citric acid imparted negative zeta-potential values and electrosteric stabilisation to particles in suspensions at all pH levels. The examination of additions of carboxylic acids to ZrO2 suspensions revealed that these reagents cause a sharp drop in zeta-potential at distinct addition levels, which correspond to surface saturation of the particles with negatively charged carboxylate groups. Adsorption cross sections of citric acid, EDTA and oxalic acid were evaluated from these results, showing that both citric acid and EDTA coordinate to ZrO2 surfaces by two carboxylate groups while oxalic acid is coordinated by one group. The use of carboxylic acids was shown to facilitate superior electrophoretic deposition in comparison with zeta-potential modification by conventional pH adjustment through improved suspension stability

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

The Effects of Firing Conditions on the Properties of Electrophoretically Deposited Titanium Dioxide Films on Graphite Substrates

Thick anatase films were fabricated on graphite substrates using a method of anodic aqueous electrophoretic-deposition using oxalic acid as a dispersant. Thick films were subsequently fired in air and in nitrogen at a range of temperatures. The morphology and phase composition were assessed and the photocatalytic performance was examined by the inactivation of Escherichia coli in water. It was found that the transformation of anatase to rutile is enhanced by the presence of a graphite substrate through reduction effects. The use of a nitrogen atmosphere allows higher firing temperatures, results in less cracking of the films and yields superior bactericidal performance in comparison with firing in air. The beneficial effects of a nitrogen firing atmosphere on the photocatalytic performance of the material are likely to be a result of the diffusion of nitrogen and carbon into the TiO2 lattice and the consequent creation of new valence band states

Anodic Aqueous Electrophoretic Deposition of Titanium Dioxide Using Carboxylic Acids as Dispersing Agents

The dispersion of anatase phase TiO2 powder in aqueous suspensions was investigated by zeta-potential and agglomerate size analysis. The iso-electric point (IEP) of anatase was determined to be at pH 2.8 using monoprotic acids for pH adjustment. In comparison, it was found that the use of carboxylic acids, citric and oxalic, caused a decrease in zeta-potential through the adsorption of negatively charged groups to the particle surfaces. The use of these reagents was shown to enable effective anodic electrophoretic deposition (EPD) of TiO2 onto graphite substrates at low pH levels with a decreased level of bubble damage in comparison with anodic EPD from basic suspensions. The results obtained demonstrate that the IEP of TiO2 varies with the type of reagent used for pH adjustment. The low pH level of the IEP and the ability to decrease the zeta-potential through the use of carboxylic acids suggest that the anodic EPD of anatase is more readily facilitated than cathodic EPD