In this work titanium dioxide (TiO2) was synthesised using hydrothermal methods. Initial studies were based on TiO2 oxalic acid passivated sols previously synthesised by Dr Tim Kemmitt. These sols were hydrothermally treated in the presence of ammonium fluoride (NH4F) as a morphology directing agent. The hydrothermally treated titanium dioxide was characterised using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). It was found the phase, size and shape of the TiO2 nanoparticles produced could be controlled by tuning the NH4F concentration, temperature profile and time of the hydrothermal treatment.

The high-temperature hydrothermal treatment caused the decomposition of organic species present in the reaction mixture, which resulted in a partial reduction of the TiO2 observed as a blue colouration

Characterising the blue colour-causing defect and identify the mechanisms behind the formation of this defect quickly became the focus of the forthcoming studies. The hydrothermal synthesis was tuned to produce a phase-pure blue anatase TiO2, which was a better candidate for an in-depth study of the electronic features of the blue TiO2. The conditions need to generate the blue colour were studied and the blue TiO2 was characterised by the following techniques: synchrotron powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectric spectroscopy (XPS), valence band spectroscopy (VBS), near edge absorption fine structure spectroscopy (NEXAFS), electron paramagnetic resonance (EPR), superconducting quantum interference device (SQUID), Raman spectroscopy, ultraviolet-visible light spectroscopy diffuse reflectance (UV-VIS DRS), infrared spectroscopy (IR), thermal gravitational analysis (TGA), particle induce gamma emission spectroscopy (PIGE), nitrogen adsorption isotherms and gas mass spectrometry (GC-MS).

Evidence from the various characterisation techniques showed that bulk phase self-doping with Ti3+ occurred during synthesis of the blue TiO2. The Ti3+ was stabilised by fluorine ions (F-) substituted for oxygen in the lattice. The reduction of Ti4+ to Ti3+ was a consequence of in-situ H2 formation due to the thermal decomposition of the isopropanol liberated from the hydrolysis of titanium isopropoxide (the titanium precursor). The reduced Ti3+ ions were incorporated into the growing nanocrystals where F- would substitute for O2- forming a stable defect. The Ti3+ defect produced an intermediate band below the conduction band which allowed the absorption of red light to produce the blue colour observed.

Although the Ti3+ was causing the blue colouration, the synthesis conditions and F- doping had other effects. First, it was found the anatase crystals grew preferentially along the c axis, this was found to be as a result of ammonium titanate formation. The ammonium titanate also encouraged the growth of very large crystals in a narrow concentration range. Secondly, the Ti3+ defects where found to be mostly located close to the surface. Finally, the TiO2 was very deficient in Ti as a result of Ti vacancies possibly due to hydroxyl inclusions.

Having determined the synthetic methodology to produce the blue colouration, the synthesis was reworked to produce even more novel blue brookite and blue rutile phase TiO2 which was characterised using many of the same methods as above.

It was found that the brookite was formed via the conversion of anatase via a sodium titanate mechanism. The temperature of hydrothermal treatment and NaF ratio could be used to control the aspect ratio of the brookite formed. Finally, it was found that Ti3+ was present as both a bulk and surface dopant causing the blue colour.

Blue rutile was formed by adding oxalic acid to the synthesis. The NH4F was found to change the shape of the rutile crystallites so there was a greater exposure of the {001} facet. The blue rutile was found to have the highest concentration of Ti3+ defects and was hence very blue.

Lastly, studies were carried out on the TiO2 to assess the effects the synthesis conditions and blue colour on the photocatalytic activity of the TiO2. In particular, there was an interest as to whether the absorption of red light could drive photocatalytic activity into the visible-light range (>400 nm). Reactive blue (RB-19) and methylene blue (MB) was used as a model dyes to examine the photocatalytic activity of the TiO2. The blue TiO2 was found to be more active than commercial photocatalyst under broad-spectrum light. The TiO2 was also capable of harnessing the energy of visible-light to degrade the dye, making it visible light active. Furthermore, the blue brookite and rutile were found to be effective photocatalysts.