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Abstract

The link between TiO2 and photocatalysis was recognized a long time ago. However, there was originally no scientific knowledge relating to the process of photocatalysis. Given this, it is impressive that the most significant advances in the properties of TiO2 in photocatalytic processes have been made since the first reports of the so-called Honda-Fujishima effect, roughly 50 years ago.
Fast-developing technological progress over the last two hundred years is taking a heavy toll in many areas, including in environmental pollution issues. Although the problem of pollution is very complex and requires a comprehensive and unified approach, the use of TiO2 can partly contribute to the solution. The most favourable properties of TiO2 are its non-toxicity, inertness, and high availability. It also has very interesting optical properties under UV illumination, which enable the use of TiO2 in photocatalytically based processes, such as the decomposition of persistent organic pollutants in wastewater and air. The final products of the process are mostly CO2 and H2O in the case of complete mineralization.
When TiO2 is illuminated by UV light, electron-hole pairs are formed, which are consequently present in the formation of reactive oxygen species. However, there may be up to 90% loss of these pairs due to their rapid recombination. This recombination yield is lower in one-dimensional structures of TiO2, such as in TiO2 nanotubes. These can be produced by various synthesis methods. However, electrochemical oxidation or anodic oxidation is the most promising process. Anodic oxidation is a highly controllable process influenced by the composition of the electrolyte, its age, the applied voltage, the time of the process and the choice of the initial substrate. In this process, rigidly attached TiO2 nanotubes are formed on the titanium metal substrate. Titanium of various shapes and purities can be used for the substrate.
The present thesis discusses three aspects of TiO2 nanotubes: their synthesis, their optical properties under UV and their photocatalytic efficiency in the degradation of a model organic compound. Each chapter of the thesis is dedicated to one of the above-stated aspects. First, it is shown that the roughness of the titanium surface significantly impacts on the final surface area of photocatalytically active TiO2 nanotube layers. Namely, we showed that titanium substrate pre-treatment, such as electropolishing, influences the TiO2 nanotubes' growth and photocatalytic activity. Next, by studying the influence of the titanium substrate’s thickness from 30 to 600 μm, we successfully determined which thickness resulted in nanotube layers that are rigidly attached and can thus be bent without risk of losing the material. And finally, we also focused on investigating repetitive anodization and change in electrolyte composition. In this aspect, it was concluded that fresh electrolyte results in uniform nanotube morphology and consequently better photocatalytic activity.
Systematic investigations of the relevant parameters were performed to gain knowledge about the material’s preparation, the importance of understanding the behaviour of nanotube growth and to determine the properties of an ideal photocatalyst in the remediation of organic pollutants. The knowledge described in those sections can also be easily applied at the practical industry level.