When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Photocatalytic decomposition involves the degradation of organic contaminants to reduce environmental pollution near the photocatalyst’s surface by transforming the optical energy of absorbed light into chemical energy for redox reactions. Recently the advantages of titanium dioxide (TiO2), as one of the most recognized photocatalysts for the degradation of organic compounds in wastewater, have prompted a great deal of research. Moreover, TiO2, as an efficient photocatalyst, has attracted ever-growing attention as a good candidate for viral disinfection under visible light conditions. However, its unsatisfactory efficiency caused by the large bandgap of TiO2 and high recombination rate of photo-generated e-/h+ has limited its application. It has been indicated that doping titania with different dopants and using a sub-stoichiometric form of titania are promising approaches to reduce the bandgap of TiO2 to activate under visible light and increase charge carriers’ lifetime, thus enhancing the interfacial charge transfer rate. Although TiO2 in powder form has superior photocatalytic performance compared to its alternatives, TiO2 coatings are more convenient as they can easily be separated from the liquid. To counterbalance the effect of surface reduction, nano- and micro-structured surfaces could be produced to increase the active surface area of the coatings efficiently. In the last decades, thermal spray processes as environmentally friendly technologies appear as a versatile and rapid processing approach compatible with industrial production. Suspension plasma spray (SPS) deposition could be used to obtain TiO2 deposits with an effective photoactive performance to decompose organic compounds and antiviral applications. Furthermore, TiO2 coatings with various degrees of sub-stoichiometric oxidation can be achieved with plasma spray deposition.
In this project, coatings with different anatase phase contents were produced. Results showed the relation of anatase concentration to the plasma gas mixture. The lower the H2 flow rate was, the higher anatase was retained in the coating. So, TiO2 coatings containing a high percentage of anatase phase (~85 %) were produced. The results show no direct correlation between anatase content and the photocatalytic activity of as-sprayed coatings due to their unique microstructure and the presence of oxygen vacancies. In comparison, anatase phase positively enhanced the photocatalytic activity in the post-treated coatings with similar coating characteristics and oxygen contents.
Moreover, oxygen vacancy presented an influential role in improving the photocatalytic activity of sub-stoichiometric TiO2-x coatings produced by SPS. The energy levels introduced by oxygen vacancies and Ti3+ ions in TiO2 lattice sites decreased the bandgap energy and shifted the absorption edge to visible light. The energy levels can also improve the charge carriers’ lifetime by acting as traps for electrons and holes. Results showed that the photocatalytic activity of as-sprayed sub-stoichiometric TiO2-x coatings was 2-3 times higher than that of post-treated stoichiometric TiO2 coatings.
Further studies on the cerium-doped TiO2 showed SPS-SPPS technique as a promising method for doping TiO2. However, results revealed the importance of adding an optimum amount of dopant to reach the highest photoactivity. Moreover, a nanocomposite of TiO2- CeO2 can help photocatalytic activity. Otherwise, sub-micron CeO2 particles in the composite matrix could cover the active surface of TiO2 particles and decrease the photocatalytic efficiency of the coatings.
Finally, the antiviral performance of the coatings was assessed. The results show that thermally sprayed coatings can introduce a potentially cost-effective solution to produce efficient antiviral high-touch coatings for indoor/ outdoor applications.