Alotaibi, Abdullah Mohmmed; (2019) Aerosol-Assisted Chemical Vapour Deposition of Doped Titania films; Characterisation and Functional Properties. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Titanium dioxide (TiO2) is the leading material for self-cleaning applications due to its unique properties¸ including the fact that it exhibits high photocatalysis, mechanical robustness, chemical inertness, low cost, environmentally friendliness and abundance. The bandgap of TiO2 is relatively large, and this limits its outdoor applications. Another obstacle for its use as a photocatalyst is the high level of electron–hole recombination and low rate of photoreaction with reactants. There have been great efforts to improve the photocatalytic properties of TiO2. These have involved searching for ways of decreasing the bandgap structure and recombination rate, as well as improving the electronic structure to enhance its functional properties under visible light. One useful approach for achieving a suitable bandgap and improving electron–hole separation is combining or doping TiO2 with anionic and/or cationic species. In this study, Cu-doped TiO2 films were deposited via aerosol-assisted chemical vapour deposition (AACVD). The Cu-doped TiO2 films system in both phases (anatase and rutile) were specifically investigated for improved photocatalytic and antimicrobial properties of TiO2 under UVA compared with pure TiO2 thin films. Interactions between substitutional (replacing oxygen sites) and interstitial (sitting in the TiO2 lattice) Cu in the anatase lattice may also explain the enhancement in exciton lifetimes. A range of copper concentrations (2, 5, 10 and 20%) was investigated so that the photocatalytic and antibacterial abilities (vs. S. aureus and E. coli) could be determined. Effective dopant selection and concentration control is key to providing the maximum efficiency in terms of carrier lifetimes for migration to the surface for the necessary reactions to take place in photocatalysis and antibacterial activity. Interestingly, the AACVD system could be used to deposit TiO2 in rutile form on a thin layer of ZrO2 at 500°C. Cu-doped rutile–TiO2 films using a range of copper concentrations (2, 5, 10 and 20%) were investigated as well. The films showed surface plasmon resonance (SPR). In addition, these films exhibited enhanced photocatalytic activity under visible light irradiation, which could have been due to SPR. To the best of our knowledge, this is the first time that the brookite thin film form has been deposited by AACVD; in using AACVD to deposit the brookite TiO2 thin films, the band structure and photocatalytic properties were investigated. The brookite films grown by AACVD showed a direct bandgap of 3.4 eV. It was found that the photocatalytic properties of the brookite form, in comparison with degradation of stearic acid, were greater than the activity of anatase TiO2 thin films, as well as active glass. In addition, transient absorption spectroscopy (TAS) measurements showed that the hole–electron recombination dynamics are similar in both phases. The high surface area of the brookite form compared with the surface area of the anatase thin film could be the primary reason for the super-photocatalytic properties. Surprisingly, the brookite film exhibited superhydrophilic properties prior to any irradiation. The addition of Zn and nitrogen into the matrix of TiO2 films by AACVD was studied most extensively to improve the functional properties of TiO2 and achieve its activity under visible light. The oxygen atom in TiO2 lattice can be replaced by a nitrogen atom, which is called Ns (substitutional doping) in this case; alternatively, nitrogen atoms can be set in the TiO2 lattice, and this is called Ni (interstitial doping). These approaches create NHX and NOX surface species, which were observed using the X-ray photoelectron spectroscopy (XPS) results in this research. Transient absorption spectroscopy (TAS) was used to investigate the addition of (N + Zn) on the charge carrier dynamics of TiO2. Heterojunction systems of semiconductor materials are employed in different applications, such as water splitting, catalysis and electronic devices. These systems strengthen the synergistic effect, electron tunnelling and electron transfer, thereby leading to improved performance compared with the individual components. By using AACVD and APCVD processes with heterojunction systems, TiO2/Fe2O3 films were deposited, and different thicknesses of TiO2 were used on the Fe2O3 films. The resulting TiO2/Fe2O3 films exhibited enhanced performance in terms of the photocatalytic properties for the degradation of stearic acid under white light, as well as better photocurrent density and stability of the TiO2/α-Fe2O3 heterojunction. The TAS measurements showed the extent of its lifetime photogenerated charges.

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