Sotelo-Vazquez, C; (2017) Modification of titania films by chemical vapour deposition for enhanced photocatalysis. Doctoral thesis , UCL (University College London).

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Abstract

Titanium dioxide (TiO2) is the leading material for self-cleaning applications due to its intrinsic properties, such as chemical inertness, mechanical robustness, high photocatalytic activity and durability to extend photocatalytic cycling. However, its relatively wide bandgap limits its outdoor applications. There has been a strenuous effort to try and improve the photocatalytic efficiency of TiO2, in particular by modifying its electronic structure to enhance its function under solar illumination. The most commonly studied approaches for achieving this have been to incorporate anionic and/or cationic species into the TiO2 structure and the design of TiO2-based heterojunction systems. The addition of nitrogen, phosphorus and sulfur species into the matrix of TiO2 was investigated. Films were grown using atmospheric-pressure chemical vapour deposition (APCVD). The nitrogen-doped system has been investigated most prominently to enhance and extend the photocatalytic response of TiO2 materials into the visible region of the electromagnetic spectrum. Nitrogen can either replace oxygen sites (Ns, substitutional doping) or sit within the TiO2 structure (Ni, interstitial doping) and form N-O groups with lattice oxygen. Interestingly, these NOx groups, as well as NHx surface species present similar binding energies, ca. 400 eV, hindering the identification of the nitrogen species and their role in the photocatalytic response of the material. Various synthesis conditions were experimented using different nitrogen precursors (tert-butylamine, benzylamine and ammonia), which were used to establish a correlation between surface and bulk nitrogen species and the photocatalytic behaviour of the N-TiO2 films. A loss of the Ni environment (as observed by X-ray photoelectron spectroscopy), as well as a decrease in photoactivity over time was observed, suggesting a direct participation of the nitrogen species in photocatalytic iv processes. In addition to traditional CVD methods, a pulse precursor approach was used for the first time, to the best of our knowledge, to synthesise stratified N-doped TiO2 thin films, by adding nitrogen into specific regions of the N-TiO2 film. Physical and functional comparison of stratified and non-stratified N-TiO2 films with similar structural and morphological features allowed us to evaluate the benefits of this synthetic approach, which not only resulted in an increase in the photocatalytic efficiency of the stratified N-TiO2 films but also did not affect the overall crystallinity of the films. The addition of phosphorus and sulfur was investigated as the most promising alternative to the use of nitrogen doping, as both could be added to the lattice of TiO2 either as cations or anions. Through functional testing, it was found that both dopant species were beneficial from a photocatalytic point of view. Interestingly, the use of APCVD techniques to deposit P-TiO2 thin films resulted in the addition for the first time, to the best of our knowledge, of P3- species, as well as P5+, to the TiO2 structure with the relative proportion being determined by the synthesis conditions. Through Hall effect probe, photocatalytic testing and transient absorption spectroscopy (TAS) analyses, it was found that the incorporation of P3- species was detrimental from an electrical conduction and photocatalytic point of view; however, the presence of solely P5+ species resulted in P-TiO2 films with enhanced self-cleaning and TCO properties. These results provide important insights on the influence of dopant nature and its location within a semiconductor’s structure. Heterojunction semiconductor materials are used in a wide range of applications including catalysis, electronic devices, sensors and solar-to-chemical energy conversion. These materials benefit from effective electron transfer processes, electron tunnelling, surface passivation and other synergistic effects to enhance their performance beyond the individual components. By using CVD methods, two different v heterojunction systems, rutile/ anatase TiO2 and WO3/TiO2, were grown. The interposition of an amorphous TiO2-based interlayer allowed direct vapour deposition of anatase on a rutile substrate, which is otherwise hindered by templating. The subsequent crystallisation of the amorphous interlayer after annealing, allowed us to investigate the impact of an efficient interface between the two rutile-anatase phases in the photodegradation of an organic model pollutant, stearic acid. Clear evidence on the synergy between the two polymorphs and more importantly, on the charge flow across the interface, which is against much conventional understanding, was evaluated through the photoreduction of silver particles. This charge flow involves electron transfer from rutile to anatase. Likewise, a conformal coating of WO3 nanorods with TiO2 was performed using APCVD techniques. The resulting WO3/TiO2 heterojunction films showed an electron transfer phenomenon, where electrons moved from WO3 into TiO2, against widely reported observation. State-of-the-art hybrid density functional theory (DFT) and hard X-ray photoelectron spectroscopy (HAXPES) were employed to elucidate the electronic interaction at the heterojunction of the WO3 and TiO2 crystalline phases. This vectorial charge separation reduces electron-hole recombination and most likely extends the lifetime and relative population of photogenerated charges. These results provide important insights on the influence of vectorial charge separation in heterojunctions. These phenomena had a dramatic impact on the photocatalytic efficiency of the heterojunction films, which are among the very highest ever reported by a thin film.

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