TY  - UNPB
N1  - Unpublished
Y1  - 2017/11/01/
AV  - public
EP  - 188
TI  - Modification of titania films by chemical vapour deposition for enhanced photocatalysis
A1  - Sotelo-Vazquez, C
M1  - Doctoral
UR  - https://discovery.ucl.ac.uk/id/eprint/1553144/
PB  - UCL (University College London)
N2  - 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 
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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.
ID  - discovery1553144
ER  -