Impact of TiO2 Surface Defects on the Mechanism of Acetaldehyde Decomposition under Irradiation of a Fluorescent Lamp

TiO2 was placed in heat-treatment at the temperature of 400–500 °C under flow of hydrogen gas in order to introduce some titania surface defects. It was observed that hole centers in TiO2 were created during its heat treatment up to 450 °C, whereas at 500 °C some Ti3+ electron surface defects appeared. The type of titania surface defects had a great impact on the mechanism of acetaldehyde decomposition under irradiation of artificial visible light. Formation of O•− defects improved both acetaldehyde decomposition and mineralization due to the increased oxidation of adsorbed acetaldehyde molecules by holes. Contrary to that, the presence of electron traps and oxygen vacancies in titania (Ti3+ centers) was detrimental for its photocatalytic properties towards acetaldehyde decomposition. It was proved that transformation of acetaldehyde on the TiO2 with Ti3+ defects proceeded through formation of butene complexes, similar as on rutile-type TiO2. Formed acetic acid, upon further oxidation of butene complexes, was strongly bound with the titania surface and showed high stability under photocatalytic process. Therefore, titania sample heat-treated with H2 at 500 °C showed much lower photocatalytic activity than that prepared at 450 °C. This study indicated the great impact of titania surface defects (hole traps) in the oxidation of acetaldehyde and opposed one in the case of defects in the form of Ti3+ and oxygen vacancies. Oxidation abilities of TiO2 seem to be important in the photocatalytic decomposition of volatile organic compounds (VOCs) such as acetaldehyde.

Investigating and Correlating Photoelectrochemical, Photocatalytic, and Antimicrobial Properties of TiO2 Nanolayers

Semiconducting transition metal oxides such as TiO2 are promising photo(electro)catalysts for solar water splitting and photoreduction of CO2. Titania admixtures are also used in paints and building materials or as coating on window glass and medical devices, giving the modified materials antimicrobial, self-or even air-cleaning properties. Although TiO2 is an effective catalyst for all these applications, it is mechanistically important to distinguish between photoelectrocatalytic, photocatalytic and antimicrobial processes. In the former, TiO2 is usually electrically contacted as photoanode, i.e. only the oxidation reaction takes place at the titania surface. In the two latter applications, TiO2 works as heterogeneous catalyst and has to catalyze a complete redox cycle. The underlying common and diverging rate-determining photochemical and photoelectrochemical mechanisms are still not well understood. Here, we thus present a systematic structural, photoelectrocatalytic, photocatalytic and antimicrobial study to directly compare and correlate these properties. We prepared TiO2 thin films on flourine-doped tin oxide (FTO) substrates by a sol-gel spin-coating technique. The materials were annealed at temperatures between 200 and 600°C and their morphologies were studied by GIXRD, FESEM and EDX. Photoelectrochemical properties were measured by linear sweep voltammetry, photoelectrochemical impedance spectroscopy, chopped light chronoamperometry, and intensity modulated photocurrent/ photovoltage spectroscopy. For comparison, photocatalytic rate constants were determined by methylene blue and Escherichea coli degradation and correlated with the deduced photoelectrocatalytic parameters.

Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach

Vanadia/titania catalysts were synthesized by the equilibrium deposition filtration (EDF) method, which is a synthesis route that follows a molecular-level approach. The type of interfacial deposition as well as the interfacial speciation of the deposited oxo-V(V) species were determined by means of a model that takes into account experimental “proton-ion” curves and “adsorption edges”. It is shown that at pH ≥ 9.5, the deposition proceeds exclusively through the formation of mono-substituted inner sphere monomeric species in an “umbrella”-like Ti–OV(OH)2O configuration, whilst with lowering of the pH, a second species, namely the disubstituted inner sphere quadrameric species in a (Ti-O)2V4O10 configuration possessing two mono-oxo V=O and two di-oxo V(=O)2 terminations gradually prevails, which is in co-existence with the monomeric species. Raman spectroscopy is used for verifying the solution speciation, which is different compared to the interfacial speciation of the deposited oxo-V(V) species. Furthermore, in situ Raman spectroscopy was used to verify the model-predicted interfacial speciation of the deposited oxo-V(V) species and to monitor the temperature-dependent evolution up to 430 °C. Hence, a controlled formation of a specific vanadia species on a titania surface is enabled, which, depending on the synthesis conditions, can result in specific catalyst characteristics and thus possibly different catalytic behavior for a specific reaction.

The Cerium/Boron Insertion Impact in Anatase Nano-structures on the Photo-Electrochemical and Photocatalytic Response

Boron- and cerium-doped titania (Anatase) were prepared via sol-gel method. Phase composition and morphology were assessed by X-ray diffraction (XRD), scanning electronic microscopy (SEM), BET, diffuse reflectance spectra (DRS), and XPS. Photo-electrochemistry of these materials, deposited onto fluorine-doped SnO2 (FTO), was investigated in acid and acid-containing methanol. The boron-doped sample showed the best opto-electronic properties among the investigated samples. On the other hand, the cerium-doped titania samples annihilate to a certain extent the titania surface states, however, photogenerated charge separation was limited, and certainly associated to surface Ce3+/Ce4+ species. The substitutional effect of boron ions for O sites and interstitial sites was confirmed by XRD and XPS analyses.

Studies of Dye-Titania Interactions in Dye-sensitised Solar Cells

This work details the synthesis of several bespoke materials to derivatise the surface of titania (TiO2) in order to obtain greater understanding of the sensitisation process in dye-sensitised solar cells and how this can influence device performance. In particular, this work has combined synthetic (dye synthesis), experimental (AR-XPS) and theoretical experiments (computer modelling) in order to investigate the self-assembly of organic dyes onto titania surfaces in dye-sensitised solar cell (DSC) devices. To test the dyes and other materials in this thesis both liquid and solid-state dye-sensitised solar devices were made. The standard liquid DSC device utilised TEC 8 glass (3 cm x 1.5 cm) for both the photo and counter electrode. On the photoelectrode a mesoporous TiO2 layer is deposited and sensitised with N719 dye. The counter electrode is coated with a thin layer of platinum PT-1 paste (GreatCellsolar). The two electrodes are bound together by melting 20 µm surlyn and a standard triiodide electrolyte is injected to the cell and the cell sealed. With this method we have managed to achieve 6.5 % using N719 dye. Solid-state dye-sensitised solar cell (ssDSC) devices we made using TEC 7 glass (2.8 cm2). Onto the glass a compact layer of TiO2 has been deposited via spray coating, thereafter a thin layer of mesoporous TiO2 is spin coated and sensitised overnight in a dye bath. After which, a thin layer of Spiro-OMeTAD is deposited via spin coating, after which Au is deposited under vacuum. With this method 4.0 % has been achieved using LEG4 dye. This thesis reports the first synthesis of ten novel half-squaraine (HfSQ) dyes, five containing a benzothiazole backbone and another five identical dyes with a dimethyl-derivatised carbon. These new dyes have been purified using various preparative (column and flash) chromatography. Once purified the dyes have been analysed using nuclear magnetic resonance (NMR), attenuated total reflectance infra-red (ATR-IR), ultraviolet-visible spectroscopy (UV-Vis) and mass spectrometry. These novel dye materials have then been used to derivatise the titania surface in DSC devices. The dyes have been designed so that a sulphur heteroatom can be used as an atomic probe of the dye-TiO2 interface in order to study surface coverage, dye orientation and dye-electrolyte interactions in dye-sensitized solar cell (DSC) devices. X-ray single crystal structural and opto-electronic data have also been correlated with density functional theory/computer modelling to better understand the widely reported but poorly understood influence of S heteroatoms on dye photochemistry, DSC device performance and lifetime. To the best of our knowledge, for the first time, the S heteroatom in the dyes has been used as a molecular probe along with angle-resolved X-ray photoelectron (ARXPS) data to study how the dye-TiO2 orientation varies with the position of the carboxylate linker on the dye periphery. In further work, a one-step synthesis has been used to link two dye-sensitized solar cell (DSC) dyes together through an anhydride moiety. The dyes we have attempted to combine are triphenylamine dye “Yellow dye” to another Yellow dye and squaraine dye SQ2 to other moieties such as Chenodeoxycholic acid and stearic acid. To the best of our knowledge, this is also the first time this has been attempted in DSC dyes. The resulting anhydride precursor has been used to self-assemble the two dyes onto TiO2 surfaces in a controlled manner. Hence, this represents a brand-new method of sensitising two dyes, in a single sorption process; providing a new method to control dye-sorption kinetics, dye loading and surface organisation. When two dyes are introduced to the titania surface at the same time it is possible to control the positions of the two dyes without the need of bulky co-sorbents (e.g. CDCA) to supress dye aggregation. This one-step synthesis method was applied to combine two different dye materials into one anhydride moiety. Attempts have been made to combine SQ2 dye to co-adsorbent CDCA and SQ2 dye to stearic acid. The photo-physical properties of the anhydride and its precursor dyes have been studied (via UV-Vis, ATR-IR NMR and tested in ssDSC devices) and compared highlighting the potential of this new method for co-sensitisation.

The influence of the morphology of titania and hydroxyapatite on the proliferation and osteogenic differentiation of human mesenchymal stem cells

Herein, the proliferation and osteogenic potential of human mesenchymal stem cells (hMSCs) on the disordered and ordered porous morphology of the titania surface and titania surface modified by hydroxyapatite (HA) are compared for the first time.