Thesis (Ph.D. (Physics)) -- University of Limpopo, 2021 / Titanium dioxide is among the most abundant materials and it has many of interesting physical and chemical properties (i.e., low density, high thermal and mechanical strength, insensitivity to corrosion) that make it a compound of choice for electrodes materials in energy storage. There are, however, limitations on the theoretical side to using the main electronic structure theories such as Hartree-Fock (HF) or densityfunctional (DFT) especially for large periodic and molecular systems. The aim of the theses is to develop a new, widely transferable, self-consistent density functional tight binding SCC-DFTB data base of TiO2, which could be applied in energy storage anodes with a large number of atoms. The TiO2, LiTiO2 and NaTiO2 potentials were derived following the SCC-DFTB parameterization procedure; where the generalized gradient approximation (GGA) and local density approximation (LDA) exchange-correlation functional were employed yielding Slater-Koster DFTB parameters. The results of derived parameters were validated by being compared with those of the bulk rutile and brookite polymorphs. The structural lattice parameters and electronic properties, such as the bandgaps were well reproduced. Most mechanical properties were close to those in literature, except mainly for C33 which tended to be underestimated due to the choice of exchange-correlation functional. The variation of the bulk lattice parameter and volume with lithiation and sodiation were predicted and compared reasonably with those in literature. The newly derived DFTB parameters were further used to calculate bulk properties of the anatase, which is chemically more stable than other polymorphs. Generally, the accuracy of the lattice structural, electronic and mechanical properties of the bulk anantase were consistent with those of the rutile and brookite polymorphs. Furthermore, nanostructures consisting of a large number of atoms, which extend beyond the normal scope of the conventional DFT calculations, were modelled both structurally and electronically. Structural variations with lithiation was consistent with experimental and sodiation tends to enhance volume expansion than lithiation. Anatase TiO2 and LiTO2 nanotubes of various diameters were generated using NanoWrap builder within MedeA® software. Its outstanding resistance to expansion during lithium insertion and larger surface area make the TiO2 nanotube a promising candidate for rechargeable lithium ion batteries. The outcomes of this study will be beneficial to future development of TiO2 nanotube and other nanostructures. Lastly, our DFTB Slater-Koster potentials were applied to recently discovered trigonal bipyramid (TB), i.e. TiO2 (TB)-I and TiO2 (TB)-II polymorphs, which have enormous 1D channels that provide suitable pathways for mobile ion transport. All structural, electronic properties were consistent with those in literature and all elastic properties agreed excellently with those that were calculated using DFT methods. Finally, the bulk structures of the two polymorphs, were lithiated and sodiated versions and electronic and structural properties were studied, together with the lithiated versions of associated nanostructures consisting of a large number of atoms. Generally, the TiO2 (TB)-I structure was found to be mechanically, energetically more stable and ductile than TiO2 (TB)-II. Hence, it will be beneficial to use TiO2 (TB)-I as an anode material for sodium ion batteries (SIB), due to its unique ductility and larger 1D channels. / National Research Fund (NRF), Department of Science and Innovation (DSI), Material Modelling Centre

Thesis (Ph. D. (Physics)) -- University of Limpopo, 2020 / Titanium dioxide is among the most abundant materials and it has many of interesting physical and chemical properties (i.e., low density, high thermal and mechanical strength, insensitivity to corrosion) that make it a compound of choice for electrodes materials in energy storage. There are, however, limitations on the theoretical side to using the main electronic structure theories such as Hartree-Fock (HF) or density functional (DFT) especially for large periodic and molecular systems. The aim of the theses is to develop a new, widely transferable, self-consistent density functional tight binding SCC-DFTB data base of TiO2, which could be applied in energy storage anodes with a large number of atoms. The TiO2, LiTiO2 and NaTiO2 potentials were derived following the SCC-DFTB parameterization procedure; where the generalized gradient approximation (GGA) and local density approximation (LDA) exchange-correlation functional were employed yielding Slater-Koster DFTB parameters. The results of derived parameters were validated by being compared with those of the bulk rutile and brookite polymorphs. The structural lattice parameters and electronic properties, such as the bandgaps were well reproduced. Most mechanical properties were close to those in literature, except mainly for C33 which tended to be underestimated due to the choice of exchange-correlation functional. The variation of the bulk lattice parameter and volume with lithiation and sodiation were predicted and compared reasonably with those in literature. The newly derived DFTB parameters were further used to calculate bulk properties of the anatase, which is chemically more stable than other polymorphs. Generally, the accuracy of the lattice structural, electronic and mechanical properties of the bulk anantase were consistent with those of the rutile and brookite polymorphs. Furthermore, nanostructures consisting of a large number of atoms, which extend beyond the normal scope of the conventional DFT calculations, were modelled both structurally and electronically. Structural variations with lithiation was consistent with experimental and sodiation tends to enhance volume expansion than lithiation. Anatase TiO2 and LiTO2 nanotubes of various diameters were generated using NanoWrap builder within MedeA® software. Its outstanding resistance to expansion during lithium insertion and larger surface area make the TiO2 nanotube a promising candidate for rechargeable lithium ion batteries. The outcomes of this study will be beneficial to future development of TiO2 nanotube and other nanostructures. Lastly, our DFTB Slater-Koster potentials were applied to recently discovered trigonal bipyramid (TB), i.e. TiO2 (TB)-I and TiO2 (TB)-II polymorphs, which have enormous 1- D channels that provide suitable pathways for mobile ion transport. All structural, electronic properties were consistent with those in literature and all elastic properties agreed excellently with those that were calculated using DFT methods. Finally, the bulk structures of the two polymorphs, were lithiated and sodiated versions and electronic and structural properties were studied, together with the lithiated versions of associated nanostructures consisting of a large number of atoms. Generally, the TiO2 (TB)-I structure was found to be mechanically, energetically more stable and ductile than TiO2 (TB)-II. Hence, it will be beneficial to use TiO2 (TB)-I as an anode material for sodium ion batteries (SIB), due to its unique ductility and larger 1D channels. / National Research Fund (NRF), the Department of Science and Innovation (DSI) Energy Storage Research Development and Innovation initiative and Materials Modeling Centre

The objective of this project was the modification of TiO₂ electrodes with various <u>silanes</u> in order to evaluate the stability of modified layers when they are used on <u>photoelectrodes</u> in SEMICONDUCTOR LIQUID-JUNCTION SOLAR CELLS (SLJSC). To determine the nature of the reactivity of surface hydroxyl groups towards different <u>silanes</u>, a surface IR study was carried out on TiO₂ powders. The powder (TiO₂) was pressed into a pellet and subjected to reactions with various silanes under different reaction conditions. All of these reactions were carried out in a <u>vacuum line</u> under very anhydrous conditions in order to prevent polymerization of the silanes. This study provided an understanding of the reactivity of different silanes towards surface hydroxyl groups of TiO₂ and the best reaction conditions for this purpose. With this information in hand we studied the TiO₂ (rutile) <u>single crystal</u> electrodes. These electrodes were subjected to reaction with silanes under the same conditions as the powders. Then the modified surface was studied using ESCA. These electrodes were subsequently subjected to photoelectrochemical conditions (photocurrent generation) and were reexamined using ESCA in order to evaluate the stability of the modified layer. A reaction scheme which was devised to induce crosslinking in the modified layer was shown to enhance the stability of the surface bound silane during the photocurrent generation. In order to form more homogeneous modified surfaces electrochemically derived polymer coatings were synthesized from divinylbenzene, 4-vinylpyridine, N-methyl-4-vinylpyridinium salts, and phenol. Except for polymers formed from N-methyl-4-vinyl pyridinium salts, other coatings were shown to be neutral. An anomalous pre-wave, and potential-induced polymer swelling and shrinking phenomena were observed in these coatings. The photocorrosion of small bandgap n-type semiconductor electrodes is a serious impediment to the development of efficient and durable conversion (photoelectrochemical) devices. Our objective in this investigation was to develop newer modified surfaces that are useful for the inhibition of this photocorrosion, and enhance the performance of n-type small bandgap semiconductors in the photoelectrochemical systems. / Ph. D. / incomplete_metadata

Inorganic UV-filters in use today often occur as nanoparticles and have a photocatalytic effect, which can be a problem since they can cause negative health effects. This is why Upsalite®, a mesoporous magnesium carbonate recently has been investigated as a UV-filter. Upsalite® itself is however not suitable as a UV-filter since it mainly protects in the UVC range and hence it needs to be complemented by other substances. The substance studied to functionalize Upsalite® in this thesis is titanium dioxide which is an inorganic UV-filter commonly used in sunscreens. In this work two different sol-gel synthesis routes of titanium dioxide have been investigated as well as a co-synthesis of Upsalite® and titanium dioxide. In the first synthesis route already synthesized Upsalite® was mixed with titanium tetra-isopropoxide and 1-propanol. The second synthesis route was a modified version of synthesis routes described in literature where methanol solvent was used and the pressure was altered by CO2. This route was explored due to its resemblance with the Upsalite® synthesis. Pressure, temperature and amount of water were varied to optimize incorporation of Upsalite® and investigate possibilities for a co-synthesis. Subsequently a co-synthesis of Upsalite® and titanium dioxide was performed that resulted in two amorphous composite materials depending on if water was added in the drying procedure or not. When mixing Upsalite® in the synthesis liquid of titanium dioxide, titanium dioxide seems to be deposited on Upsalite®. It is however difficult to determine whether the pores of Upsalite® have been completely filled or if they have collapsed. The acid catalyst HCl promotes formation to crystalline titanium dioxide but Upsalite® instead prevents it. The limited crystallization of titanium dioxide when synthesized with Upsalite® may be due to confinement in the Upsalite® pores. The UV-blocking properties of the TiO2-Upsalite® without HCl are good with an sun protection factor (SPF) of 27 for a 10 wt% blend in a lotion and an SPF of 7 for the sample with HCl. The modified synthesis route of TiO2 showed that it is possible to perform a sol-gel synthesis with a considerably lower amount of water than found in literature and that alteration of temperature and pressure during the synthesis does not affect the crystallization temperature noteworthy. The materials obtained from the co-synthesis are slightly porous, probably consisting of one or several magnesium titanium oxides and a carbonate phase and showed a transmission cutoff in between Upsalite® and titanium dioxide corresponding to an SPF of 5.

Thesis (MScEng)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: During the production of pure Ti02 for the pigment industry, ilmenite, containing 35 - 60 % Ti02, is reduced to high titania slag, containing 85 - 95 % Ti02 and pig iron. These ilmenite smelters are operated in very tight operating windows. Over reducing the slag may lead to the formation of TiC and reducing much of the Ti02 to Th03. According to Namakwa Sands furnace operators, this does not only affect the grade of the product, but it can cause slag foaming and furnace eruptions. In under reducing conditions, the liquid slag is fluxed by the FeO and may corrode the furnace lining and consequently lead to run-outs. The reducing conditions in the furnace are not only controlled by carbon addition, but also by temperature. Standard practise in industrial ilmenite smelters is to operate the furnace with a slag freeze lining to protect the refractory lining from chemical and physical attack by the slag. It is therefore clear that it is of great importance to be able to predict the slag liquidus temperature at different compositions. This can help the operator to avoid dangerous operating conditions. Over the past few decades, a number of solution models have been developed to describe non-ideal solutions. With the rapid increase in computer power, these models became more valuable and practical to use in advanced control and decisionsupport. In this study, some of the better-known models are discussed and evaluated for the Ti02 - Th03 - FeO system, based on a critical review of properties and measurements published in literature. Two of these models, the "modified quasi-chemical" model and the "cell" model were chosen to be applied to the high-titania slag system. Both these models are based on statistical thermodynamics with some differences in the initial assumptions. In this study, the model parameters for the cell model were regressed from experimental data. The high-titania slag produced, consists mainly of titanium in different oxidation states and FeO, placing its composition inside the Ti02 - Th03 - FeO ternary system. Reliable experimental data for this system are very limited. All three binary systems contained in the Ti02 - ThO) - FeO system were considered, namely FeO - Ti02, Ti02 - ThO) and FeO - ThO). Only liquidus data for these three binaries were used to regress the model parameters. Accuracy of the models was determined by calculating the root mean square (RMS) error between the experimental data point and the value calculated using the model and the newly determined model parameters. These errors corresponded weil with the reported experimental error of the datasets for both the models and all the binary systems. Due to the fact that this study focussed on the liquidus surface of the system, the results were also plotted in the form of binary phase diagrams and ternary liquidus isotherms. The cell model uses only binary interaction parameters to describe the ternary system. These parameters are not expanded to higher order polynomials, which makes this model more robust, but also less accurate than other models such as the modified quasi-chemical model. / AFRIKAANSE OPSOMMING: Tydens die produksie van suiwer Ti02 vir die pigmentbedryf, word ilmeniet, wat 35 tot 60 % Ti02 bevat, gereduseer tot 'n hoë titaan slak, met 'n Ti02 inhoud van 85 tot 95 % Ti02, en potyster. Hierdie ilmeniet smeltoonde word binne baie nou bedryfskondisies beheer. Oor-redusering van die slak kan lei tot the formasie van TiC en die redusering van Ti02 tot Th03. Dit affekteer nie net die produk se kwaliteit nie, maar kan volgens Namakwa Sands oond operateurs ook slak skuiming en ontploffings tot gevolg hê. Gedurende onder-reduserende omstandighede in die oond, word die vloeibaarheid van die slak verhoog deur die hoër FeO inhoud in die slak. Dit maak die slak meer korrosief en kan lei tot faling van die vuurvaste stene. Die mate van redusering in die oond word nie net bepaal deur die toevoeging van koolstof nie, maar ook deur die temperatuur van die slak. Dit is 'n standaard praktyk van die industrie om die oond te bedryf met 'n gevriesde slak laag om sodoende die vuurvaste stene te beskerm teen chemiese en fisiese aanval van die slak. Dit is dus duidelik dat dit baie belangrik is om die slak se smeltpunt by verskillende samestellings te kan voorspel. Dit kan die operateur help om die oond binne veilige bedryfskondisies te hou. 'n Hele aantaloplossingsmodelle is oor die afgelope paar dekades ontwikkel vir die beskrywing van nie-ideale oplossings. Hierdie modelle het oor die afgelope paar jaar baie toegeneem in praktiese waarde as gevolg van die snelle toename in rekenaarkapasiteit en -spoed. Dit het veral groot waarde in gevorderde beheerstelsels en besluitneming steun. Sommige van die meer bekende modelle word in hierdie studie bespreek en ge-evalueer vir die Ti02 - Th03 - FeO stelsel, gebaseer op 'n kritiese evaluasie van eienskappe en eksperimentele data gepubliseer in die literatuur. Twee van hierdie modelle, die "gemodifiseerde kwasi-chemiese" model en die "sel" model, is gebruik om die hoë titaan slak stelsel te beskryf. Beide hierdie modelle is gebaseer op statistiese termodinamika en het klein verskille m.b.t. die aanvanklike aannames. Die model veranderlikes vir die sel model is in hierdie studie afgelei vanaf die eksperimentele data. Die hoë titaan slak wat tydens hierdie proses geproduseer word, bestaan hoofsaaklik uit FeO en titaan in sy verskillende oksidasie toestande. Dit plaas die samestelling van die slak reg binne die Ti02 - Th03 - FeO temêre stelsel. Betroubare eksperimentele data vir hierdie stelsel is baie beperk. In hierdie studie word daar gekyk al drie binêre stelsels binne die Ti02 - Th03 - FeO temêre stelsel, naamlik: FeO - Ti02, Ti02 - Th03 en FeO - Th03. Slegs die smeltpunt temperatuur data vir hierdie twee binêre is gebruik in die afskatting van die model veranderlikes. Die akkuraatheid van die modelle is bepaal deur die wortel van die gemiddelde kwadraat van die fout tussen die eksperimentele waardes en die berekende waardes te bepaal. Albei die modelle het 'n relatiewe klein fout in vergelyking met die geraporteerde eksprimentele fout gehad vir al die binêre stelsels. Hierdie studie het gefokus op die smeltpunt temperatuur van die slak en die resultate is daarom ook in die vorm van binêre fasediagramme en isoterme projeksies op die temêre fasediagramme gestip. Die "sel" model gebruik slegs binêre interaksie parameters om die temêre stelsel te beskryf Hierdie parameters word vir die "sel" model nie uitgebrei tot hoër order polinome en dit maak die "sel" model meer robuust, maar minder akkuraat as ander modelle soos byvoorbeeld die "kwasi-chemiese" model.

The preparation of visible light driven photocatalysts for photocatalytic water splitting has been achieved by a CO₂ free, low cost and simple novel method. Combination of peroxide based route with organic free solvent and titanium nitride, carbon free precursor and air and moisture stable, would be useful. Clear red-brown solution of titanium peroxo species was obtained by dissolution of TiN in H₂O₂ and HNO₃ acid at room temperature without stirring. The resultant red brown solution is then used as a titanium solution precursor for yellow amorphous and yellow crystalline TiO₂ synthesis. Visible light photoactivity of the samples was evaluated by photooxidation of methylene blue and photoreduction producing hydrogen from water splitting. The high surface area of yellow amorphous TiO₂ exhibits an interesting property of being both surface adsorbent and photoactive under visible light for photodecolourisation of aqueous solution of methylene blue. However, it might not appropriate for hydrogen production. Nanoparticulate yellow crystalline TiO₂ with defect disorder of Ti³⁺ and oxygen vacancies depending upon synthesis conditions has been characterised by ESR, XPS, CHN analysis and SQUID. Single phase rutile can be produced at low temperature. It is stable at high temperature and the red shift of absorption edge increases with the treatment temperature. Yellow crystalline TiO₂ exhibits an interesting property of being photoactive under visible light. The best photocatalytic performance was observed for 600°C calcination, probably reflecting a compromise between red shift and surface area with changing temperature. Moreover, overall water splitting into hydrogen and oxygen might be obtained by using this material even in air atmosphere. Photoactivity can be improved by testing under anaerobic atmosphere and/or adding sacrificial agent. Quantum efficiency under visible light is still low but comparable to other reports. The maximum efficiency varies from 0.03 % to 0.37 % for hydrogen production and from 0.03 % to 0.12 % for oxygen production, depending on photon energy and sacrificial agents.

The integration of photocatalytic advanced oxidation into solar disinfection is a robust method of improving the microbial and chemical quality of treated water. This study evaluates the performance of photocatalytic solar irradiated batch reactors through an analytical model that reduces treatment parameters by simplifying photoreactor geometry and relating performance to reactor configuration. Accompanying experiments compare the performance of titanium dioxide coated foams of varying pore size to suspended and fixed film configurations through degradation of organic dyes (acid orange 24 and methylene blue), Escherichia coli, and 1,4-dioxane. Results indicate that a catalyst immobilized on a foam support can match the performance of a suspension due to effective mass transport and association between analyte and foam. Additionally, the potential treatment capacity of solar photocatalysis was compared to conventional treatment methods. Results of this comparison stress the fundamental limitation of solar photocatalysis if visible light wavelengths are not harnessed.

The integration of photocatalytic advanced oxidation into solar disinfection is a robust method of improving the microbial and chemical quality of treated water. This study evaluates the performance of photocatalytic solar irradiated batch reactors through an analytical model that reduces treatment parameters by simplifying photoreactor geometry and relating performance to reactor configuration. Accompanying experiments compare the performance of titanium dioxide coated foams of varying pore size to suspended and fixed film configurations through degradation of organic dyes (acid orange 24 and methylene blue), Escherichia coli, and 1,4-dioxane. Results indicate that a catalyst immobilized on a foam support can match the performance of a suspension due to effective mass transport and association between analyte and foam. Additionally, the potential treatment capacity of solar photocatalysis was compared to conventional treatment methods. Results of this comparison stress the fundamental limitation of solar photocatalysis if visible light wavelengths are not harnessed.

No abstract available. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.

PhD. (Chemistry) / Pre-synthesised gadolinium oxide decorated multiwalled carbon nanotubes (MWCNT-Gd) were coupled with titania to form nanocomposite photocatalysts (MWCNT-Gd/TiO2) using a sol-gel method. Rare earth metal ions (Eu, Nd and Gd), nitrogen and sulphur tridoped titania were decorated on MWCNT-Gd to yield composite photocatalysts (MWCNT-Gd/Eu/Nd/Gd/N,S-TiO2) by a similar method, using thiourea as nitrogen and sulphur source. Different carbon nanomaterials were incorporated into tridoped titania to form various composite photocatalysts (MWCNT/Gd,N,S-TiO2, MWCNT/Nd,N,S-TiO2, SWCNT (single walled carbon nanotube)/Nd,N,S-TiO2 and rGO (reduced graphene oxide)/Nd,N,S-TiO2) via the sol-gel method. Likewise, gadolinium doped graphitic carbon nitride (g-C3N4-Gd3+) was obtained by heating a mixture of gadolinium nitrate hexahydrate and cyanoguanidine and subsequently hybridised with MWCNT/TiO2 using the sol-gel method to yield composite photocatalysts with varying g-C3N4-Gd3+ loadings. All the prepared photocatalysts were characterised by microscopic tools (FE/FIB-SEM-EDX, TEM), crystallographic technique (XRD), spectroscopic tools (UV-Vis, Raman and FT-IR) and nitrogen sorption technique (BET).