- Title
- Development of titanium dioxide for photo-electrochemical hydrogen production
- Creator
- Mbulanga, Crispin Munyelele
- Subject
- Titanium dioxide
- Subject
- Nanostructured materials Water chemistry Environmental chemistry
- Date Issued
- 2019
- Date
- 2019
- Type
- Thesis
- Type
- Doctoral
- Type
- PhD
- Identifier
- http://hdl.handle.net/10948/40735
- Identifier
- vital:36231
- Description
- TiO2 is an attractive material for photo electrochemical hydrogen production and in this work the synthesis of this compound by hydrothermal, gel-calcination and one-step templating methods is investigated. Rutile-phase TiO2 rods, which are known to offer direct electrical pathways for photo generated electrons when using TiO2 as a photo anode in a photo-electrochemical hydrogen cell, were prepared on both F:SnO2 (FTO) coated glass and Ti foil substrates. Anatase-phase TiO2 tubes and mixed rutile-anatase-phase TiO2 nanostructured films were developed on FTO-coated glass substrates. Rutile-phase TiO2 rod-like structures, were synthesized hydrothermally at 150 oC for different times (6 – 20 hours) on FTO-coated glass substrate, in an aqueous solution of hydrochloric acid and titanium butoxide. The rod-like structures were found to comprise bundles of crystalline prismatic TiO2 nanorods, each approximately 4 nm in width. Each bundle was tetragonal in shape and highly oriented with respect to the substrate surface. The average diameter of the bundles varied depending on the growth time, and reached a mean diameter of ~175 nm after 20 hours of growth. In terms of Raman scattering, these bundles of nanorods acted as single entities, appearing to act as “larger” crystals to the lattice phonons. Hence, phonon confinement effects could not be observed, because the translational symmetry is preserved at the boundaries between individual rods. Moreover, as the preferential perpendicular orientation of the bundles improved with growth time, an unusual increase in the room and low (77K) temperature Eg/A1g Raman band intensity ratios was observed. The low temperature Raman peak position and peak width data was interpreted as supporting the hypothesis that the bundles of nanorods acting as single entities from the point of view of the lattice phonons. The phonon symmetries and frequencies (in cm-1) of these bundles of rutile-phase TiO2 were found to be consistent with rutile-phase TiO2 phonon symmetries and frequencies. Rutile-phase TiO2 rod-like structures were prepared on Ti foil following a two-step gel-calcination method, which involves a gel-deposition process in 5 M NaOH solution at 76 oC for 24 hours to produce a Na- (and Ti-) based layer of gelatinous material, followed by calcination at 600 oC or 800 oC for 1 hour. It is shown that the use of an alkali-based solution such as NaOH and KOH during gel-deposition, leads to the formation of faceted nanorods of rutile-phase TiO2 upon calcination at high temperature. When a solution that does not contain any alkali element, such as H2O2, was used, the material formed upon calcination at 800 oC were clustered nanoparticles, rather than nanorods. From the experiments it was deduced that the high temperature calcination step converted the Na(or K)-based amorphous gel (formed on the Ti surface during a 24-h soak in NaOH (KOH) solution) into faceted Na-titanate rods, which converted into nanorods of rutile-phase TiO2 when Na(or K) evaporates in the form of an oxide. Anatase-phase TiO2 tube-like structures were produced by a one-step templating solution approach, on FTO-coated glass substrate. ZnO nanorod templates were prepared by chemical bath deposition on FTO-coated glass substrate, and then treated in an aqueous mixture of ammonium hexafluorotitanate and boric acid. To study the effect of the template morphology and deposition processes on the formation of anatase TiO2 tubes, different times (10 – 60 minutes) and concentrations of the precursors are used. Calcination at 550 oC converted the Ti-based material developed on the ZnO rods into TiO2 nanostructures. A 10 minute deposition yielded tubes with dimensions resembling those of the ZnO template. However, the tube walls still contained Zn traces. Based on experimental observations, it was concluded that the production of titanium hydroxide complexes on ZnO surfaces takes place through two competing processes: the development of a Ti-based material and partial dissolution of ZnO along the c-axis. Calcination at 550 oC in air finally yielded anatase TiO2. Mixed rutile-anatase-phase TiO2 nanostructured films on FTO-coated glass substrate were prepared by decorating bundles of crystalline prismatic TiO2 nanorods (prepared hydrothermally) with anatase-phase TiO2 particles, using the same precursors mentioned above. The effect of reaction time and precursor concentrations were investigated. It was found that the precursor concentration ratio and reaction time played key roles in controlling the decoration process. Optimal ratios and decoration times were established based on the density of anatase-phase TiO2 decorating particles on the surface of bundles of rutile-phase TiO2 nanorods. The optical properties of rutile-phase TiO2 rods were investigated. The thickness of the TiO2 layer was calculated from reflectance fringes, and agreed well with the length of rods observed using SEM. The room temperature absorption edge of Eg=2.90 eV extracted from the transmittance spectrum correlated with typical values reported for TiO2. The room temperature absorption edge of the conductive layer of F:SnO2 (Eg=3.56 eV) could also be extracted from the transmittance spectrum. Finally, the absorption of white light by rutile-phase TiO2 rods was confirmed to be enhanced by annealing the rods in either hydrogen or nitrogen at 600 oC. Defects (possibly oxygen vacancies) or disorder in the near surface layers of TiO2 induced during the reduction experiments, created new electronic states in the band gap, as reported in literature.
- Format
- vi, 140 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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