Processing and properties of titania nanostructures

Saima Ali

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Properties of nanostructured materials differ vastly from those of bulk materials and modifications of the nanostructures may be used to develop novel functional materials with unique properties. This thesis focuses on the preparation of titania nanostructures with different crystal structures and morphologies to investigate their thermal conductivity and photocatalytic properties. Template-free synthesis methods, such as chemical processing and rapid breakdown anodization (RBA), have been used for the preparation of titania nanotubes (TNTs) and the nanolaminate thin films have been deposited by atomic layer deposition. Thermal conductivity of the nanostructures with different dimensions, crystallinity and phase structure is investigated. Both as-prepared and annealed TNTs are also tested for the photocatalytic degradation of organic pollutants using model dyes. The TNTs synthesized by chemical processing are multiwalled, open-ended, and have a wall thickness of 4-5 nm with mixed anatase/titanate crystal structure, while the TNTs prepared by RBA are single-walled with one end open and the other end closed. Amorphous TNTs with a wall thickness of 15-30 nm are obtained using an organic electrolyte and crystalline TNTs with a wall thickness of 7-12 nm are prepared by an aqueous electrolyte. When annealed at higher temperatures the TNTs diffuse to nanorods with a modified crystal structure and chemical composition. The wall thickness is seen to have a clear influence on the thermal conductivity of the crystalline TNTs, which is reduced by decreasing the wall thickness. The thermal conductivity of amorphous TNTs is slightly lower than that of the crystalline nanotube and comparison with the literature values reveal the impact of wall dimensions on the net thermal conductivity, also in case of amorphous TNTs. The thermal conductivity of amorphous Al2O3/TiO2 nanolaminates is lower than that of titania thin films. It is found that the thermal conductivity decreases by increasing the interface density, revealing the influence of non-negligible Kapitza resistance on the overall thermal conductivity in amorphous nanolaminates. Of the chemically processed TNTs, the as-synthesized TNTs are the most efficient catalysts under the UV radiation due to a higher specific surface area and a large number of hydroxyl groups on the surface. However, the TNTs prepared by RBA aqueous electrolyte show a complete decolorization of dyes under the solar irradiation. As-prepared TNTs and TNTs annealed at 250 and 450 oC are found to be the most efficient catalysts. The number of reactive surface sites, band gap, specific surface area, photocatalytic mechanism and crystal structure of the TNTs are all seen to influence the overall photocatalytic efficiency. The findings presented in the thesis also support the understanding of thermal properties of titania nanostructures for number of potential applications.
Translated title of the contributionProcessing and properties of titania nanostructures
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Hannula, Simo-Pekka, Supervising Professor
Publisher
Print ISBNs978-952-60-8301-8
Electronic ISBNs978-952-60-8302-5
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • titania nanotubes
  • nanolaminates
  • thermal conductivity
  • photocatalytic activity

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