Processing and properties of titania nanostructures
School of Chemical Technology | Doctoral thesis (article-based) | Defence date: 2018-11-23
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Aalto University publication series DOCTORAL DISSERTATIONS, 228/2018
AbstractProperties 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.
Supervising professorHannula, Simo-Pekka, Prof., Aalto University, Department of Chemistry and Materials Science, Finland
titania nanotubes, nanolaminates, thermal conductivity, photocatalytic activity
[Publication 1]: Saima Ali, Henrika Granbohm, Yanling Ge, Vivek Kumar Singh, Frans Nilsén, Simo-Pekka Hannula, Crystal structure and photocatalytic properties of titanate nanotubes prepared by chemical processing andsubsequent annealing, Journal of Materials Science, 51 (15) (2016) 7322-7335.
DOI: 10.1007/s10853-016-0014-5 View at publisher
[Publication 2]: Saima Ali, Simo-Pekka Hannula, Titania nanotube powders obtained by rapid breakdown anodization in perchloric acid electrolytes, Journal of Solid State Chemistry, 249 (2017) 189-198.
DOI: 10.1016/j.jssc.2017.03.007 View at publisher
[Publication 3]: Saima Ali, Henrika Granbohm, Jouko Lahtinen, Simo-Pekka Hannula, Titania nanotubes prepared by rapid breakdown anodization for photocatalytic decolorization of organic dyes under UV and natural solarlight, Nanoscale Research Letters, 13 (2018) 179. Full Text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201808014125.
DOI: 10.1186/s11671-018-2591-5 View at publisher
[Publication 4]: Saima Ali, Olli Orell, Mikko Kanerva, Simo-Pekka Hannula, Effect of morphology and crystal structure on the thermal conductivity of titania nanotubes, Nanoscale Research Letters, 13 (2018) 212. Full Text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201808084486.
DOI: 10.1186/s11671-018-2613-3 View at publisher
[Publication 5]: Saima Ali, Taneli Juntunen, Sakari Sintonen, Oili M E Ylivaara, Riikka L Puurunen, Harri Lipsanen, Ilkka Tittonen, Simo-Pekka Hannula, Thermal conductivity of amorphous Al2O3/TiO2 nanolaminates deposited by atomic layer deposition, Nanotechnology 27 (2016) 445704.
DOI: 10.1088/0957-4484/27/44/445704 View at publisher
- [Errata file]: Errata of P. 2, 4