Titania is an attractive material due to the diverse and versatile applications, from water purification to the more delicate application in energy conversion devices. This thesis presents an experimental study of the production of mesostructured titania for photoanode in photoelectrochemical solar cells (dye-sensitised solar cells, DSSCs). Different synthesis routes were investigated; from the conventional precipitation technique to the sol-gel method applying evaporation induced self-assembly technique. Hydrothermal technique is then proposed to accomplish the sol-gel technique and obtain better material characteristics.
Templating-precipitation technique termed as two-step sol-gel (TSS) technique was firstly investigated to produce mesostructured titania powders by involving a titanium(IV) tetraisopropoxide precursor and a neutral surfactant, dodecylamine, as a template. It has been shown that the resultant materials have improved structural properties, comparable band gaps and specific surface areas with respect to Degussa P25 titania (commonly used as the wide band gap semiconductor in DSSCs). It was shown that a highly acidic environment and higher loading of TTIP precursor in Step- 2 can improve the structural properties as well as the crystallinity of the resultant powders. However, the TSS derived films are mechanically unstable, which precludes their use in solar cell applications.
A sol-gel technique applying evaporation-induced self-assembly (EISA) was then adopted to directly form a thin film on the substrate. Systematic study in assessing the role of synthesis parameters was performed on the basis of xerogel production, which was produced in such a way to mimic the film formation. It is found that the alternation of the chemical precursors composition, either the template (Pluronic P123, EO20PO70EO20), the chelating agent (acetylacetone), or the organic solvent (isopropanol) affected the extent of the resultant mesophase ordering and degree of crystallinity. All alteration led to the formation of worm-hole like pore structure with variable degree of mesophase ordering at system condensed at 60°C. While, high amount of template and alcohol retard the formation of anatase polymorph of TiO2, thus decreasing the degree of crystallinity.
The as-synthesised films subjected to the room-temperature condensation was proven to enhance the mesophase ordering of the mesostructured hybrid titania. Progression towards lower interfacial curvature was observed under increased water content system, which was consistent with geometrical consideration and liquid-crystal templating mechanism. However, the mesostructured hybrid titania was not thermally stable upon template removal by calcination. The room temperature condensation mode was also not feasibly reproducible due to the uncontrolled interaction with atmospheric humidity.
The controlled humidity environment showed to offer feasible ways in controlling desired mesophases, anatase crystallinity, and mesoporosity. Similar progression towards the formation of higher interfacial curvature mesophases for the system condensed at almost "dry" environment was observed. Careful choice on parameters was systematically considered, since the condensation mode resulted in lower mesoporosity materials.
However, the EISA films did not show a good adsorption capability to the Ru-dye that was employed to sensitise the titania photoanodes to generate current in a photoelectrochemical solar cells due to their low porosity. Therefore, different techniques based on conventional precipitation and sol-gel process accomplished by hydrothermal treatment was then proposed.
Hydrothermal process showed great promise in terms of reproducibility. Anatase seeds, which are hydrothermally produced and have crystal size less than 5 nm, were interacted with a block copolymer solution and treated hydrothermally. It was found that the block copolymer controlled the pore formation as well as the crystal growth at a given condition. Acid route derived powders showed higher porosity and pore size with comparable surface areas to the pure neutral route. A rutile phase was also formed instead of the anatase phase in acidic route. Higher adsorption capability compare with the EISA xerogels was demonstrated.
Acid modification to the surface of the resultant powders from the latest technique has offered a way to produce a highly mechanically stable film on the glass substrate, either by using simple doctor blade (spread and drag) or spray coating technique. The selected powder derived film was assembled and tested as a photoanode in photolectrochemical solar cells. It resulted in an appreciable response and efficiency by using a Ru-"red dye" as a photosensitiser.