Immobilised TiO₂ mesocrystals and their application in solar cells and photocatalysis

Abstract

Semiconductors play a vital role in many energy- and environment-related areas, and their performance is often affected by their morphology and architecture. Mesocrystal is a hierarchical structure developed from natural minerals which has been demonstrated to bring unique properties and excellent performance to different materials. Many strategies, including bottom-up assembly of nano-sized building blocks and top-down topotactic transformation have been reported to synthesize a range of materials with mesocrystal structures. However, studies on immobilised mesocrystals are poorly developed, despite the importance of semiconductors in film form for applications. This thesis aims to propose general and facile strategies to prepare immobilized semiconductor mesocrystals using TiO₂ as the model material considering the wide applicability of TiO₂ films.

In the first strategy (Chapter 3), TiO₂ mesocrystal films were prepared by simply making a printable paste of the topotactic precursor, NH₄TiOF₃, followed by in-situ topotactic transformation of printed films. The obtained TiO₂ mesocrystal possesses typical structural features of mesocrystals, which brings properties associated with individual nanoparticles and advantages from the crystallographic orientation of nanoparticles. In a specific application, dye-sensitised solar cells (DSSCs), the TiO₂ mesocrystal films were applied as single-layer photoanodes, in which the properties from the structural features were discussed and compared with commercial TiO₂ in detail; and also as scattering layer to make better use of all the structural characteristics. Compared with commercial scattering TiO₂, TiO₂ mesocrystals showed better performance as the scattering layer owing to high dye loading capacity, superior visible light scattering and reasonable charge transport. It is believed that this method simplifies manufacturing processes for mesocrystal films and opens up general opportunities for other functional ceramic films.

The second strategy adopts a two-step fabrication route including a near room temperature hydrolysis reaction to produce immobilized rod-like (NH₄)₂TiOF₄ or flower-like NH₄TiOF₃, followed by a topotactic transformation to immobilised TiO₂ mesocrystals. The immobilised TiO₂ mesocrystals were also applied in DSSCs as photoanodes (Chapter 4). Due to the unique structural features of mesocrystals and strong adhesion to the substrate inherited from the immobilized precursors, the immobilised TiO₂ mesocrystals possess a high dye pickup capability and optimal electron transport ability. The average power conversion efficiency from DSSCs based on immobilised TiO₂ mesocrystals is comparable to devices with 2 layers of printed commercial TiO₂ under the same heating condition, with the balanced short-circuit current densities, similar open-circuit voltages and outstanding fill factors.

The immobilised TiO₂ mesocrystals synthesized from the second strategy were also applied as photocatalysts for water purification (Chapter 5). The photocatalytic performances were tested by the photodecomposition of different dyes and electrochemical characterisation. The heating programme for the topotactic transformation step has been demonstrated to control the structural and surface properties and thus the electronic and optical properties of the immobilised TiO₂ mesocrystals, leading to different photocatalytic performance towards different dyes. Moreover, the immobilisation of all samples provides generally high reusability of the immobilised TiO₂ mesocrystals.

In addition, the precursor materials used in this thesis for the preparation of TiO₂ mesocrystals, (NH₄)₂TiOF₄ and NH₄TiOF₃, have been studied computationally and experimentally (Chapter 6). Attempts have been made to apply the immobilized NH₄TiOF₃ as a photocatalyst for dye degradation. A reasonable degradation efficiency was obtained as a proof of concept.