Synthesis of novel porous nanostructures via template-directed methods and applications in photovoltaics

First, PMMA (poly(methyl methacrylate)) colloidal spheres were synthesised using surfactant free emulsion polymerisation (SFEP) process. The effects of temperature, monomer concentration and seeding in the SFEP process were investigated. PMMA colloidal crystals were fabricated using two different self-assembly techniques; the vertical deposition via evaporation and a modified floating (air-water interface) technique. The floating technique made it possible to fabricate 2D and 3D colloidal crystals with controlled thickness through multiple depositions. Once self-assembled, the PMMA colloidal crystals were used as templates to synthesise different 2D and 3D metal oxide inverse opal structures. Different colloidal crystal templating techniques including vacuum assisted and horizontal templating via sol-gel infiltration were used to produce highly ordered inverse opal structures. A comprehensive temperature dependent study on the formation of 3D TiO2 inverse opals was carried out. Successful synthesis of different metal oxide hollow spheres was made possible using a simple sol-gel templating approach. By using seeded polymerisation combined with template-directed synthesis, sphere-in-sphere hollow spheres were successfully synthesised, with independent compositions for both the inner and outer spheres. By using a modified templating technique, it was possible to synthesise bilayered inverse opals with different metal oxide layers. A successful production of such a bilayered/heterojunction system was realised. By using secondary templating combined with a chemical bath deposition (CBD) process, it was also possible to grow ZnO nanorods onto this bilayered inverse opal structure producing a hierarchical hybrid nanostructure. This novel structure was further sensitised by narrow band gap CdSe/ZnS core-shell quantum dots and used in PEC water splitting experiments. The results were very promising and showed stepwise increase in photoefficiency for every step in the synthesis of the novel hierarchical structure of quantum dot sensitised ZnO nanorods on bilayered TiO2/ZnO inverse opal. Increasing surface area, enhancing charge separation, faster charge transport, better light scattering and visible light absorption all played their parts in such a sequential photoenhancing system. Bilayered TiO2/ZnO inverse opal was also used as a photoanode material in dye sensitised solar cell (DSSC) devices and showed improved photoenhancement. The photonic crystal properties of ZnO inverse opal was investigated by coupling it to potassium titanate (K2Ti4O9) nanobelts. Such configuration showed higher photoefficiency in DSSC devices compare to a single system of titanate. In summary, these strategies offer a novel approach for the synthesis of hierarchical structures with each part playing a role in enhancing light harvesting for better energy conversion.