School of Chemistry - Theses
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ItemCore-shell structures in dye-sensitised solar cells( 2014)The growing energy demand in the world combined with global warming and pollution is increasing the need for new energy sources. Energy production through capturing sun light is emerging as a good and feasible alternative to fossil fuels. An alternative to the conventional silicon-based devices is the dye-sensitised solar cells (DSCs) with a photoelectrode generally made from nanoparticles of titania. This type of solar cell offers many advantages such as low materials and manufacturing cost, light weight, flexibility and transparency. However, so far only 12% efficiency has been achieved for laboratory cells with a liquid hole conductor. The cell efficiency can be increased by enhancing the internal light-scattering ability of the porous semiconductor photoelectrode. The overall aim of this thesis was to increase the light scattering in the photoelectrode of the DSC by introducing core-shell structures into the photoelectrode. The objective was to improve the light-scattering ability of current mesoporous structures by synthesising core-shell structures where a core of different refractive index was introduced into mesoporous spheroids to increase their light-scattering power. A semi-batch coating method for coating particles with amorphous titania was developed with which particles of varying sizes (80-300 nm) and materials could be successfully coated. Under optimal conditions the thickness of the coating (50-220 nm) could be controlled to a great extent by varying the feed time of the water and precursor feed solutions as well as by varying the concentration of the core particles. Coating thicknesses of up to 550 nm could be achieved when four coating layers were grown on the same particles. A batch method for the synthesis of monodisperse amorphous titania was developed to produce titania cores with low porosity. A solvothermal treatment method was developed where the morphology of the crystallised particles together with their pore size distribution was controlled by the oven temperature and the ammonia concentration. Successful synthesis of core-shell structures of crystalline titania core-particles and a thick amorphous titania coating was obtained by first priming the crystalline particle surface with a thin layer of amorphous titania. A solvothermal treatment method was developed for the crystallisation of the thick titania coating. The specific surface area of the solvothermally treated coatings was significantly higher (higher than 60 m2 g-1) than for the calcined coatings. The specific surface area, pore size and morphology of the solvothermally treated coatings were dependent on ammonia concentration, treatment time and temperature. The silica core-particles dissolved during the solvothermal treatment. The amount of Si incorporated into the titania coating depended on the treatment temperature. Six different core-shell structured materials were synthesised (three dense-core and three hollow-core materials) for testing of their performance in DSCs. All the core-shell structured materials had a better light-scattering ability than the control photoelectrode consisting of only nanoparticles. The dense-core materials had the lowest solar cell performance with no difference in diffuse reflectance observed for the different core-shell structures with dense cores for a 6.5 µm photoelectrode film. The similarity in the diffuse reflectance was attributed to the low structural contrast between the core and the mesoporous coating. For a 6.5 µm film, the hollow-core materials displayed a higher diffuse reflectance compared to the dense-core materials with the silica-templated material reaching more than 75% for light with a wavelength of 400 nm. The better light-scattering ability was attributed to the higher structural contrast between the core and the shell compared to the dense-core materials. The polystyrene-templated material with a thick mesoporous coating (240 nm) had the best photovoltaic performance of all the tested materials. The cell efficiency of 8.0 ± 0.3% was attributed to the high incident-photon-to-current conversion efficiency given by a high dye concentration, good light-scattering properties and low mass transport limitations for the electrolyte.
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ItemOptical properties of silica-coated metal particles( 1999)The significance of silica coating technology for stabilising metal semiconductor nanoparticles was demonstrated. The physicochemical properties of silica coated colloidal silver (Ag@Si02) and the optical properties of films containing uncoated and silica coated colloidal gold (Au@Si02) were investigated. The factors governing the deposition of silica onto silane-primed silver particles were examined and the deposition conditions (pH and reagent concentrations) optimised. Thin silica shells deposited from aqueous/ethanolic sodium silicate solutions were porous and non-rigid. The shells remained insensitive to boiling in an aqueous-ethanolic solution but were substantially dissolved when boiled in pure water. The silver cores participated in a variety of chemical reactions with charged and neutral molecules to form new structures such as hollow silica nonobubbles, semiconductor cores and silica coated alloys of silver and gold. The rate of the reactions of the core with solution phase substrates decreased with increasing silica shell thickness. The silver cores of Ag@Si02 still remained catalytically active and colloidally stable during the reduction of water. Homogeneous films containing uncoated and silica coated gold particles were prepared using a self-assembly technique. The optical properties of the films were investigated as a function of film thickness and the separation between core particles. For interparticle separations below a silica shell thickness of ≈ 6.3 nm, the absorption band red-shifted and narrowed with increasing film thickness. Moreover, with increasing interparticle separation, the band blue-shifted, and both the transmittance and reflectance spectra increasingly began to resemble those of the same isolated particles in solution, in agreement with the Maxwell-Garnett model. In contrast, above a shell thickness of ≈ 6.3 nm, the absorption peak position remained insensitive to film thickness and interparticle separation, suggesting that the gold cores were no longer electromagnetically coupled. For silica shell thicknesses above 12.2 nm, the dependence of the reflectance spectra o interparticle separation and film thickness still needs to be quantified. Possible scientific and industrial applications associated with the Ag@Si02 particles and Au@Si02 particle films are suggested.