Synthesis and characterisation of copper (I) oxide and zinc oxide quantum dots and development of new fluorescent fibre composites

Abstract

The use of nanotechnology in the fabrication of everyday products has increased significantly in the last years. Materials prepared in the form of very small particles, at nano-scale, change their physical and chemical properties and differ significantly from their bulk form. These new, size-dependent properties can improve and enhance the properties and functionality of different materials. Nanotechnology, however, is a relatively new science and many of its potential applications are just being explored. This thesis deals with the development and characterisation of new fluorescent hybrid materials made of quantum dots and NZ wool and paper fibres for potential use in fabrics, and packaging and labelling papers, respectively. To develop these hybrid materials, the metal oxide semiconductors Cu₂O and ZnO were chosen because they can easily be synthesised using aqueous solution and sol-gel methods, and they are likely to be compatible with human skin avoiding negative reactions such as skin irritation. Fluorescence of these quantum dots was tuned by varying the particle size of the quantum dot (controlled by changes in process parameters such as reaction concentrations, temperature, time, pH and use of stabilizer agent) or by introducing small quantities of dopants into the crystal lattice of the quantum dot material. Cu₂O quantum dots were synthesised after developing a wet chemical reduction method. The method allows the controlled formation of cubic shaped Cu₂O nanocrystals in aqueous solution. TEM images revealed that these nanocubes were formed by an assemblage of small spherical nanocrystals (5 nm) with a super lattice structure. This arrangement has a strong effect on the light absorption and light scattering properties of the Cu₂O suspension. Suspensions containing mainly large polycrystalline nanocubes showed a dichroic effect; the suspension was yellow-green in reflected light and red-purple and transparent in transmitted light. This phenomenon is caused by Surface Plasmon Resonance (SPR). Thereby, the size of the nanocubes and the intermolecular interaction between the nanocubes determined the wavelength (range) of the adsorbed and scattered light and the ratio of light absorption and light scattering. Fluorescence of these quantum dots was of very low intensity and therefore not suitable for the development of hybrids materials. A sol-gel method was developed to prepare ZnO quantum dots. This method produced polycrystalline particles in the shape of round rice grains of 100 nm and 1μm in the size. TEM images revealed that these polycrystalline particles consist of small spherical and ellipsoidal nanocrystals 10 - 12 nm. The ZnO suspension was white under ambient light and yellow-orange under UV light. When excited with UV-light (λexc = 260 nm), the ZnO suspension showed two distinct emission peaks in the UV (~ 380 nm) and visible range (~ 560 nm). To develop quantum dots –fibre composites, the properties of the ZnO quantum dots were combined with the bulk properties of NZ merino wool and paper fibre substrates. This was achieved by using the in-situ and building block approaches. In the in-situ approach, quantum dots were formed in the chemical surrounding of the substrate material. This method relied on cross-link reactions between the quantum dots and wool fibre during nucleation and particle growth. On the other hand, in the building block approach, quantum dots were directly attached to the substrate material or with the use of linker molecules. To ensure long-life stability of the composite material this approach relies on the creation of strong chemical bonding between both components. Different processing strategies were conducted to improve the chemical bonding between the inorganic and organic phase. This was achieved by modifying the interface between both components either by functionalising the surface area of the substrate material or exchanging functional groups at the quantum dot surface. The building block and in-situ approach produced ZnO quantum dot – wool composites with a sharp emission peak in the UV range. This emission peak (c. 380 nm) referred to the band-edge emission of the ZnO quantum dots. The attachment of ZnO quantum dots to bleached Kraft paper did not result in the functionalisation of the paper fibre with fluorescence; however, the fibres were covered with a surface layer that exhibit UV filtering properties. The use of 8-Hydroxyquinoline (8-HQ) to obtain fluorescence in the visible range of the spectrum was also explored here. 8-HQ was used as a fluorescent chelate agent for zinc cations at the ZnO quantum dot surface. ZnO quantum dots functionalised with 8-HQ conferred the wool and paper fibre with strong fluorescence in the visible range. Depending on the concentration of 8-HQ and other parameters (e.g. choice of solvent, soaking temperature and time) the fluorescence of ZnO quantum dot – wool and paper composites were tuned from white-light green to yellow-green. An alternative procedure for the development of fluorescent wool fibre composites was also developed. Wool was functionalised by the uptake of zinc and 8-HQ in two separate batch processes. 8-HQ is bi-dentate chelating agent and forms with zinc cations a planar coordination complex, chelate. This complex formation creates fluorescence in the blue and green region of the visible spectrum. Two methods, which differed in the order of uptake of the precursor material onto the wool, were used to dye the wool. The optical fluorescence of the wool fibre was tuned between white-bluish, white, white-greenish and green-yellowish by modifying the dyeing bath conditions used (e.g. concentration of 8-HQ and choice of solvent). Fluorescent composite materials have numerous potential applications and further research is necessary to optimise and develop preparation procedures. Composite materials comprising quantum dots and wool and paper were developed as part of this thesis. However, it is evident from this study that the synthesis of metal oxide quantum dots with intense, stable, spectroscopically pure fluorescent colours is difficult to achieve. This is because the synthesis of size and shaped controlled metal oxides at nanoscale is challenging and can be affected by agglomeration reactions in aqueous solution. Furthermore, it should be kept in mind that chemical interaction between quantum dots and organic substrate material can create different non-radiative energy transfer processes which will quench the fluorescence of the quantum dots. Finally, future studies should focus on developing methods to successfully incorporate dopants inside the lattice structure of metal oxide nanoparticles to obtain and tune the fluorescence at different wavelengths and hence colours in the visible range.