Nanostructured zinc oxide films were produced through anodization of zinc foil by using different electrolytes at different voltages, temperatures and over different time periods. The ZnO films were characterised by studying their surface morphology using FESEM, crystal structure using XRD, wetting behaviour through contact angle measurement, and also measuring the profile of the ZnO layer and band gap. Results show that the type of electrolyte and its concentration determine the morphology and size of the nanostructures. Voltage, time and temperature affect the distribution and density of the nanostructures along the surface. The band gaps of the films were in the range of 3.27 to 3.50 eV. Although ZnO is a hydrophilic material, some of the films display hydrophobic and super-hydrophobic behaviour. The data obtained in this study was combined with literature studies and used to devise design guidelines to obtain ZnO films with specific nanostructures and macroscopic properties by controlling the anodization parameters.
A systematic investigation of the photocatalytic activity (PCA) of nanostructured ZnO films showed how this is directly affected by the films' morphology at different scales, from the macroscale morphology of films (e.g. thickness and surface area), to the microscale feature arrangement (e.g. aligned vs. randomly oriented structures or interpenetrated ones), to the nanoscale structure (e.g. crystal size and orientation). The interest in immobilizing photocatalysts in water treatment stems from concerns about the potential toxicity of their slurry form, which requires expensive downstream removal. Immobilisation, though, leads to a reduction in PCA, generally attributed to a lower surface area. By reducing the films' feature size to the nanoscale, an immobilized photocatalyst with high surface area can be achieved. At this scale, however, feature structuring and morphology become important as they determine the interaction between light and the photocatalytic material. In this work, nanostructured ZnO films with different morphology, arrangement and structure were produced by electrochemical anodization of zinc and were tested using the degradation of phenol in a batch reactor as a model system. Results show that the PCA for immobilized catalysts can be optimised by controlling microscale arrangement (light absorbance capacity) and nanoscale structure (crystal size and orientation) rather than macroscale morphology (surface area). These results provide a clear direction to maximising the photocatalytic activity of immobilised photocatalysts for the removal of organic pollutants from water.
Abstract:The disposal of discarded tyres represents an environmental challenge for solid waste management entities. The need to reduce solid waste in urban areas along with the depletion of natural resources have made it necessary to reincorporate used materials into productive processes, giving value to what is considered waste, and minimizing the requirement of natural resources. In this study, pyrolysis was selected to thermally decompose used ground waste vulcanized rubber from automobile tyres. This rubber was exposed to the pyrolytic process in an indirectly heated batch reactor at three different temperatures. Three fractions (i.e., gas, liquid and solid) were obtained during the process. The effect of a hydrogen stream on the properties of the liquid fraction was analysed and characterized following the American Society for Testing and Materials procedures (ASTM) for the pyrolysis of liquid fuels. A multifactorial statistical analysis was used to evaluate the experimental data and thermographs of the process were recorded. Differences in thermographs suggest a different degradation pathway for the rubber exposed to 600 • C compared to the rubber exposed to lower temperatures. Temperatures in the range of 450 to 500 • C favored the production of carbon black regardless of the use of a hydrogen stream. In contrast, high temperatures favored the production of liquid and gas fractions. The highest production of liquid fraction was obtained at 550 • C, where 37% of the rubber was turned into liquid. Results also showed that a constant flow of hydrogen improves the appearance of the pyrolysis liquid. Furthermore, the hydrogen atmosphere reduces the sulphur content, water and sediments; and increases the values for the heat of combustion and the liquid fraction.
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