Parisi, Arianna (2023) Electric field-assisted flame synthesis of Carbon Nanoparticle films. [Tesi di dottorato]

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In the last years, the scientific discovery of the many special features of nanoparticles has driven the research toward the production of Carbon Nanoparticles (CNPs) as promising solutions in the field of bioimaging, nanomedicine, catalysis and energy storage applications, with some advantages compared to the conventional carbon-based materials, such as candle soot and carbon black. The easiest way to produce CNPs is the flame synthesis with carbon-to-oxygen ratios slightly higher than the stoichiometric one. In this process, the deposition mechanism ‒ known as thermophoresis ‒ is very slow, and gives rise to high porosity, low-density deposits of agglomerates composed of the pristine CNPs found in the flame at the harvesting position of the substrate. These films are characterized by low mechanical resistance and low temperature-dependent conductivity which limit their direct application. However, the primary CNPs particles can be easily detached from the support and suspended in liquids or other means, since they are deposited in the form of unstable agglomerates. The longstanding experience in the field of particle filtration by electrostatic forces suggested that the superimposition of an electric field to the flame may fasten the deposition rate of CNPs during flame harvesting, improving the process efficiency. The new process, named Electric field-assisted flame synthesis, is the topic of this PhD thesis. In this research work, a mathematical model has been developed to investigate the dynamics of particle deposition under the effect of the electric field. The model reveals that, compared to the conventional thermophoretic deposition, the electric field-assisted flame synthesis has a faster deposition rate, preferentially capturing particles larger than 5 nm, and producing more compact films. The experimental results provide an extensive and brand-new set of data on the characteristics of aged CNP films and agglomerates that can be formed: UV-vis Absorption and Raman spectroscopy, Atomic Force Microscopy, Size Exclusion Chromatography, Surface Texture Analysis, Contact Angle and Current Voltage Measurements have been used to highlight the similitudes and the differences between thermophoretic and electric field-assisted CNP films. The results confirm the increased harvesting rate and the selective deposition of non-fluorescent CNPs larger than 5 nm. The film morphology is irregular with large mesoscopic structures, the deposits have higher compactness and stability. Finally, the experiments indicate that the hydrophilic character of the thermophoretic deposits is preserved in the electric field-assisted flame synthesis deposits, despite their larger roughness and graphitic behaviour, and the higher and temperature-independent electrical conductivity. In conclusion, electric field-assisted flame synthesis emerges as a technology able to overcome the limits of conventional flame synthesis methods and to add a new degree of freedom in synthesising nanoparticles and thin films with new and tunable properties, that can be proficiently used to produce electrodes for fuel cells and hydrolysers, as heat exchange surfaces for reboilers, as scaffolds for active phases, and as noble metal catalysts.

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