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Opbouwwerk --- Creativiteit --- SCW --- Internationale stage

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Graphite oxide (GO) is a carbon material with unique properties and numerous applications in fields varying from biomedicine to energy storage. It is also an important precursor for graphene, the ‘wonder material of the 21st century’. Nowadays, GO is most commonly synthesised using methods dating back to the previous century, involving an acidic, wet chemical oxidation of graphite. Although effectively modifying the graphite structure by adding oxygen-containing functional groups, these classical oxidation methods go paired with environmental and safety issues and do not allow GO to reach its full potential. In this master’s thesis research, building further upon previous observations, photocatalytic oxidation (PCO) of graphite is researched as an alternative synthesis route towards GO. This technique is based on the use of a photocatalyst (here anatase TiO2) and ultraviolet light to generate oxidants from an oxidative gas mixture. These oxidants can then potentially introduce oxygen-containing functional groups on graphite, leading to the desired GO. It was found that it is not essential for NOX gases to be present in the oxidative gas mixture to affect the molecular structure and morphology of graphite during a PCO reaction. An introduction of defects in the graphite structure, associated with oxidation, can be obtained with a gas mixture consisting of only molecular oxygen and water vapour. This allows to avoid the use of toxic and unsafe NOX gases. A phenomenon of bubble-like features was observed on the graphite surface after a NOX free PCO reaction of both natural and synthetic graphite (HOPG). Comparison with the literature leads to assume that the bubble-like features are most likely blisters, that also occur during the first stages of a wet chemical oxidation and are the result of a subsurface oxidation reaction with gas evolution. This differs from a PCO reaction in the presence of NOX gases, that leads to a more general erosion of graphite, where a significantly larger fraction of the carbon material is removed from graphite in the form of CO and CO2 (overoxidation). It was observed that higher concentrations of O2, longer illumination times and higher reaction temperatures also lead to an increased amount of overoxidation. Increasing the illumination time and concentration of O2 appears to go paired with a higher degree of oxidation, while increasing the reaction temperature seems to decrease the degree of oxidation. This last observation suggests the possibility of a room-temperature PCO of graphite. This provides information on ways to combine the reaction parameters to maximise the product yield. A significantly decreased thermal stability and increased fluorescent behaviour of photocatalytically oxidised natural graphite provide more evidence for the promising nature of PCO as a controlled, environmentally friendly and safe synthesis route towards GO.

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