Synthese, Charakterisierung und Applikationen von unbeschichteten, silan-beschichteten und UV-modifizierten Nano-Zinkoxiden
In the course of the research for this dissertation uncoated, silane-coated and UV-modified nanoscale zinc oxides with a high surface area (130 m²/g) have been synthesised for the first time. These nano-porous materials are more robust than the so far described zinc oxides. Also the UV treated material is more effective than nanoscale titanium dioxide for the photocatalytic mineralisation of chlorinated hydrocarbons. Uncoated and silane-coated nanoscale zinc oxides are synthesised by a sol-gel process in solvents containing a high zinc-ion concentration. Such high zinc concentrations in the sol-gel process have not been used before because of the strong aggregation of zinc oxide particles that result. To affect the aggregation alkoxyalkylsilanes with organic functional groups were used as coating molecules. Previously known formulations of the synthesis of uncoated nanomaterials were optimized and the use of silanes was adjusted as coating substrates. Special for the synthesis of coated zinc oxide is the presence of the coating substrate during the precipitation of the zinc oxide particles. Thus a surface modification of the primary particles takes place in statu nascendi. In this state a competition takes place between primary particle aggregation and the reaction of the surface of the primary particles with the silane coating molecules. The high reactivity of the ZnO particle surface is used to form covalent bonds between the silanes and the zinc oxide surface. Depending on the silane concentration different types of bonds between the silanes and the surface of the particles (T0, T1 and T2) have been characterised by spectroscopical techniques. (e.g. 13C- and 29Si solid state NMR-studies) In a second step the silane-coated surface is photooxidized by UV-irradiation. This results in a high BET-surface (130 m2/g), purely inorganic porous oxidic network composed of polycyclic silasesquioxane-bridged zinc oxide particles. Furthermore, properties of the obtained material, like pH-stability and storage stability, as well as the possible use as an heterogenous photocatalyst for the photo-oxidative mineralisation of chlorinated hydrocarbons (e.g. ethylene tetrachloride) and in photovoltaic application (e.g. thin films in Graetzel-cells) have been investigated.
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