Zvýšení fotokatalytické účinnosti disociací excitonů v grafén-silikátových nanostrukturách

Abstract

This dissertation thesis studies electrochemical properties of graphene and its influence on the process of photocatalytic reactions. The focus is primary at delocalization of electron, coming from photocatalyst, on graphene layer. The theoretical part consists of introduction at solid state physics, where the electron structure of semiconductors crystal and the electron structure of graphene are explained in detail. At the experimental part there is presented a patented method of conducted vacuum sublimation, which we use to synthesis of non aglomerated nanostructures with high specific surface area. The next is presented a new experimental reactor, which allows us to measure in situ the progress of photocatalytic reaction, so we do not have to interact with the system by sample extraction. Those methods had been used for material characterization: UV-Vis spectroscopy, photoluminescence spectroscopy, scanning and transmission electron microscopy, DLS, EDX and the specific surface. Established physical and chemical properties were then compared to the photocatalytic activity of materials. Prepared photocatalysts were tested in a reactor of own construction by degradation of methylene blue (MB) in water dispersion in the presence of UVA radiation with maximum emissivity at 365 nm (3,4 eV) and they were compared to standard TiO2 P25 Degussa. There was first tried the synthesis of photocatalytic silicate nanoparticles using sodium water glass and zinc acetate (dihydrate). Resulting nanoparticles ZnO-mSiO2 (where m is the module of water glass 3,1) were more efficient in degradation than TiO2, which confirmed their photocatalytic activity. So the addition of graphene to the photocatalytic structue was then investigated and so the graphene's optimal concentration in the next step. Graphene for the experiment was exfoliated from natural graphite by sonification at power density of 1 kW/l. Then there were preliminary tests of synthesis of silicate structure with graphene and the result was material with 8,5 wt% of graphene. The material was 5 % worse at degradation of N2O gas than TiO2, but it was 3 times better at MB degradation. The next material was photocatalytic sorption fabric with 5 wt% of graphene, which was 5 times more photoactive at MB degradation than TiO2. For the observation of influence of graphene presence in photocatalytic silicate structures on their photocatalytic activity, there were chosen concentrations of 0; 0,25; 0,5 a 1 wt% of graphene. From the decay of MB concentraion there were calculated reaction kinetic constants for each material. The best photocatalytic material was specimen with 0,5 wt% of graphene, which was with its kinetic constant of 11,1·10-3 s-1 3,4 times more effective than the pure silicate structure (3,3·10-3 s-1) and more than 10 times more effective than TiO2 P25 Degussa (1,2·10-3 s-1). Looking at the trend of values of kinetics constants with respect to graphene amount, we predicted the optimal graphene concentration to be between 0,5 a 1 wt%. There were also measured photoluminescence response and specific surface area for each specimen. The presence of only one peak with maximum at 380 nm of photoluminescence spectra indicates pure physical connection of graphene with the ZnO-mSiO2 without additional chemicals bonds. The luminescence intensity and the specific surface area then showed no direct correlation with the photocatalytic activity of specimens.