Photocatalytically Active Aerogels – Development and Characterization of (Semi-)Crystalline Titania

Kurzfassung

Novel sol-gel based synthesis routes for (semi-)crystalline titania (TiO2) aerogels were developed to achieve materials for photocatalytic applications. The influence of different synthesis parameters, e.g. amount of acid, type and amount of solvent, and the amount of water, was investigated with regard to adjust the crystallinity and maximize the surface area. TiO2 aerogels are mainly amorphous and are commonly calcinated to obtain crystalline phases which are requirements for an efficient photocatalyst. This thesis provides an alternative strategy to synthesize (semi-)crystalline TiO2 aerogels without calcination and thus to maintain the porous 3D structure. The synthesized aerogels were characterized regarding the crystallinity, phase composition, pore characteristics, and electronic properties to evaluate the impact of the synthesis parameters. Besides the structural properties, the crystallization process and formation of different phases were investigated. It was found that the aerogel properties can be selectively controlled and adjusted by the chosen synthesis parameters. The crystallinity increases with the amount of acid used during the synthesis, while the surface area decreases; however, in much less extent compared to calcination in air or vacuum. The use of concentrated hydrochloric acid (conc. HCl) leads to macropores and broadens the pore size distribution compared to the narrow pore size distribution achieved in mesoporous calcined aerogels. Supported by scattering experiments, the increasing HCl content leads to changes in the hydrolysis and condensation reaction kinetics, by protons and chloride ions which alter the three-dimensional (3D) gel network. It is assumed that chloride ions from the HCl coordinate differently to the used titantetraisopropoxide (TTIP) precursor dependent on the steric hindrance of the alkoxy ligands. The usage of different solvents led to a ligand exchange of the titanium-based precursor. This was assumed to lead to different intermediate complexes with coordinated chloride ions which act as template for the formation of either nanocrystalline anatase or mixtures of anatasebrookite. Dependent on the used solvent, semi- or completely crystalline aerogels could be achieved. High water:precursor ratios were observed to influence the formed phases. Analyses performed using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) experiments indicate the presence of defects and Ti3+ in the aerogel and support the assumption that chloride ions coordinate to the titanium-based precursor but were also incorporated into the TiO2 crystal lattice leading to differently favored phases. In-situ analysis methods revealed that both the anatase and brookite nanocrystals grow during aging of the wet gels within a time of three to seven days. The ratio of amorphous to crystallineAbstract X content of the wet gels can be controlled by varying the amount of acid and the aging time to achieve precisely a crystallinity from completely amorphous to completely crystalline. A defined pore size distribution with pore diameters between 10 and 50 nm can be achieved by usage of pure solvents or ethanol-isopropanol (EtOH-iPrOH) solvent mixtures, based on the ligand exchange at the titanium-based precursor and presumably on changes in precursor cluster size. The coordination of both isopropoxy (OiPr) and ethoxy (OEt) groups to the titanium precursor are assumed to stabilize also the structural integrity of the obtained aerogel. TiO2 aerogels processed with a calcination step and the newly developed synthesis route, respectively, were successfully applied, in cooperation with the University of Bayreuth, for photocatalytic hydrogen generation and for the nitrogen reduction reaction to ammonia. The calcined aerogels, which were completely crystalline, were able to produce more hydrogen compared to the non-calcined semi-crystalline aerogel, that can be assigned to the higher charge carrier mobility and the narrower band gap of crystalline aerogels and consequently improved generation of electron-hole pairs. On the other hand, the non-calcined semi-crystalline TiO2 aerogel exhibited superior storage properties of photogenerated electrons after irradiation with ultraviolet (UV) light because of the improved charge carrier separation based on structural properties of the aerogel. Based on the developed TiO2 aerogel synthesis, mixed metal oxide-based and doped aerogels were successfully synthesized with a very homogeneous elemental distribution, confirmed by transmission electron microscopy (TEM). The measured positions and estimated width of the band gaps of the synthesized mixed metal oxide-based and doped aerogels fit to the reduction potential of hydrogen, so that they are theoretically as well able to photocatalytically generate hydrogen.