Publikationen

Six new methyl silicon (IV) precursors of the type [MeSi{ON=C(R)Ar}3] [when R = Me, Ar = 2-C5H4N (1), 2-C4H3O (2) or 2-C4H3S (3); and when R = H, Ar = 2-C5H4N (4), 2-C4H3O (5) or 2-C4H3S (6)] were prepared and structurally characterized by various spectroscopic techniques. Molecular weight measurements and FAB (Fast Atomic Bombardment) mass spectral studies indicated their monomeric nature. 1H and 13C{1H} NMR spectral studies suggested the oximate ligands to be monodentate in solution, which was confirmed by 29Si{1H} NMR signals in the region expected for tetra-coordinated methylsilicon (IV) derivatives. Thermogravimetric analysis of 1 revealed the complex to be thermally labile, decomposing to a hybrid material of definite composition. Two representative compounds (2 and 4) were studied as single source molecular precursor for low-temperature transformation to silica-based hybrid materials using sol-gel technique. Formation of homogenous methyl-bonded silica materials (MeSiO3/2) at low sintering temperature was observed. The thermogravimetric analysis of the methylsilica material indicated that silicon-methyl bond is thermally stable up to a temperature of 400 °C. Reaction of 2 and Al(OPri)3 in equimolar ratio in anhydrous toluene yielded a brown-colored viscous liquid of the composition [MeSi{ON=C(CH3)C4H3O}3.Al(OPri)3]. Spectroscopic techniques 1H, 13C{1H}, 27Al{1H} and 29Si{1H} NMR spectra of the viscous product indicated the presence of tetracoordination around both silicon and aluminum atoms. On hydrolysis it yielded methylated aluminosilicate material with high specific surface area (464 m2/g). Scanning electron micrography confirmed a regular porous structure with porosity in the nanometric range.

6×8 cm2 electrochromic devices (ECDs) with the configuration K-glass/EC-layer/electrolyte/ion-storage (IS) layer/K-glass, have been assembled using Nb2O5:Mo EC layers, a (CeO2)0.81-TiO2 IS-layer and a new gelatin electrolyte containing Li+ ions. The structure of the electrolyte is X-ray amorphous. Its ionic conductivity passed by a maximum of 1.5×10−5 S/cm for a lithium concentration of 0.3 g/15 ml. The value increases with temperature and follows an Arrhenius law with an activation energy of 49.5 kJ/mol. All solid-state devices show a reversible gray coloration, a long-term stability of more than 25,000 switching cycles (±2.0 V/90 s), a transmission change at 550 nm between 60% (bleached state) and 40% (colored state) corresponding to a change of the optical density (ΔOD=0.15) with a coloration efficiency increasing from 10 cm2/C (initial cycle) to 23 cm2/C (25,000th cycle).

Dispersions of three different types of zirconia nanoparticles were treated in a stirred media mill. The deployed surface modifier was present during milling and it established separating mechanisms between the particles. The combination of mechanical deagglomeration and chemical surface modification results in stable zirconia colloids with average particle sizes down to 9 nm. In addition to deagglomeration, the milling treatment also causes comminution of nanoparticles. This was indicated for the two coarser types of the examined particles, by increasing surface areas and decreasing primary crystallite sizes. Transmission electron microscopy of the particles confirmed the creation of smaller primary crystallites and a minority of small fragments, but the majority of particles do not undergo comminution into halves or fragments with similar size. Changes of the particles’ phase composition, wear of milling media, amorphization of the particles to a small extent, and leaching of Y2O3 dopant have been observed as side effects in the process and are characterized quantitatively. This work describes a process for nanoparticle deagglomeration and preparation of high quality colloids, and informs about occurring side effects, including approaches for their minimization.

Nanocomposites of titanium dioxide (TiO2) and multi-walled carbon nanotubes (MWNTs) were prepared and deposited by sol-gel spin coating on borosilicate substrates and sintered in air at 300 °C for 15 min. Further irradiation of the films with different CO2 laser intensities (4.3-17 W m-2) was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12.5 W m-2 was used. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is even improved when high CO2 laser intensities are used during the sintering of the coatings.

The role of oxygen diffusion in the response of individual SnO2 nanowires to this gas is studied. Different oxygen partial pressures lead to strong changes of their electrical resistance, even at room temperature. Since surface models fail to explain the experimentally observed long-term resistance transients, it is necessary to make a description of the interaction mechanisms between oxygen species and SnO2 nanowires by taking ion diffusion into account. Our model correctly describes the experimentally measured dependence of the nanowire resistance with oxygen partial pressure, and it can be applied to the characterization of other metal oxide materials.

Photoluminescence and X-ray photoelectron spectroscopy studies of Mn2+ doped ZnS nanocrystals in inorganic-organic hybrid coatings prepared by a sol-gel process are presented. A 25-fold enhancement of photoluminescence was observed after UV irradiation for 6 h in an ambient atmosphere. X-ray photoelectron spectroscopy results indicate a chemical shift of binding energy from ZnS to ZnSO4 after UV irradiation. X-ray diffraction results show a decrease of ZnS nanocrystal size during UV irradiation. The cause of these phenomena was discussed based on a photochemical reaction on ZnS nanocrystal surface.

The synthesis of BaSnO3 powders has been investigated at lyothermal conditions (temperature of 250 °C; t = 6 h), starting from SnO2·xH2O and Ba(OH)2 and methanol, ethanol, isopropanol and acetone as solvents. Among them isopropanol was found to be the most suitable medium for preparing BaSnO3. By addition of the modifier Genapol X-080 during the processing, the BET specific surface area of the end-powder was increased by a factor of 10. The as-prepared powder consisted of BaSn(OH)6. The thermal behavior, the crystallization behavior and the structure evolution of the powder during heating treatment have been studied with the TG-DTA-MS, XRD and FTIR. The weight loss of the as-prepared powder of about 12 wt% heated up to 1200 °C is mainly attributed to the dehydration around 260 °C which leads to the structure rearrangement and the building of the [SnO6] octahedra. At this temperature BaSn(OH)6 converts to an amorphous phase, from which BaSnO3 nucleates and grows with increasing temperature. The obtained BaSnO3 powders had a BET specific surface area of 16.56 m2/g and a primary crystallite size of 49 nm.

Perovskite-type BaSnO3 powders with a particle size of 30-60 nm have been prepared from tin oxide hydrate (SnO2 ∙ xH2O) gel via the hydrothermal synthesis route. The reactivity of the synthesized SnO2 ∙ xH2O was found to be dependent on the pH value of the mixture of tin chloride and the ammonia solution. The most reactive SnO2 ∙ xH2O gel was obtained at a pH value near 2. The hydrothermal products derived from this reactive gel were characterized with thermograrimetric/differential thermal analysis, Fourier transform infrared spectrometry, X-ray diffraction and high-resolution scanning electron microscopy. The BaSn(OH)6 phase in the as-prepared powder was found to convert into an amorphous phase other than into the BaSnO3 phase. The BaSnO3 powder crystallizing from the amorphous phase at 260°C for 4 h has better sintering properties than the commercial one.