Kerstin Scheurell

MgF2 antireflective coatings by sol–gel processing: film preparation and thermal densification

Magnesium fluoride sols for the wet chemical processing of porous MgF2 antireflective coatings were prepared by the reaction of MgCl2 with HF. The formation and crystallisation of MgF2 nanoparticles were followed by 19F NMR spectroscopy, X-ray diffraction (XRD) and dynamic light scattering (DLS) in the liquid phase. The crystallization of the resulting films was monitored by XRD experiments. At temperatures exceeding 550 °C the film material and glass substrates undergo a chemical reaction, MgO is formed and SiF4 evaporates as a volatile product. Microstructure and optical properties were characterized as a function of the annealing temperature. The mechanical stability of MgF2 films was evaluated by the Crockmeter test using both felt and steel wool. It is shown that porous MgF2 films prepared by this synthesis have a vast potential for the large-area processing of antireflective coatings.

Amorphous aluminium fluoride as new matrix for vanadium-containing catalysts

A new sol–gel technique was developed for preparing vanadium-containing aluminium fluoride catalysts for selective oxidation reactions. With this new synthesis, highly dispersed vanadium species can be obtained in the metal fluoride host lattice. The X-ray amorphous solids were investigated by conventional and advanced analytical methods such as for instance N2 adsorption experiments, FTIR spectroscopy, pyridine adsorption and NH3-TPD. The surface properties, acidity and catalytic performance were all dependent on the vanadium content of the samples. EDX, FTIR and Raman spectroscopy confirmed the high degree of dispersion of vanadium (oxidation state: III or IV or both) species in the aluminium fluoride matrix. The samples had relatively high BET surface areas and medium strength Lewis acid sites. Additional Bronsted acid sites were observed on samples with higher vanadium contents only. The catalytic performance of the vanadium-containing aluminium fluorides was investigated for the oxidative dehydrogenation (ODH) of propane. Depending on the vanadium content, the new catalyst system offers a surprisingly high catalytic activity and selectivity towards the desired product, propylene. The high degree of dispersion of the active vanadium species in the aluminium fluoride matrix and the type of the acid sites on the solid surface are crucial to the selectivity of the catalyst in the selective oxidation reaction.

Bulk and surface properties of highly dispersed VOx/ZrO2, VOx/SiO2 and VOx/TiO2/SiO2 systems and their relevance for propane oxidation

Highly dispersed vanadium-doped metal oxides such as VOx/ZrO2, VOx/SiO2 and VOx/TiO2/SiO2 with vanadium contents between 0 and 25 mole% were prepared by special bulk preparation methods (coprecipitation and sol–gel, followed by freeze-drying). Bulk and surface properties of the obtained mixed oxide solid solutions were thoroughly investigated by different analytical methods (Raman and FTIR spectroscopy, TPD, H2-TPR, oxygen isotope measurements etc.). Moreover, the catalytic behaviour of the oxides was studied for the example of the oxidative dehydrogenation (ODH) of propane to propylene. Independent of the preparation method, the catalytic behaviour of vanadium-doped ZrO2 and TiO2 phases is very similar. Both metal oxide solid solutions are very active in propane ODH whereas the catalytic activity of VOx/SiO2 is relatively low. On the other hand, the reduction of the catalytic activity is accompanied by an improved selectivity for the formation of propylene. The correlation between the catalytic activity and the acidity of the oxide systems is discussed. Oxidation experiments with 18O2 clearly show that the ODH reaction occurs according to the Mars–van Krevelen mechanism.

Novel aspects in the chemistry of the non-aqueous fluorolytic sol–gel synthesis of nanoscaled homodisperse MgF2 sols for antireflective coatings

Water-clear transparent sols of nano-MgF2 and nano-MgF2–CaF2 composites in ethanol were synthesized through the fluorolytic sol–gel synthesis by the reaction of the insoluble precursor magnesium ethoxide with anhydrous hydrogen fluoride. CO2, MgCl2 or CaCl2 are used as auxiliary reagents. The sols contain monodisperse nanoparticles with a size between 5 and 10 nm, depending on the particular synthesis method used. In contrast to all previous synthesis methods, the sols possess a low viscosity and offer a remarkably high long-time stability for more than one year. It has been demonstrated that they are very suitable for manufacturing antireflective porous coatings on glass substrates with remaining reflectivity below 1%, and high mechanical stability.

Porous MgF2 antireflective λ/4 films prepared by sol–gel processing: comparison of synthesis approaches

Porous MgF2 is a highly promising film material for the preparation of durable antireflective coatings. Synthesis routes based on Mg(OMe)2 and Mg(OAc)2 precursors are described, and the resulting sol–gel films and their thermal consolidation are compared using thermal analysis (TGA/DTA), scanning electron microscopy, ellipsometric porosimetry and X-ray diffraction. Due to their constant viscosity, Mg(OAc)2-based coating solutions are superior from the practical point of view. The porous films are stable against degradation under the steady-state temperature humidity life test (85/85), and they can successfully be integrated into TiO2–MgF2–TiO2 interference filters.Graphical AbstractPorous MgF2 antireflective λ/4 coatings were prepared by sol–gel processing. Synthesis routes based on Mg(OMe)2 and Mg(OAc)2 precursors are established, and the properties of the resulting film materials are compared, respectively.

Formation of nanoscopic CaF2via a fluorolytic sol–gel process for antireflective coatings

The synthesis of nanoscopic calcium fluoride was performed by the fluorolytic sol–gel process. Antireflective coatings of CaF2 were prepared from sols obtained by the reaction of CaCl2 with HF and subsequent dip coating. The addition of tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) after fluorination promotes the formation of transparent sols. The formation and crystallisation of CaF2 nanoparticles was studied by 19F liquid and solid state NMR spectroscopy, dynamic light scattering (DLS) and X-ray powder diffraction (XRD). The morphology of a CaF2-film was analysed by high resolution scanning electron microscopy (HR-SEM) and the mechanical stability of a CaF2-film was evaluated by the Crockmeter test using both felt and steel wool. The refractive index for a CaF2-film was measured by ellipsometry. The synthesis of CaF2 nanoparticles derived from CaCl2 is a good way to achieve porous antireflective coating layers.

Chlorination-Promoted Skeletal-Cage Transformations of C88 Fullerene by C2 Losses and a C-C Bond Rotation.

High-temperature chlorination of fullerene C88 (isomer 33) with VCl4 gives rise to skeletal transformations affording several nonclassical (NC) fullerene chlorides, C86 (NC1)Cl24/26 and C84 (NC2)Cl26 , with one and two heptagons, respectively, in the carbon cages. The branched skeletal transformation including C2 losses as well as a Stone-Wales rearrangement has been comprehensively characterized by the structure determination of two intermediates and three final chlorination products. Quantum-chemical calculations demonstrate that the average energy of the C-Cl bond is significantly increased in chlorides of nonclassical fullerenes with a large number of chlorinated sites of pentagon-pentagon adjacency.

Synthesis and structures of trifluoromethyl derivatives of fullerenes C84(16) and C84(18)

Trifluoromethyl derivatives of fullerene C84 minor cage isomers Cs-C84(16) and C2v-C84(18) of general formulas C84(16)(CF3)2m (2m = 8, 10, 12, 14, 18) and C84(18)(CF3)2m (2m = 10, 12) were isolated by a multi-step HPLC from the products of high-temperature trifluoromethylation of a mixture of higher fullerenes. Molecular structures of these compounds were determined by single crystal X-ray diffraction using synchrotron radiation. The experimentally obtained structural data and the results of quantum chemical calculations were discussed in terms of general regularities of trifluoromethylation of C84 isomers in a wide range of compositions. The addition patterns of CF3 groups in C84(16/18)(CF3)2m molecules were found to depend on particular structural features of isomeric C84 cages.

Specificity of the Local Structure of Nanocrystalline Doped Ceria Solid Electrolytes

This work presents results of studies of the structural features of nanocrystalline doped Ce1xMexO2-y samples (Me = Sm, Bi, Gd; x = 0-0.5) prepared via the polymerised precursor (Pechini) route by using a combination of diffraction (XRD, TEM, WAXS on SR) and spectroscopic (EXAFS, Raman, FTIRS of lattice modes, XPS, SIMS) methods.

Development of complex magnesium fluoro aluminates via the fluorolytic sol–gel synthesis

The diverse industry for optical applications demands materials with many exceptional properties, which often can only be ensured by the development of novel compounds. Specifically, the stability towards abrasion or moisture should go hand in hand with optical characteristics like a low refractive index or high transmittance. Hence, we present fluorolytic sol–gel syntheses of complex magnesium fluoro aluminates and examine resulting layer applicability. Using 19F static as well as 19F MAS NMR, XRD, and TG/DTA analysis, miscellaneous synthetic methods concerning different molar ratios between Mg and Al were discussed. We determine that two phases are predominantly in xerogels employing the sol–gel synthesis: MgAlF5·2H2O as well as MgAl2F8·2H2O. In addition, the formation of strongly disordered structures can be observed within the sol. Furthermore, thermal treatment induces a decomposition of the complex to the particular compounds, MgF2 and AlF3. Layers, which were applied by dip coating, show antireflective behavior and vastly increased stability towards moisture.