TY - JOUR A1 - Krüger, Hannes A1 - Hertwig, Andreas A1 - Beck, Uwe A1 - Kemnitz, E. T1 - Low temperature sol-gel metal oxide and fluoride layer stacks for optical applications N2 - MgF2 and TiO2 single layers and layer stacks were produced by a spin-coating sol-gel process. The final temperature treatment was carried out at 100 °C. The layers were deposited onto silicon and fused silica substrates and were analysed by means of atomic force microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ellipsometry, and UV-vis transmission spectroscopy. MgF2 and TiO2 single layers have morphological and optical properties comparable with physical vapour deposited layers. By using spectroscopic mapping ellipsometry, a good inter- and intra-sample homogeneity was confirmed. Multiple deposition steps result in a linear increase of layer thickness. Various films were deposited with thicknesses between 25 nm and 350 nm. It was shown that the low temperature sol-gel process results in films of optical quality. Anti-reflective and high reflective layer stacks consisting of MgF2 and TiO2 were designed and can be produced now by a sol-gel process, whereas the MgF2 layers in the layer stacks contains also traces of MgF2-2xOx. KW - Magnesium fluoride KW - Titanium oxide KW - Sol-gel KW - Spin-coating KW - Optical properties KW - Optical layer stacks KW - MgF2 KW - TiO2 PY - 2010 DO - https://doi.org/10.1016/j.tsf.2010.06.025 SN - 0040-6090 VL - 518 IS - 21 SP - 6080 EP - 6086 PB - Elsevier CY - Amsterdam AN - OPUS4-21516 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Noack, J. A1 - Emmerling, Franziska A1 - Kirmse, H. A1 - Kemnitz, E. T1 - Sols of nanosized magnesium fluoride: formation and stabilisation of nanoparticles N2 - The formation of magnesium fluoride sols and xerogels according to the fluorolytic sol–gel synthesis based on the reaction of Mg(OMe)2 with non-aqueous HF has been thoroughly investigated by X-ray scattering (WAXS/XRD), TEM, SAXS, DLS and 19F MAS NMR spectroscopy. Mechanistic insights were gained by following the reaction progress and formation of intermediate phases of the fluorination of magnesium alkoxides. For F:Mg ratios of 0.3 and 0.4 the formation of two crystalline phases was observed containing the recently obtained compound [Mg6F2(OCH3)10(CH3OH)14] hexanuclear dicubane units. The stoichiometric reaction yields magnesium fluoride nanoparticles with crystallite sizes below 5 nm, which show broad reflections in the X-ray diffraction pattern. Metal fluoride sols prepared by this way undergo tremendous changes over the first several weeks after synthesis. Immediately after the fluorination, particles of about 120 nm—probably agglomerates—are formed, which break apart in the course of about one month of ageing and low-viscous, transparent sols with particles of about 12 nm are obtained. At the same time structural re-organisation processes within the magnesium fluoride particles are observed by an increase of the (110) reflection in WAXS. KW - Surface modification KW - Colloidal particles KW - Gold nanoparticles KW - Gel synthesis KW - Small-angle scattering KW - Oxide KW - Polyelectrolytes KW - Aggregation KW - Dispersion PY - 2011 DO - https://doi.org/10.1039/c1jm11943e SN - 0959-9428 SN - 1364-5501 VL - 21 IS - 38 SP - 15015 EP - 15021 PB - Royal Society of Chemistry CY - Cambridge AN - OPUS4-26183 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - König, R. A1 - Scholz, G. A1 - Veiczi, M. A1 - Jäger, Christian A1 - Troyanov, S.I. A1 - Kemnitz, E. T1 - New crystalline aluminium alkoxide oxide fluorides: evidence of the mechanism of the fluorolytic sol-gel reaction N2 - This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2] contains a tetranuclear unit consisting of two different five fold coordinated AlFO4-units, with F exclusively in the terminal position. Compound 2, Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2]·Al5F5(µ5-O)(µ-O'Pr)8, contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al16F20(µ4-O)4(µ-O'Pr)20·2('PrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including 27Al triple quantum MAS NMR and 1H, 1H→13C CP, 19F and 27Al MAS NMR. On the basis of the collected data, a reliable decomposition of 27Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO4-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol–gel mechanism previously discussed. KW - Aluminium fluorine KW - MAS NMR KW - Qudrupolar nuclei KW - Crystalline aluminium isopropoxide fluoride PY - 2011 DO - https://doi.org/10.1039/c1dt10514k SN - 1477-9226 SN - 1477-9234 SN - 1364-5447 VL - 40 IS - 34 SP - 8701 EP - 8710 PB - RSC CY - Cambridge AN - OPUS4-26245 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Scholz, G. A1 - Krahl, Thoralf A1 - Ahrens, M. A1 - Martineau, C. A1 - Buzaré, J.Y. A1 - Jäger, Christian A1 - Kemnitz, E. T1 - 115In and 19F MAS NMR study of (NH4)3InF6 phases N2 - This study presents for the first time an NMR spectroscopic characterization of the room and high temperature phases of (NH4)3InF6 using 19F and 115In as probe nuclei. The reversible phase transition to the cubic phase at 353 K was followed by MAS NMR in situ. Static NMR experiments of the room temperature phase and MAS NMR experiments of the high temperature phase allowed the determination of the NMR parameters of both nuclei. Finally, the scalar In–F coupling, rarely observed in solid state NMR, is evidenced in both room and high temperature phases of (NH4)3InF6, and measured in the high temperature phase. KW - (NH4)3InF6 KW - 115In and 19F solid state NMR KW - XRD PY - 2011 DO - https://doi.org/10.1016/j.jfluchem.2011.01.010 SN - 0022-1139 SN - 1873-3328 VL - 132 IS - 4 SP - 244 EP - 249 PB - Elsevier CY - Amsterdam AN - OPUS4-23475 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hemmann, F. A1 - Scholz, G. A1 - Scheurell, K. A1 - Kemnitz, E. A1 - Jäger, Christian T1 - Time-optimized 15N qNMR determination of Lewis and Brønsted site concentrations and chemical bonding of pyridine at Brønsted sites in amorphous aluminium hydroxide fluoride N2 - Time-optimized 15N quantitative NMR (qNMR) is described for the precise determination of Lewis and Brønsted site concentrations per gram catalyst, enabling a measurement time reduction of more than a factor of 10. The proof-of-principle is demonstrated for amorphous aluminum hydroxide fluorides with their tunable Lewis to Brønsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Brønsted sites. Two different Lewis centers were found, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, 1H{15N} REDOR NMR for the acid proton at the Brønsted site yields a distance of the 15N atom of bonded pyridine to this acid proton of (0.112 ± 0.002) nm, such that fast motion/diffusion of the Brønsted site-bonded pyridine at the surface can be excluded. KW - Catalysis KW - Lewis/Brønsted acid sites KW - Acid site concentration KW - Pyridine KW - Quantitative NMR KW - 15N KW - REDOR PY - 2012 DO - https://doi.org/10.1021/jp212045w SN - 1932-7447 SN - 1089-5639 VL - 116 IS - 19 SP - 10580 EP - 10585 PB - Soc. CY - Washington, DC AN - OPUS4-27724 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Scholz, G. A1 - Emmerling, Franziska A1 - Dreger, M. A1 - Kemnitz, E. T1 - Mechanochemical synthesis and characterization of hydrated and dehydrated crystalline strontium terephtalates N2 - A successful mechanochemical synthesis of strontium terephthalate trihydrate is described for the first time. The dehydration of Sr(C8H4O4)·3H2O occurs at about 100 °C and results in a well-defined strontium terephtalate, Sr(C8H4O4), thermally stable up to 550 °C. Both compounds are not described so far in the literature. Their structures were solved by ab initio structure determination and subsequent Rietveld refinement of the powder diffraction data. Further methods like DTA-TG, MAS NMR and FT-IR spectroscopy, and BET measurements were used to characterize these compounds. KW - Mechanochemical synthesis KW - Ab initio structure determination KW - MAS NMR spectroscopy KW - Strontium PY - 2013 DO - https://doi.org/10.1002/zaac.201200550 SN - 0044-2313 SN - 1521-3749 SN - 0372-7874 SN - 0863-1786 SN - 0863-1778 VL - 639 IS - 5 SP - 689 EP - 693 PB - Wiley-VCH CY - Weinheim AN - OPUS4-29117 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Agirrezabal-Telleria, Iker A1 - Hemmann, Felix A1 - Jäger, Christian A1 - Arias, P. L. A1 - Kemnitz, E. T1 - Functionalized partially hydroxylated MgF2 as catalysts for the dehydration of d-xylose to furfural N2 - Among the furan-based compounds, furfural (FUR) shows interesting properties as building-block or industrial solvent. It is produced from pentosan-rich biomass via xylose cyclodehydration. The current FUR production uses homogeneous catalysts and steam. According to recent studies, the reaction mechanism is different in the presence of Lewis (L) or Brønsted (B) sites. In this work, partially hydroxylated MgF2 catalysts, containing bifunctional Lewis and Brønsted properties, were further functionalized using different fluorosulfonic precursors. Its main objective was to test these solids as catalysts for xylose conversion to furfural. Extensive characterization techniques using TG–MS, 19F-MAS-NMR, or pyridine adsorption confirmed the substitution of surface OH groups by stronger Brønsted sites. The activity data showed a considerable change of the reaction kinetics and a final furfural selectivity of 90% at 160 °C in water/toluene for optimized L/B ratios by the one-step grafting technique. Moreover, the reaction analyses and the change of conversion pathways were studied. KW - Hydroxylated MgF2 KW - Functionalization KW - Fluorosulfonic acid KW - Lewis KW - Brønsted KW - Xylose KW - Furfural PY - 2013 DO - https://doi.org/10.1016/j.jcat.2013.05.005 VL - 305 SP - 81 EP - 91 AN - OPUS4-30444 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hemmann, Felix A1 - Agirrezabal-Telleria, Iker A1 - Kemnitz, E. A1 - Jäger, Christian T1 - Probing slow chemical exchange of pyridine molecules at acid magnesium hydroxide fluoride surfaces by 15N NMR N2 - 15N NMR of pyridine has been used to study Lewis and Brønsted sites at the acid surface of magnesium hydroxide fluoride at two different pyridine loadings with ratios of excess pyridine compared to pyridine molecules bound to Lewis and Brønsted sites of about 1:1 (highly loaded) and 0.15:1 (low loaded), respectively. The Lewis and Brønsted sites can be distinguished by the 15N chemical shift of pyridine at about –100 ppm (LPy) and –175 ppm (BPy). In both samples a very broad 15N resonance is found around –88 ppm assigned to weakly adsorbed (excess) pyridine molecules via hydrogen bridges (HPy). Additionally, another signal at about –49 ppm is observed in the highly loaded sample only representing physisorbed pyridine (PPy). A slow chemical exchange process of strongly bound LPy and BPy molecules with excess HPy and PPy molecules can be monitored using 15N exchange NMR. It takes place on a time scale of about 50–100 ms at room temperature. All pyridine molecules have the same 15N T1 and its value increases from 800 ms (highly loaded sample) to 5.3 s (low loading) hinting on changed pyridine mobility depending on the loading level. KW - Catalysis KW - Acid surfaces KW - 15N NMR KW - Chemical exchange KW - Pyridine PY - 2014 DO - https://doi.org/10.1021/jp405213x SN - 1932-7447 SN - 1089-5639 VL - 117 IS - 28 SP - 14710 EP - 14716 PB - Soc. CY - Washington, DC AN - OPUS4-30439 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Agirrezabal-Telleria, Iker A1 - Guo, Y. A1 - Hemmann, Felix A1 - Arias, P. L. A1 - Kemnitz, E. T1 - Dehydration of xylose and glucose to furan derivatives using bifunctional partially hydroxylated MgF2 catalysts and N2-stripping N2 - The current furfural production yield is low due to the use of non-selective homogeneous catalysts and expensive separation. In this work, partially hydroxylated MgF2 catalysts, synthesized using different water contents, were screened during xylose dehydration in water–toluene at 160 °C. The different Lewis/Brønsted ratios on the MgF2 catalysts showed that under-coordinated Mg can isomerize xylose to xylulose, whilst the surface OH-groups were responsible for the dehydration reactions. The presence of glucose as a co-carbohydrate reduced the furfural selectivity from 86 to 81%, whilst it also led to high 5-hydroxymethylfurfural selectivity. The tests catalyzed by MgF2 in combination with simultaneous N2-stripping showed that a furfural selectivity of 87% could be achieved using low xylose loadings. Moreover, the catalysts regenerated by H2O2 showed high activity during the dehydration tests in water–toluene at 160 °C. KW - Bifunctional MgF2 KW - Lewis/Bronsted KW - Carbohydrates KW - Furan derivatives KW - N2-stripping PY - 2014 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-305441 DO - https://doi.org/10.1039/c4cy00129j SN - 2044-4753 SN - 2044-4761 VL - 4 IS - 5 SP - 1357 EP - 1368 PB - RSC Publ. CY - Cambridge AN - OPUS4-30544 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Ritter, B. A1 - Krahl, T. A1 - Rurack, Knut A1 - Kemnitz, E. T1 - Nanoscale CaF2 doped with Eu3+ and Tb3+ through fluorolytic sol-gel synthesis N2 - In this article, the high potential of the fluorolytic sol–gel process to synthesize nanoscopic rare earth-doped calcium fluoride sols is shown. Through a fluorolytic sol–gel process we manage to achieve spherical monodisperse ~5 nm sized nanoparticles using a simple and reproducible one-pot-wet chemical route at room temperature. The as-synthesized clear sols exhibit an intense red and green luminescence under UV excitation at room temperature. A spectroscopic study of the sols revealed the characteristic transitions 5D0 → 7FJ of Eu3+ and 5D4 → 7FJ of Tb3+, with 5D0 → 7F2 (611 nm) of Eu3+ and 5D4 → 7F4 (581 nm) of Tb3+ as the most prominent transitions. This facile synthetic strategy is also valuable for developing other luminescent nanoparticles. KW - Calciumfluorid KW - Europium KW - Lumineszenz KW - Nanopartikel KW - Sol-Gel-Synthese KW - Terbium PY - 2014 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-320120 DO - https://doi.org/10.1039/c4tc01073f SN - 2050-7526 SN - 2050-7534 VL - 2 IS - 40 SP - 8607 EP - 8613 PB - Royal Soc. of Chemistry CY - Cambridge AN - OPUS4-32012 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -