Nanometer-sized zinc aluminate (ZnAl 2 O 4 ) particles were synthesized from heterometal alkoxides, [ZnAl 2 (OR) 8 ], possessing an ideal cation stoichiometry for the ZnAl 2 O 4 spinel. ZnAl 2 O 4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl 2 O 4 . 27 Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl 2 O 4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl 2 O 4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20 -250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.

Nanostructured ZnFe2O4 and zinc oxide−iron oxide composites were obtained by sol−gel processing of a hetero-bimetallic alkoxide. For this purpose the novel molecular precursors Fe2Zn(OR)8 (R = tBu, iPr), fully characterized by spectroscopic and single-crystal diffraction techniques, have been synthesized and their applications in the sol−gel procedure were investigated. The obtained xerogels were annealed at very low temperatures (down to 200 °C) yielding crystalline phases with very small crystallite grain sizes. The phase compositions of the material strongly depend on the polarity of the solvents used in the hydrolysis reaction. When the hydrolysis was performed in the polar propan-2-ol, the spinel ZnFe2O4 (franklinite) formed at annealing temperature of 200 °C with a mean crystallite grain size Dvol of 2.24 nm. However, when using the less polar toluene as solvent in the sol−gel process, iron oxide and zinc oxide are formed together with the franklinite phase. This mixture of phases transforms to the spinel as unique crystalline phase with increasing sintering temperature.

Agricultural production systems are recognised as a major source of atmospheric ammonia. Deposition of ammonia and ammonium may contribute to undesired changes in oligotrophic ecosystems. The continuous measurement of atmospheric ammonia requires expensive and sophisticated techniques and is performed only in a very restrict number of ambient air stations in Europe. Therefore, the application of passive samplers, which have the advantage of being easy to handle and cost-efficient, is useful. In the past the comparability of different passive samplers must be considered as rather scarce. In a joint European project under the leadership of the GSF-Forschungszentrum für Umwelt und Gesundheit, Neuherberg, in 1997 a comparison of different passive ammonia monitoring methods was carried out in a prealpine rural site near Garmisch-Partenkirchen. It was considered valuable to include not only well established systems but also methods still being developed. For the comparative test ten working groups with different methods took part. A wet annular denuder system, which has been developed by the Netherlands Energy Research Foundation for on-line measurement of atmospheric ammonia, served as reference of passive methods. The experiment, which started in June and finished in December, showed that most of the passive samplers fulfil the requirements and can be recommended for further measurements. Additional measurements of meteorological parameters were performed to check the influences of different weather conditions on passive sampling.

Abstract

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl 2 O 4 : Structural and Optical Properties.-Nanometer-sized particles of the title compound are synthesized from ZnAl 2 (OR) 8 (R: iPr, tBu) by a sol-gel process (iPrOH, 50 • C, 12 h) followed by calcination at 400-1400 • C. As revealed by single crystal XRD the precursor Zn[Al(OtBu) 4 ] 2 crystallizes in the orthorhombic space group P2 1 2 1 2 1 with Z = 4. 27 Al MAS NMR spectroscopy indicates that ZnAl 2 O 4 possesses an inverse spinel structure below 900 • C, while the normal spinel phase is observed at higher temperatures. The average crystallite size shows an exponential relationship to the calcination temperature. Optical spectroscopy reveals a blue shift of the absorption edge with decreasing particle size. The optical properties show that ZnAl 2 O 4 is a better candidate than ZnO for UV reflective coatings.