Hematites precipitated in alkaline precursors: Comparison of structural and textural properties for methane oxidation

Hematite (alpha-Fe2O3) catalysts prepared using the precipitation methods was found to be highly effective, and therefore, it was studied with methane (CH4), showing an excellent stable performance below 500 degrees C. This study investigates hematite nanoparticles (NPs) obtained by precipitation in water from the precursor of ferric chloride hexahydrate using precipitating agents NaOH or NH4OH at maintained pH 11 and calcined up to 500 degrees C for the catalytic oxidation of low concentrations of CH4 (5% by volume in air) at 500 degrees C to compare their structural state in a CH4 reducing environment. The conversion (%) of CH4 values decreasing with time was discussed according to the course of different transformation of goethite and hydrohematites NPs precursors to magnetite and the structural state of the calcined hydrohematites. The phase composition, the size and morphology of nanocrystallites, thermal transformation of precipitates and the specific surface area of the NPs were characterized in detail by X-ray powder diffraction, transmission electron microscopy, infrared spectroscopy, thermal TG/DTA analysis and nitrogen physisorption measurements. The results support the finding that after goethite dehydration, transformation to hydrohematite due to structurally incorporated water and vacancies is different from hydrohematite alpha-Fe2O3. The surface area SBET of Fe2O3_NH-70 precipitate composed of protohematite was larger by about 53 m(2)/g in comparison with Fe2O3_Na-70 precipitate composed of goethite. The oxidation of methane was positively influenced by the hydrohematites of the smaller particle size and the largest lattice volume containing structurally incorporated water and vacancies.Web of Science2315art. no. 816

Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates

The interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of thecrystal quality. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown usinghigh-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The GaN crystals under study were deposited in threesteps with different process temperatures, growth rates, and ammonia flows. The first GaN layer accommodates the lattice misfit betweensapphire and gallium nitride. Thus, it contains a high number of randomly distributed threading dislocations. The next GaN layer, which isinternally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. The uppermost GaN layermainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. In order to be able to visualize and toquantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotrondiffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The transmission electron microscopy providedinformation about the kind of microstructure defects and their mutual interaction. The synchrotron diffraction and the micro-Ramanspectroscopy revealed the depth profiles of dislocation density and lattice parameters

Extreme biomimetic approach for developing novel chitin-GeO2GeO_{2} nanocomposites with photoluminescent properties

This work presents an extreme biomimetics route for the creation of nanostructured biocomposites utilizing a chitinous template of poriferan origin. The specific thermal stability of the nanostructured chitinous template allowed for the formation under hydrothermal conditions of a novel germanium oxide-chitin composite with a defined nanoscale structure. Using a variety of analytical techniques (FTIR, Raman, energy dispersive X-ray (EDX), near-edge X-ray absorption fine structure (NEXAFS), and photoluminescence (PL) spectroscopy, EDS-mapping, selected area for the electron diffraction pattern (SAEDP), and transmission electron microscopy (TEM)), we showed that this bioorganic scaffold induces the growth of $GeO_{2}$ nanocrystals with a narrow (150–300 nm) size distribution and predominantly hexagonal phase, demonstrating the chitin template’s control over the crystal morphology. The formed $GeO_{2}$–chitin composite showed several specific physical properties, such as a striking enhancement in photoluminescence exceeding values previously reported in $GeO_{2}$-based biomaterials. These data demonstrate the potential of extreme biomimetics for developing new-generation nanostructured materials

Extreme biomimetics: Preservation of molecular detail in centimeter-scale samples of biological meshes laid down by sponges

International audienceFabrication of biomimetic materials and scaffolds is usually a micro- or even nanoscale process; however, most testing and all manufacturing require larger-scale synthesis of nanoscale features. Here, we propose the utilization of naturally prefabricated three-dimensional (3D) spongin scaffolds that preserve molecular detail across centimeter-scale samples. The fine-scale structure of this collagenous resource is stable at temperatures of up to 1200°C and can produce up to 4 × 10–cm–large 3D microfibrous and nanoporous turbostratic graphite. Our findings highlight the fact that this turbostratic graphite is exceptional at preserving the nanostructural features typical for triple-helix collagen. The resulting carbon sponge resembles the shape and unique microarchitecture of the original spongin scaffold. Copper electroplating of the obtained composite leads to a hybrid material with excellent catalytic performance with respect to the reduction of p-nitrophenol in both freshwater and marine environments