Nanoporous SnO(2)-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO(2) particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO(2) particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO(2)-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO(2) nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV-visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO(2)-ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO(2), that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO(2)-ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO(2)-ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO(2) and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO(2)-ZnO photocatalyst because of the energy difference between the conduction band edges of SnO(2) and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO(2)-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.

Polymer‐derived SiOC/ZrO2 ceramic nanocomposites have been prepared using two synthetic approaches. A commercially available polymethylsilsesquioxane (MK Belsil PMS) was filled with nanocrystalline zirconia particles in the first approach. The second method involved the addition of zirconium tetra(n‐propoxide), Zr(OnPr)4, as zirconia precursor to polysilsesquioxane. The prepared materials have been subsequently cross‐linked and pyrolyzed at 1100°C in argon atmosphere to provide SiOC/ZrO2 ceramics. The obtained SiOC/ZrO2 materials were characterized by means of X‐ray diffraction, elemental analysis, Raman spectroscopy as well as transmission electron microscopy. Furthermore, annealing experiments at temperatures from 1300° to 1600°C have been performed. The annealing experiments revealed that the incorporation of ZrO2 into the SiOC matrix remarkably increases the thermal stability of the composites with respect to crystallization and decomposition at temperatures exceeding 1300°C. The results obtained within this study emphasize the enormous potential of polymer‐derived SiOC/ZrO2 composites for high‐temperature applications.

Nanoporous RuO 2 /TiO 2 heterostructures, in which ruthenium oxide acts as a quasi-metallic contact material enhancing charge separation under illumination, were prepared by impregnation of anatase TiO 2 nanoparticles in a ruthenium-(III) acetylacetonate solution followed by thermal annealing at 400 °C. Regardless of the RuO 2 amount (0.5−5 wt %), the asprepared nanocatalyst was made of a mesoporous network of aggregated 18 nm anatase TiO 2 nanocrystallites modified with RuO 2 according to N 2 sorption, TEM, and XRD analyses. Furthermore, a careful attention has been paid to determine the energy band alignment diagram by XPS and UPS in order to rationalize charge separation at the interface of RuO 2 /TiO 2 heterojunction. At first, a model experiment involving stepwise deposition of RuO 2 on the TiO 2 film and an in situ XPS measurement showed a shift of Ti 2p 3/2 core level spectra toward lower binding energy of 1.22 eV which was ascribed to upward band bending at the interface of RuO 2 /TiO 2 heterojunction. The band bending for the heterostructure RuO 2 /TiO 2 nanocomposites was then found to be 0.2 ± 0.05 eV. Photocatalytic decomposition of methylene blue (MB) in solution under UV light irradiation revealed that the 1 wt % RuO 2 /TiO 2 nanocatalyst led to twice higher activities than pure anatase TiO 2 and reference commercial TiO 2 P25 nanoparticles. This higher photocatalytic activity for the decomposition of organic dyes was related to the higher charge separation resulting from built-in potential developed at the interface of RuO 2 /TiO 2 heterojunction. Finally, these mesoporous RuO 2 −TiO 2 heterojunction nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant that opens new avenues toward potential industrial applications.

The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near-theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis-processing-property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green-body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an expos e of the state of the art in transparent polycrystalline spinel fabrication.

The effect of impurities and LiF sintering additive on the microstructure and optical properties of hot‐pressed transparent MgAl2O4 spinel was investigated. A lower and a higher purity powder were hot pressed with and without LiF and process parameters varied. Microstructure was examined using optical and electron microscopy, optical properties using spectrophotometry, and chemistry using various spectroscopic techniques. Impurities present in parts‐per million were found to segregate at grain boundaries and form an amorphous phase, restricting grain growth and causing scatter and opacity. It was found that LiF reacts with impurities to form volatile species that can be removed with proper processing, resulting in larger grain size and increased transmittance. LiF also counteracts absorption caused by reduction of spinel, but results in MgO loss, grain boundary embrittlement, and if trapped in higher concentration, restricts grain growth and causes scatter and opacity.

We have synthesized highly oxygen deficient HfO2−x thin films by controlled oxygen engineering using reactive molecular beam epitaxy. Above a threshold value of oxygen vacancies, p-type conductivity sets in with up to 6 times 10 21 charge carriers per cm 3 . At the same time, the band-gap is reduced continuously by more than 1 eV. We suggest an oxygen vacancy induced p-type defect band as origin of the observed behavior.

The microstructures of two pressureless sintered ceramics, ZrB2 and HfB2 with 20 vol% MoSi2 added, were analyzed by scanning and transmission electron microscopies. Carbides and oxides of the transition metals and MoB were observed to be well dispersed within the boride matrix. Mo5Si3 and Mo5SiB2, with Zr or Hf impurities, were observed at triple grain junctions and showed a partial wetting of the matrix. It was also noticed that the borides had a core-shell structure, which was especially pronounced in the ZrB2-based composite. The experimental results suggest the formation of a Mo–Si–B liquid phase at high temperature, which strongly promoted the densification. The densification mechanisms are discussed in light of the microstructure evolution on sintering, thermodynamic considerations, and the phase diagrams of the species involved.

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