Key Engineering Materials Vol. 617

Abstract: The luminescence properties of Eu²⁺-doped barium calcium silicon oxynitrides (general formula: Ba_4-xCa_xSi₆ON₁₀: Eu²⁺ (x = 1.8 ~ 2.2)) were examined, together with the preparation conditions for the solid state reaction. Firstly, the stoichiometric amounts of Ba, Ca₃N₂, SiO₂ and Si₃N₄ were mixed with Eu₂O₃ (activator; the molar ratios of Eu to (Ca + Ba + Eu) (= m; 0.05 to 1.0)) for the preparation of Ba_1.8Ca_2.2Si₆ON₁₀: Eu²⁺, Ba_2.0Ca_2.0Si₆ON₁₀: Eu²⁺ and Ba_2.2Ca_1.8Si₆ON₁₀: Eu²⁺, and then the mixtures were heated at a temperature between 1300 and 1550°C for 10 h in N₂ atmosphere. The single phase compounds, which were prepared by the heating at and above 1350°C, emitted the red color light whose wavelength could be changed from 638 to 653 nm with increasing m value from 0.05 to 1.00, due to the excitation at the wavelength of 528 nm.

Abstract: The susceptibility of MAX phases to thermal dissociation at 1300-1550 °C in high vacuum has been studied using in-situ neutron diffraction. Above 1400 °C, MAX phases decomposed to binary carbide (e.g. TiC_x) or binary nitride (e.g. TiN_x), primarily through the sublimation of A-elements such as Al or Si, which results in a porous surface layer of MX_x being formed. Positive activation energies were determined for decomposed MAX phases with coarse pores but a negative activation energy when the pore size was less than 1.0 μm. The insights for tailor-design of MAX phases with controlled thermal stability and intercalated MXenes for energy storage are addressed.

Abstract: One of the largest application areas of sol-gel chemistry is thin-film preparation. Using this approach, we started to synthesize M@ZnO colloidal solutions for the preparation of functional thin films. ZnO is a wide band-gap (3.37 eV) semiconductor with large exciton binding energy. In the bulk or in nanosized form, it could be used in a wide range of applications such as UV light emitters, spin functional devices, gas sensors, transparent electronics or surface acoustic wave devices. Since recently, the preparation of innovative functional M@ZnO materials by doping or functionalizing nanocolloids constitutes a new challenge. Using high concentrations of the different Ti@ZnO nanocolloids, we were able to prepare various functional colloidal solutions with tunable emission and thin films, such as red-luminescent Eu³⁺@ZnTiO₃ or versatile ZnTiON colored nanomaterials.

Abstract: Herein, we investigated the influence of the shell thickness of hollow silica-alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane (NH₃BH₃). Silica-alumina composite shells were fabricated by coating on polystyrene (PS) template particles and then completely removing the PS by calcination. Based on the transmission electron microscopy images, the shell thicknesses of the hollow spheres prepared using 5, 10, and 15 g of PS suspensions and coating times of 24.0, 7.0, and 1.5 h were 20, 13, and 5 nm, respectively. The results indicated that the shell thickness of homogeneous hollow spheres was controlled by adjusting the amount of PS suspension and the coating time. In the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm, 10.0, 9.5, and 9.0 mL of hydrogen was evolved from aqueous NH₃BH₃ solutions for 13, 12, and 13 min, respectively. The molar ratios of the hydrolytically generated hydrogen to the initial NH₃BH₃ in the presence of the hollow spheres with shell thicknesses of 5, 13, and 20 nm were 2.6, 2.5, and 2.3, respectively. From these results, it can be inferred that the activity for the hydrolytic dehydrogenation of NH₃BH₃ increase with decrease of the shell thickness of the hollow spheres. The result of the temperature-programmed desorption profile of ammonia showed that in the number of amount of acid sites of the hollow spheres increase with decrease of the shell thickness of the hollow spheres. The results indicate the activity depends on the number of acid sites of the hollow spheres.

Abstract: The present study reports a facile and effective approach for fabrication of hollow silica-alumina composite spheres. In this approach, silica-alumina composite walls were coated on polystyrene template by the sol-gel method using L(+)-arginine as a promoter for the reaction followed by calcination procedure. Using L(+)-arginine as a promoter of coating process, homogeneous hollow silica-alumina composite spheres are obtained and the wall thickness is larger than that of the hollow spheres prepared with ammonia. The hollow spheres shows high activity for hydrolytic dehydrogenation of ammonia borane compared with spherical silica-alumina composite particles without hollow structure, the hollow spheres prepared with ammonia, and conventional H-BEA zeolite. The results indicate that hollow structure plays important role to show high activity.

Abstract: The design, synthesis and characterization of magnetic-luminescent nanoparticles is now more and more studied since the last decade. However, optimizing the design of such materials requires a deep understanding of their physico-chemical properties. In this paper are reported extended investigations on luminescent and magnetic @SiO₂ Nps prepared by a water-in-oil microemulsion technique. The Cs₂[Mo₆Br₁₄] cluster compound is used as red-NIR phosphor and is prepared by solid state chemistry. We bring here new insights into the structure of such Nps and its interplay with their optical or magnetic properties.

Abstract: We report on the interest of luminescent and nanosized metal atom clusters for the development of functional silica nanoparticles. Furthermore, multi-functional hybrid nanoparticles have been achieved by associating these luminescent clusters with magnetic γ-Fe₂O₃ nanocrystals and plasmonic gold nanocrystals. In addition, as part of the risk evaluation before potential applications, the toxicity of the nanoparticles was evaluated both on plants and on human cells.

Abstract: This paper demonstrates nanostructure formation performed by supercritical fluid chemical deposition. Pt was deposited through hydrogen reduction of a Pt chelate being dissolved in supercritical carbon dioxide. Pt was deposited conformally inside nanotrences, however Pt did not grow thick due to a very long incubation time for the nucleation. Interestingly, SFCD Pt grew thick on already-existing noble metals. A two-step SFCD process was employed, where a Pt seed layer was deposited first and Pt was grown thicker in the subsequent run. This enabled to fill Pt in nanotrenches and nanoholes.

Abstract: In this study, the effects of glycine-nitrate ratios and postcombustion chemical treatment on the phase evolution and surface area of CuCrO₂ powders were investigated. The pure phase of CuCrO₂ powders was obtained at a glycine-nitrate ratio of 1.2–1.4. When the glycine-nitrate ratio was higher than 1.9, the Cu ions were reduced to Cu(0) and the phase of Cu metal and Cr₂O₃ were observed. However, when the glycine-nitrate ratio was lower than 1.1, the Cu ions were partially maintained as Cu(2⁺), and a bluish residue was observed. As-combusted CuCrO₂ powder with a high surface area (50 m²/g) was obtained at a glycine-nitrate ratio of 1.2. Furthermore, a high surface area (> 60 m²/g) was obtained by leaching as-combusted CuCrO₂ powder with diluted nitric acid.

Abstract: Commercially available TiO₂ (Degussa, P-25) is well-known as a highly active photocatalyst. However, in order to work as a photocatalyst, TiO₂ must be dispersed onto a substrate without any binders but this still remains very difficult. We thus attempted to develop a novel TiO₂ system with high photocatalytic activity. TiO₂ anchored silica fibers (TAS) were prepared by chemical vapor deposition using titanium (IV) ethoxide and 1,3-butanediol. The content of TiO₂ in TAS was varied from 0.5 to 5 mass%. It was observed that the surface of TAS was covered with anatase TiO₂ particles 5-10 nm in diameter. TAS shows higher activity at a lower loading amount compared to P-25. Measurement of the acetaldehyde decomposition rate constant of P-25 and TAS carrying 2 mass% of titanium oxide under ultraviolet irradiation showed a value for TAS that was 1.4 times that of P-25. TAS showed a solid acidity amount higher than the P-25 in NH₃-TPD measurements. It was considered that some Si species from the silica fibers migrated into the TiO₂ layer by calcination, increasing the solid acidity of TAS. As a result, the adsorption of acetaldehyde increased, and the photocatalyst activity improved.