Mesoporous semiconducting oxides for gas sensor application

A fabrication procedure of thermally stable mesoporous SnO₂ and TiO₂ powders has been overviewed along with their gas-sensing properties. Treatment of an as-prepared composite material of a supramolecule surfactant and SnO₂, i.e. a self-assembly of the surfactant fringed with a SnO₂ thin wall, with phosphoric acid enabled us to fabricate thermally stable ordered mesoporous SnO₂ powder having a d₁₀₀ value of 3.2 nm, a crystallite size of 2.0 nm and a large specific surface area of 305 m² g⁻¹ even after calcination at 600 °C for 5 h. A thick film sensor fabricated with the ordered mesoporous SnO₂ powder exhibited higher sensing performance than that fabricated with SnO₂ powder prepared by a conventional method and therefore having a lower specific surface area. Surface modification of the conventional SnO₂ powder with a mesoporous SnO₂ layer was also found to be effective for improving the sensing properties. Mesoporous TiO₂ powder could be prepared by employing a modified sol-gel method with Ti(NO₃)₄ and polyethylene glycol having different molecular weights. Higher sensitivity was achieved with a disc-type sensor fabricated with mesoporous TiO₂ powder, in comparison with one fabricated with commercially available TiO₂ powder in the same form, but its sensing properties needed to be further modified.     

Negative thermal quenching CsPbBr3 glass-ceramic based on intrinsic radiation and vacancy defect co-induced dual-emission

Halide perovskite glass-ceramic has recently moved into the center of the attention of perovskite research due to their potential for temperature sensing. However, quantum dots glass-ceramic with excellent luminescence performance still needs to be combined with rare-earth (RE) ions to accurately measure temperature. In this work, a novel non-RE doped dual-emission (460 nm and 512 nm) CsPbBr₃ quantum dots was obtained in telluride glass via the friction crystallization method, where 512 nm was derived from intrinsic luminescence of quantum dots, and 460 nm was originated from thermally induced bromine vacancy, which can be used for temperature sensing. Fluorescence intensity ratio results indicate that the relative sensitivity of dual-emission could reach 5.6 % K^?1 at 323 K. The discovery of non-RE doped CsPbBr₃ QDs glass-ceramic with negative thermal quenching uncovers a new optional sensing glass material that surpass traditional RE-doped QDs glass by their tunability and sensitivity.     

Structural and spectroscopic properties of some neodymium-boro-germanate glasses and glass ceramics embedded with silver nanoparticles

Neodymium-boro-germanate glasses and glass ceramics (with Nd₂O₃ contents up to 40 mol%) embedded with silver metallic nanoparticles (AgNPs) were prepared by the melt-quenching technique. Two series of samples (with AgNPs and without AgNPs) were investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, diffuse reflectance ultraviolet–visible (DR-UV–vis) spectroscopy and photoluminescence (PL) spectroscopy. XRD data reveal that for both series the samples with x < 40 mol% Nd₂O₃ are basically amorphous containing only small amounts of a crystalline phase (identified as crystalline B₂O₃) while for samples x = 40 mol% Nd₂O₃ an important amount of a crystalline phase (identified as the NdBO₃ orthorhombic phase) is present. FT-IR spectroscopy data show that addition of controlled amounts of Nd₂O₃ and AgNPs changes the structural units that build up the host glass ceramic network. These changes were confirmed also by the photoluminescence spectra that show that addition of AgNPs to the host matrix produces changes at the level of emission peaks. The positive values of bonding parameter (δ) calculated based on DR-UV–vis data indicate a covalent character of the bonds from the studied samples.     

Self-limited growth of large-area monolayer graphene films by low pressure chemical vapor deposition for graphene-based field effect transistors

During the last decade, fabrication of high-quality graphene films by chemical vapor deposition (CVD) for nanoelectronics and optoelectronic applications has attracted increasing attention. However, processing of large-area monolayer and defect-free graphene films is still challenging. In this work, we have studied the effect of processing conditions on the self-limited growth of graphene monolayers on copper foils during low pressure CVD both experimentally and theoretically based on thermokinetics and kinetics of Langmuir adsorption. The effect of copper pre-treatment, growth time, and carbon potential of the atmosphere (indicated by the methane-to-hydrogen gas ratio, r) on the quality of graphene nanosheets (number of layers, surface roughness and the lateral size) were studied. Microscopic studies show that careful pre-treatment of the copper foil by electropolishing provides a suitable condition for the self-limited growth of graphene with minimum surface roughness and defects. Raman spectroscopy and atomic force microscopy determine that the number of graphene sheets decreases with increasing the carbon potential while smother surfaces are attained. Large-area monolayer graphene films are obtained at relatively high carbon potential (r=1) and controlled growth time (10 min) at 1000 °C. Measurement of the electrical response of the prepared monolayer graphene films on SiO₂ (300 nm)/Si substrates in a field effect transistor (FET) device shows a high mobility of 2780 cm² V⁻¹ s⁻¹. Interestingly, the device exhibits p-type semiconducting behavior with the Dirac point at a gate voltage of 25 V. The finding show a great promise for graphene-based FET devices for future nanoelectronics.     

Batch monoethylamine degradation via nitrate respiration

Monoethylamine (MEA) degradation via nitrate respiration was evaluated in batch experiments using suspended growth bacterial cultures grown under low growth rate conditions. It was found that, under the conditions tested, MEA was highly degradable when the initial TOC/MLVSS ratio used in a batch experiment was < 0.35, beyond which, however, MEA inhibition was evident. The composition of the medium solution used to cultivate the bacterial cultures was critical in MEA degradation via nitrate respiration. In this study, the best MEA degradation was attained when cobalt (0.45 mg/l), copper (0.5 mg/l), molybdenum (0.5 mg/l), and yeast extract (1.0 mg/l) were all present in the medium solution. Ammonia was formed as an end product from MEA degradation via nitrate respiration. The MEA-N initially added in a batch experiment could be accounted for as NH⁺₄-N when the assimilatory requirements for nitrogen were negligible during the sampling period.     

Controlled synthesis and tunable luminescence of NaYF4:Eu3+

High quality NaYF₄:Eu³⁺ luminescent materials were successfully synthesized via a facile template technique by hydrothermal method. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and fluorescence spectroscopy (FS). The incorporating of Eu³⁺ ions into NaYF₄ crystal lattice influenced the symmetry types of NaYF₄ crystals, resulting in phase transformation of NaYF₄ crystals between α and β phase. The pure hexagonal phase of branched NaYF₄: Eu³⁺ was obtained as the Eu³⁺ concentration reached 15 mol.%. In addition, the luminescence color was tuned by changing the doping concentration of Eu³⁺ ions.     

Improved hydrogenation properties of Mg-x(Ti0.9 Zr0.2 Mn1.5 Cr0.3) composites

Mg-x(Ti_0.9 Zr_0.2 Mn_1.5 Cr_0.3)(x=20%, 30%, 40%) (mass fraction) composite powders were prepared by reactive ball milling with hydrogen and their hydrogen storage properties and microstructure were investigated by XRD, SEM and pressure-composition-temperature measurement. The results show that the composites have 3.83%-5.07% hydrogen capacity at 553 K and good hydrogenation kinetics, even at room temperature. Among them, the milled Mg-30%(Ti_0.9Zr_0.2Mn_1.5Cr_0.3) composite has the highest hydrogenation kinetics as it can quickly absorb 2.1% hydrogen at 373 K, 3.5% in 2 000 s at 473 K, even 3.26% in 60 s at 553 K under 3 MPa hydrogen pressure. The improved hydrogenation properties come from the catalytic effect of Ti_0.9 Zr_0.2 Mn_1.5 Cr_0.3 particles dispersed uniformly on the surface of Mg particles.