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ZnAl2O4 and CuAl2O4 spinel nanoparticles were prepared by a modified Pechini method and used with the natural chitosan (CS) and shellac (SH) polymers to form novel composite membranes as promising food packaging materials. The selection of ZnAl2O4 and CuAl2O4 spinel nanoparticles was based on their antibacterial characteristics, availability, and economy. Using a straightforward and adaptable solution mixing and casting method, the bio-composites were created. The mechanical, physical, antibacterial, homogeneity and air permeability properties of composite films were investigated. The film structure was evaluated in terms of component interactions using FTIR spectra. The addition of 10% SH increased the tensile strength, percentage strain at maximum load, Young's modulus, and burst strength by 114–101%, 3.6–8.4, 103–119, and 179–153% for low and middle M.wt./CS respectively. Chitosan/shellac-CuAl2O4 composite has superior properties compared to ZnAl2O4 composite. In general, 0.05% spinel provides a composite having better qualities than that of 0.1 additions. Middle M.wt. chitosan provides a composite with superior properties compared to that of low M.wt. The incorporation of ZnAl2O4 or CuAl2O4 enhanced the thermal stability of the SH/CS composite. ZnAl2O4 provides superior thermal stability than CuAl2O4. When shellac/CS film structure is treated with the previously indicated ZnAl2O4 or CuAl2O4 formulation, the % swelling decreases along with an increasing in the gel fraction. The antimicrobial assessment using inhibition zone diameter and shake flask methods showed that a composite of 1:9 shellac/chitosan/0.05% of CuAl2O4 exerted the highest Gram-positive antibacterial activity against B. mycoides (21 mm), and C. albicans (22 mm). So, these enhancements make chitosan/shellac/ZnAl2O4 or CuAl2O4 composite films a good alternative to producing food packaging materials. [ABSTRACT FROM AUTHOR]

The ZnAl2-xCrxO4 (x = 0.0–0.1) ceramics were synthesized using microwave-hydrothermal method. The as synthesized powders were sintered at 1250 °C/4 h. All samples exhibit a cubic crystal structure and belong to Fd-3m space group. The lattice constant (a) increased from 8.085 Å (x = 0.0) to 8.128 Å (x = 0.04) and decreased for higher concentrations. The Fourier transform infrared spectra (FTIR) show tetrahedral and octahedral absorption bands in the region, 400 - 480 cm−1 and 480 – 682 cm−1, respectively. The X-ray photoelectron spectroscopy confirmed the valence states of Zn, Al, Cr, and O. The UV-vis spectra reveal a red shift with Cr3+ doping, reducing the bandgap energy (Eg) from 3.66 eV (x = 0.02) to 3.39 eV (x = 0.10). The electrochemical performance of the present samples was investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge (GDC). The sample x = 0.10 exhibits high specific capacity of 143.671 F/g at a scan rate 5 mV/s. Therefore, the present samples may be useful for energy storage devices. Highlights: The ZnAl2-xCrxO4 (x = 0.0–0.1) ceramics were synthesized using microwave-hydrothermal method. The UV-vis spectra reveal a red shift with Cr3+ doping, reducing the bandgap energy (Eg) from 3.66 eV (x = 0.02) to 3.39 eV (x = 0.10). The sample x = 0.10 exhibits high specific capacity of 143.671 F/g at a scan rate 5 mV/s. [ABSTRACT FROM AUTHOR]

Palladium (Pd)-based catalysts are widely used for CO2 hydrogenation reactions due to the capability to activate H2, which enhances the overall CO2 conversion. However, most of the current Pd catalysts reported show high CO2 conversion only at higher temperature, and the exploration of efficient catalyst enabling low temperature conversion (180–300 °C) is of importance. In this study, a series of Pd-B/ZnAl2O4 catalysts were synthesized via in-situ doping, resulting in the Pd crystal transformation induced from crystal to amorphous. The catalytic hydrogenation activity of the amorphous Pd-B catalyst reaches its zenith at a Pd-B mole ratio of 1:2, accompanied by the superior catalysis stability in CO2 hydrogenation. Various characterization techniques, including XRD, CO-DRIFTS, H2-TPR, XPS and TEM, were used to investigate the properties of the Pd-B/ZnAl2O4 catalyst. The results showed that the Pd-B/ZnAl2O4 catalyst exhibited higher dispersion and better CO2 conversion efficiency compared to unmodified Pd catalyst. Additionally, the methanol space–time yields are nearly doubled when the B decoration is employed. At 280 °C, CO2 conversion was improved and methanol selectivity remained at ~ 65%. This study provides valuable insights into the design and optimization of Pd-B based catalysts for CO2 hydrogenation at low temperatures. [ABSTRACT FROM AUTHOR]

Three series of nanocrystalline phosphors based on ZnAl2O4 spinel co-doped with Cr3+, Er3+ and Yb3+ ions in the dopant concentration ratios 1:1:3, 1:1:5 and 1:1:7 were obtained by sol–gel citrate method. Unlike conventional high-temperature solid-state synthesis, the applied method makes it possible to obtain single-phase cubic spinel materials for all three series at a minimal annealing temperature of 700 °C. In this way, fine powders with an average grain size of 31–33 nm were obtained. The lattice parameters of the phosphors triply co-doped with Cr3+, Er3+ and Yb3+ ions are in the range of 8.094–8.087 Å, systematically higher than the corresponding values for single Cr-doped ZnAl2O4, thus proving incorporation of larger Er and Yb ions into the ZnAl2O4 spinel structure. Photoluminescence properties of ZnAl2O4:Cr,Er,Yb nanocrystalline phosphors were studied at the ambient conditions. The phosphors exhibit a characteristic photoluminescence related to d–d transitions in Cr3+ ions and up-conversion emission of Er3+ ions. The intensity of these emissions is dependent on the concentrations ratio of the activators and the annealing temperature. [ABSTRACT FROM AUTHOR]

ZnAl2O4 nanocrystallites doped with Cr3+ ions with mean sizes ranging from 2 to 16 nm were synthesized by the hydrothermal method. Chromium ions occupy the aluminum positions, which symmetry depends on the crystallite size. The smallest nanocrystals have a much larger unit cell than the bigger ones. The metal to ligand distance increases when the size of the nanocrystals decreases. This causes the nephelauxetic effect, which is for the first time (to our knowledge) observed as a size effect. It was also observed that ZnAl2O4:Cr3+ nanocrystals with size larger than 10 nm possesses the same spectroscopic properties as monocrystal. [ABSTRACT FROM AUTHOR]

Photoactive Cu-doped ZnO–ZnAl2O4 ceramic nanocomposites and coatings were prepared by polymer-salt method. The luminescent spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction methods were used for the study the structure, morphology and materials properties. The nanocomposites consist of small hexagonal ZnO and cubic ZnAl2O4 nanocrystals having size about 10 nm. The study of luminescence properties shows that prepared nanocomposites can be used as light down-converters that transfer short-wave ultraviolet (UV-C) radiation into long-wave ultraviolet (UV-A) and visible spectral range. These nanocomposites can be very attractive in photovoltaic applications as spectral down-converters. Also obtained nanocomposites demonstrate the ability to generate chemically active singlet oxygen under UV-A radiation and blue light. The experiments show that Cu-doped ZnO–ZnAl2O4 materials demonstrate antibacterial activity against gram-positive bacteria. [ABSTRACT FROM AUTHOR]

Pristine and Cu-doped ZAO nanosheets have been successfully synthesized using the hydrothermal path. XRD analysis confirmed ZAO's single-phase cubic spinel structure. The morphology of ZAO changed significantly from micro-hexagons to nanosheets with the doping of Cu ions. The XPS spectra of Cu doped ZAO confirm the presence of Zn, Al, O and Cu at 2+ , 3+ , 2− and 2+ oxidation states, respectively. The bandgap of Cu-doped ZAO nanosheets is less than that of pure ZAO. Due to the tunable bandgap, the synthesized material can be used as a potential candidate for energy-based applications. [ABSTRACT FROM AUTHOR]

In this study, well-separated Fe-doped ZnAl2O4 nanosheets were synthesized by a co-precipitation assisted hydrothermal method (220 °C). The as-synthesized nanosheets were characterized by X-ray diffraction, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV- diffuse reflectance spectroscopy and vibrating sample magnetometer studies. The diffraction patterns reveal that there is no noticeable effect of doped Fe ions on the crystal structure of ZnAl2O4. This also confirms the crystallinity and pure spinel structure formation. In addition, no characteristic diffraction peaks related to any impurity, cluster formation, or secondary phase of Fe were detected. The XPS spectra of Fe-doped ZnAl2O4 confirms the presence target elements such as Zn, Al, O, and Fe. It is also confirmed the existence of mixed oxidation states, i.e., Fe2+ and Fe3+, in the optimized sample ZAO-Fe-3 (i.e., 3% of Fe ions doped ZnAl2O4). TEM images of pure ZnAl2O4 show a micro-hexagonal structure, whereas the Fe-doped ZnAl2O4 exhibits nanosheet-like structure. DRS analysis confirms that Fe doping modifies the bandgap energy of ZnAl2O4 with increasing Fe content. The M–H hysteresis curves of Fe-doped ZnAl2O4 exhibited clear room temperature ferromagnetism. [ABSTRACT FROM AUTHOR]

Highly efficient photocatalytic degradation of Congo red dye under simulated sunlight irradiation was achieved on a ZnAl2O4/15%CeO2 photocatalyst. The results of structural and functional group characterization of ZnAl2O4/CeO2 photocatalyst shows that the sample only contain X-ray diffraction peaks of ZnAl2O4 and CeO2, no other impurities, which can increase light absorption capacity, increase specific surface area and suppress charge recombination than that of single phase of ZnAl2O4 or CeO2. Compared to pure ZnAl2O4 and CeO2 or ZnAl2O4/5%CeO2 and ZnAl2O4/10%CeO2 composites, the ZnAl2O4/15%CeO2 photocatalyst exhibited substantially improved simulated sunlight photocatalytic activity for photocatalytic degradation of Congo red dye under simulated sunlight irradiation, with the optimal dye concentration of 25 mg/L, optimal catalyst content of 50 mg/L and optimal pH value of 3 were obtained. The enhanced photocatalytic activity of ZnAl2O4/15%CeO2 photocatalyst can be attributed to the synergistic effect of light absorption capacity, specific surface area, and active site. Finally, the photocatalytic mechanism of ZnAl2O4/CeO2 photocatalyst was proposed on the basis of energy band theory and characterization results. [ABSTRACT FROM AUTHOR]

In this study, we report the photodegradation of a pharmaceutical pollutant namely the tetracycline (TC) onto the spinel ZnAl2O4. The semiconductor ZnAl2O4 was synthesized by chemical route and characterized by X-ray diffraction, scanning electron microscopy and optical analysis. The capacitance potential (C−2–E) measurements showed that the oxide exhibit n-type conduction, supported by a negative thermo-power. The degradation of TC was performed by combined adsorption/photocatalysis processes. The result showed that 30 min of contact time was sufficient to reach 42% of the adsorption capacity; the photodegradation rate of TC was evaluated on ZnAl2O4 under optimal operating parameters pH ~ 4.4, catalyst dose (1 g/L), and initial TC concentration (20 mg/L). The kinetic shows that the TC disappears almost completely (92%) and follows a second order model with a half photocatalytic lifetime of 21 min for a TC concentration of 20 mg/L. The mechanism of the antibiotic tetracycline photodegradation on ZnAl2O4 indicated that the O 2 · - radicals and the holes are mainly responsible for the oxidation of TC. The process can be qualified as clean remediation and is a part of a sustainable development perspective. [ABSTRACT FROM AUTHOR]

Zn0.9Mg0.1Al2O4 + x wt.% TiO2 (0 ≤ x ≤ 3) microwave dielectric ceramics with relatively high thermal conductivity were prepared by solid-phase reaction. The effects of TiO2 on the crystal phase, micromorphology, microwave dielectric properties, and thermal conductivity of the Zn0.9Mg0.1Al2O4 ceramics were studied. X-ray diffraction analysis, scanning electron microscopy, laser flash measurements, and network analysis were used in this study. The results showed that a small amount of nonstoichiometric compounds appeared in the Zn0.9Mg0.1Al2O4 ceramics with increasing TiO2 content. At the same time, TiO2 doping promoted growth of crystal grains, opened the ion-exchange channels in the system, promoted the sintering reaction, and reduced the sintering temperature of the Zn0.9Mg0.1Al2O4 ceramics. Moreover, addition of TiO2 suppressed the microscopic lattice disorder and macroscopic segregation, leading to an improvement in the dielectric properties and thermal conductivity of the composite ceramics. The Zn0.9Mg0.1Al2O4 + 2 wt.% TiO2 ceramic sintered at 1500°C exhibited the best dielectric performance and thermal conductivity with a dielectric constant of 8.57, Q × f product of 18,0861 GHz, temperature coefficient of resonance frequency of −41.5 ppm/°C, and thermal conductivity of 19.67 W/(m K). [ABSTRACT FROM AUTHOR]

Magnetically recyclable Zn0.95−xCr0.05MnxAl2O4 nanoparticles were synthesized via hydrothermal and heat treatment techniques. The crystal structure, morphology, and photoelectrochemical and magnetic properties of the samples were characterized by x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray energy-dispersive spectroscopy (XEDS), ultraviolet–visible (UV–Vis) spectroscopy, photoluminescence spectra (PL), vibrating-sample magnetometer (VSM), electrochemical impedance spectra (EIS) and transient photocurrent responses, and the photocatalytic activity of the samples were evaluated by the photodegradation of methyl orange (MO) dye under simulated sunlight irradiation. The experimental results show that Zn0.95−xCr0.05MnxAl2O4 nanoparticles possess a cubic spinel structure without impurity phases. Cr and Mn ions are successfully substituted in the lattice site of Zn2+ and enter the ZnAl2O4 matrix. The morphology of the samples exhibits irregular spherical or ellipsoid particles with small particle size and good dispersion. The average crystallite size and the bandgap decrease with an increase in Mn ion concentration. Meanwhile, the co-doped samples exhibit obvious room-temperature ferromagnetism, the photoluminescence intensity decreases and a luminescence quenching phenomenon occurs. In addition, Zn0.95−xCr0.05MnxAl2O4 nanoparticles exhibit excellent photocatalytic activity and possess better photocatalytic reusability in degradation of MO aqueous solution. [ABSTRACT FROM AUTHOR]

Low-temperature sintering of gahnite (ZnAl2O4) ceramic, which has the potential to exhibit higher thermal conductivity than alumina but sinters densely at a moderate temperature of approximately 1500°C, has been investigated. When adding only 5 wt.% Cu-Nb-O additive, the sintering temperature was lowered significantly to 960°C. The sample fired at 960°C for 2 h exhibited a relative permittivity (εr) of 9.1, a quality factor multiplied by resonant frequency (Q × f) value of 30,000 GHz (at a measurement frequency of approximately 13 GHz), and a temperature coefficient of resonant frequency (τf) of −69 ppm/K, being relatively satisfactory values. However, the thermal conductivity of the sample was 9.3 W/m-K, which exceeds that of conventional low-temperature co-fired ceramic (LTCC) materials but is only one-third of that of the pure gahnite sample (27 W/m-K). Our analysis revealed that the main cause was the incorporation of the Cu component into the gahnite lattice. These results enable the proposal of guidelines for the development of new LTCC materials with high thermal conductivity. [ABSTRACT FROM AUTHOR]

We report the preparation of 0.1% Eu3+; x% Gd3+ (0 ≤ x ≤ 1.2) ZnAl2O4 phosphors using the citrate sol–gel method. X-ray diffraction (XRD) data revealed that all annealed samples consisted of the single phase of cubic ZnAl2O4 structure. The scanning electron microscopic (SEM) images indicated pronounced effect of dual doping on the surface morphology of the phosphor. The estimated crystal sizes estimated by the XRD was confirmed by the high-resolution transmission electron microscopy (HR-TEM) to be approximately around 20 nm. The photoluminescence (PL) spectroscopy results revealed four distinct emission peaks located at 393, 400, 578 and 618 nm. The peaks at around 393 and 400 nm were attributed to the defect levels located at different positions on the host material (ZnAl2O4). The emission peak at 578 and 618 nm were attributed to the 5D0 → 7F1 and 2D0 → 7F2 characteristic transition within the Eu3+ ions. The International Commission on Illumination (CIE) colour coordinates revealed that the emission colour was not influenced by varying the concentration of Gd3+. [ABSTRACT FROM AUTHOR]

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al2O3 substrate, ZnAl2O4/α-Al2O3 complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl2O4 changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl2O4 transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl2O4 prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl2O4/α-Al2O3 substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes from c-axis single crystal to poly-crystal as ZnAl2O4 layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl2O4 layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film. [ABSTRACT FROM AUTHOR]