Spinel ZnCr2O4 Research Articles

Spinel ZnCr2O4 nanorods synthesized by facile sol-gel auto combustion method with biomedical properties.

In this study, spinel zinc chromite nanorods (ZnCr2O4 NRs) were successfully manipulated by a simple sol-gel auto combustion process employing urea as fuel. The sample was only required to sinter at 500 °C for 2 h to obtain the single crystalline phase. The phase formation, crystallinity, and surface topography of synthesized ZnCr2O4 NRs were explored by X-ray diffraction (XRD), UV-Vis reflectance spectroscopy (UVDRS), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray (EDX) spectroscopy, and vibrating sample magnetometry (VSM). XRD analysis confirms the formation of spinel ZnCr2O4 NRs. The FTIR spectrum displays the two vibrational peaks of Cr-O, and Zn-O at 489 and 615 cm-1, correspondingly. These vibrational bonds were correlated with ZnCr2O4 and revealed the production of cubic spinel ZnCr2O4 NRs. FESEM indicates the presence of hexagonal-rod-shaped particles. EDX spectrum demonstrates the elemental composition of the ZnCr2O4 NRs and confirms the primary peak of Zn, Cr, and O. The obtained ZnCr2O4 NRs exhibit an antiferromagnetic behavior. The bandgap energy of ZnCr2O4 NRs was ascertained and was shown to be 3.45 eV. Furthermore, the antifungal and antibacterial effect of ZnCr2O4 NRs was examined against pathogenic strains by disc diffusion technique. Besides these, the antimalarial activity of ZnCr2O4 NRs was studied against Plasmodium falciparum. Thus, the as-synthesized ZnCr2O4 NRs showed significant antibacterial, antifungal and antimalarial activity and may be helpful for research opening a novel horizon in nanomedicine. Graphical abstract.

Factors determining surface oxygen vacancy formation energy in ternary spinel structure oxides with zinc.

Spinel oxides are an important class of materials for heterogeneous catalysis including photocatalysis and electrocatalysis. The surface O vacancy formation energy (EOvac) is a critical quantity for catalyst performance because the surface of metal oxide catalysts often acts as a reaction site, for example, in the Mars-van Krevelen mechanism. However, experimental evaluation of EOvac is very challenging. We obtained the EOvac for (100), (110), and (111) surfaces of normal zinc-based spinel oxides ZnAl2O4, ZnGa2O4, ZnIn2O4, ZnV2O4, ZnCr2O4, ZnMn2O4, ZnFe2O4, and ZnCo2O4. The most stable surface is (100) for all compounds. The smallest EOvac for a surface is the largest in the (100) surface except for ZnCo2O4. For (100) and (110) surfaces, there is a good correlation, over all spinels, between the smallest EOvac for the surface and bulk formation energy, while the ionization potential correlates well in (111) surfaces. Machine learning over EOvac of all surface sites in all orientations and for all compounds to find the important factors, or descriptors, that decide the EOvac revealed that bulk and surface-dependent descriptors are the most important, namely the bulk formation energy, a Boolean descriptor of whether the surface is (111) or not, and the ionization potential, followed by geometrical descriptors that are different in each O site.

Exceptional stability and chemical mechanism over spinel ZnCr2O4 catalyst for HCl oxidation to Cl2

Deacon process, one-step catalytic oxidation of HCl to Cl2, is the most effective way to realize the recycling utilization of chlorine element in industry since 1868. However, the stability of non-noble metal catalysts, e.g. Cr-based catalysts, is problematic to be commercialized due to the Cr loss in reaction atmosphere. Herein, we reported a spinel ZnCr2O4 catalyst exhibiting a high activity as well as an unprecedented long-term stability, with a sustainable HCl conversion rate (χHCl) of ˜78% and the space time yield (STY) of 2.25 gCl2 gcat−1 h−1 even after 300 h run at 390 °C and feed O2/HCl (1:1). In addition, we conceived the control experiments and density functional theory (DFT) calculations to deeply understand the chemical mechanisms over spinel CoCr2O4 and ZnCr2O4. The XPS, O2-TPD and H2-TPR characterizations suggested the different surface redox behaviors of both catalysts. DFT calculations indicated that a defect surface of ZnCr2O4 was more active in HCl catalyzed oxidation and HCl preferred to be absorbed on the Zn top site and Cl* coupling to Cl2 was the△G-determining step, which led to the excellent stability of ZnCr2O4 by efficiently avoiding the Cr loss. On the contrary, the higher reaction energy of HCl adsorption on the Co-Cr bridge sites of CoCr2O4 caused the decreasing of activity and stability. Consequently, the different stability performance over CoCr2O4 and ZnCr2O4 was well correlated with the two distinct chemical mechanisms and therefore an outstanding agreement was obtained from the theoretical calculations and experimental behaviors.

Substitution effects on frustrated magnetism of chromite spinel ACr2O4 (A = Zn, Cd)

Abstract We investigate magnetic properties of mixed crystals of spinel oxides Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4 to study Cr-site substitution effects on the geometrically-frustrated magnetism of chromite spinels ZnCr2O4 and CdCr2O4. Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4 with above x ∼ 0.1 exhibit spin-glass-like behavior below ∼10 K which is suppressed by magnetic field H > 1 kOe. Moreover, the Fe concentration x dependence of Weiss temperature ΘW in Zn(Cr1−xFex)2O4 (Cd(Cr1−xFex)2O4) exhibits the sign change of ΘW from negative below x ∼ 0.4 (x ∼ 0.1) to positive above x ∼ 0.4 (x ∼ 0.1). This suggests that, in Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4, the competition of antiferromagnetic and ferromagnetic interactions (bond frustration) becomes stronger with increasing the Fe concentration x, while the geometrical frustration becomes weaker with increasing x.

Physical and photoelectrochemical properties of the spinel ZnCr2O4 prepared by sol gel: Application to Orange II degradation under solar light

The physical and photoelectrochemical properties of the spinel ZnCr2O4 prepared by Sol Gel method are investigated. The thermal analysis indicates that the cubic spinel is formed beyond 500 °C with a lattice constant of 0.8328(3) nm. The Rietveld texture analysis indicates the absence of inter-granular residual stresses. The attenuated transmission reflectance confirmed the phase formation and forbidden band is found to be 1.85 eV. The valence and conduction bands have respectively Cr3+-t2g Cr3+-eg character. The magnetic curves M(H) of ZnCr2O4 exhibit a low hysteresis and the magnetization rises as the applied field up to 19 kOe with no saturation. The electrical properties of ZnCr2O4 indicate semiconducting behavior with activation energy of 43 meV while the thermo-power characterizes hole doped spinel. The capacitance measurement (C−2 - E) in Na2SO4 solution (0.1 M) shows p type conduction with a flat band potential (Efb) of −0.061 VSCE and a hole density (NA) of 2.7 × 1018 cm-3. The semicircle in the Nyquist plot is assigned to the bulk contribution (7700 kΩ cm2) with a depletion angle of 4° due to a constant phase element. As application, orange II (OII) is successfully oxidized on the spinel under solar and artificial lights. The conduction band is cathodic enough to generate radicals O2 which mineralizes OII. The best activity occurs at pH ∼7 under solar light (44%) and the process follows a first order kinetic with a half-life of 170 min. As expected, the COD removal efficiency increases, with irradiation time and reaches 20% after 1 h under solar light.

Synthesis, structural, optical, and magnetic properties of ZnCr2−xFexO4 (0 ≤ x ≤ 0.8) nanoparticles

ZnCr2−xFexO4 nanoparticles with different compositions (0 ≤ x ≤ 0.8) were successfully synthesized using the microwave combustion method. The effects of Fe ions substitution with Cr ions on the spinel ZnCr2O4 structure, optical, and magnetic properties are investigated. The nanoparticles size was found to increase as a result of Fe substitution, as confirmed by the high resolution transmission electron microscope (HR-TEM) images. A reduction of the optical energy gap and a transition from paramagnetic to superparamagnetic behavior are observed with increasing Fe ratio indicating a single domain ZnCr2−xFexO4 nanoparticle. The uncompensated surface magnetic moments results in an enhancement in paramagnetic behavior compared with bulk ZnCr2O4 spinel. The saturation magnetization increases by increasing Fe ions in the spinel structure which is expected to originate from a larger surface area.

Coexisting exchange bias effect and ferroelectricity in geometrically frustrated ZnCr2O4

Concomitant occurrence of exchange bias effect and ferroelectric order is revealed in antiferromagnetic spinel ZnCr2O4. The exchange bias effect is observed below antiferromagnetic Neél temperature (TN) with a reasonable value of exchange bias field ( Oe at 2 K). Intriguingly, the ratio is found unusually high as ∼2.2, where HC is the coercivity. This indicates that large HC is not always primary for obtaining large exchange bias effect. Ferroelectric order is observed at TN, where non-centrosymmetric magnetic structure with space group associated with the magnetoelectric coupling correlates the ferroelectric order, proposing that, ZnCr2O4 is an improper multiferroic material. Rare occurrence of exchange bias effect and ferroelectric order in ZnCr2O4 attracts the community for fundamental interest and draws special attention in designing new materials for possible electric field control of exchange bias effect.

Geometric Spin Frustration in Zn1-xNixCr2O4 System

At room temperature, the cubic spinel ZnCr2O4 is known as the typical geometrically frustrated spin system. Upon cooling this compound undergoes a crystal structure change driven by removing the frustrated degenerated states, accompanying with the magnetic ordering. In the tetragonal phase the c-axis is larger than the a-axis. Another spinel compound NiCr2O4 takes the cubic structure above 310 K. Below 310 K NiCr2O4 is tetragonally distorted (c/a < 1) due to cooperative Jahn-Teller ordering. In the previous reports, the geometrical frustrated spin fluctuations still exist in NiCr2O4. In this report we investigated how the geometrical frustration changes and how the ratio of the crystal axis, c/a changes in Zn1-xNixCr2O4 system by the low temperature x-ray diffraction measurement and the SQUID magnetometer.

RETRACTED: Fabrication and Characterization of Nanospinel ZnCr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Using Thermal Treatement Method

RETRACTED PAPER: The fabrication of nanospinel zinc chromite (ZnCr2O4) crystals by the means of thermal treatment method from an aqueous solution containing metal nitrates, polyvinyl pyrrolidone (PVP), and deionized water was described in this study. The samples were calcined at various temperatures ranging from 773 to 973 K for the decomposition of the organic compounds and crystallization of the nanocrystals. PVP was used as capping agent to control the agglomeration of the particles. The characterization studies of the fabricated samples were carried out by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM), energy dispersed X-ray spectroscopy (EDX) and electron spin resonance spectroscopy (ESR). The corresponding peaks of Zn, Cr and O were observed in the EDX spectrum of the sample which confirms the formation of ZnCr2O4. The XRD patterns also confirmed the formation of the single faced nanocrystallines of spinel ZnCr2O4 with a face-centered cubic structure. The average particle size of the synthesized crystals was also determined from the XRD patterns using the Scherers formula which shows that the crystallite sizes increases with increase in calcination temperature and was in good agreement with the TEM images which shows cubical ZnCr2O4 nanocrystals with uniform morphology and particle size distributions. The ESR spectra confirmed the existence of unpaired electron in the fabricated samples and the increase in g-factor and decreases in resonant magnetic field (Hr) were observed as the calcination temperature increases.

Hydrothermal synthesis, structural and impedance studies of nanocrystalline zinc chromite spinel oxide material

Zinc chromite (ZnCr2O4) nanocrystalline spinel material was synthesized through hydrothermal method by using zinc nitrate 6-hydrate and chromium nitrate 9-hydrate as precursors. The synthesized material was characterized for phase identification, crystallinity and surface morphology by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD results showed that hydrothermally as-synthesized products remained poorly crystalline up to 300°C. Zinc chromite cubic spinel structure developed after calcination at 600°C for 4h. Formation of single-phase cubic structure of ZnCr2O4 was also confirmed by Rietveld refinement study with lattice parameter a=8.2874Å, and Fd3m space group. Thermal stability of the developed material was observed by thermogravimetry (TG) and differential thermal analysis (DTA). Thermal study reveals that ZnCr2O4 is thermally stable above 700°C. The Fourier transform infrared (FTIR) spectra show the two absorption bands of Cr–O and Zn–O at 490 and 616cm−1 respectively. These bonds were associated with ZnCr2O4 and indicate the formation of cubic spinel ZnCr2O4 material. The band gap energy of ZnCr2O4 powder was determined by absorption spectroscopy in ultraviolet–visible range and was found to be 3.771eV and 3.147eV for direct and indirect band gap respectively. SEM images show well-faceted crystals of ZnCr2O4, with the grain size of 50–80nm. An equivalent circuit model (R1Q1)(R2Q2)(R3Q3) was employed to explain three relaxation processes associated with bulk and grain boundaries in ZnCr2O4 nanoparticles, and electrodes. Conductivity of the nanoparticles increased with frequency, and dielectric constant of the material showed dispersion at comparatively lower frequencies. At higher frequencies, the dielectric constant remained independent of frequency as attributable to the atomic and electronic polarizations.

Investigation on the influence of dichromate ion on the ZnO nano-dumbbells and ZnCr2O4 nano-walls

The effect of in situ addition of K2Cr2O7 in controlling morphology, composition, structural and optical properties of ZnO nanostructures prepared by hydrothermal technique have been investigated. The effect of increasing dichromate ion concentration on the size and shapes of the ZnO nanostructures and spinel ZnCr2O4 nanowalls has been studied using field emission scanning electron microscopy and X-ray diffractometer. Further the optical properties of these nanostructures were investigated using photoluminescence spectroscopy, which showed emission in UV and visible regions. From diffused reflectance spectroscopic analysis the direct band gap of ZnO nanodumbbells and ZnCr2O4 nanowalls was estimated to be 3.27 and 3.7 eV respectively. The observed results have been explained on the basis of anisotropic growth of ZnO nanostructures in the presence of dichromate ion. Thus, we report a method to control the shape and dimension of nanostructures by varying concentration of dichromate ion, the reported nanostructures have tremendous potential for future development of ZnO based optoelectronic devices.

Effect of Cr-doping on dielectric, electric and magnetic properties of Zn0.96Cu0.04O nanopowders

Cr doped Zn0.96Cu0.04O nanopowders with different Cr concentrations from 0% to 4% were successfully synthesized by a simple co-precipitation method. X-ray diffraction patterns revealed that all the samples had hexagonal crystal structure and co-doping could not alter its hexagonal structure. The dielectric dispersion and dielectric loss were increased with Cr concentrations due to the increase of charge carrier density, dipoles and vacancies within the grains. At higher frequency, the dielectric loss was found to be low because of the inhibition of domain wall motion. The low dielectric constant and dielectric loss at high frequency region makes it to be a suitable material for high frequency applications. The conductivity was increased by Cr-doping due to the enhanced transition of charge carriers between the grains. The strong suppression of ultraviolet emission at higher Cr revealed the promotion of non-radiative recombination. A broad defect-related visible luminescence band gave evidence for an increase in optically active deep level defects with increased Cr doping level. The combined effect of intrinsic defects and exchange interactions involving Cr ions substituted for Zn in the Zn-Cu-O lattice were responsible for the observed room temperature ferromagnetism. The saturation magnetization of the Cu doped ZnO was decreased by Cr co-doping, which occurred due to the antiferromagnetic coupling of neighboring Cu-Cu ions and Cr-Cr ions. The segregation of spinel ZnCr2O4 phase is responsible for the enhanced coercivity at higher Cr concentration (4%).

Tuning of energy gap, microstructure, optical and structural properties of Cr doped Zn0.96Cu0.04O nanoparticles

Zn0.96−xCu0.04CrxO nanoparticles with different Cr concentrations from 0 to 4% were successfully synthesized by co-precipitation method. The X-ray diffraction pattern showed the crystalline nature with hexagonal wurtzite structure. The diminution in activation energy and the enhanced strain by Cr-doping from 0 to 3% reduced the average crystal size from 27 to 23.8nm while the formation of secondary phase such as spinel ZnCr2O4 enhanced it to 25.6nm after Cr=3% via increasing the activation energy. The solubility limit of Cr in the present system is fixed as 3%. The energy dispersive X-ray spectra confirmed the presence of Cu and Cr in Zn-O. The optical absorption spectra showed the three distinct bands corresponding to UV region (305–324nm) due to electronic transition between the bands; blue region (382–386nm) mainly due to the Zn/Cr interstitials and bluish green (481–482nm) region due oxygen vacancies and interstitials. The noticed poor transmittance throughout the region and the broader absorption bands in Cr=1% doped Zn0.96Cu0.04O confirmed that it had more defects such as oxygen vacancies, Zn interstitials and other defects. The blue shift of energy gap from 3.68eV (Cr=0%) to 3.71eV (Cr=1%) was due to the creations of more defects. The red shift of Eg from 3.71eV (Cr=1%) to 3.6eV (Cr=3%) was explained by sp–d exchange interactions between the band electrons and the localized d-electrons of the Cr3+ ions. Presence of chemical bonding was confirmed by FTIR spectra.

Crystal structures of spin-Jahn–Teller-ordered MgCr2O4 and ZnCr2O4

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr2O4 and ZnCr2O4 is accompanied by a structural change that helps to relieve the frustration. Analysis of high-resolution synchrotron x-ray scattering reveals that the low-temperature structures are well described by a two-phase model of tetragonal I41/amd and orthorhombic Fddd symmetries. The Cr4 tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I41/amd phase. The spin-Jahn–Teller distortion is approximately one order of magnitude smaller than is observed in first-order Jahn–Teller spinels such as NiCr2O4 and CuCr2O4. In analogy with NiCr2O4 and CuCr2O4, we further suggest that the precise nature of magnetic ordering can itself provide a second driving force for structural change.

Preparation and electrical properties of nanostructured spinel ZnCr2O4 by combustion route

Semiconductive nanometer-sized ZnCr2O4 material synthesized by a solution combustion method using inorganic reagents as Zn(NO3)3·6H2O, Cr(NO3)3·6H2O, and glycine as a fuel. This process is a convenient, environment friendly, inexpensive and efficient for the preparation of ZnCr2O4 nanomaterial. The synthesized material is characterized by thermo gravimetric differential analysis, X-ray diffraction studied, energy dispersive X-ray microanalysis studies and transmission electron microscopy techniques. The increase in conductivity with increasing temperature could be attributed to negative temperature coefficient of resistance and semiconducting nature of ZnCr2O4.

Oxidation and Thermal Shock Behavior of a Glass-Alumina Composite Coating on K38G Superalloy at 1000 °C

The glass-alumina composite coatings were successfully prepared on the K38G superalloy substrates. Their isothermal oxidation and thermal shock behavior at 1000 degrees C were characterized. With a post-annealing process at 850 degrees C, the composite coatings possessed an improved protective effect for the alloy substrates from isothermal oxidation and a higher resistance to thermal shock. Crystallization from the glass matrix and interfacial reaction between the matrix and alumina inclusions, which caused the composites more refractory and tough, accounted for this improvement. The micromechanisms for the formation of oxidation results of spinel ZnCr2O4 were also discussed.

Electrospun PAN nanofiber process control by ionic liquids and electromagnetic wave absorbing properties

Polyacrylonitrile (PAN) fibers involving Cr(NO3)3/Zn(CH3COO)2/IL additives were prepared as an electromagnetic interference shielding material by electrospinning. Adding an IL (ionic liquid) as an electrolyte to PAN solutions has been shown to facilitate the electrospinning process and significantly reduce electrospun fiber sizes. The influence of two different ILs, [C18 (mim)]+ Br− and [C18 (mim)]+ HSO4−, on electrospun fiber sizes was investigated. It was observed that the electrospun fiber sizes were strongly influenced by the type of anion. Epoxy resin-based one-layer composite coatings 4mm thick were prepared employing electrospun fibers as absorbents in the matching and absorption layer. The spinel ZnCr2O4 nanofibers obtained after annealing at 600°C for 3h, exhibited markedly lower reflection loss than was obtained non-calcined samples. The microwave absorption properties were measured by a network vector analyzer working over a 5–12GHz band.

First-principles study of the structural, mechanical and electronic properties of ZnX2O4 (X=Al, Cr and Ga)

This paper performs first-principles calculations to study the structural, mechanical and electronic properties of the spinels ZnAl2O4, ZnGa2O4 and ZnCr2O4, using density functional theory with the plane-wave pseudopotential method. Our calculations are in good agreement with previous theoretical calculations and the available experimental data. The studies in this paper focus on the evolution of the mechanical properties of ZnAl2O4, ZnGa2O4 and ZnCr2O4 under hydrostatic pressure. The results show that the cubic phases of ZnAl2O4, ZnGa2O4 and ZnCr2O4 become unstable at about 50 GPa, 40 GPa and 25 GPa, respectively. From analysis of the band structure of the three compounds at equilibrium volume, it obtains a direct band gap of 4.35 eV for ZnAl2O4 and 0.89 eV for ZnCr2O4, while ZnGa2O4 has an indirect band gap of 2.73 eV.

Electronic Structures and Optical Properties of Spinel ZnCr2O4

Electronic Structures and Optical Properties of Spinel ZnCr2O4

Fabrication of nanospinel ZnCr2O4 using sol–gel method and its application on removal of azo dye from aqueous solution

For the first time, nanoparticles of zinc chromite, spinel ZnCr2O4 have been fabricated by the thermal decomposition of Zn–Cr gel prepared by sol–gel method in the presence of oxalic acid as a chelating agent. It was shown that the well-crystallized spinel structure is formed after calcination at 450°C. The nanospinel has been characterized by differential thermal analysis (DTA), X-ray powder diffraction (XRD), infrared spectroscopy (IR), and transmission electron microscope (TEM). The average particle size is approximately 13nm according to the TEM image. The nanoparticles of zinc chromites showed excellent adsorption properties towards reactive dye, reactive blue 5 (RB5). The adsorption studies have been carried out for contact time, different pH values, different temperatures, and adsorbent doses. The investigation of removal kinetics of RB5 indicates that the removal process obeys the rate of second-order kinetic equation. The results indicate that the Langmuir adsorption isotherm fitted the data better than the Freundlich. Also, the photocatalytic degradation of RB5 using spinel ZnCr2O4 under UV irradiation at pH=1 has been also examined. The results showed that the degradation of RB5 dye follows merely an adsorption process.