The microwave-assisted synthesis and characterization of Zn1 − xCoxO nanopowders

In this paper, we present a simple microwave-assisted synthesis of Zn_1 − xCo_xO nanopowders. With the advantages of the microwave-assisted method, we have successfully synthesized good crystalline quality and good surface morphology Zn_1 − xCo_xO nanopowders. The nanopowders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-VIS absorption, and micro-Raman spectroscopy. We found, in the synthesis process, the surfactant Triethanolamine (TEA) plays an important role on the morphology of Zn_1 − xCo_xO nanoparticles. The XRD study shows that for Co doping up to 5%, Co²⁺ ions are successfully incorporated into the ZnO host matrix. The absorption spectra of Zn_1 − xCo_xO (x = 1-5%) nanopowders show several peaks at 660, 611 and 565 nm, indicating the presence of Co²⁺ ions in the tetrahedral sites. The Raman study shows that the linewidth of E₂^low mode increases with Co concentration, which further indicates the incorporation of Co²⁺ ions into the ZnO host matrix.     

Mössbauer and magnetic studies of Mg1+2xSbxFe2−3xO4 spinel ferrites

Spinel-related Mg_1+2xSb_xFe_2−3xO₄ samples (x = 0.0, 0.05, 0.10, 0.15, 0.20, and 0.30) prepared using the conventional double sintering technique were investigated using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra favor a cationic distribution of the form (Mg_δFe_1−δ)^A[Mg_1+2x−δSb_xFe_1+δ−3x]^BO₄ among the tetrahedral-A and octahedral-B sites of the spinel structure. The cation distribution parameter (δ) was found to vary with the Sb⁵⁺ concentration (x). The Mössbauer hyperfine magnetic fields at both sites and the Curie temperatures of the ferrites decrease as x increases. This was attributed to gradual weakening in the magnetic exchange interaction as more Fe³⁺ ions are substituted by diamagnetic Sb⁵⁺ and Mg²⁺ ones. The sample with x = 0.30 exhibits short range magnetic order due to cationic clustering and/or superparamagnetism. The magnetization of all samples was found to be temperature-dependent implying that δ depends on temperature in addition to x. At low temperatures the substituted ferrites (x ≠ 0.0) unexpectedly exhibit higher magnetization values relative to that of the pure ferrite MgFe₂O₄. This behavior, while at variance with the Néel's model for ferrimagnetism, is explicable in terms of the spin canting mechanism proposed in the Yafet–Kittel model.     

Studies on the structure and gas sensing properties of nickel–cobalt ferrite thin films prepared by spin coating

The influence of Co²⁺ ions content on structure and sensing properties of Ni_1−xCo_xFe₂O₄ (x = 0.25, 0.5, 0.75) thin films deposited on glass substrates by spin coating is presented. Structural characterization evidenced thin films with cubic spinel structures and morphologies dependent on cobalt content. Repartition of cations in spinel tetrahedral and octahedral sites was determined and was found that the presence of Co²⁺ ions in octahedral sites favor the formation of Fe²⁺ species. The sensitivity to some reducing vapor gases (acetone, liquefied petroleum gas LPG, ethyl alcohol and methyl alcohol) was investigated and was found that thin films with x = 0.75 exhibit high sensitivity to ethyl alcohol and thin films with x = 0.25 have high sensitivity to acetone. This sensitivity largely depends on the temperature and test gas concentration and was related to the Fe²⁺ species formed in octahedral sites.     

Spectroscopic study of Zn1−xCoxO thin films showing intrinsic ferromagnetism

Dilute magnetic semiconductors are widely studied due to their potential applications in spin-resolved electronics. We report the direct evidences of intrinsic ferromagnetism in the primarily ferromagnetic ZnO:Co thin films using near-edge X-ray absorption fine structure (NEXAFS) and soft X-ray magnetic circular dichroism (XMCD). The single phase Zn_1−xCo_xO thin films with nominal compositions (0.00 ≤ x ≤ 0.15) were synthesized by a spray pyrolysis technique, which exhibit room temperature ferromagnetism as revealed by alternating gradient force magnetometer (AGFM) measurements. The spectroscopic measurements indicate that most of Co dopants have substituted for Zn sites in ZnO matrix and they are present in divalent Co²⁺ (d⁷) state with tetrahedral symmetry according to the atomic multiplet calculations. The O 1s NEXAFS spectra suggest strong hybridization between O 2p and Co 3d electrons within ZnO matrix. The Co 2p XMCD measurements rule out the magnetism due to the presence of Co clusters, and show that Co–O–Co bonding provides localized magnetic moments leading to ferromagnetism.     

Characterizations of ferromagnetic Zn1−xCoxO thin films grown on Al2O3 (0001) by reactive radio-frequency magnetron sputtering coupled with post-growth annealing

High-quality ferromagnetic Zn_1−xCo_xO thin films were deposited on a sapphire (0001) substrate at 600 °C by using reactive radio-frequency magnetron sputtering coupled with post-annealing treatment for 1 h at 580 °C under an Ar atmosphere. High resolution X-ray diffraction patterns show that hexagonal wurzite crystal structures of undoped ZnO film were maintained even after Co doping up to 4.5 at.% without forming Co clusters or oxides. X-ray photoelectron spectroscopy spectra represent the energy difference of 15.42 eV between Co2p_3/2 and Co2p_1/2, which is different from 15.05 eV of Co clusters. The characteristic absorption bands near 658, 616, and 568 nm wavelengths out of UV-VIS-IR spectroscopy spectra are correlated with the d-d transitions of tetrahedrally coordinated Co²⁺ ions. The low temperature photoluminescence spectrum for undoped ZnO shows a strong near-band edge (NBE) emission peak of 3.42 eV without deep level emission peaks. But, Co content increases in Zn_1−xCo_xO film, the NBE emission peak intensity decreases and another emission peak at 3.37 eV as well as a broad green emission peak at around 2.5 eV starts to appear with larger intensity due to the more actively creating oxygen vacancies. The emission peak at 3.37 eV proves the interaction between Co ions and the hydrogenic electrons in the impurity band and also supports the typical ferromagnetic hysteresis curves obtained by superconducting quantum interface device magnetometry at 300 K for Zn_1−xCo_xO films. High insulator characteristics are observed for as-grown Zn_1−xCo_xO films whereas it exhibits n-type characteristics with the increased carrier concentration, mobility, and resistivity after post-growth annealing. The spintronic devices could be fabricated with the utilization of Zn_1−xCo_xO films grown by the economically feasible reactive radio-frequency magnetron sputtering coupled with the post annealing treatment.     

Single domain limit for NixCo1−xFe2O4 (0 ≤ x ≤ 1) nanoparticles synthesized by coprecipitation route

Ni_xCo_1−xFe₂O₄ (0 ≤ x ≤ 1) nanoparticles (sizes: 8–52 nm) were synthesized by chemical coprecipitation route. Single domain limit (d_sdl) for the nanoparticles, determined from the coercivity (H_C) versus particle-size curves, was explored as a function of nickel concentration (x). The coercivity of the particles attains a peak value at d_sdl, and it was found that coercivity decreases linearly with increasing nickel concentration in the samples. The saturation magnetization (M_S) and blocking temperature (T_b) of the system show increasing trends with increasing cobalt concentration in the nanoparticles.     

Synthesis and characterizations of Nd doped SrFe12O19 nanoparticles

This is the first report ever on Nd³⁺ doped M-type hexaferrite nanoparticles: SrNd_xFe_12−xO₁₉ (0 ≤ x ≤ 1) prepared by citrate precursor using the sol–gel technique followed by gel to crystallization. The influence of the Nd³⁺ substitution, Fe³⁺/Sr²⁺ molar ratio and the calcination temperature on the crystallization of ferrite phase have been examined using powder X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), inductance capacitance resistance meter bridge (LCR) and vibrating sample magnetometer (VSM). The structural analysis reveals that the Nd³⁺ ions rearrange themselves in the host lattice without disturbing the parent lattice and Fe³⁺/Sr²⁺ molar ratio less than 12 is more favorable to achieve single phase hexaferrite at calcination temperature 900 °C for 4 h. Mid-IR analysis confirms that Nd³⁺ occupies the octahedral site. Detailed studies of electrical properties of prepared materials have been investigated in the frequency range 100–1000 Hz at room temperature by LCR meter and two probe technique. The result shows that the electrical properties strongly depend upon the frequency of applied field and dopant concentration. The magnetic measurements showing a considerable improvement in coercivity with the substitution of Nd³⁺ on iron sites, while the unsubstituted hexaferrites have highest value of specific saturation magnetization.     

Synthesis of nanosized (Zn1−xCox)Al2O4 spinels: New pink ceramic pigments

The Zn_1?xCo_xAl₂O₄ system has been reported to show a blue color with particles sizes that range from 36 nm to 30 μm. However, we have observed a pink color in this system with nanoparticles of an average size of 10 nm.Such differences in the optical properties of this nanosystem are related to the distribution of the cobalt cations in octahedral sites rather than in tetrahedral sites as in standard cobaltite spinels.The solid solution Zn_1?xCo_xAl₂O₄ (x = 0.2, 0.4, 0.6, 0.8 and 1) was synthesized by a co-precipitation reaction from the metallic salts and subsequent calcinations.The color properties and the high thermal stability (1400 °C) of these nanostructures suggest that they have the potential to be applied as satisfactory ceramic pigments.     

Studies on LiNi0.7Al0.3−xCoxO2 solid solutions as alternative cathode materials for lithium batteries

A series of phase-pure Co- and Al-substituted lithium nickel oxide solid solutions of the composition LiNi_0.7Al_0.3−xCo_xO₂ with x=0.0, 0.1, 0.15, 0.2, and 0.3, has been synthesized by adopting urea-assisted combustion (UAC) route. The structure and the physico-electrochemical features of the doped materials have been evaluated through PXRD, FTIR, SEM, CV, and charge/discharge studies. The stabilization of Ni in the +3 state and the existence of enhanced 2D-layered structure without any cation mixing have been substantiated from XRD. The results of the XRD and FTIR studies have established the complete mixing of Al and Co with Ni, especially at the various levels and the combinations of the dopants attempted in the present study. The enhanced electrochemical performance of LiNi_0.7Al_0.3−xCo_xO₂ may be attributed to the “synergetic effect” resulting from the presence of both Al³⁺ and Co³⁺ dopants in the LiNiO₂ matrix. From CV studies, it was understood that the addition of 10% Co is effective in suppressing the phase transformation during Li⁺ intercalation process that leads to better electrochemical properties. The effect and the extent of substitution of Ni with Al and Co on the structural and electrochemical performance of LiNi_0.7Al_0.3−xCo_xO₂ are discussed elaborately in this communication.     

NMR study of the cation distribution in the systems Li1+5xTa1−xO3 and Li1+xTa1−xTixO3

A ⁷Li NMR study of members of the solid-solution systems Li_1+5xTa_1?xO₃ and Li_1+xTa_1?xTi_xO₃ has indicated that the excess Li⁺ ions occupy interstitial tetrahedral sites in the former. In the latter system, the Li⁺ ions appear to occupy interstitial tetrahedral sites for small values of x, but mostly octahedral sites for x > 0.1. Defect-cluster models are proposed that rationalize these findings as well as the evolution with x of the ferroelectric Curie temperature.     

Aluminum‐Tailored Energy Level and Morphology of Co3−xAlxO4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation

Tuning energy levels plays a crucial role in developing cost‐effective, earth‐abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom‐occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom‐engineered energy levels of spinel Co₃O₄ for highly‐effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co²⁺ ions in the tetrahedral sites and Co³⁺ ions in the octahedral sites (denoted as Co²⁺_Td and Co³⁺_Oh, respectively). Experimental and theoretical simulations demonstrate that Al³⁺ ions substituting Co²⁺_Td and Co³⁺_Oh active sites, especially Al³⁺ ions occupying the Co²⁺_Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co_1.75Al_1.25O₄ nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm^–2, among the best Co‐based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al‐occupied different Co sites in Co_3–xAl_xO₄ composites on OER performance, but also inspire the design of low‐cost, earth‐abundant, and high‐active electrocatalysts toward water oxidation.     

Role of additives; sodium dodecyl sulphate and manganese chloride on morphology of Zn1−xMnxO nanoparticles and their photoluminescence properties

In the present study Zn_1−xMn_xO (x = 0, 0.05 and 0.1) nanoparticles (NPs) have been synthesised in aqueous solution phase at mild reaction temperature 100 °C in moderate alkaline medium (pH = 9.5), and the role of external additives; like sodium dodecyl sulphate and manganese chloride on the morphology and size of the products has been explored on the basis of transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectral analyses data. ZnO hexagonal nano-plates, core–shell like spherical/ellipsoidal Zn_0.95Mn_0.05O structures and thin sheets, thorn/needle mixed shaped Zn_0.9Mn_0.1O structures have been observed in TEM and SEM images. Zn(OH)₂ formed in moderate alkaline medium, converted to Zn(II) hydroxo complex ions on dissolution, which further recrystallizes to produce wurtzite ZnO at 100 °C. From XRD and EDX analysis, successful doping of Mn²⁺ ions at the Zn²⁺ sites in ZnO host has been proved. In the photoluminescence spectra, the observed blue shifts in NBE peaks and decrease of emissions intensity on Mn doping have thoroughly been discussed in the present investigation.     

Structural study of CdxNi1−xAl2O4 spinels

NiAl₂O₄ and several mixed spinels Cd_xNi_1?xAl₂O₄ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, CdO and Al₂O₃ at 1273 K. X-ray diffraction analysis was used to determine the lattice parameter, a_O, oxygen parameter, u, and cation distribution for each spinel. The lattice parameter was found to increase continuously from 804.9 pm for x = 0 up to 824.6 pm for x = 0.65. The u-parameter also increases, along the same composition range, from 0.380 to 0.391. A solubility limit for Cd²⁺ in the spinel phase is reached at x = 0.68 (at 1273 K). Cd²⁺ was found to occupy almost exclusively tetrahedral sites in all cases, whilst Ni²⁺ showed a strong octahedral preference.     

Compositionally controlled rhombohedral to cubic phase transition in CsTe2−xWxO6 related to pyrochlore structure

CsTe₂O₆ adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te⁴⁺ and Te⁶⁺ in the octahedral sites respectively. Phases of the type CsTe_2−xW_xO₆ were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te⁴⁺ and Te⁶⁺ (mixed with W⁶⁺) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.     

Crystallographic and magnetic properties of the spinel type solid solution Zn0.4Co0.6AlxFe2−xO4 (0≤x≤1)

The spinel system Zn_0.4Co_0.6Al_xFe_2−xO₄ (x=0.0, 0.25, 0.50, 0.75, and 1.0) has been prepared in air at 1300 °C by standard solid state sintering method. X-ray and neutron powder diffraction measurements have been performed to characterize the materials. The crystal structure of the system has been found out by refining neutron diffraction data and cubic symmetry corresponding to the space group Fd3m has been confirmed for all the samples of the series. Zn and Al ions exclusively enter into the tetrahedral (A) and octahedral (B) sites, respectively, while Co and Fe ions are distributed over both the A and B sites for the whole compositional range investigated. With increasing x, the occupation of Co gradually increases in A site and that in B site decreases, and the Fe ions gradually decrease in both the sites. The lattice constant decreases and the oxygen position parameter increases with increasing Al content in the system. The moment distributions in the two sublattices for different compositions have also been determined. The magnetization measurements show that the saturation magnetization and the Néel temperature decreases with increasing Al contents and the coercivity decreases initially and then increases when x>0.75.     

Dependence of optical properties on the composition of (Ba1−x−ySrxEuy)Si2O2N2 phosphors for white light emitting diodes

BaSi₂O₂N₂: Eu²⁺ is an efficient phosphor because of its high quantum yield and quenching temperature. Partial substitution of Ba²⁺ by Sr²⁺ is the most promising approach to tune the color of phosphors. In this study, a series of (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ (x = 0.0–0.97, y = 0.00–0.10) phosphors are synthesized via high-temperature solid-state reactions. Intense green to yellow phosphors can be obtained by the partial substitution of the host lattice cation Ba²⁺ by either Sr²⁺ or Eu²⁺. The luminescent properties and the relationships among the lowest 5d absorption bands, Stokes shifts, centroid shifts, and the splitting of Eu²⁺ are studied systematically. Then, based on (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors and near-ultraviolet (∼395 nm)/blue (460 nm) InGaN chips, intense green–yellow light emitting diodes (LEDs) and white LEDs are fabricated. (Ba_0.37Sr_0.60)Si₂O₂N₂: 0.03Eu²⁺ phosphors present the highest efficiency, and the luminous efficiency of white LEDs can reach 17 lm/w. These results indicate that (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors are promising candidates for solid-state lighting.     

Magnetic softness and interparticle exchange interactions of (Fe65Co35)1 − x(Al2O3)x (x = 0-0.50) nanogranular films

The effect of Al₂O₃ content on the structure, electrical properties, magnetic properties, and interparticle exchange interactions of (Fe₆₅Co₃₅)_1 − x(Al₂O₃)_x films with Al₂O₃ volume fractions x ranging from 0 to 0.50 was systematically investigated. Among the films with x between 0 and 0.25, the lowest coercivity of 0.56 kA/m was achieved in the (Fe₆₅Co₃₅)_0.82(Al₂O₃)_0.18 film. This is ascribed to the strongest exchange interactions between the Fe₆₅Co₃₅ nanoparticles in this film. Combined with the microstructure analysis of the (Fe₆₅Co₃₅)_1 − x(Al₂O₃)_x films, the modified Herzer's model was extended to interpret the variation of the coercivity with x and analyze the effect of the exchange interactions between the Fe₆₅Co₃₅ nanoparticles on the magnetic softness. The remanence curves confirm the existence of the exchange interactions and reveal the evolution of the exchange interaction strength with Al₂O₃ content.     

Structural,transport and spectroscopic properties of Ti substituted magnetite: Fe3−xTixO4

The effect of Ti⁴⁺ ion on the formation of magnetite, which were prepared by solid-state route reaction method, were studied by resistivity, Raman and ⁵⁷Fe Mössbauer spectrometry. Resistivity measured in the range of 10 < T < 300 K for Ti⁴⁺ magnetite Fe_3−xTi_xO₄ exhibit first order phase transformations at the Verwey transition T_v for Fe₃O₄, Fe_2.98Ti_0.02O₄ and Fe_2.97Ti_0.03O₄ at 123 K, 121 K and 118 K, respectively. No first order phase transition was observed for Fe_2.9Ti_0.1O₄ and small polaron model retraces the semiconducting resistivity behavior with activation energy of about 72 meV. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Ti doping. The Raman active mode for Fe_2.9Ti_0.1O₄ at ≅634.4 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅670 cm⁻¹, inferring that Mn²⁺ ions are located mostly on the octahedral sites. ⁵⁷Fe Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Ti⁴⁺ ions are located mostly on the octahedral sites of the Fe_3−xTi_xO₄ spinel structure.     

Luminescent properties of Eu-activated (Sr1−z, Caz)(Al1−y, By)2O4 phosphors for UV LEDs

A series of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺ phosphors were synthesized by the sol–gel process and were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and photoluminescence (PL) excitation and emission spectra. The experiment results revealed that the highest intensity of Sr(Al_1.98, B_0.02)O₄:Eu²⁺ phosphor with pure monoclinic SrAl₂O₄ was achieved by annealing at the temperature of 1200 °C and the Eu²⁺ content of 8 mol%. However, when the post-treatment temperature for Sr(Al_1.98, B_0.02)O₄: Eu²⁺ was over 1200 °C, the Sr₄Al₁₄O₂₅ phase appeared as a minor phase, inducing small blue-shift in the emission peak (520–509 nm). Doping higher content of B³⁺ (y = 0.02–0.40) into SrAl₂O₄:Eu²⁺ at 1200 °C resulted in the transformation of phase from SrAl₂O₄ to Sr₄Al₁₄O₂₅ as well as to SrB₂Al₂O₇, which made the emission intensity enhance and the emission shift to a much shorter wavelength region (λ_p = 467 nm). It was found that, instead of purely using Sr atoms, Ca atoms with content of 20–40% could induce the crystal structure of (Sr_1−z, Ca_z)(Al_1−y, B_y)₂O₄:xEu²⁺, which led to SrAl₂O₄ from monoclinic to hexagonal phase. As a result, SrAl₂O₄ solid solution was obtained and then SrAl₂O₄:Eu²⁺ to emit 518 nm green light. At higher Ca content (z > 40%), a new CaAl₂O₄ solid solution was formed and a blue emission of CaAl₂O₄:Eu²⁺ was obtained.