Effects of Cd concentration on structure and optical properties of the ternary Zn1−xCdxO nanopowder prepared by sol–gel method

Zn_1−xCd_xO nanocrystalline powder with different Cd contents (0≤x≤1) has been prepared by new facile sol–gel route. The crystal structure and optical properties were investigated by X-ray diffraction patterns, Transmission electron microscope, X-ray photoelectron spectroscopy, Photoluminescence. As x varied from x=0 to 0.25, the Zn_1−xCd_xO nanopowder exhibits a hexagonal wurtzite structure of pure ZnO without any significant formation of a separated CdO phase. For the samples with 0.5≤x≤0.85, the Zn_1−xCd_xO nanopowder exhibits the coexistence of hexagonal ZnO and cubic CdO phase, meanwhile, the content of ZnO phase decreases while that of CdO increases with increasing the Cd content x. The ultra-violet near-band-edge emission of the Zn_1−xCd_xO nanopowder was monotonously red-shifted from 389 nm (x=0) to 406 nm (x=0.25) due to the direct modulation of band gap caused by Cd substitution.     

Synthesis and magnetic properties of Ni-doped zinc oxide powders

Polycrystalline Zn_1−xNi_xO diluted magnetic semiconductors have been successfully synthesized by an auto-combustion method. X-ray diffraction measurements indicated that the 5 at% Ni-doped ZnO had the pure wurtzite structure. Refinements of cell parameters from powder diffraction data revealed that the cell parameters of Zn_0.95Ni_0.05O were a little bit larger than ZnO. Transmission electron microscopy observation showed that the as-synthesized powders were of the size ∼60 nm. Magnetic investigations showed that the nanocystalline Zn_0.95Ni_0.05O possessed room temperature ferromagnetism with the saturation magnetic moment of 0.1 emu/g (0.29 μ_B/Ni²⁺).     

Synthesis and photoluminescence properties of YVO4:Eu3+, Al3+ phosphor

The Y_0.95?xAl_xVO₄:5%Eu³⁺ (0≤x≤0.1) phosphors were successfully synthesized by solid state reaction at 900 °C for 6 h, and their luminescence properties were investigated under UV and VUV excitation. Monitoring at 619 nm, a strong broad absorption was enhanced by co-doping of Al³⁺ into the YVO₄:Eu³⁺ lattices at 256 nm under UV excitation. The VUV excitation spectra also showed the enhanced excitation bands at about 156 and 200 nm. Under 254 or 147 nm excitation, it was found that Y_0.95?xAl_xVO₄:Eu³⁺(0≤x≤0.1) phosphors showed strong red emission at about 619 nm corresponding to the electric dipole ⁵D₀^–7F₂ transition of Eu³⁺. The improvement of luminescence intensity of YVO₄:Eu³⁺ was also observed after partial substituting Y³⁺ by Al³⁺ and the optimal luminescence intensity appeared with incorporation of 2.5 mol% Al³⁺.     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.     

Influence of V2O5 ion addition on the conductivity and grain growth of Ni–Zn–Cu ferrites

Polycrystalline nickel–zinc–copper ferrites with chemical formula Ni_0.6+xZn_0.2Cu_0.2V_xFe_2−2xO₄,(0.0 ⩽ x ⩽ 0.25) were prepared by the ceramic route. The X-ray diffraction (XRD) analysis of the samples results confirms single-phase spinel structure. Scanning electron microscopy (SEM) of the prepared ferrites reveal that vanadium addition resulted in a rapid grain growth with large pores trapped inside the grains as the vanadium concentration increases. The ac conductivity σ_ac has been studied as a function of frequency and temperature over the temperature range (300–600 K). The results obtained for these materials reveal a semiconductor – to semimetal transition as V⁵⁺ content increases. All studies composition exhibit a transition with change in the slope of conductivity. The obtained temperature T_c is found to be decrease with the increasing vanadium content. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as the hole hopping between Ni³⁺ and Ni²⁺ are found to responsible for the conduction mechanism. The relation of the universal exponent s with temperature gives evidence for the presence of the correlation barrier hopping (CHB) mechanism in these compounds. The impedance technique has been used to study effect of grain and grain boundary on the electrical properties. The analysis data show only one semi-circle for all samples except for sample with x = 0.05. The results suggested that the conduction mechanism takes place predominantly through the grain in the studied samples.     

Synthesis of Ni1−xZnxO hollow structures by a facile method

By a sol–gel method, we obtained Ni_1−xZn_xO polycrystalline powders. The obtained Ni_1−xZn_xO was characterized by scanning electron microscope and transmission electron microscope techniques. The characterized results showed that the obtained Ni_1−xZn_xO showed hollow structure. The grain sizes of the obtained hollow Ni_1−xZn_xO were quite uniform, typically in the range of 400–500 nm. This method is a simple and effective method for large-scale synthesis of hollow Ni_1−xZn_xO quasi-spheres. The present method is very promising for large-scale production because the method is only involves commercial Zn(AC)₂·2H₂O and NiCl₂·6H₂O powders and the reaction is achieved in an open system free of autoclave and organic chemical reagents. This template-free method is facile but effective and therefore it is very promising for large-scale industrial production.     

Influence of Cu doping on the microstructure,optical properties and photoluminescence features of Cd0.9Zn0.1S nanoparticles

Cd_0.9−xZn_0.1Cu_xS (0≤x≤0.06) nanoparticles were successfully synthesized by a conventional chemical co-precipitation method at room temperature. Crystalline phases and optical absorption of the nanoparticles have been studied by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirms the phase singularity of the synthesized material, which also confirmed the formation of Cd–Zn–Cu–S alloy nanocrystals rather than separate nucleation or phase formation. Elemental composition was examined by the energy dispersive X-ray analysis and the microstructure was examined by scanning electron microscope. The blue shift of absorption edge below Cu=2% is responsible for dominance of Cu⁺ while at higher Cu concentration dominated Cu²⁺, d–d transition may exist. It is suggested that the addition of third metal ion (Cu²⁺/Cu⁺) is an effective way to improve the optical property and stability of the Cd_0.9Zn_0.1S solid solutions. When Cu is introduced, stretching of Cd–Zn–Cu–S bond is shifted lower wave number side from 678 cm⁻¹ (Cu=0%) to 671 cm⁻¹ (Cu=6%) due to the presence of Cu in Cd–Zn–S lattice and also the size effect. The variation in blue band emission peak from 456 nm (∼2.72 eV) to 482 nm (∼2.58 eV) by Cu-doping is corresponding to the inter-band radiation combination of photo-generated electrons and holes. Intensity of red band emission centered at 656 nm significantly increased up to Cu=4%; beyond 4% it is decreased due to the quenching of Cu concentration.     

Ferromagnetism in Zn1−xCrxTe (x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn_1−xCr_xTe (x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d² electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10¹² to 6.7 × 10¹² m⁻³ when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.     

Effect of Eu3+ concentration on microstructure and photoluminescence of Sr2SiO4:Eu3+ phosphors prepared by microwave assisted sintering

In this paper, effect of Eu³⁺ doping concentrations on microstructure and photoluminescence of Sr₂SiO₄ phosphors was investigated. The Sr_2?xSiO₄:xEu³⁺ phosphors with x=0.05, 0.1, 0.2, 0.3 were synthesized by microwave assisted sintering at 1200 °C for 60 min in air. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials irrespective of the adding amount of Eu³⁺. From scanning electron microscopy image, it is found that with more Eu³⁺ ions introduced to Sr₂SiO₄, the shape of the particles is not much different from each other, but the particle size decreases significantly from 1 to 2 μm (when x=0.05) to less than 500 nm (when x=0.3). The emission spectrum was located obviously at 617 nm as the excitation spectrum at λ_ex=395 nm, and it had best emission intensity when x=0.1.     

Influence of substrate temperature on physical properties of sprayed Zn0.85Mn0.15O films

Zn_1−xMn_xO thin films have been synthesized by chemical spray pyrolysis at different substrate temperatures in the range, 250–450 °C for a manganese composition, x = 15%, on corning 7059 glass substrates. The as-grown layers were characterized to evaluate their chemical and physical behaviour with substrate temperature. The change of dopant level in ZnO films with substrate temperature was analysed using X-ray photoelectron spectroscope measurements. The X-ray diffraction studies revealed that all the films were strongly oriented along the (0 0 2) orientation that correspond to the hexagonal wurtzite structure. The crystalline quality of the layers increased with the increase of substrate temperature up to 400 °C and decreased thereafter. The crystallite size of the films varied in the range, 14–24 nm. The surface morphological studies were carried out using atomic force microscope and the layers showed a lower surface roughness of 4.1 nm. The optical band gap of the films was ∼3.35 eV and the electrical resistivity was found to be high, ∼10⁴ Ω cm. The films deposited at higher temperatures (>350 °C) showed ferromagnetic behaviour at 10 K.     

The studies on structural and thermal properties of delithiated LixNi1/3Co1/3Mn1/3O2 (0 < x ≤ 1) as a cathode material in lithium ion batteries

The structural and thermal properties of the delithiated Li_xNi_1/3Co_1/3Mn_1/3O₂ (0 < x ≤ 1) material have been investigated by using diffraction and thermoanalytical techniques such as XRD and TG-DSC methods. XRD result shows that the delithiated materials maintain the O3-type structure with defined stoichiometric number at the range of 0.24 < x ≤ 1, exhibiting good crystal structural stability. The cobalt and nickel ions in the delithiated materials change their valence state (i.e. Co³⁺ to Co⁴⁺ and Ni³⁺ to Ni⁴⁺) when x < 0.49; the irreversible changes of the transformation may affect the first cycle of charge–discharge efficiency of the materials. A comparison of the results of TG-DSC with TPD-MS shows that the irreversible change of oxygen species during the delithiation process of Li_xNi_1/3Co_1/3Mn_1/3O₂ have great influence on the structural and thermal stability and reversibility of the materials.     

Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O^?) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al₂O₃ (C12A7) with a composition of (12 ? x)CaO·7Al₂O₃·xCaF₂ (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO?3Al₂O₃ (C5A3) and 3CaO?Al₂O₃ (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F^? ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O^? ions (～ 92%) together with a small amount of O₂^? ions (～ 4%) and F^? ions (～ 4%). The absence of an O₂ gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O^? ions and electrons with a ratio of < 1/1. The O₂ gas supply markedly enhanced the O^? ion emission, and suppressed the electron emission. A sustainable and high-purity O^? ion emission with a current density of 11 nA cm^? 2 was achieved at 830 °C with the supply of 40 Pa O₂ gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F^? ion-substituted C12A7 is a promising material for higher intensity O^? ion emission at higher temperatures.     

Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Paramagnetic properties of the spinel-type Al1−xMnx-sulphate salts for its pillared intercalation into a clay mineral

Paramagnetic interactions on Manganese ions substituted for Aluminium ions by x-mole fraction (x=0.01–0.1) for the spinel-type Al-complexes, [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺, were investigated. The superconducting quantum interference detection (SQUID) measurements showed a small susceptibility of the order of 10⁻⁶ emu/g originated from the Mn ion, and from the saturation value of the DC-magnetization for the 10 mol% Mn concentration, the spin S-value was determined as 2.8 close to its divalent state spin 5/2. The 9 GHz electron paramagnetic resonance (EPR) study revealed that the number of Mn²⁺ ions embedded in the axial crystal field had a characteristic maximum at the stoichiometric molar concentration Al/(Al+Al₁₂) equal to 7.69%, and the Infrared absorption (IR) experimental result verified this evidence. A theoretical approach was made separately for the concentrations below and above 7.69%. The higher-frequency 24 and 97 GHz EPR measurements examined effectively the result of the crystal field analysis in the 9 GHz, except for the small line splittings due to the higher-order Zeeman and hyperfine effects of the S-state ion. The intercalation of Al_1−xMn_x-sulphate (x=0.07) as a pillaring agent into a clay mineral, montmorillonite, was confirmed from the X-ray diffraction results.     

Novel clays: Solid-state synthesis,characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na₂Si₆Al₂Mg₆O₂₀F₄ · xH₂O (Na-2-mica), Na₃Si₅Al₃Mg₆O₂₀F₄ · xH₂O (Na-3-mica) and Na₄Si₄Al₄Mg₆O₂₀F₄ · xH₂O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water swollen micas with c-axis spacing of ∼1.2 nm except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state ²⁷Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state ²⁹Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr²⁺ or 8.12 ppm of La³⁺. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Formation of nickel and nickel,zinc-bearing ferrite in aqueous suspension by air oxidation

Stoichiometric Ni-bearing ferrite was formed by air oxidation of an iron(II) hydroxide suspension at an initial Ni : Fe_tot mol ratio (r_Ni) of 0.20 : 2.80 at pH 10.0 and 65°C. Most of products formed at r_Ni=0.40 : 2.60 and 0.60 : 2.40 were Ni-bearing ferrites, of which vacancies of Fe³⁺ ion on the lattice points may be considered. Only Ni, Zn-bearing ferrites were formed in the suspensions at initial (Ni + Zn)  : Fe_tot mol ratios (r_{Ni + Zn}) of 0.20 : 2.80–0.60 : 2.40 at pH 10.0 and 65°C. At higher r_Ni or r_{Ni + Zn} by-products containing Ni, Fe and O₄²⁻ were formed. The formation of the by-products was depressed in the suspensions containing chloride ions in the place of sulfate ions.     

Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

A study on the Raman spectra of Al-doped and Ga-doped ZnO ceramics

Al_xZn_1−xO and Ga_yZn_1−yO ceramics were synthesized through a solid-state reaction technique. The crystal phase of the samples was identified by an X-ray diffraction experiment. For each sample, the electrical resistivity was determined. The Al 2-mol%-doped and Ga 0.5-mol%-doped ZnO ceramics had the lowest resistivity. Raman measurement was performed to study the doping effects in the ZnO ceramics including ZnO single crystal as a reference. The line-shape parameters, q₁ and Γ₁, at the same certain doping rate and the solubility limit of Al (2 mol%) and Ga (0.5 mol%) in ZnO ceramics, are strongly related to the each other, and that the solubility limit plays an important role. The second-order Raman peak at 1162 cm⁻¹ of the ZnO ceramics was fitted by Fano formalism. The Fano’s fitting parameters, such as the lifetime of phonon and the degree of asymmetry degree of the second-order Raman peak changed as the amounts of dopants were varied.