Nanocrystalline Zn Research Articles

Structural, optical and electron paramagnetic resonance studies on Cu-doped ZnO nanoparticles synthesized using a novel auto-combustion method

Nanocrystalline Zn1 − x Cu x O (x = 0, 0.02, 0.04, 0.06, 0.08) samples were synthesized by a novel auto-combustion method using glycine as the fuel material. The structural, optical and magnetic properties of the samples were characterized using XRD, SEM, photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies. The XRD spectra of samples reveal the hexagonal wurtzite structures of ZnO. As the copper content increases, a diffraction peak at 2θ = 39° corresponding to secondary phase of CuO ([111] crystalline face) appears when x ⩽ 6 mol.%. PL spectra of the samples show a strong ultraviolet (UV) emission and defect related visible emissions. Cu-doping in ZnO can effectively adjust the energy level in ZnO, which leads to red shift in the emission peak position in UV region. The EPR spectra of Cu-doped ZnO nanoparticles show a distinct and broad signal at room temperature, suggesting that it may be attributed to the exchange interactions within Cu2+ ions.

Mechanical behaviour and underlying deformation mechanisms in coarse- and ultrafine-grained Zn over a wide range of strain rates

Spark plasma sintering was used to process bulk ultrafine-grained (average grain size of ∼250nm) samples from high purity nanocrystalline Zn powder. The microstructure of the consolidated samples was investigated and the mechanical behaviour was characterised by means of quasistatic and dynamic compression tests at room temperature and compared to that of coarse-grained counterparts. For both materials in the strain rate regime of 10−4–101s−1 a linear relationship between the logarithm of flow stress and strain rate values was observed since room temperature corresponds to a relatively high homologous temperature of 0.43. The strain rate sensitivity was higher for ultrafine-grained Zn (∼0.18) than for coarse-grained Zn (∼0.12) and twinning in the former sample was not observed in the entire strain rate range. For strain rates higher than 103s−1 the plasticity in coarse-grained Zn was controlled by dislocation drag but partial recrystallization was also observed. In the latter regime the mobile dislocation density was about 1011m−2. However, in ultrafine-grained Zn the relatively large dislocation density (∼1014m−2) and the small grain size limit the dislocation velocity yielding the lack of dislocation drag effects up to 104s−1.

Structural, optical and antibacterial properties of nanocrystalline Zn1–xLaxO compound semiconductor

The Zn1–xLaxO powders were synthesized by the planetary ball milling method. An accumulated milling time of 15h with a milling speed of 400rpm were found to be the optimum milling conditions. The crystal structure, morphology, selected area electron diffraction, and elemental analysis were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The solubility limit of La that could substitute at the Zn sites was only 5mol% or x=0.05. The dependence of the crystallite size on the La doping content can be explained by the Zener pinning effect. The particle size of the milled ZnO powder was about 34.03nm and the particle size was reduced to 30.90nm when doped with 10mol% La. The particles of all milled samples could agglomerate as a cluster. The largest Eg value of 3.14eV was obtained from the Zn0.97La0.03O powder due to this sample having the smallest crystallite size. The Zn1–xLaxO powders can inhibit Staphylococcus aureus better than Escherichia coli due to the presence of an extra lipopolysaccharide layer on the outer surface of the latter.

Preparation of Freestanding Zn Nanocrystallites by Combined Milling at Cryogenic and Room Temperatures

The present investigation reports the preparation of freestanding nanocrystalline Zn by combined mechanical milling at cryogenic and room temperatures. The cryomilling is used as an effective means of rapid fracturing. The detailed scanning electron microscopy and transmission electron microscopy observations indicate that the minimum crystallite size is 6 ± 2 nm after 3 hours of cryomilling. The crystallite size increases to 30 ± 2 nm after 3 hours of room temperature milling of the cryomilled powder due to deformation-induced sintering. Detailed theoretical analysis allows us to obtain a diagram of size of the nanoparticles formed vs temperature to explain the experimental findings.

Spectroscopic ellipsometry and magneto-transport investigations of Mn-doped ZnO nanocrystalline films deposited by a non-vacuum sol–gel spin-coating method

Abstract Nanocrystalline Zn 1− x Mn x O films ( x = 0, 0.05, 0.1, 0.15, and 0.2) were deposited onto corning glass substrates by a non-vacuum sol–gel spin coating method. All of the films were annealed at 450 °C for 2 h. The structural, optical and magneto-transport properties were investigated by X-ray diffraction, spectroscopic ellipsometry and a system for the measurement of the physical properties. X-ray diffraction analysis of the films reveals that the Mn-doped ZnO films crystallize in the form of a hexagonal wurtzite-type structure with a crystallite size decreases with an increase of the Mn concentration. It was also found that the microstrain increases with the increase of the Mn content. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using transmission, scanning electron microscopy and atomic force microscopy. The optical constants and film thicknesses of nanocrystalline Zn 1− x Mn x O films were obtained by fitting the spectroscopic ellipsometric data ( ψ and Δ ) using a three-layer model system in the wavelength range from 300 to 1200 nm. The refractive index was observed to increase with increasing Mn concentration. This increase in the refractive index with increasing Mn content may be attributed to the increase in the polarizability due to the large ionic radius of Mn 2+ compared to the ionic radius of Zn 2+ . The optical band gap of the nanocrystalline Mn–ZnO films was determined by an analysis of the absorption coefficient. The direct transition of the series of films was observed to have energies increasing linearly from 3.17 eV ( x = 0) to 3.55 eV ( x = 0.2). Magnetoresistance (MR) was measured from 5 K to 300 K in a magnetic field of up to 6 T. Low-field positive MR and high-field negative MR were detected in Mn-doped ZnO at 5 K. Only negative MR was observed for temperatures above 200 K. The positive MR in Mn-doped ZnO films was observed to decrease drastically when the temperature increased from 5 K to 100 K. The isothermal MR of Zn 1− x Mn x O films with different Mn concentrations at 5 K reveals that the increase of the Mn content induces a giant positive MR above x = 0.05 and reaches up to 55% at an applied field of 30 kOe for x = 0.2.

Structural, morphological, and gas response properties of citrate gel synthesized nanocrystalline ZnO and Zn0.9Cd0.1O materials

Abstract ZnO and Zn 0.9 Cd 0.1 O powders were synthesized using a citrate gel combustion method. The citrate gel combustion process provides homogeneous and phase pure nanocrystalline powders through a single step exothermic reaction. The pellets of ZnO and Zn 0.9 Cd 0.1 O were sintered at 600 °C for 6 h and further used to study their structural, morphological, and gas response properties. The XRD and TEM study reveals formation of nanocrystaline powders for both compositions. The gas sensing properties of the pelletized sensor materials were studied at operating temperatures from 200 to 450 °C. It has been observed that gas response considerably depends upon the operating temperature and test gas species. The operating temperature of the ZnO sensor significantly decreased with addition of cadmium. The reproducibility and stability study of Zn 0.9 Cd 0.1 O confirmed its candidature for acetone sensor related applications.

Absence of room temperature ferromagnetism in Mn-doped ZnS nanocrystalline thin film

Mn-doped ZnS nanocrystalline thin films having composition Zn1−xMnxS (x=0.02, 0.05, 0.1 and 0.15) were prepared by the electron beam evaporation technique. Their structural and magnetic properties had been investigated. X-ray diffraction results show single phase of hexagonal type structure samples without any detectable secondary phase providing the evidence that Mn2+ ions are incorporated into the ZnS lattice. Evidence of nanocrystalline nature of the films was observed from the investigation of surface morphology using atomic force microscopy. The magnetic measurements at 5K reveal superparamagnetic behavior. The field dependent magnetization measurements confirm the absence of the room temperature ferromagnetism in the single phase electron beam evaporated Zn1−xMnxS (x=0.02, 0.05, 0.1 and 0.15) nanocrystalline films. Furthermore, the inverse susceptibility versus T confirms paramagnetic behavior for the Mn-doped films in the temperature range 100K≤T≤300K. The absence of significant impurities and/or defects could be attributed to explanation for the absence of free carriers and consequently the absence of room temperature ferromagnetism in all nanocrystalline Zn1−xMnxS films. The absence of room temperature ferromagnetism is evidence by marked concave nature of Arrott's plot without any spontaneous magnetization.

Temperature dependence anomalous dielectric relaxation in Co doped ZnO nanoparticles

We have reported temperature and frequency dependence of dielectric behavior of nanocrystalline Zn1−xCoxO (x=0.0, 0.01, 0.05 and 0.1) samples prepared by gel-combustion method. The synthesized samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and LCR-meter, respectively. The XRD analysis reveals that ZnO has a hexagonal (wurtzite) crystal structure. The morphology and size of the nanoparticles (∼10–25nm) were observed by SEM for 5% Co doped ZnO sample. In dielectric properties, complex permittivity (ɛ*=ɛ′−jɛ″), loss tangent (tanδ) and ac conductivity (σac) in the frequency range 75kHz to 5MHz were analyzed with temperature range 150–400°C. The experimental results indicate that ɛ′, ɛ″, tanδ and σac decreases with increase in frequency and temperature. The transition temperature as obtained in dispersion curve of dielectric constant shifts toward higher temperature with increase Co content.

Photoelectrochemical water splitting with nanocrystalline Zn1−xRuxO thin films

Thin films of nanocrystalline Zn1−xRuxO are deposited on ITO substrate by sol–gel. XRD and EDX analysis indicated dominant evolution of wurtzite ZnO with crystallite size in the range 26–43 nm. With no evidence of phase segregation, Ru insertion in the host lattice is probably indicated by distortion in lattice parameters and concomitant rise in microstrain and dislocation density. SEM images indicated homogenous and continuous growth of nanocrystallites. AFM images confirmed pillar like growth of crystallites along c-axis. Ru incorporation (1, 3, 5 and 7% at.) made film surface rougher, nevertheless roughness decreased with rise in Ru concentration. Ru incorporation at low concentrations significantly improved PEC response of films.

Effects of Sintering Temperature on Structural and Electrical Transport Properties of Zinc Ferrites Prepared by Sol-Gel Route

The effects of sintering temperature on the structural and electrical transport properties of nanocrystalline zinc ferrites are reported. The zinc ferrites were prepared by WOWS sol-gel synthesis route. The prepared sample was sintered at temperatures 500°C, 700°C and 900°C respectively for 2 h. X-ray Diffraction (XRD) technique was used to describe the structural properties. The crystallite size, lattice parameters and porosity of samples were measured from the analysis of XRD data. The average crystallite size for each sample was measured using the Scherrer formula by considering the most intense (3 1 1) peak. The dielectric constant (ε), dielectric loss tangent (tan ) and AC electrical conductivity of nanocrystalline Zn ferrites are investigated as a function of frequency and sintering temperature. All the electrical properties are explained in accordance with MaxwellWagner model and Koops phenomenological theory.

Effect of Fe Concentration in ZnO Powders on Ferromagnetic Resonance Spectra

Ferromagnetic resonance (FMR) investigations of nanocrystalline Zn1−xFexO (x = 0.01, 0.03 and 0.05) powders are reported. Broad FMR signals with g factor >2 representing the signature of ferromagnetism were identified. In the temperature range of 110–300 K, the magnetic field position of the resonance line shifts to lower fields and the shift increases with increasing x. The observed shift was attributed to an internal field arising from the spin sublattice within the host. The FMR line widths increase with the temperature decrease and increase with the increase in the Fe content x. This broadening is due to the presence of the non-homogeneous local magnetic fields in the investigated samples. A deviation of the experimental line shapes from the Lorentzian shape was identified in the low-temperature range.

Structural and optical investigation of nanocrystalline Zn1−xNixS diluted magnetic semiconductor thin films

We have investigated the structural and optical properties of nanocrystalline Zn1−xNixS (0.005≤x≤0.2) diluted magnetic semiconductor thin films synthesized by electron beam deposition technique. All the deposited films crystallize in zincblend type structure. The structure parameters of the nanosize deposited films are extracted by X-ray diffraction and atomic force microscopy techniques and found to vary from 14nm to 17.5nm. The X-ray diffraction measurements show that the highest solubility limit of Ni in ZnS matrix is found to be ≈15%. In a wide spectral range, the refractive index and refractive index dispersion of the nanocrystalline Zn1−xNixS films have been calculated from transmission or reflection spectra using wavenumber method and found to increase with increasing Ni concentration. The increase of the refractive index dispersion of Zn1−xNixS with Ni concentration has been ascribed to the corresponding change in the net polarizability of the whole system. Through the higher wavenumber region (2×10+4cm−1 to 2.5×10+4cm−1), the group velocity factor is found to increase with increasing Ni concentration. In addition, for fixed Ni concentration (x=0.005) the refractive index and refractive index dispersion are found to increase with increasing film thickness. Wemple and DiDomenico single oscillator model was used to explain the increases of the refractive index dispersion with Ni concentration based on variation of coordination number of the Zn1−xNixS system. The optical transition is found to be direct transition with optical energy gap Egopt decreases with increasing Ni concentration. The reduction of Egopt with increasing Ni concentration is explained by considering the sp–d interaction of the Ni ions in ZnS matrix. The results reported here show that Ni doped ZnS nanocrystalline films can be employed in the fabrication of nanoscale optical and magneto-optical devices.

Study of corrosion performance of electrodeposited nanocrystalline Zn–Ni alloy coatings

The corrosion performance of nanocrystalline Zn–Ni alloy coatings electrodeposited from an alkaline bath using direct current has been investigated using EIS and polarization curves technique. Performance of these nanocrystalline alloy coatings with various Ni content was compared to that of microcrystalline Zn–Ni alloy. The corrosion product formed on the nanocrystalline Zn–Ni alloy coatings is stable and protective which proves to be better compared with microcrystalline. The nanocrystalline Zn–Ni alloy with 17.62wt.% Ni and grain size 37nm gives the best performance of the Zn–Ni alloy coatings tested. The corrosion resistance of nanocrystalline alloy coatings depends on the chemical compositions.

Superparamagnetic behavior in chemically synthesized nanocrystalline Zn0.99Ni0.01O powders

Hexagonal shaped Zn0.99Ni0.01O nanoparticles having an average size of ~50nm were synthesized by autocombustion technique. The incorporation of Ni in the Zn site is clearly reflected from the (101) major X-ray diffraction peak shift. The presence/development of oxygen vacancies are evidenced from the micro-Raman spectrum. The M–H curve of Zn0.99Ni0.01O shows superparamagnetic behavior at room temperature which is well proven from the decreasing order of zero field cooled magnetization with the decrease in temperature. The simultaneous competition between the oxygen vacancies induced ferromagnetism and the antiferromagnetic interactions between Ni ions gives rise to the superparamagnetic state in nanocrystalline Ni doped ZnO.

Distinct magnetic response of nanograined Zn1−xMnxO (x = 0, 0.02, 0.1) powders and thin films: Focus on the effect of the working atmosphere

Nanocrystalline Zn1−xMnxO (where x = 0, 0.02, 0.1) powders and thin films were prepared by a polymeric precursor method and pulsed laser deposition, respectively. The wet chemistry method was chosen to synthesize the powders in order to improve key parameters as purity and grain-size distribution. Optimization procedures, encompassing substrate temperature, laser fluence and background gas conditions were performed and are intended to improve the crystalline quality of the films. X-ray diffraction studies reveal no clear evidence (within the detection limit of the technique) of affectation by impurities generated during the synthesis. Films prepared under optimized conditions feature a mosaic spread <0.3° and nearly bulk-like c-axis lattice parameter (5.198 Å). The substitution of Mn2+ cations in the tetrahedral sites of the wurtzite structure of pristine ZnO is confirmed both X-ray diffraction and Raman spectroscopy. Magnetic measurements, in turn, show that the powders are paramagnetic for temperatures above 3 K, while the thin films, grown in vacuum, are ferromagnetic at room temperature. These results indicate that variations in the preparation parameters have a marked influence on the magnetic responses of the Zn1−xMnxO system.

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55nm at 15MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350°C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410°C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ1). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τav), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.

On the structural analysis and magnetic behavior of heavily doped Zn1−xMnxO nanopowders synthesized by wet chemistry method

Nanocrystalline Zn1−xMnxO(x=0−0.1) powders are prepared by polymeric precursor method and their structural and magnetic properties carefully studied. X-ray diffraction studies and Raman spectroscopy reveal that Mn2+ ions have substituted the Zn2+ ion without changing the würtzite structure of pristine ZnO up to Mn concentrations x≤0.05. The presence of a secondary phase, related to the solubility of Mn in ZnO is evident for higher Mn-doping concentrations. The negative value obtained for the Curie–Weiss temperature indicates that the interactions between the Mn ions are predominantly antiferromagnetic. Thus, no bulk ferromagnetism is evident in any of the studied samples.

Nanostructured Zn-based composite anodes for rechargeable Li-ion batteries

The mechanism of the electrochemical reaction of Zn with Li was investigated by ex situ X-ray diffraction (XRD) analysis combined with a differential capacity plot of the Zn electrode at a low current of 10 mA g−1. The pure Zn electrode showed a high reactivity with Li, with first discharge and charge capacities of 574 and 351 mA h g−1, respectively. In addition, Zn–C and Zn–Al2O3–C composites prepared by simple high-energy mechanical milling were evaluated for use as anode materials in rechargeable Li-ion batteries. The Zn–C nanocomposite was composed of nanosized Zn in an amorphous C matrix, while the Zn–Al2O3–C nanocomposite (obtained by the mechanochemical reduction of ZnO and Al) was composed of nanocrystalline Zn and amorphous Al2O3 in the amorphous C matrix. Electrochemical tests showed that the Zn–Al2O3–C nanocomposite electrode exhibited a high volumetric capacity of more than 1800 mA h cm−3 over 100 cycles.

Microstructural, magnetic and optical characterizations of nanocrystalline Zn1−xMnxO dilute magnetic semiconductors synthesized by mechanical alloying

Nanocrystalline Mn doped Zn1−xMnxO (x=0.02, 0.05, 0.07, 0.10) dilute magnetic semiconductors (DMSs) have been synthesized by mechanical alloying the stoichiometric mixtures of ZnO and MnO powders for different durations in a high energy planetary ball mill at room temperature under air medium. In the course of milling, the peak broadening of ZnO reflections increases continuously and MnO reflections disappear completely after 2h of milling. Microstructure characterization of ball milled samples employing the Rietveld structure refinement using X-ray diffraction data reveals that the Zn2+ ions are replaced by Mn2+ ions and lattice parameters of ZnO matrix increase slowly due to this substitution. Particle size of these DMS decreases continuously with increasing Mn concentrations. XPS measurement indicates that the doped Mn ions possess both Mn2+ and Mn4+ valance states. Magnetic characterization of DMS samples shows room temperature ferromagnetism (RTF), which confirms the inclusion of Mn2+ in ZnO lattice and the RTF can be explained in the light of RKKY exchange interaction. Optical characterization of these DMS by UV–vis spectroscopy clearly reveals a red-shift and consequently the optical band-gap decreases with increasing Mn concentrations.

Magnetic properties and crystallization kinetics of Zn0.5Ni0.5Fe2O4

Precursor of nanocrystalline Zn0.5Ni0.5Fe2O4 was obtained by grinding mixture of ZnSO4·7H2O, NiSO4·6H2O, FeSO4·7H2O, and Na2CO3·10H2O under the condition of surfactant polyethylene glycol (PEG)-400 being present at room temperature, washing the mixture with water to remove soluble inorganic salts and drying it at 373 K. The spinel Zn0.5Ni0.5Fe2O4 was obtained via calcining precursor above 773 K. The precursor and its calcined products were characterized by differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). The result showed that Zn0.5Ni0.5Fe2O4 obtained at 1073 K had a saturation magnetization of 74 A·m2·kg−1. Kinetics of the crystallization process of Zn0.5Ni0.5Fe2O4 was studied using DSC technique, and kinetic parameters were determined by Kissinger equation and Moynihan et al. equation. The value of the activation energy associated with the crystallization process of Zn0.5Ni0.5Fe2O4 is 220.89 kJ·mol−1. The average value of the Avrami exponent, n, is equal to 1.59±0.13, which suggests that crystallization process of Zn0.5Ni0.5Fe2O4 is the random nucleation and growth of nuclei reaction.