xCoxO Nanoparticles Research Articles

The role of Co in doping ZnO nano-particles in enhancement the structural, optical and magnetic properties for spintronics

Zn1−xCoxO (0 ≤ x ≤ 0.10) nano-particles were fabricated successfully via ball milling procedure. The influence of [Co]/[Zn] on the properties of nano-particles was examined employing X-ray diffraction, the energy dispersion of the X-ray (EDX), Scan Electron Microscope (SEM), these measurements exhibit the growth hexagonal wurtzite crystal phase Zn1−xCoxO nano-particles with the successfully incorporation of Co+2 ions in the ZnO lattice. The optical characterization investigated utilizing Ultraviolet-Visible (UV) absorption spectroscopy for Zn1−xCoxO nano-particles refers to a red-shift in the optical bandgap with increasing Co ion inside ZnO matrix, this result confirm the bandgap is considerably narrowed with increasing Co ratio. The measurements of magnetization applying vibrating sample magnetometer illustrated a hysteresis loop in Co-doped ZnO nano-particles. Thanks to the variation values of bandgap Eg opt and magnetic measurements of ZnO nano-particles doped Co% plays a major role for selective coatings of optoelectronics; use as antireflective coating materials, and fabrication of optoelectronic devices.

Structural, Optical and Magnetic Properties of Zn1-xCoxO Nanoparticles.

Zn1-xCoxO nanoparticles with three different values of 'x' (x = 0.05, 0.10, 0.15) were prepared by chemical co-precipitation process without any further heat treatment. The X-ray diffraction studies confirmed the wurtzite hexagonal crystal structure for synthesized Zn1-xCoxO nanoparticles. The dislocation density results reveal that there is an increase in the concentration of lattice imperfections with increasing the concentration of Co ions. The true values of lattice constants were calculated by using Nelson-Riley Function (NRF). Further, the average bond length (BL) were also calculated and presented. The optical and magnetic properties of Zn1-xCoxO nanoparticles were examined by room-temperature photoluminescence (PL) spectroscopy and vibrating sample magnetometer (VSM), respectively. The calculated values of magnetic susceptibility for Zn1-xCoxO nanoparticles with x = 0.05, 0.10, 0.15 were found to be 9.883×10-4, 2.29×10-2 and 2.37×10-2, respectively.

Investigations on microstructure, optical, magnetic, photocatalytic, and dielectric behaviours of pure and Co-doped ZnO NPs

In this research, an effective usage of quantum size effect is a foregrounded strategy to produce and employ excellent visible light-driven photocatalytic materials. Zn100−xCoxO (x = 0, 2.5, 7.5) particles at nanoscale were synthesized successfully by chemical co-precipitation method and then investigated by some complimentary analytical techniques. Physical structure and chemical states were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. XRD and HRTEM analysis confirmed the formation of wurtzite-type structure without any traces of secondary phase. XPS spectra exhibited the incorporation of Co2+ ions into ZnO lattice. Optical spectra of Zn100−xCoxO nanoparticles (NPs) indicated that bandgap is significantly narrowed with increasing Co concentration. Ferromagnetic behaviour was observed at room temperature in pure and Co-doped ZnO, due to the effect of quantum confinement and of Co2+ ions incorporation. The frequency and composition dependence of dielectric constant and dielectric loss have been analysed in detail using Maxwell–Wagner model and Cole–Cole plots. The activity of synthesized photocatalysts under simulated sunlight irradiation was characterized through decomposition of methylene blue (MB) dye in aqueous suspension, which revealed that Co2+ ions incorporation in ZnO greatly enhanced the photocatalytic degradation efficiency and hence employable in environmental cleaning.

Evolution of Optical Band Gap and Related Properties in Ni1−xCoxO Nanoparticles

Ni1−xCoxO Nanoparticles with x varying from 0 to 0.05 were synthesized by co-precipitation method. The effect of Co concentration on the optical band gap and related properties of NiO nanoparticles were studied by UV–visible spectroscopy. UV–visible characterization of these samples indicated that the band gap of NiO decreased from 3.65 to 3.38 eV with increasing Co concentration from 0 to 5% in NiO. It has been reported that the band gap of ternary compounds show band gap bowing with composition instead of linear variation. In our case it is found to be ~ 5.28 ± 0.5 eV. The refractive index and electron polarizability with Co doping concentration in NiO is calculated from the band gap. Both the parameters found to increase with increasing Co concentration in NiO. This study suggests the usefulness of these materials for optoelectronic devices.

Effect of cobalt alloying on the electrochemical performance of manganese oxide nanoparticles nucleated on multiwalled carbon nanotubes

MnO is an electrically insulating material which limits its usefulness in lithium ion batteries. We demonstrate that the electrochemical performance of MnO can be greatly improved by using oxygen-functional groups created on the outer walls of multiwalled carbon nanotubes (MWCNTs) as nucleation sites for metal oxide nanoparticles. Based on the mass of the active material used in the preparation of electrodes, the composite conversion-reaction anode material Mn1−xCoxO/MWCNT with x = 0.2 exhibited the highest reversible specific capacity, 790 and 553 mAhg−1 at current densities of 40 and 1600 mAg−1, respectively. This is 3.1 times higher than that of MnO/MWCNT at a charge rate of 1600 mAg−1. Phase segregation in the Mn1−xCoxO nanoparticles was not observed for x ≤ 0.15. Capacity retention in x = 0, 0.2, and 1 electrodes showed that the corresponding specific capacities were stabilized at 478, 709 and 602 mAhg−1 respectively, after 55 cycles at a current density of 400 mAg−1. As both MnO and CoO exhibit similar theoretical capacities and MnO/MWCNT and CoO/MWCNT anodes both exhibit lower performance than Mn0.8Co0.2O/MWCNT, the improved performance of the Mn1−xCoxO/MWCNT alloy likely arises from beneficial synergistic interactions in the bimetallic system.

Multi-band visible photoresponse study of Co2+ doped ZnO nanoparticles

Cobalt doping enhances visible absorption in ZnO nanoparticles as a result of d–d transitions. By co-precipitation method, Zn1−xCoxO nanoparticles had been synthesised and multiband photodetectors were fabricated after characteristic analysis.

Investigations on absorption, photoluminescence and magnetic properties of ZnO: Co nanoparticles

We have investigated the consequences of cobalt (Co) incorporation with different doping concentrations (0, 2, 4 and 6 %) on structural, optical and magnetic properties of ZnO nanoparticles. The results of X-ray diffraction spectra (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern of single particle, energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared (FT-IR) authenticate the substitution of cobalt and hexagonal crystal structure without any secondary phase formation of all the samples under investigation. The ultraviolet–visible (UV–Vis) absorption study indicates that increase in Co concentration improves the visible region absorption (550–700 nm). The absorption edge of Co-doped ZnO shifts towards visible region with increase in Co concentration. The band gap of samples shows a red shift with increase in Co percentage. The photoluminescence (PL) study of the samples indicates that Co doping shifts the intense peak position of ZnO from violet to blue colour. The weak emission peak at 572 nm is also observed in all the samples. The emission is represented by chromaticity diagram. The room temperature magnetic properties have been studied using vibrating sample magnetometer (VSM). The room temperature magnetisation curve shows that an increase in Co concentration increases the linear behaviour of M–H loop. The magnetic susceptibility results indicate that all the samples have Curie–Weiss behaviour. The coercive field (Hc) and the remanence magnetisation (Mr) increase with Co doping concentration. Fig. M–H curve of pure and cobalt-doped ZnO nanoparticles at 300 K. Inset (a) shows absorption spectra, and inset (b) shows the emission spectra of Zn1−x Co x O nanoparticles.

Identifying the sources of ferromagnetism in sol-gel synthesized Zn1−xCoxO (0≤x≤0.10) nanoparticles

We have carefully investigated the structural, optical and electronic properties and related them with changes in the magnetism of sol-gel synthesized Zn1−xCoxO (0≤x≤0.10) nanoparticles. Samples with x≤0.05 were free of spurious phases. Samples with x≤0.03 were found to be with only high spin Co2+ ions into ZnO structure, whereas sample with x=0.05, exhibited the presence of high spin Co2+ and low spin Co3+. We found that the intensity of the main EPR peak associated with Co2+ varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of divalent cobalt ions. Bound magnetic polaron (BMP) model and the charge transfer model are insufficient to explain the ferromagnetic properties of Zn1−xCoxO nanoparticles. The room temperature ferromagnetism (RTFM) may be originated from a combination of several factors such as the interaction of high spin Co2+ ions, perturbation/alteration and/or changes in the electronic structure of ZnO close to the valence band edge and grain boundary effects.

Growth of Co-doped ZnO nanoparticles by porous alumina assisted sol–gel route: Structural optical and magnetic properties

We report a simple sol–gel method with spin coating to grow Co-doped ZnO nanoparticles (NPs) using porous alumina template. X-ray diffraction analysis confirms the wurtzite hexagonal structure for the Zn1−xCoxO NPs. Transmission electron microscopy (TEM) micrograph reveals clearly the formation of NPs with average diameter in the range 40–50 nm. High resolution TEM lattice images and the electron diffraction (SAED) pattern show that the NPs are single crystalline with lattice plane spacing of 2.47 Å for the (101) plane. UV–Vis absorption spectra show a slight blue shift in the absorption edge due to doping related modification in the band structure. PL spectra show a band edge related UV emission around 378 nm at low temperature, while defect (Zni, OV) related visible emission dominates at room temperature. Room temperature ferromagnetism (RTFM) is observed in Zn1−xCoxO NPs with the moment of 450–750 memu/g. The temperature dependent magnetization (M-T) curve shows high transition temperature (Tc) as 750 K. The observed FM is explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Co ions and Zni, OV related defects. This study provides an effective way to obtain HTFM with lower doping concentration as well as to have control over NP size by alumina pores in the Zn1−xCoxO system.

Sol–gel synthesis, structural, optical and magnetic properties of Co-doped ZnO nanoparticles

We report the synthesis of Zn1−x Co x O nanoparticles prepared by a sol–gel processing technique for x ranging from 0 to 0.05. The structural, morphological, optical and magnetic properties of the as-prepared nanoparticles were investigated by XRD, XPS, transmission electron microscopy, UV measurements, photoluminescence and superconducting quantum interference device. The structural properties showed that the obtained nanoparticles are single phase wurtzite structure and no secondary phases were detected which indicated that Co ions substituted for Zn ions. The energy gap decreased gradually with increasing doping concentration of Co. The photoluminescence spectra show a shift of the position of the ultraviolet emission to long wavelength and the intensity decreases with increasing Co. The Magnetic measurements at room temperature reveal diamagnetic behavior for sample with lower concentration and the presence of both paramagnetic and ferromagnetic behavior for increasing concentration.

Correlation between magnetism and electronic structure of Zn1−xCoxO nanoparticles

Zn1−xCoxO nanoparticles (∼9 nm) were produced with x ranging from 0 to 0.2 using a forced hydrolysis method. X-ray diffraction measurements confirm the samples to be single phase, and reveal a systematic change in the lattice parameters upon cobalt doping. The unit cell volume V decreases up to x = 0.025 after which it stays roughly constant. The band gap energy (Eg), determined from the photoluminescence spectra gradually increases from x = 0 to 0.025 and then remains nearly constant for x > 0.025. Room temperature hysteresis loops, obtained using vibrating sample magnetometry, show a similar trend in the saturation magnetization (Ms). Undoped ZnO nanoparticles show a weak magnetic hysteresis; doping causes an increase in Ms up to x = 0.025 and then decreases to lower values for x > 0.025. The magnetic moment per Co ion μ decreases rapidly with x nearly following μ(x) ∝ 1/x, indicating that the moments from the Co ions have little impact on the observed magnetic properties. Electron paramagnetic resonance (EPR) data confirmed that the pure samples are free of any magnetic impurities, while all the doped samples show spectra corresponding to Co2+. The variation of the integrated EPR signal intensity with x also shows a maximum at x = 0.025. X-ray photoelectron spectroscopy confirm that the dopant is incorporated as high spin Co2+ ions for low x, but increasing fractions of the dopant ions change to Co3+ as x increases to 0.2. These results along with the strong correlation observed between the structural (V), electronic (Eg), and magnetic (Ms) properties of Zn1−xCoxO nanoparticles, and the rapid decrease in magnetic moment with increasing x, indicate that the observed changes in the magnetic properties are related to changes in the electronic structure of ZnO nanoparticles caused by dopant incorporation.

Dopant position of Co atom in Zn1−xCoxO nanoparticles studied by extended X-ray absorption fine structure

Zn1−xCoxO nanoparticles were prepared by sol–gel method. The microstructure and dopant position were studied by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and extended X-ray absorption fine structure (EXAFS). The XRD patterns did not show any signal of impurity in the Zn1−xCoxO nanoparticles with Co concentration from x = 0.01 to 0.08. Neither did the HRTEM image for the highest concentration sample with x = 0.08. The nanoparticles have also been investigated by the EXAFS measurements at Co k-edge. The radical distribution functions, the fitting result of bond length and coordination numbers, indicated there was an impurity in the highest Co concentration sample with x = 0.08. Although most of the Co atoms were substituted for Zn sites in ZnO with x = 0.08, a few of Co atoms formed a microstructure similar to Co3O4, which was not found in the XRD and HRTEM. The room temperature ferromagnetic behaviour was found for x = 0.01 sample by superconducting quantum interference device .

Synthesis and structural characterization of ZnO doped with Co

Co-doped Zn1−xCoxO nanoparticles were prepared by hydrothermal synthesis. The morphology, solid solubility and local atomic structure around the doping Co atoms were investigated. The morphologies of Co-doped ZnO nanoparticles were observed by scanning electron microscopy. The crystal structure of nanoparticles and the possible impurity phase were analyzed by X-ray diffraction patterns. The local atomic arrangements were studied by X-ray absorption fine structure spectroscopy. The substitution of Co for Zn was confirmed by diffraction anomalous fine structure spectra. The results demonstrated that Zn1−xCoxO can form a single phase of Co-doped ZnO with x⩽0.03. But it is a mixture of Co-doped ZnO and Co(OH)2 phases with x⩾0.05. Hydrothermal temperature can decrease the solid solubility of Co in ZnO lattice. Polar molecules can control the morphology of the nanoparticles. The Co-doped ZnO nanoparticles take a rod-like shape and the axial direction of the nanorods is along the c-axis direction of hexagonal wurtzite unit cell. A mixed-valence state of Co2+ and Co3+ is identified for the Co-doped ZnO nanoparticles. Diffraction anomalous fine structure technique could be used to determine the solid solubility of impurity when only one equivalent crystalline site in the host crystal is replaced by the impurity atoms. The Co-doped ZnO particles were identified to have room-temperature ferromagnetic properties.

Comparative Study of Ni- and Co-Substituted ZnO Nanoparticles: Synthesis, Optical, and Magnetic Properties

Ni- and Co-substituted ZnO nanoparticles were synthesized using forced hydrolysis of acetate metallic salts in a polyol medium. The X-ray diffraction patterns show a hexagonal wurtzite structure (space group P63mc). The characteristic absorption bands of UV–vis-IR spectra are correlated with the d–d transitions of tetrahedrally coordinated Co2+ and Ni2+ ions in octahedral and tetrahedral sites. The photoluminescence spectra exhibited a typical ZnO UV-exitonic emission band around 380 nm and a broad band between 400 and 500 nm that might be ascribed to the intrinsic defects in the ZnO material. The transmission electron microscopy displays spherical particles with a diameter between 20 and 30 nm. The magnetic measurements reveal that Zn1–xNixO and Zn1–xCoxO nanoparticles show, respectively, ferromagnetic and paramagnetic behavior at 5 K. Homogeneous distributions of Co and Ni ions in the particles observed by filter imaging analysis indicates that there is no evidence of Co or Ni metal throughout the powders.

Effect of atmosphere on the magnetic properties of the Co-doped ZnO magnetic semiconductors

Zn1−xCoxO nanoparticles were synthesized by rheological phase reaction-precursor method in air and Ar, respectively. No secondary phase was observed with the sensitivity of XRD measurement. X-ray photoelectron spectroscopy and absorption spectra results indicated that Co2+ successfully incorporated into the wurtzite lattice at the Zn2+ sites. Raman spectra of Zn1−xCoxO obtained at air showed an additional broad vibrational mode, which can be attributed to disordered ZnOCo local vibrations; the mode of the samples obtained at Ar was not appeared. Magnetization measurements reveal ferromagnetism is only found in samples prepared in Ar revealing a close correlation between the defects and magnetic properties.

Effect of reducing atmosphere on the magnetism ofZn1−xCoxO (0≤x≤0.10) nanoparticles

We report the crystal structure and magnetic properties ofZn1−xCoxO (0≤x≤0.10) nanoparticles synthesized by heating metal acetates in organic solvent. The nanoparticleswere crystallized in the wurtzite ZnO structure after annealing in air and in a forming gas(Ar95% + H5%). The x-ray diffraction and x-ray photoemission spectroscopy(XPS) data for different Co content show clear evidence for theCo2+ ions in tetrahedral symmetry, indicating the substitution ofCo2+ in the ZnO lattice.However, samples with x = 0.08 and higher cobalt content also indicate the presence of Co metal clusters.Only those samples annealed in the reducing atmosphere of the forming gas,that showed the presence of oxygen vacancies, exhibited ferromagnetism at roomtemperature. The air annealed samples remained non-magnetic down to 77 K.The essential ingredient in achieving room temperature ferromagnetism in theseZn1−xCoxO nanoparticles was found to be the presence of additional carriers generated by the presenceof the oxygen vacancies.