Cobalt Doped ZnO Nanoparticles Research Articles

Correlation among oxygen vacancy and doping concentration in controlling the properties of cobalt doped ZnO nanoparticles

We have observed ferromagnetism at room temperature in 2% cobalt doped ZnO nanoparticles prepared by a simple chemical precipitation method. This can be described as a consequence of bound magnetic polaron (BMP) resulting from the occurrence of oxygen vacancies and surface defects in the sample. Photoluminescence and Raman measurements indicate the existence of such defects in the sample. Electron Spin Resonance (ESR) studies are done to probe into the magnetic nature of the doped samples further. Redshift in the band gap and formation of characteristic absorption bands after Co doping shows that Co ions are incorporated in the host ZnO lattice. The infrared sensitive vibrational modes are identified with Fourier Transform Infrared Spectroscopy (FTIR) measurements. The spin-orbital splitting in X- ray Photoelectron Spectroscopic (XPS) measurements show the matching oxidation (divalent) state among the dopant and the host. Local structural studies after Co doping by the Extended X-ray Absorption Fine Structure (EXAFS), shows the Co2+ integration in the ZnO lattice without any impurity states. Photocatalytic measurements show that ZnO is a very promising degrading agent for Rhodamine B dye solution under UV as well as solar irradiation.

Effect of cobalt doping on microstructures and dielectric properties of ZnO

Pure and cobalt-doped ZnO nanoparticles (2.5, 5, 7.5, and 10 atom % Co) are synthesized by sol–gel method. The as-synthesized nanoparticles are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (FE-SEM) analysis. The nanoparticles of 0, 2.5, and 5 atom % Co-doped ZnO exhibited hexagonal wurtzite structure and have no other phases. Moreover, the (101) diffraction peaks position of Co-doped ZnO shift toward a smaller value of diffraction angle compared with pure ZnO powders. The results confirm that Co ions were well incorporated into ZnO crystal lattice. Simultaneously, Co doping also inhibited the growth of particles, and the crystallite size decreased from 43.11 nm to 36.63 nm with the increase in doping concentration from 0 to 10 atom %. The values of the optical band gap of all Co-doped ZnO nanoparticles gradually decreased from 3.09 eV to 2.66 eV with increasing Co content. Particular, the dielectric constant of all Co-doped ZnO ceramics gradually increased from 1.62 × 103 to 20.52 × 103, and the dielectric loss decreased from 2.36 to 1.28 when Co content increased from 0 to 10 atom %.

Photo-inactivation and efflux pump inhibition of methicillin resistant Staphylococcus aureus using thiolated cobalt doped ZnO nanoparticles

Multidrug resistance (MDR) in bacteria is a major concern these days. One of the reasons is the mutation in efflux pump that prevents the retention of antibiotics and drugs in the bacterial cell. The current work is a step to overcome MDR in bacteria via inhibition of efflux pump and further photoinhibition by thiolated chitosan coated cobalt doped zinc oxide nanoparticles (Co-ZnO) in visible light. Co-ZnO were synthesized in a size range of 40–60 nm. Antibacterial activity of the Co-ZnO against methicillin resistant Staphylococcus aureus (MRSA) was found 100% at a concentration of 10 μg/ml upon activation in sunlight for 15 min. Interestingly, it was found that cobalt as a dopant was able to increase the photodynamic and photothermal activity of Co-ZnO, as in dark conditions, there was only 3–5% of inhibition at 10 μg/ml of nanoparticle concentration. Upon excitation in light, these nanoparticles were able to generate reactive oxygen species (ROS) with a quantum yield of 0.23 ± 0.034. The nanoparticles were also generating heat, Because of the magnetic nature, thus helping in more killing. Thiolated chitosan further helped in blocking the efflux pump of MRSA. The current nanoparticles were also found biocompatible on human red blood cells (LD50 = 214 μg/ml). These data suggest that the MRSA killing ability was facilitated through efflux inhibition and oxidative stress upon excitation in visible light hence, were in accordance with previous findings.

Preparation characterization and magnetic properties of Zn(1-x)CoxO nanoparticle Dilute magnetic semiconductors

Cobalt-doped ZnO nanoparticles having chemical formula Zn(1-x)CoxO (x = 0.05, 0.10, 0.15, 0.20) were prepared by means of auto-combustion synthesis. Formation of monophasic wurtzite structure material was confirmed from XRD data. Inclusion of the Co2+ ions in the ZnO matrix produced several fascinating effects in the structural, physical and magnetic properties of the nanoparticles. The lattice constants ‘a’ and ‘c’ got inflated and the average crystallite size shriveled from 25 nm to 17 nm with the increase of Co concentration ‘x’ in the sample. The FT-IR spectra showed the characteristic absorption bands in the range of 400 cm−1 to 576 cm−1 due to formation of ZnO and CoO stretching bonds in the samples. Scanning Electron Microscope (SEM) images show agglomerates of particles that are spherical in nature. Energy dispersive spectroscopy (EDS) spectra of the samples giving molecular weight percentages of the elements Zn, Co and O in the samples are in agreement with the calculated percentages within the limit of permissible error. Hysteresis, FC, and ZFC measurements made on the samples, exhibit presence of ferromagnetic phase in the materials. Energy band gap estimates made with the help of UV spectroscopy indicated a decreasing trend of band gap with increasing ‘x’ in the sample. Photoluminescence studies carried out with excitation wavelength of 318 nm gave several emission peaks ranging from green to violet emissions. This observation confirms the presence of vacancies (defects) in the nanoparticle samples. Luminescence decay time recorded was in the range microseconds for all the samples.

Precipitated cobalt doped ZnO nanoparticles with enhanced low temperature xylene sensing properties

Metal oxide nanoparticles are fabricated by co-precipitation method. The structural, morphological, optical and room temperature magnetic studies are discussed and outlined the end results along with sensing properties of ZnO and Co doped ZnO nanoparticles. Further, insights of theoretical and experimental aspects analogous with such systems are also discussed. Photoluminescence property indicates that cobalt doping improves the formation of Oxygen species which results in the enhancement of the magnetism and sensor response. The sensitive and selective detection of p-Xylene, particularly distinguishing between Ethanol and Acetone can be attributed to the tuned catalytic activity of Cobalt components, which induce preferential dissociation of p-Xylene into more active species, as well as the substantial increase of resistance with the variation in the nearby chemical environment because of large formation of Schottky barriers. This confirms that Co doped ZnO nanoparticles found to be a good candidate for detecting p-Xylene gas at low concentrations as well as at low optimum temperature.

Colloidal cobalt-doped ZnO nanoparticles by microwave-assisted synthesis and their utilization in thin composite layers with MEH-PPV as an electroluminescent material for polymer light emitting diodes

Abstract CoxZn1-xO (x = 0.01, 0.05 and 0.1) nanoparticles were prepared by microwave-assisted polyol method from zinc acetate dihydrate and Co(II) acetylacetonate. The reactions were performed in diethylene glycol (DEG) at 250 °C with the use of oleic acid as a surfactant. Resulting nanoparticle (ca. 10 nm) precipitates were washed with methanol and dried or kept as colloidal solutions redispersed in toluene. Colloidal solutions were mixed with the Poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer to produce thin nanocomposite layers with specific optoelectronic properties. The electronic structure in terms of density of states (DOS) of MEH-PPV and MEH-PPV/nanocomposite layers was investigated by recently proposed energy resolved electrochemical impedance spectroscopy method. The MEH-PPV polymer and its nanocomposites with ZnO or CoxZn1-xO nanoparticles were used as thin active layers in polymer light emitting diodes (PLED). The nanocomposite layers exhibited optoelectronic properties which were found to be beneficial as the active layer in PLEDs, exhibiting an order of magnitude enhancement in electroluminescence intensity. A pronounced effect on the opening bias voltage of final devices with CoxZn1-xO nanoparticles was also observed.

Size Control of Cobalt-Doped ZnO Nanoparticles Obtained in Microwave Solvothermal Synthesis

This article presents the method of size control of cobalt-doped zinc oxide nanoparticles (Zn1−xCoxO NPs) obtained by means of the microwave solvothermal synthesis. Zinc acetate dihydrate and cobalt(II) acetate tetrahydrate dissolved in ethylene glycol were used as the precursor. It has been proved by the example of Zn0.9Co0.1O NPs (x = 10 mol %) that by controlling the water quantity in the precursor it is possible to precisely control the size of the obtained Zn1−xCoxO NPs. The following properties of the obtained Zn0.9Co0.1O NPs were tested: skeleton density (helium pycnometry), specific surface area (BET), dopant content (ICP-OES), morphology (SEM), phase purity (XRD), lattice parameter (Rietveld method), average crystallite size (FW1/5/4/5M method and Scherrer’s formula), crystallite size distribution (FW1/5/4/5M method), and average particle size (from TEM and SSA). An increase in the water content in the precursor between 1.5% and 5% resulted in the increase in Zn0.9Co0.1O NPs size between 28 nm and 53 nm. The X-ray diffraction revealed the presence of only one hexagonal phase of ZnO in all samples. Scanning electron microscope images indicated an impact of the increase in water content in the precursor on the change of size and shape of the obtained Zn0.9Co0.1O NPs. The developed method of NPs size control in the microwave solvothermal synthesis was used for the first time for controlling the size of Zn1−xCoxO NPs.

Ultrafine Co-doped ZnO nanoparticles on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction

In this work, we developed a novel hierarchical hybrid nanostructure of reduced graphene oxide (rGO) supported cobalt-doped ZnO nanoparticles (Co-doped ZnO NPs@rGO) via a simple solution-refluxing strategy. The ultrafine CoZn-glycolate nanoparticles precursor was obtained from the alkoxide reaction among ethylene glycol and Co2+/Zn2+ ions absorbed onto GO support. The post-annealing process makes the precursor easily transferred into crystalline Co-doped ZnO NPs@rGO with well-retained morphology and the oxidation state of Co element to be Co2+ in the final hybrid. When evaluated as an oxygen reduction reaction (ORR) electrocatalyst in an O2-saturated 0.1M KOH solution, the Co-doped ZnO NPs@rGO hybrid with the optimal Co-doping concentration of 0.38 (i.e., Co0.38Zn0.62O NPs@rGO) exhibits highly enhanced ORR electrocatalytic activity compared to the ZnO@rGO composite with an ORR peak potential of 0.74V (vs. RHE), an onset potential of 0.90V, a current density of 3.94mAcm−2 (at 0.4V (vs. RHE)) and a nearly four-electron catalytic pathway. The good electrochemical stability and excellent methanol-tolerance capability of the resultant Co0.38Zn0.62O NPs@rGO hybrid, superior to those of the commercial Pt/C catalyst, further demonstrate its great potential as an efficient ORR electrocatalyst. This work proposes the feasibility of the semiconducting ZnO nanomaterials as novel ORR electrocatalysts via doping active oxygen catalytic element, such as Co2+, for the first time.

Influence of Co doping on combined photocatalytic and antibacterial activity of ZnO nanoparticles

The present work aims to investigate the structural, optical, photocatalyst and antibacterial properties of bare and cobalt doped ZnO nanoparticles (NPs) with different concentrations Zn1−xCoxO (x = 0, 0.03, 0.06 and 0.09) synthesized by co-precipitation method. The XRD patterns confirmed that all samples of cobalt doped ZnO nanostructures revealed the formation of single phase having hexagonal wurtzite structure with crystallite size in the range of 31–41 nm. Further, the decreasing trend in lattice parameters and grain sizes were also seen with increasing doping concentrations which confirms the incorporation of Co ions into the ZnO lattice. This result was further supported by the FT-IR data. HR-TEM images demonstrated the distinct hexagonal like morphology with small agglomeration. The UV–visible absorption spectra exhibits red shift with increase in Co doping concentration in ZnO while corresponding bandgap energy of cobalt doped ZnO NPs decreased with increased Co doping concentration. PL spectra showed a weak UV and visible emission band which may be ascribed to the reduction in oxygen vacancy and defects by cobalt doping. XPS and EDX spectral results confirm the composition and the purity of Co doped ZnO NPs. Furthermore, the Co doped ZnO NPs were found to exhibit lesser photocatalytic activity for the degradation of methyl green dye under UV light illumination in comparison with the bare ZnO NPs. Moreover, anti-bacterial studies reveals that the Co doped ZnO NPs possess more antibacterial effect against gram positive Basillus subtills and gram negative Klebsiella pneumoniae bacterial strains than the bare ZnO NPs.

Structural, optical, magnetic and antibacterial study of pure and cobalt doped ZnO nanoparticles

In this research paper, pure and Co-doped ZnO nanoparticles were synthesized by using the wet precipitation method. The structural, morphological, optical and magnetic properties of the pure and Co doped ZnO nanoparticles were investigated. X-ray diffraction (XRD) spectra of Co doped ZnO NPs shows the shifting of characteristic XRD peak toward the higher angle which revealed that dopant Co is successfully incorporated into the lattice structure of ZnO nanoparticles. Structural morphology of pure and Co-doped ZnO nanoparticles samples was ascertained by using the scanning electron microscopy which confirms the formation of fine and clear spherical nanocrystallites with clear and distinctive boundaries. Energy-dispersive X-ray spectroscopy spectra show the elemental composition of Co2+ ions effectively in lattice site of Zn2+ ions. Photoluminescence and Raman spectra indicate the presence of oxygen vacancies and donor defects in the doped samples. UV–visible absorption spectroscopy showed the blue shifting of absorption edge as compared to pure ZnO nanoparticles sample. Pure and Co-doped ZnO nanoparticles revealed considerable changes in the M–H loop, particularly the diamagnetic behavior changed into ferromagnetic for Co-doped samples in vibrating sample magnetometer investigation. In this study, the antibacterial properties of the cobalt doped zinc oxide were further studied against three Gram-negative pathogens via using agar well diffusion technique. Co doped nanoparticles samples were then investigated as antibacterial agent to control the bacterial growth. It is further confirmed from our study that Co-doped ZnO NPs and their exposure to sunlight enhanced the antibacterial activity against three bacterial pathogens under study.

Synthesis of ZnO Nanoparticles Doped with Cobalt: Influence of Doping on the Magnetic and Fluorescent Properties

We have investigated the influence of cobalt doping on the luminescent and magnetic properties of ZnO nanoparticles prepared by co-precipitation. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) and by use of a vibrating sample magnetometer (VSM). XRD analyses revealed a standard ZnO wurtzite crystal structure and showed that the cobalt doped ZnO nanoparticles (ZnO:Co_NPs) were synthetized without impurities. The calculation based on the XRD shows the average crystallite sizes of ZnO to be in the 8-11 nm range. TEM images of ZnO and ZnO:Co indicated that these nanoparticles are nearly uniformly spherical with a diameter of about 10 - 15 nm. The PL spectra exhibited high [low] intensity in the UV [visible] region. While the PL shows a decreasing intensity with increasing doping doses, the peak at 544nm is not present in the cobalt-doped zinc oxide, and a the peak at 378 nm shifts to 390nm. The VSM measurement confirmed the presence of ferromagnetism and we observed an increase in the values of the saturation magnetization with increasing Co concentration.

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.

Optical Properties and FTIR Studies of Cobalt Doped ZnO Nanoparticles by Simple Solution Method

Objectives: In this study, pure and cobalt doped ZnO nanoparticles are synthesized by simple solution method and doping effect of cobalt on the optical properties ZnO nanoparticles is studied at room temperature to enhance the optical properties by varying the electrons of appropriate concentration. Methods: Synthesis of ZnO nanoparticles was achieved by the simple and effective solution method using zinc nitrate tetra hydrate and cobalt nitrate tetra hydrate. The defects and impurities were investigated by Fourier transform infrared spectroscopy. Findings: The FTIR spectrum shows the characteristics peak of ZnO at 490 cm -1 . In cobalt doping ZnO nanoparticles, the entire peak transmittance % got quenched. Together these results identified the impurities which exist near ZnO surface. No other peak was not observed in the spectra confirms that final products is ZnO nanoparticles. we measured the absorption spectra for pure and cobalt doped ZnO nanoparticles to study the effect of doping concentration of cobalt on the absorption of ZnO, After being doped with Co, the absorption peaks shifted towards longer wavelength region from 358 nm to 372nm. Band gap for Co doped ZnO nanoparticles are found to be between 3.28–3.05 eV range. This is due to the increase in particle size that results the less quantum confinement within the nanoparticles. Applications: Zinc Oxide semiconductor material is applied to many applications like solar cells, transparent electrode, transducer, and laser diodes due to its enhanced optical properties by doping transition materials.

Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis.

Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.

Glucose sensing behavior of cobalt doped ZnO nanoparticles synthesized by co-precipitation method

Pure and Zn1−xCoxO (x = 0.05, 0.10 and 0.15) nanoparticles were synthesized by co-precipitation method. Powder X-ray diffraction studies confirmed that pure and Co doped ZnO have a single phase nature with hexagonal wurtzite structure and Co2+ ions were successfully incorporated into the lattice position of Zn2+ ions in ZnO matrix. FTIR and EDS spectra results confirmed the incorporation of the dopants into the ZnO lattice structure. FESEM images showed the flower like morphology in ZnO and flowers are dispersed to grains in Co doped ZnO nanoparticles. The chemical composition of Zn0.90Co0.10O nanoparticles were confirmed by XPS. The XPS results clearly showed the existence of Co as a doping element into the ZnO crystalline lattice. Further, the synthesized Co doped ZnO nanoparticles have been used for electrochemical non-enzymatic glucose biosensing. The lowest detection limit of the proposed sensor was found to be 5 µM. The sensor selectively oxidizes glucose in the presence of other interfering compounds like l-dopa, ascorbic acid, hydrogen peroxide and uric acid.

Synthesis of Co-doped ZnO nanoparticles by sol–gel method and its characterization

Cobalt doped ZnO nanoparticles with different Co contents have been synthesized by a sol–gel processing technique. In our approach, the water for hydrolysis was slowly released by esterification reaction followed by a supercritical drying in ethyl alcohol. The structural, morphological, optical and magnetic properties of the as-prepared nanoparticles were investigated by XRD, transmission electron microscopy, UV measurements, photoluminescence and superconducting quantum interference device. The structural properties showed that the obtained nanoparticles are in wurtzite single crystalline phase and no secondary phases were detected which indicated that Co substituted Zn ions. The energy band gap of the ZnO host matrix decreases gradually by increasing the doping concentration. The photoluminescence spectra exhibit intensive emission in the UV range. This emission presents a small shift to longer wavelengths and remarkable decreases in the intensity with increasing Co content. The Magnetic measurements at room temperature reveal diamagnetic behavior for the samples with lower doping concentrations; however, at higher Co content, we noted the presence of both paramagnetic and ferromagnetic behaviors.

Effect of Calcination on Properties of Cobalt Doped ZnO Nanoparticles

Diluted magnetic semiconductors are of great interest in spintronic applications due to the involvement of both spin and charge. We have investigated effect of calcination on structural and magnetic properties of cobalt doped zinc oxide nanoparticles prepared by sol-gel method. Cobalt doped ZnO nanoparticles are calcined at different temperatures ranging from 100°C to 500°C. X-ray diffraction analyses (XRD) and vibrating sample magnetometer (VSM) was used to characterize the calcined cobalt doped ZnO nanoparticles. XRD results confirm the formation of hexagonal wurtzite structure of the calcined cobalt doped ZnO. Decrease in crystallite size (34 to 27.7nm) is observed with increasing calcination temperature. Shrinkage in lattice parameters has been observed with calcination temperature. Ferromagnetic behavior was observed with increase in saturation magnetization till calcination temperature of 300°C.

On the preparation, structural and magnetic properties of ZnO:Co nanoparticles

Cobalt doped ZnO nanoparticles were synthesized by microwave irradiation technique for their structural and magnetic properties. Initially the samples were exhibit single phase standard ZnO wurtzite structure ( x ≤ 0.1) further Co concentration increase Co 3 O 4 phase was confirmed by X-ray diffraction analysis. The average crystallite sizes of cobalt doped ZnO particles were calculated by Debye-Scherrer formula and are nearly 25 nm. The hexagonal wurtzite structure of ZnO and destroyed cubical Co 3 O 4 were identified in the TEM images. The presence of metal oxide was confirmed by Fourier transform infrared spectral analysis result. The micro-Raman analysis confirmed the existence of Co 3 O 4 phase. Magnetization measurements performed by using vibrating sample magnetometer determine the paramagnetic behavior for the synthesized samples.

Cobalt doped ZnO quantum dots in a monolayer: do the bands depend on the alignment of the magnetic domain?

We show that energy of conduction and valence band-edges of ferromagnetic quantum dots in a monolayer changes when the nanocrystals' magnetic moments are aligned or oriented. Here we characterize a monolayer of cobalt-doped ZnO nanoparticles with scanning tunnelling microscopy. We observed that when their magnetic domains are aligned or oriented, transport gap of the quantum dots decreases. The decrease in the gap is primarily due to an increase in valence band-edge energy. We vary the content of cobalt in the doped-nanoparticles and also the degree of alignment of their magnetic moments. Measurements of band-edges and transport gap of such nanocrystals in a monolayer support our observation. The results hence provide a new route to control transport gap and band-edge energies of ferromagnetic quantum dots through the process of alignment of magnetic domains of the nanocrystals.

Absence of free carrier and paramagnetism in cobalt-doped ZnO nanoparticles synthesized at low temperature using citrate sol–gel route

Cobalt-doped ZnO nanoparticles have been synthesized using a simple citrate sol–gel auto-combustion method. The XRD confirms nano-single phase and Wurtzite structure. Increased cobalt solubility from 15 to 20 % was observed in ZnO matrix due to low temperature synthesis. Room temperature paramagnetic contribution is observed for all the samples, since cobalt is a neutral dopant and no free carriers are produced. Normally, ferromagnetism is observed in ZnO due to carrier mediated interaction between transition metal ions and free carriers. Hence no ferromagnetism was observed and only paramagnetism was observed due to non-availability of free carriers for long range ferromagnetic interaction in our system. At higher doping, peak broadening of the highly intense XRD peak (101) was observed indicating formation of cobalt cluster (metal–metal) which reduces paramagnetism due to antiferromagnetic interaction and the magnetization value decreases to 0.00456 from 0.0076 emu/g. Absence of photoluminescence peak at 520 nm due to oxygen related defects also supports, the presence of paramagnetism in our samples, since oxygen defects are the another source of ferromagnetism in ZnO. Indirect evidence for the presence of cobalt clustering is also obtained from the photoluminescence studies which lead to concentration quenching of peaks. Photoluminescence studies exhibit NBE peak at 412 nm and defect peaks at 471 and 672 nm. The intensity of red emission peak at 672 nm remains constant whereas the intensity of the peaks at 412 and 471 nm increases and then decreases due to doping induced disorder leading to concentration quenching.