Properties Of Co-doped ZnO Nanoparticles Research Articles

Structural, morphological, and gas sensing properties of Co-doped ZnO nanoparticles

Structural, morphological, and gas sensing properties of Co-doped ZnO nanoparticles

Optimizing physical properties of Co-doped ZnO nanoparticles: Controlling oxygen vacancy and carrier concentration

The origination of magnetism in Co-doped ZnO-based dilute magnetic semiconductor (DMS) with room temperature ferromagnetism (RTFM) has some controversy. In this paper, Zn1-xCoxO nanoparticles (x = 0.00, 0.01, 0.03 and 0.05) were prepared using sol-gel method. The relationship between oxygen vacancy or carrier concentration and magnetic properties were investigated. The hexagonal wurtzite structure of all the samples was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The morphological features were investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the optical properties, including energy gap, were analyzed by ultraviolet–visible spectroscopy (UV–vis). The number of oxygen vacancies (VO) was measured by X-ray photoelectron spectroscopy (XPS), which is positively correlated with the Co content. The RTFM properties were analyzed by physical properties measurement system (PPMS). The results reveal that a higher carrier concentration is critical to promoting ferromagnetism. And, the magnetism of Co-doped ZnO may originate from VO defects or the carrier.

Structural, chemical and optical properties of Co-doped ZnO nanoparticles obtained from an aqueous method

Co-doped ZnO nanoparticles were obtained by aqueous synthesis with an annealing at 400 °C. The incorporation efficiency of Co into the ZnO lattice was found to be around 100%. The nanoparticles keep the wurzite phase in all the Co-doping range. However, the lattice parameters and strain change monotonously mainly up to about 1 at% Co doping. The luminescence, which was signed by a wide emission band that rises from 2.2 eV to 3.3 eV, reduces the intensity as the Co-doping increases. That behavior was related to the strong influence of Co after substituting Zn in any place of the lattice. Additionally, a different approach was made on the O 1s XPS peak at binding energy BE ≈ 531.5 eV. We associate this peak to O–O re-bond caused by inward relaxation due to Zn vacancy. Such consideration is valid for doped and non-doped ZnO.

Hydrogen Sensing Properties of Co-Doped ZnO Nanoparticles

In this study, the gas sensing properties of Co-doped ZnO nanoparticles (Co-ZnO NPs) synthesized via a simple sol-gel method are reported. The microstructure and morphology of the synthesized Co-ZnO NPs were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. Co-ZnO NPs were then used for developing a conductometric gas sensor for the detection, at mild temperature, of low concentration of hydrogen (H2) in air. To evaluate the selectivity of the sensor, the sensing behavior toward some VOCs such as ethanol and acetone, which represent the most important interferents for breath hydrogen analysis, was also investigated in detail. Results reported demonstrated better selectivity toward hydrogen of the Co-ZnO NPs sensor when compared to pure ZnO. The main factors contributing to this behavior, i.e., the transition from n-type behavior of pristine ZnO to p-type behavior upon Co-doping, the modification of oxygen vacancies and acid-base characteristics have been considered. Hence, this study highlights the importance of Co doping of ZnO to realize a high performance breath hydrogen sensor.

Structure, photoluminescence, and magnetic properties of Co-doped ZnO nanoparticles

Co-doped ZnO-nanoparticles were synthesized using the sol–gel method. The microstructure, morphology, and physical properties of the particles were studied using several analytical methods, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–vis absorption spectroscopy, photoluminescence spectrometry, and vibrating sample magnetometry. The structure of the Co-doped ZnO nanoparticles was identified as hexagonal wurtzite, which suggests that Co2+ can replace the Zn2+ sites in the ZnO crystal lattice without forming a secondary phase. For Co-doped samples, the optical energy bandgap decreased with an increase in the Co content. The absorption-band edges were 565, 610, and 653 nm, which correspond to the d–d transition of Co2+ ions in the tetrahedral field of ZnO. The PL spectra revealed a strong defect-emission, which indicates that defects may stabilize the ferromagnetic order. Room-temperature ferromagnetism was observed in Co-doped ZnO nanoparticles according to M–H measurements. Furthermore, the magnetization increased with increasing Co concentration. These findings suggest that Co-doped ZnO nanoparticles could promote the development of semiconductor devices with ferromagnetic properties above room temperature in magneto-optical and spintronic applications.

The influence of Co content on the luminescence properties of Co-doped ZnO nanoparticles

Co-doped ZnO nanoparticles have been synthesized by co-precipitation technique. Photoluminescence spectra change in the range from 350 nm to 600 nm and remain unchanged at about 690 nm with the Co content increase. The UV emission is assigned to exciton emission. The density of band-edge states increases with Co content. The blue emission could be ascribed to the recombination of electrons in Co[Formula: see text] ions and holes in the valence band, whose relative intensity and full-width at half-maximum (FWHM) increase with the increase of cobalt concentration. The red emission results from the intra-d-shell emission at Co, which is independent of Co content. The relative density and energy-level position of green emission centers are also influenced by Co content.

Optical and magnetic properties of Co-doped ZnO nanoparticles and the onset of ferromagnetic order

In this study, we report on the optical and magnetic properties of Co-doped ZnO nanoparticles with increasing Co-content (CoxZn1−xO; x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.070, and 0.100) synthesized by the combustion reaction method. The X-ray diffraction patterns and the Raman spectra of all samples indicated the formation of the ZnO hexagonal wurtzite phase (space group C46V). The Raman data also show the formation of a secondary Co3O4 phase, which is barely seen in the X-ray spectra. Photoacoustic spectroscopy and electron paramagnetic resonance confirm the presence of the two phases (CoxZn1−xO and Co3O4). Vibrating sample magnetometer measurements performed at room temperature exhibited hysteresis loops, indicating the presence of long-range magnetic ordering in the samples. Analysis of the magnetization as a function of magnetic field and temperature shows that the ferromagnetism in the as-synthesized samples comes from small Co-metallic inclusions, with an estimated radius of about 4.8 nm and blocking temperature around 595 K.

Effect of Co doping on the physical properties of Co-doped ZnO nanoparticles

Cobalt-doped (1–5 wt%) ZnO nano-particles have been fabricated by sol–gel route. Fourier Transform Infrared Spectroscopy shows that Co has successfully substituted Zn in ZnO lattice. The average optical transmittance of the nanoparticles is found to be >75% in the visible region. These transparent semiconductor nanoparticles retain a band gap in the ultraviolet (3.99–3.2 eV) and high transparency across the Co doping range of (1–5 wt%). Co doped ZnO nanoparticles show room-temperature ferromagnetism. Surface study shows that nanoparticles are porous in nature. The grain size of the nanoparticles is varied between 33 and 63 nm.

Magnetic properties of Co-doped ZnO nanoparticles

The magnetism in nanoparticulate powders of Zn1-xCoxO with 0≤x≤0.09 synthesized by a combustion reaction technique is investigated in a broad range of temperatures (5≤T≤750K) for applied magnetic fields up to 85kOe. The hysteresis loops indicated the presence of both ferromagnetic and paramagnetic ordering at room temperature. An additional antiferromagnetic phase was observed for temperatures below 260K. A particle model that can account for the results is that the some of doping Co2+ ions are not interagent among themselves, a small quantity form clusters, leading to the ferromagnetic ordering with some of the particles in the superparamagnetic state, and few others Co2+ ions form CoO at the grain boundary yielding the antiferromagnetic phase. It was also found that a modified Langevin function can be used for describing the H− dependence for magnetization data.

Structural, optical, dielectric, magnetic and magnetoelectric properties of Co-doped ZnO nanoparticles

Zn1–xCoxO (x = 0.0, 0.06, 0.12, 0.18) nanoparticles were prepared by chemical precipitation method using hydroxyoxalate type precursors and thermal decomposition process. All the samples crystallize in hexagonal wurtzite structure. Cobalt incorporation into the ZnO structure was confirmed by electron diffraction X-ray, transmission electron microscopy and UV–Vis spectroscopy. The increase in the observed band gap of the ZnO nanoparticles compared to that of the bulk ZnO is explained on the basis of size effect. The fluorescence spectra confirm the excitonic peak at 366 nm corresponding to near band edge emission for undoped and cobalt doped ZnO. The ferromagnetism of the lowest concentration cobalt doped ZnO nanoparticle has been evidenced by SQUID. Dielectric properties and magnetoelectric coupling have also been analyzed.

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.

Structural and Optical Properties of Co-Doped ZnO Nanoparticles Synthesized by Precipitation Method

This article deals with the synthesis and characterization of cobalt-doped zinc oxide nanopowder prepared by precipitation method. Prepared nanopowders were analyzed for morphological, structural and optical properties. X-ray diffraction study confirmed the substitution of cobalt ion without disturbing the basic wurtzite structure of zinc oxide. The average particle size was found to increase on cobalt doping as compared to undoped zinc oxide. The lattice constants increased with composition. Scanning electron microscope study also confirmed the existence of particles in nanometer size. Furthermore, scanning electron micrograph confirmed the increase of particle size on cobalt doping. Optical transmittance spectrum shows that cobalt dopant decreases the transmittance of zinc oxide less than 60%. Energy gap of the samples determined from the absorption spectrum shows that it decreases with the increase of cobalt content. The blue and green emission lines of photoluminescence spectrum also confirmed the substitution of cobalt ion in zinc oxide.

Physical structure and optical properties of Co-doped ZnO nanoparticles prepared by co-precipitation

The structural and optical properties of cobalt-doped zinc oxide (Co-doped ZnO) nanoparticles have been investigated. The nanopowder with Co concentrations up to 5 at% was synthesized by a co-precipitation method. The physical structure and the chemical states of the Co-doped ZnO were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV–Visible reflectance and cathodoluminescence (CL) spectroscopy. The results show that cobalt ions predominantly occupy Zn2+ sites in the wurtzite crystal lattice and possess a valence state of 2+. CL analysis revealed that the incorporation of Co2+ creates a new emission band at 1.85 eV, but quenched the near-band-edge luminescence.

Synthesis and Optical Properties of Co-doped ZnO Nanoparticles

Abstract Pure and Co-doped ZnO nanoparticles were synthesized by a sol–gel method. The structure, morphology, and properties of as-prepared samples have been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and ultraviolet–visible absorption spectroscopy (UV–vis) as well as by photoluminescence (PL) spectrometry. The results showed that the diameter of the well-dispersed Co-doped ZnO nanoparticles is about 5 nm and that dopant Co2+ ions substitute Zn2+ ions sites in ZnO nanocrystals without forming any secondary phase. The optical measurements show that the Co doping can effectively tune energy band structure and enrich surface states in both UV and VL regions, which lead to novel PL properties of ZnO nanoparticles. Compared to the undoped sample, a new emission caused by transitions between localized Co2+ d levels appears at 624 nm.

Structure and magnetic properties of Co-doped ZnO nanoparticles

In this work we carefully analyze the role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. We show that a close correlation exists between microstructural features and the appearance of ferromagnetism. Both shape and size of the particles, as well as the microstructure, can be controlled by changing the pressure inside the evaporation chamber, as evidenced by transmission electron microscopy micrographs and high resolution electron microscopy (HREM). X-ray diffraction patterns and HREM make evident the absence of any significant Co segregation or any other phase different from w\"urtzite type ZnO. On the other hand, electron energy loss spectroscopy analyses performed on several particles of w\"urtzite type ZnO yielded an average Co concentration in good agreement with the nominal composition. Samples prepared in low pressure $(\ensuremath{\approx}10\phantom{\rule{0.3em}{0ex}}\text{Torr})$ exhibit a very homogeneous microstructure and are ferromagnetic at low temperature but they have a very small saturation moment, well below that expected for a ${\mathrm{Co}}^{2+}$ ion. Conversely, samples prepared at higher pressure conditions $(\ensuremath{\approx}70--100\phantom{\rule{0.3em}{0ex}}\text{Torr})$ show a defective microstructure and are paramagnetic and increasing the Co content does not induce ferromagnetism.

Magnetic properties of Co-doped ZnO nanoparticles prepared by vaporization-condensation in a solar reactor

We report on the magnetic properties of Co-doped (5% and 10% molar) ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. X-ray diffraction patterns do not show evidence of Co segregation or any other phase different from wurtzite-type ZnO. High-resolution electron microscopy shows no traces of Co clusters or any other secondary phase. Samples prepared at 70–100Torr inside glass reactors are paramagnetic and the dominant magnetic interactions are antiferromagnetic in nature. Increasing the Co content promotes a reinforcement of the antiferromagnetic interactions. A reduction of the high-temperature effective paramagnetic moment is also observed.

Role of the microstructure on the magnetic properties of Co-doped ZnO nanoparticles

We report on the magnetic and structural properties of Co-doped ZnO nanoparticles prepared by the vaporization-condensation method in a solar reactor. X-ray diffraction data and high-resolution electron microscopy (HREM) confirm the total absence of metallic Co clusters or any other phase different from würtzite-type ZnO. Electron energy loss spectroscopy analyses performed on several particles indicate that the oxidation state of Co is +2 and yield an average Co concentration of 4.5at.%, in good agreement with the nominal composition. Transmission electron microscopy micrographs show that shape and size of the particles are strongly dependent on the preparation conditions, as well as the microstructure as evidenced by HREM. Ferromagnetism is only found in samples prepared in vacuum revealing a close correlation between microstructure and magnetic properties.