Increase In Co Concentration Research Articles

Structural, optical, magnetic and photocatalytic properties of Co doped CuS diluted magnetic semiconductor nanoparticles

Pristine and Co doped covellite CuS nanoparticles were synthesized in aqueous solution by facile chemical co-precipitation method with Ethylene Diamine Tetra Acetic Acid (EDTA) as a stabilizing agent. EDAX measurements confirmed the presence of Co in the CuS host lattice. Hexagonal crystal structure of pure and Co doped CuS nanoparticles were authenticated by XRD patterns. TEM images indicated that sphere-shape of nanoparticles through a size ranging from 5 to 8nm. The optical absorption edge moved to higher energies with increase in Co concentration as indicated by UV–vis spectroscopy. Magnetic measurements revealed that bare CuS sample show sign of diamagnetic character where as in Co doped nanoparticles augmentation of room temperature ferromagnetism was observed with increasing doping precursor concentrations. Photocatalytic performance of the pure and Co doped CuS nanoparticles were assessed by evaluating the degradation rate of rhodamine B solution under sun light irradiation. The 5% Co doped CuS nanoparticles provide evidence for high-quality photocatalytic activity.

Effect of cobalt concentration on morphology of Co-doped SnO2 nanostructures synthesized by solution combustion method

Cobalt (Co) doped SnO2 nanostructures were synthesized by solution combustion synthesis and structural and optical properties were investigated. The morphology of SnO2 nanostructure was changed from nanoflakes to nanospheres via nanorods and crystallite size was reduced from 4.7 nm to 2.7 nm as increase of Co doping concentration. The obvious diffraction peaks of X-ray diffraction and clear lattice fringes confirmed the formation of rutile phase of SnO2. The absorption edge transfers to upper wavelengths and energy bandgap was found to be decreasing by increase of Co concentration. The observed absorption band shows the characteristic band of d–d transitions of Co2+ ions in octahedral site symmetry. Vibrational spectroscopy shows the characteristic absorption peaks of Sn–O and confirms the incorporation of Co ions in SnO2 lattice.

The Effect of Co Doping on the Structural, Optical, and Magnetic Properties of Fe-Doped ZnO Nanoparticles

In the present work, pure ZnO, Zn0.99Fe0.01O (ZFO), and Zn1−x−yFexCoyO (x = 0.01, y = 0.01, 0.03, 0.05) Fe–Co co-doped ZnO dilute magnetic semiconductors were successfully synthesized by using the wet co-precipitation method. Pure and doped samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, UV–Vis spectroscopy and vibrating sample magnetometer. The X-ray diffraction (XRD) analysis of pure and doped samples confirms the formation of hexagonal wurtzite structure, without formation of any other secondary and impurity phases. The surface morphology of pure and doped ZnO nanoparticles samples was performed by scanning electron microscopy (SEM) and reveals the formation of spherical nanoparticles with clear and well-defined boundaries. Energy dispersive X-ray spectrometry (EDX) indicates the substitution of dopant Fe2+ and Co2+ successfully in the lattice site of Zn2+ and results in the formation of single-phase Zn1−x−yFexCoyO. The UV–Visible absorption spectra of all doped and co-doped samples showed blueshift in absorption edge as compared to undoped (pure) ZnO nanoparticles. The magnetic characterization reveals and confirms the room-temperature ferromagnetism (RTFM) in all doped and co-doped samples. Magnetization saturation increases with increase in Co concentration in Fe-doped sample as compared with individual Fe-doped ZnO sample which further reveals that exchange interaction between Fe and Co ions dominates over the Fe–Fe ion interaction.

Glass-forming Ability and Corrosion Resistance of Zr[sbnd]Cu[sbnd]Al[sbnd]Co Bulk Metallic Glass

Investigations were carried out to examine the effects of Co addition on the glass-forming ability (GFA) and corrosion resistance of Zr46Cu46Al8 bulk metallic glass in chloride-containing solution. It is found that the GFA of (Zr46Cu46Al8)100−xCox (x = 0, 1, 2, and 4 at.%) alloys reduces with the increase in Co content and correlates well with the parameters, such as the supercooled liquid region width ΔTx, the reduced glass transition temperature Trg and γ. The corrosion resistance is however found to be enhanced with the increase in Co concentration. The addition of Co causes the enrichment of Zr and Al, but depletes Cu in the surface films, which effectively enhances the corrosion potential and lowers the corrosion current density.

Half-metallicity in Heusler-type Fe2Cr1−xCoxSi alloys

The effects of the substitution of Cr with Co on microstructure, phase composition, structure, magnetic, and electrical properties in (0 ⩽ x ⩽ 1) alloys was investigated to identify the compositions with the potential to exhibit half-metallicity. The microstructural and structural studies revealed that only alloys with x ⩾ 0.5 exhibited the desired single phase L21 full Heusler alloy structure. Both the saturation magnetization (Ms) and Curie temperature (TC) were found to increase with the increase in Co concentration. The experimentally measured Ms values are in good agreement with the Slater–Pauling rule. The electrical resistivity measurements in the temperature range 10–300 K gives indirect evidence of half-metallic behaviour in these alloys at low temperatures. The temperature range in which the half-metallic behaviour was observed also increased with an increase in Co concentration.

The electrochemical hydrogen storage performances of MgxCo100–x (x = 40, 45, 50, 55, 60, 63) body-centered cubic alloys and their Pd-doped system

In this work, a serial of Mg–Co binary alloys with body-centered cubic (BCC) phase were prepared for anode materials of Ni-MH hydrogen storage battery system. TEM/SAED analyses on binary Mg–Co alloys demonstrated that their grains were all in nano-size (∼5 nm) with BCC structure. Electrochemical tests found that, with increase of the concentration of Mg in alloys, theoretical charge–discharge capacities would increase accordingly, discharge kinetics (exchange current densities and hydrogen diffusion abilities) were improved as well. However, the tested capacity will attain a maximum value (325 mAh g−1) at the point of Mg55Co45 and deteriorate subsequently with continuous increase of Mg content, which was possibly due to the optimized lattice parameter value and Mg corrosion. The increase of Co concentration in the binary alloys effectively inhibited the corrosion of the electrodes. On the basis of Mg–Co binary alloys, the Pd-doped ternary alloys with BCC structure were also prepared by means of ball milling. These alloys possess greatly enhanced discharge capacities (280–500 mAh g−1) from binary ones. It shows that with increase of the Mg content, the discharge capacities of Mg–Co–Pd ternary alloys will increase monotonously. The additional Pd also improved cycle stabilities, kinetics, and corrosion potentials of the alloys, which was beneficial to improving the properties of Mg–Co hydrogen storage electrodes.

Auto-ignition of biomass synthesis gas in shock tube at elevated temperature and pressure

Ignition delay times of multi-component biomass synthesis gas (bio-syngas) diluted in argon were measured in a shock tube at elevated pressure (5, 10 and 15bar, 1bar = 105Pa), wide temperature ranges (1,100–1,700K) and various equivalence ratios (0.5, 1.0, 2.0). Additionally, the effects of the variations of main constituents (H2:CO = 0.125–8) on ignition delays were investigated. The experimental results indicated that the ignition delay decreases as the pressure increases above certain temperature (around 1,200K) and vice versa. The ignition delays were also found to rise as CO concentration increases, which is in good agreement with the literature. In addition, the ignition delays of bio-syngas were found increasing as the equivalence ratio rises. This behavior was primarily discussed in present work. Experimental results were also compared with numerical predictions of multiple chemical kinetic mechanisms and Li’s mechanism was found having the best accuracy. The logarithmic ignition delays were found nonlinearly decrease with the H2 concentration under various conditions, and the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable. However, the effect of pressure is relatively smaller under current conditions. Sensitivity analysis and reaction pathway analysis of methane showed that R1 (H + O2 = O + OH) is the most sensitive reaction promoting ignition and R13 (H + O2 (+M) = HO2 (+M)), R53 (CH3 + H(+M) = CH4 (+M)), R54 (CH4 + H = CH3 + H2) as well as R56 (CH4 + OH = CH3 + H2O) are key reaction, R53 (CH3 + H (+M) = CH4 (+M)), R54 (CH4 + H = CH3 + H2) have the largest positive sensitivities and the high contribution rate in rich mixture.The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases sharply as the mixture turns rich. Therefore, the ignition delays become longer as the equivalence ratio increases.

Effect of secondary air mass flow rate on the airflow and combustion characteristics and NOx formation of the low‐volatile coal‐fired swirl burner

AbstractLaboratory experiments and industrial‐scale experiments on 300‐MW low‐volatile coal‐fired boiler were performed under deep air staging. Aerodynamic characteristics, gas temperature and concentrations, furnace temperature, and boiler efficiency were measured for various secondary air mass flow rates. Under deep air staging, a steady central recirculation zone forms near the burner nozzle. With a decreasing flow rate, the swirl intensity and maximum axial, radial, and tangential velocities decrease; between X/D = 0–0.8, the velocity decay rate decreases, and the relative reverse flow rate decreases. In the early stage, the maximum axial mixing rate decreases; the primary air concentration increases slightly. A decrease in secondary air‐box damper opening decreases the gas temperature and its increasing rate, generating a farther ignition position. In the initial stage, the O2 consumption rate and CO concentration increase, while the NOx concentration decreases. In the later stage, O2 concentration remains almost constant below 2%, and the CO concentrations exceed 15 000 ppm, which restrain NOx formation notably. Near the water‐cooled wall, flue gas temperature varies slightly, and O2 concentration exceeds 3%. Along radial direction 1.5364 ≤ R0/D0 ≤ 1.8708, the O2 concentrations are below 2%, and CO concentrations exceed 6000 ppm. A decreasing damper opening decreases furnace temperature in the primary combustion zone slightly. NOx emission decreases from 833.4 to 769.9 mg/m3 (6% O2), unburnt carbon increases from 5.18% to 6.84%, and boiler efficiency decreases from 91.53% to 90.99%. Copyright © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.

STUDY OF MARTENSITIC TRANSFORMATION AND STRAIN BEHAVIOR IN Ni50-xCoxMn39Sn11 (x=0, 2, 4, 6) HEUSLER ALLOYS

The crystal structure, phase transformations and magnetic properties for Ni50xCoxMn39Sn11 (x=0, 2, 4, 6) Heusler alloys have been systematically studied by means of structure and magnetism measurements. The results show that with increase of Co concentration, the martensitic transformation temperatures are obviously decreased, while the Curie temperatures of austenite are gradually increased, and they present different structures at room temperature. At the same time, with increasing Co content, the austenitic magnetism rapidly enhances, while the martensitic magnetism almost keeps unchanged. This leads to a significant improvement of difference magneti*国家自然科学基金项目11364035, 11404186和51371111,云南省科技厅应用基础研究项目2013FZ110和2012FD051,以及曲靖师范 学院创新团队研究计划项目TD201301资助 收到初稿日期: 2015-01-26,收到修改稿日期: 2015-04-28 作者简介:李 哲,男, 1981年生,副教授 DOI: 10.11900/0412.1961.2015.00071 第1010-1016页 pp.1010-1016

Enhanced room temperature ferromagnetism in electrodeposited Co-doped ZnO nanostructured thin films by controlling the oxygen vacancy defects

We report the growth of un-doped and cobalt doped ZnO nanostructures fabricated on FTO coated glass substrates using electrodeposition method. A detailed study on the effects of dopant concentration on morphology, structural, optical, and magnetic properties of the ZnO nanostructures has been carried out systematically by varying the Co concentration (c.Co) from 0.01 to 1 mM. For c.Co ≤ 0.2 mM, h-wurtzite phase with no secondary phases of Co were present in the ZnO nanostructures. For c.Co ≤ 0.2 mM, the photoluminescence spectra exhibited a decrease in the intensity of ultraviolet emission as well as band-gap narrowing with an increase in dopant concentration. All the doped samples displayed a broad emission in the visible range and its intensity increased with an increase in Co concentration. It was found that the defect centers such as oxygen vacancies and zinc interstitials were the source of the visible emission. The X-ray photoelectron spectroscopy studies revealed, Co was primarily in the divalent state, replacing the Zn ion inside the tetrahedral crystal site of ZnO without forming any cluster or secondary phases of Co. The un-doped ZnO nanorods exhibited diamagnetic behavior and it remained up to a c.Co of 0.05 mM, while for c.Co > 0.05 mM, the ZnO nanostructures exhibited ferromagnetic behavior at room temperature. The coercivity increased to 695 G for 0.2 mM Co-doped sample and then it decreased for c.Co > 0.2 mM. Our results illustrate that up to a threshold concentration of 0.2 mM, the strong ferromagnetism is due to the oxygen vacancy defects centers, which exist in the Co-doped ZnO nanostructures. The origin of strong ferromagnetism at room temperature in Co-doped ZnO nanostructures is attributed to the s-d exchange interaction between the localized spin moments resulting from the oxygen vacancies and d electrons of Co2+ ions. Our findings provide a new insight for tuning the defect density by precisely controlling the dopant concentration in order to get the desired magnetic behavior at room temperature.

Lean Atmospheres for Sintering of Chromium Alloyed Powder Metallurgy Steels

Thermodynamic evaluation of the reducing and carburizing activities of N2‐based lean sintering atmospheres containing CO, H2, and small amounts of hydrocarbons with their total content not exceeding 5 vol% was performed with regard to sintering of chromium alloyed PM steels. Based on the thermodynamic evaluation, three sintering atmosphere compositions were chosen for experimental studies. For the chosen sintering atmospheres, the effect of graphite addition, sintering temperature, holding time and cooling rate on the microstructure, and final oxygen and carbon contents was studied. The compacts without admixed graphite exhibited a microstructure with pearlitic, and in some cases bainitic regions, with their content varying according to the temperature profile and increasing with increase in CO concentration in the atmosphere. These results are indicative of the carburizing ability of the atmospheres which was also supported by the results from chemical analysis. Pearlitic–bainitic microstructures were observed in the compacts admixed with graphite after sintering which suggests that the proposed atmospheres provide conditions sufficient for at least partial reduction of the surface iron oxide during sintering. The final oxygen content obtained after sintering in the proposed lean atmospheres was higher than that obtained in the case of nitrogen/hydrogen mix but still was within acceptable limits. The results obtained suggest that the proposed lean CO and H2 containing sintering atmosphere compositions are promising candidates for robust sintering of chromium alloyed PM steels.

Nanocasting synthesis of co-doped In2O3: a 3D diluted magnetic semiconductor composed of nanospheres

Mesoporous 3D nanosphere arrays of In2−x Co x O3 (x = 0, 0.01, 0.03, 0.05, and 0.07) were synthesized via nanocasting using the mesoporous silica LP-FDU-12 as a hard template. The mesostructure, morphology, optical properties, and magnetic properties of the materials were determined. The diameter of the nanospheres was about 15–22 nm, and the nanospheres stacked into 0.5–5 μm arrays (particles). The data revealed that the Co ions entered the lattice of the In2O3 bixbyite phase leading to a reduction of the cell parameter. The result also demonstrated that the size of the mesostructured ordering was approximately the same as the particle diameter. Moreover, the optical band gap of Co-doped In2O3 decreased monotonically with the increase of Co concentration and the room-temperature photoluminescence was also observed. The un-doped In2O3 exhibited a ferromagnetic behavior superimposed on a diamagnetic background, while the doped In2O3 displayed a room-temperature ferromagnetic behavior superimposed on a paramagnetic background, which may be correlated with the surface texture of the mesostructure. The mesoporous diluted magnetic semiconductors may find their applications in spintronic nanodevices because of their 3D uniform arrangement of nanospheres and their room-temperature ferromagnetic behavior.

Investigations on a new internally-heated tubular packed-bed methanol–steam reformer

Small-scale reformers for hydrogen production through steam reforming of methanol can provide an alternative solution to the demand of continuous supply of hydrogen gas for the operation of Proton Exchange Membrane Fuel Cells (PEMFCs). A packed-bed type reformer is one of the potential designs for such purpose. An externally heated reformer has issues of adverse lower temperature in the core of the reformer and significant heat loss to the environment thus impacting its performance. Experimental and numerical studies on a new concept of internally heated tubular packed-bed methanol–steam reformer have been reported in this paper with improved performance in terms of higher methanol conversion and reduced heat losses to surroundings. CuO/ZnO/Al2O3 is used as the catalyst for the methanol–steam reforming reaction and a rod-type electric heater at the center of the reactor is used for supplying necessary heat for endothermic steam reforming reaction. The vaporizer and the reformer unit with a constant volume catalyst bed are integrated in the annular section of a tubular reformer unit. The performance of the reformer was investigated at various operating conditions like feed rate of water-methanol mixture, mass of the catalyst and reforming temperature. The experimental and numerical results show that the methanol conversion and CO concentration increase with internal heating for a wide range of operating conditions. The developed reformer unit generates 50–80 W (based on lower heating value) of hydrogen gas for applications in PEMFCs. For optimized design and operating conditions, the reformer unit produced 298 sccm reformed gas containing 70% H2, 27% CO2 and 3% CO at 200–240 °C which can produce a power output of 25–32 W assuming 60% fuel cell efficiency and 80% of hydrogen utilization in a PEMFC.

The effect of the starting powders particle size on the electrical properties of sintered Co doped ZnO varistors

Pure and Co doped zinc oxide nanopowders have been synthesized by sol–gel route. The obtained powders, after calcination in air at 550, 650, 750 and 850 °C, were consolidated and sintered using microwaves. The effect of sintering temperature on the density and electrical properties was investigated. The best characteristics are obtained using a sintering temperature equal to 1,075 °C. The powders and sintered samples are characterized by X-ray diffraction, microstructure images, density and electrical measurements. The studied nanopowders have: a density of 5.22 (g/cm3), a breakdown voltage of 446.5 V/mm and a coefficient of non-linearity of 11.48 for ZnO doped with 7 mol% Co, and a density of 5.19 (g/cm3), a breakdown voltage of 292.5 V/mm and a non-linearity coefficient of 11.62 for ZnO doped with 5 mol% Co. The XRD results indicate that pure and Co doped ZnO powders are solid solutions crystallizing in pure wurtzite structure, and consisted of a mixture of nanoparticles with an average grain size between 36 and 210 nm. The grain size decreases with increasing Co concentration and reaches its lowest value at 7 mol% Co and increases with increasing temperature. On the other hand the increase of Co concentration leads to a decrease of the porosity and an increase of the density of samples sintered at 1,070 °C and leads also to an increase of the electric field (E) in the non-linear area. The samples of 5 mol% Co sintered at 1,075 °C, show that the non-linear coefficient α decreases by increasing of calcined temperature, therefore increases with decreasing of grain and particle sizes.

Effect of Co Doping on Structural and Optical Properties of NiO Thin Films Prepared by Chemical Spray Pyrolysis Method

In this work, nickel-cobalt oxide (Ni(1-x)CoxO) thin films, where x = 0, 4, 6 and 8 % have been successfully deposited on glass substrates by chemical spray pyrolysis (CSP) technique at substrate temperature of (400 °C) and thickness of about 300 nm. The structural and optical properties of these films have been studied using XRD, AFM, and UV-Visible spectroscopy. The XRD results showed that all films are polycrystalline in nature with cubic structure and preferred orientation along (111) plane. The crystallite size was calculated using Scherrer formula and it is found that the undoped NiO sample has maximum crystallite size (51.16 nm). AFM results showed homogenous and smooth thin films. The absorbance and transmittance spectra have been recorded in the wavelength range of (300-900) nm in order to study the optical properties. The optical energy gap for allowed direct electronic transition was calculated using Tauc equation. It is found that the band gap decreases as the Co-concentration increases and the band gap values were in the range of 3.58-3.66 eV and 3.58 eV for the prepared Nickel-Cobalt Oxide thin films. The Urbach energy increases as the Co-concentration increases and the Urbach. The optical constants including (absorption coefficient, real and imaginary parts of dielectric constant) were also calculated as a function of photon energy. Refractive index and extinction coefficient for Nickel-Cobalt Oxide thin films were estimated as a function of wavelength.

Effect of Co Doping on Structural and Optical Properties of NiO Thin Films Prepared by Chemical Spray Pyrolysis Method

In this work, nickel-cobalt oxide (Ni(1-x)CoxO) thin films, where x = 0, 4, 6 and 8 % have been successfully deposited on glass substrates by chemical spray pyrolysis (CSP) technique at substrate temperature of (400 °C) and thickness of about 300 nm. The structural and optical properties of these films have been studied using XRD, AFM, and UV-Visible spectroscopy. The XRD results showed that all films are polycrystalline in nature with cubic structure and preferred orientation along (111) plane. The crystallite size was calculated using Scherrer formula and it is found that the undoped NiO sample has maximum crystallite size (51.16 nm). AFM results showed homogenous and smooth thin films. The absorbance and transmittance spectra have been recorded in the wavelength range of (300-900) nm in order to study the optical properties. The optical energy gap for allowed direct electronic transition was calculated using Tauc equation. It is found that the band gap decreases as the Co-concentration increases and the band gap values were in the range of 3.58-3.66 eV and 3.58 eV for the prepared Nickel-Cobalt Oxide thin films. The Urbach energy increases as the Co-concentration increases and the Urbach. The optical constants including (absorption coefficient, real and imaginary parts of dielectric constant) were also calculated as a function of photon energy. Refractive index and extinction coefficient for Nickel-Cobalt Oxide thin films were estimated as a function of wavelength.

Room temperature magnetoelectric coupling in Zn1−xCoxO/BaTiO3 bilayer system

We report on room temperature magnetoelectric coupling in Zn1−xCoxO/BaTiO3 (x = 0.02, 0.05, and 0.10) bilayer thinfilm multiferroic system (BLS) grown on SrTiO3 (100) substrate. All the BLSs exhibit room temperature ferroelectric response. The BLS with x = 0.02 is paramagnetic, while the BLS with x = 0.05 and 0.10 is weakly ferromagnetic. Increase in Co concentration of the BLS results in reduction of permittivity and electric polarization along with increase of coercive voltage, coercive field, and magnetic moment. The d33 value change from 23 pm/V to 30 pm/V with increase in external magnetic field from 1500 G to 2500 G for BLS with x = 0.05. This shows that Zn1−xCoxO/BaTiO3 is magnetoelectrically coupled at room temperature.

Investigation of multifunctional properties of Mn50Ni40−xCoxSn10 (x = 0–6) Heusler alloys

A series of Co doped Mn50Ni40−xCoxSn10 (x=0, 1, 2, 2.5, 3, 4 and 6) Heusler alloys has been investigated for their structural, magnetic, magnetocaloric and exchange bias properties. The martensitic transition temperatures are found to decrease with the increase in Co concentration due to the decrease in valence electron concentration (e/a ratio). The Curie temperature of austenite phase increases significantly with increasing Co concentration. A large positive magnetic entropy change (ΔSM) of 8.6 and 10.5J/kgK, for a magnetic field change of 50kOe is observed for x=0 and 1 alloys, and ΔSM values decreases for higher Co concentrations. The relative cooling power shows a monotonic increase with the increase in Co concentration. Large exchange bias fields of 920 Oe and 833 Oe have been observed in the alloys with compositions x=0 and 1, after field cooling in presence of 10kOe. The unidirectional anisotropy arising at the interface between the frustrated and ferromagnetic phases is responsible for the large exchange bias observed in these alloys. With increase in Co, the magnetically frustrated phase diminishes in strength, giving rise to a decrease in the exchange bias effect for larger Co concentration. The exchange bias fields observed for compositions x=0 and 1, in the present case are larger than that reported for Co doped Ni–Mn–Z (Z=Sn, Sb, and Ga) alloys. Temperature and cooling field dependence of the exchange bias field and coercivity have been related to the change in the unidirectional anisotropy. This study shows that an optimum Co doping results in the tuning and enhancement of the multifunctional properties in Mn rich Ni–Mn–Sn alloys.

Quasi-one-dimensional nanostructured cobalt (Co) intercalated vanadium oxide (V2O5): Peroxovanadate sol gel synthesis and structural study

Nanostructured cobalt vanadium oxide (V2O5) xerogels spread onto crystalline Si substrates were synthesized via peroxovanadate sol gel route. The resulting products were characterized by distinct experimental techniques. The surface morphology and the nanostructure of xerogels correlate with Co concentration. The decrease of the structural coherence length is followed by the formation of a loose network of nanopores when the concentration of intercalated species was greater than 4at% of Co. The efficiency of the synthesis route also drops with the increase of Co concentration. The interaction between the Co(OH2)62+ cations and the (H2V10O28)4− anions during the synthesis was suggested as a possible explanation for the incomplete condensation of the V2O5 gel. Finally the experimental results points for the intercalation of Co between the bilayers of the V2O5. In this scenario two possible preferential occupation sites for the metallic atoms in the framework of the xerogel were proposed.

Structural, morphological and optical properties of undoped and Co-doped ZnO thin films prepared by sol–gel process

Undoped and Co-doped ZnO thin films with different amounts of Co have been deposited onto glass substrates by sol–gel spin coating method. Zinc acetate dihydrate, cobalt acetate tetrahydrate, isopropanol and monoethanolamine (MEA) were used as a precursor, doping source, solvent and stabilizer, respectively. The molar ratio of MEA to metal ions was maintained at 1.0 and a concentration of metal ions is 0.6 mol L−1. The Co dopant level was defined by the Co/(Co + Zn) ratio it varied from 0 to 7 % mol. The structure, morphology and optical properties of the thin films thus obtained were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis), photoluminescence (PL) and Raman. The XRD results showed that all films crystallized under hexagonal wurtzite structure and presented a preferential orientation along the c-axis with the maximum crystallite size was found is 23.5 nm for undoped film. The results of SEM indicate that the undoped ZnO thin film has smooth and uniform surface with small ZnO grains, and the doped ZnO films shows irregular fiber-like stripes and wrinkle network structure. The average transmittance of all films is about 72–97 % in the visible range and the band gap energy decreased from 3.28 to 3.02 eV with increase of Co concentration. DRX, EDX and optical transmission confirm the substitution of Co2+ for Zn2+ at the tetrahedral sites of ZnO. In addition to the vibrational modes from ZnO, the Raman spectra show prominent mode representative of ZnyCo3−yO4 secondary phase at larger values of Co concentration. PL of the films showed a UV and defect related visible emissions like violet, blue and green, and indicated that cobalt doping resulted in red shifting of UV emission and the reduction in the UV and visible emissions intensity.