Co3O4 Nanopowders Research Articles

The evaluation of the electrochemical properties of Co3O4 nanopowders synthesized by autocombustion and sol–gel methods

The evaluation of the electrochemical properties of Co3O4 nanopowders synthesized by autocombustion and sol–gel methods

Microplotter Printing of Co3O4 Films as Receptor Component of Hydrogen Sulfide-Sensitive Gas Sensors

A hierarchically organized Co3O4 nanopowder was obtained via programmed chemical precipitation, exhibiting several levels of microstructural self-organization: the initial particles are 40 ± 5 nm in size (average CSR size is 32 ± 3 nm), have a somewhat distorted rounded shape and are combined into curved chains, which, in turn, form flat agglomerates of approximately 350 ± 50 nm in diameter. The thermal behavior of the semiproduct (β-Co(OH)2) was studied by means of a synchronous thermal analysis (TGA/DSC). The obtained powders were examined by X-ray diffraction analysis (XRD) and Fourier-transform infrared spectroscopy (FTIR). Nanopowder of cobalt(II,III) oxide was employed as a functional ink component for the microplotter printing of the corresponding film on the chip surface, and the preservation of the material’s crystal structure was confirmed by XRD and Raman spectroscopy (RS). The microstructural features of the resulting film were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Kelvin probe force microscopy (KPFM) was utilized to estimate the work function of the material surface, and the scanning capacitance microscopy (SCM) data indicated the intergranular conductivity type. The results of the conducted chemosensor measurements demonstrate that the printed Co3O4 film exhibits hydrogen sulfide selectivity and a rather high sensory response (S = 131% for 100 ppm) to this analyte at an operating temperature of 250 °C. The dependence of the sensor response value and time when detecting H2S in the concentration range of 4–200 ppm was determined and the high reproducibility of the signal was demonstrated.

Unveiling the effect of metallic and oxidized phases of cobalt on acetone hydrodeoxygenation

The impact of metallic and oxidized phases of cobalt and the nature of different catalytic sites in the gas-phase acetone hydrodeoxygenation were evaluated using Co3O4 nanopowder as catalyst. Mild pretreatments (reduction/oxidation up to 400 °C) were performed to vary the oxidation degree of the catalyst, aiming to favor the selective CO bond cleavage. We found that the Co3O4 nanopowder was not able to deoxygenate acetone at 200 °C, even by halving the acetone weight hourly space velocity (WHSV). At this temperature, only products of hydrogenation (isopropanol) and condensation (mesityl oxide) were obtained. A different selectivity was achieved by the pretreated catalysts. Methane formation was enhanced in the prereduced catalyst whereas isopropanol was the favored product after reoxidizing the catalyst at 400 °C. The best compromise was achieved when the prereduced catalyst was calcined at a lower temperature, 200 °C, achieving about 1:1 CC: CO bond cleavage ratio. The results pointed out the nature of different catalytic sites, and their correlation with the reaction pathways was proposed.

Кинетические параметры процесса термического разложения нанопорошка гидроксида кобальта при изотермических условиях

The paper focuses on the kinetic parameters of synthesizing Co3O4 nano-powder by thermal decomposition of hydroxide Co(OH)2 under isothermal conditions. The thermal decomposition of Co(OH)2 nano-powder under isothermal conditions was carried out in a tube furnace in the temperature range from 150 to 200 °C. Findings of research show that the thermal decomposition rate constant at 200 °C is approximately 2.7 times higher than that at 150 °C. Accordingly, for 80 min of thermal decomposition, the process accelerates 1.8 times. The activation energy of this process is approximately 33 kJ/mol, which indicates a mixed reaction mode. The study shows that Co3O4 nanoparticles obtained at the temperature of the maximum thermal decomposition rate, i.e. 180 °C, mainly consist of elongated ovoid and acicular aggregates with an average particle value of 47 nm and the length of up to 200 nm.

Water Oxidation Reaction Catalyzed by Co3O4 Treated with Organic Compounds

This work describes the water oxidation reaction by the spinel Co3O4 nanopowder treated with coordinating organic compounds. Treatment of Co3O4 led to an increase in the Co3+ species at the oxide surface, probably due to stabilization by the adsorbed organic compounds, as well as partial oxidation of Co2+ to Co3+ ions by oxygen from air. Oxygen evolution reaction (OER) tests were carried out against the sacrificial oxidant cerium(IV) ammonium nitrate (CAN), and higher TOF values were obtained for the treated samples in relation to the pure oxide, showing an influence of the Co3+/Co2+ ratio in the catalytic activity. The catalysts could be recycled up to five times without considerable loss of activity.

Hexamine Role on Pseudocapacitive Behaviour of Cobalt Oxide (Co₃O₄) Nanopowders.

Influence of hexamine surfactant concentration on crystallite size, structure, morphology, vibrational and optical properties of cobalt oxide nanopowders were explored. The cobalt oxide nanopowders were synthesized by employing a simple chemical reduction method using cobalt (II) nitrate hexahydrate (Co(NO3)2 ·6H2O) as precursor and sodium borohydride (NaBH4) as reducing agent along with hexamine as surfactant. XRD studies revealed the formation of face-centered cubic (Fd3m) crystalline structure of Co3O4 with an average crystallite size of 8-18 nm. The observed prominent characteristic Raman active modes of A1g, Eg, and F2g revealed the formation of Co3O4 nanopowders. The optical properties of Co3O4 nanopowders are examined by photoluminescence spectra. The obtained IR results confirmed the formation of Co3O4 nanopowders. The band observed 569 cm-1 is the characteristic of the Co3+ ions in the octahedral hole vibration and the 665 cm-1 band is attributable to the Co2+ ions in the tetrahedral hole vibration in the cubic lattice. The estimated specific capacitance of the obtained Co3O4 nanopowders was 291 F/g at 10 mV/s which could be a potential candidate for pseudo capacitive nature of the active materials.

Influence of a Rotary Electromagnetic Field and Mechanical Stimulation on the Production of Cobalt Nanopowder by Reduction with Hydrogen

Cobalt nanopowder is very promising for use in science, engineering, industry, and medicine. The present study focuses on the kinetics of nanopowder production by the reduction of cobalt oxide with hydrogen in an electromagnetic field, under the action of a bed of ferromagnetic particles rotating in the electromagnetic field. Cobalt oxide (Co3O4) nanopowder is obtained by the thermal decomposition of cobalt hydroxide Co(OH)2 deposited from 10% ionic solutions of cobalt nitrate Co(NO3)2 and sodium hydroxide NaOH at 20°C, with pH 9. Cobalt nanoparticles are produced from Co3O4 nanopowder in a modified UAP-3 vortical bed system with a built-in furnace and flow-through reactor. The amplitude of the magnetic induction within the reactor is 0.16 T. The experimental reduction temperature is selected on the basis of thermogravimetric analysis of the initial Co(OH)2 sample. The kinetic parameters of hydrogen reduction with linear heating and in isothermal conditions are calculated on the basis of the Freeman–Carroll and McKewan models, respectively. The rate of cobalt- nanopowder production slows in an electromagnetic field (by up to 14% at 250°C), on account of hindrance to the adsorption of hydrogen atoms at the surface of the metallic nanoparticles formed. However, mechanical stimulation in a vortical bed increases the rate of the process by a factor of 4–5 as a result of activation. The properties of the initial material and the products are studied by thermogravimetric analysis, X-ray diffraction analysis, scanning electron microscopy, and measurement of the specific surface by low-temperature nitrogen absorption. In the reduction of the samples, smaller cobalt nanoparticles are produced in the presence of an electromagnetic field. Mechanical stimulation in a vortical bed results in aggregation of the cobalt nanoparticles and the formation of microgranules.

Series resistance and interface states effects on the C–V and G/w–V characteristics in Au/(Co3O4-doped PVA)/n-Si structures at room temperature

Microwave-assisted method has been used for preparation of Co3O4 nanopowders. Then, Co3O4–doped polyvinyl alcohol (PVA) composites are deposited on n-Si wafer using spin coating method. The main electrical parameters of the Au/(Co3O4-doped PVA)/n-Si type Schotkky barrier diodes (SBDs) have been investigated by capacitance–voltage-frequency (C–V-f) and conductance–voltage-frequency (G/ω–V-f) measurements at room temperature and they were performed in the frequency range of 2 kHz–2 MHz and at (±4.5 V) by 50 mV steps. The XRD pattern of the as-prepared sample show that all diffraction peaks appeared in this pattern match very well with ICDD data and they can be readily indexed to the Co3O4 nanostructures, no other peaks were observed, which indicates the high purity of the sample. The effects of (Co3O4-doped PVA), series resistance (Rs) and surface states (Nss) on electrical characteristics have been investigated in detail. In order to eliminate the effect of Rs on high frequencies these measurements, the measured Cm and Gm/ω values were adjustment. The high values of Nss at low frequencies are responsible for the non-ideal behavior of C–V and G/ω–V characteristics. The obtained value of ND exponentially decreases; the value of ΦB (C–V) exponentially increases with increases frequency. Such behavior of ND and ΦB (C–V) are expected behavior and it is attributed to the particular density distribution of Nss, polarization processes and interfacial layer. Experimental results show that the C–V and G/ω–V characteristics of SBDs are affected not only in Nss and Rs but also interfacial polymer (Co3O4-doped PVA) layer.

Production of Nano-Sized Co<sub>3</sub>O<sub>4</sub> by Pyrolysis of Organic Extracts

The most promising application field of materials based on nano-sized Co3O4 is catalysis. The method of production is one of the factors, which greatly affects the catalytic activity of Co3O4 catalysts. The aim of this research is to study possibilities of a new promising extractive-pyrolytic method (EPM) for the production of Co3O4 nanopowders and silica- and ceria-supported Co3O4 nanocomposites. Solutions of cobalt hexanoate in hexanoic acid and trioctylammonium tetrachlorocobaltate in toluene preliminary produced by solvent extraction were used as precursors. The precursors’ thermal stability, phase composition, morphology and the magnetic properties of the final products of pyrolysis were studied. The performed investigations have shown that the mean size of the Co3O4 crystallites in the materials produced by the EPM varies from amorphous to 55 nm due to the production conditions.

Combustion Syntheses of Co3O4 Powders Using Different Fuels

Co3O4 powders are used as pigment in solar selective paints. In this work, two new gel-combustion processes for the synthesis of Co3O4 nanopowders with lysine (Lys) or ethylenediaminetetraacetic acid (Edta) as fuel are presented. The first route is a conventional, stoichiometric process, while the second one is a non-stoichiometric, pH-controlled process. The samples were calcined in air at 500°C. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrum (FTIR), Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and the optical properties of the pigments were assessed with a spectrophotometer. In all cases, powders exhibited the Co3O4 crystalline structure. A minimum crystallite average size of 19nm for powders obtained by the “stoichiometric/Lys” combustion route was observed, meanwhile, a maximum value of 47nm was stated for powders obtained by the “non-stoichiometric/Edta” combustion process. The average particle size ranged between 50 and 100nm. The powders obtained by the stoichiometric nitrate–lysine route were selected to study its optical properties, their solar absorption was 88%, compared with the references, evidencing their aptitude to be used in solar absorbent paints.

Solution combustion synthesized cobalt oxide catalyst precursor for NaBH4 hydrolysis

In this paper we report the solution combustion synthesis of cobalt oxide nanofoam from solutions of cobalt nitrate and glycine and subsequent use as an effective catalyst precursor for NaBH4 hydrolysis. The catalytic activity results show that the hydrogen generation rate (HGR) at room temperature was much higher for the solution combustion synthesized material than for commercial Co3O4 nanopowder, though their specific surface areas were comparable (∼26–32 m2/g). Using a 0.6 wt.% aqueous solution of NaBH4 at 20 °C and a 5 wt.% catalyst precursor loading, a HGR of 1.93 L min−1 gcat−1 was achieved for solution combustion synthesized Co3O4. In contrast, at the same conditions, for commercial Co3O4 and elemental Co powders HGRs of 0.98 and 0.49 L min−1 gcat−1 were achieved respectively. This type of synthesis is amenable to many complex metal oxide catalysts as well, such as LiCoO2, which have also been shown to be good catalyst precursors for hydrolysis of NaBH4.

Positron annihilation study of the microstructure of Co doped ZnO nanocrystals

ZnO nanopowders are mechanically mixed with a certain quantity of Co3O4 nanopowders to obtain 10at.% Co doped Co3O4/ZnO composites. The nanocomposites are annealed in argon atmosphere at different temperatures between 100 ℃ and 1200 ℃. The structure and the grain size of the nanocomposite are investigated by X-ray diffraction 2 scans. With annealing temperature increasing up to 700 ℃, Co3O4 phase gradually disappears, and ZnO grain size begins to increase significantly. After annealing at above 1000 ℃, Co3O4 phase completely disappears, and CoO phase (rock-salt crystal structure) appears. Positron annihilation lifetime measurements reveal a large number of Zn vacancies and vacancy clusters existing in the interface region of the Co3O4 /ZnO nanocomposites. These defects are gradually recovered after annealing at above 700 ℃, and their number is under the detection limit after annealing at 900 ℃. The same conclusion can be drawn from the coincidence Doppler broadening (CDB) measurements.

Oxygen evolution on Ti/Co3O4-coated electrodes in alkaline solution

AbstractThe electrochemical performances of Co3O4 nanopowders, obtained by the sol-gel method, were investigated and compared with those of commercial Co3O4 powders, for oxygen evolution reaction in alkaline solution. The active oxide powder was mixed with teflon and assembled on Ti substrate to form thin catalyst film. Cyclic voltammetry, polarization curves, and electrochemical impedance spectroscopy were used to assess the mechanism of oxygen evolution reaction, chemical structure, and morphology of the catalyst.