Synthesis of cobalt boride nanoparticles using RF thermal plasma

Nanosize cobalt boride particles were synthesized from the vapor phase using a 30 kW–4 MHz radio frequency (RF) thermal plasma. Cobalt and boron powder mixtures used as precursors in different composition and feed rate were evaporated immediately in the high temperature plasma and cobalt boride nanoparticles were produced through the quenching process. The X-ray diffractometry (XRD) patterns of cobalt boride nanoparticles prepared from the feed powder ratio of 1:2 and 1:3 for Co:B showed peaks that are associated with the Co₂B and CoB crystal phases of cobalt boride. The XRD analysis revealed that increasing the powder feed rate results in a higher mass fraction and a larger crystalline diameter of cobalt boride nanoparticles. The images obtained by field emission scanning electron microscopy (FE-SEM) revealed that cobalt boride nanoparticles have a spherical morphology. The crystallite size of the particles estimated with XRD was found to be 18–22 nm.     

Preparation of cobalt ferrite nanoparticles by using spent Li-ion batteries

Cobalt ferrite nanoparticles are a soft magnetic material have been extensively used in many electronic and magnetic applications. In this study, Co_0.8Fe_2.2O₄ nanoparticles with particle size of about 23.5 nm were directly synthesized by sol–gel auto-combustion and calcination methods using spent Li-ion batteries as raw materials. The overall process involves four steps: formation of homogeneous sols; formation of dried gels; combustion of the dried gels; and calcination of the dried gels after combustion at 1173 K for 2 h. The DTA–TG and IR were used to study the auto-combustion and thermal decomposition of the precursor, the morphology and structure of cobalt ferrite nanoparticles were characterized by XRD and TEM techniques. Moreover, the precise metal ion stoichiometry of cobalt ferrite nanoparticles was analyzed by ICP. The results revealed that the auto-combustion process was considered as a heat-induced exothermic oxidation–reduction reaction between nitrate ions and carboxyl group. The XRD patterns of calcination the dried gels after combustion confirmed the single phase spinel structure for the synthesized materials. The crystallite size was calculated from the most intense peak (3 1 1) using the Scherrer equation. The TEM photograph also shown that cobalt ferrite nanoparticles were well-dispersed and with little aggregation.     

Low temperature vapor-phase preparation of TiO2 nanopowders

The preparation of TiO₂ nanopowders by vapor-phase hydrolysis of TiCl₄ below 600°C is studied in this paper. Influences of preparation variables, such as preparative temperature, residence time, reactant concentration, and H₂O/TiCl₄ mole ratio, on TiO₂ particle size, morphology and chlorine contamination are investigated, followed by discussion. It shows that the hydrolysis temperature exerts greatest influences, while the residence time hardly have impact on product particles below 400°C, among the hydrolysis variables investigated. The chlorine contamination on nanopowders occurs during the preparation which can be greatly reduced by proper control on preparation variables. Unlike the high temperature gas-phase processes such as oxidation route and flame synthesis, low-temperature route shows the ready control on product powders, and thus obtains titania powders with small size, narrow size distribution and very weak agglomeration. In addition, the decreased energy consumption, retarded corrosion on the reactor and the reduced operation problems would be expected for the low temperature processes.     

Preparation and characteristics of core–shell structure cobalt/silica nanoparticles

The silica nanolayer with different thickness was coated on the spherical cobalt nanoparticles (an average diameter of 67 nm) to form core–shell structure by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). This coating process was based on the use of silane coupling agent 3-mercaptopropyltrimethoxysilane (HS-(CH₂)₃Si(OCH₃)₃, MPTS) as a primer to render the cobalt surface vitreophilic, thus rendering cobalt surface compatible with silica. The control over the silica coating layer thickness can be achieved by varying the reaction time. The cobalt nanoparticles and the cobalt coated with silica shell were confirmed by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) was used to gain insight into the way in which the MPTS is bound to the surface of the cobalt nanoparticles. Result of the thermogravimetric analysis (TGA) and differential thermal analysis (DTA) indicate that the thermal stability of cobalt/silica is better than that of pure cobalt nanoparticles. Magnetic properties of these powders have been evaluated. These cobalt/silica core–shell nanoparticles can be utilized as precursors for making property-tunable magnetic nanoparticles, thin films, and multilayered core–shell structure nanocomposites.     

Size-controlled synthesis of ZrO2-TiO2 nanoparticles prepared via reverse micelle method: Investigation of particle size effect on the catalytic performance in vapor phase Beckmann rearrangement

Zirconium-titanium mixed oxide nanoparticles have been synthesized using microreactors made of bis-(2-ethylhexyl) sulfosuccinate (AOT)/water/n-hexane microemulsions. The control of particle size was achieved by varying the process variables, such as water-to-surfactant molar ratio and reagent concentration. Their sizes, appearances, crystal structures, pore diameter and surface area were characterized by TEM, XRD, N₂ adsorption/desorption methods. The results revealed that samples prepared in reverse micelles had no crystalline phase. The Beckmann rearrangement of cyclohexanone oxime on ZrO₂-TiO₂ nanoparticles was carried out in a fixed-bed down flow reactor to investigate the effect of particle size on catalytic activity and selectivity. Samples synthesized in reverse micelles had better reaction performance than samples prepared via sol-gel method. A parallel relationship could be drawn between the catalytic activity and the particle size as well as the selectivity of the catalyst.     

Cobalt nanoparticles prepared by three different methods

This work aimed to study cobalt nanoparticles (Co-NPs) preparation using three different methods in order to evaluate the effect of synthesis variables that can influence the nanoparticle size distribution and particle shape. The synthesised nanoparticles were characterised by Transmission Electron Microscopy. The first synthesis employed decomposition of Co₂(CO)₈, at high temperatures. This procedure resulted in spherical nanoparticles with low size distribution. The size of Co-NPs could be tuned by modification of precursor/surfactant, nevertheless the stirring and injection time influenced the size distribution. Using polyol process, at high temperatures, it was produced undefined-shape nanoparticles. This result suggests that the solution composition, i.e. the amount of trioctylphosphine and oleic acid was not suitable to control both size and shape of nanoparticles. Finally, the method based on reduction with NaBH₄ resulted spherical nanoparticles with tiny sizes, indicating that in this case a variation on amount of reductant would be more efficient on the particle size control than a variation in concentration of oleic acid. These results indicated that, for each method, a different variable exists for the control of the distribution size and the shape of the formed particles.     

Study of preparation and magnetic properties of silica-coated cobalt ferrite nanocomposites

The structure and the magnetic properties of silica-coated cobalt ferrite nanoparticles (80 wt% CoFe₂O₄), prepared by sol–gel method and submitted to thermal treatments in the range 800–1,000 °C, were investigated through XRD, FT-IR, TEM and VSM. The effects of thermal treatment temperatures on the structure and magnetic properties of nanoparticles were examined. A silica shell thickness of about 5 nm was synthesized on top of cobalt ferrite nanoparticles. The non-crystalline silica confines the growth of cobalt ferrite nanoparticles, i.e., the particle sizes are almost independent of the thermal treatment. Saturation magnetization (Ms) was decreased slightly and coercivity (Hc) was increased, when the non-crystalline silica was coated on the surface of cobalt ferrite nanoparticles.     

Template-free hydrothermal derived cobalt oxide nanopowders: Synthesis,characterization, and removal of organic dyes

Pure spinel cobalt oxide nanoparticles were prepared through hydrothermal approach using different counter ions. First, the pure and uniform cobalt carbonate (with particle size of 21.8–29.8 nm) were prepared in high yield (94%) in an autoclave in absence unfriendly organic surfactants or solvents by adjusting different experimental parameters such as: pH, reaction time, temperature, counter ions, and (Co²⁺:CO₃^2?) molar ratios. Thence, the spinel Co₃O₄ (with mean particle size of 30.5–47.35 nm) was produced by thermal decomposition of cobalt carbonate in air at 500 °C for 3 h. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and thermal analysis (TA). Also, the optical characteristics of the as-prepared Co₃O₄ nanoparticles revealed the presence of two band gaps (1.45–1.47, and 1.83–1.93 eV). Additionally, adsorption of methylene blue dye on Co₃O₄ nanoparticles was investigated and the uptake% was found to be >99% in 24 h.     

Synthesis of superparamagnetic cobalt nanoparticles through solvothermal process

The superparamagnetic (SPM) cobalt nanoparticles with an average size of 2 nm have been prepared through a solvothermal process in the presence of triethanolamine. The synthesized nanoparticles are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and superconducting quantum interference device magnetometer. X-ray diffraction analysis confirms the face centered cubic phase of as-prepared cobalt nanoparticles. Transmission electron microscopy was employed to study the morphology of the as-prepared product, which exhibit spherical-like shape with size around 2 nm. The high resolution transmission electron microscopy image of cobalt nanoparticles shows the lattice spacing value of 0.204 nm. This is well matched with the (111) lattice spacing of fcc Co. XPS revealed the prepared product is pure cobalt. The blocking temperature of 17 K was obtained and confirmed by field-cooled and zero-field-cooled plots. The hysteresis loop revealed the synthesized nanoparticles have SPM character at room temperature.     

Microwave-assisted synthesis and magnetic studies of cobalt oxide nanoparticles

An efficient microwave-assisted route has been used to synthesize nanoparticles of cobalt oxide. The particles were well characterized by transmission electron microscopy (TEM) which showed that the average diameter of the particles is around 6 nm. X-ray diffraction (XRD) studies further confirmed the formation of the spinel Co₃O₄. Purity of the products was detected by Fourier transform infrared spectroscopy (FTIR) combined with thermal gravimetric analysis (TG/DTG). The magnetic measurements revealed a small hysteresis loop at room temperature indicating a weak ferromagnetic nature of the synthesized Co₃O₄ nanoparticles. The magnetic moment of the particles was measured to be 4.27 μ_eff.     

Effect of temperature in synthesis of silver nanoparticles in triblock copolymer micellar solution

Knowing that poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) in aqueous solution is thermoresponsive, the effect of temperature on formation and stabilisation of silver nanoparticles has been investigated systematically. Synthesis of silver nanoparticles was achieved from silver ammonia complex [Ag(NH₃)₂]⁺ in aqueous solution of hydroxyl terminated PEO–PPO–PEO at four different temperatures. A non-Arrhenius behaviour for the rate of silver reduction with temperature was observed. The hydrodynamic diameter of the composite coils suddenly increased at certain intermediate time indicating sudden agglomeration of individual micelles to form bigger network structures. The size and the distribution of the nanoparticles show a bimodal distribution at the lowest temperature. At intermediate temperature, particles of the smallest size with a narrow distribution was achieved. At the highest temperature, a bunch-like particle morphology was found. Chemical changes in polymer properties were observed at higher temperatures. The results suggest that at a lower temperature, a change in polymer morphology play an important role in controlling the particle size and their distribution, whereas at a higher temperature, this role is shifted to the chemical change of the polymer. At an intermediate temperature, a balance between the two effects provides the optimum condition for formation of silver nanoparticles of small size and narrow distribution.     

Size variation and magnetic dilution effects on the structure and magnetic properties of cobalt zinc ferrite

Nanocrystalline cobalt zinc ferrites Co_1?xZn_xFe₂O₄ (x?=?0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0), have been prepared by employing a precursor combustion method via decomposition of the metal carboxylato hydrazinate precursors. This synthesis technique yields nanoparticles with particle size between 12 and 15 nm as determined from transmission electron microscopy (TEM) studies. The nanoferrites were then sintered at 1000 °C for 15 h to obtain micrometer size ‘bulk’ ferrites in the range of 0.3–0.8 μm. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) Spectroscopy confirmed the formation of the mixed ferrites without any impurities. Addition of non-magnetic ion like Zn²⁺ into the crystal structure of cobalt ferrite leads to a prominent change in the size, structure and properties. The saturation magnetization values (M_S) increases upto x?=?0.4 and then decreases with further increase in Zn concentration. A maximum M_S value of 90.85 emu/g and 79.59 emu/g for x?=?0.4 was obtained for the sintered and nanoferrite sample, respectively. The lower M_S and higher coercivity (H_C) values for nanoferrites than the sintered ferrites exhibited a strong dependence on the particle size due to the cation distribution and surface effects. The Curie temperature (T_C) was found to decrease appreciably with the reduction in particle size and with increasing concentration of Zn. The room temperature Mössbauer spectra showed a transition from ferrimagnetic to a paramagnetic state with increasing zinc concentration along with superparamagnetic features which was in corroboration with VSM studies.

     

Microwave-hydrothermal synthesis of γ-Fe2O3 nanoparticles and their magnetic properties

Nanosized γ-Fe₂O₃ is synthesized by the microwave-hydrothermal method. Powder X-ray diffraction and transmission electron microscopic studies showed that the average particle size is 10 nm. Magnetic studies reveal that the γ-Fe₂O₃ nanoparticles are superparamagnetic at room temperature, with a superparamagnetic blocking temperature of 200 K. The magnetic characteristics of the nanoparticles indicate their strongly interacting nature.     

Synthesis of TbMnO3 nanoparticles via a polyacrylamide gel route

We report here the synthesis of TbMnO₃ nanoparticles via an acrylamide gel route. XRD, TG analysis, DSC analysis, and FTIR spectroscopy are combinatively used to investigate the thermal decomposition process of precursor xerogels and the formation of TbMnO₃ phase. It is demonstrated that high-phase-purity TbMnO₃ nanoparticles can be prepared by using different chelating agents at a sintering temperature of 800 °C. SEM observation and XRD analysis reveal that the particle size and morphology of the products have a dependence on the chelating agent. The particles prepared using citric acid as the chelating agent appear to be regularly spherical in shape and highly uniform in size with a diameter of ～67 nm, while the sample prepared by using the chelating agent EDTA mainly consists of sphere-, ellipsoid-, and rod-like particles and exhibits a relatively broad particle size distribution with an average particle size centered around 115 nm. The use of a combination of citric acid and EDTA generally results in sphere- and ellipsoid-like particles with an average particle size between those of the samples prepared separately by using the two chelating agents.     

Reactive deposition of ultrafine cobalt powders

The particle size distributions of cobalt powders prepared by reactive deposition were examined and correlated with the deposition condition. In reactive deposition, a Co(OH)₂ colloid layer was formed at the electrode surface upon reduction of dissolved oxygen. The colloid layer accelerated the decrease in interfacial Co²⁺ concentration and inhibited crystal growth of the metal, and was therefore instrumental in the formation of fine cobalt particles. The powders from reactive deposition were statistically a factor of two finer and of more uniform size distribution than those from normal electrodeposition. It was found that both the reactive deposition of cobalt powders and the reactive deposition of porous cobalt structures were driven by the same kinetic factors, namely the requirement of maintaining an effective presence of the Co(OH)₂ colloid layer at the electrode surface. Hence a similar dependence on deposition conditions for both processes was obtained.     

Facile and green synthesis of highly dispersed cobalt oxide (Co3O4) nano powder: Characterization and screening of its eco-toxicity

A novel green synthesis of cobalt oxide (Co₃O₄) nanoparticles using latex of Calotropis procera via simple precipitation method at room temperature was investigated. An extensive characterization of the product was carried out using X-ray diffractometry (XRD), Differential scanning calorimetry (DSC), Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and UV–Visible spectroscopy. The results of the characterization confirmed that the synthesized nanomaterial is highly dispersed. TEM analysis revealed that the nano particles are having an average size around 10?nm. The eco-toxic investigation suggested that the particles are non-toxic and safe towards the environment. This green strategy proves to be an effective, fast, simple and cost-effective approach for the synthesis of Co₃O₄ nanoparticles for various applications.     

Laser synthesis of magnetic iron–carbon nanocomposites with size dependent properties

Iron–carbon nanocomposites have gained interest due to their new engineering and biomedical applications. Carbon coated iron nanoparticles (Fe@C) were obtained continuously and in a single step using the laser pyrolysis method. The continuous wave CO₂ laser beam was used to continuously heat a sensitized (with ethylene) precursor gas mixture, in which iron pentacarbonyl (vapor) and acetylene were the iron and carbon donors, respectively. The effect of varying the residence time in the reaction zone through the variation of the internal nozzle diameter was explored in order to improve the particle size and the phase distributions. At increased nozzle diameter, (i) the particle mean diameter increases (from about 3.5 to 10.5 nm), (ii) higher ordering of the crystallographic network seems to occur, (iii) the dominance of the α-Fe and iron carbide phases is revealed. Onion-like graphenic layers often cover the buried iron cores. Magnetic measurements and temperature dependent Mössbauer spectroscopy were used in order to find correlations concerning the magnetic behavior and the Fe phase composition of samples. Preliminary experiments for obtaining stable water-based magnetic nanofluids are discussed.     

Effect of process parameters on size,shape, and distribution of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Antimony trioxide (Sb₂O₃) nanoparticles with particle sizes ranging from 2 to 12 nm, spherical in shape, and well-distributed were successfully synthesized by chemical reduction method. The nanoparticles were synthesized in the presence of hydrazine as a reduction agent in ethylene glycol through the reaction between antimony trichloride and sodium hydroxide. Effects of reaction temperature, reaction time, precursor concentration and boiling temperature on the particle size, shape, and distribution of the Sb₂O₃ nanoparticles were investigated. Morphology of the nanoparticles was examined by transmission electron microscope (TEM). It was revealed that the particle size increased when reaction temperature, reaction time and concentration of precursor were increased. Moreover, the mixture needed to be boiled prior to the addition of hydrazine as a reduction agent, in order to obtain an optimum reduction. X-ray diffraction (XRD) was employed to study the crystallinity and phase of the nanoparticles. The nanoparticles were determined as cubic phase of Sb₂O₃ (ICDD file no. 00-043-1071) by XRD. Interrelation between UV–vis absorption spectra of the nanoparticles and their particle size were obtained.     

Influence of dispersant on the morphology of silica-coated cobalt nanoparticles synthesized by a wet based method

Co/SiO₂ nanospheres with nearly perfect core–shell structure were prepared by an improved sol–gel method combined with hydrogen reduction. The products were characterized by X-ray diffraction spectra (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectra. The results indicated that polyethylene glycol (PEG 8000) could act as a more efficient dispersive reagent than citric acid monohydrate (CAM). The average size of Co nanoparticles can be well controlled with optimum concentration of PEG 8000 and the average diameter of Co nanoparticles reaches about 30 nm with PEG 8000 concentration of 50 mg ml⁻¹.     

High-purity disperse α-Al2O3 nanoparticles synthesized by high-energy ball milling

The preparation of disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution, high purity, and high yield is essential for producing Al₂O₃ nanocrystalline ceramic of fine grains which may exhibit a good toughness. In this work, micron-sized α-Al₂O₃ particles were directly ball-milled and subsequently washed with hydrochloric acid at room temperature. Fracture of large α-Al₂O₃ particles and cold welding of fine α-Al₂O₃ nanoparticles occur simultaneously during ball milling. It leads to the reduction of particle size with increasing milling duration below 80?h and reaches to a dynamic equilibrium with a minimal average particle size of 6.4?nm for milling durations over 80?h. Using the optimized high-energy ball milling parameters, we prepared high-purity disperse equiaxed α-Al₂O₃ nanoparticles with an average particle size of 8?nm and a purity of 99.96% (mass percent) in a high yield. After fractionated coagulation separations, disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution were obtained. Finally, Al₂O₃ nanocrystalline ceramic with a relative density of 99.8% and an average grain size of 34?nm was sintered from the disperse fine equiaxed α-Al₂O₃ nanoparticles with an average particle size of 4.8?nm and a size distribution of 2–10?nm by pressureless two-step sintering.