Microstructural properties and local atomic structures of cobalt oxide nanoparticles synthesised by mechanical ball-milling process

In this study, facile preparation of pure and nano-sized cobalt oxides particles was achieved using low-cost mechanical ball-milling synthesis route. Microstructural and morphological properties of synthesised products were characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD results indicated that the fabricated samples composed of cubic pure phase CoO and Co₃O₄ nanocrystalline particles with an average crystallite size of 37.2 and 31.8 nm, respectively. TEM images showed that the resulting samples consisted of agglomerates of particles with average diameter of about 37.6 nm for CoO and 31.9 nm for Co₃O₄. Phase purity of the prepared samples was further investigated due to their promising technological applications. Local atomic structure properties of the prepared nanoparticles were probed using synchrotron radiation-based X-ray absorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). EXAFS data analysis further confirmed the formation of single-phase CoO and Co₃O₄ nanoparticles. In addition, structural properties of cobalt oxide nanoparticles were investigated by performing density functional theory calculations at B3LYP/TZVP level and Born–Oppenheimer molecular dynamics. Theoretical calculations for both prepared samples were found to be consistent with the experimental results derived from EXAFS analysis. Obtained results herein reveals that highly crystalline and pure phase CoO and Co₃O₄ nanoparticles can be synthesised using simple, inexpensive and eco-friendly ball-milling method for renewable energy applications involving fuel cells and water splitting devices.     

Promising anode material for lithium-ion cells based on cobalt oxide synthesized by microwave heating

High-performance anode material for lithium-ion cell based on cobalt oxide was synthesized through a combination of sol-gel route and subsequent microwave heating. The influence of microwave irradiation temperature of the precursor on the characteristics of the active materials formed was studied. The physicochemical, structural, and morphological properties of the materials were studied in addition to the electrochemical performance by cyclic voltammetry and charge-discharge cycling vs. Li⁺/Li. Microwave heating at 350 °C resulted in the formation of Co₃O₄, whereas at 450 and 550 °C, a mixture of Co₃O₄, CoO, and Co was formed. Co₃O₄ synthesized at 350 °C possessed porous morphology with high specific surface area and exhibited superior electrochemical performance with initial specific capacity of 982 mAh g^?1 and coulombic efficiency of ~75% along with good cycle performance retaining ~87% of initial capacity after 60 cycles.     

Fast electrochromic response of porous-structured cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) thin films by novel nebulizer spray pyrolysis technique

Electrochromic effect of cobalt oxide thin films was studied as a function of substrate temperature (573–673 K). Tricobalt tetraoxide (Co₃O₄) thin films were deposited on glass and fluorine-doped tin oxide (FTO) substrates by nebulized spray technique using cobalt nitrate as precursor material. The XRD patterns indicated (311) plane was dominant for all the films irrespective of the deposition temperature. Williamson-Hall (W-H) analysis was made to understand the strain variation in the prepared Co₃O₄ films under different deposition temperature by employing uniform deformation model (UDM). Scanning electron microscopy images revealed porous morphology for the film prepared at 623 K. The optical parameters such as refractive index (n), extinction coefficient (k), and band gap were derived from UV-visible spectra of Co₃O₄ films. The electrochromic performance of the deposited Co₃O₄ films was analyzed through cyclic voltammetry, chronocoulometry, chronoamperometry, and iono-optical studies.     

Ion beam shaping of embedded metal nanoparticles by Si+ ion irradiation

Fine Co and Pt nanoparticles are nucleated when a silica sample is implanted with 400 keV Co⁺ and 1370 keV Pt⁺ ions. At the implanted range, Co and Pt react to form small Co _x Pt_(1?x) nanoparticles during Si⁺ ion irradiation at 300 °C. Thermal annealing of the pre-implanted silica substrate at 1000 °C results in the formation of spherical nanoparticles of various sizes. When irradiated with Si⁺ ions at 300 °C, particles in the size range of 5–17 nm undergo rod-like shape transformation with an elongation in the direction of the incident ion beam, while those particles in the size range of 17–26 nm turn into elliptical shape. Moreover, it is suspected that very big nanoparticles (size >26 nm) decrease in size, while small nanoparticles (size <5 nm) do not undergo any transformation. During Si⁺ ion irradiation, the crystalline nature of the nanoparticles is preserved. The results are discussed in the light of the thermal spike model.     

Synthesis and characterization of Mn<Superscript>2+</Superscript>-doped ZnS nanoparticles

Mn²⁺-doped ZnS nanoparticles were prepared by chemical arrested precipitation method. The samples were heated at 300, 500, 700 and 900°C. The average particle size was determined from the X-ray line broadening. Samples were characterized by XRD, FTIR and UV. The composition was verified by EDAX spectrum. The hexagonal structure of the sample was identified. The size of the particles increased as the annealing temperature was increased. The crystallite size varied from 5 nm to 34 nm as the calcination temperature increased. At around 700°C, ZnS is converted into ZnO phase due to oxidation. The emission peak of the sample is observed at 300 nm resulting in blue emission. The solid state theory based on the delocalized electron and hole within the confined volume can explain the blue-shifted optical absorption spectra. UV-VIS spectro-photometric measurement shows an indirect allowed band gap of 3.65 eV.      

Controlled synthesis of photoluminescent Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> nanoparticles from metal-organic polymeric precursor

Bi₄Ti₃O₁₂ (BIT) nanoparticles with a narrow average particle size distribution in the range of 11–46 nm was synthesized via a metal-organic polymeric precursor process. The crystallite size and lattice parameter of BIT were determined by XRD analysis. At annealing temperatures >550 °C, the orthorhombic BIT compound with lattice parameters a = 5.4489 Å, b = 5.4147 Å, and c = 32.8362 Å was formed while at lower annealing temperatures orthorhombicity was absent. Reaction proceeded via the formation of an intermediate phase at 500 °C with a stoichiometry close to Bi₂Ti₂O₇. The particle size and the agglomerates of the primary particles have been confirmed by FESEM and TEM. The decomposition of the polymeric gel was ascertained in order to evaluate the crystallization process from TG-DSC analysis. Raman spectroscopy was used to investigate the lattice dynamics in BIT nanoparticles. In addition, investigation of the dependence of the visible emission band around the blue–green color emission on annealing temperatures and grain sizes showed that the effect of grain size plays important roles, and that oxygen vacancies may act as the radiative centers responsible for the observed visible emission band.     

Surfactant-free synthesis of novel copper oxide (CuO) nanowire–cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) nanoparticle heterostructures and their morphological control

A simple and surfactant-free synthesis of novel heterostructures comprising of copper oxide (CuO) nanowires uniformly decorated with cobalt oxide (Co₃O₄) nanoparticles was demonstrated by combining thermal growth and wet-coating method. The heterostructures were synthesized by thermally decomposing cobalt salt (cobalt nitrate) into Co₃O₄ nanoparticles onto vapor–solid (VS)-grown CuO nanowires. X-ray diffraction (XRD) and high resolution transmission electron microscopy (TEM) confirmed the presence of CuO and Co₃O₄ phases as well as a narrow size distribution of Co₃O₄ nanoparticles (average diameter ~7.0 ± 1.5 nm) on CuO nanowires (average diameter of nanowire tips ~67.9 ± 18.6 nm). Unique interfacial lattice relationship was observed for (111) Co₃O₄ nanoparticles on (200) CuO nanowire surface resulting in hemispherical shape of the former. For the first time, further systematic studies were performed to understand the influence of various parameters (cobalt salt concentration and annealing temperature, atmosphere, and time) on the morphological evolution of Co₃O₄ nanoparticles on CuO nanowires. Interestingly, by varying these parameters, it was possible to grow Co₃O₄ in different shapes (spherical, triangular, rectangular, cubical, and hexagonal nanoparticles) and forms (shells and nanorods). It was observed that all these parameters play a critical role in influencing the surface migration, nucleation, and growth of Co₃O₄ nanoparticles on CuO nanowires and this assisted in understanding the involved growth mechanisms. Finally, UV–vis–NIR spectroscopy and band gap energies for these heterostructures were evaluated that showed higher photocatalytic degradation efficiency for Rhodamine B under low-power visible-light illumination.     

Sonochemical synthesis and characterization of Co3O4 nanocrystals in the presence of the ionic liquid [EMIM][BF4]

For the first time, a sonochemical process has been used to synthesis cobalt oxide Co₃O₄ nanoflowers and nanorods morphology in the presence of the ionic liquid 1-Ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄] as reaction media and morphology template. Different sonication time periods and different molar ratios of the ionic liquid (IL) were used to investigate their effects on the structural, optical, chemical and magnetic properties of the produced Co₃O₄ nanoparticles. During synthesis process brown powder contains cobalt hydroxide Co(OH)₂ and cobalt oxyhydroxide (Cobalt hydroxide oxide) CoO(OH) was formed, after calcination in air for 4 h at 400 °C a black powder of Co₃O₄ nanoparticles was produced. The produced Co₃O₄ nanoparticles properties were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), transmission electron microscopy (TEM), FTIR spectroscopy, UV–vis spectroscopy, and Vibrating Sample Magnetometer (VSM). To explain the formation mechanism of Co₃O₄ NPs some investigations were carried on the brown powder before calcination.     

Optical,structural and magnetic properties of Zn0.9Cd0.1S:yCo nanoparticles

Zn_0.9Cd_0.1S:yCo nanoparticles were prepared by a co-precipitation method at low temperature. The obtained products were identified to be of cubic structure without any impurity phase. Cobalt incorporation leads to an increase in the local strain value and a decrease in the lattice constants as measured from XRD. Magnetic measurements showed that cobalt was incorporated in the Zn_0.9Cd_0.1S lattice as Co²⁺ and substituted for the Zn site as there was no evidence of the presence of metallic cobalt. Transmission electron microscopy suggests the crystalline nature of nanoparticles, with average particle size of ∼3.5 nm. UV-Vis measurements showed a red shift with respect to undoped nanoparticles in energy band gap with increasing cobalt concentration. Photoluminescence spectra reveal the defect-related emissions. The decay time constant is found to be in the nanosecond regime and is attributed to the spatial confinement of photo generated electron–hole pairs.     

Effect of implantation temperature on the blistering behavior of hydrogen implanted GaN

GaN epitaxial layers were implanted by 100 keV H⁺ ions at different implantation temperatures (LN₂, RT and 300 °C) with a fluence of 2.5×10¹⁷ cm^?2. The implanted samples were characterized using Nomarski optical microscopy, AFM, XRD, and TEM. Topographical investigations of the implanted surface revealed the formation of surface blistering in the as-implanted samples at 300 °C and after annealing at higher temperature for the implantation at LN₂ and RT. The physical dimensions of the surface blisters/craters were dependent on the implantation temperature. XRD showed the dependence of damage-induced stress on the implantation temperature with higher stress for the implantation at 300 °C. TEM investigations revealed the formation of a damage band in all the cases. The damage band was filled with large area microcracks for the implantation at 300 °C, which were responsible for the as-implanted surface blistering.     

Growth of CuPd nanoalloys encapsulated in carbon-shell

Phase characteristics of the partially oxidized cobalt particles were investigated. It is shown that the studied system consists of Co, CoO, and Co₃O₄ phases. The paper presents results for the magnetic characteristics of Co particles covered by a CoO shell. Magnetic measurements were performed at 77 K. It is shown that the shapes of the hysteresis loop of the investigated system are different for different cases of cooling, with and without applications of external magnetic field. Structural investigations were performed on single particles.     

Valence control of cobalt oxide thin films by annealing atmosphere

The cobalt oxide (CoO and Co₃O₄) thin films were successfully prepared using a spin-coating technique by a chemical solution method with CH₃OCH₂CH₂OH and Co(NO₃)₂·6H₂O as starting materials. The grayish cobalt oxide films had uniform crystalline grains with less than 50 nm in diameter. The phase structure is able to tailor by controlling the annealing atmosphere and temperature, in which Co₃O₄ thin film was obtained by annealing in air at 300-600, and N₂ at 300, and transferred to CoO thin film by raising annealing temperature in N₂. The fitted X-ray photoelectron spectroscopy (XPS) spectra of the Co2p electrons are distinguishable from different valence states of cobalt oxide especially for their satellite structure. The valence control of cobalt oxide thin films by annealing atmosphere contributes to the tailored optical absorption property.     

Preparation of (As x Co3−x O4)(x=0–0.024) nanoparticles by rheological phase reaction method

Nanoparticles of a series of arsenic–cobalt mixed valency spinel oxides of theoretical formula As _x Co_3?x O₄, (x=0, 0.005, 0.01, 0.015, 0.024) have been successfully prepared by the rheological phase reaction and the pyrolysis method. The products were characterized by X-ray powder diffraction, scanning electron microscope, thermogravimetric analysis and simultaneous differential thermal analysis. Calcination of the precursor at 500 ^°C resulted in the formation of arsenic-doped cobalt oxide nanoparticles of 48 nm in crystal size. The effect of the calcination temperature on the crystal size of arsenic-doped Co₃O₄ was discussed.     

Effect of calcination conditions on lithium conductivity in Li<Subscript>1.3</Subscript>Ti<Subscript>1.7</Subscript>Al<Subscript>0.3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> prepared by sol-gel route

In order to study the influence of powder calcination temperature on lithium ion conductivity, synthesized Li_1.3Ti_1.7Al_0.3(PO₄)₃ (LATP) was calcined at temperatures between 750 and 900 °C. The shape and size of the particles were characterized employing scanning electron microscopy (SEM), and specific surface area of the obtained powder was measured. The crystallinity grade of different heat-treated powders was calculated from XRD spectra. Posteriorly, all powders were sintered at 1100 °C employing field-assisted sintering (SPS), and the electrical properties were correlated to the calcination conditions. The highest ionic conductivity was observed for samples made out of powders calcined at 900 °C.     

Synthesis and physical properties of mixed Co3O4/CoO nanorods by microwave hydrothermal technique

A mixture of crystalline Co₃O₄/CoO nanorods with non-uniform dense distribution has been successfully synthesized by microwave hydrothermal technique. The synthesized nanorods have been characterized by several techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transforms infrared spectroscopy (FT-IR). The results showed that the as synthesized specimens contained mixed crystalline Co₃O₄/CoO nanorods with an average length of around 80 nm and an average diameter of 42 nm. UV–Vis spectrum of the nanorods exhibited a strong UV emission. The band energy gap of the product was 1.79 eV which lies between the energy gap of CoO and that for Co₃O₄. The obtained carrier concentration is of the order 4.32 × 10²⁷ m⁻³ and the dielectric constant is found to be 4.89. The electrical conductivity increases with increasing temperature and behaves as a semiconducting material with an activation energy of a bout 0.26 eV. This makes the as synthesized mixed Co₃O₄/CoO nanorods very useful for supercapacitor devices application. Magnetic hysteresis loops at room temperature of the as synthesized mixed oxides (Co₃O₄/CoO) nanorods exhibit typical soft magnetic behavior.     

Surface-induced structural modification in ZnO nanoparticles

Cobalt?Ccobalt carbide [Co _x C (x?=?2 or 3)] and cobalt (FCC-Co) microwires have been synthesized using a polyol method in the presence of a high external magnetic field of 4.3?kOe. It was reported before that the synthesis of these particles in the absence of magnetic field leads to the formation of spherical particles. Analysis of the X-ray diffraction (XRD) scans indicates that the synthesized Co _x C wires consist of four phases?? ??-Co, ??-Co, Co₃C, and Co₂C. The percent composition of these phases was 53.3?% Co₃C, 26.8?% Co₂C, 12.5?% ??-Co, and 7.4?% ??-Co. XRD analysis of the as-synthesized cobalt wires shows that it consists of single-phase FCC-Co. Based on Scherrer analysis of the XRD data, the average crystallite sizes of the cobalt carbide and the cobalt particles are 18.5 and 16.3?nm, respectively. Scanning electron microscopy (SEM) studies show that the diameter of Co _x C wires is in the range of 1.6(±0.2)???m with their length varying between 18 and 30???m while the diameter of the cobalt wires is 1.65(±0.1). The SEM results infer that the morphology of the growing particles was controlled by the magnetic field with the applied field directs the growth of the particles into wires. The magnetic measurements indicate a superparamagnetic character of the cobalt wires and a soft ferromagnetic nature of the synthesized Co _x C chains. The degree and field range of the interactions between magnetic domains have been investigated using a Henkel plot.     

Enhanced photocatalytic performance of TiO2 particles via effect of anatase–rutile ratio

In the present work anatase–rutile transformation temperature and its effect on physical/chemical properties as well as photocatalytic activity of TiO₂ particles were investigated. The characterisation of the synthesised and annealed TiO₂ particles were determined by X-Ray Powder Diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS) and Brunauer–Emmett–Teller surface area analysis (BET). The refraction in the ultraviolet–visible (UV–vis) range was assessed using a dual-beam spectrophotometer. The photocatalytic performance of the particles was tested on methylene blue solution. The XRD data indicated that the percentage of rutile increased with the annealing temperature and almost 100% of anatase transformed to rutile at 1000 °C. In addition, the phase transformation was a linear function of annealing temperature so phase composition of TiO₂ can be controlled by changing the annealing temperature. The SEM and BET results presented the increase of agglomerate size and the decrease of specific surface area with the increasing annealing temperature. This proved that anatase has smaller particle size and higher surface area than rutile. The photocatalytic activity of the annealed TiO₂ powders reduced with the increase of annealing temperature. The samples annealed at 900 °C and 925 °C with anatase: rutile ratio of 92:8 and 77:23, respectively, showed the best activity. These results suggested that the photocatalytic activity of TiO₂ particles is a function of phase composition. Thus it can be enhanced by changing its phase composition which can be controlled by annealing temperature.     

Sunlight assisted photodegradation by tin oxide quantum dots

Rutile phase of SnO₂ quantum dots of average size of 2.5 nm were synthesized at a growth temperature of 70 °C and characterized with XRD, TEM, FTIR and Raman analysis. The effective strain within the lattice of SnO₂ quantum dots was calculated by Williamson–Hall method. The broad peaks in XRD as well as Raman spectra and the presence of Raman bands at 569 and 432 cm⁻¹ are due to lower crystallinity of nanoparticles. The optical band gap of SnO₂ quantum dots was increased to 3.75 eV attributed to the quantum size effect. SnO₂ quantum dots were annealed in air atmosphere and the crystallite size of the particles increased with annealing temperature. Sunlight assisted photodegration property of SnO₂ quantum dots was investigated with vanillin as a model system and it shows the photodegradation efficiency of 87%. The photoluminescence and photodegradation efficiency of nanocrystallite SnO₂ decreases with increase of crystallite size contributed to the reduction in population of defects and surface area.     

Genesis of supported carbon-coated Co nanoparticles with controlled magnetic properties,prepared by decomposition of chelate complexes

Following procedures formerly developed for the preparation of supported heterogeneous catalysts, carbon-coated cobalt nanoparticles dispersed on porous alumina have been prepared by impregnation of γ-Al₂O₃ with (NH₄)₂[Co(EDTA)] and thermal decomposition in inert atmosphere. Below 350 °C, Co(II) ions are complexed in a hexa-coordinated way by the EDTA ligand. The thermal treatment at 400–900 °C leads to the EDTA ligand decomposition and recovering of the support porosity, initially clogged by the impregnated salt. According to X-ray absorption spectroscopy, and due to in situ redox reactions between the organic ligand and Co(II), both oxidic and metallic cobalt phases are formed. Characterisation by transmission electron microscopy, X-ray diffraction and magnetic measurements reveals that an increase in the treatment temperature leads to an increase of the degree of cobalt reduction as well as to a growth of the cobalt metal particles. As a consequence, the samples prepared at 400–700 °C exhibit superparamagnetism and a saturation magnetisation of 1.7–6.5 emu g⁻¹ at room temperature, whilst the sample prepared at 900 °C has a weak coercivity (0.1 kOe) and a saturation magnetisation of 12 emu g⁻¹. Metal particles are homogeneously dispersed on the support and appear to be protected by carbon; its elimination by a heating in H₂ at 400 °C is demonstrated to cause sintering of the metal particles. The route investigated here can be of interest for obtaining porous magnetic adsorbents or carriers with high magnetic moments and low coercivities, in which the magnetic nanoparticles are protected from chemical aggression and sintering by their coating.     

M?ssbauer and magnetic studies of Mn0.1Sr0.2Co0.7Fe2O4 nanoferrite

The magnetic properties of Mn_0.1Sr_0.2Co_0.7Fe₂O₄ nanoferrite with particle size of about 8 nm were investigated using magnetization and Mössbauer spectroscopy measurements. The sample shows a large increase in coercive field from 0.045 kOe at room temperature to about 3.00 kOe at 4 K. Room temperature coercive fields increased with increase in the annealing temperature between 300°C and 800°C. Our results show evidence of transformation from single domain to multi-domain structure with thermal annealing.