Preparation of cobalt oxide films by plasma-enhanced metalorganic chemical vapour deposition

Co oxide films were prepared on glass substrates at 150–400°C by plasma-enhanced metalorganic chemical vapour deposition using cobalt (II) acetylacetonate as a source material. NaCl-type CoO films were formed at low O₂ flow rate of 7cm³ min^–1 and at a substrate temperature of 150–400°C. The CoO films possessed (100) orientation, independent of substrate temperature. Deposition rates of the CoO films were 40–47 nm min^–1. The CoO film deposited at 400 °C was composed of closely packed columnar grains and average diameter size at film surface was 60 nm. At high O₂ flow rate of 20–50 cm³ min^–1, high crystalline spinel-type Co₃O₄ films were formed at a substrate temperature of 150–400°C. The Co₃O₄ film deposited at 400°C possessed (100) preferred orientation and the film deposited at 150°C possessed (111) preferred orientation. Deposition rates of the Co₃O₄ films were 20–41 nm min^–1. Both Co₃O₄ films with (100) and (111) orientation had columnar structure. The shape and average size of the columnar grains at the film surface were different; a square shape and 35 nm for (100)-oriented Co₃O₄ film and a hexagonal shape and 60 nm for (111)-oriented film, respectively.     

Physiochemical phase transformations in Co/CoO nanoparticles prepared by inert gas Condensation

We present results of the studies of structural and chemical transformations in Co/CoO nanoparticles prepared by inert gas condensation. The effect of the morphology and agglomeration on the phase transformation reaction path in self-oxidation and in controlled reduction processes are discussed in detail. As-prepared samples show self-oxidation related to the non-core/shell morphology of the particles. Annealing of particles at 250 °C in reducing atmosphere leads to the oxidation of the particles showing coexistence of CoO and Co₃O₄ structures. This is explained by the diffusion of oxygen from the amorphous oxide surface to the bulk of the nanoparticles. Upon increasing the reaction temperature beyond 250 °C, reductive transformation of the samples occurs systematically, from CoO/Co₃O₄ to CoO to Co (HCP + FCC) and eventually to Co (FCC). We have presented X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and magnetic data to track the structural and chemical transformation paths. We found strong correlation between structural and magnetic properties. Thermodynamic stability as a function of reaction temperature on the phase/chemical transformation is also discussed.     

Mechanochemical synthesis of Al2O3/Co nanocomposite by aluminothermic reaction

In this work, Al₂O₃/Co nanocomposite was successfully prepared by mechanochemical reaction between Co₃O₄ and Al powders in a planetary high energy ball mill. The mechanism of the reaction was dealt using X-ray diffraction (XRD), differential thermal analysis (DTA), and thermodynamics calculations. It was found that Co₃O₄ reacts with Al through a self-sustaining combustion reaction after an incubation period of 50 min and the reaction between Co₃O₄ and Al involves two steps. First, Co₃O₄ reacts with Al to form CoO and Al₂O₃ at the temperature around melting point of Al, and at higher temperature, CoO reacts with remaining Al to form Co and Al₂O₃. Mechanical activation process decreases the reaction temperature from 1041 °C for as-received Co₃O₄ and Al powder mixture to 869 °C for 45 min milled powders. After annealing of powder milled for 12 h, no phase transformation has been detected. The crystallite sizes of both α-Al₂O₃ and Co remained in nanometeric scale after annealing at 1000 °C for 1 h.     

Effect of fuel/oxidizer ratio and the calcination temperature on the preparation of microporous-nanostructured tricobalt tetraoxide

Microporous tricobalt tetraoxide, Co₃O₄, nanoparticles (NPs) clusters have been successfully fabricated using a simple but efficient controlled solution combustion route. Such a synthesis involves combustion reaction of cobalt nitrate with cetyl trimethylammonium bromide (CTAB). The combustion process has been analyzed by simultaneous thermal analysis. The resultant powders were characterized by means of X-ray diffraction technique (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and nitrogen adsorption at −196 °C. The morphology and specific surface area of the obtained Co₃O₄ nanoparticles clusters have proved to be strongly dependent on the fuel (F)/oxidizer (O) molar ratio and the calcination temperature. It was found that both the crystallite size and the lattice parameter nanocrystalline Co₃O₄ increase with increasing the F/O molar ratio as well as the calcination temperature. X-ray diffraction confirmed the formation of CoO phase together with spinel Co₃O₄ using F/O ratio of 1. The concentration of such phase increases with increasing the F/O ratio. Moreover, when the calcination is applied at 900–1000 °C traces of CoO was obtained together with Co₃O₄ as a major phase.     

Synthesis of cobalt-aluminum spinels via EDTA chelating precursors

In this work, synthesis of nanocrystalline Co^||Co^||| _x Al_2–x O₄ (where x = 0, 0.5, 0.8, 1, 1.18 and 2) spinels by the thermal decomposition of complex precursors derived from metal nitrate salts and ethylenediaminetetraacetic acid (EDTA) has been systematically studied. Based on XRD, FTIR, DTA, TGA, SEM and TEM analyses, it has been found that the as-prepared metal organic precursors with different Co and Al contents display different thermal behavior and crystal development and particle morphology. Increment of Co content in precursors improves the degree of crystallinity of the powder. Treated at 400°C, in contrast to the well formed Co₃O₄ or Co₃O₄-like spinel structure for the powder precursors containing higher Co content, most of the powders from precursors containing lower Co content are still amorphous. Higher Co content in the precursors slightly decreases the unit cell parameters of the resulting Co^||Co^||| _x Al_2–x O₄ spinels. Prepared at 900°C, the unit cell parameters of Co^||Al₂O₄, Co^||Co^||| _0.5Al_1.5O₄, Co^||Co^|||AlO₄, Co^||Co^||| ₂O₄ are 8.1038, 8.1007, 8.0867, and 8.0834 Å, respectively. Based on this work, preparation of a series of the Co^||Co^||| _x Al_2–x O₄ spinels including black or bright blue Co^||Al₂O₄, Co^||Co^||| _0.5Al_1.5O₄, Co^||Co^|||AlO₄, Co^||Co^||| ₂O₄ from EDTA-Co(Al) complexing precursor has been established.     

Defect-Induced Atomic Arrangement in CoFe Bimetallic Heterostructures with Boosted Oxygen Evolution Activity

Three CoFe-bimetallic oxides with different compositions (termed as CoFeO_x-A/N/H) are prepared by thermally treating metal-organic-framework (MOF) precursors under different atmospheres (air, N_2, and NaBH₄/N₂), respectively. With the aid of vast oxygen vacancies (O_v), cobalt at tetrahedral sites (Co²⁺(Th)) in spinel Co₃O₄ is diffused into interstitial octahedral sites (Oh) to form rocksalt CoO and ternary oxide CoFe₂O₄ has been induced to give the unique defective CoO/CoFe₂O₄ heterostructure. The resultant CoFeO_x-H exhibits superb electrocatalytic activity toward water oxidation: overpotential at 10 mA cm⁻² is 192 mV, which is 122 mV smaller than that of CoFeO_x-A. The smaller Tafel slope (42.53 mV dec⁻¹) and higher turnover frequency (785.5 h⁻¹) suggest fast reaction kinetics. X-ray absorption spectroscopy, ex situ characterizations, and theoretical calculations reveal that defect engineering effectively tunes the electronic configuration to a more active state, resulting in the greatly decreased binding energy of oxo intermediates, and consequently much lower catalytic overpotential. Moreover, the construction of hetero-interface in CoFeO_x-H can provide rich active sites and promote efficient electron transfer. This work may shed light on a comprehensive understanding of the modulation of electron configuration of bimetallic oxides and inspire the smart design of high-performance electrocatalysts.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

Étude expérimentale de la pulvérisation cathodique réactive du cobalt

The reactive d.c. sputtering of a Co target in argon-oxygen mixtures has been studied. The chemical structure of the thin films formed (Co-β, CoO and Co₃O₄) has been studied by X-ray diffraction, and particles ionized in the plasma have been detected with a quadrupole mass filter. The mechanisms suggested are: CoO formation on the surface of the target and sputtering of CoO; CoO production in the plasma and Co₃O₄ production on the substrate.     

Nanostructural and optical properties of cobalt and nickel–oxide/silica nanocomposites

Cobalt, nickel and mixed cobalt–nickel nanostructured oxides have been obtained by thermal treatments in oxygen of Co, Ni and Co₅₀Ni₅₀/silica nanocomposites produced by ion implantation technique. The thermal treatment produces the total or partial oxidation of the original metallic species, which diffuse towards the surface and in the depth of the substrate. This process gives rise to the growth of different oxides in both regions depending on the kind of metal. In the Co sample, mainly Co₃O₄ is formed; in the Ni sample, Ni and NiO nanoparticles are present, while in the Co–Ni sample Co₃O₄, Ni₂SiO₄ and mainly (Co_xNi_1−x)O nanostructures are formed. The optical features of cobalt and nickel samples are determined by the optical properties of the Co₃O₄ and NiO p-type semiconductor phases, respectively, with energy gaps littler than the corresponding bulk ones. The Co–Ni mixed nanostructured oxide is characterized by a direct absorption gap at 3.4 eV that is smaller than the band gaps of CoO and NiO oxides.     

Hierarchical laminated Al2O3 in-situ integrated with high-dispersed Co3O4 for improved toluene catalytic combustion

Hierarchical structure and surface properties of selective support afford some special effects on the catalytic activity, which could be tuned to achieve improved performance. Herein, using a combination of hydrothermal coprecipitation and thermal processing, we integrated highly-distributed Co₃O₄ spinel nanospecies on laminated hierarchically structured Al₂O₃ which could be used as a highly efficient VOCs treatment catalyst. Impressively, compared to Co-free Al₂O₃ counterpart (S_BET = 188.2 m²·g^?1), these obtained Co₃O₄ spinel functionalized catalysts are endowed with adjustable Co loading, optimized Co activity state, and obviously improved hierarchically structural properties (S_BET = 274.7 m²·g^?1). The microporous and mesoporous structures both existed in the obtained Al₂O₃, which is beneficial to the heterogeneous catalytic reaction process. The results reveal that the proper Co loading in the hierarchically structured Al₂O₃ could enable the rational modulation of catalytic activity in the combustion of toluene and exceeds the commercial 5 wt% Pd/C catalyst in the light of total catalytic oxidation ability. This developed heterogeneous Co₃O₄ compositing hierarchically structured Al₂O₃ provides a significant potential value for practical VOCs treatment.     

Facile Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoO Nanoparticles by Thermal Treatment of Ball-Milled Precursors

Co₃O₄/CoO nanoparticles have been synthesized by a simple method which is based on the ball-milling and calcination of cobalt acetate and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, and Fourier transform infrared spectroscopy. The results show that Co₃O₄ nanoparticles with an average particle size of ∼40 nm can be obtained by calcination of ball-milled precursors at relatively low temperature (350 °C) for 3 hours. It should be noted that it is possible to control the size of Co₃O₄ particles by calcination temperature, calcination time and also by ball-milling duration using this method. Meanwhile, the pure CoO nanoparticles were obtained successfully by thermal decomposition of Co₃O₄ at 950 °C and quickly quenching to liquid nitrogen.     

Oxidation behavior of obliquely deposited CoNi magnetic thin films

The oxidation behavior of Co₈₀Ni₂₀ thin films used for magnetic recording was studied using Auger and X-ray photoelectron spectroscopy. At 500 °C and under an oxygen pressure of 1 × 10⁻⁵ Torr cobalt oxidized to CoO but nickel remained in the metallic state. The growth of CoO followed a parabolic dependence on the oxygen exposure time, suggesting diffusion-controlled kinetics for oxidation. The [Co]/[Ni] ratio at the surface increased with oxidation indicating an oxygen-induced segregation of cobalt in the alloy.     

Defects‐Induced In‐Plane Heterophase in Cobalt Oxide Nanosheets for Oxygen Evolution Reaction

Cobalt oxides as efficient oxygen evolution reaction (OER) electrocatalysts have received much attention because of their rich reserves and cheap cost. There are two common cobalt oxides, Co₃O₄ (spinel phase, stable but poor intrinsic activity) and CoO (rocksalt phase, active but easily be oxidatized). Constructing Co₃O₄/CoO heterophase can inherit both characteristic features of each component and form a heterophase interface facilitating charge transfer, which is believed to be an effective strategy in designing excellent electrocatalysts. Herein, an atomic arrangement engineering strategy is applied to improve electrocatalytic activity of Co₃O₄ for the OER. With the presence of oxygen vacancies, cobalt atoms at tetrahedral sites in Co₃O₄ can more easily diffuse into interstitial octahedral sites to form CoO phase structure as revealed by periodic density functional theory computations. The Co₃O₄/CoO spinel/rocksalt heterophase can be in situ fabricated at the atomic scale in plane. The overpotential to reach 10 mA cm^?2 of Co₃O₄/CoO is 1.532 V, which is 92 mV smaller than that of Co₃O₄. Theoretical calculations confirm that the excellent electrochemical activity is corresponding to a decline in average p‐state energy of adsorbed‐O on the Co₃O₄/CoO heterophase interface. The reaction Gibbs energy barrier has been significantly decreased with the construction of the heterophase interface.     

Rare Earth Doping Engineering Tailoring Advanced Oxygen-Vacancy Co3O4 with Tunable Structures for High-Efficiency Energy Storage

Co₃O₄ with high theoretical capacitance is a promising electrode material for high-end energy applications, yet the unexcited bulk electrochemical activity, low conductivity, and poor kinetics of Co₃O₄ lead to unsatisfactory charge storage capacity. For boosting its energy storage capability, rare earth (RE)-doped Co₃O₄ nanostructures with abundant oxygen vacancies are constructed by simple, economical, and universal chemical precipitation. By changing different types of RE (RE = La, Yb, Y, Ce, Er, Ho, Nd, Eu) as dopants, the RE-doped Co₃O₄ nanostructures can be well transformed from large nanosheets to coiled tiny nanosheets and finally to ultrafine nanoparticles, meanwhile, their specific surface area, pore distribution, the ratio of Co²⁺/Co³⁺, oxygen vacancy content, crystalline phase, microstrain parameter, and the capacitance performance are regularly affected. Notably, Eu-doped Co₃O₄ nanoparticles with good cycle stability show a maximum specific capacitance of 1021.3 F g⁻¹ (90.78 mAh g^-1) at 2 A g^-1, higher than 388 F g^-1 (34.49 mAh g^-1) of pristine Co₃O₄ nanosheets. The assembling asymmetric supercapacitor delivers a high energy density of 48.23 Wh kg^-1 at high power density of 1.2 kW kg^-1. These findings denote the significance and great potential of RE-doped Co₃O₄ in the development of high-efficiency energy storage.     

X-ray study of gadolinium gallium garnet epitaxial layers containing divalent Co ions

Structural perfection of gadolinium gallium garnet (GGG; Gd₃Ga₅O₁₂) epitaxial layers with incorporated divalent Co ions has been studied by means of high resolution X-ray diffraction, X-ray topography, transmission electron microscopy and optical spectroscopy. Epitaxial layers were grown by liquid phase epitaxy from super-cooled high temperature solution with different concentration of Co₃O₄ and GeO₂ on both sides of the polished < 111> oriented GGG substrates. In order to facilitate incorporation of Co²⁺ ions into the garnet lattice optically inert Ge⁴⁺ ions have to be introduced first. High structural perfection is a prerequisite to obtain absorption spectra required for the use GGG epitaxial layers as tunable infrared absorbers.     

Morphology-controlled synthesis of octahedron and hexagonal plate of Co3O4

Hexagonal plate-like Co₃O₄ was prepared by the thermal oxidation of a Co(OH)₂ precursor with a hexagonal plate morphology at 600 °C in air. The Co(OH)₂ precursor was obtained by hydrothermal synthesis from CoCl₂, KCN and hydrazine at 180 °C. The hexagonal plates had a side length of approximately 3.5 μm and 1 μm in thickness. Octahedral Co₃O₄ was prepared by a hydrothermal method with CoCl₂, KCN, and H₂O₂ at 180 °C. H₂O₂ played an important role in the formation of Co₃O₄ with an octahedral morphology. A hierarchical hexagonal skeleton-like morphology assembled with small octahedral Co₃O₄ particles was also prepared using a hydrothermal method with CoCl₂ and KCN. The mechanism of the morphology-controlled synthesis of Co₃O₄ is discussed.     

Uniformly grown PtCo-modified Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanosheets as a highly efficient catalyst for sodium borohydride electrooxidation

A facile hydrothermal synthetic method, followed by in situ reduction and galvanic replacement processes, is used to prepare PtCo-modified Co₃O₄ nanosheets (PtCo/Co₃O₄ NSs) supported on Ni foam. The prepared nanomaterial is used as an electrocatalyst for NaBH₄ oxidation in alkaline solution. The morphology and phase composition of PtCo/Co₃O₄ NSs are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of PtCo/Co₃O₄ NSs is investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in a standard three-electrode system. Current densities of 70 and 850 mA·cm^–2 were obtained at–0.4 V for Co/Co₃O₄ and PtCo/Co₃O₄ NSs, respectively, in a solution containing 2 mol·L^–1 NaOH and 0.2 mol·L^–1 NaBH₄. The use of a noble metal (Pt) greatly enhances the catalytic activity of the transition metal (Co) and Co₃O₄. Besides, both Co and Co₃O₄ exhibit good B–H bond breaking ability (in NaBH₄), which leads to better electrocatalytic activity and stability of PtCo/Co₃O₄ NSs in NaBH₄ electrooxidation compared to pure Pt. The results demonstrate that the as-prepared PtCo/Co₃O₄ NSs can be a promising electrocatalyst for borohydride oxidation.

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Fabrication of single-crystalline Co3O4 nanorods via a low-temperature solvothermal process

Co₃O₄ nanorods with average diameter and length of ∼ 50 nm and 1 μm were successfully prepared via a simple surfactant-assisted solvothermal method at 160 °C for 12 h. The formation of Co₃O₄ nanorods is attributed to alcoholysis of cobalt ions dispersed in ethanol in the presence of a capping agent—CTAB. The composition and purity of the sample were characterized by X-ray diffraction (XRD). Transmission and scanning electron microscopy images show that the particles are homogenous and have the shape of rods. The mechanism of forming Co₃O₄ nanorods is also discussed.     

Facile and green synthesis of cobalt oxide nanoparticles using ethanolic extract of Trigonella foenumgraceum (Fenugreek) leaves

Cobalt oxide nanoparticles (Co₃O₄ NPs) were successfully synthesized using ethanolic extract of Trigonella foenumgraceum L. (fenugreek) leaves as a green, potentially low cost, and easily biosynthesized method. The organic bioactive compounds present in fenugreek leaves extract acted as both reducing agents and stabilizing agents for synthesizing metal NPs from cobalt chloride hexahydrate as a precursor. As evidence from UV/Visible spectroscopy, energy dispersive spectroscopy (EDS), and X-ray diffraction analysis (XRD) studies, high alkaline pH was found favorable for the preparation of pure and crystallized single-phase Co₃O₄ NPs. The interaction of biomolecules from fenugreek leaves extract with Co₃O₄ NPs was defined by Fourier transform infrared spectroscopy (FTIR) analysis and X-ray photoelectron spectroscopy (XPS). The hydrodynamic size and surface charge of the biosynthesized NPs were measured using light-scattering (DLS) and zeta potential analyses; revealed the formation of negative charged Co₃O₄ NPs with uniform hydrodynamic size distribution. According to transmission electron microscopy (TEM) analysis, quasi-spherical Co₃O₄ NPs were synthesized with an average size of 13.2 nm under the modified condition of pH 12 and reaction time of 2 h through inexpensive, environmental friendly benign synthesis process without the use of any additional toxic chemical.     

Phosphorus Regulated Cobalt Oxide@Nitrogen‐Doped Carbon Nanowires for Flexible Quasi‐Solid‐State Supercapacitors

Battery‐type materials are promising candidates for achieving high specific capacity for supercapacitors. However, their slow reaction kinetics hinders the improvement in electrochemical performance. Herein, a hybrid structure of P‐doped Co₃O₄ (P‐Co₃O₄) ultrafine nanoparticles in situ encapsulated into P, N co‐doped carbon (P, N‐C) nanowires by a pyrolysis–oxidation–phosphorization of 1D metal–organic frameworks derived from Co‐layered double hydroxide as self‐template and reactant is reported. This hybrid structure prevents active material agglomeration and maintains a 1D oriented arrangement, which exhibits a large accessible surface area and hierarchically porous feature, enabling sufficient permeation and transfer of electrolyte ions. Theoretical calculations demonstrate that the P dopants in P‐Co₃O₄@P, N‐C could reduce the adsorption energy of OH^? and regulate the electrical properties. Accordingly, the P‐Co₃O₄@P, N‐C delivers a high specific capacity of 669 mC cm^?2 at 1 mA cm^?2 and an ultralong cycle life with only 4.8% loss over 5000 cycles at 30 mA cm^?2. During the fabrication of P‐Co₃O₄@P, N‐C, Co@P, N‐C is simultaneously developed, which can be integrated with P‐Co₃O₄@P, N‐C for the assembly of asymmetric supercapacitors. These devices achieve a high energy density of 47.6 W h kg^?1 at 750 W kg^?1 and impressive flexibility, exhibiting a great potential in practical applications.