Crystallinity, magnetic and electrochemical studies of PVDF/Co3O4 polymer electrolyte

Organic-inorganic nanocomposites are gaining importance in the recent times as polymer electrolyte membranes. In the present work, composites were prepared by combining nano sized Co₃O₄ and poly(vinyledene fluoride) (PVDF), using spin coating technique. The surface of the PVDF/Co₃O₄ system characterized through field emission scanning electron microscopy (FESEM) revealed a porous structure of the films. The nanoparticles tend to aggregate on the surface and inside the pores, leading to a decrease in the porosity with an increase in Co₃O₄ content. Co₃O₄ nanoparticles prohibit crystallization of the polymer. Differential scanning calorimetry (DSC) studies revealed a decrease in crystallinity of PVDF/Co₃O₄ system with an increase in the oxide content. Magnetic property studies of the composite films revealed that with an increase in Co₃O₄ content, the saturation magnetization values of the nanocomposites increased linearly, showing successful incorporation of the nanoparticles in the polymer matrix. Further, ionic conductivity of the composite films was evaluated from electrochemical impedance spectroscopy. Addition of Co₃O₄ nanoparticles enhanced the conductivity of PVDF/Co₃O₄ system.     

An anionic surfactant-templated synthesis and lamellar ordering of Co(OH)2 and Co3O4 nanodisks

Co(OH)₂ nanodisks were synthesized using a surfactant lamellar mesophase as a soft template. From the Co(OH)₂ nanodisks, porous Co₃O₄ nanodisks could also be obtained after a heat treatment. Co(OH)₂ nanodisks were successfully converted to porous Co₃O₄ nanodisks via thermal oxidation. Using the Co₃O₄ nanodisks as a basic building block and sodium dodecyl sulfate as a structure-directing agent, the lamellar mesostructures could be formed. The lamellar ordering is expected to originate through cooperative interaction-induced self-assembly and evaporation-induced self-assembly.     

Synthesis and characterization of Ca3Co4O9 thin films prepared by sol-gel spin-coating technique on Al2O3(001)

Ca₃Co₄O₉ thin films are deposited on Al₂O₃(001) substrates using a sol-gel spin-coating process. X-ray diffraction shows that the film exhibits a single phase of Ca₃Co₄O₉ with the (00l) planes parallel to the film surface. The temperature dependence of magnetic susceptibility showed as expected the existence of two magnetic transitions similar to those observed in bulk samples: a ferrimagnetic and a spin-state transition around 19 and 375 K, respectively. At 5 K the magnetization curves along the c-axis of the Al₂O₃(001) show that the remanent magnetization and coercive field are close to those obtained for films grown by pulsed laser deposition, which evidences the interest to use such an easy technique to grow complex thin films oxides.     

Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi₅Ti₃FeO₁₅ thin films were prepared on Pt/Ti/SiO₂/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi_4.15Nd_0.85Ti₃Fe_0.5Co_0.5O₁₅ films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2P_r and 2M_r are 38 μC/cm² and 3 kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.     

Third-order nonlinear optical responses of nanoparticulate Co3O4 films

Third-order nonlinear optical responses of nanoparticulate Co₃O₄ films, prepared by sputtering and pyrolysis, were investigated. The sputtered Co₃O₄ film showed a large third-order nonlinear susceptibility (|χ⁽³⁾|) of 3.2×10⁻⁸ esu, determined by degenerate four-wave mixing (DFWM). This |χ⁽³⁾| is 7.1 times greater than that of the pyrolyzed Co₃O₄ film, attributable to the greater density and refractive index of the sputtered film. Time-resolved DFWM experiments show a slowly decaying component with a lifetime of approximately 100 ps existing for both films in addition to a fast decaying component. Nonlinear transmission experiments revealed both films exhibiting significant nonlinear absorption. The imaginary part of the susceptibility (Im[χ⁽³⁾]) found for each film was of the same order of magnitude as the |χ⁽³⁾| found by the DFWM method, showing that the major part of the fast decaying component of |χ⁽³⁾| comes from the nonlinear absorption.     

Microstructure and Thermoelectric Properties of Bi- and Cu-Substituted Ca3Co4O9 Oxides

Bi- and Cu-substituted Ca₃Co₄O₉ samples were prepared by conventional solid-state reaction method and the e?ect of element substitution on the microstructures and thermoelectric properties was investigated. Partial substitution of Cu for Co leads to an increase in electrical conductivity and a decrease in Seebeck coe±cient due to the rise of hole concentration. The microstructure of Cu-substituted sample is almost unchanged compared with undoped Ca₃Co₄O₉. On the other hand, partial substitution of Bi for Ca gives rise to a significant increase in the grain size, and c-axis-oriented structure can be formed in Ca_2.7Bi_0.3Co₄O₉, resulting in an obvious increase in electrical conductivity. Cu and Bi co-substitution further increases the grain growth and the electrical conductivity of Ca_2.7Bi_0.3Co_3.7Cu_0.3O₉. Thus, Cu and Bi co-substitution samples possess the optimal thermoelectric performance at high temperature and the highest value of power factor can reach 3.1×10^-4 Wm^-1?K^-2 at 1000 K.     

Electrochemical behavior and multilayer films of the sandwich-type polyoxotungstate complex {K10Co4(H2O)2(PW9O34)2}

The electrochemical behavior of the sandwich-type polyoxotungstate complex of K₁₀Co₄(H₂O)₂(PW₉O₃₄)₂ (Co₄(PW₉)₂) was investigated by cyclic voltammetry in aqueous solutions. The polyoxoanion exhibits two successive redox peaks originating from the tungsten-oxo framework. The electrochemical behavior for the Co^II species was not detected. Moreover, the multilayer films consisting of PEI and Co₄(PW₉)₂ were prepared by the layer-by-layer self-assembly method. The films were characterized by UV–vis spectroscopy, cyclic voltammetry and atomic force image. The films exhibit the electrochemical behavior of Co₄(PW₉)₂ polyoxoanion. Moreover, the films can catalyze the electrochemical reduction of NO₂⁻. Electron transfer to Fe(CN)₆^3−/4− redox probe was also investigated.     

Solvothermal synthesis of sheet-like Co3O4 and Ag/Co3O4 nanocomposites and their electrocatalysis performances

Precursors of Co₃O₄ and Ag/Co₃O₄ composites with sheet-like shape were synthesized with assistance of ethylene glycol via a solvothermal process. The final samples were obtained by calcining each precursor at 400 °C. The as-prepared samples were identified and characterized by thermogravimetric analysis (TG) and differential thermal gravimetric (DTG) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM). The Co₃O₄ and Ag/Co₃O₄ composite nanosheets were used as electrocatalysts modified on a glassy carbon electrode for p-nitrophenol and H₂O₂ reduction respectively in a basic solution. The electrocatalytic results showed that p-nitrophenol could be reduced by pure Co₃O₄ at a large peak current but a rather higher peak potential, and could be reduced effectively by Ag/Co₃O₄ composites at lower potential. Ag/Co₃O₄ composites with 6% Ag displayed the highest electrocatalytic activity for H₂O₂ reduction at the largest peak current and a lower peak potential. The reduction peak potentials of H₂O₂ all reduced a great deal using Ag/Co₃O₄ composite.     

Solvothermal synthesis of Co/CoFe2O4 nanobelts

A facile solvothermal route is used for the synthesis of Co/CoFe₂O₄ nanobelts by rationally manipulating the dosages of a surfactant Poly(vinylpyrrolidone) (PVP, MW 40000). PVP plays a pivotal role in preparing the Co/CoFe₂O₄ nanobelts. The optimal dosage for the synthesis of the Co/CoFe₂O₄ nanobelts is 0.5 g. Lower than this value, octahedral particles and nanobelts were coexistent; higher than this value, octahedral particles were obtained. Furthermore, the possible formation mechanism of Co/CoFe₂O₄ nanobelts was proposed. A small quantity of Co²⁺ ions are reduced by glycerol, which is the reason for the presence of metallic Co in the CoFe₂O₄ ferrite. The Co/CoFe₂O₄ nanobelts may be very attractive for potential applications because of their outstanding magnetic properties (Ms = 110 emu/g, Hc = 387 Oe).     

Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)

Epitaxial Bi₂Sr₂Co₂O_y thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO₃ substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi₂Sr₂Co₂O_y films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step-terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi₂Sr₂Co₂O_y films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals.     

Co3O4 protective coatings prepared by Pulsed Injection Metal Organic Chemical Vapour Deposition

Cobalt oxide films were grown by Pulsed Injection Metal Organic Chemical Vapour Deposition (PI-MOCVD) using Co(acac)₃ (acac=acetylacetonate) precursor dissolved in toluene. The structure, morphology and growth rate of the layers deposited on silicon substrates were studied as a function of deposition temperature. Pure Co₃O₄ spinel structure was found for deposition temperatures ranging from 360 to 540 °C. The optimum experimental parameters to prepare dense layers with a high growth rate were determined and used to prepare corrosion protective coatings for Fe-22Cr metallic interconnects, to be used in Intermediate Temperature Solid Oxide Fuel Cells.     

A simple way to achieve tunable adhesion in superhydrophobic nanostructured Co3O4 films

Tunable adhesive forces on the superhydrophobic nanostructured Co₃O₄ films have been achieved by simply rubbing the as-prepared films. Rubbing makes the change of morphologies of the films and leads to the tuning of adhesion. This morphology-dependent adhesive property is attributed to the difference in kinetic barriers resulting from the contact-state change. This simple and practical method can provide an important strategy for the adhesion adjustment on superhydrophobic surfaces.     

Enhancement of bifunctional catalysis by Ir doping of La0.6Ca0.4CoO3 perovskites

La_0.6Ca_0.4Co_0.8Ir_0.2O₃ was prepared by solid state reaction synthesis from mixtures of La(NO₃)₃·6H₂O, Ca(NO₃)₂·4H₂O, Co(NO₃)₃·6H₂O, and IrO₂ at stoichiometric ratio. Images from scanning electron microscopy on the as-synthesized powders exhibited particles in irregular shape at 100-250 nm in size. X-ray diffraction analysis confirmed the formation of perovskite without the rutile signals from IrO₂, suggesting successful incorporation of Ir⁴⁺ at the Co cation sites. The carbon nanocapsules (CNCs) were used as an electrocatalyst support. Electrodes for electrochemical measurements were fabricated by depositing mixtures of CNCs, La_0.6Ca_0.4Co_0.8Ir_0.2O₃, and polymeric binders on commercially available noncatalyzed gas diffusion electrodes. In alkaline electrolyte, the La_0.6Ca_0.4Co_0.8Ir_0.2O₃/CNCs demonstrated superior performances than those of La_0.6Ca_0.4CoO₃/CNCs and IrO₂/CNCs in both charging and discharging current-potential polarization measurements.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Nanocrystalline chemically modified CdIn2O4 thick films for H2S gas sensor

CdIn₂O₄ sensor with high sensitivity and excellent selectivity for H₂S gas was synthesized by using sol-gel technique. Structural, electrical and gas sensing properties of doped and undoped CdIn₂O₄ thick films were studied. XRD revealed the single-phase polycrystalline nature of the synthesized CdIn₂O₄ nanomaterials. Since the resistance change of a sensing material is the measure of its response, selectivity and sensitivity was found to be enhanced by doping different concentrations of cobalt in CdIn₂O₄ thick films. The sensor exhibits high response and selectivity toward H₂S for 10 wt.% Co doped CdIn₂O₄ thick films. The current-voltage characteristics of 10 wt.% Co doped CdIn₂O₄ calcined at 650 °C shows one order increase in current with change in the bias voltage at an operating temperature of 200 °C for 1000 ppm H₂S gas.     

The optical properties of sputtered Co3O4 films

Films of cobalt oxide were prepared by r.f. reactive sputtering of a cobalt target in Ar-O₂ mixtures, and their optical properties were determined by absorption spectroscopy and scanning ellipsometry. The films were further characterized by electron microscopy and X-ray diffraction. The films were found to consist of Co₃O₄ when the oxygen flow rate exceeded a critical value, in agreement with previous studies. The optical properties of this oxide are due to intense ligand field absorption by Co²⁺ and Co³⁺ ions in a spinel structure. The optical properties of cobalt films oxidized near 500 °C were very similar to those of the sputtered films. Reactive co-sputtering of nickel and cobalt blackened the films for low Ni:Co ratios as a result of charge transfer absorption. Smooth films of Co₃O₄ did not function well as selective thermal absorbers of solar radiation.     

Fabrication of Co3O4 cubic nanoframes: Facet-preferential chemical etching of Fe ions to Co3O4 nanocubes

The novel Co₃O₄ cubic nanoframes, sized in ca. 30 nm, were firstly fabricated via a facile solvothermal route. Based on the transmission electron microscopy and the powder X-ray diffraction analyses of the time-dependent products, a mechanism of facet-preferential chemical etching of Fe³⁺ ions to the pre-synthesized Co₃O₄ nanocubes is proposed for the formation of Co₃O₄ cubic nanoframes. This synthetic strategy can probably be extended to fabricate nanoframes of some other binary metal oxides, by designing similar chemical etching process.     

Synthesis and characterization of Co3O4 hollow spheres

Co₃O₄ hollow spheres were hydrothermally prepared at 130 °C for 16 h in the presence of Poly-vinylpyrrolidone (PVP). The as-prepared products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), infrared spectrum (IR), X-ray photoelectron spectrum (XPS), and optical absorption spectrum. PVP surfactant plays important roles in the formation of Co₃O₄ hollow spheres. These Co₃O₄ hollow spheres have average diameters of ca. 350 nm, and the wall thickness around the shell is about 42 nm. The possible formation mechanism of hollow Co₃O₄ spherical structures has simply been proposed.     

Thermal stability against reduction of LaMn1−yCoyO3 perovskites

Thermal and reduction-oxidation stability of substituted LaMn_1−yCo_yO₃ perovskite-type oxides (0.0 ≤ y_Co ≤ 1.0) prepared by the citrate route have been studied by means of surface area, X-ray diffraction, FTIR spectroscopy and magnetic properties. The perovskite orthorhombic structure is found for y_Co ≤ 0.5, with the exception of y_Co = 0.1, which corresponds better to rhombohedral LaMnO_3.15. For y_Co > 0.5 the diffraction profiles are quite similar to the cobaltite’s rhombohedral structure. Magnetic iso-field studies (ZFC-FC) reveal that, for y_Co ≤ 0.50, the system presents an antiferromagnetic canted-like ordering of the Mn/Co sublattice, in which the presence of divalent Co ion creates Mn³⁺-Mn⁴⁺ pairs that interact ferromagnetically through the oxygen orbital. This interpretation is confirmed by the magnetization loops, in which the magnetic moment increases when substituting Mn for Co. Therefore, the general trend is: for y_Co ≤ 0.5, the Co ions are inserted in the manganite structure and for y_Co > 0.5, the Mn ions are inserted in cobaltite structure. The enhancement of the ferromagnetic properties and the thermal stability against reduction for y_Co = 0.5 is attributed to optimized Co²⁺-Mn⁴⁺ interactions.     

The electrochemical behaviour of Co3O4 and CoO cathodes in high-temperature cells

Preliminary investigations of the electrochemical behaviour of $\text{LiA}\text{l}\text{LiC}\text{l}$, $\text{KC}\text{l}\text{Co}{}_3\text{O}{}_4$ and $\text{LiA}\text{l}\text{LiC}\text{l}$, $\text{KC}\text{l}\text{CoO}$ cells are reported. At an operating temperature of 420°C, maximum discharge capacities of up to 546 mA-h per gram of cobalt oxide were recorded during the early charge/discharge cycles. Cell performance deteriorated with further cycling due to (a) an instability of the cobalt oxides in the electrolyte, and (b) dispersion of the discharge product Li₂O into the electrolyte. During discharge, Co₃O₄ was reduced via CoO to Co metal. If excess Li₂O was added to the cathodes, CoO was oxidized to Co₃O₄ on charging, and further oxidation of Co₃O₄ gave rise to an unstable compound with postulated formula Li_2xCo₂O_3+x for $\text{x}\text{≧ 0}$. CoO was the most stable cobalt-oxide phase in the molten LiCl, KCl electrolyte.