Efficient solar photocatalyst based on cobalt oxide/iron oxide composite nanofibers for the detoxification of organic pollutants

A Co₃O₄/Fe₂O₃ composite nanofiber-based solar photocatalyst has been prepared, and its catalytic performance was evaluated by degrading acridine orange (AO) and brilliant cresyl blue (BCB) beneath solar light. The morphological and physiochemical structure of the synthesized solar photocatalyst was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). FESEM indicates that the Co₃O₄/Fe₂O₃ composite has fiber-like nanostructures with an average diameter of approximately 20 nm. These nanofibers are made of aggregated nanoparticles having approximately 8.0 nm of average diameter. The optical properties were examined by UV-visible spectrophotometry, and the band gap of the solar photocatalyst was found to be 2.12 eV. The as-grown solar photocatalyst exhibited high catalytic degradation in a short time by applying to degrade AO and BCB. The pH had an effect on the catalytic performance of the as-grown solar photocatalyst, and it was found that the synthesized solar photocatalyst is more efficient at high pH. The kinetics study of both AO and BCB degradation indicates that the as-grown nanocatalyst would be a talented and efficient solar photocatalyst for the removal of hazardous and toxic organic materials.     

Synthesis and electrochemical characteristics of Al2O3-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium ion batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials have been coated with Al₂O₃ nano-particles using sol-gel processing to improve its electrochemical properties. The X-ray diffraction (XRD) pattern of the as-prepared Al₂O₃ nano-particles was indexed to the cubic structure of the γ-Al₂O₃ phase and had an average size of ∼4 nm. The XRD showed that the structure of LiNi_1/3Co_1/3Mn_1/3O₂ was not affected by the Al₂O₃ coating. However, the Al₂O₃ coatings on LiNi_1/3Co_1/3Mn_1/3O₂ improved the cyclic life performance and rate capability without decreasing its initial discharge capacity. These electrochemical properties were also compared with those of LiAlO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode material. The electrochemical impedance spectroscopy (EIS) was studied to understand the enhanced electrochemical properties of the Al₂O₃-coated LiNi_1/3Co_1/3Mn_1/3O₂ compared to uncoated LiNi_1/3Co_1/3Mn_1/3O₂.     

The first cycle characteristics of Li[Ni1/3Co1/3Mn1/3]O2 charged up to 4.7 V

In this research, we studied the first cycle characteristics of Li[Ni_1/3Co_1/3Mn_1/3]O₂ charged up to 4.7 V. Properties, such as valence state of the transition metals and crystallographic features, were analyzed by X-ray absorption spectroscopy and X-ray and neutron diffractions. Especially, two plateaus observed around 3.75 and 4.54 V were investigated by ex situ X-ray absorption spectroscopy. XANES studies showed that the oxidation states of transition metals in Li[Ni_1/3Co_1/3Mn_1/3]O₂ are mostly Ni²⁺, Co³⁺ and Mn⁴⁺. Based on neutron diffraction Rietveld analysis, there is about 6% of all nickel divalent (Ni²⁺) ions mixed with lithium ions (cation mixing). Meanwhile, it was found that the oxidation reaction of Ni²⁺/Ni⁴⁺ is related to the lower plateau around 3.75 V, but that of Co³⁺/Co⁴⁺ seems to occur entire range of x in Li_1−x[Ni_1/3Co_1/3Mn_1/3]O₂. Small volume change during cycling was attributed to the opposite variation of lattice parameter “c” and “a” with charging-discharging.     

Co3O4 thin film prepared by a chemical bath deposition for electrochemical capacitors

Co₃O₄ thin film is synthesized on ITO by a chemical bath deposition. The prepared Co₃O₄ thin film is characterized by X-ray diffraction, and scanning electron microscopy. Electrochemical capacitive behavior of synthesized Co₃O₄ thin film is investigated by cyclic voltammetry, constant current charge/discharge and electrochemical impedance spectroscopy. Scanning electron microscopy images show that Co₃O₄ thin film is composed of spherical-like coarse particles, together with some pores among particles. Electrochemical studies reveal that capacitive characteristic of Co₃O₄ thin film mainly results from pseudocapacitance. Co₃O₄ thin film exhibits a maximum specific capacitance of 227 F g⁻¹ at the specific current of 0.2 A g⁻¹. The specific capacitance reduces to 152 F g⁻¹ when the specific current increases to 1.4 A g⁻¹. The specific capacitance retention ratio is 67% at the specific current range from 0.2 to 1.4 A g⁻¹.     

La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.     

Phase transformation in CeO2–Co3O4 binary oxide under reduction and calcination pretreatments

The CeO₂–Co₃O₄ binary oxide was prepared by impregnation of the high surface area Co₃O₄ support (S.A. = 100m² g⁻¹) with cerium nitrate (20 wt% cerium loading on Co₃O₄). Pretreatment of CeO₂–Co₃O₄ binary oxide was divided both methods: reduction (under 200 and 400 °C, assigned as CeO₂–Co₃O₄–R200 and CeO₂–Co₃O₄–R400 and calcination (under 350 and 550 °C, assigned as CeO₂–Co₃O₄–C350 and CeO₂–Co₃O₄–C550). The binary oxides were investigated by means of X-ray diffraction (XRD), nitrogen adsorption at −196 °C, infrared (IR), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and temperature programmed reduction (TPR). The results showed that the binary oxides pretreatment under low-temperatures possessed larger surface area. The cobalt phase of binary oxides also was transferred upon the treating temperature, i.e., the CeO₂–Co₃O₄–R200 binary oxide exhibited higher surface area (S.A. = 109m² g⁻¹) and the main phase was CeO₂,Co₃O₄ and CoO. While, the CeO₂–Co₃O₄–R400 binary oxide exhibited lower surface area (S.A. = 40m² g⁻¹) and the main phase was CeO₂, CoO and Co. Apparently, the optimized pretreatment of CeO₂–Co₃O₄ binary oxide can control both the phases and surface area.     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.     

Rapid synthesis of nanocrystalline Co3O4 by a microwave-assisted combustion method

A facile and rapid microwave-assisted combustion method was developed to synthesize the nanocrystalline Co₃O₄. The study suggested that application of microwave heating to produce the homogeneous porous Co₃O₄ was achieved in a few minutes. The structure and morphology of the as-prepared nanocrystalline Co₃O₄ were investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Raman spectra, UV-vis absorbance spectra and scanning electron microscopy (SEM). The XRD, FTIR and Raman spectra confirmed the formation of spinel structural Co₃O₄, and the SEM results indicated the porous surface characteristic of the products. Magnetic measurement was carried out using a vibrating sample magnetometer (VSM). The field dependence of the magnetization at room temperature showed a tiny hysteresis loop with a coercivity of 56.7 Oe.     

Effect of ZnO modification on the performance of LiNi0.5Co0.25Mn0.25O2 cathode material

ZnO was coated on LiNi_0.5Co_0.25Mn_0.25O₂ cathode (positive electrode) material for lithium ion battery via sol–gel method to improve the performance of LiNi_0.5Co_0.25Mn_0.25O₂. The X-ray diffraction (XRD) results indicated that the lattice structure of LiNi_0.5Co_0.25Mn_0.25O₂ was not changed distinctly after surface coating and part of Zn²⁺ might dope into the lattice of the material. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) proved that ZnO existed on the surface of LiNi_0.5Co_0.25Mn_0.25O₂. Charge and discharge tests showed that the cycle performance and rate capability were improved by ZnO coating, however, the initial capacity decreased dramatically with increasing the amount of ZnO. Differential scanning calorimetry (DSC) results showed that thermal stability of the materials was improved. The XPS spectra after charge–discharge cycles showed that ZnO coated on LiNi_0.5Co_0.25Mn_0.25O₂ promoted the decomposition of the electrolyte at the early stage of charge–discharge cycle to form more stable SEI layer, and it also can scavenge the free acidic HF species from the electrolyte. The electrochemical impedance spectroscopy (EIS) results showed ZnO coating could suppress the augment of charge transfer resistance upon cycling.     

The effect of zirconium oxide coating on the lithium nickel cobalt oxide for lithium secondary batteries

The electrochemical properties of LiNi_0.8Co_0.2O₂ coated with ZrO₂ by three different coating processes (ball-milling, sol-gel method, simple grinding) were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Results showed that the ZrO₂ coating significantly improved the capacity retention of the cathode by suppressing the impedance growth at the interface between electrodes and electrolyte and the best cyclability was obtained in the case of employing the simple grinding for the ZrO₂ coating. On the other hand, the initial capacities of the ZrO₂-coated LiNi_0.8Co_0.2O₂ cathode were slightly decreased.     

Preparation and electrochemical capacitance of cobalt oxide (Co3O4) nanotubes as supercapacitor material

Cobalt oxide (Co₃O₄) nanotubes have been successfully synthesized by chemically depositing cobalt hydroxide in anodic aluminum oxide (AAO) templates and thermally annealing at 500 °C. The synthesized nanotubes have been characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The electrochemical capacitance behavior of the Co₃O₄ nanotubes electrode was investigated by cyclic voltammetry, galvanostatic charge-discharge studies and electrochemical impedance spectroscopy in 6 mol L⁻¹ KOH solution. The electrochemical data demonstrate that the Co₃O₄ nanotubes display good capacitive behavior with a specific capacitance of 574 F g⁻¹ at a current density of 0.1 A g⁻¹ and a good specific capacitance retention of ca. 95% after 1000 continuous charge-discharge cycles, indicating that the Co₃O₄ nanotubes can be promising electroactive materials for supercapacitor.     

Enhanced photoluminescence property of Dy co-doped BaAl2O4:Eu green phosphors

Eu²⁺-doped BaAl₂O₄ green phosphors were prepared by a conventional solid-state reaction and the effects of Dy³⁺ co-doping on the photoluminescence property were investigated. The phosphors were characterized by X-ray powder diffraction (XRD), fluorescence spectroscopy, field-emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). XRD showed that all prepared samples exhibited a hexagonal BaAl₂O₄ phase. Fluorescence spectroscopy showed that the photoluminescence efficiency increased with increasing Eu²⁺ concentration until 3 mol% then decreased at higher concentrations due to concentration quenching effect. Moreover, Dy³⁺ co-doping increased the photoluminescence efficiency of the Eu²⁺-doped BaAl₂O₄ phosphor.     

Synthesis and electrochemical performance of carbon nanofiber-cobalt oxide composites

Carbon nanofiber (CNF) supported cobalt oxide composites as high-capacity anode materials were prepared through a facile, effective method for potential use in rechargeable lithium-ion batteries. The effects of the calcining temperature on the crystallinity, grain size, specific surface area of Co₃O₄ and phase transformation from Co₃O₄ to CoO were studied in detail. Both the specific surface area and CNF content in CNF-cobalt oxide composites strongly affect the electrochemical performance of these series composites. The CNF-Co₃O₄ composite with 24.3% CNF pyrolyzed at 500 °C in Ar shows an excellent cycling performance, retaining a specific capacity of 881 mAh g⁻¹ beyond 100 cycles. Homogeneous deposition and distribution of nanosized Co₃O₄ particles on the surface of CNF can stabilize the electronic and ionic conductivity as well as the morphology of Co₃O₄ phase, which may be the main reason for the markedly improved electrochemical performance.     

Synthesis of LiNi0.9Co0.1O2 from Li2CO3, NiO or NiCO3, and CoCO3 or Co3O4 and their electrochemical properties

Cathode active materials with a composition of LiNi_0.9Co_0.1O₂ were synthesized by a solid-state reaction method at 850 °C using Li₂CO₃, NiO or NiCO₃, and CoCO₃ or Co₃O₄, as the sources of Li, Ni, and Co, respectively. Electrochemical properties, structure, and microstructure of the synthesized LiNi_0.9Co_0.1O₂ samples were analyzed. The curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂ for the first charge–discharge and the intercalated and deintercalated Li quantity Δx were studied. The destruction of unstable 3b sites and phase transitions were discussed from the first and second charge–discharge curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂. The LiNi_0.9Co_0.1O₂ sample synthesized from Li₂CO₃, NiO, and Co₃O₄ had the largest first discharge capacity (151 mA h/g), with a discharge capacity deterioration rate of −0.8 mA h/g/cycle (that is, a discharge capacity increasing 0.8 mA h/g per cycle).     

Preparation of novel Ni/Co co-doping Fe3O4/TiO2 core–shell nanocomposites and their use in effective photocatalytic degradation of Amlodipine drug.

Ni/Co co-doping Fe₃O₄/TiO₂ magnetic core–shell nanocomposites (wt% varied amount of dopants) have been prepared by sol-gel method at low temperature. X-ray diffraction, Fourier transform infrared, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma optical emission spectroscopy and vibrating sample magnetometry studies have been made to investigate the crystalline structure, morphology and magnetic properties of these composites. The prepared Ni/Co co-doping Fe₃O₄/TiO₂ nanocomposites exhibit high degree of crystallinity and suitable magnetic properties at room temperature. Their use has been made in effective photocatalytic degradation of Amlodipine a pharmaceutical contaminant under UV light irradiation at 365 nm. The results have shown that wt% amount of dopants, calcination time, calcination temperature and pH of the Amlodipine aqueous solution are important factors in degradation efficiency of Amlodipine. The optimal weight ratios of Ni and Co to Ti were 0.015%. The nanocomposites can be recovered from the aqueous system easily by using a magnet. Their photocatalytic degradation activity for Amlodipine drug remained 94.43% after five times of repetitive use.     

Mixed oxides Ca2Fe2O5 and Ca2Co2O5 as anode materials for Li-ion batteries

The electrochemical performance of mixed oxides, Ca₂Fe₂O₅ and Ca₂Co₂O₅ for use in Li-ion batteries was studied with Li as the counter electrode. The compounds were prepared and characterized by X-ray diffraction and SEM. Ca₂Fe₂O₅ showed a reversible capacity of 226 mAh/g at the 14th cycle and retained 183 mAh/g at the end of 50 cycles at 60 mA/g in the voltage window 0.005-2.5 V. A reversible capacity in the range, 365-380 mAh/g, which is stable up to 50 charge-discharge cycles is exhibited by Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V and at 60 mA/g. This corresponds to recycleable moles of Li of 3.9±0.1 (theoretical: 4.0). Significant improvement in the cycling performance and attainable reversible capacity were noted for Ca₂Co₂O₅ on cycling to an upper cut-off voltage of 3.0 V as compared to 2.5 V. Coulombic efficiency for both compounds is >98%. Electrochemical impedance spectroscopy (EIS) data clearly indicate the reversible formation/decomposition of polymeric surface film on the electrode surface of Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V. Cyclic voltammetry results compliment the galvanostatic cycling data.     

Microwave- or conventional–hydrothermal synthesis of Co-based materials for electrochemical energy storage

Crystalline Co₃O₄ and Co(OH)₂ were synthesized using Co(NO₃)₂ as a precursor by conventional–hydrothermal and microwave–hydrothermal routes, respectively. The Co₃O₄ phase showed cubic morphologies while the β-Co(OH)₂ phase exhibited plate-like shapes. The electrochemical performances of Co₃O₄ and Co(OH)₂ phases were evaluated as electrode materials for lithium-ion battery anodes, cathodes and supercapacitors. Both Co₃O₄ and Co(OH)₂ phases showed pseudocapacitive performances in Li₂SO₄ and KOH electrolytes. The Co₃O₄ and Co(OH)₂ phases were found to be more promising as anodes than as cathodes in lithium-ion batteries. The Co(OH)₂ electrodes showed higher specific capacitances than those of Co₃O₄ materials.     

Influence of Co3O4, Fe2O3 and SiC on microstructure and properties of glass foam from waste cathode ray tube display panel (CRT)

Abstract

In the present study, the effect of temperature and oxidising agents such as Fe₂O₃ and Co₃O₄ on physical and mechanical properties of glass foam is investigated. The glass foam is made of panel glass from dismantled cathode ray tubes and SiC as a foaming agent. In the process, powdered waste glass (mean particle size below 63 μm) in addition to 4 wt-% SiC powder (mean particle size below 45 μm) are combined with Fe₂O₃ and Co₃O₄ (0·4, 0·8 and 1·2 wt-%) have been sintered at 950 and 1050°C. The glass foamed containing 1·2 wt-% Co₃O₄ has good physical properties, with porosity more than 80% and bending strength more than 1·57±0·12 MPa. However, by adding different amounts of Fe₂O₃ in comparison with samples without iron oxide, little changes in porosity and strength are obtained.     

Calibrated Co3O4 nanoparticles patterned in SBA-15 silicas: Accessibility and activity for CO oxidation

The catalytic performances in CO oxidation of Co₃O₄ nanoparticles patterned in the porosity of SBA-15 silicas are investigated. Accessibility limitations of the reactants to the catalytic sites are clearly revealed, when the Co₃O₄ nanoparticles are embedded in the SBA-15 pores. Despite these limitations, the synthesised Co₃O₄ nanoparticles exhibit promising CO oxidation properties.     

Low-temperature Oxidation of Carbon Monoxide on Co/ZrO2

A 10%Co/ZrO₂ catalyst prepared by impregnation was tested for its activity for the oxidation of CO to CO₂ in excess oxygen. Activity tests showed that conversion could be obtained at temperatures as low as 20 °C. Time-on-stream studies showed no loss of activity in these experiments, indicating that this catalyst is stable in the experimental oxidizing conditions. The activation energy for the CO to CO₂ oxidation reaction was calculated as E_a = 54 kJ/mol over this catalyst. Characterization of the material by thermogravimetric analysis, temperature-programmed techniques, X-ray photoelectron spectroscopy, and laser Raman spectroscopy indicate that Co₃O₄ is present on monoclinic ZrO₂ after the calcination of the catalyst.