Bifunctional Co3O4/ZSM-5 Mesoporous Catalysts for Biodiesel Production via Esterification of Unsaturated Omega-9 Oleic Acid

In the present work, two sets of the Co/ZSM-5 mesoporous catalysts with different acidity and Co loadings varying from 1 to 5 and 10 wt% were prepared using mesoporous ZSM-5-A (Si/Al = 50) and ZSM-5-B (Si/Al = 150) as support. X-ray diffraction (XRD) analysis showed that the Co3O4 phase was formed in the surface of catalysts and the reducibility of Co3O4 nanoparticles on the ZSM-5-B was greater in comparison with that on the ZSM-5-A solid. In situ FTIR of pyridine adsorption characterization confirmed that all of the Co/ZSM-5 catalysts contained both Lewis (L) and Brønsted (B) acid sites, with a relatively balanced B/L ratio ranging from 0.61 to 1.94. Therefore, the Si/Al molar ratio in ZSM-5 affected both the surface acidity and the cobalt oxide reducibility. In the esterification of unsaturated omega-9 oleic acid with methanol, under the optimal reaction conditions (temperature 160 °C, catalyst concentration 2 g/L, methanol/oleic acid molar ratio 30, and reaction time 180 min), the biodiesel selectivity reached 95.1% over the most active 10 wt% Co/ZSM-5-B catalyst. The higher esterification activity of the Co/ZSM-5-B catalysts can be correlated with the greater amount of B and L acid sites, the balanced B/L ratio, and the higher reducibility of Co3O4 nanoparticles. The oleic acid esterification reaction followed the bifunctional mechanism of combining metal function (dispersed Co3O4 with a greater reducibility) with the acidity function (both B and L acid sites with a relative balanced B/L ratio) on the catalysts, which may help in providing a deep understanding of the esterification pathways and benefiting the design of novel bifunctional catalysts for biofuel production.

Influence of the Defect Stability on n-Type Conductivity in Electron-Doped α- and β-Co(OH)2 Nanosheets

Electronic doping of transition-metal oxides (TMOs) is typically accomplished through the synthesis of nonstoichiometric oxide compositions and the subsequent ionization of intrinsic lattice defects. As a result, ambipolar doping of wide-band-gap TMOs is difficult to achieve because the formation energies and stabilities of vacancy and interstitial defects vary widely as a function of the oxide composition and crystal structure. The facile formation of lattice defects for one carrier type is frequently paired with the high-energy and unstable generation of defects required for the opposite carrier polarity. Previous work from our group showed that the brucite (β-phase) layered metal hydroxides of Co and Ni, intrinsically p-type materials in their anhydrous three-dimensional forms, could be n-doped using a strong chemical reductant. In this work, we extend the electron-doping study to the α polymorph of Co(OH)₂ and elucidate the defects responsible for n-type doping in these two-dimensional materials. Through structural and electronic comparisons between the α, β, and rock-salt structures within the cobalt (hydr)oxide family of materials, we show that both layered structures exhibit facile formation of anion vacancies, the necessary defect for n-type doping, that are not accessible in the cubic CoO structure. However, the brucite polymorph is much more stable to reductive decomposition in the presence of doped electrons because of its tighter layer-to-layer stacking and octahedral coordination geometry, which results in a maximum conductivity of 10^-4 S/cm, 2 orders of magnitude higher than the maximum value attainable on the α-Co(OH)₂ structure.

A novel ethanol gas sensor based on α-Bi2Mo3O12/Co3O4 nanotube-decorated particles

A novel composite based on α-Bi₂Mo₃O₁₂/Co₃O₄ nanotube-decorated particles was successfully synthesized using a highly efficient and facile two step system using electrospinning and hydrothermal techniques. The small size Co₃O₄ nanoparticles were uniformly and hydrothermally developed on the electrospun α-Bi₂Mo₃O₁₂ nanotubes. The pure α-Bi₂Mo₃O₁₂ nanofibers and composite based on α-Bi₂Mo₃O₁₂/Co₃O₄ were examined using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analyses. From the BET measurements, the composite based on α-Bi₂Mo₃O₁₂/Co₃O₄ exhibits a large specific surface area of 54 m² g⁻¹ with mesopore diameter ranges of 2–10 nm, which is mainly attributed to the remarkable and dominant enhancement in gas sensing as compared to that of the pure α-Bi₂Mo₃O₁₂ nanofibers (38 m² g⁻¹) and Co₃O₄ nanoparticles (32 m² g⁻¹), respectively. In this work, the novel composite based on α-Bi₂Mo₃O₁₂/Co₃O₄ presented a high sensitivity of 30.25 with a quick response/recovery speed towards 100 ppm ethanol at an optimal working temperature of 170 °C, as compared to the pure α-Bi₂Mo₃O₁₂ nanofibers and Co₃O₄ nanoparticles, which display a sensitivity of 13.10 and 2.99 at an optimal working temperature of 220 °C and 280 °C. The sensing performance of the composite based on the α-Bi₂Mo₃O₁₂/Co₃O₄ sensor exhibits a superior sensing performance towards ethanol, which might be owed to the enormous number of superficial oxygen species, the small size catalytic effect of the Co₃O₄ nanoparticles and the interfacial effect formed between the n-type α-Bi₂Mo₃O₁₂ and p-type Co₃O₄ leading to a high charge carrier concentration. This is a novel investigation of a composite based on an α-Bi₂Mo₃O₁₂/Co₃O₄ sensor in the gas sensing era, which might be of vital importance in applications in the advanced gas sensing field.

A novel composite based on α-Bi₂Mo₃O₁₂/Co₃O₄ nanotube-decorated particles was successfully synthesized using a highly efficient and facile two step system using electrospinning and hydrothermal techniques.

Growth of faceted pores in a multi-component crystal by applying mechanical stress

The theory for controllable growth of pores in a multicomponent crystal using mechanical stress is proposed.

Fabrication of TiO2-Nanotube-Array-Based Supercapacitors

In this work, a simple and cost-effective electrochemical anodization technique was adopted to rapidly grow TiO₂ nanotube arrays on a Ti current collector and to utilize the synthesized materials as potential electrodes for supercapacitors. To accelerate the growth of the TiO₂ nanotube arrays, lactic acid was used as an electrolyte additive. The as-prepared TiO₂ nanotube arrays with a high aspect ratio were strongly adhered to the Ti substrate. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirmed that the TiO₂ nanotube arrays were crystallized in the anatase phase. TEM images confirmed the nanotublar-like morphology of the TiO₂ nanotubes, which had a tube length and a diameter of ~16 and ~80 nm, respectively. The electrochemical performance of the TiO₂ nanotube array electrodes was evaluated using the cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements. Excellent electrochemical response was observed for the electrodes based on the TiO₂ nanotube arrays, as the cells delivered a high specific capacitance of 5.12 mF/cm² at a scan rate of 100 mV/s and a current density of 100 µA/cm². The initial capacity was maintained for more than 250 cycles. Further, a remarkable rate capability response was observed, as the cell retained 88% of the initial areal capacitance when the scan rate was increased from 10 to 500 mV/s. The results suggest the suitability of TiO₂ nanotube arrays as electrode materials for commercial supercapacitor applications.

Rapid growth of fully-inorganic flexible CaxCoO2 thin films from a ligand free aqueous precursor ink for thermoelectric applications

We report a ligand-free green approach to rapidly grow flexible nanoporous Ca_0.35CoO₂ thin films from a stable precursor-ink for wide temperature-range thermoelectric applications.

Incorporation of Carbon Quantum Dots for Improvement of Supercapacitor Performance of Nickel Sulfide

A facile hydrothermal method is adopted for the synthesis of hierarchical flowerlike nickel sulfide nanostructure materials and their composite with carbon quantum dot (NiS/C-dot) composite. The composite material exhibited good performance for electrochemical energy-storage devices as supercapacitor with a specific capacity of 880 F g-1 at a current density of 2 A g-1. The material remained stable up to the tested 2000 charge-discharge cycles. Carbon quantum dots of size 1.3 nm were synthesized from natural sources and the favorable electronic and surface property of C-dots were utilized for improvement of the supercapacitor performance of NiS. The results from Tafel analysis, double-layer capacitance, and the impedance measurement reveal that the incorporation of C-dots inside the NiS matrix has improved the charge-transfer process, which is mainly responsible for the enhancement of the supercapacitive property of the composite materials.

A Scalable Synthesis Pathway to Nanoporous Metal Structures

A variety of nanoporous transition metals, Fe, Co, Au, Cu, and others, have been readily formed by a scalable, room-temperature synthesis process. Metal halide compounds are reacted with organolithium reductants in a nonpolar solvent to form metal/lithium halide nanocomposites. The lithium halide is then dissolved out of the nanocomposite with a common organic solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a nanoporous structure. This approach is applicable to both noble metals (Cu, Au, Ag) and less-noble transition metals (Co, Fe, Ni). The microstructures of these nanoporous transition metals are tunable, as controlling the formation of the metal structure in the nanocomposite dictates the final metal structure. Microscopy studies and nitrogen adsorption analysis show these materials form pores ranging from 2 to 50 nm with specific surface areas from 1.0 m²/g to 160 m²/g. Our analysis also shows that pore size, pore volume, and filament size of the nanoporous metal networks depend on the mobility of target metal and the amount of lithium halide produced by the conversion reaction. Further, it has been demonstrated that hybrid nanoporous structures of two or more metals could be synthesized by performing the same process on mixtures of precursor compounds. Metals (e.g., Co and Cu) have been found to stabilize each other in nanoporous forms, resulting in smaller pore sizes and higher surface areas than each element in their pure forms. This scalable and versatile synthesis pathway greatly expands our access to additional compositions and microstructures of nanoporous metals.

Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode

In the present work, we have synthesized three-dimensional (3D) reduced graphene oxide nanosheets (rGO NSs) containing iron oxide nanoparticles (Fe₃O₄ NPs) hybrids (3D Fe₃O₄/rGO) by one-pot microwave approach. Structural and morphological studies reveal that the as-synthesized Fe₃O₄/rGO hybrids were composed of faceted Fe₃O₄ NPs induced into the interconnected network of rGO NSs. The morphologies and structures of the 3D hybrids have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectrometer (XPS). The electrochemical studies were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which demonstrate superior electrochemical performance as supercapacitors electrode application. The specific capacitances of 3D hybrid materials was 455 F g^-1 at the scan rate of 8 mV s^-1, which is superior to that of bare Fe₃O₄ NPs. Additionally, the 3D hybrid shows good cycling stability with a retention ratio of 91.4 after starting from ∼190 cycles up to 9600 cycles. These attractive results suggest that this 3D Fe₃O₄/rGO hybrid shows better performance as an electrode material for high-performance supercapacitors.

Electrochemical growth of Co(OH)2 nanoflakes on Ni foam for methanol electro-oxidation

Co(OH)₂ nanoflakes directly grown on Ni foam using an electrodeposition route exhibit a promising performance for electrocatalytic oxidation of methanol.

A nanocrystalline Co3O4@polypyrrole/MWCNT hybrid nanocomposite for high performance electrochemical supercapacitors

In this study, a ternary hybrid nanocomposite of Co₃O₄@polypyrrole/MWCNT was prepared via oxidative polymerization of pyrrole monomer and a hybrid composite by a hydrothermal process.

Bath temperature controlled phase stability of hierarchical nanoflakes CoS2 thin films for supercapacitor application

In the present study, CoS₂ thin-film electrodes are synthesized at different bath temperatures using a simple chemical bath deposition (CBD) method.

Facile hydrothermal synthesis of NiS hollow microspheres with mesoporous shells for high-performance supercapacitors

Hollow NiS microspheres with excellent capacitive performance were prepared by a facile hydrothermal route without any surfactant or template.

Recent Advances in Synthesis, Properties and Applications of Magnetic Oxide Nanomaterials

Oxide nanomaterials are in great demand due to their unique physical, chemical and structural properties. The nanostructured materials with desired magnetic properties are the future of power electronics. Unique magnetic properties and excellent biocompatibility of these materials found applications in pharmaceutical field also. For these applications, the synthesis of magnetic oxide nanomaterials with required properties is highly desirable. Till now, various techniques have been evolved for the synthesis of oxide nanomaterials with full control over their shape, size, morphology and magnetic properties. In nanoscale, the magnetic properties are totally different from their bulk counterparts. In this range, each nanoparticle acts as a single magnetic domain and shows fast response to applied magnetic field. This review article discusses the synthesis techniques, properties and the applications of magnetic oxide nanomaterials. Various characterization techniques for magnetic materials have been discussed along with the literature of iron oxide, nickel oxide, and cobalt oxide nanomaterials. The challenges for further development of these materials have also been presented to broaden their rapidly emerging applications.

Flexible and ultralong-life cuprous oxide microsphere-nanosheets with superior pseudocapacitive properties

A facile electrochemical two-step strategy was adopted to fabricate Cu₂O microsphere-nanosheets on flexible Cu foil with superior pseudocapacitive properties.

Metal–organic frameworks based on rigid ligands as separator membranes in supercapacitor

Two MOFs are used as separator membranes in a supercapacitor, and after a charge–discharge experiment the separator membrane of the Co compound becomes more porous.

Rapid and large-scale synthesis of Co3O4 octahedron particles with very high catalytic activity, good supercapacitance and unique magnetic properties

Octahedron Co₃O₄ particles with very high catalytic activity, good supercapacitance, and unique magnetic properties.

Porous hollow Co₃O₄ with rhombic dodecahedral structures for high-performance supercapacitors

Porous hollow Co₃O₄ with rhombic dodecahedral structures were prepared by the calcination of ZIF-67 ([Co(mim)2; mim = 2-methylimidazolate]) rhombic dodecahedral microcrystals. A supercapacitor was successfully constructed by adopting the resulting porous hollow Co₃O₄ rhombic dodecahedral structure as the electrode material, which showed a large specific capacitance of 1100 F g(-1) and retained more than 95.1% of the specific capacitance after 6000 continuous charge-discharge cycles. The excellent capacitive properties and stability mark the porous hollow Co₃O₄ with the rhombic dodecahedral structure as one of the most promising electrode materials for high-performance supercapacitors.

Synthesis of 3D-nanonet hollow structured Co3O4 for high capacity supercapacitor

A 3D-nanonet structured cobalt-basic-carbonate precursor has been obtained by a facile, low cost and eco-friendly route under ambient temperature and pressure. After calcination in air, the as-prepared precursor was converted to a 3D-nanonet hollow structured Co3O4 with its original frame structure almost preserved. Encouragingly, by alternating experimental parameters (Table S1 in the Supporting Information ), such as concentration of the starting reagents and calcination temperature, we got the optimized condition for the final product with desirable electrochemical performance (Figure S1 in the Supporting Information ). The pseudocapacitive properties of the obtained Co3O4 were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge measurement and electrochemical impedance spectroscopy in 6.0 M KOH solution. At different scan rates of 5, 10, 20, and 30 mV s(-1), the corresponding specific capacitances were 820, 755, 693, and 656 F g(-1), respectively. The material also exhibited superior charge-discharge stability and maintained 90.2% of its initial capacitance after 1000 continuous charge-discharge cycles at a current density of 5 A g(-1). From a broad view, our research and the outstanding results not only present a feasible access to nanostructured Co3O4 but also remind us of paying more attention to the simple synthetic methods without complex processes and sophisticated instruments.

Recent development of metal hydroxides as electrode material of electrochemical capacitors

Recent research on electrochemical capacitors using transition metal hydroxides as electrode materials is reviewed.