Cobalt Hydroxide Nanosheets and Their Thermal Decomposition to Cobalt Oxide Nanorings

We demonstrate herein that single‐crystalline β‐cobalt hydroxide (β‐Co(OH)₂) nanosheets can be successfully synthesized in large quantities by a facile hydrothermal synthetic method with aqueous cobalt nitrate as the cobalt source and triethylamine as both an alkaline and a complexing reagent. This synthetic method has good prospects for the future large‐scale production of single‐crystalline β‐Co(OH)₂ nanosheets owing to its high yield, low cost, and simple reaction apparatus. Single‐crystalline porous nanosheets and nanorings of cobalt oxide (Co₃O₄) were obtained by a thermal‐decomposition method with single‐crystalline β‐Co(OH)₂ nanosheets as the precursor. A probable mechanism of formation of β‐Co(OH)₂ nanosheets, porous Co₃O₄ nanosheets, and Co₃O₄ nanorings was proposed on the basis of the experimental results.     

Zn3(OH)2V2O7·2H2O纳米片的水热制备

采用硝酸锌、五氧化二钒和氢氧化钠作为反应物,通过一个简单的CTAB辅助的水热方法制备了Zn₃(OH)₂V₂O₇·2H₂O纳米片。运用XRD,ICP-AES,FTIR,HRTEM,EDS,FE-SEM对产物的晶相和形貌进行了表征。结果表明CTAB在控制产物的形貌、尺寸分布和自组装过程中起着关键作用。同时我们研究了产物的晶体生长行为和自组装过程。     

Surface analysis and electrochemical characterization of palladium–cobalt nanoring formation from molecular precursor, [Et3NH]2[CoPd2(μ‐4‐I‐3,5‐Me2pz)4Cl4], on highly ordered pyrolytic graphite

A Pd₂Co precursor, [Et₃NH]₂[CoPd₂(μ‐4‐I‐3,5‐Me₂pz)₄Cl₄], was used to synthesize palladium–cobalt nanorings and nanoparticles on highly ordered pyrolytic graphite (HOPG) surface. Different types of nanostructures were formed on HOPG surfaces and were controlled by relative humidity (%RH). These structures included Pd₂Co nanorings on HOPG surface by self‐assembly with humidity control. The %RH affects the size and dispersion of the self‐formation of the Pd₂Co rings on HOPG surfaces. The modified HOPG surface with Pd₂Co precursor at 80%RH has rings of similar sizes, while modification at 76%RH gives well‐formed rings and 70%RH with smaller diameters. After thermal reduction of the Pd₂Co precursor on HOPG, bimetallic nanostructures were formed. X‐ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy with energy‐dispersive X‐ray fluorescence spectroscopy techniques were employed to study the composition and morphology of the nanostructures formations on the HOPG surface. Electrochemical characterization of the Pd₂Co nanostructures was performed. Moreover, the bimetallic catalyst has electrocatalytic activity for the oxygen reduction reaction. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis and characterization of Co(OH)<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> nanotube composites as supercapacitor materials

A novel composite of Co(OH)₂ and TiO₂ nanotubes was synthesized by a chemical precipitation method. Co(OH)₂/TiO₂ nanotube composites and its microstructure were characterized by transmission electron microscopy (TEM), X-ray diffraction pattern (XRD). The electrochemical capacitance performance of this composite was investigated by cyclic voltammetry and charge–discharge tests with a three-electrode system in 6 M KOH solution. We synthesized different weight ratios of Co(OH)₂/TiO₂ nanotubes, a maximum specific capacitance of 229 F/g was obtained for the composite. Based on these tests, we propose that TiO₂ nanotubes provide the three-dimensional nanotube network structure for the composite and make the Co(OH)₂ dispersed. For these reasons, the TiO₂ nanotubes used as a framework for Co(OH)₂ improve the utilization of Co(OH)₂ greatly.     

Nanocrystalline nickel cobalt hydroxides/ultrastable Y zeolite composite for electrochemical capacitors

A novel nanocomposite of Co(OH)₂−Ni(OH)₂ and ultrastable Y molecular sieves was synthesized by an improved chemical precipitation method for electrochemical capacitors. The Co(OH)₂−Ni(OH)₂/ultrastable Y zeolite (USY) composite and its microstructure were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Electrochemical characterization was performed by cyclic voltammetry and galvanostatic charge–discharge measurements. The results show that Co(OH)₂−Ni(OH)₂/USY microstructure applied for the electrochemical energy storage has displayed superior capacitive performance. The effect of heat treatment conditions on specific capacitance properties was also systemically explored. Upon annealing at 250 °C, the maximum specific capacitance was up to 479 F/g (or 1,710 F/g after correcting for the weight percent of Co(OH)₂−Ni(OH)₂ phase). Annealing temperatures higher than 250 °C may cause the hydroxide to form oxide phase and decrease the surface activity of the oxide, thereby leading to a decline of the specific capacitance.     

A Co(OH)2−graphene nanosheets composite as a high performance anode material for rechargeable lithium batteries

A Co(OH)₂?graphene nanosheets (Co(OH)₂?GNS) composite as a high performance anode material was firstly prepared through a simultaneous hydrothermal method. The structure, morphology and electrochemical performance of the obtained samples were systematically investigated by X-ray diffraction (XRD), transmission electron microscope (TEM) and electrochemical measurements. According to the TEM analysis, the surface of the Co(OH)₂ is surrounded with GNS in the Co(OH)₂?GNS composite. The specific discharge (lithiation) and charge (delithiation) capacities of Co(OH)₂?GNS attain to 1599 and 1120 mAh/g at a current density of 200 mA/g in the first cycle, respectively. After 30 cycles, the reversible capacity of Co(OH)₂?GNS is still 910 mAh/g with the retention of 82%. The particular structure of Co(OH)₂ particles surrounded by the GNS could limit the volume change during cycling and provide an excellent electronic conduction pathway, which could be the main reason for the remarkable improvement of electrochemical performance.     

Facile Synthesis of Leaf-like Cu(OH)2 and Its Conversion into CuO with Nanopores

Leaf-like Cu(OH)₂ single crystals were synthesized via the controlled emulsion interface method using Span80 (sorbitan monooleate) as the stabilizer of the emulsion system. CuO products with nanopores could be simply obtained by the dehydration of Cu(OH)₂, while maintaining the strip-shaped architecture. The phase structures and morphologies were measured by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results showed that Cu(OH)₂ microleaves were single crystals and the growth direction seemed to be in [111] crystal plane of the orthorhombic Cu(OH)₂. The formation of the nanopores should be attributed to the water loss of the transformation from Cu(OH)₂ to CuO. The formation process of Cu(OH)₂ was investigated by taking TEM images at different stages of the reaction. The formed nanoparticles began to rearrange to form nanorods and microleaves possibly via edge-by-edge and side-by-side oriented-attachments because of the formation of larger crystals greatly reducing the interfacial energy. Besides, CuO microarchitectures exhibit blue shifts in UV-Vis spectra and possess larger band gaps compared with those of bulk crystals.     

Synthesis and Characterization of β‐Co(OH)2, CuO and ZnO Nanostructures by Solvothermal Method without Any Additive

β‐Co(OH)₂, CuO and ZnO nanostructures with plate‐like, particle‐like and flower‐like morphologies were prepared through the use of simple solvothermal method using of melt salt and 1,10‐phenanthroline as complexing agent and sodium hydroxide. β‐Co(OH)₂ consisted of a plate‐like structure, and the nanoplates size was about 29 nm. The structure was comprised of regular sheets which were assembled together. Furthermore, the as‐obtained β‐Co(OH)₂ nanoplates can be easily converted into Co₃O₄ nanoplates by calcining in air at 500 °C for 2 h. The results indicate that ZnO powder is of hexagonal wurtzite structure and well crystallized with high purity. CuO powder is pure monoclinic‐structured crystalline. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT‐IR) spectra. Possible formation mechanism of the nanostructures is proposed.     

Controlled-synthesis of ZnO nanorings

ZnO nanorings were synthesized on a large scale by an easy solution-based method at 70°C for 5 h using hexamethylenetramine (C₆H₁₂N₄, HMT) and Zn (NO₃)₂·2H₂O as raw materials in the presence of surfactant poly(acrylamide-co-diallyldimethylammonium chloride) (PAM-CTAC). The structure and morphology of the products were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The influence of experimental conditions such as concentration of surfactant and reactants, reaction temperature on the structure and morphology of the products were investigated. A probable formation mechanism of ZnO nanorings in the presence of surfactant PAM-CTAC was discussed. The results show that the products are wurtzite hexagonal ZnO nanorings with an inner diameter of 220 nm and a wall thickness of 70 nm. Reaction temperature and concentration of reactants influence the shape and size of ZnO nanorings but PAM-CTAC plays the key role in the formation of ZnO nanorings. Through adjusting the concentration of PAM-CTAC, controlled-synthesis of ZnO nanorings can be realized. A room temperature photoluminescence (PL) spectrum of ZnO obtained shows that the full width at half maximum (FWHM) of the UV emission (∼7 nm) is much narrower than that of commercial ZnO bulk crystals (∼18 nm). The narrow FWHM confirms the uniformity and narrow size distribution of the synthesized ZnO crystals. __________ Translated from Chemical Journal of Chinese Universities, 2008, 29(1): 28–32     

Nanoflake-like cobalt hydroxide/ordered mesoporous carbon composite for electrochemical capacitors

Nanocomposites consisting of mesoporous carbon CMK-3 and cobalt hydroxide nanoflakes are synthesized by a chemical precipitation method. The successful growth of nanometer-sized Co(OH)₂ flakes on the surface of CMK-3 is confirmed by scanning electron microscopy. The Co(OH)₂/CMK-3 composite electrodes are investigated for its use in the electrochemical capacitors with cyclic voltammograms, chronopotentiometric measurements, and electrochemical impedance spectroscopy. Experimental studies reveal that the Co(OH)₂/CMK-3 composite electrode with the 20 wt.% CMK-3 presents excellent electrochemical performance with specific capacitance of 750 F/g (or 910 F/g after being corrected for the weight percentage of the Co(OH)₂ phase). The overall improved electrochemical behavior accounts for the unique structure design in the Co(OH)₂/CMK-3 composite in terms of porous nanostructure, large specific surface area, and good electrical conductance. The Co(OH)₂/CMK-3 composite electrode also shows better rate capability and cyclic stability, suggesting its potential applications as the electrode materials for electrochemical capacitors.     

In situ electrochemical dehydrogenation of ultrathin Co(OH)2 nanosheets for enhanced hydrogen evolution

Transition metal hydroxides/oxyhydroxides have recently emerged as highly active electrocatalysts for oxygen evolution reaction in alkaline water electrolysis, while have not yet been widely investigated for hydrogen evolution electrocatalysts owing to their unfavorable H*-adsorption, making it difficult to construct an overall-water-splitting cell for hydrogen production. In this work, we proposed a straightforward and effective approach to develop an efficient in-plane heterostructured CoOOH/Co(OH)₂ catalyst via in-situ electrochemical dehydrogenation method, in which the dehydrogenated –CoOOH and Co(OH)₂ at the surface synergistically boost the hydrogen evolution reaction (HER) kinetics in base as confirmed by high-resolution transmission electron microscope, synchrotron X-ray absorption spectroscopy, and electron energy loss spectroscopy. Due to the in-situ dehydrogenation of ultrathin Co(OH)₂ nanosheets, the catalytic activity of the CoOOH/Co(OH)₂ heterostructures is progressively improved, which exhibit outstanding hydrogen-evolving activity in base requiring a low overpotential of 132 mV to afford 10 mA/cm² with very fast reaction kinetics after 60 h dehydrogenation. The gradually improved catalytic performance for the CoOOH/Co(OH)₂ is probably due to the enhanced H*-adsorption induced by the synergistic effect of heterostructures and better conductivity of CoOOH relative to electrically insulating Co(OH)₂. This work will open the opportunity for a new family of transition metal hydroxides/oxyhydroxides as active HER catalysts, and also highlight the importance of using in situ techniques to construct precious metal-free efficient catalysts for alkaline hydrogen evolution.     

Controlled synthesis of single-crystalline Mg(OH)2 nanotubes and nanorods via a solvothermal process

The synthesis of Mg(OH)₂ one-dimensional (1D) nanostructures was systematically investigated in different solvents at various temperatures with Mg₁₀OH₁₈Cl₂·5H₂O nanowires as source materials. The results showed that the characters of the products, such as crystal size, shape, and structure, were strongly influenced by the solvent and temperature during the solvothermal process. 1D nanotubes of Mg(OH)₂, with 80-300 nm outer diameter, 30-80 nm wall thickness, and several tens of micrometers in length were obtained by choosing bidentate ligand solvents such as ethylenediamine and 1,6-diaminohexane as the reaction solvent. But when using monodentate ligand pyridine as the reaction solvent, the obtained samples showed nanorods morphology. The Mg(OH)₂ thus produced was analyzed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), and selected-area electron diffraction (SAED). The possible growth mechanism of the 1D nanostructure Mg(OH)₂ was discussed.     

Water-Assisted Concerted Proton-Electron Transfer at Co(II)-Aquo Sites in Polyoxotungstates With Photogenerated RuIII(bpy)33+ Oxidant

The cobalt substituted polyoxotungstate [Co₆(H₂O)₂(α-B-PW₉O₃₄)₂(PW₆O₂₆)]¹⁷⁻ ( Co6 ) displays fast electron transfer (ET) kinetics to photogenerated Ru^III(bpy)₃³⁺, 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II) −OH ₂ and Co6(II) −OH, respectively, whose speciation in aqueous solution is associated to a pK_a of 7.6), and generates a Co(III)−OH moiety ( Co6(III) −OH), as proven by transient absorption spectroscopy; (ii) at pH>pK_a, the Co6(II) −OH→Ru^III(bpy)₃³⁺ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH <pK_a, the process involves Co6(II) − OH₂ → Co6(III)−OH transformation and proceeds via a multiple-site, concerted proton electron transfer (CPET) where water assists the transfer of the proton, as proven by the absence of effect of buffer base concentrations on the rate of the ET and by a H/D kinetic isotope in a range of 1.2–1.4. The reactivity of water is ascribed to its organization on the surface of the polyanionic scaffold through hydrogen bond networking involving the Co(II)−OH₂ group.     

Gold and silver nanorings formed at the air/water interface

Gold nanorings were prepared at the air/water interface through reduction of AuCl₄⁻ ions by UV-light irradiation or formaldehyde gas treatment at room temperature templated by thin films of phthalocyanine derivatives. Silver nanorings were produced at the air/water interface via reduction of Ag⁺ ions by UV-light irradiation templated by poly(9-vinylcarbazole) (PVK) thin films. These nanostructures were investigated by transmission electron microscopy (TEM), selective-area electron diffractometry (SAED), and high-resolution TEM (HRTEM). It was found that the gold nanorings are composed of close-packed nanoplates whose (1 1 0) crystal planes are parallel to the air/water interface; while silver rings are composed of nanoparticles. It is demonstrated that the ring-like aggregates formed by parallel linear supramolecules of the phthalocyanine derivatives and the ring-like structure of PVK supramolecules are responsible for the formation of the gold and silver nanorings, respectively.     

Pt‐Pd‐Co Trimetallic Alloy Network Nanostructures with Superior Electrocatalytic Activity towards the Oxygen Reduction Reaction

Pt alloy nanostructures show great promise as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. Herein, three‐dimensional (3D) Pt‐Pd‐Co trimetallic network nanostructures (TNNs) with a high degree of alloying are synthesized through a room temperature wet chemical synthetic method by using K₂PtCl₄/K₃Co(CN)₆–K₂PdCl₄/K₃Co(CN)₆ mixed cyanogels as the reaction precursor in the absence of surfactants and templates. The size, morphology, and surface composition of the Pt‐Pd‐Co TNNs are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected‐area electron diffraction (SAED), energy dispersive spectroscopy (EDS), EDS mapping, X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS). The 3D backbone structure, solid nature, and trimetallic properties of the mixed cyanogels are responsible for the 3D structure and high degree of alloying of the as‐prepared products. Compared with commercially available Pt black, the Pt‐Pd‐Co TNNs exhibit superior electrocatalytic activity and stability towards the ORR, which is ascribed to their unique 3D structure, low hydroxyl surface coverage and alloy properties.     

Preparation of Mesoporous 1,4‐Phenylene‐silica Nanorings through a Dual‐templating Approach

Mesoporous 1,4‐phenylene‐silica nanorings were prepared using cetyltrimethylammonium bromide (CTAB) and (S)‐2‐methyl‐1‐butanol as a chiral dopant in concentrated aqueous NH₃ solutions. Transmission electron microscopy images of the samples indicated that the nanorings were formed by bending nanorods 360°. With increasing the stirring speed or the (S)‐2‐methyl‐1‐butanol/CTAB molar ratio, the morphologies of mesoporous 1,4‐phenylene‐silicas changed from helical nanofibers to nanorings, and then to nano‐saddles. Circular dichroism spectra of these hybrid silicas indicated that they were chiral.     

Investigation of the interaction between cobalt sulfide coatings and Ag(I) ions using cyclic voltammetry in KClO4 and NaOH aqueous solutions and X-ray photoelectron spectroscopy

Cobalt sulfide coatings have been investigated by means of cyclic voltammetry in 0.1 M KClO₄ and 0.1 M NaOH solutions and analyzed using X-ray photoelectron spectroscopy. They have been shown to contain CoS(OH), CoS and Co(OH)₂. After treating such Co sulfide coatings with AgNO₃ solution, their composition changes: both the cobalt and oxygen content decreases and Ag (up to 85 at%) appears in the coating as Ag₂S, Ag₂O and metallic Ag. Co(II) compounds react with Ag⁺ ions according to an exchange reaction [CoS+2Ag⁺+2H₂O→Ag₂S+Co(OH)₂+2H⁺]. In the course of the reaction of Co(OH)₂ with silver ions, a redox process occurs, giving metallic silver [Co(OH)₂+Ag⁺+H₂O→Ag°+Co(OH)₃+H⁺ or Co(OH)₂+Ag⁺→Ag°+CoO(OH)+H⁺]. Ag₂S reduction takes place at more positive potentials than Cu reduction; therefore sulfide layers of cobalt modified with silver ions, unlike unmodified ones, may be plated with Cu from both acid and alkaline electrolytes. Electronic Publication     

Hybrid porous Ni(OH)2-MnO2 nanosheets/plasma-grafted MWCNTs for boosted supercapacitor performance

The utilization of nickel hydroxide and manganese dioxide solely as high-performance supercapacitive materials is hindered by their low capacitance retention and electrical conductivity. As Ni(OH)₂ and MnO₂ give a synergistic effect, porous Ni(OH)₂-MnO₂ nanosheets with a thickness of 9 nm are successfully grown on carbon fiber (CF) via a single-step hydrothermal co-deposition method. Multi-walled carbon nanotubes (CNT) are grafted with maleic anhydride (MA) through plasma-grafted process, followed by thiol-ene reaction to synthesize CNT-MA−S (CMS) to increase their aqueous dispersion behavior. The electrochemical properties of Ni(OH)₂-MnO₂ are further enhanced by dip-coating CMS on nanosheets. The composition and morphology of CMS and Ni(OH)₂-MnO₂ nanosheets are characterized using scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR), electron spectroscopy for chemical analysis (ESCA), transmission electron microscopy (TEM), thermogravimetric analyses (TGA), nuclear magnetic resonance (NMR), and Raman spectroscopy. The electrochemical characteristics of fabricated electrodes are analyzed using cyclic voltammetry and chronopotentiometry methods. CF−Ni(OH)₂-MnO₂/CMS electrode is successfully synthesized without using any binder, exhibited ultrahigh specific capacitance (2049 F g⁻¹ at a current density of 1 A g⁻¹), and excellent capacitance retention (>80 %) at 2 A g⁻¹ charge/discharge rate after 5000 cycles.     

Ball-milling processing of nanocrystalline nickel hydroxide and its effects in pasted nickel electrodes for rechargeable nickel batteries

Nanocrystalline Ni(OH)₂ powder synthesized by a chemical precipitation method was processed using the planetary ball milling (PBM), and the physical properties of both the ball-milled and unmilled Ni(OH)₂ were characterized by scanning electron microscopy (SEM), specific surface area, particle size distribution, and X-ray diffraction. It was found that the PBM processing could significantly break up the agglomeration, uniformize the particle size distribution, increase the surface area, decrease the crystallite size, and reduce the crystallinity of nanocrystalline β-Ni(OH)₂, which were advantageous to the improvement of the electrochemical activity of Ni(OH)₂. The ball-milled nanocrystalline (BMN) Ni(OH)₂ was then used to alter the microstructure of pasted nickel electrodes and improve the distribution of the active material in the porous electrode substrate. Electrochemical performances of pasted nickel electrodes with a mixture of BMN and spherical Ni(OH)₂ as the active material were investigated, and were compared with those of pure spherical Ni(OH)₂ electrodes. Charge/discharge tests showed that BMN Ni(OH)₂ addition could enhance the charging efficiency, specific discharge capacity, discharge voltage, and high-rate capability of pasted nickel electrodes. This performance improvement could be attributed to a more compact electrode microstructure, better reaction reversibility, and lower electrochemical impedance, as indicated by SEM, cyclic voltammetry, and electrochemical impedance spectroscopy. Thus, it was an effective method to modify the microstructure and improve the electrochemical properties of pasted nickel electrodes by adding an appropriate amount of BMN Ni(OH)₂ to spherical Ni(OH)₂ as the active material.     

Cobalt Element Effect of Ternary Mesoporous Cerium Lanthanum Solid Solution for the Catalytic Conversion of Methanol and CO2 into Dimethyl Carbonate

A citric acid ligand assisted self-assembly method is used for the synthesis of ternary mesoporous cerium lanthanum solid solution doped with metal elements (Co, Zr, Mg). Their textural property was characterized by X-ray diffraction, transmission electron microscopy, N₂ adsorption-desorption, X-ray photoelectron spectroscopy and TPD techniques, and so on. The results of catalytic testing for synthesis of dimethyl carbonate (DMC) from CH₃OH and CO₂ indicated that the DMC yield reached 316 mmol/g on Ce-La-Co solid solution when the reaction temperature was 413 K and the reaction pressure was 8.0 MPa. It was found that Co had synergistic effect with La and Ce, doping of Co on the mesoporous Ce-La solid solution was helpful to increase the surface area of the catalyst, promote CO₂ adsorption and activation, and improve the redox performance of solid solution catalyst. The conversion of Co²⁺ to Co³⁺ resulted in the continuous redox cycle between Ce⁴⁺ and Ce³⁺, and the oxygen vacancy content of the catalyst was increased. Studies have shown that the catalytic performance of Ce-La-Co solid solution is positively correlated with oxygen vacancy content. On this basis, the reaction mechanism of DMC synthesis from CO₂ and CH₃OH on the catalyst was speculated.