Preparation of β-MnO2 nanorods through a γ-MnOOH precursor route

A new MnOOH precursor route has been developed to synthesize single-crystalline nanorods of tetragonal β-MnO₂. Uniform γ-MnOOH nanorods were first prepared by reducing KMnO₄ with KI under hydrothermal conditions at 120 °C. After calcination of the γ-MnOOH nanorods at 250 °C for 2 h in air, β-MnO₂ nanorods retaining the morphologies of γ-MnOOH nanorods were obtained. The temperature, time and heating speed of calcination were found to be important for the morphologies of the β-MnO₂ nanorods.     

Preparation and characterization of porous ultrafine Fe2O3 particles

Porous ultrafine Fe₂O₃ particles were prepared by homogeneous precipitation method. Fe³⁺ and urea were chosen as starting materials and anionic surfactant as the template. It is shown that the reaction results in the precipitation of a gelatinous hydrous iron oxide/surfactant mixture, which gives ultrafine Fe₂O₃ particles after drying and calcinations. The products were characterized by XRD, TEM, TG/DTA and BET. Conventional XRD patterns show that the products are mixture of γ-Fe₂O₃ and α-Fe₂O₃ phase after being sintered at 350 °C, and γ-Fe₂O₃ transforms entirely to α-Fe₂O₃ when sintered at 650 °C. The low-angle XRD patterns indicate that the mesostructure can only exist between 350 and 400 °C. TEM results show that the Fe₂O₃ particles have diameters of about 30 nm and lengths ranging from 100 to 120 nm; in each particle, there are several vermiculate-like mesopores with diameter of about 20-25 nm. The BET surface areas in excess of 50 m²/g are obtained after calcinations at 350 °C. The BJH desorption average pore width is around 22 nm, which is in agreement with the TEM results. The results show that anionic surfactant and sintering temperature are important to obtain this special morphology.     

Study of Ln0.6Sr0.4Co0.8Mn0.2O3-δ (Ln=La, Gd, Sm or Nd) as the cathode materials for intermediate temperature SOFC

Perovskite type oxides Ln_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ (Ln=La, Gd, Sm, or Nd) have been prepared by the solid state reaction of corresponding oxides. The crystal parameters of the compositions were determined by XRD powder diffraction, which revealed that all the compositions have orthorhombic structure. The reaction test of all samples with Ce_0.8Gd_0.2O_1.9 was carried out at 1200 °C for 48 h, and no reaction product was detected by XRD. The change in mass of La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ as a function of temperature was determined by thermogravimetric analysis (TGA). The electrical conductivity of the sintered samples were measured as a function of temperature from 200 to 1000 °C. The highest conductivity of about 1400 S cm⁻¹ was found in La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ. The cathodic polarization of these oxides electrodes deposited on Ce_0.8Gd_0.2O_1.9 tablet was studied at 500-800 °C in air.     

Synthesis and characterization of β-MnO2 single crystals with novel tetragonous morphology

β-MnO₂ single crystals with novel tetragonous morphology have been hydrothermally prepared in an HCl solution at 180 °C for 24 h without using templates, catalysts, and organic reagents. The structure of the obtained β-MnO₂ was systematically investigated by X-ray diffraction, SEM and TEM microscopy, FT-IR spectroscopy, DSC-TGA analyses, and chemical compositional analyses. The morphology of β-MnO₂ could be controlled by the hydrothermal treatment temperatures and the hydrothermal reaction times. β-MnO₂ single crystals with diameters 200-600 nm and lengths up to 1-5 μm not only had novel tetragonous morphology, but also had high purity.     

Different types of MnO2 recovered from spent LiMn2O4 batteries and their application in electrochemical capacitors

Three types (λ, γ, and β) of MnO₂ have been successfully prepared from spent LiMn₂O₄ electrode materials and utilized as electrode materials for electrochemical capacitors. Experiments show that the obtained λ-MnO₂ sample has particle sizes ranging from 0.2 to 3 μm and the as-prepared β-MnO₂ and γ-MnO₂ samples both exist in nanorods with diameters of about 100 nm. The length of β-MnO₂ nanorods varies from 0.2 to 3 μm, while γ-MnO₂ nanorods have lengths of 0.2–2 μm. The main results of electrochemical tests indicate that these MnO₂ electrodes exhibit good supercapacitive performances which are dependent on their morphology and microstructure. As electrode material, λ-MnO₂ sample presents a specific capacitance of 200.55 F g^?1, which is higher than those of other two types of MnO₂. However, β-MnO₂ electrode shows superior long-term cyclic stability and rate performance to other two MnO₂ samples. These results indicate that manganese dioxides recycled from spent LiMn₂O₄ batteries show great potential in application as the active materials for electrochemical capacitors. This study could provide a novel and useful approach to the recovery and reuse of spent LiMn₂O₄ batteries.     

Synthesis of flower-like Boehmite (AlOOH) via a simple solvothermal process without surfactant

Boehmite (AlOOH) with hierarchical flower-like structures was synthesized by the solvothermal reaction of AlCl₃·6H₂O in the presence of ethanol and toluene at 200 °C for 24 h. The product was characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that boehmite with flower-like nanostructures, which aggregated together by the weak hydrogen bonds, was formed through dissolution-deposition process of boehmite microcrystals and the toluene has a great effect on the morphology of product in the reaction system. Meanwhile, the γ-Al₂O₃ was also obtained by calcination of above product at 500 °C for 2 h, and the flower-like morphology kept no change. The surface area of γ-Al₂O₃ powder was determined to be 166.8 m²/g by N₂ adsorption measurement. The possible formation mechanism of flower-like boehmite nanostructures was proposed and discussed.     

Solvothermal synthesis of nanorods of ZnO, N-doped ZnO and CdO

ZnO nanorods with diameters in the 80-800 nm range are readily synthesized by the reaction of zinc acetate, ethanol and ethylenediamine under solvothermal conditions. The best products are obtained at 330 °C with a slow heating rate. Addition of the surfactant Triton^®-X 100 gave nanorods of uniform (300 nm) diameter. By adding a small amount of liquid NH₃ to the reaction mixture, N-doped ZnO nanorods, with distinct spectroscopic features are obtained. CdO nanorods of 80 nm diameter have been prepared under solvothermal conditions using a mixture of cadmium cupferronate, ethylenediamine and ethanol at 330 °C. Similarly, Zn_1−xCd_xO nanorods of a 70 nm diameter are obtained under solvothermal conditions starting with a mixture of zinc acetate, cadmium cupferronate, ethanol and ethylenediamine.     

Preparation of ultra-fine cobalt-nickel manganite powders and ceramics derived from mixed oxalate

Co_0.30Ni_0.66Mn_2.04O₄ negative temperature coefficient ceramics were derived from mixed oxalate Co_0.30Ni_0.66Mn_2.04(C₂O₄)₃·nH₂O. The mixed oxalate was synthesized by milling a mixture of cobalt acetate, nickel acetate, manganese acetate, and oxalic acid at room temperature. An ultra-fine Co_0.30Ni_0.66Mn_2.04O₄ powder was obtained by calcining the mixed oxalate in air at 800 °C for 3 h. The oxide powder compact was sintered at a relatively low temperature of 1100 °C for 5 h, achieving a relative density of ∼98%. The specific resistivity ρ_25 °C and the thermal constant B_25/85 °C were 765.2 Ω cm and 3604 K, respectively. The resistance drift after aging at 150 °C for 500 h was 1.5%.     

Fast synthesis and formation mechanism of γ-MnO2 hollow nanospheres for aerobic oxidation of alcohols

γ-MnO₂ hollow nanospheres of about 300-800 nm in size have been synthesized by a fast 1-h 2-step process in the presence of an excess amount of Mn²⁺ in aqueous solution without using any templates, hydrothermal processes and catalytic routes. The evolution of morphologies evidenced that the fast formation mechanism of the γ-MnO₂ hollow nanospheres in the presence of the excess amount of Mn²⁺ in solution followed the “Ostward ripening” process. The as-synthesized γ-MnO₂ hollow nanospheres showed high catalytic activity and selectivity in aerobic oxidation of various alcohols which was attributed to their hollow nature and larger BET specific surface area.     

Synthesis of γ-MnOOH nanorods and their isomorphous transformation into β-MnO<Subscript>2</Subscript> and α-Mn<Subscript>2</Subscript>O<Subscript>3</Subscript> nanorods

γ-MnOOH nanorods with different diameters were synthesized by a simple one-step polymer-assisted hydrothermal method using 50% (wt.%) Mn(NO₃)₂ solution and PEG-10000 as reagents. The diameters of as-synthesized γ-MnOOH nanorods were well controlled by simply varying the volume of the 50% Mn(NO₃)₂ solution. The calcination behavior of the as-synthesized γ-MnOOH nanorods was studied. Nanorods of β-MnO₂ and α-Mn₂O₃ were synthesized by calcination at 350 and 600 °C for 1 h respectively.     

Synthesis and electrochemical performances of spinel LiCr0.1Ni0.4Mn1.5O4 compound

The spinel compound LiCr_0.1Ni_0.4Mn_1.5O₄ was synthesized by a solid reaction method and a sol-gel method using citric acid as chelating agent. The pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ was obtained by the wet method. The electrochemical performances of the pure phase sample were measured at different current rates. There were three voltage plateaus at about 4.9, 4.7 and 4.0 V in the charge-discharge curves, which were attributed to the oxidation/reduction of chromium, nickel and manganese respectively. In the range of 3.5-5.0 V, its first discharge capacity was 143, 118 and 111 mAh/g corresponding to current densities of 1.0, 4.0 and 5.0 mA/cm², respectively. After 50 cycles, the capacity retention remained well at the current densities of 1.0, 4.0 and 5.0 mA/cm². The electrochemical performances of pure phase LiCr_0.1Ni_0.4Mn_1.5O₄ at 55 °C was also measured, and the results were discussed.     

Structural and magnetic properties of Mn nanoparticles prepared by arc-discharge

Mn nanoparticles are prepared by arc discharge technique. MnO, α-Mn, β-Mn, and γ-Mn are detected by X-ray diffraction, while the presence of Mn₃O₄ and MnO₂ is revealed by X-ray photoelectron spectroscopy. Transmission electron microscopy observations show that most of the Mn nanoparticles have irregular shapes, rough surfaces and a shell/core structure, with sizes ranging from several nanometers to 80 nm. The magnetic properties of the Mn nanoparticles are investigated between 2 and 350 K at magnetic fields up to 5 T. A magnetic transition occurring near 43 K is attributed to the formation of the ferrimagnetic Mn₃O₄. The coercivity of the Mn nanoparticles, arising mainly from Mn₃O₄, decreases linearly with increasing temperature below 40 K. Below the blocking temperature T_B ≈ 34 K, the hysteresis loops exhibit large coercivity (up to 500 kA/m), owing to finite size effects, and irreversibility in the loops is found up to 4 T, and magnetization is not saturated up to 5 T. The relationship between structure and the magnetic properties are discussed.     

Synthesis of nanocrystalline Mn3O4 at 100 °C

A simple gel to crystal conversion route has been followed for the preparation of nanocrystalline tetragonal Mn₃O₄ powders at 80-100 °C under refluxing conditions. Freshly prepared manganese hydroxide gel is allowed to crystallize under refluxing and stirring conditions for 4-6 h. Formation of nano crystallites of Mn₃O₄ is confirmed by X-ray diffraction (XRD) study. Transmission electron microscope (TEM) investigations revealed that the average particle size is 50 nm for these powders.     

Crystallization kinetics and thermal properties of 20Li2O-80TeO2 glass

In this work, X-ray diffraction, Raman spectroscopy and differential scanning calorimetry techniques were used to understand the crystallization process on 20Li₂O-80TeO₂ glass. X-ray diffraction results reveal the presence of three distinct alpha γ-TeO₂, α-TeO₂ and α-Li₂Te₂O₅ crystalline phases in the glass matrix. The Raman spectroscopy band structure of this glass is similar to the one observed in glassy TeO₂. Raman results clearly reveal the metastable character of the γ-TeO₂ phase in the 20Li₂O-80TeO₂ glass, whose associated vibration modes disappear completely at temperatures higher than 315 °C. On the other hand, the Raman modes associated to α-TeO₂ and α-Li₂Te₂O₅ phases persists up to temperatures close to the final stages of the crystallization in the studied glass (around 420 °C). From DSC measurements, the activation energies 296 ± 3 and 298 ± 1 kJ mol⁻¹ were associated to γ-TeO₂ and α-TeO₂ phases crystallization, indicating that these phases crystallizes at temperatures very close in the studied glass.     

Alanine-assisted low-temperature combustion synthesis of nanocrystalline LiMn2O4 for lithium-ion batteries

Nanocrystalline LiMn₂O₄ powders have been synthesized by combustion process in a single step using a novel fuel, l-alanine. Thermogravimetric analysis and differential thermal analysis of the gel indicate a sharp combustion at a temperature as low as 149 °C. Quantitative phase analysis of X-ray diffraction data shows about 97% of phase purity in the as-synthesized powder, which on further calcination at 700 °C becomes single phase LiMn₂O₄. High Brunauer, Emmett, and Teller surface area values obtained for ash (53 m²/g) and calcined powder (23 m²/g) indicate the ultrafine nature of the powder. Average crystallite size is found to be ∼60-70 nm from X-ray diffraction analysis and transmission electron microscopy. Fourier transformed infra-red spectrum shows two strong bands at 615 and 511 cm⁻¹ originating from asymmetrical stretching of MnO₆ octahedra. A nominal composition of Li_0.88 Mn₂O₄ is calculated from the inductive coupled plasma analysis. From UV-vis spectroscopy, an optical band gap of 1.43 eV is estimated which is assigned to a transition between t_2g and e_g bands of Mn 3d. Electrochemical charge-discharge profiles show typical LiMn₂O₄ behavior with a specific capacity of 76 mAh/g.     

Synthesis of BaTi4O9 ceramics by reaction-sintering process

Synthesis of BaTi₄O₉ ceramics by a reaction-sintering process was investigated. The mixture of raw materials for stoichiometric BaTi₄O₉ were pressed and sintered into ceramics without any calcination stage involved. Pure BaTi₄O₉ phases were obtained at 1150-1280 °C. High-sintered density, 98.2-99.5% of theoretical value (4.533 g/cm³), can be obtained for pellets sintered at 1200-1280 °C for 2-6 h. Some rod-shaped grains 3-7 μm in the longitudinal axis appear in pellets sintered at 1230 °C. Both the size and the amount of these rod-shaped grains increase at higher sintering temperature.     

Synthesis of samarium sesquioxide from the thermal decomposition of samarium oxychloride

In this work, Sm₂O₃ was synthesized from the thermal decomposition of SmOCl (P4/nmm). The process was studied by gravimetry under flowing air between 600 °C and 950 °C. Reactants and products were identified and analyzed by X-ray diffraction and energy dispersive spectroscopy. From the results, the stoichiometry of the reaction was obtained. Between 800 °C and 950 °C, the successive formation of both cubic Sm₂O₃ (Ia3) and monoclinic Sm₂O₃ (C2/m) phases was observed. Below 800 °C, no transformation of cubic Sm₂O₃ to monoclinic Sm₂O₃ was detected. Quantification of phases was made using the Rietveld Method. Combined with the analysis of the evolution of the microstructure by scanning electron microscopy, an elemental analysis of the kinetics of the reaction was made and a reaction scheme was proposed.     

Hydrothermal synthesis of nanocrystalline VO2 from poly(diallyldimethylammonium) chloride and V2O5

Novel vanadium dioxide nanorods were fabricated from V₂O₅ in the presence of a reducing agent, the poly(diallyldimethylammonium chloride) (PDDA) via a hydrothermal method at 180 °C for 48 h. The samples produced were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (FTIR), nitrogen adsorption (BET) and thermogravimetry (TG/DTG). The nanorods obtained are approximately 50 nm wide and from 300 to 500 nm long and presents high surface area (42 m² g⁻¹). The nanocrystalline B phase VO₂ is not produced by hydrothermal treatment in the absence of the PDDA polyelectrolyte.     

Preparation and electrochemical properties of lamellar MnO2 for supercapacitors

Lamellar birnessite-type MnO₂ materials were prepared by changing the pH of the initial reaction system via hydrothermal synthesis. The interlayer spacing of MnO₂ with a layered structure increased gradually when the initial pH value varied from 12.43 to 2.81, while the MnO₂, composed of α-MnO₂ and γ-MnO₂, had a rod-like structure at pH 0.63. Electrochemical studies indicated that the specific capacitance of birnessite-type MnO₂ was much higher than that of rod-like MnO₂ at high discharge current densities due to the lamellar structure with fast intercalation/deintercalation of protons and high utilization of MnO₂. The initial specific capacitance of MnO₂ prepared at pH 2.81 was 242.1 F g⁻¹ at 2 mA cm⁻² in 2 mol L⁻¹ (NH₄)₂SO₄ aqueous electrolyte. The capacitance increased by about 8.1% of initial capacitance after 200 cycles at a current density of 100 mA cm⁻².