Synthesis and electrochemical performance of β-MnO2 with semi-tubular morphology

β-MnO₂ with semi-tubular morphology has been prepared in a mixed solution of KMnO₄ and MnCl₂ by a facile hydrothermal route without using templates, catalysts, and organic reagents. The structure of the obtained β-MnO₂ is systematically investigated by XRD, SEM, and TEM. Results show that the as-prepared β-MnO₂ has novel semi-tubular morphology, and its particle shows a diameter of 300–400 nm and length up to 1–4 μm. The prepared β-MnO₂ shows a good electrochemical performance, and delivers a discharge capacity of 195 mAh g⁻¹ after 40 cycles between voltage limit of 1.5 and 4.5 V at a constant current density of 20 mA g⁻¹.     

Low-temperature hydrothermal synthesis of α-MnO2 three-dimensional nanostructures

Single-crystalline α-MnO₂ three-dimensional nanostructures were synthesized via a novel redox reaction of KMnO₄ and Cr(NO₃)₃ under hydrothermal conditions. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). The addition of HNO₃ into the reaction has a significant effect on the morphologies of the final products. The α-MnO₂ three-dimensional nanostructures were obtained under the acidic condition, while α-MnO₂ nanowires were obtained without the addition of HNO₃. A mechanism for the growth of α-MnO₂ three-dimensional nanostructures was proposed.     

Hydrothermal synthesis and catalytic properties of α- and β-MnO2 nanorods

One-dimensional α-MnO₂ and β-MnO₂ single-crystalline nanostructures were prepared by hydrothermal process. The products were characterized in detail by multiform techniques: X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Experimental results indicate that the temperature plays important roles in determining produce α-MnO₂ and β-MnO₂ nanorods. In addition, the as-prepared α-MnO₂ and β-MnO₂ nanorods showed excellent catalytic performance in the Fenton-like reaction.     

Solution-combustion synthesis of ε-MnO2 for supercapacitors

ε-MnO₂ nanoparticles were synthesized through a single step solution combustion process without using any template or surfactants. Plate-like ε-MnO₂ materials, 50-150 nm in diameter and 20-25 nm in thickness, were obtained at a higher Mn(NO₃)₂:C₂H₅NO₂ molar ratio (i.e., 2:1), whereas spherical ε-MnO₂ particles (about 60 nm in diameter) were obtained as the Mn(NO₃)₂:C₂H₅NO₂ ratio is lower (e.g., 1:2). Electrochemical performance of the as-prepared ε-MnO₂ nanoparticles was examined. The spherical ε-MnO₂ nanoparticle sample shows a relatively higher specific capacitance of 123 F g^− 1 at the current density of 1 A g^− 1 in 1 M NaSO₄ electrolyte solution, probably due to its porous structures and higher surface areas in comparison with the plate-like counterparties.     

Facile and template-free preparation of α-MnO2 nanostructures and their enhanced adsorbability

In this paper, nanostructured MnO₂ materials were successfully prepared through a simple and template-free hydrothermal method. X-ray diffraction pattern indicates that the as-prepared nanomaterials are α-MnO₂. Transmission Electron Microscopy and Scanning Electron Microscopy images demonstrate that nanostructured rod-clusters α-MnO₂ could be evolved from the nanorods. Brunauer-Emmett-Teller (BET) surface area measurement was employed to characterize the surface property. Moreover, the as-obtained (α-MnO₂) nanomaterials could act as an efficient adsorbent to remove Congo Red and Methlylene Blue. More significantly, the nanomaterials are nontoxic and environmentally friendly via biological methylthiazolyldiphenyltetrazoliumbromide assay experiments. Its nontoxic and enhanced adsorbability properties guarantee their safe applications in environmental protection and industrial aspects.     

Synthesis of single-crystalline α-MnO2 nanotubes and structural characterization by HRTEM

Single-crystalline α-MnO₂ nanotubes were synthesized by hydrothermal method. The growth of α-MnO₂ nanotubes is through the formation of the core (γ-MnO₂)-shell (α-MnO₂) nanofibers, and then through the formation of the cavity by the dissolution of the core. The outer and the inner diameters of as-synthesized nanotubes are in the range from 13.3 to 39.2 nm, and from 2.0 to 10.8 nm, respectively. The lattice images on the wall and in the center correspond to the (2 2 0), and the (2 1 1) interplanar spacing of the tetragonal-structure α-MnO₂.     

A new asymmetric supercapacitor based on λ-MnO2 and activated carbon electrodes

Lithium-ion intercalated compound λ-MnO₂ was used as positive electrode in asymmetric supercapacitor with activated carbon used as negative electrode in 1 mol L^− 1 Li₂SO₄ aqueous electrolyte solution. Phase composition, morphology and particle sizes of λ-MnO₂ were studied by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrochemical capacitive performance of the asymmetric supercapacitor was tested by cyclic voltammetry and galvanostatic charge-discharge tests. The results show that the asymmetric supercapacitor has electrochemical capacitance performance within wide potential range of 0-2.2 V. The specific capacitance is 53 F g^− 1 at a constant current density of 10 mA cm^− 2. The energy density is 36 W h kg^− 1 with a power density of 314 W kg^− 1. It is obvious that λ-MnO₂ is a potential electrode material for asymmetric supercapacitor.     

Facile synthesis of ultra-long α-MnO2 nanowires and their microwave absorption properties

Well crystallized α-MnO₂ nanowires (NWs) with an average diameter of about 40 nm and an average length of about 30 μm were successfully synthesized by hydrothermal method. The complex permittivity and permeability of α-MnO₂ NWs/paraffin composites with 20 vol.% α-MnO₂ NWs were measured in a frequency region from 0.1 to 13 GHz. The value of maximum reflection loss of the composites with 20 vol.% α-MnO₂ NWs is approximately − 35 dB at 3.13 GHz with a thickness of 3.6 mm, and the bandwidth corresponding to reflection loss below − 10 dB is higher than 1.8 GHz with a lower thickness of 1.2 mm.     

Preparation and characterization of hollow α-Fe2O3 irregular microspheres

Hollow α-Fe₂O₃ irregular microspheres were prepared at 160 °C from a hydrolyzing Fe(ClO₄)₃ solution by adding sodium polyanethol sulphonate. The particles were characterized by ⁵⁷Fe Mössbauer, X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. The walls of these hollow particles consisted of elongated subunits composed of elongated and thin α-Fe₂O₃ rods. The precipitation of hollow α-Fe₂O₃ irregular microspheres was governed by the preferential adsorption of sulphonate/sulphate groups. The lateral aggregation of elongated thin rods and subunits also played an important role in the formation of hollow α-Fe₂O₃ irregular microspheres.     

Preparation and combustion properties of α-Fe2O3 coated Zr particles

Zirconium particles with irregular morphology and broad size distribution were uniformly coated by spherical α-Fe₂O₃ crystal grain via a facile route without polymer or surfactant as directing agents. The synthesized α-Fe₂O₃/Zr composite particles were characterized by X-ray diffraction, scanning electron microscopy, energy dispersion X-ray, UV-vis spectroscopy and Raman spectroscopy. The synthesis mechanism could be explained by cooperated heterogeneous nucleation and solid state transformation reaction. The combustion properties of α-Fe₂O₃/Zr composite particles were investigated. Compared with Zr particles, the combustion lasting time decreased from 16 s of Zr particles to 0.13 s of α-Fe₂O₃/Zr composite particles, and the top point of temperature reached in combustion increased from 2004 °C of Zr particles to 2378 °C of α-Fe₂O₃/Zr particles.     

Facile synthesis of α-MnO2 nanorods at low temperature and their microwave absorption properties

One-dimensional α-MnO₂ nanorods were fabricated by using low-temperature water-bathing chemical precipitation method at 80 °C. The crystalline structures, morphological evolution process and microwave absorption properties were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and measurement of electromagnetic parameters. The results show that the morphological structures and electromagnetic properties have close relationship with the reaction time. With the prolonging of the treatment time, the as-synthesized products turn from microspheres constituted of tiny dendrites to nanorods with diameters of 20–30 nm and lengths up to 1–2 μm. The electromagnetic characterization shows that the dielectric constants and magnetic permeability values show decreasing trends with the increasing frequency, however, the dielectric and magnetic loss tangents all increase with frequency. The electromagnetic absorption properties of the products have close relationship with the morphologies and thicknesses of the samples. With a thickness of 3 mm, an absorbing peak value of −25 dB was achieved for the sample treated for 24 h. The microwave absorption properties of MnO₂ can be attributed mainly to interfacial polarization, space charge polarization and relaxation phenomena.     

Preparation and characterization of α-Fe2O3 polyhedral nanocrystals via annealing technique

Polyhedral nanocrystals of α-Fe₂O₃ are successfully synthesized by annealing FeCl₃ on silicon substrate at 1000 °C in the presence of H₂ gas diluted with argon (Ar). Uniformly shaped polyhedral nanoparticles (diameter ~ 50-100 nm) are observed at 1000 °C and gases flow rate such as; Ar = 200 ml/min and H₂ = 150 ml/min. Non-uniform shaped nanoparticles (diameter ~ 20-70 nm) are also observed at an annealing temperature of 950 °C with lower gases flow rate (Ar = 100 ml/min and H₂ = 75 ml/min). Nanoparticles are characterized in detail by field-emission electron microscopy (FE-SEM), energy dispersive X-ray (EDX) and high resolution transmission electron microscopy (HRTEM) techniques. HRTEM study shows well resolved (110) fringes corresponding to α-Fe₂O₃, and selected area diffraction pattern (SADP) confirms the crystalline nature of α-Fe₂O₃ polyhedral nanoparticles. It is observed that polyhedral formation of α-Fe₂O₃ nanocrystals depends upon annealing temperature and the surface morphology highly rely on the gas flow rate inside the reaction chamber.     

Synthesis,characterization and electrochemical properties of three-dimensionally ordered macroporous α-Fe2O3

Three-dimensionally ordered macroporous (3DOM) α-Fe₂O₃ electrode materials with large pore sizes and interconnected macroporous frameworks were successfully synthesized by a simply modified colloidal crystal templating strategy. The obtained samples were characterized by means of thermogravimetry, powder X-ray diffraction, nitrogen physisorption, scanning and transmission electron microscopy. The electrochemical properties of the 3DOM α-Fe₂O₃ were evaluated with cyclic voltammetry and discharge–charge experiments in an organic electrolyte containing a lithium salt. The results showed that the 3DOM α-Fe₂O₃ possessed a potential to be used as an anode material for lithium ion batteries with high initial discharge and charge capacities of 1883 and 1139 mAh g⁻¹, respectively. After 60th cycle, the reversible capacity could still be as high as 681 mAh g⁻¹ with a stable Coulombic efficiency of around 95%.     

Facile synthesis of α-MnO2/graphene nanocomposites and their high performance as lithium-ion battery anode

α-MnO₂/graphene nanocomposites are synthesized via a facile wet-chemical route, and α-MnO₂ nanosheets are uniformly distributed on the surface of graphene. Their high performance as lithium ion battery anodes is obtained. Their reversible capacity at C/10 rate is up to 726.5 mA h/g, and maintains up to 635.5 mA h/g after 30 cycles. Such a performance can be partly attributed to high electron conductivity, excellent flexibility and high specific surface area of graphene. Also, α-MnO₂ nanostructures can play a role in preventing the pile of graphene nanosheets with the loss of their active surface area. The present results indicate that α-MnO₂/graphene nanocomposites have potential applications in lithium-ion battery anodes.     

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.     

Preparation and characterization of single-phase α-Fe2O3 nano-powders by Pechini sol-gel method

In this work, single-phase α-Fe₂O₃ nano-particles were first synthesized via Pechini sol-gel method using citric acid and polyethylene glycol-6000 as chelating agents. The structural coordination of as-prepared polymeric intermediates was investigated by FTIR analysis. Thermal behavior of the polymeric intermediates was studied by TG-DTG-DSC thermograms. The structure of the powders calcined at different temperatures was characterized by XRD and FESEM. The single-phase α-Fe₂O₃ nano-powders with uniform size were prepared when the polymeric intermediate calcined at 600 °C, and the lowest particle size was found to be 30 nm.     

Preparation and characterization of Y2O3–Sm2O3 co-doped ceria electrolyte for IT-SOFCs

Y₂O₃–Sm₂O₃ co-doped ceria (YSDC) powder was synthesized by a gel-casting method using Ce(NO₃)₃·6H₂O, Sm₂O₃ and Y₂O₃ as raw materials. Phase structure of the synthesized powders was characterized by X-Ray diffraction analysis. Sinterability of the powders was investigated by testing the relative density and observing the microstructure of the sintered YSDC samples. Electrical conductivity of the sintered YSDC samples was measured using impedance spectra method. Single solid oxide fuel cells based on the YSDC electrolyte were also assembled and tested. The results showed that YSDC powders with single-phase fluorite structure can be obtained by calcining the dried gelcasts at temperature above 800 °C. Average particle size of the YSDC powder is 50–100 nm. Relative density of more than 95% of the theoretical can be achieved by sintering the YSDC compacts at temperature above 1400 °C. The sintered YSDC sample has an ionic conductivity of 4.74 × 10⁻² S cm⁻¹ at 800 °C in air. Single fuel cells based on the YSDC electrolyte with 50 μm in thickness were tested using humidified hydrogen as fuel and air as oxidant, and maximum power densities of about 190 and 112 mW cm⁻² were achieved at 700 and 600 °C, respectively.     

The control of the diameter and length of α-MnO2 nanorods by regulation of reaction parameters and their thermogravimetric properties

α-MnO₂-type single-crystal nanorods were synthesized under hydrothermal conditions based on the redox reaction of KMnO₄ in an acidic environment. Several reaction parameters, like the reaction temperature, the reaction time and the concentration of KMnO₄ in the reaction mixture, were varied in order to determine their impact on the structure, the dimensions of the synthesized nanorods, and as well on their thermogravimetric properties. It was found that the reaction time has no significant influence on the diameter, although it has a strong influence on the length of the obtained nanorods. On the other hand, the concentration of KMnO₄ in the reaction mixture has a strong impact on both the diameter and the length. With an increasing concentration of KMnO₄ in the reaction mixture the average lengths and diameters of the isolated MnO₂ nanorods are reduced. The change in dimensions of the synthesized nanorods is reflected in their thermogravimetric properties.     

Growth process and microwave absorption properties of nanostructured γ-MnO2 urchins

A low-temperature hydrothermal method was developed to synthesize urchinlike γ-MnO₂ nanostructures. Time-dependent evolutions of morphology and crystallinity were investigated to explore the growth mechanism of the γ-MnO₂ urchins. The results show that the growth process of the γ-MnO₂ urchins occurs in two main stages, which are the generation of γ-MnO₂ microspheres and the following epitaxial growth of γ-MnO₂ nanoneedles on the surface of the initial microspheres. Microwave absorption properties of the urchinlike γ-MnO₂ nanostructures were studied in terms of complex permittivity and permeability. An effective absorption bandwidth (reflection loss lower than −10 dB) of 8.8 GHz was achieved from the γ-MnO₂/paraffin wax composite.     

Preparation and characterization of α-Fe2O3 nanorod-thin film by metal-organic chemical vapor deposition

Alpha iron oxide (α-Fe₂O₃) films were grown on catalyst-free silicon substrate using a vertical type metal-organic chemical vapor deposition process. X-ray powder diffraction and field-emission transmission electron microscopy measurements showed that these α-Fe₂O₃ films consisted of bundles of one dimensional (1D) nanorods and the nanorods in these α-Fe₂O₃ films were single crystalline with a well-ordered rhombohedral structure. The nanorods showed a preferred growth orientation in the [104] direction. Magnetic force microscopy image suggests that spin domains were formed in the α-Fe₂O₃ nanorods. Photo-catalytic property of these nanorod films was confirmed through the photo-degradation of Rhodamine B by UV irradiation. These α-Fe₂O₃ film/nanorod materials could be used as building blocks for nanodevice applications.