MnO2 Microspheres Research Articles

Self-sacrificial template method to MnO2 microspheres as highly efficient electrocatalyst for oxygen evolution reaction

e-MnO2 microspheres have been synthesized using a self-sacrificial template method by annealing MnCO3 microspheres. The transformation process from MnCO3 to MnOx at four different annealing temperatures has been investigated. XRD data show that MnOx samples obtained at 250, 350, 450, and 550 °C are corresponding to MnCO3, the mixture of MnCO3 and e-MnO2, pure e-MnO2 and pure Mn3O4, respectively. SEM images show that the spherical morphology of e-MnO2 microspheres is kept with the diameter about 4 μm. The electrocatalytic measurements of different MnOx samples for oxygen evolution reaction (OER) have been systematically studied. The results show that e-MnO2 microspheres exhibit excellent OER activity, with an overpotential of 0.42 V (at j = 10 mA cm−2) and a low Tafel slope of 75.7 mV dec−1, which is much better than the other MnOx samples. The enhanced activity may be attributed to the crystal phase and the spherical morphology of e-MnO2.

Enhanced microwave absorption properties of MnO2 hollow microspheres consisted of MnO2 nanoribbons synthesized by a facile hydrothermal method

MnO2 hollow microspheres consisted of nanoribbons were successfully fabricated via a facile hydrothermal method with SiO2 sphere templates. The crystal structure, morphology and microwave absorption properties in X and Ku band of the as-synthesized samples were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and a vector network analyzer. The results show that the three-dimensional (3D) hollow microspheres are assembled by ultra thin and narrow one-dimensional (1D) nanoribbons. A rational process for the formation of hollow microspheres is proposed. The 3D MnO2 hollow microspheres possess improved dielectric and magnetic properties than the 1D nanoribbons prepared by the same procedures with the absence of SiO2 hard templates, which are closely related to their special nanostructures. The MnO2 microspheres also show much better microwave absorption properties in X (8–12 GHz) and Ku (12–18 GHz) microwave band compared with 1D MnO2 nanoribbons. The minimum reflection loss of −40 dB for hollow microsphere can be observed at 14.2 GHz and reflection loss below −10 dB is 3.5 GHz with a thickness of only 4 mm. The possible mechanism for the enhanced microwave absorption properties is also discussed.

Synthesis, Characterization, and Photocatalytic Performance of Mesoporous α-Mn2O3 Microspheres Prepared via a Precipitation Route

α-Mn2O3 microspheres with high phase purity, crystallinity, and surface area were synthesized by the thermal decomposition of precipitated MnCO3 microspheres without the use of any structure directing agents and tedious reaction conditions. The prepared Mn2O3 microspheres were characterized by Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) and photoluminescence (PL) studies. The complete thermal transformation of MnCO3 to Mn2O3 was clearly shown by the FTIR and XRD analysis. The electron microscopic images clearly confirmed the microsphere-like morphology of the products with some structural deformation for the calcined Mn2O3 sample. The mesoporous texture generated from the interaggregation of subnanoparticles in the microstructures is visibly evident from the TEM and BET studies. Moreover, the Mn2O3 microstructures showed a moderate photocatalytic activity for the degradation of methylene blue dye pollutant under UV light irradiation, using air as the potential oxidizing agent.

ChemInform Abstract: Hierarchically Structured MnO2‐Co/C Nanocomposites: Highly Efficient and Magnetically Recyclable Catalysts for the Aerobic Oxidation of Alcohols.

Porous MnO2 microspheres distributed around magnetic carbon-coated cobalt nanoparticles were developed. The performance of these nanoparticles rivals that of Pd-based catalysts for the selective oxidation of alcohols. Excellent recyclability was conveniently achieved by magnetic decantation and is demonstrated in ten consecutive cycles with no apparent material or performance losses.

Facile Synthesis of Porous Mn₂O₃ Microspheres as Anode Materials for Lithium Ion Batteries.

Porous Mn₂O₃ microspheres with a diameter of 0.5-2 µm were synthesized by a thermal decomposition of MnCO₃ micospheres precursor. The effects of annealing time on the morphologies and electrochemical properties of the final products were systematically investigated. The porous Mn2O₃ microspheres prepared at 600 °C for 4 h exhibit the best electrochemical properties with a high reversible capability (925 mAh/g at current density of 100 mA/g) and cycling stability. It still retains a high capacity of 565 mAh/g, even after 100 cycles, as anode materials for lithium-ion batteries. The good electrochemical performance for the porous Mn₂O₃microspheres can be attributed to its high surface area and mesoporous structure.

General Synthesis of MnOx (MnO2, Mn2O3, Mn3O4, MnO) Hierarchical Microspheres as Lithium-ion Battery Anodes

Four type of MnOx (MnO2, Mn2O3, Mn3O4, MnO) hierarchical microspheres assembled by rod-like building blocks are synthesized by a facile hydrothermal process with/without a consequent calcination. The morphology and structure of these hierarchical microspheres are confirmed by XRD, SEM, TEM, HRTEM, XPS and BET measurements. The electrochemical properties of the four hierarchical microspheres are investigated in terms of cycling stability and rate capability. Specific capacities of 240, 396, 271 and 810mAhg−1 can be achieved after 100 cycles at 0.5C for MnO2, Mn2O3, Mn3O4 and MnO, respectively. Even at a high rate of 2C, MnO microspheres can still deliver a reversible capacity of 406mAhg−1. Their superior electrochemical properties might be attributed to the secondary nanostructure in the MnOx microspheres, which could effectively shorten the diffusion pathway of Li+, tolerate the structural stress caused by Li+ insertion/extraction, reduce the side reactions with electrolyte, and restrain the self-aggregation of nanomaterials.

Hierarchically Structured MnO2-Co/C Nanocomposites: Highly Efficient and Magnetically Recyclable Catalysts for the Aerobic Oxidation of Alcohols

Porous MnO2 microspheres distributed around magnetic carbon-coated cobalt nanoparticles were developed. The performance of these nanoparticles rivals that of Pd-based catalysts for the selective oxidation of alcohols. Excellent recyclability was conveniently achieved by magnetic decantation and is demonstrated in ten consecutive cycles with no apparent material or performance losses.

Sensitive electrochemical detection of bisphenol A with chitosan and urchinlike MnO2 microsphere modified carbon ionic liquid electrode

In this article, a carbon ionic liquid electrode (CILE) was fabricated by using ionic liquid N-hexylpyridinium hexafluorophosphate as the binder and the modifier. Then urchinlike MnO2 microsphere and chitosan (CTS) was further casted on the CILE surface step-by-step to get a modified electrode that was denoted as CTS/MnO2/CILE. Cyclic voltammetric studies indicated that bisphenol A (BPA) exhibited a well-defined oxidation peak at 0.486 V in 22.83 g L−1 pH 8.0 Britton‒Robinson buffer solution, which was attributed to the electro-oxidation of BPA on the modified electrode. The presence of urchinlike MnO2 microsphere on the electrode surface could increase the oxidation peak current (Ipa) greatly, which may be due to the larger surface area that could adsorb more BPA on the electrode surface. Electrochemical parameters of BPA on the modified electrode were calculated with the electron transfer coefficient (α) as 0.66 and the apparent heterogeneous electron transfer rate constant (ks) as 0.50 s−1. Under the optimal conditions, a linear relationship between the Ipa of BPA and its concentration was obtained in the range from 1.37 × 10–1 mg L−1 to 182.6 mg L−1 with the detection limit as 7.31 × 10–3 mg L−1 (3σ). The CTS/MnO2/CILE was applied to the detection of BPA content in different kinds of samples with satisfactory results.

Catalytic activities and mechanism of formaldehyde oxidation over gold supported on MnO2 microsphere catalysts at room temperature

MnO2 microspheres with various surface structures were prepared using the hydrothermal method, and Au/MnO2 catalysts were synthesized using the sol-gel method. We obtained three MnO2 microspheres and Au/MnO2 samples: coherent solid spheres covered with wire-like nanostructures, solid spheres with nanosheets, and hierarchical hollow microspheres with nanoplatelets and nanorods. We investigated the properties and catalytic activities of formaldehyde oxidation at room temperature. Crystalline structures of MnO2 are the main factor affecting the catalytic activities of these samples, and γ-MnO2 shows high catalytic performance. The excellent redox properties are responsible for the catalytic ability of γ-MnO2. The gold-supported interaction can change the redox properties of catalysts and accelerate surface oxygen species transition, which can account for the catalytic activity enhancement of Au/MnO2. We also studied intermediate species. The dioxymethylene (DOM) and formate species formed on the catalyst surface were considered intermediates, and were ultimately transformed into hydrocarbonate and carbonate and then decomposed into CO2. A proposed mechanism of formaldehyde oxidation over Au/MnO2 catalysts was also obtained.

Synthesis of Manganese Oxide Microspheres by Ultrasonic Spray Pyrolysis and Their Application as Supercapacitors

Manganese oxide (MnO2) microspheres are prepared using an ultrasonic spray pyrolysis (USP) process. A mixed solution of potassium permanganate and hydrochloric acid is nebulized into microsized droplets, which are then carried by air flow through a furnace tube. Each microdroplet serves as one microreactor and produces one microsphere. Upon heating, KMnO4 is decomposed into MnO2 microspheres; this synthetic process can easily be scaled up. Characterization of the MnO2 microspheres by scanning electron microscopy, transmission electron microscopy, powder X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectra is described. Different morphologies of MnO2 microspheres can be controlled by tuning the precursor concentrations (and ratios) and furnace temperatures. Microspheres synthesized at 150 °C give amorphous MnO2 while synthesis at 500 °C yields crystalline α‐MnO2. The electrochemical properties investigated by cyclic voltammetry give specific capacitance as high as 320 F g−1, demonstrating promising properties as supercapacitors. In addition, these microspheres can be directly sprayed on conductive substrates, such as carbon fiber paper, and may have useful applications as a supercapacitor electrode coating. The supercapacitive properties of MnO2 microspheres at higher charge and discharge rates can be improved by increasing the surface area coverage or coating them with a thin layer of conductive polymer.

Morphologically controlled synthesis of porous Mn2O3 microspheres and their catalytic applications on the degradation of methylene blue

Porous Mn2O3 microspheres are controllably synthesized by selectively etching MnCO3 precursor with HCl solution. Morphologies and microstructures of Mn2O3 microspheres are analyzed by SEM, TEM, XRD, and N2 sorption technique. The catalytic performances of Mn2O3 microspheres for the degradation of methylene blue (MB) are investigated, and the reaction kinetics of MB degradation is also studied. The results show it is feasible to control the morphologies of Mn2O3 microspheres by adjusting the concentration of HCl solution, and the well-developed porous Mn2O3 microspheres demonstrate good potential on MB degradation. The degradation reactions follow the pseudo-first-order kinetic model, and the degradation capabilities for MB are great dependent on BET surface areas and pore volumes of Mn2O3 microspheres.

Highly dispersed Mn2O3 microspheres: Facile solvothermal synthesis and their application as Li-ion battery anodes

Nanostructured transition metal oxides are promising alternative anodes for lithium ion batteries. Li-ion storage performance is expected to improve if high packing density energy particles are available. Herein, Mn2O3 microspheres with a ca. 18μm diameter and a tapped density of 1.33g/cm3 were synthesized by a facile solvothermal–thermal coversion route. Spherical MnCO3 precursors were obtained through solvothermal treatment and they decomposed and converted into Mn2O3 microspheres at an annealing temperature of 700°C. The Mn2O3 microspheres consisted of Mn2O3 nanoparticles with an average 40nm diameter. These porous Mn2O3 microspheres allow good electrolyte penetration and provide an ion buffer reservoir to ensure a constant electrolyte supply. The Mn2O3 microspheres have reversible capacities of 590 and 320mAh/g at 50 and 400mA/g, respectively. We thus report an efficient route for the fabrication of energy particles for advanced energy storage.

Pristine hollow microspheres of Mn2O3 as a potential anode for lithium-ion batteries

Ascorbic acid aided inside-out Ostwald ripening promotes the formation of Mn2O3 microspheres with a hollow interior surface that exhibits an appreciable capacity of 610 mA h g−1, even after completing 100 cycles.

Hierarchically porous MnO2 microspheres doped with homogeneously distributed Fe3O4 nanoparticles for supercapacitors.

Hierarchically porous yet densely packed MnO2 microspheres doped with Fe3O4 nanoparticles are synthesized via a one-step and low-cost ultrasound assisted method. The scalable synthesis is based on Fe(2+) and ultrasound assisted nucleation and growth at a constant temperature in a range of 25-70 °C. Single-crystalline Fe3O4 particles of 3-5 nm in diameter are homogeneously distributed throughout the spheres and none are on the surface. A systematic optimization of reaction parameters results in isolated, porous, and uniform Fe3O4-MnO2 composite spheres. The spheres' average diameter is dependent on the temperature, and thus is controllable in a range of 0.7-1.28 μm. The involved growth mechanism is discussed. The specific capacitance is optimized at an Fe/Mn atomic ratio of r = 0.075 to be 448 F/g at a scan rate of 5 mV/s, which is nearly 1.5 times that of the extremely high reported value for MnO2 nanostructures (309 F/g). Especially, such a structure allows significantly improved stability at high charging rates. The composite has a capacitance of 367.4 F/g at a high scan rate of 100 mV/s, which is 82% of that at 5 mV/s. Also, it has an excellent cycling performance with a capacitance retention of 76% after 5000 charge/discharge cycles at 5 A/g.

Facile synthesis of mesoporous α-Mn2O3 microspheres via morphology conserved thermal decomposition of MnCO3 microspheres

In this article, we report on the synthesis and characterisation of novel mesoporous α-Mn2O3 microspheres obtained by the thermal decomposition of hydrothermally grown MnCO3. The porous, hierarchical Mn2O3 microspheres were obtained without the use of any structural directing agents or templates. The self-sacrificing template character of the hydrothermally grown MnCO3 is clearly presented here. Body centred cubic phase pure crystalline structure of Mn2O3 was acquired at 600°C, whereas a minor Mn3O4 phase was detected at 500°C. The microsphere-like morphology of the intermediate precursor and the final products was clearly observed in the field emission scanning electron microscopy (FESEM) images, which depicted the morphology conserved thermal decomposition process. Roughly spherical and elongated Mn2O3 sub-nanoparticles were hierarchically gathered inside the microspheres, which points to the development of a mesoporous texture, as confirmed by the transmission electron microscopy (TEM) and porosity studies. The optical properties studied using photoluminescence spectroscopy indicated its applications under UV-light.

Facile Synthesis of Porous MnO Microspheres for High‐Performance Lithium‐Ion Batteries

Manganese oxide is a highly promising anode material of lithium‐ion batteries (LIBs) for its low insertion voltage and high reversible capacity. Porous MnO microspheres are prepared by a facile method in this work. As an anode material of LIB, it can deliver a high reversible capacity up to 1234.2 mA h g−1 after 300 cycles at 0.2 C, and a capacity of 690.0 mA h g−1 in the 500th cycle at 2 C. The capacity increase with cycling can be attributed to the growth of reversible polymer/gel‐like film, and the better cycling stability and the superior rate performance can be attributed to the featured structure of the microspheres composed of nanoparticles with a short transport path for lithium ions, a large specific surface, and material/electrolyte contact area. The results suggest that the porous MnO microspheres can function as a promising anode material for high‐performance LIBs.

Controlled synthesis of hierarchical MnO2 microspheres with hollow interiors for the removal of benzene

Three types of MnO2 microspheres, viz., hierarchical hollow beta-MnO2 microspheres consisting of uniform nanorods, hierarchical double-walled hollow beta-MnO2 microspheres assembled by two-categorical nanorods, and hierarchical hollow alpha-MnO2 microspheres constructed by nanorods and nanowires, have been synthesized by a facile hydrothermal method without employing any templates, catalysts, surfactants or calcinations. A number of characterization techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), nitrogen adsorption-desorption measurements, and X-ray photoelectron spectroscopy (XPS) are used to characterize the formed hierarchical products, and to address the following critical issues: (i) effect of the precursor concentration; (ii) the role of HCl for the formation of the hierarchical hollow structure; and (iii) the possible mechanism accounting for the formation of the hierarchical hollow microspheres. The hierarchical hollow MnO2 microspheres can be used as catalysts for the catalytic oxidation of benzene, of which the hierarchical hollow alpha-MnO2 microspheres exhibit the highest catalytic ability.

Synergistic effect for the preparation of LiMn2O4microspheres with high electrochemical performance

Under the synergistic promotion of chemical reactions, MnO2 microspheres are successfully prepared using MnCO3 precursor as self-template, with maximum use of the raw materials, and are then converted into spinel LiMn2O4 microspheres via a solid-state reaction. When applied as cathode materials for rechargeable lithium-ion batteries, the LiMn2O4 microspheres deliver a discharge capacity of 133.2 mA h g−1 and 109.8 mA h g−1 at 0.1 C and 10 C rates, respectively. More than 91.3% of the initial storage capacity is maintained after 1000 cycles at a 1 C charge and 10 C discharge rate. Furthermore, even when cycled at elevated temperature (55 °C) at a 1 C charge and 10 C discharge rate, this material showes 86.2% capacity retention after 100 cycles, which reveals a high reversible capacity, superior rate capability and excellent cycling stability under high rates both at room temperature and high temperature. The performance is comparable to the best results shown in the literature so far. This structured LiMn2O4 microspheres in the present work are very promising for large-scale commercialization of high-power lithium ion batteries.

Designed synthesis of LiNi0.5Mn1.5O4 hollow microspheres with superior electrochemical properties as high-voltage cathode materials for lithium-ion batteries

LiNi0.5Mn1.5O4 hollow microspheres with designed subunits as high-voltage cathode materials for lithium-ion batteries were synthesized using dense or hollow Mn2O3 porous-spheres as self-templates. Dense Mn2O3 porous-spheres with mesosized subunits (>100 nm) were prepared by the direct decomposition of microspherical MnCO3, while the hollow and porous Mn2O3 microspheres with nanosized subunits (<30 nm) were synthesized by temperature-controlled decomposition of Mn–Ca bicarbonates followed by the selective removal of carbonates with HCl. Through a solid state reaction between Li/Ni precursors with meso/nanoporous Mn2O3, LiNi0.5Mn1.5O4 hollow spheres with different cavity size and wall structure were prepared. The as-synthesized hollow microspheres exhibited superior cyclability and high-rate capability. The best sample delivered a high and reversible discharge capacity of around 130 mA h g−1 with a capacity retention efficiency of 98.6% after 60 cycles at 1 C rate. The sample also showed high reversible capacities of 100.5 mA h g−1 even at a high current rate of 5 C. As a comparison, LiNi0.5Mn1.5O4 powders were also produced by a conventional solid state process using ball-milled Li–Ni–Mn hydroxide-oxide precursors, which showed low capacities of around 110 mA h g−1 at 1 C and greatly degraded capacities at higher current rates.

Designed hierarchical MnO2 microspheres assembled from nanofilms for removal of heavy metal ions

Hierarchical MnO2 microspheres assembled by nano thin film are fabricated through an environmental route with subsequent drying under vacuum, and exhibit remarkable removal efficiency of heavy metal ions in water treatment.