Nanostructured Manganese Oxide Research Articles

Facile Synthesis of Monodisperse Manganese Oxide Nanostructures and Their Application in Water Treatment

Different manganese oxide nanomaterials were prepared by treating their precursor, which had been prepared by mixing KMnO4 solution and oleic acid at room temperature, at low temperatures (≤200 °C). While the hierarchical morphology was kept, the phase structure was transformed from layered manganese oxide to tetragonal hausmannite. The manganese oxide nanostructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and nitrogen adsorption−desorption measurements. These nanostructures showed better adsorption capacity of organic polluents (methylene blue) than existing MCM-22, Red mud, and other synthesized manganese oxide (including α-, β-, and γ-) materials. The adsorption capacity of the nanomaterials did not largely depend on their surface area. The possible adsorption mechanisms are also discussed.

Controlled synthesis and characterization of layered manganese oxide nanostructures with different morphologies

Layered manganese oxide nanostructures with different morphologies, such as nanowire bundles, cotton agglomerates, and platelikes were successfully fabricated by a simple and template-free hydrothermal method based on a reaction of KMnO4 and KOH solutions with different concentrations. The obtained nanowire bundles were assembled by nanowires with diameters of 10 to 200 nm and lengths up to 5–10 μm. The cotton agglomerates were composed of manganese oxide layers with a thickness of about 10 nm. Both the concentration of KOH solutions and the reaction temperature played an important role in the formation of layered manganese oxide nanostructures with different morphologies. XRD, SEM, TEM, HRTEM, SAED, TG-DTA, and chemical analysis were employed to characterize these materials. On the basis of the experimental results, a possible formation mechanism of layered manganese oxide nanostructures with different morphologies was presented.

Effects of vanadium- and iron-doping on crystal morphology and electrochemical properties of 1D nanostructured manganese oxides

One-dimensional (1D) nanostructures of vanadium- and iron-doped manganese oxides, Mn 1− x M x O 2 (M = V and Fe), are synthesized via one-pot hydrothermal reactions. The results of X-ray diffraction studies and electron microscopic analyses demonstrate that all the present 1D nanostructured materials possess α-MnO 2-type structure. While the vanadium dopants produce 1D nanorods with a smaller aspect ratio of ∼3–5, iron dopants produce 1D nanowires with a high aspect ratio of >20. X-ray absorption spectroscopy clearly shows that the dopant vanadium ions are stabilized in tetravalent oxidation state with distorted octahedral symmetry, while the iron ions are stabilized in trivalent oxidation state with regular octahedral symmetry. Significant local structural distortion and size mismatch of dopant vanadium ions are responsible for the low aspect ratio of the vanadium-doped nanorods through the less effective growth of a 1D nanostructure. According to electrochemical measurements, doping with Fe and V can improve the electrode performance of 1D nanostructured manganate and such a positive effect is much more prominent for the iron dopant. The present study clearly indicates that doping with Fe and V provides an effective way of tailoring the crystal dimension and electrochemical properties of 1D nanostructured manganese oxides.

Interfacial Forces are Modified by the Growth of Surface Nanostructures

Nanostructures formed by chemical reaction can modify the interfacial forces present in aqueous solution near a surface. This study uses force-volume microscopyto explore this phenomenon for the growth of manganese oxide nanostructures on rhodochrosite. The interfacial forces above the oxide nanostructures are dominated by electrostatic repulsion for probe-surface separations greater than ca. 2 nm but are overtaken by van der Waals attraction for shorter distances. Across the investigated pH range 5.0-9.7, the maximum repulsive force occurs 2.4 (+/-1.1) nm above the oxide nanostructures. The magnitude of the repulsive force decreases from pH 5.0 to 6.5, reaches its minimum at 6.5, and then increases steadily up to pH 9.7. Specifically, fmax(pN) = 23(+/-4)[6.8(+/-2.1) - pH] for pH < 6.5 and fmax(pN)= 19(+/-2)[pH - 6.1(+/-1.0)] for pH > or = 6.5. This dependence indicates that oxide nanostructures have a point of zero charge in the pH range 6-7. In comparison to the nanostructures, the rhodochrosite substrate induces only small interfacial forces in the same pH range, suggesting a neutral or weakly charged surface. The quantitative mapping of interfacial forces, along with the associated influencing factors such as pH or growth of nanostructures, provides a basis for more sophisticated and accurate modeling of processes affecting contaminant immobilization and bacterial attachment on mineral surfaces under natural conditions.

Petal-Shaped Manganese Oxide Nanostructure on Carbon Cloth as Electrodes for Aqueous Electrochemical Supercapacitors

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Direct Soft‐Chemical Synthesis of Chalcogen‐Doped Manganese Oxide 1D Nanostructures: Influence of Chalcogen Doping on Electrode Performance

We have developed a direct nonhydrothermal route to nanostructured chalcogen-doped manganese oxides; K(x)MnO(2)Q(y) (Q = S, Se, and Te). According to combinative diffraction and microscopic analyses, the S- and Se-doped manganese oxides exhibit 1D nanowire-type morphology with layered delta-MnO(2)- and alpha-MnO(2)-structures, respectively, whereas the Te-doped compound consists of 3D nanospheres that are amorphous according to X-ray diffraction. X-ray absorption and X-ray photoelectron spectroscopy analyses clearly demonstrate that the doped chalcogen ions exist in the form of hexavalent chalcogenate clusters mainly on the sample surface or grain boundary. According to electrochemical and ex situ X-ray absorption spectroscopy investigations, the Se-doped manganate nanowires show higher structural stability and better electrode performance with excellent rate characteristics compared to the S-/Te-doped and undoped manganate nanostructures. This is attributed to the presence of chemically stable SeO4(2-) species, leading to enhanced stability of the manganate lattice through the prevention of structural deformation during cycling and/or to the improvement of Li(+) ion transport through the maintenance of intercrystallite voids. Based on the present experimental findings, we are able to conclude that the present one-pot soft-chemical route with chalcogen dopants can provide a simple method not only to economically synthesize 1D nanostructured manganese oxides but also to finely control their electrode performance, crystal structure and morphology, and lattice stability.

Controllable synthesis, characterization, and electrochemical properties of manganese oxide nanoarchitectures

Flowerlike manganese oxide microspheres and cryptomelane-type manganese oxide nanobelts were selectively synthesized by a simple decomposition of KMnO4 under mild hydrothermal conditions without using template or cross-linking reagents. The effect of varying the hydrothermal times and temperatures on the nanostructure, morphology, compositional, and electrochemical properties of the obtained manganese oxides was investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies showed that the flowerlike manganese oxide microspheres could be obtained at relatively low hydrothermal temperatures, while high hydrothermal temperatures were favorable for the formation of cryptomelane-type manganese oxide nanobelts. A morphology and crystalline evolution of the nanostructures was observed as the hydrothermal temperature was increased from 180 to 240 °C. On the basis of changing the temperatures and hydrothermal reaction times, the formation mechanism of cryptomelane-type manganese oxide nanobelts is discussed. Cyclic voltammetry (CV) was used to evaluate the electrochemical properties of the obtained manganese oxide nanostructures, and the results show that the electrochemical properties depend on their shape and crystalline structure. This easily controllable, template-free, and environmentally friendly method has the potential for being used in syntheses of manganese oxide nanomaterials with uniform morphologies and crystal structures.

Electrical Properties of Mineral Surfaces for Increasing Water Sorption

Thin films of nanostructures alter the electrical properties of mineral surfaces and thereby affect reactions with charged species such as metal ions and biological cells. In this study, electric-force microscopy is used to probe the electrical properties of a heterogeneous layout of manganese oxide nanostructures grown as a film on a MnCO3 substrate. The role of water sorption is examined by carrying out experiments for increasing relative humidity (RH). Electric-force images collected with a negative dc tip bias show that the apparent heights of the nanostructures decrease from +3.4 nm at 16% RH to +0.7 nm at 33% RH to -5.6 nm at 74% RH, although the topographic height is 2.3 nm regardless of RH. The apparent heights for a positive dc bias also decrease with increasing RH from -3.5 nm at 16% RH to -8.9 nm at 74%. The explanation for these trends is that the dominant electric-force transitions with increasing RH from an electrostatic force attributable to surface potential to a polarization force arising from hydrated, mobile surface ions including Mn2+ and CO3(2-). The positive-to-negative trend in apparent heights implies that either the density or the intrinsic mobility (or both) of mobile ions over the substrate exceeds that over the nanostructures, implying increased water sorption over the former compared to the latter. Ridges around the perimeter of the nanostructures also develop above 40% RH for images collected using a negative dc tip bias. A tip-induced gradient of net positive charge near the nanostructure edges, which implies the nonequivalence of cations and anions there, explain this observation. The findings of this study show that thin films of nanostructures on mineral surfaces have complex but measurable RH-dependent electrical properties.

Alcohol-assisted room temperature synthesis of different nanostructured manganese oxides and their pseudocapacitance properties in neutral electrolyte

Different nanostructured manganese oxides have been synthesized by a simple precipitation technique using KMnO4 and different alcohols. The synthesized manganese oxides were extensively studied using TEM, XRD, XPS, surface area measurements and electrochemical studies. TEM observations showed a range of nanostructures from nanowiskers to nanoparticles. This synthesis method promises the tuning of electronic and structural properties of the nanostructured manganese oxides by simply varying the alcohols used in the reactions. MnO2 shows more whisker-like morphology while the Mn2O3 shows particle morphology. The nanostructured manganese oxides showed excellent performance as a pseudocapacitor electrode in a neutral electrolyte.

Synthesis, Characterization and Kinetic Evaluation of Manganese Oxide Nanoparticles for the H2O2 Catalytic Decomposition

Nanostructured manganese oxides were synthesized towards the catalytic decomposition of hydrogen peroxide. The effect of the type of precipitant (Na2CO3 and NH4OH) over the particle size and crystallographic phase was determined. Also, the influence of particle size and crystal phase over the catalytic activity was established. Characterization of the obtained nanoparticles was performed through XRD analysis, BET surface area and TEM microscopy. Catalytic activity was followed through gasometry of the oxygen evolution [H2O2(l) ? H2O(l) + O2 (g)] at 25°C. Na2CO3 (Mn-1) and NH4OH (Mn-2) precipitants produced nanoparticles with average sizes of 5-10 nm and 20 nm, respectively. This result was attributed to higher pH stability of precipitant Na2CO3 during the synthesis process. XRD results revealed the presence of ß-MnO2, Mn5O8 and Mn3O4 crystal phase mixture for Mn-1 and Mn5O8 for Mn-2 catalyst. Catalytic activity resulted in intrinsic rate constants (kint = min-1m-2) of 5.2 and 2.3 for Mn-1 and Mn-2, respectively. The greater catalytic activity of Mn-2 catalyst was attributed to the presence of the Mn55O8 phase.

Nanostructured manganese oxides and their composites with carbon nanotubes as electrode materials for energy storage devices

Abstract Manganese oxides have been synthesized by a variety of techniques in different nanostructures and studied for their properties as electrode materials in two different storage applications, supercapacitors (SCs) and Li-ion batteries. The composites involving carbon nanotubes (CNTs) and manganese oxides were also prepared by a simple room-temperature method and evaluated as electrode materials in the above applications. The synthesis of nanostructured manganese oxides was carried out by simple soft chemical methods without any structure directing agents or surfactants. The prepared materials were well characterized using different analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), surface area studies, etc. The electrochemical properties of the nanostructured manganese oxides and their composites were studied using cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopic (EIS) studies. The influence of structural/surface properties on the electrochemical performance of the synthesized manganese oxides is reviewed.

Electrosynthesis of manganese oxide films

Nanostructured manganese oxide films for electrochemical supercapacitors were obtained by cathodic electrosynthesis from aqueous NaMnO4 solutions. The proposed deposition mechanism is based on cathodic reduction of MnO4− species. The amount of the deposited material has been controlled by the variation of deposition time at a constant current density. Obtained films were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction analysis, thermogravimetric and differential thermal analysis. It was found that the composition and microstructure of the films depend on the concentration of NaMnO4 in the solutions used for deposition. The films prepared from the 0˙02M NaMnO4 solutions were crack free and porous with typical pore size of about 100–200 nm. Cyclic voltammetry and chronopotentiometry data for the films tested in the 0˙1M Na2SO4 solutions showed capacitive behaviour in the voltage window of 0–1˙0 V versus standard calomel reference electrode. The highest specific capacitance of ∼220 F g−1 was obtained for 90 μg cm−2 films at a scan rate of 2 mV s−1 using Ni substrates. The capacitance decreased with increasing scan rate. The charge storage mechanism is discussed.

Self-Assembly of Novel Mesoporous Manganese Oxide Nanostructures and Their Application in Oxidative Decomposition of Formaldehyde

Monodisperse manganese oxide honeycomb and hollow nanospheres have been prepared facilely at room temperature by varying the molar ratio of KMnO4 and oleic acid. These new nanomaterials were characterized by XRD, SEM, EDS, TEM, and BET measurements. They had robust nanostructures and were stable even after ultrasonic treatment (40 kHz, 120 W) for 30 min. A plausible mechanism of the formation of manganese oxide nanostructures was proposed. The manganese oxide nanomaterials showed high catalytic activities for oxidative decomposition of formaldehyde at low temperatures. Complete conversion of formaldehyde to CO2 and H2O could be achieved, and harmful byproducts were not detected in effluent gases. The catalytic activity of manganese oxide hollow nanospheres was much higher than that of honeycomb nanospheres, although the surface area of the latter was nearly 2 times as high as that of the former. The mechanism of such morphology-dependent catalytic activity was discussed in detail. The catalytic activities of the obtained manganese oxide nanospheres were also significantly higher than those of previously reported manganese oxide octahedral molecular sieve (OMS-2) nanorods, MnOx powders, and alumina-supported manganese-palladium oxide catalysts. Potential applications and future research efforts were proposed.

Effect of Copper Doping on the Crystal Structure and Morphology of 1D Nanostructured Manganese Oxides

We have tried to control the aspect ratio and physicochemical properties of 1D nanostructured manganese oxides through copper doping. Copper-doped manganese oxide nanostructures have been synthesized by one-pot hydrothermal treatment for the mixed solution of permanganate anions and copper cations. According to powder X-ray diffraction and electron microscopic analyses, all the present materials commonly crystallize with alpha-MnO2-type structure but their aspect ratio decreases significantly with increasing the content of copper. Such a variation of crystallite dimension is attributable to the limitation of crystal growth by the incorporation of copper ions. X-ray absorption spectroscopic studies at Mn K- and Cu K-edges clearly demonstrate that the average oxidation state of manganese ions is increased by the substitution of divalent copper ions. Electrochemical measurements reveal the improvement of the electrode performance of nanostructured manganate upon copper doping, which can be interpreted as a result of the decrease of aspect ratio and the increase of Mn valence state. From the present experimental findings, it becomes certain that the present Cu doping method can provide an effective way of controlling the crystal dimension and electrochemical property of 1D nanostructured manganese oxide.

Electrosynthesis of Single-Crystalline MnOOH Nanorods onto Pt Electrodes: Electrocatalytic Activity toward Reduction of Oxygen

This study concerns the synthesis and fabrication of manganese oxide nanostructures via electrodeposition onto Pt electrodes from aqueous solution of containing . The structural and morphological characterization of the electrodeposited manganese oxide was performed with scanning electron microscopy and high-resolution transmission electron microscopy. These techniques revealed the formation of intersected nanorods of single-crystalline (manganite phase), in agreement with the results of the X-ray diffraction data. Furthermore, a significant enhancement of the electrocatalytic activity of Pt toward the oxygen reduction reaction (ORR) has been observed in -saturated KOH upon the electrodeposition of nanorods. A surface coverage of MnOOH of ca. 0.3 caused a positive shift of the peak potential of the ORR by about compared to that obtained at bare Pt electrode. Similar Tafel slopes were obtained at the bare and the modified Pt electrodes, indicating that the ORR proceeds via the same rate-determining step at the aforementioned electrodes.

Nanostructured manganese oxide electrodes for lithium-ion storage in aqueous lithium sulfate electrolyte

Nanostructured manganese oxide electrodes are fabricated directly by electrochemical deposition. Surface morphology of the electrode deposited at high-current density shows nanowires with diameter 12–16 nm distributed randomly. Nanowires tend to aggregate to clumps when the deposition current density is low. Both annealing temperature and deposition current density affect the electrochemical performance of the deposited manganese oxide electrode in an aqueous lithium sulfate electrolyte. An optimal annealing temperature is found to be 300 °C in terms of the electrode's specific capacity during high-rate charging/discharging. An electrode with thinner nanowires deposited at high-current density has a high-specific capacity because thinner nanowires shorten the diffusion of lithium ions and in favor of high-rate charging/discharging.

Effects of thickness and electrolytes on the capacitive characteristics of anodically deposited hydrous manganese oxide coatings

Abstract An amorphous nanostructured hydrous manganese oxide (MnO 2 · n H 2 O) coating was anodically deposited onto a graphite substrate from an aqueous MnSO 4 solution. Various deposition charges were employed to achieve different coating thickness. The capacitive characteristics of the as-grown MnO 2 coatings were evaluated in various neutral alkali metal chloride solutions by cyclic voltammetry and chronopotentiometry. The as-grown MnO 2 coatings exhibited a capacitance of 275 F g −1 in a 0.5 M KCl solution. Dependence of coating thickness and electrolytes on the capacitive properties of MnO 2 coatings was demonstrated. The relationship of C a with the deposition charge exhibits linearity in the range of 4.0 C cm −2 , and there is a peak at around 4.5 C cm −2 , however, the value of C m keeps diminished. With the increase of the discharge current density, more rapid decrease of the specific capacitance was found when measured in the electrolyte with larger hydrated alkali metal cation in the order of Li + > Na + > K + . This was especially noted for thicker MnO 2 coatings.

Single-Step Synthesis, Characterization, and Application of Nanostructured KxMn1-yCoyO2-δ with Controllable Chemical Compositions and Crystal Structures

Cobalt-substituted manganese oxide one-dimensional (1D) nanowires have been synthesized through one-pot hydrothermal treatment of the ion pair solution of Co2+ and MnO4-, along with their three-dimensional (3D) hierarchically assembled microspheres. The stabilization of cobalt ion in the manganese sites of α- and δ-MnO2 structures was clearly evidenced by Mn K-edge and Co K-edge X-ray absorption spectroscopy as well as by powder X-ray diffraction and elemental analyses. The chemical composition, crystal structure, and morphology of the resultant nanostructures can be easily tailored through the control of reaction condition and precursor composition. That is, 1D nanowires with the tunnel-type α-MnO2 structure can be prepared by the reaction at 140 °C, whereas the lowering of reaction temperature or the increase of Co content gives rise to the synthesis of 3D hierarchical nanostructured microspheres with the layered δ-MnO2 structure. It has been obviously demonstrated that the partial Co substitution for the nanostructured manganese oxides causes the improvement of their catalytic activity with respect to olefin oxidation as well as affects significantly their electrode performances. The present study provides an effective way of controlling the chemical properties and functionalities of nanostructured manganese oxides through cation substitution.

The cathodic electrodeposition of manganese oxide films for electrochemical supercapacitors

Electrochemical supercapacitors (ES) are urgently needed as components in many advanced power systems. The development of advanced ES is expected to enable radical innovation in the area of hybrid vehicles and electronic devices. Electrochemical supercapacitors become of interest in hybrid electric vehicles as auxiliary power sources in combination with fuel cells or batteries. Nanostructured manganese oxides have been found to be promising electrode materials for ES. This paper describes new electrochemical methods for the fabrication of nanostructured manganese oxide electrodes for ES.

Shape-Controllable Synthesis and Electrochemical Properties of Nanostructured Manganese Oxides

Several nanostructured manganese oxides with novel shapes were controllably synthesized from the same precursor Na−BirMO via a facile, direct hydrothermal or surging method. The synthesis parameters for nanostructured manganese oxides were investigated, along with their influences on structure, morphology, composition, and electrochemical properties on glass carbon electrode. XRD diffraction showed that the prepared nanostructured manganese oxides belonged to different polymorphs. SEM, TEM, and HRTEM revealed that they had a variety of morphologies, including 0-dimension microspheres, 1-dimension distinct tetragonous and semi-tubular structures, nanorolls, and spindly nanowire bundles. Basis on the experimental results, a rolling rice glue ball formation mechanism for manganese oxide microspheres was proposed. Electrochemical properties were studied on glass carbon electrode (GCE) using cyclic voltammetry (CV). The results showed the electrochemical properties depended on the shape, size and crystal structure of the prepared nanostructured manganese oxides.