One-step electrosynthesis of MnO2/rGO nanocomposite and its enhanced electrochemical performance

We present a facile one-step electrochemical approach to generate MnO₂/rGO nanocomposite from a mixture of Mn₃O₄ and graphene oxide (GO). The electrochemical conversion of Mn₃O₄ into MnO₂ through potential cycling is expedited in the presence of GO while the GO is reduced into reduced graphene oxide (rGO). The MnO₂ nanoparticles are evenly distributed on the rGO nanosheets and act as the spacer to prevent rGO nanosheets from restacking. This unique structure provides high electroactive surface area (1173?m² g^?1) that improves ions diffusion within the MnO₂/rGO structure. As a result, the MnO₂/rGO nanocomposite exhibits high specific capacitance of 473?F?g^?1 at 0.25?A?g^?1, which is remarkably higher (3 times) than the Mn₃O₄/GO prior conversion. In addition, the electrosynthesized nanocomposite shows higher conductivity and excellent potential cycling stability of 95% at 2000 cycles.     

Gold nanoparticles mediate the assembly of manganese dioxide nanoparticles for H2O2 amperometric sensing

Aggregates of gold nanoparticles (AuNPs) that mediate the assembly of manganese dioxide nanoparticles (nano-MnO₂) for hydrogen peroxide (H₂O₂) amperometric sensing have been developed. The aggregates were prepared by directly mixing citric-capped AuNPs and poly(allylamine hydrochloride) (PAH)-capped nano-MnO₂ using an electrostatic self-assembly strategy. The prepared sensor exhibited excellent electrochemical behaviors and a wide linear range from 7.80 × 10⁻⁷ to 8.36 × 10⁻⁴ M with a detection limit of 4.68 × 10⁻⁸ M (S/N = 3) because of the synergistic influence of excellent catalytic ability of MnO₂ and good electrical conductivity of AuNPs. In addition, its applicability to practical samples for measuring H₂O₂ in toothpastes has obtained a satisfactory result. Due to the ease of preparation and excellent properties of the sensor, indicating the MnO₂-AuNP material may be a potential H₂O₂ sensor.     

Enhanced gyromagnetic properties of NiCuZn ferrite ceramics for LTCC applications by adjusting MnO2-Bi2O3 substitution

The demand for high performance microwave devices is increasingly promoting the development of miniaturization, integration and multifunctionalization. Here, a uniform and dense NiCuZn ferrite ceramic with high saturation magnetization and low ferromagnetic resonance linewidth was obtained at 950?°C by adjusting the MnO₂-Bi₂O₃ composite additives. The MnO₂-Bi₂O₃ composite additives were composed of 0.5?wt% MnO₂ and x wt% Bi₂O₃ (x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 3.0). The phase structure, microstructures and magnetic properties were systematically studied by means of modern measurement techniques. SEM images reveal that appropriate MnO₂-Bi₂O₃ additions can promote grain growth and reduce sintering temperatures, which is very advantageous for LTCC technology. In addition, the content of MnO₂-Bi₂O₃ additives can significantly reduce ferromagnetic resonance linewidth (FMR) by promoting grain growth and densification at low temperatures. Finally, a uniform and compact NiCuZn ferrite ceramic with an improved 4πM_s (~?3812.5 Gauss), a narrow ΔH (~?144.6?Oe), and a reduced H_c (~?85.2?A/m) were obtained (at 950?°C) by adding the optimal volume of Bi₂O₃ additive. It is expected that the improved gyromagnetic performances will allow the NiCuZn ferrite ceramics to be promising candidates for X-band microwave devices.     

A novel LiTi2(PO4)3/MnO2 hybrid supercapacitor in lithium sulfate aqueous electrolyte

In this work, carbon-coated lithium-ion intercalated compound LiTi₂(PO₄)₃ and MnO₂ have been synthesized and they deliver a capacity of 90 and 60 mAh/g in 1 M Li₂SO₄ neutral aqueous electrolyte within safe potentials without O₂ and H₂ evolution, respectively. The novel hybrid supercapacitor in which MnO₂ was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode was assembled and the LiTi₂(PO₄)₃/MnO₂ hybrid supercapacitor showed a sloping voltage profile from 0.7 to 1.9 V, at an average voltage near 1.3 V, and delivers a capacity of 36 mAh/g and an energy density of 47 Wh/kg based on the total weight of the active electrode materials. It exhibits a desirable profile and maintains over 80% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibit an excellent rate capability, even at a power density of 1000 W/kg, it has a specific energy 25 Wh/kg compared with 43 Wh/kg at the power density about 200 W/kg.     

Enhanced electrochemical stability and charge storage of MnO2/carbon nanotubes composite modified by polyaniline coating layer in acidic electrolytes

Manganese dioxide/multiwalled carbon nanotubes (MnO₂/MWCNTs) were synthesized by chemically depositing MnO₂ onto the surface of MWCNTs wrapped with poly(sodium-p-styrenesulfonate). Then, polyaniline (PANI) with good supercapacitive performance was further coated onto the MnO₂/MWCNTs composite to form PANI/MnO₂/MWCNTs organic-inorganic hybrid nanoarchitecture. Electrochemical performance of the hybrid in Na₂SO₄-H₂SO₄ mixed acidic electrolytes was evaluated by cyclic voltammetry (CV) and chronopotentiometry (CP) in detail. Comparative electrochemical tests revealed that the hybrid nanoarchitecture could operate in the acidic medium due to the protective modification of PANI coating layer onto the MnO₂/MWCNTs composite, and that its electrochemical behavior was greatly dependent upon the concentration of protons in the acidic electrolytes. Here, PANI not only served as a physical barrier to restrain the underlying MnO₂/MWCNTs composite from reductive-dissolution process so as to make the novel ternary hybrid material work in acidic medium to enhance the utilization of manganese oxide as much as possible, but also was another electroactive material for energy storage in the acidic mixed electrolytes. It was due to the existence of PNAI layer that an even larger specific capacitance (SC) of 384 F g⁻¹ and a much better SC retention of 79.9% over 1000 continuous charge/discharge cycles than those for the MnO₂/MWCNTs nanocomposite were delivered for the hybrid in the optimum 0.5 M Na₂SO₄-0.5 M H₂SO₄ mixed acidic electrolyte.     

A low-potential, H2O2-assisted electrodeposition of cobalt oxide/hydroxide nanostructures onto vertically-aligned multi-walled carbon nanotube arrays for glucose sensing

A novel nanocomposite was synthesized using a cathodic, low-potential, electrochemical reduction of H₂O₂ to homogeneously deposit cobalt oxide/hydroxide (denoted as CoOx·nH₂O) nanostructures onto vertically well-aligned multi-walled carbon nanotube arrays (MWCNTs), while the MWCNTs were prepared by catalytic chemical vapor deposition (CVD) on a tantalum (Ta) substrate. The CoOx·nH₂O–MWCNTs nanocomposite exhibits much higher electrocatalytic activity towards glucose (Glc) after modification with CoOx·nH₂O than before. This non-enzymatic Glc sensor has a high sensitivity (162.8 μA mM⁻¹ cm⁻²), fast response time (<4 s) and low detection limit (2.0 μM at signal/noise ratio = 3), and a linear dynamic range up to 4.5 mM. The sensor output is stable over 30 days and unaffected by common interferents that co-exist with Glc in analytical samples; it is also resistant to chloride poisoning. These features make the CoOx·nH₂O–MWCNTs nanocomposite a promising electrode material for non-enzymatic Glc sensing in routine analysis.     

Efficient photocatalytic activity of MnO2-loaded ZrO2/carbon cluster nanocomposite materials under visible light irradiation

Nano-sized ZrO₂/carbon cluster nanocomposite material was successfully prepared by the calcination of Zr(acac)₄/epoxy resin complex in air. The composite material obtained by calcining at 200 °C was treated with hydrogen hexachloroplatinate hexahydrate (H₂PtCl₆) to obtain Pt-loaded materials denoted as Ic₂₀₀Pt'sH's. The Pt-loaded material modified with MnO₂ particles efficiently decompose water into H₂ and O₂ with a [H₂]/[O₂] ratio of 2 under the irradiation of visible light (λ > 460 nm) through the electron transfer process of MnO₂ → carbon clusters → ZrO₂ → Pt.     

Exceptional supercapacitive performance of bicontinuous carbon/MnO2 composite electrodes

A new type of porous carbon/MnO₂ composites, having bicontinuous structures, i.e., continuous channels and carbon skeletons, was prepared using a phase separation method, followed by a carbonization procedure and a subsequent redox reaction. In this work, such composite electrodes show a high specific capacitance of ca. 260?F?g^?1 at 0.5?A?g^?1 in 1?M Na₂SO₄ aqueous solution, a superior cycling stability (~80% retention after 2000 cycles) and a distinctive high-rate performance. Especially, unique bicontinuous structures endow such composites with a great specific capacitance of the constituent MnO₂ (~1100?F?g^?1), very close to the theoretical value. These excellent electrochemical behaviors may render this material a promising candidate as high-performance electrodes in supercapacitors. Therefore, our findings suggest that the strategy for constructing bicontinuous hybrid electrodes represents an exciting direction for designing next-generation supercapacitors.     

Textural and capacitive characteristics of MnO2 nanocrystals derived from a novel solid-reaction route

Nanostructured manganese dioxide (MnO₂) materials were synthesized via a novel room-temperature solid-reaction route starting with Mn(OAc)₂·4H₂O and (NH₄)₂C₂O₄·H₂O raw materials. In brief, the various MnO₂ materials were obtained by air-calcination (oxidation decomposition) of the MnC₂O₄ precursor at different temperatures followed by acid-treatment in 2 M H₂SO₄ solution. The influence of calcination temperature on the structural characteristics and capacitive properties in 1 M LiOH electrolyte of the MnO₂ materials were investigated by X-ray diffraction (XRD), infrared spectrum (IR), transmission electron microscope (TEM) and Brunauer-Emmett-Teller (BET) surface area analysis, cyclic voltammetry, ac impedance and galvanostatic charge/discharge electrochemical methods. Experimental results showed that calcination temperature has a significant influence on the textural and capacitive characteristics of the products. The MnO₂ material obtained at the calcination temperature of 300 °C followed by acid-treatment belongs to nano-scale column-like (or needle-like) γ,α-type MnO₂ mischcrystals. While, the MnO₂ materials obtained at the calcination temperatures of 400, 500, and 600 °C followed by acid-treatment, respectively, belong to γ-type MnO₂ with the morphology of aggregates of crystallites. The γ,α-MnO₂ derived from calcination temperature of 300 °C exhibited a initial specific capacitance lower than that of the γ-MnO₂ derived from the elevated temperatures, but presented a better high-rate charge/discharge cyclability.     

Ordered mesoporous Co3O4 nanospheres / reduced graphene oxide composites for rutin detection

The ordered mesoporous Co₃O₄ nanospheres encapsulated with reduced graphene oxide (denoted as meso-Co₃O₄ / RGO) were synthesized via electrostatic interaction and firstly for the electrochemical detection of rutin with good sensing effects. The resultant meso-Co₃O₄ / RGO nanocatalyst not only possesses more active sites due to the high surface area deriving from the mesoporous structure, but also has benign conductibility due to the presence of RGO, both of which enhance the sensing properties for the electrochemical detection of rutin. The developed sensor displays low detection limit (0.03?μM) and large sensitivity (74.85?μA?μM^?1 cm^?2). Besides, the rutin sensor possesses good selectivity, stability and reproducibility.     

Studies on MnO2—III. The kinetics and the mechanism for the catalytic decomposition of H2O2 over different crystalline modifications of MnO2

The catalytic decomposition of aqueous H₂O₂ solution over MnO₂ of various crystalline modifications has been studied in the temperature and the pH ranges of 20–35°C and 3–14 respectively. The active site in the pH range of 3–8 is considered to be Mn⁴⁺ and in the range 8–14 is Mn³⁺ ion. Catalytic activity attains maximum at pH of about 11.5–12.0 and then decreases with further increase in pH. The experimental rate data agrees satisfactorily with the proposed mechanism based on the above considerations. Catalytic activity has been correlated with some physico-chemical characteristics of the samples such as surface oxygen, surface OH groups, lattice parameters and in somewhat complex manner with electrochemical or discharge capacity of MnO₂. Although the discharge capacity increases with increase in catalytic activity for some samples (mostly γ-MnO₂) a number of samples belonging to α, β and δ-variety show sharp deviation from the above behaviour. This is explained from the state of occurrence of MnOOH in the host lattice.     

Silver-catalysed MnO2 as hydrogen absorber

Silver-catalyzed MnO₂ has been found to be a good hydrogen absorber. The hydrogen absorber can be prepared either by mixing battery grade MnO₂ and 1–10% Ag₂O or AgO powder or by mixing the MnO₂ and AgNO₃ solution. The hydrogen gas absorption is a reaction between MnO₂ and the H₂ gas. Performance and characteristics of the silver-catalysed hydrogen absorber are described in relation to the preparation conditions and the silver content.     

A novel kojic acid amperometric sensor based on hollow CuO/Fe2O3 hybrid microspheres immobilized in chitosan matrix

Hollow CuO/Fe₂O₃ hybrid microspheres with small uniform holes were synthesized using a convenient hydrothermal method and were applied to fabricated an amperometric sensor for kojic acid. The resulting materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) and then were immobilized into the chitosan (Chi) matrix onto a glassy carbon electrode to obtain CuO/Fe₂O₃–Chi/GCE. The potential utility of the constructed electrodes were demonstrated by applying them to the analytical determination of kojic acid concentration. The electrochemical behavior of kojic acid on CuO/Fe₂O₃–Chi/GCE was explored. The modified electrode displayed excellent amperometric response for kojic acid with a linear range from 0.2 μM to 674 μM with a detection limit of 0.08 μM at a signal-to-noise ratio of 3. In order to validate feasibility, the CuO/Fe₂O₃–Chi/GCE has been used for quantitative detecting kojic acid in real samples.     

Bismuth oxide cocatalyst and copper oxide sensitizer in Cu2O/TiO2/Bi2O3 ternary photocatalyst for efficient hydrogen production under solar light irradiation

A novel Cu₂O/TiO₂/Bi₂O₃ ternary nanocomposite was prepared, in which copper oxide improves the visible light absorption of TiO₂ and bismuth oxide improves electron–hole separation. The ternary composite exhibited extended absorption in the visible region, as determined by UV–Vis diffuse reflectance spectroscopy. High-resolution transmission electron microscopy images showed close contact among the individual semiconductor oxides in the ternary Cu₂O/TiO₂/Bi₂O₃ nanocomposite. Improved charge carrier separation and transport were observed in the Cu₂O/TiO₂/Bi₂O₃ ternary composite using electrochemical impedance spectroscopy and photocurrent analysis. TiO₂ modified with bismuth and copper oxides showed exceptional photocatalytic activity for hydrogen production under natural solar light. With optimum bismuth and copper oxide loadings, the Cu₂O/TiO₂/Bi₂O₃ ternary nanocomposite exhibited an H₂ production (3678 μmol/h) 35 times higher than that of bare TiO₂ (105?μmol/h). The synergistic effect of improved visible absorption and minimal recombination was responsible for the enhanced performance of the as-synthesized ternary nanocomposite.     

Development of nonenzymatic hydrogen peroxide sensor based on catalytic properties of copper nanoparticles/Rutin/MWCNTs/IL/Chit

A new electrochemical sensor based on copper nanoparticles for detection of hydrogen peroxide has been developed. Copper nanoparticles/Rutin/Multiwall Carbon Nanotubes/Ionic liquid/Chitosan modified glassy carbon electrode (CuNPs/Rutin/MWCNTs/IL/Chit/GCE) prepared by consecutive coating of MWCNTs/IL/Chit nanocomposite and rutin on the GCE, followed by the electrodeposition of copper. Surface physical characteristics of modified electrode were studied by scanning electron microscopy (SEM). The electrochemical performance of the sensor for detection of H₂O₂ was investigated by cyclic voltammetry and chronoamperometry techniques. The modified electrode exhibits an enhanced electrocatalytic property, low working potential, high sensitivity, excellent selectivity, good stability, and fast amperometric sensing towards reduction of hydrogen peroxide. The response to H₂O₂ is linear in the range between 0.35 μM to 2500 μM, and the detection limit is 0.11 μM.     

Studies on MnO2—II. Relationship between physicochemical properties and electrochemical activity of some synthetic MnO2 of different crystallographic forms

The electrochemical activity of a number of synthetic MnO₂ samples of various crystalline modifications has been determined by discharging a thin pellet containing 0.150 g MnO₂ and 0.015 g acetylene black in both NH₄ClZnCl₂ (pH 4.1) and 9M KOH medium. An attempt has been made to find correlations between depolarizing capacity (time to reach 0.6 V) and various physico-chemical properties; such as, density, surface area, lattice parameters, surface OH groups, surface oxygen, magnetic susceptibility etc. Whereas depolarizing capacity for γ-MnO₂ increases with increase in both true and packing (powder) density, capacity tends to decrease with increase in surface area. For other modifications of MnO₂ there exist direct relationships between depolarizing capacity and the various properties of the samples mentioned above. The relationships between lattice parameters and depolarizing capacity indicate that samples with an extended ab plane in general possess higher activity. It has been observed that surface OH groups and surface oxygen when normalised to unit weight are not only important properties in active MnO₂ but are also interrelated.     

Fabrication and characterization of MnO2 based composite sheets for development of flexible energy storage electrodes

Development of cost efficient, flexible and light weight paper electrodes for high-tech applications is high in demand in era of modern disposable technology. In this study α-MnO₂ nanorods were fabricated through hydrothermal method by varying growth time and further combined with lignocelluloses fibers extracted from self growing plant, Monochoria Vaginalis. Crystal structure, morphology and thermal properties of MnO₂ nanorods were characterized by X. Ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM) and Thermogravimetric Analysis (TGA), respectively. FESEM image analysis revealed the highest aspect ratio of 48.016 for 4?h treated MnO₂ sample and high purity level was confirmed by XRD. MnO₂ sample with high aspect ratio, relatively pure and larger yield was selected for incorporation of lignocelluloses fibers to fabricate flexible, light-weight and environmentally safe LC/MnO₂ composite paper sheet. Furthermore, LC/MnO₂ composite sheet was employed as working electrode in 2?M sodium sulfate electrolyte for cyclic voltammetry measurements. Presented LC/MnO₂ composite sheet revealed specific capacitances 117, 59, 39, 25 and 23?F/g at scan rates of 5, 10, 20, 50 and 100?mV/s, respectively. Incorporation of LC fibers within MnO₂ nanorods as binders will open the possibilities to fabricate the flexible paper electrode for application in supercapacitors and batteries due to facile synthesis, light-weight and environmentally friendly aspects.     

Fabrication of graphene/prussian blue composite nanosheets and their electrocatalytic reduction of H2O2

In this work, graphene/prussian blue (PB) composite nanosheets with good dispersibility in aqueous solutions have been synthesized by mixing ferric-(III) chloride and potassium ferricyanide in the presence of graphene under ambient conditions. Transmission electron microscopy (TEM) shows that the average size of the as-synthesized PB nanoparticles on the surface of graphene nanosheets is about 20 nm. Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) patterns have been used to characterize the chemical composition of the obtained graphene/PB composite nanosheets. The graphene/PB composite nanosheets exhibit good electrocatalytic behavior to detection of H₂O₂ at an applied potential of −0.05 V. The sensor shows a good linear dependence on H₂O₂ concentration in the range of 0.02-0.2 mM with a sensitivity of 196.6 μA mM⁻¹ cm⁻². The detection limit is 1.9 μM at the signal-to-noise ratio of 3. Furthermore, the graphene/PB modified electrode exhibits freedom of interference from other co-existing electroactive species. This work provides a new kind of composite modified electrode for amperometric biosensors.     

Effect of MnO2 addition on the structure and electric properties of (Ba0.94Ca0.06)(Ti0.9Sn0.1)O3 ceramics

The (Ba_0.94Ca_0.06)(Ti_0.9Sn_0.1)O₃ (BCTS) ceramics with pure perovskite structure were prepared by conventional solid-state reaction route with the addition of 0–0.8?mol% MnO₂. The crystal structure, microstructure, and electric properties were investigated systematically. X-ray diffraction patterns showed that the addition of MnO₂ changed the ratio of the coexistence of orthorhombic and tetragonal phases, which had apparent influences on the piezoelectric properties of ceramics. When the addition amount is 0.2?mol%, the average grain size increases from ~41.88–52.24?µm, and, however, the average grain size decreases with further addition > 0.2?mol%. A good combination of properties and performance could be achieved with the addition of 0.4?mol% MnO₂. The mechanical quality factor Q_m, dielectric loss tanδ, piezoelectric constant d₃₃, and planar electromechanical coefficient k_p measured are 216, 0.011, 578?pC/N, and 0.39, respectively. Therefore, results of this study suggest that the BCTS-Mn ceramics synthesized could exhibit a great potential for piezoelectric component applications.     

Effect of MnO2 addition on properties of NiFe2O4-based cermets

The impact of MnO₂ as an additive on the properties of NiFe₂O₄-based cermets prepared by the two-steps sintering method has been investigated. The new material was characterized in terms of the crystal structure, microstructure, linear shrinkage, relative density and porosity. Moreover, the bending strength of NiFe₂O₄-based cermets was measured. Differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD) shows that the addition of MnO₂ has no obvious influence on the crystal structure of the cermets. Scanning electron microscope (SEM) studies reveals that the grain sizes of cermets decreases slightly with doped MnO₂. The results show that the linear shrinkage, relative density and bending strength increase at first and then decrease slightly. A high-density (99.56%) and high-strength (84.28 MPa) NiFe₂O₄-based cermets has been obtained by adding 0.50 wt% MnO₂ into the matrix.