MnO2 Layer Research Articles

Modification of surfaces of silver nanoparticles for controlled deposition of silicon, manganese, and titanium dioxides

In this work we show that nanometric-thick layers of SiO2, MnO2, and TiO2 may be effectively deposited on various silver nanoparticles (including cubic Ag nanoparticles) covered by a very thin (below 0.4nm) layer of silver sulphide. The background in Raman measurements generated by sulphide-protected Ag nanoparticles is significantly smaller than that for analogous Ag nanoparticles protected by a monolayer formed from alkanethiols – depositing alkanethiols on a surface of anisotropic silver nanoparticles is the current standard method used for protecting a surface of Ag nanoparticles before depositing a layer of silica. Because of significantly smaller generated Raman background, Ag@SiO2 nanostructures with an Ag2S linkage layer between the silver core and the silica shell are very promising low-background electromagnetic nanoresonators for carrying out Raman analysis of various surfaces – especially using what is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Sample SHINERS analyses of various surfaces (including pesticide-contaminated surfaces of tomatoes) using cubic-Ag@SiO2 nanoparticles as electromagnetic nanoresonators are also presented.

Synthetic Control of Manganese Birnessite: Impact of Crystallite Size on Li, Na, and Mg Based Electrochemistry

Manganese oxide materials are desirable cathode materials due to their low cost and environmentally benign character. Specifically, 2-D layered birnessite (RyMnO2·xH2O, R = monovalent or divalent metal ions) materials are conceptually appealing because of their excellent ion-exchange properties. Edge-sharing MnO6 octahedra constitute the MnO2 layers where the interlayer spacing of 7 Å is occupied by metal ions and water molecules. Herein, we designed a study of a Mg cation containing birnessite with different crystallite sizes synthesized through a room temperature precipitation and ion exchange reaction, and characterized to determine structure and magnesium content. Electrochemical behavior was tested in lithium ion, sodium ion and magnesium ion based electrolytes, where the influence of crystallite size on electrochemical activity of Mg-birnessite was evaluated. Results and implications for future materials design will be discussed.

In Situ TEM Investigation of the Electrochemical Behavior in CNTs/MnO2-Based Energy Storage Devices.

Transition metal oxides have attracted much interest owing to their ability to provide high power density in lithium batteries; therefore, it is important to understand the electrochemical behavior and mechanism of lithiation-delithiation processes. In this study, we successfully and directly observed the structural evolution of CNTs/MnO2 during the lithiation process using transmission electron microscopy (TEM). CNTs/MnO2 were selected due to their high surface area and capacitance effect, and the lithiation mechanism of the CNT wall expansion was systematically analyzed. Interestingly, the wall spacings of CNTs/MnO2 and CNTs were obviously expanded by 10.92% and 2.59%, respectively. The MnO2 layer caused structural defects on the CNTs surface that could allow penetration of Li+ and Mn4+ through the tube wall and hence improve the ionic transportation speed. This study provided direct evidence for understanding the role of CNTs/MnO2 in the lithiation process used in lithium ion batteries and also offers potential benefits for applications and development of supercapacitors.

Enhanced Cycleability of Amorphous MnO₂ by Covering on α-MnO₂ Needles in an Electrochemical Capacitor.

An allomorph MnO2@MnO2 core-shell nanostructure was developed via a two-step aqueous reaction method. The data analysis of Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Diffraction and N2 adsorption-desorption isotherms experiments indicated that this unique architecture consisted of a porous layer of amorphous-MnO2 nano-sheets which were well grown onto the surface of α-MnO2 nano-needles. Cyclic voltammetry experiments revealed that the double-layer charging and Faradaic pseudo-capacity of the MnO2@MnO2 capacitor electrode contributed to a specific capacitance of 150.3 F·g−1 at a current density of 0.1 A·g−1. Long cycle life experiments on the as-prepared MnO2@MnO2 sample showed nearly a 99.3% retention after 5000 cycles at a current density of 2 A·g−1. This retention value was found to be significantly higher than those reported for amorphous MnO2-based capacitor electrodes. It was also found that the remarkable cycleability of the MnO2@MnO2 was due to the supporting role of α-MnO2 nano-needle core and the outer amorphous MnO2 layer.

Redox properties of birnessite from a defect perspective

Birnessite, a layered-structure MnO2, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.

Magnetic Effects on the Dielectric and Polarization Properties in BiAlO3/La0.67Sr0.33MnO3Heterostructure

BiAlO3/La0.67Sr0.33MnO3 heterostructure was fabricated on LaAlO3 (111) substrate by pulsed laser deposition technology. A remarkable magneto-resistivity effect was detected at H = 1 T near the metal–insulator phase transition temperature (TMI) of La0.67Sr0.33MnO3. The magneto-dielectric constant shows an anomaly near TMI of La0.67Sr0.33MnO3, whereas the ferroelectric polarization increases under magnetic field and its variation shows the similar anomaly near TMI of La0.67Sr0.33MnO3, which could be assigned to the important role of the phase transformation of La0.67Sr0.33MnO3. The magnetic moment of the BiAlO3/La0.67Sr0.33MnO3 heterostructure distinctly arises compared to the single La0.67Sr0.33MnO3 layer, probably on account of the spin–orbit coupling effect at the interface, and the relative magnetization variation also shows an anomaly near TMI of La0.67Sr0.33MnO3.

Manipulating magnetoelectric properties by interfacial coupling in La0.3Sr0.7MnO3/Ba0.7Sr0.3TiO3 superlattices

Artificial superlattices constructed with ferromagnetic La0.7Sr0.3MnO3 layer and ferroelectric Ba0.7Sr0.3TiO3 layer were designed and fabricated on SrTiO3 substrates. An epitaxial growth with sharp interfaces between La0.7Sr0.3MnO3 and Ba0.7Sr0.3TiO3 layers was confirmed by scanning transmission electron microscopy and x-ray diffraction. An unambiguous charge transfer involving an electron transferring from the La0.7Sr0.3MnO3 layers to Ba0.7Sr0.3TiO3 layers (Mn3+→Mn4+; Ti4+→Ti3+) across the interface were resolved by electron energy loss spectra analysis. These observations are attributed to the possible modification in the stereochemistry of the Ti and Mn ions in the interfacial region. The out-of-plane lattice parameter, Curie temperature, and magnetoresistance are strongly affected by the thicknesses of the La0.7Sr0.3MnO3 and Ba0.7Sr0.3TiO3 layers. Huge magnetoresistance subsisting to low temperature was also observed in the La0.7Sr0.3MnO3/Ba0.7Sr0.3TiO3 superlattices. All spectral changes identified at a nanometer scale and their potential effect on the degradation of magnetic and transport properties at a macroscopic level. These findings highlight the importance of dependence on sublayer thickness, illustrating the high degree of tenability in these artificially low-dimensional oxide materials.

Electric field modulated conduction mechanism in Al/BaTiO3/La0.67Sr0.33MnO3 heterostructures

Mediating a metastable state is a promising way to achieve a giant modulation of physical properties in artificial heterostructures. A metastable state La0.67Sr0.33MnO3 (LSMO) layer suffering tensile strain was grown on MgO substrates. Incorporating with the ferroelectric BaTiO3 (BTO) layer, an accumulation or depletion state controlled by electric fields can be formed at the BTO/LSMO interface, which drives a switching of the conduction mechanism between space charge limited conduction and Poole-Frenkel emission, corresponding to the low and high resistance states. Our results lighten an effective way for electric-field modulated resistance states in multiferroic magnetoelectric devices.

A key role of tensile strain and surface termination in formation and properties of La0.7Sr0.3MnO3 composites with carbon nanotubes

Atomic and electronic structure of LSMO-based composites with carbon nanotubes were studied by means of density functional theory with respect to the termination of LSMO surface. The deformation of the tubes caused by the lattice mismatch with the substrate leads to a major change in their electronic structure. The surface terminated with Mn-O layer provides much stronger interaction with carbon nanotubes than Sr-O terminated one does. The interaction with transition metal atoms is essential for spin polarization of the nanotube while no spin injection was observed for Sr-O-supported tubes.

Two-Dimensional Planar Supercapacitor Based on Zinc Oxide/Manganese Oxide Core/Shell Nano-architecture

We report the fabrication and performance of zinc oxide core/manganese oxide shell (ZnO/MnO2) three-dimensional nanostructured planar supercapacitor. In this particular fabrication, ZnO nanorods were grown over FTO substrate followed by the successive ionic layer adsorption and reaction (SILAR) deposition of MnO2 nanostructure on ZnO nanorods. The quality of the ZnO/MnO2 electrode was optimized by performing various cycles of SILAR deposition of MnO2 layer over ZnO core. The electrode with 7 cycles of MnO2 deposition demonstrated maximum electrochemical performance with area capacitance of 14mFcm−2 and the same condition was utilized for the fabrication of a symmetric planar supercapacitor. This ZnO/MnO2 3D architecture planar supercapacitor showed maximum area capacitance of 5mFcm−2 in 1M Na2SO4 aqueous electrolyte with good energy density of 4.07×10−7Whcm−2 and power density of 3.19mWcm−2. Moreover, this planar supercapacitor shows an excellent electrochemical performance with comparable long term cyclic stability.

Surface modification effects on coercivity of the CoFe2O4 thin films with different thickness La0.7Sr0.3MnO3 layers

High coercivity CoFe2O4 (CFO) thin films with large-area are very desirable for applications. Here, we report La0.7Sr0.3MnO3 (LSMO) layer decoration effects on coercivity in CFO thin films by solution processing. With LSMO layer decoration, the coercivity increases initially and then decreases with increasing LSMO layer thickness. Strain induced by the mismatched thermal expansion coefficient and magnetostrictive coefficient as well as the interfacial magnetic exchange coupling between CFO and LSMO are investigated. The room temperature out-of-plane and in-plane coercivity reached 7.6 and 4.9 kOe, respectively, are obtained, which are the highest values in solution derived CFO thin films. The improved coercivity is beneficial for applications and provides a facile route to enhance coercivity of CFO thin films through surface modification by soft magnetic layers.

Surface Selective Oxide Reduction During the Intercritical Annealing of Medium Mn Steel

Third generation advanced high-strength steels achieve an excellent strength–ductility balance using a cost-effective alloy composition. During the continuous annealing of medium Mn steel, the formation of an external selective oxide layer of MnO has a negative impact on the coating quality after galvanizing. A procedure to reduce the selective oxide was therefore developed. It involves annealing in the temperature range of 1073 K to 1323 K (800 °C to 1050 °C) in a HNx gas atmosphere. Annealing at higher temperatures and the use of larger H2 volume fractions are shown to make the gas atmosphere reducing with respect to MnO. The reduction of the surface MnO layer was observed by SEM, GDOES, and cross-sectional TEM analysis.

Time-efficient synthesis of MnO2 encapsulated α-Fe2O3 ellipsoids for lithium ion battery applications

Herein, Fe2O3 was enlisted as an environmentally benign and inexpensive anode active material. A facile time-efficient synthesis strategy was developed to prepare hematite Fe2O3 ellipsoid particles via hydrothermal method. The structural and morphological properties of the prepared materials were characterized using XRD, FE-SEM, HR-Raman, and HR-TEM. The electrochemical properties were investigated using galvanostatic cycling, cyclic voltammetry, and impedance spectrum analysis. To further overcome the drawbacks of α-Fe2O3 ellipsoids, a MnO2 coating was applied. The encapsulated α-Fe2O3 ellipsoids exhibited improved electrochemical properties, such as a high specific capacity, high coulombic efficiency, stable cyclic performance, and a high rate capability. The improved electrochemical performance of the ellipsoids could be attributed to the uniform and continuous MnO2 layer coatings, yielding an enormous advantage through the sustained integrity of nanoparticles, stimulating the electronic conductivity of electrode materials leading to the formation of a thin SEI (Solid Electrolyte Interphase) layer on the electrode surface, and by maintaining the as-formed layer. The obtained results demonstrated MnO2 encapsulated α-Fe2O3 ellipsoid particles as a superior anode material for lithium ion batteries compared to bare α-Fe2O3 and commercially available graphite electrodes.

Electrosynthesis of Layered Organo-Manganese Dioxide Framework-Doped with Cobalt for Iodide Sensing.

Aqueous Mn2+ ions were anodized at 70 °C with Co2+ in the presence of cationic surfactant, cetyltrimethylammonium (CTA). X-ray diffraction (XRD) analysis revealed that the deposited film possesses a layered structure of MnO2, the interlayer of which is occupied with the assembled CTA molecules. Inclusion of Co ions in the MnO2 film was evidenced by X-ray photoelectron spectroscopy (XPS). They were located in the MnO2 framework, not in the interlayer. The thus-obtained film, CTA-intercalated Co-framework-doped layered MnO2 (CTA/Co-MnO2), was applied as an electrochemical sensor toward iodide (I-), a hydrophobic anion. The organic phase between MnO2 layers could extract I- ions from solution, providing a better sensitivity than a film consisting of layered MnO2 with hydrated alkali metals. On the other hand, the Co-doped layers of MnO2 achieved faster electron transfer kinetics for the oxidation of I-, which resulted in a drastic reduction in response time compared to the nondoped CTA/MnO2.

Electrochemical capacitor performance of TiO2 nanostructures and porous MnO2 composite supported on carbon fiber paper

Abstract Titania-based nanostructures and porous MnO2 layer are deposited onto carbon fiber paper (CFP) conductive support using hydrothermal method. TiO2 nanoparticles and leaf-like Na2Ti3O7 nanosheets are synthesized through the hydrolysis of TiF4 and post-treatment in NaOH aqueous solution. Electrochemical tests reveal that CFP@TiO2@MnO2 has a better performance with areal specific capacitance of 744 mF cm−2 at 0.25 mA cm−2, compared with that (600 mF cm−2) of CFP@Na2Ti3O7@MnO2. The possible mechanism is that TiO2 has higher surface area and better electronic conductivity than those of Na2Ti3O7. However, the cycling performance of CFP@TiO2@MnO2 is limited with capacitance retention of 17.7% after 1000 cycles while CFP@Na2Ti3O7@MnO2 keeps this retention up to 59.9%.

Fabrication of porous polyaniline modified MWNTs core-shell structure for high performance supercapacitors with high rate capability

Novel porous polyaniline (PANI) modified MWNTs core-shell structure materials (porous CNTs-PANI) were fabricated by in situ polymerization using the “worm-like” MnO2 modified MWNTs (CNTs-MnO2) as the reactive template. The MnO2 layer on surface of MWNTs can act as the reactive template in the polymerization process. Meanwhile, the reactive etching of the MnO2 can form porous structure of PANI for fabrication of porous CNTs-PANI core-shell structure, which exhibits outstanding electrochemical performance among the CNTs-PANI and pure PANI materials. The porous CNTs-PANI materials possess an excellent specific capacitance of 406.5F/g at 0.5A/g, exhibit good rate capability of 84.7% even up to 50A/g, and keep 72% of its original value up to 1000 charge-discharge cycles at the current density of 10A/g, also maintain 55% capacitance retention at 2200 charge-discharge cycles. Furthermore, the porous CNTs-PANI materials exhibit an extremely high energy density of 55.6Wh/kg at the power density of 0.25kW/kg. The novel porous CNTs-PANI core-shell structure materials reveal promising applications in high performance supercapacitors.

Hydrothermal vs electrodeposition: How does deposition method affect the electrochemical capacitor performance of manganese dioxide?

Abstract Hydrothermal and electrodeposition are widely utilized methods in the preparation of MnO 2 nanomaterials for electrochemical capacitor application. In this paper, these two approaches were applied to synthesize nanoscale MnO 2, and their electrochemical properties were fully investigated in different electrolyte environments. Because of different mechanisms, the hydrothermal method gave an ultrathin layer of MnO 2 nanosheets with a large accessible electrochemical surface area, while the electrodeposition method resulted in the formation of MnO 2 nanorods with a rather limited opening area. The electrochemical performance of hydrothermally prepared MnO 2 was much better in specific capacitance and overall resistance, except the cycling durability, as a result of its unique structure.

Tetramethyl ammonium cation intercalated layered birnessite manganese dioxide for high-performance intercalation pseudocapacitor

Abstract We develop an effective strategy to prepare intercalation pseudocapacitor by enlarging interlayer spacing of layered birnessite MnO2 through a controllable intercalation of K+, H+, TMA+/H+, TMA+ cations. The interlayer spacing of K-birnessite is swollen from ∼0.52 nm to 0.77 nm of TMA-birnessite. TMA+/H+ = 1000 sample exhibits an impressively high specific capacitance of 580.05 F g−1, much larger than that of K-birnessite, H-birnessite, TMA+/H+ = 800 (352.8, 414.9, 497.25 F g−1) at current density of 2 A g−1, showing excellent cycling performance with 112% capacitance retention after 10000 cycles. Even upon 20 A g−1, the capacity of TMA+/H+ = 1000 still remains a high level of 503 F g−1, while K-birnessite, H-birnessite, TMA+/H+ = 800 undergo a severe degradation to 180, 216, 433 F g−1. The expanded interlayer spacing enlarges the ion diffusion tunnels, weakens the layer interaction and decreases the resistance of charge diffusion, which easily enables the sufficient utilization of the active material through rapid ion intercalation. More importantly, the molecular-level thickness of host MnO2 layers that expose almost all the active sites on both sides of diffusion tunnels, thus greatly enhances the electrochemical storage capacitance. The results prove the feasibility of this method to prepare new intercalation pseudocapacitor and provide hints for other layered materials.

Development of flexible supercapacitors using an inexpensive graphene/PEDOT/MnO2 sponge composite

It remains a formidable challenge to develop supercapacitors having both high stretchability and electrochemical performance. A ternary polymer composite was herein developed by engineering graphene platelets (GnPs), poly (3,4-ethylenedioxythiophene) (PEDOT) and manganese dioxide (MnO2) into a kitchen sponge. Cost-effective GnPs having an electrical conductivity of 1460Scm−1 were deposited onto the sponge pore surface by a simple dipping and drying process, followed by in situ polymerization of 3,4-ethylenedioxythiophene. MnO2 was then deposited onto the GnPs and PEDOT layers through soaking the GnP/PEDOT sponge into KMnO4 solution. When the composite was used as the electrodes for a flexible, stretchable supercapacitor, we obtained not only high specific capacitance but a high stretchability up to 400%. The supercapacitor demonstrated a specific capacitance of 802.99Fg−1 and an energy density of 55.76Whkg−1, with 99% capacitance retention over 1000 stretching cycles; these are attributed to the synergy between the composite components as well as the wavy device structure. The mass and thickness of the MnO2 layer was found to increase with the soak, leading to the enhancement of electrochemical performance of GnP/PEDOT/MnO2 sponge electrodes, i.e. 368.96Fg−1 for 5min and 740.25Fg−1 for 10min.

Composite materials based on oxides of d and f elements and carbon layers

Nanocrystalline powder (~20 nm) with the composition of (ZrO2)0.6(In2O3)0.04 is synthesized on the basis of a co-precipitation method. After its consolidation, a dense and porous ceramic matrix for supercapacitor electrodes is obtained. The deposition conditions are determined for thin nanocarbon and MnO2, Co3O4 layers on the porous ceramic or metal matrix. It is shown that the model supercapacitor with composite electrodes based on nickel foam and thin layers of nanocarbon and MnO2 has the highest average specific capacitance.