Facile controlled synthesis and growth mechanisms of flower-like and tubular MnO2 nanostructures by microwave-assisted hydrothermal method

High-performance microwave absorption by optimizing hydrothermal synthesis of BaFe12O19@MnO2 core-shell composites

Stealth technology advances in radar-absorbing materials (RAMs) continue to grow rapidly. Barium hexaferrite is the best candidate for RAMs applications. Manganese dioxide (MnO2) is a transition metal with high dielectric loss and can be used as a booster for changing polarization and reducing reflection loss. The advantages of BaFe12O19 and MnO2 can be combined in a core-shell BaFe12O19@MnO2 composite to improve the material's performance. MnO2 composition, temperature, hydrothermal holding time, and sample thickness all have an impact on the core-shell structure. In this study, a core-shell BaFe12O19@MnO2 composite is synthesized in two stages: molten salt synthesis to produce BaFe12O19 as the core and hydrothermal synthesis to synthesize MnO2 as the shell. In the hydrothermal synthesis, BaFe12O19 and KMnO4 were mixed in deionized water using different mass ratios of BaFe12O19 to KMnO4 (1 : 0.25, 1 : 0.5, 1 : 0.75, and 1 : 1). The main goal of the analysis was to figure out how well the hydrothermal synthesis method worked at different temperatures (140 °C, 160 °C, and 180 °C) and holding times (9 h, 12 h, and 15 h). The composite material was subjected to characterization using a vector network analyzer, specifically at thicknesses of 1.5 mm, 2 mm, 2.5 mm, and 3 mm. The hydrothermal temperature and composition ratio of BaFe12O19 : MnO2 are the most influential parameters in reducing reflection loss. Accurate control of the parameters makes a BaFe12O19@MnO2 core-shell composite structure with a lot of sheets. The structure is capable of absorbing 99.99% of electromagnetic waves up to a sample thickness of 1.5 mm. The novelty of this study is its ability to achieve maximal absorptions on a sample with minimal thickness through precise parametric control. This characteristic makes it highly suitable for practical applications, such as performing as an anti-radar coating material. BaFe12O19@MnO2 demonstrates performance as a reliable electromagnetic wave absorber material with simple fabrication, producing absorption at C and X band frequencies.

Ultralight Hierarchically Structured RGO Composite Aerogels Embedded with MnO2/Ti3C2Tx for Efficient Microwave Absorption

Although intensive efforts have been devoted to fabricating Ti₃C₂T_x MXene composites for microwave absorption, it remains a great challenge to achieve excellent MA performance at low loading and thin thickness. Herein, a three-dimensional (3D) lightweight hierarchically structured MnO₂/Ti₃C₂T_x/RGO composite aerogel with abundant heterointerfaces was fabricated via a hydrothermal and chemical reduction self-assembly method. The RGO aerogel embedded with laminated MnO₂/Ti₃C₂T_x provides a lot of heterogeneous interfaces, 3D porous interconnected conductive networks, and reasonable combination of various loss materials for rich interfacial polarization, conductivity loss, multiple reflections and scattering, and good impedance matching. Benefiting from the synergy of different loss mechanisms, the maximum reflection loss (RL) is up to -66.5 dB (>99.9999% energy absorption) at only 10 wt % loading and 2.0 mm thickness, and even at only 1.5 mm thickness, the maximum RL value remains at -36 dB (>99.9% energy absorption). The work provides a promising route to construct 3D hierarchically heterogeneous composite aerogels for efficient MA at thin thickness and low loading.

α-MnO2 Nanowire Structure Obtained at Low Temperature with Aspects in Environmental Remediation and Sustainable Energy Applications

Hydrothermally obtained α-MnO2 nanowire characterizations confirm the tetragonal crystalline structure that is several micrometers long and 20–30 nm in diameter with narrow distributions in their dimensions. The absorption calculated from diffuse reflectance of α-MnO2 occurred in the visible region ranging from 400 to 550 nm. The calculated band gap with Quantum Espresso using HSE approximation is ~2.4 eV for the ferromagnetic case, with a slightly larger gap of 2.7 eV for the antiferromagnetic case, which is blue-shifted as compared to the experimental. The current work also illustrates the transformations that occur in the material under heat treatment during TGA analysis, with the underlying mechanism. Electrochemical studies on graphite supports modified with α-MnO2 compositions revealed the modified electrode with the highest electric double-layer capacitance of 3.444 mF cm−2. The degradation rate of an organic dye—rhodamine B (RhB)—over the compound in an acidic medium was used to examine the catalytic and photocatalytic activities of α-MnO2. The peak shape changes in the time-dependent visible spectra of RhB during the photocatalytic reaction were more complex and progressive. In two hours, RhB degradation reached 97% under sun irradiation and 74% in the dark.

Tuning the structural stability and spin-glass behavior in α-MnO2 nanotubes by Sn ion doping

A series of α-Mn_1-xSn_xO₂ was synthesized by a simple hydrothermal method to shed light on the effect of substitution. Powder X-ray diffraction and scanning electron microscopy indicated that the particle size, crystal structure and morphology of the samples did not change with an increase of the Sn content. Sn, Mn, O and K elements were all uniformly distributed in the particles, which was observed using energy-dispersive X-ray spectroscopy. However, thermogravimetric analysis showed that the structural stability increased, and an increase of the Mn oxidation state from 3.8+ to nearly 4.0+ was observed by X-ray absorption spectroscopy. Besides, ¹¹⁹Sn Mössbauer spectroscopy revealed that the Sn ions are all 4+ and incorporate into the lattice by replacing the Mn ions. The DC and AC magnetic susceptibility measurements down to 2 K exhibited a spin-glass phenomenon, and the freezing temperature, T_f, decreased from 44 K to 30.5 K with increasing Sn content. This indicates that increased disorder by nonmagnetic substitution results in the enhancement of the frustration in the lattice. Meanwhile, with doping of Sn⁴⁺ ions, the Curie-Weiss temperature increased, indicating enhanced antiferromagnetic interaction. Although the mixed valence of Mn³⁺ and Mn⁴⁺ almost disappeared, the reduction of charge disorder did not lead to the magnetic ordering in the sample. Since the Sn⁴⁺ ions are diamagnetic and have the same magnetic effect as cation vacancies in the lattice, so it is reasonable to believe that the spin-glass transition in α-MnO₂ results from the cation vacancies rather than the mixture of Mn³⁺ and Mn⁴⁺.

Long cyclic stability of acidic aqueous zinc-ion batteries achieved by atomic layer deposition: the effect of the induced orientation growth of the Zn anode

Aqueous Zn-ion batteries with economical ZnSO₄ solution as the electrolyte suffer from a tremendous tendency of dendrite formation under mildly acidic conditions; moreover, utilization of Zn(CF₃SO₃)₂ delivers superior performance, but is expensive. Herein, we optimize the ZnSO₄ electrolyte by inducing 50 μL of 10 M sulfuric acid in 10 mL electrolyte, which can achieve long cycle life (1000 h at 0.1 mA cm^-2, 300 h at 1 mA cm^-2 and 250 h at 10 mA cm^-2) when the Zn foil is protected by three metallic oxides deposited by atomic layer deposition (ALD). The nucleation behaviour of the (002) facet has proved to play a critical role in the reversible lifespan. The Al₂O₃ layer would restrict the stripping procedure, leading to the highest overpotential, while the TiO₂ layer and Fe₂O₃ layer tended to strip all orientations but the (002) facet. Al₂O₃@Zn demonstrated a preference for a compact hillock-like (101) orientation texture in the deposition procedure, while TiO₂@Zn and Fe₂O₃@Zn were favourable to obtain a smooth terrace texture. Additionally, symmetric cells with Fe₂O₃@Zn expressed the lowest overpotential (31.64 mV) and minimal voltage hysteresis (23.6 mV) at 1 mA cm^-2. A Zn-MnO₂ battery with Fe₂O₃@Zn also displayed superior capacity, which could reach 280 mA h g^-1 at a current density of 1 A g^-1. The diffusion coefficient of Zn²⁺ discloses that among the three ALD layers, full cells with Fe₂O₃@Zn are the most favourable for diffusion of Zn²⁺ in acidic electrolyte.

Synthesis of Carbon-Supported MnO2 Nanocomposites for Supercapacitors Application

In this study, carbon-supported MnO2 nanocomposites have been prepared using the microwave-assisted heating method followed by two different approaches. The MnO2/C nanocomposite, labeled as sample S1, was prepared directly by the microwave-assisted synthesis of mixed KMnO4 and carbon powder components. Meanwhile, the other MnO2/C nanocomposite sample labeled as S2 was prepared indirectly via a two-step procedure that involves the microwave-assisted synthesis of mixed KMnO4 and MnSO4 components to generate MnO2 and subsequent secondary microwave heating of synthesized MnO2 species coupled with graphite powder. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma optical emission spectroscopy have been used for characterization of MnO2/C nanocomposites morphology, structure, and composition. The electrochemical performance of nanocomposites has been investigated using cyclic voltammetry and galvanostatic charge/discharge measurements in a 1 M Na2SO4 solution. The MnO2/C nanocomposite, prepared indirectly via a two-step procedure, displays substantially enhanced electrochemical characteristics. The high specific capacitance of 980.7 F g−1 has been achieved from cyclic voltammetry measurements, whereas specific capacitance of 949.3 F g−1 at 1 A g−1 has been obtained from galvanostatic charge/discharge test for sample S2. In addition, the specific capacitance retention was 93% after 100 cycles at 20 A g−1, indicating good electrochemical stability.

Structural and morphological tuning of iron oxide polymorphs by ECR plasma-assisted thermal oxidation

The work presented involves the generation of oxygen plasma species at low pressure utilizing an Electron Cyclotron Resonance (ECR) plasma reactor, and their interactions with micron- and nano-sized iron films (M-Fe and N-Fe film respectively) prepared using ethyl cellulose processed at high temperature. A specially designed radiation heater (RH) was used to raise the surface temperature of the film rapidly, exactly at the film interface, where the plasma species interact with the surface. As a result of the interaction of oxygen plasma species and temperature, iron is oxidized to different polymorphs depending on the operating pressure and hence oxygen gas flow rate. The phase, as well as the morphology of the film was controlled by monitoring the oxygen flow rate using the unique Plasma-Assisted Thermal Oxidation (PATO) process. Different polymorphs, viz., Fe3O4, γ-Fe2O3, α-Fe2O3 and different morphologies, such as polygonal, compact facets, wire-like (1D) nanostructures at the surface were obtained for the films processed using PATO. The selected PATO-processed films were investigated for Field Electron Emission (FEE) properties. The 1D-grown surface of iron oxide obtained from the M-Fe film showed a turn-on field of 3 MV m-1 and emission current of 337 μA cm-2, whereas the pyramidal surface morphology obtained using N-Fe film gives a turn-on field of 3.3 MV m-1 with an emission current of 578 μA cm-2.

Shape-controllable synthesis of MnO2 nanostructures from manganese-contained wastewater for phenol degradation by activating peroxymonosulphate: performance and mechanism

Nanostructured manganese oxide materials were prepared from manganese-contained wastewater (MW) using a facile hydrothermal method and adopted as a catalyst to degrade phenol via activation of peroxymonosulphate (PMS). In the WM environment, δ-MnO₂ (flower-like Mn-2 with nanosheets) was transformed to α-MnO₂ (needle-like Mn-4 with nanowires). Catalytic evaluation experiments demonstrated that the needle-like MnO₂ was highly efficient for phenol removal, with a degradation efficiency of 100% within 15 min at the optimal conditions of catalyst dosage 0.2 g/L, PMS dosage 1.5 g/L, initial phenol concentration 0.025 g/L, initial pH 3 and temperature 25°C. Moreover, the needle-like MnO₂ catalyst could be recycled and the regenerated material after calcination remained excellent catalytic activity. On the surface of catalysts, PMS was activated by Mn^IV to generate [Formula: see text] which was the major reactive species attacking phenol. Overall, the needle-like MnO₂ prepared from MW was an efficient catalyst with low cost for organic wastewater treatment, realizing both Mn resource recycle and organic wastewater treatment.

Recent trends of MnO2-derived adsorbents for water treatment: a review

Over the years, manganese dioxide (MnO₂) and its different allotropes have gained significant research attention in the field of wastewater treatment because of their exciting physicochemical properties.

Microwave-assisted hydrothermal synthesis of manganate nanoflowers for selective retention of strontium

An alternative microwave-assisted hydrothermal route for the preparation of manganate nanoflowers under basic conditions has been proposed in view of potential uses in selective retention of strontium from multicomponent aqueous streams. Based on the combination of such characterization techniques as Scanning and Transmission Electronic Microscopy, X-ray photoelectron spectroscopy, and X-ray Diffraction, as well as taking advantage of the computer-aided structure simulation, homogeneous nanoflower morphology possessing a layered structure and K⁺ compensating cations was evidenced as corresponding to the KMn₄O₈ chemical formula. The nanoflower sample was subsequently tested for the selective adsorption of strontium and cesium by measuring the individual adsorption isotherms from single-solute and multicomponent aqueous solutions. The material appeared selective towards strontium against cesium even in multicomponent solutions provided that the concentration of calcium remained low. This difference in the retention selectivity was rationalized based on the Density Functional Theory (DFT) calculations of the energy of adsorption and direct calorimetry measurements of the enthalpy of displacement for the individual cations.

Conventional and Microwave Hydrothermal Synthesis and Application of Functional Materials: A Review

With the continuous development and progress of materials science, increasingly more attention has been paid to the new technology of powder synthesis and material preparation. The hydrothermal method is a promising liquid phase preparation technology that has developed rapidly during recent years. It is widely used in many fields, such as the piezoelectric, ferroelectric, ceramic powder, and oxide film fields. The hydrothermal method has resulted in many new methods during the long-term research process, such as adding other force fields to the hydrothermal condition reaction system. These force fields mainly include direct current, electric, magnetic (autoclaves composed of non-ferroelectric materials), and microwave fields. Among them, the microwave hydrothermal method, as an extension of the hydrothermal reaction, cleverly uses the microwave temperature to compensate for the lack of temperature in the hydrothermal method, allowing better practical application. This paper reviews the development of the hydrothermal and microwave hydrothermal methods, introduces their reaction mechanisms, and focuses on the practical application of the two methods.

A Styrofoam-nano manganese oxide based composite: Preparation and application for the treatment of wastewater

Nano-composites were synthesized by the reaction of waste polystyrene (PS) and KMnO₄. The structure of the composite was controlled by the solvent/non-solvent system and the concentration of KMnO₄. The FTIR spectra indicated the functionalization of PS and the attachment of NMO with the polymer chains. The maximum adsorption capacities (q_max) were 10,000 and 5000 Bq g^-1, for U and Th respectively. Different but controllable sorption/desorption behaviours were noted between Th and U, which could be promising in the separation of Th and U from their mixture.

rGO/MnO2 nanowires for ultrasonic-combined Fenton assisted efficient degradation of Reactive Black 5

Reduced graphene oxide (rGO) coated manganese dioxide (MnO₂) nanowires (NWs) were prepared by the hydrothermal method. Raman spectra confirmed the presence of rGO and the Brunauer-Emmett-Teller surface area of rGO/MnO₂ NWs was found to be 59.1 m²g^-1. The physico-chemical properties of prepared catalysts for the degradation of Reactive Black 5 (RB5) dye were investigated. 84% of RB5 dye in hydrogen peroxide solution was successfully degraded using rGO/MnO₂ NWs, while only 63% was successfully degraded with pristine α-MnO₂ NWs in 60 min owing to the smaller crystallite size and large surface area. Further, the ultrasonic-combined Fenton process significantly enhanced the degradation rate to 95% of RB5 by the catalyst rGO/MnO₂ NWs due to synergistic effects. The decomposition products identified using gas chromatography-mass spectrometry revealed a higher production rate of fragments in the ultrasonic-combined Fenton process. Therefore, rGO/MnO₂ NWs with the ultrasonic-combined Fenton process is an efficient catalyst for the degradation of RB5, and may be used for environmental protection.

Facile synthesis of hierarchical mesoporous weirds-like morphological MnO2 thin films on carbon cloth for high performance supercapacitor application

The mesoporous nanostructured metal oxides have a lot of capabilities to upsurge the energy storing capacity of the supercapacitor. In present work, different nanostructured morphologies of MnO₂ have been successfully fabricated on flexible carbon cloth by simple but capable hydrothermal method at different deposition temperatures. The deposition temperature has strong influence on reaction kinetics, which subsequently alters the morphology and electrochemical performance. Among different nanostructured MnO₂ thin films, the mesoporous weirds composed thin film obtained at temperature of 453K exhibits excellent physical and electrochemical features for supercapacitor application. The weirds composed MnO₂ thin film exhibits specific surface area of 109m²g^-1, high specific capacitance of 595Fg^-1 with areal capacitance of 4.16Fcm^-2 at a scan rate of 5mVs^-1 and high specific energy of 56.32Whkg^-1. In addition to this, MnO₂ weirds attain capacity retention of 87 % over 2000 CV cycles, representing better cycling stability. The enhanced electrochemical performance could be ascribed to direct growth of highly porous MnO₂ weirds on carbon cloth which provide more pathways for easy diffusion of electrolyte into the interior of electroactive material. The as-fabricated electrode with improved performance could be ascribed as a potential electrode material for energy storage devices.

High temperature ferroelectric behaviour in α-MnO2 nanorods realised through enriched oxygen vacancy induced non-stoichiometry

Here, we observe high temperature ferroelectric behaviour in non-stoichiometric α-MnO₂ nanorods due to combined effects of charge and orbital ordering.

Fabrication of Ag nanoparticles supported on one-dimensional (1D) Mn3O4 spinel nanorods for selective oxidation of cyclohexane at room temperature

Silver nanoparticles supported on spinel Mn₃O₄ nanorods efficiently catalyze cyclohexane to cyclohexanone with high yield at room-temperature.

Enhanced photoelectrochemical performance of novel p-type MoBiCuSe4 thin films deposited by a simple surfactant-mediated solution route

Nanostructured MoBiCuSe₄ thin films with different morphologies deposited using different surfactants, such as PEG, SDS and TOPO, through a surfactant-mediated modified chemical route for PEC application.

Microwave-assisted synthesis of porous Mn2O3 nanoballs as bifunctional electrocatalyst for oxygen reduction and evolution reaction

A facile and effective microwave-assisted route has been developed to synthesize electrochemically active pure and transition metal-doped manganese oxide nanoballs for fuel cell applications.

Controlled synthesis and morphology dependent luminescence of Lu2O2S:Eu3+ phosphors

The dependence of Lu₂O₂S:Eu³⁺ luminescence performance on different morphologies has been explored and possible reasons are proposed.

Bi-template assisted synthesis of mesoporous manganese oxide nanostructures: Tuning properties for efficient CO oxidation

Bi-template assisted synthesis of mesoporous manganese oxide catalysts with tuned morphology, crystal structure, and textural properties for use in efficient catalysis of CO oxidation.

Template-Free Synthesis of Ruthenium Oxide Nanotubes for High-Performance Electrochemical Capacitors

One-dimensional, hydrous ruthenium oxide nanotubes (RuO2·1.84H2O) have been successfully achieved using a template-free, microwave-hydrothermal process. These were found to be amorphous in nature and have a large specific surface area of 250 m(2)·g(-1), producing a specific and volumetric capacitance of 511 F·g(-1) and 531 F·cm(-3), respectively, at a discharging current density of 0.5 A·g(-1). When used as an electrode material in an electrochemical capacitor or ultracapacitor, they produced a significant improvement in capacitance, rate capability, and cyclability that can be attributed to the hollow nature of tubes allowing greater contact between the active surface of the electrode and the electrolyte.

Merging of Kirkendall growth and Ostwald ripening: CuO@MnO2 core-shell architectures for asymmetric supercapacitors

Fabricating hierarchical core-shell nanostructures is currently the subject of intensive research in the electrochemical field owing to the hopes it raises for making efficient electrodes for high-performance supercapacitors. Here, we develop a simple and cost-effective approach to prepare CuO@MnO₂ core-shell nanostructures without any surfactants and report their applications as electrodes for supercapacitors. An asymmetric supercapacitor with CuO@MnO₂ core-shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode yields an energy density of 22.1 Wh kg⁻¹ and a maximum power density of 85.6 kW kg⁻¹; the device shows a long-term cycling stability which retains 101.5% of its initial capacitance even after 10000 cycles. Such a facile strategy to fabricate the hierarchical CuO@MnO₂ core-shell nanostructure with significantly improved functionalities opens up a novel avenue to design electrode materials on demand for high-performance supercapacitor applications.

Nanosheet-assembled 3D nanoflowers of ruthenium oxide with superior rate performance for supercapacitor applications

Novel ruthenium oxide nanostructures are synthesized via a microwave-hydrothermal process. An electrode based on the 3D nanoflowers exhibits high specific capacitance and superior rate capability, resulting from the hierarchical 3D porous nanostructures.

Highly sensitive p-nitrophenol chemical sensor based on crystalline α-MnO2 nanotubes

Crystalline α-MnO₂ nanotubes were synthesized and used as a potential scaffold for the efficient detection of p-nitrophenol.