Urchin-like α-MnO2 Research Articles

Morphological engineering of 3D hollow urchin-like MnO2 spheres as bifunctional oxygen electrocatalyst for zinc-air batteries

Favorable stability and cost-efficient bifunctional catalysts are highly desirable for rechargeable zinc-air batteries. α-MnO2 has been considered a promising candidate with moderate electrochemical performance. Morphological engineering offers a feasible approach to further optimize the intrinsic and specific activities of α-MnO2. Herein, three-dimensional hollow Urchin-like α-MnO2 microspheres (denoted as the hollow α-MnO2) were successfully prepared by a facile hydrothermal reaction. The unique structure endows α-MnO2 with abundant accessible active sites and enhanced intrinsic activity. Profited from surface reconstruction, abundant oxygen vacancies were created and maintained high activity during electrocatalytic processes. As a result, the hollow α-MnO2 displays excellent oxygen reduction reaction activity with the half-wave potential at 0.80 V versus reversible hydrogen electrode (vs RHE), oxygen evolution reaction with an overpotential of 465 mV to reach the current density of 10 mA·cm−2, and remarkable rechargeable zinc-air battery performance with an open-circuit voltage of 1.48 V and a higher power density of 110.9 mW cm−2 at 0.64 V (vs zinc).

Singlet oxygen-dominated activation of peroxymonosulfate by 3D hierarchical MnO2 nanostructures for degradation of organic pollutants in water: Surface defect and catalytic mechanism

Manganese dioxide (MnO2) is attracting significant attention in the activation of persulfate for the degradation of organic pollutants in water. But the factor determining the catalytic efficiency of MnO2, and the evolution of reactive oxygen species (ROS) remain equivocal and elusive. Herein, three-dimensional (3D) hierarchical MnO2 nanostructures are synthesized via the calcination of hydrothermal product and used to active peroxymonosulfate (PMS) for degradation of acid orange 7 (AO7) in water. The results show that the calcination temperature can tune the morphology of 3D hierarchical MnO2 nanostructures from flower to urchin-like as well as phase transformation from δ-MnO2 to α-MnO2. Owing to the presence of more surface defects and surface adsorbed oxygen species, the urchin-like α-MnO2 nanostructures prepared at 500 °C (MnO2-500) deliver the best catalytic activity in activation of PMS, and 98.3 % of AO7 is degraded in 300 s. Additionally, the MnO2-500 shows wide pH applicability (3.5–9.6). Quenching tests and electron paramagnetic resonance spectra reveal that the catalytic oxidation of AO7 in the MnO2-500/PMS system is mainly mediated by the non-radical pathway, and the dominated ROS is 1O2. Besides, electrochemical experiment confirms the presence of electron transfer in the catalytic process. Density functional theory calculations indicate that the CN and CS groups of AO7 with high electron density are easily attacked by electrophilic ROS, which is further confirmed by liquid chromatography-mass spectrometry. Finally, MnO2-500 exhibits gradually enhanced catalytic activity with the increased cycle number due to the increased surface defects and the surface adsorbed oxygen. This study provides new insight into the PMS activation by MnO2-based catalyst for the remediation of organic pollutants in water.

Ag doped urchin-like α-MnO2 toward efficient and bifunctional electrocatalysts for Li-O2 batteries

Rechargeable Li-O2 batteries (LOBs) have been receiving intensive attention because of their ultra-high theoretical energy density close to the gasoline. Herein, Ag modified urchin-like α-MnO2 (Ag-MnO2) material with hierarchical porous structure is obtained by a facile one-step hydrothermal method. Ag-MnO2 possesses thick nanowires and presents hierarchical porous structure of mesopores and macropores. The unique structure can expose more active sites, and provide continuous pathways for O2 and discharge products as well. The doping of Ag leads to the change of electronic distribution in a-MnO2 (i.e., more oxygen vacancies), which play important roles in improving their intrinsic catalytic activity and conductivity. As a result, LOBs with Ag-MnO2 catalysts exhibit lower overpotential, higher discharge specific capacity and much better cycle stability compared to pure α-MnO2. LOBs with Ag-MnO2 catalysts exhibit a superior discharge specific capacity of 13,131 mAhg−1 at a current density of 200 mAg−1, a good cycle stability of 500 cycles at the capacity of 500 mAhg−1. When current density is increased to 400 mAg−1, LOBs still retain a long lifespan of 170 cycles at a limited capacity of 1,000 mAhg−1.

Ultrasound-assisted heterogeneous Fenton-like process for bisphenol A removal at neutral pH using hierarchically structured manganese dioxide/biochar nanocomposites as catalysts.

Bisphenol A (BPA) is an important emerging contaminant with endocrine-disrupting potential that has frequently been detected in aquatic environments. In this study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO2/BCs) were prepared for the first time via facile hydrothermal synthesis. The hydrothermal reaction was maintained at 100 °C for 6 h or 12 h, after which an ultrasound-assisted heterogeneous Fenton-like process was used to catalyze the removal of BPA under neutral pH condition. The characterization results indicated that MnO2 nanoparticles were successfully formed on the nanocomposite surfaces and had flower-like (δ-MnO2, 6 h) and urchin-like (α-MnO2, 12 h) morphology. This enabled a significant improvement in the catalytic activity of BPA removal by the reversible redox reaction. A series of experiments confirmed that the crystalline properties of the nanocomposites affected their catalytic activity. In particular, the α-MnO2/BCs exhibited catalytic activity in the ultrasound-assisted heterogeneous Fenton-like process and completely removed BPA within 20 min under the following conditions: [BPA]0 = 100 μM; [H2O2]0 = 10 mM; [catalyst]0 = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. This efficiency may have been due to the synergistic effect of ultrasound and α-MnO2/BCs, which simultaneously induce the effective generation of reactive free radicals and increase the mass transfer rate at the solid-liquid interface. Overall, these results demonstrated that hierarchical urchin-like α-MnO2/BCs have significant potential as an efficient and low-cost catalyst in ultrasound-assisted heterogeneous Fenton-like systems.

A hollow urchin-like α-MnO2 as an electrochemical sensor for hydrogen peroxide and dopamine with high selectivity and sensitivity.

A hollow urchin-like α-MnO2 material has been synthesized by ahydrothermal method starting from MnSO4·H2Oand (NH4)2S2O8, and byusing silver(I) as the catalyst. It has a hollow morphology with diameters of 5-6μm and consists of densely aligned nanowires, with a 30-40nm width and a 1.5μm length. The diameter of the cavities and the thickness of the shell are about 1.2μm and 300nm, respectively. The material was placed on a glassy carbon electrode (GCE), and electrochemical experiments showed the respective sensor to possess good stability and reproducibility. The modified GCE displays response to both hydrogen peroxide (H2O2) and dopamine (DA) at working potentials of -0.40V and +0.4V, respectively (both versus SCE). H2O2 can be detected with an 80nM detection limit, and DA with a 12nM detection limit (at S/N = 3). Graphical abstract Schematic presentation of synthesis of a hollow urchin-like α-MnO2, which taking MnSO4.H2O and (NH4)2S2O8 as raw materials and was further used to construct an electrocatalytic sensor for hydrogen peroxide and dopamine with low detection limit, desirable selectivity and sensitivity.

Enhanced microwave absorption properties of novel hierarchical core-shell δ/α MnO2 composites

Three types of hierarchical MnO2 nanostructures were prepared by a facile one-pot hydrothermal process. The crystal phase and microstructure were characterized by XRD, SEM, TEM and HRTEM. The microsphere/nanosheets structures were found to be layered δ-MnO2 at 120 °C. The dual-phase core-shell structure with the core of δ-MnO2 microsphere/nanosheets and the shell of α-MnO2 nanorods was synthesized at 150 °C. At 180 °C, a hollow urchin-like α-MnO2 structure was obtained. By analysis of the microwave absorption properties of these MnO2, the enhanced microwave absorption properties were observed in core-shell composites, and the minimum reflection loss (RLmin) of −45.2 dB at 4.5 GHz with the thickness of 3.9 mm has been achieved with 40 wt% absorbers in wax composites. The excellent microwave absorption properties of the composites are originated from the good impedance match caused by the core–shell heterostructure and more interfacial polarizations between δ-MnO2 microsphere/nanosheets and α-MnO2 nanorods.

Three-dimensional radial α-MnO2 synthesized from different redox potential for bifunctional oxygen electrocatalytic activities

The morphology of manganese dioxides has great effects on their electrocatalytic activities. Herein, two different types of three-dimensional (3D) radial α-MnO2 (dandelion- and urchin-like) catalysts are readily fabricated via two different redox reactions through the hydrothermal method, and their electrocatalytic activities are studied for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). As a result, the dandelion-like α-MnO2 exhibits a higher electrocatalytic performance than urchin-like α-MnO2, with a lower overpotential (20 mV lower for ORR and 90 mV lower for OER), higher mass activity and lower Tafel slope. The excellent electrocatalytic activity of dandelion-like α-MnO2 can be attributed to its relatively larger BET and electrochemical active surface area, richer defects, higher amount of Mn3+, and less charge transfer resistance. Our results shed insights into developing new way in constructing the deficient α-MnO2 bifunctional oxygen electrocatalysts.

Urchin-like α-MnO2 formed by nanoneedles for high-performance lithium batteries

MnO2 nanoneedles (NNs) were synthesized by sol-gel assisted by a redox reaction between ascorbic acid and KMnO4. X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), Raman, far-infrared spectroscopy, and magnetic measurements confirm the tunnel structure of the tetragonal α-MnO2 phase. The MnO2 NNs prepared by sol-gel at moderate temperature (T ≈ 350 °C) aggregate with an urchin-like morphology observed by scanning electron (SEM) and high-resolution transmission electron (TEM) microscopy. Electrochemical investigations show an outstanding initial specific capacity ca. 230 mAh g−1 and 45 % capacity retention at 100th cycle was obtained for these MnO2 nanoneedles.

Hydrothermal synthesis of urchin-like MnO2 nanostructures and its electrochemical character for supercapacitor

Urchin-like α-MnO2 nanostructures were successfully synthesized via a simple hydrothermal process at different synthesis temperatures without using any template or surfactant. The microstructure and morphology of as-synthesized products were systematically investigated by focused ion beam, transmission electron microscopy, and high-resolution transmission electron microscopy. The electrochemical test of the prepared MnO2 exhibits ideal cyclic voltammetry behavior, high specific capacitance (151.5Fg−1 at a current density of 1Ag−1) and excellent cycling stability (93.4% capacitance retention after 1000 cycles) which suggests its promising application as supercapacitor.

Efficient catalytic ozonation of bisphenol-A over reduced graphene oxide modified sea urchin-like α-MnO2 architectures

Considering the biological deleterious effect of bisphenol-A (BPA) in water to the human beings, great efforts have been made for the elimination of this contaminant from water sources. Herein, we report a novel nanocomposite composed of three-dimensional (3D) sea urchin-like α-MnO2 nanoarchitectures and reduced graphene oxide (RGO) for the elimination of BPA in water in the presence of ozone. The synthesis of the as-prepared nanocomposite is facile, and the nanocomposites were well characterized by SEM, TEM, XRD, and Raman spectra. The as-synthesized α-MnO2/RGO nanocomposite was found to be a highly efficient catalyst to eliminate BPA from water in the presence of ozone. The intermediates of ozonzation were also detected by gas chromatography–mass spectrometry (GC–MS). Our investigation initiates a new opportunity to explore the high-performance catalysts for the removal of the organic pollutions in water.

MnO2 prepared by hydrothermal method and electrochemical performance as anode for lithium-ion battery.

Two α-MnO2 crystals with caddice-clew-like and urchin-like morphologies are prepared by the hydrothermal method, and their structure and electrochemical performance are characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), galvanostatic cell cycling, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The morphology of the MnO2 prepared under acidic condition is urchin-like, while the one prepared under neutral condition is caddice-clew-like. The identical crystalline phase of MnO2 crystals is essential to evaluate the relationship between electrochemical performances and morphologies for lithium-ion battery application. In this study, urchin-like α-MnO2 crystals with compact structure have better electrochemical performance due to the higher specific capacity and lower impedance. We find that the relationship between electrochemical performance and morphology is different when MnO2 material used as electrochemical supercapacitor or as anode of lithium-ion battery. For lithium-ion battery application, urchin-like MnO2 material has better electrochemical performance.

Hierarchical LiMn2O4 microspheres for high rate lithium ion batteries, and direct electrochemistry and electrocatalysis

Hierarchical LiMn2O4 microspheres were prepared using urchin-like α-MnO2 as a self-sacrificial template. As the cathode material for lithium ion batteries, the electrochemical properties were characterized by cyclic voltammetry and charge/discharge experiments. The results indicated that the LiMn2O4 microspheres had high discharge capacity and excellent cycle stability. Furthermore, a new electrochemical biosensor was constructed by immobilization of bovine myoglobin (Mb) and LiMn2O4 in a thin film of chitosan (CTS) on the surface of carbon ionic liquid electrode (CILE). Electrochemical behaviors of Mb entrapped in the film were investigated by cyclic voltammetry. A pair of well-defined redox peaks appears, which indicates a fast electron transfer rate of Mb with the underlying CILE. The electron transfer coefficient and electron transfer rate constant of Mb were calculated to be 0.47 and 1.04s−1, respectively. The Mb modified electrode showed good electrocatalytic ability toward the reduction of trichloroacetic acid (TCA) with a detection limit of 0.16mmolL−1 (3σ).

Urchin-like α-MnO2 decorated with Au and Pd as a bi-functional catalyst for rechargeable lithium–oxygen batteries

Rechargeable lithium–oxygen batteries have attracted considerable attention due to their high energy density. The critical challenges that limit the practical use of this technology include sluggish kinetics of the electrochemical oxygen reactions on the cathode during discharging and charging. Here, urchin-like α-MnO2 materials decorated with Au and Pd nanoparticles are developed for use as a cathode catalyst for rechargeable Li–O2 batteries with hybrid electrolytes. Au and Pd particles as large as 3–8 nm are uniformly dispersed on the vertically aligned nanorods of α-MnO2. The Au/α-MnO2 and Pd/α-MnO2 catalysts show excellent bi-functional activity for both oxygen reduction and evolution. A rechargeable Li–O2 battery with a hybrid electrolyte is constructed using the nanostructured composite catalysts. Charging and discharging experiments of the batteries indicate that the metal-decorated, urchin-like α-MnO2 can be used as an efficient bi-functional catalyst for rechargeable hybrid Li–O2 batteries.

Synthesis of sea urchin-like LiMn2O4 hollow macrospheres via in situ conversion for rechargeable lithium-ion batteries

Among several materials (transition metal oxide) under development for use as a cathode in lithium-ion batteries, cubic spinel LiMn2O4 is one of the most promising cathode materials. In this study, the sea urchin-like LiMn2O4 hollow macrospheres were synthesized by using sea urchin-like α-MnO2 precursors through solid-state in situ self-sacrificing conversion route. The as-prepared LiMn2O4 was assembled by many single-crystalline “thorns” of ca.10–20 nm in diameter and ca. 400–500 nm in length. Galvanostatic battery testing showed that sea urchin-like LiMn2O4 had an initial discharge capacity of 126.8 mAh/g at the rate of 0.2 C in the potential range between 3.0 and 4.5 V. More than 96.67 % of the initial discharge capacity was maintained for over 50 cycles. The improved electrochemical properties were attributed to the reduced particle size and enhanced electrical contacts by the materials. This particular sea urchin-like structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.

One-step synthesis of sea urchin-like α-MnO2 using KIO4 as the oxidant and its oxidation of arsenite

Among several types of manganese oxides α-MnO2 is the most active due to its good catalytic, adsorption and ion exchange properties. Sea urchin-like α-MnO2 particles were synthesized by a one-step chemistry route at room temperature using MnSO4 in combination with KIO4 as oxidant. The obtained α-MnO2 was characterized using X-ray powder diffraction (XRD), structure refinement, electron microscopy and N2 gas adsorption and used to investigate the property of the As (III) oxidation. Results revealed that the space group of the prepared product is I4/m with lattice constants a=9.840Å and c=2.856Å, and the average grain diameter is around 6.7nm. The BET surface area is 201m2·g−1. Its maximum oxidation capacity of arsenite is up to 1086mmol·kg−1.

Morphology-Controlled Synthesis and Novel Microwave Absorption Properties of Hollow Urchinlike α-MnO2 Nanostructures

Three types of hollow urchinlike α-MnO2 nanostructures, namely, columnar nanorod clusters, tetragonal nanotube clusters, and tetragonal nanorod clusters, have been synthesized through a facile hydrothermal method. The microstructure and morphologies of the resulting materials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction, and the microwave absorption properties of these nanostructures were investigated in terms of complex permittivity and permeability. The results indicate an obvious magnetic loss in the manganese oxide/paraffin wax composites. The tetragonal nanorod clusters exhibit enhanced microwave absorption properties compared with columnar nanorod clusters and tetragonal nanotube clusters, which result from proper electromagnetic impedance matching. These urchinlike manganese oxide nanostructures are considered to have great potential applications as microwave absorbents.