Bang Lan

Controlled synthesis of nanostructured manganese oxide: crystalline evolution and catalytic activities

A hydrothermal process has been used to synthesize manganese oxides of various crystalline structures and morphologies, such as α-, β-, γ-MnO2, MnOOH and Mn3O4. The nanostructured materials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H2-TPR) techniques. The crystalline evolution mechanism of converting β-MnO2 or MnOOH to α-MnO2 was studied. The crystalline-dependent effect of the nanostructured manganese oxides was explored by using toluene combustion as a probe reaction. Results indicated that the catalytic activity of α-MnO2 with ultra-long nanowires is higher than that of manganese oxides with other crystalline structures. The catalytic activity was correlated with the H2-TPR and XPS results.

Multifunctional free-standing membrane from the self-assembly of ultralong MnO2 nanowires.

In this work, we report the preparation of a free-standing membrane with strong mechanical stability and flexibility through a facile vacuum filtration approach. A field-emission scanning electron microscopy image demonstrates that the membrane composed of MnO2 nanowires is 50 nm in width and up to 100 μm long and the nanowires are assembled in parallel into bundles. A possible formation mechanism for the ultralong nanowires and the free-standing membrane has been proposed. Meanwhile, the properties of the membrane could be controlled by incorporating different materials to achieve composite membranes. In order to demonstrate the broad applicability of the MnO2 membrane, we fabricate a variety of composite membranes exhibiting various novel properties including magnetism and reversibly switchable wettability between hydrophilicity and hydrophobicity through various material modification, including CoFe2O4 nanoparticles and organic triethoxy(octyl)silane. Furthermore, the free-standing membrane could also simultaneously be functionalized with two materials, which reveal multiple properties. The synthesis method of a free-standing MnO2 membrane is simple and environmentally friendly, and it is easily scalable for industry. These composite membranes constitute a significant contribution to advanced technology.

Phase controllable synthesis of three-dimensional star-like MnO2 hierarchical architectures as highly efficient and stable oxygen reduction electrocatalysts

To achieve high-performance fuel cells and metal–air batteries, inexpensive and earth-abundant catalysts with enhanced activity and durability for the oxygen reduction reaction (ORR) are currently sought after. Herein, three-dimensional (3D) α-MnO2 and e-MnO2 hierarchical star-like architectures with tunable crystal phases and desirable ORR activity were readily prepared by a facile hydrothermal method with no surfactants or templates. The effects of reaction temperature, anion type, and dwell time on the morphologies of the MnO2 products were studied in detail, and the possible formation mechanism of the 3D MnO2 hierarchical stars was proposed. Due to the improved electrical conductivity and O2 adsorption ability, the resulting α-MnO2 catalyst showed substantially enhanced ORR activity, compared to the e-MnO2 and bulk MnO2 catalysts, with a more positive onset potential, a larger limiting current density, and better durability. Our results provide a facile chemical route towards the phase-controlled synthesis of 3D MnO2 architectures, which can serve as efficient catalysts for ORR-based applications.

Crystallization design of MnO2via acid towards better oxygen reduction activity

Manganese dioxide is one of the important electrode materials used for oxygen reduction reaction (ORR) in fuel-cell and metal-air batteries, and its electrochemical activity is greatly influenced by its crystalline structure. To elucidate the crystalline structure effect on the ORR, herein, we designed a facile method to obtain α-MnO2 and γ-MnO2via adjusting the acid under hydrothermal conditions using the same starting materials and reagents. The prepared α-MnO2 and γ-MnO2 have a similar morphology composed of 3D microscopic spheres made up of numerous nanorods. X-ray photoelectron spectroscopy, temperature programmed desorption of oxygen characterization and conductivity measurements demonstrate that the α-MnO2 material has a relatively higher ratio of surface oxygen species, higher oxygen mobility and higher electrical conductivity. When compared with γ-MnO2, α-MnO2 displays superior ORR activity with a higher electron transfer number and lower yield of peroxide species in alkaline solutions. The obtained results shed new light on the crystalline structure-dependent ORR activity of MnO2.

Low-cost superior solid-state symmetric supercapacitors based on hematite nanocrystals

We present a facile method for the fabrication of hematite nanocrystal-carbon cloth (Fe2O3-CC) composite. Hierarchical manganite is chosen as the sacrificial precursor, that does not contribute to the component of final iron oxide but can be in situ dissolved by the acid produced from the Fe3+ hydrolysis. This method effectively enhances the specific surface area and conductivity of hematite (Fe2O3) by attaching Fe2O3 nanocrystals (around 5 nm) firmly on the surface of carbon fibers. The obtained Fe2O3-CC can be directly used as a binder-free electrode for a supercapacitor. Interestingly, the composite electrode exhibits synergistic electrochemical capacitance (electrochemical double-layer capacitance and pseudo-capacitance). It manifests a very high areal capacitance of 1.66 F cm-2 (1660 F g-1) at 2 mA cm-2 and excellent cycling performance at large current densities (88.6% retention at 30 mA cm-2 after 5000 cycles) in a three-electrode testing system, which is among the best performances reported in the literature. Importantly, when fabricated as a solid-state flexible symmetric supercapacitor it still shows a maximum energy density of 8.74 mW h cm-3 and power density of 253.9 mW cm-3. Additionally, its good flexibility makes it suitable for portable devices.

Hierarchical branched α-MnO2: one-step synthesis and catalytic activity

Hierarchical pine tree-like α-MnO2 architectures are prepared by a facile one-step method without any template. The hierarchical α-MnO2 is composed of branched α-MnO2 short nanorods assembled onto the backbone of an α-MnO2 long nanowire. Time-dependent experiment shows that the growth process of the branched α-MnO2 nanostructures is the formation of β-MnOOH and then the transformation to a α-MnO2 branched structure in a step by step manner. Owing to its specific structure features, the hierarchical pine tree-like α-MnO2 exhibits superior catalytic performance in the catalytic combustion of dimethyl ether with a light-off temperature (T10) at 169 °C and a complete conversion temperature (T90) of 235 °C, which is far better than commercial MnO2.

Adsorption and Oxidation of Arsenic by Ultra-long α-MnO2 Nanowires with the (1 1 0) Surface

AbstractThere are many areas in the world where the ground water has been contaminated by arsenic. MnO2 is one of the most cheap materials which can both adsorb arsenic and oxide arsenite [As(III)]...