MnO2 Crystals Research Articles

Nucleation and growth of feitknechtite from nanocrystalline vernadite precursor

ernadite is a nanocrystalline manganese oxide, which controls the fate of many trace elements in soils and sediments through sorption and oxidative-degradation mechanisms. This exceptional reactivity directly results from its crystal structure, which may however evolve upon contact with redox-sensitive species. Understanding these changes is a prerequisite to predict and model the geochemical cycle of trace elements in the environment. Here, the structural and morphological modifications affecting synthetic nanocrystalline vernadite (δ-MnO2) upon contact with increasing concentrations of Mn2+ were investigated using wet chemistry, synchrotron X-ray diffraction and transmission electron microscopy. Fresh δ-MnO2 crystals had an Mn oxidation state of 3.94 ± 0.05 and a ~10 A layer-to-layer distance. Crystal size was ~10 nm in the layer plane, and ~1 nm perpendicular to that. Upon contact with aqueous Mn2+ under anoxic conditions, δ-MnO2 crystals underwent several morphological and mineral evolutions, starting with the stacking, perpendicular to the layer plane, of d -MnO2 crystals to form crystals ~10 nm 2 nm which were then subjected to oriented aggregation both along and perpendicular to the layer plane to form lath-like crystals with dimensions o ~100 nm 20 nm. Finally, these laths stacked perpendicular to the layer plane to form synthetic feitknechtite (β-MnOOH) crystals with sizes up to ~100 nm 500 nm when the Mn2+ loading reached 31.9 mmol g1. Structural transformation from d -MnO2 to synthetic feitknechtite was detected at Mn2+ loading equal to or higher than 3.27 mmol g-1. These mechanisms are likely to influence the geochemical fate of trace elements in natural settings where Mn2+ is abundant. Firstly, the systematic increase in crystal size with increasing Mn2+ loading may impact the sorption capacity of vernadite and feitknechtite by reducing the density of reactive edge sites. Secondly, the fate of trace elements initially sorbed at the vernadite surface is unclear, as they could either be released in solution or incorporated into the feitknechtite lattice.

Laser-driven coating of vertically aligned carbon nanotubes with manganese oxide from metal organic precursors for energy storage

Carbon nanotubes-transition metal oxide systems are intensively studied due to their excellent properties for electrochemical applications. In this work, an innovative procedure is developed for the synthesis of vertically aligned multi-walled carbon nanotubes (VACNTs) coated with transition metal oxide nanostructures. VACNTs are grown by plasma enhanced chemical vapor deposition and coated with a manganese-based metal organic precursor (MOP) film based on manganese acetate solution. Subsequent UV pulsed laser irradiation induces the effective heating-decomposition of the MOP leading to the crystallization of manganese oxide nanostructures on the VACNT surface. The study of the morphology, structure and composition of the synthesized materials shows the formation of randomly oriented MnO2 crystals, with few nanometers in size, and to their alignment in hundreds of nm long filament-like structures, parallel to the CNT’s long axis. Electrochemical measurements reveal a significant increase of the specific capacitance of the MnO2-VACNT system (100 F g−1) as compared to the initial VACNT one (21 F g−1).

Cerium modified birnessite-type MnO2 for gaseous formaldehyde oxidation at low temperature

Cerium modified birnessite-type manganese dioxides (Ce-MnO2) with different doping ratios were prepared by a redox reaction of KMnO4 with (NH4)2C2O4 in the presence of Ce(NO3)3. The as-synthesized Ce-MnO2 samples were characterized by XRD, SEM, TEM, BET, XPS, ICP-AES, H2-TPR, Raman and in situ DRIFTS. The results indicate that doping of cerium significantly enhance the performance of birnessite for HCHO removal at low temperature. Ce-MnO2(1:10) with the nominal Ce/Mn ratio of 1:10 exhibited the best activity and achieved complete HCHO conversion at 100°C and better activity at room temperature than undoped birnessite. Upon the doping of cerium, the growth of MnO2 crystal was inhibited, leading to smaller particle size and higher specific surface. In addition, CeO2 nanocrystal formed even at low doping ratio (0.1:10), resulting in close contact between CeO2 and MnO2 nanocrystals. As a result, the doped material owned higher content of oxygen vacancies and surface adsorbed oxygen species, which contributed to its high activity for HCHO oxidation. This investigation provides a new point of view about how to design an inexpensive efficient catalyst for indoor air purification via introducing appropriate content of cerium.

MnO2 Submicroparticles from Chinese Brush and Their Application in Treatment of Methylene Blue Contaminated Wastewater.

Eggshell membrane (ESM) is selected as biotemplate to prepare MnO2 submicroparticles (SMPs) using Chinese Brush with sodium hydroxide solution. The size with average 710 nm of the obtained materials is in good consistency with the microsructured biotemplate. An efficient and convenient absorbent for methylene blue (MB) is developed. The removal efficiency could reach up to 93% in 35 min under room temperature without pH adjusting owing to the excellent adsorption from ESM itself and hydroxyl group formed on the surface of MnO2 crystal in the aqueous solution. Materials on the membrane can be separated from the wastewater simply to avoid the secondary pollution caused by the leak of material. This interesting approach to MnO2 SMPs and facile operation for MB adsorption could open a new path to the submicro-materials based wastewater treatment.

A comparative study of precursor effect on manganese post-synthetic incorporation into the T-sites of dealuminated β-zeolite

Manganese sites on zeolites are known to be catalytically active for a range of chemical reactions. The work here describes the post-synthetic incorporation of manganese (Mn) into the vacant silanol nests of β-zeolite. We show that markedly different material properties are obtained when the Mn(NO3)2 precursor is used as opposed to the Mn(Ac)2 precursor. Our DRIFT and XRD results show that the Mn(NO3)2 does not promote the incorporation of the Mn into the vacant silanol nests. Rather we find, by XRD and HRSEM, that the Mn(NO3)2 leads to the formation of poorly dispersed 14 ± 1 nm MnO2 crystals on the external surface of the dealuminated β-zeolite. On the contrary, when Mn(Ac)2 is used the XRD and DRIFT results clearly show the almost complete incorporation of Mn into the silanol nests and a significant effect on the microporous structure. HRSEM, DR-UV-Vis, H2-TPR and N2-physisorption show that the use of Mn(Ac)2 produces highly dispersed extra-framework Mn2O3 and Mn3O4 species inside and outside the pores of the dealuminated zeolite, where as in the case of Mn(NO3)2 the pores maintain their original properties. The difference in behavior between the two precursors is correlated to the intrinsic properties of the respective precursor and its interactions with the underlying zeolite support.

Floating zone growth of α-Na0.90MnO2 single crystals

Single crystal growth of α-NaxMnO2 (x=0.90) is reported via the floating zone technique. The conditions required for stable growth and intergrowth-free crystals are described along with the results of trials under alternate growth atmospheres. Chemical and structural characterizations of the resulting α-Na0.90MnO2 crystals are performed using ICP-AES NMR, XANES, XPS, and neutron diffraction measurements. As a layered transition metal oxide with large ionic mobility and strong correlation effects, α-NaxMnO2 is of interest to many communities, and the implications of large volume, high purity, single crystal growth are discussed.

In-situ growth of MnO2 crystals under nanopore-constraint in carbon nanofibers and their electrochemical performance.

Growing MnO2 nanocrystals in the bulk of porous carbon nanofibers is conducted in a KMnO4 aqueous solution aimed to enhance the electrochemical performance of MnO2. The rate of redox reaction between KMnO4 and carbon was controlled by the concentration of KMnO4 in a neutral solution. The MnO2 nanoparticles grow along with (211) crystal faces when the redox reaction happens on the surface of fibers under 1D constraint, while the nanoparticles grow along with (200) crystal faces when the redox reaction happens in the bulk of fibers under 3D constraint. The composite, where MnO2 nanoparticles are formed in the bulk under a constraint, yields an electrode material for supercapacitors showing good electron transport, rapid ion penetration, fast and reversible Faradaic reaction, and excellent rate performance. The capacitance of the composite electrode could be 1282 F g−1 under a current density of 0.2 A g−1 in 1 M Na2SO4 electrolyte. A symmetric supercapacitor delivers energy density of 36 Wh kg−1 with power density of 39 W kg−1, and can maintain 7.5 Wh kg−1 at 10.3 kW kg−1. It exhibits an excellent electrochemical cycling stability with 101% initial capacitance and 95% columbic efficiency even after 1000 cycles of charge/discharge.

Genetically Engineered Phage-Templated MnO2 Nanowires: Synthesis and Their Application in Electrochemical Glucose Biosensor Operated at Neutral pH Condition.

To conveniently obtain one-dimensional MnO2 nanowires (NWs) with controlled structure and unique properties for electron transfer, the genetically engineered M13 phages were used as templates for precise nucleation and growth of MnO2 crystals in filamentous phage scaffolds, via the spontaneous oxidation of Mn(2+) in alkaline solution. It was found that the morphology of NWs could be tailored by the surface charge of M13 mutants. MnO2 crystals were uniformly distributed on the surface of negatively charged tetraglutamate-fused phage (M13-E4), significantly different from irregular MnO2 agglomeration on the weakly negatively charged wild-type phage and positively charged tetraarginine-fused phage. The as-synthesized M13-E4@MnO2 NWs could catalyze the electro-oxidation of H2O2 at neutral pH. To demonstrate the superiority of the electrocatalytic activity in the solution containing plenty of chloride ions at neutral pH, both glucose oxidase and as-prepared MnO2 NWs were used for fabricating the glucose biosensor. The proposed biosensor showed a wide linear range (5 μM to 2 mM glucose), a low limit of detection of 1.8 μM glucose (S/N = 3), good interassay and intra-assay reproducibility and satisfactory storage stability. Due to the superiorities of synthesis and electrochemical performance, the as-prepared MnO2 NWs are promising for applications in electrocatalysis, electrochemical sensor, and supercapacitor.

Construction of Hierarchical α-MnO2 Nanowires@Ultrathin δ-MnO2 Nanosheets Core-Shell Nanostructure with Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes.

Poor electrical conductivity and mechanical instability are two major obstacles to realizing high performance of MnO2 as pseudocapacitor material. The construction of unique hierarchical core-shell nanostructures, therefore, plays an important role in the efficient enhancement of the rate capacity and the stability of this material. We herein report the fabrication of a hierarchical α-MnO2 nanowires@ultrathin δ-MnO2 nanosheets core-shell nanostructure by adopting a facile and practical solution-phase technique. The novel hierarchical nanostructures are composed of ultrathin δ-MnO2 nanosheets with a few atomic layers growing well on the surface of the ultralong α-MnO2 nanowires. The first specific capacitance of hierarchical core-shell nanostructure reached 153.8 F g(-1) at the discharge current density of as high as 20 A g(-1), and the cycling stability is retained at 98.1% after 10,000 charge-discharge cycles, higher than those in the literature. The excellent rate capacity and stability of the hierarchical core-shell nanostructures can be attributed to the structural features of the two MnO2 crystals, in which a 1D α-MnO2 nanowire core provides a stable structural backbone and the ultrathin 2D δ-MnO2 nanosheet shell creates more reactive active sites. The synergistic effects of different dimensions also contribute to the superior rate capability.

Importance of Metal Cations and Water for Stability of MnO2 Crystals

Metal cations as well as water are important factors to control the synthesis of MnO2 crystal nanostructures. In this work, systemic Density functional theory calculations about α, β, δ-MnO2 are presented to show the importance of metal cations and water for the structure stability and energy stability of MnO2. It is shown that the α-MnO2 crystal and its (110) surface will crash without the tunnel cations such as K+, and the distance between the layers of the δ-MnO2 will be significantly lower than that of the experimental results without the interlayer metal cations and water. At the same time, α-MnO2 and δ-MnO2 can be more stable than β-MnO2 with metal cations and water, and vice versa.

Deposition of MnO2 nanoneedles on carbon nanotubes and graphene nanosheets as electrode materials for electrochemical capacitors

Hierarchical carbon nanotubes (CNTs) and graphene nanosheets (GNs) supported MnO2 nanoneedles have been synthesized through a facile chemical-wet route without any further thermal treatment. The electrochemical capacitive performances of MnO2-based composite electrodes are analyzed using cyclic voltammetry, galvanostatic charge–discharge cycling, and impedance spectroscopy. It is found that the electrochemical utilization of MnO2 nanoneedles is effectively enhanced by the synergistic effect that combines the MnO2 crystals and the carbon supports, raising more active sites available for formation of charge transfer and electric double-layer in the hybrid architecture. The maximal specific capacitance of capacitors attains as high as 440 F g−1. The specific energy of MnO2/GN capacitor can reach as high as ∼40 Wh kg−1 at a specific power of 20,000 W kg−1, analyzed by the Ragone plot. The improved performance can described to the fact that the robust design of hybrid structure is capable of (i) maximizing the utilization of MnO2 nanoneedles and (ii) leading to fast chemical reaction kinetics including ionic electro-sorption and charge transfer. This method provides a straightforward approach to deposit MnO2 onto GNs as electrode materials for various energy-storage devices.

Antimicrobial activity of silver loaded MnO2 nanomaterials with different crystal phases against Escherichia coli

Silver-loaded MnO2 nanomaterials (Ag/MnO2), including Ag/α-MnO2, Ag/β-MnO2, Ag/γ-MnO2 and Ag/δ-MnO2 nanorods, were prepared with hydrothermal and impregnation methods. The bactericidal activities of four types of Ag/MnO2 nanomaterials against Escherichia coli were investigated and an inactivation mechanism involving Ag+ and reactive oxygen species (ROS) was also proposed. The bactericidal activities of Ag/MnO2 depended on the MnO2 crystal phase. Among these nanomaterials, Ag/β-MnO2 showed the highest bactericidal activity. There was a 6-log decrease in E. coli survival number after treatment with Ag/β-MnO2 for 120min. The results of 5,5-dimethyl-1-pyrroline-N-oxide spin-trapping measurements by electron spin resonance indicate OH and O2‾ formation with addition of Ag/β-MnO2, Ag/γ-MnO2 or Ag/δ-MnO2. The strongest peak of OH appeared for Ag/β-MnO2, while no OH or O2‾ signal was found over Ag/α-MnO2. Through analysis of electron spin resonance (ESR) and Ag+ elution results, it could be deduced that the toxicity of Ag+ eluted from Ag/MnO2 nanomaterials and ROS played the main roles during the bactericidal process. Silver showed the highest dispersion on the surface of β-MnO2, which promoted ROS formation and the increase of bactericidal activity. Experimental results also indicated that Ag/MnO2 induced the production of intracellular ROS and disruption of the cell wall and cell membrane.

Graphitic biochar as a cathode electrocatalyst support for microbial fuel cells

Graphitic biochar (BC) was generated using high temperature gasification and alkaline post-treatment (BCw) of wood-based biomass. The BCw was evaluated as a manganese oxide electrocatalytic support (MnO/BCw) and microbial fuel cell (MFC) air cathode. Nano-structured MnO2 crystals were successfully immobilized on biomass-based graphitic sheets and characterized using physical, chemical, and electrochemical analyses. Cyclic voltammetry of MnO/BCw/Nafion inks showed electrochemical features typical of β-MnO2 with a current density of 0.9mAcm−2. BC showed satisfactory maximum power densities of 146.7mWm−2 (BCw) and 187.8Wm−2 (MnO/BCw), compared with Vulcan Carbon (VC) (156.8mWm−2) and manganese oxide VC composites (MnO/VC) (606.1mWm−2). These materials were also tested as oxygen reduction reaction (ORR) catalysts for single chamber MFCs inoculated with anaerobic sludge. Our results demonstrate that BC can serve as an effective, low cost, and scalable material for MFC application.

Boron-doped MnO2/carbon fiber composite electrode for supercapacitor

The boron-doped MnO2/carbon fiber composite electrode has been prepared via in situ redox reaction between potassium permanganate and carbon fibers in the presence of boric acid. The addition of boron as dopant results in the increase of growth-rate of MnO2 crystal and the formation of worm-like nanostructure. Based on the analysis of binding energy, element boron incorporates into the MnO2 lattice through interstitial mode. The doped electrode with porous framework is beneficial to pseudocapacitive reaction and surface charge storage, leading to higher specific capacitance and superior rate capability. After experienced 1000 cycles, the boron-doped MnO2 still retain a higher specific capacitance by about 80% of its initial value. The fall in capacitance is blamed to be the combination of the formation of soluble Mn2+ and the absence of active site on the outer surface.

Manganese oxide/functionalised carbon nanotubes nanocomposite as catalyst for oxygen reduction reaction in microbial fuel cell

Manganese oxide/functionalised carbon nanotubes (MnO2/f-CNT) nanocomposite is believed to be a good catalyst for oxygen reduction reaction (ORR) in a neutral solution; especially in a microbial fuel cell. The unique interaction between MnO2 and f-CNT will enhance the electron transfer process and facilitate the ORR. MnO2/f-CNT nanocomposite was fabricated by in-situ hydrothermal synthesis method. The MnO2/f-CNT was characterised by the field emission scanning electron microscope (FESEM). Raman spectra has proved there is more carbon defect on the surface of CNT after functionalization. XRD pattern showed amorphous MnO2 crystal existed in MnO2/f-CNT catalyst. This study further employed cyclic voltammetry (CV) techniques to investigate catalytic activity. Lastly, microbial fuel cell test showed that MnO2/f-CNT displayed higher power density (520 mW m−2) compared to CNT (275 mW m−2) and f-CNT (440 mW m−2). Moreover, the coulombic efficiency (28.65%) and COD removal value (86.6%) of MFC with MnO2/f-CNT catalyst was higher amongst the three catalysts tested. This study concluded that MnO2/f-CNT can be a good catalyst in microbial fuel cell application.

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.

Carnation-like MnO2 modified activated carbon air cathode improve power generation in microbial fuel cells

Highly active and low-cost electrocatalysts are of great importance for large-scale commercial applications of microbial fuel cells (MFCs). In this work, we prepared an activated carbon (AC) air cathode containing electrodeposited γ-MnO2 using a potentiostatic method. The results indicated that carnation-like MnO2 crystals were bound to the surface of the AC air cathode after a deposition time of 10 min, which greatly improved the performance of the cathode. BET analysis results demonstrated that the electrodeposition of MnO2 decreased the micropore surface area of the cathode but increased the mesopore surface area. When compared with a bare AC air cathode, the electrodeposited MnO2 cathode exhibited higher catalytic activity for oxygen reduction reaction. The maximum power density of the MFC equipped with the electrodeposited MnO2 AC air cathode was 1554 mW m−2, which is 1.5 times higher than the control cathode.

Amorphous Carbon Nanotube/MnO<SUB>2</SUB>/Graphene Oxide Ternary Composite Electrodes for Electrochemical Capacitors

Ternary composites of amorphous carbon nanotube/MnO2/graphene oxide (a-CNT/MnO2/GO) were synthesized by a facile direct redox reaction between potassium permanganate and a-CNT, which was prepared by anodic aluminum oxide template method following co-filtration with GO. Needle-like, 100-nm-thick, MnO2 crystals were homogeneously coated on the a-CNT surface, which was then covered with GO. The electrochemical performance of the resulting MnO2-coated a-CNTs exhibited a specific capacitance of 473 F/g at a scan rate of 5 mV/s, and excellent charge/discharge stability after 500 cycles.

The Key Role of pH Value in the Synthesis of Titanate Nanotubes‐Loaded Manganese Oxides as a Superior Catalyst for the Selective Catalytic Reduction of NO with NH3

Titanate nanotubes (TNTs) synthesized by hydrothermal method were increasingly used as the catalyst support for the selective catalytic reduction (SCR) of NO with ammonia. This paper reports the critical process of postwashing to prepare satisfactory TNTs for the uses of SCR catalysts. Herein, alkaline TNTs (TNTs‐AL), acidic TNTs (TNTs‐AC), and neutral TNTs (TNTs‐NE) were synthesized by controlling washing pH value. When these TNTs were utilized as the catalyst supports for manganese oxides (Mn/TNTs‐AL, Mn/TNTs‐AC, and Mn/TNTs‐NE), the key role of pH value was found. Titanate nanosheets, titanate nanorods and titanate nanotubes were dominated in Mn/TNTs‐AL, Mn/TNTs‐AC, and Mn/TNTs‐NE, respectively. MnO2 crystal was observed when using TNTs‐AC or TNTs‐NE as the support. By contrast, Mn3O4 and NaNO3 were observed when using TNTs‐AL as the support. Mn/TNTs‐NE showed the best SCR activity, in line with the largest surface area, the best dispersion, and the most active redox property of manganese oxides. Mn/TNTs‐AL showed negligible SCR activity, resulting from the minimum surface area, the Mn3O4‐dominating crystal structure, and the bad dispersion of manganese oxides.

Electrolytic codeposition of Mn and Cr oxides

Using the method of polarization curves (E-lgIa), the kinetics of the anodic processes during the electrolytic codeposition of MnO2 and chromium oxides is studied. It is found that the codeposition of chro� mium oxides leads to a substantial reduction in the current efficiency and the rate of formation of the basic anodic product. The structure of the combined residues of Mn and Cr formed on the anode considerably dif� fers from the structure of MnO2, which is shown in the reduction in the size of the crystalline grains with their simultaneous transformation into a columnar configuration. The formation of a columnar (filamentary) structure is explained by the advanced nucleation and growth of the MnO2 crystals, the development of which along the surface of the base is hindered by molecules and microcrystalline aggregates of chromium oxide compounds. Owing to this, the resulting grain grows normal to the substrate surface via the superposition of twodimensional MnO 2 nuclei.