Birnessite Nanosheets Research Articles

Nature of High- and Low-Affinity Metal Surface Sites on Birnessite Nanosheets

Birnessite nanosheets (δ-MnO2) are key reactive nanoparticles that regulate metal cycling in terrestrial and marine settings, yet there is no molecular explanation for the sorption selectivity of metals which controls their enrichment. This fundamental question was addressed by optimizing the structure of Ni, Cu, Zn, and Pb surface complexes on δ-MnO2 and by calculating the Gibbs free-energy change (ΔG) of the sorption reactions with density functional theory. The sorption selectivity follows the order Pb > Cu > Ni > Zn in good agreement with experimental data. Cu, Ni, and Zn bind preferentially to layer edges at low surface coverage forming double-edge-sharing (DES) complexes, whereas Pb binds extensively with high selectivity over the three transition metals to both layer edges (DES bonding) and to basal planes forming triple-corner-sharing (TCS) complexes. Pb has a similar affinity for the DES and TCS sites at pH 5 and a higher affinity for the TCS sites at circumneutral pH. The Pb DES and TCS complexes are both dehydrated at the δ-MnO2–water interface and feature a trigonal pyramidal geometry with three surface O atoms. The high stability of the two new Pb complexes arises from the hybridization between the Pb 6s/6p and O 2p states, forming a strong covalent Pb–O/OH bond at the δ-MnO2 surface. The quantum chemical results provide a mechanistic and energetics insight into the metal uptake on δ-MnO2 that extends what extended X-ray absorption fine structure (EXAFS) spectroscopy alone can provide.

Recrystallization-induced capacity fading of birnessite-type MnO2 nanosheets for pseudocapacitors

Birnessite-type MnO2 materials are widely studied as electrodes for pseudocapacitors. However, the poor cycling stability limits their applications. Here, wrinkle birnessite-type MnO2 nanosheet array has been synthesized and taken as an example to discover the reason of capacity fading for pseudocapacitors. It is discovered that in the the galvanostatic charge-discharge process, the smalle nanosheets blocking the macropores of the array framework have been dissolved and recrystallized on the large wrinkle birnessite nanosheet array framework, finally forming a hierarchical structure of nanosheets@nanosheets of birnessite. The recrystallizaion of the small birnessite nanosheets may decrease the specific capacity through the decreasement of the surface area of the electrode.

The effect of electrolyte cation on electrochemically induced activation and capacitive performance of Mn3O4 electrodes

A phase transition from spinel Mn3O4 to layered birnessite has been reported through electrochemically induced activation in aqueous electrolyte for high performance supercapacitor, battery and water splitting applications. Here, Li2SO4, MgSO4 and K2SO4 aqueous electrolytes were used for the activation of Mn3O4 electrode to investigate the effect of electrolyte cation on the electrochemical activation. The Mn3O4 electrode exhibited different activated results in the three electrolytes, including the efficiency of phase transition, morphology of resulting birnessite and capacitive performance of the activated electrode. The cations affect the proceeding of H+ insertion or the peeling-off of birnessite nanosheets during the phase transition process, resulting in the formation of nanosheet materials with different lateral dimensions. Effect of electrolyte cation on the electrochemical behavior of nanosheets electrode is further investigated. The nanosheets electrode activated in K2SO4 exhibits capable capacitive behavior with reasonable specific capacitance and cycling stability. Understanding how different cations affect the phase transition of Mn3O4 together with its capacitive performance will conduce to the development of manganese oxide electrode systems with high performance.

Effective Zinc Adsorption Driven by Electrochemical Redox Reactions of Birnessite Nanosheets Generated by Solar Photochemistry

The surface properties, chemical compositions, and crystal structures of manganese oxides can be altered by redox reactions, which affect their heavy metal ion adsorption capacities. Here, birnessite nanosheets (δ-MnO2) were synthesized from the photochemical reaction of Mn2+aq and nitrate under solar irradiation, and Zn2+ was electrochemically adsorbed using the as-obtained birnessite nanosheets by galvanostatic charge–discharge. The effects of current density and electrochemical techniques (symmetric electrode and three-electrode systems) on Zn2+ adsorption capacity were also investigated. The results showed that the maximum Zn2+ adsorption capacity of the birnessite in the presence of electrochemical redox reactions could reach 383.2 mg g–1 (589.0 mmol mol–1) and 442.6 mg g–1 (680.3 mmol mol–1) in the symmetric electrode and three-electrode system, respectively; however, the Mn2+ release capacity in the three-electrode system was higher than that in the symmetric electrode system. With increasing curre...

Birnessite manganese oxide nanosheets assembled on Ni foam as high-performance pseudocapacitor electrodes: Electrochemical oxidation driven porous honeycomb architecture formation

It has not been considered in the reported literatures that the current or the cyclic number of galvanostatic charge-discharge process controls the morphology growth of manganese oxide materials. Here, standing birnessite nanosheets assembled porous honeycomb architecture is in situ formed on Ni foam by electrochemical oxidation of hydrothermally synthesized Mn3O4 nanospheres electrodes via a electrochemical cycling technique. The birnessite morphology much depends on the conditions of the electrochemical cycling. By tuning the current and cycle number, Mn3O4 nanospheres are converted to birnessite with optimal morphology, while a high pseudocapacitive performance is achieved. The optimized birnessite electrodes exhibit a high level of specific capacitance (377 F g−1 at 1 A g−1), a superior rate capability (227 F g−1 at 30 A g−1) and a good cycling stability (75% retention after 4000 cycles). This work provides useful strategies for developing high-performance manganese oxide electrodes.

Photochemically-Assisted Synthesis of Birnessite Nanosheets and Their Structural Alteration in the Presence of Pyrophosphate

Recent findings have showed the environmental abundance of soluble Mn(III)-complexes (e.g., Mn-siderophore and Mn(III)-pyrophosphate (Mn(III)-PP)). Specifically, Mn(III)-PP is of emerging interest in understanding Mn redox cycling and phosphorus cycling in catalysis and environmental systems. Curiously, however, further oxidation of Mn(III)-PP and consequent formation of a MnO2 solid phase have not been considered yet. Here, using photochemically-assisted fast oxidation of Mn2+ (aq) by the reaction with superoxide, generated from nitrate photolysis, we report the role of PP in formation of disordered δ-MnO2 nanosheets and changes in their stacking and structures. With increasing PP concentrations from 0.3 to 2 mM, δ-MnO2 nanosheets showed better stacking (thicker) layers and an alteration from hexagonal to orthogonal structure (more Mn(III) in layers). We found that the amount of Mn(III)-PP complex and the formation rate of δ-MnO2 nanosheets under the different PP concentrations affected both their layer ...

Three-dimensional honeycomb-like networks of birnessite manganese oxide assembled by ultrathin two-dimensional nanosheets with enhanced Li-ion battery performances.

Three-dimensional (3D) honeycomb-like birnessite networks composed of ultrathin two-dimensional (2D) nanosheets were firstly synthesized through a facile and low-cost synthetic route. By using carbon microspheres as a template instead of graphene, hierarchical birnessite structures assembled by ultrathin nanosheets including york-shell and hollow structures were obtained besides the ultrathin birnessite nanosheets with a thickness of about 0.7 nm. By assembling carbon spheres into an ordered 3D array, novel 3D honeycomb-like birnessite structures assembled by ultrathin nanosheets were firstly prepared. When evaluated as an anode material for Li-ion batteries, the 3D honeycomb-like networks show enhanced electrochemical performances with high capacities, excellent cycling stability and good rate capability, which can be ascribed to the novel 3D honeycomb-like macroporous structure with a 3D inverse opal structure, well-ordered macropores, interconnected walls and a regular periodicity.

Ultra-rapid microwave-assisted synthesis of layered ultrathin birnessite K0.17MnO2 nanosheets for efficient energy storage

Inorganic graphene analogues (IGAs) are currently in the spotlight of nanotechnology with the aim of achieving superior energy storage performance. However, cumbersome, expensive and time-consuming synthetic routes have definitely hindered further study of these species, thus the development of facile, low-cost and ultra-rapid synthetic approaches to these species is urgently needed and has met with limited success so far. Herein, we put forward an ultra-rapid, low-cost and facile microwave-assisted strategy to achieve this goal without exfoliation for the first time. This protocol relies on microwave dielectric heating, a reducing reagent and reactive medium, which will efficiently lower the Gibbs activation energy and polarize the conducting electrons. As an example, (001)-oriented ultrathin birnessite K0.17MnO2 nanosheets with a thickness of only 2 nm were successfully synthesized within 5 min, much faster than presently known routes. Notably, this novel route can also be extended to the synthesis of ultrathin Na-type birnessite nanosheets, revealing the universality of this synthetic strategy by extending to those layered compounds. The layered ultrathin birnessite K0.17MnO2 nanosheet-based electrode exhibits remarkably improved electrochemical characteristics compared with its bulk counterpart, showing a high specific capacitance of 206 F g−1 at 1 A g−1 and an excellent cycling performance at a large current density of 5 A g−1 (>93% retention over 1000 cycles). Even in lithium ion battery cathodes, the reversible capacity of 167.4 mA h g−1 is still retained with a negligible capacity loss per cycle (0.25%), which is superior to most reported birnessite nanostructures, suggesting a remarkably promising candidate for energy storage. Such intriguing behavior is mainly attributed to the intrinsic crystal structure and the synergistic effect of IGAs, such as huge surface area, facile guest ion diffusion and electron transport. This work opens the door for the ultra-rapid and facile preparation of ultrathin nanosheets, which will significantly expand the studies of IGAs and optimize energy storage through rational materials design and synthesis.

Formation of Na0.44MnO2 nanowires via stress-induced splitting of birnessite nanosheets

High aspect ratio Na0.44MnO2 nanowires with a complex one-dimensional (1-D) tunnel structure have been synthesized. We found that the reaction went through layered birnessite nanosheet intermediates, and that their conversion to the final product involved splitting of the nanosheets into nanowires. Based on our observations, a stress-induced splitting mechanism for conversion of birnessite nanosheets to Na0.44MnO2 nanowires is proposed. The final and intermediate phases show topotaxy with 〈001〉f // 〈020〉b or 〈110〉b where f represents the final Na0.44MnO2 phase and b the intermediate birnessite phase. As a result of their high surface areas, the nanowires are efficient catalysts for the oxidation of pinacyanol chloride dye.