Bi-MOF Modulating MnO2 Deposition Enables Ultra-Stable Cathode-Free Aqueous Zinc-Ion Batteries

The Mn-based materials are considered as the most promising cathodes for zinc-ion batteries (ZIBs) due to their inherent advantages of safety, sustainability and high energy density, however suffer from poor cyclability caused by gradual Mn²⁺ dissolution and irreversible structural transformation. The mainstream solution is pre-adding Mn²⁺ into the electrolyte, nevertheless faces the challenge of irreversible Mn²⁺ consumption results from the MnO₂ electrodeposition reaction (Mn²⁺ → MnO₂). This work proposes a “MOFs as the electrodeposition surface” strategy, rather than blocking it. The bismuth (III) pyridine-3,5-dicarboxylate (Bi-PYDC) is selected as the typical electrodeposition surface to regulate the deposition reaction from Mn²⁺ to MnO₂. Because of the unique less hydrophilic and manganophilic nature of Bi-PYDC for Mn²⁺, a moderate MnO₂ deposition rate is achieved, preventing the electrolyte from rapidly exhausting Mn²⁺. Simultaneously, the intrinsic stability of deposited R-MnO₂ is enhanced by the slowly released Bi³⁺ from Bi-PYDC reservoir. Furthermore, Bi-PYDC shows the ability to accommodate H⁺ insertion/extraction. Benefiting from these merits, the cathode-free ZIB using Bi-PYDC as the electrodeposition surface for MnO₂ shows an outstanding cycle lifespan of more than 10 000 cycles at 1 mA cm^-2. This electrode design may stimulate a new pathway for developing cathode free long-life rechargeable ZIBs.     

Graphene/MnO2 hybrid nanosheets as high performance electrode materials for supercapacitors

Graphene/MnO₂ hybrid nanosheets were prepared by incorporating graphene and MnO₂ nanosheets in ethylene glycol. Scanning electron microscopy and transmission electron microscopy analyses confirmed nanosheet morphology of the hybrid materials. Graphene/MnO₂ hybrid nanosheets with different ratios were investigated as electrode materials for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge in 1 M Na₂SO₄ electrolyte. We found that the graphene/MnO₂ hybrid nanosheets with a weight ratio of 1:4 (graphene:MnO₂) delivered the highest specific capacitance of 320 F g⁻¹. Graphene/MnO₂ hybrid nanosheets also exhibited good capacitance retention on 2000 cycles.     

Elucidating the Reaction Mechanism of Mn2+ Electrolyte Additives in Aqueous Zinc Batteries

Aqueous zinc batteries (ZIBs) have attracted considerable attention in recent years because of their high safety and eco-friendly features. Numerous studies have shown that adding Mn²⁺ salts to ZnSO₄ electrolytes enhanced overall energy densities and extended the cycling life of Zn/MnO₂ batteries. It is commonly believed that Mn²⁺ additives in the electrolyte inhibit the dissolution of MnO₂ cathode. To better understand the role of Mn²⁺ electrolyte additives, the ZIB using a Co₃O₄ cathode instead of MnO₂ in 0.3 m MnSO₄ + 3 m ZnSO₄ electrolyte is built to avoid interference from MnO₂ cathode. As expected, the Zn/Co₃O₄ battery exhibits electrochemical characteristics nearly identical to those of Zn/MnO₂ batteries. Operando synchrotron X-ray diffraction (XRD), ex situ X-ray absorption spectroscopy (XAS), and electrochemical analyses are carried out to determine the reaction mechanism and pathway. This work demonstrates that the electrochemical reaction occurring at cathode involves a reversible Mn²⁺/MnO₂ deposition/dissolution process, while a chemical reaction of Zn²⁺/Zn₄SO₄(OH)₆∙5H₂O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn²⁺/Zn₄SO₄(OH)₆∙5H₂O reaction contributes no capacity and lowers the diffusion kinetics of the Mn²⁺/MnO₂ reaction, which prevents the operation of ZIBs at high current densities.     

Core–Shell Structure and Interaction Mechanism of γ‐MnO2 Coated Sulfur for Improved Lithium‐Sulfur Batteries

Lithium‐sulfur batteries have attracted worldwide interest due to their high theoretical capacity of 1672 mAh g^?1 and low cost. However, the practical applications are hampered by capacity decay, mainly attributed to the polysulfide shuttle. Here, the authors have fabricated a solid core–shell γ‐MnO₂‐coated sulfur nanocomposite through the redox reaction between KMnO₄ and MnSO₄. The multifunctional MnO₂ shell facilitates electron and Li⁺ transport as well as efficiently prevents polysulfide dissolution via physical confinement and chemical interaction. Moreover, the γ‐MnO₂ crystallographic form also provides one‐dimensional (1D) tunnels for the Li⁺ incorporation to alleviate insoluble Li₂S₂/Li₂S deposition at high discharge rate. More importantly, the MnO₂ phase transformation to Mn₃O₄ occurs during the redox reaction between polysulfides and γ‐MnO₂ is first thoroughly investigated. The S@γ‐MnO₂ composite exhibits a good capacity retention of 82% after 300 cycles (0.5 C) and a fade rate of 0.07% per cycle over 600 cycles (1 C). The degradation mechanism can probably be elucidated that the decomposition of the surface Mn₃O₄ phase is the cause of polysulfide dissolution. The recent work thus sheds new light on the hitherto unknown surface interaction mechanism and the degradation mechanism of Li‐S cells.     

Reducing dissolution of MnO2 nanofibers by doping with ferric ion

MnO₂ nanofiber was found to possess high adsorption capacities for heavy metal ions such as, arsenic and lead, in water due to its high specific surface area (SSA) and high surface activity. However, a significant amount of manganese was found to leach from MnO₂ nanofibers. Reducing MnO₂ dissolution is very important for improving its applications in drinking water treatment. In this study, MnO₂ nanofiber was doped with Fe³⁺ to reduce its dissolution in water. Dissolution tests were conducted on un-doped and Fe-doped MnO₂ nanofibers. The results revealed that doping with Fe³⁺ significantly reduced MnO₂ dissolution. SSA and defects of MnO₂ materials were analyzed by BET and XRD methods. The effects of Fe³⁺ on MnO₂ dissolution were discussed and the optimal dopant amount was identified.     

Hierarchical α‐MnO2 Nanowires@Ni1‐xMnxOy Nanoflakes Core–Shell Nanostructures for Supercapacitors

A facile two‐step solution‐phase method has been developed for the preparation of hierarchical α‐MnO₂ nanowires@Ni_1‐xMn_xO_y nanoflakes core–shell nanostructures. Ultralong α‐MnO₂ nanowires were synthesized by a hydrothermal method in the first step. Subsequently, Ni_1‐xMn_xO_y nanoflakes were grown on α‐MnO₂ nanowires to form core–shell nanostructures using chemical bath deposition followed by thermal annealing. Both solution‐phase methods can be easily scaled up for mass production. We have evaluated their application in supercapacitors. The ultralong one‐dimensional (1D) α‐MnO₂ nanowires in hierarchical core–shell nanostructures offer a stable and efficient backbone for charge transport; while the two‐dimensional (2D) Ni_1‐xMn_xO_y nanoflakes on α‐MnO₂ nanowires provide high accessible surface to ions in the electrolyte. These beneficial features enable the electrode with high capacitance and reliable stability. The capacitance of the core–shell α‐MnO₂@Ni_1‐xMn_xO_y nanostructures (x = 0.75) is as high as 657 F g^?1 at a current density of 250 mA g^?1, and stable charging‐discharging cycling over 1000 times at a current density of 2000 mA g^?1 has been realized.     

Biodegradable Nanoprodrugs: “Delivering” ROS to Cancer Cells for Molecular Dynamic Therapy

Biodegradable nanoprodrugs, inheriting the antitumor effects of chemotherapy drugs and overcoming the inevitable drawback of side effects on normal tissues, hold promise as next-generation cancer therapy candidates. Biodegradable nanoprodrugs of transferrin-modified MgO₂ nanosheets are developed to selectively deliver reactive oxygen species to cancer cells for molecular dynamic therapy strategy. The nanosheets favor the acidic and low catalase activity tumor microenvironment to react with proton and release nontoxic Mg²⁺. This reaction simultaneously produces abundant H₂O₂ to induce cell death and damage the structure of transferrin to release Fe³⁺, which will react with H₂O₂ to produce highly toxic ·OH to kill tumor cells.     

Fabrication and capacitance of Ni-Fe LDHs/MnO2 layered nanocomposite via an exfoliation/reassembling process

Ni²⁺-Fe³⁺ layered double hydroxides (LDHs)/MnO₂ layered nanocomposite has been fabricated by using both layer-by-layer self-assembly method and flocculated technology, based on electrostatic interaction of positively charged Ni²⁺-Fe³⁺ LDHs nanosheets and negatively charged MnO₂ nanosheets. Ultraviolet-visible spectroscopy is used to probe the dynamic growth of the multilayer film, exhibiting progressive enhancement of optical absorption due to the assembly of Ni²⁺-Fe³⁺ LDHs nanosheets and MnO₂ nanosheets. The assembled Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite has been characterized by XRD, SEM and TEM. The electrochemical property of the synthesized Ni²⁺-Fe³⁺ LDHs/MnO₂ layered nanocomposite has been studied using cyclic voltammetry in a mild aqueous electrolyte. The Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite exhibits a relative good capacitive behavior in a neutral electrolyte system, and its initial capacitance value is 104 F g⁻¹.     

High‐Stability MnOx Nanowires@C@MnOx Nanosheet Core–Shell Heterostructure Pseudocapacitance Electrode Based on Reversible Phase Transition Mechanism

A stable MnO_x@C@MnO_x core–shell heterostructure consisting of vertical MnO_x nanosheets grown evenly on the surface of the MnO_x@carbon nanowires are obtained by simple liquid phase method combined with thermal treatment. The hierarchical MnO_x@C@MnO_x heterostructure electrode possesses a high specific capacitance of 350 F g^?1 and an excellent cycle performance owing to the existence of the pore structure among the ultrasmall MnO_x nanoparticles and the rapid transmission of electrons between the active material and carbon coating layer. Particularly, according to the in situ Raman spectra analysis, no characteristic peaks corresponding to MnOOH are found during charging/discharging, indicating that pseudocapacitive behavior of the MnO_x electrode have no relevance to the intercalation/deintercalation of protons (H⁺) in the electrolyte. Further combining in situ X‐ray powder diffraction analysis, the diffraction peak of α‐MnO₂ can be detected in the process of charging, while Mn₃O₄ phase is found in discharge products. Therefore, these results demonstrate that the MnO_x undergoes a reversible phase transformation reaction of Mn₃O₄?α‐MnO₂. Moreover, the assembled all‐solid‐state asymmetric supercapacitor with a MnO_x@C@MnO_x electrode delivers a high energy density of 23 Wh kg^?1, an acceptable power density of 2500 W kg^?1, and an excellent cyclic stability performance of 94% after 2000 cycles, showing the potential for practical application.     

Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and their Extra‐High Electrical Conductivity

2D metallic transition‐metal dichalcogenides (MTMDs) have recently emerged as a new class of materials for the engineering of novel electronic phases, 2D superconductors, magnets, as well as novel electronic applications. However, the mechanical exfoliation route is predominantly used to obtain such metallic 2D flakes, but the batch production remains challenging. Herein, the van der Waals epitaxial growth of monocrystalline, 1T‐phase, few‐layer metallic VSe₂ nanosheets on an atomically flat mica substrate via a “one‐step” chemical vapor deposition method is reported. The thickness of the VSe₂ nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe₂ single crystals are found to present an excellent metallic feature, as evidenced by the extra‐high electrical conductivity of up to 10⁶ S m^?1, 1–4 orders of magnitude higher than that of various conductive 2D materials. The thickness‐dependent charge‐density‐wave phase transitions are also examined through low‐temperature transport measurements, which reveal that the synthesized 2D metallic 1T‐VSe₂ nanosheets should serve as good research platforms for the detecting novel many‐body states. These results open a new path for the synthesis and property investigations of nanoscale‐thickness 2D MTMDs crystals.     

Novel Iron Oxyhydroxide Lepidocrocite Nanosheet as Ultrahigh Power Density Anode Material for Asymmetric Supercapacitors

A simple one‐step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ‐FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ‐FeOOH nanosheets is investigated via in situ X‐ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ‐FeOOH nanosheets show that a Li⁺ can reversibly insert/desert into/from the 2D channels between the [FeO₆] octahedral subunits depending on the applied potential. This process charge compensates the Fe²⁺/Fe³⁺ redox transition upon charging–discharging and thus contributes to an ideal pseudocapacitive behavior of the γ‐FeOOH electrode. Electrochemical results indicate that the γ‐FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg^?1 with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO₂ and γ‐FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg^?1 at an energy density of 37.4 Wh kg^?1.     

High‐Lithium‐Affinity Chemically Exfoliated 2D Covalent Organic Frameworks

Covalent organic frameworks (COFs) with reversible redox behaviors are potential electrode materials for lithium‐ion batteries (LIBs). However, the sluggish lithium diffusion kinetics, poor electronic conductivity, low reversible capacities, and poor rate performance for most reported COF materials limit their further application. Herein, a new 2D COF (TFPB‐COF) with six unsaturated benzene rings per repeating unit and ordered mesoporous pores (≈2.1 nm) is designed. A chemical stripping strategy is developed to obtain exfoliated few‐layered COF nanosheets (E‐TFPB‐COF), whose restacking is prevented by the in situ formed MnO₂ nanoparticles. Compared with the bulk TFPB‐COF, the exfoliated TFPB‐COF exhibits new active Li‐storage sites associated with conjugated aromatic π electrons by facilitating faster ion/electron kinetics. The E‐TFPB‐COF/MnO₂ and E‐TFPB‐COF electrodes exhibit large reversible capacities of 1359 and 968 mAh g^?1 after 300 cycles with good high‐rate capability.     

Flexible Graphene‐Wrapped Carbon Nanotube/Graphene@MnO2 3D Multilevel Porous Film for High‐Performance Lithium‐Ion Batteries

The ingenious design of a freestanding flexible electrode brings the possibility for power sources in emerging wearable electronic devices. Here, reduced graphene oxide (rGO) wraps carbon nanotubes (CNTs) and rGO tightly surrounded by MnO₂ nanosheets, forming a 3D multilevel porous conductive structure via vacuum freeze‐drying. The sandwich‐like architecture possesses multiple functions as a flexible anode for lithium‐ion batteries. Micrometer‐sized pores among the continuously waved rGO layers could extraordinarily improve ion diffusion. Nano‐sized pores among the MnO₂ nanosheets and CNT/rGO@MnO₂ particles could provide vast accessible active sites and alleviate volume change. The tight connection between MnO₂ and carbon skeleton could facilitate electron transportation and enhance structural stability. Due to the special structure, the rGO‐wrapped CNT/rGO@MnO₂ porous film as an anode shows a high capacity, excellent rate performance, and superior cycling stability (1344.2 mAh g⁻¹ over 630 cycles at 2 A g⁻¹, 608.5 mAh g⁻¹ over 1000 cycles at 7.5 A g⁻¹). Furthermore, the evolutions of microstructure and chemical valence occurring inside the electrode after cycling are investigated to illuminate the structural superiority for energy storage. The excellent electrochemical performance of this freestanding flexible electrode makes it an attractive candidate for practical application in flexible energy storage.     

One‐Step Synthesis of Single‐Layer MnO2 Nanosheets with Multi‐Role Sodium Dodecyl Sulfate for High‐Performance Pseudocapacitors

A template‐free, one‐step and one‐phase synthesis of single‐layer MnO₂ nanosheets has been developed via a redox reaction between KMnO₄ and sodium dodecyl sulfate (SDS). The successful formation of single‐layer MnO₂ nanosheets has been confirmed by the characteristic absorption around 374 nm and the typical thickness of ~0.95 nm. The slow redox reaction controlled by the gradual hydrolysis of SDS is found to be the key factor for the successful formation of single‐layer nanosheets. SDS not only serves as the precursor of dodecanol to reduce KMnO₄, but also aids the formation of single‐layer MnO₂ nanosheets as a structure‐inducing agent. The resultant single‐layer MnO₂ nanosheets possess superior specific capacitance, which can be attributed to the extended surface and high porosity of MnO₂ nanosheets on the electrode. The MnO₂ nanosheets also show excellent durability, retaining 91% of the starting capacitance after 10 000 charge/discharge cycles. Moreover, the symmetric pseudocapacitor based on the synthesized single‐layer MnO₂ nanosheets exhibits a high specific capacitance, indicating great potential for real energy storage. Therefore, it has been demonstrated for the first time that a single readily available reagent, SDS, can play multiple roles in reducing KMnO₄ to conveniently yield single‐layer MnO₂ nanosheets as a high‐performance pseudocapacitive material.     

Synergistically Coupling Black Phosphorus Quantum Dots with MnO2 Nanosheets for Efficient Electrochemical Nitrogen Reduction Under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) is a promising strategy of nitrogen fixation into ammonia under ambient conditions. However, the development of electrochemical NRR is highly bottlenecked by the expensive noble metal catalysts. As a representative 2D nonmetallic material, black phosphorus (BP) has the valence electron structure similar to nitrogen, which can effectively adsorb the inactive nitrogen molecule and activate its triple bond. In addition, the relatively weak hydrogen adsorption can restrict the competitive and vigorous hydrogen evolution reaction. Herein, ultrafine BP quantum dots (QDs) are prepared via liquid‐phase exfoliation and then assembled on catalytically active MnO₂ nanosheets through van der Waals interactions. The obtained BP QDs/MnO₂ catalyst demonstrates admirable synergetic effects in electrochemical NRR. The monodisperse BP QDs providing major activity manifest excellent ammonia production steadily with high selectivity, which benefits from the robust confinement of the BP QDs on the wrinkled MnO₂ nanosheets with decent activity. A high ammonia yield rate of 25.3 µg h^?1 mg_cat.^?1 and faradic efficiency of 6.7% can be achieved at ?0.5 V (vs RHE) in 0.1 m Na₂SO₄ electrolyte, which are dramatically superior to either component. The isotopic labelling and other control tests further exclude the external contamination possibility and attest the genuine activity.     

Graphene Scroll‐Coated α‐MnO2 Nanowires as High‐Performance Cathode Materials for Aqueous Zn‐Ion Battery

The development of manganese dioxide as the cathode for aqueous Zn‐ion battery (ZIB) is limited by the rapid capacity fading and material dissolution. Here, a highly reversible aqueous ZIB using graphene scroll‐coated α‐MnO₂ as the cathode is proposed. The graphene scroll is uniformly coated on the MnO₂ nanowire with an average width of 5 nm, which increases the electrical conductivity of the MnO₂ nanowire and relieves the dissolution of the cathode material during cycling. An energy density of 406.6 Wh kg^?1 (382.2 mA h g^?1) at 0.3 A g^?1 can be reached, which is the highest specific energy value among all the cathode materials for aqueous Zn‐ion battery so far, and good long‐term cycling stability with 94% capacity retention after 3000 cycles at 3 A g^?1 are achieved. Meanwhile, a two‐step intercalation mechanism that Zn ions first insert into the layers and then the tunnels of MnO₂ framework is proved by in situ X‐ray diffraction, galvanostatic intermittent titration technique, and X‐ray photoelectron spectroscopy characterizations. The graphene scroll‐coated metallic oxide strategy can also bring intensive interests for other energy storage systems.     

Rational Design of Na(Li1/3Mn2/3)O2 Operated by Anionic Redox Reactions for Advanced Sodium‐Ion Batteries

In an effort to develop high‐energy‐density cathodes for sodium‐ion batteries (SIBs), low‐cost, high capacity Na(Li_1/3Mn_2/3)O₂ is discovered, which utilizes the labile O 2p‐electron for charge compensation during the intercalation process, inspired by Li₂MnO₃ redox reactions. Na(Li_1/3Mn_2/3)O₂ is systematically designed by first‐principles calculations considering the Li/Na mixing enthalpy based on the site preference of Na in the Li sites of Li₂MnO₃. Using the anionic redox reaction (O^2?/O^?), this Mn‐oxide is predicted to show high redox potentials (≈4.2 V vs Na/Na⁺) with high charge capacity (190 mAh g^?1). Predicted cathode performance is validated by experimental synthesis, characterization, and cyclic performance studies. Through a fundamental understanding of the redox reaction mechanism in Li₂MnO₃, Na(Li_1/3Mn_2/3)O₂ is designed as an example of a new class of promising cathode materials, Na(Li_1/3M_2/3)O₂ (M: transition metals featuring stabilized M⁴⁺), for further advances in SIBs.     

Tumor Microenvironment‐Triggered Supramolecular System as an In Situ Nanotheranostic Generator for Cancer Phototherapy

The efficacy of photosensitizers in cancer phototherapy is often limited by photobleaching, low tumor selectivity, and tumor hypoxia. Assembling photosensitizers into nanostructures can improve photodynamic therapy efficacy and the safety profile of photosensitizers. Herein by employing supramolecular assembly, enhanced theranostic capability of Mn²⁺‐assisted assembly of a photosensitizer (sinoporphyrin sodium, DVDMS) is demonstrated. A tumor environment‐triggered coassembly strategy is further developed to form Mn/DVDMS nanotheranostics (nanoDVD) for cancer phototherapy. MnO₂ nanosheets serve as a highly effective DVDMS carrier and in situ oxygen and nanoDVD generator. In MCF‐7 cells and xenograft tumors, MnO₂/DVDMS is reduced by glutathione (GSH) and H₂O₂ and reassembled into nanoDVD, which can be monitored by activated magnetic resonance/fluorescence/photoacoustic signals. Intriguingly, the decrease of GSH, the production of O₂, and the formation of nanoDVD are shown to be synergistic with phototherapy to improve antitumor efficacy in vitro and in vivo, offering a new avenue for cancer theranostics.     

Nano-domain structure of Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript> spinel

XRD-pure Li₄Mn₅O₁₂ spinels are obtained below 600 °C from oxalate and acetate precursors. The morphology consists of nanometric particles (about 25 nm) with a narrow particle size distribution. HRTEM and electron paramagnetic resonance (EPR) spectroscopy of Mn⁴⁺ are employed for local structure analysis. The HRTEM images recorded on nano-domains in Li₄Mn₅O₁₂ reveal its complex structure. HRTEM shows one-dimensional structure images, which are compatible with the (111) plane of the cubic spinel structure and the (001) plane of monoclinic Li₂MnO₃. For Li₄Mn₅O₁₂ compositions annealed between 400 and 800 °C, EPR spectroscopy shows the appearance of two types of Mn⁴⁺ ions having different metal environments: (i) Mn⁴⁺ ions surrounded by Li⁺ and Mn⁴⁺ and (ii) Mn⁴⁺ ions in Mn⁴⁺-rich environment. The composition of the Li⁺, Mn⁴⁺-shell around Mn⁴⁺ mimics the local environment of Mn⁴⁺ in monoclinic Li₂MnO₃, while the Mn⁴⁺-rich environment is related with that of the spinel phase. The structure of XRD-pure Li₄Mn₅O₁₂ comprises nano-domains with a Li₂MnO₃-like and a Li_4/3−x Mn_5/3+x O₄ composition rather than a single spinel phase with Li in tetrahedral and Li_1/3Mn_5/3 in octahedral spinel sites. The annealing of Li₄Mn₅O₁₂ at temperature higher than 600 °C leads to its decomposition into monoclinic Li₂MnO₃ and spinel Li_4/3−x Mn_5/3+x O₄.     

α-MnO2 nanowires transformed from precursor δ-MnO2 by refluxing under ambient pressure: The key role of pH and growth mechanism

α-MnO₂ nanowires were obtained by reflux treatment of precursor δ-MnO₂ in acidic medium under ambient pressure. The great effects of pH on the transformation of δ-MnO₂ to α-MnO₂ and the concentration of coexistent cations (K⁺, Mn²⁺) was investigated in systematically designed experiments by using powder X-ray diffraction and atomic absorption spectrometry analysis. The specific surface area of the products could be simply controlled by adjusting the initial pH value of the suspension. The micro-morphologies during the transition process from the precursors to final products were characterized by SEM and TEM. A dissolution–recrystallization mechanism was proposed to describe the growth process of the one-dimensional nanowire. MnO_x units or MnO₆ octahedra was formed firstly from the dissolution of outmost surfaces of δ-MnO₂, followed by a rearrangement/crystallization to form one-dimensional α-MnO₂ nanowire. In addition, the time-dependent process of dissolution would take place gradually from the external to internal of the precursor.