α-MnO2 Nanowires Research Articles

Tailoring the Ag+ Content within the Tunnels and on the Exposed Surfaces of α-MnO2 Nanowires: Impact on Impedance and Electrochemistry

Efficient conduction of both electrons and cations (e.g., Li+) has a profound effect on the current and capacity of lithium-based batteries. With this study, we focus on cathode effects, with the preparation of pure silver hollandite materials with variable silver ion content within (intra-tunnel) and on the surface of α-MnO2 tunneled materials, followed by the measurement and analysis of impedance and electrochemistry data. Specifically, pure AgxMn8O16-y materials with low (x = 1.13) and high (x = 1.54) intra-tunnel silver content are compared with AgxMn8O16-y·aAg2O (a = 0.25, 0.63, 1.43) composites prepared via a new Ag2O coating strategy. When the Ag2O (a = 0, 0.25) content is low, the material with higher intra-tunnel silver (x = 1.53) content delivers up to ∼5-fold higher capacity accounted for by a ∼10-fold lower impedance than its lower intra-tunnel silver (x = 1.13) counterpart. In the presence of high Ag2O content (a = 0.63, 1.43), both composites exhibit comparable impedance but the lower intra-tunnel silver (x = 1.13) composite delivers up to ∼1.5-fold higher capacity than higher intra-tunnel silver composite, highlighting the key role of Li+ transport under those conditions. Our results demonstrate material design strategies which can significantly increase electronic and ionic conductivities.

Spinel-layered integrate structured nanorods with both high capacity and superior high-rate capability as cathode material for lithium-ion batteries

Spinel phase LiMn2O4 was successfully embedded into monoclinic phase layeredstructured Li2MnO3 nanorods, and these spinel-layered integrate structured nanorods showed both high capacities and superior high-rate capabilities as cathode material for lithium-ion batteries (LIBs). Pristine Li2MnO3 nanorods were synthesized by a simple rheological phase method using α-MnO2 nanowires as precursors. The spinel-layered integrate structured nanorods were fabricated by a facile partial reduction reaction using stearic acid as the reductant. Both structural characterizations and electrochemical properties of the integrate structured nanorods verified that LiMn2O4 nanodomains were embedded inside the pristine Li2MnO3 nanorods. When used as cathode materials for LIBs, the spinel-layered integrate structured Li2MnO3 nanorods (SL-Li2MnO3) showed much better performances than the pristine layered-structured Li2MnO3 nanorods (L-Li2MnO3). When charge–discharged at 20 mA·g−1 in a voltage window of 2.0–4.8 V, the SL-Li2MnO3 showed discharge capacities of 272.3 and 228.4 mAh·g−1 in the first and the 60th cycles, respectively, with capacity retention of 83.8%. The SL-Li2MnO3 also showed superior high-rate performances. When cycled at rates of 1 C, 2 C, 5 C, and 10 C (1 C = 200 mA·g−1) for hundreds of cycles, the discharge capacities of the SL-Li2MnO3 reached 218.9, 200.5, 147.1, and 123.9 mAh·g−1, respectively. The superior performances of the SL-Li2MnO3 are ascribed to the spinel-layered integrated structures. With large capacities and superior high-rate performances, these spinel-layered integrate structured materials are good candidates for cathodes of next-generation high-power LIBs.

Decoration of one-dimensional MnO2 with Co3O4 nanoparticles: A heterogeneous interface for remarkably promoting catalytic oxidation activity

Heterogeneous interface in materials has recently attracted much attention because of its crucial role in catalytic reaction. Herein, we report the preparation of high-quality Co3O4-decorated one dimensional (1D) MnO2 hybrids and their application in catalytic degradation of carcinogenic benzene. Three kinds of 1D MnO2 including α-MnO2 nanowires, α-MnO2 nanorods and β-MnO2 microrods were firstly synthesized by different hydrothermal method, followed by a controlled Co3O4 functionalized process. The homogeneous coating of the Co3O4 nanoparticles on the surface of the MnO2 created large amounts of remarkable MnO2-Co3O4 heterogeneous interface which was characterized by SEM and TEM techniques. XPS and H2-TPR studies revealed the decorated hybrids had better low-temperature reducibility and more active surface adsorbed oxygen species. Compared with the bare MnO2, these hybrid MnO2@Co3O4 nanaostructures showed improved catalytic reactivity toward gaseous benzene oxidation, and the enhancement had different dependence upon the various MnO2. Under high space velocity of 120,000mLg−1h−1, the best-performed α-MnO2 nanowires@Co3O4 can decrease the T90 (temperature of 90% benzene conversion) to 247°C which was almost 100°C lower than the pure α-MnO2 nanowires. Their significantly promoted catalytic activity should be attributed to the strong synergistic effect due to the existence of remarkable MnO2-Co3O4 hetero-interface.

Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials.

Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h(-1)·gAu(-1) at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst.

Tuning chemical bonding of MnO2 through transition-metal doping for enhanced CO oxidation

Replacing a small fraction of cations in a host metal oxide with a different cation (also known as doping) provides a useful strategy for improving the catalytic activity. Here, we report transition metal (Fe, Co, Ni, and Cu)-doped α-MnO2 nanowires synthesized by a one-step hydrothermal method as CO oxidation catalysts. The as-prepared catalysts displayed morphology, crystal structure, and specific surface area similar to those of the pure MnO2 nanowires. A catalytic activity test showed that all doped MnO2 nanowires exhibited much enhanced CO oxidation activity, with the Cu-doped ones being the most active (TOF of 9.1×10−3s−1 at 70°C). The Cu-doped MnO2 nanowires showed nearly 100% conversion of CO at 100°C at an hourly gas space velocity of 36,000mLg−1h−1, which could last for 50h without obvious deactivation even in the presence of 2% water vapor. Density functional theory calculations suggested that Cu doping could make the formation of oxygen vacancies in MnO2, which is the rate-determining step for CO oxidation reaction, easier than for Fe-, Co-, and Ni-doped and pristine MnO2. Our work demonstrates a facile and promising strategy for improving the catalytic activity for oxide-based catalysts, which should be applicable for a variety of different chemical reactions.

Dynamic Study of (De)Sodiation in Tunnel-Based Manganese Oxides and the Capacity Retention Via Voltage Control

Developing advanced cathode materials for sodium ion batteries (NaIBs) with stable host structure and reliable capacity performance during cycling is crucial for the commercialization of NaIBs.[1] Nanostructured MnO2 is one competitive cathode candidate due to its low cost, environmental friendliness, and various open-framed polymorphs to facilitate cation insertion and extraction.[2] α-MnO2, which features 2×2 tunnels with 4.6×4.6 Å2 cavity, has recently become one research focus as such a tunneled structure in nanoscale was reported to be suitable for Na+ transport.[3] However, α-MnO2 suffers from structure instability, low coulombic efficiency and quick capacity decay during cycling, which prevent its further application to a large extent. The causes of these problems should be directly related to the repetitive tunnel-Na+interactions during (de)sodiation, which is currently poorly understood. In this work, the (de)sodiation of α-MnO2 nanowires is dynamically studied and well understood using in situ TEM with an open cell design. The morphology, phase and structure of the electrode are characterized and recorded in real time. It is found that the sodiation starts with Na+ intercalation into the 2×2 tunnels without tunnel degradation up to Na/Mn equaling 0.5 (Mn3.5+). Then α-MnO2 phase gradually evolves into an intermediate Na0.5MnO2 phase. Upon deep sodiation to Na/Mn equaling 1 (Mn3+), the structure totally collapses and degrades to polycrystalline Mn2O3 and Na2O. The following (de)sodiation cycles confirm that the tunneled structure, once destroyed, can not be recovered, and the following cycles are dominated by the conversion reaction between Na0.5MnO2 and Mn2O3, whose reversibility is poor. The galvanostatic voltage profile at bulk level coin cells shows that the intercalation reaction featuring a slope profile happens between 4-1.4 V, while the conversion reaction featuring a plateau region is below 1.4 V. Based on this, the cycling voltage windows of the Na/MnO2 coin cells are set to above 1.5 V to maintain the original tunneled structures of α-MnO2 for reversible Na+(de)intercalation. This voltage control results in the prominent capacity retention of the batteries compared to those cycled below 1.5 V, whose capacity decays very fast. The coulombic efficiency is also obviously improved when the cells are cycled above 1.5 V.

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.

Fabrication, characterization and catalytic activity of α-MnO2 nanowires for dye degradation of reactive black 5

α-MnO2 nanowires (NWs) prepared by hydrothermal method are characterized using XRD and FT-IR. The crystallite size, surface area of NWs increases, whereas dislocation density and band gap decreases with an increase in oxidizer molarity. The band gap decreases from 2.55 to 1.27eV. The above observations correlate well with the enhanced catalytic activity of MnO2 NWs for degradation of azo dye reactive black 5 (RB5). About 70% of the dye were successfully removed in 60min using 20mg of MnO2 NWs in the presence of 6mL of H2O2. MnO2 NWs show a good reusability, suggesting it as an effective and recyclable catalyst.

Free standing flexible graphene oxide + α-MnO2 composite cathodes for Li–Air batteries

The most important factor regarding to influence the electrochemical performance of lithium–air battery is to choose an effective catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, α-MnO2 (manganese oxide) nanowires were used as an active electrocatalyst synthesized by microwave hydrothermal method. Graphene oxide (GO) and α-MnO2 nanowires were combined to obtain free standing paper cathodes and α-MnO2+GO free standing paper cathodes were produced with dispersing different amounts of α-MnO2 nanowires (10 wt.%, 30 wt.%, 50 wt.%, and 70 wt.%) by using vacuum filtration method. The morphologies and structures of cathodes are evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. The electrochemical tests were carried out at the voltage window between 1.0 and 4.5V in ECC-Air test cell. α-MnO2+GO composite paper containing 70wt.% α-MnO2 catalyst showed the highest specific discharge capacity value of 2900mAhg−1. Moreover, the experimental findings showed that dispersing of α-MnO2 catalyst in GO structure results in remarkable improvement of the electrochemical performance of Li–air battery cathodes.

Insights into the degradation of 2,4-dichlorophenol in aqueous solution by α-MnO2 nanowire activated persulfate: catalytic performance and kinetic modeling

α-MnO2 nanowires were synthesized through a hydrothermal method. Sulfate and hydroxyl radicals were produced in α-MnO2-activated persulfate system to degrade 2,4-dichlorophenol.

Graphene supported α-MnO2 nanocomposite cathodes for lithium ion batteries

In this study, Graphene/α-MnO2 nanocomposite cathodes were produced using ultrasonication and mechanical alloying facilities. Before production of Graphene/α-MnO2 composites, Graphene structure was fabricated by reduction of Graphene Oxide produced using Hummer's Method and α-MnO2 nanowires was synthesized using microwave hydrothermal synthesis method. The effect of Graphene support on the electrochemical performance of the Graphene/α-MnO2 nanocomposite cathodes were investigated by dispersing different amounts of Graphene (10 wt.%, 30 wt.% and 50 wt.%) in the composite cathodes. Structures of the Graphene/α-MnO2 nanocomposites were characterized using X-ray diffraction (XRD) and Raman Spectroscopy techniques. Surface morphology of the electrodes was characterized using scanning electron microscopy (SEM). Electrochemical cycling tests of the Graphene/α-MnO2 nanocomposite cathodes were performed in the voltage range of 1.5 V–4.5 V at a constant current density of 50 mA/g (C/5). This study proved that Graphene enhance the electrochemical reaction of α-MnO2 and provide stable cathode materials. Eventually, Graphene/α-MnO2 cathode containing 50 wt.% Graphene showed 225 mAh/g discharge capacity even after 200 cycles.

Atomistic Insights into the Oriented Attachment of Tunnel-Based Oxide Nanostructures.

Controlled synthesis of nanomaterials is one of the grand challenges facing materials scientists. In particular, how tunnel-based nanomaterials aggregate during synthesis while maintaining their well-aligned tunneled structure is not fully understood. Here, we describe the atomistic mechanism of oriented attachment (OA) during solution synthesis of tunneled α-MnO2 nanowires based on a combination of in situ liquid cell transmission electron microscopy (TEM), aberration-corrected scanning TEM with subangstrom spatial resolution, and first-principles calculations. It is found that primary tunnels (1 × 1 and 2 × 2) attach along their common {110} lateral surfaces to form interfaces corresponding to 2 × 3 tunnels that facilitate their short-range ordering. The OA growth of α-MnO2 nanowires is driven by the stability gained from elimination of {110} surfaces and saturation of Mn atoms at {110}-edges. During this process, extra [MnOx] radicals in solution link the two adjacent {110} surfaces and bond with the unsaturated Mn atoms from both surface edges to produce stable nanowire interfaces. Our results provide insights into the controlled synthesis and design of nanomaterials in which tunneled structures can be tailored for use in catalysis, ion exchange, and energy storage applications.

Controlled synthesis of α-MnO2 nanowires and their catalytic performance for toluene combustion

α-MnO2 nanowires with a length about 6–10μm and an average diameter of 20nm were synthesized through a facile hydrothermal process without any templates or surfactants. The products were characterized by X-ray powder diffraction, Raman spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, hydrogen temperature-programmed reduction techniques, X-ray photoelectron spectroscopy and surface area analysis. The effects of the hydrothermal temperature and the concentration of CH3COOH on the crystal phase and morphology of the final products were studied in detail. The hydrothermal temperature and the concentration of CH3COOH play crucial roles in determining the crystal phase and morphology of the products. The possible formation mechanism of the α-MnO2 nanowires was investigated and discussed. Additionally, the as-prepared α-MnO2 nanowires showed higher catalytic activity for toluene combustion than the commercial MnO2, suggesting their potential applications in the elimination of volatile organic compounds.

Controllable synthesis and characterization of α-MnO2 nanowires

Size controllable α-MnO2 nanowires were synthesized from KMnO4 and ethanol via a low-temperature hydrothermal route assisted with subsequently heat treatment. No acid or alkali was added. XRD and SEM results clarified α-MnO2 nanoparticles (5–30nm) began to form at room temperature, and then grew up to nanowires (~12nm in diameter) when temperature increasing up to 120–160°C for an appropriate reaction time. Otherwise, octahedron-like Mn3O4 began to form at 120°C more than 4h or at 160°C more than 2h. Hydrothermal-synthesized α-MnO2 show well thermal-stability even it was heated at 900°C. And various size α-MnO2 nanowires were controllable prepared by adjusting heat treatment temperature of hydrothermal-synthesized α-MnO2 at range from 150°C to 750°C for 3h, which show the strongest diffraction peak (211) plane. The capacity of hydrothermal-synthesized α-MnO2 at 120°C for 2h was about 125F/g, which exhibited a promising electrochemical property.

Multi-Wall Carbon Nanotubes As Cathodes for Lithium-Air Batteries: Effect of Catalysts and Additives

Batteries with high specific energy densities have attracted a lot of attention due to their demand in electric vehicles (EVs). Due to the increase in demand for energy storage devices for applications such as load-levelling of electrical energy supply and demand [1], propulsion for electric vehicles, and microelectro-mechanical devices, the research for new classes of electrode materials that provide high energy, high power and cycle life longer than lithium-ion batteries is being motivated [2,3]. Lithium air batteries have high specific energy density and they can achieve four times higher energy density than the current lithium ion batteries [2, 3]. Lithium air batteries also have higher theoretical energy density than lithium ion batteries [4]. Catalysts have been shown to improve both the battery capacity and the recyclability of these batteries when used in cathodes. Several catalysts have been discovered for lithium air batteries. Debart et al. used several catalysts for lithium oxygen batteries with non-aqueous electrolyte. The electro-catalysts used by them included Pt, La0.8Sr0.2MnO3, Fe2O3, NiO, CuO, CoFe2O4. Co3O4 exhibited highest discharge capacity and best cycling performance. Later Debart et al. examined manganese oxides as catalysts for lithium oxygen batteries. Mn3O4, bulk Mn2O3, bulk α, β, λ, γ-MnO2, α-MnO2 nanowires and β-MnO2 nanowires were used as catalysts [6]. The air cathode with α-MnO2 as the catalyst showed the highest capacity of about 3000 mAh/g (carbon) Lu et al. reported ORR studies on polycrystalline metal catalysts of palladium, platinum, ruthenium, gold and glassy carbon surfaces in non-aqueous electrolyte [5]. In this study, we investigate various modes of loading palladium-based catalysts and their effect on the capacity and round-trip efficiency of lithium-air batteries. The aim of our study is to increase the first discharge capacity, improve the recyclability, increase capacity retention and improve the recyclability of the battery. The loading and morphology the catalyst-CNT cathodes were studied using transmission electron microscopy. Electrochemical testing using electrochemical impedance spectroscopy and charge-discharge cycling were also investigated. The cathode consisted of conductive carbon cloth coated with MWCNT or palladium inside MWCNT. Celgard 2040 was used as a separator. LiTFSi (Bis Trifluoromethane sulfonamide Lithium salt) in TEGDME (Tetraethyl glycol di-methyl ether) was used as electrolyte. The battery was assembled in the Swagelok cell. Figure 1 shows the assembly of the battery. Figure 2 shows the TEM images of palladium nanoparticles inside MWCNT. Figure 3 shows Pristine CNT with first cycle at a current rate of 200 mA/g showing a discharge capacity of 9300 mAh/g and a charge capacity of 8250 mAh/g. Figure 4 shows catalyst-loaded CNT with first cycle at a current rate of 200 mA/g showing a discharge capacity of 13800 mAh/g and a charge capacity of 3000 mAh/g.

Morphological analysis of ultra fine α-MnO2 nanowires under different reaction conditions

A simple hydrothermal method was developed for the synthesis of ultra fine single-crystal α-MnO2 nanowires by only using potassium permanganate and sodium nitrite in acidic solution, without any seed or template. Detailed analysis of the obtained nanowires was done using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and high resolution transmission electron microscopy (HRTEM) measurements. The as-prepared α-MnO2 nanowires have the average diameter of 10–40nm and a length up to 0.1–2µm. Moreover, the effects of reaction temperature, time and reactant molar ratios on the morphology of the final product were studied in detail using field emission scanning electron microscopy (FESEM) measurements.

Coating of sulfur particles with manganese oxide nanowires as a cathode material in lithium–sulfur batteries

α-MnO2 nanowires were synthesized by a hydrothermal method and then mixed with agglomerated sulfur powders. Homogeneous α-MnO2-coated sulfur particles were obtained by utilizing heat treatment intended to melt and recrystallize the sulfur. α-MnO2 nanowires lead to the suppression in re-agglomeration of sulfur powders during heat treatment and result in successful coating as well as decreased size in parent sulfur powders. The α-MnO2-coated sulfur particles were used as the cathode material for lithium-ion battery applications. Electrochemical evaluation such as the rate capability characteristics and impedance study indicates that the α-MnO2 coated sulfur cathode shows lower resistance and improved rate capability compared to a pristine sulfur cathode.

In-Situ TEM Study of Intercalation- and Conversion-Driven Lithiation in Single α-MnO2 Nanowire

Nano-sized transitional metal oxides (TMOs, M=Fe, Co, Cu, Mn, etc.) have been increasingly recognized as promising candidate anode for lithium ion battery (LIB) with high specific capacity (700–1200 mAh/g). The high capacity is achieved via a so-called reversible “conversion reaction” with Li+, as shown in Equation (1): MxOy + 2yLi+ + 2ye- = xM + yLi2O (1) where TMO, after reaction with Li+, is reduced to metal (M), and Li2O also forms via combination of Li+ and O2-. In spite of their high capacity, most TMOs suffer from poor rate performance, poor cycle performance and low coulombic efficiency in 1st cycle, preventing its commercialization. The origins and mechanisms underlying these problems are still unclear. In this study, single crystalline α-MnO2 nanowire is used as a TMO representative to study the electrochemical lithiation mechanism by constructing an all-solid open-cell inside Transmission Electron Microscope (TEM) with MnO2 as the target electrode and Li metal as Li+ source. Morphological evolution, structural change and phase transition are simultaneously characterized inside TEM. A novel two-step expansion behavior of α-MnO2 nanowire is first visually observed during lithiation. These two expansion stages are sharply different in terms of expansion-needed time, propagation speed of each expansion front and volume change during each expansion. High resolution TEM and Selected Area Electron Diffraction analysis confirm that the first expansion stage is lead by 2×2 tunnel-driven lithiation and formation of LixMnO2 solid solution, while the second expansion stage is caused by thorough conversion of LixMnO2 to Mn and Li2O accompanied by fracture formation and electrode pulverization. These findings provide possible origins for electrochemical drawbacks of TMOs as LIB electrode and also cast light on potential electrode improvement by proper structural and compositional modification.

Physiochemical Investigation of Shape-Designed MnO2 Nanostructures and Their Influence on Oxygen Reduction Reaction Activity in Alkaline Solution

In this work, five types of MnO2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky–Levich plots and % of H2O2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nanowire > nanorod > nanotube > nanoparticle > nanoflower. α-MnO2 nanowires possess enhanced electrocatalytic activity compared to other shapes, even though the nanotubes possess a much higher BET surface area. In the ORR studies, α-MnO2 nanowires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn–O bonds along with a Mn–Mn distance of 2.824 Å, which provides an optimum requirement for the adsorbed oxygen in a bridge mode favoring the direct 4 electron reduction. In accordance with the first principles based density functional theory (DFT), the enhancement in ORR activity is due to the less activation energy needed for the reaction by the (211) surface than all other surfaces.

Performance modulation of α-MnO₂ nanowires by crystal facet engineering.

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.