α-MnO2 Nanowires Research Articles

Mechanistic Understanding of the Underlying Energy Storage Mechanism of α-MnO2-based Pseudo-Supercapacitors.

Manganese dioxide (α-MnO2) has attracted significant research interest in supercapacitors recently. However, the reaction mechanism of α-MnO2 in supercapacitors remains unclear. Therefore, a nano-supercapacitor using Environmental transmission electron microscopy (ETEM) is conducted and investigated the reaction mechanism of α-MnO2 based on three ionic liquids (ILs). It found that in the aprotic ionic liquid (AIL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIMOTF), α-MnO2 nanowires (NWs) undergo an oxidation reaction due to the presence of an active proton at the second position (H2) of the imidazole ring. As a result, α-MnO2 NWs undergo a phase transition and transform into Mn3O4, exhibiting pseudo-capacitive properties. Furthermore, characterization of the macroscopic α-MnO2 electrodes after cycling reveals that after the initial charging cycles, the dominant energy storage mechanism of the supercapacitor transitions from pseudo-capacitance to a dual-layer capacitance formed by the combination of Mn3O4 and unreacted α-MnO2. Simultaneously, due to the coexistence of these two energy storage mechanisms, the specific capacitance of the supercapacitor in EMIMOTF electrolyte reaches up to 80 F g-1, and the cycle number reaches as high as 1000 cycles. The results are expected to provide insights into the selection of electrolytes in supercapacitors and offer a fundamental understanding of the internal reaction mechanisms in capacitors.

Biochar assisted synthesis of α-MnO2 nanowires with superior performance for non-radical activation of peroxymonosulfate

Non-radical activation of peroxymonosulfate (PMS) is a promising water treatment technology for removing refractory organics, but its mechanism is still ambiguous and the development of highly efficient catalyst is still a challenge. Herein, α-MnO2 nanowires were synthesized by a hydrothermal method with the assistance of biochar and their performance for PMS activation was studied. The characterization results show that the addition of biochar had little effect on the crystal phase and valence state of MnO2 but significantly reduced the size of nanowires. These nanowires had mesoporous structure, high surface area and oxidation ability, resulting in their superior performance for removing organic pollutants with PMS activation. The degradation of phenol by α-MnO2-PMS followed first order kinetics with an activation energy of 14.82 kJ mol−1. Electron paramagnetic resonance and radical scavenging experiments demonstrated that this activation process followed the non-radical mechanism. The quenching effect of scavengers led to the misleading role of singlet oxygen. In fact, surface activated PMS (MnO2–PMS*) rather than singlet oxygen was the reactive species. Weak acidic condition was conducive for the formation of MnO2–PMS* complexes. This catalyst exhibited good reusability and high activity for removing various organic pollutants, and the common substances in real water had little effect on its performance, suggesting its potential application in wastewater treatment.

Improved near-resonant two photon absorption of α-MnO2 nanowires surface functionalized reduced graphene oxide sheets

The versatile study of graphene derivatives has resurgence of interest on understanding its nonlinear optical response at different nanoscales. Nanostructured manganese dioxide: reduced graphene oxide (MnO2: rGO) hybrids were synthesized by reduction method by varying the content of rGO (5, 10, 15 & 20 wt%). X-ray Photoelectron Spectroscopy studies confirmed the surface functionalization of MnO2 on rGO. Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Tunnelling Electron Microscopy (HRTEM) witness that with respect to the content of rGO, the morphology of MnO2 varies radically from clusters to nanorods to nanowires over the surface of rGO sheets. Ground state absorption spectra of the hybrids have absorption edge at 268 nm corresponding to combined metal oxide charge transfer between rGO (π-π* transition of CC and n-π transition of C-O bonds) and MnO2 (d-d transition). Moreover, hierarchical MnO2 nanowire network resided on 2D rGO sheets involves broad absorption at the visible spectral domain due to polaron transition as well the localization of π- π* transited electrons that facilitates the probability of occurrence of excited state absorption. Intensity dependent open aperture Z-scan technique of the prepared hybrids using Q-switched Nd: YAG 532 nm nanopulsed laser reveals unusual transitions and enhanced nonlinear absorption. MnO2:(5 wt%) rGO shows saturable absorption (SA) at low intensity and changeover dynamically to reverse saturable absorption (RSA) at higher intensity. MnO2:(10 & 15 wt%) rGO exposes coupled nonlinear absorption (SA + RSA) owing to the combined effect of graphene (π-states) and MnO2 (σ-states). MnO2:(20 wt%) rGO exhibits a stronger RSA ascribed to sequential two photon absorption. The third-order nonlinear absorption coefficient and limiting threshold of MnO2: rGO (20 wt%) are 4.04 ×10-10 m/W and 0.84 ×1012 W/m2, which shows five-fold increase compared to bare rGO and MnO2. The obtained results propose that the optical nonlinearity of MnO2 nanowires surface functionalized rGO sheets can significantly be tuned to meet the requirements for fabricating laser safety devices.

Advanced three-dimensional hierarchical porous α-MnO2 nanowires network toward enhanced supercapacitive performance

Hierarchical α-MnO2 nanowires with oxygen vacancies grown on carbon fiber have been synthesized by a simple hydrothermal method with the assistance of Ti4+ ions. Ti4+ ions play an important role in controlling the morphology and crystalline structure of MnO2. The morphology and structure of the as-synthesized MnO2 could be tuned from δ-MnO2 nanosheets to hierarchical α-MnO2 nanowires with the help of Ti4+ ions. Based on its fascinating properties, such as many oxygen vacancies, high specific surface area and the interconnected porous structure, the α-MnO2 electrode delivers a high specific capacitance of 472 F g−1 at a current density of 1 A g−1 and the rate capability of 57.6% (from 1 to 16 A g−1). The assembled symmetric supercapacitor based on α-MnO2 electrode exhibits remarkable performance with a high energy density of 44.5 Wh kg−1 at a power density of 2.0 kW kg−1 and good cyclic stability (92.6% after 10 000 cycles). This work will provide a reference for exploring and designing high-performance MnO2 materials.

High mass loading potassium ion stabilized manganese dioxide nanowire forests for rechargeable Zn batteries

Manganese dioxide (MnO2) represents an ideal cathode material for rechargeable aqueous Zn batteries due to its high theoretical capacity (308 mAh g−1), suitable potential (1.4 V vs. Zn2+/Zn), natural abundance, and negligible toxicity. However, the capacity and rate capability of MnO2 deteriorate significantly in thick electrodes owing to its low electrical and ionic conductivities. Herein, we report the design of high mass loading potassium ion stabilized α-MnO2 (K0.133MnO2) nanowire forests on carbon cloth through a seed-assisted hydrothermal method for Zn batteries. The vertically aligned K0.133MnO2 nanowire forests with uninterrupted charge transport afford a high area capacity of 3.54 mAh cm−2 and a capacity retention of 79.2% over 1000 cycles in aqueous electrolyte. Moreover, the high area capacity and cyclability can be readily transferred to quasi-solid-state devices.

Efficiency and synergy of MnO2@LDO for arsenic and fluoride simultaneous sorption from water

High levels of groundwater containing both arsenic and fluorine are prevalent, resulting in serious health problems when consumed as drinking water. This co-pollution phenomenon is widespread and requires urgent attention. The multiple forms of arsenic and arsenic–fluorine co-contamination pose a significant challenge to efficiently co-remove both substances. This research utilized a green and stable synthesis approach to create MgLaFe layered double oxide (LDO) heterostructures, which were anchored on α-MnO2 nanowires. The materials comprise magnesium and lanthanum elements with a powerful attraction toward fluoride ions; elemental iron, which can establish stable compounds with arsenate; and MnO2, which can effectively oxidize arsenous acid, thereby enabling efficient co-removal of arsenic and fluorine. The efficient oxidation process of the MnO2 nanowire and the prompt ion adsorption process of the LDO work together synergistically. The adsorption performance was assessed through isotherms and kinetic fitting. Chemisorption was found to be the process for As(Ⅲ), As(V), and F− adsorption, with As(Ⅲ) going through monolayer adsorption on the surface of MnO2 nanowires, while As(V) and F− were mainly adsorbed by multilayer process on LDO. The maximum adsorption capacities were 111.76, 230.51, and 765.10 mg/g for As(Ⅲ), As(V), and F−, respectively. The x-ray photo-electronic spectroscopy analysis provided further elucidation on the adsorption mechanism of the MnO2@LDO heterostructure, detailing each component's role in the process. The results confirm the successful construction of the heterostructure and the efficient coupling of oxidation and adsorption.

In situ TEM observation of the (De)potassiation process of α-MnO2 nanowires

Based on lithium-ion batteries (LIBs), a new type of electrochemical device known as potassium-ion batteries was developed. Potassium-ion batteries potentially offer several key benefits over LIBs in Non-aqueous and several electrode materials, including steady discharge, high rate performance, cheap cost and lower charge density. Therefore, Potassium-ion batteries (PIBs) are highly likely to become the best candidate for low-cost, long-life batteries in the future, and are expected to extensively be employed in a variety of industries, including mobile devices and new energy vehicles. In recent years, research on the electrochemistry of potassium-ion batteries is still in its initial stage. Here, we use in situ transmission electron microscopy to observe the irreversible volume expansion of α-MnO2 nanowire during potassiation and depotassiation. During the first potassiation, the single crystalline α-MnO2 nanowire transformed to K1.33Mn8O16, causing a volume expansion of 35.6 %, which was unable to recover during the depotasiation process. The irreversible volume expansion suggests that most potassium ions inserted into the α-MnO2 nanowire cannot be extracted during the depotassiation process, which leads to low Coulombic efficiency in the first cycle. The cycle performance and Coulombic efficiency of α-MnO2 maybe still limited by insufficient electronic conductivity. Composite Au on the surface of α-MnO2 maybe improves its electronic conductivity. The α-MnO2 on Au (α-MnO2/Au) nanowires’ irreversible and cycle performance was better than pristine materials by the first principle calculations. Traditionally the first cycle capacity loss is attributed to the formation of the solid electrolyte interface (SEI). This research provides a new approach of the first cycle capacity loss mechanism in potassiation of α-MnO2 cathode. Also, the α-MnO2/Au improves the cycle performance and Coulombic efficiency of the K-(α-MnO2) nanobattery.

Gallium regulated MnO2 toward high performance Zn ion batteries

α-MnO2 with open tunnel structure has received ever-increasing attention because of its superior specific capacity, yet generally undergoes unstable phase transition from the tunnel architecture to layered structure upon Zn2+ insertion as well as the disadvantages of low electrical conductivity and slow Zn2+ diffusion kinetics, thereafter triggering inactive reaction dynamics. In this regard, a gallium (Ga) doped α-MnO2 nanowires (denoted as MnO2-Ga) is firstly built in this work to address the abovementioned issue and the MnO2-Ga2 displays enhanced performance with reversible capacity of 205.1 ± 5 mAh g−1 undergoing 200 cycles at 0.2 A g−1. After repeated 2000 cycles at 1 A g−1, a capacity of 123.7 ± 5 mAh g−1 can be still obtained. Validated by the theoretical calculation, it is noted that the Ga could effectively regulate the electronic distribution of α-MnO2 and narrows the bandgap, prompting the electronic cloud polarization of O and therafter accelerating ion diffusion efficiency. Fundamentally, the Ga intercalation results in the formation of Ga–O bonds. This special building not only guarantees the reversible phase transition and alleviates the Mn dissolution of Ga-MnO2, but also weakens the Zn2+-O2- interactions, leading to elevated cyclic durability and capacity.

Synthesis, characterization, functional testing and ageing analysis of bifunctional Zn-air battery GDEs, based on α-MnO2 nanowires and Ni/NiO nanoparticle electrocatalysts

Electrically rechargeable alkaline zinc air batteries (RZAB) – currently still at the R&D stage –, have great potential for stationary, as well as prospectively mobile, electrochemical energy storage applications. Their chief appeal is that they are made of abundant, environmentally friendly, intrinsically safe, and cheap materials, with established recycling concepts and auspicious life-cycle costs. One of the key weak points of present-generation RZAB programs is the air gas-diffusion electrode (GDE). In fact, on the one hand, GDE fabrication and testing are generally based on poorly understood protocols, and, on the other hand, performance is challenged by efficiency and durability issues. This work is centred on the fabrication of a novel bifunctional GDE for the air side of RZABs, on the assessment of its electrochemical performance and on the identification of factors impacting its efficiency and durability. The electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are α-MnO2 nanowires and Ni/NiO nanoparticles, respectively. The composition of the active layer was optimized with rotating ring-disc electrode (RRDE) electrocatalysts tests. The GDEs were fabricated by spray-coating an ink, formulated with the electrocatalysts and the PTFE binder in an aqueous matrix. Fabrication and functional performance of GDEs – in pristine form and after ageing under realistic RZAB conditions – are rationalized on the basis of Scanning Electron Microscopy (SEM), Scanning Transmission X-ray SpectroMicroscopy (STXSM) at the Mn l-edge and Transmission Electron Microscopy (TEM) analyses. Imaging and spectral imaging disclosed the morphological and chemical-state evolution, brought about by electrochemical cycling. Special attention was devoted to the understanding of the role played by the presence of zincate in the electrolyte on the performance and ageing of the reversible air electrodes.

Integrated α-MnO2 nanowires PVDF catalytic membranes for efficient purification of dye wastewater with high permeability

Catalytic membranes have been studied and reported widely, but ideal catalytic membranes with high rejection ratio and high permeability simultaneously are still a challenge to develop. Herein, integrated α-MnO2 nanowires polyvinylidene fluoride (PVDF) catalytic membranes were fabricated by a vacuum filtration method. Based on the intercalation structures of GO formed by α-MnO2 nanowires and the spatial stacking of nanowires, filter channels with high water flux were constructed. While CS displayed as anchor point to fix α-MnO2 nanowires and also endow the membrane surface with electro-positivity. Then, a sequence of catalytic membranes with different content of α-MnO2 were prepared, and detailed characterizations of structure were taken out. Finally, all of the membranes displayed excellent stability in harsh conditions (pH values from 3 to 11), and M-5 with the most content of α-MnO2 displayed the best separation ability, for which the rejection ratio reached 99.26 % along with a high permeability about 923.72 L·m−2·h−1·Bar−1, when it was used to separate methyl blue (MBE) wastewater. In addition, M-5 also displayed excellent reusability, such as the rejection ratio still maintained above 92.62 % after 9th cycles and reduced to 86.74 % after 10th cycle, while the permeability still remained 612.09 L·m−2·h−1·Bar−1 after the 10th cycle. Therefore, this work developed an outstanding catalytic membrane, which displayed good application prospect for treatment of dye wastewater.

Scalable synthesis of K+/Na+ pre-intercalated α-MnO2 via Taylor fluid flow-assisted hydrothermal reaction for high-performance asymmetric supercapacitors

In this work, a two-step synthetic method: Couette-Taylor flow mixing, and hydrothermal method have been used to synthesize α-manganese oxide pre-intercalated with sodium and potassium ions (Na+/K+@α-MnO2) for supercapacitor application. In addition, the effects of different aqueous electrolytes on the energy storage properties of the Na+/K+@α-MnO2 nanowires are investigated. The results show that KOH is a better electrolyte than NaOH and Na2SO4, with Na+/K+@α-MnO2 nanowires exhibiting a higher specific capacitance of 124 Fg−1 in KOH electrolyte. This is attributed to the high ionic conductivity and smaller hydrate ion of K+ ions. DFT calculations reveal that K+ ions have a higher adsorption energy and a higher partial density of states than Na+ ions, indicating more favourable pseudocapacitive behaviour for K+ ions compared to Na+ ions. Furthermore, the asymmetric capacitor device based on N-CNTs@CF//Na+/K+@α-MnO2 delivers a specific energy density of 21 Wh kg−1 and a specific power density of 450 W kg−1, with excellent cycling performance (∼94 % capacity retention after 5,000 cycles). Our findings demonstrate that the pre-intercalation of α-MnO2 nanowires account for enhanced cycling stability, as well as higher capacitance.

Enhanced High-Performance Aqueous Zinc Ion Batteries with Copper-Doped α-MnO2 Nanosheets Cathodes

Aqueous zinc ion batteries (ZIBs) have garnered considerable interest due to their eco-friendly nature, cost-efficiency, and remarkable safety features, making them a compelling contender for next-generation energy storage systems. Within the extensive array of cathode materials investigated for ZIBs, manganese-based materials stand out for their notable attributes, including low toxicity and high voltage. Nevertheless, their widespread application has been impeded by challenges related to poor cycling stability, low electrical conductivity, and intricate energy storage mechanisms. In this study, we present a novel approach to address these challenges by synthesizing copper-doped α-MnO2 nanosheets through a facile hydrothermal route. The resulting cathode material exhibits remarkable electrochemical properties when integrated into Zn/MnO2 batteries. At a low current density of 0.1 Ag−1, these batteries demonstrate an impressive reversible capacity of 445 mAh g−1, signifying their substantial energy storage capabilities. Furthermore, even when subjected to a demanding high current density of 1 Ag−1, they exhibit an exceptional cycling life of up to 1000 cycles, highlighting the enhanced durability of the copper-doped α-MnO2 nanowire cathode. This research paves the way for the development of high-performance Zn/MnO2 batteries, leveraging the advantages of manganese-based cathode materials while mitigating their inherent limitations. These findings represent a significant step forward in the development of environmentally sustainable and economically viable energy storage solutions, offering hope for a more sustainable energy future.

Simple construction of the α-MnO2@ZIF-8 nanomaterial with highly enhanced U(VI) adsorption performance and assessed removal mechanism

In this study, the one-dimensional nanomaterial α-MnO2@ZIF-8 was satisfactorily fabricated using a simple hydrothermal method by modifying ZIF-8 on α-MnO2 nanowires for high-performance extraction of U(VI) out of water solution. The experimental data indicated that the adsorption of U(VI) on α-MnO2@ZIF-8 was intensely dependent on solution pH but not on ionic strength. The maximum adsorption of U(VI) on α-MnO2@ZIF-8 at pH ​= ​5.0 and T ​= ​298 ​K was 802.36 ​mg/g, substantially higher than the adsorption on α-MnO2(22.32 ​mg/g) or ZIF-8(551.31 ​mg/g) under the same conditions, indicating that the performance of the modified α-MnO2 was greatly enhanced. Attaining adsorption equilibrium within 45 ​min, the analytical kinetic process of α-MnO2@ZIF-8 indicated that its adsorption on U(VI) was monolayer chemisorption. The adsorption experiments and XPS analyzed the possible adsorption mechanism of U(VI) in the reaction with α-MnO2@ZIF-8 during the adsorption process, namely intra-sphere surface complexation, chemisorption and electrostatic interaction. The simple preparation and high removal efficiency of α-MnO2@ZIF-8 indicated that this adsorbent had good prospects for application in wastewater.

Construction of Nb2CTx/α-MnO2 composites by electrostatic self-assembly for controllable impedance matching and enhanced electromagnetic absorption

Nb2CTx/α-MnO2 composites have been constructed by the deposition of α-MnO2 nanowires on the surface of the layered Nb2CTx matrix through an electrostatic self-assembly strategy. The increase of α-MnO2 content can lead to an improvement of permittivity and a shift of the reflection peak towards low frequency at a fixed thickness. Theory analysis speculates that the synergistic effect of lattice defect and electron hopping of Mn4+-O-Mn3+ chains in α-MnO2 nanowires as well as heterogeneous interfaces between Nb2CTx and α-MnO2 are predominant accounts for the enhancement of permittivity. The effective complementary of impedance matching and microwave attenuation can be responsible for the displacement of the reflection peak towards low frequency and a wide-frequency absorption. Accordingly, Nb2CTx/α-MnO2 composites exhibit an optimal reflection loss of -24.7 dB and an absorption bandwidth of 5 GHz at a thin matching thickness of 1.8 mm, which are better than those of most Nb2CTx-based absorbents.

Construction of α-MnO2/g-C3N4 Z-scheme heterojunction for photothermal synergistic catalytic decomposition of formaldehyde

Formaldehyde (HCHO), as the most common indoor air pollutant, undoubtedly poses a huge threat to people's health. It is of practical importance to achieve complete decomposition of HCHO to harmless CO2 at room temperature. Herein, α-MnO2/g-C3N4 Z-scheme heterojunction was constructed through the in-situ growth of α-MnO2 nanowires on the surface of g-C3N4 nanosheets. Owing to the construction of Z-scheme heterojunction, α-MnO2/g-C3N4 composite exhibits excellent photothermal catalytic activity, achieving complete mineralization of HCHO with the illumination of 191 mW/cm2 solar-light at GHSV of 180 L/gcat·h. Particularly, even under one sun and visible light radiation, the α-MnO2/g-C3N4 composite can still maintain 60% and 68% HCHO conversion, respectively. The formation of α-MnO2/g-C3N4 Z-scheme heterojunction not only effectively inhibits the photogenerated electron-hole pairs recombination, but also overcomes the band defects of the individual component. Furthermore, the mechanism of the photothermal catalysis of α-MnO2/g-C3N4 composite was discovered to be thermal-assisted photocatalytic process rather than solar-light driven thermalcatalysis. On the one hand, the thermal energy generated from light radiation can accelerate the migration of photo-generated carriers and improve the separation efficiency of photogenerated electrons from vacancies; on the other hand, it can decrease the activation energy of lattice oxygen and produce more surface-active oxygen species. This work provides cost-effective strategies and suggestions for decomposing HCHO and other VOCs in indoor air at room temperature.

Revealing alkali metal ions transport mechanism in the atomic channels of Au@α-MnO2

Understanding alkali metal ions' (e.g., Li+/Na+/K+) transport mechanism is challenging but critical to improving the performance of alkali metal batteries. Herein using α-MnO2 nanowires as cathodes, the transport kinetics of Li+/Na+/K+ in the 2 × 2 channels of α-MnO2 with a growth direction of [001] is revealed. We show that ion radius plays a decisive role in determining the ion transport and electrochemistry. Regardless of the ion radii, Li+/Na+/K+ can all go through the 2 × 2 channels of α-MnO2, generating large stress and causing channel merging or opening. However, smaller ions such as Li+ and Na+ cannot only transport along the [001] direction but also migrate along the < 110 > direction to the nanowire surface; for large ion such as K+, diffusion along the < 110 > direction is prohibited. The different ion transport behavior has grand consequences in the electrochemistry of metal oxygen batteries (MOBs). For Li-O2 battery, Li+ transports uniformly to the nanowire surface, forming a uniform layer of oxide; Na+ also transports to the nanowire surface but may be clogged locally due to its larger radius, therefore sporadic pearl-like oxides form on the nanowire surface; K+ cannot transport to the nanowire surface due to its large radius, instead, it breaks the nanowire locally, causing local deposition of potassium oxides. The study provides atomic scale understanding of the alkali metal ion transport mechanism which may be harnessed to improve the performance of MOBs.

Transition Metal (Fe2O3, Co3O4 and NiO)-Promoted CuO-Based α-MnO2 Nanowire Catalysts for Low-Temperature CO Oxidation

As a toxic pollutant, carbon monoxide (CO) usually causes harmful effects on human health. Therefore, the thermally catalytic oxidation of CO has received extensive attention in recent years. The CuO-based catalysts have been widely investigated due to their availability. In this study, a series of transition metal oxides (Fe2O3, Co3O4 and NiO) promoted CuO-based catalysts supported on the α-MnO2 nanowire catalysts were prepared by the deposition precipitation method for catalytic CO oxidation reactions. The effects of the loaded transition metal type, the loading amount, and the calcination temperature on the catalytic performances were systematically investigated. Further catalyst characterization showed that the CuO/α-MnO2 catalyst modified with 3 wt% Co3O4 and calcined at 400 °C performed the highest CO catalytic activity (T90 = 75 °C) among the investigated catalysts. It was supposed that the loading of the Co3O4 dopant not only increased the content of oxygen vacancies in the catalyst but also increased the specific surface area and pore volume of the CuO/α-MnO2 nanowire catalyst, which would further enhance the catalytic activity. The CuO/α-MnO2 catalyst modified with 3 wt% NiO and calcined at 400 °C exhibited the highest surface adsorbed oxygen content and the best normalized reaction rate, but the specific surface area limited its activity. Therefore, the appropriate loading of the Co3O4 modifier could greatly enhance the activity of CuO/α-MnO2. This research could provide a reference method for constructing efficient low-temperature CO oxidation catalysts.

Thermal Annealing Induced Surface Oxygen Vacancy Clusters in α-MnO2 Nanowires for Catalytic Ozonation of VOCs at Ambient Temperature.

Catalytic ozonation has gained considerable interest in volatile organic compound (VOC) elimination due to its mild reaction conditions. However, the low activity and mineralization rate of VOCs over catalysts hinder its practical application. Herein, a series of α-MnO2 nanowire catalysts were prepared via thermal annealing treatment at various temperatures to tailor defect species. Numerous characterization techniques were used and combined to investigate the relationship between activity and microstructure. PALS and XAFS indicated that more unsaturated manganese and oxygen vacancies, especially surface oxygen vacancy clusters, were produced in α-MnO2 under the optimal high calcination temperature. As a result, MnO2-600 was found to exhibit the best-ever performance in toluene conversion (95%) and mineralization rate (89.5%) at 20 °C, making it a promising candidate for practical use. The roles of these defects in manipulating the reactive oxygen species of α-MnO2 were clarified by quantifying the amounts of reactive oxygen species by quenching experiments and density functional theory calculations. 1O2 and ·OH species generated in the vicinity of oxygen vacancy clusters, especially the dimer oxygen vacancy cluster, were identified as key oxygen species in the abatement of toluene. This study provides a facile method to engineer the microstructure of MnO2 by means of the manipulation of oxygen vacancies and an in-depth understanding of their roles in the catalytic ozonation of VOC.

Preparation of α- MnO2 Nanowires Coated with Hybrid of Manganese and Cerium Oxides and Studying Their Catalytic Performance Toward Electro-Oxidation of L- Methionine

A facile synthetic method for mixed transition metal oxide, with a desirable potential for electrochemical determination of L-methionine, was proposed. The hierarchical nanostructure, MnO2@MnCeO, was prepared by a hydrothermal process followed by calcination at 350 °C. The structure has a backbone made of α- MnO2 nanowires covered with a hybrid of manganese and cerium oxides. The crystallographic analysis demonstrated that the phases of MnO2 formed on the backbone surface and the backbone MnO2 are the same. The synthesized material was employed for the modification of a carbon paste electrode to design an outstanding sensor for L-methionine determination. The electrocatalytic activity of α-MnO2 nanowires covered by mixed oxides of MnCeO and single oxide of CeO toward L-methionine electrooxidation were compared to each other. It was revealed that the MnO2@MnCeO modified carbon paste elecrode exhibited better analytical performance than the one modified with MnO2@CeO. This composite electrode was successfully applied in L-methionine assessments in two ranges of concentration, 1000–10000 and 1–750 μmol l−1 with a detection limit as low as 0.16 μmol l−1. Owing to the remarkable sensitivity and promising selectivity of the prepared electrode, it could assess methionine content in complex matrices of human plasma samples.

Implications of morphology on excited state absorption of α-MnO2 nanostructures

Nanoclusters, nanorods and nanowires of α-MnO2 were synthesized by reduction method at different annealing temperature (100 °C–500 °C). X-ray diffraction and Raman studies confirms the formation of pure tetragonal phase of α-MnO2. Surface morphology using SEM and HRTEM reveals the gradual transformation of α-MnO2 nanoclusters (100 °C–300 °C) to nanorods (400 °C) to well defined nanowires (500 °C). While increasing the annealing temperature, Ostwald ripening process initially withholds the agglomerated clusters and its size get reduced and fused that leads to orientation along particular direction. HRTEM clearly displays the influence of temperature on favoured growth plane where (411) of nanoclusters changed to (211) plane of nanorods and nanowires. Optical band edge absorption at 268 nm arising from d–d transitions of Mn4+ in MnO2 got blue shifted due to variations in morphology. Here with increase in temperature, aspect ratio increases which alters the number of surface atoms and consequently the cohesive energy put on effect in the widening of bandgap of the material from 4.31 eV (nanoclusters) to 4.63 eV (nanowires). Open aperture Z-scan technique using Nd: YAG laser (532 nm, 10 Hz & 9 ns) revealed all MnO2 nanostructures involve reverse saturable absorption ascribed due to excited state absorption and its strength varies with morphology. Significant absorption in the visible region (532 nm) provoked excited state absorption and intensity dependent variation of nonlinear absorption coefficient confirms the existence of sequential two photon absorption process. α-MnO2 nanowires with extended interconnected network and larger aspect ratio exhibits wider excited state cross-section (6.64 × 10−2 GM), higher two-photon absorption coefficient (2.01 × 10−10 m/W) and lower onset optical limiting threshold (2.13 × 1012 W/m2). This stronger optical limiting performance of α-MnO2 nanowires symbolizes the scope of utilizing optimal α-MnO2 nanostructure for optical limiting applications.