Controllably Doping Nitrogen into 1T/2H MoS2 Heterostructure Nanosheets for Enhanced Supercapacitive and Electrocatalytic Performance by Low-Power N2 Plasma

N-doped MoS2 nanoflowers for the ultrasonic-vibration-driven high piezoelectric catalytic degradation

In this study, N-doped few-layer MoS2 piezocatalysts were successfully prepared by a one-pot hydrothermal method with urea as a nitrogen source. Benefiting from the optimized proportion of minority layers at edge positions and higher conductivity by N doping, the optimal N-doped few-layer MoS2 (120 mg of added urea) sample showed the optimal piezocatalytic activity for Rhodamine B (RhB) and levofloxacin (LEV), reaching 84.6 and 73.1% with the reaction kinetic rate constant of 0.020 86 and 0.017 05 min-1, respectively. In addition, the generation of superoxide radicals (·O2-) from the 120-MoS2 sample was determined to be greater than that from the 0-MoS2 sample in the piezocatalyst process by free radical scavenging experiments and electron paramagnetic resonance tests. Based on experimental data, a potential mechanism has been proposed to explain the enhanced piezocatalyst performance of N-doped few-layer MoS2. This research sheds new light on the development of efficient, cost-effective MoS2 piezoelectric catalysts through the doping of non-metallic dopants.

Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

Plasma Technology for Advanced Electrochemical Energy Storage

"Carbon Peak and Carbon Neutrality" is an important strategic goal for the sustainable development of human society. Typically, a key means to achieve these goals is through electrochemical energy storage technologies and materials. In this context, the rational synthesis and modification of battery materials through new technologies play critical roles. Plasma technology, based on the principles of free radical chemistry, is considered a promising alternative for the construction of advanced battery materials due to its inherent advantages such as superior versatility, high reactivity, excellent conformal properties, low consumption and environmental friendliness. In this perspective paper, we discuss the working principle of plasma and its applied research on battery materials based on plasma conversion, deposition, etching, doping, etc. Furthermore, the new application directions of multiphase plasma associated with solid, liquid and gas sources are proposed and their application examples for batteries (e. g. lithium-ion batteries, lithium-sulfur batteries, zinc-air batteries) are given. Finally, the current challenges and future development trends of plasma technology are briefly summarized to provide guidance for the next generation of energy technologies.

Nitrogen doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets for enhanced peroxidase activity

Heteroatom doping and phase engineering are effective ways to promote the catalytic activity of nanoenzymes. Nitrogen-doped 1 T/2H mixed phase MoS2/CuS heterostructure nanosheets N-1 T/2H-MoS2/CuS are prepared by a simple hydrothermal approach using polyoxometalate (POM)-based metal-organic frameworks (MOFs) (NENU-5) as a precursor and urea as nitrogen doping reagent. The XPS spectroscopy (XPS) and Raman spectrum of N-1 T/2H-MoS2/CuS prove the successful N-doping. NENU-5 was used as the template to prepare 1 T/2H-MoS2/CuS with high content of 1 T phase by optimizing the reaction time. The use of urea as nitrogen dopant added to 1 T/2H-MoS2/CuS, resulted in N-1 T/2H-MoS2/CuS with an increase in the content of the 1 T phase from 80 % to 84 % and higher number of defects. N-1 T/2H-MoS2/CuS shows higher peroxidase activity than 1 T/2H-MoS2/CuS and a catalytic efficiency (Kcat/Km) for H2O2 twice as high as that of 1 T/2H-MoS2/CuS. The enhanced catalytic activity has probably been attributed to several reasons: (i) the insertion of urea during the hydrothermal process in the S-Mo-S layer of MoS2, causing an increase in the interlayer spacing and in 1 T phase content, (ii) the replacement of S atoms in MoS2 by N atoms from the urea decomposition, resulting in more defects and more active sites. As far as we know, N-1 T/2H-MoS2/CuS nanosheets have the lowest detection limit (0.16 µm) for the colorimetric detection of hydroquinone among molybdenum disulfide-based catalysts. This study affords a new approach for the fabrication of high-performance nanoenzyme catalysts.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Synthesis of multiphase MoS2 heterostructures using temperature-controlled plasma-sulfurization for photodetector applications

Two-dimensional (2D) materials exhibit outstanding performance in photodetectors because of their excellent optical and electronic properties. Specifically, 2D-MoS2, a transition metal dichalcogenide, is a prominent candidate for flexible and portable photodetectors based on its inherent phase-dependent tunable optical band gap properties. This research focused on creating high-performance photodetectors by carefully arranging out-of-plane 2D heterostructures. The process involved stacking different phases of MoS2 (1T and 2H) using controlled temperature during plasma-enhanced chemical vapor deposition. Among the various phase combinations, the best photocurrent response was obtained for the 1T/2H-MoS2 heterostructure, which exhibited an approximately two-fold higher photocurrent than the 2H/1T-MoS2 heterostructure and 2H/2H-MoS2 monostructure. The 1T/2H-MoS2 heterostructure exhibited a higher photoresponse than the monostructured MoS2 of the same thickness (1T/1T- and 2H/2H-MoS2, respectively). The effect of the stacking sequences of different phases was examined, and their photoperformances were investigated. This study demonstrates that phase engineering in 2D-MoS2 van der Waals heterostructures has significant potential for developing high-performance photodetectors.

Phosphorus doped molybdenum disulfide regulated by sodium chloride for advanced supercapacitor electrodes

As a pseudocapacitor electrode material, molybdenum disulfide (MoS2) usually shows inferior capacity, rate capability and cyclability. Structural regulation and heteroatom doping are the available methods to ameliorate the electrochemical properties of MoS2. Herein, phosphorus doped molybdenum disulfide regulated by sodium chloride (SP-MoS2) is successfully synthesized using phosphomolybdate acid as a molybdenum source and an in situ dopant and sodium chloride (NaCl) as a structural regulator. Under the structural regulation of NaCl, the SP-MoS2 nanosheets exhibit an interweaved architecture with a large interlayer spacing of 0.68 nm. Owing to the in situ P doping and large specific surface area (21.0 m2 g-1), the SP-MoS2 electrode possesses a maximum capacity of 564.8 F g-1 at 1 A g-1 and retains 56.3% of the original capacity at 20 A g-1. Density functional theory (DFT) calculations indicate that SP-MoS2 displays a high K+ average adsorption energy of -3.636 eV. In addition, the fabricated SP-MoS2//AC asymmetric supercapacitor device displays an energy density of 22.8 W h kg-1 at 759 W kg-1.

Molybdenum disulfide (MoS2) along with graphene nanoplatelets (GNPs) utilized to enhance the capacitance of conducting polymers (PANI and PPy)

Hybrid composites of molybdenum disulfide (MoS2), graphene nanoplatelets (GNPs) and polyaniline (PANI)/polypyrrole (PPy) have been synthesized as cost-effective electrode materials for supercapacitors. We have produced MoS2 from molybdenum dithiocarbamate by a melt method in an inert environment and then used a liquid exfoliation method to form its composite with graphene nanoplatelets (GNPs) and polymers (PANI and PPy). The MoS2 melt/GNP ratio in the resultant composites was 1 : 3 and the polymer was 10% by wt. of the original composite. XRD (X-ray diffraction analysis) confirmed the formation of MoS2 and SEM (scanning electron microscopy) revealed the morphology of the synthesized materials. The electrochemical charge storage performance of the synthesized composite materials was assessed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCCD) measurements. Resultant composites showed enhanced electrochemical performances (specific capacitance = 236.23 F g-1, energy density = 64.31 W h kg-1 and power density = 3858.42 W kg-1 for MoS2 melt 5 mPP at a current density of 0.57 A g-1 and had 91.87% capacitance retention after 10 000 charge-discharge cycles) as compared to the produced MoS2; thus, they can be utilized as electrode materials for supercapacitors.

Self-assembled molybdenum disulfide nanoflowers regulated by lithium sulfate for high performance supercapacitors

Recently, molybdenum disulfide (MoS2) has been extensively investigated as a promising pseudocapacitor electrode material. However, MoS2 usually exhibits inferior rate capability and cyclability, which restrain its practical application in energy storage. In this work, MoS2 nanoflowers regulated by Li2SO4 (L-MoS2) are successfully fabricated via intercalating solvated Li ions. Via appropriate intercalation of Li2SO4, MoS2 nanosheets could self-assemble to form L-MoS2 nanoflowers with an interlayer spacing of 0.65 nm. Due to the large specific surface area (23.7 m2 g-1) and high 1T phase content (77.5%), L-MoS2 as supercapacitor electrode delivers a maximum specific capacitance of 356.7 F g-1 at 1 A g-1 and maintains 49.8% of capacitance retention at 20 A g-1. Moreover, the assembled L-MoS2 symmetric supercapacitor (SSC) device displays an energy density of 6.5 W h kg-1 and 79.6% of capacitance retention after 3000 cycles.

MoS2 @Polyaniline for Aqueous Ammonium-Ion Supercapacitors

Ammonium-ion aqueous supercapacitors are raising notable attention owing to their cost, safety, and environmental advantages, but the development of optimized electrode materials for ammonium-ion storage still lacks behind expectations. To overcome current challenges, here, a sulfide-based composite electrode based on MoS2 and polyaniline (MoS2 @PANI) is proposed as an ammonium-ion host. The optimized composite possesses specific capacitances above 450 F g-1 at 1 A g-1 , and 86.3% capacitance retention after 5000 cycles in a three-electrode configuration. PANI not only contributes to the electrochemical performance but also plays a key role in defining the final MoS2 architecture. Symmetric supercapacitors assembled with such electrodes display energy densities above 60 Wh kg-1 at a power density of 725 W kg-1 . Compared with Li+ and K+ ions, the surface capacitive contribution in NH4 + -based devices is lower at every scan rate, which points to an effective generation/breaking of H-bonds as the mechanism controlling the rate of NH4 + insertion/de-insertion. This result is supported by density functional theory calculations, which also show that sulfur vacancies effectively enhance the NH4 + adsorption energy and improve the electrical conductivity of the whole composite. Overall, this work demonstrates the great potential of composite engineering in optimizing the performance of ammonium-ion insertion electrodes.

Heteroatom-Doped Molybdenum Disulfide Nanomaterials for Gas Sensors, Alkali Metal-Ion Batteries and Supercapacitors

Molybdenum disulfide (MoS2) is the second two-dimensional material after graphene that received a lot of attention from the research community. Strong S-Mo-S bonds make the sandwich-like layer mechanically and chemically stable, while the abundance of precursors and several developed synthesis methods allow obtaining various MoS2 architectures, including those in combinations with a carbon component. Doping of MoS2 with heteroatom substituents can occur by replacing Mo and S with other cations and anions. This creates active sites on the basal plane, which is important for the adsorption of reactive species. Adsorption is a key step in the gas detection and electrochemical energy storage processes discussed in this review. The literature data were analyzed in the light of the influence of a substitutional heteroatom on the interaction of MoS2 with gas molecules and electrolyte ions. Theory predicts that the binding energy of molecules to a MoS2 surface increases in the presence of heteroatoms, and experiments showed that such surfaces are more sensitive to certain gases. The best electrochemical performance of MoS2-based nanomaterials is usually achieved by including foreign metals. Heteroatoms improve the electrical conductivity of MoS2, which is a semiconductor in a thermodynamically stable hexagonal form, increase the distance between layers, and cause lattice deformation and electronic density redistribution. An analysis of literature data showed that co-doping with various elements is most attractive for improving the performance of MoS2 in sensor and electrochemical applications. This is the first comprehensive review on the influence of foreign elements inserted into MoS2 lattice on the performance of a nanomaterial in chemiresistive gas sensors, lithium-, sodium-, and potassium-ion batteries, and supercapacitors. The collected data can serve as a guide to determine which elements and combinations of elements can be used to obtain a MoS2-based nanomaterial with the properties required for a particular application.

Hybrid G/BN@2H-MoS2 Nanomaterial Composites: Structural, Electronic and Molecular Adsorption Properties

Hybrid structures often possess superior properties to those of their component materials. This arises from changes in the structural or physical properties of the new materials. Here, we investigate the structural, electronic, and gas-adsorption properties of hybrid structures made from graphene/hexagonal boron nitride and 2H-molybdenum disulfide (G/BN@MoS₂) monolayers. We consider hybrid systems in which the G/BN patch is at the Mo plane (model I) and the S plane (model II). We find that the implanted hexagon of G or BN in MoS₂ alters its electronic properties: G@MoS₂ (I,II) are metallic, while BN@MoS₂ (I) is an n-type conducting and BN@MoS₂ (II) is semiconducting. We study the molecular adsorption of some diatomic gases (H₂, OH, N₂, NO, CO), triatomic gases (CO₂, NO₂, H₂S, SO₂), and polyatomic gases (COOH, CH₄, and NH₃) on our hybrid structures while considering multiple initial adsorption sites. Our results suggest that the hybrid systems may be suitable materials for some applications: G@MOS₂ (I) for oxygen reduction reactions, BN@MoS₂ (I,II) for NH₃-based hydrogen production, and G@MoS₂ (I) and BN@MoS₂ (I,II) for filtration of No, Co, SO₂, H₂S, and NO₂.

One-Step Hydrothermal Synthesis of MoO2/MoS2 Nanocomposites as High-Performance Electrode Material for Supercapacitors

By only changing the ratio of Mo to S source, a distinctive single phase MoO2 or MoS2 and MoO2/MoS2 nanocomposites (NCs) are obtained through a simple one-step hydrothermal method based on CH4N2S as a sulfur source and (NH4)6Mo7O24·4H2O as a source of Mo in oxalic acid. The effect of ratio of Mo to S source on the composition, structure, and electrochemical performance are systematically researched. Due to its unique design, abundant macropores active sites in MoO2/MoS2 NCs induce superior rate property (55.30% capacitance retention to 20 from 1 A g-1) and larger specific capacitance (1667.3 F g-1 at 1 A g-1) and longer cycle life (94.75% after 5000 cycles) as used directly as an electrode. Furthermore, at a power density of 225 W kg-1, a maximal energy density of 21.85 Wh kg-1 is provided by the asymmetric supercapacitor (MoO2/MoS2//AC). The capacitance of asymmetric supercapacitor (ASC) is remarkably enhanced by 129.02% under 5000 cycles at a current density of 1.5 A g-1, demonstrating outstanding cycle property. These results imply the prepared MoO2/MoS2 NCs have promising applications in advanced energy storages. It is important and should be noted that NCs of oxide and sulfide are prepared with only a simple one-step process.

Rational design of selenium inserted 1T/2H mixed-phase molybdenum disulfide for energy storage and pollutant degradation applications

MoS₂based materials are recognized as the promising candidate for multifunctional applications due to its unique physicochemical properties. But presence of lower number of active sites, poor electrical conductivity, and less stability of 2H and 1T MoS₂inherits its practical applications. Herein, we synthesized the Se inserted mixed-phase 2H/1T MoS₂nanosheets with abundant defects sites to achieve improved overall electrochemical activity. Moreover, the chalcogen insertion induces the recombination of photogenerated excitons and enhances the life of carriers. The bifunctional energy storage and photocatalytic pollutant degradation studies of the prepare materials are carried out. Fabricated symmetric solid-state supercapacitor showed an exceptional capacitance of 178 mF cm^-2with an excellent energy density of 8μWh cm^-2and power density of 137 mW cm^-2, with remarkable capacitance retention of 86.34% after successive 8000 charge-discharge cycles. The photocatalytic dye degradation experiments demonstrate that the prepared Se incorporated 1T/2H MoS₂is a promising candidate for dye degradation applications. Further, the DFT studies confirmed that the Se inserted MoS₂is a promising electrode material for supercapacitor applications with higherC_Qdue to a larger density of states near Fermi level as compared to pristine MoS₂.

High-Performance MnO2 Nanowire/MoS2 Nanosheet Composite for a Symmetrical Solid-State Supercapacitor

To improve the production rate of MoS₂ nanosheets as an excellent supercapacitor (SC) material and enhance the performance of the MoS₂-based solid-state SC, a liquid phase exfoliation method is used to prepare MoS₂ nanosheets on a large scale. Then, the MnO₂ nanowire sample is synthesized by a one-step hydrothermal method to make a composite with the as-synthesized MoS₂ nanosheets to achieve a better performance of the solid-state SC. The interaction between the MoS₂ nanosheets and MnO₂ nanowires produces a synergistic effect, resulting in a decent energy storage performance. For practical applications, all-solid-state SC devices are fabricated with different molar ratios of MoS₂ nanosheets and MnO₂ nanowires. From the experimental results, it can be seen that the synthesized nanocomposite with a 1:4 M ratio of MoS₂ nanosheets and MnO₂ nanowires exhibits a high Brunauer-Emmett-Teller surface area (∼118 m²/g), optimum pore size distribution, a specific capacitance value of 212 F/g at 0.8 A/g, an energy density of 29.5 W h/kg, and a power density of 1316 W/kg. Besides, cyclic charging-discharging and retention tests manifest significant cycling stability with 84.1% capacitive retention after completing 5000 rapid charge-discharge cycles. It is believed that this unique, symmetric, lightweight, solid-state SC device may help accomplish a scalable approach toward powering forthcoming portable energy storage applications.

Even Incorporation of Nitrogen into Fe0 Nanoparticles as Crystalline Fe4N for Efficient and Selective Trichloroethylene Degradation

Surface modification of microscale Fe powder with nitrogen has emerged recently to improve the reactivity of Fe⁰ for dechlorination. However, it is unclear how an even incorporation of a crystalline iron nitride phase into Fe⁰ nanoparticles affects their physicochemical properties and performance, or if Fe⁰ nanoparticles with a varied nitridation degree will act differently. Here, we synthesized nitridated Fe⁰ nanoparticles with an even distribution of N via a sol-gel and pyrolysis method. Nitridation expanded the Fe⁰ lattice and provided the Fe₄N species, making the materials more hydrophobic and accelerating the electron transfer, compared to un-nitridated Fe⁰. These properties well explain their reactivity and selectivity toward trichloroethylene (TCE). The TCE degradation rate by nitridated Fe⁰ (up to 4.8 × 10^-2 L m^-2 h^-1) was much higher (up to 27-fold) than that by un-nitridated Fe⁰, depending on the nitridation degree. The materials maintained a high electron efficiency (87-95%) due to the greatly suppressed water reactivity (109-127 times lower than un-nitridated Fe⁰). Acetylene was accumulated as the major product of TCE dechlorination via β-elimination. These findings suggest that the nitridation of Fe⁰ nanoparticles can change the materials' physicochemical properties, providing high reactivity and selectivity toward chlorinated contaminants for in situ groundwater remediation.

B, N Co-Doping Sequence: An Efficient Electronic Modulation of the Pd/MXene Interface with Enhanced Electrocatalytic Properties for Ethanol Electrooxidation

Improving the electrocatalytic properties by regulating the surface electronic structure of supported metals has always been a hot issue in electrocatalysis. Herein, two novel catalysts Pd/B-N-Ti₃C₂ and Pd/N-B-Ti₃C₂ are used as the models to explore the effect of the B and N co-doping sequence on the surface electronic structure of metals, together with the electrocatalytic properties of ethanol oxidation reaction. The two catalysts exhibit obviously stratified morphology, and the Pd nanoparticles having the same amount are both uniformly distributed on the surface. However, the electron binding energy of Ti and Pd elements of Pd/B-N-Ti₃C₂ is smaller than that of Pd/N-B-Ti₃C₂. By exploring the electrocatalytic properties for EOR, it can be seen that all the electrochemical surface area, maximum peak current density, and antitoxicity of the Pd/B-N-Ti₃C₂ catalyst are much better than its counterpart. Such different properties of the catalysts can be attributed to the various doping species of B and N introduced by the doping sequence, which significantly affect the surface electronic structure and size distribution of supported metal Pd. Density functional theory calculations demonstrate that different B-doped species can offer sites for the H atom from CH₃CH₂OH of dehydrogenation in Pd/B-N-Ti₃C₂, thereby facilitating the progress of the EOR to a favorable pathway. This work provides a new insight into synthesizing the high-performance anode materials for ethanol fuel cells by regulating the supported metal catalyst with multielement doping.

Boosting the photocatalytic performance of Cu2O for hydrogen generation by Au nanostructures and rGO nanosheets

Au@Cu2O/rGO exhibited boosting photocatalytic performance due to the yolk–shelled structure, abundant hot charges on Au, and quick charge transfer by rGO.