Aqueous Cathodic Exfoliation Strategy toward Solution-Processable and Phase-Preserved MoS2 Nanosheets for Energy Storage and Catalytic Applications

Two-Dimensional MoS2 Nanosheets Derived from Cathodic Exfoliation for Lithium Storage Applications

The preparation of 2H-phase MoS2 thin nanosheets by electrochemical delamination remains a challenge, despite numerous efforts in this direction. In this work, by choosing appropriate intercalating cations for cathodic delamination, the insertion process was facilitated, leading to a higher degree of exfoliation while maintaining the original 2H-phase of the starting bulk MoS2 material. Specifically, trimethylalkylammonium cations were tested as electrolytes, outperforming their bulkier tetraalkylammonium counterparts, which have been the focus of past studies. The performance of novel electrochemically derived 2H-phase MoS2 nanosheets as electrode material for electrochemical energy storage in lithium-ion batteries was investigated. The lower thickness and thus higher flexibility of cathodically exfoliated MoS2 promoted better electrochemical performance compared to liquid-phase and ultrasonically assisted exfoliated MoS2, both in terms of capacity (447 vs. 371 mA·h·g-1 at 0.2 A·g-1) and rate capability (30% vs. 8% capacity retained when the current density was increased from 0.2 A·g-1 to 5 A·g-1), as well as cycle life (44% vs. 17% capacity retention at 0.2 A·g-1 after 580 cycles). Overall, the present work provides a convenient route for obtaining MoS2 thin nanosheets for their advantageous use as anode material for lithium storage.

From Bulk Molybdenum Disulfide (MoS2) to Suspensions of Exfoliated MoS2 in an Aqueous Medium and Their Applications

Two-dimensional (2D) layered materials like graphene, transition-metal dichalcogenides (TMDs), boron nitrides, etc., exhibit unique and fascinating properties, such as high surface-to-volume ratio, inherent mechanical flexibility and robustness, tunable bandgap, and high carrier mobility, which makes them an apt candidate for flexible electronics with low consumption of power. Because of these properties, they are in tremendous demand for advancement in energy, environmental, and biomedical sectors developed through various technologies. The production and scalability of these materials must be sustainable and ecofriendly to utilize these unique properties in the real world. Here, in this current review, we review molybdenum disulfide (MoS2 nanosheets) in detail, focusing on exfoliated MoS2 in water and the applicability of aqueous MoS2 suspensions in various fields. The exfoliation of MoS2 results in the formation of single or few-layered MoS2. Therefore, this Review focuses on the few layers of exfoliated MoS2 that have the additional properties of 2D layered materials and higher excellent compatibility for integration than existing conventional Si tools. Hence, a few layers of exfoliated MoS2 are widely explored in biosensing, gas sensing, catalysis, photodetectors, energy storage devices, a light-emitting diode (LED), adsorption, etc. This review covers the numerous methodologies to exfoliate MoS2, focusing on the various published methodologies to obtain nanosheets of MoS2 from water solutions and their use.

Interlayer Engineering of Layered Materials for Efficient Ion Separation and Storage

Layered materials are characterized by strong in-plane covalent chemical bonds within each atomic layer and weak out-of-plane van der Waals (vdW) interactions between adjacent layers. The non-bonding nature between neighboring layers naturally results in a vdW gap, which enables the insertion of guest species into the interlayer gap. Rational design and regulation of interlayer nanochannels are crucial for converting these layered materials and their 2D derivatives into ion separation membranes or battery electrodes. Herein, based on the latest progress in layered materials and their derivative nanosheets, various interlayer engineering methods are briefly introduced, along with the effects of intercalated species on the crystal structure and interlayer coupling of the host layered materials. Their applications in the ion separation and energy storage fields are then summarized, with a focus on interlayer engineering to improve selective ion transport and ion storage performance. Finally, future research opportunities and challenges in this emerging field are comprehensively discussed.

Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Directed morphology engineering of 2D MoS2 nanosheets to 1D nanoscrolls with enhanced hydrogen evolution and specific capacitance

1D-molybdenum disulfide (MoS2) nanoscrolls displayed enhanced electrochemical properties compared to 2D-MoS2 nanosheet counterparts. Rolling of nanosheets is the main fabrication route to nanoscrolls. However, owing to the conflict between chemical stability and multiple bending, the morphology transition from nanosheets to nanoscrolls is quite challenging. Herein we describe a reversible morphology transition from nanosheets to nanoscrolls by utilizing non-covalent interactions between MoS2 nanosheets and phenothiazine based organic dye. Interestingly, nanoscrolls can easily be opened back into nanosheets by destroying the non-covalent interactions with organic solvents. The prepared nanoscrolls exhibited enhanced electrochemical properties than nanosheets. Compared to nanosheets, nanoscrolls exhibited comparatively lower overpotential with a Tafel slope of 141 mV dec-1 and high specific capacitance of 1868 F g-1. Hydrogen evolution by the Volmer-Heyrovsky mechanism being superior for the nanoscrolls is envisaged by the relatively increased availability of Hads sites at MoS2 edges induced by scrolling. Whereas the high specific capacitance value of nanoscrolls is ascribed to the enhanced electrical double-layer capacitance mediated charge storage, which arises due to the synergistic effect of both scrolled structure and the electron-rich phenothiazine-based dye.

Molybdenum disulfide, exfoliation methods and applications to photocatalysis: a review

This review provides a deep analysis of the mechanical and optoelectronic characteristics of MoS2. It offers a comprehensive assessment of diverse exfoliation methods, encompassing chemical, liquid-phase, mechanical, and microwave-driven techniques. The review also explores MoS2's versatile applications across various domains and meticulously examines its significance as a photocatalyst. Notably, it highlights key factors influencing the photocatalytic process. Indeed, the enhanced visible light responsiveness of materials like MoS2 holds immense potential across a wide range of applications. MoS2's remarkable photocatalytic response to visible light, coupled with its notable stability, opens up numerous possibilities in various fields. This unique combination makes MoS2 a promising candidate for applications that require efficient and stable photocatalytic processes, such as environmental remediation, water purification, and energy generation. Its attributes contribute significantly to addressing contemporary challenges and advancing sustainable technologies.

Nanoarchitected graphene/copper oxide nanoparticles/MoS2 ternary thin films as highly efficient electrodes for aqueous sodium-ion batteries

Sodium-ion batteries (SIBs) operating in aqueous electrolyte are an emerging technology that promises to be safer, cheaper, more sustainable and more efficient than their lithium-based counterparts. One of the great challenges associated with this technology is the development of advanced materials with high specific capacity to be used as electrodes. Herein, we describe an ingenious strategy to prepare unprecedented tri-component nanoarchitected thin films with superior performance when applied as anodes in aqueous SIBs. Taking advantage of the broadness and versatility of the liquid-liquid interfacial route, three transparent nanocomposite films comprising graphene, molybdenum sulphide and copper oxide nanoparticles have been prepared. The samples were characterized using several techniques, and the results demonstrated that depending on the specific experimental strategy, different nanoarchitectures are achieved, resulting in different and improved properties. An astonishing capacity of 1377 mA h g-1 at 0.1 A g-1 and a degree of recovery of 100% were observed for the film in which the interactions among the components were optimized. This is among the highest capacity values reported in the literature and demonstrates the potential of these tri-component materials to be used as anodes in aqueous sodium-ion batteries.

Two-dimensional materials for high density, safe and robust metal anodes batteries

With a high specific capacity and low electrochemical potentials, metal anode batteries that use lithium, sodium and zinc metal anodes, have gained great research interest in recent years, as a potential candidate for high-energy-density storage systems. However, the uncontainable dendrite growth during the repeated charging process, deteriorates the battery performance, reduces the battery life and more importantly, raises safety concerns. With their unique properties, two-dimensional (2D) materials, can be used to modify various components in metal batteries, eventually mitigating the dendrite growth, enhancing the cycling stability and rate capability, thus leading to safe and robust metal anodes. In this paper, we review the recent advances of 2D materials and summarize current research progress of using 2D materials in the applications of (i) anode design, (ii) separator engineering, and (iii) electrolyte modifications by guiding metal ion nucleation, increasing ion conductivity, homogenizing the electric field and ion flux, and enhancing the mechanical strength for safe metal anodes. The 2D material modifications provide the ultimate solution for obtaining dendrite-free metal anodes, realizes the high energy storage application, and indicates the importance of 2D materials development. Finally, in-depth understandings of subsequent metal growth are lacking due to research limitations, while more advanced characterizations are welcome for investigating the metal deposition mechanism. The more facile and simplified preparation of 2D materials possess great prospects in high energy density metal anode batteries, and thus fulfils the development of EVs.

NbSe2 Nanosheets/Nanorolls Obtained via Fast and Direct Aqueous Electrochemical Exfoliation for High-Capacity Lithium Storage

Layered transition-metal dichalcogenides (LTMDs) in two-dimensional (2D) form are attractive for electrochemical energy storage, but research efforts in this realm have so far largely focused on the best-known members of such a family of materials, mainly MoS₂, MoSe₂, and WS₂. To exploit the potential of further, currently less-studied 2D LTMDs, targeted methods for their production, preferably by cost-effective and sustainable means, as well as control over their nanomorphology, are highly desirable. Here, we report a quick and straightforward route for the preparation of 2D NbSe₂ and other metallic 2D LTMDs that relies on delaminating their bulk parent solid under aqueous cathodic conditions. Unlike typical electrochemical exfoliation methods for 2D materials, which generally require an additional processing step (e.g., sonication) to complete delamination, the present electrolytic strategy yielded directly exfoliated nano-objects in a very short time (1-2 min) and with significant yields (∼16 wt %). Moreover, the dominant morphology of the exfoliated 2D NbSe₂ products could be tuned between rolled-up nanosheets (nanorolls) and unfolded nanosheets, depending on the solvent where the nano-objects were dispersed (water or isopropanol). This rather unusual delamination behavior of NbSe₂ was explored and concluded to occur via a redox mechanism that involves some degree of hydrolytic oxidation of the material triggered by the cathodic treatment. The delamination strategy could be extended to other metallic LTMDs, such as NbS₂ and VSe₂. When tested toward electrochemical lithium storage, electrodes based on the exfoliated NbSe₂ products delivered very high capacity values, up to 750-800 mA h g^-1 at 0.5 A g^-1, where the positive effect of the nanoroll morphology, associated to increased accessibility of the lithium storage sites, was made apparent. Overall, these results are expected to expand the availability of fit-for-purpose 2D LTMDs by resorting to simple and expeditious production strategies of low environmental impact.

Boosting the Catalytic Performance of AuAg Alloyed Nanoparticles Grafted on MoS2 Nanoflowers through NIR-Induced Light-to-Thermal Energy Conversion

MoS₂ nanoflowers (NFs) obtained through a hydrothermal approach were used as the substrate for the deposition of tiny spherical bimetallic AuAg or monometallic Au nanoparticles (NPs), leading to novel photothermal-assisted catalysts with different hybrid nanostructures and showing improved catalytic performance under NIR laser irradiation. The catalytic reduction of pollutant 4-nitrophenol (4-NF) to the valuable product 4-aminophenol (4-AF) was evaluated. The hydrothermal synthesis of MoS₂ NFs provides a material with a broad absorption in the Vis-NIR region of the electromagnetic spectrum. The in situ grafting of alloyed AuAg and Au NPs of very small size (2.0-2.5 nm) was possible through the decomposition of organometallic complexes [Au₂Ag₂(C₆F₅)₄(OEt₂)₂]_n and [Au(C₆F₅)(tht)] (tht = tetrahydrothiophene) using triisopropilsilane as reducing agent, leading to nanohybrids 1-4. The new nanohybrid materials display photothermal properties arising from NIR light absorption of the MoS₂ NFs component. The AuAg-MoS₂ nanohybrid 2 showed excellent photothermal-assisted catalytic activity for the reduction of 4-NF, which is better than that of the monometallic Au-MoS₂ nanohybrid 4. The obtained nanohybrids were characterised by transmission electron microscopy (TEM), High Angle Annular Dark Field-Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (HAADF-STEM-EDS), X-ray photoelectron spectroscopy and UV-Vis-NIR spectroscopy.

Template synthesis of dual-functional porous MoS2 nanoparticles with photothermal conversion and catalytic properties

Advanced catalysis triggered by photothermal conversion effects has aroused increasing interest due to its huge potential in environmental purification. In this work, we developed a novel approach to the fast degradation of 4-nitrophenol (4-Nip) using porous MoS₂ nanoparticles as catalysts, which integrate the intrinsic catalytic property of MoS₂ with its photothermal conversion capability. Using assembled polystyrene-b-poly(2-vinylpyridine) block copolymers as soft templates, various MoS₂ particles were prepared, which exhibited tailored morphologies (e.g., pomegranate-like, hollow, and open porous structures). The photothermal conversion performance of these featured particles was compared under near-infrared (NIR) light irradiation. Intriguingly, when these porous MoS₂ particles were further employed as catalysts for the reduction of 4-Nip, the reaction rate constant was increased by a factor of 1.5 under NIR illumination. We attribute this catalytic enhancement to the open porous architecture and light-to-heat conversion performance of the MoS₂ particles. This contribution offers new opportunities for efficient photothermal-assisted catalysis.

Fast and efficient electrochemical thinning of ultra-large supported and free-standing MoS2 layers on gold surfaces

Molybdenum disulfide (MoS₂) is a very promising layered material for electrical, optical, and electrochemical applications because of its unique and outstanding properties. To unlock its full potential, among different preparation routes, electrochemistry has gain interest due to its simple, fast, scalable and simple instrumentation. However, obtaining large-area monolayer MoS₂ that will enable the fabrication of novel electronic and electrochemical devices is still challenging. In this work, we reported a simple and fast electrochemical thinning process that results in ultra-large MoS₂ down to monolayer on Au surfaces. The high affinity of MoS₂ by Au surfaces enables the removal of bulk layers while preserving the first layer attached to the electrode. With a proper choice of the applied potential, more than 90% of the bulk regions can be removed from large-area MoS₂ crystals, as confirmed by atomic force microscopy, photoluminescence, and Raman spectroscopy. We further address a set of contributions that are helpful to elucidate the features of MoS₂, namely, the hyphenation of electrochemistry and optical microscopy for real-time observation of the thinning process that was revealed to occur from the edges to the center of the flake, an image treatment to estimate the thinning area and thinning rate, and the preparation of free-standing MoS₂ layers by electrochemically thinning bulk flakes on microhole-structured Ni/Au meshes.

2D Molybdenum Sulfide-Based Materials for Photo-Excited Antibacterial Application

Bacterial infections have seriously threatened human health and the abuse of natural or artificial antibiotics leads to bacterial resistance, so development of a new generation of antibacterial agents and treatment methods is urgent. 2D molybdenum sulfide (MoS₂ ) has good biocompatibility, high specific surface area to facilitate surface modification and drug loading, adjustable energy bandgap, and high near-infrared photothermal conversion efficiency (PCE), so it is often used for antibacterial application through its photothermal or photodynamic effects. This review comprehensively summarizes and discusses the fabrication processes, structural characteristics, antibacterial performance, and the corresponding mechanisms of MoS₂ -based materials as well as their representative antibacterial applications. In addition, the outlooks on the remaining challenges that should be addressed in the field of MoS₂ are also proposed.

High-efficiency 2D nanosheet exfoliation by a solid suspension-improving method

Two-dimensional (2D) materials with mono or few layers have wide application prospects, including electronic, optoelectronic, and interface functional coatings in addition to energy conversion and storage applications. However, the exfoliation of such materials is still challenging due to their low yield, high cost, and poor ecological safety in preparation. Herein, a safe and efficient solid suspension-improving method was proposed to exfoliate hexagonal boron nitride nanosheets (hBNNSs) in a large yield. The method entails adding a permeation barrier layer in the solvothermal kettle, thus prolonging the contact time between the solvent and hexagonal boron nitride (hBN) nanosheet and improving the stripping efficiency without the need for mechanical agitation. In addition, the proposed method selectively utilizes a matching solvent that can reduce the stripping energy of the material and employs a high-temperature steam shearing process. Compared with other methods, the exfoliating yield ofhBNNSs is up to 42.3% at 150 °C for 12 h, and the strategy is applicable to other 2D materials. In application, the ionic conductivity of a PEO/hBNNSs composite electrolytes reached 2.18 × 10^-4S cm^-1at 60 °C. Overall, a versatile and effective method for stripping 2D materials in addition to a new safe energy management strategy were provided.

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

Molybdenum disulfide (MoS₂) is a promising candidate for use as a supercapacitor electrode material and non-noble-metal electrocatalyst owing to its relatively high theoretical specific capacitance, Pt-like electronic feature, and graphene-like structure. However, insufficient electrochemically active sites along with poor conductivity significantly hinder its practical application. Heteroatom doping and phase engineering have been regarded as effective ways to overcome the inherent limitations of MoS₂ and enhance its ion storage and electrocatalytic performance. In this study, a plasma-assisted nitrogen-doped 1T/2H MoS₂ heterostructure has been proposed for the first time, resulting in excellent supercapacitor performance and hydrogen evolution reaction activity. XPS, Raman, and TEM analysis results indicate that N atoms have been successfully doped into MoS₂ nanosheets via room-temperature low-power N₂ plasma, and the 1T/2H hybrid phase is maintained. As expected, the 1T/2H MoS₂ heterostructure after a 10 min plasma treatment displayed a much boosted supercapacitive performance with a high specific capacitance of 410 F g^-1 at 1 A g^-1 and an excellent hydrogen evolution property with a low overpotential of 131 mV vs RHE at 10 mA cm^-2 for hydrogen evolution reaction. The excellent performance is superior to most of the recently reported outstanding MoS₂-based electrode and electrocatalytic materials. Moreover, the as-assembled flexible symmetric supercapacitor shows a high specific capacitance of 84.8 F g^-1 and superior mechanical robustness with 84.5% capacity retention after 2000 bending cycles.

Molecular Functionalization of 2H-Phase MoS2 Nanosheets via an Electrolytic Route for Enhanced Catalytic Performance

The molecular functionalization of two-dimensional MoS₂ is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target applications. While derivatization of metallic 1T-phase MoS₂ nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS₂ nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS₂ crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS₂ lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS₂ materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS₂ catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.

Effect of Fe, Co and Ni promoters on MoS2 based catalysts for chemoselective hydrogenation of nitroarenes

The effect of Fe, Co and Ni promoters on supported MoS₂ catalysts for hydrogenation of nitroarenes were systematically investigated via experiment, characterization and DFT calculation. It was found that the addition of promoters remarkably improved the reaction activity in a sequence of Ni > Co > Fe > Mo. Meanwhile Ni promoted catalyst with the best performance showed good recyclability and chemoselectivity for a wide substrate scope. The characterization results revealed that the addition of promoters decreased the interaction between Mo and support and facilitated the reductive sulfidation of Mo species to produce more coordinated unsaturated sites (CUS). DFT calculations showed that the addition of promoters increased the formation of CUS, and enhanced the adsorption of hydrogen. The influence degree of promoters followed the sequence Ni > Co > Fe > Mo, which was consistent with those of the activities. Nitrobenzene hydrogenation and hydrogen activation occurred at the S and Mo edge, respectively. The adsorbed hydrogen diffused from the Mo edge to the S edge to participate in the hydrogenation reaction. Mechanism investigation showed that the main reason for increased activity by the addition of promoters was the increase of amounts of CUS and the secondary reason was the augmentation of intrinsic activity of CUS. The present studies give a new understanding for promoter modified MoS₂ catalysts applied for hydrogenation of nitroarenes.

The addition of promoters remarkably improved the activity for hydrogenation of nitroarenes in a sequence of Ni > Co > Fe > Mo and the amount of CUS active center was supposed to be the main reason to influence the reaction activity.