Liquid phase exfoliation of MoO2 nanosheets for lithium ion battery applications

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties' associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition.

The Dynamic Interaction of Surfactants with Colloidal Molybdenum Disulfide Nanosheets Calls for Thermodynamic Stabilization by Solvents

Top-down liquid-phase exfoliation (LPE) and bottom-up hot-injection synthesis are scalable methods to produce colloids of two-dimensional (2D) van der Waals (vdW) solids. Generally thought off as two entirely different fields, we show that similar stabilization mechanisms apply to colloids of molybdenum disulfide (MoS₂) produced by both methods. By screening the colloidal stability of MoS₂ produced in a hot-injection synthesis in a wide range of solvents, we observe that colloidal stability can be understood based on solution thermodynamics, wherein matching the solubility parameter of solvent and nanomaterial maximizes colloidal stability. Identical to MoS₂ produced through LPE, optimal solvents to disperse MoS₂ produced from the bottom-up have similar solubility parameters of ≈22 MPa^1/2 and include aromatic solvents with polar functionalities, such as o-dichlorobenzene, and polar aprotic solvents, such as N,N-dimethylformamide. We further complemented our findings by nuclear magnetic resonance (NMR) spectrscopy, highlighting that organic surfactants, such as oleylamine and oleic acid, have a minimal affinity toward the nanocrystal surface and engage in a highly dynamic adsorption/desorption equilibrium. We thus conclude that hot injection yields MoS₂ colloids with comparable surfaces as those produced by LPE. These similarities might offer the prospect of using established procedures developed for LPE nanomaterials to postprocess colloidally synthesized dispersions of 2D colloids as processable inks.

Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

Size selection and thin-film assembly of MoS2 elucidates thousandfold conductivity enhancement in few-layer nanosheet networks

Printed electronics based on liquid-exfoliated nanosheet networks are limited by inter-nanosheet junctions and thick films which hinder field-effect gating. Here, few-layer molybdenum disulfide nanosheets are assembled by Langmuir deposition into thin films, and size selection is shown to lead to a thousandfold conductivity enhancement with potential applicability to all nanosheet networks.

Complementary combinative strategy of defect engineering and graphene coupling for efficient energy-functional materials

The synergetic combination of defect engineering and graphene coupling enables to develop an effective way of exploring efficient bifunctional electrocatalyst/electrode materials. Defect-engineered amorphous MoO₂ -reduced graphene oxide (rGO) nanohybrid was synthesized by soft-chemical reduction of K₂ MoO₄ in graphene oxide colloids. Mo K-edge X-ray absorption spectroscopy clearly demonstrates the rutile-type local atomic structure of amorphous MoO₂ with significant oxygen vacancies and intimate electronic coupling with rGO. The defect-introduced MoO₂ -rGO nanohybrid shows excellent bifunctionality as electrocatalyst for hydrogen evolution reaction and electrode for sodium-ion batteries, which are superior to those of crystalline MoO₂ -rGO homologue. The beneficial effect of simultaneous defect control and rGO coupling can be ascribed to the provision of oxygen vacancies acting as active sites, the increase of electrical conductivity, and the improvement of reaction kinetics.

The Role of Additives in Suppressing the Degradation of Liquid-Exfoliated WS2 Monolayers

Group VI transition metal dichalcogenides (TMDs) are considered to be chemically widely inert, but recent reports point toward an oxidation of monolayered sheets in ambient conditions, due to defects. To date, the degradation of monolayered TMDs is only studied on individual, substrate-supported nanosheets with varying defect type and concentration, strain, and in an inhomogeneous environment. Here, degradation kinetics of WS₂ nanosheet ensembles in the liquid phase are investigated through photoluminescence measurements, which selectively probe the monolayers. Monolayer-enriched WS₂ dispersions are produced with varying lateral sizes in the two common surfactant stabilizers sodium cholate (SC) and sodium dodecyl sulfate (SDS). Well-defined degradation kinetics are observed, which enable the determination of activation energies of the degradation and decouple photoinduced and thermal degradation. The thermal degradation is slower than the photoinduced degradation and requires higher activation energy. Using SC as surfactant, it is sufficiently suppressed. The photoinduced degradation can be widely prevented through chemical passivation achieved through the addition of cysteine which, on the one hand, coordinates to defects on the nanosheets and, on the other hand, stabilizes oxides on the surface, which shield the nanosheets from further degradation.

Crystalline Free-Standing Two-Dimensional Zwitterionic Organic Nanosheets for Efficient Conduction of Lithium Ions

Crystalline two-dimensional organic nanosheets (2D-ONs) having atomic or near-atomic thickness with infinite lateral dimensions are of crucial significance for their possible application as a material for energy storage. The presence of nanofluidic channels with a designed array of molecular interlayers in such 2D-ONs, for a favorable lithium-ion transport, has special significance for improving the efficacy of lithium-ion batteries. However, the rational design of crystalline 2D-ONs remains a challenge because of the lack of appropriate monomers and convenient preparation methods. Herein, we report a unique lithium-ion conducting behavior of zwitterionic 2D-ONs, formed through self-assembly of a small organic molecule AM-1. Different microscopic studies confirm the near-atomic thickness (∼3.5 nm) of these 2D-ONs. Results of the single-crystal X-ray diffraction studies confirm the presence of a one-dimensional (1D) channel in crystalline 2D-ONs, which was generated during the self-assembly process of the zwitterionic monomer scaffold. The presence of immobilized ionic centers with well-defined directional channels in the 2D-ONs favors the transportation of lithium ions with a room-temperature lithium-ion conductivity of 5.14 × 10^-5 S cm^-1, which is rather unique for self-assembled 2D-ONs.

Nitrogen-doped carbon encapsulated hollow ZnSe/CoSe2 nanospheres as high performance anodes for lithium-ion batteries

Hierarchical nitrogen-doped carbon encapsulated hollow ZnSe/CoSe2 (ZnSe/CoSe2@N-C) nanospheres are fabricated by a convenient solvothermal and selenization approach, followed by a carbonization process. The as-obtained ZnSe/CoSe2@N-C possesses a multilevel nanoscale architecture composed of a thin carbon shell with a size of around 12 nm and hollow selenide nanoparticles as the core with tiny rough grains and rich voids as the subunits. The robust carbon protective shell and synergistic effect between double metal ions boost the electron and ion transportation as well as promote effective extraction and insertion of lithium ions. Hollow ZnSe/CoSe2@N-C spheres show high reversible capacity with 1153 mA h g-1 remaining over 100 cycles at 100 mA g-1. In particular, the hollow ZnSe/CoSe2@N-C spheres show an outstanding cycling stability at a high rate of 2000 mA g-1 with the reversible capacity of up to 966 mA h g-1 remaining after 500 cycles. As an advanced anode, ZnSe/CoSe2@N-C composite shows remarkable cycling stability and exceptional rate capability in the field of energy storage technologies.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS₂), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 10¹⁸ groups/m². They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS₂. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

Extraction of Two-Dimensional Aluminum Alloys from Decagonal Quasicrystals

Atomically thin metallic alloys are receiving increased attention due to their prospective applications as interconnects/contacts in two-dimensional (2D) circuits, sensors, and catalysts, among others. In this work, we demonstrate an easily scalable technique for the synthesis of 2D metallic alloys from their 3D quasicrystalline precursors. We have used aluminum (Al)-based single-phase decagonal quasicrystal Al₆₆Co₁₇Cu₁₇ alloy to extract the corresponding 2D alloy structure. The 2D layered Al alloy possesses 2-fold decagonal quasicrystalline symmetry and consists of two- or three-layer-thick sheets with a lateral dimension of microns. These 2D metallic layers were combined with the atomic layers of tungsten disulfide to form the stacked heterostructures, which is demonstrated to be a stable and efficient catalyst for hydrogen evolution reaction.

Chemically anchored two-dimensional-SiO x /zero-dimensional-MoO2 nanocomposites for high-capacity lithium storage materials

Silicon oxides are promising alternatives for graphite anodes in lithium-ion batteries. SiO_x nanosheets exhibit favorable anodic performances, including outstanding capacity retention and dimensional stability, due to their unique two-dimensional (2D) microstructures, but suffer from low specific capacity and poor initial coulombic efficiency. Here we demonstrate that chemically anchoring of molybdenum dioxide (MoO₂) nanoparticles on the surface of 2D-SiO_x nanosheets via a Mo–O–Si bond boosts both the reversible capacity and initial coloumbic efficiency without sacrificing the useful properties of 2D-SiO_x nanosheets. The enhancements can be attributed to the introduction of a zero-dimensional MoO₂ nano-object, which offers abnormal storage sites for lithium. The proposed nano-architecturing shows how we can maximize the advantages of 2D nanomaterials for energy storage applications.

By the synergistic effect of the unique properties of 2D-SiO_x and 0D-MoO₂, the 2D-SiO_x/0D-MoO₂ nanocomposites show enhancement in Li-storage properties.

Enriching and Quantifying Porous Single Layer 2D Polymers by Exfoliation of Chemically Modified van der Waals Crystals

2D polymer sheets with six positively charged pyrylium groups at each pore edge in a stacked single crystal can be transformed into a 2D polymer with six pyridines per pore by exposure to gaseous ammonia. This reaction furnishes still a crystalline material with tunable protonation degree at regular nano-sized pores promising as separation membrane. The exfoliation is compared for both 2D polymers with the latter being superior. Its liquid phase exfoliation yields nanosheet dispersions, which can be size-selected using centrifugation cascades. Monolayer contents of ≈30 % are achieved with ≈130 nm sized sheets in mg quantities, corresponding to tens of trillions of monolayers. Quantification of nanosheet sizes, layer number and mass shows that this exfoliation is comparable to graphite. Thus, we expect that recent advances in exfoliation of graphite or inorganic crystals (e.g. scale-up, printing etc.) can be directly applied to this 2D polymer as well as to covalent organic frameworks.

Three-dimensional hierarchical ZnCo2O4@C3N4-B nanoflowers as high-performance anode materials for lithium-ion batteries

ZnCo₂O₄ has become one of the most widely used anode materials due to its good specific capacity, cost-efficiency, high thermal stability and environmental benignity.