Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Coordination-Driven Hierarchical Assembly of Hybrid Nanostructures Based on 2D Materials

2D materials have received tremendous scientific and engineering interests due to their remarkable properties and broad-ranging applications such as energy storage and conversion, catalysis, biomedicine, electronics, and so forth. To further enhance their performance and endow them with new functions, 2D materials are proposed to hybridize with other nanostructured building blocks, resulting in hybrid nanostructures with various morphologies and structures. The properties and functions of these hybrid nanostructures depend strongly on the interfacial interactions between 2D materials and other building blocks. Covalent and coordination bonds are two strong interactions that hold high potential in constructing these robust hybrid nanostructures based on 2D materials. However, most 2D materials are chemically inert, posing problems for the covalent assembly with other building blocks. There are usually coordination atoms in most of 2D materials and their derivatives, thus coordination interaction as a strong interfacial interaction has attracted much attention. In this review, recent progress on the coordination-driven hierarchical assembly based on 2D materials is summarized, focusing on the synthesis approaches, various architectures, and structure-property relationship. Furthermore, insights into the present challenges and future research directions are also presented.

Few-Layer Clayenes for Material and Environmental Applications

Here, we coined the term "clayene" for a single layer of clay and "few-layer clayene" for clays with 2-10 layers. Few-layer clayenes, which are Fe²⁺-rich and mica-type, were prepared hydrothermally at 200 °C and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM)/high-resolution transmission electron microscopy (HRTEM) to determine the crystalline phases and morphology, respectively. Chemical composition by energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy confirmed the iron-rich mica composition, and the latter also revealed the presence of both Fe²⁺ and Fe³⁺. Mössbauer spectroscopy further confirmed the presence of Fe²⁺ and Fe³⁺ and their proportions in the mica-type few-layer clayenes. All of the synthesized mica-type few-layer clayenes except one exhibited high specific surface areas (SSAs) ranging from 94 to 149 m²/g as determined by N₂ adsorption-desorption isotherms and the Brunauer-Emmett-Teller (BET) equation. The high surface areas are in conformity with the crystal sizes calculated from XRD peaks and also as revealed by HRTEM. Taking advantage of the interfacial reactions of the high surface area of few-layer clayenes, two potential applications of clayenes were demonstrated in materials and environmental fields.

Mild Covalent Functionalization of Transition Metal Dichalcogenides with Maleimides: A "Click" Reaction for 2H-MoS2 and WS2

The physical properties of ultrathin transition metal dichalcogenides (2D-TMDCs) make them promising candidates as active nanomaterials for catalysis, optoelectronics, and biomedical applications. Chemical modification of TMDCs is expected to be key in modifying/adding new functions that will help make such promise a reality. We present a mild method for the modification of the basal planes of 2H-MoS₂ and WS₂. We exploit the soft nucleophilicity of sulfur to react it with maleimide derivatives, achieving covalent functionalization of 2H-TMDCs under very mild conditions. Extensive characterization proves that the reaction occurs through Michael addition. The orthogonality and versatility of the thiol-ene "click" chemistry is expected to allow the à la carte chemical manipulation of TMDCs.

Tailoring the physicochemical properties of solution-processed transition metal dichalcogenides via molecular approaches

In this Feature Article we highlight the tremendous progress in solution-processed transition metal dichalcogenides and the molecular approaches employed to finely tune their physicochemical properties.

High-performance multi-functional graphene/hexagonal boron nitride/poly(ethylene oxide) nanocomposites through enhanced interfacial interaction by coordination

In this study, multi-functional nanocomposites with excellent mechanical, electrical and thermal properties were prepared through metal-ion coordination. Reduced graphene oxide (rGO) and hexagonal boron nitride (h-BN) interacted through calcium coordination bonding. Poly(ethylene oxide) (PEO) was added to bridge these two nanomaterials, providing more resistance to tensile deformation. The results of UV-Vis and FTIR spectra proved that coordination bonding was successfully formed among the three compounds. SEM images showed homogenous dispersions of the nanocomposite. After calcium-ion coordination, the mechanical, electrical and thermal properties of Ca²⁺-coordinated rGO/BN/PEO composite improved significantly, indicating that metal-ion coordination is a potential method for multi-functional nanocomposite fabrication.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

Metal-Organic-Compound-Modified MoS2 with Enhanced Solubility for High-Performance Perovskite Solar Cells

MoS₂ as a graphene-like 2 D material shows a large potential to replace and even overcome graphene in various important applications owing to its ideal properties of electrical, optical, frictional, and tunable band gap. However, its low solubility in the most of common solvents makes it difficult to prepare by a simple solution process. Here, we introduce a metal-organic compound to modify MoS₂ . Phenyl acetylene silver (PAS)-functionalized MoS₂ is easily dispersed in solvents like DMF and water. A conductive polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) blend with the MoS₂ leads to a significant enhancement of the performance of planar heterojunction perovskite solar cells. The solar cells have a high power conversion efficiency of 16.47 % as well as largely increased stability. This provides a feasible method for large-scale production of MoS₂ for wide applications in various electric devices.

Colloidal 2D nanosheets of MoS2 and other transition metal dichalcogenides through liquid-phase exfoliation

This review focuses on the exfoliation of transition metal dichalcogenides MQ₂ (TMD, M=Mo, W, etc., Q=S, Se, Te) in liquid media, leading to the formation of 2D nanosheets dispersed in colloids. Nowadays, colloidal dispersions of MoS₂, MoSe₂, WS₂ and other related materials are considered for a wide range of applications, including electronic and optoelectronic devices, energy storage and conversion, sensors for gases, catalysts and catalyst supports, biomedicine, etc. We address various methods developed so far for transferring these materials from bulk to nanoscale thickness, and discuss their stabilization and factors influencing it. Long-time known exfoliation through Li intercalation has received renewed attention in recent years, and is recognized as a method yielding highest dispersed concentrations of single-layer MoS₂ and related materials. Latest trends in the intercalation/exfoliation approach include electrochemical lithium intercalation, experimenting with various intercalating agents, multi-step intercalation, etc. On the other hand, direct sonication in solvents is a much simpler technique that allows one to avoid dangerous reagents, long reaction times and purifying steps. The influence of the solvent characteristics on the colloid formation was closely investigated in numerous recent studies. Moreover, it is being recognized that, besides solvent properties, sonication parameters and solvent transformations may affect the process in a crucial way. The latest data on the interaction of MoS₂ with solvents evidence that not only solution thermodynamics should be employed to understand the formation and stabilization of such colloids, but also general and organic chemistry. It appears that due to the sonolysis of the solvents and cutting of the MoS₂ layers in various directions, the reactive edges of the colloidal nanosheets may bear various functionalities, which participate in their stabilization in the colloidal state. In most cases, direct exfoliation of MQ₂ into colloidal nanosheets is conducted in organic solvents, while a small amount of works report low-concentrated colloids in pure water. To improve the dispersion abilities of transition metal dichalcogenides in water, various stabilizers are often introduced into the reaction media, and their interactions with nanosheets play an important role in the stabilization of the dispersions. Surfactants, polymers and biomolecules usually interact with transition metal dichalcogenide nanosheets through non-covalent mechanisms, similarly to the cases of graphene and carbon nanotubes. Finally, we survey covalent chemical modification of colloidal MQ₂ nanosheets, a special and different approach, consisting in the functionalization of MQ₂ surfaces with help of thiol chemistry, interaction with electrophiles, or formation of inorganic coordination complexes. The intentional design of surface chemistry of the nanosheets is a very promising way to control their solubility, compatibility with other moieties and incorporation into hybrid structures. Although the scope of the present review is limited to transition metal dichalcogenides, the dispersion in colloids of other chalcogenides (such as NbS₃, VS₄, Mo₂S₃, etc.) in many ways follows similar trends. We conclude the review by discussing current challenges in the area of exfoliation of MoS₂ and its related materials.

Facile and Green Production of Impurity-Free Aqueous Solutions of WS2 Nanosheets by Direct Exfoliation in Water

To obtain 2D materials with large quantity, low cost, and little pollution, liquid-phase exfoliation of their bulk form in water is a particularly fascinating concept. However, the current strategies for water-borne exfoliation exclusively employ stabilizers, such as surfactants, polymers, or inorganic salts, to minimize the extremely high surface energy of these nanosheets and stabilize them by steric repulsion. It is worth noting, however, that the remaining impurities inevitably bring about adverse effects to the ultimate performances of 2D materials. Here, a facile and green route to large-scale production of impurity-free aqueous solutions of WS₂ nanosheets is reported by direct exfoliation in water. Crucial parameters such as initial concentration, sonication time, centrifugation speed, and centrifugation time are systematically evaluated to screen out an optimized condition for scaling up. Statistics based on morphological characterization prove that substantial fraction (66%) of the obtained WS₂ nanosheets are one to five layers. X-ray diffraction and Raman characterizations reveal a high quality with few, if any, structural distortions. The water-borne exfoliation route opens up new opportunities for easy, clean processing of WS₂ -based film devices that may shine in the fields of, e.g., energy storage and functional nanocomposites owing to their excellent electrochemical, mechanical, and thermal properties.

Enabling Design of Advanced Elastomer with Bioinspired Metal-Oxygen Coordination

It poses a huge challenge to expand the application gallery of rubbers into advanced smart materials and achieve the reinforcement simultaneously. In the present work, inspired by the metal-ligand complexations of mussel byssus, ferric ion was introduced into an oxygen-abundant rubber network to create additional metal-oxygen coordination cross-links. Such complexation has been revealed to be highly efficient in enhancing the strength and toughness of the rubbers. Significantly, such complexation also enables the functionalization of the rubber into highly damping or excellent multishape memory materials. We envision that the present work offers an efficient yet facile way of creating advanced elastomers based on industrially available diene-based rubber.

Single step, bulk synthesis of engineered MoS2 quantum dots for multifunctional electrocatalysis

Bi- or tri- functional catalysts based on atomic layers are receiving tremendous scientific attention due to their importance in various energy technologies. Recent studies on molybdenum disulphide (MoS2) nanosheets revealed that controlling the edge states and doping/modifying with suitable elements are highly important in tuning the catalytic activities of MoS2. Here we report a bulk, single step method to synthesize metal modified MoS2 quantum dots (QDs). Three elements, namely Fe, Mg and Li, are chosen to study the effects of dopants in the catalytic activities of MoS2. Fe and Mg are found to act like dopants in the MoS2 lattice forming respective doped MoS2 QDs, while Li formed an intercalated MoS2 QD. The efficacy and tunability of these luminescent doped QDs towards various electrocatalytic activities (hydrogen evolution reaction, oxygen evolution reaction and oxygen reduction action) are reported here.

Self-healable, super tough graphene oxide–poly(acrylic acid) nanocomposite hydrogels facilitated by dual cross-linking effects through dynamic ionic interactions

Self-healable, super tough nanocomposite hydrogels facilitated by dual cross-linking effects through dynamic ionic interactions.

Improved Mechanical Properties of Graphene Oxide/Poly(ethylene oxide) Nanocomposites by Dynamic Interfacial Interaction of Coordination

A simple and cost-effective strategy to create a strong interfacial interaction of coordination bonds in graphene oxide/poly(ethylene oxide) (GO/PEO) nanocomposites by divalent metal ions are demonstrated in this study. The strong interfacial interaction realizes efficient load transfer during the tensile process to significantly improve the mechanical properties of PEO. In addition, the dynamic interfacial interaction of coordination bonds minimizes the elongation loss. This strategy is applicable to a variety of polymer matrices containing coordination atoms, thus opens up a new opportunity for high-performance GO/polymer nanocomposites with significantly improved mechanical properties.