A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The world of two-dimensional carbides and nitrides (MXenes)

Two-dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface-to-volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure-properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high-performance gas sensors devices.

Two-Dimensional Nanostructured Materials for Gas Sensing

The unique properties and great variety of two-dimensional (2D) nanomaterials make them highly attractive for energy storage applications. Here, an insight into the progress made towards the application of 2D nanomaterials for capacitive energy storage is provided. Synthesis methods, and electrochemical performance of various classes of 2D nanomaterials, particularly based on graphene, transition metal oxides, dichalcogenides, and carbides, are presented. The factors that directly influence capacitive performance are discussed throughout the text and include nanosheet composition, morphology and texture, electrode architecture, and device configuration. Recent progress in the fabrication of 2D-nanomaterials-based microsupercapacitors and flexible and free-standing supercapacitors is presented. The main electrode manufacturing techniques with emphasis on scalability and cost-effectiveness are discussed, and include laser scribing, printing, and roll-to-roll manufacture. Various issues that prevent the use of the full energy-storage potential of 2D nanomaterials and how they have been tackled are discussed, and include nanosheet aggregation and the low electrical conductivity of some 2D nanomaterials. Particularly, the design of hybrid and hierarchical 2D and 3D structures based on 2D nanomaterials is presented. Other challenges and opportunities are discussed and include: control of nanosheets size and thickness, chemical and electrochemical instability, and scale-up of electrode films.

Synthesis of Two-Dimensional Materials for Capacitive Energy Storage.

New ultrathin and multifunctional electromagnetic interference (EMI) shielding materials are required for protecting electronics against electromagnetic pollution in the fifth-generation networks a...

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Beyond Ti3C2Tx: MXenes for Electromagnetic Interference Shielding.

MXenes refer to a family of 2D transition metal carbides/nitrides that are rich in chemistry. The first member of the family, Ti3C2Tx, was reported in 2011. Since then MXenes have opened up an exciting new field in 2D inorganic functional materials by virtue of their intrinsic electronic conductivity, superior hydrophilicity, rich surface chemistry and layered structure, as evidenced by the fact that the number of papers on MXenes has increased exponentially. The unique properties and ease of processing have positioned them as promising materials for a variety of applications including energy storage, especially for supercapacitors. In this review, we aim to summarize the current advances in MXene research on supercapacitors. We begin by reviewing various fabrication routes and their influence on the structure and surface chemistry of MXenes. The structure, properties, stability, and species of layered MXenes are then introduced. The focus then turns to the capacitive energy-storage mechanisms and the factors determining the electrochemical behavior and performance in supercapacitors. Besides, various types of MXene-based supercapacitors are summarized to highlight the significance of MXenes in constructing energy storage devices. Finally, challenges and prospects in this booming field are proposed to promote further development of MXenes in supercapacitors.

Emerging 2D MXenes for supercapacitors: status, challenges and prospects.

Direct printing of functional inks is critical for applications in diverse areas including electrochemical energy storage, smart electronics and healthcare. However, the available printable ink formulations are far from ideal. Either surfactants/additives are typically involved or the ink concentration is low, which add complexity to the manufacturing and compromises the printing resolution. Here, we demonstrate two types of two-dimensional titanium carbide (Ti3C2Tx) MXene inks, aqueous and organic in the absence of any additive or binary-solvent systems, for extrusion printing and inkjet printing, respectively. We show examples of all-MXene-printed structures, such as micro-supercapacitors, conductive tracks and ohmic resistors on untreated plastic and paper substrates, with high printing resolution and spatial uniformity. The volumetric capacitance and energy density of the all-MXene-printed micro-supercapacitors are orders of magnitude greater than existing inkjet/extrusion-printed active materials. The versatile direct-ink-printing technique highlights the promise of additive-free MXene inks for scalable fabrication of easy-to-integrate components of printable electronics.

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Additive-free MXene inks and direct printing of micro-supercapacitors

Manganese oxide nanosheets were synthesized using liquid-phase exfoliation that achieved suspensions in isopropanol (IPA) with concentrations of up to 0.45 mg ml−1. A study of solubility parameters showed that the exfoliation was optimum in N,N-dimethylformamide followed by IPA and diethylene glycol. IPA was the solvent of choice due to its environmentally friendly nature and ease of use for further processing. For the first time, a hybrid of graphene and manganese oxide nanosheets was synthesized using a single-step co-exfoliation process. The two-dimensional (2D) hybrid was synthesized in IPA suspensions with concentrations of up to 0.5 mg ml−1 and demonstrated stability against re-aggregation for up to six months. The co-exfoliation was found to be a energetically favorable process in which both solutes, graphene and manganese oxide nanosheets, exfoliate with an improved yield as compared to the single-solute exfoliation procedure. This work demonstrates the remarkable versatility of liquid-phase exfoliation with respect to the synthesis of hybrids with tailored properties, and it provides proof-of-concept ground work for further future investigation and exploitation of hybrids made of two or more 2D nanomaterials that have key complementary properties for various technological applications.

http://iopscience.iop.org/article/10.1088/2053-1583/2/2/025005/pdf

Manganese oxide nanosheets and a 2D hybrid of graphene–manganese oxide nanosheets synthesized by liquid-phase exfoliation

Manganese oxide nanosheets were synthesized using liquid phase exfoliation that achieved suspensions in isopropanol with concentrations of up to 0.45 mg/ml. A study of solubility parameters showed that the exfoliation was optimum in DMF followed by isopropanol and diethylene glycol. Isopropanol was the solvent of choice due to its environmentally friendly nature and ease of use for further processing. The 2D hybrid was synthesized in isopropanol suspensions with concentrations of up to 0.5 mg/ml and demonstrated stability against reaggregation for up to six months. The coexfoliation was found to be an energetically favorable process in which both solutes, graphene and manganese oxide nanosheets, exfoliate with an improved yield as compared to the single solute exfoliation procedure. This work demonstrates the remarkable versatility of liquid phase exfoliation with respect to the synthesis of hybrids with tailored properties, and it provides proof of concept ground work for further future investigation and exploitation of hybrids made of two or more 2D nanomaterials that have key complementary properties for various technological applications.

Manganese oxide nanosheets and a 2D hybrid of graphene-manganese oxide nanosheets synthesized by liquid-phase exfoliation

Conductive inks allow for low cost and scalable deposition of conductive tracks and patterns for printed electronics. Metal nanoparticle colloids are a novel form for producing conductive inks. Laser Ablation Synthesis in Solution (LASiS) is a “green” method for the production of metal nanoparticle colloids without the need for environmentally hazardous chemicals, however the method has typically been limited by its low production rates. This study reports on the generation of an additive free silver nanocolloid with maximized productivity using a flow-based LASiS system and its characterization using dynamic light scattering, UV–VIS, transmission electron microscopy and field emission scanning electron microscopy. The productivity of the LASiS silver nanoparticle (size ∼34 ± 5 nm) was ∼0.9 mg mL−1. While the flow-based system achieves high laser ablation rates in the mass of nanomaterial generated per unit time, the volume of liquid required for the flow leads to relatively low concentrations. Therefore, in this work, LASiS concentrated ink was formulated via a centrifugal method, which was then drop-cast and heat treated to produce a conductive silver layer. Centrifuging to concentrate the ink was shown to be a necessary step to achieve good results, with the lowest resistance across the drop-cast material of 60.2 Ω after annealing. 

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Silver nanocolloid generation using dynamic Laser Ablation Synthesis in Solution system and drop-casting

transform the printed objects directly into highly conductive and robust hydrogels with high shape fidelity on both macro- and microscales. The as-obtained hydrogel shows ultrahigh conductivity, outstanding durability, and good mechanical properties with even reversible stretchability and compressibility at extremely high water content. Furthermore, the combination of high conductivity, porous structure, and internal water environment endows the hydrogel with outstanding EMI-shielding performance at small thickness, and its printable feature promises customizable applications for satisfying the trend in the miniaturization of electronic devices. We foresee that, these highly conductive hydrogels with open porous structure, fabricated by additive manufacturing technique, also hold great potential in the fields of energy storage, water desalination

Lorcan McKeon

Papers

Additive-free MXene inks and direct printing of micro-supercapacitors

Manganese oxide nanosheets and a 2D hybrid of graphene–manganese oxide nanosheets synthesized by liquid-phase exfoliation

Manganese oxide nanosheets and a 2D hybrid of graphene-manganese oxide nanosheets synthesized by liquid-phase exfoliation

Silver nanocolloid generation using dynamic Laser Ablation Synthesis in Solution system and drop-casting

Additive Manufacturing of Ti 3 C 2 -MXene-Functionalized Conductive Polymer Hydrogels for ElectromagneticInterference Shielding