MXenes: From Discovery to Applications

Since the first report on 2D Ti3C2 in 2011, an escalating number of studies have been targeting synthesis, compelling properties, and versatile applications of MXenes. MXenes have the general formula Mn+1XnTx, where M is an early transition metal, such as Ti, Mo, Nb, V, Cr, Zr, Ta, etc., X is carbon and/or nitrogen, n = 1 – 4, and Tx represents surface terminations. The structural configuration in which transitional metal atoms are arranged in a layered fashion with carbon or nitrogen atoms, allows MXenes to enjoy extraordinary compositional diversity and adjustable properties. Moreover, those atomic sandwiches allow arrangement of transition metals in such a way that one transition metal can be in the surface layer and another in the middle in compositions like Mo2TiC2 or Mo2Ti2C3. More than 30 stoichiometric MXenes have been reported so far, and dozens of solid solutions on M and X sites have been synthesized. Taking into account that more than a hundred stoichiometric structures have been predicted to exist, and an unlimited number of solid solutions can be made, this may well be the largest family of 2D materials known so far. A large variety of surface terminations, such as O, OH, F, Cl, Br, S, and Te, further increase the number of possible compositions and allow fine tuning of physical and chemical properties of MXenes. As a result of the high electronic conductivity (up to 20,000 S/cm), hydrophilic surfaces, biocompatibility, optical absorption bands in the visible and IR range, and ability of reversible surface redox reaction, MXenes have been explored in applications ranging from electrical energy storage and electrocatalysis to optoelectronics, communication, water and gas purification, and medicine. The 2nd International Conference on MXenes hosted by the Beijing University of Chemical Technology in 2019 unlocked MXenes’ new research domains with growing scope for different technology sectors. This special issue “MXenes – From Discovery to Applications” celebrates this conference by gathering reviews and progress reports that discuss the progression and future of MXene fabrication and applications, primarily in the energy related areas, along with original contributions that also define other cutting-edge MXenes applications. The control of synthesis and structure of MXenes using various strategies holds the promise of producing the largest ever family of 2D materials with adjustable composition and desired surface properties. Wang et al. (article number 2000869) discuss the use of the synchrotron radiation for characterization during the synthesis of MXenes with tunable surface characteristics. The review mainly focuses on the understanding of surface tunability and interlayer engineering, which govern the chemical/physical properties of MXenes. Orangi et al. (article number 2005305) summarize the latest assembly and fabrication routes available to overcome the major challenges, such as layer-restacking in MXene-based electrodes, with emphasis on morphological engineering to achieve improved characteristics for advanced energy storage applications. The high conductivity, superior flexibility, and capacitive storage capability of MXenes make them a suitable candidate to be easily integrated into fibers and yarn with the promise of wearable energy devices. Levitt et al. (article number 2000739) provide a detailed progress report concerning the rapidly advancing field of MXene derived textile-based supercapacitors and manufacturing of MXene-enabled electronic textiles. Dong et al. (article number 2000706) review the use of MXene and MXene-based nanostructures in different types of metal ion batteries, where MXene participates as an active component, conductive substrate, binder and current collector in lithium-ion and beyond- lithium (Na+, K+, Mg2+, Zn2+, Ca2+) ion batteries. Ahmed et al. (article number 2000894) discuss a new type of MXene (i-MXene) obtained from in-plane chemically ordered quaternary MAX phases known as i-MAX. The article projects the growing potential of i-MXenes in the areas of energy storage and catalysis. The restacking of MXene sheets is an issue that has a profound effect on its energy storage and catalytic capability. In this context, Li et al. (article number 2000842) review the improved strategies to transform 2D MXenes into 3D architectures with their efficient use in energy storage and conversion, including supercapacitors, rechargeable batteries, and electrocatalysis. The field of electromagnetic interference shielding (EMI) is among the most rapidly growing application domains for MXenes. It is truly dominated by MXenes nowadays, with record performances of Ti3C2Tx and Ti3CNTx. Iqbal et al. (article number 2000883) debate the influence of MXene and MXene-based composite's morphology on EMI shielding, with an in-depth assessment of various morphologies, such as layer-by-layer assemblies, porous foams and aerogels, and segregated structures. The planar morphology, plasmon resonance in the visible and infrared range, and chemical diversity of MXenes, which allows tunability of optical properties, have been used in the conversion of sunlight into thermal energy, widening the applications to solar steam generation and biomedical sciences, including photothermal therapy. MXenes are among the most efficient materials for light-to-heat conversion. Xu et al. (article number 2000712) discuss the recent advancement in MXenes’ use for photothermal conversion, including their growing usage in solar water desalination, wearable devices, and solar photothermal electrodes. The high conductivity coupled with energy conversion capability and outstanding flexibility make MXenes an ideal candidate for actuators. Nguyen et al. (article number 1909504) review the inherent advantages of MXenes in actuator devices, covering the structural and compositional superiority of MXenes with a focus on state-of-the-art photothermal, electrothermal, and humidity-responsive MXene-based actuators. The MXene surface terminations can control its oxidation and hold the promise of producing tailored MXenes for advanced applications. In the original research contribution, Persson et al. (article number 1909005) reveal the science behind the MXenes surface termination and to which extent a MXene sheet could be saturated before the 2D sheet breaks. The preeminence of MXene in the electrochemical capacitors arises from its distinctive electrical double-layer and redox capacitive behavior owing to its transition metal oxide-like surface. Ando et al. (article number 2000820) used density functional theory in combination with the implicit solvation model to explore the distinctive behavior of MXene in different electrolytes and identified the key mechanisms that differentiate capacitive from pseudocapacitive (surface redox) behavior. The engineering of interlayer spacing is yet another route to improve the capacitive storage and cycling performance of MXene-based supercapacitor devices. Zhao et al. (article number 2000815) proposed the superiority of Nb4C3Tx-based freestanding film in accommodating cations to achieve high volumetric capacitance and prolonged cycle stability of the devised super capacitor. Li et al. (article number 2002739) attempted to improve the poor interlayer and interparticle conductivity of Ti3C2Tx by in-situ homogeneous growth of MWCNTs on carbon cloth-supported MXenes for high-performance flexible supercapacitors. Zhang et al. (article number 2000922) report an in-situ ice templating approach, where ice crystals formed from the residual water within the Ti3C2Tx/CNT act as a sacrificial template to transform the 2D layer structure into a 3D porous architecture with superior capacitive performance compared to its stacked counterpart. Nitrogen doping is yet another facile route to improve the inherent electrochemical performance of 2D MXenes. Lu et al. (article number 2000852) reveal the nitrogen doping mechanism in Ti3C2 MXenes, with the identification of main sites responsible for accommodating these dopant species, which influence the capacitive performance and conductivity of the devised supercapacitor electrode. In the field of renewable energy, water splitting reactions such as HER and OER are considered the key processes, with their efficiencies directly related to the chemistry of the electrocatalyst. Yu et al. (article number 2000570) explore the suitability of new hypothetical (not yet produced) 2D transition metal carbides, such as NbC2, TaC2, and MoC2, for overall water splitting and other associated reactions, with TaC2 as a suitable bifunctional electrocatalyst for HER/ORR. Cui et al. (article number 2000693) propose a new strategy to minimize the amount of the noble metal Pt-based catalyst for the HER process without losing the catalytic efficiency by constructing a hierarchical Pt-MXene-SWCNT composite. The immobilization of atom-sized metallic Pt over SWCNTs connected to the Ti3C2Tx sheet configured a robust HER electrocatalyst with prolonged working stability. Taking advantage of structural engineering, Xiu et al. (article number 1910028) propose a template-based ultra-fast approach to construct a multilevel hollow MXene architecture for HER process. The hollow interior with aggregation-resistant structures, when coupled with ultra-fine Pt, resulted in a catalytic interface with superior working durability for direct seawater electrolysis. To better understand the mechanism of the HER catalysis on MXene, Djire et al. (article number 2001136) utilized scanning electrochemical microscopy. They showed that, unlike in dichalcogenides, where only edges are catalytically active, the basal plane contributes to the electrocatalytic behavior of mixed transition metal nitride MXenes. The high conductivity and transparency of MXenes have extended their use to the field of optoelectronics. However, they can also be used as precursors for making other materials. Tu et al. (article number 1909843) demonstrate the versatility of MXenes, particularly, Nb2CTx to synthesize plate-like LiNbO3 and Pr3+-doped LiNbO3 crystals for ferroelectric and luminescent applications. In a theoretical analysis, Di Vito et al. (article number 1909028) identify the relationship between interfacial work function and MXene surface termination. A nonlinear dependence was defined between the work function of the perovskite/MXene interface and the nature of MXene surface termination with the most substantial influence resulting from OH terminations. In the area of biomedical applications, MXenes’ flexibility and transparency enable their use in the engineering of adjustable (tunable) intraocular lenses. Ward et al. (article number 2000841) report the spin-coating of Ti3C2Tx over a hydrophobic acrylate intraocular lens, which results in a lower sheet resistance with visible region transmission. The MXenes coated lens exhibited no biotoxicity towards epithelial cells or inflammatory cytokine expressions, demonstrating MXene's potential in optics with a promise of improved vision for cataract-patients. This special issue brings the recent advancement in the field of MXenes, expanding their applications from energy and catalysis to optics and biomedical sciences. We hope that the latest state-of-the-art MXene science collected in this special issue will inspire the journal readership and motivate interaction among scientists from diverse disciplines. We are thankful to the editor, Dr. Francesca Riboni, as well as the editorial team of Advanced Functional Materials and all authors for their support and contributions. Bin Xu is a full professor in the College of Materials Science & Engineering and State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology. He received his Ph.D. degree in 2006 from Beijing Institute of Technology. He worked in Drexel University as a visiting professor in 2016–2017. His current research interests focus on carbon and MXene-based materials for energy storage, including supercapacitors and secondary batteries. Yury Gogotsi is a Distinguished University Professor and Charles T. and Ruth M. Bach Professor of Materials Science and Engineering at Drexel University. He also serves as Director of the A.J. Drexel Nanomaterials Institute. He received his MS (1984) and Ph.D. (1986) from Kiev Polytechnic and a D.Sc. degree from the Ukrainian Academy of Sciences in 1995. His research group works on MXenes, nanostructured carbons, and other nanomaterials for energy, water and biomedical applications.