Symposium S.NM10 : Synthesis, Properties and Applications of 2D MXenes

The largest family of 2D materials, known as transition metal carbides and/or nitrides (MXenes), have a chemical formula of M_n+1X_nT_x, where M represents a transition metal (Ti, Mo, Nb, V, Cr, etc.), X is either carbon and/or nitrogen, and T_x represents surface terminations. The diversity in composition (~30 MXenes synthesized so far), availability of solid solutions on M and X sites, and control of surface terminations offers a plethora of structures and chemistries to investigate.¹
The first discovered and most studied MXene, titanium carbide (Ti₃C₂T_x), has shown interaction with a wide range of the electromagnetic spectrum, including visible light transparency (>97% visible light transmittance per nanometer), a transverse surface plasmon in the near-infrared range (750-800 nm), and to capability to perform as an electromagnetic shield or dipole antenna in microwave or radio frequencies.² Combining the optical properties with ease in processing, high electronic conductivity and mechanical strength, MXenes have the characteristics necessary to develop as optical materials. This talk will provide insight into the optical properties and potential applications of MXenes as well as spectroscopic information which can be applied to designing photonic and optoelectronic devices, such as electron transport layers for solar cells, optical sensors, random or femtosecond lasers, electrochromic devices, photonic diodes, metamaterials, photothermal therapy agents, and more.

References
2D Metal Carbides and Nitrides (MXenes): Structure, Properties and Applications; Anasori, B., Gogotsi, Y., Eds.; Springer International Publishing, 2019.
Hantanasirisakul, K.; Gogotsi, Y. Adv Mater 2018, 30 (52), e1804779.

MXenes is a large family of two-dimensional (2D) transition metal carbides and nitrides of composition M_n+1X_nT_z; M is an early transition metal (e.g. Ti, V, Nb, Mo), X is carbon or nitrogen, n =1-3 and T_z stands for mixture of surface terminations (e.g. O, OH, F). With more than two dozen of MXene reported in the recent years, MXenes has expanded significantly the chemical space of 2D materials. Most of studies on MXenes focused on the first reported MXene viz. Ti₃C₂T_z, much smaller number of studies focused on other transition metals including ordered double transition metal and solid solutions at the M sites in MXenes. While nitrogen doping nanomaterials including transition metal carbides and oxides are known to be a very promising approach for altering materials properties and performance, very limited number of studies focused on carbonitride MXene. Herein, we use the titanium carbonitride as a model system to highlight the importance of studying the role of "X" in MXene. The characteristic differences between Ti₃C₂T_z and Ti₃CNT_z MXenes from both structure and surface chemistries points of views are discussed based on experimental results obtained using multiple techniques and theoretical calculations. Also, the performances of Ti₃CNT_z as electrode materials for electrochemical energy storage electrocatalysis in addition to their performance as catalysts will be discussed and compared to that of Ti₃C₂T_z.

Renewable energy systems are in urgent demand; however, integration of renewable energy into electrical grids requires rapid load-leveling of abrupt power spikes/drops and a wide distribution of high-power energy storage devices represents the most promising solution. However, current electrochemical energy storage devices do not meet all the requirements for grid-scale use, particularly because of slow charge/discharge rates caused by limited ion transport. Supercapacitors have rapid operation owing to the formation of an electric double-layer (EDL) at the electrode surface, which contributes to a higher power density and longer cycle lifetime than achievable in conventional batteries. In particular, supercapacitors based on aqueous electrolytes are attractive because of their low cost, high ion conductivity, non-flammability, and eco-friendliness. However, the energy density of such devices is severely limited by the narrow potential window of water (1.23 V).
To overcome this obstacle, we focus on a highly-concentrated aqueous electrolyte known as a hydrate melt (Li(TFSI)0.7(BETI)0.3/2H2O, TFSI: bis(trifluoromethanesulfonyl)imide, BETI: bis(pentafluoroethanesulfonyl)imide). Owing to the unique local coordination structure of water, hydrate melts show an exceptionally wide electrochemical potential window (> 3 V) as electrolytes, which far exceeds the thermodynamic hydrogen/oxygen evolution limits of water (1.23 V). Aqueous supercapacitors based on hydrate-melt electrolytes are expected to realize much higher voltage operation and hence much higher energy densities with an appropriate choice of electrode materials.
As an electrode material that can maximize use of the wide electrochemical window provided by hydrate-melt electrolytes, we used transition-metal carbides MXene. Because MXenes are reported to provide a large specific capacitance greater than 300 F/g in conventional aqueous electrolytes, their combination with a hydrate-melt electrolyte should realize a much higher energy density.
Here, we demonstrate the high-voltage operation of an aqueous supercapacitor consisting of an MXene Ti2CTx electrode and a hydrate melt electrolyte.

MXenes, 2D transition metal carbides, has emerged as a light-weight, ultrathin electromagnetic shielding materials for mobile telecommunication devices and advanced 5G smart electronics, since it offers an attractive combination of high electronic conductivity (~5000 S/cm), low density, hydrophilicity, and processability. In this presentation, we demonstrate that MXene is an ideal candidate for light-weight shielding with minimal thickness. The MXene thin film shows the extraordinary EMI shielding behavior due to its large electrical conductivity and 2D nano structure. These unique experimental results will be understood by theoretical calculations. Additionally, this presentation demonstrates that the simple post-treatment of MXene thin film deduces an unusual strong interaction with electromagnetic wave, resulting in strong absorption-induced attenuation.

In this work, a new MXene is experimentally described which exhibits uniform functionality on MX layers (-Cl), contrary to the mixed functionality (-F, -O, -OH) of traditional MXenes. Most MXenes contain random surface terminations, arising from the vigorous etching process of the MAX phase with acids such as HF. However, the present MXene Y₂CCl₂ is synthesized by a direct high temperature method followed by liquid exfoliation, which results in highly crystalline 2D flakes. This MXene has well-defined properties due to single moiety surface terminations, allowing for construction of simple computational models. Previous research in the field highlights the difficulty of predicting properties such as band gap and surface energy due to the unpredictable arrangement of surface terminations and complexity of computational models. A crystalline MXene with ordered terminations offers an easily synthesizable target with properties that can be predictably tuned.

First, Y₂CCl₂ was directly synthesized by heating a pellet composed of Y, C, and YCl₃ in a sealed tantalum tube at 950^oC for 5 days according to literature procedures. Next, density functional theory (DFT) calculations were performed to predict the energy required to exfoliate Y₂CCl₂ into an MXene. The binding energy of Y₂CCl₂ (0.14 J/m²) is less than half the binding energy of graphite, indicating that Y₂CCl₂ should be easier to exfoliate than graphite. A common method for exfoliating graphite is sonication in a liquid with similar surface properties, so the material was sonicated in the polar aprotic solvent propylene carbonate. The mixture quickly forms a well-dispersed solution of 2D Y₂CCl₂ flakes. Transmission electron microscopy (TEM) shows flakes with lateral sizes of 2-5 µm, and electron diffraction confirms retention of the crystalline Y₂CCl₂ structure. The optical properties were evaluated by UV-Vis spectroscopy, which reveals an absorption peak at 1.64 eV that is close to the expected band gap of 1.56 eV, calculated using the HSE06 functional. Furthermore, X-ray photoelectron spectroscopy measurements reveal unusual oxidation states associated with yttrium (2+) and carbon (2-), suggesting interesting electrical properties.

This material has several attractive features that may be utilized. Y₂CCl₂ is easily exfoliated in mild conditions, avoiding the hazards of corrosive media such as HF and HCl. Also, the material absorbs light in the visible, making Y₂CCl₂ potentially useful for light harvesting and photocatalytic applications. More broadly, Y₂CCl₂ is part of a larger class of metal carbide halide materials with the stoichiometry M₂CX₂ (M = Y, Sc, Gd; X = F, Cl, Br, I) that share similar structure and properties. Through preliminary calculations, we predict that other metal carbide halides may be exfoliable and contain similar properties to Y₂CCl₂. Overall, Y₂CCl₂ and other similar metal carbide halides offer a new class of MXenes with ordered surface terminations.

MXenes are a new family of two-dimensional (2D) materials discovered in 2011. Because of their 2D structure and many outstanding properties [1], MXenes have raised significant interest for various applications, such as batteries, optoelectronic devices [2], supercapacitors, lasers [3,4], sensors [5], etc. It was found that titanium carbide MXene flakes in aqueous solutions spontaneously transform into TiO₂ and recently, it has been demonstrated that water plays the main role in the reactions leading to this chemical transformation [6]. Thus, studying MXene reactivity is important for prolonged shelf life of MXene colloidal solutions, as well as for robust performance of MXene based devices. In this work, we demonstrate the use of gas chromatography technique to study reactivity of MXenes. Several hypotheses have been put forward to explain the degradation of MXenes in aqueous environments. However, no studies of the gaseous products of this reaction have been reported. The analysis of gases produced during MXene degradation, using gas chromatography, allows for a better understanding of the degradation process. The chemical reactivity of MXenes with different monolayer thickness was also investigated at different pH and temperatures. Our results have led to new, important conclusions about the chemical reactivity of MXenes in aqueous solutions. This knowledge will have significant impacts on the development of MXenes and other 2D materials for many applications.
References
1. Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y., 25th anniversary article: MXenes: a new family of two-dimensional materials. Advanced Materials 2014, 26(7), 992-1005
2. Dong, Y.; Chertopalov, S.; Maleski, K.; Anasori, B.; Hu, L.; Bhattacharya, S.; Rao, A. M.; Gogotsi, Y.; Mochalin, V. N.; Podila, R., Saturable Absorption in 2D Ti3C2 MXene Thin Films for Passive Photonic Diodes. Advanced Materials 2018, 30 (10), 1705714.
3. Yi, J.; Du, L.; Li, J.; Yang, L.; Hu, L.; Huang, S.; Dong, Y.; Miao, L.; Wen, S.; Mochalin, V. N., Unleashing the potential of Ti2CT x MXene as a pulse modulator for mid-infrared fiber lasers. 2D Materials 2019, 6 (4), 045038.
4. Yi, J.; Li, J.; Huang, S.; Hu, L.; Miao, L.; Zhao, C.; Wen, S.; Mochalin, V.; Rao, A., Ti2CtX MXene-based all-optical modulator. InfoMat 2019, DOI:10.1002/inf2.12052
5. Chertopalov, S.; Mochalin, V. N., Environment-Sensitive Photoresponse of Spontaneously Partially Oxidized Ti3C2 MXene Thin Films. ACS Nano 2018, 12 (6), 6109-6116.
6. Huang, S.; Mochalin, V. N., Hydrolysis of 2D transition-metal carbides (MXenes) in colloidal solutions. Inorganic chemistry 2019, 58 (3), 1958-1966.

Raman spectroscopy has been one of the most helpful tools for analysis of two-dimensional (2D) materials^1-2. While newly discovered materials should be probed using this technique, there have been just a few studies available for a large family of 2D transition metal carbides and nitrides known as MXenes^3-5. To see which effects Raman spectroscopy can detect for MXenes we performed a systematic study of the mostly used MXene to date - Ti₃C₂T_x. By synthesizing material in different ways, we monitored changes in Raman spectra. we have synthesized Ti₃C₂ via MILD method, where HF is formed in-situ by mixing LiF and HCl, HF-HCl (mixture of two acids), and by using various concentration of HF: 5%, 10%, 20% and 30% solutions in water³. We collected Raman spectra from samples collected at different stages of MXene synthesis: from multilayer powders to delaminated flakes, in order to observe the effect of stacking. The whole spectrum of Ti₃C₂ is divided into 4 regions: a resonant peak, A_1g out-of-plane vibrations of Ti, C and O, the surface group vibrations region and carbon vibrations region. Those regions change based on the synthesis method and flake surrounding. All those peaks are affected not only by surface functional groups⁵, but also by stacking of the flakes. Due to a plasmonic peak of Ti₃C₂T_x around 785 nm, resonant conditions⁶ are achieved enabling an extra peak at around 119-123 cm^-1 when using a red diode laser. Lastly, we showed how photoluminescence, usually considered as an unwanted background of Raman spectra, can be used to detect the early stages of MXene degradation.
1. Paillet, M et al., Graphene and related 2D materials: An overview of the Raman studies. Journal of Raman Spectroscopy 2018, 49 (1), 8-12.
2. Ferrari A. C. et al., Raman Spectrum of Graphene and Graphene Layers. Physical Review Letters 2006, 97 (18), 187401.
3. Alhabeb, M et al., Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti₃C₂T_x MXene). Chemistry of Materials 2017, 29 (18), 7633-7644.
4. Naguib, M et al., Two-dimensional nanocrystals produced by exfoliation of Ti₃AlC₂. Advanced Materials 2011, 23 (37), 4248-53.
5. Hu, M et al., High-Capacitance Mechanism for Ti₃C₂T_x MXene by in Situ Electrochemical Raman Spectroscopy Investigation. ACS Nano 2016,10 (12), 11344-11350.
6. Lioi, D. B. et al., Electron-Withdrawing Effect of Native Terminal Groups on the Lattice Structure of Ti₃C₂T_x MXenes Studied by Resonance Raman Scattering: Implications for Embedding MXenes in Electronic Composites. ACS Applied Nano Materials 2019, 2 (10), 6087-6091

In a broad range of applications, including energy storage, flexible electronics, and sensors, additive manufacturing is considered as a promising device fabrication technique due to its potential for the development of complex architectures and using a variety of materials. Extrusion-based three-dimensional (3D) printing is a low-cost and straightforward method that offers rapid and precise fabrication of “on-chip” batteries and supercapacitors with 3D architectures. In this presentation, we report on our recent research on the development of functional nanocomposite inks based on two-dimensional (2D) MXenes, with the application in lithium-ion storage. MXenes are a new class of 2D materials with high electronic conductivity and hydrophilicity and have gained interest as high-performance electrode materials for electrochemical energy storage. The developed nanocomposite inks are incorporated with the MXene sheet as a conductive additive that can also provide the rheological properties required for extrusion-based 3D printing without the need for any other additive or binder. The inks were prepared by mixing lithium iron phosphate and lithium titanate nanoparticles with a highly concentrated water-based MXene ink and were directly used for printing of the cathode and the anode of a microbattery, respectively. A programmable printing machine was used in the fabrication process that follows the layer-by-layer deposition of the nanocomposite ink. The developed inks and printing methodologies facilitate the rapid fabrication of microbatteries on a variety of substrates, while the loading of active material per area of the device can be controlled by the number of deposited layers. The electrochemical test results show the excellent performance of 3D printed batteries.

2D magnetic materials have received tremendous attention recently due to their fundamentally unique properties from their bulk counterparts and promises in data storage, sensing, and magneto-opto-electronic applications. Following the discovery of 2D ferromagnetic ordering of monolayer CrI₃ and ultrathin Cr₂Ge₂Te₆, several 2D magnetic materials including halides, chalcogenides, and sulphides, have been reported. MXenes are 2D transition metal carbides, carbonitrides, and nitrides with a general formula M_n+1X_nT_x where M is early transition metal (such as Ti, Cr, Mn), X is C and/or N, n = 1-3, and T_x represents surface terminations (such as O and F). Although there are several DFT calculations that suggest magnetic ordering in 2D MXene flakes, there is no experimental report on magnetic ordering of MXenes. In this talk, magnetic and electronic properties of Cr₂TiC₂T_x, a double-metal ordered transition metal carbide MXene will be presented and discussed. We used a combination of vibrating sample magnetometry (VSM) and synchrotron X-ray magnetic linear dichroism (XMLD) to obtain evidence of antiferromagnetic order below 40 K in a free-standing film and a single flake of this MXene. Moreover, the temperature-dependence of resistivity and angular-dependent magnetoresistance measurements reveal distinct changes below 40 K, suggesting a coupling between the magnetic order and electronic transport.

Material modeling methods are now widely applied to investigate and design new materials. This is especially helpful in cases where different compositions are possible and structure-property relationships need to be predicted. This presentation will discuss the application of material modeling methods to MXenes. The methods used include density functional theory calculations and atomic-scale simulations with reactive force fields.

Small scale energy storage devices are expected to play a significant role in the future advances of portable electronics, wireless sensors, and multifunctional micro/nanosystems. A crucial requirement for the fabrication of energy storage devices with high energy and power densities is using electrode materials with superior electrochemical properties. Another important requirement is the assembly of the electrode materials into structures that promote high ionic and electronic conductivities. In this talk, we report a scalable printing method for fabrication of on-chip three-dimensional (3D) micro-supercapacitors based on a class of 2D transition metal carbides called MXenes as electrode materials. MXenes offer high electronic conductivity and high specific capacitance and therefore, have attracted much interest as high-performance electrode materials for supercapacitors. In our work, the fabrication of MXene electrodes and device assembly is achieved using an extrusion-based 3D printing process that uses viscoelastic water-based MXene ink. A programmable printing machine was used in the printing process which involved layer-by-layer deposition of the MXene ink to fabricate the 3D electrodes. The developed printing method allows rapid fabrication of micro-supercapacitors on a variety of substrates, while electrode height can be controlled by the number of deposited layers. All-solid-state devices printed on flexible substrates showed excellent electrochemical performances under different bending conditions and delivered extremely high areal capacitances (over 800 mF cm-2). Our study suggests that due to its high electronic conductivity and electrochemical properties, MXene is an excellent electrode material for the fabrication of 3D energy storage devices.

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a large class of materials that are finding numerous applications ranging from energy storage and electromagnetic interference shielding to water purification and antibacterial coatings. However, while bulk applications of MXenes are rapidly developing, their intrinsic physical property characterization through single-flake measurements remains a largely unexplored area of research. Here, we report the elastic properties of monolayers and bilayers of the most important MXene material to date, Ti₃C₂T_x (T stands for surface termination) measured using nanoindentation with the tip of an atomic force microscope. The effective Young’s modulus of a single layer of Ti₃C₂T_x was found to be 0.33 ± 0.03 TPa, which is the highest among the reported values for solution-processed 2D materials, including graphene oxide. Individual Ti₃C₂T_x flakes also exhibit a high conductivity of 4600±1100 S/cm and field-effect electron mobility of 2.6±0.7 cm²/Vs. We found that the resistivity of individual flakes is only one order of magnitude lower than the resistivity of multilayer Ti₃C₂T_x films, which indicates a surprisingly good electron transport through the surface terminations of different flakes, unlike in many other 2D materials.

MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their suitability for solution processing, hydrophilic surfaces, metallic conduction, and versatility in hosting a range of intercalant ions and molecules. Numerous additional properties such as magnetic ordering, semiconductor band gaps, and presence of topological band features have been computationally predicted but remain largely unrealized due to the experimental difficulty in obtaining uniform surface terminations (T_x). To better understand the impact of surface terminations and to develop design strategies that are independent of T_x, a layer-resolved understanding of electronic properties is needed. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-based MXenes (Ti₂CT_x, Ti₃C₂T_x, Cr₂TiC₂T_x, and Mo₂TiC₂T_x) using soft x-ray absorption spectroscopy, revealing minimal changes in the spectral features between the parent MAX phase and its MXene counterpart when no surface Ti atoms are present in the MXene, such as the M-site ordered Mo₂TiC₂T_x and Cr₂TiC₂T_x. In contrast, for MXenes with surface Ti atoms, here Ti₃C₂T_x and Ti₂CT_x, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds, indicative of a decrease in the electron count per Ti atom upon conversion to MXene. First principles calculations provide similar trends in the partial density of states derived from surface and interior Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface termination, indicating a relatively short length scale over which the T_x groups alter the nominal electron count associated with surface atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts. This work not only represents the first systematic L-edge transition metal XAS study in multi-element MXenes, but also points to the importance of deriving desired function from sub-surface M-site layers, to realize robust electronic, magnetic or topological behavior in MXenes. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, grant #DE-SC0018618.

Since their discovery in 2011 [1], MXenes have outperformed existing materials for a range of applications such as energy storage [2], water filtering [3], electromagnetic shielding [4], as catalysts for H₂ evolution from water [5] and as astonishingly effective materials for capturing CO₂ [6] to name but a few examples. Their outstanding performance is accredited to a range of properties, e.g. hydrophilic and conductive, that can be attributed to a rich transition metal chemistry. Ultimately, the range of properties are dictated by the tailoring potential of the MXenes. In this respect, MXenes stand out in stark contrast to commonly employed 2-dimensional structures.
The general formula to describe MXenes, M_n+1XT_x , identifies that the tailoring potential in the MXene family is vast. In addition to choice of thickness (n) and X element, for instance, M can be a range of single transition metal elements, or an extensive set of combinations between multiple M elements in an ordered or disordered condition. Similarly, the range and corresponding potential mix of terminating elements or molecules is an apparent key in determining the final MXene properties. On top of this, the surfaces may be further decorated by functional elements.
The present contribution primarily adopts advanced electron microscopy methods to reveal the state of the art available tailoring of the MXene structure and chemistry, owing to recent advances in tuning of surface terminations, surface decoration and corresponding properties.

MXenes exhibit excellent capacitance at high rates in aqueous electrolytes, especially in H₂SO₄ aqueous electrolyte, but in a narrow potential window, which limits the energy density. Moreover, oxidation of Ti₃C₂ under high anodic potentials in aqueous electrolytes further limits its use to cathodes of asymmetric devices. Organic electrolyte and room temperature ionic liquids (RTIL) can provide a higher potential window, leading to higher energy density and open circuit potential. While they can host other ions beyond protons, the rate capability of MXenes is not at the same level as what has been achieved in aqueous electrolytes. We hypothesis that there should be an optimum d-spacing that can allow for fast ions (beyond protons) for high power with high energy considering the larger voltage window offered by both organic and RTIL electrolytes. To test this hypothesis, we are studying the effect of interlayer spacing on the overall electrochemical performance of the electrodes to define. The specific capacitances from intercalated Ti₃C₂T_z for both Li⁺ and EMI⁺ were higher than those from non-intercalated Ti₃C₂T_z. The intercalated MXene shows a higher specific capacitance of 137 and 98 F g^-1 in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) and EMI-TFSI/AN electrolyte, respectively. The diffusion coefficient of EMI⁺ in intercalated Ti₃C₂T_z was higher than that from non-intercalated Ti₃C₂T_z, and the larger d-spacing, the higher the diffusion coefficient. Moreover, quasi-elastic neutron scattering was used to study MXene with different d-spacing, and it showed high mobile EMI⁺ confined in-between the layers of MXene with large d-spacing.

2D transition metal carbides and nitrides (MXenes) have accumulated tremendous interest recently as a result of their high conductivity, aspect ratio, and excellent figures of merit in numerous application areas. Freestanding films of MXenes are important for versatility in their incorporation into roll-to-roll production to allow for large-scale fabrication. Vacuum assisted filtration is currently the state-of-the-art for fabrication of freestanding MXene films, but this method does not project well for large-scale production due to limitations on film areas and thicknesses as well as the required use of a vacuum system. Aqueous-based solution casting onto hydrophilic substrates is a useful way to make MXene coatings but they cannot be delaminated from the substrate to yield a freestanding film. In this work, we show that it is possible to fabricate Ti₃C₂ MXene freestanding films through simple drop-casting onto hydrophobic substrates. MXene films prepared using this technique have greater alignment of MXene flakes due to repulsive interactions between the substrate and the Ti₃C₂ MXenes and allows for facile delamination of the film from the substrate. We show that freestanding drop-cast MXene films can be fabricated in large areas (125 cm²) and thicknesses (23.2 µm), have high electronic conductivity and smooth surfaces as a result of improved flake alignment. Drop-cast MXene films can also be micropatterned in three dimensions simply by using commercially available mirostructured plastics as substrates. The results presented here suggest a new scalable path towards creating MXene freestanding films for novel prototypes and industrialization.

Discovered in 2011, the 2D early transition metal carbides known as MXenes - obtained by etching the A-layers from the MAX phases - have generated substantial interest in the scientific community because of their potential in an ever-expanding host of applications. Whether during etching or use, it is critical to understand what happens in theinterlayerspace.In most applications, the first step is to etch and wash MXene multilayers, MLs, until they disperse. Using primarily XRD diffraction, the relationship between etchant used and washing protocols and the swelling of the interlayer space of Ti₃C₂T_x MLs is elucidated. How changing the intercalant cations can change the spacing, and even the nature of Ti₃C₂T_xMLs from hydrophilic to hydrophobic, is discussed. How to render MXenes oxidation resistant in aqueous solutions is described. Lastly, the similarities of MXenes and clays are overviewed.

New ultrathin and multifunctional electromagnetic interference (EMI) shielding materials are required for protecting electronics against electromagnetic pollution in the fifth-generation (5G) era. Micrometer-thin films built of 2D Ti₃C₂T_xMXene nanosheets have shown extraordinary promise for EMI shielding. Yet, EMI shielding properties of other MXenes have not been explored, despite the fact that more than 30 different stoichiometric MXenes have been reported and many more are possible, including an infinite number of solid solutions. We report on a systematic study of EMI shielding properties of 16 single-metal, ordered double-metal and random solid solution MXenes. Films with thickness ranging from nanometers to micrometers were produced by spin-casting, spray-coating and vaccum filtration. EMI shielding effectiveness of the sprayed Ti₃C₂T_xfilm with a thickness of ~40 nm reaches 21 dB (shielding >99% EM wave). Adjustable EMI shielding properties were achieved in solid solution MXenes with different ratios of elements. This work opens a pathway for designing ultrathin, flexible and multifunctional MXene films with outstanding EMI shielding performance coupled with other specific characteristics of particular MXenes.

A large family of two-dimensional transition metal carbides and nitrides (MXenes) raises interest for many applications due to their high electrical conductivity, mechanical properties [1], potentially tunable electronic structure [2], nonlinear optical properties [3], and the ability to be manufactured in the thin film state [4]. However, their chemistry that is key to development of these applications, still remains largely terra incognita [5]. In this presentation we will discuss recent progress in understanding fundamental MXene chemistry and harnessing it for development of applications.
For example, during delamination and storage in ambient air environment, spontaneous oxidation of MXene flakes leads to formation of titanium oxide, a process that can be harnessed for simple, inexpensive, and environmentally benign manufacturing MXene–titania composites for optoelectronics, sensing, and other applications [6]. We show that partially oxidized MXene thin films containing the in situ formed phase of titanium oxide have a significant photoresponse in the UV region of the spectrum. The relaxation process of photoexcited charge carriers, which takes a long time (∼24 h), can be accelerated in the presence of oxygen and water vapor in the atmosphere. These properties of spontaneously formed MXene-titania thin films make them attractive materials for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications.
Other selected examples illustrating connections between understanding MXene chemistry and development of their applications will also be considered.



References
1. Y. Li, S. Huang, C. Wei, C. Wu, V. N. Mochalin, Nature Communications, 10, 3014 (2019)
2. M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, Advanced Materials, 26(7), 992-1005 (2014)
3. J. Yi, L. Du, J. Li, L. Yang, L. Hu, S. Huang, Y. Dong, L. Miao, S. Wen, V. N. Mochalin, et al., 2D Materials, 6, 045038 (2019)
4. Y. Dong, S. Chertopalov, K. Maleski, B. Anasori, L. Hu, S. Bhattacharya, A. M. Rao, Y. Gogotsi, . N. Mochalin, R. Podila, Advanced Materials, 30(10), 1705714 (2018)
5. S. Huang, V. N. Mochalin, Inorganic Chemistry, 58(3), 1958 (2019)
6. S. Chertopalov, V. N. Mochalin, ACS Nano, 12(6), 6109-6116 (2018)

The capacitive properties of MXenes are high tunable based on their composition, terminal groups, electrolyte, choice of intercalated ions, and nanoscale confinement. To enhance the energy stored and power that can be delivered in these materials requires a significantly improved understanding of the MXene properties, intercalation chemistry, and determining the relevant processes. I will first describe a first-principles computational study focused on predicting the achievable capacitance in different MXenes with a simplified model aqueous electrolyte. Second, I will describe a highly integrated and high fidelity study between experimental and modeling approaches to investigate the intercalation processes for aqueous Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺ into Ti₃C₂ MXenes. Experiments include microcalorimetry, atomic force microscopy and cyclic voltammetry whose results are directly linked to the results of detailed first-principles modeling. Our integrated analysis allows us to understand the energy storage processes and highlights the importance of the dynamics of cations and positionings and their role in capacitive energy storage properties. These findings will expedite the evolutions of energy related functional devices by enabling improved design of higher-performing membranes and two-dimensional materials.

This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Electrically conductive 2D transition metal carbides and nitrides known as MXenes, discovered in 2011, have already shown an enormous potential in the field of electrochemical energy storage, due to their capabilities to host ions and protons in addition to their high electrical conductivity. MXenes are synthesized by selective etching of atomically thin metal layers from layered ceramics called MAX phases. The as synthesized MXene is in the form of multilayers held together by hydrogen and van der Waals bonds. The multilayers can be delaminated via intercalation followed by mechanical agitation to separate the layers from each other forming colloidal solution that can then be filtered forming freestanding MXene paper. While delaminated Ti₃C₂ MXene has shown high capacitance, multilayer Ti₃C₂ has shown more modest values. One limitation for using the free-standing MXene paper is the thickness, which results in low areal capacitance.

Herein, we will present on the electrochemical performance of multilayer MXene that was pre-intercalated with metal cations (Na⁺, K⁺, Mg²⁺) in between layers of Ti₃C₂ in H₂SO₄ aqueous electrolyte. At 5 A/g a gravimetric capacitance of 325 F/g was achieved, and it was stable over thousands of cycles. This is comparable to electrochemical performance of freestanding MXene paper but much higher than what can be achieved for conventional multilayer MXene without pre-intercalation. More interestingly, an areal capacitance up to 3 F/cm² was achieved for cation pre-intercalated multilayer Ti₃C₂. This simple modification of Ti₃C₂ MXene in its multilayer form yielded an easier avenue for making Ti₃C₂ supercapacitor electrodes with high areal capacitance. X-ray absorption spectroscopy (XAS) shows that changing cations in-between the layers alters the oxidation state of the titanium atoms in Ti3C2 MXene, whereas dispersing MXene in an H₂SO₄ environment resulted in reducing the surface titanium atoms of MXene. These results also encourage further investigation on the effects of intercalation and other modifications to Ti₃C₂ interlayer chemistry.

Raman spectroscopy is known as a fast and non-invasive tool to characterize materials, in particular their atomic bonding. In the recent years, it was widely used in the field of 2D materials, including graphene¹, transition metal dichalcogenides², boron nitride³ and others⁴. It has also been used to evaluate the composition and defects in the structure of those materials. Moreover, Raman spectra could be collected even from a monolayer of 2D material. This impact on the field shows that Raman spectroscopy can give a lot of information about 2D materials, therefore is an essential tool for studying the new families of 2D materials. One of those families are MXenes, 2-dimensional transition metal carbides and nitrides, discovered at Drexel University at 2011⁵, which gained already a lot of interest in a variety of applications.
There is a limited number of Raman spectroscopy studies of MXenes studies^6-10, therefore a database for quick assessment of a MXene is needed. We obtained Raman spectra and assigned peaks for M₂X members of the family: Nb₂C, Mo₂C, V₂C and Ti₂C, as well as M₂X₃: Ti₃C₂, Mo₂TiC₂ and M₄X₃: Cr₂TiC₂ and Nb₄C₃, Mo₂Ti₂C₄, covering the most widely used MXenes. We showed the difference between multilayer form (powder), delaminated form (deposited thin film or filtrated free-standing film). In the collected library of Raman spectra of MXenes we showed the change in the atomic vibrations with changing or substituting M element(s).

1. Ferrari A. C. et al., Raman Spectrum of Graphene and Graphene Layers. Physical Review Letters 2006, 97 (18), 187401.
2. Zhang, X et al., Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material. Chemical Society Reviews 2015, 44 (9), 2757-85.
3. Gorbachev, R. V. et al., Hunting for monolayer boron nitride: optical and Raman signatures. Small 2011, 7 (4), 465-8.
4. Ling, X. et al., Anisotropic Electron-Photon and Electron-Phonon Interactions in Black Phosphorus. Nano Letters 2016, 16(4), 2260-7.
5. Naguib, M., et al., Two-dimensional nanocrystals produced by exfoliation of Ti₃AlC₂. Advanced Materials 2011, 23 (37), 4248-53.
6. Champagne, A. et al., Electronic and vibrational properties of V₂C-based MXenes: from experiments to first-principles modeling. Physical review B 2018, 97 (11).
7. Li, L., Lattice dynamics and electronic structures of Ti₃C₂O₂ and Mo₂TiC₂O ₂ (MXenes): The effect of Mo substitution. Computational Materials Science 2016, 124, 8-14.
8. Tao Hu et al., Vibrational properties of Ti₃C₂ and Ti₃C₂T₂ (T = O, F, OH) monosheets by first- principles calculations: a comparative study. Physical Chemistry Chemical Physics 2015, 17 (15), 9997 - 10003.
9. Hu, T et al., Covalency-Dependent Vibrational Dynamics in Two-Dimensional Titanium Carbides. J Phys Chem A 2015, 119(52), 12977-84.
10. Yorulmaz, U. et al., Vibrational and mechanical properties of single layer MXene structures: a first-principles investigation. Nanotechnology 2016, 27 (33), 335702

Scaling the production of synthetic two-dimensional (2D) materials to industrial quantities has faced significant challenges due to synthesis bottlenecks whereby few have been produced in large volumes. These challenges typically stem from bottom-up approaches limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. MXenes are a large family of 2D carbides and/or nitrides that have applications in electrochemical energy storage, electromagnetic interface shielding, electrocatalysis, gas sensing, electrochromic devices, and many others. In contrast to other 2D materials, MXenes are produced via a top-down synthesis approach. The selective wet etching process does not have similar synthesis constraints as some other 2D materials. The reaction occurs in the whole volume; therefore the process can be readily scaled with reactor volume. In this study, the synthesis of 2D titanium carbide MXene (Ti₃C₂T_x) was studied in two batch sizes, 1 and 50 g, to determine if large-volume synthesis affects the resultant structure or composition of MXene flakes. Characterization of the morphology and properties of the produced MXene using scanning electron microscopy, X-ray diffraction (XRD), dynamic light scattering (DLS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, and conductivity measurements showed that the materials produced in both batch sizes are essentially identical. This illustrates that MXenes experience no change in structure or properties when scaling synthesis, making them viable for further scale-up and commercialization.

Textile devices integrate exciting functionalities into clothing, including the ability to sense, actuate, monitor health signals, and communicate with nearby electronics. These devices require power to function, motivating researchers to develop energy storage devices that can be seamlessly integrated into textiles. In the last ten years, several works have been published on the development of fiber- and yarn-based supercapacitors using various carbon-based active materials such as activated carbon, graphene, and intrinsically conducting polymers. These devices demonstrate high energy density below ~4 cm in length but suffer from high resistance at longer lengths leading to reduced performance. For this reason, new active materials with higher conductivity and advanced device architectures are needed to improve the capacitance and rate performance of textile supercapacitors.

We pioneer the incorporation of Ti₃C₂T_x, a two-dimensional carbide and member of the MXene family, into commercial natural and synthetic fibers to develop highly conductive and electrochemically active yarn electrodes. The outstanding electrical and electrochemical performance of these yarns produced by several techniques, from electrospinning to infiltration, can be attributed to the high conductivity (10,000 S cm^-1 as a thin film) and high capacitance of Ti₃C₂T_x (1500 F cm^-3) due to redox reactions of surface titanium atoms. These yarns were knitted into a new architecture of textile energy storage devices, 3D knitted supercapacitors, using industrial knitting machines. Unlike fiber- and yarn-based supercapacitors, knitted supercapacitors consist of interconnected loops of fiber/yarn electrodes, providing multiple pathways for electron transport and reducing resistance relative to a freestanding fiber/yarn. By tuning the knit structure and geometry, such as stitch density and fabric dimensionality, the spacing between electrodes reduced 4-fold, increasing the areal capacitance from 402 mF cm^-2 to 707 mF cm^-2 at 2 mV s^-1. This is the first work to study industrially relevant knitting parameters in designing textile-based supercapacitors and their effects on electrochemical performance. The ability to rapidly fabricate meters of electrochemically active yarns and energy storing textiles enables the design and study of fully integrated textile circuits that could one day be developed into garments capable of sensing and computing.

Salinity-gradient is emerging as one of promising clean and renewable sources of energy, but its energy efficiency is severely limited by unsatisfactory performance of available semipermeable membranes [1]. Most recently, nanoconfined two-dimensional channels, as fluidic conduits for harnessing the Gibbs free energy of mixing under salinity gradient, have exhibited their superior energy conversion performance to conventional technologies [2]. Here, the lamellar Ti₃C₂T_x MXene membrane is presented as a high-performance salinity battery, directly converting the stored chemical potential energy to electricity [3]. Subnanometer 2D channels in the MXene Ti₃C₂T_x membranes enable cation-selective passage, assisted with tailored surface functional groups, under salinity gradient, and consequentially produce diffusion potential. A record-high output power density of 21 W-m^-2 at room temperature with an energy conversion efficiency of up to 40.6 % is achieved by controlled surface charges at a 1000-fold salinity gradient. Furthermore, thermal control of surface charges and ionic mobility yields a power density of up to 54 W-m^-2. Our findings advance the fundamental understanding of nanoionic transport through MXene 2D channels and thus offer a new opportunity to develop MXene membranes as a promising platform for the high-performance salinity battery.

[1] Siria, A et al., Nat. Rev. Chem. 1, 0091 (2017) [2] Xiao, K. et al., Joule 3, 1 (2019) [3] Hong, S. et al., ACS Nano, 13, 8917 (2019)

MXenes have recently shown impressive optical and plasmonic properties associated with its ultrathin atomic layer structure.^[1] However, their use in photonic and plasmonic devices is still marginally explored. Herein, we have fabricated various flexible photodetectors made of different MXenes, i.e., Mo₂CT_x, Ti₃C₂T_x, Nb₂CT_x, T₂CT_x and V₂CT_x. The surface chemistry and the optoelectronic properties of the mechanically flexible arrays of two-terminal, parallel-type photodetectors deposited on paper substrates are thoroughly investigated. Amongst the five studied MXenes, Mo₂CT_x have exhibited the best performance owing to its relative stability against oxidation, moderately high free carrier density and electrical conductivity,^[2] though not as high as that of Ti₃C₂T_x.^[1,3,4]
The Mo₂CT_x flexible devices have exhibited broad photoresponse in the range of 400-800 nm with high responsivity (R, up to 9 A W^-1), detectivity (D*, ~5x10¹¹ Jones) and reliable photoswitching characteristics at a wavelength of 660 nm. It is worth mentioning that despite being the first demonstration of MXene-based photodetection, the performance of Mo₂CT_x thin films considering their R and D* is surpassing the majority of previously reported visible-band photodetectors based on solution-processed 2D materials. In particular, the responsivity of Mo₂CT_x devices were found to be ~18000 and ~1200 times higher than that of the first reported graphene^[5] and MoS₂^[6] photodetectors, respectively. In addition to their attractive performance and solution processability, we have revealed that our MXene-based devices possess an additional set of advantages including full visible spectrum coverage, highly stable operation and mechanical flexibility. Furthermore, micro-Raman spectroscopy conducted on bare and on gold-coated Mo₂CT_x films allowing for surface-enhanced Raman scattering demonstrated a surface chemistry and a specific low-frequency band that we have related to the vibrational modes of the single Mo₂CT_x nanosheets.
The photodetection mechanism of our devices was unveiled using spatially-resolved STEM-EELS and ultrafast femtosecond transient absorption spectroscopy. We show that the photoresponse is dominated by the intrinsic plasmon-assisted hot carrier generation. The demonstrated ability of coupling with light and dephasing of surface plasmons with a short lifetime, without the need of integration with other metallic plasmonic structures, as previously demonstrated with several 2D materials,^[7] have led to a photoresponse outperforming that of photoelectron-based devices.
Our findings shed light on the knowledge of photocurrent generation mechanisms in MXenes, making them much more viable for many photonic and plasmonic device applications. Moreover, the specific ability to detect and excite individual surface plasmon modes, provide a viable platform for various MXene-based optoelectronic applications.

Transition metal carbides, carbonitrides and nitrides, also known as MXenes, attract significant attention due to their unique combination of properties. In addition to interest from scientific community, MXenes hold a great promise for multiple applications, such as energy storage, electromagnetic interference shielding, water purification and others. In order to transition from the lab bench to industrial scale applications, however, it becomes increasingly important to understand how stable MXenes are under different conditions and to find approaches to optimize shelf life of the material. That is why a number of recent reports focused on oxidation stability of titanium carbide (Ti₃C₂T_x, where T is termination, such as -OH, =O, -F) as the most studied member of MXene family, suggesting different strategies to inhibit its degradation. Despite the recent advances in improving of Ti₃C₂T_x stability, understanding degradation process for other members of MXene family remains elusive. With the wide variety of MXene compositions available (>30 reported, about 100 of possible stoichiometric compositions and infinite number of solid solutions) it is critical to investigate how MXenes other than Ti₃C₂T_x behave and how their chemical composition affects stability.
In this presentation we report on our studies on stability of MXenes against hydrolysis and oxidation in aqueous solutions. We evaluate different approaches suggested in literature and numerically compare their efficiency. For instance, efficiency of different additives, such as antioxidants and edge capping agents, will be discussed using Ti₃C₂T_x MXene as a model system, as well as other MXene compositions. Significant attention will be paid to effect of synthesis method and surface groups on MXene stability. Finally, the effect of M element in MXene will be discussed using the M₂CT_x MXene family (M=Ti, Nb or V) as an example.

We will discuss our recent progress on direct fiber extrusion from graphene-oxide (GO) and MXene dispersions in water, and the corresponding fiber performance as supercapacitor electrodes in supercapacitor yarns. The flake sizes of these 2D sheets turn out to impact the resulted fiber properties and performance to a great extent.
Supercapacitors are known to have superior power densities over batteries due to their surface-centered charge-discharge processes. Recently, increasing attention has been attracted to yarn-shaped supercapacitor systems, mainly due to their high flexibility and pliable nature, leading to potential applications in wearable electronics, microrobots, medical implants, and smart textiles. However, one of the major limitations that occurs when supercapacitors are made into fiber/yarn shapes, is the increased equivalent series resistance (ESR) due to the fibrous geometry adopted here. For instance, even the best supercapacitor yarns reported will exhibit a few hundred ohms of resistance at one-meter length, which is usually neglected by researchers in their publication. To directly tackle this issue, here we report our strategies of using a metallic core filament in the yarn supercapacitors developed in our lab. With detailed materials and process engineering, we can boost the power density by one order of magnitude.

The past few years have witnessed significant development in the chemistry and potential biological applications of two-dimensional (2D) materials. Innovative 2D carbides, nitrides and carbonitrides of early transition metals, called MXenes, have been also extensively studied for several years on the applications in human health protection. The recent advances include biotechnological and biomedical applications such as anticancer treatment, photothermal therapy, drug delivery platforms, or nano-drugs without any additional modification. These can be combined with bioimaging, magnetic resonance or photoacoustic techniques. It is currently accepted that the specific functionalities of the MXenes can result in wide spectrum of bio-activities. In this context, MXenes are currently being carefully studied, with strong attention to mechanisms of action and biocompatibility features. We already know that MXenes exhibit different cellular effects which can be additionally induced by multiple external or internal factors. These effects are in turn strictly dependent on MXenes structure, chemical composition, and surface characteristics which positively support their interesting potential in nano-therapies. In fact, MXenes are promising anticancer agents, which can not only inhibit proliferation of cancer cells but also induce oxidative stress, and even influence the cell functioning and cell cycle through inducing programmed cellular death (apoptosis). On the other hand, such a wide range of bio-activities may inevitably cause unexpected toxicological effects that require a broader understanding. The obtained results clearly indicate that in this respect, the surface chemistry of MXenes has a significant impact on their biological properties. This is closely related to the synthesis methods and also oxidizing properties, leading to the occurrence of potentially toxic superficial metal oxides. In the case of MXenes, the challenge now relates not only to obtain the needed biological properties but above all, to understand and maintain (stabilize) them in the desired environment. The presented studies will shed some light on the issues raised above and also outline new directions in toxicology of the MXene phases. They are focused on elucidating antiproliferative, pro-oxidative, and pro-apoptotic mechanisms of action. The specific features of MXenes such as a high degree of morphological anisotropy, specific chemical functionalities, surface oxides, and unique surface charges undoubtedly define their biological properties.

Recent experimental success in the realization of two-dimensional (2D) magnetism has invigorated the search for low-dimensional material systems with tunable magnetic anisotropy that exhibit intrinsic long-range ferromagnetic order. In this talk, I will present a rational design approach for studying and engineering magnetism in MXenes. I will also discuss a recently developed model for applying machine learning to accelerate MXene synthesis. Using a crystal field theory model and first-principles simulations, we demonstrate intrinsic ferromagnetism, high magnetic moments, high Curie temperatures, and intrinsic semiconducting and half-metallic transport behavior in nitride and ordered double-transition-metal MXenes. We report that modifying the surface termination and transition metal in monolayer M₂NT_x nitride MXenes gives rise to a rich diversity of noncollinear spin structures and finely tunable magnetic anisotropy. We predict that manipulating the strength of the spin-orbit interaction and electron localization via the chemical degrees of freedom can induce sufficient anisotropy to counteract thermal fluctuations that suppress long-range magnetic order. Further, surface engineering and applied electric fields enable robust switching and stabilization of magnetic behavior in MXenes. We extend this approach to study recently synthesized non-van der Waals 2D transition metal oxide materials, showing an observable net magnetic moment in 2D iron and chromium oxides. Our work suggests that MXenes and transition metal oxides offer a promising avenue for achieving practical solid-state devices with 2D magnetic materials.

The family of two-dimensional (2D) transition metal carbides and nitrides, MXenes, is one of the largest families of 2D materials with more than thirty synthesized compositions (e.g., Ti₂C, Ti₃C₂, Nb₂C, Mo₂C) and several more that are predicted to exist. MXenes with two transition metals exist in either random solid solutions or ordered forms. The latter MXenes are called ordered double-transition metals carbides, in which two transition metals form either in-plane or out-of-plane atomic ordering, such as in Mo_1.33Y_0.66C, Cr₂TiC₂, and Mo₂Ti₂C₃. In the ordered double-transition metal carbides (MXenes), physical, electrochemical, and mechanical properties can be tuned by controlling the transition metals ordered chemistry.
Here, by combining two relatively new fields of double-transition metal MXenes and high entropy alloys and ceramics, a new area in the MXenes field has emerged as multi-principal element MXenes. In these MXenes, four transition metals are combined in the form of solid-solutions in the 2D atomic planes of MXenes. In this talk, the rationale behind high entropy MXenes, their synthesis conditions, and their properties will be discussed.

MXenes, a class of two-dimensional transition metal carbides, nitrides and carbonitrides, have been extensively studied since their discovery. Because of their superior conductivity, large specific surface area and tunable physic-chemical properties, MXenes have been demonstrated to be promising electrode materials for various batteries such as: Li ion batteries, Na ion batteries and Lithium-sulfur batteries etc. So far, more than thirty different kinds of MXenes have been successfully synthesized in experiment and at the same time more members were predicted to exist by theoretical calculations. Since MXenes possess rich chemical compositions and their family are still growing, it is meaningful to further exploit more high-performance MXenes-based electrode materials for various batteries. In this talk, some strategies including changing the functional groups and transition metals of MXenes for designing high-capacity anode materials in Na ion batteries will be discussed. Specifically, the structural, electronic, adsorption and diffusion properties of Ti₃C₄, Ti₃C₂S₂and Zr-based MXenes as anode materials for Na ion batteries by using density functional theory (DFT) calculations will be included. More importantly, a correlation between charge transfer, lattice mismatch and the capacity of MXenes were identified, which will be useful for designing other 2D anode materials for Na ion batteries [1-2]. At the meantime, the potential of a series of MXenes as anchoring materials for Lithium-sulfur batteries and structure-properties correlation will also be presented [3]. Moreover, the strain effects on electronic properties of MXenes with a focus on lithium adsorption and structural transformation will also be discussed [4].

[1] Meng, Q. Q.; Ma, J. L.; Zhang, Y. H.; Li, Z.; Zhi, C. Y.; Hu, A.; Fan, J. The S-functionalized Ti₃C₂MXene as a High Capacity Electrode Material for Na-ion Batteries: a DFT study. Nanoscale. 2018, 10, 3385-3392.
[2] Meng, Q. Q.; Ma, J. L.; Zhang, Y. H.; Li, Z.; Hu, A.; Kai, J. J.; Fan, J. Theoretical Investigation of Zirconium Carbide MXenes as Prospective High Capacity Anode Materials for Na-ion Batteries. Journal of Materials Chemistry A. 2018, 6, 13652-13660.
[3] Li, N.; Meng, Q.; Zhu, X.; Li, Z.; Ma, J.; Huang, C.; Song, J.; Fan, J. Lattice constant-dependent Anchoring Effect of MXenes for Lithium-sulfur (Li-S) Batteries: a DFT study. Nanoscale. 2019, 11, 8485-8493.
[4] Li, Y.R.; Li, N; Zhao S.J.; Fan, J.; Kai J.J. Strain-Tunable Electronic Properties and Lithium Storage of 2D Transition Metal Carbide (MXene) Ti₂CO₂as Flexible Electrodes. Journal of Materials Chemistry A (under revision).

Acknowledgement: Research Grant Council of Hong Kong under Grant No. 11306517 & 11305919.

Cancer phototherapy (PT) including photodynamic therapy (PDT) and photothermal therapy (PTT) has attracted extensive interest due to its non-invasive and region-specific nature. In order to enhance the efficacy, we have fabricated various nanocomposites consisting of a photosensitizer (chlorin e₆ (Ce6), pheophorbide a (Pa) or bacteriopheophorbide a (bPa)) for PDT and a two-dimensional nanosheet (graphene or MoS₂) for PTT by a one-pot protocol through liquid phase exfoliation. In addition, an anticancer drug (doxorubicin (DOX) or irinotecan (Ir)) was loaded on the nanosheet in the same way. All the nanocomposites were stably dispersed in water except for MoS₂-Ir. Among the nanocomposites, MoS₂-Ce6 killed cancer cells more than ten times more effective than Ce6 loaded other nanomaterials [1].
[1] G. Liu,* P. Zhao, N. Liu, F. Yoshino, H. Qin, Y. Zou, S. Shi, T. Amano, J. R. Aguilar Cosme, Y. Nagano, H. Tamiaki, N. Komatsu* "Photosensitizer and anticancer drug-loaded 2D nanosheet: Preparation, stability and anticancer property" 2D Materials, 6(4), 045035 (2019).

Two-dimensional (2D) transition metal carbides (called MXenes) are among the most promising candidates for supercapacitor electrodes because of their ability to intercalate a variety of cations and high rate pseudocapacitive properties in acidic and neutral electrolytes. However, the research on 2D MXenes is just at its early stages, and out of ~30 MXene compositions that are experimentally made, only Ti₃C₂T_x MXene has been extensively studied. A few reports have also shown the promise of other MXenes, such as 2D V₂CT_x in aqueous supercapacitors. Recently, we have shown high rate pseudocapacitance and exceptional (electro)chemical stability of cation-pillared delaminated V₂CT_x in neutral aqueous electrolytes as well as highly acidic electrolytes with capacitances as high as ~ 1300 F cm^-3 (obtained in 3M H₂SO₄).^[2] Despite these promising properties, the peak capacitance of Ti₃C₂T_x and V₂CT_x in 3M H₂SO₄ can only be achieved in certain potentials (~ -0.3 V vs. Ag/AgCl for Ti₃C₂T_x and ~ 0V vs. Ag/AgCl for V₂CT_x). In addition, the cycle life of V₂CT_x electrodes in the highly acidic sulfuric acid electrolyte is not on par with the Ti₃C₂T_x. Also, similar to the other 2D materials, the self-restacking of MXenes and their lower capacitances compared to other pseudocapacitive materials have hindered their further development. Controlled pillaring of 2D materials into multilayers and vertical stacking of different 2D layers into heterostructures have shown to be effective methods to avoid their self-restacking and overcome drawbacks of individual 2D materials.^[1] Such pillared 2D multilayers or superlattice heterostructures built by sequential stacking of multiple 2D layers can display new and improved electrochemical responses and offer the combination of their building blocks’ best properties. In this talk, we argue that combining different delaminated MXenes into stacked heterostructures can result in new electrode structures that have superior properties in supercapacitors. We demonstrated this by fabricating all MXene heterostructures from delaminated Ti₃C₂T_x and V₂CT_x, which showed new features in their cyclic voltammetry profiles and could deliver the highest volumetric capacitance of ~1470 F cm^-3 in 3M H₂SO₄ electrolyte. This study paves the way for the preparation of high-performance hetero-layered MXene structures for electrochemical energy storage applications and beyond.
References
[1] E. Pomerantseva, Y. Gogotsi, Nat. Energy 2017, 2, 17089.
[2] A. VahidMohammadi, M. Mojtabavi, N. M. Caffrey, M. Wanunu, M. Beidaghi, Adv. Mater. 2019, 31, 1806931.

Here we show that MXene flakes can be uniformly coated with amorphous recombinant silk fibroin in aqueous solution under ambient conditions. X-ray photoelectron spectroscopy has revealed a reduction of TiO₂ formation by half on modified flakes after 21 days in water, suggesting the silk coating provides some protection against oxidation of the MXene surface, a common issue of MXene dispersions. High resolution atomic force microscopy was used to map the morphological changes of these modified flakes over time and showed silk fibril formation on flake surfaces. This spontaneous reorganization of recombinant silk, a material that typically has difficulty reorganizing after the harsh chemical treatment used to solubilize spun fibers, suggesting some chemical reduction provided by the MXene flake.

Two-dimensional (2D) materials have demonstrated unique friction and wear properties compared with their bulk (3D) counterparts. Some of the well-known 2D materials such as graphene, MoS₂ and h-BN have shown outstanding tribological properties as additives to solvents, nanofillers in composite materials, and solid lubricants. A relatively new, large and quickly growing family of two-dimensional (2D) early transition metal carbides and nitrides (MXenes) [1] possess a unique combination of high electrical conductivity and hydrophilicity [2], which offers great potential in different applications such as energy generation [3] and storage, sensors [4], composites, and biomedical applications.
There is an increased interest to mechanical properties of MXenes [5]. However, their tribological behavior has not been sufficiently studied yet. Here we investigate the tribological properties of Ti₃C₂ MXene with different flake sizes deposited on SiO₂ coted Si substrates subjected to wear by sliding against a diamond-like carbon (DLC) coated steel-ball using a ball-on-disc type tribometer operating in dry nitrogen and in ambient air environments. We have observed that the friction is reduced to superlubric regime in dry nitrogen environment. Moreover, a combination of MXene and graphene has also been studied and the results show that with the addition of graphene, the abrasion was further reduced while the superlubricity behavior was maintained. This work opens new opportunities for exploring the potential of MXenes and MXene/graphene coatings as novel solid lubricants for various applications.

References
[1] Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y., 25th anniversary article: MXenes: a new family of two-dimensional materials. Advanced Materials 2014, 26 (7), 992-1005.
[2] Ghidiu, M.; Kota, S.; Halim, J.; Sherwood, A. W.; Nedfors, N.; Rosen, J.; Mochalin, V. N.; Barsoum, M. W., Alkylammonium cation intercalation into Ti3C2 (MXene): Effects on properties and ion-exchange capacity estimation. Chemistry of Materials 2017, 29 (3), 1099-1106.
[3] Dong, Y.; Mallineni, S. S. K.; Maleski, K.; Behlow, H.; Mochalin, V. N.; Rao, A. M.; Gogotsi, Y.; Podila, R., Metallic MXenes: A new family of materials for flexible triboelectric nanogenerators. Nano Energy 2018, 44, 103-110.
[4] Chertopalov, S.; Mochalin, V. N., Environment Sensitive Photoresponse of Spontaneously Partially Oxidized Ti3C2Tx MXene Thin Films. ACS nano 2018.
[5] Li, Y.; Huang, S.; Wei, C.; Wu, C.; Mochalin, V. N., Adhesion of two-dimensional titanium carbides (MXenes) and graphene to silicon. Nature communications 2019, 10 (1), 3014.