authors contributed equally to this work. One Sentence Summary: Lewis acidic molten salts etching is an effective and promising route for producing MXenes with superior electrochemical performance in non-aqueous electrolyte. Abstract: Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases

During the past decade, two-dimensional (2D) materials have attracted extensive attention due to their high surface area to volume ratio, unique electronic structures and physiochemical properties derived from their low dimensionality. [1][2][3][4][5][6][7] Graphene, the most studied 2D material with ultrahigh mechanical strength, [8][9] excellent electronic and thermal conductivities, [1,[10][11] exhibits potential applications in electrochemical energy storage, transparent electrodes, and nano-composites. [12] However, due to its intrinsic zero bandgap and simple chemistry, applications of graphene are restricted in some aspects, such as field effect transistors.[10] Thus, investigations on other 2D materials are performed, especially for those 2D materials with two or more composition elements, such as metal oxide, layered metal chalcogenides (LMDCs), hexagonal boron nitride(BN), hydroxides, etc. [6,13] In recent years, a new class of 2D materials called MXenes, have emerged [14] Members in this family are described by the general formula of Mn+1XnTz (wherein M is an early transition metal, X is C and/or N, n is 1, 2, 3, and Tz denotes surface terminated functional groups). [14] Generally, MXenes are produced by the selective etching of Al layers from their parental layered ternary MAX phases, a large group which comprises more than 70 members. [15][16] Through selective etching of aluminium layers, experimental investigations have successfully identified about 10 different MXenes, Ti3C2Tz, Ti2CTz, Ta4C3Tz, TiNbCTz, (V0.5,Cr0.5)3C2Tz, Ti3CNTz, Nb2CTz, V2CTz, and Nb4C3Tz. [14,17] Most of the synthesized MXenes are metallic, [18] hydrophilic, and predicted to have high elastic moduli, implying potential application as reinforcement of polymer. [19][20] The existance of Dirac electrons in some MXenes has also been theoretically predicted .[21] Moreover, similar to graphene, MXenes are promising candidate electrode materials for lithium-ion batteries and supercapacitors by facile intercalation of Li ions into the MXene layers. [22][23][24][25][26][27] Recently, considerable efforts have been made to further expand the family of 2D carbides. On the basis of a substitutional solid solution method, Gogotsi and Barsoum et al. [28] successfully synthesized Mo2TiC2Tz, Mo2Ti2C3Tz, and Cr2TiC2TzMXenes, and surface dependent electrochemical behaviors in the case of Mo2TiC2Tz have been revealed. Moreover, Mo2CTz MXene [29] has been synthesized through selective etching of gallium (Ga) from a thin film of the new ternary nanolaminated Mo2Ga2C. [30][31] Besides, large-area high-quality 2D α-Mo2C, WC, and TaC crystals have been fabricated by a chemical vapour deposition (CVD) process.[32] However, potential MXene compounds in materials systems where Al-containing MAX phases are not established, such as Hf2C and Zr2C, are yet to be produced.Herein, for the first time, we report the preparation of Zr-containing 2D carbide based on selective extraction of Al-C units from an alternative layered ternary Zr3Al3C5, benefiting from ...

We demonstrate fabrication of a two-dimensional Hf-containing MXene, HfCT, by selective etching of a layered parent Hf[Al(Si)]C compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm for lithium and sodium ions batteries, respectively, at a current density of 200 mAg after 200 cycles. Thus, HfCT MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

The two-dimensional material MXene has recently attracted interest for its excellent performance in diverse perspectives. Etched from the parental MAX phase with hydrofluoric acid, the synthesized MXene surface is normally functionalized by oxygen (-O), fluorine (-F) or hydroxyl (-OH) groups. Herein, using first-principles density functional calculations, we investigate the structural, mechanical and electronic properties of the carbide MXene M2CT2 (M=Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; T=-O, -F, -OH). Both the M atom and the surface group T have a significant effect on the MXenes properties. Generally, oxygen functionalized MXenes present smaller lattice parameters and stronger mechanical strength compared to those functionalized by fluorine and hydroxyl groups. exhibits the smallest interlayer thickness and shows the strongest mechanical strength. In regard to electronic properties, five oxygen functionalized members M2CO2 (M=Sc, Ti, Zr, Hf, W), two fluorine functionalized members M2CF2 (M=Sc, Mo), and hydroxyl functionalized Sc2C(OH)2 present semiconducting characteristics, but only Sc2C(OH)2 exhibits a direct band gap.

With the growing interest in low dimensional materials, MXenes have also attracted considerable attention recently. In this work, the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes are investigated using first-principles calculations. Hf2CO2 is determined to exhibit a thermal conductivity better than MoS2 and phosphorene. The room-temperature thermal conductivity along the armchair direction is determined to be 86.25~131.2 Wm−1 K−1 with a flake length of 5~100 μm. The room temperature thermal expansion coefficient of Hf2CO2 is 6.094 × 10−6 K−1, which is lower than that of most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility. At room temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag direction) is determined to be as high as 13.5 × 103 cm2V−1s−1 (17.6 × 103 cm2V−1s−1). Thus, broader utilization of Hf2CO2, such as the material for nanoelectronics, is likely. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided. Notably, Ti2CO2 presents relatively lower thermal conductivity but much higher carrier mobility than Hf2CO2. According to the present results, the design and application of MXene based devices are expected to be promising.

MXenes, the new 2D transition metal carbides and nitrides, have recently attracted extensive attention due to their diverse applications and excellent performances. However, the thermal and electrical properties of most MXene materials are yet to be studied. In this work, we investigate the electrical and thermal properties of semiconducting Sc2CT2 (T = F, OH) MXenes using first-principles calculations. Both of the Sc2CT2 (T = F, OH) MXenes are determined to show excellent carrier mobilities. The electron mobility in the Sc2CF2 MXene is found to be strongly anisotropic at room temperature, with values of 5.03 × 10(3) and 1.07 × 10(3) cm(2) V(-1) s(-1) in the zigzag and armchair directions, respectively. The predicted electron mobility in the zigzag direction of the Sc2CF2 is nearly four-fold that in the armchair direction of the promising semiconductor phosphorene. In contrast to Sc2CF2, Sc2C(OH)2 presents approximately isotropic electron mobility. The values at room temperature in the zigzag and armchair directions are calculated as 2.06 × 10(3) cm(2) V(-1) s(-1) and 2.19 × 10(3) cm(2) V(-1) s(-1), respectively. In regard to the thermal properties, the thermal conductivities of the Sc2CT2 (T = F, OH) MXenes have been determined. The predicted values are higher than those of most metals and semiconducting low-dimensional materials, such as monolayer MoS2 and phosphorene. In particular, the room-temperature thermal conductivity along the Sc2CF2 armchair direction has been determined to be as high as 472 W m(-1) K(-1) based on a flake length of 5 μm, which is even higher than that of the best traditional conductor silver. The corresponding value in the zigzag direction of Sc2CF2 is calculated to be 178 W m(-1) K(-1). The thermal conductivity in Sc2C(OH)2 is less anisotropic and lower compared to that in Sc2CF2. The room-temperature value in the armchair (zigzag) direction is determined to be 173 W m(-1) K(-1) (107 W m(-1) K(-1)). Based on their excellent electron mobilities and high thermal conductivities, both of the Sc2CT2 (T = F, OH) MXenes could be promising candidate materials for the next generation of electronic devices.

Mo2C, the newly synthesized MXene with a large lateral size and superconductivity property, has attracted increasing interest in material science. Employing first-principles density functional calculations, its intrinsic structural, electrical, thermal, and mechanical properties are investigated in this work. It is found that this MXene is nonmagnetic with a small molar volume. The electrical conductivity is predicted in the order of 106 Ω–1m–1, and its value is significantly influenced by doping. For thermal conductivity, both of the electron and phonon contributions are studied. At room temperature, the Mo2C’s thermal conductivity is determined to be 48.4 Wm–1 K–1, which can be further enhanced by increasing temperature and introducing n-type dopants. The specific heat and thermal expansion coefficient are also assessed, and their values at room temperature are calculated as 290 Jkg–1 K–1 and 2.26 × 10–6 K–1, respectively. Moreover, the thermal contraction of the MXene is found at low temperatures. Under biaxial strains, the elastic modulus is predicted as 312 ± 10 GPa, and the ideal strength is determined to be 20.8 GPa at a critical strain of 0.086. In view of the small molar volume, superhigh electrical conductivity, favorable thermal conductivity, low thermal expansion coefficient, and high mechanical strength, the Mo2C MXene generally merits more widespread applications besides superconductors, such as applying to substrates for other layer materials, and candidate materials for batteries and supercapacitors.

The room-temperature synthesis of an ew twodimensional (2D) zirconium-containing carbide,Z r 3 C 2 T z MXene is presented. In contrast to traditional preparation of MXene,the layered ternary Zr 3 Al 3 C 5 material instead of MAX phases is used as source under hydrofluoric acid treatment. The structural, mechanical, and electronic properties of the synthesized 2D carbide are investigated, combined with first-principles density functional calculations.Acomparative study on the structrual stability of our obtained 2D Zr 3 C 2 T z and Ti 3 C 2 T z MXenes at elevated temperatures is performed. The obtained 2D Zr 3 C 2 T z exhibits relatively better ability to maintain 2D nature and strucural integrity compared to Ti-based Mxene. The difference in structural stability under high temperature condition is explained by atheoretical investigation on binding energy.During the past decade,t wo-dimensional (2D) materials have attracted extensive attention owing to their high surface area to volume ratio,u nique electronic structures,a nd physiochemical properties derived from their low dimensionality. [1][2][3][4][5][6][7] Graphene,t he most studied 2D material with ultrahigh mechanical strength [8,9] and excellent electronic and thermal conductivities, [1,10,11] exhibits potential applications in electrochemical energy storage,t ransparent electrodes,a nd nano-composites.[12] However,owing to its intrinsic zero band gap and simple chemistry,a pplications of graphene are restricted in some aspects,s uch as field-effect transistors. [10] Thus,i nvestigations on other 2D materials are performed, especially for those 2D materials with two or more composition elements,s uch as metal oxides,l ayered metal chalcogenides (LMDCs), hexagonal boron nitride (BN), and hydroxides. [6,13] In recent years,an ew class of 2D materials, called MXenes,have emerged. [14] Members in this family are described by the general formula of M n+1 X n T z (wherein Mis an early transition metal, XisCand/or N, n is 1, 2, 3, and T z denotes surface terminated functional groups).[14] Generally, MXenes are produced by the selective etching of Al layers from their parental layered ternary MAX phases,alarge group which comprises more than 70 members. [15,16] Through selective etching of aluminum layers,experimental investigations have successfully identified about 10 different MXenes: Ti 3 C 2 T z ,Ti 2 CT z ,T a 4 C 3 T z ,TiNbCT z ,(V 0.5 ,Cr 0.5 ) 3 C 2 T z ,Ti 3 CNT z , Nb 2 CT z ,V 2 CT z ,a nd Nb 4 C 3 T z . [14,17] Most of the synthesized MXenes are metallic, [18] hydrophilic,a nd predicted to have high elastic moduli, implying potential application as reinforcement of polymer. [19,20] Theexistance of Dirac electrons in some MXenes has also been theoretically predicted.[21] Moreover, similar to graphene,M Xenes are promising candidate electrode materials for lithium-ion batteries and supercapacitors by facile intercalation of Li ions into the MXene layers.[ [29] has been synthesized through selective etching of gallium (Ga) from athin film of ...