Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between Zn element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach.In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to prepare MXenes through an HF-free chemical approach.

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.

The class of two-dimensional metal carbides and nitrides known as MXenes offer a distinct manner of property tailoring for a wide range of applications. The ability to tune the surface chemistry for expanding the property space of MXenes is thus an important topic, although experimental exploration of new surface terminals remains a challenge. Here, we synthesized Ti3C2 MXene with unitary, binary and ternary halogen terminals, e.g. -Cl, -Br, -I, -BrI and -ClBrI, to investigate the effect of surface chemistry on the properties of MXenes. The electrochemical activity of Br and I element result in the extraordinary electrochemical performance of the MXenes as cathodes for aqueous zinc ion batteries. The -Br and -I containing MXenes, e.g. Ti3C2Br2 and Ti3C2I2, exhibit distinct discharge platforms with considerable capacities of 97.6 mAh• g -1 and 135 mAh• g -1 . Ti3C2(BrI) and Ti3C2(ClBrI) exhibit dual discharge platforms with capacities of 117.2 mAh• g -1 and 106.7 mAh• g -1 . In contrast, the previously discovered MXenes Ti3C2Cl2 and Ti3C2(OF) exhibit no discharge platforms, and only ~50% of capacities and energy densities of Ti3C2Br2. These results emphasize the effectiveness of the Lewis-acidic-melt etching route for tuning the surface chemistry of MXenes, and also show promise for expanding the MXene family towards various applications.

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.

Dendrite growth and low Coulombic efficiency caused by uneven diffusion and electrodeposition of Zn2+ ions have emerged as a barrier to exploit the Zn metal anode. In this work, we demonstrate the stoichiometric halogenated MXenes (Ti3C2Cl2, Ti3C2Br2, and Ti3C2I2) as an artificial layer that can induce the uniform Zn deposition. The efficient redistribution effect results from the coherent heterogeneous interface reconstruction and regulated ion tiling by halogen surficial termination. The synergetic effects of high lattice matching (90%) between the adopted MXenes and Zn, as well as the positive halogen regulation, Zn2+ ions are guided to nucleate uniformly on the most extensive (000l) crystal plane of the MXene matrix and grow in a planar manner. In terms of Zn ion regulation, Cl termination is found to be more effective than O/F, Br, and I due to its moderate adsorption and diffusion coefficiency for Zn2+ ions. The Ti3C2Cl2–Zn anode achieves a life extension of over 12 times (840 h at 2 mA cm–2//1 mAh cm–2) over that of the bare Zn anode and serves more than 9000 cycles in a battery with a Ti3C2I2 cathode at a high rate of 3 A g–1. Given the abundance of lattice parameters and terminations of MXene materials, the developed strategy is expected to be extended to other metal anode systems.

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