Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

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.

In 2017, Fashandi et al. synthesized MAX phases with noble-metal elements in the A site, obtained through a replacement reaction. 18,19 The replacement reaction was achieved by replacement of Si by Au in the A layer of Ti3SiC2 at high annealing temperature with a thermodynamic driving force for separation of Au and Si at moderate temperature, as determined from the Au-Si binary phase diagram. The formation of Ti3AuC2 is driven by an A-layer diffusion process. Its formation is preferred over competing phases (e.g. Au-Ti alloys), and the MAX phase can be obtained at a moderate temperature. Similarly, Yang et al. also synthesized Ti3SnC2 by a replacement reaction between Al atom in Ti3AlC2 and SnO2, 20 although Ti3SnC2 can also be synthesized by a hot isostatic pressing (HIP) route. 21 Their work implies the feasibility of synthesis of novel MAX phases through replacement reactions.
It is worth noting that the synthesis of MAX phases by the A-site element exchange approach is similar to the preparation of MXenes by A-site element etching process. Both are top-down routes that make modification on the A atom layer of preexisting structure of MAX phase, which involve the extraction of A-site atoms and intercalation of new species (e.g. metallic atoms or functional terminals) at particular lattice position. Based on this idea, we introduce here a general approach to synthesize a series of novel nanolaminated MAX phases and MXenes based on the element exchange approach in the A-layer of traditional MAX phase. The late transition metal halides, e.g. ZnCl2 in this study, are so-called Lewis acids in their molten state. [22][23][24][25] These molten salts can produce strong electron-accepting ligands, which can thermodynamically react with the A element in the MAX phases. Simultaneously, certain types of atoms or ions can diffuse into the two-dimensional atomic plane and bond with the unsaturated Mn+1Xn sheet to form corresponding MAX phases or MXenes. The flexible selection of salt constituents can provide sufficient room to control the reaction temperature and the type of intercalated ions. In the present work, a variety of novel MAX phase (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) and MXenes (Ti3C2Cl2 and Ti2CCl2) were synthesized by elemental replacement in the A atomic plane of traditional MAX phases in ZnCl2 molten salts. The results indicate a general and controllable approach to synthesize novel nanolaminated MAX phases and the derivation of halide group-terminated MXenes from its respective parent MAX phase.

Experimental and Computational Details
Preparation of MAX phases and MXenes. All the obtained new MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN and V2ZnC, denoted in brief as Zn-MAX) and MXenes (Ti3C2Cl2 and Ti2CCl2, denoted in brief as Cl-MXenes) were prepared by a reaction between the traditional MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC, denoted as Al-MAX) and ZnCl2. For synthesizing the Zn-MAX, a mixture powder composed of the Al-MAX: ZnCl2=1:1.5 (molar ratio) was used as the starting material.
For synthesizing the Cl-MXenes, a mixture powder composed of Al-MAX:ZnCl2=1:6 (molar ratio) was used as the starting material.
The starting material was mixed thoroughly using a mortar under the protection of nitrogen in a glovebox. Then the as-obtained mixture powder was taken out from the glove box and placed into an alumina crucible. The alumina crucible was loaded into a tube furnace and heat-treated at 550 o C for 5 h under the protection of Ar gas.
After the reaction, the product was washed by de-ionized water to remove the residual ZnCl2 and the final product was dried at 40 o C. Finally, the target reaction product was obtained. The weight change during various steps of the process was provided in the supporting information (Table S1, Table S2, and Fig. S1).
In addition, the starting materials of Ti3AlC2:ZnCl2=1:1 to 1:6 were heat-treated at 550 o C for 5 h to investigate the influence of Ti3AlC2/ZnCl2 ratio on the composition of product. The starting materials of Ti3AlC2:ZnCl2=1:6 was heat-treated at 550 o C for different time (0.5 h to 5h) to investigate the phase evolution of the reaction product.
Preparation of MAX phase precursors. The Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC MAX phase precursors were powders prepared by a molten-salt method as was reported previously. 14,26,27 Metal carbides/nitrides powders (TiC, TiN, and VC), M-site metal powders (Ti and V) and A-site metal powders (Al) were used as the source for synthesizing the target MAX phases. NaCl and KCl with the molar ratio of 1:1 was used as the source for the molten salt bath. All these metal carbides/nitrides were ~10 μm in particle size and were purchased from Pantian Nano Materials Co.
Ltd. (Shanghai, China). Ti, V, and Al powders were 300 mesh in particle size and were purchased from Yunfu Nanotech Co. Ltd. (Shanghai, China). NaCl and KCl at analytical grade were purchased from Aladdin Industrial Co. Ltd. (Shanghai, China).
The starting materials, with the composition shown in Table 1, were mixed in a mortar and placed in an alumina crucible. Under the protection of argon, the alumina crucible was packed in a tube furnace and heat-treated to 1100 o C with the rate of 4 °C/min and kept 3 h to accomplish the reaction. After reaction, the tube furnace was cooled to room temperature with the rate of 4 °C/min. Then the product was washed by deionized water to remove the NaCl and KCl, and the residual product was dried at 60 °C. As a result, the target MAX phase precursors were obtained. Characterization. Scanning electron microscopy (SEM) was performed in a thermal field emission scanning electron microscope (Thermo scientific, Verios G4 UC) equipped with an energy-dispersive spectroscopy (EDS) system. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) was performed in the Linköping monochromated, high-brightness, double-corrected FEI Titan 3 60-300 operated at 300 kV, equipped with a SuperX EDS system. X-ray diffraction (XRD) analysis of the composite powders was performed using a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation at a scan rate of 2 °/min. X-ray photoelectron spectra (XPS) were performed in an XPS system (Axis Ultra DLD, Kratos, UK) with a monochromatic Al X-ray source. The binding energy (BE) scale were assigned by adjusting the C 1s peak at 284.8 eV.
Computational Methods. First-principles density-functional theory (DFT) calculations were carried out using the CASTEP module. 28 The CALPHAD approach was applied to calculate the phase diagrams of the Ti-Al-C and Ti-Zn-C systems. The Ti-Al-C system has been well assessed by Witusiewicz et al. 37 and the thermodynamic dataset were adopted in this work. Due to lack of experimental data on the ternary Ti-Zn-C compounds, first-principles calculations were conducted to support the CALPHAD work. 38 The formation enthalpies of the Ti2ZnC and Ti3ZnC2 ternary compounds were computed to be -414.63 and -230.16 kJ/mol, respectively. Their Gibbs free energy functions were then determined with the Neumann-Kopp rule and added in the CALPHAD-type dataset of the Ti-Zn-C system, which included the thermodynamic parameters of the binary Ti-Zn and Zn-C systems. 39 With the established thermodynamic dataset, the isothermal sections of the Ti-Al-C and Ti-Zn-C systems at 1300 and 550 °C were computed. All the calculations were performed with the Thermo-Calc software.

Results and discussion
Zn-MAX phases. Ti3ZnC2 was prepared by using the starting materials of Ti3AlC2 and ZnCl2 with mole ratio of 1:1.5. Fig.1a shows the XRD patterns of the initial phase Ti3AlC2 and the final product Ti3ZnC2. Compared to Ti3AlC2, the XRD peaks of Ti3ZnC2, e.g., (103), (104), (105), and (000l) peaks, are shifted towards lower angles, indicating a larger lattice constant caused by the replacement of Al atoms by Zn atoms. Note that the relative intensity of (0004), (0006) peaks increased while that of (0002) peaks decreased. This is caused by the change in structure factor by the replacement of the A atoms. According to a Rietveld refinement of the XRD pattern  Atomically-resolved STEM in combination with lattice resolved EDS was employed to demonstrate the elemental organization of Ti3ZnC2. Fig. 1d    Cl-MXenes. A mixture of Ti3C2Cl2 MXene sheets and Zn spheres were obtained by using the starting materials of Ti3AlC2 and ZnCl2 with mole ratio of 1:6. The SEM image (Fig. 3a) and TEM images (Fig. S5) of Ti3C2Cl2 shows exfoliation along the basal planes. The corresponding EDS analysis (Fig. S6) 40 Thus, a small part of Cl terminations might be replaced by O-containing terminals during the producing process like water washing, which could also contribute to the oxygen element detected on the surface. In addition, large Zn spheres can also be observed in the product, which can be easily distinguished from the MXenes (Fig. S7). The as-reacted product was treated by a 5 wt.% HCl solution at 25 o C for 2h and washed by de-ionized water to remove Zn according to reaction 1.
After HCl treating, the XRD peaks corresponding to Zn disappeared, while the peaks corresponding to Ti3C2Cl2 remains unchanged. The SEM and EDS analysis also confirmed that the morphology and composition of Ti3C2Cl2 was not influenced by the HCl treatment (Fig. S8), indicating that Ti3C2Cl2 is stable in the HCl solution.
A STEM/EDS study was further employed to investigate the as-synthesized MXenes. As shown in Fig. 3c  A Ti 2p X-ray photoelectron spectrum of the as-reacted sample was shown in  Table S4).
The electronic band structures and phonon spectra of single-layer Ti3C2Cl2 calculated by DFT is shown in Fig. 2f. The layer is metallic in nature with a finite density of states at the Fermi level. The phonon spectra show that all phonon frequencies are positive (Fig S11), i.e., Ti3C2Cl2 MXene is dynamically stable.
Similar to the production of Ti3C2Cl2, Ti2CCl2 was also obtained by using Ti2AlC and ZnCl2 with mole ratio of 1:6 as the staring materials. Characterization is provided in supporting information (Fig. S12). The phonon spectra of Ti2CCl2 show that all phonon frequencies are positive (supporting information, Fig. S13a). The calculated band structure indicates that Ti2CCl2 has a metallic nature, similar to Ti3C2Cl2 (Fig.   S13b).
Formation Mechanism. Taking Ti3ZnC2 as example, the formation of the Zn-MAX phase can be stated as the following simplified reaction: Ti3AlC2 + 1.5ZnCl2 = Ti3ZnC2 + 0.5Zn + AlCl3 ↑ Ti3AlC2 + 1.5ZnCl2 = Ti3C2 + 1.5Zn + AlCl3 ↑ Ti3C2 + Zn = Ti3ZnC2 (4) Reaction (2) is the general reaction that can be divided into two sub-reactions as reaction (3)-(4). It is well known that ZnCl2 has a melting pointing of ~280 o C and is ionized into Zn 2+ and ZnCl4 2tetrahedron in its molten state. 23,24 The coordinately unsaturated Zn 2+ is a strong acceptor for Cland electrons, which act as the Lewis acidity in ZnCl2 molten salt. In this acidic environment, the weakly bonded Al atoms in Ti3AlC2 can be easily converted into Al 3+ by a redox reaction (reaction 3). The as- ZnCl2, also accelerate the in-plane diffusion of Al and Zn atoms. Actually, the calculated Ti-Zn-C phase diagram indicates that Ti3ZnC2 is not stable at 1300 o C due to the existence of competitive phases (Fig. S15a). This is the main reason why the Zn-MAX phases are unavailable through traditional solid reaction methods.
Otherwise, the main composition at high temperature should be Ti-Zn alloy and TiC phase. However, the calculated phase diagram at 550 o C suggests that Ti3ZnC2 is thermodynamically competitive to the Ti-Zn alloys and should be equilibrium at this temperature (Fig. S15b). In contrast, the well-known stable Al-MAX phase exists in the Ti-Al-C phase diagrams both at 1300 o C and 550 o C ( Fig. S15c and d). In order to confirm this hypothesis, the starting materials of Ti3AlC2 and ZnCl2 with molar ratio of 1:6 were heat-treated at 550 o C for different annealing times to show the phase evolution of the reaction product. As shown in Fig. 4a, the reaction product shows a variation tendency from Ti3AlC2 to Ti3ZnC2 and next to Ti3C2Cl2 with increasing reaction time. The product after 0.5 h annealing remained Ti3AlC2, while after 1.0 h, it has converted into Ti3ZnC2. Note that for the 1.0 h sample, the (000l) peaks of Ti3ZnC2 are less obvious than that in Fig.1a, indicating that the crystal ordering along (000l) plane was reduced. This is because the fact that part of Zn atoms were already extracted from the A-layer. When the reaction time was further increased to 1.5 h, the non-basal-plane peaks of Ti3ZnC2 nearly diminished, and the (000l) of Ti3C2Cl2 emerged. Note that intense peaks of Zn were also observed in this sample, indicating that the formation of Ti3C2Cl2 is accompanied by the generation of free Zn.
The 3.0 h sample is the same as the result in Fig.3, a mixture of highly crystallized Ti3C2Cl2 and Zn were obtained. The phase evolution was further confirmed by the EDS mapping analysis of the 1.5 h sample. As shown in Fig. 4b, this sample shows a core-shell structure, in which the core region is dense and rich of Zn, and the edge region is delaminated along the in-plane direction and rich of Cl. This core-shell microstructure well shows an intermediate state of the conversion from Ti3ZnC2 to Ti3C2Cl2. The above results confirmed that Ti3C2Cl2 was the exfoliation product of Ti3ZnC2, not directly derived from Ti3AlC2. This process can be reasonably described by the following simplified equations: Ti3ZnC2 + ZnCl2 = Ti3C2Cl2+ 2Zn (5) Ti3ZnC2 + Zn 2+ = Ti3C2 + Zn2 2+ (6) Ti3C2 + 2Cl -= Ti3C2Cl2 + 2e -(7) Zn2 2+ + 2e -=2Zn (8) Reaction (5) is the general reaction that can be divided into three sub-reactions as reactions (6)- (8). Reaction (6) can be explained by the dissolution of Zn in molten ZnCl2. Previous research has demonstrated that Zn dissolved well in molten ZnCl2, which is attributed to a redox reaction between Zn and Zn 2+ . 45,46 As mentioned above, the Zn 2+ cation is a strong electron acceptor that can react with Zn to form lower valent Zinc cations (Zn + or Zn2 2+ ). Therefore, the weakly bonded Zn atoms in Ti3ZnC2 were easily extracted from the two-dimensional A-site plane and dissolved in the ZnCl2 molten salt. Meanwhile, according to reaction (7), the Clanions spontaneously intercalated into the A-site plane and bonded with the specific site in between Ti3C2 sublayers to form a more stable phase, i.e. Ti3C2Cl2. The strong electronegativity of Cl results in the increasing valence of the Ti3C2 unit, which means that extra electrons were released. These were captured by Zn + or Zn2 2+ ions and formed pure metal Zn (reaction (8)). Note that the valence state of Ti is also confirmed by the above XPS analysis (Fig. 3d). In other words, the formation mechanism of Ti3C2Cl2 is comparable to the formation chemistry of HF etching on Ti3AlC2, 3 in which Zn 2+ and Clplays a similar role as H + and as F -, respectively.   and Ti2ZnC. Significantly, this is also the first time that exclusively Cl-terminated MXenes were obtained through a non-Fluorine chemistry. The Cl-terminated MXenes are expected to be more stable than F-terminated MXenes which implicates promising applications such as energy storage. 7,8 Actually, a few recent reports have indicated enhanced electrochemical behavior in Cl functionalized MXenes. 47,48 The moderate molten-salt environment also warranty an viable and green chemistry for fabrication of MXenes that paths the way for their scale-up and even commercial application.