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

Abstract

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.

Synthesis of MAX phases Nb2CuC and Ti2(Al0.1Cu0.9)N by A-site replacement reaction in molten salts

“…Figure 3 In order to further determine the structure of Nb2CuC, STEM was performed. Figure 4 The incorporation of transition elements (Zn, Cu) into A site of MAX phase has been discussed in our previous reports where chloride salts were used [10,14]. Here, we used an alternative molten salt CuI, which has a melting point of 600°C.…”

“…Compared to Ti2AlN, the (103), (104), and (106) peaks are shifted towards lower angles. The (002) peak is almost vanished and the (004) peak becomes strong, indicating a change in periodic symmetry along the c axis and corresponding change in structure factor [10]. The EDS measurement (Figure 1(b)) on the resultant particles provide a semi-quantitative estimation of the atomic ratios, with of Ti:(Cu+Al) and…”

“…In 2017, Ti3AuC2 and Ti3Au2C2 were synthesized by replacing Si with Au in Ti3SiC2, and Ti3IrC2 was identified by replacing Au with Ir in obtained Ti3Au2C2 [7]. Recently, our group reported a series of new Zn-containing MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) obtained by a replacement reaction between MAX phase precursors and ZnCl2 molten salt [10]. In these phases, Zn atoms occupy the original Al position at the A site in the MAX phase structure.…”

MXene-based 3D porous macrostructures for electrochemical energy storage

“…For example, a nuclear magnetic resonance (NMR) spectroscopic study showed that Ti 3 C 2 T x produced by in situ formation of HF (from HCl and LiF reaction) contains considerably fewer fluorine terminations than that synthesized with HF solution directly [28]. Recently, alternative synthesis routes were able to produce fluorine-free terminations [29][30][31][32][33][34][35] and even fully chlorinated MXenes [34].…”

“…The few nitride MXenes reported were obtained using a molten eutectic fluoride salt mixture of LiF, NaF and KF at high temperatures [37] or by ammoniation of previously obtained carbide MXenes [38]. Fluorine-free syntheses are still arguably still barely explored by the literature, with only a few methods reported such as electrochemical etching [29][30][31], hydrothermal alkali treatment [32,33,35] and reaction with Lewis acidic molten salt [34]. For a more complete overview of the different synthesis' routes toward MXenes and other ultrathin 2D transition metal carbides and nitrides to date, the reader can refer to Verger et al works [10,39].…”