Résumé
MXenes, a family of two-dimensional (2D) transition metal carbides and carbonitrides prepared from MAX phase precursors by selective etching of the A-site element, are promising materials for energy storage applications. [1,2] The general formula of MXenes is M n+1 X n T x (n = 1, 2, 3, or 4), where M is a transition metal(s), X is carbon and/or nitrogen, and T x represents the surface terminations (-OH,-O,-F,-Cl, etc.). [3] MXene synthesis is based on the selective etching of the A-layer of the corresponding MAX phase in F-containing solutions, such as HF or a LiF + HCl mixture. [4] However, these methods pose significant risks to human safety and the environment owing to the formation of toxic and corrosive HF; thus, HF-free methods for MXene preparation are urgently needed. HF-free methods, such as electrochemical etching, [5] molten salt etching, [6] and hydrothermal routes, [7] have been reported, but they require hours to days to prepare MXenes. In addition, most of the synthesis methods are only effective for etching Al-containing MAX phases, making them unsuitable for MAX phase precursors with other A-site elements (such as Si, Ga, and Zn). In 2020, a Lewis molten salt etching method was proposed for etching MAX phases with various A-site elements, which expands the potential range of MAX phase precursors. [8] However, because of their oxidation sensitivity, MXenes are chemically unstable, [9] particularly at high temperatures. Consequently, inert gas protection is required during synthesis that may cause complexity in synthesis procedures and experimental facilities, resulting in increased production costs. In 2019, a molten salt-shielded synthesis (MS 3) method was proposed to prepare various oxidation-sensitive materials, such as porous Ti, Ti 3 SiC 2 , and Ti 2 AlN MAX phases, under an air atmosphere. [10] In this process, low-melting-point KBr is used as a reaction medium to protect products from oxidation during high-temperature synthesis by forming a protective molten salt layer around the sample at low temperatures. Inspiring by the MS 3 method of MAX phases synthesis and Lewis salt etching synthesis of MXenes, we recently proposed the preparation of MXenes by a one-pot molten salt synthesis method using M, A, and X (carbon) elements as precursors. [11] Ti 3 C 2 T x and Ti 2 CT x MXenes were obtained by two consecutive steps that occurred in the reactor: in-situ MAX phase synthesis at 1300 °C followed by in situ Lewis salt etching at 700 °C. This one-pot method enables the synthesis of carbide MXenes directly from elemental precursors for the first time, and the entire process was achieved in <8 h without inert gas protection. The one-pot synthesis strategy relies on the in-situ synthesis of MAX phases in molten salts, however, some MAX phases, such as Nbbased, most 413 and 514 MAX phases, have not been achieved by the molten salt method due to the severe volatilization of molten salts at the high synthesis temperatures near or beyond their boiling points, [12-14]