Layered Transition Metal Carbides/Nitrides: From Chemical Etching to Chemical Editing

Topotactic transformations between related crystal structures, involving etching, replacement, and intercalation, are increasingly recognized in the design and tuning of material properties. These transformations reveal the fundamental principles of material structural changes, paving the way for creating novel materials with unique properties. Layered materials readily undergo structural or compositional changes due to their stacked atomic layers and bonding features. MAX phases, as nonvan der Waals (non-vdW) layered compounds, exhibit distinctive elemental compositions and bonding characters that make them suitable for topotactic transformations. A notable example is the typical transformation from MAX phases to MXenes, a new addition to two-dimensional (2D) materials, through A-site etching within MAX phases. In turn, the 2D structure of MXenes further promoted versatile topotactic transformations utilizing the interlayer space and tunable surfaces. This Account comprehensively reviews the topotactic transformation in MXenes and MAX phases, covering aspects from chemical etching to versatile chemical editing. We commence with an analysis of MAX phase degradation, examining the corrosion resistance of MAX phases in liquid metals and molten salts, which is crucial for their application as nuclear materials. This leads us to introduce the novel concept of precise A-site etching in MAX phases, which has paved the way for the groundbreaking discovery of 2D MXene. Given the important effect of etching methods on MXenes, we then delve into the various etching methods employed in preparing MXene and explore the detailed processes and mechanisms behind each method. Additionally, we highlight the recent advancements made by our research group regarding the Lewis acidic molten salt (LAMS) method. This method utilizes LAMSs as etching agents to selectively etch the A-site atomic layer, creating opportunities for the subsequent intercalation of atoms or anions to achieve isomorphous replacement of A-site atoms and surface modification with novel terminations. The strong oxidation ability of LAMSs also offers versatility in selectively etching A-site atomic species, particularly confined to the Al element. The LAMS method shows potential for synthesizing and controlling the structure of MXene and MAX phases, albeit with limitations. Its success depends on the properties of LAMSs, which must facilitate both etching and intercalation. However, some LAMSs are unsuitable due to their low redox potential, low boiling points, and instability at high temperatures. Therefore, we propose a versatile chemical scissor-mediated structural editing strategy. This strategy decouples etching from intercalation, using Lewis acidic cations or reduced metal atoms as chemical scissors to create space between MX sublayers, allowing atoms or anions to diffuse and enable topotactic transitions. This approach has facilitated the intercalation of various A-site atoms, expanded MXene surface termination options, and even enabled the conversion of 2D MXene into 3D MAX phases by combining termination removal with atom intercalation. Finally, we offer insights into the future of topotactic transformations in these materials, aiming to inspire further innovative progress in this field. A deeper understanding of the topotactic transformation process holds the promise of broadening the applications of layered materials, providing a solid foundation for advancements in related areas.