Lewis-Acidic Metal Cation Etching for General Aqueous Preparation of 2D MBenes (MB) and MB/Metal Heterostructures

Emerging 2D transition metal borides (MBenes) have attracted attention following the development of MXenes. However, the absence of effective preparation methods seriously hinders their fundamental research and application exploration. Herein, we report a mild and general aqueous Lewis acidic metal cation etching strategy that efficiently converts MAB precursors into 2D MBene and MBene-based heterostructures under neutral aqueous conditions, guided by the redox potential difference between the A-element and the metal-cation etchant (such as Ag⁺, Zn²⁺, and Cu²⁺). During etching, A-atoms are oxidatively removed while X-cations are reduced to metallic X⁰, which adsorbs on the MB layer surfaces and yields a layered MB/X heterostructure. Notably, the metallic X can be retained as a functional active component or selectively removed to obtain pure 2D MBenes. The universality of this strategy is demonstrated across structurally and compositionally diverse MAB phases, including MoAlB, Cr₂AlB₂, and (Mo_2/3Y_1/3)₂AlB₂, achieving successful preparation of various MB/X composites and MBenes in as fast as 2 h. As a representative example, the prepared MoB/Ag nanosheets demonstrate ultrahigh electronic conductivity and improved electrochemical Li storage capability. This work provides a facile and universal route for the rational design and preparation of 2D MBene-based functional materials.