Synthesis and Surface Engineering of Two-Dimensional MXenes for Advanced Functional Applications

Two-dimensional MXenes (2DMxn), a rapidly expanding family of transition metal carbides, nitrides, and carbonitrides with the general formula (M_n + 1X_nT_x), have attracted significant attention for their metallic conductivity, hydrophilic surfaces, high surface area, and tunable surface terminations. These unique properties make MXenes promising candidates for applications in catalysis, sensing, energy storage, and environmental remediation. This review provides a comprehensive overview of recent advances in MXene research, particularly developments reported since 2020. The discussion begins with synthesis strategies, including conventional top-down etching of MAX phases using hydrofluoric acid and in situ fluoride-based methods, as well as safer fluoride-free approaches such as electrochemical and hydrothermal techniques and Lewis-acid molten-salt etching. Emerging bottom-up fabrication routes, including chemical vapor deposition (CVD) and template-assisted growth, are also highlighted for their potential to provide improved structural control. Furthermore, advanced postsynthesis modification methods such as solvothermal and microwave-assisted treatments, atomic layer deposition (ALD), electrochemical engineering, and ball-milling are discussed as effective strategies for tailoring interlayer spacing, surface chemistry, and hierarchical architectures. The review also summarizes the recent progress of MXene-based materials in photocatalysis, electrocatalysis, energy storage devices, and electrochemical sensors, with particular emphasis on structure-property-performance relationships that govern charge transfer, active-site accessibility, and stability. Finally, key challenges including oxidation susceptibility, nanosheet restacking, and the need for scalable and safe production methods are critically discussed, together with future prospects for the development of MXene-based multifunctional materials and devices for next-generation technologies.