Transition metal Carbides/Nitrides (MXenes): Properties, synthesis, functional modification and photocatalytic application

•MXenes is highly concerned metal carbides/nitrides photocatalyst with 2D structure.•Various etching methods have been used to obtain MXenes with different structures and morphologies.•Functional modification of MXenes can improve its photocatalytic adaptability.•Enhanced photocatalysis of MXenes-based materials is attributed to the promoted separation of photogenerated carriers. As a new metal carbides/nitrides material, MXenes behaves unique crystal and electronic structure, showing the advantages of metal-like conductivity, prominent optical properties, flexible adjustability of elemental composition, rich surface chemical activity, and regular layered structure. Hence, this review firstly provides a detailed investigation of the structural and optical characteristics of MXenes to determine its feasibility as photocatalysts. Then, the preparation methods of MXenes are summarized, among which the etching methods are the most commonly used, mainly including fluorine-containing solution etching, alkali solution etching, electrochemical etching, and molten salt etching etc. Different preparation methods can obtain MXenes with differentiated structural morphologies and photocatalytic activity. However, the poor oxidation stability of MXenes during photocatalytic reactions conducted in aqueous media remains a challenging issue. Excitedly, MXenes with abundant suspended bonds on its surfaces are also easily functionalized. Therefore, the next section is a systematical summary of the functional modification strategies of MXenes, and after modification with organic, inorganic, organic–inorganic hybrid, or ion intercalation, MXenes can exhibit various properties and superior stability in aqueous solutions, further enhancing its photocatalytic adaptability. Basedonthesepoints, the review also systematically discusses the application progress of MXenes-based materials in photocatalysis, including degradation of organic pollutants, hydrogen evolution via the decomposition of water, carbon dioxide reduction, nitrogen fixation, reduction and removal of heavy metals, as well as sterilization and disinfection. Overall, the introduction of MXenes into photocatalysts provides rich photogenerated electron transfer channels and promotes the efficient separation of photogenerated carriers, thereby improving the photocatalytic oxidation or reduction reactions. Lastly, the development outlook of MXenes-based materials is also prospected.