Substitutional Engineering in 2D MXenes: Advanced Characteristics for Performance in Photocatalytic Hydrogen Production and CO ₂ Reduction

In the past few years, MXene has attracted considerable interest in photocatalysis because of its unique physicochemical properties including strong electrical conductivity, tunable surface chemistry, large surface area, structural flexibility, hydrophilicity, etc. These properties make MXenes highly suitable for energy conversion and ecological applications. This review spotlights the development of MXene-based systems and highlights their various families and their top-down and bottom-up synthesis approaches. The preparation strategies for MXenes are outlined, with etching techniques like electrochemical etching, alkali-assisted etching, fluorine-based solution etching, molten salt etching, etc. The review highlights how the substitutional MXenes possess superior photocatalytic performance toward H 2 production and CO 2 reduction as compared to bare MXenes. It was revealed that increased active sites and adsorption capacity and efficient heterojunction formation in MXene-based composites are the crucial factors for such dazzling results toward H 2 production and CO 2 reduction. Despite these advances, challenges such as limited lasting durability, oxidation instability, complicated synthesis methods, and scalability are the major obstacles for their utilization in practical applications. The forthcoming work should focus on the design of commercial, chemically and thermodynamically stable MXene-based systems through controlled functionalization, advanced composite engineering, and mechanistic understanding to realize their potential in next-generation photocatalytic and renewable energy technologies.