Titanium Carbide MXene Nanostructures as Catalysts and Cocatalysts for Photocatalytic Fuel Production: A Review

Efficient nanomaterials are in high demand in photocatalytic applications to maximize solar energy conversion to renewable fuels. There is growing research on the use of metals as cocatalysts to promote photocatalyst efficiency, but they are expensive. Recently, titanium carbide (Ti₃C₂T_x) MXenes as layered materials have attracted attention to investigate energy conversion applications. The distinguishing characteristics of Ti₃C₂T_x are higher specific surface area, tunable terminal functional groups (−OH, −O, and −F), exposed metallic active sites, and excellent electrical conductivity. MXenes can be combined with other semiconductors as cocatalysts to improve charge carrier separation. This review discusses various synthesis routes to fabricate Ti₃C₂T_x MXenes as single materials, their surface functionalization, and as cocatalysts to construct a heterojunction for photocatalytic CO₂ conversion and H₂ production. The different synthesis approaches, such as the HF, halogen, alkali, molten salt, and electrochemical etching routes, to regulate structure, morphology, and efficiency are systematically described. Moreover, synthesis of various morphologies of Ti₃C₂T_x MXenes in terms of dimensions, sizes, and their effect on the performance of energy conversion reactions are systematically discussed. Furthermore, various synthesis routes with regard to fabrication of Ti₃C₂T_x MXene-based nanocomposites for stimulating photocatalytic efficiency with solar energy is elaborated on. The critical analysis and discussion is included on select suitable structures and morphologies of MXenes as cocatalysts and as a support to stimulate the energy harvesting efficiency. Finally, a discussion related to challenges and further developments for exploring pathways in the contexts of synthesis and production of promising renewable fuels is presented.