Effect of Substitutional Oxygen on Properties of Ti₃C₂T_x MXene Produced Using Recycled TiO₂ Source

MXenes are an emerging class of 2D materials with unique properties including metallic conductivity, mechanical flexibility, and surface tunability, which ensure their utility for diverse applications. However, the synthesis of MXenes with high crystallinity and atomic stoichiometry in a low-cost process is still challenging because of the difficulty in controlling the oxygen substitute in the precursors and final products of MXenes, which limits their academic understanding and practical applications. Here, a novel cost-effective method is reported to synthesize a highly crystalline and stoichiometric Ti₃C₂T_x MXene with minimum substitutional oxygen impurities by controlling the amount of excess carbon and time of high-energy milling in carbothermal reduction of recycled TiO₂ source. The highest used content (2 wt%) of excess-carbon yields TiC with the highest carbon content and minimal oxygen substitutes, which leads to the Ti₃AlC₂ MAX phase with improved crystallinity and atomic stoichiometry, and finally Ti₃C₂T_x MXene with the highest electrical conductivity (11738 S cm⁻¹) and superior electromagnetic shielding effectiveness. Additionally, the effects of carbon content and substitutional oxygen on the physical properties of TiC and Ti₃AlC₂ are elucidated by density-functional-theory calculations. This inexpensive TiO₂-based method of synthesizing high-quality Ti₃C₂T_x MXene can facilitate large-scale production and thus accelerate global research on MXenes.