Synthesis-Controlled Phase Evolution and Implications for the Stability of Ti2CTx MXenes

MXenes have emerged as a versatile class of two-dimensional materials with exceptional electrical, chemical, and surface properties. However, thinner members, such as Ti2CTx MXene, remain far less explored because of their limited structural stability and high susceptibility to oxidation. In this study, we investigate how excess aluminum during the MAX phase synthesis induces a molten state that governs the formation of the Ti2AlC MAX phase and significantly affects the stability of the resulting Ti2CTx MXene. Excess aluminum promotes more stoichiometric Ti2AlC formation by eliminating residual oxygen and improving powder homogeneity. Quantitative X-ray diffraction phase analysis shows that aluminum content beyond 1.6× progressively increases the fraction of secondary phases, including Ti3AlC2. Adding excess Al produces Al2O3 by eliminating residual oxygen, which may play a crucial role by forming a passive protective layer on the MAX phase to slow etching-reaction kinetics and improve the performance of the resulting Ti2CTx MXene. We identified 1.6 times the stoichiometric aluminum content as the optimal stoichiometric composition for producing the Ti2AlC MAX phase for subsequent MXene synthesis. The optimized 1.6× Al-derived Ti2CTx MXene has a conductivity of approximately 7431 S/cm and exhibits a 75.6% reduction in degradation under ambient conditions compared to the standard 1.2× Al-derived Ti2CTx. Despite these improvements, Ti2CTx MXene remains intrinsically more prone to oxidation than Ti3C2Tx due to the absence of the additional electron-donating Ti layer. These findings highlight the importance of alternative synthesis strategies, such as molten-salt etching, to enable the practical use of thinner Ti2CTx MXenes in applications that require oxidation resistance.