Green synthesis of reduced Ti₃C₂T_{: X} MXene nanosheets with enhanced conductivity, oxidation stability, and SERS activity

Transition metal carbides (MXenes) are an emerging family of highly conductive two-dimensional materials with additional functional properties introduced by surface terminations. Further modification of the surface terminations makes MXenes even more appealing for practical applications. Herein, we report a facile and environmentally benign synthesis of reduced Ti₃C₂T_x MXene (r-Ti₃C₂T_x) via a simple treatment with l-ascorbic acid at room temperature. r-Ti₃C₂T_x shows a six-fold increase in electrical conductivity, from 471 ± 49 for regular Ti₃C₂T_x to 2819 ± 306 S m^-1 for the reduced version. Additionally, we show an enhanced oxidation stability of r-Ti₃C₂T_x as compared to regular Ti₃C₂T_x. An examination of the surface-enhanced Raman scattering (SERS) activity reveals that the SERS enhancement factor of r-Ti₃C₂T_x is an order of magnitude higher than that of regular Ti₃C₂T_x. The improved SERS activity of r-Ti₃C₂T_x is attributed to the charge transfer interaction between the MXene surface and probe molecules, re-enforced by an increased electronic density of states (DOS) at the Fermi level of r-Ti₃C₂T_x. The findings of this study suggest that reduced MXene could be a superior choice over regular MXene, especially for the applications that employ high electronic conductivity, such as electrode materials for batteries and supercapacitors, photodetectors, and SERS-based sensors.