X‑Site Dependency of Optical and Electronic Properties in Ti ₃ (C_{2- y} N _y )T _x Carbonitride MXenes

MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides with the general formula of M _n+1X _n T _x (where M represents an early transition metal, X is C and/or N, and T _x is the surface functional groups), offer exceptional tailorability in structure, composition, and surface chemistry. Among them, nitrogen-containing MXenes have enhanced electronic and optical properties compared to their carbon analogs. Yet, challenges in synthesizing them have made nitride and carbonitride MXenes the least explored class. Herein, we report the synthesis of Ti₃Al-(C_2-y N _y ) MAX phases using a high-aluminum method to minimize oxygen impurities as well as other competing binary and ternary phases. Therein, subsequent etching and delamination of Ti₃Al-(C_2-y N _y ) MAX phases into Ti₃(C_2-y N _y )-T _x MXenes were done using a coupled HF/HCl/LiCl method. Systematic variation of X-site chemistry (Ti₃C₂T _x , Ti₃C_1.75N_0.25T _x , Ti₃C_1.5N_0.5T _x , Ti₃C_1.25N_0.75T _x, and Ti₃CNT _x ) enabled direct correlations between chemistry and optoelectronic properties. Increased nitrogen content leads to increased preference for halogen terminations, stronger light-matter interaction, blue-shifted optical absorbance, and decreased electrical conductivity. Despite these variations, the work function remains nearly constant across all compositions, indicating that it is primarily dictated by M and T _x chemistries. These findings demonstrate that solid-solution carbonitride MXenes provide a platform to independently control optical and electronic behaviors, offering opportunities for MXene-based optoelectronic and energy applications.