Engineering hot carrier dynamics in Ti ₂ CO ₂ MXene via methane adsorption

Hot carrier (HC) dynamics play a crucial role in determining the efficiency of light-driven processes such as photocatalysis, photodetection, and gas sensing. We present a comprehensive first-principles study on the impact of methane (CH₄) adsorption on the optoelectronic properties of oxygen-functionalized Ti₂CO₂ MXene. Using density functional theory (DFT) and real-time time-dependent DFT (rt-TDDFT), we explore how different CH₄ adsorption geometries modulate the optical properties. Our results reveal that the bridge configuration is energetically favored by 1.50 eV compared to the on-top site, leading to notable orbital hybridization between CH₄ and surface Ti/O atoms. This interaction results in a red-shifted absorption onset and significant enhancement in visible-light absorption. Transition contribution maps and hot carrier analysis further confirm that CH₄ molecules at bridge sites actively participate in photoexcitation processes, particularly by increasing hole generation in the valence band. In contrast, the on-top configuration exhibits weak physisorption and negligible optical response. These findings demonstrate the critical role of adsorption geometry in tuning MXene optoelectronics and offer a mechanistic foundation for designing CH₄-responsive materials for applications in photocatalysis, gas sensing, and solar energy harvesting.