Titanium Carbide MXene as NH₃ Sensor: Realistic First-Principles Study

Abstract

This work presents a more realistic study on the potential of titanium carbide MXene (Ti₃C₂Tₓ) for gas sensing, by employing first principle calculations. The effects of different ratios of different functional groups on the adsorption of NH₃, NO, NO₂, N₂O, CO, CO₂, CH₄, and H₂S gas molecules on Ti₃C₂Tₓ were analyzed. The results indicated that Ti₃C₂Tₓ is considerably more sensitive to NH₃, among the studied gas molecules, with a charge transfer of -0.098 e and an adsorption energy of -0.36 eV. By analyzing the electrostatic surface potential (ESP) and the projected density of states (PDOS), important physical and mechanical properties that determine the strength and nature of gas-substrate interactions were investigated, and also, the significant role of electrostatic effects on the charge transfer mechanism was revealed. Further, the Bader charge analysis for the closest oxygen and fluorine atoms to NH₃ molecule showed that oxygen atoms have 60% to 180% larger charge transfer than fluorine atoms, supporting that Ti₃C₂Tₓ substrate with a relatively lower ratio of fluorine surface terminations has a stronger interaction with NH₃ gas molecules. The calculations show that in the presence of water molecules, approximately 90% smaller charge transfer between NH₃ molecule and the Ti₃C₂Tₓ occurs. Ab initio molecular dynamics simulations (AIMD) were also carried out to evaluate the thermal stabilities of Mxenes. The comprehensive study presented in this work provides insights and paves the way for realizing sensitive NH₃ sensors based on Ti₃C₂Tₓ that can be tuned by the ratio of surface termination groups.