Superior structural, elastic and electronic properties of 2D titanium nitride MXenes over carbide MXenes: a comprehensive first principles study

Abstract The structural, elastic and electronic properties of two-dimensional (2D) titanium carbide/nitride based pristine (Ti _n+1 C _n /Ti _n+1 N _n ) and functionalized MXenes (Ti _n+1 C _n T ₂ /Ti _n+1 N _n T ₂ , T stands for the terminal groups: –F, –O and –OH, n = 1, 2, 3) are investigated by density functional theory calculations. Carbide-based MXenes possess larger lattice constants and monolayer thicknesses than nitride-based MXenes. The in-plane Young’s moduli of Ti _n+1 N _n are larger than those of Ti _n+1 C _n , whereas in both systems they decrease with the increase of the monolayer thickness. Cohesive energy calculations indicate that MXenes with a larger monolayer thickness have a better structural stability. Adsorption energy calculations imply that Ti _n+1 N _n have stronger preference to adhere to the terminal groups, which suggests more active surfaces for nitride-based MXenes. More importantly, nearly free electron states are observed to exist outside the surfaces of –OH functionalized carbide/nitride based MXenes, especially in Ti _n+1 N _n (OH) ₂ , which provide almost perfect transmission channels without nuclear scattering for electron transport. The overall electrical conductivity of nitride-based MXenes is determined to be higher than that of carbide-based MXenes. The exceptional properties of titanium nitride-based MXenes, including strong surface adsorption, high elastic constant and Young’s modulus, and good metallic conductivity, make them promising materials for catalysis and energy storage applications.