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

The structural, elastic and electronic properties of two-dimensional (2D)\ntitanium carbide/nitride based pristine (Tin+1Cn/Tin+1Nn) and functionalized\nMXenes (Tin+1CnT2/Tin+1NnT2, T stands for the terminal groups: -F, -O and -OH,\nn = 1, 2, 3) are investigated by density functional theory calculations.\nCarbide-based MXenes possess larger lattice constants and monolayer thicknesses\nthan nitride-based MXenes. The in-plane Young's moduli of Tin+1Nn are larger\nthan those of Tin+1Cn, whereas in both systems they decrease with the increase\nof the monolayer thickness. Cohesive energy calculations indicate that MXenes\nwith a larger monolayer thickness have a better structural stability.\nAdsorption energy calculations imply that Tin+1Nn have stronger preference to\nadhere to the terminal groups, which suggests more active surfaces for\nnitride-based MXenes. More importantly, nearly free electron states are\nobserved to exist outside the surfaces of -OH functionalized carbide/nitride\nbased MXenes, especially in Tin+1Nn(OH)2, which provide almost perfect\ntransmission channels without nuclear scattering for electron transport. The\noverall electrical conductivity of nitride-based MXenes is determined to be\nhigher than that of carbide-based MXenes. The exceptional properties of\ntitanium nitride-based MXenes, including strong surface adsorption, high\nelastic constant and Young's modulus, and good metallic conductivity, make them\npromising materials for catalysis and energy storage applications.\n