  The structural, elastic and electronic properties of two-dimensional (2D)
titanium carbide/nitride based pristine (Tin+1Cn/Tin+1Nn) and functionalized
MXenes (Tin+1CnT2/Tin+1NnT2, 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 Tin+1Nn are larger
than those of Tin+1Cn, 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 Tin+1Nn 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 Tin+1Nn(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.
