Highly negative Poisson's ratio in a flexible two-dimensional tungsten carbide monolayer

Abstract

Auxetic materials have numerous promising engineering applications such as fracture resistance and energy storage due to their negative Poisson's ratios (NPRs). However, compared to materials possessing positive Poisson's ratios (PPRs), auxetic materials are rare. In this paper, by employing first principles calculations, we found a high NPR two-dimensional (2D) material, tungsten carbide (W2C), in the transition metal carbides (MXenes). Our results of the relatively moderate Young's modulus and fracture strength as well as the critical strain showed that the 2D monolayer W2C is an extraordinary flexible material. Our DFT results also demonstrated that W2C possesses high NPRs while Hf2C and Ta2C have PPRs. Furthermore, the mechanically induced deformation mechanism and the NPR formation mechanism of W2C have been proposed. Such an intrinsic NPR in W2C is attributed to the strong coupling between the C-p and W-d orbitals in the pyramid structural unit. The mechanically induced deformation mechanism and the PPR formation mechanism of Hf2C have also been determined. The intrinsic NPR for W2C transforms to PPR upon the surface functionalization induced. The behavior occurs due to the W-C interaction weakening. The excellent NPR in the 2D MXene material combined with other outstanding properties such as the metallic state would bring about its promising engineering prospects, ranging from the metal-ion battery, to automobiles and aircraft.

The Recent Advances in the Mechanical Properties of Self-Standing Two-Dimensional MXene-Based Nanostructures: Deep Insights into the Supercapacitor

“…Similar findings on the effect of monolayer thickness on the elastic properties of the carbide (Ti n+1 C n ) and nitride-based (Ti n+1 N n ) MXenes, by DFT calculations, were reported in another study [ 53 ]. It was shown that increasing the monolayer thickness decreases Young’s moduli of MXenes.…”

“…Another study [ 53 ] predicted enhanced elastic properties of a 2D Tungsten Carbide (W 2 C) monolayer by DFT calculations. The calculated c 11 of 781.9 GPa indicated that W 2 C is mechanically stable as it satisfies the 2D materials criteria for mechanical stability [ 36 ].…”

“…The Young’s modulus of W 2 C along with the armchair and zigzag directions are 648 and 645 GPa, respectively, compared to graphene (1000 GPa) [ 43 ] and Ti 2 C (600 GPa) [ 37 ]. Furthermore, W 2 C was observed to have a high negative Poisson’s ratio (NPR) as they exhibited a positive strain along the longitudinal direction while applying stretching force on the transverse direction ( Figure 6 b) [ 53 ]. This intrinsic NPR of W 2 C could be explained by the robust coupling between C-p and W-d orbitals in the pyramid structural unit.…”

“…This intrinsic NPR of W 2 C could be explained by the robust coupling between C-p and W-d orbitals in the pyramid structural unit. Additionally, incorporating the surface functional groups to the calculations show that the NPR of W 2 C was turned into PPR due to the weakening of M–C interactions [ 53 ].…”

“… ( a ) Stress-strain curves of intrinsic W 2 C under uniaxial stretching, ( b ) Poisson’s ratios of W 2 C stretched along with different directions. Reproduced with permission from [ 53 ]. Copyright RSC, 2018.…”