Flexible two-dimensional Ti_n+1C_n (n = 1, 2 and 3) and their functionalized MXenes predicted by density functional theories

Two-dimensional (2D) transition metal carbides/nitrides M_n+1X_n labeled as MXenes are attracting increasing interest due to promising applications as Li-ion battery anodes and hybrid electro-chemical capacitors. To realize MXenes devices in future flexible practical applications, it is necessary to have a full understanding of the mechanical properties of MXenes under deformation. In this study, we extensively investigated the stress-strain curves and the deformation mechanisms in response to tensile stress by first principles calculations using 2D Ti_n+1C_n (n = 1, 2 and/or 3) as examples. Our results show that 2D Ti₂C can sustain large strains of 9.5%, 18% and 17% under tensions of biaxial and uniaxial along x and y, respectively, which respectively increase to 20%, 28% and 26.5% for 2D Ti₂CO₂ due to surface functionalizing oxygen, which is much better than graphene (15% biaxial). The failure of 2D Ti_n+1C_n MXene is due to the significant collapse of the surface atomic layer; however, surface functionalization could slow down this collapse, resulting in the improvement of mechanical flexibility. We have also discussed the critical strains and Young's modulus of 2D Ti_n+1C_n and Ti_n+1C_nO₂. Our results provide an insight into the microscopic deformation mechanism of MXenes and hence benefit their applications in flexible electronic devices.