Tailoring mechanical properties of MXenes by composition ratio control of surface terminations: Reactive molecular dynamics simulation

Emerging 2D transition-metal carbide materials called MXenes inherently have surface terminations differently from other 2D materials. Consequently, surface termination engineering was proven to effectively and predictably modulate their electronic, optical, and mechanical properties, enabling significant diversification of MXenes’ characteristics. However, surface termination effects of MXenes have mostly been investigated for single-component termination, and theoretical studies with multi-component terminations were rarely studied despite their common occurrence. For the first time, by using reactive molecular dynamics simulations based on a first-principles-derived force field, we investigated the tensile mechanical responses of Ti₃C₂T_x MXenes under the systematic composition variation of surface terminations by considering two prevailing oxygen (O-) and hydroxyl (OH–) terminations. We found that Ti₃C₂T_x MXenes with a mixed state of O- and OH-terminations are always mechanically weaker than MXenes with single O- or OH-terminations, exhibiting the smallest tensile strength and/or strain at a molar percentage of 60%OH and 40%OH for armchair and zigzag direction elongation, respectively. In contrast to such crossover dynamics, the elastic modulus was linearly tunable in a straightforward manner for OH increase of surface OH/O-terminations, differing from exponential Boltzmann decay observed for the surface coverage reduction of single-component O-termination. Finally, we showed that oxygen/hydrogen plasma treatment in a vacuum environment could be a powerful means of making single component O-terminated MXenes and/or controlling the composition ratio of surface terminations.