Phys. Chem. Chem. Phys. 18, 11073-11080 (2016) [pdf]

Isolation of Pristine MXene from Nb₄AlC₃ MAX Phase: A First-Principles Study

Avanish mishra, Pooja Srivastava, KHiroshi Mizuseki, Kwang-Ryeol Lee, Abhishek K. Singh

Synthesis of pristine MXene sheet from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 20-material. Efforts to exfoliate Nb₄AIC₃ MAX phase always lead to Nb₄C₃ MXene sheets, which are functionalized and have several AI atoms attached. Using first-principles calculations, we perform an intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali-atoms and LiF in Nb₄AIC₃ MAX phase. Calculated bond-dissociation energy (BDE) shows that presence of HF in MAX phase always results in functionalized MXene, as the binding of H with MXene is quite strong while that with F is weak. Alkali insertion do not facilitate pristine MXene isolation due to the presence of chemical bonds of almost equal strengths. In contrast, weak Li-MXene and strong Li-F bonding in Nb₄AIC₃ with LiF ensured strong anisotropy in BDE, which will result into the dissociation of Li-MXene bond. Ab-initio molecular dynamics calculations capture these features and show that at 500-650 K, Li-MXene bond indeed breaks leaving a pristine MXene sheet behind. The approach and insights developed here for chemical exfoliation of layered materials bonded by chemical bonds instead of van der Waals can promote their experimental realization.