MXene*

MXenes are chemically exfoliated from the bulk MAX phase using HF, however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. To emphasize the presence of the functional group, MXene is often presented as M_n+1C_nT_x, where T=F, OH, O. Using density-functional theory we show that exfoliation of Ti₃C₂ MXene proceeds via HF insertion through edges of Ti₃AlC₂ MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al−MXene bonds. The consequent opening of interlayer gap allows further insertion of HF that leads to the formation of AlF₃ and H₂, which eventually come out of the MAX. Analysis of the calculated Gibbs free energy (△G) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. The△G for the mixed functionalized MXenes are close, indicating the non-uniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation of MXene.