  Topochemically transforming layered non-van der Waals solid into two
dimensional (2D) materials involves selective etching reactions with atomic
precision. The element-specific, structure-sensitive etching at nanoscale
urgently requires in-depth understanding. Here, by means of density functional
theory calculations and a laboratory-made operando reaction monitoring
platform, the mechanism of instantaneous transforming MAX phase into MXenes is
unraveled. The overall etching kinetics exhibits a sigmoidal curve, following
self-accelerating reaction character with a small activation energy of ca. 60
kJ/mol. Interestingly, this activation energy corresponds to the Al transport
through Ti3C2 slits. Therefore the reaction of Al with hydrofluoric acid
solution in the confined interlayer space is recognized as the rate-determining
step. Last but not the least, we found that the match of A element and etchants
to form stable products is critical for the etching reaction, and reaction
energy derived from the thermodynamics provides an easy yet effective
descriptor for screening efficient etchants.
