  Ultrasonic liquid phase exfoliation is a promising method for the production
of two-dimensional (2D) layered materials. A large number of studies have been
made in investigating the underlying ultrasound exfoliation mechanisms.
However, due to the experimental challenges for capturing the highly transient
and dynamic phenomena in real-time at sub-microsecond time and micrometer
length scales simultaneously, most theories reported to date still remain
elusive. Here, using the ultra-short X-ray Free Electron Laser pulses (~25ps)
with a unique pulse train structure, we applied MHz X-ray Microscopy and
machine-learning technique to reveal unambiguously the full cycles of the
ultrasound cavitation and graphite layer exfoliation dynamics with
sub-microsecond and micrometer resolution. Cyclic fatigue shock wave impacts
produced by ultrasound cloud implosion were identified as the dominant
mechanism to deflect and exfoliate graphite layers mechanically. For the
graphite flakes, exfoliation rate as high as ~5 angstroms per shock wave impact
was observed. For the HOPG graphite, the highest exfoliation rate was ~0.15
angstroms per impact. These new findings are scientifically and technologically
important for developing industrial upscaling strategies for ultrasonic
exfoliation of 2D materials.
