Ultrasound cavitation and exfoliation dynamics of 2D materials re-vealed in operando by X-ray free electron laser megahertz imaging

TL;DR

This study uses ultrafast X-ray laser imaging combined with machine learning to observe real-time cavitation and exfoliation of graphite layers during ultrasonic processing, revealing key mechanisms and rates.

Contribution

It provides the first direct, high-resolution observation of ultrasound cavitation and exfoliation dynamics in 2D materials using MHz X-ray microscopy and machine learning.

Findings

Cyclic shock waves are the main exfoliation mechanism.

Graphite exfoliation rate up to ~5 Å per shock.

HOPG exfoliation rate up to ~0.15 Å per impact.

Abstract

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.