Environment friendly dual-frequency ultrasonic exfoliation of few-layer graphene

Ultrasound-aided liquid phase exfoliation (ULPE) of graphene in pure water is environment-friendly. Two limiting factors of ULPE are the non-uniform thickness of few-layer graphene (FLG) and a relatively low graphene yield. Here we describe ULPE in water that enables us to produce FLG flakes with a thickness of 3 layers and the flake sizes exceeding 1 μm2 in just 2 h. This process is based on using a combination of two ultrasound sources of high and low frequencies: 1174 kHz and 20 kHz. Two different frequencies generate a wider population and size distribution of cavitation bubbles that act through a number of mechanisms towards the exfoliation of graphene. For the first time ULPE was characterized by acoustic measurements. Results show that a high graphene yield (10%) can be achieved. This study demonstrates that the use of a dual frequency ultrasonic source and control of acoustic pressure is critical in optimizing the quality and yield of the cavitation assisted LPE of graphene in pure water. It is suggested that the width of the acoustic pressure peak reflecting shock-wave emissions can be used as an indicator of ULPE completeness, opening for the first time a way of in-situ monitoring of the process.