title: On the fundamental mechanisms & optimisation of ultrasonic assisted liquid phase exfoliation of two dimensional nanomaterials
Creators: name: Morton, Justin
Publisher: name: Oxford Brookes University
Year: 2023

description: The application of ultrasound and acoustic cavitation in liquid exfoliation of bulk layered materials is a widely used method. Ultrasonic assisted liquid phase exfoliation (ULPE) is a promising procedure for the large scale production of two dimensional (2D) materials. As of this date, toxic solvents such as N-Methyl-2-pyrrolidone (NMP) are commonly used for the production of graphene and other 2D layers. This research was intended to find a suitable eco-friendly liquid media alternative to NMP, such as water or water-based solutions for ULPE. However, in order to propose such a solution, the fundamental mechanisms pertaining to ULPE must first be fully understood. There are a multitude of complexities of liquid exfoliation of bulk layered materials facilitated by ultrasonic cavitation. In addition to the solvent choice, other necessary factors to consider include the source material, solution temperature, input transducer power, sonotrode geometry and operating frequency. Before these parameters could be considered, the first task was to interpret and understand the interactions between cavitation and bulk layered materials and hence to explain the mechanisms of ULPE. Unfortunately, most of the research currently reported has been ambiguous or inconclusive due to lack of direct real-time experimental evidence. Therefore, systematic work characterising cavitation emissions and observing the exfoliation of graphite in situ, in deionised water under the dynamic interaction with laser and ultrasound induced cavitation bubbles was conducted. Using ultra-high-speed optical imaging, we were able to determine the dynamic sequence of graphite exfoliation events on a time scale never reported before. Real-time observations revealed that shock wave surges predominantly with a pressure magnitude up to 5 MPa and liquid-jets secondary in the range of 80 ms-1, from transient cavitation bubble implosions, were essential for the initiation and propagation of the exfoliation process. Additionally, bubble oscillations associated with stable cavitation were beneficial for promoting a gentler delamination of graphite layers. The evidence also demonstrated that proliferation of exfoliation occurred under the sonotrode within the cavitation zone. With these fundamentals elucidated, the next step was to analyse solution temperature and input power on the developed pressure field, to optimise ULPE conditions. A high- ii temperature cavitometer (calibrated in the National Physical Laboratory (NPL) UK) in the range of 8-400 kHz was used to measure the acoustic pressure generated from a 20 kHz acoustic source in different graphite solutions in deionised water at various temperatures (10-70 °C) and input power conditions (20-100%). In addition, high-speed optical imaging revealed insight on shock wave generation from transient bubble collapses in different sonication conditions. The optimal sono-exfoliation parameters were determined using 20% input power in 10 °C for a graphite flake solution, (of size 149 μm) and 100% input power in 40-50 °C for a graphite powder solution (of size 56 μm). With the optimal temperature and power conditions identified, experiments were then conducted on solvent selection. Four solvents; three green solvents (water, ethanol and water/ethanol) plus NMP for comparison, were sonicated and examined in terms of their cavitation zone development, bubble dynamics, acoustic emissions and pressure distribution. Advanced fundamental analysis was conducted using high-speed imaging synchronised with acoustic pressure measurements complemented by shadowgraphic photography of the emitted shockwaves, in order to determine a suitable eco-friendly solvent medium from a cavitation bubbles dynamics perspective. Thereafter, ULPE of graphite in the optimum solvent took place for up to 2 h under controlled ultrasonication parameters of input power and solution temperature. The produced graphene samples were characterised by employing a series of techniques consisting of UV-Vis spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM). A mixture of deionised water and ethanol was shown to produce high quality graphene with a stability of ~ 78 % for the duration of six months. Further optimization of our ULPE procedure continued with analysis on the driving ultrasound frequency. We investigated the effect of implementing a dual-frequency system, where bubble dynamics, acoustic pressure, spectra and induced shock waves could be elucidated and compared with a single, low frequency sonotrode. Using ultra-high-speed imagining and synchronised acoustic pressure measurements, a dual-frequency system and its effect on bubble dynamics was investigated. A high frequency transducer (1174 kHz) showed that bubble fragments and satellite bubbles induced from a low frequency transducer (24 kHz) were able to extend their lifecycle sustaining an active cavitation regime in the bulk solution. In addition, this combination of ultrasonic frequencies generated higher acoustic pressures than the sum of the individual transducers, and enhanced the associated shock wave spectrum peaks, indicating an increase bubble activity with further collapses and the iii generation of additional shock waves surges. The dual-frequency system also produced an enlargement in the cavitation zone size compared to the low frequency sonotrode. These data were put to the test where ULPE of graphite was conducted using a low frequency source of either a horn (22 mm diameter) or larger bell (40 mm diameter) shaped sonotrode. The produced graphene samples were characterised by employing a series of techniques consisting of UV-Vis spectroscopy, Raman spectroscopy and TEM. DIW:EtOH produced an average area of 1.5 μm with 10±5 layers and 5.8% yield with the horn sonotrode. When using the bell sonotrode the yield was approximately the same (5.75%) in half the sonication duration, although, at the expense of smaller sized area flakes (0.3 μm). This research has led to a deeper understanding of the ULPE physics and dynamics that improved optimization and control of the exfoliation process for the generation of high quality 2D materials. The combination of parameters presented in this thesis additionally provides a promising eco-friendly substitute for future commercial manufacturing of graphene.