Real-time monitoring and hydrodynamic scaling of shear exfoliated graphene

Abstract

Shear-assisted liquid exfoliation is a primary candidate for producing defect-free two-dimensional (2D) materials. A range of approaches that delaminate nanosheets from layered precursors in solution have emerged in recent years. Diverse hydrodynamic conditions exist across these methods, and combined with low-throughput, high-cost characterization techniques, strongly contribute to the wide variability in performance and material quality. Nanosheet concentration and production rate are usually correlated against operating parameters unique to each production method, making it difficult to compare, optimize and predict scale-up performance. Here, we reveal the shear exfoliation mechanism from precursor to 2D material and extract the derived hydrodynamic parameters and scaling relationship that are key to nanomaterial output and common to all shear exfoliation processes. Our investigations use conditions created from two different hydrodynamic instabilities-Taylor vortices and interfacial waves-and combine materials characterization, fluid dynamics experiments and numerical simulations. Using graphene as the prototypical 2D material, we find that scaling of concentration of few-layer nanosheets depends on local strain rate distribution, relationship to the critical exfoliation criterion, and precursor residence time. We report a transmission-reflectance method to measure concentration profiles in real-time, using low-cost optoelectronics and without the need to remove the layered precursor material from the dispersion. We show that our high-throughput, in situ approach has broad uses by controlling the number of atomic layers on-the-fly, rapidly optimizing green solvent design to maximize yield, and viewing live production rates. Combining the findings on the hydrodynamics of exfoliation with this monitoring technique, we unlock targeted process intensification, quality control, batch traceability and individually customizable 2D materials on-demand.

Large‐Scale Syntheses of 2D Materials: Flash Joule Heating and Other Methods

“…One recent example used real‐time in situ optical microscopy and high‐speed cameras with image processing scripts to track the graphite precursor and the exfoliated sheets to study the speed, agglomeration, and live production rates during shear exfoliation. [ 91 ] The production of few‐layer graphene mechanistically depends on local strain rate distribution and graphite residence time, providing critical exfoliation criteria for scaling. High‐resolution 3D flow simulations were also carried out to further study how strain rate and topology impact exfoliation.…”

Foam Flows in Turbulent Liquid Exfoliation of Layered Materials and Implications for Graphene Production and Inline Characterisation

“…Mechanical methods that rely predominantly on shear-driven exfoliation are beneficial as they typically produce defect-free 2D materials without oxidation. Sonication [5], high-shear mixing [6], microfluidisation [7], Taylor-Couette and thin film reactors [8,9] are a few of many examples that have been used successfully.…”

“…Experiments by Paton et al [6] and Biccai et al [12] showed there is a minimum shear rate required to overcome van der Waals attractive forces and exfoliate layered materials. Stafford et al [8] recently uncovered the general scaling relationship governing production for continuous flow, shear exfoliation systems. Combining high fidelity numerical simulations (DNS, LES), fluid dynamic experiments and nanomaterial characterisation, it was found that graphene concentration depends on derived parameters of shear rate and particle residence time.…”

“…Many high performance solvents are toxic and have high boiling points which has led to research on greener alternatives [4,22]. Water can function as a suitable solvent for few-layer graphene dispersions when modified through co-solvent strategies [8] or with the addition of surfactants [23,24]. In the latter approach, surface-active agents adsorb to nanosheets and prevent re-stacking through steric and electrostatic repulsion mechanisms.…”

“…This system is capable of exfoliating 2D materials [10,11] and can serve as a template for studying contemporary stirred tank reactors that are used at larger scales for production and material blending opera-tions. A reliance on slow and expensive ex situ materials characterisation tools also presents a bottleneck for realising large-scale high-throughput material quality control [25,8]. Here, turbulent mixing and shear exfoliation have been investigated using inline spectroscopy to probe the process characteristics in situ and exfoliation performance on-thefly.…”