Scalable, Flow-Based Processing of 2D Exfoliated Nanosheets via Cross-Flow Filtration

Two-dimensional (2D) materials are highly attractive for device applications due to their size-dependent optical properties, unique charge transport characteristics, and abundant surface area for chemical functionalization. When formulated into printable inks, these 2D materials can be leveraged in additive manufacturing schemes to realize large-area, flexible devices for sensing, energy storage, and computing technologies. However, producing sufficiently large quantities of electronic-grade ultrathin nanosheets on the consumer scale remains a grand challenge in the 2D material field. While top-down, liquid-phase exfoliation allows high-volume processing of layered bulk crystals, this approach produces nanosheets that are co-mingled with meso-exfoliated bulk particles, thus necessitating post-exfoliation separation prior to ink formulation. This separation is traditionally achieved by centrifuge-based sedimentation, which is mostly confined to labor-intensive batch processing that limits overall manufacturing throughput. Even when continuous-flow rotors are implemented, rapid pellet formation from nanomaterial dispersions will eventually require operator intervention and make material recycling difficult.<br/>In this presentation, we will introduce a strategy for overcoming this separation bottleneck by adapting cross-flow filtration (CFF), a process commonly used in protein purification, to enable the first flow-based, centrifuge-free separation method for liquid-phase exfoliated 2D nanosheets. CFF features a process dispersion flowing tangentially to the surface of a porous membrane, which limits material buildup on the membrane surface, while the target material (the ‘permeate’) passes through the filter. Using a shear-exfoliated graphite mixture as a material testbed, we have successfully isolated graphene nanosheets from the poorly exfoliated graphite particles using this method. Stable separation was achieved over extended periods by identifying an optimal flow regime to prevent entrapment of graphene nanosheets in the porous membrane structure. Furthermore, a range of pore sizes in the hollow fiber membranes offered additional control over graphene flake size distributions, resulting in material dispersions with comparable size statistics to centrifuged samples, but with a ~1000-fold improvement in process throughput. Moreover, the graphene nanosheets obtained via CFF have been processed into functional inks that are highly conductive (~10⁴ S/m) and printable across multiple additive manufacturing platforms. Since this CFF process is generalizable to other layered materials, it can become a platform technology for the high-throughput production of electronic-grade 2D material inks for large-area printed electronics.