From Layered Crystals to Permselective Membranes: History, Fundamentals, and Opportunities

Membranes are crucial in permselective processes, such as water purification, gas sequestration, ion separation, membrane distillation and pervaporation, and energy conversion and storage. The two-decade development of two-dimensional (2D) membranes has shifted the membrane transport from solution-diffusion in the polymer matrix to size-dependent migration in Ångström-to-nanoscale slit-like channels, delivering distinctive fundamentals within nanoconfinements and potential implementations. In this review, we delve into the comprehensive information that facilitates the transformation of layered crystals into continuous thin-film architectures boasting well-defined nanopores. We trace the evolutionary history of these technologies, engage in debates on their underlying principles, and reveal obstacles that demand attention. Starting with the preparation of "single-layer carbon," we explore an array of protocols designed for the sizable production of 2D nanosheets. Our focus then shifts toward unveiling the solutions available for membrane fabrication and control, specifically focusing on the chemical and structural properties of 2D membranes that dictate their mass transportation behaviors in various scenarios. This is followed by a brief comparative discussion on the development of atomically thin and 2D-based mixed matrix membranes. Finally, we conclude by outlining the challenges and limitations associated with streamlining the interlocked exfoliation-reconstruction-control processes to consolidate their practicalities for use in environment and energy-related applications.