Single-Layered Nanosheets of Covalent Triazine Frameworks (CTFs) by Mild Oxidation for Molecular-Sieving Membranes

Covalent triazine framework (CTF) nanosheets featured with uniform intrinsic nanoporosity and excellent stability are promising building blocks for fast, selective membranes. However, it remains challenging to produce ultrathin CTF nanosheets, significantly hindering the development of CTF-based membranes. Herein, we develop a mild oxidation strategy to exfoliate CTFs, enabling the preparation of highly permeable membranes with stacked CTF nanosheets as the selective layers. The interlamellar spacing of CTF is effectively expanded following the mechanism of "proton donating-accepting" in which dimethyl sulfoxide (DMSO) works as a soft oxidant, leading to ultrathin CTF nanosheets with the assistance of ultrasonication. Furthermore, oxygen-containing functional groups are also introduced onto the CTF nanosheets through mild oxidation, improving surface hydrophilicity. The CTF nanosheet can be stacked onto porous substrates by vacuum filtration to form composite membranes with the thickness of the stacked CTF nanosheets down to ∼30 nm. Thus-obtained membranes exhibit impressive dye separation performances with both high water permeance and high rejection. This work provides not only an efficient method to synthesize ultrathin CTF nanosheets but also a process to prepare fast but selective membranes for molecular separations.