Ultrathin nanofilm with tailored pore size fabricated by metal-phenolic network for precise and rapid molecular separation

The tailored nanofilms have attracted a rapidly growing interest in various separation processes. Recently, microporous materials have provided a promising way to meet this requirement due to their distinct structures. However, suffering from the uncontrollable heterogeneous nucleation, crystalline materials are difficult to form the ultrathin and defect-free nanofilms. Herein, tannic acid (TA)/Fe3+ complex, which was microporous amorphous material, was fabricated on the ultrafiltration membrane via the layer-by-layer technique. Interestingly, in addition to the pH value mentioned in the literature, the aggregation of Fe3+ in the aqueous solution was also found to have significant influence on chemical composition and physical structure of the complex. Through the strict control on the Fe3+ aggregation, the defect-free TA/Fe3+ nanofilm has an average pore size of 0.7 nm, which was the smallest among the reported TA/Fe3+ nanofilms. More importantly, the pore size of nanofilm was in accordance with our expectation, and the molecular dynamic simulation further confirmed the result. It demonstrated that the tailored pore structure of the nanofilm can be designed on the basis of the molecular structure. Moreover, the nanofilm has an ultrathin thickness of 5 nm. To the best of our knowledge, the nanofilm was the thinnest nanofilm used for liquid separation. Compared with the nanofilms made by various materials, the TA/Fe3+ nanofilm obviously broke through the selectivity-permeability trade-off of the nanofilms, with the ultrathin thickness and the narrow pore size distribution. Additionally, the TA/Fe3+ nanofilm can precisely separate molecules that were close in size. This work provides a facile method to fabricate tailored nanofilms with high performance designed at a molecular level.