ZIF-8 heterogeneous nucleation and growth mechanism on Zn(II)-doped polydopamine for composite membrane fabrication

Abstract

Continuous ZIF-8 membranes have showed great potential for gas separation. The prospect is subjected to two pivotal issues: shorten the time to construct the ZIF-8 selective layer and tightly bond ZIF-8 layer onto the substrate. In our previous work, Zn(II)-doped polydopamine (Zn-PDA) linkage layer was proposed and effectual to solve these problems. In this research, the mechanism likely to support the improvements is studied intensively. The dominant factor is that Zn(II) ions can be chelating and depositing sufficiently in the linkage layer. The strongly alkaline condition for dopamine polymerization is also beneficial to Zn(II) chelation and deposition, which has been confirmed by DFT simulation, together with characterization tests, e.g., XPS, FTIR, and Zn(OH)2 precipitation for Zn(II)–O coordination. Phenolic hydroxyl groups are deprotonated in the strongly alkaline environment with high Mulliken charge (−0.515 & −0.524 e) and become strong chelation sites to Zn(II). Subsequently, Zn(II) clusters extensively filled in the PDA linkage layer are acting as the starting sites for ZIF-8 heterogeneous nucleation and growth. The correspondence between Zn(II) clusters probed by EDX and nascent ZIF-8 crystals visualized by FE-SEM can support this phenomenon clearly. On the whole, the abundant starting sites in Zn-PDA enhanced ZIF-8 growth, and ZIF-8 growth from the rooting-in Zn(II) clusters created an indented and hinged boundary for tightly bonding ZIF-8 layer onto the substrate.