Cathodic Deposition of Thickness-Controllable ZIF-62 Film and Its Vitrification

Glass metal-organic framework (MOF) membrane is a newly developed material that is promising for separation works like ionic sieving and gas separation. Compared with its crystalline counterpart, the glass MOF membrane takes the advantage of its grain-boundary-free nature that greatly benefits the separation performance. However, the fabrication of the glass MOF membrane requires the pre-preparation of the mother crystalline MOF film, which remains a challenge. Taking the deposition of the iconic ZIF-62 (Zn-imidazole, melting point: 711 K) film as an example, this area remained silent until Jiang et al. reported the first example in 2018. 1,2 They achieved the ZIF-62 deposition on anodic aluminum oxide (AAO) membrane by a solvothermal method. Nevertheless, this approach is limited by the low yield of the generated MOF crystals on the substrate and the time/energy-consuming process. To mitigate the existing dilemma, here we show an economic cathodic deposition approach to the preparation of ZIF-62 film with a controllable thickness. Thanks to the addition of hydrogen peroxide as a probase, the pure ZIF-62 film can be deposited without the co-deposition of metallic Zn. In the proposed deposition, a nucleation-Ostwald ripening-crystallization ternary process can be observed, which is different from the cathodic deposition of other MOF species like MOF-5. Also, it is found that the crystallinity of the deposited ZIF-62 film improves with the deposition temperature and the thickness of it can be well-tuned by the deposition time from a range of ~1 µm to ~15 µm. The cathodically deposited ZIF-62 film can be directly used as the mother material for the preparation of glass MOF membrane via a vitrification process. Systematic annealing experiments indicate that the heating temperature and dwelling time are crucial to the quality of the glass MOF membrane. At the optimized conditions (713 K, 4 hours), the dense and bulb-free glass MOF membrane with a flat surface can be obtained, which is ideal for separation works. Wang, Y.; Jin, H.; Ma, Q.; Mo, K.; Mao, H.; Feldhoff, A.; Cao, X.; Li, Y.; Pan, F.; Jiang, Z., A MOF glass membrane for gas separation. Angewandte Chemie 2020, 132 (11), 4395-4399. Bennett, T. D.; Yue, Y. Z.; Li, P.; Qiao, A.; Tao, H. Z.; Greaves, N. G.; Richards, T.; Lampronti, G. I.; Redfern, S. A. T.; Blanc, F.; Farha, O. K.; Hupp, J. T.; Cheetharm, A. K.; Keen, D. A., Melt-Quenched Glasses of Metal-Organic Frameworks. Journal of the American Chemical Society 2016, 138 (10), 3484-3492.