2.2 Synthesis of MOF nanosheets
The MOF nanosheets were solvothermally synthesized under continuous ultrasonication as reported in literature.[37]Primarily, 0.375 mmol CoCl2·6H2O and 0.375 mmol NiCl2·6H2O were fully dissolved in 32 mL DMF. Then, ethanol (2 mL), water (2 mL), and TEA (0.8 mL) were added into the solution under constant stirring, followed by adding 0.375 mmol organic ligands (BDC, NH2-BDC, or CH3-BDC). The organic ligands were completely dissolved in solution and it was further stirred for 2 h to obtain a uniform colloidal suspension. Whereafter, the colloidal suspension was continuously ultrasonicated for 20 h under airtight condition and the precipitates were collected via centrifugation. The obtained MOF nanosheets were then washed with ethanol by 4 times to remove the residual reactants, and vacuum dried at room temperature. Finally, MOF nanosheets were dispersed in ethanol for further utilization.
2.3 Preparation of hierarchical MOF lamellar membranes
Hierarchical MOF lamellar membranes were prepared by double-needled electrostatic atomization technology as shown in Scheme 1. Primarily, Nylon substrates were smoothly adhered on the receiver roller. MOF-CH3nanosheet dispersion (0.1 g L-1) was injected into one syringe (A), and MOF-CH3, MOF-BDC or MOF-NH2nanosheet dispersion (0.1 g L-1) was injected into another syringe (B). The two syringes both connected with a high voltage of 18 kV, and the tip-to-collector distance was 15 cm. Subsequently, 240 mL MOF-CH3 nanosheet dispersion in A syringe was pumped out at a speed of 0.005 mm s-1 along with scanning cyclically along roller surface at a speed of 160 rpm to prepare support layer. After that, 3 mL solution in B syringe was sprayed on the support layer under the same conditions, to construct ultrathin membrane surface layer.