(725a) Direct Synthesis and Morphology Control of Metal Organic Framework Nanosheets

Metal-organic frameworks (MOFs) are promising candidates for gas separation membranes due to their regularly arranged pores, tunable pore sizes and functionalities. In particular, MOFs nanosheets, which possess few nanometer thickness and large lateral dimensions, are highly desirable for membranes as mass transport is increased along the thin dimension and the orientation of pores can be readily controlled during the fabrication. However, it remains challenging to synthesize free-standing MOF nanosheets with uniform size and shape. MOFs nanosheets are usually fabricated through delamination of layered MOFs. This top-down method has a low yield meanwhile the resultant nanosheets often suffer from fragmentation and reaggregation. Herein, we demonstrate a direct synthesis approach for zinc benzimidazole-acetate (Zn(Bim)OAc) MOF nanosheets. Surfactant and judicious selection of dispersing solvents are found critical to prevent reaggregation of Zn(Bim)OAc during the synthesis. The obtained nanosheets have a lateral dimension of 500 nm when synthesized at room temperature. The morphology of thin Zn(Bim)OAc can be tuned from nanosheets to nanobelts by adjusting the synthesis temperature. To measure the thickness of nanosheet, a method was developed using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A thickness of 7 nm is determined for Zn(Bim)OAc nanosheets which would be promising building blocks of gas separation membranes.