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Abstract:
While downscaling metal-organic frameworks (MOFs) into a nanosize regime is highly relevant to meet their growing demand in various potential applications, a simple synthesis of nano-MOF under ambient conditions still remains a difficult task. Here we report a room temperature synthesis of 3D MOF, [Zn2(bdc)2dabco]n (ZBD) (bdc = benzene-1,4-dicarboxylic acid and dabco = 1,4-diazabicyclo[2.2.2]octane) with controlled polymorphism, size, and morphology by changing the kind and composition of the solvents. Solvent functions as both a template and a crystal modulator. Dimethylformamide (DMF) preferably forms hexagonal rod MOF (ZBDh) while methanol (MeOH) leads to tetragonal plate MOF (ZBDt) via a solvent template effect (i.e., polymorph control). The size and morphology can be further controlled by using a cosolvent of DMF and MeOH with various volume ratios. DMF and MeOH work competitively, and the solvent with weaker template effect at the given condition acts as a crystal modulator that lowers the rate of nucleation and increases the size of crystals. With an increase of MeOH amount, morphology changes from 1D rods to 2D plates. Protic MeOH reduces the reactivity of nucleophilic dabco and suppresses the crystal growth along Zn-dabco [001], thereby leading to formation of 2D ZBDt plates. To help understanding fundamental morphology-volumetric capacitance relations in energy storage devices, the resulting ZBDs are conformally pyrolyzed to hexagonal rod- and tetragonal plate-nanoporous carbons and used as electrodes for supercapacitors. Thanks to a 2D morphology and a relatively high packing density, tetragonal plate carbon delivers two times higher volumetric capacitance than hexagonal rod carbon, despite of their similar gravimetric capacitance.
