Solvent mediated morphology control of zinc MOFs as carbon templates for application in supercapacitors

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 solvents. The solvents function as both templates and crystal modulators. Dimethylformamide (DMF) preferably forms a hexagonal rod MOF (ZBDh) while methanol (MeOH) leads to the formation of a tetragonal plate MOF (ZBDt) via a solvent template effect (i.e., polymorph control). The size and morphology can be further controlled using DMF and MeOH as cosolvents with various volume ratios. DMF and MeOH work competitively, and the solvent with a weaker template effect under the given conditions acts as a crystal modulator that lowers the rate of nucleation and increases the size of the crystals. With an increase of MeOH amount, the morphology changes from 1D rods to 2D plates. Protic MeOH reduces the reactivity of nucleophilic dabco and suppresses crystal growth along Zn-dabco [001], thereby leading to the formation of 2D ZBDt plates. To help understand the fundamental morphology–volumetric capacitance relationships 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 relatively high packing density, tetragonal plate carbon delivers two times higher volumetric capacitance than hexagonal rod carbon, despite their nearly similar gravimetric capacitances.