Bottom-up synthesis of 2D Co-based metal-organic framework nanosheets by an ammonia-assisted strategy for tuning the crystal morphology

Direct synthesis of 2D metal-organic framework (MOF) nanosheets is extremely challenging. In this work, ammonia (NH4OH) was used as a modulator to synthesize 2D cobalt-based MOF Co-2(bim)(4) (bim = benzimidazole) nanosheets. Two main factors including Co/bim molar ratio and NH4OH/bim molar ratio were investigated in detail to study their effects on the morphology and structure of Co-based MOFs. It is found that the amount of NH4OH had a significant impact on the morphology of Co-2(bim)(4). NH4OH added to the synthesis solution could coordinately interact with CO2+ and restrict the crystal growth along the vertical direction by competitive intercalation with bim, resulting in the formation of 2D Co-2(bim)(4) nanosheets. Simultaneously, the addition of NH4OH also helps to deprotonate the bim ligand in solution, thus mediating the nucleation and growth of Co-2(bim)(4). As a result, well-defined 2D Co-2(bim)(4) nanosheets can be easily synthesized with the aid of NH4OH. Moreover, with the aid of NH4OH, the low Co2+/bim molar ratio (<1) in the synthesis solution favors the formation of Co-2(bim)(4) nanosheets, in contrast, for the synthesis solution with a high Co2+/bim molar ratio (>= 1), it is easy to form another 2D Co-based MOF nanosheet structure, Co(bim)(OAc). Therefore, the ammonia-assisted strategy is a simple and adjustable bottom-up method for the synthesis of 2D Co-based MOF nanosheets.