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

Abstract

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 Co₂(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 Co₂(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 Co₂(bim)₄ nanosheets. Simultaneously, the addition of NH₄OH also helps to deprotonate the bim ligand in solution, thus mediating the nucleation and growth of Co₂(bim)₄. As a result, well-defined 2D Co₂(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 Co₂(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.