Versatile Morphological Control of Cobalt Metal-Organic Frameworks with Enhanced Catalytic Activity by Simple Tuning...

Metal-organic frameworks (MOFs) present a huge chemical and structural tuneability with enhanced porosity and surface area, offering new applications in the fields of catalysis, drug delivery, chemical/biosensing, or rechargeable batteries. However, their properties not only reside on their composition and structure but also strongly depend on their morphology, introducing a new important parameter during their synthesis. For this reason, a precise morphological control is crucial for applications where the shape and size of MOF crystals directly determine their specific surface area, accessibility of active sites, and adsorption capacity. This work presents a rational study to optimize the morphology of MOFs by using as a playground zeolitic imidazolate frameworks (ZIFs), an emerging subclass of MOFs. The most important synthetic parameters, namely, the molar ratio between the metal precursor (Co2+) and the organic ligand (2-methylimidazole) as well as the concentration of the surfactant used as a modulator and stabilizer (cetyltrimethylammonium bromide, CTAB), are controlled to optimize the size, shape, distribution, and structure of the obtained ZIF nanoMOFs. We demonstrate that keeping the organic ligand as the excess reagent is crucial for achieving stable and uniform porous nanocrystals, highlighting the importance of precise stoichiometric control during the nanoMOF synthesis. When maintaining a constant Co2+:Hmim molar ratio, the use of high CTAB concentrations (4.0 mM) in the synthesis results in the formation of cubic ZIF-67 nanostructures with sizes around 80 nm. As the concentration of CTAB decreases, the nanocubes develop spikes, and at concentrations below 1.5 mM, these evolve into flower-like structures with crossed spikes. This morphological transition underscores the critical role of CTAB concentration in dictating the final shape and size of the porous nanostructures. Moreover, at low metal-to-ligand ratios, a potential new structural polymorph emerges. All these findings are corroborated by a complete structural, imaging, composition, and functional characterization that confirms a notable catalytic activity of MOFs to degrade the dye RhB in aqueous solution.