Synthesis and electromagnetic properties of NH2-MIL-88B(Fe) crystals with morphology and size controllable through synergistic effects of surfactant and water

Metal-organic framework (MOF) crystals have promising applications in a number of different fields, but the growth of MOF crystals with controllable morphology and high uniformity remains a major challenge. By taking the advantage of the synergistic effects of water molecules in the mixed solvent and polyvinylpyrrolidone (PVP) surfactants, this work demonstrates an approach for the preparation of NH2-MIL-88B(Fe) MOF crystals with both highly customizable size and aspect ratios and high uniformity. The effects of the concentration of water in the mixed solvent and the amount of PVP surfactants on the morphology and size of the MOF crystals were investigated. The morphology and crystalline properties were characterized by scanning electron microscopy and X-ray diffraction techniques. The data showed that through controlling the concentration of water in the solvent and the amount of the PVP surfactant, the size of the MOF crystals can be tuned rather widely, over 344–5.14 μm, while the aspect ratio of the crystals can be varied widely over 1.0–7.1. Possible mechanisms for such high tunability were discussed. As an example of potential applications, the complex permeability and permittivity of MOF-derived composites (25 wt%) were measured over a wide frequency range of 2–18 GHz. The parameters were then used to evaluate the potential for electromagnetic wave absorption. The results indicate that the composite with a thickness of only 1.2 mm exhibits a “− 10 dB” absorption bandwidth of 4.1 GHz and a minimum reflection of − 20 dB. Such bandwidth and reflection properties provide inspiration for future design of lightweight, broadband electromagnetic wave absorbers from the perspective of fine morphology control.