Surfactant-assisted ZnO processing as a versatile route to ZIF composites and hollow architectures with enhanced dye adsorption

Metal oxides can be used as hard sacrificial templates for the preparation of multifunctional core-shell MOF-based composites following reaction with an organic linker. This is a facile method, but often structures of well-defined shape are only obtained under narrow ranges of conditions, the shape can be lost completely and low levels of MOF conversion observed. Using the prototypical framework ZIF-8 we present an alternative surfactant-assisted surface passivation strategy where the ZnO precursor particles are first coated with a guanidinium-based amphiphile. The surfactant interacts strongly with the oxide surface and allows fine-tuning of the release of Zn(II) and ZIF-8 nucleation by the level of surface coverage permitting a range of well-defined ZnO@ZIF-8 core-shell architectures to be prepared including in water. Further, selective base-etching of the oxide core provides facile access to hollow ZIF-8 and yolk-shell structures. We also demonstrate enhanced dye adsorption and recovery from aqueous mixtures using ZnO@ZIF-8 composite microspheres.