Guiding Metal Organic Framework Morphology via Monolayer Artificial Defect-Induced Preferential Facet Selection

Abstract

Guiding metal organic framework (MOF) morphology, especially without the need for chemical additives, still remains a challenge. For the first time, we report a unique surface guiding approach in controlling the crystal morphology formation of zeolitic imidazole framework-8 (ZIF-8) and HKUST-1 MOFs on disrupted alkanethiol self-assembled monolayer (SAM)-covered Au substrates. Selective molecule removal is applied to generate diverse SAM matrices rich in artificial molecular defects in a monolayer to direct the dynamic crystal growth process. When a 11-mercaptoundecanol alkanethiol monolayer is ruptured, the hydroxyl tail groups of surface residue molecules act as nucleating sites by coordination with precursor metal ions. Meanwhile, the exposed alkane chain backbones stabilize a particular facet of MOF nuclei in the dynamic growth by slowing down their crystal growth rates along a specific direction. The competitive formation between the [110] and [100] planes of ZIF-8 ultimately regulates the crystal shapes from rhombic dodecahedron, truncated rhombic dodecahedron, and truncated cube to cube. Similarly, changeable morphologies of HKUST-1 crystals are also achieved from cube and tetrakaidekahedron to octahedron, originating from the competitive selection between the [100] and [111] planes. In addition to the artificial matrix preferred orientation of initial nucleation, parameters such as temperature also play a crucial role in the resulting crystal morphology. Standing on the additive-free MOF crystal morphology growth control, porous architectures prepared in this approach can act as templates for ligand-free metal (Au, Ag, and Cu) nanocluster synthesis. The nanocluster-embedded MOF structures represent distinct crystal morphology-dependent optical properties, and interestingly, their fluorescence emission can be highly enhanced by facet-induced nanocluster packing alignments. These findings not only provide a unique thought on MOF crystal morphology guidance but also pave a new route for the accompanied property investigation and further application.