Crystal growth of the core and rotated epitaxial shell of a heterometallic metal-organic framework revealed with atomic force microscopy.

Atomic force microscopy has been used to determine the surface crystal growth of two isostructural metal-organic frameworks, [Zn2(ndc)2(dabco)] (ndc = 1,4-naphthalenedicarboxylate, dabco = 4-diazabicyclo[2.2.2]octane) (1) and [Cu2(ndc)2(dabco)] (2), from a core crystal of 1 for the former and a core-shell 1@2 crystal for the latter. AFM studies show that the surface terrace morphology expressed is a function of supersaturation, with steps parallel to both the <100> and <110> directions being expressed at higher supersaturations for 1, and steps parallel to the <110> direction being expressed solely at low supersaturation for 1 and 2. The crystal growth mechanisms for both 1 and 2 are essentially identical and involve 2D nucleation and spreading of 0.5 nm high metastable sub-layers of the stable extended 1.0 nm high growth terrace. Surface growth features of 2 indicate that there is an in-plane rotational epitaxy between 2 and 1 of 5.9(7)° that may be directed by the synthesis conditions and that intimate mixtures of different domains of ±5.9(7)° rotational epitaxy are not observed to coexist on the several micron scale on the shell surface. The results provide potential routes and understanding to fabricate MOFs of different crystal forms and defect structures, which are necessary for future advanced function of these versatile materials.

Unveiling the mechanism of lattice-mismatched crystal growth of a core-shell metal-organic framework.

Determining the effect of severe lattice mismatch on the crystal growth mechanism and form of epitaxially grown materials is vital to understand and direct the form and function of such materials. Herein, we report the use of atomic force microscopy to reveal the growth of a shell metal-organic framework (MOF) on all faces of a core MOF that has similar a, b-lattice parameters but a ∼32% mismatch in the c-lattice parameter. The work shows the mechanism through which the shell MOF overcomes the core terrace height mismatch depends on that mismatch being reduced before overgrowth of continuous shell layers can occur. This reduction is achieved via a process of growth of non-continuous shell layers that are terminated by terrace edges of the core. The crystal form of the shell MOF is heavily influenced by the lattice mismatch which hinders continuous spreading of the interfacial and subsequent shell layers on some facets. The results exemplify the crystal growth versatility of MOFs to accommodate large lattice mismatch, to house many more functional defects in a core-shell MOF than either of the component MOFs, and has broader implications for engineering lattice-mismatched core-shell materials in general.