Influence of Isomorphous Substituting Cobalt Ions on the Crystal Growth of the MOF-5 Framework Determined by Atomic Force Microscopy of Growing Core-Shell Crystals

Atomic force microscopy is used to conduct the first detailed nanoscopic study on the crystal growth of a complex mixed metal/metal-organic framework based on the MOF-5 framework topology. Shells of isomorphously substituted Co/Zn-MOF-5 and MOF-5 were epitaxially grown on MOF-5 core crystals at room temperature and low supersaturation to produce complex core shell shell structures with a hierarchal mixed metal nature involving mixing at the atomic level in the isomorphously substituted Co/Zn-MOF-5 shell and at the nanometer level through segregation of the Co/Zn-MOF-5 and MOF-5 layers. The presence cobalt in the growth solutions was found to retard the overall rate of surface growth in comparison to a cobalt free growth solution and stop growth entirely for a growth solution containing a Zn/Co ratio = 0.6. The presence of cobalt in the growth solutions was also found to affect the relative rates of terrace spreading in different crystallographic directions compared to cobalt free growth with spreading in the < 110 > directions decreasing relative to the rate along the < 100 > directions. The work provides new understanding of the crystal growth of complex mixed metal/metal-organic frameworks and a methodology to prepare these complex forms in a more controlled manner.