Adam Cahn

Alternating zirconium/hafnium layers as markers of crystal growth in the metal-organic framework UiO-66

Metal-organic frameworks (MOFs) are a diverse family of porous materials comprising metal nodes (which form the vertices of a 3D crystal lattice) and organic linkers (which connect the nodes and form the edges of the lattice). Their porosity and crystallinity make MOFs good candidates for catalysis, gas storage, drug delivery, biological imaging, and other applications. However, to reproducibly tune particle properties such as size, shape, and composition for these applications, a better understanding of their growth mechanisms is needed. This work presents a novel method of monitoring crystal growth in the archetypal MOF UiO-66. By adding alternating spikes of zirconium and hafnium nodes to the reaction mixture, “onion layers” of each metal were sequentially incorporated onto the framework, acting as markers of the location and extent of particle growth. Scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM-EDS) was used to generate elemental maps of zirconium and hafnium and measure their spatial distributions in the MOF. X-ray powder diffraction (XRD) patterns were consistent with highly crystalline samples, and transmission electron microscopy (TEM) was used to measure the size of each layer. These results demonstrate the potential of zirconium/hafnium or other metal combinations to visualize crystallization mechanisms of a variety of MOFs.