Competing phases in the room-temperature M2(2,6-ndc)2(dabco) metal-organic framework thin film synthesis

Abstract

The targeted synthesis of DUT-8 metal-organic framework thin films of composition M2(2,6-ndc)2(dabco), where 2,6-ndc = 2,6-naphthalenedicarboxylate, and dabco = 1,4-diazabicyclo[2.2.2]octane were performed using a dip-coating synthesis strategy on functionalised gold-coated silicon substrates at room temperature. The thin films were characterised using atomic force microscopy (AFM), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD), revealing a crystalline and oriented thin film with a homogeneous nanoparticle formation when M = Cu, and an inhomogeneous distribution of micron-sized crystals for M = Zn. However, the refined unit cell parameters indicated a large reduction to the known DUT-8 unit cells (−5% and −13% in the ab-plane for M = Cu and Zn, respectively). Three-dimensional electron diffraction (3D ED) was performed on the crystals from the thin film synthesis of M = Zn, revealing the formation of Zn(2,6-ndc)(H2O), instead of the intended Zn2(2,6-ndc)2(dabco) compound. Similarly, in the case of M = Cu thin films, Rietveld refinements of the powder X-ray diffraction data indicated that Cu(2,6-ndc) was the most likely phase grown. Our results highlight the competing phases possible for a room-temperature dip-coating synthesis strategy of DUT-8, and demonstrate the advantage of using 3D ED in thin film research.