Insights into the Morphology and Kinetics of Growth of Silver Metal-Organic Nanotubes

The kinetics of the formation of novel porous metal organic nanotubes, [Ag₂(4,4′-(1,4-(xylene)diyl)bis(1,2,4-triazole) (NO₃)₂]·NMP, was investigated by means of ex-situ time-resolved small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). The SAXS results were modeled using the Gualtieri model, which decouples the nucleation and growth processes giving additional insight into the crystal formation mechanism. The results show that the semirigid 4,4′-(1,4-(xylene)diyl)bis(1,2,4-triazole) ligand (L) binds with silver ions, adopting a seesaw geometry to form a polydisperse isotropic framework immediately after mixing the ligands and metal ions. In addition, the SEM imaging demonstrates that the microcrystals grow anisotropically, with nucleation along the edge of the 3D aggregate. These combined data demonstrate that the growth of this MONT occurs in two steps: a rapid formation of an isotropic porous structure immediately after mixing the reactant, which then develops anisotropically as the aggregates of nanorods grow in a preferred direction. The anisotropic growth of the crystal is autocatalytic and determined by the rate of nucleation of new growth sites on the crystals. Moreover, the results of this analysis elucidate, for the first time, the exact order of the competing processes that occur in the synthesis of these MONTs, showing that their anisotropic growth occurs on the initial 3D aggregate and appears to be directed by the interplay between the surface energies that exist during the MONT formation process. This insight is crucial to the use of crystal engineering to guide the crystal formation processes in MONTs to targeted structures, properties, and applications.