Evaporative drying of sessile droplets as a means of monitoring UiO-66 MOF synthesis and quality

We demonstrate a simple droplet drying based diagnostic approach to monitor the UiO-66 MOF (metal organic framework) synthesis and it’s quality using the sessile drop drying phenomenon. Drying a sessile droplet involves evaporation-driven hydrodynamic flow and particle nature-dependent self-assembled deposition. In general, the MOF synthesis process involves different sizes and physicochemical natures of particles in every stage of the synthesis. Equivalent quantities of each of purifed pore-activated UiO-66 MOF, yet to be purifed pore-inactivated UiO-66 MOF, and reaction precursors of UiO-66 MOF give different deposition patterns when a well-dispersed aqueous droplet of these materials undergo drying over substrates of varying stiffness and wettability. Yet to be purifed pore-inactivated UiO-66 MOF nanoparticles undergo transport towards the droplet periphery leading to a thick ring-like deposition at the dried droplet edge. Under appropriate condition of drying, such deposit leads to desiccationtype mud-like reticular cracking. We study the origin of such ring-like deposits and cracks to understand how it get influenced by the presence of thermal Marangoni flow feld and the addition of salts contributing to surface charge density of UiO-66 particles controlling their stability. We clearly demonstrate ZrOCl2 salt trapped in non-purifed pore-inactivated UiO-66 MOF moiety is the principal reason for ring-like deposit formation and subsequent cracking in its drying aqueous droplet edge. In general, Lewis acid salts (like FeCl3, SnCl2, ZrOCl2), those also act as Brønsted-acid upon hydrolysis, influence surface charge density and colloidal stability of dispersed UiO-66 MOF particles. As a result immediate particle coagulation gets avoided, so those travel to the droplet edge leading to the formation of ring-like deposition and subsequent cracking upon drying. Further we show crack patterns on such deposit are highly dependent on the stiffness and temperature of depositing substrates via a competition between axial and lateral strain at the deposit-substrate interface.