Zr6O4(OH)4(O2CR)12 Precursors Uncover How Modulators Govern Supersaturation, Nucleation, and Growth of UiO-66 Nanocrystals

Abstract

Pure Zr6O4(OH)4(O2C-t-Bu)12 (Zr6-Pivalate) is used to prepare UiO-66 nanocrystals (d = 19–186 nm, σ = 10–30%) in a hot injection synthesis. In situ dynamic light scattering (DLS) measurements show that nanocrystal formation is complete in 2–20 minutes, much faster than the 12–24 hours required by conventional syntheses from zirconyl chloride. The nanocrystal size monotonically increases as the benzoic acid modulator concentration and/or reaction temperature are increased and when solutes are supplied slowly. 13C labeled UiO-66 nanocrystals are found to dissolve in benzoic acid modulator solution using nuclear magnetic resonance (NMR) spectroscopy. The solubility is size dependent (Ksp(d)= 0.7–5.7 × 10-22, d = 26–108 nm; Ksp(bulk) = 6.8 × 10-23), corresponding to a low surface tension (0.02 ± 0.03 to -0.001 ± 0.001 mJ/m2). The low surface tension explains the lack of Ostwald ripening under the synthesis conditions. The kinetics of MOF formation and the final nanocrystal size can be predicted using a homogeneous nucleation and growth mass balance model where the supersaturation is governed by a competition between the modulator and linker for the nodal cluster.