Zr₆O₄(OH)₄(O₂C-t-Bu)₁₂ precursor uncovers how modulators govern supersaturation, nucleation, and growth of UiO-66...

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 dissolve in benzoic acid modulator solution liberating terephthalic acid that is observed using nuclear magnetic resonance (NMR) spectroscopy. The solubility is size dependent (K _sp(d) = 0.7-5.7 × 10^-22, d = 26-108 nm; K _sp(bulk) = 6.8 × 10^-23), corresponding to a low surface energy (0.02 ± 0.03 to -0.001 ± 0.001 mJ m^-2). The low surface energy 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.