The optimal conditions found using the above screening route was ultradilute precursor solution consisting of Zr 4+ and BDC-NH2 at an equimolar concentration of 1 mM, acetic acid concentration of 108 mM, and a solution pH of 3.3 (Table ). This delayed homogeneous nucleation and no crystals were formed in the bulk for up to 30 min. This strategy of delaying the reaction kinetics within a relatively low pH regime deviates from the usual approach to maximize the yield of crystalline UiO-66-NH2 involving a synthesis solution pH of ~4.3. 39 Within 5 min of synthesis at room temperature in aqueous solution, high-quality, continuous, nanometer-thick UiO-66-NH2 films (see discussion below) could be obtained on highly-ordered pyrolytic graphite (HOPG) or graphene film as a substrate, as indicated by AFM analysis (Figure ).