Ammonia assisted formation of tubular MOP-18 crystals

Abstract

Tubular MOP-18 crystals have been successfully prepared by cooling-induced crystallization from copper hydroxide nanostrands and 5-dodecyloxybenzene-1,3-dicarboxylic acid DMF (N,N-dimethyl-formamide)–ammonia solution. The dependence of the crystal morphologies on the cooling rate is investigated in detail. A slower cooling rate results in larger and longer MOP-18 tubes. The ammonia in the DMF solution plays a crucial role in the formation of MOP-18 tubes. It deprotonates the acid ligand to trigger the nucleation of MOP-18 and leads the growth of tubular crystals. A “diffusion driven instability” growth mechanism is proposed. The outside surfaces of the crystals have a better supply of material from the surrounding solution and maintain continuous growth, while the inner cavity part not only have a worse supply of material but the inner walls also dissolve in order to balance the local saturated solubility of MOP-18 in DMF at a certain temperature, and results in the formation of tubes.