Non-classical crystal growth of metal organic frameworks MIL-68(In) and ZIF-67

Abstract

This project investigated the non-classical crystal growth of two different metal-organic frameworks. The growth of crystal was studied by carrying out solvothermal or room temperature growth methods across a range of reaction times, with the samples produced at each time analysed using scanning electron microscopy. Supporting evidence for their growth was provided by transmission electron microscopy, powder X-ray diffraction and thermogravimetric analysis.

MIL-68(In) under solvothermal conditions revealed a non-classical crystal growth mechanism, with the metal-organic framework growth proceeding via a route of nanocrystallite – aggregation – surface recrystallisation – extension of recrystallisation from the surface to the core of the particle, resulting in single-crystal hexagonal microrods. Terephthalic acid molecules which are adsorbed onto the surface of nanocrystallites are believed to strengthen the inter-particle interaction, leading to a non-classical growth mechanism. Initially crystal growth may be inhibited by blocking agents such as pyridine, but if the reaction is allowed to reach equilibria under thermodynamic control, this effect will be minimised, and growth will proceed as would otherwise be expected.

ZIF-67 was synthesised by two routes, one solvothermal, and one room temperature. Unlike the MIL-68(In) studied in this project, for ZIF-67 the solvothermal route revealed a classical crystal growth mechanism. However, the room temperature method appears to follow a non-classical crystal growth mechanism.