Understanding Crystal Polymorph Control in Confinement using In-situ TEM (PolyTEM)
Funded by a Horizon 2020 Marie Skłodowska-Curie Individual Fellowship (H2020-MSCA-IF-2019-885795) granted to Dr. Yifei Xu

Introduction

 

Controlling the polymorph (crystal structure) of crystalline materials is of vital importance to both material science and the pharmaceutical industry. Many crystal polymorphs are difficult to access, however, as polymorph is determined by both kinetics and thermodynamics. Recently, it has been observed that precipitation of crystals in confinement often leads to the formation of unusual polymorphs. For example, CaCO3 forms purely as aragonite when it is precipitated in small nanopores. These observations suggest that confinement could offer a generic route to polymorph control. However, the fundamental mechanisms underlying this confinement effect are poorly understood. In this project, Dr. Yifei Xu uses in-situ transmission electron microscopy (TEM) to study how confinement effects give rise to polymorph control. CaCO3 forms the principal focus of the study, and a graphene pocket (GP) is used as the confinement system.

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Results

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We have confirmed that GP can significantly facilitate the formation of aragonite, a metastable polymorph of CaCO3 (Figure 1a). Using cryogenic TEM (cryoTEM) and electron tomography (cryoET), we have thoroughly studied the 3D morphology of GP in aqueous solution (Figures 1b to 1d), and in-situ studied the nucleation of aragonite within the pockets. (Figures 1e to 1g). In combination with X-ray photoelectron spectroscopy (XPS) analysis, we have systematically investigated the relationship between the morphology and surface chemistry of GP and its polymorph control ability. Furthermore, we have demonstrated that GP is able to control the polymorph of several other crystals as well. A publication reporting our findings is now in preparation.

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Summary & Outlook

 

Our results demonstrate the strong ability of GP in controlling the polymorph of many crystals, which originate from its confinement effect. The next step would be exploring whether such an ability could be applied for a more efficient synthesis of useful pharmaceutical products. Another interesting question would be whether pockets formed by other 2D materials such as MoS2, BN or clay can also control polymorph so effectively, which will also be investigated in our future researches.