Electrochemically Assisted Growth of Hopper and Tabular Calcite under Confinement

Abstract

Crystallization under confinement is commonplace in nature and offers new pathways to engineer crystals with the desired morphologies in a controlled and reproducible manner. In this work, we demonstrate the electrochemically assisted, in situ formation of hopper and tabular calcite crystals within a wedge-like pore formed by confinement induced by the surface force apparatus (SFA). In this geometry, the distance between the confining surfaces decreases continuously from hundreds of micrometers to a few nanometers in a single experimental setup. Calcium carbonate precipitation followed in real-time is triggered by elevating the pH and thus supersaturation directly inside the pore. The pH increase is tracked with a fluorophore tracer and calculated analytically. The unusual calcite crystal habits are obtained in the absence of precipitation-modifying additives and vary as a function of the distance between the confining walls, where at the largest surface separations, hopper crystals form on the gold surface, whereas flat, tabular calcite forms on mica when the confining distance was below 5 μm. Hexagonal or triangular calcite plates exhibit surfaces dominantly defined by rough (001) planes and increase in their surface area-to-volume ratios with decreasing SFA pore thickness. Stabilization of calcite plates bound by (001) faces is a cooperative effect of the oriented growth of calcite on the confining, negatively charged mica walls and the confinement shape, which promotes the development of calcite morphology in the rapid crystal growth directions. The heterogeneous crystallization of calcite is preceded by the nucleation and spreading of a front composed of submicrometer-sized calcium carbonate particles visualized in situ in the SFA pore. Our work demonstrates abundant crystal growth under nanoscale confinement facilitated by elevated pH and a high surface charge of the confining walls.