Alternate Path to the Formation of Co-aligned Hierarchical Mesocrystals

A diverse class of materials occur as hierarchical mesocrystals, which are single crystal structures consisting of atomically-aligned domains — a characteristic attributed to crystallization through oriented attachment (OA). However, many questions about the alignment mechanisms remain unanswered. Here we focus on crystallization of spindle-shaped hematite (Fe₂O₃) from poorly ordered ferrihydrite (5Fe₂O₃ ●9H2O, fh). Spindle structure, crystallization, and dissolution were probed by high resolution transmission electron microscopy (HR-TEM), cryo-TEM and liquid cell TEM. In particular, we developed a method for tracking regions by HR-TEM during the crystallization. Our results show the spindles consist of atomically aligned domains organized into a hierarchical, rod-like structure penetrated by nm-scale pores. This structure forms as follows: First, the fh-to-hematite transformation is highly localized, with the initial hematite nucleating on fh surfaces. Second, spindles grow by addition of new hematite particles that nucleate from the solution. This process can lead to formation of half-spindles with the growth direction pointing into the bulk solution. Consequently, OA occurs via an autocatalytic process in which the hematite-solution interface drives the nucleation and growth.