INTERFACIALLY-DRIVEN NANOPARTICLE NUCLEATION BIASES HEMATITE CRYSTALLIZATION TOWARDS ORIENTED ATTACHMENT

A diverse class of materials exhibit characteristics of mesocrystals: single crystals composed of distinct nano-sized domains that are atomically aligned. The formation of such structures is often attributed to crystallization through oriented attachment (OA). However, many questions about the fundamental drivers and dynamic progression of this phenomenon remain. Here we focus on the crystallization of hematite (hm, Fe₂O₃) mesocrystals from ferrihydrite (fh) nanoparticles. In the oxalate-free solution, the resulting hm crystals are well faceted rhombohedron, while in the presence of oxalate hm forms a nanoporous spindle-shaped single crystal elongated along [001]. The spindles are hierarchically organized across two length scales. At the shortest length scale, they consist of atomically aligned nanometer size domains. These then form a second order structure consisting of chains of the hm domains. To investigate the formation process, we applied in situ liquid phase TEM with heating on solutions of fh held at temperatures of 80 °C into which hm seeds were added. We directly observed both the dissolution of fh and the nucleation of new hm particles, which all formed within close proximity (~ 1 nm) of the hm/solution interface. Immediately after nucleation, the hm particles attached to the nearby seed to form a hm mesocrystal. Post growth analyses using EDX mapping and electron diffraction after disassembling the liquid cell confirmed the growth of spindle-shaped hematite during the liquid phase TEM experiments. In addition, we utilized a freeze-and-look approach in which indexed TEM grids were used to cycle samples between the growth reactor and the TEM in order to track the pathway of crystallization over long periods of time. The results were consistent with those of the in situ experiments and confirmed that the fh serves as a buffer that sets the Fe³⁺ concentration and the spindles grow by creation of new particles in the solution near the hm interface. Based on ATR FTIR measurements of the relative binding strength of oxalate to the (001) and (012) faces and calculations of chemical potential gradients near the interface, we propose that oxalate plays the role of inhibiting classical monomer-by-monomer growth of the hematite particles while promoting the nucleation of new hm particles at the hm/solution interface by creating a gradient in Fe³⁺ chemical potential. In this way, the oxalate ligands bias the growth process away from classical mechanisms and towards oriented attachment.