A diverse class of materials exhibit characteristics of mesocrystals: single crystals constituting distinct nano-sized domains that are atomically aligned. The formation of such structures is often attributed to crystallization through oriented attachment. However, many questions about the fundamental drivers and dynamic progression of this phenomenon remain. And the process become more complexed when coupled with phase transformation. Here we focus on the crystallization of hematite (hm, Fe<sub>2</sub>O<sub>3</sub>) 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 nanoporous spindle-shaped mesocrystals. We applied in situ liquid phase TEM with heating temperatures of 80 °C to investigate the formation. We directly observed both the dissolution of fh and the nucleation of new hm particles, which 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 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 quasi in situ approach of using indexed TEM grids to cycle samples between the growth reactor and the TEM in order to track the pathway of crystallization. The results were consistent with those of the in situ experiments and confirmed that the fh serves as a buffer and the Hm grow by creation of new particles in the solution near the hm interface. Based on ATR FTIR measurements, ion chromatography and classic DFT calculation 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 modifying the interfacial energy of hm close to the hm/solution interface. In this way, the oxalate ligands bias the growth process away from classical mechanisms and towards oriented attachment.

Hematite mesocrystal formation through near interface nucleation and oriented aggregation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.