Towards a Predictive Understanding of Hematite Crystallization in the Solution

Abstract

Crystallization by particle attachment (CPA) is a common mechanism of colloidal crystallization resulting in hierarchical morphologies1-4. It has been exploited to create nanomaterials with unique, emergent properties4-6 and implicated in the development of complex mineral textures1, 7. Oriented attachment (OA)7-8, a form of CPA in which crystalline primary particles align and attach along specific crystallographic directions, produces structures (typically referred to as mesocrystals) that diffract like single crystals, even though the constituent particle domains are still discernable2, 9. While the existence of mesocrystals has been well documented in a wide range of crystal systems1-9 and individual particle attachment events have been directly visualized10, the mechanism by which these seemingly random events lead to well-defined, self-similar morphologies remains a mystery, as does the role of organic ligands, which are ubiquitous in nanoparticle systems3, 9, 11. In chapter 2, we focused on the crystallization of hematite (Hm, Fe2O3) from ferrihydrite (Fh) as an example to understand the formation mechanism of mesocrystal and role of starting phase. Multiple techniques, primarily TEM, have been applied to understand the crystallization mechanism. We applied ex-TEM to characterize the Hm formation, the results of which are verified by cryo-TEM on the crystallization process. The Hm mesocrystal has a spindle shape consisting of nm sized primary particles that are atomically aligned, as observed by HRTEM, HRSTEM, cross section HRTEM, and 3D STEM tomography. We developed a freeze-and-look approach using indexed TEM grids to cycle samples between the growth reactor and the TEM in order to track the pathway of crystallization at identical positions over time. We found that Hm grows into the solution with a half-spindle shape suggesting that Hm is growing from the solution, and Fh is dissolving to supply the solutes. Most importantly, we applied in situ liquid phase TEM (LP-TEM) at 80°C to visualize the crystallization process. We found that Fh is dissolving and Hm grows by nucleation of new-born particles a few nanometers away from Hm/solution interface, and then attach to the surface. 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 proposed that oxalate plays the role of inhibiting classical monomer-by-monomer growth of the hematite particles while promoting the nucleation of new hm particles close to the hm/solution interface. In chapter 3, we discussed several limitations that face the powerful LP-TEM technique which includes beam induced particle dissolution and motion, particle-membrane adhesion, contamination, triggering of reaction and replication difficulties. We described our efforts to address these challenges by modifying solution and interface chemistry, changing beam parameters, maintaining solution flow, performing post in situ analyses and applying controlled heating. We believe overcoming these limitations will largely empower the technique in many fields. In chapter 4, we listed two examples of applying in situ TEM to investigate dissolution and deformation properties of Hm materials. Further structure characterization of the Hm structure were rationalized to their dissolution and deformation properties. Chapter 5 is the summary of the thesis, along with my vision of utilizing and developing in situ methods to explore the synthesis-structure-property relationship of materials.