PNNL

Abstract

Crystallization by particle attachment (CPA), which is a common mechanism of colloidal crystallization resulting in hierarchical morphologies1-4, has been both 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. Combining in situ TEM at 80°C with “freeze-and-look” TEM using indexed grids, we tracked formation of hematite (Hm) mesocrystals in the presence of oxalate and interpreted the results using classical density functional theory. The results show that formation of isolated Hm particles rarely occurs. However, once formed, interfacial gradients created by hematite-bound oxalate drive new hematite particles to repeatedly nucleate about 2 nm away from the new interface and then immediately undergo OA. Because Hm nucleation rates are statistically deterministic and direction-specific, the resulting mesocrystals are self-similar. Comparison to natural and synthetic systems suggests interface-driven pathways are widespread.