Self-similar mesocrystals form via interface-driven nucleation and assembly

• Published in: Nature (London) 2021-02, Vol.590 (7846), p.416-422
• Main Authors: Zhu, Guomin, Sushko, Maria L., Loring, John S., Legg, Benjamin A., Song, Miao, Soltis, Jennifer A., Huang, Xiaopeng, Rosso, Kevin M., De Yoreo, James J.
• Format: Article
• Language: English
• Subjects: 119/118; 639/301; 639/301/357; 639/301/357/551; 639/638/298; 639/925; Aggregates; Attachment; Brownian motion; Composition; Contact angle; Crystallization; Crystallography; Crystals; Dimensional analysis; Electron microscopy; Hematite; Humanities and Social Sciences; Iron constituents; Iron oxides; Materials chemistry; MATERIALS SCIENCE; Methods; Morphology; multidisciplinary; Nanomaterials; Nanoparticles; Nanoscale materials; Nanoscience and technology; Nanotechnology; Nucleation; Oxalates; Oxalic acid; Phase transitions; Precursors; Properties; Science; Science (multidisciplinary); Seeds; Self-similarity; Single crystals; Structure; Synthesis and processing; Transmission electron microscopy
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03300-0

Crystallization by particle attachment (CPA) is a frequently occurring mechanism of colloidal crystallization that results in hierarchical morphologies 1 – 4 . CPA has been exploited to create nanomaterials with unusual properties 4 – 6 and is implicated in the development of complex mineral textures 1 , 7 . Oriented attachment 7 , 8 —a form of CPA in which particles align along specific crystallographic directions—produces mesocrystals that diffract as single crystals do, although the constituent particles are still discernible 2 , 9 . The conventional view of CPA is that nucleation provides a supply of particles that aggregate via Brownian motion biased by attractive interparticle potentials 1 , 9 – 12 . However, mesocrystals often exhibit regular morphologies and uniform sizes. Although many crystal systems form mesocrystals 1 – 9 and individual attachment events have been directly visualized 10 , how random attachment events lead to well defined, self-similar morphologies remains unknown, as does the role of surface-bound ligands, which are ubiquitous in nanoparticle systems 3 , 9 , 11 . Attempts to understand mesocrystal formation are further complicated in many systems by the presence of precursor nanoparticles with a phase distinct from that of the bulk 1 , 13 , 14 . Some studies propose that such particles convert before attachment 15 , whereas others attribute conversion to the attachment process itself 16 and yet others conclude that transformation occurs after the mesocrystals exceed a characteristic size 14 , 17 . Here we investigate mesocrystal formation by iron oxides, which are important colloidal phases in natural environments 18 , 19 and classic examples of systems forming ubiquitous precursor phases and undergoing CPA accompanied by phase transformations 15 , 19 – 21 . Combining in situ transmission electron microscopy (TEM) at 80 degrees Celsius with ‘freeze-and-look’ TEM, we tracked the formation of haematite (Hm) mesocrystals in the presence of oxalate (Ox), which is abundant in soils, where iron oxides are common. We find that isolated Hm particles rarely appear, but once formed, interfacial gradients at the Ox-covered surfaces drive Hm particles to nucleate repeatedly about two nanometres from the surfaces, to which they then attach, thereby generating mesocrystals. Comparison to natural and synthetic systems suggests that interface-driven pathways are widespread. Mesocrystal formation is investigated for haematite in the presence of