Crystallization by particle attachment (CPA) is a frequently occurring mechanism of colloidal crystallization that results in hierarchical morphologies^1,2,3,4. CPA has been exploited to create nanomaterials with unusual properties^4,5,6 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^1,9,10,11,12. However, mesocrystals often exhibit regular morphologies and uniform sizes. Although many crystal systems form mesocrystals^{1,2,3,4,5,6,7,8,9} and individual attachment events have been directly visualized¹⁰, 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 attachment¹⁵, whereas others attribute conversion to the attachment process itself¹⁶ 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^15,19,20,21. 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.

通过粒子附件 (CPA) 结晶是胶体结晶的常见机制，导致分层形态^1,2,3,4。CPA 已被用于制造具有不寻常特性的纳米材料^4,5,6，并与复杂矿物质地^1,7的发展有关。定向附件^7,8（一种 CPA 形式，其中粒子沿特定的晶体学方向排列）产生像单晶一样衍射的介晶，尽管组成粒子仍然可辨别^2,9。CPA 的传统观点是，成核提供了通过布朗运动聚集的粒子供应，这些粒子由有吸引力的粒子间势^{1,9,10,11,12 偏置}. 然而，介晶通常表现出规则的形态和均匀的尺寸。尽管许多晶体系统形成介晶^{1,2,3,4,5,6,7,8,9}并且单个附着事件已被直接可视化¹⁰，但随机附着事件如何导致明确定义的自相似形态仍然未知，因为没有表面结合配体的作用，这些配体在纳米粒子系统^3,9,11中无处不在。在许多系统中，由于前体纳米粒子的存在与体^1,13,14的相不同，试图了解介晶形成的尝试更加复杂。一些研究提出，此类粒子在附着之前发生转化¹⁵，而其他研究则将转化归因于附着过程本身^{16和其他人得出结论，在介晶超过特征尺寸14,17}后发生转变。在这里，我们研究了氧化铁形成的介晶，这是自然环境中重要的胶体相^18,19以及形成普遍存在的前体相并经历 CPA 伴随相变^{15,19,20,21的系统的经典例子}. 将 80 摄氏度的原位透射电子显微镜 (TEM) 与“冷冻观察”TEM 相结合，我们追踪了在草酸盐 (Ox) 存在下赤铁矿 (Hm) 介晶的形成，草酸盐 (Ox) 在土壤中含量丰富，其中铁氧化物很常见。我们发现孤立的 Hm 粒子很少出现，但一旦形成，Ox 覆盖表面的界面梯度会驱使 Hm 粒子在距离表面约 2 纳米处重复成核，然后它们附着在其上，从而产生介晶。与天然和合成系统的比较表明，界面驱动的途径很普遍。