Leffler Vanessa, Ehlert Sascha, Förster Beate, Dulle Martin, Förster Stephan
Jülich Centre for Neutron Science (JCNS-1/IBI-8), Forschungszentrum Jülich, 52425 Jülich, Germany.
Institute of Physical Chemistry, RWTH Aachen University, 52074 Aachen, Germany.
ACS Nano. 2021 Jan 26;15(1):840-856. doi: 10.1021/acsnano.0c07359. Epub 2021 Jan 4.
Heat-up synthesis routes are very commonly used for the controlled large-scale production of semiconductor and magnetic nanoparticles with narrow size distribution and high crystallinity. To obtain fundamental insights into the nucleation and growth kinetics is particularly demanding, because these procedures involve heating to temperatures above 300 °C. We designed a sample environment to perform SAXS/WAXS experiments to investigate the nucleation and growth kinetics of iron oxide nanoparticles during heat-up synthesis up to 320 °C. The analysis of the growth curves for varying heating rates, Fe/ligand ratios, and plateau temperatures shows that the kinetics proceeds a characteristic sequence of three phases: an induction Phase I, a final growth Phase III, and an intermediate Phase II, which can be divided into an early phase with the evolution and subsequent dissolution of an amorphous transient state, and a late phase, where crystalline particle nucleation and aggregation occurs. We extended classical nucleation and growth theory to account for an amorphous transient state and particle aggregation during the nucleation and growth phases. We find that this nonclassical theory is able to quantitatively describe all measured growth curves. The model provides fundamental insights into the underlying kinetic processes especially in the complex Phase II with the occurrence of a transient amorphous state, the nucleation of crystalline primary particles, particle growth, and particle aggregation proceeding on overlapping time scales. The described experiments together with the extension of the classical nucleation and growth model highlight the two most important features of nonclassical nucleation and growth routes, i.e., the formation of intermediate or transient species and particle aggregation processes. They thus allow us to quantitatively understand, predict, and control nanoparticle nucleation and growth kinetics for a wide range of nanoparticle systems and synthetic procedures.
热合成路线非常普遍地用于大规模可控生产尺寸分布窄且结晶度高的半导体和磁性纳米粒子。要深入了解成核和生长动力学尤其具有挑战性，因为这些过程涉及加热到300°C以上的温度。我们设计了一个样品环境来进行小角X射线散射/广角X射线散射（SAXS/WAXS）实验，以研究在高达320°C的热合成过程中氧化铁纳米粒子的成核和生长动力学。对不同加热速率、铁/配体比和平台温度下的生长曲线分析表明，动力学过程呈现出三个特征阶段：诱导阶段I、最终生长阶段III和中间阶段II，中间阶段II又可分为一个早期阶段，在此阶段会形成并随后溶解无定形瞬态，以及一个后期阶段，在此阶段会发生晶体颗粒的成核和聚集。我们扩展了经典的成核和生长理论，以解释在成核和生长阶段出现的无定形瞬态和颗粒聚集现象。我们发现这种非经典理论能够定量描述所有测量的生长曲线。该模型为潜在的动力学过程提供了深入的见解，特别是在复杂的阶段II中，其中会出现瞬态无定形状态、晶体初级颗粒的成核、颗粒生长以及颗粒聚集在重叠的时间尺度上进行。所描述的实验以及经典成核和生长模型的扩展突出了非经典成核和生长路线的两个最重要特征，即中间或瞬态物种的形成以及颗粒聚集过程。因此，它们使我们能够定量地理解、预测和控制广泛的纳米颗粒系统和合成过程中的纳米颗粒成核和生长动力学。
