Distinguishing Classical from Non-Classical Nanoparticle Growth Mechanisms by In-Situ X-Ray Diffraction

Hot-injection and heat-up synthesis routes are very commonly used for the controlled synthesis of nanoparticles with narrow size distribution and high crystallinity. These synthesis routes can proceed via classical or different non-classical nucleation and growth pathways, the latter often involving pre-nucleation clusters or oriented attachment. For synthetic control is important to have knowledge about the reaction pathways. To obtain fundamental insights into the nanoparticle nucleation and growth kinetics for these reactions is particularly demanding, because these synthesis routes typically involve heating to temperatures above 200°C which is challenging for many in-situ experimental techniques.<br/><br/>We designed glass flask reactors to perform <i>in situ</i> X-ray diffraction (SAXS/WAXS) experiments to investigate the nucleation and growth kinetics of noble metal, semiconductor and oxidic nanoparticles during heat-up synthesis up to 340°C. The analysis of the growth curves for varying heating rates, precursor/ligand ratios and plateau temperatures shows that the kinetics proceeds <i>via</i> non-classical reaction paths involving the formation of amorphous prenucleation clusters, nucleation, oriented attachment and final growth by monomer consumption. [1,2] Although these pathways are more complicated compared to classical nucleation and growth, one can identify kinetic phases of induction, nucleation and growth that are typically transected during the synthesis. <br/><br/>For a quantitative analysis of the growth curves we extended classical nucleation and growth theory to account for amorphous transient or precursor states and particle aggregation e.g. by oriented attachment during the nucleation and growth phases. We find that this non-classical model is able to quantitatively describe all experimental growth curves. The model provides fundamental insights into the underlying kinetic processes especially in the nucleation and growths phases with the occurrence of a transient amorphous state, the nucleation of crystalline primary particles, particle growth and particle aggregation, all proceeding on overlapping time scales.<br/>The described <i>in situ</i> X-ray scattering experiments together with the extension of the classical nucleation and growth model quantitatively describe the two most important mechanisms of non-classical nucleation and growth routes, <i>i.e.</i> the formation of intermediate or transient species, and the particle aggregation processes.<br/> <br/>[1] V. Leffler, S. Ehlert, B. Förster, M. Dulle, S. Förster, <i>ACS Nano</i> 2021, 15, 840-856<br/>[2] V. Fokina, M. Wilke, M. Dulle, S. Ehlert, S. Förster, <i>J Phys Chem C</i> 2022, 126, 50