Reduction Rate as a Quantitative Identification Toward Growth Pathway and Size Control in Low-Polydisperse Colloidal Metal Nanocrystals

:kinetic control is a powerful means for achieving thedesired size and size distribution of metal nanocrystals and thustailoring their properties in different applications. However, the explicitrole of kinetics in the synthesis of metal nanocrystals is still elusive dueto the lack of quantitative correlation among the reduction kinetics,monomer concentration, growth pathways, andfinal products. With Auas an example, here, we quantitatively correlate the initial reduction rateof the Au3+precursor with monomer concentration, growth pathways,and thefinal size/size distribution of products by simply manipulatingthe aging time of the Au3+precursor and volume ratio of oleylamine totoluene during the reaction based on quantitative analysis of in situsmall-angle X-ray scattering/UV-vis measurements and transmissionelectron microscopy results. When the initial reduction rate of Au3+isin the range of 1.45-2.26x10-5Mmiddots-1, corresponding to a slowdepletion of monomers, Au nanoparticles with larger size are formed through a sequence of growth events comprising coalescence ofthe nuclei into bigger particles, intraparticle growth within coalesced particles, atomic deposition, and subsequent focusing of sizedistribution. It is found that if the initial reduction rate is equal to or faster than 3.47x10-5Mmiddots-1, the growth pathway of Aunanoparticles is gradually changed owing to the disappearance of the atomic deposition event in the later stage caused by asignificant reduction in monomer concentration. Interestingly, focusing of the size distribution event is governed in the later growthstage and can take place during the intraparticle growth process as the initial growth rate reaches 5.55x10-5Mmiddots-1, giving rise tomuch smaller products with low polydispersity. This work will not only offer a mechanistic understanding of the nonclassical growthmechanism but also provide a general guideline for precisely controlling the size and size distribution of noble-metal nanocrystals.