Nonclassical Nucleation and Growth of Pd Nanocrystals from Aqueous Solution Studied by In Situ Liquid Transmission Electron Microscopy

Abstract

Direct visualization and understanding of the atomic mechanisms governing the growth of nanomaterials are crucial for designing synthesis strategies of high specificity. Aside from playing a key role in numerous technological applications, palladium clusters and nanoparticles are particularly valuable due to their outstanding catalytic activity. Studies show that the properties of Pd nanomaterials depend on shape and size. Therefore, optimizing the synthesis to control the final size and shape of Pd nanoparticles is important for a large number of current and future applications. In this work, we exploit in situ liquid cell scanning transmission electron microscopy to track at the atomic scale the growth of Pd nanoparticles from the very early stage to mature, crystalline nanoparticles. We find that the formation of Pd nanoparticles consists of multiple steps. The first step in nanoparticle formation, representing a nonclassical nucleation step, can be described by the formation of agglomerates of Pd atoms. In the second step, these agglomerates grow via atomic addition to form primary nanoclusters, which coalesce to form amorphous clusters. In the third stage, these clusters continue to coalesce, leading to the formation of amorphous Pd NPs, while in parallel, growth by monomer attachment continues. Then, in the fourth step, the amorphous nanoparticles undergo a nanocrystallization process, where the continuous improvement of crystallinity and the establishment of a distinct morphology eventually give rise to the formation of facetted, crystalline nanoparticles. Similar to our earlier work with Au and Pt nanoparticles, these results confirm that even for simple systems, nonclassical nucleation and growth processes dominate and that these multi-step mechanisms are highly element-specific. Despite the fact that the synthesis conditions are identical, the element-specific interactions define the pathway of the formation of crystalline nanoparticles.

Unraveling chemical processes during nanoparticle synthesis with liquid phase electron microscopy and correlative techniques

“…Erni and co-workers investigated nanoparticle coalescencebased growth mechanisms using GLC-TEM to visualize nanoparticle formation at the atomic scale. 33,[97][98][99][100] They observed that metal atom aggregation into metal clusters played a vital role in facilitating attachment-based growth of PtPd nanoparticles (Fig. 5f).…”

“…5g. 100 Initially, Pd atom aggregation formed amorphous metal clusters, which continuously aggregated and coalesced to form large amorphous particles. Finally, the amorphous particles crystallized with defined facets.…”

Emerging transmission electron microscopy solutions for electrocatalysts: from synthesis to deactivation

“…The study of Pt nanoparticles using LC-TEM has inspired the study of other noble metal particles, including Au, [58][59][60][61] Ag, 62,63 and Pd 64 and their nano-alloy. [65][66][67] Zhu et al 68 found that the nucleation and growth of Au nanoparticles in a mixed solution of 34 mM sodium citrate and 0.24 mM HAuCl 4 had the same mechanism, and noticed that the agglomeration of particles was preferred on {111} planes due to the lower ligand binding energy.…”

Establishment of the Most Effective Methods of Obtaining Nanosize Magnesium Oxide

“…Two technological approaches are used: self-propagating synthesis and volumetric combustion synthesis. The methodology of the first one provides for the mechanism of spontaneous redox endothermic reactions between the oxidiser and the fuel, which are mixed at the molecular level in the corresponding solution with the gradual establishment of the target product in the solid aggregate state [33]. The second technological approach involves external heat supply to the entire volume of the reagent solution.…”