Growth mechanisms of nanomaterials studied by atomic-resolution liquid-phase scanning transmission electron microscopy

Abstract text (incl. figure legends and references)

Understanding the chemical and the physical processes of nanomaterials (NMs) in liquid media is a crucial challenge and of fundamental interest to modern materials science, chemistry and biology.1 For example, observing the initial steps of nucleation and the growth of solids and following the exact pathway of the formation of bimetallic NMs are of highest importance for the development of novel materials. This requires techniques allowing the study of NMs in their liquid environments with the corresponding resolution necessary to observe processes at the atomic scale. The most attractive and unique capabilities of a scanning/transmission electron microscope (S/TEM) is its high spatial resolution, enabling the study of materials at the atomic scale. Nevertheless, a key limitation of this technique is the need of high vacuum conditions to analyse samples. In recent years, a new frontier in the field of TEM has emerged to observe specimens in liquid environments under high-vacuum conditions, by separating the liquid sample from the vacuum column using thin impermeable/electron transparent membranes.2 Recent advances in microfabrication technology have led to a number of commercially available liquid cell TEM holders, where the liquid cells are made of two micro-fabricated silicon nitride electron transparent thin films. This liquid cell consists of liquids normally exceeding 100 nm in thickness encapsulated by SiN windows that are several tens of nanometer in thickness as well, and thus are unsuitable for atomic scale observations. Graphene-based liquid cell electron microscopy (GLC) has demonstrated a better imaging resolution3 (Fig 1).

In this study, we optimized graphene-based liquid cells to realize in real time atomic scale observations of nucleation and growth of Platinum (Pt) nanoparticles (NPs) and the formation of Pt-Pd core-shell NPs in liquid mode. We investigate the exact growth mechanisms of platinum NPs from single atoms to final crystals by in-situ liquid phase STEM at atomic-scale. After nucleation, we show that the nanocrystals grow via two main stages: atomic attachment in the first stage, followed by the second stage of growth, which is based on particle attachment by different atomic pathways (Fig 2). In addition, we investigated the exact atomic mechanisms underlying the growth of Pt-Pd core-shell NPs, where growth mechanisms of the Pt shell on Pd nanocubes are studied in aqueous solution at the atomic level. We found that Pd-Pt core-shell NPs are formed via two distinct mechanisms: (i) at low concentration of Pt atoms, an ultra-thin skin of only a few atomic layers is formed via atom-by-atom deposition and (ii) at higher concentration of Pt atoms, inhomogeneous islands and thick shells are formed via attachment of Pt clusters (Fig 3).3, 4

Figure 1: Encapsulated Pt precursor with GLC: (a) schematic illustration, (b) formation of Pt atoms in liquid.

Figure 2: Nucleation and growth of nanoparticles: (a) ADF-STEM time lapse and (b) schematic illustration. (Reproduced from ref. 3)

Figure 3: Pd NCs core and Pt shell STEM analysis. Pd nanocube (a) with thin layer of Pt shell (b) with Pt shell islands. (Reproduced from ref.4)

REFERENCES

1Unocic, R. R. UMicrosc. Microanal 2015, 21, 1972-1973

2Alloyeau, D.; Dachraoui, W.; Javed, Y.; et al. Nano Letters 2015, 5, 2574-2581

3Dachraoui, W.; Henninen, T. R..; Keller D.; Erni, R. Scientific Reports 2021, 11, 23965

4Dachraoui, W.; Bodnarchuk, M. I.; Vogel, A.; Kovalenko, M. V.; Erni, R. Applied Physics Reviews 2021, 8, 041407