Published:
[Erscheinungsort nicht ermittelbar]: ETH Zurich, 2021
Language:
English
Identifier:
Origination:
University thesis:
Dissertation, ETH Zurich, 2021
Footnote:
Description:
When a solid crystalline material forms, such as a nanoparticle, this process starts with a nucleation reaction, where a handful of atoms come together to form a cluster of atoms. Understanding and controlling this nucleation process and the subsequent particle growth, is essential in materials science. Though, it has been challenging to directly observe the nucleation process experimentally, due to the short space and time scales involved. In this thesis, we demonstrate observing the initial steps of sub-nm Pt clusters nucleating from individual Pt atoms dispersed in ionic liquid. We observe the nucleation at atomic scale by using scanning transmission electron microscopy to study Pt deposited into suspended nanofilms and carbon-supported nanodroplets of the ionic liquid 1-butyl-3-methyl imidazolium chloride. The nucleation is hard to induce and rarely observed in the suspended films due to the high energy barrier of homogeneous nucleation. In contrast, nucleation is easily induced in supported nanodroplets, as the Pt heterogeneously nucleates on the ionic liquid-carbon film interface. By using neural-network image denoising, we can minimise the electron beam dose required to interpret the structure of the nucleating clusters. The Pt atoms first nucleate into few-atom clusters, which then either redissolve, or coalesce and grow into cluster agglomerates or nanoparticles. Among the sub-nm clusters we often find close-packed crystalline structures, in particular face-centred cubic (fcc) based structures, such as cuboids and cuboctahedrons. These crystalline clusters can be stabilised above 300°C on a carbon surface without liquid. Though, in liquid they they appear only as brief metastable states during the nucleation reactions, as no long-term stability has been observed until the particles grew above 1 nm in size. Ultimately, our results show that the nucleation pathways of nanoparticles are not just determined by the local chemical environment, but are also influenced by the size and structure of the initially formed clusters.