Abstract
Bimetallic nanoparticles are often superior candidates for a wide range of technological and biomedical applications owing to their enhanced catalytic, optical, and magnetic properties, which are often better than their monometallic counterparts. Most of their properties strongly depend on their chemical composition, crystallographic structure, and phase distribution. However, little is known of how their crystal structure, on the nanoscale, transforms over time at elevated temperatures, even though this knowledge is highly relevant in case nanoparticles are used in, e.g., high-temperature catalysis. Au-Fe is a promising bimetallic system where the low-cost and magnetic Fe is combined with catalytically active and plasmonic Au. Here, we report on the in situ temporal evolution of the crystalline ordering in Au-Fe nanoparticles, obtained from a modern laser ablation in liquids synthesis. Our in-depth analysis, complemented by dedicated atomistic simulations, includes a detailed structural characterization by X-ray diffraction and transmission electron microscopy as well as atom probe tomography to reveal elemental distributions down to a single atom resolution. We show that the Au-Fe nanoparticles initially exhibit highly complex internal nested nanostructures with a wide range of compositions, phase distributions, and size-depended microstrains. The elevated temperature induces a diffusion-controlled recrystallization and phase merging, resulting in the formation of a single face-centered-cubic ultrastructure in contact with a body-centered cubic phase, which demonstrates the metastability of these structures. Uncovering these unique nanostructures with nested features could be highly attractive from a fundamental viewpoint as they could give further insights into the nanoparticle formation mechanism under non-equilibrium conditions. Furthermore, the in situ evaluation of the crystal structure changes upon heating is potentially relevant for high-temperature process utilization of bimetallic nanoparticles, e.g., during catalysis.
Highlights
Heterogeneous catalysis is one of the fields where bimetallic nanoparticles (NPs) have attracted much attention due to their enhanced physicochemical properties compared to their monometallic counterparts [1,2,3,4]
Special attention was paid to individual shoulder reflections that emerged due to multiple fcc phases, which www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research originated from different local compositions and crystallite sizes inside Au–Fe NPs
The existence of multiple fcc phases might indicate the presence of complex structures inside Au–Fe NPs produced by pulsed laser ablation in liquid (PLAL)
Summary
Heterogeneous catalysis is one of the fields where bimetallic nanoparticles (NPs) have attracted much attention due to their enhanced physicochemical properties compared to their monometallic counterparts [1,2,3,4]. Such thermal treatments will lead to enhanced atomic mobility in and between the metallic constituents by overcoming the diffusion activation energy barrier [7], thereby influencing the resulting properties This makes it highly interesting to investigate the in situ temporal evolution of the structure, composition, and phase inhomogeneities of the bimetallic NPs at catalytically relevant temperature regimes to further fine-tune their performance. Since the physicochemical properties of Au–Fe NPs potentially depend on the inhomogeneous phase distribution [24, 48, 49], a detailed analysis of such irregularities at the near-atomic scale and their temporal evolution is highly relevant for fundamental studies of NP formation far from room temperature (RT) thermodynamic equilibrium This kind of metastable structural complexity may play a significant role in their high-temperature catalytic performance [10, 11, 24, 50, 51]. Monitoring structural evolution, recrystallization, and relaxation enables the detection of the temperature regions, where such inhomogeneous formations (nests) exist in Au–Fe NPs before the thermally induced transformations set in
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