Tailoring the crystalline structure of multinary alloy nanoparticles

Tailoring the crystalline structure of multinary alloy nanoparticles  

Multinary transition metal nanoparticles are promising candidates for energy applications. Our research is based on the synthesis and atomic-scale characterization of such multinary systems. The nanoparticles are prepared by sputtering of elements into ionic liquids. Depending on the synthesis conditions, they grow either in an amorphous or crystalline state. The amorphous particles can be transformed to different crystalline phases via electron beam bombardment or post annealing.

The multinary nanoparticles are synthesized by our project partners using direct current sputtering and high-power impulse magnetron sputtering from five elemental targets (Cr-Mn-Fe-Co-Ni) into ionic liquids. Due to their low vapor pressure, ionic liquids (salts with a melting point < 100°C) can be used as liquid substrates in sputter processes. Furthermore, the ionic liquid itself can act as an electronic as well as a steric stabilizer preventing particle growth and particle aggregation leading to the formation of extremely small nanoparticles and with a strong effect on the structure of the formed nanoparticles.

The main focus of our work is set on the investigation of multinary nanoparticles by using various microscopy techniques, such as Cs corrected high-resolution transmission electron microscopy (HRTEM), (scanning) transmission electron microscopy ((S)TEM) and energy dispersive X-ray spectroscopy (EDS). With these techniques the size, shape, crystallinity, defects and chemical composition of the nanoparticles are deduced.

Direct current sputtering technique leads to amorphous multinary nanoparticles as observed by HRTEM. Crystalline nanoparticles can be achieved by in situ TEM experiments using illumination with 300 keV electrons, ex situ annealing in vacuum at 100°C during 10h and high-power impulse magnetron sputtering process. A body-centered cubic structure is generated during in situ electron beam illumination and high-power impulse magnetron sputtering technique while longer ex situ annealing leads to a face-centered cubic structure. Finally, EDS elemental mapping of multinary nanoparticles provides an effective way to study the chemical composition and the distribution of each element (Cr-Mn-Co-Fe-Ni).

Cs corrected high-resolution TEM images of MNPs after their (a) in-situ and (b) ex-situ crystallization or synthesis via (c) HiPIMS (d) Cs corrected high-resolution scanning TEM image of MNPs after the ex-situ crystallization.

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