TEM characterization of laser-generated colloidal high-entropy alloy nanoparticles

Abstract number
European Microscopy Congress 2020
Corresponding Email
[email protected]
PSA.5 - Nanoparticles & Catalysts
Marius Kamp (1), Viola Duppel (2), Friedrich Waag (4), Yao Li (4), Anna Rosa Ziefuß (4), Erwan Bertin (3), Galina Marzun (4), Stephan Barcikowski (4), Bilal Gökce (4), Lorenz Kienle (1)
1. Kiel University
2. Max Planck Institute for Solid State Research
3. St Francis Xavier University
4. University of Duisburg-Essen

HEA, nanoparticles, High-entropy alloys, TEM, laser ablation in liquid

Abstract text

A novel synthesis approach, called laser ablation in liquid, opens the field for a unique nanoparticulate material system to a wide range of applications. TEM characterization underlines the kinetically-controlled synthesis of high-entropy alloy nanoparticles by laser ablation in liquid. Defect-rich crystalline phases with complex metastable ultrastructures, e.g., core shell NPs are observed [1].

The unique mechanical, electromagnetic, and electrochemical properties of bulk high-entropy alloys are frequently studied [2]–[4]. The alloys are stabilized by a high-entropy effect, which is induced by a large number (≥5) of alloying elements. Due to limitations of state of the art synthesis methods, nanoparticulate high-entropy alloys are rarely reported, although the high surface to volume ratio of nanoparticles offers applications in additive manufacturing, sensors, and energy storage as well as enhanced catalytic properties [5], [6]. Challenging preconditions like the ligand-free surface and large scale productivity, limit the synthesis methods [7]. Laser ablation in liquid is a suitable one-step synthesis method of colloidal high entropy nanoparticles. The demobilization on various substrates offers excellent flexibility.

In laser ablation in liquid, an ultrashort-pulsed laser is focused on a bulk high-entropy alloy target. Under the formation of a cavitation bubble, nanoparticles are generated and quenched rapidly in the surrounding liquid. The establishment of a unique synthesis method needs to be supported by sample characterization with (scanning) transmission electron microscopy applying various methods, like high-resolution phase-contrast imaging, Z-contrast imaging, and selected area electron diffraction (Tecnai F30 STwin G2 300 kV). Nanoprobe elemental mapping is performed with a Si(Li) detector (EDAX system). The nanoparticles are generated in ethanol with a 10 ps pulsed Nd:YAG laser (Atlantic, Ekspla, Vilnius, Lithuania) at a wavelength of 1064 nm, a repetition rate of 100 kHz, and a power of 8.8 W and are dispersed on carbon-coated copper grids for sample preparation. 

Laser generated high-entropy alloy nanoparticles show two major size fractions. The highest frequency is observed for NPs <10 nm in diameter. Electron diffraction and high-resolution TEM investigations show a single crystal fcc lattice with a lattice constant of a = 3.62 Å. An equiatomic and homogeneous chemical composition CoCrFeMnNi is detected by STEM-EDX, which underlines the formation of high-entropy alloy nanoparticles. The heterogeneous catalysis of the alkaline oxidation reaction is demonstrated and dominated by NPs of this small size fraction. Interestingly, the stability of the nanoparticles seems to be size-dependent. A size fraction with diameters >10 nm, shows segregated ultrastructures with high defect density. Elemental mapping illustrates a Mn- and Cr-rich oxide spinel formation that results in partial core shell nanoparticles. Oxygen may be generated by the splitting of the solution (Ethanol) during synthesis. The core represents an fcc crystal structure with a high defect density of Σ3-twins and domain sizes in the range of a few atomic layers [1]. In further experiments, we observed that variations in the liquid medium and the stoichiometry of Mn seem to control the size distribution, degree of oxide spinel formation, and NPs ultrastructures. 

In conclusion, we established laser ablation in liquid as a new kinetically-controlled synthesis method for high-entropy alloy nanoparticles with enhanced heterogeneous catalytic activity. The fcc single crystal structure is shown by XRD and SAED characterization. Furthermore, segregated ultrastructures with high defect concentrations and oxide spinel phases are analyzed for respective NPs size fractions. The results allow an application of the synthesis method to other unique material systems like high-entropy oxides, which are expected to be less sensitive to oxidation.


[1]        F. Waag et al.,  RSC Adv. 9 (2019), p. 18547.

[2]        D. B. Miracle and O. N. Senkov, Acta Mater. 122 (2017), p. 448.

[3]        A. Sarkar et al., Adv. Mater. 31 (2019), p. 1806236.

[4]        Z. Li et al., Nature, 534 (2016), p.227.

[5]        A. Sarkar et al., Nat. Commun. 9 (2018), p. 3400.

[6]        P. Xie et al., Nat. Commun., 10 (2019), p. 1.

[7]       Y. Yao et al., Science, 359 (2018), p. 1489.

[8]         Funding by the German Research Foundation (Project KI 1263/15-1) is acknowledged.