Evolution of Interfaces in Multi-Principal Element Alloy Nanoparticles

Multi-principal element alloy nanoparticles (NPs), composed of multiple elements arranged in an ordered intermetallic crystal structure, pose unique properties for a variety of applications. They can be used as catalysts, where their unique composition and structure can improve catalytic activity and selectivity. They also hold promise in energy storage and conversion, sensing technologies, and biomedical applications, among others. In this work, we seek to understand the structure and stability of grain boundaries (GBs) formed between various medium entropy alloy NPs, the mechanisms of metals diffusion through them, and its influence on NPs coalescence. The density functional theory (DFT) calculations supported by the high-resolution transmission electron microscopy (HRTEM) measurements are employed to understand the GB atomic structures, their evolution, and their chemo-mechanical properties. The DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package) code with the plane wave basis sets and the projector augmented wave (PAW) pseudopotentials in the framework of the Perdew-Burke-Ernzerhof generalized-gradient approximation (GGA). For GBs structure creation, the Atomistic Tool Kit (ATK) software is used. The crystalline phase structure and GBs evolution of nanoparticles and their elemental composition and distribution are characterized by annular dark field scanning transmission electron microcopy (ADF-STEM) images with 512x512 pixels scanning resolution and energy-dispersive spectroscopy (EDS) analysis performed at 8c probe size on a JEOL ARM200CF scanning electron microscope operating at 200 kV.<br/>Different size medium entropy alloy NPs containing various compositions of Pt, Cu, Ir and Ni elements, have been obtained through electron beam-induced co-reduction of mixed metal salts solution inside the liquid-cell TEM holder. The ADF-STEM images of the formed colloidal NPs were obtained at the end of the process on dry sample after liquid was removed. It was observed that there is an aggregate of multiple crystalline medium entropy alloy NPs of different sizes and orientations attached by amorphous grain boundaries and loosely bonded atoms at the interface. The colloidal medium entropy alloy NPs have <i>fcc</i> structure with lattice constant of 3.89 Å. STEM-EDS analysis indicates a homogenous distribution of Pt, Cu, Ir, and Ni elements in NPs with atomic composition of (Pt_0.46Cu_0.3Ir_0.16Ni_0.06). The HRTEM measurements revealed the diverse GB structures consisting of (100), (110), (121), etc., surface orientations of the above-mentioned NPs composition. Based on the HRTEM results, the corresponding computational slabs have been built and optimized using ATK and VASP. Pt, Cu, Ir and Ni adatom diffusion inside grain and GB, after the optimization of all structures, were calculated. For each GB, several adatom positions with multi-atom bonding were identified. The activation energy of metal diffusion varies significantly depending on the slab’s structure and the number of adatoms. A non-symmetric metal transition state for all considered structures and atoms, which indicates a complex multi-atom hopping mechanism, was identified. In addition, the DFT calculations are used to obtain electron density and the density of states for corresponding GB structures. Based upon the electron density profile the stability of analyzed interface with and without metallic adatoms is quantified.<br/>Our combined experimental and modeling study provides valuable understandings into developing mitigation strategies towards a better understanding of medium and high entropy alloys for a variety of applications.<br/><br/>Authors acknowledge the financial support from the National Science Foundation Award DMR-2311104.