Designing Material Properties by Controlling Impurity Content in Structural Defects
We study the impurity incorporation mechanisms in metallic nano-aerogels is expected to identify routes to the targeted design of specimens with desired concentrations of impurities.
Controlling impurities in solid materials is a key to designing materials’ properties, which puts doping at the core of modern materials technology. Increasing dopant concentration and reducing unwanted impurities in synthesized materials will enable targeted design and optimization of device performance. Establishing reliable design routes for materials with specified impurities content requires diverse information ranging from atomistic insights to experimental validations. For this reason, several groups and different departments at the MPIE joined forces to identify mechanisms of impurities incorporation in solid materials by combining state-of-the-art experimental and theoretical approaches.
Developments related to colloidal nanoparticles during the last 150 years enabled materials scientists to synthesize nano-crystals, which are promising candidates for catalysis. Due to its excellent reducing properties, NaBH4 is commonly used as a reducing agent for forming a metallic nano-aerogel (MNA). These MNAs are often perceived as being purely metallic and free of any impurity. Our direct observation of the incorporation of alkali atoms (i.e., Na, K) at the grain boundaries (GB) in Pd-MNAs contradicts this perseption. To rationalize our observations, we carried out a thermodynamic concentration analysis based on state-of-the-art density-functional theory calculations and taking the experimental conditions into account. Through a direct comparison of theoretically calculated and experimentally observed concentrations of alkali atoms in various adsorption places on the Pd surface, in the Pd bulk, or in Pd GBs, we demonstrate that the comparatively large K atoms are kinetically trapped at GBs. The smaller Na atoms achieve, in contrast, thermodynamic equilibrium during the coalescence of two nanoparticles .
Given that our aforementioned work showed that MNAs synthesized using NaBH4 can no longer be considered pristine metallic nanostructures, we also measured the content of boron (B) incorporated in the Pd-MNAs from the NaBH4 reductant and found it non-negligible . The B was found in the sub-surface regions of the MNAs far from the surface. Based on ab-initio computed concentrations of B species at surfaces, in the sub-surface layers, and in the bulk, and taking into account the conditions during synthesis, we currently study B incorporation mechanisms. In this ongoing project we collaborate with different groups, aiming to provide direct insights into the question how controlling B impurities in Pd nanoparticles affects their catalytic performance for the hydrogen oxidation reaction, for which Pd nanoparticles are considered promising candidates for Pt-free catalysts.
Both projects are carried out in collaboration with the Atom Probe Tomography and the Nanoanalytics and Interfaces groups at MPIE. In addition we collaborate with the groups of Prof. O. Kasian (MPIE & HZB) and Prof. Hyun-Joo Lee (KAIST) on the B/Pd catalysis oriented project.