In Situ Observation of Bimetallic Alloy Nanoparticle Formation and Growth Using High-Temperature XRD

Bimetallic alloy nanoparticles exhibit a complex, (for the most part) poorly understood, crystallographic phase behavior, rarely following their macroscopic counterparts. We have studied formation kinetics, time scales of individual processes, compositional changes, and particle growth rates of Pt bimetallic alloy nanoparticles. We chose the Pt-Cu system, because of its technological importance as a precursor for core shell nanoparticle electrocatalysts. We provide correlation of annealing control parameters, such as heating rate, temperature, and time, with microscopic alloy structure, composition, and particle size. We have clarified the roles of annealing temperature and time in the alloy formation process and traced out entire Vegard-type structure composition relationships over a wide temperature range. We have found that, during heating ramps, the annealing temperature essentially controls the Cu content of the resulting disordered Pt-Cu lattices. Increasing annealing times, in contrast, leads primarily to particle growth. Phase ordering occurs only during cooling. Our insight offers practical synthetic guidelines toward single-phase ordered and disordered PtCu3 alloy nanoparticles with optimized particle dispersion.