We are interested in phase transformations in metallic nanoparticles. Using in-situ time-resolved synchrotron-based x-ray diffraction, we have investigated the structural properties of nanoscale catalysts as they undergo various heat treatments.
Example of the structural evolution of PtNiCo nanoparticles annealed at 700C.
We recently performed a thorough investigation of high-temperature sintering of ternary PtxNi100-x-yCox (x = 40-54%) nanoparticles. In the early stage of thermal treatment, the nanoparticles were found to undergo a two-step coalescence process similar to the behavior observed in AuPt nanoparticles. This process was explained by a model involving crystallization and subsequent dissolution of metastable structures. In the late stage of sintering, evidence was found for self-limited grain growth and L10 chemical ordering. Fitting of the experimental data with the model for grain growth with size-dependent impediment led to an activation-energy for mass transport that is consistent with other reports.
We have been also studied the structural transformations occurring during thermal treatment of nanoparticle mixtures. This is an original, alternative path to synthesize alloy nanoparticles with novel structures and enhanced catalytic activities. For example, Au-Cu nanoparticle mixtures were found to form alloyed nanocubes when annealed around 200°C, as shown in the figure. The nano-alloying process is particularly well illustrated by the Pd-Cu nanoparticle mixtues. In this material system, heat treatment results in the formation of an unusual ordered bcc-like structure, unique to the Pd-Cu alloys. Upon heat-treatment up to 700°C, the ordered PdCu phase undergoes a solid-phase transformation to a disordered fcc phase.