KANGAROO-TYPE Ni-Cr₂O₃ NANOPARTICLES OBTAINED BY SOLID STATE DEWETTING OF Ni-Cr MULTILAYERS

Thin metal films deposited on ceramic substrates tend to agglomerate (dewet) upon thermal annealing due to their drive to decrease the total surface and interface energy of the system. The final result of this process is an array of isolated faceted metal particles. This process can be disadvantageous in terms of thermal stability of thin metal films, but on the other hand it can be employed for the synthesis of catalytic nanoparticles, i.e. for the catalytic growth of carbon nanotubes.

Little is known about dewetting of thin films of multicomponent alloys due to the intrinsic complexity of the system. Here we studied the dewetting behavior of multilayers of Nickel and Chromium oxide deposited on sapphire substrate. The multilayers were heat treated at elevated temperatures and their microstructure was characterized with the aid of SE, BSE and EDS detectors in the HRSEM. We observed a phase separation and a unique morphology of the nano-particles formed at the late stages of dewetting. These particles consisted of a main single crystalline Ni particle residing upon a Cr₂O₃ tail embedded between the Ni particle and the sapphire substrate and protruding outwards. TEM cross section samples of these particles were prepared by the FIB, and a careful high-resolution examination with the HAADF, EDS and EELS detectors revealed a surprising morphology of Cr₂O₃ particle with a larger Ni particle hanging over it. Therefore, the name of “Kangaroo-type” nanoparticles was given to these configurations (i.e. Ni being a “mother” Kangaroo, and Cr₂O₃ being its “baby”). The orientation relationships between the Ni and Cr₂O₃ particles was further characterized, both with the TEM and the XRD methods, and a Cr-Al spinel was also observed at the interface with sapphire.

We proposed a model explaining our experimental results, which is based on the analysis of the surface and interfacial energies of various configurations of composite nanoparticles.