Temperature-induced restructuring of self-assembled PtPd nanoparticle superlattices

PtPd nanoparticle superlattices were prepared at room temperature and investigated for their restructuring at elevated temperatures. The PtPd nanoparticles were spherical with a diameter of 3.5 +/- 0.3 nm and were capped with surfactants of oleic acids and oleylamine. Images of the nanoparticle superlattices were taken using a transmission electron microscope. Analysis of the images with fast Fourier transform (FFT) revealed whether the superlattices were crystalline or amorphous. It was found that the critical temperature that induced a phase transition of PtPd superlattices from a crystalline state to an amorphous state falls in the range of 100-110 degrees C. Meanwhile, heat-induced restructuring transformed less-ordered superlattices into better-ordered ones, similar to annealing. It was especially pronounced for 3D superlattices, which also showed well-defined grain boundaries after restructuring.