Revealing the unusual grain growth of nanoparticles in calcination: oriented attachment in the solid state

Crystals usually grow by attachment of ions or atoms to seed crystals. The explorations two decades ago showed that oriented attachment, an aggregation-based crystal growth, can be an alternative process in nanocrystal solutions. However, oriented attachment is thought to be not operative in solids as the rotational alignment is not favored in the absence of a liquid medium. Here, we report the unusual grain growth of nickel nanoparticles during calcination. The nickel nanocrystals calcined at 600 °C were coarsened by nearly two folds, much larger than the coarsening amount at lower and higher temperatures, which suggests that directional growth based on oriented attachment wins out over the classical crystal growing mechanism at a certain temperature. Varied temperature transmission electron microscope (TEM) observation confirmed the oriented attachment behavior in the growth of nanocrystals. This finding reshapes our view on the mechanism of grain growth at the nanoscale and helps us to better design the synthesis and application of nanomaterials.