Self-assembly behavior of hematite nanoparticles with controllable anisotropic morphology

In this work, complex structures such as long chain, "semi-flexible" chain, "threefold junction" and network were formed by self-assembly of colloidal hematite nanoparticles. Morphology of these colloidal nanoparticles used as building blocks transformed from truncated rhombohedra, hexagon to pseudo-hexagon by altering reaction time and surfactants. By further observation using HRTEM, these nanoparticles were confirmed to grow along the c-axis. It was found that the molecular structures of surfactants make great influence on the transformation of bonding modes between carboxyl and iron atom on the surface. Then crystal growth rate was changed. It led to two opposite growth trends along the c-axis. More interestingly, the chains formed by these colloidal nanoparticles were also assembled along the c-axis. Meanwhile, configuration diversity seemed related to the morphological anisotropy along the c-axis. It was believed that two main forces between the nanoparticles were responsible for the various configurations, magnetic dipole-dipole and exchange-coupling interaction. The morphological anisotropy was considered to play a key role in the coordination of the two interactions which led to different complex structures by self-assembly. Discussion was taken to explain the formation of these interesting configurations.