Fatty acid as structure directing agent for controlled secondary growth of CoFe2O4 nanoparticles to achieve mesoscale assemblies: A facile approach for developing hierarchical structures

Abstract Mesoscale hierarchical assemblies have emerged out as a new class of structures between fine dimension nanoparticles and bulk structures, having distinctly different physical properties from either side. Controlling the self-assembly process of primary nanoparticles and subsequent secondary growth mechanism is the key aspect for achieving such ordered structures. In this work, we introduce a new insight on achieving hierarchical assemblies of CoFe 2 O 4 nanoparticles based on the temporal stability of the primary nanoparticles, where, the growth and stability of the primary particles are controlled by using oleic acid. It is found that the developed particles, at a critical concentration of oleic acid, prefer a secondary growth process, rather than promoting their individual growth. Domination of the attractive hydrophobic interaction over steric repulsion among the primary particles at this critical concentration of oleic acid is found to be the key factor for the initial aggregation of the primary particles, which eventually leads to the formation of spherical hierarchical assemblies via oriented attachment. It is also realized that the extremely well or poor stability conditions of the primary particles do not allow this secondary growth process. Estimated values of Co 2+ distribution factor show that the cation distribution factor of CoFe 2 O 4 system is not affected by the nature of dominant growth processes, when these are controlled. Interestingly, magnetic measurements reflect the stronger interparticle interaction in the hierarchical system and high magnetic moment values at low magnetic field.