Uniform spheroidal nanoassemblies of magnetite using Tween surfactants: influence of surfactant structure on the morphology and electrochemical performance

Abstract

We report solvothermal synthesis of uniform spheroidal nanoassemblies of magnetite (Fe₃O₄) in ethylene glycol medium by using polyethoxylated surfactants viz. Tween 80 and Tween 20, having oleyl (18 carbon) and lauryl (12 carbon) hydrophobic tails, respectively as structure directing agents. Uniform nanoassemblies could be obtained only in a narrow range of water content in the reaction medium (between 20 and 50 ppt). Outside this window, heterogeneous assemblies were obtained. Within the optimized water content regime, varying the water content allowed easy modulation of assembly dimensions between 80 and 200 nm. Under similar synthetic conditions, Tween 80 yielded smaller nanoassemblies having larger crystalline domain sizes when compared to Tween 20. Dynamic light scattering (DLS) confirmed the dimensional uniformity and stability of these assemblies in ethanol dispersions. High resolution scanning electron microscopy (HRSEM) showed the presence of smaller nanoparticles within each assembly while transmission electron microscopy (TEM) revealed the densely packed internal architecture of these assemblies. Powder X-ray diffraction (PXRD) confirmed that the nanoassemblies were exclusively composed of the magnetite phase. The size of crystalline domains present within the nanoassemblies (as determined by the Scherrer equation) could be varied from ∼10 to 25 nm based on synthetic conditions employed. Formation of small nanoparticles precludes the assembly formation that requires ≥8 h to mature. Magnetic measurements revealed very high saturation magnetization values even at room temperature, as well as hysteresis behavior for all the nanoassemblies. Electrochemical measurements showed that Tween 20 derived samples had the highest specific capacitance of 75 F g⁻¹ and their performance was also more stable in cyclic tests compared to Tween 80 derived materials. This is attributed to the greater physical stability of Tween 20 derived nanoassemblies over Tween 80 derived assemblies, as indicated by SEM studies.