2, Pittsburg State University, Pittsburg, Kansas, United States
Among various morphologies of nanomaterials, hollow spheres are of great interest because of their high ratio of surface to volume, large pore volume and low density, which could be exploited for applications in controlled encapsulation-release of drugs and medical diagnostic, energy storage and conversion, photocatalysis, chemical sensors, and photonic crystals. In the context of magnetism, magnetic hollow spheres can show unique physical properties compared to those of flat thin films and their solid counterparts of the same sizes, due to their confined hollow geometry and curved surfaces. It is known, that coercivity is dependent on domain-wall motion and the barrier to domain-wall propagation along a curved surface is larger than that of a flat surface. Due to growing application of nanoscale magnetic hollow spheres in biomedical end energy fields it remains important to understand the influence of growth parameters to prepare Fe3O4 with highly homogeneous features in terms of size and shape.
In this work, effect of hydrolyzing agents such as urea, ammonium bicarbonate (ABC), dodecylamine (DDA) on morphology, size and electrochemical activity of Fe3O4 nanospheres was investigated. For comparison, Fe3O4 nanospheres were also synthesized without hydrolyzing agent. The structural and morphological assessment of the synthesized Fe3O4 nanopowder was performed using x-ray diffraction, scanning electron microscopy and surface area analysis. The room temperature magnetic properties were studied via vibrating sample magnetometer. The scanning electron microscopy images showed nanospheres of Fe3O4 with a range of sizes (150-330 nm) which depend on hydrolyzing agents used. All the synthesized samples were crystalline in structure with distinct signature of magnetite phase. The surface area analysis indicated that these particles were mesoporous in nature. VSM measurement show that Fe3O4 prepared via hydrolyzing agent display high magnetization ~85 emu/g with average coercivity in the range of 150 Oe, Different hydrolyzing agents were observed to have minimum influence on the magnetic property of Fe3O4 hollow spheres. Electrochemical characteristics were investigated using cyclic voltammetry and galvanostatic measurements. Cyclic voltammetry measurements were performed in three different electrolytes viz. KOH, NaOH, and LiOH and observed that specific capacitance of the synthesized Fe3O4 depend on electrolyte used. Relatively high specific capacitance of 173.8 F/g was observed for Fe3O4 prepared using DDA in 3M KOH electrolyte. Fe3O4-DDA also showed excellent cyclic stability as well, retaining 107% of specific capacitance value at up to 5,000 cycles measured. The study clearly elucidates the effect of hydrolyzing agent on physical and morphological properties of Fe3O4. In addition, through electrochemical testing the study illustrates the choice of aqueous electrolyte in optimizing the electocapactive performance of Fe3O4 nanospheres.