Synthesis of structure-controlled hematite nanoparticles by a surfactant-assisted hydrothermal method and property analysis

Abstract

Abstract The impact of the type and concentration of surfactant on the synthesis of iron oxide nanoparticles (IONPs) has been investigated. Three types of surfactant — anionic, cationic, and nonionic surfactants (sodium dodecyl sulfate, SDS; cetyltrimethylammonium bromide, CTAB; and Triton X-100, respectively) — have been selected to control the phase, shape, and size of the IONPs. The synthesis was carried out with varying concentrations at 2, 4, and 6 times the critical micelle concentration (CMC) of each surfactant in a hydrothermal system at 180\xa0°C. XRD and FE-SEM with the Microsoft Visio 2010 program have been applied for characterization and nanoparticle analysis. All synthesized IONPs, without and with the different surfactants, were characterized as spherical hematite with diameters in the range 15–205\xa0nm, depending on the surfactant type and concentration. Surfactant type and concentration influenced the nature of the ferrous hexahydrate intermediate through acid-base equilibria and the Le Châtelier principle. The magnetic properties of the hematite nanoparticles have been examined by means of a vibrating sample magnetometer (VSM). The highest magneticity of synthesized hematite occurred at the transition from a single monolithic structure to one with multiple domains at a diameter of approximately 70\xa0nm with a coercivity (Hc) of 225\xa0Oe. The smallest hematite particles were chosen to prepare water-based nanofluids using the surfactants as stabilizing agents. Colloidal stability of the nanofluids has been studied by sedimentation, turbidity, and ζ-potential analyses. Nanofluid thermal conductivity has been determined by transient heat-transfer experiments. A nanofluid prepared in 2 CMC SDS solution showed the highest stabilization and a thermal conductivity of 0.4787\xa0W/m.K.