Effect of phosphate and oleic acid capping on structure, magnetic properties and thermal stability of iron oxide nanoparticles

The thermal stability and magnetic properties of iron oxide nanomaterials under different environmental conditions are important for technological applications. Herein, we report the influence of dynamic capping of iron oxide nanoparticles with phosphate and oleic acid, on their structure, magnetic properties and thermal stability and compare the results with that of uncoated nanoparticles. The composition, structure, morphology, quality of capping, thermal stability and magnetic properties of prepared magnetite nanoparticles are evaluated by using X-Ray Diffractometer (XRD), Small angle X-ray scattering (SAXS), High resolution transmission electron microscopy (HRTEM), Energy dispersive X-ray (EDX) analysis, Fourier transform infrared (FTIR) spectrometer, Thermo gravimetric analyzer (TGA), Differential scanning calorimetry (DSC) and Vibrating Sample Magnetometer (VSM). Our results suggest that the rapid adsorption of phosphate ions onto specific crystal planes of nucleated magnetite enabled the growth of magnetite core into nearly cubic shape while the oleic acid capping enabled a homogeneous growth of magnetite core along all the crystallographic directions, enabling the formation of nearly spherical particles with narrow size distribution. On the contrary, the phosphate adsorbs as multilayer islands on nanoparticles, leading to a broader size distribution, identical to the uncoated particles. The HRTEM analysis showed dark areas with asymmetric contrast in uncoated nanoparticles, which was not observed in phosphate and oleic acid capped magnetite. The FTIR study on phosphate coated magnetite particles confirms that the surface capping is a mixture of mono and non-protonated monodentate phosphate complexes. The DSC studies confirm a 100 degrees C enhancement in the magnetic phase thermal stability in phosphate-coated particles, as compared to uncoated and oleic acid coated magnetite. The temperature dependant magnetization results showed a lower blocking temperature for phosphate capped nanoparticles due to the denser multilayer islands of phosphate over magnetite as compared to a uniform close packed monolayer coverage of oleic acid. These results are useful for tailoring stable superparamagnetic nanoparticles for technological applications. (C) 2016 Elsevier B.V. All rights reserved.