Towards 3-D spherical self-assembly by ternary surfactant combinations: The case of magnetite nanoparticles

Here we report on a facile one-step route to assemble magnetite nanoparticles into 3D-spherical aggregates with an average diameter of 100 nm. The self-assembly approach was achieved in a solvothermal reduction reaction with a ternary surfactant combination comprising trioctylphosphane oxide (TOPO), polyvinylpyrrolidone (PVP), and oleic acid as the "mortar". Transmission electron microscopy (TEM) investigations indicated that the spherical aggregates were composed of magnetite nanoparticles. The mean size of the individual magnetite particles was estimated to be 6 nm, calculated by the Scherrer equation with the powder X-ray diffraction (XRD) data. The formation mechanism of magnetite spheres can be attributed to the cooperation of the ternary surfactants described above. Magnetic measurements performed by a Quantum Design SQUID magnetometer on the nanoparticles exhibited superparamagnetic behavior above liquid-nitrogen temperatures. Fourier-transform infrared spectroscopy (FTIR), inductively coupled plasma-atomic emission spectrometer (ICP-AE) and X-ray photoelectron spectroscopy (XPS) were also used in the characterization of the assembled magnetite spheres.