Colloidal properties of water dispersible magnetite nanoparticles by photon correlation spectroscopy

Abstract

We report the development of ultra-stable aqueous colloidal dispersion of magnetite nanocrystals produced by aqueous ‘coprecipitation method’. Magnetic nanofluids were prepared by dispersing the Fe₃O₄ NPs in water medium in the presence of tetramethylammonium hydroxide (TMAH). The synthesized nanocrystals were characterized by XRD, TG-DTA, XPS and TEM for evaluating the phase, crystal structure and morphology. FTIR spectroscopy was used to shed light onto the nature of the interactions between TMAH and Fe₃O₄ NPs. The TMAH peptized nanofluids was clear translucent colloidal dispersion found to contain spheroidal nanoparticles of average size 13 nm with very narrow size distribution similar to TEM size. High-resolution microscopy indicated that all the NPs are indeed single crystals with truncated octahedral shape. Lattice fringes belonging to predominant (111), (220) and (311) planes could be identified. The M_s values estimated are 64.68 and 57.92 emu g⁻¹ at room temperature for NPs before and after peptization respectively and they are superparamagnetic. The key colloidal properties such as charge, hydrodynamic size, photon counts, dispersion stability and surface chemistry have been analyzed and compared with a dispersion of aqueous precipitated magnetite. The TMA suspensions are stable over a year without any loss due to precipitation. Photon scattering experiments have indicated the presence of very small NP clusters of 28 nm in aqueous suspensions. The lower extent of agglomeration in TMA promotes the one-shell clusters of primary nanoparticles, a fact which can forecast the stability of the ferrofluid. The change in surface charge of the magnetic fluid from −44 to +49 mV while varying the pH indicated the PZC at pH 5.98. The dynamic processes were investigated during the photon scattering experiment against time, temperature and concentration. The stability of the ferrofluid against time, temperature and concentration indicates the great potentials in biotechnology, selective catalysis and other industrial applications.