Nature of Alkali Used in the Co-Precipitation of Superparamagnetic Nanoparticles on Their Thermal Stability and the Properties of Ferrofluids

We investigate the nature of alkali used in the synthesis of Fe3O4 nanoparticles on the phase transformation temperatures of nanoparticles and the properties of ferrofluids produced with those nanoparticles. We tailored nanoparticles of average size similar to 9 nm by co-precipitation approach using two different alkaline media, sodium hydroxide (NaOH) and ammonium hydroxide. The produced nanoparticles are functionalized with a fatty acid for production of ferrofluid. We investigate the phase transformation of these particles using in-situ high temperature X-ray diffraction (HTXRD). Thermogravimetry cum differential scanning calorimetry is used to carry out weight loss and heat flow measurements simultaneously in the temperature range 30-1000 degrees C under Argon and air atmosphere. The amount of sodium ions present in the samples was measured using a double beam atomic absorption spectrometer (AAS) and the magnetization measurements are carried out using a vibrating sample magnetometer. Fourier transform infrared spectrometer (FTIR) is used to confirm the fatty acid coating over magnetite nanoparticles. Size and distribution of the nanoparticles are measured using Zeta nanosizer. Under vacuum annealing, Fe3O4 nanoparticles prepared using ammonium hydroxide and NaOH are found to decompose into gamma-Fe2O3/FeO in the temperature range of 700-1000 degrees C and 600-1000 degrees C respectively. Interestingly, under air annealing, the gamma-Fe2O3 to alpha-Fe2O3 phase transition temperature is found to enhance by 100 degrees C for NaOH based sample, possibly due to increased activation energy due to trapped sodium ions. These results show that the nature of alkali used in the synthesis of nanoparticles plays an important role in the phase stability of transition iron oxide. Fatty acid molecules form monolayers on both these particles (ammonia and sodium alkali based) and are found to be equally good for the production of ferrofluids for various applications such as response stimuli sensors, advanced coolants for electronics cooling and biomedical applications.