Engineering chemical pathways for phase-tuned nanocrystalline iron oxides in microwave-assisted solvothermal synthesis

Abstract

In microwave-assisted solvothermal synthesis (MASS) of nanocrystalline iron oxides, we have analysed chemical pathways to explore the role played by the temperature and heating rate in deciding the phases and crystallographic orientations. We have achieved control over synthesis parameters by using tailored mixtures of solvents. These kinetically controlled pathways are engineered by suitably mixing high microwave dielectric loss solvents – ethanol and water, with 1-decanol. We propose two new pathways, and combinations of different pathways, to explain the observed phases in reactions between metal acetylacetonates and these solvents, using Fe(acac)₃ as a representative. The proposed pathways also explain the changes in the local geometry from octahedral to square pyramidal, enroute to tetrahedral in oxides. Mixing solvents to alter tan δ is shown to provide an energy-efficient way to control the specific geometry of grown phases. The results demonstrate new ways of analysing and tuning the synthesis of complex oxides in pure and mixed solvents, under microwave irradiation.