Citrate milling of oxides: from poly-dispersed micron scale to nearly mono-dispersed nanoscale

Abstract

Complex (multivalent/mixed valent) oxides involving two or more cations (e.g. ABO₃, AB₂O₄ and A₂B₂O₇) exhibit the most fascinating range of physical and chemical properties amongst the family of materials systems. There is growing interest in nanoscale forms of such oxides which emanates from the novel changes in their properties with size. To obtain nanomaterials with a high degree of crystallinity it is desirable to first make crystalline oxide powders by high temperature processing and then mill them down to nanometer size. In this paper we show that simple citric acid treatment of BiFeO₃ and Bi₂O₃ powders leads to the desired micron-scale to nanoscale transformation, yielding nearly monodispersed nanoparticles. Importantly, these are highly dispersible and stable in water. By performing similar experiments on Fe₃O₄ and Fe₂O₃ we have elucidated the possible mechanism, which hinges on valence-controlled dissolution and ripening phenomena.