Phase transformation of magnetite and goethite nanoparticles controlled by pH: Experimental and simulation study of cuboid magnetic nanoparticles prepared with NaOH

The phase transformation between magnetite and goethite nanoparticles (NPs) as a function of pH is investigated. This is done via molecular dynamics and by the co-precipitation method. The binding energy (BE) of cuboid and spherical shaped NPs are compared, and it is shown that cuboid shaped NPs are favored above spherical when NaOH is used to control the pH, since these morphologies are more stable. Furthermore, the ratio of excess and desorbed hydroxyls to free protons (i.e., hydrogen ions) is used to qualitatively study the role that pH plays in controlling this phase transformation. Neutral to acidic solutions lead to the formation of stable goethite cuboid NPs whereas basic solutions lead to the formation of magnetite NPs.