Spectroscopic and electron microscopic investigation of iron oxides formed in a highly alkaline medium in the presence of rhodium ions

The effect of the presence of rhodium ions on the formation of iron oxides in a highly alkaline precipitation system was investigated using X-ray powder diffraction (XRD), 57Fe Mössbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular α-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as reference material. Characterization of α-FeOOH samples formed in the presence of rhodium ions showed a somewhat smaller mean crystallite size, increased unit-cell dimensions, a reduced average hyperfine magnetic field and a slight shift in the position of IR absorption bands in comparison with the reference α-FeOOH sample. By additional heating of the precipitation system, α-FeOOH precipitated in the presence of rhodium ions transformed to α-Fe2O3 crystals in the form of hexagonal bipyramids via a dissolution–recrystallization process. Metallic rhodium nanoparticles were formed simultaneously by the reduction of Rh3+ ions in the presence of the products of TMAH thermal decomposition (trimethylamine and methanol). These rhodium nanoparticles acted as a catalyst for the reductive dissolution of α-Fe2O3 particles and the formation of Fe3O4 crystals in the form of octahedrons.