Optical properties of hydrothermally synthesized hematite particulate pigments

Iron oxide hematite particles with various shapes (platelet, polyhedron, pseudocube, and peanut-like) have been synthesized by hydrothermal treatment of a Fe(OH)xOy precursor under various conditions. The size and shape of hematite particles can be adjusted by carefully controlling the processing parameters such as holding time, temperature, and adsorption ions present in the system. The nearly monosized α-Fe2O3 platelets possess face diameters of approximately 3 μm and a thickness of 0.5 μm under a scanning electron microscope. The apparent color of the particles changes as particle size and shape varies. Munsell color notation was employed to compare the color of hematite particles with various sizes and shapes. Diffuse reflectance spectra show that a "red-shift" of 40 nm is observed in platelet, pseudocube, and peanut-like particles compared with conventional particles. The band at 850 nm for the 6A1→4T1 transition was split in the pseudocubic and peanut-like particles. Raman spectra of the hematite particles also reveal that the vibrational modes of α-Fe2O3 particles diminish as particle size decreases, and dependence of vibrational band intensity on frequency is also observed. The spectral profiles demonstrate significant difference as excitation radiation lines changes from blue (457 nm) to red (647 nm). Possible mechanisms responsible for the optical properties of hematite particles are postulated based on the findings of the experiments.