Synthesis of hierarchically organized α-Fe₂O₃ nanostructures for the photocatalytic degradation of methylene blue

The solvothermal synthesis of hierarchically organized hematite nanoparticles is reported. The nanoparticles with an average diameter of ≈ 55 nm consist of self-assembled small sub-particles with average sizes of ≈ 5 nm. They were synthesized by controlling the humidity and reaction temperature under solvothermal conditions using an especially designed set-up in a Teflon-lined stainless steel autoclave. Under optimized humidity conditions, the morphology of the nanoparticles changed from self-assembled poly-crystalline to monocrystalline nanoparticles by varying the temperature from 80 to 150 °C. The overall size of the nanoparticles and sub-particles’ domain remained the same, i.e., 55 nm and 5 nm for the product synthesized at 80 °C and 120 °C. Hematite nanoparticles prepared at 150 °C temperature consisted of single-domain nanoparticles with an average size of ≈ 60 nm. The size and crystallinity of these nanoparticles was monitored by transmission electron microscopy (TEM) and powder X-ray diffractometry (PXRD). The optical properties of the as-synthesized hematite nanoparticles were measured by UV-visible spectroscopy. Nanoparticles with an absorption in the visible range were tested for the photocatalytic degradation of organic pollutant using methylene blue as model compound. Hematite nanoparticles synthesized at lower temperatures, i.e., 80 °C, showed higher photocatalytic activities compared with nanomaterials synthesized at 120 °C and 150 °C.