Solvothermal synthesis of hematite (α-Fe2O3) nanoparticles: Influence of surfactants on morphology, magnetic anisotropy, MRI relaxivity and biocompatibility

Abstract

Recent advancements in synthesis of hematite (α-Fe₂O₃) nanocrystals have enabled precise control of particle size and shape, thereby enhancing their use in catalysis, photonics, magnetic devices, electronics, medical diagnostics, magneto-mechanical actuators, solar cells and sensors. This study emphasizes the role of surfactants in the solvothermal synthesis of α-Fe₂O₃ nanoparticles, uncovering a novel connection between particle morphology and functional properties. By precisely adjusting surfactant concentrations (sodium hydroxide and acetic acid), the hematite particle morphology transitions from irregular shapes to uniform, plate-like structures. This transformation significantly enhances magnetic anisotropy, resulting in high coercivity (H_C = 1725 Oe, above the Morin transition T_M = 263 K) in the plate-like hematite nanoparticles, whereas irregularly shaped hematite nanoparticles exhibit coercivities of 235 Oe and 256 Oe (above the Morin transition T_M = 251 and T_M = 239 K, respectively). Furthermore, the study establishes a clear relationship between hematite nanoparticle shape and MRI relaxivity. The MRI results indicated that hematite nanoplates had superior transverse relaxivity rates (r₂ = 8.68 mM⁻¹ s⁻¹) compared to the irregular hematite nanoparticles (r₂ = 5.28 mM⁻¹ s⁻¹). Conversely, the irregular hematite nanoparticles displayed better longitudinal relaxivity (r₁ = 0.304 mM⁻¹ s⁻¹) than the hematite nanoplates (r₁ = 0.041 mM⁻¹ s⁻¹). Furthermore, the hematite nanoparticles demonstrated low cytotoxicity, indicating they are safe for cells. These findings suggest that controlling iron oxide nanoparticle morphology is crucial for enhancing their applications in magnetic materials and in biomedicine.