Silica-free hydrothermal synthesis of ε-Fe2O3 nanoparticles and their oriented attachment to nanoflakes with unique magnetism evolution

Abstract

It is a general consensus that ε-polymorph of Fe2O3 is not easy to synthesize in the form of bare nano-sized objects owing to its significant thermal instability. Thus, a supporting medium (e.g., silica matrix in most cases) is always necessary for its successful high-temperature synthesis at the sacrifice of its yield. In this study, optimized amounts of protons and nitrate ions were employed in a hydrothermal synthetic route to facilitate the growth of pure ε-Fe2O3 nanoparticles. The absence of silica matrix ensured a high yield of ε-Fe2O3 product, and moderate reaction temperature of mere 200 °C was conducive to its scalable energy-efficient production. These ε-Fe2O3 nanoparticles exhibited large room-temperature coercivity of ca. 16.3 kOe, which was found to be degenerated to 36 Oe as prolonging the reaction time due to the phase and structure evolution. The preformed ε-Fe2O3 nanoparticles and post-formed α-Fe2O3 phase would co-align with each other to form nanoflakes for long-time reaction, being of a character of oriented attachment. The concomitant distorted lattice spacing and disordered atomic configuration not only contributed to the degeneration of ferromagnetism of ε-Fe2O3 phase, but also induced unique spin glass behavior.