Continuous aspect-ratio tuning and fine shape control of monodisperse α-Fe2O3 nanocrystals by a programmed microwave-hydrothermal method

A rapid and economical route based on an efficient microwave-hydrothermal process has been developed to synthesize monodisperse alpha-Fe2O3 nanocrystals with continuous aspect-ratio tuning and fine shape control, which takes advantage of microwave irradiation and hydrothermal effects. This method easily programs the experimental conditions (e.g., temperature and time) and significantly shortens the synthesis time to minutes. It allows the creation of numerous recipes for optimizing and scaling up production. The effects of experimental conditions including reaction temperature and reactant concentration on the morphology of alpha-Fe2O3 have been investigated systematically. Results reveal that the initial molar ratio of Fe3+ to PO43- plays a crucial role in the final morphology of the alpha-Fe2O3 products. Several morphologies, which include ellipsoids/spindles with aspect ratios that range from 1.1 to 6.3, nanosheets, nanorings, and spheres can be obtained. The as-formed alpha-Fe2O3 exhibits shape-dependent infrared optical properties. The growth process of colloidal alpha-Fe2O3 crystals in the presence of phosphate ions is discussed. The products have been characterized by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy. This work presents an efficient and cost-effective approach that is potentially competitive for scaling-up industrial production. The as-formed alpha-Fe2O3 crystals with controllable morphologies not only provide flexible building blocks for advanced functional devices, but are also ideal candidates for studying their nanoarchitecture-dependent performance in optical, catalytic, and magnetic applications.