not-yet-known not-yet-known not-yet-known unknown The FVIOs were synthesized according to our previously reported method. Briefly, the hematite α-Fe2O3 nanorings were prepared through hydrothermal treatment of FeCl3·6H2O and NH4H2PO4 solutions at 220 °C for 48 h. The α-Fe2O3 nanorings were transformed to FVIOs (Fe3O4 nanorings) in a 20% H2/80% Ar atmosphere at 400 °C for 3 h. The FVIOs were dispersed into 30 mL of oleic acid, and the mixture was heated and maintained at 260 °C for 1 hour under an Ar flow. After cooling to room temperature, the oleic acid-coated FVIOs were obtained after washing and centrifuging with an ethanol/hexane solution.