Synthesis of Hematite and Magnetite Nanodiscs

Synthesis process of nanodiscs follows the protocol described in the previous study^{11 (link)}. The synthesis process consists of two steps. First, hematite nanodiscs were synthesized by mixing 10 ml 99.5% ethanol, 0.6 ml ddH₂O (or 0.8 ml ddH₂O for smaller nanodiscs), 0.8 g of anhydrous sodium acetate (Sigma-Aldrich) and 0.273 g of FeCl₃·6H₂O (Sigma-Aldrich). After the mixture was homogenized by stirring, the mixture was transferred and sealed into a Teflon-lined steel vessel. The vessel was heated in the oven at 180 °C for 18 h. The hematite nanodiscs were washed with ddH₂O twice. Then nanodiscs were washed with ethanol twice. Nanodiscs were dried in a vacuum desiccator. Hematite nanodiscs were further converted into magnetite nanodiscs or used for control experiments. For reduction, 1 mg hematite nanodiscs were mixed with 20 ml of tri-octylamine (Acros Organics) and 1 g of oleic acid (Sigma-Aldrich). The mixture was placed in a three-neck flask connected to a Schlenk line to heated to 370 °C for 25 min in an atmosphere of H₂ (5% with 95% Argon, Chiah Lung) and N₂ (99.9%, Chiah Lung). During reduction, the red hematite nanodiscs turned to dark gray. After cooled down, the discs were washed with hexane (Alfa Aesar). The magnetite nanodiscs were then dispersed in chloroform (J.T. Baker) and stored in a glass vial at 4 °C.