Micro Single Crystals of Hematite with Nearly 100% Exposed {104} Facets: Preferred Etching and Lithium Storage

The controlled synthesis of inorganic single crystals with a large percentage of exposed high-index facets has attracted much attention. However, high-index facets usually disappear during the early stage of crystal growth due to the minimization of surface energy and typically facet-controlling agents are employed. Here a facile fast hydrothermal method for the preparation of microsize α-Fe₂O₃ rhombohedra with nearly 100% exposed {104} facets was developed in a simple formulated solvent without any additives. The hydrothermal reaction time could be as short as 75 min, in contrast to typical hydrothermal reactions over days. The preferred etching edges along the diagonal axis of microsize rhombohedra by the self-generated ions was observed, which could be potentially extended to synthesize and tailor other transition metal oxides. The formation mechanism was revealed by ex situ FESEM observations of the samples prepared at different reaction times. Improved electrochemical performances in terms of cyclability, specific capacity, and high rate were achieved. The specific capacity was maintained at 550 mAh/g after 120 cycles at a rate of 200 mA/g. Experimental evidence clearly shows that the as-designed solid microsize α-Fe₂O₃ can effectively and reversibly store lithium ions with performance comparable to nanosize α-Fe₂O₃, suggesting electrode materials with particle size at the microscale will be worth further exploration