Template-free solvothermal synthesis of hollow hematite spheres and their applications in gas sensors and Li-ion batteries

Magnetite (Fe₃O₄) hollow spheres were prepared by solvothermal reaction of ethanol solution containing Fe-acetate and l-lysine, and were subsequently transformed into hematite (Fe₂O₃) hollow spheres with nanoscale (20-30 nm) thin shells by heat treatment at 500 °C for 2 h. Both the as-prepared and heat-treated hollow spheres contained another small sphere within each shell, which was attributed to the following solvothermal self-assembly reactions: (1) the nucleation of Fe₃O ₄ spheres, (2) lysine capping on the nuclei, (3) the growth of lysine-capped particles by cross-linking between lysine molecules, and (4) the formation of Fe shell layers by the interaction between Fe ions and outer lysine molecules. In the assembly reaction, l-lysine with amino and carboxyl radicals played the key role. The heat-treated Fe₂O₃ hollow spheres showed significantly enhanced C₂H₅OH sensing characteristics and promising Li-ion intercalation behaviors.