Cooperative effect of pH value and anions on single-crystalline hexagonal and circular α-Fe2O3 nanorings

► Single-crystalline α-Fe2O3 nanorings were synthesized by a solvothermal approach. ► The cooperative action of the VO43− and SO42− is a crucial factor in the formation of the ring-like structures. ► The Hc value of circular nanorings is larger than that of the hexagonal ones.