Controlled synthesis of mesoporous hematite nanostructures and their application as electrochemical capacitor electrodes.

In this work, iron oxalate (FeC₂O₄·2H₂O) with different morphologies was synthesized through a simple solution-based direct precipitation process. Three samples with distinct morphologies, i.e., microrods with a parallelogram-like cross-section, nanorods, and multi-layered nanosheets, could be obtained in a selective manner. We found that the shapes of the iron oxalate could be controlled just through simply altering the solvents used. The one-dimensional (1D) characteristic of the infinite linear chains and the selective interaction between solvents and various crystallographic planes of FeC₂O₄·2H₂O played an important role in the formation of FeC₂O₄·2H₂O with different morphologies. Phase-pure hematite (α-Fe₂O₃) had be obtained by annealing these as-prepared FeC₂O₄·2H₂O precursors without significant alterations in morphology. The as-obtained mesoporous α-Fe₂O₃ products had high specific surface areas with narrow pore size distribution. The electrochemical properties of the α-Fe₂O₃ electrodes were investigated using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements by a three electrode system. The electrochemical experiments revealed that they showed a structure-dependence in their specific capacitances. The mesoporous multi-layered nanosheets exhibited a significant structurally induced enhancement of capacity properties associated with their novel structure characteristic in addition to the high specific surface area. They can present the highest specific capacitance value (116.25 F g⁻¹) and excellent long cycle life within the voltage window from - 0.6 to 0 V. This method can be easily controlled and is expected to be extended to produce other functional materials with controlled structure.