Synthesis and Photoelectrochemical Activity of α-Fe₂O₃−CdFe₂O₄ Hybrid Structure for the Water Oxidation Reaction

The electrochemical properties of spinel ferrites have been studied since the 1970s after the pioneering discovery of water electrolysis by using TiO₂ as the photoanode in a photoelectrochemical cell. Several approaches have been developed to fabricate ferrites of different sizes and shapes but most of these methods suffer from several disadvantages, such as the difficulty to produce the ferrite in a thin layer form or on top of another (pre-prepared) structure as a second layer. Moreover, the formation process requires calcination at high temperatures, and the electrode fabrication requires an additional assembly method. Herein, a CdFe₂O₄ ferrite was formed via chemical vapor deposition of a single-source precursor directly on a hematite structure followed by an annealing process. This thermal treatment simultaneously achieves both activation of the hematite and the formation of CdFe₂O₄. Three different iron oxide (rectangular, nanobelt, and mesoporous) structures were examined as photoanode electrodes. The maximum current at 1.65 V vs. RHE of the mesoporous film was more than two times higher than the rectangular and the nanobelt films. Therefore, iron oxide with mesoporous structures was used to form the hybrid structure of α-Fe₂O₃−CdFe₂O₄. The hybrid structure presents stable and high photocurrent density (about 20 % enhancement in the measured maximum current density at 1.65 V vs. RHE).