Enhanced photoelectrochemical water oxidation on nanostructured hematite photoanodes via p-CaFe₂O₄/n-Fe₂O₃ heterojunction formation

In this paper, nanostructured hematite p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanodes have been fabricated employing a facile template-less film processing technique by controlling the chemical bath. Anisotropic growth of a β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl₃ solution containing CaCl₂ followed by a two-step thermal annealing at 550 and 800 °C induces the formation of a p-CaFe₂O₄/n-Fe₂O₃ heterojunction. The structural, morphological, electronic states, and electrochemical characteristics of the films have been investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and impedance spectroscopy, respectively. The heterojunction photoanode showed 100% higher photocurrent response than that obtained using a bare hematite electrode under simulated 1-sun illumination (100 mW/cm²). The photocurrent enhancement is attributed to the enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and the electrolyte as revealed by electrochemical impedance spectroscopy analysis. The modification of the p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanode with CoPi cocatalyst further facilitates the electron transfer at the electrode/electrolyte interface and thus enhances the photoelectrochemical water oxidation. Since cheap and abundant materials have been employed for the synthesis of the heterojunction photoanode via a simple route, the current results have great importance, both from a scientific and an economical point of view.