Enhanced photoelectrochemical water splitting performance of α-Fe2O3 photoanodes through Co-modification with Co single atoms and g-C3N4

Abstract

The practical application of α-Fe₂O₃ in water splitting is hindered by significant charge recombination and slow water oxidation. To address this issue, a CoSAs-g-C₃N₄/Fe₂O₃ (CoSAs: cobalt single atoms) photoanode was fabricated in this study through the co-modification of CoSAs and g-C₃N₄ to enhance photoelectrochemical (PEC) water splitting. The coupling between g-C₃N₄ and α-Fe₂O₃ resulted in the formation of a heterojunction, which provided a strong built-in electric field and an additional driving force to mitigate charge recombination. Moreover, g-C₃N₄ served as a suitable carrier for single atoms, which effectively anchored CoSAs through N/C coordination. The highly dispersed CoSAs provided abundant active sites, which further promoted surface holes extraction and oxidation kinetics, resulting in higher PEC performance and photostability. This study indicates the benefits of these collaborative strategies and provides more efficient designs for solar energy conversion in PEC systems.

Fig. 1. (a) Schematic of the CoSAs–g-C₃N₄/Fe₂O₃ NRA photoanode preparation process. Top-view SEM images of (b) α-Fe₂O₃; (c) g-C₃N₄/Fe₂O₃; (d) CoSAs–g-C₃N₄/Fe₂O₃.

Fig. 2. (a)–(c) Atomic-resolution TEM images of CoSAs–g-C₃N₄/Fe₂O₃ NR (CoSAs are highlighted by red circles); (d) and (e) HRTEM image and EDS mapping of Co, Fe, O, C, and N for CoSAs–g-C₃N₄/Fe₂O₃ NR.

Fig. 3. High-resolution XPS spectra of (a) C 1s; (b) N 1s; (c) O 1s; (d) Co 2p, for CoSAs–g-C₃N₄/Fe₂O₃.

Fig. 4. (a) XANES spectra; (b) R-space Co K-edge EXAFS spectra; (c) EXAFS R-space fitting curve; (d) WT-EXAFS signals of Co foil, CoO, CoPc and CoSAs–g-C₃N₄/Fe₂O₃.

Fig. 5. (a) LSV curves; (b) transient photocurrent measurements at 1.23 V_RHE; (c) IPCE curves; (d) ABPE curves of α-Fe₂O₃, g-C₃N₄/Fe₂O₃, and CoSAs–g-C₃N₄/Fe₂O₃ photoanodes.

Fig. 6. (a) M–S plots; (b) OCP transient decay curves; (c) potential (V_RHE) versus time for the photoelectrodes at 1 mA cm⁻²; (d) η_bulk; (e) η_surface; (f) EIS of α-Fe₂O₃, g-C₃N₄/Fe₂O₃, CoSAs–g-C₃N₄/Fe₂O₃ photoanodes.

Fig. 7. (a) Photocurrent density stability of CoSAs–g-C₃N₄/Fe₂O₃ photoanodes at 1.23 V_RHE; (b) XRD pattern; (c) SEM; (d) HRTEM of CoSAs–g-C₃N₄/Fe₂O₃ photoanodes after long-term stability tests.

Fig. 8. (a) Calculated band gap values (based on UV-Vis absorption spectra) of α-Fe₂O₃ and g-C₃N₄. Ultra UV photoelectron spectroscopy (UPS) and work function of (b) α-Fe₂O₃ and (c) g-C₃N₄; (d) energy band diagram of the g-C₃N₄/Fe₂O₃ heterojunction; (e) Schematic illustration of electron–hole separation of CoSAs–g-C₃N₄/Fe₂O₃ photoanode for PEC water splitting.