Aliovalent Na-Substituted Calcium Ferrite Photocathodes for Photoelectrochemical Water Splitting

Aliovalent Na-substituted calcium ferrite photocathode films with Ca_1-xNa_xFe₂O₄ (x = 0, 0.02, 0.04, 0.06, 0.08) composition were fabricated on fluorine-doped tin oxide (FTO) coated glass substrates via a facile drop-casting method followed by thermal annealing. Powder x-ray diffraction (XRD) combined with Rietveld refinement confirmed the formation of single-phase orthorhombic CaFe₂O₄ with a Pnma space group for all compositions, while systematic lattice expansion suggested the successful incorporation of Na into the CaFe₂O₄ framework. Field-emission scanning electron microscopy (FESEM) revealed that Na substitution promoted grain growth and improved film connectivity compared with pristine CaFe₂O₄. Optical absorption spectra demonstrated a gradual bandgap narrowing from 2.20 to 1.90 eV with increasing Na content, accompanied by enhanced visible-light absorption. Photoelectrochemical measurements revealed cathodic photocurrent responses characteristic of p-type semiconductor behavior. The optimized Ca_0.94Na_0.06Fe₂O₄ photocathode achieved the highest photocurrent density of −192.6 µA cm⁻² at 0.2 V vs. RHE, representing a 3.4-fold enhancement compared with pristine CaFe₂O₄, together with an onset potential of 0.88 V vs. RHE. Mott–Schottky and impedance analyses indicated increased acceptor density and reduced charge-transfer resistance after Na incorporation. These improvements are attributed to Na-induced electronic structure modulation and defect-mediated charge compensation, facilitating carrier transport and interfacial charge-transfer kinetics.