Aliovalent Na‐Substituted Calcium Ferrite Photocathodes for Photoelectrochemical Water Splitting

ABSTRACT

Aliovalent Na‐substituted calcium ferrite photocathode films with Ca _{1‐ x} Na _x Fe ₂ O ₄ ( 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 CaFe ₂ O ₄ with a Pnma space group for all compositions, while systematic lattice expansion suggested the successful incorporation of Na into the CaFe ₂ O ₄ framework. Field‐emission scanning electron microscopy (FESEM) revealed that Na substitution promoted grain growth and improved film connectivity compared with pristine CaFe ₂ O ₄ . 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 _0.94 Na _0.06 Fe ₂ O ₄ 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 CaFe ₂ O ₄ , 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.