Band‐Edge Engineering of BiFeO ₃ –KTaO ₃ Solid Solutions toward Efficient Solar Hydrogen Production

BFO 1− x KTO x solid solutions are synthesized and evaluated as perovskite oxide‐based photocathodes for solar hydrogen production. X‐ray diffraction and Raman spectroscopy confirm the presence of single‐phase perovskites for 0 ≤ x ≤ 0.25 with systematic crystal lattice expansion and symmetry evolution. Optical spectroscopy indicates that the direct bandgap increases from 1.94 eV (BFO) to 2.05 eV (BFO 75 KTO 25 ). Mott–Schottky analysis shows a downward shift of the conduction band edge to −0.14 V relative to reversible hydrogen electrode (RHE), which is more negative than the H + /H 2 redox potential and therefore thermodynamically suitable for the hydrogen evolution reaction (HER). Under AM 1.5 G illumination, the optimized BFO 75 KTO 25 photocathode exhibits a photocurrent density of −90 mA·cm −2 at −0.86 V vs. RHE, a maximum photon‐to‐current efficiency under applied polarization of 0.10%, and a hydrogen production rate of 2909 μmol·L −1 ·h −1 . Electrochemical impedance spectroscopy attributes the improved activity to a significantly reduced interfacial charge‐transfer resistance of 1.25 Ω and improved carrier transport. The findings validate composition‐driven band‐edge tuning in BFO–KTO as a viable route to HER in the absence of ferroelectric poling, noble‐metal cocatalysts, and multiphase heterojunction architectures.