Quenching–induced defect engineering and Sn–assisted charge collection in CuBi2O4 for solar hydrogen production

The photoelectrochemical performance of CuBi 2 O 4 (CBO) photocathodes is strongly affected by intrinsic point defects. Here, defect–engineered CBO was prepared by rapid quenching from 700°C, preserving a non–equilibrium defect configuration while inducing Sn 4+ diffusion from FTO. XPS and EPR confirm oxygen–vacancy (V O ) and reduced–Cu (Cu + ) related states, including a V O –associated EPR signal at g = 2.003. Mott–Schottky and impedance analyses reveal an apparent acceptor density of 3.48 × 10 17 cm −3 and a reduced charge–transfer resistance of 415 Ω. CBO–700Q exhibits bias–dependent defect functionality: V O /Cu + states mainly promote recombination at low cathodic bias, whereas stronger polarization favors charge separation and electron injection through enhanced p –type character, enlarged active area, and possible Sn–assisted charge collection. Consequently, CBO–700Q delivers ∼1.17 mA cm −2 at 0.4 V vs RHE and ∼0.36 mA cm −2 under zero–bias tandem operation.