Enabling Directional Hole Extraction via NiO x Nanoseeding of Localized Gradient Oxygen Vacancies in BiVO 4 for Underwater Ultraweak Light Photoelectrochemical Detection

ABSTRACT Self‐powered photoelectrochemical photodetectors are promising for sustainable underwater optoelectronics. However, severe charge recombination under zero bias hinders their ultraweak light (<0.05 mW/cm 2 ) detection in marine environments. While oxygen vacancy (V O ) gradients improve carrier direction by deepening band bending to synergize with junction‐built‐in fields, existing studies focus on homogeneous gradient V O , causing chaotic lateral diffusion and non‐negligible recombination. Here, we propose a bio‐inspired “seeds‐taking‐root” strategy to construct localized V O gradients within 3D nanocoral‐like BiVO 4 . Through one‐step sintering, Ni seeds selectively extract O from BiVO 4 via strong Ni‐O interactions, simultaneously forming NiO x nanoclusters and NiO x ‐anchored localized gradient V O , leading to confined charge transfer pathways with localized space charge regions and unidirectional band bending for highly directional hole extraction. The structure achieves 82.5% charge separation efficiency at 1.23 V RHE (1.7 times than pristine BiVO 4 ), enabling efficient self‐powered broadband operation (365–940 nm). Even at 0.02 mW/cm 2 (matching real underwater light levels) in artificial seawater, it achieves a photoresponsivity of 43.82 ± 0.52 mA/W. It exhibits robust stability with over 50 h continuous operation and 98.5% photocurrent retention after one week in seawater. This seed‐mediated strategy for constructing directional charge transport channels offers a viable pathway for high‐performance optoelectronics in energy‐limited environments.