Incorporating carbon into Bi3O4 Cl enhances its internal electric field by 126 times, which induces a bulk charge separation efficiency (eta(bulk)) of 80%. This ultrahigh eta(bulk) value presents a state-of-the-art result in tuning the bulk charge separation. The generated C-doped Bi3O4Cl has a noble-metal-and electron-scavenger-free water-oxidation ability under visible light, which is difficult to achieve with most existing photocatalysts.