Significance: Hydrogen peroxide is a highly competitive ready-to-use product for solar energy transformation. Nevertheless, the contemporary photosynthetic systems are not efficient enough, due to severe charge recombination caused by high activation energy and binding energy of the exciton. Herein, we achieve spontaneous exciton dissociation at room temperature. Moreover, the photosynthesis of H ₂ O ₂ reaches between 9,366 and 12,324 µmol·g ⁻¹ from 9 AM to 4 PM in ambient conditions, that is, sunlight irradiation, real water including fresh water and seawater, room temperature, and open air. The ultrahigh photocatalytic efficiency in ambient conditions allows the solar-to-chemical conversion in a real cost-effective and sustainable way, which represents an important step toward real applications.