General Observation of Photocatalytic Oxygen Reduction to Hydrogen Peroxide by Organic Semiconductor Thin Films and Colloidal Crystals

Low-cost semiconductor photocatalysts offer unique possibilities for industrial chemical transformations and energy conversion applications. We report that a range of organic semiconductors are capable of efficient photocatalytic oxygen reduction to H2O2 in aqueous conditions. These semiconductors, in the form of thin films, support a 2-electron/2-proton redox cycle involving photoreduction of dissolved O2 to H2O2, with the concurrent photooxidation of organic substrates: formate, oxalate, and phenol. Photochemical oxygen reduction is observed in a pH range from 2 to 12. In cases where valence band energy of the semiconductor is energetically high, autoxidation competes with oxidation of the donors, and thus turnover numbers are low. Materials with deeper valence band energies afford higher stability and also oxidation of H2O to O2. We found increased H2O2 evolution rate for surfactant-stabilized nanoparticles versus planar thin films. These results evidence that photochemical O2 reduction may be a widespread feature of organic semiconductors, and open potential avenues for organic semiconductors for catalytic applications.