Visible Light-Active Ternary Heterojunction Photocatalyst for Efficient CO2 Reduction with Simultaneous Amine Oxidation and Sustainable H2O2 Production

Interface coupling to improve O2 adsorption and accelerate charge separation for boosting photocatalytic performance of ZnO/ZnIn2S4 composite in H2O2 production and environmental remediation

The key to heterogeneous photo-Fenton technology lies in the efficient generation of hydrogen peroxide (H2O2). Herein, a newly-designed ZnO/ZnIn2S4 composite with heterostructure is synthesized. Benefiting from the formation of built-in electric field, the recombination of photoinduced electrons and holes is suppressed and interfacial charge transfer resistance is reduced. Importantly, the embedding of ZnO in ZnIn2S4 can improve the hydrophobicity and create microscopic three-phase interface, thereby boosting the capture capability for O2 and providing the convenience for the occurrence of O2 reduction reaction. More interestingly, the existence of ZnIn2S4 in the ZnO/ZnIn2S4 composite can reduce the Gibbs free energy (ΔG) of key intermediate (OOH*) formation, which will accelerate the generation of H2O2. As a result, the ZnO/ZnIn2S4 composite displays excellent performance in photocatalytic H2O2 production, and the highest yield was about 897.6 μmol/g/h within 60 min under visible light irradiation. The transfer of photoinduced carriers follows the S-scheme type mechanism. The photogenerated holes can be captured by drug residues (i.e., diclofenac sodium) to accelerate H2O2 production, while generated H2O2 can combine with Fe2+ to construct photo-Fenton system for achieving the advanced degradation of diclofenac sodium, which was mainly related to the formation of OH•. Furthermore, generated H2O2 can be applied for performing the inactivation of pathogenic bacteria. In short, current work will provide a valuable reference for future research.

Visible-Light Photocatalytic CO2-to-CO and H2O-to-H2O2 by g-C3N4/Cu2O-Pd S-Scheme Heterojunctions

Visible-light photocatalytic conversion of CO₂-to-fuels for green electricity is sustainably attractive for alleviating carbon emissions. Photocatalytic CO₂-to-CO frequently suffered from relatively low yields, mainly due to ineffective charge transfer rates. A new approach for photocatalytic CO₂-to-CO enhanced with effective H⁺ from H₂O-to-H₂O₂ through the water oxidation reaction (WOR) has been studied in the present work. Here, the nano palladium (9 wt %), serving as a cocatalyst, dispersed on the g-C₃N₄/Cu₂O heterojunctions (i.e., g-C₃N₄/Cu₂O-Pd) has been prepared to facilitate charge separation for the two-electron reduction of CO₂ to CO. Experimentally, the g-C₃N₄/Cu₂O-Pd heterojunctions have a higher photocatalytic H₂O-to-H₂O₂ yield than the g-C₃N₄/Cu₂O heterojunction by 5.3 times. The photocatalytic WOR provides sufficient electrons (e^-) and H⁺ (2H₂O → H₂O₂ + 2H⁺) for CO₂-to-CO (CO_2(aq) + 2H⁺ + 2e^- → CO_(g) + H₂O_(l)). Relatively high photocatalytic yields of H₂O₂ (34.0 μmol/mg) and CO (14.6 μmol/mg) affected by the heterojunctions can be achieved. Also, the heterojunctions have a high photostability with a photocatalytic generated CO/H₂ ratio of 1.75 approximately. This visible-light photocatalytic CO₂-to-CO and H₂O-to-H₂O₂ by the new g-C₃N₄/Cu₂O-Pd S-scheme heterojunctions demonstrates the feasibility of the zero carbon emission approach with additional green oxidant (H₂O₂) generation.