Simultaneously Utilizing Excited Holes and Electrons for Piezoelectric-Enhanced Photoproduction of H₂O₂ from S-Scheme 2D S-Doped VO_x/g-C₃N₄ Nanostructures

2D/2D step-scheme (S-scheme) piezo-photocatalysts for the production of fine chemicals, such as hydrogen peroxide (H₂O₂), have attracted significant attention of global scientists owing to the efficiency in utilizing surface piezoelectric effects from 2D materials to overcome rapid charge recombination in photocatalytic processes. In this research, we reported the fabrication of 2D S-doped VO_x deposited on 2D g-C₃N₄ to produce H₂O₂ via the piezo-photocatalytic process with high production yields at 20.19 mmol g^-1 h^-1, which was 1.75 and 4.87 times higher than that from solely piezo-catalytic and photocatalytic H₂O₂ generation. The finding pointed out that adding sulfur (S) to VO_x can help to improve the catalytic outcomes by modifying the electronic properties of pristine VO_x. In addition, when coupled with g-C₃N₄, the presence of S limits the formation of graphene in the VO_x/g-C₃N₄ composites, causing shielding effects and pushing the cascade reactions toward water generation in the materials. Besides, the research also sheds light on the charge transport between g-C₃N₄ and S-VO_x under irradiation and how the composites work to trigger the formation of H₂O₂. The presence of S in the composite systems enhances charge transfer between two semiconductors by strengthening the internal electric fields (IEF) to drive electrons moving in one direction, as demonstrated by density functional theory (DFT) calculations. Moreover, the formation of H₂O₂ significantly relies on the reduction of oxygen to generate oxygenic radical species at the g-C₃N₄ sites. Meanwhile, S-VO_x provides oxidative sites in the composites to oxidize water molecules to directly or indirectly generate H₂O₂ or O₂, which will further participate in the reactions to produce the final products. This study confirms the validation of S-scheme piezo-photocatalysts, thus encouraging further research on developing heterojunction materials with high catalytic efficiency, which can be used in practical conditions.