What Is the Transfer Mechanism of Photoexcited Charge Carriers for g-C₃N₄/TiO₂ Heterojunction Photocatalysts? Verification of the Relative p–n Junction Theory

Investigation of photoexcited charge transfer mechanism has always been one of the hotspots of the photocatalysis field. In our recent studies, the relative p–n junction was proposed as a new concept, and the built-in electric field formed in the heterojunction is the inner impetus for driving photoexcited charge transfer. Here, a series of g-C₃N₄/TiO₂ samples with different mass percentage contents were synthesized and further characterized by physical and chemical techniques for the investigation of the charge transfer mechanism and internal natural law. The results state clearly that the migration of photoexcited charges belongs to Z-scheme mechanism, which is suitable for as-synthesized g-C₃N₄/TiO₂ samples, whether the main part of the g-C₃N₄/TiO₂ is TiO₂ or g-C₃N₄. The photoexcited electrons enriched in g-C₃N₄ with a higher negative conduction band (CB) potential have reduction ability to convert O₂ into superoxide radicals (•O₂–). Meanwhile, the photoexcited holes in TiO₂ with a higher positive valence band (VB) potential have oxidation ability to activate H₂O or hydroxyl ions (OH–) to hydroxyl radicals (•OH). Furthermore, the g-C₃N₄/TiO₂ photocatalyst exhibits better photocatalytic performance than TiO₂ and g-C₃N₄. It is encouraging that the abovementioned Z-scheme mechanism of photoexcited charge transfer can also be explained and confirmed by the relative p–n junction theory. The built-in electric field promotes the migration of the photoexcited charges in the heterojunction, and its migration direction is opposite to that of the photoexcited charge in the CB and VB of g-C₃N₄ and TiO₂. Therefore, the relative p–n junction theory not only is used to explicate the migration mechanism and internal natural law of the photoexcited charge in the heterojunction photocatalysts but also has crucial guiding significance for the theoretical design and practical construction of composite photocatalysts.