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 O-2 into superoxide radicals (center dot O-2(-)). 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 (center dot 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.