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What Is the Transfer Mechanism of Photoexcited Charge Carriers for g‑C3N4/TiO2 Heterojunction Photocatalysts? Verification of the Relative p–n Junction Theory
journal contribution
posted on 2020-04-09, 12:34 authored by Nannan Yuan, Jinfeng Zhang, Sujuan Zhang, Gaoli Chen, Sugang Meng, You Fan, Xiuzhen Zheng, Shifu ChenInvestigation
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.