Fabrication Of G-C₃N₄ And MoSe₂-based Heterojunction,photocatalytic Enhanced Mechanism,and Its Interface Behavior Of Charge Carriers

Two dimensional（2D）layered materials,such as graphitic carbon nitride（g-C₃N₄）and Molybdenum diselenide（MoSe₂）,have been considered as the promising solar-light-driven photocatalysts owing to their earth-abundant element,controllable morphology,good visible-light absorption,adjustable band position,controllable phase structure transformation,excellent carrier mobility,and good anti-photo-corrosion.Nonetheless,g-C₃N₄ and MoSe₂ still suffer from their low specific surface area,high recombination efficiency of photogenerated charge carriers,and insufficient redox capability,which limits their practical application.Herein,g-C₃N₄ and MoSe₂ were investigated in this work,constructing different kinds of g-C₃N₄ and MoSe₂-based heterojunction photocatalysts via morphology controlling,phase tuning,and heterojunction fabricating.And the composition,morphology,and photoelectrochemical properties of the as-prepared heterojunction photocatalysts were systematically studied by various characterization methods.According to the band structures of semiconductors and active species capture experimental results,the interface carrier behaviors were investigated in detail and the photocatalytic enhanced mechanisms of g-C₃N₄ and MoSe₂-based heterojunctions were revealed,providing experimental support for the design of efficient heterojunction photocatalysts.And the main research contents were present as follows:（1）Fabrication of g-C₃N₄/rGO/Ag₂WO₄ composite with enhanced photocatalytic activity.g-C₃N₄/rGO/Ag₂WO₄ composite with 0D/2D structure was fabricated via an ultrasonic-assisted self-assembly method and a rGO-assisted in-situ deposited method.Benefited from the introduction of rGO,g-C₃N₄/rGO/Ag₂WO₄ composite possessed the better light-harvest capability and the highest separation efficiency of charge carriers.The optimal g-C₃N₄/rGO/Ag₂WO₄ composite exhibited the enhanced Rhodamine B（Rh B）photodegrading rate of 91%,within 60 min of simulated solar light irradiation.And its maximum k was 0.0364min^-1,being 2.4,4.8,and 13.5 times than that of g-C₃N₄/Ag₂WO₄,g-C₃N₄/rGO,and g-C₃N₄,respectively.Based on the energy band the theory of semiconductors,it could be reasonably inferred that:Schottky barrier and Ohmic contact at the interface of rGO-semiconductor provided unidirectional electron transfer routes for charge carriers,ensuring electron-hole pairs migrated and separated via the Z-scheme charge transfer pathway.And rGO could act as the charge-transfer-highway between g-C₃N₄ and Ag₂WO₄ for the migration of carriers.Then a novel Schottky barrier and Ohmic contact synergistically boosting Z-scheme heterojunction-based photocatalytic mechanism was proposed.（2）Fabrication of Ag Br and GO co-decorated g-C₃N₄/Ag₂WO₄ composite and its charge carrier behavior.In order to further improve the photocatalytic activity and recyclability of g-C₃N₄/Ag₂WO₄ composite,a novel g-C₃N₄/Ag₂WO₄/Ag Br/GO composite was successfully fabricated by hexadecyl trimethyl ammonium bromide（CTAB）-assisted co-deposition method.With the synergy of Ag Br and GO,the g-C₃N₄/Ag₂WO₄/Ag Br/GO composite possessed the enhanced photocatalytic efficiency for Rh B and tetracycline（TC）degradation as well as good recyclability,compared than that of g-C₃N₄/rGO/Ag₂WO₄ and g-C₃N₄/Ag₂WO₄ samples,respectively.Based on the energy band theory,electron-hole pairs would transfer and separate following a dual Z-scheme based carrier migration pathway,with the effect of continuous unidirectional internal electric field at the interface of g-C₃N₄-Ag₂WO₄-Ag Br and Ohmic contact at the semiconductor-GO interface.Accordingly,a reasonable Ohmic contact enhanced dual Z-scheme heterojunction-based photocatalytic mechanism was proposed.（3）Fabrication and charge carrier behavior of rGO-MoSe₂/g-C₃N₄ 3D/2D heterostructure.Employing GO as a self-support template,a rGO-MoSe₂ composite with 3D network hierarchical structure was fabricated by the solvothermal method.Then the rGO-MoSe₂ was combined with g-C₃N₄ nanosheet with porous structures,constructing a rGO-MoSe₂/g-C₃N₄ 3D/2D hierarchical structure.The introduced rGO could act as efficient transfer channels for charge carriers between MoSe₂ and g-C₃N₄;and 3D/2D hierarchical structure endow rGO-MoSe₂/g-C₃N₄ composite with the improved light-harvesting and abundant active sites,which is beneficial for the promoted photocatalytic activity.For the carrier behavior in the rGO-MoSe₂/g-C₃N₄,with the synergy of the internal electric field and band bending,the spatial behavior of electron-hole pairs would comply to a Z-scheme transfer pathway using rGO as the charge transfer channel,promoting photocatalytic performance.（4）Construction and interface carrier behavior of ordered 1T/2H MoSe₂/pg-C₃N₄heterojunction.An ordered 1T/2H phase MoSe₂ with few-layers structure was obtained by a Na BH₄-assisted hydrothermal method without any template,improving the morphology and photoelectrochemical activity of MoSe₂.Further,1T/2H MoSe₂ was integrated with proton g-C₃N₄ via solvothermal method,forming a direct Z-scheme 1T/2H MoSe₂/pg-C₃N₄ composite with 2D/2D heterostructure.As for the carrier behavior in the ordered 1T/2H MoSe₂/pg-C₃N₄heterojunction,the electron-hole pairs would transfer following a direct Z-scheme carrier migration route,with the effect of internal electric field and band bending.And the 1T phase structure from the surface of ordered 1T/2H MoSe₂ would be intimate contacted with pg-C₃N₄,further enhancing the transfer and separation of electron-hole pairs at interface as well as improving the photocatalytic activity for degrading Rh B and TC.（5）Fabrication of Ag I/1T-2H MoSe₂/Bi₄O₅Br₂ dual Z-scheme heterojunction and its charge carrier behavior.A flower-like mixed 1T-2H phase MoSe₂ was synthesized via a Na BH₄-assisted solvothermal method.Further,the 1T-2H MoSe₂ was selected as a“charge-transfer-bridge”to fabricate a dual Z-scheme Ag I/1T-2H MoSe₂/Bi₄O₅Br₂ heterojunction via the combine of solvothermal method and in-situ deposited method,boosting the carrier separation efficiency and redox activity of 1T-2H MoSe₂.The optimal Ag I/1T-2H MoSe₂/Bi₄O₅Br₂ exhibited the highest photocatalytic TC degradation performance than that of Ag I/2H MoSe₂/Bi₄O₅Br₂ and 1T/2H MoSe2/pg-C3N4,respectively.According to the VB offset(ΔE_VBO),VB offset(ΔE_CBO),and band alignment of Ag I/1T-2H MoSe₂/Bi₄O₅Br₂heterojunction,the carrier behavior at the interface of Ag I-1T-2H MoSe₂-Bi₄O₅Br₂ was explored,proposing a mixed 1T-2H MoSe₂ boosting dual Z-scheme photocatalytic mechanism.