Construction Of G-C₃N₄-based 2D/0D Nanocmposites And Their Photocatalytic Activity

Semiconductor-based photocatalysis has been recognized as an environmental friendly and promising technology to solve energy crisis and issues of environment pollution.And it has been used in organic contaminants purification,water splitting,CO₂ photoreduction.Graphitic carbon nitride（g-C₃N₄）is a novel visible-light-driven metal-free polymeric semiconducting material,which possesses a two-dimensional structure.The advantages of a relatively narrow band gap（E_g=2.7 eV）,suitable band positions and excellent chemical stability make g-C₃N₄ a promising photocatalyst.Unfortunately,pristine g-C₃N₄ exhibits a limited visible light absorption region as well as a poor quantum efficiency,leading to its low photocatalytic activity,which seriously limit the practical application.An effective strategy of coupling zero-dimensional nanomaterials with two-dimensional g-C₃N₄ nanosheets was unveiled for reducing the recombination rate of photogenerated electron-hole pairs,extending the lifetime of charge carriers.In this work,zero-dimensional materials were introduced to two-dimensional layered g-C₃N₄ photocatalyst,aim to form a good contact interface and accelerate the separation and transmission of photoelectron-hole,thereby significantly enhancing the photocatalytic performance of g-C₃N₄.The g-C₃N₄-based 2D/0D hybrid composites of MoS₂ QDs/g-C₃N₄,Ag/g-C₃N₄,MoS₂QDs-g-C₃N₄/Ag and Fe（Ⅲ）/Ag/g-C₃N₄ were constructed.The structure,morphology,optical properties and photo-generated carriers transport of the prepared photocatalysts were investigated,and discussed the possible photocatalytic mechanism.The detailed contents of this thesis were as follows:（1）According to the energy band matching principle,a novel visible-light-driven MoS₂QD/g-C₃N₄ composites synthesized through the combination of a hydrothermal process and impregnation preparation.AFM and TEM characterization showed that the size of MoS₂ QDs were less than 5 nm.The photocatalytic activity was evaluated by the photodegradation of rhodamine B（RhB）under visible light irradiation.Trapping experiment,TA-assisted PL spectroscopy and NBT transformation revealed the effects of different scavengers in the photocatalytic reaction.The possible enhanced photocatalytic mechanism was attributed to a direct Z-scheme system,which not only can increase the separation efficiency of photogenerated electron-hole pairs but also possesses excellent oxidation and reduction ability for high photocatalytic performances.（2）Zero-dimensional Ag nanoparticles were deposited on the surface of g-C₃N₄ by chemical reduction method by using NaBH₄ and AgNO₃ to obtain Ag/g-C₃N₄ heterojunction composites with good interface contact.Photocurrent,electro-chemical impedance spectroscopy（EIS）and fluorescence spectra（PL）were used to analyze the separation,transmission and recombination of charge carriers of Ag/g-C₃N₄ composites.It was found that the surface plasmonic resonance（SPR）effect of Ag nanoparticles apparently increased separation efficiency of photogenerated carriers and inhibited the recombination of electron-hole pairs.（3）A novel Z-scheme photocatalytic system MoS₂ QDs decorated g-C₃N₄/Ag heterostructured photocatalyst were synthesized via a two-step method including in situ microemulsion-assisted reduction and wetness impregnation method.The surface morphology,phase structure,chemical elements and optical properties of the photocatalysts were analyzed by TEM,XRD,XPS and DRS.Furthermore,The photocatalytic activities of g-C₃N₄ with different MoS₂ QDs and Ag content were investigated based on the degradation of MO under visible-light irradiation.Trapping experiment implyed that·O₂^-and h⁺played a major role in the enhanced photocatalytic performance,and analyzed the migration route of the photogenerated charge.The enhanced photocatalytic activity can be mainly ascribed to enhanced visible light absorption,the efficient separation of charge carriers and the stronger oxidation and reduction ability through a Z-scheme system composed of g-C₃N₄,Ag and MoS₂ QDs,in which Ag nanoparticles act as the charge separation center.（4）The Fe（Ⅲ）/Ag/g-C₃N₄ photocatalyst was prepared by using Ag and Fe（Ⅲ）as cocatalysis.The phase structure and chemical status of Fe（Ⅲ）/Ag/g-C₃N₄ composites were analyzed by XRD and XPS and the photocatalytic activity of g-C₃N₄ with different amount of Ag and Fe（Ⅲ）were investigated by degradating MO under visible light irradiation.It can be seen that the Fe（Ⅲ）/Ag/g-C₃N₄ composites exhibit higher photocatalytic activity than Ag/g-C₃N₄,Fe（Ⅲ）/g-C₃N₄ and pure g-C₃N₄ photocatalysts.The synergistic effect of Ag nanoparticles and Fe（Ⅲ）on the photocatalytic activity of g-C₃N₄ was discussed in detail.