Controllable Synthesis Of G-C₃N₄ Materials And Their Photocatalytic Activities In Degradation For Organic Pollutants

Semiconductor photocatalysis has great advantages in solving the energy and environmental crisis. Photocatalysis can convert the clean and renewable solar energy to the chemical energy or degrade environmental organic pollutants or reduct CO2 to olefin under a facile reaction condition. The fabrications of the photocatalysts with high activity and stability are the core of photocatalysis. The traditional photocatalysts(such as TiO2, ZnO and so on) can only be excitated by the ultraviolet light due to the large band gaps, which hinders their developments in the field of photocatalysis. Thus,the visible-light-driven photocatalysts with low cost, high efficiency and stability have attracted much attention. However, the fabrication technologies and photocatalytic mechanism studies have restricted the developments of the photocatalysis.In this dissertation, it is the emphasis that the design and preparation of the low-cast, high-efficiency and stable g-C3N4 materials with the combinative use of the controlled preparation, advanced characterization technologies and mechanism studies,and their application for the photocatalytic degradation of environmental organic pollutants. In the meanwhile, the relationship between the photocatalytic performance,structure and composition provides a well-defined platform for further understanding the photocatalytic meachanisms. The composition and structure of g-C3N4 materials have been tailored toward the higher photocatalytic activity for photocatalytic degradation of the environmental organic pollutants. To improve the efficiency of the photongenerated carriers separation, the dimensionality of g-C3N4 has been also tailored. The transfer direction of the photogenerated carriers of g-C3N4 has been tailored toward better producting the photogenerated carriers so that boost the photocatalytic activity. The main results are summarized as follows:1. The cubic CeO2 is loaded on the g-C3N4 to form the heterogeneous photocatalysts by in-situ growth method. For CeO2/g-C3N4, the content of CeO2 is optimized so that maximize the photocatalytic activity for photocatalytic degradation of the environmental organic pollutants. In CeO2/g-C3N4 composites, the sizes of cubic CeO2 are 3-10 nm, indicating that composites possess quantum confinement effect. The effective separation of the photogenerated carriers and the quantum confinement effect improve the photocatalytic activity.2. Graphene-Like C3N4 is prepared via organic solvent liquid-phase exfoliation method. The efficiency of preparing graphene-Like C3N4 is optimized by the different organic solvents. Graphene-Like C3N4 is used in photocatalytic degradation of the environmental organic pollutants. It is found that graphene-Like C3N4 consists of several layers of C-N, which shortens transfer distance of the photogenerated carriers from inside to surface of graphene-Like C3N4. Thus, the photogenerated carries of graphene-Like C3N4 can separate more efficiently. The specific surface area of graphene-Like C3N4 is also improved, showing that it has more adsorption sites and active sites. Thus, the performances of photocatalytic degradation of pollutants are improved.3. Monolayer O-g-C3N4 is prepared by thermal oxidation method. The time and number of the calcination are tailored toward higher yield and better quality for two-dimensional materials. And the photocatalytic performance for degrading organic pollutants of monolayer O-g-C3N4 is investigated. It is found that the thickness of g-C3N4 is gradually decreased during calcining process, finally forming the single atomic layer. However, g-C3N4 is calcined in the air. Thus, a small quantity of O is doped in samples to form monolayer O-g-C3N4. The two-dimensional structure and introduction of O result in the shorter transfer distance, higher diffusion rate and separation efficiency of the photogenerated carriers. Thus the photocatalytic activity of monolayer O-g-C3N4 increases. Compared with P25, the photocatalytic performances of degradation of 4-CP is obviously improved under UV light irradiation.4. Two-dimensional porous ultrathin O-g-C3N4 nanosheets (PUOCNs) are prepared via a facile template-free strategy. The relationships between the photocatalytic performance and two-dimensional porous structure/ introduction of O are studied. The calcination makes g-C3N4 expand, and then expanded g-C3N4 forms two-dimensional porous structure via chemical oxidation method at room temperature.It is found that two-dimensional porous structure and introduction of O boost the photocatalytic performances of degradation of pollutants. Thereinto, two-dimensional porous structure shortens transfer distance of the photogenerated carriers and provides more adsorption sites and active sites. The introduction of O changes charge distribution of materials and improves the life time of the photogenerated carriers.Thus, the photocatalytic activity of PUOCNs increases. Under the visible light irradiation, the photocatalytic activity for degrading MO of PUOCNs is almost 71 times higher than that of g-C3N4.5. The method to synergetic utilization of plasmonic effect of Ag and electroconductibility of CNTs for visible-light-driven photocatalysts is developed by employing two-dimensional g-C3N4 (2D-C3N4) nanosheets. By in-situ growth method,CNTs/2D-C3N4 is firstly prepared. And then the cubic Ag is assembled on CNTs/2D-C3N4 to form Ag/2D-C3N4/CNTs composite by self-assembly method.Under the visible light irradiation, cubic Ag and 2D-C3N4 in Ag/2D-C3N4/CNTs composite are excited and produce the hot electrons and photogenerated electrons.And then the hot electrons can be injected to CB of 2D-C3N4. Whereafter, the merged electrons can transfer by CNTs with good electrical conductivity and participate in the photocatalytic reaction. The synergistic effects of plasmonic effect of Ag and electroconductibility of CNTs improve the photocatalytic activity of Ag/2D-C3N4/CNTs composite.6. Two dimensional a-Fe2O3/2D g-C3N4 composite is prepared via the catalytic synthesis method. The hexagonal a-Fe2O3 synthesized by the hydrothermal method is added in melamine and the mixture is calcined to form Z-Scheme a-Fe2O3/2D g-C3N4 composite photocatalyst. Under the visible light irradiation, ?-Fe2O3 and 2D g-C3N4 in composite can be excited and the electrons at CB of ?-Fe2O3 and the holes at VB of 2D g-C3N4 can recombine. The holes at VB of ?-Fe2O3 and the electrons at CB of 2D g-C3N4 can participate in the photocatalytic reaction, which not only suppresses the recombination of the photogenerated carriers but also enhances redox ability of the composite. By XANES characterization, it is found that the interaction between?-Fe2O3 and 2D g-C3N4 occur through the C sites from 2D g-C3N4 to form the compact interface. The above collectively improve the photocatalytic activity of?-Fe2O3/2D g-C3N4 composite.