Design And Synthesis Of Carbon Nitride Based Photocatalysts And Studies Of Their Performance

Semiconductor-based photocatalysis technology is one of the best ways to address the energy and environmental problems,because it can not only photodegrade environmental pollutants but also drive water splitting to produce H₂ using solar energy.Graphitic carbon nitride?g-C₃N₄,CN?is a completely metal-free semiconductor photocatalyst constitued of earth abundant C and N elements which can be facilely synthezed synthesized by simple hydrothermal methods.It has high chemical and thermal stability and hence considered as a promising semiconductor photocatalyst for photodegradation environmental pollutants and photocatalytic splitting of water.In this thesis,a series of carbon nitride-based photocatalysts have been synthesized and studied for splitting of water to hydrogen and photodegradation organic pollutants under visible light irradiation.The main achievements of this thesis can be summarized as follows:1)An efficient Fe₂O₃/CN visible light photocatalyst has been synthesized by loading a small amount of Fe₂O₃ with a diameter of 2-3 nm onto the CN nanosheets through a facile deposition-precipitation method.While a novel dual function photocatalyst of Z-scheme heterojunction WO3/CN with excellent adsorption performance and photocatalytic degradation performance for the high concentration level of methylene blue?MB?were successfully prepared by loading a small amount of highly distributed WO3 nanoparticles with a diameter of 10 nm on the CN nanosheets through a facile preparation method.The crystal structure,morphology,chemical states and optical absorption of the composites were investigated,and the results confirmed the strong interaction and the formation of heterojunction structure between highly dispersed Fe₂O₃ or WO3 nanoparticles and CN nanosheets.The results of photocatalytic reactions demonstrated that introducing of Fe₂O₃ or WO3 nanoparticles brought about remarkable improvement of photocatalytic performance of CN.0.1 wt%Fe₂O₃/CN composite had the optimum photocatalytic activity,which is much higher than that of pure CN and Fe₂O₃ under visible-light irradiation.The dual-function photocatalyst of Z-scheme heterojunction WO3/CN with excellent adsorption and photocatalytic performance.The experimental results showed that the introduction of WO3 not only improved the adsorption performance of CN for methylene blue?MB?but also greatly improved the photocatalytic degradation performance under visible light.The 30 wt%WO3/CN composite heterojunction catalyst showed both the highest adsorption capacity(Qe=97.00 mg g^-1)and the best photocatalytic degradation capability for the removal of MB,which is much higher than most of the heterojunction photocatalysts reported in the literature.Baed on the band structure and the results of the photocatalytic activities,a possible new Z-scheme photocatalytic mechanism was tentatively proposed.2)With the purpose of greatly promoting the charge transfer and separation of CN-based semiconductor photocatalyst,an in situ molten salt strategy has been developed to construct polytriazine/heptazine based CN isotype heterojunctions through an in situ ionothermal molten salt method by using low cost and earth-abundant urea as the single-source precursor for the first time.The engineering of crystallinity and phase structure of CN has been attempted through facile tailoring of the condensation conditions in a molten salt medium.Increasing the synthetic temperature and eutectic salts/urea molar ratio leads to the formation of CN from bulk heptazine phase to crystalline polytriazine imide?PTI?phase,while CN isotype heterojunctions are in situ created at moderate synthetic temperature and salt amount.The conduction band potentials can be tuned in a wide range from-1.51 to-0.96 V by controlling the synthetic temperature and salt amount,and the apparent band gap energies are reduced accordingly.These metal-free CN heterojunctions demonstrate a well-ordered needle-like morphology,and the optimal sample yields a remarkable hydrogen evolution rate(4813.2?mol h^-1 g^-1)under visible-light irradiation,improved by a factor of 12 over that of bulk heptazine-based CN and a factor of 4 over that of PTI.The enhancement of the photocatalytic activity was attributed to the synergistic effect of the improved crystallinity with reduced structural defects,the decreased band gap energy,and the efficient separation of charge carriers induced by the formation of heterostructures.3)In order to design and synthesis of CN-based visible-light photocatalyst with efficient photocatalysts for removal of toxic heavy metal and organic pollutant in a single or coexisting system,the ionothermal synthesis conditions were further optimized on the base of our previous work.The as-obtained ms-CN-x composite material appears as well-ordered nanorod structure with high crystallinity and consists of phases of both heptazine-based CN and triazine-based CN.The ms-CN-0.5photocatalyst with structural and morphological merits exhibits remarkable adsorption-photodegradation performance for highlevel methylene blue?MB?.Moreover,simultaneously enhanced photocatalytic reaction rate of Cr???reduction and MB oxidation over ms-CN-0.5 are achieved and systematically investigated,which benefits from improved crystallinity,increased visible-light absorption,and promoted charge-carrier separation via one-dimensional heterojunction nanorods.On the basis of the reactive species study,a synergistic reduction-oxidation elimination mechanism is revealed,indicating that the photogenerated holes are primary active species for MB degradation,which are rapidly scavenged by MB in the Cr???/MB coexisting solution,and the photoexcited electrons are responsible for the reduction of Cr???.