Surface Structure Modulation Of Nitride Catalysts And Their Performance In Photolysis Of Water And Degradation Of Environmental Organic Pollutants

The widespread use of fossil fuels has led to not only an energy crisis but also increasingly severe environmental problems,which has prompted people to design and develop green and clean energy and environmental purification technologies.Using solar energy photocatalysis technology to splite water into renewable hydrogen while simultaneously achieving the effective degradation of pollutants is an effective way to solve the environmental pollution and energy problems.However,the bottleneck that currently restricts the application of photocatalytsis is the low separation efficiency of photocatalytic electron-hole pairs,i.e.,the low photocatalytic quantum efficiency.Therefore,there is an urgent need to develop and expand new and high-performance photocatalytic systems.By effectively modulating the surface structure of photocatalytic materials,the energy efficiency of light absorption,the separation of photogenerated carriers,and the charge transfer efficiency can be significantly improved,thus enhancing the reactivity of photocatalytic materials.In addition,due to the shortcomings in preparation approaches,characterization techniques and exploration of reaction mechanisms,the scope of the applications of photocatalysts may be limited.Therefore,the investigation and construction of efficient photocatalytic materials,the modulation and optimization of the surface structure,and the enhancement of the photocatalytic application efficiency have become critical scientific issues in the field of water photolysis for hydrogen/oxygen production and photocatalytic pollutant degradation.In this regard,typical nitride catalysts（tantalum nitride and porous ultra-thin carbon nitride nanosheets）with visible light response,high structural tunability and stable physicochemical structure were selected as the target subjects in this thesis.Aiming to improve the catalytic activity various co-catalyst systems were designed for the construction of new-type high-efficient photocatalysts..The surface modulation and response mechanism of the microstructure of nitride catalysts by various co-catalyst loadings were explored,the modifying effects of surface manipulating strategy in improving charge separation efficiency,increasing molecular adsorption and activation,improving photoelectric properties and lowering the reaction energy barrier of materials were systematically investigated.Moreover,advanced spectroscopic characterization techniques were applied to study the effects of different co-catalysts on the performance of water photolysis and pollutant degradation.The research provides indexing guidance and identifiable basis for the mechanistic study of different surface strategies on the activity enhancement of semiconductor photocatalyst.The following aspects of works have been carried out and completed:1.Aiming at addressing the high carrier recombination rate and the resultant low photocatalytic performance of tantalum nitride（Ta₃N₅）,iron-nitrogen co-doped graphene oxide loaded tantalum nitride（Fe-NGO/Ta₃N₅）photocatalysts were constructed by high temperature calcination under ammonia gas（NH₃）.By loading Fe-NGO co-catalyst on the surface of Ta₃N₅,the Fe-N_x sites were formed by the coordination of Fe single atoms with pyridine nitrogen.thus increasing the number of oxygen-producing active centers.Meanwhile,Fe-NGO serves as an electron conductor and accelerates the carrier migration and separation,thereby significantly improving the photocatalytic performance of the material.It is shown that Fe-NGO co-catalysts with large specific surface area and excellent electrical conductivity facilitate the electron transfer from the semiconductor Ta₃N₅ to Fe-NGO;the uniformly distributed Fe-NGO traps electrons and leaves holes in the VB of Ta₃N₅,which effectively promotes the enhancement of oxygen-producing activity of Ta₃N₅.The experimental results show that the oxygen production rate reached 184.7μmol g^-1 for 5 h of operation at the optimal ratio of 5 wt.%Fe-NGO/Ta₃N₅,which is about 3.5 times that of Ta₃N₅.2.To address the problems of low photogenerated electron-hole pair separation ability and poor photocatalytic reduction ability of carbon nitride（C₃N₄）materials,the surface-loaded co-catalyst was used to improve their photocatalytic hydrogen production and synergistic pollutant degradation performance.The metal-free photocatalytic system was constructed by loading zero-dimensional nitrogen-doped graphene quantum dots（N-GQDs）on porous ultrathin carbon nitride（PUCN）.The N-GQDs co-catalyst formsπ-πconjugation with PUCN and produces a strong mutual coupling effect.With PUCN as the carrier and N-GQDs as an electron collector,the charge migration was promoted and the light absorption region of PUCN was extended to improve the photocatalytic performance of the catalyst as a whole.The photocatalytic performance test revealed that the hydrogen production rate of the N-GQDs/PUCN photocatalyst reached 1248μmol g^-1 h^-1,208 times higher than that of PUCN,with a quantum efficiency of0.114%in the 405 nm band.With less doping amount,the photocatalytic efficiency of N-GQDs/PUCN can reach half of the photocatalytic efficiency of low-loading of noble metal Pt.In the synergistic hydrogen generation with the degradation of bisphenol A（BPA）,67μmol g^-1hydrogen was achieved in 5 h,while 21%of BPA was degraded.,In the efficiency test of the degradation of rhodamine B（Rh B）,it was found that the degradation rate of 2 wt.%N-GQDs/PUCN was twice that of PUCN.3.Based on the idea that co-catalysts can enhance the catalytic activity of PUCN,graphene quantum dots（NHNH₂-GQDs）containing-NHNH₂ sites were developed,focusing on the exploration of activity enhancement mechanism of photocatalytic co-catalysts by loading this functional group on graphene quantum.The NHNH₂-GQDs were prepared by the"molecular melting"method;the NHNH₂-GQDs/PUCN photocatalysts were constructed using PUCN as the carrier;the mechanism of the photocatalytic performance enhancement of NHNH₂-GQDs on PUCN was systematically investigated by spectroscopic characterizations.It was found that the NHNH₂-GQDs own high electrical conductivity and the ability of providing more active centers.At the same time,the loaded NHNH₂ groups act as Lewis base sites to anchor H⁺（Lewis acid）in solution to capture electrons,thus promoting water photolysis hydrogen generation performance.3 wt.%NHNH₂-GQDs/PUCN has better photocatalytic performance than PUCN,with the hydrogen production of 1023μmol g^-1 in 5 h,which was 93 times higher than that of PUCN;the performance of the degradation of methylene blue（MB）was 3.5 times better than PUCN in 150min.4.In view of the limited spectral utilization and low surface charge density of current PUCN photocatalytic materials,chalcogen-doped graphene quantum dots（O-GQDs,S-GQDs）with efficientπ-conjugation system were loaded onto PUCN catalysts.The enhancement mechanism of photogenerated carriers and the special role played by chalcogen-doped graphene quantum dots were clarified in the catalytic system.It was found that the chalcogen group elements could change the energy barrier of electron transport in GQDs,and chalcogen-doped GQDs could significantly improve the photocatalytic performance of hydrogen production and synergistic degradation of MB in PUCN nanosheets.The efficientπ-conjugated effect of chalcogen-doped GQDs was utilized to generate high-energy hot electrons for the directional migration to the conduction band of carbon nitride during the light illumination.The photocatalytic hydrogen production test revealed that 1 wt.%O-GQDs/PUCN and 1 wt.%S-GQDs/PUCN produced 87.2and 69.5μmol g^-1 of hydrogen in 5 h,respectively,8 and 6.3 times higher than that of PUCN.In the degradation MB test,the degradation rate of O-GQDs/PUCN and S-GQDs/PUCN were 2.2and 1.9 times of PUCN in 150 min.5.The effects of boron-or chlorine-doped graphene quantum dots（B-GQDs,Cl-GQDs）on the performance of the co-catalysts were further explored The one-step solvothermal method was used to construct B-GQDs or Cl-GQDs with hydrophilic effect and fluorescence effect,the edge functionalization of GQDs with B or Cl ligands caused the hydrophilicity and positively charged.More electrons were generated in the photocatalytic process and migrated to the conduction band of PUCN,finally significantly enhanced the overall activity of the catalyst.Meanwhile,B-or Cl-doped GQDs can provide more active sites to manifest its co-catalytic function.The hydrogen production performance of PUCN could be significantly enhanced by doping a small amount of B-GQDs and Cl-GQDs.The 5-h hydrogen production of 3 wt.%B-GQDs/PUCN was 112.4μmol g^-1,which was 10.2 times higher than that of PUCN,while the 5-h hydrogen production of 3 wt.%Cl-GQDs/PUCN was 157.2μmol g^-1,which was 14.2 times higher than that of PUCN.Moreover,MB pollutant degradation rates of B-GQDs/PUCN and Cl-GQDs/PUCN were 2.9 and 1.8 times higher than those of PUCN alone,respectively.