In-situ Construction Of G-C₃N₄ Quantum Dot-based Composite Materials And Their Mechanisms Of Photocatalytic Degradation

Photocatalysis technology,which could convert solar energy into chemical energy and at the same time save energy and solve the problem of water pollution,is the pollution control technology with the most development potential and industrialization prospect.A single semiconductor has some shortcomings,such as low utilization of light,insufficient driving force,and degradation only a few of organic compounds,which cannot satisfy the requirements of the practical applications.Due to its excellent fluorescence performance,moderate band gap,high chemical activity and unique electronic structure,g-C₃N₄ quantum dots can greatly enhance the utilization of sunlight,forms active sites,and improve the separation of photogenerated carriers in semiconductors,and thus the photocatalytic activity of the semiconductor is enhanced.Due to the diversity of raw materials and preparation methods,g-C₃N₄ quantum dots have different elemental composition and functional groups,thus physicochemical property of them are different.To construct high-efficiency g-C₃N₄quantum dots-based composite materials,five different types of g-C₃N₄ quantum dots were synthesized,and their physical and chemical properties were also studied in this paper;Then the quantum dots were combined with the semiconductors FeOOH,TiO₂ and Bi₂WO₆ to construct g-C₃N₄ quantum dots-based composite materials;By adjusting the optical properties,composition and structure,the influence of g-C₃N₄ quantum dots on the photodegradability of the composite was analyzed;Finally,the degradation mechanism of g-C₃N₄ quantum dots-based composites were systematically studied.This research provides theoretical basis and new ideas for the construction of high-performance g-C₃N₄ quantum dots-based photocatalysts.The specific research work is as follows:(1)Firstly,using urea as nitrogen source,and sodium citrate,citric acid and ammonium citrate as carbon sources,three kinds of g-C₃N₄ quantum dots,marked as A,B and C,were prepared by hydrothermal method,respectively;Secondly,using ammonium citrate as raw material,the quantum dots,marked as E,were synthesized;Finally,using barbituric acid and urea as raw materials,the g-C₃N₄ quantum dots,marked as L,were obtained by solid phase method.The results of fluorescence testing show that,the A,B,C,and E have excitation-independent emission behavior,and the emission wavelength is 440 nm,which was independent of the nitrogen source,raw material ratio,and quantum dot particle size,indicating that the fluorescent structures of the four quantum dots were formed by the condensation and polymerization of citrate;When the percentage of barbituric acid rose from10%to 16%,the emission wavelength of quantum dot L was red-shifted from 376 nm to 400nm,and it has excitation-dependent emission behavior;The experiment of photocatalytic degradation of methylene blue show that the photocatalytic performance of the five quantum dots was proportional to their pyrrole N content.(2)Ferrous sulfate,A and L quantum dots were used as source materials,and then the L/FeOOH and A/FeOOH composites were constructed in situ by hydrolysis method,respectively.SEM results show that quantum dots had an inducing effect on the morphological changes of FeOOH.L/FeOOH composite was rod-shaped,while A/FeOOH composite was banded structure;The results of XPS and TEM tests show that the g-C₃N₄ quantum dot/FeOOH was successfully synthesized,and there was electron transfer between quantum dots and FeOOH;Using antibiotics and aromatic hydrocarbon nitro compounds as simulated pollutant,the results of degradation experiments show that,the degradation efficiency of the two compounds could reach as high as more than 80%in a few minutes;The results of photocatalysis mechanism experiments of composites showed that,·OH was the main offensive effect,and·O₂^-were secondly offensive effect in the photocatalytic degradation.The photoelectric test results showed that,A and L quantum dots can promot the separation and transfer of photogenerated carriers in FeOOH.(3)Using TiO₂as the raw material,combined with the quantum dots of C and E,two kinds of quantum dots/TiO₂ photocatalysts,namely C/TiO₂ and E/TiO₂ composites,were obtained by hydrothermal method,respectively.The TEM demonstrated that the quantum dots were only loaded on the surface of TiO₂,and has no effect on the crystal lattice;The photocatalytic experiments prove that g-C₃N₄ quantum dots could enhance the photocatalytic activity of TiO₂,and make it selective for the photo-degradation of organic pollutants;The mechanism experiments show that,compared with pure TiO₂,loading C on TiO₂ can improve the generation of·O₂^-radicals,and E can increase the production of·OH of TiO₂.(4)Using Bi(NO₃)₃·5H₂O,Na₂WO₄·2H₂O,A quantum dots,and g-C₃N₄ nanosheets as raw material,A/Bi₂WO₆ composite and g-C₃N₄ nanosheets/Bi₂WO₆ composite were synthesized by hydrothermal method,respectively.The SEM results show that pure Bi₂WO₆was sheet structure,and the size of Bi₂WO₆ was decreased after modified with A quantum dots,which is attributed to the fact that g-C₃N₄ quantum dots could prevent Bi₂WO₆ crystal growth.The Bi₂WO₆ particle growed larger and formed compact and flower-like structure after modified with g-C₃N₄ nanosheets,resulting from the stronger electrostatic interaction between g-C₃N₄ nanosheets and Bi₂WO₆.The mechanism experiments results show that h⁺was the main offensive effects in the photocatalytic degradation of antibiotic;the photoelectric experiments results revealed that,compared with g-C₃N₄ quantum dots,g-C₃N₄ nanosheets can improve the photogenerated carrier separation of Bi₂WO₆,since there was the electron transfer between two of them.