Preparation Of Graphite G-C₃N₄ Based Photocatalysts And Their Photocatalytic Performance

Photocatalytic technology is one of green technologies, which has been widely used in environmental pollutant control and solar energy conversion. Graphite carbon nitride (g-C3N4), a novel polymer semiconductor photocatalyst, has drawn much attention for its unique properties such as visible responded, high chemical inertness and thermal stability. However, the photocatalytic activity of g-C3N4 is low due to its high photogenerated carriers recombination rate and the weak oxidation ability of photogenerated hole. As we know, the heterojuction can provide driving force for the photogenerated carrier separation, thus improve the quantum yield. In this dissertation, we mainly focus on designing high-efficiency g-C3N4-based heterojunction to enhance the photocatalytic activity of g-C3N4, and several works have been done as follows:(1) A novel polyimide (PI)/g-C3N4 heterojunction was prepared through a facile chemisorption approach. The PI/g-C3N4 composite exhibited extraordinary high photogenerated carrier separation efficiency and the highest photocurrent density was 4 times as high as g-CsN4.2,4-dichlorophenol (2,4-DCP) was chosen as the target to investigate the photocatalytic performance of the composite. And the results showed that the PI/g-C3N4 composites sequentially presented high-efficiency photocatalytic activity in degradation of 2,4-dichlorophenol (2,4-DCP) for 4 hours under visible light irradiation. In particular, the optimum 2,4-DCP removal rate of the PI/g-C3N4 composites with weight ratio of PI at 30% was 98%which was almost 3.8 times as high as that over g-C3N4. Furthermore, it was found by experimental analysis and density functional theory (DFT) calculations that the superior photocatalytic performance of the composites can be attributed to the facile band alignment and different electronic structure of g-C3N4 and PI components in the heterojunction for efficient charge separation and transfer.(2) The Ag2Cr04/g-C3N4 heterojunction photocatalyst was synthesized using a simple in situ chemical precipitation method. The introduction of Ag2CrO4 on g-C3N4 promoted its light absorption performance. And the Ag2CrO4/g-C3N4 composite displayed high photogenerated carrier separation ability and the highest photocurrent density is 4.4 times as high as that of g-C3N4. The Ag2CrO4/Vg-C3N4 composites showed excellent photocatalytic capability of 2,4-dichlorophenol (2,4-DCP) degradation for 2 hours under visible light irradiation. The 10% Ag2CrO4/g-C3N4 exhibited the highest photocatalytic performance which the 2,4-DCP removal rate can reach 94% after 2 hour. And the kinetics constant was 5.2 times as high as that over g-C3N4. DFT results showed that Ag2CrO4 exhibit high-efficiency charge separation and transfer ability, which can boost the photogenerated carrier transfer to the surface of photocatalyst to participate in the reaction. And base on the active species capture experiment and the electron spin resonance (ESR) technology, the Z-scheme charge transfer mechanism was identified. The photogenerated charge carriers in the Ag2CrO4/g-C3N4 heterojunction displayed a higher separation compared to those in carbon nitride alone, and the heterojunction system can maintain the high redox ability simultaneously.(3) A two-dimensional BiOIO3/g-C3N4 layer heterojunction photocatalyst was fabricated by using the electrostatic self-assembly and heat treatment method. The efficient BiOIO3/g-C3N4 heterojunction photocatalyst was constructed through a face to face combination with an intimate contact, and the effective interfaces can provide a huge surface and active sites enhance the adsorption capacity and speed the surface reaction rate. And the build-in electric field at the interface can boost the photogenerated carrier separation, thus enhance the photocatalytic performance of the composite. The photoelectric test result showed that the BiOIO3/g-C3N4 composite exhibited a high photocurrent density which is 4.1 times as high as that of g-C3N4. The photocatalytic hydrogen production test and the 2,4,6-trichlorophenol (2,4,6-TCP) photocatalytic degradation experiment were conducted to evaluate the photocatalytic performance of the composite in solar energy conversion and environment pollution control. And the results showed that the composite displayed high photocatalytic activities in both aspects, the hydrogen evolution rate over BiOIO3/g-C3N4 was 3.5 times as high as g-C3N4 alone. And the 2,4,6-TCP removal rate is 4.8 times as high as g-C3N4 alone under simulated solar irradiation. The active species capture experiment demonstrate that the hydroxyl radical and the superoxide radical were the main active species in this reaction, and high-performance liquid chromatography-mass spectrometry (LC-MS) analysis result showed that 2,4,6-TCP photodegradation pathway of the BiOIO3/g-C3N4 composite was the dechlorination process of the 2,4,6-TCP.The hydroxyl radical attacked on the chlorine atoms of benzene ring, thus decomposing the 2,4,6-TCP step by step. By employing the electron spin resonance technology and the chemical fluorescence technology, a Z-scheme charge transfer mechanism was confirmed. The improved performance of the BiOIO3/g-C3N4 hybrid nanocomposite photocatalyst was attributed by a synergistic effect, including the relatively high surface area, high charge separation efficiency of the photoinduced charge carriers and the maintained high redox ability of the photoinduced electron and hole.