Synthesis and photocatalytic evaluation of graphitic carbon nitride based ternary heterostructure systems.

Abstract

The problem of water pollution continues to be of significant global concern, which has made water availability one of the biggest humanitarian issues. The use of dyes by the textile industries is considered to be one of the major causes of water pollution, which significantly hamper the availability and the aesthetic of water resources. It is therefore imperative that wastewater treatment techniques be enhanced to deal with this problem. Photocatalysis based on semiconductor heterojunction systems has shown to be a promising technique for the removal of these dyes from wastewater. Therefore, in this thesis, different heterostructures based on graphitic carbon nitride (g-C3N4) were synthesized as visible light photocatalysts for dye degradation. The as-synthesized materials were assessed for their structural, optical, and optical characteristics using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and ultraviolet (UV) techniques, and their photocatalytic potentials were evaluated against methyl orange. Firstly, Bi2S3 nanostructures were prepared using bismuth(III) dithiocarbamate as a singlesource precursor and in different solvents. Then, the effect of Cu doping on the structural and optical properties of Bi2S3 was explored, as this is very important in improving the photocatalytic activity of developed heterostructures. The formation of bismuth sulphide (Bi2S3) and copper-doped bismuth sulphide (Cu-Bi2S3) with different amounts of Cu2+ dopant was successfully achieved through the solvothermal method. Structural analysis of the nanorods, using the Scherrer equation and Williamson-Hall technique showed a direct correlation between dopant concentration and crystallite size, which was in the range of 23.49 to 34.16 nm (Scherrer's) and 27.19 to 35.92 nm (Williamson-Hall's). The dislocation density, however, showed an inverse correlation with the dopant's concentration. The Cu doping shifted the bandgap of Bi2S3 between 1.35-1.78 eV, which was confirmed by first principle calculations. Subsequently, the photocatalytic degradation of methyl orange dye was carried out with Bi2S3 and Cu-doped Bi2S3 nanorods under visible light irradiation. The Cu-doped Bi2S3 showed a photocatalytic efficiency of 98% compared to the 88% recorded for the pristine materials. Further investigation into the mechanism was conducted on the possible mechanism involved in the MO degradation and the possible active species involved. In the degradation process, the most active species appears to be the radical through radical scavenging experiments. In order to improve charge transfer, it is essential to reduce electron-hole recombination by controlling the charge transfer process and light absorption property of developed photocatalysts. Therefore, the reported copper-doped bismuth sulphide nanoparticles (CuBi2S3) were further incorporated into graphitic carbon nitride nanosheet (g-C3N4) using the solvothermal method. By forming heterostructures between g-C3N4 and Cu-Bi2S3, the band gaps were narrowed compared to the pristine materials. The obtained S-scheme heterostructure showed a Cu-Bi2S3 concentration-dependent photocatalytic efficiency, with the highest efficiency of 94% achieved for MO degradation, showing the high synergy between the components. Studies on radical scavenging indicated that superoxide radicals and photogenerated holes had the greatest impact on the process. This study confirms that the heterostructure formed from g-C3N4 and semiconductor nanomaterials could enhance dye degradation in an efficient and facile manner.Furthermore, g-C3N4-based ternary heterojunctions (g-C3N4/Bi2S3/CuS and g-C3N4/CuBi2O4) were developed and their photocatalytic performance was evaluated under visible light. A novel g-C3N4/Bi2S3/CuS ternary heterostructure material was successfully achieved by the solvothermal method, which comprises p-type CuS, n-type Bi2S3, and n-type g-C3N4. The produced g-C3N4/Bi2S3/CuS heterojunction system was able to minimize the recombination rate of the photogenerated charge carriers, enhancing charge carrier separation. Compared to pristine materials (g-C3N4, Bi2S3, CuS) and binary composite (g-C3N4/Bi2S3, g-C3N4/CuS), the g-C3N4/Bi2S3/CuS heterostructure showed an improved photocatalytic performance for MO degradation, attaining 98% degradation efficiency and a reaction rate constant of 8.10 × 10-2min-1. The improved photocatalytic performance of the ternary structure was due to the effective separation of charge carriers and enhanced visible light absorption. The ternary composite provides effective electron separation and transportation, as well as efficient application of visible light, making it a good photocatalyst for MO dye degradation. Based on the effect of the radical scavengers, the photogenerated hydroxyl radicals (⦁OH), electrons (e-), and holes (h+) were observed to play major roles during the MO degradation process. A charge transfer scheme and mechanism of reaction were proposed on this basis. The ternary composite could possibly present new perspectives for developing new, effective and stable g-C3N4 based photocatalysts for dye removal applications. Finally, g-C3N4/CuBi2O4 p-n heterojunction was developed and evaluated for its photocatalytic activity. The TEM and SEM images showed that CuBi2O4 was successfully incorporated into g-C3N4 nanosheets. The ternary g-C3N4/CuBi2O4 heterojunction showed good photocatalytic performance in the visible light irradiation compared with pristine g-C3N4 and CuBi2O4. The pseudo first-order rate constant was also considerably improved by the nanocomposite during the process. The highest photocatalytic performance was achieved by g-C3N4/CuBi2O4(20%) composite, which is due to improved visible light absorption, and enhanced charge carrier separation between CuBi2O4 and g-C3N4 during photocatalysis. Moreover, g-C3N4/CuBi2O4 heterostructure showed great activity across different pH, concentration, and catalyst dosage values indicating that the system may be applicable to a variety of wastewater sources. The photogenerated holes (h+) and superoxide radicals (O2- −), were observed to be the main active radical species involved in photocatalytic MO degradation by the g-C3N4/CuBi2O4 p-n heterojunction photocatalyst. The results from this study show that g-C3N4 based heterostructured photocatalysts are effective for the photocatalytic degradation of dye pollutants in wastewater. Therefore, the use of these technologies can be extended to other organic pollutants.