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The Effect of Graphene Quantum Dots Addition in Modifying 1D ZnO Nanostructured Photocatalysts

Ebrahimi, Mahdi | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50749 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Moshfegh, Alireza
  7. Abstract:
  8. In this research, two type of graphene quantum dots (GQDs) have synthesized including, normal GQDs and nitrogen-doped GQDs (N-GQDs). Then, each of GQDs were deposited on the 1 dimensional zinc oxide (ZnO) nanostructures namely nanowires (NWs) and branched hierarchical (BH) systems. Then, photocatalytic activity of the samples were studied in photodegradation of methylene blue (MB) and methylene orange (MO) under visible light. First, ZnO nanowires (ZnO NWs) were fabricated by anodization of zinc sheet possessing wurtzite crystal structure with their average length and diameter measured in the range of 50-60 μm and 170-300 nm, respectively. Then, normal GQDs were synthesized by electrochemical method in a range of 12-22 nm. Different amounts of GQDs including 5, 10, 20 and 40 μL with concentration of 2 mg/mL were decorated uniformly on the surface of ZnO NWs forming GQDs/ZnO NWs. The GQDs/ZnO NWs exhibited a considerable enhancment on the photocatalytic degradation of MB under solar irradiation.The results indicated that the optimized GQDs (10 μL)/ZnO NWs showed the highest photoactivity with about 2.8-fold increase as compared to the pure ZnO NWs. In addition, a good photostability was found for the GQDs (10 μL)/ZnO NWs samples due to formation of Zn−O−C bonds, which reduced the activation of the surface oxygen against corrosion as comared to the ZnO NWs. Finally, a mechanism of charge carrier transport and separation was proposed using different scavengers (KI and AgNO3) during photocatalytic degradation reaction. In the next study, Branched hierarchical ZnO NWs (BH-ZnO NWs) were fabricated successfully on the ZnO NWs by a solution based method at very low temperature (31 oC). The BH- ZnO NWs with different aspect ratio were obtained by varying growth time (0.5, 2, 5, 10 h). Photocatalytic activity of the samples was investigated under both UV and visible light. The results indicated that the optimized BH-ZnO NWs (5 h) as a photocatalyst exhibited the highest photoactivity with about 3 times higher than the ZnO NWs under UV light. Then, N-GQDs were synthesized in a size range of 2–12 nm by using hydrothermal method. Subsequently, different amounts of N-GQDs including 10, 20 and 40 μL with concentration of 2 mg/mL were deposited on the surface of BH-ZnO NWs (5 h) and forming N-GQDs/BH-ZnO NWs (5 h). Based on photocatalytic data analysis, it was found that the optimized N-GQDs(20 μL)/BH-ZnO NWs (5 h) sample exhibted about 1.8 times enhancement as compared to the pure BH-ZnO NWs (5 h) during MB photodegradation reaction under visible light. Finally, charge transfer mechanism was proposed by using different scavengers in photocatalytic degradation process under UV-visble irradation
  9. Keywords:
  10. Nanowire ; Photocatalyst ; Graphene Quantum Dots (GQDs) ; Zinc Oxide ; Quantum Dot

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