Graphite Carbon Nitride: Preparation, Modification And Enhanced Photocatalytic Activities

Some weakness in the g-C3N4 prepared by common methods such as high surface area, low crystallinity, low quantum efficiency and photoinduced charge separation are intended to have hindering effects in the photocatalytic activity, which greatly limited the applications of photocatalysis technology. To solve this problem, two aspect of promoting the photocatalytic performance of g-C3N4 were proposed. First, oxidation etching, Ag loading and mechanical milling were employed to modify the photocatalysts to promote their dispersity and quantum efficiency. Second, with melamine and urea as precursors and narrow-mouthed bottle as the reaction vessel, the g-C3N4 nanosheets was synthesized with large specific surface area and high crystallinity.The detailed research contents are as follows:(1) The bulk g-C3N4 photocatalysts were synthesized by thermal condensation of melamine at 550℃. Then, oxidation etching and dispersion of bulk g-C3N4 materials was employed with hydrogen peroxide as oxidant, acetic acid and concentrated sulfuric acid as dispersant. The effects of the amount of H2O2, acetic acid and synthesis temperature on the photocatalytic performance of g-C3N4 were systemic studied. The photocatalytic activities of g-C3N4 samples were investigated based on the decomposition of methyl orange under simulated sunlight irradiation. The results revealed that, when the volume ratio of H2SO4:CH3COOH:H202was 20:20:1, the modified g-C3N4 sample performed the highest photocatalytic activity, and the photocatalyst could degrade methyl orange by 93% in 1 h. On the other hand, the higher photocatalytic activity the modified g-C3N4 sample performed with the higher synthesis temperature, but the lower the yield. XRD results showed that the crystallinity of the sample was improved, which indicated oxidation etching could remove the region with low crystallinity and high energy. IR and EDS characterization results show that a small amount of oxygen was doped in sample. TEM and SEM results show that oxidation etching modification can greatly improve the dispersion properties of the sample. High crystallinity, high dispersibility and oxygen doping were proposed to be responsible for the enhancement of photocatalytic properties.(2) The bulk g-C3N4 photocatalysts were synthesized by thermal condensation of melamine at 550℃. Then, g-C3N4 nanosheets were prepared by exfoliation of as-prepared bulk g-C3N4 in deionized water. Ag/g-C3N4 nanocomposites were synthesized by photodepositing Ag nanoparticles onto the g-C3N4 nanosheets. The samples were characterized by X-ray diffraction (XRD), infrared spectral radiometry (IR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-vis diffuse reflection spectra (DRS) and photoluminescence spectra (PL). The photocatalytic activities of Ag/g-C3N4 nanocomposites samples were investigated based on the decomposition of Rhodamine B (RhB) under visible light irradiation. The results revealed that, The Ag/g-C3N4 nanocomposites exhibited significantly enhanced photocatalytic performance for the degradation of RhB compared with pure g-C3N4 nanosheets. The optimal Ag content was determined to be 8 wt%, and the photocatalyst could degrade RhB by 93% in 30 min. Efficient separation of photogenerated electron-hole pairs at the interfaces of Ag/g-C3N4 composite nanosheets and surface plasmon resonance of Ag nanoparticles were proposed to be responsible for the enhancement of photocatalytic properties.(3) The bulk g-C3N4 photocatalysts were synthesized by thermal condensation of melamine at 550℃. Then, a simple one-step ball milling method was presented for the modification of g-C3N4. The g-C3N4 powder was milled for selected milling times of 12 h,1 day,3 days, and 8 days in this experiment. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflection spectra (DRS). The photocatalytic activities of g-C3N4 samples were investigated based on the decomposition of RhB under simulated sunlight irradiation. The results revealed that, as the milling time increases, gradual size reduction, accompanied by continual bandgap absorption shift, occurred accordingly. The as-prepared g-C3N4 nanosheets exhibited much higher photocatalytic activity than that of pristine bulk g-C3N4 for the degradation of RhB. The large surface areas are believed to contribute to the efficient visible light photocatalytic activity.(4) g-C3N4 nanosheets with excellent photocatalytic performance were synthesized by improving the g-C3N4 preparation process. In brief, with melamine and urea as precursors and narrow-mouthed bottle as the reaction vessel, the g-C3N4 nanosheets was synthesized under the non isothermal condition. The effects of the mass ratio of melamine and urea, synthesis temperature, holding time and airtight sex of the reaction vessel were studied. The samples were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), UV-vis diffuse reflection spectra (DRS) and photoluminescence spectra (PL). The photocatalytic activities of the samples were investigated based on the decomposition of RhB and methylene blue (MB) under visible light irradiation. The results revealed that, when the mass ratio of urea to melamine was 5:2, the calcine temperature and lasting time were selected as 575℃ and 6 h, the prepared g-C3N4 nanosheets with high crystallinity, large specific surface area and low resistance performed the highest photocatalytic activity. The results also showed that, the better closure of the reaction container had, the better the photocatalytic activity the photocatalyst performed. By comparing the results from the degradation of RhB and MB, g-C3N4 nanosheets exert better photocatalytic activity for RhB as its dye sensitization effect.