Synthesis And Performance Study Of G-C₃N₄-Based Composite Photocatlysts

As a metal-free visible-light-excitable semiconductor, graphitic carbon nitride （g-C₃N₄） has been widely used in photocatalytic fields such as splitting of water into hydrogen, degradation of organic pollutants, and reduction of carbon dioxide, owing to its moderate band gap （2.7eV）, high stability, cheap and facile production. But the prepared g-C₃N₄ nanomaterials with traditional high-temperature thermal polymerization method have many disadvantages such as low surface area, fast photogenerated electron-hole recombination, low quantum efficiency, and so on, and these weaknesses seriously hampered their practical applications in energy- and environment-related areas. To enhance the photocatalytic activity of g-C₃N₄ nanomaterials, our work aims to design and synthesize g-C₃N₄-based composite photocatalysts, in order to enhance the electron-hole separation and broaden their light absorption. The developed photocatalysts have been characterized by transmission electron microscopy （TEM）, X-ray diffraction （XRD）, infrared spectroscopy （FT-IR）, X-ray photoelectron spectroscopy （XPS）, UV-Vis diffuse reflectance spectroscopy （UV-vis）, photoluminescence （PL）, photocurrent （i-t）. Their photocatalytic activities were also assessed by the degradation of an organic pollutant Rodamine B （RhB）. The main contents of this thesis are summarized as follows:1. g-C₃N₄ nanosheets were obtained with a one-step calcination method under the assistance of water. The g-C₃N₄/Bi2WO6 composite photocatalysts were prepared by in situ growth of Bi2WO4 nanocrystals on the g-C₃N₄ nanosheets through solvothermal method. The XRD, XPS, FT-IR data confirmed the hybridation of g-C₃N₄ and Bi2WO6 in the composites; TEM data revealed a tightly-connected interface between Bi2WO6 and g-C₃N₄; UV-vis analysis indicated that the composites had enhanced absorption in the visible range; The decrease in PL intensity and increase in photocurrent data implied that the electron-hole recombination has been significantly restrained in the composites. Fast degradation of RhB under visible-light showed that the g-C₃N₄/Bi2WO6 composites had excellent photocatalytic activity and its degradation rate was almost 5 and 9 times as high as pure g-C₃N₄ and respectively. The enhanced visible-light photocatalytic activity in the composite can be assigned to the formation of heteroj unction structure between the g-and Bi2WO6, which could effectively promote the separation of photogenerated electrons and holes.2. NaYF4:Yb,Tm upconversion nanocrystals were prepared by high-temperature thermal decomposition and hydrophobic ligands were removed from them via HCl protonation. Ultrathin g-C₃N₄ nanosheets were obtained by liquid exfoliating of bulk g-C₃N₄ in water which was initially prepared by thermal condensation of melamine. These two components were self-assembled together by electrostatic interactions and the g-C₃N₄/NaYF4:Yb,Tm composite photocatalysts were formed. Uponversion PL spectra showed that the NaYF4:Yb,Tm can absorb NIR light and give UV and Vis emissions to be absorbed by the g-C₃N₄. Under NIR exposure （780nm< λ< 2500 nm） or a 980-nm NIR diode laser, the g-C₃N₄/NaYF4:Yb,Tm composite photocatalysts exhibited obvious activity in response to the NIR light, and showed enhanced activity under the full solar spectrum. Through controlling the ratio of NaYF4:Yb,Tm to g-C₃N₄, we also investigated the payload effect of upconversion nanocrystals on their total activities in the composites. Both theoretical and experimental analyses showed that the enhance activity in the composite photocatalysts was attributed to the upconversion nanocrystals which proved extra UV and Vis lights for the g-C₃N₄, leading to a broad utilization of the solar spectrum in the NIR region.