Synthesis, Characterization And Photocatalytic Activity Of Graphene-or Graphitic Carbon Nitride-based Photocatalysts

Environment pollution and energy crisis are two of the most serious problems facinghumanity today. As a hot research issue, photocatalytic technology provides a possible routeto solve the environmental pollution and energy crisis. The fast development in thephotocatalytic technology field has suggested the great potential of photocatalysts for theremoval of organic pollutants and water splitting. However, the limited utilization of sunlightand high recombination rate of the photogenerated electron-hole pairs of the many reportedphotocatalysts have restricted the applications of photocatalysis. So far, various approacheshave been introduced to improve the photocatalytic activity for photocatalysts, such as doping,loading metals as a co-catalyst and coupling with other semiconductors.Two-dimensional materials with layed structure, such as graphene and graphitic-likecarbon nitride have draw much attentions due to their excellent physical and chemicalproperties. Nanocomposite photocatalysts based on metal oxides and graphene or g-C3N4possess the properties of individual components or even with a synergistic effect. The highphotocatalytic activity of the catalysts could be a result of the adsorption of contaminantmolecules form the supporter. Nanocomposite photocatalysts also can accelerate theinterfacial electron-transfer process and suppresses the recombination of electrons and holes,thus improving the photocatalytic activity. In this dissertation, we focus on the synthesis,characterization and photocatalytic activity of graphene-or graphitic carbon nitride-basedphotocatalysts. The main points of this thesis are summaried as follows:(1) SnO2nanorods-GR and SnO2nanosheets-GR composites were prepared viahydrothermal and solvothermal process respectively. The samples were characterized bypowder X-ray diffraction, scanning electron microscopy, transmission electron microscopy,UV-vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy andthermogravimetric analysis. By incorporation of graphene, SnO2-GR show enhancedphotocatalytic activity towards the decomposition of Rhodamine B. The mechanism of theirhigh photocatalytic activity is mainly ascribed to the synergy effect between SnO2andgraphene, resulting in the superior adsorption and the improved electric conductivity.(2) Composite photocatalysts based on g-C3N4and SnO2nanoparticles were prepared byheating the mixture of SnO2nanoparticles and melamine in a muffle furnace. Theas-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy,transmission electron microscopy, ultraviolet-visible light absorbance spectra, FT-IR and TG analysis. The significantly enhanced photocatalytic activity of the composite photocatalystswas attributed to the enhancement of electron-hole separation at the interface. It was foundthat holes (h+) was the main reactive species in the photocatalytic degradation of RhB.Furthermore, the dye-sensitised photocatalysis would not happen in this system, althoughRhB was active to visible light.(3) A novel photocatalyst In2O3/g-C3N4(HT) was prepared through a simple method. Thecomposite catalysts displayed higher photocatalytic performance towards the decompositionof RhB and4-NP, which can be attributed to superior adsorption from g-C3N4and the synergyeffect between In2O3and g-C3N4. The highest photocatalytic activity of In2O3/g-C3N4(HT)composite was obtained with19.0wt%In2O3content. It was found that holes (h+) andsuperoxide (O2-) were the main reactive species in the photocatalytic degradation of RhB. Inaddition, The In2O3/g-C3N4(HT) composite catalysts are stable during the reaction and can beused repeatedly.(4) BaTiO3/g-C3N4was prepared based on bandgap engineering between carbon nitrideand BaTiO3semiconductor. The dye-sensitised photocatalysis did not happen inBaTiO3/g-C3N4, due to the lowest unoccupied molecular orbital (LUMO) potential of RhBwas more positive than the CB of BaTiO3. Under visible light irradiation, g-C3N4was excitedto form the photo-generated electron-hole pairs. The most excited electrons in the CB ofg-C3N4were quickly transferred to the CB of BaTiO3.The excited electrons further react withthe absorbed O2to form O2-and H2O2. Thus, more electrons react with the absorbed O2, andthe higher photocatalytic activity was achieved.In summary, construction of heterostructured semiconductor photocatalysts is a promisingway to enhance the separation rate of electons-holes and improve the photocatalyticperformance.