Preparation And Photocatalytic Activity Regulation Of Graphitic Carbon Nitride

Nowadays,with the deepening of the industrialization process,human beings are facing the great challenges of the environment pollution and the energy shortage,especially the water pollutants,most of which are organic,toxic,with extensive source and very stable.Thus,the organic pollutants in water are very difficult to degrade.Recently,photocatalytic technology has been applied to the environment remediation and sewage treatment,and the effect is remarkable.However,traditional semiconductor titanium dioxide?TiO2?cannot be widely used in the practical application because of its poor response to visible light.Therefore,as a novel polymeric semiconductor photocatalyst,graphite carbon nitride?g-C₃N₄?has attracted considerable attention because of the characteristics of the metal-free,non-toxic and easy modification.Unfortunately,the photocatalytic efficiency of g-C₃N₄ is far from commercial application prospects due to not only the low surface area and low photocatalytic efficiency but the high recombination rate of photo-excited charge carrier.In this paper,some strategies were used to improve the performance of graphite carbon nitride photocatalyst such as heterojunction construction,noble metal deposition,surface area increasing and non-metal doping.The main contents were as followings:1.The Bi₂S₃/g-C₃N₄ composite photocatalysts with different Bi₂S₃ contents were prepared by in-situ synthesis method.The phase,morphology,structure and optical property of as-obtained photocatalysts were studied by X-ray diffraction?XRD?,Fourier-transform infrared?FT-IR?spectroscopy,scanning electron microscopy?SEM?,transmissionelectron microscopy?TEM?,UV-vis diffuse reflectance spectroscopy?DRS?,photoluminescence?PL?spectroscopy measurements and time-resolved fluorescence decay spectra.Organic pollutants?Rhodamine B?were selected as pollutant models to evaluate the visible-light photocatalytic activity of Bi₂S₃/g-C₃N₄ composite photocatalysts.As a result,the visible-light photocatalytic activity of g-C₃N₄ was greatly improved by decorating the short rod-like Bi₂S₃ on surface.Besides,the photocatalytic activity of Bi₂S₃/g-C₃N₄ composite photocatalysts was obviously influenced with the adding amount of narrow-band-gap semiconductor Bi₂S₃,and the optimal amount of Bi₂S₃ was determined to be 5 wt%.The roles of reactive species during photocatalysis were verified by a series of combined techniques,including the active species trapping experiments,NBT transformation and terephalic acid photoluminescence probing technique over Bi₂S₃/g-C₃N₄ composites and pristine samples.The results suggested that h+ played the major role and ·O-2 was the secondary active species.Furthermore,the mechanism for photocatalytic activity enhancement of Bi₂S₃/g-C₃N₄ composite photocatalysts was also revealed.It may be ascribed to the increasing visible-light absorption and the formation of heterostructures at the Bi₂S₃/g-C₃N₄ interface,which can facilitate the separation of photogenerated electrons and holes and extend the carrier lifetime.2.g-C₃N₄/PPy/Ag ternary photocatalyst was synthesized by a three-step method.Firstly,graphitic carbon nitride photocatalyst was obtained by sintering melamine at high temperature,then series of g-C₃N₄/PPy composite photocatalysts were prepared by in-situ polymerization of pyrrole,and finally the g-C₃N₄/PPy/Ag ternary composite photocatalyst was obtained by depositing Ag on the surface of g-C₃N₄/PPy.The photocatalytic activities of as-prepared samples were evaluated by degradation of antibiotic tetracycline?TC?under visible light.The influence of the amounts of polypyrrole?PPy?and Ag on the photocatalytic activity of g-C₃N₄/PPY/Ag ternary photocatalyst was investigated.Through the analysis of the optical properties and electronic structure of the as-prepared samples,both PPy and Ag played critical roles for the enhanced photocatalytic activity of g-C₃N₄/PPy/Ag.Therefore,the g-C₃N₄/PPy/Ag composites exhibited remarkably improved photocatalytic activities for degrading TC compared with g-C₃N₄,g-C₃N₄/PPy and g-C₃N₄/Ag.Besides,the highly efficient photocatalytic mechanism of the g-C₃N₄/PPy/Ag photocatalyst was also analyzed.The band match of PPy with g-C₃N₄ benefited the separation of photo-generated carriers in g-C₃N₄,and the deposited Ag nanoparticles played important roles as an electron mediator due to the surface plasmon resonance?SPR?effects.3.The phenolic resin spheres were used as a template,and then mixed with melamine liquid.The obtained mixture was calcined in nitrogen and oxygen respectively to remove the template,and finally the porous g-C₃N₄?CN?structure was successfully prepared.Meanwhile,through a series of characterization methods,the phase structure,morphology and optical properties of the obtained products were analyzed.It showed the obtained g-C₃N₄ sample was porous structure,and significantly increased the absorption intensity in the visible light region and sharply decreased the photogenerated recombination rate.The photocatalytic activity of the obtained samples was evaluated by degradation of RhB under visible light irradiation.The results indicated that the photocatalytic activity of the porous g-C₃N₄ templated by phenolic resin had been improved,and the photocatalytic activity of samples calcined in O2 was much better than that of samples calcined only in N2.Moreover,the photocatalytic activity of the porous g-C₃N₄?CN-0.021?was the highest when the amount of template was 0.021 g,which was 7.8 and 2.2 times higher than that of bulk g-C₃N₄ and C/CN-0.021,respectively.Besides,the reason for the improvement of porous g-C₃N₄ was analyzed,and the result showed that the large surface area?150.5 m2/g?and internal pore structure were the main reasons.4.On the basis of the synthesis of porous g-C₃N₄,phosphorus-doped porous carbon nitride?PCN?was simultaneously prepared by using diammonium hydrogen phosphate??NH4?2HPO4?as phosphorus source.By changing the amount of?NH4?2HPO4 added,a series of PCN photocatalysts were prepared by fixing the amount of melamine and phenolic resin spheres.The photocatalytic activity of the as-prepared samples was evaluated by using Rh B as the degradation model.The surface area?131.2 m2/g?of PCN-0.3 was less than that of CN-0.021?150.5 m2/g?,but its photocatalytic activity did not decrease.The optimal doping amount of phosphorus was discussed.When the amount of?NH4?2HPO4 was 0.3 g,the obtained phosphorus-doped porous carbon nitride?PCN-0.3?had the best photocatalytic performance,which was about 13.4,2.9 and 1.7 times higher than that of bulk g-C₃N₄,P-C₃N₄,CN-0.021,respectively.The optical properties of the as-prepard bulk g-C₃N₄,P-C₃N₄,CN-0.021 and PCN-0.3 samples were studied.The reason for the increased photocatalytic activity of the phosphorus-doped porous carbon nitride was due to the large surface areas providing more reaction sites and the doping phosphorus reducing the bandgap.