Synthesis And Properties Of Graphitic Carbon Nitride Based Photocatalytic Materials

Photocatalytic technology,started in the 1970s,has broad application prospects in the fields of new energy and environmental purification because it cannot only convert solar energy into electric energy,chemical energy,but also catalyze the degradation of organic pollutants.The key to photocatalytic technology is to design and develop efficient,stable and inexpensive photocatalysts.Graphic carbon nitride(g-C3N4)is one kind of semiconductor of nonmetallic organic polymer.It has attracted wide attention due to its chemical stability and visible light excitability.However,photocatalytic property of pure g-C3N4 is not good enough because of the rapid recombination of photogenerated electron-hole pairs and low utilization efficiency of solar energy.In this paper,several methods,such as noble metal loading,metal cationic doping,heterojunction constructing with other semiconductors with suitable band structure,etc.were used to improve photocatalytic property by accelerating the transfer of photogenerated electrons to the reaction system,widening the light responsive range,optimizing the transmission path of photogenerated charges and enhancing the separation efficiency of photogenerated electron-hole pairs.The crystal phases,morphologies,chemical compositions,energy states and optical properties of synthesized g-C3N4 based catalysts were characterized by X-ray diffraction(XRD),UV-Vis spectroscopy,Scanning electron microscopy(SEM),N2 adsorption,electrochemical impedance spectra(EIS)and X-ray photoelectron spectroscopy(XPS).The catalytic activities in the nitrogen fixation,hydrogen production,phenol degradation and Rhodamine(RhB)degradation were evaluated,and the photocatalytic mechanisms were discussed.The main findings are listed as follow:(1)A novel Ag-g-C3N4/W18O49 heterojunction catalyst was prepared through noble metal loading and heterojunction constructing.And it was the first time to be used in full-spectrum-driven N2 photofixation from ultraviolet(UV)to near infrared(NIR)region.The results showed that Ag-g-C3N4/W18O49 performed as a Z-scheme photocatalytic system.The loaded Ag advanced the separation of electron-hole pairs and it acted as a bridge to transmission of photogenerated electrons due to its surface plasma effect.g-C3N4 was the active component in the catalyst for N2 photofixation.W18O49 with plenty of vacancy oxygen played a role in light absorber in the full-spectrum to form more photogenerated electrons.These photogenerated electrons crossed the interface along "Z" type path andrecombined with the holes in the valence band(VB)of g-C3N4.The reduction was completed by the electrons in the conduction band(CB)of g-C3N4 being transferred quickly by Ag to N2 then.The Ag-g-C3N4/W18O49(1:1)heterojunction catalyst displayed much higher N2 photofixation performance and excellent stability due to the better separation rate of electron-hole pairs and more efficient light utilization.The highest r(NH4+)was 3.20 mg-L-1·h-1·gcat-1),which is 8.9 times higher than that of pure g-C3N4.(2)Two types of visible-light-driven(BiO)2CO3/g-C3N4 composites were prepared by self-assembly and chemical precipitation.Photochemical performance was studied in phenol and RhB degradation.The results showed that this type of catalyst belongs to heterojunction catalysts with(BiO)2CO3 2D nanosheets decorating on the g-C3N4 sheets instead of self-assembling into spheres.This structure can enlarge the contact area between two semiconductors to facilitate the transfer of photogenerated charges through the interface.The synthetic methods had no influence on the crystal phase,morphology and optical property of the obtained composite catalysts.But there exists a great influence in the interaction strength between g-C3N4 and(BiO)2CO3,resulting an obvious difference in the separation rate of photogenerated electrons and holes.With the photosensitization of rhodamine B(RhB),the(BiO)2CO3/g-C3N4 composites prepared by self-assembly method showed the best photocatalytic performance and stability.The degradation rate constant was 7.6,5.9 and 3.8 times higher than that of single g-C3N4,(BiO)2CO3 and their mechanical mixture,respectively.The study on active species in the reaction showed that the ·O2-radicals were confirmed its responsibility for RhB visible-light-driven degradation.(3)Three g-C3N4/WO3 heterojunctions were prepared through ultrasonic dispersion,hydrothermal method and calcination,respectively.The Z-scheme photocatalytic system was formed by two semiconductors of g-C3N4 and WO3 with suitable band structures,in which the recombination of photogenerated electron-hole pairs was depressed and the high reducibility of electrons in the conduction band(CB)of g-C3N4 and the high oxidizability of holes in the valence band(VB)of WO3 were maintained.The ·O2-and ·OH radicals were responsible for the high efficiency of RhB degradation.The g-C3N4/WO3 composites prepared by hydrothermal method showed the highest photocatalytic activity and stability due to the strongest interaction between g-C3N4 and WO3.RhB degradation rate was 2.1 which was 3.8 times higher than that of pure g-C3N4 and WO3,respectively,and H2 production was 295.5?mol·h-1,15.7 times higher than that of pure g-C3N4.(4)A series of alkali metals co-doped graphitic carbon nitrides(g-C3N4)were prepared using melamine as precursor,KCl and NaCl as molten salt.The K and Na were well-distributed on the surface of g-C3N4,and the alkali metals co-doping leaded to restraingrowth of carbon nitride grains,reduce recombination of photogenerated electron-hole pairs and increase absorption of visible light.By controlling the weight ratio of eutectic salts to melamine,the VB and CB potentials of prepared graphitic carbon nitride products could be tuned from+1.55 to+2.30 V,and from-1.13 to-0.26 V,respectively.The RhB photo-degradation mineralization performance were improved by ·O2-and ·OH radicals,and CDCN-15 exhibited the highest photocatalytic performance and stability,its degradation rate was 6 times higher than that of neat g-C3N4.