Synthesis And Photocatalytic Activities Over G-C₃N₄-Based Composite Semiconductor Photocatalysts

The increasing global environmental pollution and energy issues are the two daunting threats to the long-term development modern human society.Due to the large-scale use of fossil fuels,both serious environmental pollution and depletion of energy sources have occurred.Among various potential solutions,semiconductor-based photocatalysis has been considered as the most promising way.On one hand,it can take advantage of the solar energy to split water to produce hydrogen energy;on the other hand,the solar energy can also be used to degrade pollutants,such as dyes,antibiotics,nitrophenols and so on.The graphitic carbon nitride（g-C₃N₄）,as a new type non-metal material,has been catching unprecedented attention of researchers.However,its reaction activity is restricted by the weak visible-light absorption and low charge separation efficiency.Fabricating composites based on g-C₃N₄has been proved to be an effective route to enhanced the visible light absorption of g-C₃N₄and improve photocatalytic performance.In this paper,three types of composite semiconductor photocatalysts were constructed based on g-C₃N₄.Photocatalytic water splitting and pollutants degradation performances were tested,and the mechanism was also investigated.Specifically,our work involves the following three aspects:（1）g-C₃N₄nanosheets were synthesized by calcining urea at high temperature,and Zn_0.8Cd_0.2S/g-C₃N₄composites were successfully obtained via hydrothermal method.XRD,XPS,TEM and SEM were used to characterized their crystal structure,chemical composition and morphology.The optical properties of the samples were characterized by UV-vis DRS and PL measurements.The results show that Zn_0.8Cd_0.2S/g-C₃N₄composites presented a core-shell structure.Compared with pure g-C₃N₄,the visible light absorption of the composites was increased,while the recombination of photo-generated electrons and holes was decreased.The photocatalytic hydrogen production activities of these samples were evaluated.It is found that the H₂ production rates of Zn_0.8Cd_0.2S/g-C₃N₄-10wt% sample was 2351.18μmol·h^-1·g^-1,which is 146.0 and 5.7 times higher than that of pristine g-C₃N₄nanosheets and twinned Zn_0.8Cd_0.2S.According to the Motte-Schottky result,the band structures of g-C₃N₄and Zn_0.8Cd_0.2S were determined and the electron and hole transfer process was analyzed.It is revealed that Zn_0.8Cd_0.2S/g-C₃N₄is a typical typeⅡheterostructure which enables the effective separation of carriers.（2）S-doped ZnO/g-C₃N₄composite photocatalyst was synthesized by one-step thermal condensation using flaky ZnS（en）_0.5and urea as precursors.All the samples were characterized using XRD、XPS、SEM、TEM、UV-vis DRS、PL.Characterization results showed that S-doped ZnO nanoparticles were uniformly distributed on the g-C₃N₄support,and the size ranges from 2 to 6 nm.The visible light absorption of the composite was enhanced,and the recombination of photo-generated electrons and holes was suppressed.Photocatalytic degradation of methylene blue（MB）experiment showed that the photocatalytic activities of the prepared S-doped ZnO/g-C₃N₄composite were significantly improved.It is found that S-doped ZnO/g-C₃N₄compound semiconductors are direct Z type heterostructures.The carriers are separated effectively,and a strong redox capability is also remained.（3）Cu₂O/g-C₃N₄composites were synthesized using g-C₃N₄nanosheets and copper nitrate as raw materials.It is revealed that Cu₂O nanospheres with a diameter of about150-200 nm were composed of smaller nanoparticles.After the loading of Cu₂O nanospheres,the visible light absorption of g-C₃N₄was greatly enhanced,and the suppression of photoinduced electrons and holes was achieved.The photocatalytic H₂production of the as-prepared photocatalysts were tested and it turned out that the composites photocatalyst showed excellent photocatalytic performance in methanol solution under visible light.The average H₂production rate was up to 637μmol·g^-1·h^-1when the added amount of g-C₃N₄was 150 mg.It is found that Cu₂O/g-C₃N₄presents a special p-n junction,which inhibits the recombination of photogenerated carriers.At the same time,the built-in electric field in the p-n junction can promote the transfer of electrons and holes.