Preparation Of Ag₃PO₄-based Composite Photocatalyst And Its Photodegradation Behavior And Mechanism Of Dyes

In recent years the coating,textile,printing and dyeing.and other industries has developed a lot,and simultaneous the discharge of dye wastewater has caused more and more serious pollution to the environment.Semiconductor photocatalysis shows great potential in the treatment of dye wastewater and other pollutants which are difficult to be treated by traditional methods.Among them,silver phosphate(Ag3PO4),as the third generation of visible-light response photocatalytic material,has become a research hotspot in the removal of refractory organic pollutants in water due to its high efficiency and low toxicity.Unfortunately,due to the limitation of thermodynamics,the crystal surface with high catalytic activity is often not easy to form during the preparation of Ag3PO4.What’s more,Ag3PO4 has a non-negligible problem of photocorrosion in the process of catalysis,which greatly restricts its practical application.In view of the above two problems,this paper conducts an in-depth study on the synthesis of Ag3PO4-based composite photocatalyst and its photodegradation behavior and mechanism of dyes by means of controlling morphologies and the separation path of photogenic carriers.The research results are as follows:we successfully synthesized rhombic dodecahedrons CQDs/Ag3PO4 composites with only {110} facets exposed through electrochemical oxidation and chemical deposition.Thanks to the unique up-converted emissions of CQDs,it can absorb visible light with a longer wavelength and emit shorter wavelength,thus making CQDs/Ag3PO4 composites harness more incident light.As a result,more photogenerated electrons and holes are formed.The optimal content of CQDs in CQDs/Ag3PO4 composites is 1.2%,and the bandgap of it is 2.29eV,which is lower than that of pure Ag3PO4(2.45eV).CQDs/Ag3PO4 composites showed great photocatalytic activity and more than 98.22%of rhodamine-B(Rh-B)could be photodegraded within 15min.After 5 cycling runs,CQDs/Ag3PO4 composites could still degrade 92.22%RhB,whereas the degradation rate of pure Ag3PO4 decreased only to 39.08%.CQDs played the role as electron acceptors,as a result the photogenerated electrons could easily transfer from the surface of Ag3PO4 to CQDs,inhibiting the corrosion of Ag3PO4 in the catalytic process.The trapping experiments were carried out to investigate the photodegradation behavior and mechanism of Rh-B,and it demonstrated that the holes were the main active oxidizing species in the reaction process.In order to further inhibit the photocorrosion in the process of photocatalysis thus to improve the stability of Ag3PO4,CNQDs/Ag3PO4 composites were successfully prepared by gradually stripping bulk g-C3N4 and chemical deposition.Similar to CQDs,CNQDs also played an important role in controlling the morphology of Ag3PO4.Due to the existence of unbonded ’N’ atoms in-NH2 in the CNQDs,it formed a complex with Ag+in Ag3PO4,which promoted the self-assembly process of the catalysts and further formed the octopods shape on the basis of fully exposing the {110} crystal surface.The optimal content of CNQDs in CNQDs/Ag3PO4 composites is 12.5%.Under this circumstance,it showed stronger catalytic activity compared with pure Ag3PO4.The degradation rate of 12.5%CNQDs/Ag3PO4 was 5.1 times higher than that of Ag3PO4.After continuous cyclic degradation of 6 times,the catalytic activity only decreased slightly,which indicated the corrosion resistance was greatly improved.In order to elucidate the degradation mechanism,the trapping experiments the DMPOESR measurements were conducted.It is proposed that binary CNQDs/Ag3PO4 could be direct Z-scheme photocatalysts.When the composites were photoexcited,Ag3PO4 and CNQDs generated their own photogenic carriers.Then the photogenerated electrons in the Conductive band(CB)of Ag3PO4 quickly transferred to CNQDs and combined with the holes in the Valence band(VB)of CNQDs,leading to the accumulation of electrons in the CB of CNQDs and holes in the VB of Ag3PO4.The electrons holes in the VB of Ag3PO4 would react with pollutants directly,and the electrons in the CB of CNQDs could reduce O2 into ·O2-.So the holes and ·O2-were the main active oxidizing species in the reaction process.