Syntheses And Visible-light Driven Photocatalytic Properties Of Silver-containing Semiconductors Based Ternary Composites

In recent decades,with the rapid development of industry,environmental problems,especially water pollution,have become an increasing challenge.Semiconductor photocatalysis is a very promising way to deal with the environmental pollution by using solar energy to drive a series of redox reactions to degrade organic pollutants.Semiconductor material itself is not involved in a chemical change in the process of photocatalytic reaction,but it occurs electron transition under light irradiation to form photogenerated electronic-hole pairs,which further generate the active species such as super oxygen anion?·O₂^-?,holes?h⁺?and hydroxyl radicals?·OH?.These active species can degrade pollutant molecules by redox reactions.In this paper,the research progress,the catalytic mechanism and the application prospect of photocatalytic materials are reviewed.On this basis,four new ternary visible-light driven photocatalysts are reported,including g-C₃N₄/GO/AgBr,g-C₃N₄/NCDs/AgBr,g-C₃N₄/Ag₃PO₄/NCDs and Ag₃PO₄/GO/NCDs.The composition,structure,morphology as well as the light absorption of the as-prepared photocatalysts are investigated by XRD,SEM,TEM,Raman,FT-IR,UV-vis and XPS,etc.The band structures are investigated by VB-XPS?Valence band X-ray photoelectron spectroscopy?and UPS?Ultraviolet photoelectron spectroscopy?.The photocatalytic activities of the ternary photocatalysts are evaluated by the degradation of organic pollutants of MB,RhB and phenol under visible-light irradiation.The photocatalytic degradation mechanisms are studied by trapping experiment and ESR measurement.In addition,the degradation mechanisms of MB and RhB are studied by GC-MS and ion chromatography.The main research contents of this paper are as follows:1.An excellent ternary visible-light all solid Z-scheme photocatalytic heterojunction of g-C₃N₄/GO/AgBr has been prepared by the attachment of graphene oxide?GO?to the surface of g-C₃N₄,followed by in-situ growth of AgBr nanoparticles on GO sheets.The g-C₃N₄/GO/AgBr heterojunction exhibits excellent photocatalytic efficiency for RhB degradation.In the photocatalytic process of g-C₃N₄/GO/AgBr,GO acts as the charge transmission bridge between g-C₃N₄ and AgBr,which is beneficial to the separation of electrons and holes to enhance the photodegradation efficiency and to improve the stability of the photocatalyst.In addition,GO is low-cost as compared with precious metals.Therefore,the development of GO inserted Z-scheme photocatalytic heterojunctions could be a promising approach to high-performance photocatalysts for the environmental and energy applications.2.A new ternary visible-light driven photocatalyst of g-C₃N₄/NCDs/AgBr has been prepared by the introduction of nitrogen-doped carbon dots?NCDs?onto the surface of g-C₃N₄,followed by in-situ growth of AgBr nanoparticles on NCDs-modified g-C₃N₄ nanosheets.The g-C₃N₄/NCDs/AgBr nanocomposite exhibits excellent photocatalytic efficiency for organic pollutant degradation,which is about4.0,5.3 and 2.3 times higher than that of AgBr,g-C₃N₄ and g-C₃N₄/AgBr,respectively.The result indicates the introduction of NCDs into g-C₃N₄/AgBr can largely improve the photocatalytic activity since NCDs act as the light absorber and the electron mediator between g-C₃N₄ and AgBr,which effectively promote the separation of photogenerated charge carriers and the utilization of visible light.This study highlights the potential application of highly efficient NCDs decorated photocatalysts in waste water purification.3.An all solid Z-scheme g-C₃N₄/Ag₃PO₄/NCDs photocatalyst has been prepared through decorating the direct Z-scheme g-C₃N₄/Ag₃PO₄ photocatalyst with nitrogen-doped carbon dots?NCDs?.The g-C₃N₄/Ag₃PO₄/NCDs photocatalyst exhibits excellent photocatalytic activity for the degradation of methylene blue?MB?,rhodamine B?RhB?and phenol under visible light irradiation.The solutions of MB(10 mg L^-1)and RhB(10 mg L^-1)can be efficiently degraded within 20 min and 15min,respectively,and the phenol(50 mg L^-1)can be degraded to 36%within 80 min,which are much better than those of Ag₃PO₄ and g-C₃N₄/Ag₃PO₄,indicating that the introduction of NCDs into g-C₃N₄/Ag₃PO₄ can effectively improve the photocatalytic activity.A possible photocatalytic mechanism based on the experimental results is proposed.It is revealed that NCDs on the ternary g-C₃N₄/Ag₃PO₄/NCDs can enhance the light harvesting capacity and molecular oxygen activation ability of the photocatalyst,and serve as excellent electronic transmission medium to promote the transfer and separation of photo-generated electron-hole pairs.4.An all solid Z-scheme Ag₃PO₄/GO/NCDs photocatalyst has been prepared through anchoring NCDs on Ag₃PO₄/GO?GO=graphene oxide?composite.The Ag₃PO₄/GO/NCDs photocatalyst exhibits excellent photocatalytic activity for the degradation of organic pollutants of methylene blue?MB?,rhodamine B?RhB?and phenol under visible-light irradiation.The pollutants of MB(10 mg L^-1),RhB(10 mg L^-1)and phenol(50 mg L^-1)could be efficiently degraded within 2.5 min,5 min and120 min,respectively,which are much better than that of Ag₃PO₄ and Ag₃PO₄/GO,indicating that the introduction of NCDs can further improve the photocatalytic activity of Ag₃PO₄/GO.Moreover,after four photocatalytic cycles,the photocatalytic activity of Ag₃PO₄/GO/NCDs shows only slight decrease,demonstrating a high photocatalytic stability.It is revealed that GO in the photocatalyst acts as a semiconductor and forms Z-scheme heterojunction with Ag₃PO₄,while NCDs improve the oxygen activation,enhance the light harvesting capacity and serve as the reaction sites during the photocatalytic process.