Preparation And Photocatalytic Properties Of Several Silver-containing Composites

Since the 21st century,science and technology have developed rapidly.It not only brings comfort and convenience to mankind,but also results in a series of serious environmental problems.Semiconductor photocatalysis technology exhibits a broad application prospect in the field of environment protection and energy conservation,and has attracted great interest.Since the discovery of photocatalytic properties of titanium dioxide?TiO2?by Fujishima et al in 1972,there has been an upsurge in the research on photocatalytic materials.In recent years,researchers developed oxide,sulfide,carbon nitride and other new photocatalytic materials to improve the utilization of sunlight.In order to further improve photocatalytic efficiency,researchers have carried out various ways on photocatalytic materials,such as metal nanoparticle modification,ion doping and construction of heterojunction structure,etc.These methods improved the photocatalytic activity to some extent,but the high electron-hole recombination still couldn't be solved.Silver vanadium phosphorous oxide?Ag2VO2PO4?is a monoclinic compound,consisting of edge shared VO₆ octahedron and dimer layer of PO4 tetrahedron,separated by silver ions and paralleled with?001?crystal plane.Like most silver-based compounds,it has a narrow band gap of 2.2 eV,and can respond to visible light.Due to positive conduction band?+2.72 eV?,it has a high oxidation potential.At present,it is mainly used in the cathode material of high-energy lithium ion batteries.To use Ag2VO2PO4 as a better photocatalytic material,a series of work should be carried out.Chemical etching method is gradually applied to the modification of materials,which can change the crystal structure and surface morphology of materials.In addition,new phase or heterostructure can be obtained through the reaction of materials with solution containing ions of products.Therefore,Ag2VO2PO4 and its composites were chemically modified by chemical etching,and the degradation performance of organic pollutants was tested.The main research includes the following three aspects:?1?Direct Z-scheme Ag2VO2PO4/g-C3N4 photocatalysts were successfully synthesized.Their crystalline structure,morphology,optical,and electrochemical properties were analyzed by X-ray diffractometry,scanning electron microscopy,X-ray photoelectron spectroscopy,UV–vis diffuse reflectance spectroscopy,and electrochemical measurements.The visible light photocatalytic activities of as prepared samples were evaluated by the photodegradation of methyl orange?MO?and phenol in the aqueous phase.Compared with pure g-C3N4 and Ag2VO2PO4,the Ag2VO2PO4/g-C3N4 composites exhibited the higher photocatalytic activity under visible light irradiation.When the mass ratio is 1:0.4,the photoactivity of Ag2VO2PO4/g-C3N4 is nearly 4 and 3 times higher than Ag2VO2PO4 and g-C3N4 for removal of MO,respectively.The degradation of phenol was also significantly improved.Base on the result of rapping experiments of the active specie,the mechanistic was proposed.?2?Ag3VO4/Ag2VO2PO4 heterostructure composites with highly enhanced visible light photocatalytic activity were synthesized by using Ag2VO2PO4 as the sacrificed template in different concentration of ammonia solution.The experimental results proved that in the process of chemical etching,Ag2VO2PO4 reacted with ammonia and a new phase of Ag3VO4was formed.Concluded from the photoelectric performance test and photocatalytic degradation efficiency of methylene blue?MB?,MO and imidacloprin in aqueous phase,the formation of new heterojunction greatly improved the performance of pure Ag2VO2PO4phase.The Ag3VO4/Ag2VO2PO4 with 0.15 M ammonia solution exhibits the highest activity,which are nearly 3.8,8.6 and 9 times higher than bare Ag2VO2PO4 for removal of MB,MO and imidacloprid,respectively.Based on the experimental results,the highly efficient photoactivity can be attributed to the construction of heterostructure,and the corresponding plausible mechanism was proposed.?3?Using Ag2VO2PO4/g-C3N4 as a self-sacrifice precursor,the effect of the etching process on the properties of Ag2VO2PO4/g-C3N4 in ammonia solution was further investigated,and the Ag4V2O7/g-C3N4 composite was obtained.As the concentration of ammonia in the etching solution increases from 0.5 M to 2 M,a phase transition took place.The phase transition process can be described as Ag2VO2PO4/g-C3N4?Ag2VO2PO4/Ag4V2O7/g-C3N4?Ag4V2O7/g-C3N4.Compared with bare Ag2VO2PO4/g-C3N4,the novel Ag2VO2PO4/Ag4V2O7/g-C3N4 and Ag4V2O7/g-C3N4 heterojunctions show excellent visible light degradation performance.At the etching concentration of 0.15 M,the obtained Ag4V2O7/g-C3N4 heterojunction exhibited the highest photoactivity,which is 9.1,3.0 and 24.3 times more efficient than pristine Ag2VO2PO4/g-C3N4 in the degradation of MB,MO and imidacloprid,respectively.The improvement of photocatalytic performance is attributed to the higher separation efficiency and longer lifetime of photogenerated electron holes in the novel heterojunctions.