Ag₃PO₄ Nanophase Materials Prepared By Controllable Microemulsion System And Photocatalysis Performance Study

The photocatalytic technology is a novel technology for water purification and hydrogen production via absorption of photon energy by semiconductor materials to produce electrons-hole pairs and oxidation-reduction reaction with given target objects,showing a promising application perspective in clean energy and environmental governance.At present,titanium dioxide?Ti O₂?is an available semiconductor photocatalytic material closed to the commercialization.Due to the band gap of 3.2 e V and low quantum efficiency of Ti O₂,it has become a key issue in the field of semiconductor photocatalysis research to design and develop a high-efficient photocatalytic material with visible light response and improved utilization ratio of solar energy.Owing to the band gap of 2.36 e V,the strong photo-oxidation capacity in the wavelength of less than 530 nm,high-efficient degradation of organic pollutant,and high quantum efficiency?above 90%?,silver phosphate?Ag₃PO₄?is regared as a kind of visible light response photocatalytic materials with high catalytic activity.In this paper,basing on the synthsis of nanostructured Ag₃PO₄ with different morphology by micro-emulsion method,a series of Ag₃PO₄ materials based nanomaterical by doping and compositing with other materials,including SCNTs/Ag₃PO₄?GO/Ag₃PO₄?Ba₃?PO₄?₂/Ag₃PO₄,Ca₃?PO₄?₂/Ag₃PO₄,Mg₃?PO₄?₂/Ag₃PO₄ were fabricated.And then,the morphology,formation mechanism,photocatalytic activity and stability performance were studied.Furthermore,in order to clarify the weak light stability of Ag₃PO₄,the stability and thermodynamics properties of Ag₃PO₄ from the perspective of chemical and electronic structure were researched by using the first principle calculation based on density function theory.The main research contents are as follows:?1?Silver phosphate?Ag₃PO₄?materials with different nanostructures,including rod,cambiform and tetrahedron with round edges and corners,were synthesized by adjusting the molar ratio?W?of water to surfactant in a reverse microemulsion,which consists of sodium dodecyl sulfate?SDS?,isopentyl alcohol,cyclohexane,and aqueous solution,using silver nitrate and potassium dihydrogen phosphate as starting materials.The structure,morphology and visible-light-response of the obtained samples were characterized by XRD,SEM,HRTEM,and UV-Vis-DRS spectra.The formation mechanisms of nanostructured Ag₃PO₄ were discussed.Moreover,the photocatalytic activity was also evaluated by means of degradation of organic dye methylene blue?MB?in water.The results show that all the samples have body-centred cubic crystal structure,and the W values have obvious effects on sizes and morphology of nanostructured Ag₃PO₄.The varied morphologies can be attributed to the change of diameter and interfacial film strength of water nuclear influenced by SDS content.In addition,all the obtained samples exhibit excellent efficient photocatalytic activity for the photo-degradation of methylene blue?MB?under visible light irradiation,and the tetrahedral Ag₃PO₄ exhibits the best photocatalytic activity due to the exposion of{111}crystal surface.?2?To resolve the problem of impurity formation during the synthesis process of nano-Ag₃PO₄ by microemulsion method,the influences of experimental factors including types and concentration of reactants,aging times of products and doping of the second phase?such as carbon nanotubes and graphene?on impurity formation were studied.On this basis,a“Ag⁺drift-fixed”theory used to explain the reason of impurities formation andfloating in the micro-emulsion droplets due to the relatively low collision probability,resulting in the formation of Ag PO₃ or Ag₂HPO₄.Although impurity formation probability can be reduced to a certain extent by optimizing experimental parameters,impurity still cannot be completely eliminated due to the floating property of Ag⁺.Ag⁺can be fixed on the surface of carbon tube due to the physical adsorption by adding carbon tube into micro-emulsion droplets,and the floatability of Ag⁺is subsequently decreased.Thus,impurities are eliminated completely due to the complete reaction between Ag⁺and PO₄^3-.Photocatalytic experimental results show that the photocatalytic activity of nano-Ag₃PO₄ is improved effectively with the elimination of impurities.?3?The composite photocatalytic materials,including Ba₃?PO₄?₂/Ag₃PO₄,Ca₃?PO₄?₂/Ag₃PO₄ and Mg₃?PO₄?₂/Ag₃PO₄,were prepared by microemulsion method.The effects of Mg₃?PO₄?₂,Ca₃?PO₄?₂ and Ba₃?PO₄?₂ on structure,morphology and photocatalysis of Ag₃PO₄ were studied.And the mechanism of the impact on properties was also discussed.The results show that the Mg₃?PO₄?₂,Ca₃?PO₄?₂,Ba₃?PO₄?₂ do not affect the crystal structure of Ag₃PO₄,but decrease the formation probability and size of tetrahedral morphology of Ag₃PO₄,leading to a weaker photocatalytic activity of Mg₃?PO₄?₂/Ag₃PO₄ and Ca₃?PO₄?₂/Ag₃PO₄ compared with Ag₃PO₄.Ba₃?PO₄?₂/Ag₃PO₄ has a similar photocatalytic activity with Ag₃PO₄,which may be attributed to the stronger light absorbability?Eg=2.10 e V?.?4?The Mulliken populations,energy band structures,density of states,phonon and phonon density of silver phosphate?Ag₃PO₄?were calculated and analyzed by the first principles method using plane-wave norm conserving pseudo-potential calculation based on density functional theory.The results show that Ag ion plays an important role in the crystal.The formation of stable PO₄ tetrahedral units with P-O and O-O bonds weakens the covalent nature of Ag-O bonds,pumping Ag ion into a metastable state.Then,the metastable Ag ion will be easy to get rid of the oxygen ions and obtain electron to form metallic silver.Therefore,Ag₃PO₄ shows photodegradation.Ag₃PO₄ exhibits dynamic stability.The enthalpy and entropy of Ag₃PO₄ nonlinearly increase with increasing temperature in the range of 300-3000 K,while free energy decrease.When temperature reaches 1200K,Cv reaches to a stable value of about 93cal/cell·K.Theconclusions mentioned above are in good agreement with experimental results.