Study On Preparation And Visible Light Photocatalytic Activity Of Ag₆Si₂O₇/WO₃ Photocatalyst

The unique silicate tetrahedral structure of the silicate is easily distorted and would lead polarization,and these polar tetrahedrons are arranged inside the silicate material would construct a polarized electric field.When transition metal ions existed,the transition metal coordination complex environment will provide more possibilities for photogenerated electron transfer between silicate-linked polyhedra.So some materials with high photocatalytic activity would likely to be found in the silicate compounds.Among them,Ag₆Si₂O₇ is a new type of non-metal oxyacid salt photocatalyst,which has good photocatalytic activity.However,due to high cost,easy recombination of photogenerated electron-hole pair and instability of physical and chemical properties,its practical application is limited.In this paper,a series of Ag₆Si₂O₇/WO₃ composite photocatalysts were prepared by simple ultrasonic assisted method based on energy level characeristics of Ag₆Si₂O₇ and WO₃.Ag₆Si₂O₇/WO₃ photocatalysts were characterized by X-ray diffraction（XRD）,field emission scanning electron microscopy（FE-SEM）,energy dispersive spectroscopy（EDS）,X-ray photoelectron spectroscopy（XPS）,UV-vis diffuse reflectance spectra（DRS）,inductively coupled plasma optical emission spectrometer（ICP-OES）,and so on to determine the phase,composition,morphology and optical absorption properties of the as-prepared catalysts.The results show that the Ag₆Si₂O₇/WO₃ composite photocatalysts were successfully prepared by ultrasonic assisted precipitation method.In the composite samples,the Ag element exists in the form of Ag⁺,the W element exists in the form of W⁶⁺,and the Si element exists in the form of Si⁴⁺.Ag₆Si₂O₇ and WO₃ are uniformly doped in composite,the absorption edge has a red shift compared with the pure WO₃,and the visible light photocatalytic performance is enhanced.To investigate the influence of molar ratio of Ag₆Si₂O₇ and WO₃ on photocatalytic activity of Ag₆Si₂O₇/WO₃,MB was chosen as the model pollutant to evaluate the photocatalytic activity.the photocatalytic activities of Ag₆Si₂O₇/WO₃ improve generally with the increase of molar ratio of Ag₆Si₂O₇ and WO₃.The Ag₆Si₂O₇/WO₃（1:1）exhibits the most significantly enhanced photocatalytic performance.The photocatalytic degradation efficiency of MB by Ag₆Si₂O₇/WO₃（1:1）attains 97.4%,which is much higher than（87.3%）by sole Ag₆Si₂O₇ and（14.0%）by sole WO₃ after visible light irradiation for 30 min.Ag₆Si₂O₇/WO₃（1:1）has the highest apparent rate constant,which was about 1.59 times as that of sole Ag₆Si₂O₇ sample and 44.6 times as that of sole WO₃.48.7% of the organic carbon content of MB was degraded in 1 h under visible light irradiation and is still much higher than that by sole Ag₆Si₂O₇（27.7%）and sole WO₃（14.9%）.All the results proved the superior photocatalytic activity of Ag₆Si₂O₇/WO₃（1:1）.The photocatalytic activity of Ag₆Si₂O₇/WO₃（1:1）composite is also much higher than that of simple mixture of Ag₆Si₂O₇ and WO₃ with the same weight of active component.The apparent rate constant at 30 min is about 2.88 times as that simple mixture of Ag₆Si₂O₇ and WO₃.Ag₆Si₂O₇/WO₃（1:1）also showed excellent photocatalytic activity in degradation of（MO）and rhodamine B（RhB）.The degradation activity order is MB> RhB> MO.The degradation of colorless compound 2,4-dichlorophenol also showed a higher activity than single Ag₆Si₂O₇ and WO₃.After degradation for 12 h under visible light irradiation,the mineralization rate to 2,4-dichlorophenol was 40%,indicating that the sensitization of dye was not main action in photocatalytic degradation of three dyes by Ag₆Si₂O₇/WO₃（1:1）.The photocatalytic activity of Ag₆Si₂O₇/WO₃ in degradation of MB and 2,4-dichlorophenol did not significantly decrease after repeated use of three times.After 60 min,the removal rate to MB is still above 90% and that to 2,4-dichlorophenol removal rate is still up to 70%.It is shown that Ag₆Si₂O₇/WO₃ is a stable and efficient photocatalyst.Combining the results of photoluminescence and free radical capture experiment,the possible reason of high photocatalyst activity of Ag₆Si₂O₇/WO₃ composite in degradation of organic compounds under visible light irradiation can be attributed to that when the molar ratio of Ag₆Si₂O₇ to WO₃ is 1: 1,heterojunction can be formed on the contact interface due to Ag₆Si₂O₇ being deposited to the surface of WO₃ and heterojunction can effectively suppresses the recombination of photo-generated electron-hole pairs,thus promoting the photocatalytic degradation of organic pollutants.·OH is the main active species of Ag₆Si₂O₇/WO₃（1:1）photocatalyst in degradation of organic pollutants.