Preparation Of Ag₂CrO₄ Multi-pod-like Magnetic Photocatalysts And Their Performance For Degradation Of Organic Pollutants

Silver chromate exhibits high utilization of visible light and excellent visible light photocatalytic activity due to its relatively narrow band gap（～1.8 e V）.However,the practical application of silver chromate in photocatalysis is still limited due to its high photogenerated carrier recombination rate and susceptibility to photocorrosion under continuous photocatalytic reaction conditions.In addition,silver chromate can be generated rapidly at room temperature,which makes its morphology difficult to be regulated.In this study,from the perspectives of morphology control,crystal surface exposure,heterojunction construction,easy recycling and green synthesis,tannic acid（TA）,manganese ferrate（Mn Fe₂O₄）and silver chromate（Ag₂Cr O₄）were combined together for the first time to synthesize magnetic multi-pod-like TA-Mn Fe₂O₄/Ag₂Cr O₄（TA-MF/AC）heterojunction photocatalysts,in which Ag₂Cr O₄exposed high surface energy（011）crystal surface.TA-MF/AC exhibits excellent visible-light photocatalytic degradation of organic pollutants such as methyl orange（MO）,gentian violet,rhodamine B（Rh B）and tetracycline（TC）.Due to its special polyphenol structure,TA acts as bridging agent,capping agent and electron trap in the whole photocatalytic process,which can inhibit the aggregation of Mn Fe₂O₄and regulate the shape of Ag₂Cr O₄,and promote the separation of electron-hole pairs,thus improving the photocatalytic performance of TA-MF/AC composites.The main work and results of this paper are as follows.（1）TA-MF/AC composite photocatalysts were prepared by co-precipitation method,and the phase composition,morphology,microstructure,surface chemical state,and photoelectric properties of TA-MF/AC heterojunction were characterized by electron microscopy and energy spectroscopy.The results show that the magnetic multi-pods-like Z-scheme TA-MF/AC heterojunction photocatalysts were successfully constructed,in which Mn Fe₂O₄agglomeration was suppressed and uniformly dispersed on the Ag₂Cr O₄surface.The TA-MF/AC composite photocatalysts have larger surface area,higher visible light utilization and more efficient photo-generated electron-hole pair separation efficiency.（2）TA-MF/AC photocatalyst exhibited the highest photocatalytic activity for MO with an apparent rate constant of 0.0374 min^-1,which was 2.6,8.5,37.4 and 53.4times higher than that of Mn Fe₂O₄/Ag₂Cr O₄（MF/AC）,Ag₂Cr O₄,TA-Mn Fe₂O₄（TA-MF）and Mn Fe₂O₄,respectively.And it showed enhanced photocatalytic degradation activity for gentian violet,Rh B and TC.In addition,the initial concentration of MO,the initial p H of the solution,the coexistence of inorganic anions and humic acid could affect the photocatalytic degradation activity of TA-MF/AC.TA-MF/AC had good stability and recoverability,and its MO removal rate remained at 74%after four cycles of degradation.（3）Based on radical capture experiments and ESR analysis,·O₂^-,h⁺and·OH reactive species were involved in the photocatalytic reaction of TA-MF/AC,among which·O₂^-and h⁺played a major role.Charge transfer mechanism during the photocatalytic degradation of TA-MF/AC composites can be well described by the Z-scheme heterojunction mechanism.By transferring the photogenerated electrons of Ag₂Cr O₄to Mn Fe₂O₄,the photocorrosion of TA-MF/AC was effectively inhibited,and the photocatalytic stability of the composite photocatalyst was realized.