Silver-based Visible-light-response Photocatalvst:Preparation,Modification,Photocatalytic Properties And Reaction Mechanism

Semiconductor photocatalytic technology has the potential of being an new technology in pollution treatment and attracted a lot of attention because of its advantages such as mild reaction condition,low energy-consumption and high efficiency.Photocatalyst is the key in this technology thus developing high-efficiency photocatalyst with full potential of utilizing sunlight has become the hot topic.Ag-based photocatalysts attracted lots of attention because of its high photocatalytic activity under visible light.However,the application of Ag-based photocatalysts was limited by it poor stability.Moreover,most photocatalysts are powder-like and have difficulty to recover which is also disadvantage for application.This paper aimed at developing an efficient,stable,seperable photocatalyst with visible-light response.Four Ag-based visible-light-response photocatalysts,AgFeO₂,Ag₂CO₃,AgBr/Ag₂CO₃ and Fe₃O₄/AgBr/Ag₂CO₃ were prepared.The degradation properties of those samples toward methyl orange（MO）were discussed and photocatalytic mechanisms were proposed.Here are the main conclusions:1.A novel visible-light-response AgFeO₂ photocatalyst was synthesized via hydrothermal method at 160℃and 6h was proved to be the optimal hydrothermal time.The sample was able to degrade 97%of MO（10mg/L）within 180min.Relatively large BET surface area and high electron-hole separation efficiency were the main reason why this photocatalyst possessed higher photodegradation activity than samples with other hydrothermal time.Holes and·O₂^-were the main active species and holes contributed most to the degradation of pollutant.2.Ag₂CO₃ prepared via simple precipitation reaction was a high-efficiency photocatalyst.The sample prepared by NaHCO₃ possessed higher purity and photocatalytic activity than the one prepared by Na₂CO₃.It was able to decomposed 96.2%of MO（20mg/L）with 30min.However,the stability of Ag₂CO₃ was poor.Dissolution of Ag₂CO₃ was the main reason and photocorrosion was also an important factor.Holes were the dominant degradation agent and·O₂^-also contributed to the degradation of pollutant.3.AgBr/Ag₂CO₃ photocatalyst was prepared via ion-exchange reaction.It possessed higher reaction efficiency and stability.AgBr/Ag₂CO₃-60%was the most efficient photocatalyst and could remove 97.3%of MO（20mg/L）within 9min.Its high activity was attributed to better light adsorption and larger contact interface area between AgBr and Ag₂CO₃.The improved stability resulted from the formation of Ag/AgBr/Ag₂CO₃ system and the decreased dissolved quantity of Ag₂CO₃ achieved by AgBr coating.Holes and ·O₂-were the main reactive species responsible for pollutant degradation and holes were dominant.Photocatalytic process could be divided into two stages.The photo-generated electrons trapped by Ag+ to form Ag⁰ in the first stage and reacted with O₂ on the surface of Ag⁰ in the second stage.Holes could be generated in both stages whereas ·O₂-could only be produced in the second one.4.Fe₃O₄/AgBr/Ag₂CO₃ prepared by in-situ ion-exchange process was an efficient,stable,seperable photocatalyst with visible-light response.The photocatalyst have fine magnetic separation property and recovered from solution within 15s with magnet.Fe₃O₄/AgBr/Ag₂CO₃-5%was the most efficient and achieved 95.6%degradation of MO（20mg/L）within 6min.Within certain limits,the coupled Fe₃O₄ was beneficial for electron-hole separation and could provide larger surface area which could improve the reaction activity.However,excess amount of Fe₃O₄ would act as recombination center for electron-hole pairs thus decrease reaction activity.Holes were the main reactive species for pollutant degradation.Multiple reactions existed during the degradation of MO.N-CH₃ bond,azo bond and C-N bond between benzene ring and azo structure were the main target for reactive species.