Preparation Of Cs_xPW₁₁O₃₉M?H₂O?/TiO₂ Nanotubes Composites And Their Visible-light Photocatalytic Properties

Using photocatalytic technology to treat organic pollutants in wastewater is an effective way to solve the energy crisis and environmental pollution.In photocatalysis,the catalyst plays an important role because there is no need to consume energy other than solar energy.TiO₂ is the most widely used semiconductor photocatalytic material.As TiO₂ has a wide band gap value of 3.2 eV and a high photoelectron-hole recombination rate,its light energy utilization rate and quantum efficiency are both low.TiO₂ can be compounded with semiconductors to improve the utilization rate of light energy and promote the separation of photogenerated carriers at the same time,thereby improving photocatalytic efficiency.so as to improve the photocatalytic efficiency.Polyoxometalate?POM?has been proved to be a novel and efficient photocatalyst with strong oxidative reducibility and semiconductor like photochemical behavior.In this paper,we selected a series of Keggin-type transition metal substituted polyoxometalates(Cs_xPW₁₁M,x=4,5)to use as visible photoactive components.Insoluble polyoxometalates substituted with different transition metals(Fe³⁺,Cu²⁺,Mn²⁺)were synthesized,forexample:(Cs₄PW₁₁O₃₉Fe?III??H₂O?[Cs₄PW₁₁Fe],Cs₅PW₁₁O₃₉Cu?II??H₂O?[Cs₅PW₁₁Cu],Cs₅PW₁₁O₃₉Mn?II??H₂O?[Cs₅PW₁₁Mn],and immobilized them on TiO₂ through the hydrothermal method.Considering that the one-dimensional structure of nanomaterials has excellent electron transport capability,the microscopic morphology of TiO₂ is regulated into nanotube arrays?TNAs?.The photocatalytic properties of the composites were evaluated by the degradation of the dye RhB under visible light.The experimental results are as follows:The results of UV-vis DRS show that the light absorption properties of Cs_xPW₁₁M/TNAs composites retain the advantages of their single components,and they absorb both the ultraviolet and visible light,which broaden the light absorption range of the material.The SEM results show that the size of Cs_xPW₁₁M crystals supported on TNAs decreases from the original micron order to the nanometer level after hydrothermal reaction.In addition,the crystal morphology of Cs_xPW₁₁M is related to the reaction time in the hydrothermal process.The proper time is beneficial for the Cs_xPW₁₁M crystal to grow on the surface of TNAs with uniform size and regular structure.The Cs_xPW₁₁M/TNAs composites showed high photocatalytic activity in the degradation of RhB under visible light.Among them,the degradation rate of RhB by Cs₄PW₁₁Fe/TNAs was 88.74%after 180 min reaction,and the COD value decreased by 66%after 4 h reaction.The degradation rate of RhB by Cs₅PW₁₁Cu/TNAs after 180 min was 88.53%,and the COD value decreased by 24%after 4 h reaction.The degradation rate of RhB by Cs₅PW₁₁Mn/TNAs was 68.15%after 180 min,and the COD value decreased by 33%after 4 h reaction.The degradation mechanism of RhB was investigated by methanol quenching hydroxyl radical experiment.The results showed that RhB degradation process produced hydroxyl radical with strong oxidability,which accelerated the oxidation of RhB.In addition,the effects of different experimental conditions on the degradation of RhB were investigated.The results show that increasing the loading of Cs_xPW₁₁M and the acidity of the reaction system are beneficial to the photocatalytic degradation of RhB.The results of cyclic degradation experiments show that Cs_xPW₁₁M/TNAs composites have good photocatalytic stability.