Preparation And Properties Of Vanadium And Molybdenum Based Visible Light Catalysts

Over the past few decades,global energy shortages and serious environmental pollution have raised awareness of a potential crisis.For the social stability and sustainable development of human society,the development of non-pollution and low-energy consumption technologies is an urgent task.Among the renewable energy resources,solar energy,being free and green,is an ideal energy source for overcoming current environmental challenges.Photocatalytic H₂ generation or reduction of CO₂ into carbon sources by the semiconductor photocatalysts is a potential way to solve both energy conversion and environmental problems,which shows good application prospects and thus has been widely studied by a great deal of research.Besides traditional TiO2 and ZnO,hundreds of photocatalysts were developed over the past decades,such as Bi₂WO₆?Bi₂MoO₆?SrTiO₃?BiFeO₃ etc.However,the limitation of photocatalytic efficiency of the photocatalyst with a small specific surface area and low adsorption capacity in the visible light range is relatively low in very dilute solution.In addition,most nanoscale photocatalysts are not easily separated in suspension,which hinders their large-scale industrial application.To solve these problems,lots of methods have been developed.In this paper,the novel vanadate semiconductor materials?such as BiVO₄?and molybdenum semiconductor materials?such as SrMoO₄,Sr₂FeMoO₆ and their doped semiconductors?have been synthesized.The structure,morphology and properties of the pholocatalyst were studied by XRD,SEM,TEM,BET.XPS,Raman,UV-vis,PL,ESR,etc.The photocatalytic properties were evaluated by the degradation of methyl blue?MB?under visible light irradiation.Meanwhile,the photocatalytic reaction kinetics was investigated.The possible formation mechanism of morphology and photocatalytic degradation mechanism were also put forward on the basis of experimental results.The main research work of this paper could be described as follows:?1?Synthesis and photocatalytic properties of reticular BiVO₄ nanoparticles prepared via a modified sol-gel methodReticular BiVO₄ catalysts were successfully synthesized via a modified sol-gel method.Here,citric acid?CA?was used as the chelating agent and ethylenediaminetetraacetic acid?EDTA?was used as the chelating agent and template.Furthermore,the effects of pH values and EDTA on the structure and morphology of the samples were studied.We determined that EDTA and pH played important roles on the determination of the morphology of the as-prepared BiVO₄ samples.Photocatalytic evaluation revealed that the reticular BiVO₄ exhibited superior photocatalytic performance characteristics for the degradation of methylene blue?MB?under visible-light??>400 nm?exposure,about 98%of the MB was found to degrade within 50 min.Moreover,the degradation kinetics of MB was in good agreement with pseudo-first-order kinetics.The obtained apparent reaction rate constant kapp of reticular BiVO₄ was about 10 times higher than that of BiVO₄ synthesized by a conventional sol-gel method.?2?Morphology and luminescent properties of SrMoO₄:Eu³⁺,Dy³⁺ nanoparticles prepared via co-precipitation methodSrMoO₄ and SrMoO₄:xEu³⁺(yDy³⁺)phosphors were synthesized via co-precipitation method.Two influence factors on the luminescent and morphology properties of the phosphors have been discussed in detail.One is pH value of the precursor solution,and the other is the Eu?Dy?doping content.The SEM morphology changes from spindle-like to cylindrical shape with pH value increasing from 6 to 8.When pH value reaches 10,these cylindrical yoked each other and formed the flower-like structure.The SrMoO₄:xEu samples exhibit spindle-like,uniform,and well-dispersed nanorods with the length and outer diameter approximately 10 and 2 lm.However,for SrMoO₄:yDy samples,the flower-like structure that composed of spindle-like rods is observed.The possible formation mechanism is also put forward on the basis of experimental results.The most intense emission is located around 616 and 576 nm due to 5D0?7F2 and ⁴F_9/2?⁶H_13/2 transition of Eu³⁺ and Dy³⁺,respectively.The emission intensity is enhanced with Eu?Dy?doping and the related mechanism is discussed in detail.?3?Effect of surfactants on morphology and photoluminescence properties of SrMO4:0.01Dy³⁺ nanoparticlesUsing sodium citrate?Na₃Cit?and polyethylene glycol?PEG4000?as surfactants,SrMoO₄:0.01Dy³⁺ phosphors were prepared by hydrothermal method.The structure,morphology and luminescent properties are investigated by X-ray diffraction?XRD?,scanning electron microscope?SEM?,and fluorescence spectrometer.Our results suggest that these surfactants do not change the crystal structure of SrMoO₄ phosphors.However,the morphologies of SrMoO₄ phosphors are greatly influenced with the introduction of these surfactants.With Na₃Cit or PEG addition,the flake-like morphology of SrMoO₄:0.01Dy³⁺is changed into the spindle-and flower-like one.The most intense emission is observed to be located around 576 nm due to ⁴F_9/2?⁶H_13/2 transition of Dy³⁺.The emission intensity is quenched for Na₃Cit-added sample.For PEG-added samples,with increasing amounts of PEG,the emission intensity is firstly enhanced and then lowered.The maximum emission intensity is achieved when the molar ratio of PEG to Sr is 0.5:1.?4?Synthesis and photocatalytic performance of Cr and Co doping SrMoO₄ via hydrothermal methodThe Cr-doped and Co-doped SrMoO₄ samples with visible-light-driven photocatalyst were successfully synthesized by one facile hydrothermal preparation technique.The results show that the Cr?Co?doping have litter effect on the crystal structure of SrMoO₄.However,the Cr?Co?-doping play an important role on sample morphology.The photocatalytic properties are evaluated by the degradation of methyl blue?MB?under visible light irradiation.Electron spin resonant?ESR?measurements show that the ·O₂^-radical is the major active species in the photodegradation process.The photocatalytic measurements indicate that Cr³⁺ and Co²⁺ doping can effectively enhance the photocatalytic activites of the SrMoO₄.When the Cr doping content and Co doping content are 7%and 5%respectively,the doped SrMoO₄ exhibits best photocatalytic performance.The enhancement of photocatalytic activity is attributed to the following two factors.One is the decrease of sample band gap,and the other is the lower recombination ratio of photo-induced electron-hole pairs with Cr?Co?doping.The current experimental investigations clearly suggest that Cr?Co?doping is one useful way to improve the photocatatlytic properties of SrMoo4 material.?5?Structural,magnetic,and photocatalytic performance of Sr2FeMoo6:Eu³⁺,Dy³⁺ nanoparticles prepared via sol-gel methodThe Eu and Dy-doped Sr₂FeMoO₆ photocatalysts with rod-like and flower-like structures were successfully synthesized by sol-gel method.The results show that that the crystal structure of the sample is tetragonal double perovskite.The Eu?Dy?doping have litter effect on the crystal structure of photocatalysts.However,the doping play an important role on sample morphology.With the increase of Eu?Dy?doping,the order degree of Fe/Mo decreases,the oxygen vacancy concentration in the structure increases,and the equilibrium between Fe²⁺/³⁺-Mo⁵⁺/⁶⁺ is gradually broken down.The magnetic properties show that the saturation magnetization?Ms?decreases and the coercivity?Hc?increases with the increase of doping amount of Eu?Dy?.Photocatalytic evaluation reveal that the Eu and Dy-doped Sr₂FeMoO₆ exhibited excellent photocatalytic performance characteristics for the degradation of methylene blue?MB?under visible-light exposure,about 98%of the MB was found to degrade within 35 min.When the Eu doping content and Dy doping content are 7%and 5%respectively,the doped Sr₂FeMoO₆ exhibits best photocatalytic performance.It is found that the doped Sr₂FeMoO₆ with magnetic properties and high photocatalytic activity has been obtained,which provides the possibility for the recovery and reuse of the catalyst.