Design And Synthesis Of Novel Ti-based/Polyoxometalates Visible-Light-Response Photocatalyst And Their Photocatalytic Properties In Environmental Remediation

Photocatalysis,which can convert low density solar energy into high density chemical energy by direct utilization of solar energy,plays more and more important roles in solving current urgent energy shortage and environmental pollution issues.Ti O₂ is the most widely used and studied photocatalyst owing to its low cost,nontoxicity,high oxidation ability,efficient photocatalytic properties,and photochemical stability.However,its relatively wide band gap（3.2 e V）and low photoinduced charge carriers separation efficiency hinder the practical application of Ti O₂.Therefore,the modification of Ti-based photocatalysts to expand the spectral absorption range and enhance the separation efficiency of photogenerated carrier,and the development of novel and efficient visible-light photocatalysts have become a hot topic in the photocatalytic field.Based on the above considerations,we carried out the following two researches: on the one hand,we have enhanced the photocatalytic performance of bismuth titanate and titanium dioxide by element doping and semiconductor coupling methods.On the other hand,we have developed one kind of novel and efficient POMs-based visible-light-response photocatalyst.The main contents are as follows:1.A series of Cr-doped Bi₄Ti₃O₁₂/Bi₂Ti₂O₇（BTO-x;x stands for the amount of Cr（NO₃）₃·9H₂O;x=0.01,0.02,0.04,0.06,0.08 g,respectively）heterostructures fibers have been synthesized by one-step of facile and economical electrospinning/calcination routem.SEM and TEM results reveal that the diameters of the as-prepared fibers are 100 ± 30 nm in size.By regulating the amount of Cr doping,we can not only adjust the band structure of Bi₄Ti₃O₁₂ and Bi₂Ti₂O₇ in the fibers to enhance its visible-light absorption,but change the ratio of Bi₂Ti₂O₇ component in Bi₄Ti₃O₁₂/Bi₂Ti₂O₇ heterostructures.With increasing the content of Cr doping,XRD and XPS shows that the amount of Bi₂Ti₂O₇ increases in the fibers.As a result,the Bi₄Ti₃O₁₂/Bi₂Ti₂O₇ heterojunction structure has been enhanced,which further promote the charge separation of photogenerated charge carriers.Photocatalytic tests displayed that the as-prepared Cr-doped Bi₄Ti₃O₁₂/Bi₂Ti₂O₇ fibers exhibit good photocatalytic activity for photodegradation of methyl orange（MO）under visible-light irradiation.We also investigated the effect of different amount of Cr doping on the photocatalytic activity,and the BTO-0.08 had the highest photocatalytic activity.The significance of this work is that the spectral absorption of bismuth titanate has been largely enhanced,and the Bi₄Ti₃O₁₂/Bi₂Ti₂O₇ heterostructures was also constructed by simple Cr doping method,which effectively promoted the separation of photogenerated carriers.This work provides some new ideas for the design and preparation of novel visible-light-response photocatalyst.2.We successfully fabricated Ag/Ag_xH_3-xPMo₁₂O40-x(Ag/Ag HPMo₁₂-x;x stands for the irradiation time;x = 2,4,6,8 h,respectively)nanowires composites through a facile solid reaction route and in-situ photoreduction method.In Ag/Ag HPMo₁₂,Ag+ counter cation effectively reduces the solubility of H₃PMo₁₂O₄₀ and the formed Ag HPMo₁₂ possesses nanowire morphology,and their diameters are about 45±10 nm.The Ag content in the Ag/Ag HPMo₁₂ composite was manipulated by the light irradiation time,and the size of Ag NPs ranges from 5 to 15 nm.Ag NPs exhibit surface plasmon resonance（SPR）absorption,thus enhancing the visible light absorption of Ag/Ag HPMo₁₂.In addition,it also causes intense local electromagnetic fields by SPR,which accelerates the charge separation of photogenerated e-and h+ in Ag/Ag HPMo₁₂.These factors are beneficial for the improvement the photocatalytic activity of Ag/Ag HPMo₁₂.The photocatalytic tests show that the as-synthesized Ag/Ag HPMo₁₂ exhibits highly efficient and recyclable photocatalytic performance for the degradation of MO and reduction of Cr2O72-under visible light（λ > 420 nm）irradiation.Among them,Ag/Ag HPMo₁₂-4 has the highest photocatalytic activity.The successful preparation of Ag/Ag HPMo₁₂ not only compensates the shortcomings of the traditional POMs-based photocatalyst,which only works under UV light,but also solves the problem that the surface plasmonic photocatalyst is unstable and easily photocorroded with light irradiation.The work has extended the application of POMs as photocatalyst,which can provide some reference for the design and preparation of novel visible-light-response POMs-based photocatalyst in future.3.A series of Ag nanoparticles（NPs）loaded PMo₁₂/Ti O₂(PMo₁₂=H₃PMo₁₂O₄₀)nanofibers PMo₁₂/Ti O₂/Agx（x=0.05,0.1,0.2,0.3g,respectively）have been synthesized by a facile electrospinning/calcination and in situ photodeposited route.The effects of PMo₁₂ content,calcination temperature and Ag amount on the photocatalytic activity of the composites were also investigated.The SEM and TEM images manifest the diameters of the PMo₁₂/Ti O₂ fibers are about 130 ± 30 nm,and the average diameters of Ag NPs are about 15 ± 5 nm in size.The light absorption of the composites in visible region has been largely enhanced due to the narrow band gap of PMo₁₂ and the SPR effect of Ag NPs.The results of PL,EIS and i-t curves reveal that the separation rate of photogenerated charge carriers has been largely improved by the introduction of PMo₁₂ and Ag NPs.Photocatalytic experiments indicate the PMo₁₂/Ti O₂/Ag composites exhibit highly efficient photocatalytic performance for degradation of methyl orange（MO）and photoreduction of Cr（VI）under visible light irradiation（λ> 420 nm）,which is obviously higher than that of precursor Ti O₂,PMo₁₂/Ti O₂ and Ti O₂/Ag.Among the composites,PMo₁₂/Ti O₂/Ag0.1 exhibits the best photocatalytic performances.The successful preparation of PMo₁₂/Ti O₂/Ag,on the one hand,it has enhanced the visible-light absorption of Ti O₂ and improved the separation efficiency of photogenerated carriers.On the other hand,it has solved the problem that POMs are difficult to recover when used as homogeneous catalysts,and POMs/Ti O₂ system can only use the UV light.The research provides some new ideas for the development of other novel composite photocatalysts.