Preparation Of Bi₂WO₆ Based Visible-Light-Driven Photocatalysts For Degradation Of Organic Pollutants

Semiconductor photocatalytic technology is a "green" and promising way for solar energy conversion.As a novel narrow bandgap semiconductor,Bi2WO6 can be used for the treatment of pollutants in water due to its good visible light response and high photocatalytic activity.However,the practical application of Bi2WO6 is seriously restricted by the high carrier recombination rate and low carrier mobility.For this reason,several bismuth-based photocatalysts with excellent visible light response have been successfully synthesized.The influences of forming solid solution,morphology modification,ion doping,and building heterostructure on the photocatalytic activity of Bi2WO6 were investigated through the structural analysis and photocatalytic degradation test.The main research contents and conclusions are as follows:(1)The enhanced visible light photocatalytic degradation activity of Bi2WO6 was achieved through forming solid solution with Bi2MoO6.BiOBr microspheres were prepared by solvothermal method.Bi2MoxW1-xO6 solid solutions with uniform 3D hierarchical hollow microsphere structure were first time prepared by a facile anions exchange method.The results show that continuous Bi2MOxW1-xO6 solid solutions form in the range of x=0?0.8,and all the Bi2MOxW1-xO6 samples show an orthorhombic Bi2WO6 phase.The substitution of W sites in Bi2WO6 with Mo6+ ions effectively reduce the bandgap of the Bi2MoxW1-xO6 solid solutions and enhance their visible light response.While excess Mo6+ ions replacing W6+ ions in Bi2WO6 cause lattice distortion,which acts as the capture center and reduces photocatalytic activity of samples.Under the visible light irradiation,Bi2Mo0.4W0.6O6 showed excellent photocatalytic activity and cycling stability for the degradation of RhB,phenol and tetracycline hydrochloride.The corresponding pseudo-first-order reaction kinetic constants k were 0.024,0.016,and 0.007 min-1,respectively.The formation of hollow Bi2MoxW1-xO6 microspheres are achieved by the microscale Kirkendall effect.That is,mass migration occurs due to the velocity difference between the MO42-(M = W or Mo)ions and the Br-ions.(2)Bi2WO6 with different morphology were prepared by the PVP assisted hydrothermal method.The influences of PVP addition,PVP molecular weight and pH on the morphology and structure of Bi2WO6 were investigated systematically.The results show that PVP plays an important role in the formation of 3D Bi2WO6 nanoflowers.During the hydrothermal reaction,PVP long molecular chains absorbed on surface of Bi2WO6 nucleus induces the formation of more and more new nanoplates on the surface,which finally forms 3D microsphere.At a higher pH value,the formation rate of Bi2WO6 nuclei reduces and adsorption ability between PVP molecular chains and nucleus surface weak,which makes Bi2WO6 anisotropically grow along(001)crystal plane.0.2 g PVP assisted Bi2WO6 nanoflowers exhibited the best photocatalytic degradation rate of RhB.The pseudo-first-order reaction kinetic k was 0.003 min-1.The enhanced photocatalytic activity of PVP assisted Bi2WO6 nanoflowers can be attributed to the high surface area and good adsorption capacity.(3)The enhanced visible light response and photocatalytic activity of Bi2WO6 was achieved through Er3+/Nd3+ co-doping.Er/Nd co-doped Bi2WO6 nanosheets were prepared by hydrothermal method.The visible light response of Bi2WO6 was extend by Er and Nd doping,which introduced new impurity levels in the Bi2WO6 band structure.UV-vis DRS results show that Er3+ and Nd3+ ions have two up-conversion peaks at 524 nm and 585 nm.It means Er3+ and Nd3+ ions can be excited by visible light of 524 and 585 nm and then transform the visible light into ultraviolet light,which can activate Bi2WO6 to generate electron-hole pairs.The Er/Nd co-doped Bi2WO6 photocatalysts exhibited improved photocatalytic degradation rate of RhB,MO,phenol,and tetracycline hydrochloride.Their corresponding pseudo-first-order kinetic constants k were 0.034 min-1,0.026 min-1,0.013 min-1 and 0.013 min-1,respectively.In addition,photocatalytic mechanism of Er/Nd co-doped Bi2WO6 was proposed.That is,the complementary distribution of electrons between Er3+ and Nd3+ions in the Er3+/Nd3+ redox couple acts as an electron trapping center and prolongs the lifetime of photogenerated carrier.(4)The enhanced carrier separation rate of Bi2WO6 was achieved through forming heterojunction with TiO2.Highly dispersed TiO2/Bi2WO6 microflowers were synthesized by using hydrothermal method.During the hydrothermal process,the TiO2 nanoparticles were tightly coupled to the surface of the Bi2WO6 nanoflowers,which facilitated the migration and separation rate of photogenerated carriers.The amount of TiO2 nanoparticles directly affects the photocatalytic activity of TiO2/Bi2WO6 nanoflowers,of which 60 wt%TiO2 loaded Bi2WO6 has the highest photocatalytic activity.Specifically,60 wt%TiO2/Bi2WO6 photocatalyst showed the excellent photocatalytic activity and cycling stability for the degradation of RhB and MO under the visible light irradiation.Their corresponding pseudo-first-order reaction kinetic constants k were 0.075 min-1 and 0.056 min-1 respectively.Time-resolved fluorescence spectroscopy(TR-PL)studies show that the lifetime of photogenerated carriers in TiO2/Bi2WO6 is prolonged,which is due to the heterojunction between semiconductors facilitating the migration and separation rate of photogenerated carriers.(5)TiO2/Bi2WO6 photocatalysts loaded concrete was successfully prepared by a simple spraying technology,and the influences of dispersant,components and load amount on photocatalytic degradation of formaldehyde gas were preliminarily studied.The results show that the addition of CTAB as a dispersant effectively improve the dispersibility of TiO2/Bi2WO6 photocatalysts in the solution,and then the solution can be evenly sprayed on the surface of concrete.After a series of controlled experiments,the best load density of photocatalysts is determined to 5 g/m2.Moreover,after 24 h fluorescent light irradiation,the photocatalytic degradation rate of HCHO is 83%by 90 wt%TiO2/Bi2WO6 loaded concrete,which is 2.1 times higher than that of TiO2 loaded concrete.