The Preparation Of PAN Nanofibers Loaded With Bi-based Photocatalysts And Their Application In Degradation Of Isoproturon

Photocatalysis technology could harness solar energy to drive thermodynamic uphill chemical reactions.Photocatalysts can absorb photons to produce many active species,which can decompose a variety of toxic chemical agents.So they have potential application for environmental purification.However,there are two inherent defects for photocatalysts including the wide band gap energy and the recombination of photo-generated charge carriers.Another technical limitation in application is the difficult separation of catalyst from the slurry,which reduces the recycle utilization of photocatalysts.How to immobilize the high efficient photocatalysts on proper support materials was a huge challenge.The electrospun fibers have smaller fiber diameters,smaller pore diameters,larger specific surface areas and higher porosity.They can be used as suitable supports for photocatalysts to provide much more photocatalytic reaction site.Furthermore,the electrospun nanofiber mats modified by photocatalysts possess the properties of both photocatalysis and high filtration efficiency.The functional nanofiber mats with light weight and high moisture permeability were suitable for chemical protective materials,solving these problems including high thermal loads,high weight and bulkiness burden and secondary contamination.In this work,two kinds of visible-light responsive photocatalysts,Ag-Ag Br@Bi₂₀TiO₃₂ and CQDs-Bi₂₀TiO₃₂,were prepared successfully.Their photocatalytic activities were studied by photocatalysis degradation of the phenylurea herbicides isoproturon.Furthurmore,the photocatalysts were loaded on the surface of electrospun polyacrylonitrile（PAN）fibers through coaxial electrospinning technology.The photocatalytic activity of the obtained composite fiber mats was also evaluated by the degradation of isoproturon under the irridation of visible light.（1）Fabrication of Z-scheme photocatalyst Ag-Ag Br@Bi₂₀TiO₃₂ and its visible-light photocatalytic activity for the degradation of isoproturon herbicideThe visible-light-driven photocatalyst Bi₂₀TiO₃₂ was prepared by solvothermal method.The Zscheme photocatalyst Ag-Ag Br@Bi₂₀TiO₃₂ was successfully fabricated by a simple depositionprecipitation method combined with photo-reduction route.In this photocatalytic system,Bi₂₀TiO₃₂ and Ag Br could be simultaneously excited to generate e CB-and h VB+,respectively.The e CB-of Bi₂₀TiO₃₂ could flow into metal Ag and then transferred to the VB of Ag Br.As a result,the e CB-and h VB+ in these two photocatalysts could be separated effcctively.At the same time,the Z-scheme photocatalyst reserved e CB-with high reduction capability and h VB+ with strong oxidation capability.XRD spectra indicated a coexistence of both Ag-Ag Br and Bi₂₀TiO₃₂ phases.The elemental composition and chemical states of Ag-Ag Br@Bi₂₀TiO₃₂ were studied by XPS.The SEM and TEM images showed that Ag-Ag Br@Bi₂₀TiO₃₂ displayed the wire-like morphology with diameter 30-50 nm and their surfaces were decorated with the Ag-Ag Br spherical particles.N₂ adsorption-desorption analysis demonstrated the photocatalysts powder kept mesoporous structure with BET surface area 35.82 m2/g.UV-vis diffuse reflectance spectra showed that Ag-Ag Br@Bi₂₀TiO₃₂ photocatalysts exhibited much broader absorption in the region of visible light.The photocatalytic activity of Ag-Ag Br@Bi₂₀TiO₃₂ was evaluated by the degradation of chemical agent isoproturon under visible-light irradiation.Results showed that the sample Ag-Ag Br@Bi₂₀TiO₃₂-1exhibited the highest photocatalytic activity with a degradation rate of 93.2% for isoproturon due to its efficient visible-light utilization.The degradation reaction of isoproturon by Ag-Ag Br@Bi₂₀TiO₃₂ could be described by pseudo-first-order kinetics.The degradation pathway of isoproturon herbicide was also proposed according to the intermediates identified by LC-MS-MS.The free radical and hole scavenging experiments indicated that h+ and ·O2-were the active species for isoproturon degradation.A possible catalytic mechanism for Ag-Ag Br@Bi₂₀TiO₃₂ photocatalyst was put forward based on band gap structure of Ag Br and Bi₂₀TiO₃₂.（2）PAN supported Ag-Ag Br@Bi₂₀TiO₃₂ electrospun fiber mats with efficient visible light photocatalytic activity and antibacterial capabilityPAN-supported Ag-Ag Br@Bi₂₀TiO₃₂ electrospun fiber mats with different photocatalyst contents were successfully fabricated by coaxial electrospinning technology.The phase compositions of AgAg Br@Bi₂₀TiO₃₂/PAN electrospun fiber mats were characterized by XRD.The intensity of main diffraction peaks indexed to Ag-Ag Br@Bi₂₀TiO₃₂ increased gradually with the increase contents of photocatalysts,indicating the successful coating of Ag-Ag Br@Bi₂₀TiO₃₂ photocatalyst on PAN electrospun fibers.The surface morphologies and structures of the electrospun fiber mats were further characterized by SEM and FESEM.It was observed that the electrospun fibers were evenly coated by the Ag-Ag Br@Bi₂₀TiO₃₂ particles.The fiber surfaces become much rougher and the diameters increased with the increase amounts of photocatalysts.The UV-vis diffuse reflectance spectroscopy of the PAN/Ag-Ag Br@Bi₂₀TiO₃₂ electrospun fiber mats showed that the absorption in the UV-vis light spectrum was largely improved by increasing the amount of photocatalyst,whicn was in agreement with the darker color of the samples.N₂ adsorption-desorption analysis indicated that the Ag-Ag Br@Bi₂₀TiO₃₂/PAN electrospun fiber mats exhibited the hierarchical macro-and mesoporous structure and the BET specific surface area increased as the amount of photocatalyst increased from 0%（w/v）to 15%（w/v）and decreased as the amount reached 20%（w/v）.The photocatalytic activity of electrospun composite fiber mats was evaluated by degrading the herbicide isoproturon under visible light irradiation.The change of isoproturon concentration was monitored by UV-vis spectrophotometer and HPLC.Results showed that after being coated with Ag-Ag Br@Bi₂₀TiO₃₂ the composite fiber mats removed much more isoproturon than pure PAN and Ti O2-P25/PAN electrospun fiber mats.The sample S4 exhibited the highest degradation rate 87.9% for isoproturon.After four cycles,the photodegradation rates of isoproturon could reach to 75%.The Ag-Ag Br@Bi₂₀TiO₃₂/PAN electrospun fiber mats had remarkable recyclability.But the addition of Ag-Ag Br@Bi₂₀TiO₃₂ led to a higher viscosity of sheath fluid inhibiting the formation of electrospun fibers.In our experiments the amount of photocatalysts in the sheath fluid cannot exceeded 20%（w/v）.In addition,the PAN/Ag-Ag Br@Bi₂₀TiO₃₂ electrospun fiber mat was endowed with good antibacterial activity.Compared with the pristine PAN electrospun fiber mat,the bacteriostatic rates of PAN/Ag-Ag Br@Bi₂₀TiO₃₂ electrospun fiber mat S4 against the Gramnegative bacteria E.coli and Gram-positive bacteria S.aureus were 99.79% and 99.42%,respectively.（3）Construction of up-converting fluorescent carbon quantum dots/Bi₂₀TiO₃₂ composites with enhanced photocatalytic properties under visible lightCQDs-Bi₂₀TiO₃₂ photocatalysis system was designed due to the excellent electron transfer ability and up-converting photoluminescence of CQDs.On the one hand,CQDs can be excited by nearinfrared light and emit visible light,which will be absorbed by Bi₂₀TiO₃₂ to generate more e CB-and h VB+ pairs.On the other hand,the excellent electron transfer ability was beneficial for the separation of photo-generated e CB-and h VB+ pairs,improving the utilization efficiency of light energy.CQDs were prepared by facile hydrothermal method and up-converting fluorescent CQDs/Bi₂₀TiO₃₂ was prepared by oil bath reflux method.Photoluminescence spectra demonstrated that CQDs have remarkable photoluminescence property.HRTEM images showed CQDs was near spherical particles with size of approximately 5 nm.The crystal structures,morphologies,surface electronic states of CQDs-Bi₂₀TiO₃₂ were further characterized by XRD,HRTEM,XPS and FT-IR.Due to the low content and amorphous nature of CQDs,the XRD spectrum of CQDs/Bi₂₀TiO₃₂ was well matched to that of Bi₂₀TiO₃₂ but no obvious peaks of CQDs were observed,indicating that the crystalline structure of Bi₂₀TiO₃₂ did not change during the preparation process.From HRTEM images,Bi₂₀TiO₃₂ nanowires were decorated evenly by CQDs particles.The XPS results confirmed the coexistence of CQDs and Bi₂₀TiO₃₂ in the CQDs/Bi₂₀TiO₃₂ composite photocatalyst.Compared with Bi₂₀TiO₃₂,the characteristic peaks Bi,O,Ti and C elements in CQDs/Bi₂₀TiO₃₂ showed slight shifts,indicating their changing chemical environment.In the FT-IR spectra,the characteristic absorption bands of CQDs appeared for 3%-CQDs/Bi₂₀TiO₃₂ and 5%-CQDs/Bi₂₀TiO₃₂,confirming the Bi₂₀TiO₃₂ photocatalyst was modified by CQDs successfully.Photoluminescence spectra intensity of CQDs/Bi₂₀TiO₃₂ was much lower than that of Bi₂₀TiO₃₂ demonstrating its lower recombination rate of photo-generated hole-electron pairs.The electrochemistry analysis showed CQDs/Bi₂₀TiO₃₂ had a higher photocurrent signal and lower electron-transfer resistance,further indicating the better transfer action of photo-generated electrons because the introduction of CQDs can accelerate the separation of photo-generated hole-electron pairs.The photocatalytic activity was evaluated by photocatalysis degradation of isoproturon and 1%-CQDs-Bi₂₀TiO₃₂ exhibited the highest photocatalytic activity and could degradate 98.1% of isoproturon.The changes of the isoproturon concentration versus irradiation time followed the pseudo-first-order kinetics plot and the degradation rate constant of 1%-CQDs/Bi₂₀TiO₃₂ was approximately 5 times of that of pure Bi₂₀TiO₃₂ and 1.6 times of Ag-Ag Br@Bi₂₀TiO₃₂.Free radicals trapping experiments showed h+ and ·O2-were the main activespecies in this photocatalysis system and ·O2-played a comparatively major role in photo-degradation of isoproturon.The intermediate products were detected and identified by LC-MS-MS and a plausible photocatalytic degradation pathway of the isoproturon was put forward.Compared with the Ag-Ag Br@Bi₂₀TiO₃₂ photocatalytic system,the intermediate products of isoproturon degraded by CQDs/Bi₂₀TiO₃₂ have simpler molecular structure,indicating its much higher photocatalytic activity.（4）CQDs-Bi₂₀TiO₃₂/PAN electrospun fiber mats and their photocatalytic activity towards the degradation of isoproturonBased on the detailed study on CQDs-Bi₂₀TiO₃₂ photocatalysts,CQDs-Bi₂₀TiO₃₂/PAN composite fiber mats with different photocatalyst contents were prepared by coaxial electrospinning technology.They were characterized by XRD,SEM,TEM,FT-IR,UV-vis diffuse reflectance spectroscopy and N₂ adsorption-desorption analysis.The main diffraction peaks indexed to CQDs-Bi₂₀TiO₃₂ could be clearly found in the XRD spectra of CQDs-Bi₂₀TiO₃₂/PAN electrospun fiber mats.SEM and TEM images showed that the electrospun composite fibers were evenly loaded with photocatalyst particles and the average diameter increased from 665 nm to 1069 nm with the increasing photocatalyst contents.The EDX spectrum showed that there were C,N,O,Bi and Ti elements on the surface of electrospun fibers.The characteristic peaks of CQDs-Bi₂₀TiO₃₂ in FT-IR spectra indicated the successful modification of PAN electrospun fibers.UV-vis diffuse reflectance spectroscopy showed that after being coated with CQDs-Bi₂₀TiO₃₂ photocatalysts the composite fiber mats exhibited stronger absorption intensity in the UV-vis light spectrum.The N₂ adsorptiondesorption analysis showed that there were hierarchical macro-and mesoporous structure in the fiber mats,which could promote the transfer of isoproturon molecular.The composite fiber mats exhibited obvious photocatalytic activity towards the degradation of isoproturon.The sample S3’ with 15%w/v content of CQDs-Bi₂₀TiO₃₂ photocatalysts showed the highest photocatalysis degradation rates 90.4% for isoproturon.The composite fiber mats with more photocatalysts showed a downward trend for the degradation of isoproturon due to the thicker fiber diameter and low specific surface area.It can still degrade 76.9% of isoproturon after four cycles proving good recyclability.