Construction Of Graphene-based Semiconductor Composite Nanofiber Membranes And Their Photocatalytic Performance

In recent years,semiconductor photocatalysis has gradually become a research hotspot in the field of water purification due to its green,efficient and sustainable oxidative degradation.Among the semiconductor photocatalysts,titanium dioxide（TiO₂）and iron-based metal-organic frameworks（Fe-MOFs）,as representatives of traditional and novel photocatalysts,have been widely noticed for their stable chemical properties,excellent photocatalytic performance and good biocompatibility.However,there are some problems such as low visible light utilization,low quantum efficiency and difficult separation and recovery.Significantly,graphene is a two-dimensional material with single atomic thickness,which has an ultra-high specific surface area,stable chemical structure and superior electron transfer capability.It can not only effectively load semiconductor nanoparticles,but also promote the separation and transfer of photogenerated carriers,thus obviously enhancing the photocatalytic activity of semiconductor materials.Thus,graphene-based semiconductor composite nanofiber membranes with visible light drive,high catalytic activity and easy recovery were developed by in-situ compounding of TiO₂ and Fe-MOFs with graphene in combination with electrostatic spinning technology.The main research elements are as follows.Firstly,a new strategy for the preparation of high quality graphene by chemical-hydrothermal synergistic reduction was developed.Graphene oxide（GO）was removed from carbonyl groups using sodium borohydride,followed by hydrothermal treatment at 180°C to remove the residual epoxy groups and hydroxyl groups.The electrical conductivity of the prepared graphene was as high as 1249.7 S·m^-1,much higher than that of the graphene prepared by the one-step reduction method.It was found that the restoration of sp² hybridisation structure on graphene significantly enhanced the electron transport capacity.This study reveals the intrinsic connection between the structure and electrical properties of graphene,and provides an important theoretical basis for the construction of graphene-based semiconductor composites.Then,a three-dimensional porous TiO₂/graphene composite aerogel was constructed by in-situ growth method under hydrothermal conditions using reduced graphene oxide（r GO）and titanium sulphate as precursors of graphene and TiO₂,respectively.It was observed that TiO₂ nanoparticles with a particle size of 9 nm were uniformly grown on graphene sheets,which significantly enhanced their interfacial charge transfer and photocatalytic activity.The degradation efficiency of TiO₂/graphene composite aerogel for methylene blue（MB）and C.I.reactive red 24（RR24）exceeded 99%within 120 min under UV light.In addition,the introduction of graphene reduced bandgap of semiconductor photocatalysts and effectively promoted the separation and transfer of photogenerated charges,thus significantly increasing the photocatalytic activityIn order to achieve efficient degradation of dyes in visible light,three kind of visible light-driven Fe-MOFs/graphene composites were prepared by combining Fe-MOFs（MIL-53,MIL-88A and MIL-100）with graphene oxide（GO）using an in-situ growth method.Compared with Fe-MOFs,the Fe-MOFs/graphene composites exhibited stronger photocatalytic activity in the visible light/H₂O₂ system.Among them,the MIL-88A/graphene composite possessed the highest activity,with a degradation efficiency of nearly 100%for the dye within 60 min.In addition,the photocatalytic mechanism of Fe-MOFs/graphene composites in the visible light/H₂O₂system was explained by energy band structure and active substance analysis.In order to solve the recycling problem of nano-photocatalysts,MIL-88A semiconductors were synthesized on polyvinyl alcohol/sodium alginate/graphene oxide（PVA-SA-GO）electrospun nanofiber membranes using an in-situ synthesis method to construct the visible light-driven MIL-88A@graphene-based composite fiber membrane.It was shown that the composite membrane completely degraded dyes after 90 min of visible light radiation.Moreover,the membrane can be regenerated by a simple photocatalytic self-cleaning process.To further improve the photogenerated carrier utilisation efficiency,graphene-based Z-type heterojunction composite nanofiber membrane was constructed by in-situ synthesis of TiO₂/r GO/MIL-100 heterojunction on polyvinylidene fluoride/polyvinylpyrrolidone（PVDF-PVP）nanofiber membrane.It was found that the graphene-based Z-type heterojunction not only effectively suppressed the complexation of photogenerated electron-hole pairs,but also retained the stronger redox potential on TiO₂ and MIL-100,thus significantly enhancing the oxidative decomposition for organic pollutants.Under simulated sunlight,the degradation efficiency of the composite nanofiber membrane for dyes was close to 100%.In addition,the composite nanofiber membrane has good screening performance for cationic dyes MB and can efficiently separate oil-water emulsions,which has a promising application in the field of water purification.