Controllable Preparation Of Carbon Nitride-based Composite Membranes And Their Photocatalytic Performance And Mechanism

Global energy shortage and environmental pollution are the two major obstacles to the green and sustainable development of modern society.Using solar energy to realize clean energy conversion and solve water pollution is an important measure.In recent years,photocatalytic composite membrane prepared by coupling photocatalytic technology and membrane separation technology can effectively photocatalytic decomposition of aquatic hydrogen and degradation of organic pollutants,which solves the problems of difficult recovery and reuse of powder catalyst and membrane fouling resulting in the decrease of membrane flux.The key to the successful application of photocatalytic membrane technology is the design of efficient photocatalyst.Graphitic carbon nitride（g-C₃N₄）is an ideal material for the preparation of photocatalytic membranes due to its advantages of low cost,environmentally friendly,good stability and easy synthesis.At present,the performance of most photocatalytic membrane systems cannot exceed the corresponding powder system.In general,polymer substrates are only considered as a support for catalyst nanoparticles,which holds no substantial promotive effects on reaction efficiencies.Moreover,the entrapment of catalysts inside polymer membrane vastly limits the exposed surface area of catalyst particles,which is unfavorable to the reaction efficiency.Based on this,in this work,the g-C₃N₄ photocatalytic composite materials are used as the active component and PVDF is selected as the membrane matrix to construct porous g-C₃N₄-based PVDF photocatalytic composite membrane and g-C₃N₄-based PVDF piezoelectric photocatalytic composite membrane system.By regulating the composition and structure ratio of g-C₃N₄photocatalytic composite material,the effects of g-C₃N₄ photocatalytic composite material on the morphology,flux,fouling resistance,catalytic activity and regeneration of the membrane were investigated,and the mechanism of the photocatalytic degradation of tetracycline wastewater and the mechanism of hydrogen production of g-C₃N₄-based PVDF photocatalytic composite membrane enhanced by fluid-induced piezoelectric field were studied.The main contents of this paper are as follows:1.Preparation of g-C3N4/RGO/AgVO3/PVDF composite membrane and their performance and mechanism investigation of photocatalytic degradation of tetracyclineAn alternative silver vanadate quantum dots（AgVO₃ QDs）doped reduced graphene oxide（RGO）and graphitic carbon nitride（g-C₃N₄）nanocomposites modified polyvinylidene fluoride（PVDF）membrane（g-C₃N₄/RGO/AgVO₃/PVDF）was successfully fabricated to enhance the photocatalytic activity.The g-C₃N₄/RGO/AgVO₃ nanocomposites were functioned as the active component for the photocatalytic membrane.The unique Z-scheme heterostructure of g-C₃N₄/RGO/AgVO₃ enhanced the separation and transport efficiency of photogenerated carriers and the porous PVDF framework facilitated the interaction between the photocatalyst and the pollutant.As a result,the degradation efficiency of TC for the g-C₃N₄/RGO/AgVO₃/PVDF reached 88.53%within 120 min,which was higher than that of g-C₃N₄/RGO/PVDF（64.8%）and g-C₃N₄/AgVO₃/PVDF（79.18%）.In addition,g-C₃N₄/RGO/AgVO₃/PVDF exhibited high permeability(1977 L m^-2 h^-1 bar^-1)and excellent antifouling activity.Under visible-light irradiation,the flux recovery rate（FRR）increased from 92.4%to 99.1%.Furthermore,g-C₃N₄/RGO/AgVO₃/PVDF could kill 98.2%of Escherichia coli（E.coli）under visible-light irradiation.2.Preparation of g-C3N4/RSF/BiVO4/PVDF composite membrane and their performance and mechanism investigation of photocatalytic degradation of tetracyclineUsing g-C₃N₄/BiVO₄ heterojunction modified by regenerated silk fibroin（RSF）nanofibers derived carbon as the catalytic unit,g-C₃N₄/RSF/BiVO₄/PVDF photocatalytic nanofiber membranes were successfully prepared by electrospinning.The introduction of RSF can not only improved the visible light absorption capacity,but also facilitated the separation of photogenerated carriers.Therefore,g-C₃N₄/RSF/BiVO₄/PVDF photocatalytic nanofiber membrane had higher degradation efficiency of TC（83.33%）than that of g-C₃N₄/PVDF（53.68%）and g-C₃N₄/RSF/PVDF（73.58%）.Under gravity driven,g-C₃N₄/RSF/BiVO₄/PVDF had high permeability(226 L m^-2 h^-1).The flux recovery rate（FRR）can reach 100%under visible light irradiation.In addition,g-C₃N₄/RSF/BiVO₄/PVDF exhibited excellent photocatalytic antibacterial performance,and the inactivation rate of bacteria was more than 99%after illumination for 150 min.In addition,free radical trapping tests and ESR results indicated that hole was the main active specie,and superoxide radical played a secondary role in the degradation process of g-C₃N₄/RSF/BiVO₄/PVDF photocatalytic nanofiber membrane.3.Preparation of g-C3N4/Bi OI/PVDF composite membrane and their performance and mechanism investigation of photocatalytic degradation of tetracyclineZ-scheme g-C₃N₄/Bi OI heterojunction was successfully prepared by hydrothermal method,and Z-scheme g-C₃N₄/Bi OI heterojunction as the active component blended in polyvinylidene fluoride membrane（PVDF）was prepared via freezing phase inversion technique.The construction of the Z-scheme g-C₃N₄/Bi OI heterojunction increased the face-to-face contact area,the interfacial structure exhibited a higher carrier transfer rate,and the as-prepared macroporous photocatalytic membrane had high porosity,which was beneficial to the pollutants rapidly transferred to the active site of the photocatalyst,thereby improving the photocatalytic activity.As expected,the prepared g-C₃N₄/Bi OI/PVDF photocatalytic membrane achieved exceptional photocatalytic degradation efficiency for tetracycline（94.6%）as compared to the g-C₃N₄/Bi OI heterojunction power（84.0%）and two other control membrane matrixes（g-C₃N₄/Bi OI/PAN and g-C₃N₄/Bi OI/CA）within 120 min.Meanwhile,the dynamic cyclic degradation test showed the degradation efficiency of g-C₃N₄/Bi OI/PVDF could reached 94.8%in 80 min.Besides,the g-C₃N₄/Bi OI/PVDF not only had outstanding self-cleaning activity and remarkable permeability(up to 30688 L m^-2 h^-1 bar^-1),but also had high stability and reusability even after five runs.Importantly,the hydroxyl radical detection and ESR analysis identified that theβ-phase PVDF membrane could promote photoinduced carrier separation efficiency of g-C₃N₄/Bi OI heterojunction.4.Preparation of g-C3N4/Li NbO3/PVDF composite membrane and their performance and mechanism investigation of piezoelectric photocatalytic co-decomposition of water for hydrogen productionHerein,we report a three-dimensional（3D）porous g-C₃N₄/Li NbO₃/PVDF membrane with an enhanced fluid-induced piezoelectric field for photocatalytic hydrogen evolution reaction（HER）.The freezing phase inversion strategy is critical to the formation of piezoelectric sensitiveβphase PVDF.The unique 3D porous membrane structure and the fluid-induced piezoelectric potential synergistically contribute to the HER efficiency under white light illumination.The as-prepared g-C₃N₄/Li NbO₃/PVDF piezoelectric photocatalytic membrane（19.6875 cm² in size）exhibited HER rate of 2720.40μmol h^-1 g^-1,which was even higher than that of the g-C₃N₄/Li NbO₃ powder sample(2236.00μmol h^-1 g^-1)with the identical mass of catalyst particles.In-situ diffuse reflection infrared Fourier transform spectroscopy（DRIFTS）demonstrates a better water affinity of the porous g-C₃N₄/Li NbO₃/PVDF piezoelectric membrane prepared from the freezing phase inversion.Piezoelectric force microscopy（PFM）,in-situ piezoelectric and electrochemical measurements further visualized the fluid-induced piezoelectric field on 3D g-C₃N₄/Li NbO₃/PVDF membrane,by which boosting the HER efficiency.5.Preparation of N-g-C3N4/PVDF composite membrane and their performance and mechanism investigation of piezoelectric photocatalytic co-decomposition of water for hydrogen productionThe porous N-g-C₃N₄ nanosheet was prepared by copolymerization of histidine（His）as precursor with urea.The porous N-g-C₃N₄ nanosheet was used as the active component,and the N-g-C₃N₄/PVDF piezoelectric photocatalytic membrane was prepared by electrospinning method.The prepared N-g-C₃N₄/PVDF piezoelectric photocatalytic membrane not only broadened the absorption range of visible light,but also significantly improved the separation efficiency of photogenerated charge carriers and generated a sustainable electric field driven by water flow,further improved the photocatalytic HER performance.As a result,the prepared N-g-C₃N₄/PVDF piezoelectric photocatalytic nanofiber membrane had a higher photocatalytic HER activity(1376.54μmol h^-1 g^-1),which was higher than that of the same mass of N-g-C₃N₄powder catalyst(860.34μmol h^-1 g^-1)and N-g-C₃N₄/PVDF piezoelectric photocatalytic membrane prepared by freezing phase conversion method(924.14μmol h^-1 g^-1).In addition,compared with N-g-C₃N₄/PVDF prepared by freezing phase conversion method,N-g-C₃N₄/PVDF prepared by electrospinning had better mechanical properties.In situ piezoelectric and electrochemical measurements showed that the piezoelectric field on the surface of N-g-C₃N₄/PVDF piezoelectric photocatalytic nanofiber membrane can be induced by water flow,which promoted the separation efficiency of photocarriers and enhanced the photocatalytic HER rate.