G-C₃N₄-based Composites For Improved Visible-light Photodegradation Of Enrofloxacin

In recent years,photocatalytic technology has attracted tremendous attention to realize the conversion and utilization of renewable solar energy into green sustainable energy.Graphite phase carbon nitride（g-C₃N₄）,as a new two-dimensional non-metal semiconductor material,has a visible light response and makes good prospects in photocatalytic field.It is studied that g-C₃N₄ has a high photocatalytic performance in the degradation of organic pollutants under visible light irradiation.However,the higher recombination rate of photogenerated carriers severely limits the application of g-C₃N₄ in the field of photocatalysis.To overcome the defect of low quantum efficiency of g-C₃N₄ itself,TiO₂/g-C₃N₄composites and K-doped g-C₃N₄ photocatalysts were designed and prepared by calcination.The photocatalytic performance and related mechanism of the composites were also studied.Meanwhile,K-doped g-C₃N₄ composites were also used in photocatalytic-Fenton co-catalytic systems to achieve Fe（III）/Fe（II）cycle conversion in Fenton reactions.The specific experimental results are as follows:Herein,mesoporous TiO₂/g-C₃N₄ composites were fabricated with 2D TiO₂（B）nanosheets regulating thermal condensation process of g-C₃N₄ nanosheets.FT-IR and XPS results suggested that the formation of O-Ti-N chemical bond increases the percentage of N-（C）³ in the conjugated system,accelerating the transportation of photo-induced electrons.The optical property and PL results illustrated that the formed interface heterojunction with chemical bond facilitates the separation and transfer of photo-induced charge carriers.Hence,TCN-12 exhibited the largest removal rate constant（k）of 0.0555 min^-1,46.3 times higher than unmodified g-C₃N₄(0.0012 min^-1).Moreover,ESR and HPLC-Q-TOF-MS experiments indicated that·O₂^-radicals played dominant role in the ENR photodegradation for TiO₂/g-C₃N₄ composites.In this work,K-doped g-C₃N₄ was successfully prepared by simple calcination of a K⁺/urea precursor.The morphology,structure,chemical composites and photoactivity were investigated.BET results confirmed that the doping of potassium metal had effect on the incomplete condensation of g-C₃N₄,leading to improved pore structure and more active sites for the photocatalytic reaction.FT-IR and XPS results demonstrated that the interaction of doped K and urea resulted in introduced O atoms entered into the framework of g-C₃N₄.Due to mental doping,g-CN-K composites exhibited the broaden visible light absorption range and improved efficiency of photo-induced electrons and holes separation.Therefore,the removal efficiency of ENR by doped g-C₃N₄ was obviously improved.Moreover,quenching experiments and ESR results further showed that the main active species of K-doped g-C₃N₄under visible light irradiation were produced to degrade pollutants,including·O₂^-,·OH and h⁺.Considering the low Fe（III）/Fe（II）cycle conversion in the traditional Fenton reaction,the K-doped g-C₃N₄ photocatalysts were applied to the photocatalysis-Fenton co-catalytic system in this paper.For g-CN-3.9%K composite with optimal K loading,the degradation kinetic constant was 0.2690 min^-1 in the co-catalytic system,31.0 times higher than pure photocatalytic reaction.The enhancement of photoactivity may be due to the fact that Fe（III）was easily adsorbed on the surface of g-CN-K composites to form complex through electrostatic interaction.Under visible light irradiation,absorbed Fe（III）captured photo-generated electrons（e^-）from g-CN-K and was reduced to Fe（II）,accelerating the cyclic conversion of Fe（III）/Fe（II）.The generated Fe（II）was used to catalyze the decomposition of H₂O₂ to generate·OH radicals.In addition,the complex was coordinated with the O-O bond in H₂O₂,and the heterolytic cleavage of oxidation was to form Fe（V）,which directly oxidation of ENR.Therefore,the main active species in the photocatalysis-Fenton co-catalytic system were Fe（V）and·OH radicals under visible irradiation.