Preparation Of ML-WO₃/TiO₂ Heterojunction And Its Photocatalytic Degradation Of Dye

Due to its large volume,high stability,complex composition,high colour,high alkalinity,high toxicity and biochemical properties,printing and dyeing wastewater has been one of the hardest to treat in China.The environment and people’s health are greatly endangered by it.Among the many treatment methods,photocatalytic technology has gradually become a research hotspot in advanced oxidation technology due to its advantages of high efficiency,easy operation,safety and low energy consumption.Ti O₂-based catalysts,which has been researched extensively,has been found to be highly active,stable and inexpensive.But the large forbidden band width,low specific surface area and high complexation rate of photogenerated electron-hole pairs make further research of this system very restricted.This requires that modifications such as ion doping,non-metallic doping,precious metal deposition,and semiconductor compounding be explored.Among them,semiconductor compounding to form heterogeneous structures and to explore the mechanism of action is a frontier research in the field of photocatalysis.Therefore,in this study,monolayer two-dimensional（2D）shaped WO₃ nanosheets with Ti O₂ composite modification were synthesized as photocatalysts by spatial confinement method using methylene blue represented by thiazine dyes and rhodamine B represented by arylmethane as substrates and explored for photocatalytic experiments.Firstly,monolayer WO₃ nanosheets（called ML-WO₃）were prepared by the space-limited method and combined with Ti O₂ as ML-WO₃/Ti O₂ heterojunctions,and the composite conditions were optimized by orthogonal experiments,and the optimum conditions were the composite ratio of ML-WO₃ and Ti O₂ of 1 mol%;calcination temperature of 320°C;and calcination time of 2 h.The ML-WO₃/Ti O₂ composites prepared by AFM,SEM（HR）TEM,XPS,XRD,UV-Vis,BET,PL were used to characterize the microscopic morphology,lattice spacing,chemical composition,optical response and specific surface area distribution of the prepared ML-WO₃/Ti O₂composites,and the characterization results were analyzed.The AFM analysis showed that the monolayer WO₃ nanosheets had been successfully prepared,and the longitudinal dimensions of the selected ML-WO₃ regions The SEM analysis showed that Ti O₂ was in the form of regular spherical nanoparticles,ML-WO₃ showed an irregular flake-like structure,and ML-WO₃ was tightly connected to Ti O₂.TEM and HRTEM analysis showed that the average diameter of Ti O₂ was about 20～45 nm,ML-WO₃ exhibited a low lining,with grain size only ranging from 50 to 100 nm and ML-WO₃/Ti O₂ size around 95～100 nm.and the formation of heterojunctions can be clearly observed.XRD analysis shows that the Ti O₂ characteristic peak gradually decreases and the ML-WO₃ characteristic peak gradually increases with increasing ML-WO₃doping.surface chemical elements and electronic valence states shown by XPS indicate that The composites are mainly composed of oxygen,tungsten,titanium and a small amount of carbon.The N₂ adsorption-desorption isotherms and the BHJ pore size distribution detected by BET indicate the possession of a high specific surface area and a typical mesoporous structure.UV-vis demonstrates that the nano ML-WO₃/Ti O₂overcomes the defect of the large band gap width of pure Ti O₂ and broadens the absorption performance of Ti O₂ for visible light.PL spectra show that 1 mol%ML-WO₃/Ti O₂ exhibits the lowest luminescence intensity,confirming that the formation of heterojunctions alters the carrier migration path,limiting and retarding the electron-hole complexation.ML-WO₃/Ti O₂ photocatalysts were used as a carrier to investigate its application in the photocatalytic degradation of methylene blue under UV light,and the experimental results showed that:（1）due to the large specific surface area and dense adsorption sites of the monolayer WO₃ nanosheets,the adsorption of the composite photocatalytic materials was significantly improved with the increase of the ML-WO₃doping ratio;（2）the photocatalytic activities of the composite systems were all better than those of the single system,and they had significant synergistic effects;（3）the photocatalytic performance of 1 mol%ML-WO₃/Ti O₂ was the best at various composite ratios,and its degradation rate could reach 99.1%within 10 min;（4）the best photocatalytic performance was achieved when the catalyst dosage was 60 mg;（5）the UV-Vis absorption spectra showed that with the progress of the oxidation reaction The results of the photocatalytic degradation of rhodamine B by ML-WO₃/Ti O₂photocatalyst under daylight showed that:（1）ML-WO₃ and Ti O₂ had a significant synergistic effect,and the ML-WO₃/Ti O₂ composite system had better photocatalytic performance than conventional WO₃/Ti O₂;（2）the best photocatalytic performance was achieved when the catalyst dosage was 60 mg and the p H value was 5;（3）using H₂O₂,PDS and PMS as oxidants,all three oxidants had certain catalytic activity for 1 mol%ML-WO₃/Ti O₂,but their There were differences in the activation catalytic activity,and the degradation activity for Rh B was:H₂O₂<PDS<PMS;（4）After 5 cycles,the photocatalysis performance of the material is still around 80%,which shows that it is a highly effective,stable and Recyclability.The mechanism of photocatalytic degradation of dyes by ML-WO₃/Ti O₂photocatalysts was explored:（1）The active species that play an important role in Rh B/MB were identified by radical trapping tests and electron paramagnetic resonance（EPR）analysis:·OH and·O₂^-.Combining the radical results with the energy band structures of both ML-WO₃ and Ti O₂ materials confirms that the studied ML-WO₃/Ti O₂composite photocatalyst is consistent with the transport mechanism of a Z-scheme heterojunction.（2）The LC-MS identification of the intermediate products indicated that the degradation pathways of Rh B are:N-demethylation,cleavage of the conjugated chromophoric group,ring opening of the benzene ring and mineralisation.