Preparation Of TiO₂-Based And G-C₃N₄-Based Composites And Their Photocatalytic Degradation Of Organic Pollutants

In recent years,the treatment of organic pollutants in water bodies has become one of the hot topics in the field of environmental protection.Photocatalytic technology is widely used in the removal of organic pollutants（such as dyes,pesticides,antibiotics,etc.）in water,conversion and degradation of atmospheric pollutants,reduction of carbon dioxide,and catalytic hydrogen production.A high-efficiency photocatalyst needs to have the characteristics of wide visible light absorption edge,high carrier separation rate,strong redox ability,and high cycle stability,but it is difficult for a single TiO₂ or g-C₃N₄semiconductor photocatalyst to have these characteristics at the same time.Therefore,the design and research of TiO₂-based and g-C₃N₄-based composite photocatalysts have attracted much attention.（1）Four kinds of rare earth ion(RE³⁺=Ce³⁺,Sm³⁺,Pr³⁺,La³⁺)doped and modified titanium dioxide（TiO₂）catalysts were prepared by the sol-hydrothermal method.They were characterized by X-ray diffraction（XRD）,field emission scanning electron microscopy（FESEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,specific surface area（BET）and UV vis diffuse reflectance spectroscopy（UV-vis DRS）.The characterization results show that part of the doped RE³⁺enters the TiO₂ lattice by replacing Ti⁴⁺.Under ultraviolet light irradiation,the degradation efficiency of Sm³⁺doped TiO₂catalyst for methyl orange（MO）reached 100%in 60 min,and the components of the mixed pesticides all reached more than 90%degradation within 180 min.Sm-TiO₂ was the best catalyst,followed by Pr-TiO₂.The enhancement of activity can be attributed to the combined effect of impurity levels introduced by a small amount of RE³⁺doping and a large number of surface oxygen vacancies（OVs）.Finally,through the combination of experimental results and density functional theory（DFT）calculation results,a possible catalytic mechanism of RE³⁺-doped TiO₂ catalysts for photocatalytic degradation of organic pollutants is proposed.（2）Constructing nanocomposite structures with good charge transfer pathways is an effective way to obtain high-efficiency photocatalysts.Therefore,we prepared a direct Z-scheme heterojunction photocatalyst CNS/Sm-TiO₂ by recombining Sm³⁺-doped TiO₂ and g-C₃N₄ thermally oxidized and etched two-dimensional nanosheets CNS with a narrow band gap.The results show that the specific surface area of the nanosheets CNS obtained by the thermal oxidation etching of bulk g-C₃N₄ increases obviously.The photocatalytic activity ofα%CNS/Sm-TiO₂ composite is significantly higher than that of a single semiconductor.When the content of CNS is 40%,the efficiency of visible light degradation of Rhodamine B（Rh B）is 96.53%within 300 minutes.The reaction rate of 40%CNS/Sm-TiO₂ degradation of tetracycline hydrochloride（TC-HCl）(0.18215 h^-1)was 3.6 times higher than that of bulk g-C₃N₄(0.05085 h^-1).CNS/Sm-TiO₂ composite catalyst has a significant multi-purpose photocatalytic activity for removing organic dyes,antibiotics and pesticides.The enhanced photocatalytic activity ofα%CNS/Sm-TiO₂ composites can be attributed to the formation of Z-scheme heterojunction and the accelerated photogenerated carrier separation by Sm³⁺doping.Based on the results of the fluorescent hydroxyl radical capture experiment,we proposed a possible degradation mechanism for the direct Z-scheme CNS/Sm-TiO₂ heterojunction photocatalyst to degrade organic pollutants.（3）A one-step solvothermal method is used to synthesize CdS/Sm-TiO₂/CNS ternary nanocomposites with good uniformity and dispersion.Under simulated sunlight,the photocatalytic activity was evaluated by degrading organic pollutants Rh B and TC-HCl.The results show that the reaction rate(0.2107 min^-1)of the CdS/Sm-TiO₂/CNS ternary composite for degradation of TC-HCl is 20 times higher than the reaction rate of pure CNS(0.01020 min^-1).This enhanced performance can be attributed to the strong interfacial interaction between the three components and the enhanced visible light absorption ability after recombination with narrow band gap cadmium sulfide（CdS）.Besides,the matched energy levels of CdS,Sm-TiO₂,and CNS are conducive to the effective charge separation of electron-hole bidirectional transfer in the ternary system.