Specific Analysis Of The Main Active Species And Mechanism In Degradation Of Organic Pollutants By Aqueous-phase Photocatalysis

In recent years, the use of semiconductor materials as photocatalysts has attracted greatattention for the removal of organic and inorganic pollutants in aqueous phase. Themain researches of the mechanism in aqueous-phase photocatalysis have focusedmainly on the role of active species leading to the initial photoreaction process.However, the specific mechanisms in the photocatalytic process still remain obscureand controversial. To recognize the photocatalytic process systematically in thedifferent kinds of systems would be helpful to explain the photocatalytic mechanismand develop new type visible light photocatalysts.In this thesis, the degradation process of methyl orange （MO） solution over TiO₂wasfirst chosen as our object to identify the main active species and determine their roles inthe photodegradation of MO under UV light irradiation. Then through the comparisonand investigation of the active species during the degradation of MO on Zn_xCd_1-xS andTiO₂in liquid-phase, the differences of the active species between the visible lightsystem and the UV light system were revealed. To further realize the mechanism in thedegradation of MO, ZnO as a UV-light response catalyst was also applied to comparewith TiO₂. The active species and their sources in the two systems were exploredconcretely. At last, in order to decompose organic compounds efficiently, a novelapproach to enhance photosensitized degradation of dyes under visible light irradiationby the Zn_xCd_1-xS/TiO₂nanocomposites was explored. The composition, structures, andmorphologies of the samples have been investigated through XRD, UV–Vis DRS, XPS,TEM, and et al. characterizations. ESR, NMR, PL, electrochemical analysis and LCMStechniques were used to characterize the active species and the generated byproducts inthe reaction process. The main conclusions are as follows:（1） Through the detection of the active species in the degradation process of MO onTiO₂, the MO oxidation by photocatalysis was mainly due to the participation of O₂radicals and slightly to the contribution of holes and OH radicals. The dissolvedoxygen and OH groups were important to the generation of these active species.（2） The Zn_xCd_1-xS nanoparticles and nanorods with visible-light response were synthesizedby hydrothermal process and microwave solvothermal method, respectively. Comparedwith TiO₂, it was found that O₂, OH and holes played much more role inZn_0.28Cd_0.72S-UV and TiO₂-UV systems. O₂and holes contributed to the degradation inZn_0.28Cd_0.72S-visible light system.（3） With the similar band gap, ZnO and TiO₂showeddifferent properties in the photocatalytic process. After the addition of different types ofactive species scavengers, it was found that in TiO₂system, OH was generated byholes; while in ZnO system, OH was generated by holes and O₂. The larger numbersof the active species and their incessant sources in ZnO system contributed to the betteractivity.（4） Zn_xCd_1-xS/TiO₂nanocomposites were synthesized by a simplehydrothermal method. The results showed that the composite of the two inorganicsemiconductors largely enhanced the photosensitized degradation of rhodamine B （RhB）under visible light irradiation. These photocatalytic reactions were supposed to arisemainly from the electron that transferred from the adsorbed dye in its singlet excitedstate to the conduction band of Zn_xCd_1-xS and TiO₂.The features and innovations of this thesis are as follows:（1） the relationshipbetween the active species and the degradation process was revealed by theelectrochemical method and other techniques in the TiO₂/MO system.（2） Zn_xCd_1-xSnanocrystals were first used and exhibited good photocatalytic activity in thedegradation of dyes.（3） The role of the active species involved in the degradation ofMO in Zn_0.28Cd_0.72S and TiO₂systems was first compared and realized.（4） Thedifferent sources of the active species in ZnO and TiO₂systems were investigated indetail.（5） The novel approach to enhance the photosensitized degradation was exploredby the composite of two inorganic semiconductors. And the Zn_xCd_1-xS/TiO₂nanocomposites with good activity were synthesized for the first time.