Synthesis,Applications And Mechanisms Of Metal (Titanium)-organic Framework-based Photocatalyst For The Removal Of Pollutant From Aqueous Solution Under The Visible Light Exposure

Environmental pollution and energy shortage have become important and urgent global problems due to urbanization,industrialization and the changing lifestyles.Photocatalysis has attracted extensive attention in the field of environmental remediation.In order to improve the photocatalytic efficiency for the removal of environmental pollutants,a large amount of photocatalysts have been developed.In recent years,metal-orgainc frameworks（MOF）attest fastidious concern in the field of photocatalysis due to the tailorable chemistry,large specific surface area,high pore volume,structural adaptivity and biological compatibility.In this study,MIL-125（Ti）,a highly porous and crystalline titanium dicarboxylate with high thermal stability,accessible pore diameter,large surface area and photochemical property,had been chosen as the photocatalyst for the removal of pollutants from aqeous solution.In order to tune the photocatalytic activity in visible light irradiation,various MIL-125（Ti）-based composites had been synthesized via solvothermal method,in situ photodeposition and self-assemble photoreduction method.Then,the as-obtained samples had been characterized via a variety of techniques for the analysis of microstructure and physical-chemical property.Finally,the photocatalyst were applied to remove persistent organic pollutants or heavy metals from water under the visible light exposure,and the mechanisms were also explored.This dissertation is mainly composed of the following five sections.In the first section,highly active g-C₃N₄/MIL-125（Ti）hybrids were successfully prepared on the basis of solvothermal strategy where g-C₃N₄ acted as the supporter for solving the problem of no visible-light response of MIL-125（Ti）.The as-obtained materials were characterized to investigate the crystal structure,morphology,size,composition,optical properties and surface pore structure.It was indicated that the hybrids had large surface area,mesoporous heterostructure,thermal stability,and enhanced visible-light absorption.The improved photocatalytic performance had been obtained for the degradation of Rhodamine B from water under visible-light irradiation.The photodegradation rate of RhB on g-C₃N₄/MIL-125（Ti）composites with the g-C₃N₄ content of 7.0 wt%was 0.0624 min^-1,about 2.1 and 24 times higher than that of pure g-C₃N₄ and MIL-125（Ti）,respectively.RhB degradation and mineralization depended on both holes（h⁺）and·O₂ˉ.The improved photocatalytic performance was ascribed to the indirect dye photosensitization,the Ti³⁺–Ti⁴⁺intervalence electron transfer and the synergistic effect between MIL-125（Ti）and g-C₃N₄.Cyclic experiments also indicated the reusability and stability of photocatalysts for the dye degradation.In the second section,a novel core-shell In₂S₃@MIL-125（Ti）（MLS）photocatalytic adsorbent was successfully synthesized by a facile solvothermal method.The composites had large surface area(303.9 m² g^-1),mesoporous structure（pore diameter is 3.7³.8 nm）and narrow band gap（2.28 eV）with visible light response.The maximum adsorption capacity for tetracycline（TC）was up to 157.2 mg g^-1 based on the Langmuir adsorption equilibrium.The TC adsorption onto MLS was mainly ascribed to surface complexation,π-πinteractions,hydrogen bonding and electrostatic interaction.It also strongly depended on solution pH,initial TC concentration and ion strength.Further,the TC degradation experiments under visible light irradiation showed that the additive content of optimal MIL-125（Ti）in synthesis process was determined to be 0.1 g,and the corresponding photodegradation efficiency for TC was 63.3%,which was higher than that of pure In ₂S₃ and pure MIL-125（Ti）,respectively.The opened porous structure,the effective transfer of photo-generated carriers,the Ti³⁺–Ti⁴⁺intervalence electron transfer and the synergistic effect between MIL-125（Ti）and In₂S₃ were the main reasons for the improvementofphoto-degradationperformance.Three-dimensional excitation-emission matrix fluorescence spectra showed that the by-products after photocatalytic process might be ascribe to humic acids-like and fulvic acids-like matters.Parts of them were also mineralized into CO₂ and H₂O.More than 81%of TC was removed by the integrated photocatalytic adsorbent for TC-obtained medical wastewater after the adsorption and photocatalytic process.In the third section,metal sulfides quantum dot（QD）sensitized MIL-125（Ti）heterostructure hybrids were fabricated via a facile photodeposition strategy in order to maintain the structure and morphology of MIL-125（Ti）.From the results of characterization,it could be inferred that Ag₂S,CdS and CuS QD were uniformly deposited on the MIL-125（Ti）,in which no change occurred in terms of structure and morphology.MIL-125（Ti）only had absorption in the ultraviolet region,whereas loading metal sulfides QD onto the MIL-125（Ti）extended the absorption from ultraviolet to visible region.It was also found that the heterostructure hybrids can be utilized as efficient photocatalysts for Cr（VI）reduction under visible light irradiation,and the highest removal rate was 0.0101 min^-1,which was more than that of MIL-125（Ti）.The improved visible light absorption and the electro-hole pairs separation were responsible for the remarkably enhanced visible photocatalytic performance.What’s more,the mechanisms of the hybrids preparation and photocatalysis were also discussed.In the fourth section,an efficient one-pot self-assembly and photo-reduction strategy had been developed for the construction of Ag/rGO/MIL-125（Ti）composites heterostructure,by which negatively charged GO was intimately integrated with the positively charged MIL-125（Ti）based on electrostatic attractive interactions followed by GO photo-reduction and Ag nanoparticles deposition,thus giving rise to a Ag/rGO/MIL-125（Ti）ternary heterostructure.The ternary composites showed a broad absorption band from 400 to 750 nm.Such band was attributed to the localized surface plasmon resonance absorption of Ag nanoparticles.The heterostructure photocatalyst exhibited an improved photocatalytic performance for the degradation of RhB under visible-light irradiation.A degradation efficiency of 95.7%RhB for Ag/rGO/MIL-125（Ti）was obtained,and the photodegradation rate was 0.0644 min^-1,which was 1.62 times higher than that of the pure MIL-125（Ti）.The better photocatalytic performance had been obtained owing primarily to the imperative role of Ag NPs acting as“electron reservoir”.Indispensably,rGO served as an excellent electron transporter,therefore facilitating the separation of photogenerated electron–hole pairs over the rGO/MIL-125（Ti）binary nanocomposite.In the fifth section,a highly active amino-functionalized titanium MOF with well crystalline lattice,large surface area and mesoporous structure were successfully prepared by a facile solvothermal route using 2-aminoterephthalic acid as the organic link and tetrabutyl titanate as the titanium source.The photo-absorption edge of MIL-125（Ti）could be shifted to the visible light region using H₂ATA units as the organic ligand.Because increasing the electron density（N 2p electrons）of the aromatic motifs through polyamination of the 2-aminoterephthalic acid would modified the value band of MIL-125 while the lower conduction band was unchanged.So a band gap of 2.65 eV was obtained for NH₂-MIL-125（Ti）.Compared with MIL-125（Ti）,the NH₂-MIL-125（Ti）exhibited an improved photocatalytic reduction of Cr（VI）to Cr（III）under visible light irradiation.A photocatalytic reduction efficiency of 97%was shown under the condition of pH 2.1 in solution and ethanol as the hole sacrificial agent.Moreover,photocatalytic Cr（VI）reduction depended on the hole sacrificial agent species and the corresponding addition amount.The presence of Ti³⁺-Ti⁴⁺intervalence electron transfer was the main reason for photo-excited electrons transportation from titanium-oxo clusters to Cr（VI）,resulting in the high-toxic Cr（VI）reduction under the acid medium.