Controllable Synthesis And Band-structure Modulation Of Mesoporous TiO₂ Microspheres And Their Photo-thermal Catalytic Performance

Mesoporous TiO₂ has the characteristics of uniform mesopore structure,excellent photoelectrochemical properties,non-toxicity,wide availability,low cost,etc.It has a wide range of application prospects in the fields of photocatalysis,photothermal conversion,photovoltaic and adsorption and separation.As a popular material,people have conducted in-depth investigations on mesoporous TiO₂ materias,including controllable synthesis,control of morphology,microscopic pore structure and surface property,as well as various applications in many fields.At present,TiO₂ materials with a single structure and/or a simple composition cannot easily meet the needs of high-performance applications.Controllable constricution of uniform morphologies,and creation of diversified compositions and complex microstructures are major trends in the field of TiO₂ research.The key issue is how to improve the charge transfer and mass transfer efficiency of the TiO₂-based materials,thereby improving the performance of the material.The mesoporous TiO₂-based microspheres with high specific surface areas,open mesoporous channels and rich and adjustable microstructures are a potential solution.In view of the above consideration,the present dissertation focues on the synthesis and application of mesoporous TiO₂-based microspheres.The research was carried out according to the following ideas:?)Guided by controllable material synthesis,simple and efficient routes were designed to synthesize three-dimensional mesoporous TiO₂-based microspheres.By optimizing the synthesis conditions,the size,specific surface area and surface structure of the microspheres can be effectively controlled,which provides a basis for realizing its high-efficiency catalytic performance;?)Guided by structural controlling,simple and efficient straetgies,including semiconductor combination,metal loading and defect engineering,were proposed for optimizing the energy band structures,light absorption range,charge separation efficiency and the number of active sites of the TiO₂-based materials;?)With application as the guidance,the use of the obtained mesoporous TiO₂-based microspheres in the fields of photocatalysis,photothermal conversion,adsorption and separation were explored to provide some theoretical guidance for the potential applications of the mesoporous TiO₂ microspheres.In the second charpter,for the problems of low photo-generated charge separation efficiency and few catalytic active sites of pure TiO₂,we propose to improve these disadvantages by building up defects in TiO₂.By using absolute ethanol as the solvent,TBOT as the titanium source,and triblock copolymer F127 as the template,and concentrated HCl as the angent for controlling the concentrations of H+ and Cl-ions,we synthesized novel mesoporous TiO₂ microspheres rich in defects by using a simple one-step solvothermal method followed by a high temperature calcination process.In the solvothermal process,Ti and Cl in the reaction solution coordinate to form Ti-Cl bonds,and defects are formed after Cl is removed by high temperature calcination The synthesized large-size TiO₂ microspheres have rich mesoporous structures,rough surfaces and a certain concentration of defects.The size,specific surface area,surface structure and defect concentration of the TiO₂ microspheres were controlled by changing the calcination temperature,HCl concentration,solvothermal temperature,solvothermal time and the adding amount of surfactant F127.The final microspheres were applied to photo catalytic N2 reduction reaction.The mesoporous TiO₂ microspheres synthesized at the HCl concentration of 5.5 mol/L and the calcination temperature of 500? have the best photofixation performance.Under simulated sunlight,the rate of NH3 synthesis is 284?molh^-1g^-1,and the quantum efficiency under visible light is up to 0.05%.The preparation of this type of mesoporous TiO₂ microsphere catalyst with rich surface interface structure and adjustable defect concentration can provide certain reference significance for realizing high performance photocatalytic nitrogen fixation.In the third chapter,based on the theory that Au can effectively broaden the light absorption range of mesoporous TiO₂ microspheres and promote the separation of photogenerated charges,we propose to construct of Au@TiO₂ composite structure to further improve the photocatalytic nitrogen fixation performance of defect-rich mesoporous TiO₂ microspheres.Mesoporous TiO₂ microspheres with the optimized surface structure and defect concentration synthesized in the second chapter were used as carriers,and different amounts of Au nanoparticles were loaded on their surfaces by wet impregnation and high temperature hydrogen reduction.Based on the plasma resonance effect of Au nanoparticles and the synergistic effect of defect sites,Au nanoparticles have excellent absorption in visible light region.Then the defect concentration,light absorption intensity and the number of active sites in the microspheres are controlled by changing the loading amount of Au nanoparticles.The role and internal mechanism of hot electrons generated by Au in the photonitrogen fixation process on the surface of TiO₂ microspheres are analyzed.Under simulated sunlight irradiation,the rate of NH3 production is up to 1040?molh^-1g^-1,and the quantum efficiency under visible light is up to 1.5%.In the fourth chapter,considering the relative complexity and low yield of solvothermal synthesis of mesoporous TiO₂ microspheres,as well as the relatively low photogenerated charge separation efficiency of pure TiO₂,we propose to improve these disadvantages by constructing heterostructures using a novel microfluidic jet spray drying technology.Iron-loaded homogeneous mesoporous TiO₂ microspheres were rapidly prepared by a one-step microfluidic spray drying technology,and anatase/rutile/?-Fe₂O₃ ternary heterostructure(Meso-ARH)can be formed by calcining at 400?.The doping of iron promoted the phase transition of TiO₂ at a lower temperature.The extremely fast solvent evaporation process led to the assembly of mesoporous structures and induced the formation of a large number of oxygen defects.The ternary heterostructure particles could improve the absorption of light,improve the separation efficiency of photo-generated carriers,as well as promote the reduction of ?Fe(?)/?Fe(?).With the addition of low amount of H2O2 and under the irradiation of low power ultraviolet lamp,the ternary heterostructure showed excellent heterogeneous photocatalytic performance.The 20 ppm MB solution can be completely degraded in 5 minutes,and also can be completely mineralized in 30 minutes.In the fifth chapter,we further contruct heterogeneous structures with multiple components to improve the utilization of solar energy.A kind of monodisperse Cu/C/TiO₂/SiO₂ composite microsphere with a nano-engineered rich surface composition and surface structure was developed by rapid spray drying technology,and was specifically applied to photothermal water evaporation.Such microspheres have the characteristics of wrinkled surface and hollow interior,and can float stably on the water surface for a long time.Cu nanoparticles enriched on the surface of the microsphere,and TiO₂ nanoparticles rich in defects evenly distributed in the shell layer and amorphous C layer can effectively broaden the light absorption range of the microsphere and improve its photothermal conversion efficiency.The amorphous SiO₂ component can be used as a thermal insulation layer to promote heat localization.The folded surface of the composite microspheres and the gaps between the microspheres can provide a large specific surface area for water heating.Under 1 Sun irradiation,the water evaporation rate of the composite material was up to 1.5 kg m-2 h-1,and the photothermal conversion efficiency was up to 92%In the last chapter,we summarized the major results,conslusions and novelty of the thesis.A certain outlook for the future work was also proposed.