A Study On The Preparation,photocatalytic And Photo-electrochemical Properties Of Modified TiO₂ Nanomaterials And Its Hierarchical Structures

In recent years,with the rapid development in the society and industrial progress,environment pollution and energy shortage have become two of the most vital problems that threatened sustainable development of the society.Semiconductor photocatalyst has been considered as one of the effective approches to solve the two major crises of environmental problems and energy shortage due to its unique advantages.Photocatalysts gradually develop into a interdisciplinary science,which involves chemistry,physics,environment,materials and other related frontier interdisciplinary subjects.As the most popular and representative material,titanium dioxide（TiO₂）has been recognized as a popular photocatalyst because of its non-toxicity,low cost,admirable stability and excellent activities.Until now,many studies have been reported in the literature about the well-known semiconductor.However,large band gap energy（3.2 e V for anatase TiO₂,3.0 e V for rutile TiO₂）and ease of electrons（e^-）and holes（h⁺）recombination which seriously limit its practical application to solve environmental problems and to improve energy conversion.The properties of TiO₂nanomaterials are closely related to their morphology,size,crystallinity,crystal structure,electronic structure,energy band structure and surface active sites,etc.In order to improve its performance,modification of TiO₂is one of the common methods to enhance their performance for nanomaterials.Therefore,more researches should be focued on defects to further understand the relationship of structure and characteristics of the defects in TiO₂semiconductors,and to design defects reasonably to obtain high-performance semiconductor photocatalysts.In addition,the fabrication of hierarchical heterogenous structures of nanomaterials is considered as another approach to improve the photocatalytic performance of TiO₂nanomaterials.For this kind of structure can not only avoid the agglomeration and sintering of traditional nanomaterials,but also help to enhance the specific surface area of the nanomaterial,promote the orderly migration of charge carriers and enhance the scattering and harvest of the light on the surface of the material.In this thesis,TiO₂-based semiconductors were modified and structured to optimize their photocatalytic performance based on the above strategies.In order to discuss the effect of transition metal defects on the band gap and performance of photocatalyst,Zn_1-xFe_xO nanoparticles were studied in detail for Fe ions were well-matched with and easy to dope in Zn O lattice.In addition,due to the limited improvement of the wide band gap photocatalysts,a novel narrow band gap semiconductor Ag₃PO₄was studied and defect-related research was discussed.The details were listed as follows:（1）TiO₂microspheres with good dispersibility were successfully prepared by hydrothermal method.The effects of annealing atmosphere on the phase composition and intrinsic defects of TiO₂microsphere were studied.The research results are as follows:The anatase phase could gradually transform to rutile phase with the temperature increasing from 450℃to 600℃under air atmosphere.However,the phase transition was inhabited to the range of 500-650℃under N₂atmosphere.Moreover,due to the effect of the oxygen-deficient atmosphere,N₂annealing could bring about the formation of charged oxygen vacancies and Ti³⁺,which further prolonged the life time of charge carriers and optimized the photocatalytic performance.When TiO₂microspheres were annealed at 550℃under N₂atmosphere,the photo-catalytic activity of TON-550（84.7%anatase phase and15.3%rutile phase）showed the best photocatalytic performance.（2）TiO₂microspheres and TiO₂/carbon quantum dots（CQDs）composites with different CQDs contents were successfully synthesized.The phenomenon of carrier migration on the interface was discussed in detail.The conclusions were as below:Due to the presence of carbon-containing groups during the formation of the precursor sol,carbon elements were successfully doped into the TiO₂lattice,which could broaden their response to the spectrum.In addition,CQDs were hybrid with C-TiO₂microspheres by polycondensation reaction of-OH from C-TiO₂microspheres and-OH from glucose precursor.The C-TiO₂/CQDs-2（glucose concentration was 0.02%wt）sample had the largest photocatalytic degradation rate k under visible light,which was almost 3 times than that of as-prepared TiO₂microspheres.（3）Zn_1-xFe_xO nanoparticles were prepared by combustion method.The effects of the valence of Fe ions on the energy band structure and photocatalytic performance of Zn_1-xFe_xO nanoparticles were studied:The photocatalytic performance was enhanced with appropriate content compared with that of the undoped Zn O nanoparticles.Fe²⁺played an important role in acting as a charge capture state,preventing electrons from recombining with holes.When the doping content was4%,the maximum photocatalytic activity was obtained due to the highest ratio of Fe²⁺/Fe³⁺.（4）Spherical Ag₃PO₄crystal and Ag₃PO₄@GO hierarchical heterogenous core-shell structure were prepared by co-precipitation method.The intrinsic defects in the spherical Ag₃PO₄crystal and the dual effects of GO on the improvement of photocatalytic performance in Ag₃PO₄@GO composites were studied.The details were as follows:The weak Ag-O bond led to silver vacancies(V_Ag)in Ag₃PO₄lattice,which acted as trapping centers of photo-generated holes.In the Ag₃PO₄@GO core-shell structure,the amount of silver vacancies in the Ag₃PO₄varied with the change of GO content due to the protection of the defect state at the interface.Among them,Ag₃PO₄@10GO turned out to had the highest content of V_Ag,thus led to the optimum improvement of photocatalytic activity;while the Ag₃PO₄@20GO sample had the best removal effect（including adsorption and degradation）on rhodamine B.（5）Hierarchical and ordered TiO₂-based structures were successfully fabricated by anodic oxidation and boron doping in the section.Based on B-TiO₂/TiNTAs-0.1 electrode,a unique flow-photoelectrochemical（flow-PEC）decoupling water splitting device was designed to generate H₂fuels from waste water decomposition at an infinitely low input,which can satisfy the needs of wastewater treatment and energy requirements at the same time.The results were as follows:TiO₂nanotube arrays were in-situ grown on the Timesh by anodization method to form TiO₂/TiSchottky junction.After that,B-TiO₂/TiNTAs electrodes were synthesized by boron doping.Among them,B-TiO₂/TiNTAs-0.1 exhibited the best photocatalytic performance:the output voltage was only 110 m V when the output constant current density was 2 m A cm^-2.Based on the basis of the conventional photoelectrochemical system,B-TiO₂/TiNTAs-0.1 photocathode was applied to a self-designed flow-cell under simulated sunlight(AM 1.5 G,100 m W cm^-2).The system ran continuously for 12 hours at the constant current density of 1 m A cm^-2runs,while the output voltage was only about 500 m V.After calculation,it could be concluded that the coulombic efficiency was greater than 1100%in the COD removal of brewery wastewater,while the Faraday efficiency of H₂production was higher than 94%in the flow-photoelectrocatalytic cell system.This system could treat brewery wastewater and at the same time evolute hydrogen at an extremely low potential,which could simultaneously solve two major problems of environment pollution and energy shortage.