One Dimensional Titanium-Dioxide Base Nanostructured Composites For High Photocatalytic And Photoelectric Properties

One-dimensional TiO₂ nanomaterials are widely developed and applied in different research fields(such as photolysis of water,photodegradation,CO₂conversion,solar cells,photovoltaic devices,lithium-ion batteries and super capacitors)due to its excellent physical and chemical properties.However,there are two extremely important breakthroughs for the practical application of one-dimentional TiO₂ nanomaterials,that is,the thermodynamics and reaction kinetics.On one hand,the modulation of its thermodynamics(band structure)not only regulate its response to different wavelength of light but also construct new energy levels to meet carrier transitions,thereby improving the carrier separation efficiency.On the other hand,the change of its reaction kinetics not only promotes its absorption of photons but also promotes the diffusion and adsorption of reactants,improves the surface reactivity and its performance in optoelectronic applications.In this work,based on the fully understanding of one-dimensional TiO₂nanomaterials and graphene/nanoparticle composites,and combine with the current hot issues of research on these two materials.The in-depth studies of one-dimensional TiO₂-based heterogeneous nanocomposites and graphene/nanoparticles composites have been carried out on their preparation,characterization,performance and application.Firstly,a physical method(ion sputtering)was proposed to prepare graphene/silver hybrid.Secondly,the the use of graphene/silver hybrid for the modulation of photogenerated electrons in one-dimensional TiO₂ nanofibers was explored.In addition,based on the traditional TiO₂/ZnO heterojunction,the effects of graphene as a carrier bridge layer on the carrier separation process and photocatalytic performance of TiO₂/ZnO was investigated.Finally,based on the one-dimensional TiO₂ nanowire array,a TiO₂/graphene/Cu Sb S₂ photoelectrode with high photoelectric conversion efficiency was developed.These studies have made useful explorations for the development of one-dimensional TiO₂-based heterogeneous composites and graphene/nanoparticle hybrid.This dissertation includes seven chapters.The first chapter introduces the significance and importance of the topic of this thesis.And then the basic knowledge and research status of TiO₂ nanomaterials,one-dimensional TiO₂ nanomaterials,graphene and TiO₂/graphene composites was introduced.Subsequently,the basic knowledge,research status and progress of graphene/nanoparticles hybrid and the TiO_2/graphene/nanoparticle heterogeneous nanocomposites are reviewed.Finally,the research background,source of this research direction,the main contents and innovations of this research work are introduced.The experimental materials,testing methods and instruments used in the experiments are introduced in this chapter.Which include the experimental methods(ion sputtering)for preparing graphene/silver nanoparticles hybrid;preparation and testing methods of TiO₂/graphene/AgNPs photocatalytic fibers;preparation and testing methods of TiO₂/graphene/ZnO nanocomposite photocatalysis;preparation and testing methods of TiO₂/graphene/Cu Sb S₂ composite photoelectrode,in addition,the characterization and testing techniques such as the electron microscopes(SEM/TEM/OM),X-ray instruments(XRD/XPS)and optical instruments(Raman/UV-Vis/FTIR/Photoluminescence)are introduced.In the third chapter,the preparation of graphene/silver nanoparticle nanocomposite by a physical method(ion sputtering)was proposed.It is found that the microstructure of graphene in the graphene/silver hybrid prepared by physical method is more integrate than that prepared by traditional chemical methods.Moreover,the average particle size of silver nanoparticles is smaller(10 nm)and the dispersion is higher.Secondly,the factors affecting the average particle size and dispersion of silver nanoparticles on the surface of graphene were discussed.Finally,the mechanism of physical method for preparing graphene/silver nanoparticle hybrid is proposed.In the forth chapter,we introduces the method of spinning the graphene/silver nanoparticle hybrid prepared by physical method and traditional chemical method into TiO₂ nanofibers.Experiments revealed that the surface plasmon effect produced by these nanostructures can reduce the band gap energy of TiO₂,thereby improving the photoresponse of TiO₂ nanofibers in the visible region.Comparing these two fibers,it was found that TiO₂/graphene/AgNPs(physical method)nanofiber was of higher photocurrent intensity and better photocatalytic performance.It is considered that graphene with a lower surface defect density in graphene/silver nanoparticle hybrid(physical method),silver nanoparticle with smaller particle size(10 nm)and higher dispersion improves the separation efficiency of photogenerated electron holes in TiO₂ fiber.Finally,by analyzing the energy band structure and work function,the separation and transmission process of photogenerated carriers are discussed.In the fifth chapter,a ternary TiO₂/graphene/ZnO nanocomposite is prepared by using graphene sheets as bridge between TiO₂ nanorod arrays and ZnO nanoparticles.The experimental study reveals that the graphene sheets provide a barrier-free access to transport photo-excited electrons from TiO₂ nanorod arrays and ZnO nanoparticles.In addition,there generates an interface scattering effect of visible light as the graphene sheets provide appreciable nucleation sites for ZnO nanoparticles.This synergistic effect in the ternary nanocomposite gives rise to a largely enhanced photocurrent density and visible-light driven photocatalytic activity,which is 2.6times higher than that of regular TiO₂/ZnO heterostructure.Finally,based on this ternary composite material,the energy band structure and work function are calculated,and the separation and transmission process of the photogenerated carriers are analyzed.In the sixth chapter,we report a ternary material system to simultaneously enhance the charge separation and light-absorption of the photoanodes by introducing reduced graphite oxide and Cu Sb S₂ quantum dots on TiO₂ arrays.The experimental results reveal that the graphene sheets with a low Femi level and a superior electron mobility accept photo-excited electrons from TiO₂ and enable fast electron transportation;While Cu Sb S₂quantum dots promotes the visible light-absorption of the photoanode.This synergistic effect in this ternary TiO₂/graphene/Cu Sb S₂photoanode gives rise to the largely enhanced photoconversion efficiency(1.2%at0.3 V)and photocurrent density(5.5 m A cm^-2 at 0.4 V).It should be pointed out that the photocurrent density of ternary TiO₂/graphene/Cu Sb S₂ composite,as far as we know,is superior to previously reported TiO₂-based photoanodes in neutral media.Finally,based on this three-phase material,the energy band structure and work function are calculated,and the separation and transmission characteristics of the photogenerated carriers are analyzed.Chapter seven is the summary.Finally,a brief introduction is given for author's published papers,academic conference,scientific research projects,honors and awards.The acknowledgement and the resume are attached as well.