Photovoltaic Properties And Photocatalytic Performance Of Noble Metal/Titanium Dioxide Nanocomposites

At present,the world is facing two major problems of environmental pollution and energy crisis.Semiconductor photocatalyst can convert solar energy into chemical energy,is one of the effective ways to control environmental pollution and solve the energy crisis.Among them,TiO₂,as a representative of the photocatalyst,has been widely used in photocatalytic decomposition of organic pollutants,anti-bacterial anti-virus,photodegradation of water and solar energy-sensitive batteries and other fields due to its excellent oxidation capacity and chemical stability.However,the wide band gap semiconductor TiO₂?anatase 3.2 eV?can only absorb ultraviolet light and the quantum efficiency is low,which greatly limits its practical applications.Recent works have shown that the modification of noble metal nanoparticles,especially the ones with surface plasmonics,is one of the effective ways to improve the photocatalytic performance of TiO₂.Through the modification of the noble metal nanoparticles with the surface plasmon effect,the photocatalytic activity of the TiO₂ is enhanced.This is due to the fact that the Fermi level of the metal is lower than that of the TiO₂ conduction band,which promotes the separation and transfer of the photo-generated electrons and inhibits the carrier recombination.On the other hand,due to the surface plasmon resonance effect of noble metal nanoparticles,the photocatalytic activity of TiO₂ in the visible region is enhanced by the excited electrons being injected into the TiO₂ conduction band from the noble metal nanoparticles.The research work of this dissertation mainly expands the range of TiO₂ light response by regulating the crystal structure,metal composition and morphology of TiO₂-based photocatalyst,and improves the formation,separation and transfer efficiency of photo-generated carriers,And the photocatalytic activity of the catalyst was improved.At the same time,the relationship between photogenerated charge behavior and photocatalytic activity was studied and discussed,which provided a strong basis for the design and preparation of high performance photocatalyst.The specific work mainly includes:?1?In the first chapter,we firstly introduced the basic information of the crystal structure,energy band structure,photogenerated charge behavior and photocatalytic mechanism of TiO₂ photocatalyst.Then,the influence factors of photo-induced charge transfer behavior and photocatalytic performance of TiO₂ photocatalyst are also described,which includes the structural design,doping ions,metal nanoparticles as well as the surface heterojunction structure.Furthermore,the applications of noble metal modified nano-Ti O₂ photocatalyst in the photodegradation of environmental pollutants,photocatalytic hydrogen production,photocatalytic reduction of carbon dioxide and so on are introduced.?2?In chapter ?,we synthesis of one-dimensional TiO₂ nanowires with different crystal structures and further studies of photogenerate charge carriers transfer properties.TiO₂ nanowires with different crystal structures were prepared by simple and environmentally friendly methods.XRD,TEM and SEM results show that the sample exhibits a nanometer linear morphology with a length of about ten micrometers and a width of about 60-80 nm.With the increase of annealing temperature,the crystal structure is changed from anatase,to anatase/rutile and finally the rutile phase.By using the surface photovoltage,transient surface photovoltage and surface photocurrent techniques,the charge transfer properties of the nanowires with different crystal phases were studied.The results show that the surface states and charge transfer rate of different TiO₂ nanowires decrease with the transformation of anatase,anatase/rutile and rutile,and more importantly,our findings provide the theoretical basis for the design and preparation of photocatalyst.?3?In chapter ?,we studies of photogenerated charge transfer and photocatalytic degradation properties of spherical Ag/TiO₂ nanocomposites.The Ag/TiO₂ nanocomposite complex with uniform size and good dispersibility was prepared by a simple and low temperature solvothermal method.The morphology,structure and composition of the composites were characterized by XRD,TEM,STEM,SEM and EDX.The results show that the composite is composed of 5-10 nm Ag and TiO₂ nanoparticles with a size of about 500 nm.The Ag particles are homogeneously dispersed in the nanocomposite.UV-vis,SPV,TPV and SPC were used to study the charge transfer properties between TiO₂ and Ag.The results showed that under the excitation of ultraviolet light and visible light,the mechanism of photo-electron transfer was different.Under the irradiation of ultraviolet light,electrons were transferred from TiO₂ to Ag,and the electrons will be long-term stored in Ag nanoparticles.Under the visible light irradiation,the surface plasmas of Ag nanoparticles are excited,and electrons are transferred from Ag nanoparticles to TiO₂.Thus,photogenerated electrons are effectively separated and transferred,resulting in a stronger PV response than TiO₂.Through the diffusion or transfer of electrons in TiO₂ or on the surface of Ag nanoparticles,the electrons-holes are effectively separated,and therefore,the photocatalytic activity is effectively improved.In addition,we designed the control experiments to degrade acetaldehyde in dark rooms and visible light conditions,further demonstrating the above mechanism,which provides a valuable strategy for solar energy storage and utilization.?4?In chapter ?,we synthesis and studies of photocatalytic properties of Au/TiO₂ composite with three-dimensional flower-like structure.Au/TiO₂ composites with three-dimensional flower-like structure were prepared by a simple and green solvothermal method.The structure,morphology,composition and photocatalytic activity of the samples were characterized by TEM,STEM,SEM,XRD,XPS and UV-vis techniques.The results show that the Au/TiO₂ composite is a with a particle size of 2-3?m,which is composed of random packed one-dimensional nanowires,and the Au is uniformly distributed in the composite.Under the visible light irradiation??> 420 nm?,the photocatalytic degradation of methyl orange by Au/TiO₂ complex exhibits excellent catalytic activity and stability,which is far better than that of pure TiO₂.There are two reasons: one is its three-dimensional layered structure with appropriate light absorption depth,large surface area,more surface activity sites,thereby enhancing the reactivity;the other is due to the surface plasmon resonance effect of Au,Au/TiO₂ composite has a surface light response range in the visible region,excited electrons transferred from Au to TiO₂ conduction band,which promotes the separation and transfer of carriers.Futhermore,more electrons are captured,resulting in the degradation of active substances?,?,thereby enhancing the visible photocatalytic oxidation of TiO₂.?5?In chapter ?,we synthesis and studies of photocatalytic properties of Pd/TiO₂ composite with three-dimensional spherical grading structure.A Pd/TiO₂ composite,which is with three-dimensional spherical structure composed of one-dimensional nanorods,with different Pd content was prepared by one-step solvothermal method.The results of XPS and UV-Vis show that the interaction between Pd and TiO₂ is strong,which results in the change of the electron density near Ti4+,and therefore the absorbance of TiO₂ is obviously red shifted.At the same time,due to the surface plasmon resonance effect of Pd nanoparticles,the absorption spectrum of Pd/TiO₂ composite is extended to 600 nm visible region,which improves the light utilization efficiency.Under the irradiation of xenon lamp,the Pd/TiO₂ composite exhibited better photocatalytic activity for the degradation of Rhodamine B,and the photodegradation efficiency of rhodamine B was 10 times of that of TiO₂.This is because: first,under the irradiation of xenon lamp,the electron of TiO₂ is excited from the valence band to the conduction band and transferred to Pd nanoparticles.Second,the surface plasmon resonance induced electrons is considered to overcome Schottky barrier at the metal/semiconducting interface into the TiO₂ conduction band.The synergistic effect of two separate transfer mechanisms of photogenerated electrons promotes the formation,separation and transfer of photogenerated electron-hole pairs,inhibits the recombination of carriers,and more electrons and holes will diffuse into the surface of the sample to participate in the catalytic reaction,and therefore,the activity was significantly enhanced.