Influences Of Ag And Au On The Photocatalytic And Electrochemical Properties Of Titanium Dioxide

As one of the most important non-functional materials,nanocrystalline titanium dioxide?TiO₂?materials have been widely used and extensively investigated in many areas such as solar energy storage and utilization,photocatalytic,photocatalytic degradation of pollutants and lithium ion batteries due to its excellent physicochemical properties,relatively high photocatalytic activity,economic feasibility and non-toxicity.TiO₂-based materials have been considered to be one of the most promising materials for solving environmental pollution and energy shortages.However,the fast recombination of the photo-generated electron-hole pairs,large optical-band gap?3.2 eV for anatase and 3.0 eV for rutile?have limited the applications of TiO₂ in photocatalysis,and its low conductivity and electron mobility have limited its applications in lithium-ion batteries.In this thesis,the method of loading noble metal was used to form TiO₂-based composite to reduce the recombination probability of electron-hole pairs,decrease its band gap and increase electric conductivity.?1?Cu-doped TiO₂ nanofilms were prepared by a sol-gel method on different thickness?10 nm,30 nm,50 nm and 100 nm?of Ag film,and then heat treatment at 500 oC for 2 hours.Scanning electron microscope?SEM?and X-ray photoelectron spectroscopy?XPS?analysis indicated that the Cu-doped Ti O2 nanofilms with appropriate Ag film and amount of Cu ions greatly improved the dispersion of nanoparticles on the surface of the nanofilms.At the same time,some of Ag+ diffused into the lattice of Ti O2 wahich accompanied by the formation of Cu O particles on the surface of the film.Adding of Cu ions reduces the band gap of TiO₂ and increases the absorption of visible light,and Ag+ acts mainly as the traps of electrons to suppress the recombination probability of electron-hole pairs.Compared with only Cu doping,Ag films greatly improve the photocatalytic efficiency of the Cu-doped TiO₂ nanofilms under visible light.?2?N-doped TiO₂ thin films were prepared by magnetron sputtering on Ag substrates with different thicknesses.The morphology and properties of the N-doped Ti O2 thin films with Ag films of different thicknesses were analyzed.Ag diffuses into the TiO₂ film during the annealing process,leading to the formation of Ag nanoparticles.The Ag nanoparticles distributed in the TiO₂ film as well as on the surface of the film.These Ag nanoparticles were not oxidized during heat treatment.?3?TiO₂ nanotube arrays with different lengths and diameters were prepared by anodic oxidation method,and their electrochemical properties were investigated.SEM morphology analysis indicated that the diameters and lengths of the nanotubes increased from 21.6 nm and 5.1 ?m to 51.0 nm and 12.5 ?m when the anodization time increased from 0.5 h to 4 h.And the diameters and lengths of the nanotubes grow almost linearly with the increase of the anodization time.Rate capability performance show that the all of the nanotube electrodes have good cycle properties,relatively high capability still be maintained even after 200 cycles.And it is showed that the longer the nanotube electrodes,the higher capacity at high current density.The best capacity ratio?capacity/diameter ratio?2.72 could be obtained when the diameter is 25 nm and length is 6 ?m.?4?TiO₂ nanotube arrays with 75 ± 5 nm in diameter and 6.5 ?m in length were prepared by anodization,and a thickness of 225 nm Ag film was sputtered on the surface of the nanotube arrays.Then the nanotube arrays with Ag films were heat treatment at 300,400 and 500 oC for 1,2 and 3 hours respectively.The structural characterization and performance analysis show that Ag will diffuse on the outer wall surface of TiO₂ nanotubes due to the curvature effect,and the diffusion depth is closely related to the annealing temperature and the holding time.The diffusivity for the diffusion of Ag atoms on the outmost surface of the Ti O2 nanotubes at 400 oC is 6.87×10-18 m2/s,which is three orders of magnitude larger than the diffusivities for the diffusion of Ag through amorphous TiO₂ films.The activation energy for the diffusion of Ag atoms on the outmost surface of the TiO₂ nanotubes in the temperature range of 300 to 500 oC is 157 kJ/mol,which is less than that for the lattice diffusion of Ag and larger than that for the grain boundary diffusion.?5?The diffusion of Au in the Ti O2 nanotube array was studied by the same method,and the electrochemical performance of the Au loaded Ti O2 as anode materials of lithium ion electrode were investigated.The diffusivity for the diffusion of Au atoms on the outer surface of the TiO₂ nanotubes is in the range of 4.14¹9.4×10-18 m2 s-1 for temperature in the range of 400⁵00 oC.The activation energy for the migration/diffusion of Au on the outer surface of the TiO₂ nanotubes in the temperature range of 400 to 500 oC is 67.2 kJ mol-1.The diffusion of Au into the TiO₂ nanotube arrays involves the grain boundary diffusion and surface diffusion of Au and the interface diffusion on the surface of the TiO₂ nanotubes.Measuring the size of Au nanoparticles after heat treatment,it was found that the growth of Au nanocrystals on the surface of the TiO₂ nanotubes can be described as the first order reaction.The effect of Au on the electrochemical performance of TiO₂ nanotubes as electric anode in lithium-ion batteries was investigated.The results show that under the same conditions,the conductivity of TiO₂ nanotubes with Au has been improved,and the capacity has also been increased remarkably.