Multi-doping Nanocrystalline TiO₂ Composite Films For The Photodegradation Of Organic Pollutants

Titanium dioxide is a typical photocatalyst,due to its good chemical stability,excellent photo-catalytic performance,clean and eco-friendly,low cost,which has been widely used in sewage treatment,air purification,antibacterial disinfection and other fields.However,TiO₂materials still have some limitations in large-scale practical applications.For example,the anatase TiO₂ can respond only to ultraviolet light with an incident light wavelength of less than 387.5 nm,while ultraviolet light only accounts for 3%⁵%of natural light,resulting in very low utilization of natural light;moreover,the key of photocatalytic reaction is the migration and diffusion of photonic carriers,but their composition is rapid,resulting in low light yield.Therefore,it is an important breakthrough to improve the photo-catalytic performance of TiO₂ and expand its practical application range by improving the utilization of natural light and photon quantum yield.For these goals,we must seek effective ways to modify TiO₂ materials.There are many kinds of modification methods that scientists have explored.In this paper,the effects of ion doping and oxide compositing on the photo-catalytic properties of TiO₂ materials were described,and the mechanism of multi-doping nanocrystalline TiO₂ composite film was explored.The main research content is as follows:?1?The preparation and performance tests of multi-doping nanocrystalline TiO₂composite films.The colloids used in this experiment are TiO₂,ZnO,SiO₂ and SnO₂.They are all prepared by sol-gel process.The single thin film and composite film are prepared by dip-coating process on the glass substrate with corresponding colloid.In this paper,the selected elements of ion doping modification research include non-metals B,F,transition metals Fe,Co,rare earth metals La,Y,and noble metal Ag.Based on the photocatalytic degradation of methyl green in TiO₂ thin film,the optimum concentration of the seven ions alone was determined and the best single-doped ion was selected as the first ion of the three-ion doping system.According to the influence of doping the other six ions and the first ion on the photo-catalytic activity of TiO₂,the optimal concentration of the six ions in the double-doped combination is determined and the best double-doped ion is selected as the second ion of the three-ion doping system.Similarly,we can select the third doping ion.The study of-oxide composite doping modification is to select ZnO,SiO₂ and SnO₂ colloid to be mixed with TiO₂ in different volume ratios respectively,so as to determine the optimal ratio of each composite and select the best combination.Multi-doping nanocrystalline TiO₂ composite films were prepared,and the photo-degradation experiments of oxytetracycline and formaldehyde were tested in the visible light,respectively.The experimental results show that the photo-catalytic activity of TiO₂ can be improved to a certain extent by ion doping or oxide compositing,and the photo-response range of the multi-doping composite film is obviously enlarged,so the utilization of visible light is significantly improved.?2?The characterization of multi-doping nanocrystalline TiO₂ composite samplesIn this paper,UV visible spectrophotometer?UV-Vis?,fluorescence emission spectrometer?PL?,X-ray diffractometer?XRD?,differential thermal-thermogravimetry analyzer?DTA-TG?,field emission scanning electron microscope?FE-SEM?and nitrogen adsorption-desorption instrument?BET?were used to characterize the multi-doping nanocrystalline TiO₂ composite films,including the structure and photo-catalytic behavior of the composite films.The co-doping modification was explained based on the characterization results.UV-Vis and PL results show that the modified TiO₂ thin film light response range has been extended to the visible region,and the recombination rate of photo-generated electrons and holes was significantly decreased;the XRD and DTA-TG results show that the crystalline form of modified TiO₂ is mainly anatase with the best photo-catalytic activity,and the thermal stability of materials are also enhanced;FE-SEM and BET results show that TiO₂ shows a uniform and uniform morphology after co-doping modification.Moreover,the particles are continuously refined to have a larger specific surface area,which is favorable for the increase of the photo-catalytic reaction rate.?3?The description of photocatalytic mechanismsThe photo catalytic performance of the nanocrystalline TiO₂ films is obviously improved after modification.According to the performance tests and characterization results,the mechanisms of modification can be further speculated as follows:a.Non-metallic elements B and F are doped into the TiO₂ energy band to form the doping energy level.This acts as an electron donor and acceptor,which reduces the band gap energy of TiO₂ and enhances the absorption of visible light.The doping of transition metal elements Fe and Co can significantly increase the degradation rate of organic matter under UV light and broaden the light absorption of TiO₂.The rare-earth metal elements La and Y have large ionic radii and can be embedded in the lattice of TiO₂ to expand the crystal lattice forming defects,thereby effectively reducing the recombination rate of photo-generated electrons and holes.The introduction of precious metal element Ag into TiO₂ can serve as the electron storage and transport medium of the material surface,so as to increase the lifetime of electron-hole pairs.b.When semiconductor ZnO is doped with TiO₂,they are easy to form a staggered energy level due to the similar band gap energy between them,so that the band gap width of TiO₂ is narrowed,thereby effectively increasing the absorption rate of visible light;When semiconductor TiO₂ and SnO₂ are doped together,due to their band gap energy are greatly different,the difference in energy levels is likely to cause the reverse transfer and diffusion of photo electrons and holes,thereby prolonging the lifetime of carriers and promoting the separation rate;Insulator SiO₂ material has a huge surface area which can increase the photocatalytic degradation rate,and when the SiO₂ is introduced,the electric field force generated by the interface effect so that the photo generated charge can be effectively transferred and the recombination opportunities of electrons and holes can be reduced.c.There are synergistic effects among non-metal ion,metal ion,and semiconductor composite in the multi-doping nanocrystalline TiO₂ composite film,which shows that the photo catalytic behavior and structure of TiO₂ are significantly improved.