Fabrication,Tunable Microstructure And Photocatalytic Performance Of Modified TiO₂-based Nanomaterials

Environmental pollution not only restricts the economic development of society,but also poses a threat to human health.Semiconductor photodegradation technology is considered to be an effective way to solve the problem of environmental governance.The photocatalysts can be used to absorb solar energy to completely degrade organic pollutants into non-toxic small molecules such as H2O and CO₂.Among the various applied photocatalysts,TiO₂-base materials have been the widely studied one,owing to the characteristics of high efficiency,nontoxicity,low cost and good stability.Although TiO₂-based photocatalysts has so many advantages,the application of TiO₂-based photocatalytic materials is still restricted by some aspects such as low photocatalytic efficiency,only excitated by UV-visible light,small specific surface area etc.Based on the fully understanding of the background and combination with the existed problems,this paper aim to solve the above problems with TiO₂-based materials by non-metal anions modification,composite and doping methods,so as to obtain higher specific surface area and better photocatalytic performance.Besides,various analytical techniques are conducted to analyze the adsorption behavior of photocatalysts,the migration behavior of photoinduced electron-hole,morphology,structure and the surface physicochemical properties.Furthermore,the defects and crystal structure changes of TiO₂ during the photocatalytic process was investigated by annihilation positron lifetime and HRTEM techniques.It is hoped that through our researches,we could seek new methods to synthesize TiO₂-based photocatalysts with high photocatalytic activity and large surface area,and deep our understanding of the photocatalytic mechanism.The main contents of our studies are as follows:1)The surface active sulfate promoted TiO₂-based nanotubes?TNT?was prepared by one-step alkaline hydrothermal process.XPS,FT-IR and Raman characterizations show that the sulfate was assigned to Ti4+ coordinated SO42-species,and formed the S-O-Ti bonds,which led to the tensile stress Ti-O bonds in TiO6 units after the hydrothermal process.The sulfate modification of TNT could promote the formation of adsorption sites on the surface and significantly impove the adsorption capacity.Besides,the formation of S-O-Ti could improve the separation ratios and inhibit the recombination rate of the photoinduced electron-hole pairs,and finally improve the photocatalytic ability,which is proved by the higher photocurrent intensity and lower fluorescence intensity.Therefore,sulfate promoted TNT exhibit higher photocatalytic activity for photodegradation of cationic dye MB and anion dye RhB under visible light irradiation.The interaction mechanism between sulfate and TNT was theoretically investigate by density functional theory calculation?DFT?.It finds out that the sulfate ions could form the stable S-O-Ti bond model,which could increase the length of Ti-O bond in TNT and lead to 0.1 eV decrease of bandgap.2)As the TiO₂-based nanotubes?TNT?and TiO₂-based nanotubes?TNS?have the advantages of large surface area,strong adsorption capacity,negative charge and abundant hydroxyl,they could be uased as the sutstrates to synthesize the visible light-driven structures of C₃N₄/TNT and C₃N₄/TNS by a facial water bath method.The shift in the Ti 2p core level in the XPS spectrum of C₃N₄/TNT and C₃N₄/TNS composites indicate that there is an increase in the net positive charge of the Ti ions,which has proved the formation of good interface interaction between TNT,TNS and g-C₃N₄.The appropriate amount of g-C₃N₄ in the composite materials could improve the photocurrent intensity,thus effectively prevented the recombination rate of the photoinduced electron-hole pairs and enhance the photoactivity.With the increase ratio of g-C₃N₄,the photocatalytic ability of the C₃N₄/TNT and C₃N₄/TNS composites increase at first and then decreased.3%-C₃N₄/TNT and 6%-C₃N₄/TNS possesse the highest visible light photocatalytic ability,and the reaction rate is 2 times and 1.66 times than that of TNT and TNS,respectively..3)The CetOy/TNT composite was successfully prepared via a hydrothermal process.It is found that the composite still possess a well-defined one-dimention nanotube morphology with high specific area of about 300 m2/g.The XPS results indicates that Ce ions are in the form of Ce3+ and Ce4+ and form the Ti-O-Ce bonds,which indicates the formation of heterojunction interface between CexOy and TNT.The experiment results suggest that the appropriate amount of CexOy could promote the formation of adsorption sites on the surface and significantly impove the adsorption capacity.The 0.05%CexOy/TNT has exhibits the higher adsorption ability of crystal violet than that of TNT,which the adsoption behaviors conform the Langmuir adsorption model.According to the model,the maximum amount of monolayer adsorption of 0.05%CexOy/TNT and TNT are 215 mg/g and 171.2 mg/g,respectively.The photoactivity of 0.05%CexOy/TNT is greatly enhanced in degradating RhB under visible and Uv-visible light,which is 4 times and 6.25 times of TNT,repectively.4)The unique sheet-like S-N co-doped TiO₂ was successfully prepared via anodic oxidation and calcination processes.The sheet-like morphology is uniformly composed with a diameter of 12 nm nanoparticles,which could provide larger specific surface area of 105.25 m2/g.The XPS results confirm that the S elements exist in the form of S4+ and substituted Ti ion,and the N elements seems to have occupied the oxygen interstitial sites.S-N co-doped catalysts show stronger light adsorption and improve the separation ratios of electron-hole pairs,which exhibits superb photocatalytic activity almost 5 times and 3 times than that of pure P25 and N-doped TiO₂ under visible light irradiation.5)The existence of the surface lattice distortion and defects variations during the differents stages of TiO₂ photocatalytic process by using positron annihilation and HRTEM.The experimental results reveal that:during adsorption process,distortion of the surface lattice finally resulted in increase of small-size defects in TiO₂ nanoparticles.After the degradation,the greatly prolonged ?1 demonstrated that a huge number of small defects would gather and be introduced into the TiO₂ lattice.Finally,fully degradated TiO was exposed in the air for 30 days,the defects formed during the photocatalysis with higher energy have vanished,and the distorted lattice has recovered to the more integrated lattice with lower energy states.Photoluminescence spectroscopy and UV Raman spectroscopy reulsts were further proved the conclucions that the TiO₂ surface distortion plays an important role during photocatalysis,which induces various defects as electron capture sites for improving electron-hole separation.In other words,the lattice distortion relaxation provides the energy to cut the chemical bonds and defect sites to accomplish the degradation during photocatalysis.This mechanism give a novel point of view for understanding the degradation and reactivation of TiO₂ photocatalysis from atomic scales.