The Study On The Mechanism Of Synergistic Effect Between Oxygen Vacancy And Doped Metals/surface Modified Fe(?)cocatalyst For Transition Metal Doped Titanium Dioxide

Doped TiO₂ with metal ions,are attracting more and more attention as an important method to extend the photo-responsive range of TiO₂ into visible spectral region.This is because doping metal ions may introduce defect sites in surface or bulk of semiconductor and change crystallinity.Especially for transition metal ions,due to multi-valence of them,doped TiO₂ with a small amount of transition metal ions which can become shallow potential capture trap of photogenerated electron-hole pairs,can prolong recombination time of electron–hole and extend the absorption spectrum of TiO₂ toward the visible light range,so it will utilize solar energy more effectively,thereby improving photocatalytic activity of TiO₂.Studies suggests that oxygen vacancy generated during doping can also improve visible light absorption of metal ions doped TiO₂,and thus enhance photocatalytic activity,but mechanism of action of oxygen vacancy don't be discussed in detail.Although the introduction of metal ions in the TiO₂ lattice can extend visible-light absorption of TiO₂ to some extent,meanwhile it also suffers from another important problem that this method introduces a new carrier recombination center at the same time of increasing in light absorption.It is a deserving research that how to utilize efficiently visible light at same time decrease the introduction of photo-generated charge carriers.Nanotube titanic acid?NTA?is a traditional material studied by our research group.NTA can generate a large amount of oxygen vacancy defect: single-electron-trapped oxygen vacancy under hydro-thermal treatment or heat treatment as well as vacuum treatment.A transition of crystal phase from orthorhombic system to anatase TiO₂ is companied during the above treatment and the as-prepared sample are called as novel TiO₂.An inter-band contributed by a large amount of single-electron-trapped oxygen vacancies is introduced between the band gap and thereby induce visible-light absorption.But the photocatalytic activity of this novel TiO₂ is very low,so how to utilize its visible-light absorption and prepare photocatalyst owning higher photocatalytic activity is of great importance.The topic is aimed at preparing metal-doped TiO₂ nanoparticles containing a large amount of single-electron-trapped oxygen vacancy?SETOV?using nanotubular titanic acid?NTA?as precursor to make use of the larger specific surface area and the visible light absorption preformance to prepare novel photocatalyst with visible light response and study its application in elimination of environmentalpollutants through photocatalytic method under the solar spectra.According to the purpose,transition metal ions Fe,Cr were selected as doptant,the photo-degradation of the gas propylene was chose as evaluation system,and adopting UV-Vis,XRD,TEM,XPS and ESR characterization technique and combining with the theoretical calculation,we carried out a series of studies centering on the preparation of metal ions doped TiO₂,characterization of physical and chemical properties,the evaluation of visible light catalytic performance and the photocatalytic mechanism.The optimal concentrations of doped metal ions and the optimal calcination temperature were studied.The mechanism of the reaction between SETOV and doped metal ions and the effect on visible light photocatalytic properties were discussed.The oxygen vacancy derived from the calcination of nanotubular titanic acid under the high temperature,therefore,oxygen vacancy can be considered as a kind of self-doping.Meanwhile,Oxygen vacancy is a donor level,and the metal ion is a receptor dopant,thus the co-compensation between donor and acceptor may decrease the introduced carrier recombination center by single doping.On this basis,Fe?III?cocatalyst loaded SETOV-TiO₂ and metal doped TiO₂ were further studied.The result revealed that the surface modification of Fe?III?cocatalyst greatly improved photocatalytic activity of sample,because Fe?III?cocatalyst loaded on TiO₂ surface can further decrease the recombination rate of photo-generated charge carriers.Hence the as-prepared sample showed improved photocatalytic activity than single-metal doping.Some conclusions were obtained as follows:1?Using nanotube titanic acid?NTA?as titanium precursor and Fe Cl₃ × 6H₂ O as Fe source,the co-doped TiO₂ with SETOV and Fe are prepared via a facile solid state sintering method.We have researched the effect of the amount of Fe doping and calcination temperature on structure and photocatalytic performance.The results revealed that the optimal doped amount and calcination temperature is 0.5 at%?Fe/Ti atom ratio?and 400°C,respectively.The result indicates that the co-doped TiO₂ with SETOV and Fe improve greatly photooxidation rate of propylene under visible light irradiation.Combining with theoretical calculation,we have studied synergistic effect mechanism between single-electron-trapped oxygen vacancy and Fe doptant.DFT calculation result shows that the existence of SETOV make Fe 3d energy level more delocalized,which can immensely increase mobility of charge carrier.And,combined theoretical calculation with experimental result,we propose a novel photocatalytic mechanism,namely the spin-flip mechanism.Under visible light illumination,the presence of spin-flipinduces the two spin channel photo-absorption,while the spin-inhibition effect provide one spin channel recombination congestion,thereby reducing recombination rate of electron-hole pairs,and synergistic effect between SETOV and doped Fe improve enhancement of photocatalytic property.2?In order to study the mechanism of action between oxygen vacancies and Cr doptant,we prepared Cr-doped TiO₂ by impregnation-calcination method.The influence of the doping amount of Cr and calcination temperature on the crystal structure,the optical properties,and the photocatalytic properties of the catalyst were studied.The best optimized doping content and calcination temperature were 0.1at% and400°C,respectively.When the doping content of Cr is controlled as 0.1%,titanium dioxide exist as anatase in the calcination temperature in the range of 300-600°C.An equivalent proportion of anatase and rutile phase was produced at 700°C,and only rutile at 800°C.This is different from the Fe-doped TiO₂.The visible light absorption of co-doped samples was derived from the interaction of Cr doptant and single-electron-trapped oxygen vacancies.The theoretical results showed that the hybrid occurs between Cr3 d orbit and O 2p orbit as well as Cr 3d and Ti 3d orbit,thereby resulting in the energy level splitting of Cr3 d.And this hybrid effect improves the absorption of low energy photons,and further promotes the absorption of visible light,and the migration of photo-generated carriers and the photocatalytic processes.Energy states of impurity above Fermi level in the forbidden band could capture photo-generated electrons and while photo-generated holes were captured by the energy states of impurity below Fermi level,which effectively promote the transfer of photo-generated carriers to the active sites on the surface and the enhancement of the photocatalytic activity,and a possible visible light catalytic mechanism was proposed.3?A novel anatase phase TiO₂ containing a large amount of SETOV was achieved upon annealing NTA at 600°C in air for 2h.Although the existence of SETOV extends its absorption into visible light region to some extent,this novel anatase phase TiO₂ has no photocatalytic activity for the photo-degradation of propylene owing to the lack of effective capture center of electronic.In this part,Fe?III?cocatalyst was grafted onto the surface of TiO₂ by using a simple and economical impregnation method to make it an effective visible light sensitive catalyst.The influence of Fe?III?cocatalyst on the structure,morphology and photo-absorption performance of the samples were studied.The results indicated that the grafting of Fe?III?nanoclusters on the surface has little influence on the morphology and particle size of the samples.The light absorption of Fe?III?grafted V0·-TiO₂ was mainly attributed to V0·.Thephotodegradation of propylene shows that the optimum loaded mass of Fe?III?is 0.1wt%.The synergistic effect between Fe?III?and V0· promotes the enhancement of photocatalytic activity.As an effective electronic capture center,Fe?III?can make the electronic transfer to the surface of catalyst effectively and fast,which restrains significantly the recombination of photo-generated carries.And the matching energy level between Fe?III?and V0· is the key of the electron transfer fast and effectively.4?On the basis of the previous chapter studies,we carry out surface modification to metal-doped TiO₂ using Fe?III?as cocatalyst.The results show that Fe?III?cocatalyst loaded on surface produced different effects on the absorption spectra of three different metals ion-doped systems.For Fe and V doped systems,the interfacial charge transfer?IFCT?absorption can be observed obviously,while there was no obvious IFCT absorption for Cr doped system.The loading of Fe?III?cocatalyst on the surface improved the enhancement of photocatalytic activity of the three doped systems.The degree of the enhancement of photocatalytic activity is different for the different metal doped system.The photocatalytic activity of V-doped system was the greatest increased.Combined with density of states of the different doping systems,we proposed a different photocatalytic mechanism.Similar to the oxygen vacancy system,the electrons in the valence band and the Cr 3d energy levels firstly transfer to the energy states of oxygen vacancies,and further transfer to the co-catalyst.Therefore,Cr-doped system grafted with Fe?III?was no significant IFCT absorption.The Fe-doped system was similar to V-doped system where the IFCT absorption was obvious due to electrons on the valence band and the metal doped energy level direct transfer to cocatalyst.Fe?III?cocatalyst is an effective electron capture centers that can improve effectively the electron in bulk and conduction band transfer to surface,thereby reducing the recombination rate of the carriers in bulk and surface,and enhancing the photocatalytic activity.