Modified Anatase TiO₂ For Environmental And Photonic/photovoltaic Applications

Multi-functional metal oxides TiO₂ has shown distinguish advantages and great potential in environmental/photonic applications due to its novel optical and electrical properties,non-toxicity,non-secondary-pollution,high chemical stability and abundance of raw materials etc.However,studies have shown considerable limitation on further application of raw TiO2,such as wide bandgap,low mobility and high resistivity etc.What's more,the market evolution with growing demands have stipulated new requirement of Ti O₂ in these fields.Therefore,modifying pristine TiO₂ to improve its optical and electrical properties to suit targeted fileds,making it more effective in the applications above or even expanding its applications is the main foucus of current research in TiO₂ material.On the other hand,a systematic study of modified material and its association between properties and application can shed some light on the general principles of material design.Focusing on environmental/photonic applications,the work in this thesis was list as follws:1.Self-aligned anatase films with c axis orientation were successfully fabricated by hydrothermal method on FTO?F-doped SnO₂?substrate for the first time.The selection growth of the targeted phase?either rutile or anatase?in the thin films was controlled by adjusting concentration and the types of acidic anions such as Cl^-and SO₄^2-.A thorough study between hydrothermal parameters and TiO₂ product?crystal structure,phase formation and film morphology?was conducted to help tailor the microstructure of titanium oxide to improve the practical application of TiO₂.Results show that the growth mechanism of anatase film on FTO substrate was transient nucleation and a competition between rutile and anatase existed in the growth process,Thus leads to the fact that anatse films can only growth under specific conditions.The hydrogen sensing characteristics of films of both rutile and anatase phases were investigated and improvement on sensitivity and response time over reported data was presented.It was found that rutile films had higher sensitivity while anatase films had faster response and the sensing mechanism was closely related to the exposed surfaces of the film:the close-assembled rutile films were composed high energy surface-?001??anatase of low energy plane{101}and{001}?which allows more capacity to absorb hydrogen atoms thus longer time to reponse.2.Mn doped TiO₂ photocatalyst with visible light response were prepared via hydrothermal method.The successful incorporation of Mn into TiO₂ lattice was acheieved by using ethanol and HNO3 in the precursor.Mn doping effectively reduce the bandgap of TiO2and extend its light absorption range to visible or near Infrared Radiation?IR?range.A lattice contraction along c axis and decreasing in particle size?the average size of Mn-TiO₂nanoparticle was of 6nm?was observed after doping.Discussion in doping concentration revealed the the solubility of Mn in anatase structure was about 35%,beyond which excess Mn precipated as magnese oxide-Mn₃O₄ in this case.X-ray photoelectron spectroscopy?XPS?showed a possible Ti⁴⁺with a small shift toward lower binding energy with Mn doped anatse Ti O₂.More definitive results were give by XANES,which confirmed a 4+valence,suggesting the shift in binding energy was only caused by weaking bonding of Ti and O.A mixture of 3+and 4+valence states exsited for all levels of Mn doping?except for the one doped beyond 35%,which showed a mixted 2+,3+and 4+?and the percentage of 3+increases with increasing doping concentration.Mn L edge XANES spectrum suggested that the main phase of Mn is Mn₃O₄ when doping beyond35%.With the help of EELS calculation,it is then confirmed that the changes of Mn valence was displayed as variations of the relative intensity of the two peaks in Mn L3 edge.DFT calucation revealed an average valence of 3.26-3.4,which was consistent with XPS results.Mn doping introduced two curvy band?caused by the interaction of Mn 3d and Ti 2p?into anatase band diagram which were believed to be the origination of its red-shift in optical absorption.An improved photocatalytic was found on doped TiO2 powders.Results show that a better photocatalytic performance can be achieved by a relatively low/moderate dosage.And Mn⁴⁺was believed to be the key to improved photocatalytic performance.3.Nb doped TiO₂?TNO?films was prepared and its potential as transparent conductive oxides?TCO?was discussed.In this work the remote plasma sputtering technology was first employed in depositing TNO thin films.A High quality of TNO films were achieved with a significantly enhanced conductivity of an order of 10^-4?·cm and a high transparency above80%?comparable with ITO?with a thickness of 445nm.The films were deposited at room temperature and then followed by annealing.The annealing temperature of the TNO films deposited by this method was significantly reduced compared to previous reports.The as-deposited films were amorphous which were then crystallized into anatase nanocrystals after a short thermal exposure of 30 minutes at a moderate temperature of 280oC in air.Such low temperature crystallization induced remarkable enhancement of both conductivity and transparency,with the annealed samples demonstrating low resistivity of 6.4x10^-4?·cm at room temperature and up to 87%optical transmittance.Regarding to carrier concentration and mobility,a 5 orders higher carrier concentration(to 10²¹)and an increasing from 1.96 to 17.817.8cmV^-1s^-1 in mobility has been observed for the annealed samples.EDX measurement suggested a 6.7at%Nb concentration in metallic species.Further didcussion in microstructure found some nano-crystal of anatase phase existing in the as-prepared films,which were believed to be the promoter of lower annealing temperature in this work.The doping chemistry of TNO films was studied by XPS and a mixed valence for both Ti(Ti³⁺and Ti⁴⁺)and Nb(Nb⁵⁺and Nb⁴⁺)was observed,with Ti³⁺as an indication of defects.The overall valence of Ti?3.77 to 3.81?increased while a decrease was seen in Mn?4.78 to4.75?valence,suggesting a charge transfer from Ti to Mn.The Oxygen content of TNO films was increased to 64.4at%from 63.9at%,providing further evdience that lattice defects such as Oxygen vacancies were removed during annealing.Fundamentally,the excellent transparent conductivity from the current work can attributed to the interplay between doping Nb to the Ti sub-lattice sites of the anatase phase and energetic/electronic effects due to formation of native defects.In case of depositing conditions,a slightly oxygen-poor sputtering condition was found to be helpful to avoid donor-killer defects such as Ti vacancies.The achieved optical transparency and electrical conductivity for TNO are comparable to those for tin-doped indium oxide?ITO?,thus demonstrating great potential in low-carbon processing of TNO to substitute the ITO that is based on the depleting and expensive indium resource.