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Magnetic properties of W doped and WO_3 composite TiO_2 and optical properties

Author: JiaoJunRong
Tutor: GongChangWei
School: Taiyuan University of Science and Technology
Course: Materials Science
Keywords: TiO2 dopant Composite First principle Optical properties magneticproperties
CLC: TN304.7
Type: Master's thesis
Year: 2014
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Abstract

The TiO2based diluted magnetic semiconductors were given attention because of theirsuperior performance than the traditional semiconductors, the applying under visible light ofTiO2is limited because of the large band gap. So it is important in preparing the TiO2baseddiluted magnetic semiconductors and improving the photocatalytic activities of TiO2undervisible light.In this paper, the electronic structures, optical properties and magnetic properties of pureTiO2, O vacancies (Ov)-doped TiO2(Ov-TiO2),(W Ov) co-doped TiO2((W Ov)-TiO2),W-doper TiO2(W-TiO2) and coupling TiO2with WO3(WO3/TiO2) systems were preparedrespectively by sol-gel method and magnetron sputtering method, and the phase structures,surface morphology, optical properties and magnetic properties were characterized andanalyzed. The results indicated that list below.Comparing with the pure TiO2system,the Ovdecrease the band gap of TiO2by formingimpurity bands over the valance band. W decrease the band gap of TiO2by forming impuritybands under the conduct band. For (W Ov)-TiO2system, the band gad decrease dramaticallybecause of the combined interaction of both. For Ov-TiO2,the Ovbound the surroundingelectrons and form the bound magnetic polarons and lead to the magnetism occurrence. ForW-TiO2,the magnetism derived from the p-d orbit hybridization.For (W,Ov)–TiO2, the extraelectrons which led into by W doping prevent the Ovfrom forming the band magneticpolarons, and lead to the magnetic moment decreasing. So we should avoid the generation ofOvin the process of preparing W-TiO2diluted magnetic semiconductor.For WO3/TiO2systems, the interact of W and O, Ti and O, W and Ti at the interface leadto the impurity bands appear under the conduction band and decrease the band gap. Theelectrons of O2p orbit do not contribute to the magnetic moment, but transport the spinangular momentum between the W5d and Ti3d orbits, and lead to the electrons of d orbitsspin polarization. The results indicate that coupling TiO2with WO3could reduce the band gapeffectivity and shift the absorption region to the visible light region, and couping TiO2withWO3could induce the magnetism. The band gap are reduced with the increased molar ratio ofTi and W, but the different ratios have not effect on the magnetic moment. The experiment results of W-doped TiO2thin films with different molar ratios of W andTi show that pure TiO2sample has only one single phase and the ferromagnetism has notobserved in pure TiO2sample. W6+ions enter into TiO2lattice by replacing the Ti4+ions, andgrain size decreased at first and then increased with the increased doping ratio. The grain sizeincreased may because of two reasons, one is the grain size increasing aim to eliminate thedistortional strain energy which induced by W6+ions doping, another one is part of W6+ionsenter into the gap position and lead to the grain size increased. W doping enhancedramatically the photocatalytic efficiency of methyl orange under visible light irradiate, andinduced the room temperature ferromagnetism. With the increased doping ratio, thephotocatalytic activity and the coercive force are increasing, but the band gap is decreasing.The experiment results of WO3/TiO2composite thin films indicate that there is only onesingle anatase TiO2and it is not found room-temperature ferromagnetism in the pure TiO2film. In the WO3/TiO2composite films, with the sputtering times increased, the degree ofoxidation reduced and the WO3-xformed. Comparing to the pure TiO2, in the WO3/TiO2composite films, the optical absorption spectrum shifted to red and the magnetic propertiesenhanced with the sputtering times increased. The transfer of electron-holes in the WO3/TiO2interfaces decreased the recombination rate of electron-hole pairs and the energy of electronsexcitation and shift the absorption region to the visible light region. The interact between theelectrons at the interface lead to producing a collective spin-split. The oxygen vacancies thatare initiated by the uncompleted oxidation act as n-type dopant and lead to appearing impuritybands over the valance band and shift the optical absorption spectrum to red. The oxygenvacancies form the Ov-center bound magnetic polarons and enhanced the ferromagneticproperties.

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