Study Of Optical And Magnetic Properties For TiO₂ Based Semiconductors

The research and development of TiO₂ based semiconductor films and nanostructures have potential applications in the fields of luminescent semiconductor materials, spintronics, catalysts and dye-sensitized solar cells. Strong luminescence from visible to near-infrared range can be obtained by incorporation of different rare earth ions into TiO₂ systems; Co-doped TiO₂ nanomaterials is a promising nano diluted magnetic semiconductors (DMS) with room temperature ferromagnetism; The incorporation of N make the absorption band of TiO₂ nanofibres shift from ultraviolet to visible range. The thesis is mainly concentrated on the preparation of TiO₂ materials and the incorporation of some metal and nonmetal ions into them, and investigated their structure, optical, luminescence as well as magnetic properties. The main content and conclusions of the thesis are as follows:(1) TiO₂: Tb films ions are obtained by sol-gel method and their photoluminescence (PL) properties were also investigated. Strong PL peaking at 410, 432, 467, 493, 550, 590 and 624 nm were found from PL spectra, which are due to the intra-4f transitions of ⁵D₃→⁷F₅, ⁵D₃→⁷F₄, ⁵D₃→⁷F₃, ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄ and ⁵D₄→⁷F₃ of Tb³⁺ ions. Among them, the strongest emission is green PL at 550 nm and it can be observe with necked eyes. PL intensity attaches maximum when Tb³⁺ ions concentration is about 9.5 mol% and concentration quenching occurs when more Tb³⁺ ions are incorporated. PL intensity increases remarkably when Ce³⁺ ions are introduced into TiO₂: Tb film, this result is mainly due to the activation of Tb³⁺ ions by Ce³⁺ ions. The obtained films have potential application in luminescent semiconductor materials.(2) We also fabricated TiO₂: Eu films by sol-gel method. The samples were annealed at various temperatures in oxygen atmosphere for 1 h. With the increase of annealing temperature, PL intensity of visible emissions peaking at 620 nm due to Eu³⁺ ions increases firstly but then decreases, and reaches maximum when annealing temperature is 700℃. Energy transfer from STE state to Eu³⁺ ions is considered to exist in TiO₂: Eu system compared with PL spectra of pure TiO₂. PL intensity of 815 nm emission due to defect states associated with Ti³⁺ ions of TiO₂ host appears when annealing temperature is higher than 700°C and increases rapidly with increasing of annealing temperatures. We conclude that the energy back transfer from Eu³⁺ ions to defect level associated with Ti³⁺ ions dominants the emissions at high annealing temperatures, and leads to the decrease of visible emissions and the increase of near-infrared emission.(3) TiO₂ nanofibers with the diameter of～75 nm were fabricated by electrospinning method. Er³⁺ ions were successfully incorporated into TiO₂ nanofibres and the effect of annealing temperature on the morphology, structure and PL properties of the nanofibres was investigated. The strong green PL of Er³⁺ ions peaking at 566.6 nm was detected and the PL intensities increase with the increase of annealing temperature. At higher annealing temperatures of 600 and 800°C, a near-infrared emission peaking at 815 nm appear, which is ascribed to the defect states associated with Ti³⁺ ions. TiO₂: Er nanofibers can be used as a good candidate for nanophosphors because of the strong green photoluminescence of Er³⁺ ions.(4) We prepared TiO₂: Co nanofibers with an average diameter of -70 nm by electrospinning. XRD measurements showed that the nanofibres present anatase to rutile phase at annealing temperature of 420 to 800°C. The obtained nanofibres present room temperature ferromagnetism and the ferromagnetic moments decrease with increasing of annealing temperature. TiO₂: Co nanofibers show a PL band peaking at 473 nm composed of two emission bands due to STEs and oxygen vacancies through multipeak Gaussian fitting, which is different from that of pure TiO₂ nanofibers. These indicate that the oxygen vacancies may play an important role for ferromagnetism in TiO₂: Co nanofibers. The ferromagnetism may come from the ferromagnetic coupling of CO²⁺ ions via F-center which is the result of partially substitute of Ti⁴⁺ ions by CO²⁺ ions. From these results, we introduced a way to investigate the structure and origin of ferromagnetism in DMSs by PL measurements.(5) We fabricated N-doped TiO₂ nanofibers by electrospinning with annealing TiO₂ nanofibers under NH₃ flow. Their morphology, crystal structure and UV-Vis absorption properties were investigated. It is found that the optical band-gap of TiO₂: N nanofibres present a blueshift for about 0.46eV compared with that of pure TiO₂ nanofibers. This indicate that the incorporation of N extend the absorption band from UV to visible region. The obtained materials have potential application in the field of photocatalysts.