Synthesis and characterization of zinc oxide-titanium dioxide nanocomposites co-doped with dysprosium and europium
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Date
2016-01
Authors
Mofokeng, Sefako John
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
Recently, the essence of nanocrystalline phosphor materials to enhance the efficiency of solar cells have been the subject of interest in research. To improve the efficiency of solar cells, special attention has been paid to down-conversion based on nanomaterial phosphor doped with rare earth ions which absorb ultraviolet (UV) from the sun and down-convert them into visible photons that could subsequently be absorbed by solar cells.
Titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticulate phosphor were activated with Eu3+ ions. These nanoparticles powders were successfully synthesized by co-precipitation method. The structural properties of TiO2 nanoparticulate phosphor were examined with X-ray diffraction (XRD). The XRD confirmed crystallization of both tetragonal anatase and rutile phases and the average crystallite size of undoped and Eu3+ doped TiO2 were 21 and 8 nm, respectively. Scanning electron microscopy showed that the morphology of TiO2 nanoparticles composed of nanorods with average length and diameter of approximately 410 nm and 73 nm respectively. The optical properties of TiO2 nanoparticulate phosphors were studied using photoluminescence (PL) spectroscopy and ultraviolet-visible (UV-Vis) spectroscopy. At 325 nm excitation wavelength, PL data showed a broad emission from undoped TiO2 centred at 455 nm. This broad emission band was assigned to defects in TiO2. Eu3+ doped TiO2 nanoparticulate phosphors exhibited five emissions which are associated with f→f transitions of Eu3+ ions when excited at 466 nm. The band gaps of the nanophosphors were also determined from the UV-Vis reflectance measurement using Tauc’s plot.
The XRD analysis of Eu3+ doped zinc oxide (ZnO) nanoparticulate phosphor was consistent with wurtzite hexagonal structure of ZnO. In addition, the XRD patterns confirmed the presence of secondary phase of Eu2O3. The morphological changes of ZnO nanoparticles due to incorporation of Eu3+ ions were observed from the SEM micrographs. The PL emission of undoped ZnO nanoparticulate phosphor excited at 325 nm exhibited weak ultraviolet emission and an intense broad deep level emission (DLE). This DLE is normally related to green, yellow and blue luminescence. The PL emission of ZnO:Eu3+ nanoparticulate phosphor excited at 466 nm showed weak and intense emissions at 593 nm, 618 nm, 646 m and 682-696 nm which are attributed to Eu3+ transitions: 5D0→
7FJ (J = 1, 2, 3 and 4) respectively.
Nanocomposite (ZnO-TiO2) phosphors single doped with europium (Eu3+) and co-activated with 0.4 mol% Dy3+-Eu3+ with different concentrations of Eu3+ ions were synthesized via sol-gel method. The X-ray diffraction (XRD) confirmed crystallization of the wurtzite hexagonal ZnO and tetragonal TiO2 (anatase and rutile) phases. In addition, the XRD data confirmed that secondary phases of ZnTiO3 and Zn2TiO4 were formed. ZnO–TiO2 nanocomposites exhibited a broad band emission ranging from 400 nm to 900 nm and represent the combined emission band of both hexagonal wurtzite ZnO and tetragonal TiO2 phases when excited at 325 nm. The coactivated nanocomposite were excited in the UV region with excitation wavelength of 248 nm and the corresponding emissions were observed in the visible region at 496, 584, 593 and 614 nm. The emissions at 496 nm and 584 nm were assigned to 4F9/2→ 6H15/2 and 4F9/2→ 6H13/2 f→f transitions of Dy3+ transitions while those at 593 nm and 614 nm were assigned to 5D0→ 5F1 and 5D0→ 5F2
f→f transitions of Eu3+ activator, respectively. Energy transfer mechanism between host and dopants (Dy3+ and Eu3+) was discussed.
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Keywords
Nanocomposites, Down-conversions, Energy transfer, Europium, Dysprosium, Zinc oxide, Titanium dioxide, Dissertation (M.Sc. (Physics))--University of the Free State, 2016