Optical band-gap of TiO 2 nanopowders doped with Al 2 O 3
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Optical band-gap of TiO2 nanopowders doped with Al2O3 Keisuke Yoshimura1, Tetsuya Hashimoto and Hiroshi Katsumata 1 School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan ABSTRACT Optical band-gap and cathode luminescence (CL) properties of anatase TiO2 nanopowders mixed with Ȗ- Al2O3 powders by planetary ball mill were evaluated as a function of a powder mass ratio (x=Al2O3/TiO2) of 0 to 0.5 and their correlation with XRD spectra was also investigated. The optical band-gap of TiO2 increased from 3.36 eV to 3.41eV with increasing milling time (tm) up to 600 min, which was in good agreement with the blue shifts observed in the CL spectra with increasing tm and it was interpreted as a quantum size effect. In addition, the optical band-gap of TiO2 powders mixed with Al2O3 with tm=60min greatly increased from 3.36 eV to 3.48 eV with increasing x up to x=0.5. On the other hand, the optical band-gap of all the powders was decreased by annealing at temperatures above 600 oC, which was evidenced by the XRD spectra to be due to the growth of grain size. INTRODUCTION Dye-sensitized solar cells (DSSCs) have been widely studied as a potential alternative for the conventional inorganic solar cells. However, the highest efficiency for a DSSC is still 11.4 %. One of the approaches to increase the efficiency is to enhance open circuit voltage (Voc). Voc for DSSC is determined by the energy difference between the oxidation-reduction states of an electrolysis solution and the Fermi level of an oxide semiconductor. It is possible to enhance the Voc by widening the band-gap of TiO2 [1]. However, the energetic positions of the dye excited state must lie above the TiO2 conduction band to keep driving force for electron injection, and their energy difference between dye (e.g., N-719) and TiO2 has been reported to be about 0.3eV [2]. Therefore, the band-gap widening of TiO2 should be controlled below 0.3eV when using N719 as a dye. The purpose of this study is to control the optical band-gap of TiO2 powders toward the higher energies. It is examined by adding Al2O3 powders to TiO2 powders and by changing their particle size using planetary ball mill. EXPERIMENTAL TiO2 powders with an average grain size (Gs) of 7 nm and Ȗ- Al2O3 powders with a Gs of 23 ȝm were used as starting materials. TiO2 powders were mixed with Ȗ- Al2O3 powders in mass ratio (x=Al2O3/TiO2) of 0 to 0.5. They were milled with planetary ball mill system (Planet M23F, Nagao system Inc.) for 60 min -300 min at the orbital speed of 650rpm in ethanol in the
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alumina crucible using alumina balls (ij2mm, 50g) with a mass ratio of alumina-balls/mixedpowders= 25/3. Then they were annealed in air at 450 oC – 1150 oC for 1 hr. These powders were evaluated by UV-vis reflection spectroscopy (V-670, Jasco Co., Ltd.), photoluminescence (PL) (Triax320, Horiba Ltd.), CL (CL-4, Oxford Ltd.), XRD and SEM (JSM-7600F, JEOL Ltd.). All the measurements were performed at room temperature. Optical band-gap of processed powders was derived from the measured reflection
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