Fabrication, characterization, and photoluminescence properties of highly ordered TiO 2 nanowire arrays
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Highly ordered TiO2 nanowire arrays were prepared in anodic alumina membranes by a sol-gel method. The nanowires are single-crystalline anatase phase with uniform diameters around 50 nm. At room temperature, photoluminescence (PL) measurements of these TiO2 nanowire arrays showed a visible broad band with three peaks, which were located at about 425, 460, and 530 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. A model is also presented to explain the PL intensity drop-down of the TiO2 nanowire arrays embedded in the alumina membrane: the blue PL band of the anodic alumina membranes arises from the F+ centers on the pore walls, and the TiO2 nanowires first form in the center area of the pores and then extend to the pore walls.
I. INTRODUCTION
The porous anodic alumina membrane (AAM) has been studied for more than 40 years.1 In the last decade, the template synthesis method has been playing an important role in the fabrication of many kinds of nanowires2,3 and nanotubes4,5 for its interesting and useful features. 6 Recently, self-organization during pore growth, leading to a densely packed hexagonal pore structure of AAMs, has been reported for anodization in both oxalic7 and sulfuric8 acid solutions. Meanwhile, the recent interest in anatase TiO2 was motivated by its key role in the injection process in a photochemical solar cell with a high conversion efficiency.9 The optical10 –16 properties based on anatase TiO2 materials have been the subject of intensive research. Anatase TiO2 nanowires (TNWs)17 and nanotubes4,17 have also been widely synthesized. However, according to available literature, no research has been reported about the photoluminescence (PL) properties of TNWs prepared in highly ordered through-hole AAMs. In this paper, singlecrystalline anatase TNW arrays were prepared in AAMs by a sol-gel process and the PL phenomenon of these highly ordered TNW arrays was investigated at room temperature. PL spectrum analysis indicates that the physical origins of the three peaks are most probably self-trapped excitons, F, and F+ centers. Two conclusions can be drawn from the formation analysis of the TNWs within the AAM: First, there have many F+ centers (V●O in Kro¨ger notion, one electron trapped on an oxygen vacancy)18 on the pore wall of the AAM; i.e., the strong blue PL band of AAMs is due to the F+ center emission on the pore wall. Second, it is very interesting to find that 1138
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J. Mater. Res., Vol. 16, No. 4, Apr 2001 Downloaded: 28 Jan 2015
the TNWs first form in the center area of the pores and then extend to the pore walls, which is different from that in the previous works.17 II. EXPERIMENTAL
The anodic alumina membranes were prepared in a two-step anodizing process7 and have a densely packed hexagonal pore structure with notable regularity. Highpurity (99.999%) aluminum foil was used as the starting material. Before anodization the aluminum foil was first degreased with acetone, then annealed at 400 °C under vacuum condition to remov
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