Tailored Anodization Leads to the Formation of Long, High-Aspect-Ratio TiO 2 Nanotube Arrays for Dye-Sensitized Solar Ce
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Tailored Anodization Leads to the Formation of Long, High-AspectRatio TiO2 Nanotube Arrays for Dye-Sensitized Solar Cells As dye-sensitized solar cells (DSCs) continue to receive significant attention for their potential as low-cost alternatives to crystalline silicon photovoltaics, it appears that arrays of nanotubular TiO2 may hold the key to their performance. In particular, the use of long, smooth TiO2 nanotube arrays instead of disordered nanoparticulate TiO2 can enhance DSC efficiencies by absorbing larger amounts of photosen sitizing dye, leading to increased photo electron generation. Now, work reported in the September issue of the Journal of the American Ceramic Society (DOI: 10.1111/j.1551-2916.2008.02546.x; p. 3086) by Y. Yang, X. Wang, and L. Li at Tsinghua University in Beijing demonstrates that careful control of the conditions of anodization can lead to the formation of smooth TiO2 nanotube arrays as long as 22 μm, with aspect ratios up to 200. The researchers began the nanotube formation process by potentiostatically anodizing 99.5%-pure titanium foils in a dimethylformamide electrolyte solution containing 0.5 wt% hydrofluoric acid, under an applied voltage of 30 V. This
resulted in arrays of TiO 2 nano tubes whose length increased with anodization time, saturating at 22 μm after 40 h, which the researchers identify as the point at which the oxide growth and dissolution rates equalize. The nanotubes’ wall smoothness also increased with anodization time, and their aspect ratio was as high as 200, as revealed in fieldemission scanning electron microscope images. The researchers next created samples using 50 h of anodization time and different (fixed) applied voltages, finding that the average nanotube diameter increased from 110 nm at 20 V to 130 nm at 30 V. A key observation of these experiments was that the nanotubes were covered by a thick protective oxide layer during the anodization, enabling the long growth. In order to investigate the effectiveness of these long nanotube arrays in solar cells, the researchers first calcined them at 400ºC for 3 h and then used them as photoanodes in DSC devices. Although the devices employed an inexpensive and less-effective dye (Indoline dye D102) than the state of the art, the device’s conversion efficiency was as high as 3.4%. In light of these results, long, high-aspectratio TiO2 nanotube arrays may bring the
MRS BULLETIN • VOLUME 33 • NOVEMBER 2008 • www.mrs.org/bulletin
commercial viability of DSCs one step closer to reality. COLIN MCCORMICK
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