Facile Synthesis and Morphology Control of Bamboo-Type TiO2 Nanotube Arrays for High-Efficiency Dye-Sensitized Solar Cells

We report fast synthesis of bamboo-type TiO2 nanotube arrays via anodization of Ti in nonhazardous electrolyte under alternating voltage condition for applications as photoanodes in high efficiency dye sensitized solar cells (DSSCs). Formation mechanism of bamboo type nanotubes is also explored. The low voltage step reduces pH and ion diffusion gradient inside TiO2 nanotubes and induces formation of bamboo ridges on outer tube walls when a second high-voltage step is conducted. Ridge spacing and length of bamboo-type nanotubes can be facilely tuned by adjusting time of high-voltage step and electrolyte composition. All DSSCs based on bamboo-type TiO2 nanotube arrays show higher efficiencies than those based on smooth-walled nanotubes with the same tube length due to enhanced surface area of bamboo-type nanotubes for dye loading. It is also noted that DSSC efficiency increases with the ridge density of bamboo-type nanotubes of the same length. For example, DSSC based on bamboo-type nanotubes (8 mu m long) with the highest ridge density exhibits an efficiency of 5.64%, higher than the efficiency of 3.90% exhibited by DSSC based on smooth walled nanotubes of the same length. Moreover, the DSSC efficiency can be further increased to 6.8% by growing significantly longer bamboo type TiO2 nanotubes (16.5 mu m long) via decreasing water content in electrolyte.