Morphological Control of Anodic TiO2 Nanotubes by the Modulation of Applied Potential

Bamboo-type TiO2 nanotube (BT-TONT) arrays were developed by biasing the facile alternating voltage (AV, 80-20 V) and compared to smooth-walled TiO2 nanotube (SW-TONT) arrays grown by forcing a constant voltage (CV). The duration at 20 V in the AV mode had a considerable influence on the morphological properties such as the thickness, ridge spacing, and pore size for the same cycles. As the duration at 20V was increased, the length and ridge spacing decreased, while the pore size increased. This is due to the formation of a thick blocking layer that blocks the growth of TONT. However, the repetitive formation of ridges in the BT-TONT arrays induced an enlargement of the surface area, resulting in an improvement of the short-circuit current (J(sc)) in dye-sensitized solar cells (DSSCs) in comparison with that of the SW-TONT arrays. In particular, the 20 V (20 min dwell time) TONTs exhibited a photoconversion efficiency (eta) of 0.20% per thickness, compared with 0.077% for the SW-TONTs. Therefore, the formation of BT-TONT arrays can give an increased surface area for dye uptake, leading to an improved photoconversion efficiency.