Hydrothermal growth of TiO₂ nanorod arrays and in situ conversion to nanotube arrays for highly efficient quantum dot-sensitized solar cells

TiO₂ nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO₂ NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO₂ NR arrays are reported as single crystalline, but the TiO₂ NR arrays are demonstrated to be polycrystalline with a bundle of 2-5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO₂ NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot-sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO₂ arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double. TiO₂ nanorod (NR) arrays are hydrothermally grown on FTO substrate, and can be converted to nanotubes (NTs) by selective hydrothermal etching of the (001) core and remaining the inert sidewall of (110) face. After conversion from NRs to NTs, the CdSe quantum dot-sensitized solar cells almost double in energy conversion efficiency.