Two-Stage Hydrothermal Growth of Long ZnO Nanowires for Efficient TiO2 Nanotube-Based Dye-Sensitized Solar Cells

Vertically aligned TiO2 1-D nanostructures synthesized directly on transparent conducting oxide (TCO) have shown great promise to overcome the limitation of the random network of TiO2 nanoparticles currently used in dye-sensitized solar cells (DSCs). A promising process for synthesis of such structures is to first grow aligned ZnO nanowire arrays and then convert them to TiO2 nanotubes. Growth of ZnO nanowires with a sufficient length and density but without fusion of their roots, however, remains challenging. In this paper, we investigate the dependence of wire density and aspect ratio on the temperature, concentrations of reagents, and seeding crystals used in a liquid-phase ZnO nanowire synthesis process. On the basis of the results of the investigation, we introduce a two-step hydrothermal process for synthesis of long and vertically aligned ZnO nanowire arrays, and demonstrate that, by dividing the wire growth process into two steps at two different temperatures and reducing Zn2+ concentration, ZnO nanowires up to 20 mu m long can be obtained without fusing their roots. The resulting ZnO nanowires can be converted into TiO2 nanotubes, and DSCs fabricated using such TiO2 nanotubes yield power conversion efficiencies up to 6.1%.