Growth mechanism of titanium dioxide nanowires for dye-sensitized solar cells

Mesoporous films made of titanium dioxide nanowires are desirable for dye-sensitized solar cells because nanowires provide direct conduction pathways for photogenerated electrons. Anatase titanium dioxide nanowires with polycrystalline microstructure were synthesized on titanium foil using a three-step process. First, the top surface of the titanium foil was transformed to Na2Ti2O4(OH)2 nanotubes through hydrothermal oxidation in NaOH. Next, the Na2Ti2O4(OH)2 nanotubes were converted to H2Ti2O4(OH)2 nanotubes by ion exchange. Finally, the H2Ti2O4(OH)2 nanotubes were converted to polycrystalline anatase nanowires through a topotactic transformation. The film morphology evolution, crystal structure transformations and growth mechanism are described in detail. Titanium foil reacts with NaOH to form Na2Ti2O4(OH)2 sheets, which exfoliate and spiral into nanotubes. The Na2Ti2O4(OH)2 nanotubes are immersed in HCl solution to replace the Na+ ions with H+ ions. During the topotactic transformation of H2Ti2O4(OH)2 nanotubes to anatase TiO2 nanowires, the sheets made of edge bonded TiO6 octahedra in the H2Ti2O4(OH)2 nanotubes dehydrate and move towards each other to form anatase crystals oriented along the nanotube axis which creates a polycrystalline nanowire. These mesoporous TiO2 nanowire films were suitable for use as dye-sensitized solar cell photoanodes