[en] 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