TiO₂ nanowire/nanorod arrays grown on FTO substrate by hydrothermal reaction have attracted great attention because of favorable applications in dye-sensitized solar cells and quantum dot solar cells. In this paper, discrete vertically aligned TiO₂ nanowire arrays as long as 7.2 μm and diameter of less than 100 nm were successfully synthesized on FTO substrate via the hydrothermal method. The influence of hydrothermal precursor composition on morphologies of the grown wires was investigated. Photoelectrochemical water-splitting performance for nanowires with different lengths was studied; the best performance was observed for the longest array without a compact layer. The growth mechanism in concentrated HCl was also examined. The TiO₂ nanowire array developed in this paper provides an optimal structure for energy-harvesting applications requiring long and discrete nanowires as electron collectors.