Highly ordered TiO2 nanotube arrays (TiO2-NTAs) are produced by electrochemical anodization of a Ti foil in ammonium fluoride (NH4F)-ethylene glycol solution. Photocatalytic (PC) investigations indicate that the length of the NTAs plays an important role in their photoactivity. The PC activity increases initially with the NT length and then decreases and the optimum length that yields the highest PC is 6.2 mu m for TiO2-NTAs. The TiO2-NTAs are further converted to heterojunction strontium titanate (SrTiO3)/TiO2-NTAs by a hydrothermal reaction in Sr(OH)(2) solution. As the hydrothermal reaction proceeds, more TiO2 is converted into SrTiO3 and the thickness of the SrTiO3 layer increases. The SrTiO3/TiO2-NTAs exhibit variable PC activities that depend on the hydrothermal reaction time, and the SrTiO3/TiO2-NTAs hydrothermally treated for 1 h or less have enhanced PC properties. The advantage of combining TiO2 and SrTiO3 stems from the difference in the flatband potential, thereby improving the separation of the photogenerated electron-hole pairs and consequently the PC activity.