Synthesis and Photocatalytic Activity of Highly Ordered TiO ₂ and SrTiO ₃ /TiO ₂ Nanotube Arrays on Ti Substrates

Highly ordered TiO 2 nanotube arrays (TiO 2 ‐NTAs) are produced by electrochemical anodization of a Ti foil in ammonium fluoride (NH 4 F)–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 μm for TiO 2 ‐NTAs. The TiO 2 ‐NTAs are further converted to heterojunction strontium titanate (SrTiO 3 )/TiO 2 ‐NTAs by a hydrothermal reaction in Sr(OH) 2 solution. As the hydrothermal reaction proceeds, more TiO 2 is converted into SrTiO 3 and the thickness of the SrTiO 3 layer increases. The SrTiO 3 /TiO 2 ‐NTAs exhibit variable PC activities that depend on the hydrothermal reaction time, and the SrTiO 3 /TiO 2 ‐NTAs hydrothermally treated for 1 h or less have enhanced PC properties. The advantage of combining TiO 2 and SrTiO 3 stems from the difference in the flatband potential, thereby improving the separation of the photogenerated electron–hole pairs and consequently the PC activity.