TiO2 nanotube arrays formed on Ti substrate by electrochemical anodization have been converted into TiO2-SrTiO3 heterostructures by controlled substitution of Sr under hydrothermal conditions. The growth of SrTiO3 crystallites on the nanotube array electrode was probed by electron microscopy and X-ray diffraction. As the degree of Sr substitution increases with the duration of hydrothermal treatment, an increase in the size of SrTiO3 crystallites was observed. Consequently, with increasing...