Heterojunction of SrTiO3/TiO2 nanotubes with dominant (001) facets: Synthesis, formation mechanism and photoelectrochemical properties

Controllable growth of anatase TiO2 with dominant high reactive crystal facets has attracted intense interest in the past few years due to their unique structure-dependent properties. In this work, SrTiO3/TiO2 heterostructure nanotubes (SrT) arrays films with high reactive dominant (001) facets of anatase TiO2 were successfully prepared through the part conversion of TiO2 by a hydrothermal method. Various characterizations were used to investigate the morphologies, crystal phases and chemical compositions of the prepared samples. Structure analyses using X-ray diffraction and a high-resolution transmission electron microscope revealed that, the treatment sequence of hydrothermal reaction and annealing was a crucial influence factor on controlling the growth of preferred orientation of (001) facets of TiO2 in SrTiO3/TiO2 heterojunction, whereas the inverse treatment sequence yielded randomly oriented facets. The formation mechanism of dominant (001) facets of TiO2 in the heterojunction was related to crystal structures of TiO2 and SrTiO3. The heteroepitaxial growth on (001) plane of SrTiO3 led to dominant (001) facets of anatase TiO2 attributing to their excellent surface lattices match. The photoelectric performances of pure TiO2 nanotubes and SrT were investigated by photoluminescence, UV-visible diffuse reflectance, photocurrent and current-voltage response. The semiconductor characteristics of them were studied by electrochemical impedance spectroscopy and Mott-Schottky analysis in detail. The results demonstrated that the heterostructure of SrT not only increased light absorption, but also effectively shifted the Fermi level and promoted charge transfer. Among all the samples, the SrTiO3/TiO2 heterojunction prepared by 1 h hydrothermal treatment (1SrT) exhibited the best photoelectrochemical performances due to the synergetic contributions of the high active (001) facets of anatase TiO2 and the charge transport properties of the heterostructure.