Comprehensive Study on the Morphology Control of TiO₂ Nanorods on Foreign Substrates by the Hydrothermal Method

The hydrothermal method is a facile route for the synthesis of TiO2 nanostructured materials, but it requires accurate process optimizations and adjustments to circumvent the undesirable morphology products. In this study, systematically controlled experimental studies are carried out under thermodynamic and kinetic considerations to understand the formation of well-aligned TiO2 nanorods during the hydrothermal reaction. In this regard, TiO2 nanorods are synthesized on various types of substrates, including single crystal TiO2 and sapphire, fluorine doped tin oxide, and silicon. Variable growth parameters are classified and investigated for their effects on the morphological evolution of TiO2 nanorods. The preferred morphology of TiO2 nanorods with the {110} facet is confirmed based on the crystallographic results for TiO2 nanorods acquired by extensive transmission electron microscopy studies during the entire growth processes. The presence of the seeds on the substrates is found to be mandatory for the formation, growth, and strong adhesion of TiO2 on the applied foreign substrates. The results elucidate that the growth kinetics of the process is significantly governed by the amount of applied HCl concentration, which is highly influential on the morphology of the synthesized TiO2 nanorods. Accordingly, the growth mechanism for the preferential growth maintaining the rod shape is derived.