Formation and crystallization of TiO₂ nanostructures on various surfaces

A comparative study of the synthesis of TiO₂ nanorods on fluorine-doped tin oxide (FTO) glass, Si, SiO₂, Si/Ta, Si/TiN, Si/TiN/Ti and Si/HFO₂ substrates by hydrothermal reaction is presented. Detailed analysis on the growth of TiO₂ on pre-annealed Si/TiN/Ti and HfO₂ (HFO) surfaces is also given. For Si/TiN/Ti, a pre-annealing procedure led to the transformation of Ti to a TiO₂ layer which acts as a seed for aligned growth of TiO₂ nanorods. In contrast, Si/HFO does not provide a nucleation site for the formation of aligned nanorods. Various samples were prepared by varying the synthesis conditions, i.e. pre- and post-annealing temperatures and hydrothermal reaction time to figure out the optimum conditions which lead to unidirectional and highly aligned nanorods. X-ray diffraction, scanning electron microscopy, ultraviolet-visible spectroscopy and Raman spectroscopy were used to study structural, morphological and optical properties of synthesized samples. It is found that TiO₂ nanorods exhibit a rutile phase on the Si/Ti/TiN and Si/HFO substrates, but highly oriented vertical growth of nanorods has been observed only on pre-annealed Si/TiN/Ti substrates. On the other hand, TiO₂ nanorods form dandelion-like structures on Si/HFO substrates. Growth of vertically oriented TiO₂ nanorods on Si/TiN/Ti is attributed to the TiO₂ seed layer which forms after the process of pre-annealing of substrates at a suitable temperature. Variation in hydrothermal reaction time and post-annealing temperature brings further improvement in crystallinity and morphology of nanorods. This work shows that the pre-annealed Si/TiN/Ti substrate is the optimal choice to achieve vertically oriented, highly aligned TiO₂ nanorods.