The effect of morphology on the optical and electrical properties of sodium titanate nanostructures

Sodium based titanate nanostructures (NaTNT) are promising materials for energy storage and photoelectrode applications. We present an experimental work describing the structural, morphological and electronic properties of hydrothermally synthesized NaTNT, evaluating different synthesis conditions (synthesis temperature and time). We found a clear dependence between the synthesis condition and the observed, morphological, electrical and optical features. Increasing synthesis time and decreasing temperature allow the nanotubes to be filled. Local and extended structural analysis, and Raman spectroscopy, supported the samples stoichiometry with a pentacoordinated Ti-O polyhedral. Electrochemical impedance spectroscopy analysis revealed a predominant two-transport mechanism; however, a third process was observed for two samples with particular morphology (nanorods and nanoparticles co-existing with nanotubes). Finally, the influence on the optical properties of the structural and morphological characteristic is determined and discussed. We found a reduction of the gap energies when compared with anatase. Additionally, from the photoluminescence response we suggest different kind of defects must be present in the samples. This work presents a very complete set of experimental tools for the analysis of NaTNT, showing how the synthesis conditions allow tuning the electrical and optical responses of NaTNT for energy storage and photoelectrode applications.