Surface engineering of TiO₂ nanotubes with tantalum for improved electrochemical performance

Abstract

In this work, TiO2 anatase nanotubes were synthesized using a straightforward, two-step anodic oxidation method. To tackle with the optical and electrical properties of the material, a thin layer of tantalum was sputtered onto the nanotube surface. The microstructure of the modified material was analyzed using scanning and transmission electron microscopy (SEM and TEM), while changes in chemical bonding were examined through X-ray photoelectron spectroscopy (XPS). Structural analysis found formation of β-Ta2O5 phase on the surface of deposited TiO2 NTs. Electrical resistivity, done by 4-point probe technique, showed a reduction in resistivity for the modified material, implying an increase in conductivity. Diffuse reflectance spectroscopy (DRS) showed an increase in the energy gap from 3.05 eV to 3.85 eV, while photoluminescence (PL) spectra revealed a suppression of deep-level trap states within the bandgap for modified NTs. These results indicate that increased conductivity can most probably be attributed to the reduction of Ti4+ to Ti3+ as confirmed by XPS, modification of surface oxygen states and suppression of deep-level trap states within the bandgap for Ta deposited nanotubes. Electrochemical tests further revealed improved capacity for Li-ion intercalation, as well as Coulombic efficiency, particularly at elevated temperatures.