Enhancing electrical and optical properties of anatase TiO2 nanotubes through electrochemical lithiation

Abstract

In presented paper, anatase TiO<inf>2</inf> nanotubes (NTs) were obtained by anodic oxidation of Ti-foil followed by subsequent annealing in air at 400 °C. After thermal treatment, TiO<inf>2</inf> nanotubes (NTs) were electrochemically lithiated by means of galvanostatic (GS) discharge, as a part of GS cycling, at 25°C and 55 °C. Microstructural properties of the as-prepared material were observed by scanning and transmission electron microscopy (SEM, TEM), while changes in chemical bonding with lithiation were examined by X-ray photoelectron spectroscopy (XPS). Specific electrical conductivity, obtained by 4-point probe method, showed multifold increase after Li-ion insertion in TiO<inf>2</inf> NTs, at both temperatures. By using diffuse reflectance spectroscopy (DRS), the decrease in energy gap, from 3.04 eV for the as-prepared TiO<inf>2</inf> NTs to 2.81 eV for lithiated TiO<inf>2</inf> NTs, was observed. The photoluminescence (PL) measurements suggest that Li-ion intercalation led to suppression of deep-level trap states within the bandgap, of TiO<inf>2</inf> NTs, and promoted shallow defects associated to F-centers. Open-circuit photovoltage decay (OCVD) measurements confirm the promotion of shallow defect states. Furthermore, HER measurements indicate that the electrochemical lithiation is a promising strategy for enhancing the catalytic performance of TiO<inf>2</inf>.