Carbon treated self-ordered TiO₂ nanotube arrays with enhanced lithium-ion intercalation performance

Vertically aligned TiO₂ nanotube (TONT) arrays on titanium substrate developed by facile electrochemical anodization in an aqueous solution of 0.5 M Na₂SO₄, 0.5 M H₃PO₄, 0.2 M sodium citrate, and 0.5 wt% NaF were prepared having a pore diameter and thickness of 100 nm and 1.2 μm, respectively. The undoped (u-doped) TONT arrays possessing an anatase phase were again annealed at 500 C under a mixed gas flux of nitrogen (N₂) and acetylene (C₂H₂), to induce the enhancement of electrical conductivity. It was designated as carbon-doped (c-doped) TONT arrays. Undoped and c-doped TONT arrays were compared using various characterization tools, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS). Furthermore, based on several electrochemical tests (galvanostatic charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS)), it was observed that c-doped TONT arrays revealed improved charge/discharge capacity, cycle stability, and rate capability, due to the enhanced electrical conductivity of c-doped TONT arrays.