Sulfur-Doped Highly Ordered TiO₂ Nanotubular Arrays with Visible Light Response

The future of TiO 2 -based photocatalysts strongly depends on their structural optimization so as to obtain high activity as well as visible light response. Recently we successfully fabricated sulfur-doped highly ordered TiO 2 nanotubular arrays by potentiostatic anodization of titanium foils, followed by annealing in a flow of H 2 S at 380 °C. The as-prepared arrays were characterized using field emission scanning electron microscopy, differential scanning calorimetry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS), and photoelectrochemical measurements. The results clearly show that the vertically oriented nanotubular arrays are highly ordered with a wall thickness of 10 nm. After being annealed at 380 °C, the original TiO 2 nanotubular arrays were transformed from an amorphous structure to an anatase phase with a crystallization enthalpy of 324.6 J/g. With treatment in a H 2 S atmosphere, sulfur ions were incorporated into oxygen atom sites in the TiO 2 nanotubes to form O−Ti−S bonds, confirmed by high-resolution XPS of S2p and Ti2p and XRD analysis. The sulfur doping caused the absorption edge of TiO 2 to shift into the lower energy region, making the nanotubular arrays active under visible light irradiation up to 650 nm, demonstrated by UV−vis DRS and photocurrent measurements.