Visible light absorbing TiO₂ nanotube arrays by sulfur treatment for photoelectrochemical water splitting

Herein, we report the preparation and characterizations of the sulfur (S)-doped TiO₂ nanotube (TONT) arrays prepared by a sulfurization process of TONT arrays via electrochemical anodization on a Ti substrate with pure TONT arrays. The S-doped TONT arrays were prepared with the annealing temperature from 450 to 550 °C under H₂S gas for 10 min, and these reaction conditions corresponded to no modification of the morphological features relative to that of the TONT arrays. Furthermore, the 500 °C annealed S-doped TONT arrays showed enhanced visible light absorption and high electric conductivity, thus resulting in the most improved photocurrent density (2.92 mA cm^-2 at 1.0 V vs sat. Ag/AgCl) in the 0.1 M KOH solution as compared with that (0.965 mA cm^-2 at 1.0 V vs sat. Ag/AgCl) of TONT arrays. In addition, the incident photon-to-electron conversion efficiency (IPCE) of S-doped TONT arrays exhibited approximately 43% in the UV region, whereas TONT arrays had 32% IPCE in the UV region. In addition, the small photoactivity in the visible light region for the S-doped TONT arrays was observed up to a 600 nm wavelength, where IPCE value of 2.4% at 500 nm was achieved in the S-doped TONT arrays, in contrast to the negligible IPCE values for the TONT arrays. However, the relatively reduced photocurrent density (2.04 mA cm^-2 at 1.0 V vs sat. Ag/AgCl) was achieved at further sulfurization temperature at 550 °C for the S-doped TONT arrays; this value is attributed to the rough tube shape and atomic level defects in the edge region for the excessively S-doped TONT array, which indicated a role as the light scattering centers and the electron-hole trap sites.