TiO2 Nanotube / Chalcogenide-Based Photoelectrochemical Cell: Nanotube Diameter Dependence Study

Abstract

We present photo-electrochemical results for anodic TiO2 nanotube layers grown with different diameter sizes (21, 35, 56 and 95 nm) with a thickness of approx. 560 nm using a novel anodization protocol. These tube layers were utilized as highly ordered n-type conductive scaffold for the inorganic chromophore Sn-S-Se. While downscaling the nanotube diameter significantly increased the number of nanotubes per square unit (from 5.6E+09 to 7.2E+10 pcs/cm2) and thus the active surface area increased as well, we found that the photo-electrochemical response in the UV light was identical and thus independent of the TiO2 nanotube diameter in the range of nanotube diameters from 35 to 95 nm. Further, we demonstrate that a heterostructured photo-electrochemical cell consisting of TiO2 nanotubes sensitized with crystalline Sn-S-Se chromophore showed higher photocurrent density (from 6 to 32 µA/cm2 for the wavelength of 460 nm) with increasing nanotube diameter size. Upon detailed SEM analyses it was revealed that the Sn-S-Se was infilled in all nanotube layers approximately to one third of the thickness. Therefore, this photocurrent increase with increasing tube diameter can be ascribed to better interfacial contact (and improved charge transport) facilitated between the chromophore and nanotube walls.