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

We present photoelectrochemical results for anodic TiO 2 nanotube layers grown with different diameter sizes (21, 35, 56, and 95 nm) with a thickness of approximately 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 5.6 × 10 9 to 7.2 × 10 10 pcs/cm 2 ) and thus the active surface area increased as well, we found that the photoelectrochemical response in the UV light was identical and thus independent of the TiO 2 nanotube diameter in the range of nanotube diameters from 35 to 95 nm. Further, we demonstrate that a heterostructured photoelectrochemical cell consisting of TiO 2 nanotubes sensitized with crystalline Sn–S–Se chromophore showed higher photocurrent density (from 6 to 32 μA/cm 2 for the wavelength of 460 nm) with increasing nanotube diameter size. Upon detailed SEM analyses it was revealed that 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.