Chalcogenide photosensitized titania nanotube arrays

• CuInS2 and In2S3 nanoparticles deposited on/into the TNAs by drop-casting.
• Suitable photosensitizer coverage exposed to light generated photoelectrons.
• Oversaturated CIS produced defects for electron–hole pair recombination.
• Quick mass transfer in polysulfide electrolyte neglected Nernst diffusion impedance.
• InS3 buffer layer enhanced longer lifetime of photoelectrons in TNA/InS3CIS5.

The spatially ordered and vertically oriented TiO2 nanotube (TNT) arrays were prepared by a two-step anodization process. The TNT arrays (TNAs) were annealed at 450 °C for 30 min, yielding discrete, hollow nanotube structures with anatase phase. CuInS2 (CIS) and In2S3 (InS) chalcogenide nanoparticles were deposited on the top surface, walls, and bottom of the TNAs by repeated drop-casting in ambient atmosphere. Suitable photosensitizer coverage was achieved with CIS5 and InS3CIS5, when exposed to visible light, efficiently generated photoelectrons that traveled uniformly into TNAs. The TNA/CIS10 nanostructures were oversaturated with CIS nanoparticles, resulting in more defects and an overly thick absorption layer promoting premature electron–hole pair recombination. Electrochemical impedance spectroscopy (EIS) analysis indicated that the mass transfer in polysulfide electrolyte occurred sufficiently quickly for the Nernst diffusion impedance (Zd) to be neglected. The equivalent circuit of the TNA nanostructures in polysulfide electrolyte exposed to light was modeled by two serial RQ circuits. The impedance in lower frequency regions contributed to the mechanism of the interface charge transfer when exposed to light. When a positive bias was applied, the photosensitized electrons could travel from the CIS photoabsorption layer through the TNAs, and into Ti electrode more efficiently. The relaxation behavior of open circuit potential exhibited by the TNA/InS3CIS5 nanostructures showing that the enhanced lifetime of photo-induced electrons was a function of the InS3 buffer layer. The novelty of facile drop-casting chalcogenide photosensitive composition containing an excess of sulfur avoided the use of toxic H2S gas and other toxic solvents. The charge transfer mechanisms were established on the basis of the behaviors of suitable photosensitive layers and function of buffer layer.

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