Single Crystalline-Like TiO2 Nanotube Fabrication with Dominant (001) Facets Using Poly(vinylpyrrolidone) for High Efficiency Solar Cells

The single crystalline-like TiO2 nanotubes (SC-TiO2 NTs) are prepared in the anodization process containing poly(vinylpyrrolidone) (PVP). The PVP in the electrolyte solution functions as a surfactant and controller of the crystal growth. The PVP is favorably adsorbed onto the (001) surfaces rather than onto the (101) facet during the TiO2 nanotube (TiO2 NTs) synthesis because of the high absorption energy (81.1 kcal/mol) on the (001) facets. PVP adsorbed (001) facets are protected during the synthesis, and a single crystalline anatase that primarily exposes the (001) plane is prepared. Furthermore, the overall synthetic mechanism of the fabricated crystalline anatase is clarified using computational calculations. It is clear that there is a difference in the binding interactions between the PVP and TiO2 facets depending on the type of surface. It is concluded that the differences in the binding energies might cause the generation of the SC-TiO2 NTs with exposed (001) facets. In the photovoltaic performance results, the dye-sensitized solar cells (DSSCs) based on the SC-TiO2 NTs show higher photocurrent density because of the large amount of adsorbed dye and the high crystallinity in comparison with that of TiO2 NTs. Because most of the crystalline SC-TiO2 NTs with the active (001) facets have low recombination sites, this results in the effective charge separation and electron transport in the SC-TiO2 NTs DSSCs, leading to the high efficiency solar cell devices.