Deposition of TiO₂ photocatalyst on polyethylene terephthalate or polyimide polymer films by reactive sputtering for flexible photocatalytic sheets

Polycrystalline anatase TiO₂ films were deposited by rf reactive magnetron sputtering on polyethylene terephthalate (PET) or polyimide (PI) films with different buffer layers, such as SiO₂ or Zn₂SnO _x (ZTO), between the TiO₂ film and the substrate. Such TiO₂/(SiO₂ or ZTO)/(PET or PI) sheets with high transmittance in the visible region of light showed high photo-decomposition ability towards CH₃CHO under UV irradiation by a black light centered at 352 nm. When SiO₂ was used as the buffer layer, a "nano-pore layer" was observed clearly over the full area of the polymer substrate close to the buffer layer after UV irradiation, whereas such degradation was suppressed drastically when ZTO was used as the buffer layer. Such photocatalytic sheets also demonstrated photo-induced hydrophilicity upon UV irradiation, with the contact angle to pure water becoming around 5° under UV irradiation. After additional deposition of SiO₂ on the outermost surface of the TiO₂ films by reactive sputtering under rather a high total gas pressure of 3.0 Pa, the photo-induced hydrophilicity was maintained for more than 98 days in the dark. Such three-layered SiO₂/TiO₂/ZTO films on PET or PI substrates could be promising flexible photocatalytic sheets that retain photo-induced hydrophilicity.