Titanium films were deposited on ITO (indium tin oxide)-coated PEN (polyethylene naphthalate) and flexible clay substrates by ion beam sputter deposition method. The surface morphology of the deposited films was smooth on PEN and rough on clay substrates. The titanium film deposited on Clay-mo (98% montmorillonite) substrate was anodized in ethylene glycol + 2 vol% H2O + 0.3 wt% NH4F solution, and the titanium films deposited on Clay-st (99% stevensite) substrate was anodized in 2-propanol + 16 vol% H2O + 0.14 M NH4F solution. Then nanohole-structured titania (TiO2) films were firstly and successfully fabricated on the flexible transparent clay substrates. The nanohole structures of TiO2 on both clay substrates were similar to those on PEN and glass substrates. The TiO2 nanohole structure was almost maintained after annealing at 450 °C for 4 h in air. The optical transmittance of the nanohole-structured TiO2 films on Clay-st increased from 26% to 54% at 800 nm in wavelength after annealing at 450 °C for 1 h in air.
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