Patterning Effect of a Sol-Gel TiO2 Overlayer on the Photocatalytic Activity of a TiO2/SnO2 Bilayer-Type Photocatalyst

TiO2 films with a thickness of 75 ± 5 nm (anatase) were formed on SnO2-film (580 ± 80 nm) coated soda-lime glass substrates (SnO2/SL-glass) by a sol-gel method. Although the photocatalytic activity for CH3CHO oxidation (λex > 300 nm) significantly exceeded that of a standard TiO2/quartz sample, it decayed with illumination time (t) at t > 0.75 h. Stripes of anatase TiO2 films of 40 nm in thickness and 1 mm in width were prepared on the SnO2/SL-glass substrate in a 1-mm pitch by photolysis of an organically modified sol-gel film. The TiO2 patterning further increased the photocatalytic activity by a factor of 4.1 as compared to the non-patterned sample, and it was also maintained at 0 < t < 2 h. The flat band potentials of the TiO2 and SnO2 films are determined to be −0.34 and +0.07 V (vs. SHE), respectively, at pH = 7 by the Mott-Schottky plots. On the basis of the results, the outstanding patterning effects could be rationalized in terms of the vectorial charge separation at the interface between TiO2 and SnO2.