Transparent Micrometer-Thick TiO₂ Films on SiO₂-Coated Glass Prepared by Repeated Dip-Coating/Calcination: Characteristics and Photocatalytic Activities for Removing Acetaldehyde or Toluene in Air

TiO 2 films on silica-coated glass plates were obtained through dip-coating and subsequent calcination at 753 K; this temperature and the proportions of the dip-coating solution components (tetraisopropyl orthotitanate, ethoxyethoxyethanol, propan-2-ol, α-terpineol, and a hydroxypropyl cellulose) were optimized. Depending on the number N (5 ≤ N ≤ 30) of dip-coating/calcination procedures, the films characteristics were hardness, >9 H; thickness, 0.9−5.4 μm; transmitted light, ca. 70% (visible spectral region); absorbed light, 90−100% at 358 nm; surface area, 40−80 m 2 g - 1; pore volume 7−20 mL g - 1; anatase particle size, 3.5−4 nm (elemental crystallites) and 12−16 nm (agglomerates). These tiny sizes, which did not increase for N ≥ 20, were obtained because of the use of hydroxypropyl cellulose. The photocatalytic removal rate of acetaldehyde (3.25 ppmv) or toluene (1 ppmv) in flowing air (50% RH) showed that the TiO 2 minimum amount varied with the pollutant and that a 1−2 μm thickness was sufficient to achieve a near-maximum efficacy. In summary, this preparation produces anatase films having the mechanical, optical, and photocatalytic properties required to obtain self-cleaning (water contact angle test) and air depolluting (at least in relatively confined spaces) glass without using a binder, which can restrict the accessibility to TiO 2 and alter the photocatalytic activity.