2.2. Preparation of TiO2 photocatalyst suspension andTiO2 coatingThe TiO2 suspension was prepared by mixing deionizedwater with 2.5 mass % of TiO2 (colloidal dispersion – VPDisp.W 2730 X, Degussa, Frankfurt, Germany, grain size<100 nm, dry matter content 30.0±1.0 mass%, pH 5–7) and0.208 mass % of polyethylene glycol (PEG) (molecular weight600, Baker, Germany) in an ultrasonic bath (40 min).The freshly prepared suspension was sprayed on a mechanicallycleaned surface of the fired clay roofing tiles. The procedurewas carried out in semi-industrial conditions by using the alreadyexisting production line for glazing of ceramic wall tiles (speedof line – 6.1 m/s, nozzle pressure 8 bar, the loading ratio of TiO2was 0.885 mg/cm2). The applied photocatalytic suspensionwas fixed onto ceramic roofing tiles in an industrial furnace(290 ◦C for 40 min), roller kiln “Sacmi” KAT 2000, by usingwaste heat.2.3. Characterization of the tile surface without and withTiO2 coating2.3.1. Textural propertiesThe total porosity was determined by mercury intrusionporosimetry (Hg Porosimeter Carlo Erba 2000 WS, Italia), whilethe BET surface area and pore size distribution (BHJ) weredetermined by low-temperature nitrogen adsorption-desorptionmeasurements at−77 K on a nitrogen adsorption apparatus(Micromeritics ASAP 2010). The modification of the specificsurface area and pore size distribution of the tile surface, causedby the application of the photocatalytic coating, was determinedon the same samples before and after the application of thecoating.2.3.2. Contact angleThe samples (4 cm×4 cm×1 cm) of the industrially coatedtiles were used for the determination of the photo-inducedhydrophilicity. The changes of the water contact angle weremonitored by a sessile drop method (Surface Energy EvaluationSystem, Advex Instruments). The measurements were performedby putting∼10 m3 water droplets at ten different spotson each tile surface in environmental conditions, and the measurementresults were averaged. During the measurements thesamples were illuminated by UV lamp (Osram Eversun, 40 W,320–400 nm), light intensity 0.80 mW/cm2 at a distance of 33 cm
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