*2.4. Photocatalytic Measurements*

Photodegradation experiments of methylene blue (MB) dye in solution were used to quantify the photocatalytic activity of the two TiO2 thin films with different preferred grain orientation. They will be referred to as sample "<101>" corresponding to the sample with 2% <001> orientation (*i.e.*, almost randomly oriented grains dominated by <101> facets), and sample "<001>" with 25% <001> orientation. The experiments with MB were performed in a liquid-phase reactor employing in situ laser colorimetry for chemical analysis of the MB concentration employing a Ȝ = 365 nm diode laser, as described in detail elsewhere [29]. A standard 4 W UV tube (Ȝ = 365 nm) was used as a light source, positioned above the sample and attached on a stand, allowing for the distance between the tube and the reaction cell to be changed, as well as the illumination intensity. The intensity of the UV tube measured with a calibrated thermopile detector (Ophir, North Andover, MA, USA) was 2.44 mW cm–2 at the UV tube's wall. Using the linear-source spherical emission (LSSE) model, the intensity distributions at the position of the film in the reactor was estimated to be 0.38 mW cm<sup>í</sup><sup>2</sup> .

Samples were placed on a sample holder at the bottom of the reaction cell, which was filled with 100 mL of distilled water and circulated with a magnetic stirrer. Background absorption in the reactor was measured, and then 1 mL of 100 ppm MB stock solution was added, leading to an initial concentration of 0.99 ppm. The system was allowed to reach equilibrium during a time period of 40 min allowing adsorption-desorption equilibrium between reactor walls and sample to be obtained. The MB concentration was measured in situ every 2 min during this equilibration and the increase of the laser signal at Ȝ = 365 nm (due to MB adsorption in the reactor) was used to determine that equilibration was reached.
