Preparation, kinetics, mechanism and properties of semi-transparent photocatalytic stable films active in dye degradation

• This study presents the stable repetitive degradation of MB on PE-TiO2 films.
• By scavenging experiments ROS-radicals leading to MB-degradation were identified.
• The ROS-radicals were monitored as a function of time during MB-degradation.
• The PE-TiO2 hydrophilic-hydrophobic conversion under UV-light was evaluated.

The photocatalytic activity, mechanism and characterization of polyethylene-TiO2 films (PE-TiO2) during the discoloration/degradation of methylene blue (MB) are reported in this study. PE-TiO2 films were prepared at moderate temperatures (∼90 °C). TiO2 did not leach out during discoloration/degradation of MB. This is a significant finding for a low temperature catalyst-polymer preparation leading to a stable repetitive MB degradation kinetics. MB discoloration on PE-TiO2 films proceeded showing a quantum yield of 0.25%. Band-gap irradiation of TiO2 was applied by way of a mercury medium pressure lamp to excite only TiO2 and avoid MB photosensitization. The nature of the oxidative radicals intervening in MB discoloration was investigated by scavenging experiments. The relative importance of the oxidative species during MB discoloration is: TiO2vbh+ > HO2/O2− > 1O2 > OH. Evidence is presented for the superoxide anion-radical O2− at pH 6, as the main radical species leading to MB-discoloration. The MB- discoloration kinetic trend on PE-TiO2 followed the behavior observed for diffusion-controlled reactions. The diffusion distance of the HO2 and OH radicals during the discoloration of a solution MB 1 × 10−5 mol L−1 were estimated as 71 and 2.2 μm respectively. The OH formation was assessed quantitatively during MB degradation by fluorescence measurements. The PE-TiO2 film at time zero presented a contact angle (CA) of ∼92°, decreasing within the five hour MB discoloration period to <5°. Reversibility from the hydrophilic nature of the film to the initial hydrophobic state proceeded in the dark within few days. The implications of the reversibility time on the MB-discoloration performance are discussed. Evidence for redox catalysis during MB-discoloration was found by XPS measurements. The surface properties of the (PE-TiO2) films were characterized by X-ray fluorescence (XRF), UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass-spectrometry (ICP-MS).

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