Aging of photocatalytic coatings under a water flow: Long run performance and TiO ₂ nanoparticles release

Although photocatalytic coatings may experience severe wearing in most of their application, little work has been done to investigate their aging in a comprehensive way. In this article, we present an original experimental protocol to simulate an accelerated aging of photocatalytic coatings under a water flow, and test it on two materials: a well-known commercial product, Pilkington Activ™, and an experimental coating. The influence of intrinsic properties of the coatings (chemical nature, thickness) as well as environmental parameters (water matrix, UV-light) is investigated while the consequences of aging are evaluated under three different endpoints, related either to the long run performance of photocatalytic coatings or their environmental impact: (i) loss of the photocatalytic activity, (ii) degradation of mechanical properties, and (iii) release of TiO ₂ nanoparticles. It is observed that both photocatalytic coatings experienced a deactivation of their active sites upon prolonged immersion. The extent of deactivation varies depending on the coating, being around 20% for experimental coatings and 65% for Pilkington Activ™ but shows little dependency on water matrix or illumination. An alteration of mechanical properties is seen on experimental coatings, which was accompanied by TiO ₂ emissions as high as 150.5μgL ^-1. Although no reduction in film hardness or adhesion could be evidenced for Pilkington Activ™, TiO ₂ concentrations up to 30.8μgL ^-1 was detected in the aging water showing that some release of TiO ₂ nanoparticles also took place on this material. Interestingly, a common mechanism of release, triggered by an interaction between TiO ₂, NaCl and UVA could be identified. Most severe damages were observed in presence of sodium chloride. These results suggest that the use of photocatalytic coatings with surface-bound nanoparticles in environmental applications may entail new entries of nanomaterials into the aqueous medium. They also prove that aging assays are an effective way of assessing the emissions.