On the Difference Between Air-cleaning and Self-cleaning

Abstract

Abstract There is a common notion in the field of photocatalysis that the same requirements for air purification hold also for self-cleaning surfaces, and that the same product may be adequate for the two purposes. This notion was tested on cementitious objects by comparing the activity of a series of commercially available TiO2 products in oxidizing NO versus their ability to de-coloring adsorbed dyestuff. Care was made to perform these measurements under UV light in order to avoid coupling with sensitization effects due to absorption by the dyes. In general, introducing TiO2 powder into the matrix (type II samples) was found to yield higher activities in comparison with over-coating with a film made from colloidal nano-particles (type I samples), despite the larger surface coverage of the latter. Within the type II samples a negative correlation between self-cleaning and air-decontamination was found, suggesting that some products are more adequate for self-cleaning while others are more adequate for NO oxidation. In addition, macro-corrugation may play a large role; the higher the corrugation of the surface was, the more active were the samples in the oxidation of NO. The opposite was found in the de-coloring of rhodamine B. Strange enough, 20% of the coated films prepared from colloidal nano-particulate suspensions showed very poor performance towards NO oxidation. This finding is disturbing, since at this stage, bad experience from a specific product might be translated into a disappointment from photocatalysis per-se.