Virtually Transparent TiO₂/Polyelectrolyte Thin Multilayer Films as High-Efficiency Nanoporous Photocatalytic Coatings for Breaking Down Formic Acid and for Escherichia coli Removal

Virtually transparent photocatalytic multilayer films composed of TiO 2 nanoparticles and polyelectrolytes were built on model surfaces using layer-by-layer assembly and investigated as photocatalytic nanoporous coatings. Formic acid (HCOOH) and Escherichia coli were used as models for the degradation of gaseous pollutants and for studying antibacterial properties. Positively charged TiO 2 nanoparticles were coassembled with negatively charged poly(sodium 4-styrenesulfonate) (NaPSS) which leads to highly transparent nanoscale coatings in which the content of TiO 2 particles is controlled mainly by the number of deposition cycles and the enhanced translucency with respect to titania powders is likely due to the presence of the polyelectrolytes in the interstitial space between the particles. Build-up and structural properties of the films were determined by ellipsometry, quartz crystal microbalance (QCM-D, with dissipation monitoring), and UV–vis spectrophotometry in transmission and scanning electron microscopy. Complementary photophysical and activity tests of (PSS/TiO 2 ) n multilayer films were performed in the gas-phase under UV-A light and revealed a peculiar dependence on the number of layer pairs (LPs), corresponding to a clear deviation from the usual observations in photocatalysis with increasing TiO 2 amounts. Most notably, a single LP film showed a strongly enhanced HCOOH mineralization and outperformed films with a higher number of LPs, with respect to the quantity of TiO 2 catalyst present in the films. It is believed that the high quantum yield (8.1%) of a coating consisting of a single TiO 2 layer which is 6–7 times higher than that of a 6–10 LP film could be due to the optimum accessibility of the TiO 2 crystallites toward both HCOOH and water molecules. In thicker films, while no detrimental light screening was observed with increasing the number of LPs, diffusion phenomena could cap the efficiency of the access of the pollutant and water to the catalytic surface. Unlike for HCOOH mineralization, three PSS/TiO 2 LPs were required for observing a maximum antibacterial activity of the nanocomposite coatings. This is likely due to the fact that micrometer-sized E. coli bacteria do not enter into the interstitial space between the TiO 2 particles and require a different surface morphology with respect to the number of active contact points for optimum degradation.