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 TiO2 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 TiO2 nanoparticles were coassembled with negatively charged poly(sodium 4-styrenesulfonate) (NaPSS) which leads to highly transparent nanoscale coatings in which the content of TiO2 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/TiO2)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 TiO2 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 TiO2 catalyst present in the films. It is believed that the high quantum yield (8.1%) of a coating consisting of a single TiO2 layer which is 6-7 times higher than that of a 6-10 LP film could be due to the optimum accessibility of the TiO2 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/TiO2 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 TiO2 particles and require a different surface morphology with respect to the number of active contact points for optimum degradation.

Nanoscale precipitation coating: the deposition of inorganic films through step-by-step spray-assembly.

Thin films and surface coatings play an important role in basic and applied research. Here we report on a new, versatile, and simple method ("precipitation coating") for the preparation of inorganic films, based on the alternate spraying of complementary inorganic salt solutions against a receiving surface on which the inorganic deposit forms. The method applies whenever the solubility of the deposited material is smaller than that of the salts in the solutions of the reactants. The film thickness is controlled from nanometers to hundreds of micrometers simply by varying the number of spraying steps; 200 spray cycles, corresponding to less than 15 min deposition time, yield films with thicknesses exceeding one micrometer and reaching tens of micrometers in some cases. The new solution-based process is also compatible with conventional layer-by-layer assembly and permits the fabrication of multimaterial sandwich-like coatings.