Kevin Castelló Lux

L’objectif de ce travail de thèse est de mettre au point des matériaux photocatalytiques permettant de lutter contre la pollution de l’air dans des conditions se rapprochant de celles d’un environnement intérieur (50% humidité relative, faible intensités d’irradiation UV-A et visible, faible quantité de matériau) et offrant une alternative au TiO2, produit de référence dans ce domaine. Dans ce but, trois oxydes métalliques semi-conducteurs, TiO2, WO3 et ZnO, et le nano-composite ZnWO4/WO3 ont été synthétisés et leur activité photocatalytique a été évaluée par rapport à la dégradation des NOx (NO et NO2) sur des échantillons de verre et mortier à l’échelle d’un réacteur de laboratoire. Les oxydes les plus performants, TiO2 et ZnO, ont été ensuite décorés par des nanoparticules métalliques d’or (Au) afin d’améliorer leur activité photocatalytique (séparation des charges photogénérées) et leur propriété d’absorption (effet de résonance plasmon), et les rendre ainsi actifs sous lumière visible artificielle. Les performances des oxydes décorés par l’or (1% massique), Au/TiO2 et Au/ZnO, et non décorés, TiO2 et ZnO, ont été comparées sur verre et sur mortier pour l’abattement du NO et du NO2. Les résultats ont montré l’intérêt qu’offre le matériau Au/ZnO par rapport au TiO2 avec des performances de dégradation accrues sous NO2 et une sélectivité supérieure sous NO. Enfin, des essais dans une chambre expérimentale de 10 m3, servant ici de réacteur à une échelle intermédiaire, ont été réalisés pour comparer les performances de TiO2 et de Au/ZnO sous lumière artificielle (tubes fluorescents UV-A et visible) et lumière solaire indirecte (rayonnement à travers la surface vitrée).Nos études ont permis de montrer que la séparation des charges photogénérées est l’effet prédominant à l’origine de l’augmentation des propriétés photocatalytiques de l’oxyde décoré Au/ZnO dans nos conditions d’essais. L’utilisation d’une irradiation uniquement dans le domaine visible (400 – 800 nm) de faible intensité (6 W/m2) ne permet pas d’activer ce photocatalyseur de manière significative. L’oxyde ZnO décoré à l’or est en revanche très efficace dès de très faibles irradiations UV-A (≤ 0,1 W/m2, 315 – 400 nm), ce qui montre ainsi la possibilité d’utiliser le matériau Au/ZnO en intérieur sous irradiation solaire indirecte et/ou avec certains illuminants d’intérieur (tubes fluorescents) émettant une fraction minime d’UV-A.

Self-cleaning products are commercially available to protect surfaces against soiling and avoid the high consumption of energy and chemical detergents necessary for cleaning. They are based on semiconductor oxides, mostly titanium dioxide (TiO₂), which induce photocatalytic oxidation activity and superhydrophilicity. Therefore, we present an experimental procedure at a lab scale to assess the self-cleaning ability of various photocatalytic coatings (five TiO₂-based commercial products and one lab-grade zinc oxide (ZnO) product) applied to mortar surfaces. The samples were artificially stained with three types of soiling: Congo red dye, diesel soot, and motor oil. They were exposed to the environmental cycle of UV illumination and water flow for two weeks and the changes in stain colors were first assessed with visual inspection. Then, spectrophotometry measurements were conducted before and after the self-cleaning experiment to calculate the color differences for each stain in the CIELab color space data. In addition, the coatings were characterized via X-ray diffraction analyses and water contact angle measurements. Results highlighted color changes for each stain and higher wettability (induced by OH radicals) of the coated surfaces, which favored surface washing and thus stain removal. Light also had a positive effect on the attenuation of the stains, particularly for the Congo red dye.

The nanometer size Cu₂O@WO₃·H₂O composite material has been prepared by the direct hydrolysis of mesitylcopper (I) on WO₃·2H₂O nanoleaves. The synthesis has been performed in toluene without the addition of any ancillary ligands. The prepared nanocomposite has been deposited as a gas-sensitive layer on miniaturized silicon devices and heated up gradually to 500 °C in the ambient air. During the heating, the CuWO₄ phase is formed upon the reaction of Cu₂O with the WO₃ support as revealed by the XRD analyses. The as-prepared CuWO₄@WO₃ sensors have been exposed to 10 ppm of CO or 0.4 ppm of NO₂ (RH = 50%). At the operating temperature of 445 °C, a normalized response of 620% towards NO₂ is obtained whereas the response to CO is significantly lower (S = 30%). Under these conditions, the sensors prepared either with pristine CuO or WO₃ nanostructures are sensitive to only one of the two investigated gases, i.e., CO and NO₂, respectively. Interestingly, when the CuWO₄@WO₃ sensitive layer is exposed to UV light emitted from a 365 nm Schottky diode, its sensitivity towards CO vanishes whereas the response towards NO₂ remains high. Thus, the application of UV illumination allowed us to modify the selectivity of the device. This new nanocomposite sensor is a versatile sensitive layer that will be integrated into a gas sensor array dedicated to electronic nose platforms.

Microbial contamination of the surface of building materials and subsequent release of microbial particles into the air can significantly affect indoor air quality. Avoiding the development or, at least, reducing the quantity of microorganisms growing on building materials is a key point to reduce health risks for building occupiers. In that context, the antimicrobial activity of TiO₂, ZnO and Au/ZnO was assessed by measuring log reductions of Escherichia coli and Aspergillus niger populations both in the dark and under a light intensity close to real-life conditions. The bactericidal activities (≥2.3 log reduction) of tested products were stronger than their fungicidal activities (≤1.4 log reduction) after 2 h of contact. Different parameters including concentration of photocatalyst, intensity of light (dark vs. 5 W/m² UV-A), and duration of contact between photocatalyst and microbial cells and spores were investigated. Results of this study confirmed bactericidal activities of TiO₂, ZnO and AuZnO on E. coli and brought new insight on their fungicidal activity on the spores of A. niger. They also confirmed the greatest antimicrobial efficiency of ZnO compared to TiO₂ and its increased photocatalytic activity when decorated with Au, leading to the highest log reductions detected after 2 h of contact for both tested microorganisms (4 and 1.4 for E. coli and A. niger, respectively). The antimicrobial activity was enhanced by the duration of contact between microorganisms and nanoparticles of the different tested photocatalytic products.

WO₃ is a known photocatalytic metal oxide frequently studied for its depollution properties. However, it suffers from a high recombination rate of the photogenerated electron/holes pair that is detrimental to its performance. In this paper, we present a new chemical method to decorate WO₃ nanoleaves (NLs) with a complementary metal oxide (ZnWO₄) in order to improve the photocatalytic performance of the composite material for the abatement of 400 ppb NO₂ under mild UV exposure. Our strategy was to synthesize WO₃·2H₂O nanoleaves, then, to expose them, in water-free organic solution, to an organometallic precursor of Zn(Cy)₂. A structural water molecule from WO₃·2H₂O spontaneously decomposes Zn(Cy)₂ and induces the formation of the ZnO@WO₃·H₂O nanocomposite. The material was characterized by electronic microscopy (SEM, TEM), TGA, XRD, Raman and solid NMR spectroscopies. A simple thermal treatment under air at 500 °C affords the ZnWO₄@WO₃ nanocomposite. The resulting material, additionally decorated with 1% wt. Au, presents a remarkable increase (+166%) in the photocatalytic abatement of NO₂ under UV compared to the pristine WO₃ NLs. This synthesis method paves the way to the versatile preparation of a wide range of MOx@WO₃ nanocomposites (MOx = metal oxide).