Lionel Nicole

Optimised photocatalytic activity of grid-like mesoporous TiO2 films: effect of crystallinity, pore size distribution, and pore accessibility

Photocatalytic activity of anatase cubic-based ordered mesoporous thin films was related to the morphology of the crystalline porous network obtained upon thermal treatment with a specific incremented sequence. The porosity and pore size distribution of these thin films were investigated with a novel Environmental Ellipsometric Porosimetry (EEP) technique. Network crystallinity was assessed by XRD. In parallel, the evolution of the photocatalytic activity was studied through UV-induced photodegradation of methylene blue and lauric acid within the films at the various steps of the temperature treatment. The photoactivity was linked to the porous characteristics of the films and we concluded that the activity is optimal when the porosity is high and completely accessible, and when the nanoparticle and pore size have dimensions of 7.5 and 5.5 nm respectively. Such an optimal system was obtained after a sequential thermal treatment ending with 10 min at 600 °C in air, for which the films adopts an ordered bidirectional grid-like structure.

Mesostructured hybrid organic–inorganic thin films

This review discussed the design and different synthetic routes that lead to hybrid thin films that have a periodically organised porosity. The mechanism of formation, the role of the organic functionality and the resulting mesophase, the comparison between one-pot and post-synthesis approaches and the applications domain of this mesostructured hybrid film are described and discussed in this article.

Advanced selective optical sensors based on periodically organized mesoporous hybrid silica thin films.

Mesoporous thin films functionalized with silylated [small beta]-diketone compounds with symmetry mesostructure dependent on the probe quantity were used as fast uranyl species sensors with high selectivity and sensitivity.

Rare earth doped mesoporous hybrid thin films with tunable optical responses

Optical quality mesostructured silica thin films functionalized with three large hydrophobic organosilylated quinizarin derivatives are prepared via evaporation induced self-assembly (EISA). Incorporation of Eu3+ is performed by post-reacting the functionalized layers with several europium precursors. Unambiguous location of the quinizarin moieties inside the porosity and their accessibility to Eu3+ ions are demonstrated using XRD, SAXS and fluorescence measurements. Complexation of Eu3+ reduces the fluorescence of quinizarin; for some europium precursors an energy transfer between the grafted organic dye and the lanthanide is clearly observed. The luminescence intensity of Eu3+ can be tuned by varying the nature of the rare earth precursor, the mesophase and the chelate itself. The resulting optical responses differ with respect to concentration, lifetime and local environment of Eu3+ inside the thin films. Additionally, efficient energy transfer from Tb3+ to Eu3+ and electronic coupling probed by EPR between Cu2+ cations trapped in such mesoporous hybrid films give strong evidence of the presence of several metal ions per pore.

Distance dependence of the photocatalytic efficiency of TiO2 revealed by in situ ellipsometry.

Spectroscopic ellipsometry was utilized to follow in situ photodegradation of organic species in the vicinity of TiO(2) nanoparticles during UV irradiation. Stacked layers composed of TiO(2), mesoporous SiO(2), and mixed mesoporous SiO(2)/TiO(2) nanocomposites with controlled thickness and porosity were used as model materials. Lauric acid molecules and poly(vinyl chloride) (PVC) layers were used as model mobile and immobile pollutants, respectively. The local photocatalytic activity was deduced by monitoring the variation of the thickness and refractive index of each independent layer. We show that the photocatalyzed degradation of an organic pollutant takes place only when the latter is located in close vicinity to the TiO(2) nanoparticle surface or can naturally diffuse toward it. As a result, the reaction efficiency is directly related to the organic pollutant diffusion. We also show that the distance of photocatalysis efficiency (d(s)) at which radical intermediates are still present and active is <10 nm from the TiO(2) surface under the conditions of the experiments. This was confirmed by the fact that an immobile condensed organic phase such as PVC was protected from the photocatalytic degradation when separated from the TiO(2) by a 20 nm layer of mesoporous silica.

Photochromic hybrid organic-inorganic liquid- crystalline materials built from nonionic surfactants and polyoxometalates: elaboration and structural study

This work reports the elaboration and structural study of new hybrid organic-inorganic materials constructed via the coupling of liquid-crystalline nonionic surfactants and polyoxometalates (POMs). X-ray scattering and polarized light microscopy demonstrate that these hybrid materials, highly loaded with POMs (up to 18 wt %), are nanocomposites of liquid-crystalline lamellar structure (Lalpha), with viscoelastic properties close to those of gels. The interpretation of X-ray scattering data strongly suggests that the POMs are located close to the terminal -OH groups of the nonionic surfactants, within the aqueous sublayers. Moreover, these materials exhibit a reversible photochromism associated to the photoreduction of the polyanion. The photoinduced mixed-valence behavior has been characterized through ESR and UV-visible-near-IR spectroscopies that demonstrate the presence of W(V) metal cations and of the characteristic intervalence charge transfer band in the near-IR region, respectively. These hybrid nanocomposites exhibit optical properties that may be useful for applications involving UV-light-sensitive coatings or liquid-crystal-based photochromic switches. From a more fundamental point of view, these hybrid materials should be very helpful models for the study of both the static and dynamic properties of nano-objects confined within soft lamellar structures.

Highly ordered transparent mesoporous TiO2 thin films: an attractive matrix for efficient immobilization and spectroelectrochemical characterization of cytochrome c

We demonstrate remarkably fast incorporation and high loading of cytochrome c within thin films of periodically ordered nanocrystalline TiO(2) deposited on transparent electrodes. The immobilized cytochrome c is not denaturated and it can be reversibly reduced without mediator over the time scale of a few seconds as evidenced by spectroelectrochemistry.

Sol–gel route to advanced nanoelectrode arrays (NEA) based on titania gold nanocomposites

A simple sol–gel-coating strategy has been used to prepare below 10 nm thin nanostructured oxide (TiO2) membranes on conducting surfaces. Well calibrated (20 nm in diameter) and homogeneously dispersed nanoperforations are embedded into the membranes, altogether forming highly ordered and dense nanoelectrode arrays (NEAs) or patterns. Controlling the deposition conditions and the solution chemistry allowed for the formation of homogeneous membranes on very hydrophobic, and difficult to wet surfaces, such as gold. Calibrated pore size and interpore spacing are controlled through the self-assembly of macromolecular templates with the inorganic precursors upon evaporation. Structures were assessed by AFM and SEM-FEG, while XPS allowed us to estimate surface chemical state and composition. Cyclic voltammetry was used to describe the diffusion regime and the accessibility of the conducting nanosurfaces. We also show, using surface-tension measurement, that the ceramic matrix can be selectively chemically modified, which is an easy method to adjust the surface chemical nature of an electrode without altering its electron-transfer properties. It thus constitutes a novel route to hybrid organic–inorganic nanostructured surfaces with extended multifunctionality.

Hierarchical inorganic nanopatterning (INP) through direct easy block-copolymer templating

We introduce a simple route towards hierarchical TiO2 nanopatterns using a block-copolymer template approach combined with a dip-coating process and soft inorganic chemistry. The bimodal characteristic is associated to the preparation of solutions that contain two distinct populations of micelles PB-b-PEO (with the same chemical nature but different sizes) that do not mix or aggregate. The relative quantity of each population can be precisely adjusted in the initial solution composition by mixing the corresponding proportion of parent solutions, and is recovered in the final TiO2 nanopattern. The latter hierarchical nanopatterning exhibits homogeneous distribution of 2 distinct sizes of nanoperforations in relation to the presence of both micelles. We show that the bimodal micellar solution is stable for more than four weeks at RT, revealing an unexpected dynamic stability. We believe that the resulting nanopatterns could be useful for investigating the behaviour of block copolymer micelles in solutions. The present study is mainly supported by DLS and AFM analyses.

Mesoporous SiO2 thin films containing photoluminescent ZnO nanoparticles and simultaneous SAXS/WAXS/ellipsometry experiments

We report the formation of ZnO/SiO2 nanocomposite thin films. They are composed of 5 nm sized-wurtzite ZnO nanoparticles nucleated within vitreous SiO2 which was in the form of either an ordered mesoporous or a dense matrix. Mesoporous thin films were obtained by evaporation induced self-assembly from the chemical solution deposition of solutions containing TEOS, ZnCl2, and Pluronic-type block copolymers as structuring agent. Direct thermal treatment in air was applied to achieve the desired film characteristics. Dense or highly ordered mesoporous (pore size = 6 nm and pore volume = 60%) films can be achieved with high optical quality. When 50 mol% ZnO is used, the mesoporous films exhibit a very low refractive index of 1.17, which suggests a high accessible porosity that is ideal for molecular diffusion towards ZnO nanoparticles. In these films, the similar intensity of the UV and visible emission bands reveals a high amount of oxygen defects, making them well suited for applications in stimulated emitters, actuators, lasing cavities, or optical sensors. The present results are supported by in situ time-resolved grazing incident small angle X-ray scattering/wide angle X-ray scattering/thermal ellipsometry analysis experiments, which allow for the study of the crystallization kinetics of ZnO within amorphous matrices. The present work constitutes an example of a one-pot synthesis of divalent oxide nanoparticles dispersed in mesoporous silica that can virtually be extended to most divalent metal oxides, but also to other processing such as aerosol pulverisation of powders.