Galo J. A. A. Soler-Illia

Block copolymer-templated mesoporous oxides

Block copolymers (BC) are indeed suitable and versatile templates for the creation of mesostructured and mesoporous materials. Great advances have been achieved in the last 3 years. Nowadays, it is possible to obtain highly controlled large-pore and highly stable mesostructured and mesoporous materials (silica, non-silica oxides, carbons,…) shaped as powders, films, monoliths or aerosols. This paper reviews mainly the synthesis of BC-templated mesostructured oxides, stressing in the physical, chemical and processing parameters, which have to be thoroughly controlled to reproducibly obtain mesoporous materials.

Highly porous TiO2 anatase optical thin films with cubic mesostructure stabilized at 700 °C

TiO2 optical thin films stable to 700 °C, exhibiting 35% volume porosity, more than 100 m2·g-1 in surface area, fully nanocrystalline anatase framework, and organized mesostructure (cubic Im3m derived), have been stabilized by careful delayed rapid crystallization (DRC) thermal treatments. In-situ time-resolved SAXS and WAXS investigations were simultaneously performed during such treatments. They revealed that a slow and progressive heating to a temperature just below that of the formation of anatase (Tc ≈ 400 °C), followed by a long pretreatment at this temperature, stabilizes the amorphous network. A following rapid increase of temperature up to temperatures as high as typically 700 °C, followed by a short residence time at this high temperature, provokes the homogeneous formation of crystalline small nanoparticles and the total elimination of organic residues. The crystallization is accompanied by matter migration through diffusing sintering and pore merging along the [111] directions of the cubic str...

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.

Humidity-controlled mesostructuration in CTAB-templated silica thin film processing. The existence of a modulable steady state

A study is presented of the self-assembly process that takes place during CTAB-templated silica film formation through in situ SAXS, interferometry and water titration investigations during evaporation associated with dip coating under various conditions. This work shows that the quantity of water present in the film when the mesostructuration takes place depends on the relative humidity (RH) during deposition. Indeed, the system contains considerable quantities of water at high RH, while it loses water at low RH. The water content is demonstrated to be a critical parameter, as poorly ordered, 2D-hexagonal or 3D-cubic final structures are obtained, depending on the RH, in agreement with the general physico-chemical laws of CTAB mesophases. Furthermore, changing the RH or the solvent vapour pressure just after evaporation induces a film composition change and a potential mesostructure modification, evidencing a modulable steady state, during which the mesostructure can be modified by external influences. The present study pinpoints the role of processing conditions that are often considered secondary to chemical conditions. The conclusions of this study into the CTAB–TEOS system are also relevant to other surfactant-templated systems which undergo evaporation-controlled self-assembly.

Nanocrystallised titania and zirconia mesoporous thin films exhibiting enhanced thermal stability

Tuned chemical conditions complemented with a set of treatments have been developed to enhance the thermal stability of transition metal oxide mesoporous thin films (up to 500 °C for TiO2 and 450 °C for ZrO2), allowing the formation of crack-free coatings presenting excellent optical quality (transmittance higher than 85% in the visible region), tailored thickness (from 50 to 700 nm), a highly organised cubic (Im3m space group for TiO2) or 2D-hexagonal (P6m space group for ZrO2) mesostructure, high surface area (140–200 m2 g−1), controlled and narrowly distributed mesopore size (25–80 A), and a nanocrystalline inorganic framework (anatase or tetragonal zirconia).

Design of meso-structured titanium oxo based hybrid organic–inorganic networks

Titanium oxo clusters or nanosized Ti–O particles with hydrophobic or hydrophilic character can be obtained by varying the sol–gel synthesis conditions. These species are potentially interesting nano-building blocks (NBB) in the design of textured materials. The reactivity of well-defined hydrophobic NBB towards different nucleophiles has been characterised and discussed, in order to understand the processes taking place in the formation of meso-organised hybrids. Subsequently, different synthesis conditions were used to generate textured titania-based hybrid phases, using PEO-based surfactants as templating agents. The tuning of the interactions between the template and the different kinds of nano-building blocks allow worm-like and hexagonal titania-based hybrid phases to be reproducibly obtained.

Adsorption properties in high optical quality nanoZIF-8 thin films with tunable thickness

We report processing of a thin film by chemical solution deposition of a microporous ZIF-8 nanoparticle dispersion. By using the drain and capillary regimes involved in the dip-coating process, we tuned the thickness of the films from 40 nm to 1 μm and controlled the packing of the nanoparticles on the substrate. The high optical quality thin films show a dual hierarchical porous structure from the micropores of the framework and the mesoporous interparticular voids. Moreover, vapor sorption properties of the microporous ZIF-8 based thin films have been evaluated by ellipsometric porosimetry. The hydrophobic films show alcohol, THF and hydrocarbon adsorption. Experiments for isopropanol/water separation have been carried out and the selective adsorption of the alcohol versus the water makes these thin films good candidates for vapor sensors.

Critical aspects in the production of periodically ordered mesoporous titania thin films

Periodically ordered mesoporous titania thin films (MTTF) present a high surface area, controlled porosity in the 2-20 nm pore diameter range and an amorphous or crystalline inorganic framework. These materials are nowadays routinely prepared by combining soft chemistry and supramolecular templating. Photocatalytic transparent coatings and titania-based solar cells are the immediate promising applications. However, a wealth of new prospective uses have emerged on the horizon, such as advanced catalysts, perm-selective membranes, optical materials based on plasmonics and photonics, metamaterials, biomaterials or new magnetic nanocomposites. Current and novel applications rely on the ultimate control of the materials features such as pore size and geometry, surface functionality and wall structure. Even if a certain control of these characteristics has been provided by the methods reported so far, the needs for the next generation of MTTF require a deeper insight in the physical and chemical processes taking place in their preparation and processing. This article presents a critical discussion of these aspects. This discussion is essential to evolve from know-how to sound knowledge, aiming at a rational materials design of these fascinating systems.

Controlled Deposition of Silver Nanoparticles in Mesoporous Single‐ or Multilayer Thin Films: From Tuned Pore Filling to Selective Spatial Location of Nanometric Objects

Silver nanoparticle assemblies are embedded within mesoporous oxide thin films by an in situ mild reduction leading to nanoparticle-mesoporous oxide thin-film composites (NP@MOTF). A quantitative method based on X-ray reflectivity is developed and validated with energy dispersive spectroscopy in order to assess pore filling. The use of dilute formaldehyde solutions leads to control over the formation of silver nanoparticles within mesoporous titania films. Inclusion of silver nanoparticles in mesoporous silica requires more drastic conditions. This difference in reactivity can be exploited to selectively synthesize nanoparticles in a predetermined layer of a multilayered mesoporous stack leading to complex 1D-ordered multilayers with precise spatial location of nanometric objects. The metal oxide nanocomposites synthesized have potential applications in catalysis, optical devices, surface-enhanced Raman scattering, and metal enhancement fluorescence.

Aminopropyl-modified mesoporous silica SBA-15 as recovery agents of Cu(II)-sulfate solutions: Adsorption efficiency, functional stability and reusability aspects

Hybrid mesoporous materials are potentially useful for metal ion scavenging and retrieval because of their high surface areas, controlled accessibility and tailored functionalization. Some aspects that are linked to the performance of HMM include pore accessibility, stability of the organic functions and reusability. Knowledge of these aspects is critical in the design of adsorption-desorption protocols. In this work we produce and characterize propylamino-substituted large pore silica (SBA-15-N), which is submitted to Cu(II) adsorption from copper sulfate solutions, followed by desorption in acid media and material regeneration. We find that the hybrid material is an efficient adsorbent (1.15-1.75mmol Cu(II)g(-1)), although a fraction of the organic groups is lost during the adsorption process. An X-ray photoelectron spectroscopy (XPS) study demonstrates that the contents of amino groups are higher in the material surface, leading to different behaviors in Cu(II) complexation along the material. These materials can be regenerated by exposure to acidic media. Thermal processing of the hybrid materials leads to better durability in aqueous solutions during reprocessing, due to enhanced polycondensation of the inorganic framework. Thermally treated samples, once regenerated, are efficient adsorbents in a second step of Cu(II) adsorption. We discuss the materials processing factors involved in the improved adsorption of Cu(II), its quantitative release and reusability of the material.