In this paper, we report the complete synthesis and characterization procedures to generate highly organized and oriented mesoporous titania thin films, using poly(ethylene oxide) (PEO)-based templates. Controlled conditions in the deposition, postsynthesis, and thermal treatment steps allow one to tailor the final mesostructure (2D hexagonal, p6m, or 3D cubic, Im3m). Various techniques were used to determine the time evolution of the mesostructure. Spectroscopic techniques (UV/vis, (17)O NMR) and EXAFS/XANES have been used to follow the chemical changes in the Ti(IV) environment. Crossing these techniques spanning all ranges permits a complete description of the chemistry all the way from solution to the mesostructured metal oxide. A critical discussion on all important chemical and processing parameters is provided; the understanding of these features is essential for a rational design and the reproducible construction of mesoporous materials.

This review presents an extensive discussion on the major advances in the field of periodically organized mesoporous thin films (POMTFs) obtained via surfactant templated growth of inorganic or hybrid polymers. A large variety of templating agents can be coupled with inorganic polymerization reactions for the design of periodically organized nanostructured hybrid phases that yield POMTFs. The tuning of the interface between the template and the polymerizing phase and the control over chemical and processing conditions are the key parameters in producing tailor-made POMTFs with a high degree of reproducibility. This dynamic coupling between chemical and processing conditions dictates extensive use of complementary ex situ measurements with in situ characterization techniques that follow, in real time, film formation from the molecular precursor solutions to the final stabilized POMTF. Among modern analytical tools, 2D-GISAXS, ellipsoporosimetry, HRTEM, X-ray reflectometry, WAXS, time-resolved infrared spectroscopy, SAW, and optically polarized xenon NMR have proved to be highly relevant for this purpose. POMTFs combine the intrinsic physical and chemical properties of the inorganic or hybrid matrices with a highly defined nanoporous network having a tunable pore size and connectivity, high surface area and accessibility, and a specific orientation with respect to the substrate. As such, POMTFs are a promising family of advanced materials for a host of future applications including micro-optics and photonic devices, microelectronics, nanoionics and energy, environment, functional and protective coatings, biomaterials, environmentally responsive materials, and biomicrofluidics, among others.

This article gives an overall view of the mechanisms involved in the mesostructuring that takes place during the formation of surfactant‐templated inorganic materials by evaporation. Since such a method of preparation is well suited to fabricating thin films by dip coating, spin coating, casting, or spraying, it is of paramount interest to draw a general description of the processes occurring during the formation of self‐assembled hybrid organic/inorganic materials, taking into account all critical parameters. The following study is based on very recent works on the meso‐organization of thin silica films using tetraethylorthosilicate (TEOS) as the inorganic source and cetyltrimethylammonium bromide (CTAB) as the structuring agent, but we will show that the method can also be extended to other systems based on non‐silica oxides and block copolymer surfactants. We demonstrate that the organization depends mainly on the chemical composition of the film when it reaches the modulable steady state (MSS), where the inorganic framework is still flexible and the composition is stable after reaching an equilibrium in the diffusion of volatile species. This MSS state is generally attained seconds after the drying line, and the film's composition depends on various parameters: the relative vapor pressures in the environment, the evaporation conditions, and the chemical conditions in the initial solution. Diagrams of textures, in which the stabilized structures are controlled by local minima, are proposed to explain the complex phenomena associated with mesostructuring induced by evaporation.

Mesoordered silica thin films with cubic structures were prepared by evaporation induced self-assembly (EISA) with two types of structuring agent (CTAB and block copolymer F127). A complete and accurate description of these films was obtained by combining 2D-SAXS analyses, variable angle spectroscopic ellipsometry, and a specially designed environmental ellipsometric porosimetry (EEP) experiment. The EEP analysis is rapid and cheap and operates at ambient pressure and temperature. This latter experiment was performed with water and produced a set of water adsorption-desorption isotherms. A modified Kelvin equation, coupled with a modelisation of pores contraction, enabled the determination of the structural parameters of films porous networks: ellipsoidal pore diameters, porous volume, and surface area. Young moduli of films in the direction perpendicular to the substrates were calculated from these parameters.

TiO 2 optical thin films stable to 700 °C, exhibiting 35% volume porosity, more than 100 m 2 ‚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 (T c ≈ 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 structure, leading to a novel grid-like mesostructure with open porosity. This DRC treatment allows the preparation of highly porous and crystalline anatase films, with thermal stability 200 °C higher than previously reported, that are ideal for energy transfer applications. This emphasizes the role of the treatment method to stabilize transition metal oxide mesoporous materials over extended crystallization at high temperatures. These films exhibit excellent long time stability below 500 °C.

No abstract

Dip-coating is an ideal method to prepare thin layers from chemical solutions since it is a lowcost and waste-free process that is easy to scale up and offers a good control on thickness. For such reasons, it is becoming more and more popular not only in research and development laboratories, but also in industrial production, as testified by the increasing number of annual publications (9, 180, and 480 articles in 1990, 2000, and 2010, respectively). Even so, the full potential of dip-coating has not yet been fully explored and exploited. This article highlights the recent progresses made by tuning the processing conditions beyond conventional ranges to prepare more and more complex and controlled nanostructured layers. Especially, we will see how one can take advantage of an accurate tuning of the withdrawal speed and of the atmosphere to control the nanostructuration originating from evaporation-induced-selfassembly (EISA), together with the final thickness from a few nm up to 1 mm from the same initial solution. A new regime of deposition, involving capillary induced convective coating that is highly suitable for the deposition from aqueous and/or highly diluted solutions, will be described. Finally, it will be demonstrated that dip-coating is also a well suited method to impregnate porosity, to make nanocomposites, or to perform nanocasting. The present discussion is illustrated with systems of interests in domains such as optics, energies, nanoelectronics, nanofluidics, etc.

Dip-coating of sol-gel solutions is a complex dynamic process that is difficult to model because it is associated with time-dependent evaporation-induced concentration and viscosity gradients in the solution. It is, however, highly used in the coating technology because it is simple and provides excellent reproducibility. Existing fair models have been proposed some decades ago to describe this method, but they are based on Newtonian and nonevaporating liquids and require several important assumptions and simplifications. In this work, we present a simple experimental study of sol-gel film formation by dip-coating, through which we propose a general semiexperimental model to predict the final film thickness. Spectroscopic ellipsometry was used as the main technique to obtain the film physical thickness and optical density for various dip-coating processing conditions (withdrawal speeds from 0.01 to 20 mm · s -1 and temperatures from 25 to 60°C) and for several different chemical solutions (TiCl 4 , TEOS, and MTEOS, all in the presence, or not, of block PEO-b-PPO copolymer templates in EtOH/H 2 O, with concentrations from 10 -1 to 10 -3 mol · L -1 ). We show that phenomena that are difficult to assess during deposition, such as viscosity variation, evaporation cooling, chemical reaction, and thermal Marangoni flow, may not have to be taken into account. The influences of various experimental parameters are discussed together with the limitations and the full potentiality of the dip-coating technique. We show that two regimes of film formation independently exist at extreme withdrawal speeds, while they combine into a third regime at intermediate speeds. Although the first regime is well-known and is governed by gravity-induced viscous drag at higher speeds, the second one is barely used and is governed by interdependent evaporation and capillarity rise at lower speeds. We show that both regimes can be selected to build up films with a tunable thickness and that a minimum thickness exists for each given solution at a critical speed for which we believe that the capillarity rise effect perfectly counterbalances the viscous drag. We also show that the capillarity regime is well-suited when one needs to deposit thick films from highly diluted solutions.