C. Sanchez_2010.pdf

C. R. Chimie 13 (2010) 3–39 Account/Revue ‘‘Chimie douce’’: A land of opportunities for the designed construction of functional inorganic and hybrid...

‘‘Chimie douce’’: A land of construction of functional inorganic organic-inorganic nanomaterials C. Sanchez *, L. D. Grosso, C. UMR CNRS 7574, Chimie Collège de France, Received Abstract ‘‘Chimie douce’’ based strategies molecular engineering of nanomaterials. This performed during the last years. We describe Blocks (NBBs) based hybrid materials, nanostructured processed mesoporous powders and finally hierarchically interfaces via the use of modern tools as DOSY interfaces and the self-assembly processes mechanical properties of the resulting inorganic R. Chimie 13 (2010). # 2009 Published by Elsevier Masson SAS on Keywords: Hybrid; Nanomaterials; Mesoporous; 1. Introduction In all living organisms nature provides a multiplicity of materials (organic, inorganic, hybrids and compo- sites), architectures, systems and functions. Natural composites, such as mollusk shells or crustacean carapaces and in the human body, bone or tooth tissues, are characterized by hierarchical constructions at scales ranging from nanometers, micrometers, to * Corresponding author. E-mail address: [email protected] (C. Sanchez). soft chemical conditions. It is a synergic material that by Elsevier Masson SAS on behalf of Académie des sciences. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 We will emphasise the importance of the control of the hybrid interfaces to be able to tailor made nanomaterials. In this context tools as DOSY NMR, SAXS, Ellipso- metry, etc. are very useful to evaluate in situ the hybrid interfaces and the self-assembly processes. These comprehensive studies are very important to design functional hybrid materials. Some examples of the optical, photocatalytic, electrochemical and mechanical properties of the resulting inorganic or hybrid nanoma- terials will also be presented and briefly discussed. ‘‘chimie 2. Hybrid materials and sol–gel chemistry: general background Hybrid inorganic-organic materials can be broadly defined as synthetic materials with organic and inorganic components, intimately mixed. Such materi- als could be either homogeneous derived from monomers and miscible organic and inorganic compo- nents, or heterogeneous and phase-separated where at least one of the component domains has a dimension ranging from few Å to several nanometers [7,23,24]. Hybrid nanocomposites had an explosive development since the 1980s, with the expansion of soft inorganic chemistry processes. The mild synthetic conditions provided by the sol–gel process such as metallo-organic precursors, low processing temperatures and the versatility of the colloidal state allow for the mixing of the organic and inorganic components at the nanometer scale in virtually any ratio [9,10,14,19,25]. 2.1. Sol–gel chemistry in brief Sol–gel chemistry is central in the development of hybrid materials. Typically, M(OR)n, MXn, R’-M(OR)n- 1 precursors are commonly used, where M represents a metal center, n is its oxidation state and X and RO are common ‘‘leaving groups’’ present in metallic salts or metal alkoxides. X can for example represent a chloride anion, as in metal halides. R’ is any organic functionality anchored to the metallic center via covalent bonds (Si–C, Sn–C) or via complexing ligands (M = Ti, Zr...). The sol–gel process implies connecting the metal centers with oxo- or hydroxo- bridges, therefore generating metal-oxo or metal-hydroxopoly- mers in solution. Hydrolysis of an alkoxy group or depronation of a water molecule attached to a metal center leads to a hydroxyl-metal specie: MOR þ H2 O ! MOH þ ROH (1) The hydroxylated metal species can react with other metal centers leading to condensation reactions, where C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 5 broadly classified in two main classes, depending on the nature of bonds and interactions existing at the hybrid interface: [9,27] Class I hybrids include all systems where there are no covalent or iono-covalent bonds between the organic and inorganic components. In this (2) class, only Van der Waals, hydrogen bonding or electrostatic forces are present. On the contrary, in takes Class II hybrids, at least parts of the inorganic and organic components are linked through strong covalent or iono-covalent bonds. Hybrids can also be character- ized by the type and size of the organic or the inorganic precursors. Precursors can be two separate monomers or polymers, or they can be covalently linked. Generally, (3) phase separation between the organic and the inorganic components will occur, due to mutual insolubility. However, it is possible to obtain homogeneous or single-phased hybrids by choosing bifunctional mono- mers exhibiting organic and inorganic components, or by combining both types of components in phases where one of them is in large excess. The general chemical pathways used to obtain hybrid materials are schematized in Fig. 1. 2.2.1. Path A Path A corresponds to very versatile soft chemistry based routes including conventional sol–gel chemistry, the use of specific bridged and polyfunctional pre- cursors and hydrothermal synthesis. Via conventional sol–gel pathways amorphous hybrid networks are obtained through hydrolysis–condensation of various metal species (alkoxides / halides derivatives) which could be organically modified. The sol could also contain an organic component (specific organic molecule, a biocomponent or polyfunctional polymers) that can interact more or less strongly with the inorganic components leading to class I or class II hybrid materials. Alternatively, solvothermal synthesis has allowed the elaboration of numerous crystalline microporous hybrid solids such as zeolites and, more recently, Metal organic frameworks (MOF). 2.2.2. Path B Path B corresponds to the assembling or the dispersion of well-defined nanobuilding blocks (NBB) which are perfectly calibrated preformed objects that keep their integrity in the final material [10,22]. These NBB can be clusters, organically pre- or postfunctio- nalized nanoparticles (NP) (metallic oxides, metals, chalcogenides, etc.), nano-core shells or layered compounds (clays, layered double hydroxides, lamellar phosphates, oxides or chalcogenides) able to intercalate organic components. C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 routes leading to nanostructured inorganic and hybrid organic-inorganic materials. objects at the micron scale [11,16,33–35]. The combination between these strategies and those described above along paths A, B, and C allow the construction of hierarchically organized materials in terms of structure and functions [11,12]. The chemical construction and patterning of materials with long- range order architectures (beyond nanometer size) remains an important challenge [2,10,12,16,36]. These versatile features, and the advancement of organometallic chemistry, and polymer and sol–gel processing, allow a high degree of control over both composition and structure (including nanostructure) of these materials, which present tunable structure– property relationships. This, in turn, allows the tailoring and the fine-tuning of properties (mechanical, optical, electronic, thermal, chemical...) in a very broad range, and the design of specific systems for applications. Hybrid materials can be processed as gels, monoliths, thin films, fibers, particles or powders. The seemingly unlimited variety, unique structure–property control, Sanchez et al. / C. R. Chimie 13 (2010) 3–39 7 (Nylon, Kevlar) to produce new materials. However, this research field is still in its early stage of development. Bottom-up approaches based on ‘‘chimie douce’’. Examples include materials from electronics to developed by Jacques Livage are opening a land of opportunities for the design of advanced functional inorganic and hybrid materials in which positioning of functionalities at different length scales are well- controlled. There is no doubt, the 21st century through a better knowledge and development of a bio-inspired integrative and green chemistry will give birth to numerous tailor made functional inorganic and hybrid materials and systems. The subjects presented in this short background part have been extensively reviewed in their different fields, such as synthesis, properties [24,42] and applications, [19,39] textured hybrids, [11,12] and biohybrids [3,8,43]. Therefore, in the present feature article we will mainly refer to some comprehensive reviews, in order to encourage the interested reader to explore into them for more specific sources. 3. Sol derived hybrids versus NBB approaches 3.1. Introduction cements in Sol–gel processes, and more generally, soft chem- istry routes have been used to elaborate a great variety of hybrid organic-inorganic materials, which exhibit many different interesting and useful properties [19,24] In parallel to these routes based on molecular precursors, the NBB approach has developed with the goal of achieving hybrid materials in which the spatial extension and the architecture of the inorganic domains, as well as the nature of the interface between the organic and inorganic components, would be better defined. NBB approach is based on the assembling of well- defined and preformed objects such as metal oxo- clusters (MOCs) and NPs, that keep their integrity in the final materials [10,44]. An important point in this approach is a careful control of the NBBs to provide reactive groups on their surface or anchoring sites that can react with complementary NBBs. This required functionalization can be achieved directly during the synthesis of the NBBs or in a subsequent step. Another important point is to use NBBs as well-defined as possible, i.e. with the highest possible control on their composition, size, shape, number and position of the. During the last years the field of research associated functional groups. With this respect, NBBs based on of materials MOCs, which are molecular species and can generally be purified by crystallization, are definitely more appealing than NPs to elaborate model systems. However, the diversity of the latter as well as the C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 of hybrid materials applications in different fields. oxo-clusters species (polynuclear oxo-complexes), [TixOy(OR)z(CL)w], whose size ranged from 5 to 25 Å. They exhibit a rich variety of structural types characterized by different coordination modes of the ligands (O, OR, and CL); a large diversity in linkages of the coordination polyhedra and the coordination number of titanium atoms; the degree of condensation O/Ti differs also in oxo-complexes. Their nuclearity (number of titanium atoms) reported in literature varies from two to 18. The metallic centers that exhibit coordination numbers between four and six are coordinated to bridging oxo-ligands and terminal or bridging alkoxy OR groups. A large variety of oxo bridges are encountered in the known clusters from m2- O to m5-O. Furthermore, numerous titanium oxo- clusters bear strongly complexing oxo-ligands CL (b- diketone, carboxylic acid, phosphonates...) with a chelating or a bridging coordination mode. Among the numerous titanium oxo-clusters, we especially focused our interests on three titanium oxo-clusters, the Ti16 [Ti16O16(OEt)32], the Ti8 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 9 cluster to form functional nano-briks and to build hybrid materials; b. Representation of the of the Ti6O6(OiPr)6(OCR)6 oxo-clusters. gated and leads to nanostructured hybrid materials assign- ment of the 32 ligands in correlation with the crystallographic data, and in a second hand allowed to justify and quantify the formation of postmodified Ti16 derivatives [50,52]. The eight-membered-ring [Ti8O8(OOCR)16] oxo- cluster is a very attractive NBB. First, it presents no residual OR group that considerably increase its stability toward nucleophilic species. Second, the ring shape of this purely carboxylate oxo-cluster allows the building of organized hybrid edifices by the choice of carboxylate ligands. The last presented candidate for the elaboration of hybrids by the NBBs approach, is the Ti6 oxo-cluster [Ti6O6(OR)6(OOCR’)6]. Many times described in the literature, we can still take advantage of its particular structure. Indeed, this cluster presents at once carbox- ylate ligands in the equatorial position and alcoxy ligands in the axial position. Many different carboxylate ligands can be located in the six equatorial positions during the synthesis of the clusters from titanium alkoxide and carboxylic acid. In the axial position the iso-propoxide ligands on preformed Ti6 clusters can be postmodified by transalcoholysis reactions as demon- strated for the Ti16 oxo-clusters. The simultaneous presence of carboxylate located perpendicularly to the alkoxo ligands allows the building of directional hybrid edifices. C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 surface-plasmon band, with respect to the gold NPs capped by only one of the two ligands. Finally, when an hydrophilic thiol was used (HS-(CH2)10-CO2H), self- organization behavior was also evidenced. 3.4. Mechanical and optical properties of the resulting hybrids The introduction of fillers in polymer matrices meets a well-established industrial scheme that aims at enhancing specific properties of polymers while reducing production costs. The regularity of the dispersion of the organic and inorganic phases and the extended and controlled interfaces between the two phases encountered in hybrid materials offer the possibility to control the mechanical properties. High performance in term of mechanical responses has been reported for hybrids based on calibrated nano-objects (MOCs and NPs) which allow the control of the degree of organization and the size and the nature of the hybrid interfaces. Functionalized titanium oxo-clusters (Ti16O16 (OEt)32-x(OR)x, x = metacrylate or styrenic groups) were copolymerized with organic monomers (metacry- late or styrene) to obtain organic-inorganic hybrid networks. The postmodification of the NBBs allows limiting phase segregation between both components; hybrid networks are formed in which the oxo-clusters are covalently linked to the organic matrices. These oxo-clusters play the role of nano-sized cross-linkers, type NPs, well dispersed in the host media. A significant reinforcement of polymers has been observed, espe- cially in the rubbery regime (Fig. 4). Coatings with efficient mechanical properties, investigated by the nanoindentation technique, have been elaborated by introducing goethite (a-FeOOH) nanorods or maghemite (g-Fe2O3) nanospheres in a poly(hydroxyethyl methacrylate) (PHEMA) matrix. Both goethite nanorods and maghemite nanospheres yield a strong reinforcement effect of PHEMA, even at low volume fraction. The origin of such reinforcement is attributed to the existence of strong interactions at the iron oxide-PHEMA interface combined with a homo- geneous dispersion of the particles. The chemical modifications of the organic component by the iron oxide particles (hydrolysis of the ester yields carbox- ylate which complexes iron sites) favor the high mechanical performance of the resulting hybrids (Fig. 4). By the control of the specific interactions between the particles and the polymer matrix, the particles are likely to be used as functional reinforcing elements. Nanocomposites are then synthesized in C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 11 bonded nano-crosslinkers. b. Mechanical properties by nanoindentation of functional nanoparticles dispersed in poly(hydroxylethyl methacrylate). The incorporation of titanium oxo-clusters as additive into already known ORMOCER1 matrix materials offers a variety of new hybrid materials for high added-value applications, for example in micro- optical devices. The refractive index and the optical loss measurements, key parameters for micro-optical appli- cations, of the resulting materials have been recently investigated. The refractive index of a material, in close relation to its permittivity, depends on the polarizability of a substance. The use of a titanium oxo-cluster, [Ti6O4(OOCC6H5)8(OnPr)8], which bears both aromatic groups and titanium, a transition metal with high polarizability, results in a significant increase of the refractive index. Moreover, the nanometer size of the titanium oxo-clusters limit the scattering of the resulting nanocomposites, and the very high optical qualities of the ORMOCER1, used for optical waveguides, is not affected. NBBs based hybrids are also useful models to understand the optical behavior of conventional nanocomposites. Thus, the [Ti16O16(OEt)32] oxo-clus- ters based hybrid materials were used as a reference sample to compare its photosensitivity with that of TiO2 gel-based hybrid materials. Less attention is paid to the photochromic properties of TiO2 based materials. Nevertheless, it is well known that the reduction of Ti4+ into Ti3+ undergoes strong darkening of TiO2 based C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 capping agents to control the size, the size-dispersity, the morphology, the crystalline structure, and the stability. Such syntheses yield suspensions in which the NPs are protected by a shell of capping molecules that, in addition to the role played during the synthesis, can also modulate the properties of the NPs. The use of NPs also often relies on the functionalization of their surface. This step can be required to make them compatible with the application medium or to create new functionalities at their surface (sites with specific molecular recognition properties, unsaturated groups that can polymerize, etc.). Indeed, many NPs based suspensions can be considered as true hybrid organic- inorganic systems. A fine characterization of the surface chemistry of NPs is therefore highly necessary to understand and improve the synthesis and to rationalize the utilization of NPs. In particular, measuring the affinity of organic molecules for NPs, in suspension, is of prime importance. However, evaluating this affinity in situ is not a simple task, especially when the capping molecules are involved in a dynamic equilibrium between a free form and a grafted form (Fig. 6). In such a context, Diffusion Ordered Spectroscopy (DOSY) NMR has been evidenced as an efficient tool. This technique, which belongs to pulsed field gradient methods, provides in addition to the classical data of solution NMR, i.e. chemical shifts and scalar coupling constants, a way to measure self-diffusion coefficients, which can be used to sort species according to their size, as the diffusion coefficient is inversely propor- tional to the hydrodynamic radius. Accordingly, free molecules can be differentiated from those that interact with NPs as their diffusion coefficients are different. Moreover, a careful exploitation of the data allows one to quantify the populations associated to the different self-diffusion coefficients. This technique combines the well-known high selectivity of NMR, able to spectroscopically differentiate many components in a complex mixture, with the absence of any physical separation step. The first uses of DOSY NMR in the context of inorganic NPs were on monolayer-protected gold clusters (MPCs). In these studies, DOSY NMR was only used to measure the hydrodynamic size of the MPCs. In 2005, the first use of DOSY NMR to prove the functionalization of NPs was reported. This study was performed on nano-CeO2 functiona- lized by carboxylic acid, and while standard 1H NMR gave no information on functionalization, grafted and TiO2- free ligands were clearly differentiated and quantified electron transfer using DOSY NMR experiments. The same year, the use of DOSY NMR to assign the 1H resonances of C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 13 NMR can be used to analyzed in situ the functionalization of nano-objects, such as NPS or MOCs. from reference. the enol form of acac was present as grafted and free species while its diketone form was found only as free molecules; pTs was distributed between strongly interacting molecules, weakly interacting molecules and free ones, the two last types being in rapid equilibrium; butanol was not interacting with the NPs. All these species were also quantified. More recently, other. The free energy (DG0) for the adsorption was reports on the use of DOSY NMR for NPs have been published. When the lifetimes of species involved in a equilibrium ranges from ca. 10 ms to ca. 1 s, information on the dynamic of the systems can be extracted from DOSY NMR. Recently, we have used this approach for TOPO molecules adsorbed on CdSe QDs and we have been able to estimate the lifetime of the bound species to ca 0.6 s. In conclusion, for favorable cases, not only Sanchez et al. / C. R. Chimie 13 (2010) 3–39 us to improve the synthesis of silica-based meso- structured materials (in particular the influence of various organosilica compounds on the final meso- structure) and to explore the field of non-silicate, transition metal oxide, rare earth oxide and multi- metallic based mesostructured materials (ZrO2, TiO2, [81,82] V2O5, ZrO2-CeO2, [84,85] ZrO2- Y2O3, TiO2-Nb2O5, HfO2, WO3, SnO2,...). These latter materials could be designed with amorphous or nanocrystalline walls opening new opportunities for the development of many applications arising from the active properties (optic, electronic, magnetic...) due to the presence of metallic centers having d or f orbitals. However, the crystallization process is not straightforward [15,76]. The crystal- lization of the inorganic walls often leads to a collapse of the mesostructure due to nucleation-growth mechan- isms. Then the experimental conditions (maximum temperatures, time of the treatment, heating rates, sequential steps, storage conditions, nature of block copolymers...) are critical in order to promote crystal- lization while avoiding mesostructure collapse. The optimal experimental conditions, for a given system, were determined precisely via in situ SAXS-WAXS experiments [82,90,91]. Briefly we analyzed simulta- neously during the thermal treatment the evolution of the mesostructure by in situ SAXS and the crystallinity of the network composed of crystalline NPs by in situ WAXS (Fig. 7). It was then possible to synthesize new crystalline mesostructured thin films such as SrTiO3 perovskite, tetragonal MgTa2O6, MTiO3 – TiO2 (M = CoII or NiII) (ilmenite – anatase/rutile), [90,91] g -Al2O3, cubic Eu2O3, cubic Y2O3, Nb2O5 , etc. In order to fully understand crystallization mechan- isms, the in situ SAXS-WAXS experiments were recently completed by in situ thermal ellipsometric analysis. This lab-scale technique, more accessible than synchrotron experiments, gives access to the index of refraction and film thickness during heating. These informations were used to evaluate the evolution of the porosity and material composition as a function of temperature, thus identifying key parameters that affect densification, pyrolysis, crystallization, and sintering (an example is shown in part 5). The influence of the heating schedule, the initial film thickness, the nature of the substrate, the solution aging, the presence of water during calcination, the nature of the templating agent, and the influence of additives (H2O, HCl) in the calcination environment were studied on the index refraction profiles as a function of temperature. These C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 15 experiment. Left: SAXS-WAXS patterns of mesostructured TiO2 thin films during thermal Im3m mesostructure into a ‘‘grid-like’’ nanocrystalline structure [82,90,91]. Examples of different . HCl, H2O) triggers the self-assembly process by concentrating progressively the system in inorganic precursors and surfactant. In due time, the concentration in surfactant reaches the equivalent of a critical micellar concentration for the system, and then micelles start to form, surrounded by the inorganic precursors. An alternative way has been recently published implying block copolymers PS-b-PEO of high molecular weight which exist in the micellar state in the starting solution. This process was called Evaporation Induced Micellar Packing (EIMP). EIMP is particularly useful to create mesoporous layer with difficult systems (RuO2 and multicationic oxides) [90,100]. Depending on the processing conditions (i.e. water and solvent relative pressures, temperature), the micelles could self-orga- nize on a large scale leading to a liquid-crystalline phase before the extended condensation of inorganic species will lock the system. This step is addressed as the C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 of EISA mechanism via dip-coating and spray-drying processes [15,75,77,79,98,99]. dilution (crucial parameters in sol–gel chemistry) have to be carefully controlled in order to generate hydrolyzed but slightly condensed inorganic species in the starting sol. These prerequisites could be difficult to manage with the synthesis of (multi)metallic oxides- based or hybrid silica-based POMTFs. Despite the well- established conditions for pure silica and titania , the situation becomes much more complicated with multimetallic oxides and organosilanes species. For metal oxides, experimental conditions have to be selected with respect to the speciation diagram of cations in aqueous solution. Generally, with trivalent to pentavalent cations (e.g. TiO2, ZrO2, Al2O3, Nb2O5...), high hydrochloric acid content in the solution is required to inhibit an extended condensation of the inorganic species. The difficulty arises for the synthesis of multimetallic oxides-based POMTFs (perovskite SrTiO3, tetragonal MgTa2O6, ilmenite MTiO3 (M = CoII or NiII)) since these compounds correspond to solid solutions. It is then necessary to Sanchez et al. / C. R. Chimie 13 (2010) 3–39 17 Steady State where the system is flexible enough for further modifications (slight condensation of inorganic species) and where its composition is in equilibrium with the environment. At this stage, changes in the atmosphere composition (i.e. relative humidity) could lead to modifications of the mesostructure. For example, it is usually observed that a high relative humidity within the deposition chamber favors mesophases with high micellar curvature (cubic structure) for low surfactant content. This general tendency could be somehow modified by the incorporation of functional organosilanes in the starting sol (‘‘one-pot’’ synthesis). Depending on the nature of the functional group (hydrophobic, hydrophilic, ionic, aromatic, polar, apolar) and the length of the organic chain between the functionality and the anchoring group (–Si(OR)3), various modifications of the mesostructure could be observed from simple variations of the lattice para- meters to phase transitions, [102,105] and even the formation of a mesophase for a (very) low molar ratio of surfactant/silica which leads usually to amorphous thin films without organosilanes. Since TSS is a relatively slow stage for metal oxides (it can last a few hours while it takes few seconds with silica-based systems due to different condensation rates of inorganic precursors), the mesostructure is retained by aging thin films in a humidity-controlled atmosphere before a prethermal treatment step. Indeed, it has been shown that a long prethermal treatment below the crystallization temperature is beneficial since the inorganic network slowly condenses by dehydration and full departure of the remaining condensation inhibitor, HCl. For multimetallic oxides, this humid- ity-controlled aging step must be limited sometimes, in order to prevent demixing and aggregated clusters formation due to the differences in solubility and condensation kinetics of metallic centers. The use of surfactants presenting a high hydrophobic-hydro- philic contrast such as KLE, PS-b-PEO, PIB-b-PEO block copolymers, which favor fast micellization and mesostructuration, is then required. In this case, the slight thermal treatment applied just after TSS is preferred. (i.e. cations 5.1.3. The thermal treatment The aim of this step is to consolidate the inorganic network, to remove the templating agent (once the inorganic walls are sufficiently rigid to prevent collapsing of the mesostructure) and eventually to promote crystallization of the inorganic walls. It has to be modulated following the nature of thin films. With hybrid mesostructured thin films, temperature and C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 chamber and the optical density value is recorded once the absorbate pressure is stabilized [112,114]. The second one (environmental ellipsometric porosimetry) works under atmospheric conditions where a continuous flux of air containing a controlled relative pressure of gas (usually water or alcohol) is applied at the sample surface. This last setup allows a shorter analysis time (1 h for a complete adsorption–desorption isotherm instead of around 8 h with sealed chamber) and is very advantageous for the simulation of environmental adsorption of gas. EP and EEP offer the opportunity to determine not only classical textural properties but also the thickness of thin films during absorption– desorption of gas and then their mechanical properties (transverse Young’s modulus E). EEP was successfully used to investigate the stability of mesostructured thin films. It was, for example, observed that pure silica meso-organized thin films were poorly stable in simulated body fluid on the contrary to Zr (or Al)- loaded SiO2 mesoporous thin films, TiO2 films being the more stable. The stability of mesostructured thin films is not only environmental conditions dependent but is also function of the mesostructure’s geometry. For SiO2 POMTFs, the most often obtained and reported 2D- hexagonal structure collapses in the presence of water, while tridimensional phases (cubic and 3D-hexagonal ones) exhibit longer lifetimes. 5.2. Properties of mesoporous thin films by EISA This part concerns the properties of periodically organized mesoporous architectures including thin films and membranes. These materials exhibit unex- pected electrochemical, catalysis and photocatalysis properties because the mesoporous network serves as efficient pathways for molecular transport or/and the connected solid network allow fast transport of electrons, oxygen ions, protons throughout the nanoarchitecture. Because these nanoarchitectures exhibit high surface-to-volume ratios, they also serve as tools to amplify the critical phase of any electro- chemical system: the interface. The sol–gel based chemistry and the processing involved for their synthesis allow a facile control of the solid and pore network with tuned properties. We will describe in this paragraph the recent research on mesoporous thin films as new materials in photocatalysis, photovoltaic, electrochemical/sensing and, fuel cell applications. 5.2.1. Photocatalyst The continuous organized mesoporous network on TiO2 mesoporous thin films facilitates the flux of C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 19 inorganic network forming the pore wall influence the photon-energy conversion. In TiO2 mesoporous film, high crystallinity with an open grid-like mesophase structure seems to be the best conditions for efficient. For example, in TiO2 mesoporous thin films, photoactivity. The efficiency of photon-energy conversion can be increased by using visible wavelength photons. Their captation is promoted by doping the TiOX (x < 2) structure with nitrogen centres, the presence of which decreases the anatase band-gap. The decomposi- tion of lauric acid coated on mesoporous N-doped titania films is highly efficient for films nitrided at 500 8C. These mesoporous films contain an optimal surface concentra- tion of nitrogen since the oxygen vacancies are still not important enough to promote the recombination of the photogenerated electrons and holes. blue over TiO2 / N-doped TiO2 films calcined at various temperatures and under UV light or Transmission Electron micrograph of a mesostructured anatase film. Right part: A schematic for testing mesostructured TiO2 films. Power energy conversion efficiency for different colloidal brookite-based (&), and mesostructured anatase-based TiO2 film (*) as a function of porous C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 very interesting for the fabrication of robust membranes or hybrid silica films selective to probes. Indeed, it was observed that the meso-organization of POMTFs offers a better environment for sensing than their corresponding non-organized thin films. Briefly, hybrid POMTF-based optical sensors are characterized by a shorter response time, a whole accessibility of the sensing molecules, a higher sensitivity, an ability to work in both gas or liquid phase without synthesis modifications (due to the large opened and fully accessible porosity), and a response depending on the mesostructure (better with a 3D mesophase compared to a 2D one). Then various optical sensors resulting from the functionalization (one-pot, post-functionalization or combined route ) of POMTFs have been elaborated. Scientists synthesized interesting pH, [123,124] O2, uranyl (UO22+), explosives (vapor of TNT), and BF3 (a gas commonly used in semiconductor industries) sensors. Although most of the POMTF-based optical sensors are class II hybrid materials, class I meso- organized sensors have been also reported for the detection of methanol vapor, the determination of pH and the detection of Cu2+ in aqueous phase has also been investigated. For example, phosphine-stabilized gold NPs encapsulated inside silica POMTFs allowed the detection of thiols and small phosphines due to plasmon band position shifts of gold NPs. Multilayer stacks of TiO2 and SiO2 POMTFs functionalized , or not , were used for the detection of water, alkanes and alcohols via photonic bandgap shifts of these ordered mesoporous Bragg reflectors. species. In this way, fast 5.2.4. Fuel cells Mesoporous materials are also used and developed for hydrogen economy. The high surface area-to- volume ratio present in a mesoporous silica network has been used to construct an efficient proton pathway in hybrid organic/inorganic membranes for PEMFC working at high temperature [134,135]. These mem- branes are constituted of a functionalized mesoporous silica network embedded in a polymer which can be proton conductive or not. The inner surface area of the porous silica network accommodates functional groups i.e. –SO3H and –PO3H2, the connected pore network allow fast transport of hydrated protons and the pore volume serves as a ‘‘reservoir’’ for water. These characteristics permit fast transport for protons throughout the nanoarchitecture while the mechanical strength and gas tightness are ensured by the polymer. Preliminary results have shown that the growth of a functionalized mesoporous silica network in conductive Sanchez et al. / C. R. Chimie 13 (2010) 3–39 21 conditions. Hydrophilic chains must ideally be com- posed of chelating groups, such as PEO, so as to stabilize the inorganic precursors within the shell of the micelles. The principle of pattern formation is illustrated in Fig. 10. The nanostructure periodicity is associated to the self-assembly of copolymer micelles on the surface, while their thermal decomposition is eventually responsible for the creation of the void motifs such as the nano perforations. Deposition is performed by dip- coating in a dry and warm atmosphere at a withdrawal rate that is adjusted so as to obtain a monolayer of hybrid micelles on the surface. Typical solutions contain around 1% weight of non volatile compounds and require to be deposited between 1 and 2 mm s1. The coating is thus directly transferred below an IR lamp to be thermally cured. In the first step, the temperature rise up to 200 8C induces inorganic condensation while the PEO polymer chains become less hydrophilic due to a conformation change of the EO units. Both effects promote the depletion of hydrosoluble species (water and inorganic precursors) from the micelle shell region. The inorganic phase condenses between the polymer micelles on the surface of the substrate. Increasing the temperature up to 500 8C causes the complete degrada- tion of the copolymer, leading to the formation of nano cavities at the locations formerly occupied by polymer micelles. The present systems constitute original surfaces bearing ultrathin membranes of inorganic oxide with nanoperforations through which the surface of the substrate is accessible. Similar systems can be obtained with polymers using PS-PMMA or PS-PLA block copolymers, the preparation of which is more delicate and longer. Their inorganic homologues are chemically, thermally and mechanically more stable than the polymers which make them complementary materials. According to our investigations, proportion and size of the bare substrate surface area can be tuned between 25 and 75% and 12 and 60 nm in diameter, respectively, by adjusting the chemical and processing conditions. These ceramic based nanopatterns can be seen as novel highly ordered heterogeneous substrates that contain well-dispersed nano sites for heterogeneous nucleation. These sites, or nano beakers, are not only defined by their chemical heterogeneity, but are mechanically confined by the presence of the surround- ing ceramic nano walls. When deposited on conductive surfaces, the perforations constitute a nanoelectrode array (NEA) within which polymers (i.e. polyphenol ) of metals (i.e. platinum) can be electrochemically and locally generated leading to nanocomposite surfaces. Both membrane and substrate can be of very C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 of nanopatterned surface by EIMP. AFM, MET, SEM and GISAXS investigation were produced in presence of PB-b-PEO block copolymer onto Si wafer substrates. These revealed that over the treated surface, has a thickness of 7 nm, and is composed of circular perforations of 27 nm in hexagonal structure of 39 nm of periodic distance. The substrate surface is accessible through the with a thiol group promotes the selective addressing of gold nanoparticles inside the perforations. If the reduction of salt can be performed inside the perforation only [15,138–142]. nano bubble entrapment fitting with a Cassie-Baxter model) was observed. using 5.4. Aerosol route to mesoporous submicronic spheres Evaporation Induced Self Assembly (EISA) applied to the spray drying process involves a similar structura- tion mechanism as that described above for the preparation of nanoporous thin films (Fig. 8, right part). Although, in this case, the mother solution containing inorganic precursors, condensation catalyst, structuring agent and volatile solvent is not deposited onto a substrate but is atomized to form an aerosol made of sub-micron size to micron size droplets. The fast drying of these spherical droplets in an oven (the temperature of which Sanchez et al. / C. R. Chimie 13 (2010) 3–39 23 structuration (that is the oven temperature has to be higher than the evaporation temperature of the solvent). As a consequence, the critical parameters here are the solvents’ partial pressures in the drying chamber (and not the relative humidity observed for film formation at ambient temperature). Finally, the powder being collected by frontal filtration of the aerosol flux, the drying temperature has to be high enough to create rigid submicronic particles in order to avoid the coalescence of the product onto the filter. This rigidifying temperature is very sensitive to the inorganic precursor used and the sol–gel condensation pathway selected [75,147,148]. In parallel, we developed a large spectrum of mesostructured materials (Fig. 11), both for funda- mental knowledge development and industrial applica- tions. The simplest metal-oxides based particles with amorphous walls could be obtained by this way: SiO2 TiO2, ZrO2, Al2O3 with various mesostructures: 2D- hexagonal, cubic or lamellar [75,107,145,147–149]. These systems were then extended to more complex mixed metal oxides such as SiO2-ZrO2, SiO2-Al2O3, SiO2-P2O5... [150–152]. In these cases, we observed that a careful tuning of the sol–gel reactivity and degree of condensation of inorganic precursors is needed for achieving a homogeneous incorporation of both metal- oxides. Indeed, whenever the mobility of one inorganic precursor is very different from the other into the surfactant-solvent medium, the drying of the aerosol. However, in spray-drying, the 2D hexagonal (starting from the periphery and progressing toward the centre of the droplet) may promote an inorganic- inorganic separation. For example, a gradient of inorganic composition appears for SiO2-ZrO2 of SiO2-Al2O3 powders prepared in acidic media when- ever more than 10 to 20% of one element is dispersed in the other. The more mobile the precursor, the more concentrated the particle centre. Such inhomogeneity being linked to a diffusion phenomenon, it is more pronounced for big particles than for small ones [151,152]. The concentration gradient created by the solvent evaporation may also be used for promoting hierarch- ical mesostructures. Accordingly, we used a solution containing a mixture of surfactant, F127 block copolymers and cationic fluorocarbon surfactant IC- 11 C8F17-CH2-OHCH2-NH(C2H5)2-Cl to prepared core-shell silica mesostructured particles with a bimodal porous structure. The high surface activity of the fluorocarbon surfactant, the alkyl- fluoroalkyl microphase separation and the solvent concentration gradient associated to the evaporation at the droplet interface were found to be responsible for C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 made by aerosol process via EISA. A. Aluminosilicate particles with Si/Al molar ratio of 10 using treated at 300 8C. C. Hierarchical core-shell silica powders prepared via dual templating strategy particles nanocrystallised at 900 8C. E. CeO2 nanoparticles encapsulated in functionalized by phenylphosphonate species embedded in mesos

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