C. Sanchez 2010 PDF: ""Chimie douce"": Designed Inorganic & Hybrid Nanomaterials
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Université de Montpellier
2010
C. Sanchez et al
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This article focuses on "Chimie douce" strategies for designing functional inorganic and hybrid organic-inorganic nanomaterials. It covers various aspects like sol-gel derived hybrids, nanostructured materials, and hierarchical structures. The authors emphasize the importance of controlling hybrid interfaces using tools like DOSY NMR, SAXS and ellipsometry, highlighting diverse optical, photocatalytic, electrochemical and mechanical properties of resulting nanomaterials.
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C. R. Chimie 13 (2010) 3–39 Account/Revue ‘‘Chimie douce’’: A land of opportunities for the designed construction of functional inorganic and hybrid...
C. R. Chimie 13 (2010) 3–39 Account/Revue ‘‘Chimie douce’’: A land of opportunities for the designed construction of functional inorganic and hybrid organic-inorganic nanomaterials C. Sanchez *, L. Rozes, F. Ribot, C. Laberty-Robert, D. Grosso, C. Sassoye, C. Boissiere, L. Nicole UMR CNRS 7574, Chimie de la matière condensée de Paris, université Paris 6 (UPMC), Collège de France, bâtiment C–D, 11, place Marcelin-Berthelot, 75231 Paris, France Received 14 April 2009; accepted after revision 2 June 2009 Available online 17 July 2009 Abstract ‘‘Chimie douce’’ based strategies allow, through the deep knowledge of materials chemistry and processing, the birth of the molecular engineering of nanomaterials. This feature article will highlight some of the main research accomplishments we have performed during the last years. We describe successively the design and properties of: sol–gel derived hybrids, Nano Building Blocks (NBBs) based hybrid materials, nanostructured porous materials proceeds as thin films and ultra-thin films, aerosol processed mesoporous powders and finally hierarchically structured materials. The importance of the control of the hybrid interfaces via the use of modern tools as DOSY NMR, SAXS, WAXS, Ellipsometry that are very useful to evaluate in situ the hybrid interfaces and the self-assembly processes is emphasized. Some examples of the optical, photocatalytic, electrochemical and mechanical properties of the resulting inorganic or hybrid nanomaterials are also presented. To cite this article: C. Sanchez et al., C. R. Chimie 13 (2010). # 2009 Published by Elsevier Masson SAS on behalf of Académie des sciences. Keywords: Hybrid; Nanomaterials; Mesoporous; Photocatalysis; Fuel cells; Sensors; Hierarchical structures 1. Introduction millimeters that introduce the capacity to answer the physical or chemical demands occurring at these In all living organisms nature provides a multiplicity different levels [1–4]. Indeed, many natural materials of materials (organic, inorganic, hybrids and compo- are highly integrated hybrid systems synthesized in sites), architectures, systems and functions. Natural mild chemical conditions and have found the best composites, such as mollusk shells or crustacean compromise between different properties or functions carapaces and in the human body, bone or tooth tissues, (mechanics, density, permeability, colour, hydro- are characterized by hierarchical constructions at phobia...). Such highly complex and aesthetic struc- scales ranging from nanometers, micrometers, to tures pass well beyond current man-made materials accomplishments. Maya blue is a beautiful example of a very old and * Corresponding author. remarkable man-made hybrid material processed via E-mail address: [email protected] (C. Sanchez). soft chemical conditions. It is a synergic material that 1631-0748/$ – see front matter # 2009 Published by Elsevier Masson SAS on behalf of Académie des sciences. doi:10.1016/j.crci.2009.06.001 4 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 combines the color of the organic pigment (indigo) and We will emphasise the importance of the control of the the resistance of the inorganic host (clay mineral known hybrid interfaces to be able to tailor made nanomaterials. as palygorskite), with properties and performance well In this context tools as DOSY NMR, SAXS, Ellipso- beyond those of a simple mixture of its components. But metry, etc. are very useful to evaluate in situ the hybrid as for many other ancient materials, its conception was interfaces and the self-assembly processes. These most likely the fruit of a fortunate accident, an ancient comprehensive studies are very important to design serendipitous discovery. functional hybrid materials. Some examples of the More than twelve centuries later, our refined optical, photocatalytic, electrochemical and mechanical analytical techniques have allowed us to understand properties of the resulting inorganic or hybrid nanoma- the true nature and atomic and molecular structure of terials will also be presented and briefly discussed. many materials. In the 1970s, the concept of ‘‘chimie douce’’ pioneered by Livage [6–8] opened up a new 2. Hybrid materials and sol–gel chemistry: ‘‘école de pensée’’. The controlled design of novel general background advanced inorganic or hybrid materials was born replacing the trial and error tradition. Hybrid inorganic-organic materials can be broadly Indeed ‘‘chimie douce’’ based strategies allow, defined as synthetic materials with organic and through the deep knowledge of materials chemistry inorganic components, intimately mixed. Such materi- and processing, the birth of the molecular engineering als could be either homogeneous derived from of nanomaterials [7,9–14]. Today with the recent monomers and miscible organic and inorganic compo- advances made in the field of materials chemistry the nents, or heterogeneous and phase-separated where at ‘‘biomimetic and bioinspired’’ chemical construction least one of the component domains has a dimension and patterning of materials with long-range order ranging from few Å to several nanometers [7,23,24]. architectures (beyond nanometer size) is an important Hybrid nanocomposites had an explosive development scientific and technological challenge which becomes since the 1980s, with the expansion of soft inorganic accessible [11,15–17]. chemistry processes. The mild synthetic conditions For many years the ‘‘Hybrid Materials group’’ of provided by the sol–gel process such as metallo-organic the Laboratoire chimie de la matière condensée de precursors, low processing temperatures and the Paris at the University Pierre et Marie Curie has versatility of the colloidal state allow for the mixing been working on the field of ‘‘chimie douce’’ based of the organic and inorganic components at the construction of advanced materials and related areas nanometer scale in virtually any ratio [9,10,14,19,25]. (hybrids and nanomaterials, organized matter chem- istry, integrative chemistry...). The interested reader 2.1. Sol–gel chemistry in brief will find in the following set of comprehensive reviews the different research topics explored by the Sol–gel chemistry is central in the development of group [9–11,15,18–22]. hybrid materials. Typically, M(OR)n, MXn, R’-M(OR)n- The present feature article will highlight some of the 1 precursors are commonly used, where M represents a main research accomplishments we have performed metal center, n is its oxidation state and X and RO are during the last years. Indeed, through a few selected common ‘‘leaving groups’’ present in metallic salts examples we try to demonstrate that ‘‘chimie douce’’ or metal alkoxides. X can for example represent a has opened a land of opportunities for the designed chloride anion, as in metal halides. R’ is any organic construction of functional inorganic and hybrid organic- functionality anchored to the metallic center via inorganic nano materials. After a brief reminder of the covalent bonds (Si–C, Sn–C) or via complexing ligands chemical background and general synthesis strategies (M = Ti, Zr...). The sol–gel process implies connecting we will describe: the metal centers with oxo- or hydroxo- bridges, therefore generating metal-oxo or metal-hydroxopoly- the sol–gel derived hybrids that will be compared to mers in solution. Hydrolysis of an alkoxy group or the hybrid designed through the Nano Building depronation of a water molecule attached to a metal Blocks (NBBs) approaches; center leads to a hydroxyl-metal specie: the elaboration of nanostructured porous materials MOR þ H2 O ! MOH þ ROH (1) proceeds as thin films and ultra-thin films; the synthesis of aerosol processed mesoporous The hydroxylated metal species can react with other powders and hierarchically structured materials. metal centers leading to condensation reactions, where C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 5 an oligomer is formed by bridging two metal centers. In broadly classified in two main classes, depending on the the case of oxolation, condensation leads to an oxo nature of bonds and interactions existing at the hybrid bridge, and water or alcohol is eliminated: interface: [9,27] Class I hybrids include all systems where there are no covalent or iono-covalent bonds M OH þ XO M ! M O M þ X OH between the organic and inorganic components. In this X ¼ H or R (2) class, only Van der Waals, hydrogen bonding or electrostatic forces are present. On the contrary, in In the case of olation, an addition reaction takes Class II hybrids, at least parts of the inorganic and place, and a hydroxo bridge is formed. This reaction organic components are linked through strong covalent takes place when metal centers may have coordination or iono-covalent bonds. Hybrids can also be character- higher than their valence, as in the case of Ti(IV), ized by the type and size of the organic or the inorganic Zr(IV), or related cations, precursors. Precursors can be two separate monomers or M OH þ XO M ! M ðOHÞ M OX polymers, or they can be covalently linked. Generally, X ¼ H or R (3) phase separation between the organic and the inorganic components will occur, due to mutual insolubility. In most sol–gel processing, the inorganic framework However, it is possible to obtain homogeneous or is built by successive hydrolysis and condensation single-phased hybrids by choosing bifunctional mono- reactions. The structure, connectivity and mor- mers exhibiting organic and inorganic components, or phology of the final inorganic network depend strongly by combining both types of components in phases on the relative contribution of reactions 1–3. Depending where one of them is in large excess. The general on intrinsic (i.e. metal center features such as chemical pathways used to obtain hybrid materials are coordination, acidity, lability...) or extrinsic (i.e. schematized in Fig. 1. tunable reaction conditions such as solvent, water contents, pH, catalysts, reaction time...) conditions, the 2.2.1. Path A hydroxo-oxo-polymers thus formed can exhibit a Path A corresponds to very versatile soft chemistry variety of structures, from branched arrangements to based routes including conventional sol–gel chemistry, compact clusters. The growth of these structures can be the use of specific bridged and polyfunctional pre- controlled (for example, by poisoning or by limiting cursors and hydrothermal synthesis. Via conventional aggregation), and thus sols made of suspended colloidal sol–gel pathways amorphous hybrid networks are size entities are obtained. Some of these structures may obtained through hydrolysis–condensation of various grow extensively or aggregate until reaching macro- metal species (alkoxides / halides derivatives) which scopic dimensions, trapping solvent and smaller could be organically modified. The sol could also monomers; a gel is thus obtained. Chemical control contain an organic component (specific organic of activation/polymerization reactions (1)–(3) allows molecule, a biocomponent or polyfunctional polymers) the tuning of the size and the shape of the inorganic that can interact more or less strongly with the inorganic polymers or colloids, as well as their miscibility (i.e. the components leading to class I or class II hybrid interactions, for example hydrophobic/hydrophilic) materials. Alternatively, solvothermal synthesis has with the organic counterparts. Functional precursors allowed the elaboration of numerous crystalline R’-M(OR)n-1 can also be used to co-condense with microporous hybrid solids such as zeolites and, more M(OR)n or MXn precursors, or to modify the surface of recently, Metal organic frameworks (MOF). inorganic entities, therefore enhancing the compat- ibility with (bio)organic components. 2.2.2. Path B Path B corresponds to the assembling or the 2.2. Hybrid materials: classification, synthesis dispersion of well-defined nanobuilding blocks (NBB) strategies, applications which are perfectly calibrated preformed objects that keep their integrity in the final material [10,22]. These A distinct characteristic of hybrid materials is that NBB can be clusters, organically pre- or postfunctio- their properties are related not only to the chemical nalized nanoparticles (NP) (metallic oxides, metals, nature of the inorganic and organic components, but chalcogenides, etc.), nano-core shells or layered also rely heavily on their synergy. Therefore, the compounds (clays, layered double hydroxides, lamellar interface between inorganic and organic does influence phosphates, oxides or chalcogenides) able to intercalate strongly their properties. Hybrid materials can be thus organic components. 6 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 Fig. 1. Scheme of the main chemical routes leading to nanostructured inorganic and hybrid organic-inorganic materials. 2.2.3. Path C objects at the micron scale [11,16,33–35]. The Path C corresponds to the well-known self-assem- combination between these strategies and those bling procedures which consist in the organization or described above along paths A, B, and C allow the the texturation of growing inorganic or hybrid networks, construction of hierarchically organized materials in templated by organic surfactants [11–13,15,21,31,32]. terms of structure and functions [11,12]. The chemical This strategy allows a high control and tuning of the construction and patterning of materials with long- hybrid interfaces and leads to a whole continuous range range order architectures (beyond nanometer size) of nanocomposites, from ordered dispersions of remains an important challenge [2,10,12,16,36]. inorganic bricks in a hybrid matrix to highly controlled These versatile features, and the advancement of nanosegregation of organic polymers within inorganic organometallic chemistry, and polymer and sol–gel matrices. processing, allow a high degree of control over both composition and structure (including nanostructure) of 2.2.4. Path D, Integrative synthesis these materials, which present tunable structure– The strategies reported above mainly offer the property relationships. This, in turn, allows the tailoring controlled design and assembling of hybrid materials in and the fine-tuning of properties (mechanical, optical, the 1 to 500 Å range. Recently, micromolding methods electronic, thermal, chemical...) in a very broad range, have been developed, in which the use of controlled and the design of specific systems for applications. phase separation phenomena, emulsion droplets, latex Hybrid materials can be processed as gels, monoliths, beads, bacterial threads, colloidal templates or orga- thin films, fibers, particles or powders. The seemingly nogelators leads to controlling the shapes of complex unlimited variety, unique structure–property control, C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 7 and the compositional and shaping flexibility give these (Nylon, Kevlar) to produce new materials. However, materials a high potential. Indeed, many hybrid this research field is still in its early stage of development. materials have already reached a commercial status Bottom-up approaches based on ‘‘chimie douce’’. Examples include materials from electronics to developed by Jacques Livage are opening a land of automotive coatings with varied mechanical and opportunities for the design of advanced functional optical properties, adhesives and composites. inorganic and hybrid materials in which positioning of Quite recent examples are the one million TV sets functionalities at different length scales are well- sold annually by Toshiba, the screens of which are controlled. There is no doubt, the 21st century through coated with hybrids made of indigo dyes embedded in a a better knowledge and development of a bio-inspired silica/zirconia matrix, organically doped sol–gel glass- integrative and green chemistry will give birth to ware sold by Spiegelau and sol–gel entrapped numerous tailor made functional inorganic and hybrid enzymes sold by Fluka. Today hybrids play an materials and systems. important role in the development of many functional The subjects presented in this short background part systems as in: have been extensively reviewed in their different fields, such as synthesis, properties [24,42] and applications, microelectronics (resistors and molding compounds, [19,39] textured hybrids, [11,12] and biohybrids spin-on dielectrics in microelectronic interlayer, [3,8,43]. Therefore, in the present feature article we multilayer dielectric, planarization applications); will mainly refer to some comprehensive reviews, in optical systems (luminescent solar concentrators, order to encourage the interested reader to explore into laser dyes, sensors, photochromic, NLO or photo- them for more specific sources. voltaic devices); micro-optics (free-standing micro-optical elements, 3. Sol derived hybrids versus NBB approaches such as a lens array on VCSEL elements for fiber coupling, diffractive lenses); 3.1. Introduction bioactive hybrids (biosensors, bioreactors, cements in dental applications, controlled release of drugs and Sol–gel processes, and more generally, soft chem- molecules); istry routes have been used to elaborate a great variety decorative and protective coatings involving scratch of hybrid organic-inorganic materials, which exhibit and abrasion resistance applications and barrier many different interesting and useful properties [19,24] systems (solar cells, optics, electronics, food In parallel to these routes based on molecular packaging...), electrochemical devices (batteries, precursors, the NBB approach has developed with the photovoltaic and fuel cell application). goal of achieving hybrid materials in which the spatial extension and the architecture of the inorganic domains, Hybrid nanocomposites based on clay-polymers are as well as the nature of the interface between the organic also used as effective reinforcement agents in the so- and inorganic components, would be better defined. called ‘‘green’’ nanocomposites, with gas barrier NBB approach is based on the assembling of well- properties and fireproof enhancements. Furthermore, defined and preformed objects such as metal oxo- these NBB based hybrids are also used in cosmetics, clusters (MOCs) and NPs, that keep their integrity in the water-repellents, additives for paper, pressure-sensitive final materials [10,44]. An important point in this adhesive release coatings. Some examples of applica- approach is a careful control of the NBBs to provide tions or systems where at least one component is based reactive groups on their surface or anchoring sites that on hybrid materials are gathered in Fig. 2. can react with complementary NBBs. This required An eclectic approach to conceiving and manufactur- functionalization can be achieved directly during the ing advanced hybrid materials necessarily includes synthesis of the NBBs or in a subsequent step. Another biology, as a remarkable particularity of biological important point is to use NBBs as well-defined as systems is their capacity to integrate molecular synthesis possible, i.e. with the highest possible control on their at very high levels of organization, structure and dynamic 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 with the ‘‘biomimetic or bionspired design’’ of materials MOCs, which are molecular species and can generally much increased. Industrial technologies have already be purified by crystallization, are definitely more been inspired by dolphin skin (Riblets in air plane wings), appealing than NPs to elaborate model systems. lily leafs (superhydrophobic coatings) and spider threads However, the diversity of the latter as well as the 8 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 Fig. 2. Some examples of hybrid materials applications in different fields. larger sizes that can be reached makes NPs still oxo-clusters species (polynuclear oxo-complexes), interesting candidates to develop hybrid materials. [TixOy(OR)z(CL)w], whose size ranged from 5 to As for classical hybrid materials prepared from 25 Å. They exhibit a rich variety of structural types molecular precursors, the vast majority of the NBB characterized by different coordination modes of the derived hybrid materials are based on silicon and ligands (O, OR, and CL); a large diversity in linkages of polyhedral oligo silsesquioxanes (POSS), many of the coordination polyhedra and the coordination which are now commercially available, can be number of titanium atoms; the degree of condensation considered as the archetype of covalently functionalized O/Ti differs also in oxo-complexes. Their nuclearity NBBs. However, NBBs based on transition metals (number of titanium atoms) reported in literature varies or tin oxo-clusters are interesting because they can from two to 18. The metallic centers that exhibit offer, in addition to the specific magnetic, electronic, coordination numbers between four and six are catalytic, etc. properties associated to the metal centers, coordinated to bridging oxo-ligands and terminal or different functionalization processes that use complex- bridging alkoxy OR groups. A large variety of oxo ing ligands or charge compensating anions [22,48,49]. bridges are encountered in the known clusters from m2- O to m5-O. Furthermore, numerous titanium oxo- 3.2. Metal-oxo clusters clusters bear strongly complexing oxo-ligands CL (b- diketone, carboxylic acid, phosphonates...) with a Transition metal oxo-clusters and especially tita- chelating or a bridging coordination mode. nium oxo-clusters offer a large variety of candidates to Among the numerous titanium oxo-clusters, we elaborate hybrid organic-inorganic materials by the especially focused our interests on three titanium NBBs approach. The literature describes many titanium oxo-clusters, the Ti16 [Ti16O16(OEt)32], the Ti8 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 9 Fig. 3. a. Postmodification of the Ti16O16(OEt)32 cluster to form functional nano-briks and to build hybrid materials; b. Representation of the Ti8O8(OOCC6H4)16 oxo-clusters; c. Representations of the Ti6O6(OiPr)6(OCR)6 oxo-clusters. [Ti8O8(OOCR)16], and the Ti6 [Ti6O6(OR)6(OOCR’)6] gated and leads to nanostructured hybrid materials (Fig. 3).. NMR studies allowed first the complete assign- The Ti16, oxo-alkoxo cluster, is built of an inorganic ment of the 32 ligands in correlation with the oxo-core of 16 TiO6 octahedra, surrounded by an crystallographic data, and in a second hand allowed organic shell of 32 ethoxy groups. The specificity of to justify and quantify the formation of postmodified this oxo-alkoxo cluster is to present labile ethoxide Ti16 derivatives [50,52]. groups which can be selectively exchanged by The eight-membered-ring [Ti8O8(OOCR)16] oxo- transalcoholysis or transesterification reactions while cluster is a very attractive NBB. First, it presents no keeping the titanium oxo-core. We demonstrated the residual OR group that considerably increase its possibility to generate new titanium oxo-clusters stability toward nucleophilic species. Second, the ring [Ti16O16(OEt)32-x(OR’)x] by postmodification of the shape of this purely carboxylate oxo-cluster allows the Ti16 cluster. Both the kinetics and the number of building of organized hybrid edifices by the choice of modified titanium atoms are strongly dependent on the carboxylate ligands. nature of the new introduced ligands. The reaction The last presented candidate for the elaboration of between the cluster and aliphatic alcohols (such as n- hybrids by the NBBs approach, is the Ti6 oxo-cluster propanol or n-butanol) leads to the introduction of eight [Ti6O6(OR)6(OOCR’)6]. Many times described in the new ligands on well-defined positions in the structure, literature, we can still take advantage of its particular even with an excess of alcohol. The use of more acidic structure. Indeed, this cluster presents at once carbox- (such as phenol derivatives), or less sterically hindered ylate ligands in the equatorial position and alcoxy (such as methanol) reactants leads to the substitution of ligands in the axial position. Many different carboxylate all the 16 terminal ethoxy by the new ligands. This ligands can be located in the six equatorial positions behavior can be extended with the aim of forming during the synthesis of the clusters from titanium functional titanium oxo-clusters. Based on results alkoxide and carboxylic acid. In the axial position the obtained on model reactions, NBBs with polymeriz- iso-propoxide ligands on preformed Ti6 clusters can be able ligands (methacrylate or styrenic), have been postmodified by transalcoholysis reactions as demon- obtained. By a judicious choice of the organic strated for the Ti16 oxo-clusters. The simultaneous component, original architectures can also be elabo- presence of carboxylate located perpendicularly to the rated. As an example, the assembly of well-defined alkoxo ligands allows the building of directional hybrid dendrimers with Ti16 oxo-clusters has been investi- edifices. 10 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 3.3. Nanoparticles surface-plasmon band, with respect to the gold NPs capped by only one of the two ligands. Finally, when an Though less well-defined, nanoparticles (NPs) hydrophilic thiol was used (HS-(CH2)10-CO2H), self- represent a rich and versatile alternative to metallic organization behavior was also evidenced. oxo-clusters in the NBB approach to hybrid organic- inorganic materials. Many compositions, crystalline 3.4. Mechanical and optical properties of the structures and shapes can be obtained with size ranging resulting hybrids from few nanometers to few tenths of microns. As for clusters, a crucial point remains a careful control of their The introduction of fillers in polymer matrices meets functionalization, which is generally necessary to a well-established industrial scheme that aims at stabilize, to give new properties and to make them enhancing specific properties of polymers while compatible with the application medium. reducing production costs. The regularity of the As mentioned in the general background (part 2), the dispersion of the organic and inorganic phases and sol–gel process can be conveniently used to prepare a the extended and controlled interfaces between the two stable suspension of metal oxide NPs. One way is to phases encountered in hybrid materials offer the control the growth of the oxo-framework through a possibility to control the mechanical properties. High poisoning process with complexing ligands that, at the performance in term of mechanical responses has been end, remain on the surface of the particle and provide a reported for hybrids based on calibrated nano-objects way to functionalize them. Such an approach was (MOCs and NPs) which allow the control of the degree used with a pyrrole functionalized ß-diketone as of organization and the size and the nature of the hybrid complexing ligand and TiO2 anatase NPs with pyrrole interfaces. groups grafted on the surface were produced from Functionalized titanium oxo-clusters (Ti16O16 titanium alkoxides in a one step process. The (OEt)32-x(OR)x, x = metacrylate or styrenic groups) dangling pyrrole groups were subsequently chemically were copolymerized with organic monomers (metacry- or electrochemically polymerized, and films made of late or styrene) to obtain organic-inorganic hybrid TiO2 anatase NPs (2–4 nm) homogenously distributed networks. The postmodification of the NBBs allows in a matrix of short polypyrrole chains were prepared on limiting phase segregation between both components; ITO coated glass. The cyclic voltametric response of hybrid networks are formed in which the oxo-clusters these films was stable and reversible. are covalently linked to the organic matrices. These Another family of NPs that has attracted attention in oxo-clusters play the role of nano-sized cross-linkers, the last few years is the so-called ‘‘Janus-head’’ type NPs, well dispersed in the host media. A significant which exhibit a dissymmetric distribution of two reinforcement of polymers has been observed, espe- functions at their surface and are accordingly named cially in the rubbery regime (Fig. 4). from the two faced Roman god. These objects are Coatings with efficient mechanical properties, especially interesting for their potential properties in the investigated by the nanoindentation technique, have field of self-organization. However, their elaboration been elaborated by introducing goethite (a-FeOOH) usually requires a multistep process that involves reaction nanorods or maghemite (g-Fe2O3) nanospheres in a at an interface or the fusion of two different hemi-objects. poly(hydroxyethyl methacrylate) (PHEMA) matrix. Moreover, the stability of the dissymmetry relies on a Both goethite nanorods and maghemite nanospheres strong and irreversible anchoring of the functions on the yield a strong reinforcement effect of PHEMA, even at surface of the final object in order to prevent any low volume fraction. The origin of such reinforcement relaxation towards a homogeneous distribution. is attributed to the existence of strong interactions at the Recently, thermodynamic Janus NPs were prepared in iron oxide-PHEMA interface combined with a homo- one step from gold cores functionalized with two ligands: geneous dispersion of the particles. The chemical a diphenylphosphinine, 3,5-(C6H5)2(C5H3P), and a thiol modifications of the organic component by the iron (RSH). The quite different physicochemical proper- oxide particles (hydrolysis of the ester yields carbox- ties of these two ligands cause their phase separation on ylate which complexes iron sites) favor the high the surface of the NPs and create a dissymmetric mechanical performance of the resulting hybrids distribution of the capping molecule. Moreover, the (Fig. 4). By the control of the specific interactions different electron donating-electron withdrawing abil- between the particles and the polymer matrix, the ities of these ligands, in association with their dissym- particles are likely to be used as functional reinforcing metric distribution, cause a blue shift of the localized elements. Nanocomposites are then synthesized in C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 11 Fig. 4. a. Reinforcement of polymers by covalently bonded nano-crosslinkers. b. Mechanical properties by nanoindentation of functional hybrid coatings based on anisotropic iron oxide nanoparticles dispersed in poly(hydroxylethyl methacrylate). which the polymer acts as binder that fixes the The incorporation of titanium oxo-clusters as functional nanoparticles while the particles bring the additive into already known ORMOCER1 matrix functionality of the hybrid material and participate to materials offers a variety of new hybrid materials for the mechanical reinforcement. high added-value applications, for example in micro- Nowadays, abundant optical devices, luminescent optical devices. The refractive index and the optical loss solar concentrators, dye laser, sensors, photochromic, measurements, key parameters for micro-optical appli- NLO and photovoltaic devices, based on sol–gel cations, of the resulting materials have been recently derived hybrid materials have been referenced investigated. The refractive index of a material, in [18,59]. Most of the optical properties are brought close relation to its permittivity, depends on the by the incorporation of organic molecules into polarizability of a substance. The use of a titanium inorganic oxide matrices. The optical behavior oxo-cluster, [Ti6O4(OOCC6H5)8(OnPr)8], which bears of hybrids can also evolve from the inorganic both aromatic groups and titanium, a transition metal component of the hybrids. We recently reported with high polarizability, results in a significant increase hybrid coatings based on goethite iron oxide nano- of the refractive index. Moreover, the nanometer size of particles (cf. mechanical properties of hybrids). In the titanium oxo-clusters limit the scattering of the addition to the coloration of the samples, due to the resulting nanocomposites, and the very high optical presence of the nanoparticles, the coatings exhibit qualities of the ORMOCER1, used for optical interesting birefringent properties associated to the waveguides, is not affected. stabilization inside a polymeric organic matrix, the NBBs based hybrids are also useful models to poly(hydroxyethyl metacrylate) PHEMA, of a liquid understand the optical behavior of conventional crystal type organization resulting from the self- nanocomposites. Thus, the [Ti16O16(OEt)32] oxo-clus- organization of the highly anisotropic goethite nanor- ters based hybrid materials were used as a reference ods (Fig. 4). Concentrated and stable aqueous disper- sample to compare its photosensitivity with that of TiO2 sion of goethite particles behave as nematic lyotropic gel-based hybrid materials. Less attention is paid to the liquid crystals with interesting magnetic properties. photochromic properties of TiO2 based materials. The same behaviour is also encountered in a glassy Nevertheless, it is well known that the reduction of polymeric medium. Ti4+ into Ti3+ undergoes strong darkening of TiO2 based 12 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 samples. However, in most cases spontaneous reoxida- capping agents to control the size, the size-dispersity, tion occurs, leading to the concomitant discoloration of the morphology, the crystalline structure, and the the samples. The oxidation of the Ti3+ sites can be stability. Such syntheses yield suspensions in which the restricted and the coloration stabilized by the elabora- NPs are protected by a shell of capping molecules that, tion of TiO2 based hybrids, in which the inorganic in addition to the role played during the synthesis, can photoactive component is generated simultaneously also modulate the properties of the NPs. The use of NPs with a polymeric component. We showed that the also often relies on the functionalization of their electronic coupling along the extended interface surface. This step can be required to make them between the inorganic, and the organic, poly(hydrox- compatible with the application medium or to create yethyl methacrylate), interpenetrated networks allows: new functionalities at their surface (sites with specific molecular recognition properties, unsaturated groups a rapid scavenging of the photo-excited holes by the that can polymerize, etc.). Indeed, many NPs based polymer; suspensions can be considered as true hybrid organic- an efficient trapping of the photo-excited electrons as inorganic systems. A fine characterization of the surface small polarons (Ti3+) that develop ‘‘dark’’ absorption chemistry of NPs is therefore highly necessary to continuum covering the spectral range from 350 nm understand and improve the synthesis and to rationalize (UV) to 2.5 mm (IR); the utilization of NPs. In particular, measuring the their long-term conservation over months at high affinity of organic molecules for NPs, in suspension, is number density (Fig. 5) [61,62]. of prime importance. However, evaluating this affinity in situ is not a simple task, especially when the capping Furthermore, we observed that the electron transfer molecules are involved in a dynamic equilibrium depends on the material microstructure, which can be between a free form and a grafted form (Fig. 6). affected by the materials chemistry and processing. In such a context, Diffusion Ordered Spectroscopy Undeniably, tuning the delay between the gelation of (DOSY) NMR has been evidenced as an efficient tool. the system and the organic polymerization step, or using This technique, which belongs to pulsed field gradient NBBs as an inorganic reference species allows the methods, provides in addition to the classical data of optimisation of the photochromic responses of the solution NMR, i.e. chemical shifts and scalar coupling resulting nanocomposites. constants, a way to measure self-diffusion coefficients, which can be used to sort species according to their 4. In situ evaluation of hybrid interfaces and size, as the diffusion coefficient is inversely propor- their evolution tional to the hydrodynamic radius. Accordingly, free molecules can be differentiated from those that 4.1. Affinity by DOSY NMR in the colloidal state interact with NPs as their diffusion coefficients are different. Moreover, a careful exploitation of the data The chemical syntheses of nanoparticles (NPs), allows one to quantify the populations associated to the through a bottom-up approach, very often use organic 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 Fig. 5. Laser-induced photopatterning of organic–inorganic TiO2- free ligands were clearly differentiated and quantified based hybrid materials with tunable interfacial electron transfer using DOSY NMR experiments. The same year, the [61,62]. use of DOSY NMR to assign the 1H resonances of C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 13 Fig. 6. Diffusion Ordered Spectroscopy (DOSY) NMR can be used to analyzed in situ the functionalization of nano-objects, such as NPS or MOCs. The example on TiO2 nanoparticles is taken from reference. TOPO molecules interacting with InP quantum dots the enol form of acac was present as grafted and free (QDs) was also reported. In this system species while its diketone form was found only as free (TOPO@InP in toluene), free and bound TOPO molecules; molecules exhibit different 1H chemical shifts and, pTs was distributed between strongly interacting therefore, can be quantified ‘‘classically’’ by integra- molecules, weakly interacting molecules and free tion in the spectroscopic dimension. In 2006, this ones, the two last types being in rapid equilibrium; quantification method was used to report the adsorp- butanol was not interacting with the NPs. All these tion isotherm of TOPO on 4.7 nm InP QDs at 295K 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 measured and a Fowler isotherm was shown to better published. describe the adsorption than a simple Langmuir isotherm, pointing out the non-negligible and favor- When the lifetimes of species involved in a able role of TOPO–TOPO interactions in the process. equilibrium ranges from ca. 10 ms to ca. 1 s, information In 2007, a complex multicomponent sol–gel derived- on the dynamic of the systems can be extracted from system containing TiO2 NPs (2–6 nm), acetylacetone DOSY NMR. Recently, we have used this approach (acac), p-toluenesulfonic acid (pTs), and butanol was for TOPO molecules adsorbed on CdSe QDs and we have investigated (Fig. 6). DOSY NMR clearly showed been able to estimate the lifetime of the bound species to that: ca 0.6 s. In conclusion, for favorable cases, not only 14 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 thermodynamics data can be evaluated but information us to improve the synthesis of silica-based meso- on the dynamic can also be obtained. structured materials (in particular the influence of In the context of hybrid materials built from well- various organosilica compounds on the final meso- defined metallic oxo-clusters, [10,22] DOSY NMR is structure) and to explore the field of non-silicate, also a valuable tool to study the functionalization of transition metal oxide, rare earth oxide and multi- these molecular NBB. In the case of the macro-cation metallic based mesostructured materials (ZrO2, {(RSn)12O14(OH)6}2+, which can be functionalized TiO2, [81,82] V2O5, ZrO2-CeO2, [84,85] ZrO2- and/or assembled through its charge compensating Y2O3, TiO2-Nb2O5, HfO2, WO3, anions, [48,73] DOSY NMR was used to study the SnO2,...). influence of the solvent on the ionic dissociation and These latter materials could be designed with anion exchange. amorphous or nanocrystalline walls opening new opportunities for the development of many applications 4.2. Self assembly: Interface class arising from the active properties (optic, electronic, I – SAXS / Ellipsometry magnetic...) due to the presence of metallic centers having d or f orbitals. However, the crystallization As mentioned previously, one of the major issues process is not straightforward [15,76]. The crystal- leading to a high control of the final nanomaterials lies lization of the inorganic walls often leads to a collapse in the characterization of the hybrid interfaces all along of the mesostructure due to nucleation-growth mechan- the formation mechanisms. However, in the case of isms. Then the experimental conditions (maximum mesostructured thin films coated onto a substrate or temperatures, time of the treatment, heating rates, meso-organized particles prepared by spray-drying, the sequential steps, storage conditions, nature of block very small amount of matter (usually for thin films copolymers...) are critical in order to promote crystal- between 10 and 500 mg/cm2 and for particles 6 mg/cm3 lization while avoiding mesostructure collapse. The in the gas phase ) requires very sensitive techniques optimal experimental conditions, for a given system, for their whole characterization. In this way, synchro- were determined precisely via in situ SAXS-WAXS tron 2D-SAXS techniques, used for the structural experiments [82,90,91]. Briefly we analyzed simulta- characterization of mesophases, are very efficient for neously during the thermal treatment the evolution of three main reasons. This technique is not limited by the the mesostructure by in situ SAXS and the crystallinity low amount of matter and then could be applied for the of the network composed of crystalline NPs by in situ analysis of mesostructured materials shaped as (ultra) WAXS (Fig. 7). It was then possible to synthesize new thin films [15,76,77] and particles generated by crystalline mesostructured thin films such as SrTiO3 spray-drying. It also leads to a straightforward and non- perovskite, tetragonal MgTa2O6, MTiO3 – ambiguous characterization of the structural periodic TiO2 (M = CoII or NiII) (ilmenite – anatase/rutile), organization of the mesoporous network on the contrary [90,91] g -Al2O3, cubic Eu2O3, cubic Y2O3, to the conventional (1D) laboratory XRD techniques. Nb2O5 , etc. Moreover, the high X-ray flux provided by synchrotron In order to fully understand crystallization mechan- sources allows in situ time-resolved SAXS experiments isms, the in situ SAXS-WAXS experiments were which are of great importance for a deep understanding recently completed by in situ thermal ellipsometric of self-assembly mechanisms and (rapid) mesophase analysis. This lab-scale technique, more accessible transformation specially when coupled with other than synchrotron experiments, gives access to the index techniques as interferometry, WAXS or ellipsometry. of refraction and film thickness during heating. These For example, we studied the structural evolution of informations were used to evaluate the evolution of the meso-organized thin films just after coating by in situ porosity and material composition as a function of time-resolved SAXS experiments coupled with inter- temperature, thus identifying key parameters that affect ferometry [78,79]. By recording 2D-SAXS diagrams densification, pyrolysis, crystallization, and sintering every 20 seconds or 5 minutes (depending on the (an example is shown in part 5). The influence of the system studied), we investigated independently the heating schedule, the initial film thickness, the nature of influence of the chemical parameters (e.g. pH, aging the substrate, the solution aging, the presence of water time of the solution, initial sol composition...) and during calcination, the nature of the templating agent, processing conditions (e.g. partial vapor pressures, and the influence of additives (H2O, HCl) in the temperature...) on the self-assembly mechanisms calcination environment were studied on the index [76,77]. These specially designed experiments allowed refraction profiles as a function of temperature. These C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 15 Fig. 7. Top: Experimental scheme of the SAXS-WAXS experiment. Left: SAXS-WAXS patterns of mesostructured TiO2 thin films during thermal treatment. Right: Transformation of the intial Im3m mesostructure into a ‘‘grid-like’’ nanocrystalline structure [82,90,91]. Examples of different nanocrystalline mesostructured thin films [86,92,93]. parameters are shown to have unique and often HCl, H2O) triggers the self-assembly process by substantial effects on the final film structure. concentrating progressively the system in inorganic precursors and surfactant. In due time, the concentration 5. Nanoporous materials in surfactant reaches the equivalent of a critical micellar concentration for the system, and then micelles start to 5.1. Periodically Organized Mesoporous Thin form, surrounded by the inorganic precursors. An Films (POMTFs) by EISA alternative way has been recently published implying block copolymers PS-b-PEO of high molecular weight Up to now, the preferential way of POMTFs which exist in the micellar state in the starting solution. synthesis is the EISA approach (for more detailed This process was called Evaporation Induced Micellar information about POMTFs, we recommend the Packing (EIMP). EIMP is particularly useful to create following reviews [15,21,76,77,96,97]). Briefly, the mesoporous layer with difficult systems (RuO2 and EISA process (Fig. 8) implies first the elaboration of an multicationic oxides) [90,100]. Depending on the isotropic dilute solution containing mainly inorganic (or processing conditions (i.e. water and solvent relative hybrid) precursors, solvents, catalysts and molecular pressures, temperature), the micelles could self-orga- templating agents. nize on a large scale leading to a liquid-crystalline phase In a second step during coating processing, the fast before the extended condensation of inorganic species evaporation of volatile species (typically ethanol, THF, will lock the system. This step is addressed as the 16 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 Fig. 8. Schematic representation of EISA mechanism via dip-coating and spray-drying processes [15,75,77,79,98,99]. Tunable Steady State (TSS). The third step is dedicated dilution (crucial parameters in sol–gel chemistry) have to the removal of the surfactant, the stiffening of the to be carefully controlled in order to generate inorganic network and eventually the crystallization of hydrolyzed but slightly condensed inorganic species the inorganic walls. The EISA approach is not in the starting sol. These prerequisites could be difficult straightforward since it implies competitive processes: to manage with the synthesis of (multi)metallic oxides- inorganic condensation versus organic meso-organiza- based or hybrid silica-based POMTFs. Despite the well- tion, both of them being influenced by the diffusion of established conditions for pure silica and titania , the volatile species. In the following paragraphs, we will the situation becomes much more complicated with emphasize some critical aspects of the synthesis of multimetallic oxides and organosilanes species. nanocrystalline mesostructured thin films and hybrid For metal oxides, experimental conditions have to be thin films synthesized via EISA approach. selected with respect to the speciation diagram of cations in aqueous solution. Generally, with trivalent to 5.1.1. The initial solution pentavalent cations (e.g. TiO2, ZrO2, Al2O3, Nb2O5...), The prerequisites concerning the initial solution are high hydrochloric acid content in the solution is numerous. This latter has first to be highly homo- required to inhibit an extended condensation of geneous and stable with time. Self-assembly requires the inorganic species. The difficulty arises for the mobile and flexible inorganic species able to accom- synthesis of multimetallic oxides-based POMTFs modate to the curved surface of micelles and to interact (perovskite SrTiO3, tetragonal MgTa2O6, ilmenite with the hydrophilic part of the surfactant. pH, aging MTiO3 (M = CoII or NiII)) since these compounds time, water content, solvent and to a lesser extent correspond to solid solutions. It is then necessary to C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 17 adjust the solution conditions to solubilize conveniently Steady State where the system is flexible enough for all the species (a wise choice of the metallic precursors, further modifications (slight condensation of inorganic salt or alkoxides, must be done) and to avoid condensed species) and where its composition is in equilibrium species which could favor nanocrystalline phase with the environment. At this stage, changes in the segregation in the final material. However, it atmosphere composition (i.e. relative humidity) could could happen that ideal solution conditions, suitable for lead to modifications of the mesostructure. For example, all the cations, do not exist reducing then considerably it is usually observed that a high relative humidity the time window of solution coating. within the deposition chamber favors mesophases with Difference of reactivity and solubilization difficul- high micellar curvature (cubic structure) for low ties are also encountered with hybrid silica-based surfactant content. This general tendency could be mesostructured materials synthesized by a ‘‘one-pot’’ somehow modified by the incorporation of functional procedure [15,96,101]. With hydrophobic species, organosilanes in the starting sol (‘‘one-pot’’ synthesis). hardly soluble in ethanol, the replacement of a part Depending on the nature of the functional group of ethanol by THF is the solution widespread due to (hydrophobic, hydrophilic, ionic, aromatic, polar, both its high solubilization abilities and physicochem- apolar) and the length of the organic chain between ical properties close to those of ethanol [15,102]. It is the functionality and the anchoring group (–Si(OR)3), also important to point out that the amount of cosolvent various modifications of the mesostructure could be in the initial sol should be adapted to avoid phase observed from simple variations of the lattice para- separation during the evaporation step (higher than the meters to phase transitions, [102,105] and even quantity necessary to solubilize the organosilane the formation of a mesophase for a (very) low molar especially when its boiling point is lower than that of ratio of surfactant/silica which leads usually to ethanol). The presence of organosilanes could also amorphous thin films without organosilanes. modify the hydrolysis-condensation kinetics of silica Since TSS is a relatively slow stage for metal oxides (it species modifying then the self-assembly process or can last a few hours while it takes few seconds with even hindering mesostructuration [103,104]. silica-based systems due to different condensation rates of inorganic precursors), the mesostructure is retained 5.1.2. Self-assembly induced by evaporation by aging thin films in a humidity-controlled atmosphere The self-assembly constitutes also an intricate before a prethermal treatment step. Indeed, it has been stage leading to the formation of a hybrid mesophase shown that a long prethermal treatment below the composed of an ordered micellar arrangement crystallization temperature is beneficial since the embedded within a stable amorphous inorganic inorganic network slowly condenses by dehydration medium. Indeed, it can be divided in four main steps, and full departure of the remaining condensation kinetically (steps 1 and 4) or thermodynamically (steps inhibitor, HCl. For multimetallic oxides, this humid- 2 and 3) governed, which could overlap all along the ity-controlled aging step must be limited sometimes, in coating process: (1) fast evaporation of solvent(s), (2) order to prevent demixing and aggregated clusters equilibrium of atmospheric water (relative humidity) formation due to the differences in solubility and with water inside thin film, (3) formation and condensation kinetics of metallic centers. The use stabilization of the hybrid mesophase and (4) stiffening of surfactants presenting a high hydrophobic-hydro- of the network by extended condensation. philic contrast such as KLE, PS-b-PEO, PIB-b-PEO Solvent evaporation and water equilibrium (steps 1 block copolymers, which favor fast micellization and and 2) must be as fast as possible. Indeed the aim is to mesostructuration, is then required. In this case, the limit as far as possible chemical modifications and slight thermal treatment applied just after TSS is possible phase separation resulting from solubility preferred. heterogeneity between non volatile species (i.e. cations or hydrophobic organosilanes aggregation). Moreover, 5.1.3. The thermal treatment since water molecules inside thin films play an The aim of this step is to consolidate the inorganic important role (micellar curvature and viscosity network, to remove the templating agent (once the variations, hydrolysis of the remaining non-hydrolyzed inorganic walls are sufficiently rigid to prevent precursor) in the formation of the liquid-crystalline collapsing of the mesostructure) and eventually to phase, the fast water equilibrium appears to be a critical promote crystallization of the inorganic walls. It has to point due to the competition of this process with the be modulated following the nature of thin films. With inorganic condensation. The third step is the Tunable hybrid mesostructured thin films, temperature and 18 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 duration of the thermal treatment have to be adapted: chamber and the optical density value is recorded once stiffening the inorganic network without decomposing the absorbate pressure is stabilized [112,114]. The functional organosilanes. The thermal treatment is second one (environmental ellipsometric porosimetry) usually followed by a washing step, which allows the works under atmospheric conditions where a continuous extraction of surfactant [102,105]. For metallic oxides, flux of air containing a controlled relative pressure of the processes involved are more complicated especially gas (usually water or alcohol) is applied at the sample when crystallization of inorganic walls is the aim. As it surface. This last setup allows a shorter analysis was mentioned previously, a prethermal treatment at time (1 h for a complete adsorption–desorption isotherm low temperature is usually applied after TSS. It prevents instead of around 8 h with sealed chamber) and is very a too high deformation of the mesostructure upon advantageous for the simulation of environmental removal of the organic phase and further crystallization. adsorption of gas. EP and EEP offer the opportunity This prethermal treatment is then followed by the to determine not only classical textural properties but decomposition of the template at higher temperatures. also the thickness of thin films during absorption– The third step of thermal treatment is about the desorption of gas and then their mechanical properties crystallization process. One must keep in mind that (transverse Young’s modulus E). EEP was successfully crystallization occurs through nucleation and growth of used to investigate the stability of mesostructured thin crystallized seeds and is followed by diffusive sintering films. It was, for example, observed that pure silica if a sufficiently high temperature is maintained long meso-organized thin films were poorly stable in enough. To retain a highly organized mesostructure, simulated body fluid on the contrary to Zr (or Al)- generally a fast homogeneous nucleation must be loaded SiO2 mesoporous thin films, TiO2 films being the favored at the same time that an extensive diffuse more stable. The stability of mesostructured thin sintering must be avoided. It was also observed that the films is not only environmental conditions dependent but presence of a high surface area associated to thin is also function of the mesostructure’s geometry. For inorganic walls tends to delay crystallization and SiO2 POMTFs, the most often obtained and reported 2D- stabilize metastable phases. Moreover, the role of the hexagonal structure collapses in the presence of water, surfactant during the crystallization process is of great while tridimensional phases (cubic and 3D-hexagonal importance since mesostructure collapsing is observed ones) exhibit longer lifetimes. when the thermal decomposition of surfactant occurs before dehydration process is completed (as in the case 5.2. Properties of mesoporous thin films by EISA of Al2O3) [107,108]. KLE or PS-b-PEO surfactants are then preferred to pluronic block copolymers since their This part concerns the properties of periodically hydrophobic part (polyethylenecobutylene or polystyr- organized mesoporous architectures including thin ene) decomposes at higher temperature than the films and membranes. These materials exhibit unex- hydrophobic polypropylene part of pluronic templates. pected electrochemical, catalysis and photocatalysis Although a deep knowledge of formation mechan- properties because the mesoporous network serves as isms is a first requirement for designing well-adapted efficient pathways for molecular transport or/and the materials for a given application, many applications connected solid network allow fast transport of require a real assessment of the textural characteristics of electrons, oxygen ions, protons throughout the materials (i.e. surface area, porous volume, pore size nanoarchitecture. Because these nanoarchitectures distribution, pore connectivity, diffusion phenomena). exhibit high surface-to-volume ratios, they also serve For powders, such investigations are usually done via gas as tools to amplify the critical phase of any electro- adsorption/desorption experiments. However, the small chemical system: the interface. The sol–gel based amount of matter and the presence of substrate limit chemistry and the processing involved for their considerably such analyses. Among textural techniques synthesis allow a facile control of the solid and pore dedicated to thin films (Surface Acoustic Waves , network with tuned properties. We will describe in this krypton physisorption , X-Ray Reflectivity por- paragraph the recent research on mesoporous thin films osimetry ) spectroscopic ellipsometry porosimetry as new materials in photocatalysis, photovoltaic, [112,113] is widely used. Two possible setups are electrochemical/sensing and, fuel cell applications. nowadays accessible. The first one (ellipsometric porosimetry) consists of a spectroscopic ellipsometer 5.2.1. Photocatalyst on which a vacuum chamber is fixed. A known and The continuous organized mesoporous network on progressive quantity of absorbate is introduced into the TiO2 mesoporous thin films facilitates the flux of C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 19 reactant molecules that access the catalytically active inorganic network forming the pore wall influence the ‘‘internal surface’’ area. The efficiency of photon– photon-energy conversion. In TiO2 mesoporous film, energy conversion in these systems has been shown to high crystallinity with an open grid-like mesophase readily depend on the pore-size and its distribution structure seems to be the best conditions for efficient. For example, in TiO2 mesoporous thin films, photoactivity. efficient photocatalytic properties are reached for The efficiency of photon-energy conversion can be mesoporous films exhibiting a cubic mesophase with increased by using visible wavelength photons. Their pores of 7.5 and 5.5 nm, respectively. In these studies, captation is promoted by doping the TiOX (x < 2) the modification of the film porosity with the heat structure with nitrogen centres, the presence of which treatment was followed by Ellipsometry Porosimetry. decreases the anatase band-gap. The decomposi- The corresponding isotherm shows the opening of the tion of lauric acid coated on mesoporous N-doped titania structure and the stabilization of the porous network films is highly efficient for films nitrided at 500 8C. These between 500 and 600 8C. The highest decomposition mesoporous films contain an optimal surface concentra- rate of methylene blue (MB) is obtained for mesoporous tion of nitrogen since the oxygen vacancies are still not TiO2 films heat-treated at 600 8C (Fig. 9 left part). important enough to promote the recombination of the Additionally, the cristallinity and the structure of the photogenerated electrons and holes. Fig. 9. Left part: Photodegradation of methylene blue over TiO2 / N-doped TiO2 films calcined at various temperatures and under UV light or visible irradiation [117,118]. Middle part: Transmission Electron micrograph of a mesostructured anatase film. Right part: A schematic representation of the experimental set-up used for testing mesostructured TiO2 films. Power energy conversion efficiency for different types of TiO2: colloidal anatase-based (^), colloidal brookite-based (&), and mesostructured anatase-based TiO2 film (*) as a function of porous TiO2 layer thickness. 20 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 5.2.2. Photovoltaic very interesting for the fabrication of robust membranes The literature covering the photovoltaic properties of or hybrid silica films selective to probes. Indeed, it was mesoporous TiO2 indicates that the solid-pore archi- observed that the meso-organization of POMTFs offers a tecture of the mesoporous films contributes to a facile better environment for sensing than their corresponding electron transfer to the collector electrode, to an non-organized thin films. Briefly, hybrid POMTF-based increase of the electron-hole pair density generated at optical sensors are characterized by a shorter response the hybrid interface and to a better hole-transporting time, a whole accessibility of the sensing molecules, a organic material impregnation. These mesoporous higher sensitivity, an ability to work in both gas or liquid films constitute a ‘‘host material’’ for polymers that phase without synthesis modifications (due to the large differ from nanocrystalline films with the same nominal opened and fully accessible porosity), and a response compositions and structure. In these nanocrystalline depending on the mesostructure (better with a 3D films, the highest photovoltaic properties for dye- mesophase compared to a 2D one). Then various optical sensitized solar cells (DSSC) are reached for fully sensors resulting from the functionalization (one-pot, nanocrystalline TiO2 films exhibiting the highest post-functionalization or combined route ) of specific surface area (Fig. 9, right part). These POMTFs have been elaborated. Scientists synthesized characteristics correspond to the largest polymer-TiO2 interesting pH, [123,124] O2, uranyl (UO22+), interface. An improvement in solar conversion is explosives (vapor of TNT), and BF3 (a gas possible if the thickness of the TiO2 mesoporous commonly used in semiconductor industries) films is increased and the quality of the hybrid sensors. Although most of the POMTF-based optical interpenetrating semiconductor nanostructure is opti- sensors are class II hybrid materials, class I meso- mized. For example, an increase of solar conversion organized sensors have been also reported for the efficient by 50% is obtained for cells containing TiO2 detection of methanol vapor, the determination of mesoporous films with a thickness greater than or equal pH and the detection of Cu2+ in aqueous phase to 1 mm.. An alternative way to dye-based sensors has also been investigated. For example, phosphine-stabilized 5.2.3. Electrochemical properties/Sensing gold NPs encapsulated inside silica POMTFs allowed the Mesoporous nanocrystalline thin films offer all the detection of thiols and small phosphines due to plasmon attributes for efficient electrochemical reactions. They band position shifts of gold NPs. Multilayer stacks exhibit a network of nanoscopic oxides, which can serve of TiO2 and SiO2 POMTFs functionalized , or not as an uninterrupted three-dimensional (3-D) pathway for , were used for the detection of water, alkanes and ions and electrons conduction, and an interpenetrated alcohols via photonic bandgap shifts of these ordered through-connected mesoporous network which serves as mesoporous Bragg reflectors. ‘‘host material’’ for electroactive species. In this way, fast ionic or electronic carriers transport in the internal 5.2.4. Fuel cells surface area of the mesoporous network may occur. For Mesoporous materials are also used and developed example, an improvement of proton conduction in for hydrogen economy. The high surface area-to- mesoporous zirconium phosphate films compared to the volume ratio present in a mesoporous silica network has ones with disordered porosity has been observed. This been used to construct an efficient proton pathway in behavior is correlated to the high concentration of –OH hybrid organic/inorganic membranes for PEMFC entities grafted on the pores’ inner surface and the working at high temperature [134,135]. These mem- efficient proton diffusion pathways achieved by the branes are constituted of a functionalized mesoporous organized and well-connected porous network. Further- silica network embedded in a polymer which can be more, organic entities grafted at the surface of these proton conductive or not. The inner surface area of the mesoporous thin films exhibit a pseudodiffusive electron porous silica network accommodates functional groups hopping between the redox species and the inorganic i.e. –SO3H and –PO3H2, the connected pore network TiO2 network. The transport of molecules in these allow fast transport of hydrated protons and the pore mesoporous architectures can, however, be tuned and volume serves as a ‘‘reservoir’’ for water. These depends on the nature of the functional groups attached to characteristics permit fast transport for protons the pore surface (the length of organic chains, the throughout the nanoarchitecture while the mechanical functionality size or polarity) and on the geometrical strength and gas tightness are ensured by the polymer. constraints including the pore size, the pore orientation Preliminary results have shown that the growth of a and their interconnection [116,122]. This capability is functionalized mesoporous silica network in conductive C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 21 polymer such as Nafion improves its water retention, its conditions. Hydrophilic chains must ideally be com- thermal stability and, its proton conductivity at high posed of chelating groups, such as PEO, so as to temperatures. Further, the structure of Nafion1 is stabilize the inorganic precursors within the shell of the to some extend reproduced in these hybrid membranes, micelles. The principle of pattern formation is i.e. there is a phase separation between hydrophilic and illustrated in Fig. 10. hydrophic domains at the micron-scale. Because this The nanostructure periodicity is associated to the phase separation can be tuned through the physical self-assembly of copolymer micelles on the surface, parameter of the sol–gel chemistry and the processing while their thermal decomposition is eventually techniques (spray, evaporation, electrospinning), such responsible for the creation of the void motifs such as hybrid organic/inorganic membranes constitute an the nano perforations. Deposition is performed by dip- interesting tool for understanding proton conduction coating in a dry and warm atmosphere at a withdrawal mechanisms at high temperature and low humidity. rate that is adjusted so as to obtain a monolayer of hybrid micelles on the surface. Typical solutions contain 5.3. Nanopatterned surfaces by EIMP around 1% weight of non volatile compounds and require to be deposited between 1 and 2 mm s1. The TiO2, Al2O3, and ZrO2 patterns (or masks) composed coating is thus directly transferred below an IR lamp to of ordered nano motifs of various morphologies and be thermally cured. In the first step, the temperature rise typical lateral dimension of less than 60 nm and up to 200 8C induces inorganic condensation while the thickness below 15 nm have been prepared by dip- PEO polymer chains become less hydrophilic due to a coating Si-wafer, FTO, glass or even Au-coated conformation change of the EO units. Both effects substrates in corresponding metal chloride ethanolic promote the depletion of hydrosoluble species (water solutions in the presence of commercial PB-b-PEO or and inorganic precursors) from the micelle shell region. PS-b-PEO block copolymer micelles [137–140]. The The inorganic phase condenses between the polymer various nanostructures are obtained by self-assembly micelles on the surface of the substrate. Increasing the during evaporation and are subsequently stabilized at temperature up to 500 8C causes the complete degrada- 500 8C. The nano pattern structures and morphologies tion of the copolymer, leading to the formation of nano are usually investigated by combined techniques such as cavities at the locations formerly occupied by polymer TEM, SEM, AFM, Ellipsometry, Electrochemistry, and micelles. The present systems constitute original GI-SAXS. Here, we chose to illustrate them by surfaces bearing ultrathin membranes of inorganic AFM images (Fig. 10). Each one of the images is oxide with nanoperforations through which the surface representative of the whole sample surface. They of the substrate is accessible. Similar systems can be confirm that, by varying the utilized copolymer as well obtained with polymers using PS-PMMA or PS-PLA as the chemical and processing conditions applied, block copolymers, the preparation of which is various degree of organization and morphologies can be more delicate and longer. Their inorganic homologues formed at the substrate interface. These various are chemically, thermally and mechanically more stable morphologies mainly depend on the solution composi- than the polymers which make them complementary tion and conditioning that governs the micellisation and materials. According to our investigations, proportion the stabilization of the inorganic precursor and the and size of the bare substrate surface area can be tuned deposition conditions. Most of the work was focused on between 25 and 75% and 12 and 60 nm in diameter, nanopatterns made of well-ordered and dispersed respectively, by adjusting the chemical and processing discoid perforations crossing the thin inorganic layer conditions. These ceramic based nanopatterns can be to give accessibility to the substrate surface, because of seen as novel highly ordered heterogeneous substrates its interest in multiple domains of nanotechnology. The that contain well-dispersed nano sites for heterogeneous present approach is simple, cheap, scalable, and gives nucleation. These sites, or nano beakers, are not only high reproducibility without expensive, specialized defined by their chemical heterogeneity, but are equipment. On the other hand, chemical and processing mechanically confined by the presence of the surround- conditions must be well controlled in order to optimally ing ceramic nano walls. When deposited on conductive achieve highly ordered patterns. Typical requirements surfaces, the perforations constitute a nanoelectrode are utilizing block copolymers with high molecular array (NEA) within which polymers (i.e. polyphenol weight (i.e. MW > 20.000 g mol1) and with strong ) of metals (i.e. platinum) can be electrochemically hydrophilicity contrast. These characteristics allows for and locally generated leading to nanocomposite the micelles to form and stabilize in very dilute surfaces. Both membrane and substrate can be of very 22 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 Fig. 10. Scheme describing the formation mechanism of nanopatterned surface by EIMP. AFM, MET, SEM and GISAXS investigation were effectuated onto 500 8C treated TiO2 nanopattern produced in presence of PB-b-PEO block copolymer onto Si wafer substrates. These revealed that the so-formed nanopattern is homogeneous all over the treated surface, has a thickness of 7 nm, and is composed of circular perforations of 27 nm in diameter which are organised into a 2D compact hexagonal structure of 39 nm of periodic distance. The substrate surface is accessible through the perforations since selective fonctionalisation with a thiol group promotes the selective addressing of gold nanoparticles inside the perforations. If the substrate is conductive, such as gold, selective reduction of salt can be performed inside the perforation only [15,138–142]. different materials with different surface chemical nano bubble entrapment fitting with a Cassie-Baxter affinities such as TiO2 on gold or ZrO2 on Si wafers. model) was observed. Both materials can be selectively functionalized using appropriated coupling agents such as thiol for noble 5.4. Aerosol route to mesoporous submicronic metals, triethoxysilane for Si-based surfaces, or com- spheres plexing carboxylic acid or phosphate groups for transition metal oxides. It is thus possible to attach Evaporation Induced Self Assembly (EISA) applied any type of molecule specifically on one of the materials to the spray drying process involves a similar structura- if the molecule is bound to the proper coupling agent. tion mechanism as that described above for the We have, in this way, prepared heterogeneous hydro- preparation of nanoporous thin films (Fig. 8, right part). philic and hydrophobic surfaces composed of perfluoro Although, in this case, the mother solution containing groups attached to the inorganic continuous networks inorganic precursors, condensation catalyst, structuring or on the second version on the SiO2 discrete nano agent and volatile solvent is not deposited onto a substrate patches. Depending on the position of the hydrophobic but is atomized to form an aerosol made of sub-micron perfluoro groups, superhydrophilicity (due to 3D size to micron size droplets. The fast drying of these enhanced capillarity) or hydrophobicity (due to air spherical droplets in an oven (the temperature of which C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 23 ranges usually between 70 and 500 8C) leads to the structuration (that is the oven temperature has to be generation of mesostructured spherical particles contain- higher than the evaporation temperature of the solvent). ing all non volatile compounds of the homogeneous As a consequence, the critical parameters here are the mother solution. First applied to the generation of solvents’ partial pressures in the drying chamber (and mesostructured silica powders at the end of the 1990s not the relative humidity observed for film formation at , this process is nowadays developing very fast, for it ambient temperature). Finally, the powder being presents numerous advantages for industrial production collected by frontal filtration of the aerosol flux, the over conventional solution powder generation processes, drying temperature has to be high enough to create rigid such as the batch precipitation: submicronic particles in order to avoid the coalescence of the product onto the filter. This rigidifying it is a continuous process; temperature is very sensitive to the inorganic precursor it eliminates time consuming and costly steps such as used and the sol–gel condensation pathway selected separation of the powder from the mother solution and [75,147,148]. washing that produces large volume of effluents to be In parallel, we developed a large spectrum of disposed; mesostructured materials (Fig. 11), both for funda- it is fast (a few seconds are needed to prepare dry mental knowledge development and industrial applica- mesostructured particles); tions. The simplest metal-oxides based particles with it is easily scalable. amorphous walls could be obtained by this way: SiO2 TiO2, ZrO2, Al2O3 with various mesostructures: 2D- Soon after the initial work proving the feasibility of hexagonal, cubic or lamellar [75,107,145,147–149]. EISA via the aerosol process [99,144], we investigated These systems were then extended to more complex in detail the influence of processing parameters over the mixed metal oxides such as SiO2-ZrO2, SiO2-Al2O3, mesostructuration phenomena that are significantly SiO2-P2O5... [150–152]. In these cases, we observed different from those of coating processes. The final that a careful tuning of the sol–gel reactivity and degree mesostructure obtained by EISA is generally mainly of condensation of inorganic precursors is needed for governed by tuning the surfactant to inorganic ratio. achieving a homogeneous incorporation of both metal- Indeed, for CTAB-SiO2 films for example, the entire oxides. Indeed, whenever the mobility of one inorganic range of mesostructures (from cubic to lamellar) has precursor is very different from the other into the been obtained by increasing the CTAB/SiO2 molar ratio surfactant-solvent medium, the drying of the aerosol. However, in spray-drying, the 2D hexagonal (starting from the periphery and progressing toward the structure is systematically encountered in a wide range centre of the droplet) may promote an inorganic- of CTAB/ SiO2 ratios (0.8 to 0.28) [145,146]. This inorganic separation. For example, a gradient of difference is related to processing parameters and was inorganic composition appears for SiO2-ZrO2 of shown for the first time by SAXS investigations. SiO2-Al2O3 powders prepared in acidic media when- We designed an unique and very sensitive experiment ever more than 10 to 20% of one element is dispersed in that involves in situ SAXS characterisation, using a high the other. The more mobile the precursor, the more flux third generation synchrotron (ELETTRA-Italy); concentrated the particle centre. Such inhomogeneity This experiment is able to detect meso-order within the being linked to a diffusion phenomenon, it is more aerosol mist (5 108 g of matter is analysed). By pronounced for big particles than for small ones systematically varying the residence time of aerosol in [151,152]. the pre-evaporation chamber, the temperature of the The concentration gradient created by the solvent oven (from 25 to 500 8C), the inorganic precursor (TiCl4 evaporation may also be used for promoting hierarch- or silicon alcoxide), and the water-alcohol content of the ical mesostructures. Accordingly, we used a solution initial solution, we proved that the resting time before containing a mixture of surfactant, F127 block heating chamber has no effect on the mesostructuration, copolymers and cationic fluorocarbon surfactant IC- while better organisation is obtained with alcohol-rich 11 C8F17-CH2-OHCH2-NH(C2H5)2-Cl to prepared solvent systems. The tuning of this parameter is of core-shell silica mesostructured particles with a importance since the organic domain structuration has bimodal porous structure. The high surface to occur before the inorganic matter condensation activity of the fluorocarbon surfactant, the alkyl- rigidifies the structure. Since the evaporation is fluoroalkyl microphase separation and the solvent occurring in a confined environment, alcohol evapora- concentration gradient associated to the evaporation tion conditions have to be fulfilled for reaching at the droplet interface were found to be responsible for 24 C. Sanchez et al. / C. R. Chimie 13 (2010) 3–39 Fig. 11. TEM pictures of mesoporous particles made by aerosol process via EISA. A. Aluminosilicate particles with Si/Al molar ratio of 10 using CTAB surfactant [152,153]. B. ZrO2 particles treated at 300 8C. C. Hierarchical core-shell silica powders prepared via dual templating strategy using IC 11 and Pluronic surfactants. D. g-alumina particles nanocrystallised at 900 8C. E. CeO2 nanoparticles encapsulated in mesostructured SiO2 particles. F. CeO2 nanoparticles functionalized by phenylphosphonate species embedded in mesos