Independently of the types or applications, as well as the nature of the interface between organic and inorganic components, a second important feature in the tailoring of hybrid networks concerns the chemical pathways that are used to design a given hybrid material. General strategies for the synthesis of sol–gel derived hybrid materials have been already discussed in details in several reviews.7,8,11 The main chemical routes for all type of hybrids are schematically represented in Fig. 3.
Path A corresponds to very convenient soft chemistry based routes including conventional sol–gel chemistry, the use of specific bridged and polyfunctional precursors and hydrothermal synthesis.
Route A1: Via conventional sol–gel pathways amorphous hybrid networks are obtained through hydrolysis of organically modified metal alkoxides (vide infra section III) or metal halides condensed with or without simple metallic alkoxides. The solvent may or may not contain a specific organic molecule, a biocomponent or polyfunctional polymers that can be crosslinkable or that can interact or be trapped within the inorganic components through a large set of fuzzy interactions (H-bonds, p–p interactions, van der Waals). These strategies are simple, low cost and yield amorphous nanocomposite hybrid materials. These materials, exhibiting infinite microstructures, can be transparent and easily shaped as films or bulks. They are generally polydisperse in size and locally heterogeneous in chemical composition. However, they are cheap, very versatile, present many interesting properties and consequently they give rise to many commercial products shaped as films, powders or monoliths. These commercial products and their field of application will be discussed in section III-2. Better academic understanding and control of the local and semi-local structure of the hybrid materials and their degree of organization are important issues, especially if in the future tailored properties are sought. The main approaches that are used to achieve such a control of the materials structure are also schematized in Fig. 3.
Route A2: The use of bridged precursors such as silsesquioxanes X3Si–R9–SiX3 (R9 is an organic spacer, X = Cl, Br, OR) allow the formation of homogeneous molecular hybrid organic–inorganic materials which have a better degree of local organisation.8g,e,15 In recent work, the organic spacer has been complemented by using two terminal functional groups (urea type).15–17 The combination within the organic bridging component of aromatic or alkyl groups and urea groups allows better self-assembly through the capability of the organic moieties to establish both strong hydrogen bond networks and efficient packing via p–p or hydrophobic interactions.15–17 Route A3: Hydrothermal synthesis in polar solvents (water, formamide, etc.) in the presence of organic templates had given rise to numerous zeolites with an extensive number of applications in the domain of adsorbents or catalysts. More recently a new generation of crystalline microporous hybrid solids have been discovered by several groups (Yaghi,18 Ferey,19–23 Cheetham and Rao24). These hybrid materials exhibit very high surface areas (from 1000 to 4500 m2 g21) and present hydrogen uptakes of about 3.8 wt% at 77 K.18–24 Moreover, some of these new hybrids can also present magnetic or electronic properties.20,25 These hybrid MOF (Metal Organic Frameworks) are very promising candidates for catalytic and gas adsorption based applications.18
Path B corresponds to the assembling (route B1) or the dispersion (route B2) of well-defined nanobuilding blocks (NBB) which consists of perfectly calibrated preformed objects that keep their integrity in the final material.7 This is a suitable method to reach a better definition of the inorganic component. These NBB can be clusters, organically pre- or post- functionalized nanoparticles (metallic oxides, metals, chalcogenides, etc.), nano-core–shells26 or layered compounds (clays, layered double hydroxides, lamellar phosphates, oxides or chalcogenides) able to intercalate organic components.27–29 These NBB can be capped with polymerizable ligands or connected through organic spacers, like telechelic molecules or polymers, or functional dendrimers (Fig. 3). The use of highly pre-condensed species presents several advantages: N they exhibit a lower reactivity towards hydrolysis or attack of nucleophilic moieties than metal alkoxides; N the nanobuilding components are nanometric, monodispersed, and with better defined structures, which facilitates the characterization of the final materials. The variety found in the nanobuilding blocks (nature, structure, and functionality) and links allows one to build an amazing range of different architectures and organic–inorganic interfaces, associated with different assembling strategies. Moreover, the step-by-step preparation of these materials usually allows for high control over their semi-local structure. One important set of the NNB based hybrid materials that are already on the market are those resulting from the intercalation, swelling, and exfoliation of nanoclays by organic polymers. Their applications will be described in section IV.
Path C Self assembling procedures. In the last ten years, a new field has been explored, which corresponds to the organization or the texturation of growing inorganic or hybrid networks, templated growth by organic surfactants (Fig. 3,Route C1).30–36 The success of this strategy is also clearly related to the ability that materials scientists have to control and tune hybrid interfaces. In this field, hybrid organic– inorganic phases are very interesting due to the versatility they demonstrate in the building of a whole continuous range of nanocomposites, from ordered dispersions of inorganic bricks in a hybrid matrix to highly controlled nanosegregation of organic polymers within inorganic matrices. In the latter case, one of the most striking examples is the synthesis of mesostructured hybrid networks.33 A recent strategy developed by several groups consists of the templated growth (with surfactants) of mesoporous hybrids by using bridged silsesquioxanes as precursors (Fig. 3, Route C2). This approach yields a new class of periodically organised mesoporous hybrid silicas with organic functionality within the walls. These nanoporous materials present a high degree of order and their mesoporosity is available for further organic functionalisation through surface grafting reactions.37
Route C3 corresponds to the combination of self-assembly and NBB approaches.7 Strategies combining the nanobuilding block approach with the use of organic templates that selfassemble and allow one to control the assembling step are also appearing (Fig. 3). This combination between the ''nanobuilding block approach'' and ''templated assembling'' will have paramount importance in exploring the theme of ''synthesis with construction''. Indeed, they exhibit a large variety of interfaces between the organic and the inorganic components (covalent bonding, complexation, electrostatic interactions, etc.). These NBB with tunable functionalities can, through molecular recognition processes, permit the development of a new vectorial chemistry.38,39
Path D Integrative synthesis (lower part of Fig. 3). The strategies reported above mainly offer the controlled design and assembling of hybrid materials in the 1 A° to 500 A° range. Recently, micro-molding methods have been developed, in which the use of controlled phase separation phenomena, emulsion droplets, latex beads, bacterial threads, colloidal templates or organogelators leads to controlling the shapes of complex objects in the micron scale.30,34 The combination between these strategies and those above described along paths A, B, and C allow the construction of hierarchically organized materials in terms of structure and functions.30,34 These synthesis procedures are inspired by those observed in natural systems for some hundreds of millions of years. Learning the ''savoir faire'' of hybrid living systems and organisms from understanding their rules and transcription modes could enable us to design and build ever more challenging and sophisticated novel hybrid materials.
Path A corresponds to very convenient soft chemistry based routes including conventional sol–gel chemistry, the use of specific bridged and polyfunctional precursors and hydrothermal synthesis.
Route A1: Via conventional sol–gel pathways amorphous hybrid networks are obtained through hydrolysis of organically modified metal alkoxides (vide infra section III) or metal halides condensed with or without simple metallic alkoxides. The solvent may or may not contain a specific organic molecule, a biocomponent or polyfunctional polymers that can be crosslinkable or that can interact or be trapped within the inorganic components through a large set of fuzzy interactions (H-bonds, p–p interactions, van der Waals). These strategies are simple, low cost and yield amorphous nanocomposite hybrid materials. These materials, exhibiting infinite microstructures, can be transparent and easily shaped as films or bulks. They are generally polydisperse in size and locally heterogeneous in chemical composition. However, they are cheap, very versatile, present many interesting properties and consequently they give rise to many commercial products shaped as films, powders or monoliths. These commercial products and their field of application will be discussed in section III-2. Better academic understanding and control of the local and semi-local structure of the hybrid materials and their degree of organization are important issues, especially if in the future tailored properties are sought. The main approaches that are used to achieve such a control of the materials structure are also schematized in Fig. 3.
Route A2: The use of bridged precursors such as silsesquioxanes X3Si–R9–SiX3 (R9 is an organic spacer, X = Cl, Br, OR) allow the formation of homogeneous molecular hybrid organic–inorganic materials which have a better degree of local organisation.8g,e,15 In recent work, the organic spacer has been complemented by using two terminal functional groups (urea type).15–17 The combination within the organic bridging component of aromatic or alkyl groups and urea groups allows better self-assembly through the capability of the organic moieties to establish both strong hydrogen bond networks and efficient packing via p–p or hydrophobic interactions.15–17 Route A3: Hydrothermal synthesis in polar solvents (water, formamide, etc.) in the presence of organic templates had given rise to numerous zeolites with an extensive number of applications in the domain of adsorbents or catalysts. More recently a new generation of crystalline microporous hybrid solids have been discovered by several groups (Yaghi,18 Ferey,19–23 Cheetham and Rao24). These hybrid materials exhibit very high surface areas (from 1000 to 4500 m2 g21) and present hydrogen uptakes of about 3.8 wt% at 77 K.18–24 Moreover, some of these new hybrids can also present magnetic or electronic properties.20,25 These hybrid MOF (Metal Organic Frameworks) are very promising candidates for catalytic and gas adsorption based applications.18
Path B corresponds to the assembling (route B1) or the dispersion (route B2) of well-defined nanobuilding blocks (NBB) which consists of perfectly calibrated preformed objects that keep their integrity in the final material.7 This is a suitable method to reach a better definition of the inorganic component. These NBB can be clusters, organically pre- or post- functionalized nanoparticles (metallic oxides, metals, chalcogenides, etc.), nano-core–shells26 or layered compounds (clays, layered double hydroxides, lamellar phosphates, oxides or chalcogenides) able to intercalate organic components.27–29 These NBB can be capped with polymerizable ligands or connected through organic spacers, like telechelic molecules or polymers, or functional dendrimers (Fig. 3). The use of highly pre-condensed species presents several advantages: N they exhibit a lower reactivity towards hydrolysis or attack of nucleophilic moieties than metal alkoxides; N the nanobuilding components are nanometric, monodispersed, and with better defined structures, which facilitates the characterization of the final materials. The variety found in the nanobuilding blocks (nature, structure, and functionality) and links allows one to build an amazing range of different architectures and organic–inorganic interfaces, associated with different assembling strategies. Moreover, the step-by-step preparation of these materials usually allows for high control over their semi-local structure. One important set of the NNB based hybrid materials that are already on the market are those resulting from the intercalation, swelling, and exfoliation of nanoclays by organic polymers. Their applications will be described in section IV.
Path C Self assembling procedures. In the last ten years, a new field has been explored, which corresponds to the organization or the texturation of growing inorganic or hybrid networks, templated growth by organic surfactants (Fig. 3,Route C1).30–36 The success of this strategy is also clearly related to the ability that materials scientists have to control and tune hybrid interfaces. In this field, hybrid organic– inorganic phases are very interesting due to the versatility they demonstrate in the building of a whole continuous range of nanocomposites, from ordered dispersions of inorganic bricks in a hybrid matrix to highly controlled nanosegregation of organic polymers within inorganic matrices. In the latter case, one of the most striking examples is the synthesis of mesostructured hybrid networks.33 A recent strategy developed by several groups consists of the templated growth (with surfactants) of mesoporous hybrids by using bridged silsesquioxanes as precursors (Fig. 3, Route C2). This approach yields a new class of periodically organised mesoporous hybrid silicas with organic functionality within the walls. These nanoporous materials present a high degree of order and their mesoporosity is available for further organic functionalisation through surface grafting reactions.37
Route C3 corresponds to the combination of self-assembly and NBB approaches.7 Strategies combining the nanobuilding block approach with the use of organic templates that selfassemble and allow one to control the assembling step are also appearing (Fig. 3). This combination between the ''nanobuilding block approach'' and ''templated assembling'' will have paramount importance in exploring the theme of ''synthesis with construction''. Indeed, they exhibit a large variety of interfaces between the organic and the inorganic components (covalent bonding, complexation, electrostatic interactions, etc.). These NBB with tunable functionalities can, through molecular recognition processes, permit the development of a new vectorial chemistry.38,39
Path D Integrative synthesis (lower part of Fig. 3). The strategies reported above mainly offer the controlled design and assembling of hybrid materials in the 1 A° to 500 A° range. Recently, micro-molding methods have been developed, in which the use of controlled phase separation phenomena, emulsion droplets, latex beads, bacterial threads, colloidal templates or organogelators leads to controlling the shapes of complex objects in the micron scale.30,34 The combination between these strategies and those above described along paths A, B, and C allow the construction of hierarchically organized materials in terms of structure and functions.30,34 These synthesis procedures are inspired by those observed in natural systems for some hundreds of millions of years. Learning the ''savoir faire'' of hybrid living systems and organisms from understanding their rules and transcription modes could enable us to design and build ever more challenging and sophisticated novel hybrid materials.
Asignatura: CRF
Fuente: www.rsc.org/materials Journal of Materials Chemistry
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