Aerogels and their applications

Aerogels and their applications

12

Susan Montesa, Hajar Malekib a Chemistry and Physics of Materials, Paris–Lodron University Salzburg, Salzburg, Austria, b Institute of Inorganic Chemistry, Department of Chemistry, University of Cologne, Cologne, Germany

12.1

Introduction

Aerogels are a fascinating type of synthesized solid materials. Their appearance attracts attention due to the way light is dispersed through the material, giving them a translucent and smoked look similar to that of glass in the case of silica aerogels. Some such as aerogels prepared from (bio)polymers look like soft, thin foams while others such as carbon-based aerogels look like crunchy black bodies. They feel fragile and extremely light to the touch because they are formed by up to 90%–99% of gas or air. But what exactly are aerogels? According to the IUPAC terminology of 2014, aerogels are a gel composed of a microporous solid in which the dispersed phase is a gas [1]. However, reconsideration in term of aerogel has been later provided as even classical silica aerogels; pore size ley on mesopore (2–50 nm) regime. Microporous glasses as well as zeolites are included in this general definition; however, aerogel preparation is conducted through a mild wet chemical synthesis route instead of hydrothermal processes or the cooling of the melts with foaming agents. Aerogels are synthesized mainly by the sol-gel process. Their composition and nanostructuration are controlled by controlling in the chemical reaction parameter as well as by a series of processing steps to obtain a dry porous body in a monolithic form or granules. They are mainly amorphous materials instead of crystalline. The microstructure of aerogels is formed by a solid continuous network of primary and secondary colloidal particles connected to each other, either by condensation or cross-linking or by the formation and aggregation of fibrils due to the rearrangement of polymer chains or macromolecules (see Fig. 12.1). Regarding the gel processing, the extraction of the liquid in the cavities of the solid wet gel is carried out through techniques that allow the gel to dry with low to minimal structural deformations. However, it is not only their appearance that makes them attractive to scientists or technology research centers, but also their outrageous physical properties. One of the most interesting property is their low density. The typical densities of stable silica aerogels show values between 0.02 and 0.2 g cm
3. The low weight/volume ratio makes them good candidate materials for lightweight applications. As a result of their continuous solid phase open micro, and mesopores, aerogels show a very high specific surface area of around 300–1500 m2 g
1. The air or gas contained in the cavities of an Colloidal Metal Oxide Nanoparticles. https://doi.org/10.1016/B978-0-12-813357-6.00015-2 © 2020 Elsevier Inc. All rights reserved.

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Fig. 12.1 Pristine silica aerogel monolith with schematic structural build up.

aerogel makes them good intrinsic thermal insulators and wave or vibration absorbers. Their thermal conductivity can have typical values of 0.005 to 0.1 W m
1 K
1, comparable to that of expanded polystyrene foam EPS (λ ¼ 0.03–0.04 W m
1 K
1) or polyurethane (λ ¼ 0.02–0.03 W m
1 K
1) [2]. Their sound transmission is around 100 m s
1, which is characteristically found in soft polymeric foams. The carbon aerogels due to their large surface area, low electrical resistivity (<40 mΩ cm), a capacitance density of 100 F g
1 and 70 F cm
3 and controllable pore size distribution can be used as supercapacitors or deionization electrodes [3–5]. In Table 12.1, the main physical properties of silica-based aerogels are shown. Since the creation of aerogels by Kistler in 1931 [6], their large-scale applications have been limited. However, after Teichner’s work [8] based on the development of silica-based aerogels, these have become one of the most studied compositions. At the end of the 1980s, Pekala developed carbon aerogels from resorcinol-formaldehyde [9]. Since the early 2000s, because of the rapid development of processing techniques, new compositions, structural reinforcement strategies, drying methods and, in general, the sustainability of their synthesis, aerogels have become an emerging class of material with great potential of advanced technological applications. In Table 12.2, the main physical properties of aerogels, their characteristics, and some of the most important applications derived from each property are shown.

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Table 12.1 Main physical properties of SiO2 aerogels Typical value

Property

Range

Bulk density (kg m
3) Skeletal density (g cm
3) Porosity (%) Mean pore diameter (nm) Inner surface area (m2 g
1) Refractive index Thermal conductivity λ (in air 300 K) (W m
1 K
1) Young’s modulus E (Mpa) Sound velocity (m s
1) Acoustic impedance Z (kg m
2 s
1)

3–150 1.7–2.1 90–99.8 20–150 500–1500 1.007–1.24 0.014–0.021

1000 1.02 0.015

[6] [7] [8] [7,8] [8] [7] [9]

0.002–100 <20–800 10
4–10
5

1 100 10
4

[7] [7,10] [7,10]

200

Reference

N. H€ using, U. Schubert, Aerogels—airy materials: chemistry, structure, and properties, Angew. Chem. Int. Ed. 37 (1–2) (1998) 22–45.

Table 12.2 Properties, features, and applications of aerogels Properties

Features

Applications

Thermal

Building insulation materials Space vehicles and detection Casting molds Lightweight composite structures Catalysts, sorbers, sensors Fuel storage, ion exchange

Acoustic

Best insulating solid Transparent High temperature Lightweight Lightest synthetic solid Homogeneous High specific surface area Multiple compositions Low refractive index solid Transparent Multiple compositions Lowest sound speed

Mechanical

Elastic lightweight

Electrical

Lowest dielectric constant High dielectric strength High surface area

Density/porosity

Optical

Cherenkov detectors Lightweight optics Special effect optics Sound-absorption materials Impedance matchers for transducers Speakers Energy absorber, hypervelocity Particle trap Dielectrics Vacuum electrodes Supercapacitors

A.C. Pierre, A. Rigacci, SiO2 aerogels, in: M.A. Aegerter, N. Leventis, M.M. Koebel (Eds.), Aerogels Handbook, 2011, Springer, New York, NY, pp. 21–45.

Numerous recent studies have been focused on preparation of aerogels out of various biopolymers. Some have been used for centuries as biomaterials for sutures, for example, most recently silk fibroin aerogels were used as biomaterial for tissue engineering [10, 11]. In tissue engineering, for example, aerogels’ composition and

340

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microstructure can be adjusted in such a way to control the biodegradation, biomineralization, and cell attachment, as well as the possibility of loading drugs on their surface or even be used as biosensors [12]. The physical and chemical properties of an aerogel depend to a large extent on its composition and all stages in its production, from the preparation of the sol and gel transition to the drying method as well as the intermediate or postsynthesis steps. According to their composition, aerogels are generally divided into the following general categories. Inorganic aerogels: composed mainly of metal oxides such as silica, titania, alumina, zirconia, niobates, tantalates, mixed metal oxides, metals, metal chalcogenides, and so on [13]. Organic aerogels: formed by synthetic or natural organic compounds. Aerogels of organic phenolic polymers synthesized mainly from resorcinol-formaldehyde [9], melamine-formaldehyde [14], phenolic-furfural [15], polyurethane-dichloromethane [16], and cresol-formaldehyde [17], among others. Biopolymers cellulose, alginates, polysaccharides, i.e., cellulose, alginates, pectin, chitosan, and proteins, i.e., soybean, silk fibroin are examples of organic polymers that can also be processed into aerogels. Hybrid aerogels: mixtures of inorganic structures and organic monomer/polymers. The interaction between the different components is based on strong covalent links, interactions, or links between the blocks of the different chemical species. The interactions or bonds formed between different chemical species are carried out through functional groups contained in both types of components [18]. Carbon and carbide aerogels: pyrolyzed organic-based aerogels result in carbonbased aerogels. Carbon aerogels can be activated after their preparation [19] and modified on their surface or doped with metals [20]. Depending on the dopant metal and the pyrolysis temperature, carbothermal process can give rise to metal-doped monoliths with nanostructured composition or carbides [21]. Table 12.3 shows some of the characteristic physical properties of aerogels with different compositions. Aerogels, generally made up of microscale secondary colloidal polymer particles which is developed from small primary nanoparticles, have a structure similar to that of a pearl necklace, where the particles are connected to each other through necks (see Fig. 12.1). However, the microstructure of aerogels of the aforementioned categories Table 12.3 Main properties and typical values of aerogels depending on their composition Composition Silica aerogel Resorcinolformaldehyde aerogel Carbon aerogel Cellulose aerogel

Bulk density (g cm23)

Specific surface area (m2 g21)

Reference

0.003–0.5 0.005–0.3

600 100–1500

[7] [9]

0.05 0.1–0.35

100–1000 200–400

[5,40] [131]

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341

is strongly influenced by the composition, starting materials, reaction parameters, and drying strategies. In this chapter, we discuss the general method of aerogel preparation from the starting molecular precursor solution as well as the mechanisms involved in gel formation, aging, and drying, as well as postsynthesis treatments that are applied after the synthesis procedure such as surface modifications, pyrolysis, and hydrothermal reduction. We also further discuss the main aerogel compositions and their resulting properties. Later on, we present some examples of applications where aerogels are used as components to increase the efficiency in thermal energy, and/or used as media to purify the environment through filtration, adsorption, catalysis, or the capture of ions in gaseous or aqueous media as well as their applications in biomedical and life science fields.

12.2

Synthesis of aerogels

There is a great variety of aerogel compositions, but all have in common the main aspects of their synthesis and processing. In general, aerogels are prepared by a wet chemical synthesis approach. The first steps of the synthesis of aerogels are of great relevance because mainly through the sol-gel process will the solid phase be formed. The solid phase constitutes the skeleton of the aerogel and each parameter during the process such as the concentration and type of reagents and solvents, the temperature, and the pH, significantly influences the gel formation. Understanding the implications and effects that occur in each stage is necessary for the manufacture of aerogels under design. Aerogels are generally prepared by following the steps outlined schematically in Fig. 12.2. The synthesis of aerogels begins with the preparation of a mixture of precursors and solvents. When hydrolyzed, these precursors and solvents react with each other and form a colloidal solution of small polymeric particles and/or dissolved polymer clusters (see Fig. 12.2). These primary particles grow and agglomerate, forming larger secondary particles that bond with each other and form the solid threedimensional (3D) network of the gel (see Fig. 12.1). Once the solid 3D network has formed, there are still unreacted species in the liquid. These species react with the hydrolyzed surface of the gel and fortify the solid structure. This stage is known as aging (see Fig. 12.2). After the formation and fortifying the 3D network structure, rest of unreacted species and products are removed. The solid 3D structure, containing a large number of interconnected pores, allows the continuous transport of liquids. The washing of the gels as well as solvent exchange is a later step that will allow preparing the gel for the drying process but also purifying the gel from nonreacted product. Some chemical modifications on the surface are carried out, followed by the exchange of solvents. Commonly, precursor species containing functionalities are dissolved in a solvent. Then, through the infiltration and transport through all the pores and cavities of the gel, the functional molecules react with the gel surface (see Fig. 12.2). The drying step is one relevant process for preserving

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Colloidal Metal Oxide Nanoparticles

Fig. 12.2 General aerogel preparation steps.

the structure of the gel. There are different techniques with which the liquid can be extracted from the gel. Each results in dry gels with different degrees of structural deformations of the solid network. Once the aerogel is obtained, there is still the possibility of further modifications, whether through changes in the chemistry of the aerogel surface or changes in the composition of the entire aerogel (see Fig. 12.2). These changes are generally based on the modification of the surface of the aerogel by the infiltration of molecules with functional groups or doping elements by means of a gas phase. For the preparation of carbon aerogels, the pyrolysis of organic aerogels is carried out under controlled atmospheres and temperature gradients. Carbothermal reduction is also a postsynthesis modification for obtaining carbides. In the following section, each of the processes and the mechanisms involved in each stage for the preparation of aerogels will be explained in more detail.

12.2.1 Sol-gel process The formation of colloidal particles occurs when molecular precursors are hydrolyzed and condensation occurs between these species. Hydrolysis and condensation reactions are initiated from an aqueous solution of ionic precursors by changing the pH

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343

and/or the temperature. The concentration of initial molecular precursors will determine the density of the resulting aerogel. Reactions (12.1)–(12.3) describe the chemical reactions during the sol-gel processing of (semi) metal alkoxides. Hydrolysis  M
OR + H2 O Ð M
OH + R
OH

(12.1)

Water condensation  M
OH + HO
M  Ð  M
O
M  + H2 O

(12.2)

Alcohol condensation  M
OR + HO
M  Ð  M
O
M  + R
OH

(12.3)

The precursor molecules dispersed in an organic solvent are transformed into species that may experience condensation. In a first stage, the active molecular species are formed by hydrolysis. In a second step, the hydrolyzed species condense to form M–O–M (metal oxide M) units. In this way, primary oligomeric particles are formed that, at the same time, still contain hydrolyzable groups on the surface. These oligomers serve as cores for further polycondensation reactions and can grow to a certain size, depending on the experimental conditions or the agglomeration. The agglomeration of primary colloidal particles does not necessarily end in the formation of gels. The primary sol particles as they grow to sizes between 2 and 10 nm finally collide and agglomerate to form secondary particles or clusters. These clusters grow larger by repeated collisions or by the entanglement of polymer chains or macromolecules. The gelation describes the transformation of a liquid into a swollen solid. A gel is then composed of solid subunits connected by bonds surrounded by a liquid phase, usually water, alcohols, and residues of the reactions. When the collision between the sol particles is no longer possible, the viscosity increases strongly and the sol behaves like a soft solid. It goes from a liquid to a viscous liquid and finally an elastic solid with a shear modulus G greater than zero [23]. The transition of the gel is strongly influenced by the reaction parameters that control the rates of hydrolysis and condensation. The gelation can be reversible, but this will depend on the type of bonds involved. The time required from the formation of the sol to the point at which it becomes a viscoelastic solid is called the gelation time. The gelation time depends on the initial concentration of molecular precursors, solvent, pH, and temperature. The solvent or aqueous medium plays an important role in the preparation of aerogels; it serves to homogenize the precursors during the formation of colloidal particles and clusters and influences the formation of particles and networks due to their polarity and viscosity. Because the density of the final aerogels depends mostly on the concentration of precursors, the solvent is then also an important component of the mixture.

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Colloidal Metal Oxide Nanoparticles

12.2.2 Aging After gelation, the gel network still contains a continuous liquid phase or pore liquid with condensable particles or monomers. These unreacted species will further condense to the already formed gel network. The gel network at this point is flexible, allowing neighboring MdOH or MdOR groups to approach each other and undergo further condensation. This may cause the spontaneous contraction of the network and expulsion of pore liquid (see Fig. 12.3B). This phenomenon is called syneresis and continues as long as the gel network exhibits sufficient flexibility. The driving force is the reduction of the large solid-liquid interface between the gel formed and the water and solvent mixture surrounding the network. Mass from the solid network dissolves into thermodynamically unfavorable regions, and the solutes condense into thermodynamically more favorable regions (see Fig. 12.3A), particularly in pores, cracks, and particle necks (see Fig. 12.3B). This process results in the reduction of the net curvature (network coarsening) between colloidal particles, the disappearance of small particles, and the filling of small pores (see Fig. 12.3C). Aging and maturation of the network can increase the rigidity of the gels.

12.2.3 Drying A critical step during the preparation of aerogels is drying. This process involves removing the liquid contained in the pores of the gel without deforming the solid structure and thus obtaining a dry aerogel with a microstructure almost identical to that of

Fig. 12.3 Ripening at the aging process. (A) Dissolution of mass from thermodynamically unfavorable region to a more favorable region, followed by (B) neck growth, and (C) disappearance of network curvature.

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the wet gel. The large deformations during drying are mainly due to the fact that the gels contain mesopores, that is, pores with diameters between 2 and 50 nm. This small diameter of the pores generates a large solid-liquid interface, which in turn generates interfacial capillary forces capable of collapsing the microstructure (see Fig. 12.4A). Especially for small pores below 10 nm, the capillary pressure reaches values of around 4 to 20 Mpa; for example, at a 10 nm pore, the pressure gradient would be 109 MPa m
1. This causes the walls of the pores to pull on themselves. Once the pore walls are close to each other, they can chemically react with each other and form new bonds or OdMdO units (see Fig. 12.4B). The drying technology of aerogels consists of avoiding the collapse of the pores due to the forces derived from the solid-liquid interface. There are different routes through which the liquid contained in the pores can be extracted. As seen in the phase diagram of Fig. 12.5, the solid-liquid interface can be evaded if the liquid contained in the pores solidifies, is subsequently sublimated, and removed from the gel. This method is known as lyophilization. Another path that is considered one of the methods that allows conserving the original volume of the gel as well as the

Fig. 12.4 Schematic representations of (A) partially filled pore and acting forces from surface tension and (B) effect of the capillary tension in the contraction of pores in aerogels.

Fig. 12.5 Phase diagram of a pure compound and overview of the different pathways that follow each of the available aerogel drying techniques.

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Colloidal Metal Oxide Nanoparticles

microstructure is supercritical drying. In the phase diagram of Fig. 12.5, the critical point of a substance is indicated above which a supercritical fluid is obtained. Supercritical fluids can diffuse as gases. In this way, through supercritical drying, the liquid in the pores is replaced with a substance, commonly carbon dioxide, the temperature and pressure rise above the critical point, and then it is extracted by effusion. If we take a look again at the phase diagram of Fig. 12.5, drying at pressure and at room temperature confronts the forces generated by the solid-liquid interface. An acceptable drying with minimal deformations and the decrease of the initial volume of the gel can be achieved by generating repulsion forces on the pore surface in combination with the use of a highly volatile solvent (see Fig. 12.5) as a pore filler solvent.

12.2.3.1 Ambient drying Ambient drying is one of the last drying techniques investigated by Smith, Brinker, and Deshpande in 1991 [22]. Nowadays, it is used for the industrial production of silica aerogels by the Cabot Corporation with super hydrophobic properties. In principle, this process involves a series of solvent exchange processes and the chemical modification of the inner surface of the aerogel with nonpolar groups. Such a surface modification with the nonpolar groups on the pore walls prevents new condensation reactions during the pore contraction upon the capillary stresses. By changing the surface chemistry of the pore walls, the reactivity of the gel surface is reduced. On the other hand, if the solvent contained in the pores of the gel is exchanged with a hydrocarbon or a low surface tension solvent, the capillary stresses and therefore the deformation of the solid phase of the gel can be reduced. At the end of drying, due to the repulsion of the nonpolar neighboring groups, the dried gel expands to almost its original size by the so-called “spring-back effect” (see Fig. 12.6). This procedure has, however, some limiting factors. The solid network must be sufficiently stable and flexible to tolerate a reversible deformation of almost 25%.

Fig. 12.6 Introduction of polar groups on pore walls and the spring-back effect after pore liquid removal at ambient conditions.

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12.2.3.2 Freeze drying or lyophilization This method is a simple way to remove the solvent from a gel. By freezing the gel, the liquid solidifies and evaporates along the sublimation line in the phase diagram (see Fig. 12.5). Commercially available freeze dryers in different sizes and capacities make this technique suitable for simple production purposes. The gels dried through this method are called cryogels. The process is carried out in a series of steps with which it is possible to extract the liquid completely from the gel. First, the gel freezes at atmospheric pressure. Then, in an initial drying step, the liquid is removed by sublimation; in the second stage, it is eliminated by desorption. The lyophilization is carried out under vacuum. The conditions under which the process is carried out will determine the quality of the structure resulting from the cryogel. However, it should be remembered that when an aqueous product cools, crystal cores are formed. The surrounding liquid will turn around these nucleation sites, resulting in crystals of different sizes and shapes. The freezing speed, the composition of the liquid phase, the water content, and the viscosity of the liquid are decisive factors to determine the shape and size of the crystal formed from the liquid and the following sublimation process. The pores of the cryogels are deformed by the crystals formed by freezing the liquid [24]. The large crystals leave a relatively open structure in the aerogel after the sublimation of the liquid while the small ice crystals leave narrow spaces in the dry gel. Cryogels compared to aerogels obtained from the same gel have 20% less porosity and almost half the specific surface area. They are required for a long aging period to strengthen the solid structure of the gel. The exchange of solvent with a low coefficient of expansion and a high sublimation pressure will help to preserve the porosity and the specific surface area.

12.2.3.3 Supercritical drying This technique allows drying gels with minimal structural changes. This method consists of bringing the liquid contained in the pores of the wet gel above its supercritical state (i.e., above the critical temperature Tc and a critical pressure Pc) and thus extracting it from the porous solid without the effects of surface tension. When there are no capillary stresses from the solid-liquid interface, dry solids can be obtained with a solid microstructure almost without deformations. Carbon dioxide is commonly used as a solvent for dry aerogels by means of this technique. The supercritical pressure and temperature conditions of carbon dioxide are relatively low compared to those of other organic solvents (see Table 12.4). Carbon dioxide generally behaves like a gas in the air at standard temperature and pressure, STP. However, at low temperatures and high pressures, the gas liquefies and has a density similar to that of water. In a liquid state, it can replace miscible organic solvents, usually acetone or ethanol. Water has a low miscibility with liquid carbon dioxide, so the water contained in the gel is usually exchanged for organic solvents before drying. Nitrous oxide has physical properties similar to those of carbon dioxide; however, it is a powerful oxidant in its supercritical state. Supercritical water is inconvenient because its supercritical temperature Tc exceeds

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Colloidal Metal Oxide Nanoparticles

Table 12.4 Critical properties for some commonly used components used as supercritical fluids Solvent Methane (CH4) Ethylene (C2H4) Carbon dioxide (CO2) Ethane (C2H6) Nitrous oxide (N2O) Propylene (C3H6) Propane (C3H8) Acetone (C3H6O) Methanol (CH3OH) Ethanol (C2H5OH) Water (H2O)

Tc (°C)

Pc (bar)

Critical density (g cm3)

16.0 28.1 44.0


82.75 9.25 30.95

45.4 49.7 72.8

0.162 0.215 0.469

30.1 44.0

32.15 33.42

48.1 72.5

0.203 0.452

42.1

91.75

45.4

0.232

44.1 58.1

96.65 234.95

41.9 46.4

0.217 0.278

32.0

239.45

79.8

0.272

46.1

240.75

60.6

0.276

180.2

373.85

217.8

0.322

Molecular mass (g mol21)

R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, McGraw Hill Book Co., New York, NY, 1987.

300°C and is highly corrosive in this state [25]. This drying technique is carried out in a pressure chamber called an autoclave (see Fig. 12.7), in which the organic solvent of the gel is exchanged for liquid carbon dioxide. Once the organic solvent has been completely replaced by liquid carbon dioxide, the temperature and pressure rise above Tc and Pc and subsequently the carbon dioxide is extracted isothermally. Supercritical drying compared to ambient pressure drying and lyophilization is a relatively fast method. The low viscosity of the supercritical fluids and the high diffusivity allow the liquid to be extracted from the porous solids in a couple of hours. Fig. 12.8 shows schematically the main components of supercritical drying system. Supercritical fluids, mainly carbon dioxide, are used today in different applications such as cleaning, extraction, impregnation, etc. They are also used to dry delicate archaeological or biological samples for the electron microscope.

12.2.4 Postsynthesis modifications 12.2.4.1 Postsynthesis surface functionalization Materials prepared from wet gels generally have external and internal surfaces covered by final hydrolyzed groups (M-OH). These hydroxyl groups on the surface can be changed with other functional groups using hydrolyzable molecular precursors

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Fig. 12.7 Schematic representation of autoclave loaded with wet gels for supercritical CO2 drying. V, valve; P, pressure; T, temperature; F, flux.

Fig. 12.8 Schematic supercritical CO2 extraction system. V, valve; P, pressure; T, temperature; F, flux.

containing the desired functionality. Through this technique, organic or inorganic groups can be incorporated in a controlled manner as well as other nanoelements [26] called particles, nanotubes, nanofibers, etc. The resultant surface modified aerogels obtained are applicable in a wide range of application fields such as adsorption,

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Colloidal Metal Oxide Nanoparticles

catalysis, microelectronics, optoelectronics, thin film technology, protective coatings, biosensors of chemical sensors, nanomaterials, and bioactive surfaces, in addition to cell adhesion and adsorption of protein adsorption/drug release, among others [27–31]. This step in the manufacture of aerogels is contributed in add value and function to the final aerogel. Many of the applications of aerogels are mainly due to the properties derived from their surface. Functionalization of the surface can be achieved by physically mixing; however, this method is limited because the involved molecules can be washed off the surface. By means of a chemical reaction, more sophisticated and selective surfaces can be obtained. The main reaction involved is that of the reactive groups (most frequent hydroxyl) condensed with molecular precursors hydrolyzed. wRMðOR0 Þn + xMðOR0 Þm + yH2 O ! Ra MOb + zR0 OH

(12.4)

The variation of the functional groups R can result in combinations of materials with interesting characteristics [32]. Some examples of the R groups are the methyl, alkyl, aryl, hydroxyl, and amino groups. Fig. 12.9 shows the incorporation of organic groups onto hydrolyzed surfaces. The incorporation of methyl groups on the surface results in hydrophobic surfaces. This is a strategy used to produce aerogels by ambient drying. The modification of silica aerogels with amino groups can improve the mechanical properties of aerogels, such as the elasticity and strength of the structure [33, 34], as well as improving the capture of chemical products through this functionality. It is also possible to introduce

Fig. 12.9 Schematic surface modification possibilities.

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carboxyl groups by conversion of preexisting amino groups into carboxylic acid functionalities after the reaction with succinic anhydride [32, 35–37]. The posttreatment of aerogels is described by gas phase or by supercritical fluid seems to be an appropriate method to maintain the original structural properties of the aerogel. This is expected because during this type of functionalization, the dry aerogel is only in contact with the vapor. If no fluid accumulates in the pores, there will be no significant changes in the structure of the solid. However, the concentration of functional groups bound to the surface may be lower than that achieved by the functionalization of the gel in liquid solutions. This may be due to transport limitations in the gas-solid reaction that directly affect the effectiveness of the functionalization. In the case of functionalization in the liquid phase, more significant structural changes are expected. During this process, additional condensation and esterification reactions may occur. Therefore, the internal gel network changes in terms of pore size, specific surface, density, etc.

12.2.4.2 Carbonization Chemical modifications and changes in the composition of the entire solid structure are possible after drying. Carbonization is an additional step in aerogel processing in addition to those synthesis step explained in the previous section. It is used to produce carbon aerogels. This stage is based on the decomposition and conversion of synthetic organic compounds such as resorcinol-formaldehyde (RF), phenolfurfural (FF), phenol-resorcinol-formaldehyde (FRF), melamine-formaldehyde (MF), and polyurethanes as well as polyureas and polyvinyl chloride compositions to obtain aerogels with a high carbon content [5]. In the carbonization stage, the organic aerogel is transformed into an aerogel composed of carbon. For this, the temperature is increased above the pyrolysis temperature of the organic compound that forms the gel in a furnace, generally. The amount of heat applied controls the degree of carbonization and the content of residual elements. Carbonization is often exothermic. The process is carried out in an oven that begins with the replacement of air under a constant and moderate flow of inert gas such as nitrogen, argon, helium, or carbon dioxide, followed by an increase in temperature that can vary between 600°C and 2100°C [38–40]. The temperature of the pyrolysis can significantly change the final properties of the processed aerogels. For example, higher temperatures can reduce the surface area and therefore the double-layer capacitance. However, a lower pyrolysis temperature (e.g., 600°C) can increase the surface area and decrease the electrical conductivity. Carbon aerogels obtained by the resorcinolformaldehyde pyrolysis may not be electrically conductive unless they are carbonized above 750°C [41].

12.2.4.3 Carbothermal reduction This reaction involves the reduction of metal oxides such as silicon oxide using carbon as a reducing agent. This reaction is used to prepare newly investigated silicon carbide aerogels. Silicon carbide SiC has a high hardness, a good resistance to thermal shock,

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high thermal conductivity and stability, a low coefficient of thermal expansion, superior chemical inertness, and a large band gap [42–46]. This is a high-temperature postsynthesis process that transforms the chemical composition of the solid phase of the synthesized aerogel. This method applied to aerogels can improve their thermal stability, but the specific surface area is drastically reduced. The overall elementary reactions involved between C and SiO2 can be described by Reactions (12.5)–(12.10). The initial elementary process to form SiO gas SiO2 ðsÞ + 3C ðsÞ ! SiC ðsÞ + 2CO ðgÞ

(12.5)

SiO2 ðsÞ + C ðsÞ ! SiO ðgÞ + CO ðgÞ

(12.6)

SiC can be then produced through SiO ðgÞ + 2C ðsÞ ! SiC ðsÞ + CO ðgÞ

(12.7)

SiO ðgÞ + 3CO ðgÞ ! SiC ðsÞ + CO2

(12.8)

CO reacting with SiO2 can produce more SiO to keep the reaction running SiO2 ðsÞ + CO ðgÞ ! SiO + CO2

(12.9)

Then CO2 is consumed by the surrounding free carbon to form COCO2 g + C ðsÞ ! 3CO ðgÞ

12.3

(12.10)

Main aerogel compositions

12.3.1 Metal oxide-based aerogels Through the sol-gel process, wide range of the metal or semimetallic oxides gels can be produced. It starts from metal organic derivatives in alcoholic solution that, after being mixed, are subjected to hydrolysis followed by condensation polymerization that leads to the formation of a gel. By the sol-gel process, it is possible to obtain compositions that require very high melting temperatures, or chemical compositions for which very high cooling rates must be used to avoid crystallization. Almost all metals have a stable coordination; however, the reactivity of many metal alkoxides to water is so high that precipitates may form instantaneously [7, 47]. In the hydrolysis reaction with water, the hydroxyl OH is bound to the metal atom M by displacing, totally or partially, the alkoxyl OR ligand (see Reaction (12.11)). MðORÞn + mH2 O ! ðHOÞmMðORÞn
m + MROH

(12.11)

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One of the most important differences between metal oxides and silicate precursors is that the metal oxides are stronger Lewis acids; the nucleophilic attack is promoted in the center and the hydrolysis rates increase. The reactivity of some tetra isopropoxides (OiMR) in the hydrolysis reactions increases in the following order.




Si Oi PR 4 ≪ Sn Oi PR 4  Ti Oi PR 4 < Zr Oi PR 4 < Ce Oi PR 4 The hydrolysis of the metal alkoxides occurs by an addition/elimination mechanism according to Reactions (12.12) and (12.13) [7, 47]. H

H O +

M - OR

M - OR

O

H

H

ð12:12Þ

H O

HO - M

M - OR

+ ROH

R

H OR

O M

M M - OH +

M - OR

ð12:13Þ

H O M

+

ROH

M

In the first stage of synthesis, the reaction between the alkoxides forms a complexe, with which two metal alkoxides can be associated through coordinated bridges (μ). The degree of association usually increases with the size of the metal and with the tendency to form oligomers. When metal alkoxides are found pure or dissolved in nonpolar solvents, the expansion of coordination occurs by association through OR bridges. The association through alkoxide bridges is for the expansion of coordination number of central atoms in nonpolar solvents, for example, Zr(OnPr)4 forms oligomeric species through OH bridges in cyclohexane solutions. The reactivity of the metal alkoxides is promoted through pH changes and catalysts. In this way, the reaction rate is controlled to produce gels instead of precipitates. It is also possible to moderate the reactivity of the metal alkoxide by replacing one or more alkoxy ligands with groups that hydrolyze less easily, and additionally to block coordination sites in the metal. The most common ligands are carboxylate or β-diketone [47] (see Reaction (12.14) in Fig. 12.10).

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Colloidal Metal Oxide Nanoparticles

CH3 CH O

Pr i O

Pr i O 2Ti(O i Pr) + 2CH3COOH

Ti

O i O Pr

(12.14)

Ti

Pr i O

i

O Pr

O Pr i

O

O

CH CH3

Fig. 12.10 Molecular structure of the acetate derivative [Ti(OiPr)3(OAc)]2 [47].

12.3.1.1 Silicate-based aerogels Silica-based aerogels have been one of the most studied compositions for the synthesis of aerogels since their invention by Kistler. Their synthesis is based on the transformation of silanol SidOR and species that contain SidOH in siloxane compounds SidOdSi by condensation reactions. The main precursors are silicon alkoxides and sodium silicates. The reaction mechanisms of both precursors are the same, but there are some differences in the synthesis parameters. The synthesis of the sodium silicates is mainly stable only in strong alkaline conditions dissolved in water while for synthesis using silicon alkoxides as precursors, organic solvents are preferred. The gelation in the case of both precursors is initiated by pH changes, generating ^SidOH groups. Gels formed from sodium metasilicate or water glass (Na2SiO3) are prepared by reaction of this salt with an acid such as HCl in aqueous solution, described in Reaction (12.15). Na2 SiO3 + 2HCl + ðX
1ÞH2 O ! SiO2 xH2 O + 2NaCl

(12.15)

In Fig. 12.11 the mechanisms of neutralization and condensation of sodium silicate are shown. The process can be carried out in a single step or by a two-step reaction. First, the compound is hydrolyzed by ion exchange by acidification, as Na+ ions are replaced by H+. It is then neutralized by the addition of a base where the sodium silicate species is converted to silicic acid H2SiO3 [6] (see Reaction (12.16) and further condensation (see Reaction 12.17) [48]. O– Na+ O

O

Si

+ 2 H+ –

+

O Na

O

H + 2 Na+

Si O

H

ð12:16Þ

Aerogels and their applications

O O

355

O

H

Si

O

+ Na2SiO3 O

O–

Si O

O

H

Si

H

H

O–

O

+ H2SiO3 O

H O–

Si O

O

(12.17)

Si

+ H2O

H

O–

Fig. 12.11 Mechanism of hydrolysis, neutralization and condensation of sodium silicate [48].

12.3.1.2 Silica aerogels from silicon alkoxides In these types of systems, gels are obtained from the generation of ^SidOH from alkoxides ^SidOR through the addition of water. The sol-gel chemistry of tetraalkoxysilanes is described by global Reaction (12.18). SiðORÞ4 + 2H2 O ! SiO2 + 4ROH

(12.18)

As the hydrolysis and condensation reactions compete throughout the process, the systems are normally complex; many different polycondensation routes are possible. The chemical parameters determine which route is taken and, consequently, the final properties of the obtained gel. The main parameters that influence hydrolysis and condensation are the type of precursor, the type of catalyst and pH, the alkoxy group-to-water ratio, the type of solvent, the temperature, and the relative concentration of the components in the precursor mixtures. These parameters are discussed below. Precursor: The most common and investigated types of alkoxides are orthosilicates: tetramethyl orthosilicate (Si(OCH3)4 and tetraethylorthosilicate (Si(OH2CH3)4. It is also possible to reduce the four siloxane bonds in the network and form gels using precursors of the type R0 Si(OR)3 where R0 is an organic group. As mentioned in the section about metal oxide aerogels, replacing Si-O bonds by nonhydrolyzable or condensable groups (e.g., Si-C) provides some flexibility to the network because the molecular structure is not densely bound. At the same time, this imparts an organic character to the gel without the need for a postsynthesis treatment [49]. The inductive effects caused by the organic groups attached to a central silicon atom can stabilize or destabilize the transition states during hydrolysis and condensation. The density of electrons in the silicon atom decreases in the following order.

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Colloidal Metal Oxide Nanoparticles

 Si
R > Si
OR > Si
OH > Si
O
Si 

(12.19)

Catalyst and pH: At pH <7, hydrolysis is favored while condensation is the step that determines the rate of reaction. At this pH, the silica dissolves, and there is no aging process so the particles grow up to a size between 2 and 10 nm. Subsequently, these particles condense and aggregate into branches and form a network of polymeric character with small pores formed by the distance between the aggregated primary particles. Under acidic conditions, organically substituted precursors of the type R0 Si(OR)3 are more reactive than Si(OR)4. At a pH > 7, condensation is favored and hydrolysis is the step that determines the reaction rate. The hydrolyzed species are consumed by a faster condensation reaction and, due to the higher solubility of the silica in this pH range, the growth of primary particles is favored. Therefore, ideally, in this pH range, you can prepare a stable sol of equal-sized particles derived from a monomeric limited reaction group [49, 50] (see Fig. 12.12). Alkoxy group/H2O ratio: In the overall reaction under the sol-gel process of silicon tetraalkoxides, Rw ¼ 2 is needed to convert Si(OR)4 species to SiO2 while four equivalents of water (RW ¼ 1) are required for the complete hydrolysis of Si(OR)4 if condensation does not occur. The decrease in RW (the increase in water content) generally favors the formation of silanol groups on the SidOdSi groups. The solvent is important to homogenize the reaction mixtures, especially at the beginning of the reaction. Polar and particularly protic solvents such as water, alcohols, and formamides 1

Molecular precursors

Condensation nrel Hydrolysis

0 0

7

14 pH

(A)

Particle growth

Network growth

3DParticle network

(B)

Sol Acidic conditions

pH

Basic conditions

Fig. 12.12 (A) Dependence of the relative rates of hydrolysis and condensation reactions of Si(OR)4 on the pH [1], (B) structural development if silica gels [2].

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357

stabilize polar siliceous species such as [Si(OR)x(OH)y]n by means of hydrogen bonds. Nonpolar solvents are used for R0 Si(OR)3 or incompletely hydrolyzed systems [47]. The sol-gel product of trifunctional alkoxysilanes of the type R0 SiX3, where 0 R ¼ alkyl, alkene, aryl, or vinyl and X ¼ hydrolyzable groups, are commonly called silsesquioxanes (R0 SiO1.5). Silsesquioxane-based aerogels are usually flexible and have reduced hydrophobicity; they also exhibit an improvement in properties such as solubility, thermal stability, and thermomechanics. This type of precursor is also used in the preparation of hybrid compounds with emergent properties of nonhydrolyzable groups [51].

12.3.1.3 Mixed metal oxide/metal-doped aerogels Mixed oxide or metal-doped aerogels are attractive materials for relevant applications such as catalysis, photocatalysis, sorption media, biosensors, and electrochemical and optical applications [13]. Binary of multinary oxides, for example, ZrO2/SiO2, TiO2/ SiO2, TiO2/ZrO2, Al2O3/ZrO2, CoTiO2/TiO2, etc., where the final properties of the aerogels obtained from these mixtures are strongly influenced by the reaction parameters. The MaOx/MbOy ratio, the chemical homogeneity, the accessibility of the different metal/semimetal centers, the structural properties, and the drying process are the parameters that most influence the resulting properties of mixed oxide aerogels [52–58]. The binary SiO2/TiO2 aerogel can serve as a model for mixed oxide systems. The degree of homogeneity is often associated with the relative amount of SidOdTi bonds in the mixed oxide. Titanium can be dispersed within the framework of silica at the atomic level as an isomorphic substitution that results in tetrahedral coordinated metal centers (see Fig. 12.13) [59].

Fig. 12.13 Schematic representation of the structure of SiO2/TiO2 aerogels with a partially crystalline TiO2 phase [7].

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Colloidal Metal Oxide Nanoparticles

In this phase-separated system, the particles can be crystalline or amorphous depending on the synthesis. Starting from the solution in the sol-gel process, the typical precursors are metal alkoxides, in this case Ti(OR)4 for titania and Si(OR)4 for silica. One of the challenges of the mixed metal oxide sol-gel process in the presence of water is the different reaction rate of hydrolysis. This reaction typically occurs through a nucleophilic substitution mechanism and, therefore, depends on the partial charge, δ+ on the metal atom. The titanium atom, Ti4+ in Ti(OR)4, is a strong Lewis acid with a significant positive partial charge that imparts a much greater reactivity toward hydrolysis and the Si atoms in tetraalkoxysilanes. In 2001, Gash et al. [60] developed a new sol-gel route based on metal ion salts in the presence of epoxides as gelling agents to synthesize different transition metal aerogels and metal oxides [61]. The use of epoxies provides good control over the composition with respect to the M/ Si ratio, which allows the synthesis of gels as the main component. Ratke et al. [62] synthesized mixed oxides of ZrO2/SiO2 by applying the epoxy base method and using commercially available low-cost precursors such as water glass or metal salts. With respect to chemical homogeneity, when the sol prepared with silicon alkoxide is mixed with that of another metal alkoxide, the heterogeneous systems comprising SiOx(OH)y(OR)z species as well as the corresponding metal oxide particles can be afforded after drying. A wide variety of aerogels have been synthesized with nanoparticles of PbS, ZnS, BaTiO3, Au, Ag, and Pt [63–65]. This approach not only offers the production of gels from amorphous and crystalline nanoparticles simultaneously, but also provides gels with a unique combination of properties such as the photocatalytic behavior of anatase particles combined with the rich surface chemistry of silica. Even when chemical homogeneity cannot be adjusted to an atomic level, this cogelation approach opens fascinating possibilities for the creation of aerogels with unprecedented architectural complexity [66–69].

12.3.2 Carbon aerogels Carbon aerogels are prepared mainly from aerogels of organic polymers such as the resorcinol-formaldehyde RF system [9, 70], melamine-formaldehyde MF [71] phenolic-furfural FF [15], polyurethane-dichloromethane [16], cresol-formaldehyde CF [17], phloroglucinol-formaldehyde epoxy-amine [72], diamino-aromatic dianhydride, 1,3-dimethoxybenzene or 1,3,5-trimethoxybenzene-hydroxylated benzene derivatives—alkyl or—aryl aldehydes, dianhydrides—aromatic diamines or a combined aromatic and aliphatic diamine, and their main structural properties are shown in Table 12.5. The pyrolysis of organic polymers in an inert atmosphere forms carbon aerogels. The structures of carbon aerogels are formed from the agglomerate of uniform spherical carbon particles. These individual carbon particles contain interlaced meshes of microcrystalline platelets with micropores of about 0.6 nm [73]. The thermal treatment during the processing has an important influence on the increase of the structural order of this type of aerogel. Table 12.7 summarizes the main structural and physical properties of a typical carbon aerogel synthesized from the pyrolysis of aerogel. The chemistry of the solution, the sol-gel process parameters, the pyrolysis temperature, and the aerogel density affect the pore structures of carbon aerogels [74]. Hanzawa et al. [75] investigated the change in the pore structure of

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359

carbon aerogels at high temperatures up to 2800°C. The microporosity disappeared with an increase in temperature up to 2000°C. Mesoporosity still remained even after heat treatment up to 2800°C, although 50% of the volume of the mesopores was also lost due to the fusion of carbon particles. RF aerogels can be dried with supercritical CO2; however, other alternative techniques have also been studied, such as supercritical acetone CO2. Some other alternative techniques have been studied, such as supercritical acetone [76]. The exchange of acetone by CO2 by this procedure is no longer necessary, so the process is shortened in comparison with drying with supercritical CO2. This technique was suggested on a large scale of production for industrial purposes. However, as shown in Table 12.4, high temperatures and pressures are needed to dry using supercritical organic solvents. Drying with controlled air temperature, speed, and humidity has also been used to produce RF aerogels. When the synthesis conditions are adequate, it is possible to produce nanoporous resorcinol-formaldehyde aerogels by convection drying to remove the solvent [77]. The variation of the initial synthesis parameters also has a great influence on the final physical and structural properties. For example, Lee et al. [78] studied the effect of resorcinol (R) on the ratio of the catalyst (C) on volume contraction (R/C), BET surface area, and the electrochemical properties of developed ambient dried aerogels. In the R/C ratios of 50–2000, they were able to prepare air-dried aerogels with BET surfaces of 706 m2 g
1 and specific capacitances of 81 F g
1 (Table 12.5). Carbon aerogel applications have places as adsorbents [79], catalysts [80], electrodes for capacitive deionization of aqueous solution, ion exchange resins [81], electrochemical double layer capacitors and supercapacitors [82, 83], gas diffusion electrodes in proton exchange membrane PEM fuel cells [84, 85], and anodes in rechargeable lithium-ion batteries [86]. However, carbon aerogels are fragile, and after pyrolysis there is a substantial collapse of their structure that limits the applications mentioned above. Recently, some techniques have been developed to improve the mechanical behavior of this aerogel composition. In this sense, Salihovic et al. [87] developed reversibly compressible carbon aerogels from polystyrene spheres loaded with resorcinol, which are then transferred to the hollow elastic spheres after the carbonization process. These hollow spheres scattered within the particles of the network provide carbon aerogels in general with pseudoelasticity (Fig. 12.14).

Table 12.5 Typical structural data of carbon aerogels [7] Characteristic

Value

Skeletal density (g cm
3) Density (g cm
3) Specific surface area (m2 g
1) Pore size (nm) Particle morphology (nm) Appearance

2.06 0.05–0.80 600–800 <50 3–20, amorphous Black, opaque

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Colloidal Metal Oxide Nanoparticles

Fig. 12.14 (A) Scanning electron micrograph of flexible carbon aerogel with incorporate hollow spheres. (B) Transmission electron micrograph of a hollow sphere. M. Salihovic, et al., Carbon aerogels with improved flexibility by sphere templating, RSC Adv. 8 (48) (2018) 27326–27331. Published by The Royal Society of Chemistry.

12.3.2.1 Silicon carbide aerogels Silicon carbide aerogels are interesting aerogel compositions with applications in catalysis and high frequency electronics, photoelectric, antiradiation, membrane holders for hydrogen separation, and wave absorption devices. Typical properties of silicon carbide aerogels are shown in Table 12.6. It has also shown better physical, mechanical, and thermal properties compared to the single component of silica or carbon [88]. These types of aerogels are prepared by carbothermal reduction of silica and carbon, according to Reaction (12.20). SiO2 + 3C ! SiC + 2CO

(12.20)

Reaction (12.20) is a complex process that starts with the generation of SiO gas and CO at the SiO2/C interface [89]. SiC is primarily the result of the gas-solid reaction between SiO (g) and C (s) [90]. Carbothermal reduction (meaning using carbon as the reducing agent) of polymeric cross-linked silica aerogels, such as polyacrylonitrile PAN [91], or silica aerogels cross-linked with resorcinol-formaldehyde RF [92]

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361

Table 12.6 Typical properties of silicon carbide aerogels

RF/ SiO2 C/SiC SiC

ρ (g cm23)

Porosity (%)

Shrinkage (%)

Specific surface area (m2 g21)

0.110

92.6

0

513

0.331 0.262

88.7 91.8

51 51

412 328

Y. Kong, et al., Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation, Ceram. Int. 40 (6) (2014) 8265–8271.

Fig. 12.15 (A) SEM micrograph of SiC aerogel and (B) monolith of SiC aerogel. Reprinted with permission from Y. Kong, et al., Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation, Ceram. Int. 40 (6) (2014) 8265–8271.

can normally result in SiC compounds preserving their initial structural properties [93] (Table 12.6; Fig. 12.15).

12.3.2.2 Graphene aerogels Graphene aerogels have excellent electrical, thermal, and mechanical properties [94, 95], high resistance to deformation, high strength/strain to failure [96], a high specific surface area [97], and chemical stability [98]. They have found important applications in composites and energy storage [95, 99–103], sensors [104], catalysis [105], and biomedicine [106]. Graphene aerogels consist of a network of aggregate sheets of graphene exhibiting a relatively high mechanical strength [107]. Energy storage is one of the most relevant applications, being used as an electrode material for electrochemical energy sources and batteries [107, 108], supercapacitors [109, 110],

362

Colloidal Metal Oxide Nanoparticles

in waste removal [111, 112], and thermal conductivity applications [113–117]. They can also be used in air purification, as solar light photocatalysts [118], and as phase change materials for better heating-cooling responses [119]. The addition of nanoparticles or dopants extends their field of applications [120]. They are prepared from a dispersion of graphene oxide GO in aqueous media. In graphene oxide, the oxygen fractions that are present both in the basal planes and in the edges are able to react covalently with different compounds and thus produce new materials made to measure. Among these compounds, biopolymers can be covalently immobilized in GO sheets, which improves their biocompatibility without affecting their overall performance. It has been shown that GO can be reduced to graphene through a hydrothermal process or by appropriate reducing agents that produce (by self-assembly) a 3D structure with micropores [121]. The self-assembly process takes place due to the hydrophobic interactions and the π-π stacking between the reduced sheets of graphene aerogels.

12.3.3 Organic aerogels Organic aerogels can be further subdivided into subclasses such as biopolymeric, phenolic, protein based, and polyols. RF aerogels belong to the phenolic type of aerogels together with melamine-formaldehyde, cresol-formaldehyde, phenol-furfural polyurethanes, and polyimides.

12.3.3.1 Resorcinol-formaldehyde RF aerogels RF aerogels were first prepared in the late 1980s by Pekala [9]. Organic aerogels can be prepared by polycondensation catalyzed by resorcinol and formaldehyde using a sol-gel process followed by drying using supercritical carbon dioxide. The RF aerogel can be pyrolyzed after drying under an inert atmosphere to produce carbon aerogels. The organic RF aerogels show an extremely low thermal conductivity of up to 0.012 W km
1 in an average specific surface area up to 1500 m2 g
1, envelope densities between 0.05 and 0.3 g cm
3, with good mechanical strength compared to that of the silica aerogels [7, 15] (Table 12.7). The mechanism, structure, and properties of resorcinol-formaldehyde aerogels can be compared to that of silica. Resorcinol (see Fig. 12.16A) has a high electron density that increases the reactivity of these positions with an electrophile, such as a carbonyl group in formaldehyde (see Fig. 12.16B). The molecules undergo an electrophilic aromatic substitution that can be catalyzed by a base or an acid (Fig. 12.17). The derivatives react in a condensation reaction to form compounds with methylene bridges (Fig. 12.18A) or methylene ether (Fig. 12.18B). The reaction rate of the sol-gel process depends to a large extent on the pH value. The gelation time has a maximum value at the neutral pH; that is, the pH value of the solution without a catalyst. In the base-catalyzed approach, resorcinol is protonated, which leads to a greater capacity for electron donation and a greater reactivity for the substitution reaction. A basic environment generates many resorcinol anions, and this initiates the formation of many small particles. The addition reaction is followed by a condensation reaction,

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363

Table 12.7 Typical structural data and properties of organic aerogels [7, 19] Parameter

RF aerogel

MF aerogel

PF aerogel

Density (g cm
1) Surface area (m2 g
1) Pore diameters (nm) Particle morphology (nm) Application

0.03–0.06 350–900

0.10–0.80 875–1025

0.10–0.25 385

50

50

3–20

2–10

Precursor for carbon aerogels Dark red, transparent

Color

Irregular shape, 10 nm Precursor for carbon aerogels Dark brown

Colorless, transparent

RF, resorcinol formaldehyde; MF, melamine formaldehyde; PF, phenolic furfural.

Fig. 12.16 Structure of (A) resorcinol and (b) formaldehyde.

OH

O

HO

OH

+ H

H

OH

OH– / H+

OH

Fig. 12.17 Electrophilic aromatic substitution to hydroxymethyl resorcinol derivates.

(A) HO

H H O O

HO

OH

OH

OH OH

–H2O HO

(B)

H O

H O O

OH OH

Fig. 12.18 Condensation reaction to methylene (A) or methylene ether (B) bridged compounds.

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Colloidal Metal Oxide Nanoparticles

OH

HO CH2

O

CH2

OH

O OH

CH2

CH2 HO

O

CH2

CH2

OH

OH OH

OH

CH2 OH

CH2 OH O

HO CH2 OH

Fig. 12.19 Resol structure.

which is responsible for particle growth and is relatively slow in the basic environment, producing branched polymer groups. The interpenetration of these groups can be manipulated under appropriate reaction conditions. In the acid-catalyzed approach, the acid induces a greater electrophilicity of formaldehyde by protonation. After the substitution reaction, the hydroxymethyl resorcinol derivatives are protonated to a compound with water as the leaving group and can easily undergo a reaction with resorcinol and other derivatives. By this pathway, groups of linear or slightly branched polymers are produced. The synthesis of aerogels requires small ratios of resorcinol to formaldehyde (R/F < 1). By this, more than a single network condensation reaction can take place. The 3D network formed by this pathway is called Resol (see Fig. 12.19). In general, RF aerogels are fragile; however, it is possible to obtain flexible aerogels or even superflexible RF aerogels through a two-stage acid-base catalyzed route. Using an acid-base route to prepare aerogels, shrinkage after drying is nevertheless reduced by approximately 50% compared to pure base-catalyzed aerogels. The critical step in RF aerogels catalyzed by base acid is the time when the base conditions are changed to acidic conditions by adding a suitable acid with an adopted concentration [122]. Various other parameters, such as molar ratios, that is, resorcinol to formaldehyde R/F, resorcinol to water R/W, resorcinol to catalyst R/C, and reaction conditions, that is pH values, catalyst, and temperature, have a great influence on the sol-gel process of RF systems and, therefore, on the final properties of the resulting aerogels as their microstructure and related physical properties. Milow et al. [123] performed several experiments to study the influence of the molar ratios of resorcinol, formaldehyde, catalyst, and pH in the structure of RF. Effect of the molar ratio R/W: They defined the ranges where rigid, flexible, and corky RF aerogels are obtained (see Fig. 12.20C). Small R/W ratios lead

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365

Fig. 12.20 Micrograph showing morphological effect of R/W molar ratio at (A) 0.005, (B) 0.007, (D) 0.012, (C) dependence of R/W molar ratio on density and flexibility. Reproduced from M. Schwan, L. Ratke, Flexibilisation of resorcinol-formaldehyde aerogels, J. Mater. Chem. A 1 (43) (2013) 13462–13468 with permission from The Royal Society of Chemistry.

to nonflexible and compact dense aerogels (see Fig. 12.20A). The RF aerogels produced with mild R/W are flexible and have pores of approximately 5–20 μm (see Fig. 12.20B). Higher R/W ratios lead to corky aerogels (see Fig. 12.20D), which show a nanostructure with small pores of approximately 0.2–1 μm [124]. The ratio of resorcinol to catalyst R/C: Organic resorcinol with a large R/C ratio is usually hard and brittle with a compressive strength of about 100 kPa and an elasticity modulus in the range of 1–2 MPa. With increasing R/C ratio, the size of the spheres is larger and the number of spheres per volume unit is less. Fig. 12.21 shows two samples prepared under similar conditions. The particle radius is 6 μm for R/C ¼ 333 (see Fig. 12.21A) and around 2.5 μm for R/C ¼ 16.4. 18 (see Fig. 12.21B). The sizes of the particles compared to base catalyzed aerogels is much bigger [124].

12.3.3.2 Polysaccharide and peptide aerogels Biopolymeric aerogels have been in the center of research for just a decade. The typical aerogels developed are from cellulose, chitosan, alginates, starch, pectin, carrageenan, gelatin, soy, and recently developed silk fibroin [10, 49].

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Colloidal Metal Oxide Nanoparticles

Cellulose aerogels: Cellulose is the substance that makes up most of plant’s cell walls (see Fig. 12.22). The versatility of cellulose has been reevaluated as a useful structural and functional material. The development of techniques for the dissolution and regeneration of cellulose makes possible the preparation of aerogels. Cellulose is rigid and has a high content of crystalline domains. The structure of cellulose makes it suitable for chemical reactions. It is insoluble in common organic solvents, and has a basic monomeric unit called D-glucose (anhydroglucose unit AGU), which links successively through a β-configuration between carbon 1 and carbon 4 of adjacent units to form a long chain of glucans [126]. Chemically, it is a chain of 1–4 linked β-D-glucopyranose, meaning the β-D-glucopyranose ring in its chair conformation is connected via oxygen bridges to a linear chain and hydrogen bonds that stiffen the chain. Cellulose can be derived from a variety of sources such as woods, plants, and microbes. Characteristics of the cellulose isolated from Dendrocalamus strictus (DCS), Lantanacamara (LC), and Parthenium

Fig. 12.21 Micrograph showing the morphological effect of the ratio of resorcinol to catalyst of a mixture of (A) R/C ¼ 333, Dparticle  6 nm and (b) R/C ¼ 16.4 Dparticle  10 nm. Reproduced from M. Schwan, L. Ratke, Flexibilisation of resorcinol-formaldehyde aerogels, J. Mater. Chem. A 1 (43) (2013) 13462–13468 with permission from The Royal Society of Chemistry.

Cell wall

Fibril

Plant cell Microfibril

Fig. 12.22 Fringe-fibril model of cellulose [125].

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hysterophorus (PH) are shown in Table 12.8. It can be extracted from cotton, soft and hard wood fibers, bast fibers such as jute and hemp, grasses such as baboo, algae (Valonica ventricosa), and bacteria (Acetobacter xylinum) [127]. However these materials also contain hemicelluloses and lignin. Cellulose fibers in the cell walls together with hemicellulose and lignin are responsible for the extremely good mechanical properties of this three-phase composite, especially allowing trees to grow to huge sizes [128]. When the cellulose molecule is fully extended, it takes the form of a flat ribbon with hydroxyl groups protruding laterally and capable of forming both inter- and intramolecular hydrogen bonds (see Fig. 12.23). Intramolecular hydrogen bonding enhances the linear integrity of the polymer chain and affects the reactivity of the hydroxyl groups. The surface of ribbons consists mainly of hydrogen atoms linked directly to carbon and is therefore hydrophobic. These two features of cellulose are responsible for its supramolecular structure and this determines many of its chemical and physical properties. [130] (Table 12.8). The dissolution of cellulose takes place by polymer swelling, often by water, sodium hydroxide, and mixtures of water with urea. During swelling, the solvent molecules penetrate and labilize the structure of cellulose. Cellulose has a content of 40%–60% amorphous and crystalline regions. The amorphous regions are important for swelling as well as for dissolution. The production of cellulose fibers starts with separation of the cellulose from lignin and hemicellulose. This can be done chemically to produce a viscous liquid. Because cellulose is a chemically stable material, the production of aerogels from cellulose needs a technology or processing route to disintegrate the cellulose into elementary fibrils or even down to the polymeric chain level without degradation or derivatization and then to rebuild them into a suitable low-density open porous gel that can be dried to obtain a 3D structure [128]. Several methods are described in the literature to prepare cellulose aerogels; many methods are newly developed. Table 12.8 Characteristics of the cellulose isolated from Dendrocalamus strictus (DCS), Lantanacamara (LC), and Parthenium hysterophorus (PH) Value (%) Composition of cellulose

DCS

LC

PH

α-Cellulose β-Cellulose g-Cellulose Lignin Ash Ave. DP

90.09 3.9 5 0.44 0.56 816

94.80 2.5 1.42 0.80 0.48 430

90.82 3.2 1.2 4 0.98 661.5

Cellulose isolated from: DCS, Dendrocalamus strictus; LC, Lantana camara; PH, Parthenium hysterophorus; DP, degree of polymerization. V.K. Varshney, S. Naithani, Chemical functionalization of cellulose derived from nonconventional sources, in: S. Kalia, B. Kaith, I. Kaur (Eds.), Cellulose Fibers: Bio- and Nano-Polymer Composites, Springer, Berlin, Heidelberg, 2011.

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H

H O HO

O

H O

OH

O

HO O

O

O O H

OH

O

O

HO

O HO

O H

OH

O H

H O

OH

HO

OH

O

O HO

O

OH

O O

HO

O O O H

HO

O

O

OH O H

Fig. 12.23 Scheme of the hydrogen bonds in cellulose [129].

The mostly used dissolving agents for the preparation of cellulose aerogels are strong bases such as NaOH or LiOH or using additives such as urea and thiourea and surfactants at low temperatures and salt hydrate melts like Ca(SCN)24H2O. Further chemical modification into either is possible, such as hydroxypropylation cyanoethylation or carboxymethylation [128]. The first cellulose aerogels that became well known and popular were prepared by Tan and coworkers [131]. Jin and coworkers [132] developed a technique to produce high-quality cellulose aerogels. Their technique avoids the utilization of toxic isocyanates and allows using lower amounts of cellulose, in contrast to the method of Tan. Their technique is based on semicrystalline raw cellulose whose morphology can be well described by a mixture or regions with a highly crystalline order and unordered intermediate areas connecting the ordered ones [132]. Alginate aerogels: Alginic acid is, after cellulose, one of the most common polysaccharides in nature. It consists of β-D mannuronic acid and α-L guluronic acid, which are linked via a 1-4 glucosidic bond (see Fig. 12.24). A rich source of alginic acid is brown algae, which are harvested dried and then extracted. The product is usually in an alkali salt such as sodium alginate, which is in the water-soluble form typically provided as a powder. Water-soluble alkali salts from alginic acid become insoluble in contact with solutions containing divalent or multivalent cations. Gelation can be pictured with the “egg-box” model where carboxyl and hydroxyl groups of four guluronic monomers bind one divalent calcium cation (see Fig. 12.25). The effect causes immobilization of the polysaccharide chains; as the viscosity of the alginate solution increases, a mechanically stable gel is formed. The properties of alginate hydrogels strongly depend on the ratio of guluronic to mannuronic acid G/M; higher ratios result in higher mechanical stiffness [133]. Alginate hydrogels are used as immobilizing agents for drugs [134], food additives, and as structures for biocatalysts [135]. They can be shaped into thin films, 3D hydrogel nanoparticles via emulsion gelation, and millimeter-sized beads. The preparation of

Aerogels and their applications

OH O

O HO O

OH

369

O O n

HO

OH OH O

O

Fig. 12.24 Alginic acid consists of 1-4 glucosidic linked consists of β-D mannuronic acid and α-L-guluronic acid.

m

Fig. 12.25 Representation of “egg-box” model binding of guluronic acid blocks to calcium ions.

alginate gels starts by aqueous solutions of the precursors in water. Gelation of alginate takes place by inducing the cross-linking of the alginate polymers with divalent cations (Ca2+ usually) following the “egg-box” model mechanism [136, 137] (see Fig. 12.25). The properties of the alginate gel are mainly influenced by the divalent cation content and the G/M ratio and sequence of addition [133]. Addition of the cross-linking ion can be carried out by dropping the alginate solution into the cation source solution, known as the diffusion method [138], or by the controlled release of the cross-linking ion already dispersed as an inert source within the alginate solution [139]. Inorganic salts such as Ca(SCN)24H2O, CaCl2, CaCO3, CaSO4, NaCl, and KCl can also be added to promote ionic bonding [132]. For some polysaccharides, the choice of monovalent or divalent/multivalent cations will control the formation of either soluble salts or gels, respectively [140]. In chemical hydrogels, the cross-linking of polysaccharide chains is strengthened by covalent bond formation assisted by coupling agents or cross-linker promoters. Ethylene glycol diglycidyl ether, glutamic acid, sucrose, and glutaraldehyde are among the biocompatible chemical cross-linkers reported in the literature for hydrogels [141, 142]. Chitin and chitosan: Chitosan is the deacetylate form of chitin, which is normally extracted from the exoskeleton of crustaceans. Chitin is a linear aminopolysaccharide

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Colloidal Metal Oxide Nanoparticles

formed by N-acetyl glucosamine units connected through β-(1-4) linkages [143]. Chitin is sparsely soluble in conventional solvents due to its high crystallinity based on strong hydrogen bonds between chains, but it can be solubilized by concentrated formic acid, calcium chloride, dehydrate-saturated methanol, or lithium chloride [144]. Chitin gels can be formed either by simple aging or by diluting the resulting chitin solutions in a large excess of water or alcohol [145]. Chitin hydrogels can also be obtained from reacetylated chitosan in an aqueous alcoholic solution [146]. After exchanging the solvent to an alcohol, the chitin gels can then be dried using supercritical carbon dioxide to obtain chitin aerogels of high porosity, high surface area, and low density, depending on the chitin concentration and the alcohol solvent used in the original wet gel [144]. Gelling properties of chitosan are mainly influenced by the degree of deacetylation and the source of the chitin precursor [147]. Hydrogels of chitosan can be obtained by precipitation of an acidic chitosan solution in an alkaline solution [140]. After aging, the chitosan gels are washed with water until a neutral pH is reached [148] After a sequential water-to-ethanol solvent exchange, they can be dried using supercritical carbon dioxide with volume shrinkages of 60% [149]. The big amount of polar hydroxyl and amino groups in the backbone of chitosan favors the application of this material as a biosorbent for removal of inorganic ions, dyes, and heavy metals. These functional groups can favor the interaction through hydrogen bonding and dipol-dipol interactions with other polar molecules such as solvents, silica, or even other chitosan units [150]. Silk fibroin: (SF) is a fibrous protein-based polymer that is isolated from the Bombyx mori silkworm cocoon [18, 151]. This biopolymer is abundant in nature and can be extracted with relatively low cost. It has shown a very good biocompatibility and biodegradability and therefore is a material often used to fabricate sutures. However, other materials can be processed from SF such as sponges, microspheres, hydrogels, and aerogels [151]. The mechanical strength and toughness of silk fibers can be compared to Kevlar or common biopolymers such as collagen and poly-L-lactic acid PLA [152, 153]. This feature of silk fibroin attracts attention to fabricate bone scaffolds [11]. However, except for a very recent report of the groups of Mallepally et al. [10], who developed an SF-based aerogel through a CO2-assisted gelation technique, and Omenetto et al., who investigated biopolymer-based hierarchical constructs, silk fibroin aerogels have not been exploited [152, 154]. Silk fibroin consists of a light L chain and a heavy H chain polypeptide which linked together via a single disulfide bond at the C-terminus of the H-chain, forming an H-L complex. The H-L complex also binds glycoprotein P25 in a ratio of 6:1 via hydrophobic interactions to form an elementary micellar unit [155–157]. In silk fibroin, H-chains form discrete β-sheet crystallites that are responsible for its toughness and good mechanical properties [156, 158]. The β-sheet crystallites are semiamorphous. In terms of amino acid composition, the H-chain comprises primarily the three simplest amino acids, that is, glycine G ( 43%–46%), alanine A ( 25%–30%), and serine S ( 12%) so as tyrosine Y ( 5%) [156, 159, 160]. In vitro cell culture studies with human foreskin, fibroblast cells demonstrate the cytocompatibility of the silk fibroin aerogel scaffolds and the presence of cells within the aerogel scaffolds [10] (Fig. 12.26).

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Fig. 12.26 Representation of the hierarchical structure of raw silkworm filament along with its different conformations and structures. Reprinted with permission from H. Maleki, et al., Compressible, thermally insulating, and fire retardant aerogels through self-assembling silk fibroin biopolymers inside a silica structure–an approach towards 3D printing of aerogels, ACS Appl. Mater. Interfaces 10 (26) (2018) 22718–22730.

12.3.4 Hybrid aerogels Organic-inorganic hybrid materials are in general definition those materials whose structure is formed by both organic and inorganic moieties interacting with each other at the molecular scale. This may be achieved through weak interactions such as hydrogen bonding, van der Waals forces, electrostatic interactions, or by strong coordinative or covalent bonds [161]. The interest in acquiring this category of materials lies in the extending and bettering of the material properties without compromising the desired existing ones. Applied to aerogels, the incorporation of organic elements such as organic groups or organic polymers may improve the mechanical stability of the fine mesoporous structure by adding some flexibility to the matrix, or vice versa, the inorganic moiety may provide some rigidity to a soft polymeric aerogel. Other properties such as hydrophobicity can also be improved or tuned by the integration of functional organic groups into the inorganic matrix, changing the surface chemistry

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Fig. 12.27 Scheme of different types of organo-substituted alkoxysilanes from R0 Si(OR)3 (A) and from alkyl bridged bis-alkoxysilane Si(OR)3 R0 Si(OR)3 (B) with their resultant hybrid network structure [162].

of the prepared aerogel. A general route to integrate organic moieties is during sol-gel processing by creating mixtures of organic polymers with or without specific interactions with the inorganic moiety. Inserting organic molecules into gels without chemical bonding can be achieved by dissolving the molecules in the precursor solution. However, in this way, the probability that the organic molecules are washed out during further processing, such as washing of the filigree gel and supercritical drying, is relatively high. Successful mixtures can be achieved with organic polymers capable of forming hydrogen bonds or covalent bonding. In silicate systems, compounds of the type R0 Si(OR)3 can be employed where R is the functional organic group. Here, the functional organic group is bonded through a hydrolytically stable SidC link to the network. Fig. 12.27 presents a schematic illustration of the different possibilities to introduce an SidC bond into the inorganic silica network. This could be achieved either through using organosilane with monofunctionality such as R0 Si(OR)3 in the network (Fig. 12.27A) or through alkyl bridged bis-alkoxysilane Si(OR)3 R0 Si (OR)3 (Fig. 12.27B). In a later case, when the bridging moiety is a long aliphatic hydrocarbon chain, the network possesses an extra mechanical resiliency. It might be important to remark that this approach is mainly exclusive for siloxane-based materials because of the high stability of SidC bonds over other metal-carbon bonds. This kind of linkage is not as successful for other metal alkoxide systems due to the hydrolytic instability of MdC bonds. However, the organic groups can be introduced with bidentate ligands [7]. Stein et al. made an interesting review about silicates with uniform channel structures as well as some of their applications. They demonstrated how functional groups are placed selectively on the internal or external pore surfaces of mesoporous solids by grafting methods or by cocondensation under surfactant control [163]. Maleki et al. developed novel multifunctional

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polymethylsilsesquioxane-silk fibroin aerogel hybrids for environmental and thermal insulation applications through simultaneous processing of silk fibroin proteins with organically substituted alkoxysilanes via a successive sol-gel approach to produce homogeneous interpenetrated IPN polymethylsilsesquioxane PMSQ–SF hybrid aerogels with significantly improved mechanical properties [18].

12.4

Applications

The peculiar aforementioned physical properties derived from their microstructure and composition, as well as the possibility of controlling these properties by regulating the synthesis parameters during preparation, has introduced aerogels as highly efficient versatile materials that can be proper alternatives for those materials that are commonly used today. Some of their applications are, however, in the development phase, and their use on a large scale is still limited. However, there are a few commercial technologies based on aerogels that make use of their superinsulating properties and thus reduce energy losses (see Table 12.9). The capture and/or catalysis of gases or liquids can also be carried out by aerogel systems with an aim at capturing or neutralizing pollutants by controlling the composition of surface and structure. Systems based on aerogels have also been developed for the cleaning of water as chemical/ physical adsorbents and clarification of sea and brackish water by adsorption and capacitive deionization. Some compositions of aerogels have also been studied as advanced materials for life science and biomedical applications. In this context, aerogels can be designed from the molecular level to their macroscopic structure to be used, for example, as bone scaffolds in bone tissue engineering, absorbable implants for controlled release of drugs and substances, and as biological sensors. This chapter presents the most important applications of relevant aerogels in energy savings, environmental remediation, and tissue engineering.

12.4.1 Thermal insulation Characteristics of heat transfer within the aerogel: As the porous media have a composition in both solid and gas phases, therefore the conduction heat transfer mode could be subdivided into gaseous heat conduction and solid heat conduction (Fig. 12.28). The conduction of heat in the gaseous medium is caused by different mechanisms such as the collision between the gas molecules [165], the convective transport by means of the pores, the radiative transport of solid surfaces toward the gas contained in the pores, and the radiative transport through the solid network [166]. An example of aerogel-based facade windows for thermal insulation and day lighting is shown in Fig. 12.29. The mean pore diameter in an aerogel is typically c. 20–40 nm, and the mean free path of air under the standard temperature and pressure STP is about 70 nm. The small diameter in pores (i.e., micro- and mesopores) restricts the free circulation of air molecules and thus reduces the conduction of heat by the gaseous medium [43]. The different modes of heat transfer within the aerogel material interact with each other to form a coupled effect [167].

374

Table 12.9 Insulation branches where aerogel products have an economic relevance Relative economic potential

Installation and assembly cost

Operating costs

Offshore oil and gas

Smaller pipe diameter Low weight More pipes per installation round trip Fewer trips Simplification of the overall design Light construction Size reduction Lower materials/assembly costs Comparable to conventional insulation More elaborated due to lack of experience Smaller overall pipe diameter or exterior dimensions Easier installation Smaller overall pipe diameter or exterior dimensions Easier installation Significantly more complex than standard technology

Superior lifetime Improved degradation resistance

High

Smaller gross weight and resulting fuel savings or additional capacity

High

Reduction of heat/cooling energy and larger usable building space

Very high

Reduced area per unit length Lower radiation losses Improved resistance and lifetime Reduced sensitivity to cryoembrittlement Increased lifetime, energy, and building space

High

Energy savings increased thermal comfort for lightweight extreme performance clothes

Middle

Aeronautics/ aerospace

Building insulation

High-temperature insulation Cryogenic applications Appliances and apparel

M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-Gel Sci. Technol. 63 (3) (2012) 315–339.

Middle

Colloidal Metal Oxide Nanoparticles

Application

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375

Fig. 12.28 Lattice vibration phenomenon in the interface of adjacent particles on an aerogel [164].

The solid heat transfer depends mainly on the reticular vibration of the molecular solid around its equilibrium positions. In analogy with the concept of a photon in the radiative heat transfer, the minimum quantizing energy of the network vibration is called a phonon. Using the silica aerogel with secondary particle diameters between 2 and 10 nm as an example, the mean free path of the phonon is about 0.58 nm, and the characteristic length of the interparticle network of the aerogel follows in the same order of the free path of the phonon [168, 169]. Due to the small size of network particles, solid heat transfer and thermal conductivity are significantly reduced [170, 171]. The nanoscale solid skeleton could reduce the solid heat transfer due to the size effect; at the same time, the complex structure of aerogel could also decrease solid heat transfer. In general, the superinsulation products based on aerogels can be categorized into the following three classes that differ in the way in which the aerogel is incorporated into a material, system, or component [166]. Table 12.9 names insulation branches where aerogels have found applications as thermal insulation materials. l

l

l

Monolithic aerogels: large (cm), homogeneous blocks of aerogel. Granules or powders: finely fragmented aerogels with typical diameters below 1 cm for granules and 1 mm for powders. Composite materials: homogeneous or heterogeneous aerogel phases with at least one additive incorporated into the gel matrix (e.g., during synthesis) or added to the gel as a second phase, such as fibers and blankets or also by a subsequent surface modification.

Silica aerogels as thermal insulation in buildings: Due to their superior physical and chemical characteristics, silica aerogels have shown very low thermal conductivity of about 0.013–0.02 W m
1 K
1 along with a translucent appearance [172]. Therefore, they are considered one of the most promising upper-insulating materials for construction in both residential and industrial buildings, ultralight structures, skyscrapers, pipes, electronic devices, and thermal clothing, to name only a few. The use of fossil fuels for the generation of electricity and heat contributes strongly to the continuous

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Colloidal Metal Oxide Nanoparticles

Fig. 12.29 Aerogel glazing integrated into the facade of the ZAE building in Wurzburg (ISOTEG. 1998–2001). Reprinted with permission from M. Reim, et al., Silica aerogel granulate material for thermal insulation and daylighting, Solar Energy 79 (2) (2005) 131–139.

increase of greenhouse gas emissions as well as the use of nonrenewable resources. Buildings with a deficient thermal insulation represent large energy consumption for the generation of heat or cooling systems. Baetens reported that 8.3 Gt of CO2 were emitted in 2005, representing more than 30% of greenhouse gas emissions in many developed countries [173]. Intensive efforts have been made especially to decrease this trend [174–176]. One alternative is to reduce energy consumption through the use of advanced insulation systems for buildings [177], such as aerogel in this regard. The market share of aerogels tripled to $83 million in 2008, and it was estimated that it would reach $646 million in 2013. In this sense, thermal insulation based on aerogels has become strategically important for the global insulation market of 29 G $ [173]. Thermal conductivity λ is an intrinsic property of a material’s ability to conduct heat. Superthermal insulation is a term used for materials with a thermal conductivity lower than 0.020 W m
1 K
1 [178]. Metals are known as good conductors, and their λ values vary from tens to hundreds of W m
1 K
1. Glass, sand, and minerals have

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thermal conductivities of less than 10 W m
1 K
1. Common thermal insulation materials such as fiberglass, expanded polystyrene, and extruded polystyrene have λ values in the range of 0.03–0.04 W m
1 K
1. High-performance foam insulating materials such as polyurethane and phenolic resins are considered transition materials between conventional and superinsulation materials [48]. The low thermal conductivity and optical transparency of aerogels allow their application in ceilings, window panes, and solar collector covers [179]. In addition to heat insulation, the acoustic properties of aerogels make them a good acoustic insulator. Moreover, the dimensional stability and the high-performance thermal insulation feature make it possible to use them for insulating facades. Further, inorganic silica-based aerogel is a noncombustible material and withstands heat up to 1200°C. Therefore, it can also be used inside buildings as a flame-retardant component (Table 12.10). Fig. 12.31 shows a granular window based on aerogel developed by ZAE Bayern (Germany). Granular silica aerogels were integrated into highly insulating translucent glass. To avoid the damage of the granules, which often occurred in previous glazing concepts and even caused the destruction of the glazing, the granules were sandwiched between a double sheet made of polymethylmethacrylate PMMA. The sheet was mounted between two low emissivity glass panels. They used krypton as a filler gas to optimize thermal insulation. This construction allowed achieving heat transfer coefficients of less than 0.4 W m
1 K
1. The optimized granular layers provided a high solar transmittance of 65% for thicknesses of 20 mm [180, 181]. The integration Table 12.10 Overview of thermal insulation materials sorted by their thermal conductivity range Insulation product

Chemical composition

λ (W m21 K21)

Mineral wool Glass wool Foam glass Expanded polystyrene EPS Extruded polystyrene XPS Phenolic resin Polyurethane foam Silica aerogels Organic aerogels Vacuum insulation panels Vacuum glazing VG

Inorganic oxides Silicon dioxide Silicon dioxide Polymer foam

0.034–0.045 0.031–0.043 0.038–0.050 0.029–0.055

Polymer foam

0.029–0.048

Polymer foam Polymer foam SiO2-based aerogel Aerogels derived from organic compounds Silica core sealed and evacuated in laminate foil Double glazing unit with evacuated space and pillars

0.021–0.025 0.020–0.029 0.012–0.020 0.012–0.020 0.003–0.011 0.003–0.008

M. Aegerter, N. Leventis, M.M. Koebel, Aerogels for superinsulation: a synoptic view, in: M.A. Aegerter, N. Leventis, M.M. Koebel (Eds.), Aerogels Handbook, Springer New York, New York, NY, 2011, pp. 607–633.

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of aerogels in window systems began to appear in the market in 2005 [182, 183]. Several aerogel glazing systems have been developed by different research groups. Crosssections of aerogel glazing systems based in granular silica aerogel between two glass planes developed by Reim et al. and Schultz, Jensen et al. are shown in Fig. 12.30. At the University of Perugia, Buratti and Moretti carried out several experimental works on aerogel-based window systems. They analyzed systems using monolithic and granular silica aerogel [184] and determined that, of the most promising materials for insulation, d is the monolithic aerogel due to the better transmittance of light (τ¼ ¼ 0.62) together with very low λ values (approximately 0.60 W m
1 K
1 in a double glazing under conditions of evacuation), a smaller thickness (14 mm), and high lightness. On the other hand, for granular systems, the reduction in light is about 60% compared to a two-pane glass system with low emissivity. The thermal conductivity of the granular system is low (>1 W m
1 K
1) with the same total thickness (Fig. 12.31). The results also indicated that innovative monolithic aerogel glazing systems allow obtaining thin windows with thermal conductivity values lower than 0.5 W m
1 K
1 without reducing the solar factor or significantly reducing the daylight transmittance [185].

Fig. 12.30 Cross-section through the aerogel glazing consisting of two glass panes. Reprinted with permission from M. Reim, et al., Silica aerogel granulate material for thermal insulation and daylighting, Solar Energy 79 (2) (2005) 131–139; J.M. Schultz, K.I. Jensen, Evacuated aerogel glazings, Vacuum 82 (7) (2008) 723–729.

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Fig. 12.31 (A) Monolithic aerogel and (B) granular aerogel glazing systems. Reprinted with permission from C. Buratti, E. Moretti, Glazing systems with silica aerogel for energy savings in buildings, Appl. Energy 98 (2012) 396–340.

Commercial materials based on aerogels: Several attempts have been made to develop new insulating products based on aerogels motivated by their low conductivity. In this regard, Spaceloft developed by Aspen Aerogels (Northborough, MA, United States) is a flexible aerogel in the form of a blanket with a thickness of 10 mm and a thermal conductivity of 0.013 W m
1 K
1 at 273 K is available. Its thermal conductivity is 2 to 2.5 times smaller than the thermal conductivity of traditional insulation materials. Although monolithic silica aerogels are very brittle, they can be incorporated into fiber matrices in the form of textile blankets. The product can be used to reduce thermal bridges in wooden or steel frames with a production cost of 25 € m
2 or 4000 $ m
3 (record of year 2008) [173]. Basogel, the BASF silica aerogel, is prepared in the form of pellets by means of spraying. The production of Basogel is a two-stage process from a low-cost sodium silicate (water glass) and sulfuric acid, where the produced aerogel is used as a separator and insulating material in windows and solar panels as well as industrial thermal insulation in heating and cooling areas [186]. Slentite is another commercially available high-performance polyurethane-based aerogel insulation panel from BASF. Another aerogel-based insulation material called Nanogel was developed by Cabot Aerogel (MA, United States) [173]. It is used for pipe insulation and its thermal conductivity is 0.014 W m
2 K
1. Nanogel could be activated after expanding to fill all gaps. These products have been developed using robust aerogel production techniques, using them as fillings or blankets. Larger silica aerogel blocks can also be prepared when their manufacturer has appropriate processing facilities for larger samples. The washing and aging treatments of silica gels are vital for developing robust aerogels because they significantly improve permeability, hydrophobicity, and the

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mechanical properties of the gels. However, an excessive washing of filigree gel can generate cracking in the gel body [183, 187–189]. Aerogel-based insulating window systems are still relatively expensive compared to insulated windows, that is, the cost of an aerogel window is six times greater than a conventional double-glazing window [172]. Yingde stated that the cost is almost £ 20 m
2 [179]. The cost is progressively decreasing. In fact, the global aerogels market from 2003 to 2008 tripled to $83 million, and was expected to reach up to $646 million in 2013. An aerogel plate with a thickness of 1 cm can be dried at a cost of US $ 2 m
2 [189]. Composite aerogels as flame retardant and insulation materials: Many of the flame retardants commonly used are halogenated or phosphorous compounds that seriously damage the environment and human health, especially the brominated flame retardants that generate furans and brominated neurotoxic dioxins and that are potentially carcinogenic. Chen and Wang made a comparison between different flame-retardant technologies used in China; they reported that the aerogel composed of the phenolformaldehyde resin PFR and silica prepared by copolymerization is a robust solid with a significantly low thermal conductivity, even lower than expanded polystyrene, with excellent resistance to fire (see Fig. 12.32). The PFR/SiO2 composite aerogel has a minimum thermal conductivity of 24 mW m
2 K
1, which is better than commercial expanded polystyrene EPS and inorganic glass wool. The PFR/SiO2 composite aerogel with a silica content of 70% can withstand approximately 1300°C without

Fig. 12.32 (A) Fire resistance test setup using a blow torch by extended heating at 1300°C, (B–E) at different time points. Reproduced with permission from Z.L. Yu, et al., Fire-retardant and thermally insulating phenolic-silica aerogels, Angew. Chem. Int. Ed. 57 (17) (2018) 4538–4542.

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disintegration and prevents the temperature on the unexposed side from increasing above 350°C, suggesting that the material could provide extended protection against the fire-induced collapse of reinforced concrete structures [190].

12.4.2 Removal of pollutants 12.4.2.1 Capture of pollutants in the gas phase The separation of components in the gas phase can be achieved by three main mechanisms: Filtration in the case of particle separation, gas sorption, and destruction of compounds by means of chemical reactions. Silica aerogels are suitable sorbent materials for this purpose as the pollutant particles in gas flows are captured by them when they pass through the aerogel due to the aerogel’s small pore sizes. The sorption method is used for molecular separation in gaseous phases and is achieved by aerogels with surface modification by appropriate functional groups. The compounds can be adsorbed on the surface and/or trapped in the pores of the aerogel and react chemically with the functional groups on the surface. Removal of solid particles in the gas phase: Filtration is a common method for the separation of gases (e.g., zeolites and molecular sieves). The silica aerogel can be used for the filtration of small particles due to the porous structure. There have been important studies to analyze the performance of aerogels in the removal of solid and oily particles from the gas phases compared with high-efficiency particle filters HEPA. The silica aerogel in the form of powder or granules of micrometric size has excellent filtration efficiency for particles with sizes <70 nm. Higher efficiencies are possible in the removal of aerosol oil particles at a speed of 2.3 cm s
1 [191]. The pore size of the aerogel can be controlled during the gel preparation and thus achieve a more selective filtration of particles. Hybrid compositions can be used to achieve a combination of meso- and macroporous structures [24]. Hybrid compositions usually possess a reinforced structure with sufficient mechanical strength to be used in systems with higher gas flow rates [192]. CO2 capture: The aerogels with amino functional groups on the surface of the solid are used to purify and separate CO2 gas followed by thermal desorption and recollection of the gas [193]. The amine functionality can be introduced into the surface of the aerogel by postsynthesis functionalization or by cocondensation during gel preparation. An alternative method is to use macromolecules that have a high content of amino groups. In the case of using silica-based aerogels as CO2 sorbent media, amino groups can be chemically linked into the network surface using an aminofunctionalized silane during the sol-gel reaction. The capture of the gas is based on the affinity of the molecular CO2 and selective reaction with the amino groups in the presence of humidity at a temperature of 100°C, as described by Reaction (12.21) [194]. CO2 adsorption and ammonium carbamate formation CO2 + RNH2 + RNH2 Ð RNHCOO
+ RNH3+

(12.21)

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Colloidal Metal Oxide Nanoparticles

Fig. 12.33 Schematic of di- and tri-amine used on the surface of silica aerogels for CO2 capture.

The advantage of physically absorbed amines to the aerogel surface is that they achieve a greater total CO2 absorption capacity as well as higher operating temperatures (useful for capturing combustion gas emissions) compared to the surface modified with chemical linkage, but they have little cyclic stability due to the evaporation of the amine during the thermal regeneration process. The covalently linked amines (e.g., mono-, di-, and trialkylaminotrimethoxysilane), on the other hand, have an excellent cyclic stability and, among them, the mono-, di-, and tri-amino moieties (Fig. 12.33) offer the best sorption/desorption kinetics because they leave more space for the gas exchange. In order to capture CO2 at a high scale applicable to industrial equipment, the capture systems need to have a low sorption of water as well as a good thermal regeneration capacity. The incorporation of silica aerogel granules with amine functionalities and a hydrophobic feature for CO2 capture systems is used based on fluidized-bed bench reactors developed by Begag and White [193]. Volatile organic compounds/catalysis: Organic compounds are easily adsorbed on the surface of silicone and silica-based aerogels largely due to the chemistry on the surface, which is rich in dOH groups. Applications in the capture of hydrocarbons are based on the adsorption of the compound and the destruction or degradation by catalysis. Sevhan has developed systems using silica aerogel in the form of particles with sizes from 20 to 60 μm mounted on polypropylene fibers for the capture of xylene. The composite material has obtained a maximum capture capacity of 3100 mg g
1 [195]. The capture of chlorinated hydrocarbons can be carried out by silica aerogels doped with Ti. The Yeung group has synthesized by impregnation and cohydrolysis titania-silica aerogels with high titanium content for the capture and photodegradation reaction of trichloroethylene [196].

12.4.2.2 Water treatment Removal of oil and other toxic organic contaminants from water is an important issue regarding environmental restoration [197, 198]. Large amounts of industrial and municipal oils are released to aquatic ecosystems without proper treatment, causing

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383

serious environmental problems and damage to the aquatic biota [199]. In this section, we discuss the use of aerogels in the removal of oils from water as well as the brackish water treatment through ionic capacitive desalinization. Oils in water: In principle, oils are divided into light hydrocarbons, heavy hydrocarbons, lubricants and cutting fluids, nonemulsified oils such as greases and emulsified oil, water-soluble oils, plants, and animal fat [199]. There are many existing techniques to separate oil and other toxic organic compounds from water. The main mechanisms involved are the direct capture through filtration by adsorption of the contaminants’ chemical and biological treatments. Current industrial sorbents have low elimination and selectivity and slow kinetics. Aerogels with hydrophobic and oleophilic surfaces offer the possibility of improving the absorption of pollutants [200–203]. Hydrophobic silica aerogels contain a high capacity for adsorption of nonsoluble organic compounds while hydrophilic silica aerogels appear to be more efficient for the adsorption of soluble organic compounds in water [204]. The Cabot Corporation developed a hydrophobic silica aerogel granule called Nanogel. The surface of the aerogel is treated with trimethylsilyl TMS groups, which gives hydrophobic character to the surface. This granulate is used to remove emulsion oils using a reverse fluidization-bed configuration. They discovered that the main factors influencing the Nanogel oil absorption capacity in the reverse fluidization-bed system are the size of the aerogel granules, the height of the bed, the water flow, and the oil-to-water ratio of the emulsion. The particles developed from Nanogel contained an oil absorption capacity of up to 2.8 times their weight [204]. Wastewater and brackish water treatment: The cleaning of sewage water as well as the treatment of brackish water near the coasts with an average salinity of 17 g L
1 in sodium and 0.2 g L
1 in chlorine is a major problem in terms of the progressive reduction of natural resources such as drinking water. Carbon aerogels due to their electrical conductivity and high capacitance have been analyzed and tested as electrode materials for capacitive deionization CDI systems. The desalination of seawater and brackish water has become the strategic water supply for many countries around the world, and for the Persian Gulf countries in particular. Desalination using CDI is an electricity-based desalination technique [205]. The CDI has demonstrated the ability to eliminate 85% of divalent ions [206]. In capacitive deionization systems, an electric potential is applied to the electrodes that produce poles with positive and negative charge. The salt molecules dissolved in water exist in the form of ions with positive and negative charges. The positive ions (cations) are attracted by the electrostatic force to the negative electrode while the negative ions (anions) are attracted by the positive electrode. In this way, the ions are separated from the water solution. The system is based on a cycle of alternating steps such as loading, purification (ion removal, Fig. 12.34A), and regeneration (ion discharge, Fig. 12.34B) to produce two streams of desalinated water and one of brine [207]. Most of the effort and focus in CDI research has been done on the development and synthesis of high-performance electrode materials. Numerous articles studied the various performances of different electrode materials with the abundance of carbonbased materials. The materials that have been reported up to now that are suitable

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Fig. 12.34 (A) Deionization and (B) regeneration step in CDI systems [207].

for the CDI system are activated carbons [208], alumina and silica nanocomposites [209], carbide-derived carbon CDCs [210], carbon aerogel [211], carbon nanotubes CNT, nanocarbon fibers CNF [212, 213], and mesoporous carbons [214]. The development of the aerogel-based CDI systems began in the early 1990s with the introduction of electrodes of activated carbon aerogel. Andelman was one of the first to use carbon aerogels on continuous flow condensers in 1990. One of the main advances in CDI came from Farmer’s pioneering work during the 1990s, when carbon aerogel electrodes in a flow condenser system were used [215]. His three consecutive patents served as a springboard for developing CDI-based systems [216]. The carbon aerogel has been the electrode of choice for CDI applications. The electrodes based on carbon exhibit superior electrical properties, a low electrical resistivity of 40 mΩ cm
1, a specific surface area of 400–1100 m2 g
1, and controllable pore size distribution of 50 nm [217]. The minimum desalination energy required for seawater at 35,000 ppm is approximately 1.09 kWh m
3 for a 50% regeneration while that of brackish water at 5000 ppm is 0.12 kWh m
3 [218]. In general, the minimum desalination energy required for the process increases with the concentration and the regeneration step [205].

12.4.3 Biomedical applications Recently, aerogels have become important in the biomedical field as (bio) materials that can be used for controlled and localized administration of drugs or proteins, tissue engineering, implantable medical devices, noninvasive imaging, bone grafting, and biosensing. All biomaterials before being used in a certain organ must be rigorously tested in order to detect adverse biological responses, such as allergies, inflammatory response, and coagulation/hemolysis as well as damage to cells, tissues, and DNA that can subsequently induce development of cancer cells. Some aerogel compositions have shown great biocompatibility through a series of in vitro and in vivo analyses. However, more developed systems are still required to be implemented in medical procedures and techniques. The large amount of research and studies conducted to improve their processing and formulation can make aerogels the next-generation material for specialized biomedical applications [49].

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12.4.3.1 Aerogels for the administration of medicines Aerogel as drug delivery systems (DDS) are based on the controlled release of medicines or substances. For this purpose, the composition of the aerogel should be able to contain the drug without major changes during its storage within the aerogel and then be able to release the drug in a controlled manner. In the case of cytotoxic drugs used for the treatment of some diseases, it is important that the drug arrives intact to the place where it is desired to be released, thus avoiding side effects and reducing the administration dose. Medicines poorly soluble in water have low performance in vivo, which leads to a low bioavailability of the drug. The controlled administration of this type of drug can be improved by using aerogels because their large specific surface area, designed porosity, and the wide variety of surface functional groups that can be incorporated into the aerogel render the effective incorporation of drugs [136, 219, 220]. The incorporation of drugs in aerogels can be carried out during the sol-gel process during the exchange of the solvents of the gels or by impregnation during supercritical carbon dioxide drying. The quality of loading will depend naturally on the affinity of adsorption to the surface of the gel, the stability of the drug in water, and the organic solvents or carbon dioxide used as media [134, 136, 219, 221, 222]. The gradient of drug release incorporated in the aerogel will depend on the composition and microstructure, for example, its rate of dissolution and reabsorption [135]. The aerogel for this purpose can be modified by the use of different methods, such as the formation of a gel with multiple layers or hybrid compositions [222–225]. Aerogels can also be endowed with magnetic properties for directed transport purposes, the subsequent destruction of malign cells or tissues through hyperthermia, or for the localized concentrated supply of drugs [226, 227]. The use of aerogels as drug carriers began by using silica aerogel as a carrier system due to the biocompatibility of silica as well as the extensive knowledge on their processing [7]. The release rate of the charged drugs on the silica aerogel is generally characterized by an immediate release of the drug due to the gradual disintegration of silica aerogel in different aqueous media [135, 136, 223, 228]. In general, the rate of drug release from the aerogel carrier increases significantly with the low solubility of silica aerogel in water compared to the dissolution gradient of the pure substance. Also, the stability of the drug as well as the release rate improve when the drug is loaded into silica aerogels, compared to the other polymeric vehicles [229]. The degradation of the silica aerogels into biological fluids can be adjusted by surface treatments [223]. An immediate or gradual release of the drug can be provided depending on the hydrophilic or hydrophobic nature of the gel surface. Silica aerogels loaded with drugs and coated with polymers can be applied for the systems requiring a controlled release of substances under a low pH media, such as for gastric or enteric administration [225]. In principle, the aerogels with bio-based compositions have been actively studied to be used for the controlled administration of drugs because they have advantages over the silica aerogels due to their biodegradation [136]. The most studied aerogels of biological composition for the controlled administration of drugs are those based on polysaccharides such as starch, alginate, pectin, chitosan, cellulose, proteins, and silk

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fibroin or hybrid compositions based on gelatin-silica, cellulose-polyethyleneimine, and chitosan-silica [10, 136, 229].

12.4.3.2 Tissue engineering The most relevant application of aerogels in tissue engineering is mainly focused on developing implantable porous materials for bone tissue repair [230, 231]. The aerogel structure full of pores imitates the natural porous structure in the cancellous bone tissue. The porosity in aerogels can promote cell binding and pore interconnectivity can provide the supply of nutrients and oxygen to cells and at the same time eliminate the metabolic byproducts of cells/tissues. An advantage of using an aerogel as the cellbased scaffold for this purpose is the opportunity to design its composition as well as its microstructure and internal architecture during the synthesis [136]. Generally, aerogels lack macroporosity, which plays an important role for the adhesion and proliferation of bone cells as well as the vascularization necessary for tissue growth. The mechanical properties of aerogels are usually not enough to withstand the mechanical stresses needed for large structural bone implants. However, numerous efforts have been made to develop mechanically reinforced aerogels sufficiently strong with strategically designed morphologies. The reinforced aerogels are commonly mixtures of biopolymers or inorganic fillers that increase not only the rigidity of the aerogel but also its ductility and at the same time its density [232]. The reinforcement of the aerogels can be carried out by soaking the wet gel in a solution or dispersion containing the reinforcing agent. The reinforcing agent, often a biopolymer, precipitates on the gel and can be subsequently dried [232, 233]. The resulting implants based on biopolymer-reinforced aerogels can favor cell adhesion and proliferation. Macroporosity can be achieved through the use of different strategies. Solid sacrificial templates or porogens with controlled size, usually with 100–600 μm, such as PMMA, paraffin, D-fructose, and NaCl salts, can be used to confer macroporosity to aerogel implants [234]. For this, the production of the gel is carried out in the presence of these porogenic agents, which serve as spacers or molds. Subsequently, when the solid structure of the gel has been fortified after aging, the porogen is removed or leached out from the material, leaving in its path macroscopic cavities in the structure of the gel. The elimination of the porogen is generally carried out by leaching [234]. An alternative method to induce macroporosity in gel body includes emulsion technique or soft templating approach that would be introduced during the preparation of the gel or after preparation of the through freeze casting or ice templating method. Once the solid structure of the gel has been aged and fortified, the template is removed before or after supercritical carbon dioxide drying [142, 230]. Using meso-/macroporous structures in combination with some mechanical reinforcement in the structure, when a biopolymer as a main component of the scaffold is used, is advantageous as not only the mechanical properties are improved, but it can also favor the biocompatibility and biodegradability [142, 234]. Biofunctionalization of aerogels is generally done using biopolymers that are biodegradable and have a composition similar to the components found in extracellular

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matrices, ECM, of tissues [235, 236]. The use of additives such as bioceramics or lignin can improve mechanical properties and osteoconductivity while increasing cell adhesion [142, 237]. Aerogel matrices have also been prepared from bioactive glasses through sintering silica aerogels with the addition of phosphate and calcium oxides [231] as well as the incorporation of these components by cross-linking, thermal treatments, and ultrasound or radiation [231].

12.4.3.3 Biosensing The possibility of hosting different compounds in the interconnected pores contained in the body of the aerogel makes them interesting as a base material for the design of biological sensors [238, 239]. Ayers used a silica aerogel with photoluminescent properties as an active element for the optical detection of gaseous molecular oxygen. The surface of the silica aerogel was treated to incorporate NH2 or H2 groups and thus obtain oxygendeficient silica SiOx on the inner surface. The oxygen-lacking sites acted as fluorophores to absorb light in the UV-visible range and emit visible light. The operation of this optical sensor was based on the collisional cooling of a fluorophore excited by molecular O2 [150]. Power reported a biosensor based on wetted silica aerogels used for the immobilization and growth of bacteria with a biological function for the detection and collection of viral aerosol particles. The silica matrix was treated with a water-soluble polymer to increase the diameter of the pores to the range of 10–100 μm, resulting in successful cell colonization. The bacteria can express the green fluorescence protein GFP when they are infected with a bacteriophage virus in the aerosol and then the fluorescent light emitted can be measured without disturbing the cells [240]. Li manufactured a silica aerogel using an ionic liquid as a porogenic agent during the preparation of the gel with the aim of generating larger pores and thus reducing the shrinkage of the aerogel during lyophilization. These cryogels were used for the molecular recognition of DNA with a specific sequence. Then, ATP5O (human ATP synthase) and CANX (Homo sapiens calnexin) were used as models for DNA recognition in a 3D aerogel biochip. The greater capacity for molecular recognition by aerogel compared to the traditional planner slide was attributed to the high internal pore volume of the aerogel and its large surface area. In the following year, the same group manufactured biochips based on silica aerogel functionalized with amino groups and a large specific surface area and porosity. These aerogels were used for the recognition of antigens of human interleukin–6 IL6 with an excellent capture capacity compared to two-dimensional biochips [241].

12.5

Conclusions

Aerogels are an interesting class of solid porous materials obtained by the sol-gel process or through the dissolution and regeneration of synthetic and natural polymers accompanied by an appropriate technique, mainly supercritical drying.

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The technological applications of aerogels are based on the exploitation of the most characteristic physical and structural properties that can be systematically tuned by their synthetic routes. The large specific surface area, high extent of porosity (90% air only 10% contained solid material), and very low density with only a few milligrams per cm
3 followed by a microstructure in the form of interconnected pores and channels make these materials very fascinating for most of their highperformance applications. The final composition of the aerogels as well as their microstructure depend strongly on the type of precursors used for their production, the chemical processes during gel formation, the process of obtaining them from the casted solution to the drying step, and posttreatments such as surface chemistry modification, carbonization, and carbothermal reduction (when carbon aerogel and other components such as SiC are involved). There are wide varieties of compositions/materials that can be processed into aerogels. Nowadays, these compositions are getting extended more and more so that it is not feasible to include all of them in this chapter. However, it is possible to say that aerogels play an important economic and technological role in recent and sustainable applications such as the thermal isolation of buildings and cooling/heating systems. Commercial products obtained using technologies based on aerogels are aimed at the reasonable use of energy resources and thus minimizing the ecological impact caused by humans during the generation of electricity and the exploitation of resources such as potable water for consumption. The capture and neutralization/elimination of compounds derived from industrial processes is possible and highly selective using capture systems based on aerogels. Applications based on the development of other technologies are, for example, the detection systems using aerogels with which it is possible to easily detect gases or organic compounds with great selectivity. Other important application areas of aerogels are in the biomedical field as highly porous biocompatible/degradable cell-based scaffolds in tissue regeneration, in the pharmaceutical field as drug delivery vehicles, and in the biosensing field as high throughput biological sensors. The use of aerogels for the production of synthetic tissues in which natural tissues can be regenerated is a fascinating topic in the field of nanomedicine and tissue engineering. The use of aerogels as a base material for the growth and proliferation of cell cultures of different cell lines makes possible a series of medical technologies such as an implant of regenerating synthetic tissues in living organisms. In Europe, the production of technologies based on aerogels is based on thermal insulation systems, mainly in the form of powders or granules mounted on fiber blankets to form sheets/panels. With 17 companies participating globally in the production of aerogels, the market estimated by Dr. Koebel for 2013 was 500 M $, and this production volume is estimated to reach to 1500 $ m
3 in 2020.

Acknowledgment Susan Montes acknowledges INTERREG Bayern–Austria 2014–2020 for financial support. Project AB29. Synthese, Charakterisierung und technologische Fertigungsans€atze f€ ur den Leichtbau “n2m” (nano-to-macro).

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Further reading [242] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, McGraw Hill Book Co., New York, NY, 1987. [243] V. Varshney, S. Naithani, Chemical functionalization of cellulose derived from nonconventional sources, in: Cellulose Fibers: Bio- and Nano-Polymer Composites, Springer, 2011, , pp. 43–60.