Nanoporous materials

Nanoporous materials

Nanoporous materials 7 Rajesh Mishra, Jiri Militky, Mohanapriya Venkataraman Department of Material Engineering, Faculty of Textile Engineering, Tec...

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Nanoporous materials

7

Rajesh Mishra, Jiri Militky, Mohanapriya Venkataraman Department of Material Engineering, Faculty of Textile Engineering, Technical University of Liberec, Liberec, Czech Republic

7.1 Introduction Porous materials are of scientific and technological importance because of the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales, enabling them to discriminate and interact with molecules and clusters. Interestingly the big deal about this class of materials is about the “nothingness” within—the pore space. The International Union of Pure and Applied Chemistry (IUPAC) classifies porous materials into three categories—micropores of <2 nm in diameter, mesopores between 2 and 50 nm, and macropores of >50 nm. In this chapter, nanoporous materials are defined as those porous materials with pore diameters <100 nm. Nanoporous materials are well known to be technologically useful for a wide spectrum of applications such as energy storage and conversion in fuel cells, solar cells, and Li-ion batteries; hydrogen storage and supercapacitors; catalysis; sorption applications; gas purification; separation technologies; drug delivery; cell biology; environmental remediation; water desalination, purification, and separation; sensors; and optical, electronic, and magnetic devices. Typical examples of natural and synthetic nanoporous solids are zeolites, activated carbon, metal-organic frameworks, covalent organic frameworks, ceramics, silicates, nonsiliceous materials, aerogels, pillared materials, various polymers, and i­norganic porous hybrid materials. However, the applicability of the porous nanomaterials depends on their targeted design at the atomic and molecular level, which controls their porosity and surface area. The nanoporous materials can be synthesized in the l­aboratories using organic or inorganic templates. The self-assembly of organic templates or the existing pore size of the inorganic templates controls the porosity of the final product. In addition, the nanometer-size pores can be utilized to impregnate nanoparticles/proteins/ ions to create multifunctional hybrids of practical and scientific interests. Considering the widespread applications of these nanoporous materials, uncovering their recent synthesis approaches, structure-dependent properties, and potential applications in various disciplines of science and engineering is necessary and urgent [1]. Nanoporous materials consist of a regular organic or inorganic framework supporting a regular, porous structure. The size of the pores is generally 100 nm or smaller. Most nanoporous materials can be classified as bulk materials or membranes. Activated carbon and zeolites are two examples of bulk nanoporous materials, while cell membranes can be thought of as nanoporous membranes. A porous medium or a porous material is a material containing pores (voids). The skeletal portion of the material is often called the “matrix” or “frame.” The pores are typically filled with a fluid (liquid or gas). There are many natural nanoporous materials, but Nanotechnology in Textiles. https://doi.org/10.1016/B978-0-08-102609-0.00007-9 Copyright © 2019 Elsevier Ltd. All rights reserved.

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artificial materials can also be manufactured. One method of doing so is to combine polymers with different melting points, so that upon heating, one polymer degrades. A nanoporous material with consistently sized pores has the property of letting only certain substances pass through while blocking others [1,2]. Over the last decade, there has been an ever-increasing interest and research effort in the synthesis, characterization, functionalization, molecular modeling, and design of nanoporous materials. The main challenges in research include the fundamental understanding of structure-property relations and tailor design of nanostructures for specific properties and applications. Research efforts in this field have been driven by the rapid growing emerging applications such as biosensor, drug delivery, gas separation, energy storage and fuel cell technology, nanocatalysis, and photonics. These applications offer exciting new opportunities for scientists to develop new strategies and techniques for the synthesis and applications of these materials [2]. Two-dimensional (2D) materials have received increasing attention in various fields such as physics, materials science, chemistry, and engineering. In particular, the graphene-based membrane is an emerging subject mainly due to the atomic thickness, simple processing, and compatibility with other materials. In the case of water treatment, stable graphene-based laminar structures and high separation performance are pursued. Nanoporous crystals embedded with graphene laminate membranes were reported for water purification. Reduced graphene oxide (rGO) nanosheets obtained from a solution chemical process act as building blocks to construct the 2D channels through a pressure-driven filtration process. By incorporating three-dimensional (3D) nanoporous crystals with subnanosized aperture size into the 2D graphene laminates, both the interlayer spacing and numbers of nanofluidic channels are increased, leading to greatly enhanced water separation performance. The optimized 3D/2D membranes exhibit 15 times higher water permeability than that of the rGO membrane with similar high dye retention rate. The significance of such 3D nanoporous structure and transport mechanism through the 3D/2D membranes is systematically studied. This general approach of enhancing the molecular transport through 2D nanofluidic channels proposed here may also find application in gas separation and battery membranes. The special features are shown in Fig. 7.1 [3]. The water purification performance of UiO-66-rGO membranes and PB-rGO membranes was finely regulated by varying the intercalation amount of these nanocrystals, while excessive embedding resulted in incompatible structure and destroyed the sieving effect of rGO membrane. It was demonstrated that high retention rate of organic dyes and greatly improved water permeability could be realized by nanoporous crystal intercalation. The proposed 3D nanoporous crystals enabling 2D graphene channel approach may be extended to other membrane fabrication and separation processes [3].

7.2 Nanoporous silica aerogel for thermal insulation Since the traditional thermal insulation materials can hardly meet the high demand in modern industrial, aerospace, and other fields, the development of the nanoporous aerogel insulation material becomes important. Because of its nanostructure, aerogel

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Fig. 7.1  Three-dimensional nanoporous membranes for water purification [3]. (A) Gas separation, (B) Battery membrane (2D channel), and (C) Battery membrane (3D channel).

has some special heat transfer phenomenon that makes heat transfer inside the material much more complex. Therefore, analyzing the heat transfer modes and the internal heat transfer mechanisms, constructing a suitable thermal conductivity model for aerogel material, and studying the influence of various influencing factors on the heat transfer performance of material have significant values in the performance prediction, optimization, and practical application for the silica aerogel composite insulation material. And this could also provide theoretical foundation for the further development of new heat insulation material with high temperature resistance, light weight, and high efficiency. A schematic of heat transfer through porous medium is shown in Fig. 7.2 [4]. Although there are a lot of literature that studied the heat transfer characteristics of nanoporous silica aerogel insulation materials, some issues still need to be investigated to better reveal the heat transfer mechanism of the material: (1) Apply suitable methods to investigate the nanoscale effect, interface effect, and coupled heat transfer effect on the aerogel material; (2) accurately calculate the nanoscale solid thermal conductivity and the total effective thermal conductivity of nanoporous aerogel material with complex particle aggregation structures; (3) study the characteristics of nanoscale radiative heat transfer and the impact of microstructure of the material on the nanoscale radiative heat transfer; (4) optimize the structure design and preparation of aerogel material on the guide of heat transfer mechanism of the material [4,5]. Aerogels are advanced materials almost like solid smoke; an aerogel resembles a hologram, appearing to be a projection rather than a solid object. They consist of >96% air. The remaining 4% is a wispy matrix of silicon dioxide. Aerogels, consequently, are

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Fig. 7.2  Modes of heat transfer through porous medium [4].

one of the lightest solids ever conceived. An aerogel is made by the so-called sol-gel process. During this process, organic compounds containing silica undergo a chemical reaction producing silicon oxide (SiO2). This mixture is a liquid at the creation of the reaction and becomes more and more viscous as the reaction proceeds. When the reaction is completed, the solution loses its fluidity, and the whole reacting mixture turns into a gel. This gel consists of a three-dimensional network of silicon oxide filled with the solvent. During the special drying procedure, the solvent is extracted from the gel body leaving the silicon oxide network filled with air. This product is called aerogel. Silica aerogels (SiO2) are highly porous, optically transparent solid materials. It is composed of individual particles only a few nanometers in size, which are linked in a three-dimensional structure. Aerogels can be synthesized not only from silicon oxide (silica aerogels) but also from different organic and inorganic substances, for example, titanium oxide, aluminum oxide, and carbon. This novel material has many unusual properties, such as a low thermal conductivity, refractive index, and sound speed, along with a high surface area and thermal stability. An aerogel can be made with a density only three times larger than that of air [5]. The nanostructure and the very high porosity in aerogels are also responsible for exceptional dielectric properties and electronic behavior. The dielectric properties of aerogels are dominated by the large volume fraction of trapped gas in the pores and the high concentration of adsorbed molecules on the abundant surfaces. This has been confirmed by measurements of the linear change of the dielectric properties with aerogel density and the large effect on these properties attributed to adsorbed water. The electric conductivity of aerogels is predictably low because the tenuous solid structure provides poor conduction paths and few charge carriers. The volume resistivity is expected to be high for the same reason. The dielectric strength of aerogels is also expected to be high due to the high volume resistivity and because the nanosized pores

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confine the charge carriers to spaces that are about the same size as the mean free path for collisions. These properties show that aerogels are unusual dielectric materials and suggest that they can be used for many interesting applications.

7.2.1 Types of aerogels The term aerogel does not refer to a particular substance, but rather to a geometry that a substance can take on—the same way a sculpture can be made out of clay, plastic, papier-mâché, etc., aerogels can be made of a wide variety of substances, including (1) (2) (3) (4) (5) (6) (7) (8) (9)

silica transition metal oxides—iron oxide lanthanide and actinide metal oxides—praseodymium oxide main group metal oxides—tin oxide organic polymers—resorcinol formaldehyde, phenol formaldehyde, polyacrylates, polystyrenes, polyurethanes, and epoxies biological polymers—gelatin, pectin, and agar-agar semiconductor nanostructures—cadmium selenide quantum dots carbon-carbon nanotubes metals—copper and gold

Aerogel composites, for example, aerogels reinforced with polymer coatings or aerogels embedded with magnetic nanoparticles, are also routinely prepared [6].

7.2.2 Commercial manufacturers and products As illustrated in Table 7.1, there are currently 16 major companies that are involved in the production of different types of aerogels. They either use aerogels for the production of their own products or distribute and supply to other industries. A list of manufacturers is given in Table 7.1.

7.2.3 Special properties of aerogels Many aerogels boast a combination of impressive material properties that no other materials possess simultaneously (Table 7.2). Specific formulations of aerogels hold records for the lowest bulk density of any known material (as low as 0.0011 g/cm3), the lowest mean free path of diffusion of any solid material, the highest specific surface area of any monolithic (nonpowder) material (up to 3200 m2/g), the lowest dielectric constant of any solid material, and the slowest speed of sound through any solid material. It is important to note that not all aerogels have record properties (in fact most don’t, although they may have very good values for many properties). By tailoring the production process, many of the properties of an aerogel can be adjusted. Bulk density is a good example of this, adjusted simply by making a more or less concentrated precursor gel. The thermal conductivity of an aerogel can also be adjusted this way, since thermal conductivity is related to density. Typically, aerogels exhibit bulk densities ranging from 0.5 to 0.01 g/cm3 and surface areas ranging from 100 to 1000 m2/g, depending of course on the composition of the aerogel and the density of the precursor

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Table 7.1  Global aerogel industry players by alphabetical order No.

Company

Annual revenuea

Base

1 2 3 4 5 6 7 8 9 10 11 12

Aerogel Composite, Inc. Airglass American Aerogel Corporation Aspen Aerogels, Inc. Cabot Corporation CDT Systems, Inc. CF Technologies Cooper Electronic Technologies, Inc. Dow Corning Corporation Honeywell International Inc. Marketech International Inc. Matsushita (Panasonic) Electric Works NanoPore Inc. Ocellus Technologies TAASI United Nuclear Scientific Equipment & Suppliers

$1–10 mil N/A $1–10 mil $3.8 mil $2.1 bil $32 K $750 K $17.5 bil $4.9 bil $34.6 bil $1.8 mil $12.9 bil

The United States Sweden The United States The United States The United States The United States The United States The United States The United States The United States The United States Japan

$1.8 mil N/A $750 K N/A

The United States The United States The United States The United States

13 14 15 16

a

Revenue figures may include income generated by other products and operations.

gel used to make the aerogel. Other properties such as transparency, color, mechanical strength, and susceptibility to water depend primarily on the composition of the aerogel. For example, silica aerogels, which are the most widely researched type of aerogel (and the type people typically see in photographs), are usually transparent with a characteristic blue cast due to Rayleigh scattering of the short wavelengths of light off of nanoparticles that make up the aerogel’s framework. Carbon aerogels, on the other hand, are totally opaque and black. Furthermore, iron oxide aerogels are just barely translucent and can be either rust-colored or yellow. As another example, low-density (<0.1 g/cm3) inorganic aerogels are both excellent thermal insulators and excellent dielectric materials (electric insulators), whereas most carbon aerogels are both good thermal insulators and electric conductors. Thus, it can be seen that by adjusting processing parameters and exploring new compositions, we can make materials with a versatile range of properties and abilities. Aerogels of all sorts hold records for different properties. Here are some records held by some specially formulated silica aerogels: ➢













Lowest density solid (0.0011 g/cm3). Lowest optical index of refraction (1.002). Lowest thermal conductivity (0.016 W/m/K). Lowest speed of sound through a material (70 m/s). Lowest dielectric constant from 3 to 40 GHz (1.008). Record held by a specially formulated carbon aerogel. Highest specific surface area for a monolithic material (3200 m/g).

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Table 7.2  Properties of aerogels Property

Value

Comments 3

Apparent density Internal surface area

0.003–0.35 g/cm 600–1000 m2/g

Percent solids Mean pore diameter

0.13%–15% ~20 nm

Primary particle diameter Index of refraction Thermal tolerance

2–5 nm

Coefficient of thermal expansion Poisson’s ratio

2.0–4.0 × 10−6

Young’s modulus

106–107 N/m2

Tensile strength Fracture toughness

16 kPa ~0.8 kPa m1/2

Dielectric constant

~1.1

Sound velocity through the medium

100 m/s

1.0–1.05 500°C

0.2

Most common density is ~0.1 g/cm3 As determined by nitrogen adsorption/ desorption Typically 5% (95% free space) As determined by nitrogen adsorption/ desorption (varies with density) Determined by electron microscopy Very low for a solid material Shrinkage begins slowly at 500°C, increases with inc. temperature. Melting point is >1200°C Determined using ultrasonic methods Independent of density. Similar to dense silica Very small (< 104×) compared with dense silica For density = 0.1 g/cm3 For density = 0.1 g/cm3. Determined by three-point bending For density = 0.1 g/cm3. Very low for a solid material For density = 0.07 g/cm3. One of the lowest velocities for a solid material

7.2.1 Thermal properties Aerogels with densities in the range 40–175 mg/cm3 were prepared using a tetraethyl orthosilicate (TEOS) ethanol-water solution as the precursor and hydrofluoric acid as the catalyst via a sol-gel process and CO2 supercritical fluid drying. The density gradient aerogels were prepared using layer-by-layer gelation, sol cogelation, and ­gradient-sol cogelation methods, and their gradient properties were studied systematically. A sample of granular silica aerogel with a particle size in the range from 1 to 2 mm and air as filling gas was used for the thermal measurements. It was first measured in an unloaded state, at 3.2 bar external load, and then once again in an unloaded state. After measuring the packed bed in a loaded state, the granulate does not return to its normal thickness, but stays more densely packed [7].

7.2.2 Transparent aerogel For thermal properties of aerogel, there are two different positions from which to optimize the performance. Aerogels can be transparent to a certain degree and thus be used as a light inlet with very low thermal conductivity compared with glass. At the same time,

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this will mean a relatively large conductivity from radiation. The aim could also be to optimize the thermal properties and thus try to minimize radiation through the material. Researchers have investigated the optical and thermal properties of aerogel windows. A transparent aerogel with a conductivity of 19 mW/(m K) was tested. Because of the brittleness of monolithic aerogel, the aerogel was fitted between two glass panes for protection. For common window glass, an 8 mm-thick aerogel layer had a transmittance of 0.6, which is similar to the transparency of a triple glass window. By exchanging the protective glass to one with lower iron content, the thickness could be increased to 14 mm keeping a transmittance of 0.6. For an aerogel layer of 6 mm the transmittance could be enhanced to 0.7, similar to a double glass window. Researchers found the 6 mm aerogel window to have a U-value of around 2 W/(m2 K) and a 14 mmthick aerogel window got a U-value close to 1 W/(m2 K). They reached a conductivity of 17 mW/(m K) for a transparent aerogel. The measurements were made by the hotstrip method. The heat source in the hot-strip method has a very low emissivity that would yield low heat transfer from radiation. In other words, the transparent aerogels might let out indoor heat radiating from interior surfaces. The conductivity in evacuated aerogel was measured with a guarded hot plate apparatus. Between the measurements, the surface in contact with the aerogel was exchanged between materials with varying emissivity. The measurements showed that the boundaries of the aerogel had an impact on the measured conductivity, which led to a formula for a pseudoconductivity that is dependent of the specimen thickness. For a 10 mm-thick evacuated aerogel tile in a low emissivity envelope, the apparent conductivity was measured to around 10 mW/(m K).

7.2.3 Thermal properties of pet nonwovens with aerogel The thermal properties of PET nonwoven thermal wraps of varying thicknesses treated with aerogel were compared. The SEM images were also taken to compare the physical structure of the aerogel-treated fabrics. Specific thermal properties like thermal conductivity, thermal resistance, thermal diffusivity, and thermal absorptivity were measured using Alambeta instrument. The air permeability of the thermal wraps was measured in air permeability tester. The relative water vapor permeability and absolute water vapor permeability were measured in Permetest. These tests were conducted to understand thermal properties and air and water vapor permeability of aerogel-treated nonwoven fabrics. The results of the experiments were statistically analyzed and found to be significant. The results were evaluated and studied for air permeability, thermal resistance, thermal conductivity, thermal diffusivity, and water vapor permeability. It was examined by one-way analysis of variance (ANOVA) with 95% confidence level. A significant difference (P < .05) has been observed in the thermal resistance, thermal conductivity, thickness, fabric weight, water vapor permeability, and air permeability properties for the three different thickness of fabrics treated with silica aerogel [8].

7.2.4 Air permeability Air permeability is the measure of air flow passed through a given area of a fabric. This parameter influences the thermal comfort properties of fabrics to a large extent.

Nanoporous materials319 Sample 1 Sample 2 Sample 3

3500

Flow rate (L/m2/s)

3000 2500 2000 1500 1000 500 0

200

400 600 Pressure range (Pa)

800

1000

Fig. 7.3  Flow rate versus pressure range [8].

It is generally accepted that the air permeability of a fabric depends on its air porosity, which in turn influences its openness. With more porosity, more permeable fabric is obtained. Statistical analysis results show that the there is a significance on the air permeability values of the aerogel-treated nonwoven fabrics (P = .000). Fig. 7.3 shows the air permeability with respect to different pressure levels of the fabrics. The result indicates that air permeability is directly proportional to the pressure level. On comparison of three fabrics, the air permeability is higher in the case of sample 1. It may be due to the fact that air permeability is related to porous structure of the fabric and is directly proportional to percentage of porosity of the fabric. It was also noticed that when the pressure level increased, the flow rate also increased. The air permeability was lower for samples with higher fabric thickness and may be attributed to the layered structure and high porosity [8].

7.2.5 Relative water vapor permeability The water vapor permeability (WVP) depends on the water vapor resistance, which indicates the amount of resistance against the transport of water through the fabric structure. To maintain the degree of comfort of the user, the amount of water present in a fabric must be minimum. Fig. 7.4 shows that water vapor permeability of the fabric has been decreased for sample 1 and increases for samples 2 and 3. Thus, the water vapor resistance of sample 2 was higher than samples 1 and 3. The decrease and increase in the water vapor permeability of the fabric may be attributed to the structure of the fabric and also the percentage of aerogel particles present in the fiber. This behavior can be explained by the moisture vapor transmission mechanism. When vapor transmits through a textile layer, two processes, namely, diffusion and sorption-desorption,

Nanotechnology in Textiles Relative water vapor permeability (%)

320

25 20 15 10 5 0

272.6

499.5

440.7

Fabric weight (g/m2)

Fig. 7.4  Relative water vapor permeability [8].

are involved. Water vapor diffuses through a textile structure in two ways, simple diffusion through the air spaces between the fibers and along the fiber itself.

7.2.6 Scanning electron microscopy (SEM) Scanning electron microscope (SEM) images were taken on microscopic scale for the cross-sectional area of the three fabrics with different magnification. The physical structure was confirmed to be different for three fabrics due to different thickness shown in Fig. 7.5. It was observed that sample 2 had higher fabric density as compared with other samples. The aerogel deposition on the fabric was also observed.

SEM MAG: 500 x HV: 20.0 kV VAC: HiVac

DET: BE Detector DATE: 03/26/13 Device: TS5130

100 um

SEM MAG: 1.00 kx DET: BE Detector DATE: 03/26/13 Vega ãTescan HV: 20.0 kV Device: TS5130 TU Liberec VAC: HiVac

Fig. 7.5  SEM images of nonwoven fabric treated with aerogel [8].

50 um

Vega ãTescan TU Liberec

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7.2.7 Fabric density Fabric density is the factor of weight and thickness. To obtain an indication of the effect of fabric density on thermal properties, nonwoven fabrics with comparable densities in different thicknesses and their corresponding weights were measured for ­aerogel-treated nonwoven fabrics. In Fig. 7.6, samples 1 and 2 have higher densities than sample 3. The density difference of samples may be attributed to the fabric structure and also the percentage of aerogel particles present in the fiber.

7.2.8 Thermal conductivity Thermal conductivity, λ, is a measure of the rate at which heat is transferred through unit area of the fabric across unit thickness under a specified temperature gradient and thus is defined by the relation Q (7.1) DT Ft h where Q, amount of conducted heat; F, area through which heat is conducted; τ, time of heat conducting; ∆T, drop of temperature; and h, fabric thickness. The analysis of variance (ANOVA) results show that the fabric density affects the thermal conductivity values of the aerogel-treated nonwoven fabric (P = .006). The thermal conductivity of nonwoven fabrics depends on many factors including environmental temperature, thermal conductivity of the solid polymer materials, and fabric dimensional and structural parameters such as fabric density, fabric porosity, and fiber arrangement. It is understood that the thermal conductivity of fibrous materials will increase with increasing environmental temperature due to the contribution of radiation, convection, and conduction. Hence, the thermal conductivity of a fabric increases significantly with increases in the heating temperature. The thermal conductivity of air is constant at a certain temperature; heat transfer in a fabric may be subject

l (W / m / K ) =

90.00 Fabric density (kg/m3)

80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00

272.6

499.5 Fabric weight (g/m2)

Fig. 7.6  Fabric density of nonwoven fabrics treated with aerogel [8].

440.7

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to some variations depending on the different thermal conductivities of the component fibers. Among the polymers, polypropylene (PP) has a low thermal conductivity (0.12 W/m/K), and polyethylene (PE) has a relative high value (0.34 W/m/K), and the thermal conductivity of silica aerogel is about 0.02 W/(m K) at normal atmospheric condition. The three different nonwoven fabrics with varying thickness considered in the study are composed of polypropylene and polyethylene fibers treated with silica aerogel. Since the difference in thermal conductivity of the two fibers is small, the conductivity of a low-density fabric of this type, corresponding to an air volume of about 90%, is practically independent of the fiber composition. The volumetric proportion of fibers in a fabric is represented by the fabric density, which relates to the volumetric proportion of air trapped in the fabric (or fabric porosity). For nonwoven fabrics, the density is the primary factor contributing to the heat transfer through fabrics. Fig. 7.7 shows that the thermal conductivity for the nonwoven fabrics is inversely proportional to fabric density. Sample 1 has the lowest thermal conductivity, and samples 2 and 3 didn’t show any difference in the thermal conductivity even though the fabric density of sample 2 is higher than sample 3. This may be attributed to the open structure of the fabrics that is shown in Fig. 7.8, percentage of fiber content, and also the aerogel particles present in the fibers. Among the three main reasons, aerogel present in the fiber plays a major role in the thermal conductivity and fabric density of the nonwoven fabrics [10].

7.2.9 Thermal diffusivity Thermal diffusivity describes the rate of temperature spread through a material. Thermal diffusivity, a, is calculated from the thermal conductivity and the heat thermal capacity as given below:

(

)

Thermal conductivity (W/m/K)

a m2 / s =

l rc

(7.2)

0.028 0.027 0.026 0.025 0.024 0.023 0.022 0.021 0.02

272.6

499.5 Fabric weight (g/m2)

Fig. 7.7  Thermal conductivity [9].

440.7

Thermal diffusivity (m2/s) x10–6

Nanoporous materials323 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

272.6

499.5

440.7

Fabric weight (g/m2)

Fig. 7.8  Thermal diffusivity [9].

where λ, thermal conductivity; ρ, fabric density; and c, specific heat capacity of fabrics. The analysis of variance (ANOVA) results show that the effect of fabric density on the thermal diffusivity is significant (P = .006). The comparisons of the average thermal diffusivity values show that the thermal diffusivity and fabric densities are inversely proportional. Fig. 7.8 shows the results of thermal diffusivity and fabric density, where the sample 1 has the lowest value of the thermal diffusivity whereas the sample 3 has the highest value of this parameter. Sample 3 shows the increase in thermal diffusivity with the decrease of fabric density. This may be attributed to the fabric structure shown in Fig. 7.8, fiber content, and the aerogel particles present in the fiber [9].

7.2.10 Thermal resistance Thermal resistance is defined as the ratio of the temperature difference between the two faces of a material to the rate of heat flow per unit area. Thermal resistance determines the heat insulation property of a textile material. The higher the thermal resistance, the lower is the heat loss. The thermal resistance, R, is connected with the thermal conductivity, λ, and the fabric thickness, h, as follows:

(

)

R m2 K / W =

h l

(7.3)

The statistical analysis shows that the fabric thickness has a highly significant influence on the thermal resistance (P = .006). Thermal resistance is a function of the thickness and thermal conductivity of a fabric, and is a very important parameter from the viewpoint of thermal insulation, and is proportional to the fabric structure also. The original thickness measurements for the three fabrics were under relaxed conditions. Fig. 7.9 indicates that the thermal resistance of sample 1 is lower than the samples 2 and 3. If the thickness is higher like in samples 2 and 3, the thermal resistance is also higher. Due to increase in thickness,

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0.24

8 0.20 7 0.16 6 0.12 5

0.08

Thickness, h (mm)

Thermal resistance, r, (K m2/W)

9

Thermal resistance, r, (K m2/W) Thickness, h (mm)

4

0.04

3

0.00

272.6

499.5 Fabric weight (g/m2)

440.7

Fig. 7.9  Thermal resistance versus thickness [9].

there is an increase in thermal insulation, and the decrease of heat losses is due to the space insulated by the fabric. This may be attributed to aerogel particles in the fabric.

7.2.11 Thermal absorptivity Thermal absorptivity is the quantity of heat penetrating a fabric during the time period when temperature is raised rapidly. The thermal absorptivity, b, is given by the following relation:

(

)

b W s1/ 2 / m / K = l × r c

(7.4)

where λ, thermal conductivity; ρ, fabric density; and c, specific heat capacity of fabrics. Thermal absorptivity allows evaluating the transient contact properties of a textile material together with maximum heat flux, Qmax, and thermal diffusivity. Transient heat transfer occurs when a fabric initially contacts with the skin. The thermal absorptivity together with the maximum heat flux is accepted as the objective measure of warm-cool feeling of fabrics. The contact area between fabric and the skin and heat capacity and fabric thermal conductivity determine the warm-cool feeling. Therefore, the surface property (roughness/smoothness) of a fabric has a great influence on this sensation. A smooth surface increases the thermal absorptivity and heat flux values due to a large area of contact with the human skin. Conversely, a rough surface reduces the thermal absorptivity and the heat flux values. According to this, the high values of thermal absorptivity and maximum heat flux provide cool feeling, whereas the low values provide the warm feeling. The experimental values were statistically analyzed and found to be significant (P = .000). In Fig. 7.10, the thermal absorptivity of sample 1 is higher than samples 2 and 3. This may be attributed to the surface roughness/smoothness of the fabrics.

Thermal absorptivity (W/m2/s1/2/K)

Nanoporous materials325 50 48 46 44 42 40 38 36

272.6

499.5 Fabric weight (g/m2)

440.7

Fig. 7.10  Thermal absorptivity of nonwoven fabrics [9].

It shows that sample 1 had higher contact area with the measuring head of the instrument and comparatively sample had lower contact area with the measuring head. Thus, the sample 1 gives cooler feeling than sample 3. As explained above, a smooth surface increases the contact area of a fabric with a skin and therefore the thermal absorptivity.

7.2.12 Applications of aerogels Aerogels can be used for the development of a wide variety novel high-performance products such as jackets for protection from extreme cold weather conditions, space suits, building and pipeline insulation, acoustic and thermal insulation blankets, ­aerogel-textile composites, and many more. Some of these applications are discussed below.

7.2.13 Aerogel as a composite As silicon alkoxide precursor is reactive enough to form gel networks with other metal oxides, several studies were carried out to synthesized silica aerogel composites for various applications. Structural and magnetic properties of silica aerogel‑iron oxide nanocomposites were studied. There are several reports that describe synthesis of ­silica-titania, silica‑carbon, and silica and alumina microfibers or activated carbon powder composite aerogels.

7.2.14 Aerogel as an absorbent Synthesis of flexible and superhydrophobic aerogels and their use in absorption of organic solvents and oils were studied. They investigated the absorption and desorption capacity of superhydrophobic silica aerogels using 11 solvents and 3 oils.

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7.2.15 Aerogel as a sensor Aerogels have high overall porosity, good pore accessibility, and high surface active sites. They are therefore potential candidates for use as sensors. A study on silica nanoparticle aerogel thin films showed that their electric resistance markedly decreases with increasing humidity. They are highly sensitive to 40% RH and greater and operate with a 3.3% hysteresis, which is attributed to their pore structure. Xerogels of the same material, on the other hand, show very low sensitivity. Surface-modified aerogels are less affected by humidity as compared with hydrophilic aerogels and can be used as anticorrosive, hydrophobic agents [11].

7.2.16 Aerogel as material with low-dielectric constant SiO2 aerogel thin films have received a significant attention in IC applications because of their unique properties such as their ultralow dielectric constants, high porosity, and high thermal stability. Park et al. investigated silica aerogel thin films for interlayer dielectrics, and the dielectric constant was measured to be approximately 1.9. They produced ultralow dielectric constant aerogel films for intermetal dielectric (IMD) materials. The SiO2 aerogel films having a thickness of 9 mm, high porosity of 99.5%, and low dielectric constant of 2.0 were obtained by a new ambient drying process using n-heptane as a drying solvent [11].

7.2.17 Aerogel as catalysts The high surface area of aerogels leads to many applications, such as a chemical absorber for cleaning up spills. This feature also gives it a great potential as a catalyst or a catalyst carrier. Aerogels aid in heterogeneous catalysis, when the reactants are in either gas or liquid phase. They are characterized by very high surface area per unit mass and high porosity that makes them a very attractive option for catalysis.

7.2.18 Aerogel as a storage medium The high porosity and very large surface area of silica aerogels can also be utilized for applications as gas filters, absorbing media for desiccation and waste containment, encapsulation media, and hydrogen fuel storage. Partially sintered aerogels can resist the tensions of a gas/liquid interface because their texture is strengthened during sintering. They can therefore be used for the storage, thickening, or transport of liquids, for example, rocket fuels. In the latter case, the low weight of aerogels is particularly advantageous. Aerogel can be used in drug delivery systems due to their biocompatibility. Carbon aerogels are used in the construction of small electrochemical double-layer supercapacitors. Due to the high surface area of the aerogel, these capacitors can be from 1/2000th to 1/5000th the size of similarly rated electrolytic capacitors. Aerogel supercapacitors can have very low impedance compared with normal supercapacitors and can absorb or produce very-highpeak currents. At present, such capacitors are polarity-sensitive and need to be wired in series if a working voltage of greater than about 2.75 V is needed [9,12–14].

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7.2.19 Aerogel as a template Silica aerogel films are used for dye-sensitized solar cells. High-surface-area mesoporous aerogel films were prepared on conductive glass substrates. Atomic layer deposition was employed to coat the aerogel template conformally with various thicknesses of TiO2 with subnanometer precision. The TiO2-coated aerogel membranes were incorporated as photoanodes in dye-sensitized solar cells. The charge diffusion length was found to increase with increasing thickness of TiO2 leading to increasing current and efficiency [15].

7.2.20 Aerogel as a thermal insulator Apart from high porosity and low density, one of the most fascinating properties of aerogels is their very low thermal conductivity. Aerogels possess a very small thermal conductivity, ~1%–10% that of a solid; additionally, they consist of very small particles linked in a three-dimensional network with many “dead ends.” Therefore, thermal transport through the solid portion of an aerogel occurs through a very tortuous path and is not particularly effective. The space not occupied by solids in an aerogel is normally filled with air (or another gas) unless the material is sealed under vacuum. These gases can also transport thermal energy through the aerogel. The pores of aerogel are open and allow the passage of gas (albeit with difficulty) through the material [16].

7.2.21 Housing, refrigerators, skylights, and windows Silica aerogels can be synthesized using low-cost precursors at ambient pressure that makes aerogels suitable for commercialization. Aerogels transmit heat only one hundredth and normal density glass. The first residential use of aerogels is as an insulator in the Georgia Institute of Technology’s Solar Decathlon House, where it is used as an insulator in the semitransparent roof. Aerogels are a more efficient, low-density form of insulation than the polyurethane foam currently used to insulate refrigerators, refrigerated vehicles, and containers. Foams are blown into refrigerator walls by chlorofluorocarbon (CFC) propellants, the chemical that is the chief cause of the depletion of the earth’s stratospheric ozone layer [16].

7.2.22 In clothing, apparel, and blankets Commercial manufacture of aerogel “blankets” began around the year 2000. An aerogel blanket is a composite of silica aerogel and fibrous reinforcement that turns the brittle aerogel into a durable and flexible material. The mechanical and thermal properties of the product may be varied based upon the choice of reinforcing fibers, aerogel matrix, and pacification additives included in the composite. Aspen Aerogels, Inc. of Marlborough, Massachusetts, has produced a Spaceloft product, an inexpensive and flexible blanket that incorporates a thin layer of aerogel embedded directly into the fabric. Another type of aerogels is organic, which is made of carbon and hydrogen atoms. Mount Everest climbers have used aerogel insoles and sleeping bags lined with the material [17–19].

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7.2.23 In space NASA used aerogels to trap space dust particles aboard the Stardust spacecraft. The particles vaporize on impact with solids and pass through gases, but can be trapped in aerogels. NASA also used aerogel for thermal insulation of the Mars Rover and space suits. The US Navy is evaluating aerogel undergarments as passive thermal protection for divers [20].

7.2.24 Electrospun nanofibrous aerogels for effective oil/water separation The separation of oil and water is a worldwide challenge due to the ever-increasing amount of oily industrial wastewater and polluted oceanic waters and the increasing frequency of oil spill accidents. As the leader of advanced fibrous materials, electrospun nanofibers combine the properties of tunable wettability, large surface area, high porosity, good connectivity, fine flexibility, and ease of scalable synthesis from various materials (polymer, ceramic, carbon, etc.), and they hold great potential for many emerging environmental applications, including the separation of oily wastewater. The recent progress in the design and fabrication of electrospun nanofibrous materials with tunable surface wettability for oil/water separation applications is highlighted. The research and development starting from the design concepts and the synthesis of nanofibrous sorbents, nanofibrous membranes, and nanofibrous aerogels for effective oil/ water separation is extensive Fig. 7.11 [21]. One of the most economical and efficient approaches for oil spill cleanup is mechanical extraction by sorbents, which involves using sorbents to concentrate and transform oil from the spilled area to the semisolid or solid phase. Therefore, the characteristics of ideal sorbent materials should include oleophilicity-hydrophobicity, high sorption rate and capacity, high buoyancy, and good reusability. Among the current oil sorbent materials, organic synthetic fibers, such as nonwoven polypropylene (PP) fibrous mats, have been widely used in oil spill cleanup; however, they suffer from a low oil sorption capacity (<30 g/g) in practical applications. Recently, electrospun nanofibers have shown great promise as sorbents for oil sorption. The oil sorption capacity has been proved to be further enhanced because the nanofibrous sorbent can drive the oil not only into the voids between fibers but also into its multipores. To date, many types of nanofibrous sorbents have been developed for oil sorption, and they can be classified into three major categories: hydrophobic-oleophilic polymer nanofibers, composite nanofibers, and carbon nanofibers [21].

7.3 Nanoporous carbon materials Nanoporous carbon materials have attracted intense interests as electrode materials for supercapacitor applications, primarily due to their various desirable advantages such as wide allotrope forms, high surface areas, large pore volumes, superior stabilities, and low costs. To efficiently engender nanoscale pores in carbon matrix, several

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Fig. 7.11  Electrospun nanofibrous aerogels [21].

synthesis protocols have been developed. The template carbonization method recently is an intriguing one, which commonly has been incorporated with soft and/or hard templates. After the removal of templates from carbonization products, nanoporous carbon materials form finally. Notably, the present carbonization/activation temperatures of 900/1000/800°C are much higher than the boiling point (756°C) of pristine ZnCl2, making it complete volatilization after reaction, without any remaining of zinc compounds. Furthermore, the ZnCl2 can be recycled for use after disposing it properly. Therefore, the present ZnCl2 activation method is expected to produce nanoporous materials in an economic and green manner. Studies of the intense laser-material interaction are of high interest due to the development of numerous laser types on one side and plenty of novel materials on the other. When laser power density is below the damage threshold, all of the absorbed laser energy will be converted into heat. This may induce evaporation, oxidation, melting, ablation, etc., which can lead to phase modifications and changes of material structure. Since carbons possess high absorptivity and high thermal diffusivity like metals, similar processes are anticipated [22,23]. Interaction of pulsed transversely excited atmospheric (TEA) CO2-laser radiation at 10.6 μm with nanoporous activated carbon cloth was investigated. Activated carbon

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cloth of different adsorption characteristics was used. Activated carbon cloth modifications were initiated by laser pulse intensities from 0.5 to 28 MW/cm2, depending on the cloth adsorption characteristics. CO2 laser radiation was effectively absorbed by the used activated carbon cloth and largely converted into thermal energy. The type of modification depended on laser power density, number of pulses, but mostly on material characteristics such as specific surface area. The higher the surface area of activated carbon cloth, the higher the damage threshold. Laser beam in the present case induced less substantial mechanical damages on the ACC compared with the previously examined unactivated material. It is evident that the TEA CO2 laser pulses (10.6 μm wavelength) produced fiber damage on the surface of an ACC upon the conversion into thermal energy. The larger the surface area of the material, the higher power density for causing mechanical damage is needed. This may be due to a higher dissipation of the absorbed energy from the affected zone in the case of the nanoporous activated cloth with larger pore volume. The efficient storage of energy combined with a minimum carbon footprint is still considered one of the major challenges toward the transition to a progressive, sustainable, and environment-friendly society on a global scale. The energy storage in pure chemical form using gas carriers with high heating values, including H2 and CH4, and via electrochemical means using state-of-the-art devices, such as batteries or supercapacitors, are two of the most attractive alternatives for the combustion of finite, carbon-rich, and environmentally harmful fossil fuels, such as diesel and gasoline. A few-step, reproducible, and scalable method is presented in this study for the preparation of an ultramicroporous (average pore size around 0.6 nm) activated carbon cloth (ACC) with large specific area (>1200 m2/g) and pore volume (~0.5 cm3/g) upon combining chemical impregnation, carbonization, and CO2 activation of a low-cost cellulose-based polymeric fabric. The ACC material shows a versatile character toward three different applications, including H2 storage via cryo-adsorption, separation of energy-dense CO2/CH4 mixtures via selective adsorption, and electrochemical energy storage using supercapacitor technology. Fully reversible H2 uptake capacities in excess of 3.1 wt% at 77 K and ~ 72 bar along with a significant heat of adsorption value of up to 8.4 kJ/mol for low surface coverage have been found. Upon incorporation of low-pressure sorption data in the ideal adsorbed solution theory model, the ACC is predicted to selectively adsorb about 4.5 times more CO2 than CH4 in ambient conditions and thus represents an appealing adsorbent for the purification of such gaseous mixtures. Finally, an electric double-layer capacitor device was assembled and tested for its electrochemical performance, constructed of binder-free and flexible ACC electrodes and aqueous electrolyte. The fuel cell exhibits a gravimetric capacitance of ~ 121 F/g for a specific current of 0.02 A/g, which, relative to the ACC’s specific area, is superior to commercially available activated carbons. A capacitance retention of >97% was observed after 10,000 charging/discharging cycles, thus indicating the ACC’s suitability for demanding and high-performance energy storage on a commercial scale. The enhanced performance in all tested applications seems to be attributed to the mean ultramicropore size of the ACC material instead of the available specific area and/or pore volume [24].

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Fig. 7.12  Nanoporous activated carbon cloth [24]. (A) Macroscopic view, (B) Yarn arrangement, (C) 50 μm magnification, (D) 40 μm magnification, (E) 10 μm magnification, and (F) Fractured surface.

Fig. 7.12 shows SEM images of the hierarchical structure of the ACC material over a wide range of magnifications that cover three orders of magnitude in length (i.e., from mm to μm). From a macroscopic view (Fig. 7.12A), the ACC appears to maintain the original woven chain-like morphology of the VCR precursor upon carbonization and CO2 activation. In Fig. 7.12B, it is clear that the material is woven from yarns that form a continuous fabric, while each yarn spans several hundreds of microns (i.e., 200–500 μm) in width and is composed of tens of individual CFs. The geometric characteristics of the main building block (i.e., CF) can be resolved at even higher magnifications (Fig. 7.12C–E). Each CF has a diameter in the range of 10–15 μm (Fig. 7.12C and D), while it exhibits a corrugated circumference with wavelengths on the order of 1–2 μm (Fig. 7.12E). Fig. 7.12F shows a fractured CF surface, which appears to be dense in the interior, suggesting that the presence of porosity should exist at even smaller scales and most probably at the level of nanometers, which however cannot be resolved by SEM. Such porous structure appears as another key factor contributing to the high specific surface area and pore volume attributes of the ACC material [24]. Nanoporous hard carbon microspheres (NHCSs) as shown in Fig.  7.13 were prepared by combination of microemulsion and polymerization methods and using phenolic resin (resol) as precursor and ethanol and ethylene glycol (EG) as solvent and soft template, respectively, followed by carbonization process. Using different amounts of EG resulted in NHCSs with different crystalline structure, surface area and pore volumes, and Li-ion storage capacity, as evidenced by physical and electrochemical measurements. Higher and lower polymerization rates were also tested on the starting resol solution with composition that led to the NHCS with the highest surface area and Li-ion storage capacity. The sample polymerized at higher rate had the

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SEM MAG: 15.00kx SEM HV: 15.00 kV

Det: SE WD: 4.499 mm

2 µm

VEGA\\ TESCAN

Fig. 7.13  Carbon microspheres with nanopores [25].

highest surface area and pore volume, as well as the best Li-ion storage performance in terms of capacity and rate capability. For all of the NHCSs, the specific surface area and Li-ion storage capacity were well correlated, and a good correlation was observed between total pore volume and rate capability. Furthermore, acceptable correlations were found between Li-ion storage capacity and either surface area or microstructure of the NHCSs [25].

7.4 Nanoporous copper structures The research on metal nanostructures is mainly centered on their nanofabrication by both physical and chemical methodologies, especially for catalysis, optical imaging, plasmon resonance, etc. Metal nanostructures have a capability to dramatically enhance the local electric field by concentrating electromagnetic energies into subwavelength volumes. This could lead toward their wide applications in various fields including cancer diagnostics, optical bioimaging, and single-molecule Raman scattering. Among these, surface-enhanced Raman scattering (SERS) is of particular research interest to detect and identify the adsorbate down to the limit of single-­ molecule detection using the enhanced Raman signal [26]. Copper nanostructures (Fig. 7.14) have aroused extensive research interests due to their large number of applications in phonology, electronics, catalysis, etc. Here, we have developed a novel single-step redox colloidal route to construct three-­dimensional (3D) nanoporous Cu leaves. By tuning the surface chemistry of copper nuclei, the

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Fig. 7.14  Nanoporous copper structures [26]. (A) Copper nanodispersion, (B) Nanoflowers, (C) Pores, (D) Multiscale pores, (E) Pore dimensions at higher scale, (F) Granular structure, (G) Individual granules, (H) Porous film, and (I) Pores distribution.

morphology of as-prepared copper nanocrystals can be adjusted from nanoparticles to leaves. The 3D Cu leaves own anisotropic nanoporous structure and exhibit ­surface-enhanced Raman scattering (SERS) performance with an enhancement factor of nearly 106. The present facile and low-cost route to develop 3D nanoporous Cu leaves provides an efficient platform for achieving high-performance nanostructures for next-generation photocathode applications. The introduction of three solutions including PEG, isooctane, and water is the key to the formation of 3D leaves with good uniformity. Ligands adsorb on copper clusters through hydrogen bonding, metal ion coordination, and even interaction between hydrophobic and hydrophilic moieties. Meanwhile, organic tartrate ions have a structure with hydrophilic carboxyl and hydroxyl on one side and hydrophobic methyne on the other side. Tartrate ions adsorb on the copper surface with hydrophilic groups and connect with PEG and isooctane via methyne group through chelating bonding. A single-step redox colloidal approach is presented to synthesize 3D porous Cu leaves and spherical Cu nanocrystals. In the Cu nanocrystal growth, the composition of the reaction solution is tuned to adjust the sample morphology. The mixed solution of isooctane/PEG/deionized water of 20:3:3 is effective for the growth of uniform Cu leaves. Spherical nanocopper assembled into 3D porous copper leaves covered

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with ligands delivers a large SERS enhancement factor due to the novel porous 3D structure and the synergistic effect of organic ligands. The results provide a new pathway to develop inexpensive Cu for applications in SERS-based sensitive devices and an efficient synthetic method for the preparation of pure metallic nanocrystals in 3D morphology [26,27]. The ultrafine nanoporous CuO ribbons modified by Au nanoparticles are prepared through simply dealloying the melt-spun AleCueAu alloys combined the subsequent calcination in air. A nanoporous Cu (Au) solid solution ribbon is constructed as the removal of Al from the precursory alloys. After calcined at high temperature, Cu is oxidized to CuO with a porous structure and the in situ-generated Au nanoparticles embed in ligaments, resulting in the formation of nanoporous CuO/Au composites with a pore size about 25 nm. The experiment results indicate that the composites have large surface area, high activity, and stability. And the CO conversion rate can reach 100.0% just at 180°C. According to X-ray photoelectron spectroscopy (XPS) results, the superior performance at low temperature is ascribed to the presence of Aut1 species and the interfacial interaction between Au nanoparticles and CuO ligaments.

7.5 Polypyrrole nanostructures SEM images of polypyrrole (PPy)-methyl orange (MO)/cotton yarns are shown in Fig. 7.15. The deposition of PPy on the cotton yarn can be clearly observed. The originally white yarn becomes black after the coating with PPy. In the case of the PPy/cotton yarn obtained in the absence of MO, the fiber surface is tightly covered with the irregular agglomerations of PPy (Fig. 7.15B). For the PPy-MO/cotton yarns prepared with lower MO concentration (PPy-MO/cotton-1 and PPy-MO/cotton-2), the PPy nanoparticles and nanotubes are deposited together on the fiber surface (Fig. 7.15A–D). Higher MO concentration produces more PPy nanotubes, leading to a three-­dimensional interconnected network structure, as indicated by Fig. 7.15E–H. However, the PPy nanotubes of the PPy-MO/cotton-5 yarn are relatively long and loose-packed as compared with the PPy-MO/cotton-10 yarn. This hierarchically structured feature is considered to favor the ionic migration and to increase the active interface of PPy. In addition, the mass loading of PPy for the PPy-MO/cotton-5 yarn (about 24%) is lower than that for the PPy/cotton yarn (about 30%). It is demonstrated that thinner coating of conductive polymers is obtained on the substrates during the polymerization in the presence of surfactants owing to the additional formation of colloidal polymers [29]. Polypyrrole (PPy), as one of the conducting polymers, has emerged as a promising active material for high-performance supercapacitor owing to its intrinsic characteristics (e.g., high electric conductivity and interesting redox properties). It’s attracting more and more attentions with the development of flexible/wearable devices thanks to the great flexibility and ductility of PPy as a polymer. Nanostructured PPy (Fig.  7.16) represents as a promising flexible electrode material for high-performance supercapacitor, owing to its intrinsic advantages, such as low cost, low toxicity, high electric conductivity, interesting redox properties, and facile fabrication [28,31,32].

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Fig. 7.15  Cotton yarn electrodes coated with polypyrrole nanotubes [28]. (A) Polypyrrole, (B) Agglomeration, (C) Nanostructures, (D) Clusters, (E) 3D network, (F) Nanorods, (G) Single nanorod, and (H) Nanoassembly.

Fig. 7.16  Nanostructures of polypyrrole [30].

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In addition, for a deeper understanding of the fundamental knowledge of nanostructured PPy and its composite as employed in the supercapacitors, advanced simulation and modeling studies are needed to uncover underlying electrochemical mechanisms in nanoscale. As a good complementary strategy, the state-of-the-art in situ microscopic and spectroscopic techniques are also needed to provide direct proofs for studying the nanoscale processes in energy storage. With these further insights, it will be easier to optimize electrochemical properties of PPy in revolutionary new nanostructured supercapacitor devices [30].

7.6 Suspended hydrophobic porous membrane for high-efficiency water desalination An ultrathin highly fluorinated porous membrane (Fig. 7.17) was designed for a large production of desalted water at very low energy consumption. Imprinting water droplets were used through a low thermally conductive tetra-fluoroethylene (TFE)/2,2,4-trifluoro5-tri-fluorometoxy-1,3-dioxol (TIT) (HYFLON 60 CE) solution, and the generated porous nanofilm was suspended onto a polyethersulfone (PES) honeycomb texture. The very tiny fluorinated thickness together with a large number of small-shaped pores provided the membrane for enhanced antiwetting surface properties, extremely reduced resistance to water vapor transfer, and outstanding thermal efficiency. Fine-material structure-transport relations let the membrane reach unusual p­roductivity-efficiency trade-off during water desalination via thermally driven membrane distillation. The exceptional performance affords this novel nanostructured membrane to catalyze the accomplishment of new-concept water desalination processes [33]. The challenge to manipulate and shape a highly hydrophobic polymer such as HYFLON AD in a very tiny porous layer can be regarded as the result of a fine manipulation of structure-transport property relationships. Compared with other homemade and commercial membranes, this novel nanostructured membrane exhibits a better performance. Higher capability to transfer large amounts of water with appreciable salt rejection is obtained when softer processing temperatures are selected, resulting in an exciting thermally high-efficiency membrane distillation process. These achievements can be regarded as the result of a directed nanoassembly of materials in a well-defined volumetric space, where suitable structure-chemistry interplay is obtained. Reduced pathways together with use of fluorinated materials at lower thermal conductivity make the designed membrane extremely promising for future competitively productive and energetically favored water desalination [34].

7.7 Nanoporous chitosan materials Up until now, numerous inorganic and organic materials have been made into nanoporous forms using a templating method. However, chitosan, which is an abundant resource of biomaterials, has not been reported. More significantly, because of

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Fig. 7.17  Nanoporous membrane for water desalination [33]. (A) Porous membrane, (B) Nanopores, (C) Micropores, (D) Cellular structure, and (E) Magnified view.

the versatility and biocompatibility of chitosan, the nanoporous chitosan material could be used for a wide variety of applications in biomaterial construction and many other areas of science as well. Nanoporous membrane is very important and has potential applications in many areas, especially in the biomedical or biomaterial sciences. With improvements in both structure and properties, nanoporous membrane design

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Fig. 7.18  Chitosan-based nanoporous materials [35].

can directly benefit the fields of chemical separation, drug delivery, and wastewater remediation. In those applications, a biocompatible material is desirable and preferable. To date, most of the studies related to this subject are focused on synthetic polymer materials. Chitosan (Fig. 7.18) is an N-deacetylated product of chitin that is one of the most abundant polysaccharides in nature and has good physical, biological, and biodegradable properties. It is readily processed into membranes from aqueous acidic solutions [35]. Fully biobased composite membranes (Fig. 7.19) for water purification were fabricated with cellulose nanocrystals (CNCs) as functional entities in chitosan matrix via freeze-drying process followed by compacting. The chitosan (10 wt%) bound the CNCs in a stable and nanoporous membrane structure with thickness of 250–270 μm, which was further stabilized by cross-linking with glutaraldehyde vapors. Scanning electron microscopy (SEM) studies revealed well-individualized CNCs embedded in a matrix of chitosan. Brunauer, Emmett, and Teller (BET) measurements showed that the membranes were nanoporous with pores in the range of 13–10 nm. In spite of the low water flux (64 L/m2/h), the membranes successfully removed 98, 84, and 70%, respectively, of positively charged dyes like Victoria Blue 2B, Methyl Violet 2B, and Rhodamine 6G, after a contact time of 24 h. The removal of dyes was expected to be driven by the electrostatic attraction between negatively charged CNCs and the positively charged dyes [36].

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Fig. 7.19  Chitosan nanoporous membranes [36]. (A) Membrane, (B) Microstructure, (C) Multilayered pattern, and (D) Cellulose nanostructures.

Isolated CNCs isolated from low-cost sludge from industrial cellulose production was used as functional entity for the preparation of fully biobased nanocomposite membranes with chitosan as a matrix. Cross-linking of nanocomposite showed positive impact on mechanical stability and dimensional stability in moist environment and also resulted in slight decrease in surface area and pore size. The pore diameter was found to be in 13–17 nm range and classifies these membranes as ultrafiltration membranes [36].

7.8 Bioinspired engineering of honeycomb structure Honeycomb structures, inspired from bee honeycombs, had found widespread applications in various fields, including architecture, transportation, mechanical engineering, chemical engineering, nanofabrication, and recently biomedicine. A major challenge in this field is to understand the unique properties of honeycomb structures, which depended on their structures, scales, and the materials used. These structural

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Fig. 7.20  Bioinspired honeycomb structures [37]. (A) Honeycomb nanostructure, (B) Cross section of nanopores, (C) Micropores, and (D) Cross section of micropores.

perspectives have led to new insights into the design of honeycomb structures ranging from macro-, to micro-, to nanoscales. Current scientific advances in micro- and nanotechnologies hold great promise for bioinspired honeycomb structures as shown in Fig.  7.20. There are emerging applications of honeycomb structures in biomedicine such as tissue engineering and regenerative medicine. Understanding the design principles underlying the creation of honeycomb structures and the related scientific discovery and technology development is critical for engineering bioinspired materials and devices designed based on honeycomb structures for a wide range of practical applications [38]. In biomedicine, the honeycomb structures also hold great potential in addressing major challenges in tissue engineering and regenerative medicine. One of the most challenging goals in tissue engineering is to design scaffolds as guidance for tissue regeneration, especially for 3D artificial thick living tissues and even organs. Threedimensional scaffold-based tissue engineering is mainly based on seeding cells on scaffold surface, followed by inducing cell to grow into the scaffold [37,38]. Recently, cytocompatible hydrogels capable of 3D cell encapsulation have attracted increasing attention due to their biocompatibility, biodegradability, processability, and advanced properties that mimic nature extracellular matrix. Directed self-assembly of honeycomb cell-laden microgels is used as building blocks to create large living tissues. Besides, bottom-up approaches are newly developed to create 3D tissue

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c­ onstructs by assembling honeycomb cell-laden microgels with magnetic, acoustic, and electrostatic forces, which holds great potential for overcoming diffusion limitation (200 μm) that is usually encountered in traditional top-down approaches. Honeycomb structures with high specific surface area as electrodes have also been applied in the field of biosensors, bioadsorption, biocatalysis, and drug release. Microsensors composed of nanohoneycomb flake ZnO or electrochemically anodized TiO2 honeycomb nanostructure have been used in various applications involving the detection and measurement of the environmental and atmospheric hazardous chemicals as well as body glucose. Nanohoneycomb structures and materials play an important role in bioadsorption and biocatalysis, such as adsorption of small biological molecules [37]. The design and fabrication of multiscaled honeycomb structures is the future developing trend for research and applications, especially the emerging applications associated with biological medicine, tissue engineering, energy conversion for high-efficiency storage battery, and multifunctional environment-friendly materials. Understanding the design principles underlying the creation of honeycomb structures and the related scientific discovery and technology development is critical for engineering bioinspired materials and devices designed based on honeycomb structures for a wide range of practical applications. The trends of these prospective are summarized as follows: Multifunctional designs on artificial honeycombs have involved 3D printing and biological mineralization. Energy conversion and environmental engineering: The bioinspired hierarchical structures (microscale honeycomb structure) and the free-standing hierarchical porous structures composed of functionalized graphene sheets, especially those multifunctional composites (hierarchical porous combined functional nanoparticles), show great potential in high-performance energy capacitors, water treatment, and solar energy conversion. Three-dimensional scaffold-based tissue engineering: the main challenge associated with this strategy is to construct thick and complex tissues/organs: (i) good mechanical properties to maintain structural integrity, (ii) sufficient nutrient supply, and (iii) controlled 3D cell distribution and microenvironmental cues (both physical and biochemical) in 3D scaffolds. Special sensing characteristics: honeycomb-derived graphenes as emerging bioelectronic platform have huge potentials for next-­generation biosensors due to its chemical robustness and sensitivity to electromagnetic, mechanical environment changes [37].

7.9 Bio-inspired superoleophobic and smart materials Through evolution, nature has arrived at what is optimal. Inspired by the biomaterials with special wettability, superhydrophobic materials have been well investigated. The construction of superoleophobicity is more difficult than that of superhydrophobicity because the surface tension of oil or other organic liquids is lower than that of water. However, superoleophobic surfaces have drawn a great deal of attention for both fundamental research and practical applications in a variety of fields. Recent r­ esearch

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has progressed in the design, fabrication, and application of bioinspired superoleophobic and smart surfaces, including superoleophobic-superhydrophobic surfaces, ­oleophobic-hydrophilic surfaces, underwater superoleophobic surfaces, and smart surfaces [39]. Although the research of bioinspired superoleophobicity is in its infancy, it is a rapidly growing and enormously promising field. The remaining challenges and future outlook of this field need to be addressed. Multifunctional integration is an inherent characteristic for biological materials. Learning from nature has long been a source of bioinspiration for scientists and engineers. Therefore, further cross-disciplinary cooperation is essential for the construction of multifunctional advanced superoleophobic surfaces through learning the optimized biological solutions from nature [39,40]. Natural materials and artificial structured materials can be used as templates for the construction of oleophobic surfaces with hierarchical structures (Fig.  7.21).

Fig. 7.21  Superoleophobic textiles [40]. (A) Oleophobic fabric, (B) After 1 h, (C) Oleophilic, (D) Microstructure of oleophilic fabric, and (E) Droplets of different solvents on fabric surface.

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Nanoenabled surfaces are used as a base material to replicate the structures of templates. After solidification, the complement of the template surface pattern is transferred on the solid negative replica. Similarly, a negative replica is used as the template to fabricate a positive replica. Nanobased biomimetic replica is prepared by using the dry natural colocasia leaf as a template. After further modification with silica nanoparticles followed by chemical vapor deposition of fluorocarbon materials, the resultant CF3-terminal, silica-modified surface exhibits both high oil-repellent and superhydrophobic properties [40]. Traditionally inorganic and organic materials such as ceramics, polystyrene, and polytetrafluoroethylene usually exhibit hydrophilic-oleophilic, hydrophobic-­oleophilic, and hydrophobic-oleophobic characteristics, respectively. Utilizing different synthetic strategies, a great number of functional surfaces with hydrophilic-oleophilic, ­hydrophobic-oleophilic, or hydrophobic-oleophobic properties have been fabricated. It is usually difficult to design and construct functional surfaces possessing both oleophobicity and hydrophilicity. For a given surface, wettability is usually dominated by the surface tension of the liquid. For example, hexadecane has a lower surface energy than water. Therefore, for the identical homogeneous substrates, hexadecane will always have a lower contact angle than water. Recently, in order to overcome the traditional limitation of thermodynamic surface energetics, many different synthesis methods have been developed to construct functional coatings with simultaneous oleophobicity and hydrophilicity, where the smart surfaces have different functional groups possessing a favorable interaction with polar liquids and an unfavorable interaction with nonpolar liquids. For these smart surfaces, the locations of the hydrophilic and oleophobic constituents are intercalated with each other. In the presence of oil droplets, the interface is occupied by the low-surface-energy component, resulting in oleophobicity. However, when water droplets are located on these surfaces, water molecules are able to penetrate these surfaces owing to the presence of hydrophilic moieties and water-induced molecular rearrangement exhibiting hydrophilicity [40]. Smart materials with special wettability have attracted more and more interest because of their unique properties and promising applications. The study of superhydrophobic surface originates from mimicking nature, but it has been extended to design new functional materials beyond those found in nature. A great variety of smart surfaces possessing reversible switching wettability between superhydrophobicity and superhydrophilicity have been fabricated through different approaches. By using these synthesis strategies in the superhydrophobic system as the reference, design and fabrication of intelligent surfaces with stimuli-responsive reversible oil wettability/ adhesion will be an exciting direction in the field of smart materials owing to their important applications in sensors, devices, lab-on-chip systems, etc. Self-healing is a unique ability for living organisms. Inspired by the self-healing function of biomaterials, a great deal of attention has been focused on the design and construction of self-healing superoleophobic materials, which can spontaneously recover their structure and wettability when damaged. Therefore, endowing superoleophobic surfaces with a self-healing function is believed to be another interesting research field for the smart materials. Bioinspired superoleophobic materials constitute a challenging domain in materials science and surface chemistry, which is experiencing explosive

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growth. Therefore, interdisciplinary cooperation is necessary for researchers in the field of materials science, chemistry, physics, biology, and engineering to further construct novel smart interfacial materials [41].

7.10 Nanoporous graphene film Continuously hierarchical nanoporous graphene (hnp-G) films are synthesized by a combination of low-temperature CVD growth of hydrogenated graphite (HG) coating on nanoporous copper (NPC) and rapid catalytic pyrolysis of HG at high temperature. Low-temperature growth of HG coating on NPC can obviously delay the coarsening evolution of NPC at high temperature, providing the precondition to obtain hnp-G with small pore size (1–150 nm) by catalytic pyrolysis at high temperature. The high specific surface area (1160 m2/g) of hnp-G are mainly originated from the external surface (954.7 m2/g), resulting in fully accessible channels for ion transport. The features are visible in Fig. 7.22 [42]. More importantly, the continuously 3D hierarchical nanoporous structure and full wettability of the hnp-G with gelled electrolyte not only effectively prevent the restacking of graphene even under dramatic squeezing but also guarantee the continuous and short electron/ion diffusion pathway in the whole electrodes, resulting in ultrahigh specific capacitance (38.2 F/cm3 based on the device) and excellent rate performance. The symmetrical SC offers ultrahigh energy density (2.65 mWh/cm3) and power density (20.8 W/cm3) and exhibits almost identical performance at various

Fig. 7.22  Nanoporous graphene films [42]. (A) Graphene pore structure, (B) Graphene film, (C) Nanopores, (D) Copper coating, (E) Copper layer on graphene, (F) Multilayered structure, and (G) Flexibility.

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curvatures and excellent lifetime (94% retention after 10,000 cycles), suggesting its wide application potential in powering wearable/miniaturized electronics [42,43]. NPGs can be fabricated through either top-down or bottom-up approaches. Upon the formation of nonporous structures, the bandgap of pristine graphene may be opened. The periodicities and neck widths of NPG control its electronic properties. Semiconducting NPG sheets have been used to fabricate FETs, and their ON/OFF ratios are comparable to those of counterparts based on GNRs. NPG materials are also promising for applications in ECs, gas sensors, DNA sequencing, and molecular sieving. However, several challenges still remain in this field. First, as described above, all the techniques developed for fabricating NPGs have their drawbacks. Thus, a lowcost and productive method for synthesizing NPGs with controllable nanostructures is still required. Second, the relationship between the nanostructures of NPGs and their properties has not yet been well revealed. The effects of sizes, number densities, and edging configurations of nanopores on the properties of NPGs need to be studied systematically. Furthermore, the properties of NPGs can also be further improved via covalent and noncovalent functionalization. Third, theoretical simulations have predicted the great potential of NPGs for practical applications. However, these applications have rarely been realized experimentally. Nevertheless, NPGs are attractive graphene materials, and they have inspired a great deal of interest in both academy and industry for filtration and water purification (Fig. 7.23) [43].

Fig. 7.23  Nanoporous graphene for water cleaning [43]. (A) Graphene structure, (B) Absorption of fresh water, and (C) Absorption of salt water.

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7.11 Decolorization of methylene blue by nanosheets Nowadays, dye pollution in water has become a major source of environmental pollution in waste fields due to the fast development of dye industry. Various strategies have been applied to remove these pollutants from the environment. For instance, physical adsorption on active carbon and coagulation by chemical agents are generally used by simply transferring organic compounds from water to another phase, which may cause second pollution. Therefore, the highly efficient pathways are needed to overcome this dilemma [44]. Owing to the unique layers or tunnels in crystal lattices and high specific areas, MnO2 is broadly applied in ion exchange, adsorption, catalysis, oxidant, electrochemical capacitor, etc. In particular, some reports have demonstrated that MnO2 is one of the outstanding candidates for the practical application in the degradation of dye wastewater in different surroundings. Currently, the MnO2 nanostructures have been prepared by reduction and deposition. But these synthetic methods are relatively tedious since they need either expensive template or carbon coating or electrochemical process. Therefore, it is still a challenge to readily and controllably fabricate well-defined MnO2 nanosheets (Fig. 7.24) on the complex three-dimensional structure [44,45].

7.12 Hierarchically nanoporous nanofibers Surface-driven charge storage materials based on both electrochemical double-layer (EDL) formation and pseudocapacitive behavior can deliver high energy and power capabilities with long-lasting cycling performance. On the other hand, the electrochemical performance is strongly dependent on the material properties, requiring sophisticated electrode design with a high active surface area and a large number of redox-active sites. Hierarchically nanoporous pyropolymer nanofibers (HN-PNFs) as shown in Fig. 7.25 were fabricated from electrospun polyacrylonitrile nanofibers by simple heating with KOH. The HN-PNFs have a hierarchically nanoporous structure and an exceptionally high specific surface area of 3950.7 m2/g as well as numerous redox-active heteroatoms (C/O and C/N ratio of 10.6 and 16.8, respectively). These unique material properties of HN-PNFs resulted in high reversible Na-ion capacity of 292 mAh/g and rapid kinetics and stable cycling performance in the cathodic potential range (1–4.5 V vs Na+/Na). Furthermore, energy storage devices based on HN-PNFs showed a remarkably high specific energy of 258 Wh/kg at 245 W/kg and a high specific power of 21,500 W/kg at 78 Wh/kg, with long and stable cycling behaviors over 2000 cycles [46]. A novel fabrication approach is adopted to highly sensitive formaldehyde sensors by the surface modification of the electrospun nanofibrous membranes. The three-dimensional fibrous membranes comprising nanoporous polystyrene (PS) fibers (Fig. 7.26) were electrospun deposition on quartz crystal microbalance (QCM), followed by the functionalization of the sensing polyethyleneimine (PEI) on the membranes. The morphology and Brunauer-Emmett-Teller (BET) surface area of

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Fig. 7.24  Nanoporous sheets for decolorization of dyes [45].

the fibrous PS membranes with fiber diameter of 110–870 nm were controllable by tuning the concentrations of PS solutions. After PEI modification, PEI particles in clusters of varying sizes (from 50 nm to 1.2 μm) were immobilized onto the surface of the bead-on-string structured nanoporous fibers. The developed formaldehyde-­ selective sensors exhibited fast response and low detection limit (3 ppm) at room temperature. This high sensitivity is attributed to the high surface-area-to-volume ratio (∼47.25 m2/g) of the electrospun porous PS membranes and efficient nucleophilic addition reaction between formaldehyde molecules and primary amine groups of PEI [46,47]. The morphology of porous 3D fibrous membranes was found strongly affected by the concentrations of PS solutions. BET surface area test and N2 adsorption/desorption isotherm demonstrated that PS fibers electrospun from higher concentration of PS solution (13 wt%) tended to possess larger pore volume and surface area, offering an

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Fig. 7.25  Nanoporous pyropolymer nanofibers [46]. (A) Nanofibrous layer, (B) Macropores, (C) Nanopores, and (D) Mesopores.

excellent sensing template for the modification of PEI. When exposed to 140 ppm of formaldehyde, QCM-based PEI-PS (13 wt%) sensor has achieved the largest response value, approximately four times as much as PEI-PS (7 wt%) sensor. Additionally, the sensing properties of the resultant sensors indicated the membrane structures and the coating load of PEI were important parameters to influence the sensitivity of the sensors for formaldehyde. Moreover, the developed sensors perform excellent selectivity toward formaldehyde when exposed to other interfering VOCs. These results demonstrate a promising approach in development and realization of a low-cost and high-performance formaldehyde sensor [47]. Porous materials can be prepared by sol-gel method, hydrothermal synthesis method, electrospinning, and other methods. Electrospun porous nanofibers were prepared by adjusting electrospinning parameters, and the properties of obtained porous nanofiber mats were investigated. Theoretical analysis and experiment research were carried out to understand mechanism of electrospun porous nanofibers and could be used to optimize and control the porous structure. The theoretical analysis results were further verified according to the experimental data. In addition, Bernoulli equation was used to study the electrospinning “splaying” process. The ratio of pore width to pore length

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Fig. 7.26  Nanoporous polystyrene fibers [47]. (A) Polystyrene fibers, (B) Granules on surface, (C) Pores on fiber surface, (D) Microgranules, (E) Pores on microgranules, and (F) Nanoporous granules.

was varied along with the variation of the internal pressure of the jet, and the internal pressure of the jet increases with the velocity of the charged jet decreases [48]. Electrospinning provides a straightforward method to produce ultrafine fibers, where electrostatic forces on a charged polymer jet elongate it into thin fibers before solidification. Charge is induced on the liquid surface by an electric field; mutual charge repulsion causes a force directly opposite to the surface tension. As the jet accelerates and thins in the electric field, radial charge repulsion results in splitting of the primary jet into multiple filaments [48,49]. The surface of fibers exhibits many isolated pores that were oval in shape and elongated in the direction of fiber axis, and the ratio of pore width to pore length was varied along with the variation of the internal pressure of the jet [49].

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Fig. 7.27  Cellulose nanowhiskers [50]. (A) Cellulose nanowhiskers, (B) Pores in membranes, (C) Micropores, (D) Naopores, (E) Mesopores, and (F) Macropores.

Cellulose nanowhiskers (Fig.  7.27) as a kind of renewable and biocompatible nanomaterials evoke much interest because of its versatility in various applications. Controllable fabrication of spiderweb-like nanoporous networks based on jute cellulose nanowhiskers (JCNs) deposited on the electrospun (ES) nanofibrous membrane by simple directly immersion-drying method is possible [50]. Jute cellulose nanowhiskers were extracted from jute fibers with a high yield (over 80%) via a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)/NaBr/NaClO system selective oxidization combined with mechanical homogenization. The morphology of JCN nanoporous networks/ES nanofibrous membrane architecture, including coverage rate, pore width, and layer-by-layer packing structure of the nanoporous networks, can be finely controlled by regulating the JCN dispersions properties and

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drying conditions. The versatile nanoporous network composites based on jute cellulose nanowhiskers with ultrathin diameters (3–10 nm) and nanofibrous membrane supports with diameters of 100–300 nm would be particularly useful for filter applications.

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