Multifunctional Silica Nanotube Aerogels Inspired by Polar Bear Hair

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Multifunctional Silica Nanotube Aerogels (SNTAs) Inspired by Polar Bear Hair for Light Management and Thermal Insulation Ai Du, Hongqiang Wang, Bin Zhou, Chen Zhang, Xueling Wu, Yingting Ge, Tingting Niu, Xiujie Ji, Ting Zhang, Zhihua Zhang, Guangming Wu, and Jun Shen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02926 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Chemistry of Materials

Multifunctional Silica Nanotube Aerogels (SNTAs) Inspired by Polar Bear Hair for Light Management and Thermal Insulation Ai Du*‡, Hongqiang Wang‡, Bin Zhou, Chen Zhang, Xueling Wu, Yingting Ge, Tingting Niu, Xiujie Ji, Ting Zhang, Zhihua Zhang, Guangming Wu, Jun Shen Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai, 200092, People’s Republic of China. ABSTRACT: Designing macroscopic, 3D porous multifunctional materials is of great importance in many fields, including energy storage, thermal insulation, sensors, and catalysis. Polar bears have hairs with a membrane-pore structure, which contributes to adoption against harsh environments. Inspired by polar bear hair, this study reports a facile route to fabricate multifunctional silica nanotube aerogels (SNTAs) via chemical vapor deposition (CVD) of silica onto the sacrificial carbon nano-skeleton of a carbon aerogel (CA). The resulting SNTAs are not only porous, nanotubular, transparent, and lightweight but also hydrophobic, thermal resistant, mechanically robust and machinable. Moreover, SNTAs show relatively high visible and near-infrared light transmittance and almost no ultraviolet and far-infrared light transmittance, which makes it an ideal material to provide greenhouse effects and prevent human beings from an overdose of ultraviolet radiation. Multifunctional SNTAs provide an integrated solution for thermal insulation, daylighting and UV protection applied in outer space or at high latitudes.

INTRODUCTION Animals living in polar regions, such as penguins and polar bears, have multifunctional hair that helps them to keep warm and survive in harsh environments.1-2 Polar bear hair is a good archetype in nature to mimic, because its membrane-pore structure leads to low thermal conductivity ranges from 0.02785 to 0.04886 W m-1 K-1.3-5 Polar bear hair also has high far infrared (FIR) reflectivity, which contributes to infrared stealth and thermal insulation.6 Inspired by the structure of polar bear fur and skin, researchers have reported artificial fur for solar-collection systems and the textile woven with excellent thermal insulation property as well as good breathability and wearability.7-8 Although the real or artificial hair of polar bear exhibits various unique properties, it is not perfectly suitable for human beings. According to Grojean’s results, polar bear hair exhibits low UV reflectance and relatively high reflectance of visible and near-infrared (NIR) light.9 This means that more UV light, less visible light and much less NIR can cross the hair, which may be harmful to human health, daylighting and heat collection, respectively. One possible solution is to design a nano-scale microstructure to induce strong Rayleigh scattering. Silica aerogels have an unordered nano-framework, which could induce strong Rayleigh scattering. Silica aerogels also have ultralow density, high porosity, low thermal conductivity, high temperature resistance, and high transparency, and have drawn great interest in a wide range of applications such as adsorption, thermal insulations, astronautical applications, and soft acoustic metamaterials.1015 However, nano-sized solid backbones consisting of weak linking of silica particles leads to the inherent friability and

relatively low Young’s modulus of the traditional silica aerogels.16-17 Many researchers have attempted to reinforce mechanical performance using fibers or introducing proper organic functional groups on the surface of silica aerogels. However, these attempts commonly lead to a white appearance of materials; in addition, organic functional groups cannot resist high temperature,18-22 which limits the wide application of silica aerogels. The design of the aerogel structure may allow for solutions to this problem. For example, nanotube structure shows better mechanical performance due to its low bending modulus.23-25 Previously, research has focused on the preparation of silica nanotubes by deposition on different templates.26-27 Organic molecular deposition is also used.28-29 Liu et al obtained CNT-silica hybrid aerogels and corresponding silica nanotube aerogels (SNTAs). SNTA shows higher mechanical strength and low density, but the sample is a whitish, non-transparent material with giant pores.30 Here, inspired by polar bear hair, we present a facile route to fabricate multifunctional SNTAs via chemical vapor deposition (CVD) of tetraethoxysilane (TEOS) onto the skeletons of carbon aerogel (CA), followed by combustion in the air. The random nano-scale tubular structure contributes to enhancing short-wave scattering, leading to the low UV transmittance and high transmittances of visible and NIR light. In addition, this type of aerogel shows comprehensive performance, including ultralow density, good mechanical properties, good machinability, low FIR transmittance and ultralow thermal transmittance. The UV blocking and green house effects make the SNTAs a perfect material for use in spacecraft and in the civil domain.

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Figure 1. Preparation, digital photographs and TEM images of samples. (a) Schematic illustration of the preparation of SNTA, (b) CA has super black appearance and slender skeletons, (c) CA/SiO2 aerogel does not show obvious surface changes, but skeletons become smooth and reinforced. (d) SNTA has high transparency, super hydrophobicity, and nanotube skeletons.

EXPERIMENTAL SECTION Materials. All reagents, including resorcinol (99%), formaldehyde (37% in water), sodium carbonate, ethanol, tetraethoxysilane, aqueous ammonia solution and 1H,1H,2H,2H-perfluorooctyl trichlorosilane, were purchased from Shanghai Chemical Reagent Company with analytical grade purity and were used without further purification. Sample Preparation. CAs in this study are derived from the carbonization of resorcinol-formaldehyde (RF) aerogels, and RF aerogels are prepared via the sol-gel polymerization of R and F with sodium carbonate as a catalyst and deionized water as a solvent.31-33 Resorcinol (3.14 g) is dissolved in deionized water (91.260 mL), and formaldehyde (4.470 mL) is added. The solution is stirred for 10 min, and sodium carbonate aqueous solution (0.735 mL, 0.05 mol L1) is added. After further stirring, the precursor solution is cured in a sealed glass vial (3.5 cm in diameter) at 85 for 72 h. The gel is removed from the vial, cut into a disk shape (1.5 cm in thickness), and washed with ethanol several times to completely remove residual water and solvents. The wet gel is dried using supercritical CO2 to obtain the RF aerogel, which is subsequently carbonized in a quartz tube furnace at 1000 for 3 h under N2 flow to obtain the CA. Carbon-Silica core-shell (CA/SiO2) aerogels are fabricated via CVD of TEOS catalyzed by ammonia. CA is placed in a vacuum desiccator together with two open glass vessels containing 2mL of TEOS and aqueous ammonia solution, respectively. The desiccator is closed, outgassed for several

seconds, and placed in a thermotank (160 ). CVD of TEOS is performed for 6 h. The desiccator is allowed to cool to room temperature to obtained a CA/SiO2 aerogel. To obtain different silica thicknesses, experiments with different deposition cycles are performed. The CA/SiO2 aerogel is named CA/SiO2-x, where x represents the number of CVD cycles. The corresponding SNTAs are fabricated via calcination of CA/SiO2 aerogels. Calcination is performed in a quartz tube furnace at 600 for 2 h under air. To reduce surface energy, hydrophilic SNTA is coated with volatile silane (1H,1H,2H,2H-perfluorooctyl trichlorosilane, 1 mL) by CVD for 4 h at room temperature. The final SNTA is named SNTA-x. Characterization. The bulk density of the SNTAs is determined by ρ=M/V, where ρ, M, and V are the bulk density, mass, and volume of the SNTA, respectively. The linear shrinkage is determined by 100·(DCA-DSNTA)/ DCA, where DCA and DSNTA are the diameters of CA and SNTA, respectively. The morphology of the sample is characterized by transmission electron microscope (TEM, Tecnai G2 F20 STwin, USA) and scanning electron microscopy (SEM, Philips-XL30FEG, USA). The average thickness of the silica nanotube wall was measured using Nano Measure software from TEM images, with 50 data points in each image. Nitrogen adsorption-desorption isotherms at 77.4 K are measured using a N2 adsorption analyzer (TriStar 3000, USA) to determine the specific surface area and pore size distribution. Before N2 adsorption/desorption measurements, the aerogel was degassed at 120 under vacuum for approximately 12 h. The total specific surface area and


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Figure 2. Digital photographs, mass, density, shrinkage and TEM images of different samples. (a) Monolithic SNTAs show different transparencies under macroscopic observation. (b) The mass of CA/SiO2 aerogel increases with CVD cycles, which leads to density ranges from 25.30 to 65.64 mg cm-3 of SNTAs. (c) Linear shrinkage of SNTAs and thickness of the silica shell. (d) CA/SiO2 aerogels show reinforced, smooth skeletons. The morphology of the crosslinked points of skeletons changes from ‘V’ to ‘U’ after repeated CVD. (e) SNTAs show smooth, nanotubular skeletons.

the total pore volume are calculated using the BET equation applied at a relative pressure ranging from 0.05-0.35. The micro specific surface area and micro pore volume are calculated using t-plot method applied with the relative pressure ranging from 0.2-0.5. The pore size distribution is obtained from the adsorption branch of the isotherms using the Barrent-Joyner-Halenda (BJH) model and HorvaihKawazoe (HK) equation. Young’s modulus is measured using an eletronic universal tensile testing machine (CMT5105, China). The transmittance spectra of SNTAs is measured using a UV/vis/NIR spectrophotometer (V-570, Japan) equipped with an integrating sphere (Figure S1) and Fourier transform infrared spectroscopy (FTIR, Tensor 27, Germany). The room temperature thermal conductivi-

ties are measured using a hotdisk thermal analyzer (TPS 3500, Sweden). The infrared image is taken with an infrared thermal imager (FLIR T630sc, China). The sleeve is processed using mini milling machine (FF 500, Germany). The images of contact angles and sleeve thickness are measured by optical microscopy. The volume of the water droplet was fixed at 3 μL. RESULTS AND DISCUSSION Synthesis and textural properties of SNTAs Our approach is illustrated in Figure 1a. Low density CA is derived from supercritical drying and carbonization of



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Table 1. Pore structural parameters of samples Sample

SBETa/m2 g-1

Smicb/m2 g-1

Vtotalb/cm3 g-1

Vmicb/cm3 g-1

Dporec/nm

CA

811.51

572.71

0.781

0.299

11.10

CA/SiO2-1

190.57

34.32

0.386

0.015

8.09

CA/SiO2-2

172.82

43.09

0.329

0.019

6.46

CA/SiO2-3

141.28

42.35

0.302

0.021

8.54

CA/SiO2-4

137.74

43.63

0.282

0.022

9.85

SNTA-1

427.36

91.94

1.152

0.043

10.79

SNTA-2

412.10

98.23

1.041

0.046

10.11

SNTA-3

370.70

92.07

0.968

0.044

10.44

SNTA-4

422.90

98.92

0.997

0.047

9.43

a Total

specific surface area and pore volume calculated using the BET method; calculated by the t-plot method; c Average pore diameter.

the RF hydrogel. We use the skeleton of CA as the template, because CAs have a three-dimensional, random nano-scale frame structure (Figure 1b, Figure S2), which contributes to improved performance. Similar to a Stöber reaction and Deng’s method,34-35 the carbon core is coated with a silica shell via CVD of the ammonia-catalyzed TEOS, obtaining the carbon-silica core-shell (CA/SiO2) aerogel. Compared with CA, the CA/SiO2 aerogel with silica coat shows smooth, reinforced skeletons (Figure 1c). There is no obvious change in the color of the CA/SiO2 aerogel, indicating that the silica shell has a very high degree of transparency. Calcinating CA/SiO2 aerogel at 600 for 2 hours in an air atmosphere leads to combustion of the carbon core and the formation of a rigid nanotubular silica aerogel (Figure 1d). The aerogel is coated with volatile silane to reduce the surface energy and enable super hydrophobicity. The final sample is named as SNTA. The SNTA shows high transparency, low density (25.30 mg cm−3) and super hydrophobicity with a static contact angle of 144º (Figure 1d (insert)). Water drops on the surface of SNTA can bounce off (Movie S1). To adjust the shell thickness of silica nanotubes in the SNTA, we coated the carbon core several times (6 h each time). To distinguish, samples are named as ‘SNTA-x’, where x represents the number of CVD cycles. SNTA (Figure 2a) shows different transparencies under macroscopic observation. Differences in transparency are related to strong Rayleigh scattering and will be discussed in the optical properties section. Figure 2b shows the mass changes of each sample in different steps and the density of SNTAs. The mass of CA/SiO2 aerogel can be precisely designed by adjusting the CVD cycles and shows a monotonously increasing trend, which leads to a monotonous increase in SNTA mass. The limited increase in mass and small linear shrinkage (Figure 2c) leads to low density in SNTAs, ranging from 25.30 to 65.64 mg cm−3. Figure 2d shows the TEM images of CA/SiO2 aerogels. CA has slender skeletons, which resemble a chain of pearls. Unlike CA, CA/SiO2 aerogels have reinforced smooth carbon-silica core-shell skele-

b Micropore

surface area and micropore volume

tons. The rough chains of CA convert to smooth core-shell clubs of CA/SiO2 aerogels, and we clearly observe the skeleton coarsening process from CA/SiO2-1 to CA/SiO2-4. Due to the effect of capillary condensation, the morphology of the crosslinked points of skeletons also changes from “V” type to “U” type after repeated CVD, especially for the CA/SiO2-4. The “U” type structure can enhance the mechanical performance of the materials. Figure 2e shows TEM images of SNTAs. After combustion of the carbon core, the silica shell maintains its roughness and network texture. The clubbed core-shell structure becomes nanotubular and still retains the “U” type morphology of crosslinked points. As shown in Figure 2c, the measured average thickness of silica shell ranges from 3.57 to 5.03 nm. Nitrogen adsorption-desorption isotherms are measured to further characterize the textural properties of different aerogels. As shown in Figure 3a, CA shows type II isotherms,36 corresponding to an interconnected macroporous system. Mercury intrusion porosimetry (MIP) (Figure S3) indicates that CA has broad pore size distribution ranges from 0.02 to 8 μm, which facilitates the transport of vapor TEOS and ammonia. CA also has several smaller feature pores (0.56, 2.79, and 11.40 nm), caused by the crosslinked points of the carbon nanospheres and their roughness. After coating, the feature pores of CA are covered by silica shell. All CA/SiO2 aerogels show a relatively low unfeatured pore size distribution (Figure 3b), indicating that the rough chains of CA turn into the smooth coreshell clubs of CA/SiO2 aerogels. Compared with CA, the specific surface area and total pore volume of CA/SiO2 aerogels decrease significantly (Table 1). These pore structural parameters have a good corresponding relationship with TEM images. After combustion of CA, SNTAs show type IV isotherms (Figure 3c),36 corresponding to an interconnected mesoporous system. The feature mesopore (Figure 3d) is formed via removal of the carbon core. The highest specific surface area and total pore volume of SNTAs can reach 427.36 m2 g−1 and 1.153 cm3 g−1, respectively. The high


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Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution of CA, CA/SiO2 aerogels and SNTAs. (a) CA shows an interconnected macroporous system. (b) Broad pore size distribution of CA contributes to transport of TEOS and ammonia. Unfeatured pore size distributions of CA/SiO2 aerogels indicate that rough chains turn into smooth core-shell clubs. (c) SNTAs show an interconnected mesoporous system. (d) SNTAs feature mesopores, formed by removal of the carbon template.

pore volume and nanotubular skeletons lead to the low density of the SNTAs. Optical properties Silica aerogels with high transparency can be used in many fields, such as thermal insulation, the aerospace program, and the Cherenkov radiation detector. Compared to the spectrum of the polar bear hair,9 SNTAs have the similar spectrum line shape in the infrared regions (Figure 4a). However, SNTAs have higher visible and NIR light transmission and ultralow UV transmission. The highest average transmittance between 400 and 780 nanometers is 53.91 %. It is known that light transmission in aerogels is dominated by Rayleigh scattering:3 , ⁄ (1) where T (λ, h) is the transmittance, A and C are constants, λ is the wavelength, and h is the thickness of the aerogel. Parameter C is the “clarity coefficient” and is the key parameter to determine the transmittance. The clarity coefficient (Figure S4) of SNTAs obtained by fitting the direct transmittance curves according to Equation (1) is larger than the traditional silica aerogel (C=0.0053).37 In other words, the thin wall (several nanometers) of SNTA leads to stronger Rayleigh scattering, especially in the UV band. The stronger scattering of UV light leads to a larger clarity coefficient and thus lower transmittance.37-38 The direct transmittance is lower than the transmittance measured with an

integrating sphere, which also demonstrates strong Rayleigh scattering in SNTAs. For different SNTAs, it is clear that a higher density aerogel has lower transmittance. This phenomenon is very easy to understand. SNTAs with larger silica shell thicknesses have greater density. First, a larger density indicates a larger equivalent refractive index. According to the Fresnel formula, materials with a large refractive index have a large reflectance. In other words, a higher density aerogel has a larger reflectance. Second, a larger density indicates larger solids content and therefore more light absorption. SNTA may be an ideal material for human beings for four reasons: (i) perfect UV blocking performance protects the human body, animals, and others from UV radiation. (ii) Relatively high visible light transmission (50.35% at 550 nm) ensures good daylighting performance. (iii) Relatively high NIR light transmission facilitates heating by solar radiation. (iv) Perfect FIR light interception leads to the greenhouse effect. According to blackbody radiation law and Wien’s displacement law: (2) where λmax is the peak wavelength of radiation, T is the absolute temperature of the blackbody, and b is a constant with a value of 2.898×106 nm K. When the temperature ranges from -50 to 40 , the peak wavelength ranges from 9.25 to 12.99 µm. Therefore, mutual FIR radiation between the human body and the harsh environment can be blocked by SNTAs, which contributes to personal thermal management, if it is used in high latitudes and polar regions.



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Figure 4. The optical and thermal performance of SNTAs. (a) Due to the strong Rayleigh scattering. SNTA, especially SNTA-1, has high visible and NIR light transmission, but ultralow UV and FIR transmission, which is totally different from the spectra of polar bear hair. (b) SNTAs have low thermal conductivity ranging from 30.25 to 32.67 mW m−1 K−1. These four low heat transfer components together contribute to the ultralow thermal conductivity of SNTAs (insert). (c) The picture and infrared images of cloth, polystyrene foam, and SNTA-3. SNTA-3 shows better thermal insulation and infrared blocking performance at 50 and 100 . (d) Changes in the temperature of the central area of the sample surface, SNTA-3 shows a slowest temperature rise process and lowest thermal equilibrium temperature.

thermal conductivity of the gas (λg) and radiative heat transfer coefficient (λr):42-45

Thermal insulation properties. Silica aerogels have been extensively studied by researchers for their good thermal insulation properties and high temperature resistance.39 Wang et al established a heat transfer model to study the heat transfer process in silica aerogels with different nanostructures.40-41 The SNTAs also have good thermal insulation properties. Its room temperature (26 ) thermal conductivity ranges from 30.25 to 32.67 mW m−1 K−1 (Figure 4b). The measured total thermal conductivity (λtotal, reflecting heat transfer coefficient in fact, Figure 4b (insert)) of highly porous materials comprises four components: convective heat transfer coefficient (λc), thermal conductivity of the solid (λs),

(3)

the convective heat transfer coefficient is negligible in the aerogel.46 Therefore, its contribution can be ignored. The solid thermal conductivity can be written as follows:42, 44 ,

! !

(4)

where λs,s is the solid thermal conductivity for the basic materials, ρ is the apparent density, ρs is the true density of the sample, ν is the velocity of phonon, and νs is the value estimated by the vibration of the phonon in the solid skele-


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ton for heat and transfer. Because SNTAs have a low solid fraction, interconnected 3D structure, nanotubular skeletons, and the thickest nanotube wall is only 5.03 nm, the SNTAs exhibits much lower solid thermal conductivity than traditional silica aerogels (with larger particle thickness: dozens of nanometers). Obviously, according to equation 4, low density (thin silica shell thickness) SNTA has low solid thermal conductivity. The gas thermal conductivity can be written as follows:42-44

"$ # %&'()* ⁄+,

(5)

where λ0g is the thermal conductivity in free air, α is the constant specific to the gas in the pores, lmfp is the mean free path of a gas molecule, and lcl is the average pore diameter in the porous materials. SNTAs have small average pore diameters, in the range of 9.43-10.79 nm. The average pore diameter is much smaller than the mean free path (70 nm) of the major molecules (N2, O2) in the atmosphere. The small average pore diameter (lcl) leads to a low λg according to equation 5. For different SNTAs, there is almost no difference in the pore size distribution curves of SNTAs, so the difference in their gas thermal conductivities is negligible. The radiative thermal conductivity can be written as follows:42, 44

%- /012 . 34 ⁄!

(6)

where σ is the Stefen-Boltzmann constant, n is the mean refractive index, and Κa is the absorption coefficient. SNTAs have negligible radiative thermal conductivity, because the contribution of radiative thermal conductivity to the total thermal conductivity is low at room temperature, and SNTAs have perfect FIR light interception (Figure 4a). These four components of heat transfer together contribute to the ultralow thermal conductivity of SNTAs at room temperatures. Infrared images are measured to compare the thermal insulation and infrared blocking performance of SNTA with other common thermal insulation materials. The thicknesses of cloth, polystyrene foam and SNTA-3 are all approximately 2.5 mm. As shown in Figure 4c, compared with cloth and polystyrene foam, SNTA-3 shows better thermal insulation and infrared blocking performance when the temperature of the heating platform is 50 and 100 . In the first 30 seconds, the temperature of cloth and polystyrene foam changes greatly. However, the temperature of SNTA-3 is always lowest and changes only a little. We can clearly see the difference between SNTA-3 and common insulation materials from the temperature change curve of the center of the material surface (Figure 4d). Unlike cloth and polystyrene foam, the temperature of SNTA-3 rises slowly and the thermal equilibrium temperature is much lower. The thermal equilibrium time of SNTA-3 at 50 and 100 is 90 seconds and 60 seconds, respectively, while that of cloth and polystyrene foam are less than 25 seconds. The thermal equilibrium temperature of SNTA-3 at 50 and 100 is 34.5 and 54.1 , respectively, which is lower than that of cloth and polystyrene foam and far lower than the temperature of the heating platform. The good thermal insulation and infrared blocking performance of SNTAs lead to ideal isolation that reduces both

body heat loss and energy release to warm the body used in building or wearable devices in the high latitudes or polar region. Mechanical properties and machinability. In practical applications, mechanical performance is crucial. SNTAs have better mechanical performance than traditional silica aerogels due to their smooth and nanotubular structure and ‘U’ type crosslinked points of the skeletons. As shown in Figure 5a, high density SNTAs present the typical behavior of brittle materials and show a similar linear elastic stage. The reversible deformation corresponds to reversible bending of the nanotubular network structure. The slope of the curve is considered Young’s modulus. For high density SNTAs, the compression stress increases proportionally with the strain and declines abruptly at the highest point when the sample is broken. This indicates that high density SNTAs are brittle. Low density SNTAs present both elastic and plastic behavior and can be significantly compressed without obvious brittle fracture (the highest compressive strain can surpass 70%, Figure S5). As shown in Figure 5b, SNTAs have a relatively high Young’s modulus (0.302-1.996 MPa) and a high linear deformation capacity (8.13-35.20%). The Young’s modulus of SNTA-4 is higher than traditional silica aerogels (78 mg cm−3, 1.2 MPa).47 SNTAs are also machinable (Figure 5c). An SNTA-3 sleeve with a wall thickness of 500 µm and outer diameter of 1cm is machined. Further detailed processing steps are given in Figure S6. The sleeve can be supported by a human hair, and the hair shows no obvious bending, demonstrating good mechanical properties and ultralow density for SNTAs. The reasons for the good mechanical properties are the following three points: (i) as discussed in the morphology section, the morphology of crosslinked points of skeletons changes from “V” to “U”, which increases the strength of the crosslinked points of skeletons. The smooth skeletons of SNTAs also show better strength, compared with the skeletons of a chain of pearls of traditional silica aerogels. (ii) SNTAs have dense, rigid nanotubular skeletons after calcination. (iii) Compared with a clubbed structure, a nanotubular structure has a low section modulus in bending, when they have the same outer diameter. The section modulus in bending (Figure 5d) of a clubbed structure (Wzc) and a nanotubular structure (Wzn) can be written as: 56 560

782

(7)

.9 782 .9

;

1 8

(8)

where D and d are the outer diameter and inner diameter of section, respectively. We calculate that the ration of Wzc/Wzn ranges from 1.09 to 1.21. In other words, a nanotubular structure is more likely to bend. For different SNTAs, we assume that the inner diameter of the nanotubes is the same. Because the same CA core was used and there was almost no difference in the pore size distribution curves of SNTAs, low density (thin silica shell thickness) SNTA therefore has a small outer diameter. According to equation 8, the small outer diameter leads to a low section



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Figure 5. Mechanical performance of SNTAs. (a) Low density SNTAs present a combined behavior of both elastic and plastic materials, but high-density SNTAs presents brittle performance. (b) SNTAs have relatively high Young’s modulus and large linear deformation capacity. (c) SNTA-3 could be easily machined into a sleeve, which can be supported by a human hair, with no obvious bending of that hair, demonstrating good mechanical properties and ultralow density. (d) The illustration shows nanotubular structure with low section modulus in bending compared to clubbed structure, contributing to elastic behavior.

modulus in bending. In other words, SNTA with thin silica walls (low density) is more likely to be compressed, in good agreement with the stress-strain curves and Young’s modulus in Figure 5a-b. Based on the performance of SNTAs analyzed above, we concluded that the thickness of the silica shell has a great effect on the performance of SNTA. As shown in Figure S7, the density, thermal conductivity, and Young’s modulus of SNTA are positively correlated with the thickness of the silica shell, and the transmittance of SNTA is negatively correlated with the thickness of the silica shell.

ASSOCIATED CONTENT Supporting Information. Schematic of the transmittance measurement setup, SEM image of CA, broad pore size distribution of CA according to Mercury intrusion porosimetry, fitting results of transmittance according to Rayleigh scattering, Full stress-strain curves of SNTA-3 and SNTA-4, processing steps of SNTA-3 sleeve, Summary of the performance of SNTAs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author CONCLUSIONS

* [email protected]

In summary, inspired by polar bear hair, we fabricated multifunctional SNTAs using a multi-step process. We coated the nano-scale carbon core of CA with a silica shell, adopting CVD for ammonia-catalyzed TEOS, followed by combustion of the carbon core in air and then hydrophobic treatment. The smooth, dense nanotube morphology and “U” type crosslinked points of the skeletons ensured the ultralow density (25.30 mg cm−3), high Young’s modulus (1.996 MPa), high linear deformation capacity (35.2%), and machinability of SNTAs. The designed SNTAs showed relatively high visibility and NIR light transmittance and almost no UV and FIR light transmittance, ideal properties to provide the greenhouse effect and protect human beings from UV radiation. Due to their versatility, SNTAs have further applications in thermal insulation, gas storage, sensors, and adsorption. The simple fabrication route also provides a sight to design aerogels with airy density and different controllable micromorphologies.

ORCID Ai Du: 0000-0002-6113-6185. Hongqiang Wang: 0000-0001-6036-9756.

Author Contributions ‡A. Du and H. Wang contributed equally. The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank Prof. Hao Bai and Prof. Ning Zhao for their constructive discussion. This work was supported by the National Key Research and Development Program of China (2017YFA0204600).

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Multifunctional Silica Nanotube Aerogels Inspired by Polar Bear Hair

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