Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Bioinspired Ultralight Inorganic Aerogel for Highly Efficient Air Filtration and Oil−Water Separation Yong-Gang Zhang,†,‡ Ying-Jie Zhu,*,†,‡ Zhi-Chao Xiong,† Jin Wu,† and Feng Chen*,† †
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Inorganic aerogels have been attracting great interest owing to their distinctive structures and properties. However, the practical applications of inorganic aerogels are greatly restricted by their high brittleness and high fabrication cost. Herein, inspired by the cancellous bone, we have developed a novel kind of hydroxyapatite (HAP) nanowire-based inorganic aerogel with excellent elasticity, which is highly porous (porosity ≈ 99.7%), ultralight (density 8.54 mg/cm3, which is about 0.854% of water density), and highly adiabatic (thermal conductivity 0.0387 W/m·K). Significantly, the as-prepared HAP nanowire aerogel can be used as the highly efficient air filter with high PM2.5 filtration efficiency. In addition, the HAP nanowire aerogel is also an ideal candidate for continuous oil−water separation, which can be used as a smart switch to separate oil from water continuously. Compared with organic aerogels, the as-prepared HAP nanowire aerogel is biocompatible, environmentally friendly, and low-cost. Moreover, the synthetic method reported in this work can be scaled up for large-scale production of HAP nanowires, free from the use of organic solvents. Therefore, the asprepared new kind of HAP nanowire aerogel is promising for the applications in various fields. KEYWORDS: aerogel, hydroxyapatite nanowire, ultralight, PM2.5, oil−water separation network of rods or plates.26,27 The cancellous bone is composed of hydroxyapatite (HAP) crystals deposited within an organic matrix.28 The biomineralized meshwork of the cancellous bone provides a large specific surface area, interconnected pores, and excellent mechanical properties. This cellular structure is also utilizable in the adsorption and storage. However, the cancellous bone-inspired inorganic functional materials have been rarely reported to date. In this work, inspired by the cancellous bone, we report a new kind of HAP nanowire aerogel with a multiple meshwork structure, high porosity (∼99.7%), ultralight density (8.54 mg/ cm3), high elasticity, and ultralow thermal conductivity (0.0387 W/m·K). The as-prepared HAP nanowire aerogel is a promising air filter with excellent PM2.5 filtration efficiency and low air flow resistance. In addition, the HAP nanowire aerogel is also an ideal material for continuous oil−water separation, which can be used as a smart switch to separate oil from water continuously. The as-prepared HAP nanowire aerogel is biocompatible, environmentally friendly, and lowcost. Moreover, the synthetic method reported herein can be scaled up for the large-scale production of HAP nanowires, free from the use of organic solvents. Therefore, the as-prepared
1. INTRODUCTION Aerogels, first reported by Kistler in 1931, are porous materials formed using air to replace the liquid component of wet gels.1 Up to now, various kinds of aerogels have been successfully fabricated, including carbon nanotube,2,3 graphene,4,5 copper,6 gold,7 silica,8 and alumina.9 These lightweight porous materials exhibit many interesting and unusual properties, including large pore volume, large specific surface area, low density, low thermal conductivity, and low acoustic propagation speed; thus, they are promising for a wide range of applications.10−12 Nevertheless, the practical applications of the existing aerogels have been constrained because of the following shortcomings: (1) high cost and complex preparation procedures for the building blocks, including carbon nanotube13 and graphene;14 (2) high brittleness of inorganic aerogels;15−18 (3) many aerogels are fabricated using the expensive and dangerous critical point drying (CPD) method to overcome the damaging effect of large capillary forces that exist during the drying process.7,19−24 In addition, the aerogels made from carbon, metals, and metallic oxides are not biodegradable and may cause healthcare and environmental problems. The rich diversities of functional biological structures that exist in nature have been studied extensively for decades, providing mechanical design principles and inspirations for the fabrication of synthetic structural materials.25 Cancellous bone is a cellular bone (porosity 40−90%) made of a connected © XXXX American Chemical Society
Received: February 4, 2018 Accepted: March 23, 2018
A
DOI: 10.1021/acsami.8b02081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
scanning electron microscopy (SEM, Hitachi S-4800, Japan). X-ray powder diffraction (XRD) patterns were obtained using an X-ray diffractometer (Rigaku D/max 2550 V, Cu Kα radiation, λ = 1.54178 Å). The Fourier transform infrared (FTIR) spectra of the HAP nanowire aerogels were performed with an FTIR spectrometer (FTIR7600, Lambda Scientific, Australia). Water contact angles were measured with an optical contact angle system (model SL200B). Thermogravimetric analysis (TGA) was carried out using a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) in flowing air at a heating rate of 10 °C min−1.
new kind of HAP nanowire aerogel is promising for the applications in various fields.
2. EXPERIMENTAL SECTION 2.1. Materials. Anhydrous calcium chloride (CaCl2), sodium oleate, sodium hexametaphosphate [(NaPO3)6], chloroform, oleic acid, cyclohexane, N,N-dimethylformamide, and ethanol were purchased from Sinopharm Chemical Reagent Co. All reagents were used as received without further purification. 2.2. Preparation of HAP Nanowire Aerogel. First, the hydrothermal product slurry containing ultralong HAP nanowires was prepared by the calcium oleate precursor hydrothermal method using an aqueous solution containing CaCl2, sodium oleate, and (NaPO3)6. In a typical experiment, 438.4 g of sodium oleate was dissolved in 2.5 L deionized water, and 2.5 L of CaCl2 (44.0 g) aqueous solution was added. After stirring for 1 h (400 rpm), 2.0 L of (NaPO3)6 (36.7 g) aqueous solution was added and stirred for 30 min (400 rpm). The resulting reaction system was transferred into a 10 L stainless steel autoclave and heated to 200 °C and maintained at that temperature for 36 h. When the autoclave cooled to room temperature, the hydrothermal product aqueous slurry containing HAP nanowires is obtained. The obtained hydrothermal product aqueous slurry with a certain volume was placed in the mold and dried by freeze-drying. After the freeze-drying, the sample was immersed in ethanol for 6 h (or in the mixture of NaOH aqueous solution and ethanol [20 mL of 10 M NaOH aqueous solution and 480 mL of ethanol] at 75 °C for 6 h) and then in deionized water for 6 h to remove impurities and was lyophilized to obtain the HAP nanowire aerogel. 2.3. Compression Tests of HAP Nanowire Aerogels. Compression tests of HAP nanowire aerogels with a height-to-width ratio of 0.6−0.8 were conducted in a single-column mechanical testing system (Instron-5566, Instron) at a constant loading rate of 2 mm/ min. 2.4. Particulate Matter Generation and Removal Efficiency Measurements. The particulate matter (PM) particles were generated by burning incenses in a hermetic room (3 m × 5 m), and the PM concentration was controlled by adjusting the amount of the burning incenses. The PM concentration was measured by a particle counter (DT-9881M, CEM), and the PM removal efficiency was calculated by comparing the PM concentrations before and after filtration. The pressure drops of the HAP nanowire aerogel at different air flow velocities were measured by a differential pressure gauge test instrument (EM201B, UEi). Each measurement was performed three times. For the model test of the air purifier, a hermetic chamber was used to simulate indoor circumstance. A small fan equipped with the HAP nanowire aerogel filter (25 mm × 17 mm) was put in the hermetic chamber. When the incenses were burnt, the fan was turned on. The digital images of the hermetic chamber were captured, and the PM concentrations in the hermetic chamber were measured at given time intervals. 2.5. Oil−Water Separation Tests. The HAP nanowire aerogel was placed in the oil or organic solvent. After absorption, the HAP nanowire aerogel was withdrawn. The absorption capacity was calculated by comparing the weight of the HAP nanowire aerogel before and after absorption. The cyclic absorption test was performed by absorption−squeeze cycles 20 times. The continuous oil−water separation was performed by an oil−water separation device made of a piece of hydrophobic HAP nanowire aerogel, a tube, and a peristaltic pump, and the tube was inserted into the HAP nanowire aerogel and immersed into the oil−water system. When the peristaltic pump was turned on, the oil or organic solvent was absorbed into and flowed through the hydrophobic HAP nanowire aerogel to the collector. Finally, the oil or organic solvent was continuously separated from the oil−water system and collected. 2.6. Characterization. The as-prepared HAP nanowires were observed with a field-emission transmission electron microscopy (TEM, JEOL JEM-2100F, Japan) under an accelerating voltage of 200 keV. The as-prepared HAP nanowire aerogels were characterized by
3. RESULTS AND DISCUSSION The schematic illustration for the structure of the cancellous bone is shown in Figure 1A. The SEM micrographs of a
Figure 1. (A) Schematic illustration of the microstructure of the cancellous bone. (B) Digital image and SEM micrographs of the cancellous bone. (C) Digital image and SEM micrographs of the asprepared HAP nanowire aerogel.
cancellous bone from the cancellous femur of a mature New Zealand white rabbit reveal the cellular structure which is composed of the interconnected porous meshwork (Figure 1B). It is well-known that the porous meshwork structure is composed of HAP nanocrystals deposited in the collagen fibril matrix. Inspired by the microstructure of the cancellous bone, a new kind of highly porous and ultralight HAP nanowire aerogel has been prepared by a self-assembly strategy using ultralong HAP nanowires. The as-prepared HAP nanowire aerogel exhibits a highly porous structure, which is similar to the porous meshwork of the cancellous bone in both nanoscale and macroscale (Figure 1C). The synthetic process of the HAP nanowire aerogel is shown in Figures 2A and S1 in the Supporting Information. The preparation method reported in this work is environmentally friendly, low-cost, and can be scaled up for a large-scale production, which is superior compared with some complex methods, such as the chemical vapor deposition29−31 and CPD.32,33 The hydrothermal product aqueous slurry containing HAP nanowires is prepared by the calcium oleate precursor hydrothermal method in a 10 L stainless steel autoclave.34 The hydrothermal product aqueous slurry is stable for a relatively B
DOI: 10.1021/acsami.8b02081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (A−C) Digital images and SEM micrographs of the freezedried HAP nanowire sample without washing (A), hydrophobic HAP nanowire aerogel (B), and hydrophilic HAP nanowire aerogel (C). (D−F) Digital images and schematic illustrations of the surface of the freeze-dried HAP nanowire sample without washing (D), hydrophobic HAP nanowire aerogel (E), and hydrophilic HAP nanowire aerogel (F) after adding a droplet of an aqueous solution of methylene blue on the top surface of the aerogel.
Figure 2. (A) Schematic illustration of the preparation of the new kind of HAP nanowire aerogel; step 1: washing with ethanol and water; step 2: washing with ethanol−NaOH−water. (B) TEM images of the as-prepared HAP nanowires, SAED pattern, and high-resolution TEM (HRTEM) image of a single HAP nanowire. (C) Schematic illustration of the surface structure of the as-prepared HAP nanowires adsorbed with oleate groups in the hydrothermal product aqueous slurry. (D) Schematic illustration of the self-assembly of HAP nanowires during washing with ethanol and water. (E) TEM image of self-assembled HAP nanowires (HAP nanowire bundles) after washing with ethanol and water.
observation (Figure 3A), XRD (Figure 4A), FTIR (Figure 4B), TGA (Figure 5A), and energy-dispersive spectroscopy (EDS) (Figure 5B). The XRD pattern of the freeze-dried sample without washing can be indexed to a mixture of HAP (JCPDS no. 09-0432) and sodium chloride (JCPDS no. 99-0059). There is also a broad hump at around 2θ = 20°. The constituents of NaCl and the organic component originate from CaCl2, (NaPO3)6, and sodium oleate, which were used as the reactants for the preparation of the hydrothermal product aqueous slurry containing HAP nanowires. Figure 4B shows the FTIR spectra of the freeze-dried HAP nanowire sample without washing, hydrophobic HAP aerogel, and hydrophilic HAP aerogel. The absorption peaks at 2923 and 2858 cm−1 in the FTIR spectrum of the freeze-dried sample without washing are attributed to the asymmetric and symmetric C−H stretching vibration of the alkyl group of the oleate. The TGA (Figure 5A) and EDS analysis (Figure 5B) indicate the existence of the oleate constituent in the freezedried sample without washing. The weight loss is as high as 84.22% in the TG curve (Figure 5A), and the weight percentage of the carbon element is 46.45% (Figure 5B) for the freeze-dried sample without washing. As a result, the freezedried HAP nanowire sample without washing is hydrophilic and can be re-dispersed in water. The surface hydrophobic/hydrophilic properties of the asprepared HAP nanowire aerogel can be adjusted by using different washing procedures. To remove the oleate groups adsorbed on the surface of HAP nanowires, the freeze-dried sample without washing is immersed in ethanol. In the freezedried sample without washing, an oleate layer is strongly
long time (Figure S2 in the Supporting Information). Figure 2B shows the TEM images of the as-prepared HAP nanowires with relatively uniform diameters. The selected-area electron diffraction (SAED) pattern of an individual HAP nanowire shows a single-crystalline structure. The as-prepared HAP nanowires have ultralong lengths and very high aspect ratios, as shown in Figure S3 in the Supporting Information. The HRTEM images in Figure 2B show a fringe spacing of 0.34 and 0.46 nm, which corresponds to the crystal plane of (002) and (110), respectively. The excellent dispersity of HAP nanowires in the hydrothermal product aqueous slurry can be explained by adsorbed oleate groups on the surface of HAP nanowires, which can prevent HAP nanowires from aggregation (Figure 2C). However, when the hydrothermal product aqueous slurry is washed with ethanol and water, the floccule-like aggregation appears, and the HAP nanowires self-assemble along the direction of the c-axis to form long fibers with a nanowire bundle structure (Figure 2D,E). Then, the hydrothermal product aqueous slurry containing HAP nanowires was placed into the mold with a well-designed shape and frozen at −20 °C. The frozen sample was subsequently dried by freeze-drying to remove ice crystals. The SEM micrographs indicate that a porous structure is formed after freeze-drying (Figure 3A). At this stage, the freezedried sample comprises HAP nanowires adsorbed with oleate groups and salts, which is characterized and confirmed by SEM C
DOI: 10.1021/acsami.8b02081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. XRD patterns (A) and FTIR spectra (B) of the freeze-dried HAP nanowire sample without washing, hydrophobic HAP aerogel, and hydrophilic HAP aerogel.
Figure 5. (A) TG curves of the freeze-dried HAP nanowire sample without washing, hydrophobic HAP aerogel, hydrophilic HAP aerogel, and commercial HAP powder. (B) Carbon weight percentage of the freeze-dried HAP nanowire sample without washing, hydrophobic HAP aerogel, and hydrophilic HAP aerogel measured by EDS.
adsorbed on the surface of HAP nanowires through the interaction between carboxylic groups and Ca2+ ions. There are also many free oleate ions stacked loosely on the surface of HAP nanowires. When the freeze-dried sample without washing is immersed in ethanol, most loosely stacked oleate ions are removed by diffusion, and only one layer of oleate ions is preserved by the electrostatic interaction between carboxylic groups and Ca2+ ions. Subsequently, the salts deposited on the surface of HAP nanowires are removed by washing with water. Finally, the hydrophobic HAP nanowire aerogel is obtained after washing with ethanol and water and then freeze-drying (Figure 3B,E). The XRD pattern of the hydrophobic HAP nanowire aerogel (Figure 4A) can be indexed to a single phase of HAP (JCPDS no. 09-0432), indicating the successful removal of NaCl in the HAP nanowire aerogel. The absorption peaks at 2923 and 2858 cm−1 in the FTIR spectrum of the hydrophobic HAP nanowire aerogel are much weaker than those of the freeze-dried sample without washing (Figure 4B), and the weight loss in the TG curve (18.38%, Figure 5A) and the weight percentage of the carbon element (8.84%, Figure 5B) of the hydrophobic HAP nanowire aerogel are also much lower than those of the freeze-dried sample without washing, implying the removal of oleate ions in the freeze-dried sample by washing with ethanol and water. The hydrophilic HAP nanowire aerogel can be easily obtained by washing the freeze-dried sample with NaOH aqueous solution + ethanol at 75 °C for 6 h and then with deionized water (Figure 3C,F). During the washing process with NaOH aqueous solution + ethanol, the layer of oleate ions adsorbed on the surface of HAP nanowires is removed by a reaction−dissolution process. The intensities of the absorption
peaks at 2923 and 2858 cm−1 in the FTIR spectrum of the hydrophilic HAP nanowire aerogel (Figure 4B) are hardly observed, the weight loss of the hydrophilic HAP nanowire aerogel in the TG curve is about 6.08% (Figure 5A), which is similar to the commercial HAP powder, and the weight percentage of the carbon element is low (6.2%, Figure 5B). The as-prepared HAP nanowire aerogels exhibit ultrahigh porosities. The porosity of the hydrophobic HAP nanowire aerogel is as high as 99.73%, and the porosity of the hydrophilic HAP nanowire aerogel is 99.68%. In addition, the as-prepared HAP nanowire aerogels have very low densities. The density of the hydrophobic HAP nanowire aerogel is as low as 8.54 mg/ cm3, and the density of the hydrophilic HAP nanowire aerogel is 10.1 mg/cm3 (Figure S4 in the Supporting Information). The calcium oleate precursor hydrothermal method adopted in this study is a large-scale synthetic technique using a 10 L autoclave, and 7 L hydrothermal product aqueous slurry containing HAP nanowires can be obtained by using a 10 L autoclave (Figure S1 in the Supporting Information). Furthermore, the shape and size of the HAP nanowire aerogel are controllable by using different molds (Figure 6A,B). The as-prepared hydrophobic HAP nanowire aerogel possesses an excellent elastic property (Figure 6C, Movie S1 in the Supporting Information). The compressed hydrophobic HAP nanowire aerogel can rapidly recover to its original shape and size after removing the load. Figure 6D shows the stress− strain curves of the hydrophobic HAP nanowire aerogel in the compressing−releasing process under different maximum D
DOI: 10.1021/acsami.8b02081 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
performance (Table S1 in the Supporting Information). The infrared image of the hydrophilic HAP nanowire aerogel with a thickness of 1 cm on a metal plate after heating by an alcohol lamp for 60 min is shown in Figure 6F. The temperature gradually decreases from 430 °C at the bottom to 55 °C on the top of the hydrophilic HAP nanowire aerogel, indicating that the hydrophilic HAP nanowire aerogel can inhibit the heat transportation efficiently. Therefore, the as-prepared hydrophilic HAP nanowire aerogel can act as an excellent heat insulator. As shown in Figure 6G,H, Movies S2 and S3 in the Supporting Information, a piece of cotton is placed on a metal plate which is heated by an alcohol lamp, and the cotton is burnt and carbonized quickly within 1 min (Figure 6G). In contrast, a piece of cotton can be well-protected by the hydrophilic HAP nanowire aerogel as a heat insulator (thickness = 5 mm) from the flame of an alcohol lamp even after heating for 5 min, implying the superior thermal insulation property of the as-prepared hydrophilic HAP nanowire aerogel. PM in polluted air is a complex mixture of solid and liquid particles that vary in size, number, shape, chemical composition, origin, and so forth. According to the size distribution of the particles, PM is divided into three categories, including coarse particles (more than 2.5 μm in diameter), fine particles (0.1 to 2.5 μm in diameter), and ultrafine particles (
