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Amphiphobic Nanofibrous Silica Mats with Flexible and High-Heat-Resistant Properties Meng Guo,†,‡,§ Bin Ding,*,†,‡ Xiaohong Li,‡,§ Xueli Wang,‡ Jianyong Yu,‡ and Moran Wang| State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua UniVersity, Shanghai 201620, China, Nanomaterials Research Center, Modern Textile Institute, Donghua UniVersity, Shanghai 200051, China, College of Textiles, Donghua UniVersity, Shanghai 201620, China, and Earth and EnVironmental Sciences DiVision and Center of Nonlinear Study (CNLS), Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: October 9, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009
In this study, for the first time, we fabricated flexible, high-heat-resistant, and amphiphobic mats by (fluoroalkyl)silane (FAS) modification of electrospun pure silica nanofibrous mats. The inorganic silica nanofibrous mats were obtained via electrospinning the blend solutions of poly(vinyl alcohol) (PVA) and silica gel, followed by calcination to remove the organic component. The PVA/silica and silica fibers were found to be randomly oriented as nonwoven mats with fiber diameters in the range of 150-500 nm. After the FAS modification, the surface wettability of the silica mats was converted from amphiphilic to amphiphobic. The fluorinated mat with the bead-on-string structure showed the highest water contact angle (WCA) of 154° and oil contact angle (OCA) of 144°. Additionally, the fluorinated inorganic fibrous mats exhibited a high heat resistance; they kept their hydrophobicity (WCA of 138°) and oleophobicity (OCA of 132°) even after the annealing treatment at 450 °C for 30 min. Potential applications of the fluorinated fibrous mats include high-temperature filtration, selective filtration, and self-cleaning coatings. 1. Introduction Amphiphobic surfaces that exhibit both water-repellent and oil-repellent properties, especially those with a contact angle for both the water and oil higher than or close to 150°, have attracted much attention for practical applications in water repellency, self-cleaning, contamination prevention, and antifouling.1,2 Inspired by the surface superhydrophobicity of natural lotus leaves, artificial amphiphobic surfaces can generally be fabricated by increasing the surface roughness and decreasing the surface energy of materials.3,4 Several approaches have been employed for making high surface roughness such as plasma etching,5 mechanical abrasion,6 anode oxidation,7 hot-water immersion,8 layer-by-layer self-assembly,9 chemical vapor deposition,10 etc. To lower the surface energy, chemicals with low surface energy such as fluorine-based or silicone-based silanes,11,12 wax,6 and polymers13,14 can be selected to modify the material surface. On the other hand, electrospinning has been proved to be a versatile and effective method for producing polymer or composite nanofibrous mats with high specific surface and high surface roughness.15,16 This technique involves applying a high voltage on a viscous solution to fabricate ultrathin fibers with diameters in the range of 20-2000 nm. Such nanofibrous mats can be used for a broad range of applications such as filtration,17 tissue engineering,18 sensors,19 solar cells,20 catalysts,21 etc. Acatay et al.22 and Jiang et al.23 first reported the mimicry of the topography of lotus leaves to achieve a superhydrophobic * To whom correspondence should be addressed. Fax: +86 21 62378392. Phone: +86 21 62378202. E-mail:
[email protected]. † College of Materials Science and Engineering, Donghua University. ‡ Modern Textile Institute, Donghua University. § College of Textiles, Donghua University. | Los Alamos National Laboratory.
surface by electrospinning a dilute polystyrene (PS) solution. Subsequently, by the inspiration of the lotus leaves, the superhydrophobicity of electrospun mat surfaces was extensively studied with other polymers such as poly(styrene-block-dimethylsiloxane),24 polyphosphazene,25 polyaniline/PS,26 Teflon/ poly(ε-caprolactone),27 etc. Moreover, inspired by the selfcleaning silver ragwort leaves, Miyauchi et al.28 designed a biomimetic superhydrophobic surface comprising micro/nanoporous PS microfibers via electrospinning. The achievement of surface superhydrophobicity of electrospun polymer mats with good flexibility is mainly attributed to the hierarchical micro/ nanostructure of the fibrous mats. However, the surface superhydrophobicity of fibrous polymer mats is not stable after the mats are treated at elevated temperature due to the meltdown of the fibrous polymer structures. Besides the fibrous polymer mats, fluorinated electrospun inorganic fibrous mats also showed superhydrophobicty such as ZnO,29 TiO2,30 ceramic,31 Fe3O4-filled carbon,32 etc., but the brittleness of inorganic fibrous mats significantly limits their practical applications. To the best of our knowledge, discussions of amphiphobic electrospun nanofibrous mats with flexible and high-heat-resistant properties are difficult to find in journals and monographs. Herein, we report the fabrication of a new class of flexible, high-heat-resistant, and amphiphobic nanofibrous mats by the combination of electrospinning, calcination, and surface modification techniques. Water-soluble poly(vinyl alcohol) (PVA) was introduced as a template material to blend with silica gel for producing the composite nanofibrous mats via electrospinning. Flexible inorganic silica mats could be obtained by the calcination of composite mats and further modified by (fluoroalkyl)silane (FAS). The surface wettability of fluorinated mats was investigated regarding their morphologies, compositions, and annealing temperatures.
10.1021/jp909672r 2010 American Chemical Society Published on Web 12/10/2009
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TABLE 1: Characteristics of the Solutions and Resultant Fibers sample name
PVA solution concn (wt %)
content of silica in PVA/silica fibers (wt %)
av fiber diam before calcination (nm)
av fiber diam after calcination (nm)
av bead width after calcinaton (nm)
PVA/S1 PVA/S2 PVA/S3
5 7 9
74.6 67.7 62.0
248 486 494
186 355 441
757 681
2. Experimental Section 2.1. Materials. The starting materials included PVA (Mn ) 86 000, Wako Pure Chemical Industries, Japan), tetraethyl orthosilicate (TEOS; Lingfeng Chemical Co., Ltd., China), phosphoric acid (85 wt %), hexane (Sinopharm Chemical Reagent Co., Ltd., China), dodecane (Aladdin Chemical Co., China), FAS (CF3(CF2)7(CH2)2Si(OCH3)3) (Gelest Inc.), and pure water. 2.2. Experimental Procedures. 2.2.1. Preparation of Precursor Solutions. PVA was dissolved in pure water at concentrations of 5, 7, and 9 wt % at 80 °C under vigorous stirring. A silica gel with a molar composition of TEOS:H3PO4:H2O ) 1:0.01:11 was prepared by hydrolysis and polycondensation by dropwise addition of H3PO4 to TEOS with stirring at room temperature for 6 h. Then 1 g of silica gel was dropped slowly into solutions of 1 g of PVA with various concentrations and the mixture stirred for another 6 h. Thus, viscous solutions of PVA/silica used as the electrospinning solutions were obtained. 2.2.2. Electrospinning. The composite solutions were placed in 10 mL plastic syringes and fixed with a syringe pump (LSP02-1B, Baoding Longer Precision Pump Co., Ltd., China). The feeding rate of the solutions by the syringe pump was 1 mL h-1. A positive electrode of a high-voltage power supply (DW-P303-1ACD8, Tianjin Dongwen High Voltage Co., China) was clamped to a metal needle tip, which was connected to the plastic syringe. A grounded stainless drum was used as the collector, which was covered by a piece of aluminum foil and rotated at 100 rpm. The applied voltage was 15 kV, and the tip-to-collector distance was 15 cm. The ambient temperature and relative humidity were kept at 25 °C and 50%, respectively. The composite PVA/silica fibrous mats prepared from 5, 7, and 9 wt % PVA solutions are denoted as PVA/S1, PVA/S2, and PVA/S3, respectively. The characteristics of the solutions and resultant fibers are listed in Table 1. 2.2.3. Fabrication of Fibrous Silica Membranes. The composite fibrous samples were calcined at 800 °C in air for 2 h to remove the organic components. The inorganic fibrous mats resulting from PVA/S1, PVA/S2, and PVA/S3 are referred to as S1, S2, and S3, respectively. 2.2.4. Fabrication of Flat Silica Films. A 1 g portion of silica gel was dropped onto the slide glass and dried at room temperature for 6 h. Then the sample was calcined at 800 °C in air for 2 h and is named S4. 2.2.5. Surface Modification of Silica Samples. Surface modification of the obtained inorganic samples was carried out by immersing the samples into 3 wt % FAS in hexane for 24 h at room temperature. Then the samples were dried in air and heated at 100 °C for 1 h in an oven. The FAS-modified S1, S2, S3, and S4 are denoted as FS1, FS2, FS3, and FS4, respectively. To investigate the effect of elevated temperature on the surface wettability of fibrous mats, the fluorinated samples were annealed for 30 min at 200, 250, 300, 350, 400, 450, and 500 °C, respectively. 2.3. Characterization. The morphology of the fibrous mats was examined by scanning electron microscopy (SEM; S-3000N, Hitachi Ltd., Japan). The diameter of the fibers was measured using an image analyzer (Adobe Photoshop CS2). Fourier
transform infrared (FT-IR) spectra were recorded using a Nicolet 8700 FT-IR spectrometer in the wavenumber range of 4000-400 cm-1. The water contact angle (WCA), WCA hysteresis (WCAH), oil contact angle (OCA), and OCA hysteresis (OCAH) of the resultant samples were measured with a contact angle meter (Contact Angle System OCA40, Dataphysics Co., Germany). Measurements from at least six droplets of 12 µL of freshly distilled pure water and dodecane were averaged. 3. Results and Discussion 3.1. Morphology, Composition, and Flexibility of Fibrous Mats. Figure 1 presents the SEM images of composite PVA/ silica fibrous mats formed with various PVA concentrations and their calcined samples. As was typical for electrospun mats, the mats showed three-dimensional (3D) structures of nanofibers and nanofibrous networks with a random fiber orientation and broad distribution of fiber diameters. In Figure 1a, the sample PVA/S1 was composed of thin fibers (average diameter of 248 nm) with numerous submicro- and microsized beads along the fiber axis. The formation of the bead-on-string structure was caused by the utilization of the solutions with low viscosity or low polymer concentration during the electrospinning process.33 The characteristics of the solutions and resultant fibers are shown in Table 1. Fibrous PVA/S1 mats formed from the PVA solution with the lowest concentration of 5 wt % exhibited the thinnest fiber diameter and highest bead density. The FT-IR spectrum of PVA/S1 is shown in Figure 2a. The absorption features of PVA were found at 3400 cm-1 (-OH), 2900 cm-1 (-CH2),
Figure 1. SEM images of various fibrous mats of (a) PVA/S1, (b) S1, (c) PVA/S2, (d) S2, (e) PVA/S3, and (f) S3.
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Figure 3. Optical image of a flexible silica fibrous mat (S1) when bent with a PET film.
Figure 2. FT-IR spectra of various fibrous mats of (a) PVA/S1 and (b) S1.
1740 cm-1 (CdO), 1450 cm-1 (OdCsOR), 1340 cm-1 (-CH2), and 1110 cm-1 (C-O-C).34 Additionally, absorption peaks at 1100, 790, and 470 cm-1 were observed in the case of the Si-O-Si bond, and the peak around 929 cm-1 was assigned to the Si-OH bond, indicating the existence of silica in the composite fibers.35 After the calcination at 800 °C, the bead-on-string structure was still maintained in S1 mats (Figure 1b). The average diameter of S1 fibers was decreased from 248 to 186 nm due to the pyrolysis of PVA in PVA/S1 mats. The complete pyrolysis of PVA in composite mats was confirmed by the FTIR result in Figure 2b. The absorption features around 1160, 1070, 790, and 470 cm-1 corresponded to the Si-O-Si bond left without showing the features of PVA. Meanwhile, the Si-OH band36 at 929 cm-1 disappeared after the calcinations. Such results indicated that the S1 mats were composed of pure SiO2 nanofibers. With increasing PVA concentration used in the solutions, the composite fibers of PVA/S2 (Figure 1c) and PVA/S3 (Figure 1e) both showed increased average fiber diameters of 486 and 494 nm as well as few beads compared with PVA/S1 fibers (Figure 1a). After the pyrolysis of PVA in the calcination process, their average fiber diameters were slightly decreased to 355 and 441 nm, respectively. It can be seen that the various structures of the fibrous mats could be fabricated by simply tuning the solution compositions for electrospinning. The flexibility of the nanofibrous silica mats could be demonstrated by putting the mats on the surface of a flexible PET film, fixing the two ends of the mats, and then bending the PET film outward (Figure 3). We noted that the silica mats were very flexible and no crack appeared during the bending process. The amorphous silica is primarily composed of an infinite network of corner-linked SiO4 tetrahedrons.37,38 The resultant silica nanofiber is a one-dimensional (1D) system formed by minimal crystal grains and the boundary component among the crystal grains. The extremely small size of the crystal grains led to an increased volume percentage of the boundary component, which means more atoms could be distributed on the boundary. Thus, the stress concentration was decreased substantially, which reduced the chance of the generation and extension of the microcracks on the fibers. Therefore, the flexibility of the 3D silica mats was a result of the extremely small size of the 1D silica nanofibers.
Figure 4. (a) Water contact angles and the corresponding shapes of water droplets for the fluorinated silica samples. (b) Water contact angle hysteresis for the fluorinated silica samples.
3.2. Amphiphobicity of Fluorinated Silica Samples. The nanofibrous silica mats without FAS modification showed superhydrophilicity with a WCA of 0°. Water droplets were absorbed by fibrous mats immediately after placement. A conversion of surface wettability was expected using an FAS monolayer modification.30 Figure 4a presents the WCAs and the corresponding shapes of a 12 µL water droplet of the fluorinated samples. The average WCAs of FS1, FS2, FS3, and FS4 were 154°, 147°, 141°, and 129°, respectively. Sample FS1 showed the highest hydrophobicity among the four samples. Its surface wettability was converted from superhydrophilic to
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Figure 6. Several water and dodecane droplets placed on the FS1 mats showing superhydrophobicity and oleophobicity.
Figure 7. Water contact angles and the corresponding shapes of water droplets for the FS1 mats annealed at different temperatures. Figure 5. (a) Oil contact angles and the corresponding shapes of dodecane droplets for the fluorinated silica samples. (b) Oil contact angle hysteresis for the fluorinated silica samples.
superhydrophobic after the surface modification with FAS. Fibrous samples of FS2 and FS3 did not show superhydrophobicity, but their WCAs were above 140°. The fluorinated flat silica film (FS4) possessed the lowest WCA of 129°. Figure 4b shows the variation of the WCAH of the fluorinated samples. The WCAH was observed to decrease concurrently with an increase in hydrophobicity. The lowest WCAH was 7°, which was observed on the FS1 sample. The OCAs and OCAH of the fluorinated samples are shown in Figure 5. Selected dodecane droplet images are shown in the inset. The oleophobicity of the fluorinated samples showed a change identical to that of the hydrophobicity of the samples. Sample FS1 had the largest average OCA of 144° and the lowest OCAH of 10°. The average OCAs of FS2 (139°) and FS3 (131°) mats were lower than that of the FS1 mat, but higher than that (121°) of the flat FS4 film. As a result, the amphiphobicity of the fluorinated samples was strongly affected by the structure of the surfaces. The fluorinated S1 mat showed the highest amphiphobicity possessing the bead-on-string structure (Figure 1b) which constructed the micro- and nanometer-scaled hierarchical structures on nanofibrous mats to the biggest extent. Therefore, the increase of the surface roughness of nanofibrous mats would be the key parameter to influence the surface wettability when the mat surfaces have the same chemical composition. On a solid surface, the binary micro- and nanometer-scaled structures could trap enough air to prevent the penetration of
the liquid droplet into the pores or grooves. The WCA and OCA of air are commonly regarded to be 180°. Cassie and Baxter proposed the following equation to describe the relationship between the contact angle on a flat surface (θ) and a rough surface (θr) composed of a solid and air:23
cos θr ) f1 cos θ - f2
(1)
In this equation, f1 and f2 are the ratios of solid surface and air in contact with liquid, respectively. In other words, f1 + f2 ) 1. This equation predicts that increasing the fraction of air (f2) would increase the contact angle of the nanofibrous mats (θr). Given the WCAs and OCAs of the fluorinated flat silica films and rough silica fibrous mats, the f2 value of the rough surface was calculated to be 0.73 and 0.61, respectively, which indicated that the enhancement of amphiphobicity by fluorinated nanofibrous silica mats was a result of the air trapped in the rough hierarchical micro/nanostructures of the mats. Several water and oil droplets were placed at the same time on the FS1 mats showing amphiphobicity in Figure 6. Brittle superhydrophobic fluorinated inorganic nanofibrous mats such as ZnO,29 TiO2,30 ceramic,31 Fe3O4-filled carbon,32 etc. have been widely reported. However, the brittleness of inorganic mats significantly limited their practical applications. In this study, we prepared the first example showing that the FAS monolayer modified inorganic silica nanofibrous masts possessed both flexibility and amphiphobicity at the same time. 3.3. High Heat Resistance of Fluorinated Fibrous Mats. The surface hydrophobicity of FS1 mats after the annealing treatment at different temperatures is presented in Figure 7. When the annealing treatment was carried out at or below 350
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Guo et al. indicating the existence of the -CH2- bond36 of FAS in the mats. However, the absorption feature of the C-F bond of FAS around 1145 cm-1 was overlapped by the broad and strong absorption peak around 1100 cm-1 in the case of the Si-O-Si bond. Additionally, a new peak at 1640 cm-1 was found which can be attributed to the Si-H2O absorption39 by the moisture absorbed in KBr during the FT-IR measurement. After the annealing treatment of FS1 mats at 450 °C, there were no evident changes in the FT-IR spectrum (Figure 9b) compared with Figure 9a. This result indicated that FS1 mat surfaces annealed at 100 and 450 °C have similar surface compositions and possess similar surface wettabilities. In Figure 9c, when the annealing temperature was up to 500 °C, the absorption peak around 2900 cm-1 disappeared, indicating the complete pyrolysis of the long chains of FAS from the fibrous silica mats. Therefore, the fluorinated fibrous sample calcined at 500 °C lost its surface amphiphobicity and became amphiphilic.
Figure 8. Oil contact angles and the corresponding shapes of dodecane droplets for the FS1 mats annealed at different temperatures.
4. Conclusions Inorganic nanofibrous mats with flexible, high-heat-resistant, and amphiphobic properties were successfully fabricated through electrospinning, calcination, and surface modification techniques. FS1 mats with the hierarchical micro/nanostructures of beadon-string showed high amphiphobicity with a WCA of 154° and an OCA of 144°. The surface amphiphobicity of FS1 mats was gradually decreased with increasing annealing temperature. After the annealing treatment at 450 °C, the FS1 mats still maintained their surface hydrophobicity (WCA of 138°) and oleophobicity (OCA of 132°). The fluorinated mats were converted to amphiphilic after the annealing treatment at 500 °C due to the complete pyrolysis of FAS from the fibrous mats, which is supported by the FT-IR results. This study opens an effective way to fabricate the flexible and high-heat-resistant fibrous mats with self-cleaning properties.
Figure 9. FT-IR spectra of the FS1 mats annealed at different temperatures: (a) 100 °C, (b) 450 °C, and (c) 500 °C.
°C, the FS1 mat surface maintained superhydrophobicity. This is the first report that the superhydrophobicity of the electrospun mat surfaces could be maintained after annealing at such high temperature. Additionally, the FS1 mat surface possessed a high hydrophobicity with WCAs of 148° and 138° after annealing at 400 and 450 °C, respectively. The surface hydrophobicity of FS1 fibrous mats was converted to superhydrophilicity when the annealing temperature was at 500°, the water droplet on the mats spread thoroughly, and the WCA was almost 0°. Figure 8 shows the surface oleophobicity of FS1 mats annealed at various temperatures. With increasing annealing temperature from 100 to 450 °C, the OCAs of FS1 mats were gradually decreased from 144° to 132°. When the annealing temperature was up to 500 °C, the FS1 mat surface lost its oleophobicity, showing an OCA of 21°. It can be seen from the results in Figures 7 and 8 that the amphiphobic FS1 nanofibrous mat surfaces have high-heat-resistant properties. The conversion of surface wettability could be illustrated from the results of FT-IR. Figure 9 presents the FT-IR spectra of the FS1 mats annealed at various temperatures. As seen in Figure 9a, the FS1 mats annealed at 100 °C maintained the absorption features at 1100, 790, and 470 cm-1 corresponding to the Si-O-Si bond. A new absorption peak appeared at 2900 cm-1,
Acknowledgment. This work was partly supported by the National Natural Science Foundation of China under Grant Nos. 50803009 and 10872048. Partial support from the Programme of Introducing Talents of Discipline to Universities (Grant Nos. 111-2-04 and B07024) is appreciated. References and Notes (1) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. AdV. Mater. 1999, 11, 1365–1368. (2) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743–1746. (3) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460. (4) Erbil, H.; Demirel, A.; Avci, Y.; Mert, O. Science 2003, 299, 1377– 1380. (5) Baldacchini, T.; Carey, J.; Zhou, M.; Mazur, E. Langmuir 2006, 22, 4917–4919. (6) Bartell, F.; Shepard, J. J. Phys. Chem. 1953, 57, 211–215. (7) Tsujii, T.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011–1012. (8) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213–3216. (9) Ogawa, T.; Ding, B.; Sone, Y.; Shiratori, S. Nanotechnology 2007, 18, 165607. (10) Lau, K.; Bico, J.; Teo, K.; Chhowalla, M.; Amaratunga, G.; Milne, W.; McKinley, G. Nano Lett. 2003, 3, 1701–1705. (11) Nakajima, A.; Abe, K.; Hashimoto, K.; Watanabe, T. Thin Solid Films 2000, 376, 140–143. (12) Li, M.; Zhai, J.; Liu, H.; Song, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. 2003, 107, 9954–9957. (13) Shiu, J.; Kuo, C.; Chen, P.; Mou, C. Chem. Mater. 2004, 16, 561– 564. (14) Takeda, K.; Sasaki, M.; Kieda, N.; Katayama, K.; Kako, T.; Hashimoto, K.; Watanabe, T.; Nakajima, A. J. Mater. Sci. Lett. 2001, 20, 2131–2133. (15) Reneker, D.; Chun, I. Nanotechnology 1996, 7, 216–223.
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