pubs.acs.org/Langmuir © 2010 American Chemical Society
Study of Superhydrophobic Electrospun Nanocomposite Fibers for Energy Systems Ramazan Asmatulu,*,† Muhammet Ceylan,† and Nurxat Nuraje*,‡ †
Department of Mechanical Engineering, Wichita State University, 1845 Fairmount, Wichita, Kansas 67260, United States, and ‡Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States Received September 14, 2010. Revised Manuscript Received December 15, 2010 Polystyrene (PS) and polyvinyl chloride (PVC) fibers incorporated into TiO2 nanoparticles and graphene nanoflakes were fabricated by an electrospinning technique, and then the surface morphology and superhydrophobicity of these electrospun nanocomposite fibers were investigated. Results indicated that the water contact angle of the nanocomposite fiber surfaces increases to 178°on the basis of the fiber diameter, material type, nanoscale inclusion, heat treatment, and surface porosity/roughness. This is a result of the formation of the Cassie-Baxter state in the fibers via the nanoparticle decoration, bead formation, and surface energy of the nanofiber surface. Consequently, these superhydrophobic nanocomposite fibers can be utilized in designing photoelectrodes of dye-sensitized solar cells (DSSCs) as self-cleaning and anti-icing materials for the long-term efficiency of the cells.
1. Introduction Research on superhydrophobicity where the contact angle exceeds 150° has accelerated around the globe, and many investigators have taken a keen interest in the fabrication of superhydrophobic materials. Superhydrophobicity, a unique property of materials, has been studied extensively in recent years because it has wide applications in many areas, such as antibiofouling paints for boats,1-3 antistick coatings to repel snow on antennas and windows,4,5 self-cleaning windshields for automobiles,6 metal refining, stain-resistant textiles, antisoiling architectural coatings,7and oil/water separation.8,9 Superhydrophobic surfaces possessing a high advancing contact angle (higher than 150°) and low water contact angle hysteresis is explained by the Cassie-Baxter model.10 Examples of superhydrophobic surfaces including lotus leaves can be widely found in nature.11,12 The surface is usually textured with 3-10-μm-sized hills and valleys that are decorated with nanometer-sized particles of a hydrophobic material;12,13 therefore, the micrometer-sized hills and valleys can provide very low area-to-water contact, and the hydrophobic nanoparticles can prevent water penetration into the hills and valleys. Nature-inspired artificial superhydrophobic *Corresponding authors. E-mail:
[email protected], nurxat@ mit.edu. (1) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350. (2) Scardino, A.; Nys, R. D.; Ison, O.; O’Connor, W.; Steinberg, P. Biofouling 2003, 19, 221. (3) Schultz, M. P.; Kavanagh, C. J.; Swain, G. W. Biofouling 1999, 13, 323. (4) Saito, H.; Takai, K.; Takazawa, H.; Yamauchi, G. Mater. Sci. Res. Int. 1997, 3, 216. (5) Kako, T.; Nakajima, A.; Irie, H.; Kato, Z.; Uematsu, K.; Watanabe, T.; Hashimoto, K. J. Mater. Sci. 2004, 39, 547. (6) David, Q. Rep. Prog. Phys. 2005, 68, 2495. (7) Zielecka, M.; Bujnowska, E. Prog. Org. Coat. 2006, 55, 160. (8) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 2012. (9) Wang, C.; Yao, T.; Wu, J.; Ma, C.; Fan, Z.; Wang, Z.; Cheng, Y.; Lin, Q.; Yang, B. ACS Appl. Mater. Interfaces 2009, 1, 2613. (10) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (11) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (12) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (13) Zhai, L.; Cebeci, F. C-.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (14) Soeno, T.; Inokuchi, K.; Shiratori, S. Trans. Mater. Res. Soc. Jpn. 2003, 38, 1207. (15) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701.
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surfaces have been fabricated by various techniques,13-22 including layer-by-layer approaches,13,14 roughening the surface of hydrophobic materials,15-18 in situ polymerization of monomers in the presence of a porogenic solvent,19 and electrospinning.20-22 Among these techniques, electrospinning is a straightforward, cost-effective method of producing novel fibers with diameters in the range of 3 nm to greater than 100 μm.23,24 The electrospinning technique is applied to fabricate the superhydrophobic surface because it can intrinsically provide at least one length scale of roughness.20-22,25-27 Most of the above techniques are applied in two ways to generate superhydrophobic surfaces. In the first case,23,25-27 roughness resulting from the small diameters of the fibers combined with hydrophobic polymers was sufficient to receive a contact angle of greater than 150°. In the latter case,20-22 the particles introduced a second length scale of structure into the fibrous membrane, which is called “beads on strings.” Electrospinning28-30 is a process in which a high electric field is directly utilized to fabricate microfibers and nanofibers from (16) Feng, L.; L., S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (17) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (18) Shiu, J.-Y.; Kuo, C.-W.; Chen, P.; Mou, C.-Y. Chem. Mater. 2004, 16, 561. (19) Levkin, P. A..; et al. Adv. Funct. Mater. 2009, 19, 1. (20) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (21) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210. (22) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K.; Cohen, R.; Rubner, M.; Rutledge, G. Adv. Mater. 2007, 19, 255. (23) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531. (24) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Appl. Phys. Lett. 2001, 78, 1149. (25) Xue, Y.; Wang, H.; Yu, D.; Feng, L.; Dai, L.; Wang, X.; Lin, T. Chem. Commun. 2009, 6418. (26) Park, S. H.; Lee, S. M.; Lim, H. S.; Han, J. T.; Lee, D. R.; Shin, H. S.; Jeong, Y.; Kim, J.; Cho, J. H. ACS Appl. Mater. Interfaces 2010, 2, 658. (27) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549. (28) Jabal, J. M. F.; McGarry, L.; Sobczyk, A.; Aston, D. E. Langmuir 2010, 26, 13550. (29) Bao, Q.; Zhang, H.; Yang, J.-x.; Wang, S.; Tang, D. Y.; Jose, R.; Ramakrishna, S.; Lim, C. T.; Loh, K. P. Adv. Funct. Mater. 2010, 20, 782. (30) Peng, M.; Li, D.; Shen, L.; Chen, Y.; Zheng, Q.; Wang, H. Langmuir 2006, 22, 9368.
Published on Web 12/20/2010
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Figure 1. SEM images of PVC and PS fibers: (A) large-diameter PVC fibers (scale bar 5 um), (B) small-diameter PVC fibers (scale bar 400 nm), (C) large-diameter PS fibers, and (D) small-diameter PS fibers.
polymeric solutions. The electrospinning process31-33 has been successfully exploited to create porous electrodes for dye-sensitized solar cells (DSSCs) and can provide porous films that consist of a controlled nanofiber size and tunable porosity, which in turn is very important for DSSC applications. From the above applications of electrospinning, it can be seen that this process has a promising potential application in photovoltaic cells because photovoltaic cells with self-cleaning and anti-icing properties are more desirable for efficient and long-term applications.19 These self-cleaning and anti-icing properties are directly linked to superhydrophobicity. In the present work, we designed and studied the superhydrophobicity of porous nanocomposite fiber films because titania and graphene are the most important materials in photoelectrodes of DSSCs. The electrospinning process can provide tunable nanofibers for porosity and superhydrophobicity.
2. Experimental Section 2.1. Materials. Poly(vinyl chloride) (PVC, Mw = 150 000 g/ mol), polystyrene (PS, Mw = 230 000 g/mol), dimethylformamide (DMF), and dimethylacetamide (DMAC) were purchased from Sigma-Aldrich. Nanoscale graphene with a thickness of less than 10 nm was purchased from Angstron Materials. These products were directly used in the electrospinning process without further purification. Using a sol-gel process, TiO2 nanoparticles were fabricated in-house. The following procedure was utilized to prepare these nanoparticles: isopropanol and titanium(IV) butoxide were mixed for about 10 min, and then 10% HCl solution was added dropwise using a pipet. As the gel formed, it was dried at 150 °C for 24 h (or until completely solidified). Following the mortar grinder step or pulverizing, the amorphous TiO2 nanoparticles were annealed at 550 °C for 2 h. 2.2. Method. PS and PVC were separately dissolved in solvents (dimethylformamide and dimethylacetamide) in a ratio (31) Drew, C.; Wang, X.; Senecal, K.; Schreuder-Gibson, H.; He, J.; Kumar, J.; Samuelson, L. A. J. Macromol. Sci., Part A: Pure Appl. Chem. 2002, 39, 1085. (32) Song, M. Y..; et al. Nanotechnology 2004, 15, 1861. (33) Kim, I.-D.; Hong, J.-M.; Lee, B. H.; Kim, D. Y.; Jeon, E.-K.; Choi, D.-K.; Yang, D.-J. Appl. Phys. Lett. 2007, 91, 163109.
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of 0.8:0.2 prior to the electrospinning process. Electrospinning experiments were conducted at various pump speeds, dc voltages, and spinning distances. Nanosize graphene (thickness of less than 10 nm with a diameter of 600 nm) and TiO2 nanoparticles (1025 nm) were added to polymeric solutions in varying percentages. The fiber size and morphology were analyzed using scanning electron microscopy (SEM). Electrospun PVC and PS fibers with different diameters were used in the contact angle measurements using a CAM 100 contact angle goniometer. The composition of the films was confirmed by thermal gravity analysis (TGA Q50 equipped with a Thermostar mass spectrometer). Because only a qualitative determination of surface enrichment is sought at this stage, no attempt was made to obtain a better estimate of the actual sampling depth.
3. Results and Discussion In photovoltaic cells, porous photoelectrodes with selfcleaning properties are more desirable. Electrospinning is an excellent technique for creating porous films containing both organic and inorganic materials. Titania and graphene are the essential materials in the fabrication of photoelectrodes in DSSCs.31-33 Therefore, we investigated the superhydrophobicity of porous films using the above components under different conditions. This study provides valuable scientific fundamentals for the fabrication of DSSC devices and evaluations of their performances. 3.1. Superhydrophobic Behavior of Electrospun Fibers without Inclusions. PVC and PS electrospun fibers were obtained at a 2.5 mL/h pump speed, 25 kV dc voltage, and 25 cm spinning distance (Figures 1 and 2). As can be seen, fiber diameters of PVC are between 200 and 500 nm and that of PS are between 400 nm and 1.25 μm. In the present study, we determined that the water contact angles were usually higher, which may be due to the fact that the beads that formed on the fiber surfaces became smaller at smaller diameters. This may also be attributed to the molecular structure, initial hydrophobicity, and surface roughness of fibers and the smaller air packets on the fiber films. Some of these findings are DOI: 10.1021/la103661c
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Figure 2. Change in contact angle values of PVC and PS fibers as a function of fiber diameter.
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Figure 3. Thermal gravimetric analysis (TGA) of both PVC fibers containing titania nanoparticles (red line) and PVC fibers containing graphene nanoparticles (blue line).
consistent with the literature.34 Ma et al. found that hydrophobicity increased with a reduction in diameter among bead-free fibers and with the introduction of a high density of relatively small diameter beads. The contact angles of bead fibers are higher than that of bead-free fibers. With the formation of beads in the fibers, the water droplets sit on the heterogeneous surface of the fiber and the air, resulting in a Cassie-Baxter state. 3.2. Superhydrophobic Behavior of Electrospun Fibers with Inclusions. Effects of nanoparticles, such as titania, and graphene nanoflakes on the superhydrophobic properties of PVC and PS fibers were investigated. The composition of nanocomposite fibers was confirmed using thermal gravity analysis after two different types of fibers were fabricated via the electrospinning process. One contains only titania nanoparticles in the fibers, and the other contains graphene nanoflakes. Usually, in the graphene-contained nanocomposite fibers, three-shouldered peak areas can be detected. However, in the titania-containing composite fibers, two-shouldered peak areas can be observed. For example, as shown in Figure 3, the blue line represents the TGA curve for graphene-containing PVC nanocomposite fibers. The brown line represents the titania-included PVC composite fibers. In both cases, the peaks around 200 and 400 °C indicated dimethylacetamide and PVC coming off of the electrospun fibers separately. However, in the graphene-accompanied PVC fibers (blue line), the shoulder at 500 °C indicates that the graphene component is removed from the fibers. In the titania-included PVC fibers, only two peaks can be detected because titania cannot be removed from the electrospun fibers in that temperature range. This is useful data for confirming the successful fabrication of nanocomposite fibers. TiO2 nanoparticles (10-25 nm) were added to PVC and PS polymeric solutions at different weight percentages (1, 2, 4, and 8) and then electrospun at a 25 cm distance, 25 kV dc voltage, and 1 mL/h pump speed to determine the effects of inclusions on the contact angle values of nanocomposite fibers. Figure 4 shows the contact angle values of an electrospun nanocomposite fiber film containing different numbers of TiO2 nanoparticles and graphene nanoflakes. The PS nanocomposite fibers obtained at 1, 2, 4, and 8% TiO2 nanoparticles give 155.5, 156.1, 164.5, and 177.5 water contact angle values, respectively, whereas the PVC nanocomposite fibers
with the same concentrations and spinning conditions give 151.3., 164.3, 168.3, and 165.0 water contact angles, respectively. The highest contact angle value (177.5) was received from the PS nanocomposite fiber obtained at 8% TiO2 nanoparticles. Additionally, increasing the number of TiO2 nanoparticles in both polymers results in higher contact angle values. The results show that TiO2 nanocomposite fibers have higher contact angle values, which may be due to the nanoscale gaps/bumps/voids formed on the fiber in the presence of TiO2 nanoparticles.35 This structure induces a Cassie-Baxter state in the film, thus reaching superhydrophobicity on the nanocomposite fiber films. We also investigated the effects when nanosized graphene flakes were added to PVC polymeric solutions at different weight percentages (0.5, 1, 2, and 4) and then electrospun at a 30 cm distance, 25 kV dc voltage, and 1.5 mL/h pump speed. The concentration of
(34) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742.
(35) Gupta, P.; Asmatulu, R.; Claus, R.; Wilkes, G. J. Appl. Polym. Sci. 2006, 100, 4935.
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Figure 4. Contact angle values of an electrospun nanocomposite fiber film as a function of the percentage of TiO2 nanoparticles or graphene nanoflakes. (Red line) PS nanocomposite fiber film containing TiO2 nanoparticles. (Blue line) PVC nanocomposite fiber films with TiO2 nanoparticles. (Green line) PVC nanocomposite fiber films with graphene nanoflakes.
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Table 1. Effect of Heat Treatment on the Contact Angle Values of PS and PVC Fiber Films contact angle values at different temperatures (°C) polymeric fibers
60
80
100
120
PVC PVC with 1% graphene PS
144 155 156
149 161 158
138 151 155
133 140 101
graphene in the polymeric solution was increased up to 4% because a graphene concentration higher than 4% adversely affected the electrospinability of the polymeric solutions, which was not the case for the TiO2 nanoparticle inclusions. The main purpose of adding graphene particles to the polymeric solution was to increase the surface roughness, voids, and hydrophobicity of the electrospun fibers to obtain superhydrophobicity. Figure 4 also shows that increasing the graphene concentration in the electrospun fibers gradually increases the water contact angle values. In the absence of graphene, the water contact angle of the fibers is below 140; however, water contact angle values of 0.5, 1, 2, and 4% graphene-added nanocomposite fibers are 141.8, 151.5, 165.3, and 166.3, respectively. This indicates that graphene inclusions in the polymeric fibers drastically change the surface morphology and chemistry, resulting in higher contact angles and superhydrophobicity. Zhai et al.13 created superhydrophobic multilayer films through the inclusion of silica nanoparticles in layer-by-layer assembled multilayers. Ma et al.22 also studied the superhydrophobicity of electrospun films created via a layer-bylayer assembly of negatively charged silica nanoparticles with positively charged polyallylamine. Both our results and the above studies conceptually mimic the hierarchically roughened nature of superhydrophobic surfaces of the lotus leaf. Our results further prove that the microscale and nanoscale beards on the morphology improved the superhydrophobicity of nanofibrous films. 3.3. Effects of Heat Treatment on Superhydrophobicity. We studied the effects of heat treatment on the contact angle values of electrospun PVC and PS fibers at 60, 80, 100, and 120 °C in the presence and absence of nanoparticle inclusions. Table 1 gives the water contact angle values of PS fibers obtained at various temperatures. The contact angle values of PVC and PS fibers without graphene inclusion decreased to 137 and 101° at 100 and 120 °C, respectively. However, the contact angle values of PVC nanocomposite fiber films initially increased from 155 to 161° at 80 °C and then decreased to 140° at 100 °C. Similar behavior can also be seen in the PS fibers. The change in the contact angles may be related to the rearrangement of fiber structures in the film at the glass-transition temperature (Tg). Both high-molecular-weight PS and PVC are hard, glassy solids at room temperature. However, when the temperature is higher than
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the Tg of the polymers, they become flexible and rubbery. Below the Tg point, polymer chains are rigid and segments of the chains cannot move. Above the Tg for the polymer, the chain segment undergoes rotation because of carbon-carbon bonds in the chain that can be twisted easily.36 Two bonds can act as an axis of rotation, permitting carbon atoms (and the groups attached to them) to rotate. Therefore, after exceeding the Tg point, the polymeric materials become softer and rubbery, where the surface roughness, morphology, and voids of the nanocomposite fibers can change considerably on the microscale and nanoscale. Perhaps the rougher surface becomes smoother in this case. However, it is very difficult to imagine nanoscale morphological changes on the surface of polymeric fibers. Tg of polystyrene is around 100 °C, and exceeding the Tg can permanently change the size, shape, porosity, and surface composition of the fibers, which results in lower contact angles on the surfaces of PS fibers. The same phenomena can occur with PVC, the Tg of which is 80 °C. However, for a graphene-mixed PVC composite film, the contact angles of the composite films are higher than the contact angles of PVC-only films. The changes in contact angle values of the nanocomposite films are not significant. These results further explain the effect of graphene on the glass-transition temperature of PVC films.
4. Conclusions In the present study, we have successfully fabricated superhydrophobic nanocomposite fibers using an electrospinning process. Titania nanoparticles and graphene nanoflakes were included in the electrospun fibers to induce superhydrophobicity on the surfaces of the fibrous films. Our studies showed that nanoscale inclusions, concentrations, and fiber diameters drastically changed the superhydrophobicity of the surfaces. The investigation of the heat treatment on the fiber film also provides valuable information on building superhydrophobic surfaces. The tunable properties of titania- and graphene-based superhydrophobic surfaces and electrodes can play an important role in improving the efficiency of DSSC devices. The superhydrophobicity properties of the photoelectrodes are likely to provide selfcleaning and anti-icing surfaces, which is the primary motivation of the present study. Acknowledgment. We gratefully acknowledge Wichita State University for the support of this work. Supporting Information Available: PVC electrospun fibers incorporated with 8% TiO2 nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. (36) Henson, J. H. L.; Whelan, A. Developments in PVC Technology; John Wiley and Sons: New York, 1973.
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