504 DOI: 10.1021/la103661c Langmuir 2011, 27(2), 504–507Published on Web 12/20/2010

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 nanoflakeswere fabricated by an electrospinning technique, and then the surface morphology and superhydrophobicity of theseelectrospun nanocomposite fibers were investigated. Results indicated that the water contact angle of the nanocompos-ite 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 nano-particle decoration, bead formation, and surface energy of the nanofiber surface. Consequently, these superhydrophobicnanocomposite fibers can be utilized in designing photoelectrodes of dye-sensitized solar cells (DSSCs) as self-cleaning andanti-icing materials for the long-term efficiency of the cells.

1. Introduction

Researchon superhydrophobicitywhere the contact angle exceeds150� has accelerated around the globe, and many investigators havetaken a keen interest in the fabrication of superhydrophobic materi-als. Superhydrophobicity, a unique property of materials, has beenstudied extensively in recent years because it has wide applications inmany areas, such as antibiofouling paints for boats,1-3 antistickcoatings to repel snow on antennas and windows,4,5 self-cleaningwindshields for automobiles,6 metal refining, stain-resistant tex-tiles, antisoiling architectural coatings,7and oil/water separation.8,9

Superhydrophobic surfaces possessing a high advancing contactangle (higher than 150�) and low water contact angle hysteresis isexplained by the Cassie-Baxter model.10 Examples of superhy-drophobic surfaces including lotus leaves can be widely found innature.11,12 The surface is usually textured with 3-10-μm-sizedhills and valleys that are decorated with nanometer-sized particlesof a hydrophobicmaterial;12,13 therefore, themicrometer-sizedhillsand valleys can provide very low area-to-water contact, and thehydrophobic nanoparticles can prevent water penetration intothe hills and valleys. Nature-inspired artificial superhydrophobic

surfaces have been fabricated by various techniques,13-22 includinglayer-by-layer approaches,13,14 roughening the surface of hydro-phobic materials,15-18 in situ polymerization of monomers in thepresence 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 inthe range of 3 nm to greater than 100 μm.23,24 The electrospinningtechnique is applied to fabricate the superhydrophobic surfacebecause it can intrinsically provide at least one length scale ofroughness.20-22,25-27Most of the above techniques are applied intwo ways to generate superhydrophobic surfaces. In the firstcase,23,25-27 roughness resulting from the small diameters of thefibers combined with hydrophobic polymers was sufficient toreceive a contact angle of greater than 150�. In the latter case,20-22

the particles introduced a second length scale of structure into thefibrous membrane, which is called “beads on strings.”

Electrospinning28-30 is a process in which a high electric field isdirectly utilized to fabricate microfibers and nanofibers from

*Corresponding authors. E-mail: ramazan.asmatulu@wichita.edu, 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,

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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,

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DOI: 10.1021/la103661c 505Langmuir 2011, 27(2), 504–507

Asmatulu et al. Letter

polymeric solutions. The electrospinning process31-33 has beensuccessfully exploited to create porous electrodes for dye-sensitizedsolar cells (DSSCs) and can provide porous films that consist of acontrollednanofiber size and tunable porosity,which in turn is veryimportant for DSSC applications. From the above applications ofelectrospinning, it can be seen that this process has a promisingpotential application in photovoltaic cells because photovoltaiccells with self-cleaning and anti-icing properties are more desirablefor efficient and long-term applications.19 These self-cleaning andanti-icing properties are directly linked to superhydrophobicity.

In the present work, we designed and studied the superhydro-phobicity of porous nanocomposite fiber films because titaniaand graphene are themost importantmaterials in photoelectrodesof DSSCs. The electrospinning process can provide tunablenanofibers for porosity and superhydrophobicity.

2. Experimental Section

2.1. Materials. Poly(vinyl chloride) (PVC,Mw = 150000 g/mol), polystyrene (PS,Mw=230000 g/mol), dimethylformamide(DMF), and dimethylacetamide (DMAC) were purchased fromSigma-Aldrich. Nanoscale graphene with a thickness of less than10 nm was purchased from Angstron Materials. These productswere directly used in the electrospinning process without furtherpurification. Using a sol-gel process, TiO2 nanoparticles werefabricated in-house. The following procedure was utilized toprepare these nanoparticles: isopropanol and titanium(IV) but-oxide were mixed for about 10 min, and then 10% HCl solutionwas added dropwise using a pipet. As the gel formed, it was driedat 150 �C for 24 h (or until completely solidified). Following themortar grinder step or pulverizing, the amorphous TiO2 nano-particles were annealed at 550 �C for 2 h.

2.2. Method. PS and PVC were separately dissolved insolvents (dimethylformamide and dimethylacetamide) in a ratio

of 0.8:0.2 prior to the electrospinning process. Electrospinningexperiments were conducted at various pump speeds, dc voltages,and spinning distances. Nanosize graphene (thickness of less than10 nm with a diameter of 600 nm) and TiO2 nanoparticles (10-25 nm) were added to polymeric solutions in varying percentages.The fiber size and morphology were analyzed using scanningelectronmicroscopy (SEM). Electrospun PVC and PS fibers withdifferent diameters were used in the contact angle measurementsusing a CAM 100 contact angle goniometer. The composition ofthe films was confirmed by thermal gravity analysis (TGA Q50equipped with a Thermostar mass spectrometer). Because only aqualitative determination of surface enrichment is sought at thisstage, no attempt was made to obtain a better estimate of theactual sampling depth.

3. Results and Discussion

In photovoltaic cells, porous photoelectrodes with self-cleaning properties are more desirable. Electrospinning is anexcellent technique for creating porous films containing bothorganic and inorganic materials. Titania and graphene are theessential materials in the fabrication of photoelectrodes inDSSCs.31-33 Therefore, we investigated the superhydropho-bicity of porous films using the above components underdifferent conditions. This study provides valuable scientificfundamentals for the fabrication of DSSC devices and evalua-tions of their performances.3.1. Superhydrophobic Behavior of Electrospun Fibers

without Inclusions. PVC and PS electrospun fibers wereobtained at a 2.5 mL/h pump speed, 25 kV dc voltage, and25 cm spinning distance (Figures 1 and 2). As can be seen, fiberdiameters of PVC are between 200 and 500 nm and that of PSare between 400 nm and 1.25 μm. In the present study, wedetermined that the water contact angles were usually higher,which may be due to the fact that the beads that formed on thefiber surfaces became smaller at smaller diameters. This mayalso be attributed to the molecular structure, initial hydro-phobicity, and surface roughness of fibers and the smallerair packets on the fiber films. Some of these findings are

Figure 1. SEM images of PVC and PS fibers: (A) large-diameter PVC fibers (scale bar 5 um), (B) small-diameter PVC fibers (scale bar400 nm), (C) large-diameter PS fibers, and (D) small-diameter PS fibers.

(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.

506 DOI: 10.1021/la103661c Langmuir 2011, 27(2), 504–507

Letter Asmatulu et al.

consistentwith the literature.34Maet al. found that hydrophobicityincreased with a reduction in diameter among bead-free fibers andwith the introduction of a high density of relatively small diameterbeads. The contact angles of bead fibers are higher than that ofbead-free fibers. With the formation of beads in the fibers, thewater droplets sit on the heterogeneous surface of the fiber and theair, resulting in a Cassie-Baxter state.3.2. Superhydrophobic Behavior of Electrospun Fibers

with Inclusions. Effects of nanoparticles, such as titania, andgraphene nanoflakes on the superhydrophobic properties of PVCand PS fibers were investigated. The composition of nanocom-posite fibers was confirmed using thermal gravity analysis aftertwo different types of fibers were fabricated via the electrospin-ning process. One contains only titania nanoparticles in the fibers,and the other contains graphene nanoflakes. Usually, in thegraphene-contained nanocomposite fibers, three-shouldered peakareas can be detected. However, in the titania-containing compo-site fibers, two-shouldered peak areas can be observed. Forexample, as shown in Figure 3, the blue line represents the TGAcurve for graphene-containing PVC nanocomposite fibers. Thebrown line represents the titania-includedPVCcomposite fibers. Inboth cases, the peaks around 200 and 400 �C indicated dimethy-lacetamide and PVC coming off of the electrospun fibers sepa-rately. However, in the graphene-accompanied PVC fibers (blueline), the shoulder at 500 �C indicates that the graphene componentis removed from the fibers. In the titania-included PVC fibers, onlytwo peaks can be detected because titania cannot be removed fromthe electrospun fibers in that temperature range. This is useful datafor confirming the successful fabrication of nanocomposite fibers.

TiO2 nanoparticles (10-25 nm) were added to PVC and PSpolymeric solutions at different weight percentages (1, 2, 4, and 8)and then electrospun at a 25 cm distance, 25 kV dc voltage, and 1mL/h pump speed to determine the effects of inclusions on thecontact angle values of nanocomposite fibers. Figure 4 shows thecontact angle values of an electrospun nanocomposite fiber filmcontaining different numbers of TiO2 nanoparticles and graphenenanoflakes.

The PS nanocomposite fibers obtained at 1, 2, 4, and 8%TiO2

nanoparticles give 155.5, 156.1, 164.5, and 177.5 water contactangle values, respectively, whereas the PVCnanocomposite fibers

with the same concentrations and spinning conditions give 151.3.,164.3, 168.3, and 165.0 water contact angles, respectively. Thehighest contact angle value (177.5) was received from the PSnanocomposite fiber obtained at 8% TiO2 nanoparticles. Addi-tionally, increasing the number of TiO2 nanoparticles in bothpolymers results in higher contact angle values. The results showthat TiO2 nanocomposite fibers have higher contact angle values,which may be due to the nanoscale gaps/bumps/voids formed onthe fiber in the presence of TiO2 nanoparticles.

35 This structureinduces a Cassie-Baxter state in the film, thus reaching super-hydrophobicity on the nanocomposite fiber films. We alsoinvestigated the effects when nanosized graphene flakes wereadded to PVC polymeric solutions at different weight percentages(0.5, 1, 2, and 4) and then electrospun at a 30 cm distance, 25 kVdc voltage, and 1.5 mL/h pump speed. The concentration of

Figure 2. Change in contact angle values of PVC and PS fibersas a function of fiber diameter.

Figure 3. Thermal gravimetric analysis (TGA) of both PVCfibers containing titania nanoparticles (red line) and PVC fiberscontaining graphene nanoparticles (blue line).

Figure 4. Contact angle values of an electrospun nanocompositefiber film as a function of the percentage of TiO2 nanoparticles orgraphene nanoflakes. (Red line) PS nanocomposite fiber filmcontaining TiO2 nanoparticles. (Blue line) PVC nanocompositefiber films with TiO2 nanoparticles. (Green line) PVC nanocom-posite fiber films with graphene nanoflakes.

(34) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macro-molecules 2005, 38, 9742.

(35) Gupta, P.; Asmatulu, R.; Claus, R.; Wilkes, G. J. Appl. Polym. Sci. 2006,100, 4935.

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graphene in the polymeric solution was increased up to 4%because a graphene concentration higher than 4% adverselyaffected the electrospinability of the polymeric solutions, whichwas not the case for the TiO2 nanoparticle inclusions. The mainpurpose of adding graphene particles to the polymeric solutionwas to increase the surface roughness, voids, and hydrophobicityof the electrospun fibers to obtain superhydrophobicity.

Figure 4 also shows that increasing the graphene concentrationin the electrospun fibers gradually increases the water contactangle values. In the absence of graphene, the water contact angleof the fibers is below 140; however, water contact angle values of0.5, 1, 2, and 4%graphene-added nanocomposite fibers are 141.8,151.5, 165.3, and 166.3, respectively. This indicates that grapheneinclusions in the polymeric fibers drastically change the surfacemorphology and chemistry, resulting in higher contact angles andsuperhydrophobicity. Zhai et al.13 created superhydrophobicmultilayer films through the inclusion of silica nanoparticles inlayer-by-layer assembled multilayers. Ma et al.22 also studied thesuperhydrophobicity of electrospun films created via a layer-by-layer assembly of negatively charged silica nanoparticles withpositively charged polyallylamine. Both our results and the abovestudies conceptuallymimic the hierarchically roughened nature ofsuperhydrophobic surfaces of the lotus leaf. Our results furtherprove that the microscale and nanoscale beards on the morphol-ogy improved the superhydrophobicity of nanofibrous films.3.3. Effects of Heat Treatment on Superhydrophobicity.

We studied the effects of heat treatment on the contact anglevalues of electrospun PVC andPS fibers at 60, 80, 100, and 120 �Cin the presence and absence of nanoparticle inclusions. Table 1gives the water contact angle values of PS fibers obtained atvarious temperatures. The contact angle values of PVC and PSfibers without graphene inclusion decreased to 137 and 101� at100 and 120 �C, respectively.However, the contact angle values ofPVC nanocomposite fiber films initially increased from 155 to161� at 80 �C and then decreased to 140� at 100 �C. Similarbehavior can also be seen in the PS fibers. The change in thecontact angles may be related to the rearrangement of fiberstructures in the film at the glass-transition temperature (Tg).Bothhigh-molecular-weight PS andPVCare hard, glassy solids atroom temperature.However, when the temperature is higher than

the Tg of the polymers, they become flexible and rubbery. Belowthe Tg point, polymer chains are rigid and segments of the chainscannot move. Above the Tg for the polymer, the chain segmentundergoes rotation because of carbon-carbon bonds in the chainthat can be twisted easily.36 Two bonds can act as an axis ofrotation, permitting carbon atoms (and the groups attached tothem) to rotate. Therefore, after exceeding the Tg point, thepolymeric materials become softer and rubbery, where the sur-face roughness, morphology, and voids of the nanocompositefibers 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 morphologicalchanges on the surface of polymeric fibers. Tg of polystyrene isaround 100 �C, and exceeding the Tg can permanently change thesize, shape, porosity, and surface composition of the fibers, whichresults in lower contact angles on the surfaces of PS fibers. The samephenomena canoccurwithPVC, theTg ofwhich is 80 �C.However,for a graphene-mixed PVCcomposite film, the contact angles of thecomposite films are higher than the contact angles of PVC-onlyfilms. The changes in contact angle values of the nanocompositefilms are not significant. These results further explain the effect ofgraphene on the glass-transition temperature of PVC films.

4. Conclusions

In the present study, we have successfully fabricated super-hydrophobic nanocomposite fibers using an electrospinning pro-cess. Titania nanoparticles and graphene nanoflakes wereincluded in the electrospun fibers to induce superhydrophobicityon the surfaces of the fibrous films. Our studies showed thatnanoscale inclusions, concentrations, and fiber diameters drasti-cally changed the superhydrophobicity of the surfaces. Theinvestigation of the heat treatment on the fiber film also providesvaluable information on building superhydrophobic surfaces.The tunable properties of titania- and graphene-based super-hydrophobic surfaces and electrodes can play an important role inimproving the efficiency of DSSC devices. The superhydropho-bicity properties of the photoelectrodes are likely to provide self-cleaning and anti-icing surfaces, which is the primary motivationof the present study.

Acknowledgment. We gratefully acknowledge Wichita StateUniversity for the support of this work.

Supporting Information Available: PVC electrospun fibersincorporatedwith 8%TiO2 nanoparticles. Thismaterial is avail-able free of charge via the Internet at http://pubs.acs.org.

Table 1. Effect of Heat Treatment on the Contact Angle Values of PS

and PVC Fiber Films

contact angle values atdifferent temperatures (�C)

polymeric fibers 60 80 100 120

PVC 144 149 138 133PVC with 1% graphene 155 161 151 140PS 156 158 155 101

(36) Henson, J. H. L.; Whelan, A.Developments in PVC Technology; JohnWileyand Sons: New York, 1973.