Photoresist Derived Electrospun Carbon Nanofibers with Tunable

(11) Recently, carbon-MEMS fabricated starting from the SU-8 precursor have also ... (1-5, 7, 17-21) During the electrospinning process, a high voltag...
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Ind. Eng. Chem. Res. 2010, 49, 2731–2739

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Photoresist Derived Electrospun Carbon Nanofibers with Tunable Morphology and Surface Properties Chandra S. Sharma,†,‡ Rajesh Vasita,‡,§ Devendra K. Upadhyay,† Ashutosh Sharma,*,† Dhirendra S. Katti,*,§ and R. Venkataraghavan| Departments of Chemical Engineering and Biological Sciences and Bioengineering, Indian Institute of Technology, Kanpur-208016, India, UnileVer Research & DeVelopment Bangalore, 64, Main Road, Whitefield, Bangalore 560066, India

A new precursor, SU-8, which is a negative photoresist, was electrospun to produce ultrafine polymeric fibers with a wide range of morphology and wettability characteristics. Electrospun nanofibers of SU-8 were pyrolyzed at 1173 K in an inert atmosphere to give carbon nanofibers. A set of parameters, including electric potential, distance between source and collector, polymer flow rate, and polymer concentration, was optimized for high-viscosity SU-8 photoresist to synthesize long continuous carbon fibers having diameters in the range of 120-600 nm. However, for the same conditions, medium- and lower-viscosity SU-8 yielded beaded fibers and isolated beads, respectively. The wettability of the carbon web was significantly influenced by its surface morphology, as shown by water contact angle measurements. These SU-8-derived carbon nanostructures with tunable surface properties and morphologies could be especially suitable for integration with photoresistbased carbon-MEMS to produce multiscale hierarchal assemblies and could be of potential use in a broad range of applications. Introduction Carbon nanofibers as a new material have the potential to play a key role because of their physical dimensions, electrical conductivity, high mechanical strength, and low weight-tostrength ratio. Uses of carbon nanofibers are presently being explored in broad-ranging applications, including composites, filtration, protective clothing, agriculture, catalysis, defense, and security.1–8 Recently, polymeric fibers including carbon fibers have also received attention in biomedical applications such as drug delivery and orthopedic/dental implants.9,10 Among the precursors [e.g., poly(acrylonitrile) (PAN), pitches, and hydrocarbon gases] used for the synthesis of carbon fibers,1–9 PAN is the most commonly employed. We report here for the first time the use of an epoxy-based negative photoresist, SU-8, as a polymer precursor to carbon nanofibers prepared by the electrospinning route. We also explore the conditions for tuning the morphology (nanofibers/particles) and surface properties of the polymer and carbon nanostructures produced by electrospinning of the SU-8 precursor. SU-8 resist is the single most important material of choice for the fabrication of opto-electronic and lab-on-a-chip devices, as well as a variety of microfluidics and microelectromechanical systems (MEMS) platforms.11 Recently, carbon-MEMS fabricated starting from the SU-8 precursor have also been shown to have potential applications in microbatteries and biological platforms.12–16 Thus, a major motivation for SU-8-derived nanofibers is their ease of integration and compatibility with the underlying device materials and processes. Further, use of a photoresist material as a precursor to carbon fibers provides * To whom correspondence should be addressed. Tel.: +91-512259 7026 (A.S.), +91-512-259 4028 (D.S.K.). Fax: +91-512-259 0104 (A.S.), +91-512-259 4029 (D.S.K.). E-mail: [email protected] (A.S.), [email protected] (D.S.K.). † Department of Chemical Engineering, Indian Institute of Technology. ‡ These authors contributed equally to this work. § Department of Biological Sciences and Bioengineering, Indian Institute of Technology. | Unilever Research & Development Bangalore.

a novel way to fabricate various kinds of high-aspect-ratio fibrous patterns by simple photolithography techniques that find direct applications in C-MEMS and Bio-MEMS.13–16 Also, the pyrolysis of SU-8 nanofibers to produce carbon nanofibers could allow their facile integration with other SU-8-derived carbon mesostructures to produce multiscale hierarchal assemblies where high external surface areas are of importance, for example, in microbatteries and supercapacitors.13–15 Photoresistderived carbon nanofibers could also be of potential use in a broad range of bulk applications such as filtration systems, composites, and biomedical applications. Biocompatibility of SU-8-derived carbon16 also provides a motivation to produce nanoscaffolds of this material, which could provide effective bioplatforms for cell support and tissue repairs. A number of methods, including drawing, phase separation, self-assembly, template synthesis, and electrospinning, have been employed in different contexts to fabricate polymer nanofibers.17 The electrospinning process is a simple, versatile, and widely used method to produce nanofibers at large scale. A large number of polymers have been electrospun into nanofibers.1–5,7,17–21 During the electrospinning process, a high voltage is used to create an electrically charged jet of polymer.22–29 Once the electric field overcomes a threshold value, this charged jet is ejected from the tip of the electrospinning apparatus, forming a conical shape known as a Taylor cone.22–25 This charged polymer jet undergoes instabilities that lead to a whipping motion in which the liquid polymer jet traverses a lengthy path that enables elongation as well as solvent evaporation, thereby leading to a charged and dry polymeric fiber with ultrafine diameters. The trajectory of this charged polymer jet can be controlled by the applied electric field.22–30 To the best of our knowledge, the study reported herein is the first to consider the electrospinning of a negative photoresist, SU-8, to produce its nanofibers and particles and their pyrolysis to produce carbon nanostructures. The aim of the present work is to study the effects of various electrospinning parameters on fiber morphology and then to modulate and characterize the wetting behavior of variously prepared carbon nanostructures.

10.1021/ie901312j  2010 American Chemical Society Published on Web 02/05/2010

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Figure 1. Schematic of electrospinning setup.

Wettability is greatly affected by the surface texture of the electrospun carbon nanofibers. We show that the surface texture alone can modulate the wetting behavior of SU-8-derived carbon from mildly hydrophilic to nearly superhydrophobic for a beady surface. Further, we propose surface treatments to make the surface very hydrophilic that can be employed to fabricate hydrophilic carbon nanofiber scaffolds. A precise tuning of nanomorphology, roughness, and surface/wetting properties should allow a greater flexibility in the use of SU-8-derived carbon nanostructures, including in biomedical devices, satellite dishes, solar energy panels, photovoltaics, and so on.31,32 Experimental Section Materials. An epoxy-based negative photoresist, SU-8 2000 series formulation (cyclopentanone-based), was obtained from MicroChem, Newton, MA. It consists of eight epoxy groups per molecule.33 This photoresist is available in many different compositions that provide a control on viscosity. Three different resists of this series, namely, SU-8 2002, SU-8 2007, and SU-8 2015, with respective viscosities of 7, 140, and 1250 cSt at 25 °C were used in this study. To dilute the photoresist, cyclopentanone (Sigma-Aldrich, St. Louis, MO) was used as a thinner. Electrospinning Setup. A schematic diagram of the electrospinning setup used in this study is shown in Figure 1. It consists mainly of three components: a high-voltage dc power supply (Glassman High Voltage, Inc., High Bridge, NJ), a syringe pump (Harvard Apparatus, Holliston, MA) mounted with a needle, and a grounded collector plate. The flow rate of the polymer solution could be modulated using a syringe pump. Preparation of SU-8 Nanofibers. SU-8 nanofibers were synthesized using the electrospinning process. Electrospinning uses an electric field to create a charged jet of polymer solution. The polymer solution was placed in a stainless steel needle (nozzle) connected to a glass syringe and held at the tip of the nozzle by its surface tension. Electrostatic charge was induced on the solution by applying an external electric field between the needle and a grounded plate. When the electric field was increased above a threshold value, a charged jet of the polymer solution formed. As this jet traversed the air, the solvent evaporated, leaving behind a charged polymer fiber that was collected on a silicon wafer attached to the grounded copper screen. On the silicon wafer, continuous fibers were laid to form a nonwoven fiber mesh (fabric). Samples were then placed in a vacuum desiccator for 6 h to ensure complete drying/removal of the solvent. The applied electric potential and distance between the needle and collector screen were varied from 1.2 to 2.4 kV cm-1 and

from 6.0 to 20.0 cm, respectively, to examine the effects on the diameter and morphology of as-spun SU-8 nanofibers. The SU-8 feed rate was also varied from 0.1 to 0.5 mL/h to control the fiber diameter. In all experiments, a 26-gauge needle (internal diameter of 0.26 mm) was used. Once the synthesis conditions to yield continuous nanofibers were optimized with respect to the three processing parameters mentioned above, the effects of the SU-8 photoresist concentration and viscosity on morphology were studied. Preparation of Carbon Nanofibers. After being dried, electrospun SU-8 nanofibers deposited on a silicon wafer were exposed to 365-nm UV light for 1 min in order to cross-link the fibers. These cross-linked photoresist nanofibers samples were then placed in a quartz boat and heated to 1173 K under inert nitrogen (N2) atmosphere in a tubular high-temperature furnace for carbonization of the SU-8 polymer. Before the pyrolysis process started, nitrogen gas with a 0.5 L/min flow rate was purged for about 15 min into the quartz tube to displace unwanted air or oxygen. The rate of heating was optimized to be 5.0 K/min, and the N2 gas flow during the heating was kept constant at 0.3 L/min. Once the maximum temperature was reached, it was kept constant for 60 min. The furnace was then cooled to room temperature in about 10 h to obtain SU-8-derived carbon nanofibers. The inert atmosphere was maintained by purging N2 gas until the furnace attained room temperature. Surface Treatment. The chemical functionality of the carbon fabric was modified by exposing the fabric to ultraviolet ozone (UVO) for 30 min. The surface of these carbon nanofibers was also treated by low-temperature oxygen plasma with a plasma power of 18 W. The flow rate of carrier gas (oxygen) was maintained at 0.3 mL/h at 0.2 mbar for 5 min. Characterization. Scanning electron microscopy (SEM) (Quanta 200, FEI, Frankfurt am Main, Germany; SUPRA 40 VP, Gemini, Carl Zeiss, Oberkochen, Germany) was used to observe the surface morphology of the nonwoven fiber meshes and to determine the range of diameters produced for all of the above studies. All samples were first sputter-coated with a thin layer of Au-Pd to reduce the surface charging during electron beam scanning. The average diameter of the nanofibers was obtained for each sample by measuring the diameters of 50 nanofibers in six different fields of view and calculating their average. In the case of beaded fibers, both the nanofibers and beads were measured (fibers were on the nanometer scale, and beads were on the micrometer scale). The wetting behavior of the carbon nanofiber surfaces was characterized by measuring static contact angles and contact angle hysteresis with a contact angle goniometer (Rame´-Hart, Netcong, NJ). In most of the measurements, 5 µL (∼3-mm spherical drop diameter) water droplets were used. The functional groups on the surface of the carbon nanofibers before and after plasma and UVO treatment were characterized by Fourier transform infrared attenuated total reflection (FTIRATR) spectroscopy (Bruker Optik GmbH, Ettlingen, Germany). A confocal micro-Raman microscope (CRM 200, WiTec GmbH, Ulm, Germany, with λ ) 543 nm) was used to record the Raman spectra of the carbon nanofibers before and after plasma treatment. This technique allowed for the recording of the Raman spectra of individual fibers to characterize the types of bonds between the elements constituting the material. Results and Discussion Effects of Applied Electric Potential on Polymer Fiber Morphology and Size. The effects of the applied electric potential on the average fiber diameter and morphology are

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Figure 2. SEM images showing the effect of applied electric potential on SU-8 fibers: (a) 1.2, (b) 2.0, and (c) 2.4 kV cm-1. (d) Graphical representation of the influence of the electrospinning voltage (V) on the average fiber diameter (d). The flow rate and distance between the source needle and collector were 0.3 mL/h and 10 cm, respectively, for all cases.

summarized in Figure 2a-d. For these experiments, SU-8 2015 photoresist with the highest viscosity was used, and the applied electric potential was varied in the range of 1.2-2.4 kV cm-1. The volumetric feed rate of polymer and the distance between the source and collector were fixed at 0.3 mL/h and 10 cm, respectively. At the lowest value of the electric potential, nonuniformly distributed fibers with large diameters (average fiber diameter ) 1480 nm) were formed, as shown in Figure 2a. At 2.0 kV cm-1, long continuous SU-8 electrospun fibers with an average diameter of 800 nm were formed (Figure 2b). When the electric potential was increased to 2.4 kV cm-1, smaller-diameter fibers (average fiber diameter ) 190 nm) were obtained. However, as shown in Figure 2c, these fibers were not continuous and broke at shorter lengths. Upon increasing the electric potential from 1.2 to 2.4 kV cm-1, the average SU-8 fiber diameter decreased from 1480 ( 90 to 190 ( 40 nm. A decrease in the average fiber diameter with increasing applied electric potential, as shown in Figure 2d, is generally consistent with the electrospinning process.28 The final diameter of the polymer jet is governed by the outcome of a combination of forces generated by surface tension, viscosity, and electrical potential (field configuration and surface charges on the polymer solution that lead to electrostatic charge repulsion).23,26 With increasing electric potential, the density of charge on the polymer jet increases, thereby leading to a higher electrostatic repulsion that enables larger drawing forces and greater stretching of the jet and a reduction in the fiber diameter. Interestingly, even though the applied electric potential significantly influenced the fiber diameter of electrospun SU-8 nanofibers, it did not seem to influence the morphology of the nanofibers.

Effects of Source-Collector Distance on Polymer Fiber Morphology and Size. Based on the results for the effect of electric potential on fiber diameter, the applied electric potential was fixed at 2.0 kV cm-1. The volumetric flow rate of highly viscous SU-8 2015-type photoresist was kept constant at 0.3 mL/h, and the distance between the source needle and collector screen was varied from 6 to 20 cm. The effect of varying the distance on fiber diameter is summarized in Figure 3. At a distance of less than 6.0 cm, the polymer jet did not have sufficient time for evaporation of the solvent, thus resulting in inadequate drying of the fibers. This led to spreading of partially dried fibers on the collector screen. Figure 3a shows the as-electrospun fibers at a distance of 6.0 cm that had an average fiber diameter of 1600 ( 260 nm. Upon careful observation, it can be concluded that the fibers obtained were not uniform and that the length of the path was not sufficient for complete solvent evaporation, thereby leading to thick, fused fibers. When the distance was increased to 10 cm, long, continuous, and uniform fibers with an average diameter of 285 ( 78 nm were obtained (Figure 3b). A further increase in the distance to 12 cm reduced the diameter (155 nm) of the asspun Su-8 fibers by 1 order of magnitude (Figure 3c) as compared to the fibers obtained at 6.0 cm (1600 nm). A further increase in the electrospinning distance did not produce any significant change in the fiber diameter. We also observed an uneven deposition of fibers once the distance became larger than 12 cm because of different rates of drying before deposition on the collector screen. At a distance of 20 cm (not shown in Figure 3), the polymer jet became too short and unsteady, thus leading to the formation of very short fibers.

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Figure 3. SEM images showing the effect of the distance between the source needle and collector on the SU-8 fibers: (a) 6, (b) 10, and (c) 20 cm. (d) Graphical representation of the influence of the electrospinning distance (D) on the average fiber diameter (d). The flow rate and applied electric potential were 0.3 mL/h and 2.0 kV cm-1, respectively, for all cases.

Figure 4. SEM images showing the effect of the volumetric flow rate of the SU-8 photoresist on the SU-8 fibers: (a) 0.2, (b) 0.3, and (c) 0.4 mL/h. The electric potential and the distance between the source needle and collector were 2.0 kV cm-1 and 10 cm, respectively, for all cases.

Thus, in addition to its influence on the electric field intensity, the distance between the source needle and collector screen also significantly influences the average diameter and uniformity of the fibers by changing their path length and solvent evaporation. Larger electrospinning distances enabled an increase in the total path length covered by the jet, which, in turn, allowed more time for thinning of the polymer jet.2,22,26,29 At a fixed electric potential, an increased distance also led to a reduction of electrostatic force, causing the jet to travel in linear path without any bending instability.23,25 Effects of SU-8 Volumetric Flow Rate on Polymer Fiber Morphology and Size. The volumetric flow rate of highly viscous SU-8 2015 photoresist was varied from 0.1 to 0.5 mL/h while the other parameters were held as optimized in the previous sections: electric potential of 2.0 kV cm-1 and source needle-collector screen distance of 10 cm. At very low flow rate of 0.1 mL/h, the yield of fibers was not significant. After

the flow rate had been increased to 0.2 mL/h, nanofibers with an average fiber diameter of 119 ( 18 nm were produced. However, these fibers were neither continuous nor sufficiently long, as shown in Figure 4a. When the flow rate was increased further to 0.3 mL/h, continuous long fibers with increased average diameters (192 ( 16 nm) could be produced (Figure 4b). A further increase in the flow rate to 0.4 mL/h yielded fibers with a broad distribution of diameters, as shown in Figure 4c. This resulted because of an increase in the mass throughput of the jet, which led to the formation of multiple jets of different diameters and different levels of instability. Highly nonuniform, thick fibers along with ultrathin fibers were produced at very high flow rate, 0.5 mL/h (data not shown in Figure 4). Based on the results presented in this section, the electric potential and distance between the source needle and collector were found to play significant roles in controlling the average diameter of electrospun fibers, whereas the polymer flow rate

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Figure 5. SEM images showing the effect of viscosity on the morphology of the carbon nanofibers: (a,b) highest viscosity, 1250 cSt; (c,d) medium viscosity, 140 cSt; and (e,f) lowest viscosity, 7 cSt. The applied electric potential, flow rate, and distance between the source needle and collector were 2.0 kV cm-1, 0.3 mL/h, and 10 cm, respectively, for a-e. The applied electric potential was 1.0 kV cm-1 for f, with the other conditions remaining the same. b and d are high-magnification images of a and c, respectively.

had relatively less influence.28 The following set of parameters yielded continuous ultrathin uniform fibers with an average diameter of 190 ( 40 nm: electric potential, 2.0 kV/cm; electrospinning distance, 10 cm; and flow rate, 0.3 mL/h. The fiber diameter could be reduced by decreasing the flow rate, but the yield became poor. Effects of Viscosity of SU-8 Solution on Polymer and Carbon Fiber Morphology and Size. Other than the aforementioned parameters, the influence of the viscosity of the photoresist on nanofiber formation was also studied. Experiments were performed using two other solutions having viscosities of 7 and 140 cSt while maintaining the other parameters at the values that were optimized previously for the highest viscosity SU-8 (electric potential of 2.0 kV/cm, electrospinning distance of 10 cm, and flow rate of 0.3 mL/h). The results demonstrated that, for the lowest-viscosity SU-8 (7 cSt), clusters of beads were formed with an average diameter in the range of 180-620 nm whereas, for the medium-viscosity SU-8 (140 cSt), long uniform carbon fibers connected through beads (beaded fibers) were produced. These results indicate that the viscosity of the solution used can significantly influence the fiber morphology.2,23,29 At very low viscosity, the Coulombic forces that tend to stretch the jet dominate the viscoelastic forces that resist the jet breakup. Low viscosity thus aids in a rapid breakup of the charged polymer jet at shorter distances, thus leading to the formation of nanometer-size beads rather than long fibers.2

At moderate viscosity (140 cSt), the charged polymer jet does not break up into beads completely because of the increased viscoelasticity engendered by chain entanglements (as a consequence of the presence of an increased number of polymer chains per unit volume of polymer solution).2 As a result, beaded fibers form. It was found that the morphology and diameters of the SU-8 fibers were preserved during pyrolysis under the conditions reported. This was because of the cross-linked nature of the photoresist used. Figure 5 summarizes the findings for the effects of viscosity on the morphology of carbon nanofibers that were formed from as-electrospun SU-8 fibers after pyrolysis at 900 °C in the presence of inert atmosphere. Figure 5a shows the carbon nanofibers produced at optimized parameters with an average fiber diameter of 120 nm. Figure 5b shows a highermagnification representation of smooth continuous carbon fibers. Whereas parts c and d of Figure 5 show the formation of beaded carbon fibers for medium viscous SU-8 photoresist, part e shows the formation of small clusters of uniform spherical beads only. By changing the applied electric potential to 1.0 kV cm-1 for the lowest-viscosity SU-8, we observed that uniform and discrete carbon beads could also be produced, as shown in Figure 5f. This change in fiber morphology added a certain roughness to the surface that was further characterized by the contact angle measurements discussed later in this study.

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Figure 6. SEM images showing the effect of solvent concentration for (a) 20% and (b) 40% (v/v) solvent as a thinner. The high-magnification images in the insets show the macropores on the surfaces.

Effects of SU-8 Solvent Concentration on Polymer Fiber Morphology. Cyclopentanone was used as a thinner to dilute the SU-8. Two different solutions of SU-8 2015 with 20% and 40% (v/v) thinner were electrospun to observe the effects of solvent concentration as summarized in Figure 6. The operating parameters were the same as optimized above. Interestingly, SU-8 resist with 20% (v/v) thinner produced beaded fibers showing macropores on the surface (Figure 6a). The fibers appeared porous because of the evaporation and phase separation of solvent from SU-8. However, with the increase in dilution to 40% (v/v), porous beads were formed and then became agglomerated to form clusters (Figure 6b). The mechanism for the formation of porous fibers in a polymer-solvent system undergoing solvent evaporation has been explained theoretically in literature.34 Such porous beaded fibers could find applications in filtration, drug delivery, and fuel cell membranes, among others. Wetting Properties of Carbon Fibers. To better understand the effects of carbon nanofiber morphology on the surface roughness and thus on the wetting behavior of these fabrics, water contact angles (WCAs) and contact angle hysteresis (CAH) were measured. Smooth films of SU-8 2015 photoresist were also prepared by spin coating at 3000 rpm and subsequent soft baking at 333 K for 20 min for comparison with the electrospun fabric. Smooth carbon films were obtained after UV exposure followed by pyrolysis at the same conditions as mentioned earlier. Figure 7 shows images of water droplets on different surfaces (polymer or carbon) with different morphologies, namely, beads, beaded fibers, fibers, and smooth films, that are with and without surface treatment. The first row of images is for the SU-8 surfaces, the second row depicts untreated carbon surfaces, third row is for the UV treated carbon and the fourth row is for the plasma-treated carbon. Images in the first to fourth columns correspond to different substrate morphologiessbeads, beaded fibers, fibers and smooth film, respectively. Thus, an understanding of the effect of surface treatment on equilibrium water contact angle can be developed by comparing the images in different rows of Figure 7, whereas the effect of substrate morphology can be seen by comparing the images in different columns of Figure 7. A smooth film of the SU-8 negative photoresist is weakly hydrophilic. The water contact angle of SU-8-derived smooth carbon film increased slightly to 70.8° ((1.6°) (Figure 7h) as compared to that of an SU-8 photoresist film, 61.2° ((1.3°) (Figure 7d). There was a significant increase of the contact angle to 127.9° ((3.1°) (Figure 7g) for carbon fibers, and for carbon

Figure 7. Images of a water droplet on surfaces with various morphology and chemical treatments: (a-d) SU-8, (e-h) SU-8-derived carbon, (i-l) SU-8-derived carbon after 30 min of UVO treatment, and (m-p) SU-8derived carbon after 5 min of oxygen plasma treatment. Morphology changes occurred in the following order (from left to right): beads, beaded fibers, fibers, and films. The drop volume was ∼5 µL for all samples.

beaded fibers, it increased further to 138.4° ((2.7°) (Figure 7f). The formation of carbon beads imparted even more hydrophobicity, as shown by the contact angle of water on the carbon nanobead surface, 142.7° ((2.8°) (Figure 7e). Thus, surface morphology and roughness were found to be important factors in controlling the wettability of the electrospun fabrics.20,32,35,36 Interestingly, tailoring of the carbon substrate morphology alone without any surface chemical modifications could control the wetting properties in a large rangesfrom mildly hydrophilic (