Journal of Undergraduate Research
Volume 6, Issue 7 - May/June 2005

Controlled Growth of Linear Carbon Nanofibers

Ian McKenna and Dr. Charles Beatty

ABSTRACT

This study investigates how changes to the tip surface area and deposition surface affect the growth of linear nanofibers using a standard electrospinning procedure with modification to the apparatus to include a step-down voltage collection system. The apparatus consists of a tip, a syringe pump, a rotating deposition disk, a rotating uptake disk, and two voltage supplies. The system works in the following manner: a high voltage is applied to the tip, a low voltage is applied to the deposition disk, and the uptake disk is grounded. Polymer solution from the tip is elongated by an applied electric field which is collected on the rotating deposition disk and transferred to the uptake disk by the change in voltage. Coupling this apparatus with the developments made in electrospinning, temperature-controlled air blowing, which facilitates additional shearing-stretching forces of the nanofibers and controlled solvent evaporation rates, will result in linear carbon nanofibers.

INTRODUCTION

Electrospinning is currently a highly investigated method used to fabricate nanofibers, and research is underway to extend this methodology to nanotubes. The process has had great success in producing nonwoven mats that have applications in filtration devices, high surface-area-to-weight materials, electroactive foams, bandages, substrates for tissue regeneration, artificial blood vessels, and other similar applications. Much research has been performed to examine the changes physical parameters induce on the morphology and properties of the structure, such as field strength, fluid viscosity, and various solvent compositions. However, little development has been achieved in producing oriented films or linear nanofibers. By introducing a novel approach to the electrospinning collection apparatus, we hope to produce linear carbon nanofibers thus broadening the current scope of electrospinning implementation to include, but not to be limited to, atomic force microscopy (AFM) tip generation, hard-drive probes, oriented membranes, nano-sensors, high axial strength ropes and wires, and nanoscale circuitry. This also has potential to be used in bio-filtration devices that are electrically induced to release growth factors in the human body at specified intervals.

Electrospinning works by applying a voltage to a source such as a tip or syringe needle. The polymer fluid elongates into a jet that produces a cone-like shape, called a “Taylor cone.” Due to the low weight and small size, coulombic forces between the needle and other portions of the jet result in instabilities in the fiber that produce this cone shape. The highly complex three-dimensional shape of the cone restricts the continuous winding of the fibers. However, the key is to control the nanofiber placement by using electrostatic and magnetic forces. Reduction of the cone can be achieved by controlling the surface area of the deposition, successfully shown by Theron et al [1]. This experiment extends their research with various modifications to the apparatus.

During the course of this experiment changes in the geometry, surface area of the tip, and the collection surface will be investigated. The goal is to create continuous nanofibers of any length by spinning the fibers, then collecting them. In order to do this, implementation of an uptake mechanism consisting of a step-down voltage will be applied to a second rotating disk.

EXPERIMENTAL SETUP

Figure 1 demonstrates the final setup of the equipment to generate potentially aligned and linear nanofibers. The apparatus works by applying a high voltage to the tip or syringe and a moderate voltage to the deposition disk. The uptake disk is then grounded such that the fibers follow the path of potential from the highest point to the lowest point. Depending upon the source of fiber generation, a micron-sized silicon arrow tip or syringe needle, the polymer solution is placed on the tip of a micro-fabricated silicon arrow or within a syringe. The deposition disk is set to rotate at a specific constant speed, and a voltage is applied to the tip. A syringe pump is then engaged if a syringe is used, and, eventually, when the voltage reaches approximately 5 kV per centimeter, the solution draws out into a fiber and deposits on the disk. The heated air flow system aids in removal of excess solvent and increases shear which augments the drawing of the fibers as deposition continues. At this point the deposition disk eventually rotates so that the nanofiber on the disk can be drawn onto the uptake disk by a potential drop. The uptake disk continues to collect the nanofiber until the experiment concludes. Both disks’ outer edges are coated in Teflon in order to facilitate the transfer of the nanofiber from one disk to the other. The relative speeds of the disks are given careful consideration such that the fiber does not break during the transfer. In order to help control nanofiber alignment various deposition disks with varying sizes and thicknesses are employed. Deposition on gear-shaped disks is also underway due to the unique magnetic field the surface will produce when a moderate voltage is applied. This helps to quantify any effects that surface geometry has on alignment. The entire system consists of two high voltage DC sources, two rotating disks, a syringe pump, a syringe and needle or silicon arrow tip, select polymer solutions, two motors, and a heated air system.

In order to apply a voltage to the rotating disk a mounted spring attached to the voltage supply maintains continual contact with the rotating disk. Isolation of the disk from the motor is then required such that a voltage can build up on the deposition disk. In order to isolate the disk from the motor, a nonconductive connection is utilized.

Current setup differs from the final setup at this time. Figure 2 shows the system that has been built to date and is currently being used to gather results as the final portions of the apparatus are being constructed. The system consists of a syringe pump, a rotating deposition disk, one motor, and one voltage supply. Although not shown in Figure 2, a second mounted motor and deposition surface have been built; however, no trial runs have been executed due to the fact that the moderate voltage connection by mounted spring is still currently being constructed. Solutions of polyacrylonitrile in dimethylformamide, ethyl-vinyl alcohol in isopropyl alcohol, and PZT in a poly-vinyl alcohol and deionized water mixture have been electrospun. All electrospinning experiments were carried out at ambient room temperatures. Disk rotation speeds were limited to within a range of 100 to 700 rpm, and a 22 gauge stainless-steel 90 degree blunt end pipette was used as the tip source during the course of all experiments. A potential between 5 kV and 10 kV was used on the samples with a consistent distance of 1 cm between the source and deposition disk. One run of 8% ethyl-vinyl alchohol and one run of 8% Celvol 425 poly-vinyl alcohol with 1 weight percent PZT sol gel were initially performed. Scanning electron microscopy (SEM) analysis was performed on both samples. Neither of the first two runs of ethyl-vinyl alcohol or PZT had any method of sample removal implemented. So SEM analysis was limited to sides of the disk and slight portions of the edges. Two runs of 6% polyacrylonitrile were run afterwards. These subsequent runs had a Teflon coating on the edge and the samples were carefully removed by tweezers. SEM analysis was then executed on the deposited fibers on the edges and the surface of these samples. All experiments utilized ordinary washer deposition disks with approximately a 2 mm edge thickness.

Figure 1. Schematic of eventual apparatus setup.

Figure 2. Current apparatus setup in the stage of being reconstructed to final system.

Gear-shaped disks were produced by cutting eight symmetrical grooves into the surface of the brass rod. These disks were made such that the distance between teeth is approximately 1 mm in length. Figures 3 and 4 show the fabrication process undertaken to produce these disks. In order to mount them into the electrospinning apparatus, a hole was then drilled out of the center of the brass rod. The rod was then cut into 1/8-inch pieces and tapered to a fine point. The resultant disks are shown in Figures 5 and 6. These disks will be compared with conventional washers that were previously used to deposit the nanofiber.

Figure 3. Setup used to fabricate the deposition surface for the disks.

Figure 4. Side view revealing the 1 mil cuts into the surface of the brass.

Figure 5. Resultant disks produced after drilling out the middle, using a lathe to cut the brass into disks, and then tapering the edges. Disks shown have differing deposition surfaces based on the mil cut depth.

Figure 6. Side view showing the 1 mil cuts in the brass disk to produce a gear-type deposition surface.

RESULTS AND DISCUSSION

The first two samples run are shown in Figure 7. The samples both had limited deposition on the edge of the disks, possibly due to the fact that these runs were initial trials. Samples were mostly electrospun onto the sides of the disk because the syringe needle’s alignment was somewhat off center. Approximately 1 mL of solution was electrospun onto each of these two disks. The samples were analyzed using an SEM to examine any possible orientation that may have developed. Both samples showed no orientation, as expected because the portion of the sample being analyzed was the side of the disk, not the edge. Figure 8 shows an SEM photo that is representative of the results of both sample runs, and Figure 9 shows the outer edge of the PZT sample with a microfiber protruding off the edge of the disk. This photo shows initial results that are promising for future runs because there is a single microfiber of a very limited length deposited on the edge of the disk.

Figure 7. To the left is an 8% concentration of ethyl-vinyl alcohol in a solution composition of 70 percent isopropyl alcohol and 30 percent water. To the right is an 8% concentration Celvol 425 poly-vinyl alcohol in deionized water with 1 weight percent PZT sol gel precursor stirred into solution. Sample 1 and 2 runs.

Figure 8. Ethyl-vinyl alcohol mat of nanofibers and microfibers unaligned.

Figure 9. A single PZT microfiber protruding off the side of the deposition disk.

The third run used the polyacrylonitrile solution and the fourth run was a replication of the third run. Figure 10 shows the third run in action as the deposition occurs while the disk is being rotated. What is noticeable on the third sample and in the picture is a grooved pattern on the deposition. The pattern occurs because of the rotation and the large amount of deposited solution in excess of 3 mL. The reason for this occurrence is currently unknown; however, with future analysis and runs, the cause of this phenomenon can be determined if future occurrences are reproducible and desirable.

Figure 10. Demonstration of 6% polyacrylonitrile solution in dimethylformamide being electrospun onto a single rotating disk. Sample 3 run.

Figure 11 shows the fourth sample which does not have the grooved pattern that was evident on the third sample run. Instead a lip is evident on the disk. Both the third and fourth samples using polyacrylonitrile solution showed similar results as the previous two samples with very limited alignment when analyzed with an SEM.

Figure 11. Second run using 6% polyacrylonitrile solution spun onto a disk. Sample 4 run.

SUMMARY AND CONCLUSION

The electrospinning process is driven by the electrical charges that build up on the surface of the polymeric liquid to produce nanofibers. Aligning and controlling the nanofiber growth is a highly delicate procedure of adjusting electromagnetic forces. Currently, results show little to no alignment of nanofibers. Introducing disks with decreased edge thickness may lead to increased alignment on disks. Such disks have just recently been fabricated as shown in Figures 5 and 6. More trials and analysis of current electrospinning samples are underway. Consecutive samples should prove more successful in alignment. Eventually, with the complete construction of the final apparatus continuous growth of aligned nanofibers should result. Future experimentation may incorporate nanoparticles into the polymer solution so that aligned composite nanofibers can be produced. Machining the disk to resemble a gear or sprocket, as shown in Figure 6, and then applying the same procedure outlined in the experimentation section may help influence alignment by combining the works of Li et al [3] and Theron et al [1]. The distance between teeth and the height of the teeth would have to be on the order of millimeters or less to achieve any desired results based upon prior research performed by Li et al [3]. As the length of the pit between the teeth exceeds 1 cm, the nanofiber’s weight shears the delicate fiber in two. Future experimentation will use micro-fabricated silicon arrow tips and various deposition disks with varying geometries. The heated air flow system should also help stabilize the fiber and possibly decrease the size of the Taylor cone.

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REFERENCES

1. Theron, A., Zussman, E., and Yarin, A. L., Electrostatic field-assisted alignment of electrospun nanofibres, Nanotechnology, 12, 384-390 (2001).
   
2. Kameoka, J., Orth, R., Yang, Y., Czaplewski, D., Mathers, R., Coates, G. W., and Craighead, H. G., A scanning tip electrospinning source for deposition of oriented nanofibres, Nanotechnology, 14, 1124-1129 (2003).
   
3. Li, D., Wang, Y., and Xia, Y., Electrospinning of polymeric and ceramic nanofibers as uniaxially Aligned arrays, Nano Letters, 3, 1167-1171 (2003).
   
4. Ko, F., Gogotsi, Y., Ali, A., Naguib, N., Ye, H., Yang, G., Li, C., and Willis, P., Electrospinning of continuous carbon nanotube-filled nanofiber yarns, Advanced Materials, 15, 1161-1165 (2003).

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