NANO LETTERS
Platinum Nanowires Produced by Electrospinning
2009 Vol. 9, No. 4 1307-1314
Jianglan Shui and James C. M. Li* Materials Science Program, Department of Mechanical Engineering, UniVersity of Rochester, Rochester, New York 14627 Received September 24, 2008; Revised Manuscript Received January 5, 2009
ABSTRACT A method of making long (cm) Pt nanowires of a few nanometers diameter from electrospinning is described. A major problem of avoiding bead formation along the nanofibers is analyzed, and the conditions under which the bead formation is minimized are investigated. Our ultimate purpose is to make free-standing fuel cell electrodes from these nanowires.
Electrospinning is now a well-known technique for the production of nanofibers. Combining with a metal compound, it is possible to make nanowires of a metal. But the formation of beads along the nanofibers is a nuisance that should be avoided before a uniform metal wire can be made. Exactly how the beads form is not yet known. But in general, it is due to the instability of a liquid jet which has a tendency of reducing its surface energy. For example per unit length of a liquid jet of radius r, the surface energy per unit volume is (γ is the surface energy per unit area) 2πrγ 2γ ) r πr2
(1)
4R 1 2 πR 3π 2
[
]
πr2L ) 2π r +
(3)
which gives a relation between R and L L R )π r r
4R ( ) (1 + 3πr ) 2
(4)
The driving force for this process or the reduction of free energy is approximately ∆G ) 2π(r + R)πRγ - 2πr(L + 2R)γ ) R 4 R 2πr2γ (π - 2) r 3 r
[
3
( )]
(5)
which is plotted in Figure 1. It is seen that there is a critical length of
[
]
For a sphere of radius r, the surface energy per unit volume is
L π(π - 2) 2√π - 2 ) 1+ ) 1.10 r 4 3π
4πr2γ 3γ ) r (4 ⁄ 3)πr3
corresponding to a radius of R/r ) (π - 2)1/2/2 ) 0.534 below which the growth is unstable. So the nucleation of a bead along the jet requires an activation energy of
(2)
So a liquid jet will not change into beads of the same radius but it will change into beads of larger than three-halves of its radius, and the thinner the jet the more driving force is available for bead formation. We report here the conditions we used to minimize the formation of beads and a success of obtaining the nanowires of Pt by electrospinning. Thermodynamics of Bead Formation. In eletrospinning, the liquid jet does not break up into drops of liquid. It forms beads along the jet and solidifies into nanowires with beads on them. The energetics of the formation process can be viewed as follows: To start to form a bead along a jet of radius r, a short length L of the jet may spread its volume around the jet in the form of a ring of a half-circle cross section of radius R with the following volume balance * To whom correspondence should be addressed. 10.1021/nl802910h CCC: $40.75 Published on Web 03/04/2009
2009 American Chemical Society
∆G * )
16π π - 2 3 4
(
3⁄2 2
)
r γ ) 2.55r2γ
(6)
(7)
This activation energy increases with the radius of the jet and with the surface energy. Hence thinner jets with lower surface
Figure 1. Thermodynamics of bead formation along a liquid jet.
energy are easier to nucleate beads even though higher surface energy gives larger driving force for bead formation. But the activation energy is too large to overcome by thermal activation. So the nucleation of beads must depend on a macroscopic fluctuation of a length shortening of more than
[
]
L 3π(π - 2) 2√3(π - 2) ) 1+ ) 3.746 r 4 3π
(8)
corresponding to a radius of R/r ) [3(π - 2)]1/2/2 ) 0.925 above which a stable bead can be formed and grow. Again thinner jets require shorter length fluctuations and hence easier to form beads. But the fluctuation is against the pulling force for extensional flow of the jet. Hence the nucleation is difficult if the pulling force is high, the viscosity is high, the surface energy is high, the electric field is high, or the feeding rate is low. On the other hand, high pulling forces, high surface energies, and high electric fields will result in thinner jets which favor bead formation. Another consideration is that the equivalent pressure inside the liquid jet due to the surface energy is also 2γ/r and so the pressure, like the surface energy, increases the free energy of the system and hence the vapor pressure of the liquid jet increases with decreasing radius. As a result, the thinner jets may evaporate faster and hence solidify earlier. Once the jet is solidified into a nanowire, the tendency of bead formation is much reduced. As a result, thinner jets may form less beads even though the driving force is higher, the activation energy is lower, and the macroscopic fluctuation needed is smaller. It is seen that bead formation is a complex process, and it competes with solidification. So it gives us some room to play experimentally in an attempt to avoid it. For example, Fong et al.1 in the electrospinning of PEO (polyethylene oxide) in water found that by increasing the viscosity of the solution (higher polymer concentration), increasing the charge density (adding NaCl), or reducing the surface tension (adding ethanol to water) could all avoid bead formation. Lin et al.2 eliminated bead formation on the polystyrene nanowires by adding a small amount of cation surfactants during electrospinning. Um et al.3 blew hot air at 57 °C and 70 ft3/hr (about 550 mL/s) at the orifice and successfully made high quality hyaluronic acid nanofibers by increasing the rate of evaporation of the solvent. Tripatanasuwan et al.4 found beads as the partial pressure of solvent approached saturation in the environment so the rate of evaporation reduced. Beads could be avoided by reducing the partial pressure of solvent in the environment to speed up evaporation. Eda et al.5 used N,N-dimethylformamide (DMF) as solvent for PS and obtained continuous fibers without beads. They tried four other solvents all resulting in beaded fibers. In this paper, we investigated several parameters for experimental control of electrospinning of poly(vinyl pyrrolidone) (PVP) with the addition of H2PtCl6 to see whether we can produce thin nanowires without beads. Our ultimate purpose is to make nanowires of Pt for fuel cell catalysts. Experimental Setup. The setup for electrospinning is shown in Figure 2. It consisted of a high voltage supplier (Dongwen, Tienjing, China), a syringe pump (Harvard Pump 11, U.S.A.), and a plastic syringe equipped with a 22 gauge stainless steel needle. Carbon paper was used to collect the composite fiber. The polymer used was PVP of 1.3 × 106 1308
Figure 2. The setup of electrospinning used in our laboratory.
MW, obtained from Aldrich. The platinum salt was hexachloroplatinic acid (H2PtCl6·6H2O) from Aldrich. Solvent was a mixture of deionized water and ethanol with a typical volume ratio of H2O/C2H5OH ) 0.12. The distance between the orifice and the carbon paper collector was 6 cm and the applied field was 1kV/cm. Solution feeding speed was 0.25 mL/h. Temperature was around 25 °C in room environment. Field emission source scanning electron microscope (FESEM, Zeiss-Leo DSM982 model) was used to observe the morphologies of all wires produced. The surface tension of precursor solution was measured by a capillarity method. Experimental Results. We report here the effects of concentration of polymer, the concentration of H2PtCl6, the water content in the solvent, as well as the electric field strength and feeding rate. 1. The Effect of PVP Concentration. We chose four PVP concentrations 35, 24.5, 17.5, and 10.5 mg/ml. The H2PtCl6 concentration was fixed at 5.6 mg/ml for all four experiments. As shown in Figure 3, at higher polymer concentration the fiber was smooth but thick with a diameter around 140 nm. Upon decreasing the polymer concentration, the fiber diameter decreased to 86, 46, and 26 nm, but with increasing occurrence of beads. Such phenomena were also observed in pure polymers. For example, Li and Xia6 found no beads when PVP was dissolved in 16:3 ethanol/water at a concentration of 7% by weight but many beads at concentrations of 3 and 5% by weight. Shenoy et al.7 found a minimum of 9 wt % of PVP in ethanol to avoid beads altogether. Kim et al.8 found beads when the concentration of poly(2-acrylamido-2-methyl-1-propane sulfonic acid) in ethanol was 2 or 4 wt % but no beads at 6 and 8 wt %. Yu et al.9 found beads when the concentration of polyaniline in 4% sulfuric acid was 10.6 and 11.5% but no beads at 14.0 and 17.9%. Zong et al.10 found a lot of beads for poly(D,L-lactic acid) in dimethyl formamide at a concentration of 20 wt % but no beads when the concentration was 35 wt %. The wires were thicker at higher concentrations. They found a 50-fold increase in viscosity from 20 to 36 wt %. They argued that low polymer concentration has more solvent to evaporate and hence takes longer to solidify. Of course higher viscosity requires larger force to spin and hence harder to nucleate a bead and thinner wires have higher driving force and are easier to nucleate beads. Gupta et al.11 investigated the effect of chain entanglement on the bead formation. They used the Berry number as an indication of chain entanglement. Berry number is the product of intrinsic viscosity and concentration. Intrinsic viscosity is the initial slope when specific viscosity is plotted Nano Lett., Vol. 9, No. 4, 2009
Figure 3. The morphology of electrospun PVP/H2PtCl6 composite nanofiber at different PVP concentrations. (a) 35, (b) 24.5, (c) 17.5, and (d) 10.5 mg/ml.
Figure 4. The morphology of electrospun PVP/H2PtCl6 composite wire at different H2PtCl6 concentrations (a) 16.8, (b) 11.2, (c) 5.6, and (d) 2.8 mg/ml. Corresponding high magnification pictures are inserted.
against concentration. Specific viscosity is relative viscosity (ratio of solution viscosity to solvent viscosity) minus one. For the electrospinning of PMMA from DMF solution Nano Lett., Vol. 9, No. 4, 2009
without beads, they found necessary to have a Berry number of 7. According to Eda et al.,5 Megelski et al.12 produced polystyrene fibers with beads from a tetrahydrofuran (THF) 1309
Figure 5. The morphology of electrospun PVP/H2PtCl6 composite wire at ultra combination of PVP/H2PtCl6. They are (a) 33.1 mg/ml PVP, 10.6 mg/ml H2PtCl6, (b) 33.1 mg/ml PVP, 2.6 mg/ml H2PtCl6, (c) 9.9 mg/mL PVP, 10.6 mg/ml H2PtCl6, and (d) 9.9 mg/mL PVP, 2.6 mg/mL H2PtCl6.
Figure 6. Surface tension of PVP/H2PtCl6 precursor solution vs H2O/C2H5OH (v/v) ratio.
solution with a Berry number of 14 but no beads for a Berry number of 35. For the same system, Eda et al.5 found a minimum Berry number of 9 to produce fibers without beads. Lee et al.13 found no beads when the concentration of PS in a mixture of THF and DMF exceeded 15% by weight. According to Eda et al.,5 this solution had a Berry number of about 8. Tao and Shivkumar14 found stable fibers without beads when the Berry number was about 9 and the round fibers changed to ribbons at a Berry number of about 12 in the electrospinning of PVA (polyvinyl alcohol) in water. They found that the wire diameter in the stable region was a power function of Berry number with an exponent of about 1.1. For the same system, Koski et al.15 found for the same weight percent of PVA concentration, low molecular weight 1310
favored thin fibers with beads and high molecular weight favored thicker fibers without beads. For the same molecular weight, larger weight fraction of PVA produced thicker fibers. For the same Berry number, high molecular weight produced thicker fibers with beads. 2. The Effect of H2PtCl6 Concentration. Next the H2PtCl6 concentration was varied from 16.8, 11.2, 5.6, to 2.8 mg/ ml. PVP concentration was fixed at 17.5 mg/ml for all four experiments. The feeding rate was 0.25 mL/h except for Figure 4d which was 0.15 mL/h to stabilize the spinning condition. We observed that H2PtCl6 concentration mainly influenced bead size in Figure 4. Beads were much larger but fewer per unit length with increasing H2PtCl6 concentration. Wire diameter decreased a little with decreasing acid concentration. The charge density effect was reported by Fong et al.1 They found that increasing charge density reduced the occurrence of beads but the size of beads was not affected. Jun et al.16 found that the addition of an organic salt, pyridinium formiat, to poly-L-lactide solution caused a significant reduction of beads. Son et al.17 found beads in the acid solution but not in alkaline solution in the electrospinning of PVA in water with the pH adjusted by adding HCl or NaOH. In their experiments, neither the viscosity nor the surface tension were affected by pH. Here we saw an additional effect, larger beads for higher concentrations of acid. It is possible that our concentration of acid was so high that the vapor pressure of solvent was lowered resulting in a slower vaporization to allow time to grow the beads to very large sizes. Nano Lett., Vol. 9, No. 4, 2009
Figure 7. The morphology of electrospun PVP/H2PtCl6 composite nanowire with following H2O/C2H5OH ratios: (a) 0.036, (b) 0.075, (c) 0.117, (d) 0.163, (e) 0.212, and (f) 0.265.
Some more experiments on different concentrations of H2PtCl6 and PVP are shown in Figure 5. The various concentrations in the two figures are listed here in the following table:
figure
PVP, mg/ml
H2PtCl6, mg/ml
figure
PVP, mg/ml
H2PtCl6, mg/ml
4a 4b 4c 4d
17.5 17.5 17.5 17.5
16.8 11.2 5.6 2.8
5a 5b 5c 5d
33.1 33.1 9.9 9.9
10.6 2.6 10.6 2.6
By comparing Figures 5b, 4d, and 5d, the effect of polymer concentration at low acid concentration can be seen. High polymer concentration produces thicker wires without beads and low polymer concentration produces thinner wires with beads. By comparing Figures 5a, 4b, and 4c, the effect of polymer concentration at high acid concentration can be seen. Again high polymer concentration produced thicker wires Nano Lett., Vol. 9, No. 4, 2009
without beads and low polymer concentration produced thinner wires with beads. When the polymer concentration was high, such as in Figure 5a,b, smooth wires were produced despite the difference in acid concentration. On the other hand, when the polymer concentration was low, such as Figure 5c,d, beads were produced no matter what was the acid concentration. 3. The Effect of Water Content. Next we checked the effect of the ratio of H2O/C2H5OH on the final composite fiber morphology. When we dropped H2PtCl6 solution into PVP solution in ethanol, floccules were produced that needed a little water to disappear. We also observed that when replacing ethanol with methanol, a clear solution could be obtained without water. So the water was necessary for the ethanol/H2PtCl6 solution. Besides, mixing water in ethanol was supposed to increase surface tension since water has a surface tension of 72.75 mN/m versus 22.32 for ethanol. We chose the following six volume ratios of H2O/C2H5OH: 1311
Figure 8. The morphology of electrospun PVP/H2PtCl6 composite nanofiber at following feeding speeds. (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mL/h.
Figure 9. The morphology of electrospun PVP/H2PtCl6 composite nanowire at following electric field strengths. (a) 5, (b) 6, (c) 7, and (d) 8 kV/6 cm. Corresponding high magnification pictures are inserted.
0.036, 0.075, 0.117, 0.163, 0.212, and 0.265. PVP concentration was 17.5 mg/ml and H2PtCl6 was 5.6 mg/ml in all the experiments. The surface tension of the precursor solution 1312
is plotted in Figure 6 including those with pure ethanol (20.1 mN/m) and pure deionized water (66.6 mN/m). We observed that precursor solution surface tension increased gradually Nano Lett., Vol. 9, No. 4, 2009
Figure 10. An example of some Pt wires obtained under the following conditions: 17.5 mg/ml (PVP), 5.6 mg/ml H2PtCl6, H2O/C2H5OH ) 0.12, feeding rate 0.25 mL/hr, spinning distance 6 cm, applied voltage 1 kV/cm, R.H. 30%, temperature 20 C.
Figure 11. Diameter distribution of Pt nanowires showing a possible bimodal behavior.
from 21.1 to 24.5 mN/m with water/ethanol ratio increasing from 0.036 to 0.265. In Figure 7, we observed that in the low water/ethanol ratio range increasing the water content made wires thinner. In the high water/ethanol ratio range, fiber diameter did not change much with increasing water content, but bead density increased. Surface tension is the driving force for bead formation. Fong et al.1 also found in an experiment in which the ethanol/water ratio was changed, the beads along the poly(ethylene oxide) fiber increased with increasing water content. Yang et al.18 also found a solvent ratio effect for the electrospinning of PVP in an ethanol/DMF mixture 50/50 by weight. They obtained 20 nm wires without beads and attributed to the combined effects of viscosity (0.014 Pa.s) and surface tension (35mN/m). For the same 4 wt % of PVP but dissolved in pure ethanol, the diameter of the wire was 250 nm. Instead of 50/50, if the weight ratio of the solvents was 35/65 beads were observed. 4. The Effect of Feeding Rate. Then we checked the influence of the solution feeding rate on the morphology of the final composite fiber. We chose four feeding rates of 0.1, 0.2, 0.3, and 0.4 mL/h. PVP concentration was 17.5 mg/ml, and H2PtCl6 was 5.6 mg/ml for all the experiements. At 0.1 mL/h feeding was discontinuous, while at 0.4 mL/h big drops of liquid fell onto the collecting plate during spinning. Mejelski et al.12 found no beads along PS fibers in THF when Nano Lett., Vol. 9, No. 4, 2009
the feeding rate was less than 0.1 mL/m. Zong et al.10 observed relatively thicker fiber diameters and larger beads in fibers spun from a higher feeding rate (75 µL/min) and thinner fibers and smaller beads for lower feeding rate (20 µL/min). But we did not see any obvious effect of feeding rate on the wire diameter but the beads were larger at higher feeding rate as shown in Figure 8 in agreement with Zong et al.10 They thought that at higher feeding rate the size of the drop was larger and the liquid jet moved faster so there was not enough time to dry before it reached the collection plate. As a result, the bead grew larger. 5. The Effect of Electric Field. Next we checked the influence of the electric field on the morphology of the composite fiber. We chose four voltages of 5, 6, 7, and 8 kV across a distance of 6 cm from the orifice to the collecting plate. Since different a electric field means a different drawing force that will cause a different shape of the suspending droplet and the initiation position of the jet at the orifice,9,11 we used different feeding rates for different electric fields in an attempt to make the stable jet initiation condition similar for all the samples. So the feeding rates used were 0.1, 0.15, 0.3, and 0.4 mL/h corresponding to the above voltages. PVP concentration was 16.5 mg/ml, and H2PtCl6 concentration was 5.15 mg/ml for all the experiments. As shown in Figure 9, we observed thinner wires and larger beads at higher electric field strength. We can use the idea of Zong et al.10 With increasing electric field, the jet speed is faster so the jet has less time to evaporate, which allowed the bead to grow larger before solidification. Deitzel et al.19 found no beads along the poly(ethylene oxide) fiber when 5.5 kV was applied across a distance of 6.5 in or 16.5 cm but found beads at 7 kV and the bead density increased 10-fold at 9 kV. However the size of the beads and the diameters of the fibers remained similar. Zong et al.10 also found more beads at higher electric fields. However, Helgeson et al.20 found no beads at high electric fields but beads at low fields when the polymer (polyethylene oxide in water) concentration exceeded 2 wt %. Mejelski et al.12 found beads when the distance between the needle and the collecting plate was reduced from 35 to 30 cm for the same applied voltage. 1313
Platinum Nanowires. As pointed out by Lieber21 in his 2003 MRS medalist lecture, nanowires can be used as building blocks in nanocomputers, nanoscale sensors, nanophotonics, and so forth. Here we envision another use in fuel cell electrodes. Most nanowires are made chemically by controlled growth in the length direction and hence are not very long. For example, Chen et al.22 were able to grow Pt nanowires of 500 nm long and single crystal nanorods of 100 nm long. These short wires still need a support. For example, Zhao et al.23 must use a Ti/Si support for their nanowires of 30 nm diameter and 1 µm long spaced about 100 nm apart. Yan et al.24 must use oxide surfaces such as zirconia to grow Pt nanowires of 20 nm × 5 nm × 12 µm dimensions. Lee et al.25-27 used Pt and W gauzes to support their Pt nanowires of about 8 nm diameters. Our efforts are directed to make very long Pt polycrystalline nanowires so that no support is needed in the fuel cell electrodes. These free-standing electrodes are expected to have longer durability and stability and more efficient use of Pt without loss. An example of the Pt nanowires obtained is shown in Figure 10 and a diameter distribution is shown in Figure 11. X-ray diffraction showed pure Pt and the diameter distribution showed a possible bimodal behavior. We are hopeful that further optimization of conditions could allow us to make Pt nanowires with an average diameter of only a few nanometers useful for fuel cells. Conclusion. We investigated the effect of several operating parameters on the morphology of the composite nanowires made by electrospinning. We chose PVP/H2PtCl6 system with the ultimate goal of making Pt nanowires for fuel cell applications. We found that in order to obtain a uniform fiber structure without beads, certain high polymer concentration is necessary such as 20 mg/ml, a low water/ethanol ratio such as 0.1, a low feeding rate such as 0.1 mL/h, and a low electric field strength such as 5 kV across a distance of 6 cm. These conditions must be further optimized together with the heat treatment in the making the Pt nanowires. Acknowledgment. Work supported by NSF through DMR-0801402 monitored by Drs. Harsh Chopra and Bruce MacDonald. References (1) Fong, H.; Chun, I.; Reneker, D. H. Beaded Nanofibers Formed during Electrospinning. Polymer 1999, 40, 4585–4592. (2) Lin, T.; Wang, H.; Wang, H.; Wang, X. The Charge Effect of Cation Surfactants on the Elimination of Fiber Beads in the Electrospinning of Polystyrene. Nanotechnology 2004, 15, 1375–1381. (3) Um, I. C.; Fang, D.; Hsiao, B. S.; Okamoto, A.; Chu, B. ElectroSpinning and Electro-Blowing of Hyaluronic Acid. Biomacromolecules 2004, 5, 1428–1436. (4) Tripatanasuwan, S.; Zhong, Z.; Reneker, D. H. Effect of Evaporation and Solidification of the Charged Jet in Electrospinning of Poly(ethylene oxide) Aqueous Solution. Polymer 2007, 48, 5742–5746. (5) Eda, G.; Liu, J.; Shivkumar, S. Solvent Effect on Jet Evolution during Electrospinning of Semi-dilute Polystyrene Solutions. Eur. Polym. J. 2007, 43, 1154–1167. (6) Li, D.; Xia, Y. Electrospinning of Nanofibers: Reinventing the Wheel? AdV. Mater. 2004, 16, 1151–1170.
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NL802910H
Nano Lett., Vol. 9, No. 4, 2009
