Microcapsules containing suspensions of carbon ... - Semantic Scholar
COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Microcapsules containing suspensions of carbon nanotubes† Mary M. Caruso,a Stuart R. Schelkopf,a Aaron C. Jackson,b Alexandra M. Landry,c Paul V. Braunab and Jeffrey S. Moore*ab Received 1st June 2009, Accepted 16th July 2009 First published as an Advance Article on the web 24th July 2009 DOI: 10.1039/b910673a
Single-walled nanotubes (SWNTs) suspended in chlorobenzene (PhCl) and ethyl phenylacetate (EPA) were incorporated into microcapsules using an in situ emulsification polymerization of ureaformaldehyde; the capsules release the SWNTs upon mechanical rupture. Carbon nanotubes (CNTs) have proven valuable components of composites due to their exceptional mechanical and electronic properties.1 CNTs have been incorporated into polymer composites in order to produce materials with unique optical and electronic properties not present in the polymer alone.2 Structural composites have been made stiffer3 and tougher4 by the incorporation of carbon nanotubes. Their application has also been explored in the field of self-healing composites, where the embedded CNTs serve as damage sensors.5,6 Toward related goals, Bielawski and coworkers have used N-heterocyclic carbenes and transition metals in the pursuit of restoring electrical conductivity to a self-healing system.7 To date, self-healing materials systems have largely focused on restoring functions such as mechanical properties of structural composites8 and barrier properties of protective coatings.9 The self-healing concept based on release of liquid content from microcapsules is a general one that may potentially apply to other functionality such as restoration of electrical properties in damaged electronics. Here, we demonstrate the preparation of microcapsules containing a suspension of CNTs in organic solvents and their conductive behavior when ruptured. These healing agents could be used in a system where mechanical damage to the microcapsules results in the release of CNTs, and thus restores electrical conductivity. CNTs have been ‘‘encapsulated’’ with polymers at the molecular level in an effort to enhance dispersion in composites10,11 and with peptide amphiphiles for use in bioactive nanomaterials.12,13 To the best of our knowledge, no reports of microcapsules with robust shell walls containing suspensions of CNTs have been published. Due to the hydrophobic character of unfunctionalized CNTs, it has been hypothesized that the CNTs will remain suspended in the organic phase of the solvent and therefore can be encapsulated using the in situ polymerization method employed for the microencapsulation of nonpolar phases.14 a Department of Chemistry and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, U.S.A. E-mail: jsmoore@illinois. edu; Fax: (+217) 244-0181 b Department of Materials Science and Engineering and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA c Department of Chemical Engineering, North Carolina State University, Raleigh, NC, 27695, USA † Electronic supplementary information (ESI) available: Experimental details, thermogravimetric analysis (TGA) scans of the microcapsules, and additional SEMs. See DOI: 10.1039/b910673a
This journal is ª The Royal Society of Chemistry 2009
We began by studying the dispersion of the CNTs in various solvents. Although surfactants would normally be used to disperse the CNTs,15 these stabilizers can adversely affect capsule synthesis. Therefore, unfunctionalized CNTs and a number of nonpolar solvents were screened to maximize dispersability. CNT suspensions were reasonably stable in chlorobenzene (PhCl) and ethyl phenylacetate (EPA). Solutions of commercially available unfunctionalized single-walled nanotubes (SWNTs) in both PhCl and EPA were prepared at a concentration of 0.5 mg/mL (0.05 wt%) and were mildly sonicated for 30 minutes using an ultrasonic cleaner to achieve complete dispersion. Fig. 1 shows images acquired by transmission electron microscopy (TEM) of the unmodified CNTs before encapsulation. The impurities present in the as-received CNTs are apparent in the SWNT suspension. Whereas the pure solvents EPA and PhCl are routinely encapsulated,16,17 secondary nucleation of the poly(urea-formaldehyde) particles became a problem when encapsulating CNTs with these solvents. Therefore, the quantity of shell wall components was reduced in order to minimize particle agglomeration on the outer walls of the capsules.18 Representative SEM images of the resulting capsules containing SWNTs are in Fig. 2. Optical microscopy was also used to show the formation of spherical capsules. It is evident that the interior contains bundles of CNTs (Fig. 2b and d). These capsules were imaged in an index-matched fluid. This fluid was selected based on the respective core solvent (i.e. SWNT-EPA capsules were imaged in EPA). When crushed between glass slides, all prepared dry microcapsules were observed to release the suspended CNT bundles (ESI†). The average diameters of the prepared microcapsules ranged from 280–350 mm using a stir rate of 300 RPM during encapsulation. The
Fig. 1 TEM images of SWNTs deposited from an EPA suspension.
J. Mater. Chem., 2009, 19, 6093–6096 | 6093
Fig. 3 SEM showing bundles of SWNTs released from a ruptured microcapsule. Remnants of the microcapsule are visible on the right hand side and lower edge of the large image.
capsule diameter depends on the stir rate in the encapsulation procedure and can be modified as desired for specific applications. When capsules were prepared at stir rates above 300 RPM (e.g., 400–600 RPM), capsules continued to form, but their small size (40–90 mm in diameter) made characterization of the core material and release of the CNTs more difficult. As a result, larger capsules prepared at 300 RPM were used in the subsequent studies. The release of CNTs from these capsules was monitored when crushed onto a silicon wafer mounted on top of a carbon tape-coated stage, which was observed using SEM (Fig. 3). The nanotubes appear to
Fig. 2 Representative SEM images and optical micrographs of microcapsules containing SWNTs (0.05 wt%) suspended in (a, b) EPA and (c, d) PhCl. Capsules were prepared at a stir rate of 300 RPM.
6094 | J. Mater. Chem., 2009, 19, 6093–6096
Fig. 4 (a) Experimental setup showing probe tips submerged into the solution droplet containing CNTs. (b) Representative current–voltage plots (with a DC electric field) showing conductivity measurements of solutions from capsules containing SWNT in EPA at various concentrations (inset: optical micrograph of CNT bundles bridging the probe tips).
This journal is ª The Royal Society of Chemistry 2009
Table 1 Average conductivity measurements for microcapsules Capsule core contents
Method of mixinga
Average conductivity (S/cm)b
0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA PhCl only (no SWNT) EPA only (no SWNT) 0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA 0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA
destructivec destructive destructive destructive non-destructivee non-destructive solution onlyf solution only
1.8 1.3 0 0 0 0 7.7 1.4
10 10
4
10 10
4
4
4
1.2 10 4.7 10
5d
2.7 10 8.6 10
4d
6d
5d
a Solutions were mixed for 30 minutes. b Measurements taken at +50 V. c The destructive method consisted of mixing with a stir bar. d The standard deviation is based on 4–10 measurements for each sample. e The non-destructive method consisted of mixing with a vortex shaker and capsules did not break open to release core material. f Control solutions of just SWNT in each solvent (not from microcapsules).
contain particulate impurities (inset), consistent with the observation using TEM in Fig. 1 of the CNT-EPA solution. Indirect evidence of the CNT encapsulation was shown using conductivity measurements. Encapsulated SWNTs suspended in EPA and PhCl were first ruptured with a destructive method of mixing (under magnetic stirring for 30 minutes). The solutions of microcapsules were imaged using optical microscopy to confirm the capsule rupture after stirring. The core materials were then analyzed at room temperature and the voltage was swept from 50 V to +50 V. When the voltage was ramped between these values, the bundles of CNTs migrated towards the probe tips (separated by a distance of 100–150 mm) until enough material had collected that a bridge was formed from tip to tip (see Fig. 4). The migration of CNTs in an organic solvent in the presence of an external electrical field has been previously reported.15,19–21 In our case, enough CNTs in solution had released from the microcapsules containing SWNTs that it was possible to measure the resulting current. A summary of these results is given in Table 1. The observed current for these solutions indeed resulted from a bundle of SWNTs bridging the probe tips based on the result that an individual SWNT shell displays a current of 20 mA.22 A series of controls were also done. First, solvent capsules without CNTs were tested to show that the solution contained no conductive species. Next, SWNT capsules were mixed in a non-destructive manner (without rupturing the capsules) by vortexing for 30 minutes to show that the CNTs in solution were those that were encapsulated and not from CNTs adsorbed to the outside of the capsules. In order to confirm the non-destructive nature of mixing, multiple aliquots of the solutions of microcapsules were imaged optically after vortexing, and no ruptured capsules were observed. All of these control experiments had no measured current observed. For a comparison, solutions of the same concentration of CNTs (0.05 wt%) in each solvent were deposited onto a glass slide to determine their average conductivity. These values are given in Table 1 and are on the same order of magnitude for those solutions from ruptured microcapsules. As the amount of CNTs in the core of the microcapsules increases, the measured current is expected to increase. Due to a lower dispersability of CNTs in chlorobenzene, conductivity measurements for PhCl-CNT capsules as a function of weight fraction of CNTs could not be performed reliably. In addition, the lower vapor pressure of EPA allowed longer experiments to be run while evaporation led to inconsistent current readings for experiments done with PhCl solutions. As a result, only EPA-CNT capsules were tested for their resistance as a function of CNT weight fraction. In order to analyze This journal is ª The Royal Society of Chemistry 2009
the effect of CNT weight fraction in the core material of the microcapsules on the resulting current, microcapsules were prepared containing core suspensions of SWNTs in EPA that varied from 0.025–0.1 wt% CNTs. The current carrying capabilities of core material suspensions with different concentrations of CNTs were then studied. For solutions with a smaller concentration of CNTs, the voltage was swept 5 times before a significant current was observed (multiple sweeps were necessary in order for a sufficient concentration of CNTs to multiple bridges between the probe tips). Representative plots at each concentration are shown in Fig. 4. As expected, the current increased as a larger amount of CNTs was released from the microcapsules. In summary, we have demonstrated the microencapsulation and efficacy of CNTs for use in autonomic electronic materials. Many devices utilizing the alignment of carbon nanotubes for energy conduction are fabricated from a suspension of CNTs.19 We hope to employ a passive self-healing system for these capsules where CNTs bridge a broken circuit upon release. In order to make full use of the high aspect ratio of the CNTs, a more active system can also be envisioned in which a smart circuit applies an appropriate aligning potential across the failed circuit. By developing capsules that release their contents under various conditions, the technology could furthermore be extended to the encapsulation of functionalized CNTs and nanoparticles for a variety of applications including safer and longer-lifetime batteries.23–26 Future investigations will test the ability to encapsulate other components including nanoparticles and carbon-rich molecular fragments that may undergo electric field triggered nanowire self-assembly.
Acknowledgements This work was supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0642573, the Department of Defense (National Defense Science and Engineering Graduate Fellowship), and Center for Microanalysis of Materials, UIUC, which is partially supported by the U.S. Department of Energy (grant no. DE-FG0207ER46453 and DE-FG02-07ER46471). The authors gratefully acknowledge Benjamin J. Blaiszik, Joshua A. Ritchey, and Stefanie Sydlik for insightful discussions, Scott Robinson for assistance with the electron microscopy, Tony Banks for assisting with the conductivity measurements, and Eric Shoemaker of the Imaging Technology Group (Beckman Institute) for graphics design. J. Mater. Chem., 2009, 19, 6093–6096 | 6095
Notes and references 1 P. Avouris, Acc. Chem. Res., 2002, 35, 1026–1034. 2 R. Martel, ACS Nano, 2008, 2, 2195–2199. 3 R. H. Baughman, A. A. Zakhidov and W. A. de Heer, Science, 2002, 297, 787–792. 4 W. Jeong and M. R. Kessler, Chem. Mater., 2008, 20, 7060–7068. 5 E. T. Thostenson and T.-W. Chou, Adv. Mater., 2006, 18, 2837–2841. 6 W. Zhang, V. Sakalkar and N. Koratkar, Appl. Phys. Lett., 2007, 91, 133102. 7 K. A. Williams, A. J. Boydston and C. W. Bielawski, J.R. Soc. Interface, 2007, 4, 359–362. 8 S. R. White, M. M. Caruso and J. S. Moore, MRS Bull., 2008, 33, 766–769. 9 S. H. Cho, S. R. White and P. V. Braun, Adv. Mater., 2009, 21, 645– 649. 10 X. Wang, Z. Du, C. Zhang, C. Li, X. Yang and H. Li, J. Polym. Sci. Part A: Polym. Chem., 2008, 46, 4857–4865. 11 H. Xia, Q. Wang and G. Qiu, Chem. Mater., 2003, 15, 3879–3886. 12 M. S. Arnold, M. O. Guler, M. C. Hersam and S. I. Stupp, Langmuir, 2005, 21, 4705–4709. 13 M. S. Arnold, J. Suntivich, S. I. Stupp and M. C. Hersam, ACS Nano, 2008, 2, 2291–2300. 14 E. N. Brown, M. R. Kessler, N. R. Sottos and S. R. White, J. Microencapsulation, 2003, 20, 719–730.
6096 | J. Mater. Chem., 2009, 19, 6093–6096
15 P. V. Kamat, K. G. Thomas, S. Barazzouk, G. Girishkumar, K. Vinodgopal and D. Meisel, J. Am. Chem. Soc., 2004, 126, 10757–10762. 16 M. M. Caruso, D. A. Delafuente, V. Ho, N. R. Sottos, J. S. Moore and S. R. White, Macromolecules, 2007, 40, 8830–8832. 17 M. M. Caruso, B. J. Blaiszik, S. R. White, N. R. Sottos and J. S. Moore, Advanced Functional Materials, 2008, 18, 1898–1904. 18 B. J. Blaiszik, M. M. Caruso, D. A. McIlroy, J. S. Moore, S. R. White and N. R. Sottos, Polymer, 2009, 50, 990–997. 19 M. S. Kumar, S. H. Lee, T. Y. Kim, T. H. Kim, S. M. Song, J. W. Yang, K. S. Nahm and E. K. Suh, Solid-State Electronics, 2003, 47, 2075–2080. 20 S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke, Appl. Phys. Lett., 2001, 79, 2252–2254. 21 Z.-B. Zhang, X.-J. Liu, E. E. B. Campbell and S.-L. Zhang, J. Appl. Phys., 2005, 98, 056103-1-3. 22 P. G. Collins, M. S. Arnold and P. Avouris, Science, 2001, 292, 706– 709. 23 M. S. Whittingham, MRS Bull., 2008, 33, 411–419. 24 M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652–657. 25 L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause and J. R. Dahn, Electrochem. Solid-State Lett., 2001, 4, A137–A140. 26 P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem. Int. Ed., 2008, 47, 2930–2946.
This journal is ª The Royal Society of Chemistry 2009
www.rsc.org/materials | Journal of Materials Chemistry
Microcapsules containing suspensions of carbon nanotubes† Mary M. Caruso,a Stuart R. Schelkopf,a Aaron C. Jackson,b Alexandra M. Landry,c Paul V. Braunab and Jeffrey S. Moore*ab Received 1st June 2009, Accepted 16th July 2009 First published as an Advance Article on the web 24th July 2009 DOI: 10.1039/b910673a
Single-walled nanotubes (SWNTs) suspended in chlorobenzene (PhCl) and ethyl phenylacetate (EPA) were incorporated into microcapsules using an in situ emulsification polymerization of ureaformaldehyde; the capsules release the SWNTs upon mechanical rupture. Carbon nanotubes (CNTs) have proven valuable components of composites due to their exceptional mechanical and electronic properties.1 CNTs have been incorporated into polymer composites in order to produce materials with unique optical and electronic properties not present in the polymer alone.2 Structural composites have been made stiffer3 and tougher4 by the incorporation of carbon nanotubes. Their application has also been explored in the field of self-healing composites, where the embedded CNTs serve as damage sensors.5,6 Toward related goals, Bielawski and coworkers have used N-heterocyclic carbenes and transition metals in the pursuit of restoring electrical conductivity to a self-healing system.7 To date, self-healing materials systems have largely focused on restoring functions such as mechanical properties of structural composites8 and barrier properties of protective coatings.9 The self-healing concept based on release of liquid content from microcapsules is a general one that may potentially apply to other functionality such as restoration of electrical properties in damaged electronics. Here, we demonstrate the preparation of microcapsules containing a suspension of CNTs in organic solvents and their conductive behavior when ruptured. These healing agents could be used in a system where mechanical damage to the microcapsules results in the release of CNTs, and thus restores electrical conductivity. CNTs have been ‘‘encapsulated’’ with polymers at the molecular level in an effort to enhance dispersion in composites10,11 and with peptide amphiphiles for use in bioactive nanomaterials.12,13 To the best of our knowledge, no reports of microcapsules with robust shell walls containing suspensions of CNTs have been published. Due to the hydrophobic character of unfunctionalized CNTs, it has been hypothesized that the CNTs will remain suspended in the organic phase of the solvent and therefore can be encapsulated using the in situ polymerization method employed for the microencapsulation of nonpolar phases.14 a Department of Chemistry and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, U.S.A. E-mail: jsmoore@illinois. edu; Fax: (+217) 244-0181 b Department of Materials Science and Engineering and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA c Department of Chemical Engineering, North Carolina State University, Raleigh, NC, 27695, USA † Electronic supplementary information (ESI) available: Experimental details, thermogravimetric analysis (TGA) scans of the microcapsules, and additional SEMs. See DOI: 10.1039/b910673a
This journal is ª The Royal Society of Chemistry 2009
We began by studying the dispersion of the CNTs in various solvents. Although surfactants would normally be used to disperse the CNTs,15 these stabilizers can adversely affect capsule synthesis. Therefore, unfunctionalized CNTs and a number of nonpolar solvents were screened to maximize dispersability. CNT suspensions were reasonably stable in chlorobenzene (PhCl) and ethyl phenylacetate (EPA). Solutions of commercially available unfunctionalized single-walled nanotubes (SWNTs) in both PhCl and EPA were prepared at a concentration of 0.5 mg/mL (0.05 wt%) and were mildly sonicated for 30 minutes using an ultrasonic cleaner to achieve complete dispersion. Fig. 1 shows images acquired by transmission electron microscopy (TEM) of the unmodified CNTs before encapsulation. The impurities present in the as-received CNTs are apparent in the SWNT suspension. Whereas the pure solvents EPA and PhCl are routinely encapsulated,16,17 secondary nucleation of the poly(urea-formaldehyde) particles became a problem when encapsulating CNTs with these solvents. Therefore, the quantity of shell wall components was reduced in order to minimize particle agglomeration on the outer walls of the capsules.18 Representative SEM images of the resulting capsules containing SWNTs are in Fig. 2. Optical microscopy was also used to show the formation of spherical capsules. It is evident that the interior contains bundles of CNTs (Fig. 2b and d). These capsules were imaged in an index-matched fluid. This fluid was selected based on the respective core solvent (i.e. SWNT-EPA capsules were imaged in EPA). When crushed between glass slides, all prepared dry microcapsules were observed to release the suspended CNT bundles (ESI†). The average diameters of the prepared microcapsules ranged from 280–350 mm using a stir rate of 300 RPM during encapsulation. The
Fig. 1 TEM images of SWNTs deposited from an EPA suspension.
J. Mater. Chem., 2009, 19, 6093–6096 | 6093
Fig. 3 SEM showing bundles of SWNTs released from a ruptured microcapsule. Remnants of the microcapsule are visible on the right hand side and lower edge of the large image.
capsule diameter depends on the stir rate in the encapsulation procedure and can be modified as desired for specific applications. When capsules were prepared at stir rates above 300 RPM (e.g., 400–600 RPM), capsules continued to form, but their small size (40–90 mm in diameter) made characterization of the core material and release of the CNTs more difficult. As a result, larger capsules prepared at 300 RPM were used in the subsequent studies. The release of CNTs from these capsules was monitored when crushed onto a silicon wafer mounted on top of a carbon tape-coated stage, which was observed using SEM (Fig. 3). The nanotubes appear to
Fig. 2 Representative SEM images and optical micrographs of microcapsules containing SWNTs (0.05 wt%) suspended in (a, b) EPA and (c, d) PhCl. Capsules were prepared at a stir rate of 300 RPM.
6094 | J. Mater. Chem., 2009, 19, 6093–6096
Fig. 4 (a) Experimental setup showing probe tips submerged into the solution droplet containing CNTs. (b) Representative current–voltage plots (with a DC electric field) showing conductivity measurements of solutions from capsules containing SWNT in EPA at various concentrations (inset: optical micrograph of CNT bundles bridging the probe tips).
This journal is ª The Royal Society of Chemistry 2009
Table 1 Average conductivity measurements for microcapsules Capsule core contents
Method of mixinga
Average conductivity (S/cm)b
0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA PhCl only (no SWNT) EPA only (no SWNT) 0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA 0.05 wt% SWNT in PhCl 0.05 wt% SWNT in EPA
destructivec destructive destructive destructive non-destructivee non-destructive solution onlyf solution only
1.8 1.3 0 0 0 0 7.7 1.4
10 10
4
10 10
4
4
4
1.2 10 4.7 10
5d
2.7 10 8.6 10
4d
6d
5d
a Solutions were mixed for 30 minutes. b Measurements taken at +50 V. c The destructive method consisted of mixing with a stir bar. d The standard deviation is based on 4–10 measurements for each sample. e The non-destructive method consisted of mixing with a vortex shaker and capsules did not break open to release core material. f Control solutions of just SWNT in each solvent (not from microcapsules).
contain particulate impurities (inset), consistent with the observation using TEM in Fig. 1 of the CNT-EPA solution. Indirect evidence of the CNT encapsulation was shown using conductivity measurements. Encapsulated SWNTs suspended in EPA and PhCl were first ruptured with a destructive method of mixing (under magnetic stirring for 30 minutes). The solutions of microcapsules were imaged using optical microscopy to confirm the capsule rupture after stirring. The core materials were then analyzed at room temperature and the voltage was swept from 50 V to +50 V. When the voltage was ramped between these values, the bundles of CNTs migrated towards the probe tips (separated by a distance of 100–150 mm) until enough material had collected that a bridge was formed from tip to tip (see Fig. 4). The migration of CNTs in an organic solvent in the presence of an external electrical field has been previously reported.15,19–21 In our case, enough CNTs in solution had released from the microcapsules containing SWNTs that it was possible to measure the resulting current. A summary of these results is given in Table 1. The observed current for these solutions indeed resulted from a bundle of SWNTs bridging the probe tips based on the result that an individual SWNT shell displays a current of 20 mA.22 A series of controls were also done. First, solvent capsules without CNTs were tested to show that the solution contained no conductive species. Next, SWNT capsules were mixed in a non-destructive manner (without rupturing the capsules) by vortexing for 30 minutes to show that the CNTs in solution were those that were encapsulated and not from CNTs adsorbed to the outside of the capsules. In order to confirm the non-destructive nature of mixing, multiple aliquots of the solutions of microcapsules were imaged optically after vortexing, and no ruptured capsules were observed. All of these control experiments had no measured current observed. For a comparison, solutions of the same concentration of CNTs (0.05 wt%) in each solvent were deposited onto a glass slide to determine their average conductivity. These values are given in Table 1 and are on the same order of magnitude for those solutions from ruptured microcapsules. As the amount of CNTs in the core of the microcapsules increases, the measured current is expected to increase. Due to a lower dispersability of CNTs in chlorobenzene, conductivity measurements for PhCl-CNT capsules as a function of weight fraction of CNTs could not be performed reliably. In addition, the lower vapor pressure of EPA allowed longer experiments to be run while evaporation led to inconsistent current readings for experiments done with PhCl solutions. As a result, only EPA-CNT capsules were tested for their resistance as a function of CNT weight fraction. In order to analyze This journal is ª The Royal Society of Chemistry 2009
the effect of CNT weight fraction in the core material of the microcapsules on the resulting current, microcapsules were prepared containing core suspensions of SWNTs in EPA that varied from 0.025–0.1 wt% CNTs. The current carrying capabilities of core material suspensions with different concentrations of CNTs were then studied. For solutions with a smaller concentration of CNTs, the voltage was swept 5 times before a significant current was observed (multiple sweeps were necessary in order for a sufficient concentration of CNTs to multiple bridges between the probe tips). Representative plots at each concentration are shown in Fig. 4. As expected, the current increased as a larger amount of CNTs was released from the microcapsules. In summary, we have demonstrated the microencapsulation and efficacy of CNTs for use in autonomic electronic materials. Many devices utilizing the alignment of carbon nanotubes for energy conduction are fabricated from a suspension of CNTs.19 We hope to employ a passive self-healing system for these capsules where CNTs bridge a broken circuit upon release. In order to make full use of the high aspect ratio of the CNTs, a more active system can also be envisioned in which a smart circuit applies an appropriate aligning potential across the failed circuit. By developing capsules that release their contents under various conditions, the technology could furthermore be extended to the encapsulation of functionalized CNTs and nanoparticles for a variety of applications including safer and longer-lifetime batteries.23–26 Future investigations will test the ability to encapsulate other components including nanoparticles and carbon-rich molecular fragments that may undergo electric field triggered nanowire self-assembly.
Acknowledgements This work was supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0642573, the Department of Defense (National Defense Science and Engineering Graduate Fellowship), and Center for Microanalysis of Materials, UIUC, which is partially supported by the U.S. Department of Energy (grant no. DE-FG0207ER46453 and DE-FG02-07ER46471). The authors gratefully acknowledge Benjamin J. Blaiszik, Joshua A. Ritchey, and Stefanie Sydlik for insightful discussions, Scott Robinson for assistance with the electron microscopy, Tony Banks for assisting with the conductivity measurements, and Eric Shoemaker of the Imaging Technology Group (Beckman Institute) for graphics design. J. Mater. Chem., 2009, 19, 6093–6096 | 6095
Notes and references 1 P. Avouris, Acc. Chem. Res., 2002, 35, 1026–1034. 2 R. Martel, ACS Nano, 2008, 2, 2195–2199. 3 R. H. Baughman, A. A. Zakhidov and W. A. de Heer, Science, 2002, 297, 787–792. 4 W. Jeong and M. R. Kessler, Chem. Mater., 2008, 20, 7060–7068. 5 E. T. Thostenson and T.-W. Chou, Adv. Mater., 2006, 18, 2837–2841. 6 W. Zhang, V. Sakalkar and N. Koratkar, Appl. Phys. Lett., 2007, 91, 133102. 7 K. A. Williams, A. J. Boydston and C. W. Bielawski, J.R. Soc. Interface, 2007, 4, 359–362. 8 S. R. White, M. M. Caruso and J. S. Moore, MRS Bull., 2008, 33, 766–769. 9 S. H. Cho, S. R. White and P. V. Braun, Adv. Mater., 2009, 21, 645– 649. 10 X. Wang, Z. Du, C. Zhang, C. Li, X. Yang and H. Li, J. Polym. Sci. Part A: Polym. Chem., 2008, 46, 4857–4865. 11 H. Xia, Q. Wang and G. Qiu, Chem. Mater., 2003, 15, 3879–3886. 12 M. S. Arnold, M. O. Guler, M. C. Hersam and S. I. Stupp, Langmuir, 2005, 21, 4705–4709. 13 M. S. Arnold, J. Suntivich, S. I. Stupp and M. C. Hersam, ACS Nano, 2008, 2, 2291–2300. 14 E. N. Brown, M. R. Kessler, N. R. Sottos and S. R. White, J. Microencapsulation, 2003, 20, 719–730.
6096 | J. Mater. Chem., 2009, 19, 6093–6096
15 P. V. Kamat, K. G. Thomas, S. Barazzouk, G. Girishkumar, K. Vinodgopal and D. Meisel, J. Am. Chem. Soc., 2004, 126, 10757–10762. 16 M. M. Caruso, D. A. Delafuente, V. Ho, N. R. Sottos, J. S. Moore and S. R. White, Macromolecules, 2007, 40, 8830–8832. 17 M. M. Caruso, B. J. Blaiszik, S. R. White, N. R. Sottos and J. S. Moore, Advanced Functional Materials, 2008, 18, 1898–1904. 18 B. J. Blaiszik, M. M. Caruso, D. A. McIlroy, J. S. Moore, S. R. White and N. R. Sottos, Polymer, 2009, 50, 990–997. 19 M. S. Kumar, S. H. Lee, T. Y. Kim, T. H. Kim, S. M. Song, J. W. Yang, K. S. Nahm and E. K. Suh, Solid-State Electronics, 2003, 47, 2075–2080. 20 S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood and E. A. Grulke, Appl. Phys. Lett., 2001, 79, 2252–2254. 21 Z.-B. Zhang, X.-J. Liu, E. E. B. Campbell and S.-L. Zhang, J. Appl. Phys., 2005, 98, 056103-1-3. 22 P. G. Collins, M. S. Arnold and P. Avouris, Science, 2001, 292, 706– 709. 23 M. S. Whittingham, MRS Bull., 2008, 33, 411–419. 24 M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652–657. 25 L. Y. Beaulieu, K. W. Eberman, R. L. Turner, L. J. Krause and J. R. Dahn, Electrochem. Solid-State Lett., 2001, 4, A137–A140. 26 P. G. Bruce, B. Scrosati and J.-M. Tarascon, Angew. Chem. Int. Ed., 2008, 47, 2930–2946.
This journal is ª The Royal Society of Chemistry 2009
