LETTER pubs.acs.org/NanoLett
Carbon Nanotube/Polyaniline Composite Nanofibers: Facile Synthesis and Chemosensors Yaozu Liao,†,‡ Chen Zhang,§ Ya Zhang,† Veronica Strong,† Jianshi Tang,|| Xin-Gui Li,*,‡ Kourosh Kalantar-zadeh,§ Eric M. V. Hoek,^ Kang L. Wang,|| and Richard B. Kaner*,† †
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Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California, 90095-1569 ‡ Institute of Materials Chemistry, College of Materials Science and Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092, China § School of Electrical and Computer Engineering, RMIT University, City Campus, P.O. Box 2476 V, Melbourne 3001, Victoria, Australia Department of Electrical Engineering, University of California, Los Angeles, Los Angeles, California 90095-1969, United States ^ Department of Civil and Environmental Engineering, University of California, Los Angeles, Los Angeles, California 90095-1969, United States
bS Supporting Information ABSTRACT: An initiator is applied to synthesize single-walled carbon nanotube/polyaniline composite nanofibers for use as high-performance chemosensors. The composite nanofibers possess widely tunable conductivities (10-4 to 102 S/cm) with up to 5.0 wt % single-walled carbon nanotube (SWCNT) loadings. Chemosensors fabricated from the composite nanofibers synthesized with a 1.0 wt % SWCNT loading respond much more rapidly to low concentrations (100 ppb) of HCl and NH3 vapors compared to polyaniline nanofibers alone (120 s vs 1000 s). These nanofibrillar SWCNT/polyaniline composite nanostructures are promising materials for use as low-cost disposable sensors and as electrodes due to their widely tunable conductivities. KEYWORDS: Carbon nanotubes, conducting polymers, composite, nanofibers, chemosensors
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relatively high loadings of carbon nanotubes (relative to the aniline content) have generally resulted in low conductivities (often 200 MΩ to 430 kΩ, and then increases to 3.8 MΩ after 3 days of storage in N2. After two successive 1 min exposures of 100 ppb NH3 vapor, the resistances increase to 27.5 and 74.0 MΩ, respectively. Upon exposure to HCl vapor for another minute, the resistance decreases dramatically from 74.0 MΩ to 610 kΩ. Continued gas phase cycling (i.e., doping/dedoping) experiments indicate that the nanofibrillar composite ultrathin films show highly reversible sensitivity to acid doping and base dedoping. A chemosensor made with a nanofibrillar composite film exhibits a comparable sensitivity to acid doping, but responds much faster to base dedoping compared to that made with a PANi nanofiber film (120 s vs 1000 s).38 For example, upon exposure to NH3 vapor at the same concentration (100 ppb), a 20-fold increase in resistance takes only 120 s for the nanofibrillar composite film, while the PANi nanofiber film takes 1000 s to respond. Both the increase in the conductivity and the enhancement in the response of the composite nanofibers can be strongly attributed to the increase in the charge-carrier mobility. As discussed above, a large number of redox sites have been found existing at the junctions between SWCNTs and PANi nanofibers leading to an extended conjugated network. Since PANi nanofibers are a well-known sensing material for H2 gas,46 the above hypothesis can be further confirmed by measuring the redox response of pristine PANi and SWCNT/PANi composites to a 1% concentration of H2 gas. Pristine PANi nanofibers show >5 times the resistance change when compared to the composite nanofibers (Figure S7, Supporting Information), indicating that many more reduced emeraldine segments exist in the composites. As a result, the SWCNTs appear to interact with polymer chains, which can efficiently lower the hopping length that exists in the PANi part of the composite nanofibers in comparison to that of the pristine PANi nanofibers. It has been demonstrated that electron transfer occurs from the carbon nanotubes to the lowest unoccupied molecular orbital (LUMO) of the conjugated polymer, leading to an increase in overall charge-carrier mobility of the system via the SWCNTs.12,47 On the other hand, carbon nanotubes have a high affinity for both NH3 and HCl.48 Consequently, the presence of SWCNTs in the composite may promote the absorption of more vapor molecules. We also note that the PANi layer coating on the carbon nanotubes is 10-30 nm thick, while pristine PANi nanofibers are 30-60 nm thick as determined by TEM (Figure 1). The thin PANi layer coating surrounding the SWCNTs has a higher surface area than pristine PANi nanofibers. This may explain why the 1.0 wt % SWCNT/ PANi composite nanofiber chemosensors demonstrate a much improved sensitivity compared to the chemosensors fabricated by PANi nanofibers alone. Both the doped and dedoped composite nanofibers can be dispersed in aqueous solvents or dissolved in polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), etc. Processable
Figure 4. (a) Typical I-V curves of nanofibrillar 1.0 wt % SWCNT/ polyaniline composite films upon exposure to alternating vapors of HCl and NH3. The insets show the geometry of the device: (left) front view and (right) top view. (b) A zoomed-in SEM image of the device.
5 wt % SWCNT loading has been increased by greater than 50 times compared to that of the doped PANi nanofibers alone (1.7 S/cm). The nanofibrillar composite material synthesized with a low concentration of SWCNT (0.5 wt %) shows a dramatic decrease in conductivity above pH 2.1, comparable to pristine PANi.45 However, the maximum conductivity remains slightly higher than that of pristine PANi (2.8 vs 1.7 S/cm). As the pH increases, the conductivity of the three other composite nanofibers with increasing SWCNT content decreases gradually, similar to an acid-base titration curve. Even when the composite nanofibers are completely dedoped with strong base (pH = 12.8), they still show a relatively high conductivity (∼0.3 S/cm), especially when compared to that of pristine PANi (200 MΩ due to the use of ultrathin (50-100 nm) fully dedoped films 957
dx.doi.org/10.1021/nl103322b |Nano Lett. 2011, 11, 954–959
Nano Letters
LETTER
SWCNT/PANi composite nanofibers with enhanced electrical and thermal properties can be potentially useful in many other applications. For example, the bucky-like films made from the composite nanofiber aqueous dispersions could provide a simple route to conducting asymmetric films by using flash welding.49 Additionally, the solubility of the composite nanofibers opens a window to prepare electrically conductive ultrafiltration membranes by using a nonsolvent induced phase separation (NIPS) method. In summary, bulk quantities of SWCNT/PANi composite nanofibers are readily synthesized with the addition of an initiator to a rapidly mixed polymerization of aniline in the presence of carbon nanotubes. The SWCNT content in the final dedoped products ranges from 4 to 40 wt %, as determined by thermal and elemental analyses. Nanofibrillar composite materials exhibit enhanced conductivities with values up to 95 and 0.3 S/cm in their doped and dedoped states, respectively, compared to 1.7 and
