Single Walled Carbon Nanotube

NANO LETTERS

Electrostatic Assembly of Polymer/ Single Walled Carbon Nanotube Multilayer Films

2003 Vol. 3, No. 1 59-62

Jason H. Rouse*,† and Peter T. Lillehei‡ ICASE, AdVanced Materials and Processing Branch, NASA-Langley Research Center, Hampton, Virginia 23681 Received September 3, 2002; Revised Manuscript Received November 18, 2002

ABSTRACT Polymer/carbon nanotube films have been formed by the alternate adsorption of the polyelectrolyte poly(diallyldimethylammonium chloride) and single walled carbon nanotubes (SWNT) onto substrates. Atomic force and scanning electron microscopies indicated that the adsorbed SWNTs were mostly in the form of 5−10 nm bundles and that uniform substrate coverage occurred. Absorbance spectrophotometry (UV− vis−NIR) confirmed that the adsorption technique resulted in uniform film growth. Characterization of the adsorbed SWNTs by X-ray photoelectron, Raman, and UV−vis−NIR spectroscopies suggested that they have a core of well ordered nanotubes covered by a layer of oxidized carbon nanotubes.

Given the unique electrical, mechanical, and optical properties of single walled carbon nanotubes (SWNT),1 their incorporation into materials has been an area of significant interest. The formation of SWNT materials, however, has been hampered by the poor solubility of pristine SWNTs in most solvents.2 To improve SWNT processability, considerable effort has been focused on either the direct functionalization of the nanotube with groups that improve solubility,3,4 or the treatment of nanotubes with dispersing agents.5 One method that has proven very useful for both the bulk processing of nanotubes and their manipulation, is chemical oxidation. The formation of carboxylic acid sites either along the length of the nanotube or at their ends has allowed the attachment of various moieties through amidization and esterification reactions.4 Researchers have also shown that the terminal carboxylic groups strongly interact with metal surfaces either directly or after functionalization,6 allowing the formation of self-assembled arrays of vertically aligned shortened nanotubes. In this communication, we report the ability to use the oxidized groups present on SWNTs to allow the electrostatic formation of polyelectrolyte/carbon nanotube multilayer films. The formation of thin, uniform films by the sequential adsorption of polyionic species of opposite charge has been an area of significant interest over the past decade.7 Films have been formed using synthetic polyelectrolytes, biomacromolecules, and various polyionic inorganic species (exfoliated clay sheets, colloidal metal, and metal oxides, * Corresponding author. Tel: (757) 864-4248. Fax: (757) 864-8312. E-mail: [email protected]. † ICASE. ‡ Advanced Materials and Processing Branch. 10.1021/nl025780j CCC: $25.00 Published on Web 12/05/2002

© 2003 American Chemical Society

etc.), allowing the fabrication of materials such as lightemitting diodes (LEDs),8 ion-selective membranes,9 and humidity sensors.10 While researchers have shown that carbon nanotubes can be incorporated within polymer11 or surfactant films12 using the Langmuir-Blodgett technique, to the authors’ knowledge, a technique for the stepwise formation of polymer/carbon nanotube films has not been reported.13 Polyelectrolyte/carbon nanotube multilayer films were prepared by first dipping a clean hydroxy-bearing silicon wafer into a 1 wt % aqueous solution of polycationic poly(diallyldimethylammonium chloride) (PDDA, 200 000350 000 Mw, Aldrich). After 10 min, the wafer was removed, rinsed with deionized water (DI), and dried with nitrogen. Rinsing and drying were repeated a second time for each adsorption treatment. The PDDA-treated wafer was then placed horizontally, face down, into a dispersion of purified SWNTs produced by the high pressure CO disproportionation (HiPco) process in dimethylforamide (DMF) (0.005 mg SWNT (Tubes@Rice)/1.0 mL DMF, 6 h prior sonication) for 100 min, removed, rinsed with DMF, and dried with nitrogen. The SWNT-terminated film was then dipped into a 1 wt % aqueous solution of PDDA in 1.0 M NaCl for 10 min, followed by rinsing with deionized water and drying with nitrogen. The addition of 1.0 M NaCl to the PDDA was required for uniform film growth as attempts to form films with only 1 wt % PDDA resulted in little sequential adsorption. Studies on polyelectrolyte multilayer films have shown that the addition of salt causes a dramatic increase in the amount of polyelectrolyte deposited.14 After PDDA treatment, the surface was dipped into the SWNT solution for 100 min, rinsed, and then dried. The treatment of the

Figure 2. Scanning electron micrograph of a (PDDA/SWNT)9 film adsorbed onto a silicon wafer.

Figure 1. Tapping-mode AFM images of a PDDA/SWNT multilayer film prepared on a silicon wafer after various numbers of adsorption treatments: (a) PDDA/SWNT, (b) (PDDA/SWNT)3, (c) (PDDA/SWNT)6, (d) (PDDA/SWNT)9. The scale bar is 1.25 micron and the z-scale in all images is 50 nm.

film with the PDDA solution followed by the SWNT solution will be referred to as an “adsorption cycle,” and the total number of adsorption cycles noted includes the initial treatment of the substrate with PDDA in the absence of NaCl. Atomic force microscopy (AFM, Digital Instruments Nanoscope IIIa, tapping mode) of a silicon wafer that had been treated with PDDA and then the SWNT solution revealed a uniform covering of randomly oriented SWNTs mostly 1-3 µm in length (Figure 1a). Height-profile measurements indicated that the adsorbed SWNTs were mostly in the form of bundles having diameters in the 5-10 nm range, although smaller bundles and/or single tubes were also present. Images obtained after the silicon wafer was treated with three PDDA/SWNT adsorption cycles displayed a dramatic increase in nanotube coverage (Figure 1b). Close inspection of the carbon nanotube bundles revealed that compared to the smooth surfaces observed for the bundles adsorbed onto the initial PDDA-treated surface, the carbon nanotube bundles in this film were covered with what appeared to be beads along their length. The presence of such beads suggests that after the initial SWNT adsorption step, the mechanism for film growth involves the coating of the SWNTs with globular PDDA domains, possibly the result of using 1.0 M NaCl during the PDDA adsorption step.14a With the upper surface of the bundles covered with PDDA, subsequent treatment of the film with the SWNT solution results in adsorption of the SWNTs onto the PDDA-covered bundles. Given the overlapping morphology of the film, nanotube adsorption occurs as a result of the adsorbing SWNT interacting with multiple bundles already on the surface. As a consequence a porous film, containing voids between the adsorbed layers of SWNT bundles, is formed. 60

Figure 3. UV-vis-NIR absorbance measured after each PDDA/ SWNT treatment for a 10-cycle film deposited on a quartz slide. The inset shows the measured absorbance at 1349 nm versus the number of adsorption treatments; the line is a linear least-squares fit to the data.

As the number of adsorption cycles was increased to 6 and then 9, the increased layering of the film was clearly evident (Figures 1c and 1d, respectively). Even after 9 adsorption cycles films were uniform, with no indication of any “island formation.” Scanning electron micrographs (Hitachi S-5200 FESEM) of a 9-bilayer film further confirmed that the adsorption technique produced a uniform film over the entire surface of the silicon wafer (Figure 2), except for a few areas where micron-size surface debris were seen. Given the fibrous appearance of the debris, it may be inferred that some of the carbon nanotube ropes could not be dispersed by sonication into small bundles (5-10 nm in diameter). To determine if the amount of nanotubes adsorbed per adsorption cycle was uniform, film growth was monitored by ultraviolet-visible-near-infrared spectrophotometry (UVVis-NIR, Perkin-Elmer Lambda 900). Figure 3 shows the increase in absorbance after each SWNT treatment for a 10cycle PDDA/SWNT film grown on a quartz slide. Since PDDA does not absorb in the spectral region monitored, the measured absorbance is due only to the SWNTs. The spectra Nano Lett., Vol. 3, No. 1, 2003

Figure 4. High-resolution X-ray photoelectron spectra of the C 1s region for carbon nanotubes adsorbed onto a PDDA-treated silicon wafer (solid line) and the bulk carbon nanotube sample (dotted line).

Figure 5. Raman spectra of carbon nanotubes adsorbed onto a PDDA-treated silicon wafer (solid line) and the bulk carbon nanotube sample (dotted line). The peak at 303 cm-1 is due to the silicon substrate.

of the absorbed carbon nanotubes contained a strong adsorption peak at 278 nm, a number of unresolved electronic transitions from 350 to 900 nm, and three unresolved transitions between 1100 and 1500 nm, positions that agree with reported literature spectra for purified HiPco SWNTs.15 Reproducible adsorption of carbon nanotubes from cycle to cycle was confirmed by the linear increase in film absorbance at 1326 nm as a function of the number of adsorption cycles (Figure 3, inset),16 thus verifying the increase in film thickness suggested by the AFM study. The ability of the SWNT bundles to adsorb onto the positively charged PDDA surface suggested an electrostatic mechanism for nanotube adsorption, with the nanotube bundles acting as polyioinic species. Further support for the nanotubes bearing a negative charge was their failure to adsorb onto the negatively charged surface of poly(styrene sulfonate). Since it is possible that the oxidative procedure used to remove metal catalyst and amorphous carbon also caused partial oxidation of the tubes themselves,15 X-ray photoelectron spectroscopy (XPS, Scienta ESCA 300, Lehigh University, Bethlehem, PA) was employed to identify the presence of oxidized defects in the adsorbing tubes. A highresolution XPS spectrum of the C 1s region for carbon nanotubes adsorbed onto a PDDA-treated silicon wafer is shown in Figure 4 (solid line). Aside from the main peak at 284.5 eV, significant photoemission was present at higher binding energies, indicating the presence of various oxidized carbon species (alcohols, aldeyhdes, acids, etc.).17 In comparison, the C 1s region for the bulk SWNT material (Figure 4, dotted line) contained a narrow intense peak at 284.5 eV (∼0.7 eV, full-width at half-maximum) for the main carbon component, photoemission above 290 eV from the πfπ* shake-up, and a small shoulder centered at approximately 286.2 eV, a location where carbon atoms with singly bound oxygen (alcohols, ethers) are normally found.17 Interestingly, XPS indicated the presence of only 2% oxygen in the nanotube sample (determined from a high-resolution spectrum of the oxygen 1s region), a value substantially lower than expected if the shoulder was due solely to oxidation. With the C 1s region for purified nanotubes containing a

significant amount of photoemission at higher binding energies from non-oxidized moieties,18 the increase in the amount of oxidation present in the adsorbed nanotubes compared to the bulk sample was determined. Subtraction of the normalized photoemission of the bulk sample from the normalized photoemission of the adsorbed nanotubes revealed that those nanotubes contained approximately 42% more oxidized carbon than the bulk sample. This indicates that the more oxidized bundles are preferentially adsorbed onto the polyelectrolyte-treated surface during film formation. Given the presence of oxidation in the adsorbed SWNTs, Raman spectroscopy was employed to confirm that the linear structures present in the AFM and SEM images contained highly structured carbon nanotubes. Raman spectra obtained for the SWNTs adsorbed onto a PDDA-treated silicon wafer and of the bulk SWNTs are shown in Figure 5 (Nicolet Almega dispersive spectrometer, 785 nm laser). It can clearly be seen that the adsorbed SWNTs (solid line) have Raman signatures nearly identical to the bulk sample (dotted line). Both spectra contained peaks at 1564 and 1592 cm-1 (tangential modes) and at 208, 229, 233, and 269 cm-1 (radial breathing modes).15,19 Taken together, XPS, Raman, and UV-vis-NIR data suggest that the adsorbed bundles have a core of well ordered nanotubes covered by a layer of oxidized carbon nanotubes. In summary, a method of assembling uniform polymer/ single walled carbon nanotube films via the sequential adsorption of the polyelectrolyte PDDA followed by carbon nanotubes onto a substrate has been reported. The adsorption technique results in the uniform growth of films containing a high concentration of SWNTs. As carbon nanotubes possess many interesting properties, the use of these films as flexible electrodes and as high-strength thin films is being explored.

Nano Lett., Vol. 3, No. 1, 2003

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NL025780J

Nano Lett., Vol. 3, No. 1, 2003