Enhanced Electrical Conductivities of Transparent Double-Walled

Jul 9, 2009 - The DWNT-Au nanoparticle hybrid films exhibited the best transmittance-conductivity performance among the evaluated network films, i.e.,...
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Enhanced Electrical Conductivities of Transparent Double-Walled Carbon Nanotube Network Films by Post-treatment Seung Bo Yang, Byung-Seon Kong, Jianxin Geng, and Hee-Tae Jung* Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea ReceiVed: April 20, 2009; ReVised Manuscript ReceiVed: June 14, 2009

We investigated the electrical conductivities of double-walled carbon nanotube (DWNT) network films upon post-treatment via HNO3 or gold ions, particularly with respect to their potential use as transparent conducting films. Post-treatment of DWNT films with HNO3 and deposition of gold nanoparticles, respectively, resulted in a significant decrease of the electrical resistance, while the initial value of the transmittance of pristineDWNT films was maintained. The DWNT-Au nanoparticle hybrid films exhibited the best transmittanceconductivity performance among the evaluated network films, i.e., pristine-SWNT films, pristine-DWNT films, SWNT-Au nanoparticle hybrid films, and HNO3-treated DWNT films. The electrical behaviors of the gold nanoparticle-coated and HNO3-treated DWNT films were examined by an analysis of the work functions of the DWNT films. The DWNT-Au nanoparticle hybrid films are expected to be applicable to transparent electrodes of various optoelectronic devices such as solar cells, light-emitting diodes, sensors, and fieldeffect transistors. Introduction On the basis of their superior electrical, optical, and mechanical properties,1 carbon nanotubes (CNTs) are expected to find applications in touch screens, smart windows, solar cells,1-4 organic light emitting diodes (OLEDs),1,5 biosensors,6 field effect transistors (FETs),1,7 and field emission displays (FEDs).8,9 Recently, considerable efforts have been directed toward developing flexible transparent conducting films by using CNTs, which may replace the currently available indium tin oxide (ITO) electrodes. Singled-walled carbon nanotube (SWNT) network films have been primarily used for such applications owing to the relatively high intrinsic mobilities and conductivities of SWNTs,11 and also since multiwalled carbon nanotubes (MWNTs) suffer from deteriorated transmittance by Rayleigh scattering due to the multiplicity of their walls.10 Various approaches using SWNTs have been developed to enhance their conductivities with little change in the transmittance, including simple immersion of SWNT films in HNO3,12 SOCl2,13 HNO3 followed by SOCl2,14 and Au salt solution.15 On the other hand, DWNTs have unique characteristics differentiating them from SWNTs and MWNTs. It is wellknown that they exhibit a high structural stability in comparison with that of SWNTs.18 The band structure of DWNTs is rarely affected by interlayer interaction, but their potential barrier depends on the chirality pairs.18 Furthermore, independent doping of inner and outer tubes is possible for DWNTs.18 Recently, several researchers reported on the electrical properties of DWNT network films through a comparison of the resistance according to shell numbers,11 types of nanotubes, and film preparation methods.16 Pristine DWNT films have been reported to show greater conductance than pristine SWNT films.11,16 Furthermore, among pristine SWNT films, unsorted DWNT films, and sorted DWNT films and their SOCl2-doped films, * To whom correspondence should be addressed. Phone: +82-42-3503931. Fax: +82-42-350-3910. E-mail: [email protected].

sorted DWNT films after SOCl2 doping showed the lowest electrical resistance.17 In the present work, we investigate the conductivity and transmittance of pristine DWNT network films and acid-treated and gold ion-treated DWNT network films in terms of their potential use as transparent conducting films. As in SWNT films, it is shown that acid and gold ion treatment enhance the conductivity of the network films without a considerable change in their transmittance. Notably, the gold nanoparticle-coated DWNT films exhibit superior transmittance-conductivity performance in comparison with that of the acid-treated DWNT networks, making them a potential candidate for application in transparent conducting films. Experimental Section Materials. Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) fabricated by a highpressure CO disproportionation (HiPco) process were purchased from Unidym (purified grades). Additional purification with dry oxidation and acid treatment was not carried out in order to avoid creation of defects on the CNT sidewalls. The length of DWNTs is in the range of 10-30 µm, which was determined from multiple TEM observations. The diameter of DWNT estimated from the RBM mode of Raman spectra with an 633 nm excitation laser source is approximately 0.9-1.2 nm. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O, g99.9% pure) and Nafion perfluorinated ion-exchange resin, 5 wt % in a mixture of lower aliphatic alcohols and H2O (contains 45% water), were purchased from Aldrich. NaOH (97%) and ethanol (99.9% pure) were purchased from Merck. HNO3 (60% pure) was purchased from Junsei Chemicals. A porous alumina membrane filter (200 nm pore size, 47 nm diameter) was purchased from Whatman International Ltd., England. For the substrates, untreated slide glasses (∼75 × 25 × 1.1 mm3) were obtained from Paul Marienfeld GmbH & Co. KG. Deionized water (18 MΩ cm) was purified by using an ultrapure water system (Milli-Q, Milli-pore.).

10.1021/jp903605p CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

DWNT-Gold Nanohybrids as Transparent and Conductive Films

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SCHEME 1: Schematic of the Fabrication of CNT Films by the Vacuum Filtration Method and Their Post-treatment15

Preparation of CNT Films and Their Post-treatments (Scheme 1). First, the CNT dispersion solution was prepared as follows.19 (1) DWNT and SWNT (20 mg) were dispersed in a mixture of H2O (160 mL), 2-propanpol (40 mL), and Nafion (4 g). The solution was ultrasonicated by using a probe-type sonicator [Ultrasonic processors VCX 750, 750W] with an amplitude of 90% for 10 min. (2) The extracted solution from the suspension was centrifuged at 15 000 rpm for 1 h to remove bundles of CNTs. DWNT and SWNT films were fabricated on a glass substrate with supernatants (upper 50%) by vacuum filtration methods, as reported previously.15,20,21 Ti-Au contact electrodes (10 and 90 nm thick, respectively) were formed on CNT films (15 mm spacing) by using an electron-beam evaporator. The DWNT and SWNT films were immersed in a 1 mM gold salt solution for 10 min. The DWNT films were immersed in concentrated HNO3 for 1 h.12 The CNT films after post-treatment were rinsed extensively with deionized water and dried under nitrogen gas. All procedures were carried out under ambient temperature in air. Characterization. SEM images of DWNT films were obtained with use of a field emission scanning electron microscope (FESEM, Philips, XL30SFEG). The structural change of the DWNT films after gold nanoparticle deposition was observed by using a JEM-2100F field emission transmission electron microscope (JEOL). TEM samples were prepared as reported before,15 and the structure of the gold particles was analyzed by TEM performed in bright field mode at 200 kV. Current-voltage curves were obtained via measurements in the linear sweep voltammetry mode (3-electrode mode (working electrode, reference and counter electrode system)), using a CHI 600C potentiostat (CH Instruments Inc.) at ambient temperature.

We measured the electrical resistance of the CNT films from the reciprocal of the slope from the current-voltage curve. The potential was swept at a scan rate of 0.1 V s-1 from -3 to +3 V. The light transmittance at 550 nm of the CNT films was measured by a UV/vis/NIR spectrophotometer (V-570, JASCO). An ultraviolet photoelectron spectrophotometer (AC-2, RKI Instruments, Inc.) was used for the work function measurement of the DWNT films. Results and Discussion Scheme 1 represents the overall procedure for fabricating the nanotube network films including the HNO3 and gold ion posttreatments. The nanotubes (SWNTs and DWNTs) were dispersed in a mixture of water and propanol, with the help of Nafion (step 1). The Nafion induces electrostatic and steric stabilization of the nanotubes due to their hydrophobic backbone and hydrophilic tail, leading to a good dispersion of the nanotubes in the solvent mixture.18,21 The vacuum filtration method was used to prepare uniform nanotube network films (steps 2-4).15 Ti/Au, employed as a contact electrode, was deposited on the nanotube films to measure the conductivities of the films (step 5). To enhance the transmittance of the nanotube films without considerable loss of their optical transmittance, the DWNT and SWNT films were further posttreated by two different methods (step 6), gold ion and HNO3 addition onto the nanotube networks. The gold salt was dissolved at 1 mM in HAuCl4 · 3H2O in 50 vol % ethanol, and the nanotube films were subsequently immersed in a gold salt solution for 10 min.15 In the case of HNO3 post-treatment, the nanotube films were soaked in concentrated HNO3 (60% pure)

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Figure 1. SEM images of (a) pristine-DWNT films, (b) DWNT-HNO3 films, and (c) DWNT-Au films with 83% transmittance at 550 nm. The scale bar represents 500 nm.

for 60 min.12 The nanotube films were then extensively washed with deionized water and dried under nitrogen gas (step 7). Figure 1 shows scanning electron microscope (SEM) images of the nanotube network films at ∼82% transmittance. DWNT network films (Figure 1a), the acid (HNO3)-treated DWNT films (Figure 1b), and the Au nanoparticle-coated DWNT films (Figure 1c) are shown. Pristine DWNT networks uniformly covered the glass substrate (Figure 1a). Similar images were observed for the HNO3-treated DWNT films (Figure 1b), although they were treated with concentrated HNO3. In the case of DWNT films from immersion of pristine DWNT films in 1 mM gold salt solution, gold nanoparticles were observed after the electroless reduction of gold ions on the DWNT networks (indicated by a red circle in Figure 1c). A transmission electron microscopy (TEM) image (Figure 2a) further verifies that the gold nanoparticles are formed by the electroless reduction of gold ions (Au3+) on the DWNT films. A magnified image clearly shows the formation of gold nanocrystals after reduction of gold ions (inset of Figure 2a), with regular d-spacing of 2.36 ( 0.02 Å, which corresponds to the spacing between the {111} planes of crystalline gold (2.355 Å).15,23 These particles were spherical in shape, having a mean diameter of 10 nm with a standard deviation of 4.2, as determined from a total of ∼160 gold particles in multiple TEM images (Figure 2b). Figure 3 shows the electrical resistance of five different nanotube network films as a function of the transmittance at 550 nm: pristine SWNT network films (0), pristine DWNT films (b), Au-treated SWNT films (2), Au-treated DWNT films (1),

Yang et al. and HNO3-treated DWNT films ([). The network densities of the nanotube films were controlled by precise extraction of the dispersed nanotube solutions, and the electrical resistances were obtained in the linear sweep voltammetry mode by using a CHI 600C potentiostat. As expected, the resistances of the nanotube network films increase gradually as the transmittance (at 550 nm) increases. However, we found that the nanotube network films have distinctly different resistances depending on the types of nanotubes and the post-treatment methods, and they are ranked in the following order: pristine-SWNTs > pristineDWNTs > SWNT-Au > DWNT-HNO3 > DWNT-Au. The resistances of the pristine-DWNTs were lower than those of the pristine-SWNTs, indicating better performance of the transparent conductive film as compared with that of the pristineSWNTs. This is due to higher conducting π pathways of DWNTs relative to those of SWNTs.11 The electrical resistances of both pristine DWNTs (b) and pristine SWNTs (0) decreased significantly after Au nanoparticle hybridization. This is attributed to an electron depletion mechanism that results from a doping effect.15 However, the reduction ratio of the electrical resistances is slightly different in both films. The reduction ratio of the resistances for pristine-DWNTs after gold nanoparticle deposition is ∼58%, while that for pristine-SWNTs is ∼66%. The average reduction ratio was calculated from r ) [1 - (R/ R0)] × 100(*), where r, R0, and R denote the reduction ratio, the resistance of pristine-nanotube films, and the resistance of nanotube films after post-treatment, respectively. Similarly, the resistances for pristine-DWNTs after HNO3 treatment decreased significantly owing to a doping effect. However, it is of note that DWNT-Au has lower electrical resistance than that of DWNT-HNO3, particularly above 80% transmittance. The reduction ratio of the electrical resistance of DWNT-Au is 58.16%, while that of DWNT-HNO3 is 50.04%. As a result, DWNT-Au exhibits the highest conductance without substantial loss of the transmittance among the DWNT network films evaluated in this study. Figure 3b shows a representative optical image of a transparent DWNT-Au film with 83% transmittance. To investigate the effect of immersion time in gold salt solution and HNO3 on the conducting performance of the DWNT-Au (Figure 4a) and DWNT-HNO3 (Figure 4b) and thereby evaluate their potential transparent conducting films, we measured the electrical resistance and transmittance at 550 nm as a function of post-treatment time. Figure 4a shows the electrical resistance and light transmittance (550 nm) of DWNT films according to the immersion time in 1 mM concentrations of gold salt solution. The electrical resistance of DWNT films at 83% transmittance decreased significantly after immersion for 10 min, but the values were maintained constantly from 10 to 30 min. However, the optical transmittance at 550 nm of the DWNT films was not strongly influenced by immersion time in the 1 mM gold salt solution, similar to observations for SWNT-Au.15 The resistance and transmittance results of DWNTHNO3 according to immersion time of HNO3 were also plotted in order to determine the optimum post-treatment conditions. The electrical resistance of DWNT-HNO3 dramatically decreased at 30 min, and reached a minimum at 60 min with constant transmittance. Note that the resistance increased slightly after 60 min of immersion (Figure 4b), although a precise explanation for this has not yet been found. To further understand the role of the gold reduction and HNO3 treatments on the pristine-DWNTs, we measured the currentvoltage curve and the work function (Figure 5b-d) of the DWNT films at each stage using linear sweep voltammetry and ultraviolet photoelectron spectroscopy, respectively. The

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Figure 2. (a) TEM images of DWNT-Au films. The scale bar represents 100 nm. The inset in panel a is a TEM image of the same film, and the scale bar represents 3 nm. Note that the inset in panel a shows regular d-spacing (2.36 ( 0.02 Å) of the observed planes of the gold lattice. (b) Histogram of diameters of gold nanoparticles on pristine-DWNTs.

Figure 3. (a) Electrical resistance of pristine-SWNT, pristine-DWNT, SWNT-Au, DWNT-Au, and DWNT-HNO3 films. (b) Photographic images of a DWNT-Au hybrid film.

current-voltage curves (Figure 5a) show a linear relationship for the pristine-DWNTs (b), DWNT-Au (1), and DWNT-HNO3 ([) at 93% transmittance. The electrical resistances of various DWNT films are calculated from the reciprocal of slope of the current-voltage curve. The slopes of the current-voltage curve increased after the post-treatment of the gold nanoparticles and HNO3, respectively, on the pristine-DWNTs, indicating an increase in the electrical conductance, although the degree of increased conductance of DWNT-HNO3 was slightly lower than that of DWNT-Au. It has been reported that the increase in the electrical conductivity of HNO3-treated SWNT films is likely due to the effect of removal of sodium dodecyl sulfate (SDS) by acid treatment, while improvement of the electrical conductivities in our DWNT-Au is presumably due to a doping effect.15,27

Figure 4. (a) Changes of electrical resistance and light transmittance at 550 nm of DWNT films according to the time of immersion in 1 mM concentrations of gold salt solution. (b) Changes of electrical resistance and light transmittance at 550 nm of DWNT films according to the time of immersion in concentrated HNO3 solution.

We measured the work function using a UPS with a UV source (AC-2 photoelectron spectrometer, Riken Keiki) to analyze the electrical behaviors on different DWNT films. We found that the work functions of the pristine-DWNTs (Figure 5b) were considerably changed after HNO3 treatment (Figure 5c) and gold ion treatment (Figure 5d), where the work function was calculated from the crossing point of the background and the yield line of the photoemission energy.24 It was found that the work functions of DWNT-Au (5.79 eV) and DWNT-HNO3 (5.84 eV) were higher than that of the pristine-DWNTs (5.24 eV). The increase of the work function after post-treatment indicates significant changes in the surface potential of the

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Yang et al.

Figure 5. (a) Current-voltage curves of pristine-DWNTs, DWNT-Au, and DWNT-HNO3 at 93% transmittance. UPS spectra according to posttreatment of pristine-DWNTs: (b) pristine-DWNTs, (c) DWNT- HNO3, and (d) DWNT-Au.

pristine-DWNTs.25,26 The photon energy shift of DWNT-Au films is due to electron transfer from the pristine-DWNTs to gold ions, indicating a decrease in the density of the Fermi level states. In the case of DWNT-HNO3, however, it is assumed that oxygen, as an electron acceptor, having higher electron negativity is physisorbed on the pristine-DWNTs.27 Also, the decrease of the electrical resistance is attributed to electron depletion, i.e., an increase of the hole concentration, from specific bands in the mainly semiconducting outer wall of the DWNTs due to doping with electron acceptors such as Au3+ and oxygen.

HNO3 prepared by a Nafion/water-propanol dispersion agent would be applicable to transparent electrodes of various optoelectronic devices such as solar cells, light-emitting diodes, sensors, and field effect transistors owing to their enhanced electrical properties. Acknowledgment. This work was supported by the National Research Laboratory Program (R0A-2007-000-20037-0, KOSEF), the Center for Nanoscale Mechatronics & Manufacturing (08K140100414, CNMM), and the Blue Ocean Program of Small and Medium Business Administration (S1025548).

Conclusions We investigated the electrical resistance and transmittance of pristine-DWNTs, DWNT-HNO3, and DWNT-Au for potential application in transparent conducting films. We found that the electrical conductance of the pristine-DWNTs was higher than that of pristine-SWNTs. Post-treatment of the pristine-DWNTs with gold nanoparticles and HNO3, respectively, resulted in a significant decrease of the electrical resistance without substantial loss of transmittance. More importantly, DWNT-Au exhibited the best transmittance-conductivity performance among the nanotube network films. The films could be ranked in the following order: pristine-SWNTs > pristine-DWNTs > SWNTAu > DWNT-HNO3 > DWNT-Au. The work function results of the DWNT-Au and DWNT-HNO3 films show that the variation of the electrical resistance in the DWNT films is likely due to a doping effect. Therefore, our DWNT-Au and DWNT-

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