J. Phys. Chem. C 2007, 111, 8377-8382
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Single-Walled Carbon Nanotube Gold Nanohybrids: Application in Highly Effective Transparent and Conductive Films Byung-Seon Kong,† Dae-Hwan Jung,‡ Sang-Keun Oh,§ Chang-Soo Han,‡ and Hee-Tae Jung*,† Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, Korea Institute of Machinery and Materials, Daejeon 305-343, Korea, and Top Nanosys Incorporated, Suwon 443-749, Korea ReceiVed: February 15, 2007
We report a new technique to enhance the electrical conductivity of transparent single-walled carbon nanotube (SWNT) films with a negligible loss of their optical transmittance. Hybridization of the SWNT films with gold nanoparticles increased the electrical conductivity 2-fold to a maximum of 2.0 × 105 S/m while maintaining the transmittance of the initial value. The same trends are shown for other SWNT films with various initial conductivities and transparency levels. Functional changes in the gold-nanoparticle-coated SWNT films suggest that the electrical conductivity change is due to an electron depletion mechanism as a result of a doping effect.
Introduction Surface modification plays an important role in enhancing the properties of single-walled carbon nanotubes (SWNTs) for use in optoelectronics, energy storage, catalysis, and biotechnology. Various guest hybrid materials, including metallic crystals, ions, biomolecules, small amphiphiles, and polymers, have been incorporated into SWNTs by chemical or physical methods. There has been a great deal of recent interest in the use of gold nanoparticles as guest hybrid materials because of their potential applications in sensors and as catalysts. Gold guests have been employed in probes for a DNA microarray,1 and gold nanoparticle-oligonucleotide complexes have been used as intracellular gene-regulating agents.2 Also, gold-coated SWNTs have been investigated for the detection of CO, H2, H2S, NH3, and NO2 gases,3 and gold nanoparticles can be employed as high-activity catalysts for CO oxidation4 and as catalysts for the polymerization of alkylsilanes on siloxane nanowires, filaments, and tubes.5 Here, we show for the first time that electroless reduction of gold ions on SWNT networks can enhance their electrical conductivity without a substantial change in their transmittance of visible light. This is a promising strategy for the creation of transparent electrodes as touch screens, large-area flexible displays, flexible solar panels,6,7 and thin film transistors.8,9 Previous transparent conducting SWNT films have been limited by a tradeoff between the electrical conductivity and the optical transparency, which are controlled by the SWNT density in the film.10,11 In this study, we show that the reduction conditions have a large effect on the conductivity of the films but little effect on the transparency of the films. In addition, we investigated the nature of these changes. In contrast to previous methods,12 the fabrication of the transparent conducting SWNT films by gold-hybridization is easy, cost-effective, and easily * To whom correspondence should be addressed. Phone: +82-42-8693931. Fax: +82-42-869-3910. E-mail:
[email protected]. † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Machinery and Materials. § Top Nanosys Incorporated.
scaled up because it uses environmentally friendly reactants under ambient conditions. Experimental Section Materials. Single-walled carbon nanotubes (SWNTs; 90 vol % pure) made by arc discharge were purchased from ILJIN NanoTech Co., Ltd. Sodium dodecyl sulfate (SDS; g 99.0% pure) and NaOH were purchased from Fluka. For transparent plastic substrates, untreated poly(ethylene terephthalate) (PET) sheets (thickness, 100 µm) were purchased from SKC. HAuCl4‚ 3H2O (g99.9% pure) and ethanol (99.9% pure) were purchased from Aldrich and Merck, respectively. All chemicals were used without purification or treatment. Deionized water (18 MΩ cm) was purified using an ultrapure water system (Milli-Q, Millipore). Preparation of SWNT Films. Scheme 1 shows the overall process of fabricating transparent conducting SWNT films on a flexible substrate using the vacuum filtration method. The SWNT films were prepared on a poly(ethylene terephthalate) (PET) substrate as described previously (steps 1-3).10,13 SWNT powders (1.5 mg) were dispersed in aqueous 1 wt % SDS. The solution was sonicated for 2 h at a power of 135 W and centrifuged for 1 h at 20 000 g (Step 1). Only well-dispersed and separated SWNT solutions lacking flocculated SWNT particles were used. Vacuum filtration of SWNT solutions was performed using a porous alumina membrane filter (200 nm pore size, 47 mm diameter; Whatman), and the SDS was removed by rinsing the filtered film with deionized water until bubbles were no longer seen under the filter (Step 2). The rinsed SWNT film was placed directly into a bath of 3 M NaOH. The alumina membrane was then dissolved, and the thin SWNT film was floated on the surface of the NaOH solution. The NaOH solution was exchanged with deionized water by recirculation until the pH was near 7.0. A piece of the PET sheet was immersed into the water and allowed to sink to the bottom of bath. As the water was drained, the floating SWNT film slowly descended and attached onto the PET sheet. The SWNT filmPET composite was dried in an oven below the glass transition temperature of PET (Step 3). Contact electrodes separated by
10.1021/jp071297r CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
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Figure 1. Photographic (a and b) and SEM images (c and d) of a SWNT film. Images were taken (a and c) before (0 min) and (b and d) after formation of gold nanoparticles by immersing the SWNT material in a 1 mM gold salt solution for 10 min. The scale bar represents 500 nm. The inset in d is a TEM image of the same film, and the scale bar represents 200 nm.
SCHEME 1: Schematic of the Fabrication of SWNT Films by the Vacuum Filtration Method
15 mm were patterned on the SWNT films from Ti-Au films (10 and 90 nm thick, respectively) using the electron-beam evaporation technique (Step 4). To form gold nanoparticles onto the SWNT films, gold salt was dissolved at 1, 5, and 10 mM in HAuCl4‚3H2O in 50 vol % ethanol. The SWNT film specimen was immersed in the gold salt solution for various amounts of time (Step 5). The sample was rinsed extensively with deionized
water and dried under nitrogen gas (Step 6). All procedures were performed under ambient temperature in air. Characterization. Images of SWNT films before and after formation of gold nanoparticles were taken at various magnifications using a XL30SFEG field emission scanning electron microscope (Philips) equipped with a Schottky based field emission gun. The gold nanoparticles on the SWNT films were
SWNT-Gold Nanohybrids as Transparent and Conductive Films
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TABLE 1: Electrical Conductivity for Various Transparent Gold-Nanoparticle-Coated SWNT Films conductivity (S/m)
transmittance (%, 550 nm)
specimen no.
beforea (σ0)
afterb (σ)
∆σ/σ0 (%)
beforea
afterb
1 2 3 4c 5
9.83 × 103 4.40 × 104 9.13 × 104 2.93 × 103 2.28 × 10
2.47 × 104 1.01 × 105 2.01 × 105 1.09 × 104 8.47 × 102
151 130 120 272 3615
85 82 81 84 92
84 80 79 82 91
film preparation method vacuum filtration
chemical assemblingd
a Initial conductivities before the formation of gold nanoparticles on the SWNT films. b Conductivities after the formation of gold nanoparticles by immersing the SWNT film in a 1 mM gold salt solution for 10 min. c This film was made by HiPco SWNTs, same batch product as SWNTs used for the specimen no. 5. d This SWNT film was fabricated by covalent bonding of functionalized SWNTs and a substrate.15-18
analyzed using a Tecnai G2 F30 super-twin field emission transmission electron microscope (FEI). The structure of the gold particles was analyzed by TEM performed in bright field mode at 300 kV. Scanning transmission electron microscopy (STEM) and EDS modes were used for elemental analysis. TEM samples were prepared by detaching SWNT films from PET substrates.14 The SWNT film was covered with a 25% aqueous solution of poly(acrylic acid) (PAA, Aldrich). After drying in air, the PAA/SWNT film composite was easily peeled off of the PET substrate and then floated on deionized water PAA side down for several hours. When the PAA layer was completely dissolved in the water, the floating SWNT film was recovered on a copper TEM grid. Current-voltage measurements were carried out in the 3-electrode mode using a CHI 600C potentiostat (CH Instruments Inc.) in air at ambient temperature. A schematic of the 3-electrode measuring setup is shown in Figure S3. For the linear sweep voltammetry mode, the working electrode of the potentiostat was connected to one contact electrode of a specimen, and both reference and counterelectrodes were connected to the other contact electrode. The potential was swept at a scan rate of 0.1 V s-1 from -3 to +3 V. The electrical conductivity (σ) was calculated using the equation σ ) G(l/tw), where G is the electrical conductance obtained from the slope of the linear current-voltage curve, l is the distance between two electrodes (15 mm), t is the thickness of SWNT film measured by atomic force microscopy (AFM; SPA 400; Seiko Instruments Inc.), and w is the width of the SWNT film (15 mm). The light transmittance of SWNT films was measured by a V-570 UV/vis/NIR spectrophotometer (JASCO) that a bare PET sheet was used for the reference. Result data represent the transmittance of a SWNT film excluding the transmittance of a PET sheet. An AXIS-NOVA ultraviolet photoelectron spectrophotometer (Kratos) was used for the work function measurement of SWNT films. The ultraviolet source was He I (21.2 eV) and the measurement was performed at 8.9 × 10-8 Torr under -20 V sample bias. The surfaces of the specimens were not cleaned before the measurement, and the work functions were determined from the secondary electron cutoff of ultraviolet photoelectron He I spectra using silver metal (4.2 eV) as a reference. Results and Discussion To prepare high-density SWNT films with a wide range of conductivities, we used two different methods: chemical assembling15-18 and vacuum filtration.10,13 We prepared a highdensity, low-conductive SWNT film by covalent bonding of carboxyl group-functionalized SWNTs to an aminopropylsilanemodified glass substrate using consecutive amidation reactions (see Supporting Information).15-18 A high-density multilayer was then fabricated by means of repeated reactions between the carboxylic acid groups of the shortened SWNTs and the amino
Figure 2. (a) Rate of electrical conductivity change by SWNT films according to the time of immersion in three different concentrations of gold salt solution. σ0 is the initial electrical conductivity, and ∆σ is the change from the initial electrical conductivity after formation of gold nanoparticles for various amounts of time, respectively. (b) Light transmittance at 550 nm by the gold-nanoparticle-coated SWNT films according to the time of immersion in a 1 mM gold salt solution.
groups of the linker (4,4′-oxydianiline). The resulting SWNT layers were found to have a high density and excellent surface adhesion due to their direct chemical bonding to the substrate surface. The vacuum filtration method was used for preparing highly conductive, high-density SWNT films.10,13 During the immersion process, gold nanoparticles were formed by the electroless reduction of gold ions (Au3+) on the SWNT networks due to a redox reaction between Au3+ and SWNT.19 Figure 1 shows optical and scanning electron microscope (SEM) images of the SWNT films made by the vacuum filtration
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Figure 3. TEM images of SWNT films after immersion for (a) 0, (b) 10, (c) 60, and (d) 180 min in a 1 mM gold salt solution containing 50 vol % ethanol as a solvent. The insets show magnified images of selected regions. Note that the inset in b shows regular d spacing (2.36 ( 0.02 Å) of the observed planes of the gold lattice.
method before (parts a and c of Figure 1) and after (parts b and d of Figure 1) the electroless reduction of gold ions on the SWNT films. The SWNT films thoroughly and uniformly covered the substrate surface (parts a and c of Figure 1), and similar images were observed in the chemically anchored SWNT films after six successive reaction cycles (see Supporting Information); however, current-voltage measurements (3electrode mode using a potentiostat) showed that, at similar transparencies, the conductivities of the vacuum-filtered films are much higher than that of the SWNT film generated by chemical assembling (Table 1). This might be due to the very few defects in the sidewalls of the vacuum-filtered SWNT. The large difference in the D-band of the Raman spectrum of the chemically anchored SWNT film compared to the vacuumfiltered SWNT film indicates that a large structural modification of the SWNT sidewalls is induced by the introduction of defects (see Supporting Information). This is confirmed by the difference in the electrical conductivity between the vacuum-filtered film and the chemically anchored film, which are made by the same batch product of HiPco SWNTs (Table 1). Visual inspection shows that the transparency of the uncoated SWNT films (Figure 1a) is essentially the same as the SWNT films generated by 10 min of electroless reduction in 1 mM gold salt solution (Figure 1b). SEM (Figure 1d) and corresponding TEM images (Figure 1d, inset) clearly show that the gold nanoparticles are formed by the electroless reduction of gold ions (Au3+) on SWNT films.
We found that the conductivity of the SWNT films is strongly influenced by the duration of the electroless reduction of gold ions (Figure 2a). We measured the initial electrical conductivity (σ0) and the difference between the electrical conductivity before and after the reduction of gold ions (∆σ) (see Supporting Information). Regardless of the concentration of the gold salt solution, the electrical conductivities of the films increased dramatically (>100%) at 10 min and reached a maximum at 60 min. Note that the conductivities slowly decreased after 60 min of reduction (Figure 2a), although the precise reason for this is not clear. We set the concentration at 1 mM for all subsequent experiments because it gave a slightly higher conductivity than the other concentrations tested. We found that the transmittance of the SWNT films in the visible light region slightly decreased as the reduction time increased (Figure 2b). The transmittance at 550 nm decreased by ∼80% after 10 min of reduction. Further reduction resulted in little difference in the transparency of the films. As a result, the SWNT films generated by the vacuum filtration method showed a large increase in conductivity (120-150%) and a negligible change in optical transmittance at wavelengths in the visible range after 10 min of reduction (Table 1). This method appears to be widely applicable because we were able to use it to generate SWNT films with wide range of conductivities, and there was no an obvious restriction to the types of SWNT films to which it can be applied. Overall, the chemical assembling method generated SWNT films with a much greater enhance-
SWNT-Gold Nanohybrids as Transparent and Conductive Films
Figure 4. (a) Current-voltage curves and (b) UPS spectra according to the time of immersion in a 1 mM gold salt solution containing 50 vol % ethanol as a solvent. The inset in b shows the UPS spectra around the secondary electron threshold region for each SWNT film.
ment of conductivity and with little change in the transmittance of visible light compared to films generated by the vacuum filtration method. We used TEM to analyze the specific optoelectronic changes that occurred at each stage for SWNT films reduced for various times in 1 mM gold salt in 50 vol % ethanol. A number of gold nanoparticles were observed on the SWNT networks after 10 min of immersion (Figure 3b). These particles were spherical and highly monodisperse, with a mean diameter of 25 ( 5 nm as determined from a total of approximately 200 gold particles in multiple TEM images. The average particle size increased with the time of reduction: the diameter was 25 nm after 10 min, ∼1 µm after 60 min, and ∼2 µm after 180 min of immersion (parts c and d of Figure 3). Importantly, as shown in Figure 3c, the gold nanoparticles coexisted with triangular and hexagonal plates after 60 min, although the growth mechanism is not fully understood. The formation of gold nanocrystals at reduction times longer than 10 min is readily observed at higher magnification (Figure 3b, inset). The measured regular d spacing of the observed planes of the lattice is 2.36 ( 0.02 Å, which corresponds to the spacing between the {111} planes of crystalline gold (2.355 Å).20 Further
J. Phys. Chem. C, Vol. 111, No. 23, 2007 8381 analysis of the gold nanoparticles on SWNT films was made by energy-dispersive X-ray spectroscopy (see Supporting Information). The size and shape of the gold nanoparticles are responsible for their optoelectronic properties. The sharp increase in the conductivity after 10 min of immersion is likely due to the formation of gold hybrids that are reduced from the gold ions (Au3+) on the SWNT networks; however, further growth in Au0 particles has no significant effect on the electrical conductivity. The optical transmittance of the SWNT film after immersion over 10 min may be slightly lower than that of a bare SWNT film due to the agglomerated gold particles on the SWNT film being larger than the wavelength of visible light. If the size of the gold particle becomes larger than the wavelength of visible light, the transmittance will be reduced due to light scattering.21 Therefore, a 10-min immersion is optimal because it results in a 2-fold enhancement of electrical conductivity and 80% transmittance of visible light. To further understand the role of the gold reduction on the SWNT films, we measured the current-voltage curve and the work function of the SWNT films at each stage using linear sweep voltammetry and ultraviolet photoelectron spectroscopy, respectively. The current-voltage curves (Figure 4a) were linear for both the uncoated SWNT films and gold hybrid SWNT films, indicating metallic characteristics due to metallic intertube junctions in the SWNT networks.11 Modification of the SWNT film surface with gold nanohybrids, however, caused an increase in the slope of current-voltage curve. This indicates an increase in the electrical conductivity after gold nanoparticles are formed on the SWNT network. These phenomena were analyzed according to the work function change of the gold hybrid SWNT films. The ultraviolet photoelectron spectra for gold hybrid SWNT films showed shifts toward a higher binding energy (rightward shift of the curve) at -11 eV for the uncoated SWNT film (Figure 4b). The binding energy shifts were 0.58 and 0.75 eV for immersion times of 10 and 60 min, respectively. The ultraviolet photoelectron spectra of the second electron threshold region show the same trend (Figure 4b, inset). The work functions of each SWNT film, which were determined from these spectra, increased with the immersion time, from 3.8 eV for the uncoated SWNT film to 4.2 eV after 10 min of immersion and to 4.4 eV after 60 min of immersion. The increase of the work function with immersion time indicates a change in the surface potential due to the formation of gold nanoparticles on the SWNT networks.22 During the process of gold nanoparticle formation, gold ions (Au3+) and SWNTs behave as electron acceptors and donors, respectively. The binding energy shifts indicate a downward shift of the Fermi level due to electron transfer from SWNTs to gold ions, indicating a decrease in the density of the Fermi level states.23 As a result, the increase of the electrical conductivity is attributed to electron depletion, i.e., the increase of the hole concentration, from specific bands in the semiconducting SWNTs due to doping with electron acceptors (Au3+);24 however, we found a lower work function for the uncoated SWNT film (3.8 eV) than previously reported (4.8 eV),23 a finding that needs to be investigated further. It is possible that the work function was lower because of the surface condition of the SWNT films; for example, O2 adsorption and impurities such as amorphous carbons and metallic catalysts are likely to lower the work function. Conclusions We have demonstrated a simple and versatile process for enhancing the electrical conductivity of transparent SWNT
8382 J. Phys. Chem. C, Vol. 111, No. 23, 2007 films without a substantial loss in the optical transmittance. Specifically, electroless reduction of gold ions onto the SWNT networks resulted in a 2-fold increase in the electrical conductivity (maximum of 2.0 × 105 S/m) and ∼80% transmittance. The trend was shown for other SWNT films with various initial conductivities and transparencies. The electrical conductivity change of the SWNT films appeared to be due to an electron depletion mechanism as a result of a doping effect. This was further supported by analysis of the work function for the gold nanoparticle-coated SWNT films. Moreover, this process results in an electrical conductivity over 1.0 × 105 S/m, which is recommended for transparent conducting films.25 Therefore, this method should allow production of transparent electrodes with excellent potential for use in a variety of optoelectronic devices such as solar cells, light-emitting diodes, sensors, and field effect transistors. Acknowledgment. This research was supported by a grant (M102KN010004-06K1401-00413) from Center for Nanoscale Mechatronics and Manufacturing, one of the 21st Century Frontier Research Programs, and the CUPS-ERC. Ultraviolet photoelectron spectroscopy experiments were performed at KBSI. Supporting Information Available: The detailed fabrication procedures using chemical assembling method and Raman spectra of SWNT films; SEM images of a SWNT film fabricated by the chemical assembling method; schematic of the electrical conductivity measuring setup; atomic force microscopy analysis of SWNT film thickness; electrical conductivity changing curve; scanning TEM images and an EDS spectrum of a gold nanoparticle-coated SWNT film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Park, S.-J.; Taton, T. A.; Mirkin, C. A. Science 2002, 295, 1503. (2) Rosi, N. L.; Giljohann, D. A.; Thaxton, C. S.; Lytton-Jean, A. K. R.; Han, M. S.; Mirkin, C. A. Science 2006, 312, 1027. (3) Star, A.; Joshi, V.; Skarupo, S.; Thomas, D.; Gabriel, J.-C. P. J. Phys. Chem. B 2006, 110, 21014.
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