pubs.acs.org/Langmuir © 2009 American Chemical Society
Electrical Conductivity of Graphene Films with a Poly(allylamine hydrochloride) Supporting Layer Byung-Seon Kong,†,‡ Hae-Wook Yoo,† and Hee-Tae Jung*,† †
National Research Laboratory for Organic Opto-Electronic Materials, Department of Chemical and Biomolecular Engineering (BK-21), Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Korea, and ‡KCC Central Research Institute, 83 Mabook-dong, Giheung-gu, Yongin-si, Gyunggi-do 446-912, Korea Received April 14, 2009. Revised Manuscript Received July 14, 2009 The electrical conductivity of graphene oxide (GO) and reduced graphene oxide (RGO) films with poly(allylamine hydrochloride) (PAH) supporting layers is investigated. Graphene-PAH hybrid films were produced in a two-step procedure that consisted of vacuum filtration for GO (or RGO) dispersion to fabricate the graphene thin films on quartz substrates, followed by the deposition of PAH onto the graphene films via solution casting. Highly selective deposition of the PAH layer on the graphene sheets was confirmed through the detection of the fluorescence signals of hybridized Cy3-DNA onto the PAH-coated graphene surfaces. In this case, electrostatic interaction plays an important role in the selective deposition process. Interestingly, it was found that the electrical conductivity of RGO films was significantly enhanced by 120% after PAH treatment, whereas that of the GO films was reduced by 98% of its initial conductivity. This finding was interpreted in terms of the molecular structure and oxygen functionalities of GO and RGO films combined with the ionic conduction characteristics of hydrated PAH on the RGO film.
Introduction Graphene has recently attracted considerable attention because of unique electronic properties that include its room-temperature quantum Hall effect and the massless Dirac fermion behavior of its charge carriers.1-3 To date, graphene has been primarily prepared by reducing graphene oxide (GO) because this is a cost-effective, reliable manufacturing process.4-6 In this process, GO is produced by the exfoliation of graphite oxide via a modified Hummers method.7-9 It has been reported that the hybridization of graphene (or its derivatives) and functional nanomaterials enhances the combined attributes of each component. Several examples exist in the literature. First, transparent, conductive silica-graphene composite films were prepared by incorporating individual GO sheets into silica sols.10 Second, the electronic and *Corresponding author. Tel: þ82-42-350-3931. Fax: þ82-42-350-3910. E-mail:
[email protected]. (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (4) Stankovich, S; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558. (5) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (6) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Nat. Nanotechnol. 2009, 4, 25. (7) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (8) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (9) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394. (10) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S.-E.; Chen, S.-F.; Liu, C.-P.; Nguyen, S. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888. (11) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (12) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’homme, R. K.; Brinson, L. C. Nat. Nanotechnol. 2008, 3, 327.
11008 DOI: 10.1021/la901310g
barrier properties of polymers were enhanced when graphene was well dispersed in the polymers.11-14 Finally, graphene-SnO2 nanohybrid electrodes were reported that showed enhanced cyclic performance and lithium storage capacity in lithium ion batteries.15 Recently, gold nanoparticles were hybridized with graphene in the form of layer-by-layer (LbL) films, which may be useful for the realization of biosensing devices.16 Among graphene-functional hybrid systems, graphene-polyelectrolyte hybrid composites in particular have garnered keen interest owing to their potential for application to the highcapacity intercalating cathode of lithium ion batteries,17 conductive films,7,18,19 and biomolecular sensing devices.20 This is due to the superior electrical properties, 3-D features, and flexibility with regard to chemicals (and therefore the surface functionalities) of graphene. Moreover, polyelectrolytes have excellent binding capacities with graphene. Self-assembled poly(ethylene oxide), poly(diallyldimethylammonium chloride), and GO cathodes were used for a rechargeable lithium ion battery, providing a large surface area per volume for the penetration of lithium ions.17 LbL films composed of negatively charged GO sheets and cationic polymers such as poly(allylamine hydrochloride)7 and poly(diallyldimethylammonium chloride)18 or the anionic polymer of poly(acrylic acid)19 have been reported, showing an enhancement of the electrical conductivity of GO-polyelectrolyte LbL films after an electrochemical7 or chemical reduction process18 or after increasing the number of bilayers.19 Additionally, the electrical conductivity of GO-poly(allylamine hydrochloride) hybrid thin film transistors was characterized to examine (13) Eda, G.; Chhowalla, M. Nano Lett. 2009, 9, 814. (14) Kalaitzidou, K.; Fukushima, H.; Drzal, L. T. Carbon 2007, 45, 1446. (15) Paek, S.-M.; Yoo, E.; Honma, I. Nano Lett. 2009, 9, 72. (16) Kong, B.-S.; Geng, J.; Jung, H.-T. Chem. Commun. 2009, 2174. (17) Cassagneau, T.; Fendler, J. H. Adv. Mater. 1998, 10, 877. (18) Szabo, T.; Szeri, A.; Dekany, I. Carbon 2005, 43, 87. (19) Wu, J.; Tang, Q.; Sun, H.; Lin, J.; Ao, H.; Huang, M.; Huang, Y. Langmuir 2008, 24, 4800. (20) Mohanty, N.; Berry, V. Nano Lett. 2008, 8, 4469.
Published on Web 08/06/2009
Langmuir 2009, 25(18), 11008–11013
Kong et al.
Article
Scheme 1. Schematic of the Steps Involved in the Fabrication of Graphene Films and Post Treatment with Poly(allylamine hydrochloride)
a chemically modified graphene (CMG) transistor’s specificity to polarity, which is applicable to CMG-based label-free DNA sensors.20 As with other conductive additives, variations in the electrical properties of GO (or RGO) films hybridized with polyelectrolytes is essential to realizing their applications. In spite of the great activity currently taking place in the field of hybrid films of graphene and polyelectrolytes, the effect of an incorporated polyelectrolyte on the electrical behavior of hybrid films has not been reported. The results of this study demonstrate a number of interesting electrical properties of graphene-polyelectrolyte hybrid films. It was found that the electrical conductivity levels of GO and RGO are significantly affected by the poly(allylamine hydrochloride) (PAH) supporting layer. The conductivity of GO films decreases after PAH treatment on the surface, whereas that of RGO increases significantly. PAH was selectively deposited onto graphene sheets, which was confirmed by the fluorescence results of Cy3-DNA hybridization onto the PAH-supported graphene surfaces. These results are likely due to the oxygen functionalities and molecular structures of the GO and RGO sheets, which interact with the PAH.
Experimental Section General. All chemicals were used without purification or treatment, and deionized water (18 MΩ 3 cm) purified by an ultrapure water system (Milli-Q, Millipore) was used in all experiments. All procedures and characterizations were performed Langmuir 2009, 25(18), 11008–11013
under an environment with a temperature of 20 ( 5 °C and a relative humidity of 60 ( 10%. Preparation of GO and RGO Films. Scheme 1 illustrates the overall fabrication process of the hybrid films of GO (or RGO) with PAH supporting layers. Graphite oxide was prepared from natural graphite (FP 99.95% pure, Graphit Kropfm€ uhl AG) via a modification of the method described by Hummers.7-9 For the exfoliation of the graphite oxide, sonication by a bath sonicator (FS140H, 135 W, 42 kHz; Fisher Scientific) was carried out for 30 min to yield a 0.5 mg/mL GO aqueous dispersion. To prepare the GO films, the GO aqueous dispersion was diluted with deionized water to adjust the GO concentration. The procedure reported by Li et al.5 was used for the reduction of GO (i.e., RGO). The homogeneous GO dispersion (20 mL) was mixed with 20 mL of water, 20 μL of hydrazine solution (35 wt % in water, Aldrich), and 400 μL of ammonia solution (28 wt % in water, Junsei Chemical). The hydrazine-to-GO weight ratio was 7:10, which was suggested to be optimal by Li et al.5 The mixture was stirred at 95 °C for 1 h (RGO-1) and 3 h (RGO-3). The dispersion was centrifuged at 15 000 rpm for 30 min to remove flocculated graphene using a Sorvall Biofuge Stratos centrifuge with a no. 3335 rotor (step 1 in Scheme 1). The GO or RGO films were prepared on a quartz substrate using a previously described vacuum filtration method.16 The vacuum filtration of the GO or RGO dispersion was performed using a porous alumina membrane filter (200 nm pore size, 25 mm diameter; Whatman) (step 2). The GO or RGO films were placed directly into a bath of 3 M NaOH. The alumina membrane was then dissolved, and the thin GO or RGO films were floated on the surface of the NaOH solution. The NaOH solution was exchanged DOI: 10.1021/la901310g
11009
Article
Kong et al.
with deionized water by recirculation until the pH was ca. 7. A piece of the quartz substrate was immersed in the water and allowed to sink to the bottom of the bath. As the water was drained, the floating GO or RGO films slowly descended and attached onto the quartz substrate. The GO or RGO films on the quartz were dried in an oven at 60 °C for 12 h (step 3). Contact electrodes separated by 12.5 mm were patterned onto the GO or RGO films from Ti-Au films (10 and 90 nm thick, respectively) using the electron-beam evaporation technique (step 4). To attach poly(allylamine hydrochloride) (PAH; Mw=56 000, Aldrich) onto the GO or RGO films, a PAH aqueous solution (50 mg in 15 mL of water) was dropped onto GO or RGO film surfaces between Ti-Au contact electrodes to prevent electrode contamination, and this was stored for 60 min (step 5). The samples were rinsed with deionized water and then dried under nitrogen gas (step 6). DNA Hybridization on a GO or RGO Film. DNA arrays were fabricated on a glass substrate with a GO or RGO film. To compare the effects of the PAH treatment, a PAH aqueous solution (3 mg/mL) was dropped onto half of the GO or RGO film, and this was stored for 60 min. Fifty-mer single-stranded DNA (GenoTech Co.) solutions without Cy3 labeling were prepared in a solution of DMSO/H2O 1:1 (v/v) and were deposited onto entire GO or RGO films. The probe DNA-immobilized specimens were incubated overnight at room temperature. The substrates were washed twice with a solution of 10 mM NaOH and 50 mM Na2CO3 for 2 min each, which was followed by washing with water for 2 min to remove unbound molecules. For hybridization, blocking was achieved with 5 Denhardt’s solution (containing 0.1 mg/mL each of Ficoll, poly(vinylpyrrolidone) and bovine serum albumin, all from Sigma) for 30 min, which was followed by washing with water. Complementary DNA labeled with Cy3 (GenoTech Co.) was dissolved in a hybridization buffer containing 6 SSC and 0.5% (v/v) Triton X-100. This solution was deposited onto a probe DNA array, and all specimens were sealed. After hybridization for 14 h, the substrate was washed with 6 SSC and 0.1% (v/v) Triton X-100, then with 3 SSC and 0.1% (v/v) Triton X-100 for 5 min each, and finally with 0.1 SSC for 30 s. The substrates were then dried by centrifugation at 500g. Characterization. An ESCA 2000 X-ray photoelectron spectrophotometer (XPS; VG Scientific) with monochromated Mg KR radiation (1253.6 eV) was used for the elemental analyses of the GO or RGO films. Peak deconvolution was performed using PeakFit version 4.11 (SYSTAT software), and the area of each peak was calculated using OriginPro 8 (OriginLab Corporation). The optical absorbance of the GO or RGO films was measured using a V-570 UV-vis-NIR spectrophotometer (Jasco) with a quartz sheet used for the reference. The resulting data represent the absorbance of GO or RGO films excluding the absorbance of the quartz substrate. The current-voltage curve was measured using a probe station (M150; Cascade Microtech) and a parameter analyzer (4200-SCS/F; Keithley Instruments) at room temperature. 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 (12.5 mm), t is the thickness of the GO or RGO films measured by a surface profiler (Alpha-Step IQ; KLA Tencor), and w is the width of the GO or RGO films (15 mm). The topologies of the GO or RGO films before and after PAH treatment were measured using an atomic force microscope (AFM, SPA 400, Seiko Instruments) in noncontact mode. The rms roughness of the films was calculated with NanoNavi version 5.00A (SII NanoTechnology). The DNA array was scanned with a GenePix 4000A fluorescence scanner (Axon Instruments). The 16-bit TIFF images obtained were analyzed using GenePix Pro 3.0 (Axon Instruments).
Results and Discussion UV-vis-NIR absorption spectroscopy and XPS were used to investigate the structures of graphene before and after the 11010 DOI: 10.1021/la901310g
reduction process. Reduction of GO was done at two different reduction times (1 and 3 h). These are denoted as RGO-1 and RGO-3, respectively. The UV absorption maxima of the GO, RGO-1, and RGO-3 films were observed at 227, 255, and 264 nm, respectively (Figure 1a). This indicates that the electronic conjugation within the graphene sheets is restored during the reduction process.5 In contrast to the C 1s XPS spectrum of the GO film (Figure 1b), however, the RGO-1 and RGO-3 films (Figure 1c,d) showed weaker C-O (286.5 eV), CdO (287.4 eV), and O-CdO (289.0 eV) spectral intensities, indicating that considerable deoxygenation was caused by the reduction process.4,21 The relative ratios of the C-O absorption peaks of GO, RGO-1, and RGO-3, as calculated from the areas of the deconvoluted peaks (Table 1), were 26.6, 9.3, and 7.5%, respectively. However, the CdO and O-CdO peaks remained in the RGO-1 (23.2%) and RGO-3 (14.4%) samples. This suggests that a certain number of carboxyl groups existed at this point on the RGO sheets.4,22 The XPS peak at 285.8 eV for RGO-1 and RGO-3 corresponds to the carbon in CdN bonds of the hydrazone (as shown in Figure 1c,d as well as in Table 1).21,23 To investigate the electrical conductivity of GO, RGO, and their hybrid films with PAH, the current-voltage curve of the films was measured using a probe station (M150; Cascade Microtech) and a parameter analyzer (4200-SCS/F; Keithley Instruments). To ignore the effect of film density on the electrical conductivity, three films were fabricated at the same transmittance (∼70% at 550 nm) by controlling the concentration of graphene in an aqueous solution. Before the PAH treatment, the current-voltage curves showed a linear relationship in all three films (Figure 2). The GO film exhibited the lowest electrical conductivity of the films (Figures 2a and Table 2) as a result of the defect sites composed of sp3-hybridized carbons.4 As expected, the conductivities of the RGO-1 (3.82 10-3 S/m) and RGO-3 (9.38 10-1 S/m) films were much higher than that of the GO films (2.58 10-4 S/m) (Figure 2 and Table 2), indicating that the sp2-hybridized carbon atoms were restored upon reduction,5,11,24,25 as shown in Figure 1a. After PAH treatment, however, the electrical conductivity of the GO film (GO-PAH) decreased considerably by 96% in comparison with the initial conductivity, which is in the range of insulating materials (Figure 2a). More surprisingly, the RGO-1 (8.19 10-3 S/m) and RGO-3 films (1.1410° S/m) after PAH treatment exhibited significant increases in their conductivities, by 114 and 22%, as compared to the RGO-1 (3.82 10-3 S/m) and RGO-3 films (9.38 10-1 S/m) without PAH supporting layers. In a further investigation of the effect of the initial conductivity of the GO and RGO films on the conductivity of the films after PAH treatment, GO and RGO films were prepared with five different electrical conductivity values by varying the amounts of GO, RGO-1, and RGO-3 aqueous dispersions. The initial conductivities of the GO films are in the range of 9.99 10-4 to 8.14 10-3 S/m whereas those of the RGO films are in the range of 3.82 10-3 to 9.38 10-1 S/m. Figure 3 shows the variation of the conductivities in the GO (Figure 3a) and RGO (Figure 3b) films after PAH treatment as a function of the initial conductivities (σ0) of the films before PAH treatment. The conductivities of all five GO films were considerably reduced, by 68 to 98%, after (21) Stankovich, S.; Piner, R. D.; Chen, X.; Wu, N.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155. (22) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Nat. Nanotechnol. 2008, 3, 538. (23) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856. (24) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463. (25) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008, 2, 1487.
Langmuir 2009, 25(18), 11008–11013
Kong et al.
Article
Figure 1. (a) UV-vis-NIR spectra of GO, RGO-1, and RGO-3 films. C 1s XPS spectra of (b) GO, (c) RGO-1, and (d) RGO-3 films. Table 1. Portion of Carbon Bonds Calculated from Deconvoluting the C 1s XPS Spectra of GO, RGO-1, and RGO-3 Films carbon bonds
GO (%)
RGO-1 (%)
RGO-3 (%)
C-C CdN C-O CdO O-CdO
52.4 0 26.6 14.1 6.9
61.3 6.1 9.3 13.2 10.0
71.6 6.5 7.5 11.1 3.3
PAH treatment (Figure 3a). Surprisingly, however, the conductivities of the five RGO films increased significantly, by 22 to 120%, after PAH treatment, depending on the initial conductivity of the RGO film (Figure 3b). RGO films with relatively low conductivities resulted in a greater enhancement in the conductivity after PAH treatment: 114 and 81% increases in the initial conductivity values of 3.82 10-3 and 4.75 10-3 S/m, respectively. However, less of an enhancement (
