Organo- and Water-Dispersible Graphene Oxide−Polymer

Jun 28, 2010 - The grafted hydrophobic polymer brushes substantially enhance the solubility of GO in organic solvents, and the GO-g-PtBA nanosheets ca...
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Organo- and Water-Dispersible Graphene Oxide-Polymer Nanosheets for Organic Electronic Memory and Gold Nanocomposites Guo Liang Li, Gang Liu, Min Li, Dong Wan, K. G. Neoh, and E. T. Kang* Department of Chemical & Biomolecular Engineering, National UniVersity of Singapore, Kent Ridge, Singapore 119260 ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: June 11, 2010

A solution-processable graphene oxide (GO)-polymer complex, consisting of GO nanosheets with covalently grafted poly(tert-butyl acrylate) (PtBA) brushes, was prepared via surface-initiated atom transfer radical polymerization (ATRP). The grafted hydrophobic polymer brushes substantially enhance the solubility of GO in organic solvents, and the GO-g-PtBA nanosheets can form a uniform and stable dispersion in toluene. The functionalized GO nanosheets can also be integrated into an electroactive polymer matrix. Bistable electrical conductivity switching behavior and a nonvolatile electronic memory effect were demonstrated in a composite thin film of poly(3-hexylthiophene) (P3HT) containing 5 wt % GO-g-PtBA in an Al/GO-g-PtBA+P3HT/ ITO sandwich structure (ITO ) indium-tin oxide). Hydrolysis of GO-g-PtBA produces water-dispersible GO-g-PAAc (PAAc ) poly(acrylic acid)) nanosheets and allows the preparation of gold nanoparticle-decorated GO-g-PAAc nanofilms from aqueous dispersions. Introduction An important strategy for fabricating flexible organic electronics is to utilize the solution-processing technique. To achieve this goal, the electroactive materials must be organosoluble1 and allow integration into an active/passive matrix. Graphene, a single graphite layer of sp2-hybridized carbon atoms in a twodimensional (2D) crystal, is of great interest for electronic applications, such as transistors, sensors, and electrodes, because of its distinct electronic and mechanical properties.2-6 However, neither graphene nor graphene oxide (GO), which is exfoliated from chemical oxidation of graphite,7,8 is soluble in organic solvents. The poor dispersion of graphene and GO can lead to their aggregation in an organic medium, difficulty in material processing, and high cost of device fabrication. Similar to carbon nanotubes, chemical functionalization is expected to play a key role in tailoring the solubility and electronic properties of graphene nanosheets. Numerous efforts have been made to solubilize or disperse graphene-based materials in common organic solvents. For instance, graphite was functionalized with a long-chain alkylamine. The resulting octadecylamide graphite (G-CONH(CH2)17CH3) forms stable solutions in THF, CCl4, and DMF.9 Surfactant-supported graphene nanosheets have also been prepared for transferring graphene sheets from water to chloroform solutions by ionic interactions.10 Stable aqueous dispersions of polymer-modified graphene sheets have been prepared by in situ reduction of GO in the presence of cationic poly(ethyleneimine).11 The dispersity of graphene can also be increased through noncovalent functionalization of graphene with a water-soluble aromatic electroactive dye, methylene green.12 Surface-initiated atom transfer radical polymerization (ATRP) is a convenient “grafting from” technique, which allows the propagation of polymer chains of controlled molecular weight and dispersity from various substrate surfaces.13,14 The polymer brushes grafted on a substrate via surface-initiated ATRP * To whom correspondence should be addressed. E-mail: cheket@ nus.edu.sg. Tel: (65) 65162189. Fax: (65) 67791936.

technique, in turn, can offer a variety of active sites for further functionalization of the surface. In this work, we describe the functionalization of GO nanosheets with grafted polymer brushes via surface-initiated ATRP. The GO nanosheets with grafted polymer brushes can be dispersed in organic solvents, thus allowing the convenient fabrication of electronic devices. Rewritable electronic memory, based on bistable electrical conductivity switching behavior of the composite thin film of poly(3-hexylthiophene) (P3HT) with poly(tert-butyl acrylate)grafted GO nanosheets (GO-g-PtBA), is demonstrated in a Al/ GO-g-PtBA+P3HT/ITO sandwich device (ITO ) indium-tin oxide). Furthermore, hydrolysis of GO-g-PtBA produces the water-dispersible GO-g-PAAc (PAAc ) poly(acrylic acid)) nanosheets. Experimental Section Surface-Initiated Atom Transfer Radical Polymerization (ATRP) of tert-Butyl Acrylate from Graphene Oxide Nanosheets. Graphene oxide (GO) was synthesized from graphite powder by the modified Hummers-Offeman method.8,15 About 1.2 g of graphite was suspended in 50 mL of concentrated H2SO4 under vigorous stirring. About 6 g of KMnO4 was then added gradually with stirring and cooling so that the temperature was maintained below 10 °C. The stirring was continued for 2 h at 35 °C, followed by the addition of 300 mL of deionized water and stirring for another 15 min. Finally, the content of the flask was poured into 500 mL of deionized water and a sufficient amount of H2O2 (25 mL of a 30% aqueous solution) was added to destroy the excess permanganate. Graphene oxide was isolated by centrifugation, filtered through a sintered glass filter, and washed with dilute HCl until no sulfates were detected. For the preparation of ATRP initiator-coupled GO nanosheets, about 50 mg of GO and 0.5 mL of trichloro(4-chloromethylphenyl)silane (97%) were introduced into 5 mL of absolutely dried THF in a two-necked flask. Triethylamine (0.1 mL, 0.72 mmol) was then added dropwise, with vigorous stirring under

10.1021/jp102640s  2010 American Chemical Society Published on Web 06/28/2010

Graphene Oxide-Polymer Nanosheets an argon environment. The reaction mixture was left to stand for 12 h and then exposed to air for another 36 h. After five cycles of THF rinsing and separation by centrifugation, the benzyl chloride initiator-immobilized GO nanosheets (GO-Cl) were dried in a vacuum oven at 50 °C until a constant weight was obtained. Surface-initiated ATRP of t-BA from GO-Cl was carried out as follows. About 14 mg of GO-Cl, t-BA (3 mL, 0.02 mol), CuCl (24 mg, 0.24 mmol), and 8 mL of dry THF were introduced into a dry Pyrex test tube. After the test tube was purged with argon for 30 min, about 60 µL (0.24 mmol) of N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA, 99%) was added and the tube was sealed under an atmosphere of argon. Surface-initiated ATRP was carried out under stirring at 80 °C for 20 h to produce GO nanosheets with covalently grafted PtBA hydrophobic polymer brushes (GO-g-PtBA). The initial product of GO-g-PtBA was washed with an aqueous solution of ethylenediaminetetraacetic acid (EDTA), to remove the copper catalyst, followed by repeated washing with ethanol and acetone, prior to being dried in the vacuum oven. The GO-Cl provided a common platform for graft polymerization, via surface-initiated ATRP, of various other functional monomers on the GO nanosheets. For example, poly(methyl methacrylate) (PMMA) brushes were also grafted from the GO-Cl nanosheets via the surface-initiated ATRP process. Preparation of the Al/GO-g-PtBA+P3HT/ITO Sandwich Device. The Al/GO-g-PtBA+P3HT/ITO sandwich device was fabricated as described below. The indium tin oxide (ITO)-glass substrate was precleaned by ultrasonication in deionized water, acetone, and 2-propanol, in that order, each for 15 min. A 50 µL toluene solution of P3HT (10 mg/mL), containing 5 wt % GO-g-PtBA, was spin-coated onto the precleaned ITO substrate at a spinning speed of 2000 rpm and a duration of 60 s, using a G3P-8 spin-coater (Specialty Coating Systems, Inc.). The film was dried under reduced pressure (10-5 Torr) at room temperature overnight, to give a typical thickness of around 240 nm, as revealed by the FE-SEM cross-sectional view image (Figure 2d). Finally, Al was thermally evaporated onto the film surface at 10-7 Torr through a shadow mask, to form 0.4 × 0.4 mm2, 0.2 × 0.2 mm2, and 0.15 × 0.15 mm2 top electrodes with a thickness of about 300 nm, in a Leybold UHV Univex 350 Thermal Evaporation System. Preparation of the GO-g-PAAc/Au NP Composite Nanosheets. Narrowly dispersed gold nanoprticles were prepared according to the standard sodium citrate reduction method. Briefly, an aqueous solution of hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O) was prepared and heated to reflux under magnetic stirring, followed by addition of a given volume freshly prepared trisodium citrate aqueous solution. The reduction of HAuCl4 was initiated by trisodium citrate. The reaction was allowed to proceed under reflux for approximately 30 min. Gold nanoparticles (Au NPs) of 18, 23, and 45 nm in average diameters were obtained at sodium citrate concentrations of 3.5, 2, and 1.5 mM, respectively. To prepare the Au NP-decorated GO-g-PAAc nanosheets, the aqueous dispersion of Au NPs was mixed with the aqueous dispersion of GO-g-PAAc under sonication for 15 min. After five cycles of washing with a water/ ethanol (1:1, v/v) mixture and centrifugation to remove the free Au NPs, the GO-g-PAAc/Au NP composite nanosheets were redispersed in deionized water. Characterization. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos AXIS HSi spectrometer equipped with a monochromatized Al KR X-ray source (1468.6 eV photons). Field-emission scanning electron micros-

J. Phys. Chem. C, Vol. 114, No. 29, 2010 12743 copy (FE-SEM) images were obtained on a JEOL JSM-6700 SEM. Fourier transform infrared (FT-IR) spectroscopy analysis was carried out on a Bio-Rad FTS-135 spectrophotometer. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F field emission TEM (FE-TEM). The electronic properties of the as-fabricated devices were characterized under ambient conditions on a Cascade Probe Station, using a Keithley 4200 Semiconductor Parameter Analyzer. Molecular Simulation. Molecular simulations of the electronic properties, including the optimized geometry, dipole moment, total charge density, electrostatic potential surface, and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the basic unit (BU) of GOg-PtBA, were carried out using the Gaussian 03 (Revision E.01) program package on a Compaq GS320 alpha server with 8 CPUs and up to 4 GB memory.16 In order to reduce the time-cost of calculation, a simplified basic unit of GO-g-PtBA, which includes 16 benzene rings representing the GO nanosheet and 1 tBA repeating unit representing the grafted polymer chains, was used instead of the comprehensive and complicated GOg-PtBA molecular structure. It is reasonably to believe that by shrinking the GO nanosheet size and shortening the polymer chain length simultaneously in the simplified model, the calculated electronic properties would be similar to those of the actual system and are representative enough for studying the switching behavior of the GO-g-PtBA complex. The calculation was based on density function theory (DFT), using the Becke’s three-parameter functional with the Lee, Yang, and Parr correlation functional method (B3LYP) and the basis set 6-31G with d function added to heavy atoms (in short, DFT B3LYP/6-31G(d)).17 The density function theory at B3LYP/631G(d) level is accurate enough for calculating the required electronic properties. Results and Discussion The poly(tert-butyl acrylate)-grafted graphene oxide (GO) nanosheets, or GO-g-PtBA nanosheets, were prepared via (i) immobilization of the benzyl chloride initiator on GO nanosheets through the action of a silane coupling agent, and (ii) surfaceinitiated atom transfer radical polymerization (ATPR) of tertbutyl acrylate (t-BA), as illustrated in Scheme 1. Considering the chemical structure of GO,18,19 which contains hydroxyl groups on the surface and edge of atomic carbon sheets, the ATRP initiator, trichloro(4-chloromethylphenyl)silane, can be readily coupled to the GO nanosheets. In comparison with that of the pristine GO (Figure 1a), the X-ray photoelectron spectroscopy (XPS) wide scan spectrum of the initiatorimmobilized GO (GO-Cl) exhibits a new peak at the binding energy (BE) of 200 eV (Figure 1b), attributable to the Cl 2p species of covalently bonded chloride,14 indicating that the benzyl chloride initiators have been successfully immobilized on the GO nanosheets. The XPS wide scan and C 1s core-level spectra of GO-g-PtBA nanosheets are shown in Figure 1c and 1d, respectively. The persistence of Cl 2p signal with reduced relative intensity in the wide scan spectrum of GO-g-PtBA nanosheets is consistent with the preservation of dormant alkyl halide species at the graft PtBA chain ends from the ATRP process. In comparison with that of GO (Supporting Information, Figure S1), the C1s core-level spectral line shape of the GOg-PtBA nanosheets is characteristic predominantly for that of the PtBA brushes and can be curve-fitted into three peak components with BEs at 284.6, 286.4, and 288.3 eV, associated, respectively, with the C-C/C-H, C-O/C-Cl, and O-CdO species.20 The area ratio of the three peak components is

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SCHEME 1: Functionalization of Graphene Oxide (GO) with Poly(tert-butyl acrylate) (PtBA) Brushes via Surface-Initiated ATRP

9:1:0.7, which is close to the theoretical ratio of 7:1:1 expected for PtBA.21 The higher ratio of C-C species indicates that the PtBA layers on the surface of the GO nanosheets is less than the XPS probing depth (∼8 nm in an organic matrix22). The Fourier-transformed infrared (FT-IR) spectrum of GO-g-PtBA nanosheets is also consistent with the presence of grafted PtBA on GO nanosheets (Supporting Information, Figure S2). The pristine GO nanosheets adsorb weakly throughout the IR region. The respective aromatic and carbonyl absorption bands at 1600 and 1725 cm-1 are discernible (Figure S2a). Upon grafting of PtBA, the FT-IR spectrum of GO-g-PtBA is dominated by that of the PtBA brushes (Figures S2b and S2c). The characteristic absorption bands at 1396/1365 and 1729 cm-1 are associated, respectively, with bending vibrations of the CH3 group and the CdO (ester) groups of the PtBA brushes.23 These absorption bands have disappeared completely upon hydrolysis of the tertbutyl group of PtBA to produce the PAAc brushes (Figure S2d). The weight loss in the thermal gravimetric analysis (TGA) curve suggests that the amount of grafted PtBA brushes on the GO nanosheets is around 12.6 wt % (Supporting Information, Figure S3). Figure 2a and 2b show the respective TEM images of the pristine GO and the GO-g-PtBA nanosheets. Chemical functionalization of the hydroxyl groups at the edge or surface defects does not alter the physical structure or morphology of GO nanosheets significantly. Due to the improved compatibility

between the hydrophobic tert-butyl (C(CH3)3) moieties and the solvent molecules, the dispersity of GO nanosheets in organic solvents has been substantially enhanced. Consequently, the GOg-PtBA nanosheets can be readily dispersed in toluene while the pristine GO nanosheets remain as a heterogeneous suspension in toluene (1 mg/mL), even after 2 h of continuous agitation in an ultrasonic bath (Figure 2c). As shown in the FE-SEM image of Figure 2d, the GO-g-PtBA nanosheets can be uniformly dispersed into the poly(3-hexylthiophene) (P3HT) matrix and it is difficult to distinguish the individual components in the composite film. The ester groups of the grafted PtBA brushes on GO surfaces have also improved substantially the compatibility and processability of GO-g-PtBA with inorganic salts (Figure 2e). Pristine GO can hardly be dispersed in potassium bromide (KBr) to form a uniform KBr pellet for FTIR spectroscopy study (Figures 2e-1, 2e-2, and 2e-3 show the corresponding KBr pellets with 0 wt %, 0.5 wt %, and 1 wt % pristine GO). The dispersity of GO-g-PtBA complex in KBr pellets is greatly enhanced, leading to homegeneous composite pellets. Figures 2e-4 and 2e-5 show, respectively, KBr pellets with 0.5 wt % and 1 wt % GO-g-PtBA. Electronic memory devices based on the electrical bistability of organic and polymeric materials have been of great interest in recent years.24-26 Bistable electrical conductivity switching behavior and nonvolatile rewritable memory effects are also demonstrated in the current density-voltage (J-V) character-

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Figure 1. XPS wide-scan spectra of the (a) pristine GO, (b) GO-Cl, and (c) GO-g-PtBA complex. (d) XPS C1s core-level spectrum of the GOg-PtBA nanosheets.

Figure 2. TEM micrographs of the (a) pristine GO and (b) GO-g-PtBA nanosheets. (c) Pristine GO (c1) and GO-g-PtBA (c2) dispersed in toluene. (d) FE-SEM image (cross-section view) of the GO-g-PtBA+P3HT composite film containing 5 wt % GO-g-PtBA. (e) Pristine GO and GO-g-PtBA nanosheets dispersed in KBr pellets: (e1) blank KBr, (e2) 0.5 wt % pristine GO, (e3) 1 wt % pristine GO, (e4) 0.5 wt % GO-g-PtBA, (e5) 1 wt % GO-g-PtBA, and (e6) 1 wt % GO-g-PMMA. The respective scale bars for a, b, and d are 1 µm, 100 nm, and 100 nm.

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Figure 4. (a) Molecular orbitals of the basic unit (BU) of GO-g-PtBA complex. (b) Electronic transition from the ground state to the charge transfer state of the GO-g-PtBA+P3HT composite film.

Figure 3. (a) Current density-voltage (J-V) characteristics of the Al/GO-g-PtBA+P3HT/ITO device (with 5 wt % GO-g-PtBA content). Stability of the device in the ON and OFF state (b) under a constant stress of -1 V and (c) under a continuous read pulse with a peak voltage of -1 V, a pulse width of 1 µs, and a pulse period of 2 µs. The inset of Figure 3a shows a schematic diagram of the Al/GO-g-PtBA+P3HT/ ITO device. The inset of Figure 3c shows the voltage pulse employed for the device stability test.

istics of the composite thin film of P3HT with 5 wt % GO-gPtBA in an Al/GO-g-PtBA+P3HT/ITO (ITO ) indium-tin oxide) sandwich structure (Figure 3). Starting with the low conductivity (OFF) state in the as-fabricated device, the current density increases gradually with the increase in applied negative voltage (Al as cathode). The current density remains low (e.g., 7.25 × 10-3 A/cm2 at -1 V), until the applied voltage exceeds the threshold voltage of -1.6 V. At the threshold (turn-on) voltage, the current density increases abruptly from 10-2 to 10 A/cm2 (Sweep 1), switching the device from the low conductivity (OFF) state to the high conductivity (ON) state (write process). The ON state can be retained after removing the power supply (Sweep 2) and can be programmed back to the initial OFF state by a positively biased sweep with sufficient magnitude of 2.4 V (erase process, Sweep 3). The OFF state of the device can be read (Sweep 4) and reprogrammed to the ON state in the subsequent negative sweep (Sweep 5), thus completing the “write-read-erase-read-rewrite” cycle for a nonvolatile rewritable memory device. Both the OFF and ON states of the present device are accessible and stable under a constant voltage stress of -1 V for up to 3 h (Figure 3b), or under a pulse voltage stress of -1 V for up to 108 continuous read cycles (pulse period ) 2 µs, pulse width ) 1 µs, Figure 3c), with the ON/OFF state current ratio in excess of 103 well retained. The J-V characteristics of the present GO-g-PtBA+P3HT composite film are independent of the active device area, thus ruling out the possibility of random metallic filamentary conduction or leakage

current under high electric field that might give rise to the electrical bistability.27 To better understand the electronic process in GO-gPtBA+P3HT composite film, molecular simulation using the Gaussian 03 (Revision E. 01) program package16 and the density function theory (DFT) at B3LYP/6-31G(d) level17 was employed to calculate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the basic unit (BU) of GO-g-PtBA (Figure 4a). In order to reduce the CPU time, a simplified model with 16 aromatic rings and 1 tBA unit was employed to represent the BU of GO-g-PtBA complex. Due to scattering at the surface or edge defects of epoxide and carbonyl groups, the ballistic charge carrier transport on a single GO nanosheet is significantly hindered. Thus, the electrical conductivity of GO is several orders of magnitude lower than that of graphene or graphite.28 After functionalization with PtBA, the grafted polymer brushes form an energy barrier for charge transport between neighboring nanosheets, limiting the overall electrical conductivity of the composite film and leading to the low conductivity (OFF) state of the as-fabricated Al/GO-g-PtBA+P3HT/ITO device.29 At the switching voltage, electrons are excited under the high electric field and injected from the HOMO of P3HT into the LUMO of GO-g-PtBA complex (or the conduction band of GO) via intermolecular charge transfer (CT) interaction (Figure 4b). The transferred electrons can delocalize effectively in the giant π-conjugation system and reduce GO to the more conductive form.30-33 Charge carriers can migrate with less scattering in the reduced GO nanosheet, as well as transfer readily among neighboring graphene nanosheets via interplane hopping. The increase in both density and mobility of the free charge carriers significantly enhances the electrical conductivity of the composite film and switches the device from the OFF state to the ON state. The charge transfer states of the GO-g-PtBA+P3HT composite are effectively stabilized by electron delocalization in the GO nanosheet (Figure 4a). The grafted PtBA layer can act as an energy barrier for the back transfer of electrons. Thus, the memory device exhibits nonvolatile nature of the electrical bistability. However, application of a reverse positive bias with sufficient magnitude can extract electrons from the graphene

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J. Phys. Chem. C, Vol. 114, No. 29, 2010 12747 Gold nanoparticles (Au NPs), because of their fascinating optical and surface properties for catalytic and nanotechnology applications, can be loaded onto the GO-g-PAAc nanosheets. Narrowly distributed Au NPs of 18, 23, and 45 nm in mean diameters were prepared by the standard sodium citrate reduction method (Supporting Information, Figure S4).36,37 Decoration of the GO-g-PAAc nanosheets with Au NPs can be achieved from a mixture of aqueous dispersions of the GO-g-PAAc nanosheets and Au NPs. The TEM images of 18-nm Au NP-decorated GOg-PAAc/Au nanosheets are shown in Figure 5b and 5c. GO-gPAAc nanosheets decorated with larger Au NPs (23 and 45 nm) do not form uniform nanofilms with sufficient mechanical integrity. Conclusions Functionalization of graphene oxide nanosheets with hydrophobic polymer brushes via surface-initiated ATRP can give rise to composite nanosheets with good dispersity in common organic solvents. The poly(tert-butyl acrylate)-grafted graphene oxide nanosheets (GO-g-PtBA) can be readily dispersed in toluene and uniformly integrated into poly(3-hexylthiophene) (P3HT) thin films. Bistable electrical conductivity switching behavior and nonvolatile rewritable memory effects were demonstrated in the composite thin film of P3HT containing 5 wt % GO-g-PtBA in an Al/GO-g-PtBA+P3HT/ITO sandwich device. The bistability electrical conductivity switching in the present device can be ascribed to the field-induced charge transfer between the electron-donating P3HT matrix and the electron-accepting GO-g-PtBA nanosheets. Acid hydrolysis of GO-g-PtBA nanosheets can give rise to water-dispersible GOg-PAAc nanosheets (PAAc ) poly(acrylic acid)). Gold nanoparticle-decorated GO-g-PAAc nanosheets can be readily prepared from a mixture of aqueous dispersions of the nanosheets and nanoparticles. Acknowledgment. G. L. Li and G. Liu contributed equally to this work. Supporting Information Available: XPS C1s core-level spectrum of the pristine GO nanosheets, FT-IR spectra the GO, GO-g-PtBA, and GO-g-PAAc nanosheets and PtBA homopolymer, TGA curves of the GO and GO-g-PtBA nanosheets, and TEM images of the Au NPs of differential sizes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 5. TEM images of the (a) GO-PAAc nanosheets, (b) Au NPs of 18 nm in diameter and (c) GO-g-PAAc nanosheets decorated with 18-nm Au NPs processed from aqueous dispersions. The respective scale bars for a, b, and c are 200, 20, and 500 nm.

nanosheet, returning the complex to the initial less conductive form and programming the device back to the OFF state. Furthermore, the functionalized GO nanosheets can also be used as a support for immobilizing noble metal/metal oxide catalysts.34,35 Water-dispersible GO-g-PAAc (PAAc ) poly(acrylic acid)) nanosheets can be obtained via hydrolysis of the tert-butyl groups (C(CH3)3) of PtBA brushes of the GO-g-PtBA complex in the presence of trifluoroacetic acid (Supporting Information, Figure S2d). Figure 5a shows the TEM image of GO-g-PAAc nanofilm obtained from an aqueous dispersion.

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