Controllable Adsorption of Reduced Graphene Oxide onto Self

Feb 18, 2010 - College of Chemistry and Chemical Engineering, South China University of Technology, Wushan, Guangzhou 510640, School of Chemistry ...
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J. Phys. Chem. C 2010, 114, 4389–4393

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Controllable Adsorption of Reduced Graphene Oxide onto Self-Assembled Alkanethiol Monolayers on Gold Electrodes: Tunable Electrode Dimension and Potential Electrochemical Applications Shunlong Yang,⊥,† Baofeng Xu,⊥,‡ Jiaqi Zhang,§ Xiaodan Huang,| Jianshan Ye,*,† and Chenzhong Yu*,| College of Chemistry and Chemical Engineering, South China UniVersity of Technology, Wushan, Guangzhou 510640, School of Chemistry and Resources EnVironment, Linyi Normal UniVersity, Shandong 276005, College of EnVironmental Science and Safety Engineering, Tianjin UniVersity of Technology, Tianjin 300071, and Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed: December 12, 2009; ReVised Manuscript ReceiVed: January 25, 2010

This paper describes a facile and effective method to construct graphene nanosheet film (GNF) by controllable adsorption of reduced graphene oxide (rGO) onto the self-assembled monolayer (SAM) of n-octadecyl mercaptan (C18H37SH) at Au electrodes. Nyquist plots show a gradual decrease of the charge transfer resistance (Rct) of the [Fe(CN)6]3-/4- redox couple at the GNF/SAM electrode with prolonging the self-assembly time, suggesting the controllable adsorption of rGO onto the SAM. Cyclic voltammetry (CV) studies reveal that the GNF/SAM electrodes have tunable dimensions ranging from a nanoelectrode ensemble to a conventional electrode, depending on the self-assembly time of rGO. The excellent electrocatalytic activity of the GNF/ SAM electrode toward ascorbic acid, dopamine, and uric acid further indicates that our approach is successful for the fabrication of stable GNF with excellent electrochemical properties, which is very attractive for electrochemical studies and electroanalytical applications. At the same time, as a new kind of nanosheet film electrode, the GNF electrode could be exploited in a new field for micro- and nanoelectrodes in electrochemical investigations and practical applications, e.g., electroanalysis in vivo and in vitro. Introduction Recently, self-assembled monolayer (SAM) films have received considerable interest because of their potential applications in sensor fabrication and as active surfaces for patterning and chemical architecture of solid supports.1-3 In the electrochemical fields, the strong interest in SAM films based on thiols and related molecules is due to the following aspects: (1) they can be employed as insulating barriers between an electrode and a redox couple to study long-range electron transfer; (2) they can be used to fabricate supramolecular assemblies with tailored architecture and properties for creating selective voltammetric detectors or for measuring very fast electron transfer kinetics.4-8 However, the electrochemical properties of these kinds of SAM films are strongly dependent on the surface termination employed: when the surface is terminated with alkyl chain, inhibition of electron transfer is observed, but if the surface is terminated with well conductive material, such as Au,9,10 Pt,11 and carbon-based materials,12-14 fast electron transfer to aqueous redox couples will take place. Very recently, the Gooding group explored the electrochemical performance of carbon nanotube arrays or ferrocene-modified alkanethiols attached to the SAM with different lengths of the carbon chain, suggesting that the rate * Corresponding authors. (J.S.Y.) Tel: 86-20-8711 3241. Fax: 86-208711 2906. E-mail: [email protected]. (C.Z.Y.) Tel: 86-21-5566-5103. Fax: 86-21-6564-1740. E-mail: [email protected]. † South China University of Technology. ‡ Linyi Normal University. § Tianjin University of Technology. | Fudan University. ⊥ These authors contributed equally to this work.

of electron transfer may be influenced both by SAM length and the polarity of the surface.15,16 As a recently discovered member in the carbon family, graphene has triggered enormous interest based on fundamental studies and practical aspects because of its distinctive electrical conductivity and novel physicochemical properties.17-19 For example, because of its novel electrical conductivity properties, graphene has been proposed for future applications in electronics and transistors.20,21 In electrochemical studies, graphene is particularly promising for supercapacitors, lithium batteries, solar cells, and hydrogen storage and is thus potentially useful in the practical applications.22-25 Moreover, graphene has recently been demonstrated to show excellent electrochemical catalytic activities toward physiologically important species, such as neurotransmitters26-28 and the species involved in oxidase/ dehydrogenase enzymatic reactions.29-31 This property essentially enables this kind of carbon nanostructure to be potentially used as a new kind of electrode materials with potential applications in electrochemical sensing and biosensing. It is expected that by forming novel graphene-SAM nanocomposites and taking advantages of the striking properties of both graphene and SAM film, fundamental understanding in hybrid material manipulation and new electrochemical properties can be obtained.32,33 However, so far there have been few reports concerning the preparation, electrochemical characterization, and potential electrochemical applications of the graphene-SAM film electrodes. On the basis of our previous research on reduced graphene oxide (rGO),34,35 this paper demonstrates a new kind of

10.1021/jp911760b  2010 American Chemical Society Published on Web 02/18/2010

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graphene-SAM film prepared by adsorbing rGO onto SAMmodified Au electrodes. The method present here not only provides a facile protocol for the preparation of graphene nanosheet film (GNF) with controlled thickness, but also efficiently integrates the advantages of both SAM and GNF, leading to novel GNF/SAM electordes particularly useful for electrochemical studies. More notably, the GNF/SAM electrodes show another advantage, i.e., tunable electrode dimension from conventional to nanoelectrode ensemble. While the GNF/SAM with conventional dimensions is useful, e.g., for electrochemical determinations and energy conversion and storage, the GNF/ SAM with nanoelectrode ensemble is believed to be very attractive for electrochemical study on nanoscale. Experimental Section Reagents and Materials. n-Octadecyl mercaptan (C18H37SH) was purchased from Aldrich. Potassium ferricyanide, potassium ferrocyanide, N,N-dimethylformamide (DMF), ascorbic acid (AA), dopamine (DA), uric acid (UA), and other chemicals of at least analytical reagent grade were obtained from Beijing Chemical Corporation (Beijing, China). Pristine graphene oxide was prepared according to the modified Hummers method,36 and then graphene was prepared by the chemical reduction of graphene oxide with hydrazine, as reported previously.37 Doubly distilled water was used throughout the experiments. Electrode Preparation and Modification. Au electrodes (2 mm diameter, Ingsens Instruments (Guangzhou) Co. Ltd., China) were used as the substrate to prepare the self-assembled graphene film electrodes. Au electrodes were first polished with aqueous slurries of fine alumina powders (0.3 and 0.05 µm) on a polishing cloth, and then the electrodes were rinsed with doubly distilled water and acetone in an ultrasonic bath, each for 5 min, and were finally rinsed with doubly distilled water. Then the electrodes were subjected to electrochemical pretreatment by consecutive potential cycling in 0.50 M H2SO4 within a potential range between -0.20 and +1.60 at 0.20 V s-1 until a cyclic voltammogram characteristic of a clean Au electrode was obtained. In a typical experiment, Au electrodes modified with SAM of C18H37SH were prepared by immersing the electrodes into ethanol solution of C18H37SH (10 mM) for at least 24 h at room temperature. The SAM electrodes were then thoroughly rinsed with ethanol and dried with pure N2. For adsorption of the rGO onto the SAM electrodes, the SAM electrodes were immersed into rGO dispersion in DMF (1 mg/mL). The self-assembly time was adjusted to achieve electrodes with different surface coverage of rGO. The electrodes (denoted as GNF/SAM electrodes) were thoroughly rinsed with DMF and doubly distilled water to remove rGO unstably adsorbed onto the electrode surface and then dried with pure N2 before use. Apparatus and Measurements. Cyclic voltammetry (CV) experiments were carried out on a Ingsens-1010 Electrochemical Workstation (Ingsens Instruments (Guangzhou) Co. Ltd., China) and electrochemical impedance spectroscopy (EIS) experiments were carried out on a CHI 660C electrochemical workstation (Shanghai Chenhua Ltd., China) with a bare or modified Au as working electrode, a platinum disk as counter electrode, and a Ag/ AgCl (1.0 M KCl) electrode as reference electrode. All experiments were performed at room temperature. Results and Discussion Controllable Adsorption of the GNF onto the SAM Electrode. It is well known that C18H37SH can form a stable SAM on a Au substrate through the formation of a Au-S bond,

Figure 1. Nyquist plots obtained with the GNF/SAM electrode in 0.50 M KCl of 5.0 mM [Fe(CN)6]3-/4-. GNF/SAM electrode was prepared by immersing the SAM electrode into a rGO dispersion in DMF (1 mg/mL) for 0 (curve a), 3 (curve b), 5 (curve c), 15 (curve d), 30 (curve e), 60 (curve f), and 120 min (curve g). EIS conditions: open potential; alternative voltage, 5 mV; frequency range, 100 kHz to 0.01 Hz.

and that these compact SAMs can block the electron transfer between bare Au electrode and redox species in solution phase. According to recent reports, carbon nanotubes (CNTs) can be tunneled through SAMs with different lengths of carbon chains and undergo a charge transfer interaction with the based electrode.12,15 Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has an electric conductivity similar to that of CNTs. So research on the electrochemical properties of the GNF/SAM electrode is believed to be particularly useful for fundamental studies on graphene-based electrochemistry and for practical electrochemical applications. Figure 1 depicts typical EIS spectra obtained in 0.50 M KCl solution containing 5.0 mM [Fe(CN)6]3-/4- at the GNF/SAM electrode prepared by immersing the SAM electrodes into the graphene dispersion for different times. As shown in Figure 1, the charge-transfer resistance (Rct) of the [Fe(CN)6]3-/4- redox couple at the SAM electrode (without the adsorption of rGO) is very large (i.e., >3.0 × 106 Ω) (Figure 1a), indicating the insulating feature of the SAM of the C18H37SH used in this study. The Rct is decreased to 2.2 × 106 Ω, only for 5 min, upon the adsorption of rGO (Figure 1c), which is indicative of the restoration of electron transfer with rGO assembled onto the SAM electrode. The Rct of GNF/SAM electrodes gradually decreases with increasing self-assembly time, which is again indicative of the adsorption of rGO onto the SAM electrode with a tunable surface coverage of rGO. This phenomenon observed in the GNF/SAM electrodes can be explained clearly as shown in Figure 2. rGO adsorbed onto the SAM of C18H37SH may also be able to relay the electron transfer between bare Au electrode and the redox species in the solution phase through the insulating SAM, probably similar to the CNTs and mesoporous carbon demonstrated recently.12-15 In our system, the electron transfer at the GNF/SAM electrodes is believed to proceed through two steps, the first between the bare Au and rGO, and the second between rGO and the redox species in the solution phase, as illustrated in Figure 2A. The electron transfer in the first step is believed to proceed essentially quickly, and as a result, the flux of electron transferred can well satisfy the Nernstian equilibrium, at the same time, leading to a gradual decrease in RG-SAM and Rct of the GNF/SAM electrodes, as shown in Figure 2b. These demonstrations substantially suggest that the hydrophobic interaction between rGO and the SAM of C18H37SH essentially makes it possible to adsorb rGO stably onto the SAM in a controllable manner and thus to fabricate the GNF with a

Self-Assembled Alkanethiol Monolayers on Gold Electrodes

Figure 2. (A) Schematic illustration of fabrication of the rGO film electrode through controllable adsorption of rGO onto the SAM electrode. (B) The equivalent circuit of the prepared GNF/SAM electrodes. (Rr, RG-SAM, Rsol, and RSAM is responsible, respectively, for the electrochemical resistance of the soluble redox species, the GNF/SAM, the solution, and the SAM. CG-SAM and CSAM is responsible, respectively, for the capacitance of the GNF/SAM and the SAM).

Figure 3. CVs at the GNF/SAM electrode in 0.50 M KCl of 5.0 mM [Fe(CN)6]3-. The GNF/SAM electrode was prepared by immersing the SAM electrode into a rGO dispersion in DMF (1 mg/mL) for 120 min (curve d), 15 min (curve c), 5 min (curve b) and 0 min (curve a). Scan rate, 50 mV s-1.

controllable surface coverage of rGO. This property is very similar to the attachment of Au and Pt nanoparticles, CNTs, and mesoporous carbon9-15 onto an electrode surface modified with, for example, sol-gel38 and SAM technology. The controllable surface coverage of rGO on the SAM electrode will cause the fabrication of GNF with tunable dimensions ranging from a conventional electrode to a nanoelectrode ensemble, discussed as follows, and have potential uses in other research fields, such as graphene-based nanodevices. Tunable Electrode Dimensions Ranging from a Nanoelectrode Ensemble to a Conventional Electrode. Typical CVs obtained in 0.50 M KCl solution containing 5.0 mM [Fe(CN)6]3at the GNF/SAM electrode prepared by immersing in rGO dispersion for different times is shown in Figure 3. It is known that a bare Au electrode has a pair of well-defined redox peaks, showing a reversible process for [Fe(CN)6]3- in solution phase. The first study of the SAM electrode (without rGO adsorption on the electrode) is shown in Figure 3a. No voltammetric

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Figure 4. CVs at the GNF/SAM electrode in 0.50 M KCl of 5.0 mM [Fe(CN)6]3-. The GNF/SAM electrode was prepared by immersing the SAM electrode into a rGO dispersion in DMF for 120 min (A and C) and 5 min (B and D). (A) Scan rate is 5, 20, 50 mV s-1, from inner to outer. (B) Scan rate is 5 (short-dotted blue curve), 20 (short-dashed red curve), 50 (solid black curve) mV s-1. (C, D) Consecutive CVs at the GNF/SAM electrodes in 0.50 M KCl of 5.0 mM [Fe(CN)6]3- for 50 cycles. Scan rate, 50 mV s-1.

responses were recorded for the redox couple of [Fe(CN)6]3-/4at the SAM electrodes, indicating that electron transfer between the Au electrode and the redox species was largely blocked by the SAM of the long hydrocarbon chain. This observation further indicates that the SAM of C18H37SH confined on the Au electrode is compact and essentially pinhole-free. A large difference in the CV shape was clearly observed at the GNF/ SAM electrodes. A typical sigmoidal-shaped voltammetric response was recorded at the electrodes when the self-assembly time of the SAM electrode in the dispersed rGO solution was 5 min (Figure 3b), indicating that the process of the redox couple at such electrodes was a nonlinear diffusion process and that the film electrodes essentially act as a nanoelectrode ensemble.38,39 This demonstration reveals that the heterogeneous electron transfer process blocked by the SAMs of the C18H37SH (Figure 3a) was restored upon the adsorption of rGO onto the SAM electrode, which was consistent with the results of EIS (Figure 1). Similarly, when the self-assembly time is about 120 min, the CVs obtained at the as-prepared GNF/SAM electrodes for the [Fe(CN)6]3-/4- redox couple changed dramatically as shown in Figure 3d. Interestingly, a pair of well-defined and peak-shaped redox waves was recorded under this condition, indicating that the process of the redox couple at such electrodes was semi-infinite linear diffusion-controlled and that the film electrodes essentially act as a conventional electrode. In addition, the near unity peak current ratio (ipa/ipc ) 0.98 at 50 mV s-1) and the small peak-to-peak separation (73 mV at 50 mV s-1) essentially suggests a fast electron transfer process for [Fe(CN)6]3-/4at the GNF/SAM electrodes prepared here. The formation of the GNF/SAM electrodes with tunable dimensions were further verified with the CVs obtained with the electrodes for the [Fe(CN)6]3-/4- redox couple at various potential scan rates, as shown in Figure 4. At the GNF/SAM electrodes prepared by immersing the SAM electrode into a rGO

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dispersion for 120 min (Figure 4A), both anodic and cathodic peak currents clearly increase with increasing potential scan rate and are linear with the square root of the potential scan rate within a range from 5 to 100 mV s-1 (data not shown here). These again demonstrate a semi-infinite linear diffusioncontrolled feature of the redox process of the [Fe(CN)6]3-/4couple at the electrodes, which is characteristic of the electrodes of a conventional dimension.39,40 The slope of the straight line (r ) 0.9995) of ipa vs V1/2 is 1.5335 × 10-4 A s1/2 V-1/2 for GNF/SAM electrodes. For a reversible process,39 1/2 ip ) (2.69 × 105)n3/2AD1/2 o V Co

(1)

where n is the number of electrons transferred, A is the effective area, Do is the diffusion coefficient of [Fe(CN)6]3-, V is the scan rate, and Co is the bulk concentration of [Fe(CN)6]3-. For 5 mM Fe(CN)63- in 0.5 M KCl, n ) 1, Do ) 7.7 × 10-6 cm2 s-1, and then A is estimated to be 1.4807 × 10-2 cm2. This large effective coverage of rGO on the SAM electrodes will result in close spacing among rGO nanosheets and thereby the possibility of overlap of the diffusion layer of each graphene nanoelectrode, so that a peak-shaped CV was obtained. Meanwhile, at the SAM film electrodes prepared with a short self-assembly time of rGO (i.e., 5 min), the limiting currents are almost independent of potential scan rate up to 100 mV s-1 (Figure 4B), further suggesting that the electrodes behave as a nanoelectrode ensemble. It is well known that the enhanced mass transport, the reduced iR drop, and the increased faradic-to-charging current ratio at the microelectrodes and their ensembles have substantially made these kinds of electrodes advantageous over the electrodes with a conventional dimension both in the fundamental and practical aspects.39-41 Here, a facile and effective method to construct the GNF/SAM electrode by adjusting electrode dimension that ranges from a nanoelectrode ensemble to a conventional electrode provides a new kind of method for constructing the nanoelectrode ensemble and may even allow development of a new field for rGO in electrochemical investigations and practical applications of micro- and nanoelectrodes. Moreover, the prepared GNF/SAM electrodes in this study, both the conventional electrode and the nanoelectrode ensemble, were found to be very durable for consecutive potential cycling, and almost no decrease in the currents was observed after continuously scanning the electrodes in 0.50 M KCl solution containing 5.0 mM [Fe(CN)6]3- for at least 50 cycles, suggesting that the nanocomposite is electrochemically stable in such media (Figure 4C and 4D). At the same time, after immersion in different pH buffer solutions (from pH 1 to pH 13) for 12 h, the prepared GNF/SAM electrodes were not obviously changed in CV response, showing chemical stability of the nanocomposite in the progress of self-assembly (data not shown here). All these properties of the graphene film electrodes are envisaged to substantially enable them to be very useful not only for fundamental electrochemical studies but also for electroanalytical applications, for example, for the development of electrochemical sensors, biosensors, and biofuel cells. The Electrocatalysis Activity of the GNF/SAM Electrode toward AA, DA, and UA. The GNF/SAM electrodes with tunable dimensions are believed to possess similar electrochemical properties as reported for the electrode modified by dipcoating or other traditional methods, e.g., encapsulation of enzymes or proteins for biosensor development.26-31 Demonstrated here is a typical example of the conventional dimensions

Figure 5. CVs at the GNF/SAM electrode in 0.10 M PBS (pH 7.0) without (dotted line) or with (solid line) 0.5 mM AA (A), 0.5 mM AA and 0.25 mM UA (B), 0.5 mM AA, 0.25 mM UA, and 0.05 mM DA (C). GNF/SAM electrode was prepared by immersing the SAM electrode into a rGO dispersion in DMF for a sufficiently long time. Scan rate, 50 mV s-1.

of the GNF/SAM electrode for electrocatalysis toward AA, DA, and UA. Figure 5 shows the CVs at the GNF/SAM electrodes in 0.10 M PBS (pH 7.0) containing AA, DA, and UA. As illustrated in Figure 5, the current response of the GNF/SAM electrode toward AA, DA, and UA is much larger, and the electrocatalytic peak potential toward the oxidation of AA, DA, and UA is about -0.01 V, 019 V, and 0.35 V, respectively. These observations suggest that the GNF/SAM electrodes exhibit higher electrocatalytic activity compared with SAM electrodes (data not shown here). More importantly, the mixed biomolecules display three well-resolved voltammetric responses from each other at the GNF/SAM electrode (Figure 5B). All of the phenomena may result from the unique microstructure of rGO with a large density of edge-plane-like defective sites and a large surface area.42-44 The greatly enhanced electrochemical reactivity again demonstrates the excellent electrochemical activity of the graphene-based film electrode. Based on the convenient construction method together with the excellent electrochemical behavior toward AA, DA and UA, this prepared GNF/SAM electrode may benefit the development of novel electrochemical sensors and biosensors. Conclusions The fabrication of a stable GNF/SAM electrode with excellent electrochemical properties by controllable adsorption of graphene onto the SAM of C18H37SH at Au electrodes is achieved. The dimensions of the as-prepared film electrodes could be readily ranged from a conventional electrode to a nanoelectrode ensemble by adjusting the self-assembly time of graphene onto the SAM electrode. Moreover, these prepared GNF/SAM electrodes are found to have excellent electrochemical properties, such as heterogeneous electron transfer and electrocatalysis activity. These demonstrations offer a facile approach to fabrication of stable graphene-based film electrodes with excellent electrochemical properties that are believed to be particularly useful for fundamental studies on carbon-based electrochemistry and for practical electrochemical applications. Acknowledgment. The authors gratefully acknowledge the financial support of the High Technology Research Program, Ministry of Science and Technology of China (2008AA06Z311), the State Key Research Program (2006CB932302), NSFC (20721063, 20945004), SLADP (B108), and Shanghai Science Committee (06JC14011).

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