Electrochemistry at CVD Grown Multilayer Graphene Transferred onto

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Electrochemistry at CVD Grown Multilayer Graphene Transferred onto Flexible Substrates Adriano Ambrosi and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ABSTRACT: The application of graphene and graphene-related materials for the fabrication of improved electrochemical devices is a very active research field. Because of the need of bulk quantities of graphene for such applications, chemical vapor deposition (CVD) represents one of the most promising fabrication processes to obtain large area and good quality graphene films with controlled thickness. Here we investigated the electrochemical properties of a multilayer graphene film grown by CVD and transferred to an insulating flexible poly(ethylene terephthalate) (PET) sheet. After a careful characterization with optical microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy of the transferred graphene film, we tested its heterogeneous electron transfer properties with Fe(CN)63−/4− and Ru(NH3)62+/3+ redox mediators and its sensing capability toward dopamine (DA), ascorbic acid (AA), and the reduced form of β-nicotinamide adenine dinucleotide (NADH) as biological relevant molecules. The CVD grown multilayer graphene film transferred to a PET substrate showed an electrochemical behavior that resembles that of basal plane graphite with a low density of edge-plane defects sites. The possibility to retain electrochemical properties of graphene after the transfer to flexible and insulating supports is of high importance for the fabrication of novel electrochemical devices.

1. INTRODUCTION Graphene and graphene-derived materials have been the focus of a massive research interest in the past few years.1,2 This is due to the unusual electronic,3 mechanical,4 optical,5 and electrochemical6 properties they have demonstrated. One very active research area deals with the use of graphene materials for electrochemical applications including the fabrication of novel and improved energy storage and energy production devices7 as well as sensing and biosensing systems.8,9 In order to be adopted in electrochemical applications, bulk quantities and large volume production of graphene and graphene materials are normally required. Two different fabrication methodologies are available to meet such requirements: (i) oxidation of graphite followed by exfoliation and oxygen elimination10 and (ii) chemical vapor deposition (CVD) onto catalytic metallic substrates (typically Ni and Cu).11−13 The first method produces bulk quantities of graphene materials which however present several structural defects in addition to a significant grade of chemical fuctionalization which varies in relation to the agents or processes used. Electrochemical characterizations of graphene materials produced with this method showed that the electron transfer properties are strongly influenced by the structural features of the graphene as well as by the functional groups bonded to its basal and edge sites.14 It has also been discovered that, similarly to the carbon nanotubes, metallic15,16 and carbonaceous17 impurities have an enormous effect on the electrocatalytic properties of the graphene material.18 Such variability on the electrochemical properties of graphene materials influenced by structural defects, chemical functional © 2013 American Chemical Society

groups, and metallic and carbon-based impurities raised doubts on the effective applicability of such materials for electrochemical applications lacking homogeneity of behavior and clear understanding. Chemical vapor deposition represents a promising fabrication process able to produce large area graphene in a single, few, or multilayer structure in a controlled manner and with a low level of structural defects and chemical fuctionalization. In addition, such a method offers the possibility to transfer the graphene film onto arbitrary substrates after the metal catalyst etching.19,20 Electrochemical properties of graphene have been investigated recently using micrometer size electrode based on exfoliated single and bilayer graphene flakes.21 Large area electrochemical studies have been performed using epitaxial graphene grown on silicon carbide.22 The authors demonstrated that the heterogeneous electron transfer kinetics and sensing capabilities of the graphene film were strongly enhanced after applying an anodization procedure which creates a high density of edge-plane defects sites.22 Here we performed electrochemical characterizations of a multilayer graphene film grown by CVD on a Ni foil after the transfer to a flexible nonconductive PET sheet. To the best of our knowledge, this is the first macroscopic evaluation of the electrochemical properties of a CVD grown graphene after the transfer process over an insulating and flexible substrate. We Received: November 29, 2012 Revised: January 10, 2013 Published: January 14, 2013 2053

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Figure 1. (A) Multilayer graphene film on Ni foil. (B) Backside of Ni foil after removing multilayer graphene film. (C) Ni foil etching in a Fe(III) chloride solution and floating multilayer graphene film. (D) Multilayer graphene film transferred onto a PET sheet. (E) Schematic of the electrochemical cell used for the measurements. A rubber O-ring (i.d. = 4.5 mm; o.d. = 8.1 mm) and four screws ensure a secure and sealed chamber for a total surface of 17 mm2 for the graphene sample to be examined. The transferred graphene acts as the working electrode while a Pt and Ag/ AgCl wire act as the auxiliary and the reference electrode, respectively. A Cu tape was used to ensure the electrical connection between the CVD graphene and the electrochemical analyzer.

first characterized the graphene film by means of optical microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) before and after the etching of the Ni foil and transfer to the PET sheet. This is to evaluate possible structural and chemical alteration to the graphene film during the transfer process. Finally, we tested the heterogeneous electron transfer properties of the graphene film with both an inner-sphere (Fe(CN) 6 3−/4− ) and outer-sphere (Ru(NH3)62+/3+) redox mediator and also its sensing capabilities toward the analysis of dopamine (DA), ascorbic acid (AA), and the reduced form of β-nicotinamide adenine dinucleotide (NADH) as biological relevant molecules.

of 1 cm2 were first carefully polished at one side with fine abrasive paper to remove graphene film and expose one side of the Ni foil, leaving untouched the graphene layer at the other side. The CVD graphene samples were then let floating on a FeCl3 (0.1 g/mL) solution for at least 2 h or until complete dissolution of the Ni metal is achieved. The floating graphene film is then transferred with a spoon to a distilled water solution to remove the excess of the FeCl3 solution. Transferring of the graphene film to a clean DI water solution is repeated at least three times to ensure complete washing. A clean PET sheet is then used to pull out the graphene film from the solution for the final stage of the transfer process. The PET substrate with transferred graphene film is then gently heated at 50 °C in a vacuum oven for 5 h to eliminate residual water and facilitate the adhesion. Electrochemical Characterizations. Electrochemical experiments were performed at room temperature by using a threeelectrode configuration. A platinum electrode (Autolab) served as an auxiliary electrode, while an Ag/AgCl electrode (CH Instruments, Austin, TX) served as a reference electrode. A custom-made voltammetric cell (see Figure 1) was used to allocate the CVD graphene−PET sheet as working electrode. A normal 5 mL glass cell was adopted to test EPPG, GC, and BPPG working electrodes. All electrochemical potentials in this paper are stated versus Ag/AgCl reference electrode. The EPPG and GC electrode surfaces were renewed by polishing with 0.05 mm alumina particles on a cloth and sonicated in ethanol and deionized water for 10 min each to remove any adhered microparticles of alumina. The BPPG electrode surface was renewed by pressing the surface onto an adhesive tape and removing the top few layers of graphite. This was repeated several times before rinsing the electrode surface in acetone to remove any residual adhesive glue. Cyclic voltammetry experiments were performed at a scan rate of 100 mV s−1 by using 50 mM phosphate buffer (pH 7.2) for DA, AA, and NADH (1 mM); 0.1 M KCl solution was employed as supporting electrolyte when ferro/ferrycyanide (5 mM) and ruthenium(III) hexamine chloride (1 mM) redox probes were 0 used. The kobs values were determined using a method

2. EXPERIMENTAL SECTION 2.1. Materials. CVD multilayer graphene grown on Ni foil was purchased from Graphene Laboratories Inc., Calverton, NY. Potassium chloride, potassium phosphate disodium salt, iron(III) chloride, ruthenium(III) hexamine trichloride, dopamine (DA), ascorbic acid (AA), the reduced form of βnicotinamide adenine dinucleotide (NADH), and potassium ferricyanide were purchased from Sigma-Aldrich, Singapore. Glassy carbon (GC), edge-plane pyrolytic graphite (EPPG), and basal-plane pyrolytic graphite (BPPG) electrodes with a diameter of 3 mm were obtained from Autolab, Japan. 2.2. Apparatus. All voltammetric experiments were performed on a Autolab PGSTAT101 electrochemical analyzer (Methrom Autolab B. V., The Netherlands) connected to a personal computer and controlled by NOVA software Version 1.8 (Methrom Autolab B. V.). X-ray photoelectron spectroscopy (XPS) was performed with a Phoibos 100 spectrometer and a monochromatic Mg X-ray radiation source (SPECS, Germany). Raman spectroscopy analysis was performed using a confocal micro-Raman LabRam HR instrument (Horiba Scientific, Japan) in backscattering geometry with a CCD detector, a 514.5 nm Ar laser, and a 100× objective mounted on an Olympus optical microscope. 2.3. Procedures. Transfer of CVD Multilayer Graphene onto PET Sheets. CVD graphene samples of approximate size 2054

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in both cases an intense G band at about 1580 cm−1 and a 2D band at about 2710 cm−1. The ratio 2D/G < 1 confirms also the multilayer conformation of the graphene film.25 The absence of the D band at about 1360 cm−1, which is related to disorder/defects in graphitic structures, indicates also a wellordered and homogeneous sp2-hybridized carbon surface. Beside the possibility of structural alteration/damages deriving by the transfer process, it is important to evaluate also possible chemical modifications of the film surface. We characterized the graphene film before and after the transfer process by means of XPS. Figure 3A shows the wide XPS

developed by Nicholson that relates the anodic/cathodic peak separation (ΔEp) to dimensionless parameter ψ and consequently to k0obs. The roughness factor was not taken into account. The diffusion coefficients used for the calculations were Fe(CN)63−/4−, DO = DR = 7.26 × 10−6 cm2 s−1; Ru(NH3)62+/3+, DO = DR = 6.5 × 10−6 cm2 s−1, in 0.1 M KCl.23,24

3. RESULTS AND DISCUSSION Multilayer graphene films can be grown by CVD process using Ni foil (25 μm thickness) as catalytic substrates. In this work commercially available samples were cut in small pieces of about 1 cm2 area before the transfer of the graphene film onto PET sheets. Figure 1 shows pictures of the transfer process which is also described in more detail in the Experimental Section. One side of the CVD graphene sample was polished with a fine abrasive paper in order to remove the graphene film and have a better exposure of the Ni metal (Figure 1A,B). After the etching of the Ni metal by using a Fe(III) chloride solution, followed by several washing steps in pure water, the multilayer graphene film was carefully transferred to a PET sheet (Figure 1C,D). Gentle heating in a vacuum oven at 40 °C for 2 h ensured complete adhesion to the substrate and elimination of residual water. Graphene samples so prepared were first characterized by Raman spectroscopy and XPS to evaluate possible structural and chemical alterations occurring during the transfer process and finally tested for electrochemical applications performing cyclic voltammetric measurements with common redox probes using the setup shown in Figure 1E. It is important to make sure that the transfer process does not alter the structural conformation of the graphene film in order to maintain its original electrochemical behavior. We extensively examined our multilayer graphene samples as received and after the transfer process by means of optical microscopy and Raman spectroscopy. Figure 2A,B shows

Figure 3. (C) XPS spectra of multilayer graphene film before (top) and after (bottom) the transfer process. (A) Survey spectra. (B) Highresolution core level spectra of C 1s signal. Both spectra have been normalized according to the intensity of C 1s signal.

spectra of the graphene film. It can be noticed that after the transfer process a slight increment of the oxygen content was recorded from a value of 2% to about 6%. In addition, a nitrogen signal appeared resulting in a relative amount of about 1%. The high-resolution C 1s spectra on the contrary presented almost overlapping profiles (Figure 3B). The chemical composition of the graphene film resulted therefore almost perfectly preserved without major alterations during the transfer process. In order to investigate the heterogeneous electron transfer properties of the transferred CVD graphene film, we performed cyclic voltammetric measurements in the presence of two redox couples: Fe(CN)63−/4− and Ru(NH3)62+/3+ which follow an inner-sphere and outer-sphere charge transfer mechanism, respectively. The electron transfer kinetics with the Fe(CN)63−/4− redox couple is influenced by the density of electronic states near the Fermi level of the carbon material and more significantly by the surface microstructure. In particular, it is known that the presence of edge-plane defects on graphitic materials accelerates significantly the electron transfer process.26,27 Figure 4 shows cyclic voltammograms recorded in the presence of 1 mM Fe(CN)63−/4− and using multilayer graphene transferred to PET sheet (ML-G-PET) as working electrode. Cyclic voltammograms using EPPG and BPPG electrode are also shown as comparison. It can be seen that the ML-G-PET electrode showed the largest peak-to-peak separation, indicating the slowest electron transfer kinetics. Interestingly, it should be noticed that BPPG electrode showed faster electron transfer kinetics than the transferred

Figure 2. Optical microscopy images of multilayer graphene film before (A) and after (B) the transfer process. (C) Raman spectra of multilayer graphene film before (top) and after (bottom) the transfer process. Both spectra have been normalized according to the intensity of the G band.

typical optical images of a large area of the graphene film before and after the transfer process. It appears clear that an almost identical surface conformation is visualized after the transfer process showing no damage or significant alteration. Raman spectra in Figure 2C confirm that the graphene film surface structure remained unaltered after the transfer process, showing 2055

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Figure 4. Representative cyclic voltammograms in the presence of (A) 5 mM Fe(CN)63−/4− and (B) 1 mM Ru(NH3)62+/3+ redox probes and using BPPG (blue), EPPG (black), and multilayer graphene transferred to PET (ML-G-PET) (red) electrodes. Supporting electrolyte 0.1 M KCl; scan rate 0.1 V s−1 (vs Ag/AgCl).

Figure 5. Representative cyclic voltammograms in the presence of 1 mM of (A) dopamine, (B) ascorbic acid, and (C) NADH probes and using BPPG (blue), EPPG (black), and multilayer graphene transferred to PET (ML-G-PET) (red) electrodes. Supporting electrolyte 50 mM PBS, pH 7.2; scan rate 0.1 V s−1 (vs Ag/AgCl).

In order to evaluate the performance of the transferred multilayer graphene film for sensing applications, we investigated the electrochemistry of biologically relevant molecules, such as dopamine (DA), ascorbic acid (AA), and the reduced form of β-nicotinamide adenine dinucleotide (NADH). Figure 5 summarizes the cyclic voltammograms for the three biomolecules using the ML-G-PET electrode in addition to BPPG and EPPG electrodes as reference materials. It can be seen that the ML-G-PET electrode showed an oxidation wave at about +0.53 V toward DA which is more positive than those obtained using EPPG and BPPG electrodes with values of +0.29 and +0.43 V, respectively. Again it can be said that the multilayer graphene film presented a behavior comparable to that of the basal plane graphite showing however a lower density of edge plane defects which resulted in a slower electron transfer. Similar conclusions can be drawn by observing the response toward the oxidation of AA and NADH (Figure 5B,C). EPPG electrode with the highest density of edge plane sites showed the lowest oxidation potentials for AA and NADH with values of +0.40 and +0.61 V, followed by the BPPG electrode with values of oxidation potential of +0.71 and +0.81 V, respectively. These values are in agreement with previous reports.30,31 The ML-G-PET electrode showed a poorer electrochemical response, in both cases giving oxidation waves at about +1.05 and +1.08 V for AA and NADH, respectively.

ML-G-PET electrodes. This indicates that ML-G-PET electrode possesses a lower density of edge plane defects than the basal plane highly oriented pyrolytic graphite electrode (BPPG) which is generally adopted as reference material with a low level of edge plane defects. Using the method of Nicholson,28 the peak-to-peak separation can be correlated to the heterogeneous electron transfer rate constant (k0obs). EPPG, BPPG, and ML-G-PET electrode resulted with k0obs of 2.36 × 10−3, 4.6 × 10−8, and 2.8 × 10−10 cm s−1, respectively. The low value of electron transfer rate at ML-G-PET electrode is similar to reported values for basal plane graphite.29 The Ru(NH3)62+/3+ redox probe following an outer-sphere mechanism is influenced mainly by the density of electronic states and not by the microstructure of the electrode material. Figure 4B compares cyclic voltammograms recorded in the presence of 1 mM Ru(NH3)62+/3+ with EPPG, BPPG, and MLG-PET electrodes. It can be seen that ML-G-PET electrode showed a slightly larger peak-to-peak separation compared to EPPG and BPPG electrodes which have an almost overlapping voltammetric profiles. The heterogeneous electron transfer rate constant for EPPG and BPPG resulted in 1.7 × 10−2 cm s−1 while for ML-G-PET electrode resulted in 5.3 × 10−3 cm s−1. From the cyclic voltammetric measurements with Ru(NH3)62+/3+ it can be concluded that a slower electron transfer process occurs on the transferred graphene film attributable to a slightly lower density of electronic states. 2056

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Considering globally the responses for the multilayer graphene film, it can be said that from an electrochemical point of view it showed poor electrocatalytic properties with slow electron transfer kinetics and large overpotentials. This indicates that such type of material has no advantages over the commonly used EPPG or BPPG electrodes in particular for catalytic and biosensing applications. However, it should be noted that the electrochemical behavior of such graphene film resembles that of basal plane pyrolitic graphite with no edge plane defect sites. This can be seen by the slower electron transfer rate measured with Fe(CN)63−/4− redox probe in comparison to that of the BPPG electrode. BPPG electrodes normally used for fundamental electrochemical studies are known to possess a certain density of edge-plane sites which alter their electrochemical properties.32,33 This seems to be the most significant difference between the CVD grown graphene film and the BPPG electrode since the density of electronic states tested with Ru(NH3)62+/3+ probe resulted almost equal. Chemical or electrochemical activations can be performed to alter the density of electronic states as well as the density of edge-plane defects sites in order to improve electron transfer kinetics.

4. CONCLUSIONS We investigated in this work the electrochemical behavior of CVD grown multilayer graphene film transferred on a PET flexible substrate. The simple transfer process while ensuring a complete removal of the Ni catalyst foil preserves chemical and structural properties of the graphene film as demonstrated with Raman and XPS analysis. The electron transfer kinetics measured for the transferred graphene film resembled that of a defect-less basal plane graphite as observed by the slower electron transfer toward the Fe(CN)63−/4− redox probe compared to that of the BPPG electrode which is known to possess a certain density of edge-plane/defect sites. While there is no advantage in using this type of electrode material for electrocatalytic and biosensing applications which demand fast electron transfer kinetics, the CVD grown multilayer graphene film with no activation treatment performed may represent an interesting reference material for fundamental electrochemical investigations. Current investigations are now focused on optimizing different activation procedures to be performed in order to enhance the electron transfer kinetics of this type of electrode material for possible applications in electrocatalysis and biosensing.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by an NAP start-up fund grant (no. M58110066) provided by NTU. REFERENCES

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