New Insights into the Electronic Transport of Reduced Graphene

Letter pubs.acs.org/JPCL

New Insights into the Electronic Transport of Reduced Graphene Oxide Using Scanning Electrochemical Microscopy Tiphaine Bourgeteau, Steven Le Vot, Michael Bertucchi, Vincent Derycke, Bruno Jousselme, Stéphane Campidelli,* and Renaud Cornut* CEA Saclay, IRAMIS, NIMBE, LICSEN, Bat. 466, Gif-sur-Yvette, Cedex F-91191, France S Supporting Information *

ABSTRACT: The present work investigates the electronic conduction of reduced graphene oxide flakes and the coupling between flakes through a combined SECM (scanning electrochemical microscopy), AFM, and SEM analysis. Images of individual and interconnected flakes directly reveal the signature of the contact resistance between flakes in a noncontact and substrate-independent way. Quantitative evaluation of the parameters is achieved with the support of numerical simulations to interpret the experimental results. The interflakes contact resistance importantly impacts the transport of electrons, which can be anticipated as a key parameter in r-GO-based materials used in fuel cells, lithium batteries, supercapacitors, and organic electronic devices.

SECTION: Energy Conversion and Storage; Energy and Charge Transport contact resistance between flakes is available. Scanning electrochemical microscopy (SECM) is a method that permits to image and characterize surfaces without any contact.15−19 The distinctive feature of SECM is its ability to investigate the local electrochemical response and reactivity of a wide range of substrates. In particular, we and others have investigated r-GO films or flakes, and their ability to transport electrons or exchange charges with the solution was imaged.20−23 SECM24 and SECM variants25,26 have been used to investigate the interfacial behavior of the parent material, graphene. These studies focused on the charge transfer kinetics, while the present work investigates the electronic transport. Herein, we show how SECM can be used to investigatein situ and without contact associated with the measurementthe electronic conduction of individual and interconnected r-GO flakes. The method is based on the measure of the microelectrode current at different redox mediator concentrations: if the substrate underneath the probe (r-GO, here) is able to transport electrons at the appropriate level, the response is the signature of the complex electronic pathways at the substrate. With the support of numerical simulation, the different contributions can be isolated. In particular, the intrinsic resistance and the contact resistances between the objects can be individually evaluated. As a starting point of the study, we investigated a simple situation, that is, that of individual flakes showing no overlap.

T

he future of energy supply depends on innovative breakthroughs regarding the design of efficient systems for the conversion and the storage of energy. In this context, porous electrodes and transparent electrodes are intensively investigated because they represent the functional entity of a large diversity of energy conversion and storage devices.1,2 Porous electrodes are complex systems made out of multiple components: conductive additive, binder, pores filled with electrolyte, and so on, in addition to the active centers. Among different nanomaterials, graphene and graphene analogues such as graphene oxide (GO) or reduced-GO (r-GO) are very promising building blocks for transparent electrodes3−5 and for porous electrodes6−8 used in supercapacitors,9 fuel cells,10 or lithium batteries.11 Unlike graphene, whose synthesis and processing require expensive setups and treatments, GO can be produced very easily in large amounts and at a low cost.12,13 GO is an insulating material, and to recover some of the outstanding properties of graphene, a reduction of GO into rGO is usually performed. Unfortunately, r-GO exhibits a lower electronic conductivity than graphene, and this may be detrimental for the final performances of porous electrodes or transparent ones. This aspect may be specifically investigated with individual objects or films deposited on a substrate. Typically, r-GO flakes are imaged by SEM and AFM and then contacted with electrodes fabricated by lithography to measure their conductivity.14 Unfortunately, the accuracy of the measurement is limited, notably because of the presence of contact resistances between the monolayer and the metal. This complicates the interpretation, and no information on the © XXXX American Chemical Society

Received: October 21, 2014 Accepted: November 13, 2014

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same r-GO flakes and shows that the flake has a uniform thickness of ca. 1.5 nm, as expected for a monolayer of r-GO.31 The SECM image (Figure 1c) was obtained using the SECM feedback mode32 with ferrocene (Fc) as redox mediator and using N,N-dimethylformamide as solvent. This solvent was chosen to achieve high redox mediator concentration without saturation issues as well as for its very slow evaporation speed. In the feedback mode, the measured signal, namely, the probe current, comes from the regeneration at the substrate of the species consumed at the probe. Figure 1c shows that when the probe is located above the flake the current is raised by 60% up to 4.1 nA due to the redox mediator regeneration at the substrate. (The current value above the bare substrate, SiO2/Si, is 2.5 nA at [Fc] = 3.5 mM and corresponds to the so-called “negative feedback current”, Ineg, which comes from lateral diffusion of the redox mediator occurring directly and only from the solution to the probe.33 This signal is used to evaluate the probe substrate distance). In fact, the r-GO flake is clearly visible in Figure 1c because of a local enhancement of the electronic conduction.20,34−36 Figure 1c also shows that the SECM response of the flake at the top left corner is higher. The SEM image taken at lower magnification over the entire area (see Figure S1 in the Supporting Information) shows that this flake is larger. This explains the higher current in SECM. The feedback reaction at the substrate is necessarily associated with a counter reaction occurring elsewhere on the substrate, as depicted in Figure 1d. A larger flake provides a larger area for the counter reaction, and the reactant supply is thus enhanced.37 The fact that the total size of the conducting zone impacts the measure illustrates that advanced SECM investigations can provide information on complex electronic pathways, as detailed in the following. For this purpose assemblies of r-GO flakes were investigated as presented in Figure 2. Figure 2a,b shows SEM and SECM images of the same group of flakes, respectively. The SECM image was obtained using a low redox mediator concentration, namely, 0.04 mM of ferrocene (leading to a probe current measured far from the substrate, Iinf, equal to 0.07 nA). It shows the relative current increase normalized to the bulk current (I − Ineg)/Iinf. The SEM image (Figure 2a) allows us to verify that the flakes are monolayered and to evaluate the exact geometry of each element. It also permits the visualization of overlapping between flakes when occurring thanks to a clear change in the contrast. Here also it can be noticed that for the SECM image (Figure 2b) the response varies among the flakes. The large flake (α) in the center shows the highest response, with a

Different imaging techniques were used: Figure 1 shows SEM (panel a), AFM (panel b), and SECM (panel c) images of the same r-GO flake deposited on p-doped silicon wafer (oxide thickness ca. 150 nm).

Figure 1. (a) SEM at 4 kV and (b) AFM (tapping mode). Inset: height profile used for the evaluation of the flake thickness. (c) SECM images of the same r-GO flake. SECM color bar: Current probe (nA). Probe active radius: rT = 5 μm. Redox mediator concentration ([Fc]): 3.5 mM. (d) Scheme presenting the different steps involved in the feedback loop between the probe and the microelectrode.

Graphene oxide was prepared using the standard Hummers method followed by spontaneous exfoliation in water.27,28 It was then deposited on a substrate using the bubble deposition method,29 then reduced using hydrogen iodide (HI) vapors. (See the Supporting Information (SI) for more details.)30 Remarkably, the established preparation procedure produces extremely large (up to 5000 μm2) and unwrinkled r-GO flakes. SEM analysis enables us to detect superimposed GO or r-GO flakes through contrast evolution.28 Such a contrast variation can be seen on the top and bottom of the central flakes in the SEM images (Figure 1a and Figure S1 in the Supporting Information). It is caused in this case by the folding of some extremities of the flake and not by the presence of multilayered r-GO. Figure 1b displays an AFM (tapping mode) image of the

Figure 2. Image of an agglomerate of r-GO flakes deposited on Si-SiO2 substrate obtained by (a) SEM at 15 kV and (b) SECM. [Fc] = 0.04 mM, rT = 5 μm. The color scale represents the probe current increase normalized to the bulk probe current (0.07 nA). (c) Schematic representation of the electronic pathway occurring during SECM feedback measurements when the flakes are in contact. 4163

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Figure 3. SECM response obtained with a redox mediator concentration of (a) 0.25, (b) 0.8, and (c) 15 mM. The color scale represents the probe current increase normalized by the bulk probe current (Iinf) ((a):0.44, (b): 1.28, and (c): 25 nA).

Figure 4. Experimental data extracted from Figure 3 (dots) and theoretical results (lines) showing the current increase as a function of the redox mediator concentration. (a) (flake γ): green curve: R■ = 300 MΩ, black curve: R■ = 30 MΩ, pink curve R■ = 3 MΩ. (b,c) (respectively, flakes α and β): R■ = 30 MΩ with red curve: no contact resistance between flakes, black curve: Rcontact = 150 MΩ, blue curve: no contact at all. The theoretical results have been obtained for d = 4 μm, rglass = 75 μm, and rT = 5 μm.

normalized current increase equal to 0.95. Two smaller flakes β and β’ overlap the flake α, and their high response level, similar to that of the flake α, can be explained by their connection to the flake α, which provides a larger area for the counter reaction to take place, as depicted in Figure 2c. Isolated flakes (for example, γ) do not benefit from such a connection, and their response shows a lower current (as depicted by the green color). Thus, SECM experiments are directly able to detect the overlaps enabling an electrical contact between the flakes. The following shows how the associated resistance can be evaluated. The same flakes as in Figure 2 were imaged with different redox mediator concentrations, as presented in Figure 3. Increasing the redox mediator concentration leads to a decrease in the SECM response: the maximal current variation normalized to the bulk current goes from 0.95 (Figure 2b) at low concentration to 0.22 (Figure 3c) at the highest concentration. Indeed, increasing the redox mediator concentration increases the solicitation, meaning that the substrate must transport more electrons to efficiently perform the feedback loop. The fact that the response is depending on the redox mediator concentration is not compatible with exclusive limitation by the charge-transfer kinetics20 and confirms that the ability of the assembly to conduct the electrons is impacting the current response. In the following, a fast charge transfer has been considered, and verified, in accordance with the outer sphere nature of the electron-exchange process that occurs with ferrocene. The results presented in Figure 3 show that the response of each flake is evolving differently. In particular, Figure 3 shows that upon increasing the redox mediator concentration, the small flake β, which has a high current response at low concentration (Figure 2b), tends to have a medium response

level at intermediate concentration (Figure 3a,b). This is the experimental proof that upon concentration increase, the electronic communication between the flakes α and β starts to be a limiting factor. At the highest concentration (Figure 3c), the response of all of the flakes tends to be similar: the solicitation is such that the measure is dominated by the conductivity in the immediate vicinity of the probe, and the domain size or the interconnections have little impact on the results. To interpret quantitatively these observations, we performed numerical simulations with the software Comsol Multiphysics and compared them with the experimental results. The result is shown in Figure 4, with the presentation of the experimental and theoretical normalized current increase as a function of the redox mediator concentration. The calculations involved a similar equation system as the one developed in ref 20 (local Ohm’s law at the substrate and diffusion inside the solution)20 but with the specific geometry extracted from the SEM image (see the SI for details). The theoretical results were obtained for d = 4 μm, rglass = 75 μm, and rT = 5 μm. The working distance was determined using the response level measured above the naked substrate and compared with the theoretical negative feedback response. First of all, the evolution of the current above the flake γ, which is isolated from the other flakes, can be interpreted without any consideration of the contact resistance. This is shown in Figure 4a. A good fit between experimental and theoretical results is obtained for R■ (= 1/σe, with σ the conductivity and e the thickness of the flake) approximately equal to 30 MΩ (a change in the intrinsic square resistance R■ results in a lateral translation of the theoretical curves, as illustrated in Figure 4a). This value, combined with the thickness of the flake as evaluated by AFM (see Figure 1b), 4164

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leads to a conductivity equal to 0.2 S cm−1, in agreement with the results obtained in the literature by deposition of metallic electrodes on the flakes.14 Next, the response evolution of flakes α and β has been analyzed with the support of numerical simulations, considering both the intrinsic and the contact resistances between the flakes. (See the SI.) The results are presented in Figure 4b,c. The intrinsic resistance of the flakes determines the concentration where the response is decreasing, similarly as for flake γ, while the low concentration response level of flake β is governed by the contact resistance. As shown in Figure 4b, these parameters have little impact on the low concentration response of flake α. The satisfying fit that is obtained there confirms that a fast charge transfer is occurring between the solution and the sample. Then, by adjustment of the simulation parameters with the experiments, it turns out that the intrinsic resistance of the interconnected flakes is the same as that observed with the isolated flake γ (30 MΩ) and that the contact resistance between the flakes α and β is about five times larger, equal to 150 MΩ. This indicates that the interflakes contact resistance importantly impacts the transport of electrons in rGO-based materials. This may be the source of performance limitations for both transparent electrodes and porous ones. In summary, we showed how scanning electrochemical microscopy measurements can readily provide very diverse information regarding the electronic conductivity of reduced graphene oxide flakes. In particular, thanks to the support of numerical modeling, the signature of the interflakes contact resistance can be unambiguously evaluated, apart from the other contributions that are the intrinsic conductivity and the size of the conducting areas. In a future work, we are going to use this method to investigate the impact of different posttreatments on the electronic conduction of the flakes, with a precise quantification of both the intrinsic resistance and the contact resistances.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and numerical simulation details and an additional SEM image are presented in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.C.). *E-mail: [email protected] (R.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the French ANR through the COPEL project (ANRPDOC2011-Copel). We also thank the Labex Nanosaclay for partial support through the FIGARO grant.

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