Preparation of a Three-Dimensional Reduced Graphene Oxide Film

Article pubs.acs.org/Langmuir

Preparation of a Three-Dimensional Reduced Graphene Oxide Film by Using the Langmuir−Blodgett Method M. Musoddiq Jaafar,*,† Gustavo P. M. K. Ciniciato,∥ S. Aisyah Ibrahim,† S. M. Phang,‡,§ K. Yunus,∥ Adrian C. Fisher,∥ M. Iwamoto,⊥ and P. Vengadesh*,† †

Low Dimensional Materials Research Centre (LDMRC), Department of Physics, ‡Institute of Ocean and Earth Sciences (IOES), and §Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia ∥ Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, New Museum Site, CB2 3RA Cambridge, United Kingdom ⊥ Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT: The Langmuir−Blodgett method has always been traditionally utilized in the deposition of two-dimensional structures. In this work, however, we employed the method to deposit three-dimensional reduced graphene oxide layers using an unconventional protocol for the first time. This was achieved by carrying out the dipping process after the collapse pressure or breaking point, which results in the formation of a highly porous three-dimensional surface topography. By varying the number of deposition layers, the porosity could be optimized from nanometer to micrometer dimensions. Employed as bioelectrodes, these three-dimensional reduced graphene oxide layers may allow improved adhesion and biocompatibility compared to the conventional two-dimensional surfaces. A larger number of pores also improves the mass transport of materials and therefore increases the charge-sustaining capacity and sensitivity. This could ultimately improve the performance of biofuel cells and other electrode-based systems.

1. INTRODUCTION Graphene represents a new class of materials composed of a periodic array of carbon atoms densely packed in a very regular honeycomb two-dimensional (2D) structure1 first prepared by Brodie in 1859.2 However, the very first theoretical description of graphene was only reported in 2003.3 One year later, Geim and Novoselov successfully synthesized the 2D structure of graphene using a simple technique.4 Since then, graphene has been widely used due to its chemical and physical properties that come from its highly hybridized sp2 bonds, resulting in strong mechanical resistance as well as heat and electrical properties and high optical transmittance.5 Graphene also finds use in nonvolatile memory application,6 infrared optoelectronic sensors,7 thin film transistors,8 transparent solar cells,9 and biosensors.10 The mechanical properties and biocompatibility11−13 of reduced graphene oxide (rGO)-reinforced nanofiber mats have been identified to be an important feature for the development of electrodes for biosensors14−16 and biofuel cells.17 This particular advantage is related to the intrinsic threedimensional (3D) structure of graphene that may play a role in cellular interactions.12 The low surface energy and hydrophobicity play important roles in the interaction of biocatalysts with the electrode surface in the formation of the biofilm. Also, substrate topography such as pore size18−20 and surface roughness21−24 significantly influence cell behavior in the long run. Control of pore size that can range from the micrometer to nanometer scale may prove vital for cell © 2015 American Chemical Society

attachment. This could also be beneficial for biological microenvironments, which is very important for the health of cells in the biofilm. Many reports and methods for fabricating thin films of graphene oxide or rGO such as spin coating, dip coating, vacuum filtration, liquid−liquid assembly, and a chemical vapor deposition technique5 have been proposed. However, most procedures involve complicated and costly steps such as chemical processes, expensive and hazardous gases, high pressure, long deposition times, and catalysts that limit its use at the commercial level. The Langmuir−Blodgett (LB) assembly technique is wellknown as a powerful tool to assemble homogeneous and uniform 2D thin films.25 It presents the possibility of developing films of both organic26 and inorganic27 compounds with the desired structures and functionality on the molecular scale.28 It is also considered to be one of the most promising techniques for preparing nanometer-order thin films as it enables effective control of the monolayer thickness, large-area homogeneous deposition, and the possibility to deposit multilayers with varying layer composition.29 A typical surface-pressure isotherm profile for any material prepared by this method usually consists of three distinct regions ranging from gaseous, liquid, and solid states, which correlate to the molecular arrangement on top of the liquid subphase.30 Received: April 23, 2015 Revised: September 7, 2015 Published: September 8, 2015 10426

DOI: 10.1021/acs.langmuir.5b02708 Langmuir 2015, 31, 10426−10434

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Figure 1. Flowchart for the preparation of the rGO LB film. maintained in a class 1000 clean room. An aliquot of 1000 μL of rGO and methanol solvent was spread on a deionized water (DI) surface by using a microsyringe (±2 μL) to form a monolayer. Whatman filter paper with dimensions of 1.0 × 2.2 cm2 was attached to a tensiometer to monitor the surface tension during compression at a speed of 15.0 cm2/min. The monolayer was then left to self-stabilize for about 15 min. (iv) A tissue was wetted with methanol and carefully used to spread the methanol vapor when placed up to 0.5 cm from the monolayer surface to help reduce the clouded area of the rGO Langmuir film at the water surface. Upon compression, an isotherm graph was continuously recorded and monitored up to the target pressure. (v) At the target pressure of 15 mN/m, a glass substrate (2.5 × 2.5 cm2) was vertically dipped at a speed of 20 mm/min. If the dipping speed is faster than this, then the film will not attach effectively to the glass. (vi) The rGO monolayer was then transferred onto the substrate during the dip-coating process. (vii) The prepared substrate was dried in an oven overnight (8 h) at 80 °C to remove water and stabilize the adhesion of the rGO layer. Subsequent layer deposition was achieved after overnight air drying to prevent peeling off of the underlying rGO layers. This process was repeated for another 10 dipping times. 2.2. Characterization. A field-emission scanning electron microscope (FESEM, Quanta FEG-450 and Hitachi S-4500) was used to study the surface morphology and structure of rGO. An atomic force microscopy (AFM) and a scanning probe microscope−nanoscope in noncontact mode (AMBIOS v5.0.0) were used to study the thickness and porosity of the film, respectively. The transparency of the film was measured using UV−vis spectroscopy (PerkinElmer Lambda 750), while sheet resistance was measured by the four-point probe method (Jandel Universal Probe Station).

Overcompressing the monolayer leads to monolayer collapse or the breaking point, which results in uncontrolled multilayer formation. This is usually considered to be detrimental to film formation and as such is avoided during conventional deposition processes.25 Surprisingly, recent investigations in the development of rGO electrodes for biophotovoltaic (BPV) fuel cells31 in our laboratory yielded interesting result. By exploiting the traditional method of the dipping mechanism in an LB trough, the monolayer was compressed beyond the collapse pressure state. Interestingly, the resulting layers exhibit unique structures, and subsequent deposition at this stage provided a novel method of producing 3D rGO structures. We believe that this technique could be exploited to achieve highly biocompatible and porous 3D rGO films. In this report, a novel method of layer-by-layer assembly deposition at collapse pressure was carried out to successfully reengineer the micrometer- to nanometer-scale porosity dimensions of the rGO thin film.

2. EXPERIMENTAL SECTION 2.1. Experimental Method. The preparation of the rGO film using the LB method is presented step by step as shown in Figure 1: (i) 4.0 mg of high-surface-area rGO (specific surface area of 833 m2/g, Graphene Supermarket, USA) was mixed with 2.0 mL of methanol in a 5.0 mL vial. (ii) The vial was sealed and sonicated for 10 h, resulting in the formation of an rGO suspension. (iii) rGO LB film deposition was achieved using a round-type NIMA LB trough (model 2200) from NIMA Technology, U.K. The experimental environment was 10427


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Langmuir The electrochemical measurements were performed in a 30 mL single-compartment electrochemical cell containing three electrodes using a micropotentiostat (μAUTOLABIII/FRA2, Metrohm Autolab B.V., U.K.) connected to a computer. The 3D RGO film deposited with six deposition layers on glass was used as the working electrode, while a 5.0 × 5.0 cm2 platinum foil was used as the counter electrode. A silver/silver chloride (Ag/AgCl/Cl−) electrode immersed in a compartment containing saturated potassium chloride from Bioanalytical Systems, Inc., USA, was used as a reference electrode. The solution used for these experiments was a 0.1 M potassium phosphate buffer solution at pH 7.0 prepared with potassium phosphate monobasic and potassium phosphate dibasic trihydrate (SigmaAldrich, U.K.).

3. RESULTS AND DISCUSSIONS 3.1. Preparation of the rGO LB Film. Methanol (polar alcohol) was chosen because rGO tends to collapse and adopt a 3D compact confirmation in nonpolar solvents.32 It assists in the rapid spreading of surfactants on the water surface.33 The size of rGO used was varied, and sometimes the smaller flakes were observed to be attached to larger-area flakes, forming a cloudy area on the Langmuir film.33,34 To prevent this, methanol vapor was applied to the cloudy areas and helped to evenly disperse the rGO flakes, which could be due to the effect of nonpolar molecules with respect to the rGO flakes.32 The surface pressure−area isotherm recorded three major phase transitions from gaseous to liquid states (Figure 2). From

Figure 3. Surface pressure−area isotherm and elastic modulus of rGO at the air−water interface.

Figure 4. Surface morphology of one layer of the rGO film deposited at the solid state (a) and the breaking point (b).

At stage D, density increasing with compression beyond the close-packed stage results in collapse, creating multilayer formation and wrinkling. The wrinkling effect is observed to be more significant than in the solid state as shown in Figure 4(b). The manipulation from 2D structures into the 3D film can be carried out at this point by performing the dipping process at this collapse state followed by layer-by-layer deposition. However, multilayer deposition cannot be carried out immediately after the first dip since the wet rGO film tends to peel off of the glass substrate upon insertion or withdrawal into the water subphase. This is because rGO flakes still contain water and tend to spread back from the substrate into the water following its natural polar behavior if it was dipped immediately. After overnight drying at 80 °C, deposition of the second layer can be carried out, as the adhesion of the rGO film will increase after the heat treatment. This process of drying and deposition was repeated for each subsequent layer formation until the required number of multilayers is achieved. Even though the surface pressure mean molecular area isotherm profile did not show a significant difference between the breaking point and the solid state or show a crashing pattern, the rGO layer on top of the water can be seen clearly to collapse at certain pressure using the naked eye. Furthermore, It was shown that the value of the intermolecular force between molecules could be extrapolated from the isotherm graph as shown by the isotherm graph in Figure 3.30 From the intermolecular force pattern, it can be clearly shown that the breaking point occurs around 12.5 mN/m and above or in region D in Figure 3. The values obtained in this work also agree well with Yang et al., who also reported a similar surface pressure versus area graph.34,37,38 3.2. Surface Morphology Studies. rGO flakes were obtained from Graphene Supermarket USA (average particle (lateral) size about 3−5 μm and specific surface area 833 m2/g)

Figure 2. Surface pressure mean molecular area isotherm showing the four stages of compression: gaseous state (A), liquid (B), solid states (C), and breaking point (D).

the beginning of stage A, the monolayer exists in gaseous states, and the compression of the surface pressure remains constant. The pressure starts to increase (stage B) as the rGO layers begin to touch each other and intermolecular forces become apparent, increasing the monolayer density at the water surface. There are two fundamental types of geometric interactions between the layers: edge-to-edge and face-to-face.34 Upon continuous compression, the liquid phase undergoes a transition to an even higher density and the monolayer reaches the solid state (stage C) as the rGO flakes appear to be in closepacked arrangements. The uniformity of the film can be observed in Figure 4(a), which shows a wrinkle that normally is observed in any graphene35 or graphene oxide36 films prepared using the LB method. 10428


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Langmuir During compression at the critical point, the electrostatic repulsion between 2D layers34 creates wrinkles in the rGO flakes and can be observed by the FESEM image in Figure 6(a). Capillary and gravitational forces also contributed to the wrinkle effects when water evaporates during the drying process (Figure 5).33 While transferring onto the glass substrate,

Figure 6. FESEM and AFM images of rGO sheet deposited on a glass surface: (a) wrinkle effect of rGO, (b) wrinkle effect after thermal annealing, (c) low magnification of one deposition layer, and (d) AFM image of the wrinkle effect with a thickness of 6.18 nm.

flat, rippled, standing collapsed, folded, overfolded, and scrolled.37 This may affect the average roughness of the sample. The compression of rGO flakes on top of water will make the flakes crumble and wrinkle. Upon drying, the wrinkle dimension becomes smaller. The thickness not only is attributed to multilayer deposition but also is due to the wrinkle effect, which increases the thickness as a result of ripple and mountainlike structures on the flakes. After the second deposition, the roughness and thickness increased to 197.90 and 219.90 ± 13.22 nm, respectively. The surface morphology of the film (Figure 7(a)) was observed to be more porous and rougher. Further layers of deposition show quite predictable

Figure 5. Schematic diagram explaining the occurrence of the wrinkling effect due to capillary and gravitational forces.

molecules of water also adhere to the glass, allowing the rGO monolayer to float as shown in Figure 5(a). Water provides a higher surface area prior to drying, which results in more space for the flakes to float and rearrange themselves more uniformly as illustrated in Figure 5(b). However, after complete drying, rGO compresses and creates wrinkles on the nanoscale due to insufficient space and gravitational rearrangement33 as illustrated in Figure 5(c), resulting in surface porosity as seen in the FESEM image in Figure 4(b). A highly-porous rGO film (Figure 6(c)) can be instrumental in the attachment of biomaterials such as deoxyribonucleic acid (DNA), enzymes, and protein molecules.39−41 The uniformity of the micrometer-scale film porosity was observed to be prominent probably due to the manipulation of the compressive pressure. The AFM image in Figure 6(d) clearly shows the 3D wrinkle structure, which results in an increase in the average thickness of a single flake of the graphene film to 6.18 nm compared to the theoretical value, which is 0.5 nm.34 The wrinkle effect can be reduced (Figure 6(b)) by means of thermal annealing at 350 °C for 20 h under vacuum conditions to achieve a flatter film for other applications.8,42,43 Layer-by-layer deposition using the LB technique is capable of producing high porosity and controlling the roughness of rGO films from nano- to macroscales. The average thickness of the rGO LB film after one deposition increased to 89.5 ± 4.43 nm while the mean roughness of the film was 87.4 nm. Compared to the single flakes, the average thickness increases due to patterns of wrinkling over different areas, which could be

Figure 7. FESEM images of multilayer rGO films show (a) 2, (b) 4, (c) 6, and (d) 10 depositions. 10429


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The pores were also well distributed and may be a factor in the successful encapsulation of the Chlorella sp. algae or any other biomaterials of similar size. The cross-sectional view in Figure 8(b) shows the formation of six depositions of rGO film on top of glass. Figure 8 clearly shows the 3D surface morphology structure with the porosity measured at around 1.207−3.810 μm with an average pore diameter of 2.147 um.31 The lateral specific surface area was 833 m2/g (Graphene Supermarkat, USA). 3.3. Comparison of Optical and Electrical Properties. Comparisons of optical and electrical parameters between different numbers of rGO layers demonstrate very similar patterns (Figure 9(b)). The sheet resistance was improved as the thickness increased, but as expected, it lowers the optical transmission. Sheet resistance values of the rGO film were measured in the range of 2.63 × 107 to 3.03 × 105 Ω/sq for a film thickness of 6.18 to 2700 nm. The presence of functionalized groups in graphene reduced the electron mobility, resulting in insulating behavior.47 The conductivity was improved by increasing the number of layers. At 6.18 nm, the sheet resistance value was 2.63 × 107 Ω/sq with 56.66% transmittance. Higher numbers of deposition layers or thickness registers increase and decrease the sheet resistant and transmittance values, respectively. For example, 555 nm (or four layers of deposition) records 438 × 106 Ω/sq and 37% corresponding to the thickness and transmittance, respectively. However, for six deposition layers and above, the values of transmittance and sheet resistance were almost saturated. At 6 deposition layers, transmittance values reach 5.4%, while for 8 and 10 deposition layers it was 2.4 and 1.9%, respectively. Corresponding sheet resistance values for 6, 8, and 10 deposition layers are 4.51 × 105, 3.07 × 105, and 3.03 × 105 Ω/sq, respectively. Due to the deposition at the breaking point of the isotherm graph, six deposition layers of 3D rGO creates a pore size similar to that of UMACC 313 Chorella sp. algae as shown in our earlier work. The work reported good biocompatibility properties, which was instrumental in improving the current density during the light cycle by approximately 120% compared to when it was used with conventional electrode material, indium tin oxide (ITO).31 3.4. Surface Chemistry. The carbon/oxygen ratio according to the manufacturer is 10:5. The G band of Raman spectra (Figure 10) shows the sp2-hybridized carbon-based material. The D band was stimulated because of the participation of the double-resonance Raman scattering near the K point of the Brillion zone, which indicated the presence of functional groups. The presence of the D band is important because functional groups are needed for the detection and attachment of DNA, enzymes, hormones, and biomaterials for colorimetric biosensors. The 2D band is an indicator of crystalline graphitic material and is stimulated because of the π band in the graphitic electronic structure. Furthermore, the D + G combination peak is prompted by disorder.48 3.5. Electrochemical Properties. As discussed previously, the scope of this research is focused on the development of new material for the construction of bioelectrodes. These electrodes will be modified with active biocatalyst to be applied in the area of biosensors, as well as for biocathodes or bioanodes in biological fuel cells. For this reason, it is important to investigate the electrical characteristics of the material developed along with the electrochemical properties that govern the interface between this material and the electrolyte. The latter properties are responsible for the diffusion of species

values, as the slope of the graph was comparable for each different time of deposition, which shows almost a straight line. As such, it can be concluded that the roughness of each new layer is predictable. Hence the roughness of the six depositions was 489.4 nm with the thickness at 2.24 ± 0.08 μm. For six depositions, micrometer-scale porous structures could be clearly observed as shown in Figure 7(c). This dimension fits various micrometer-sized biomaterials and organisms such as chloroplasts, algae, and most bacteria. At greater numbers of layers such as 8 deposition layers and above, the roughness reaches the submicrometer scale (1.1 μm) with a thickness of 6.40 ± 0.06 μm, while 10 layers of deposition equals about 1.6 μm. As can been seen in Figure 7(d), 10 depositions of rGO on top of glass produces larger roughness and porosity values, on the micrometer scale (4 to >10 μm). Hence, the manipulation of layer by layer deposition might be useful for targeting the desired porous and roughness structures for different applications. Overall, thickness values in Figure 9(a) do not follow a linear trend and become unpredictable due to overcompression of the LB barrier. Even though the barrier compression at the collapse state was maintained at a constant speed, control of the layer thickness was unpredictable due to different wrinkling mechanisms at different dipping times. The complex 3D structures show interconnected pores, creating microscale cavities suitable for securely harboring biomaterials within the porous cavities, unlike graphene films with relatively smooth 2D surfaces.12 Two-dimensional structures of graphene thin films have narrow bacteria loading capacities, and the stacking between individual sheets largely places the high intrinsic specific area of graphene at a disadvantage.44 The 3D structures of rGO meanwhile aid cellular communication, the transportation of oxygen and nutrients, the removal of waste, and cellular metabolism more competently than does a 2D graphene film.12 Besides that, a lot of evidence suggest that the high-porosity film platform demonstrates a remarkably positive difference in adhesion and structure for growing biomaterials.45 Furthermore, for microbial fuel cell applications, the 3D porous rGO anodes provide a larger surface area to interface with biomaterials compared to the 2D structures. These latter structures may include low specific surface area due to very small pore sizes for bacterial penetration, poor conductivity, disruption of the bacterial membrane, or any sharp nanomaterials.46 The LB technique employed in this work does not contribute to such negative influences because in this case the roughness can be controlled from submicrometer to micrometer dimensions. A greater number of layer depositions, for example, 6 to 10 layers, enable the formation of macrosized porosity. For rGO with six depositions as shown in Figure 8(a), the porously structured nature of the film can be clearly observed.

Figure 8. FESEM images of higher magnification of multilayer rGO films (six depositions) (a) and the cross-sectional view of the film (b). 10430


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Figure 9. Graphs showing a comparison of surface roughness and thickness (a) and transmittance spectra and sheet resistance (b) for different deposition times.

Figure 11. Cyclic voltammograms of the 3D rGO in a 0.1 M potassium phosphate buffer solution at pH 7.0 running under a scan rate of 1 mV·s−1.

potential window of 900 mV is the region of interest for the development of biosensors since the influence of the material is minimized, and the obtained signal can be entirely related to the interaction between the biocatalyst and the analyte of interest. The electrode surface acts exclusively as a current collector or as an electrochemical potential probe. Kuyilazhagan et al. suggests that the oxygen groups present at the edges of graphene influence the electric double layer in graphene and similar materials.49 These oxygen groups enhance the accessibility to the hydrophilic surface in aqueous electrolytes. From the two extremes outside of this quasi-rectangular region, it is possible to observe the leakage of current starting from the double-layer region, where the potential is high enough for electrochemical reactions to happen. The current obtained in this case is designed to be Faradaic. At potential values around −100 mV vs Ag/AgCl/Cl−, it is possible to observe the appearance of a first reduction peak. It was demonstrated for a similar case with electrochemically active rGO that this Faradaic current is typically related to the electrochemical oxygen reduction reaction on the surface of rGO. Carbon materials typically present quinone functional groups that are responsible for this partial reduction reaction involving only two electrons per molecule of oxygen, resulting in the formation of peroxide. However, rGO presents additional functional groups, such as hydroxyl, carboxylic, and carbonyl groups, that may catalyze the further reduction of the formed peroxide at more negative potentials.50 In fact, a second step in the reduction of the molecular oxygen at potentials of around −520 mV vs Ag/ AgCl/Cl− has been observed, confirming the capacity of the 3D

Figure 10. Raman spectra for the deposited rGO film.

from the bulk solution to the electrode surface and the subsequent adsorption with the following electrochemical reaction of these species. Cyclic voltammetry experiments were conducted with the 3D rGO film deposited on glass as the working electrode in a threeelectrode electrochemical cell. The material obtained with six deposition layers was chosen due to the combination of having both better stability and low resistance in comparison to the other. The experimental data shows that the shape of the current−potential curves reaches a pseudosteady state after the 3rd cycle; therefore, the 10th cycle was chosen for representation. Figure 11 presents the results from the experiment performed at a very slow scan rate (1 mV·s−1). From Figure 11, it is possible to observe a quasi-rectangular region (represented by the dashed lines) when the potential window is maintained from approximately −100 to 800 mV vs Ag/AgCl/Cl−. The distortion in the quasi-rectangular region as observed by the bend in the curves results from the material’s electrical resistance. This represents the region where no electrochemical reactions occur with considerable intensity, and the dominant effect is the charging of the interfacial double layer resulting from the adsorption and reorganization of water molecules and charged species present in the electrolyte due to the applied potential. The measured current in this case is designed to be purely capacitive. The symmetrical shape indicates characteristics of ideal capacitive behavior. This 10431


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Figure 12. Cyclic voltammograms of 3D rGO in a 0.1 M potassium phosphate buffer solution at pH 7.0 with changing (a) scan rate and (b) scan size at a fixed scan rate of 10 mV s−1.

capacitance behavior can be artificially made by annealing graphene at high temperatures to promote the removal of water molecules from the material interface. But differently from what is presented in the literature, our material can present the same behavior without having the decomposition of oxygen functional groups resulting from the high-temperature treatment, which might decrease the lifespan of the electrode. A similar conclusion can be obtained from Figure 12(b), where the increase in the scan size leads to an increase in the concentration of ions at the surface of the electrode, resulting in an increase in the dielectric capacitive current. Furthermore, the symmetry of the curves even over a very long range of potentials, from −2.5 to 2.5 V vs Ag/AgCl/Cl−, suggests that the charge and discharge of the material happen at a pseudoconstant rate. These results suggest that the developed 3D rGO presents the possibility of application as a material for low-temperature capacitors, where the presence of pores improves the mass transport of materials present in the solution and increases the amount of charge that the material can hold. Also, the improved mass transport can increase the amount of substrate to be consumed by biocatalysts immobilized or adsorbed on the 3D rGO surface, leading to an increase in its performance as a bioelectrode for biofuel cells as well as an increase in the sensitivity of the material as a biosensor.

rGO to fully reduce the molecular oxygen through a two-step reduction involving two electrons each. On the other extreme region of the double layer, at values of potential around 800 mV vs Ag/AgCl/Cl−, it is possible to observe the leakage of current related to an oxidation reaction occurring on the surface of the 3D rGO. The exponential increase in the Faradaic current, rather than the appearance of a peak-shaped oxidation, indicate that this reaction does not involve the mass transport of any species present in the solution. Instead, it appears to be related to the oxidation of the material itself or even the oxidation of the solution (electrolysis). The fact that successive cycles of 3D rGO do not present any observable change in this region suggests that the material itself is electrochemically stable and is not consumed or damaged. Wang et al. using a hydrated graphene oxide film as a dielectric spacer for capacitors showed that this oxidation is related to a voltage-induced dehydration, which can in turn reduce the gap in the graphene oxide and facilitate electron hopping between adjacent graphene oxide sheets. Here, the capacitive current of this hydrated film starts to be partially transformed to resistive current due to the material polarization.51 This result suggests that the transport of electrolyte plays an important role in the electrochemical surface availability and activity of 3D rGO. With an attempt to study the influence of the electrolyte within the formation of the electrochemical double layer, experiments of cyclic voltammetry changing the scan rate and the potential range were performed with 3D rGO, and the results obtained are presented in Figure 12. From this figure, it is possible to observe that increasing the scan rate causes the cyclic voltammograms of 3D rGO to become skewed, resulting in the change in the capacitance of the electrochemical double layer. This represents the typical electrochemical behavior of porous materials.52 At a low scan rate, ions present in solution can easily diffuse into almost all available spaces on the material surface. At a high scan rate, ions (mostly cations) can approach only the outer surface of the electrode, and the material embedded in the inner space makes a smaller contribution to the capacitance behavior, thus leading to a deviation from ideal capacitance.53 This mass transport differentiation of the ions present in the solution to the pores causes changes in the way the double layer is charged, and this difference causes the interface of the material to behave as a dielectric interface.51 This dielectric

4. CONCLUSIONS A novel method for fabricating the 3D rGO film was achieved by exploiting the traditional method of LB assembly. This was achieved by means of unconventional dipping at the breaking point of the Langmuir surface−pressure profile, which was able to optimize the porosity from nanometer to micrometer scales during layer-by-layer deposition. These biocompatible 3D rGO film electrodes could be useful in the development of bioelectrodes, biosensor platforms, materials for low-temperature capacitors, and others. The work demonstrated an effective and cheaper way to customize the 3D porosity profile compared to other conventional methods.

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The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistances provided by FRGS (FP004-2013A), UMRG (RG321-15AFR), PPP (PG111-2014A), CAPES ́ (Coordenaçaõ de Aperfeiçoamento de Pessoal de Nivel Superior), and the Ministry of Education of Brazil (Process 9275/13-4) grants are greatly appreciated. We also acknowledge the Campus for Research Excellence and Technological Enterprise (CREATE) in Singapore for research support.

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