Dispersion Stability and Surface Morphology Study of

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Cite This: J. Phys. Chem. C 2019, 123, 15122−15130

Dispersion Stability and Surface Morphology Study of Electrochemically Exfoliated Bilayer Graphene Oxide Zhenyuan Xia,*,†,‡ Giulio Maccaferri,‡,§ Chiara Zanardi,‡,§ Meganne Christian,∥ Luca Ortolani,∥ Vittorio Morandi,∥ Vittorio Bellani,⊥ Alessandro Kovtun,‡ Simone Dell’Elce,‡ Andrea Candini,‡ Andrea Liscio,‡,# and Vincenzo Palermo†,‡ †

Industrial and Materials Science, Chalmers University of Technology, 41258 Göteborg, Sweden Istituto per la Sintesi Organica e la Fotoreattività, CNR, 40129 Bologna, Italy § Department of Chemical and Geological Sciences, Università di Modena e Reggio Emilia, 41125 Modena, Italy ∥ Istituto per la Microelettronica e Microsistemi, CNR, 40129 Bologna, Italy ⊥ Dipartimento di Fisica, Universitá degli Studi di Pavia, 27100 Pavia, Italy # Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, 00133 Roma, Italy

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S Supporting Information *

ABSTRACT: During the last decade, electrochemical exfoliation of graphite has aroused great interest from both academia and industry for mass production of graphene sheets. Electrochemically exfoliated graphene oxide (EGO) features a much better tunability than chemically EGO, or even than graphene obtained with ultrasonic exfoliation. Chemical and electrical properties of EGO can be modified extensively, thanks to its step-controllable oxidation process, varying the electrolytes and/or the applied potential. It is thus possible, using tunable electrochemical oxidation, to produce low-defect EGO sheets, featuring both good electric conductivity and good dispersibility in common solvents (e.g., acetonitrile or isopropanol). This greatly facilitates its application in several fields, for example, in flexible electronics. In this work, we correlate the dispersion behavior of EGO with its chemical properties using the relative Hansen solubility parameter, zeta potential values, X-ray photoemission spectroscopy, and Raman analysis. A surface morphology study by atomic force microscopy and transmission electron microscopy analyses also reveals that EGO sheets are multiple structures of (partially oxidized) graphene bilayers. Conductive EGO films could be easily prepared by vacuum filtration on different substrates, obtaining electrical conductivity values of up to ∼104 S/m with no need for further reduction procedures.

1. INTRODUCTION Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has attracted tremendous interest in the academic and industrial communities in the last 10 years. This two-dimensional carbon material exhibits unique properties, such as extremely high surface area, excellent electrical and thermal conductivity, and outstanding mechanical strength. Nowadays, numerous strategies based on bottom-up or topdown synthesis approaches have been extensively explored to obtain graphene sheets. High-quality graphene can be synthesized via bottom-up approaches, which use surfaceassisted coupling of molecular precursors on metal catalyst substrates by chemical vapor deposition and epitaxial growth on the wafer scale.1,2 The large-grain size graphene prepared by bottom-up approaches is extremely valuable for flexible transparent conductors or field-effect transistors in the electronics industry. On the other hand, top-down approaches enable direct exfoliation of natural or synthetic graphite to single or few-layer graphene in the liquid phase. Several of the top-down approaches such as liquid sonication, high-shear © 2019 American Chemical Society

mixing, reduction of graphite oxide, and electrochemical exfoliation have been employed for the bulk synthesis of graphene sheets on the gram-scale and even the kilogramscale.3−7 For top-down approaches, a stable suspension of graphene sheets is critical to the successful application of graphene in printed electronics, conductive coatings, and functional composites. However, the low colloidal stability of pristine graphene results in a poor dispersion in common organic solvents because of the small mixing entropy gain and significant van der Waals forces between graphene sheets.8,9 Sonication-assisted liquid-phase exfoliation (LPE) has been employed to obtain pristine graphene sheets in certain organic solvents. Nevertheless, the most effective solvents such as 1,3dichlorobenzene (DCB, Tb = 181 °C) or N-methyl-2pyrrolidone (NMP, Tb = 203 °C) are carcinogenic and Received: April 11, 2019 Revised: May 17, 2019 Published: May 22, 2019 15122

DOI: 10.1021/acs.jpcc.9b03395 J. Phys. Chem. C 2019, 123, 15122−15130

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The Journal of Physical Chemistry C

(ACN), Tb = 82 °C, or isopropanol (IPA), Tb = 83 °C], conductive EGO films could be easily prepared by vacuum filtration on different polymer substrates, obtaining electrical conductivity values of up to ∼104 S/m.

nonvolatile with high boiling points. Besides this, graphene obtained by LPE suffers from low yield and small lateral size.8,10 Pristine graphene sheets can also be dispersible in aqueous solution by the addition of certain aromatic/ nonaromatic surfactants, such as sodium dodecyl benzene sulfonate or sodium chlorate.11,12 However, the presence of such insulating stabilizers is not desirable for most electronic applications. Therefore, it is still a major challenge to obtain the stable dispersion of graphene in environmentally benign, easily-processable, and low-boiling point solvents. Unlike pristine graphene, graphene oxide (GO), a fully oxidized single-layer graphene sheet, is a widely-used solutionprocessable 2D material. Oxygen functional groups such as hydroxyl, carbonyl, and carboxyl groups decorated on the graphene surface make GO strongly hydrophilic and easily dispersed in aqueous solutions.13,14 The electrically insulating GO sheets can be partially reduced to give conductive reduced GO (rGO).15,16 However, the chemical reduction process in solution is always accompanied by undesirable aggregation and precipitation of rGO flakes. This issue could be solved either by chemical functionalization of rGO with organic molecules or by precise control of the rGO dispersion in an organic/ water mixed solvent system, but these complicated multistep synthesis routes with cumbersome purification processes hamper its final use in the industrial arena.17−20 Compared with the LPE approaches mentioned above, electrochemical exfoliation has also attracted intensive interest because of its advantages of high yield, low cost, and fast production. The use of electrochemistry allows a better control over the oxidative damage of the sheets and faster production rates as compared to the standard chemical oxidation approach. For example, the oxidation degree of graphite could be step-controlled by varying whether the electrode is positively or negatively charged,21,22 by choosing an aqueous or nonaqueous solution system,23,24 and by tuning the concentration of certain electrolytes.25−29 Recently, we have utilized a step-controlled electrochemical method to produce high-yield electrochemically exfoliated graphene oxides (EGOs) with a moderate oxidation degree.7 The EGO obtained is highly dispersible few-layer flakes with good electrical properties, and they can be easily processed into conductive coating films or high-capacitance electrodes.14,27,30 Herein, a systematic study based on dispersion stability and surface morphology was performed to explore the attractive physical and chemical properties of EGO. Usually, high-boiling point aprotic solvents, such as N,N-dimethylformamide (DMF, Tb = 153 °C), are used to disperse EGO sheets. In this work, a solvent exchange process was adopted to transfer EGO from DMF to a series of organic solvents including polar protic, polar aprotic, and nonpolar solvents. Stable colloidal suspensions of EGO free of any surfactant were achieved in all of the polar solvents we tested. The relation of the dispersion behavior to the chemical properties was correlated using the relative Hansen solubility parameter, zeta potential data, X-ray photoemission spectroscopy (XPS), and Raman analyses. In addition, a precise surface morphology study by atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis reveals that the vertical structure of dispersible EGO flakes is a multiple of (partially oxidized) graphene bilayers. The thinnest flakes are 2−4 bilayers covered by residual fragments (corresponding to a thickness of 1.5−2.5 nm). Thanks to the possibility of EGO dispersion in low-boiling point solvents [e.g., acetonitrile

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite flakes (Sigma-Aldrich, +100 mesh), NMP (Acros Organics, 99%), DMF (99.8%, SigmaAldrich), ethylene glycol (EGC, Sigma-Aldrich), ethanol (EtOH, 99.8%, Fluka), methanol (MeOH, 99.8%, Lab-Scan), IPA (99.8%, Sigma-Aldrich), acetone (ACE, 99.9%, SigmaAldrich), chloroform (CHCl3, 99.8%, Sigma-Aldrich), tetrahydrofuran (THF, 99.8%, Sigma-Aldrich), 1,4-dioxane (DOX, 99.8%, Sigma-Aldrich), cyclohexane (CHX, 99%, SigmaAldrich), DCB (99.9%, Sigma-Aldrich), toluene (TOL, 99.9%, Sigma-Aldrich), and ACN (99.9%, Sigma-Aldrich) were used as received without further purification. Type of filter membrane used: nylon 66 (pore size 0.20 μm, diameter 47 mm, Whatman), polyvinylidene difluoride (PVDF) (pore size 0.22 μm, diameter 47 mm, Durapore), cellulose acetate (pore size 0.20 μm, diameter 47 mm, Whatman), and cellulose nitrate (pore size 0.20 μm, diameter 47 mm, Whatman). 2.2. Preparation of EGO. Graphite flakes (1 g) were put into a nylon filter bag (200 mesh porosity) and inserted into a porous plastic tube to keep the whole material coherent. The flakes were compressed by pressing a plastic cap on the top of the tube. The plastic grid with a filter mesh had a mechanical containment role, keeping the graphite flakes compressed inside the tube to ensure that they were all electrically connected to the metal electrode and thus subject to exfoliation. This method allowed electrochemical exfoliation to be performed on nonmonolithic powder samples. A platinum foil was connected to the graphite on the top of the tube through a slot in the plastic cap, to act as a working electrode. Another platinum wire was used as a counter electrode. The ionic solution was prepared by dissolving 1.3 g of sodium perchlorate in 10 mL of ACN (1 mol L−1). Then, exfoliation was performed following an approach already described in the previous work.7 In brief, uncharged ACN molecules were intercalated in the graphite by the electrochemical treatment because of the synergistic action of perchlorate ions dissolved in the ACN. Then, the ACN molecules were decomposed with microwaves, causing gas production and rapid graphite exfoliation. This method uses the gas produced by the decomposition of ACN molecules as a powerful blowing agent to promote graphite exfoliation in a few seconds. The first electrochemical intercalation/exfoliation stage was carried out for 30 min by applying a DC bias on the graphite electrode at a voltage of +5 V. After reaction, the partially exfoliated graphite samples were washed with ACN several times and blow-dried with dry nitrogen for 2 min. A commercial microwave oven (Whirlpool JT379) with a rotating tray was used for further expansion. The graphite samples were placed in a porcelain crucible (capacity 50 mL) and heated in the microwave oven for 30 s under 90 W. Then, the expanded foam-like graphite was compressed again to make the whole material coherent. The material for the second time was fixed in a porous plastic tube, connected to a Pt foil, and further exfoliated in 0.1 M H2SO4 solution as an anode at +10 V for 2 h. Then, the graphene sheets were collected by vacuum filtration onto a polytetrafluoroethylene membrane and cleaned several times by repeated washing with deionized 15123


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Figure 1. (a−c) Photographs of EGO dispersion in different solvents, prepared by a high-speed centrifugation-assisted solvent-exchange process from DMF, after 2 months of storage. (d) UV−vis absorption spectra of EGO in the polar solvents; UV cutoff wavelengths of the selected solvents were limited to avoid the UV detection of certain solvents.

water. Afterward, the exfoliated flakes were redispersed in DMF by sonication for 30 min (37 kHz, ≈30 W effective ultrasonic power, model ELMA S10H Ultrasonic). The EGO solution was centrifuged at 2000 rpm for 30 min to remove any re-aggregated particles. In comparison, direct dispersion of EGO in different low-boiling point solvents (IPA, EtOH, and ACN) instead of DMF results in serious aggregation after standing for 2 months. 2.3. Preparation of EGO in Different Solvents by the Solvent Exchange Process. EGO in DMF (20 mL; 0.33 g/ L) was first separated by high-speed centrifugation at 20 000 rpm for 5 min to sediment the graphene flakes. Then, the upper part of the clean solution was decanted and refilled with 20 mL of the target solvent. The mixture was dispersible with the assistance of mild sonication for 5 min. This process was repeated three times. Less than 0.1% of DMF remained in all of the solvents as confirmed by UV absorption of control samples. The organic solvents used were ACE, MeOH, EtOH, CHCl3, IPA, EGC, DMF, NMP, THF, DOX, CHX, DCB, TOL, and ACN. 2.4. Preparation of EGO films on different membrane substrates. The EGO solution (20 mL) in IPA (0.02 g/L) was sonicated in a bath ultrasonicator for 10 min and then filtered on different filtration membranes (nylon 66, PVDF, cellulose acetate, and cellulose nitrate) to obtain a graphenebased thin film. The obtained EGO film was dried overnight at 100 °C to remove the residual solvent. 2.5. Characterization. Water contact angle (WCA) measurements were performed with a CAM 200 instrument (KVS Instruments, Biolin AB, Sweden), where a 6 μL drop of MQ water (Millipore) was put on the surface of the EGOnylon film, and a series of five pictures were taken with a 1 s interval. The reported WCA values were determined as the average of the measurements. The electrical resistance of the graphene films was measured with a four-point probe system (Keithley 2400 Multimeter). The morphology and the corresponding surface potential of

the deposited films were mapped at the nanoscale with AFM techniques. Both measurements were performed in air by employing a commercial digital microscope MultiMode 8 (Bruker). Zeta potential determination was performed using a NanoBrook OMNI (Brookhaven Instruments; quartz cell model AQ-1289 was used for water, and SR-663 was used for organic solvents). We used RTESP (Bruker) tips for AFM measurements. Scanning probe microscopy images were acquired in the same measurement; a topographic line scan was first obtained by AFM operating in the semicontact mode and then that same line was rescanned in the lift mode with the tip raised to a lift height of 20 nm. Raw AFM data were filtered to remove experimental artifacts by using histogram flattening procedures.31 Scanning electron microscopy (SEM) images were obtained with a ZEISS 1530 instrument. TEM observations were carried out with a Fei Tecnai F20 TEM operated at 80 keV, equipped with an energy-dispersive spectrometer. XPS was performed exploiting an ultrahigh vacuum apparatus (base pressure 1 × 10−10 mbar) using a nonmonochromatic Mg Kα excitation source (XR-50, Specs) and a hemispherical energy analyzer (Phoibos 100, Specs). Quantitative chemical analysis and determination of the C/O ratio were performed using a self-consistent protocol recently developed to deconvolve the XPS C 1s surveys of graphenerelated materials.32 UV−vis absorption spectra were recorded using a PerkinElmer Lambda 20 spectrometer, and the cutoff wavelengths of the selected solvents were considered to avoid the influence of UV absorption from the solvents. Raman scattering measurements were carried out with a micro-Raman spectrometer (model: LabRAM from HORIBA Jobin-Yvon), using a 50× objective (laser spot diameter ≈ 2 μm), laser excitation wavelength of 632.8 nm, and laser power ∼4 mW.

3. RESULTS AND DISCUSSION Starting from natural graphite flakes, we obtained EGO flakes featuring both nanoscale thickness and micron-scale lateral size by the electrochemical intercalation, expansion, and exfoliation 15124


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Figure 2. (a) Plot of polar and hydrogen-bonding Hansen solubility parameters vs surface tension of various solvents and (b) zeta potential of EGO dispersion in various polar solvents.

steps in our previously published work (Scheme S1).7,8 Pretreatment of intercalation in organic electrolytes (1 M NaClO4 in ACN) and microwave expansion significantly improved the yield of EGO (50%), as compared with that of one-step electrochemical exfoliation (∼20%).27 Similar to the other reports about electrochemically exfoliated graphene,26,33 the as-prepared EGO could form a stable colloidal dispersion in typically high-boiling point solvents such as DMF and NMP. However, these low-volatility and toxic solvents limit the use of EGO in the synthesis of polymer composites and in surface deposition processes. To solve this issue, a solvent exchange process was utilized to transfer EGO from DMF into a variety of organic solvents. After repeated centrifugation, decantation, and redispersion steps, EGO suspensions were obtained in a series of organic solvents, including: (1) polar protic solvents such as MeOH, EtOH, IPA, water, and EGC; (2) polar aprotic solvents such as THF, ACE, ACN, DMF, and NMP; and (3) nonpolar solvents such as CHCl3, DOX, CHX, TOL, and DCB. As shown in Figure 1a−c, stable dispersions of EGO were obtained in all polar solvents tested after 2 months of storage at room temperature. On the other hand, EGO was not dispersible in nonpolar solvents, which led to a poor colloidal suspension of EGO, with obvious agglomeration and sediments. Figure 1d shows UV−vis spectra of EGO in all of the dispersible solvents. The absorption peak in all cases was centered at ∼266 nm, which is related to the π−π* transition for the sp2 conjugation system on the basal plane of EGO.34 This value is red shifted around 30 nm compared to the reported GO absorption peak (∼233 nm), and it is similar to the value of rGO and pristine graphene.35,36 It is known that the UV adsorption of GO usually contains a broad shoulder peak at around 290−305 nm, which is due to the n−π* transition of epoxide (C−O−C) bonds.35,37 These features were not observed in the case of EGO, indicating a low amount of oxygen functional groups (especially epoxide groups) distributed on the surface of the EGO sheets. The above results suggest that EGO has a larger electronic conjugation and lower oxidation degree compared to GO. Therefore, EGO might be directly used as a conductive 2D material without any reduction treatment. The concentration of EGO in the solutions was also estimated from the UV absorption spectra (Figure 1d) measured at 660 nm, using a molar extinction value of α660 = 2460 L/g·m.10 The concentration of EGO in other solvents was lower than the initial solution in DMF (0.33 mg/mL) (Figure S1) because a small amount of EGO was discarded from the supernatant of the corresponding solvents during the centrifugation process,

and slight aggregation of EGO sheets was observed in some low-boiling point organic solvents. In order to understand the dispersion behavior of EGO in these polar solvents, surface tension and Hansen solubility parameters were used. They include the dispersion cohesion parameter (δd), the polarity cohesion parameter (δp), and the hydrogen-bonding cohesion parameter (δh).38 The δd, δp, and δh values of the corresponding solvents tested are summarized in Table S1. Previous studies indicate that the δp and δh Hansen parameters are two key factors for stable graphene dispersions. For example, Hernandez et al. demonstrated that good solvents for pristine graphene have nonzero values of δp and δh.9 Park et al. also reported that rGO can be dispersed in solvents defined by 10 < (δh + δp) < 30.39 In our case, a stable dispersion of EGO was obtained with the sum δh + δp in the wide range of 13.7−37 in organic solvents and extended to 60 in the case of water (Figure 2a and Table S1). The dispersibility of EGO in organic solvents, and especially in water, might due to the polar (or hydrophilic) nature of EGO prepared from mild electrochemical oxidation because the oxygen functional groups on EGO, even in low amounts, can stabilize these sheets in polar solvents. On the other hand, EGO was not dispersed in nonpolar solvents with δh + δp < 11.4, which is consistent with the reported rGO results.39 Meanwhile, the surface tensions of the solvents, as shown in the x-axis of Figure 2a, are not directly correlated with the dispersibility behavior of the EGO sheets. The set of good solvents for EGO covers a wide range of Hansen solubility parameters, which strongly facilitates the solution processability of graphene in various low-boiling point solvents. The stability of EGO dispersion not only depends on the properties of the liquid but also on the charge properties of the sheet surface. Therefore, zeta potential measurements were performed to evaluate the electrostatic behavior of EGO sheets in these polar solvents. As shown in Figure 2b, EGO sheets always showed a negative charge, with zeta potential values ranging from −25 to −46 mV in all of these polar solvents; these values are enough to maintain stable graphene dispersion by electrostatic repulsion. Similar to the zeta potential value of GO, these negative potential values could be attributed to the charged oxygen functional groups such as carboxyl and hydroxyl groups on the basal or edge plane of the EGO sheets. Hence, the stable dispersion of EGO in organic solvents is mainly controlled by electrostatic repulsion between the charged graphene surfaces. Because oxygen functional groups play an important role in the stable suspension of EGO, it is worth studying the fundamental chemical functional groups by XPS measure15125


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Figure 3. (a) ID/IG Raman mapping image and (c) corresponding Raman profile of selected points on EGO sheets; (b) I2D Raman mapping image and (d) corresponding profile of selected points on (b).

hand, the intensity of the 2D peak (the second order of the D peak, Figure 3b,d) at 2660 cm−1 was too weak to evaluate the layer number information of graphene.42 In addition, there were two shoulder peaks at ∼2920 cm−1 that are attributed to the D + D′ scattering. The low intensity of the 2D peaks is due to the breaking of the stacking order associated with the electrochemical oxidation, which agrees with GO or rGO data from the literature.14,42 To monitor the morphology of EGO flakes after the solvent exchange process, we combined TEM and AFM. The first technique allowed the interlayer distance of the few-layer EGO flakes to be directly measured, whereas the latter was utilized to characterize the length, width, and thickness of flakes. TEM analysis was used to analyze EGO suspended on a TEM grid substrate. From the low-magnification TEM image (Figure 4a), the suspended EGO samples consist of homogeneous micrometer-size flakes with low contrast. A selected-area electron diffraction (SAED) pattern from the EGO flake (shown in the inset of Figure 4a) shows the crystalline structure with sharp and clear diffraction spots for the hexagonal phase. This crystallographic orientation of EGO indicates that it does not have a completely amorphous

ments. As shown in the high-resolution C 1s core-level spectra (Figure S2 and Table S2), the EGO signal can be fitted with six peaks corresponding to carbon atoms in different functional groups: 284.4 eV (CC, sp2-bonded carbon, 56.3 ± 0.9%), 284.9 eV (C−C, sp3-bonded carbon, 13.2 ± 0.5%), 285.7 eV (C−OH, hydroxyl group, 3.9 ± 0.3%), 286.8 eV (C−O−C, epoxy group, 19.5 ± 0.6%), 288.0 eV (CO, carbonyl group, 4.3 ± 0.3%), and 289.2 eV (O−CO, carboxylate carbon, 2.7 ± 0.3%). The C/O value, amounting to 3.8 ± 0.2 (Figure S3 and Table S2, peak area ratio obtained from C 1s/O 1s), is lower than that of graphene exfoliated in solvents by sonication, but still higher than the typical C/O ratio of GO (about 2).14,40 It is noteworthy that the surface chemistry and oxidation degree of EGO are similar to those of rGO obtained by electrochemical reduction at −1.0 V of normal GO.14 Thus, EGO prepared by moderate electrochemical oxidation should have comparable conductivity and electrochemical activity to rGO, even without any reduction treatment. The average oxidation degree of the EGO flakes was further confirmed by Raman spectroscopy, which was performed by monitoring the relative area integral intensity of the D peak (intervalley double resonance, usually from defect-related scattering) and the G peak (in-plane E2g optical phonon mode). Figure 3a,b presents the ID/IG area ratio and I2D area intensity mapping of EGO sheets with a scan area of 10 × 10 μm2, containing 400 spectra. The ID/IG peak area intensity ratio has an average value of 1.4, a value overall compatible with the typical oxidation of EGO flakes.14,25 Figure 3c shows the typical EGO Raman spectrum with two distinct and broad peaks, the D peak (∼1330 cm−1) and the G peak (∼1598 cm−1),41 selected from three different locations on the Raman map (Figure 3a, P1, P2, and P3 are the selected locations on the EGO flakes, and P4 is the location on the bare SiOx substrate). The almost identical Raman peak position and shape highlight that the oxidation-induced defects were uniformly distributed on the EGO surface. On the other

Figure 4. (a) TEM image of EGO sheets on a holey carbon support, inset of (a), SAED pattern from the EGO sheet; (b) HRTEM image of bilayer EGO sheets on the cross-sectional part. 15126


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Figure 5. (a) AFM image of EGO sheets from DMF spin-coated on a SiOx substrate, and (b) corresponding histogram analysis with heights (b inset) measured at different regions marked in the AFM image; (c) schematic cartoon of the surface morphology from (a). Z-range: (a) 4 nm.

two surface levels: a basic structure (L1, height of 20 ± 3 Å) covered by small fragments of approximately 4 Å thickness on the top (L2, a height of 24 ± 3 Å). The same behavior is observed in the other regions (II and III). The resulting heights are plotted in Figure 5c showing a linear behavior in which region II (III) is Δ = 8 ± 2 Å higher than region I (II). The linear increase of the thickness and the presence of a “double peak” (L1 and L2) with fixed distance for each region strongly suggest that EGO flakes consist of “bilayer” repeating units, having the same thickness and the same surface morphology. A thickness of approximately 8 Å allows a trilayer (or more) structure to be excluded. Moreover, UV−vis spectra and XPS analysis revealed that layered EGO is partially oxidized and similar to rGO whose thickness is about 5 Å allowing us to rule out that the repeating units could be composed of a single sheet. In summary, the experimental data can be explained, assuming that each EGO structure is composed of a partially oxidized graphene bilayer (G̃ bilayer). In particular, because the starting material is bulk graphite, we should consider that the partially oxidized graphene sheets are still stacked in an AB configuration and thus the bilayer thickness should be somewhat larger than that of Bernal-stacked bilayer graphene: G̃ bilayer ≥ 6.7 Å. Using this scheme, the regions I, II, and III marked in Figure 5b correspond to the stacking of 2, 3, and 4 G̃ bilayer, respectively, and on their external surfaces, there are graphene fragments of few nanometer size, with a thickness of ca. 4 Å. The thickness of N-region can be generalized by the expression: (N + 1)·G̃ bilayer + 2·fragment. Combining the results obtained by the histogram analysis with the XPS and TEM characterization, we can describe the complex structural morphology of EGO, as depicted in the cartoon in Figure 5c. EGO is a few-layer “sandwich-like” structure consisting of bilayer graphene stacking (partially oxidized with an effective measured thickness of 8 ± 2 Å) and small fragments on both sides of the EGO flakes. Therefore, a minimum of bilayer graphene with nanoscale fragments on both sides is expected to be the most reasonable EGO structure. Interestingly, our AFM observation on EGO is consistent with the previously reported two-component model of GO that is composed of functionalized graphene sheets decorated by strongly oxidative debris (OD).45−47 However, even in the case of GO modeling, different synthetic methods will lead to different structure formations. For example, the Hummers−

structure; the lack of other diffraction spots except those from graphite means that there is no influence from the oxygen functional groups with the formation of any superlattice-type ordered arrays. Figure 4b shows a high-resolution TEM (HRTEM) image of the folded edge of an EGO sheet. The edge of the thin-layer EGO sheet was not simply single layer or few layers; rather, it was a mixture of defective few layer structures. Because the electrochemical exfoliation process also produced non-fully exfoliated graphene oxide, defective multilayer structures of up to 4 layers were observed in all of the TEM images (Figure S4). The lattice fringe of graphene showed an interplanar distance d002 of ∼4.0 Å, which is slightly higher than the value of pristine graphite (3.4 Å). This may be attributed to partial oxidation of the defective graphene sheets on edge sites, and the result is consistent with our previous Xray diffraction analysis with a spacing of ≈4.1 Å (2θ = 22°).8 AFM analysis was performed on flakes deposited on SiOx substrates from different polar solvents (Figure 5a and Figure S5). In all cases, we observed thinnest flakes of around 1.7−2.0 nm thickness, with a lateral size ranging between 1 and 10 μm. These values are consistent with most of the electrochemically exfoliated graphene-based materials reported in the literature (as summarized in Table S3). The measured thickness is at least twice that of a GO monolayer (>0.9 nm),8,43,44 indicating that EGO might be a few-layered structure instead of single layer.14,25 We then performed a statistical analysis of the AFM images with a total of 75 flakes including the height distribution of the flakes (Figure S6a,c) to understand the precise surface morphology of the EGO layers. Figure 5a shows a representative AFM image of a partially exfoliated EGO where the layered fragments are well preserved. We can clearly distinguish three regions on the layered structure of EGO with different heights (Figure 5a, marked with I, II, and III, whereas the substrate is indicated with 0). Figure 5b shows a histogram of the averaged height of the sample, showing only peaks 0 and I for clarity; the histograms of all of the regions are reported in Figure S6d. The high quality of the AFM images allows a quantitative study of the thickness of the mapped flakes. In particular, we studied the peak position and the shape of the histogram, allowing a deconvolution analysis to be performed on each peak. The peak 0 is the silicon substrate, corresponding to a symmetric Gaussian function (root-meansquare roughness, Rrms = 3 ± 1 Å), in agreement with the values reported in the literature.31 In contrast, peak I is asymmetric and can be deconvoluted as the sum of two symmetric peaks, indicated with L1 and L2 corresponding to 15127


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The Journal of Physical Chemistry C Offeman (KMnO4/H2SO4/NaNO3) method can obtain monolayer GO sheets adhered with OD (mainly from humic- and fulvic-like structures and oxygen functional groups), whereas the Brodie method can produce mainly few-layer GO with much less OD.47 In our case, EGO obtained by mild anodic exfoliation is very similar to the proposed GO model prepared from the Brodie method. Meanwhile, there are still some differences between the EGO and GO models: (i) we can observe a repeatable bilayer structure on EGO, which helps us to identify the nanoscale fragments from a precise histogram analysis. (ii) The surface roughness of EGO is much higher than that of a GO sheet because of the contribution of small fragments,44,46 and these fragments arise mainly from the residue of EGO extralayers during the violent electrochemical exfoliation process.8 However, we still cannot exclude the existence of OD (ca. 5 Å) physically absorbed on the EGO surface. It is worth recalling that EGO is the product of a top-down approach in which the starting bulk graphite material is oxidized, fragmented, and exfoliated into small pieces, starting from the external part. Thus, electrochemical exfoliation, especially in the case of anodic oxidation with a dilute sulfuric acid electrolyte, leads to the formation of EGO sheets with a complex surface morphology.8,48 As we know, the successful exfoliation of GO includes the following steps: (1) efficient intercalation with the formation of graphite intercalation compounds (GICs) as intermediates under a critical potential; (2) oxidative cleavage of carbon atoms from the edges or defects of the graphite; and (3) fast detachment of the defective graphene sheets by interlayer gas expansion, mainly from electrolysis of the co-intercalated water molecules.48,49 The water molecule plays an important role in the production of bilayer structures. On one hand, the intercalate anions are limited to high stage (n ≥ 2; n is the number of graphene layers between the adjacent intercalated layers) GICs because of the penetration of H2O molecules and the high stage GIC intermediates determine the layer thickness of the final product. On the other hand, the oxidative hydrolysis of the GICs comes from the nucleophilic attack of water and the subsequent production of oxygen is caused by water electrolysis, which mechanically peels off the EGO flakes, also leading to the incomplete exfoliation of upper graphite layers with the formation of small fragments.8 The presence of water guarantees the rapid process of intercalation, oxidation, and expansion steps without serious damage to the graphene sheets. As a result, moderately oxidized EGO with minimum bilayer structures was obtained by anodic oxidation. Because the hydrophilic nature of these oxygen functional moieties on EGO facilitates its dispersibility in different polar solvents, some eco-friendly and volatile solvents (e.g., EtOH or IPA) could be used as substitutes for the solvents most commonly used for exfoliation such as NMP or DMF with high boiling points. To demonstrate the good processability of the material presented herein, the EGO solution after a solvent exchange process from DMF to IPA was filtered on various filtration membranes (nylon 66, PVDF, cellulose acetate, and cellulose nitrate) to obtain a graphene thin film and the film was dried overnight at 100 °C to thoroughly remove the residual solvent (Figure 6a). Uniform graphene films with a thickness of ca. 60−100 nm were obtained in all cases, as confirmed by the SEM analysis in Figures 6b and S7a−c. In addition, water droplet contact angle (WCA) measurements were performed on the surface of the EGO-nylon membrane

Figure 6. (a) Photograph of EGO transferred from IPA solution to different filter membranes and (b) SEM images of the EGO film transferred from IPA solution onto cellulose nitrate membranes.

(Figure S7d). The results further confirm that EGO is hydrophilic with an average WCA value of 51.8° ± 2°. Because of the good stacking of the individual EGO sheets and the moderate oxidation degree of the bilayer structure, EGO films exhibit a high bulk conductivity (>42 000 S/m). Higher bulk conductivity can be obtained in rGO films (76 000 S/m) but this requires thermal annealing at 1000 °C,50 whereas our process avoids any chemical or high-temperature postreduction treatment: EGO can be processed and directly used as a conductive material, with no post-deposition treatment needed. Thanks to the solubility of EGO in lowboiling point and eco-friendly solvents, the fabrication process of the EGO film could be simply performed by vacuum filtration on various kinds of filter membranes without solvent compatibility issues. Moreover, there is a strong adhesion between the cellulose-based membranes and EGO sheets because of their hydrophilic nature. Considering the high strength and flexibility of the cellulose film and the good conductivity from the EGO thin film, the EGO/cellulose-based membrane obtained could potentially be used as a flexible conductive substrate for electrochemical (bio)sensors and supercapacitors.

4. CONCLUSIONS In summary, we have studied the dispersion of EGO sheets in various organic solvents without the use of surfactants/ stabilizers. Thanks to the electrostatic repulsion from the oxygen functional groups on the EGO surface, EGO shows good solubility in the various polar solvents that we tested. The surface morphology was analyzed on both the nano- and macroscale with the assistance of XPS, TEM, AFM, and Raman spectroscopy. Unlike the classical monolayer structure of fully oxidized GO, EGO sheets obtained by anodic oxidation in dilute electrolyte conditions have a mainly bilayer structure with a rough basal surface. The moderate electrochemical oxidation process maintains the intrinsic sp2 conjugation system on bilayer EGO, and the oxygen functional groups created by anodic oxidation also facilitate the good dispersibility of EGO in a range of low-boiling point solvents. Thus, combining the suitable processability of GO and good conductive properties, EGO is a versatile material for solution processing that is extremely attractive for a range of flexible electronics applications. 15128


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03395.

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Concentration of EGO in different solvents after solvent exchange processes, high-resolution core level C 1s and full survey XPS spectra of EGO, HRTEM images of fewlayer EGO sheets, AFM images of EGO dispersion from various polar solvents, and histogram analysis of EGO from DMF, SEM images of EGO coated on cellulose acetate, Nylon and PVDF membranes, and water droplet contact angle measurement of the EGO-nylon membrane (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhenyuan Xia: 0000-0003-2227-3598 Chiara Zanardi: 0000-0002-2091-3398 Vittorio Morandi: 0000-0002-8533-1540 Vittorio Bellani: 0000-0003-2914-1459 Alessandro Kovtun: 0000-0002-7614-7100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under GrapheneCore2 785219 − Graphene Flagship and from the Swedish Research Council under project (project Janus 2017-04456).

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REFERENCES

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