Simple and cost-effective synthesis of graphene by electrochemical exfoliation of graphite rods

Abstract

We report a simple and efficient approach for graphene production, by electrochemical exfoliation of graphite rods, in acidic solutions. The applied bias and electrolyte concentration were optimized. The X-ray powder diffraction (XRD) patterns of nanosheets reveal the decreasing, or even the disappearance, of the graphene oxide peak by reducing the electrochemical exfoliation bias from +6 to +2.5 V. Thus, graphene with different oxidation degrees can be obtained by controlling the applied bias. An optimal graphene sample was prepared in a mixture of H₂SO₄ : HNO₃ (3 : 1 ratio; 1 M each) at a bias of +3 V. The interlayer spacing, crystallite size and the average number of layers were determined by XRD. The structural and morphological characteristics of the prepared samples were also investigated by Raman Spectroscopy, FTIR Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM).

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1. Introduction

Graphene, a two-dimensional material formed from sp²-bonded carbon atoms packed in a honeycomb crystal lattice, has become one of the most exciting research topics in the last decade. Its unique properties including high surface area,¹ high chemical stability,² and elasticity,³ along with exceptional electronic properties⁴ makes it a suitable material for numerous applications.⁵ Regarding its synthesis, some important features like production cost, scalability, reproducibility and the quality of the obtained material should be considered. In order to create graphene with few defects from abundant and inexpensive carbon sources, new versatile processes have been pursued. Hence, few-layer graphene were produced by micromechanical exfoliation⁶ but, due to the low productivity of the method, it was considered unsuitable for large scale production. Exfoliation and reduction of graphite oxide^7,8 appears to be a more efficient approach for bulk production of graphene sheets, but the sheets generally have residual oxygen functionalities and network defects. Chemical vapour deposition (CVD) using transition metals as catalysts^9–11 is another method employed for obtaining high-quality graphene, but the major disadvantage is the high costs associated with it. So far, the direct electrochemical exfoliation of graphite into graphene appears to meet most of the requirements, except the quality of the obtained materials.^12–14 Various electrolytes were used in the electrochemical approach. For example, Liu et al. reported the preparation for the first time of graphene by electrochemical exfoliation of a graphite anode, using a mixed solution of water and ionic liquid (1-octyl-3-methylimidazolium hexafluorophosphate) as electrolyte.¹⁵ Sodium dodecyl sulphate was used as intercalation agent for the synthesis of graphene sheets with controlled thickness.¹⁶ A simple, fast and cost-effective approach for the synthesis of few-layer graphene from pyrolytic graphite sheet in a protic ionic liquid, by applying a suitable DC bias was also reported.¹⁷ A multiple electrochemical exfoliation strategy was designed for graphene production, both in high quality and quantity using aqueous solutions of protic acids (i.e., H₂SO₄, H₃PO₄ or H₂C₂O₄) as electrolytes. The as-prepared graphene proved to have excellent electrochemical performance when used as anode material in lithium ion batteries.¹⁸ Another simple and fast electrochemical method, employed to exfoliate graphite into thin graphene sheets, mainly AB-stacked bi-layered graphene with a large lateral size (several tens of micrometers) was reported. The electrochemical exfoliation of graphite was performed in sulfuric acid.¹⁹ By electrochemical intercalation of Na⁺/dimethyl sulfoxide complex and subsequent addition of thionin acetate salt into the electrolyte, few-layer graphene were obtained.²⁰ A non-oxidative route to produce few-layer graphene was elaborated by the electrochemical intercalation of tetraalkylammonium cations into a graphite rod. The target graphene was obtained directly, without any additional exfoliation step.²¹

The process of electrochemical exfoliation and the effect of the exfoliation conditions on the size, thickness, and functionalization of produced graphene flakes were also investigated. The exfoliation was carried out using natural graphite powder. It was found that during the exfoliation, two distinct processes controlled the properties of the produced graphene. The intercalation of solvent (1-methyl-2-pyrrolidone, NMP) in the graphite lattice affected the thickness of the graphene while the expansion of the intercalated solvent into gas bubbles determined the lateral flake size and the exfoliation onset.²²

The size, shape and number of the graphene layers were poorly controlled by the presented synthetic methods due to the random growth or exfoliation process. Thus, a facile and efficient strategy for producing high quality graphene in large yield is still required.

In this study, we report a simple, cost-effective electrochemical approach to produce graphene by electrochemical exfoliation of graphite rods, in acidic electrolytes. As far as we know, this is the first paper which reports the use of a diluted H₂SO₄ : HNO₃ mixture as electrolyte. The size of graphene flakes and the exfoliation/oxidation level were studied by varying the electrochemical parameters (e.g. applied bias, electrolyte concentration). X-ray powder diffraction (XRD) was used as the main technique for graphene structural characterization. XRD provides three types of structural information: the number of graphene/graphene oxide layers, the interlayer distance, and the size of graphene crystallites. The obtained materials were also investigated by Raman Spectroscopy, FTIR Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM).

2. Experimental

2.1. Materials

All reagents were of analytical grade and used without further purification. All solutions were prepared using double distilled water. Sulfuric acid, nitric acid and ethanol were purchased from Merck chemicals. Two different graphite rods with high purity were used: a graphite rod obtained from SPI Supplies-Germany (denoted R1, with ∼50 ppm impurities), and a graphite rod from Pierce eurochemie b.v. – The Netherlands (denoted R2, <2 ppm impurities).

2.2. Characterization

The X-ray powder diffraction patterns were obtained with a Bruker D8 Advance diffractometer, using CuK_α1 radiation (λ = 1.5406 Å). In order to increase the resolution, a Ge(111) monochromator in the incident beam was used, to filter out the K_α2 radiation. Before plotting the experimental results, all spectra were background corrected.

Raman spectra were collected at room temperature using a JASCO (NRS 3300) spectrophotometer arranged in a backscattering geometry and equipped with a CCD detector (−69 °C) using a 600 l mm⁻¹ grid. The incident laser beam (approx. 1 μm²) was focused on the sample surface with an Olympus microscope and a 100× objective. For the calibration, we used the Si 521 cm⁻¹ peak. Excitation was done with an Ar-ion (514 nm) laser with a power at the sample surface of 1 mW and a spectral resolution of 14.5 cm⁻¹.

Fourier Transformed Infrared measurements were performed with a JASCO 6100 spectrometer; the infrared spectra were recorded with a resolution of 4 cm⁻¹ from 4000 to 500 cm⁻¹, by using KBr pellet technique. 0.2 mg of each material were mixed with KBr powder and pressed in a pellet.

X-ray Photoelectron Spectroscopy technique has been used for the characterization of chemical composition and state of elements present in the investigated graphene samples. XPS spectra were recorded using a SPECS spectrometer, equipped with a dual-anode X-ray source Al/Mg, a PHOIBOS 150 2DCCD hemispherical energy analyzer and a multi-channeltron detector. The pressure inside the measurement chamber was maintained constant at about 1 × 10⁻⁹ torr. The sample, as a colloidal suspension in methanol, was dried in successive layers on indium foil stacked on wolfram sample holder. Irradiation was made with an Al_Kα X-ray source (1486.6 eV) operated at 200 W. The XPS survey spectra were recorded at 30 eV pass energy, 0.5 eV per step. The high resolution spectra for individual elements were recorded by accumulating 10–15 scans at 30 eV pass energy and 0.1 eV per step. The surface cleaning was ensured through argon ion bombardment at 300 V for 5 minutes. Data analysis and experimental curve fitting of the C 1s, and O 1s spectra was performed using Casa XPS software with a Gaussian–Lorentzian product function and a non-linear Shirley background correction.

The morphology of obtained materials was analyzed by Transmission Electron Microscopy (H-7650 120 kV Automatic TEM, Hitachi, Japan) and Scanning Electron Microscopy (SU-8230 STEM – Hitachi, Japan). For TEM images, samples were dispersed in ethanol and sonicated with a cup-horn sonicator (Sonics, Vibra-Cell VC 505, 500 W, 20 kHz) for 3 minutes. Few drops of these suspensions were dried on holey carbon grids.

The Atomic Force Microscopy analysis of graphene morphology was carried out with a Keysight 9500 Scanning Probe Microscope. The system was equipped with a standard 90 μm scanner. Imaging mode: acoustic AC mode (tapping); k = 7 N m⁻¹, f = 1500 kHz; all closed loop control imaging; Quickscan nose cone. Graphene samples were dispersed by sonication in ethanol, drop-casted on a freshly cleaved mica surface and dried at room temperature.

A Christ Alpha 1-4 LSC Freeze Dryer was used for sample lyophilisation.

2.3. Sample preparation

The materials were prepared via an electrochemical exfoliation method: in a typical synthesis process, a graphite rod was connected to the anode of a power supply and placed along with a platinum foil cathode in the reaction vessel. A schematic representation of graphite electrochemical exfoliation is depicted in Fig. 1. All experiments were performed under ambient conditions.

Fig. 1 Schematic illustration of graphite electrochemical exfoliation: (a) experimental setup; (b) schematic representation of graphite exfoliation.

Mixtures of H₂SO₄ : HNO₃ (3 : 1, various concentrations, 65 ml total volume in each experiment) were used as electrolytes. Low bias (2–10 V) was applied to the electrodes, using a DC power supply (Mesit, Slovakia). Shortly after the bias was applied, the electrochemical exfoliation rapidly started; the graphite rod surface became rough in time, and spread into the solution. The release of gaseous bubbles, which became more intense with the increase of electrolyte concentration, was observed during the entire electrochemical process. Afterwards, the exfoliated graphitic material was repeatedly washed with double distilled water to remove the acidic electrolyte, and then sonicated for 2 h. Subsequently, the sonicated solution containing the graphite flakes was filtered with a Whatman qualitative filter paper (589⁵). It was observed that most of the thick graphite flakes remained on the filter. In order to further separate the small graphite nanosheets from the large ones, the filtrate was centrifuged (at 500 rpm), and the obtained material was dried by lyophilization.

Because the presented samples were obtained in various conditions, the following codification was used: GO/G-bias (V) – electrolyte concentration (M), for a mixture of graphene oxide (GO) with graphene (G). When only graphene is present in the sample, the codification was G-bias (V) – electrolyte concentration (M). For example GO/G-6-0.5 represents a mixture of GO and G obtained at 6 V in 0.5 M electrolyte concentration.

3. Results and discussions

The electrochemical exfoliation of graphite (Fig. 1) proved to be significantly dependent on a couple of parameters which affects the quality of the obtained materials: the electrolyte concentration and the applied bias. The changes in the reaction time mostly influenced the final amount of exfoliated material. Hence, after 4 hours of exfoliation in concentrated electrolyte (2 and 3 M) more graphite was present in the final samples and the separation was very difficult. For this reason, when the influence of the electrolyte concentration was studied, all samples were collected after 2 hours. The amount (%) of each type of graphene, e.g. graphene oxide (GO), few-layer graphene (FLG) and multi-layer graphene (MLG), was determined from XRD, expressed as the ratio of the fitted peak area to the total area of the diffractogram.²³ In each set of experiments, the optimum samples were selected according to the average number of layers and the amount (%) of each type of graphene.

3.1. Optimization of the exfoliation process

The XRD analysis was used to characterize the crystalline nature and phase purity of the synthesized materials. The peak around 25° (corresponding to (002) reflections) is asymmetric and displays two components which were separated by fitting the XRD pattern into two theoretical Gaussian peaks. The mean crystallites size (D) for each sample was calculated from full width at half maximum (FWHM) of the XRD peak, using the Debye–Scherrer equation, as previously reported by other research groups.^24,25 The interlayer distance (d) was found using Bragg equation.²⁶

The average number of graphene layers (n) was obtained using eqn (1):

3.1.1. Selection of the graphite rod. It was previously reported that the electrochemical exfoliation of graphite is strongly dependent on the graphite structure. A successful exfoliation was observed only on crystalline graphite such as highly oriented pyrolytic graphite (HOPG), natural graphite flakes or graphite paper.¹⁹ It is interesting to reveal the importance of the graphite rod used in our exfoliation process. For this purpose, the properties of the materials obtained with two different high purity graphite rods, were investigated (3 V applied bias, 0.5 M electrolyte concentration, 2 h reaction time) (ESI, Fig. S1†). Samples were denoted as GO/G-3-0.5-R1 and GO/G-3-0.5-R2, where R1 and R2 represent the used graphite rods, according to the notations from paragraph 2.1.

The comparison of the obtained results reveals significant differences between the samples. Hence, GO/G-3-0.5-R1 sample consists of 46% FLG (4 layers), while GO/G-3-0.5-R2 is formed of 71% bi-layer graphene (ESI, Tables S1 and S2†). In case of GO/G-3-0.5-R2 sample it can be noticed (ESI, Fig. S1†) that the MLG peak is well diminished, and the number of layers is smaller (n = 13).

Based on the number of layers and the distribution of each type of graphene, the GO/G-3-0.5-R2 sample was considered to be the optimum material. Therefore, the next set of experiments were made using graphite rod R2.

3.1.2. Influence of the applied bias. The influence of the applied bias was studied in the 2–10 V range, while the rest of the conditions were kept constant (0.5 M solution concentration; 4 h reaction time). We have observed that a 2 V bias was not enough for the exfoliation process, since only a small amount of graphene was collected from the solution. On the other hand, a 10 V bias was found to be excessive, since the graphite rod was severely damaged within 2 minutes time.

The fitted XRD spectra of GO/G-6-0.5, GO/G-5.5-0.5, G-3-0.5 and G-2.5-0.5 samples are presented in Fig. 2. One can see that with the decrease of the applied bias (from 6 to 2.5 V, respectively), the intensity of the peak located at 2θ ∼11° (GO) decreases, and finally the peak disappears, indicating that a low bias is benefic for graphene preparation. The position of this peak, at low 2θ values, indicates a large interlayer distance (see Table 1).

Fig. 2 The XRD patterns of the samples obtained at various biases: (a) GO/G-6-0.5; (b) GO/G-5.5-0.5; (c) G-3-0.5; (d) G-2.5-0.5.

Table 1 The number of layers (n) and interlayer distance (d) of the samples prepared at various biases

Sample	n			d (nm)
	GO	FLG	MLG	GO	FLG	MLG
GO/G-6-0.5	3	4	14	0.747	0.379	0.34
GO/G-5.5-0.5	2	4	17	0.777	0.356	0.34
GO/G-5-0.5	3	3	25	0.85	0.394	0.339
GO/G-4-0.5	3	3	18	0.905	0.370	0.34
G-3-0.5	—	2	15	—	0.395	0.34
G-2.5-0.5	—	3	30	—	0.410	0.34

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As a result of the oxidation and exfoliation processes, the obtained powder is actually a mixture of graphene oxide (having 2–3 layers) and graphene (with different number of layers, from 2 to 30). By employing our procedure, the average number of layers was steadily reduced from 123 (raw graphite) to few-layers (graphene). Table 1 summarizes the number of layers and interlayer distance of the samples prepared at different biases. As expected, the interlayer distance of the graphene-based samples is proportional to the sample oxidation degree. For example, in the case of GO/G samples the interlayer spacing has increased from 0.747 to 0.905 nm. The change may be attributed to the various amounts of oxygen containing functional groups attached to graphene.

In the case of MLG, the interlayer distance (0.34 nm) is comparable with that of bulk graphite (0.335 nm), while for FLG, this varies between 0.356 (GO/G-5.5-0.5) and 0.41 nm (G-2.5-0.5), suggesting the presence of more wrinkled or disordered graphene sheets.

Table 2 presents the amount (%) of different types of graphene, found in the exfoliated samples. At low bias (≤4 V) more than half of the samples consist of few-layer graphene (2–3 layers, as can be seen for GO/G-4-0.5; G-3-0.5; and G-2.5-0.5). At high bias (5–6 V) the amount of FLG in the samples decreases while that of GO increases (GO/G-6-0.5; GO/G-5.5-0.5; GO/G-5-0.5). GO/G-6-0.5 contains the highest amount of GO (55%).

Table 2 The amount (%) of each type of graphene in the exfoliated samples

Sample	GO/G-6-0.5	GO/G-5.5-0.5	GO/G-5-0.5	GO/G-4-0.5	G-3-0.5	G-2.5-0.5
% of GO	55	30	26	10	—	—
% of FLG	23	45	45	57	73	81
% of MLG	22	25	29	33	27	19

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Based on the obtained results, the graphene sample synthesized at 3 V bias (G-3-0.5) was selected as reference. 73% of this sample is formed of high quality bi-layer graphene. The next sets of experiments were performed at this bias.

3.1.3. Influence of the electrolyte concentration. The influence of the electrolyte concentration was studied in the 0.5–3 M concentration range, while the rest of the conditions were kept constant (3 V applied bias; 2 h reaction time). The presence of a sharp peak corresponding to the (002) reflections of graphitic structure (2θ ∼ 25°) can be noticed in all XRD patterns, but this is near to that of (002) reflections of graphene (2θ ∼ 21°) (see Fig. 3 and Table 3).

Fig. 3 The XRD patterns of the samples prepared in various electrolytes: (a) 0.5 M (G-3-0.5); (b) 1 M (G-3-1); (c) 2 M (GO/G-3-2) and (d) 3 M (GO/G-3-3).

Table 3 The amount (%) of each type of graphene in the samples prepared at various electrolyte concentrations

Sample	G-3-0.5	G-3-1	GO/G-3-2	GO/G-3-3
% of GO	—	—	7	9
% of FLG	77	93	59	79
% of MLG	23	7	34	12

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It can be observed that at a high electrolyte concentration, only small amounts of GO were obtained (7–9%) (see Fig. 3c and d, and Table 3).

Table 4 lists the number of layers, the interlayer distance and the crystallite size, for the optimum samples. At the first look, sample G-3-0.5 appears to be better than G-3-1. The number of layers of G-3-0.5 sample was found to be 3 (FLG) and 15 (MLG), while G-3-1 sample has 2 (FLG) and 25 (MLG). However, based on the percentage of FLG, sample G-3-1, synthesized in 1 M electrolyte, was selected as the best material prepared in this work: 93% of this sample is composed of high quality bi-layer graphene (Tables 3 and 4).

Table 4 The number of layers (n), crystallite size (D) and interlayer distance (d) of the optimum samples

Sample	n			D (nm)			d (nm)
	GO	FLG	MLG	GO	FLG	MLG	GO	FLG	MLG
G-3-0.5	—	3	15	—	1.1	5.0	—	0.418	0.343
G-3-1	—	2	25	—	1.0	8.5	—	0.418	0.339
GO/G-3-2	4	3	16	4.1	1.1	5.6	0.939	0.375	0.341
GO/G-3-3	3	2	13	3.0	1.0	4.8	0.939	0.409	0.345

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Based on the above results, the samples GO/G-6-0.5, G-3-0.5, GO/G-3-3 and G-3-1 were considered to have the best properties, and were further characterized by Raman spectroscopy, FTIR spectroscopy, XPS, TEM, SEM and AFM.

3.2. Structural characterization by Raman, FTIR and XPS

Raman spectroscopy is a practical tool for studying the number of layers and disorder in graphene.²⁷ This technique has been commonly used to investigate the quality of the electrochemically produced graphene sheets in various electrolytes.¹⁶ In graphene Raman spectrum, one can notice three important bands: the G band, located at ∼1595 cm⁻¹, the 2D band at 2700 cm⁻¹ and the D band at ∼1360 cm⁻¹. The D band appears in graphitic materials with small crystallites and is associated with the number of structural defects in the graphitic layers whereas the G-band arises from the vibrations of the graphite sp² carbon atom domains. According to Ferrari et al.²⁸ the evolution of the 2D band clearly indicates the transformation of graphite to graphene, after electrochemical exfoliation. However, the same authors stated that if the number of layers is higher than 5, the Raman spectrum becomes less distinguishable from that of bulk graphite. The XRD results indicate that the exfoliated graphene samples are not homogenous, and so the identification of the number of layers by Raman spectroscopy is not possible. The representative Raman spectrum of graphite and that corresponding to GO/G-6-0.5, G-3-0.5, and G-3-1 samples are shown in Fig. 4. It can be seen that the Raman features of the exfoliated samples are different from those of the parent graphite.

Fig. 4 Raman spectrum of: (a) graphite and of the chosen optimum samples (b) GO/G-6-0.5; (c) G-3-0.5 and (d) G-3-1.

The shifting of the G band (1591–1607 cm⁻¹) may be explained by the presence of isolated double bonds, which resonate at higher frequencies than the G band corresponding to graphite (1585 cm⁻¹).²⁹ The high fluorescence signal of sample GO/G-6-0.5 suggests a more oxidized material, being in accordance with the XRD data. In case of G-3-0.5 and G-3-1 samples, an increase of the 2D band intensity can be observed, indicating that the number of layers tends to decrease, in conformity with the XRD data. An overview of the Raman bands for graphite and GO/G-6-0.5, G-3-0.5, G-3-1 samples are shown in Table 5.

Table 5 Raman bands for graphite, GO/G-6-0.5, G-3-0.5, and G-3-1 samples

Sample	G band (cm^−1)	D band (cm^−1)	2D band	IG/ID
Graphite	1585	1363	2731	16.95
GO/G-6-0.5	1606.9	1357.5	2712	1.09
G-3-0.5	1600.1	1363.2	2714	1.22
G-3-1	1591.2	1357.6	2712	1.24

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The intensity ratio of the G and D bands (I_G/I_D) gives information about the amount of defects in graphene samples. The increasing of the I_G/I_D ratio (from GO/G-6-0.5 to G-3-1) indicates a decrease in the amount of defects present in the carbon structure. However, the obtained values indicate a relatively high defect density, which may be due to the presence of the MLG in the samples.

FTIR study was next employed to identify the oxygen-containing functional groups attached to the graphene sheets. In the GO/G-6-0.5 sample, it is expected to see the characteristic vibrations of the carboxylic groups, based on the Raman and XRD measurements. Fig. 5a and b shows the FTIR spectrum of GO/G-6-0.5 and G-3-1, respectively. The intense broad band at 3430 cm⁻¹ is attributed to the characteristic vibration of the OH group. The bands at 2922 and 2850 cm⁻¹ are the stretching vibrations of CH₂ groups. The characteristic bands from the vibrations of different oxygen functional groups such as C=O (carboxyl 1729 cm⁻¹), the O–H deformation (1401 cm⁻¹) or C–O–C stretching vibrations (1111 cm⁻¹) are weaker or has disappeared in the G-3-1 spectrum (Fig. 5b).³⁰

Fig. 5 FTIR spectra of samples (a) GO/G-6-0.5 and (b) G-3-1.

XPS measurements for two different samples (G-3-1 and GO/G-3-3) were performed in order to gain information about their chemical composition, the oxidation degree and the type of oxygen species present at the surface of investigated graphene structures. Carbon atoms have different binding energies depending on whether they are linked to one oxygen atom by a single or double bond, or if they are connected to two different oxygen atoms. For both samples the deconvolution of C 1s band revealed six individual component groups (Fig. 6a and 7a). The main contribution comes from the sp² carbon framework (surface and bulk), located at binding energies (BE) of 293.97 eV (G-3-1) and 284.08 eV (GO/G-3-3), respectively. Beside this main line, five other spectral features, dependent on the chemical environment experienced by the carbon atoms, could be observed and, as a result, they can be assigned to different carbon containing groups (C–O–C, C–OH, C=O, O=C–O and COOH). Table 6 summarizes the calculated percentages of graphitic and functional carbon atoms, where the values were given in atomic% of total intensity. The presence of structural disorder, carbon–hydrogen bonded groups in a sp³ hybridized state (which appear at the edges of the sp² network) and the high content of oxidized species are a clear proof for the changes of graphite structure due to the variation of electrochemical exfoliation process parameters. In the high binding energy region of the spectra at about 291.89 eV (G-3-1) and 290.30 eV (GO/G-3-3), respectively, a small contribution assigned to the π → π* shake-up satellite band of graphitic carbons appears.³¹

Fig. 6 High resolution XPS C 1s and O 1s core level spectra for sample G-3-1.

Fig. 7 High resolution XPS C 1s and O 1s core level spectra for GO/G-3-3.

Table 6 XPS fitting results of the C 1s bands, values given in at% of total intensity for G-3-1 and GO/G-3-3

	Binding energy (eV)/AC (%)
	Carbon atoms in polyaromatic structures C sp^2	Carbon atoms in aliphatic structures C sp^3	C–OH	C=O	O–C=O	π–π*
G-3-1	293.97 (43.23%)	284.96 (18.33%)	286.26 (19.92%)	287.89 (11.91%)	289.84 (4.27%)	291.89 (2.34%)
GO/G-3-3	284.08 (27.69%)	284.46 (21.55%)	285.42 (16.88%)	286.62 (18.12%)	288.18 (11.15%)	290.30 (4.61%)

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The shape of the experimental O 1s bands obtained for investigated materials suggests a deconvolution into four components corresponding to single-bonded oxygens, double-bonded oxygens, oxygen atoms from water and a small intensity component located in the low binding energy region of the spectra corresponding to the oxidized indium support used for measurement (Fig. 6b and 7b). Single-bonded oxygen in hydroxyls and epoxies has been found to be the dominant component, which is in agreement with the accepted models for graphene oxide.³² The water contribution was found out to be 9.31% for G-3-1 and 13.51% for GO/G-3-3, respectively.

The C/O atomic ratio is 0.345 in case of G-3-1 and 0.297 for GO/G-3-3, confirming the higher oxidation degree in the second material which consists in a mixture of graphene oxide (GO) with graphene (G) (see Table 3). The oxidation degree for the two samples is clearly different as one can also see from the carbon content. In G-3-1 sample, 38.44% of the total carbon content is oxidized, while for GO/G-3-3 the combined intensity of signals for ether/phenolic and carboxylic/ester functionality types reach levels of 50.76% of the total C 1s intensity.

3.3. Morphological characterization by TEM, SEM and AFM

TEM images offer valuable information regarding the exfoliation degree and the morphological aspects of the graphene sheets. TEM images of the samples prepared at various biases are shown in Fig. 8. All nanomaterials obtained by electrochemical exfoliation of graphite rods possess a sheet like morphology, with different transparency. The morphological aspects of the electrochemically exfoliated graphene are similar to those prepared by other approaches.³³ In all figures, the graphene flakes reveal a mixture of multi-layer graphene (black areas) with few-layer graphene (more transparent zones).

Fig. 8 Representative TEM images of the samples: (a) GO/G-6-0.5, (b) G-3-0.5 and (c) G-3-1; scale bar 200 nm.

The graphene morphology was further confirmed by SEM analysis. Fig. 9 shows typical SEM images of the GO/G-6-0.5, G-3-0.5 and G-3-1 samples. In all images one can see the thin and crumpled nanosheets which are randomly arranged and overlapped with each other. Actually, the morphology of the synthesized materials is quite similar with those previously reported.^34,35

Fig. 9 Representative SEM images of the samples: (a) GO/G-6-0.5, (b) G-3-0.5 and (c) G-3-1.

The AFM investigation of graphene samples confirms the results obtained by TEM/SEM as well as those provided by XRD measurements. The typical AFM image of the GO/G-6-0.5 sample (Fig. 10a) reveals a graphene flake with an average thickness of approximately 5 nm, corresponding to multi-layer graphene (∼14 layers, see Table 1; D = 4.76 nm according to XRD). The topography image of the G-3-1 sample reveals two graphene flakes, formed by thin and stacked carbon layers (Fig. 10b). From the corresponding profile line it is clear that the flakes have various thicknesses: the thinner one is around 9 nm which is in good agreement with the crystallite size determined by XRD (8.5 nm for MLG, see Table 4); the thicker one is around 15 nm; this is probably due to the re-stacking of layers, during the measurements.

Fig. 10 (a) Topography image at the edge of a graphene flake (GO/G-6-0.5 sample), revealing a thickness of about 5 nm; (b) topography image of two multi-layer graphene flakes (G-3-1 sample). The first step height of the layered sample is about 9 nm.

A comparison with other graphene samples (Table 7) obtained by electrochemical exfoliation of graphite in acidic solutions, reveal similar or even better properties of our materials.

Table 7 The comparison of different graphene samples prepared by electrochemical exfoliation of graphite in acidic solutions

Anode	Cathode	Electrolyte	Applied bias (V)	Properties of graphene	Ref.
Pyrolytic graphite sheet	Pyrolytic graphite sheet	0.5 M H2SO4	0–10 V	4–6 stacked graphene layers	17
Graphite rod	Pt wire	H2SO4, H3PO4 or H2C2O4	6–8 V	∼80% with D = 4–8 nm	18
Graphite flakes or HOPG	Grounded Pt wire	4.8 g of H2SO4 (98%) in 100 ml of deionized (DI) water	−10 to +10 V	D = 1.5 nm (FLG)	19
Graphite foil	Pt sheet	1 M NaOH, then 0.5 M H2SO4	+10 V	D = 1.8 nm (five-layer graphene)	34
HOPG foil	Pt wire	5.4 ml of 96 wt% H2SO4 in 200 ml of DI water	+3 to +6 V	The average layer number of the graphite nano-sheets vary from 6 to 22 as the bias enlarges from 3 to 6 V	36
Graphite rod	Pt foil	1 M H2SO4 : HNO3 (3 : 1)	+3 V	93% D = 1 nm (bi-layer graphene)	This work

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4. Conclusions

This paper presents the synthesis of high quality graphene by electrochemical exfoliation of graphite rods, using mixed (H₂SO₄ : HNO₃) electrolyte. Different electrochemical parameters (e.g. applied bias, electrolyte concentration) were studied in details and was demonstrated that the size of graphene flakes and the exfoliation/oxidation level can be controlled. Thus, in all of the as-prepared materials a mixture of few-layer and multi-layer graphene was present. However, as the exfoliation parameters were optimized, the graphene quality increased. The best sample contained 93% bi-layer graphene. The experimental conditions for the optimum sample were as follows: 1 M electrolyte concentration, 3 V applied bias and 2 h exfoliation time. Our present study is promising for scaling-up the graphene synthesis. The method has the advantage of using cheap reagents, ambient reaction conditions, a simple reaction set-up, and mild oxidation of the graphene layers, even in more concentrated electrolytes. Obviously, the reported method can be optimized to obtain only bi-layer graphene. Our objective was to show that the electrochemical exfoliation of graphite rods in acidic media is an interesting method to produce high quality graphene. Also, if some degree of oxidation is required, it can be obtained by choosing the appropriate electrolyte concentration and applied bias.

Acknowledgements

This work was supported by grants of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, Projects Number PN-II-ID-PCE-2011-3-0125 and PN-II-ID-PCE-2011-3-0129. We are very grateful to Gerald Kada (Keysight Technologies GmbH, Linz, Austria) for providing the AFM images. The authors are also grateful to Dr Ovidiu Pană and Dr Cristian Leoştean, for performing the XPS measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra19277c

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