Plasma electrolysis can be used to enhance the chemical or physical processes occurring at the electrodes. In this chapter, we describe a new plasma-assisted electrochemical exfoliation method, involving a plasma-generated graphite cathode and a graphite anode, for the production of graphene sheets from graphite electrodes in a basic electrolyte solution in a short reaction time. This method provides a green and fast step toward the mass production of two dimension materials such as graphene.
4-1. Introduction
Graphene sheets (GSs), including graphite nanoplatelets (GNPs), graphite nanosheets (GNs), exfoliated graphite (EG), and multiple-layer graphene, are nanometer-scale platelets that comprise of a few layers of planar graphene with platelet thicknesses ranging from 0.34 to 100 nm.16,18,87 Recently, GSs have attracted great interest because of their extraordinary properties and their potential applications in energy storage,4,46-49 composites materials,12,50,51,55,88
and polymer/GNP nanocomposites.59,89-91 The methods through which GSs are synthesized determine their structures and, therefore, can influence their practical applications.12,88,92-94
Several methods have been developed for the preparation of GSs, including chemical vapor deposition,62 discharging,7,8 mechanical milling,9,10 liquid-phase exfoliation of graphite,68,95 chemical reduction of exfoliated graphite oxide (GO),69,96 and electrochemical exfoliation.5,6,22,30,97-101
Among them, the electrochemical exfoliation of graphite is one of the simplest and most convenient methods for the large-scale production of GSs.
38
Plasma electrolysis, when the applied voltage is greater than the threshold voltage in electrochemical reactions, a strong electric field was generated near the working electrode, can be used to enhance the chemical or physical processes occurring at the electrodes. Herein, we describe a highly efficient plasma-assisted electrochemical exfoliation method, involving a plasma-generated graphite cathode and a graphite anode, for the production of graphene sheets from electrodes in a basic electrolyte solution in a short reaction time. This method provides a greener step toward the mass production of graphene. Up to now, we are unaware of any previous reports of the preparation of graphene sheets through electrochemical exfoliation involving plasma.
In the present study, a much larger potential, 60V, as compared to 5-10V in the case of conventional electrochemical method, is applied to the high-purity graphite (HG) cathode that maintain its tip barely above the basic electrolyte, a solution containing a mixture of KOH and (NH4)2SO4, different from conventional electrochemical cells,6,22,25 while the rod-HG anode is submerged in the electrolyte.
Discharge can be initiated by lowering the cathode to touch the surface of the electrolyte. Because the surface contact area of cathode in the electrolyte is much smaller than that of the anode, an extremely high electric field is generated near the cathode surface that is in contact with the electrolyte. Simultaneously, hydrogen gas bubbles evolved from the hydrolysis of the water in the electrolyte near the surface contact area of the cathode tip. As a result of an instant ionization of hydrogen gas bubbles by the presence the high electric field near the cathode tip, the onset of plasma around the cathode tip takes place (see experimental section).
4-2. Experimental
4-2.1. Preparation of plasma- electrochemically exfoliated graphene (PEEG)
39
The electrolytic solution, comprising KOH (5%, 200 mL) and (NH4)2SO4
(2.5%, 40 mL) at a pH of approximately 12, was preheated to an initial temperature of 70 °C. A cylindrical high-purity graphite rod (HG) was used as the cathode connected to a voltage supply unit (negative voltage output); the cathode diameter and length were 6 and 100 mm, respectively. Another HG rod (diameter: 6 mm;
length: 150 mm) was used as the anode in the electrochemical system for the plasma-assisted electrochemical exfoliation process (PEEP). The HG tip surface cathode was placed about 1 mm above the surface of the electrolytic solution, while the anode was submerged in the electrolytic solution. Both electrodes were connected to a regulated DC power supply (TES-6220) with the bias voltage increased gradually to 60 V, resulting in discharging plasma in the area adjacent to the HG tip surface cathode and the electrolytic solution. The temperature of the solution within the beaker was measured during the process using a conventional mercury thermometer; it was maintained at approximately 70–80 °C. To enhance exfoliation and the homogeneity of the reaction, a magnetic stirrer was placed in the beaker with its rate of spinning maintained at 200 rpm. When a sufficiently high potential of 60 V was applied across the two electrodes, electrochemical oxidation reactions were triggered with the simultaneous release of gases on the surface of anode and the generation of plasma on the HG tip surface cathode (see movie in SI); as a result, the surface of the graphite electrodes slowly disintegrated into micrometer-sized sheets and dispersed into the electrolyte. The tip position of the cathode was lowered to maintain an approximate current range from 0.6 to 1.2 A. The length of time in which the samples experienced simultaneous treatment was 5 min. Fig.4-1a provides a schematic representation of the equipment setup.
After cooling to room temperature, the resulting exfoliated graphite flakes were collected through vacuum filtration of the solution through PVDF membranes (average pore size: 0.2 µm) supported on a fritted glass holder. The prepared
40
products were washed with DI water and dried at 50 °C under vacuum for 24 h.
After peeling off the PVDF membrane, the powder prepared from PEEP, described herein as plasma- electrochemically exfoliated graphene (PEEG) was stored in a drying box at 50 °C until required for use. For conventional electrochemical method, two HG rod is dipped into electrolyte, one electrode is biased positively, and other electrode is biased negatively. The graphene sheets produced from this mode named EEG.
4-2.2. Preparation of PEEG dispersion
The obtained PEEG (15 mg) was added to N-methyl-2-pyrrolidone (NMP, 15 mL) to create PEEG dispersion (1 mg/mL) when treated with an ultrasonic cleaning bath, operated at 20 kHz and a power of 130 W for 10 min.
4-2.3. Measurements and Characterization
The structures of the HG and the PEEG powders were examined using a D2 X-ray diffractometer equipped with a Cu K tube and a Ni filter ( = 0.1542 nm).
Raman spectra of these samples were recorded using a high-resolution confocal Raman microscope (HOROBA, Lab RAM HR) and a 514.5 nm Ar laser source.
High-resolution transmission electron microscopy (HRTEM) images were recorded using a JEOL 2100F apparatus operated at 200 kV; for HRTEM measurement, a few drops of the HG or PEEG dispersion were placed on a Cu grid presenting an ultrathin holey C film. Scanning electron microscopy (SEM) was performed using a JEOL JSM-6500F scanning electron microscope operated at 15 kV. For preparation of the SEM sample, a PEEG dispersion was filtered through an AAO membrane (Anodisc; diameter: 47 mm; nominal pore size: 0.02 µm); the solids were then dipped in EtOH to remove residual NMP; the flakes that floated on the surface of the EtOH were collected on a Si substrate for SEM measurement.
41
4-2.3. Results and discussion
Figure 4-1. (a) Schematic representation of the equipment used for PEEG. (b–e) Photographs of (b, c) the electrolytic solution (b) before and (c) after plasma-assisted electrochemical exfoliation process; (d) the PEEG-based graphene film prepared through vacuum filtering of the electrolyte after plasma-assisted electrochemical exfoliation process; and (e) a dispersion of PEEG in an NMP solution.
Fig. 4-1(a) show a schematic representation of the equipment used for production of the plasma-electrochemically exfoliated graphene sheets (PEEG).
Figs. 4-1b and 4-1c display images of the electrolyte at various times during the PEEG, revealing a change in color, from colorless to dark, after only 5 min (see movie in the SI), suggesting a high exfoliation rate when using this method. Fig. 4-1e displays a photograph of the PEEG dispersion in NMP after settling for 6 h; this sample exhibited good dispersion and homogeneity and was stable for a period of
42
72 h. In addition, the PEEG-based graphene film prepared through vacuum-filtering of the electrolyte after plasma-assisted electrochemical exfoliation process exhibited good conductive properties with resistance of 130 Ω (Fig.4-1d) and sheet resistance of 121 Ω. per square area with a four-probe method. This graphene film might have been compressed during filtration, resulting in re-agglomeration of graphene nanosheets.
Figure 4-2. (a) XRD patterns and (b) Raman spectra of HG and PEEG and (c, d) XPS spectra (C 1s signal) of (c) HG and (d) PEEG.
43
Fig. 4-2a displays X-ray diffraction (XRD) patterns of the high purity graphite (HG) and PEEG; that of the HG displays a characteristic sharp, high-intensity (002) peak at a value of 2 of 26.6° and three small peaks at values of 2 of 42.8, 44.6, and 54.6°, corresponding to the 100, 101, and 004 reflections, respectively. After the HG had experienced plasma-assisted electrochemical exfoliation process, the intensity of the characteristic (002) peak at 26.6° decreased significantly, suggesting that the graphitic lattice of HG had changed its periodic arrangement in the z-direction after exfoliation into graphite flakes. Additionally, the Raman spectra of HG and PEEG (Fig. 4-2b) reveal a structural change from HG to PEEG. The Raman spectrum of HG features a weak D (defect) band, a prominent G (graphite) band, and a broad 2D (doubly generated G) band at 1353, 1579, and 2706 cm–1, respectively, which are characteristic bands of graphite.83,84 In the Raman spectrum of PEEG, the G band was broader and had shifted to a lower frequency and the D band was more pronounced than that in the spectrum of HG, implying that defects or structural disorder had occurred in the graphitic lattice of PEEG after PEEP. Notably, the 2D band of PEEG had shifted to a lower frequency with a significant increase in intensity relative to that of HG, indicating the formation of graphene structures in PEEG after PEEP.24,95,99,102,103
Fig. 4-2c displays the C 1s XPS spectrum of HG; we de-convoluted the C 1s XPS signal into one major peak at 284.4 eV representing C=C bonds (sp2 -hybridized carbon atoms) and a minor peak at 285.5 eV representing C–C bonds (sp3-hybridized carbon atoms).79 Fig. 4-2d presents the C 1s XPS spectrum for PEEG; we could de-convolute it into three peaks: the signals of PEEG at binding energies of 284.4 and 285.5 eV represent non-oxygenated carbon components—
C=C (sp2-hybridized carbon atoms) and C–C (sp3-hybridized carbon atoms) bonds, respectively—while the other at 286.4 eV represents oxygenated C–O components
44
(i.e., hydroxyl and epoxy units). The intensity and sharpness of the graphitic C=C/C–C signals decreased and widened significantly relative to those of the HG sample, implying that some oxygen-containing functional groups had attacked the C=C bonds in the graphite to generate C–O bonds, resulting in their partial oxidation.79
Table 4-1 lists the calculated amounts of the various functional groups in the HG and PEEG samples, based on the areas under these XPS peaks. The atomic percentage ratio of oxygen to carbon atoms in the PEEG [as determined from the small content of C–O bonds (ca. 4.2%)] was approximately 1:22.8, confirming that very slight oxidation occurred during our PEEP process. Notably, strong oxidation could yield graphite oxide–like structures when increasing concentration of electrolyte, changing HG anode into a stainless-steel grid with much higher surface area than that of cathode in which the vapor plasma envelope calorific effect provided instant oxidation and expansion of graphite for producing plasma-expanded graphite oxides from HG.104
Table 4-1. Relative atomic percentages of carbon atoms in various functional groups in HG and PEEG, estimated based on the areas under the C 1s peaks.
C=C (%) C–C (%) C–O (%)
HG 84 16
PEEG 68 27.8 4.2
45
Figure 4-3. SEM (a, c) and TEM (b, d) images of (a, b) HG and (c, d) PEEG.
46
Figure 4-4. SEM images of flattened or scrolled PEEG (b, c, d) high-magnification images; Two arrows pointing in opposite directions indicate the thickness of PEEG that was on the surface of the Si/SiO2 substrate.
47
Figure 4-5. TEM images of flattened or scrolled PEEG, inset: corresponding SAED pattern. Two arrows pointing in opposite directions indicate the thickness of PEEG that was on the top of the copper grid.
48
Figure 4-6. AFM images and height profile of a PEEG sample deposited on a Si/SiO2 substrate.
Fig. 4-3 presents SEM and TEM images of the HG and PEEG samples that were collected after filtration with PVDF film and dispersed in NMP, respectively;
49
they indicate a dramatic change in morphology in the prepared samples. The HG comprised thick, bulk graphite flakes that transformed into thinner nanometer-scale platelets after after plasma-assisted electrochemical exfoliation process. We also observed scrolled graphene sheets, as indicates by the arrows in Fig. 4-4 and Fig.
4-5, where in the folded regions, the thickness of layers could be approximately determined (see high-magnification images of the folded regions in Fig. 4-4 and Fig. 4-5). Image analyses based on PEEG sheets revealed that the average lateral dimensions was approximately 2.5 µm with a thickness of approximately 10–30 nm, close to the dimensions of a nanoplatelet;16,89 based on cross-sectional imaging of the folded edges of PEEG after tilting the sample from 0 to 25°; furthermore, Fig.4-6 shows the AFM image of the samples prepared from a diluted solution, revealing in a lateral dimension of approximately 0.5–2.5 µm and a thickness of approximately 2.5 nm, corresponding to approximately seven layers of graphene, based on an interlayer spacing of 0.34 nm, confirming the formation of graphene sheets. The exfoliated graphite flakes have a tendency to aggregate and stack on each other after drying on the surface of Si/SiO2 substrate and copper grid when they are used for SEM and TEM observation, resulting in different thickness values obtained using SEM/TEM and AFM imaging.
50
Table 4-2. Comparison between graphene sheets produced with plasma-assisted and conventional electrochemical exfoliation methods
Table 4-2 displays a comparison between the PEEG that were produced with the plasma-assisted electrochemical method and a conventional electrochemical method and indicates the production rate of graphene sheets is six times as fast as that obtained from the conventional electrochemical method, 32mg vs. 5mg in five minutes. Moreover, the PEEG produced are larger and thinner than that fabricated by the conventional electrochemical method (see Fig. 4-7), suggesting that the plasma-assisted electrochemical exfoliation method appears to be a very efficient means of synthesizing graphene sheets.
51
Figure 4-7. (a) SEM high-magnification and (b) TEM images of electrochemically exfoliated graphene sheets (EEG), (c) Raman spectra of EEG, and (d) AFM image and height profile of a EEG sample deposited on a Si/SiO2
substrate.
52
Figure 4-8. Proposed mechanisms for the formation of PEEG.
Fig. 4-8 depicts the mechanism of the formation of graphene sheets from graphite. We suspect that rapid hydrogen bubble evolution (hydrolysis of water) in the cathode played a role in the exfoliation of the graphite as depicted in Fig 4-8:
first, a violent release of hydrogen gas on the tip of the HG surface cathode results in opened up the edge sheets of the its surface and facilitated the intercalation of hydrogen gas into the graphite layers, forming graphite intercalation compounds, as a result of these expansion processes, the van der Waals forces between the graphitic sheets weakened. Second, in the plasma the temperature could instantaneously reach above 2000 °C within a short period of time (e.g., <10–6 s)35 on the HG tip surface during discharging, thermo-mechanical stresses occurred on this surface. A combination of these two factors induces the expansion and sequential exfoliation of the surface layer of the thick graphite cathode into smaller and thinner graphene sheets. Furthermore, electrochemical reactions on the surface of anode could induce exfoliation directly from graphite electrode, producing a small portion of graphene sheets from the starting graphite. Thus, our
53
method is a new route toward the production of graphene sheets from both electrodes simultaneously in a basic electrolyte medium.
4-3. Conclusion
Graphene sheets with good dispersion in solvents can be prepared through plasma-assisted electrochemical exfoliation process at moderate temperatures without the need for acidic media. This method is quite promising because of its simple setup, environmentally benign, and rapid throughput.
54
Chapter 5: The influence of electrolytic concentration on morphological and