Synthesis and Characteristics of Graphene–Graphene Oxide Material Obtained by an Underwater Impulse Direct Current Discharge

This work proposed a novel approach, in which an impulse underwater discharge was used to produce graphene–graphene oxide material in distilled water. The characteristics of this discharge were presented, including electrical parameters, plasma composition, and electron concentration. Graphene-based material produced using this approach can be evenly dispersed in water without the use of a surfactant or stabilizer, and is suitable for storage at room temperature. Ultraviolet–visible spectroscopy was employed to analyze the optical properties of the graphene-based structures. Scanning electron microscopy was adopted to explore the morphology and size of the particles. The FTIR spectroscopy confirms the formation of graphene oxide. The Raman spectroscopy demonstrates the formation of a graphene-containing multilayer material. The results of this study confirmed that graphene-based material production by impulse underwater discharge is a low-cost, fast, and effective manufacturing method.


Introduction
Graphene is a two-dimensional (2D) layer of sp 2 -hybridized carbon atoms arranged in a hexagonal structure. In the 2D plane, strong σ-bonds form the graphene skeleton, and perpendicular π-bonds form a two-dimensional electron gas with linear band dispersion near the Fermi level. The unique lattice and electronic structure determine the unique electrical, mechanical and physicochemical properties of graphene [1,2] and the huge potential for its application in new generation electronic devices, composite materials, energy storage devices, the aerospace sector, the automotive industry, electronics, small-scale energy, ecology, and biomedicine [3][4][5].
Graphene can be obtained by various processes, including chemical vapor deposition [6], micromechanical cracking of graphite [7] and exfoliation of graphite [8]. All existing 1 3 methods for producing graphene have a number of significant drawbacks. These are the high cost of the final product, the significant energy and labor intensity, the use of the strongest oxidizing and reducing agents, the multilayer nature of the final graphene structures with numerous defects. Currently, graphene is mainly produced by chemically oxidizing graphite to graphene oxide (GO) and then reduction to graphene [9]. Graphene oxide is currently used in biotechnology and medicine for cancer therapy, drug delivery, and cell imaging [10]. In addition, GO has various physicochemical properties, including nanoscale size, high surface area, and charge [11,12]. Therefore, both graphene oxide and graphene have unique properties and are future materials with broad application prospects.
It was found that low-layer graphene was successfully prepared by an arc discharge burning process. The first experimental work on the production of carbon materials using an electric arc discharge, in particular fullerene (C 60 ), was carried out by Krastchmer and Hoffman [13]. Subsequently, this method was widely used to obtain nanocarbon materials [14][15][16][17]. In these works, the process of obtaining graphene took place in an atmosphere of hydrogen, ammonia, helium, or air when an arc discharge occurred between graphite electrodes at a discharge current of 100-150 A. The surface area of graphene samples, determined by the Brunauer-Emmett-Teller (BET) method, was in the range 270-680 m 2 /g [18]. However, gas-phase plasma processes usually require complex gas path connection systems, which increase operating costs. Furthermore, the gaseous precursors used (including hydrogen, ammonia, methane) are explosive or corrosive in nature, which poses safety concerns and limits their industrial scalability for practical applications. Although the plasma-solution modification of the surfaces of various materials has been used since the middle of the twentieth century [19], there are practically no works on the synthesis of graphene using low-temperature plasma in liquid media. Experimental results show that liquid microwave discharge plasma can effectively reduce GO solution, further restore the π-conjugated structure of graphene at low temperature, and prepare plasma-treated reduced graphene oxide with low oxygen and high conductivity [20]. But in this work, graphene was reduced from chemically synthesized graphene oxide. In [21] graphite electrodes were placed in deionized water for ignition of the electric arc discharge to obtain multilayer graphene. The discharge current was maintained at the level of 40-140A. The authors obtained samples of graphene with the number of layers from 2 to 8.
The studies found that impulse underwater discharge initiated by a direct current source is an efficient method to obtain various nanoparticles [22][23][24]. It is a simple and fast method that does not require sophisticated equipment, harmful chemicals or the use of an inert atmosphere.
In this study, we carried out and analyzed the results of a one-step synthesis of graphene-containing particles, using the low current underwater impulse discharge in liquid. Figure 1 presented the scheme of the setup for the synthesis of graphene-containing materials using the impulse underwater discharge. The impulse underwater discharge was excited between two graphite rods 5 mm thick. The graphite rods (CJSC "Grafitservis", Russia, and graphite content 99.99%, density 1.814 g cm −3 ) were placed in heat-resistant glass tubes 7 mm in diameter. The DC power supply BP-0.25-2 (LLC 1 3 "TD ARS THERM", Russia) with output voltage up to 5 kV and 0.5 kΩ ballast resistor excites the discharge. The voltage and current waveforms were recorded using the multichannel digital oscilloscope Hantek-4104B (Hantek, China). The average discharge current is 0.25 A. The experiments were performed in a plasma cell with a constant solution volume of 200 ml and flat quartz optical windows. Real-time images of the discharge were captured by a Baumer VCXU-04M high-speed camera with an AZURE-0918M3M lens at a frame rate of 430 frames per second. Discharge emission spectra (λ = 200-950 nm) were recorded using an AvaSpec ULS-3648 spectrometer (Avantes, The Netherlands) at a resolution of 0.3 nm. The electrodes were weighed before and after discharge ignition on an analytical balance to determine the mass production rate of graphene-containing particles. The resulting suspension of graphenecontaining particles was dried at room temperature for further physical investigation.

Characterization
The surface morphology of graphene-containing was obtained with the scanning electron microscopy (Quattro S, Thermo Fisher Scientific, Czech Republic).
The phase composition of obtained structures was analyzed by X-ray diffraction (X-ray diffractometer D2 Advance, Brucker, CuK α source). The interpretation of diffractograms was performed with the use of the COD open crystallographic database.
FTIR spectra of synthesized particles were recorded using the VERTEX-80v spectrometer (Brucker Optics, Germany) with 0.2 cm −1 resolution in the range of 4000-400 cm −1 .
Raman spectra of the samples were recorded with Confotec NR500 Raman microscope by using 532 nm excitation wavelengths.
Measurements of the specific surface area by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were conducted on NOVAtouch NT LX-1 Quantachrome analyzer at 77 K (USA).
The spectra of aqueous suspension were analyzed by a spectrophotometer SF-56 (Spectr, Russia) in the range of wavelengths 210-850 nm.
Thermal characteristics of powder were studied utilizing differential scanning calorimetry (DSC) measurements using Dynamic Heat Flux Differential Scanning Calorimeter DSC 204 F1 Phoenix (NETZSCH). The sample (1.51 mg) was scanned in the temperature range of 25-240 °C with a heating rate of 1 °C min −1 under an argon atmosphere.

Preparation of Graphene/GO-Aluminum Electrode
The graphene/GO powder was dispersed in distilled water to achieve a concentration of 0.5 mg/ml. The obtained dispersion was then cast directly onto aluminum (Al) foil in a Petri dish and dried in an oven at 40 °C. The dried powder on aluminum foil was then cut into two rectangles and used as working electrodes. The electrodes were then sandwiched and separated with filter paper soaked in sodium sulfate solution. The electrodes were clamped to the assembly using a microscope slides and clamps. Cyclic voltammetry measurements were performed on the electrodes in the voltage range of 0-1 V using an electrochemical workstation (P-20X, Electrochemical Instruments, Russia). The potential sampling rate was set to 0.1 V s −1 .
The specific capacitance of the electrodes could be calculated by the following equation: where C (Fg -1 ) is the specific capacitance, m is the mass of the active materials, Q is the average charge during the charging and discharging process, and V is the potential window. The discharge specific capacitance could also be calculated from the discharge curves by the equation: where I, t, m and V are the current, time consumed in the potential range of V, mass of the active materials, and the potential windows, respectively.

The Characteristics of a Pulsed Underwater Discharge During the Synthesis of Graphene-Containing Particles
The discharge starts with the formation of air bubbles near the electrodes, as shown in Fig. 2. During the initial stages of electrolysis, the chemical process of releasing oxygen and hydrogen occurs at the electrodes. The solution was still not sufficiently heatedAt the beginning. It can be assumed that it is not water vapor bubbles but oxygen or at least mainly composed of oxygen. When the interelectrode space is filled with bubbles, a breakdown occurs, resulting in a sharp increase in current. Increasing the current will cause the solution to overheat and the diameter of the bubbles will increase due to the formation of water vapor. In contrast to a discharge initiated between two metal electrodes, two discharge pulses follow one another (Fig. 3). This can be explained by the relative positions of the electrodes. Initial breakdown occurs at the tip of the electrode, where the gap is the smallest. The first discharge pulse initiates the development of the second pulse. Furthermore, the current decreases and the process repeats. The active burning time of the discharge is about 7 ms. A breakdown of liquid causes a sharp increase in the amplitude of the voltage and current. The amplitude value of the current is 3 A. The amplitude value of the voltage is 1.8 kV. The data obtained from the multichannel digital oscilloscope was processed on a computer according to the ratios presented in the study [25]. The obtained discharge energy characteristics are shown in Table 1. The power value of the discharge is obtained higher than in the case of underwater impulse discharge burning between two metal electrodes, where the discharge power is 22 W.
The estimates of the energy consumption for the formation of mixed oxides particle were made using the relationship: η = P d /v, where v is the rate of formation of graphene powder [26]. The values of the average rate were determined from the difference of the masses of the electrodes before and after the discharge burning. The average energy consumption is about 950 eV atom −1 ( Table 1). The obtained results of energy consumption are comparable with the results on the synthesis of metal oxide particles using nanosecond plasma (75-600 eV atom −1 ) presented in [27]. It should be noted that the energy consumption for producing graphene in arc discharges in water [21] is larger.
The emission spectrum of a discharge is presented in Fig. 4. The atomic oxygen lines at 777 and 844 nm and hydrogen Balmer lines of H γ at 434 nm, H β at 486 nm and H α at 656 nm are registered. The bands of OH radicals in the range of 280-340 nm are represented. "Swan-band" emission from C 2 has been registered to range from 461 to 481 nm for the Δv = 1 sequence, from 501 to 517 nm for the Δv = 0 sequence [28]. In addition, the emission lines of atomic carbon at 472 nm and 494 nm and the bands of the CH radicals  Graphite electrodes 24 ± 3 6.7 ± 0.9 150 ± 15 1.2 ± 0.5 4500 ± 300 950 ± 100 (A 2 Δ-X 2 П) in the region of 425-430 nm are present in the emission spectrum of the discharge [29]. The presence of lines and bands of carbon-containing particles in the emission spectrum may indicate the sputtering of the electrode material and the reactions occurring in the plasma with the participation of these particles. The temperature was determined by fitting the synthetic spectrum to the experimental spectrum of the hydroxyl radical transition emission band in the 280-310 nm (A 2 Σ → X 2 Π) range using the CyberWit Diatomic 1.4.1.1 [30]. The rotational temperature of OH in the plasma is about 4500 K. The value of the rotational temperature of hydroxyl radicals can be identified with the gas temperature in the plasma bulk. The Stark broadening of the H α line is used to determine electron densities n e . An approximation formula for the electron density n e (in cm −3 ) dependent on the full width of half area (FWHA) of the Stark profile Δλ FWHA S (in nm) is given by Gigosos et al. [31]: For the Lorentzian distribution, van der Waals broadening and Stark broadening are considered only. Using data on the molar fractions of components in the plasma, taking into account the energies of possible transitions and reduced masses, the final expression for the van der Waals FWHA of H α can be written as: The obtained data are presented in Table 1.

The Characteristics of Obtained Graphene-Containing Particles
The UV-Visible absorption spectra of a synthesized suspension with graphene-containing particles (red curve) and graphite powder (black curve) dispersed in deionized water is shown in Fig. 5. The particles were stably suspended in water instead of depositing as sediment.
As seen from the Fig. 5, the graphite powder has only a shoulder at about 275 nm, whereas graphene-containing particles have a shoulder peak at 298 nm. The UV-Vis spectra obtained in the present study match well with the literature data [32]. An absorption peak shoulder at 298 nm agrees to the n-π* transition of the C=O group. The obtained observation confirms the formation of graphene oxide (GO) [33]. Powder XRD measurements were carried out to describe and observe the structure and formed phases of the fabricated materials. The average crystalline size and microstrain values of the materials were calculated via Scherrer's equations, where, d and ε are the crystalline size and micro-strain of the materials respectively, λ is the wavelength of X-ray radiation, β is the full width half maximum value (FWHM) and θ is the diffraction angle [34]. The Bragg's equation was applied to (002) reflection for evaluating the distance between graphene layers, denoted as d:d = 2sin , (6) Figure 6 displays The graphite shows a sharp and narrow peak (2θ = 26.5°) which corresponds to the diffraction line C (002) with the intercellular spacing in the crystal is 3.2 Å. The data shows the typical crystal structure of graphite. The GO shows two diffraction peaks at 2θ = 10.4° and 42.2° that correspond to (001) and (100) diffraction planes with the spacing between plane is about 8.8 Å. [34]. Increasing the distance between layers in the graphene oxide is due to the presence of oxygen-functional groups and water molecules in the carbon layer structure. The peak is observed at 2θ = 23.9° which indicates that graphene oxide is not fully interconnected with oxygen atoms. Thus, graphene is also present in the sample in a fairly large amount. The resulting graphene has a structure between the crystalline and amorphous structures. This is evidenced by the appearance of the diffraction line C (002) which looks wider and the intensity is lower than the peak obtained in the graphite powder. The Scherrer equation with Warren constant of 1.84 [35] was applied to two-dimensional (100) reflection for estimating the average size of stacking layers, denoted as L. If each parallel layer consists of n layers, L for a parallel layer group is defined as L = (n − 1)·d, or n = (L + d)/d [36]. The obtained graphene-containing particles consist of 7-10 layers in a stacking nanostructure with layer distance about of 0.75 nm. XRD analysis data are presented in Table 2. SEM images of the obtained sample are shown in Fig. 7. The average size of the scales can be estimated at several micrometers, although smaller scales may be observed. The number of layers corresponds to the results obtained from X-ray phase analysis. FTIR spectra of initial graphite and obtained graphene-containing powder are presented in Fig. 8. The spectrum of the initial graphite shows only bands related to C=C and C-H bond vibrations. Figure 8 shows that the synthesized powder has peaks at 1081 cm −1 which is attributed to the C-O bond of GO, confirming the presence of oxide functional groups after the oxidation process in discharge plasma. It can be noted the peaks appear in graphene oxide at 3422, 1737, 1639, 1380, and 1250 cm −1 due to -OH stretching, C=O  (carboxyl) stretching, C=O, -OH bending and C-OH stretching [37]. The absorbed water in GO is shown by a broad peak at 2885 cm −1 to 3715 cm −1 , contributed by the O-H stretch of H 2 O molecules [38]. This supports the fact that GO is a highly absorptive material.
DSC measurements have been further performed to examine the thermal properties of the prepared powder, containing graphene oxide (Fig. 9). An irreversible endothermic peak is observed in the range between 39 and 158 °C and is attributed to elimination of water entrapped between the expanded layers of graphene oxide and graphene [39]. An endothermic peak around 250 °C indicated the thermal decomposition of the oxygenated functional groups in GO [40]. The value of the specific heat capacity of the sample in the indicated temperature range varies from 0.6 to 2.0 J g −1 K −1 , which is consistent with the data from [41].
The N 2 adsorption-desorption technique was used to characterize the porous structure of the powder. Figure 10 shows the isotherms and the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution curve.
The BET surface area, BJH surface area, BJH desorption average pore diameter, and pore volume of the synthesized powder are summarized in Table 3. The pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method, using the adsorption branch of the isotherm. The obtained particles have an average mesopore diameter of about 5 nm. The data indicate a layered structure of the obtained powder containing graphene/ graphene oxide.
Since graphite consists of multilayer graphene structures, their Raman spectra include the same groups of bands characteristic of graphite, graphene, and graphene oxide; therefore, it is appropriate to interpret the position and shape of the bands simultaneously for three related structures-graphite, graphene, and graphene oxide (Fig. 11). The most intense band in these structures (~ 1582 cm −1 ) is known as the G band [42]. In graphite G-the band is narrower, its intensity is much higher than in graphene. In GO, the band broadens and shifts towards lower energy; in graphene/GO, this is associated with the influence of the layer thickness, i.e., number of layers. The shoulder about ~ 1600 cm −1 in graphene/GO is associated with defects in their structure. The next band in the ~ 1350 cm −1 region is known as the D band. In graphite D, the band is weak and narrower in comparison with graphene/OG. In graphene D, the band is known as the disorder band or defect band [43]. In single-layer graphene, the band is weak compared to graphene oxide. The intensity of the band increases with the number of layers. The intensity of the D band is also directly proportional to the number of defects. In the Raman spectra, the I D /I G band intensity ratio close to unity is typical for GO [44]. The third band in the considered structures is known as the 2 D band ~ 2685 cm −1 [45]. In graphene/ GO, the intensity of the 2D band is much lower than in graphite. The band in the OG is wider. The broadening of the band is associated with the presence of defects in the sample and, to a greater extent, with the number of GO layers. As the number of layers increases, the maximum shifts towards an increase in the wavenumber.

The Electrochemical Performance of Obtained Graphene-Containing Particles
The cyclic voltammetry of Al foil and prepared graphene/GO-Al foil is shown in Fig. 12. Due to the porous structure of filter paper and the good capacitive behavior of   [46]. Due to the larger current response value and CV range the electrochemical performance of graphene/GO-Al is much better than that of Al foil. The quasi-rectangular area suggests high double layer capacitance [47]. The capacitance can be calculated by integrating the curve area of the CV curve [48]. The graphene/GO-Al capacitance is 100.5 mF, while the aluminum foil capacitance is 0.02 mF. The capacity of Graphene/ GO-Al foil is four orders of magnitude higher than that of Al, so GO greatly improves the capacitance performance.

Conclusion
In this work, we demonstrated a fast, effective, and environmentally friendly process for producing graphene-based material by using underwater impulse discharge. UV-vis, FTIR, Raman spectroscopy and DSC, BET, XRD analyzes confirmed the presence of graphene and graphene oxide in the samples. Finally, a material containing up to 10 layers of graphene and graphene oxide was obtained. The discharge used in the synthesis of graphene-containing material is a pulsed discharge with a pulse duration of about 7 ms. The emission spectra of the plasma confirm the sputtering of graphite electrodes during discharge combustion and the occurrence of chemical processes involving carbon particles. The rate of formation of graphene-containing material is about 0.5 g/h. The obtained graphene/GO was found to enhance the electrochemical behaviors of the supercapacitor set-up. Further research in this area will be aimed at selecting experimental conditions, including the discharge current, varying the material of graphite electrodes, additional exposure to ultrasonic waves, in order to obtain pure graphene or pure graphene oxide.