Research Note pubs.acs.org/IECR
Ultrasonic-Assisted Production of Graphene with High Yield in Supercritical CO2 and Its High Electrical Conductivity Film Yahui Gao,†,‡ Wen Shi,† Wucong Wang,† Yan Wang,† Yaping Zhao,*,† Zhihong Lei,† and Rongrong Miao† †
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, China Department of Environmental Engineering and Chemistry, Luoyang Institute of Science and Technology, 90 Wang Cheng Road, Luoyang 471023, China
‡
ABSTRACT: A simple, cost-effective approach is presented for producing exfoliated films of pure graphene or polymer− graphene composite with high yield, high conductivity, and processability. The approach combines supercritical CO2 with ultrasonics. Characterization by Raman spectroscopy combined with atom force field microscopy demonstrates that the graphene sheets were obtained with 24% as monolayers, 44% as bilayers, and 26% as trilayers. The layer number and lateral size of graphene sheets can be controlled by adjusting the process parameters. The yield of graphene sheets with a lateral size of about 0.5−5.0 μm is about 16.7 wt % under optimum conditions, which can be easily raised to 40−50 wt % by repeated exfoliation of the sediment that remained in the reactor. The resultant pure graphene film made by filtration has a high electrical conductivity of 2.8 × 107 S/m. The electrical conductivity of the film of polyvinyl alcohol−graphene composite is 300 S/m.
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INTRODUCTION Since its discovery, graphene has garnered attention worldwide because of the excellent physical and chemical properties it possesses.1−5 At present, a variety of methods for the preparation of graphene have been reported, all with the overarching goal of exploring graphene’s unique properties in order to increase potential practical applications. However, these methods each have their own unique disadvantages. For instance, high-quality graphene can be obtained through micromechanical cleavage,1 but this method is both too expensive and time-consuming to be applicable for mass production. Furthermore, graphene can also be prepared by epitaxial growth on a SiC substrate6 and by chemical vapor deposition (CVD) on metal substrates;7−9 however, these methods result in spatially nonuniform products, make transferring difficult, and are expensive. Another technique for graphene production is the chemical reduction of graphene oxide (RGO),10−13 yet it too has its drawbacks. The aggressive oxidation process of graphite introduces oxygen containing groups, such as hydroxyl (−OH), carbonyl (−COOH), and epoxide groups, into the basal plane of graphene, which then introduces structural defects into the sp2-bonded network and results in the network’s disruption. As a result, its electronic performance is severely damaged. Finally, the recently developed method of liquid phase exfoliation (LPE) of graphite in various organic solvents or a surfactant−water solution assisted by ultrasonication produces oxygen-free graphene.14−17 However, completely removing the residual solvent or surfactant from the resultant graphene is difficult, and as a result, there is a deterioration of the product’s electrical properties. In addition, this method is time-consuming. Recently, supercritical fluid is applied to exfoliate graphite for preparing graphene because of its zero interfacial tension, wonderful surface wettability, and high diffusion coefficient. Pu et al.18 first reported that graphite could be intercalated and © 2014 American Chemical Society
delaminated into graphene sheets in supercritical CO2, but the exfoliated graphene sheets dispersed throughout the surfactant solution each had at least 10 layers. Then, Rangappa et al.19 studied the conversion of graphite crystals into graphene using organic solvents under their supercritical conditions for the first time. Using ethanol, N-methylpyrrolidone (NMP), and N,Ndimethylformamide (DMF) as the supercritical solvents, 90− 95% of the exfoliated graphene sheets had less than eight layers, and about 6−10% were monolayer graphene. However, the processing conditions are exceptionally severe as it requires high temperatures of 300−400 °C and high pressures of 38−40 MPa. Furthermore, the high boiling solvents of NMP and DMF are toxic and difficult to remove from the final product. In addition, Liu et al.20 performed similar studies using DMF as a supercritical solvent, and their yield was only 7−8%. Although graphene sheets composed of less than 10 layers can be obtained by using supercritical organic solvents, it is advantageous to use CO2 as the supercritical solvent for graphene production because doing so requires a lower critical point, is nontoxic, and is economically and environmentally friendly. It was first reported by Wu et al.21 that graphene sheets could be stable in supercritical CO2 fluid because the repulsive nature of the free energy barrier generated in supercritical CO2 could prevent the aggregation of graphene. Recently, our group has developed a simple ultrasonic exfoliation method in supercritical CO2 for the fabrication of two-dimensional BN, WoS2 and MoS2 layers.22 In this work, we systematically investigated the ultrasonic exfoliation of graphite into graphene sheets in supercritical CO2. The pure graphene film and polymer−graphene composite film were also Received: Revised: Accepted: Published: 2839
September 2, 2013 January 28, 2014 January 30, 2014 January 30, 2014 dx.doi.org/10.1021/ie402889s | Ind. Eng. Chem. Res. 2014, 53, 2839−2845
Industrial & Engineering Chemistry Research
Research Note
Figure 1. Schematic drawing of exfoliation graphene sheets with supercritical CO2 assisted by ultrasonication: (a) layered graphite crystal; (b) layered graphite immersed in supercritical CO2; (c) CO2 molecules penetrate and intercalate between interlayer of graphite; (d) forming mono- or few-layer graphene sheets; (e) graphene sheets dispersed in 40% ethanol aqueous solution.
using a UV−vis spectrophotometer (PC 756, Shanghai Spectrum Instruments Co., Ltd., China), and its concentration was calculated according to Lambert−Beer law, A/l = αC, similar to that in previously reported work.15 Herein, A/l (m−1) is the absorbance per cell length, α (mL·mg−1·m−1) is the absorption coefficient (α = 1505 mL·mg−1·m−1 is the mean value of triplicate tests), and C (mg·mL−1) is the dispersion concentration. The yield of graphene was calculated by the formula: Y (wt %) = (CV/m) × 100 (V is the volume, m is the initial graphite mass). 2.3.2. Transmission Electron Microscopy. Transmission electron microscopy (TEM), a JEOL JEM-2100F operated at an accelerating voltage of 200 kV, was used to identify the morphology and layer number of the resultant samples. A few milliliters of the graphene dispersion were deposited dropwise onto standard copper grids covered by thin holey carbon films (400 mesh) and then dried prior to characterization. 2.3.3. Atomic Force Microscopy. The layer number and morphology of the resultant samples were probed by atomic force microscopy (AFM), a scanning probe microscope instrument, a Veeco Dimension 3100 in tapping mode. Several microliters of graphene dispersion were deposited onto freshly cleaved and heated mica substrates. The mica surface was dried at 110 °C to evaporate ethanol and water. 2.3.4. Raman Spectra. Raman spectra, Renishaw inVia Reflex Raman System equipped with a 532 nm laser source and 100× objective lens, was used to identify the layer number and quality of the obtained samples. Several milliliters of graphene dispersion were deposited on the substrates of Si/SiO2 (300 nm) and then dried under vacuum (10−3 mbar) at room temperature. 2.3.5. X-ray Diffraction (XRD). The graphite and graphene dispersions were deposited individually onto glass slides and then dried under vacuum (10−3mbar) at room temperature. The samples were examined on a model of D/max-2200/PC X-
fabricated by as-exfoliated graphene sheets in order to study its electrical properties.
2. EXPERIMENTAL SECTION 2.1. Materials. The graphite powders used in all experiments were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) (product number 20019128). They were passed through a 100 mesh sieve before use. Carbon dioxide (99.9%) was obtained from shanghai high-tech Co., Ltd. (China). Absolute ethanol (99.5%) and polyvinyl alcohol (PVA) (average degree of polymerization 1750 ± 50) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used as received. 2.2. Experimental Methods. Supercritical CO2 exfoliation was performed in a 200 mL high-pressure stainless steel reactor housing an ultrasonic probe. The procedures are similar to previously reported work.22 In brief, a certain amount of graphite powder was put into the reactor and heated by an electric heating sleeve. CO2 was then pumped into the reactor by a manual pump. After the pressure and the temperature reached the preset values, the ultrasonic generator started to run for a certain time under the predetermined power. The graphite was exfoliated into the graphene sheets under the coupled effect of the ultrasonication and supercritical CO2. After exfoliation, the graphene flakes were collected by opening a vent valve and directly spraying the exfoliated products with CO2 into a 200 mL conical flask containing 40 wt % ethanol solution.23 The resulting dispersion was left to stand for approximately 2 h and then centrifuged at a certain rate for 30 min to remove any graphite that was not exfoliated. The supernatant was collected by a pipet for later analysis and characterization. 2.3. Analysis and Characterization. 2.3.1. UV−Vis Absorption Spectroscopy. The optical density (λ = 660 nm) of the graphene−ethanol aqueous dispersion was measured 2840
dx.doi.org/10.1021/ie402889s | Ind. Eng. Chem. Res. 2014, 53, 2839−2845
Industrial & Engineering Chemistry Research
Research Note
ray diffractometer (XRD, Rigaku Co., Japan). The analysis conditions were irradiated at 10−70° with a scan rate of 5°/ min. The patterns were recorded under the acceleration voltage of 40 kV and a current of 20 mA. 2.3.6. Fourier Transform Infrared Spectra (FTIR). The graphite and graphene dispersions were deposited individually onto aluminum foil and then dried under vacuum (10−3 mbar) at room temperature. The samples were analyzed on a Spectrum 100 FT-IR spectrophotometer (FTIR, Perkin-Elmer Inc., USA) under attenuated total reflectance (ATR) mode. 2.3.7. Electrical Conductivity Measurement. The resistances of graphene films and PVA−graphene composite films were measured using a RTS-8 digital four-point probe resistivity measurement system with silver paint as electrodes (Guangzhou Four-Point Probe Technology Co., Ltd., China). Graphene films were prepared by the same method as the SEM samples. The thickness of the films was controlled by the amount of deposited graphene. A graphite film with thickness of 300 nm was made similarly. The prepared films were flattened and compacted by a 50 kN force before testing. The PVA/graphene film was prepared by dropping the PVA/graphene dispersion on a flat glass container and drying overnight at 40 °C in a vacuum oven. The film with a thickness of about 60 μm was obtained by carefully peeling it off from the flat glass. PVA/ graphene dispersion was prepared by directly spraying the exfoliated graphene sheets with CO2 into a 200 mL conical flask containing 2.0 wt % PVA aqueous solutions and ultrasonicating for 10 min. The content of the graphene in the resulted solution was about 1.0 wt %.
Figure 2. Effects of different factors on the yield of graphene: (A) supercritical CO2 pressure; (B) ultrasonic treatment time; (C) ultrasonic power; (D) initial graphite mass.
accumulated in the interlayer of graphite due to a good compressibility of supercritical CO2, which then generates a stronger repulsive free energy barrier that expands the space between the interlayers and, therefore, results in an easier formation of graphene sheets. The space of the interlayer of graphite (0.335 nm) is large enough for the CO2 molecule (0.233 nm) to enter. The size of the gap is around 0.102 nm. Therefore, the CO2 molecule can easily diffuse into the interlayers of graphite. Moreover, this increase in pressure and the resulting higher free energy barrier and reduced attraction can also enhance the dispersion stability of graphene sheets.21 As a result, the yield is enhanced with increasing pressure. However, it is noticed that the yield decreases to 15.63% and 14.29% when the pressure increases from 12 to 15 MPa and 18 MPa, respectively, which might be able to be attributed to the counteraction between the higher pressure and the cavitation effect of ultrasonication. In other words, higher acoustic pressure and increasing power are needed to produce the desired cavitation. The ultrasound role is suppressed at the higher pressure, so the yield decreases. 3.1.2. Ultrasonic Treatment Time. Different ultrasonication treatment times ranging from 15 to 120 min were applied to study its influence on the graphene yield. The other conditions were kept constant: such as a pressure of 12 MPa, ultrasonic power of 120 W, and initial graphite amount of 0.5 g. As shown in Figure 2B, the yield increases from 2.4% to 21.5% with increasing ultrasonic treatment time from 15 to 120 min, but the growth rate slows after 60 min. Under the supercritical condition, CO2 molecules can easily diffuse and intercalate to the interlayers of graphite (Figure 1c) due to its high diffusivity, low interfacial tension, and low viscosity. Under ultrasonic condition, both CO2 molecules and graphite absorb more energy from the collapse of cavitation bubbles, and then the space between the interlayers of graphite is further expanded until it is exfoliated into monolayer and/or few-layer graphene (Figure 1d). Apparently, with increasing ultrasonic treatment time, CO2 molecules and graphite absorb more energy, leading to more exfoliation of graphite. The slow yield growth after 60 min could be attributed to the limitation of CO2 amount in the reactor. 3.1.3. Ultrasonic Power. Ultrasonic power is another important parameter that provides energy for exfoliation and
3. RESULTS AND DISCUSSION The schematic drawing for exfoliation procedure is illustrated in Figure 1. When graphite is immersed in supercritical CO2 (Figure 1b), CO2 molecules penetrate and intercalate into interlayers of graphite (Figure 1c) because of their high diffusivity, small molecule size, low interfacial tension, and low viscosity. With more and more CO2 molecules accumulating in the graphite interlayers, the van der Waals’ force between each interlayer of graphite is weakened. Under ultrasonic conditions, both CO2 molecules and graphite absorb more energy from the collapse of cavitation bubbles, leading to the exfoliation of the graphite layer into few-layer graphene (Figure 1d). Exfoliated graphene sheets can coat any substrate by spraying or dispersing into a solvent/solution forming dispersions as shown in Figure 1e. The parameters’ effects during the exfoliation procedure are discussed in detail in section 3.1 3.1. Effects of Process Parameters on the Yield. One purpose of this work is to develop a simple technique for the mass production of graphene that results in high yield and high quality. First, the influences of process parameters on graphene yield including supercritical CO2 pressure, ultrasonic treatment time, ultrasonic power, and initial graphite amount, are systemically investigated. The temperature was fixed at 40 °C in all experiments. 3.1.1. Pressure. As pressure is a crucial parameter for the exfoliation of graphite into graphene, its influence on graphene yield was investigated. Other parameters were fixed in order to isolate the role that supercritical CO2 pressure plays, such as a graphite amount of 0.5 g, an ultrasonic power of 120 W, and an ultrasonic time of 60 min. As shown in Figure 2A, the yield increases from 4.64% to 16.7% with an increase of the pressure from 8 to 12 MPa. This increase in yield results from, as the pressure is increased, more CO2 molecules intercalated and 2841
dx.doi.org/10.1021/ie402889s | Ind. Eng. Chem. Res. 2014, 53, 2839−2845
Industrial & Engineering Chemistry Research
Research Note
Figure 3. TEM images of exfoliated graphene sheets: (A) folded monolayer; inset, electron diffraction pattern; (B) bilayer; (C) trilayer; (D) multilayer (
