Microwave-Assisted Rapid Exfoliation of Graphite into Graphene by

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Microwave-assisted rapid exfoliation of graphite into graphene by using ammonium bicarbonate as the intercalation agent Juxiang Lin, Yajing Huang, Shi Wang, and Guohua Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01302 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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Microwave-assisted rapid exfoliation of graphite into graphene by using ammonium bicarbonate as the intercalation agent Juxiang Lin, Ya Jing Huang, Shi Wang and Guohua Chen* Department of Polymer Science & Engineering Huaqiao University, Xiamen 361021, People’ s Republic of China Guohua Chen (Corresponding Author) Tel.: +86-592-6162280; fax: +86-592-6162280. E-mail: [email protected]

ABSTRACT :

Making full use of the unique properties of graphene have

required a new route for its scalable production. Here we demonstrate a green and fast approach to prepare high quality few-layer graphene sheets (GS) based on microwave irradiation (MWI) method. This method takes advantage of solvent-free microwave directly and violently exfoliate pristine graphite into GS by using ammonium bicarbonate (NH4HCO3)-assisted as peeling media. In the microwave heating process, the decomposition of NH4HCO3 into H2O steam, CO2 and NH3 can generate strong pressure that exceeds the van der Waals force between the sheet layer of graphite, so as to achieve effective peeling off the sheets to very large, fewer defects graphene.

1. Introduction Since the discovery of graphene by Novoselov and Geim in 20041, graphene as the first two-dimensional atomic crystal with outstanding properties has received tremendous interest in recent years2-4. The practical application of graphene sheets


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(GS) needs to develop a method that allows cost-effective mass-production of the high-quality material. Currently, the preparation of graphene can be roughly divided into micromechanical cleavage by Scotch tape1, chemical vapor deposition (CVD)5-8, epitaxial growth9-11, oxidation-reduction method12and mechanical exfoliation of graphite13-16, etc. All the methods above have their own advantages and shortcomings according to each application. For example, micromechanical cleavage is not suitable for the large-scale production of graphene to fulfill the requirements in different areas17. CVD, as a high-yield approach, has been widely used to prepare large-area GS. But the CVD process is expensive for mass production due to its extremely careful fabrication process and large energy consumption18. On the other hand, oxidation-reduction method is one of the most commonly used methods to prepare graphene and considered to be the most promising way for the industrial production of graphene. However, this process is still exist a lot of problems in the expansion of production, such as control the degree of reducing, product purification and environmental pollution19. Fortunately, the mechanical exfoliation of graphite as one of the most promising method has been paid more and more attentions by researchers, as it can produce high quality of GS. But the preparation method has limitations of rapid preparation of this material at present. Therefore, in order to develop a rapid, facile and large-scale approach to produce high quality graphene, faster and more effective methods are urgently needed. Recently, as one kind of mechanical exfoliation method to produce graphene, microwave-assisted exfoliation of graphite nanoplatelets has attracted extensive



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attention for its many advantages20. GS produced under MWI have been reported in the previous papers21-24. For instance, A. A. Pirzado et al.25 have obtained the electrical property of large few layer graphene flakes by microwaves assisted exfoliation of expanded graphite. Specifically, the few layer graphene was synthesized by µ-wave assisted exfoliation of expanded graphite in toluene with an overall yield. Typically, the previous studies have been mainly proposed to effectively reduce the oxidation of graphene by MWI26. Most of these reduction methods assisted by MWI used pretreated graphite oxide as the precursor or synthesized in liquid phase, which cause the pretreatment process inevitably involve the use of chemical reagents. Moreover, the preparation methods have limitation of rapid preparation of graphene at present. Herein, a more rapid, facile and large-scale mechanical exfoliation of graphite approach to produce high quality graphene (microwave-assisted rapid exfoliation graphite into GS) has been studied. In this paper, flake graphite powder and ammonium bicarbonate were used as raw materials without adding any solvents as liquid medium. Compared with previous methods, the preparation method via peeling from bulk graphite assisted by MWI in solid state shows many advantages like maneuverable, time-saving and scalable, which may find practical applications in the preparation of graphene-base materials.

2. Experimental section 2.1 Chemicals and materials Graphite powder (8000 mesh) were placed in a desiccator prior. Ammonium


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bicarbonate (NH4HCO3) was obtained from Sinopharm Chemical Reagent Co., Ltd, China. Anhydrous ethanol was purchased from Guangdong Xilong Chemical Reagent Co., Ltd. Deionized water, other raw materials were used as received. 2.2

The preparation of graphene and its mechanism In a typical proceduce, pristine graphite (5 g) and ammonium bicarbonate (10 g)

were placed into a stainless steel container with zirconia milling beads (400 g, 0.5 mm to 5 mm in diameter). Then the container was agitated at 90 rpm for 2 h. After mixing, the mixture powder was placed in an airtight container which can resist high temperature and pressure. Then the mixture was heated in a vacuum oven at 60 ℃ for 20 min and subsequently cooled to room temperature to ensure obtaining the ammonium bicarbonate sufficiently intercalated graphite. Thereafter the intercalated graphite was put into a microwave oven (Microwave oven (WP800P23-K, 2450 MHz, 800 W) MWI for 60 s. Under MWI, the precursor (graphite) reacted rapidly, accompanied by lightening.

Figure 1: Procedures used for the preparation of graphene sheets (GS).

On completion of the reaction, the product was heated in muffle furnace at 500 ℃ for 5 min without any special treatment so that it can completely remove the undecomposed ammonium bicarbonate. The finally obtained product was collected



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for the next characterization. The whole process is described in Figure 1. As a control experiment, graphite powder was treated as the aforementioned procedure without using ammonium bicarbonate. According to the report by Y. Tang et al.27, the possible reaction mechanism of this research may be as follows: Ammonium bicarbonate can effectively disperse graphite powder and prevent graphene forming agglomeration, as ammonium bicarbonate has the reversible thermal decomposition behavior. Ammonium bicarbonate was inserted into the edge defects of graphite by heating and cooling treatment. Moreover, Under MWI, the van der Waals force of intercalated graphite interlayer was broken and the graphite layer was exfoliated violently and transformed to GS due to gaseous species (such as carbon dioxide, water vapor and ammonia) released by the thermal decomposition of NH4HCO3. 2.3 Characterization Scanning electron microscopy (SEM) was taken on a JSM-6700F field-emitting scanning electron microscope with an operating voltage of 5 kV. The distance between the sample and detector was 7.5 mm. The microstructure of the as-prepared samples were investigated by transmission electron microscopy (TEM) on a JEOL 2010 microscope operating at 160 kV with a point-to-point resolution of 1.9 Å. High-resolution transmission electron microscopy (HRTEM) images were taken by an H-7650 (HITACHI). Atomic Force Microscopy(AFM) measurement were performed in air using a Bruker Multimode 8 instrument. Experiments were carried out in tapping mode using a V-shaped ‘RTESPA-300’ probe B (Bruker Instruments,


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antimony(n) doped Si with frequency 300kHz,spring constants 40 N/m). X-ray diffraction (XRD) patterns were recorded with a D8-Advance instrument (Bruker AXS) using Cu-Ka radiation generated at a voltage of 40 kV and a current of 40 mA. The range of 2θ was from 5° to 40° with a scanning rate of 5° /min. Raman spectra were taken with a He−Ne laser (532 nm) as the excitation source by using Labram spectrometer (Super LabRam II system).

3. Results and discussion Compared with pristine graphite (Figure 2(a)), the flake structure appeared (Figure 2(b)) instead of granule-like feature at the same magnification as pristine graphite after it was solvent-free microwave exfoliated by using NH4HCO3 as a peeling media. As shown in the inset of Figure 2b, the thin GS can be more clearer observed, confirming the effectiveness of the exfoliation method.



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Figure 2: (a) SEM image of graphite particles; (b) SEM images of GS; (c) TEM image of GS. The inset is the selected area electron diffraction pattern (SAED), which confirms the crystalline nature of the graphene sheet; (d) HRTEM image of graphene sheet.

The typically flat graphene retain its planar morphology on the TEM grid after evaporation of the solution (Figure 2(c)). However, as the GS have a significant tendency to coalesce into overlapped structure, the edges of the sheets are partially folded and somewhat wrinkled on the grid. In order to determine the exact layer of the sheets, HRTEM measurement was carried out. The dark lines running in a parallel direction in the HRTEM image of the slice shown in Figure 2(d) reveal the cross-sections of the obtained GS, in which layer of the graphene nanoplatelets can be clearly illustrated. The one dark line (thickness of ~0.5 nm) indicates the few-layer GS have been obtained.


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Figure 3: (a) AFM topography image of graphene sheets (GS). (b) Corresponding cross-section profiles along the dotted line in (a).

In an effort to determining the size and thickness of the as-synthesized GS, atomic force microscopy (AFM) imaging was carried out. Figure 3(a) shows the AFM image of GS and its corresponding AFM height images are shown in Figure 3(b). As AFM images in tapping mode on a 1.2 µm× 1.2 µm area in Figure 3(a), GS with larger lateral sizes, i.e., 0.4 ~ 0.8 µm, has been observed. Figure 3(b) shows cross-section profiles along the dotted line across the GS and substrate in Figure 3(a). As shown in Figure 3(b), it is obvious that the thickness of the GS has been measured be 2 ~ 2.8 nm through the cross-section profiles. According to the thickness of a single graphene layer is around 0.34 nm, the GS obtained in the present work contain between five and eight graphene layers. As a result, the AFM topographic images in Figure 3 show the obtained product is few-layer graphene.



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Figure 4: XRD diffraction patterns of pristine graphite, the mixture of graphite powder and ammonium bicarbonate under the microwave treatment, and graphite powder treated with the same method without using ammonium bicarbonate.

At the same time, X-ray diffraction (XRD) and Raman spectroscopy were employed to further characterize the crystal structure of GS. Figure 4 shows the XRD patterns of the pristine graphite, the microwave-treated graphite and the GS. As can be seen in Figure 4, the appearance of a sharp and strong diffraction peak (002) at 26.47°of graphite in XRD spectrum (corresponding to the full width at half maximum (FWHM)=0.194), indicating the highly crystalline nature of graphite. However, compared with the diffraction peak of the pristine graphite, the microwave-treated graphite has a broad and weak diffraction peak (corresponding to the FWHM =0.206) at the same position of graphite. The reason may be that the layer of the sheets are smaller, increasing the degree of disorder28. In addition, calculated from XRD results, the obtained GS have a totally different diffraction peak and full width at half maximum ( FWHM =0.312). As shown in Figure 4, it is obvious that the diffraction peak intensity of graphite powder treated by microwave in presence of ammonium bicarbonate as intercalation agent is much lower than graphite powder treated with the same method without using ammonium bicarbonate, and the grain size decreases with the FWHM increased, indicating that graphite are sufficiently peeled into GS in this process. Besides, the broadening of the d (002) peak is representative for randomly ordered GS, demonstrating that the re-aggregation of GS can be strongly limited because of the effectiveness of the exfoliation method.


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Moreover, XRD analysis also is in accordance with the HRTEM characterization (Figure 2d). Raman spectroscopy is one of the most widely used techniques to identify the disorder and defect structures of graphene28. It is a powerful tool to determine the exact number of graphene layers. Figure 5 compares the Raman spectra of GS prepared by MWI in the presence of ammonium bicarbonate, graphite powder under the MWI without ammonium bicarbonate and bulk graphite. Pristine graphite shows a strong G band at ~1580 cm-1 and a 2D band at ~2717 cm-1 with a very weak D band at ~1350 cm-1 (Figure 5). The result indicates that the pristine graphite has a highly ordered structure with low defects. It should be noted that the frequency of the G and D bands in the GS are very similar to that observed in the graphite. And the ratios of the D- to G-band intensities (ID/IG) of pristine graphite and GS have been no significant differences. While GS display a main 2D band locates at 2690 cm-1 (Compared with pristine graphite, that is strongly red shifted), which is 27 cm-1 lower than that of pristine graphite located at 2717 cm-1. This is consistent with few-layer features of GS29, which is in accordance with the HRTEM analysis. Interestingly, compared with graphite powder, the shape of 2D bands of graphite powder treated with the same method without using ammonium bicarbonate are similar with graphite powder. It suggests that graphite powder under MWI without ammonium bicarbonate has not been clearly peeled. The comparative study of Raman spectra further confirms the formation of graphene from pristine graphite by Microwave-assisted exfoliation in the presence of ammonium bicarbonate as intercalation agent.



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Figure 5: Raman spectra of the pristine graphite and graphene nanosheets (GS) by MWI in the presence of ammonium bicarbonate, with a laser excitation wavelength of 532 nm.

XPS can visually evaluate the defects and the presence of any oxygenated groups of the as-prepared samples (GS). As revealed in Figure 6, the XPS survey spectrum of GS shows the O/C ratio of the sample is 0.168 (peak area), indicating only a few oxygen-containing groups are attached to the GS, which is in accordance with the Raman analysis. Characteristic C 1s core level spectrum is performed in Figure 6. The C 1s spectrum of GS clearly indicates a certain degree of oxidation with four components corresponding to carbon atoms in different functional groups: the non-oxygenated ring C (284.4 eV and 285.1 eV ), the C in C-O bonds (286.2 eV), and the carboxylate carbon (C=O, 290.5 eV). The largest contribution in the sample comes from the non-oxygenated ring C. Except the binding energy of 284.4 eV is attributed


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to the sp2 hybridized C-C bonds and the band at 285.1 eV can be associated with the sp3 hybridized C, typically assigned to graphite31. Moreover, the bands at 286.2 and 290.5 eV can be assigned to C-O and C=O species, respectively. Thus far, it can be concluded that the obtained GS are high quality with fewer defects.

Figure 6: XPS survey spectra of the GS and XPS C 1s spectra of the GS.

4. Conclusion In summary, a new method for preparing graphene has been developed by making use of microwave assisted rapid exfoliation of graphite into graphene with ammonium bicarbonate as the intercalation agent. In the pretreatment process, ammonium bicarbonate was infiltrated into the expanded graphite by ball milling. Then the decomposition of ammonium bicarbonate by microwave radiation (MWI) can form gaseous carbon dioxide, water vapor and ammonia, resulting of the strongly exfoliation. A combination of SEM, TEM, AFM, XRD diffraction patterns and Raman spectra analyses performed on samples indicated that graphite had been successfully exfoliated into graphene by this method. XPS survey spectra of the GS revealed only a few defects and oxygenated groups appeared on graphene. Such an easy,



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time-saving and green approach to prepare graphene sheets (GS) is suitable for the high-efficiency and large-scale production of graphene to fulfill the requirements in different areas.

Author information Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by Natural Science Foundation of China (Grants 51373059), Science Foundation of Fujian Province (Grants 2013H6014), Science Foundation of Xiamen (Grants 3502Z20150046) and Science and technology innovation team of Huaqiao University (Grants Z14X0046).

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Microwave-Assisted Rapid Exfoliation of Graphite into Graphene by

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