Scalable Production of Hydrophilic Graphene Nanosheets via in Situ

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Scalable Production of Hydrophilic Graphene Nanosheets via in situ Ball-milling assisted Supercritical CO Exfoliation 2

Zhuo Chen, Huadi Miao, Jiaye Wu, Yushu Tang, Wang Yang, Liqiang Hou, Fan Yang, Xiaojuan Tian, Liqiang Zhang, and Yongfeng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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Industrial & Engineering Chemistry Research

Scalable Production of Hydrophilic Graphene Nanosheets via in situ Ball-milling Assisted Supercritical CO2 Exfoliation Zhuo Chen, Huadi Miao, Jiaye Wu, Yushu Tang, Wang Yang, Liqiang Hou, Fan Yang, Xiaojuan Tian, Liqiang Zhang and Yongfeng Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing Changping, 102249, P. R. China

ABSTRACT: The scalable production of large quantities of defect-free graphene nanosheets (GNs) with low cost and excellent properties is essential for practical applications. Despite the high intense research of this area, the mass production of graphene nanosheets with high solubility remains a key challenge. In the present work, we propose a scalable exfoliation process for hydrophilic GNs by ball-milling assisted supercritical CO2 exfoliation in the presence of polyvinylpyrrolidone via the synergetic effect of chemical peeling and mechanical shear forces. The exfoliation difficulty has been reduced due to the intercalation effects of supercritical CO2 molecules. With the ball milling assistance, the modifier has been introduced onto the edge or/and surface of the GNs. The process results in the hydrophilic GNs with little damage to the in-plane structure. The GNs can be dispersed in various solvents with concentration up to 0.854 mg/mL (water) and remained stable for several months.

Keywords: Graphene nanosheets, hydrophilic, ball milling, supercritical CO2 exfoliation

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1. INTRODUCTION Graphene, a star material with preeminent optical, mechanical, electrical properties, has attracted tremendous research attentions since it was discovered by Novoselov and Geim in 2004 1-2. Several techniques have been developed to prepare graphene nanosheets (GNs), such as peel-off by Scotch tape1, chemical vapor deposition (CVD)3, epitaxial growth on SiC4 and solution exfoliation of graphite oxide (GO)5. Although the graphene obtained by the Scotch tape method has high quality and great properties, it is unsuitable for large-quantity preparation of GNs due to its technique difficulties. Even if the CVD method can obtain large-area thin GNs, the experimental condition must be controlled extremely strict and the amount is too little. The most common method to prepare GNs is solution exfoliation of graphite oxide followed by the process of solution reduction. In this method, tedious multistep process is needed and hazardous strong oxidizing and reducing reagents are required to break the strong interactions between the hexagonally sp2-bond carbon layers in graphite and restore the graphitic basal plane6-9. Because of the limited reduction conversion of the post-exfoliation reduction process, a further high-temperature thermal annealing step is required to dislodge the abundant oxygenated groups and structural defects in as-prepared reduced GO (rGO)8. A benign method for large-quantity production of graphene is needed. Then, effective exfoliation of graphite by supercritical fluid from graphite sprung up10. Many substances can be used as exfoliation media under the supercritical state, such as DMF11, NMP10, ethanol12, carbon dioxide (CO2)13, etc. Comparing with others, CO2 has easily reached critical point with lower temperature and pressure13. Also, the molecule size of CO2 is 0.33 nm closing to the distance (0.34nm) between the graphene layers14. The reasons all above made CO2 an excellent intercalative medium to exfoliate graphite into GNs. Recent years, supercritical CO2 exfoliation has been developed assisted with many methods, 2


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like ultrasonic15 or shear force16.

Unfortunately, the GNs prepared by above-mentioned methods have a high surface area and surface energy, easily agglomerated like other nanomaterials9, 17. The low wetting behavior of GNs hinders many practical applications18. Poulikakos et al. showed that dopants can indeed affect the hydrophilicity of graphene19. Chemical dopants often involved with high energy or high temperature to increase the difficulty for mass production. Xu et al. have developed reverse-micelle-induced exfoliation of graphite assisted supercritical CO2 to improve the dispersity of graphene in the presence of polyvinylpyrrolidone (PVP) and ethanol20. The process is not suitable for mass production as the use of large amount ethanol and little graphite. Xu et al. also developed an exfoliation process assisted supercritical CO2 to form stable graphene dispersions in different solvents using pyrene-polymers as modifier21-22. Baek et al. proposed that ball milling can introduce few defects on the edge of graphene and increase the dispersity of graphene in water23-25. In addition, Chen et al. performed a long-time ball milling process of graphite to form hydrophilic graphene in the presence of PVP and ethanol26. However, the defects introduced by long-time ball milling are of large amounts confirmed by the high intensity ratio of D/G peak during the Raman measurement. Balancing the quality and dispersity of graphene is the key point to produce hydrophilic GNs.

To overcome the difficulties as mentioned above, a simple and effective method has been developed to produce hydrophilic GNs with low defects in the presence of PVP by short-time ball milling assisted supercritical CO2 exfoliation (BSCE). The PVP-GNs obtained by the process of BSCE can make a homogenous dispersion in water and the dispersion can remain stable for several months. Apart from water dispersion, the PVP-GNs prepared by BSCE can also disperse well in 3



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other polar organic solvent, e.g. ethanol, isopropanol (IPA), N-methyl pyrrolidone (NMP), dimethylacetamide (DMAC), etc.

2. EXPERIMENTAL SECTION 2.1 Materials Graphite flakes (10000 mesh, 99.5%) was obtained from Jinrilai Graphite Co., Ltd, Qingdao, China. Polyvinylpyrrolidone (PVP) was purchased from Aladdin Chemical Reagent Co., Ltd, China. Carbon dioxide, anhydrous ethanol and other materials were used as received.

2.2 Synthesis of hydrophilic graphene nanosheets Hydrophilic GNs were produced by BSCE in the presence of PVP in a stirring ball-mill machine. Typical process is as the following. Graphite flakes (50g) and PVP (5g) were directly added into a supercritical CO2 apparatus, containing stainless steel balls of 5 mm in diameter and composed of stainless steel autoclave (2L) with a heating jacket and a temperature controller. The heating temperature ranged from 35 to 55 ℃ and the pressure ranged from 7.5 to 12 MPa. The operation condition needs to be chosen by considering all the properties presented above in order to be safe. The autoclave was heated to 55 ℃, and then CO2 was charged into the autoclave to the desired pressure (10 MPa) under milling (400 rpm). The gas was released after reaction in 4h and the lid was opened in atmospheric conditions. Then, by filtering with a sieve, the balls and products were separated. In order to remove the extra PVP, the as-prepared GNs were filtered and washed with deionized water and anhydrous ethanol three times. Final products were dried at 70 ℃ for 12 hours.

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2.3 Characterization The morphology and structure of the samples were obtained by field-emission scanning electron microscopy (SEM) (Hitachi SU8010), scanasyst-mode atomic force microscopy (AFM) (Brucker Multimode 8) and high-resolution transmission electron microscopy (TEM) (Hitachi F20). UV-Vis spectra (Macy UV-1800s) were performed to evaluate the graphene dispersions concentration. Raman spectroscopy was measured on a Renishaw inVia Reflex with laser wavelength 532nm. Powder X-ray diffraction (XRD) patterns were recorded on Brucker D8 Focus. Contact angle goniometry was carried out under ambient conditions on KINO SL200KS. The X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB MK II system with Al/K (photon energy =1486.6eV) anode mono X-ray source to semi-quantitate the containing elements. Thermogravimetric analysis (TGA) was performed with Hitachi STA 7200.

3. RESULTS & DISCUSSION 3.1 Formation mechanism Our purpose is to develop a high-efficiency technique for scalable production of hydrophilic GNs. The exfoliation process of GNs by ball milling assisted supercritical CO2 is schematically illustrated in Figure 1. PVP is used as an additive, which was put in the system of ball-milling assisted supercritical CO2 exfoliation. Supercritical CO2 (SC) molecules as effective intercalative media can be used to effectively exfoliate the graphite into GNs. SC diffuses in the graphite interlayer during incubation period and its expansion upon decompression pushes graphene layers away from each other. The ball milling process can provide the vertical press force and the horizontal shear force. Especially, the horizontal shear force plays a critical role in exfoliating the graphite. With the 5



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combining of the SC and ball milling processes, the graphite layer with enlarged distance caused by the force of SC will be more easily exfoliated by the ball milling process. After the ball milling process, the PVP may be anchored on the increased edge of GNs or adhere on the surface of GNs, as shown in Figures 1a and b. The PVP on the edge or/and surface makes GNs hydrophilic to form a stable dispersion in water. 3.2 Characterization of graphene nanosheets The morphological characterization of the pristine graphite and PVP-GNs was carried on by SEM, TEM and AFM. Figure 2a presents a typical SEM image of the pristine graphite flakes, showing that the graphite stacks together densely with smooth surface. After the BSCE process, the formation of wrinkles is clearly visualized in the SEM image in Figure 2b which indicates graphite has been effectively exfoliated into GNs with a thinner thickness. Figure 2c shows the low-magnification TEM of as-prepared PVP-GNs. The corresponding selective area electron diffraction (SAED) patterns (inset of Figure 2c) which present the typical 6-fold symmetry lattices prove the crystallinity of graphene has been retained well without any damages. Then, the layer number of GNs is clearly obtained in the HRTEM image (Figure 2d). To confirm the exfoliation effect of the BSCE process, the thickness measurement of the pristine graphite and as-prepared PVP-GNs has been further extended with examined by the detailed scanasyst-mode AFM (Figure 3). The pristine graphite shows an average thickness of about 60 nm, as seen in Figure 3a, corresponding to more than 100 layers. After the BSCE process, the as-prepared PVP-GNs have an average thickness of about 4.4 nm, as shown in Figure 3b, corresponding to 6-7 layers. Based on the thickness difference before and after the BSCE process, the graphite has been effectively exfoliated. The above results indicate that the crystallinity of the pristine graphite remains after low-speed 6


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ball-milling process, as confirmed by the results of SAED patterns. To determine the effects of the BSCE process on the structure of the GNs, the samples were further characterized by Raman spectroscopy and X-ray diffraction (XRD). Figure 4a displays the Raman spectra of pristine graphite and PVP-GNs with excitation at 532 nm. Owing to the small size of graphite, its D band at 1346 cm-1 is non-negligible with a ratio of the D-band to G-band intensity (ID/IG) to be approximately 0.13 (Figure 4a). In contrast, PVP-GNs shows a stronger D band over 1351 cm-1 with ID/IG =0.23, indicating that little defects have been introduced into the graphite structure during the BSCE process. A slight shoulder of the G band at 1621 cm-1 corresponding to D’ band is observed, which is consistent with the strong D band in the PVP-GNs sample. We calculated the in-plane crystallite sizes (La) of the pristine graphite and PVP-GNs based on Raman data by the formula27: 𝐿𝑎 (nm) = (2.4 × 10−10 )𝜆4 (𝐼𝐷 /𝐼𝐺 )−1

(1)

Here, λ is the wavelength used for Raman measurements while the ID and IG represent the intensity of the D- and G-bands, respectively. The crystallite sizes of the pristine graphite and PVP-GNs are 144 and 84nm. The PVP-GNs produced by the BSCE process exhibits a smaller crystallite size compared to the graphite carbon source. XRD patterns of graphite precursor and PVP-GNs are displayed in Figure 4b. Two main peaks are showed in the XRD patterns. For pristine graphite, it shows a strong intensity diffraction peak at 2θ = 26.5° (d-spacing = 0.34nm), which corresponds to the (002) diffraction peak of the graphite structure (PDF 41-1487). Moreover, the pattern of PVP-GNs has no shift in the position and d-spacing of (002) compared with pristine graphite, indicating that the BSCE process introduces little structural defects and the PVP-GNs remains the original graphite structure. After the BSCE process with PVP, the graphite has been exfoliated into the hydrophilic 7



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PVP-GNs. The hydrophilicity of the samples has been measured by the contact angle test and the UV-Vis spectra. Contact angle goniometry of pristine graphite and PVP-GNs were carried out under ambient conditions, as shown in Figures 5a and b. A 1.5 μL deionized water droplet was released onto the surface of the sample from a syringe needle. The images of liquid droplet were captured in real time by a CCD camera. The angle between the tangent line and the baseline indicates the contact angle of the solid and liquid interface. The contact angle of PVP-GNs (60°, Figure 5b) is obviously smaller than pristine graphite (90°, Figure 5a), implying that the hydrophilicity of as-prepared PVP-GNs has been effectively improved. Owing to the large demands to GNs in many areas, the GNs need to be dispersed into different solvents. To confirm the excellent dispersity of GNs, the PVP-GNs as-prepared were dispersed into several solvents, such as water, ethanol, IPA, etc. The digital picture of the dispersions mentioned above is shown in Figure 5c indicating the PVP-GNs can make homogenous dispersions in many polar solvents. UV-Vis adsorption spectroscopy is used to determine the concentration of the PVP-GNs dispersions. The absorption coefficient, α, which is related to the absorbance, A, through the Lambert-Beer Law (A=αCλ，where C is the concentration and λ is the wavelength) is essential for characterizing any dispersion. In order to accurately determine α, several dispersions were prepared with different concentrations. The obtained absorbance curves of different-concentration dispersions are shown in Figure 6a. An excellent linear relationship (Figure 6b) is obtained between the absorbance and the concentration of PVP-GNs with wavelength at 269 nm. By measuring the absorbance of the PVP-GNs dispersion which has been diluted 15 times in volume (owing to the opacity of the origin dispersion, an inset of Figure 6b), the final concentration of the stable dispersion after several-month placement is found to be as high as 0.854 mg/mL. The concentration of 8


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PVP-GNs in ethanol taken by the same method is 1.577 mg/mL (Figure S2). The hydrophilicity of PVP-GNs is originated from the PVP on the edge or/and surface of the GNs, as mentioned above. To confirm the existence of PVP, thermogravimetric analysis and XPS analysis were performed. TGA curves of pristine graphite, PVP-GNs and PVP shown in Figure 7a were tested from 20 to 900 oC under argon atmosphere at a flow rate of 100 mL/min. In TGA curve, a little loss of 2.77% can be attributed to the adsorbed water or the few liable functional groups, indicates that the pristine graphite has a good stable thermal structure. As seen, the pure PVP can be completely degraded at 460 oC. By comparison, the PVP-GNs show a rapid and significant weight loss after 460 oC, which can be regarded as the decomposition of the grafted PVP on graphene. The elemental elucidation of our material has been further investigated with the aid of XPS analysis (Figure 7b). The long range spectrum of the PVP-GNs clearly portrays the presence of carbon (C), nitrogen (N), and oxygen (O) with atomic percentages of 93.27%, 2.18% and 4.55%, respectively. The dominant peak at 284.4 eV is attributed to the graphitic carbon. The C/O ratio of 20.50 for graphene is significantly high, indicating lower oxidation of graphene than the rGO or other method exfoliated graphene28-29. The peak at 531.5 eV is corresponding to the oxygen, which is presumably originated from the adsorbed oxygen or the functional group containing oxygen formed during the BSCE process. The asymmetric shape of the N1s peak indicates the existence of at least two components. By further analysis, the N1s peak in the inset of Figure 7b indicates that three components corresponding to nitrogen atoms in different states: the graphitic N (401.2 eV), the pyrrolic N (400.2 eV) and the pyridinic N (399.6 eV), which validates the existence of the coordination between the nitrogen and the carbon in the PVP-GNs.

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4. CONCLUSION In summary, we have developed a simple and eco-friendly ball-milling assisted supercritical CO2 exfoliation process to efficiently produce large-scale low-defect hydrophilic GNs in the presence of PVP in solid state without involving hazardous chemicals. Our results confirmed that the ball-milling assisted supercritical CO2 exfoliation has significantly improved the hydrophilicity with little damage to the in-plane structure of the GNs in the presence of PVP. The hydrophilicity of PVP-GNs has been confirmed to be improved and the final concentration can reach up to 0.854 mg/mL after several-month placement according to the results of contact angle and UV-Vis spectra. The exfoliation process is not only easily scalable for graphene but also applicable to other layered materials. Additionally, the water dispersion remains stable formed by the PVP-GNs, which gives a potential use in the field of the conductive ink/paint, battery electrode and other composite materials. It is believed that this technique is very suitable and prospective for mass production of high-quality hydrophilic graphene for commercialization without any pollution to the environment.

ASOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.XXXXXXX Illustration of the supercritical apparatus. UV-Vis measurement of PVP-GNs ethanol dispersion. High resolution of XPS peaks.

AUTHOR INFORMATION 10


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

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21576289), Science Foundation of China University of Petroleum, Beijing (No. C201603), Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (2462014QZDX01) and Thousand Talents Program.

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Figures captions Figure 1. Schematically illustration of exfoliation process of graphite into GNs with supercritical CO2 assisted with ball-milling. (a) PVP decorated on the edge of GN, (b) PVP combined on the surface of GN. Figure 2. SEM images of (a) pristine graphite, (b) exfoliated graphene; TEM images of (c-d) exfoliated graphene, (c) (the inset showing SAED pattern) in low resolution, (d) in high resolution. Figure 3. AFM images and height profile along the line of (a) pristine graphite, (b) exfoliated graphene. Figure 4. (a) Raman spectra of pristine graphite and exfoliated graphene, (b) XRD patterns of pristine graphite and exfoliated graphene. Figure 5. Contact angle images of (a) pristine graphite, (b) PVP-GNs; Digital picture (c) of the dispersions of PVP-GNs in water, ethanol, IPA, NMP, DMAC. Figure 6. (a) UV-Vis spectra of PVP-GNs water dispersion with different concentrations (0.01-0.10 mg/mL), (b) Linear plot of absorbance vs. concentration of PVP-GNs (the inset showing a digital image of the solution after months and the UV-Vis of the solution diluted 15 times). Figure 7. (a) TGA curves of the pristine graphite, PVP-GNs and pure PVP, (b) XPS spectra of PVP-GNs, and the inset shows the N1s peak resolved.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 6

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Scalable Production of Hydrophilic Graphene Nanosheets via in Situ

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