Efficient Fractionation of Graphene Oxide Based on Reversible

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Functional Nanostructured Materials (including low-D carbon)

Efficient Fractionation of Graphene Oxide Based on Reversible Adsorption of Polymer and Size-Dependent Sodium Ion Storage Liangrong Zhu, Runze Liu, Zebo Fang, Phillips O. Agboola, Najeeb Fuad Al-Khalli, Imran Shakir, and Yuxi Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16188 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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ACS Applied Materials & Interfaces

Efficient Fractionation of Graphene Oxide Based on Reversible Adsorption of Polymer and SizeDependent Sodium Ion Storage Liangrong Zhuab, Runze Liub, Zebo Fangb, Phillips O. Agboolac, Najeeb Fuad Al-Khallid, Imran Shakir*e, Yuxi Xu*b aZhejiang bState

Industry Polytechnic College, Shaoxing 312000, China

Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular

Science, Fudan University, Shanghai 200433, China cMechanical

Engineering Department, College of Applied Engineering, King Saud University

(Al Muzahimiyah Branch), Riyadh 11421, Saudi Arabia dDepartment

of Electrical Engineering, King Saud University, Riyadh 11421, Kingdom of Saudi

Arabia eSustainable

Energy Technologies Center, College of Engineering, King Saud University,

Riyadh 11421, Kingdom of Saudi Arabia KEYWORDS: size fractionation, graphene oxide, reversible adsorption, responsive polymer, sodium ion storage

ABSTRACT: Graphene oxide (GO) is not only a uniqe class of two-dimensional materials but also an important precursor for scalable preparation of graphene. The efficient size fractionation

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of graphene oxide is of great importance to the fundamental and applied studies of chemically modified graphene, however, remains a great challenge. Herein we report an efficient and scalable fractionation method of GO employing reversible adsorption/desorption of temperatueresponsive poly(N-isopropylacrylamide) on GO to amplify their mass difference and significantly improve the fractionation efficiency. Furthermore, size-dependent sodium ion storage of the resulting fractionated reduced GO is revealed for the first time with high sodiumstorage performance achieved for the smallest reduced GO due to its largest d-spacing and most defect sites. This work provides valuable insights to the size fractionation and size-dependent electrochemical performance of graphene, which can be potentially extended to other twodimensional materials.

1. INTRODUCTION Recent studies demonstrate that the lateral dimensions of graphene sheets are of great importance to controll their applications. For example, the graphene sheets with large sizes are highly desirable for constructing 3D graphene-based network structure,1 2D film structures,2 and fabricating optoelectronic devices,3-7 while small-sized ones make it possible to form the biocompatible functionalized surface, and is especially useful in biosensing and drug delivery.8-10 Therefore, obtaining the size uniform graphene sheets is of great significance. Currently, the most frequently used strategy for scalable production of graphene sheets is the reduction of graphene oxide (GO).11 Therefore, the lateral dimensions of finally obtained graphene i.e. reduced GO (RGO) are mainly determined by their corresponding GO precursors. Unfortunately, GO was usually synthesized by strong oxidation and exfoliation of graphite,12 which leads to a wide size distribution. Although the size distribution of GO could be narrowed to a limited extent


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by carefully choosing the oxidant and experimental condition, the final outcome is proved to be unsatisfied. Another solution is to separate GO with a wide distribution into several fractions of similar sizes. Recently, a pH-assisted selective sedimentation was used to separate GO into two different sizes.13 Elham Bidram et al. also reported a gradient centrifugation method to make four fractions even available.14 Besides, anion-exchange HPLC,15 track-etched membrane filtration,16 spherical particles adhesion17, controlled directional freezing,18 polar solvent-selective natural deposition,19 and electrophoresis20 have all been explored for GO sheets’ fractionation. However, these methods are either complicated or low throughput, which put forward challenge for developing new efficient and scalable fractionation method. Furthermore, because of lacking of efficient fractionation method, the study of size-dependent performance of graphene in the electrochemical energy storage remains largely unexplored.21,22 Herein we report a simple, efficient and recyclable size separation strategy based on reversible adsorption/desorption of temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) on GO. We found the adsorbed PNIPAM could significantly amplify the mass difference between GO sheets of different sizes. After simple centrifugation, we could separate primary GO into three fractions within 4000 r/min which is highly efficient but low cost. After further thermal reduction of the fractionated GO, they could be used as anodes for sodium ion battery (SIBs), which showed significantly different sodium ion storage ability. Thermally reduced graphene stored Na+ via adsorption on its surface and defect sites, and insertion/extraction to its interlayers. 23-25

The large d-spacings of host materials were necessary for Na+ insertion/extraction to

accommodate more Na+ and gain fast kinetics process. The defect sites could facilitate ion diffusion from the vacancy of graphene sheets due to the lower diffusion barrier. Therefore, the


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fractionated GO with the smallest size delivered the best electrochemical performance due to their larger d-spacing and more defect sites, outperforming many previously reported graphenebased SIB anodes. 2. EXPERIMENTAL SECTION 2.1 Synthesis of GO. The natural graphite powder (325 mesh, Aladdin) was used to prepare GO through modified Hummers’ method.1 2.2 Size fractionation of GO. First, completely soluble PNIPAM aqueous solution (10 mg/ml) was added into as prepared GO solution (2 mg/ml) (equal volume ratio) under room temperature and stirring to form a homogenous clear solution. Then, the mixed solution was heated to 40 °C with violent stirring for about 10 min to form a translucent dispersion. The asprepared PNIPAM/GO dispersion was then transferred into 50 ml centrifuge tube while the pure GO solution with the same concentration was also used for centrifugation as the control group. After that, the resultant solution was centrifuged for 10 min at 250 r/min. The precipitate in PNIPAM/GO group was collected as the F1 part and the upper solution part was further centrifuged at 2000 r/min and 4000 r/min repeating the above operation to obtain F2 and F3 part, respectively. 40 °C has always been kept during the centrifugation process in order to avoid the desorption process of PNIPAM. After the completion of centrifugation process, F1, F2, and F3 solution was collected. These solutions were cooled to 0 °C before filtration so that PNIPAM chains could efficiently desorb from GO sheets. Then the solid state GO sheets collected on the filtration membrane were washed by deionized water. The filtrate collected from filtration process was dried at 60 °C overnight to recycle the PNIPAM.


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2.3 Reduction of fractionated GO. The obtained three GO fractions were subjected to thermal reduction at 400 °C under nitrogen atmosphere for 3 hours to yield black reduced GO (RGO) which could be used as anodes for sodium ion batteries. 2.4 Preparation of sodium-ion batteries’ electrode. Working electrodes were prepared by mixing RGO 40 mg reduction from F1, F2, and F3 fraction with 5 mg acetylene black (Super P) and 5 mg PVDF (8:1:1) dissolved in N-methyl-2-pyrrolidinone (NMP). After coating the above mixture on Cu foils, the electrodes were dried at 120 °C under vacuum for 6 hours to remove the NMP. The electrodes were then cut into disks and dried at 120 °C for 12 hours under vacuum. The Na/RGO cells were assembled in an argon-filled glovebox with less than 0.1 ppm of oxygen and water, using sodium foil as the counter/reference electrode, glass fibers as separator, and 1 M NaPF6 electrolyte solution dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 v/v). CR2032 (3 V) coin-type cells were used for electrochemical measurements. The window of voltage is 0.01-2.5 V. 2.5 Characterizations. The morphology of three portions F1, F2, and F3 were characterized by high-resolution transmission electron microscopy (HR-TEM, Nova NanoSem 450) and field emission scanning electron microscope (SEM, Zeiss Ultra-55). Dynamic light scattering (DLS) measurements were conducted on an ALV/CGS-3 Gonipmeter system. Atomic force microscope measurements (AFM) were conducted by a scanning probe microscope (Multimode 8). FTIR spectral measurements were performed using a Perkin-Elmer Paragon 1000. Raman measurements were recorded on an Invia/Reflrx Lasser Micro-Raman spectroscope (Horiba Jobin Yvon, Franch) with an excitation laser beam wavelength of 532 nm. XRD analysis was performed on a Rigaku D/Max 2500 X-ray diffractometer. XPS data were obtained by a PerkinElmer PHI 5300. CV measurements were performed at a scan rate of 0.1 mV s-1 on a CHI 660D


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electrochemical workstation. The galvanostatic charge–discharge experiments were performed on an LANHE instruments testing system in the potential range from 0.01 to 2.5 V for NIBs at different current rates. 3. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the size fractionation of GO based on reversible adsorption/desorption of PNIPAM on GO. The fractionation procedure of GO is shown in Figure 1. Under stirring and the reaction temperature above the low critical solution temperature (LCST) of PNIPAM, the PNIPAM undergo a hydrophilic-to-hydrophobic transition and can rapidly adsorb onto GO surface via hydrophobic interaction between hydrophobic PNIPAM chains and the aromatic domain within GO based on our recent study (Figure 2a).26 Therefore, larger GO sheet could provide more hydrophobic sites to adsorb more PNIPAM chains, in other words, the adsorption capacity of PNIPAM actually depends on the size of GO sheet. It could be further described as follows:


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In this formula, M is for the GO sheet adsorbed with PNIPAM, while m0 is for the original mass of GO sheet, and α is for the mass of one PNIPAM chain assuming the monodispersity of PNIPAM. The good monodispersity of PNIPAM colloids (T>LCST) has been demonstrated by DLS test result, as shown in Figure S1. The parameter N is number of the adsorbed PNIPAM chains and is proportional to the lateral dimension of a specific GO sheet. That means the larger a GO sheet is, the more PNIPAM it will be adsorbed. As a result, the adsorption of PNIPAM on GO could greatly amplify the mass difference of those GO sheets, which make the fractionation of GO much more efficient and scalable via low-speed centrifugation at different rates (Figure 2b). More importantly, when the temperature returned to be below LCST again, the PNIPAM could be desorbed from GO and recycled from the aqueous mixture, which makes our method more cost-effective.


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Figure 2. (a) Photographs of the homogenous dispersion/solution of GO and PNIPAM and the clear dispersion of GO/PNIPAM mixture at room temperature. Upon the temperature increased above LCST (e.g. 40 °C), the dispersion became translucent, which was reversible once the temperature returned to below LCST. (b) Photos taken from PNIPAM/GO group (left) and the control GO group (right) at each centrifugation rate, which show that the low-rate centrifugation can efficiently fractionate the PNIPAM-adsorbed GO while the pristine GO cannot be separated at all in the same condition. The successful size fractionation of crude GO (cGO) into three parts can be demonstrated by the scanning electron microscope (SEM) images shown in Figure 3. According to the SEM images and the histogram of size distribution, we can find that the cGO has a wide size distribution from 200 μm2. In contrast, the three fractionated GO named F1, F2 and F3 respectively by centrifugation at 250 rpm, 2000 rpm and 4000 rpm respectively, show obviously different and narrowed size distribution. The Gauss fits of cGO, F1, F2, and F3 indicates their maximum distribution of sheet sizes are 14, 239, 100 and 7 μm2, respectively. And the corresponding thickness of F1, F2 and F3 measured by AFM are 0.91, 0.96 and 0.99 nm, respectively, as shown in Figure S2. The yields of F1, F2 and F3 are 48.1%, 28.3% and 17.6%, respectively. These results indicated that cGO has been successfully separated into three fractions with large (mostly>200 μm2), medium (mostly 50~150 μm2) and small (mostly 5~10 μm2) sizes, respectively.


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Figure 3. SEM images and corresponding histogram of GO size distribution of crude graphene oxide (cGO, a, b), large-area GO sheet (F1, c, d), medium-area GO sheet (F2, e, f), and smallarea GO sheet (F3, g, h) on a silicon substrate. The Gaussian fit curve of each sample is colored in blue.


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The successful size fractionation of GO can also be reflected by the spectroscopy characterizations of F1, F2, and F3. The UV-vis spectra (Figure 4a) shows that F1, F2 and F3 dispersions all have an absorption peak at 230 nm corresponding to the pi-pi* transitions of conjugated domain, and a shoulder peak at 305 nm corresponding to n-pi* electron transitions of C=O bonds, which agrees well with previous documents.27 Compared with F2 and F3, F1 has the strongest adsorption at 230 nm, which indicates the most retention of aromatic rings in their basal plane and the fewest oxidation in their microscopic structure. Figure 4b shows the fluorescence emission spectra of F1, F2, and F3. Compared with F1 and F2, the peak at about 405 nm of F3 shows the smallest intensity and the peak at about 428 nm of F3 shows the obvious shift to low wavelength, which indicates the higher oxidation degree.28 This result is also disclosed by Fourier transform IR spectra (Figure 4c). The intensity of peaks at around 1050 cm-1 and 1730 cm-1 corresponding to C-O-C and C=O bonds decreased by the sequence of F3, F2 and F1,29 which indicates that larger F1 sheets have a lower oxidation degree. As GO nanosheets have been prepared by modified Hummers’ method, strong acids and strong oxidizer ions need to diffuse and insert into the 2D layered structure of graphite. Along with the diffusion of strong oxidizer ions, the oxidation of graphite takes place. The near-edge of graphite flakes will be highly oxidized since they first contact with high concentration of strong oxidizer. At the same time, excessive oxidation of the near-edge of graphite flakes will break their C-C bonds and produce GO nanosheets with small sizes. Therefore, larger F1 sheets show a lower oxidation degree. XPS also confirmed increased oxidation degree with the decreased size (Figure S3 and Figure 4d). The intensity of the peak at 286.6 eV corresponding to C-O is in the following sequence: F1F3 under the same pH condition, proving that sheets in F3 have the smallest size and the highest solubility.


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Figure 4. (a) UV-vis spectra of F1, F2, and F3 in ethanol solvent with the same concentration (0.05 mg/ml). (b) Fluorescence emission spectra, (c) FT-IR spectra, (d) XPS C1s spectra, (e) XRD patterns, (f) Raman spectra of F1, F2, and F3 samples. With efficient fractionation of GO, we further studied the size-dependent sodium ion storage of graphene using the three fractionated GO as the precursors for RGO samples. Figure 5a shows XRD patterns of the thermal reduction of F1, F2 and F3 samples, and they all exhibit the typical diffraction peaks of RGO. The thermal reduction of F1, F2, and F3 show peaks at


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2θ=25.8°, 23.4°, and 22.3°, respectively, and their d-spacings are calculated to be 0.345, 0.38, and 0.398 nm, indicating the reduced F3 shows the largest d-spacing. By means of HR-TEM (Figure S5), the corresponding d-spacings of reduced F1, F2 and F3 were measured to be 0.35, 0.38 and 0.40 nm, respectively, which are very close to results of XRD patterns (Figure 5a). Besides, the Raman spectra showed the higher ID/IG for the F3 compared to F1 and F2 (Figure S6), indicating that F3 has the most defect sites. As the large d-spacing facilitate fast insertion/extraction of Na+ and defect sites benefit ion diffusion, F3 delivers the best sodium ion storage ability (Figure 5b-d). Figure 5b shows the initial discharge and charge specific capacities of F3 anode are 446 and 244 mAh g-1, respectively, which corresponds to a limited initial Coulomb efficiency of 54.7%. It can be ascribed to the degradation of electrolyte and the formation of solid electrolyte interface (SEI).34-36 In addition, the CV curves of F3 anode collected at the scan rate of 0.2 mV s-1 between 0.01 and 2.5 V are showed in Figure S7. During the first cycle, a cathodic peak located at about 0.7 V has been detected, which can be ascribed to the Na+ insertion into the F3 layer structure and the formation of SEI. It is consistent with the voltage plateau of the first discharge curve of F3 anode in Figure 5b. After several cycles, the discharge-charge curves of the F3 anode almost overlapped, demonstrating its excellent cycling stability. The reversible capacity of F3 anode is 206 mAh g-1 at 0.1 A g-1, which is obviously larger than that of F1 anode (116 mAh g-1) and F2 anode (157 mAh g-1), as shown in Figure 5c. It is worth noting that the capacity of F3 anode is higher than many graphene-based anodes for SIBs (Table S1). The rate performance of these anodes was also studied, as shown in Figure 5d. The corresponding capacities of F3 anode are 206, 180, 149, 120 and 98 mAh g-1 at 0.1, 0.5, 1, 2 and 5 A g-1, respectively, which are obviously larger than those of F1 and F2 anodes. Even at a high current density of 10 A g-1, the F3 anode can still achieve a high capacity of 65 mAh g-1,


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while the corresponding capacities of F1 and F2 anodes are merely 40 and 22 mAh g-1, respectively, demonstrating the extraordinary rate performance of the F3 anode. Electrochemical impedance spectroscopy (EIS) of these electrodes were also performed and further analyzed, as shown in Figure 5e. EIS plots can be well fitted by an equivalent circuit (the inset of Figure 5e), where Rs is the equivalent circuit resistance, Rct represents the charge-transfer resistance, W represents the Warburg impedance, CdI represents the constant phase element and CL represents the intercalation capacitance. The fitted Rct values of F1, F2 and F3 anodes are 389.1, 322.6 and 201.3 Ω, respectively, which are close to their experimental values. The F3 anode shows the smallest charge-transfer resistance and the lowest Warburg impedance, which results in the best electrochemical performance among these three anodes.37-39 The long-term cycling performance of F1, F2 and F3 anodes at 5 A/g was also studied (Figure 5f). After 1000 cycles, the F3 anode still achieves a reversible capacity of 93 mAh g-1 with a high capacity retention of 95.8%, which is higher than those of F1 and F2 anodes. All these results demonstrate F3 anode with the largest d-spacing and the most defect sites delivers the best electrochemical performance.


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Figure 5. (a) XRD patterns of the thermally reduced F1, F2, and F3. Electrochemical performances of the reduced F1, F2, and F3 as anodes for SIBs. (b) Discharge-charge curves of F3 at the current density of 0.1 A g-1. (c) Cycling performance at 0.1 A g-1. (d) Rate performance, (e) EIS plots, the inset is equivalent circuit, and (f) Cycling performance at 5 A g-1 for 1000 cycles. 4. CONCLUSIONS


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In summary, we successfully develop an original method for the size fractionation of GO based on the reversible adsorption/desorption of PNIPAM/GO systems. The obtained three portions are proved to have different microscopic structures and properties owning to the difference of their lateral dimensions. Furthermore, the size-dependent sodium ion storage of graphene has been studied for the first time and the smallest graphene sheets with largest dspacing and most defect sites show the best performance as anode materials for SIBs including a high capacity of 206 mAh g-1 at 0.1 A g-1, excellent rate capability of 65 mAh g-1 at 10 A g-1 and superior cycling stability of 95.8% capacity retention after 1000 cycles. This fractionation technique is simple, efficient and scalable, and has the potential to be further used in size fractionation of other 2D materials which is essential for their basic and applied research. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Supplementary figures and tables AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Yuxi Xu: 0000-0003-0318-8515 Author Contributions L. Zhu and R. Liu contribute equally


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the support by the National Natural Science Foundation of China (51673042, 51873039, 51872186), and the Young Elite Scientist Sponsorship Program by CAST (2017QNRC001). The authors would like extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project no RGP-VPP-312. REFERENCES [1] Xu, Y. X.; Wu, Q. O. ; Sun, Y. Q.; H, Bai.; Shi, G. Q. Three-Dimensional Self-Assembly of Graphene Oxide and DNA into Multifunctional Hydrogels. ACS Nano 2010, 4, 7358-7362. [2] Yousefi, N.; Gudarzi M. M.; Zheng, Q.; Zheng, S. H.; Aboutalebi, F.; Sharif, J. Kim. SelfAlignment and High Electrical Conductivity of Ultralarge Graphene Oxide-Polyurethane Nanocomposites. J. Mater. Chem. 2012, 22, 12709-12717. [3] Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008, 2, 463-470. [4] Wang, X.; Zhi, L.; Müllen, K. Transparent, Conductive Graphene Electrodes for DyeSensitized Solar Cells. Nano Lett. 2008, 8, 323-327. [5] Zhao, J.; Pei, S.; Ren, W.; L, Gao.; Cheng, H. Efficient Preparation of Large-Area Graphene oxide sheets for transparent conductive films. ACS Nano 2010, 4, 5245-5252.


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Table of Graphic


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Efficient Fractionation of Graphene Oxide Based on Reversible

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