An alternative pathway to water soluble functionalized graphene from the defluorination of graphite fluoride

Carbon 96 (2016) 1022e1027

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

An alternative pathway to water soluble functionalized graphene from the defluorination of graphite fluoride Guoxin Zhang, Kang Zhou, Ruoyu Xu, Hekai Chen, Xiaoke Ma, Biao Zhang, Zheng Chang, Xiaoming Sun* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2015 Received in revised form 24 September 2015 Accepted 8 October 2015 Available online 22 October 2015

In this study, we report a potentially scalable strategy for the cost/time-efficient production of watersoluble functionalized few layered graphene (FcG) through the mild defluorination of graphite fluoride (GF) at room temperature (RT). The strategy includes mechanical milling which is of high simplicity and operability, and subsequently water purification. By using heteroatom-containing alkaline such as NaNH2 and Na2S, N or S dopants can be functionalized into the defluorinated graphene, leading to the formation of N- and S-doped FcG, respectively. Our methodology of defluorination of GF allows the fabrication of water soluble FcG in much safer ways relative to the conventional Hummers' method that involves strong acidic/oxidative ambience and disposal of large amount of salt wastes. Meanwhile, the abundance of GF and the simplicity of processibility may also facilitate the practical uses of FcG obtained via our methodology. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Graphene, one-atom thin carbon allotrope, has been regarded star material by its exciting properties especially unique physical and electrical properties (high tensile strength, Young's modulus, electron mobility, and thermal conductivity) [1e3]. Besides, its chemically functionalized derivatives are also fascinating the material scientists [3e5]. Graphene oxide (GO), which is commonly fabricated through Hummers' method involves concentrated H2SO4 and KMnO4, is one main type of graphene derivatives [6]. Stepwise intercalation and oxidation can effectively exfoliate thick layered graphite and thus endows graphene with abundant functional groups such as carboxyl and hydroxyl groups [4,5,7]. The reason of using strong oxidative agents like H2SO4 and KMnO4 is due to the high chemical stability of graphite which usually denies direct functionalization and exfoliation. Thanks to the functionality abundance of GO, large proportion of functionalized graphene (FcG) can be converted from the stepwise functionalization of GO; i.e., converting graphite to before coupling GO with other functional groups [3e5,7e9]. Another effective strategy to fabricate FcG is through the solution exfoliation aided with surfactants under

* Corresponding author. E-mail address: [email protected] (X. Sun). http://dx.doi.org/10.1016/j.carbon.2015.10.020 0008-6223/© 2015 Elsevier Ltd. All rights reserved.

mechanical force of sonication [10e12]. Despite the abovementioned strategies which are accessible to FcG, we are still facing the bottleneck of safe production of FcG. For the GO synthesis, strong oxidative agents are inevitably used, with the problem that these chemicals are of great hazard, which potentially threatens the human production and life. Also, these hazardous oxidants demand rigidly safe container to transport, store and carry related reactions, which hinders the practical implantations of FcG. Meanwhile, the solution-based exfoliation of graphite also faces many other problems including large amount of liquid wastes and low yield of single or few layered graphene due to incapability of physical forces like sonication cavitation or mechanical grinding in nanometer scale [10,12e14]. Thus, mass production of FcG with simple and convenient way is highly appealing to both scientific and practical worlds. Halogenated graphene and graphite are newly explored graphene materials and revealed to be promising in many fields such as insulating 2D Teflon [15e17]. Recent years, the focus of graphite fluoride (GF) was partially transferred into the fields of chemical materials [17]. The conversion reactions of graphite fluoride have been explored using defluorination agents [18e21], while rigid pretreatment such as sonication for reducing the thickness of graphite fluoride (GF) was occasionally needed [18,20,22]. Otherwise, rigorous conditions such as special solvent (sulfolane or liquid NH3), molten alkaline (KOH and NaOH) or thermal treatment (T > 160
C) were employed [19,23,24]. Also, too strong the

G. Zhang et al. / Carbon 96 (2016) 1022e1027

defluorination agent and too drastic the performing conditions would crack the 2D morphology into particle-like form which greatly limited further applications of FcG [18,20,21,23e25]. Here in this study, mild defluorination of GF by commercially available agents KOH, NaNH2 and Na2S were employed to manufacture water soluble functionalized graphene (FcG) and alter the dopant type of FcG. The defluorination reaction can be performed at room temperature (RT) which possesses high simplicity and operability, Only small amount of strong polar solvents was required to accelerate the defluorination reaction by forming a mixed slurry in the manufacturing process. All procedures can be accomplished in ~3e4 h and not restricted by production capacity as long as there is a large enough reaction ball milling jar. During the whole processes, no gaseous reactant or product would be generated, and the byproduct NaF can be easily removed by simply washing with water, which means that reasonably high safety can thus be secured. More importantly, the yield of single and fewelayer graphene was comparative to the yield of GO from Hummer's method. Final FcG can be functionalized with multiple choices of alien atoms, i.e., NaNH2 for N and Na2S for S. Considering its simplicity, operability, yield, and safety, our methodology of fabricating water soluble FcG was of high potential for instant practical implantations. 2. Experimental 2.1. Material preparation Typically, 50.0 mg graphite fluoride (CFx, x ~ 1.0, white colour, J&K Scientific Ltd) together with 0.56 g KOH, (~10.0 mmol, MW ¼ 56.1, Sinopharm Chemical Reagent Beijing Co., Ltd), 0.5 g NaNH2 (~12.8 mmol, MW ¼ 39.01, Aladdin Industrial Inc.) or 2.4 g Na2S$9H2O (~10 mmol, MW ¼ 240.2, Sinopharm Chemical Reagent Beijing Co., Ltd) were added into the ball milling jar, followed by 5.0e10.0 mL N, N-Dimethyl formamide (DMF, A.R. grade, Sinopharm Chemical Reagent Beijing Co., Ltd). The mixture was milled at 30 Hz for 2 h (QM-3SP04, Nanjing Laibu industrial Science and technology Co., Ltd), resulting dark black slurry. Controlled experiment was set without any dehalogenation agents, going through the whole ball milling procedure. The slurry was then transferred into 20.0 mL-packed container and the demanding volume was complemented by adding DMF. The 20.0 mL materials/DMF suspension was submitted to be sonicated using an ultrasonic cleaner (250 W, KQ-250DE, Kunshan Co., Ltd) for 30 h, resulted uniform suspension was then centrifuged and washed with deionized water for 2e3 times. Afterwards, the black precipitates were dried at 60
C oven or transferred to desired solvent such as water. 2.2. Characterizations The morphologies of samples were examined using scanning electron microscope (SEM, Zeiss SUPRA-55) and transmission electron microscopy (TEM, Hitachi-800). Layer thickness analysis was done using atomic force microscope (AFM, Nanoscope V multimode). Powder X-ray diffraction (XRD) patterns of samples were recorded on a Shimadzu XRD-6000. Raman spectra were recorded on a LabRAM Aramis Raman spectrometer (HORIBA Jobin Yvon). Chemical compositions were tested using XPS (Thermo Electron ESCALAB-250). Solid samples were ultrasonicated in deionized water for more than 10 min s to form a transparent solution and measured by a Zetasizer instrument (ZS90 Malvern instruments) to define their surface charge properties. The surface composition of the nanoparticles was studied by FT-IR spectroscopy with a Varian 3100 FT-IR in the 400e4000 cm1 spectral range and a resolution of 2 cm1. The hydrophilic magnetic nanoparticles

1023

were mixed with KBr powder, then grounded and compacted into thin disk-shaped pellets. To avoid water molecule contamination, near infrared lamp was used to dry the disk-shaped pellets. 3. Results and discussions As graphically depicted in Fig. 1, the fabrication of functionalized graphene (FcG) initially consists mild dehalogenation of graphite fluoride (GF) by strong alkaline such as KOH, NaNH2 or Na2S at room temperature (RT). GF (white colour, whose diameter is in several micrometers, Fig. S1(a)) and alkaline powder were mixed and immersed in minor amount (~5.0 mL) of N, N-dimethyl formamide (DMF, can be substituted to other strong polar solvents such as N, N-dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP), or dimethyl sulfoxide (DMSO)). The mixture/DMF was then set to ball milling (30 Hz) for ~ 2 h. Resulted dark black slurry was transferred into DMF solution (~15.0 mL) and sonicated for another 0.5 h to fully exfoliate the attached graphene layers after the defluorination reaction. Finally, FcG products were purified by water and dried in 60
C-oven. Due to the non-use of strong oxidative agents, this processing is much safer than the Hummers' method which is one general way to obtain GO. Also, the yield can be guaranteed because the reaction can be performed in seconds due to the high activity between strong alkaline and F on GF. Also, no high temperature treatment was involved in the whole manufacturing process which could greatly reduce the fragmentation of resulted FcG [21]. Therefore, by performing the mild defluorination of GF, 2D layered morphology can be well maintained. As observed in Fig. 2(a) and (d), N- and S-FcG are in lateral size of 1e2 um and all of thin layered morphology, which are distinctly different from its GF source and the controlled sample of GF with no alkaline (thick carbon sheet, Fig. S1(a) and (b)). If the moderate dehalogenation reactions were substituted, for instance, fast defluorination takes charge upon high temperature ambience, the already twisted 2D sheet will probably be heavily cracked, leading the formation of abundant holes or fragmented carbon particles [21]. The layered structure needs to self-balance into stable configuration after Fleaving and being peeled off by either forming aromatic structure or reacting with nearby active components like oxygen, nitrogen, or sulphur, and these processes can be done in seconds following the thermodynamic principles. While for the controlled experiment without any alkaline, the size of sheet-like particles undergoes obvious decreasing but not so big change happens for the thickness (Fig. S1(b)). However, the thickness of alkaline-treated GF found distinguishable change, as revealed by TEM characterizations. As shown in Fig. 2(b) and (e), thin carbon layer can be clearly observed. Further enlarged edge section in Fig. 2(c) and (f) revealed that layer number is lying in 3e6 layers for both N-FcG and S-FcG, confirming the effectiveness of preparing few layered FcG through our strategy. The thickness of FcG, characterized by AFM (Fig. S2) was revealed to be 4.6 nme5.7 nm, roughly fitting 4e7 layers according to previous report that the thickness of single layer FcG was ~0.8 nm [5]. Fig. 3(a) shows the XRD profiles of N-FcG and S-FcG, providing information of crystallinity of FcG. The expanded bands at ~12.5
and 24.5
are indexed to the (001) and the (002) crystal lattice of FcG [21,22,25]. While for the controlled samples: GF and GF with no alkaline, no typical graphitization peaks were detected at ~26
(Fig. 3(a)). Due to the massive disordered carbon in the starting GF (no carbonization band was found in Raman spectrum of GF, Fig. 3(a)), resulted FcG is inevitably possessing defects but can be partially reduced using mild treatment. As shown in Fig. 3(b), the ID/IG ratio are in between 1.02 and 1.08 for our FcG, which are comparable to that of FcG via the functionalization of GO [26]. Detailed functionalities were analysed using FT-IR and XPS spectra,

1024

G. Zhang et al. / Carbon 96 (2016) 1022e1027 HO

0.71 nm

F

F

F

F F

HO

HO

F

F

F F

F

F

F F

OH OH

F

F

F

HO

KOH

HO

OH

OH

F F

F F F

F

F

F F F

F

F

F F

F F

F F

F F

F

F

F

F

F

F F F

F

F

OH

O-FcG O

F

NaNH2

HO NH

Room T Ball milling

F F F

NH2

N HN

N O

F

F

F

O

Na2S

OH

S HO

F

F

F

F

N-FcG

HO OH

F

OH

F

HO

F

O

O HO

F F

F

OH HO

SH HO

S

S

S HO

Graphite Fluoride

S

OH

SH

S-FcG Fig. 1. Schematic illustration of the preparation procedures of water soluble functionalized graphene via the dehalogenation of graphite fluoride (GF) by strong inorganic alkaline such as KOH, NaNH2, and Na2S. (A colour version of this figure can be viewed online)

Fig. 2. Morphology characterization: SEM images of (a) N- and (d) S-FcG, TEM images of (b) N- and (e) S-FcG. HRTEM images of (c) N- and (f) S-FcG. Bars for (a), (b), (d), and (e) are of 200 nm, for (c) and (f), bars ¼ 5 nm.

which are depicted in Fig. 3(c). In Fig. 3(c), band 2 located at 1236 cm1 is ascribed to the stretching mode of CeF. After dehalogenation, absorbance of CeF was greatly suppressed, instead, two new types of vibration emerged, locating at ~1020, and ~1558 cm1,

which are indexed to vibrations of CeO/N/S, and sp2 hybridized C, respectively. In Fig. 3(d), XPS element survey is displayed, indicating the presence of C, O, and N in both N-FcG and S-FcG. Intriguing part of this figure lies in the N existence in Na2S

G. Zhang et al. / Carbon 96 (2016) 1022e1027

Fig. 3. (a) XRD profiles, (b) Raman spectra, (c) FT-IR spectra, and (d) XPS survey spectra of GF, GF-with-no-alkaline, N-FcG, and S-FcG.

Fig. 4. XPS analysis: (a) bar chart of element survey, (b) C1s spectra, (c) N1s spectra, and (d) S2p spectra and their deconvoluted curves of N- and S-FcG.

1025

1026

G. Zhang et al. / Carbon 96 (2016) 1022e1027

zeta potentials of each FcG samples, we found the values of FcG samples were all ~30 mV, highly comparable to that of Hummers' GO which was massively reported to be water soluble. 4. Conclusions In conclusion, we developed an alternative way to water soluble few layer graphene via the dehalogenation of graphite fluoride. This highly promising alternative way should fit for mass production due to its attributes of simple operation and safe conversion. Room temperature ball milling treatment was engaged and aided with minor amount of DMF to facilitate the defluorination reaction. The whole process is much safer than other pathways involving strong oxidative agents and subsequent functionalization. Moreover, the graphene fabricated via our strategy could be simultaneously doped with O, N, or S, and showed very high doping efficiency. Consequently, our methodology can serve as a highly potential alternative way for the mass production of water soluble functional graphene and might find its advantages in the near future. Acknowledgements

Fig. 5. Water solubility measurements: Zeta potential measurements of (a) N-FcG and (b) S-FcG, (c) water solubility tests: the upper half is for N-FcG, the lower part is for SFcG. Figures on the head of bottle was indexed to the hours elapsed after 30-min sonication in water. (A colour version of this figure can be viewed online)

defluorinated GF. The only N source in this experiment is DMF which is one very stable organic solvent (boiling point ¼ 152.8
C). This uncovered the extremely high reactivity the F-leaving C sites, any nearby reactants can be connected to the carbon matrix, for instance, N in DMF can be extracted and O2 in air ambience and DMF can be doped in the FcG. It can be seen more clearly in summarized bar chart of Fig. 4(a) and (c), without addition of other N source except the stable solvent DMF, N content of 0.82 at% can be achieved in the final S-FcG, it will be slightly elevated to 1.47 at% if with additional NaNH2. Due to high alkalinity of NaNH2 and Na2S, the defluorination-reaction-aided exfoliation can take place fast, leading to the quick coupling between the fresh F-leaving C sites with oxygen in O2 and DMF. Therefore, these highly reactive Fleaving C sites in N-FcG and S-FcG can accommodate high oxygen content, as shown in Fig. 4(a). Carbon contents were respectively achieving 75.2 at% and 84.2 at% for N- and S-FCG, the high C content can be also revealed by the sharp graphitic-C peaks in Raman spectra (Fig. 3(b)). Judging from XPS, NaNH2, who has higher alkalinity, achieved better defluorination effect. Detailed functionalities were investigated by deconvolution of XPS fine scanning of specific elements. As shown in Fig. 4(b), (c), and (d), the functional groups in N-FcG mainly include CeO and CeN single bond, C]N double bonds, agreeing to the N deconvolution (Fig. 4 (c)) in which graphitic N take main positions. While in the S-FcG, the functionalities are in the forms of CeO, CeS, and C]S, as shown in Fig. 4(d). Over 4.0 at% S was revealed by XPS, facilitating the S-FcG taking crumpled form through SeS bonding [26,27]. Taking advantage of abundant functionalities, good water solubility of FcG can be speculated [13]. The solubility test was conducted by dissolving FcG in water through a 10-min sonication, forming a homogeneous colloid suspension of 1 mg/mL concentration. The containers were set still to observe the sedimentation behaviour of FcG. Fig. 5(c) shows the digital images FcG/water suspension against elapsing time from 0 to 24 h. During the concerned time of 24 h, no obvious precipitates were observed, substantiating very good solubility of our N- and S-FcG. By measuring

This work was supported by National Natural Science Foundation of China, 973 Program (2011CBA00503, 2011CB932403), and the Program for Changjiang Scholars and Innovative Research Team in the University, and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.10.020. References [1] D.K. James, J.M. Tour, Graphene: powder, flakes, ribbons, and sheets, Acc. Chem. Res. 46 (10) (2013) 2307e2318. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [3] Y. Zhu, S. Murali, W. Cai, X. Li, J. Suk, J.R. Potts, et al., Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (35) (2010) 3906e3924. [4] J. Shen, Y. Hu, C. Li, C. Qin, M. Ye, Synthesis of amphiphilic graphene nanoplatelets, Small 5 (1) (2009) 82e85. [5] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (6) (2008) 1679e1682. [6] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339. [7] C.K. Chua, M. Pumera, Covalent chemistry on graphene, Chem. Soc. Rev. 8 (2013) 3222e3233. [8] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (1) (2009) 228e240. [9] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nat. Nanotechnol. 4 (4) (2009) 217e224. [10] J.N. Coleman, Liquid exfoliation of defect-free graphene, Acc. Chem. Res. 46 (1) (2012) 14e22. [11] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O'Neill, et al., Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids, Nat. Mater. 13 (6) (2014) 624e630. [12] A. Ciesielski, P. Samori, Graphene via sonication assisted liquid-phase exfoliation, Chem. Soc. Rev. 43 (1) (2014) 381e398. [13] A. Lucas, C. Zakri, M. Maugey, M. Pasquali, Schoot Pvd, P. Poulin, Kinetics of nanotube and microfiber scission under sonication, J. Phys. Chem. C 113 (48) (2009) 20599e20605. [14] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z. Sun, S. De, et al., High-yield production of graphene by liquid-phase exfoliation of graphite, Nat. Nanotechnol. 3 (9) (2008) 563e568. [15] R.R. Nair, W. Ren, R. Jalil, I. Riaz, V.G. Kravets, L. Britnell, et al., Fluorographene: a two-dimensional counterpart of teflon, Small 6 (24) (2010) 2877e2884. [16] L. Bulusheva, V. Tur, E. Fedorovskaya, I. Asanov, D. Pontiroli, M. Ricco, et al., Structure and supercapacitor performance of graphene materials obtained from brominated and fluorinated graphites, Carbon 78 (2014) 137e146. [17] F. Karlický, K. Kumara Ramanatha Datta, M. Otyepka, R. Zboril, Halogenated

G. Zhang et al. / Carbon 96 (2016) 1022e1027

[18]

[19]

[20]

[21]

[22]

graphenes: rapidly growing family of graphene derivatives, ACS Nano 7 (8) (2013) 6434e6464. e , A.B. Bourlinos, K. C pe, A. Ambrosi, A.H. Loo, et al., V. Urbanov a, K. Hola Thiofluorographene-hydrophilic graphene derivative with semiconducting and genosensing properties, Adv. Mater. 27 (14) (2015) 2305e2310. X. Liang, J. Guo, S. Liang, C. Hong, Y. Zhao, Synthesis of soluble graphene nanosheets from graphite fluoride in low-temperature molten hydroxides, Mater. Lett. 135 (2014) 92e95. A.B. Bourlinos, K. Safarova, K. Siskova, R. Zboril, The production of chemically converted graphenes from graphite fluoride, Carbon 50 (3) (2012) 1425e1428. K.A. Worsley, P. Ramesh, S.K. Mandal, S. Niyogi, M.E. Itkis, R.C. Haddon, Soluble graphene derived from graphite fluoride, Chem. Phys. Lett. 445 (1) (2007) 51e56. L. Liu, M. Zhang, Z. Xiong, D. Hu, G. Wu, P. Chen, Ammonia borane assisted solid exfoliation of graphite fluoride for facile preparation of fluorinated

1027

graphene nanosheets, Carbon 81 (2015) 702e709. [23] S. Chakraborty, W. Guo, R.H. Hauge, W. Billups, Reductive alkylation of fluorinated graphite, Chem. Mater. 20 (9) (2008) 3134e3136. [24] R. Zboril, F. Karlický, A.B. Bourlinos, T.A. Steriotis, A.K. Stubos, V. Georgakilas, et al., Graphene fluoride: a stable stoichiometric graphene derivative and its chemical conversion to graphene, Small 6 (24) (2010) 2885e2891. [25] A.B. Bourlinos, V. Georgakilas, R. Zboril, D. Jancik, M.A. Karakassides, A. Stassinopoulos, et al., Reaction of graphite fluoride with NaOHeKOH eutectic, J. Fluor. Chem. 129 (8) (2008) 720e724. [26] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, et al., Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (1) (2011) 205e211. [27] M. Seredych, J.-C. Idrobo, T.J. Bandosz, Effect of confined space reduction of graphite oxide followed by sulfur doping on oxygen reduction reaction in neutral electrolyte, J. Mater Chem. A 1 (24) (2013) 7059e7067.