Materials Chemistry and Physics 206 (2018) 7e11
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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of graphene via ultra-sonic exfoliation of graphite oxide and its electrochemical characterization
Hassnain Asgar a, K.M. Deen b, Usman Riaz a, Zia Ur Rahman c, Umair Hussain Shah a, * Waseem Haider a, c, a School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI 48859, USA b Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada c Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI 48859, USA highlights
A relatively direct synthesis method for production of graphene is presented. IR, Raman and XRD analyses confirmed formation of graphene starting from graphite. XPS and TEM characterization validated the formation of graphene. Electrochemical response of GO and graphene was evaluated in de-aerated 0.5M KOH.
Presence of functional groups in GO resulted in improved values of Rct and Ceff,p. abstract
A direct method of producing graphene from graphite oxide (GO) via ultra-sonication is presented in this work. The synthesis of graphene was validated through IR, XRD, Raman, and XPS analyses. Moreover, the diffraction pattern obtained from TEM also validated the formation of graphene with char- acteristics (002) plane. The electrochemical behavior of GO and graphene was evaluated by electrochemical impedance spectroscopy and linear sweep voltammetry in 0.5M KOH solution. The relatively larger effective pseudocapacitance and broad current peak exhibited by GO in the LSV plots was related with the dominant adsorption of ‘Hads’ during reduction of water. It has been considered that large overpotential and relatively higher current response exhibited by GO compared to graphene was associated with the preferential adsorption of Hads in the presence of surface functional groups. © 2017 Elsevier B.V. All rights reserved.
1. Introduction processes [5e7]. But the large-scale use of graphene is still hin- dered due to extensive and time-consuming production methods. Graphene and its related materials have unique physicochem- In this work, a relatively direct and efficient route is presented to ical properties to support several electrochemical processes produce graphene from graphite and the effect of surface functional involving electro-catalysis [1], electrochemical sensing [2], super- groups on electrochemical properties has been investigated. capacitance [3] etc. It is well established that the heterogeneous electron transfer, required for these processes, from/to a graphene 2. Experimental sheet takes place on the edges and is affected by the attached functional groups. The presence of functional groups also facilitates Graphite powder (Asbury Carbon Inc.) was oxidized to graphite the adsorption/desorption of molecules on the graphene planes [4]. oxide (GO) using improved Hummer's method as explained in The production method for graphene greatly influences these Ref. [8]. After the reaction, GO was dried overnight at 80 C. To properties. A lot of work is going on with graphene as core interest produce graphene, GO was ultra-sonicated in DI water for 12 h and its production is being reported continuously via various ® using Branson M2800H ultrasonic bath operating at the frequency of 40 kHz. The powder from the suspension was obtained via centrifugation with a g-force of ~2285 using the Rotofix 32A ® Benchtop Centrifuge by Helmer Scientific. * Corresponding author. School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI 48859, USA. Infrared (IR) spectra of graphite, GO and graphene was acquired E-mail address: [email protected] (W. Haider). from FTIR-ATR spectrometer (NicoletTMiSTM 50) in Attenuated https://doi.org/10.1016/j.matchemphys.2017.11.062 0254-0584/© 2017 Elsevier B.V. All rights reserved. 8 H. Asgar et al. / Materials Chemistry and Physics 206 (2018) 7e11
Fig. 1. IR (a), XRD (b), and Raman (c) trends of graphite, graphite oxide, and graphene.
Total Reflection (ATR) mode. X-ray diffraction (XRD) patterns 3. Results and discussion (Rigaku Mini Flex II) were obtained by using Cu Ka (l ¼ 1.54 Å) radiation source. For Raman spectra, the laser beam excitation of IR, XRD and Raman spectra of graphite, GO and graphene are 532 nm (Kaiser Optical Systems Inc.) was used. The XPS spectra shown in Fig. 1. Graphite exhibited an insensitive behavior in the (Thermo Scientific K-Alpha) of graphene were gathered by using Al infrared range (Fig. 1a). The transmittance peaks at 2325- Ka irradiation source. The morphology and structure of graphene 1981 cm 1 could be associated with the diamond crystal used in sheets were observed in transmission electron microscope (TEM) ATR mode [9]. A broad peak at 3405 cm 1 presented by GO could be (HT7700). attributed to the -OH bond stretching vibrations belonging to C-OH For electrochemical studies, the working electrodes were and/or adsorbed moisture. Similarly, the peak at 1173 cm 1 could prepared by casting the paste made of an active material; GO be related to the stretching of C-O bond [10]. Graphene demon- and/or graphene (85 wt%), carbon black (5 wt%), binder; poly(- strated similar behavior as graphite in the IR range; no signatures vinylidene fluoride) (10 wt%) and curing agent; 1-methyl-2-pyr- for functional groups which could be removed during ultra-sonic rolidinone into the electrode cavity. The as-cast paste was cured cleavage of GO. The XRD patterns (Fig. 1b), represented the char- overnight at room temperature before electrochemical in- acteristic peak of graphite (2
Fig. 2. XPS high-resolution (inset; survey) spectra (a) and TEM micrograph (inset; diffraction pattern) (b) of graphene. H. Asgar et al. / Materials Chemistry and Physics 206 (2018) 7e11 9
Fig. 3. Nyquist plots (a), bode plots (b), and linear sweep voltammograms for GOPE (c) and GrPE (d) in de-aerated 0.5M KOH solution.
calibrated in accordance with the C1s peak at 285.08 eV. The O1s [16]. The effective capacitance (Ceff) was calculated from the Hsu- peak was observed at 533.08 eV. In the high-resolution spectra, the Mansfeld model [17] which could be simulated with the normal peaks at 285.98 and 288.78 eV were attributed to the remaining/ distribution of time constants and originated from the additive unremoved epoxide (-C-O-C-) and carboxyl (-COOH) groups [14], effect of double layer and pseudocapacitance. From Table 1, the respectively. The spectra of C1s has depicted the same trend as Ceff,dl for the double layer in case of GOPE and GrPE was almost reported in the literature for reduced-GO/graphene [14,15]. similar, however, the effective pseudocapacitance (Ceff,p) related The morphology and diffraction pattern of graphene were with the adsorption of ionic species was relatively higher in GOPE. evaluated from TEM images. The sheets of graphene were observed To further confirm this behavior, the LSV curves (Fig. 3c and d) containing wrinkles (Fig. 2b), which could form due to the were obtained at three different sweep rates. The existence of increased surface energy and/or drying front of the suspension sweep rate dependent reduction peak and relatively larger current during sample preparation for microscopic analysis. The electron response at large overpotential was evident in the case of GOPE diffraction pattern of graphene (inset), also validated the existence compared to GrPE which represented large polarization under the of characteristic peak corresponding to (002) plane which was in same conditions without any reduction peak. confirmation with the XRD pattern. The impedance spectra of GOPE and GrPE are shown in Fig. 3a and b. The spectra were simulated with the equivalent circuit Table 1 fi model (ECM) (inset; Fig. 3a). The kinetic performance of the sam- Simulated electrochemical parameters calculated from the ECM tting to the experimental impedance spectra of GOPE and GrPE. ples could be evaluated from the high-frequency depressed semi- circle and a slight decrease in phase angle (inset; Fig. 3b) at low Parameters GOPE GrPE 2 frequency could be associated with the heterogeneous distribution Rs (U-cm ) 10.87 54.09 F n 2 5 4 of charge due to adsorption of ionic species. This adsorption can be dl (S-s /cm ) 42.69 10 12.00 10 n 0.596 0.368 termed as pseudocapacitance and is represented as constant phase 2 Rct (U-cm ) 82.75 117.74 element (Fp) in series with the parallel combination of the double m 2 3 3 Fp (S-s /cm ) 19.05 10 21.89 10 layer (Fdl) and charge transfer resistance (Rct). The relatively lower m 0.270 0.414 2 m 2 Rct observed in case of GOPE (82.75 U-cm ) than GrPE (117.74 U- Ceff,dl ( F/cm ) 44.27 41.56 2 2 Ceff,p (mF/cm ) 1.43 1.11 cm ) was attributed to the interaction of ionic species with the Goodness of Fit 114.9 10 6 138.9 10 6 functional groups, present on GOPE as identified in the IR spectrum 10 H. Asgar et al. / Materials Chemistry and Physics 206 (2018) 7e11
Fig. 4. The proposed reaction sequence for the electron transfer from the surface functional groups and adsorption of Hads species from the dissociation of water.
e þ e H2O þ e ¼ H þ OH (1) current density with increase in the overpotential (h) without any observable reduction current peak by GrPE was related to the þ e > C¼O þ H þ e 4 ≡C e OH (2) structural characteristics of the graphene. e þ þ 4 e COO H COOH (3) 4. Conclusions
The origin of this current peak at relatively lower overpotential Graphene was synthesized via a direct route and its electro- predicts the electron transfer (pseudocapacitive response) reaction chemical response in de-aerated 0.5M KOH solution was evaluated. possibly either due to the adsorption of Hads species by the disso- Conversion of graphite to graphite oxide and then to graphene was ciation of water molecules over the surface of GOPE or to the validated by XRD, IR and Raman spectroscopy. XPS spectra pro- e reduction of surface functional groups as given in reactions (1) (3). vided the insight into the surface chemistry of the graphene. h The current peak was also shifted to larger overpotential ( ) with Moreover, the formation of few layer graphene ((002)-plane) was an increase in sweep rate which corresponded to the delayed validated by TEM micrograph and diffraction pattern. Finally, the electrochemical response or to the quasi-reversible character of the effect of functional groups on the electrochemical properties of electrode material. Fang et al. [18] and some other studies [19,20] U þ GOPE and GrPE was evaluated. The lower value for Rct (82.75 - report the preferential adsorption of H ions on the active sites 2 2 cm ) and larger Ceff,p (1.43 mF/cm ) exhibited by GOPE was (present as surface functional groups) by the reduction of water attributed to the presence of surface functional groups. It was over graphitic materials. However, the possibility of the reduction concluded that the presence of surface functional groups i.e of surface functional groups cannot be overruled under these carbonyl and epoxide etc. on the graphene oxide could improve the conditions. The presence of epoxide and carbonyl functional groups pseudocapacitive electrochemical response in de-aerated 0.5M fi was con rmed from the IR analysis of GOPE in our case as shown in KOH solution. Fig. 1. Based on the experimental evidence, the possible reaction sequence of hydrogen adsorption has also been postulated as References shown in Fig. 4. fl Brie y, under applied negative potential the activation energy [1] P. Fernandez, E. Castro, S. Real, M.E. Martins, Inter. J. Hydrogen Ener 34 (2009) required for the electron tunneling through the surface functional 8115e8126. groups e.g. carbonyl and epoxides over GOPE would decrease, [2] M. Pumera, A. Ambrosi, A. Bonanni, E. Chng, H. Poh, Trends Anal. Chem. 29 (9) þ (2010) 954e965. leading to the adsorption of (H ) species from the water. This could [3] C. Liu, Z. Yu, D. Neff, A. Zhamu, B. Jang, Nano Lett. 10 (2010) 4863e4868. also increase the active site for hydrogen adsorption and hence [4] M. Pumera, Chem. Rec. 9 (2009) 211e223. would enhance the pseudo-capacitive response in the alkaline [5] G. Nandamuri, S. Roumimov, R. Solanki, Nanotechnology 21 (14) (2010). aqueous solution as observed in Fig. 3c. Therefore, the overpotential [6] S. Stonkovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. Nguyen, R. Ruoff, Carbon 45 (7) (2007) 1558e1565. prior to the current peak could be related to the activation energy [7] F. Johra, J. Lee, W. Jung, J. Ind. Eng. Chem. 20 (2014) 2883e2887. required for the electron transfer reaction which was found to be [8] D. Marcano, D. Kosynkln, J. Berlin, A. Sinitskll, Z. Sun, A. Slesarev, L. Alemany, e potential sweep rate dependent. W. Lu, J. Tour, ACS Nano 4 (2010) 4806 4814. [9] H. Lu, Y. Peng, M. Lin, S. Chou, J. Lo, B. Cheng, Opt. Photonics J. 3 (6A) (2013). The limited but larger current response provided by GOPE [10] C. Uthaisar, V. Barone, B. Fahlman, Carbon 61 (2013) 558e567. compared to GrPE even at very large h (~ 1.0 V) and the existence [11] C. Willemse, K. Tlhomelang, N. Jahed, P. Baker, E. Iwuoha, Sensors 11 (2011) 3970e3987. of broad current peak was assigned to the adsorption of Hads spe- [12] D. Wang, W. Niu, M. Tan, M. Wu, X. Zheng, Y. Li, N. Tsubaki, ChemSusChem 7 cies by the reduction of water. Also, the continuous increase in (2014) 1398e1406. H. Asgar et al. / Materials Chemistry and Physics 206 (2018) 7e11 11
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