Applied Surface Science 264 (2013) 329–334 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Graphene sheets synthesized by ionic-liquid-assisted electrolysis for application in water purification Chia-Feng Chang a , Quang Duc Truong b,c,∗ , Jiann-Ruey Chen a a b c Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Chemistry, Vietnam National University, Hanoi, Viet Nam Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan a r t i c l e i n f o Article history: Received 22 March 2012 Received in revised form 20 July 2012 Accepted October 2012 Available online 13 October 2012 Keywords: Graphene Water purification Heavy metal Ionic liquids-assisted electrolysis a b s t r a c t A facile and green synthesis of graphene sheets by ionic-liquid-assisted electrolysis was investigated in this work The synthesized graphene sheets have been studied using transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray powder diffraction (XRD), Raman spectroscopy (Raman) and Fourier transform infrared (FTIR) analysis The obtained graphene was used for the adsorption of Fe2+ whose presence in the drinking water in wide areas of South Asia has been widely known The result shows that the graphene could absorb Fe2+ with a capacity of 299.3 mg/g which is times higher than that of graphite oxide The adsorption properties of metal ions on graphene and the effects of various factors on the adsorption capacity were also investigated in detail The research results suggest a novel material for developing highly efficient water purification materials for the developing economies © 2012 Elsevier B.V All rights reserved Introduction Elevated concentrations of geogenic As and related heavy metal in groundwater raises a threat to the health of tens of millions of people living in the large delta areas of South Asia In the Red River delta, Vietnam, an estimated 11 million people who are using groundwater as the major domestic water source for daily life are at risk [1] Although it has been found that several geochemical processes involve in the release of As into groundwater, the reduction of As-containing Fe-oxides with natural organic matter is generally considered the most important mobilization mechanism [2,3] Therefore, the contamination of As always associates with the elevated concentration of Fe(II) in the groundwater The investigation of As and Fe contaminations in Red River aquifer using random monitoring wells in Northern Vietnam showed that the concentration of Fe increases linearly with that of As (Fig 1) Thus, beside the task of removing As from groundwater, the removal of Fe in groundwater is still challenging the engineer in the environmental research Many approaches have been employed to remove metal ions from contaminated water including precipitation, ion-exchange, membrane filtration, and adsorption [4–7] However, adsorption is ∗ Corresponding author at: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577 Japan Tel.: +81 22 2175651; fax: +81 22 2175651 E-mail address: tqduc@mail.tagen.tohoku.ac.jp (Q.D Truong) 0169-4332/$ – see front matter © 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.apsusc.2012.10.022 most promising approach for the removal of metal ions from water owing to its simplicity, cost-effectiveness, and enabling large-scale application Therefore, adsorption method is widely used for processes of the magnitude of municipal water supplies to small domestic water filters, particularly as packed bed filters For practical use, there are several materials which could absorb heavy metals effectively, such as activated carbon, fly ash, prawn shell and peanut hull pellet [8–10] In fact, people in the Red River delta, Vietnam is currently using hand-made filter containing natural sand and activated carbon for cleaning the domestic water The current research has been focused and developed new adsorbents with better absorptive capacity for water purification To date, the adsorption of metal ions onto carbon nanotubes (CNT) has been investigated extensively Recently, it has been suggested that graphene oxide and graphite oxide can be practically used for separation of arsenic and heavy metal from water [11,12] As far as the authors know, the adsorption properties of graphene, a newly emerging member of carbon materials, for heavy metal ions have not been reported up to now Graphene with hexagonally, sp2 -hybridized and one-atomthick layer structures, has attracted extraordinary research interest since Geim and Novoselov, awarded the Nobel Prize in Physics 2010, explored for the first time [13–17] Theoretical and experimental studies have proved its outstanding mechanics, electricity, chemicals, thermal stability, specific surface areas, mobility of charge carriers, and thermal conductivity [14–17] Therefore, great attention has been devoted on the study the applications of graphene such as solar cells, sensors, transistors, supercapacitors, 330 C.-F Chang et al / Applied Surface Science 264 (2013) 329–334 Fig The relationship between Fe(II) and As(III) concentration in groundwater and composite materials In comparison with well-known CNTs, graphene could be soluble in normal solvents or polymers without reducing ideal intrinsic properties by chemical variations Hence, graphene provides many advantages over other carbon materials In this study, we focused on studying the removal of high concentration of Fe2+ by using graphene as adsorbent High concentration of heavy metal such as Fe2+ intake adverse health effects and long-term exposure to metal can cause acute gastrointestinal effects [18,19] Therefore, removal of iron from ground water is highly desirable, especially, for South Asia where contamination of As and Fe in groundwater is a serious problem Experimental 2.1 Synthesis of graphite oxide and graphene sheets Graphene was synthesized by ionic-liquid-assisted electrolysis according to a procedure reported by Liu et al [20] The electrolytes consist of ionic liquid, 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6 ]), and water In typical experiment, two high-purity graphite oxide rods as the electrodes were inserted into the solution of ([C8mim][PF6 ])/water (1 g in 15 mL water), paralleled with a distance of 4.0 cm DC voltage of 15 V was applied to two graphite rods by Keithley 2400 model potentiostat After electrolyzing for 0.5 h, the anode graphite rod was corroded and black precipitate appeared in the reactor After electrolyzing for 12 h, the precipitate was taken out of the reactor, and was washed thoroughly with deionized water Then it was dried at 60 ◦ C in the electric oven for h Finally, the mixture of graphene and graphite oxide was obtained Since the graphite oxide is more hydrophilic than graphene, it can be separated from graphene by using polar and non polar solvent dispersion Particularly, mixture of toluene (90 vol.%) and H2 O (10 vol.%) was used to separate graphite oxide and graphene Typically, mixture of graphite oxide and graphene was dispersed in water The precipitation of agglomerated graphite oxide sheets was immediately observed in water after the addition of toluene to the aqueous solution After collecting the aqueous suspension three times, graphite oxide and graphene could be isolated Finally, graphite oxide and graphene sheets were dried at 60 ◦ C in the electric oven for another h 2.2 Characterization The morphology of the obtained structures was examined using transmission electron microscopy (TEM) of JEM-2100F The thickness measurement of Graphene sheets was conducted with tapping mode atom force microscopy (AFM, SPA-400 SPM unit from Seiko, Japan) 10 mg graphite oxide and graphene sheets were dispersed in DMF for TEM and AFM measurements The X-ray powder diffraction (XRD) measurements on a Rigaku D/max-IIB X-ray ˚ was performed diffractometer with Cu K␣ radiation ( = 1.5418 A) to identify the crystallite phase of the sample In order to identify the graphene material in the synthesized products, Raman measurement with 514 nm excitation wavelength at room temperature was carried out The spectra were acquired by a Jasco R-3000 Raman spectrometer in the backscattering geometry using 514 nm wavelength and laser power of 90 W for excitation The incident beam was focused on the sample through the 100× lenses of microscope into the spot of m size The FTIR measurement was carried out using universal attenuated total reflectance Fourier transform infrared (UATR-FTIR) over the range from 4000 to 650 cm−1 For specific surface area determination, 25 mL dye solutions with 20–120 mg L−1 was introduced separately into 25 mL (adsorbent: 0.5 mg mL−1 ) conical flasks and proceeded methylene blue (MB)relative surface area measurement 2.3 Batch adsorption experiments Batch adsorption experiments were conducted to determine the Fe(II) adsorption capacity of graphene Iron stock solution (1000 mg/L) was prepared by dissolving Fe(NO3 )2 (Merk, Germany) in deionized water and further diluted to the required concentrations before use The effect of initial and final pH on metal ion adsorption by graphene was conducted in a pH range of 5.0–9.0 The pH of the solution was adjusted to a desired value using appropriate concentrations of HNO3 or NaOH solutions For the batch tests, about 20 mg graphite, graphite oxide or graphene sheets were added to 20 mL solution containing Fe(II) (20 mg L−1 ) while the pH of the solution was varied from 5.0 to 9.0 After the suspensions were shaken for 24 h to achieve sorption equilibrium, the solid phase was separated from the solution by syringe filtration The results of kinetic sorption suggested that Fe(II) sorption on graphite oxide and graphene sheets achieved equilibrium after several hours The concentrations of Fe(II) in the filtrate were determined by atomic absorption spectroscopy (PerkinElmer 3110) All the experimental data were the average of duplicate determinations, and the relative errors were about 5% The adsorption experiment was also performed with Co(II) The amounts of Fe(II) or Co(II) ions adsorbed on graphite, graphite oxide and graphene sheets (mg/g) were calculated from the initial concentration (C0 ) and the equilibrium one (Ce ) (qe = (C0 − Ce ) × V/m, where V is the volume of the suspension, and m is the mass of graphite, graphite oxide or graphene sheets) Results and discussion 3.1 Characterization of the synthesized materials The XRD pattern of graphite (Fig 2) presents very sharp diffraction peak at 2Â = 26.5◦ , which can be indexed to the {0 2} peak of graphitic structure of carbon This peak only appears in the cases of graphene sheets and graphite but not in graphite oxide sheets In particular, when graphite was oxidized to graphite oxide, a new diffraction peak appears at 2Â = 10.5◦ , along with the disappearance of the diffraction peak {0 2} in the XRD pattern of the product In the XRD pattern of graphene, a typical diffraction peak at 2Â = 27.5◦ [21,22] was observed which is broaden, indicating the smaller crystalline size of graphene in single layer or few layers structure The calculated d-spacing between layers of graphite oxide and graphene sheets are 0.84 nm and 0.32 nm respectively C.-F Chang et al / Applied Surface Science 264 (2013) 329–334 Fig The XRD patterns of graphite, graphite oxide and graphene sheets The crystalline nature and morphology of graphite oxide and graphene sheets was investigated using TEM Fig shows TEM images and corresponding selected-area electron diffraction (SAED) of the obtained graphite oxide and graphene sheets The thinner layers and higher crystalline nature of graphene sheets than graphite oxide is revealed Selected area electron diffraction was performed on the graphite oxide and graphene sheets and the corresponding SAED patterns are shown as the inset in Fig The diffraction rings can be fully indexed to the hexagonal graphitic structure, indicating the crystalline nature of the graphite oxide and graphene sheets The obtained graphite oxide and graphene sheets were further analyzed by AFM and the results are shown in Fig The AFM analysis reveals the presence of graphite oxide and graphene sheets with average thickness of 3.0 and 0.98 nm respectively, which are consistent with result of previous report [20] Particularly, the graphene sheet thickness of 0.98 nm is characterized of single layer graphene synthesized by ionic-liquid-assisted method [20] This thickness is thicker than the theoretical value of a single layer (0.34 nm), which may be due to the presence of functionalized hydrocarbon chains and PF6 − attached to graphene sheets These 331 AFM images also reveal that the exfoliation of graphite has been occurred X-ray photoelectron spectroscopy (XPS) was used to verify the element binding configuration in graphite oxide and graphene sheets (Fig 5) The presence of C and O elements from the survey scan of graphite oxide and graphene sheets is revealed The presence of O proves that graphite has been oxidized by hydroxyl and oxygen radicals at the electrode The characterization peak of C O, C O, COOH and the peak area in XPS spectra indicate the change of composition during the electrolysis (Fig 5) The relative peak area of the C 1s to O 1s in graphene is higher than that in graphite oxide This evidence indicates that the concentration of the C C moiety of graphene is higher than that of graphite oxide, suggesting the smaller amount of oxygenated functional groups in exfoliated graphene sheets The successful exfoliation of graphite into graphene was further verified by Raman spectra In the Raman spectrum of natural graphite (Fig 6), there are two peaks at 1580 cm−1 and 2736 cm−1 respectively The peak at 1580 cm−1 corresponds to an E2g mode related to the sp2-bonded carbon atom’s vibration in a 2D hexagonal lattice And the peak at 2736 cm−1 arises from the second order of zone-boundary phonons of graphite However, the major features, commonly observed in all chemically processed graphene, are the D band at 1354 cm−1 , G band at 1567 cm−1 , and 2D band at 2736 cm−1 The D band shows the presence of sp3 defects The presence of the sharp 2D band of graphene at 2723 cm−1 is presumably due to two phonon double resonances Raman process The shape of the 2D band of graphene is similar to that of previously characterized graphene, indicating the presence of few layers for the synthesized graphene The exfoliated graphite oxide has a high density of defects as evident from the stronger D peak in the Raman spectrum in Fig Due to the presence of oxygenated functional groups, surface defects are formed It is also suggested that the D band arises from edge defects as well as the larger surfaceto-volume ratio The extracted graphene has sharp sp2 bonds in 2D system and the graphene provides high aspect ratio structure are evidenced We observed the different functional groups of graphite, graphite oxide and graphene sheets in the FTIR spectra as shown in Fig The absorption peaks are observed at 3398–3428 cm−1 ( OH), 1681 cm−1 (C O), 1598–1603 cm−1 (skeletal vibrations from oxidized graphite domains), 1529–1543 cm−1 ( ph), 1205–1279 cm−1 (C OH stretching vibrations) and 1086 cm−1 (C O stretching vibrations) C O Fig TEM images and SAED of graphite oxide (a) and graphene sheets (b) 332 C.-F Chang et al / Applied Surface Science 264 (2013) 329–334 Fig AFM images of graphite oxide (a) and graphene sheets (b) Fig XPS spectra of graphite oxide (a, c) and graphene sheets (b, d) C.-F Chang et al / Applied Surface Science 264 (2013) 329–334 333 Fig Raman spectra of graphite, graphite oxide and graphene sheets group was only found in the spectrum of graphite oxide structure From FTIR investigation, it has been proved that graphite was transferred to the graphite oxide and graphene structures On the basis of above result, the formation of graphene by electrolysis can be understood as follows Typically, after applying voltage, hydroxyl and oxygen radicals were produced inevitably These radicals were formed by oxidation of dissociated water Consequently, the graphite anode was oxidized to produce the graphite oxide At the same time, the reduction of the cation from ionic liquid, leads to the formation of the 1-octyl-3-methylimidazolium free radical The radical can combine with one of the electrons of the p-bond of the graphite [20] The complicated interactions between ionic liquid and graphite, including Coulomb interaction and − interactions, could exfoliate graphite into graphene sheets In addition, ionic liquid also provides the anionic ion to open layers between the graphite structures Thus, the production of graphite oxide and graphene occurred through several steps as follows: (a) oxidation of water and ionic liquid at anode to produce the hydroxyl, oxygen radicals and free radical from ionic-liquid; (b) the anionic BF6 − and free radical from ionic Fig Effect of pH on the sorption (qe ) of Fe(II) (a), and Co(II) (b) on graphite, graphite oxide, and graphene sheets liquid depolarized and expanded the graphite into graphene sheets; (c) the radicals oxidized the graphite into the graphite oxide 3.2 The adsorption of heavy metals Fig FTIR spectra of graphite, graphite oxide and graphene sheets The adsorption of Fe2+ and Co2+ on graphite, graphite oxide and graphene sheets under the different pH values were studied (Fig 8) The adsorption capacity is characterized by adsorptions amount of metal ion/mass of adsorbent (qe , mg/g) It was found that the adsorption capacity of graphene sheets is 6–7 times higher than that of graphite oxide sheets The adsorption capacities of graphene sheets increased sharply vs pH value in range of 5.0–8.0 and reach maximum value of 299.3 and 370 mg/g for Fe(II) and Co(II), respectively The lower adsorption capacity at pH 9.0 may be due to the formation of hydroxide Fe(OH)2 and Co(OH)2 , which reduced number of metal ions adsorbed on the graphene surface On the other hand, the adsorption competition of H+ with metal ions for adsorption sites resulted in low adsorption capacity at low pH condition It was reported that the isoelectric point (IEP) of graphene where the zeta potential equals zero is 4.7 [23] This indicates that at pH > IEP, the graphene has negative surface charge, which benefits for adsorbing cations Thus, the increase of the adsorption capacity in pH range of 5.0–8.0 may be attributed to the electrostatic interaction between negatively charged surface of graphene and metal cation The highest adsorption of Fe2+ and Co2+ on graphene sheets at pH 8.0 is presumably due to negatively charged surface of 334 C.-F Chang et al / Applied Surface Science 264 (2013) 329–334 graphene Thus, it can be concluded that the electron rich surfaces of the graphene sheets offered the strongest attraction to metal cations at pH 8.0 The adsorption capacity of graphene is higher than those of SWCNTs and MWCNTs [24] In comparison of the adsorption rate, the free energy (Ea ) of metal ions to be adsorbed onto material surface should be considered The faster rate of adsorption indicates the lower free energy Ea Thus, it is proposed that the Ea of metal ions adsorbed on graphene is less than those on SWCNTs and MWCNTs [24] Due to the structure of multiple atomic layers (MWCNT) or curly ones (SWCNTs), the metal ions require more energy (higher Ea ) to diffuse and adsorb on these structures It was also found that the adsorption capacity to Co2+ is slightly higher than that to Fe2+ ions The specific cation formed electric double layer complex on the surface of graphene, which blocked the adsorption of other ions The different adsorption capacities to different metal ions are mainly depended on the metal properties The adsorption capacity to Co2+ is slightly higher than that to Fe2+ ions is presumably due to the smaller atom size of the Co(II) cation Furthermore, graphene showed higher adsorption capacity of metal ions than those of graphite and graphite oxide is presumably due to its single layer structure and the anionic surface properties of graphene The methylene blue (MB)-relative surface area of graphene was very high (605.4 ± 0.7 m2 /g, n = 3) which is expected to be higher than those of graphite and the graphite oxide It is well-known that the adsorption capacity has a direct correlation with the specific surface area The single layer structure could provide the high surface area which contributed to high sorption properties Moreover, more defects occurred after the chemical adjustment, which also reflected the interaction between metal ions and graphite oxide structures In summary, single layer structure could provide the high surface area which contributed to high sorption properties The functional group of graphite oxide may reduce their ability for adsorption of metal ions such as Fe2+ and Co2+ Conclusions The graphite oxide and graphene sheets were successfully synthesized by the ionic-liquid-assisted electrochemical method The maximum adsorption capacity (qe ) for Fe2+ and Co2+ on graphene is 299.3 and 370 mg/g respectively (pH 8.0), which are much higher than those on other reported carbon materials The adsorption capacity of graphene sheets is also 6–7 times higher than that of graphite oxide sheets The research results suggest a novel material for developing highly efficient water purification materials for the developing economies in South Asia Acknowledgement This work was supported by National Science Council of the Republic of China through the Contract No NSC 95-2221-E-007 174–MY3 and Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan References [1] M Berg, H.C Tran, T.C Nguyen, P.H Viet, R Schertenleib, W Giger, 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characterization of multi-walled carbon nanotubes/chitosan nanocomposite and its application for the removal of heavy metals from aqueous solution, Journal of Alloys and Compounds 509 (2011) 2582–2587 ... separate graphite oxide and graphene Typically, mixture of graphite oxide and graphene was dispersed in water The precipitation of agglomerated graphite oxide sheets was immediately observed in water. .. concentrations of Fe(II) in the filtrate were determined by atomic absorption spectroscopy (PerkinElmer 3110) All the experimental data were the average of duplicate determinations, and the relative... structure, indicating the crystalline nature of the graphite oxide and graphene sheets The obtained graphite oxide and graphene sheets were further analyzed by AFM and the results are shown in Fig