Ultrasonication Induces Oxygenated Species and Defects onto

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Ultrasonication Induces Oxygenated Species and Defects onto Exfoliated Graphene Theodosis Skaltsas, Xiaoxing Ke, Carla Bittencourt, and Nikos Tagmatarchis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4057048 • Publication Date (Web): 03 Oct 2013 Downloaded from http://pubs.acs.org on October 8, 2013

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The Journal of Physical Chemistry

Ultrasonication Induces Oxygenated Species and Defects onto Exfoliated Graphene Theodosis Skaltsas†, Xiaoxing Ke‡, Carla Bittencourt§, and Nikos Tagmatarchis†* †

Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece ‡

EMAT, University of Antwerp, B-2020, Antwerp, Belgium §

ChIPS, Université de Mons, B-7000, Mons, Belgium

AUTHOR EMAIL ADDRESS. [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) TITLE RUNNING HEAD. Exfoliated Graphene CORRESPONDING AUTHOR FOOTNOTE. Tel.: + 30 210 7273835; Fax: + 30 210 7273794; email: [email protected] (N. Tagmatarchis)

ABSTRACT. The effect of ultrasonication parameters, such as time and power applied, to exfoliate graphite in ortho-dichlorobenzene (o-DCB) and N-methyl-1,2-pyrolidone (NMP) was examined. It was found that the concentration of graphene was higher in o-DCB, while its dispersibility was increased when sonication was applied for longer period and/or at higher power. However, spectroscopic examination by X-ray photoelectron spectroscopy (XPS) revealed that ultrasonication causes defects and

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induces oxygen functional groups in the form of carboxylic acids and ethers/epoxides onto the graphene lattice. Additional proof for the latter arose from Raman, IR and thermogravimetry studies. The carboxylic acids and ethers/epoxides onto exfoliated graphene were derived from air during ultrasonication and found independent of the solvent used for the exfoliation and the power and/or time ultrasonication applied. Quantitative evaluation of the amount of oxygenated species present on exfoliated graphene as performed by high-resolution XPS revealed that the relative oxygen percentage was higher when exfoliation was performed in NMP. Finally, the sonication time and/or power affected the oxygen content on exfoliated graphene, since extended ultrasonication resulted on a decrease in the oxygen content on exfoliated graphene, with a simultaneous increase of defected sp3 carbon atoms.

KEYWORDS. graphite; exfoliation; sonication; defects; carboxylic acids; ethers. INTRODUCTION Graphite is the starting material en-route to graphene, however, sufficient energy must be given to surpass the thermodynamic stability of the former due to the plethora of interlayer van der Waals interactions that tightly hold graphene sheets in graphite. The amount of energy needed to overcome the corresponding barrier and liberate graphene from graphite can be reached by either a physical process (mechanical cleavage,1, 2 epitaxial growth3), or by chemical means (intercalation,4 oxidation-reduction,5 surfactants,6-8 polymers,9 functionalization10,

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), with each method having their own scope and

limitations. For example, exfoliation of graphite by physical means lacks access to macroscopic quantities of material and can only provide sufficient amounts of highly pure graphene, though, suitable only for certain studies. On the other hand, exfoliated graphene by wet chemistry, via oxidation under harsh conditions, results on graphene oxide (GO),12,

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however, with a disrupted electronic network

deficient of novel electronic properties. In addition, as reduction of GO partially occurs, sometimes leading to amorphous carbon,14 the restoration of the sp2 network is incomplete and therefore the properties of the resulting reduced graphene oxide (RGO) significantly deviate from those of pristine graphene. Along the same lines, milder liquid-phase approaches developed for exfoliating graphite with ACS Paragon Plus Environment

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the aid of ultrasonication,15 however, stabilizer additives are needed to prevent reaggregation. This stabilization of exfoliated graphene can be achieved by either covalent or non-covalent functionalization approaches, with the former leading to saturation of carbon-carbon double bonds, thus introducing sp3 defects onto the lattice at sites where organic addends are located.16 These lattice defects, similarly present in GO and RGO, on covalently functionalized graphene, considerably affect graphene’s remarkably high electron mobility.17, 18 Nevertheless, graphene retains its pristine nature, with all novel electronic properties remained unaffected, when it is stabilized upon non-covalent methodologies,19-21 such as π-π stacking and/or electrostatic interactions. In fact, we recently succeeded on the exfoliation of graphite by ultrasonication and stabilization of graphene sheets in either organic solvents or water in the absence or presence of ionized block copolymers, respectively.22 A critical parameter on the production of exfoliated graphene based on non-covalent approaches is ultrasonication. However, in most, if not at all, studies in which ultrasonication has been applied, a careful examination on the quality of the exfoliated graphene shows the presence of defects, which, however, have been neglected from being extensively evaluated. Therefore, it is absolutely timely to comprehensively assess the nature of those defects. Herein, it is presented that ultrasonication of graphite induces oxygen containing functionalities onto the exfoliated graphene lattice. Especially, with the aid of X-ray photoelectron spectroscopy (XPS) and IR, information regarding the nature and amount of these oxygenated species become available. In the current study, the effect of ultrasonication, in terms of power and time applied, on the induction of defects – in the form of oxygenated species – onto exfoliated graphene is pursued. On top of the information obtained from XPS and IR measurements, Raman spectroscopy, thermogravimetry (TGA) and microscopy imaging (TEM) were also applied to characterize in detail and evaluate the quality of exfoliated graphene. EXPERIMENTAL SECTION Materials. Graphite flakes (75+ mesh, >75%, batch no: 13802EH) and ortho-dichlorobenzene (oDCB) were obtained from Aldrich, while N-methyl-1,2-pyrolidone (NMP) was obtained from SDS and used as received without further purification. ACS Paragon Plus Environment

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Ultrasonication. Tip sonication was performed with a Bandelin Sonoplus Ultrasonic Homogenizer HD 3200 equipped with a flat head probe (VS70T), running at 10% or 20% of the maximum operating power (200 Watt). Centrifugation was performed by an Eppendorf 5702 at 2500 rpm. Steady-State Absorption Spectroscopy. The electronic absorption spectra were recorded on a Perkin Elmer (Lambda 19) UV-Vis-NIR spectrophotometer. Attenuated-Total-Reflectance Infrared Spectroscopy. Mid-infrared spectra in the region 550-4000 cm-1 were obtained on a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics) equipped with a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies). A drop of the solution was placed on the diamond surface, followed by evaporation of the solvent, in a stream of nitrogen, before recording the spectrum. Typically, 100 scans were acquired at 4 cm-1 resolution. Raman Spectroscopy. Micro-Raman scattering measurements were performed at room temperature in the backscattering geometry using a RENISHAW inVia Raman microscope equipped with a CCD camera and a Leica microscope. A 2400 lines mm-1 grating was used for all measurements, providing a spectral resolution of ± 1 cm-1. As an excitation source the Ar+ laser (514 nm with less than 0.5 mW laser power) was used. Measurements were taken with 60 seconds of exposure times at varying numbers of accumulations. The laser spot was focused on the sample surface using a long working distance 50x objective. Raman spectra were collected on numerous spots on the sample and recorded with Peltier cooled CCD camera. The intensity ratio ID/IG was obtained by taking the peak intensities following any baseline corrections. The data were collected and analyzed with Renishaw Wire and Origin software. X-ray Photoelectron Spectroscopy (XPS). The chemical composition of the samples was investigated using XPS VERSAPROBE PHI 5000 from Physical Electronics, equipped with a Monochromatic Al Kα X-Ray. The energy resolution was 0.7 eV. For the compensation of built up charge on the sample surface during the measurements, a dual beam charge neutralization composed of an electron gun (~1 eV) and the Argon Ion gun (≤10 eV) was used. To support the interpretation of the XPS spectra, standard graphite was analyzed.


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TEM Analysis. Images were acquired using a conventional TEM microscope (model FEI-Tecnai G2F20, with point resolution of 2.5 Å operated at 200 KV). Imaging process was based on single image acquisition using typical low current density (~ 15 A/cm2) and acquisition times of 5s, which represent a good compromise between obtaining an image with acceptable good signal-to-noise ratio and reducing damage by irradiation. The graphene specimen was dispersed in ethanol and then fixed on a copper TEM grid coated with holey carbon film. Exfoliated graphene in organic solvents. Graphite flakes (50 mg) were added to 100 mL of solvent (i.e. o-DCB or NMP) and the mixture was sonicated at different times (5, 15, 30 and 60 minutes) and at different sonication powers (20 and 40 Watt). The so-produced ink-like graphene dispersion was centrifuged for 15 minutes at 2500 rpm and the supernatant layer was collected. In order to characterize the samples, the dispersions were filtered through a nylon membrane (pore size 0.2 µm) and the solid material remained on top of the filter was extensively washed, collected and dried overnight in a vacuum oven at 70 ºC. RESULTS AND DISCUSSION Initially, ultrasonication was applied to exfoliate graphite flakes and produce stable graphene dispersions in NMP or o-DCB.22 Time and power variation of ultrasonication was investigated, searching for optimum conditions to achieve maximum exfoliation, while producing highly concentrated graphene dispersions. At this point it should be pointed that o-DCB is sensitive to sonication,23-25 however, similarly with other studies conducted in o-DCB and graphene, the minute amounts of decomposed o-DCB negligibly affects exfoliated graphene.26 Then, electronic absorption spectroscopy was used to estimate solubility values for the graphene dispersions in both o-DCB and NMP. By monitoring the absorbance of exfoliated graphene at 660 nm and applying the Beer-Lambert law, the solubility values for each solvent and for all different sonication periods and power applied (Table 1), were obtained. The trend is that the longer the period and/or the stronger the power of ultrasonication, the higher the concentration of the as-prepared exfoliated graphene. Evidently, the highest solubility value for exfoliated graphene is obtained after 60 minutes of ACS Paragon Plus Environment

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ultrasonication at 40 W. In addition, o-DCB seems to be more efficient to exfoliate graphite and disperse graphene sheets than NMP, since the highest concentration obtained in the former is more than double as compared to that for the latter (sample EG16 possessing 43.4 µg/mL in o-DCB vs sample EG8 possessing 18.6 µg/mL in NMP – see Table 1). Table 1. Solubility values of exfoliated graphene. Sample

Solvent

Solubility (µg/mL)

Sonication Time (min) Sonication Power (W)

EG1

NMP

1.2

5

20

EG2

NMP

2.6

15

20

EG3

NMP

4.8

30

20

EG4

NMP

12.2

60

20

EG5

NMP

4.3

5

40

EG6

NMP

8.8

15

40

EG7

NMP

11.6

30

40

EG8

NMP

18.6

60

40

EG9

o-DCB

5.6

5

20

EG10

o-DCB

13.6

15

20

EG11

o-DCB

16.0

30

20

EG12

o-DCB

32.8

60

20

EG13

o-DCB

10.2

5

40

EG14

o-DCB

16.3

15

40

EG15

o-DCB

26.3

30

40

EG16

o-DCB

43.4

60

40

Initially, the as-prepared exfoliated graphene material was morphologically examined by TEM. In Figure 1, typical TEM images of graphite and graphene obtained upon 60 minutes of tip sonication in NMP are shown. Evidently, oligolayered graphene sheets are present, while large graphitic aggregates are absent. This is in accordance with previous AFM studies which indicated that the oligolayered graphene sheets are small-sized of around 50 nm.22 Particularly, TEM imaging of monolayered graphene 6 ACS Paragon Plus Environment

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sheets was possible, which were identified from selected area electron diffraction (SAED) and fast Fourier transform (FFT) in the selected areas (see inset of Fig. 1b). Similar TEM images obtained from exfoliated graphene after ultrasonication in o-DCB. Furthermore, imaging of exfoliated graphene samples as obtained at different sonication conditions (i.e. time and operating power) did not reveal significant dissimilarities. All in all, the current TEM studies are indicative for the success and efficiency of exfoliation of graphite upon ultrasonication.

Figure 1. Representative TEM images of (a) intact graphite, and (b) exfoliated graphene as obtained after 60 minutes of ultrasonication in NMP. The inset image shows the FFT from the specific selected area electron diffraction. Raman spectroscopy is a powerful tool to evaluate the extent of exfoliation in graphite.27, 28 In Figure 2, the Raman spectrum of intact graphite is compared with the one obtained from exfoliated graphene. Three bands are of importance, namely D, G and 2D, and special attention is given to each one, since useful information regarding the nature and properties of graphene can be obtained. The frequency of the strongest G-band for exfoliated graphene, due to the presence of sp2 hybridized carbon atoms, remains virtually unchanged relative to that of intact graphite (i.e. 1582 cm-1), thus suggesting the absence of doping (p- or n- type) on exfoliated graphene,29 signifying the lack of appreciable electronic interactions ACS Paragon Plus Environment

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between the solvents used and graphite. In other words, the exfoliation of graphite by ultrasonication in NMP or o-DCB is only a physical process, taking place without the interference of electronic interactions from the solvents. Then, focusing in the 2D band, which its shape and position are characteristic of the exfoliated graphene layers, an asymmetrical band is observed, thus attributed to rather oligolayered than monolayered graphene sheets. In addition, a 27 cm-1 down-shift relative to that of intact graphite (i.e. 2699 cm-1 for exfoliated graphene vs 2726 cm-1 for intact graphite) is discernible in the 2D-band for exfoliated graphene. As far as the 2D/G ratio concerns, it is found almost unaffected when comparing the exfoliated graphene sample with that of intact graphite (i.e. around 0.55). The latter comes as additional support of negligible electronic interactions between graphene sheets and the solvents used for the exfoliation, since the 2D/G ratio is sensitive in cases where electron or hole doping occurs.30 For example, assuming that o-DCB (or NMP) as electron-donating species is attached onto the exfoliated graphene network, a decrease of the 2D-band should have been expected.29 Finally, it is interesting to note the evolution of the D-band at 1350 cm-1, in the exfoliated graphene sample, with a relatively high intensity as compared with that of the G-band (i.e. ID/IG = 0.6), characteristic of defects due to sp3 hybridization of carbon atoms present at graphene lattice.31 Although formation of monolayered graphene sheets possessing sharp and symmetric 2D-band cannot be identified in these Raman bulk measurements, the current spectrum is indicative of exfoliated oligolayered graphene sheets. Keeping in mind that only ultrasonication was applied to exfoliate graphite, in the absence of any reactive organic additives which could result on the covalent modification of the graphitic skeleton and functionalization of the material,32 the enhanced ID/IG ratio observed suggests the presence of defective carbon atoms in the lattice of exfoliated graphene. Those defective carbon atoms may well not only be defects situated mostly at the edges of graphene sheets created by the exfoliation, but also sp3 hybridized carbon atoms carrying oxygenated species, as it is shown and justified below with additional spectroscopic tools and thermal analysis.


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Figure 2. Raman spectra of intact graphite flakes (black) and exfoliated graphene (red), as obtained by ultrasonication for 60 minutes in NMP (λexc. = 514 nm). The spectra are shifted for clarity. Thermogravimetric analysis studies revealed the thermal stability of intact graphite up at least 900 ºC in the presence of nitrogen. On the other hand, exfoliated graphene starts to lose weight at 230 ºC. At this point, it should be noted that prior of the thermal studies, exfoliated graphene was initially treated at 130 ºC for 30 minutes, to ensure that the presence of any solvent impurity trapped within the graphitic lattice is evaporated and does not interfere with the measurement. According to Figure 3, exfoliated graphene shows a weight loss of 14% in the temperature range 230-350 ºC, always under an inert atmosphere, while the weight loss observed at higher temperature is attributed to the pyrolysis of defective graphitic lattice (Figure 3). It should be noted that same TGA traces were obtained when measurements performed under Ar atmosphere, thus, suggesting the absence of N-doping in the presence of nitrogen during thermal heating.33 Therefore, the 14% weight loss occurring at relatively lower temperature is credited to the presence of functionalities introduced onto the graphene sheets during the exfoliation process upon the ultrasonication treatment.


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Figure 3. Thermographs of intact graphite (black) and exfoliated graphene, obtained after 60 minutes of ultrasonication in NMP (red). The decisive information for the nature of the introduced functionalities on exfoliated graphene samples is brought by XPS – a technique for surface rather than bulk analysis of the samples. Figure 4a, shows typical survey spectra for graphite and exfoliated graphene as obtained in o-DCB and in NMP. For graphite, a main peak near 284.6 eV (C 1s) associated to photoelectrons emitted from sp2 bonding carbon atoms is observed. On the other hand, for the exfoliated graphene samples, in addition to the main peak (C 1s), a low intensity peak near 520 eV related to the presence of oxygen is identified. The elemental relative atomic percentages found for exfoliated graphene samples as well as intact graphite are summarized in Table 2. In more detail, intact graphite shows a high carbon percentage of 98.2 %, while also all exfoliated graphene samples examined, show high degree of carbon content. Moreover, in the samples of exfoliated graphene as obtained by ultrasonication in the presence of o-DCB, high oxygen percentage (i.e. 8.0-11.0 %), accompanied by a small amount of chlorine (i.e. 1.4-2.0 %, derived from decomposed o-DCB) was identified (Table 2). Similar, high values for oxygen were found in the exfoliated graphene samples when NMP was used as solvent, however, this time nitrogen was also identified in small quantities (i.e. 1.0-1.5 %, derived from decomposed NMP) instead of chlorine. Interesting observations derived from these XPS results are the following (a) in o-DCB exfoliated graphene, the amount of oxygen was found to decrease, while simultaneously the amount of carbon content found higher when (i) the sonication time increased from 30 to 60 minutes (sonication power ACS Paragon Plus Environment

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kept constant at 20 W), (ii) the sonication power was increased from 20 to 40 W (sonication period kept constant at 30 minutes), (b) in NMP, the amount of oxygen (and carbon) found to be sensitive only with the sonication power, independently of the period ultrasonication was applied. Moreover, it was found that the oxygen percentage in the exfoliated graphene was higher when NMP was used as compared with the case of o-DCB. Since o-DCB does not possess oxygen in its structure, it is reasonable to assume at this stage, that the oxygen found in the corresponding sample of exfoliated graphene is derived from air. Notably, high-resolution XPS data (analyzed below), suggest the presence of carboxylic acids and ethers/epoxides on the exfoliated graphene. The same holds also true for NMP, in which case carbonyl is also not found, regardless the fact that NMP possesses oxygen in the form of a carbonyl, thus indicating again that the oxygen species found on exfoliated graphene derive from the air and not from the solvent. Focusing on exfoliated graphene sample obtained in o-DCB by ultrasonication for 30 minutes at 20 Watt, the high resolution X-ray photoelectron spectrum of C 1s was deconvoluted into five components (Figure 4b). The main band at 284.6 eV is attributed to sp2 hybridized carbon in the graphene lattice, while the band at 285.2 eV is assigned to sp3 carbon-carbon single bonds on structural defects derived upon ultrasonication. The latter is in full agreement with the Raman spectrum (cf. Fig. 2) in which an enhanced disorder band was identified. Moreover, the components at 286.1 eV and 289.2 eV are due to carbon atoms in ether or epoxy (C-O-C) and carboxylic groups (-COOH), respectively.34 Moreover, these two components can be observed in the O 1s core level spectra (Figure 4c). The fifth component deconvoluted in the C 1s core corresponds to the characteristic satellite peak from the sp2 hybridized carbon atoms due to π-π* shake-up features at 290.6 eV.34 Based on the high-resolution XPS data analysis, the main trend observed for exfoliated graphene in both solvents o-DCB and NMP is that as sonication time and/or power increases, a relative increase in the intensity of the component corresponding to sp3 carbons (i.e. structural defects) in the expense of both oxygenated species (C-O-C and –COOH) occurs – see Table 3. Thus, from the XPS results acquired, it is interesting to note the following points (a) ultrasonication induces not only defects but also oxygenated species onto exfoliated ACS Paragon Plus Environment

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graphene lattice, and (b) extensive ultrasonication or sonication with enhanced power, results on decomposition (to some degree) of those oxygenated species, by introducing additional defects onto the graphene plane. In addition, although ultrasonication induces defects and oxygenated species on exfoliated graphene, the C/O ratio as derived from the surface atomic percentage for carbon and oxygen calculated from XPS cannot be used to evaluate the number of layers of exfoliated graphene, because the amount of oxygen is related to the number of defects at the surface of graphene but not to the number of layers as in a recent study.35 Nevertheless, we cannot exclude the possibility that the sonication treatment is further separating the flakes, which can have an influence on the XPS analysis of an individual flake. Table 2. Surface atomic percentage (experimental error is 10%) for the elements calculated from XPS as a function of ultrasonication time (T) and power (P) applied as well as solvent used for exfoliated graphene. Sample

Solvent

T (min)

P (W)

C 1s %

O 1s %

N 1s %

Cl 2p %

Graphite

-

-

-

98.2

1.8

0

0

GR1

o-DCB

30

20

87.0

11.0

0

2.0

GR2

o-DCB

30

40

90.0

8.0

0

2.0

GR3

o-DCB

60

20

88.7

9.9

0

1.4

GR4

o-DCB

60

40

87.6

11.0

0

1.4

GR5

NMP

30

20

74.0

25.0

1.0

0

GR6

NMP

30

40

85.0

13.5

1.5

0

GR7

NMP

60

20

74.0

25.0

1.0

0

GR8

NMP

60

40

87.7

11.0

1.3

0


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Table 3. XPS Analysis of C 1s peak recorded as a function of ultrasonication time (T) and power (P) applied as well as solvent used for exfoliated graphene. Sample

Solvent

T (min)

P (W)

sp2 %

sp3 %

ether %

acid %

π-π %

Graphite

-

-

-

93.2

-

2.4

-

4.4

GR1

o-DCB

30

20

46.5

14.2

24.7

11.6

3.0

GR2

o-DCB

30

40

54.6

23.7

15.6

4.3

1.8

GR3

o-DCB

60

20

46.4

28.0

16.0

8.0

1.6

GR4

o-DCB

60

40

28.0

47.3

17.2

6.2

1.0

GR5

NMP

30

20

45.0

17.2

28.0

7.0

2.8

GR6

NMP

30

40

48.0

26.0

19.0

6.0

1.0

GR7

NMP

60

20

44.6

17.0

26.0

10.4

2.0

GR8

NMP

60

40

32.0

31.5

22.0

13.0

1.5


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Figure 4. Representative X-ray photoelectron spectra. (a) Survey spectra of pristine graphite (black) and exfoliated graphene as obtained upon ultrasonication in o-DCB (grey) and in NMP (light grey). The spectra are shifted for clarity. (b) C 1s, and (c) O 1s, high-resolution deconvoluted spectra. Finally, ATR-IR spectroscopy further justified the XPS findings, namely the introduction of ether/epoxy units together with carboxylic moieties on exfoliated graphene after ultrasonication. Thus, while intact graphite does not really possess any characteristic fingerprint modes in the IR spectrum,


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exfoliated graphene samples showed relative strong bands at 1717 cm-1 and 1250 cm-1, corresponding to carboxylic and ether/epoxy stretching vibrations, respectively (Figure 5). Without a doubt, ultrasonication of graphite not only is effective on the exfoliation of graphene sheets but also most importantly introduces oxygenated species, mainly in the form of carboxylic and ether oxides (or epoxides) onto the graphene skeleton. Moreover, the presence of these oxygenated species found to be independent of the solvent used for the exfoliation (i.e. o-DCB or NMP), the sonication period (i.e. 5-60 minutes) or power applied (i.e. 20-40 Watts), since virtually the same IR spectra were recorded.

Figure 5. ATR-IR spectra of intact graphite (black) and exfoliated graphene obtained after ultrasonication in NMP (red) and o-DCB (blue). CONCLUSION In summary, exfoliation of graphite by ultrasonication in o-DCB and NMP was achieved and the concentration of exfoliated graphene in both solvents was calculated. By monitoring the absorbance of exfoliated graphene at 660 nm and applying the Beer-Lambert law, higher dispersibility values for exfoliated graphene in o-DCB as compared with NMP were found. Importantly, spectroscopic insight on exfoliated graphene revealed the presence of oxygenated species. Based on extensive XPS studies, the effect of ultrasonication power and time on the quality of exfoliated graphene was examined. The main outcome was the presence of defects and moreover the identification of carboxylic acids and ethers/epoxides onto the graphene lattice. This was further confirmed by vibrational spectroscopy, since an enhanced D-band was evolved in the Raman spectrum of exfoliated graphene and characteristic ACS Paragon Plus Environment

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modes for –COOH and C-O-C were evident in the IR. Notably, the presence of carboxylic acids and ethers/epoxides onto exfoliated graphene, most likely derived from the presence of air during ultrasonication, was found independent of the solvent used for the exfoliation (either o-DCB or NMP) and the power and/or time ultrasonication applied. However, the relative oxygen percentage in the exfoliated graphene was higher in NMP as compared with the case in o-DCB. Furthermore, it was found that the oxygen concentration on exfoliated graphene appears relatively insensitive to ultrasonication time, but highly sensitive to the choice of solvent (o-DCB or NMP) and sonication power (20 or 40 W), especially for the case when NMP is used. However, it should be noted that the precise nature of the oxygen-carbon bonding does appear to be sensitive to the ultrasonication time as shown in the complex variations presented in Table 3. The findings of the current study suggest that improvement of ultrasonication as a strategy toward exfoliation of graphite is needed, especially when applications of defect-free graphene are considered. ACKNOWLEDGMENT. Partial financial support by the Greek General Secretariat for Research and Technology and the European Commission, through the European Fund for Regional Development, NSRF 2007-2013 action “Development of Research Centers – ΚΡΗΠΙΣ”, project “New Multifunctional Nanostructured Materials and Devices – POLYNANO”, the Belgian Fund for Scientific Research (FRSFNRS) under FRFC contract “Chemographene” (convention N°2.4577.11) and the COST action MP0901 are gratefully acknowledged. REFERENCES (1). Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.


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Ultrasonication Induces Oxygenated Species and Defects onto

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