Permanent adsorption of organic solvents in graphite oxide and its effect on the thermal exfoliation

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Permanent adsorption of organic solvents in graphite oxide and its effect on the thermal exfoliation F. Barroso-Bujans a,*, S. Cerveny a, R. Verdejo b, J.J. del Val A. Alegrı´a a,c, J. Colmenero a,c,d

a,c

, J.M. Alberdi

a,c

,

a

Centro de Fı´sica de Materiales (CSIC-UPV/EHU), Material Physics Center, Avda. Tolosa 72, 20018 San Sebastia´n, Spain Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC. Juan de la Cierva 3, 28006 Madrid, Spain c Departamento de Fı´sica de Materiales, Universidad del Paı´s Vasco (UPV/EHU) Apartado 1072, 20080 San Sebastia´n, Spain d Donostia International Physics Center, Paseo Manuel de Lardiza´bal 4, 20018 San Sebastia´n, Spain b

A R T I C L E I N F O

A B S T R A C T

Article history:

The dispersion of graphite oxide (GO) in organic solvents followed by their evaporation at

Received 7 September 2009

relative high temperature resulted in a strong adsorption of the solvent molecules in the

Accepted 13 November 2009

graphitic interlayers as confirmed by

Available online 16 December 2009

solvents, alcohols (1-methanol, 1-propanol, 1-pentanol, 1-heptanol), aromatics (benzene,

13

C magic-angle-spinning NMR. Three series of

toluene, p-xylene, chlorobenzene) and chloride compounds (dichloromethane, chloroform, carbon tetrachloride) were studied to understand the interaction of graphite oxide with solvents. The distribution of basal interlayer spacing changed due to solvent intercalation and this distribution was particularly different for each solvent series. Even though there was on average 1 solvent molecule per 100 carbon atoms of GO, they had a profound effect on the thermal properties of the resulting GO. The exfoliation temperature was drastically reduced by the presence of the solvents due to the increase of the interlayer spacing and the reducing power of the solvent. Ó 2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphite oxide (GO) is an oxygen rich derivative of graphite. GO has mainly been prepared by three different oxidative procedures: Brodie [1], Hummer and Offeman [2], and Staudenmaier [3] methods. The oxidation process of these methods involves the use of an inorganic salt in a strong acid medium such as nitric acid, sulfuric acid or a mixture of them, respectively. The resulting GO contains large quantities of oxygen, in the form of epoxy, hydroxyl and carboxyl groups. Lerf et al. [4,5] established a structural model of GO based on solid state nuclear magnetic resonance (NMR), where the epoxy and hydroxyl groups decorated the surface of the graphene sheets and the carboxyl groups the edges of these nanosheets. Cai et al. [6] confirmed the assignment

of the NMR spectra made by Lerf et al., using an enhanced sensitivity 13C-labeled GO. However, Szabo´ et al. [7,8] reported the presence of other functional groups such as ketones. The presence on the GO of all these functional groups makes it extremely hydrophilic. Hence, GO absorbs atmospheric moisture which is strongly adsorbed on the graphene sheets and impossible to remove with a normal drying process [9]. The amount of intercalated water affects both the interlayer distance and the chemical composition of GO. GO is an efficient adsorbent where polar and certain nonpolar solvents can be accommodated in the interlayer space. Recent studies on the swelling of GO in benzene/n-heptane mixture showed that benzene was preferentially absorbed on the external surface of GO and in ethanol/cyclohexane mixture, the ethanol was preferentially absorbed in the

* Corresponding author: Fax: +34 943 212236. E-mail address: [email protected] (F. Barroso-Bujans). 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.11.029

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interlayer [10]. Hence, GO adsorbs polar adsorbents more readily than the non-polar ones due to the specific interactions with the polar groups on GO. Specifically, ammonia was strongly adsorbed on GO via hydrogen bonding [11,12] and about 50% of ammonia was retained [13]. Other polar molecules like alcohols [14], amines [14,15] and diamines [16], as well as amphiphilic molecules like quaternary alkylammonium and alkylpyridinium with variable aliphatic chain length [17] were successfully intercalated into GO. Although common organic solvents are frequently used both as transporter vehicles for intercalants [18] and to produce exfoliated graphite oxide via ultrasonic treatments [19], their retention in GO have been poorly explored. For instance, Matsuo et al. [15] reported the presence of remaining hexane in the intercalation of alkylamines. The presence of solvent molecules after deintercalation processes could explain the interactions with matrices, which are not totally understood up to date. Additionally, the polarity of the solvent does not totally explain the dispersability of the exfoliated nanosheets into specific solvents [20]. The present study provides comprehensive evidences, for the first time, of the solvent–GO interactions through the interpretation of interspacing distribution and thermal behavior of the retained-solvent/GO compounds. Surprisingly, small quantities of solvent molecules adsorbed in GO can modify the interspacing distribution of GO and even more, significantly reduce the exfoliation temperature of GO. Although this effect has been previously suggested in intercalated-GO systems with large amount of intercalates [15,21,22], the thermal exfoliation behavior of GO modified by the presence of trace amounts of adsorbed solvents has never been studied.

2.

environment with 100% humidity for one week. The second and third samples were prepared by soaking in water (Fluka) for a week and then dried to the desired hydration levels. Calorimetric measurements were carried out by means of a differential scanning calorimeter (DSC-Q2000) from TAInstruments. Measurements were performed in a standard mode at a scanning rate of 10 °C/min from 150 °C to 350 °C. GO was heated from room temperature to 500 °C, at a heating rate of 1 °C/min under a constant He flow of 90 mL/min, with a thermogravimetric analyzer (TGA) Q500 TA Instruments coupled to a Pfeiffer Vacuum ThermoStarTM mass spectrometer. The weight loss was measured as a function of temperature, and the evolved gas masses were directly monitored. 13 C magic-angle-spinning (MAS) NMR spectra were recorded using a Bruker Avance DSX300 spectrometer operating at 74.488 MHz and a 7 mm MAS probe at room temperature (about 22 °C). 13C Bloch decay spectra with high-power (30 kHz) 1H decoupling were recorded with spinning at 7.5 kHz, 90° pulses of 7.5 ls duration, and 20 s recycle delays. MestReC NMR data processing was used to fit and integrate the peaks. X-ray diffraction (XRD) was performed in a Philips X’Pert Pro powder diffractometer working with Cu Ka radiation ˚ ). This equipment allowed registering the diffracto(k = 1.54 A grams in the 2h range from 5 to 60° in 2.3 min, avoiding the significant absorption of the atmospheric water during the measurements. LaB6 was used as internal standard for all samples.

3.

Results and discussion

3.1.

Thermal behavior of GO

Experimental

A large graphite plate (Alfa Aesar) was cut into smaller pieces and oxidized using the Brodie method [1]. A reaction flask with 200 mL fuming nitric acid (Fluka) was cooled to 0 °C with a cryostat bath for 20 min and then the pieces of graphite plate (10 g) were introduced. Next, 80 g of potassium chlorate (Fluka) was slowly added over 1 h, to avoid sudden increases in temperature, and the reaction mixture was stirred for 21 h at 0 °C. During the course of the reaction, the small pieces were disintegrated to form a fine powder with particle size lower than 63 lm (80%). Once the reaction was finished, the mixture was diluted in distilled water and filtered until the supernatant had a nitrate content lower than 1 mg/L (AQUANALÒ-plus nitrate (NO3) 1–50 mg/L). The GO slurry was dried at 110 °C for 72 h and stored in a vacuum oven at room temperature until use. Solvent–intercalated GO (solvent/GO) were prepared by stirring 0.5 g of GO in 20 mL of solvent (CCl4, CHCl3, CH2Cl2, benzene, toluene, p-xylene, chlorobenzene, methanol, 1-propanol, 1-pentanol and 1-heptanol) for 72 h. Then, the solid was filtrated through a fritted funnel and dried at 110 °C for 48 h and stored in a vacuum oven at room temperature until use. The solvents were purchased from Sigma–Aldrich and were used as received. GO was hydrated to 30, 55 and 80 wt.% hydration levels. The first hydration level was saturated in a closed humid

GO was dispersed in a series of solvents such as alcohols (1methanol, 1-propanol, 1-pentanol, 1-heptanol), aromatics (benzene, toluene, p-xylene, chlorobenzene) and chloride compounds (dichloromethane, chloroform, carbon tetrachloride). After that, GO compounds were separated by filtration and dried at 110 °C for 72 h. The selection of this temperature was based on the requirement of removing the free solvent from the solid without decomposing the GO. The remaining solvent found in our experiments was associated to adsorbed solvent. Although, it has been reported that GO should be dried below 70 °C to avoid decomposition [23], our results of TGA–mass spectrometry (TGA–MS) (Fig. 1) and DSC (Fig. 2) suggested that no decomposition occurred before 110 °C. Both the weight loss from room temperature to 125 °C in the TGA and the endothermic peak at about 60 °C in the DSC were associated to the absorbed moisture during sample preparation which is present in both GO and retained-solvent GO (representative samples are shown in Fig. 2). A heating–cooling–heating cycle in the temperature range from 150 to 150 °C on GO samples (not shown) confirmed the elimination of water in the first heating step. A second experiment to corroborate the evaporation of water in this region was done by analyzing the GO samples with different hydration levels (Fig. 3). The area of the endothermic peak increased with the hydration level and the maximum temperature shifted to higher temperatures.

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The reductive exfoliation of GO occurred between 252 °C and 267 °C, showed in the TGA profile of Fig. 1, accompanied with the evolution of C (m/z = 12), H2O (m/z = 17, 18), CO2 (m/ z = 44) and likely, CxHyO species (m/z = 43, 44, 45, 46). Although the release of CO2 and water is known [24,25], to our knowledge, the evolution of CxHyO species has not been reported yet. The gases formed during this decomposition cause an increase of the pressure in the interlayer and therefore the exfoliation of graphene sheets. The effect of the solvents in the thermal exfoliation of GO will be analyzed later on.

100 m/z = 46

95

m/z = 45

Weight (%)

90 m/z = 44

85

m/z = 43

80

m/z = 18

1081

m/z = 17

75

3.2. GO

m/z = 12

Detection and quantification of retained solvents in

70 0

50

100

150

200 250 T(ºC)

300

350

400

450

Fig. 1 – TGA–MS of graphite oxide.

0.0

Heat Flow (W/g)

-0.5 CCl4/GO

-1.0 GO methanol/GO

-1.5

toluene/GO

-2.0

-2.5 -40

-20

0

20

40 60 T(ºC)

80

100

120

140

Fig. 2 – Region of DSC profiles where the adsorbed water is evaporated from GO and the retained-solvent/GO.

Fig. 3 – DSC experiments carried out on GO hydrated with different water content (wt.%). In the table it is reported the temperature of the endothermic peak (Tendo) related to the hydration degree of GO.

The composition of GO and the retained-solvent/GO (Table 1) showed changes on the C/O ratio. This effect is more evident both in the aromatic series, where the solvents contribute with 6–8 additional carbon atoms, and in the alcohol series, where there was a proportional increase of C/O ratio with the amount of alcohol carbon atoms. Those solvents containing only one carbon atom did not significantly changed the C/ O ratio. Chlorine was detected in those samples treated with chloride solvents, allowing the determination of the amount of solvent molecules per carbon atoms of GO (Table 1). Chlorobenzene was the most retained of chloride solvents, followed by dichloromethane, chloroform and carbon tetrachloride. Although chlorobenzene has the highest dipole moment of all these solvents (Table 2), it cannot be compared with the rest of tetragonal shaped solvents due to its planar structure. The forces involved in the retention of aromatic compounds might be different than those in the non-aromatics. Thereby, chlorobenzene was placed in the aromatic series. The retained solvent molecules were also detected by solid state 13C NMR (Fig. 4). Three main peaks were clearly detected at around 127, 68 and 54 ppm in the spectrum of non-treated GO. Following the work of Lerf et al. [4], those signals were assigned to C@C, C–OH, and epoxide 1,2-ether, respectively. In addition, some small signals appeared in the spectrum of the solvent treated GO and were coincident with the signals coming from the bulk solvent. Thus, methanol/GO showed a peak at 49 ppm, chloroform/GO at 77 ppm and carbon tetrachloride/GO at 96 ppm. Toluene/GO only showed a peak at 20 ppm since the toluene signals in the region from 126 to 138 ppm were overlapped with the C@C signal of GO. Hence, NMR experiments confirmed the presence of solvents within GO after the drying procedures, demonstrating that a strong adsorption takes place in these graphitic materials. Note that the broadening of the solvent signals in the solvent/GO spectra could be attributed to a restricted solvent mobility, confirming that the solvent is trapped and strongly adsorbed in GO. Since CP/MAS 13C NMR is in principle not a quantitative reliable measure, the recycle delays were evaluated in each sample for the acquisition of semiquantitative MAS 13C NMR spectra. Then, the amount of non-chloride solvents was estimated by integration of the solvent signal in the 13C NMR spectrum (Table 1). The results suggested that in average 1 solvent molecule is accommodated in 100 GO–carbon atoms. Again, the retention of such molecules depends on

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Table 1 – Composition of non-treated GO and retained-solvent/GO by elemental analysis. Quantification of the adsorbed solvent molecules per 100 GO-carbon atoms by elemental analysis and 13C NMR. Composition by elemental analysis

GO CH2Cl2/GO CHCl3/GO CCl4/GO Benzene/GOa Toluene/GO p-Xylene/GO Cl–benzene/GO Methanol/GO 1-Propanol/GO 1-Pentanol/GO 1-Heptanol/GO

Solvent molecules per 100 GO–carbon atoms

wt %C

wt %H

wt %O

wt %Cl

C/Oat

Elemental analysis

13

66.76 63.23 62.94 64.79 70.36 69.47 69.37 67.41 66.51 67.91 69.23 69.88

0.60 1.14 0.87 1.03 1.56 1.11 1.16 1.49 1.34 1.43 1.69 1.76

32.64 30.60 30.22 30.00 28.08 29.42 29.47 27.96 32.15 30.66 29.07 28.36

– 5.03 5.97 4.18 – – – 3.14 – – – –

2.73 2.76 2.78 2.88 3.33 3.14 3.14 3.22 2.76 2.96 3.17 3.29

– 1.4 1.1 0.6 – – – 1.8 – – – –

– – – – – 1.1 0.4 – 0.9 0.9 1.1 1.0

C NMR

a It was not possible to quantify the amount of benzene by 13C NMR since the C@C benzene signals were superposed to the C@C signals of GO.

Table 2 – Properties of the chloride solvents [33]. Solvent

Molar volume (cm3)

Dipole moment (D)

CH2Cl2 CHCl3 CCl4 Chlorobenzene

64.0 80.7 96.5 101.8

1.60 1.04 0 1.69

*

*

CCl4/GO * CHCl3/GO

*

toluene/GO

methanol/GO

*

250

225

200

175

3.3.

*

GO

*

* 150 125 δ (ppm)

100

75

50

and molar volume (Table 2). Hence, the most retained is that having larger dipole moment and lower molar volume. It is worthy to note that all the chloride compounds, included the chlorobenzene, are effectively adsorbed independently of their size and shape. This could be associated with the positive polarization of the carbon atom bonded to the chlorine atom, being in the case of chlorobenzene the aromatic ring which becomes positively charged. This favors the interactions with the p-electron clouds of the GO aromatic rings. Additionally, aromatics interact preferentially with the nonoxidized carbon surface, i.e., with the graphene aromatic rings, in a p-stacking arrangement [26]. On the other hand, the methyl groups on the aromatic ring have the opposite effect, as suggested by the adsorption values of toluene and pxylene of Table 1. These groups not only impose steric restrictions but also increase the electronic density of the solvent ring. As a result, the most negatively charged ring, i.e., p-xylene, is the less retained from the series.

25

0

Fig. 4 – 13C NMR spectra of representative retained-solvent GO and non-treated GO (black line). The corresponding solvent spectra are shown in grey color. (*) spinning side bands. their chemical structure and geometry. Alcohols are able to interact via hydrogen bond with the functional groups of GO and the retained amounts seem to be independent of the alcohol chain lengths, as observed in Table 1. The chloride compounds were also adsorbed on GO but in different amount. Thus, CCl4 was the less retained followed by CH2Cl2 and CHCl3, respectively. CCl4 was excluded from the GO galleries, as discussed below, and its adsorption was then limited to the external surfaces. Concerning the adsorption of CH2Cl2 and CHCl3, it seems that it is related to their dipole moment

Structure of retained-solvent/GO compounds

XRD experiments were conducted to study the effect of the retained molecules on the graphitic interlayer (Figs. 5–7). GO showed a main diffraction peak at 2h = 16°, which is attributed to the reflection 0 0 2 [27]. According to this assignation, ˚ (Table 3). Most of the crystallographic c axis spacing is 5.55 A the solvent treated GO showed a peak broadening and an intensity decrease of the 0 0 2 reflection. This result indicated that the presence of solvents altered the crystalline structure of GO probably due either to disorientations around the c axis or to displacements of carbon atoms from their mean positions [28]. The diffractograms of dichloromethane/GO and chloroform/GO (Fig. 5) showed a displacement of the 0 0 2 peak to lower angles, which indicated the formation of an intercalated structure. Meanwhile, the appearance of humps at higher angles suggested interstratifications (Table 3). This last phenomenon could occur as a result of contractions of unoccupied layers upon intercalation or of chemical reduction of GO upon drying. The second consideration was rejected since the relative intensity of the peak at 17.4° of the CHCl3/GO after

CARBON

GO CH2Cl2/GO

2500

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2500

GO methanol/GO 1-propanol/GO 1-pentanol/GO 1-heptanol/GO

CHCl3/GO 2000

CCl4/GO Lin (Counts)

Lin (Counts)

2000

1500

1000

500

1000

500

0

0 5

10

15

20

25

2 theta (deg)

Fig. 5 – Effect of the retained chloride solvents (CH2Cl2, CHCl3 and CCl4) in the main diffraction peak of GO. The peak centered in 21.41° is a reflection of LiB6, which was used as internal standard.

GO benzene/GO toluene/GO p-xylene/GO chlorobenzene/GO

2500

2000 Lin (Counts)

1500

1500

1000

500

0 5

10

15 2 theta (deg)

20

25

Fig. 6 – Effect of the retained aromatic solvents (benzene, toluene, p-xylene and chlorobenzene) in the main diffraction peak of GO. The peak centered in 21.41° is a reflection of LiB6, which was used as internal standard. heating at 110 °C for 2, 4 and 6 days in vacuum did not change. Therefore, the origin of interstratification could be the contraction of some layers caused by the expansion of adjacent layers. The ratios of the intercalated peak area to the interstratified peak area (Table 3) showed an important contribution of the interstratified peak. Previous works on the calculation of interstratified systems, in particular clays, were based on well-defined basal planes reflections [29,30]. However, in our case, the successive order reflections of the 0 0 2 peak are not detectable due to the low intensity signal of this peak. In addition, the peak indicating interstratifications is also rather broad and weak which suggests a complex interstratified mixture. Therefore, these profiles are too ill-defined to be exploitable and thereby no further consideration to interstratification can be done in this study. On the other hand, the poorly retained CCl4 (0.6 molecules per 100 GO–carbon atoms) did not appreciably change the

5

10

15 2 theta (deg)

20

25

Fig. 7 – Effect of the retained alcohols (1-methanol, 1propanol, 1-pentanol, 1-heptanol) in the main diffraction peak of GO. The peak centered in 21.41° is a reflection of LiB6, which was used as internal standard.

basal spacing of GO. This could be due to the fact that the ˚ [31]) is much larger molecular diameter of CCl4 (5.37–5.42 A ˚ , see Fig. 8 than the inner space between GO layers (4.1 A and the explanation below), excluding the entrance to the GO galleries. As a consequence, CCl4 is likely adsorbed on the external layers and defects of GO. The desorption of aromatic solvents resulted in occupied and unoccupied galleries in GO as suggested by the presence of two XRD reflection peaks, one shifted to lower angles and other at the same position than that of GO (Fig. 6). These reflections provide information on the interactions between solvent and GO. In the case of benzene and chlorobenzene, the main peak is at lower angles, indicating full intercalation. On the other hand, for toluene and p-xylene the main peak remains at the same position than that of GO and there are only small humps at lower 2h angles indicating partial intercalation. As mentioned above, the aromatic compounds interact with graphene layers via p-stacking and thereby the substituent groups affect these interactions. This can be reflected in the amount of adsorbed molecules, the interlayer expansion and the solvent accessibility to the graphitic galleries. These results are correlated and therefore it is difficult to establish a direct relationship between molecule type and interlayer expansion. For example, chlorobenzene was the most adsorbed producing the larger interlayer expansion. Although p-xylene was the less retained, it however produced larger interlayer expansion than toluene. This effect is probably related to the larger volume that p-xylene occupies. The increase of the interlayer spacing due to the retention of about one alcohol molecule per 100 GO-carbon atoms was proportional to the increase of the alcohol chain length (Fig. 7). This result would confirm the fact that the alcohols are effectively adsorbed in the interlayer. As in chloride solvents, an additional contraction of the basal spacing is observed. The ratios of the intercalated peak area to interstratified peak area (Table 3) indicate that the interstratified contribution become more pronounced as a function of the chain length. Thus, the amount of compressed layers increases

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Table 3 – Calculated interlayer distances (d) from 2h values of the X-ray diffraction peaks. Ratios of the intercalated peak area to the interstratified peak area (AIC/AIS). Intercalated

GO CH2Cl2/GO CHCl3/GO CCl4/GO Benzene/GO Toluene/GO p-Xylene/GO Chlorobenzene Methanol/GO 1-Propanol/GO 1-Pentanol/GO 1-Heptanol

Interstratified

0 0 2GO

2h (deg.)

˚) d (A

– 12.9 14.3 – 13.0 13.6 13.2 12.7 14.3 13.5 13.0 12.7

– 6.86 6.18 – 6.81 6.51 6.71 6.94 6.18 6.55 6.82 6.98

AIC/AIS

2h (deg.)

˚) d (A

2h (deg.)

˚) d (A

16.0 – – 16.0 15.9 – 16.1 16.1 – – – –

5.55 – – 5.55 5.57 – 5.50 5.50 – – – –

– 18.7 17.4 – – – – – – 17.8 17.8 17.8

– 4.75 5.10 – – – – – – 4.99 4.99 4.99

– 0.18 0.36 – – – – – – 1.14 0.71 0.33

Fig. 8 – Schematic representation of the solvent intercalation in the GO interlayer and the maximum expansion obtained in our results.

with the molecule size. This would corroborate the hypothesis of the origin of interstratification in GO. The origin of the GO interlayer expansion can be understood by means of the diagram showed in Fig. 8. The distance between continuous lines represents the interlayer distance ˚ in GO and 7.0 A ˚ in one of the interdetermined by XRD (5.5 A calated GO). The difference between dot-dashed lines dis˚ ) after subtracting two plays the inner interspace (4.1 A ˚ ) from the interlayer distance. In carbon atom radiuses (0.7 A this picture only the maximum interlayer distance obtained ˚ ) and therefore the maximum interlayer expansion by us (7.0 A ˚ ) are represented. The heights of hydroxyl and epoxy (1.5 A groups were taken from Ref. [25]. The explanation given here is based only on the geometrical sizes and does not take into account the molecular orbital interactions. First of all, the entrance allowance of a molecule will be given by the inner interspace of the graphite layers. Thus, those molecules having at least one of their dimensions lower or similar than ˚ could enter in the interspace. We have also to take into 4.1 A account that this is only an average value coming from the maximum peak position of XRD analysis. If we consider the width of the diffraction peak, a range of interlayer distances ˚ . This value explains the penecan be estimated as 4.1 ± 0.3 A ˚ and tration of rod-shaped cylinder with diameter 2.9 A length l (long chain alcohols) [31], disk-shaped molecules

(aromatics) [32] and small globular molecules. On the other hand, the positioning of such molecules in the interlayer will be determined by the type of interaction with GO. Thus, alcohols probably go to hydroxyl and epoxy functional groups, aromatics to graphene aromatic rings and chloride compounds to both functional groups and graphene rings. Fig. 8 shows three cases where the solvent or a group of molecules are located at (a) the hydroxyl group, (b) epoxy group and (c) graphene rings. As observed, the height of a molecule or a group of molecules located at the hydroxyl group to give a ˚ might be 3.4 A ˚ . Similarly, at maximum expansion of 1.5 A ˚ and between graphene rings the epoxy group might be 3.7 A ˚. 5.6 A The high angle region of the diffractograms showed a ˚ ), which is related to the lattice small peak at 2h = 42.2° (2.14 A parameter a = b from a hexagonal structure [28]. The position of this peak did not change with the solvent intercalation. A final and worthy comment about the interlayer expansion is that the method used to introduce the solvents in the galleries did not lead to the exfoliation of the GO as it occurs by sonication [19,20]. This has been confirmed by comparing the results obtained by the solvent absorption from liquid phase with that obtained from vapor phase, which does not involve any mechanical stimuli. We obtained similar XRD patterns in both cases.

CARBON

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120

GO 100

chlorobenzene/GO

Heat Flow (W/g)

80 60

benzene/GO

40

Fig. 11 – Decrease of the reducing power of the aromatic solvents. Inductive effects of substituent on the aromatic ring.

toluene/GO 20

p-xylene/GO

0 -20 180

200

220

240 260 T (ºC)

280

300

320

Fig. 9 – DSC of aromatic series. Exothermic peaks of GO exfoliation.

3.4.

Thermal behavior of retained-solvent/GO compounds

The exfoliation temperatures for all compounds were obtained using DSC. As shown in Fig. 9, the reductive exfoliation of GO is evidenced by an exothermic peak at 293 °C. All intercalated solvents decreased the exfoliation temperature of GO, as it is shown as example in Fig. 9 for aromatic solvents. It is noteworthy to say that the unoccupied galleries previously detected in the XRD experiments expanded at similar temperature than the non-treated GO. Therefore, there would exist a narrow relationship between structure and decomposition temperature of the graphitic network in GO. The exfoliation temperature values corresponding to the solvent-occupied interlayers were plotted as a function of the interlayer distance (Fig. 10). In general, the increase of the interlayer distance produced a decrease of the exfoliation temperature. This is a sensible suggestion since the molecule

300 280

non-treated

a)

CCl 4

Exfoliation temperature (ºC)

CH 2 Cl 2

CHCl 3

260 240 300

diffusion requires a lower energy to go through higher dimensions. In case of the alcohol series, the temperature decrease showed a linear relationship with the interlayer distance. In the chloride series, the dependence with the interlayer distance was less evident, and in the case of CCl4, which seems to be adsorbed in the external layers, the temperature was not changed. In the aromatic series, there is an apparent scattering of the data likely due to the particular reducing power (electron donation) of each solvent. Nethravathi and Rajamathi [21] associated the decrease of the decomposition temperature of colloidal dispersions of GO in organic solvent with the reducing power of three solvents: ethylene glycol > ethanol > 1-butanol. In Fig. 11, it is represented the decrease of the reducing power of the aromatic solvents, which is related to the inductive effect of the substituent. Evidently, methyl groups exert inductive electron donation to the aromatic ring through sigma bond causing the activation of the ring while chlorine exerts a withdrawal effect causing the deactivation of the ring. Thus, the increase of the ring electronic density increases the reducing power of the solvent and thereby, the decrease of the exfoliation temperature of GO. Consequently, p-xylene caused the largest decrease of temperature (42 °C) in these series. It is worthy to note that the only presence of solvent, either in few amounts as adsorbate or in huge amounts

non-treated

b) chlorobenzene

280

benzene

260 240 300

toluene

p-xylene

non-treated

c)

280

methanol

1-propanol

260

1-pentanol 1-heptanol

240 5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

7.2

Interlayer distance (Aº)

Fig. 10 – Decrease of the exfoliation temperature of GO with the increase of interlayer distance by the retention of chloride solvents, aromatic solvents and alcohols.

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as dispersant [21], has a notable effect in the reduction of the exfoliation temperature. As a conclusion, both effects the interlayer distance and the reducing power of the solvent affected the exfoliation temperature of GO. For that reason, the reductive nature of solvents from different families is not easily comparable.

4.

Conclusions

In this study we have systematically demonstrated that common organic solvents penetrate into the graphitic layers of GO and remain adsorbed in a permanent way. The intercalation process was simply done by stirring in the organic solvent without sonication and then dried by filtration and evaporation in vacuum at 110 °C. Three series of solvents were studied, chloride compounds, aromatic compounds and alcohols. Most of the solvents, from polar to non-polar, modified the interfacial distribution of the graphitic galleries and completely removed the crystalline structure of GO. After the drying process, the diffraction pattern of GO indicated a non organized structure and the 13C NMR showed the presence of remaining solvents. Carbon tetrachloride was the only solvent that was not able to penetrate into the GO interlayer. However, some preliminary results in the intercalation of polymers dissolved in CCl4 demonstrated that it is not a good solvent to promote intercalation but it can be used for desorbing or cleaning external species. The modification of the interlayer structure by the adsorbed solvents caused a notable effect in the exfoliation temperature of GO. In general, the higher the graphitic interlayer distance and higher the reducing power of the solvent, the lower the exfoliation temperature of GO was. The dependence of the reducing power of the solvent with the exfoliation temperature of GO was clearly demonstrated with the aromatic series. The ring substituent having an electrodonor effect, i.e. p-xylene, caused the largest decrease of the exfoliation temperature in its series. These observations highlight the challenges and limitations of graphene chemistry in intercalation procedures since it is difficult to eliminate completely the used solvents. An adequate solvent for intercalation should promote the intercalation of the desired species without affecting the host structure.

Acknowledgments The authors gratefully acknowledge the support of the Spanish Ministry of Education, Project CSD2006-00053; the European Union, Project 502235-2 and SOFTCOMP Program and the Basque Government, Project IT-436-07. F. Barroso-Bujans and R. Verdejo also acknowledge a JAE-Doc contract from CSIC.

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