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Green and Facile Esterification Procedure Leading to CrystallineFunctionalized Graphite Oxide M. Rosaria Acocella,*,† Luciana D’Urso,† Mario Maggio,† Roberto Avolio,‡ M. Emanuela Errico,‡ and Gaetano Guerra*,† †

Dipartimento di Chimica e Biologia, Università degli Studi di Salerno, Via Giovanni Paolo II, 132-84084 Fisciano, Salerno, Italy Istituto per i Polimeri, Compositi e Biomateriali (IPCB-CNR) Via Campi Flegrei 34, 80078-Pozzuoli, Napoli, Italy

‡

S Supporting Information *

ABSTRACT: A simple and eco-friendly procedure of esterification of graphite oxide (GO), which uses acetic anhydride as a model reagent and ethyl acetate as a green solvent, is reported. The procedure leads to high functionalization degrees (at least up to 4.5 mol % of acetyl groups, referred to as graphitic C atoms) and it is much more effective than the literature method based on pure acetic anhydride. Surprisingly, our acetylation procedure does not destroy or reduce GO crystallinity but, irrespective of a substantial increase of distance between GO layers (from 0.84 nm up to 0.95 nm), leads to an increased order in the direction perpendicular to the graphitic planes. This phenomenon indicates that acetyl groups of acetylated GO (AcGO) are easily accommodated between graphitic layers, even improving interlayer order. The esterification procedure is generally applicable with various anhydrides providing differently functionalized graphite oxide. Dispersion of crystalline functionalized GO in volatile organic solvents followed by solvent fast removal, can easily lead to complete exfoliation. nucleophilic substitution to epoxy groups,13 diazonium salts coupling,14 or esterification reaction of carboxyl groups.15 In particular, the esterification reaction has attracted much attention due to the possibility of easily modifing the compatibility of GO in a wide range of media and to acquire improved stability also in physiological environments.16−18 All the functionalizations reported, although able to efficiently modify the surface of graphene oxide, are usually performed by using toxic reagents and solvents in harsh conditions. Especially for graphene oxide esterification, the most common procedures require the activation of carboxylic group through the wellknown coupling agent, N,N′-dicyclohexylcarbodiimide (DCC) in the presence of 4-dimethylaminopyridine (DMAP)19−21 or thionyl chloride (SOCl2),22 to favor alcoholic nucleophilic attack and ensure a high degree of esterification (Figure 1A). Although the DCC treatment can sufficiently activate the carboxylic group, DMAP, a highly toxic reagent, as base, can react with other functional groups present on a graphene oxide surface (acid−base reaction with carboxylic groups, dehydration, etc.). Definitely more efficient is SOCl2 activation, which is able to convert the carboxylic groups into more reactive acylic groups. Even if the strong activation can ensure a high degree of functionalization, the hydrochloric acid evolving during the reaction can significantly reduce the starting carbon

1. INTRODUCTION Graphene has attracted much attention in recent years owing to its exceptional thermal, mechanical, and electrical properties.1−3 Because of these features, graphene is a candidate for several applications, including single molecule gas detection,4,5 biosensors,6 and as filler for a broad range of polymer composites.7 As a matter of fact, with only a small quantity of nanofiller incorporated into the composite, high mechanical, thermal and electrical properties can be obtained. Due to graphene’s poor solubility and difficult dispersibility in polymer matrices, incorporation usually requires its oxidation followed by exfoliation8 and reduction,9,10 or some modifications to provide, with adequate functional groups, suitable interactions with specific chemical moieties of the used polymer matrices. In this framework, many studies have been devoted to graphene oxide as a nanofiller precursor, which contains several kinds of oxygen functional groups on its basal planes and edges: hydroxyl, carbonyl, epoxide, carboxylic, ether (pyran or furan like), and anhydride.11 For this oxidized nanofiller, control of surface chemistry is essential to achieve a homogeneous dispersion in suitable matrices and maximum improvement in final properties, mainly by preventing filler aggregation. A useful tool to control surface chemistry of oxidized carbon nanofillers is chemical functionalization. As for graphene oxide, several chemical methods have been already explored: reduction of graphite oxide (GO) in a stabilization medium,10 covalent modification by amidation of carboxylic groups,12 © 2017 American Chemical Society

Received: April 20, 2017 Revised: June 14, 2017 Published: June 19, 2017 6819

DOI: 10.1021/acs.langmuir.7b01356 Langmuir 2017, 33, 6819−6825

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Figure 1. Esterification reactions performed by two different approaches: (A) nucleophilic attack of alcohol to activated carboxyl groups; (B) esterification of hydroxyl functionalities by acetic anhydride. The presently proposed green procedure uses ethyl acetate solvent. h, and then 45 mL of acetic anhydride was added. After 2 h under stirring, the obtained sample was poured into 500 mL of deionized water, and then centrifuged at 10000 rpm for 10 min with a Hermle Z 323 K centrifuge. The isolated acetylated powders were dried at 60 °C for 12 h. 2.4. Characterizations. 2.4.1. Wide Angle X-ray Diffraction. Wide-angle X-ray diffraction (WAXD) patterns were obtained by an automatic Bruker D8 Advance diffractometer, in reflection, at 35 kV and 40 mA, using nickel filtered Cu−Kα radiation (1.5418 Å). The dspacings were calculated using Bragg’s law and the observed integral breadths (βobs) were determined by a fit with a Lorentzian function of the intensity corrected diffraction patterns. The instrumental broadening (βinst) was also determined by fitting of Lorentzian function to line profiles of a standard silicon powder 325 mesh (99%). For each observed reflection, the corrected integral breadths were determined by subtracting the instrumental broadening of the closest silicon reflection from the observed integral breadths, β = βobs − βinst. The correlation lengths (D) were determined using Scherrer’s equation.

material, providing a graphene surface modification with a simultaneous sensible reduction of the O/C ratio. A possible alternative to this approach is the esterification of hydroxyl functionalities already present on the graphitic surface and of those derived from the epoxide ring opening. Although this way was already reported in 1946,23 poor interest was devoted to this procedure, possibly due to the long reaction times, low levels of functionalization, and mainly to competitive reduction pathways. Here we propose a new green procedure of esterification of GO, which uses acetic anhydride as a model reagent and ethyl acetate as an ecofriendly solvent (Figure 1B), which is much more effective than the literature method.23 It will be shown that the procedure is generally applicable with various anhydrides providing differently functionalized GO. Furthermore, it will be shown that the esterification procedure generally leads to crystalline and functionalized graphite oxide, which, differently from the starting GO, can be easily exfoliated and dispersed in less polar environments.

D=

Kλ β cos θ

(1)

where λ is the wavelength of the incident X-rays and θ the diffraction angle, assuming the Scherrer constant K = 1. 2.4.2. Elemental Analysis. Elemental analysis was performed with a Thermo FlashEA 1112 Series CHNS-O analyzer, after pretreating samples in an oven at 100 °C for 12 h. 2.4.3. Infrared Spectroscopy. FTIR spectra were obtained at a resolution of 2.0 cm−1 with a FTIR (BRUKER Vertex70) spectrometer equipped with deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter, using KBr pellets. The frequency scale was internally calibrated to 0.01 cm−1 using a He−Ne laser. 32 scans were signal-averaged to reduce the noise. 2.4.4. Thermogravimetric Analysis. The thermogravimetric (TG) analysis was carried out on a TG 209 F1, manufactured by Netzsch Geraetebau, from 20 to 200 °C at a heating rate of 10 °C, under N2 flow. The water content was determined from the weight decrease below 100 °C. 2.4.5. NMR. Solid-state 13C magic angle spinning (MAS) spectra were collected on a Bruker Avance II 400 spectrometer operating at a static field of 9.4 T, equipped with a 4 mm MAS probe. Samples were packed into 4 mm zirconia rotors sealed with Kel-F caps and spun at a spinning speed of 12 kHz. Direct 13C single-pulse (SP) spectra were recorded using a 13C π/2 pulse width of 3.6 μs and a repetition time of 10s, under high power proton decoupling. All spectra were referenced to external adamantane (CH signal at 38.48 ppm downfield of tetramethylsilane (TMS), set at 0.0 ppm). Line fitting procedure was carried out by means of the GRAMS/AI software, using Gaussian and mixed Gaussian/Lorentzian line shapes.

2. EXPERIMENTAL SECTION 2.1. Materials. High surface area graphite, with a minimum carbon wt % of 99.8 and a surface area of 330 m2/g, has been purchased from Asbury Graphite Mills, Inc. Sulfuric acid, sodium nitrate, and potassium permanganate were purchased from Sigma−Aldrich. All reagents were used as received, without purification. 2.2. Oxidation of Graphite. Graphite Oxide (GO) was prepared by Hummers’ method.18 120 mL of sulfuric acid and 2.5 g of sodium nitrate were introduced into a 2000 mL three-neck round bottomed flask immersed into an ice bath, and 5 g of carbon samples were added, with magnetic stirring. After obtaining a uniform dispersion, 15 g of potassium permanganate were added very slowly to minimize the risk of explosion. The reaction mixture was thus heated to 35 °C and stirred for 24 h. Deionized water (700 mL) was added in small amounts into the resulting dark green slurry, under stirring and, finally, gradually adding 5 mL of H2O2 (30 wt %). The obtained sample was poured into 7 L of deionized water, and then centrifuged at 10000 rpm for 15 min with a Hermle Z 323 K centrifuge. The isolated GO powders were first washed twice with 100 mL of a 5 wt % HCl aqueous solution and subsequently washed with 500 mL of deionized water. Finally, powders were dried at 60 °C for 12 h. About 7.5 g of GO powders was obtained. The obtained oxygen/carbon weight ratio was 0.71. 2.3. Acetylation Procedure. In a 250 mL three-neck round bottomed flask, 375 mg of GO was dispersed in 75 mL of ethyl acetate and left for 1 h in a ultrasonic bath at room temperature. After obtaining a uniform dispersion, the mixture was heated to 80 °C for 2 6820

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Figure 2. (a−c) X-ray diffraction patterns (CuKα) and (a′−c′) FTIR spectra in the range 2000−400 cm−1 of (a,a′) GO (with O/C, wt/wt = 0.71); (b,b′) GO as acetylated by the present procedure (AcGO); (c,c′) GO as acetylated by the literature procedure (LitAcGO). It is worth adding that the reduction of intensity of the 100 peak observed for the crystalline AcGO panel b is due to a preferential orientation of the AcGO nanoplatelets parallel to the solid substrate

Table 1. Elemental Analysisa

3. RESULTS AND DISCUSSION The described esterification procedure has been applied to a GO sample, exhibiting a O/C molar ratio of 0.71, as obtained

GO GOb AcGO eAcGO LitAcGOc

C (wt %)

H (wt %)

O (wt %)

S (wt %)

O/C

57.2 59.0 63 63 71

1.4 1.3 1.0 1.3 2.4

39.2 39.7 36.0 35.3 26.2

2.2 / / / /

0.71 0.67 0.59 0.56 0.37

a

Elemental composition of anhydrous samples: water contents (as evaluated by TGA, after equilibration at room temperature in air with a relative humidity of 45%) was in the range 13 ± 2 wt %, for all the examined samples. bGO sample under thermal treatment at 80 °C for 24 h in ethyl acetate. cLitAcGO obtained with the procedure reported in ref 23.

acetylation procedure not only does not destroy or reduce GO crystallinity but also, irrespective of a substantial increase of distance between GO layers, leads to an increased order in the direction perpendicular to the graphitic planes.‡ This phenomenon indicates that acetyl groups are easily accommodated between graphitic layers of GO, even contributing to the interlayer order. A comparison with the literature acetylation procedure made by analyzing the WAXD pattern of the crystalline GO powder (of Figure 2a), after the literature acetylation procedure, involving the direct use of acetic anhydride (solvent free conditions, at 80 °C for 24 h),23 is shown in Figure 2c. From the WAXD pattern it is immediately apparent that literature acetylation increases the d001 spacing only up to 0.89 nm rather than up to 0.95 nm, as for the presently proposed acetylation procedure. Correspondingly, the half-height width of the 001 peak increases, indicating a reduction of the correlation length perpendicular to the graphitic layers from 3.9 nm down to 2.9 nm, while with the procedure of this paper it increases from 3.9 nm up to 4.8 nm. Moreover, the crystallinity of the GO powder acetylated by the literature procedure reduces down to less than 40% (Figure 2c), while the crystallinity of the AcGO obtained by the present procedure remains high (Figure 2b). FTIR spectra of powders (dried at 80 °C for 12h) of starting GO and of AcGO, (whose X-ray diffraction patterns are in

Figure 3. Solid State 13C NMR SP spectra of GO and AcGO and LitAcGO with assignment to specific carbons of the observed resonances. Spinning sidebands are marked by a dot.

by Hummers oxidation24 of a high surface area graphite,25 using acetic anhydride as model reagent. The X-ray diffraction pattern of this GO powder (Figure 2a) shows interlayer distance d001 = 0.84 nm, i.e., much higher than the interlayer spacing of the starting graphite (0.339 nm) and a well-defined 100 peak at d100 = 0.21 nm, whose broadness indicates a large correlation length parallel to the graphitic planes (D100 = 11 nm). The X-ray diffraction pattern of acetylated GO (AcGO, Figure 2b) shows substantial changes with respect to the starting GO (Figure 2a). In fact, the 001 peak not only is remarkably shifted (from d001 = 0.84 nm up to d001 = 0.95 nm) but also becomes narrower with correlation length D001 increasing from 3.9 nm up to 4.8 nm. Hence surprisingly, our 6821

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Figure 4. (a−d) X-ray diffraction patterns (CuKα) and (a′−d′) FTIR spectra in the range 2000−400 cm−1 of (a,a′) GO with O/C, wt/wt = 0.71); (b,b′) GO functionalized with phthalic anhydride by the present procedure (PhGO); (c,c′) GO funtionalized with valeric anhydride by the present procedure (VGO); (d,d′) VGO exfoliated by dispersion in acetone, followed by fast solvent desorption (eVGO).

Figure 5. Schematic presentations of crystallites of starting GO (A) and of the derived esterified GO (VGO, B) and esterified and exfoliated GO (eVGO, C).

Figure 2a,b) are shown in Figures 2a′,b′, respectively. The occurrence of acetylation is clearly confirmed by decrease of intensity of hydroxyl, epoxide, and peroxide peaks26 at 1041, 855, and 581 and mainly by a shift of the maximum of the carbonyl peak from 1718 cm−1 (Figure 2a′, typical of carboxylic groups), up to 1735 cm−1 (typical values for acetyl groups)19 for AcGO sample (Figures 2b′). It is worth adding that an additional peak appears around 800 cm−1, which could be possibly attributed to C−O−C edge ether termination.11 A FTIR spectrum of the crystalline GO powder, after the literature acetylation procedure, is shown for comparison in Figure 2c′. Although WAXD patterns indicate that the literature acetylation reduces the size and eventually disrupts GO crystallites (Figure 2c), the corresponding FTIR spectrum (Figure 2c′) clearly indicates that the acquired degree of

acetylation is definitely lower. Particularly informative is the carbonyl peak at 1718 cm−1, which only shifts up to 1721 cm−1. To gather more detailed information on the acetylation reaction, solid state NMR experiments were performed. The analysis of the SP 13C spectra of AcGO sample reveals, in addition to the typical signals of GO, the appearance of two resonances centered at 19 and 175 ppm, attributable to the CH3− and COO− of the acetoxy moieties, respectively (Figure 3). The degree of acetylation was evaluated by line fitting of the solid-state NMR spectra, as detailed in the Supporting Information. In particular, comparing the integral of acetoxy peaks with the integrals of GO carbons, a degree of acetylation of 4.5 mol % was calculated, corresponding to ≈10 wt %. Moreover, it is worth noting that, in AcGO, the intensity of signals in the range 50−80 ppm, diagnostic for the alcoholic and epoxide groups, is reduced by less than 20% with respect to 6822

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The X-ray diffraction pattern of phthalic-GO (PhGO) and of valeric-GO (VGO) (Figure 4b,c) again shows substantial changes with respect to the starting GO (Figure 4a). In fact, 001 peak is remarkably shifted from d001 = 0.84 nm up to d001 = 0.92 nm and d001 = 1.03 nm for PhGO and VGO, respectively. The occurrence of esterification is further confirmed by FTIR spectra of powders (dried at 80 °C for 12 h) shown in Figure 4a′−c′. The consistent shift of the maximum of the carbonyl peak from 1718 cm−1 up to 1737 cm−1 (typical values for ester groups),19 for both PhGO and VGO, and the decrease of intensity of hydroxyl, epoxide, and peroxide peaks at 1041, 855, and 581 cm−1, clearly verified the functionalization. (See more in the Supporting Information.) Schematic presentations of crystallites of starting GO and of the derived functionalized GO are shown in Figure 5A,B, respectively. A comparison between Figure 5A and 5B shows increase of interlayer spacing as well as increase of correlation length perpendicular to the graphitic layers, as induced by esterification. Crystalline esterified GO can be exfoliated by dispersion in volatile organic solvents followed by fast solvent removal. This is shown, for instance, by WAXD pattern in Figure 4d of VGO dispersed in acetone at room temperature by sonication for 5 min and then treated at 35 °C under vacuum. The disappearance of the 001 peak clearly indicates a complete loss of order perpendicular to the graphitic layers, i.e., complete exfoliation of crystalline VGO, as schematically shown in Figure 5C. It is worth adding that the same esterification procedure, when applied directly to exfoliated graphite oxide, does not allow one to obtain the exfoliated functionalized graphite oxide because of the prevalence of the competitive reduction pathway. Dispersibility tests of the esterified samples were performed in water and organic solvents. As a consequence of esterification, the well-known good dispersion of GO in water is lost (upper part of Figure 6) while, correspondingly, AcGO and VGO are easily dispersed in ethyl acetate, even in the absence of sonication (lower part of Figure 6). On the contrary, PhGO after sonication and 24 h of storage, has reduced dispersibility in ethyl acetate with respect to AcGO and VGO due to the chemical nature of the introduced substituent. Consequently, the choice of an appropriate anhydride allows one to conveniently modify the compatibility of graphite oxide in different media, as, for example, polymer matrixes with different polarities.

Figure 6. GO, AcGO, PhGO, and VGO suspension in water and ethyl acetate solution 5 mg/mL, after 5 min of sonication and 24 h of storage.

that of neat GO (Figure 3, AcGO vs GO). This suggests that our acetylation procedure induces only a limited GO reduction. The SS 13C NMR spectrum of the acetylated GO, as obtained from the literature procedure (LitAcGO, upper curve in Figure 3), as already suggested by the FTIR spectra of Figure 2, clearly indicates a much lower degree of acetylation. In fact, the CH3− signal of the acetoxy moiety is broader and weaker while the COO− signal of the acetoxy moiety is barely detectable. The NMR spectrum of LitAcGO also shows a strong decrease of the epoxy and hydroxyl signals, indicating that the literature procedure leads to a substantial reduction of GO. Additional information comes from elemental analyses of starting GO and of acetylated samples, as reported in Table 1. In agreement with the NMR spectra of Figure 3, elemental analysis indicates that GO acetylation by the literature method is associated with a substantial reduction of GO (with O/C going from 0.71 wt/wt down to 0.37 wt/wt, cfr. first and last rows of Table 1). With the present acetylation procedure, it is possible to reach higher functionalization degrees with a much lower contribution of competitive reduction reactions (O/C ratio wt/wt = 0.59, third row of Table 1), in agreement with results of the NMR analysis. The remarkable chemical reduction, observed for the literature procedure, is not due to the thermal treatment, but rather to competitive reduction pathways induced by acetic acid evolving during the acetylation reaction. This is clearly demonstrated by the negligible contribution to chemical reduction by the sole thermal treatment at 80 °C for 24 h (O/C ratio of 0.67, 2 rd row of Table 1). It is also worth adding that, for the case of GO acetylated by the presently proposed procedure, even the exfoliation procedure only leads to an additional minor reduction process (O/C ratio wt/wt = 0.56, fourth row of Table 1). The procedure was further tested with phthalic and valeric anhydrides, providing differently functionalized GO.

4. CONCLUSIONS A simple and eco-friendly procedure of esterification of GO, which uses acetic anhydride as the acetylating agent in ethyl acetate, i.e., much milder conditions with respect to those reported in the literature (acetic anhydride in solvent free conditions at 80 °C for 24 h), is described. The new procedure is much more effective than the literature method because the latter occurs with a simultaneous reduction of graphite oxide (with O/C going from 0.71 wt/wt of GO down to 0.37 wt/wt). Hence, milder acetylation conditions limit reduction side reactions, generally occurring in the presence of acidic reagents. High functionalization degrees (up to 4.5 mol % or 10 wt %, as determined by SP MAS 13C NMR) are easily reached by the new acetylation procedure, when applied to crystalline GO, thus transforming crystalline GO into crystalline acetylated graphite oxide (AcGO). The occurrence of the esterification 6823

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reaction is associated with a marked increase of the GO interlayer spacing (from 0.84 nm up to 1.03 nm) and with an increase of order in the direction perpendicular to the graphitic planes (correlation length, D001, increasing from 3.9 nm up to 4.8 nm). The corresponding products can be easily dispersed in suitable organic solvents (like, e.g., acetone and ethyl acetate) after sonication, while differently from the starting GO they have poor dispersibility in water. The choice of an appropriate anhydride allows one to conveniently modify the compatibility of graphite oxide in different media, as, for example, polymer matrixes with different polarities. The dispersion of functionalized GO, in volatile organic solvents followed by fast solvent removal, can lead to complete exfoliation, i.e., to the formation of functionalized graphene oxide. This result is particularly relevant because direct esterification of exfoliated graphite oxide does not lead to functionalization of graphene oxide, due to the prevalence of competitive reduction pathways.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01356. Experimental procedures, elemental analysis, thermogravimetric analysis, and NMR and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

M. Rosaria Acocella: 0000-0001-6917-2271 Gaetano Guerra: 0000-0003-1576-9384 Author Contributions

M.R.A. conceived and designed the experiments and participated in the interpretation of the results and the writing of the paper. L.D’U. and M.M. were responsible for the experiments and data analysis. M.E.E. and R.A. were responsible for MASNMR experiments. G.G. supervised the research and participated in the interpretation of results and the writing of the paper. All the authors contributed to the realization of the manuscript and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ministero dell’ Istruzione, dell’ Università e della Ricerca, and (INSTM) are gratefully acknowledged. ABBREVIATIONS GO, graphite oxide; AcGO, acetylated graphite oxide; DCC, N,N′-dicyclohexylcarbodiimide; DMAP, 4-dimethylaminopyridine; SOCl2, thionyl chloride; PhGO, phtalic graphite oxide; VGO, valeric graphite oxide

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REFERENCES

(1) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. 6824

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Langmuir (24) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (25) Mauro, M.; Cipolletti, V.; Galimberti, M.; Longo, P.; Guerra, G. Chemically Reduced Graphite Oxide with Improved Shape Anisotropy. J. Phys. Chem. C 2012, 116, 24809−24813. (26) Maggio, M.; Mauro, M.; Acocella, M. R.; Guerra, G. Thermally Stable, Solvent Resistant and Flexible Graphene Oxide Paper. RSC Adv. 2016, 6, 44522−44530.

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