Simultaneous Reduction and Functionalization of Graphene Oxide via

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Simultaneous Reduction and Functionalization of Graphene Oxide via Ritter Reaction Al C. de Leon,† Laura Alonso,‡ Joey Dacula Mangadlao,§ Rigoberto C. Advincula,∥ and Emily Pentzer*,† †

Department Department § Department ∥ Department ‡

of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States of Chemistry, Elmhurst College, Elmhurst, Illinois 60126, United States of Radiology, Case Western Reserve University, Cleveland, Ohio 44106, United States of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States

S Supporting Information *

ABSTRACT: Graphene oxide, the oxidized form of graphite, is a common precursor to conductive nanosheets and used widely in the preparation of composite materials. GO has the benefits of easy exfoliation and handling, but it tends to aggregate and restack when reduced. One approach to overcoming this undesired aggregation is covalent modification of the nanosheets; however, this typically requires additional reagents and time. Herein, we report the simultaneous reduction and functionalization of graphene oxide using the Ritter reaction such that reduced nanosheets show good conductivity without the aggregation typical of unmodified material. GO reacts with nitriles in strongly acidic conditions to give highly reduced graphene oxide (C:O of 4.38:1) with covalently attached amides, which compatibilizes it to a number of organic solvents. This Ritter-type reaction produces carbocations on the basal plane of graphene oxide, which allows nucleophilic attack by the nitrogen of the nitrile and produces amides upon hydrolysis. The product has sheet resistance (57.60 ± 4.04 kΩ/sq) substantially lower than that of the starting graphene oxide (529.60 ± 10.04 kΩ/sq) and, more importantly, can easily be dispersed in various organic solvents and does not restack into graphite-like materials upon drying. This method yields individual conductive nanosheets that can be readily incorporated into a number of different systems. KEYWORDS: graphene oxide, functionalization, reduction, Ritter reaction, nitrile

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tion.23−25 While the pristine network of graphene should theoretically be accessible by the graphite → GO → rGO conversion, current methods fall short due to residual defects in the basal plane.26 Moreover, rGO sheets tend to aggregate due to van der Waals interactions between graphitic domains.27 Such aggregation is undesirable for applications that require the sheets to be dispersed, and thus, an ideal synthetic protocol would allow the preparation of rGO without aggregation so that conductive, processable nanosheets are available. In addition to accessing exfoliated sheets, use of GO offers opportunities to tailor surface energy and tune solubility by chemical modification of the oxygen functionalities.2,28−32 Various routes have been developed to introduce different functional groups, make the material soluble in organic solvents or compatible with polymers, enhance mechanical properties, add sensing capabilities, and regulate conductivity.33−35 In contrast to noncovalent modification of GO by π−π

INTRODUCTION Graphene is a high surface area, electrically and thermally conductive sheet of conjugated sp2-hybridized carbon atoms arranged in a honeycomb structure;1−5 this material has been used in polymer nanocomposites,6−8 membranes,9−11 chemical and biological sensors,12−14 field-effect transistors,15,16 and various biomedical applications.17−20 Chemical exfoliation of graphite using a strong oxidizing agent is one of the most costeffective methods to access large amounts of individual graphene-based sheets.21 In this process, graphene, as stacked sheets in graphite, is oxidized to graphene oxide (GO) by conversion of sp2-hybridized carbon atoms to sp3-hybridized carbon atoms and introduction of oxygen atoms on the basal plane as epoxide and hydroxyl functionalities. While this chemical change diminishes some of the desirable thermal, electrical, and mechanical properties inherent to graphene,22 these properties can be recovered by the chemical, electrochemical, or thermal reduction of GO, which removes some of the oxygen functionalities and partially restores the conjugated structure. This pathway produces reduced GO (rGO), with different methods resulting in different extents to reduc© 2017 American Chemical Society

Received: February 8, 2017 Accepted: April 10, 2017 Published: April 10, 2017 14265

DOI: 10.1021/acsami.7b01890 ACS Appl. Mater. Interfaces 2017, 9, 14265−14272

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ACS Applied Materials & Interfaces

Figure 1. (A) Overview of the work reported in which the basal plane of GO is functionalized by the Ritter reaction with a nitrile in under strongly acidic conditions (only a small portion of the nanosheet is shown for clarity). (B) Mechanism of the Ritter reaction, showing the reaction of tertiary alcohol with nitrile in a strong acid, producing amide.

interactions, covalent chemical functionalization strategies include ring opening of the epoxides, reaction of isocyanate with alcohols, and esterification of hydroxyl groups or carboxylic acids, among others.7 Unfortunately, many of these functionalization approaches occur without reduction of GO, and thus, the desired thermal and electrical conductivity and mechanical properties are unrealized without further processing.33 Indeed, simultaneous reduction and functionalization of GO could produce chemically modified rGO that has desired electrical properties in addition to compatibility with different solvents and polymer matrixes. To the best of our knowledge, there are only a few synthetic approaches that can simultaneously reduce and functionalize graphene oxide.34−36 Those reports, however, require either extreme temperatures, complex synthetic routes, or utilize relatively weak covalent bonds to graphene oxide. In this report, GO is simultaneously reduced and functionalized using nitriles in a strongly acidic solution (Figure 1A). The Ritter reaction, first reported in 1948, involves the reaction of a tertiary alcohol or alkene with a nitrile in a strongly acidic environment to form an amide; the hydroxyl groups of GO are protonated, and then water is expelled to yield a carbocation which undergoes nucleophilic attack by the nitrogen of a nitrile.37−40 This intermediate is then hydrolyzed to form an amide (Figure 1B). While the Ritter reaction leads to functionalization of the GO nanosheets, reduction also occurs during the reaction due to the removal of the oxygen functionalities, as previously reported under strongly acidic conditions (e.g., hydroiodic acid). We note that the chemistry of GO is complicated due to the influence of the reaction of one functional group to the others; as such, formation of carbocations on the surface is favorable, as it leads to delocalization of electrons, and can occur both from protonation and disassociation of hydroxyl or epoxide groups. Products are characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), contact angle, Raman spectroscopy, thermogravimetric

analysis (TGA), and four-point probe resistivity measurements, and control experiments indicate that temperature, nitrile, and acidic environment are all vital for this transformation to occur.

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EXPERIMENTAL SECTION

Materials and Instrumentations. Graphite flakes, potassium permanganate, sulfuric acid, aqueous hydrogen peroxide solution (30 wt %), isopropanol, acetic acid, and acrylonitrile were purchased from Sigma-Aldrich and were used as received. Atomic force microscopy (AFM) was performed with NX-10 Park System in tapping mode. FTIR was done using a Cary 600 by Agilent Technologies in ATR mode. Survey and high resolution XPS were performed using PHI Versaprobe 5000 X-ray photoelectron spectrometer with Al Kα radiation and referenced to internal SiO2. Contact angles were measured using CAM 200 optical goniometer by KSV Instrument, Ltd. Raman spectra were acquired using Jasco Analytic Instruments NRS4100 with 532 nm excitation. SEM data were collected from a FEI Helios 650 field emission scanning electron microscope with focused ion beam with EDS. 2D X-ray diffraction experiments were performed using a Bruker Discover D8 X-ray diffractometer, which has a monochromated X-ray source (normally used with a Co Kα X-ray tube) configured in point focus mode. Thermogravimetric analysis was done using TGA 2050 system by TA Instruments set at 10 °C/min. UV−vis spectra were collected using an Agilent Cary 5000 UV−vis− NIR instrument in a quartz cuvette. Fluorescence studies were performed using Agilent Cary Eclipse fluorescence spectrophotometer. Sheet resistances were measured using an EDTM R-Chek four-point probe. Preparation of Graphene Oxide. Typical preparation of graphene oxide involved the dispersion of 6 g of graphite flakes in 800 mL of sulfuric acid with magnetic stirring at room temperature. After 10 min of stirring, 6 g of potassium permanganate was added. The dispersion was allowed to stir for 1 day, after which another 6 g of potassium permanganate was added. This was repeated two more times. One day after the addition of the fourth batch of potassium permanganate, the reaction mixture was quenched in ice-water. Hydrogen peroxide solution was added while stirring until the pink color of excess potassium permanganate disappeared. The GO dispersion was centrifuged to separate the acidic supernatant. The solid pellet was then washed seven times with isopropyl alcohol by 14266


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ACS Applied Materials & Interfaces centrifugation. The washed GO was dried in vacuum until the weight remained constant. Ritter Reaction with GO. In a typical reaction, the graphene oxide (as prepared above) was dispersed in acetic acid and sulfuric acid via sonication for 1 h. Acrylonitrile was then added, and the reaction mixture was heated to the prescribed temperature in Table 1. After 24

2A shows the AFM image of a single sheet GO which was dispersed in water and then drop-cast on freshly cleaved mica. GO nanosheets are micrometers in diameter and have a height of about ∼1.2 nm (line profile shown in Figure 2B), consistent with single layer GO.10,41 Further evidence for the preparation of GO can be found in the FTIR spectrum in Figure 2C, which shows characteristic peaks corresponding to CO at 1745 cm−1, C−O at 1050 cm−1, CC at 1620 cm−1, and the broad O−H stretching frequency at 2500−3700 cm−1, attributed to carboxylic acids, alcohols, and adsorbed water.21,42 The survey XPS spectrum in Figure 2D further confirms the preparation of GO and shows only peaks corresponding to carbon (284.4 eV) and oxygen (531.5 eV),43 with 1.54 carbon atoms for every oxygen (Figure 2E and Table S1), which reflects its highly oxidized nature. Also notable is the absence of any nitrogen in the spectra of as prepared GO, as expected. GO was then exposed to conditions of the Ritter reaction (acrylonitrile and sulfuric and acetic acids at 70 °C, product denoted rGO-ACN, and spectra shown as black traces in Figure 2). Under these conditions, the oxygen functionalities disappear due to reduction and functionalization of GO. The FTIR spectrum of rGO-ACN in Figure 2C shows a substantial decrease in intensity of the O−H stretching frequency compared to that of GO as well as the appearance of a broad peak from 1630 to 1695 cm−1, attributed to the CO of the amide (amide I band) and emergence of a peak centered at 1565 cm−1, corresponding to the N−H stretch (amide II band). XPS further confirms the simultaneous functionalization and reduction of GO via Ritter reaction. From the survey spectrum in Figure 2D, rGO-ACN contains 2.6% nitrogen, equivalent to incorporation of 1 amide for every 27 graphitic carbons (Table S1). The high resolution nitrogen scan of rGO-ACN (Figure 2E) shows that there is a strong signal at 399.9 eV (amide N 1s).44 The absence of a peak at 398.7 eV (nitrile N 1s) confirms acrylonitrile is not adsorbed to the surface of GO and that the

Table 1. Experimental Conditions for Preparation of rGOACN and Control Experiments

GO control 1 control 2 control 3 rGO-ACN

graphene oxide (mg)

acetic acid (mL)

sulfuric acid (mL)

acrylonitrile (mL)

temp (°C)

100 100 100 100 100

0 50 50 50 50

0 5 5 5 5

0 5 0 0 5

RT RT 70 RT 70

h of reaction, the mixture was centrifuged to separate the product from the acidic supernatant. The solid pellet was washed with isopropyl alcohol seven times. The washed product was dried under reduced pressure until the weight remained constant.

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RESULTS AND DISCUSSION GO was prepared by the chemical oxidation of expanded graphite using the modified Hummers method with extended oxidation,21 and characterization by a number of techniques demonstrated the successful preparation of GO (Figure 2, orange traces). Extended addition of oxidizing agent to graphite yields highly oxidized graphene oxide that is more stable to storage under ambient conditions than graphene oxide prepared by other methods. This stability is likely due to covalently bound sulfates that stabilize oxygen functionalities and thus not only allow for GO to be stored as a solid over extended periods of time but likely also impacts its patterns of reactivity compared to GO prepared by other methods. Figure

Figure 2. Characterization of the GO, controls 1−3, and rGO-ACN: (A) AFM image of GO, (B) line profile of GO, (C) FTIR spectra, (D) survey XPS scans, (E) high resolution nitrogen scans, and (F) high resolution carbon scans. 14267


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Figure 3. (A) Solubility profile of GO (left) and rGO-ACN (right) in different solvents (polarity index shown). (B) Contact angle measurement of different nanosheet films prepared by drop casting.

Figure 4. Characterization of materials by (A) Raman spectroscopy, (B) XRD, (C) TGA, (D) UV−vis spectroscopy, and 2D fluorescence spectra of (E) GO and (F) rGO-ACN.

as evidenced by the substantial decrease in the peak intensity corresponding to the oxidized carbon (C−O and CO) at 286−289 eV (Figure S1A−E).27,32 Control reactions performed at room temperature, with acid only at room temperature, and with acid only at 70 °C (denoted controls 1−3, respectively) did not show similar reduction or functionalization of GO. Under no control conditions were stretching frequencies attributed to amides observed in FTIR, nor was nitrogen

Ritter reaction took place. This degree of functionalization is comparable to that reported by Collins et al. using the Claisen rearrangement.44 In addition to the incorporation of nitrogen by the Ritter reaction, a significant decrease in oxygen content is observed for rGO-ACN compared to that for GO; the C:O ratio increased from 1.54 to 4.38 (Table S1), indicating removal of oxygen atoms.24,45 The high resolution carbon scan (Figure 2F) of rGO-ACN confirms that GO was significantly reduced 14268


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utilized. Graphite shows the expected peak at 2θ = 30.869°, which corresponds to an interlayer distance of 3.36 Å, as previously reported.60 In contrast, the XRD spectrum of GO is featureless, signifying that GO nanosheets are exfoliated and do not stack together.21 Importantly, the XRD spectrum of rGOACN is also featureless and indicates that the nanosheets do not aggregate upon reduction and functionalization. Thus, the Ritter reaction can be used to produce nanosheets that do not stack together, which is ideal for dispersibility in nanocomposites.27 Figure 4C compares the thermal stability of graphite, GO, rGO-ACN, and the control samples. At the extremes, graphite starts to degrade at 700 °C, and 62% of the original mass is left after heating to 900 °C, while GO loses 8 wt % after heating to 100 °C (loss of water), 27% from 100 to 200 °C (loss of labile oxygen-containing functional groups), 12% between 200 and 500 °C (pyrolysis of the carbon skeleton), and ∼36% between 500 and 800 °C (detachment of covalently bound sulfates).21 In contrast, the TGA profile of rGO-ACN has a much smaller weight loss contribution from oxygen functionality (100−200 °C), and no apparent weight loss due to removal of sulfates (i.e., over 500 °C).61 The general shape of the thermograms for all control samples are intermediate to those of GO and rGO-ACN as they show weight loss due to oxygen-based functional groups but not covalently bound sulfates, indicating sulfates may be removed in the acidic reaction conditions. Recently, absorption and emission spectroscopies have been used to determine the optoelectronic properties of GO and its reduced analogues. As seen in the photographs of Figure 3, GO disperses in DMSO to give a yellow-brownish solution, while rGO-ACN disperses in DMSO to give a black solution. This difference in color can be more thoroughly characterized using UV−vis absorption measurements (Figure S3 and 4D); in water, GO has a global absorption maximum at 229 nm and a local maximum at 304 nm with absorption trailing to longer wavelengths. The absorption at 229 nm has been attributed to ketones, dienes, and the π−π* transition of CC, while the local maximum at 304 nm is attributed to the n−π* transition of CO.62 In DMSO, the absorption profile of GO does not change at wavelengths longer than 280 nm, though shorter wavelengths cannot be evaluated due to absorption of the solvent. The absorption spectrum of rGO-ACN in DMSO has a maximum at 279 nm that is due to restoration of the conjugated structure upon reduction and is accompanied by a disappearance of the shoulder at 304 nm due to the decreased concentration of carbonyls.63,64 The absorption spectra of all control samples did not significantly change from that of GO. In addition to the difference in absorption spectra, a broad fluorescence emission spectrum has been reported for GO but not for its reduced analogue. Emission of GO is attributed to electronic transitions of unoxidized graphitic domains at the boundary of oxidized regions with epoxides, alcohols, and carboxylic acids all involved in the process. As seen in Figure 4E, GO in DMSO has a broad excitation and emission between 500 and 800 nm with a maximum at 660 nm. This fluorescence is red-shifted compared to that in previous reports, likely because of the solvent in which the graphene oxide is dispersed (water in previous reports vs DMSO in this study).65,66 In contrast, the excitation and emission spectrum of rGO-ACN in DMSO shows only a faint fluorescence between 300 and 400 nm (Figure 4F), which is attributed to the emission of DMSO (Figure S4C). This background fluorescence is not observed in

incorporated into the product (i.e., no XPS signal), nor was the C:O ratio significantly changed. GO is hydrophilic and can be readily dispersed in highly polar solvents such as water, DMSO, and DMF.46,47 A drastic change in solubility and dispersibility has been reported based on either oxidation or reduction due to changes in the number of hydrogen-bonding functional groups.48−50 As seen in Figure 3A, as-synthesized GO is highly dispersible in water, DMSO, and DMF, marginally soluble in methanol (MeOH) and chloroform (CHCl3), and poorly soluble in toluene and hexanes. Upon sitting in solution for 24 h, reduction of GO is clearly indicated by the color change (as commonly observed under ambient conditions), though its dispersibility remains unchanged. In contrast, after modification by the Ritter reaction, rGO-ACN is not dispersible in water but is dispersible in DMSO and DMF as well as the solvents of lower polarity index, MeOH and CHCl3 (after mild sonication). rGO-ACN is also much darker in color than GO, which is visual proof of its reduced nature.51 These differences in solubility between GO and rGO-ACN indicate they will show different compatibility with polymers and small molecules. To further confirm the transformation of GO by the Ritter reaction, water contact angle measurements were collected for each sample (Figure 3B). A film of GO is highly hydrophilic and therefore absorbs water to give a contact angle of 0°; in contrast, a film of rGOACN showed a contact angle of 30 ± 1° and thus is substantially more hydrophobic than GO. This further proves the successful reduction and functionalization of GO via Ritter reaction, as hydrophilic oxygen-based functionalities are removed.48,52 All control samples showed contact angles similar to those of GO (0°), again indicating the impact of the Ritter reaction on the properties of the nanosheets. Moreover, scanning electron microscopy (SEM) images of GO and rGO-ACN show little difference in morphology of the dried samples (Figure S5), indicating that a difference in surface functionalization of the two materials is responsible for difference in solubility and wettability. The surface areas of GO and rGO-ACN powder, determined by the BET method using N2 adsorption, are 219.03 and 244.85 m2/g, respectively. Raman spectroscopy is a valuable tool for determining the structural characteristics of carbon nanomaterials, including GO, due to the high Raman intensities of conjugated structures.53−55 The Raman spectrum of GO contains two main peaks (Figure 4A): the G band (1590 cm−1), which corresponds to the primary in-plane vibrational mode of the sp2 carbon atoms, and the D band (1350 cm−1), which corresponds to the disorder caused by the graphitic edges.56,57 The Raman spectrum of rGO-ACN shows an increase in the intensity of the D band relative to that of the G band (from ID/IG = 0.969 for GO to ID/IG = 0.886 for rGO-ACN, Table S2), which has previously been attributed to smaller graphitic domains and an increase in disorder due to functionalization.58 Moreover, relative to GO, the G band of rGO-ACN is red-shifted by ∼13 cm−1, which may be attributed to the interaction between the nanosheet and the newly formed amide. A similar phenomenon was observed by Lin et al. and Ye et al. upon introduction of different functional groups onto GO.52,59 For all control samples, neither the intensity ratio between D and G bands nor the peak position of the G band changed compared to the starting material. To determine if rGO-ACN stacks into graphite-like structures and to establish its thermal properties, XRD (Figure 4B, 2D-XRD in Figure S2), and TGA (Figure 4C) were 14269



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the spectrum of GO due to the much higher emission intensity of the material itself. Reduction of GO restores electrical conductivity by removing oxygen-containing functional groups and partially reinstating a conjugated structure, though this does not yield complete reduction or return to a pristine graphene framework. Fourpoint probe measurements were used to determine sheet resistance of films of GO, rGO-ACN, and the control samples (Table 2). Sheet resistivity of GO was 529.60 ± 10.04 kΩ/sq

GO control 1 control 2 control 3 rGO-ACN

*E-mail: [email protected]. ORCID

Al C. de Leon: 0000-0002-9449-0519 Laura Alonso: 0000-0003-3336-7522 Joey Dacula Mangadlao: 0000-0002-2649-8527 Rigoberto C. Advincula: 0000-0002-2899-4778 Emily Pentzer: 0000-0001-6187-6135

a

Notes

The authors declare no competing financial interest.

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sheet resistance (kΩ/sq) 529.60 509.80 522.20 528.60 57.60

± ± ± ± ±

ACKNOWLEDGMENTS The authors thank CWRU College of Arts and Sciences for financial support and Jasco Analytical Instruments for Raman. L.A. was supported through the NSF REU in Chemistry at CWRU Award 1359022. A.D. thanks Brad Rodier and Peiran Wei for discussion. The authors thank NSF CAREER Award 1551943.

10.04 11.17 8.35 5.89 4.04

Averages and standard deviations based on five measurements.

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and is higher than typically reported.30 This difference in measured resistance is likely due to the extended oxidation of graphite that introduces more oxygen functionalities and defects on the basal plane of GO than other methods. After the Ritter reaction, the nanosheets become much less resistive; the sheet resistance of rGO-ACN was 57.60 ± 4.04 kΩ/sq, an order of magnitude lower than GO, though slightly higher than those reported other studies.2,45 The control samples all had sheet resistivities similar to those of GO.

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CONCLUSION Herein, we reported the simultaneous reduction and functionalization of graphene oxide in the presence of a nitrile in an acidic environment via the Ritter reaction. Functionalization of the nanosheets was verified by FTIR spectroscopy as well as both survey and high resolution nitrogen XPS data. Reduction of the nanosheets was determined by high resolution carbon XPS spectra as well as Raman and UV−vis spectroscopy and conductivity measurements. Moreover, upon Ritter reaction, the solubility and contact angle of the nanosheets changed, demonstrating a material more hydrophobic than the parent GO. Importantly, preparation of rGO-ACN did not sacrifice dispersibility and compatibility with various organic solvents, and XRD revealed that the material does not stack into wellorganized graphite-like structures, even when dried into a solid sample. In this work, the Ritter reaction was used for the simultaneous reduction and functionalization of GO using acrylonitrile, and other nitriles should also prove fruitful provided appropriate boiling points and solubility. The novel method reported herein provides a fast and efficient route to access conductive, processable materials that can be incorporated into a variety of applications.

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AUTHOR INFORMATION

Corresponding Author

Table 2. Sheet Resistance of GO, rGO-ACN, and Control Samplesa sample

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01890. Elemental analysis, Raman spectra, XRD spectra, UV−vis and fluorescence spectra, and SEM and AFM images (PDF) 14270


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