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Single Stage Simultaneous Electrochemical Exfoliation and Functionalization of Graphene Andinet Ejigu, Ian A. Kinloch, and Robert A.W. Dryfe ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12868 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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ACS Applied Materials & Interfaces

Single Stage Simultaneous Electrochemical Exfoliation and Functionalization of Graphene Andinet Ejigu*a,c, Ian A. Kinlochb,c and Robert A.W. Dryfe*a,c a

School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK c National Graphene Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, UK b

ABSTRACT: Development of applications for graphene are currently hampered by its poor dispersion in common, low-boiling point solvents. Covalent functionalization is considered as one method for addressing this challenge. To date, approaches have tended to focus upon producing the graphene and functionalizing subsequently. Herein, we describe simultaneous electrochemical exfoliation and functionalization of graphite using diazonium salts at a single applied potential for the first time. Such an approach is advantageous, compared to post functionalisation of pre-made graphene, as both functionalisation and exfoliation occur at the same time meaning that monolayer or few layer graphene can be functionalized and stabilized in situ before they aggregate. Furthermore, the N2 generated during in situ diazonium reduction is found to aid the separation of functionalised graphene sheets. The degree of graphene functionalisation was controlled by varying the concentration of the diazonium species in the exfoliation solution. The formation of functionalised graphene was confirmed using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The functionalized graphene was soluble in aqueous systems and its solubility was two orders of magnitude higher than the non-functionalized electrochemically exfoliated graphene sheets. Moreover, the functionalization enhanced the charge storage capacity when used as an electrode in supercapacitor devices with the specific capacitance being highly dependent on the degree of graphene functionalization. This simple method of in situ simultaneous exfoliation and functionaliztion may aid the processing of graphene for various applications. KEYWORDS: : Simultaneous exfoliation and functionalization; electrochemical exfoliation; functionalization of graphene; Cs+ intercalation; reductive exfoliation; graphene dispersion; capacitance of functionalized graphene Introduction. The electrochemical intercalation of ions into the galleries of graphite has been studied for decades, mainly with regard of the field of battery technology.1 Following the discovery of graphene,2 this technique has been used as a method to produce few layer graphene by intercalating graphite with ions that are larger than the

interlayer spacing (0.335 nm).3-6 This intercalation expands the electrode, and this expansion is often accompanied by observable electrode exfoliation.5 Cathodic exfoliation in a non-aqueous system involves nonoxidative processes and hence it avoids the formation of oxygen containing functional groups in the final product. Inspired by the high intercalation density of Li+ to graphite in battery technology, some researchers investigated the potential use of Li+ intercalation for synthesis of graphene in organic electrolytes.7-8 Wang and co-workers electrochemically intercalated Li+ into graphite in propylene carbonate (PC) and only obtained an expanded electrode.7 Direct exfoliation of graphite was not observed as Li+ was too small to exfoliate graphite since the crystallographic diameter of Li+ is 0.146 nm.9 As a result, an intense subsequent sonication step was needed to exfoliate the expanded graphite to few layer graphene. Swager and Zhong used a two-stage electrochemical process where first they intercalate Li+ from PC solution followed by a larger cation, tetrabutylammonium ([TBA], the ionic diameter is 0.9 nm) to achieve exfoliation.10 Cooper et al. studied the intercalation behaviour of various tetraalkylammonium ([TAA]+) cations in 1-methyl-2pyrrolidone using HOPG and obtained few layer nonoxidised graphene.5,11 However, the yield and thickness of graphene production might be affected as the intercalation density of [TAA]+ ions into graphite is generally low. For example, the stoichiometry of tetraethylammonium (TEA) to graphite was C39[TEA]11 while it was C6[Li] in lithium.1 With the aim of getting high intercalation density, Abdelkader et al. used a single stage exfoliation process in an electrolyte containing a mixture of Li salt and trimethylamine hydrochloride to produce graphene.12 One of the major challenges that hinder the technological uptake of graphene is the difficulty of dispersing it in low-boiling point, environmentally benign solvents.13 Solvents that tend to disperse graphene in reasonable concentrations are usually toxic, and have a high boiling point, presenting problems for deposition of flakes and formation of composite materials.13 Covalent functionalizations of graphene have been used to enhance the solubility and to prevent the agglomeration of graphene sheets.14-15 Diazonium functionalisation of graphene has been widely used because of its versatility, simplicity and

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high reactivity towards sp2 hybridised carbon.16-17 Upon interaction with electron rich surfaces diazonium compounds readily generates an aryl radical, which then rapidly reacts with sp2-hybridized C-atoms. In previous work a variety of graphene substrates prepared by a range of techniques were used for diazonium grafting including mechanical exfoliation,18 chemical vapour deposition,19 epitaxially grown graphene,20 chemical reduction of graphene oxide21 and electrochemical exfoliation.10 Sun et al.15 chemically functionalized thermally expanded graphite with 4-bromobenzene diazonium and then sonicated to exfoliate the expanded functionalized graphite. This protocol was shown to enhance the solubility of graphene in N,N’-dimethylformamide (DMF). Englert et al.14 reported the bulk functionalization of chemically exfoliated graphene with 4-tert-buytlbenzene diazonium /4sulfonylphenyldiazonium chloride. The functionalisation prevented graphene aggregation and improved its solubility in chloroform. Alternatively, Swager et al.10 electrochemically intercalated Li+ and [TBA]+ in to graphite in a separate two stage process in PC electrolyte. They subsequently electrochemically functionalized the graphene with 4-bromobenzenediazonium. Within battery research the formation of a protective layer on the graphite surface from the decomposition of PC during electrolysis is widely known.22 Therefore, this protective layer could affect the efficiency of exfoliation and functionalization. It is notable that the studies reported to date used either pre-made graphene or expanded graphite for functionalization and there are no studies on bulk simultaneous electrochemical exfoliation and functionalization of graphene in a single pot at a single applied potential. The major advantage of such an approach is that it allows functionalization of the graphene sheets before they get the chance to re-aggregate. The N2 generated during in situ diazonium reduction may also aid the separation of functionalized graphene sheets. Furthermore, such processes may present an opportunity to selectively functionalise the more reactive graphene edge. Edge functionalization prevents damage to the basal plane which largely retains the physicochemical properties of pristine graphene.23 Edge functionalisation also increases the dispersibility of graphene and is attractive for various potential applications including catalysis, polymer composite formation and energy storage applications.23 In this contribution, simultaneous electrochemical exfoliation of graphite and functionalization in a single pot process is described for the first time. Electrochemical exfoliation of graphite is performed in the presence of a diazonium compound in the exfoliation solution by applying a single cathodic potential (−4.0 V vs Ag wire). This potential was sufficient to graft the graphite surface with diazonium species as well as for exfoliation of the already functionalized graphite edge. The diazonium species selected for our study were 4nitrobenzenediazonium tetrafluoroborate (NBD) and 4bromobenzenediazonium tetrafluoroborate (BBD) (see

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Scheme 1). As we will show, detailed electrochemical analysis indicated that the functionalized graphite surface did not hinder the intercalation of cation for exfoliation. We report for the first time the use of Cs+ as an exfoliating cation: its intercalation density is comparable to Li (as the reported stoichiometry of Cs to carbon was C8[Cs]) and its ionic diameter (0.338 nm) is similar to the interlayer spacing of graphite.1 Characterization of the exfoliated product using Raman spectroscopy, scanning electron microscopy transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy confirmed the formation of few layer functionalized graphene. Significantly, because of the surface functionalization, the resulting graphene sheets were found to be soluble in water and isopropanol mixture (1:1 v/v) and their solubility is two orders of magnitude higher than nonfunctionalized electrochemically exfoliated graphene sheets. Finally, the prospect of using these materials for supercapacitor applications will be discussed.

Scheme 1 The chemical structure of 4-nitrobenzenediazonium tetrafluoroborate (left) and 4-bromobenzenediazonium tetrafluoroborate (right)

Results and discussion Electrochemistry of Diazonium Salt. The aim of the current work is to simultaneously exfoliate and functionalize graphite in a single stage process as such, first it is important to examine if the diazonium species attached to the graphite hinders the intercalation of Cs+. It is commonly known that grafting carbon electrodes with diazonium species passivates the surface and often the grafted surface tends to be inactive towards simple redox mediators such as ferrocene and ferricyanide.24 Figure 1A shows cyclic voltammograms (CVs) recorded under N2 atmosphere at HOPG with and without NBD. In blank electrolytes (0.1 M CsClO4 in DMSO), as the potential of the electrode is scanned in a negative direction, a cathodic current started to flow at approximately −1.3 V and a broad peak was observed at −3.5 V (see Figure S1A for full scan). This was due to the intercalation of Cs+: it has been reported that Cs+ forms a complex with eight carbon atoms during intercalation.25 A sharp increase in current was seen due to the reduction of DMSO as the electrode potential was scanned further negative of −4.0 V (Figure S1A). During the return positive scan (Figure S1A), a single anodic peak was observed with a peak current centered at 0.3 V due to the de-intercalation of Cs. This deintercalation process depends on the magnitude of the applied cathodic potential. As the negative potential limit is decreased, the magnitude of the oxidation current increased with decreasing cathodic reversal potential,

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which indicates the increase in quantity of Cs+ intercalation. The CV response containing NBD, in contrast, showed a series of oxidation and reduction processes. Scanning the electrode potential in the negative direction yielded four reduction peaks. Peak clipping experiments revealed that C2, C3, and C4 were related to A2, A3 and A4 respectively (see Figure S1B). C1 was found to be irreversible indicating that the electrogenerated product was unstable and reacted with the surface. C1 was attributed to the formation of the nitrobenzene radical according to Equation 1 (Scheme 2), and this radical was highly reactive towards the carbon-based electrode.26-27 C2 and C3 were attributed to the formation of the nitro radical and dianion species respectively (Equation 2 and 3 of Scheme 2). Nitrobenzene reduces reversibly in a one-electron transfer process to yield a stable radical anion whereas the dianion species is susceptible to rapid protonation in the presence of a weak acid.24,28 Since the peak current ratio of A3 to C3 in our CV is less than ∼0.5, the dianion might be protonated to form unstable species by accepting proton from trace water or from DMSO itself (Equation 4). Bard et al. reported that the dianion can decompose to nitrosobenzene after the loss of hydroxyl ion in the presence of weak acid (isopropyl alcohol).28 Compton et al. also reported the instability of the dianion species in an aprotic room temperature ionic liquid.29 The nitrosobenzene formed, after the decomposition of the dianion, can reduce to phenylhydroxylamine via two-electron and twoproton transfer processes in aprotic electrolytes,28-29 or to aniline30 via four-electron and four-proton transfer processes in aqueous system. The current measured due to C4 is approximately twice that of the current measured due to either C2 or C3 suggesting that C4 is a two-electron transfer process and therefore we attributed C4 to the formation of phenylhydroxylamine as shown in Equation 5. Perhaps the most important observation from the CV is that the grafted HOPG surface by the nitrobenzene species did not hinder the intercalation of Cs, only its overpotential increased by 0.3 V compared to Cs+ intercalation in blank electrolyte, demonstrating the possibility of obtaining functionalized graphene in a single electrochemical step.

spacing of graphite. Moreover, the size of solvated Cs+ is expected to be higher than the interlayer distance of graphite. In fact, the Cs−O bond length in DMSO solvated Cs+ is 0.306 nm and each Cs+ forms a solvation shell with eight DMSO molecules.31 It is likely that the intercalation of DMSO-solvated Cs+ can easily expand and exfoliate graphite. To demonstrate whether the electrochemical intercalation of Cs+ expands and exfoliates graphite or not, electrolysis was carried out at −4.0 V for 1 hr using graphite electrode in 0.3 M CsClO4. During electrolysis, visual electrode expansion was observed and the SEM image shows a clear expansion of the electrode (See Figure S2), and the expansion occurs uniformly throughout the surface in thin layers. Graphite expansion followed by exfoliation within a few minutes of electrolysis demonstrates that the intercalation of Cs+ can expand and exfoliate graphite in DMSO electrolyte (see the characterization section).

Scheme 2 Reactions that represents the electrochemical reduction of NBD cations at HOPG electrode and the subsequent grafting and reduction steps

The electrochemistry of BBD is different from the electrochemistry of NBD in that only two reduction peaks and one oxidation peak were observed (see Figure 1B). The first reduction peak (C1) was due to the formation of the bromobenzene radical which rapidly reacts with HOPG surface as discussed previously (Equation 1). C2 was related to A2 and could be due to the reversible one electron transfer to form radical species. The intercalation of Cs+ also occurred at the same potential as in the blank electrolyte suggesting that the presence of bromobenzene on HOPG surface did not change the potential for Cs+ insertion. As previously mentioned, the crystallographic diameter of Cs+ is 0.338 nm which is similar to the interlayer

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after 30 min indicating that 85 % of the diazonium had already reacted within 30 min of electrolysis. Complete diazonium reaction was realised at t=2 hr. Moreover, a new absorption peak had emerged at 256 nm for t >30 min and a modest constant absorption in the visible light range was also noted. This new absorption peak is likely due to the electronic transition from exfoliated functionalised graphene like materials. All in situ electrochemical exfoliation and functionalization, therefore, were carried out for 2 h.

Figure 1. Cyclic voltammograms recorded at HOPG electrode in 0.1 M CsClO4 in DMSO under N2 atmosphere containing (A) 1 mM NBD and (B) 1 mM BBD. In each case, the potential was swept between −1.8 and -0.3 V from an initial potential of -0.3 V at 100 mV s-1. The black line in each case shows the data obtained in the absence of diazonium species and corresponds to the right axis while the red line shows the data obtained in the presence of diazonium species and corresponds to the left axis.

Diazonium Reaction Progress during Electrochemical Exfoliation. Graphite rod or foil was used for simultaneous exfoliation and functionalization, following the understanding of Cs+ and diazonium electrochemistry, at a single applied potential. −4.0 V vs Ag was chosen as the exfoliation and functionalization potential since at this potential the Cs+ intercalation occurs at a diffusion controlled rate and also the reduction of diazonium species occurs rapidly. During electrolysis, a significant amount of gas evolution was detected at the surface of the electrode, partly due to the reduction of diazonium which generates N2 gas (see Equation 1). The gas evolution was accompanied by the exfoliation of graphite within a few minutes of electrolysis. The rate of the diazonium reaction during exfoliation was monitored by UV-vis spectroscopy. Figure 2 shows the reaction progress of BBD in a solution containing 0.3 M of CsClO4 and 40 mM BBD as function of electrolysis time (t). A broad absorption peak was observed at 287 nm which was due to the electronic transitions from the diazonium at t=0.32 The intensity of this peak decreased gradually over the test to 10 min and rapidly decreased

Figure 2 UV–visible spectra of the electrolyte recorded after applying -4.0 V to an iso-moulded graphite working electrode at the times indicated. The initial electrolyte compromised 40 mM BBD in 0.3 M CsClO4 in DMSO

Although Cs+ alone has been demonstrated to exfoliate graphite in an electrochemical cell, we conclude that the reduction of diazonium products also assist the exfoliation process. As described in the previous section when the diazonium salt reduces i) it generates an aryl radical that attacks the graphite edge, which then causes a limited degree of expansion that may even assist intercalation of the cationic species; ii) it generates N2 gas which may assist the separation of the functionalised graphene layers. Some of the N2 gas may diffuse into the interstitial spaces between the sheets to help drive apart the already weakened van der Waals forces by the presence of the intercalating species. It has been suggested that the stress caused by the injection of gas between the layers of graphite is sufficient to overcome the van der Waals force that holds the graphene sheets together. For example, Kamali and Fray showed that that the diffusion of hydrogen in the interlayer space of graphite can lead to the exfoliation of graphite.33 Significantly, the assistance of the exfoliation process by diazonium species even permits very small intercalating species to be used. For example, Li+ can intercalate between layers in graphite but does not cause exfoliation to produce graphene because of its rather small size. Indeed, no exfoliation was observed during electrolysis when Li+ was used and only some physical detachment of graphite was noted without

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expansion. However, we observed that a combination of a diazonium species (40 mM NBD) with Li+ resulted in both functionalisation and exfoliation of the graphitic electrode, producing functionalised few layer graphene (Figure S3A). This demonstrates that the presence of diazonium salt in the exfoliation solution not only plays a role in functionalisation but also facilitate the exfoliation process. However, it should be noted that the use of diazonium salt (0.1 M in DMSO) without CsClO4/LiClO4 did not result in any graphite exfoliation despite significant gas evolution as functionalisation occurred. This confirms the necessity of an intercalating species when performing the simultaneous exfoliation and functionalization process.

salt concentrations and they reported an iD/iG of between 0.1 and 3.1.37 The intensity of the 2D band also decreased and broadened due to the electron withdrawing (p-type doping) nature of the nitro and bromo aromatic group.17,40-41 The p-type doping is more evident by the upshift in the peak position of the G band and 2D band with increasing diazonium concentration and this observation is consistent with reported literature.17,41-42 The Raman spectra also did not show any physisorbed diazonium related bands. In previous works it was reported that surface adsorbed diazonium molecules usually appeared between 1400 and 1440 cm-1.43 This result suggests that the absence of any such non-covalently bonded diazonium molecules and that our electrochemical functionalization exclusively forms covalent bonds.

Characterisation of the Exfoliated Product. Raman spectroscopy was used to confirm the formation of few layer functionalised graphene and Figure 3 shows Raman spectra of functionalized graphene at various diazonium concentrations. For comparison, the electrochemically exfoliated graphene (EEG) using Cs+ in the absence of diazonium is also presented and compared with the Raman spectrum of the starting graphite material. The Raman spectrum of graphite showed two intense peaks at 1579 cm-1 and 2719 cm-1 that correspond to the G band and 2D bands, respectively. The G band is due to the E2g vibrational mode of sp2 hybridised carbon and the 2D band is a second order vibration caused by the scattering of two phonons with opposite wave vectors.17,34 An additional small peak is seen at 1349 cm-1 due to the D band, this band is active when there are defects in graphitic materials.34-35 The lattice defect can be caused by formation of sp3 hybridisation through covalent chemistry or by physical defects or edges in sp2-conjugated carbon.17,34 After electrochemical exfoliation in the absence of diazonium, the peak position of the 2D band and the G band red shifted by about 33 cm-1 and 8 cm-1 respectively; as shown in previous reports.36 Moreover, the shape of the 2D peak changed from the typical broad asymmetric shape of graphite to a symmetric line shape indicating the formation of few layer graphene sheets.5,34,36

X-ray photoelectron spectroscopy (XPS) was also used to confirm the electrochemical functionalisation of graphene as well as to evaluate the resultant chemical compositions. Figure 4 displays the wide scan spectrum of EEG, graphene-functionalized with nitrobenzenediazonium (G-NBD) and graphene functionalized with bromobenzenediazonium (G-BBD). In each case, the signal due to C1s and O1s was observed and all peak positions are charge corrected by setting the binding energy of the C1s signal equal to 285 eV.44 The presence of N1s in GNBD and Br3p in G-BBD in the survey scan confirms the functionalisation with the desired phenyl moieties. Moreover, the atomic concentration of N increased from 0.5 % in 1 mM of NBD to 4.8 % in 100 mM NBD while the atomic concentration of Br increased from 0.4 % in 1mM BBD to 5.2 % in 100 mM BBD (see Table S1). The high resolution N1s signal only showed one peak at 399.3 eV in 1 mM NBD and previous literature attributed this feature to NH2 group.30,45 Although our electrochemical analysis indicated that NHOH is the most likely surface moieties at -1.5 V, the NHOH group might be further reduced to NH2 group as exfoliation and functionalisation was carried out at -4.0 V. However, as the concentration of NBD was increased to 40 mM and above, a small shoulder peak at 405.6 eV emerged at the characteristic binding energy of nitrogen in a NO2 group.20,30 The evolution of the NO2 group depends on the concentration of NBD. In 40 mM NBD, the NO2 group accounts for 19 % and it increased to 29 % in 100 mM NBD. The presence of the NH2 group in significant concentrations (>70 %) in all the samples studied supports the conclusions drawn from the electrochemical analysis of NBD. On the other hand, the existence of NO2 group may suggest diazonium functionalization occurred in the solution by spontaneous in situ chemical reaction after electrochemical exfoliation.46

A substantial increase in the intensity of the D band was noted relative to EEG for the samples that were exfoliated in the presence of diazonium salt. The intensity of the D-band was found to be strongly dependent on the concentration of diazonium salt. The evolution of the Dband was accompanied by an increase in the intensity of the D′ band at 1614 cm-1. For example, the intensity ratio of the D band to the G band (iD/iG) in EEG is 0.28, this value increased to 2 and 3 respectively when 100 mM NBD and 100 mM BBD were used for in situ functionalisation. It is widely accepted that the intensity ratio of the D band to the G band is a measure of disorder or defects within graphene flakes.37-38 It can also be used to characterise the degree of covalent functionalization by the diazonium species.17,37,39 Greenwood et al. electrochemically functionalized pristine graphene with a range of diazonium

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Figure 3. Representative Raman spectra of in situ electrochemically exfoliated and functionalized graphene sheets with various concentrations of (A) BBD and (B) NBD. The electrochemical exfoliation and functionalization of iso-moulded graphite was performed at -4.0 V vs Ag wire for 2h at indicated diazonium concentrations. The samples for Raman analysis were prepared by drop coating the dispersion of graphene on to Si/SiO2 wafer and dried on hot plate at 100 °C to evaporate the solvent

Figure 5A shows representative SEM micrograph of the functionalised graphene flakes (40 mM G-NBD) when the starting raw material was graphite foil. The lateral size measurement of 200 flakes indicated that the flake size varies between 0.5 µm and 3.5 µm, with the majority of the flakes being ∼1 µm. When an iso-moulded graphite rod was used, however, the majority of flakes are below 0.3 µm, indicating that flake size depends on the source material. There is a clear difference between the morphology of functionalized graphene and nonfunctionalized graphene as shown in SEM, TEM and AFM images. The functionalized graphene flakes displayed a network of intense wrinkles and ripples on their surface whereas the non-functionalized graphene flakes exhibited

Figure 4. (A) Wide-scan XP spectrum of EEG, G-NBD and G-BBD. (B) High-resolution XP spectrum of G-NBD in the N1s region and (C) High-resolution XP spectrum of G-BBD in the B3p region. All peak positions were charge-corrected by setting the binding energy of the C 1s signal to 285 eV. The samples for XPS were prepared by filtering the dispersions on PTFE membrane.

a folded flat surface (see Figure 5C and 5E). Close examination of the TEM and AFM images also show that the wrinkles are more intense on folded edge than the basal plane. Such observations have been noted previously for functionalized graphene and were attributed to the strain generated by an intense functionalization.47-48. Nanoscale wrinkles on graphene sheets reduce restacking between

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individual sheets, offer fast ion diffusion channels, provide more active sites for catalytic reactions, and are attractive for energy storage devices such as supercapacitors.49 This feature also provides improved adhesion and better interlocking property within polymer composites.48-49 The measured flake thickness for the non-functionalized graphene using AFM varies between 1.7 nm to 5 nm indicating the formation of few layer graphene to multilayer graphene whereas it fluctuates from 2.4 nm to 30 nm for functionalised graphene because of the substantial wrinkling and functionalisation. Dispersibility of Functionalised Graphene. The dispersibility of graphene in a solvent is dictated by the match between the surface energy of graphene and the solvent, whereas its long term stability is determined by the solvents ability to stabilise the graphene sheets via electrostatic repulsion or steric hindrance.50-51 Solvents with similar surface energies to that of graphene are considered to be the most efficient solvents for dispersing graphene as the interfacial tension between those solvents and graphene is minimal.51 Organic solvents such as N-methyl-2-pyrrolidone and N, N-dimethylformamide have been found to be the best solvents for dispersing graphene.51-52 However, these organic solvents are toxic to multiple organs and also the high boiling points pose problems for deposition of flakes and formation of composite materials.52 Low boiling point solvents such as chloroform and isopropanol (IPA) were used for graphene dispersion and exfoliation, but they suffered from poor dispersion stability and exfoliation quality.53 In this regard, improved dispersibility in water and low toxicity organic solvents are of paramount importance for commercial applications. The solubility of graphene in water is very low because of its strong hydrophobicity while graphene oxide does disperse in water due to its surface functional groups.54 In order to assess the dispersibility of our functionalised graphene in water and IPA mixtures, samples were centrifuged at 8000 rpm for 30 min and the supernatant was analysed by UV-vis spectroscopy. It is widely accepted that the concentration of graphene can be estimated using Beer-Lambert law by taking the value of the absorbance at 660 nm (A660).51 Figure 6 shows UV-visible spectra obtained from a dispersion of G-NBD and G-BBD in water, IPA and water-IPA mixes. Both G-NBD and G-BBD showed a low value of A660 when dispersed in either neat water or IPA. However, the mixture that contained IPA and water in one to one volume ratio gave the highest value of A660 for each sample, indicating that this solvent mix is the best for dispersing G-NBD and G-BBD. Wateralcohol mixtures were used for exfoliation of graphite and in this case the dispersion of graphene was found to be much higher in the mixtures than either of the individual solvents.55 The enhanced dispersion of graphene was attributed to the modification of Hansen solubility parameters (HSP).55 It has been also shown that the fraction of

IPA in the water/IPA mixture was crucial for the dispersibility of graphene, and the optimum mass fraction of isopropanol in IPA-water mixture was 55 % which is very close to the mass ratio of IPA (∼60 %) we used in the this work.55 The UV-visible spectra in Figures 6A and 6C show that increasing the concentration of water in the dispersion solution impacted the BBD functionalised graphene more than the NBD functionalised graphene, suggesting that brominated graphene is more polar than the nitrogenated graphene. The dispersion of G-BBD was found to be strongly dependent on the extent of functionalisation (Figure 6D), showing an increase in solubility as the BBD concentration was increased from 1 mM to 100 mM. In contrast, the solubility of G-NBD was independent of NBD concentration and one can get a higher dispersion concentration with only 1 mM NBD compared to the dispersion obtained in 100 mM BBD. The absorption coefficient (α) of G-NBD and G-BBD dispersion was determined as per the method given by Coleman et al.51 using IPA/water mixture in 1:1 volume ratio. As shown in Figure S4, each dispersion follows the Beer-Lambert law as the absorbance increases linearly with increasing concentration, and α values of 2978 ± 125 mL mg-1 m-1 and 2853 ± 268 mL mg-1 m-1 were obtained for G-NBD and G-BBD respectively. A range of α values was reported previously in literature. Coleman et al.51 reported a value of 2460 mL·mg−1 m−1 in different solvents and Raju et al.56 reported 4632 mL·mg−1 m−1 for dispersions of solution exfoliated graphene. Lotya et al.57 reported 6600 mL mg−1 m−1 for dispersions of graphene that were stabilized by surfactant. Konios et al. reported a value of 3592 mL mg-1 m-1 for graphene oxide dispersion in water.58 The solubility of G-NBD and G-BBD were determined using the α value we obtained. G-NBD showed the highest solubility (250 µg mL−1) in IPA/H2O compared to 150 µg mL−1 for G-BBD and only 5 µg mL−1 for EEG. Kim et al. 59 reported the exfoliation and dispersion of graphene in water at elevated temperature and obtained a maximum solubility of 6.5 µg mL−1 while Wu et al. reported 50 µg mL−1 in ethanol and water mixtures. Clearly the surface functionalisation of graphene with phenyl moieties enhanced the solubility of graphene by more than two orders of magnitude. We believe that this simple method of in situ simultaneous exfoliation and functionalization of graphene may aid its processing in various applications.

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Figure 5 SEM images of G-NBD that were obtained by electrochemical exfoliation of graphite foil at -4.0 V vs Ag wire for 2h in 40 mM NBD and 0.3 M of CsClO4 in DMSO dilute dispersion deposited on Si/SiO2 wafer (A) and restacked films deposited on Si/SiO2 (B), TEM image of EEG flake (C), TEM image of 40 mM G-NBD (D) AFM image of EEG (E) and AFM image of 40 mM G-NBD (F). The inset in Figure 5E and 5F show the height profile for the selected region

Capacitance of Functionalised Graphene. The capacitance of functionalised graphene was investigated using cyclic voltammetry and chronopotentiometry using symmetrical coin cell architecture (CR2032). The electrodes were made by filtering a known volume of the dis-

persion on pre-weighed PTFE membrane where the PTFE act as both an electrode and charge separator.60 The typical mass loading of each electrode was approximately 0.5 mg cm-2. Figure 7A compares the CV obtained at EEG and G-NBD electrodes in deoxygenated 6M KOH (aq) at

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Figure 6 Typical UV–vis absorption spectra of (A) 100 mM G-NBD dispersed at indicated solvent/s (B) G-NBD dispersion in water/IPA (1:1 v/v), (C) 100 mM GBBD dispersed at indicated solvent/s and (D) G-BBD dispersion in water/IPA (1:1 v/v). In each case, the functionalised graphene was diluted by a factor of five before measurement

0.1 V s −1. The CV obtained using EEG electrodes displayed the typical capacitive behaviour with rectangular shape, and no notable faradaic reaction was observed when scanning the voltage up to 0.9 V. In contrast, two well defined transient-shaped oxidation peaks (EP) at 0.3 V (O1) and 0.61 V (O2), along with two corresponding reduction peaks at 0.2 V (R1) and 0.54 V (R2) emerged as the functionalization concentration of NBD increased from 1 mM to 40 mM. The ratio of the anodic peak current, ip,a, to that of the cathodic peak current, ip,c, was approximately 1:1 for each redox couple. In addition, the plot of ip,a increased linearly with increasing scan rate for each redox reactions (Figure 7B), confirming the redox reactions are a surface confined process.61 Given that the XPS shows that G-NBD contained predominately aniline groups and the two redox peaks may be attributed to the electrooxidation of aniline. Literatures report showed that electrochemical oxidation of aniline may produce a variety of redox couples that were attributed to the formation of nitrene cations, p-aminodiphenylamine, benzidine and degradation of polyaniline films.62-63 At 100 mM NBD, the O2/R2 redox peaks become widely separated when compared to the CV obtained using 40 mM NBD and the O1/R1 redox reaction disappeared This suggests

that the intense functionalization may lower the conductivity of graphene and has shifted the O1/R1 reaction to more negative potentials. The presence of faradaic reactions is also more evident in the charge-discharge curves when moving from EEG to G-NBD (Figure 7C). In EEG, the charge-discharge curve displayed symmetrical triangular shape whereas in the G-NBD electrodes the curve deviated from the ideal linear shape and two plateau regions at ∼0.2 and ∼0.6 V were observed that correspond to the redox reactions. On the other hand, G-BBD predominately displayed faradaic reactions with a sharp oxidation and reduction current in the potential range studied which is presumably due to the redox reaction of bromine moieties (Figure S5). This demonstrated that Brfunctionalised graphene may therefore be less preferred than NBD for applications in supercapacitor technology.

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The specific capacitance of each electrode was calculated from the CV at 0.1 V s-1 and a value of 19 F g-1 was obtained for EEG. The capacitance value of EEG correlate well with reported value for electrochemically exfoliated graphene films.4 The functionalised electrodes exhibit much larger specific capacitance than EEG, and its capacitance increases with increasing degree of functionalisation (due to the contribution from faradaic reactions). Specific capacitance of 31.4, 56.9 and 71.3 F g-1 were obtained in electrodes that were functionalised in 1 mM, 40 mM and 100 mM NBD, respectively. Bissett et al.60 reported a gravimetric capacitance of 21 F g-1 using commercial graphene and Parvez et al.4 reported 19 F g-1 using electrochemically exfoliated graphene with a similar graphene loading to our work. On the other hand, graphene oxide-based electrodes can show capacitances of over 100 F g-1 because of their pseudocapacitive behaviour.64 Although the overall capacitance (double layer capacitance and pseudo capacitance) of functionalised electrodes increased more than three times compared to EEG, the significant proportion of capacitance increment was obtained from the contribution of the faradaic process. Closer inspection of the CVs also indicate that the charging current measured using 40 mM G-NBD and 1 mM GNBD electrodes was not changed in any notable amount. These observations suggest that the functionalisation of graphene preferably occurred at the edge rather than the basal plane, and this argument was supported by analysis of powder X-ray diffraction data of restacked functionalised graphene. As shown in Figure S6, the (002) peak of G-NBD appeared at a similar position(26.6°) to that of EEG demonstrating that functionalisation did not change the interlayer distance. A change in the interlayer distance would be anticipated if functionalization occurred on the basal plane, as is the case for graphene oxide.65 It is commonly acknowledged that graphene edge functionalization is the preferred site for diazonium chemistry.15,6667 For example, Sun et al.15 studied the functionalization of graphene with 4-bromodiazonium using energy filtered transmission electron microscopy and showed that graphene edges were selectively functionalized. Lim et al.41 reported that diazonium species was covalently bound to the graphene edge, while it was non-covalently adsorbed on the basal plane. Strano et al.18,67 also showed that the reactivity of graphene edge towards diazonium was greater than the basal plane. In general the graphene edge is found to be more reactive towards a variety of reactions compared to its basal plane as the dangling bonds at the edge promote covalent bonding with various chemical species.68-69 In our experimental set-up in particular, functionalization continues to be preferred at the edge sites since already edge-functionalised graphene sheets are constantly extracted from the electrode by Cs+ intercalation. However, it is difficult to rule out the functionalisation of the basal plane to a certain degree given that Raman spectroscopy showed an increase in intensity of

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the D-band up on increasing the degree of functionalisation (Figure 3).

Figure 7 (A) Cyclic voltammograms recorded at 100 mV s-1 in 6.0 M KOH (aq) using symmetrical coin cells constructed from indicated samples. The voltage was swept between 0.0 V to 1.0 V (B) ) CVs obtained at 40 mM G-NBD coin cells in 6.0 M KOH at (from top to bottom) 100, 85, 60, 45 and 20 mV s−1 between 0.0 V (initial potential) and 1.0 V (C) charge-discharge curve obtained at indicated elec-1 trodes at 0.5 A g

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Conclusions We have developed a simple and cost effective process that involves in situ electrochemical exfoliation and functionalization of graphene in a single stage procedure. The success of this process arises from the fact that pregrafting the graphite surface with diazonium species did not prevent the intercalation of Cs+ into graphite, and thus Cs+ can intercalate and exfoliate already functionalized graphite. The two most significant findings emerging from this study are i) the presence of diazonium salt in the exfoliation solution not only plays a role in functionalisation but also facilitates the exfoliation process potentially through the generation of N2 gas and by expanding the graphite edge, so that even ions normally too small to exfoliate graphite to produce graphene and graphitic nanoplatelet structures may be used in combination with diazonium salt ii) the functionalized graphene sheets are soluble in water-based systems and this may increase the processability of graphene in composite formulation, ink formation, catalysis and energy storage devices. Fine tuning the functional groups using various diazonium species could further enhance its solubility and future work should focus on this. The formation of functionalized graphene was confirmed using a variety of techniques, including Raman spectroscopy and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy confirmed the presence of the desired functional moieties and in particular showed that aniline-based surface moieties were the dominant surface groups when using nitro-substituted diazonium reagents. The degree of functionalization increased as the concentration of diazonium salt in the exfoliation solution was increased and it is possible to further fine tune the degree of surface grafting by controlling the exfoliation time and diazonium concentration. The functionalization also enhanced the charge storage capacity when used as supercapacitor electrode because of the redox reactions. This technique can be extended to redox active electrolyte such as anthraquinone9 diazonium to further enhance the capacitance as well as to produce metal nanoparticles decorated graphene. Experimental Methods Materials and Reagents. 4nitrobenzenediazoniumtetrafluoroborate (97 %), 4bromobenzenediazonium tetrafluoroborate (96 %), anhydrous dimethyl sulfoxide (99.9 %) was obtained from Sigma-Aldrich. Caesium perchlorate (99 %) was obtained from Fisher Scientific. All electrochemical measurements were performed using an Autolab potentiostat model PGSTAT302N (Metrohm Autolab, The Netherlands). Isomolded graphite (>99.95%) rods were purchased from GraphiteStore and graphite foil (99.8 %) was obtained from Alefa Aesar. Polytetrafluroethylene was obtained from omnipore membrane filters (JVWP01300) with pore size of 0.1 µm. Millipore water (18.2 MΩ) was obtained

from Milli-Q water purification system. Highly oriented pyrolytic graphite (HOPG) ZYB quality was purchased from Micromechanics Ltd (Hong Kong) Electrochemistry of Cs+ and Diazonium salt. A freshly cleaved HOPG working electrode, a Pt mesh counter electrode and an Ag wire reference electrode were used for the electrochemical measurements. The potential of an Ag wire was stable within few mV for over 4 hr. Prior to performing cyclic voltammetry, N2 gas was bubbled into the electrolyte for 30 min and during electrochemical measurements an atmosphere of N2 was maintained above the electrolyte. The electrolyte consists of either 0.1 M CsClO4 and 1mM 4nitrobenzenediazoniumtetrafluoroborate (NBD) in an anhydrous dimethyl sulfoxide (DMSO) or 0.1 M CsClO4, and 1mM 4-bromobenzenediazonium tetrafluoroborate (BBD ) in DMSO. Electrochemical Exfoliation and Functionalisation. Electrochemical exfoliation and functionalisation of graphene was performed using a three electrode setup consisting of an isomolded graphite rod/graphite foil working electrode, a silver wire reference electrode and an isomolded graphite rod counter electrode. The effective area of the working electrode that was exposed to the electrolyte was ∼12 cm2. The electrolyte was prepared by dissolving 0.3 M CsClO4 and various concentrations (1 mM, 40 mM and 100 mM) of either NBD or BBD in anhydrous DMSO. Simultaneous electrochemical exfoliation and functionalisation was performed using chronoamperometry by applying a potential of -4.0 V vs Ag wire for 2 hr under constant stirring. Similarly, non-functionalised graphene was exfoliated at the same potential in solution that only contained 0.3 M CsClO4 in DMSO. The exfoliated product was then washed with plenty of acetone and ultra-pure water, and dried under vacuum at 60 °C overnight. The functionalised powder was dispersed in the desired solvent (water, isopropanol and a mixtures of water and isopropanol alcohol) by sonicating for 30 min. The resulting mixture was centrifuged at 4000 rpm for 30 min, and the supernatant was extracted using a pipette without disturbing the residue. Characterisation of the Exfoliated Product. Raman spectra were obtained using Renishaw inVia microscope with a 532 nm excitation laser operated at power of 3.32 mW with a grating of 1800 l/mm and 100× objective. The sample for Raman and scanning electron microscopy (SEM) measurement were prepared by drop coating the dispersion of graphene on to Si/SiO2 wafer and dried on hot plate at 100 °C to evaporate the solvent. SEM analysis was carried out using XL30 FEI Environmental scanning electron microscope operated at 15 kV The samples for AFM measurements were prepared by spray coating the dispersion of graphene on Si/SiO2 wafer and the AFM operates in tapping mode under ambient conditions. Transmission electron microscopy (TEM) images were recorded using a JEOL 2000FX TEM, operated at 200 kV and the samples were prepared by drop casting the dis-

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persion on holy carbon film (300 mesh Cu, Agar Scientific) TEM grid. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer with a monochromated Al Kα X-ray source (E = 1486.6 eV, 10 mA emission), a hemispherical electron energy analyser and a multichannel plate and delay line detector (DLD). The dispersion concentration of graphene was measured using UV-visible spectroscopy using a model DH-2000-BAL (ocean optics). The extinction coefficient of functionalised graphene was determined following the method described by Coleman et al.51 Electrode Preparation for Supercapacitor testing. Films of non-functionalised graphene, NBD functionalised graphene or BBD functionalised graphene were prepared by filtering a known volume of the dispersions over polytetrafluroethylene (PTFE) membrane using a syringe pump dispenser (New Era Pump Systems, Inc, NY) at a rate of 10 mL hr-1. The membrane is then dried in an air oven at 80 °C overnight. Coin cell assembly was prepared in standard CR2032 coin cell hardware with symmetrical active materials. The cells were assembled by stacking the two symmetrical membranes back-to-back with the active material contacting the current collector.60 A few drops of deoxygenated 6 M KOH (aq) were added to fill the electrode before the coin cell was sealed using a hydraulic crimping machine (MSK-160D). The specific capacitance was calculated using the best practice methods established by Stoller and Ruoff.70

Supporting Information The supporting information is available free of charge on ACS publication website at http://pubs.acs.org. Supplementary materials on cyclic voltammograms of CsClO4, Raman spectra, XPS analysis for atomic concentration of functionalized graphene samples, graph of absorbance per unit path length and Powder X-ray diffraction spectrum

AUTHOR INFORMATION Corresponding Author * Email: [email protected], Tel: +44 (0)161-3064522. Fax: +44 (0)161-275-4598. * Email: [email protected]

Notes The authors declare the following competing financial interest: the University of Manchester has filed a patent application in the area of electrochemical exfoliation and functionalization of graphene.

ACKNOWLEDGMENT We would like to thank the European Union Seventh Framework Programme under grant agreement no. 604391 Graphene Flagship and EPSRC (UK) for funding (Grant refs

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EP/K016954/1, EP/I023879/1) and Samuel G. Booth for assistance with TEM.

Notes Source data files can be obtained http://www.mub.eps.manchester.ac.uk/robertdryfeelectrochemistry/.

from

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Single Stage Simultaneous Electrochemical ... - ACS Publications

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