Kar High-yield Graphene Production by Electrochemical Exfoliation of Graphite Novel Ionic Liquid...

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High-yield graphene production by electrochemical

exfoliation of graphite: Novel ionic liquid

(IL)acetonitrile electrolyte with low IL content

Amin Taheri Najafabadi, El}od Gyenge *

Department of Chemical and Biological Engineering, Clean Energy Research Centre, The University of British Columbia, 2360 East Mall,

Vancouver, BC V6T 1Z3, Canada

A R T I C L E I N F O

Article history:

Received 10 October 2013

Accepted 7 January 2014

Available online 15 January 2014

A B S T R A C T

Electrochemical exfoliation of graphite assisted by ionic liquids (ILs) has been proposed as a

high-throughput, green and scalable graphene production technique. Previous research has

focused on IL/water electrolytes with high IL content (from 1:0.1 to 1:1 IL/water volume

ratios). Here, we introduce and investigate a novel IL/acetonitrile electrolyte with dramat-

ically lower loads of ionic liquids (1:50 IL/acetonitrile vol. ratio). Our approach provides

three main advantages: cost efficiency due to low IL content, extended electrochemical sta-

bility in a non-aqueous electrolyte, and high exfoliation yield by effective anionic interca-

lation within the graphitic layers. Using iso-molded graphite rod as the anode, we achieved

up to 86% of exfoliation with the majority of the products as graphene flakes in addition to

smaller quantities of carbonaceous particles and rolled sheets. We also demonstrate byRaman spectroscopy the beneficial sonication effect on improving the quality of the graph-

ene-based products. Moreover, in contrast with previous literature, we prove that the elec-

trolyte coloration during electro-exfoliation in the IL media is related to the occurrence of

diverse reactions involving the IL moieties and cannot be associated with different stages

of graphene formation. The cathodically generated species can also interfere with the anio-

nic intercalation in the graphite anode.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, as the rising star in the world of advanced materi-

als, offers game-changing prospects for many key areas of

research including energy storage and conversion, biotech-

nology, novel electronic devices and so forth[14]. This two-

dimensional planar sheet of sp2-bonded carbon atoms is

among the most widely researched nanomaterials since its

isolation by Geim and Novoselov in 2004 [5,6]. Graphene has

a large theoretical specific surface area (2630 m2 g1)[7], high

intrinsic electron mobility (200,000 cm2 V1s1) [8], high

Youngs modulus (1.0 TPa) [9], and thermal conductivity

(5,000 Wm1K1) [10], while showing noticeable optical

transmittance (97.7%)[11,12]. Nonetheless, the commercial

success of many proposed graphene-based applications is

still unproven and among a number of factors, the need to

competitively produce it in large quantities is a major imped-

iment. The latter necessity created a veritable graphene syn-

thesis rush leading to the blooming state of publications on

various graphene production techniques with associated rosy

promises.

Graphene sheets (GNs) in scalable top-down approaches

(i.e. exfoliation of graphitic structures to produce ultrathin

layers) are usually produced as a mixture of monolayers,

0008-6223/$ - see front matter

2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2014.01.012

* Corresponding author: Fax: +1 604 822 6003.E-mail address:[email protected](E. Gyenge).

C A R B O N 7 1 ( 2 0 1 4 ) 5 86 9

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bi-layers and multilayers (310 layers), in the form of irregu-

larly structured flakes or flat (or folded) sheets [13,14]. GNs

can be also produced as buckyballs (fullerenes) or as 1 D

nanotubes[15,16]. However, a key challenge here is to over-

come the resilient exfoliation energy of the graphites p-

stacked layers originated from the high cohesive van der

Waals energy (5.9 kJ mol1 carbon) among graphene sheets

[17,18]. In fact, Geim and Novoselov developed a top-down ap-

proach so-called micromechanical cleavage to extract sin-

gle sheets of atoms from three dimensional graphitic

crystals using scotch tape exfoliation [6]. Graphite as an

earth-abundant starting material for the top-down prepara-

tion of graphene offers a cost-efficient and environmentally-

friendly alternative to bottom-up nanocarbon synthesis (i.e.

growing graphene monolayers from carbon atoms). Other

top-down methods include wet-chemical synthesis (or elec-

trosynthesis) from graphite intercalation compounds (GICs)

[1921], direct liquid phase exfoliation [22], and solution-

based chemical reduction of graphene oxide (GO)[23,24].

At present, chemical reduction of GO, obtained from

graphite oxide exfoliation, to graphene sheets has emerged

as a promising method due to its low-cost and mass produc-

tion potential [19,2531]. Graphite oxide is usually synthe-

sized through oxidation of graphite via concentrated

sulphuric acid, nitric acid and potassium permanganate

based on the Hummers method [32,33]. Although graphite

oxide and GO share some similar chemical properties (i.e. sur-

face functional groups), their structures are different. GO is a

monolayer material produced by the exfoliation of graphite

oxide[30]. Several reducing agents are proposed for chemical

reduction of GO sheets including hydrazine [3437], and so-

dium borohydride [38,39]. Hydrazine, unlike other strong

reductants, is nonreactive with water and is suggested to be

the most effective in synthesizing ultrathin and fine graphene

nanosheets [30]. Throughout the reduction process, the

brownish GO dispersion in water turns black and the reduced

sheets begin to agglomerate and precipitate in the reaction

vessel[29,40]. Such trends in color changes and dispersibility

of the reduced GO (rGO) in water is rendered by the removal of

oxygen atoms from the GO sheets, making the final product

more blackish and less hydrophilic[30].

Nonetheless, GNs synthesis via GO reduction leaves a sig-

nificant amount of oxygen impurities behind, resulting in

numerous lattice defects [41,42]. Thermal annealing of rGO

sheets is reported to lower the GN network defects [24,43],

and finding more effective routes for complete reestablish-

ment of the GNs sp2-bonded structure is of great interest

[26]. Moreover, most of the reported chemical production

techniques use harsh oxidizers (e.g. H2SO4/KMnO4), and an

excess of organic solvents (e.g. dimethylformamide or tetra-

hydrofuran), which are not environmentally benign [26,44].

Besides, the successive reduction of GO sheets to graphene

typically requires a strong chemical reducer (e.g. hydrazine

or sodium borohydride), and high temperature heating in or-

der to recover the graphitic structure[45]. Thus, sever safety

measures must be taken when large quantities of these

reducing agents are used, making the scale-up of the process

challenging [46]. A handful of environmentally friendly pro-

cesses are available [47,48]to reduce GO to graphene either

by chemical or electrochemical pathways; however, an

integrated green approach to the synthesis of graphene has

not been extensively explored.

To address the above-mentioned issues, researchers have

utilized electrochemical methods as a part of the GN fabrica-

tion process in the past few years [20,21,4954]. In principle,

GN electrosynthesis employs an ionically conductive solution

(electrolyte) and a direct current (DC) power source to prompt

the structural changes within the graphitic precursor (e.g. rod,

plate, or wire) used as the electrode. This offers a number of

potential advantages including ease of operation and control

over the entire synthesis process, being more environmen-

tally benign with elimination of harsh oxidizers/reducers, rel-

atively fast fabrication rates, and high mass production

potential at ambient pressure/temperature. Presumably, di-

rect exfoliation of the graphene sheets from graphite would

overcome the low electronic conductivity of graphene films

chemically reduced from GO derivatives[29,54].

Considering the limitation of water electrolysis in aqueous

electrolytes, non-aqueous solvents are generally employed to

provide a wide electrochemical window [21,5254]. Among

those, ionic liquidshaveexhibited remarkable tendencyto inter-

calate graphitic electrodes and yield gram-scale quantities of

carbon nanostructures (CNS) and GNs[20,54]. Air- and mois-

ture-stable room temperature ionic liquids (RTILs), salts of large

organic cationswith relatively bulky inorganic counter-ions, are

molten salts with melting points close to room temperature

[55,56]. Owing to lowvaporpressure,high chemical and thermal

stability, solvating capability, non-flammability with potential

recyclability, RTILs have received pronounced interest as green

solvents in organic synthetic processes to replace classic, toxic

and volatile molecular solvents[57,58].

Due to the interdisciplinary nature of IL-assisted graphene

electrosynthesis, relatively little attention has been paid to

the reactivity of RTILs[59,60], and their unusual optical proper-

ties accompanied by a complex electrochemical behavior

[61,62]. Recently, the reactive nature of some specific RTILs

has been discussed by several research groups[6365]. Accord-

ingly, the possible utilization of RTILs as catalysts or reagents,

besides their application as mere solvents, has been proposed

[56,60,66]. The concept of employing co-solvents to modify ILs

physicochemical properties [67,68], and the inevitable water

presence, coupled via hydrogen bonding with ILs anion

[69,70], are among other factors that induce more complexity.

The first published work on graphene electrosynthesis

using ILs goes back to 2008 when Liu et al. reported production

of functionalized GNs with 1:1 IL/water volume ratios [53].

They applied 1015 V to an electrochemical cell with graphitic

anode and cathode; where the exfoliation was observed at the

anode. This line of research was then pursued by Lu et al.,

focusing mainly on the complementary role of water during

the exfoliation process [54]. The latter authors showed that

replacing the graphitic cathode with a platinum wire does

not affect the electrosynthesis since the main processes

related to GN electrosynthesis occur solely at the anode. Fur-

thermore, they attributed the wide range of produced nano-

structured carbon materials to the fine tuning of the IL/water

ratios. In addition, the fluorescence behavior of the product

solution was assigned to the presence of carbon nanostruc-

tures. Ultimately, they suggested IL/water ratios of 1:0.1 for

the maximum production of sheet-like structures[54].

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Importantly, none of the aforementioned studies dis-

cussed nor defined the yield for GN electrosynthesis for the

various types of employed ILs and different IL/water ratios.

Thus, it is uncertain how practically feasible the electrosyn-

thesis process could be for the production of specific nano-

structures. Another aspect that has been previously

neglected is related to the possible parasitic reactions of the

IL molecules on the electrode surface. The adsorption, inter-

calation and possible electrode reactions of the IL moieties

can have a major impact both on the exfoliation process

and the overall efficiency.

In this research, we introduce and investigate a non-aque-

ous electrolyte composed of IL and acetonitrile with dramat-

ically lower loads of ionic liquids (1:50 volume ratio)

compared with previously reported water-based systems. By

carrying out pertinent control experiments, we also reveal

the background electrode processes involving IL moieties that

could interfere with the anodic exfoliation process yet were

neglected in previous studies.

2. Materials and methods

Fig. 1shows the schematic of the experimental setup used for

graphite electro-exfoliation in the presence of ionic liquid

electrolytes. High purity iso-molded graphite rod (6.35 mm

diameter with 4 cm effective length exposed to the electro-

lyte, Graphite Store) was used as the anode with a platinum

wire as the counter-electrode (1.6 mm diameter and 4 cm

effective length). As electrolyte, both IL/water solutions and

the newly proposed IL/acetonitrile were tested (15 ml total

volume in each trial), at constant voltage of 7 V (B&K Preci-

sion-9110 DC power supply), and room temperature (293 K).

The passed current was 100 mA for all the cases with slight

decrease in the course of experiments. In our main trials,

0.1 M ionic liquid solutions in acetonitrile were used as the

electrolyte (1:50 IL/solvent volume ratio). It should be noted

that depending on the IL type, the exfoliation could happen at

lower potentials as well (e.g. 5 V). However, the operating con-

ditions were fixed at the previously mentioned levels for the

sake of consistent comparison between different electrolytes

during 4 h of synthesis. The extent of graphite anode

exfoliation was evaluated by measuring the electrode volume

changes. This variable was used to quantify the exfoliation

yield in the presence of different ILs. The exfoliation solid

products were washed with copious amounts of water and

ethanol followed by separation by filtration and ultracentrifu-

gation with rotational speed of 104 RPM at 20 C. Sample sus-

pensions inN-methyl-2-pyrrolidone (NMP) were sonicated for

1 h to reach stable suspensions using a VWR Scientific table-

top ultrasonic cleaner (B3500-MTH). All the chemicals were of

the analytical grade, and double-distilled water was used dur-

ing all of the preparation steps.

Iso-molded graphite is the iso-statically cold-pressed

graphite that guarantees uniform properties in all grain direc-

tions [71]. This differs from extruded graphite, which is

pushed through a die and has varying properties within its

structure. There is also no binder used in the manufacturing

process which enables more facile flake detachment upon

electro-expansion. It is our observation that in some previous

studies, ill-characterized graphitic precursors are employed

such as pencil lead. The latter is one of the most unreliable

choices since a typical 9B lead consists of more than 20 com-

pounds including clays and binders, along with undefined

graphitic phases, that differ among various manufacturers

[72]. This significantly undermines the reproducibility of the

results when pencil lead is used as the working electrode

(i.e. graphene precursor).

As for the investigated RTILs, Fig. 2shows the four types

tested in this research along with their electrochemical stabil-

ity on platinum electrode at 293 K. The electrochemical win-

dow data was provided by the supplier (Iolitec Company)

and it was double-checked by us to reassure the accuracy.

These candidates noticeably possess high ionic conductivity,

electrochemical stability, non-inflammability, and very low

vapor pressures, making them an ideal medium for a vast ar-

ray of syntheses. We selected them in a way to afford the eval-

uation of the roles played by different anions and cations

during graphene electrosynthesis. Compared toFig. 2, most

of the previous researchers have examined only imidazoli-

um-based ILs[53,54]. Furthermore, to lower the IL loading in

the electrolyte, it was dissolved in acetonitrile with 1:50

volume ratio (0.1 M). It is noteworthy that acetonitrile is

Fig. 1 Experimental setup used for graphite anodic exfoliation in the presence of ionic liquids. (A color version of this figure

can be viewed online.)

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commonly used as a nonreactive solvent for ionic liquids

especially in supercapacitors as it assures remarkable ionic

transport and conductivity while preserving the electrochem-

ical stability window of the RTILs [73,74].

As for product characterization, transmission electronmicroscopy (TEM) images were obtained with a Tecnai G2

microscope at acceleration voltages of 20200 kV. TEM grids

were covered with 5 microliters of the diluted products in

water/isopropanol mixtures to provide more facile evapora-

tion and product visibility in imaging. Field emission scan-

ning electron microscopy (FESEM) was performed using a

Hitachi S-4700 FESEM at 330 kV. X-ray photoelectron spec-

troscopy (XPS) was done by a Leybold Max200 for the surface

elemental composition and binding energy analysis with five

channeltrons, using an unmonochromated Mg Ka X-ray

source (1253.6 eV). A Micromeritics surface area analyzer

(ASAP2020) was utilized to determine the specific surface area

of the electro-exfoliated flakes. The fluorescence spectra ofthe diluted electrosynthesis solution in water were collected

using a Varian Eclipse fluorescence spectrophotometer. The

Raman spectra were recorded via a LabRAM ARAMIS micro-

scope Raman spectrometer with an argon-ion laser at an exci-

tation wavelength of 632 nm. The samples deposited on

aluminum substrates were exposed to the laser beam (1 lm

diameter at the focus) with the exposure time of 1 s. The data

was collected from 3 to 5 different regions of each sample to

ensure the consistency of the results. All the measurements

were taken at room temperature without special mention.

3. Results and discussion

3.1. Electrosynthesis experiments

According to Lu et al., the GNs synthesis process by ILs elec-

tro-intercalation can be monitored based on the color

changes of the electrolyte [54]. They used BF4- as the anion

with an imidazolium-based cation similar to EMIM (Fig. 2,

left). Furthermore, these authors claimed that the specific

solution colors can be assigned to different categories of prod-

ucts generated including fluorescent carbon nanostructures

and graphene sheets [54]. In Fig. 3, we compare the color

changes during different stages of graphite electro-exfoliation

using EMIM BF4(Fig. 3a) with control electrolysis experiments

where the graphite electrode was replaced by platinum wire

with a similar electrolyte (Fig. 3b).

Lu et al. proposed a three-stage color change classification

[54], which is in good agreement with our images shown in

Fig. 3a. They speculated that in stage I, there is an inductionperiod before visible signs of exfoliation can be detected. In

stage II, the electrolyte color changes from colorless to yellow

and then dark brown which is accompanied by a visible

expansion of the graphite anode (Fig. 3a). Finally, in stage

III, the expanded flakes peel off from the anode and form

the black slurry with the electrolyte [54]. However, nearly

identical color changes were observed in the control experi-

ment, where we replaced the graphite anode with a platinum

wire and repeated the electrolysis under exactly the same

conditions (Fig. 3b). Therefore, clearly the color changes can-

not be used as an indicator of various carbon nanostructure

formation suggested by Lu and coworkers[54]. Evidently, dif-

ferent color changes are solely originated from IL interactionwith the electrode surface (Pt or graphite).

In fact, one of the key properties of the imidazolium-

based RTILs, the most extensively studied class of ionic

liquids, is their Brnsted acidity[75]related to the C-2 hydro-

gen of the 1,3-dialkylimidazolium cation [62]. The electro-

chemical reduction of the C-2 proton initiates H2 evolution

at the cathode. This reaction also leads to the formation of

the electro-generated N-heterocyclic carbenes (NHCs) and

their derivatives via subsequent chain and/or decomposition

reactions [59,61,76]. Therefore, these species are

mainly responsible for the solution coloration (Fig. 3). NHCs

are highly reactive, as base/nucleophile, and can also react

with the graphite anode (see Section 3.2, XPS analysisresults).

In addition to the visible color changes of the electrolyte,

another discussion has emerged with respect to the fluores-

cent nature of some of the carbon nanostructures (e.g. nano-

particles and bucky gels) produced during electrosynthesis

[54]. Lu et al. described the obtained spectra of the alleged

fluorescent carbonaceous particles as generally broad and

excitation-wavelength-dependent; attributed to the size het-

erogeneity and distribution of different emissive sites on the

carbon nanoparticles [54]. This false presumption can also

be objected by comparing the fluorescence spectra of the

electrolyte solutions obtained from the two parallel

electrolysis experiments, with and without graphite anode,

Fig. 2 Investigated ionic liquids and their respective electrochemical stability windows on platinum at 293 K versus normal

hydrogen electrode (NHE).

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respectively (Fig. 4a and b). When the final solutions from the

two sets of experiments (a and b) were analyzed in a fluores-

cence spectrophotometer, they both exhibited similar excita-

tion and emission trends (Fig. 4).

As mentioned earlier, another complexity of studyingRTILs, especially in the case of imidazolium-based cations,

is their unusual optical properties in the ultraviolet (UV) and

near visible regions[77,78]. Imidazolium containing ILs exhi-

bit significant UV absorption, which extends into the early

part of the visible region. Their fluorescence trend covers a

large portion of the visible domain characterized by a signifi-

cant excitation-wavelength-dependence. Such shifts of the

fluorescence maximum peaks are attributed to the various

associated structures present in the ionic liquids and the

incongruous excitation energy transfer process among them

[77,78]. In general, wavelength-dependent fluorescence

behavior of the organized assemblies such as membranes,

proteins and RTILs is observed when their dipolar moietiesget excited at the red edge of the absorption band, so-called

red-edge effect (REE)[79,80]. The described optical abnormal-

ities become more severe under electrosynthesis conditions

where NHCs instigate a series of side reactions and further

affect the unusual excitation wavelength-dependent

tendencies. Thus, the complex fluorescence behavior of the

IL-containing solutions, especially under electrolysis

conditions, undermines any direct connection between the

fluorescence spectra and release of various carbon nanostruc-

tures to the solution during electrosynthesis.

As shown in Fig. 5, we next carried out parallel experi-

ments using the four ILs illustrated in Fig. 2 to assess the

influence of the IL structure on the graphite electrochemical

exfoliation. InFig. 5, first it can be observed that the solution

coloration cannot be correlated with the graphite exfoliation

yield. In other words, as shown in Fig. 5, about 86% of the

graphite electrode was exfoliated when BMPyrr BTA was used

as IL, while the final solution remained virtually colorless.Conversely, when the exfoliation rate was at the lowest

(29%), in the case of EMIM BF4, the solution coloration was

the most severe, as discussed previously in relation toFig. 3.

Furthermore, it is important to note that no color change of

the solution does not imply that the IL remains unchanged

during the electrosynthesis process. In the case of BMPyrr

BTA, where over 85% of the graphite anode peeled off with

minimum solution coloration, we still witnessed gas evolu-

tion at the Pt cathode; probably due to the cathodic decompo-

sition of BMPyrr into methylpyrrolidine and a butyl radical

with its subsequent disproportionation leading to gaseous

product species formation[81].

The low exfoliation rate in the presence of EMIM BF4 canbe explained by the strong adsorption of EMIM on the cathode

surface followed by the electrode reaction leading to the for-

mation of NHCs and subsequent chain reaction products.

These species formed at the cathode dissolve back to the

solution and can re-adsorb on the graphite anode, in compe-

tition with BF4-, hindering the BF4

- adsorption and intercala-

tion into the graphite layers. Thus, its inferred that the

cathodic by-products are hampering the anodic graphite

exfoliation. This hypothesis is further supported by the exper-

iments using the same anion while changing the cations,

where a dramatic improvement in exfoliation rate was

obtained with minimum coloration in the case of pyrrolidini-

um-based cations (Fig. 5).

(a)

(b)

Fig. 3 Electrolyte color changes during (a) electrochemical graphite exfoliation, and (b) control electrolysis experiment

without graphite electrode in 0.1 M EMIM BF4/acetonitrile solution at 7 V and 293 K in the course of 4 h (anode is on the left

side in all images). (A color version of this figure can be viewed online.)

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Switching from BF4- to more oxygenated anions such as

bis(trifluoromethylsulfonyl)imide (BTA) increases the extent

of complexities but also enhances the exfoliation yield

(Fig. 5).Fig. 6demonstrates how the initial brownish exfolia-

tion of the graphite anode in the first 15 min is followed by

an intense blackish flake precipitation during four hours ofelectrosynthesis. Due to the oxygenated nature of the BTA an-

ions (Fig. 2), there is a higher chance of reactivity with the vul-

nerable sites of graphite anode. Similar impact was reported

during graphite exfoliation with SO42 intercalation[50]. We

concluded again by control experiments utilizing only plati-

num electrodes that the mild coloration of the electrolyte ob-

served inFig. 6after 2 h is originated from the products of the

BMPyrr BTA electrode reactions such as cathodic reduction

followed by chain reactions and re-adsorption of the products

on the anode.

In the previous studies, there has been an overemphasis

on the critical role of water in water-based IL mixtures

[53,54]. Thus, it is necessary to make a few comments basedon our own experiments. First of all, no notable exfoliation

was observed with equivalent IL/water ratios compared to

that of the newly introduced IL/acetonitrile systems

(1:50 volume ratios). Thus, in order to induce any electro-

exfoliation in the water-containing systems much higher IL

content is necessary (e.g. 1:0.11 IL/water vol. ratios [53,54]),

which substantially increases the cost of the entire process.

Furthermore, high water contents significantly narrow the

electrochemical stability window, thus reducing the exfolia-

tion power of the anionic intercalation compounds. On the

other hand, the water presence in RTILs is inevitable as most

of the ionic liquids are hygroscopic [82,83]. However, such

water traces are rapidly removed by the electrode reactions

Fig. 4 Fluorescence spectra comparison of the resulting electrolyte from (a) graphite electro-exfoliation experiment, and (b)

control experiment both in 0.1 M EMIM BF4/acetonitrile solution at 7 V and 293 K after 4 h. (A color version of this figure can be

viewed online.)

Fig. 5 Color changes and the exfoliation yield after 4 h of

ionic-liquid-assisted electrochemical graphite exfoliation at

7 V using 0.1 M IL/acetonitrile (1:50 IL/solvent vol. ratio) at

293 K (the shows 95% confidence interval from 3

experiments). (A color version of this figure can be viewed

online.)

C A R B O N 7 1 ( 2 0 1 4 ) 5 86 9 63


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during electrolysis in the IL/acetonitrile electrolyte and will

not have any long-lasting impact. Therefore, water presence

is unnecessary for the electrochemical graphite exfoliation.

Instead, we proved that other co-solvents such as acetonitrile

that preserve the ILs potential window serve as much more

advantageous alternatives by affording the dramatic lowering

of the IL content with high rate of exfoliation.

3.2. Products characterization

After 4 h of electrosynthesis, the exfoliated precipitates were

collected and thoroughly washed using deionized water and

ethanol. The solid products were then separated by filtration

and ultracentrifugation followed by drying in a vacuum oven

for 24 h at 85 C. The dried products were subjected to one

hour sonication in NMP.Fig. 7shows the corresponding trans-

mission electron microscopy (TEM) and field-emission scan-

ning electron microscopy (FESEM) images. Depending on the

sampling depth and duration, we were able to capture a vari-

ety of structures from ultrathin graphene flakes (Fig. 7ad) to

semi-transparent carbonaceous particles (Fig. 7e), and rolled

sheets (Fig. 7f). After taking 250 TEM and SEM images of more

than 15 samples, we inferred that the majority of the pro-

duced materials are of the ultrathin irregularly shaped sheets

rather than the other described morphologies. Based on our

statistical analysis of the TEM images, the average length of

the flakes was in the order of 500 nm. In terms of thickness,

according to Meyer et al.[84]the folding fashion of the edges

is an important indication for the number of layers. There-

fore, from the folded edges shown inFig. 7b and c, formation

of monolayers and bilayers is inferable. Moreover, we did not

notice any major deviation from the planar topographies to

other described by-products when various IL structures were

employed for the synthesis.

Nonetheless, it is oversimplification to assume that the

rigorous electro-intercalation of energetic IL anions only leads

to planar morphologies. Isolation of the particles and rolled

sheets, even in trace amounts, which was successfully

achieved in our work certainly sheds some light on the exfo-

liation mechanism. Presumably, the initial anionic intercala-

tion/expansion poses a strong mechanical stress on the

vertices and edges of the graphitic precursor which ultimately

results in breakage of the vertices (i.e. particle formation) and

bending the edges (i.e. rolled sheet formation). It should be

noted that such structures were separated in trace amounts

from filtration and centrifuge disposals, which might be typ-

ically overlooked by the researchers. Thus, we believe report-

ing these morphologies is a significant step towards

conducting more detailed studies in the future and is out of

the current focus of this paper.

Raman spectroscopy is another useful probe to diagnose

the number of layers and disorder in graphene [8589]. This

technique has also been commonly used to investigate the

quality of the electrochemically produced GNs in several

electrolytes[21,51]. The Raman spectra presented in Fig. 8,

obtained from EMIM BF4/acetonitrile electrosynthesis, dis-

play the well-known graphitic G peak at 1581 cm1 (from

bond stretching of sp2 pairs in rings and chains), and a D

band at 1335 cm1 (corresponding to the breathing modes

of sp2 atoms in rings) [21,90]. According to Ferrari et al.

[86], the evolution of the 2D peak (second order of D peak)

is a clear indication of structural transformation from

graphite to graphene heterostructures after electro-exfolia-

tion. The sonication impact is also noticeable by increasing

the G/D peak ratios, which represent lower disorders there-

fore better graphene quality [91]. Importantly, G/D peak

intensity ratios represent the disorder in graphene in the

widest possible meaning [86,92]. In fact, there is a certain

chance for the incomplete electro-exfoliation or aftermath

agglomeration of the products which induces the disorder

effect. NMP is considered among the most promising candi-

dates for the solvent-assisted graphite exfoliation [22,93],

and its effectiveness was evident in improving the overall

quality of our products. Brief sonication can also help to

more rigorously remove any physically adsorbed IL rem-

nant/contamination on the surface of graphene which can

be another contribution to the quality enhancement. More-

over, these observations highlight the importance of imple-

menting a systematic approach for isolating the effect of

all the processing and treatments methods performed on

the GNs in the course of synthesis. Similar trends were

observed for the exfoliation products synthesized with the

other investigated IL structures in this work (Fig. 2).

Lastly, X-ray photoelectron spectroscopy (XPS) analysis re-

vealed nitrogen incorporation into the exfoliation products

via NHCs parasitic adsorption on the anode during electro-

exfoliation in EMIM BF4 electrolytes (Fig. 9). The presence of

the imidazolium moiety as well as the BF4 counter-ion is

inferable considering the XPS survey scan presented in

Fig. 9. A well-defined peak at 400.2 eV can be assigned to the

Fig. 6 Progressive exfoliation of the graphite anode in the BMPyrr BTA during 4 h with 0.1 M IL/acetonitrile at 7 V and 293 K.

(A color version of this figure can be viewed online.)

64 C A R B O N 7 1 ( 2 0 1 4 ) 5 86 9


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N 1s of the imidazolium ring[54](Fig. 9). The presence of B 1s

peak at 193.2 eV and F 1s at 686.6 eV represent the fingerprint

of BF4 anion remnants on the exfoliated flakes (8 h) [98,99]. Furthermore, the resulting suspensions

produced in this work remain stable after centrifugation and

sedimentation over a period of few months. We believe that,

(a) (b)

(c) (d)

(e) (f)

Fig. 7 TEM and SEM images showing the variety of products generated by electrochemical exfoliation of iso-molded graphitein EMIM BF4/acetonitrile electrolyte at 7 V and 293 K: (ad) crumpled/folded sheets, (e) semi-transparent carbonaceous

particles, and (f) rolled sheets.

C A R B O N 7 1 ( 2 0 1 4 ) 5 86 9 65


9/12

besides functionalization, the high surface areaof the products

generated by electrochemical exfoliation (over 250 m2 g1) is

anotherkeyfactorforfast andstable dispersion formation. An-

other possible explanation for this observation might be the

presence of amorphous carbon structures. However, Raman

analysisfrom over fiveregions of each sampleoffered a consis-

tent association with GNs characteristics rather than other

carbon types including amorphous. Additionally, we did not

capture any notable amorphous particle in our imaging exper-

iments either.

4. Summary and conclusions

We investigated the room temperature electrochemical exfoli-ation of iso-molded graphite anodes in a novel electrolyte

composed of IL and acetonitrile (1:50 vol. IL/solvent ratios)

using four different IL structures. Previous research was

focused only on IL/water mixtures with much higher IL

content (e.g. up to 1:0.1 IL/water vol. ratios). Our approach

provides three main advantages: cost efficiency due to low IL

content, extended electrochemical stability in the non-

aqueous electrolytes, and much higher exfoliation yields

caused by the effective anionic intercalation within the

graphitic layers. Thus, up to 86% exfoliation yield was

achieved in 4 h using BMPyrr BTA/acetonitrile (1:50 vol.

ratio). The anodic intercalation of the large oxygenated BTA

anions wasobserved to be promising in overcoming the strongvan der Waals cohesion among the graphitic layers. The major

products of the graphite exfoliation, for all the four tested ILs,

were crumpled and folded graphene sheets. In addition, some

by-products such as carbonaceous particles and rolled sheets

were isolated but in much smaller quantities. Furthermore, we

demonstrated the complementary role of sonication after

electrosynthesis for improving the overall quality of the graph-

ene-based products characterized by the Raman spectra.

Previous research had also placed a great deal of emphasis

on the electrolyte coloration during electrosynthesis as an

indicator of various graphite exfoliation stages and nano-

structure formations. However, using control experiments

(i.e. replacing graphite anode by a platinum wire), we revealedthat the reactions of the IL moieties with the electrodes along

with their unusual fluorescence behavior are mainly respon-

sible for this false presumption. The cathodic reduction of

the cationic moieties such as imidazolium rings generates

carbene species that are reactive with the graphite anode

and interfere with the anionic intercalation. On the other

hand, the electrochemical reactions involving the IL provide

the opportunity for in situ functionalization of the graphene

sheets via redox species produced during the electrosynthesis

process. The resulting functionalized GNs can be used for a

variety of applications including energy storage/generation,

environmental remediation, biomedical applications and so

forth. In any case, via facile post-treatment techniques, anyfunctional group can be removed from the products if not de-

sired. Finally, the research presented here could pave the way

for future studies on ionic-liquid-assisted electrochemical

production of GNs on large scales.

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