Soft processing of graphene nanosheets by glycine-bisulfate ionic-complex-assisted electrochemical exfoliation of graphite for reduction catalysis

Article abstract of DOI:10.1002/adfm.201402621

This study demonstrates a mild, environmentally friendly, and cost-effective soft processing approach for the continuous synthesis of high-quality, few-layer graphene nanosheets. This has been achieved via electrochemical exfoliation of graphite, using an

Full text of DOI:10.1002/adfm.201402621

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Soft Processing of Graphene Nanosheets by Glycine-Bisulfate
Ionic-Complex-Assisted Electrochemical Exfoliation of
Graphite for Reduction Catalysis
Kodepelly Sanjeeva Rao, Jaganathan Sentilnathan, Hsun-Wei Cho, Jih-Jen Wu, and
Masahiro Yoshimura*
and volatile-agent-assisted intercalation-
[
9,12]
This study demonstrates a mild, environmentally friendly, and cost-effective
soft processing approach for the continuous synthesis of high-quality,
few-layer graphene nanosheets. This has been achieved via electrochemical
exfoliation of graphite, using an environmentally friendly glycine-bisulfate
ionic complex and was performed under ambient reaction conditions. Gra-
phene nanosheets with 2–5 layers were obtained under optimized exfoliation
conditions using a 15 wt% glycine-bisulfate (aqueous) solution, with working
biases of +1 V and +3 V applied for 5 min. The role of the glycine-bisulfate
ionic complex in the electrochemical exfoliation process was confirmed
through comparison with a control experiment using only sulfuric acid as the
electrolyte. A plausible electrochemical exfoliation mechanism that involves
the formation of surface molecule nuclei via the polymerization of intercalated
monomeric HSO4− and SO42− ions is proposed. The ionic complex plays a key
role in the anodic graphite exfoliation via electrochemical-potential-induced
intercalation, leading to an efficient expansion of graphite sheets via the
insertion of oxygen functional groups.
expansion,
have been applied in the
preparation of these graphene-based
materials.
Solution processing[8,9,12–22] is a par-
ticularly attractive approach due to its
low cost and environmental friendli-
ness. The electrochemical exfoliation
[
23–34]
of graphite
is one such example of
solution based processing that has been
used for the direct preparation of low-
defect graphene nanosheets (GNSs).
Within this broader processing regime
[23–25]
the use of ionic liquids
are of par-
ticular interest due to their suitability as
[23]
electrolytes. Previously, Liu et al.
and
[
24]
Lu et al. have demonstrated the prepa-
ration of graphene via the electrochem-
ical intercalation of PF6 and BF4− anions
into graphite layers using alkylimida-
zolium hexafluorophosphate (RIMPF6)
and alkylimidazolium tetrafluoroborate
−
(
RIMBF ) ionic liquids. Ionic liquids have several merits,
1
. Introduction
4
including negligible vapor pressure, low toxicity, high chem-
ical and thermal stability, and ease of availability.
[
23–25]
The outstanding mechanical, electrical, optical, and thermal
Whilst
properties[
1–4]
of graphene, along with its many potential appli-
ionic-liquid-assisted electrochemical methods have several
advantages, they also suffer from a number of drawbacks.
Ionic liquids require non-soft (uncommon in natural/bio-
logical systems, expensive/complexity in handling) starting
materials along with complex/additional procedures, higher
voltages, higher costs, and suffer some disruption to the elec-
tronic properties of produced graphene as a result of ionic
liquid functionalization.
in acidic electrolytes provides good quality graphene, but a
large number of oxygen functional groups are inserted via
the acid-assisted heavy oxidation of graphite. Our recently
cations, has led to an exponential growth in graphene-based
materials research over the past few years. Numerous func-
tional graphene materials have now been synthesized using
different structural motifs including metal-organic frame-
works, polymers, biomaterials, organic crystals, and inor-
ganic nanostructures. Several synthetic approaches, such as
[
5–7]
[23]
chemical vapor deposition (CVD),
micromechanical exfoli-
Alternatively, graphite exfoliation
[
5,8]
[8,9]
ation,
epitaxial growth on SiC,
solution route wet chem-
ical exfoliation,[ sonochemical liquid-phase exfoliation,
10]
[8,11]
[
32]
[
31]
dveloped
peroxide ions assisted exfoliation method has
Dr. K. S. Rao, Dr. J. S. Nathan, Prof. M. Yoshimura
Promotion Center for Global Materials
Research (PCGMR)
Department of Material Science and Engineering
National Cheng Kung University, 70101, Taiwan
E-mail: yoshimur@mail.ncku.edu.tw
several advantages, although it does require the use of strong
alkaline solutions. Therefore, the development of simple, low-
cost, environmental friendly, yet highly efficient methods for
the preparation of good quality graphene is still required for
practical applications. We have found that glycine-bisulfate
Mr. H-W. Cho, Prof. J-J. Wu
Department of Chemical Engineering
National Cheng Kung University, 70101, Taiwan
(
Gly·HSO ) is such a candidate, particularly as glycine is a
4
natural biocompatible material.
The formation of Gly·HSO ionic complex in aqueous solu-
4
DOI: 10.1002/adfm.201402621
tions is a 100% atom-economical process (the molecular mass
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DOI: 10.1002/adfm.201402621
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of Gly·HSO is equal to the total molecular mass of starting
under ambient reaction conditions. Initial experiments were
conducted to compare the use of H SO alone as the electro-
4
glycine and H SO ), being low-cost, and environmentally
2
4
2
4
friendly, and has been applied previously for organocatal-
lyte with that of the Gly·HSO ionic complex, these were done
4
ysis.[
35–37]
To date, it has not been applied in the electrochem-
using a Gly·HSO to water (by weight) ratio of 4:96 under dif-
4
ical processing of graphene. The study presented here utilizes
Gly·HSO4 ionic complex as a mild, cost-effective chemical
agent for the efficient electrochemical exfoliation of graphite
for the first time. We present a comprehensive characteriza-
tion of the exfoliated GNSs including a plausible and detailed
proposal explaining the exfoliation mechanism. This work
ferent bias voltages. Experiments were then performed across
a further range of Gly·HSO to water weight ratios (4:96, 9:91,
4
15:85, and 40:60). The corresponding products are denoted as
GNS4, GNS9, GNS15, and GNS40, with obtained yields of 0.8,
–
1
1.3, 2.2, and 2.4 g L , respectively (see Table S1, Supporting
Information for full details).
[
38]
illustrates an efficient soft processing method for the prepa-
ration of high quality GNSs.
The optimal exfoliation conditions for the preparation of
high-quality, few-layer GNSs were found to be a Gly·HSO4
to water ratio of 15:85 and working biases of +1 V and +3 V
applied for 5 min (GNS15, Table S1, ESI). Photographs of the
exfoliated GNS15 directly in electrolyte solution and also re-
dispersed in N-methyl-1-pyrrolidone (NMP) after purification
are shown in Figure 1c. No exfoliation was observed in a con-
trol experiment conducted with a graphite cathode under the
optimal anodic exfoliation conditions. Another control experi-
ment, which utilized only H SO as the electrolyte, did not
2
. Results and Discussion
2
.1. Optimization of Exfoliation Conditions
The process developed for the preparation of GNSs is pre-
sented in Figure 1. To prepare the Gly·HSO ionic complex
4
2
4
used for exfoliation experiments, an equimolar mixture of
glycine and H SO in aqueous solution was stirred for 1 h at
produce GNSs, but instead formed a graphene oxide (GO)-like
material (Table S1, Supporting Information). The Gly·HSO4
ionic complex is mild yet superior to the mixed electrolyte
2
4
[
35,39 ]
room temperature (Figure 1a).
A schematic diagram of
[
32]
the electrochemical setup is presented in Figure 1b. Within
the reaction vessel a graphite anode was placed parallel to a
platinum cathode and exfoliation experiments were performed
H SO + KOH (pH 1.2) for the exfoliation of graphite and
2 4
better than aqueous NaOH + H O reported in our previous
2
2
[
31]
work.
[
35]
Figure 1. Preparation process of GNSs. a) A simple solution-process approach for Gly·HSO ionic complex preparation in aqueous solution.
4
b) Diagram of electrochemical experimental setup. c) Photographs of GNS15 directly in electrolyte (left) and re-dispersed in NMP after purification
right). d) Exfoliation yields versus weight ratios of Gly·HSO ionic complex to water used in exfoliation experiments.
(
4
2
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edge carbons via the formation of rip-
[
26]
ples, dislocations or charged impurities.
The 2D bands (related to disorder-induced
defects) of graphite and GNS15 appear at
−
1
−1
2
664 cm and 2655 cm , respectively. The
defects formed in the GNS15 during exfoli-
ation process increase the D band intensity
and reduce the 2D and G bands intensi-
[
14]
ties.
The 2D/G ratio of GNS15 (1.1) is
good agreement with the graphene (1.2)
prepared via submerged liquid plasma pro-
[
14]
cess by our group. The D band to G band
[
40,41]
ratio (I /I )
of graphite and GNS15
D
G
Figure 2. Raman spectra of a) graphite and b) GNS15 recorded at a laser excitation wavelength
of 633 nm.
are 0.54 and 0.70, respectively. The higher
I /I value of GNS15 is likely due to struc-
D
G
tural defects formed during the exfoliation
2
.2. Properties of GNS15
process. Comparison of the Raman spectra of graphite and
GNS15 suggest the formation of high-quality graphene under
the optimal mild exfoliation conditions. The Raman results
of GNS15 are also in good agreement with high-quality gra-
The Raman spectrum of GNS15 was compared with that
of pure graphite (Figure 2). The D band (related to disor-
dered SP structures) and the doubly degenerate (TO and
3
[24]
phene reported by Lu et al.
LO) phonon E mode G band (related to vibration of in-
Transmission electron microscopy (TEM) results of
GNS15 are summarized in Figure 3. Figure 3a shows a
low-magnification TEM image of GNS15 drop-casted onto a
lacey carbon grid. The high-resolution TEM (HR-TEM) image
of GNS15 shows the intermixing of defect-free and disor-
dered domains (Figure 3b). These defect-free and disordered
domains are shown at higher magnification in Figure 3c.
Regarding the quality of GNS15, π-bonded carbon frameworks
with few defects can be seen in the carbon lattice patterns
(Figure 3d). The HR-TEM analysis also indicates the presence
of few-layer graphene with 2–5 layers (Figures 3e-f). The sta-
tistical distribution of graphene sheet layers was obtained by
2
g
2
plane SP π-network structure in a two-dimensional hex-
−
1
agonal lattice) of graphite normally appear at 1337 cm
−
1
and 1581 cm , respectively. Whereas the D and G bands
of GNS15 appear at 1334 cm−1 and 1578 cm , respectively.
−1
The increased D band intensity of GNS15 indicates the for-
mation of defects[
14,26]
such as adatoms, vacancies, and grain
boundaries, formed most likely during electrochemical exfo-
liation process. The D′ shoulder bands (corresponding to dis-
order of edge carbons) appears at 1621 cm−1 and 1615 cm−1
for graphite and GNS15, respectively. The increased D′ band
intensity of GNS15 indicates an increase in disorder of the
Figure 3. TEM analysis of GNS15 structure. a) Low-magnification TEM image of GNS15 drop-casted on lacey carbon grid. b) HR-TEM image of GNS15.
c) High magnification of b) showing defect-free domains. d) Selected-area electron diffraction (SAED) pattern of carbon lattice. e, f) HR-TEM images
of few-layer graphene and statistical distribution of graphene sheet layers (Inset of figure 3f).
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evaluating a large number of HR-TEM images obtained from
almost 70 graphene sheets. The distribution data shows that
GNS15 consists of approximately 13% bi-layer, 48% tri-layer,
oxygen functional groups during the exfoliation process. These
results are in good agreement with the literature reports
[
31,43–45]
and suggest that GNS15 is graphene in nature as it appears
to consist of graphene sheets with low level (11%) of oxygen
3
2% four-layer and 7% fiver-layer graphene nanosheets (Inset
[
22]
of Figure 3f). The carbon lattice spacing is around 0.35 nm,
which is close to that reported in the literature.[24] This indi-
cates the presence of few oxygen functional groups in GNS15.
The appearance of an intense 2D band at 2660 cm− in the
Raman spectrum of GNS15 also indicates the presence of few-
layer graphene, further supported in literature,[ leading us
to believe the nanoscale morphology and the atomic structure
of the carbon frameworks of GNS15 are of high quality.
X-ray photoelectron spectroscopy (XPS) studies also suggest
the high quality of GNS15, the level and kind of functional
groups inserted (Figure 4). The presence of carbon and oxygen
in graphite and GNS15 is confirmed by a wide-scan XPS
spectrum (Figure 4a). The starting graphite material contains
carbon and oxygen in a ratio of 98.5:1.5, whereas exfoliated
GNS15 contains carbon and oxygen in a ratio of 89:11, indi-
cating the insertion of some oxygen functional groups during
the exfoliation process.
groups unlike graphene oxide or even graphene.
The Fourier transform infrared (FT-IR) spectrum of the
starting graphite indicates the presence of very few oxygen
1
−1
−1
functional groups, with peaks at 1000 cm and 3340 cm cor-
responding to C-O/CO-H (stretching) and CO-H (bending),
respectively (Figure 4d, I). The peak corresponding to the
42]
−1
π-carbon framework is at approximately 1580 cm (C = C
stretching). The FT-IR spectrum of GNS15 shows corresponding
−
1
−1
peaks at 1580 cm (C = C, C = O stretching) and 3340 cm
(CO-H bending) (Figure 4d, II). The relatively high intensity
−1
of the peak at around 1000 cm (C-O/CO-H stretching) indi-
cates the presence of the hydroxyl/phenolic/alkoxy functional
groups, which are in good agreement with the research work
of others. These results confirm the insertion of oxygen func-
tional groups, such as hydroxyl/phenolic, alkoxy, and carboxyl
groups, during the exfoliation process. These FT-IR results con-
firm our findings from our XPS results. Therefore, based on
our XPS and FT-IR results, we propose the structural features
of GNS15 contain epoxide, hydroxyl, phenolic, and carboxylic
functional groups, as illustrated in Figure S3, ESI.
[46]
The C1s spectrum of graphite and GNS15 shows an intense
band at 284.8 eV corresponding to C-C bonds (Figure 4b-c).
Appearance of a band at 285.9 eV in C1s spectrum of graphite
indicating the presence of C-O functional groups (Figure 4b).
For the GNS15, an increased intensity of the band at 286.3 eV
corresponds to C-O bonds (related to hydroxyl/phenolic/epoxide
groups) and the appearance of a new band (in comparison to
the starting graphite) at 288.8 eV corresponds to C = O bonds
2.3. Proposing Exfoliation Mechanism
Various research groups have proposed anodic graphite
electrochemical exfoliation mechanisms using a range of
(related to carboxylic acid groups) indicating the insertion of
Figure 4. XPS analysis results. a) XPS spectra of i) graphite and ii) GNS15 in wide-range scan. C1s core-level spectra of b) graphite and c) exfoliated
GNS15, showing insertion of some oxygen functional groups. d) FT-IR spectra of I) graphite and II) GNS15, showing good agreement with XPS results.
4
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processing parameters.[25,27–34] Most electrochemical mecha-
nisms assume that simple anions generated from electrolytes
can directly undergo intercalation when a potential is applied
d) When such ion clusters exceed a certain size, the internal spe-
cies enveloped by surface ions becomes isolated, and they be-
come reduced to neutral molecules.
across two electrodes,[ i.e.,HSO4− and SO4 anions gen-
28]
2−
e) Surface molecule nuclei form via the addition of many mo-
erated from H SO electrolyte undergo intercalation at the
nomeric ions, eventually forming large (SO ) gas bubbles,
where n is in thousands.
2
4
2 n
graphite anode.[ The intercalation of these anions may also
28]
promote depolarization and the subsequent expansion of the
f) The oxidation of graphite by such ionic species leads to the
formation of oxygen functional groups on the graphite sur-
faces, which in turn lead to the evolution of large (SO ) gas
graphite anode,[
48,49]
leading to further exfoliation. Exfoliation
can also be induced by electrochemical potentials (electric field
2
n
force),[ therefore the controlled synthesis of graphene can
25]
bubbles.
[47]
also be achieved by varying the potential. Recently, Parvez
et al.[ studied the electrochemical exfoliation of graphite and
proposed a plausible exfoliation mechanism. They suggested
g) The large forces among graphite layers created by large
(SO2)n gas bubbles facilitate the efficient exfoliation of
graphite sheets into GNSs.
50]
that the reduction of intercalated SO42 ions produces SO gas,
−
h) Additionally, hydroxyl (·OH) and oxygen (·O) radicals
2
[
14]
and the evolution of gas bubbles creates large forces among
graphite layers, leading to exfoliation. However, it is not clear
generated from the anodic oxidation of water are involved
in the opening of graphite edge sheets via the oxidation of
−
how enough monomeric SO molecules are able to accumulate
edge sites and facilitate the intercalation of HSO and
2
4
HSO42 ions. Subsequently, depolarization and expansion of
−
to create large enough bubbles and in turn large forces among
the graphite layers to cause exfoliation. The formation of an
[
31]
the graphite anode occur.
2
−
SO gas bubble requires a huge number of SO ions (e.g., a
2
4
3
1
00-nm SO bubble comprises 10 monomeric SO molecules).
This exfoliation mechanism includes several hypotheses.
Experimental results reveals that aqueous Gly·HSO4 ionic
complex is an efficient exfoliating agent. The conversion
process of SO gas into Na SO by passing through aqueous
2
2
The present study proposes an electrochemical exfoliation
mechanism based on the experimental results and published
literature.[
25,27–35,47–49]
The mechanism is shown in Figure 5.
2
2
3
NaOH solutions is one of the best methods to avoid the envi-
ronmental pollution. Moreover H SO itself is one of a very
a) The aqueous solution of glycine reacts with equimolar H SO
2
4
[35]
2
4
to form the Gly·HSO ionic complex.
4
common chemicals, then it can be rather easily precipitated
as CaSO by Ca(OH) or even CaCO , which are cheaper and
4
2
3
safer chemicals. The present method is more efficient than
[
31]
our recently reported exfoliation method that uses aqueous
NaOH + H O2 because it is faster, low-cost, environmental
2
b) The Gly·HSO ionic complex facilitates the intercalation
of HSO4 and SO4 ions into graphite (anode) sheets. The
4
−
2−
friendly and a simple production process. We believe our exfo-
liation method is one of the most efficient methods available
when considering our systematic analysis and comparison
with other reported methods shown in Table S2, Supporting
Information.
electrochemical potential also helps the intercalation of these
ions, leading to exfoliation.[31]
c) The intercalated monomeric HSO4− and SO42− ions undergo
polymerization, forming dimers, trimmers, and their clusters
via repeated addition of monomeric ions.
2
.4. Reduction Catalysis
The reduction of carbonyl compounds such as esters to their
corresponding alcohols is a common reaction in organic
[
51]
chemistry
and important to the pharmaceutical industry.
[
51]
Traditional reduction methods such as catalytic hydrogena-
tion and chemical reduction often involve complex operational
procedures, long reaction times, toxic solvents/reagents, and
sensitive to processing conditions. Here, we demonstrate the
reduction of a carbonyl compound (easily detectable ethyl ben-
zoate) using aqueous NaBH4 catalyzed by gold nanocrystals
(
AuNCs) and a GNS-AuNCs hybrid (Figure 6). Aqueous NaBH
4
52]
[
has been applied in our previous work for benzaldehyde
reduction catalyzed by a nitrogen-functionalized graphene-
AuNCs hybrid.
The reaction evolution was monitored using time-dependent
ultraviolet-visible (UV–Vis) spectroscopy. It was found that
only AuNCs were catalytically inactive (Figures 6a–b), whilst
enhanced catalytic efficiency of the GNS-AuNCs hybrid was
observed (Figure 6c). The GNS-AuNCs hybrid catalysis required
only 100 seconds for completion, whereas AuNCs required
Figure 5. Schematic of proposed electrochemical exfoliation mechanism.
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Figure 6. Investigation of ethyl benzoate reduction catalysis by time-dependent UV–Vis spectra for a) blank, b) AuNCs, and c) GNS-AuNCs hybrid in
aqueous media at 25 °C. d) Structures of bioactive molecules that contain benzyl alcohol core moiety.
4
0 min. Initially the ethyl benzoate UV–Vis spectra showed
ions and the reduction of ionic species that form large (SO2)n
gas bubbles, which create large forces inside graphite layers,
leading to effective exfoliation. The present method is useful
for the development of new graphene-based materials for
future technological applications.
peaks at 230 (C = C) and 275 nm (C = O). After the reaction was
complete, these peaks disappeared and a new peak appeared
at 259 nm (C = C) corresponding to benzyl alcohol. Ethyl ben-
zoate molecules adsorbed onto the surface of GNSs and were
subsequently reduced by AuNCs catalysis, therefore, the GNS-
AuNCs hybrid shows enhanced catalytic efficiency towards
ethyl benzoate reduction. The proposed reduction mechanism
is supported by the catalysis results and a study on nitroarene
reduction catalyzed by a GO-AuNCs hybrid.[ Importantly, the
product benzyl alcohol is a core moiety in several anti-tumor
bioactive molecules (Figure 6d).[ Thus we have demonstrated
the high-performance of GNS-AuNCs hybrid as an active cata-
lyst for ethyl benzoate reduction.
4
. Experimental Section
53]
Materials: All chemicals were used as received from Sigma-Aldrich.
High-purity graphite rods (99.9995%, 6.15 mm in diameter, 152 mm
in length) were obtained from Alfa Aesar. HPLC-grade solvents were
used in this study. Aqueous solutions were purified using Milli-Q water
54]
(
>18.2 MΩ) by a Roda purification system (Te Chen Co. Ltd).
Preparation of GNSs: The Gly·HSO ionic complex was prepared
4
using a standard procedure.[ Typically, glycine (1.30 g, 17.3 mmol)
is dissolved in 75 mL of water and sulfuric acid (1.69 g, 17.3 mmol)
was slowly added, then reaction proceeds for a period of 1 h at room
temperature. Throughout the procedure the solution was kept stirring.
The reaction produces a 4:96 weight ratio of Gly·HSO /H O. To perform
35]
3
. Conclusions
In summary, an efficient in situ solution-processed aqueous
glycine-bisulfate ionic-complex-assisted electrochemical exfo-
liation method for the synthesis of GNSs was developed. The
method has major advantages over existing methods, such as
a) environmental friendliness, b) low cost, c) facile operational
procedure, d) efficient exfoliation, e) low operational voltages
4
2
the exfoliation experiments, a graphite rod as anode was placed in
parallel to a platinum sheet cathode in the above solution with a gap
of 3 cm. The applied static potentials were managed using a DC power
supply potentiostat (TES-6200, USA). The exfoliation experiments
were conducted at ambient conditions. After an appropriate time, a
black precipitate was collected through 100-nm porous filters, washed
with excess aqueous solution and ethanol, and then re-dispersed into
NMP solvent using water-bath sonication for 10 min. The resulting
graphene dispersion was directly used for further characterizations.
These exfoliation experiments can also be conducted using graphite
(+1 V for 5 min and +3 V for 5 min), f) one-pot soft processing
approach, g) formation of high-quality, few-layer GNSs with a
–
1
yield of 2.2 g L , and h) an observed highly catalytic efficiency
when employed GNS-AuNCs hybrid for benzoate reduction.
The proposed exfoliation mechanism involves the formation of
surface molecule nuclei via the polymerization of intercalated
[29]
rod anode with graphite rod or Al or Ni electrode as the cathode. The
experimental procedure of reduction catalysis is same as we reported in
the elswhere.[
52]
6
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Characterizations: The structural investigation and surface
morphology analysis of GNS15 was carried out using HR-TEM (JEOL,
JSM-2100F) with an acceleration voltage of 200 kV. The XPS analysis
was performed using PHI Quantera SXM (ULVAC Inc.) to measure
the binding energies of carbon and oxygen. A confocal Raman
spectrometer (Renishaw inVia) was used to collect Raman spectra
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peak at 520 cm was used as a reference for wavenumber calibration.
2
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−
1
FT-IR spectra in a range of 400 to 4000 cm were collected using an
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0 min). The KBr disc method was used for the preparation of FT-IR
samples. The concentration of the GNS dispersion was analyzed
as follows: 100 µL of GNS dispersion in a pre-weighted differential
scanning calorimetry (DSC) crucible was evaporated under vacuum
[
[
[
[
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