Simple preparation of high-quality graphene flakes without oxidation using potassium salts

Article abstract of DOI:10.1002/smll.201002005

Preparation of graphene flakes without oxidation is important to preserve the superb intrinsic properties of graphene. A new and simple route to produce few-layer, flake-type graphene with minimized oxidation (<2%) by the intercalation of potassium into raw graphite using potassium salts is proposed. Copyright

Full text of DOI:10.1002/smll.201002005

communications
Graphene Flakes
Simple Preparation of High-Quality Graphene Flakes
without Oxidation Using Potassium Salts
Jiyoung Kwon, Sun Hwa Lee, Kwang-Hyun Park, Dong-Hwa Seo, Jinsup Lee,
Byung-Seon Kong, Kisuk Kang,* and Seokwoo Jeon*
Graphene is a 2D sheet of sp2-hybridized carbon with inter- of Li in its crystal structure, and therefore serve as anodes with
esting properties,including exceptionally high thermal/electrical high energy density.[22,23] Alkali metal atoms (Li, Na, K, Rb, Cs)
conductivity, surface area, and mechanical strength.[1–5] Many and alkali earth metal atoms (Be, Mg, Ca, Sr, Ba, Ra) are well-
fabrication methods have been proposed to obtain graphene known intercalants of graphite. Generally, when the ionization
so as to utilize these interesting properties. The approaches to potential of intercalants is lower than the electron affinity of
fabricate graphene can be roughly categorized into two classes: graphite (4.6 eV), intercalation occurs spontaneously.[24] While
i) top-down exfoliation of multilayer graphene or graphite the intercalation kinetics is dependent on the ionic size and the
by breaking pi-bonding between carbon atoms,[6–10] and valence of the intercalants, potassium, rubidium, and cesium
ii) bottom-up formation of sp2-bonding between carbon atoms can intercalate into graphite at relatively low temperatures
in a monolayer.[11–16] Mechanical and chemical exfoliation (<300 °C) and ambient pressure.[25] When they form GICs, the
methods fall into the first category, while chemical vapor depo- interlayer distance of graphite increases above 0.5 nm (K-GICs:
sition (CVD) and epitaxial growth from silicon carbide (SiC) 0.53 nm, Rb-GICs: 0.56 nm, Cs-GICs: 0.59 nm), which is about
belong to the second. Flake-types of graphene promise poten- 1.5 times larger than that of Li-GICs and approaches the value
tial applications in the fields of transparent electrodes, energy of graphite oxide.[26] The interlayer distance of GICs can sur-
storage, electromagnetic (EM) shields, etc.[17–19] Dispersed pass 0.5 nm when co-intercalation simultaneously involves both
graphene flakes are mostly produced by a chemical exfoliation metals and organic molecules.[27] In this work, taking into con-
method, through chemical oxidation and reduction, known sideration availability and cost-effectiveness, we use K-GICs to
as Hummers’ method.[20] Hummers’ method is a low- cost produce few-layer graphene flakes.
process applicable to mass production. However, the quality
In most previous reports on alkali-metal-GICs, reac-
of the achieved graphene is often below desired levels, mainly tive alkali metal has been directly used as a source to form
due to the presence of residual oxygen, even after a sufficient GICs.[6,25] However, because alkali metals are typically
reduction process.[21] Here, we firstly introduce a new method unstable in ambient conditions and even become explosive,
to acquire high-quality graphene flakes by simply using metal the graphene fabrication process becomes more complicated,
salts without oxidation.
and safety is a greater concern. The easy surface oxidation of
Graphite intercalation compounds (GICs) are typically alkali metals also makes the control of oxygen content difficult.
formed by the insertion of atomic or molecular species, called This adds considerable complexity in the preparation of GIC
intercalants, between layers in a graphite host.The formation of and, therefore, increases the fabrication cost. Here, we show
GICs has been an active field of research especially in relation to that the use of salts is a promising alternative. Although alkali
lithium ion batteries because graphite can store a large amount metal salts have a strong catalytic influence upon the reac-
tions of carbonaceous materials with oxygen, carbon dioxide,
and water vapor at elevated temperatures,[28,29] the graphite
J. Kwon, Dr. S. H. Lee, K. H. Park, J. Lee, Prof. S. Jeon
Department of Materials Science and Engineering
Institute for Nanocentury
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon, 305–701, Republic of Korea
E-mail: jeon39@kaist.ac
source in the proposed method undergoes a mild intercala-
tion process below 500 °C. The formed graphite intercalation
compound, without further degradation or reaction, trans-
forms into graphene flakes without oxidation. The process is
fully mass producible, and the graphene flakes obtained from
the GICs have better material properties because the overall
process excludes the introduction of oxygen.
Figure 1 shows a schematic illustration of the overall
process to obtain graphene flakes in a pressured vessel (Parr
4651). First, the following chemical reaction (Equation 1)
occurs between 1,2-dichlorobenzene (1,2-DCB) and potas-
sium iodide at 300 °C in an encapsulated system for 10 h.
Prof. K. Kang
Department of Materials Science and Engineering
Institute for Eco-Energy
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon, 305–701, Republic of Korea
E-mail: matlgen1@kaist.ac.kr
Dr. D.-H. Seo, Dr. B.-S. Kong
KCC Central Research Institute
Gyunggi-Do, 446–912, Republic of Korea
2 (1,2-DCB) +2 (K+) + 2 (I−)
(1)
DOI: 10.1002/smll.201002005
→ 2 (1-iodo-2-chloro-benzene)+ 2(K) + Cl2
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Preparation of High-Quality Graphene Flakes Using Potassium Salts
KC8 + CH3 CH2OH → 8 (C) + KOCH2 CH3 +1/2(H2
(2)
)
Residual potassium metal is further removed by
sonication.[25]
Figure 2a and b show a high-resolution transmission
electron microscopy (HRTEM) image of single-layer
graphene and an image of multilayer graphene, respectively.
The number of fringes at the edge of the graphene flake is
proportional to the number of graphene layers at the given
locations. The electron contrast of few-layer graphene sig-
nificantly depends on the incidence angle, and consequently
only relatively small variations become visible in the surface
normal.[31] Figure 2a (red arrow) shows a single, observ-
able fringe, but Figure 2b (blue arrow) has multiple fringes.
Insets are electron diffraction images corresponding to each
HRTEM image.An intensity analysis of hexagonal diffraction
peaks provides information on both the number of layers and
stacking nature. The innermost diffraction spots are from the
(100) planes, while second innermost spots are from the (110)
planes. According to computational studies of the diffraction
intensity (I100 and I110) for the Bernal stacking model, the
intensity ratio I100/I110 of single layer graphene is greater
than1.[32,33] Incontrast,theratioI100/I110 ofmultilayergraphene
is smaller than 0.4.[32,33] The intensity ratio I100/I110 of the inset
of Figure 2a is 1.2 (see Figure S2 in the SI). Brighter spots
from the 100 planes relative to those from the 110 planes in
the inset of Figure 2a verify that the sample in Figure 2a com-
prises a single layer. On the contrary, the sample in Figure 2b
has multilayers, since the 110 spots are brighter than the
100 spots in the inset of Figure 2b.
In addition, the shape of the diffraction pattern from
the graphene flake, determined by TEM analysis, indi-
rectly verifies that the graphene flake is not oxidized; per-
fect hexagonal diffraction patterns are clearly shown in the
insets.[34]
Figure 3a and b are Raman mapping images showing the
number of layers achieved from the shape analysis of 2D
peaks and from the IDpeak/IGpeak value, IDpeak and IGpeak rep-
resent the intensity of the D and G peaks, respectively. Two
regions of a graphene sheet, where the size of each region is
5 μm × 3 μm, are used for the Raman mapping. The film is
prepared from a dispersed graphene solution after centrifu-
gation (8000 g, 30 min). Shape analysis of 2D peaks from
Figure 1. Schematic illustration for forming the graphene. Synthesis of
free potassium metal, formation of graphite intercalation compounds,
and exfoliation by ethanol are included. Dispersed graphene in
1,2-dichlorobenzene is filtered by vacuum filtration in order to make
flexible and transparent graphene sheets.
The formation of 1-iodo-2-chlorobenzene is confirmed by
gas chromatography/mass spectroscopy (GC/MS) (Figure S1
in the Supporting Information (SI)). In Figure S1a, a new
substance peak at 21.92 min that does not belong to 1,2-DCB
is detected. From a mass spectroscopy analysis (Figure S1b)
the substance matches well with 1-iodo-2-chlorobenzene.
The produced chlorine gas is dissolved in solution after the
reaction, and is verified by a pH analysis of the resulting
solution. The 1,2-DCB or 1-iodo-2-chloro-benzene solu-
tion is initially neutral (Table S1 in the SI). However, when
deionized (DI) water is added to the reacted solution, the
pH value decreases due to the reaction of Cl2 + H2O → HCl
+ HClO (Table S2 in the SI). Over time, Cl2 evaporates from
the solution in an open vessel, and the pH value recovers to
the original value (∼6.56) from a value of ∼3.15 within a day
(Table S3 in the SI).
On addition of graphite, the free potassium metal atoms
produced from the reaction above start to intercalate into
graphene layers and spontaneously form stage I K-GICs at
300 °C.[24,27] The partial exfoliation of graphite in 1,2-DCB
further promotes the intercalation of potassium.[30] A control
experiment without the intercalant was performed to help elu-
cidate the role and strong points of the inter-
calation for generating graphene (Table S4
in the SI). Achieving KC8 with minimum
degradation of the graphitic network is a
key step in achieving high-quality graphene
flakes.The pressured vessel is transferred to
a glove box, and the following procedures
are performed in the glove box to minimize
oxygen exposure. The generated K-GIC
solution is mixed with ethanol and finally
produces graphene flakes in a solution
from spontaneous reaction between inter-
Figure 2. TEM images and diffraction peaks of graphene flakes. a) HRTEM image of single-
layer graphene. No wrinkles are observed. b) HRTEM image of multilayer graphene. Many
wrinkles are detected. Inset: diffraction pattern by TEM. If graphene is a single layer, the
innermost diffraction spots from the (100) planes are brighter than the second innermost
spots from the (110) planes.
calated potassium and the ethanol. Ethanol
is an effective exfoliating solvent of K-GIC
and removes potassium from graphene, as
delineated in Equation 2.[25]
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J. Kwon et al.
communications
because the D peak mostly originates
from defects of graphene. Furthermore,
two main reasons that increase the D
peak intensity could be basal defects in
the sp2 network and edge defects of the
graphene flakes. The contribution from
the latter will be dominant as the flake
size becomes smaller. Considering the
small size of our flakes, the small value of
the D to G ratio may prove the low defect
density in the basal plane of graphene
flakes.[9,37] From this ratio, the prepared
graphene has significantly less defects,
thus promising better material proper-
ties, compared to flakes prepared from
the conventional oxidation method. The
above full plot of Figure 3b is used for the
analysis. It is worth noting that a smaller
number of layers is associated with a
smaller IDpeak/IGpeak ratio in general.
Although Hummers’ method is an
easy, low-cost process applicable to
mass production, the conduction prop-
erties of the achieved graphene flakes
is significantly worse than that of pris-
tine graphene because residual oxygen
atoms disrupt the graphene network
and serve as scattering centers for elec-
tronic conduction. In this sense, stable
methods for synthesizing graphene flakes
with minimum oxidation are necessary.
a
b
IDpeak / IGpeak
Number of Layers
Single layer
2 layers
N=1
N=2
N>2
G
D
D'
2600
2700
2800
1200
1400
1600
1800
Raman shift (cm-1)
Raman shift (cm-1)
Mapping
Mapping
0.7
3
2
Region 1
Region 1
0.65
1
0.6
0
3
0.55
0.5
Region 2
Region 2
2
1
0
N=1
0.45
0.4
N=2
N>2
0
1
2
3
4
5
0
1
2
3
4
5
x position (μm)
Figure 3. Raman mapping analysis in two regions of a graphene sheet centrifuged at 8000 g
for 30 min. Size of each region is 5 μm × 3 μm. Raman laser whose spot size is 1 μm × 1 μm
is moved by 500 nm. a) Analysis of the number of layers by considering the shape of the 2D
peak. b) Intensity ratio, IDpeak/IGpeak
.
Raman spectroscopy is a well-known indicator to deter- Figure 4a is a plot of wide-scanning X-ray photoelectron
mine the precise number of graphene layers.[35] To confirm spectroscopy (XPS) results. Graphene flakes prepared by
relations between the shape of 2D peak and the number our method have a high carbon peak and extremely low
of graphene layers in our graphene flakes, we performed oxygen peak (<2%) compared to those by Hummers’
simultaneously atomic force microscopy (AFM) and Raman method. A detailed analysis of the carbon peak through
spectroscopy analysis. Figure S3a in the SI shows an AFM narrow scanning and peak decompositions (Figure 4b)
topography image of prepared graphene flakes and their verifies that the final graphene flakes undergo very little
measured heights. Figure S3b (SI) is the result of Raman oxidation from the original graphite source (SP-1 graphite,
spectroscopy on the flakes. From these results, the correla- Bay Carbon) even after all the processing steps. The C–C
tion between the shape of the 2D peak and the number of bonding peak of graphene flakes (black line) remains
graphene layers produced by this method becomes clear; unchanged from that of the graphite source (red dotted
the shape of the 2D peak reveals that the flakes are bilayer line) and the peak from C–O bonding (blue dotted line)
graphene, and the measured height of ∼0.85 nm is in agree- is nearly negligible. Traceable amounts of potassium, chlo-
ment with this finding. The above graph of Figure 3a (and rine, or iodine are not found from a further analysis of
Figure S3c, SI) presents a series of representative 2D peak XPS peaks (Figure S4 in the SI).
shapes measured from the produced graphene according to
Dispersed graphene flakes can be useful for many
the number of layers. Because bilayer graphene has a much applications, especially for transparent and flexible elec-
broader and up-shifted 2D band compared to single-layer trodes.[38,39] The dispersed graphene solution produced in
graphene, the 2D peak analysis can easily distinguish single- this work can be of practical use for making large-area,
layer graphene from the bi- or few-layer graphene produced low-cost transparent electrodes through a solution process.
in this work. Figure 3a is the corresponding mapping image, However, the dissolved salts in the solution need to be puri-
illustrating the number of graphene layers at given loca- fied. To verify the possibility of application to transparent
tions. The overall amount of single- or bi-layer graphene electrodes, the solution was first filtered by an aluminum
is above 80% in the sheets. In Figure 3b, the average D to oxide membrane filter with 0.1 μm pores (Whatman) and
G intensity ratio of single-layer graphene is 0.549, which is washed three times with 1,2-DCB and ethanol. After full
about half of the value achieved from reduced graphene.[36] drying in a vacuum oven at 80 °C, the anodic aluminum
A smaller ratio of IDpeak/IGpeak in Raman spectroscopy can oxide (AAO) filter was etched by NaOH; and to remove the
be an important index to predict the quality of graphene, NaOH residue of the film, it went through dilution by DI
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Preparation of High-Quality Graphene Flakes Using Potassium Salts
a low annealing temperature is a key processing condition
for flexible electronics. As shown in Figure 5b, the film is
transparent and flexible. The successive sheet surface was
observed using scanning electron microscopy (SEM; inset
of Figure 5b). Conductivity can be calculated from sheet
resistance through a four-point probe measurement and
the thickness of the graphene sheet. The conductivity of the
transparent film is 11 033 S m−1 (Figure 5c). The best con-
ductivity of reduced graphene oxide film annealed below
150 °C is smaller than 8500 S m−1 [9,21,40] even after com-
plex reduction processes.[21] Considering the low content of
oxygen and the low D peak measured from Raman spec-
troscopy however, we believe that the conductivity of our
sample can be significantly improved with further research.
The suggested routes to achieve this are i) increasing the
average size of the graphene flakes, which reduces the
number of resistive contacts between flakes in the same con-
ducting path length;[9] ii) minimizing possible damage from
the etching process of the AAO filter; and iii) improving the
transfer process from the AAO filter to other substrates.
In conclusion, metal salts are promising intercalation
agents to obtain metal-GICs and to exfoliate graphite in a
simple, low-cost process. The proposed process produces high-
quality graphene flakes (verified by TEM, Raman, and XPS
analyses) in solvent, which is of great use in large-area appli-
cations with a solution process.The conductivity of our sample
exceeds the highest conductivity reported to date for reduced
graphene annealed below 150 °C. Since the use of KI is eco-
nomically viable and allows ease of handling, the proposed
method may become an alternative to the graphite oxidation
and reduction method. Further experimental and theoretical
work is necessary, however, to achieve optimized material
properties (i.e., larger average size of graphene flakes).
a
C
O
1000
800
600
400
200
0
Binding Energy (eV)
Graphene
Graphite Source
C-OR (286.3 eV)
b
Graphite Source (Red)
Graphene (Black)
C-OR (Blue)
280 281 282 283 284 285 286 287 288 289 290
Binding Energy (eV)
Figure 4. Oxygen analysis results by wide-scanned XPS (a) and narrow-
scanned XPS in the oxide region (b).
water. The film was transferred to a polyethyleneterephtha-
late (PET) plastic substrate after etching the AAO filter.The
film on the PET plastic substrate was annealed at 150 °C
under H2 atmosphere for 1 h. (Figure 5a) Because most
plastic films are deformed or damaged above ∼150–200 °C,
a
AAO Filter Etching
Transfer
Supporting Information
Washing
by NaOH
DI water
Injection
NaOH
Imbibition
Supporting Information is available from the
Wiley Online Library or from the author.
150oC
Annealing
NaOH Dilution
Acknowledgements
100
80
60
40
20
0
c
b
μm
1
This research was supported by WCU (World
Class University) program through the National
Research Foundation of Korea funded by the
Ministry of Education, Science and Technology
6.23k Ω/sq
810 Ω/sq
(R32–10051), the National Research Foun-
dation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (No.
2009–0083380), and Basic Science Research
Program through the National Research Foun-
dation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (CAFDC-
20100009890).
88Ω/sq,1030 nm,11,033 S/m
5 cm
400
500
600
700
Wavelength (nm)
Figure 5. a)Schematic illustration of graphene sheet forming process. b)Transferred graphene
sheet on PET film. Inset: SEM Image of the graphene sheet surface. c) Transmittance analysis
of the graphene sheet.
small 2011, 7, No. 7, 864–868
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J. Kwon et al.
communications
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Received: November 8, 2010
Revised: December 22, 2010
Published online: February 25, 2011
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