Supplementary Data
Large-scale production of highly conductive reduced
graphene oxide sheets by a solvent-free low temperature
reduction
Kyu Hyung Lee a, Byeongno Lee a, Son-Jong Hwang b, Jae-Ung Lee c, Hyeonsik Cheong c,
Oh-Sun Kwon a, Kwanwoo Shin a, and Nam Hwi Hur a,*
a
b
Department of Chemistry, Sogang University, Seoul 121-742, Korea
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 92115, USA
c
Department of Physics, Sogang University, Seoul 121-742, Korea
*
Corresponding author: Tel: +82-2-705-8440; Fax: +82-2-701-0967
E-mail address: nhhur@sogang.ac.kr (N. H. Hur).
1. General Information
Materials. Graphite (<20 μm), potassium permanganate (KMnO4, ≥99.0%), heptane
(CH3(CH2)5CH3, 99%), dodecane (CH3(CH2)10CH3, ≥99.0%), chloroform (CHCl3, ≥99.0%),
dimethyl sulfoxide ((CH3)2SO, ≥99.9%), tetrahydrofuran (C4H8O, ≥99.0%), glycerol
(HOCH2CH(OH)CH2OH, ≥99.9%) and hydrazine monohydrate (NH2NH2·H2O, 98%) were
purchased from Sigma-Aldrich. Silicon oil was purchased from Shin-Etsu Chemical Co., Ltd.
Concentrated sulfuric acid (H2SO4, assay 95%) and hydrogen peroxide (H2O2, assay 35%) was
purchased from Jin Chemical Co., Ltd. All reagents were used without any further purification.
Anodized aluminum oxide membranes (AAO, 200 nm pore size, 47 mm diameter) were
purchased from Whatman. An inkjet printer was purchased from EPSON Korea Co., Ltd
(EPSON Stylus T22). A4 size papers for inkjet printing were purchased from EPSON Korea
Co., Ltd. (S042187) and Hyundai Printec Co., Ltd. (V2300). Solid hydrazine (H3N+NHCO2-)
was prepared by the literature method [S1]. Briefly, solid hydrazine was obtained from the
reaction of hydrazine hydrate with dry ice in an autoclave.
Precaution.
Solid hydrazine is very stable in a closed bottle. However, it could be harmful for health when
it exposes in air. The equipment of effective ventilation is highly recommended for handling
solid hydrazine to avoid vapor inhalation. Handle solid hydrazine inside a fume hood
whenever it is grinding.
Synthesis of graphene oxide (GO). Graphene oxide was prepared from graphite powder
(Aldrich, <20um) by the modified Hummers method [S2,S3]. Briefly, graphite powder (5.0 g)
and 130 mL of concentrated H2SO4 were added into a 1 L flask until the powder was
completely dispersed. The flask was then cooled to 0 oC using a water-ice bath. A 15.0 g of
KMnO4 powder was added to the cold reaction mixture, which was allowed to warm to room
temperature. The temperature was then raised to 35 oC and the mixture was stirred for 2 h.
The reaction mixture was cooled with an ice bath again, which was then diluted with 230 mL
of water. To the diluted mixture, about 10 mL of H2O2 was added until evolution of gas was
ceased. The mixture is allowed to settle for about 30 h. After settling, the clear supernatant
was decanted. The remaining mixture was centrifuged and washed with a diluted HCl solution
(10 % v/v) and a mixed solution containing CH3OH and water (50% v/v) several times. The
resulting graphene oxide (GO) was dried under vacuum at room temperature for 24 h,
yielding about 7.0 g of dark brown powders.
Synthesis of reduced graphene oxide (RGO) by solid state reaction of GO with solid
hydrazine. In a typical procedure, a 0.5 g of GO was mixed with 0.1 g of H3N+NHCO2-,
which was then ground using a pestle and mortar. After grinding at ambient temperature, the
ground powder was stored in a closed vessel. The ground mixture was allowed to react at 25
o
C for 24 h or 50 oC for 10 min. Approximately 0.4 g of RGO was obtained from the vessel
stored at 25 oC for 24 h while about 0.41 g of RGO was collected from the vessel stored at 50
o
C for 10 min. Yields based on GO are 80 (25 oC) and 82 % (50 oC).
Fabrication of freestanding RGO films. To prepare the GO films, the GO aqueous
dispersion (3.5 g/L) was diluted with water to adjust the GO concentration (0.035 wt%). The
GO films were prepared on a porous alumina membrane filter (200 nm pore size, 47 mm
diameter; Whatman) with vacuum filtration method [S4]. Film thickness was tuned by the
volume of the GO solution. Typically, about 10 mL of the GO solution was filtrated, followed
by drying in the oven. The GO films were reduced by the vapor of solid hydrazine at 80 oC
for 12 h in a closed bottle, which resulted in about 16 micron thick film.
Preparation of GO dispersions for inkjet printing. To obtain uniform dispersions for inkjet
printing, the GO dispersion (3.5 g/L) in water was mixed with glycerol. The optimal weight
ratio of GO to glycerol is 5:4 (25 g of GO dispersion to a 20 g of glycerol). The mixed
solution was sonicated for 1 h to obtain a very uniform GO dispersion suitable for inkjet
printing.
Ink-jet printing of GO dispersions. An EPSON Stylus T22 printer was used for dispensing
the GO ink onto paper substrates. The printer employs the so-called drop-on-demand method
in which the size and ejection of a drop is controlled by a piezoelectric transducer, compresses
the fluid contained in a micro-capillary channel, jetting a submicron-sized (minimum size: 4
pL) drop from the orifice of the nozzle in a few microseconds [S5]. The designed images
were ink-jet printed on A4 papers (EPSON, S042187) and PET films (Hyundai Printec Co.,
Ltd., V2300). Film thickness was controlled by printing repetitions. The printed films were
placed in a closed bottle containing solid hydrazine, which were annealed at 80 oC for 12 h.
Efficiency of solvent absorption by RGO. The efficiency of solvent absorption was
evaluated by weight gain using a balance, where weight gain is defined as the weight of
absorbed solvent per unit weight of RGO. Organic solvents were used for this experiment. For
instance, a 0.53 g of RGO was immersed in heptane for 5 min. The weight of RGO absorbing
heptane was 10.083 g. The weight gain for heptane was about 1,810%. Similar experiments
were performed with dodecane, chloroform, silicon oil, dimethyl sulfoxide (DMSO), and
tetrahydrofuran (THF). Their weight gains were also calculated and illustrated in Fig. S6.
Instruments. Powder X-ray diffraction patterns were recorded with a Rigaku Miniflex
diffractometer (Cu Kα) operating at 40 kV and 150 mA. Transmission electron microscope
(TEM) was carried out on a JEOL JEM-2100F. Raman spectra of powder samples were
obtained using a Jobin-Yvon Triax 550 spectrometer (1200 grooves/mm) equipped with a
liquid-nitrogen-cooled charge-coupled-device (CCD) detector. A conventional confocal
micro-Raman system was used. The 514.5 nm line of an Ar ion laser, with a total power of 1
mW, was focused onto the sample using a microscope objective (0.8 NA). The spectral
resolution was ~1.2 cm−1. High resolution scanning electron microscope (HR-SEM) analyses
were carried out using a Hitachi s-5500 microscope (Hitachi, Tokyo, Japan). X-ray
photoelectron spectra (XPS) were carried out on an AXIS-NOVA (Kratos) with
monochromatic Al Kα radiation. Thermo gravimetric analysis (TGA) was carried out using a
TGA 2050 instrument. The sample was placed on a platinum pan for each run. The data were
collected under N2 atmosphere from 25 ºC to 700 ºC at the rate of 5 ºC/min. Nicolet 205
instrument was used to measure infrared spectra. The elemental analysis was performed using
a Vario Micro Cube, in which about 2.0 mg of each sample was subjected to 1,150 oC with
surfanilic acid used as the standard. Adsorption and desorption measurements were carried
out using an ASAP 2420 instrument (Micromeritics), with nitrogen as the adsorptive, at 77 K.
The Brunauer-Emmett-Teller (BET) surface areas were calculated using p/p0 = 0.05-0.3 in
the adsorption curve using the BET equation. The pore size distributions were obtained from
the desorption curve using the density functional theory method. Prior to each sorption
measurement, the sample was out-gassed at 300 oC for 24 h under a vacuum to remove all the
impurities completely. 1H magic angle spinning (MAS) and
13
C MAS spectra with 1H
decoupling were recorded at two different fields (4.7 T and 11.7 T) equipped with a Bruker
DSX 200 console and a Bruker 7 mm CPMAS probe and a Bruker DSX 500 MHz console
and a Bruker 4 mm CPMAS probe, respectively. All data acquisition was performed at room
temperature. For single pulse experiments, a 4 microsecond 90 degree pulse was used for all
nuclei. Sample spinning rates were 12-14 kHz at the 500 MHz spectrometer while 5-6 kHz
was employed for 7 mm MAS probe. The chemical shifts were reported with respect to
external references of tetramethylsilane for both 1H and
13
C nuclei. The conductivity was
measured using a four-probe conductivity test meter (Keithely 2400) at room temperature.
Measurements were carried out with a pressed pellet, a thick freestanding film, and an ink-jet
printed paper with a rectangular shape. The topographic image was obtained using an AFM
(NT-MDT NTEGRA Spectra).
2. Supplementary Figures
Fig. S1 Time-dependent FT-IR spectra of mixed powders obtained from grinding GO and
solid hydrazine powders. After grinding, the mixed powder was stored in a vial at room
temperature without any agitation. Measurements were done after 12, 24, 36 and 48 h. In
addition, FT-IR spectra are shown in the bottom and top panels corresponding to those of GO
and graphite, respectively. They were included for comparison. Spike peaks, marked as
asterisks, at approximately 2,300 cm-1 are due to CO2 in air.
Fig. S2 FT-IR spectra of (a) graphite, (b) GO, and (c) reduced graphene oxide. Reduced
graphene oxide was prepared by storing the ground powder of GO and solid hydrazine at 50
o
C. FT-IR spectra of graphite and GO were included for comparison. Spike peaks, marked as
asterisks, at approximately 2,300 cm-1 are due to CO2 in air.
Fig. S3 C/O and C/N ratios of GO and RGO prepared by storing the ground powder of GO
and solid hydrazine at 50 oC. Square symbols are obtained from elemental analysis data of
GO and RGO.
Fig. S4 TGA data of graphite (black line), GO (red line) and RGO (blue line) prepared by
storing the ground powder of GO and solid hydrazine at 50 oC. For the measurements,
temperature was increased by 5 oC per minute from 25 to 700 oC under N2 atmosphere.
Fig. S5 Brunauer-Emmett-Teller N2 adsorption-desorption isotherms of RGO prepared by
storing the ground powder of GO and solid hydrazine at 50 oC.
Fig. S6 Cross-sectional SEM images of (a) GO layered film prepared by vacuum filtration of
GO dispersion and (b) RGO film obtained from the reduction of the GO layered film by solid
hydrazine at 80oC in a closed bottle. After reduction, volume was drastically expanded to
produce foams between the layers. (c) A photograph showing that the resulting RGO film is
easy to bend and is very flexible. In addition, the RGO film exhibits a shiny metallic luster.
Fig. S7 Powder XRD patterns of (a) GO annealed at 50 oC in the absence of solid hydrazine
and (b) RGO sheets obtained from the reduction of GO with solid hydrazine at 50 oC.
3. Supplementary Tables
Table S1. Comparison of various solvent-based methods to produce RGO materials.
Description
Reagents
Processable
graphene sheets
NH2NH2
(35 wt% in H2O)
NH3
(28 wt% in H2O)
Solvothermal
reduction
1-methyl-2pyrrolidinone
(NMP)
Solvents useda)
Ref.
H2 O
(~12,000 mL)
[S6]
refluxed at 205 C for 24 h
annealed at 250 – 1000 oC
H2 O
(~6,000 mL)
NMP
(~6,000 mL)
[S7]
[S8]
Reducing conditions
heated at 95 oC for 1 h
o
Hydrazine
reduction
NH2NH2·H2O
heated at 100 oC for 24 h
H2 O
(~7,500 mL)
CH3OH
(~4,500 mL)
Chemical
conversion
NaBH4/H2SO4
heated at 80 oC for 1 h
annealed at 1100 oC
H2 O
(>3,000 mL)
[S9]
Microwave
exfoliation
KOH
(7 M in H2O)
annealed at 800 oC for 1 h
KOH
(>150 mL)
[S10]
stored at 40 oC for 40 h
CH3COOH
(~1,125 mL)
NaHCO3
(~375 mL),
H2 O
(~375 mL)
(CH3)2CO
(~150 mL)
[S11]
hydrothermally treated at
180 oC for 12 h
annealed at 1050 oC for 3 h
C4H5N
(~375 mL)
H2 O
(~7,500 mL)
[S12]
Chemical
graphitization
Graphene
framework
HI
CH3COOH
C4H5N
(pyrrole)
a)
Volumes of solvents were estimated assuming that 3.0 g of GO was used on the basis of
corresponding procedures.
4. Supplementary References
[S1]
Lee B, Kang SH, Kang D, Lee KH, Cho J, Nam W, et al. Isolation and structural
characterization of the elusive 1:1 adduct of hydrazine and carbon dioxide. Chem
Commun 2011;47(40):11219-21.
[S2]
Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc
1958;80(6):1339.
[S3]
Park S, An H, Piner RD, Jung I, Yang D, Velamakanni A, et al. Aqueous Suspension
and Characterization of Chemically Modified Graphene Sheets. Chem Mater
2008;20(21):6592-4.
[S4]
Kong B-S, Yoo H-W, Jung H-T. Electrical conductivity of graphene films with a
poly(allylamine hydrochloride) supporting layer. Langmuir 2009;25(18):11008-13.
[S5]
Kwon O-S, Kim H, Ko H, Lee J, Lee B, Jung C-H, et al. Fabrication and
characterization of inkjet-printed carbon nanotube electrode patterns on paper.
Carbon 2013;58:116-27.
[S6]
Li D, Müller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions
of graphene nanosheets. Nat Nanotechnol 2008;3(2):101–5.
[S7]
Dubin S, Gilje S, Wang K, Tung VC, Cha K, Hall AS, et al. A one-step, solvothermal
reduction method for producing reduced graphene oxide dispersions in organic
solvents. ACS Nano 2010;4(7):3845-52
[S8]
Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al.
Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite
oxide. Carbon 2007;45(7):1558–65.
[S9]
Gao W, Alemany LB, Ci L, Ajayan PM. New insights into the structure and
reduction of graphite oxide. Nat Chem 2009;1(5):403–8.
[S10]
Zhu Y, Murali S, Stoller MD, Ganesh KJ, Cai W, Ferreira PJ, et al. Carbon-based
supercapacitors produced by activation of graphene. Science 2011;332(6037):153741.
[S11]
Moon IK, Lee J, Ruoff RS, Lee H. Reduced graphene oxide by chemical graphitization. Nat Commun 2010;1:73.
[S12]
Zhao Y, Hu C, Hu Y, Cheng H, Shi G, Qu L. A versatile, ultralight, nitrogen-doped
graphene framework. Angew Chem Int Ed 2012;51(45):11371-5.