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 Department of Chemistry, Sogang University, Seoul 121-742, Korea

b 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

oC 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

oC 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 13C 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 13C 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 oC. 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 Reducing conditions Solvents useda) Ref.

Processable graphene sheets

NH2NH2

(35 wt% in H2O)NH3

(28 wt% in H2O)

heated at 95 oC for 1 hH2O

(~12,000 mL)[S6]

Solvothermal reduction

1-methyl-2-pyrrolidinone

(NMP)

refluxed at 205 oC for 24 hannealed at 250 – 1000 oC

H2O(~6,000 mL)

NMP(~6,000 mL)

[S7]

Hydrazine reduction

NH2NH2·H2O heated at 100 oC for 24 h

H2O(~7,500 mL)

CH3OH(~4,500 mL)

[S8]

Chemical conversion

NaBH4/H2SO4heated at 80 oC for 1 hannealed at 1100 oC

H2O(>3,000 mL)

[S9]

Microwave exfoliation

KOH(7 M in H2O)

annealed at 800 oC for 1 hKOH

(>150 mL)[S10]

Chemical graphitization

HICH3COOH

stored at 40 oC for 40 h

CH3COOH(~1,125 mL)

NaHCO3

(~375 mL), H2O

(~375 mL)(CH3)2CO(~150 mL)

[S11]

Graphene framework

C4H5N(pyrrole)

hydrothermally treated at 180 oC for 12 hannealed at 1050 oC for 3 h

C4H5N(~375 mL)

H2O(~7,500 mL)

[S12]

a)Volumes of solvents were estimated assuming that 3.0 g of GO was used on the basis of

corresponding procedures.

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