Exfoliation of Graphite Oxide in Propylene Carbonate and Thermal Reduction of the Resulting Graphene Oxide Platelets by Yanwu Zhu , Meryl D

Supporting Information

Exfoliation of graphite oxide in propylene carbonate and thermal reduction of the resulting graphene oxide platelets By Yanwu Zhu , Meryl D. Stoller , Weiwei Cai, Aruna Velamakanni , Richard D. Piner , David Chen , and Rodney S. Ruoff *

1. Effect of pH values on the dispersion of GO in PC

It was found that the pH of the graphite oxide (GO) has a significant effect on its dispersion extent in propylene carbonate (PC). During the preparation of GO powder,

10% HCl solution in water (200 ml) was used to wash the GO slurry before drying in vacuum. Thus the as-made GO dispersed in water or PC has a pH of about 3 (for 1 mg/ml) (hereafter we shall label it as pH3 GO). To tune the pH of GO, pH3 GO was re- dispersed in water by magnetic stirring (no sonication) and diluted ammonium hydroxide

(15% concentration) was added with a drop step till the pH reached 5, 7 or 10, respectively.

Then, the GO suspensions with pH 5, pH 7 and pH 10 were filtered and dried in air; thus we obtained pH5 GO, pH7 GO and pH10 GO plate-like samples on filter membranes.

The pH5 GO, pH7 GO and pH10 GO samples were subject to further drying in vacuum for two days. Following the same process described in the experimental section, 20 mg of dry pH5 GO, pH7 GO and pH10 GO were dispersed in 20 ml PC each and sonicated for

2 hours in the bath sonicator (VWR B2500A-MT). As can be seen from Fig. S1, pH10

GO has almost no dispersion in PC while pH7 GO has a small degree of dispersion degree in PC as observed by changing the color of PC to light brown. The pH5 GO showed a similar dispersion with pH3 GO at this concentration (1 mg/ml). The highest concentrations of dispersed/exfoliated pH3 GO, pH7 GO and pH10 GO in PC were estimated as 1.3, 0.05 mg/ml and 0.01 mg/ml, respectively. As a comparison, pH3 GO dispersion has a highest concentration of about 3 mg/ml.

Fig. S1 Optical images of GO with variable pH values in PC, after 2 hours of bath sonication.

2. High-resolution transmission electron microscopy of the reduced graphene oxide

Fig. S2 Typical HRTEM image of the RG-O platelets from the 150 oC treatment. From the fringes of the folded regions, we can see that this region of the sample is composed of a stack of RG-O platelets.

3. FT-IR of pure PC, GO and RG-O paper

FT-IR was done to investigate the functional groups and to detect any residual PC in the

RG-O paper obtained by heating graphene oxide suspension in PC at 150 oC followed by drying in vacuum at 80 oC for 2 days. Fig. S3 shows the spectra of pure PC, as-made GO and RG-O paper. All samples were made by mixing (for PC) or grounding (for GO and

RG-O) with KBr. Those features at around 1044, 1610, 1708 and 3400 cm -1 were observed for the GO sample, corresponding to the C-O ( v(epoxy or alkoxy)), C=C

(possibly due to skeletal vibrations of unoxidized graphitic domains), C=O in carboxylic acid and carbonyl moieties ( v(carbonyl)), and O-H stretching mode of intercalated water, respectively. 1, 2 Clearly, after thermal treatment in PC followed by drying, most of these transmission features were nearly eliminated in the spectrum of RG-O sample. Moreover, no significant PC trace was observed in the RG-O spectrum.

Fig. S3 FT-IR spectra of PC, GO and RG-O. 4. Comparison of the conductivity of paper-like samples composed of ‘RG-O’ from different fabrication/reduction methods

Table S1

Materials/reduction Method Electrical properties Reference Heating GO (50-90 oC) in alkaline conditions Unknown 3 Hydrothermal treatment of GO in supercritical Conductive (powder resistance of ~40 4 water (120-180 oC) Ω) Reduction of GO by sodium borohydride Highest conductivity of 45 S/m 5

Flash reduction of GO Conductivity of ~ 1000 S/m 2 Hydrazine reduction of GO Powder conductivity of 200 S/m 6 Hydrazine reduction of KOH-modified Conductivity of 690 S/m 7 graphene oxide Hydrazine reduction of GO in DMF/water Air drying 1700 S/m 8 mixture (HRG) Annealing at 150 oC 16000 S/m Hydrazine reduction of GO in ammonia Conductivity of ~ 7200 S/m 9 solution 10 Pre-reduction of GO by NaBH 4 --- Conductivity of 1250 S/m sulphonation --- Post reduction with hydrazine 11 Pre-redcution of GO by NaBH 4 --- Heating in Conductivity of 20200 S/m o H2SO 4 --- Post annealing at 1100 C in Ar/H 2 Rapid heating of GO up to 1050 oC Powder conductivity of 1000-2300 S/m 12 Reduced graphene oxide in PC at 200 oC ~ 1800 S/m Present work Annealing at 250 oC ~2640 S/m Reduced graphene oxide in PC at 150 oC ~ 2100 S/m Present work Annealing at 250 oC ~ 5230 S/m

5. Calculation of capacitance from CV and galvanostatic curves Capacitance values were calculated from the CV curves by dividing the current by the voltage scan rate, C = I/(dV/dt). Specific capacitance reported is the capacitance for the mass of RG-O in one electrode (specific capacitance = capacitance of single electrode / weight RG-O material of a single electrode), as per the normal convention. Capacitance as determined from galvanostatic charge/discharge was measured using C = I/(dV/dt) with dV/dt calculated from the slope of the discharge curve.

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