High Quality Thin Graphene Films from Fast. Research Center for Applied Sciences, Academia Sinica, Taipei, 11529, Taiwan

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1 Supporting Materials High Quality Thin Graphene Films from Fast Electrochemical Exfoliation Ching-Yuan Su, Ang-Yu Lu #, Yanping Xu, Fu-Rong Chen #, Andrei N. Khlobystov $ and Lain-Jong Li * Research Center for Applied Sciences, Academia Sinica, Taipei, 11529, Taiwan # Dept. of Engineering and System Science, National Tsing Hua University, 101, section 2 Kuang Fu Road, Hsinchu 300, Taiwan $ School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK. Table S1. Summary for the electrolytes tested in our exfoliation experiments Electrolyte Voltage Results ph=2 buffer solution (Sigma Aldrich) +5V No obvious exfoliation. Only ph=4 buffer solution (Sigma Aldrich) +5V No obvious exfoliation. Only PBS buffer 1:1 +5V (also try up to +30V) No obvious exfoliation. Only KOH 30% (in DI water) +5V No obvious exfoliation. Only HCl 37%(in DI water) +5V No obvious exfoliation. Only HBr(10ml HBr+5ml DI water) +10 V We can get exfoliated sheets but low yield. Raman data exhibits

2 H2SO4 (2.4g H 2 SO 4, + KOH(30%): 9/1 H2SO4+KOH(pH~8.96) H2SO4+KOH(pH~ 7.19) rgo like features (2D/G<0.3) +5V/ 5V switching We can get large amounts of exfoliated sheets +6V We can get large amounts of exfoliated sheets (1) 1V, 30 min We can get large amounts of (2) +5V, 1min exfoliated sheets (1) 1V, 30 min We can get large amounts of (2) +10V, 1min exfoliated sheets (as discussed in text) (1) 1V, 30min We can get large amounts of (2) +3V, 10min exfoliated sheets. The exfoliated sheets consist of GO and rgo like materials. (1) 1V, 30 min We can get large amounts of (2) +10V, 1min exfoliated sheets. The exfoliated sheets consist of GO and rgo like materials. (1) +2.5V, 1 min As described in text (2) Switching (+10V, 2s; 10V, 5s) (1) +25V/ 25V Obtained sheets are relatively switching thick (~ 3nm) (1) +10V/ 10V High percentage bilayered films switching are obtained but the films are not uniform. Conclusions: We observed that only the electrolytes containing H 2 SO 4 exhibit ideal exfoliation efficiency. However, the exfoliation using only H 2 SO 4 shall produce the graphene sheets with large amounts of defects as demonstrated in Figure S1. Therefore, KOH was added to lower the exfoliation rate. For the case of exfoliation using H 2 SO 4 + KOH as electrolytes, we obtained the following conclusions (1) Working Bias Dependence: If the working bias voltage is small (<10V), the exfoliation process becomes very slow and inefficient. When the bias is increased to the value larger than 10V, the exfoliation rate is fast and large graphite particles and thick ( >3nm) graphene layers are easily observed. Therefore working bias voltage is optimiszed at around 10V.

3 (2) Concentration Dependence: If the concentration of the electrolyte is changed (from ph~1.2 to ph~7.2 while the working voltage is fixed at +10/-10 V), the obtained product still exhibits highly-percentage of bilayer sheets. However, the film quality is not uniform among the sheets. Figure S1. (a) Raman spectrum (excited by 473 nm laser) for a selected graphene electrochemically exfoliated using H 2 SO 4 as the electrolyte. he measured thickness for this film is ~1.6nm. (b) The corresponding STM image on exfoliated graphene. The fuzzy portions, enclosed by solid line, show the domains of the graphene with 2- functional groups. (c) The ATR-FTIR spectrum illustrates the presence of free SO 4 (at 983 and 1002 cm -1 ), C-O-C (at 1062 and 1257 cm -1 ), C-OH (at 1368 and a broad absorption band at cm -1 ) and C=O(at 1671 cm -1 ) S1,S2. The peak at ~1600 cm -1 is from the skeletal vibrations of un-oxidized graphitic domains.

4 Figure S2. Statistical analysis for the lateral size of the graphene sheets electrochemically exfoliated from graphite as described in text. Figure S3. The high resolution-tem characterizations for exfoliated graphene: (a) the low-magnification image for graphene sheet on lacy-carbon. To identify the

5 number of graphene layers, the images are taken at the wrinkled area (as indicated with arrows). (b) A corresponding electron diffraction (ED) pattern of the exfoliated graphene. (c)-(f) The bilayer graphene taken from various exfoliated graphene sheets. The inset in (c) illustrates the lattice fringe which can be imaged by TEM. (g)-(i) The observed 3- and 4- layered graphene. Figure S4. The statistical analysis for the interlayer distance of exfoliated graphene sheets by employing a TEM imaging software ImageGauge (FUJIFILM).

6 Figure S5. (a) The low-magnification OM image of a conducting thin-film assembled from graphene sheets (b) The AFM image showing that the junctions were nicely formed between the edges of graphene sheets. Figure S6. (a) The AFM image shows that when the water/dmf volume ratio is increased to 600µL/500µL, the graphene sheets start to stack in a layer-by-layer manner, where the film thickness is ~3.81 nm) by using the interface aggregation method (b) The morphology of film made by using the drop-casting method. The film is thicker but rather un-uniform (thickness ranged from 2.13 to 9.25 nm).

7 Figure S7. (a) A roll-to-roll process for transferring our conductive film from glass substrate onto an EVA/PET substrate at 100oC. (b) The graphene thin-film was well transferred onto EVA/PET after removing the glass substrate. (c), (d) and (e) The graphene on EVA/PET exhibits excellent mechanical flexibility, optical transparency and electrical conductivity. Reference [S1] Si, Y.; Samulski, E. T., Nano Lett. 2008, 8, [S2] Guo, X.; Xiao, H. S.; Wang, F.; Zhang, Y. H., J. Phys. Chem. A 2010, 114,