Efficient Graphene Production by Combined Bipolar Electrochemistry and High-Shear Exfoliation

Transcription

1 Supporting Information Efficient Graphene Production by Combined Bipolar Electrochemistry and High-Shear Exfoliation Emil Tveden Bjerglund, Michael Ellevang Pagh Kristensen,, Samantha Stambula, Gianluigi A. Botton,,,# Steen Uttrup Pedersen,*, and Kim Daasbjerg*, Carbon Dioxide Activation Center. Department of Chemistry and Interdisciplinary Nanoscience Center (inano), Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4L7, Canada Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, L9S 4M1, Canada # Canadian Centre for Electron Microscopy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada Radisurf Aps, Arresøvej 5B, 8240 Risskov, Denmark * Corresponding authors: Steen Uttrup Pedersen (sup@chem.au.dk) and Kim Daasbjerg (kdaa@chem.au.dk) S1

2 Table of contents S1: Materials... 2 S2: Methods... 2 S3: Measuring flake sizes... 4 S4: Principal component analysis scree plot... 4 S5: EELS elemental maps... 5 S6: Determination of the number of graphene layers from Raman data... 6 S7: Calculation of the mass fraction of graphene from XPS... 7 S8: XPS N 1s high-resolution spectrum... 7 S9: Electrochemistry with graphene... 8 References... 9 S1: Materials 1-Methyl-2-pyrrolidinone (NMP; reagent grade, 99%) and graphite flakes (no , +100 mesh) were obtained from Sigma-Aldrich. The graphite foil (1 mm, 97 %) was purchased from Alfa Aesar. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was prepared by mixing a 1:1 molar ratio of sodium tetrafluoroborate (Sigma-Aldrich) and tetrabutylammonium hydrogensulfate (Fluka). The precipitate was recrystallized in ethyl acetate and pentane, and dried under vacuum at 80 C. S2: Methods Exfoliation using bipolar electrochemistry and high-shear Caution! Working with high voltages is extremely dangerous if proper precautions are not taken. All high-voltage electrochemistry in this study was performed in a grounded cage (see Figure S1 below) with an interlock system ensuring no access to the electrodes while high voltage is applied. S2

3 Figure S1. Picture of the grounded cage (right) used for high-voltage bipolar electrochemistry. The Heinzinger PNC power supply (left) can only be connected if the door is properly closed, and the high-voltage can only be applied when three interlock switches attached to the door are all activated. Photograph courtesy of Emil Tveden Bjerglund. Copyright Intercalation via bipolar electrochemistry Figure S2. Typical read-out of the voltage between FEs (top), current between FEs (middle), and the temperature of the solution (bottom) recorded while applying voltage in a bipolar electrolysis induced intercalation of Bu 4 N + in graphite flakes in NMP. The time was not recorded while system was allowed to cool down. S3

4 S3: Measuring flake sizes A total of 412 graphene flakes were measured (maximum length) as illustrated in Figure S3. Figure S3. Example of flake size measurements on graphene flakes. S4: Principal component analysis scree plot To eliminate noise in the data set principal component analysis was used. Figure S4 shows the explained variance ratio for the first ten principal components. Since the components are ordered by explained variance, keeping the first 7 components retains 97.8% of the variance in the data set. Figure S4. Scree plot showing the explained variance ratio for the first 10 principal components. S4

5 S5: EELS elemental maps Elemental maps were acquired with Gatan DigitalMicrograph by performing a power-law subtraction on the pre-edge region of the spectra. Maps were then formed using the spectrum image from the specific collection windows outlined in Figure S5. Figure S5. Elemental maps from EELS analysis. S5

6 S6: Determination of the number of graphene layers from Raman data The number of layers in the graphene was estimated using the approach devised by Paton et al. S1 A simple Raman metric,, is defined based on intensity ratios in two positions on the 2D-peak,,,,, where, is the peak position of the 2D peak in the source graphite,,, 30 cm, and denotes the spectral intensity in the specified position. The number of layers,, can be estimated by the relation:.. 10 Figure S6 displays the distribution of and for the graphene produced in this work. As seen, the majority (96%) of the flakes contains fewer than 10 layers with 4 6 layers being the most common. The estimated mean layer number = Figure S6: Histograms of (a) the calculated Raman metric, M, and the (b) estimated number of layers in the graphene sample,. S6

7 S7: Calculation of the mass fraction of graphene from XPS Figure S7 gives the structure and the masses of NMP and Bu4NBF4. Figure S7. Structure and masses of NMP and Bu 4 NBF 4. The mass fraction of graphene was calculated from the relative amounts of C (96.6%), N (1.5%), and F (0.3%). Amount of N in Bu4NBF4:. % 0.075% Amount of N in NMP: 1.5% 0.075% 1.43% Amount of C in Bu4NBF4: % 1.20% Amount of C in NMP: % 7.13% Remaining amount of C in graphene: 96.6% 1.20% 7.13% 88.27% From this the mass fraction of graphene in relation to the total amount of carbon can be calculated as: corresponding to 84.5 % S8: XPS N 1s high-resolution spectrum A high-resolution N 1s XPS spectrum was acquired. It was not possible to determine, if several nitrogen-containing compounds were present based on this, due to the low N content. Figure S8. High-resolution N 1s spectrum of graphene produced by combined bipolar electrochemical intercalation and high-shear exfoliation. S7

8 S9: Electrochemistry with graphene A graphene electrode was produced by filtering a graphene suspension in NMP on a 0.45 μm nylon membrane filter and washing with ethanol. The filter was cut into 5 mm strips. To ensure a good electrical connection with the graphene a small amount of silver paint was brushed onto one end of the electrode. The electrochemical properties were examined in an ordinary three-electrode setup using graphene as the working electrode, a Pt counter electrode, and a Ag/AgI reference electrode in 0.1 M Bu4NBF4/acetonitrile. First, voltammograms of a blank and a 2 mm ferrocene solution were recorded using a normal glassy carbon working electrode as illustrated in Figure S9 (top). In Figure S9 (bottom) the same experiments were performed, now using the graphene electrode. In this case the current grows essentially linearly with the potential and no specific oxidation peak pertaining to ferrocene is seen. Figure S9. Cyclic voltammograms recorded on a blank (red) and 2 mm ferrocene (green) solution with a glassy carbon electrode (top) and graphene electrode (bottom) using a sweep rate of 0.1 V s -1 in 0.1 M Bu 4 NBF 4 /acetonitrile. Figure S10 shows measurements of the resistance between two points on the graphene electrode at different distances, yielding an internal resistance in the film of 1.04 ± 0.19 kohm mm -1. Presumably, this high value can be attributed to a large contact resistance between the flakes. This interpretation is supported by the TEM images (see Figure 6), in that the amorphous carbon residues seen present on the graphene sheets are expected to limit the electrical contact between the sheets. Furthermore, this would explain the absence of distinct redox features in the cyclic voltammogram of ferrocene (Figure S9). The graphene films could be compressed to increase the conductivity. S2 Also, in future experiments it would be interesting to perform conductivity measurements on single flakes. S8

9 Figure S10. Electrical resistance as a function of distance for graphene deposited on a nylon membrane. References S1. Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M.; King, P.; Higgins, T.; Barwich, S.; May, P.; Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.; Long, E.; Coelho, J.; O Brien, S. E.; McGuire, E. K.; Sanchez, B. M.; Duesberg, G. S.; McEvoy, N.; Pennycook, T. J.; Downing, C.; Crossley, A.; Nicolosi, V.; Coleman, J. N. Nat. Mater. 2014, 13, S2. Huang, X.; Leng, T.; Zhang, X.; Chen, J. C.; Chang, K. H.; Geim, A. K.; Novoselov, K. S.; Hu, Z. Appl. Phys. Lett. 2015, 106, S9