Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pka Measurements

Transcription

1 Understanding Aqueous Dispersibility of Graphene xide and Reduced Graphene xide through pka Measurements Bharathi Konkena and Sukumaran Vasudevan * Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore , INDIA Supporting Information CNTENTS: S1. Preparation of Graphene xide and reduced Graphene xide S2. X-ray Photoelectron Spectra of Graphene xide and reduced Graphene xide S3. 13 C CP-MAS NMR spectra of Graphene xide S4. Raman Spectra of Graphene xide and reduced Graphene xide S5. ph- Titration method S6. Scheme for the ionization of carboxylic groups on Graphene xide S7. Scheme for ketone formation on ionization of hydroxyl groups on Graphene xide

2 S1. Preparation of Graphene oxide and reduced Graphene oxide Graphene oxide (G) was prepared from graphite by a modified Hummers method. 19,20 In a typical reaction, 0.25 g of graphite(alfa-aesar) and 46 ml of H 2 S 4 were stirred in an ice bath followed by slow addition of 6g of KMn 4. The solution was transferred to a water bath at 37 o C, diluted, and 30% H 2 2 added with stirring at which time the solution turns from dark brown to yellow. The mixture was allowed to settle and the precipitate washed till free from sulfate, re-dispersed and then centrifuged. The process was repeated till the dispersion was neutral. The preparation of reduced graphene oxide (r-g) typically involved dispersing 200mg of G in 200mL of water by sonicating followed by addition of 1mL of Hydrazine hydrate ( 32.1 mmol) and refluxing in an oil bath at 95 o C for 24 hours at which time the solution turns black, and the reduced G gradually precipitates as a black solid. The precipitate was removed by centrifuging and washed with distilled water. The G and r-g samples were characterized by X-Ray Photoelectron, Raman and 13 C CP- MAS NMR spectroscopy. Raman spectra recorded on a Horiba Jobin-Yvon Raman Microscope using a 514 nm excitation. The 13 C CP-MAS NMR spectra of G was recorded on a Bruker AV 500S 500 MHz High Resolution Multinuclear FT-NMR Spectrometer at a spinning speed of 9.4 KHz. The zeta potential of the G and r-g dispersions, at different values of ph, were determined using a Zetasizer Nanoseries-ZEN 3690 (Malvern) instruments. IR spectra were recorded on a Perkin Elmer FT-IR spectrometer in the DRIFT mode using KBr as a reference. For recording spectra at different values of ph the graphene samples were filtered from the titration experiment (described below) at select values of ph and dried under vacuum.

3 S2. X-ray Photoelectron Spectra of Graphene xide and reduced Graphene xide counts (a) H-C= C C= C=C Binding energy / ev counts (b) 285.6eV C- C-N -C= C= 286.5eV 289.1eV 287.8eV 284.5eV C=C Binding Energy / ev Figure S1. Core level C 1s XPS spectra of (a) Graphene oxide, (b) reduced Graphene oxide X-ray Photoelectron Spectroscopy was employed to analyze the G and r-g. Spectra were recorded on a Thermo Fisher Scientific Multilab 2000 Spectrometer using an Mg Kα source. The C 1s XPS spectrum of graphene oxide (Figure S1a) indicates considerable degree of oxidation with deconvolution indicating four components that correspond to carbon atoms in different functional groups. The non-oxygenated ring C = C appears at ev, the C in C bonds at ev, the carbonyl C (C=) at ev and the carboxylate carbon (H C=) at, ev. The spectra are in good agreement with the literature. S1 The C1s XPS spectrum of the rg (Figure S1b) exhibits these same oxygen functionalities as G but the peak intensities are much weaker than those in G except for the peak at ev corresponding to the C=C bond that is strongly enhanced indicating the restoration of sp 2 domains. The spectra of r-g shows an additional component at ev corresponding to the C=N bonds of hydrazones. S2

4 S3. 13 C CP-MAS NMR spectra of Graphene xide Intensity =C- 172 sp C- 101 C--C 59.7 C-H 69.8 CH δ (ppm) Figure S2. Solid state 13 C CP-MAS NMR spectra of G The 13 C MAS NMR spectrum of G (Figure S2) shows a number of resonances corresponding to different oxygen functionalities. The peaks at 59.7 and 69.8 ppm represent the 13 C nuclei in the epoxide and hydroxyl groups, respectively.the peak at ppm belongs to the unoxidized sp 2 carbons of the graphene network and the peak at 172 ppm arises from carboxyl groups.

5 S4. Raman Spectra of Graphene xide and reduced Graphene xide (a) 1578cm -1 G band Intensity 1350 cm -1 D band Graphite 1595cm -1 G band G rg Raman shift / cm -1 Figure S3. Raman spectra of Graphite, G, rg The Raman spectrum of the pristine graphite shows the G band at 1578cm -1 corresponding to the first-order scattering of the E 2g mode. In the Raman spectrum of G, the G band is broadened and shifted to 1595 cm -1. In addition, the D band at 1350 cm -1 becomes prominent, indicating the reduction in size of the in-plane sp 2 domains due to oxidation. The Raman spectrum of the r- G also shows both G and D bands but with an increased D/G intensity ratio as compared to G indicating the restoration of sp 2 domains on reduction with hydrazine.

6 S5. ph- Titration method. S3 ph Blank G (a) ph 12 (b) 10 8 Blank rg Amount of HCl/ ml Amount of HCl/ ml Figure S4. The ph titration curves (red) of (a) G and (b). r-g. The black curve is the blank titration. is the difference in the volume of HCl added in the two titrations at the same value of ph. The concentration of ionized groups present on G and r-g sheets at different values of ph were determined using a ph titration. In this experiment 0.1g of G was taken in a beaker containing 20 ml of 0.1 M NaH solution and 0.1mol HCl solution added in incremental steps of 0.25ml. At each step the ph of the solution was recorded after ensuring that equilibrium had been attained (Figure S4 a; red curve). The experiment was repeated with the same volume of NaH but now without addition of G (Figure S4 a; black curve). The difference in the volumes of HCl in the two titration curves for the same value of ph, indicated as in the figure, gives the concentration of the ionized groups per gm of G at that ph. A similar titration ( Figure S4 b) was performed using r-g to determine the concentration of ionized groups on r-g as function of ph.

7 S6. Scheme for the ionization of carboxylic groups on Graphene xide H H H ph 4.3 ph 6.6 H - H + - H + H H S7. Scheme for ketone formation on ionization of hydroxyl groups on Graphene xide H ph H +

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