Electrochemical Polymerization of. Functionalized Graphene Quantum Dots

Transcription

1 Electrochemical Polymerization of Functionalized Graphene Quantum Dots Mark B. Miltenburg, Tyler B. Schon, Emily L. Kynaston, Joseph G. Manion, and Dwight S. Seferos* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Corresponding Author * KEYWORDS graphene quantum dots, nanostructures, carbon materials, organic electrodes, catalyst supports, functionalized graphene Experimental

2 General Considerations All reagents were used as received unless otherwise noted. Graphite, oxalyl chloride, 4- (dimethylamino)pyridine, triethylamine, and hydroxymethyl EDOT were purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF) was purchased from Caledon Laboratories Ltd. and dried using an Innovative Technologies solvent purification system. All reactions were carried out under argon. Instrumentation Reactions were performed in a Biotage Initiator microwave synthesizer. Atomic force microscopy (AFM) imaging was carried out in tapping-mode on a Bruker Dimension Icon microscope. XPS was performed using a Thermo Scientific K-Alpha spectrometer with a monochromated Al Kα source. Absorption spectra were obtained using a Varian Cary 5000 spectrophotometer. Fluorescence spectra were collected using a Photon Technology International (PTI) QuantaMaster 40-F NA spectrofluorometer with a photomultiplier detector and a xenon arc lamp source. Raman spectroscopy was performed on a Bruker SENTERRA dispersive Raman Microscope using a 532 nm laser at 20 mw with a resolution of 3 5 cm -1. FTIR was carried out using a Perkin Elmer Spectrum 100 FTIR spectrometer equipped with a 10-bounce diamond/znse ATR accessory. All electrochemical measurements were recorded in an argon-filled glovebox (mbraun) with oxygen and moisture levels below 1.0 ppm at room temperature (23.5 C). All electrochemistry experiments were performed using a Biologic SP-200 Potentiostat/Galvanostat/FRA. Impedance spectra were measured with an excitation amplitude of 10 mv from a frequency range of 7 MHz 10 mhz. Transmission electron microscopy (TEM) imaging was performed on a LEO 912B microscope operating in bright-field mode at 120 kv. Scanning electron microscopy (SEM) was performed on a Quanta FEG 250 ESEM at 20 kv. Profilometry was performed using a KLA-Tencore P16+ profilometer and averaged over five measurements. Page S2

3 Synthesis of Graphene Quantum Dots (GQDs) The synthesis of GQDs was performed as described by Sekiya et al. 1 Graphite (7.50 g) was added to a mixture of sulfuric acid (360 ml) and nitric acid (120 ml). The mixture was sonicated for 3 h, and stirred at 120 C for 24 h. The mixture was cooled to room temperature, diluted with distilled water, neutralized with Na 2 CO 3 to ph 5-6, and filtered. The solution was then filtered through 0.22 µm PTFE filters, and dialyzed (Spectra/Por 7 2kD), for 1 week to remove excess salts. The GQDs were freeze-dried to remove water. Synthesis of GQD-EDOT GQDs (80 mg) were added to a flame-dried 5 ml microwave vial and dispersed in oxalyl chloride (3.5 ml). The mixture was sonicated for 10 minutes and dry DMF (10 µl) was added. The mixture was sonicated for 1 h. The mixture was placed in a microwave reactor at 90 C for 90 min. The oxalyl chloride was removed under vacuum. In a separate flame-dried round bottom flask, 4-dimethylaminopyridine (0.9 mg, mmol) and hydroxymethyl EDOT ( mg, 1.50 mmol) were combined. Triethylamine (0.6 ml) was added to the flask, and the reagents were dissolved in dry DMF (5 ml). The solution was transferred to the microwave vial containing the reaction mixture. The mixture was placed in a microwave reactor at 120 C for 3 h. The reaction mixture was concentrated via rotary evaporator, re-dissolved in dichloromethane, washed three times with 0.1 M HCl (aq), and three times with distilled water. The organic phase was dried with MgSO 4 and the solvent was removed via rotary evaporator, affording mg of GQD-EDOT as a dark brown solid. Electrochemical Polymerization of GQD-EDOT A solution containing GQD-EDOT (6.9 mg/ml) and tetrabutylammonium hexafluorophosphate (TBAPF 6 ) (0.1 M) in dichloromethane was placed in a custommade Teflon electrochemical cell sealed with a Viton O-ring, and cycled from 1.14 V to V vs. ferrocene/ferrocenium (Fc/Fc + ) redox couple using a three-electrode configuration. A gold-coated silicon wafer (Platypus Technologies) with a surface area of cm 2 was used as the working electrode, a platinum mesh was used as the counter electrode and a silver wire was used as a pseudoreference electrode. After 20 cycles the film was rinsed five times with dichloromethane and left in the glove-box for further electrochemical characterization. Electrochemical Polymerization of hydroxymethyl EDOT A solution containing hydroxymethyl EDOT (0.01 M) and TBAPF 6 (0.1 M) in dichloromethane was placed in a custom-made Teflon electrochemical cell sealed with a Viton O-ring, and cycled from 1.14 V to V vs. Fc/Fc + using a three-electrode configuration. A gold-coated silicon wafer (Platypus Technologies) with a surface area of cm 2 was used as the working electrode, a platinum mesh was used as the counter electrode and a silver wire was used as a pseudoreference electrode. After 20 cycles the film was rinsed five times with dichloromethane and left in the glove-box for further electrochemical characterization. Page S3

4 Fabrication of Symmetric Supercapacitors: Films of GQD/PEDOT or poly(hydroxymethyl EDOT) on Au-coated Kapton (Astral Technology Unlimited) were separated via a Celgard polypropylene separator. The films and separator were placed inside an Al-laminated polymer film (MTI Corporation) pouch. The electrodes and separator were soaked with electrolyte (1.0 M TBAPF 6 in acetonitrile), and the pouch was heat-sealed. Page S4

5 Figure S1. a) Atomic force microscopy (AFM) height image in tapping mode of graphene quantum dots on mica substrate. b) Height profiles of indicated GQDs (A- E in (a)). c) Histogram of GQD sizes. Page S5

6 Figure S2. C1s XPS spectrum of GQDs (black line). The four main components are sp 2 C=C ( ev, red line), C-OH ( ev, green line), C=O ( ev, blue line), and COOH ( ev, magenta line). The asymmetry at high binding energies is attributed to shake-up features arising from the conjugated graphene core. 2 Page S6

7 Figure S3. UV-Vis spectra of GQDs in water. Figure S4. Fluorescence emission spectra of GQDs in water. The GQDs show excitation-dependent emission. Due to low fluorescence intensity, Raman scattering peaks from the water are observed in each spectrum. Page S7

8 Figure S5. UV-Vis spectra of the films obtained via electropolymerization on an indium tin oxide (ITO) coated glass electrode. Figure S6. Raman spectra of GQD/PEDOT and poly(hydroxymethyl EDOT) films. Page S8

9 Figure S7. C1s XPS spectra of a) GQD/PEDOT and b) poly(hydroxymethyl EDOT) films. S2p XPS spectra of c) GQD/PEDOT and d) poly(hydroxymethyl EDOT) films. Page S9

10 Figure S8. Galvanostatic charge-discharge (GCD) curves of a) GQD/PEDOT film and b) poly(hydroxymethyl EDOT) film. c) Capacitance of films as a function of current density. Volumetric current densities were calculated using an electrode area of cm 2 and thicknesses obtained via profilometry (GQD/PEDOT: nm, poly(hydroxymethyl EDOT): nm). Page S10

11 Figure S9. Coulombic efficiency (left axis, square symbols) and capacitance retention (right axis, circular symbols) of poly(hydroxymethyl EDOT) and GQD/PEDOT films. Figure S10. a) Nyquist plot of films obtained at 0.75 V vs. Fc/Fc +. Inset shows the high frequency region. Solid lines are fitted data. b) Equivalent circuit used to fit the Nyquist spectra. Page S11

12 Circuit element Table S1. Parameters for equivalent circuit elements GQD/PEDOT film poly(hydroxymethyl EDOT) film R1 (Ω) CPE1-T (F) 1.08E E-03 CPE1-P R2 (Ω) CPE2-T (F) 8.84E E-04 CPE2-P R3 (Ω) Figure S11. a) GCD of symmetric poly(hydroxymethyl EDOT) supercapacitor. (b) CV of symmetric poly(hydroxymethyl EDOT) supercapacitor. REFERENCES (1) Sekiya, R.; Uemura, Y.; Murakami, H.; Haino, T. Angew. Chem. Int. Ed. 2014, 53 (22), (2) Briggs, D. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge University Press: Cambridge, Page S12