Supporting Information Self-assembled graphene/carbon nanotube ...
Supporting Information Self-assembled graphene/carbon nanotube hybrid films for supercapacitors Dingshan Yu and Liming Dai*
Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
* To whom correspondence should be addressed. Tel: 1-216-3684151
E-mail: [email protected]
Experimental Details Preparation of poly(ethyleneimine)-modified graphene nanosheets: Graphene oxide (GO) was prepared by acid oxidation of natural graphite according to a Hummers method.1 The as-prepared GO sheets were then dispersed in water to form a stable dispersion (0.25 mg/mL) and purified by ultrasonication in an ultrasonic bath (VWR model 75D) for 30 min. The resultant homogeneous GO dispersion (10 mL) was mixed with 10.0 mL of 4 mg/mL poly(ethyleneimine) (PEI) aqueous solution, and the mixed solution was then stirred at 60 °C for 12 h. After cooled to room temperature, to the resulting dispersion was added 25 µL of hydrazine (hydrazine monohydrate, 98%) solution. After being vigorously shaken or stirred for a few minutes, the vial was put in an oil bath (95 °C) for 12 h. Excess polymer and hydrazine hydrate were removed by repeated centrifugation at 20000g (15000 rpm, 20 min) and washing cycles, followed by
redispersion of the PEI-stabilized graphene sheets in de-ionized (DI) water.
Preparation of surface-functionalized MWNTs: MWNTs were obtained from Helix Materials Solution, Inc. (purity >95%, length 0.5-40 µm, diameter < 20 nm). MWNTs were refluxed in concentrated H2SO4/HNO3 (3/1 v/v) at 70 °C for 1 h to produce carboxylic acid functionalized MWNTs (MWNT-COOH), and then filtrated through nylon membrane filter (0.2 µm) and washed with DI water several times. Finally, MWNT-COOH powder was obtained by drying the collected materials on the membrane at 50 °C in vacuum for 24 hrs.
Self-assembly of graphene/MWNT hybrid films: The silicon wafer and ITO glasses were used as substrates to grow hybrid films by sequential self-assembling. Prior to use, the substrates were cleaned in piranha solution (a 1:3 mixture of 30% H2O2 and concentrated H2SO4 (Caution: piranha solution is exothermic and strongly reacts with organics and should be handled with extreme care!), rinsed with DI water, sonicated for 15 min and again thoroughly rinsed with DI water. The pHs of the PEI modified graphene (PEI-GN, 0.25 mg/mL) and acid-treated MWNT (MWNT-COOH, 0.5 mg/mL) solutions were adjusted to 8 and 6.5, respectively; and the resulting solutions were sonicated briefly prior to assembly. Substrates were first dipped into a PEI-GN solution for 30 min, then washed by DI water three times and dried in N2 flow. The substrates were then dipped in a MWNT-COOH solution for 30 min, followed by washing with DI water three times and drying in N2 flow. This cycle makes one bilayer of PEI-GN and MWNT-COOH, denoted [PEI-GN/MWNT-COOH]1. The cycle was repeated to reach other desired bilayer-number for the [PEI-GN/MWNT-COOH]n hybrid films.
Characterization:
The microstructure and surface morphology of the samples were investigated using a scanning electron microscope (Hitachi S-4800, SEM) operating at 5.0 kV. The surface topology was examined using an AFM microscope (Micro 40, Pacific Technology) in the tapping mode in the air. The surface chemistry was analyzed using a VG Micro Tech ESCA 2000 X-ray photoelectron spectrometer (XPS). Surface composition of the samples (atom %) was also determined by XPS. For the XPS measurements, a GO (0.25 mg/mL, pH = 6.5) and PEI-GN (0.25 mg/ml, pH = 8) solution was deposited onto silicon substrates to form films, respectively. Fourier transform infrared (FTIR) spectra were recorded for vacuum-dried GO and PEI-GN powders on a Perkin Elmer FTIR spectrometer (Spectrum ONE). Optical absorption spectra were recorded on a Perkin Elmer Lambda 900 Uv-vis-NIR spectrophotometer. Raman spectra were taken on a Renishaw micro-Raman setup by using an Ar laser at 514.5 nm. A three-electrode cell was employed for electrochemical measurements, where a saturated Ag/AgCl electrode and Pt wire were used as the reference and counter electrode, respectively. [PEI-GN/MWNT-COOH] n hybrid films on ITO coated glass slides were used as the working electrode. Cyclic voltammetry was performed between -0.4 and 0.6 V at room temperature.
Figure S1. (a) Raman spectra of GO and PEI-GN excited by 514.5 nm and (b) FTIR spectra of GO and PEI-GN.
Figure S1a shows the Raman spectra of GO and PEI-GN, both exhibit two peaks at 1348 cm-1 and 1592 cm-1 corresponding to the D and G bands, respectively. It is well known that G band is assigned to the first-order scattering of the E2g mode observed for sp2 carbon domains and the pronounced D band is associated with disordered structural defects (e.g., amorphous carbon or edges that can break the symmetry and selection rule).2 Upon the hydrazine reduction and PEI absorption, the D/G intensity ratio increases from 0.82 to 0.95. Although an increase in the D/G ratio has been reported for reduced GO due to a decrease in the average size of the sp2 domains,3 the above observed increase in the D/G ratio could be at least partially attributed to the formation of the sp3 carbon after polymer functionalization.4 FTIR spectra given in Figure S1b shows that the characteristic band of the carboxyl group in GO appears at 1730 cm-1 (C=O stretching vibration),5 along with the C–O vibrations of epoxy groups at 1058 and 857 cm-1.5 After the PEI-GN adsorption, a doublet at 2847 and 2921 cm-1 corresponding to symmetric νs (CH2) and asymmetric νas (CH2) of the PEI chains appeared.6 Moreover, the N-H bending vibration were detected at 1630 and 1557 cm-1, 7
along with the C-N stretching mode at 1223 cm-1,6 confirming, once again, the successful
attachment of PEI chains onto the graphene surface.
Figure S2. (a) UV-vis spectra of LBL assembled (PEI-GN/MWNT-COOH)n on an ITO glass, the inset showing the absorbance at 600 nm versus the number. (b) Optical images of the multilayer
films on Si wafer and ITO glasses.
The as-obtained films were characterized by UV-vis spectroscopy. Figure S2a shows an increase in optical absorbance upon assembly of [PEI-GN/MWNT-COOH]n in each bilayer on the glass slide, while Figure S2b shows the dependence of the absorption (at wavelength of 600 nm) on the bilayer number for the multilayer films. The clear increase in the absorbance with the assembly cycle is a clear indication of the film deposition on the substrate. In addition, the linear relationship between the absorbance and the bilayer number (the inset) further suggests that a controllable amount of graphene and MWNTs was loaded in each deposition cycle. Figure S2b shows representative optical images of the multilayer films on Si wafer and ITO glasses where each film has a characteristic color corresponding to its thickness. The film darkened with increasing thickness and eventually appeared black. As for the 15-bilayer hybrid film, the thickness has reached about 350 nm. Additionally, the electrical conductivity for these hybrid films with different bilayers (1∼15) has been measured to be in the range of 1∼10 S/cm.
REFERENCES (1) Hummers, W. S.; Offman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (2) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (3) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499–3503.
(4) Shen, J.; Hu, Y.; Li, C.; Qin, C.; Shi, M.; Ye, M. Layer-by-Layer Self-Assembly of Multiwalled
Carbon Nanotube Polyelectrolytes Prepared by in Situ Radical Polymerization. Langmuir 2009, 25, 6122–6128. (5) Yang, H.; Li, F.; Shan, C.; Han, D.; Zhang, Q.; Niu L.; Ivaska A. Covalent Functionalization of Chemically Converted Graphene Sheets via Silane and its Reinforcement. J. Mater. Chem. 2009, 19, 4632–4638. (6) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 17, 1290–1295. (7) Liu, Y.; Gao, L. In situ Coating Multiwalled Carbon Nanotubes with CdS Nanoparticles. Mater. Chem. Phys. 2005, 91, 365-369.
Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
* To whom correspondence should be addressed. Tel: 1-216-3684151
E-mail: [email protected]
Experimental Details Preparation of poly(ethyleneimine)-modified graphene nanosheets: Graphene oxide (GO) was prepared by acid oxidation of natural graphite according to a Hummers method.1 The as-prepared GO sheets were then dispersed in water to form a stable dispersion (0.25 mg/mL) and purified by ultrasonication in an ultrasonic bath (VWR model 75D) for 30 min. The resultant homogeneous GO dispersion (10 mL) was mixed with 10.0 mL of 4 mg/mL poly(ethyleneimine) (PEI) aqueous solution, and the mixed solution was then stirred at 60 °C for 12 h. After cooled to room temperature, to the resulting dispersion was added 25 µL of hydrazine (hydrazine monohydrate, 98%) solution. After being vigorously shaken or stirred for a few minutes, the vial was put in an oil bath (95 °C) for 12 h. Excess polymer and hydrazine hydrate were removed by repeated centrifugation at 20000g (15000 rpm, 20 min) and washing cycles, followed by
redispersion of the PEI-stabilized graphene sheets in de-ionized (DI) water.
Preparation of surface-functionalized MWNTs: MWNTs were obtained from Helix Materials Solution, Inc. (purity >95%, length 0.5-40 µm, diameter < 20 nm). MWNTs were refluxed in concentrated H2SO4/HNO3 (3/1 v/v) at 70 °C for 1 h to produce carboxylic acid functionalized MWNTs (MWNT-COOH), and then filtrated through nylon membrane filter (0.2 µm) and washed with DI water several times. Finally, MWNT-COOH powder was obtained by drying the collected materials on the membrane at 50 °C in vacuum for 24 hrs.
Self-assembly of graphene/MWNT hybrid films: The silicon wafer and ITO glasses were used as substrates to grow hybrid films by sequential self-assembling. Prior to use, the substrates were cleaned in piranha solution (a 1:3 mixture of 30% H2O2 and concentrated H2SO4 (Caution: piranha solution is exothermic and strongly reacts with organics and should be handled with extreme care!), rinsed with DI water, sonicated for 15 min and again thoroughly rinsed with DI water. The pHs of the PEI modified graphene (PEI-GN, 0.25 mg/mL) and acid-treated MWNT (MWNT-COOH, 0.5 mg/mL) solutions were adjusted to 8 and 6.5, respectively; and the resulting solutions were sonicated briefly prior to assembly. Substrates were first dipped into a PEI-GN solution for 30 min, then washed by DI water three times and dried in N2 flow. The substrates were then dipped in a MWNT-COOH solution for 30 min, followed by washing with DI water three times and drying in N2 flow. This cycle makes one bilayer of PEI-GN and MWNT-COOH, denoted [PEI-GN/MWNT-COOH]1. The cycle was repeated to reach other desired bilayer-number for the [PEI-GN/MWNT-COOH]n hybrid films.
Characterization:
The microstructure and surface morphology of the samples were investigated using a scanning electron microscope (Hitachi S-4800, SEM) operating at 5.0 kV. The surface topology was examined using an AFM microscope (Micro 40, Pacific Technology) in the tapping mode in the air. The surface chemistry was analyzed using a VG Micro Tech ESCA 2000 X-ray photoelectron spectrometer (XPS). Surface composition of the samples (atom %) was also determined by XPS. For the XPS measurements, a GO (0.25 mg/mL, pH = 6.5) and PEI-GN (0.25 mg/ml, pH = 8) solution was deposited onto silicon substrates to form films, respectively. Fourier transform infrared (FTIR) spectra were recorded for vacuum-dried GO and PEI-GN powders on a Perkin Elmer FTIR spectrometer (Spectrum ONE). Optical absorption spectra were recorded on a Perkin Elmer Lambda 900 Uv-vis-NIR spectrophotometer. Raman spectra were taken on a Renishaw micro-Raman setup by using an Ar laser at 514.5 nm. A three-electrode cell was employed for electrochemical measurements, where a saturated Ag/AgCl electrode and Pt wire were used as the reference and counter electrode, respectively. [PEI-GN/MWNT-COOH] n hybrid films on ITO coated glass slides were used as the working electrode. Cyclic voltammetry was performed between -0.4 and 0.6 V at room temperature.
Figure S1. (a) Raman spectra of GO and PEI-GN excited by 514.5 nm and (b) FTIR spectra of GO and PEI-GN.
Figure S1a shows the Raman spectra of GO and PEI-GN, both exhibit two peaks at 1348 cm-1 and 1592 cm-1 corresponding to the D and G bands, respectively. It is well known that G band is assigned to the first-order scattering of the E2g mode observed for sp2 carbon domains and the pronounced D band is associated with disordered structural defects (e.g., amorphous carbon or edges that can break the symmetry and selection rule).2 Upon the hydrazine reduction and PEI absorption, the D/G intensity ratio increases from 0.82 to 0.95. Although an increase in the D/G ratio has been reported for reduced GO due to a decrease in the average size of the sp2 domains,3 the above observed increase in the D/G ratio could be at least partially attributed to the formation of the sp3 carbon after polymer functionalization.4 FTIR spectra given in Figure S1b shows that the characteristic band of the carboxyl group in GO appears at 1730 cm-1 (C=O stretching vibration),5 along with the C–O vibrations of epoxy groups at 1058 and 857 cm-1.5 After the PEI-GN adsorption, a doublet at 2847 and 2921 cm-1 corresponding to symmetric νs (CH2) and asymmetric νas (CH2) of the PEI chains appeared.6 Moreover, the N-H bending vibration were detected at 1630 and 1557 cm-1, 7
along with the C-N stretching mode at 1223 cm-1,6 confirming, once again, the successful
attachment of PEI chains onto the graphene surface.
Figure S2. (a) UV-vis spectra of LBL assembled (PEI-GN/MWNT-COOH)n on an ITO glass, the inset showing the absorbance at 600 nm versus the number. (b) Optical images of the multilayer
films on Si wafer and ITO glasses.
The as-obtained films were characterized by UV-vis spectroscopy. Figure S2a shows an increase in optical absorbance upon assembly of [PEI-GN/MWNT-COOH]n in each bilayer on the glass slide, while Figure S2b shows the dependence of the absorption (at wavelength of 600 nm) on the bilayer number for the multilayer films. The clear increase in the absorbance with the assembly cycle is a clear indication of the film deposition on the substrate. In addition, the linear relationship between the absorbance and the bilayer number (the inset) further suggests that a controllable amount of graphene and MWNTs was loaded in each deposition cycle. Figure S2b shows representative optical images of the multilayer films on Si wafer and ITO glasses where each film has a characteristic color corresponding to its thickness. The film darkened with increasing thickness and eventually appeared black. As for the 15-bilayer hybrid film, the thickness has reached about 350 nm. Additionally, the electrical conductivity for these hybrid films with different bilayers (1∼15) has been measured to be in the range of 1∼10 S/cm.
REFERENCES (1) Hummers, W. S.; Offman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (2) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (3) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499–3503.
(4) Shen, J.; Hu, Y.; Li, C.; Qin, C.; Shi, M.; Ye, M. Layer-by-Layer Self-Assembly of Multiwalled
Carbon Nanotube Polyelectrolytes Prepared by in Situ Radical Polymerization. Langmuir 2009, 25, 6122–6128. (5) Yang, H.; Li, F.; Shan, C.; Han, D.; Zhang, Q.; Niu L.; Ivaska A. Covalent Functionalization of Chemically Converted Graphene Sheets via Silane and its Reinforcement. J. Mater. Chem. 2009, 19, 4632–4638. (6) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 17, 1290–1295. (7) Liu, Y.; Gao, L. In situ Coating Multiwalled Carbon Nanotubes with CdS Nanoparticles. Mater. Chem. Phys. 2005, 91, 365-369.
