- Email: [email protected]
Preparation of mesoporous polyacrylonitrile and carbon fibers by electrospinning and supercritical drying
CARBON
585
6 3 (2 0 1 3) 5 6 2–59 2
also the thickness affects the intensity of the resonance, which is not fully consistent with an excitonic resonance. However, an exciton–plasmon state could be responsible for the resonances in our system [9]. Graphene sandwiched between glass and polymer has also shown to pose an excess broadband absorption at s-polarization in TIR [10]. However, further experimental and theoretical studies are needed to confirm the underlying mechanism of the observed dispersive optical resonance. In conclusion, we have observed a clearly dispersive optical resonance in reflection spectroscopy of metallic SWCNT films. It appears only for an s-polarized excitation source, and is strongest at film thicknesses around 100 nm. The dependence of intensity and dispersion of the resonance on the thickness and the surrounding environment is consistent with creation of magnetic plasmons or MPPs, and the vicinity of M11 and M22 transitions suggests that excitons may be involved in the process. This work was supported by the Academy of Finland (Project Nos. 135193 and 218182). We thank Jaakko Koivisto for acquiring Raman spectra. T.I. thanks the Finnish National Doctoral Programme in Nanoscience.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.07.018.
R E F E R E N C E S
[1] Wang F, Dukovic G, Brus LE, Heinz TF. The optical resonances in carbon nanotubes arise from excitons. Science 2005;308(5723):838–41. [2] Kramberger C, Hambach R, Giorgetti C, Ru¨mmeli MH, Knupfer M, Fink J, et al. Linear plasmon dispersion in singlewall carbon nanotubes and the collective excitation spectrum of graphene. Phys RevLett 2008;100(19):196803. [3] Perez R, Que W. Plasmons in isolated single-walled carbon nanotubes. J Phys: Condens Matter 2006;18:3197–216. [4] Lu Q, Rao R, Sadanadan B, Que W, Rao AM, Ke PC. Coupling of photon energy via a multiwalled carbon nanotube array. Appl Phys Lett 2005;87(17):173102. [5] Shuba MV, Paddubskaya AG, Plyushch AO, Kuzhir PP, Slepyan GY, Maksimenko SA, et al. Experimental evidence of localized plasmon resonance in composite materials containing single-wall carbon nanotubes. Phys Rev B 2012;85(16):165435. [6] Sarychev AK, Shvets G, Shalaev VM. Magnetic plasmon resonance. Phys Rev E 2006;73(3):036609. [7] Ruppin R. Surface polaritons of a left-handed medium. Phys Lett A 2000;277(1):61–4. [8] Li T, Wang S-M, Liu H, Li J-Q, Wang F-M, Zhu S-N, et al. Dispersion of magnetic plasmon polaritons in perforated trilayer metamaterials. J Appl Phys 2008;103(2):023104. [9] Gru¨ning M, Marini A, Gonze X. Exciton–plasmon states in nanoscale materials: breakdown of the Tamm–Dancoff approximation. Nano Lett 2009;9(8):2820–4. [10] Ye Q, Wang J, Liu Z, Deng Z-C, Kong X-T, Xing F, et al. Polarization-dependent optical absorption of graphene under total internal reflection. Appl Phys Lett 2013;102(2):021912.
Preparation of mesoporous polyacrylonitrile and carbon fibers by electrospinning and supercritical drying Dandan Sun a, Guotong Qin
a,*
, Miao Lu¨ a, Wei Wei b, Nu¨ Wang a, Lei Jiang
a
a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, 37 Xueyuan Road, Beijing 100191, China b College of Arts and Science of Beijing Union University, 197 Beituchengxi Road, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous polyacrylonitrile and carbon fibers have been prepared by electrospinning and
Received 22 March 2013
subsequent supercritical drying and carbonization. Polyvinylpyrrolidone was used as a
Accepted 4 July 2013
template. Ambient drying, oxidation, and supercritical drying were conducted to investi-
Available online 13 July 2013
gate the effects of treatment methods on the structure of the fibers. Interesting surface morphologies of the fibers, including nanoconvexities and nanorods, were found when the different drying methods were used. The surface area of the mesoporous carbon fibers was estimated as 602.0 m2 g 1, with an average pore size of 3.6 nm. 2013 Elsevier Ltd. All rights reserved.
* Corresponding author: Fax: +86 10 82338556. E-mail addresses: [email protected], [email protected] (G. Qin). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.020
586
CARBON
6 3 ( 2 0 1 3 ) 5 6 2 –5 9 2
Porous fibers can be used in thermal insulation, adsorption, filtration, and catalysis. By means of electrospinning, the composition, characteristics, and radii of such fibers can easily be controlled. There are two main routes to porous fibers. The first route involves adjustment of the electrospinning parameters, such that phase separation processes occur due to evaporation of the solvent [1] or the presence of vapor [2]. The second route relies on phase separation between two substances such that one is dispersed in the matrix of the other, and then internal pores can be created by removal of the dispersed phase [3,4]. In the present paper, we report a novel approach for preparing mesoporous polyacrylonitrile (PAN) and carbon fibers with surface nano-roughness or high surface areas by means of electrospinning and different drying methods. PAN (Mw = 150,000) and polyvinylpyrrolidone (PVP) (Mw = 1,300,000) were purchased from Sigma–Aldrich Co. (USA). PAN and PVP were dissolved in N,N-dimethylformamide to give 10 and 20 wt% solutions, respectively. Solutions for electrospinning were prepared by mixing these polymer solutions in a PAN to PVP mass ratio of 1:1. A nozzle with an inner diameter of 0.8 mm served as an electrode. Fibers
were collected on an aluminum foil, which served as the counter electrode. The nozzle and collector were placed 20 cm apart and a potential of 25 kV was applied between them. The as-obtained electrospun fibers were treated by different methods, specifically leaching, oxidation, and drying. Fig. 1 shows the micro morphologies of the PAN and carbon fibers. The as-obtained electrospun PAN/PVP fibers were dense with some tiny cracks and a diameter of about 170 nm (Fig. 1a). These PAN/PVP fibers were then leached with water to remove the PVP, washed with ethanol, and dried. The obtained fibers did not show a porous structure, but nanoconvexities were seen on their surfaces (Fig. 1b). The leaching of PVP was expected to leave pores in the PAN fibers. The nonporous nature of the obtained fibers may be attributed to pore collapsing under drying stress. To enhance the rigidity of the PAN fibers, so as to make them more resistant to the drying stress, the PAN/PVP fibers were oxidized at 250 C in a flow of air before leaching. The morphology of the oxidized PAN/ PVP fibers was seen to be almost the same as that of the asobtained electrospun fibers, that is, without pores but with some tiny cracks (Fig. 1c). After leaching, washing with ethanol, and drying, the morphology of the oxidized fibers re-
Fig. 1 – SEM images of the fibers from different methods: as electrospun and ambient drying PAN/PVP composite fibers (a), PAN fibers from ambient drying after water and alcohol leaching (b), PAN fibers from ambient drying after pre-oxidized treatment and water-alcohol leaching (c), PAN fibers from supercritical drying after pre-oxidized treatment (d), PAN fibers from directly supercritical drying (e), carbon fibers (f).
587
6 3 (2 0 1 3) 5 6 2–59 2
600
400
1.0
dv/dlog(r)(cm3g-1)
500 3 -1
mained unchanged. The non-porous structure implies that oxidation of the PAN/PVP fibers resulted in cross-linking of either PAN or PVP, such that the PVP was not leachable. The mechanical strength of the skeleton and the drying method are the main factors affecting the drying of a porous material. We employed a supercritical drying method with a view to improving the pore structure of the PAN fibers. After leaching with water and washing with ethanol, the oxidized PAN/PVP fibers were placed in an autoclave partially filled with ethanol. The autoclave was heated to 250 C and 8 MPa and maintained under these conditions for 1 h. It was then isothermally depressurized to ambient pressure and then cooled to room temperature naturally. The resultant fibers showed an interesting morphology, with nanorods grafted onto their surfaces (Fig. 1d). These nanorods were of diameter 10–20 nm. This result might be related with the solvency of supercritical ethanol, leaching cross-linked PVP. However, these PAN fibers did not show a porous structure after carbonization. The nanorods disappeared during carbonization. We also treated the as-obtained electrospun PAN/PVP fibers with supercritical ethanol without pre-oxidation or leaching. The leaching of PVP and drying of the fibers were simultaneously conducted during the supercritical treatment. The resultant PAN fibers were seen to be mesoporous (Fig. 1e). The different drying methods resulted in a series of PAN fibers are schematically showed in Fig. S1. These mesoporous PAN fibers were carbonized in an N2 atmosphere at 800 C for 2 h. The resultant fibers retained the mesoporous structure, albeit with some shrinkage (Fig. 1f). This result indicated that mesoporous PAN and carbon fibers could be directly obtained by supercritical drying. In order to investigate the effects of PVP content in the PAN/PVP precursor on the structure of the fibers, four PAN fiber samples with different mass ratios of PAN/PVP (1:1, 1:3, 1:5, 1:7) were prepared and subjected to supercritical drying. These are designated as S11, S13, S15, and S17, respectively. The PAN fibers designated as S13 were carbonized at 800 C and the product is referred to as Sc13. N2 adsorption isotherms and pore-size distributions of the PAN and carbon fibers are shown in Fig. 2. The isotherms show hysteresis loops attributable to the mesoporous structure. The carbon fiber Sc13 showed the highest adsorption amount. The poresize distributions of the various PAN fibers were mainly in the range 10–50 nm. The pore-size distribution of the carbon fiber, Sc13, was broader than that of its precursor, S13, covering the range 2–50 nm. The pore structure characteristics of the mesoporous PAN fibers are shown in Table 1. Although the PAN fibers showed a developed porous structure in scan-
Adsorption (cm g )
CARBON
S11 S13 S15 S17 Sc13
0.8 0.6 0.4 0.2 0.0
300
1
10
100
Pore Diameter (nm)
200 100 0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2 – Nitrogen adsorption isotherms and pore size distribution (inset image) of PAN and carbon fibers.
ning electron microscope (SEM) images, their surface areas were only 10–70 m2 g 1. S13 showed the highest surface area among the PAN fibers. The carbon fiber Sc13 (obtained from S13) showed a surface area of 602 m2 g 1. The nano domains of PAN and PVP formed in the fibers formed because of incompatibility of PAN and PVP during electrospinning induced by evaporation of solvent. Leaching the PVP and keeping the skeleton structure is important to form porous fibers. Supercritical drying is an efficient method to avoid the drying stress. PVP also affects the final structure of fibers. When PAN/PVP is less than 1/3 the pores are not developed enough (Fig. S2a). When PAN/PVP is more than 1/3 the partial PAN skeletons combine together, forming thick skeleton and reducing pores (Fig. S2c, d). Mesoporous PAN and carbon fibers have been prepared by electrospinning and supercritical drying, with PVP as a template. The PAN/PVP mass ratio and drying method are the main factors affecting the pore structure and morphology of the fibers. Well developed mesoporous PAN and carbon fibers could be obtained by directly supercritical drying of PAN/PVP fibers without leaching.
Acknowledgements Project Supported by the National Basic Research Program of China (2010CB934700, 2012CB933201) and NSFC (51272014).
Table 1 – Surface and pore parameters of PAN and carbon fibers. Fibers
Surface area (m2 g 1)
Pore volume (cm3 g 1)
Average pore diameter (nm)
S11 S13 S15 S17 Sc13
36.6 70.5 10.5 19.3 602.0
0.10 0.12 0.04 0.04 0.54
11.0 6.5 15.7 9.2 3.6
588
CARBON
6 3 ( 2 0 1 3 ) 5 6 2 –5 9 2
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.07.020.
R E F E R E N C E S
[2] Casper CL, Stephens JS, Tassi NG, Chase DB, Rabolt JF. Controlling surface morphology of electrospun polystyrene fibers: effect of humidity and molecular weight in the electrospinning process. Macromolecules 2004;37(2):573–8. [3] Li X, Nie G. Nano-porous ultra-high specific surface ultrafine fibers. Chin Sci Bull 2004;49(22):2368–71. [4] Zhang Z, Li X, Wang C, Fu S, Liu Y, Shao C. Polyacrylonitrile and carbon nanofibers with controllable nanoporous structures by electrospinning. Macromol Mater Eng 2009;294(10):673–8.
[1] Bognitzki M, Czado W, Frese T, Schaper A, Hellwig M, Steinhart M, et al. Nanostructured fibers via electrospinning. Adv Mater 2001;13(1):70–2.
Enhanced electron transfer in composite films of reduced graphene oxide and poly(N-methylaniline) Tom Lindfors
a,*
, Anna O¨sterholm
a,1
, Jussi Kauppila b, Ro´bert E. Gyurcsa´nyi
c
˚ bo Akademi University, Process Chemistry Centre, Department of Chemical Engineering, Laboratory of Analytical Chemistry, A FI-20500 Turku, Finland b University of Turku, Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials Chemistry and Chemical Analysis, FI-20014 Turku, Finland c Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry Research Group of Technical Analytical Chemistry, MTA-BME ‘‘Lendu¨let’’ Chemical Nanosensors Research Group, H-1111 Budapest, Szt. Gelle´rt te´r 4, Hungary a
A R T I C L E I N F O
A B S T R A C T
Article history:
Poly(N-methylaniline) (PNMA) has been electropolymerized for the first time from a graph-
Received 18 April 2013
ene oxide (GO) dispersion containing 1.0 M HClO4. Both GO and perchlorate were incorpo-
Accepted 4 July 2013
rated in the PNMA matrix during the electropolymerization resulting in the formation of a
Available online 13 July 2013
mixed composite material of PNMA-ClO4 and PNMA-GO. Under the acidic polymerization conditions, the carboxylic groups of GO are undissociated and GO is therefore mostly mechanically entrapped in the PNMA matrix while perchlorate functions as the primary charge compensating ion. Electrochemical reduction at
0.85 V improved the electron
transfer of the composite film due to reduction of GO in the PNMA matrix. 2013 Elsevier Ltd. All rights reserved.
Polyaniline (PANI) is one of the most studied electrically conducting polymers (ECP) due to its good environmental stability, easy and cost-effective synthesis, and high electrical conductivity at acidic pH [1]. However, PANI is usually deprotonated at slightly acidic and neutral pH resulting in the loss of the conductivity [2]. This property usually excludes the use of PANI in applications operating at physiological pH. Replacing PANI in such applications with poly(N-alkylanilines) (PNANI) which maintain their electroactivity even at neutral pH has the potential to overcome this problem [3]. Despite
of their low pH sensitivity [2,4], the PNANIs have been studied to much lesser extent than PANI owing to their considerably lower electrical conductivity (10 1 to 10 6 S cm 1) [3,5] compared to PANI (1–10 S cm 1). The disadvantage of the lower conductivity can be in principle solved by the incorporation of reduced graphene oxide (rGO) having the electrical conductivity of 8.5 · 101 S cm 1 [6] or graphene (theoretically ca. 106 S cm 1) into the PNANI matrix. This is expected to improve the overall electrical conductivity and mechanical properties of the PNANI films. We have recently reported a simple elec-
* Corresponding author. E-mail address: [email protected] (T. Lindfors). 1
Current address: Georgia Institute of Technology, School of Chemistry and Biochemistry, Atlanta, GA, USA. 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.022
585
6 3 (2 0 1 3) 5 6 2–59 2
also the thickness affects the intensity of the resonance, which is not fully consistent with an excitonic resonance. However, an exciton–plasmon state could be responsible for the resonances in our system [9]. Graphene sandwiched between glass and polymer has also shown to pose an excess broadband absorption at s-polarization in TIR [10]. However, further experimental and theoretical studies are needed to confirm the underlying mechanism of the observed dispersive optical resonance. In conclusion, we have observed a clearly dispersive optical resonance in reflection spectroscopy of metallic SWCNT films. It appears only for an s-polarized excitation source, and is strongest at film thicknesses around 100 nm. The dependence of intensity and dispersion of the resonance on the thickness and the surrounding environment is consistent with creation of magnetic plasmons or MPPs, and the vicinity of M11 and M22 transitions suggests that excitons may be involved in the process. This work was supported by the Academy of Finland (Project Nos. 135193 and 218182). We thank Jaakko Koivisto for acquiring Raman spectra. T.I. thanks the Finnish National Doctoral Programme in Nanoscience.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.07.018.
R E F E R E N C E S
[1] Wang F, Dukovic G, Brus LE, Heinz TF. The optical resonances in carbon nanotubes arise from excitons. Science 2005;308(5723):838–41. [2] Kramberger C, Hambach R, Giorgetti C, Ru¨mmeli MH, Knupfer M, Fink J, et al. Linear plasmon dispersion in singlewall carbon nanotubes and the collective excitation spectrum of graphene. Phys RevLett 2008;100(19):196803. [3] Perez R, Que W. Plasmons in isolated single-walled carbon nanotubes. J Phys: Condens Matter 2006;18:3197–216. [4] Lu Q, Rao R, Sadanadan B, Que W, Rao AM, Ke PC. Coupling of photon energy via a multiwalled carbon nanotube array. Appl Phys Lett 2005;87(17):173102. [5] Shuba MV, Paddubskaya AG, Plyushch AO, Kuzhir PP, Slepyan GY, Maksimenko SA, et al. Experimental evidence of localized plasmon resonance in composite materials containing single-wall carbon nanotubes. Phys Rev B 2012;85(16):165435. [6] Sarychev AK, Shvets G, Shalaev VM. Magnetic plasmon resonance. Phys Rev E 2006;73(3):036609. [7] Ruppin R. Surface polaritons of a left-handed medium. Phys Lett A 2000;277(1):61–4. [8] Li T, Wang S-M, Liu H, Li J-Q, Wang F-M, Zhu S-N, et al. Dispersion of magnetic plasmon polaritons in perforated trilayer metamaterials. J Appl Phys 2008;103(2):023104. [9] Gru¨ning M, Marini A, Gonze X. Exciton–plasmon states in nanoscale materials: breakdown of the Tamm–Dancoff approximation. Nano Lett 2009;9(8):2820–4. [10] Ye Q, Wang J, Liu Z, Deng Z-C, Kong X-T, Xing F, et al. Polarization-dependent optical absorption of graphene under total internal reflection. Appl Phys Lett 2013;102(2):021912.
Preparation of mesoporous polyacrylonitrile and carbon fibers by electrospinning and supercritical drying Dandan Sun a, Guotong Qin
a,*
, Miao Lu¨ a, Wei Wei b, Nu¨ Wang a, Lei Jiang
a
a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, 37 Xueyuan Road, Beijing 100191, China b College of Arts and Science of Beijing Union University, 197 Beituchengxi Road, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous polyacrylonitrile and carbon fibers have been prepared by electrospinning and
Received 22 March 2013
subsequent supercritical drying and carbonization. Polyvinylpyrrolidone was used as a
Accepted 4 July 2013
template. Ambient drying, oxidation, and supercritical drying were conducted to investi-
Available online 13 July 2013
gate the effects of treatment methods on the structure of the fibers. Interesting surface morphologies of the fibers, including nanoconvexities and nanorods, were found when the different drying methods were used. The surface area of the mesoporous carbon fibers was estimated as 602.0 m2 g 1, with an average pore size of 3.6 nm. 2013 Elsevier Ltd. All rights reserved.
* Corresponding author: Fax: +86 10 82338556. E-mail addresses: [email protected], [email protected] (G. Qin). 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.020
586
CARBON
6 3 ( 2 0 1 3 ) 5 6 2 –5 9 2
Porous fibers can be used in thermal insulation, adsorption, filtration, and catalysis. By means of electrospinning, the composition, characteristics, and radii of such fibers can easily be controlled. There are two main routes to porous fibers. The first route involves adjustment of the electrospinning parameters, such that phase separation processes occur due to evaporation of the solvent [1] or the presence of vapor [2]. The second route relies on phase separation between two substances such that one is dispersed in the matrix of the other, and then internal pores can be created by removal of the dispersed phase [3,4]. In the present paper, we report a novel approach for preparing mesoporous polyacrylonitrile (PAN) and carbon fibers with surface nano-roughness or high surface areas by means of electrospinning and different drying methods. PAN (Mw = 150,000) and polyvinylpyrrolidone (PVP) (Mw = 1,300,000) were purchased from Sigma–Aldrich Co. (USA). PAN and PVP were dissolved in N,N-dimethylformamide to give 10 and 20 wt% solutions, respectively. Solutions for electrospinning were prepared by mixing these polymer solutions in a PAN to PVP mass ratio of 1:1. A nozzle with an inner diameter of 0.8 mm served as an electrode. Fibers
were collected on an aluminum foil, which served as the counter electrode. The nozzle and collector were placed 20 cm apart and a potential of 25 kV was applied between them. The as-obtained electrospun fibers were treated by different methods, specifically leaching, oxidation, and drying. Fig. 1 shows the micro morphologies of the PAN and carbon fibers. The as-obtained electrospun PAN/PVP fibers were dense with some tiny cracks and a diameter of about 170 nm (Fig. 1a). These PAN/PVP fibers were then leached with water to remove the PVP, washed with ethanol, and dried. The obtained fibers did not show a porous structure, but nanoconvexities were seen on their surfaces (Fig. 1b). The leaching of PVP was expected to leave pores in the PAN fibers. The nonporous nature of the obtained fibers may be attributed to pore collapsing under drying stress. To enhance the rigidity of the PAN fibers, so as to make them more resistant to the drying stress, the PAN/PVP fibers were oxidized at 250 C in a flow of air before leaching. The morphology of the oxidized PAN/ PVP fibers was seen to be almost the same as that of the asobtained electrospun fibers, that is, without pores but with some tiny cracks (Fig. 1c). After leaching, washing with ethanol, and drying, the morphology of the oxidized fibers re-
Fig. 1 – SEM images of the fibers from different methods: as electrospun and ambient drying PAN/PVP composite fibers (a), PAN fibers from ambient drying after water and alcohol leaching (b), PAN fibers from ambient drying after pre-oxidized treatment and water-alcohol leaching (c), PAN fibers from supercritical drying after pre-oxidized treatment (d), PAN fibers from directly supercritical drying (e), carbon fibers (f).
587
6 3 (2 0 1 3) 5 6 2–59 2
600
400
1.0
dv/dlog(r)(cm3g-1)
500 3 -1
mained unchanged. The non-porous structure implies that oxidation of the PAN/PVP fibers resulted in cross-linking of either PAN or PVP, such that the PVP was not leachable. The mechanical strength of the skeleton and the drying method are the main factors affecting the drying of a porous material. We employed a supercritical drying method with a view to improving the pore structure of the PAN fibers. After leaching with water and washing with ethanol, the oxidized PAN/PVP fibers were placed in an autoclave partially filled with ethanol. The autoclave was heated to 250 C and 8 MPa and maintained under these conditions for 1 h. It was then isothermally depressurized to ambient pressure and then cooled to room temperature naturally. The resultant fibers showed an interesting morphology, with nanorods grafted onto their surfaces (Fig. 1d). These nanorods were of diameter 10–20 nm. This result might be related with the solvency of supercritical ethanol, leaching cross-linked PVP. However, these PAN fibers did not show a porous structure after carbonization. The nanorods disappeared during carbonization. We also treated the as-obtained electrospun PAN/PVP fibers with supercritical ethanol without pre-oxidation or leaching. The leaching of PVP and drying of the fibers were simultaneously conducted during the supercritical treatment. The resultant PAN fibers were seen to be mesoporous (Fig. 1e). The different drying methods resulted in a series of PAN fibers are schematically showed in Fig. S1. These mesoporous PAN fibers were carbonized in an N2 atmosphere at 800 C for 2 h. The resultant fibers retained the mesoporous structure, albeit with some shrinkage (Fig. 1f). This result indicated that mesoporous PAN and carbon fibers could be directly obtained by supercritical drying. In order to investigate the effects of PVP content in the PAN/PVP precursor on the structure of the fibers, four PAN fiber samples with different mass ratios of PAN/PVP (1:1, 1:3, 1:5, 1:7) were prepared and subjected to supercritical drying. These are designated as S11, S13, S15, and S17, respectively. The PAN fibers designated as S13 were carbonized at 800 C and the product is referred to as Sc13. N2 adsorption isotherms and pore-size distributions of the PAN and carbon fibers are shown in Fig. 2. The isotherms show hysteresis loops attributable to the mesoporous structure. The carbon fiber Sc13 showed the highest adsorption amount. The poresize distributions of the various PAN fibers were mainly in the range 10–50 nm. The pore-size distribution of the carbon fiber, Sc13, was broader than that of its precursor, S13, covering the range 2–50 nm. The pore structure characteristics of the mesoporous PAN fibers are shown in Table 1. Although the PAN fibers showed a developed porous structure in scan-
Adsorption (cm g )
CARBON
S11 S13 S15 S17 Sc13
0.8 0.6 0.4 0.2 0.0
300
1
10
100
Pore Diameter (nm)
200 100 0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2 – Nitrogen adsorption isotherms and pore size distribution (inset image) of PAN and carbon fibers.
ning electron microscope (SEM) images, their surface areas were only 10–70 m2 g 1. S13 showed the highest surface area among the PAN fibers. The carbon fiber Sc13 (obtained from S13) showed a surface area of 602 m2 g 1. The nano domains of PAN and PVP formed in the fibers formed because of incompatibility of PAN and PVP during electrospinning induced by evaporation of solvent. Leaching the PVP and keeping the skeleton structure is important to form porous fibers. Supercritical drying is an efficient method to avoid the drying stress. PVP also affects the final structure of fibers. When PAN/PVP is less than 1/3 the pores are not developed enough (Fig. S2a). When PAN/PVP is more than 1/3 the partial PAN skeletons combine together, forming thick skeleton and reducing pores (Fig. S2c, d). Mesoporous PAN and carbon fibers have been prepared by electrospinning and supercritical drying, with PVP as a template. The PAN/PVP mass ratio and drying method are the main factors affecting the pore structure and morphology of the fibers. Well developed mesoporous PAN and carbon fibers could be obtained by directly supercritical drying of PAN/PVP fibers without leaching.
Acknowledgements Project Supported by the National Basic Research Program of China (2010CB934700, 2012CB933201) and NSFC (51272014).
Table 1 – Surface and pore parameters of PAN and carbon fibers. Fibers
Surface area (m2 g 1)
Pore volume (cm3 g 1)
Average pore diameter (nm)
S11 S13 S15 S17 Sc13
36.6 70.5 10.5 19.3 602.0
0.10 0.12 0.04 0.04 0.54
11.0 6.5 15.7 9.2 3.6
588
CARBON
6 3 ( 2 0 1 3 ) 5 6 2 –5 9 2
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.07.020.
R E F E R E N C E S
[2] Casper CL, Stephens JS, Tassi NG, Chase DB, Rabolt JF. Controlling surface morphology of electrospun polystyrene fibers: effect of humidity and molecular weight in the electrospinning process. Macromolecules 2004;37(2):573–8. [3] Li X, Nie G. Nano-porous ultra-high specific surface ultrafine fibers. Chin Sci Bull 2004;49(22):2368–71. [4] Zhang Z, Li X, Wang C, Fu S, Liu Y, Shao C. Polyacrylonitrile and carbon nanofibers with controllable nanoporous structures by electrospinning. Macromol Mater Eng 2009;294(10):673–8.
[1] Bognitzki M, Czado W, Frese T, Schaper A, Hellwig M, Steinhart M, et al. Nanostructured fibers via electrospinning. Adv Mater 2001;13(1):70–2.
Enhanced electron transfer in composite films of reduced graphene oxide and poly(N-methylaniline) Tom Lindfors
a,*
, Anna O¨sterholm
a,1
, Jussi Kauppila b, Ro´bert E. Gyurcsa´nyi
c
˚ bo Akademi University, Process Chemistry Centre, Department of Chemical Engineering, Laboratory of Analytical Chemistry, A FI-20500 Turku, Finland b University of Turku, Turku University Centre for Materials and Surfaces (MATSURF), Laboratory of Materials Chemistry and Chemical Analysis, FI-20014 Turku, Finland c Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry Research Group of Technical Analytical Chemistry, MTA-BME ‘‘Lendu¨let’’ Chemical Nanosensors Research Group, H-1111 Budapest, Szt. Gelle´rt te´r 4, Hungary a
A R T I C L E I N F O
A B S T R A C T
Article history:
Poly(N-methylaniline) (PNMA) has been electropolymerized for the first time from a graph-
Received 18 April 2013
ene oxide (GO) dispersion containing 1.0 M HClO4. Both GO and perchlorate were incorpo-
Accepted 4 July 2013
rated in the PNMA matrix during the electropolymerization resulting in the formation of a
Available online 13 July 2013
mixed composite material of PNMA-ClO4 and PNMA-GO. Under the acidic polymerization conditions, the carboxylic groups of GO are undissociated and GO is therefore mostly mechanically entrapped in the PNMA matrix while perchlorate functions as the primary charge compensating ion. Electrochemical reduction at
0.85 V improved the electron
transfer of the composite film due to reduction of GO in the PNMA matrix. 2013 Elsevier Ltd. All rights reserved.
Polyaniline (PANI) is one of the most studied electrically conducting polymers (ECP) due to its good environmental stability, easy and cost-effective synthesis, and high electrical conductivity at acidic pH [1]. However, PANI is usually deprotonated at slightly acidic and neutral pH resulting in the loss of the conductivity [2]. This property usually excludes the use of PANI in applications operating at physiological pH. Replacing PANI in such applications with poly(N-alkylanilines) (PNANI) which maintain their electroactivity even at neutral pH has the potential to overcome this problem [3]. Despite
of their low pH sensitivity [2,4], the PNANIs have been studied to much lesser extent than PANI owing to their considerably lower electrical conductivity (10 1 to 10 6 S cm 1) [3,5] compared to PANI (1–10 S cm 1). The disadvantage of the lower conductivity can be in principle solved by the incorporation of reduced graphene oxide (rGO) having the electrical conductivity of 8.5 · 101 S cm 1 [6] or graphene (theoretically ca. 106 S cm 1) into the PNANI matrix. This is expected to improve the overall electrical conductivity and mechanical properties of the PNANI films. We have recently reported a simple elec-
* Corresponding author. E-mail address: [email protected] (T. Lindfors). 1
Current address: Georgia Institute of Technology, School of Chemistry and Biochemistry, Atlanta, GA, USA. 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.022