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Aqueous-dispersible fullerol-carbon nanotube hybrids
Materials Letters 82 (2012) 48–50
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Aqueous-dispersible fullerol-carbon nanotube hybrids Athanasios B. Bourlinos a, b, Vasilios Georgakilas c, Aristides Bakandritsos d, Antonios Kouloumpis e, Dimitrios Gournis e, Radek Zboril b,⁎ a
Physics Department, University of Ioannina, 45110 Ioannina, Greece Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University in Olomouc, 77146, Czech Republic Institute of Materials Science, NCSR Demokritos, Ag. Paraskevi Attikis, 15310 Athens, Greece d Department of Materials Science, University of Patras, 26504 Rio, Patras, Greece e Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece b c
a r t i c l e
i n f o
Article history: Received 9 February 2012 Accepted 10 May 2012 Available online 22 May 2012 Keywords: AFM Aqueous colloids Carbon materials Carbon nanotubes Fullerols Hybrid materials
a b s t r a c t Fullerol-carbon nanotube hybrids have been fabricated through wet impregnation of oxidized multi-wall carbon nanotubes with fullerols and subsequent solid-state heating at 200 °C to promote interfacial bonding. Because fullerols attach to the walls as polyanions providing static repulsion, the hybrid displays high aqueous solubility. The decoration of the nanotubes with fullerols was established by infrared spectroscopy and atomic force microscopy, whereas its impact on the surface properties was studied by means of zeta-potential measurements and dynamic light scattering. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The dispersion of carbon nanotubes in a solvent is critical in applications pertaining to polymer composites, nanoparticle hybrids and biomedicine [1–3]. Generally, good dispersions can be obtained after surface modification of the nanotubes. This results in derivatives with long term colloidal stability, improved surface compatibility and exceeding solubility. Typical modifiers include organic reagents, polyaromatics, surfactants or polymers. Recently, a new class of functionalized carbon nanotubes has been introduced as an alternative to the molecularly modified adducts. These derivatives are obtained from the attachment of charged clusters on the walls of the nanotubes and may include polyoxometalate clusters or alkaline-treated silica nanoparticles [4,5]. Such ionic species provide electrostatic repulsion between decorated nanotubes and thus high dispersability in water, the latter being an ideal solvent for biomedical and environmental studies. In addition, they offer ion-exchange sites for further modification in a simple way. Fullerol [C60(OH)n] is a polyhydroxy-fullerene that holds great promise in biological applications and water treatment [6,7]. The hydroxyl groups on the surface can be neutralized by NaOH to give water-soluble clusters having a negative surface charge (−O −Na +). Such polyanions seem an elegant choice for the modification of
⁎ Corresponding author. Tel.: + 420 58 5634947; fax: + 420 585634958. E-mail address: [email protected] (R. Zboril). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.026
carbon nanotubes as above suggested. Herein we describe the synthesis and characterization of aqueous-dispersible fullerol-carbon nanotube hybrids derived from the individual counterparts using wet chemistry and solid-state heating. The decoration of the nanotubes by fullerols was established with infrared (FT-IR) spectroscopy and atomic force microscopy (AFM). The effect of the decoration on the surface properties of the nanotubes was studied by zeta-potential (ζp) and dynamic light scattering (DLS) measurements. 2. Materials and methods Multi-wall carbon nanotubes (CNTs, Aldrich) were first oxidized with concentrated HNO3 as follows: 200 mg CNTs were suspended in 50 mL HNO3 (67%) and the mixture was refluxed for 24 h. The oxidized CNTs were isolated by centrifugation and washing with deionized water prior to drying. In the next step, 20 mg oxidized CNTs were suspended by sonication in 2 mL de-ionized water containing dissolved 20 mg fullerols, sodium salt C60(OH)30·25Na·30H2O (Aldrich). Water was evaporated at 80 °C and the remaining solid was heated at 200 °C for 24 h. The product was collected by sonication using 15–20 mL de-ionized water. The as-obtained colloid was filtered off to remove un-dissolved particles and the filtrate was further centrifuged at 5000 rpm for 10 min. FT-IR spectra of samples in powder form, dispersed in KBr pellets, were measured with a Perkin-Elmer 210 GX, Fourier transform spectrometer. AFM images were obtained in tapping mode with a Multimode Nanoscope 3D using RTESP n-doped silicon cantilevels. For the
A.B. Bourlinos et al. / Materials Letters 82 (2012) 48–50
49
Fig. 1. Synthesis of the fullerol-CNTs hybrid. The inset photo shows from left-to-right: a concentrate aqueous dispersion of fullerol-CNTs hybrid, the corresponding dilute dispersion and a fullerol aqueous solution.
AFM measurements, isolated fullerol-CNTs deposited on a Si-wafer were obtained by a modified technique which combines LangmuirSchaefer (LS) deposition and self-assembly [8]. In a KSV-2000 Lagmuir-Blodgett trough, Millipore Q-grade water was used as subphase and an octadecylammonium solution (0.2 mg mL − 1) dissolved in chloroform-methanol (9:1) was spread onto the aqueous subphase.
Fig. 3. AFM images of the fullerol-CNTs hybrid.
Absorbance
1800
1600
1400
1200
1000
800
Fullerol-CNT s
Fullerol
The hydrophobic Si-wafer was then dipped horizontally at a constant surface pressure of 20 mN m − 1. After the LS deposition, the surface of the film was rinsed with pure water and dipped into an aqueous solution of fullerol-CNTs (0.2 mg mL− 1). Finally, the surface was rinsed copiously with pure water and dried with a flow of N2 gas. DLS and electrokinetic measurements for the determination of the hydrodynamic diameter and zeta-potential respectively of the dispersions were performed on a Malvern Instrument ZetaSizer Nano. 3. Results and discussion
2200
2000
1800
1600
1400
1200 -1
1000
800
Wavenumbers (cm ) Fig. 2. FT-IR spectra of fullerol and fullerol-CNTs hybrid.
600
The synthesis of the hybrid (Fig. 1) involves three steps: i) oxidation of multi-wall CNTs with concentrated HNO3, ii) wet impregnation of the oxidized CNTs with fullerol, iii) solid-state heating of the impregnated solid at 200 °C. Extraction of the product with water under sonication followed by filtration and centrifugation affords a
50
A.B. Bourlinos et al. / Materials Letters 82 (2012) 48–50
-100
Number (%)
Total Counts
Fullerol
-80
-60
-40
-20
0
35
Fullerol, D h=~3.5 nm
30
CNTs Fullerol, D h =~68 nm CNTs, D h=~90 nm
25 20 15
20 10
CNTs
Total Counts
5 0 1
10
100
1000
Hydrodynamic Diameter (nm) Fig. 5. DLS profiles of the aqueous dispersions of the materials.
-100
-80
-60
-20
0
20
-20
0
20
CNTs
Total Counts
Fullerol
-40
In contrast, oxidized CNTs without fullerols appear with lower ζp of about −26 mV (Fig. 4). Based on DLS measurements, the diameter of the dispersed particles is 3.5 nm for fullerols, 68 nm for fullerol-CNTs and 90 nm for oxidized CNTs (Fig. 5). In case of fullerol, the size of 3.5 nm is higher than that theoretically expected (ca. 1 nm) due to the ability of the hydroxylated clusters to form hydrogen bonds with each other in solution [11]. Most importantly, the small size of fullerolCNTs compared to oxidized CNTs indicates stronger electrostatic repulsion between fullerol-coated CNTs. 4. Conclusions
-100
-80
-60
-40
Zeta potential (mV) Fig. 4. Zeta-potential graphs of the aqueous dispersions of the materials.
clear black colloid with remarkable stability (inset photo). Note also the striking color difference between the dispersed hybrid and watersoluble fullerol (brown solution). In the first step, chemical oxidation with HNO3 creates -COOH and -OH surface groups [9] that increase the compatibility of carbon nanotubes with fullerols and provide reactive sites for its binding. In the second step, impregnation of the oxidized CNTs with fullerols leads to uniformly deposited clusters on the carbon surface. In the third step, solid-state heating at 200 °C promotes an interfacial bonding between fullerols and oxidized CNTs through ester, ether and hydrogen bonding formation. Ester and ether bridges have been previously proposed in carbon nanobuds (fullerene-CNTs hybrids) [10]. In the present case, such bonds are likely the result of simple esterification/etherfication reactions. On the other hand, hydrogen bonding is expected on account of the oxygen-rich surface groups. The presence of fullerols in the hybrid was evidenced by the characteristic FT-IR peaks of the cluster (Fig. 2). Accordingly, the hybrid shows absorptions between 1800 and 800 cm− 1 as follows: 1600 and 1380 cm− 1 ascribed to C C bonds in fullerol and 1100 cm− 1 attributed to the C―O stretching in C―OH [11]. Decoration of the nanotubes with fullerols was revealed by AFM (Fig. 3). Isolated CNTs covered with ballshaped objects are clearly visible in the images. The average height of these objects was within 0.8–1.1 nm, as derived from topographical height profile, which corresponds well to the diameter of fullerol [12]. The attached fullerol alters the surface properties of the hybrid rendering it ionic and highly hydrophilic. Characteristically, both fullerols and fullerol-CNTs display practically the same mean ζp of −40 mV (Fig. 4). Such value manifests a great colloidal stability in water and highly negative surface charge, indicative of the successful coating of the CNTs with fullerols. Similar effect has been observed for nanocarbon aqueous dispersions stabilized by anionic surfactants [13].
Fullerol-carbon nanotube hybrids have been prepared and characterized for the first time. The hybrid is composed of fullerol clusters attached on the surface of the nanotubes through covalent and hydrogen bonding. The robust anions alter significantly the surface properties of the support conferring ionic character and aqueous solubility. The material may find uses in medicine as a new nanopharmaceutic, in water treatment for heavy metal removal or in polymer composites as reinforcing filler. Acknowledgements The work was supported by the Operational Program Research and Development for Innovations-European Social Fund (CZ.1.05/ 2.1.00/03.0058) and the projects of Grant Agency of the Academy of Sciences of the Czech Republic (KAN115600801, KAN200380801 and P208/10/1742). References [1] Coleman JN, Khan U, Blau WJ, Gun'ko YK. Carbon 2006;44:1624–52. [2] Georgakilas V, Gournis D, Tzitzios V, Pasquato L, Guldi DM, Prato M. J Mater Chem 2007;17:2679–94. [3] Bianco A, Kostarelos K, Partidos CD, Prato M. Chem Commun 2005:571–7. [4] Fei B, Lu H, Hu Z, Xin JH. Nanotechnology 2006;17:1589–93. [5] Bourlinos AB, Georgakilas V, Zboril R, Dallas P. Carbon 2007;45:2136–9. [6] Djordjevic A, Bogdanovic G. Arch Oncol 2008;16:42–5. [7] Anderson R, Barron AR. J Am Chem Soc 2005;127:10458–9. [8] Gengler RYN, Gournis D, Aimon AH, Toma LM, Rudolf P. Chem Eur J 2012;18: 7594–600. [9] Datsyuk V, Kalyva M, Papagelis K, Parthenios J, Tasis D, Siokou A, et al. Carbon 2008;46:833–40. [10] Nasibulin AG, Pikhitsa PV, Jiang H, Brown DP, Krasheninnikov AV, Anisimov AS, et al. Nat Nanotechnol 2007;2:156–61. [11] Vileno B, Marcoux PR, Lekka M, Sienkiewicz A, Fehér T, Forró L. Adv Funct Mater 2006;16:120–8. [12] Assemi S, Tadjiki S, Donose BC, Nguyen AV, Miller JD. Langmuir 2010;26: 16063–70. [13] Smith RJ, Lotya M, Coleman JN. New J Phys 2010;12(125008) 11 pp.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Aqueous-dispersible fullerol-carbon nanotube hybrids Athanasios B. Bourlinos a, b, Vasilios Georgakilas c, Aristides Bakandritsos d, Antonios Kouloumpis e, Dimitrios Gournis e, Radek Zboril b,⁎ a
Physics Department, University of Ioannina, 45110 Ioannina, Greece Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University in Olomouc, 77146, Czech Republic Institute of Materials Science, NCSR Demokritos, Ag. Paraskevi Attikis, 15310 Athens, Greece d Department of Materials Science, University of Patras, 26504 Rio, Patras, Greece e Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece b c
a r t i c l e
i n f o
Article history: Received 9 February 2012 Accepted 10 May 2012 Available online 22 May 2012 Keywords: AFM Aqueous colloids Carbon materials Carbon nanotubes Fullerols Hybrid materials
a b s t r a c t Fullerol-carbon nanotube hybrids have been fabricated through wet impregnation of oxidized multi-wall carbon nanotubes with fullerols and subsequent solid-state heating at 200 °C to promote interfacial bonding. Because fullerols attach to the walls as polyanions providing static repulsion, the hybrid displays high aqueous solubility. The decoration of the nanotubes with fullerols was established by infrared spectroscopy and atomic force microscopy, whereas its impact on the surface properties was studied by means of zeta-potential measurements and dynamic light scattering. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The dispersion of carbon nanotubes in a solvent is critical in applications pertaining to polymer composites, nanoparticle hybrids and biomedicine [1–3]. Generally, good dispersions can be obtained after surface modification of the nanotubes. This results in derivatives with long term colloidal stability, improved surface compatibility and exceeding solubility. Typical modifiers include organic reagents, polyaromatics, surfactants or polymers. Recently, a new class of functionalized carbon nanotubes has been introduced as an alternative to the molecularly modified adducts. These derivatives are obtained from the attachment of charged clusters on the walls of the nanotubes and may include polyoxometalate clusters or alkaline-treated silica nanoparticles [4,5]. Such ionic species provide electrostatic repulsion between decorated nanotubes and thus high dispersability in water, the latter being an ideal solvent for biomedical and environmental studies. In addition, they offer ion-exchange sites for further modification in a simple way. Fullerol [C60(OH)n] is a polyhydroxy-fullerene that holds great promise in biological applications and water treatment [6,7]. The hydroxyl groups on the surface can be neutralized by NaOH to give water-soluble clusters having a negative surface charge (−O −Na +). Such polyanions seem an elegant choice for the modification of
⁎ Corresponding author. Tel.: + 420 58 5634947; fax: + 420 585634958. E-mail address: [email protected] (R. Zboril). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.026
carbon nanotubes as above suggested. Herein we describe the synthesis and characterization of aqueous-dispersible fullerol-carbon nanotube hybrids derived from the individual counterparts using wet chemistry and solid-state heating. The decoration of the nanotubes by fullerols was established with infrared (FT-IR) spectroscopy and atomic force microscopy (AFM). The effect of the decoration on the surface properties of the nanotubes was studied by zeta-potential (ζp) and dynamic light scattering (DLS) measurements. 2. Materials and methods Multi-wall carbon nanotubes (CNTs, Aldrich) were first oxidized with concentrated HNO3 as follows: 200 mg CNTs were suspended in 50 mL HNO3 (67%) and the mixture was refluxed for 24 h. The oxidized CNTs were isolated by centrifugation and washing with deionized water prior to drying. In the next step, 20 mg oxidized CNTs were suspended by sonication in 2 mL de-ionized water containing dissolved 20 mg fullerols, sodium salt C60(OH)30·25Na·30H2O (Aldrich). Water was evaporated at 80 °C and the remaining solid was heated at 200 °C for 24 h. The product was collected by sonication using 15–20 mL de-ionized water. The as-obtained colloid was filtered off to remove un-dissolved particles and the filtrate was further centrifuged at 5000 rpm for 10 min. FT-IR spectra of samples in powder form, dispersed in KBr pellets, were measured with a Perkin-Elmer 210 GX, Fourier transform spectrometer. AFM images were obtained in tapping mode with a Multimode Nanoscope 3D using RTESP n-doped silicon cantilevels. For the
A.B. Bourlinos et al. / Materials Letters 82 (2012) 48–50
49
Fig. 1. Synthesis of the fullerol-CNTs hybrid. The inset photo shows from left-to-right: a concentrate aqueous dispersion of fullerol-CNTs hybrid, the corresponding dilute dispersion and a fullerol aqueous solution.
AFM measurements, isolated fullerol-CNTs deposited on a Si-wafer were obtained by a modified technique which combines LangmuirSchaefer (LS) deposition and self-assembly [8]. In a KSV-2000 Lagmuir-Blodgett trough, Millipore Q-grade water was used as subphase and an octadecylammonium solution (0.2 mg mL − 1) dissolved in chloroform-methanol (9:1) was spread onto the aqueous subphase.
Fig. 3. AFM images of the fullerol-CNTs hybrid.
Absorbance
1800
1600
1400
1200
1000
800
Fullerol-CNT s
Fullerol
The hydrophobic Si-wafer was then dipped horizontally at a constant surface pressure of 20 mN m − 1. After the LS deposition, the surface of the film was rinsed with pure water and dipped into an aqueous solution of fullerol-CNTs (0.2 mg mL− 1). Finally, the surface was rinsed copiously with pure water and dried with a flow of N2 gas. DLS and electrokinetic measurements for the determination of the hydrodynamic diameter and zeta-potential respectively of the dispersions were performed on a Malvern Instrument ZetaSizer Nano. 3. Results and discussion
2200
2000
1800
1600
1400
1200 -1
1000
800
Wavenumbers (cm ) Fig. 2. FT-IR spectra of fullerol and fullerol-CNTs hybrid.
600
The synthesis of the hybrid (Fig. 1) involves three steps: i) oxidation of multi-wall CNTs with concentrated HNO3, ii) wet impregnation of the oxidized CNTs with fullerol, iii) solid-state heating of the impregnated solid at 200 °C. Extraction of the product with water under sonication followed by filtration and centrifugation affords a
50
A.B. Bourlinos et al. / Materials Letters 82 (2012) 48–50
-100
Number (%)
Total Counts
Fullerol
-80
-60
-40
-20
0
35
Fullerol, D h=~3.5 nm
30
CNTs Fullerol, D h =~68 nm CNTs, D h=~90 nm
25 20 15
20 10
CNTs
Total Counts
5 0 1
10
100
1000
Hydrodynamic Diameter (nm) Fig. 5. DLS profiles of the aqueous dispersions of the materials.
-100
-80
-60
-20
0
20
-20
0
20
CNTs
Total Counts
Fullerol
-40
In contrast, oxidized CNTs without fullerols appear with lower ζp of about −26 mV (Fig. 4). Based on DLS measurements, the diameter of the dispersed particles is 3.5 nm for fullerols, 68 nm for fullerol-CNTs and 90 nm for oxidized CNTs (Fig. 5). In case of fullerol, the size of 3.5 nm is higher than that theoretically expected (ca. 1 nm) due to the ability of the hydroxylated clusters to form hydrogen bonds with each other in solution [11]. Most importantly, the small size of fullerolCNTs compared to oxidized CNTs indicates stronger electrostatic repulsion between fullerol-coated CNTs. 4. Conclusions
-100
-80
-60
-40
Zeta potential (mV) Fig. 4. Zeta-potential graphs of the aqueous dispersions of the materials.
clear black colloid with remarkable stability (inset photo). Note also the striking color difference between the dispersed hybrid and watersoluble fullerol (brown solution). In the first step, chemical oxidation with HNO3 creates -COOH and -OH surface groups [9] that increase the compatibility of carbon nanotubes with fullerols and provide reactive sites for its binding. In the second step, impregnation of the oxidized CNTs with fullerols leads to uniformly deposited clusters on the carbon surface. In the third step, solid-state heating at 200 °C promotes an interfacial bonding between fullerols and oxidized CNTs through ester, ether and hydrogen bonding formation. Ester and ether bridges have been previously proposed in carbon nanobuds (fullerene-CNTs hybrids) [10]. In the present case, such bonds are likely the result of simple esterification/etherfication reactions. On the other hand, hydrogen bonding is expected on account of the oxygen-rich surface groups. The presence of fullerols in the hybrid was evidenced by the characteristic FT-IR peaks of the cluster (Fig. 2). Accordingly, the hybrid shows absorptions between 1800 and 800 cm− 1 as follows: 1600 and 1380 cm− 1 ascribed to C C bonds in fullerol and 1100 cm− 1 attributed to the C―O stretching in C―OH [11]. Decoration of the nanotubes with fullerols was revealed by AFM (Fig. 3). Isolated CNTs covered with ballshaped objects are clearly visible in the images. The average height of these objects was within 0.8–1.1 nm, as derived from topographical height profile, which corresponds well to the diameter of fullerol [12]. The attached fullerol alters the surface properties of the hybrid rendering it ionic and highly hydrophilic. Characteristically, both fullerols and fullerol-CNTs display practically the same mean ζp of −40 mV (Fig. 4). Such value manifests a great colloidal stability in water and highly negative surface charge, indicative of the successful coating of the CNTs with fullerols. Similar effect has been observed for nanocarbon aqueous dispersions stabilized by anionic surfactants [13].
Fullerol-carbon nanotube hybrids have been prepared and characterized for the first time. The hybrid is composed of fullerol clusters attached on the surface of the nanotubes through covalent and hydrogen bonding. The robust anions alter significantly the surface properties of the support conferring ionic character and aqueous solubility. The material may find uses in medicine as a new nanopharmaceutic, in water treatment for heavy metal removal or in polymer composites as reinforcing filler. Acknowledgements The work was supported by the Operational Program Research and Development for Innovations-European Social Fund (CZ.1.05/ 2.1.00/03.0058) and the projects of Grant Agency of the Academy of Sciences of the Czech Republic (KAN115600801, KAN200380801 and P208/10/1742). References [1] Coleman JN, Khan U, Blau WJ, Gun'ko YK. Carbon 2006;44:1624–52. [2] Georgakilas V, Gournis D, Tzitzios V, Pasquato L, Guldi DM, Prato M. J Mater Chem 2007;17:2679–94. [3] Bianco A, Kostarelos K, Partidos CD, Prato M. Chem Commun 2005:571–7. [4] Fei B, Lu H, Hu Z, Xin JH. Nanotechnology 2006;17:1589–93. [5] Bourlinos AB, Georgakilas V, Zboril R, Dallas P. Carbon 2007;45:2136–9. [6] Djordjevic A, Bogdanovic G. Arch Oncol 2008;16:42–5. [7] Anderson R, Barron AR. J Am Chem Soc 2005;127:10458–9. [8] Gengler RYN, Gournis D, Aimon AH, Toma LM, Rudolf P. Chem Eur J 2012;18: 7594–600. [9] Datsyuk V, Kalyva M, Papagelis K, Parthenios J, Tasis D, Siokou A, et al. Carbon 2008;46:833–40. [10] Nasibulin AG, Pikhitsa PV, Jiang H, Brown DP, Krasheninnikov AV, Anisimov AS, et al. Nat Nanotechnol 2007;2:156–61. [11] Vileno B, Marcoux PR, Lekka M, Sienkiewicz A, Fehér T, Forró L. Adv Funct Mater 2006;16:120–8. [12] Assemi S, Tadjiki S, Donose BC, Nguyen AV, Miller JD. Langmuir 2010;26: 16063–70. [13] Smith RJ, Lotya M, Coleman JN. New J Phys 2010;12(125008) 11 pp.