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Electronic structure of cyclodextrin decorated carbon nanotube films
Chemical Physics Letters 610–611 (2014) 95–97
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic structure of cyclodextrin decorated carbon nanotube films Cheol-Soo Yang a , Hae Kyung Jeong b,∗ a b
Advanced Material Division, Korea Research Institute of Chemical Technology, Daejeon 305-343, Republic of Korea Department of Physics, Center for Bio-Nanomaterials, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 May 2014 In final form 27 June 2014 Available online 8 July 2014
a b s t r a c t The electronic structure of cyclodextrin decorated carbon nanotube (CNT) films is investigated by the photoelectron spectroscopy. Typical sp2 states found in graphite are also seen in the film near the Fermi energy, and the valence band of the film is similar to that of graphite, explaining high performance of the electric double-layer capacitive behavior of the film. The work function of the film, however, is very different with those of graphite and CNT and found to be 6.1 eV, indicating a dipole layer formed on the surface of the film by the cyclodextrin. © 2014 Elsevier B.V. All rights reserved.
Cyclodextrin (CD) is a well-known biomolecule used in food, pharmaceutical, drug delivery, chemical industries, agriculture, and environmental engineering [1]. The molecule is hydrophobic inside and hydrophilic on the outside surface so that it can enhance the solubility and bioavailability. Recently CD on a graphene platform has been investigated heavily for the biosensor applications due to the possibility to obtain large surface area and high film conductivity. For example, CD functionalized graphene nanosheets have been demonstrated high supramolecular recognition capability [2]. They were soluble in water, ethanol, dimethylformamide, and dimethylsulfoxide due to the combination of hydrophobic graphene with the hydrophobic CD cavities [3], exhibiting high supramolecular recognition. Since the electrochemical performance of the composite was excellent it has been known to be used for the biomolecule recognition sensors and drug deliveries [4–6]. Graphene and multiwall carbon nanotubes (MWCNTs) were also functionalized with CD for the biosensor applications [7]. The host–guest chemical reaction ability of CD and – stacking interaction of graphene and MWCNT introduced synergetic effects on high selectivity, excellent sensitivity and good stability, providing an efficient and promising platform for the electrochemical biomolecule recognition sensors [7]. MWCNT only, without graphene, was also functionalized with CD and investigated its electrochemical properties in a 1 M H2 SO4 electrolyte for supercapacitor applications [8]. The composite was very useful for thin film applications due to the possibility of preparing easily uniform films on different substrates. It showed
∗ Corresponding author. E-mail address: [email protected] (H.K. Jeong). http://dx.doi.org/10.1016/j.cplett.2014.06.056 0009-2614/© 2014 Elsevier B.V. All rights reserved.
highly electric double layer capacitor (EDLC) behavior based on the results of cyclic voltammetry, galvanostatic charge/discharge measurements, and impedance spectroscopy. The specific capacitance of the film was about 4 F/g, and the power density was up to 4552 W/kg. Capacitance retention was more than 80% after 500 cycles due to high cyclic stability, demonstrating that the film can be used as a reliable electrode for various applications. However, fundamental studies such as electronic structure, optical properties, and chemical properties of the film have not been investigated yet. In this report, we investigate the electronic structure of the CD decorated MWCNT film by the photoelectron spectroscopy. The valence band of the film is very similar to that of graphite or multiwall carbon nanotube. We also investigate the work function of the film from the kinetic energy cutoff of the secondary electrons with respect to the Au reference. The film was synthesized in a simple chemical method as followings. The MWCNT (Hanwha, average diameter of 50 nm, length of 40 m) and the ␣-CD (Alfar Aesar) were used as purchased. The MWCNT of 40 mg and the ␣-CD of 300 mg were dispersed in deionized water of 100 mL and then the solution was sonicated for 180 min in ambient atmosphere. The solution was vacuum filtered slowly using a nylon filter. A thin film of the composite on a filter was formed and then dried overnight in a vacuum oven. Figure 1(a) shows a photo of the obtained film. Size and thickness of the film can be tuned and it can be easily cut for an electrode of any applications. The thickness and diameter of the obtained film in Figure 1(a) were 350 m and 47 mm. It is worth noting that the ␣-CD composite is likely to be a free standing film compared to or ␥-CD composites. We therefore investigate the electronic structure of ␣-CD composite only since the free standing film is good for the sampling of photoelectron spectroscopy measurement.
96
C.-S. Yang, H.K. Jeong / Chemical Physics Letters 610–611 (2014) 95–97
Figure 1. (a) A photo and (b) a SEM image of the synthesized film. The inset presents the chemical model of ␣-cyclodextrin. (For interpretation of the references to colour in text, the reader is referred to the web version of this article.)
Photoelectron spectroscopy (PES) measurements using a SCIENTA-R3000 (Gamma data) electron analyzer were performed at beam line U20 of the National Synchrotron Radiation Laboratory (NSRL) [9–11]. Valence band spectra were recorded with the photon energy of 170 eV. The photoemission spectra were recorded at normal emission and have been normalized to the photon flux which was calibrated by measuring the Au 4f spectra from a clean Au foil attached to the sample holder. Binding energies were calibrated with respect to the Au 4f7/2 feature from the Au foil, which was measured immediately after each spectrum. Chemical bonding of the carbon backbone was investigated in situ
by X-ray photoelectron spectroscopy (XPS) at beam line U20 of the NSRL [9–11]. The sample work functions were deduced from the kinetic energy cutoff of the secondary electrons when irradiated with photon energy of 130 eV—where the electrons had barely enough energy to overcome the sample work function. The sample was biased by −20 V to prevent exposing the electron detector to the intense secondary electron beam. Morphologies of the samples were investigated by scanning electron microscopy (SEM, S-4300, Hitachi, Japan), and thermogravimetric analysis (TGA Instruments, Q600) was used to measure the component and weight of the elements.
Figure 2. (a) The thermogravimetric analysis result and XPS spectra of (b) wide, (c) C 1s and (d) O 1s. The C 1s XPS spectrum is deconvoluted into two peaks originating from the sp2 aromatic carbon peak and the sp3 CD peak.
C.-S. Yang, H.K. Jeong / Chemical Physics Letters 610–611 (2014) 95–97
Figure 3. The valence band of HOPG, PG and CD + CNT film with a photon energy of 130 eV.
The synthesized film is durable and solid (Figure 1(a)), meaning MWCNTs are dispersed well throughout the film. A SEM image of the film confirms that the MWCNTs are well dispersed and wrapped with CD marked by the red circle in Figure 1(b). The external hydrophilic property of the CD molecule might improve the dispersion of MWCNT in the solution and resulted in a uniform film. Thermogravimetric analysis shows that 10 wt% of CD was included in the film as presented in Figure 2(a). The wide scan of XPS in Figure 2(b) displays an intense C 1s peak near 285 eV and a small bumpy O 1s peak near 533 eV, confirming the absence of the catalytic transition metals and contaminants in the film except for MWCNTs and CD elements. The C 1s XPS spectrum in Figure 2(c) is deconvoluted into two peaks: a sp2 peak from the MWCNT aromatic carbon rings and a sp3 peak from CD related with oxygen and hydroxyl functional groups bonded to the carbon. The C 1s XPS also shows a small feature at 5 eV below the main peak, indicating the –* transition. The small O 1s XPS spectrum in Figure 2(d) indicates the oxygen functional groups bonded to the carbon based on CD, and it is consistent with the oxygen peaks from esters, carbonates and acids (∼532.8–532.9 eV) [12]. Figure 3 shows the valence band of the samples. The electronic states of our references, HOPG and PG, were assigned based on the previous works [13–15]. The -originated density of state (DOS), compared to the -originated DOS, was enhanced in HOPG with respect to PG because of the periodic array of in-plane crystallinity in HOPG. For CD decorated MWCNT film (CD + CNT), the valence band looks similar to that of PG except for the shift of (). There was no Fermi level shift due to CD. It is quite different with oxygen functionalized carbon based systems in which Fermi level is typically shifted, causing the band gap opening [16–18]. The similar valence band of CD + CNT film with PG and HOPG near the Fermi level is a good reason for the high EDLC behavior of the film in the aqueous electrolyte [8]. The relative work function of the film was measured using the kinetic energy cutoff of the secondary electrons with respect to the Au reference, and the difference is 1.0 eV as shown in Figure 4. Since the work function of the Au reference is 5.1 eV [15] the work function of the film is estimated 6.1 ± 0.1 eV. It increased considerably compared to MWCNT (4.2 eV) and SWCNT (4.5–4.6 eV) itself [19,20]. It is also larger than those of PG and HOPG (4.4 eV) and even graphite oxide (5.9 eV) [16]. The circumambient CD on MWCNT
97
Figure 4. Kinetic energy cutoff of the secondary electron with a photon energy of 130 eV.
might introduce a dipole layer, preventing the photoelectrons from escaping the film and then resulting in the large work function. It is a quite interesting property with the conductive valence band. In summary, we investigated the valence band structure of CD decorated MWCNT film using photoelectron spectroscopy at the NSRL. The valence band of the film near the Fermi level was very similar to that of HOPG and PG, demonstrating the high EDLC behavior due to high conductivity. The work function of the film, however, was higher compared to HOPG and PG due to a dipole layer formed by CD and estimated 6.1 ± 0.1 eV using the kinetic energy cutoff of the secondary electron. The new synthesized CD decorated MWCNT film, therefore, could be a good conductive electrode for electrochemical applications with improved selectivity even though virtual counter electrodes, via solution, make CNT chemical sensors possible, without any chemical specificity. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013R1A1A3A04008714) and partly by PAL through the abroad beam time program of Synchrotron Radiation Facility Project under MEST, performed under the approval of the National Synchrotron Radiation Facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
W. Saenger, et al., Chem. Rev. 98 (1998) 1787. Y. Guo, S. Guo, J. Ren, Y. Xhai, S. Dong, E. Wang, ACS Nano 4 (2010) 4001. B. Konkena, S. Vasudevan, Langmuir 28 (2012) 12432. H. Yang, Y. Zhu, D. Chen, C. Li, S. Chen, Z. Ge, Biosens. Bioelectron. 26 (2010) 295. Y. Zhang, R. Yuan, Y. Chai, W. Li, X. Zhong, H. Zhong, Biosens. Bioelectron. 26 (2011) 3977. D. Lu, S. Lin, L. Wang, X. Shi, C. Wang, Y. Zhang, Electrochim. Acta 85 (2012) 131. T. Kuila, S. Bose, P. Khanra, A.K. Mishra, N.H. Kim, J.H. Lee, Biosens. Bioelectron. 26 (2011) 4637. Y.S. Jang, J.R. Bae, H.K. Jeong, Sae Mulli 62 (2012) 322. Y. Pan, et al., Langmuir 28 (2012) 6045. D.D. Kong, et al., J. Phys. Chem. C 115 (2011) 6715. Y.H. Pan, et al., J. Phys. Chem. C 115 (2011) 10744. D. Rosenthal, M. Ruta, R. Schlögl, L. Kiwi-Minsker, Carbon 48 (2010) 1835. B. Reihl, J.K. Gimzewski, J.M. Nicholla, E. Tosatti, Phys. Rev. B 33 (1986) 5770. N.A.W. Holzwarth, S.G. Louie, S. Rabii, Phys. Rev. B 26 (1982) 5382. D.E. Eastman, Phys. Rev. B 2 (1970) 1. H.K. Jeong, C.S. Yang, B.S. Kim, K.-J. Kim, EPL 92 (2010) 37005. H.K. Jeong, L. Hong, X. Zhang, E. Vega, P.A. Dowben, Carbon 57 (2013) 227. H.K. Jeong, X. Zhang, P.A. Dowben, Chem. Phys. Lett. 572 (2013) 58. P. Chen, X. Wu, X. Sun, J. Lin, W. Ji, K.L. Tan, Phys. Rev. Lett. 82 (1999) 2548. S. Suzuki, C. Bower, Y. Watanabe, O. Zhou, Appl. Phys. Lett. 76 (2000) 4007.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electronic structure of cyclodextrin decorated carbon nanotube films Cheol-Soo Yang a , Hae Kyung Jeong b,∗ a b
Advanced Material Division, Korea Research Institute of Chemical Technology, Daejeon 305-343, Republic of Korea Department of Physics, Center for Bio-Nanomaterials, Institute of Basic Science, Daegu University, Gyeongsan 712-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 May 2014 In final form 27 June 2014 Available online 8 July 2014
a b s t r a c t The electronic structure of cyclodextrin decorated carbon nanotube (CNT) films is investigated by the photoelectron spectroscopy. Typical sp2 states found in graphite are also seen in the film near the Fermi energy, and the valence band of the film is similar to that of graphite, explaining high performance of the electric double-layer capacitive behavior of the film. The work function of the film, however, is very different with those of graphite and CNT and found to be 6.1 eV, indicating a dipole layer formed on the surface of the film by the cyclodextrin. © 2014 Elsevier B.V. All rights reserved.
Cyclodextrin (CD) is a well-known biomolecule used in food, pharmaceutical, drug delivery, chemical industries, agriculture, and environmental engineering [1]. The molecule is hydrophobic inside and hydrophilic on the outside surface so that it can enhance the solubility and bioavailability. Recently CD on a graphene platform has been investigated heavily for the biosensor applications due to the possibility to obtain large surface area and high film conductivity. For example, CD functionalized graphene nanosheets have been demonstrated high supramolecular recognition capability [2]. They were soluble in water, ethanol, dimethylformamide, and dimethylsulfoxide due to the combination of hydrophobic graphene with the hydrophobic CD cavities [3], exhibiting high supramolecular recognition. Since the electrochemical performance of the composite was excellent it has been known to be used for the biomolecule recognition sensors and drug deliveries [4–6]. Graphene and multiwall carbon nanotubes (MWCNTs) were also functionalized with CD for the biosensor applications [7]. The host–guest chemical reaction ability of CD and – stacking interaction of graphene and MWCNT introduced synergetic effects on high selectivity, excellent sensitivity and good stability, providing an efficient and promising platform for the electrochemical biomolecule recognition sensors [7]. MWCNT only, without graphene, was also functionalized with CD and investigated its electrochemical properties in a 1 M H2 SO4 electrolyte for supercapacitor applications [8]. The composite was very useful for thin film applications due to the possibility of preparing easily uniform films on different substrates. It showed
∗ Corresponding author. E-mail address: [email protected] (H.K. Jeong). http://dx.doi.org/10.1016/j.cplett.2014.06.056 0009-2614/© 2014 Elsevier B.V. All rights reserved.
highly electric double layer capacitor (EDLC) behavior based on the results of cyclic voltammetry, galvanostatic charge/discharge measurements, and impedance spectroscopy. The specific capacitance of the film was about 4 F/g, and the power density was up to 4552 W/kg. Capacitance retention was more than 80% after 500 cycles due to high cyclic stability, demonstrating that the film can be used as a reliable electrode for various applications. However, fundamental studies such as electronic structure, optical properties, and chemical properties of the film have not been investigated yet. In this report, we investigate the electronic structure of the CD decorated MWCNT film by the photoelectron spectroscopy. The valence band of the film is very similar to that of graphite or multiwall carbon nanotube. We also investigate the work function of the film from the kinetic energy cutoff of the secondary electrons with respect to the Au reference. The film was synthesized in a simple chemical method as followings. The MWCNT (Hanwha, average diameter of 50 nm, length of 40 m) and the ␣-CD (Alfar Aesar) were used as purchased. The MWCNT of 40 mg and the ␣-CD of 300 mg were dispersed in deionized water of 100 mL and then the solution was sonicated for 180 min in ambient atmosphere. The solution was vacuum filtered slowly using a nylon filter. A thin film of the composite on a filter was formed and then dried overnight in a vacuum oven. Figure 1(a) shows a photo of the obtained film. Size and thickness of the film can be tuned and it can be easily cut for an electrode of any applications. The thickness and diameter of the obtained film in Figure 1(a) were 350 m and 47 mm. It is worth noting that the ␣-CD composite is likely to be a free standing film compared to or ␥-CD composites. We therefore investigate the electronic structure of ␣-CD composite only since the free standing film is good for the sampling of photoelectron spectroscopy measurement.
96
C.-S. Yang, H.K. Jeong / Chemical Physics Letters 610–611 (2014) 95–97
Figure 1. (a) A photo and (b) a SEM image of the synthesized film. The inset presents the chemical model of ␣-cyclodextrin. (For interpretation of the references to colour in text, the reader is referred to the web version of this article.)
Photoelectron spectroscopy (PES) measurements using a SCIENTA-R3000 (Gamma data) electron analyzer were performed at beam line U20 of the National Synchrotron Radiation Laboratory (NSRL) [9–11]. Valence band spectra were recorded with the photon energy of 170 eV. The photoemission spectra were recorded at normal emission and have been normalized to the photon flux which was calibrated by measuring the Au 4f spectra from a clean Au foil attached to the sample holder. Binding energies were calibrated with respect to the Au 4f7/2 feature from the Au foil, which was measured immediately after each spectrum. Chemical bonding of the carbon backbone was investigated in situ
by X-ray photoelectron spectroscopy (XPS) at beam line U20 of the NSRL [9–11]. The sample work functions were deduced from the kinetic energy cutoff of the secondary electrons when irradiated with photon energy of 130 eV—where the electrons had barely enough energy to overcome the sample work function. The sample was biased by −20 V to prevent exposing the electron detector to the intense secondary electron beam. Morphologies of the samples were investigated by scanning electron microscopy (SEM, S-4300, Hitachi, Japan), and thermogravimetric analysis (TGA Instruments, Q600) was used to measure the component and weight of the elements.
Figure 2. (a) The thermogravimetric analysis result and XPS spectra of (b) wide, (c) C 1s and (d) O 1s. The C 1s XPS spectrum is deconvoluted into two peaks originating from the sp2 aromatic carbon peak and the sp3 CD peak.
C.-S. Yang, H.K. Jeong / Chemical Physics Letters 610–611 (2014) 95–97
Figure 3. The valence band of HOPG, PG and CD + CNT film with a photon energy of 130 eV.
The synthesized film is durable and solid (Figure 1(a)), meaning MWCNTs are dispersed well throughout the film. A SEM image of the film confirms that the MWCNTs are well dispersed and wrapped with CD marked by the red circle in Figure 1(b). The external hydrophilic property of the CD molecule might improve the dispersion of MWCNT in the solution and resulted in a uniform film. Thermogravimetric analysis shows that 10 wt% of CD was included in the film as presented in Figure 2(a). The wide scan of XPS in Figure 2(b) displays an intense C 1s peak near 285 eV and a small bumpy O 1s peak near 533 eV, confirming the absence of the catalytic transition metals and contaminants in the film except for MWCNTs and CD elements. The C 1s XPS spectrum in Figure 2(c) is deconvoluted into two peaks: a sp2 peak from the MWCNT aromatic carbon rings and a sp3 peak from CD related with oxygen and hydroxyl functional groups bonded to the carbon. The C 1s XPS also shows a small feature at 5 eV below the main peak, indicating the –* transition. The small O 1s XPS spectrum in Figure 2(d) indicates the oxygen functional groups bonded to the carbon based on CD, and it is consistent with the oxygen peaks from esters, carbonates and acids (∼532.8–532.9 eV) [12]. Figure 3 shows the valence band of the samples. The electronic states of our references, HOPG and PG, were assigned based on the previous works [13–15]. The -originated density of state (DOS), compared to the -originated DOS, was enhanced in HOPG with respect to PG because of the periodic array of in-plane crystallinity in HOPG. For CD decorated MWCNT film (CD + CNT), the valence band looks similar to that of PG except for the shift of (). There was no Fermi level shift due to CD. It is quite different with oxygen functionalized carbon based systems in which Fermi level is typically shifted, causing the band gap opening [16–18]. The similar valence band of CD + CNT film with PG and HOPG near the Fermi level is a good reason for the high EDLC behavior of the film in the aqueous electrolyte [8]. The relative work function of the film was measured using the kinetic energy cutoff of the secondary electrons with respect to the Au reference, and the difference is 1.0 eV as shown in Figure 4. Since the work function of the Au reference is 5.1 eV [15] the work function of the film is estimated 6.1 ± 0.1 eV. It increased considerably compared to MWCNT (4.2 eV) and SWCNT (4.5–4.6 eV) itself [19,20]. It is also larger than those of PG and HOPG (4.4 eV) and even graphite oxide (5.9 eV) [16]. The circumambient CD on MWCNT
97
Figure 4. Kinetic energy cutoff of the secondary electron with a photon energy of 130 eV.
might introduce a dipole layer, preventing the photoelectrons from escaping the film and then resulting in the large work function. It is a quite interesting property with the conductive valence band. In summary, we investigated the valence band structure of CD decorated MWCNT film using photoelectron spectroscopy at the NSRL. The valence band of the film near the Fermi level was very similar to that of HOPG and PG, demonstrating the high EDLC behavior due to high conductivity. The work function of the film, however, was higher compared to HOPG and PG due to a dipole layer formed by CD and estimated 6.1 ± 0.1 eV using the kinetic energy cutoff of the secondary electron. The new synthesized CD decorated MWCNT film, therefore, could be a good conductive electrode for electrochemical applications with improved selectivity even though virtual counter electrodes, via solution, make CNT chemical sensors possible, without any chemical specificity. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013R1A1A3A04008714) and partly by PAL through the abroad beam time program of Synchrotron Radiation Facility Project under MEST, performed under the approval of the National Synchrotron Radiation Facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
W. Saenger, et al., Chem. Rev. 98 (1998) 1787. Y. Guo, S. Guo, J. Ren, Y. Xhai, S. Dong, E. Wang, ACS Nano 4 (2010) 4001. B. Konkena, S. Vasudevan, Langmuir 28 (2012) 12432. H. Yang, Y. Zhu, D. Chen, C. Li, S. Chen, Z. Ge, Biosens. Bioelectron. 26 (2010) 295. Y. Zhang, R. Yuan, Y. Chai, W. Li, X. Zhong, H. Zhong, Biosens. Bioelectron. 26 (2011) 3977. D. Lu, S. Lin, L. Wang, X. Shi, C. Wang, Y. Zhang, Electrochim. Acta 85 (2012) 131. T. Kuila, S. Bose, P. Khanra, A.K. Mishra, N.H. Kim, J.H. Lee, Biosens. Bioelectron. 26 (2011) 4637. Y.S. Jang, J.R. Bae, H.K. Jeong, Sae Mulli 62 (2012) 322. Y. Pan, et al., Langmuir 28 (2012) 6045. D.D. Kong, et al., J. Phys. Chem. C 115 (2011) 6715. Y.H. Pan, et al., J. Phys. Chem. C 115 (2011) 10744. D. Rosenthal, M. Ruta, R. Schlögl, L. Kiwi-Minsker, Carbon 48 (2010) 1835. B. Reihl, J.K. Gimzewski, J.M. Nicholla, E. Tosatti, Phys. Rev. B 33 (1986) 5770. N.A.W. Holzwarth, S.G. Louie, S. Rabii, Phys. Rev. B 26 (1982) 5382. D.E. Eastman, Phys. Rev. B 2 (1970) 1. H.K. Jeong, C.S. Yang, B.S. Kim, K.-J. Kim, EPL 92 (2010) 37005. H.K. Jeong, L. Hong, X. Zhang, E. Vega, P.A. Dowben, Carbon 57 (2013) 227. H.K. Jeong, X. Zhang, P.A. Dowben, Chem. Phys. Lett. 572 (2013) 58. P. Chen, X. Wu, X. Sun, J. Lin, W. Ji, K.L. Tan, Phys. Rev. Lett. 82 (1999) 2548. S. Suzuki, C. Bower, Y. Watanabe, O. Zhou, Appl. Phys. Lett. 76 (2000) 4007.