9012
2007, 111, 9012-9015 Published on Web 06/06/2007
Interactions of Single Wall Carbon Nanotubes with Methyl Viologen Radicals. Quantitative Estimation of Stored Electrons Anusorn Kongkanand and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry and Biochemistry and Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: April 4, 2007; In Final Form: May 22, 2007
The electron storage and discharge properties of single wall carbon nanotubes (SWCNTs) are probed by their interaction with radiolytically and photolytically produced methyl viologen (MV+•) radicals. Quantitative estimation of stored electrons amounts to one electron per ∼100 carbon atoms of SWCNT. These stored electrons are readily discharged on demand by introducing an electron acceptor such as thionine (E0TH/TH2- ) 0.064 V vs NHE). On the basis of the charge equilibration experiments, we estimate the apparent Fermi level of SWCNT as 0.039 V versus NHE.
Carbon nanostructures such as fullerenes and carbon nanotubes possess a large array of carbon-carbon double bonds and are capable of storing multiple electrons.1 C60, for example, exhibits multiple stepwise reduction and is often referred to as an electron sponge.2,3 The obvious question is whether the extended carbon-carbon network of single wall carbon nanotubes (SWCNT) can also possess a similar electron storage capability. Earlier studies have investigated the electron charging property of SWCNT by means of redox titration and electrochemical methods.4-9 Supercapacitor10,11 and Li+ ion intercala-
employed as light harvesting assemblies and assisted in improving the performance of photoelectrochemical cells.28 If indeed SWCNT have the ability to accept and store electrons, we should be able to monitor the electron storage property in a quantitative fashion. We now present here a spectroscopic method to probe the charging of SWCNT by bringing them in contact with radiolytically (and photolytically generated) methyl viologen radicals (MV+•). The stored electrons are discharged on demand by introducing an electron acceptor such as thiazine dye (Scheme 1). The interaction between MV+• and SWCNT which lead to the electron
SCHEME 1: Electron Storage and Discharge in SWCNT
tion12,13 properties of SWCNT have led their use in the anodes of a Li ion secondary battery. In addition, carbon nanotubes as support architectures for Pt electrocatalyst have shown to improve the performance of direct methanol and hydrogen fuel cells.14-21 Recent studies of SWCNT-chromophore22-24 and SWCNT-semiconductor25-27 composites have demonstrated the ability of SWCNT to accept electrons and promote photoinduced charge separation. Such composites have been successfully * Author to whom correspondence should be addressed. E-mail:
[email protected].
10.1021/jp0726541 CCC: $37.00
equilibration between the two systems are presented in this paper. SWCNT sample was purchased from SES Research and further purified by refluxing with concentrated nitric acid and was washed repeatedly with water prior to filtration. The SWCNT sample consisted of both semiconductor and metallic type nanotubes, and no additional effort was made to isolate them. The SWCNT (0.5 g/L) were suspended in ethanol with the aid of tetraoctylammonium bromide and Nafion (0.005% each). SWCNT deposited on a conducting glass exhibited © 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 26, 2007 9013
Figure 1. Difference absorption spectra of MV+• radical (0.094 mM) generated by gamma radiolysis of 0.5 mM MV2+ in ethanol: (a) before addition of SWCNT suspension and (b-g) following incremental additions of SWCNT suspension under N2 atmosphere. A solution of 0.5 mM MV2+ (non-irradiated) with corresponding concentration of SWCNT was used as a reference. Inset shows the amount of electrons transferred to SWCNT (as estimated from the decreased absorbance at 607 nm) as a function of SWCNT concentrations.
bundles of diameters 5-30 nm. Resonance Raman spectroscopy revealed dominant peaks of SWCNT associated in tight bundles29 (see Supporting Information, Figures S1 and S2). The diameter of the SWCNT as estimated from the Raman shift in the ring breathing mode was in the range of 0.8-1.5 nm.30 A deaerated methyl viologen (Aldrich) solution in ethanol was subjected to gamma radiolysis (Shepherd 109, Dose 1.8 krad/min) or to UV photolysis to produce a blue colored methyl viologen radical. The duration of radiolysis and photolysis controlled the concentration of the MV+• produced in the ethanol solution (see Supporting Information for detailed experimental procedure). Following γ irradiation, the quartz cuvette capped with a rubber septum was placed in an absorption spectrophotometer. The spectrum “a” in Figure 1 shows the absorption characteristics of MV+• formed in deaerated ethanol solution.31 The absorption maxima at 397 and 607 nm are characteristics of MV+• and often serve as a probe to elucidate the photoinduced charge-transfer processes.32 Deaerated SWCNT suspension was then introduced in small increments using a microsyringe, and the absorption spectra were recorded after each addition. A decrease in MV+• absorption (spectra b-g) was observed with increasing concentration of SWCNT. The reduction potential of MV2+/MV+• is -0.45 V versus normal hydrogen electrode (NHE)33 and thus favors electron transfer to the lower lying conduction band of SWCNT (usually around 0.0 V). The disappearance of MV+• absorption observed here confirms the electron transfer to SWCNT. With continued addition of SWCNT, we were able to transfer all of the electrons from 0.094 mM MV+• into 200 mg/L SWCNT (reaction 1).
we should be able to quantitatively estimate the number of electrons stored in SWCNT. The inset in Figure 1 shows the dependence of net stored electrons in SWCNT with increasing concentration of SWCNT. The concentration of stored electrons was estimated by the absorption decrease in MV+• absorption (extinction coefficient equal to 13 800 M-1cm-1 at 607 nm31). Because of the uncertainties in the length and diameter of SWCNT employed in our studies, we cannot directly estimate the number of electrons stored per single strand of SWCNT. However, we obtain an average estimate of the electron storage based on carbon atoms. Since 1 mg/L of SWCNT corresponds to 0.083 mM of carbon, we can calculate the molar concentration of carbon required to store one electron. The estimated value was in the range of 100-177 carbon atoms per electron for SWCNT concentrations of 57-200 mg/L (inset of Figure 1). As the electrons are transferred to SWCNT, it undergoes equilibration with the MV2+/MV+• redox couple. Thus, the maximum limit of one electron per ∼100 carbon units in the present study arises from the closer lying redox potential of the viologen redox couple. By employing stronger reducing radicals, it should be possible to store more electrons in the SWCNT. The stored electrons in SWCNT can be extracted on demand by introducing another electron acceptor that has a reduction potential less negative than the equilibrated potential of the SWCNT-viologen system. We employed thionine dye as an electron acceptor since its reduction potential (TH/TH2-) is 0.064 V.34 The dye has a ground state absorbance maximum at 600 nm and undergoes two-electron reduction to produce a colorless product (leucothionine).
MV+• + SWCNT f MV2+ + SWCNT(e)
SWCNT(2 e) + TH f SWCNT + TH2-
(1)
If indeed the decrease in absorbance of MV+• in Figure 1 reflects the concentration of electrons transferred to SWCNT,
(2)
Upon addition of thionine to the equilibrated SWCNT-MV+• solution, negative absorption at 600 nm corresponding to the
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Figure 2. Difference absorption spectra recorded following the incremental addition of deaerated thionine to SWCNT-MV+• solution g of Figure 1. Thionine concentrations were (a) 0, (b) 4.6, (c) 9.2, (d) 15.3, and (e) 23 µM. Inset shows the concentrations of discharged electrons with each addition of thionine solution as estimated from the changes in the absorbance at 600 nm.
Figure 3. Generation of MV+• radical during the gamma radiolysis of deaerated ethanol solution of 0.5 mM MV2+ and varying amounts of SWCNT.
reduction of thionine is observed as the electrons are scavenged by the dye (reaction 2). Once the electrons stored in SWCNT are titrated, further addition of thionine no longer results in a decrease in absorption at 600 nm. Absorbance changes arising from the incremental addition of thionine to the SWCNT-MV+• system are presented in Figure 2. Stored electrons in SWCNT were titrated with 10.5 µM of thionine, which is ∼22% of the transferred electrons from MV+• to SWCNT. The experiment described in Figure 2 demonstrates the ability of SWCNT to discharge stored electrons upon demand. As shown previously,35 we can employ the Nernst equation to determine the apparent Fermi level (E*F) for the equilibrium between SWCNT and TH/TH2- couple from expression 3,
E*F (SWCNT(e)) ) E0Ox/Red +
( )
[Ox]eq RT log nF [Red]eq
(3)
where E0Ox/Red is the standard reduction potential of the redox couple (TH/TH2-) and n is the stoichiometric number of electrons involved in the equilibration step (n ) 2 as per reaction 2). By substituting equilibrium concentrations of thionine and reduced thionine under equilibrium conditions and using the thionine reduction potential of E0TH/TH2- ) 0.064 V versus NHE, we estimate the apparent Fermi level of SWCNT to be 0.039 V versus NHE. This value is close to the value of the conduction band energy of ∼0.0 V obtained from other measurements.36,37 The charge equilibration property of SWCNT can also be probed by following the production of MV+• during the radiolysis experiment. The formation of MV+• was monitored during the gamma irradiation of the SWCNT-MV2+ solution at different compositions (Figure 3). With increasing duration of radiolysis, we see the accumulation of MV+• (monitored from the absorbance at 607 nm) at different concentration levels. The rate of MV+• accumulation in the presence of SWCNT is significantly lower than that observed in the absence of SWCNT (less than 30% accumulation is seen at 200 mg/L SWCNT) as the electron equilibration is achieved between the SWCNT and the MV+• system. The charge equilibration process was further probed in a laser flash photolysis study. The laser pulse (308 nm) excitation of MV2+ in deaerated ethanol yields significant amounts of MV+•.
Figure 4. Transient absorption-time profile recorded following the 308 nm laser pulse excitation of 0.08 mM MV2+ in deaerated ethanol: (a) in the absence and (b-d) in the presence of SWCNT.
The transient absorption spectra of MV2+ in the absence and in the presence of SWCNT are provided in Supporting Information (Figure S5). The details on the photochemical reduction process in alcohols can be found elsewhere.38 Electron transfer between excited MV+• and SWCNT was monitored from the absorption-time profile at 395 nm (Figure 4). The UV photoreduction of MV2+ in ethanol is completed within the laser pulse duration, and the MV+• does not exhibit any significant decay during 400 µs. Addition of the SWCNT to the MV2+ solution results in the decay of the transient absorption. This decay which represents the disappearance of MV+• is indicative of the time scale for the electron transfer to SWCNT. The decrease in absorbance at 400 µs with increasing decay parallels the steady state measurements. On the basis of these transient experiments, we estimate that the electron equilibration between MV+• and SWCNT is completed within a period of 0.4 ms. In conclusion, we have demonstrated the ability of SWCNT to interact with electron rich radicals and store the electrons under deaerated conditions. The determination of one electron per ∼100 carbon atoms of SWCNT provides a quantitative estimate of electron storage and opens up new ways to use
Letters SWCNT for promoting electron transfer in catalytic, capacitor, and hybrid electrode systems. Acknowledgment. The research described herein was supported by the Office of Basic Energy Science of the Department of the Energy. This is contribution NDRL-4722 from the Notre Dame Radiation Laboratory. Supporting Information Available: Details on the characterization of SWCNT and the production of methylviologen radical. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fowler, P. W.; Ceulemans, A. J. Phys. Chem. 1995, 99, 508. (2) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 7773. (3) Xie, Q. S.; Perezcordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992, 114, 3978. (4) Zheng, M.; Diner, B. A. J. Am. Chem. Soc. 2004, 126, 15490. (5) (a) Barisci, J. N.; Wallace, G. G.; Baughman, R. H. J. Electroanal. Chem. 2000, 488, 92. (b) Hughes, M.; Chen, G. Z.; Shaffer, M. S. P.; Fray, D. J.; Windle, A. H. Chem. Mater. 2002, 14, 1610. (6) Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937. (7) Keblinski, P.; Nayak, S. K.; Zapol, P.; Ajayan, P. M. Phys. ReV. Lett. 2002, 89, Article No. 255503. (8) Kamat, P. V.; Thomas, K. G.; Barazzouk, S.; Girishkumar, G.; Vinodgopal, K.; Meisel, D. J. Am. Chem. Soc. 2004, 126, 10757. (9) Ye, H. S.; Liu, X.; Cui, H. F.; Zhang, W. D.; Sheu, F. S.; Lim, T. M. Electrochem. Commun. 2005, 7, 249. (10) Liu, C. Y.; Bard, A. J.; Wudl, F.; Weitz, I.; Heath, J. R. Electrochem. and Solid State Lett. 1999, 2, 577. (11) An, K. H.; Kim, W. S.; Park, Y. S.; Choi, Y. C.; Lee, S. M.; Chung, D. C.; Bae, D. J.; Lim, S. C.; Lee, Y. H. AdV. Mater. 2001, 13, 497. (12) Gao, B.; Kleinhammes, A.; Tang, X. P.; Bower, C.; Fleming, L.; Wu, Y.; Zhou, O. Chem. Phys. Lett. 1999, 307, 153. (13) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Phys. ReV. Lett. 2000, 85, 1706. (14) Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Viswanathan, B. Bull. Mater. Sci. 2000, 23, 341. (15) Che, G. L.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Langmuir 1999, 15, 750.
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9015 (16) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. S. Nano Lett. 2004, 4, 345. (17) McElrath, K. O.; Smith, K. A.; Bahr, J. L.; Wainerdi, T. J.; Karohl, D. A.; Colbert, D. T.; Miller, M. A.; Oviatt, H. W.; Cline, E. D. Fuel cell electrode comprising carbon nanotubes. U. S. Patent Application, 2004, p A1. (18) Girishkumar, G.; Vinodgopal, K.; Meisel, D.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960. (19) Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. Langmuir 2005, 21, 8487. (20) Girishkumar, G.; Hall, T. D.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 107. (21) Kongkanand, A.; Kuwabata, S.; Girishkumar, G.; Kamat, P. Langmuir 2006, 21, 2392. (22) Guldi, D. M.; Rahman, A.; Sgobba, V.; Ehli, C. Chem. Soc. ReV. 2006, 35, 471. (23) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 25477. (24) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Am. Chem. Soc 2005, 127, 11884. (25) Robel, I.; Bunker, B.; Kamat, P. V. AdV. Mater. 2005, 17, 2458. (26) Kongkanand, A.; Domı´nguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676. (27) Sheeney-Haj-Khia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78. (28) (a) Kamat, P. V. Nano Today 2006, 1, 20. (b) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (29) Heller, D. A.; Barone, P. W.; Swanson, J. P.; Mayrhofer, R. M.; Strano, M. S. J. Phys. Chem. B 2004, 108, 6905. (30) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185. (31) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617. (32) Dimitrijevic, N. M.; Kamat, P. V. J. Phys. Chem. 1993, 97, 7623. (33) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637. (34) Kamat, P. V. J. Chem. Soc., Faraday Trans. 1 1985, 81, 509. (35) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (36) Kazaoui, S.; Minami, N.; Matsuda, N.; Kataura, H.; Achiba, Y. Appl. Phys. Lett. 2001, 78, 3433. (37) Zhao, J. J.; Han, J.; Lu, J. P. Phys. ReV. B 2002, 65. (38) Ebbesen, T. W.; Ferraudi, G. J. Phys. Chem. 1983, 87, 3717.
