by Molecular Oxygen at the Electrified Liquid−Liquid Interface

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Two-Phase Oxidation of C60- by Molecular Oxygen at the Electrified Liquid-Liquid Interface Peter Liljeroth, Bernadette M. Quinn,* and Kyo¨sti Kontturi Laboratory of Physical Chemistry and Electrochemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland Received December 19, 2002. In Final Form: April 3, 2003 The heterogeneous oxidation of the fullerene monoanion by aqueous-phase molecular oxygen is reported. Scanning electrochemical microscopy (SECM) is used to investigate the electron transfer (ET) kinetics at an externally biased water-chlorobenzene (CB) interface. The kinetic parameters obtained indicate that ET is fast and can be controlled by the applied interfacial potential difference. A novel means of applying SECM to an electrified liquid-liquid interface is introduced. Provided the size of the liquid-liquid interface is sufficiently small such that the current flow is in the nanoampere range, a conventional bipotentiostatic arrangement can be used to externally control both the interfacial potential difference (∆w o φ) at the liquidliquid interface and the SECM tip potential. This arrangement provides a simple and fast means of studying ET reactions at a range of ∆w o φ without the need to vary either the base electrolyte ions or their concentrations. The potential dependence of both the forward and the backward rate constants for the model ET reaction between 7,7,8,8-tetracyanoquinodimethane and hexacyanoferrate was initially investigated at the water-1,2-dichloroethane interface to demonstrate the applicability of the present technique.

Introduction The electrochemistry of fullerene (C60) and its derivatives has been extensively reviewed,1,2 and it is considered to be a good model to test electron transfer (ET) theories. Because of a degenerate ground state, it can accept up to six electrons sequentially, and these reduction states, C60n-, where n ) 1-6, can be detected electrochemically in controlled atmosphere conditions.1,2 Fullerene is expected to undergo rapid ET because of the small reorganization energy of the large anion.3,4 The reaction of C60nwith O2 has received relatively little study in a nonphotochemical context while its role both as a desired and as an undesired participant in photoconversion processes has been well-documented.1,2,5,6 The monoanion C60- can be obtained electrochemically under ambient conditions because aerobic oxidation is slow and the anion is not water-sensitive as a result of its low basicity.1,2 The reaction C60- + O2 f C60 + O2- is thermodynamically disfavored by about 0.4 V in low dielectric media but may become favored in highly protic media.1,7,8 Although molecular oxygen is a relatively good electron acceptor 0 ) -0.42 V vs saturated calomel electrode),9 there (EO •2 /O2 are few reports concerning its use as a redox species in ET studies at liquid-liquid (L-L) interfaces.10 Here, we * Corresponding author. E-mail: [email protected]. Telephone: +358 9 451 2572. Fax: +358 9 451 2580. (1) Reed, C. A.; Bolskar, R. D. Chem. Rev. 2000, 100, 1075. (2) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593. (3) Mirkin, M. V.; Bulhoes, L. O. S.; Bard, A. J. J. Am. Chem. Soc. 1993, 115, 201. (4) Zhang, J.; Unwin, P. R. J. Chem. Soc., Perkin Trans. 2 2001, 1608. (5) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (6) Smirnov, S.; Vlassiouk, I.; Kutzki, O.; Wedel, M.; Montforts, F.-P. J. Am. Chem. Soc. 2002, 124, 4212. (7) Ohlendorf, V.; Willnow, A.; Hungerbu¨hler, H.; Guldi, D. M.; Asmus, K.-D. Chem. Commun. 1995, 759. (8) Stinchcombe, J.; Pe´nicaud, A.; Bhyrappa, P.; Boyd, P. D. W.; Reed, C. A. J. Am. Chem. Soc. 1993, 115, 5212. (9) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637. (10) Ohde, H.; Maeda, K.; Yoshida, Y.; Kihara, S. J. Electroanal. Chem. 2000, 483, 108.

demonstrate that although the fulleride monoanion does not react homogeneously in solution with oxygen, it can be oxidized interfacially by aqueous-phase molecular oxygen under interfacial potential control. To the best of our knowledge, this is the first report concerning the heterogeneous oxidation of native C60- by molecular oxygen and is of relevance when the two-phase reactivity of C60 is being considered, for example, in thin films and bilayer studies. Heterogeneous ET reactions at electrified L-L interfaces between redox species located in opposing phases, Ox1(w) + Red2(o) f Red1(w) + Ox2(o), have been actively investigated in recent years. The rate has been noted to be dependent on the interfacial potential difference, ∆w oφ ) φw - φo, and at lipid-modified interfaces on the thickness of the lipid layer.11-24 The dependence of the rate of ET on the driving force has been studied in depth, and recently inverted Marcus region behavior was reported.4,12,20,25 This study was initially motivated by a recent report of scanning (11) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1996, 100, 17881. (12) Ding, Z.; Quinn, B. M.; Bard, A. J. J. Phys. Chem. B 2001, 105, 6367. (13) Barker, A. L.; Slevin, C. J.; Unwin, P. R.; Zhang, J. In Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications; Volkov, A. G., Ed.; Marcel Dekker: New York, 2001; Vol. 95, p 283. (14) Tsionsky, M.; Bard, A. J.; Mirkin, M. V. J. Am. Chem. Soc. 1997, 119, 10785. (15) Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2341. (16) Liu, B.; Mirkin, M. V. J. Am. Chem. Soc. 1999, 121, 8352. (17) Delville, M.-H.; Tsionsky, M.; Bard, A. J. Langmuir 1998, 14, 2774. (18) Zhang, Z.; Yuan, Y.; Sun, P.; Su, B.; Guo, J.; Shao, Y.; Girault, H. H. J. Phys. Chem. B 2002, 106, 6713. (19) Barker, A. L.; Unwin, P. R.; Zhang, J. Electrochem. Commun. 2001, 3, 372. (20) Barker, A. L.; Unwin, P. R.; Amemiya, S.; Zhou, J.; Bard, A. J. J. Phys. Chem. B 1999, 103, 7260. (21) Liu, B.; Mirkin, M. V. J. Phys. Chem. B 2002, 106, 3933. (22) Cheng, Y.; Schiffrin, D. J. J. Chem. Soc., Faraday Trans. 1993, 89, 199. (23) Ding, Z.; Fermin, D. J.; Brevet, P.-F.; Girault, H. H. J. Electroanal. Chem. 1998, 458, 139. (24) Eugster, N.; Fermin, D. J.; Girault, H. H. J. Phys. Chem. B 2002, 106, 3428.

10.1021/la0270338 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/02/2003

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Figure 1. Schematics of the setup used for the SECM experiments at an externally polarized L-L interface (a) and the Pt-coat micropipet assembly (b).

electrochemical microscopy (SECM) measurements of heterogeneous ET between fulleride and aqueous-phase hexacyanoferrate, in which Zhang and Unwin noted that the rate of ET was highest for the lowest driving force, indicative of Marcus inverted region kinetics.4 Here, we propose a simple means of applying SECM in feedback mode to the electrified L-L interface. The existing bipotentiostatic arrangement of the SECM instrument can be used to externally bias the L-L interface if the interfacial area is decreased such that the current flowing is in the nanoampere range. In this configuration, a two-electrode arrangement can be used, as is common in micropipet-supported L-L measurements.26,27 This arrangement does not require the use of redox reference electrodes, as in a recent report, and, thus, feedback SECM mode is possible.18 Feedback mode also ensures that the currents flowing at the polarized interface remain small because the concentration changes are located in the tipsubstrate gap and the interfacial current flow is determined by the size of the SECM tip used and not the area of the interface.28 This method has the advantage that both forward and reverse ET can be easily investigated by simply inverting the phases. As a test of the experimental arrangement, the well-studied ET reaction between organic-phase 7,7,8,8-tetracyanoquinodimethane (TCNQ) and aqueous-phase hexacyanoferrate was considered.12,15,16,19,22,23,29 Subsequently, the potential dependence of ET between the tip-generated chlorobenzene (CB)phase fulleride, C60-, and aqueous-phase molecular oxygen, was investigated. Experimental Section Chemicals. Potassium hexacyanoferrate [K3Fe(CN)6, Merck], TCNQ (Sigma), and buckminsterfullerine (C60, Aldrich) were used as redox couples. The supporting electrolyte in the organic phase was either bis-(triphenylphosphoranylidene) ammonium tetrakis(pentafluorophenyl)borate (BTPPATPBF20) or tetraphenylarsonium tetrakis(pentafluorophenyl)borate (TPAsTPBF20), which were prepared as described elsewhere.30 Aqueous solutions were prepared using MQ-treated water (Millipore, U.S.A.). 1,2Dichloroethane (1,2-DCE, Rathburn Chemicals, Scotland) and CB (Aldrich) were used as organic solvents. CB was used as the organic solvent for measurements involving C60 because of its low solubility in 1,2-DCE. All other chemicals were of the highest commercially available purity and were used as received. (25) Zu, Y.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. B 1999, 103, 6272. (26) Taylor, G.; Girault, H. H. J. Electroanal. Chem. 1986, 208, 179. (27) Evans, N. J.; Gonsalves, M.; Gray, N. J.; Barker, A. L.; Macpherson, J. V.; Unwin, P. R. Electrochem. Commun. 2000, 2, 201. (28) Bard, A. J.; Faulkner, L. R. Electrochemical methods, Fundamentals and applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (29) Zhang, J.; Unwin, P. R. Phys. Chem. Chem. Phys. 2002, 4, 3820. (30) Fermin, D. J.; Duong, H. D.; Ding, Z.; Brevet, P. F.; Girault, H. H. Phys. Chem. Chem. Phys. 1999, 1, 1461.

Electrochemical Measurements. The following electrochemical cells were used for the SECM experiments at the externally biased L-L interface, where | refers to the polarizable interface under study.

The experimental arrangement is shown schematically in Figure 1a. For cells I and III, the glass capillary (inner diameter, 400 µm) was filled with the aqueous phase, and the main reservoir was filled with the organic phase (1,2-DCE or CB), resulting in a polarizable 1,2-DCE-water or CB-water interface at the capillary tip. For cell II, the phases were inverted, resulting in a polarizable water-1,2-DCE interface at the capillary tip. Thus, for cells I and III, the SECM tip is located in the organic phase, whereas for cell II, it is in the aqueous phase. Either arrangement resulted in the formation of a stable interface at the tip of the capillary. The body of the cell was made of glass because the tip positioning over the interface was done visually with the aid of x- and y-piezoelectric drivers of the SECM. Potential control at both the SECM tip-electrolyte (WE1) and the L-L interfaces was achieved using the bipotentiostat of the SECM apparatus (CHI900, CH-Instruments, U.S.A.). Two electrodes located in opposing phases, RE1 and RE2, controlled the applied potential across the L-L interface. Because the current flow across the interface is in the nanoampere range, this two-electrode arrangement is sufficient with RE1 and RE2 acting as both reference and counter electrodes for each phase. For cells I and III, a silver wire and an Ag/AgCl electrode served as the organic (RE1) and aqueous (RE2) reference electrodes, respectively. For cell II, an ion-selective organic reference electrode (Ag/AgCl/10 mM TPAsCl) was used as RE2,31 mostly to avoid problems with evaporation of DCE from the capillary, which would have resulted in a variation of the position of the interface. RE1 served as the reference/counter electrode for WE1. Because the faradaic current flowing through the L-L interface is small, IR compensation was unnecessary. Plots of the tip current, I, normalized with respect to the current far from the interface, Iinf, versus distance, d, from the L-L interface, so-called “approach curves”, were measured at different applied ∆w o φ. The potential at the SECM tip was such that the solution redox mediator reacted at a diffusion-controlled rate. The kinetic parameters were obtained by fitting the experimental (31) Girault, H. H. In Modern Aspects of Electrochemistry; White, R. E., Conway, B. E., Bockris, J. O. M., Eds.; Plenum Press: New York, 1993; Vol. 25, p 1.

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Figure 2. Typical CVs obtained at the polarized water-1,2-DCE interface (a), where the DCE (dotted line) or aqueous phases (solid line) are located in the glass capillary, and at the water-CB interface (b). The scan rate was 100 mV/s. curves to theoretical curves generated for the reaction under investigation.28 The underlying theory has been outlined in detail previously11,19,20,28,32 and is summarized in the Supporting Information. All SECM experiments were performed in feedback mode, that is, only the oxidized or reduced species are initially present, for example, Ox1(w)/Ox2(1,2-DCE) or Red1(w)/Red2(1,2DCE), which ensures that the reaction under investigation is localized in the tip-substrate gap. This also simplifies the theoretical treatment.11,28 A complete list of the applied ∆w o φ for all systems considered and the applied SECM tip potentials is given in the Supporting Information. To convert the potential applied across the water-1,2-DCE interface from the cell scale to the absolute or Galvani scale, the transfer of a simple ion across the interface (tetraethylammonium, TEA+) was used as an internal standard.33 This involved the addition of a small amount of TEA+ to the cell after the measurement and correlating the measured midpeak potential on the cell scale with the formal transfer potential on the Galvani scale available in the literature.31 The water-CB interface has not been widely studied, and tables of formal transfer potentials are not available. In this case, the potential of zero charge determined from capacitance values, as were extracted from cyclic 31 voltammograms (CVs), was taken as ∆w o φ ) 0. Electrode Preparation. Disk-shaped Pt SECM tips were prepared as was previously described.34 Briefly, Pt wire (diameter, 25 µm; Goodfellows, U.K.) was heat-sealed in pulled borosilicate glass capillaries (Harvard GC200-10, U.S.A.) under vacuum followed by tip sharpening and polishing until the desired ratio of the overall tip radius to that of the platinum disk, RG, was achieved. The tips used had RG ) 5, as was determined from both optical micrographs and SECM approach-curve experiments to insulating, (poly(tetrafluoroethylene)) and conducting (Pt) substrates, followed by fitting the results to the approximations provided by Amphlett and Denuault.35 In separate studies, cell III was used with Pt-coat micropipetsupported L-L interfaces to probe the interfacial reactivity of the fulleride anion, C60-, as is illustrated schematically in Figure 1b. The Pt-coat micropipet was filled with the aqueous phase and dipped into the organic phase. The technique and the manufacture of the coated-pipet assembly have been described in more detail previously.33 Briefly, robust micropipets suitable for L-L electrochemistry were prepared as was described by Evans et al.27 A coned silver microelectrode was painted with bright platinum paint (PBC11611, Johnson Matthey, U.K.) using a small brush and was fired at 600 °C for 15 min. The resulting Pt-coat tip was then polished with 0.3-µm alumina grinding paper (Buehler, U.S.A.) to expose the silver wire. The silver wire was etched completely with concentrated nitric acid, resulting in a Pt-coat micropipet with a 25-µm inner diameter. An electrical (32) Barker, A. L.; Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. J. Phys. Chem. B 1998, 102, 1586. (33) Liljeroth, P.; Quinn, B. M.; Kontturi, K. Electrochem. Commun. 2002, 4, 255. (34) Bard, A. J.; Fan, F. R. F.; Mirkin, M. V. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, p 243. (35) Amphlett, J. L.; Denuault, G. J. Phys. Chem. B 1998, 102, 9946.

connection was made to the Pt coating. When a bipotentiostatic arrangement was used, fulleride was generated by in situ localized electrolysis of fullerene at the Pt-coat electrode while concurrently recording CVs at the pipet-supported interface. In this way, the reactivity of fulleride at the L-L interface was studied.

Results and Discussion Typical CVs obtained at the externally polarized interface supported at the tip of the capillary with cells I (Figure 2a, solid line), II (Figure 2a, dotted line), and III (Figure 2b) are given in Figure 2. The CV responses are as expected for a polarizable L-L interface: there exists a potential region where little or no faradaic current flows. This is the so-called potential window, and the transfer of the base electrolyte ions limits its size.31 When the CVs in Figure 2a are compared, it is clear that the arrangement where the capillary is filled with the organic phase (dotted line) is more resistive compared with that filled with the aqueous phase (solid line). However, this IR drop does not pose a problem because the current flow through the L-L interface is induced by the reaction occurring at the SECM tip and, hence, the current will be in the nanoampere range, making the IR term negligible. SECM at the Externally Biased L-L Interface. To test the proposed application of SECM to the externally biased L-L interface, the ET reaction between organicphase TCNQ and aqueous-phase hexacyanoferrate was fully characterized. The kinetic parameters for the forward and reverse ET reactions, obtained by fitting the measured approach curves at different applied interfacial potentials, were compared to theoretical expectations based on Butler-Volmer kinetics.11,28 In Figure 3a is a set of approach curves obtained with cell I at different applied interfacial ∆w o φ (from bottom to top, -0.2 to -0.03 V). In this experiment, TCNQ is reduced to its radical anion at the SECM tip located in the 1,2DCE phase and then reacts irreversibly at the interface with Fe(CN)63-: kf

TCNQ-(1,2-DCE) + Fe(CN)63-(aq) 98 TCNQ(1,2-DCE) + Fe(CN)64-(aq) (1) with an associated bimolecular rate constant kf. As can be seen from the figure, the feedback response gradually changes from negative to positive as the applied ∆w o φ is made more positive. For a low driving force,28 the ButlerVolmer formalism is valid and the dependence of the rate

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Figure 3. SECM approach curves (solid lines) obtained for different applied ∆w o φ with cell I (a) and II (b) and the theoretical fits to the experimental data (dotted lines) using the theories described in the text. The feedback response gradually changes from negative to positive with increasing (a) or decreasing (b) ∆w o φ.

constant on the applied ∆w o φ is given by

of kb can be written as follows:

RF (∆ φ - ∆E )] [RT

kf ) k0 exp

w o

0

(2)

where k0 is the standard rate constant of the reaction, R 0,o is the transfer coefficient, and ∆E° ) ETCNQ/TCNQ •- 0,w EFe(CN)63-/4- is the difference between organic and aqueous standard redox potentials. From eq 2, it can readily be seen that by increasing ∆w o φ, the measured rate constant should increase exponentially. The SECM approach curves given in Figure 3a are consistent with this expectation. The transfer of tip-generated TCNQ•- from the organic to the aqueous phase can be discounted at potentials less 0′ positive than 0.185 V (∆w o φTCNQ•- ≈ 0.215 V), and coupled ion transfer does not have to be taken into account in the analysis.33 Although reaction 1 is second order, it can be treated as pseudo first order because the concentration of b b hexacyanoferrate is in excess, Kr ) cFe(CN) 3-/cTCNQ•- > 20, 6 and approach curves obtained could be fitted to the theory developed by Tsionsky et al. to extract kinetic parameters.11,20,36 In our case, however, the RG of the tips used was rather small (RG ) 5), and, hence, so-called back diffusion (diffusion from behind the plane of the SECM tip) was taken into account in the manner described by Amphlett and Denuault35 (Supporting Information). The simulation yields the dimensionless rate constant K ) kfa b cFe(CN) 3-/DTCNQ, where a is the radius of the microelec6 trode and DTCNQ is the diffusion coefficient of TCNQ. When DTCNQ ) 1.1 × 10-5 cm2/s is used,37 families of approach curves were generated for various values of kf and compared to the experimental approach curves. The dotted lines given in Figure 3a represent the best fits to the experimental curves. In this manner, the rate constant can be obtained as a function of the applied ∆w o φ. The potential dependence of the rate of the reverse ET was investigated using cell II. In this case, Fe(CN)64- is generated at the tip located in the aqueous phase and reacts interfacially with TCNQ: kb

TCNQ(1,2-DCE) + Fe(CN)64-(aq) 98 TCNQ-(1,2-DCE) + Fe(CN)63-(aq) (3) Similar to the forward rate constant, the ∆w o φ-dependence (36) Bard, A. J.; Mirkin, M. V.; Unwin, P. R.; Wipf, D. O. J. Phys. Chem. 1992, 96, 1861. (37) Zhang, Z.; Ye, J.; Sun, P.; Yuan, Y.; Tong, Y.; Hu, J.; Shao, Y. Anal. Chem. 2002, 74, 1530.

[

kb ) k0 exp -

βF w (∆ φ - ∆E0) RT o

]

(4)

where β is the transfer coefficient for reaction 3. Approach curves obtained at different applied ∆w o φ (from bottom to top, -0.060 to -0.160 V) are given in Figure 3b (solid lines). In this case, as a result of solubility limitations of b b /cFe(CN) TCNQ in 1,2-DCE, Kr ) cTCNQ 3- ) 5; thus, the first6 order approximation used above (valid for Kr > 20) is not valid, and the diffusion of both reactants, Fe(CN)64- and TCNQ, has to be taken into account in the solution of the transport problem. This was done in a manner described by Barker et al.20 (Supporting Information). The simulation’s input values are the ratio of the concentrations, b b /cFe(CN) Kr ) cTCNQ 3- ) 5, and the ratio of the diffusion 6 coefficients, γ ) DTCNQ/DFe(CN)63- ≈ 1.403,20 where values of DTCNQ ) 1.1 × 10-5 cm2/s and DFe(CN)63- ) 7.84 × 10-6 cm2/s were used.37,38 When the theory described in detail in the Supporting Information was used, the dotted lines in Figure 3b were obtained as the best fits to the experimental results by varying the dimensionless rate b /DFe(CN)63-. constant K ) kbacTCNQ Tafel plots, where the logarithm of the experimental bimolecular rate constants for reactions 1 (circles) and 3 (squares) are plotted as a function of the applied ∆w o φ are given in Figure 4. Linear fits to the experimental data at the standard potential, that is, the potential where kf ) kb, confirm that the potential dependence of both forwardand reverse-rate-constant systems follow Butler-Volmer kinetics, as was expected. Analysis of the slope yields a transfer coefficient of R ) 0.59 for the forward reaction 1, and β ) 0.66 is obtained for the reverse reaction 3, which agree well with the values reported in the literature.12,15,19,29,39 It is apparent from Figure 4 that there is a slight curvature in the Tafel plot for the reverse reaction at negative potentials. This may be due to double-layer effects, so-called Frumkin effects, where the apparent rate constant is dependent on the potential distribution across the interface.28 The rates of the forward and reverse 0′ reactions are equal at the standard potential (∆w o φ ) of w 0′ reactions 1 and 3, and, thus, ∆o φ ()-0.092 V) and the corresponding standard bimolecular rate constant k0 ()0.51 cm s-1 M-1) can be obtained from Figure 4. The value of k0 is roughly an order of magnitude higher than (38) Heyrovsky´, J.; Ku˚ta, J. Principles of Polarography; Academic Press: New York, 1966. (39) Zhang, J.; Unwin, P. R. J. Phys. Chem. B 2001, 105, 11052.

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Figure 4. Logarithm of the forward (kf, circles) and reverse (kb, squares) bimolecular rate constants as a function of the applied potential difference. The crosses are the results of the analysis for the reverse reaction, where the constant composition approximation was used, neglecting diffusion of TCNQ in the lower DCE phase.

those reported recently by Barker et al.19 and Zhang and Unwin29 and in earlier reports.22,23 This is consistent with the low ionic strength of the aqueous phase used in this study because a high aqueous ionic strength is known to depress the rate of ET at L-L interfaces.12,19 It should be noted that, in studies where partitioning ions are used to control the interfacial potential difference, concentrations of base electrolytes added to the aqueous and organic phases are typically different for each point on the Tafel plot. With the presented experimental arrangement, the interfacial potential is controlled potentiostatically and this is avoided. The crosses in Figure 4 show the Tafel plot obtained when the first-order approximation is used for the analysis of the approach curves given in Figure 3b and the contribution of TCNQ diffusion in the lower phase to the SECM tip current response is neglected. The slope of the plot is the same as that where diffusion in the lower phase was considered (open squares), while the rate constant is underestimated by a constant factor, that is, the entire plot shifts down. This trend was also noted by Zhang et al. for cases where Kr < 20.18 Barker et al. have treated this situation theoretically, and the observed underestimation is consistent with their predictions.20 Thus, the kinetic parameters obtained for the TCNQ/ hexacyanoferrate model ET system compare well with both theoretical expectations and experimental results obtained using different experimental arrangements, demonstrating that SECM in feedback mode can be successfully applied to the electrified L-L interface. This arrangement was then applied to study the interfacial reactivity of the fulleride anion at the externally biased water-CB interface. Two-Phase Oxidation of Fulleride by Molecular Oxygen. The reaction between the tip-generated fulleride monoanion in the organic phase and dissolved oxygen in the aqueous phase can be written as follows:

Figure 5. CVs obtained at the micropipet-supported waterCB interface (cell III) when the Pt-coat electrode is biased at a potential where diffusion-limited reduction of C60 occurs [concentration of C60 is 1.0 mM (solid line) or 2.0 mM (dasheddotted line)] and in the absence of the applied coat potential (dotted line). The sweep rate was 100 mV/s.

Thermodynamically, the reduction of O2 at pH > 4, in water, can be written as follows:40

O2 + e- f O2-

(7)

O2- + H+ f HO2

(8)

O2- + 2H+ + e- f H2O2

(9)

(and then possibly)41 H2O2 + 2H+ + 2e- f 2H2O

(10)

which is most likely followed by the reaction of superoxide to water, giving, in total, a four-electron reaction according to

The overall reaction, thus, involves either two-electron or four-electron transfer depending on whether the final product is hydrogen peroxide or water. However, the overall kinetics of reaction 6 are governed by a single ET rate-determining step, which we assume is the initial ET given in reaction 5. The influence of the overall reaction stoichiometry on the interpretation of the measured kinetic parameters is discussed in more detail below. The role of reaction 8 in stabilizing the superoxide formed in reaction 5 could possibly be assessed by systematic variation of the aqueous-phase pH. This will be considered fully in a further study. Prior to the SECM experiments, initial studies on the heterogeneous oxidation of C60- with oxygen were undertaken using a combined electrolysis/cyclic voltammetric approach at a Pt-coat micropipet-supported water-CB interface, as is shown schematically in Figure 1b. This method is based on the localized electrolysis of an electroactive species at the Pt-coat electrode in the vicinity of a polarized L-L interface supported at the tip of a micropipet. Concurrent CVs recorded at the L-L interface give an indication of the lipophilicity of the generated charged species33 or, as in this case, its reactivity. The presence or absence of C60 in the organic phase did not result in a noticeable difference in the CV response at the micropipet-supported water-CB interface. The dotted line in Figure 5 shows the potential window for cell III in the presence of fullerene in the absence of applied potential to the Pt-coat electrode. When the Pt electrode is biased such that the reduction of fullerene to the fulleride

4C60-(CB) + O2(aq) + 4H+(aq) f 4C60(CB) + 2H2O(aq) (6)

(40) Suzuki, M.; Matsui, M.; Kihara, S. J. Electroanal. Chem. 1997, 438, 147. (41) Hoare, J. P. In Standard Potentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds.; Marcel Dekker: New York, 1985.

kf

C60-(CB) + O2(aq) 98 C60(CB) + O2-(aq)

(5)

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Figure 6. SECM approach curves (solid lines) obtained with cell III where tip-generated C60- reacts interfacially with O2 and theoretical fits to the experimental data (dotted lines) using the theory described in the text.

monoanion occurs at a diffusion-limited rate, a voltammetric wave is apparent in the water-CB CV response. Figure 5 shows the results of two such experiments with different concentrations of fullerene, 1 mM (solid line) and 2 mM (dashed-dotted line). The height and the halfwave potential of the wave are dependent on the concentration of fullerene. The limiting current is doubled, as was expected, as the concentration is increased from 1 to 2 mM, indicating that the limiting species for reaction 6 is C60. However, the shift of the half-wave potential is substantial (ca. 0.220 V) and cannot be explained by simple ion-pair formation between the electrogenerated C60- and the base electrolyte cation. The reason for this is currently speculative. Deoxygenating the solutions by purging with nitrogen results in a decrease in the wave-limiting current Ilim until, under oxygen-free conditions, the wave is absent and the background response is regained. Reexposing the solution to air results in the reappearance of the wave. These measurements show that reaction 6 is heterogeneous and give an indication of the potential region where the ET reaction can be switched on/off. Thus, in terms of the predicted SECM approach-curve response as a function of applied ∆w o φ, a negative feedback response is expected upon approaching the water-CB interface for ∆w oφ < -0.4 V (for a 1 mM fullerene solution) with a gradual transition to a positive feedback response as the interfacial potential is made more positive. CVs recorded at the SECM Pt microelectrode for the reduction of C60 to the monoanion in CB were unchanged in the presence and absence of oxygen, as has been previously reported.1,2 For the SECM experiments, fulleride was generated at the SECM tip dipped in the upper CB phase, and approach curves were obtained by following the tip current response as the tip approached the water phase located in the glass capillary. The tip was biased at the beginning of the limiting current region for C60 + e- f C60•- reduction because the formation of C602- under ambient conditions leads to the formation of an electroactive polymeric film of C60O.42 Experimental approach curves (full lines) for the interfacial ET reaction between tip-generated fulleride and aqueous-phase oxygen for a range of applied ∆w o φ (from bottom to top, -0.465 to -0.260 V) are given in Figure 6. The trend displayed in the approach curves is consistent with predictions based on the micopipet CV response given in Figure 5, that is, a gradual shift from negative to positive feedback as the interfacial potential is made more positive. This is a clear (42) Winkler, K.; Costa, D. A.; Fawcett, W. R.; Balch, A. L. Adv. Mater. 1997, 9, 153.

Liljeroth et al.

Figure 7. Logarithm of the bimolecular rate constant (kf) as a function of the applied potential difference for reaction 5.

illustration that the rate of ET is dependent on the applied interfacial potential difference. Kinetic parameters cannot be extracted from the approach curves based on the firstorder approximation because the concentration of dissolved O2 in water is not in excess compared to that of C60. Thus, diffusion of both reactants, fulleride and oxygen, has to be taken into account, and the theoretical treatment described in the previous section for Kr < 20 was used to extract the bimolecular rate constant. In this case, the simulation’s input values are the ratio of the concentrab /cb ) 0.25 (with the oxygen concentration tions Kr ) cO 2 C60 in water under normal atmospheric pressure taken as 0.25 mM) and γ ) 4DO2/DC60 ) 22.56, where DO2 ) 2.2 × 10-5 cm2/s, DC60 ) 3.9 × 10-6 cm2/s,4 and the factor 4 accounts for the 4C60/O2 stoichiometry. When the theory described in detail in the Supporting Information is used, the dotted lines in Figure 6 were obtained as the best fits to the experimental results by varying the dimensionless rate constant. In this way, values of kf as a function of applied ∆w o φ were obtained. Coupled ion transfer of species involved in the ET reaction, C60-, and superoxide can be discounted in the potential region investigated. Fulleride transfer was not observed in the coated-pipet measurements (Figure 5), consistent with the known hydrophobicity of this ion. From a thermodynamic point of view, superoxide transfer to the CB phase should be energetically very unfavorable in the moderate ∆w oφ range used in this study. The corresponding Tafel plot using the values of kf obtained from curve fitting is given in Figure 7, and it can be seen that the logarithm of the rate constant increases linearly with increasing ∆w o φ, as was expected for ButlerVolmer-type kinetics. The charge-transfer coefficient R ) 0.27 was estimated from the slope of the plot. The standard rate constant can be estimated based on the standard 0,w w 1 potential ∆E° ) EC0,o60/C60- - EO •- ≈ -0.394 V, with ∆o φ 2/O2 -1 M-1. ) ∆E°, where the rate constant is about 3.5 cm s This analysis was based on the assumption of an overall four-ET reaction. If n ) 2, oxygen depletion in the aqueous phase due to ET is overestimated in our analysis, resulting in an underestimation of the rate constant by a constant factor. The potential dependence of the rate would not be affected. The transfer of oxygen across the interface was not considered in the model described here. The concentration of O2 is typically greater in organic solvents compared to that in water, and the diffusion-controlled transfer of O2 across the water-1,2-DCE interface has been reported.32 Such transfer at the CB-water interface would offset aqueous oxygen depletion due to reaction 5 and act as a reservoir for the reactant. Our analysis treatment would then overestimate oxygen depletion and result in an underestimation of the rate constant by a

Two-Phase Oxidation of C60-

constant factor, whereas the potential dependence would remain unchanged. Thus, the standard rate constant may be greater than that reported here. For this initial study, a simplified approach was used because there is little information available concerning the oxygen-reduction mechanism at L-L interfaces. Results obtained indicate that the rate of reaction is dependent on the driving force, which in this case is ∆w o φ because it is the only parameter varied. In a previous report where Fe(CN)64- was used as the aqueous electron acceptor,4 the rate dependence on ∆w oφ was the inverse of that reported here with kf decreasing with increasing driving force. The authors concluded that the reaction was in the Marcus inverted region.28 In the present study, the highest driving force, ∆w o φ - ∆E°, where the ET rate was not diffusion-limited, was about 0.2 V. This value is more than 0.5 V lower compared to that of the Fe(CN)64- case [on the basis of the difference between the standard redox potentials of Fe(CN)63-/4- and C600/- couples, ∆E° ) 0.535 V]. On the basis of the calculated values for the solvent reorganization energy, the driving force should be greater than 0.8 V to be in the inverted kinetic region; thus, the Butler-Volmer or normal Marcus region kinetic response noted here is consistent with the low values of driving force. Conclusion The SECM feedback mode was successfully applied to the externally biased L-L interface, as was demonstrated by the success of the method to determine kinetic

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parameters for the model TCNQ/hexacyanoferrate ET system. While fulleride does not react homogeneously with oxygen, it can be oxidized heterogeneously at the L-L interface, as was demonstrated by the in situ reduction of fullerene at the Pt-coat micropipet-supported waterCB interface. SECM at the externally biased water-CB interface was used to determine the driving-force dependence of the rate of the interfacial reaction. The ET reaction is fast, and the rate increases with increasing driving force in line with predictions based on Butler-Volmer kinetics. The above experimental arrangements are readily applicable to the study of both the lipophilicity and reactivity of fullerene derivatives at L-L interfaces and offer a novel means of probing interfacial behavior. Furthermore, the proposed SECM setup makes it possible to conduct potential-dependentexperimentsatconstantionicstrength, which is crucial in comparing experimental kinetic parameters with theoretical predictions. Acknowledgment. Funding from the Academy of Finland and the National Technology Agency is gratefully acknowledged. The authors thank Zhifeng Ding and Lasse Murtoma¨ki for valuable discussions. Supporting Information Available: Detailed description of the SECM theory and additional experimental information. This material is available free of charge via the Internet at http://pubs.acs.org. LA0270338