Langmuir 1999, 15, 819-825
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Chemically-Selective and Spatially-Localized Redox Activity at Ta/Ta2O5 Electrodes Solomon B. Basame and Henry S. White* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received July 7, 1998. In Final Form: November 18, 1998 Electron-transfer reactions at Ta electrodes covered by a thin (∼2.5 nm) native oxide film, hereafter designated as Ta/Ta2O5, have been investigated using scanning electrochemical microscopy (SECM). Oxidation and reduction of soluble redox species (I-, Fe(CN)64-, and Ru(NH3)63+) are localized to randomlydistributed, microscopic sites (radius ) 2-50 µm) at the surface of the Ta/Ta2O5 electrode. The electroactive sites display an unusual dependence on the identity of the redox species. Specifically, SECM images show that some sites are only active for reduction of Ru(NH3)63+ while others, on the same surface, are active for both the reduction of Ru(NH3)63+ and the oxidation of I-. The rate of I- oxidation at individual sites has been quantified by SECM and shown to approach the theoretical mass-transport-limited value, suggesting that the oxide film is either absent or very thin (∼1 nm) at these electroactive sites. The influence of the Ta2O5 film thickness on electron-transfer kinetics at individual sites has also been investigated. A sudden decrease in the electron-transfer rate is observed at individual sites during controlled anodic growth of the film. The abrupt decrease in electron-transfer rate at different redox-active sites occurs at electrode potentials spanning a ∼1 V range, reflecting differences in the local energetics and kinetics associated with nucleation and growth of the Ta2O5 film.
Introduction Thin oxide films on metal surfaces are of technological importance in microelectronics, catalysis, and corrosion resistance.1 The electronic and chemical properties of oxide films are employed in chemical sensors and determine the biocompatibility of metal-based medical implants. While the properties of thick oxide films are relatively well understood, the initial stages of oxide growth and the electronic and barrier properties of very thin films have received less attention.1,2 The native oxide film that forms upon exposure of the metal to oxygen or water at room temperature is typically only a few nanometers thick on most metals. Thicker oxide films can be grown at elevated temperatures or by using electrochemical methods to increase the driving force for oxide formation.2 In a series of recent reports, our laboratory has investigated the electrochemical behavior of Ti electrodes covered by native (∼2 nm) and anodically grown (∼5 nm) films of TiO2.3 The electrochemical reactivity at the Ti/ TiO2 electrodes is determined by electron tunneling across the oxide film and by conduction of electrons through the oxide lattice. Electron tunneling across very thin oxide films is possible but is effectively blocked by films that have a thickness greater than a few nanometers. Scanning electrochemical microscopy4,5 (SECM) of Ti/TiO2 electrodes (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (2) (a) Young, L. Anodic Oxide Films; Academic Press: New York, 1961. (b) Oxides and Oxide Films; Diggle, J. W., Ed.; Marcel Dekker: New York, 1972. (3) (a) Basame, S. B.; White, H. S. J. Phys. Chem., in press. (b) Basame, S. B.; White, H. S. J. Phys. Chem. 1995, 99, 16430. (c) Casillas, N.; Charlebois, S. J.; Smyrl, W. H.; White, H. S. J. Electrochem. Soc. 1994, 141, 636. (d) Casillas, N.; Charlebois, S. J.; Smyrl, W. H.; White, H. S. J. Electrochem. Soc. 1993, 140, L143. (4) (a) Bard, A. J.; Fan, F.-R, F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61, 132. (b) 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. (5) (a) Engstrom, R. C.; Weber, M.; Wunder, D. J.; Burgess, R.; Winquist, S. Anal. Chem. 1986, 58, 844. (b) Engstrom, R. C.; Meaney, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987, 59, 2005. (c) Engstrom R. C. Anal. Chem. 1984, 56, 890.
during the oxidation or reduction of soluble redox species has demonstrated that the observed electrochemical activity on Ti/TiO2 is dominated by a small number of redox-active microscopic sites, the remainder of the surface being electrochemically silent.3a The origin of the spatial localization of the electron-transfer reactivity on Ti/TiO2 is not fully understood but is clearly related to a significant spatial variation in the electronic, structural, and chemical properties of the oxide film. In the present report, we use SECM to demonstrate that the electrochemical behavior of Ta electrodes covered by a native oxide film of Ta2O5 is also dominated by electron-transfer reactions occurring at a few microscopic sites on the surface. However, unlike the results previously reported for Ti/TiO2, the redox-active sites on Ta/Ta2O5 are selective toward the identity of the redox molecules that is, some sites are active for one type of redox-active molecule but not for other redox molecules in the same solution. SECM analysis is also used to monitor electrontransfer rates at individual redox sites during anodic growth of the oxide film. Experimental Section Chemicals. Potassium iodide, potassium ferrocyanide, potassium sulfate, hydrofluoric acid, nitric acid (all from Mallinckrodt), and hexaamineruthenium trichloride (Johnson Matthey) were used as received. All aqueous solutions were prepared using water (∼18 MΩ‚cm) purified with a Barnstead “E-pure” system. X-ray Photoelectron Spectroscopy. The XPS spectra were obtained using a VG ESCA lab 220i multianalyzing instrument with Al KR radiation of 1486.7 eV and an electron takeoff angle of 90° with respect to the sample surface. The binding energy scale was calibrated with respect to the C 1s binding energy of 284.6 ( 0.1 eV. Ta/Ta2O5 Electrodes. Ta electrodes were prepared from 25µm-thick 99.997% Ta foil (Johnson Matthey). Cu wire was contacted to an ∼1.5 cm2 piece of Ta foil sample using conductive Ag epoxy. The Ag epoxy was covered by an inert nonconductive epoxy, and the entire assembly was mounted on a glass tube that served as a sample holder for voltammetry and SECM imaging. Immediately prior to each experiment, the electrode was etched
10.1021/la9808216 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998
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Figure 2. Schematic drawing depicting the SECM carbonfiber tip positioned a few micrometers above the native-oxidecovered Ta electrode. The tip radius and tip-to-Ta separation d are drawn to scale. The Ta2O5 film (∼2.5 nm) is much thinner than that drawn.
Figure 1. X-ray photoelectron spectra of Ta electrodes covered with (a) a native oxide film and (b) a thick anodically-grown Ta2O5 layer. Peaks A (21.8 eV) and B (23.7 eV) correspond to the spin-orbit-coupled 4f7/2 and 4f5/2 photoelectron lines of elemental Ta. Peaks C (27.3 eV) and D (29.2 eV) correspond to the same lines in Ta2O5. for 3 min in an aqueous solution containing 4% HF and 6% HNO3 and then thoroughly rinsed with water. The thickness of the air-formed oxide film on Ta electrodes has been previously reported to be 2-3 nm.6a-d Sung and Bard have recently measured a value of 2.47 nm.7 The results of XPS analysis of electrodes prepared for our study are consistent with these values. Figure 1 shows spectra of two Ta electrodes, one covered by a native oxide film and the other by a thick (∼15 nm) anodically-grown8 layer of Ta2O5. The XPS spectrum of the electrode coated by the anodically-grown layer shows peaks associated with the Ta 4f7/2 and 4f5/2 lines at binding energies of 27.3 (peak C) and 29.2 eV (peak D). These peaks correspond to Ta(V) in Ta2O5.9,10 In addition to peaks C and D, the XPS spectrum of the native-oxide-covered Ta also displays peaks at 21.8 (peak A) and 23.7 eV (peak B), corresponding to elemental Ta.9 Because the escape depth of photoelectrons for the Ta 4f lines is approximately 3.4 nm,11 the appearance of peaks for Ta(0) suggests that the native oxide has an average thickness that is, at most, on the order of a few nanometers. The XPS peaks at 27.3 (C) and 29.2 eV (D) indicate that the nominal chemical composition of the native oxide is Ta2O5. However, a pair of very small peaks, located at binding energies slightly higher than peaks A and B for Ta(0), are also apparent in the XPS spectrum of the native-oxide-covered Ta electrode (marked by * in the spectrum). These peaks have not been assigned but most likely correspond to Ta(I) or Ta (II) in the oxide film. In summary, the native oxide on the Ta electrodes employed in this study has an average thickness of a few nanometers, consists primarily of Ta2O5 (>95%), but probably also contains an unidentified lower-valent oxide. All experiments were performed at room temperature using freshly-etched Ta electrodes (6) (a) Macagno, V.; Schultze, J. W. J. Electroanal. Chem. 1984, 180, 157. (b) Vermilyea, D. A. Acta Metall. 1953, 1, 282. (c) Smith, D. J.; Young, L. Thin Solid Films 1983, 101, 11. (d) Mathieu, H. J.; Landolt, D. Surf. Interface Anal. 1983, 5, 77. (7) Sung, Y.-E.; Bard, A. J. J. Phys. Chem., submitted for publication. (8) The anodic oxide film was deposited by scanning the potential of a Ta electrode immersed in a 0.5 M H2SO4 solution to 11 V at a scan rate of 10 mV/s. (9) Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., King, R. C., Eds.; Physical Electronics, Inc.: 1995. (10) McGuire, G. E.; Schweitzer, G. K. K.; Carlon, T. A. Inorg. Chem. 1973, 12, 2451. (11) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
covered by the native oxide filmsthese electrodes are referred to as Ta/Ta2O5. Scanning Electrochemical Microscopy. SECM is a scanned-probe microscopy based on the amperometric detection of redox-active molecules.4 During image acquisition, the SECM tip is scanned across the sample surface at a fixed height and the tip current it is recorded as a function of its spatial position (Figure 2). In the experiments reported here, the substrate potential Es and the SECM-tip potential Et are independently controlled by two separate potentiostatic circuits. Es is set at a value to either oxidize or reduce a redox-active species that is present in the bulk of the solution, while Et is held at a potential to detect the products that are electrogenerated at the substrate. A detailed description and schematic of the in-house-built SECM has been previously reported.12,13 Pt wire auxiliary electrodes, Ag/AgCl (3 M NaCl) reference electrodes, and a C-fiber SECM tip are used in these studies. The tip is fabricated by coating a 8-µm-diameter C fiber with an insulating polymer film. The polymer-coated fiber is cut with a razor blade to expose a 4-µm-radius disk-shaped C electrode. A detailed description of the preparation of the C-fiber SECM tip has been previously published.3a,b The radius of the C disk is determined by measuring the voltammetric limiting current ilim corresponding to the oxidation of Fe(CN)64-. The electrode radius rt is related to ilim by eq 1,14
ilim ) 4nFDC*rt
(1)
where C* is the concentration of redox species, D is the diffusivity of Fe(CN)64- (0.65 × 10-5 cm2/s),15 F is Faraday’s constant, and n is the number of electrons transferred per molecule. Following the assembly of the cell components, the SECM tip is slowly lowered onto the substrate surface (at open circuit) until a gentle contact is made. Using a video microscope with a zoom lens for viewing, the tip is then retracted in the z direction in increments of 0.2 µm until it is observed to travel freely in the x-y plane. The tip is retracted by an additional 2-5 µm in order to prevent contact with the surface during imaging. The tip is scanned during imaging at a velocity of 10 µm/s.
Results and Discussion Voltammetric Behavior of Ta/Ta2O5 Electrodes. The voltammetric responses of Ta/Ta2O5 electrodes in aqueous solutions containing either 10 mM Fe(CN)64-, 10 mM I-, or 5 mM Ru(NH3)63+ are shown in Figure 3. All solutions contained 0.1 M K2SO4 as the supporting electrolyte. The Ta/Ta2O5 electrode exhibits a slightly different behavior in each of the three redox solutions. The (12) Scott, E. R.; White, H. S.; Phipps, J. B. Anal. Chem. 1993, 65, 1537. (13) Scott, E. R.; Laplaza, A. I.; White, H. S.; Phipps, J. B. J. Pharm. Res. 1993, 10, 1699. (14) Saito, Y. Rev. Polarogr. 1968, 15, 177. (15) von Stackelberg, M.; Pilgram, M.; Toome, V. Z. Elektrochem. 1953, 57, 342.
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Figure 3. Voltammetric response (20 mV/s) of an ∼1 cm2 Ta/ Ta2O5 electrode in aqueous solutions containing either 10 mM I-, 10 mM Fe(CN)4-, or 5 mM Ru(NH3)63+; all solutions contained 0.1 M K2SO4 as the supporting electrolyte.
voltammetric response in the I- and Fe(CN)64- solutions is characterized by an irreversible wave at potentials positive of the standard redox potentials for the reactions
3I- f I3- + 2e-
(2)
Fe(CN)64- f Fe(CN)63- + e-
(3)
The voltammetric wave for I- oxidation is sigmoidal shaped, reminiscent of the voltammetric response at a disk-shaped electrode of micrometer dimensions.16 Neither of the voltammetric responses in the I- or Fe(CN)64solutions shows a cathodic wave on the reverse scan toward negative potentials. In contrast, the reduction of 5 mM Ru(NH3)63+
Ru(NH3)63+ + e- f Ru(NH3)62+
(4)
is characterized by a cathodic wave that is approximately twice as large as the oxidation waves for I- and Fe(CN)64even though the concentration of Ru(NH3)63+ is only half as large. The wave for Ru(NH3)63+ reduction has a pronounced peak shape, typical of a diffusion-controlled reaction at a large planar surface. The response of Ta/ Ta2O5 in this solution also exhibits an irreversible oxidation wave on the reverse positive-going scan. Ta2O5 is a large-band-gap (∼4 eV17), n-type semiconductor, and it is thus possible that the different behaviors described above reflect, at least in part, the positions of the conduction and valence band edges at the electrode/ electrolyte interface relative to the energy levels associated with the redox molecules. However, the SECM images presented in the following section demonstrate that the (16) (a) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmidt, P. P. Ultramicroelectrodes; Datatech Systems: Morganton, NC, 1987. (b) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1970; Vol. 4, p 129. (17) (a) Clechet, P.; Martin, J. R.; Ollies, R.; Vallouy, C. R. Acad. Sci. 1976, 282C, 887. (b) Vijh, A. K. In Oxides and Oxide Films; Diggle, J. W., Ed.; Marcel Dekker: New York, 1972; Vol. 2.
Figure 4. Top-view SECM images (x-y plane) of redox-active sites on a Ta/Ta2O5 electrode. Image a was recorded in an aqueous solution containing 10 mM Fe(CN)64- and 0.1 M K2SO4. Image b was recorded in a solution containing 10 mM I- and 0.1 M K2SO4. The images correspond to different regions on the surface of the same electrode. The tip-substrate separation d is ∼4 µm. Images were recorded at Es ) 1.0 V versus Ag/AgCl and Et ) 0.0 V.
electron-transfer reactions are localized to microscopic redox sites on the Ta/Ta2O5 surface, and that the activity of these sites displays an unusual dependency on the identity of the redox species. Thus, the voltammetric response reflects not only the electronic properties of Ta2O5 but also the spatial distribution of the reaction. SECM Images of Electroactive Sites on Ta/Ta2O5. Figure 4a shows an SECM image of a 200 × 200 µm2 area of a Ta/Ta2O5 electrode in a solution containing 10 mM Fe(CN)64- and 0.1 M K2SO4 as the supporting electrolyte. The image was obtained with the Ta/Ta2O5 potential (Es) set at 1.0 V to oxidize Fe(CN)64- and the SECM tip potential (Et) at 0.0 V to detect any Fe(CN)63- that is electrogenerated at the Ta/Ta2O5 surface (see Figure 2). The tip-substrate separation distance d was ∼4 µm. The dark regions in this image correspond to a large tip current it and thus a high local concentration of Fe(CN)63-. (The quantitative relationship between it and the redox concentration is given by eq 1.) The image in Figure 4a demonstrates that the oxidation of Fe(CN)64- is localized to two microscopic sites within the 200 × 200 µm2 region.
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The characteristic dimensions of the electroactive sites range from 2 to 50 µm, as will be discussed in a later section. Figure 4b shows an SECM image of a different region of the same surface in a solution containing 10 mM I-. The image was obtained with Es set at 1.0 V to oxidize Iand Et set at 0.0 V to detect electrogenerated I3-. The dark regions in this image correspond to high local concentrations of I3-. Thus, similar to the behavior observed for Fe(CN)64-, the oxidation of I- is localized at microscopic sites on the Ta/Ta2O5 surface. The SECM images suggest that the sigmoidal shape of the voltammetric wave observed in the I- solution (Figure 3) reflects the electron-transfer reaction taking place at an array of discrete, randomly-positioned microscopic surface sites, rather than occurring uniformly across the Ta/Ta2O5 surface. The surface density of active sites was determined in the Fe(CN)64- and I- solutions by imaging the entire surface of several Ta2O5 electrodes and counting the number of sites that were apparent in the images. The surface density was first determined in one solution, for example, the Fe(CN)64- solution, and then redetermined, using the same electrode, in the second solution using the same imaging parameters. The results from 10 experiments yielded an average of 259 ( 51 sites/cm2 for Ioxidation and 81 ( 9 sites/cm2 for Fe(CN)64- oxidation. This difference in surface activity is not yet understood but clearly reflects a difference in the electron-transfer kinetics for these two redox reactions, the oxidation of Ibeing more facile. A logical extension of this experiment would be to obtain SECM images of Ta/Ta2O5 electrodes in solutions containing both I- and Fe(CN)64- to determine if I- and Fe(CN)64- are oxidized at the same redox-active sites. Unfortunately, it is impossible to discern between oxidation of I- and Fe(CN)64- in the SECM images, since the redox-active potential regions for these species are essentially identical (Figure 3). Imaging the chemical activity associated with two different redox reactions at the same Ta2O5 electrode, however, can be performed in solutions containing Ru(NH3)63+ and I-, since these redox molecules are reduced and oxidized, respectively, at very different electrode potentials. For example, Figure 5 shows a set of SECM images of a Ta/Ta2O5 electrode that is immersed in a solution containing 2.5 mM Ru(NH3)63+, 10 mM I-, and 0.1 M K2SO4. The images were obtained sequentially in the order A, B, C, and D. Image A is obtained at Es ) 1.0 V and Et ) 0.0 V, corresponding to imaging only those sites active for the oxidation of I- (see the voltammogram below the SECM images). Two sites (labeled 1 and 2) are observed in this 200 × 200 µm2 image. The Ta/Ta2O5 and tip potentials were then switched to Es ) -0.8 V and Et ) 0.4 V, respectively, to image those sites active for the reduction of Ru(NH3)63+. Image B shows that four sites are active for Ru(NH3)63+ reduction. Two of these correspond to sites active for I- oxidation (1 and 2). The other two sites (3 and 4) were not active in the I- solution and thus are apparently active only for Ru(NH3)63+ reduction. The oxidation of I- and reduction of Ru(NH3)63+ at the active sites can be reproducibly switched “on” and “off” without any loss of activity. Image C, corresponding to Ioxidation, was obtained after recording image B; only sites 1 and 2 are active, and their positions exactly match those in the original image of I- oxidation activity (image A). Similarly, when the surface was reimaged for Ru(NH3)63+ reduction (image D), all four sites previously observed in image B were apparent. As previously noted, Ta2O5 is a large-band-gap, semi-
Basame and White
conductor material. Sung et al. have estimated the position of the conduction band Ec of the n-type native Ta2O5 layer on Ta to be ∼ -0.8 V versus Ag/AgCl in acetonitrile solutions.18 Ideally, for an n-type semiconductor, electrontransfer reactions are expected to occur rapidly when the electrode potential is biased negative of Ec. Conversely, reactions are expected to be inhibited at potentials positive of Ec, since no electron states are available in the bandgap region. Assuming that the value of Ec in aqueous solutions is not too different from the value measured by Sung et al., the behaviors of sites 3 and 4 (Figure 5) are consistent with the expected ideal behavior. Ru(NH3)3+ is reduced at potentials near Ec, while oxidation of I- does not occur at potentials positive of Ec. In contrast, sites 1 and 2 are active at all potentials and, thus, can be described as exhibiting metal-like behavior. Analogous metal-like redox sites have been previously observed on Ti electrodes that are covered by a thin native oxide.3,19 The sites observed on the Ta/Ta2O5 electrodes probably correspond to microscopic regions where the oxide film is sufficiently thin to allow electron tunneling between Ta and the redox species or to a local region where the oxide film is more electrically conductive due to a variation in oxide stoichiometry or to impurities. SECM Measurement of Reaction Kinetics at Individual Electroactive Sites. Figure 6a shows a 50 × 50 µm2 cross-sectional (i.e., x-z plane) SECM image over an active site in a 10 mM I- solution (Es ) 1.0 V, Et ) 0.0 V, scan rate ) 10 µm/s). As in the top-view (i.e., x-y plane) SECM images, the dark regions correspond to a high concentration of electrogenerated I3-. The semicircular concentration pattern that is apparent in the crosssectional SECM image reflects the radial diffusion of I3away from the active site. The concentration profile of I3- was quantified by measuring the current at the SECM tip as a function of the distance z above the electrode surface (z is equivalent to d, the tip-to-substrate distance) and using eq 1 with n ) 2 to relate it to the local concentration. A value of 0.74 × 10-5 cm2/s was employed for the diffusivity of I3-.20 The resulting experimental profile C(z) is plotted in Figure 6b. Approximating the shape of the active site as a disk and noting that transport of I3- away from the redox site is by diffusion, the theoretical concentration profile above the disk is given by eq 5,13
C(z) )
2Cs a tan-1 π z
()
(5)
where Cs is the concentration of I3- at the surface and a is the radius of the active site. Values of Cs and a for an individual site may be obtained by fitting eq 5 to the experimental data.21 For instance, the best fit of eq 5 to the data in Figure 6b yields Cs ) 1.9 mM and a ) 3.8 µm. The analysis of 14 sites on different electrodes yields values of a ranging from 2 to 46 µm and Cs values between 0.7 and 3.3 mM. The current at an active site (eq 6) may be expressed in terms of the two parameters Cs and a, determined by (18) Sung, Y.-E.; Gaillard, F.; Bard, A. J. J. Phys. Chem., in press. (19) Garfias-Mesias, L. F.; Alodan, M.; James, P. I.; Smyrl, W. H. Electrochem. Soc. 1998, 145, 2005. (20) Our measured value of the diffusivity of I- in aqueous 0.2 M K2SO4 is 1.47 × 10-5 cm2/s, slightly smaller than the literature value of 1.9 × 10-5 cm2/s (Adams, R. N., Electrochemistry of Solid Electrodes; Marcel Dekker: New York, 1969). The diffusivity of I3- is reported to be about 1/2 as large as the value for I-. Thus, we assume a value of 0.74 × 10-5 cm2/s for the diffusivity of I3-. (21) An empirical correction factor reported in ref 3a is used to correct the value of the site radius a for an error resulting from SECM-tipinterference effects. The correction factor is given by atrue ) 0.61(ameasured).
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Figure 5. (Top) SECM images of a 200 × 200 µm2 region of a Ta/Ta2O5 electrode in a solution containing both 2.5 mM Ru(NH3)63+ and 10 mM I- (supporting electrolyte: 0.1 M K2SO4). The images were recorded sequentially in the order A, B, C, and D and are of the same area of the electrode surface. Images A and C show two surface sites at which I- is oxidized (Es ) 1.0 V, Et ) 0.0 V), while images B and D show four redox sites at which Ru(NH3)63+ is reduced (Es ) -0.8 V, Et ) 0.4 V). Thus, sites 1 and 2 are active for both I- oxidation and Ru(NH3)63+ reduction, while sites 3 and 4 are active only for Ru(NH3)63+ reduction. The tip-substrate separation d is 6 µm for all images. (Bottom) Steady-state voltammetric response of Ta/Ta2O5 in the solution used for obtaining the SECM images. Scan rate ) 20 mV/s. The tip and substrate potentials used for obtaining SECM images are indicated on the voltammogram.
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Figure 7. Plot of the current density at individual redox-active sites versus the inverse of the site radii a-1. The results were obtained in a solution containing 10 mM I- and 0.1 M K2SO4. The solid line represents the expected dependence for the masstransport-limited oxidation of I- at the sites.
Figure 6. (a) Cross-sectional SECM image (x-z plane) of a redox-active site on Ta/Ta2O5 in a 10 mM I-/0.1 M K2SO4 solution. The image was obtained at Es ) 1.0 V and Et ) 0.0 V, corresponding to the oxidation of I- at the Ta/Ta2O5 substrate (3I- f I3- + 2e-) and the reduction of I3- at the SECM tip, respectively. The tip-substrate separation d is ∼4 µm. The image, including the electrochemically-active site (represented by the shaded area under the image), is drawn to scale. (The white 4 × 50 µm2 rectangle directly above the electrode represents a region of space where no data were acquired.) (b) Concentration profile of I3- measured along the center line z-axis extending above the active site (i.e., the vertical line in part a). The line drawn through the concentration profile represents the best fit of eq 5, yielding Cs ) 2.9 mM and a ) 3.8 µm.
fitting eq 5 to the concentration profile.
isite ) 4nFDCsa
(6)
Equation 6 is identical to eq 1, except that the concentration term corresponds to that of the electrogenerated product (I3-) at the surface, while D corresponds to the product species diffusivity. Figure 7 shows a plot of the current density at the 14 sites versus the inverse of the site radius, that is, a plot of isite/πa2 versus a-1, where isite is the current at the site due to I- oxidation. The straight line drawn in this plot represents the anticipated relationship between isite/πa2 and a-1, assuming that I- is oxidized at the mass-transport-limited rate. The experimental data for the smaller sites tend to fall along this line, indicating that the rate of electron transfer at these sites is large. Values of the current density for larger sites (a g 10 µm) consistently fall below the mass-transportlimited value, suggesting that the electron-transfer rate at these sites is reduced in comparison to those for the smaller sites. A similar trend was previously observed for Ti/TiO2 electrodes.3a We speculate that a slightly thicker
oxide layer covers the larger sites, thereby selectively reducing the rate of electron transfer at these sites. Since the probability of electron tunneling across the oxide film decreases exponentially with increasing film thickness, a small increase in the oxide film thickness, for example, of the dimensions of one unit cell of the oxide, is expected to result in a significant decrease in redox activity. SECM Measurement of Oxide Growth at Individual Electroactive Sites. The expected strong dependence of electron-transfer rate on the thickness of the oxide was employed to monitor the film thickness at individual sites during anodic oxide growth. SECM images of sites active for I- oxidation were recorded in a solution containing 10 mM I- and 0.1 M K2SO4, while holding the Ta/Ta2O5 potential Es at 1.0 V to effect the oxidation of I-. After an image was recorded, the electrode potential was scanned to 1.1 V at 10 mV/s and then returned to Es ) 1.0 in order to record a new SECM image. The intermediate scan to 1.1 V results in an increase in film thickness due to anodic oxide growth. This procedure was repeated with the electrode potential scanned to increasingly positive potential, Ef, between each image. For instance, the second image was recorded after scanning the electrode potential to 1.1, the third image after scanning to 1.2 V, and so on. However, the SECM images were always recorded at Es ) 1.0 V in order that the driving force for electron transfer remained constant during imaging. Figure 8 shows the results of two experiments using different Ta/Ta2O5 electrodes. The SECM-tip current measured along single-line scans is plotted, each curve corresponding to a different Ef. Two sites labeled I and II are active on each electrode, as indicated by the two peaks in the SECM line scans. For electrode 1, the peak currents at sites I and II are relatively constant for 1.0 e Ef e 1.5 V. Between 1.6 and 1.7 V, the current above site II decreases suddenly, indicating that this site is no longer active, presumably due to a local increase in the oxide thickness. However, site I, located only ∼100 µm from site II, remains electrochemically active up to Ef ∼ 2 V. Analogous behavior was observed at electrode 2, with deactivation of sites I and II occurring at 1.2 and 1.3 V, respectively. Results from experiments using seven different Ta/Ta2O5 samples show that the passivation potential at these sites ranges from 1.2 to 2.0 V. The abrupt decrease in electron-transfer rate at a microscopic redox-active site suggests that the mechanism of oxide growth involves nucleation of the oxide phase followed by rapid local growth. The fact that the redox
Redox Activity at Ta/Ta2O5 Electrodes
Langmuir, Vol. 15, No. 3, 1999 825
discontinuous over the ∼1 V range (1.0-2.2 V versus Ag/ AgCl) investigated in this experiment. Indeed, it is clear that the growth of the oxide film cannot be continuous over the ∼1 V range, since the dimensions of the orthorhombic Ta2O5 unit cell are a ) 0.62, b ) 0.37, and c ) 0.39 nm.22 Rather, within this potential range, the film thickness should increase in several discrete steps, each increment in thickness corresponding approximately to the unit cell length in the direction of oxide growth. Oxide growth at the active sites on Ta/Ta2O5 occurs over a wide range of electrode potentials (at least 0.8 V), indicating that the energetics and/or kinetics of film nucleation and growth varies significantly from one site to the next. This preliminary finding is currently being investigated in more detail in order to understand the various factors that influence oxide growth. For instance, the potential at which site deactivation occurs may correlate with the dimensions of the site, a, or with the kinetics of electron transfer at the site prior to oxide growth. SECM appears to be uniquely well suited for exploring these structure-reactivity relationships. Figure 8. (Top panels) Steady-state line plots of SECM tip current measured at neighboring redox-active sites at two Ta/ Ta2O5 electrodes (left and right) immersed in a solution containing 10 mM I- and 0.1 M K2SO4. All curves were recorded at Es ) 1.0 V and Et ) 0.0 V, corresponding to the oxidation of I- at Ta/Ta2O5 and the reduction of I3- at the SECM tip, respectively. After each SECM curve was recorded, the thickness of the Ta2O5 layer was increased by biasing the electrode potential, in incremental 0.1 V steps, between 1 and 2.2 V versus Ag/AgCl; that is, the first (top) curve corresponds to I- oxidation following oxide growth at 1.0 V, the second curve to oxide growth at 1.1 V, the third to oxide growth at 1.2 V, and so forth. (Bottom panels, right and left) Plot of the peak currents for I- oxidation at neighboring sites I and II as a function of the potential Ef used to grow the oxide film. The peak current at site II (for either electrode) disappears abruptly and at potentials prior to the disappearance of the peak current at site I.
Conclusions The results of this study demonstrate that SECM is useful in quantifying the microscopic current distribution at large-area electrodes that exhibit complex reaction behaviors. SECM images demonstrate that electrontransfer reactions at Ta/Ta2O5 electrodes occur at randomly distributed, microscopic sites, having dimensions ranging from 2 to 50 µm. The reactions at these sites are facile, approaching the diffusion-controlled rate for Ioxidation. The results suggest that the oxide film is very thin or entirely absent at the sites. The sites display selective redox activity; that is, some sites are active for one redox reaction but not for others. Further studies are underway to understand the origin of the chemical selectivity.
activity at neighboring sites remains unchanged clearly indicates that the oxide, once nucleated, does not grow beyond the area of the active site. This result is consistent with our earlier speculation that the redox activity at the site is due to the oxide film being very thinselectric-fielddriven anodic growth of the oxide film should be localized to regions where the film is initially thin. The average rate of growth of thick oxide layers on Ta is reported to be approximately 1.5 nm per volt increase in the applied potential.6b-d However, the sudden decrease in electrontransfer kinetics (Figure 8) suggests that oxide growth is
Acknowledgment. The authors thank Mr. Jodie L. Conyers for performing the XPS analysis of the Ta electrodes and Dr. Y. E. Sung and Dr. A. J. Bard for sharing preprints of articles describing their studies of Ta/Ta2O5 electrodes. This work was supported by the Office of Naval Research. The XPS analysis was performed in a National Science Foundation-funded surface analysis facility (CMS-9413498). LA9808216 (22) Lehovec, K. J. Less-Common Met. 1964, 7, 397.
