Noncontact Electrochemical Imaging with Combined Scanning

Combined scanning electrochemical atomic force micros- copy (SECM-AFM) is a ... image over the same area, but with the tip held in a fixed x-y plane a...
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Anal. Chem. 2001, 73, 550-557

Noncontact Electrochemical Imaging with Combined Scanning Electrochemical Atomic Force Microscopy Julie V. Macpherson* and Patrick R. Unwin

Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imagingsin conjunction with contact mode topographical imagingshave been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-µm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface. Scanning electrochemical microscopy (SECM) has proven a powerful scanned probe technique for the quantitative investigation of a wide range of interfacial processes.1 The probe tip in SECM is an ultramicroelectrode (UME), which typically has a characteristic dimension in the 0.1-25-µm range. SECM can be conventionally operated in one of three ways. The amperometric or potentiometric response of the UME is measured as the tip is (i) scanned over the interface, usually with the UME held in a (1) For reviews, see for example: (a) Barker, A. L.; Gonsalves, M.; Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. Anal. Chim. Acta 1999, 385, 223. (b) Unwin, P. R. J. Chem. Soc., Faraday Trans. 1998, 94, 3183. (c) Mirkin, M. V. Anal. Chem. 1996, 68, A177. (d) Bard, A. J.; Fan, F. R. F.; Mirkin, M. V. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 243-373. (e) Bard, A. J.; Fan, F. R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science 1991, 254, 68.

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fixed x-y plane above the substrate, i.e., “fixed-height” imaging; (ii) translated perpendicular to the surface of interest, to make tip approach measurements; or (iii) held in a fixed position with respect to the substrate, and the response determined as a function of time. For all three modes of operation, it is essential that the tipsubstrate separation is accurately known (and controlled), for quantitative analysis of the tip response. This is particularly important for fixed-height imaging experiments, where the tip response is influenced by both the tip-substrate separation (which in turn depends on the underlying topography and sample orientation) and the reactivity of the substrate, with a spatial resolution typically dependent on the dimension of the electrode. Indeed a drawback of fixed-height imaging with higher resolution (submicrometer) tips is that significant variations in the topography of the substrate, or even just a sample tilt, increase the chances of tip-surface crash, due to a lack of control of the separation between the tip and the substrate. Separating topographical effects from surface activity in SECM measurements has proven a challenging problem. A simple approach is to use a second electroactive mediator in solution to provide information on the tip-sample separation, from which topographical information is deduced.1e,2,3 However, this procedure still lacks a height control feedback mechanism, thus limiting the types of surfaces that can be studied to those that are relatively flat compared to the electrode dimension. As an interesting, alternative approach, vertical tip position modulation has been shown to be useful for distinguishing between contrasting conducting and insulating sites on a surface.4,5 Nonelectrochemical methods have been explored to provide feedback mechanisms, to maintain a constant tip-substrate separation (i.e., “constant distance”) during electrochemical imaging. In particular, by measuring the shear force damping of an UME, dithered laterally at resonant frequency6-8san approach similar to that used to regulate the substrate-probe separation (2) Lee, C.; Kwak, J.; Bard, A. J. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 1740. (3) (a) Gonsalves, M.; Barker, A. L.; Macpherson, J. V.; Unwin, P. R.; O’Hare, D.; Winlove, C. P. Biophys. J. 2000, 78, 1578. (b) Macpherson, J. V.; O’Hare, D.; Unwin, P. R.; Winlove, C. P. Biophys. J. 1997, 73, 2771. (c) Gonsalves, M.; Macpherson, J. V.; O’Hare, D.; Winlove, C. P.; Unwin, P. R. Biochim. Biophys. Acta 2000, 1524, 66. (4) Wipf, D. O.; Bard, A. J. Anal. Chem. 1992, 64, 1362. (5) Wipf, D. O.; Bard, A. J.; Tallman, D. E. Anal. Chem. 1993, 65, 1373. (6) Ludwig, M.; Kranz, C.; Schuhmann, W.; Gaub, H. E. Rev. Sci. Instrum. 1995, 66, 2857. (7) Hengstenberg, A.; Kranz, C.; Schuhmann, W. Chem. Eur. J. 2000, 6, 1547. 10.1021/ac001072b CCC: $20.00

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in scanning near-field optical microscopy (SNOM)9sit is possible to map sample topography, independent of the electrochemical measurement, with micrometer and submicrometer vertical resolution. In these measurements, the lateral resolution of the technique is typically lower than in SECM, limited by the diameter of the overall probe, i.e., not just the electrode but also the insulating sheath that surrounds it.7 Nonetheless, this represents a potentially interesting approach, with scope for further development and improvement.10 Atomic force microscopy (AFM) is well established as a technique for high-resolution topographical imaging of surfaces.11 In AFM, the topography of a substrate is mapped by monitoring the interaction force between the sample and a sharp tip, which is attached to the end of a force-sensing cantilever. AFM has been successfully combined with SNOM12 and scanning ion conductance microscopy (SICM),13 while our group has integrated AFM and SECM.14-18 In its initial form, combined SECM-AFM employed Pt-coated AFM tips, insulated at all but the cantilever and tip.14-18 These tips allowed simultaneous high-resolution electrochemical and topographical imaging in air,14,18 and high-resolution topographical imaging under solution, with electrochemical control of the local solution environment.15,16 To facilitate high-resolution electrochemical imaging under solution, SECM-AFM probes were custom-made from flattened and etched Pt microwires, coated with insulating, electrophoretically deposited paint.17 The flattened portion of the probe served as a force sensor (cantilever) while the coating procedure insulated the wire, such that only the tip (electrode) was exposed to solution. Simultaneously recorded current and topographical images were obtained with these probes, with a lateral and vertical resolution at the submicrometer level. Absolute measurement of the tip-substrate separation was also possible, as demonstrated by the simultaneous acquisition of tip current and cantilever deflection approach curves.17 For initial SECM-AFM studies, images were acquired with the tip in intimate contact with the surface, a scenario that does not accurately represent the imaging conditions of a true SECM experiment and that may limit the range of surfaces open to (8) James, P. I.; Garfias-Mesias, L. F.; Moyer, P. J.; Smyrl, W. H. J. Electrochem. Soc. 1998, 145, L64. (9) Shiku, H.; Dunn, R. C. Anal. Chem. 1999, 71, A23. (10) Bu ¨ chler, M.; Kelley, S. C.; Smyrl, W. H. Electrochem. Solid State Lett. 2000, 3, 35. (11) (a) Binnig, G.; Quate, C. F.; Gerber. Ch. Phys. Rev. Letts. 1992, 60, 2484. (b) Rugar, D.; Hansma, P. K. Phys. Today 1990, 43, 23. (c) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy; Cambridge University Press: Cambridge, U.K., 1994. (12) (a) Shalom, S.; Lieberman, K.; Lewis, A.; Cohen, S. R. Rev. Sci. Instrum. 1992, 63, 4061. (b) Keller, T. H.; Rayment, T.; Klenerman, D.; Stephenson, R. J. Rev. Sci. Instrum. 1993, 68, 1448. (c) Muramatsu, H.; Chiba, N.; Homma, K.; Nakajima, K.; Ohta, S.; Kusumi, A.; Fujihira, M. Appl. Phys. Letts. 1995, 66, 3245. (13) (a) Hansma, P. K.; Drake, B.; Marti, O.; Gould, S. A. C.; Prater, C. B. Science 1989, 243, 641. (b) Proksch, R.; Lal, R.; Hansma, P. K.; Morse, D.; Stucky, G. Biophys. J. 1996, 71, 2155. (c) Korchev, Y. E.; Bashford, C. L.; Milovanovic, M.; Vodyanoy, I.; Lab, M. J. Biophys. J. 1997, 73, 653. (14) Jones, C. E.; Macpherson, J. V.; Barber, Z. H.; Somekh, R. E.; Unwin, P. R. Electrochem. Commun. 1999, 1, 55. (15) Macpherson, J. V.; Unwin, P. R.; Hillier, A. C.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 6445. (16) Jones, C. E.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2351 (17) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 277. (18) Jones, C. E.; Barker, A. L.; Unwin, P. R.; Macpherson, J. V., unpublished results.

Figure 1. (a) Schematic of the experimental arrangement for SECMAFM measurements. In these studies, the substrate was a 10-µmdiameter Pt disk electrode and the tip was an SECM-AFM probe. The substrate could be biased externally or left unbiased. (b) Scanning electron micrograph of a typical SECM-AFM probe.

investigation. To further develop the capabilities of combined SECM-AFM, we demonstrate in this paper both fixed-height and constant-distance noncontact current imaging, with the collection of complementary topographical (and conductivity) data from contact mode measurements. This work is illustrated with substrate generation-tip collection and feedback studies of a 10µm-diameter disk UME, which serves as a model substrate. The approaches described allow electrochemical images, obtained with the tip above the surface, to be closely correlated with the underlying topography of the surface deduced from contact mode imaging. EXPERIMENTAL SECTION Materials. All solutions were prepared from Milli-Q reagent water (Millipore Corp.) and contained potassium hexachloride iridate(III) (IrCl63-; Sigma Aldrich) at a concentration of 0.01 mol dm-3 in 0.5 mol dm-3 potassium nitrate (Fisons, AR). The UME substrate was prepared by sealing a 10-µm-diameter Pt wire (Goodfellow, Cambridge, U.K.) in a glass capillary.19 To position the substrate in the AFM sample chamber, the UME was first reduced in length, to ∼1-2 mm, by cutting through the capillary shank with a file. A cylindrical block, cast in epoxy resin, 14 mm in diameter and 4 mm in height, into which a small hole had been drilled, was used to secure the electrode in position (see Figure 1a). The base of the hole was partly filled with silverloaded conducting epoxy resin (RS, Corby, U.K.), in which the (19) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15.

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electrode sat. External connection was then made via a length of tinned Cu wire, inserted through a small opening in the side of the base (Figure 1a). Any free space in the hole was filled with epoxy resin (Araldite Rapid, Bostik, Leceister, U.K.) to ensure the electrode was held firmly in place and that none of the silverloaded epoxy resin came into contact with the solution. The surface of the UME was polished with 0.05-µm-diameter alumina on a moistened polishing pad (Buehler, Coventry, U.K.) prior to use. Instrumentation. A Nanoscope E AFM and fluid cell (Digital Instruments, Santa Barbara CA) was employed for all measurements. The AFM was equipped with a sample scanner that facilitated a maximum scan range of 120 µm × 120 µm. The instrument was placed on a Newport Corp. (Newbury, CT) SHP series subhertz platform which, in turn, was placed on a custombuilt granite bench incorporating vibration isolators. The AFM was shielded using a home-built Faraday cage. Pt SECM-AFM tips were fabricated using a procedure outlined in detail previously.17 Electrical contact to the SECM-AFM probe was made via the metal spring clasp of a commercial fluid cell holder which rested on the exposed section of the Pt probe. This contact was then coated with a nail varnish/super glue mixture (1:1) which acted as an insulator. Visual inspection of the probes was carried out using an Olympus BH2 light microscope, equipped with a 3-CCD color video camera system (JVC, model KY-F55BE). The camera was linked to a video capture card that allowed the images to be transferred to a PC. For higher resolution imaging, a JEOL JSM-6100 scanning electron microscope was employed. Characterization of SECM-AFM Probe Geometry. SECMAFM tips with cantilever beam lengths in the range 500-700 µm, such as that shown in Figure 1b, were employed for the experiments described in this paper. Effective exposed electrode radii in the range 0.25-1.00 µm were estimated (assuming a hemispherical tip geometry), using eq 1, by measuring the steady-

i(∞) ) 2πnatipFDc*

(1)

state diffusion-limited current, i(∞), for the oxidation of aqueous 0.01 mol dm-3 IrCl63- in 0.5 mol dm-3 KNO3, where atip is the radius of the SECM-AFM tip electrode, n is the number of electrons transferred per redox event, F is Faraday’s constant, and D and c* are, respectively, the diffusion coefficient and concentration of the redox species of interest. A value of 7.5 × 10-6 cm2 s-1 was used for D.20 Further information on tip geometry was obtained by recording simultaneous cantilever deflection and tip current approach curves, as the tip approached and was retracted from an insulating glass surface, in a direction normal to the substrate. From the shape of the current response, it was possible to estimate the geometry of the exposed electrode.17,21,22 Amperometric SECM-AFM Measurements. The general experimental arrangement is shown in Figure 1a. For all electrochemical measurements, the Pt SECM-AFM tip served as a working electrode and the 10-µm-diameter Pt disk UME (exter(20) Macpherson, J. V.; Jones, C. E.; Unwin, P. R. J. Phys. Chem. B 1998, 102, 9891. (21) Mirkin, M. V.; Fan, F. R. F.; Bard, A. J. J. Electroanal. Chem. 1992, 328, 47. (22) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627.

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Figure 2. Schematic of the various imaging modes employed with SECM-AFM.

nally biased or unbiased) was the substrate. For substrate generation-tip collection (SG-TC)1d,23,24 measurements, the substrate was biased at a potential (+0.9 V), sufficient to oxidize the solution mediator (IrCl63-) at a diffusion-controlled rate, while the tip was held at a potential (+0.5 V) to reduce the oxidized species (IrCl62-) at a diffusion-limited rate. Experiments were carried out in a four-electrode mode with silver wire serving as a quasireference electrode (AgQRE) and Pt wire as a counter electrode. For feedback measurements,25 a two-electrode mode was employed. The SECM-AFM tip was held at a potential, relative to AgQRE, sufficient to oxidize the solution mediator (IrCl63-) at a diffusion-controlled rate. For these measurements, the substrate was not externally biased. The current was measured and the potential controlled using a model EI-400 bipotentiostat (Cypress Systems) with a high-gain preamplifier, connected to a purpose-built triangular wave/pulse generator (Colburn Instruments, Coventry, U.K.). The current was recorded directly to an auxiliary output using a signal access module (Digital Instruments), allowing the simultaneous acquisition of electrochemical and topographical data by the AFM instrumentation. Linear sweep voltammograms (LSVs) were recorded directly on a PC equipped with a data acquisition card (Lab-PC-1200, National Instruments Corp. Ltd.) using software written in-house. In general, electrochemical images were acquired in one of three modes (see Figure 2): (i) with the SECM-AFM tip in direct contact with the surface; (ii) with the tip at a fixed-height above the surface, in a fixed x-y plane. For these experiments, a contact mode topography image was recorded of the region of interest, the feedback loop was then disabled, and the tip retracted a known distance from the substrate, using a stepper motor. An electrochemical image was then recorded over the same area. (iii) With the tip at a constant distance above the surface, following the surface contours. In this case, the “Lift Mode” capabilities of the instrument were employed. In the first line scan, the tip, in contact mode, recorded the topography of the surface; the tip then rescanned the line and recorded the current, with the probe at a fixed-height above the surface. This mode was originally developed by Digital Instruments for surface potential imaging and (23) Engstrom, R. C.; Weber, M.; Wunder, D. J.; Burgess, R.; Winquist, S. Anal. Chem. 1986, 58, 844. (24) Amatore, C.; Szunertis, S.; Thouin, L.; Warkocz, J.-S. Electrochem. Commun. 2000, 2, 353. (25) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1221.

electric and magnetic force microscopy,26 here we show how the technique can be used for noncontact electrochemical imaging allowing the ready elimination of topographical artifacts. Simultaneous Conductivity and Topography Measurements in Air. To locate the region of interest, the tip-sample conductance was imaged with a potential difference of 1 V applied between the SECM-AFM tip and the 10-µm-diameter Pt substrate UME, across a current-limiting resistor (1 MΩ).20 Simultaneous contact mode images of the substrate topography and current, via a home-built current follower, set at 1 µA/Vsso that 1 V denotes a conducting contact between tip and substrateswere then recorded. RESULTS AND DISCUSSION In-Air Imaging. Prior to electrochemical measurements, contact mode conducting measurements were employed to locate the electrode surface, with the substrate in air. Although it is not essential to use conducting AFM (C-AFM) to determine the location of the substrate electrode, C-AFM images provide an unambiguous assignment of the location of conducting regions in an insulating surface. Parts a-c of Figure 3 show typical height, deflection, and conductivity images, respectively, of a 10-µmdiameter Pt UME sealed in glass, recorded simultaneously in air with an SECM-AFM tip of effective radius, atip ) 0.33 µm. The scan size is 50 × 50 µm (comprising 256 lines), imaged at a scan rate of 100 µm s-1. Panels a and b of Figure 3 clearly demonstrate the capabilities of the custom-made SECM-AFM probe, in producing high-resolution maps of the substrate topography. Both of these images show that the tip is capable of resolving fine structural details associated with the surface of the 10-µm-diameter Pt electrode, such as scratch lines resulting from the fine alumina polish. The conductivity map of the surface,20,27 Figure 3c, determines unequivocally the precise position and geometry of the electrode in the glass insulator and, moreover, provides evidence that the coated Pt SECM-AFM probe is exposed at the apex of the tip. Cross-section analysis of both Figure 3a and c, for this particular UME, revealed an electrode of diameter 10.9 µm, with the electrode slightly raised above the surface, to a maximum height of 75 nm, compared to the insulating glass sheath. Substrate Generation-Tip Collection Measurements. To make electrochemical measurements, solution containing 0.01 mol dm-3 IrCl63- and 0.5 mol dm-3 KNO3 was introduced into the cell. Comparison of topographical images obtained before (in air) and after (in fluid) confirmed that there was relatively little image drift, on the scale of the measurements, due to this procedure. For the SG-TC measurements, the substrate was biased at a potential of +0.9 V, sufficient to oxidize the solution mediator (IrCl63-) at a diffusion-controlled rate, while the SECM-AFM tip electrode was held at a potential (+0.5 V versus AgQRE), to collect the substrategenerated species (IrCl62-) by diffusion-limited reduction. After first obtaining substrate topography information, the tip was disengaged from the surface, by disabling the feedback loop. The stepper motor was then employed to retract the substrate known (26) (a) Memmert, U.; Muller, A. N.; Hartmann, U. Meas. Sci. Technol. 2000, 11, 1342. (b) Lee, I.; Lee, J. W.; Stubna, A.; Greenbaum, E. J. Phys. Chem. B 2000, 104. 2439. (27) Gallo, P.-J.; Kulik, A. J.; Burnham, N. A.; Oulevey, F.; Gremaud, G. Nanotechnology 1997, 8, 10.

Figure 3. Simultaneously recorded height (a), deflection (b), and conductivity (c) images of a 10-µm-diameter Pt disk UME sealed in glass, recorded in contact mode in air, with an SECM-AFM tip of effective radius, atip ) 0.33 µm. The scan size is 50 × 50 µm (comprising 256 lines), imaged at a scan rate of 100 µm s-1.

distances from the tip, with amperometric data acquired at each height. Figure 4 shows a series of line scans, representing the amperometric response of an SECM-AFM tip (s), as the tip was scanned from left to right, at different distances over the center line bisecting the substrate electrode. The measured local current, i, has been normalized with respect to the current, i(∞), the electrode would see for the diffusion-limited reduction of 10 × 10-2 mol dm-3 IrCl62-. This was taken to be the value for the diffusion-limited oxidation of IrCl63- (present in bulk solution), given that D for IrCl62- is very similar to D for IrCl63-, under identical solution conditions.20,28 The tip, characterized by an effective atip ) 0.93 µm, was scanned at a rate of 4.97 µm s-1, in a fixed x plane (with the y scan axis disabled) at distances of 1.00 (upper curve), 1.11, 1.33, 1.55, 1.77, 2.43, 6.45, and 8.43 µm (lower (28) (a) Birkin, P. R.; Silva-Martinez, S. Anal. Chem. 1997, 69, 2055. (b) Beriet, C.; Pletcher, D. J. Electroanal. Chem. 1994, 375, 213.

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tration profile, c, of the electrogenerated species (IrCl62-) in a vertical direction along the center line axis of the electrode, i.e., c(r ) 0, z), where r and z are the radial and axial coordinates, respectively, is given by30,31

c(r ) 0,z) )

Figure 4. SECM-AFM tip collection current response. A tip (atip ) 0.93 µm) was scanned, at a rate of 4.97 µm s-1, in a fixed x plane at distances of 1.00 (upper solid curve), 1.11, 1.33, 1.55, 1.77, 2.43, 6.45, and 8.43 µm (lower solid curve) from the surface of a 10-µmdiameter Pt UME, biased at a potential to oxidize IrCl63- to IrCl62- at a diffusion-controlled rate (+0.9 V versus AgQRE). The tip was held at +0.5 V sufficient to collect IrCl62- by transport-limited reduction to IrCl63-. The solution contained 0.01 mol dm-3 IrCl63- and 0.5 mol dm-3 KNO3. Also shown is the corresponding topographical data (---; flattened) recorded in contact mode just prior to tip disengagement from the surface.

curve) from the substrate surface (fixed-height imaging). Also shown in Figure 4 is the topographical data (---) recorded with the tip in contact with the surface, which has been flattened in order to emphasis the topography of the electrode surface. The location of the 10-µm-diameter UME is clearly evident and shows that the electrode protrudes no more than 70 nm from the surrounding glass surface. As the tip moves from one side of the substrate to the other, passing across the electrode, the collector current rises to a maximum over the center of the electrode. The current, due to collection at the tip of substrate-generated IrCl62-, decreases as the tip-substrate separation increases. At the closest tip-substrate distances, the current response at the left-hand side hits a plateau close to 0 nA. This is due to the underlying substrate topography. The unflattened height data reveal a slope of ∼2 µm, over the scan region considered. The top of the slope is at the left edge of the scan, and it is likely that the tip actually makes contact with the surface for the closest fixedheight scans. When the tip is further back from the substrate, this current anomaly disappears. This observation reflects one of the drawbacks of fixed-height imaging; if the sample is not relatively flat with respect to the electrode dimension, tip-sample crash may occur. For SECM, contact between the probe and the substrate often results in significant damage to the electrode; however, with SECM-AFM electrodes the tip geometry is relatively unaffected by contact, as the cantilever deflects as a result of any forces acting on the tip, as described previously.17 Moreover, very little deterioration in topographical image quality is observed between scans. The current data shown in Figure 4, recorded at fixed distances from the underlying substrate, represent the observation of concentration profiles within the diffusion field of the Pt substrate UME.23,29 For diffusion from a disk-shaped electrode, the concen(29) Engstrom, R. C.; Meaney, T.; Tople, R.; Wightman, R. M. Anal. Chem. 1987, 59, 2005.

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()

2cs -1 as tan π z

(2)

where cs represents the surface concentration of the electrogenerated species and as is the radius of the substrate electrode. Under conditions where (i) the substrate electrode potential is held at a diffusion-limiting value, (ii) tip-substrate feedback effects are negligible, and (iii) the collector electrode acts as a passive concentration probe, cs corresponds to the bulk concentration of electroactive species in solution (0.01 mol dm-3). Figure 5a displays the variation of c/cs, along the center line axis of the electrode, as a function of z/as (b). The data have been taken from experimentally recorded x-z current scans, such as those given in Figure 4, using the relation i/i (∞) ) c/cs. Also shown is the theoretical response (s) based on eq 2. As the tipsubstrate separation increases, the experimental data are found to correlate well with the theoretical response. However, at close tip-substrate distances, there is a noticeable deviation. This behavior is not unexpected, given the effective size of the SECMAFM electrode (and its associated diffusion layer length, δ, which approximates to atip for a hemispherically shaped tip) compared to the tip-substrate separations employed, which may lead to depletional effects. Similar effects have been reported by other workers, when imaging diffusion fields with UMEs.23,31-33 Moreover, diffusional feedback effects in the tip-substrate gap become increasingly important at the very closest tip-substrate separations.34,35 In the absence of diffusional feedback, the probe can be assumed to be noninteracting when the following condition is satisfied (strictly assuming a hemispherical source):36

z/(atipas)1/2 . 1

(3)

For the SECM-AFM tip used here, relation 3 suggests that at tipsubstrate separations of .2.3 µm (i.e., z/as . 0.46), the probe should become relatively noninvasive, which appears to be in fairly good agreement with the experimental data presented in Figure 5a. The entire concentration profile can be described by30,36

c(r,z) ) 2cs as21/2 tan-1 2 π [(r + z2 - as2) + ((r2 + z2 - as2)2 + 4z2as2)1/2]1/2 (4) Figure 5b shows two experimental concentration profiles (s) (30) Saito, Y. Rev. Polarogr. 1968, 15, 177. (31) Basame, S. B.; White, H. S. J. Phys. Chem. B 1998, 102, 9812. (32) Basame, S. B.; White, H. S. Langmuir 1999, 15, 819. (33) Slevin, C. J.; Unwin, P. R. Langmuir 1999, 15, 7361. (34) Amatore, C.; Bento, M. F.; Montenegro, M. I. Anal. Chem. 1995, 67, 2800. (35) Martin, R. D.; Unwin, P. R. Anal. Chem. 1998, 70, 276. (36) Scott, E. R.; White, H. S.; Phipps, J. B. Anal. Chem. 1993, 65, 1537.

Figure 5. (a) Experimental data (b) taken from along the center line axis of x-z scans such as those shown in Figure 4. Theoretical variation of concentration with distance (s), along the center line axis of a 10-µm-diameter substrate electrode, calculated via eq 2. (b) Experimental concentration profiles (s) recorded in fixed-height mode, with the same tip as for Figures 4 and 5a at tip-substrate separations of 6.45 (upper curve) and 8.43 µm (lower curve), alongside the corresponding theoretical concentration profiles (---), calculated from eq 4.

recorded at the largest tip-substrate separations of 6.45 (upper curve) and 8.43 µm (lower curve), where tip depletion and feedback effects should be minimal. The theoretical concentration profiles, simulated using eq 4, at these distances are also shown (---). It is evident that there is fairly good agreement between the shape of the experimental concentration profiles and those predicted theoretically, at these distances. Figure 6 shows a typical x-y topography and current map for the diffusion-controlled tip detection of IrCl62- (tip potential of +0.5 V versus AgQRE) generated from the transport-limited oxidation of IrCl63- at an underlying substrate UME (electrode potential of +0.9 V versus AgQRE). For both images the tip was scanned, at a scan rate of 25.4 µm s-1, in a solution containing 0.01 mol dm-3 IrCl63- and 0.5 mol dm-3 KNO3. For topographical imaging, the tip was held in contact with the surface (substrate UME unbiased), while for electrochemical data acquisition, the tip imaged at a fixed height of 1 µm from the substrate. From Figure 6, the zone over which tip collection of the electrogenerated species, IrCl62-, occurs

Figure 6. Topography (flattened) and fixed-height current maps for the diffusion-controlled tip detection of IrCl62- (tip potential, +0.5 V versus AgQRE) generated from the transport-limited oxidation of IrCl63- at a 10-µm-diameter substrate electrode (substrate potential, +0.9 V versus AgQRE). The scan size was 50.7 µm × 50.7 µm imaged at a rate of 25.4 µm s-1, in a solution containing 0.01 mol dm-3 IrCl63- and 0.5 mol dm-3 KNO3. The SECM-AFM tip was characterized by an effective atip ) 0.93 µm. Both images were recorded in the same area. For topographical imaging, the tip was held in contact with the surface (unbiased), while electrochemical data were acquired with the tip imaging at a fixed height of 1 µm from the surface of the substrate.

is clearly evident, correlating well with the underlying location of the substrate electrode, identified from topographical imaging. Feedback Measurements. Feedback measurements involved oxidizing IrCl63- to IrCl62- directly at the SECM-AFM tip and measuring the current as the tip was scanned just above the substrate of interest. Figure 7 shows simultaneously recorded x scans, with an SECM-AFM tip of effective atip ) 0.26 µm, using the Lift Mode capabilities of the instrument. For these measurements, the tip (held at a potential of +0.9 V versus AgQRE, sufficient to oxidize IrCl63- at a diffusion-controlled rate) was scanned over a distance of 40 µm, at a rate of 4 µm s-1, with the substrate electrode unbiased. Panels a and b of Figure 7 show simultaneously acquired substrate topography (height) and tip current data, respectively, recorded with the tip in intimate contact with the surface. The tip current response displayed in Figure 7c was recorded immediately after these data, with the tip held at a constant distance of 0.21 µm from the substrate, throughout the entire line scan; i.e., the tip traced the contours of the surface. This Lift Mode imaging makes use of the topographical information obtained in the first line scan in order to maintain a constant tip-substrate separation, during the subsequent line scan, with the tip just above the surface. Analytical Chemistry, Vol. 73, No. 3, February 1, 2001

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Figure 8. Experimental SECM-AFM tip current (s) approach curve, recorded as a function of the tip separation from the glass insulator surrounding the Pt substrate UME, in a solution containing 0.01 mol dm-3 IrCl63- and 0.5 mol dm-3 KNO3. The sample was translated toward the tip electrode, biased at +0.9 V versus AgQRE, at a scan speed of 0.2 µm s-1. Also shown are the theoretical negative (---) and positive (‚‚‚) feedback approach curves for a cone-shaped electrode geometry characterized by k values of 1 (lower line - - -, upper line ‚‚‚) and 2 (upper line ---, lower line ‚‚‚). The experimental data points were taken from the limiting tip currents recorded in Figure 7b in the vicinity of the insulating glass surface (b) and the conducting Pt substrate UME (9).

Figure 7. Simultaneously recorded profiles of height (a; flattened) and current (b) of a 10-µm-diameter Pt UME, with an SECM-AFM tip of effective a ) 0.26 µm. The tip was scanned over a distance of 40 µm, in contact mode, at a scan rate of 4 µm s-1, across the center of the UME substrate, in a solution containing 0.01 mol dm-3 IrCl63and 0.5 mol dm-3 KNO3. The tip potential was +0.9 V versus AgQRE, sufficient to oxidize IrCl63- at a diffusion-controlled rate, while the substrate electrode was unbiased. (c) Current profile recorded with the tip held at a constant distance of 0.21 µm from the sample surface. These data were acquired using Lift Mode; after acquiring topographical information on the substrate, the tip was rescanned over the same line, tracing the contours of the surface, but at a constant distance.

As the tip scans over the substrate surface in contact mode, from left to right, the force-sensing cantilever records the topography of the substrate (Figure 7a) while the tip measures the current (Figure 7b). The position of the electrode is clearly visible in both figures. The height image suggests a raised electrode surface of ∼90 nm; a small hole at the right edge of the electrode is just apparent. The large negative height at the left of the image is an artifact of the flattening algorithm and is not present in the unflattened data. The current at the tip is small until it touches the Pt substrate electrode, where a contact is created and the current increases significantly (Figure 7b). The initial contact of the tip with the substrate electrode effectively represents a potential step experiment at the substrate UME and results in a current spike and subsequent decay transient. The current of 12.5 nA attained as 556 Analytical Chemistry, Vol. 73, No. 3, February 1, 2001

the current approaches steady state agrees fairly well with the limiting current response of 14.7 nA which is predicted for the unhindered, diffusion-controlled, oxidation of IrCl63- at a 10-µmdiameter disk UME. This value was calculated using eq 1, but replacing the geometrical constant, 2π by 4, to reflect the disk shape geometry of the electrode. The slighter lower experimental current can be attributed to a small degree of blocking of the substrate diffusion field by the physical presence of the SECMAFM tip electrode in contact with it. With the tip retracted 0.21 µm from the surface, the tip current response in the vicinity of the glass insulator, ∼1 nA, is lower than the bulk response of ∼1.3 nA. This is expected as the insulating surface hinders diffusion of electroactive species to the tip electrode.25 With the tip over the substrate electrode, the current rises to a maximum of ∼1.7 nA, which results from a positive feedback response in the tip-substrate gap. In this case, IrCl62- electrogenerated at the tip electrode is reconverted to IrCl63- at the Pt substrate surface,25 for subsequent detection at the tip. The negative and positive feedback effects remain fairly constant over the line scan, which is expected, given that the tip images at a constant distance from the surface, thus removing topographical effects from the current response. An approach curve for the tip, against the insulating glass surface surrounding the substrate UME, is shown in Figure 8. For this measurement, the substrate was translated toward the tip electrode, biased at +0.9 V versus AgQRE, at a scan speed of 0.2 µm s-1. Also shown are the corresponding theoretical tip current-distance responses (---) for a cone-shaped electrode geometry17,21 characterized by k values of 1 (lower line) and 2 (upper line), where k ) h/r (h and r are the height and radius of the cone, respectively). The simultaneously recorded cantilever deflection approach curve (not shown) allowed the point of contact

between the tip and the surface to be determined unambiguously, as described previously.17 At close tip-substrate separations, the experimental current data lie slightly above the theoretical response for k ) 1. The steeper experimental current-distance gradient, in this region, is probably because (i) the substrate is not completely flat compared to the scale of the measurements and/or (ii) the electrode geometry is not an ideal cone, although this seems to be a reasonable approximation. The value of i/i(∞) obtained from the x scan at a fixed height of 0.21 µm, over the glass insulator (b), corresponds well with the experimental approach curve data. Moreover, the value of i/i(∞) in the zone of the substrate electrode (9) lies between the theoretical current-distance curves for positive feedback at a cone-shaped electrode characterized by k ) 2 (lower curve) and 1 (upper curve). These measurements clearly demonstrate the viability of Lift Mode for noncontact electrochemical imaging with combined SECM-AFM. CONCLUSIONS The capabilities of combined SECM-AFM have been developed, by the demonstration of noncontact electrochemical imaging modes. The studies described build significantly on previous work where data were acquired with the tip in intimate contact with the surface.17 Although these earlier studies provided interesting information, electrochemical imaging in contact mode was not representative of conventional SECM methodology and limited the applicability of the technique to insulating surfaces. Improvements in SECM-AFM electrochemical imaging methodology have been made in two ways. By disabling the feedback loop and using the stepper motor to retract the tip specified distances from the surface of a substrate, conventional SECM fixed-height imaging is possible with the combined SECM-AFM. Topographical information is collected in contact mode, just prior

to tip disengagement from the surface. Alternatively, with Lift Mode it is possible to acquire current data with the tip imaging at a constant distance above the surface. Here the probe scans one line of the surface, recording sample topography, with the tip in contact with the surface. The line scan is then repeated, but with the current-sensing tip, retracted a known distance from the sample, tracing the contours of the surface. This mode of imaging is particularly useful for samples that show variations in both the surface (electro)activity and topography. Given the current signal is now topography independent, fluctuations in the tip response must be due to surface reactivity effects alone and not changes in the tip-sample separation. Noncontact electrochemical imaging has been illustrated with both substrate generation-tip collection studies and simple feedback measurements. Alternative modes1 developed for SECM could also be used in SECM-AFM. In light of the results presented in this paper, there is now considerable scope for using SECM-AFM for the investigation of structure-activity problems at a variety of interfaces. We expect the technique to be particularly powerful for identifying and characterizing reactive sites on complex heterogeneous surfaces. ACKNOWLEDGMENT J.V.M. appreciates support through a Royal Society University Research Fellowship, a Molecular Imaging Young Scanned Microscopist Award, and the EPSRC (ROPA:GR/L71377). We also thank Anna Barker (Department of Chemistry, University of Warwick) for helpful advice.

Received for review November 6, 2000.

September

8,

2000.

Accepted

AC001072B

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