Electrochemical microscopy of polymer layers

Elecrrochimica

Pergamon

Acru. Vol. 42, Nos 23-24. pp. 3637-3640. 1997 C 1997 Elsevier Science Ltd. All rights reserved.

Printed in 0013-4686/97

PII: S0013486(97)ooo97-2

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Electrochemical microscopy of polymer layers D. Jeffrey, B. R. Horrocks, R. D. Armstrong*

Chemistry

Department,

University

of Newcastle,

(Received

Newcastle

27 January

and A. B. Walsh

upon Tyne, NEI

7RU.

UK

1997)

Abstract-Electrochemical imaging of holes in a polymer layer lying on a metal is demonstrated. Electrochemical imaging has been used to obtain high definition “pictures” of metal/insulator composite surfaces (under aqueous solutions), and porous materials, eg dentine and membranes, [I]. The principle of the method depends on scanning the surface of interest with a microelectrode (the “tip”) which either responds to changes in potential or changes in current as it is scanned [2]. We demonstrate here how the method can be used to image holes in a thin polymer coating on a metal surface. 0 1997 Elsevier Science Ltd Key words: electrochemical microscopy, pOlytner

kiyer.

The surface being investigated was in the form of a flat sheet of gold approx. 1 cm x 1 cm. The model protective coating consisted of a Nuclepore (Costar, Badhoevedorp, The Netherlands) membrane placed on the gold sheet. The membrane was 10 pm thick. It had 12 pm diameter holes in it (randomly situated). It was attached to the gold sheet by epoxy except for a small area 3 mm x 3 mm which was probed by the microdisc. The solution above the gold sheet/ membrane was 0.1 M KC1 with 10m3M Ru(NH3)h3+. The cell body was teflon with a platinum ring counter electrode. A fine silver wire coated with AgCl was used as a reference electrode. The potential of the tip (-0.6 V vs Ag/AgCl) was set so that the Ru(NH,)G~+ was reduced at the tip under diffusion controlled conditions. The tip diameter (Pt microdisc) was IO pm. When the microdisc electrode is over a part of the polymer layer which has no flaws in it, the current flow to the electrode will be lower than that found for the same electrode when it is in the bulk of the solution. This effect is due to diffusional restriction as shown in Fig. 1. When the tip is over a flaw the Ru(II) which is generated at the tip can be re-oxidised at the underlying metal (provided that the metal is held at a suitable potential) causing a sharp increase in

*Author

to whom all correspondence

should

be addressed.

current. In this way the flaws in the coating can be imaged. The increase in current due to an open pore which exposes the metal to the solution is much greater than for a closed pore because of the fact that Ru(II1) is not regenerated at the bottom of a closed pore. Thus, strong images are only obtained for open pores. This was explored quantitatively by carrying out finite difference simulations of the steady-state concentration profiles and tip currents for a range of typical pore:tip size ratios. Table 1 shows the ratio of the currents calculated for the situation where the tip (microdisc) is directly above the centre of a pore to that where the tip is at the same height over non-porous polymer. The tip-to-polymer surface distance was one tip radius and the tip-to-underlying metal distance was two tip radii. Tip current is also shown for the case where the polymer layer is absent. Typical contour maps of the steady-state concentration profiles are shown in Figs 2(a) and 2(b) for a pore half the diameter of the tip. The clustering of the concentration profiles around the polymer area is due to our setting the concentration of the redox couple to zero in the polymer and reflects the algorithm used to draw the contour lines and not the actual data from the simulation. It should be noted from Table I that when the bottom of the pore is conducting, pores of half the tip size can be easily imaged (I 8% enhancement of tip current), whereas when at the bottom of a pore where the metal is

,

3638

Preliminary note

Scanning microdisc electrode

Fig. 1. Scanning microelectrode in amperometric mode over (A) intact polymer coating, (B) over a flaw in the polymer layer which exposes the underlying metal. The arrows indicate the diffusional fluxes of the initial form of the redox couple.

covered with a thin non-conducting layer shows only a tiny enhancement of current. The tip current increases with pore radius as expected, however, little effect of delamination was observed for a delaminated volume of radius 4 tip radii and uniform thickness of 0.2 tip radii. The simulations were carried out by successive overrelaxation [3] of the standard five-point finite difference representation of Laplace’s equation in cylindrical co-ordinates. The concentration of redox couple was set to zero on the tip and 1 at large distances and exposed metal substrate. A zero flux condition was imposed on the tip sheath and polymer surfaces. A non-uniform grid was employed to allow more resolution at the discontinuity in the boundary conditions at the microdisc edge. As a calibration of our simulation we computed tip current vs distance for a bare metal and found agreement with previous workers [2] to within 5%. Most pores in real systems have a degree of delamination of the metal around the base of the pore (Fig. 3). The imaging mode used here is relatively insensitive to the wetted metal area-responding instead to the pore diameter for open pores. Table 1. Current ratios Pore radius/ tip radius 0 0.5 1 2 5 Bare metal, ie no polymer

in imaging Delamination at base of pore

Ii 1 1.002 1.054

1.480

1 1.178 1.492 2.163 2.379 2.381

II = ratio of current with pore and without for the case where the metal is covered with an insulating film. I2 = ratio of current with pore and without for the case where the exposed metal regenerates the redox couple at the diffusion controlled rate. Tip to metal distance = 2 tip radii, polymer thickness = 1 tip radius. The delaminated volume was modelled as a cylinder of radius 4 tip radii and 0.2 tip radii next to the underlying metal.

It is important that the tip is brought to a known height above the polymer layer before commencing a scan. This was achieved by moving the tip in to the surface of the polymer covered gold until the current due to Ru(lI1) reduction dropped to 50% of its value in the bulk of the solution. At this point it can be assumed that the tip is the distance of the tip radius from the surface of the polymer ie 5 pm above. The images which were obtained are shown in Fig. 4. The lighter areas correspond to higher currents at the tip and therefore are indicative of the presence of holes in the polymer. The images of the holes become sharper as the tip is brought closer to the surface, as would be expected. Note that good images of the pore are obtained even though the tip and pore are similar in size. There are a number of possible methods of electrochemical imaging which will be suitable for polymer coatings on metals. They are as follows: (i) DC measurements of tip current with potentiostatically controlled metal substrate. (The present method). (ii) AC measurements of tip current with an applied ACpotential between tip and metal substrate. (iii) DC measurements of tip potential with potentiostatically controlled metal substrate. (iv) AC lock-in measurements of tip potential with applied AC potential between the metal substrate and a remote counter electrode. The scheme for obtaining mode (i) images is described above. Mode (i) has the advantage that it can be used to distinguish blocked and open pores. Mode (ii) avoids the need to have a redox couple in the solution and allows any dc potential to be chosen (eg a potential where no or little reaction is occurring) for the metal (with a sinusoidal UCpotential of 50 mV peak to peak imposed on it). Therefore, it will be useful in imaging polymers on non-noble metals. In mode (ii) the admittance between the tip and the underlying metal is measured. Measurements can be made at frequencies up to 10 kHz and down to 10 Hz. The low frequency limit is set by the tip scan rate. At high frequencies the measurement will reflect the impedance of the solution immediately below the tip and

therefore

it will be sensitive

to the presence

of

3639

Preliminary note

0

10

20

30

40

mtencebaeenmicrqrobeandmetetsubstrate (63fbitelyunb)

0

i!

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Fig. 2. Concentration profiles when a pore is being imaged by a microprobe (in the dc amperometric mode). Concentrations of the redox couple are referenced to the bulk concentration. Thus, contour line 0.70 indicates 0.70 x the bulk concentration. In A there is a pore which runs down to the underlying metal and exposes it to the solution. In B the metal at the bottom of the pore is covered with a thin insulating film. Note that the concentration contours near the microdisc in A are much more closely spaced than in B which means that open pores give strong images whereas closed pores give weak or no images.

pores. At low frequencies it will reflect to some extent the wetted metal area at the bottom of the pores. Modes (iii) and (iv) depend on the fact that the diffusion field or electric potential emanating from a pore, in which an electrochemical reaction

is occurring, spreads in a hemispherical fashion. A significant percentage (loo/,) of the concentration or potential can be sampled at distances of 10 pore radii, increasing the chances of locating a pore by a factor x 100. Further work is needed to show which method

Metal Fig. 3. Schematic drawing of a pore with delamination occurring at the metal.

3640

Preliminary

note

C Fig. 4. Imaging holes (diameter 12 pm) in a polymer layer in the amperometric mode at different microprobe heights (A - 8, B - 7, C - 6 and D - 5 pm) above the polymer. The area in each case is 60 p x 60 pm. The contour lines correspond to different currents at the microprobe.

is best

for

coatings

the

imaging

of real

polymer

protective

on metals.

ACKNOWLEDGEMENTS We wish to thank International Paint (Courtaulds Coatings) for their support of this work. REFERENCES 1. D. 0.

Wipf,

A. J. Bard

and

D. E. Tallman,

Anal.

C/rem. 65, 1373, (1993). S. Nugues and G. Denault, J. Electroanal. Chem. 408, 125 (1996). J. V. McPherson, M. A. Beeston, P. R. Unwin, N. P. Hughes and D. Littlewood, Langmuir 11, 3959 (1995). E. R. Scott, H. S. White and J. B. Phipps, Anal. Chem. 65, 1537 (1993). 2. J. Kwak and A. J. Bard Anal. Chem. 61, 1221 (1989). K. A. Ellis, M. D. Pritzker and T. Z. Fahidy, Analyt. Chem. 67, 4500 (1995). Recipes in Fortran” Chap 19, 2nd Edition, 3. “Numerical W. H. Press, S. A. Teukolsky, W. T. Vetterling and B. P. Flannery, Cambridge University Press, Cambridge, (1992).