J. Phys. Chem. 1995, 99, 15728-15732
15728
Measuring Electrochemically Induced Surface Stress with an Atomic Force Microscope Roberto Raiteri* and Hans-Jurgen Butt Max-Planck-Institut fur Biophysik, Kennedyallee 70, 60596 Fran@rt, Germany Received: July 6, 1995; In Final Form: August 30, 1 9 9 9
Atomic force microscope cantilevers have been used to measure surface stress curves for gold and platinum while varying their surface free energies. To do this, the cantilevers were coated on one side with the chosen metal. They were then immersed in aqueous electrolyte solution in the standard fluid cell of an atomic force microscope. A variable potential was applied to the metal via a counter electrode. Depending on the surface potential of the metal the interfacial tension changed, causing a surface stress which bends the cantilever; the bending was detected using an optical lever with high sensitivity. The bending radius of the cantilever is a direct measure of the surface stress. For the given situation the surface stress is similar to the interfacial tension. Surface stress curves measured with gold and platinum in different electrolytes agree with results obtained with other techniques. One advantage of using atomic force microscope cantilevers is that kinetic measurements can be done. Only the resonance frequency of the cantilever limits the time resolution with which the surface stress can be measured. Typical resonance frequencies in water are 1- 100 kHz.
1. Introduction Information about solidlliquid interfaces have been obtained from a series of different experimental techniques both at a macroscopic level (e.g., cyclic voltammetry, capacitance measurements, and elipsometry), and at a microscopic level (e.g., infrared spectroscopy,' SERS2 and EXAFS3s4). Among the former, different methods have been proposed to measure changes in the interfacial tension of solidmetal electrodes associated with applied electric potentials. They are based either on small deformations-supposed elastic-of the surface under ~ t u d y ~or- on ~ changes in the solution meniscus at the metal surface.8 Recently the atomic force microscope (AFM) too has been used to characterize some electrical properties of the interface between solids and electrolyte Also surface energy changes due to the adsorption of different chemical vapors were detected through an AFM setup." In this work we describe a technique based on the bending of a metal-coated standard AFM microcantilever caused by an externally applied potential. This lets us obtain curves of surface stress versus applied potential for different metals such as gold and platinum. The method is, in principle, similar to the one described by Fredlein et However, since the AFM-coated cantilevers have much higher resonance frequencies than previous electrodes, the kinetic response of the metalisolution interface can be studied with a time resolution up to 10-100 ,us. Moreover, AFMs are available in many laboratories and the instrument, including electrochemical cells, works reliably. We begin with section 2 giving the relation between surface stress, surface free energy, and the AFM cantilever displacement. In section 3 the experimental method is described. The obtained results are presented and discussed in section 4.
can be related to changes in cantilever displacement that can be measured by the properly calibrated AFM. The situation of a beam-shaped substrate covered by a thin film on one side, is depicted in Figure 1. It is commonly accepted1'.'3-'7 that uniform surface stress difference, Aa, between the two surfaces of a thin beam induces a circular bending. Using the continuum elasticity theory, it is possible to relate the radius of curvature, R, to A a by: 1-v 1 = -bo
R
Et,
the substrate thickness and E and v are Young's modulus and Poisson's ratio of the substrate material respectively. This is an approximate solution valid when ts is much greater of 4, thickness of the coating film, as in our case where r, = 600 nm and 4 < 60 nm. The applicability of eq 1 has been discussed thoroughly. By geometric reasoning, one can easily verify that the relation between the displacement, z, of the free end of the cantilever, and the radius of curvature, R, for small values of z, is well approximated by: ts is
a L 6 3 ' *
2. Theoretical Background In this section we shall show how changes in surface free energy of the thin film deposited on one side of the cantilever
* To whom correspondence should be addressed. Permanent address: Department of Biophysical and Electronic Engineering (DIBE), University of Genova, via Opera Pia 1 la, 16145 Genova, Italy. E-mail:
[email protected]. @Abstractpublished in Advance ACS Absrrucrs, October 15, 1995.
1---
R
22
z 2 + L2
where L is the cantilever length. Using relations 1 and 2, it is now possible to relate the cantilever displacement to the differential surface stress. The voltage applied to the metal might also effect the surface potential on the other side of the beam. However, using an appropriate mode120 for the calculation of the surface charge and potentials of the noncoated silicon nitride surface, we found that the electrical characteristics are almost unaffected by the voltages applied to the metal surface (in the range of f1.5 V). This allows us assume that the differences in surface stress are to ascribe to the metal electrode surface only, and therefore A a = 0. Varying the potential of the metal coating immersed in the electrolyte solution one varies its surface energy y . This is related to the surface stress, (5, by the Shuttleworth
0022-3654/95/2099-15728$09.00/0 0 1995 American Chemical Society
Letters
-g____...........
J
Figure 1. Diagram showing the cantilever as a beam of thickness ts with the metal layer of thickness if on top, R represents the radius of curvature. Above: one section of the beam of length dL.Changes in surface stress deform each section by increasing or decreasing the surface area at the top. The result over the entire beam is a circular arc shape.
equation:21q22
where E = AS/S is the surface strain, and AS is the difference between the surface areas of the metal film before and after applying the voltage. For liquid electrodes, the second term of the equation is zero and one can easily derive the well-known Lipmann equation. For solids the second term takes into account the fact that when the surface is elastically strained the interatomic distance is changed from the value which would minimize y , and it therefore requires more energy to form a unit area of the strained surface, than of the unstrained one. In analogy to Mohliner and Beck23we estimated the second term of eq 3 in the case of gold (and platinum). The estimated value of dylde N/m was much smaller than the typical measured surface stresses u lo-’ N/m. Hence u y, and the surface stress diagrams can be interpreted as very close to real classical electrocapillary curves.
J. Phys. Chem., Vol. 99, No. 43, 1995 15729
Commercial V-shaped silicon nitride cantilevers (Digital Instruments) of 100 pm length, 0.6 p m thickness, and a calculated spring constant of 0.2 N/m were used. These cantilevers were first rinsed in aqua regia (1 part HNO3 3 parts HCl) in order to remove the original gold coating used to reflect the laser spot. One side of the whole chip was then coated by evaporating a ~ 5 nm 0 thick gold layer over 3-5 nm of chromium (without the Cr the gold easily comes off the substrate) or by sputtering ~ 6 nm 0 of Pt. We assumed that the metal layer did not change the elastic properties of the original Si3N4 beam. To prevent any force between the tip and the platinum CE, a stack of two silicon O-ring was used so that the distance between the tip and the Pt foil was about 2 mm. Electrical contact with the coated side of the cantilever was achieved through the standard metallic spring clip that holds the chip in the fluid cell. The spring clip and part of the chip were then covered with a two component silicone (Sylgard, Dow Corning) to avoid contamination by the spring clip metal and to be sure that the measured current is only through the deposited Au or Pt and the CE. Electrolyte solutions were prepared using purified water (18 MWcm Seral Seralpur PRO 90 CN) and p.a. chemicals. Two liters of the solution were stored in a glass reservoir. While bubbling argon and steering in order to get rid of the oxygen, the solution was continuously circulated into the fluid cell through Teflon tubes (except for a short silicon tube necessary for the peristaltic pump) in a closed circuit for at least 6 h before stopping the flux and doing the measurements. Great care was taken to minimize contaminations: cantilevers, fluid cell, O-rings, counter electrode, and Teflon tubing were all washed before any series of experiments using the following sequence: Ethanol, hexane, ethanol, pure water. All the measurements were conducted at room temperature. After each series of measurements a standard force vs distance curve was recorded on a hard sample surface (Le., the platinum counter electrode). The constant compliance zone was used to push the cantilever a known distance upward. One needs to consider that, in this case, the shape of the beam is different from the beam being shaped like a circumference with constant radius of curvature over its whole length. Since in most AFMs the deflection is measured with the optical lever t e c h n i q ~ e ~ ~ . ~ ~ the shape of the beam matters because the optical lever detects the slope of the beam. For a beam which is pushed upwards at its end the slope of the beam, d z c ~ / d xis, related to the deflection by
+
(4) For a beam with constant radius of curvature the measured slope is
3. Materials and Methods A commercial AFM (Nanoscope 11, Digital Instruments) was used with its standard electrochemical fluid cell. The voltage was applied via a function generator between the coated cantilever (Working Electrode WE) and a platinized platinum foil (Counter Electrode CE). The platinum foil was larger than the cell diameter and positioned at the bottom of the cell. A standard AgCl reference electrode (RE) was used to measure the potential of the cantilever with respect to the bulk of the solution. Deflection of the cantilever, current flow between WE and CE and potential drop between WE and RE were recorded by a PC-based acquisition board.
Equating (4) and ( 5 ) is possible now to relate the radius of curvature with the values of displacement, ~ ~ obtained 1 , by the standard calibration procedure
4. Results and Discussion
Electrocapillary Curves. Figure 2 shows two typical electrocapillary-type curves obtained with a gold and platinum
15730 J. Phys. Chem., Vol. 99,No. 43, 1995 0.I
Letters
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0.0
-$ 5
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Figure 3. Potential of surface stress maximum, W,,, for gold in 0.1 M solutions of different K+ salts. Increasing WSsm corresponds to a decreasing amount of adsorbed anions. Results are compared with those To make our results comparable we obtained with the extensi~meter.~~ referenced our WPsm values to the standard hydrogen electrode (SHE) by VSHE= V ~ ~+c 0.29 i V. TABLE 1: Potential (versus Standard Hydrogen Electrode, V S ~of )Surface Stress Maximum and Point of Zero Charge for Platinum in 0.1 N of NaC104 HClOz at Different pH
+
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Figure 2. Stress and current versus measured voltage for (a) gold in a 0.1 M solution of KCl and (b) platinum in a 0.1 M solution of NaC104 + NaOH, pH 2. Raw data are shown for both anodic and cathodic voltage sweeps. In (a) three complete sweeps are presented. Little drift is present. At the bottom right the deflection signal went out of range. In (b) only one complete sweep is displayed. The deflection curve does not close due to cantilever drift. To calculate the stress-displacement relation we took the following values for the silicon nitride cantilever: E = 1.5 x 10" N/m2, Y = 0.3, ts = 6 x lo-' m, L = m. The zero value for the stress was arbitrarily assigned to the maximum, this because the signal from the photodiode was in both cases reset before taking the calibrating force-distance curves. In principle, from the calibrating procedure it is possible to obtain absolute values for the surface stress. Current per unit surface values are only indicative because of the difficulty of calculating the working electrode surface in contact with the solution since it was partially covered with silicon in an irregular way. electrode, respectively. Raw data are presented for both current and deflection. The anodic voltage sweep (voltage going to more positive values) and the cathodic voltage sweep (voltage going to more negative values) are presented. Maxima of the cantilever deflection diagrams correspond to maximum contraction of the coated side and, therefore, maximum surface stress. The sweep frequency was 0.1 Hz for all measurements presented in this paper. Using other frequencies in the range 0.01- 1 Hz, we did not notice any important difference. Values obtained for the (relative) surface stress for gold are in good agreement with those presente by Lin and Beckz7 and Fredlein et al.6 For platinum we always obtained values between those presented by Fredlein et and by Pangarov and K ~ l a r o v . ~ In general, subsequent voltage sweeps showed the presence of a neglectable drift in the deflection detection, e.g., due to temperature changesz6. Figure 2a represents a best case, while Figure 2b can be considered as a worst case. In any case, when present, this drift shifted the diagram only vertically without changing the potential of surface stress maximum, v s s m . This
can be considered, as we argued in section 2, so close to the potential of zero charge, given by the classical electrocapillarity measurements, that we compare our results with potentials of zero charge given in literature. The current diagrams in Figure 2 do not show any peaks due to ion adsorptioddesorption since the voltage was limited to the vicinity of the surface stress maximum (double-layer regionz7). No water dissociation occurred (hydrogen region27) and usually no oxide film (oxide region27)was formed. When increasing the voltage sweep range we observed, as expected, signs of chemical reactions: peaks and a steep increases of the current, while the deflection signal went soon out of scale. When using voltage sweeps larger than 2 V, often the metal coating, after few cycles, literally dissolved. We also calculated the power dissipated by the electrode in typical experiments, in order to estimate possible bending caused by temperature changes. We found that this effect can be neglected. The v s s m values measured in anodic and cathodic sweeps were generally shifted; the anodic value of v,,, being more positive than the cathodic one. The cathodic maximum varied more and, if the oxide region was reached, depended on the range of the applied potential. For this reason we considered only the anodic sweeps to determine qssm. The graph in Figure 3 shows the values found for a gold surface in potassium salt solutions with different anions.28 Our results agree in the adsorption order for I- and Br- and C1-. We cannot yet explain the deviation for Nos- and s 0 4 ' - . Tables 1 and 2 present values obtained with Pt in different solutions. In the first experiment, the platinum electrode was immersed in solutions of NaC104 at different pH values. v s s m changed with pH. Our results agree with values reported in the literature obtained with the friction coefficients technique.29 In the second experiment the concentration of K2SO4 was changed. Here increased with the salt concentration.
vSsm
vSsm
vssm
Letters r
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350
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0.5
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Figure 5. Calculated time constants for current (open symbols) and deflection (filled symbols) for the intermediate and slow process, t2 and t3. The signals were measured after applying a step of 0.1 V to a platinum covered cantilever in KzS04 solutions of different concentrations. Results of three different measurements are plotted.
0 I
I
E 0
500
I
1
1000
1500
time [ms]
Figure 4. Different deflection responses to a voltage step of flOO
time, 73, were the fitting parameters. The sign of CZdid not change when shifting the bias voltage. In contrast, the slow component C3 changed sign when crossing the l/fssm. Results obtained with a platinum coated cantilever in a KzSO4 solution at different concentrations are shown in Figure 5 . zz increased with increasing salt concentration while the delay time z3 was unaffected by a change of the salt concentration. Probably the second component corresponds to a residual electrochemical process. The fact that the slowest component could be described by a one over square root term indicates that it is limited by a diffusion controlled process.30
mV applied at different positions in the electrocapillary curve of platinum in 10 mM of KzS04. All three different components in the response could be always detected. It is evident that the intermediate exponential decay did not change while the slow response changed when crossing qSsm.
Acknowledgment. The authors would like to express their appreciation for helpful discussions with Prof. M. Grattarola. This work was partially supported by the COMETT Program of the EC.
TABLE 2: Potential (versus Standard Hydrogen Electrode, Vsm) of Surface Stress Maximum for Platinum in KzS04 Solution at Different Salt Concentrations
References and Notes
qssm
c, mM
present method
1 10 100
-0.24
0.04 0.09
*Wll
extensiometerS
-0.13/-0.03
Kinetic Measurements. One of the unique characteristics offered by the high resonant frequency of AFM cantilevers is the possibility to measure surface stress with a high time resolution. Figure 4 shows deflection curves after applying a voltage step of constant amplitude at different voltage biases. Three components could be distinguished in both the deflection and current response: A fast component with a characteristic relaxation time, zl, of < 100 ps, one intermediate component with a characteristic relaxation time, ZZ,in the order of 10 ms, and a slow process which took roughly 0.3 s. The fast component is probably the relaxation of the electric doublelayer (estimated in the order of e l 0 pus) and was faster than the response time of the cantilever (of about 1 ms). The deflection signal for times, t > 1 ms after applying the voltage step could be fitted with
The amplitudes, C2 and C3, the relaxation time zz, and the delay
(1) Bewick A.; Pons, S. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1984; Vol. 12. (2) Chang, R. K.; Laube, B. L. CRC Crit. Rev. Solid State Mater. Sci. 1984, 12, 1. (3) Gordon, J. G.; Melroy, 0.R.; Borges, B. L.; Reisner, D. L.; Abruna, H. D.; Chandrasekhar P.; Blum, L. J. Electroanal. Chem. 1986, 210, 311. (4) Blum, L. J; Abruna, H. D.; White, 3; Gordon, J. G.; Borges, G. L.; Samant, M. G.; Melroy, 0. R. J. Chem. Phys. 1986, 85, 6732. (5) Beck, T. R. J. Phys. Chem. 1969, 75, 466. (6) Fredlein, R. A.; Damjanovic, A; Bockris, J. O'M. Surf: Sci. 1971, 25, 261. (7) Pangarov, N.; Kolarov, G. J. Electroanal. Chem. 1978, 91, 281. (8) Murphy, 0. I.; Wainright, J. S. Langmuir 1989, 5, 519. (9) Butt, H.-J.; Jaschke, M.; Ducker, W. Bioenergetics Bioelectrochem., in press. (10) Ishino, T.; Hieda, H.; Tanaka, K.; Gemma, N. Jpn. J. Appl. Phys. 1994, 33, 1552. (1 1) Chen, G. Y.; Thundat, T.; Wachter, E. A,; Warmack, R. J. J. Appl. Phys. 1995, 77, 1. (12) Fredlein, R. A,; Bockris, 3. O'M. Surf: Sci. 1974, 46, 641. (13) Flinn, P. A.; Gardner, P. S.; Nix, W. D. IEEE Trans. Electron. Dev. 1987, 34, 689. (14) Martinez, R. E.; Augustyniak, W. M.; Golovchenko, J. A. Phys. Rev. Lett. 1990, 64, 1035. (15) Hanneman, R. E.; Finn, M. C.; Gatos, C. J. Phys. Chem. Solids 1962, 23, 1553. (16) Rao, K.; Martinez, R. E.; Golovchenko, J. A. Surf: Sci. 1992,277, 323. (17) Sander, D.; Ibach, H. Phys. Rev. E 1991, 43, 4263. (18) Preissing, F. J. J. Appl. Phys. 1989, 66, 4262. (19) Muller, P.; Kern, R. Surf: Sci. 1994, 301, 386.
15732 J. Phys. Chem., Vol. 99, No. 43, 1995 (20) Raiteri, R.; Martinoia, S.; Grattarola, M. Eiosensors Eioelectron., submitted. (21) Shuttleworth, R. Proc. Phys. SOC.(London)A 1950, 63, 444. (22) Hemng, C. In Structure and Properties of Solid Sutjimes; Gomer, R., Smith, C. S., Eds.; University of Chicago Press: Chicago, 1953; p 5. (23) Mohilner, D. M.; Beck, T. R. J. Phys. Chem. 1979, 83, 1160. (24) Alexander, S.; Helleman, L.; Marti, 0.; Schneir, I.; Elings, V.; Hansma, P. K.; Gurley, I. Appl. Phys. Lett. 1989, 65, 164. (25) Meyer, G.; Amer N. M. Appl. Phys. Lett. 1989, 53, 2400. (26) Radmacher, M.; Cleveland, J. P.; Hansma, P. K. Scanning, in press.
Letters (27) Lin, K. F., Beck, T. R. J. Electrochem. Sac. 1976, 123, 1145. (28) For more values and references on point of zero charge, pzc, for gold as a function of the anion see, for instance ref 27. (29) Boclais, J. OM.; Argade, S. D.; Gileadi, E. Electrochim. Acta 1969, 14, 1259. (30) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York; Chapter 5 , pp 142-144. Jp95 19124
