AFM Study of the Behavior of Polystyrene and Glass Particles during

11030

Langmuir 2004, 20, 11030-11038

AFM Study of the Behavior of Polystyrene and Glass Particles during the Electrodeposition of Copper C. Dedeloudis† and J. Fransaer* Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, 3001 Heverlee, Belgium Received November 24, 2003. In Final Form: July 12, 2004 In this paper, the behavior of polystyrene and glass particles on a copper electrode during the electrodeposition of copper was studied using an atomic force microscope (AFM). Polystyrene or glass particles glued to the tip of the AFM cantilever were kept in contact with the surface of the electrode. The surface forces between the polystyrene or glass particle and the copper electrode were measured before, during, and after electrodeposition. These experiments revealed that glass particles do not make contact with the electrode, probably due to the repulsive hydration force. Polystyrene particles, on the other hand, make contact with the electrode, due to the attractive hydrophobic force. The AFM experiments were correlated with sedimentation co-deposition experiments of polystyrene and glass particles with copper. It was found that 80% of the polystyrene particles added to the plating solution incorporated with copper, while only 0.25% of the glass particles co-deposited under the same conditions.

Introduction The incorporation of ceramic, polymer, or metal particles suspended in an electrolytic bath during metal electrodeposition results in composite coatings with better and sometimes new mechanical and chemical properties.1,2 Despite the industrial application of this process, very little is known about the mechanism of composite plating. Initially, it was believed that a particle that touches the electrode becomes mechanically entrapped in the growing layer.7 Later on, both Guglielmi and Buelens assumed that the ionic cloud that surrounds the particle plays a role in the adsorption of the particle to the electrode.8,9 Guglielmi8 assumed that the adsorption of particles and electrochemical reduction of particle-bound ions are responsible for the encapsulation of particulate matter in a growing electrodeposit. Buelens suggested that a particle will be entrapped in the deposit if a certain amount of the adsorbed ions on the particle surface is reduced.9Valdes and Fransaer4,5,37 were the first to link the behavior of particles to the surface forces acting between the particle and the electrode. By studying the trajectory of a particle in the vicinity of a rotating disk electrode (rde), it was found that a particle will only * To whom correspondence should be addressed. E-mail: [email protected]. Fax:+30-16-32 19 91. † Presently at Cereco S.A., P.O. Box 18646, 34100 Chalkida, Greece. (1) Hovestad, A.; Janssen, L. J. J. J. Appl. Electrochem. 1995, 25, 519-527. (2) Stojak, J. L.; Fransaer, J.; Talbot, J. B. Review of electrocodeposition. In Advances in electrochemical science and engineering, 7th ed.; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinheim, Germany, 2002; pp 193-223. (3) Terzieva, V.; Fransaer, J.; Celis, J.-P. J. Electrochem. Soc. 2000, 147 (1), 198-202. (4) Fransaer, J. Study of the behaviour of particles in the vicinity of electrodes. Ph.D. Thesis, KULeuven, Heverlee, Belgium, 1994. (5) Fransaer, J.; Celis, J.-P.; Roos, J. R. J. Electrochem. Soc. 1992, 139, 413. (6) Dedeloudis, C.; Fransaer, J.; Celis, J.-P. Proceedings of the 2nd Panhellenic Scientific Conference in Chemical Engineering; Tziolas Publications: Thessaloniki, 1999; p 149. (7) Brandes, E. A.; Goldthorpe, D. Metallurgia 1967, 76, 195. (8) Guglielmi, N. J. Electrochem. Soc. 1972, 119, 8, 1009-1012. (9) Buelens, C. A model for the electrolytic codeposition of inert particles with metals. Ph.D. Thesis, KULeuven, Heverlee, Belgium, 1984.

incorporate if the normal forces, which are mainly surface forces, are large enough to compensate the hydrodynamic shear force which tends to remove the particles from the rde.4,5 The advent of the surface force apparatus (SFA)10 and of the atomic force microscope (AFM)11 allowed the direct measurement of surface forces with high precision. In recent years, many direct measurements of DerjaguinLandau-Verwey-Overbeek (DLVO) forces have been carried out between mica surfaces,12 between surfactant monolayers13,14 or bilayers,15 between silica16,17 and between alumina surfaces.18 More recently, force measurements in electrochemical systems have also been performed.19-22 So far, no one has looked at how metal deposits around a particle or has tried to measure the surface forces during electrodeposition. Also, it is not known why hydrophilic particles such as silica and most oxides have a small tendency to incorporate during metal electrodeposition3 while hydrophobic materials such as SiC and polystyrene particles co-deposit readily.4,5,22 Hence, the aim of this research is to remedy this situation by studying the incorporation of hydrophilic and hydrophobic particles during electrodeposition. Glass was selected as a hydrophilic material and polystyrene as a hydrophobic material. The co-deposition was studied in copper to avoid the (10) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1978, 74 (1), 975. (11) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (12) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (13) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (14) Claesson, P. M.; Herder, P. C.; Rutland, M. W.; Waltermo, Å., Anhede, B. Prog. Colloid Polym. Sci. 1992, 88, 64. (15) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2902. (16) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (17) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 2207. (18) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437. (19) Hillier, A. C.; Kim, S.; Bard, A. J. J. Phys. Chem. 1996, 100, 18808. (20) Raiteri, R.; Grattarola, M.; Butt, H.-J. J. Phys. Chem. 1996, 100, 16700. (21) Campbell, S. D.; Hillier, A. C. Langmuir 1999, 15, 891. (22) Dedeloudis, C.; Fransaer, J.; Celis, J.-P. J. Phys. Chem. B 2000, 104, 2060-2066.

10.1021/la0362066 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/10/2004

Behavior of PS and Glass during Electrodeposition

influence of hydrogen evolution. In situ electrodeposition experiments were carried out using an AFM, and the surface forces between a glass or a polystyrene particle with a copper electrode were measured. The results from the AFM experiments were correlated with results from co-deposition experiments of polystyrene or glass particles with copper. Experimental Section Force Measurements. Surface force measurements were performed using a Nanoscope III (Digital Instruments, Santa Barbara, CA). Commercial AFM cantilevers (ThermoMicroscopes, Sunnyvale, CA) with a high spring constant of 2.1 and 3.2 N/m were used in order to measure surface forces close to the electrode without interference of cantilever instability. Each cantilever was modified by gluing a spherical glass or polystyrene particle to it. The glass spheres were obtained from Potters-Ballotini (Valley Forge, PA). The size of the glass particles was between 20 and 30 µm, and they consisted of soda-lime glass (72.5 wt % SiO2, 13.7 wt % Na2O, 9.8 wt % CaO). Even though the particles contain other elements besides silica, the behavior of glass spheres with respect to the double layer force was found to be similar to that of pure silica particles.16 The roughness of two glass particles was measured by AFM. The first particle had an Rq roughness of 0.10 nm (500 × 500 nm) and 1.26 nm (1000 × 1000 nm). The second particle had an Rq roughness of 0.67 nm (500 × 500 nm) and 1.64 nm (1000 × 1000 nm). The polystyrene (PS) particles were prepared from bulk polystyrene by an oil-in-water emulsion technique.5 The mean diameter of the particles is 30 µm. The PS particles are dense and nonporous spheres with a specific surface area of 3.25 m2/g. The particles have a very slight surface charge of -0.85 mC/m2, which means that the particles are hydrophobic.23 The Rq roughness of one particle produced by solvent evaporation was measured by AFM, and a value of 0.87 nm was found (2000 × 2000 nm). The spheres were attached to the cantilevers with an epoxy resin (Epikote 1004, Shell) as described by Ducker et al.16 The cantilever was placed on a heating stage at a temperature above the melting point of the resin. A thin tungsten wire attached to a three-dimensional translation stage was used to position a drop of molten resin near the tip of the cantilever. With the help of another tungsten wire, a particle was positioned on the cantilever and the resin was frozen by lowering the temperature of the hot plate. After the force measurement experiments, the quality of the colloidal probe was examined by scanning electron microscope (SEM) and the radius of the particle was measured. The force between the particle and the electrode is measured as a function of the displacement of the electrode which is mounted on a piezoelectric crystal as it moves toward and away from the particle. The linear velocity of the electrode was 10 nm/s. In view of the size of the particle (20-30 µm), a low velocity was chosen in order to minimize the hydrodynamic forces between particle and surface.24 Measurements results were saved in ASCII format (512 data points). The voltage versus displacement curve was converted to a force versus separation curve. The conversion of the voltage to force data and the piezo displacement to surface separation was performed using the slope of the linear change of the cantilever deflection with respect to the piezo displacement when both are in contact (constant compliance region). This also defines the point of zero separation. The point of zero force corresponds to the deflection data when the two surfaces are far apart. Force measurements were done at different places on the surface of the electrode, yielding consistent results. The reported force curves are the average of 10 curves. Electrochemical AFM Experiments. Electrochemical AFM experiments were performed in a solution of 0.25 M CuSO4 and 0.25 M H2SO4 with a pH of 0.5. All chemicals were pro analysis (23) Hunter, R. J. Zeta potential in colloid science, 1st ed.; Academic Press: London, 1989. (24) Brenner, H. Chem. Eng. Sci. 1961, 16, 242-251.

Langmuir, Vol. 20, No. 25, 2004 11031 reagents supplied by ACROS. Milli-Q water from Millipore was used for the preparation of the solution. The electrochemical AFM experiments were performed inside the DI glass solution cell with an elastomeric O-ring. The closed electrochemical cell was used with a copper milli-electrode as working electrode, a copper ring of 5 mm diameter as counter electrode, and a platinum wire pseudo-reference electrode. The milli-electrode was produced by embedding a copper wire (99.9%, Goodfellow Cambridge Limited) of 1 mm diameter in epoxy resin. The electrode was ground and polished. AFM images showed that the copper electrode had a mean roughness of 3 nm over a 1 × 1 µm area. The electrode potential was controlled with a Nanoscope potentiostat (Digital Instruments). The open circuit potential of the copper electrode in the CuSO4 solution is -505 mV versus Pt electrode or +162 mV versus normal hydrogen electrode (NHE). In these experiments, the copper electrodeposition was performed at a constant current density of -0.4 A/dm2. The average thickness of the copper deposit was calculated using Faraday’s law assuming 100% current efficiency. However, for circular electrodes the primary current distribution is not homogeneous across the surface of the electrode. For a disk electrode of radius ae, the primary current distribution is given by

i ) iavg

0.5

x () 1-

F ae

(1)

2

with i the local current density at a distance F from the center and iavg the average current density of the electrode.25 At the center of the electrode, therefore, the local current density is about half of the average current density. Hence, the thickness of the copper deposit at the center of the electrode is about half of the average thickness. Quartz Crystal Microbalance. The dissolution of copper in the plating electrolyte was studied using a quartz crystal microbalance (QCMB). AT-cut polished crystals (6 MHz) of 14 mm diameter were used. On both sides of the crystal, circular gold electrodes of 150 nm thickness were sputtered on top of a titanium layer of 3 nm. The gold electrode was the working electrode inside the cell, while a platinum grid and a mercury/ mercurous sulfate reference electrode were used as counter and reference electrode, respectively. A 1 µm copper layer was electrodeposited on the gold electrode at a current density of -0.4 A/dm2, in an electrolyte similar to the one used in the AFM experiments. During the electrodeposition of copper, the frequency change, ∆f, was recorded. The current efficiency of the copper electrodeposition in the electrolyte used in this experiment is close to 100%. Therefore the mass of deposited copper, ∆m, can be calculated from Faraday’s law, and the sensitivity ∆f/∆m of the quartz microbalance is determined. After electrodeposition, the copper layer was kept immersed in the electrolyte under stationary conditions. Using the sensitivity of the microbalance, the frequency was transformed to mass loss. Co-deposition Experiments. Electrodeposition experiments of polystyrene or glass particles into a copper matrix were carried out under conditions similar to the AFM experiments. The electrodeposition experiments were performed in an electrolyte containing 0.25 M CuSO4 and 0.25 M H2SO4 of pH 0.5. The electrolytic cell was a glass cylinder with a diameter of 10 cm sealed on the bottom by a stainless steel electrode. The bottom of the electrolytic cell was used as the working electrode, while a copper sheet, covering the top of the cell, was used as a counter electrode. Particles were added to the plating solution and left to sediment on the bottom electrode. The electrodeposition was started after all the particles reached the bottom. The total quantity of polystyrene particles inside the composite coating was determined by carbon analysis (Coulomat 702 PC, Stro¨hlein Instruments). The weight percentage of carbon was determined by burning the coating at 1250 °C. The resulting CO2 was adsorbed in a caustic solution of Ba(ClO4)2 and coulometrically titrated. The content of SiO2 in the copper coatings was determined by X-ray fluorescence (XRF) analysis (PW2400, (25) Newman, J. S. Electrochemical systems, 2nd ed.; Prentice Hall: Englewood Cliffs, NJ, 1991.

11032

Langmuir, Vol. 20, No. 25, 2004

Figure 1. Force vs separation curve (approach and pull-off curves) measured by AFM between a polystyrene sphere and a copper electrode at open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C. The line indicates the theoretical DLVO force calculated for a polystyrene sphere versus a copper electrode at open circuit potential in the abovementioned electrolyte. The force was scaled by the radius of the particle. Philips). The samples were first oxidized in air at 1100 °C. The oxidized samples were dissolved in a flux (10 g of Fluorex, 1 g of Sr(NO3)2, and 2 drops of LiBr) at 1100 °C in a gold-platinum alloy crucible. Standards were prepared in the same way from pure CuO and SiO2 powders. The oxidation step in that case was eliminated.

Results and Discussion Surface Forces between Polystyrene and Copper. Approach and retraction curves between a polystyrene particle and a copper electrode at open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C are shown in Figure 1. The force on approach becomes attractive at a separation below 14 nm. The force between copper and polystyrene consists of the van der Waals force, the double layer interaction, the hydrophobic interaction due to the hydrophobicity of the polystyrene surface, and the hydration force. The double layer force is very small, since the Debye length is only 0.24 nm. Hence, according to the DLVO theory, the van der Waals force will dominate the interaction. The calculated DLVO force between a polystyrene particle and a copper electrode is shown in Figure 1 in comparison with the surface force measured by AFM. The DLVO force was calculated, for a Hamaker constant of 2.4 × 10-20 J for the system polystyrene/water/ copper and assuming a surface potential of 5 mV for the copper electrode and 1 mV for the polystyrene particle. It was assumed that the surface potential of copper equals the surface potential of copper at open circuit in 0.01 M MgSO4.6 The surface potential of polystyrene was assumed to be equal to the zeta potential of the polystyrene particles in a copper sulfate bath of pH 0.5. From Figure 1, it is obvious that the surface force between copper and polystyrene measured by AFM is higher than the force predicted by the DLVO theory. Since hydration forces are always repulsive, the hydrophobic interaction can be the only reason for this discrepancy. The hydrophobic interaction becomes significant at separations below 11 nm. The pull-off force (see Figure 1) measured between polystyrene and copper at open circuit potential is larger than the approach force. This hysteresis is a common phenomenon and is correlated to adhesive bonds formed

Dedeloudis and Fransaer

when two surfaces are brought in contact.21,26 When two surfaces are close together, an interaction force exists between them. This force is the approach force Fappr measured between the two surfaces by AFM. Under the influence of an attractive Fappr, the surfaces will come in contact. Especially, when a sphere just touches a flat plane, only one point of the sphere is in contact with the flat plane. The other points on the surface of the sphere are at a certain distance from the flat plane. The force acting between these points and the flat plane is identical to the approach force measured between the two surfaces by AFM and therefore will be called Fappr. When the surfaces come in contact under the influence of an attractive Fappr, the surfaces are deformed and an adhesive area is formed. Therefore, the pull-off force, Fpull-off, required to separate the two surfaces consists of two components: the force, Fappr, acting between the adherents before deformation at the instant of first contact, and the force, Fadh, necessary to break the adhesive bonds formed on the adhesive area under the load of Fappr, if Fappr is attractive.27 For the polystyrene-copper system where the approach force is attractive, the interface between copper and polystyrene deforms and an adhesive area is formed. In this area, adhesive bonds are formed and an extra force, Fadh, is necessary to separate the two surfaces. It is obvious that this Fadh depends on the size of the adhesive area.27 Therefore, a polystyrene particle that comes in the vicinity of a copper electrode will contact the electrode due to the attractive surface forces. For composite plating, it is important to see how such a particle will behave if copper electrodeposition takes place. This effect was studied by combining surface force measurements with in situ copper electrodeposition experiments inside an AFM cell. Particles glued to the AFM cantilever were brought in contact with the center of the electrode at a load of 20 nN and kept motionless on the surface while a certain amount of copper was electrodeposited. Afterward, the particle was detached from the electrode and the surface force between the particle and the electrode was measured at open circuit potential by approaching and retracting the particle from the electrode at the same point. As can be seen in Figure 2, the attractive force measured between a polystyrene particle and a copper electrode increases with the thickness of the deposited copper layer. An important clue for the mechanism of composite plating would be the force needed to detach the particle from the electrode immediately after electrodeposition, that is, during the first pull-off. However, it was not possible to measure this with the available AFM. The maximum approach and pull-off forces of Figure 2 are compiled in Figure 3 as a function of the thickness of the copper deposited in the center of the electrode. The dependence of the surface forces on the copper deposit thickness can only be explained by assuming an increase of the interaction area of the two surfaces by the copper electrodeposition. If the polystyrene particle comes in physical contact with the copper electrode, copper electrodeposition around the polystyrene particle creates a cavity whose height d equals the thickness of the deposit (Figure 4). After electrodeposition, when the sphere reapproaches the surface, all the points of this cavity are at the same separation distance from the surface of the sphere. The force measured on approach between a cavity (26) Israelachvili, J. Intermolecular and surface forces; Academic Press: London, 1992. (27) Krupp, H. Adv. Colloid Interface Sci. 1967, 1, 111-239.


Langmuir, Vol. 20, No. 25, 2004 11033

Figure 4. Interaction geometry between a cavity and a sphere. All the points of section 1 of the sphere are at the same separation distance d from the surface of the sphere. Section 2 is the remainder of the sphere.

the sphere with the flat plane (Figure 4). Therefore, the total approach force, Fcavity-sphere, is equal to

Fcavity-sphere(h) ) πd(2ap - d)f(h) + 2π(ap - d - h)W(h) (2) with f(h) the force between two flat planes per unit area and W(h) the interaction energy between two planes per unit area. According to the Derjaguin and Langbein approximation,26 the force, f(h), and the interaction energy, W(h), between two flat planes per unit area are related to the force between a sphere and a flat plane, Fplane-sphere:

f(h) )

Fplane-sphere πaph

(3)

Fplane-sphere 2πap

(4)

and

W(h) ) Figure 2. Force vs separation curve between a polystyrene sphere and a copper electrode at open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C as a function of the deposit thickness in the center of the electrode. During the electrodeposition, the polystyrene particle was kept motionless in contact with the electrode. The force was scaled by the radius of the particle.

Substituting eqs 3 and 4 in eq 2, the force, Fcavity-sphere, acting between the cavity and the sphere is obtained as a function of the force Fplane-sphere acting between the sphere and the flat surface, that is, the force between the polystyrene particle and the copper electrode before the cavity was formed.

Fcavity-sphere )

[

]

d(2ap - d) ap - d - h + Fplane-sphere aph ap (5)

Since h , ap and d , ap, eq 5 simplifies to

(

Fcavity-sphere ≈ 1 +

Figure 3. Approach and pull-off forces measured between polystyrene and copper as a function of the thickness of the copper electrodeposited in the center of the electrode. The data were deduced from Figure 2. Fadh is obtained by subtracting Fappr from Fpull-off.

of depth d and a sphere of radius ap at a separation h is the sum of the interactions between section 1 of the sphere with the cavity and the interactions between section 2 of

2d F h plane-sphere

)

(6)

Equation 6 shows that Fcavity-sphere changes linearly with the depth d of the cavity, exactly as was observed for Fappr in Figure 3. The slope of the maximum approach force, Fappr, between the sphere and the cavity, which was measured experimentally at a separation of 1 nm, versus the deposit thickness is 0.09 N/m. According to eq 6, however, the slope should be equal to 9 N/m for a Fplane-sphere of 4.5 nN (Figure 1) and for a separation of 1 nm. The large discrepancy between theoretical and experimental slope of the Fappr versus the depth of the cavity, d, can probably be attributed to the assumption that the sphere fits exactly inside the cavity, that is, that the copper deposition is conformal. If the cavity has a larger radius of curvature than the sphere, Fappr will be smaller because only a part of section 1 of the sphere will be the same distance from the copper electrode and not all as was assumed.

11034



Figure 5. Evolution of the separation distance between a polystyrene or a glass particle and a copper electrode as a function of the immersion time in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C at open circuit potential.

The pull-off force, on the other hand, consists of two components, the approach force, Fappr, between polystyrene and copper and the force Fadh, due to the adhesive area formed at the interface. Fadh, shown in Figure 3, is obtained by subtracting Fappr from Fpull-off. As can be seen, Fadh is constant and does not depend on the depth of the cavity. However, for the case of a sphere on a flat plane, Fadh is only half of the force found for the case of a sphere in contact with a cavity. As discussed above, Fadh depends on the adhesive area formed at a load of Fappr. The difference in Fadh between the two different interaction geometries is probably related to an increase of the adhesive area in the case of a sphere-cavity geometry. AFM Approach Curves. The AFM approach curves can be separated in two parts. In the first part, the sample approaches the cantilever but they are not yet in contact. In the second region, the sample is in contact with the sphere and the two surfaces travel together (constant compliance region). Therefore, the total displacement, ztot, that the sample travels during the measurement consists of the initial separation distance, d, between particle and electrode and the distance, zcompl, that the two surfaces travel in the constant compliance area:

ztot ) d + zcompl

(7)

If sequential photodiode voltage versus displacement curves are recorded and the two surfaces do not change, the separation distance d between the two surfaces remains constant for the same total displacement ztot of the sample. The separation distance d between polystyrene and copper remained constant in the case of force measurements before and after the copper deposition. This confirms our earlier assumption that polystyrene particles make physical contact with the copper electrode and that copper does not deposit below the particle. However, if the force curves between a polystyrene particle and a copper electrode at open circuit potential are recorded repeatedly over time after the copper electrodeposition is stopped, the separation distance between polystyrene particle and copper electrode increases with time (Figure 5). This means that the constant compliance region decreases, for the same total displacement of the copper electrode. This suggests that copper dissolves in the plating solution at open circuit potential. This causes an increase in separation distance between the polystyrene particle and the copper electrode with

Figure 6. Force vs separation curve on approach measured by AFM between a glass sphere and a copper electrode in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C at open circuit potential and at -50 and -200 mV overpotential.

time. The slope of the line in Figure 5 corresponds to the rate at which copper dissolves and was found to be equal to 5 nm/min. The dissolution of copper was measured using a QCMB. Therefore, a 1 µm copper layer was first deposited on a QCMB crystal from a 0.25 M CuSO4-0.25 M H2SO4 solution at a constant current density of -0.4 A/dm2. The frequency of the QCMB crystal, when left in the abovementioned solution at open circuit, increases with time. This increase in frequency is due to the redissolution of the copper deposited on top of the QCMB crystal. The dissolution rate was found to be 3.9 nm/min, in close agreement with the value found in the AFM experiment. This is absolutely reasonable since both the copper bath used in AFM experiments and the bath used in QCMB experiments were not degassed and therefore it can be considered that the same amount of oxygen was dissolved inside the bath. Moreover, this confirms the dissolution of copper as the reason for the increase in separation distance between the copper electrode and a polystyrene particle noticed in the AFM experiments (Figure 5). Surface Forces between Glass and Copper. The force-separation curves between a glass sphere and a copper electrode at open circuit potential recorded on approach and retraction indicate that the two surfaces repel each other (Figure 6). The curves in Figure 6 are the average of 10 curves obtained between 6 and 11 s after the start of the polarization. At longer polarization times,


the curves can no longer be used because the polarization creates a concentration gradient which shifts the laser beam.36 When the copper electrode is at its open circuit potential (-505 mV vs Pt) or polarized slightly cathodically at -50 mV overpotential, the force is repulsive. At a cathodic overpotential of -200 mV, the force becomes attractive. A similar behavior was found for the interactions between glass and copper in MgSO4.22 Hence, as was the case in MgSO4, the change in the force is probably due to the electrostatic forces. The isoelectric point of silica is between pH 2 and 3,29 and therefore, silica is positively charged at the pH of 0.5 used in these experiments. It was found that the potential of zero charge of copper in aqueous solutions lies around -100 mV cathodic overpotential.22 When a copper electrode is polarized at a potential higher than the potential of zero charge Epzc, it is positively charged, while at an potential lower than Epzc, it becomes negatively charged. As a consequence, silica is repelled from the copper electrode at potentials higher than Epzc and is attracted to the copper electrode at lower potentials. The force measured between glass and copper at open circuit potential after electrodeposition of 50 or 100 nm of copper remains repulsive, and both approach and pulloff curves are identical to the force curve measured before electrodeposition (Figure 7). Analysis of the photodiode voltage versus displacement curves revealed that the separation distance between the glass particle and the copper electrode decreases as a function of the thickness of the electrodeposited copper. It was found that the separation distance between the two surfaces decreases as much as the thickness of the deposited copper layer in the center of the electrode (Figure 8). The fact that the deposition process does not affect the force curves and that the separation distance between the glass particle and the copper electrode decreases when depositing copper suggests that the electrodeposition of copper occurs underneath the glass particle. The reason metal deposits underneath the particle can be found in the repulsive hydration forces acting between glass and copper at open circuit potential. This repulsive force keeps the particle a certain distance from the electrode, and consequently, a thin film of electrolyte remains between the two surfaces. Diffusion of copper ions in this liquid film allows the electrodeposition of copper underneath the particle. Even when the surface force between glass and copper is attractive, for example, for a cathodic overpotential of -200 mV (Figure 6), a thin liquid film remains between glass and copper, and copper deposits underneath the glass particle. White et al.30 observed a similar behavior when they tried to co-deposit alumina particles with copper from a copper sulfate bath at a current density of -5 A/dm2 in a 0.5 L verticalelectrode plating cell. They observed that even though a high amount of alumina covered the surface of copper in the as-deposited conditions, particles did not co-deposit. The observation that copper electrodeposition can occur underneath a particle due to a stable liquid film can explain such a riding effect. The separation distance, however, slowly increases again when the electrodeposition of copper stops (Figure 5). The rate of increase was identical to the one noticed for polystyrene. This suggests that the copper dissolves at open circuit potential in the highly acidic bath, exactly as in the case of polystyrene. Stability of the Thin Liquid Film between Glass and Copper. The stability of the thin liquid film between (28) Hunter, R. J. Foundations of colloid science, Vol. 1; Clarendon Press: Oxford, 1995. (29) Tadros, Th. F.; Lyklema, J. J. Electroanal. Chem. 1968, 17, 267.

Langmuir, Vol. 20, No. 25, 2004 11035

Figure 7. Force vs separation curve measured by AFM between a glass sphere and a copper electrode at open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C as a function of the copper deposit thickness in the center of the electrode. During the electrodeposition, the glass particle was kept motionless in contact with the electrode.

Figure 8. Evolution of the separation distance between a glass particle and a copper electrode as a function of the thickness of the electrodeposited copper layer.

copper and glass was investigated, by gradually increasing the constant compliance region. In this way, the liquid between the glass particle and the copper electrode is subjected to an increasing pressure. When the piezo crystal travels toward the particle by a distance x beyond the

11036


Figure 9. Force vs separation curve (approach and pull-off) measured by AFM between a glass sphere and a copper electrode at the open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C before and after the glass particle was pushed against the electrode with a force of 30 nN and 2.3 µN.

point of zero separation, the cantilever is pushed upward over a distance x. In this way, the cantilever exerts a force on the particle equal to Fpres ) kx, with k the spring constant of the cantilever. Force-separation curves were recorded for different Fpres values. The pull-off curves remain repulsive and almost the same as in Figure 6 until the force on the particle equals 2.3 µN. At this value, the pull-off force suddenly becomes attractive and equals 3 nN (Figure 9). It can be assumed that the thin liquid film between the particle and the electrode becomes unstable at that force and that the two surfaces come into real contact. Following real contact, bonds formed between the two surfaces produce an additional adhesive force that must be overcome on retraction of the two surfaces.21 Rupture of the thin electrolyte film with a less stiff cantilever was experimentally found to occur at the same force. This shows that a force of 2.3 µN is the critical force for the rupture of the thin film between a glass particle with a diameter of 26 µm and copper. Surface force measurements between glass and copper electrode at open circuit potential, performed after the electrodeposition of a certain amount of copper while the particle is pushed against the electrode with a force of 2.3 µN, showed that glass particles stop riding. Moreover, the pull-off force measured between a glass particle pushed against the electrode with a force of 2.3 µN and a copper electrode at open circuit potential increases as a function of the copper layer thickness (Figure 10). The increase in pull-off force suggests, exactly as in the case of the polystyrene particle, that the interaction area increases with the thickness of the copper deposit. A force of 2.3 µN probably leads to the rupture of the thin electrolyte film between glass and the copper electrode. This allows the two surfaces to come in real contact, and consequently copper no longer deposits underneath the particle but (conformably) around it. As a result, a cavity is formed around the glass particle. The maximum pull-off force, Fpull-off, measured between a glass particle that was pushed against the electrode with a force of 2.3 µN during electrodeposition and the copper cavity increases linearly with the electrodeposited copper layer thickness with a slope of 0.05 N/m (Figure 11). This pull-off force, as mentioned above, can be considered as the sum of the approach force, Fappr, and the “adhesion” force, Fadh, which is related to the adhesive area formed by deformation of the interface of the two


Figure 10. Force vs separation curve (pull-off only) measured by AFM between a glass sphere pushed against the electrode with a force of 2.3 µN and a copper electrode at open circuit potential in 0.25 M CuSO4 and 0.25 M H2SO4 at pH 0.5 and 25 °C as a function of the deposit thickness.

Figure 11. Maximum pull-off force measured between a glass particle and copper as a function of the electrodeposited copper layer thickness. During the deposition, the particle was pushed against the copper electrode with a load of 2.3 µN.

surfaces. After the possible rupture of the liquid film, the approach force Fappr acting between the two surfaces is probably attractive and brings the two surfaces in contact. This force acts only between section 1 of the sphere (Figure 4) and the electrode, since the thin liquid film will become unstable at the shortest distance between the two surfaces. The pull-off force, Fpull-off, therefore, has to overcome this attractive force between the particle and the electrode in order to separate the two surfaces. Since glass and copper are quite hard materials that will not significantly deform under the applied load of 2.3 µN, the interface between them will not deform and no extra adhesion force, Fadh, will be needed to achieve the separation of the two surfaces. Therefore, the pull-off force, Fpull-off, will be equal to the attractive approach force. As was calculated and experimentally confirmed for the case of polystyrene, the approach force between section 1 of a polystyrene sphere and the cavity increases linearly with the copper layer thickness. A linear relationship between the attractive approach force and the thickness of the copper layer should be expected also for the case of glass against copper. Therefore, the linear dependence of the pull-off force on the copper layer thickness is due to the change from a sphere-plane to a sphere-cavity interaction geometry.


Langmuir, Vol. 20, No. 25, 2004 11037

∫0∞ f(r)φ(F, r) dr

Co-deposition of Polystyrene and Glass Particles with Copper. To link the AFM results of polystyrene and glass particles to their co-deposition behavior, electrodeposition experiments with copper were performed. Polystyrene or glass particles, identical to the particles used in the AFM experiments with a mean diameter of 25 µm, were suspended in the acidic copper electrolyte used in the AFM experiments. Either 41 mg of polystyrene or 106 mg of glass particles was added to obtain a surface coverage of (15% of the electrode. After the particles settled on the stainless steel cathode, a copper layer of 15 µm was deposited at a current density of -0.4 A/dm2 without agitation. After electrolysis, the copper deposits were removed from the stainless steel electrode, rinsed thoroughly in distilled water and ethanol, and dried. The copper-polystyrene composite coating contained 32.1 ( 0.7 mg of polystyrene or 4.15 ( 0.09 wt % polystyrene. The silica content inside the copper-glass composite coating, on the other hand, is 0.26 ( 0.08 mg of silica or 0.037 ( 0.011 wt % silica. It is obvious, therefore, that around 80% of the polystyrene particles were incorporated in the copper-polystyrene coating, while only 0.25% of the glass particles were incorporated. Since the co-deposition experiments were performed without agitation, the particles had the most favorable conditions to co-deposit since the hydrodynamic shear force is zero and the particles remain motionless on the electrode. The amount of particles found in the coating, therefore, should be close to the initial amount of particles on the electrode. Even though this is the case for polystyrene particles, glass is not incorporated at all, despite the higher density of the glass particles. These results confirm the conclusions from the AFM experiments, that glass particles “ride” on the electrode during copper deposition while polystyrene particles stick to the electrode and are incorporated in the metal matrix. Influence of Adhesion-Induced Deformations. Because particles are not perfectly rigid, they deform under the influence of the forces that keep them in contact with the electrode.33-35 Since polystyrene is relatively soft compared to glass, polystyrene particles deform more than glass particles. To see if this effect can explain the differences in co-deposition behavior between equally sized polystyrene and glass particles seen in this work, the deformation of polystyrene and glass spheres against a rigid plane subject to surface forces was calculated using the method of Hughes and White.32 As the deformations are small, they were determined by integrating the stress-distribution multiplied by a Green function over the area subjected to the stress, where the Green function is the fundamental solution of the linear-elasticity equations for an applied point force of unit magnitude. Since the problem is axisymmetric, one integration was done in advance, and the surface deformation D is given by

where FLvdW is the dispersion force, and A is the crosssectional area of the deformed particle, depending on the vertical coordinate, z, and the distance h between the sphere and sample. The deformation of the sphere was found by subdividing the surface of the sphere around the contact region into linear boundary elements. The deformation of each element was calculated, starting from the undeformed state, and the position of the boundary nodes changed iteratively until the external load equaled the elastic force, resisting further deformation. In this work, we set l ) 1 nm, K ) 5 × 106 N m-2, and the Hamaker constants APS-H2O-Cu ) 3.3 × 10-20 J m-2 and Aglass-H2O-Cu ) 2.96 × 10-20 J m-2. As can be seen from Figure 12, the deformation is highest on the axis of symmetry, where a dimple is formed. The deformation was magnified for clarity. At this magnification, the center of the 30 µm particle lies 30 m above the top of this page. The radius of the deformed area is approximately equal to 22 nm for polystyrene and 8 nm for glass, resulting in an increase of the adhesion force of 0.99% and 0.16%, respectively. Although the exact values depend on l, K, and the Hamaker constants, the qualitative conclusion remains the same: the changes in adhesion force due to the adhesion-induced deformations are too small to explain the differences in co-deposition behavior between polystyrene and glass.

(30) White, C.; Foster, J. Trans. IMF 1981, 59 (1), 8-12. (31) Hertz, H. J. Reine Angew. Math. 1881, 92, 156-171. (32) Hughes, B. D.; White, L. R. Q. J. Mech. Appl. Math. 1979, 32 (4), 445-471. (33) Dahneke, B. J. Colloid Interface Sci. 1972, 40 (1), 1-13. (34) DeMejo, L. P.; Rimai, D. S.; Bowen, R. C. Adhesion induced deformations between particles and substrate. In Particles on surfaces, 2; Mittal, K. L., Ed.; Plenum: New York, 1989; pp 49-58. (35) Bowen, R. C.; Rimai, D. S.; DeMejo, L. P. J. Adhes. Sci. Technol. 1989, 3 (8), 623-636. (36) Stappers, L.; Fransaer, J. J. Appl. Phys. 2002, 92 (9), 55435549. (37) Valdes, J. L. Deposition of colloidal particles in electrochemical systems. Ph.D. Thesis, Columbia University, New York, 1987.

This study shows that the way in which a metal electrodeposits around a particle on an electrode depends strongly on the surface properties of the particle and on the surface forces acting between the particle and the electrode. For hydrophobic materials such as polystyrene, the surface force is dominated by the hydrophobic interaction and the van der Waals force. As a result, polystyrene particles that approach the electrode closer than 11 nm are attracted toward the electrode and make contact with

D(F) ) 4θ

(8)

where f(r) is the stress distribution over the solid surfaces and φ(F, r) is the Green function given by

φ(F, r) )

(

)

4Fr r K r + F F2 + r2

(9)

where K is the complete elliptic integral of the first kind. In view of the rather low values of the particle-electrode adhesion forces measured by AFM, we assumed that the solvation force prevents the sticking of particles. We therefore set the stress distribution equal to the solvation force. To complete the analysis, the integral of the stress distribution over the surface of the particle was set equal to the total normal load on the particle. This load consists of the solvation force and the London-van der Waals forces. The solvation force was found by integrating over the surface of the particle:

Fsolv ) 2πK

∫Γ exp(-h/l) dΓ

(10)

where Γ is the arc length of the particle, K is the solvation force constant, and l is the decay length of the solvation interactions. The London-van der Waals force was found by integrating the Lifshitz dispersion force gradient ∂F/∂z per unit area of the sphere:

FLvdW(h) )

A(z - h) dz ∫h∞ ∂F ∂z

(11)

Conclusions

11038


Figure 12. Surface profile of a 30 µm polystyrene (light gray) or glass (dark gray) sphere resting on a flat surface. Under the influence of the adhesion force, the polystyrene sphere deforms more than the glass sphere. The dots (b) mark the position of the boundary nodes used in the calculation.

it. A pull-off force of 0.85 mN/m is necessary to detach a 26 µm polystyrene particle from the electrode. When such a polystyrene particle is in contact with the electrode, copper deposition takes place around the particle. As a consequence, the interaction area increases due to the cavity formed by copper deposition, and as a result, the pull-off force increases with time. Hence, as copper deposition goes on, it becomes more and more difficult to detach the polystyrene particle from the electrode. The surface force that attracts polystyrene particles to the electrode, together with the formation of a cavity around


polystyrene particles, favors the incorporation of polystyrene particles in copper. For hydrophilic materials such as silica, the force between the particle and electrode is dominated at short distances by the hydration force. This repulsive force prevents the silica particle from coming in real contact with the electrode. As a result, a silica particle remains separated from the electrode by a film of electrolyte that sits between the silica particle and the copper electrode. This electrolyte film allows electrodeposition of copper to take place underneath the particle causing the particle to ride on the copper surface. This makes the incorporation of silica particles into a copper matrix unfavorable, as demonstrated by sedimentation co-deposition experiments. The different behavior of glass and polystyrene particles found in this study explains why hydrophilic materials such as silica have a small tendency to co-deposit with copper while hydrophobic materials such as polystyrene co-deposit without any trouble. From the observed differences in the growth of metal around the particle, it seems that the surface properties of the particles, influencing the surface forces between particles and electrode, are the key parameter for the incorporation of particles into a metal matrix during electrodeposition. Acknowledgment. C. Dedeloudis thanks the European Commission for a Marie-Curie Fellowship (Contract No. ERBFMBICT961034). J. Fransaer thanks FWO Vlaanderen for their financial support. Part of this research was done within the IUAP P4/33 contract funded by the Belgian government and the KAN 1.5.012.03 contract, funded by FWO Vlaanderen. LA0362066

AFM Study of the Behavior of Polystyrene and Glass Particles during

Recommend Documents