Properties of Thin Polystyrene Films Prepared on Gold Electrodes by

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Properties of Thin Polystyrene Films Prepared on Gold Electrodes by the Dip-Coating Method S. J. Xia, G. Liu, and V. I. Birss* Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4 Received June 16, 1999. In Final Form: October 11, 1999 The characterization of thin, polystyrene (PS) coatings (molecular weight ca. 90 000, up to 30 nm in thickness), deposited on polycrystalline Au surfaces, was carried out by using ellipsometry, mass measurements using the quartz crystal microbalance (QCMB) technique, and various electrochemical methods. The PS coatings were reproducibly deposited using a dip-coating technique, involving the immersion of the Au substrate for times ranging from 5 s to 2 h in 0.6-10 g of PS/(L of cyclopentane (CP)). The coverage of the Au substrates by PS in aqueous solutions, determined from the extent of suppression of the cyclic voltammetric (CV) response vs at bare electrodes, as well as from ac impedance measurements in pH 7 phosphate buffer and in pH 7 K4Fe(CN)6 (1 mM) solutions, could be controllably varied from ca. 0.1 to 0.99 by the deposition conditions employed. With the time of potential cycling in aqueous solutions, the degree of blockage of the Au surfaces by these PS films decreases, likely due to the development of an increasing number of defects in the coating and the consequent uptake of solution into the film. Evidence for the presence of both an inner and outer PS layer has been found, based on the observed dependence of the coating density, thickness, and degree of Au coverage on the PS deposition time.

Introduction A convenient method to produce thin polymer coatings on solid substrates is to immerse, and then withdraw, the substrate from a dilute polymer solution (dip-coating). In the present work, we report a thorough characterization of thin (up to 30 nm) polystyrene (PS) films, formed by the dip-coating of a variety of gold substrates in PS/cyclopentane (CP) solutions. The extent of blockage of the metal sites, as well as the coating thickness and density, have been determined, for the first time, as a function of the coating formation conditions. PS coatings were selected for this study partly for comparison purposes, as parallel studies carried out using thin, dip-coated diblock copolymeric films containing a PS block have shown that each block has a different impact on the coating properties.1 Other previous studies of thin PS films have been carried out on gold and chromium surfaces.2-8 However, in these prior works,2-8 the films were deposited from cyclohexane at the Θ temperature (35 °C) and were examined only in situ, without removal from the organic solvent. The adsorbed polymer molecules apparently change their orientation with adsorption time, having an initially flat conformation in the case of the early arrivals and changing to a more extended form as additional molecules adsorb.2,8 The main goal of this work has been to characterize thin, PS films formed on Au by the dip-coating method. Gold was chosen as the metallic substrate due to its stability and because its state (oxide-coated vs metallic) * To whom correspondence should be addressed. (1) Ding, J.; Birss, V. I.; Liu, G. Macromolecules 1997, 30, 1442. (2) Stromberg, R. R.; Tutas, D. J.; Passaglia, E. J. Phys. Chem. 1965, 69, 3955. (3) Kawaguchi, M.; Hayakawa, K.; Takahashi, A. Macromolecules 1983, 16, 631. (4) Takahashi, A.; Kawaguchi, M.; Hirota, H.; Kato, T. Macromolecules 1980, 13, 884. (5) Xu, H.; Schlenoff, J. B. Langmuir 1994, 10, 241. (6) Peyser, P.; Stromberg, R. R. J. Phys. Chem. 1967, 71, 2066. (7) Grant, W. H.; Smith, L. E.; Stromberg, R. R. Discuss. Faraday Soc. 1976, 59, 209. (8) Stromberg, R. R.; Smith, L. E. J. Phys. Chem. 1967, 71, 2470.

can be controlled easily. The extent of coverage of Au by PS in aqueous solutions was determined using four different electrochemical approaches, revealing that coverage depends on the concentration of PS in the CP solutions, the time of immersion in these solutions, and potential cycling in aqueous solutions. Ellipsometry and mass measurements have revealed the presence of two layers of PS and have also led to the determination of the PS film density. The results obtained have also demonstrated the effect of substrate geometry and surface roughness on the dip-coated PS film properties. Experimental Section 1.0. Polystyrene Coating Deposition on Gold Surfaces. PS (90 000 MW) was dissolved in ca. 5 mL of cyclopentane to form solutions ranging from 0.5 to 10 g/L. The PS coatings on Au sputter-coated quartz crystal surfaces were prepared by adding the polymer solution into a 20 mL cell in which the crystal was clamped between two Teflon-coated O-rings, with one face of the crystal exposed to solution. After times up to 2 h, the polymer solution in the cell was removed and the Au surface was allowed to dry in air prior to electrochemical or thickness measurements. PS coatings on Au wire electrodes (Aldrich, 99.99%, 0.5 mm diameter) were prepared by immersing the Au wire in the PS solution for up to 2 h. All Au surfaces were degreased with acetone prior to polymer deposition. 2.0. Equipment. An EG&G PARC 173 potentiostat was used in conjunction with a PARC 175 Universal programmer for the cyclic voltammetry (CV) experiments. All electrochemical data were recorded using either a BBC SE 780 X/Y recorder or an HP7044A X/Y recorder. A Gaertner L116C ellipsometer was employed in the monochromatic mode in all of this work. The light source employed was a He-Ne laser at a wavelength of 623.8 nm. Instrument operation and all ellipsometric data collection were controlled by an IBM computer, interfaced to the system. A Pierce-type oscillator9 and a Philips PM6654C highresolution frequency counter-timer were employed for the quartz crystal microbalance (QCMB) experiments. All frequency data were plotted on an HP7044A X/Y recorder. The impedance work (9) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 1-85.

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Figure 2. Effect of cycle number for data of Figure 1 on gold oxide reduction charge and coating coverage (degree of blockage of gold oxide response). Figure 1. Typical CV response of bare (a) and coated (b-d) gold wire electrode (0.5 cm2) cycled between 0.2 and 1.7 V at 100 mV/s in pH 7 phosphate buffer solution. The coating was deposited in a 5 g/L PS/CP solution for 2 h. was performed using a Solartron 1255 frequency response analyzer and a 1287 interface. The impedance response was recorded in the frequency range from 10 kHz to 0.01 Hz with 5 steps/decade, using a 5 mV (root mean square) perturbing signal. Z-plot software (Scribner Associates) was used to control the impedance measurements. The fit of the data to an appropriate equivalent circuit was accomplished using Z-view software (Scribner Associates). In the presentation of data in this paper, the size of the symbols used approximately represents the error in the measured values. Nonlinear best-fit curves were drawn manually, without the use of regression analysis. 3.0. Solutions, Electrodes, and Cells. All solutions were prepared using Analar-grade NaOH and KH2PO4 reagents, using triply distilled water. The working electrode (WE) was a Au wire (ca. 0.25 cm2), embedded in glass, or a Au sputter-coated quartz crystal electrode (ca. 0.45 cm2). The in situ mass measurements and ellipsometric analysis were carried out using AT-cut 5 MHz quartz crystals (2.5 cm diameter, Valpey-Fisher), first sputtercoated with an undercoat of Ti to a thickness of ca. 20 nm, followed by a coating of Au to ca. 120 nm. Electrical contact was made to each side of the crystal with Au wires using conducting silver epoxy. A two-compartment glass cell was used for all measurements. Oxygen in the cell solution was removed by bubbling nitrogen through it prior to the electrochemical measurements. In the QCMB measurements, the quartz crystal was sandwiched horizontally between two rubber O-rings at the base of the main compartment. The Pt gauze CE was placed in the same compartment as the WE, while the RHE was located in a second compartment, connected via a Luggin capacity. All current and charge density data are reported with respect to the apparent area of the WE, and all potentials are referred to the reversible hydrogen electrode (RHE). All experiments were carried out at room temperature, i.e., 21 ( 2 °C.

Results and Discussion 1.0. Determination of PS Coating Coverage on Au Wire Electrodes. 1.1. Gold Oxide Signal in pH 7 Buffer Solutions. The CV response of a bare polycrystalline gold wire electrode is dominated by the characteristic gold oxide formation and removal peaks, centered at ca. 1.3 V vs RHE (Figure 1a). These peaks are linearly proportional in size to the true area of gold in contact with solution, when the other variables are kept constant. In the case of polymer-coated electrodes, in order for the gold oxide formation signal to be seen, water must reach the underlying metal and the protons released must be able

to exit from the coating, via defects or pinholes, and vice versa during gold oxide reduction. By comparing the gold oxide formation or reduction charge (using a constant upper potential limit), observed at a polymer-coated electrode (Figure 1b-d), with that seen at the same, but bare electrode (Figure 1a), the extent of blockage of the Au surface sites by the polymer can be determined. It should be noted, however, that this may not be an accurate measurement of the degree of porosity of the coating, as it is not known how homogeneous the solution pathways to the underlying Au surface would be. Curves b-d of Figure 1 show a typical sequence of CVs, with time of cycling of the potential, for a PS-coated gold wire electrode, in a pH 7 phosphate buffer solution. This PS coating was formed by 2 h of immersion of the Au wire electrode in a 5 g/L PS solution in CP. The electrode was not rinsed with CP after its formation, to avoid excessive redissolution of the coating. It can be seen that substantially less gold oxide is formed and reduced vs at the bare electrode, although the size of these peaks increases with time of cycling, reaching a steady-state after ca. 70 cycles under these particular conditions. Also, in the first few cycles, it can be seen that the onset of Au oxidation is shifted to more positive potentials, demonstrating the presence of a physical or kinetic barrier to this process. By comparing the oxide reduction charge, for example, at the coated electrode, q, with that at the bare gold electrode, q0, the coverage of the Au surface by the PS coating can be determined, as 1 - q/q0. Figure 2 shows the charge density and the corresponding calculated coverage with cycle number for the experiment described in Figure 1. The increase in charge, ultimately reaching a steady state, cannot be due to roughening of the underlying Au surface, as the CV response obtained normally for bare Au in this pH 7 solution is very stable. There are several possible explanations for the observed decrease in the degree of blockage of the Au signal with time of cycling (Figure 2). The first involves the supposition that just time in the aqueous solution allows water (solution), needed for gold oxide to form, to penetrate into the hydrophobic PS coating. However, when the PS-coated Au electrode was immersed for 3 h in pH 7 phosphate buffer solution at open circuit, without potential cycling, no subsequent decrease in the coverage by the PS coating was seen. Therefore, either new defects/pinholes are being formed, or their average size increases, as solution is brought in and out of the coating during potential-induced gold oxide formation/removal at the underlying metal surface.

Properties of Thin PS Films on Au Electrodes

Figure 3. Relationship between degree of blockage of Au wire electrochemistry and PS concentration in the CP solution. Coverage calculated from oxide reduction charge in (a) first CV and (b) final steady-state CV obtained between 0.2 and 1.7 V at 100 mV/s, from (c) Fe2+/Fe3+ redox current peaks between 0.2 and 1.1 V at 100 mV/s in 1 mM Fe(CN)64-, pH 7 buffer solution, from (d) impedance measurements at 0.2 V, and from (e) double layer capacitance measurements made in CV mode in pH 7 phosphate buffer solution.

Similar trends to the results of Figures 1 and 2 were seen for PS coatings formed in solutions of different PS concentrations. Figure 3 (curve a) shows the extent of blockage of the Au wire electrochemistry in the first CV cycle in the pH 7 buffer solution for PS coatings formed in solutions of varying PS concentration. Curve a has the classical shape of an isotherm of the high-affinity type,3-5 with a maximum coverage of 0.98 reached at ca. 3 g/L. The fact that this curve fits this isotherm would argue that the polymer initially has the same conformation on the surface, independent of the PS concentration, and that the extension of the adsorbed polymer chains increases with adsorption time.2-5 The equilibrium extension increases with increasing solution concentration until a plateau (maximum thickness) is reached.2 As the PS solution concentration increases, the competition for available sites on Au increases, resulting in a decreasing number of attachments per PS molecule. This yields an increase in the number of individual chains in contact with the Au surface and therefore a greater blockage of gold oxide formation. This would also predict a greater polymer film thickness with increasing PS concentration. A prior study of PS coatings (MW 114 000), formed on gold in cyclohexane (CH) at 35 °C using the in situ QCMB approach in the organic polymer-containing solution, showed that the maximum amount of PS on the surface (in weight per unit area) was reached at a very low PS concentration, i.e., at 0.025 g/L.4 Also, another study of PS (MW 3 300 000) coatings on chromium, using in situ ellipsometric methods, demonstrated that the maximum coverage (in thickness) was achieved at 0.2 g/L PS in CH solutions.2 The varying degrees of blockage of Au electrochemistry, seen in the present work as a function of the PS concentration (Figure 3 (curve a)), and the fact that the maximum coverage of a freshly formed coating is not reached until 3 g/L, can be explained in several ways. First, the maximum coverage determined in CH was measured in situ by weight alone, and no information is provided regarding the degree of substrate blockage. Second, our PS coatings were dip-coated and were allowed to dry, without rinsing, in air. This could cause coating shrinkage, leading to crack and pinhole formation, yielding a lower Au blockage, overall. Third, the exposure of the

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PS-coated Au to aqueous solutions may also affect the metal coverage by the polymer. On the basis of Figure 3 (curve a), it is reasonable to suggest the following model of PS deposition. At PS concentrations less than 3 g/L, the Au surface coverage increases rapidly with PS concentration. At PS concentrations above 3 g/L, the number and structure of the monomers attached directly to the surface, in an “inner layer”, remain fixed as a function of the PS concentration. In all solutions, the overall film thickness increases with concentration, due to the evaporation of residual solution left on the Au wire electrode after removal from the PS solution. The presence of two layers of PS will be confirmed below from the results obtained using the ellipsometric and QCMB techniques. As discussed above, Figure 3 (curve a) shows the coverage in the first CV cycle in pH 7 buffer solution, and these data can be viewed as approximating the extent of blockage of Au in the as-dip-coated, and then dried, condition. The effect of potential cycling in the pH 7 buffer solutions is seen in Figure 3 (curve b). These data show that the coating coverage obtained from the final steadystate CV response still increases with increasing PS concentration. At PS concentrations greater than ca. 7 g/L, the Au signal hardly decreases with time of cycling, dropping to only ca. 0.91, indicating that these coatings may be quite thick and therefore resist pinhole formation. In other experiments, when the deposition time on Au wire electrodes was reduced to 5 min in the 5 g/L PS solution, a θ value of only 0.2 (vs 0.94 after 2 h) was seen in the first potential cycle in pH 7 phosphate buffer solution. When the deposition time was extended to 1 h in the same polymer solution, a coverage of ca. 0.80 was seen under otherwise the same conditions. Therefore, the degree of blockage of Au electrochemistry also depends on the deposition time in the polymer solution, suggesting that adsorption equilibrium must be reached. The effect of deposition time on the degree of blockage of the gold oxide formation/removal reaction will also be examined in a later section (section 2.0) using flat, Au sputter-coated quartz crystal electrodes. 1.2. Coating Coverage Determined from CV Signals in K4Fe(CN)6 Solutions. Electrochemical experiments were also carried out in K4Fe(CN)6 solutions, used often in the literature as a reliable probe of coating coverage,10,11 since the Fe2+/Fe3+ redox couple is a simple, electrochemically reversible, one electron, outer-sphere reaction. The primary intention of these experiments was to ascertain that the PS coating coverage, obtained from gold oxide formation/reduction signals (section 1.1), is reliable. Figure 4a shows the CV response for a bare gold wire in a 1 mM K4Fe(CN)6, pH 7, phosphate solution. Curves b and c of Figure 4 show the response in the same solution after deposition of a PS coating, identical to that in Figure 1, both before and after, respectively, 70 full cycles of potential (from 0.2 to 1.7 V) in the pH 7 phosphate solution. The CV response after a single cycle (curve b) is almost completely inhibited, with essentially no evidence for the Fe2+/Fe3+ redox peaks. After 70 cycles of gold oxide formation/removal in the pH 7 phosphate buffer solution and transfer back to the 1 mM K4Fe(CN)6 solution, the peaks for the Fe2+/Fe3+ reaction can now be readily seen (Figure 4c). However, the currents are still 4-5 times smaller than at the bare electrode (Figure 4a), and the shape of the CV is not that expected for a reversible, (10) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (11) Zhao, M.; Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 1388.

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circuit (EC) shown in Figure 5. This EC is typical for an electrode/solution interface with a double layer capacitance (CPE) in parallel to a charge-transfer resistance (R).13 A constant phase element (CPE), instead of a capacitor, was used to fit the data with a low error; the n values associated with the CPE were very high (0.91-0.94), indicative of a CPE which is very similar to a capacitor in its characteristics. The admittance (Y*) representation of a CPE is given by the following equation:12

Y*(ω) ) Y0(jω)n

(1)

For n ) 0, Y*(ω) represents a resistance with R ) Y-1 0 , while for n ) 1, Y*(ω) represents a perfect capacitor with C ) Y0. When n is between 0 and 1, its value can provide information about diffusion phenomena, surface morphology, and other dissipative processes.14,15 When the electrode is covered by a PS coating, the extent of blockage, θ, of the working electrode can be given by where CPE°

θ ) 1 - (CPE/CPE°)

Figure 4. CVs of (a) bare gold and PS-coated gold wire electrodes (b) before and (c) after 70 cycles between 0.2 and 1.7 V at 100 mV/s in 1 mM Fe(CN)64-, pH 7, buffer solution. The Au electrode was coated with PS using the same conditions as in Figure 1 (2 h in 5 g/L PS/CP solution).

diffusion controlled process. The fact that the reduction process, in particular, displays almost a current plateau, vs a peak, may be an indication of the small size of each current-carrying pathway, typical of the behavior of microelectrodes.10 By comparing the magnitude of the currents for the bare and coated Au electrodes, the degree of the blockage offered by this PS coating can be estimated as ca. 0.70. This is very similar to that obtained from the gold oxide reduction peak (θ ) 0.72) after a similar number of cycles in the pH 7 buffer solution (Figure 2), again demonstrating the slow breakdown of the PS film, likely by the development of pinholes. Figure 3 shows that the θ values, obtained from the Fe2+/Fe3+ redox peaks (curve c) and from the gold oxide reduction signal (curve b) after multicycling in pH 7 buffer, are very similar at all PS concentrations. 1.3. Coating Coverage Determination Using the ac Impedance Technique. This approach was also examined to determine if a reliable estimate of polymer coating coverage can be obtained from ac impedance measurements. Two different solutions, the pH 7 phosphate buffer and a pH 7 1 mM K3Fe(CN)6/1 mM K4Fe(CN)6 solution, were studied in these experiments. (a) pH 7 Phosphate Buffer Solution. Typical Bode impedance representations for a bare Au wire and for an electrode dip-coated with a PS film, after 2 h in a 5 g/L PS CP solution, are shown in Figure 5a and b respectively. All of the impedance data were gathered in pH 7 phosphate buffer solution, once a steady-state CV response was obtained for the gold oxide formation/reduction process. The data were collected at 0.2 V, in the potential range where only the Au/solution double layer charging process should occur. To obtain a numerical quantity representing coating coverage from these results, the data were best-fitted (χ2 error12 of 2 × 10-3) to the equivalent (12) Boukamp, B. A. Equivalent Circuit; University of Twente: Enschede, The Netherlands, 1988/89.

(2)

and CPE are the values of the CPE measured at the bare and polymer-coated gold electrode, respectively. For a PS coating formed on a gold wire in 2 h of immersion in a 5 g/L PS/CP solution, after cycling to steady state in the pH 7 buffer solution (see Figure 1), a θ value of 0.68 was obtained from these impedance measurements. This is very close to the values obtained from the gold oxide and Fe2+/Fe3+ peak currents (Figure 3, curves b and c, repectively). Figure 3 (curve d) shows these θ values, obtained by the impedance technique after multicycling in the pH 7 buffer solution, for all of the coatings studied. Figure 3 (curve e) also shows that the same coverages can be obtained from double layer capacitance measurements by rapid (100 mV/s) potential cycling between 0.2 and 0.6 V. (b) pH 7 Fe2+/Fe3+ Solution. Figure 6 shows the complex impedance plots obtained at 0.85 V for a bare (a) and PS-coated (b) Au wire electrode in a pH 7 phosphate buffer solution containing 1 mM of both Fe(CN)63- and Fe(CN)64-. The plot for bare Au reveals a barely discernible semicircle at high frequencies and a 45° response at lower frequencies, characteristic of a Warburg diffusion element.16 The dominance of the Warburg component indicates that the Fe2+/Fe3+ redox reaction is occurring at a diffusion controlled rate at low frequencies. For the PS-coated Au (Figure 6b), examined before potential cycling between 0.2 and 1.7 V at 100 mV/s in the pH 7 phosphate buffer solution, a larger diameter semicircle at high frequencies is seen again, accompanied by a 45° response at low frequencies. The diameter of the semicircle can be considered to be equal to the charge-transfer resistance (Rct) of the Faradaic reaction, based on a Randles’ equivalent circuit.10,11,17,18 The coating coverage can then be estimated from eq 3, where R0ct is the charge-transfer resistance measured at bare gold and Rct refers to the case of the coated electrode. This equation, giving the (13) Zhang, A. J.; Birss, V. I.; Vanysek, P. J. Electroanal. Chem. 1994, 378, 63. (14) Leibig, M.; Halsey, T. Electrochim. Acta 1993, 38, 1985. (15) Armstrong, R. D.; Burnham, R. A. J. Electroanal. Chem. 1978, 72, 257. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980; p 718. (17) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365. (18) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974.

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Figure 5. Bode plots of (a) bare gold and (b) PS-coated gold wire electrode (2 h in 5 g/L PS/CP solution) after cycling between 0.2 and 1.7 V at 100 mV/s in pH 7 phosphate buffer solution until a steady-state response was reached. Impedance measurements were carried out at 0.2 V.

extent of coverage of the surface by a coating, has been derived from a pinhole model, which assumes that the reaction occurs only at bare spots on the coated surface and that diffusion to these sites is planar in nature.10,11,18

θ ) 1 - Rct°/Rct

(3)

The fraction, θ, of the electrode area is again assumed to be the fraction of the surface unavailable for participation in the electron-transfer process due to blockage by the PS coating. For the PS coating formed in 2 h of immersion in a 2 g/L PS/CP solution, the coverage calculated from Figure 6 and using eq 3 is 0.91, very similar to that obtained from the first gold oxide formation/ removal CV in pH 7 phosphate buffer solution, which was 0.87. Since all of the electrochemical methods employed in the present work yield very similar PS coverages for films formed identically, this suggests a high degree of reliability in their values. Also, these results show that dip-coated PS coatings, formed on Au electrodes, are very reproducible in terms of their properties. 2.0. Ellipsometric Studies of PS-Coated Au Electrodes in Air. All ellipsometric measurements were

carried out in air, as preliminary experiments showed that the PS coating thickness measured in solution fluctuated with time. As a specularly reflecting surface is needed, a Au sputter-coated crystal, also used for the QCMB experiments (see below), was employed in these experiments, and its refractive index (n ) 0.30 ( 0.02) and extinction coefficient (k ) -3.26 ( 0.03) were measured prior to coating deposition. In extracting the film thickness from the experimental data, an extinction coefficient of 0 was assumed for all PS films;2 i.e., PS films are considered to be transparent in nature. Since there is no oxide film on the gold surface in air, a one layer model could be used to calculate the PS film refractive index and thickness from the experimental data.19 A refractive index of 1.47 ( 0.02 was obtained for all of the PS films studied in this work, i.e., those formed after immersion in the PS/CP solutions for between 5 s and 2 h. The PS film thickness obtained represents the average thickness measured at four different sites. The scatter in the values for the same surface was found to be ca. (5% for all of the PS films. (19) McCrackin, F. L. NBS Tech. Note, 1969, 479.

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Figure 7. Effect of PS concentration in the PS/CP solution on the ellipsometrically determined (in air) total PS coating thickness after 5 s (triangles) and 2 h (squares) of deposition on Au-coated quartz crystal surface. Inner layer thickness (circles) obtained from the difference between the 5 s and 2 h data. Figure 6. Complex impedance plots obtained at 0.85 V in a 1 mM Fe(CN)63-/1 mM Fe(CN)64- solution for (a) bare Au and (b) PS-coated Au wire electrode. The coating was formed in a 2 g/L PS solution for 2 h.

In these experiments, films were dip-coated from the PS/CP solutions in either 5 s or 2 h and then removed and air-dried for ellipsometric analysis. In comparison, in the literature study of the adsorption of PS (MW 929 000) on Au from CH at 35 °C,5 after 1 min in PS/CH, only oneeighth of the maximum coverage (weight) was obtained, while adsorption equilibrium was reached only after ca. 40 min in solution. Therefore, it was assumed in the present work that, in only 5 s of exposure of Au to the 0.6-10 g/L PS/CP solutions studied here, very little PS would be adsorbed and an inner layer would not yet be formed. However, an outer layer, produced from the evaporation of residual solution left on the surface after removal of the quartz crystal from the PS/CP solution, should still be present. In the case of a 2 h coating, it is likely that a well-formed inner, adsorbed layer, anchored to the Au surface (train-loop model and/or the looptrain-tail model20,21), will have formed. Also, a precipitated outer layer, formed from the evaporation of residual solution, will be present. Since the adsorbed layer could consist of a mixture of polymer molecules and solvent,2 removal of solvent in air could result in the collapse of some polymer loops onto the surface, thus possibly densifying the “inner” layer of the PS coating on Au. The ellipsometrically determined total thickness of both the 5 s and 2 h PS coatings, for the full range of PS concentrations studied, is shown in the top two curves in Figure 7. In both cases, the PS thickness increases with the PS concentration, although the 5 s coatings are all somewhat thinner. Both the 5 s and 2 h coatings should have the same outer layer thicknesses, arising from the evaporation of solvent. The difference between the 5 s and 2 h film thicknesses is considered to reflect the inner layer thickness, seen in Figure 7 to also increase with increasing PS concentration, to a maximum of ca. 4 nm. When formed in the more dilute PS solutions, the inner PS layer is likely to contain PS chains lying predominantly horizontally on the Au surface, with fewer loops protruding into solution than when formed in more concentrated PS (20) Silberberg, A. J. Phys. Chem. 1962, 66, 1872. (21) Silberberg, A. J. Chem. Phys. 1968, 48, 2835.

solutions. Each polymer chain will therefore occupy a greater Au surface area, resulting in fewer anchoring sites per unit area and less efficient blocking of the Au surface. With increasing PS concentration, the inner layer will contain a greater number of PS chains, so that they will necessarily extend more into solution, thus forming a thicker (up to 4 nm) and more blocking inner layer. The CV response of these PS-coated Au/quartz crystal electrodes (not shown) is very similar to that seen at Au wires (Figure 1). With the time of cycling between 0.2 and 1.7 V at 100 mV/s in the pH 7 phosphate solution, the currents also increase and reach a steady-state. Curves a and b of Figure 8 show the degree of blockage of the gold oxide formation/reduction reaction on these flat Au electrodes in the first CV cycle and final steady-state CV cycle, respectively. Very little change is seen after multiple cycles, contrary to the case at Au wire electrodes (Figure 3), especially at PS concentrations above 3 g/L. This indicates that the PS coatings on sputtered Au have a much higher resistance to potential cycling in the pH 7 phosphate buffer solution. The degree of blockage (Figure 8a and b) of Au after 2 h of deposition is only a little higher than that after 5 s for the same PS concentration, and the difference in these coverages decreases with increasing PS concentration, from 0.20 at 0.6 g/L to 0.02 at concentrations above 3 g/L. These results for the flat, Au sputter-coated crystal surface are again quite different from those for the Au wire electrode (Figure 3). In the latter case, almost no blockage of gold oxide formation/removal was observed when the deposition time was only 5 s, for all PS concentrations. It is possible that on the cylindrical, somewhat rougher Au wire electrodes, the PS coating cracks during drying in air, leading to a substantially lower degree of blockage of the Au electrochemistry. By combining the data of Figures 7 and 8b, the relationship between the extent of blockage of Au electrochemistry and the corresponding film thickness, from ellipsometry, can be derived (Figure 9). It is seen that the data for the 5 s and 2 h coatings overlap, even though an inner layer is present only for the 2 h coatings. The similarity of the plots in Figure 9 suggests that the outer layer, common to both coatings, dominates pinhole formation, as well as the time of access of the aqueous solution to the metal substrate during electrochemical reactions in aqueous solutions.

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Figure 10. Cyclic voltammetry and corresponding frequency changes in pH 7 phosphate buffer solution at (a) bare Au-coated quartz crystal and (b) PS-coated Au/quartz crystal after 10 cycles between 0.2 and 1.7 V at 100 mV/s. The PS coating was formed by immersion of Au in a 5 g/L PS/CP solution for 2 h.

Figure 8. Relationship between blockage of Au-coated quartz crystal electrochemistry and PS concentration, obtained from (a, top) first CV and (b, bottom) final steady-state CV obtained between 0.2 and 1.7 V at 100 mV/s in pH 7 phosphate buffer solution after 2 h (solid symbols) and 5 s (open symbols) of PS deposition (see Figure 3 for analogous data for Au wire electrode).

Figure 9. Relationship between PS coverage of sputtered Au surface, obtained in final steady-state CV, and coating thickness (from Figures 7 and 8b) after 2 h (solid symbols) and 5 s (open symbols) of deposition from CP solutions of varying PS concentration.

It can also be seen in Figure 9 that PS coating deposition occurs in several different stages for both the 5 s and 2

h coatings. At PS concentrations of ca. 0.5-1 g/L (2 h coatings) or 0.5-1.5 g/L (5 s coatings), the total thickness hardly changes, while the degree of Au blockage increases rapidly. This shows that PS deposition is primarily twodimensional in nature under these conditions. From 1 to 3 g/L (2 h coatings) or 1.5 to 5 g/L (5 s coatings) PS concentration, the relationship between the Au coverage and film thickness is approximately linear (θ ∼ 0.70 to 0.95, d ) 4-15 nm), suggesting a closure of solution pathways, dependent on film thickness (three-dimensional deposition). Above 3 g/L (2 h coatings) or 5 g/L (5 s coatings) PS concentration, there is no further increase in the extent of Au blockage, and Figure 9 shows that the PS coating thickens only. 3.0. Mass Measurement of PS Coating on Au (QCMB Experiments). The CV response and corresponding frequency change (inverse of mass change) with potential, obtained using a bare Au/quartz crystal in pH 7 phosphate buffer solution, is shown in Figure 10a. No change in the absolute mass of the crystal after multiple full anodic/cathodic potential scans is seen, indicating that electrode dissolution is not occurring. This would also have been detected by surface roughening (increasing currents with time). A mass gain (frequency decrease) is seen during the anodic scan as gold oxide forms, and vice versa during the cathodic scan, as expected.22 The mass increase seen in the positive scan at potentials of ca. 0.65 V before the onset of oxide formation probably reflects the adsorption of phosphate and/or hydroxide ions on the positively charged gold surface, and vice versa during film reduction. A PS film was then dip-coated on this crystal by immersion for 2 h in a 5 g/L PS/CP solution, and the coating thickness was measured with ellipsometry in air, yielding a thickness of 18 nm. The absolute frequency of the crystal dropped by 120 Hz in this solution. This shows that the density of this PS coating is 1.15 g cm-3. When then examined by CV in the pH 7 phosphate buffer solution, (22) Wilde, C. P.; Ding, T. J. Electroanal. Chem. 1992, 327, 279.

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Figure 11. Relationship between mass of PS coating and its thickness after 2 h (solid symbols) and 5 s (open symbols) of deposition on sputter Au-coated quartz crystal from PS/CP solutions of varying PS concentration.

the current and corresponding frequency response were both found to be suppressed vs in Figure 10a. For example, the data collected in the tenth cycle between 0.2 and 1.7 V at 100 mV/s are shown in Figure 10b. It can be seen that the gold oxide growth current, and the associated mass change, is substantially lower than at the bare electrode, as expected. Also, at potentials negative of the onset of Au oxidation at 1.3 V, the frequency decreases by 0.75 Hz on the coated Au surface with increasing potential, while the frequency decreases as much as 5 Hz on bare gold. These results indicate that the PS coating on Au surface substantially blocks both gold oxide formation and the adsorption of phosphate and/or hydroxide ions. Notably, the mass of the crystal measured in air after potential cycling in pH 7 phosphate buffer solution is found to increase with cycling time, and the thickness measured with ellipsometry in air increases correspondingly after each period of cycling.23 As an example, after 1 h of cycling between 0.2 and 1.7 V at 100 mV/s in pH 7 phosphate buffer solution, the PS film thickness increased by 9% and its mass increased by 12%. This demonstrates clearly that potential cycling leads to the uptake of solution into the coating. This must cause the PS coils, at areas covered with loosely packed PS, to stretch outward into solution, thickening the film somewhat and creating a greater porosity. The mass of a series of 2 h PS films, dip-coated from solutions of different PS concentrations and measured by QCMB after drying in air, was found to be linearly dependent on the ellipsometrically determined total film thickness, as shown in Figure 11. The density of these PS coatings on the Au quartz crystal, obtained from the slope of the plot, is ca. 1.2 g cm-3. For the 5 s PS coatings, however, Figure 11 shows that the density of the coatings formed at PS concentrations below 5 g/L is only ca. 1.0 g cm-3, while, for PS concentrations above 5 g/L, the density of the 5 s coatings is close to that of the 2 h coatings. As the 5 s films are not likely to have a well-developed inner layer, the overall lower density of these films implies that the outer layer has a more open structure than does the inner layer. Summary It has been found that thin PS films can be easily and reliably dip-coated, without subsequent rinsing, on Au

Xia et al.

surfaces, yielding reproducible thicknesses and coverages. Both smooth Au sputter-coated quartz crystals and polycrystalline Au wires can be coated, although PS films formed on the smoother surfaces appear to better resist pinhole formation with time of potential cycling in a variety of aqueous solutions. A key observation, made by ellipsometric measurements on sputtered Au surfaces in air (Figure 7), is that given enough time (up to 2 h), an inner adsorbed layer of PS, ca. 4-5 nm in thickness, is deposited. In addition, an outer coating, up to 25 nm in thickness, is formed by the evaporation of CP from residual solution left on the surface after withdrawal of the Au electrode from the PS/CP solution. Both the inner and outer layers increase in thickness with the PS concentration in the CP solution (Figure 7). By combining these data with mass measurements, it was found that the inner layer has a density of ca. 1.2 g/cm3, while the outer layer has a lower density of 1-1.2 g/cm3. In the present work, PS-coated Au electrodes have been exposed to pH 7 buffer solution after drying in air, and the magnitude of the gold oxide signal, or of the Fe2+/Fe3+ redox peaks, or of the ac impedance response in either of these solutions, has led to a reproducible and reasonable estimate of film coverage. The signal in the first cycle of potential has been interpreted as yielding information about the PS coverage of Au in the PS/Au interfacial region in the “as dip-coated” condition. The effect of multicycling of the potential should be viewed as revealing information about the resistance of the PS film to pinhole formation, as well as the strength of the PS/Au interactions. While it is realized that exposure to water could alter the coating thickness or density, the fact that the electrochemical response was not altered by long times of soaking of a coated Au electrode in aqueous solutions suggests that the coating is inherently quite stable in water. Figures 3 and 8 show that as the PS concentration in the CP deposition solution increases, at least from ca. 0.5 to 5 g/L, the degree of blockage of Au increases notably. At higher PS concentrations, there is little further change, and the surface coverage is close to 100%. Figure 8 also shows that the extent of blockage offered by the 2 h coating (inner and outer layers) is generally greater than that for the 5 s coating (outer layer only). These results demonstrate that allowing sufficient time in the polymer solution to produce an inner PS layer yields a greater suppression of Au electrochemistry. This is consistent with the higher density of the 2 h vs 5 s coatings (Figure 11). The overall effect of cycling of the potential of PS-coated Au crystal surfaces in pH 7 buffer solutions is to reduce the extent of blockage of the Au response (Figure 8). This pertains to both the 5 s and 2 h coatings. This suggests that some rearrangement of the inner layer (2 h coatings) and of the region of the evaporated layer directly in contact with Au (5 s coatings) occurs with time of potential cycling. It is most likely that this involves the loss of some PS/Au contact points and that some PS swelling also occurs. This is consistent with the gain in both coating mass and thickness with time of potential cycling. A more detailed summary of the electrochemical degradation and delamination of PS films, using more aggressive applied potential programs in a range of acidic and alkaline solutions, has been submitted for publication.23 A final issue to address concerns the differences seen between PS-coated Au wire and sputtered Au crystal surfaces. Figure 3 shows that, with time of cycling of the (23) Xia, S. J.; Liu, G.; Birss, V. I. Submitted for publication in Langmuir.

Properties of Thin PS Films on Au Electrodes

potential, the decrease in blockage is much more substantial for the wire vs for the smooth, sputtered Au electrodes (Figure 8). It is also noteworthy that, after 5 s of PS deposition on the Au wire electrodes, the degree of blockage is very small, compared to at the sputtered surfaces. Both of these results demonstrate the less protective properties of the outer (evaporated) PS layer on Au wire electrodes, perhaps due to their rougher surfaces, which could then allow the PS coating to crack more easily. Overall, the present study suggests that dip-coated PS films, deposited on Au, have a higher coverage when formed on smooth surfaces, from PS/CP solutions > 5 g/L in PS and when the coating process is allowed sufficient

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time for the formation of an inner, adsorbed layer. Coating thickness increases both with increasing PS concentration and time spent in the PS/CP polymer solution. The coating density is 1.2 g/cm3 on average but is somewhat higher in the inner layer, close to the metallic substrate. Acknowledgment. This work has been supported by a Strategic Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). We would also like to thank Dr. G. Borover for assistance with some of the preliminary experiments. LA9907735