J. Phys. Chem. C 2007, 111, 11329-11338
11329
Improving PPy Adhesion by Surface Roughening Yingkai Liu, Qi Gan, Shermeen Baig, and Elisabeth Smela* Mechanical Engineering, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: March 7, 2007; In Final Form: May 17, 2007
Conjugated polymers have found applications as “artificial muscles” because they undergo significant volume change upon electrochemical cycling. However, this large repeated strain also leads them to delaminate from the underlying electrode. Two methods of improving the adhesion of polypyrrole films to Au electrodes during extended electrochemical cycling were quantitatively characterized. Both involved roughening the Au surface, the first by electroplating and the second by etching. The extent of delamination was quantified using a tape test at regular intervals during switching between the fully oxidized and reduced states in an aqueous electrolyte, and the surfaces were characterized by scanning electron microscopy. Untreated smooth control surfaces were simultaneously monitored the same way. Without surface treatment, 3000 Å thick polypyrrole films doped with dodecylbenzenesulfonate, PPy(DBS), delaminated unpredictably between 1 cycle and 20 000 cycles, with almost half the samples failing before 3000 cycles. Electroplating even a thin layer of 1000 Å of Au in a low-concentration plating bath consistently improved lifetimes to at least 10 000 cycles. Thicker plating produced even better adhesion, and a 1 µm thick layer of electroplated Au virtually eliminated delamination, extending the lifetime beyond 60 000 cycles. However, this Au roughness is thicker than the PPy film itself, and is not useful for microactuator work. Etching also improved adhesion, with the best results obtained for the shortest etch times. With a roughness of only 700 Å, an undercut surface morphology was produced that allowed good mechanical interlocking of the PPy. This process is appropriate for use with thin films; however, it will be sensitive to the grain structure of the original Au film. On surfaces with no delamination, it was possible to measure the oxidative degradation of the PPy(DBS) in the aqueous electrolyte. The electroactivity decreased steadily over time, with 20% loss after 10 000 cycles, 50% at 35 000, and virtually no electroactivity by 60 000 cycles.
1 Introduction 1.1. Motivation. Conjugated polymer electrochromic windows, actuators, supercapacitors, and batteries undergo repeated electrochemical switching between oxidized and reduced states during operation. However, the very expansion and contraction during switching that is exploited for actuation also results in high shear stresses at the polymer/electrode interfaces in the device,1 which can cause the polymer to delaminate, leading to reduced performance, slow degradation in performance, or catastrophic device failure. Figure 1 shows a film of PPy (black in the figure) that has delaminated from the Au electrode leads (white) to bilayer microactuators on the surface of a silicon wafer (gray).2,3 As in the figure, delamination typically begins at corners and edges. Delamination can occur in as soon as a single cycle, or after several thousand cycles, depending on variables that are not fully understood. It is therefore critical to understand and improve adhesion between conjugated polymers and electrodes. 1.2. Prior Work on Improving Adhesion/Preventing Delamination. Previous work on improving adhesion can be divided into two approaches: mechanical interlocking and chemical bonding. The object of the former is the creation of protrusions or crevices around or into which the polymer is deposited, producing a mechanical bonding akin to the joining of puzzle pieces. The latter seeks to covalently join the polymer to the metal through an intermediating molecule with moieties * Corresponding author. E-mail:
[email protected]. Tel: 301-4055265.
Figure 1. Delamination of PPy from underlying Au electrode going to bilayer microactuators (bent at an angle of approximately 20° from the surface).
that can bond with both, or by copolymerizing with a second polymer that can be grafted to the metal surface.4 This paper builds upon earlier work on surface roughening to increase mechanical interlocking between a gold electrode and a conjugated polymer film.5-7 The adhesion of electrodeposited PPy films was improved by first electroplating a rough layer of gold over an originally smooth gold film, producing a “fuzzy” surface that resulted in greater strains and improved
10.1021/jp071871z CCC: $37.00 © 2007 American Chemical Society Published on Web 07/03/2007
11330 J. Phys. Chem. C, Vol. 111, No. 30, 2007 lifetimes. While the morphology of the plated layer was found to play an important role in adhesion, the optimal morphology to improve adhesion under long-term electrochemical cycling has not yet been determined, and is the aim of this work. Another mechanical interlocking approach that has been found to be effective is to completely enclose the electrode within the polymer. PPy was deposited around a helical electrode8 to produce a tubular actuator that did not undergo mechanical degradation and that produced greater strain. However, this approach cannot be adapted to every application, and particularly not to microfabricated bilayer actuators, so surface-treatment types of remedies are still required. The covalent bonding approach has also been investigated.9,10 Self-assembled monolayers (SAMs) of thiol-modified pyrroles were used to treat Au electrodes prior to PPy deposition and were found to improve the adhesion of as-deposited films.11-16 Unfortunately, such monolayers can be unstable to electrochemical cycling,17,18 and they have not been shown to improve the adhesion of films under cycling. A different surface treatment method that has been reported is the chemical oxidation of titanium.19 PPy films survived 6000 cycles on such Ti electrodes without delamination. While it works very well on some occasions, however, the method is difficult to control and reproduce, and therefore those who have tried to implement the method for the fabrication of devices have abandoned it. Particularly when significant investment has gone into the substrate, a method is required that is robust and reliable. Likewise, nitric acid treatment of iron20 and salicylate salt used with oxidizable metals21 have been shown to improve the adhesion of as-deposited PPy films. The success of these methods under the right conditions shows that such approaches merit further investigation. 1.3. Adhesion Testing. It has been said that, “Adhesion is one of those topics in thin-film technology which everyone wishes would go away”.22 Reviews of methods for measuring adhesion, and discussions of the difficulties and the parameters that affect the measured results, are given in refs 22-26, with an emphasis on thin films, and coating failure mechanisms are reviewed in ref 27. Basic adhesion refers to the interfacial bond strength and depends only on interfacial properties, with no contributions from film thickness, sample geometry, or measurement technique. However, experimentally one measures the work needed to separate a film from a substrate; this is known as practical adhesion and is influenced by all of these factors. The measurements are also affected by the adhesion test itself. The external stresses applied during the test vary from shear to tensile, and they are transmitted to the interface in a complex way.28 Thus, different adhesion tests give different results. Nevertheless, it is important to have some measure of adhesion. Although many tests have been devised,23,29,30 none has been found to be ideal. Therefore, the test must be chosen on a caseby-case basis. Several factors are important for measuring adhesion of thin conjugated polymer films, including the effect of the test itself on measured lifetime, the reproducibility of the results, the ease and speed of application, and the ability to continue electrochemical cycling after a test. For our applications, a test to determine whether, after a given number of cycles, a film had delaminated was deemed to be more important than one that quantified the strength of adhesion after initial deposition. Furthermore, the study of polymer delamination caused by electrochemical cycling is complicated by polymer degradation, which occurs simultaneously, since each influences the other. Namely, a drop in electroactivity diminishes the induced stress
Liu et al. and thus also the tendency to delaminate, and local delamination results in loss of electrical contact, and apparent loss of electroactivity. Thus a mechanical, rather than an electrical, test was chosen for adhesion. The tape test is a standard test of adhesion in which a piece of adhesive tape is pressed to the film and peeled back.31-33 The film is not at all removed, removed in patches, or completely removed. This is a threshold test which does not quantify adhesion strength, but instead determines whether the tensile force generated at the interface can overcome the combination of film cohesion and film-substrate bonding. The force applied by the tape has been reported to be 7.5 to 110 g/mm of width29,34 or 5-9 × 105 Pa,23,35 but this depends on the tape and how strongly it sticks to the film. Because the applied force is small, the test can be applied only to practically non-adherent films. This test was therefore expected to only minimally influence the measured lifetime. It is also relatively simple and quick, does not damage adherent PPy films, and can be applied during breaks in cycling. 1.4. Approach Used in This Paper. This paper investigates two approaches to roughening a smooth Au surface: electroplating an additional, rough layer of Au, and etching the Au. Thus, the material is the same in all cases, and only the surface morphology is altered. We show that electroplating produces a consistent lifetime even when the coating is thin, which is a significant improvement over the widely varying lifetime of untreated control samples. When the plated layer is 1 µm thick, delamination ceases for thin PPy films. Etching produces the longest lifetime when done for only a short time, with delamination completely prevented on surfaces etched 70 nm deep. For the studies reported herein, we used polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), which is employed in our microactuator devices,2,3,36-38 but the results are expected to be generally applicable to other conjugated polymers. PPy(DBS) electrodeposits as smooth, cohesive films and has an inplane strain of approximately 3%.39-41 Films were deposited to a thickness of 3000 Å because the electrochromism of the film is most apparent at this thickness, allowing a visual confirmation of the electrochemical activity. It is important to note, however, that film thickness affects measured lifetimes.42 The tape test is known, for example, to be inadequate for thick films, in which cohesion dominates the behavior. Furthermore, interfacial peeling and shear stresses are known to depend on film thickness with, in the general case, thicker films delaminating sooner. Furthermore, during electrochemical cycling the films do not experience uniform stresses, but stress gradients due to ion and solvent transport that differ upon oxidation and reduction. Finally, the ratio of the surface roughness to the film thickness will play a large role in the outcome of the test. We thus did not examine delamination as a function of PPy thickness here; understanding these issues was outside the scope of the present study. 2 Experimental Methods 2.1. Sample Preparation and Cycling. 2.1.1. Preparation of Untreated Control Electrodes. Oxidized 4′′ (100) silicon wafers were coated with 100 Å of Cr and 2000-3000 Å of Au by either thermal evaporation at pressures below 1 × 10-6 T (preliminary experiments) or by sputtering (all other experiments); the Au thickness was increased to 9000 Å for some of the surfaces that were roughened by etching. These surfaces were either used as untreated controls, or they were roughened by either electroplating or wet etching. As shown below, Au
Improving PPy Adhesion by Surface Roughening
Figure 2. A linear sweep voltammogram from 0 to -1.6 V vs Ag/ AgCl (50 mV/s) in Au plating solutions diluted 1:3, 1:10, and 1:20. The current for the latter two has been multiplied by 10 to facilitate comparison. The plating voltages that were used are indicated by arrows.
deposited in different batches varied considerably in its adhesiveness, so pieces from the same wafer were used when comparing other variables. 2.1.2. Roughening Surfaces by Electroplating. Au plating was done potentiostatically (pgstat30, EcoChemie) versus an Ag/ AgCl reference electrode at room temperature. An Au-coated Si wafer served as the counter electrode. The thickness of plated Au was controlled by the consumed charge and was measured after deposition by surface profilometry (Dektak 3ST). The plating solution was prepared by mixing, to various dilutions, a commercial plating solution (Oromerse SO Part B, Technic) with aqueous 1.7 M Na2SO3, which maintains the pH and prevents reactant decomposition.43 The plating mechanism is discussed in ref 44. The pH of the plating solution was 10 when fresh and dropped with plating time. The solution was used until its pH dropped to 8.9; below that the plated Au was nonuniform. Cleanliness is critical to electroplating, so only designated glassware and electrodes were used and the gold surfaces to be plated were cleaned in boiling “piranha” solution (H2SO4:H2O2:H2O ) 1:2:4 volume ratio). For complete details, refer to ref 45. Figure 2 shows linear voltage sweeps at 50 mV/s in plating solutions diluted 1:3, 1:10, and 1:20 with 1.7 M Na2SO3. There is a cathodic current peak near -1.2 V. For voltages more negative than -1.3 V, there is significant hydrogen evolution by hydrolysis. The basic shapes of the curves remained the same at different dilutions, with the peak position moving slightly positive with increasing dilution. 2.1.3. Roughening Surfaces by Wet-Etching. A commercially available wet gold etchant (TFA, Transene) was diluted with three parts deionized (DI) water to increase control of the timing of the etching process, which was performed at room temperature. A rocking platform (Type 100, VWR) was used to provide agitation to improve the uniformity of the etch over the entire surface. The rocking rate was 50 cycles/min, and the rocking angle was 6°. (Rocking produced more uniform surfaces than stirring.) In addition, the outer 1 mm edge, which etched more quickly, was cleaved off. The samples were etched one at a time to prevent scratching of the surfaces. The etch depth was controlled by the etch time and measured by profilometry. The etch rate was 200 nm/min. Fresh solution was used each time because the etch rate decreases as the solution is depleted of reactants. 2.1.4. PPy Deposition. For preliminary testing, smooth Aucoated Si pieces 5 mm × 35 mm were mounted with gold-
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11331 plated screws, nuts, and washers in a Teflon cell that was able to hold 8 pieces horizontally. These were the working electrodes (WE). They were degreased in-situ by rinsing in acetone, ethanol, and DI water, and cleaned in situ with the standard RCA SC1 cleaning procedure of hot H2O2 (28%), NH3 (25%), and water, 1:1:5. Immediately after cleaning and rinsing in DI water, PPy was electrochemically deposited over the entire surface at 0.6 V vs Ag/AgCl (EG&G PAR model 173 potentiostat/galvanostat, model 179 coulometer, model 178 electrometer, model 175 universal programmer, and a Graphtec WX2400 x-y recorder) to a thickness of approximately 2000 Å, as determined by color and profilometry, from a freshly prepared aqueous solution of 0.1 M pyrrole and 0.1 M sodium dodecylbenzenesulfonate (Na·DBS). The pyrrole had been stored frozen solid in an ultralow freezer. DI water was used for all solutions. An Au wire made contact to an indium tin oxide (ITO) covered glass slide, 4.8 × 13 cm in area, used as the counter electrode (CE). The ITO plate and PPy-coated strips were parallel and separated by 5 mm. An Ag/AgCl reference electrode (BAS) was placed between the samples and the ITO slide. For further information on PPy(DBS) deposition, refer to ref 38. Undoubtedly, the deposition conditions will have an effect on the adhesion of the PPy. In preliminary investigations, we also examined several deposition parameters, but they did not produce significant, consistent changes in measured lifetime on smooth control samples, so the effects of these parameters is secondary to the surface characteristics. Polymerization potential was examined since it is an important variable in determining film characteristics (morphology, crosslinking, peak positions, peak shapes, conductivity, etc.) (see, for example, refs 46 and 47 and many others). However, the higher the deposition potential, the more nonuniform the film thickness,38 and this may affect the adhesion measurements indirectly. Potentiodynamic polymerization was also investigated, since the waveform used for deposition also has a strong influence on film characteristics and adhesion.48,49 PPy(DBS) does not deposit using square waves with periods of less than 20 s, and with longer periods the deposit is patchy, cracked, and dull brown, and has little electroactivity. Deposition temperatures from 10 to 37 °C were investigated as well, but no consistent effect on lifetime were observed. For all other samples, the PPy was deposited potentiostatically at 0.47 V (pgstat30 EcoChemie or BT 2000 Arbin Instruments) versus Ag/AgCl onto a piranha-cleaned substrate that was suspended vertically to a depth of 10 mm into the monomercontaining solution, and the wafer piece was subsequently cleaved into smaller pieces 10 mm wide. The pyrrole had been stored frozen solid, and it was warmed and filtered over alumina before use. The thickness of these PPy films was 3000 Å unless otherwise specified. 2.1.5. Electrochemical Cycling. Preliminary tests were performed with the EG&G system. In the same electrochemical cell as described above, the samples were cycled in aqueous 0.1 M NaDBS by stepping (square wave) between fully oxidizing and reducing potentials (0 to -1 V vs Ag/AgCl). This electrolyte was used because (1) of our interest in cycling in aqueous electrolytes for biomedical applications,50 (2) other electrolytes that would be more appropriate for biomedical applications contain Cl, which electrochemically reacts with Au, thereby adding a complicating factor to the delamination mechanism, (3) we have extensive prior experience and data for films cycled in it, and (4) to achieve the maximum volume change, and thus a maximum stress for delamination during cycling (see for example39,51), only a single ion should be
11332 J. Phys. Chem. C, Vol. 111, No. 30, 2007 exchanged, so that solutions with small anions should not be employed. After a fixed number of electrochemical cycles, the samples were rinsed in situ in the cell under DI water and blown dry with a stream of nitrogen. It was important to remove all of the DBS, which is a surfactant, or the tape would not stick to the PPy. For all other testing, the 10 mm wide samples were suspended vertically and face-inward in a circularly symmetric electrochemical cell able to hold 8 samples simultaneously; the reference electrode was placed in the center of the cell, and the counter electrode was a piece of graphite placed at the bottom of the cell, perpendicular to the samples. As a result, each sample had an identical position with respect to the reference and counter electrodes. The samples were connected together to the WE and cycled simultaneously. The potential was stepped between 0 and -1 V, holding each for 10 s so that it underwent complete oxidation and reduction (redox) in each cycle. Stepping (voltage square waves), rather than cyclic voltammetry (triangle waves), was done with the aim of stressing the interface to the maximum extent possible upon switching by imposing the highest strain rate and holding the polymer at its extreme maximum and minimum volumes. We did not, however, measure whether films failed sooner under cyclic voltammetry or stepping. Another purpose of stepping was to complete each cycle quickly, while allowing the polymer to undergo full redox, so that the testing could be completed expeditiously. After each interval of a predetermined number of cycles, the samples were rinsed with DI water and dried with nitrogen. 2.2. Tape-Test Methodology and Validation. 2.2.1. The Tape Test. Tape tests were performed immediately after PPy deposition to ensure that the films were initially adherent. The samples were cycled until they failed, meaning that either the PPy delaminated completely or that the PPy lost all electroactivity, as determined by cyclic voltammetry. Samples that were rinsed, dried, and left in laboratory air showed no additional delamination upon retesting. The tape (Scotch Magic 810, 3M) was pressed down by hand over a 1 cm length of the PPy film (for the preliminary tests, at the end furthest from the gold-plated screws, making a test area of 50 mm2, and in other tests over the entire PPy-coated area, which was 100 mm2). The tape was pulled off at approximately 1 cm/sec, starting from the edge of the Si piece, at an angle of either 135° (preliminary tests) or 180° (all other tests). Small changes in angle did not affect the results. (Standard peel tests often use a 90° angle, but that led to breakage of the brittle Si substrates.) The standard ASTM tape-test method31,32 requires scribing the surface to form a grid for quantifying delamination. Lifetime results with and without a grid were compared to determine if the scribing step could be eliminated. The measured percentage of PPy removed versus cycle number was the same with and without a grid, so the step of scribing a grid into the PPy was removed from the protocol. 2.2.2. Tape-Test Validation. To determine the experimental protocol and ensure that it was appropriate for testing the adhesion of PPy, the following question had to be answered: does the tape-testing step shorten the measured lifetime? If tape testing exerted a significant force on the PPy films, samples that were tested more often would delaminate sooner. Sixteen PPy-coated Au surfaces were divided into four groups. After initial tape testing at 0 cycles, the first group was tape-tested after every 250 cycles, the second after every 500 cycles, and the third after every 1000. The fourth group was handled
Liu et al.
Figure 3. Percentage of film area remaining after 0, 500, and 1000 cycles (x-axis groups). Four samples each were electrochemically cycled and tape-tested at intervals of 250 (black bars), 500 (dark gray), and 1000 (white) cycles (corresponding labels are above the bars). Another group of four samples were not cycled (light gray bars, labeled nc) but were tape-tested together with the 250-cycle samples. (Note that the “1000” group was not tested at 500 cycles.)
identically to the first (immersed in electrolyte, rinsed, dried, taped) except that it was not electrochemically cycled. The results are shown in Figure 3. The samples in the three cycled groups failed completely after 1000 cycles, regardless of the testing interval (with the exception of a single sample, which had been tested the most frequently, every 250 cycles). None of the uncycled group samples suffered any delamination. These data lead to two conclusions. First, the taping process did not shorten the measured lifetime, since all the cycled samples failed at the same time. Second, delamination was primarily caused by actuation stress at the interface induced during electrochemical cycling, not by any other aspects of the cycling or testing process, since the uncycled samples did not delaminate at all even though they were immersed in electrolyte and frequently tape-tested. In a separate test, adhesion was measured as a function of the position of the sample in the electrochemical cell. The placement had no effect on the lifetime. 3 Results and Discussion In every experiment, smooth control surfaces from the same wafer as the roughened samples were included in the set of 8 samples in the cell. This section begins with a discussion of the lifetimes measured on these control surfaces. The lifetimes on electroplated surfaces are presented next, examining the effects of plating voltage, plating solution concentration, and plated layer thickness on adhesion. This is followed by lifetimes on surfaces roughened by etching, examined as a function of etch depth. We conclude with a study of electroactivity versus time due to polymer degradation under electrochemical cycling in the absence of delamination. 3.1. Lifetimes on Smooth Control Samples. In order to be able to judge whether the various roughening treatments actually improve the adhesion, one must know the lifetimes on untreated surfaces, which served as controls. Thirty control samples were cleaved from 6 wafers sputtered with Au in different batches under nominally identical deposition conditions. Samples were tape-tested in intervals of 1000 cycles. Figure 4a shows the lifetime, defined as the number of cycles at which 100% of the film had been removed from the surface, as a function of batch number. There was a significant variation in lifetime among samples from the same batch. In batch 1, the lifetimes ranged from 3000 cycles to 16 000 cycles. In batches 2, 3, and 4, there were films
Improving PPy Adhesion by Surface Roughening
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11333
Figure 4. Lifetime of control samples. (a) Lifetime vs sputtering batch. Lines indicate the average for each wafer. Triangular points (7) are from batch 6, but the tape was pressed harder onto the PPy. (b) Histogram of lifetimes of the samples in panel a, excluding 7.
that peeled off during the very first tape test; these might have been removed even sooner than 1000 cycles had they been tested earlier. There was also substantial variation in average lifetimes between different batches, with the samples in batch 6 particularly long-lived. These data clearly illustrate the problem for device manufacture: even when all process parameters are held the same, there is a large uncertainty in how long the devices will last before failure on such untreated surfaces. Although conditions are nominally the same, some depositions produce long-lived devices, and others particularly short-lived devices, and a priori there is no way of knowing which it will be. Under visual observation, scanning electron microscopy (SEM), and even standard atomic force microscopy (AFM), the gold surfaces are identically smooth and featureless. However, the microstructures (grain boundaries, orientations) are most likely different. The samples in batch 7 were the same as those in batch 6, but the tape was pressed onto the PPy with greater pressure during tape testing. This may have had the effect of lowering the lifetime somewhat, although with only 3 samples in each group this is not conclusive. These samples were also particularly long-lived compared to batches 1-5. Figure 4b shows a histogram of the data in Figure 4a. Nearly half the control samples failed within the first 3000 cycles, 70% failed within 6000 cycles, and none exceeded 22 000 cycles. The percentage of PPy remaining vs cycle number is plotted in Figure 5 for two batches of 8 samples each prepared for preliminary testing by thermally evaporating the Au. These samples were much shorter lived than those in Figure 4, again illustrating the problem of unpredictable lifetimes on control samples. Within this group, failure was catastrophic: the time between initial and total delamination was short. (The distribution of lifetimes was not well described by Weibull statistics, which are often used to characterize failure distributions.) The onset of catastrophic failure was between 200 and 1100 cycles, and lifetimes were between 500 and 1300 cycles. Looking at the shapes of the adhesion failure curves, delamination began with the removal of a small amount of PPy, and in the subsequent couple of testing intervals, this was followed by catastrophic adhesion failure in which most of the film was removed. The amounts of material removed thereafter slowed down again for the remaining PPy. This differed from failure on more adherent substrates (vide infra), in which material was gradually removed little by little. In general, the first fragment of PPy to be removed was less than 0.1 mm2, and its position was invariably at the edge of the
Figure 5. Percentage of tested PPy area remaining as a function of the number of cycles for 16 nominally identical evaporated Au samples. Samples were tape-tested every 100 cycles (points).
TABLE 1: Summary of the Factors and Values of Electroplating That Were Tested plating voltages
plated Au thicknesses
plating solution dilutions
-0.9 V -1.25 V
