Color and Volume Change in PPy(DBS) - American Chemical Society

Dec 11, 2008 - PPy(DBS), is studied during electrochemical switching. Cations enter this material during reduction, leading to an increase in volume. ...
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J. Phys. Chem. C 2009, 113, 359–368

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Color and Volume Change in PPy(DBS) Xuezheng Wang and Elisabeth Smela* Department of Mechanical Engineering, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: April 4, 2008; ReVised Manuscript ReceiVed: September 30, 2008

The optical-mechanical-electrochemical coupling in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), is studied during electrochemical switching. Cations enter this material during reduction, leading to an increase in volume. A special experimental geometry is used in which the top surface of the PPy film is covered by a transparent ion barrier, which constrains the charge-compensating cations to enter the film only at the edges. The cation concentration determines the volume of the polymer and is also directly related to the oxidation level of the polymer, which in turn determines its color. During electrochemical reduction, the edges of the film lighten, and this light color travels as a front to the center of the film. At the same time, the height of the film increases when and where the color lightens, and the increase in height is directly proportional to the change in color intensity. The height and the intensity attain their maximum values as the front passes and do not increase further thereafter, indicating that the ion concentration in the fully reduced state has a maximum value that is not exceeded. The color and volume changes are associated with only the first pair of peaks in the cyclic voltammogram, which represent only a fraction of the consumed charge in a typical electrochemical scan. 1. Introduction Since conjugated polymer actuators were first demonstrated,1-4 a large variety of proof-of-concept devices have been fabricated.5-9 However, improvements in actuation have been hampered by a lack of fundamental understanding of the underlying physical phenomena that occur during electrochemical switching. In this paper, we characterize the color and volume changes associated with redox in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), and correlate those changes with the cyclic voltammogram. The optical-mechanical-electrochemical coupling, i.e., the relationship between color, strain, and charge, have not previously been mapped out and provide important insight into the physical processes occurring during cycling. This work is part of a longer-term effort to develop a greater understanding of the coupled chemical, electrical, and mechanical processes that occur during reduction and oxidation (redox) of conjugated polymers. These studies make use of an experimental geometry, presented previously in a short communication,10 that allows one to track lateral ion transport inside the film via electrochromic color changes by constraining the ions to travel inward from the edges. We show, by studying the color change of the film and the change in film thickness, that during reduction in this geometry cations enter the PPy in a front, moving from the edges to the center. The volume increase and color change occur simultaneously, and their magnitudes are linearly related. By correlating these changes with the cyclic voltammogram, we show that they are associated with only the more cathodic pair of peaks and that these peaks represent only 20% of the total charge consumed during a typical cycle. The polymer studied in this paper is PPy(DBS), a cationtransporting material in which the DBS- anions are immobile. This polymer was chosen as our model system because of our * Corresponding author. Tel.: 301-405-5265. Fax: 301-314-9477. E-mail: [email protected].

extensive prior work with it.11-22 PPy(DBS) is anisotropic: it has a lamellar structure with the layers oriented preferentially parallel to the electrode surface.23 In the experiments in this paper, the in-plane ion transport rate parallel to the lamellae is measured, which is different from the out-of-plane transport rate.24 The electronic charge carriers in PPy are polarons and bipolarons, which are created upon oxidation of the polymer by the removal of electrons. They carry a positive charge, and we shall refer to them as “holes” in analogy with the term used in inorganic semiconductors. During reduction, the holes leave the polymer (electrons are added), and since the DBS- counterions are immobile in the PPy, hydrated cations simultaneously enter the polymer to maintain overall charge neutrality. During oxidation, holes re-enter the film, and the cations exit. Conjugated polymers are electrochromic, meaning that their color can be controlled with an applied electrochemical potential. (Oxidation introduces additional electronic states into the band gap, allowing absorption of light at lower energy.25,26) PPy changes from brown to light yellow upon reduction, but for a thin film deposited on Au, such as used in this study, the film appears red in the oxidized state and transparent in the reduced state. The schematic diagram in Figure 1 illustrates the experimental approach. A long, narrow stripe of a thin conjugated polymer film is sandwiched between an electrode on the bottom and a transparent ion-blocking layer on the top. The ion barrier is transparent to enable visualization of the colors underneath. When the conjugated polymer is switched, ions can only enter or leave it from the edges, not from the top or bottom. Holes, on the other hand, are able to enter and leave anywhere through the bottom electrode area. The distance that the cations must travel to get to the center of the strip is thus 500 times longer than the path for electrons to reach the opposite face. The higher mobility of the holes (orders of magnitude higher than that of ions) shortens their arrival time relative to the ions even further.

10.1021/jp802937v CCC: $40.75  2009 American Chemical Society Published on Web 12/11/2008

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Figure 1. Device configuration that makes ion transport the ratelimiting step during electrochemical switching of a conjugated polymer. The polymer is patterned into a long, narrow stripe over an electrode and is covered on the top side with a transparent ion-blocking layer (vertical dimensions exaggerated for clarity). Ions enter and exit the polymer from the long edges. The color of the film varies with its oxidation level, which cannot change until charge-compensating ions arrive or leave.

Because the polymer must remain charge neutral during switching, the electrochemical reaction cannot proceed at any particular location in the filmsthe oxidation level cannot change theresuntil the cations reach (or leave) that point. Thus, a key rate-limiting step in the electrochemical reaction in this device is cation transport. Also, because of the link between oxidation level and cation concentration imposed by charge neutrality, the color of the film is a direct (although not necessarily linear) indication of the cation concentration. As mentioned above, in this geometry the rate-limiting step in the electrochemical reaction is cation transport. Depending on the experimental conditions, the rate-limiting step can be ion transport, hole transport,27-29 or chain conformational changes.30,31 However, cation transport is strongly affected by the state of the polymer matrix. When the polymer is in a compacted state, ion mobility is lower than when the polymer is in a more expanded state. The ion transport rate is not specifically studied in the experiments reported below, but this is the central topic of another paper.32 Thus, cation transport is the rate-limiting step in these experiments in the following sense: although the electrochemical reaction is energetically allowed by the applied potential, it cannot proceed in a given region of the polymer until the cations arrive or depart from there. (This is discussed in detail in ref 32). However, the state of the polymer controls how rapidly the cations can move through the matrix. The more compact the polymer, the slower the ion transport, as shown below and in ref 32. Free-volume generation by electrochemical stimulation of chain conformational movements30,33 therefore plays an indirect role in the results, since higher overpotentials provide more energy to drive conformational changes, thereby opening

Wang and Smela the matrix and increasing ion mobility. The ion mobility is therefore dependent on the current local oxidation level of the polymer as well as the applied voltage. These experiments were inspired by those of Kaneto et al.27 and Tezuka et al.,28,29 who made an electrical connection to only one end of a strip of conjugated polymer, the face of which was exposed to the electrolyte. In that configuration, the path of the holes was long compared to that for the ions, the opposite of our configuration. It has been well established that volume change in conjugated polymers is primarily caused by the ingress/egress of ions, which are typically solvated. (See, for example, refs 34-64). Thus, the strain would be expected to closely follow the color change (as long as there is no significant strain due to osmotic effects62 taking place with a different time constant) since color depends on the oxidation level (i.e., the number of holes on the backbone), which is linked to the ion concentration through the requirement for charge neutrality. (Engineering strain is defined as the change in length divided by the initial length l, (lfinal l)/l ) ∆l/l, and strain is therefore unitless and is typically given in %.) Out-of-plane strain is the change in the thickness of the film upon actuation.19,65 This is different from in-plane strain, which is harnessed in bilayer actuators.20,66 Out-of-plane actuation strain is typically 10 times larger than in-plane strain and can thus be utilized for applications such as microfluidic valves.67 2. Experimental Methods 2.1. Device Fabrication. The devices illustrated in Figure 1 were fabricated using standard microfabrication techniques. The Au electrode was deposited by either sputtering (ATC 1800, AJA International, 200 W, 5 × 10-3 Torr, 5 Å/s) or thermal evaporation (Cooke, 80 A, 5 × 10-6 Torr, 5 Å/s) to a thickness of 3000 Å over an adhesion layer of Cr 150 Å thick (3 Å/s) onto 4-inch diameter oxidized silicon wafers. Film thicknesses were measured by mechanical profilometry (Dektak 3ST or Alphastep 500). The wafers were cleaved into pieces 1 cm × 3 cm. PPy was deposited potentiostatically at 0.47 V (0.1 mA/cm2) (Ecochemie pgstat30) onto the Au-coated Si working electrode (WE) to a thickness of 300-400 nm. The polymer film was kept thin so that changes in oxidation level could easily be observed. (When PPy films on Au are thicker than approximately 1 µm, they appear black even in the reduced state.) A three-electrode cell was used with Ag/AgCl (BAS) as the reference electrode (RE) and porous carbon (VWR) as the counter electrode (CE) (3 × 2 × 0.5 cm3). The WE and CE were parallel and separated by 3 cm. The deposition electrolyte contained 0.1 M pyrrole and 0.1 M NaDBS (Aldrich) in deionized (DI) water. Pyrrole (Aldrich) was stored at -40 °C and filtered through alumina (Aldrich) before use. The ion barrier material was SU8-2002 (MicroChem), a negative epoxy-based photoresist. It was spin-coated over the PPy to a thickness of 2 µm (1000 rpm, 30 s). (Note that SU8 cannot be applied to PPy(DBS) films that have already been electrochemically cycled in NaDBS since the resist dewets in this case.) The SU8 was prebaked (65 °C for 1 min, then ramped at 300 °C/h to 95 °C and baked for a further 1 min) and exposed under ultraviolet light (365 nm, 24 s) through a mask to form 300 µm wide, 8 mm long stripes. The SU8 was then postbaked (conditions the same as during prebake) and developed (MicroChem SU8 developer) for 2 min. To determine whether the redox reactions had been in any way adversely affected by exposure to these substances, cyclic

Color and Volume Change in PPy(DBS) voltammetry was performed on PPy(DBS) films deposited on identical substrates after coating the PPy with SU8 and rinsing it off in SU8 developer, without UV exposure. The cyclic voltammograms were unaltered from those of unexposed PPy(DBS). To pattern the PPy, it was etched by reactive ion etching (RIE) in an oxygen plasma (March Jupiter, 0.2 mTorr, 200 W, etch rate 300 nm/min). The SU8 served as a mask, and PPy was removed in the uncovered areas, leaving the PPy that remained under the SU8 with perpendicular edges. Etching usually produced a few star-shaped cracks in the SU8 at isolated points; these increased in number if higher power and oxygen flow rates were used. It has previously been shown that RIE and UV exposure do not affect PPy redox.14 2.2. Electrochemical Cycling. Samples were placed horizontally in a flat-bottomed electrochemical cell (12.0 mm × 8.5 mm, similar to the one shown in ref 19). Gold-plated screws, nuts, and washers were used to make electrical contact. Leads from the potentiostat were attached outside the electrolyte to the heads of the screws using alligator clips. The electrolyte was 0.1 M NaDBS in DI water and was 8 mm deep over the sample. The PPy(DBS) films were cycled in a solution of NaDBS to ensure that charge compensation was only by Na+. (In NaDBS, a small amount of OH- may also be transported, but it does not contribute to volume change.12) (In electrolytes with small anions, such as NaCl, anion transport also takes place64 and contributes to actuation strain with the opposite sign as that due to the Na+. In addition, Cl- can electrochemically react with Au. Such a situation would be too complicated for initial studies.) Experiments were carried out in a glovebox filled with argon gas to provide an oxygen-free environment. (Oxygen chemically dopes PPy and therefore interferes with the electrochemical reduction process.) To reduce the amount of oxygen in the electrolyte, argon gas (1-2 psi) was bubbled through it for 15 min prior to use. The RE and CE were the same as for PPy deposition. Data from samples that delaminated during the experiment were discarded. If there were small regions of the film that were problematic but that did not affect the rest of the film, the samples were kept, but those areas were disregarded during data analysis. For example, there was local delamination at small spots due to the accumulation of hydrogen gas in the film (from hydrolysis at some of the large negative potentials that were applied), which formed what appeared to be bubbles.68 (These circular areas remained dark red (oxidized) when a reducing potential was applied, while the rest of the film turned transparent. In thin PPy films, such bubbles were observed only after many cycles, but in films >1 µm thick, bubbles formed after only a few cycles. Bubbles first appeared during oxidation, typically at the center of the film but later spreading.) 2.3. Out-of-Plane Strain Measurement. Sample thickness was measured in situ with a Dektak 3ST mechanical profilometer using a 65 µm vertical range, a 1 mm horizontal scan length, a 20 µm/s scan rate (high resolution, taking 50 s to complete), and a 5 mg stylus force. Scans to measure thickness changes during redox were performed at a lower resolution and took only 2 s to complete. A sketch of the measurement is shown in Figure 2. Scans were performed before immersion in the electrolyte, after immersion, and during the first reduction scan. Thereafter, scans were taken in the fully oxidized and reduced states. Each steady-state height profile reported below represents an average of at least six repeated scans at the same nominal position. If bubbles appeared in the region of the film under

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Figure 2. Illustration of the thickness measurement method by mechanical profilometry, performed with the samples immersed in the electrolyte (not shown).

Figure 3. Red color intensity of only the Au areas in the image as a function of time before and after correction. The inserted overheadview photographs show the image at the times corresponding to the arrows; neither of these images required correction, but later images did.

the stylus, the stylus was repositioned to an adjacent location without bubbles. 2.4. Front Analysis. 2.4.1. Data Collection. The samples were observed from overhead through a stereomicroscope (Leica MZ12.5), inside the glovebox, with a halogen light source (ACE1, Schott-Fostec LLC). Because of the mirror surface of the substrate, coaxial illumination was used. A digital camera (Nikon Coolpix 4500) was mounted to the microscope, and it transmitted a video signal to a digital camcorder (Sony DCRTRV330) via an analog video cable to the video input port. Video was recorded on Hi8 digital tape at 30 frames per second (fps). The video was downloaded to a PC in DV format using video editing software (Studio, Ver. 8). Slower frame rates (down to 5 fps) were used for large files to reduce analysis time (e.g., for long, slow scans that did not require fast frame rates). The video was read using Matlab in RGB format with 8 bit depth (256 intensity levels), and every frame was saved to the hard disk in bitmap format. 2.4.2. Intensity Correction. The video signal taken from the digital camera was the one sent to the LCD screen on the back of the camera. To aid camera users, the brightness of this image is automatically adjusted in response to changes in brightness of the scene. In our case, unfortunately, this occurred when the PPy changed from red to yellow (see times >7 s in Figure 3). This shift could not be prevented and therefore had to be corrected. This was done prior to any data analysis and was based on the two Au regions on either side of the PPy stripe, the color of which did not change with applied potential (confirmed on substrates without PPy). The images were

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Figure 4. Overlay of an image of a partially reduced 300 nm thick PPy(DBS) film underneath a 2 µm thick SU8 ion barrier at t ) 1.5 s with an applied voltage of V ) -1 V vs Ag/AgCl. The reduced areas are red, the oxidized areas are light orange, and the Au on either side is yellow. Also shown are cross-section intensity profiles on the red, green, and blue channels.

corrected to bring the Au to a constant level by multiplying the intensity of each pixel by the normalized gold film intensity on the red channel: Icorrected ) I* (IAu/IAu final). (Only the red channel was used for data analysis since, as illustrated below, it showed the color change more clearly than the other channels.) An offset was not used. This simple linear correction was found to be appropriate in all cases based on visual inspection of the corrected images. 2.4.3. Data Analysis. As reported previously10 and as shown in Figure 4, when the PPy was reduced, an ion front moved from the edges of the PPy stripe to the center. The intensities in the red, green, and blue channels of the pixels in a line across this image are also shown in Figure 4. The intensity increased on all three channels upon reduction, but the change was largest on the red channel. Therefore, for data analysis only the red channel signal was used to reduce computation time. (We examine the correlation of charge and red channel intensity below. It should be noted that the camera represents the color of each pixel as a combination of red, green, and blue intensities, but this may not be the same as what would be recorded at red, green, and blue frequencies in a spectrophotometer. This method of image analysis did not take into account any shifts in the wavelength of the peaks associated with the polaron and bipolaron bands as a function of oxidation level.69) The difference in intensity between the fully oxidized and reduced states was approximately 100 levels on the red channel. The downward intensity spikes at either edge of the stripe were due to shadows. 2.5. Chronoamperometric Current Correction. The background current needed to be removed from the total chronoamperometric current (Figure 5) to obtain the charge consumed by the PPy upon switching. (The data in this figure were collected during reduction at -1.5 V, and the fronts reached the center at t ) 2 s.) The original current (dashed line) had a large residual component (indicated by the gray line) even after 10 s. This was subtracted to obtain a corrected current (heavy dashed line) and integrated to obtain an estimate of the charge consumed by the PPy (solid line). This subtraction removed only parasitic currents, however, and not capacitive currents on the electrodes, which cannot be readily separated.

Wang and Smela

Figure 5. Method for correcting the current and the charge obtained by integrating the current.

Figure 6. Intensity profiles at 30 s intervals during the first-ever reduction at -1 V vs Ag/AgCl. The arrow indicates the direction of front movement over time.

3. Results We first present color change during reduction, then the corresponding out-of-plane strain measurements, and finally we examine the relationships between color, strain, and redox charge. In the following experiments, the potential was stepped from 0 to -1 V vs Ag/AgCl unless otherwise noted. 3.1. Color Change During Redox. As shown in Figure 3 and Figure 4, as well as ref 10, during reduction ion fronts entered the film from the two edges and moved toward the center, where they met. During the first cycle, these fronts were sharp, and they remained sharp throughout the process (this is discussed in detail in ref 32). Red channel color intensity profiles taken at 30 s intervals during a first-ever reduction are shown in Figure 6. The profiles had nearly vertical steps, and front velocities were on the order of 1 µm/s. As shown in ref 32, in later cycles, the fronts were broader and moved more quickly. The first reduction scan of a conjugated polymer is known to be anomalous, and it can take several cycles to “break in” the polymer before it starts to show steady state behavior. This has been attributed to rearrangements of the polymer chains during the first reduction scan and the uptake of solvent that subsequently remains within the film, both of which increase the mobility of the ions.51,70,71 (As mentioned previously, the state of the polymer matrix strongly affects cation transport rates.) During oxidation, on the contrary, there were no color fronts. Instead, the entire film lightened all at once, although the changes were faster at the edges. Instead of steps,

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Figure 7. (a) Height changes during the first-ever reduction (-1 V vs Ag/AgCl). Shown are the as-deposited PPy + SU8 thickness (gray line), thickness snapshots taken every 40 s during the reduction process (thin black lines), and the completely reduced thickness (thick black line). The unchanging ion barrier thickness is indicated schematically by the dashed line (in actuality the SU8 is above, not below, the PPy.) (b) Magnified view of the subsequent height changes during later actuation cycles.

the intensity profiles were smooth, somewhat bell-shaped curves whose heights dropped over time. 3.2. Out-of-Plane Actuation Strain. The out-of-plane strain of an ion barrier-covered PPy(DBS) film is shown in Figure 7a during reduction for the first time at -1.0 V. The gray line shows the thickness of the as-deposited PPy with the overlying SU8 layer. The slow ion velocity during the first reduction scan, which took approximately 4 min, allowed snapshots of the process to be taken since the height profiles were completed in only 2 s. The thin black lines in Figure 7a are profiles taken 40 s apart, and the thick black line shows the final, completely reduced state, which was 55% thicker than the as-deposited film. The film thickness changes reflected the same ion fronts as the color intensity profiles shown in Figure 6. The height increased stepwise to a value corresponding to the maximum ion concentration in the PPy, and the steps traveled from the edges of the film to the center. The thickness at the edges did not increase further as the front advanced. It should also be noted that no separate volume-changing process was observed that had a different time constant. Different time constants have previously been observed by others due to water ingress independent of cation ingress.62 Upon reoxidation (shown in the close-up Figure 7b), the PPy thickness did not return back to its original, as-deposited value. A quasi-irreversible swelling occurred in the first cycle: the oxidized state was 25% thicker than the as-deposited film. This irreversible swelling is consistent with results obtained on uncovered PPy(DBS) films with atomic force microscopy19 and with the hypothesis that water enters the film during the first reduction scan and remains in the film during reoxidation. (This may also be the case for some fraction of the cations since the charge consumed during the first reduction scan is larger than in later scans.) In subsequent cycles, the height changed reproducibly between the reduced state value and the oxidized state value, with a reversible actuation strain, based on the oxidized film thickness, of 28%. One key thing to appreciate in Figure 7 is that the strain reaches a maximum Value beyond which it does not increase. This fact confirms that the ion density in the polymer has an upper bound, Cmax, as expected from the notion that the cations enter the material to re-establish charge neutrality by compensating the fixed density of DBS anions left behind by an equal concentration of holes upon reduction. Models of charge transport in conjugated polymers must take this Cmax into account

since it affects the driving forces that can be achieved by diffusion. Additional evidence for a maximum actuation strain, and therefore a maximum ion concentration, is that the strain did not depend on the applied potential for voltages more negative than the reduction peak (see the Supporting Information). Thus, the final ion concentration in the film is determined by the number of charges to be neutralized in the polymer and not by the driving force on the ions. Given the not-perfectly-flat-topped profiles (Figure 7b), actuation strain was determined in three ways. One was to “eyeball” the thicknesses; the second was to average the height values between x ) 50 and 250 µm (x ) 0 and 300 correspond to the edges of the film); and the third was to subtract the curves at different oxidation levels and obtain the average differences. The three methods yielded results within 5% of each other. To verify that the swelling in the first cycle shown in Figure 7 was due to water drawn in as a result of electrochemical reduction, rather than due to passive uptake, which also occurs upon initial immersion of the film into water,19,72 height changes were measured after different immersion times in the electrolyte. The thickness increased 5% within the first 10 min and then remained stable for the next 6 h. (In 11 µm thick uncovered PPy(DBS) films, the swelling was found to be 11%;19 the difference may be due to the stiffness of the ion barrier or to differences in the film itself since properties vary as a function of film thickness.20,73) All samples were therefore immersed for at least 10 min to reach the equilibrium hydration levels associated with the as-deposited state of the film before actuation strain measurements were begun. 3.3. Correlating Intensity and Strain 1. Since color is linked (although not necessarily linearly) to hole concentration H, and volume is linked to cation concentration C, and H and C are tied together by C ) A - H, where A is the DBSconcentration, then one would expect color and volume to have a correlated dependence on the reduction charge, and thus indirectly on the applied potential. The actuation strain of an ion-barrier-covered film measured by mechanical profilometry and the average red channel intensity of an uncovered film of the same thickness are shown as a function of potential in Figure 8. (An uncovered film was used to more rapidly obtain uniform color changes across the entire film surface in response to the applied voltage, without the complication of moving fronts. A barrier-covered film could also have been used.) The data were obtained by stepping between 0 V (fully oxidized state) and

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Figure 8. Red channel intensity (open circles, left axis) of an uncovered PPy film that was 400 nm thick as deposited and thickness change (black points, right axis) in an SU8-covered PPy film that was 420 nm thick as deposited vs the applied reduction potential after stepping from 0 V and holding for several minutes. The inset shows the height change versus the red channel intensity between 0 and -1 V, with a linear fit of y ) -59 + 1.14x, R ) 0.93.

the various reduction potentials indicated on the x-axis (in order of increasingly negative potentials). Each potential (both oxidizing and reducing) was held for several minutes to establish equilibrium before each measurement was taken. The strain and intensity had identical relationships with the reduction potential below -0.5 V, and the height change was linear with intensity. The onset of the increase for both occurred at -0.5 V, and the increase was linear with potential for both until they saturated at -0.85 V: increasing the cathodic potential further did not increase either one. At potentials more positive than -0.5 V, the height increased slightly, for reasons that are not clear but may involve solvation, while the intensity was flat.

Wang and Smela 3.4. Correlating Intensity and Strain 2. If volume and color changes in these devices occur simultaneously (i.e., at precisely the same time, rather than just in the same way or at the same voltages), there is even stronger evidence that both are linked to exactly the same underlying cation fluxes (the latter indirectly through the hole concentration via H ) A - C), and models that can correctly predict ion transport are therefore directly relevant for actuation. To demonstrate that this is the case, samples were fabricated to show both color and volume change in a single PPy film by depositing a gold film over part of the ion barrier surface to serve as a mirror. (This sample used a less stiff, thin Parylene C layer as the ion barrier.) Where it was parallel to the substrate surface, the mirror reflected light back to the camera, but where it was sloped over a change in height, it appeared dark. Following the dark line on the mirror thus allows one to track (but not quantify) the out-of-plane strain, just like the color, using a video camera. Different stages during the first-ever reduction step are shown in Figure 9a-c, with the mirror on the right. The left half of each image shows the color change, and the right half shows the steps over the change in height of the underlying PPy. A sequence of images from a later cycle reduction is shown in Figure 9d-f. The lines were less visible in later cycles because both the total height change and the slope were smaller. The interfaces between the red and transparent regions of the film, and the dark lines at the steps in height (indicated by the arrows), occurred at approximately the same distance from the edge and moved together in synchrony in both the first cycle and later cycles. There was, however, a small difference in the velocity of the fronts on the right and left sides of these images. During the first cycle, the ion front moved more slowly under the Au + Parylene side than on the Parylene-only side, but in subsequent cycles, it moved more quickly on that side. The difference between the two halves is that the Au + Parylene is stiffer than the Parylene alone. The difference in velocity

Figure 9. Color and volume change shown simultaneously during reduction (-1.5 V) of a PPy(DBS) film (820 nm thick as deposited) covered with a Parylene C ion barrier (1000 nm), and on the right side of the device, also by a thin gold mirror (200 nm). Arrows indicate the two parallel, inward-moving shadows resulting from the slope of the Au film above the changing PPy thickness. (a, b, c) First-ever reduction step, at 30 s intervals. (d, e, f) Later cycle reduction step, at 0.67 s intervals. Videos of these processes are available in AVI format: the files are named jp802937v_si_004.avi (speeded up 10×) and jp802937v_si_005.avi in the Supporting Information.

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Figure 10. Average intensity over the entire width of the PPy stripe (300 nm PPy, 2 µm thick SU8) and the charge consumed for (a) reduction (-1.1 V) and (b) oxidation (0 V). Note that the intensity axes have been reversed to facilitate comparison.

therefore tells us something about the electrochemicalmechanical coupling: cation movement is affected by mechanical constraints. (Otero has shown that such coupling can be exploited for tactile sensing.74) A complete model of the redox process must account for this. To explain the results in Figure 9, we reason thusly. In the first cycle, the greater barrier stiffness on the right side, and consequentially the higher force required to bend it, hinders the volume expansion associated with ion ingress and thus slows down the ion front on the right side relative to the left. Upon oxidation, however, it is likely that the higher barrier stiffness also hinders the collapse of the PPy matrix upon ion egress, thus holding it in a more open state. During the next reduction cycle, the ions on the right side therefore encounter a more open matrix than those on the left, which now facilitates their ingress. This explanation is supported by the fact that the first-cycle velocity decreases with SU8 thickness but that later-cycle velocities increase with barrier stiffness,32 and also that both first- and later-cycle velocities increase with PPy thickness.32 In Figure 9a, the smooth transition between the two front positions extends approximately 50 µm under the mirror. In Figure 9d, the transition extends approximately 75 µm in the other direction, to the left of the mirror. In both cases, the curvature occurs in the slower half. 3.5. Correlating Intensity and Charge. Not all of the charge injected into a conjugated polymer is associated with volume change (see, for example, refs 12 and 75). Capacitive charging, charge consumed by the reversible addition of OH- groups onto the backbone, and charge consumed by parasitic reactions can be significant and moreover are essentially impossible to distinguish from redox charge76 (by which we mean charge associated with the creation or annihilation of holes), particularly during chronoamperometry. Nevertheless, it is of interest to see what the correlation is between the intensity and the consumed charge, even though we do not expect a one-to-one relationship. The charge consumed by an ion-barrier-covered PPy(DBS) film and the normalized intensity of the red channel during reducing and oxidizing potential steps are compared in Figure 10. The potential was stepped between 0 and -1.5 V, and the current and color were recorded simultaneously. The normalized intensity averaged over the entire 300 µm width of the covered PPy(DBS) stripe was tracked over time to provide a single color measure for the entire film. This is essentially equivalent to tracking the flux of ions entering or exiting the edges. The charge was obtained by integrating the corrected chronoamperometric current.

During reduction, the intensity and charge curves differed: the negative charge increased rapidly initially and slowed to approach the final value, but the color change was essentially linear for the first few seconds during front propagation and then did not change further after the fronts met. During oxidation, on the other hand, the charge and color curves followed one another more closely. (A second set of data is shown in the Supporting Information.) The reason for the difference in the intensity and charge curves during reduction is not immediately obvious and must be considered further. Two possible explanations for the mismatch are (1) electrochemical processes that draw current not associated with polarons/bipolarons (only the redox charge, not the total charge consumed during switching, corresponds to the hole concentration, and thus the ion concentration, in the film) and/or (2) absorption at wavelengths the camera records on the red channel is nonlinear with charge. It should be noted that a similar noncorrelation of charge and strain was seen in polyaniline films and fibers during oxidation (the ion ingress step):75 the strain had a steplike profile, while the charge followed a slower, smooth curve. (The stress was even less correlated with the charge.) Likewise, the strain and charge curves were more similar during reduction (ion egress). To explore this issue further, the color and current during cyclic voltammetry at 20 mV/s of a PPy(DBS) film uncoVered by an ion barrier were measured simultaneously (Figure 11). The intensity was recorded once per second from video taken during the CVs. The CV was corrected by subtracting the parasitic current measured on a substrate without PPy, as shown in ref 19, to obtain a more accurate calculation of the charge. During reduction, current was consumed between 0 and -0.3 V (in the pseudocapacitive region labeled c) and during the first cathodic process associated with the shoulder at -0.47 V (possibly associated with OH- transport12 or the filling of different types of intercalation sites within the polymer,78 labeled 2), but the color did not start to change until -0.5 V (just as in Figure 8), at the foot of the cathodic peak at -0.63 V (associated with Na+ transport12 and labeled 1). (The nature of the current above the oxidation peak has been the subject of debate for many years, and it has the rectangular shape associated with the capacitive charging of an electrode surface and hence has frequently been referred to as capacitive current; however, the magnitude of this current is strongly dependent on deposition conditions, being totally absent in some films that have otherwise identical cyclic voltammograms.77) Upon oxidation, color change started at -0.73 V with the start of the first oxidation peak (1′)

366 J. Phys. Chem. C, Vol. 113, No. 1, 2009

Figure 11. Intensity as a function of potential (black line) during cyclic voltammetry at 20 mV/s (gray line, corrected CV) of an uncovered PPy(DBS) film 450 nm thick.

Wang and Smela to represent the two pairs of peaks (Figure 12). The charge in the anodic and cathodic peaks of each pair was constrained to be equal. These peaks were subtracted from the total current to show the remaining (pseudocapacitive) current. The separation between the anodic and cathodic peaks in the first pair was 0.10 V and in the second 0.11 V. It is noteworthy that the pair of peaks 1 has only 36% as much charge as is in the pair 2 and less than 20% of the total charge in the CV. Thus, 80+% of the charge consumed during redox is not going to hole annihilation (and ion transport, leading to volume change) but to other electrochemical processes (as yet unknown). This would account for a significant fraction of the inefficiency of PPy(DBS) actuators.16 This would also account for the noncorrelation of charge and intensity in Figure 10a. The total charge Q and the charge in the first Gaussian QG1 are plotted with the intensity as a function of voltage in Figure 13a; QG1 is much more closely correlated with the intensity. (A less-good fit to the charge under both Gaussians as well as a second set of data are shown in the Supporting Information.) Figure 13b shows that the intensity and QG1 are linearly related over a large range (the anodic side of the curve is less well approximated by the Gaussian). The fact that the color change is faster than the charge in Figure 10 suggests that Na+ transport during reduction is faster than the processes labeled 2 and c in Figure 11 and Figure 12 when -1 V is applied, even though peak 1 requires a more cathodic potential and thus greater energy. This is a topic that should be further investigated in future work. 4. Summary and Conclusions

Figure 12. Current components of the CV in Figure 11 (gray line). First (solid black line) and second (dotted black line) Gaussian peaks (1 and 2), their sum (dashed black line), and the remaining current (dotted gray line, c).

and was completed by -0.3 V, prior to the end of the second peak (2′) and before the pseudocapacitive current. These data are strikingly similar to previously published bilayer actuation data (Figure 10 in ref 12), which showed that there was no bilayer movement associated with either the pseudocapacitive current or the “OH-” peak. To gain qualitative insight into the processes giving rise to the cyclic voltammogram, the curve was fit using four Gaussians

We have described an experimental configuration in which ion transport is the rate-limiting step in the redox reaction so that reduction does not take place at a given location until the cations arrive there, and oxidation does not take place until they leave. It should be borne in mind that this configuration gives information about ion transport parallel to the film surface, which may be quite different from transport perpendicular to the surface if the material is anisotropic, as PPy(DBS) is known to be. The simplicity of device fabrication should allow this configuration to be used by other groups with different materials, and we particularly encourage its use to study anion-transporting conjugated polymers. Upon electrochemical reduction, there is a color front that moves into the center of the film from the edges and a strain front that moves together with it. They have an identical dependence on potential and are linearly proportional to each

Figure 13. (a) Intensity (gray line) versus potential compared with the total charge (divided by 5 for plotting purposes, dashed line) and the charge in Gaussian 1 (solid black line). (b) Intensity vs the charge in Gaussian 1. The filled symbols show the points used for the line fit.

Color and Volume Change in PPy(DBS) other. Once the film is fully reduced, neither the color nor the thickness change any further. During the first-ever reduction, the PPy film thickness increases by 55% through a combination of swelling strain and actuation strain. The film does not return to its original thickness when the cations exit during oxidation but remains swollen with water (and probably some cations), which remains semipermanently in the film. The irreversible swelling strain is ∼25%. This change is invisible in the current and color measurements since it does not involve the electronic states of the polymer. In later cycles, the actuation strain is 28%, and these thickness increases and decreases are reversible. These results are consistent with those seen previously by atomic force microscopy.19 The reversible component of out-of-plane actuation strain, which is the only component that is useful in actuators cycled more than once, is caused by solvated cation transport. The first-cycle water ingress merely causes anomalously large strains in the first cycle, or the first several cycles in thick films. In the subsequent reduction steps, the cations enter a more open, hydrated polymer. Fronts are again seen, but they move much faster. There is no evidence that in these devices water enters the film with a different time constant, i.e., there is no second strain wave. The color and the strain do not change any further once the film reaches its fully oxidized or reduced state, showing that there is a maximum ion concentration Cmax in the film, consistent with the notion that cation transport occurs solely to maintain charge neutrality. This is important to consider during model development. While color and strain are linearly correlated and reflect the oxidation level, the total consumed charge comprises currents from pseudocapacitive charging and reactions other than polaron/bipolaron creation, which can occur at different potentials and with other time constants. The electrochemical reactions occurring during the second pair of peaks and the pseudocapacitive region have yet to be identified and should be the subject of future work. Under the experimental conditions used in this paper, there were two pairs of peaks in the cyclic voltammogram, as well as pseudocapacitive current in the oxidized state. During cyclic voltammetry, color change is only associated with the more cathodic pair of redox peaks and is linearly proportional to the charge in those peaks. There are no color changes associated with the second pair of peaks or the so-called capacitive region, even though those processes consume 80% of the total charge. These results are consistent with prior work on in-plane strain, which is only associated with the more cathodic pair of peaks.12 The data in this paper do not answer the interesting question of whether injection of ions under an electric field is responsible for the volume change in the polymer or whether an initial volume increase is required first to accommodate the ions. It is likely that both are involved: some free volume is created by electrochemically stimulated conformational relaxation processes,30,33 allowing some initial ingress, but once inside, the ions hold the matrix open. The latter conclusion is based on the observation that actuation strain depends on the (solvated) ion size.79 Acknowledgment. We would like to acknowledge funding through DuPont’s Young Professor Grant and the Laboratory for Physical Science. We acknowledge Jason West for developing the CV curve-fitting algorithm. We also would like to thank Dr. Donald DeVoe and Dr. Likun Zhu for the Parylene C deposition.

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