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Electrochemistry of Conductive Polymers 47: Effects of Solubilizers on 3,4-Ethylenedixoythiophene Oxidation in Aqueous Media and Properties of Resulting Films Shin Hyo Cho,† Hyo Joong Lee,‡ Younghoon Ko,§ and Su-Moon Park*,§ †
Samsung Electronics Research Institute, Suwon, Korea Department of Chemistry, Chonbuk National University, Jeonju, Jeonbuk 561-756 Korea § Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-805, Korea ‡
ABSTRACT: Effects of a few solubilizers on electrochemical oxidation of 3,4-ethylenedioxythiophene (EDOT) in aqueous media and on electrochemical, morphological, and electronic properties of its resultant thin polymer (PEDOT) films have been investigated by Fourier transform electrochemical impedance spectroscopic and current sensing atomic force microscopic experiments. Two different types of solubilizers including hydroxypropyl-β-cyclodextrin (HpβCD) forming an inclusion complex with a hydrophobic species and two anionic surfactants forming micelles, sodium dodecylsulfate (SDS) and sodium dodecylsulfonate (SDBS), were used due to the low solubility of the EDOT monomer in aqueous media. Results indicate that anionic surfactant micelles protect electrogenerated cationic species of monomers and oligomers by encapsulating them, leading to formation of polymer films of better properties. The polymer growth characteristics were shown to be closely related to electrical properties of resultant polymer films; PEDOT films obtained in the presence of SDS showed the best growth, ion transport, electrochemical, and electronic properties.
’ INTRODUCTION For the last few decades, conducting polymers have been studied extensively and attracted intensive attention owing to a wide range of applications to numerous practical devices including field effect transistors,1 light-emitting diodes (LEDs),2 solar cells,3 electrochromic devices,4 electronic circuits,5 sensors,6 and other devices7 because of their easy processability, reasonable stability, low cost, and possibilities of tailoring the structures on a molecular scale. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most attractive conducting polymers because of its high conductivity and good stability under ambient conditions as well as its optical transparency in its oxidized state. In particular, the thin oxidized PEDOT film is a prerequisite to a few practical applications. When thin PEDOT coatings are required on a substrate, the electrochemical technique offers an efficient way to prepare a superior film. As a result, the PEDOT films are now utilized in wide variety of applications for, e.g., antistatic coatings in photographic films, conducting layers in electroluminescent lamps, hole injecting layers in polymeric light-emitting diodes, and polymer photovoltaic cells.8,9 Due to the low solubility of EDOT in aqueous solutions and its high oxidation potential, most studies on electrochemical polymerization of EDOT and subsequent characterization of PEDOT films have been carried out in organic media.1012 In efforts to overcome these problems, micellar solutions prepared using anionic surfactants such as sodium dodecylsulfate (SDS)13 r 2011 American Chemical Society
or sodium dodecylbenzenesulfonate (SDBS)14 have been employed to obtain PEDOT films in aqueous solutions. These studies showed that the micellar media not only solubilize and catalyze electropolymerization of EDOT via a soft template effect at the electrode but also lower its oxidation potential. The micellar solutions were also successfully employed to other conducting polymers such as polypyrrole and polybithiophene to make adherent polymer films with better mechanical and electrochemical properties.15 In addition, it has recently been reported that the PEDOT film was electrochemically grown in aqueous environments with the aid of a neutral molecule, hydroxypropyl-β-cyclodextrin (HpβCD),16 forming an inclusion complex with an EDOT monomer;16a this compound has a hydrophobic interior with a hydrophilic exterior, leading to formation of an inclusion complex in aqueous solution with hydrophobic species. However, despite the studies reported in the literature, overall effects of the solubilizers on PEDOT thin films have not been quantitatively compared. Electrochemical impedance spectroscopy (EIS) is a unique and powerful technique that allows an electrochemical system to be fully described.17 However, conventional EIS methods (e.g., frequency response analyzer, FRA) do not provide information Received: October 23, 2010 Revised: February 22, 2011 Published: March 18, 2011 6545
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The Journal of Physical Chemistry C on a transient system as it undergoes continuous chemical changes at the electrode/electrolyte interface during a full frame impedance measurement. Recently, a few different forms of Fourier transform electrochemical impedance spectroscopy (FTEIS) and related theory have been developed.18,19 Of these, a technique combining the staircase cyclic voltammetry and FTEIS allows impedance data to be taken in real time at a given dc bias potential from transient signals.19b Here, a series of small ascending and/or descending potential steps of 515 mV are applied, and resulting chronoamperometric currents are recorded. Subsequently, the first-derivative signals of both the voltage steps and the chronoamperometic currents obtained thereof are then converted to ac voltage and current signals in frequency domain by Fourier transformation. Impedance data are then acquired at any desired frequencies from the ac voltages and currents, and a staircase cyclic voltammogram (SCV) is constructed at the same time by connecting sampled currents at a specified step period, which is identical to the cyclic voltammogram (CV) when certain experimental conditions are met.20 Thus, the SCV-FTEIS technique is appropriate for studying a dynamic electrochemical system in which a primary electrochemical product undergoes a series of changes at the electrode.21 Along with the FTEIS, we also used current-sensing atomic force microscopy (CS-AFM) equipped with a conductive probe to characterize the polymer films thus obtained.2230 The CSAFM allows not only two-dimensional topographic and current images to be taken simultaneously but also currentvoltage characteristics to be measured at selected points of the image. The technique has been applied to various nanostructures including single molecules,23 self-assembled monolayers,24 carbon nanotubes,25 quantum dots,26 and others27 because it allows an easy and reproducible contact to be established with a variety of surfaces under a precisely controlled load force between the tip and the sample. The CS-AFM was also successfully used for investigation into conducting polymer films including polyaniline,28 polypyrrole,29 and polythiophene derivatives30 by obtaining two-dimensional current and topographic images and nanoscale electrical properties by measuring the current voltage curves. The origin of mesoscopic inhomogeneity of conducting polymers has also been discussed based on the Kelvin probe force microscopy and CS-AFM measurements as well.31 When the FTEIS and CS-AFM techniques are used in combination, overall information on PEDOT thin films starting from their electrochemical polymerization to their electrical and morphological properties can be obtained; the growth characteristics can then be related to the properties of the films thereof. In this work, thin PEDOT films are prepared electrochemically during an earlier stage of the polymerization reaction in the presence of SDS, SDBS, or HpβCD as a solubilizer, and we analyze and compare how their electrochemical growth, polymer structures, and ionic as well as electronic properties are affected.
’ EXPERIMENTAL SECTION Doubly distilled, deionized water was used for the preparation of aqueous solutions. Sodium dodecylsulfate (SDS, Fluka, 99%), sodium dodecylbenzenesulfonate (SDBS, Fluka), (2-hydoxypropyl)-β-cyclodextrin (HpβCD, Aldrich), 3,4-ethylenedioxythiophene (EDOT, Aldrich), and lithium perchlorate (LiClO4, Aldrich, 99.99%) were used as received. Solutions were prepared by dissolving 30 mM EDOT monomer, 0.10 M LiClO4, and 30 mM solubilizer (SDS, SDBS, or
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HpβCD) in water for electrochemical synthesis of PEDOT films. Prior to each experiment, the solution was purged with nitrogen but the solutions containing SDS or SDBS were not because of bubbles generated during the purging process. A single-compartment cell with a three-electrode configuration was used for all electrochemical experiments. A gold disk electrode was polished successfully with alumina slurries from 1.45 to 0.03 μm and sonicated in doubly deionized water just before it was used for FT-EIS experiments. A gold-on-silicon electrode (with Cr adhesive layers, Inosteck) was annealed for 5 min with a hydrogen flame after they had been cleaned in a piranha solution (H2SO4:H2O2 = 70:30 v/v), rinsed thoroughly with deionized water, and used for CS-AFM experiments. A piece of platinum gauze and an Ag/AgCl (in saturated KCl) electrode were used as counter and reference electrodes, respectively. Thicknesses of the films prepared on the gold-on-silicon electrodes were measured from the cross-sectional view of the SEM images. The FTEIS measurements were made with a homemade fastrise potentiostat with a rise time of SDBS > SDS, which suggests that EDOT monomers appear more strongly held in HpβCD cages leading to less effective electron transfer in comparison to SDS and SDBS. Second, the n values decrease from >0.96 to about 0.80 over the potential increase when SDS and SDBS are used as solubilizers, whereas not much potential dependency is observed for HpβCD with n of around 0.9 (Figure 3b). This indicates that nonideal capacitive dispersions increase at higher potentials due to the polymer deposition, which leads to increases in not only effective capacitances (Figure 3c) but also the porosity as evidenced by the decrease in the n value; these are attributed to the increase in the amount of the polymer. As pointed out, the n value stays about constant throughout the potential range when HpβCD is used as an additive, suggesting that its capacitance does not undergo much change upon an increase in potential. The capacitors do not show a large dispersion from ideal behavior as the contribution of pseudocapacitances arising from the faradaic charge transfer is not significant as can be seen from its CV in Figure 1b. The capacitances increase in the order of SDS > SDBS > HpβCD contrary to the Rp values, which is in the opposite sequence, as the potential increases (Figure 3c). This also indicates that the rate of monomer oxidation, which determines the amount of polymer deposition, is more efficient
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with SDS than HpβCD. The continuous increase in Cp indicates that EDOT oxidation results in formation of a thicker electrically conductive film on the surface upon an increase in potential. The differences shown during EDOT oxidation and polymer formation depend on solubilizers due to the interaction between the solubilizer and the EDOT monomer. In the case of SDS and SDBS, negatively charged micelles would wrap around radical cations of EDOT and its dimer, oligomers, and/or polymers generated upon electrochemical oxidation and stabilize them.13b,14 Interestingly, SDS shows a greater effect than SDBS, although the two have similar structures. These two are different in that SDBS has a benzene ring in its structure with its cmc of 1.2 mM, an aggregation number of 51, and a dissociation constant (R) of 0.17, whereas SDS has a cmc of 8.2 mM with an aggregation number of 62 and R = 0.30.34 Thus, SDS molecules form larger micelles and stabilize the oxidation products of EDOT better than SDBS. However, HpβCD has no charge, and the radical cations may pop out of HpβCD cavities and undergo reactions with water to produce degradation products or monomers to grow to oligomers and polymers. Another possibility is formation of the PEDOT chain through a series of HpβCD hollows connected back to back. Whichever the case, it is certain that much fewer anions would be trapped within the polymer backbone when it was obtained in the presence of HpβCD than with other anionic surfactants. The polymer films formed during the first potential cycle have been characterized by impedance measurements as well. CVs obtained for the polymer films formed in the presence of anionic surfactants show relatively large redox currents overlapped on capacitive currents, whereas that with HpβCD primarily shows capacitive currents (Figure 1b). Shown in Figure 4 are the following: a series of typical Nyquist plots recorded at a few different potentials upon potential reversal to reduce the film obtained with SDS present during the first cycle of the anodic scan (Figure 4a) and the equivalent circuit used to fit these impedance data (Figure 4b). The impedance data exhibit the usual Warburg behavior with a straight line of a 45° angle at high frequencies followed by a capacitive behavior at low frequencies. This is a typical characteristic of impedances in a thin layer cell and a conducting polymer film acting like a thin layer cell. The straight lines at high frequencies increase as the potential decreases, indicating that the electron transfer becomes more resistive and the capacitance increases (vide infra, Figure 5). To describe the electron transfer across the film, we used a T element, which is used for a thin film with a fixed amount of the electroactive substance contained. The T element is described by the equation35 Y ðωÞ ¼ Y0 ðjωÞ1=2 tanh½BðjωÞ1=2
ð2Þ
where Y0 is the Warburg admittance at ω and the time constant B has the form pffiffiffiffi B ¼ δ= D ð3Þ Here δ is the thickness of the thin layer and D is the diffusion coefficient for a mobile species within the film. The films thicknesses used for these analyses were 278, 228, and 140 nm, respectively, when prepared by a single anodic cycle in a solution containing SDS, SDBS, and HpβCD; the gold-on-silicon electrode was used instead of the gold disk for measurement of film thickness. From B’s obtained by fitting the equivalent circuit and the film thicknesses, the diffusion coefficients of ions (Dions) are obtained. 6549
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Figure 5. Dependences of (a) Rp, (b) Cp, (c) B, and (d) diffusional admittance, Y0, on the potential during the cathodic scan of the first cycle. Scan directions are indicated by arrows.
Figure 5 shows the results of data analyses; Rp values displayed in Figure 5a show an exponential increase upon decreasing potential along the scan direction, indicating that the electron transfer follows the ButlerVolmer kinetics for the polymer film reduction. Larger Rp values are obtained in the order of the solubilizer SDS > SDBS > HpβCD in the potential region studied. The difference in Rp values in the presence of a different additive is explained by the difference in film thickness for the measurements. The Rp values are roughly proportional to the thickness (Figure 5a). The Cp values shown in Figure 5b also support the thickness effect, showing larger Cp values for the films prepared with the additive SDS > SDBS > HpβCD. Also noted are steeply decreasing capacitances of the films obtained with SDS and SDBS present when the potential goes negative, while those obtained in the presence of HpβCD are nearly constant over the wide potential range. The capacitances were obtained from both n and Q values from eq 1. The results suggest that bulky anions such as DS and DBS are tightly incorporated into the polymer matrix during the polymerization process and cannot diffuse out to the solution. Thus, the anions trapped in the polymer matrix form internal capacitors when the films are doped, which decrease upon decreasing potential. However, this effect is not large for the polymer prepared in the presence of HpβCD. Similar observations were reported by other groups as well.13b,14 All these results show that the capacitance of the film is influenced by the ion transport properties of polymer film. The time constant, B, displayed in Figure 5c shows increases as the potential gets negative. This can be interpreted in two ways.
Figure 6. Topographic (top) and current (bottom) images with their cross-sectional profiles for the PEDOT films grown by one cycle scan between 0.6 and 1.05 V and back to 0.7 V in aqueous solutions containing (a) SDS, (b) SDBS, and (c) HpβCD. The scan size of all AFM images was 2 2 μm. Full scales of the vertical axis of the crosssectional profiles were 400, 500, and 120 Å (top) for topographic images, while they were 500, 400, and 300 nA (bottom) for current images for a, b, and c, respectively.
First, the time constant of an electronic device is determined by the product of the resistance, Rp, in this case and the capacitance, Cp, although B here is not exactly the same as the usual RC time constant. The trend shown by B shown in Figure 5c is in general 6550
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in agreement with the RC time constant. Second, although B here is defined by both the diffusion coefficient, Dion, and the film thickness, δ, it is reasonable to regard Dion as their mobility, uion, in cm2 s1 V1; thus, the ion transport property varies depending on the potential gradient across the film. In order to see whether the properties characterized in solution are related to those in dry films, the electronic transport and surface morphology of the films were concurrently investigated by CS-AFM. Figure 6 shows topographic (top) and current images (bottom) obtained simultaneously for the same PEDOT films as used for studies described in Figure 5, which were prepared by a single potential scan from 0.6 to 1.05 V and by stopping the scan to keep the films in fully doped states at 0.7 V in a solution containing (a) SDS, (b) SDBS, or (c) HpβCD. The granular structures of different sizes are observed in the topographic images depending on the solubilizer used. The root-mean-square (rms) roughness obtained from the topographic images in Figure 6a, 6b, and 6c were 9.1, 12.7, and 2.6 nm, respectively. Sakmeche et al. reported a columnar structure with a globular surface when PEDOT films were grown in the presence of SDS.13b They proposed from AFM results that SDS directed the PEDOT growth perpendicular to the electrode surface. We also believe that the relatively large rms roughness values in Figure 6a and 6b result from the nature of polymer chains growing perpendicular to the electrode, leading to more porous and sparse polymer structures. The larger rms roughness value in Figure 6b than the one in Figure 6a is due to their larger micellar structures than DS. On the other hand, the topographic image in Figure 6c reveals a relatively homogeneous and smoother surface made of small and regular grains, showing small rms roughness. This indicates again that smaller amounts of anions are trapped in films obtained with HpβCD present. The observations of topographic images show that different growth mechanisms and polymer structures formed thereby explain the tendency shown by migration of ions. The current images and their cross-sectional current profiles displayed below the images in Figure 6 show that current flow is in the order of additives of SDS > SDBS > HpβCD. To obtain the conductivity of the films, the contact radius, rc, of the tip with the film was first evaluated using the Hertz theory2830 according to the equation rc 3 ðFÞ ¼
3 R ðF þ Fad Þ 4 E
ð4Þ
where F is the external loading force (7 nN), Fad is the adhesion force (1 nN), and R* is an effective radius of curvature of the tipsample contact with 1/R* = 1/Rtip þ 1/Rsample. Here Rsample is the radius of the sample, resulting in R* = Rtip = 20 nm; E* is an effective Young’s modulus with 1/E* = (1 σt2)/Et þ (1 σs2)/Es, where Et and Es are the Young’s modulus of the tip (= 169 GPa) and the sample (= 3.2 GPa),33 and σt (0.38) and σs (0.38) are the corresponding Poisson’s ratios.36 Poisson ratios of most polymers are between 0.2 and 0.7,37 and we took it to be 0.38 just as for platinum. The contact area is then calculated to be 38.7 nm2 from the contact radius obtained from eq 4 using parameters described above. Then the average conductivity is calculated from the following equation σ ¼
Iavg δ V A
ð5Þ
Figure 7. Topographic (top) and current images (bottom) with their cross-sectional profiles simultaneously obtained for galvanostatically grown PEDOT films by applying 0.40 mA for 10 s (=14.2 mC/cm2) in aqueous solutions containing (a) SDS, (a) SDBS, and (c) HpβCD. The scan area was 2 2 μm.
where Iavg is the average current flowing vertically through the film calculated by averaging currents at 256 256 points across the whole surfaces, V the bias voltage used for the current measurements between the electrode and the tip (= 50 mV), δ the thickness of the film, and A the contact area just calculated above (= 38.7 nm2). Average currents were 336 ((144), 210 ((79), 139 ((57) nA, and average conductivities thus calculated were 482 ((207), 247 ((93), and 100 ((41) S/cm for films obtained in the presence of SDS, SDBS, and HpβCD, respectively. Note that the standard deviations in these measurements are rather large because deviations in both currents were obtained from spot to spot across the scanned surface. We also see this on the current profiles shown below the current images, where the currents vary wildly depending on the spots. Finally, we examined the PEDOT films galvanostatically prepared in order to confirm whether they would also show the same results as discussed thus far, independent of the electrochemical method used. Figure 7 shows the topographic (top) and current images (bottom) obtained simultaneously for PEDOT films, which were prepared by applying an anodic current of 0.4 mA for 10 s (= 1.42 mA/cm2 or 14.2 mC/cm2) in aqueous solutions containing (a) SDS, (a) SDBS, and (c) HpβCD as a solubilizer. The rms roughness values obtained from the topographic images in Figure 7a, 7b, and 7c were 12.4, 15.6, and 8.9 nm, respectively. The film thicknesses measured from cross-sectional views of SEM images were roughly 147, 126, and 77 nm, respectively, when prepared in the presence of SDS, SDBS, and HpβCD. The conductivities were also calculated to be 501 ((176), 267 ((159), and 56 ((48) S/cm for films obtained in the presence of SDS, SDBS, and HpβCD, respectively. These results indicate that the trend shown by the galvanostatically prepared polymer films matched well with that shown by the films prepared by the potentiodynamic method. Thus, we conclude from the results that the electrical and morphological properties of PEDOT films are strongly affected by which solubilizer has been used, not much by the electrochemical method used for their preparation. We also varied the current density to a few different values and found that less conductive films with lower film conductance were generally 6551
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The Journal of Physical Chemistry C obtained at lower current densities even though their thicknesses were smaller and the surfaces smoother. This is probably because the degradation reactions compete with the polymerization reaction.
’ CONCLUSIONS We conducted studies on the effects of solubilizers including SDS, SDBS, and HpβCD on electrochemical growth as well as ionic, electrochemical, electrical, and morphological properties of PEDOT thin films employing FTEIS and CS-AFM measurements. The films were shown to grow more efficiently in the presence of SDS and SDBS than with HpβCD. The situation is somewhat similar to self-doped conducting polymer films in which anionic dopants are present in the polymer matrix regardless of whether the film is doped or not.38,39 Of the two anionic surfactants, DS shows greater effects than DBS on PEDOT film growth due perhaps to its greater micelle size and higher degree of dissociation. The differences in electron transfer, ion transport, and electronic conductivity of the films arose from different growth mechanisms. Our work provided greater insights into the properties of electrode/electrolyte interfaces during electrochemical preparation of conducting polymers and how they are eventually instilled into their properties of the final products, i.e., polymer films. ’ AUTHOR INFORMATION Corresponding Author
*Fax: þ82-52-217-2909. E-mail:
[email protected].
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