Electrochemistry of Conductive Polymers. 32. Nanoscopic

Figure 2a shows the relatively thick nanoropes (40∼60 nm in diameter) in the center of the topographic image. The current image in Figure 2b shows t...
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J. Phys. Chem. B 2004, 108, 13921-13927

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Electrochemistry of Conductive Polymers. 32. Nanoscopic Examination of Conductivities of Polyaniline Films Dong-Hun Han and Su-Moon Park* Department of Chemistry and Center for Integrated Molecular Systems, Pohang UniVersity of Science and Technology, Pohang, Gyeongbuk 790-784, Korea ReceiVed: February 8, 2004; In Final Form: July 8, 2004

The conductivities of electrochemically prepared polyaniline (PAn) films have been determined using an atomic force microscopic tip with a conducting probe after the films were doped/dedoped at various potentials. The results indicate that the PAn films are rather inhomogeneous, even though the film is doped or dedoped until no further anodic or cathodic currents flow. This inhomogeneity was shown to be present even in the PAn nanostructures. The conductivities of the film ranged as widely as by three orders of magnitude on the same surface of 2 × 2 µm2 depending on the location, at which the measurements were made. The range of highest conductivities was obtained when the film was doped at +0.30 V vs Ag|AgCl (in saturated KCl) while the one dedoped at -0.30 V and the one oxidized at +0.80 V were practically insulating. It was also discovered that the film prepared in the perchloric acid medium displayed very poor electrical characteristics compared to those prepared in the nitric acid medium. It is concluded from this study that the electrical properties are determined by the morphology of the film, which in turn is determined by the preparation conditions.

Introduction Polyaniline (PAn) is an attractive material because it is easily synthesized, it is stable under ambient conditions, and it shows well-behaved electrochemistry.1 PAn has five different domains of conductivities, which are determined by pH and the potential.2 Depending on the oxidation state and the degree of protonation, PAn can be either an insulator or a conductor with a different conductivity. Chemical forms responsible for these domains have been discussed by a number of investigators.2,3 The in situ measurements of resistances made as a function of applied potential for PAn films grown between interdigitated gold microelectrodes using a bipotentiostat in an aqueous solution4 provided the basis for such discussions. However, faradaic reactions due to the presence of degradation products3a,b,5 and aniline dimers6 trapped within the polymer matrix could have affected the resistances of the PAn film deposited between the two microelectrodes in the solution. Furthermore, it has been shown that the PAn morphology is affected heavily by the electrolytes used during its preparation,7 and the current flowing across the conducting polymer matrix would depend on how the contacts are made between fibers and/or grains of PAn. For this reason, the current flowing through PAn films may not have been measured accurately and thus may not represent their true conductivities. Recently a few investigators attempted to measure the electrical property of PAn nanowires.8,9 Long et al.8 measured the conductivity of PAn nanotubes doped with camphor sulfonic acid using a 4-point probe method. They reported that the conductivity of a single polyaniline nanotube is much higher than that of nanotube pellets. He et al.9 measured charge transport properties in conducting polymers bridged between two gold nanoelectrodes separated by a nanoscale gap and * Corresponding author. E-mail: [email protected]; phone: +8254-279-2102; fax: +82-54-279-3399.

Figure 1. A cyclic voltammogram recorded for a PAn film in a solution containing 1.0 M nitric acid at a scan rate 50 mV/s; the film was potentiodynamically grown in a 1.0 M nitric acid solution containing 50 mM aniline.

showed changes in the charge transport properties with applied potentials. While these measurements would certainly represent truer conductivities of conducting polymer nanowires than those measured indirectly by methods such as optical measurements,10 they still may include experimental artifacts, as they are based on macroscopic concepts. To avoid the experimental artifacts that may be introduced by the macroscopic conductivity measurements, we used atomic force microscopy (AFM) with a conducting probe in this work. The AFM with a conducting probe, also termed a currentsensing AFM (CS-AFM), has found its applications in many areas during the past decade due to its merits, such as an easy contact with various substances including organic materials.11 A decided advantage of this method is that the load force can be controlled precisely for reproducible contacts between the tip and the sample. Electronic transports through single mol-

10.1021/jp0494279 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004

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Figure 2. (a) Deflection and (b) current images recorded for the nanostructures prepared on 4-ATP/C16-SH mixed SAM-modified gold-onsilicon.

Figure 3. (a) Topographic and (b) current images with their cross-sectional analyses, as well as (c) three-dimensional and (d) scanning electron microscopic (SEM) images, for the PAn film potentiodynamically grown in a 1.0 M nitric acid solution and doped at 0.30 V in a 1.0 M nitric acid solution for 600 s. The vertical axis of the three-dimensional image is Å.

ecules,12 self-assembled monolayers,13 carbon nanotubes,14 and quantum dots15 have been investigated by measuring the current/ voltage (I/V) traces using the CS-AFM. Most of these observations are the results of inhomogeneity of the conducting polymers for a number of reasons, which have been discussed in the recent literature by a few investigators.16 In this paper, we address three major points: (1) the evidence that such an experimental artifact may affect the conductivity measurements, (2) the evidence that conducting polymers could be significantly inhomogeneous in terms of their electrical conductivities, and (3) the question of how the polymerization conditions and morphology affect the conductivities of the conducting polymers. We take PAn to demonstrate these points.

Experimental Section PAn Film Preparation. Aniline (Aldrich, ACS grade) was used after vacuum distillation over zinc powder. Nitric acid (Matsumoen Chemicals) and perchloric acid (Samchun Chemical) were used as received. Doubly distilled, deionized water was used for the preparation of all solutions. PAn films were electrochemically prepared on gold-on-silicon electrodes (with Cr adhesive layers, LGA Films), after the solutions were deaerated with N2 gas (99.0%, BOC Gases). The gold-on-silicon electrode was annealed for 5 min with a hydrogen flame after being washed in methanol and deionized water, and was used as a working electrode. A platinum foil was used as a counter electrode, and an Ag|AgCl (in saturated KCl) electrode was used

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Figure 4. Current-voltage curves measured at the points labeled in Figure 3b.

as a reference electrode. The poteniodynamic method was used to produce films with superior adhesion and smoothness.1a,17 Thus, the PAn films were prepared by cycling potentials 10 times between -0.10 and +1.2 V at a scan rate of 50 mV/s using an EG&G Princeton Applied Research model 273A potentiostat/galvanostat. Preparation of PAn Nanostructures. Hexadecanethiol (C16-SH, Aldrich 97%) and 4-Aminothiophenol (4-ATP, 97%, Aldrich) were used as received to make up a mixed selfassembled monolayer (SAM). A gold-on-silicon electrode was used in preparing the PAn nanostructures. A mixed SAM composed of 4-ATP and C16-SH was formed on the gold-onsilicon electrode by immersing it in an ethanol solution containing 2.0 mM 4-ATP and 2.0 mM C16-SH for 5 h. PAn nanostructures on this mixed SAM modified electrode were prepared by three potential cycles between -0.10 and +1.2 V at 50 mV/s in a 1.0 M nitric acid solution containing 50 mM aniline. After the electrode was washed thoroughly with deionized water, the PAn nanostructures were doped at 0.30 V in 1.0-M nitric acid solution to make them conductive. Current Measurements by CS-AFM. The AFM with a current-sensing tip, CS-AFM (PicoSPM, Molecular Imaging Inc.), was used in a contact mode to simultaneously obtain topographical and current images. The PAn films were first treated in solution by applying various potentials for 600 s to attain certain doping levels after they were washed thoroughly with deionized water to remove any degradation products and dimers trapped during their preparation, and then were dried at

room temperature for 5 min. The platinum coated cantilevers (spring constant, 0.13 ∼ 0.20) were purchased from Nanosensors (http://www.nanosensors.com). The load force was maintained below 7 nN. A bias voltage between the substrate (gold electrode) and the conducting cantilever (which is grounded) was 50 mV during all of the imaging experiments except for the current measurements of PAn nanostructures (300 mV). Before imaging the PAn-film-covered surfaces, the surface was purged with high-purity N2 gas (99.999%, BOC Gases) to minimize the effects of moisture and water. All of the AFM experiments were carried out under the controlled environment. Results and Discussion Figure 1 shows a cyclic voltammogram (CV) recorded for a PAn film in 1.0 M nitric acid. The film was prepared by repeatedly cycling potentials between -0.10 and 1.2 V on the gold-on-silicon electrode in a 1.0-M nitric acid solution containing 50 mM aniline. The CV showing four redox processes is in excellent agreement with those reported in the literature.1,3a,b The relatively sharp anodic peak A at about 0.20 V represents a transition from leucoemeraldine to protonated emeraldine, which has been known to be the most conductive of the five different forms of PAn.1e,4 The broad peak A′ during the cathodic scan represents the reversal current for peak A. The two middle peaks (peaks B/B′ and C/C′) have been assigned to the electrochemical redox reactions of dimers as well as degradation products such as p-benzoquinone and p-aminophe-

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Figure 5. Current images for PAn films doped at: (a) -0.30, (b) -0.20, (c) -0.10, (d) 0.50, (e) 0.60, and (f) 0.80 V in 1.0 M nitric acid for 600 s. The images were recorded across an area of 2.0 × 2.0 µm2. Note that the full-scale currents are the same for (a), (b), (e), and (f), and (c) and (d), respectively. Compare these with the current image shown in Figure 3b, which has a 50-nA full-scale current.

nol, along with capacitive currents.3a The PAn film undergoes its final oxidation to the most oxidized state at peaks D and D′, a state which is known to have a quinoid structure.1c Before we examine the topological and current images of the PAn film thus prepared, we first examine the nanostructures of conducting polymers. Figure 2 shows topology (a) and current (b) images of PAn nanoropes. These nanostructures were grown by cycling the potential between -0.10 and 1.2 V in solution containing 50 mM aniline and 1.0 M nitric acid at a gold electrode modified with a two-component SAM consisting of 4-ATP (electrochemically active site) and n-alkanethiol (electrochemically inactive site).18 After washing the film thoroughly with deionized water, the polymer nanostructures were doped at 0.30 V in 1.0 M nitric acid for 600 s to make them conductive. Figure 2a shows the relatively thick nanoropes (40∼60 nm in diameter) in the center of the topographic image. The current image in Figure 2b shows that the nanoropes have relatively

inhomogeneous current distributions although they were doped for 10 min until no further anodic currents flow. We see from the current image that the upper part is made of a few pieces of PAn nanostructures grown separately. If one were to measure the conductivity across this nanorope by, for example, a fourprobe method or across two interdigitated electrodes, its values would have been irreproducible depending on where the contact is made between the probe and the nanosized sample. This explains why there could be large deviations between the “best” and “worst” values. Further, this demonstrates that the nanowires that appear to have good physical contacts in AFM or scanning electron microscopic (SEM) images may not necessarily be well connected. Figure 3 shows topographical (a) and current (b) images obtained simultaneously for the surface of a PAn film potentiodynamically grown in a 1.0-M nitric acid solution. Because the topographic image could not be sharpened any more than

Examination of Conductivities of Polyaniline Films

J. Phys. Chem. B, Vol. 108, No. 37, 2004 13925 the voltage between -1.5 and +1.0 V. As can be seen from these I-V curves, the film acts like a semiconductor rather than a well-defined resistor. Nonetheless, the conductance of the film can be calculated by using the linear part of the I-V curves, and the maximum conductance (I-V curve not shown here) was 4.21 × 10-6 S. To obtain the conductivity of the PAn film, the contact radius a of the probe with the film was calculated using the Hertz theory20 according to the equation

a3(F) )

Figure 6. Ranges of conductance of PAn films doped at various potentials.

shown, we also show its three-dimensional version in Figure 3c, which gives a much clearer picture of the surface. After the removal of degradation products and dimers, the film was doped at 0.30 V in 1.0 M nitric acid for 600 s to obtain the most conductive, protonated emeraldine film. While Choi and Park reported that the PAn film prepared under the condition similar to the one used here had an open structure with well-defined fibers of spaghetti shapes,7 the PAn shown in Figure 3a has shorter but thicker fibers. This is perhaps due to the lower aniline concentration used in our present study. We also found that the AFM images of conducting polymers are usually somewhat less well defined than those obtained by scanning electron microscopy (SEM). The current image shown in Figure 3b, which was obtained at a 50 mV bias voltage, shows bright (high currentflowing) and dark (low current-flowing) regions; so, the doping process did not take place homogeneously over the entire area even though the polymer film was doped until no further oxidation currents flow at 0.30 V. This suggests that the surface of the PAn film facing the electrode may not be, or may be poorly, connected to the substrate electrode at times (see below). The interconnection between fibers could also be poor because of the anisotropic growth of the polymer. From the SEM image of the film/electrode interface, we can see that the contacts between the two are not very uniform. Doping distributions of the current image shown here are similar to those recorded for polybithiophene and polypyrrole by the Kelvin probe method,19 although our recent results have shown that relatively homogeneous surfaces were obtained for polypyrrole.16a Current-voltage (I-V) curves were obtained to evaluate the electrical properties of the film at nine points labeled in Figure 3 and the results are shown in Figure 4. The measurements were made between the probe and the substrate electrode by scanning

3 R*(F + Fad) 4 E*

where F is the external loading force (equal to 7 nN), Fad is the adhesion force (equal to 1 nN), and R* is an effective radius of curvature of the tip-sample system with 1/R* ) (1/Rtip) + (1/ Rsample). Rsample is the radius of the sample, resulting in R* ≈ Rtip ) 10 nm; E* is the effective Young’s modulus with 1/E* ) (1 - σt2)/Et + (1 - σs2)/Es, where Et and Es are the Young’s modulus of the tip (Et ) 168 GPa) and the sample (Es ) 0.2 GPa21), and σt (0.38) and σs (0.38) are the corresponding Poisson’s ratios. Poisson’s ratios of most polymers are between 0.2 and 0.722 and we assumed the Poisson’s ratio of PAn to be 0.38, just as for platinum. The contact area between the tip and the sample was then calculated to be 127.1 nm2. The film thickness (1.02 ( 0.01 µm) was measured from the crosssectional view of the SEM image (Figure 3c). Thus, the above conductance value of 4.21 × 10-6 S translates into the maximum conductivity of 3.4 × 102 S/cm. The conductivity values can vary widely depending on the Young’s modulus of the sample; Young’s moduli of PAn fibers spun by various methods were reported to range from 0.2 to as large as 3.2 GPa depending on the dopants used,21,23 where anions such as 2-acrylamido-2methyl-propanesulfonate or chloride were used. For our calculation, we used a conservative value of these (0.2 Gpa). Nonetheless, the values obtained at the most conductive region of the film are significantly higher than those reported in the literature,1d,24 which were measured for bulk PAn films or pellets. We believe these conductivities represent the lower limits to the possible values. In other words, the conductivities we obtained from our nanoscopic measurements are significantly larger than the conductivities measured by the bulk method. Figure 5 shows the current images for the same PAn film as that used for the images shown in Figure 3. After the removal of degradation products and dimers, the potential was stepped to a given value for 600 s in nitric acid to obtain different doping levels until no further current flowed. The PAn film completely dedoped at -0.20 V would be in its neutral state (leucoemeraldine), but it still is shown to have conductive areas (bright regions) as shown in Figure 5b. At -0.10 V (Figure 5c), the film becomes significantly more conductive than those at -0.30 and -0.20 V (Figures 5a and b). The film becomes most conductive at +0.30 V as shown in Figure 3b. An important point to be noted here is that the films dedoped at -0.30 through -0.10 V have quite a bit of conductivity despite our general understanding that the film becomes insulating in this potential region. These results indicate the dedoping process is very slow, even though the process is continued until no further doping or dedoping currents flow. It is surprising that the current map of the film doped at +0.50 V (Figure 5d) is similar to that of the film treated at -0.10 V (Figure 5c). A much smaller doping (oxidation) current flows at -0.10 V compared to that at +0.50 V, as can be seen from the CV shown in Figure 1. One would expect that the current map of the film doped at +0.50 V must be much brighter than that at -0.10 V. Our observation indicates that much of the current flowing at +0.50 V must be due to

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Figure 7. (a) Topographical and (b) current images of the PAn film potentiodynamically grown in 1.0 M perchloric acid and doped at 0.30 V in 1.0 M perchloric acid for 600 s, (c) an I/V curve recorded from spot E of the film shown above, (d) a three-dimensional image for the twodimensional topographical image shown in (a).

the capacitive current and the faradaic current for oxidation of degradation products and/or dimers trapped during the film preparation and repeated doping-dedoping processes. The other reason for this is perhaps because polarons produced at +0.25 V start to undergo further oxidation to bipolarons, which are stabilized through another resonance structure, that is, the quinoid structure. This trend is even more severe at potentials beyond +0.60 V and the conductivity decreases significantly by the time the doping potential reaches +0.80 V. The formation of the quinoid structure above this potential and the ejection of the inserted counteranions are well established from electron spin resonance1a,3c,d and electrochemical quartz crystal microbalance25 experiments. In other words, PAn becomes the pernigraniline form upon oxidation at +0.80 V, which becomes more or less an insulator, and the current image has almost no current contrast. In sum, the conductivity of the film peaks out when it is doped at 0.30 V and the conductive regions in the

current images slowly disappear on both sides of the optimum doping potential, that is, 0.30 V. The observations described thus far are summarized in a more quantitative form in Figure 6. As can be seen from this figure, the conductivity of the PAn film doped or dedoped at a given potential varies even in the small area of 2 × 2 µm2 from as small as by a few times (-0.30 V) to as large as by three orders of magnitude (+0.30 V) depending on the location where the measurements were made, even though they were doped or dedoped until no further currents flow. The reason the PAn film is so inhomogeneous in its conductivity should be because no good microscopic contacts are made between the fibers or grains; thus, it takes a long time for the film to be completely doped or dedoped even after the doping or dedoping current decays to 0. Finally, we also examined the PAn films prepared in perchloric acid under otherwise identical conditions. Figure 7

Examination of Conductivities of Polyaniline Films shows topographical (a) and current (b) images for the PAn film prepared in 1.0 M perchloric acid. When compared with the current images shown earlier in Figures 3b and 5, the one in Figure 7b shows nearly the same contrast as that of the film oxidized at +0.80 V (Figure 5f). The I-V curve shown in Figure 7c shows that the film displays very low conductivities. The conductance calculated by using this I-V curve is about 5.76 × 10-11 S, with a corresponding conductivity of 4.6 × 10-3 S/cm. Zotti et al.1d reported that PAn doped with perchlorate anions had higher conductivity than that doped with nitrate anions. The huge difference in current images shown in Figures 3b and 7b, even though the two films were prepared under very similar conditions (except for the use of different supporting electrolytes), appears to have resulted from the different growth mechanism arising from different electrolytes during electrochemical polymerization. The PAn film prepared in nitric acid grows into the well-defined linear polymer shape,7 which makes interpolymer contacts easier, whereas the one prepared in perchloric acid grows into stacked globules leading to the poorer connections among the PAn globules and low conductivities. Also, PAn grows more isotropically in nitric acid in a given fiber, which makes it more conductive. Conclusions Our study demonstrated that PAn films could be more inhomogeneous than were thought, even though they were doped or dedoped until no currents were observed. Even the PAn nanostructures show inhomogeneity because they could be made of a few pieces of separately grown nanostructures. When a small area was doped at a given potential for a length of time, the conductivity varied as much as by a few orders of magnitude, depending on which location the current was measured at, as was shown in Figure 6. Further, the conditions under which the polymer film was prepared were shown to significantly affect its electrical properties. Currently, we do not have a good understanding as to why the conductivity varies so much depending on the counteranions inserted during the doping process. Finally, our results indicate that PAn films may not be appropriate for use as electronic components because of their inhomogeneity. More studies are necessary to find better materials that would display more homogeneous electrical properties. Studies are in progress along these lines in our laboratory. Acknowledgment. This work was supported by a grant from the National R&D Project for Nano Science and Technology of the Ministry of Science and Technology. Thanks are also due to the Korea Research Foundation for providing graduate stipends through its BK21 program. References and Notes (1) (a) Glarum, S. H.; Marshall, J. H. J. Electrochem. Soc. 1987, 134, 142. (b) Zotti, G.; Cattarin, S.; Comisso, N. J. Electroanal. Chem. 1987, 235, 259. (c) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 2254. (d) Zotti, G.; Cattarin, S.; Comisso, N. J. Electroanal. Chem. 1988, 239, 387. (e) Park, S.-M. In Handbook of Organic ConductiVe Molecules

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