Electrochemical in Situ Electron Spin Resonance, Conductance, and

Jun 5, 2009 - (1-3) Among all known CPs, polyaniline (PANI) has been studied most extensively because of its high conductivity, good redox reversibili...
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Electrochemical in Situ Electron Spin Resonance, Conductance, and Atomic Force Microscopy Studies of Poly-o-phenylenediamine Qin Zhou,†,‡ Lin Zhuang,† Juntao Lu,*,† and Chang Ming Li‡ Hubei Key Lab of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan UniVersity, Wuhan 430072, China, and School of Chemical and Biomedical Engineering & Center for AdVanced Bionanosystems, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457 ReceiVed: September 15, 2008; ReVised Manuscript ReceiVed: April 28, 2009

Poly-o-phenylenediamine (PoPD) has been commonly considered to be almost insulative at high electrode potentials (in the oxidized state). However, our in situ conductance and in situ electron spin resonance (ESR) results in controlled electrochemical environments suggested that PoPD at high potentials is not only still conductive but probably even more conductive than at low potentials, showing distinct contrast to the prevailing views in the literature (Yano, J.; Shimoyama, A.; Nagaoka, T.; Ogura, K. Denki Kagaku 1992, 60, 1101). The controversy is explained by the difference between the intrinsic conductivity of a material and the measured conductivity of a test sample made from this material. When internal contacts are poor in the test sample, apparent conductivity would be less than the intrinsic conductivity by a certain extent, depending on the measuring conditions. In situ atomic force microscopy (AFM) showed significant roughening of the PoPD coating with switching potentials from low to high values, implying worse internal contacts, resulting in considerably depressed apparent conductivity. This AFM observation supports well the conclusion inferred from the in situ conductance and ESR measurements. Introduction Conducting polymers (CPs) have been attracting much attention from researchers because of their unique properties and broad application potentials. In past decades, CPs have been widely used in supercapacitors, electrochromics, batteries, sensors, and for anticorrosion.1-3 Among all known CPs, polyaniline (PANI) has been studied most extensively because of its high conductivity, good redox reversibility, and stability in air. Poly-o-phenylenediamine (PoPD), a derivative of polyaniline, has become an attractive topic of research in recent years. In the 1960s, Adams and co-workers4,5 used electrochemical methods to synthesize PoPD in acid solutions for the first time. Since then, numerous papers have studied various aspects of PoPD, including its polymerization mechanism,6-8 redox transformation,7 stability,9 conductivity,10-12 structure,8 and applications.13-15 PoPD is electrically conductive but less than PANI. Its conductivity was reported as high as 0.21 S cm-1,11 which is still lower than that of PANI by 2 to 3 orders of magnitude. The potential dependence of conductivity, an important property of CPs, was reported in only a few papers for PoPD. The only detailed description of the potential dependent conductivity of PoPD was reported by Yano and co-workers.12 They measured the conductivity using a comb electrode and found that the polymer film became “almost an insulator” at potentials above 0.3 V (versus SCE). Mazeikiene and Malinauskas also reported that PoPD was conductive in a reduced state and insulative in an oxidized state but gave no data.9 To date, PoPD has been studied by using various experimental techniques, including electrochemical methods,8,10,15 in situ resonance Raman spectroscopy,17 in situ UV-vis subtractive reflectance spectroscopy,18 in situ external reflection FTIR * Corresponding author. E-mail: [email protected]. † Wuhan University. ‡ Nanyang Technological University.

spectroscopy,19,20 X-ray photoelectron spectroscopy,21 and by using a quartz crystal microbalance.7,16 However, to our best knowledge, electron spin resonance (ESR), a technique able to unambiguously detect the major charge carriers (polarons) in CPs and widely used for CP studies, has not been used to characterize PoPD. In our previous work, experimental techniques of simultaneous electrochemical, ESR, and conductance measurements22 and in situ variable temperature ESR measurements were established and successfully employed to study the conduction mechanism for PANI.23,24 In this paper, in situ ESR and conductance measurements and in situ atomic force microscopy (AFM) in controlled electrochemical environments were carried out, aiming to provide scientific insights on the potential dependence of the conductivity of PoPD. Experimental Section Electrochemical Polymerization of o-PD. PoPD was electrochemically polymerized on gold electrodes by cyclic voltammetry (from 0.05 to 1.3 V versus RHE, 50 mV/s) in a solution of 0.5 mol/L H2SO4 + 0.05 mol/L o-PD. o-Phenylenediamine (o-PD) (98%, Aldrich, U.S.A.) was recrystallized twice before solution preparation. Different electrochemical cell designs were adopted, according to experimental requirements and will be described below. Electrochemical in Situ ESR and Conductance Measurements of PoPD. For in situ ESR measurements, the cell design reported in our previous work23 was adopted with minor modifications. In brief, the working electrode was a spiral made from a piece of Au wire of 0.3 mm diameter. The outer diameter of the spiral matched the inner diameter of the ESR sample tube (about 4 mm). The counter electrode was a piece of Au wire located along the axis of the spiral. The reference electrode was a reversible hydrogen electrode (RHE) in 0.5 mol/L H2SO4 and connected to the ESR cell through a Teflon Luggin capillary (Figure 1). ESR measurements were conducted on an ESR

10.1021/jp9011153 CCC: $40.75  2009 American Chemical Society Published on Web 06/05/2009

Electrochemical Studies of Poly-o-phenylenediamine

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Figure 1. Electrode arrangement for simultaneous electrochemical, ESR, and conductance measurements. The enlargement shows the polymer links between the two working electrodes.

spectrometer (JES-FE1XG, JEOL, Japan) equipped with a TE011 cylindrical cavity working at X-band and interfaced to a personal computer using a software package developed by the authors. A Mn(II) marker was kept in the cavity to serve as the g-value reference and indicator of the quality factor (Q-factor) change of the cavity. In order to measure conductance simultaneously with ESR under controlled electrochemical conditions, a second working electrode (WE2) was introduced into the cell (Figure 1). Before cell assembly, PoPD film was grown on WE1 and WE2 separately by cyclic voltammetry. After being coated with PoPD, the two working electrodes were assembled in the ESR sample tube according to the configuration shown in Figure 1, where WE2 was kept in firm contact with WE1 through an elastic mechanism so there were stable polymer links formed between the two working electrodes. Two potentiostats were used for the electrochemical in situ conductance measurement. The first potentiostat (P1, model CHI660, Shanghai) worked in the usual three-electrode mode to control the potential (E) of WE1 with respect to the reference electrode. The second potentiostat (P2, model ZF-3, Shanghai) worked in the two-electrode mode, with its working electrode terminal being connected to WE1, and counter and reference terminals were connected to WE2 to keep a constant potential difference (∆E) between WE1 and WE2.22-24 The ratio of the current measured by P2 (I) to the voltage ∆E was taken as the conductance of the polymer links at potential E, σ ) I/∆E. Electrochemical in Situ AFM Measurements of PoPD. For electrochemical in situ AFM measurements, a homemade electrochemical cell was used in this work.25 The working electrode was a smooth Au electrode coated with electropolymerized PoPD. The topography of the PoPD surface was monitored with AFM at selected potentials. The AFM instrument was a Dimension 3100 SPM system (Veeco, U.S.A.) and was operated in tapping mode with a commercial cantilever (DNP, Veeco, U.S.A.). Results and Discussion Electrochemical Polymerization of o-PD. As described above, a PoPD film was deposited on the gold electrode by cyclic voltammetry (from 0.05 to 1.3 V versus RHE, 50 mV/s) in a conventional electrochemical cell containing 0.5 mol/L H2SO4 + 0.05 mol/L o-PD. Figure 2 shows the cyclic voltammogram of polymerization, which is in agreement with literature reports.7 It can be seen that there is a pair of peaks in the low potential region, corresponding to the redox reaction of PoPD. The oxidative peak in the high potential region was due to the oxidation of monomer o-PD, and this reaction was necessary for polymerization. The oxidative current peak was high during the first anodic potential scan but decreased quickly as the

Figure 2. Cyclic voltammogram of the eletropolymerization of PoPD (in 0.05 mol/L o-PD + 0.5 mol/L H2SO4, Au electrode, 50 mV/s).

Figure 3. Current recorded by potentiostat P2 during cyclic voltammetry of WE1 at 50 mV/s. The voltage between WE2 and WE1 was kept to 10 mV.

cycling number increased. Some researchers thought the decrease in the anodic current was caused by the low conductivity of PoPD at the high potentials.7 Because of this so-called selfblock phenomenon, it is usually impossible to obtain a thick PoPD film by electrochemical polymerization. It was reported that the thickness of PoPD was only several micrometers at the most.12,21 The quick decrease in the oxidative current, however, may also be caused by the poorer catalytic activity of the polymer surface in comparison to the Au surface for the monomer oxidation, but the polymerization mechanism was beyond the scope of this work and not pursued. Electrochemical in Situ Conductance Measurements. The real operation of conductance measurements was a little bit more complicated than that described above. According to the circuit arrangement, when the electrode potential of WE1 changed, the potential of WE2 would change accordingly. When a cyclic potential scan was imposed on WE1, the recorded current of P2 would appear similar to the one shown in Figure 3. It can be seen that a cyclic voltammetric current (charging current of WE2) was superimposed on the direct current reflecting the conductance of the polymer links. In order to avoid the interference of the charging current, the conductance was measured in a steady-state mode point by point. A required potential (E) was set on P1, and the current of P2 (I) was not taken until it reached a stable value. In this way, the charging current was minimized so that the current of P2 could reflect the polymer conductance to a good approximation. Figure 4a shows the conductance data measured at different electrode potentials. The potential was changed stepwisely from 0 to 0.7 V (forward going) and then back to 0 V (backward

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Figure 4. Potential dependence of PoPD conductance. (a) Electrochemically in situ-measured conductance in this work (4 and 0 are measured values, O are average values, and arrows indicate potential change sequences). (b) Normalized conductance σ/σmax of PoPD in this work (O) in comparison to that reported in ref 12 (b, data copied from ref 12 with permission).

going). It can be seen that the conductance only slightly changed with potential. The conductance at high potentials was only about 7% less than the maximum conductance found at low potentials. This result is distinctly different from the prevailing view in the literature, i.e., PoPD being insulative9 or almost insulative12,26 at high potentials (or at oxidized state). For example, Yano et al. reported a plot of normalized conductivity (σ/σmax) versus electrode potential, showing σ/σmax dropped to essentially zero at potentials above 0.3 V (versus SCE), and claimed the polymer to be “almost an insulator” at high potentials.12 In contrast, our result clearly showed that the polymer was definitely not insulating at high potentials (Figure 4b). The hysteresis of conductance in Figure 4a is interesting and needs discussion. The hysteresis was obviously caused by the slow response of the polymer to the stepwise change of the potential set on the potentiostat P1. The response time constant was determined by the capacitance of the polymer and resistance involved, with the latter being mostly the resistance of the polymer itself. The absence of hysteresis in the potential region above 0.45 V may be attributed, at least in part, to the capacitance in this region being much smaller than that in the lower potential region (Figure 2). However, the absence of hysteresis may also imply a higher conductance in this potential region. The assumption of higher conductance at higher potential seems to also be supported by the direction of the hysteresis. As shown in Figure 4a, the conductance measured on the backward going direction was higher than that on the forward going direction. This conductance difference between the two potential change directions cannot be interpreted by interference of the charging current. As shown in Figure 3, the charging current would lower the measured current of P2 on the backward

Figure 5. Potential dependences of (a) logarithm of normalized conductance σ/σmin, (b) ESR intensity, and (c) ESR line width of PoPD (solid lines with O) in comparison to corresponding data of PANI (dashed lines, reproduced from ref 22 with permission). Arrows indicate the applicable scales for the relevant curves. The zero point of the potential scale for PoPD is 0.22 V positively shifted with respect to the zero potential for PANI, so that the ESR intensity maxima of the two polymers appear at the same horizontal position.

going measurements and result in a lower conductance, in opposite to the hysteresis seen in Figure 4a. A reasonable explanation of the hysteresis direction is the polymer conductance being higher at higher potential. Because of the slow response of the polymer to potential changes, the true potential of the polymer at the time of using the current I to calculate the conductance was slightly deviated from the set potential to the previous potential. If the conductance at higher potential was higher, the potential delay should result in the hysteresis shown in Figure 4a. In summary, our data of electrochemical in situ conductance measurements clearly showed that the polymer was rather conductive in the potential region studied and conductivity at the higher potentials might be even higher than at the lower potentials. These results are in conflict with prevailing literature reports and will be further discussed with ESR and AFM data below. Electrochemical in Situ ESR Measurements. In situ ESR measurements were taken in the steady state at a series of selected electrode potentials simultaneously with conductance measurements. ESR spectra of PoPD showed a single line centered at g ) 2, which is characteristic for polarons.27 The potential dependent ESR intensity (secondary integration) and

Electrochemical Studies of Poly-o-phenylenediamine peak-to-peak line width are given in Figure 5, together with log(σ/σmin) for comparison. Many conducting polymers showed a similar correspondence between their conductivity and ESR signals when the oxidation state (or electrode potential) of the polymer changed.22-24 In order to infer from ESR data, the change of conductivity with electrode potential, the potentialdependent ESR intensity and line width of PoPD were compared to the corresponding data obtained with PANI (from ref 22). For this purpose, the zero points of potential scales for the two polymers were shifted from each other by 0.22 V, so that the ESR intensity maxima of the two polymers appeared at the same horizontal position. It can be seen that the trends of ESR changes of the two polymers are fairly similar. For PoPD, with increasing potential, the ESR intensity increased initially to reach a maximum and then decreased sharply; at the same time, the ESR line width decreased first to reach a minimum and then increased. These phenomena were similar to those of PANI in the corresponding potential regions (marked II and III). The potential-dependent ESR and conductivity of PANI have been thoroughly studied at room temperature and variable temperatures in our previous work.22-24 For a clear presentation, the potential scale in Figure 5 is divided into five regions. At the negative extreme potential, the ESR intensity was very low and attributable to residual polarons. In potential region I, with an increase in potential, ESR intensity increased slightly because of the increase in polaron concentration on polymer oxidation, and the ESR line became broadened because of enhanced spin-spin coupling. Because polarons are charge carriers, a slight increase in polaron concentration was accompanied by a slight increase in conductivity. In potential region II, the concentration and mobility of polarons increased quickly with potential, resulting in a sharp increase in ESR intensity, a decrease in line width (moving narrowing28), and a fast increase in conductivity. In potential region III, further oxidation of the polymer caused the polarons to merge into so-called polaron lattice and the charge carrier type to change from Curie spins to Pauli spins, resulting in decreased ESR intensity and increased ESR line width. Pauli spins behave similarly to the free electrons in a metal. Therefore, in potential region III, the conductivity increased further to a maximum. In potential regions IV and V, the polymer was excessively oxidized, and the charge carriers changed back to Curie spins with decreased concentration and conductivity decreased accordingly. The above-described correspondence between ESR and conductivity is rather common22-24 and should be applicable to PoPD, a derivative of PANI. As shown in Figure 5, the ESR behaviors of PoPD matched well those of PANI in the potential regions studied (II and III). From the similarity of the ESR data, it is inferred that the conductivity of PoPD should increase with potential in the potential window studied (0-0.7 V versus RHE). On one hand, this inference supports Figure 4a in that PoPD was conductive rather than insulating at the high potential region. On the other hand, this inference differs from the fact that in the high potential region the measured conductance decreased slightly in our case (Figure 4a) or seriously in Yano’s case. This controversy might be explained by the difference between the intrinsic conductivity of a material and apparent conductivity of a sample made from the material. For a material with high intrinsic conductivity, apparent conductivity is also high for solid samples made from this material. However, a loosely packed powder body of the same material will be found rather resistive. In our case, the directly measured value was the apparent conductance, while the ESR predicted value was the intrinsic one. If the PoPD film is assumed to have poorer internal contacts at high potentials

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Figure 6. In situ AFM images of PoPD at (a) 0 V and (b) 0.6 V (versus RHE).

than at low potentials, the above-mentioned controversy will become understandable. One way to verify this hypothesis is to monitor the structure of PoPD film while the potential changes. This was done by in situ AFM in this work as described below. Before switching to the AFM results, it might be appropriate to mention the possible reasons for the poor conductivity of PoPD in comparison to that of PANI. In principle, poor conductivity should be related to low concentration and/or low mobility of the carriers. Goyette and Leclerc studied the conductivity of PoPD derivatives and suggested that low conductivity of PoPD derivatives might be explained by low carrier concentration and poor interchain contacts.29 These explanations may also be applicable to PoPD. ESR may contribute to the study of the conduction mechanism. For example, using quantitative ESR, one may estimate the carrier concentration, and ESR line width contains information on the spin motions. However, these are beyond the scope of this paper and remain for future studies. Electrochemical in Situ AFM Measurements. In order to verify our model of intrinsic versus apparent conductivities for PoPD, the topography of a PoPD coating electropolymerized on an Au substrate was examined at selected potentials. As shown in Figure 6, the PoPD coating surface was rather smooth at the reduced state (0.0 V, versus RHE), whereas it became very rough at the oxidized state (0.6 V). This observation was in agreement with the literature.11 The increase in roughness was indicative of a worsening of internal contacts in the polymer layer. The roughness (peak to valley) of PoPD at the oxidized state may be estimated to be about 40 nm on average. However, this value should be considered as a lower limit for the real roughness. This was because the accessible depth to the AFM

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tip in a hole or a valley was limited by the finite size and pyramid shape of the AFM tip. The real roughness of the polymer coating at the high potential should be greater than 40 nm. However, the thickness of the PoPD layer was estimated to be about 100 nm, according to the cyclic voltammetric charge and assuming a density of 1.5 g cm-3. Therefore, the AFMobserved roughness may not only be a surface roughness but may also reflect deep cracks to some extent. Besides, the change of AFM image might also imply that the coating became porous. In any case, the topographic changes found by AFM implied loosened internal contacts in the polymer sample, and these changes should result in a decrease in apparent conductivity. Therefore, the increase in potential had two opposite effects on the measured conductivity: increasing intrinsic conductivity and decreasing apparent conductivity. When the negative effect of the structural changes overwhelmed the positive effect of the intrinsic conductivity increase, the directly measured conductivity would decrease rather than increase. Thus, the conflict between the directly measured and the ESR predicted conductivities is well-explained. The quantitative difference in the change of σ/σmax with potential between Yano’s result and ours (Figure 4b) may be attributed to different experimental conditions. First, the two experiments measured the conductance in different directions in the polymer film. In Yano’s work, they used a comb-shaped electrode and measured the conductance along the polymer film plane. In contrast, we measured the conductance in perpendicular to the film (see enlargement of Figure 1). If there were cracks through the film thickness, the along-plane conductance would be much smaller than the perpendicular conductance and might drop to nearly zero. This can explain why Yano and co-workers found the film “almost an insulator” at high potentials, but we did not. Second, in our experiment, the local polymer being tested (polymer links between WE1 and WE2) was under some pressure imposed between WE1 and WE2 (elastic assembly), while in Yano’s experiment and our AFM test the polymer was allowed to change structures freely. As a result, the local structural change in our conductance measurement might be notably less than in the other two cases. Therefore, our directly measured conductance showed only a marginal decrease with increasing potential. Conclusions Simultaneously conducted conductance and ESR measurements under controlled electrochemical conditions revealed that PoPD was not only definitely conductive but should be even more intrinsically conductive at high potentials (0.3-0.7 V versus RHE) than at low potentials (0-0.2 V), which is in contrast to previous reports.9,12,26 This work also discovers that it is necessary to distinguish intrinsic conductivity from apparent conductivity. When the internal contacts of the test sample is poor, the measured conductivity will be more or less lower than the intrinsic conductivity, and in extreme cases, the sample made

Zhou et al. from the material with an intrinsically high conductivity may appear almost insulating. As suggested by in situ AFM, the internal contacts for the electropolymerized PoPD coating became much worse at high potentials. When the internal contact worsens to a certain degree to roughly compensate for the increase of intrinsic conductivity, the test sample would show little change in the measured (apparent) conductivity with potential, which is supported by this work. When the worsening of the internal contact overwhelms the improvement of the intrinsic conductivity, the apparent conductivity will prominently decrease, in the worst case, to almost zero, as reported in the literature.12 Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (Project 29903008) for financial support of this work. References and Notes (1) Syed, A. A.; Dinesan, M. K. Talanta 1991, 38, 815. (2) Scott, J. C. Science 1998, 278, 2071. (3) Inzelt, G.; Pineri, M.; Schultze, J. W.; Vorotyntsev, M. A. Electrochim. Acta 2000, 45, 2403. (4) Mizoquchi, T.; Adams, R. N. J. Am. Chem. Soc. 1962, 84, 2058. (5) Lee, H. Y.; Adams, R. N. Anal. Chem. 1962, 34, 1587. (6) Chiba, K.; Ohsaka, T.; Oyama, N. J. Electroanal. Chem. 1987, 217, 239. (7) Marinusz, K.; Czirok, E.; Inzelt, G. J. Electroanal. Chem. 1994, 379, 437. (8) Yano, J. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 2435. (9) Mazeikiene, R.; Malinauskas, A. Synth. Met. 2002, 128, 121. (10) Komura, T.; Yamaguchi, T.; Takahashi, K. Electrochim. Acta 1996, 41, 2865. (11) Ogura, K.; Kokura, M.; Yano, J.; Shiigi, H. Electrochim. Acta 1995, 40, 2707. (12) Yano, J.; Shimoyama, A.; Nagaoka, T.; Ogura, K. Denki Kagaku 1992, 60, 1101. (13) Myler, S.; Eaton, S.; Higson, S. P. J. Anal Chim. Acta. 1997, 357, 55. (14) Mederos, A.; Dominguez, S.; Molina, R. H.; Sanchiz, J.; Brito, F. Coord. Chem. ReV. 1999, 193-195, 913. (15) Ekinci, E.; Erdogdu, G.; Karagozler, A. E. J. Appl. Polym. Sci. 2001, 79, 327. (16) Dai, H. P.; Wu, Q. H.; Sun, S. G.; Shiu, K. K. J. Electroanal. Chem. 1998, 456, 47. (17) Wu, L.; Luo, J.; Lin, Z. J. Electroanal. Chem. 1996, 417, 53. (18) Wu, L.; Luo, J.; Lin, Z. J. Electroanal. Chem. 1997, 440, 173. (19) D’Elia, L. F.; Ortiz, R. L.; Marquez, O. P.; Marquez, J.; Martinez, Y. J. Electrochem. Soc. 2001, 148, C297. (20) Lin, X.; Zhang, H. Electrochim. Acta 1996, 41, 2019. (21) Losito, H.; Giglio, E. D.; Cioffi, N.; Malitesta, C. J. Mater. Chem. 2001, 11, 1812. (22) Zhuang, L.; Zhou, Q.; Lu, J. J. Electroanal. Chem. 2000, 493, 135. (23) Zhou, Q.; Zhuang, L.; Lu, J. Electrochem. Commun. 2002, 4, 733. (24) Zhou, Q.; Zhuang, L.; Lu, J. Synth. Met. 2003, 135-136, 473. (25) Chen, W.; Li, C. M.; Yu, L.; Lu, Z.; Zhou, Q. Electrochem. Commun. 2008, 10, 1340. (26) Bilal, S.; Holze, R. Electrochim. Acta 2006, 52, 1247. (27) Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1987, 236, 199. (28) Bandey, H. L.; Cremins, P.; Garner, S. E.; Hillman, A. R.; Raynor, J. B.; Workman, A. D. J. Electrochem. Soc. 1995, 142, 2111. (29) Goyette, M. A.; Leclerc, M. J. Electroanal. Chem. 1995, 382, 17.

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