Effect of secondary dopants on electrochemical and spectroelectrochemical properties of polyaniline

Electrochimica Acta 51 (2006) 2756–2764

Effect of secondary dopants on electrochemical and spectroelectrochemical properties of polyaniline Li-Ming Huang a , Cheng-Hou Chen a , Ten-Chin Wen a,∗ , A. Gopalan b a

Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan b Department of Industrial Chemistry, Alagappa University, Karaikudi, India Received 23 May 2005; received in revised form 4 August 2005; accepted 9 August 2005 Available online 13 September 2005

Abstract Polyaniline (PANI) was doped with poly(styrene sulfonic acid) (PSS) via doping–dedoping–redoping procedure. Incorporation of PSS in PANI resulted modifications in electrochemical and electrochromic properties, morphology and polymer structure of the polymer film as evidenced by the results of cyclic voltammetry, in situ UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis and conductivity measurements. PANI doped with PSS was found to have a cross-link/branched structure with a minimum degradation product. The absence of degradation products improves the electrochemical, electrochromic properties and thermal stability of the PANI layer for electrochromic applications. © 2005 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; Poly(styrene sulfonic acid); Coloration efficiency; Thermal stability

1. Introduction Polyaniline (PANI) is one of the most promising conducting polymers due to its straightforward polymerization and excellent chemical stability combined with relatively high levels of conductivity [1]. For inducting processibility to polyaniline in the undoped, i.e. insulating, state, post-processing doping is required. A method refereed as “counterion-induced processibility” was used with success for the preparation of polyaniline with metallic-type conductivity. For example, PANI(CSA)0.5 (where PANI denotes polyaniline repeat unit involving one ring and one nitrogen atom and CSA denotes dl-camphor-10-sulfonic acid) processed from m-cresol [2,3] or 1,1,1,3,3,3-hexafluoro-2propanol [4] is metallic down to 230 and 200 K, respectively. In comparisons to the method adopted by Cao et al. [2], counterions of PANI–CSA were replaced with anions from conventional acids, such as HCl, HClO4 , H2 SO4 , H3 PO4 and p-toluene sulfonic acid (p-TSA) in the other procedures ∗

Corresponding author. Tel.: +886 6 2385487; fax: +886 6 2344496. E-mail address: [email protected] (T.-C. Wen).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.08.012

[5,6]. The high room temperature conductivity obtained in these cases was attributed to arise from the expanded conformation of PANI as it has been reported for PANI–CSA in m-cresol. Recently, polyacids and polyelectrolytes were employed for doping polyaniline in order to improve selected properties [7–11]. Several other studies on the properties of polyaniline prepared with different dopants can be found in the literature [12–15]. In addition, series of articles concerning the thermal stability of PANI have been reported in the literature [16–19]. It was found that thermal stability of PANI salts depends on the counterions used for doping [20]. In the present work, we investigated the effect of secondary dopants on the electrochemical, spectroelectrochemcial, surface morphology and thermal stability of CSA-doped and poly(styrene sulfonic acid) (PSS) doped PANI film. Samples of PANI doped with CSA and PSS were characterized by cyclic voltammetry, in-situ UV–vis spectroscopy, scanning electronic microscopy (SEM) and thermogravimetric analysis (TGA) for having a comparative account on the properties between PANI doped with CSA and PANI doped with PSS.

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2757

Fig. 1. Flow chart of doping–dedoping–redoping process of PANI–PSS.

2. Experimental Polyaniline was prepared by the oxidative polymerization of aniline (Merck) with ammonium peroxydisulfate (APS, Fluka) in HCl (Merck) aqueous solution at 5 ◦ C as described elsewhere [21]. Emeraldine base form of PANI was obtained by treating the PANI in 1 M ammonium hydroxide (Aldrich) for 16 h. The neutral PANI was further treated with camphor10-sulfonic acid (Fluka) in a molar ratio of CSA to ANI unit as 2:1. An appropriate quantity of the resulting mixture (PANI–CSA, 6 wt% in solution) was dissolved in m-cresol (Riedel-deHaen), treated in an ultrasonic bath for 48 h at 25 ◦ C. Solution of PANI–CSA in m-cresol was thus obtained. Thin films were formed over a cleaned 1.0 cm × 2.0 cm indium tin oxide electrode (ITO) with 1–3 layers of PANI by spin coating (3000 rpm for 40 s) technique. The process of doping–dedoping–redoping was followed as described in Fig. 1. First, the resulting films (i.e. PANI–CSA) were dedoped by 1 M ammonium hydroxide solution to get emeraldine base (EB) films, and then the EB films were redoped by poly(styrene sulfonic acid) (Mw = 75,000, Aldrich). Electrochemical studies were performed with PGSTAT20 electrochemical analyzer, AUTOLAB Electrochemical Instrument (The Netherlands). All experiments were carried out in a three-component cell. An Ag/AgNO3 (3 M TBAP/Acetonitrile) electrode, platinum wire and ITO coated glass plate (1 cm2 area) were used as reference, counter and working electrodes. A Luggin capillary, whose tip was set at a distance of 1–2 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes. Spectroelectrochemical studies were performed with Shimadzu UV-2100 UV–vis spectrophotometer to record the in situ UV–vis spectra by operating in time course mode. UV–vis spectroelectrochemical experiments were done in a quartz cuvette of 1 cm path length assembled as an electrochemical cell with an optically transparent working electrode, a platinum wire as counter electrode and Ag/AgNO3 as reference electrode. Characterization of PANI doped with CSA and PSS was performed with scanning electron microscopy, thermogravimetric analysis and four point probe conductivity measurements. SEM micrographs of PANI–CSA and PSS were taken by a JEOL JSM-6700 F HR-FESEM scanning electron microscope. Thermograms were recorded in a PerkinElmer TGA 7/DX Thermal Analyzer over a temperature

range of 50–1000 ◦ C in an inert atmosphere at a heating rate of 10 ◦ C min−1 . XPS was performed with ESCA 210 spectrometers, XPS spectra employed Mg K␣ (hν = 1253.6 eV) irradiation as the photon sources, with a primary tension of 12 kV and during the scans was approximately 10−10 mbar.

3. Results and discussion 3.1. Electrochemical studies of PANI doped with CSA and PSS For the application of PANI in an electrochromic device, it is important to select a suitable electrolyte. Non-aqueous electrolytes provide several advantages for the application in electrochromic device, such as wider electrochemical windows, absence of release of hydrogen and oxygen in the operating potentials and etc. Herein, we have investigated the effect of dopants for doping PANI in the process of determining suitable process conditions that can suit for electrochromic devices. According to this viewpoint, at first instant, non-aqueous electrolyte (LiClO4 dissolve in PC) was selected to monitor the electrochemical and spectroelectrochemical properties of PANI. Fig. 2 represents the CVs of (a) PANI–CSA and (b) PANI–PSS with 1–3 layers in 1 M LiClO4 -PC non-aqueous medium. PANI film doped with CSA and PSS shows prominent redox peaks at around 0.2 (peak I, accompanied by a color change from yellow to green), 0.57 (peak II) and higher than 0.8 V (peak III, accompanied by a color change from green to blue). Peaks I and III are assigned for the conversion of leucoemeraldine to emeraldine (peak I), emeraldine to pernigraniline (peak III). Peak II corresponds to the redox reaction of the degradation products (hydroquinone to quinone) [22,23]. Three important observations can be noticed from Fig. 2. Firstly, for PANI doped with CSA and PSS, the current density increased with increasing layers on ITO electrodes. It indicates the deposition of more amounts of electroactive species on ITO electrodes as the layer number is increased. The following equation was used to calculate the thickness of films at the different layers: d=

QMw zFAρ

(1)

where Q is the charge under first cyclic voltammetric scan, Mw the molecular weight of aniline (93.13 g mol−1 ), z = 0.5

2758

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

Fig. 3. Cyclic voltammograms of (a) PANI–CSA and (b) PANI–PSS deposited as a single layer by spin coating in 1 M LiClO4 -PC non-aqueous background electrolyte.

Fig. 2. Cyclic voltammograms of (a) PANI doped with CSA and (b) PANI doped with PSS with increasing the deposited layers.

(number of electrons/aniline unit), A the area of the electrode (1.0 cm2 ), ρ the specific density of aniline (1.02 g cm−3 ) and F is the Faraday’s constant. The thickness of 1–3 layer are 36, 60 and 95 nm for PANI–CSA and 35, 52 and 58 nm for PANI–PSS films, respectively. Secondly, the peak separation for the redox reactions for PANI doped with PSS are narrow than PANI doped with CSA. Thirdly, the current density of the middle peak between I and III in the Fig. 2, corresponding to the coexistence of degradation products in PANI film [22,23] is very low for PANI doped with PSS films. Otherwise, the peak marked II is relatively unobvious for PANI doped with PSS than for PANI doped with CSA. This indi-

cates that the PANI degradation is minimum in PSS doped with PANI and this leads to a better quality of PANI–PSS film. Fig. 3 presents the CVs of PANI (1 layer) doped with CSA and PSS in 1 M LiClO4 -PC as background electrolyte. The differences in electrochemical properties between PANI–CSA and PANI–PSS can be seen from the comparative analysis of peak potential and current density at the redox sites. Peaks I, II and III of PANI doped with PSS appear at lower anodic peak potentials in comparison with PANI doped with CSA. This is attributed to the decrease in the resistance of the polymer film as a result of incorporation of PSS having SO3 − in PANI film. PSS incorporation in PANI film makes charge neutralization easy upon oxidation of PANI. Reports on other features are also available. Other features have been noticed with PSS when it interacted with PANI. Ding and Park [9] noticed a faster polymerization for aniline in PSS. Kang et al. [24] noticed a relatively faster redox reaction at high pH values and better mechanical properties for PANI when doped with PSS. Additionally, the current density at peak III is much higher for PANI–PSS than that for PANI–CSA. This indicates that conversion of emeraldine to pernigraniline form of PANI is facile in PANI–PSS in comparison to PANI doped with CSA. Stability of the films of PANI doped with CSA and PSS were compared by recording CVs of (a) PANI–CSA and (b) PANI–PSS for various cycles. CVs of 2nd and 50th cycles (Fig. 4) are presented for comparison. Relative decrease (RD) in peak intensity for PANI films was calculated from the following equation employing the charge consumed in the cyclic

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2759

Fig. 5. UV–vis spectra of PANI doped with CSA (solid line) and PSS (dash line) recorded at (a) 0.0 V, (b) 0.6 V and (c) 0.9 V.

reduced consumption and result higher value of RD for PANI doped with PSS in comparison to PANI doped with CSA. Optical properties were also followed to obtain the effects of dopants and to get more details on stability of PANI films. From the electrochemical point of view, PSS provides the anions to compensate the positive charges generated from the oxidation of PANI and reduces the possibility for the protonated imine sites of PANI to undergo side reactions to cause degradation products of PANI. 3.2. Spectroelectrochemical studies of PANI doped with CSA and PSS

Fig. 4. Cyclic voltammograms of (a) PANI–CSA and (b) PANI–PSS deposited as a single layer by spin coating in 1 M LiClO4 -PC non-aqueous background electrolyte (2nd and 50th cycles).

voltammetric scan. RD (%) =

Qi − Qf × 100 Qi

(2)

where Qi and Qf are the charges at the initial and final cycles. RD values were calculated to be 5.73 and 9.20% for PANI doped with CSA and PSS, respectively. From the analysis of RD values, it seems that PANI doped with CSA is more stable than with PSS. However, it can be clearly seen that the peak corresponding to degradation product (peak II) disappears for PANI doped with PSS after 50th cycle scan. This causes

Fig. 5 displays the in situ UV–vis spectra for PANI doped with CSA and PSS at various applied potentials. One can notice a strong absorption peak (peak I) for the yellow colored leucoemeraldine form of PANI (benzenoid π–π* transition) doped with CSA and PSS at 313 and 350 nm, respectively. At more positive potentials, PANI becomes oxidized, causing a loss of the leucoemeraldine peak and shifting the absorption to 425 nm (PANI–CSA) and 427 nm (PANI–PSS) (peak II). At potential (>0.50 V), a strong peak at 770 nm (peak III) can be noticed. Oxidized PANI has two possible forms: the emeraldine base at pH higher than 4 and the protonated emeraldine salt at acidic pHs [25,26]. The band at around 425 nm is assigned to an intermediate state of PANI [27]. Based on detailed UV–vis and resonance Raman spectroscopic investigations, this band has been assigned by Monkman [28] to charged para-coupled phenyl structures, which correspond presumably to localized phenyl ring excitions. Comparatively, bands I and II show hypsochromic shifts when PANI was doped with PSS. This might be due to that the existence of sulfonic acid groups

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2760

Table 1 Optical and electrochemical data collected for coloration efficiency measurements T%ox T%red Qd (mC/cm2 ) OD PANI–CSA (1 layer) PANI–CSA (2 layer) PANI–CSA (3 layer) PANI–PSS (1 layer) PANI–PSS (2 layer) PANI–PSS (3 layer)

69.2 63.0 49.1 76.0 61.4 50.2

92.1 93.5 92.8 97.7 97.7 92.3

−2.1 −2.9 −4.2 −1.4 −2.1 −2.6

−0.12 −0.17 −0.28 −0.11 −0.20 −0.26

CE (cm2 /C) 57.1 58.6 66.7 78.6 95.2 100

Note: OD = log[T%ox /T%red ]; CE = OD/Qd .

along PSS main chain that provide more anions to dope with PANI and may ultimately lead to a decrease in the band gap. The main absorbance band in the red region of the UV–vis spectrum corresponds to the emeraldine state of PANI [29]. In situ colorimetry has also been utilized to precisely map the colors associated with EC polymers [30,31]. Coloration efficiency (CE) measurements can guide us to study and understand the electrochromic phenomenon. CE (η) is defined as the relationship between the injected/ejected charge as a function of electrode area (Qd ) and the charge in optical density, OD, at a specific wavelength (λmax ) as illustrated by Eq. (3) η=

OD(λ) Qd

(3)

where OD = log[Tox /Tred ] [32,33]. An ideal material or device should have a large transmittance for a small amount of charge to have large CEs. Measurements of CE for PANI doped with CSA and PSS for films having different layers of PANI have been done here to have a comparative account on the influence of changes in polymer structure on electrochromism. The transmittance (T%) of the fully reduced films (T%red ), which serve as baselines for the change in percent transmittance (T%) values, and the transmittance of the fully oxidized films (T%ox ) were recorded. The transmittance of reduced and oxidized state of PANI films and injected/ejected charge (Qd ) were listed in Table 1. As can be seen from the Table 1, the transmittance of reduced state did not vary much on increasing the layers of PANI. However, the transmittance of oxidized state decreases largely by increasing the layers of PANI. Interestingly, the oxidized transmittance of PANI (1 layer) doped with PSS is much higher than PANI-doped with CSA. By increasing layers of PANI doped with PSS, the transmittance drops rapidly and closed to the values of PANI doped with CSA. It is envisaged that relatively more amounts of electroactive species (PANI) were doped with PSS to result a composite film. PSS provides SO3 − ions to dope with PANI and the excess of SO3 − ions can be compensated for the charge balance during oxidized process. An excellent electrochromic material must have larger optical changes for a small amount of charge consumed. PANI doped with PSS shows a small

amount of charge consumed accompanying with the optical change (OD) close to PANI doped with CSA. CE increases with increasing the layers of PANI–PSS and is much higher than PANI–CSA. The lower charge consumption might be due to the absence of side product on doping PANI with PSS and this causes oxidation of PANI to occur more completely. From the comparison of CE, it is therefore concluded that films of PANI doped with PSS have improved electrochromic performance. For a more detailed analysis of electrochromic properties of PANI doped with CSA and PSS, optical properties of the PANI layers, such as response time, stability and memory effect were evaluated. A double potential step chronoamperometry was performed to estimate the response time and its stability during consecutive scans of the PANI doped with CSA and PSS. Potentials were stepped between 0.0 and 0.9 V with a residence time of 30 s. The optical contrast (T%) at 770 nm was monitored. Fig. 6 demonstrates charge-time and transmittance (at 770 nm) time profiles for the PANI doped with CSA and PSS (with 3 layers) at different repeated cycles. It reveals that the response time of PANI doped with CSA (coloring state: 10.4 s and bleaching state: 9.9 s) and PSS (coloring state: 11.8 s and bleaching state: 9.9 s) are nearly the same. Otherwise, incorporation of PSS into PANI did not affect the response time of PANI. Moreover, bleaching seems to have faster response than coloring for both dopants. This result also indicates a faster dedoping of PANI+ ClO4 − in non-aqueous PC-LiClO4 electrolyte. In comparison with the response time (15 s) in the literature [34], PANI doped with CSA and PSS has a lower response time. This might be due to the high conductivity of PANI doped with CSA (σ = 1.0 S cm−1 ) and PSS (σ = 1.2 S cm−1 ). Polyaniline salt doped with CSA and dissolved in appropriate organic reagents (m-cresol) has been demonstrated to cause a conformational transition of PANI chains from a “compact coil” to an “expanded coil”, leading to a concomitant increase in conductivity by up to several orders of magnitude [35]. MacDiarmid and Epstein [35] explained this conformational change as the “secondary doping effect”. UV–vis spectra (Fig. 5), present strong evidences for the protonation of PANI doped with CSA and PSS. At 0.6 and 0.9 V, UV–vis spectra show the presence of free tail carrier at wavelength higher than 700 nm. The presence of free tail in the NIR region for the PANI doped with CSA and PSS films indicates the lowering of band gap of PANI and this can consequently result high conductivity. The stability of the bleaching and/or coloring states toward multiple redox switches often limits the utility of electrochromic materials in electrochromic display (ECD) applications. Compared with inorganic electrochromic materials, such as the tungsten oxide, conducting polymers often show poor stability due to the poor adherence of polymers to the surface of inorganic transparent electrode surface [36]. We employed colorimetry for monitoring the transmittance changes to investigate the stability for PANI doped with CSA and PSS upon multiple deep switches. Fig. 7 shows

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2761

Fig. 6. (a) Charge-time and (b) transmittance-time profiles of the PANI doped with CSA (solid line) and PSS (dash line) recorded during double step spectrochronamperometry.

the long-term stability test at (a) coloring and (b) bleaching state for PANI doped with CSA and PSS. We noticed a loss of 9.38 and 7.78% at coloring state and 2.92 and 1.94% at bleaching state for PANI doped with CSA and PSS after switching 600 cycles. PANI doped with PSS is more stable than doped with CSA. The enhancement of stability of PANI doped with PSS is likely due to a combination of the following factors. First, incorporation of PSS into PANI structure increases the mechanical properties of PANI and improves the adhesion between PANI and ITO substrate. Second, the side products or degradation products are suppressed for PANI doped with PSS, leading to a better quality of the PANI/PSS film. From the analysis of stability test, polyelectrolytes not only provide anions to dope with conducting polymers but

Fig. 7. Test for long-term switching stability of PANI doped with CSA (solid line) and PSS (dash line) at (a) coloring state and (b) bleaching state by switching the potential in the range of 0.0–0.9 V for every 30 s.

also improve the optical and electrochemical stability for EC materials. The color persistence in the ECDs is an important feature since it is directly related to aspects involved in its utilization and energy consumption during use [37]. An experiment was performed by polarizing the PANI doped with CSA and PSS in the coloring/bleaching state and following the optical spectrum as a function of time at open circuit conditions. Fig. 8 represents the changes for PANI doped with (a) CSA and (b) PSS in the transmittance at the open circuit conditions after polarizing at +0.9 and 0.0 V. The transmittance

2762

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764 Table 2 XPS results of PANI (emeraldine base), PANI–CSA and PANI–PSS O 1s (%) N 1s (%) C 1s (%) S 2p (%) S/N C/N PANI (EB) 13.3 PANI–CSA 18.7 PANI–PSS 19.2

6.0 4.7 6.0

80.7 73.8 70.7

– 2.7 4.1

O/S

– 13.45 – 0.57 15.70 6.93 0.68 11.78 4.68

tions of C, N, S and O in the polymer film, calculated from the corresponding photoelectron peak area after sensitivity factor corrections (SF = 1.00, 1.77, 2.17 and 2.85 for C 1s, N 1s, S 2p and O 1s, respectively), are listed in Table 2. It can be seen that PANI doped with PSS shows the lowest value of C/N ratio. In general, the C/N ratio is 6 for aniline units in PANI. The excessive presence of carbon in the polymer film is attributed to the degradation of PANI via hydrolysis to benzoquinone. From C/N ratio analysis, it is consistent with the CV results that the degradation products are suppressed for PANI doped with PSS. Table 2 also reveals that O/S ratio of PANI–PSS is lower in comparison with PANI (EB) and PANI–CSA. The presence of PSS is excepted to bind PANI to get a compact network structure. This network structure of PANI–PSS inhibits degradation to hydroquinone/quinone of PANI, resulting a lower O/S ratio for PANI doped with PSS. Furthermore, conductivity of PANI is dependant on the doping level of PANI. We observed that doping level of PANI–PSS (S/N = 0.68) is higher than PANI–CSA (S/N = 0.57). It might be related to the presence of pendant SO3 − groups along PSS main chain. The excess SO3 − groups play additional role. PSS molecules are expected to be adsorbed on the positively charged electrode surface to improve the adhesion between PANI and ITO and facilitate the electron-transfer reaction across the polymer film. 3.4. Polymer morphology

Fig. 8. Variation of transmittance with time under open circuit conditions for (a) PANI–CSA and (b) PANI–PSS at coloring state and bleaching state.

showed a decay of only 4.88 and 3.85% at coloring state and bleaching state for PANI doped with PSS in comparison with PANI doped with CSA (10.05% at coloring state and 4.13% at bleaching state). Results of memory effect revealed that PANI doped with PSS showed better stability than doped with CSA. 3.3. Chemical analysis For the XPS investigation, PANI (emeraldine base), PANI–CSA and PANI–PSS were casted as thin film on glass substrate by spin coating techniques. The relative concentra-

From the above discussion, it is conceivable that surface morphology of PANI is influenced by the incorporation of PSS into PANI structure. Therefore, SEM was used to know the morphology of the films (Fig. 9). It can be seen that the PANI doped with CSA has grains with more porous structure, whereas the SEM picture of PANI doped with PSS shows better cohesion and higher aggregation (more compact morphology). The change in surface structure of the PANI–PSS in contrast to PANI–CSA arises from on the influence of PSS molecules on the orientation of PANI to the substrate (ITO), leading to improvements in electrochemical and spectroelectrochemical properties. 3.5. Thermogravimetric analysis Thermogravimetric curves for PANI doped with CSA and PSS are given in Fig. 10. PANI doped with CSA and PSS show three stages of thermal transition leading to weight losses. First thermal transitioin is related to removal of dopants [38]. The second thermal transition corresponds to the loss of low

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2763

Table 3 Thermal properties of PANI doped with CSA and PSS Polymers

Weight loss in T2b

T3c

300 ◦ C

208 331

489 534

811 914

36.6 (%) 65.1 (%) 5.5 (%) 45.3 (%)

(◦ C) PANI–CSA PANI–PSS a b c

(%) weight loss

T1a

(◦ C)

(◦ C)

600 ◦ C

1000 ◦ C 82.6 (%) 64.7 (%)

Onset temperature of first thermal event. Onset temperature of second thermal event. Onset temperature of third thermal event.

molecular weight oligormers or side products (hydroquinone or quinone) and the final transition is due to the degradation of backbone units of PANI. The percent of weight losses at the selected temperatures (corresponding to dopant removal, loss of oligomers and main chain degradation) are listed in Table 3. PANI doped with PSS has better thermal stability than PANI doped with CSA. We observed a similar thermal stability for PANI–CSA films as noticed by Cao and co-workers [19]. PANI–CSA decomposes above 200 ◦ C, corresponding to the decomposition temperature, 193–195 ◦ C [39], of the CSA itself. For PANI doped with PSS, the first thermal transition occurs at higher temperature (more about 125 ◦ C) than noticed with PANI–CSA. The incorporation of PSS into PANI resulted a compact structure with increased thermal stability.

4. Conclusions

Fig. 9. SEM photographs of (a) PANI–CSA and (b) PANI–PSS.

The approach is based on the doping of polyelectrolyte, PSS subsequently into PANI structure, through a doping–dedoping–redoping route. Incorporation of PSS into PANI resulted advantageous parameters for electrochromic applications. PANI doped with PSS has less side products and possesses better stability than PANI doped with CSA. Coloration efficiency of PANI–PSS is superior than for PANI–CSA. This type of composite material that requires low charge consumption for a large optical change is a promising candidate for electrochromic materials. XPS results revealed that PANI doped with PSS (C/N = 11.78) has less-amount of side products in comparison to PANI doped with CSA (C/N = 15.70). The lower value of the O/S ratio (4.68) indicates that a compact composite exists to result a dense fine structure. The observed modification in morphology through SEM is consistent with branched structure for PANI doped with PSS. The branched structure for PANI–PSS resulted better thermal stability as evident from TGA results.

Acknowledgements

Fig. 10. Therograms of PANI doped with (a) CSA and (b) PSS.

The financial support of this work by the National Science Council of Taiwan under NSC93-2214-E-006-025 and NSC93-2214-E-006-002 is gratefully acknowledged.

L.-M. Huang et al. / Electrochimica Acta 51 (2006) 2756–2764

2764

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14] [15] [16] [17] [18] [19] [20]

R.K. Paul, C.K.S. Pillai, Synth. Met. 114 (2000) 27. Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91. A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 (1994) 103. A.R. Hopkings, P.G. Rasmussen, R.A. Basheer, Macromolecules 29 (1996) 7838. W. Li, M. Wan, Synth. Met. 92 (1998) 121. W. Li, M. Wan, J. Appl. Polym. Sci. 71 (1999) 615. A.J. Motheo, J.R. Santos Jr., E.C. Venancio, L.H.C. Mattoso, Polymer 39 (1998) 6977. Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 57 (1993) 3514. H. Ding, S.M. Park, J. Electrochem. Soc. 150 (2003) E33. Q. Zhou, L.L. Miller, J.R. Valentine, J. Electroanal. Chem. Interfacial Electrochem. 261 (1989) 147. T. Shimidzu, A. Ohtani, K. Honda, J. Electroanal. Chem. Interfacial Electrochem. 251 (1988) 323. R.S. Kohlman, J. Joo, Y.G. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. Lett. 77 (1996) 2766. J. Joo, Z. Oblakowaki, G. Du, J.P. Pouget, E.J. Oh, J.M. Wiesinger, Y.G. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 49 (1994) 2977. J. Joo, V.N. Prigodin, Y.G. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 50 (1994) 12226. L.H.C. Mattoso, A.G. MacDiarmid, A.J. Epstein, Synth. Met. 68 (1994) 1. X.-H. Wang, Y.-H. Geng, L.-X. Wang, X.-B. Jing, F.-S. Wang, Synth. Met. 69 (1995) 263. X.-H. Wang, Y.-H. Geng, L.-X. Wang, X.-B. Jing, R.-S. Wang, Synth. Met. 69 (1995) 265. K.G. Neoh, M.Y. Pun, E.T. Kang, K.L. Tan, Synth. Met. 73 (1995) 209. C.Y. Yang, M. Reghu, A.J. Heeger, Y. Cao, Synth. Met. 79 (1996) 27. S. Palaniappan, B.H. Narayana, J. Polym. Sci., Part A: Polym. Chem. 32 (1994) 2431.

[21] V. Rajendran, A. Gopalan, T. Vasudevan, T.C. Wen, J. Electrochem. Soc. 147 (2000) 3014. [22] T.C. Wen, L.M. Huang, A. Gopalan, J. Electrochem. Soc. 148 (2001) D9. [23] W.C. Chen, T.C. Wen, A. Gopalan, J. Electrochem. Soc. 148 (2001) E427. [24] Y. Kang, M.H. Lee, S.B. Rhee, Synth. Met. 52 (1992) 319. [25] D. DeLongchamp, P.T. Hammond, Adv. Mater. 13 (2001) 1455. [26] A.G. MacDiarmid, B.D. Humphrey, W.S. Huang, J. Chem. Soc., Faraday Trans. 82 (1986) 2385. [27] A. Malinauskas, R. Holze, Synth. Met. 97 (1998) 31. [28] A.P. Monkman, in: J.L. Bredas, R.R. Chance (Eds.), Conjugated Polymeric Materials: Opportunities in Electronics, Optoelectronics, and Molecular Electronics, Klwer, Dordrecht, 1990, p. 273. [29] A. Malinauskas, R. Holze, J. Appl. Polym. Sci. 73 (1999) 287. [30] B.C. Thompson, P. Schottland, K. Zong, J.R. Reynolds, Chem. Mater. 12 (2000) 1563. [31] R.D. Rauh, F. Wang, J.R. Reynold, D.L. Meeker, Electrochim. Acta 46 (2001) 2023. [32] P.H. Aubert, A.A. Argun, A. Cirpan, D.B. Tanner, J.R. Reynolds, Chem. Mater. 16 (2004) 2386. [33] C.L. Gaupp, D.M. Welsh, R.D. Rahu, J.R. Reynolds, Chem. Mater. 14 (2002) 3964. [34] H. Hu, L. Hechavarria, J. Campos, Solid State Ionics 161 (2003) 165. [35] Y. Xia, J.M. Wiesinger, A.G. MacDiarmid, A.J. Epstein, Chem. Mater. 7 (1995) 443. [36] C.M. Lampert (Ed.), Large-Area Chromogenics: Materials and Devices for Transmittance Control, vols. IS 4, Bellingham, Washington, USA, 1988, p. 339. [37] P. Camurlu, A. Cirpan, L. Toppare, J. Electoanal. Chem. 572 (2004) 61. [38] L.M. Huang, T.C. Wen, A. Gopalan, Mater. Lett. 57 (2003) 1765. [39] The Merck Index, ninth ed., Merck & Co. Inc., USA, 1976, p. 1735.