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Multicolor electrochromic performance of electroactive poly(amic acid) containing pendant oligoaniline, azobenzene and sulfonic acid groups
Electrochimica Acta 89 (2013) 594–599
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multicolor electrochromic performance of electroactive poly(amic acid) containing pendant oligoaniline, azobenzene and sulfonic acid groups Shutao Wang a , Danming Chao a,∗ , Erik B. Berda b , Xiaoteng Jia a , Rui Yang a , Ce Wang a,∗ a b
Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, PR China Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, USA
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 30 October 2012 Accepted 30 October 2012 Available online 9 November 2012 Keywords: Electroactive polymer Polyaniline Electrochromic Oligoaniline
a b s t r a c t A novel electroactive poly(amic acid) (EDA-CON-EPAA) containing oligoaniline pendants, azobenzene and sulfonic acid groups was synthesized by copolymerization. The synergistic interplay of these three distinct functional groups results in a number of interesting and novel properties. The polymer showed photoisomerization induce by irradiation with ultraviolet light and visible light by virtue of azobenzene groups, and also revealed excellent electroactivity in acid, neutral and even in alkaline solutions (pH = 12) due to self-doping between oligoaniline and sulfonic acid (and/or carboxylic acid) groups. Moreover, EDACON-EPAA displayed acceptable electrochromic performance even in alkaline solutions and multiple colors attributed to the complementary effects of the two chromophores, which greatly enlarged the range of the electrochromic application. The coloration efficiency could still reach 93.8 cm2 /C (at 700 nm) even at pH = 10. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction As an important conducting polymer, polyaniline (PANI) has gained extensive attention due to its excellent electrochemistry [1], good environmental stability [2], simple polymerization and electrochromism [3], making it a potentially applicable in sensors, electromagnetic shielding materials, rechargeable batteries, metal anticorrosion and electrochromic devices [4–8]. PANI and its derivatives have been widely studied for electrochromic materials due to its excellent switching speed, high optical contrast, and abundant colors [9–12]. PANI and its derivatives exhibited multicolor changes depending on both pH and applied potential [13,14]. In the reduced state, the absorption occurs in the ultraviolet region due to the –* transition, which led to yellow color assigned to the radical species inserted in the polymeric sites. In the semioxidized state, the absorption peak at 425 nm (polaronic species) and 800 nm engender the green color. In the full-oxidized state, the blue-violet color is related with the transitions of the bipolaronic form [15–17]. Recently, many PANI-based electrochromic materials have been prepared and reported. The layer-by-layer assembled method was utilized by Delongchamp et al. to fabricate the multiple-color electrochromism of polyaniline/Prussian blue nanocomposite thin films, which exhibited high contrast and
∗ Corresponding authors. Tel.: +86 431 85168292; fax: +86 431 85168292. E-mail addresses: [email protected], [email protected] (D. Chao), [email protected] (C. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.10.149
fast switching speed in 0.1 M H2 SO4 + 0.1 M KCl solution [18]. An organic–organic hybrid system composed of the polyaniline (PANI) and poly-(3,4-ethylenedioxythiophene) (PEDOT) was developed by Kang et al. and the material had enhanced optical contrast in the hydrophobic lithium electrolyte than the previous PANI–PEDOT ECDs [19]. Pahal et al. synthesized an organic–inorganic hybrid thin films of PANI and K2 Co2+ [Fe2+ (CN)6 ], which revealed high coloring efficiency in the neat ionic liquid (3.3 M 1-butyl1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) [20]. The core–shell electrochromism also had been exploited with higher optical contrast and faster switching speed in 0.1 M LiClO4 /acetonitrile electrolyte [21,22]. However, due to its inherent shortcomings of solubility and processability, PANI has been greatly limited in application. Hence, polymers containing oligoaniline which feature excellent solubility and good film-forming ability are a competitive candidate for electrochromic materials. Up to now, many efforts have been attempted to synthesize the electroactive polymer containing oligoaniline groups, such as graft [23–25], alternating [26–28], block-like [29], star-like [30] and so on. To the best of our knowledge, the electrochromic materials made by conducting polymers are usually applied in acid solutions [18–22], due to the conductive essence of doping/dedoping chemistry. In our work, a novel polymer containing oligoaniline and azobenzene dye bearing sulfonate groups was synthesized. A multicolor electrochromic device was prepared using this polymer and exhibited excellent electrochromic performance in acid, neutral and alkaline solutions because of the self-doping effect
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between oligoaniline and sulfonic acid (and/or carboxylic acid) groups.
2. Experimental 2.1. Chemicals and instrumentations All chemicals, including 4-aminophenol (99%), dichloromethane (99.5%), 4,4 -oxydiphthalic anhydride (ODPA, 98%), congo red (99%) were purchased from Shanghai Chemical Factory. N,N dimethylacetamide (DMAc, 99%) and toluene (99%) were used as received without further purification. Distilled water was selfmade. Sodium hydroxide (99%), sodium chloride (99.5%) and sulfuric acid (99%) are all from Beijing Chemical Factory; optically transparent Indium–Tin Oxide (ITO) glass substrates (Reintech Electronic Technologies Co. Ltd., Beijing) with dimensions of 4.0 cm × 0.5 cm were used as electrochromic thin films electrode. Fourier-transform infrared spectra (FTIR) measurements were recorded on a Bruker vector 22 spectrometer. The nuclear magnetic resonance spectra (NMR) of samples in deuterated dimethyl sulfoxide (DMSO) were run on a Bruker-500 spectrometer. Mass spectroscopy (MS) was performed on an AXIMA-CFR laser desorption ionization flying time spectrometer (COMPACT). The number-average molecular weight (Mn ), weight-average molecular weight (Mw ), and molecular weight distribution of the polymer were measured with a gel permeation chromatography (GPC) instrument equipped with a Shimadzu GPC-802D gel column and SPD-M10AVP detector with DMF as an eluent at a flow rate of 1 mL/min. A Perkin–Elmer PYRIS 1 TGA was used to investigate the thermal stability of the polymer. The CV was investigated on a CHI 660A Electrochemical Workstation (CH Instruments, USA) with a conventional three-electrode cell, using a saturated calomel electrode (SCE) as the reference electrode, a platinum wire electrode as the counter electrode, and an ITO-coated glass as the working electrode. The film was cycled in 1.0 mol/L H2 SO4 aqueous solution in the range of 0–800 mV. Solutions of 1.0 mol/L sulfuric acid and 1.0 mol/L KCl solutions with pH ranging from 0 to 12 were prepared and tested at room temperature. Spectroelectrochemical measurements were carried out in a cell built from a 1 cm commercial cuvette using a UV-2501 PC Spectrometer (SHIMADZU). The ITOcoated glass was used as the working electrode, a Pt wire as the counter electrode, an Ag/AgCl cell as the reference electrode and 0.1 mol/L H2 SO4 was used as the electrolyte. Photoisomerization of EDA-CON-EPAA was conducted using high pressure mercury lamp in conjunction with band-pass UV filter (max = 360 nm). UV–vis spectra were recorded on a UV-2501 PC Spectrometer (SHIMADZU).
2.2. Synthesis of electroactive diamine monomer (EDA) The synthetic routes of EDA had been reported in the literature [31]. MALDI-TOF-MS: m/z calculated for C43 H36 N6 O3 = 684.8. Found 685.0. FTIR (KBr, cm−1 ): 3380 (s, N H ), 3037 (m, C H ), 1657 (vs, C O ), 1602 (s, C C of benzenoid rings), 1506 (vs, C C of benzenoid rings), 1294 (s, C N ), 1233 (m, C O C ), 829 (m, ıC H ), 750 (m, ıC H ), 696 (m, ıC H ). 1 H NMR (500 MHz, d6 -DMSO, ppm): ı = 10.24 (s, 1H, due to CONH ), ı = 7.74 (s, 1H, due to NH ), ı = 7.69 (s, 1H, due to NH ), ı = 7.60 (s, 1H, due to NH ), ı = 7.54 (d, 2H, due to Ar H), ı = 7.14 (t, 3H, due to Ar H), ı = 6.93 (d, 12H, due to Ar H), ı = 6.81 (d, 4H, due to Ar H), ı = 6.67 (t, 1H, due to Ar H), ı = 6.59 (d, 4H, due to Ar H), ı = 6.29 (d, 2H, due to Ar H), ı = 4.99 (s, 4H, due to NH).
Scheme 1. The structure of CON-EDA-EPAA.
2.3. Synthesis of EDA-CON-EPAA We adopted the one-step synthetic route to obtain EDA-CONEPAA polymer. The typical procedure is as follows. Congo red (0.348 g, 0.5 mmol), EDA (0.342 g, 0.5 mmol) and DMAc (4 mL) were added into a 25 mL three-necked flask with magnetic stirring under nitrogen at room temperature for 3 h till the solids dissolved completely. Then ODPA (0.31 g, 1 mmol) were added into the mixed solution to react for 5 h with magnetic stirring under nitrogen at room temperature. Hence, we got EDA-CON-EPAA solution with solid content 21 wt% (95% yield). The structure formula of EDACON-EPAA is shown in Scheme 1. 2.4. Fabrication of electrochromic electrodes The ITO substrates were washed ultrasonically in DMAc for 30 min and then in the ethanol for another 30 min, followed by drying in the air before use. The DMAc solution (0.5 mL of the original solution diluted in 0.5 mL DMAc) of EDA-CON-EPAA was acidulated by HCl solution. Then, the acidified solution was filtered through 0.2 m poly(tetrafluoroethylene) syringe filter, then spin-coated onto the ITO substrates. The spin-coating process started at 500 rpm for 9 s and then 1000 rpm for 30 s. 3. Results and discussion 3.1. Synthesis and characterization of EDA-CON-EPAA FTIR and 1 H NMR spectroscopy were used to study the chemical structure of the as-prepared EDA-CON-EPAA. FTIR spectra showed the characteristic peak around 3438 cm−1 belonging to N H stretching vibration, at 2925 cm−1 due to O H stretching vibration of carboxylic acid groups. The peak at 1714 cm−1 was attributed to C O stretching vibration. The stretching vibration of C C of the benzene rings located at 1606 cm−1 and 1507 cm−1 . The peak at 1404 cm−1 was ascribed to the stretching vibration of N N. The absorption peak about 1307 cm−1 derived from the stretching vibration of C N of aniline tetramer. The characteristic peak at 1217 cm−1 was assigned to the stretching vibration of C O C of the aryl ether linkages. The characteristic absorption bonds around 1135 cm−1 and 617 cm−1 were ascribed to the stretching vibration of S O and S O of sodium sulfonate, respectively. In the 1 H NMR of EDA-CON-EPAA, the signals at ı = 13.22 were ascribed to the hydroxyl protons of carboxylic acid groups, the signals around ı = 10.45 and ı = 10.35 were attributed to the amino protons of amide groups and the signals ı = 8.96–8.43 were assigned to the amino protons of oligoaniline. Moreover, other aromatic protons appeared at ı = 8.14–7.96, ı = 7.78–7.44 and ı = 7.31–7.12. All of these 1 H NMR signals were to support the expected molecular structure of EDA-CON-EPAA. The number average molecular weight (Mn ) and the polydispersity index of
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Fig. 1. TGA thermogram of CON-EDA-EPAA in N2 .
EDA-CON-EPAA, obtained by GPC, are 3.2 × 104 and 1.09, respectively. EDA-CON-EPAA exhibited excellent solubility in many polar solvents such as DMF, DMAc, DMSO and NMP; we suggest interchain entanglement is inhibited to some extent by the bulky pendant groups. The thermal properties of EDA-CON-EPAA were tested by TGA (Fig. 1). The first stage mass loss beginning at 140 ◦ C and ending at 284 ◦ C was observed corresponding to the progress of imidization and the evaporation of small amount of solvent. The decomposition of main chain of the imidized polymer occurred at 450 ◦ C. Compared with other PANI materials [32], the imidized EDA-CON-EPAA possessed a higher thermal stability. 3.2. Photoisomerization of EDA-CON-EPAA Photoisomerization of EDA-CON-EPAA was investigated by irradiating its DMAc solution. First, UV–vis absorption bands centered at 326 nm and 530 nm were observed, associated with the overlap of the –* transitions in the benzoid rings and azobenzene rings [33]. Then, with increased irradiation time, the intensity of the absorption peak at 326 nm of EDA-CON-EPAA decreased and the absorption band occurred with a gradual blue shift, while the absorption peak at about 530 nm underwent a decrease in intensity and a red shift. It was attributed to the trans-to-cis photoisomerization process of the azobenzene chromophore. The whole process was only 110 s and then it reached to a steady state (Fig. 2(a). Then the above solution was irradiated with visible light. As shown in Fig. 2(b), the two absorption bands recovered almost to its initial value prior to irradiation 150 s later. The peaks around 326 nm and 530 nm underwent a red shift and blue shift, respectively.
Fig. 3. CV of the CON-EDA-EPAA electrode in 1 mol/L H2 SO4 at different potential scan rates: 10–100 mV/s. Inset shows the relationships between the oxidation peaks and reduction current vs. potential scan rate.
3.3. Electrochemical activity The cyclic voltammetry of EDA-CON-EPAA was carried out in 1.0 M H2 SO4 at different potential scan rates (10–100 mV/s) with Ag/Ag+ as the reference electrode. Firstly, the DMAc solution of EDA-CON-EPAA was acidulated by HCl solution. Then, the acidulated solution was spin-coated on ITO-coated glass electrode and the thickness of the film was about 1 m. Fig. 3 shows two redox progresses with the oxidation peaks at 346 and 486 mV, assigned to the transition of leucoemeraldine base (LEB)/emeraldine base (EB) and emeraldine base/pernigraniline base (PNB), respectively. A linear dependence of the peak currents as a function of scan rates in the region of 10–100 mV/s (inset of Fig. 3) confirmed both a surface controlled process [34] and a well-adhesion. In order to evaluate the electroactivity of the EDA-CON-EPAA in different pH solution, the cyclic voltammetry was obtained using Ag/Ag+ as the reference electrode in 1.0 M KCl aqueous solution with pH ranging from 1 to 12 at a scan rate of 100 mV/s. The EDACON-EPAA DMAc solution was acidulated by HCl solution. Then, the acidified solution was cast on the g-c working electrode and evaporated to form a thin solid film. As shown in Fig. 4, when pH < 5, the CV curve exhibits two pairs of redox peaks corresponding to the transition of LEB/EB and EB/PNB, respectively. When pH > 5, two pairs of redox peaks overlap into one pair of redox peaks, assigned to LEB/PNB redox reaction. This is due to the pH dependence of the EB/PNB redox reaction. In addition, the as-synthesized EDA-CONEPAA could maintain its electroactivity until the pH = 12, while the response current at pH = 12 decreases to one-fifth of that at pH = 1.
Fig. 2. Changes in the UV–vis absorption spectra of CON-EDA-EPAA with different irradiation time in DMAc solution.
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Fig. 4. CV traces of the CON-EDA-EPAA at a scan rate of 100 mV/s in NaCl aqueous solution at pH values between 1 and 12.
Compared with other types of PANI and its derivatives, the EDACON-EPAA exhibited excellent electroactivity in a broader pH range (pH = 1–12) [35]. This wonderful electroactivity behavior should be attributed to the sulfonic and carboxylic groups in the polymer chain, which could impede the deprotonation of the conducting form PANI effectively and made EDA-CON-EPAA maintain the electroactivity in neutral and alkaline condition. 3.4. Spectroelectrochemistry and electrochromic performances Spectroelectrochemical studies were investigated on the film of EDA-CON-EPAA spin-coated on the ITO glass slide in different electrolytes (pH = 0, 2, 4, 7, 10, 12) coupled with applied potentials of −0.2, 0.2, 0.4, 0.8 and 1.0 V (vs. Ag/AgCl). As shown in Fig. 5, the EDA-CON-EPAA film shows different UV–vis transmittance spectra at different pH and various applied potentials. The peak around 430 nm (related to polarons of oligoaniline) enhanced gradually with the applied potential increasing from −0.2 V to 0.4 V at pH = 0–10 and underwent a red shift from 427 nm (pH = 0, 2) to 435 nm (pH = 4, 7, 10), while it disappeared as the potential
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increased above 0.4 V. When the applied potential was increased beyond 0.4 V, a new broad band at about 800 nm emerged associated with superimposed contributions of polaronic and bipolaronic species of oligoaniline [33] at pH = 0–10. The characteristic peak of EDA-CON-EPAA underwent a blue shift from 636 nm (pH = 0) to 621 nm (pH = 2), to 510 nm (pH = 4), and to 500 nm (pH = 6, 10), and strengthened with the potential increasing from −0.2 V to 0.8 V at pH < 10. Especially, the peak transferred from 620 nm (<0.4 V) to 520 nm (>0.4 V) when pH = 2. However, when pH = 12, only one peak existed and the intensity of transmittance in all wavelengths (280–800 nm) almost maintained unchanged in potential from −0.2 V to 1.0 V. Because of the controlled adjustments of color of the congo red and oligoaniline by pH and applied potential, respectively, the EDACON-EPAA film could reveal various colors. The color transformed from cyan (−0.2 V), to bottle green (0.4 V) and finally to dark blue (0.8 V) at pH = 0. When pH = 2, it generated an alternation of color from dimgray (−0.2 V), to olive green (0.4 V) to blue gray (0.8 V). The change of color occurred from pink (−0.2 V), to brown (0.4 V) and finally to plum purple (0.8 V) at pH = 4. Because of the limitation of the color range of congo red (pH = 3.5–5.2) varied with pH, the change of color remains almost the same when pH > 4, which existed a transformation from dark pink (−0.2 V), to medium brown (0.4 V) and to amaranth (0.8 V). When pH = 12, the color of the film (violet) did not change with the variation of applied potential. Compared with the traditional PANI, this kind of electrochromic device exhibited more colors because of the dye essence of azobenzene group, which greatly enlarged the range of electrochromic application. Electrochromic switching studies were carried out to monitor changes in the optical contrast at 700 nm during repeated potential stepping between reductive (−0.2 V) and oxidized state at 0.8 V. Because EDA-CON-EPAA film exhibited different electroactivity in different pH solution, the residence times were fixed at 60 s, 90 s, 120 s, 120 s when pH = 0, 4, 7, 10, respectively. Fig. 6 presents the results of the first 7 cycles. The optical contrast value (%T) were found to be 29.5%, 36.1%, 33.7%, 32.3% measured between coloring (oxidization) and bleaching (reduction) states at pH = 0, 4, 7, 10, respectively. Due to the influence of the characteristic peak at
Fig. 5. The spectral changes of CON-EDA-EPAA/ITO electrode at different potentials. Inset shows photographs of CON-EDA-EPAA/ITO electrode at different potentials. (a) pH = 0, (b) pH = 2, (c) pH = 4, (d) pH = 7, (e) pH = 10 and (f) pH = 12.
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Fig. 6. (a) Absorbance changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode for the first 7 cycles in pH = 0, 4, 7, 10. (b) Current consumption changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode for the first 7 cycles in pH = 0, 4, 7, 10.
636 nm, the optical contrast value in pH = 0 was smaller than other pH values. When pH > 4, the optical contrast value decreased following the pH increasing. Switching time was the time required to bring the polymer to its most reduced state from its most oxidized state or vice versa. It was defined as the time required for reaching 90% of the full change in coloring/bleaching process. The EDA-CON-EPAA film revealed switching times of 6.4 s, 22.6 s, 35.2 s and 35.6 s at 0.8 V for the coloring process at 700 nm and 2.2 s, 6.5 s, 10.4 s and 11.5 s for bleaching at pH = 0, 4, 7, 10, respectively. The current decreased gradually with the pH increasing, in accordance with Fig. 4. The coloration efficiency CE ( = OD/Q) was a practical tool to measure the power requirements of an electrochromic material. The electrochromic behavior of EDA-CON-EPAA exhibited CE up to 209.7, 137.2, 117.9 and 93.8 cm2 /C (at 700 nm) at the first oxidation stage at pH = 0, 4, 7, 10, respectively. The stability was another important parameter for electrochromic devices. Hence, we investigated its stability at pH = 4, 7 and 10. As shown in Fig. 7, due to the self-doping effect, the optical contrast value could maintain the value of the first 5 cycles even after 200 cycles at pH = 4 and 7, which possessed a perfect stability and revealed good potential in application. While, in the pH = 10 solution, the stability was not very good and the optical contrast value decreased to the half of original value after 50 cycles, because the self-doping effect was damaged in the pH = 10 solution due to the neutralization of sulfonic acid and carboxylic acid groups in the alkaline solution. Compared with other types of PANI and its derivatives reported, the EDA-CON-EPAA revealed an acceptable electrochromic
property even in neutral and alkaline resolutions (pH < 12), which is due to the self-doping effect originating from sulfonic and carboxylic groups. In other parts, the sulfonic acid/sulfonate and carboxylic acid/carbonate greatly influenced the UV–vis absorption in different pH, which made it revealed multicolor. 4. Conclusion A new poly(amic acid) containing oligoaniline, azobenzene groups and sulfonic acid groups was synthesized. The prepared polymer EDA-CON-EPAA showed good solubility and excellent thermal stability. Based on the existence of sulfonic acid groups, the polymer exhibits excellent electroactivity in acid, neutral and even alkaline solutions. Moreover, due to the synergistic effect of the three different chromophores, the EDA-CON-EPAA revealed acceptable electrochromic performance even in neutral and alkaline solutions (pH < 12) and multicolor with the changing of pH and applied potential. Acknowledgements This work has been supported in part by the National Natural Science Foundation of China (No. 21104024 and 50973038), and the National 973 Project (No. S2009061009). EBB acknowledges funding through the NSF Center for High Rate Nanomanufacturing (EEC-0425826). References
Fig. 7. Absorbance changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode at pH = 4, 7 for the first 5 and 201st–205th cycles and at pH = 10 for the first 5 and 51st–55th cycles when the anodic potential was switched between −0.2 V and 0.8 V with a residence time of 60 s or 120 s, respectively.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Multicolor electrochromic performance of electroactive poly(amic acid) containing pendant oligoaniline, azobenzene and sulfonic acid groups Shutao Wang a , Danming Chao a,∗ , Erik B. Berda b , Xiaoteng Jia a , Rui Yang a , Ce Wang a,∗ a b
Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, PR China Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, USA
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 30 October 2012 Accepted 30 October 2012 Available online 9 November 2012 Keywords: Electroactive polymer Polyaniline Electrochromic Oligoaniline
a b s t r a c t A novel electroactive poly(amic acid) (EDA-CON-EPAA) containing oligoaniline pendants, azobenzene and sulfonic acid groups was synthesized by copolymerization. The synergistic interplay of these three distinct functional groups results in a number of interesting and novel properties. The polymer showed photoisomerization induce by irradiation with ultraviolet light and visible light by virtue of azobenzene groups, and also revealed excellent electroactivity in acid, neutral and even in alkaline solutions (pH = 12) due to self-doping between oligoaniline and sulfonic acid (and/or carboxylic acid) groups. Moreover, EDACON-EPAA displayed acceptable electrochromic performance even in alkaline solutions and multiple colors attributed to the complementary effects of the two chromophores, which greatly enlarged the range of the electrochromic application. The coloration efficiency could still reach 93.8 cm2 /C (at 700 nm) even at pH = 10. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction As an important conducting polymer, polyaniline (PANI) has gained extensive attention due to its excellent electrochemistry [1], good environmental stability [2], simple polymerization and electrochromism [3], making it a potentially applicable in sensors, electromagnetic shielding materials, rechargeable batteries, metal anticorrosion and electrochromic devices [4–8]. PANI and its derivatives have been widely studied for electrochromic materials due to its excellent switching speed, high optical contrast, and abundant colors [9–12]. PANI and its derivatives exhibited multicolor changes depending on both pH and applied potential [13,14]. In the reduced state, the absorption occurs in the ultraviolet region due to the –* transition, which led to yellow color assigned to the radical species inserted in the polymeric sites. In the semioxidized state, the absorption peak at 425 nm (polaronic species) and 800 nm engender the green color. In the full-oxidized state, the blue-violet color is related with the transitions of the bipolaronic form [15–17]. Recently, many PANI-based electrochromic materials have been prepared and reported. The layer-by-layer assembled method was utilized by Delongchamp et al. to fabricate the multiple-color electrochromism of polyaniline/Prussian blue nanocomposite thin films, which exhibited high contrast and
∗ Corresponding authors. Tel.: +86 431 85168292; fax: +86 431 85168292. E-mail addresses: [email protected], [email protected] (D. Chao), [email protected] (C. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.10.149
fast switching speed in 0.1 M H2 SO4 + 0.1 M KCl solution [18]. An organic–organic hybrid system composed of the polyaniline (PANI) and poly-(3,4-ethylenedioxythiophene) (PEDOT) was developed by Kang et al. and the material had enhanced optical contrast in the hydrophobic lithium electrolyte than the previous PANI–PEDOT ECDs [19]. Pahal et al. synthesized an organic–inorganic hybrid thin films of PANI and K2 Co2+ [Fe2+ (CN)6 ], which revealed high coloring efficiency in the neat ionic liquid (3.3 M 1-butyl1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) [20]. The core–shell electrochromism also had been exploited with higher optical contrast and faster switching speed in 0.1 M LiClO4 /acetonitrile electrolyte [21,22]. However, due to its inherent shortcomings of solubility and processability, PANI has been greatly limited in application. Hence, polymers containing oligoaniline which feature excellent solubility and good film-forming ability are a competitive candidate for electrochromic materials. Up to now, many efforts have been attempted to synthesize the electroactive polymer containing oligoaniline groups, such as graft [23–25], alternating [26–28], block-like [29], star-like [30] and so on. To the best of our knowledge, the electrochromic materials made by conducting polymers are usually applied in acid solutions [18–22], due to the conductive essence of doping/dedoping chemistry. In our work, a novel polymer containing oligoaniline and azobenzene dye bearing sulfonate groups was synthesized. A multicolor electrochromic device was prepared using this polymer and exhibited excellent electrochromic performance in acid, neutral and alkaline solutions because of the self-doping effect
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between oligoaniline and sulfonic acid (and/or carboxylic acid) groups.
2. Experimental 2.1. Chemicals and instrumentations All chemicals, including 4-aminophenol (99%), dichloromethane (99.5%), 4,4 -oxydiphthalic anhydride (ODPA, 98%), congo red (99%) were purchased from Shanghai Chemical Factory. N,N dimethylacetamide (DMAc, 99%) and toluene (99%) were used as received without further purification. Distilled water was selfmade. Sodium hydroxide (99%), sodium chloride (99.5%) and sulfuric acid (99%) are all from Beijing Chemical Factory; optically transparent Indium–Tin Oxide (ITO) glass substrates (Reintech Electronic Technologies Co. Ltd., Beijing) with dimensions of 4.0 cm × 0.5 cm were used as electrochromic thin films electrode. Fourier-transform infrared spectra (FTIR) measurements were recorded on a Bruker vector 22 spectrometer. The nuclear magnetic resonance spectra (NMR) of samples in deuterated dimethyl sulfoxide (DMSO) were run on a Bruker-500 spectrometer. Mass spectroscopy (MS) was performed on an AXIMA-CFR laser desorption ionization flying time spectrometer (COMPACT). The number-average molecular weight (Mn ), weight-average molecular weight (Mw ), and molecular weight distribution of the polymer were measured with a gel permeation chromatography (GPC) instrument equipped with a Shimadzu GPC-802D gel column and SPD-M10AVP detector with DMF as an eluent at a flow rate of 1 mL/min. A Perkin–Elmer PYRIS 1 TGA was used to investigate the thermal stability of the polymer. The CV was investigated on a CHI 660A Electrochemical Workstation (CH Instruments, USA) with a conventional three-electrode cell, using a saturated calomel electrode (SCE) as the reference electrode, a platinum wire electrode as the counter electrode, and an ITO-coated glass as the working electrode. The film was cycled in 1.0 mol/L H2 SO4 aqueous solution in the range of 0–800 mV. Solutions of 1.0 mol/L sulfuric acid and 1.0 mol/L KCl solutions with pH ranging from 0 to 12 were prepared and tested at room temperature. Spectroelectrochemical measurements were carried out in a cell built from a 1 cm commercial cuvette using a UV-2501 PC Spectrometer (SHIMADZU). The ITOcoated glass was used as the working electrode, a Pt wire as the counter electrode, an Ag/AgCl cell as the reference electrode and 0.1 mol/L H2 SO4 was used as the electrolyte. Photoisomerization of EDA-CON-EPAA was conducted using high pressure mercury lamp in conjunction with band-pass UV filter (max = 360 nm). UV–vis spectra were recorded on a UV-2501 PC Spectrometer (SHIMADZU).
2.2. Synthesis of electroactive diamine monomer (EDA) The synthetic routes of EDA had been reported in the literature [31]. MALDI-TOF-MS: m/z calculated for C43 H36 N6 O3 = 684.8. Found 685.0. FTIR (KBr, cm−1 ): 3380 (s, N H ), 3037 (m, C H ), 1657 (vs, C O ), 1602 (s, C C of benzenoid rings), 1506 (vs, C C of benzenoid rings), 1294 (s, C N ), 1233 (m, C O C ), 829 (m, ıC H ), 750 (m, ıC H ), 696 (m, ıC H ). 1 H NMR (500 MHz, d6 -DMSO, ppm): ı = 10.24 (s, 1H, due to CONH ), ı = 7.74 (s, 1H, due to NH ), ı = 7.69 (s, 1H, due to NH ), ı = 7.60 (s, 1H, due to NH ), ı = 7.54 (d, 2H, due to Ar H), ı = 7.14 (t, 3H, due to Ar H), ı = 6.93 (d, 12H, due to Ar H), ı = 6.81 (d, 4H, due to Ar H), ı = 6.67 (t, 1H, due to Ar H), ı = 6.59 (d, 4H, due to Ar H), ı = 6.29 (d, 2H, due to Ar H), ı = 4.99 (s, 4H, due to NH).
Scheme 1. The structure of CON-EDA-EPAA.
2.3. Synthesis of EDA-CON-EPAA We adopted the one-step synthetic route to obtain EDA-CONEPAA polymer. The typical procedure is as follows. Congo red (0.348 g, 0.5 mmol), EDA (0.342 g, 0.5 mmol) and DMAc (4 mL) were added into a 25 mL three-necked flask with magnetic stirring under nitrogen at room temperature for 3 h till the solids dissolved completely. Then ODPA (0.31 g, 1 mmol) were added into the mixed solution to react for 5 h with magnetic stirring under nitrogen at room temperature. Hence, we got EDA-CON-EPAA solution with solid content 21 wt% (95% yield). The structure formula of EDACON-EPAA is shown in Scheme 1. 2.4. Fabrication of electrochromic electrodes The ITO substrates were washed ultrasonically in DMAc for 30 min and then in the ethanol for another 30 min, followed by drying in the air before use. The DMAc solution (0.5 mL of the original solution diluted in 0.5 mL DMAc) of EDA-CON-EPAA was acidulated by HCl solution. Then, the acidified solution was filtered through 0.2 m poly(tetrafluoroethylene) syringe filter, then spin-coated onto the ITO substrates. The spin-coating process started at 500 rpm for 9 s and then 1000 rpm for 30 s. 3. Results and discussion 3.1. Synthesis and characterization of EDA-CON-EPAA FTIR and 1 H NMR spectroscopy were used to study the chemical structure of the as-prepared EDA-CON-EPAA. FTIR spectra showed the characteristic peak around 3438 cm−1 belonging to N H stretching vibration, at 2925 cm−1 due to O H stretching vibration of carboxylic acid groups. The peak at 1714 cm−1 was attributed to C O stretching vibration. The stretching vibration of C C of the benzene rings located at 1606 cm−1 and 1507 cm−1 . The peak at 1404 cm−1 was ascribed to the stretching vibration of N N. The absorption peak about 1307 cm−1 derived from the stretching vibration of C N of aniline tetramer. The characteristic peak at 1217 cm−1 was assigned to the stretching vibration of C O C of the aryl ether linkages. The characteristic absorption bonds around 1135 cm−1 and 617 cm−1 were ascribed to the stretching vibration of S O and S O of sodium sulfonate, respectively. In the 1 H NMR of EDA-CON-EPAA, the signals at ı = 13.22 were ascribed to the hydroxyl protons of carboxylic acid groups, the signals around ı = 10.45 and ı = 10.35 were attributed to the amino protons of amide groups and the signals ı = 8.96–8.43 were assigned to the amino protons of oligoaniline. Moreover, other aromatic protons appeared at ı = 8.14–7.96, ı = 7.78–7.44 and ı = 7.31–7.12. All of these 1 H NMR signals were to support the expected molecular structure of EDA-CON-EPAA. The number average molecular weight (Mn ) and the polydispersity index of
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Fig. 1. TGA thermogram of CON-EDA-EPAA in N2 .
EDA-CON-EPAA, obtained by GPC, are 3.2 × 104 and 1.09, respectively. EDA-CON-EPAA exhibited excellent solubility in many polar solvents such as DMF, DMAc, DMSO and NMP; we suggest interchain entanglement is inhibited to some extent by the bulky pendant groups. The thermal properties of EDA-CON-EPAA were tested by TGA (Fig. 1). The first stage mass loss beginning at 140 ◦ C and ending at 284 ◦ C was observed corresponding to the progress of imidization and the evaporation of small amount of solvent. The decomposition of main chain of the imidized polymer occurred at 450 ◦ C. Compared with other PANI materials [32], the imidized EDA-CON-EPAA possessed a higher thermal stability. 3.2. Photoisomerization of EDA-CON-EPAA Photoisomerization of EDA-CON-EPAA was investigated by irradiating its DMAc solution. First, UV–vis absorption bands centered at 326 nm and 530 nm were observed, associated with the overlap of the –* transitions in the benzoid rings and azobenzene rings [33]. Then, with increased irradiation time, the intensity of the absorption peak at 326 nm of EDA-CON-EPAA decreased and the absorption band occurred with a gradual blue shift, while the absorption peak at about 530 nm underwent a decrease in intensity and a red shift. It was attributed to the trans-to-cis photoisomerization process of the azobenzene chromophore. The whole process was only 110 s and then it reached to a steady state (Fig. 2(a). Then the above solution was irradiated with visible light. As shown in Fig. 2(b), the two absorption bands recovered almost to its initial value prior to irradiation 150 s later. The peaks around 326 nm and 530 nm underwent a red shift and blue shift, respectively.
Fig. 3. CV of the CON-EDA-EPAA electrode in 1 mol/L H2 SO4 at different potential scan rates: 10–100 mV/s. Inset shows the relationships between the oxidation peaks and reduction current vs. potential scan rate.
3.3. Electrochemical activity The cyclic voltammetry of EDA-CON-EPAA was carried out in 1.0 M H2 SO4 at different potential scan rates (10–100 mV/s) with Ag/Ag+ as the reference electrode. Firstly, the DMAc solution of EDA-CON-EPAA was acidulated by HCl solution. Then, the acidulated solution was spin-coated on ITO-coated glass electrode and the thickness of the film was about 1 m. Fig. 3 shows two redox progresses with the oxidation peaks at 346 and 486 mV, assigned to the transition of leucoemeraldine base (LEB)/emeraldine base (EB) and emeraldine base/pernigraniline base (PNB), respectively. A linear dependence of the peak currents as a function of scan rates in the region of 10–100 mV/s (inset of Fig. 3) confirmed both a surface controlled process [34] and a well-adhesion. In order to evaluate the electroactivity of the EDA-CON-EPAA in different pH solution, the cyclic voltammetry was obtained using Ag/Ag+ as the reference electrode in 1.0 M KCl aqueous solution with pH ranging from 1 to 12 at a scan rate of 100 mV/s. The EDACON-EPAA DMAc solution was acidulated by HCl solution. Then, the acidified solution was cast on the g-c working electrode and evaporated to form a thin solid film. As shown in Fig. 4, when pH < 5, the CV curve exhibits two pairs of redox peaks corresponding to the transition of LEB/EB and EB/PNB, respectively. When pH > 5, two pairs of redox peaks overlap into one pair of redox peaks, assigned to LEB/PNB redox reaction. This is due to the pH dependence of the EB/PNB redox reaction. In addition, the as-synthesized EDA-CONEPAA could maintain its electroactivity until the pH = 12, while the response current at pH = 12 decreases to one-fifth of that at pH = 1.
Fig. 2. Changes in the UV–vis absorption spectra of CON-EDA-EPAA with different irradiation time in DMAc solution.
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Fig. 4. CV traces of the CON-EDA-EPAA at a scan rate of 100 mV/s in NaCl aqueous solution at pH values between 1 and 12.
Compared with other types of PANI and its derivatives, the EDACON-EPAA exhibited excellent electroactivity in a broader pH range (pH = 1–12) [35]. This wonderful electroactivity behavior should be attributed to the sulfonic and carboxylic groups in the polymer chain, which could impede the deprotonation of the conducting form PANI effectively and made EDA-CON-EPAA maintain the electroactivity in neutral and alkaline condition. 3.4. Spectroelectrochemistry and electrochromic performances Spectroelectrochemical studies were investigated on the film of EDA-CON-EPAA spin-coated on the ITO glass slide in different electrolytes (pH = 0, 2, 4, 7, 10, 12) coupled with applied potentials of −0.2, 0.2, 0.4, 0.8 and 1.0 V (vs. Ag/AgCl). As shown in Fig. 5, the EDA-CON-EPAA film shows different UV–vis transmittance spectra at different pH and various applied potentials. The peak around 430 nm (related to polarons of oligoaniline) enhanced gradually with the applied potential increasing from −0.2 V to 0.4 V at pH = 0–10 and underwent a red shift from 427 nm (pH = 0, 2) to 435 nm (pH = 4, 7, 10), while it disappeared as the potential
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increased above 0.4 V. When the applied potential was increased beyond 0.4 V, a new broad band at about 800 nm emerged associated with superimposed contributions of polaronic and bipolaronic species of oligoaniline [33] at pH = 0–10. The characteristic peak of EDA-CON-EPAA underwent a blue shift from 636 nm (pH = 0) to 621 nm (pH = 2), to 510 nm (pH = 4), and to 500 nm (pH = 6, 10), and strengthened with the potential increasing from −0.2 V to 0.8 V at pH < 10. Especially, the peak transferred from 620 nm (<0.4 V) to 520 nm (>0.4 V) when pH = 2. However, when pH = 12, only one peak existed and the intensity of transmittance in all wavelengths (280–800 nm) almost maintained unchanged in potential from −0.2 V to 1.0 V. Because of the controlled adjustments of color of the congo red and oligoaniline by pH and applied potential, respectively, the EDACON-EPAA film could reveal various colors. The color transformed from cyan (−0.2 V), to bottle green (0.4 V) and finally to dark blue (0.8 V) at pH = 0. When pH = 2, it generated an alternation of color from dimgray (−0.2 V), to olive green (0.4 V) to blue gray (0.8 V). The change of color occurred from pink (−0.2 V), to brown (0.4 V) and finally to plum purple (0.8 V) at pH = 4. Because of the limitation of the color range of congo red (pH = 3.5–5.2) varied with pH, the change of color remains almost the same when pH > 4, which existed a transformation from dark pink (−0.2 V), to medium brown (0.4 V) and to amaranth (0.8 V). When pH = 12, the color of the film (violet) did not change with the variation of applied potential. Compared with the traditional PANI, this kind of electrochromic device exhibited more colors because of the dye essence of azobenzene group, which greatly enlarged the range of electrochromic application. Electrochromic switching studies were carried out to monitor changes in the optical contrast at 700 nm during repeated potential stepping between reductive (−0.2 V) and oxidized state at 0.8 V. Because EDA-CON-EPAA film exhibited different electroactivity in different pH solution, the residence times were fixed at 60 s, 90 s, 120 s, 120 s when pH = 0, 4, 7, 10, respectively. Fig. 6 presents the results of the first 7 cycles. The optical contrast value (%T) were found to be 29.5%, 36.1%, 33.7%, 32.3% measured between coloring (oxidization) and bleaching (reduction) states at pH = 0, 4, 7, 10, respectively. Due to the influence of the characteristic peak at
Fig. 5. The spectral changes of CON-EDA-EPAA/ITO electrode at different potentials. Inset shows photographs of CON-EDA-EPAA/ITO electrode at different potentials. (a) pH = 0, (b) pH = 2, (c) pH = 4, (d) pH = 7, (e) pH = 10 and (f) pH = 12.
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Fig. 6. (a) Absorbance changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode for the first 7 cycles in pH = 0, 4, 7, 10. (b) Current consumption changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode for the first 7 cycles in pH = 0, 4, 7, 10.
636 nm, the optical contrast value in pH = 0 was smaller than other pH values. When pH > 4, the optical contrast value decreased following the pH increasing. Switching time was the time required to bring the polymer to its most reduced state from its most oxidized state or vice versa. It was defined as the time required for reaching 90% of the full change in coloring/bleaching process. The EDA-CON-EPAA film revealed switching times of 6.4 s, 22.6 s, 35.2 s and 35.6 s at 0.8 V for the coloring process at 700 nm and 2.2 s, 6.5 s, 10.4 s and 11.5 s for bleaching at pH = 0, 4, 7, 10, respectively. The current decreased gradually with the pH increasing, in accordance with Fig. 4. The coloration efficiency CE ( = OD/Q) was a practical tool to measure the power requirements of an electrochromic material. The electrochromic behavior of EDA-CON-EPAA exhibited CE up to 209.7, 137.2, 117.9 and 93.8 cm2 /C (at 700 nm) at the first oxidation stage at pH = 0, 4, 7, 10, respectively. The stability was another important parameter for electrochromic devices. Hence, we investigated its stability at pH = 4, 7 and 10. As shown in Fig. 7, due to the self-doping effect, the optical contrast value could maintain the value of the first 5 cycles even after 200 cycles at pH = 4 and 7, which possessed a perfect stability and revealed good potential in application. While, in the pH = 10 solution, the stability was not very good and the optical contrast value decreased to the half of original value after 50 cycles, because the self-doping effect was damaged in the pH = 10 solution due to the neutralization of sulfonic acid and carboxylic acid groups in the alkaline solution. Compared with other types of PANI and its derivatives reported, the EDA-CON-EPAA revealed an acceptable electrochromic
property even in neutral and alkaline resolutions (pH < 12), which is due to the self-doping effect originating from sulfonic and carboxylic groups. In other parts, the sulfonic acid/sulfonate and carboxylic acid/carbonate greatly influenced the UV–vis absorption in different pH, which made it revealed multicolor. 4. Conclusion A new poly(amic acid) containing oligoaniline, azobenzene groups and sulfonic acid groups was synthesized. The prepared polymer EDA-CON-EPAA showed good solubility and excellent thermal stability. Based on the existence of sulfonic acid groups, the polymer exhibits excellent electroactivity in acid, neutral and even alkaline solutions. Moreover, due to the synergistic effect of the three different chromophores, the EDA-CON-EPAA revealed acceptable electrochromic performance even in neutral and alkaline solutions (pH < 12) and multicolor with the changing of pH and applied potential. Acknowledgements This work has been supported in part by the National Natural Science Foundation of China (No. 21104024 and 50973038), and the National 973 Project (No. S2009061009). EBB acknowledges funding through the NSF Center for High Rate Nanomanufacturing (EEC-0425826). References
Fig. 7. Absorbance changes monitored at 700 nm of CON-EDA-EPAA/ITO electrode at pH = 4, 7 for the first 5 and 201st–205th cycles and at pH = 10 for the first 5 and 51st–55th cycles when the anodic potential was switched between −0.2 V and 0.8 V with a residence time of 60 s or 120 s, respectively.
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