Electrodeposition of conductive poly(3-methoxythiophene) in ionic liquid microemulsions

Journal of Electroanalytical Chemistry 628 (2009) 60–66

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Electrodeposition of conductive poly(3-methoxythiophene) in ionic liquid microemulsions Bin Dong a,b, Jingkun Xu a,*, Liqiang Zheng b,*, Jian Hou c a

Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, China c State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266071, China b

a r t i c l e

i n f o

Article history: Received 5 October 2008 Received in revised form 9 January 2009 Accepted 12 January 2009 Available online 22 January 2009 Keywords: Ionic liquid microemulsions Electropolymerization Conducting polymers Poly(3-methoxythiophene) Polythiophene

a b s t r a c t The electrosyntheses of poly(3-methoxythiophene) (PMOT) by direct anodic oxidation of MOT in novel ionic liquid microemulsions, BmimPF6/Tween 20/H2O, have been investigated. Among water-in-BmimPF6 (W/IL), bicontinuous (BC), and BmimPF6-in-water (IL/W) sub-regions, IL/W was found to be the most suitable medium for the electropolymerization of MOT. The use of IL/W microemulsions remarkably reduces the amount of IL, which is really expensive as electrolyte. BmimPF6 serves both as the core of IL/W microemulsions and as the supporting electrolyte and thus presents a novel microenvironment for the electropolymerization of MOT. Thus, MOT microdroplets were assembled on a bare ITO electrode and polymerized into PMOT microcups. In addition, the oxidation onset potential of MOT in IL/W microemulsions was lower than that in micellar aqueous solutions or conventional organic solvents. As-formed PMOT films obtained in IL/W microemulsions had an electrical conductivity of 3.8 S/cm and could be dissolved in many conventional organic solvents, including dichloromethane, chloroform, acetonitrile, and dimethyl sulfoxide with green-light emitting property. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The field of conducting polymers (CPs) has been extensively studied since 1970s due to their wide scope of possible applications in sensors, supercapacitors, and electrochromic devices [1]. Especially, the family of polythiophene (PTh) was investigated actively because of the ease to grafting substituents on the thiophene ring, and of the numerous side chains available. The electrochemical polymerization of thiophene and its derivatives in water was not so straightforward while the major studies were typically in organic solvents like acetonitrile and propylene carbonate [2], resulting from their low solubility in water, higher oxidation potential than that of water, and strong nucleophilic reactivity of thienyl cation radicals with water molecules [3]. However, water is still the ideal medium for the electropolymerization merely from both economical and ecological points. To overcome these obstacles, the use of anionic surfactants such as sodium dodecyl sulfate (SDS) was then proposed [4]. As a result, the addition of anionic surfactants increased the solubility of thiophene derivatives in water and decreased the oxidation onset potential at which the electropolymerization happened. The use of cationic or non-ionic surfactants was also attempted [4i,5]. Indeed, * Corresponding authors. Tel.: +86 791 3823320; fax: +86 791 3823357 (J. Xu). E-mail addresses: [email protected] (J. Xu), [email protected] (L. Zheng). 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.01.011

the use of surfactant-containing aqueous solutions for the electropolymerization of heteroaromatic compounds revealed several advantages. Micellar medium affected the electrochemical reactions by irreversible adsorption, changing the solution–electrode interface properties and producing some template effects [6]. Additionally, micellar medium could also stabilize charged species such as anion or cation radicals [7]. Nowadays, ionic liquids (ILs), a class of organic molten electrolytes at or near ambient temperature, have attracted a large amount of interest deriving from their specific physicochemical properties, i.e., no significant vapor pressures, fire resistance and stableness at temperatures up to 300 °C or more [8]. As electrolytes with high ionic conductivity, wide potential windows, and the ability of dissolving various compounds, the air and moisture stable ILs are expected to be peculiarly suitable media for the electropolymerization [9]. Indeed, PTh and its derivatives were already successfully electrosynthesized in diverse ILs [10]. As a result, the use of IL as both the growth medium and a supporting electrolyte led to significantly altered film morphologies and improved electrochemical activities, comparable to CPs electrosynthesized in conventional media. Although the popularity of the air and moisture stable ILs has risen steadily, they are really expensive. Microemulsions are transparent, isotropic, and thermodynamically stable dispersions of two immiscible liquids stabilized by surfactant [11]. They have extensive applications in chemical reactions and material syntheses with some peculiar advantages.

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Recently, the use of IL instead of water or conventional organic solvents to prepare novel IL-based microemulsions has been realized [12]. Similar to ‘‘classic” microemulsions, gradual sub-structural transition from microdroplets to a bicontinuous structure spans the single-phase microemulsion region. For 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6), non-ionic surfactant Tween 20, and water system, water-in-BmimPF6 (W/IL), bicontinuous (BC), and BmimPF6-in-water (IL/W) sub-regions were identified [12b]. This kind of IL microemulsions reduces the amount of IL significantly, especially for IL/W. It is believed that this new electrolyte system involving both a surfactant and an IL will be useful in the electrosyntheses of CPs. In our preliminary communication, we pointed out the feasibility of the electrosyntheses of conductive and electroactive poly(3,4-ethylenedioxythiophene) (PEDOT) films in BmimPF6/Tween 20/H2O IL microemulsions [13]. Electrochemical measurements demonstrated that among three types of IL microemulsions, IL/W was the most suitable medium for the electropolymerization of EDOT. Compared to micellar aqueous solutions with an additional supporting electrolyte, BmimPF6 serves both as the core of IL/W microemulsions and as the supporting electrolyte and thus presents a novel microenvironment for the monomer. Here, we describe in detail the electropolymerization of 3-methoxythiophene (MOT) in this new electrolyte system. The electrochemical and optical properties of as-formed poly(3-methoxythiophene) (PMOT) and its structure were investigated.

mentioned in this paper were referred to a saturated calomel electrode (SCE). All the measurements were performed under a nitrogen atmosphere to avoid the effect of oxygen. As-formed PMOT films were dedoped with 25 wt% ammonia for 3 days and then washed repeatedly with deionized water. Finally, they were dried in vacuum at 60 °C for 24 h. Note that during electrochemical polymerizations and examinations, there was no deposit at all in the monomer-free IL microemulsions, indicating that the electrolyte systems and the electrode were electrochemically inert. 2.3. Characterizations Infrared (IR) spectra were recorded by using KBr pellets on the Bruker Vertex 70 FT–IR spectrometer. 1H NMR spectra were recorded on a Bruker AV 400 NMR spectrometer. UV–vis spectra were taken by using a Perkin–Elmer Lamda 900 UV–vis–NIR spectrophotometer. The fluorescence spectra were determined with an F-4500 fluorescence spectrophotometer (Hitachi). Scanning electron microscopy (SEM) measurements were performed using a Philips XL 30 scanning electron microscope. The electrical conductivity of a dried pressed PMOT pellet was measured by the conventional four-probe technique. The thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were performed with a Netzsch TG209 thermal analyzer. 3. Results and discussion 3.1. Electrochemical syntheses of PMOT films

2. Experimental 2.1. Chemicals BmimPF6 was prepared according to the procedure reported by Dupont et al. [14]. The purity of the product was ascertained by 1H NMR spectrum in d6-DMSO (d: 0.91, t, 3 H; 1.28, m, 2 H; 1.74, m, 2 H; 3.87, s, 3 H; 4.14, t, 2 H; 7.68, s, 1 H; 7.74, s, 1 H; 9.06, s, 1 H). Before use, BmimPF6 was dried in vacuum. 3-Methoxythiophene (MOT, Acros Organics) was used as received.; its typical concentration in IL microemulsions was 0.12 mol/L. 25 wt% ammonia (analytical grade, Jinan Chemical Reagent Co., Ltd.), non-ionic surfactant Tween 20 (chemical pure grade, Tianjin Kermel Chemicals Co., Ltd.), and acetonitrile (CH3CN, HPLC grade, Tianjin Kermel Chemicals Co., Ltd.), were used directly without further purification. Deuterium-substituted dimethyl sulfoxide (d6-DMSO) and CDCl3 were purchased from Cambridge Isotope Laboratories, Inc and J&K Chemical LTD, respectively. Tetrabutylammonium tetrafluoroborate (Bu4NBF4, Acros Organics) was dried in vacuum at 60 °C for 24 h prior to use. Deionized water was used and its electro-resistivity was 18 MX cm.

In our preliminary communication [13], the ionic conductivity (j) results of BmimPF6/Tween 20/H2O IL microemulsions with BmimPF6-to-Tween 20 weight ratio, I = 0.12, as well as Tween 20 micellar aqueous solution, showed that j was evidently increased after the formation of IL microemulsions by the addition of BmimPF6 in comparison with Tween 20 micellar aqueous solution. Especially for IL/W microeumlsions, j was even comparable to pure BmimPF6 while the use of IL/W microemulsions remarkably reduces the amount of IL, which is really expensive. The elevated j of IL microemulsions would be beneficial to the electrosyntheses of CPs. Accordingly, the anodic polarization measurements of MOT were performed in three types of IL microemulsions and Tween 20 micellar aqueous solution, as shown in Fig. 1. It is obvious that for IL/W, the oxidation onset potential of MOT was the least (1.08 V vs. SCE) and the current density was the largest, together with polymer depositions on the working electrode clearly observed. As

A 3

Electrochemical polymerizations and examinations were performed in a one-compartment cell by the use of model 263 potentiostat-galvanostat (EG&G Princeton Applied Research) under computer control at 30 °C. To obtain a sufficient amount of PMOT for structural characterization, stainless steel sheets with surface areas of 10 and 12 cm2 each were employed as the working and counter electrodes, respectively. Prior to each experiment, the electrodes were polished with abrasive paper (1500 mesh) and cleaned with water and acetone successively. PMOT films were grown potentiostatically and the amount was controlled by the integrated current passing through the cell. The anodic polarization and cyclic voltammetry measurements were carried out with a platinum wire (diameter 0.5 mm) as the working electrode and a stainless steel wire (diameter 1 mm) as the counter electrode. The potentials

j / mA cm-2

2.2. Electropolymerization

2

.

B D C

1 0

0.0

0.5 1.0 Potential / V vs. SCE

1.5

Fig. 1. Anodic polarization curves of 0.12 mol/L MOT in IL microemulsions (I = 0.12) or Tween 20 micellar aqueous solution. A: IL/W, H2O% = 90 wt%; B: BC, H2O% = 55 wt%; C: W/IL, H2O% = 20 wt%; D: CTween 20 = 0.2 mol/L. Scan rate: 20 mV/s.

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reported in literature, oil-in-water microemulsion provides a spatially restricted environment and creates hydrophobic and hydrophilic regions at the metal/solution interface, inducing an electrocatalytic effect [4b,6c]. Essentially, IL/W microemulsion is a special type of oil-in-water microemulsion. In IL/W microemulsions, MOT molecules were trapped in BmimPF6, the core of IL/W microemulsions. IL/W microemulsions provide a large interfacial area and are less viscous together with relative high ionic conductivity comparable to pure BmimPF6, which are all beneficial to the oxidation of MOT. As to BC, the oxidation onset potential of MOT was initiated at 1.64 V vs. SCE, higher than that in W/IL (1.26 V vs. SCE) or in Tween 20 micellar aqueous solution (1.14 V vs. SCE). Schultze et al. reported that at large surfactant concentrations the electropolymerization was inhibited due to surfactant adsorption [5d]. It is reasonable that the oxidation onset potential of MOT was the least in IL/W microemulsions since the Tween 20 concentrations in IL/W, BC, and W/IL were 8.95 wt%, 40.26 wt%, and 71.58 wt%, respectively. Therefore, among three types of IL microemulsions, IL/W was the most suitable medium for the electropolymerization of MOT. Similar results would be obtained in the following potentiodynamic measurements. On the other hand, at the same scanning rate of 20 mV/s, the oxidation onset potential of MOT was 1.39 V vs. SCE in CH3CN + 0.1 mol/L LiClO4 and 1.19 V vs. SCE in 0.03 mol/L SDS + 0.1 mol/L LiClO4 aqueous solution [4g]. The oxidation onset potential in IL/W microemulsions decreases, which may be ascribed to the trapping of MOT molecules in BmimPF6, the core of IL/W microemulsions. Compared to micellar aqueous solutions with an additional supporting electrolyte, BmimPF6 as the core of IL/W microemulsions and the electrolyte presented a novel microenvironment for MOT, which was beneficial to the electropolymerization of MOT. Generally, the lower is the oxidation onset potential of the monomer, the less is the side reactions, such as a–b coupling and overoxidation, which lead to the formation of poor-quality polymer films. Therefore, it was expected that in IL/ W microemulsions, high-quality PMOT films would be electrosynthesized due to the relatively low oxidation potential. Fig. 2 represents successive cyclic voltammograms (CVs) of MOT in three types of IL microemulsions and Tween 20 micellar

3.2. Electrochemistry of PMOT films The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of PMOT were determined from the onset of reduction and oxidation

0.5

1.5 mA cm-2

0.0

1.5 mA cm-2

1.0

1.5

0.0

0.0

0.5

0.5

1.0

.

C

.

B

.

1.5 mA cm-2

1.5 mA cm-2

A

.

-0.5

aqueous solution. In BC, W/IL, or Tween 20 micellar aqueous solution, redox wave currents did not increase but decrease with propagation of the potential scans, and thus, almost no polymer film was formed on the working electrode. As to IL/W (Fig. 2A), such a CV is similar to that of thiomethyl substituted thiophene reported by Heinze et al. [15], indicating the oxidation of the oligomer to a monocationic and then to a dicationic redox state. Redox wave currents increased upon sequential cycles, suggesting that more and more polymer films were formed on the working electrode [16,17]. The current density in IL/W was the largest, implying that PMOT was the easiest to be formed and the polymerization rate was the highest [18]. Compared with CVs of MOT in non-ionic surfactant TX405 aqueous solutions (0.1 mol/L TX405 + 0.1 mol/L LiClO4 + 0.1 mol/L n-butanol) [4i], the higher peak current densities and the larger increasing interval reflected a comparatively faster coupling of cation radicals and the formation of PMOT films with higher electroactivity in IL/W microemulsions, indicating that BmimPF6 with high intrinsic ionic conductivity as the core of IL/W microemulsions was beneficial to the electropolymerization of MOT. Based on the above discussions, the following work was focused on the electropolymerization of MOT in IL/W microemulsions. It was worth mentioning that the monomer concentration affected the electropolymerization. Fig. 3 showed the CVs in IL/W microemulsions at different MOT concentrations of 0.04, 0.09, 0.12, and 0.145 mol/L. At the low concentrations of 0.04 and 0.09 mol/L, the curves were in disorder and there were no defined redox peaks. Although defined redox peaks existed at the high concentration of 0.145 mol/L, the original electrolyte solution was ivory-white, e.g., it never belongs to the definition of ‘‘microemulsion”, but belongs to the definition of ‘‘emulsion”. Therefore we focused on the electropolymerization of MOT in IL/ W microemulsions at the optimum condition of 0.12 mol/L MOT.

1.0

1.5

0.0

1.5

D

0.5

1.0

1.5

Potential / V vs. SCE Fig. 2. Cyclic voltammograms of 0.12 mol/L MOT in IL/W (A), BC (B), W/IL (C), and Tween 20 micellar aqueous solution (D). Scan rates: 100 mV/s.

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2.4

2.4

A

1.6

B

1.6

0.8 0.8

j / mA cm-2

0.0 -0.8

.

0.0 -0.4

0.0

0.4

0.8

1.2

0.0

C

2.4

0.4

0.8

1.2

1.6

1.6

D

0.8

0.8 0.0

0.0

-0.8 -0.8

-1.6 -2.4 -0.4

0.0

0.4

0.8

1.2

1.6

-0.4

0.0

0.4

0.8

1.2

Potential / V vs. SCE Fig. 3. Cyclic voltammograms in IL/W microemulsions at different MOT concentrations of 0.04 mol/L (A), 0.09 mol/L (B), 0.12 mol/L (C), and 0.145 mol/L (D). Scan rates: 100 mV/s.

j / mA cm

-2

0 -2

.

-4 -6

-1.6

-0.8 0.0 0.8 Potential / V vs. SCE

1.6

Fig. 4. Cyclic voltammograms of PMOT films in CH3CN + 0.1 mol/L Bu4NBF4 at a potential scan rate of 50 mV/s. The films were synthesized electrochemically in IL/ W microemulsions at a constant applied potential of 1.3 V vs. SCE.

methane, chloroform, acetonitrile, and dimethyl sulfoxide (mainly in the dedoped state). The solution of doped PMOT was baby blue in color while that of dedoped PMOT was orange. For characterization of the structure and properties of as-formed PMOT films, FT–IR, 1H NMR, UV–vis, and fluorescence spectra were performed. Fig. 5 shows the FT IR spectra of MOT (A), doped PMOT (B), and dedoped PMOT (C) obtained potentiostatically in IL/W microemulsions. The changes between the monomer and the polymer were distinct. The strong and narrow peak at 1547 cm1 in the monomer was assigned to the stretching mode of the aromatic C@C bond, which were shifted to 1508 cm1 in doped PMOT and 1518 cm1 in dedoped PMOT. The –CH3 bending bands at 1448, 1394 cm1 in the monomer were shifted to 1445, 1362 cm1 in doped PMOT and 1454, 1358 cm1 in dedoped PMOT, respectively. Two strong bands at 1238, 1159 cm1 in the monomer were attributed to Ca–H in-plane bending vibrations. They nearly disappeared in the

C

potentials, respectively, by assuming the energy level of ferrocene/ ferrocenium (Fc) to be 4.8 eV below vacuum [19]. The empirical equations are as follows:

ð1Þ ð2Þ

re ox Eec g ¼ ELUMO  EHOMO ¼ eðEonset  Eonset ÞV

ð3Þ

As shown in Fig. 4, CVs of as-formed PMOT films in CH3CN + 0.1 mol/L Bu4NBF4 were performed at a wide potential ox range, 1.7–+1.8 V. Ere onset and Eonset of PMOT in CH3CN + 0.1 mol/L Bu4NBF4 were observed at 0.78 and +1.19 V vs. SCE, respectively. Therefore, the electrochemical band gap (Eec g ) was estimated to be 1.97 eV. This result was comparable to Eg (1.86 eV) determined by Brédas et al. [20]. 3.3. Structural characterizations PMOT films electrosynthesized in IL/W microemulsions can dissolve in many conventional organic solvents, including dichloro-

B

Transmittance

ELUMO ¼ eðEre onset þ 4:8ÞV EHOMO ¼ eðEox onset þ 4:8ÞV

A

2800

2400

2000 1600 1200 Wavenumber / cm -1

800

Fig. 5. FT–IR spectra of MOT (A), doped PMOT (B), and dedoped PMOT (C) obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions.

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O 3

2 1

S

2

8

A

O 3

2

3

4 n

ppm

1

B

and to the thiophene ring local p ? p* transition, respectively [23]. The third one with lower absorption intensity could be attributed to a polarized transition parallel with the long molecular axis [24]. The maximum one at 485 nm resulted from the delocalized p ? p* electronic transition, implying the existence of conjugated segments in the polymer. On the other hand, the absorption bands of doped PMOT were different from those of dedoped PMOT. The absorption band at 517 nm assigned to the delocalized p ? p* electronic transition was red-shifted, relative to the corresponding adsorption band of dedoped PMOT at 485 nm, probably due to the extended conjugation structure in the charged PMOT main backbone. As explained elsewhere, the maximum absorption band at 676 nm can be attributed to polarons and dipolarons or to diamagnetic dimmer dications or p-dimers [25]. Similar results were reported for PMOT electrosynthesized in SDS micellar aqueous solutions [4j,k]. Generally, the larger is the wavelength of the absorption band, the higher is the conjugation length of the polymer [26]. Therefore, the UV– vis spectra results confirmed the formation of conjugated PMOT main backbone. Fig. 7 shows the UV–vis spectra of as-formed PMOT films on ITO electrode with different deposition time. Upon increasing the deposition time, the intensity of the main absorption band for doped or dedoped PMOT was gradually enhanced, indicating the increase in the amount of the polymer on ITO electrode. Doped PMOT on ITO electrode exhibited a main absorption band in the region of 520–680 nm. After undoping through electrochemical reduction until the current reached a value close to zero, dedoped PMOT on ITO electrode exhibited a strong absorption band at 490 nm. These main absorption bands of doped or dedoped PMOT in solid state were in accordance with those in solution. Moreover, PMOT films on ITO electrode represented good and reversible electrochromic properties between the doped and dedoped states, from blue to orange (inset of Fig. 7). It is well known that electro-

7

1

6

2

5

4

4

3

4

S

ppm

8

7

6

5

4

Fig. 6. 1H NMR spectra of MOT (A) and dedoped PMOT (B) obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions. Solvent: CDCl3 (A) and d6-DMSO (B).

polymer spectra. The Cb–H in-plane bending band at 1034 cm1 in the monomer was broader and shifted to 1067 cm1 in doped PMOT and 1066 cm1 in dedoped PMOT. Weak bands appearing at 1219 cm1 in doped PMOT and 1209 cm1 in dedoped PMOT were assigned to the stretching vibration of an interring C–C bond. All results demonstrated that the electropolymerization of MOT in IL/W microemulsions mainly happened through a–a0 coupling. A broad and intense peak at 845 cm1 in doped PMOT was the characteristic band of the dopant anion PF 6 [21]. In the dedoped PMOT, most dopant anions were excluded from the polymer, leading to the visible decrease in the intensity of PF 6 band. To get deep insight into the polymer structure and the polymerization mechanism, the 1H NMR spectra of MOT (Fig. 6A) and dedoped PMOT obtained potentiostatically in IL/W microemulsions (Fig. 6B) were performed. As seen clearly, proton lines of the polymer were much broader than those of the monomer due to the wide molar mass distribution or the complex structure of the polymer. The chemical shift at 3.84 ppm assigned to the C–H bond of – CH3 in the monomer were moved to 3.99 ppm in dedoped PMOT. In Fig. 6A, three peaks at 7.21, 6.78, and 6.28 ppm were assigned to the protons at the C1, C2, and C4 positions (chemical structure in the inset of Fig. 6), respectively. In the same region, there was only one broad peak at 7.14 ppm in the polymer, corresponding to the C–H bond at the C2 position. These results indicated that the polymerization should happen at C1 and C4 positions (a–a0 coupling), in good agreement with the FT–IR results. Meanwhile, the chemical shifts of the protons moving to lower fields also confirmed the formation of a conjugated delocalizing structure [22]. The UV–vis spectra of MOT, doped and dedoped PMOT obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions were measured with CH3CN as the solvent. The main electronic absorption bands are listed in Table 1. The monomer showed a characteristic absorption band at 218 nm with a shoulder at 242 nm. In contrast, dedoped PMOT exhibited four absorption bands at 235, 280, 333, and 485 nm. The first two absorption bands, deriving from the monomer, were ascribed to a charge transfer transition from the oxygen atom in the methoxy group to the thiophene ring

doped dedoped

2

Absorption (a.u.)

doped dedoped

1 400 s 200 s 100 s 400 s 200 s 100 s

0

400

600

800

1000

Wavelength / nm Fig. 7. UV–vis spectra of as-formed PMOT films on ITO electrode obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions with different deposition time. Dedoped PMOT films were prepared through electrochemical reduction at a constant applied potential of 0.8 V vs. SCE. Doped (solid); dedoped (dash).

Table 1 Electronic absorption and fluorescence properties of MOT and PMOT in CH3CN. Solvent

Sample

Electronic absorption bands k1 (nm)

CH3CN

MOT Doped PMOT Dedoped PMOT

222 238 235

FWHM: full-width-at-half-maximum of the band.

k2 (nm) 345 280

Fluorescence properties k3 (nm) 517 333

k4 (nm) 676 485

FWHMA (cm1)

kex (nm)

kem (nm)

FWHMF (cm1)

4494 5694

287 489 491

329 562 559

1996 1989

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3.4. Morphology and conductivity The surface morphology of as-formed PMOT films on ITO electrode synthesized in IL/W microemulsions with different deposition time was observed by scanning electron microscope (SEM). As shown in Fig. 8A, at the electrochemical deposition of 400 s, the microcups located densely and covered the whole surface of the working electrode. With the increase of the deposition time, the thickness of microcups increased and most microcups were turned to microballs (Fig. 8B). The morphology and its change were similar to those of PEDOT synthesized in tri(ethylene glycol) micellar aqueous solutions on a six bilayers PDDA/PSS modified ITO electrode by Shi et al. [5f]. Shi et al. also reported that PEDOT films composed of particles were formed on a bare ITO electrode. Here, on a bare ITO electrode, PMOT microcups were obtained in IL/W microemlsions, mainly due to the fact that the high viscosity of BmimPF6 reduced the diffusion rate of MOT microdroplets and also decreased their number assembled on the electrode surface at the same time. The electrical conductivity of as-formed PMOT films was determined to be ca. 3.8 S/cm. Since the electrical conductivity of PMOT films generally ranged from 103 to 15 S/cm reported in the literature [30], PMOT films with good electrical conductivity were obtained in IL/W microemulsions.

100

2.4 %

A

90

626 K

B

80

DTG

Weight / %

chromism results from the generation of different absorption bands in the visible region during shifting between redox states [27]. The fact that CPs can repeatedly undergo electrochemical doping/dedoping processes makes them the most promising class of material used in electrochromic devices [28]. The fluorescence spectra of MOT, doped and dedoped PMOT obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions were performed in CH3CN through the wavelength scans of excitation and emission. The emission and excitation wavelengths are listed in Table 1. It was evident that both emission and excitation wavelengths were largely red-shifted from the monomer to the polymer, which further proved the formation of the conjugated backbone of PMOT, in accordance with UV–vis and 1H NMR spectra results. The obvious emission peaks of doped and dedoped PMOT appearing at 562 and 559 nm also implied that PMOT was a green-light emitter, suggesting possible utilizations in organic optoelectronics. Moreover, as seen in Table 1 the fullwidth-athalf-maximum (FWHM) of the bands for the UV–vis were larger than those for the fluorescence, which indicated that more conformers were present in the ground state than in the excited singlet state. Analogous phenomena were found for PMOT electrosynthesized in SDS micellar aqueous solutions and other 3-substituted oligophenes [4j,29].

70 32.4 %

60 15.1 %

50 400

600 800 Temperature / K

1000

Fig. 9. TGA curves of dedoped PMOT obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions.

3.5. Thermal analysis The thermal stability of CPs is very important for their potential application. To investigate the thermal stability of as-formed PMOT films prepared in IL/W microemulsions, thermal analyses were performed under a nitrogen stream from 297 to 1058 K with a heating rate of 10 K/min. As seen in the thermogravimetry curve (Fig. 9A), there was a three-step loss of weight. The first one was slight from 297 to 450 K, up to 2.4%, owing to the evaporation of moisture trapped in the polymer [31]. The second one from 450 to 762 K, up to 32.4%, may be ascribed to the degradation of the skeletal PMOT backbone structure. The last one was about 15.1% from 762 to 1058 K, probably resulting from the overflow of some oligomers decomposing from the polymer. From the differential thermogravimetry curve (Fig. 9B), it can be seen that the maximum decomposition rate occurred at 626 K, much higher than that of PTh [32], implying that the electron-donating effect of the methoxy group presents a notable effect on improving the thermal stability of PMOT. 4. Conclusions Novel IL/W microemulsions (BmimPF6/Tween 20/H2O) were used as electrolytes to electrosynthesize poly(3-methoxythiophene) (PMOT). The use of IL/W microemulsions remarkably reduces the amount of IL, which is really expensive. Compared to micellar aqueous solutions with an additional supporting electrolyte, BmimPF6 serves both as the core of IL/W microemulsions and as

Fig. 8. SEM micrographs of as-formed PMOT films on ITO electrode obtained potentiostatically at 1.3 V vs. SCE in IL/W microemulsions for 400 s (A) and 600 s (B).

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