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Significant improvement in the thermoelectric properties of zwitterionic polysquaraine composite films

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Signi

ficant improvement in the thermoelectric properties of

zwitterionic polysquaraine composite

films

Mei-Chan Ho

a

, Ching-Hsun Chao

b

, An-Ya Lo

c,*

, Chun-Hua Chen

a

, Ren-Jye Wu

d

,

Mei-Hui Tsai

e

, Yi-Chia Huang

a

, Wha-Tzong Whang

a,*

aDepartment of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hsin-Chu 30010, Taiwan R.O.C

bDow Chemicals, Advanced Materials, Electronic Materials, No. 6, Kesi 2nd Road, Jhunan, Miaoli, Science-Based Industrial Park 35053, Taiwan R.O.C cDepartment of Materials Science and Engineering, Green Energy Development Center, Feng Chia University, No. 100, Wenhwa Road, Seatwen,

Taichung 40724, Taiwan R.O.C

dIndustrial Technology Research Institute, Material and Chemical Research Laboratories, Rm 104, Bldg 67, 195, Sec. 4, Chung Hsing Road, Chutung,

Hsinchu 31040, Taiwan R.O.C

eDepartment of Chemical and Materials Engineering, National Chin-Yi University of Technology, No. 57, Sec. 2, Zhongshan Road, Taiping, Taichung 41170,

Taiwan R.O.C

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Polysquaraine SQI0.1 blended with

SWNTs and CMK-3 can develop freestandingfilm.

 SWNTs well-dispersed in the SQI0.1

matrix and endowed the composite withflexibility.

 The iodine-doped SQI0.1-based films

have potential for thermoelectric application.

 Thermoelectric efficiency of com-posite can be promoted by SWNTs and CMK-3.

a r t i c l e i n f o

Article history:

Received 18 February 2013 Received in revised form 21 June 2013 Accepted 22 June 2013 Keywords: Polymer Composite materials Electrical conductivity Thermoelectric effects

a b s t r a c t

In this study, the polysquaraine SQI0.1, a zwitterionicp-conjugated polymer, was adopted as the matrix

for the preparation offlexible and freestanding films; the low band gap of this semiconducting polymer made it a natural choice for use as a thermoelectric (TE) polymer. To enhance their TE applications, both single-walled carbon nanotubes (SWNTs) and mesoporous carbon (i.e., CMK-3) were integrated into the SQI0.1-basedfilms and the effects of doping with iodine were also investigated. Using scanning electron

microscopy, the variations in morphology of these SQI0.1-basedfilms were examined. Raman

spectros-copy was used to study thepepinteractions between iodine and the carbon materials (i.e., SWNT, CMK-3); X-ray diffraction and Raman spectroscopy to investigate the intercalation of the doped iodine in the compositefilms; and X-ray photoelectron spectroscopy to determine the valence state of the doped iodine. The TE properties of these materials were characterized in terms of the electrical conductivity (s), thermal conductivity (k), and Seebeck coefficient (S). The TE properties of the iodine-doped composite film prepared from SWNTs, CMK-3, and SQI0.1 included a notable value of ZT (Figure of Merit) of

4.563 103, which was 143% of that of the corresponding iodine-doped SQI 0.1film.

Ó 2013 Elsevier B.V. All rights reserved.

* Corresponding authors.

E-mail addresses:a.y.lo1125@gmail.com(A.-Y. Lo),wtwhang@mail.nctu.edu.tw

(W.-T. Whang).

Contents lists available atSciVerse ScienceDirect

Materials Chemistry and Physics

j o u rn a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s

0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.

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1. Introduction

Because of their niche applications (e.g., wearable electronics), polymer-based thermoelectric (TE) materials have attracted wide-spread research attention. Although traditional

p

-conjugated poly-mers are excellent choices for TE applications because of their intrinsic semiconductivities[1e4], their low electrical conductivities and low Seebeck coefficients lead to small TE figures of meritdfor example, a low efficiency index of TE converters (ZT ¼ S2

s

T/

k

; where

S,

s

, T, and

k

are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively).

This problem can potentially be solved by polysquaraine, a zwitterionic

p

-conjugated polymer, because the band gap energy can be decreased effectively and strong charge-transfer interactions can be induced by its alternating donor/acceptor repeating units and resonance-stabilized zwitterionic structure[5e7]. To the best of our knowledge, however, no freestanding zwitterionic

p

-conjugated polymers have been prepared, restricting their use in TE applica-tions. Gratifyingly, this limitation can be overcome by using the ionic liquid (IL)ecoordinating method that we have proposed previously [8], which is crucial to broaden its applications in solar cells[9,10], optical data storage [11], and NIR Photography [12]. Other than

optoelectronic applications, it is worth to be mentioned that free-standing zwitterionic

p

-conjugated polymers film would create great potential on TE applications due to its intrinsic electrical conductivity, good thermal stability, and low thermal conductivity. Another approach that has been adopted widely to enhance the values of

s

of conjugated polymers has been doping with suitable agents[13e16]. Iodine, for example, can greatly improve the values of

s

by more than seven orders of magnitude[17]. In addition, car-bon nanotubes (CNTs) and mesoporous carcar-bons (e.g., CMK-3) have also been applied widely to enhance the values of

s

of polymers because of their own outstanding values of

s

and mechanical properties[14,18]. In addition, CMK-3 is a well-established material for nanoparticle dispersion because of its excellent specific surface area (ca. 1000 m2g1), nanoporosity (ca. 3 nm), and specific pore

volume (ca. 1 cc g1)[19]. Nevertheless, the potential of these ma-terials for dispersing doping agents has been overlooked until now. In this study, CMK-3 was adopted to enhance the value of

s

and, especially, the porosity for the separation of iodine in freestanding SQI0.1 films; single-walled carbon nanotubes (SWNTs) were also

integrated into freestanding SQI0.1films to enhance the values of

s

and theflexibility. Thorough investigation was carried out on the morphologies, properties, and their interrelationships. As the

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result, the SQI0.1-based compositefilms exhibiting good

thermo-electrical and mechanical properties can be obtained. Further study on structure design of compositefilms would probably improve the conductivity again[20].

2. Experimental section

2.1. Poly(bispyrrole-co-squaric acid) (SQ)

2,5-Bis(dodecyloxy)-1,4-dibenzyl phosphonate (BDCBP) was synthesized from 2,5-bis(dodecyloxy)-1,4-bis(bromomethyl)ben-zene and triethyl phosphate (97%, SigmaeAldrich) following a previously reported procedure [21]. Desired amounts of BDCBP,

squaric acid (99%, Acros), butanol, and benzene were then heated under reflux at 120C for 24 h, yielding polysquaraine from the

insoluble precipitated powder. In a previous report [8], the IL methyltrioctylammonium trifluoromethanesulfonate (99%, UR-MATOATS) was found to play a critical role in the formation of freestanding SQI0.1 films. In this study, a solution of SQI0.1

con-taining 0.1 wt% of this IL was used to prepareflexible, freestanding SQI0.1-basedfilms.

2.2. SBA-15 mesoporous silica and CMK-3 mesoporous carbon

SBA-15, the template of CMK-3, was prepared following the synthetic procedure reported by Zhao et al. [22]. Typically,

Fig. 2. SEM images of SQI0.1-basedfilms: (a) top-view image of SQ film coated on substrate (inset: irregular snipping removed from substrate), the cross-session of (b) SQI0.1,

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tetraethyl orthosilicate (TEOS;99%; SigmaeAldrich), hydrochloric acid (HCl; 37%), and the triblock copolymer P123 (PEO20-PPO70

-PEO20; Mn¼ 5800; SigmaeAldrich) were mixed at a desired ratio,

stirred at 40C for approximately 2 h, and then aged at 100C for 2 days. The solid product was thenfiltered off, washed with H2O, and

dried at 100C prior to calcination in air at 560C for 6 h. The resultant SBA-15 was then impregnated with the carbon source [D-(þ)-sucrose, 99þ%; Acros] following reported procedures

[23,24]. A viscous mixture of SBA-15, sucrose, H2O, and H2SO4

(weight ratios: 2:2.5:10:0.28) was ground, dried at 60C, and then dehydrated at 160C for 6 h; this procedure was repeated once using sucrose (1.6 g), H2O (6.4 g), and H2SO4(0.18). The obtained

mixture was carbonized under Ar at 900C for 1 h. After removing SBA-15 through HF(aq)etching, the CMK-3 powder wasfiltered off

and dried.

2.3. Specimen designation and post-treatment through iodine-doping

For the preparation of pristine/composite films, a desired amount of SWNTs (>90 wt.%, Cheap Tubes) or CMK-3 was sus-pended in the SQI0.1 solution through ultrasonication for 4e5 h

prior to casting on a glass substrate (2.5 2.5 cm2); after

evapo-ration of the solvent at 80C for 2 h, theflexible composite films wereflaked off from substrate in MeOH solution and dried under vacuum at 100C for 24 h.

The as-synthesized pristine/composite films were further doped with iodine using a previously reported procedure[25, 26]. In short, the pristine/compositefilms were placed in a sealed jar containing iodine crystals and heated for 2e24 h at 35e50 C. Finally, the residual iodine vapor was removed under vacuum for 1 h.

2.4. Characterization

Fourier-transform infrared (FTIR) spectra were recorded using a PerkinElmer Spectrum 100 spectrometer. Thermogravimetric analysis (TGA) was performed under N2 (99.99%) using a Q500

thermogravimetric analyzer operated at a heating rate of 15 C min1 (from room temperature to 700C). Powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed using a Bruker NanoStar SAXS system (Cu K

a

radiation) and a Thermo Scientific K-Alpha apparatus, respectively. The elec-trical conductivities (

s

) of the polymers were measured using a four-point probe electrical measurement analyzer (RT-80/RG-80, Napson). For Seebeck coefficient measurements, a microheater was applied to control the temperature difference across the polymer film. Two points along the heat flow direction were selected, and the corresponding temperatures were measured by calibrated thermocouples. The temperature difference (

D

T) between these two points could be derived. The Seebeck voltages (

D

V) were detected via using the data acquisition system (Keithley 2700) through a pair of thin Cu wires connected to the sample at same points as the junctions of thermocouples. The Seebeck coefficient (S¼

D

V/

D

T) could be acquired. The microstructures and morphol-ogies of the specimens were analyzed through transmission elec-tron microscopy (TEM), using a Cryo-HRTEM microscope (JEM-2010), and scanning electron microscopy (SEM), using JSM-6500 and JSM-6700 microscopes. The values of

k

of the pristine/com-positefilms were evaluated using the ASTM D5470 process and a CL-TIM-III thermal conductivity measurement instrument (Meg-tron Technical). Raman spectra were recorded in the range from 100 to 2000 cm1using a Horiba Jobin-Yvon Raman spectrometer (Ar laser; wavelength: 514.5 nm).

3. Results and discussion

3.1. Evolution and morphology of polysquaraine-basedfilms The evolution of the freestanding SQI0.1-basedfilms is presented

inFig. 1. Synthetic SQ-based polymers have, until recently[8], been prepared only in the form of powders (Fig. 1a)[5], and not in the

0

4

8

12

16

20

24

10

-5

10

-4

10

-3

10

-2

10

-1

10

0

10

1 35oC 40oC 50oC

Co

nductivity (S/cm)

Doped time (hr)

a

0

4

8

12

16

20

24

10

-3

10

-2

10

-1

10

0

10

1

10

2

Conductivity (S/cm)

Doped time (hr)

b

Fig. 3. The variation of electrical conductivity variation versus the iodine-doping conditions of SQI0.1-based polymerfilms: (a) SQI0.1film and (b) 20NT/5CMK/SQI0.1

compositefilm for iodine-doping at 35C.

Fig. 4. The variation trend of electrical conductivity versus the iodine mass fraction of SQI0.1films: ( ) pristine SQI0.1, and doped under 35C ( ), 40C ( ), and 50C ( ).

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form of freestanding films (Fig. 1b). On the other hand, the pro-posed polysquarainefilms exhibited great potential for TE appli-cations, due to the zwitterionic

p

-conjugated structure and intrinsically low band gap (Eg ¼ 0.8e1.1 eV) [5, 6]. To further

improve its potential TE applications, CMK-3 and SWNTs were in-tegrated into SQI0.1films at various ratios in this study. The

inte-gration of CMK-3 disruptedfilm formation, as revealed inFig. 1c, presumably because of the loose structure that existed after

exceeding the critical degree of porosity. In contrast, the integrated SWNTs (20 wt.%) enhanced the formation of freestanding films (Fig. 1d). Therefore, for subsequent film formation, additional SWNTs were applied to overcome the drawbacks of adding CMK-3. The optimal film, formed from 20 wt.% of SWNTs integrated together with 5 wt.% of CMK-3 in the SQI0.1film (denoted as 20NT/

5CMK/SQI0.1), exhibited metallic luster (Fig. 1e) and goodflexibility

(Fig. 1f).

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SEM images of the various materials inFig. 1are presented in Fig. 2. A top-view image of the SQI0.1film prior to its removal from

the substrate is provided inFig. 2a; once it had been removed from the substrate, only irregular/disintegrated snipping was obtained (see the inset). After the addition of the IL, the SEM image (Fig. 2b) reveals a rather smooth cross-session, result in a good freestanding film (Fig. 1b). The dispersion of CMK-3 (Fig. 2c) in the SQI0.1film was

not as good as that of the CNTs, leading to rather poorflexibility, consistent withFig. 1c. Although it is well-established that SWNTs tend to entangle with one another, the SWNTs were dispersed well in the polysquaraine matrix, as revealed inFig. 2d, presumably because of strong

pep

interactions between the SWNTs and the

p

-conjugated electrons of the SQI0.1main chain (vide infra: Raman

spectroscopic results) [27]. Well-dispersed SWNTs penetrating through thefilm appear in the cross-section of 20NT/5CMK/SQI0.1

(Fig. 2e); the CMK-3 units were bound andfixed by these SWNTs in the SQI0.1 matrix (Fig. 2f). Hence, good freestanding films were

observed inFig. 1e and f.

3.2. Effect of iodine-doping on morphology and electrical conductivity (

s

) of polysquaraine (SQI0.1)-basedfilm

To determine the effects of doping with iodine, the variations in the values of

s

of SQI0.1with respect to the doping conditions (35,

40, or 50C for 0e24 h) were investigated (Fig. 3a). The value of

s

of the iodine-doped SQI0.1 film increased rapidly from

2.589 105S cm1to approximately 101S cm1after the doping

temperature had been increased from 35 to 50 C. The highest values of

s

for each doping temperature varied depending on the doping time: 8.236  101 S cm1 after 6 h at 35 C,

8.816 101S cm1after 4 h at 40C, and 6.446 102S cm1after 2 h at 50C. The conclusion drawn from these enhanced values was that the charge carriers were incorporated well and positioned along with the

p

-bond conjugated main chain[28]. According to Zeng et al., over-doping of iodine in

p

-conjugated polymers would retain the value of

s

at the same level[29]. In case of the zwitter-ionic

p

-conjugated polymer reported herein, however, excessive amounts of iodine-doping led to decreased values of

s

, especially in the cases when doping was performed at temperatures below 50C (Fig. 3). This phenomenon has not been described previously; from the viewpoint of zwitterionic

p

-conjugation, an excessive amount of doped iodine might have provided a greater possibility of attracting the delocalization of negative charge along the backbone of the zwitterionic polymer. Finally, the stability exhibited by the iodine-doped SQI0.1 films prepared herein was excellent under

ambient conditions, even after exposure to the air for 14 days (Fig. S1). To determine the effect of the mass fraction of iodine, the relationship between the mass fraction of iodine and the value of

s

was examined (Fig. 4). The same trend was exhibited by the SQI0.1films that had been doped at different temperatures, with

the value of

s

having increased when the content of iodine was increased in the initial stage. Optimization of the value of

s

(8.816 101S cm1) occurred in the presence of 8e12 wt.% of

iodine. As a result, the highest value of

s

for the SQI0.1 system

appeared after doping at 35C for 6 h.

The effect of the iodine-doping time on the value of

s

for the 20NT/5CMK/SQI0.1compositefilm system was also examined. The

value of

s

of 20NT/5CMK/SQI0.1 increased dramatically (Fig. 3b)

from 7.63 103S cm1to 13.21 S cm1, thereafter reaching a

plateau, when the doping time was increased from 0 to 10 h (35C). This value was tenfold higher than that of the iodine-doped SQI0.1

film (cf. Fig. 3a). The better dispersion of iodine, as had been observed also through SEM analysis, appeared to be responsible for this behavior.

To determine the morphological effects of doping with iodine, SEM images of cross-sections of various SQI0.1-basedfilms were

recorded (Fig. 5). The grains of iodine were small (ca. 1

m

m) and uniform when the sample was doped at 35C for 6 h (Fig. 5a). The grains of iodine grew when either the doping time or temperature was increased (Fig. 5aef). Notably, however, no iodine crystals were evident in the SEM images (Fig. 5g and h) of the iodine-doped 20NT/5CMK/SQI0.1(35C for 6 h). The presence of iodine in the

SWNT/CMK-3/SQI0.1compositefilms was confirmed using Raman

spectroscopy. After doping at 35C, doping times of 2, 6, 12, and 24 h caused the G-band to shift from 1572 cm1to 1575, 1577, 1581, and 1584 cm1, respectively (Fig. 6). The intercalation of polyiodide into the CNTs [30,31] would be consistent with this up-shift; intercalation of polyiodide presumably occurred in the 20NT/ 5CMK/SQI0.1compositefilms. Furthermore, a broad band (ca. 100e

300 cm1) was featured in the Raman spectra after doping with iodine; this signal was attributed to the formation of charged I3and

400 800 1200 1600 2000 Raman Shift(cm-1) prinstine 1572 1575 I2-doped 2h 1577 I2-doped 6h 1581 I2-doped 12h 1584 Intensity (a.u.) I2-doped 24h

Fig. 6. Raman spectra of 20NT/5CMK/SQI0.1compositefilms doped under 35C for 0e24 h.

100

200

300

400

500

600

700

20

40

60

80

100

undoped

doped_2h

doped_6h

doped_24h

Weight (%)

Temperature (°C)

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I5polyiodide chains[30], the presence of which was also revealed

in XPS spectra (Fig. S4,Table S1). From the SEM images and Raman spectra, it was concluded that iodine was dispersed well in the SWNT/CMK-3/SQI0.1compositefilms, presumably because of the

great nanoporosity of CMK-3, which has been applied widely for nanodispersion in manyfields[19,32e34].

3.3. Influence of iodine-doping on stability and structure of polysquaraine (SQI0.1)

TGA was employed to determine the effect of iodine-doping on the stability of SQI0.1films that had been doped with iodine at 35C

for 0e24 h (Fig. 7). Good thermal stability was exhibited by the pristine SQI0.1film, with the decomposition of the polymer backbone

represented by a high decomposition onset temperature (385C)[8]. The weight losses of the iodine-doped SQI0.1films occurred at lower

temperatures, between 150 and 330C, attributable to the elimina-tion of free iodine and iodine centers[35]. For the SQI0.1films doped

for 2, 6, and 24 h, the weight losses within the range 150e330C were 8.31, 12.33, and 16.68 wt.% respectively; these values are approxi-mately equal to the mass fractions of doped iodine. Leaving the ex-istence of iodine aside, the weight loss from backbone decomposition between 330 and 520 C decreased when the doping time was increased from 2 to 6 to 24 h (62.14, 59.26, and 57.15 wt.%, respec-tively), hinting that the stability of the zwitterionic

p

-conjugated polymer SQI0.1was enhanced after doping with iodine.

To obtain more detailed understanding of the TGA data, TGA derivative curves of SQI0.1films doped at 35, 40, and 50C were

recorded (Fig. 8); for simplicity, only the curves of the samples treated at optimized doping times (i.e., 6 h at 35C, 4 h at 40C, and 2 h at 50C; refer toFig. 3a) are displayed, as well as those of the corresponding specimens doped for 24 h. Two major peaks and one minor peak appeared in each of the derivative curves. The major peaks were attributable to the elimination of iodine (between 225 and 265C) and the decomposition of the polymer backbone (be-tween 417 and 426C). The minor peak was evident in the trace of each of the iodine-doped SQI0.1 films (between 600 and 650C;

Fig. 8), but not in that of the pristine SQI0.1film (refer toFig. S2). The

existence of polyiodide intercalated in SQI0.1 is suggested by

this feature; similar phenomena have been reported previously [30,31,35]. Using XPS, the states of the intercalated polyiodide were characterized (Fig. S3); the I3d5/2spectra of the iodine-doped SQI0.1

films prepared with different doping levels could be resolved into two peaks near 618 and 620 eV, which were attributable to the I3

and I5anions, respectively. For simplicity, only the ratios of I3and

I5for these specimens have been listed inTable 1. As the doping

time increased, the iodine mass fraction increased, accompanied by an increase in the I5/I3ratio. The same trend appeared for the

iodine-doped 20NT/5CMK/SQI0.1 films (Table S1). A similar

phe-nomenon has been observed in iodine-doped polyaniline [29]. These data also confirm that the small signals in the TGA derivative curves near 650C (Fig. 8) represent the existence of polyiodide intercalated in SQI0.1.

To further understand the effect of polyiodide on the interchain packing structure, the XRD patterns of the SQI0.1films that had been

doped at 35C were recorded. Feature peaks at values of 2

q

of approximately 3.7, 19.6, and 26appear inFig. 9, corresponding to d-spacings of 23.9, 4.5, and 3.4 A, which are associated with the interchain, interlayer, and interlayer

p

-stacking packing distances, respectively, of SQI0.1[8]. These three diffraction peaks were also

present in the XRD patterns of all of the iodine-doped SQI0.1

poly-merfilms, but with values of 2qthat deviated slightly from those of the pristine film. In particular, the interchain packing distance increased pronouncedly from approximately 24 A for the pristine

100 200 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8

Temperature (°C) Temperature (°C) Temperature (°C)

Deriv. Weight ( %/ C ) 6h 24h

a

100 200 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 Deriv. Weight (%/ C ) 4h 24h

b

100 200 300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 Deriv. Weight (%/ C ) 2h 24h

c

Fig. 8. Thefirst derivative TGA curves of iodine-doped SQI0.1films doped under (a) 35C, (b) 40C and (c) 50C.

Table 1

Deconvolution data from I3d5=2XPS spectra of SQI0.1films doped with iodine at 35

C

for 0e24 h.

I2-doped SQI0.1 Binding energy (eV) Percentage of total area (%)

2 h 618.28 59.5 620.38 40.5 6 h 618.38 56.4 620.08 43.6 12 h 618.28 55.5 620.08 44.5 24 h 618.48 51.9 620.28 48.1 5 10 15 20 25 30

Normalized Intensity (a.u.)

3.5 Å 24h 4.5 Å 2 (degree) 25.7 Å 4.6 Å 6h 25.7 Å 3.5 Å 3.5 Å 4.6 Å 24.6 Å 26.9 Å 4.6 Å 4h 2h pristine 23.9 Å 3.4 Å 3.5 Å 4.5 Å

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film to approximately 27 A after doping with iodine. Similar phe-nomena have been observed previously for the intercalation of iodine influencing the interchain packing distance [29]. Taken together, strong evidence is provided by these results for the ex-istence of iodine in the chain structure of the polymer SQI0.1.

3.4.

pep

Interactions between carbon materials and polysquaraine (SQI0.1)-basedfilms

To examine the existence of

pep

interactions between SQI0.1

-basedfilms and the carbon materials, Raman spectra of the SQI0.1

film, SWNT, CMK-3, 5CMK/SQI0.1, 20NT/SQI0.1, and 20NT/5CMK/

SQI0.1were recorded (Fig. 10). Weak signals for CeOeC vibration of

the polymeric side chain (936 cm1), CeC single bond stretching (1371 cm1), and CeC stretching of the benzenoid ring (1595 cm1) appear in the Raman spectrum of the SQI0.1 film (Fig. 10a). In

addition, the G-band (1572 cm1) of the 20NT/5CMK/SQI0.1

com-positefilm had shifted to lower frequency relative to those of the SWNTs (1579 cm1;Fig. 10b) and CMK-3 (1598 cm1;Fig. 10c).

pep

Interactions between the SWNTs and the

p

-conjugated polymer were presumably responsible for this phenomenon[27,36,37]. In this present system, therefore,

pep

interactions were suggested by the shift to exist between the SQI0.1matrix and the carbon

mate-rials (i.e., SWNTs and CMK-3); these interactions were responsible for these carbon materials dispersing well in the SQI0.1matrix.

3.5. Thermoelectric properties of polysquaraine (SQI0.1)-basedfilms

The TE parameters of our SQI0.1-basedfilms are listed inTable 2;

the Seebeck coefficient (S) of the pristine SQI0.1film (78

m

V K1) was

higher than those of other conjugated polymers [3,38,39]. In addition, the pristine SQI0.1film was a p-type TE material, as

indi-cated by the positive Seebeck voltage. According to the definition of ZT, large

s

/

k

ratios are required for good TE materials. An increase in

the value of

s

has, however, always been accompanied previously by an increase in the value of

k

; this behavior can be explained by the WiedmanneFranz law [40]. Gratifyingly, this rule was not obeyed by the SQI0.1film developed herein when the value of

s

was

increased after doping with iodine. The values of

s

and

k

were improved (from 105to 101S cm1) and decreased (from 0.33 to 0.13 W mK1), respectively, after doping with iodine (until 6 h; Table 2). This special property was presumably derived from phonon scattering from the microstructure of the iodine-doped SQI0.1film (vide supra; SEM), the intercalation process (vide supra;

TGA), and the variation of the interchain packing distance (vide supra; XRD), all of which led to lower overall values of

k

. Accord-ingly, these materials appear to have great potential in TE appli-cations. A suitable level of iodine-doping can simultaneously improve the value of

s

significantly, enhance the value of S, and decrease the value of

k

. For example, for the sample prepared under the optimized conditions (35 C for 6 h), a power factor of 1.475

m

W mK2and a value of ZT of 3.175 103were obtained. Accordingly, these freestanding polysquaraine films have great potential for application inflexible thermoelectric devices. Finally, due to the

pep

interaction between SQI0.1 matrix and carbon

materials, SWNTs and CMK-3 can greatly improve theflexibility of the compositefilm (Fig. 1) and the dispersion of the doped iodine (Fig. 5), respectively. Both are responsible for the enhancement of electrical conductivity. As the result, TE properties of the 20NT/ 5CMK/SQI0.1films doped for 10 h revealed a noticeable value of ZT

of 4.563 103, which is 143% of the optimized iodine-doped SQI0.1

film, and 87% greater than that of the same sample that had been doped for 6 h.

4. Conclusions

A series of novel polysquaraine (SQI0.1)-based films has been

prepared and characterized; doping with iodine and integration of Table 2

Sample designations and the corresponding thermoelectric properties.

Sample I2-treatment SWNT/CMK-3 (wt%) s(S cm1) S (mV K1) P (mW mK2) k (W mK1) ZT (at 300 K)

A d* d 2.589 10-5 78.14 1.581 10-5 0.325 1.458 10-8 B-6 35C, 6 h d 8.236 10-1 133.84 1.475 0.139 3.175 10-3 B-24 35C, 24 h d 1.501 10-1 105.69 1.677 10-1 0.129 3.890 10-4 C-4 40C, 4 h d 8.816 10-1 90.13 7.162 10-1 0.239 8.967 10-4 C-24 40C, 24 h d 1.089 10-1 61.34 4.097 10-2 0.169 7.265 10-5 D-2 50C, 2 h d 6.446 10-2 104.12 6.988 10-2 0.158 1.325 10-4 D-24 50C, 24 h d 4.940 10-5 95.17 4.474 10-5 0.153 8.762 10-8 E d 20/0 3.586 10-2 70.53 1.784 10-2 0.351 1.523 10-5 F d 20/5 7.631 10-3 66.58 3.383 10-3 0.390 2.603 10-6 E-6 35C, 6 h 20/0 5.264 43.45 9.939 10-1 0.214 1.394 10-3 F-6 35C, 6 h 20/5 7.162 63.16 2.86 0.351 2.444 10-3 F-10 35C, 10 h 20/5 13.214 60.74 4.87 0.321 4.563 10"3 *Pristine SQI0.1. 500 750 1000 1250 1500 1750 2000 936 1371 1595 Intensity (a.u.) SQI

a

1341 1572 Raman Shift (cm ) 20NT/5CMK/SQI 1450 1500 1550 1600 1650 1700 1750 15751579 1595 SQI Raman Shift (cm ) 20NT/SQI Intensity (a.u.) SWNT

b

1450 1500 1550 1600 1650 1700 1750 15931598 SQI

c

1595 CMK-3 Intensity (a.u.) Raman Shift (cm ) 20CMK/SQI

(9)

SWNTs and CMK-3 enhanced the potential of thesefilms for TE applications. The electrical conductivities (

s

) of the SQI0.1 films

varied depending on the degree of iodine-doping, being optimized (8.816 101S cm1) in the presence of 8e12 wt.% of iodine. In the case of the 20NT/5CMK/SQI0.1film, doping with iodine provided an

excellent value of

s

of 13.21 S cm1. According to TGA, XPS, and XRD analyses, the stability of the zwitterionic

p

-conjugated polymer SQI0.1improved after doping with iodine, with the resulting

inter-calated polyiodide influencing the interchain packing distance of the polymer. Signals for

pep

interactions between the carbon materials and the polymer SQI0.1, a possible explanation for the

high dispersion of the SWNTs and CMK-3 in the SQI0.1matrix, were

evident in Raman spectra. Values of ZT of 2.603  106 and 4.563  103, suitable for TE applications, were obtained after integration of carbon materials (i.e., 20 wt.% SWNTs and 5 wt.% CMK-3) and further doping with iodine, respectively. For the iodine-doped 20NT/5CMK/SQI0.1film, the optimized value of ZT of

4.563 103was 143% of the optimized iodine-doped SQI 0.1film,

revealing the great improvements that can result from increasing the doping time together with the integration of SWNT and porous carbon in SQI0.1. The excellent dispersion of SWNTs in SQI0.1

pro-vides for other application opportunities.

Acknowledgment

We thank the National Science Council of the Republic of China, Taiwan, for supporting in this research financially under grants NSC100-2221-E009-023-MY3 and NSC 99-2221-E009-010-MY3 (W.-T. Whang) and 101-2218-E-035-005 (A.-Y. Lo).

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.matchemphys.2013.06.024.

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數據

Fig. 1. Photographs of specimens: (a) SQ powder, (b) SQI 0.1 , (c) 5CMK/SQI 0.1 , (d) 20NT/SQI 0.1 , and (e, f) 20NT/5CMK/SQI 0.1 .
Fig. 2. SEM images of SQI 0.1 -based films: (a) top-view image of SQ film coated on substrate (inset: irregular snipping removed from substrate), the cross-session of (b) SQI 0.1 ,
Fig. 3. The variation of electrical conductivity variation versus the iodine-doping conditions of SQI 0.1 -based polymer films: (a) SQI 0.1 film and (b) 20NT/5CMK/SQI 0.1
Fig. 5. Cross-section SEM images of iodine-doped (aef) SQI 0.1 films, and (g, h) 20NT/5CMK/SQI 0.1 film.
+4

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