(This is a sample cover image for this issue. The actual cover is not yet available at this time.)

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

Author's personal copy

Synthetic Metals 163 (2013) 42– 46

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h o me pag e: www.elsev ier .com/ locate /synmet

Short Communication

Facile synthesis of high-aspect-ratio PEDOT tube arrays with ultra hydrophilicproperties

Qiang Zhao, Yan Sun, Guixin Wang ∗, Wei Wu, Chunhui Luo, Kangping Yan, Houfang LuCollege of Chemical Engineering, Sichuan University, Chengdu 610065, China

a r t i c l e i n f o

Article history:Received 4 June 2012Received in revised form 4 December 2012Accepted 11 December 2012

Keywords:PEDOTETA templateHigh gravityTube arraysUltra-hydrophilic properties

a b s t r a c t

Highly ordered poly(3,4-ethylenedioxythiophene) (PEDOT) tube arrays with hierarchical structure andhigh aspect ratio were synthesized facilely by an in situ high gravity polymerization in soft Etched Tun-nel Aluminum (ETA) metal templates at room temperature. The morphology, microstructure, molecluarstructure and interface properties were evaluated by various techniques. The tube length, tube density,and aspect ratio can reach ∼43 �m, ∼107/cm2, and ∼30.7, respectively. The PEDOT arrays synthesizedwith the feasible method exhibit excellent ultra-hydrophilic properties, and the contact angle of wateron the arrays is 1–2◦ in short spreading time of less than 3s, which is very promising for improving theelcetrochemical performance of PEDOT arrays in aqueous electrolytes.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

As an important �-conjugated electronic material with smartproperties (redox chemistry, high stability, acceptable conductiv-ity, etc.), poly(3,4-ethylenedioxythiophene) (PEDOT) has receivedmuch attention for various applications, including solar cells [1],supercapacitors [2], lithium-ion batteries [3], sensors [4], elec-trochromic devices [5], and catalysts for oxygen reduction [6] orhydrogen generation [7]. As many other inherent conductive poly-mers with rigid �-conjugated bond conformation [5], PEDOT isdifficult to be processed into thin films, fibrils, or tubes [2,8,9].Therefore, different methods have been explored to synthesizePEDOT with specific morphology. PEDOT arrays were synthesizedusing ZnO arrays [10] or anodic aluminum oxide (AAO) [2] tem-plates, but the tube length is below 20 �m and the synthesis processis complex. In addition, the scale of the reported arrays is not large,and the templates have poor toughness.

High gravity field is able to greatly intensify multiphase masstransfer and micromixing by enhancing the shearing force to formthin films, threads, or tinier droplets, and a rotating packed bed(RPB) can gain such ends, so using high gravity to synthesize mate-rials is very promising [11,12]. Compared to AAO [2,9] templates,ETA templates has many advantages such as good strength andtoughness, conductivity, large area, and high thickness. Herein, the

∗ Corresponding author. Tel.: +86 28 85406192; fax: +86 28 85406192.E-mail addresses: guixin66@scu.edu.cn, guixin68@yahoo.com (G. Wang).

synthesis and characterization of PEDOT tube arrays via an in situhigh-gravity polymerization in soft ETA templates are described.

2. Experimental

ETA templates were prepared by the method of galvanostaticelectrochemical etching. The utilized aluminum foils (99.999 wt.%,∼110 �m in thickness, provided by Jiangsu United TechnologyGroup, China) were fully annealed to obtain a high cubic texture[13]. In a two-electrode compartment, the aluminum foil and highpurity graphite were used as the working electrode and the counterelectrode, respectively. The aluminum was etched in a mixed acidof H2SO4 and HCl (3:1 in mol) at ∼80 ◦C with a current density of0.2 A cm−2, and thereafter rinsed repeatedly with high purity water(18 M� cm of specific resistance) to obtain ETA templates [13].

PEDOT arrays were synthesized using the ETA templates. Thesolution of 0.5 mL EDOT monomer and 10 mL isopropyl alcohol wasadded dropwise to the solution of 10 mL Fe(III) p-toluene-sulfonate(FeTos, 55 wt.%) n-butanol and 5 mL tetrahydrofuran under contin-uous stirring conditions. After being preheated at ∼70 ◦C for 10 min,the precursor solution was transferred to the RPB bottom-inner-lined with ETA template. The RPB was rotated at room temperaturewith a rotating speed of ∼1370 rpm (the high-gravity level was cal-culated to be ∼1960 m s−2) for 10 min. The template covered withblue black materials was dried at ∼70 ◦C in air for 4–7 h, and thenenwrapped by epoxy resin. The aluminum template was dissolvedin a 0.5 mol L−1 NaOH solution at room temperature. After beingcleaned repeatedly with high purity water, PEDOT arrays wereobtained and characterized.

0379-6779/$ – see front matter © 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.synthmet.2012.12.011

Author's personal copy

Q. Zhao et al. / Synthetic Metals 163 (2013) 42– 46 43

Fig. 1. Schematic diagram of the synthetic route for PEDOT tube arrays via in situ high-gravity polymerization in an ETA template.

The morphology and tube size were observed using a field emis-sion scanning electron microscope (FESEM, FEI INSPECT F), andthe elemental composition was analyzed on an energy-dispersiveX-ray spectrometry (EDX, INCA PENTA FET×3, OXFORD Instru-ments). The chemical structure and electronic properties werecharacterized by FTIR spectrum and UV–vis absorption spectro-graph. The FTIR and UV–vis measurements were carried out on aThermo Nocolet 370 FT-IR spectrometer and a GBC Cintra 20 spec-trophotometer, respectively. The ultra-hydrophilic properties wereevaluated with a water contact angle measurement [14].

3. Results and discussion

To synthesize PEDOT arrays, high-gravity field was used tointensify the entry of precursor into the template and the in situpolymerization in the template at room temperature, and thescheme is presented in Fig. 1. Aluminum was etched to obtainnumerous parallel straight tunnel pores, and was inner-lined tothe bottom of a RPB. The precursor solution of EDOT and FeToswas mixed thoroughly and preheated at ∼70 ◦C to enhance theionic diffusion and initiation reaction. The preheated precursor wastransferred to the RPB for obtaining a high-gravity field. Such pre-cursor has difficulty in entering the pores of ETA template becauseof large surface tension, so high gravity field with strong centrifu-gal effect was adopted to push the precursor to enter the poresof template. At the same time, high-gravity field will strengthenthe polymerization reaction [11,12]. Theoretically, the polymeriza-tion reaction will start when the EDOT reductant and the FeTosoxidant meet together. However, the polymerization reaction ratedepends on the reaction conditions such as reaction tempera-ture, reaction environment, and reactant concentration. In theexperiments, the precursor can exist stably over 24 h at room tem-perature, but the time is greatly shortened at 70 ◦C. After beingpreheated at 70 ◦C for 10 min, the precursor is still a yellow solution

and fluid, indicating the polymerization reaction goes very lit-tle because the color is close to that of the oxidant and PEDOTis a black solid. On the other hand, large polymer particles can-not enter the pores of the template. Thus, the polymerizationof EDOT is initiated when the precursor is preheated at ∼70 ◦C,but the main polymerization of EDOT goes in the template underhigh-gravity field. Compared with the experiment without high-gravity field, the reaction time using high-gravity field to obtain asame status was decreased from ∼36 to ∼10 min under the sameconditions. Therefore, high-gravity field will shorten the polymer-ization time besides pushing the precursor into the template. Afterreaction under the high-gravity field, some blue black materialsappear at the template surface. Via sequential treatments of drying,enwrapping, and dissolving, the sample was cleaned and character-ized.

According to the ETA morphology shown in Fig. 2(a) and (b),great amounts of parallel tunnels with diameter ranging from 1.1to 1.5 �m and length of 40–45 �m are vertical to the aluminumsurface. Such template can be prepared in a large area of more than0.5 m × 1.0 m and rolled without being broken, which is impos-sible for AAO template. The SEM gradient view and cross-sectionmicrographs in Fig. 2(c–g) show that the sample exhibits highlyordered arrays with hierarchical structure, and the arrays consistof many parallel free-standing tubes which are vertical to theepoxy substrate, like a dense oak forest. Based on the statisticalresult from Fig. 2(c) and (d), the tube density is estimated to be∼107/cm2. According to Fig. 2(e) and (g), the tubes are hollow,and the wall thickness ranges from 100 to 200 nm, which canbe adjusted by controlling the precursor concentration, rotatingspeed and time. The average length and outer diameter are about43 and 1.4 �m, respectively, so the aspect ratio is ∼30.7, which ismuch higher than that of the reported PEDOT fibers [9], or tubes[2,10]. Furthermore, few transverse tubes are from the transversetunnels formed in the etching process of ETA template, which can

Author's personal copy

44 Q. Zhao et al. / Synthetic Metals 163 (2013) 42– 46

Fig. 2. FESEM images of the ETA template (a and b), and PEDOT arrays (gradient part (c and d), cross section (e), open tip of the tubes (f and g)), and EDX spectrum (h) of thereferred yellow line of the tubes in (g).

be adjusted by changing the etching conditions of aluminum. TheEDX spectrum of the selected section of the tubes from inner toouter (Fig. 2(g) and (h)) indicates the tubes contain elements C,O and S, which contents are almost unchanged. The atomic ratioof C:O:S is nearly 6:2:1, consistent with the empirical formula ofPEDOT. The white dots on the surface of tubes are Au particles toimprove the sample conductivity during SEM observation. Underthe high gravity field, part of the precursor solution will enter thetunnel pores of ETA template. Because the pore bottom is not open,

the entered solution will become rich at the bottom and cover theinner surface of the pores under the function of high centrifugalforce. By controlling the reactant concentration and reaction time,the entered precursor is not enough to completely fill up thepores. Consequently, the combination of strong centrifugal effect,polymerization of EDOT at the inner surface and surface shrinkageof PEDOT originating from solvent volatilization will lead to theformation of tubes with hollow structure in the template. Afterremoving the ETA template, PEDOT tube arrays will be obtained.

Author's personal copy

Q. Zhao et al. / Synthetic Metals 163 (2013) 42– 46 45

400 500 600 700 800 900 1000 1100

(b)

Ab

sorb

an

ce (

a.u

)

Wavelength (nm)

2800 2400 2000 1600 1200 800 400

29

20

.0

Tra

nsm

itta

nce (

%)

Wavenumber (cm-1

)

16

45

.4

13

92

.51

35

6.2

1189.0

11

25

.61

03

8.6 10

09

.7

81

3.1

68

2.4

56

7.9

(a)

Fig. 3. FTIR (a), and UV–vis spectra (b) of the PEDOT arrays.

Fig. 4. The images and contact angles of water droplets on the arrays.

From the FTIR result (Fig. 3(a)), typical peaks of PEDOT andstrong electronic absorption in the IR region are presented,showing weak characteristic CH2 stretching of dioxyethy-lene bridge (2990–2850 cm−1), strong doped level (1645.4 cm−1),C˛ Cˇ (1525–1475 cm−1), C˛ Cˇ (1392.5–1356.2 cm−1), alkylene-dioxy group (1200–1038.6 cm−1), PEDOT skeleton (1089 cm−1),C S (980–567.9 cm−1) and characteristic bands [10,15–17]. Thesample is blue black, referring to the color of reduced PEDOT[5], which is confirmed by the UV–vis spectrum (Fig. 3(b)).The absorption peak at ∼860 nm, ascribed to a little IR-shift, suggests the PEDOT is in an n-doped (reduced) state[16,17].

As one of the most important properties of the surface, thewettability is significant for improving performance by reducinginterface resistance. The contact angle of water on the arrays is1–2◦ (Fig. 4) in short spreading time of less than 3 s, indicating thePEDOT arrays exhibit good superhydrophilic properties and can beeasily wetted in aqueous solution. The interface behavior allowssuch PEDOT tube arrays for good electrochemical performance inaqueous solutions.

4. Conclusions

A facile and effective route has been developed to synthesizePEDOT nanowalled tube arrays by in situ high-gravity polymer-ization in soft ETA metal templates. High-gravity field is used tomake the precursor enter the tunnel pores, strengthen the poly-merization reaction, and shorten the polymerization time at roomtemperature. The tubes of the as-synthesized PEDOT arrays aresmooth and compact. The tube length and aspect ratio are ∼43 �mand ∼30.7, respectively. The arrays exhibit excellent superhy-drophilic properties, and the contact angle of water on the arraysis 1–2◦ in short spreading time of less than 3 s. The detailed influ-ence of high gravity field on the synthesis and the other electricalparameters like conductivity, charge transfer and diffusion coeffi-cient of the obtained polymeric material are under investigation.The initial results indicate that the novel method is greatly promis-ing for the production of long-tube arrays with specific morphologyand good hydrophilic properties, which allows such tube arrays forthe possible applications such as energy storage and conversion,high-sensitive electrochemical ion sensing, and electrocatalysis.

Author's personal copy

46 Q. Zhao et al. / Synthetic Metals 163 (2013) 42– 46

Acknowledgements

The authors gratefully acknowledge financial support by theNational Science Foundation (grant no. 21206099), Youth Foun-dation (07046) and helps from the Analytical & Testing Center ofSichuan University.

References

[1] X. Wang, T. Ishwara, W. Gong, M. Campoy-Quiles, J. Nelson, D. Bradley,Advanced Functional Materials 22 (2012) 1454–1460.

[2] R. Liu, S.I. Cho, S.B. Lee, Nanotechnology 19 (2008) 215710.[3] S. Kim, S. Sen, H. Song, G. Palmore, Electrochemistry Communications 12 (2010)

761–764.[4] N. Pinto, D. Rivera, A. Melendez, I. Ramos, J.H. Lim, A. Johnson, Sensors and

Actuators B 156 (2011) 849–853.[5] J. Kawahara, P. Ersman, I. Engquist, M. Berggren, Organic Electronics 13 (2012)

469–474.[6] B. Winther-Jensen, O. Winther-Jensen, M. Forsyth, D. MacFarlane, Science 321

(2008) 671–674.

[7] B. Winther-Jensen, K. Fraser, C. Ong, M. Forsyth, D.R. MacFarlane, AdvancedMaterials 22 (2010) 1727–1730.

[8] X. Zhang, J. Lee, G. Lee, D. Cha, M. Kim, D. Yang, S. Manohar, Macromolecules39 (2006) 470–472.

[9] Y. Shirai, S. Takami, S. Lasmono, H. Iwai, T. Chikyow, Y.J. Wakayama, PolymerScience Part A: Polymer Physics 49 (2011) 1762–1768.

[10] M. Döbbelin, R.T. Zaera, P.M. Carrasco, J.R. Sarasua, G. Cabanero, D.J.Mecerreyes, Polymer Science Part A: Polymer Chemistry 48 (2010)4648–4653.

[11] A.J. Nathanael, S.I. Hong, D. Mangalaraj, P.C. Chen, Chemical Engineering Journal173 (2011) 846–854.

[12] X.W. Lu, W. Wu, J.F. Chen, P.Y. Zhang, Y.B. Zhao, Industrial Engineering andChemistry Research 50 (2011) 5589–5595.

[13] R. Xiao, K. Yan, Corrosion Science 50 (2008) 3256–3260.[14] V.A. Dhumale, P.V. Shah, I.S. Mulla, R.B. Sharma, Applied Surface Science 256

(2010) 4192–4195.[15] D.K. Taggart, Y. Yang, S.-C. Kung, T.M. Mcintire, R.M. Penner, Nano Letters 11

(2011) 125–131.[16] H. Mao, X. Lu, X. Liu, J. Tang, C. Wang, W.J. Zhang, Journal of Physical Chemistry

C 113 (2009) 9465–9472.[17] H. Meng, D.F. Perepichka, M. Bendikov, F. Wudl, G.Z. Pan, W. Yu, et al., Journal

of the American Chemical Society 125 (2003) 15151–15162.