Template-Free Growth of Vertical Poly(3,4-ethylenedioxythiophene) Fiber Array By Alternating Current-Bipolar Electrolysis
Authors: Zhou Yaqian , Shida Naoki , Koizumi Yuki , Watanabe Tempei , Nishiyama Hiroki , Tomita Ikuyoshi , Inagi Shinsuke 
Published: December 2020

Conducting polymers (CPs) play an important role in the field of organic device. Electropolymerization is a very common technique for obtaining CPs, which usually gives a CP film with granular or particulate morphology on the electrode. On the other hand, CP fibers are also of great interest due to their unique properties such as efficient anisotropic carrier mobility. The electropolymerization with a template-modified electrode is regarded as one of the most promising strategies to obtain a CP fiber array. However, the attachment and removal of templates require additional working processes, and the use of a template may hinder other types of surface modification on the substrates. Therefore, a facile way to prepare CP fiber arrays without a template remains a challenging problem.

The use of an electric field as a guide for polymer formation is deemed to be efficient to grow CP fibers in a specific direction. We have pioneered the electric field-guided growth of poly(3,4-ethylenedioxythiophene) (PEDOT) fibers. In that system, dendritic PEDOT fibers grew from the terminal of Au wires that were used as bipolar electrodes (BPEs) by the electropolymerization of 3,4-ethylenedioxythiophene (EDOT) under an alternating current (AC).1

Here, we introduce the next progress of a successful template-free fabrication of PEDOT fiber arrays from a flat electrode surface using BPEs under an AC condition (Figure 1).2 An acetonitrile solution containing 50 mM EDOT, 5 mM 1,4-benzoquinone (BQ) and 1 mM Bu4NClO4 was used as an electrolyte. A cylindrical setup with driving electrodes was used here, and a conductive plate acted as a BPE. When an iterative potential was applied to the BPE, the circular anodic or cathodic area appeared alternatively underneath the region covered by the cylinder, with a surrounding cathodic or anodic area simultaneously. A potential difference generated on the BPE surface (Delta V
BPE) under an external electric field offers redox reactions on the BPE. When Delta
V
BPE exceeds Delta
V
min (1.82 V), the oxidation of EDOT proceeds in the anodic area, while the sacrificial reduction of benzoquinone occurs in the cathodic area.

Vertical fiber arrays of PEDOT and its derivatives were successfully grown from the central area of various BPE surfaces by the AC-bipolar electrolysis. The growth of fiber array follows a particle-induced growth mechanism. First, anodic oxidation of EDOT gives a PEDOT film on the BPE surface when the voltage between driving electrodes generates a sufficient Delta
V
BPE. The film growth proceeds with Stranski-Krastanov (S-K) mode as an induction period to give several particles. Once particles are formed, the deposited particles are considered as nodes to initiate subsequent PEDOT fiber growth.

Experimental conditions significantly affected the formation of fiber arrays. Higher applied voltages and shorter gap distances between the BPE and the cylinder were beneficial to form fiber arrays, which was supported by the computational simulation using COMSOL Multiphysics. Higher monomer concentration and lower applied frequency were also in favor of fiber array formation. The choice of solvents and electrolytes also influenced morphology of the fiber arrays. In addition, linear fibers were easily fabricated using a cylinder with a smaller diameter, while branching fibers formed with a larger diameter. This new methodology provides an extremely simple and straightforward way to fabricate uniform fiber arrays without the use of template, which are potentially applicable for sensors and electronic devices.

References

(1) Y. Koizumi, N. Shida, M. Ohira, H. Nishiyama, I. Tomita, S. Inagi, Nat. Commun.,
2016,
7, 10404.

(2) Y. Zhou, N. Shida, Y. Koizumi, T. Watanabe, H. Nishiyama, I. Tomita, S. Inagi, J. Mater. Chem. C,
2019,
7, 14745.

Figure 1