Electro-Responsively Reversible Transition of Polythiophene Films from Superhydrophobicity to Superhydrophilicity Lianyi Xu, Qiang Ye, Xuemin Lu, and Qinghua Lu* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China * S Supporting Information ABSTRACT: An electro-responsively reversible switching of wettability between superhydrophobicity and superhydrophi- licity has been obtained from a highly porous structured polythiophene film. The polythiophene film was prepared by two-step electrochemical deposition on an indium tin oxide (ITO) substrate. The underlying poly(3,4-ethylenedioxythio- phene) (PEDOT) provides a highly porous structured conductive support, and poly(3-methylthiophene) (P(3- MTH)) deposited thereon plays the role of a low-surface- energy conductive coating. The wettability switching of this double-layer film between superhydrophobicity and superhydrophilicity has been investigated by doping and dedoping in an electrolyte solution containing ClO 4 - . Electrochromism of the film was also seen to accompany the electrochemical process of conversion between the two superwetting states. On the basis of this porous electro-active film, an in situ electro-wetting device was also demonstrated. KEYWORDS: polythiophene film, wettability transition, superhydrophobicity, superhydrophilicity, surface chemistry 1. INTRODUCTION Control of surface wettability is of considerable technological and scientific importance, with applications in self-cleaning surfaces, 1-7 antifogging surfaces, 8,9 microfluid systems, 10,11 and biotechnology. 12 Surface wettability is usually governed by the surface geometry and chemical composition. 1,5,13 Smart surfaces with reversibly switchable wettability have received special attention due to their vital importance in many fields. Such surfaces may be constructed by introducing a stimuli- responsive material into an appropriately rough structure, 14-19 and the reversible switching feature can be realized with the help of an external stimulus such as light irradiation, 20-22 temperature, 18 pH, 23 solvent, 19,24 or electrical potential. 25-28 Among stimuli-responsive materials, π-conjugated polymers are typical electrochemical-responsive materials and can be easily prepared. 17,27,29,30 In particular, for π-conjugated polymers that can be doped (oxidized) and dedoped (reduced) by applying a voltage in an electrolyte solution, these processes are accompanied by counterions (dopants) moving into and out of the film. This may result in a conversion of the wetting properties of the π-conjugated polymer. Yan et al. employed perfluorooctanesulfonate to dope polypyrrole (PPy) film, which resulted in a change in surface wetting from superhydrophilicity to superhydrophobicity. 17 Advincula et al. electrochemically polymerized a polythiophene derivative, poly(G0-3T COOR), as a surface layer on polystyrene (PS) nanoparticles, and found that the wetting could be switched by doping and dedoping of tetrabutylammonium hexafluorophosphate. 27 Although highly porous structures of π-conjugated polymer films were successfully achieved in these two studies, which is necessary not only for the passage of counterions but also for the amplification of surface wettability properties, 17,31-36 the processes are complicated and additional substances need to be added. This may cause a reduction in the transparency of the film or a weakening of the adhesion between the polymer layer and conductive substrate. For example, in previous work, 17 Fe 3+ was included as catalyst in the plating solution, and coupling of electropolymerization and Fe 3+ -catalyzed chemical polymer- ization promoted the growth of a porous, rough PPy film. In the second study, 27 the latex assembly of PS nanoparticles on a flat conducting substrate was first carried out by an LB-like technique, and then the polythiophene derivative was electro- deposited to form a rough polymer surface. The required roughness of the PS nanoparticles may have reduced the conductivity of the electrochemically deposited film. Therefore, such electro-active π-conjugated polythiophene films, which are easy to prepare on a large scale, have remained unexplored for wetting switching application. In our earlier study, an electro-responsive double-layer polythiophene film was prepared for water-droplet adhesion switching reversibility between sliding and pinned super- hydrophobic states. 37 This preparation process did not need any additive for the creation of a rough surface and low surface energy. Poly(3,4-ethylenedioxythiophene) (PEDOT) was directly electrodeposited on an indium tin oxide (ITO)-coated glass electrode to provide a porous conducting substrate. On Received: July 4, 2014 Accepted: August 12, 2014 Published: August 12, 2014 Research Article www.acsami.org © 2014 American Chemical Society 14736 dx.doi.org/10.1021/am5043627 | ACS Appl. Mater. Interfaces 2014, 6, 14736-14743