Article pubs.acs.org/Langmuir
Electrically Conductive PEDOT Coating with Self-Healing Superhydrophobicity Dandan Zhu, Xuemin Lu,* and Qinghua Lu* School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: A self-healing electrically conductive superhydrophobic poly(3,4-ethylenedioxythiophene) (PEDOT) coating has been prepared by chemical vapor deposition of a fluoroalkylsilane (POTS) onto a PEDOT film, which was obtained by electrochemical deposition. The coating not only maintained high conductivity with a low resistivity of 3.2 × 10−4 Ω·m, but also displayed a water contact angle larger than 156° and a sliding angle smaller than 10°. After being etched with O2 plasma, the coating showed an excellent self-healing ability, spontaneously regaining its superhydrophobicity when left under ambient conditions for 20 h. This superhydrophobicity recovery process was found to be humidity-dependent, and could be accelerated and completed within 2 h under a high humidity of 84%. The coating also exhibited good superhydrophobicity recovering ability after being corroded by strong acid solution at pH 1 or strong base solution at pH 14 for 3 h.
1. INTRODUCTION Many groundbreaking studies on materials of a superwaterrepellent nature found in the leaves of plants (most typically lotus leaves), the silk of spiders, the feathers of birds, and the legs/wings/backs of insects,1−6 have inspired numerous researchers in both the scientific and industrial communities. The surfaces with this superwater-repellent nature, namely superhydrophobic surfaces, are characterized by static contact angles with water in excess of 150°. It has been revealed that a combination of surface hierarchical micro/nanoscaled structures and low-surface-energy materials can give rise to superhydrophobic surfaces.7−11 Since superhydrophobicity may lead to reductions in surface contamination, wear, and corrosion, considerable efforts have been directed toward the development of fabrication techniques to mimic such natural nonwetting surfaces. The resulting materials could prove useful in many fields, such as self-cleaning materials, anti-icing materials, antiadhesion coatings, corrosion-resistant coatings, antifogging surfaces, and reducing fluid resistance for microfluidic devices.12−18 To date, many different methods have been developed for the fabrication of artificial superhydrophobic surfaces.19 Along with the development of fabrication techniques for superhydrophobic surfaces, conductivity has been introduced to such surfaces by electrochemical techniques, which are easy to work out, relatively fast, and very reproducible. Because conducting polymers (such as polyaniline, polypyrrole, and polythiophene) have unique optical, electrical, and mechanical properties, the fabrication of electrically conductive superhydrophobic surfaces is of particular interest.20−24 Moreover, their capability of © 2014 American Chemical Society
removing static charges accumulated on surfaces makes them potentially applicable in areas such as conductive textiles,25 antistatic coatings,26 electrostatic dissipative coatings,27 electromagnetic interference (EMI) shielding materials,28 and so on. However, the poor durability of most artificial superhydrophobic surfaces has hindered them from widespread effective applications. These films were prone to lose their superhydrophobicity permanently when they are exposed to air, a special chemical environment, strong light, or damaged by physical abrading. Therefore, it is important to address this issue of poor durability, therefore extending the applications of superhydrophobic surfaces. The past two decades have witnessed an increasing development of self-healing materials, which are defined as materials possessing an intrinsic ability to heal (recover/repair) damage automatically or autonomously.29 The concept of selfhealing was inspired by natural living organisms, where minor damage (e.g., a small cut or bruise) triggers a spontaneous healing response. Moreover, some naturally hydrophobic leaves have been found to be able to regenerate their hydrophobic epicuticular wax layer, resulting in recovery of their hydrophobicity.30 Thus, mimicking living systems, imparting artificial superhydrophobic surfaces with self-healing ability seems to be a promising approach to solving their low durability problem and extending their applications. One of the general strategies for endowing a polymer material with the function of selfReceived: February 17, 2014 Revised: April 3, 2014 Published: April 4, 2014 4671
dx.doi.org/10.1021/la500603c | Langmuir 2014, 30, 4671−4677
Langmuir
Article
healing is damage-initiated in situ polymerization of monomeric “healing agents” stored in hollow fibers, particles, microcapsules, or a microvascular network embedded in the polymer matrix.31−35 Similar to this strategy, recent developments have allowed the fabrication of superhydrophobic coatings with selfhealing property, that is, an ability to regain their superhydrophobicity. Such property can be expected to prolong the lifetime of superhydrophobic coatings, allowing them to protect a hydrophilic bulk material continuously. So far, some studies on self-healing superhydrophobic surfaces/coatings/fabrics have been reported, and the restoration of superhydrophobicity could be realized by the release of low surface energy material stored in the porous surfaces, such as layer-by-layer assembled film,36 anodized alumina surfaces,37 fluorinated-decyl polyhedral oligomeric silsesquioxane (FD−POSS) coatings38,39 or mesoporous silica films,40 or by the recovery of damaged microand nanoscale topographic features after treating the crushed coating with water.41 Those studies have showed great promise for improving the durability of artificial superhydrophobic surfaces. Nevertheless, there are still a few works focusing on electrically conductive self-healing superhydrophobic coatings. Herein, we report an electrically conductive self-healing superhydrophobic poly(3,4-ethylenedioxythiophene) (PEDOT) coating, which was conveniently prepared by chemical vapor deposition (CVD) of a fluoroalkylsilane onto a PEDOT film surface obtained by electrochemical deposition of EDOT monomer onto ITO. After the CVD treatment, the PEDOT coating still showed good conductivity with a resistivity of 3.2 × 10−4 Ω·m, as compared to 2.34 × 10−4 Ω· m for a PEDOT film. A coating with a thickness of 3.5 ± 0.5 μm showed excellent self-healing ability, spontaneously regaining its superhydrophobicity over more than nine cycles of O2 plasma-etching treatment without any external stimulus. This self-healing process was found to be humidity-dependent and a coating was able to recover its superhydrophobicity after O2 plasma etching in just 2 h in an atmosphere of 84% humidity. It also exhibited good recovering ability after being corroded by strong acid solution at pH = 1 or strong base solution at pH = 14 for 3 h, demonstrating good corrosion resistance. Such conductive self-healing superhydrophobic coatings are also freestanding, so that they can be removed from the ITO substrate and transferred to another surface, such as that of a silicon wafer.
wire as quasi-reference electrode. The electrochemical deposition was performed by cyclic voltammetry as shown in Supporting Information, SI, Figure S1 with a CHI 630E electrochemical analyzer under computer control. The PEDOT polymer was thereby deposited and stacked upon the ITO surface to form a PEDOT film. By regulating the quantity of deposited charge, PEDOT films with various thicknesses and varying surface roughnesses could be obtained. 2.3. Chemical Vapor Deposition (CVD) of POTS. CVD of POTS onto the surfaces of PEDOT films gave rise to superhydrophobic PEDOT coatings. It was conducted as follows: ITO glass samples coated with PEDOT films were placed in a sealable reactor, on the bottom of which a few drops of POTS and methanol were dispensed. The reactor was then sealed and placed in an oven at 150 °C for 5 h so that the POTS vapor diffused into the porous surfaces of the PEDOT films, where it was retained. After cooling the reactor to room temperature, superhydrophobic PEDOT coatings were obtained. 2.4. Oxygen Plasma Etching. O2 plasma etchings were conducted using a DT-01 plasma instrument (Suzhou OPS Plasma Technology Co., Ltd., China) at 0.63 mbar under a power of 125 W at ambient temperature for 3 min. Such plasma etching treatment could render the surface completely hydrophilic (water contact angle 0°). 2.5. Characterization. SEM observations were carried out on a Hitachi S-4800 field-emission scanning electron microscope. To investigate changes of the surface elementary composition, EDS analysis was performed on a Horiba XMAX silicon drift X-ray detector connected to the scanning electron microscope, and X-ray photoelectron spectroscopy measurements were carried out on a Kratos AXIS Ultra (Shimadzu, Japan). Surface wettability was characterized by static contact angle measurement with 4 μL water droplets and dynamic contact angle measurements (sliding angle) with 4 μL water droplets using the tilted-drop method on a Contact Angle System OCA20 (DataPhysics Instrument GmbH, Germany) in air at room temperature. The conductivity studies were measured by the standard four-probe method on an SB120/2 four-probe-method test platform (Shanghai Qianfeng electronic instrument Co., Ltd., China) after transferring the film and superhydrophobic coating to the surface of a silicon wafer.
2. EXPERIMENTAL SECTION 2.1. Materials. 3,4-Ethylenedioxythiophene (EDOT) and anhydrous lithium perchlorate (LiClO4) were purchased from Aladdin. 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (POTS) was purchased from Alfa Aesar and used as received. The solvents, methanol, and commercial chromatographically pure acetonitrile were purchased from Shanghai Lingfeng Chemical Reagent Company and used as received. Indium tin oxide (ITO)-coated glass (
