fabrication of superhydrophobic surfaces by electropolymerization of thiophene and pyrrole...

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Fabrication of Superhydrophobic Surfacesby Electropolymerization of Thiophene andPyrrole DerivativesMael Nicolas aa Institut de Chimie de Nice, Laboratoire de Chimie des MatériauxOrganiques et Métalliques, EA 3155, Université de Nice-Sophia Antipolis,Parc Valrose, F-06108 Nice cedex 2, France;, Email: [email protected] online: 02 Apr 2012.

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Journal of Adhesion Science and Technology 22 (2008) 365–377www.brill.nl/jast

Fabrication of Superhydrophobic Surfaces byElectropolymerization of Thiophene and Pyrrole Derivatives

Mael Nicolas ∗

Institut de Chimie de Nice, Laboratoire de Chimie des Matériaux Organiques et Métalliques,EA 3155, Université de Nice-Sophia Antipolis, Parc Valrose, F-06108 Nice cedex 2, France

AbstractThe electropolymerization of five-membered heterocycles like thiophene and pyrrole leads to the deposi-tion of conductive and morphology-controlled films onto different kinds of electrodes. It involves manyexperimental parameters such as the chemical nature and concentration of the monomer and electrolyte,the solvent, the nature of electrode, the applied electrical conditions and the time considered. These elec-trosynthesis conditions determine to a large extent the structure, morphology and properties of the resultingpolymer. By controlling them, it represents a straightforward method to obtain highly porous surfaces. Theuse of low surface energy alkyl or perfluoroalkyl chains together with the electropolymerization processallows the control of the two parameters that generally govern the wettability of a surface, i.e. the chemicalcomposition and the microstructure of the surface, and affords an excellent method to reach very stablesuperhydrophobic surfaces with a low contact angle hysteresis. Koninklijke Brill NV, Leiden, 2008

KeywordsConjugated polymers, electropolymerization, rough surfaces, low surface energy, superhydrophobicity

1. Introduction

Conjugated polymers have been the subject of great interest, both theoreticallyand experimentally, since the discovery of conductivity in doped polyacetylenein the seventies [1]. Many works have been devoted to their synthesis, character-izations and properties [2]. They have found many applications, particularly inthe field of optoelectronics, as light-emitting diodes (LEDs), field-effect transis-tors (FETs), solar cells, etc., due to the semi-conducting behavior of the conju-gated backbone [3]. Among them, polythiophenes (PTh) and polypyrroles (PPy)have been extensively studied because of their synthesis versatility and environ-mental stability [4–6]. Their functionalization permits the combination of their

* Tel.: +33492076196; Fax: +33492076578; e-mail: [email protected]

Koninklijke Brill NV, Leiden, 2008 DOI:10.1163/156856108X295446

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366 M. Nicolas / Journal of Adhesion Science and Technology 22 (2008) 365–377

intrinsic conducting properties with a new specific function brought by the sub-stituent, for instance, alkyl chains [7] for solubility and processability; crownether [8] and boronic groups [9] for cationic and anionic recognition, respec-tively; chiral groups [10] for enantioselective membranes; alkylsulfonates [11]for self-doped polymers, etc. The substitution also plays a role in the controlof the electronic, optical and electrochemical properties of the conjugated back-bone.

The synthesis methods are now well-established and involve oxidative chemical(mostly with FeCl3) or electrochemical polymerization and metal-catalyzed cross-coupling polymerization (with Co, Ni, Cu, Zn, Pd, etc. as a catalyst). Compared tothe chemical synthesis of conducting poly(heterocycles), the anodic electropoly-merization presents some unique advantages since it permits to obtain, in onestep, directly at the surface of an electrode, a doped, thus conducting, film withcontrolled thickness. The electrochemical polymerization of heterocyclic aromaticcompounds, which was first reported for pyrrole [12], is proving to be a fairly gen-eral reaction for the preparation of thin film coatings on electrode surfaces. Thisis of particular interest for electrochemical applications and also represents an ele-gant way to obtain morphology-controlled surfaces by controlling electrosynthesisconditions and monomer structure. It represents an important advantage for con-trolling the properties of conducting polymer surfaces, compared to the depositionmethods (spin-coating that requires soluble polymer fractions, plasma chemicalvapor deposition, etc.) that give rise to smooth thin films. The wettability is a cru-cial parameter for the applicability of conducting polymers in interfacial science,sensors, corrosion protection, antistatic coatings, conductive textiles, bioactive sur-faces, etc. [2, 13–16]. The wettability of solid surfaces mainly depends on twofactors: the geometrical microstructure and the chemical composition of the sur-face [17–19]. The former can be controlled by optimizing the different parametersof the electropolymerization process and the latter by incorporating into the conju-gated backbone low surface energy materials such as alkyl or perfluoroalkyl chains.The combination of these two parameters should be useful to obtain super-repellentmaterials, which represents an important topic in daily life and in industry. Indeed,techniques to make superhydrophobic surfaces are mainly based on the preparationof rough surfaces with low surface energy materials [19–21]. It involves inorganicmaterials (metals, metal oxides, silica, glasses, etc.) as well as organics (polymers,self-assembled monolayers, multilayers, etc.) and many methods to make the de-posit onto substrates (conventional coating processes such as spraying or dipping,vacuum deposition techniques, electrochemical methods, plasma based techniques,etc.).

This study reports on the analysis of the wetting properties of polythiopheneand polypyrrole, by presenting the unique process of electropolymerization and therecent progress that leads to superhydrophobicity.

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M. Nicolas / Journal of Adhesion Science and Technology 22 (2008) 365–377 367

2. Results and Discussion

2.1. Mechanisms of Electropolymerization of Thiophene and Pyrrole

Electropolymerization consists in a succession of electrochemical (E) and chemi-cal (C) reactions following a general E(CE)n process (Fig. 1). It begins with theformation of a radical cation by anodic oxidation of the monomer (electrochem-ical step E), followed by the coupling between two radicals to obtain a dicationwhich leads to a dimer after loss of two protons and re-aromatization (chemicalstep C). The dimer, which is more easily oxidized than the monomer, is present inits radical form and undergoes a further coupling with a monomeric radical. It pro-ceeds through this scheme until the oligomers become insoluble in the electrolyticmedium and precipitate onto the electrode surface.

This creates nucleation sites randomly distributed on the electrode surface (Pt,ITO, Au, etc.). Then, the nucleus size increases and the nuclei coalesce. In thefirst stage of the nucleation process, the contribution of a two-dimensional nucle-ation is important, indicating that the polymeric deposit initiates by the formationof a two-dimensional film. At longer times, upper structures appear which corrob-orates a three-dimensional growth. This 2D and 3D nucleation and growth mecha-nism (NGM) of PTh and PPy has been extensively studied by different techniques[22–24] and presents similarities with the electrodeposition of metals [25, 26]. As

Figure 1. Mechanism of electropolymerization of five-membered heterocycles. Epa: oxidation peakpotential of monomer.

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Figure 2. Global electropolymerization reaction of five-membered heterocycles andp-doping/undoping reaction.

a result, thin films (�1 µm) are compact and smooth in morphology (due to a two-dimensional process). As the thickness of the film increases (up to a few µm), itssurface becomes more and more irregular and rough and exhibits a cauliflower ap-pearance or packed grain structure (typical of a three-dimensional process). Withincreasing time of deposition and thus increasing of the film, upper structures ap-pear, with less cohesion between them, that renders the surface highly porous.

The global reaction requires 2 moles of electron per mole of monomer plus anexcess of charge δ to dope (or oxidize) the polymer (Fig. 2). The thicknesses ofthe films were estimated from the total anodic charge Qs (Qs = n(2 + δ)F whereF is the faraday) passed during electrodeposition and consequently depends onexperiment time. The electropolymerized film is obtained in its oxidized state and isaccompanied by the incorporation of counter anions to assure the electroneutralityof the film. This p-doping reaction is reversible since the neutral polymer can beobtained by reduction.

2.2. Wettability of the Electropolymerized Surfaces — Influence ofElectrosynthesis Conditions

Only a few reports deal with the contact angle measurements on PTh and PPy sur-faces. It has been found that the measured contact angles of water on electrochemi-cally prepared films strongly depend on film thickness as well as on polymerizationvariables such as applied electrode potential, nature of the working electrode, elec-trolyte, solvent and temperature. It is far from obvious to conclude on the wettabilityof these surfaces but some trends can be drawn from literature.

The first study of PTh and PPy wettability was reported in 1984 [27]. Deriva-tives of thiophene and pyrrole were polymerized in a classical electrolytic medium,i.e. acetonitrile and tetrabutylammonium tetrafluoroborate as the electrolytic salt.The thickness of the film was approximatively 0.5 µm and the water contact anglesreported (86◦ for PTh and 62◦ for PPy) indicate their hydrophilicity. These elec-

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Table 1.Reported contact angles of water on electropolymerized polythiophene and polypyrrole films. Th:thiophene; Py: pyrrole

Monomer Electrolytic medium Film thickness Water contact angle Reference(µm) (◦)

Th CH3CN, Bu4NBF4 0.5 86 (neutral) [27]83 (ox)

biTh CH2Cl2, Bu4NBF4 5 45 [29]Th CH3CN, Bu4NBF4 – <75 [28]Th BFEE 0.1 82 [28]Th BFEE 1 98 [28]Th BFEE 5 116 [28]Py CH3CN, Bu4NBF4 0.5 62 (neutral) [27]

55 (ox)Py CH2Cl2, Bu4NBF4 5 57 [29]Py H2O, LiClO4 46 51 [30]Py H2O, NaCl 0.1 53 [31]Py H2O, NaSO3–C12H25 0.1 69 [31]Py H2O, NaSO3–C6H4–CH3 0.1 80 [31]

tropolymerized films have the advantage to show exceptional stability for severalmonths. In the oxidized form, the films are ionic composites. Electrochemicallydoping the polymer effectively introduces a large dipole in the backbone; thus, onewould expect doped polymers to be more hydrophilic than the same materials in theneutral state. Surprisingly, in this work, very small difference was observed.

A more recent study confirmed these results by measuring water contact angleon PTh electrosynthesized in the same medium. Values reported were less than75◦ [28].

In another article [29], lower contact angle values were reported for PTh and PPyfilms electropolymerized in dichloromethane. SEM study showed that PPy filmswere smooth and densely packed while PTh films showed a relatively porous andfibroid structure. Contact angle value of 51◦ was obtained for thick films of PPyelectropolymerized in a lithium perchlorate aqueous medium [30]. This highlightsthe differences in wettability when changing one or several conditions.

The influence of the electrolytic salt was emphasized in a study that comparedthe water contact angles on PPy electropolymerized in aqueous solutions contain-ing sodium chloride (NaCl), sodium dodecylsulfate (NaSO3–C12H25) and sodiump-toluenesulfonate (NaSO3–C6H4–CH3) as electrolytic salts [31]. Hydrophobicitywas enhanced by using sulfonate salts. Two parameters must be considered: on theone hand, the decrease of surface free energy by using hydrophobic parts (C12H25and C6H4–CH3); on the other hand, the change in microstructures morphologies.Indeed, the use of sulfonate salts permits obtaining of PPy microtubules grownperpendicularly onto the surface of the electrode [32, 33]. The formation of these

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unusual microstructures was attributed to the fact that self-assembled gas bubbleson the working electrode acted as a template. In all these cases, the surfaces showhydrophilicity with water contact angle less than 90◦ (Table 1).

Fabrication of hydrophobic surfaces of conductive polythiophene was reported in2003 [28]. The electrolytic solution was boron trifluoride-diethyl etherate BF3·OEt2(BFEE) and films of different thicknesses were obtained by electrosynthesis asshown in Fig. 3. With a flat surface obtained with thin film, water contact anglewas 82◦. But with increasing thickness and thus increasing disorder and porosity,contact angle up to 116◦ was observed. These results demonstrate that the con-tact angles of water on PTh films are, to a great extent, dependent on their surfacemorphology. For comparison, PTh films have been electropolymerized in the sameconditions but in classical electrolytic medium (acetonitrile and tetrabutylammo-nium tetrafluoroborate). Contact angle values were not higher than 75◦. The useof pure BFEE as a medium for anodic oxidative polymerization [34–36] has beendictated by the fact that BF3 is a strong Lewis acid and can coordinate a pair of

Figure 3. Scanning electron micrographs of PTh films of different thicknesses (a) 0.1 µm, (b) 1 µmand (c) 5 µm and photos of water droplets on the films (contact angle (CA) = 82◦, 98◦ and 116◦,respectively). Reproduced with permission, from reference [28].

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free electrons of the sulphur atom of the thiophene monomer and thus lower the ox-idation potential. This allows electrodeposition onto various noble and oxidizablemetallic substrates. Films obtained are more homogeneous and uniform and presentbetter mechanical properties. With these results, it also proved to be a good mediumfor the electrosynthesis of highly hydrophobic PTh films.

2.3. Towards Superhydrophobic Surfaces

Due to their unique nucleation and growth mechanism onto the electrode, elec-tropolymerized polythiophenes and polypyrroles represent promising candidatesfor coating surfaces. They present morphology-specified surfaces depending onelectrosynthesis conditions and film thickness. Rough and porous surfaces can beeasily obtained. With a view to develop super-repellent materials, they appear asexcellent matrices provided their chemical composition can be modified. Indeed, ingeneral, the wettability of solid surfaces is governed by both the chemical composi-tion and the geometrical microstructure of the surface [17–19]. The main approachfor the chemical factor is the introduction of alkyl or perfluoroalkyl chain on thepolymer backbone. To obtain super-water repellent conductive polymer films suchas polythiophene and polypyrrole films, it is necessary to associate this chemical ap-proach with the geometrical approach leading to the formation of fractal or roughsurface structures. There are three main strategies for immobilizing molecular com-ponents into or onto conducting polymers.

Perhaps the simplest is to perform the electropolymerization from solutionscontaining both the monomer and the additive of interest as the counter an-ion (Fig. 4). In this sense, Py monomers have been electropolymerized with analkyl or aryl dopant from aqueous solutions. Water contact angle increased from53◦ for Cl− doped PPy to 69◦ and 80◦, respectively, for C12H25–SO3

− andCH3–C6H4–SO3

− doped PPy [31]. PPy was also electropolymerized in water witha fluorinated dopant. The PPy film containing perfluorinated dopant exhibited hy-drophobicity (92◦) while ClO4

− doped film was hydrophilic (51◦) [30]. Hydropho-

Figure 4. Electropolymerized PPy doped with different counter anions.

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bicity was thus enhanced with the insertion of these counter anions but was far fromsuper water repellency.

Following these works, Xu et al. optimized the method conditions and reportedsuperhydrophobicity for the first time for a conducting polymer in 2005 [37]. Thelow water contact angles previously reported for fluorinated doped PPy was essen-tially due to a lack of roughness of the electroformed film. Electropolymerizationof pyrrole was this time performed in acetonitrile, with perfluorooctanesulfonate(C8F17–SO3

−) as the fluorinated dopant. The key feature consists in adding a tinyamount of FeCl3 in the electrolytic medium, which permits to obtain highly poroussurfaces. FeCl3 is a classical oxidant for the chemical polymerization of pyrrole.Thus the formation of porous PPy may be attributed to the simultaneous occurrenceof electropolymerization and chemical polymerization processes. Without FeCl3,the PPy had a compact structure. The as-prepared highly porous films showed su-perhydrophobicity with a water contact angle as high as 152◦ while on the smoothcompact PPy film it was about 105◦. Two models are considered to explain the ef-fect of roughness on wettability [38]: the homogeneous wetting (Wenzel) [39] andthe heterogeneous wetting (Cassie) [40]. First, the above observation can be ex-plained by the Wenzel model describing the contact angle for a liquid droplet on arough solid surface, i.e.,

cos θr = r cos θ, (1)

where θ is the intrinsic contact angle on a smooth surface, θr that on a rough sur-face made on the same material and r the roughness factor. In this approach, it isassumed that the liquid fills the grooves on the rough surface. This equation in-dicates that the surface roughness enhances both the hydrophilicity of hydrophilicsurfaces (θ < 90◦) and the hydrophobicity of hydrophobic ones (θ > 90◦), sincer is always larger than unity. This is the case here and the use of Wenzel’s equationgives a roughness factor r = 3.41. To thoroughly understand the hydrophobicity ofthis rough surface, we introduce the Cassie’s equation:

cos θr = f1 cos θ − f2, (2)

that describes the contact angle of a liquid on a surface composed of solid and air, inwhich f1 is the fraction of the surface area made of material and f2 the fraction ofopen area. In Cassie’s approach, it is assumed that the drop settles on the peak of theroughness geometry, i.e. the liquid does not fill the grooves on the rough surface.It is easy to deduce that increasing f2 increases θr , that is, the fraction of air inthe surface is important to the superhydrophobicity. According to this equation, thef2 value of the rough polypyrrole film is estimated to be 0.84, which indicates thatthe fraction of air in the surface is high. Moreover, the low contact angle hysteresis(�θ = θadvancing − θreceding = 155◦ − 147◦ = 8◦) confirms the lotus effect and thestability of the surface in presence of water. This highlights the general way toobtain superhydrophobicity by combining a rough surface with low surface energymaterials such as perfluorinated components.

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Another strategy consists in chemically modifying the polymer after polymer-ization with low surface free energy materials by solid phase synthesis. Follow-ing this approach, PTh and PPy films were modified by treatment with alka-nethiol (dodecanethiol C12H25–SH) and perfluoroalkanethiol (tridecafluorooctanethiol C6F13–C2H4–SH) [41]. The grafting on polymer surfaces was followed byscanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) andenergy-dispersive X-ray spectroscopy (EDS) analyses. The nucleophilic attack bythiols results in substantial modification of the surface as evidenced by the contactangle measurements. The water contact angle increases from 45◦ for PTh to 93◦and 97◦, respectively, when treated with alkane and perfluoroalkane thiol, and from57◦ to 103◦ and 109◦ for PPy. This represents an opportunity to modify the surfacefree energy but at the expense of reduced electroactivity and some structural degra-dation of the polymers. Even if this postpolymerization derivatization permits thepolymerization of relatively inexpensive and highly reactive monomers, one of themajor disadvantages is the difficulty in controlling the homogeneity of the reactionon surfaces.

To solve this problem, the most effective and useful way to immobilize a specificsubstituent into a polymer matrix is to covalently bind it to the monomer beforepolymerization. This requires developing synthesis routes but remains massivelyemployed.

In 2005, Tsujii and co-workers produced super-water repellent poly(alkylpyrrole)films from electrochemical polymerization of N -octadecylpyrrole [42]. The elec-trochemical synthesis was performed in an acetonitrile solution containing sodiump-toluenesulfonate (NaSO3–C6H4–CH3) as electrolytic salt. In these condi-tions, as already observed for polypyrrole, “needle-like” and/or microtubulespoly(alkylpyrrole) structures grew perpendicularly to the surface of the ITO elec-trode (Fig. 5), creating fractal surfaces. These structures were approximately 5 µmin diameter and 40 µm in length. On the basis of the analysis by the box countingmethod, the surface was considered to be fractal with a dimension of 2.23. The sur-face of the film showed super water repellency with a contact angle larger than 150◦and excellent environmental stability to both heating and organic solvent treatmentsin terms of the contact angle towards water.

Most recently, our group has focused on the covalent bonding of a fluorinatedsegment on thiophene [43, 44]. The electrochemical polymerization of the chosenfluorinated monomer was carried out in thoroughly dried acetonitrile containingBu4NPF6 as the supporting electrolyte. The electrodeposited polythiophene ex-hibits a “cauliflower-like” structure (Fig. 6b) previously reported by Del Valleand co-workers [24]. The rough electrodeposited film exhibits superhydrophobicitywith a water contact angle of 153◦, in comparison with the 108◦ value obtained forthe flat chemically formed polymer sample coated on glass plate by drop casting achloroform solution (Fig. 6a). The water dewetting on this rough surface, studied bythe two models attributed to Wenzel and Cassie, gives a roughness factor r = 2.88and an f2 value of 0.84, indicating the air trapped in the rough structure is crucial

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Figure 5. SEM image of electrodeposited poly(alkylpyrrole) film on ITO plate (scale bar: 15 µm).Left inset: SEM image of the cross section of the film. Right inset: photo of a water droplet on the film(CA = 150◦). Reproduced with permission, from reference [42].

to obtain superhydrophobicity. The electrodeposited surface has very small contactangle hysteresis, less than 10◦ (�θ = θadvancing − θreceding = 151◦ − 145◦ = 6◦)while the chemically formed surface shows a high hysteresis value higher than70◦ (�θ = θadvancing − θreceding = 105◦ − 32◦ = 73◦), suggesting that the surfacechanges (reorganization of the surface, penetration of the liquid, heterogeneity, etc.)as a result of the interaction with water. Over a ten-minute period, the contact angleof water on the electrodeposited film remains constant while its rapid decrease onthe chemically deposited film is an evidence for surface instability.

In both cases, the combination of a rough structure obtained by electropolymer-ization together with the presence of low surface free energy substituent allowscontrol of the two factors — i.e. the chemical composition and the geometricalstructure — that govern the wettability of a solid surface by a liquid and leads tovery stable super water repellent films. It is noteworthy that these electrodepositedfilms demonstrate exceptional time stability since no loss of properties or degrada-tion was observed for several months. Further, the specificity of fluorinated chainadds oil-repellency to the films with contact angles of 135◦ with hexadecane [43].

3. Conclusions

The concomitant use of a low surface energy substituent such as alkyl or perflu-oroalkyl chain together with the attainment of rough surfaces by electropolymer-ization generates very stable superhydrophobic surfaces. The most efficient andstraightforward method consists in electropolymerizing the substituted monomer

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Figure 6. SEM images of the poly(fluorinated thiophene) films: (a) film formed by a drop casting ofchloroform solution of polymer on a glass plate and (b) electrodeposited film on ITO plate (scale bar:8 µm). Insets: camera images of a water droplet on the films (CA = 108◦ and 153◦, respectively).Reproduced with permission, from reference [43].

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onto a conductive electrode. The choices of the electrolytic medium (solvent andelectrolyte), the nature of the electrode (ITO, Au, Pt, vitreous C, stainless steel,etc.), the electrochemical method, the concentration of the monomer, etc., representthe many variables that control the morphology of the deposited films. Generally,these films exhibit environmental and thermal stabilities together with their classi-cal electronic and optical properties. Water repellency adds another advantage for awide variety of applications in the field of interfacial science.

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