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A One-Step and Templateless Electropolymerization Process using Thienothiophene Derivatives to Develop Arrays of Nanotubes and Tree-Like Structures with High Water Adhesion Gabriela Ramos Chagas, Thierry Darmanin, and Frederic Guittard ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08536 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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ACS Applied Materials & Interfaces

A One-Step and Templateless Electropolymerization Process using Thienothiophene Derivatives to Develop Arrays of Nanotubes and Tree-Like Structures with High Water Adhesion

Gabriela Ramos Chagas, Thierry Darmanin, Frédéric Guittard*

Univ. Nice Sophia Antipolis, CNRS, LPMC, UMR 7336, 06100 Nice, France [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT Here, we report for the first time the possibility to obtain not only arrays of nanotubes, but also tree-like structures with high water adhesion using a one-step and templateless electropolymerization process. Using thienothiophene derivatives, particularly thieno[2,3b]thiophene (Thienothiophene-1) and thieno[3,2-b]thiophene (Thienothiophene-2), we demonstrate this surface fabrication in organic solvent (dichloromethane) and without any surfactants. The formation of nanotubes is due to the stabilization by the polymer of gas bubbles produced in-situ during electropolymerization process and we show that the water content plays an important role in the formation of gas bubbles even if it is not the unique parameter. Using cyclic voltammetry as an electropolymerization method, the amount of released gas is more significant, but at constant potential it is much easier to control the nanotube formation. It is also possible to obtain arrays of tree-like structures when electropolymerizing with high deposition charges, and the resulting surfaces have high θw with extremely high water adhesion even if the polymers are intrinsically hydrophilic (θYw ≈ 70°). This work is extremely important for potential applications in water transportation and harvesting, oil/water separation membranes, energy systems and biosensing.

Keywords Nanotubes, Nanostructures, Electropolymerization, Wettability, Adhesion, Hydrophobicity

INTRODUCTION

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Controlling surface hydrophobicity and water adhesion is fundamental for various applications in water harvesting, anti-icing, microfluidic devices or oil/water separation membranes.1-7 Wetting phenomena are extremely present in nature.8 Species having superhydrophobic properties, characterized by high water contact angles (θw) and low water adhesion or hysteresis (H) are resistant to wetting by water during rainfall, are able to walk on aqueous surfaces, or can see clearly in foggy conditions.9-11 Superhydrophobic properties with high robustness can be reached by combining surface structures often at a micro/nanoscale with low surface energy materials.12,13 Other species having parahydrophobic properties,14 characterized by high θw and high H, can capture water droplets or climb on vertical substrates.15,16 High water adhesion surfaces can be obtained by using materials of higher surface energy and/or with surface structures that permit a higher solid-liquid interface. Amongst the surface structures that are of interest towards the control of water adhesion, nanotubes are among the most fascinating for their high and controllable porosity.17,18 However, the methods to develop nanotube arrays directly on substrates are not numerous. It is well known that anodization of titanium substrates can lead to nanotubes, but to our knowledge only titanium substrates can lead to nanotubes by a direct anodization process.19-21 Otherwise, in order to obtain nanotubes with various materials, anodized aluminum substrates are often used as hard templates. In this process, first aluminum substrates with cylindrical nanopores closed-packed in a hexagonal geometry are prepared. Then, the pores are filled by solution

casting22-24

or

electrodeposition25-27/electropolymerization28-30,

followed

by

membrane dissolution to yield well-structured nanotube arrays. The processes to obtain nanotubes without hard templates are extremely rare in the literature.31-39 For example, densely packed carbon nanotubes were reported by chemical vapor deposition.31-33 Nanotubes can also be obtained in one-step by electropolymerization without any template.36-41 This process is extremely interesting because the resulting 3



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dimensions of the nanotubes can be easily controlled by electrochemical process and the monomer used. Shi et al. reported this possibility by electropolymerization using pyrrole in an aqueous solution with a surfactant (β-naphthalenesulfonic acid, (+)- and (-)-camphorsulfonic acids or poly(styrene sulfonic acid).36-38 The role of the surfactant is to stabilize H2 or O2 bubbles produced in-situ either during water electrolysis or the decomposition potential of acidic water. Previously, vertically aligned nanotubes were formed by electropolymerization in organic solvent and without surfactant using naphtho[2,3-b]thieno[3,4-e][1,4]dioxine (NaphDOT) as monomer.39 Moreover, the presence of nanotubes made these surfaces parahydrophobic. Here, using thienothiophene derivatives we show for the first time the possibility to obtain not only arrays of nanotubes, but also arrays of tree-like structures. Indeed, poly(thienothiophenes) are excellent polymers for their high rigidity and optoelectronic properties.42,43 In this manuscript, five thienothiophene derivatives (Scheme 1) are tested as monomers: Thieno[2,3-b]thiophene (Thienothiophene-1), thieno[3,2-b]thiophene (Thienothiophene-2), 2,2′-bithieno[3,2-b]thiophene (Thienothiophene-3), dithieno[3,2-b:2′,3′d]thiophene (Thienothiophene-4) and naphtho[1,2-b:5,6-b′]dithiophene (Thienothiophene-5). We report the influence of the monomer, the electrodeposition method and electrolyte on the surface structures formed and resulting surface hydrophobicity.

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S S

S

S

Thienothiophene-1

Thienothiophene-2

S S

S

S

S S Thienothiophene-3

S Thienothiophene-4 S

S Thienothiophene-5

Scheme 1. Monomers studied in this manuscript.

EXPERIMENTAL SECTION Electropolymerization Conditions. The monomers were purchased from Sigma-Aldrich. For each electropolymerization experiment, 10 mL of anhydrous dichloromethane was inserted in an electrochemical cell and the solution was degassed under argon. Then, 0.1 M of electrolyte and 0.01 M of monomer were added to the solution. The electrochemical cell was connected to an Autolab potentiostat of Metrohm using three electrodes. Here, gold-coated silicon wafers were used as working electrodes while a carbon rod and a saturated calomel electrode were used as counter-electrode and reference electrode, respectively. Electrodeposition was carried out by two methods to better study the polymer growth: cyclic voltammetry (from -1.0 V to the working potential Ew νs SCE at a scan rate of 20 mV s-1 and different number of scans) and imposed potential (at the working potential of each monomer νs SCE and different imposed charges). After electrodeposition, the surfaces were washed in dichloromethane and slowly dried. 5



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Surface Characterization. The wettability of the substrates was determined by contact angle measurements using a DSA30 goniometer (Krüss). The apparent contact angles (θ) correspond to the angle tangent to the triple point between a liquid droplet and the substrate. Two liquids differing by their surface tension were chosen for the measurements: water (γLV = 72.8 mN/m) and diiodomethane (γLV = 50.0 mN/m). For the characterization of the water adhesion, a 6 µL water droplet was placed on the substrate and the substrate was inclined until the droplet moved. The advanced (θadv) and receding (θrec) contact angles are taken just before the droplet moving. θadv and θrec correspond to the angles in the moving and the opposite direction, respectively, and are used to determine the hysteresis H = θadv – θrec. The maximum surface inclination is called sliding angle (α). If the water droplet does not move even if α = 90°, the substrate is called sticky. The scanning electron microscopy (SEM) images were obtained using a 6700F microscope (JEOL). The mean (Ra) and quadratic (Rq) surface roughness were determined using a Wyko NT 1100 optical microscope (Bruker) and with these different options: High Mag Phase Shift Interference (PSI) working mode, field of view 0.5X and objective 50X.

RESULTS AND DISCUSSION Electrochemical Characterization. First, tetrabutylammonium perchlorate (Bu4NClO4) was used as electrolyte. The monomer oxidation potentials were determined by cyclic voltammetry and after the polymers were electrodeposited by scanning from -1 V to a potential slightly lower Eox called the working potential Ew, as shown in Table 1. The cyclic voltammograms are displayed in Figure 1. Thienothiophene-3 and Thienothiophene-4 polymerize perfectly and the cyclic voltammograms present several oxidation and reduction 6


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peaks. A high and relatively constant amount of polymer is also deposited after each scan. By contrast, with Thienothiophene-1, Thienothiophene-2 and Thienothiophene-5, the amount of polymer deposited is much lower. It is not surprising that Thienothiophene-3 and Thienothiophene-4 polymerize more than the other monomers, because there are at least two important parameters that can explain this behavior. First, the monomers will readily polymerize if the density of radicals formed at the polymerization sites after monomer oxidation is significant important. Usually strong donating effects favor the polymerization and reversely. Second, the polymer rigidity is also an extremely important parameter that will enhance the polymer conductivity and thus the polymer chain length. A very intense peak at about -0.5 V was observed during the back scans of Thienothiophene1, Thienothiophene-2 and Thienothiophene-5. This peak is very important for the formation of nanotubes, because it may correspond to the decomposition potential of acidic water 2 H2O → 2 H2(g) + 2 O2 (g) and as a consequence, the formation of gas bubbles. Another possibility is that H+ is released during electropolymerization and leads to H2 formation, but this is less probable since tetrabutylammonium ion is often considered as H+ ion scavenger. Hence, Thienothiophene-1, Thienothiophene-2 and Thienothiophene-5 seem to be the most appropriate monomers for the formation of nanotubes. The stability of the polymeric films was also studied by cyclic voltammetry in order to observe the mass loss during the electrochemical process. Figure 2 shows that PThienothiophene-1 and PThienothiophene-2 have a larger polymer loss in the first scans, which is then reduced in the following scans. With this behavior, both monomers are more stable than the other polymers studied. For PThienothiophene-3, PThienothiophene-4 and PThienothiophene-5 the polymer loss is much more intense and increases after each scan.

Table 1. Oxidation Potential (Eox) and the Working Potential (Ew) for Each Monomer by Electrochemical 7



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Process. Electropolymerization Dichloromethane/Bu4NClO4

Monomers Thienothiophene-1 Thienothiophene-2 Thienothiophene-3 Thienothiophene-4 Thienothiophene-5

Oxidation potential Eox (V) 2.49 2.68 2.49 2.34 2.76

in

0.1

M

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of

Working potential Ew (V) 2.28 2.46 2.13 2.06 2.45

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Figure 1. Cyclic voltammograms of the monomers in 0.1 M Bu4NClO4/anhydrous dichloromethane and at a scan rate of 20 mV s-1.

Figure 2: Cyclic voltammograms (6 scans at 20 mV s-1) for the thienothiophenes after electrodeposition of 3 scans. Solution of 0.1 M Bu4NClO4/anhydrous dichloromethane.

Surface Characterization. The SEM images for 3 deposition scans are given in Figure 3. First of all, the electropolymerization of Thienothiophene-3, Thienothiophene-4 and Thienothiophene-5 induces the agglomeration of nanostructures forming extremely rough surfaces. The nanostructures consist of nanofibers for Thienothiophene-3, mixtures of nanofibers and nanosheets for Thienothiophene-4 and mixtures of nanofibers and spherical nanoparticles for Thienothiophene-5. However, large nanotubes are obtained with Thienothiophene-1 and Thienothiophene-2, where the tops of the nanotubes are closed with Thienothiophene-1 but rest open with Thienothiophene-2. Hence, Thienothiophene-1 and Thienothiophene-2 are excellent monomers for the formation of nanotubes. The SEM images of Thienothiophene-1 and Thienothiophene-2 confirm the importance of the presence of the peak corresponding to H+ reduction during electropolymerization, while the absence of 9



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nanotubes observed on Thienothiophene-5 surfaces confirms the importance of the monomer in the stabilization of gas bubbles and the nanotube formation. Indeed, the monomer or the corresponding polymer has to be able to stabilize the gas bubbles produced during the nanotube formation. A possible explanation for the different structures obtained for the polymers in this study is the monomer structure and the number of polarizable sites of each monomer. As Thienothiophene-1 and Thienothiophene-2 are small and less complex structures, the polymerization proceeds in a more organized and aligned manner, which is favors the formation of nanotubes. Contrary to this behavior, Thienothiophene-3, Thienothiophene-4 and Thienothiophene-5 monomers are more complex and voluminous in structure, which induces a higher steric hindrance during polymerization and favors the formation of large and spaced agglomerates on the surface.

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Figure 3. SEM images at two magnifications (5000x and 25000x) of the polymers electrodeposited by cyclic voltammetry (3 scans) in Bu4NClO4/dichloromethane. 11



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The surface roughness and the apparent contact angles for water (θw) are presented in Table 2. For the majority of the surfaces, θw is the highest after 1 deposition scan and decreases with further deposition scans as the surfaces become too rough. For the surfaces with nanotubes obtained with Thienothiophene-2, the surface with highest θw (θw = 150.7°) is formed after 1 deposition scan, and also has very high water adhesion (sliding angle αw > 90º). When the number of deposition scans increases, hydrophilic behavior is observed. The same behavior is observed for Thienothiophene-3. However, after the formation of rough nanotubes by 1 deposition scan of Thienothiophene-1, the wettability remains practically the same as the number of deposition scans increases, with similar behavior observed for Thienothiophene-4. In opposition, PThienothiophene-5 presents a high θw for 1 and 3 deposition scans, while the adhesion transitions from sticky to non-sticky behavior when increasing from 1 to 3 scans. However, superhydrophilicity was observed when the number of deposition scans increased further to 5 scans, due to the increase of the surface roughness. In summary, the polymers formed with high θw are parahydrophobic (α > 90º), except for PThienothiophene-5 after 3 scans which presents a low water adhesion (H = 12; α = 11º). Figure 4 shows the dynamic properties for the polymeric surfaces. Moreover, all the surfaces are completely oleophilic using diiodomethane, independent of the number of deposition scans, which suggests good potential for application of these materials in oil/water membrane separation.

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Figure 4. Water droplet images for (a) PThienothiophene-2 after 1 scan and (b) PThienothiophene-5 after 3 scans showing a sticky and non-sticky behavior, respectively. PThienothiophene-1, PThienothiophene-3 and PThienothiophene-4 showed a similar behavior of PThienothiophene-2 as Table 2 shown. Table 2. Apparent Contact Angles of Water (θw) and Roughness Data as a Function of the Polymer and the Number of Deposition Scans (Electrolyte: Bu4NClO4) Number of Polymer deposition Ra [µm] Rq [µm] θw [deg] α [deg] scans 1 0.4 0.7 122.4 > 90 PThienothiophene-1 3 1.4 2.8 114.6 > 90 5 2.2 3.9 108.1 > 90 1 1.3 2.9 150.7 > 90 3 6.1 9.3 36.2 PThienothiophene-2 5 5.4 8.5 44.5 1 2.8 4.2 149.0 > 90 3 6.9 9.7 23.7 PThienothiophene-3 5 9.9 13.1 0 1 3.6 6.2 144.5 > 90 PThienothiophene-4 3 8.9 12.6 136.6 > 90 5 12.0 16.2 133.8 > 90 1 2.3 3.6 146.4 > 90 3 2.3 3.7 153.3 11 PThienothiophene-5 5 7.2 10.7 0 -

To better analyze the wettability behavior of the nanotubes, Figure 5 shows the SEM images of PThienothiophene-1 and PThienothiophene-2 for different number of scans. A lower density of agglomerates and a minimal amount of surface structures, as well as nanotubes that remain with the top closed as the number of deposition scans increases are observed on PThienothiophene-1 surfaces. This should keep the wettability of the surface constant as the number of deposition scans increases. The lower θw showed on Table 2 for PThienothiophene1 after 1 scan can be explained by the spaced distribution of nanotubes and confirmed by the low roughness. Indeed, the diameter of the nanotubes does not grow significantly when the number of deposition scans increases. This is not surprising since the peak at about -0.5 V (Figure 1) concerning the formation of gas bubbles decreased in intensity during the scans. 13



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The opposite behavior was found concerning PThienothiophene-2. The nanotubes showed the top closed after 1 scan, but when the number of scans increased, the top of the nanotubes opened. The porosity also increases with the number of scans as well as the diameter of the nanotubes, which is expected due to the reduction of H+ into H2 taking place during each scan when the potential is close to -0.5 V. Hence, the presence of surface porosity can have a negative effect on θw, even though the surface with Thienothiophene-2 presents rough structures.

Figure 5. SEM images of PThienothiophene-1 (left) and PThienothiophene-2 (right) electrodeposited by cyclic voltammetry in Bu4NClO4/dichloromethane for different number of scans.

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Influence of the Electropolymerization Method. To better control the formation of nanotubes, Thienothiophene-2 was also electropolymerized at constant potential using different deposition charges (Qs) in order to visualize the changes in the surface structures at different stages. Table 3 shows the surface roughness and θw for the PThienothiophene-2 as function of the electrolyte and deposition charge. Table 3. Apparent Contact Angles of Water (θw) and Roughness Data for PThienothiophene-2 as a Function of the Electrolyte and the Number of Deposition Charge. Deposition charge Electrolyte Ra [nm] Rq [nm] θw [deg] α [deg] [mC cm-2] 12.5 6.4 8.3 118.7 25 7.9 11.1 116.1 50 37.1 47.6 38.4 Bu4NClO4 > 90 100 69.4 88.1 64.3 200 190.7 264.0 0.0 400 170.1 271.1 0.0 12.5 6.6 8.4 126.0 25 7.0 9.9 126.4 50 11.9 15.0 125.5 Bu4NBF4 > 90 100 30.0 38.2 126.7 200 593.6 1046.3 137.9 400 3137.3 4733.3 137.6 12.5 7.1 9.1 119.7 25 7.7 9.5 120.6 50 23.8 30.0 121.8 > 90 Bu4NPF6 100 82.7 107.2 123.4 200 323.2 503.1 123.5 400 3436.7 5453.3 115.8 Using Bu4NClO4 as electrolyte, all the nanoseeds are formed in the first instance of the electropolymerization. Figure 6 shows that for Qs = 12.5 mC cm-2, all the surface structures are nanoporous and the diameter ∅ ≈ 120-130 nm and the height ≈ 100 nm. Here, the surface is parahydrophobic with θw = 118.7° and high adhesion. Then, as Qs increases from 12.5 to

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50 mC cm-2, the diameter and the height of the surface structures increase, but the number of surface structures having their top open decreases. This number is about 100% for Qs = 12.5 mC cm-2, 50% for Qs = 50 mC cm-2 and 0% for Qs = 100 mC cm-2, which leads to a decrease in θw. From Qs = 50 mC cm-2, the height of the structures remains quite constant, but they begin to close which explains the decrease in θw. As the deposition charge increases, a new layer of polymer is deposited on the surface covering the nanotubes. Hence, fewer structures can be seen clearly after high deposition charges. The SEM images for the Thienothiophenes 1, 3, 4 and 5 are shown in Figure S1-S4 (see Supporting Information) as well as their CA and roughness in Table S1. It was possible to see that the type of structures did not change significantly when the electrochemical method was changed. The roughness is higher for PThienothiophene-4 which mirrors what is shown by cyclic voltammetry, and the wettability follows the same trends for both electrochemical methods.

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Figure 6. SEM images (25000x; without and after substrate inclination of 60°) of PThienothiophene-2 electrodeposited at constant potential, using different deposition charges in Bu4NClO4/dichloromethane.

Influence of the electrolyte. Two other electrolytes were tested to investigate their effect on the

surface

structures:

tetrabutylammonium

tetrafluoroborate

(Bu4NBF4)

and

tetrabutylammonium hexafluorophosphate (Bu4NPF6). Indeed, it is known that an electrolyte can affect the surface morphology since counterions of the electrolyte are present as dopants in the conducting polymers during their formation. Using Bu4NBF4 as electrolyte, all the nanoseeds are formed in the first instance of the electropolymerization as observed with Bu4NClO4 (Figure 7). Indeed, for Qs = 12.5 mC cm-2, all the surface structures are nanoporous and their diameter ∅ ≈ 50-60 nm and the height ≈ 100 nm are lower than that observed with Bu4NClO4, which leads also to higher θw = 126.0°. 17



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Then, as Qs increases from 12.5 to 100 mC cm-2, the diameter and the height of the surface structures increase, but the number of surface structures having their tops open decreases and

θw does not significantly change. For Qs = 100 mC cm-2 the height of the nanoplot is higher than that with Bu4NClO4. By contrast, for Qs > 100 mC cm-2, the surface structures change dramatically from nanoplots to tree-like structures with a large increase of θw up to 137.9°. There is also a large difference in the roughness for the high deposition charges. The tree-like structures formed yield a roughness 18 times higher than the roughness of the nanoseeds observed with Bu4NClO4 as electrolyte. Using Bu4NBF4, the structuration is still clearly observed on the surface at all charges, which is quite different than the uniformly covered surface formed when Bu4NClO4 was used as an electrolyte and a charge of 400 mC cm-2 was applied.

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Figure 7. SEM images (25000x; without and after substrate inclination of 60°) of PThienothiophene-2 electrodeposited at constant potential, using different deposition charges in Bu4NBF4/dichloromethane.

Using Bu4NPF6 as electrolyte (Figure 8), the results are relatively similar to those observed with Bu4NBF4, but the porous structures close much faster between Qs = 12.5 and 25 mC cm2

. Indeed, the tree-like structures are formed more quickly (Qs = 50 mC cm-2) than the

surfaces produced with Bu4NBF4 (Qs = 100 mC cm-2). However, the tree-like structures obtained after 400 mC cm-2 of imposed charge using Bu4NPF6 present almost the same roughness and height than those using Bu4NBF4. The θw obtained with Bu4NPF6 is also lower than that obtained with Bu4NBF4.

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Tree-like structures are also obtained by cyclic voltammetry for both Bu4NBF4 and Bu4NPF6 as electrolytes, shown in Figure S5 (see in Support Information). The surfaces present a very similar roughness compared to those which used Bu4NClO4, but, opposite of the Bu4NClO4 surfaces, also present an improvement in the wettability properties for 3 and 5 scans. However, as a broad range of potential is used, it is difficult to control the amount of polymer deposited on the surface. Hence, the polymer growth is more easily evaluated using the imposed potential method than cyclic voltammetry.

Figure 8. SEM images (25000x; without and after substrate inclination of 60°) of PThienothiophene-2 electrodeposited at constant potential, using different deposition charges in Bu4NPF6/dichloromethane.

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Discussion about the mechanism of porous structures formation. Here, it is clear that the formation of porous structures is due to the formation of gas bubbles during the electropolymerization process. Several authors already reported the formation of porous structures during the electropolymerization of pyrrole in water and using a surfactant.36-39 They observed that O2 and H2 bubbles can be formed from water following the tested potentials during the electropolymerization process. In our work, anhydrous dichloromethane is used as solvent but it is impossible to completely remove “trace” water. For example, it is known that trace water can be absorbed onto glass substrates and we used a glass electrochemical cell and also a glass electrode (SCE) for our experiments. Otherwise, H2 bubbles may also be formed from H+ ions released during the electropolymerization process. As a first step towards evaluating the effects of either water or H+ ions, 0.5 % of water was added in the experiments. Because the solubility of water in dichloromethane is extremely low, the concentration could not be higher. With this modification, an increase of the porosity of the structures both by cyclic voltammetry (Figure 9) and at constant potential (Figure 10) was observed. For example, at constant potential and after adding 0.5 % of water, the diameter of the nanoporosities increased from 120-130 nm to 180-200 nm for Qs = 12.5 mC cm-2 and from 180-200 nm to 310-330 nm for Qs = 50 mC cm-2. Moreover, the number of structures having their tops open is also larger. Hence, it seems that the water content plays a very important role on the formation of surfaces structures.

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Figure 9. SEM images of PThienothiophene-2 electrodeposited by cyclic voltammetry (number of scans: 3) in Bu4NClO4/dichloromethane + 0.5 % water.

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Figure 10. SEM images (25000x; without and after substrate inclination of 60°) of PThienothiophene-2 electrodeposited at constant potential, using different deposition charges in Bu4NClO4/dichloromethane + 0.5 % water. In an attempt to evaluate the effect of H+ ions, 1 % HClO4 (70 % in water) was used. Here, we clearly see in the SEM images (Figure 11 and Figure 12) that the presence of H+ ions has a negative impact on the formation of porous structures, both by cyclic voltammetry and at constant potential. For example, at constant potential only some nanorings are observed and only for Qs = 12.5 mC cm-2.

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Figure 11. SEM images of PThienothiophene-2 electrodeposited by cyclic voltammetry (number of scans: 3) in Bu4NClO4/dichloromethane + 1 % HClO4 for different number of scans.

Figure 12. SEM images (25000x; without and after substrate inclination of 60°) of PThienothiophene-2 electrodeposited at constant potential, using different deposition charges in Bu4NClO4/dichloromethane + 1 % HClO4.

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Hence, we can now conclude that it is really the water content, which influences the structure porosity and not the H+ ions. The mechanism is the following: - At high potential around 2.0–2.5 V vs SCE (at constant potential or by cyclic voltammetry): • n Monomer → Polymer + 2 ne- + 2 nH+ • 2 H2O → O2 (bubbles) + 4 H+ + 4 eAt constant potential, the porous structures are highly ordered because the polymerization and the formation of O2 bubbles occurred at the same time. Only trace water can explain the formation of gas bubbles at constant potential. - At low potential around -0.5 V vs SCE (only by cyclic voltammetry): 2 H+ + 2 e- → H2 (bubbles) As a consequence, it is not surprising to have many more porous structures formed by cyclic voltammetry than with constant potential because both O2 and H2 bubbles can be produced by cyclic voltammetry if the potential range is sufficiently large and only O2 bubbles form with constant potential processes. By cyclic voltammetry, the structures are more porous, but are disordered because the formation of H2 bubbles can destroy the polymer films already formed.

Discussion about the wettability results. The wettability results can be explained using the Wenzel and Cassie-Baxter equations.42,43 Because both equations are dependent on the apparent contact angles of the corresponding “smooth” surfaces, also called Young angle (θYw), these angles were first determined. These smooth surfaces were prepared at constant potential, but using low Qs of 1 mC cm-2 followed by a reduction step by cyclic voltammetry from 1.5 V to -1V. Roughness measurements (Table 4) were performed to confirm their ultralow roughness. These θYw showed that all these polymers are intrinsically hydrophilic with

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θYw θw, which is not the case here. Hence, the results can be explained only with the Cassie-Baxter equation confirming the presence of air between the water droplet and the surface. The Cassie-Baxter equation is cos θ = rf.f.cos

θY + f – 1, where rf is the roughness ratio of the substrate wetted by the liquid, f is the solid fraction and (1 – f) is the air fraction. The Cassie-Baxter equation can predict extremely high

θ with low adhesion (superhydrophobic properties) if the air fraction is extremely large and close to 1, but also high adhesion (parahydrophobic properties) if the solid fraction is more significant.14 Hence, the parahydrophobic properties of the nanotubes can be explained by the presence of a composite interface. Because the polymer is intrinsically hydrophilic, the water droplet should penetrate the space between the nanotubes (Wenzel state), which increases the water adhesion, but should not penetrate inside the nanotubes (Cassie-Baxter state), as shown in Figure 13. As a consequence, it is easy to demonstrate that the parahydrophobic properties are 26


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highly dependent on the diameter, height of the nanotubes as well as the distance between them. The parahydrophobic properties of the tree-like structures can also be explained if the fractal roughness of the structures impedes their full wetting (Figure 13).

Figure 13. Schematic representation of a water droplet deposited on intrinsically hydrophilic nanotubes and tree-like structures.

CONCLUSION

In the literature vertically aligned nanotubes are often prepared using hard templates22-30 but the number of methods developed to produce nanotubes on substrates in one-step and very quickly are not numerous. Among them, the electropolymerization is a choice process.36-41 Shi et al. was the first to report this possibility by electropolymerization of pyrrole in an aqueous solution of a surfactant in order to stabilize H2 or O2 bubbles produced during the electrodeposition process.36-38 Here, we showed for the first time the possibility to obtain not only arrays of nanotubes, but also tree-like structures with high water adhesion using thienothiophene derivatives, in organic solvent (dichloromethane) and without any surfactants. The formation of nanotubes is due to the stabilization by the polymer of gas bubbles produced in-situ during

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electropolymerization, and we demonstrated that the water content plays an important role even if it is not the unique parameter. We also showed that the electrochemical parameters are extremely important in the control of the resulting nanostructures. Using cyclic voltammetry as the electropolymerization method, the amount of released gas was found to be much higher, but the electropolymerization at constant potential allows a much easier control of the nanotube formation. Moreover, it was also possible to obtain arrays of tree-like structures at high deposition charge displaying extremely high water adhesion with high θw even with intrinsically hydrophilic polymers (θYw ≈ 70°). This work is extremely important for the control of water adhesion by electropolymerization for various potential applications in water transportation and harvesting, oil/water separation membranes, energy systems and biosensing.

ACKNOWLEDGMENT

This work was supported by CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brazil (Process Nº 202280/2014-4). The authors thank Jean-Pierre Laugier and François Orange of the Centre Commun de Microscopie Appliquée (CCMA) for the SEM images.

ASSOCIATED CONTENT Supporting Information Available: Additional SEM images and wettability data. This material is available free of charge via the Internet at http://pubs.acs.org.

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SYNOPSIS

Arrays of nanotubes and tree-like structures are produced by a one-step and templateless electropolymerization process using thienothiophene derivatives. The surfaces display both high water contact angles and high adhesion with an easy control of these properties.

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One-Step and Templateless Electropolymerization ... - ACS Publications

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