TEXTURIZATION OF FLUORINATED MATERIALS

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HYDROPHOBIC AND SUPERHYDROPHOBIC SURFACES BY

MEANS OF ATMOSPHERIC PLASMAS: SYNTHESIS AND

TEXTURIZATION OF FLUORINATED MATERIALS

UNIVERSITE LIBRE DE BRUXELLES

FACULTE DES SCIENCES

SERVICE DE CHIMIE ANALYTIQUE ET CHIMIE DES INTERFACES

JULIE HUBERT

A THESIS SUBMITTED FOR THE DEGREE OF PHILOSOPHIÆ DOCTOR IN SCIENCES

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Je tiens à exprimer toute ma gratitude au professeur Claudine Buess-Herman pour m’avoir chaleureusement accueillie au sein du service de Chimie Analytique et Chimie des Interfaces, ainsi que pour son suivi tout au long de mon parcours.

Je souhaite remercier particulièrement mon promoteur de thèse, François Reniers, pour son apport tant scientifique qu’humain. Merci pour ta confiance et ton optimisme constants ; cela n’a pas été facile tous les jours mais comme tu le dis si bien : Impossible n’est pas François ! Merci également pour les congrès et séjours d’études aux quatre-coins du monde qui m’ont enrichie autant scientifiquement que culturellement. Tu n’imagines pas à quel point je ressors grandie de ces 4 années !

I would like to thank the professor David B. Graves from UC Berkeley (California, USA) for having welcomed me within his team during these few months.

Je tiens à remercier le professeur Roberto Lazzaroni ainsi que Simon Desbief, Pascal Viville et Philippe Leclère de Materia Nova/Umons pour leur collaboration fructueuse concernant la caractérisation des échantillons par AFM.

Merci aux professeurs Patrick Bertrand et Arnaud Delcorte, ainsi qu’à Claude Poleunis de l’Institut de la Matière Condensée et des Nanosciences de l’UCL pour leur aide et disponibilité dans le cadre des caractérisations par SIMS.

Un grand merci au professeur Yves Geerts et à Guillaume Schweicher du Laboratoire de Chimie des Polymères de l’ULB pour leur précieuse connaissance des polymères et des rayons-X.

I would also like to thank Priya Laha and Alexandros Kakaroglou from the group of professor Herman Terryn at the VUB for their help with the ellipsometry measurements and electrical issues.

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Je souhaite remercier les membres du personnel pour tous les services rendus avec le sourire. Merci à Sandhya pour le soutien administratif, et à Philippe et (Mac)Albert pour le soutien technique. Eric merci pour tout ! L’accueil toujours chaleureux, les petits déjeuners conviviaux et ton enthousiasme continu n’ont fait qu’embellir mes journées.

Titi, mon éternel compère, un grand merci pour la bonne humeur et les connaissances que tu m’as transmises pendant ces (premières) années de thèse ! Comme quoi un duo chimiste-physicien peut donner du bon. Dédé, un grand merci pour ta disponibilité, ta solidarité, ton éternelle gentillesse et pour m’avoir guidé vers la polymérisation plasma... Nico, merci pour ton aide ; les fluorés n’auront bientôt plus de secrets pour toi. Francis, bedankt voor de zorgvuldige correcties !

Un merci tout particulier à mes gossips adorées pour toutes ces aventures. De Bruxelles à New-York, en passant par Prague, Miami ou Disney, merci pour tous ces bons moments ! Ce n’est plus avec des collègues, mais avec des amis que j’arrive à la fin de ce petit bout de ma vie…

Petit bout de ma vie tout au long duquel m’ont accompagnée mes inconditionnels amis chimistes ; Ludo, Maudich, Steph, Cédric, Nancy, Nico, Stéphane et Alexis. Je ne vais pas m’éterniser sinon on risquerait de crier au 3615 MY LIFE mais merci pour tout ; on y sera arrivés ! Merci également à Rylou, Sofia et Matthieu; le "P2", ma deuxième famille.

Thanks to Bianca, Amy, Gérome and Walter for helping me to enjoy the California life as it should be.

Je tiens à remercier Vivi, Pintus, Elena, Max, Michel, Julie, Jérôme, Benoît, Soumia, et Anso pour avoir toujours été présents. Vous avez fait de moi ce que je suis aujourd’hui et rien que pour ça merci !

Const, merci d’être là. Tes encouragements, ta non-contradiction et ta patience à vivre avec quelqu’un de peu disponible ces derniers mois m’ont permis d’arriver jusqu’au bout.

Finalement, un grand merci à ma famille, Papa, Maman, Céline, ainsi que Paulette et Henry. Vous m’avez toujours soutenue et aidée à me surpasser ; c’est en grande partie grâce à vous si je suis arrivée jusqu’ici.

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ABSTRACT

In this thesis, we focused on the understanding of the synthesis and texturization processes of hydrophobic and (super)hydrophobic fluorinated surfaces by atmospheric plasmas.

First, we focused on the surface modifications of a model surface, the polytetrafluoroethylene (PTFE), by the post-discharge of a radio-frequency plasma torch. The post-discharge used for the surface treatment was characterized by optical emission spectroscopy (OES) and mass spectrometry (MS) as a function of the gap (torch-sample distance), and the helium and oxygen flow rates. Mechanisms explaining the production and the consumption of the identified species (N2, N2+, He, O, OH, O2m, O2+, Hem) were proposed.

The surface treatment was then investigated as a function of the kinematic parameters (from the motion robot connected to the plasma torch) and the gas flow rates. Although no change in the surface composition was recorded, oxygen is required to increase the hydrophobicity of the PTFE by increasing its roughness, while a pure helium plasma leads to a smoothing of the surface. Based on complementary experiments focused on mass losses, wettability and topography measurements coupled to the detection of fluorinated species on an aluminium foil by XPS, we highlighted an anisotropic etching oriented vertically in depth as a function of the number of scans (associated to the treatment time). Atomic oxygen is assumed to be the species responsible for the preferential etching of the amorphous phase leading to the rough surface, while the highly energetic helium metastables and/or VUV are supposed to induce the higher mass loss recorded in a pure helium plasma.

The second part of this thesis was dedicated to the deposition and the texturization of fluorinated coatings in the dielectric barrier discharge (DBD). The effects of the nature of the precursor (C6F12 and C6F14), the nature of the carrier gas (argon and helium), the plasma

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RÉSUMÉ

L’objectif de ce travail fut de comprendre les processus de synthèse et de texturation de surfaces fluorées hydrophobes et (super)hydrophobes par plasmas atmosphériques. Notre travail s’est divisé en deux parties ; l’étude de modifications de surface d’un polymère conventionnel, le polytétrafluoroéthylène (PTFE) et la synthèse et texturation de couches fluorées déposées par plasma.

Nous avons premièrement étudié la post-décharge d’une torche plasma radiofréquencée (RF) utilisée dans le traitement du polymère. La post-décharge a été caractérisée par spectroscopie d’émission optique (OES) et spectrométrie de masse (MS) en fonction de la distance torche-échantillon et des flux d’hélium et d’oxygène. Des mécanismes expliquant la production et la consommation des espèces identifiées (N2, N2+, He, O, OH, O2m, O2+, Hem) ont été proposés.

Les modifications de surfaces ont été étudiées en fonction des paramètres cinématiques (provenant du robot sur lequel est montée la torche) et des flux de gaz. Bien qu’aucun changement de composition de surface n’ait été observé, la présence d’oxygène est nécessaire à l’augmentation de rugosité induisant une augmentation d’hydrophobicité. Un plasma d’hélium pur induit, quant à lui, une légère diminution de l’angle de contact traduite par un léger lissage de la surface. Grâce à la complémentarité des mesures de perte de masse, de mouillabilité et de topographie, couplées à la détection, par XPS, d’espèces fluorées éjectées du PTFE sur une feuille d’aluminium, nous avons mis en évidence un processus de gravure orienté verticalement en fonction du nombre de balayages (associé au temps de traitement). Les atomes d’oxygène sont supposés être les espèces responsables de la gravure préférentielle de la phase amorphe par rapport à la phase cristalline, menant ainsi à une surface plus rugueuse. Dans un plasma d’hélium pur, la perte de masse plus importante est supposée être induite par les métastables d’hélium et/ou rayonnements VUV énergétiques.

La seconde partie de cette thèse fut dédiée à l’optimisation des méthodes de dépôts et de texturation des couches fluorées dans une décharge à barrière diélectrique (DBD). L’influence de la nature du précurseur (C6F12 et C6F14) et du gaz porteur (argon et hélium), ainsi que de la

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LIST OF ABBREVIATIONS

AFM: atomic force microscopy DBD: dielectric barrier discharge gs: gap (distance torch-surface sample)

HR: high resolution Ls: scanning length

MS: mass spectrometry Ns: number of scans

OES: optical emission spectroscopy (pp): plasma polymerized

PECVD: plasma-enhanced chemical vapour deposition PTFE: polytetrafluoroethylene

RF: radio-frequency RMS: root mean square

SIMS: secondary ion mass spectrometry UV: ultraviolet

VUV: vacuum ultraviolet vs: scanning velocity

WCA: water contact angle

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PUBLICATIONS

§ Plasma polymerization of C4Cl6 and C2H2Cl4 at atmospheric pressure

J. Hubert, C. Poleunis, A. Delcorte, P. Laha, J. Bossert, S. Lambeets, A. Ozkan, P. Bertrand, H. Terryn, and F. Reniers,

Polymer, vol. 54, no. 16, p. 4085, 2013

§ Competitive and synergistic effects between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He–O2 post-discharge

T. Dufour, J. Hubert, N. Vandencasteele, P. Viville, R. Lazzaroni, F. Reniers

Journal of Physics D: Applied Physics, vol. 46, 315203 (14pp), 2013

§ Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma

C.Y. Duluard, T. Dufour, J. Hubert, F. Reniers

Journal of Applied Physics, vol. 113, 093303 (12pp), 2013

§ Etching processes operating on a PTFE surface exposed to He and He-O2 atmospheric

post-discharges

J. Hubert, T. Dufour, N. Vandencasteele, S. Desbief, R. Lazzaroni, F. Reniers

Langmuir, vol. 28, no. 25, p. 9466, 2012

§ Chemical mechanisms inducing a DC current measured in the flowing post-discharge of an RF He-O2 plasma torch

T. Dufour, J. Hubert, N. Vandencasteele, F. Reniers

Plasma Sources Science & Technology, vol. 21, 045013 (10pp), 2012

§ PTFE surface etching in the post-discharge of a scanning RF plasma torch: evidence of ejected fluorinated species

T. Dufour, J. Hubert, P. Viville, C.Y. Duluard, S. Desbief, R. Lazzaroni, F. Reniers

Plasma Processes and Polymers, vol. 9, no. 8, p. 820, 2012

§ Synthesis of membrane-electrode assembly for fuel cells by means of (sub)atmospheric plasma processes

D. Merche, T. Dufour, J. Hubert, C. Poleunis, S. Yunus, A. Delcorte, P. Bertrand, F. Reniers

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§ Surface modification of PTFE using an atmospheric pressure plasma jet in argon and argon + CO2

Sarani, N. De Geyter, A. YU. Nikiforov, R. Morent, C. Leys, J. Hubert, F. Reniers

Surface and Coatings Technology, vol. 206, no. 8-9, p. 2226, 2012

§ One Step Polymerization of Sulfonated Polystyrene Films in a Dielectric Barrier Discharge

D. Merche, J. Hubert, C. Poleunis, S. Yunus, P. Bertrand, P. De Keyzer, F. Reniers

Plasma Processes and Polymers, vol. 7, no. 9-10, p. 836, 2010

PATENTS

§ Method for polymer plasma deposition F. Reniers, J. Hubert,

WO/2012/007466, 2010

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CHAPTER 1

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THESIS OUTLINE

1. Aim of the research

The main purpose of this work is to understand the synthesis and texturization processes of (super)hydrophobic fluorinated surfaces using atmospheric plasmas.

2. Context

In the last few decades, the industrial applications of polymeric materials have grown intensely thanks to their unique chemical, physical and mechanical properties. Biomedical, microelectronics, food packaging and aerospace industries are some of the sectors taking advantages of polymeric materials. However, certain applications require the optimization of the polymeric surface properties - such as wettability or biocompatibility - to interact properly with its surrounding. Among the different techniques, plasma processing presents major advantages to improve surface properties without affecting the bulk material. It is indeed a very fast method, easy to implement, as well as a low-temperature solvent-free process. For many years, plasmas have been used to modify and functionalize surfaces as well as to synthesize inorganic and organic coatings. Initially, the approach was mainly empirical and focused on plasma techniques at low pressure. However, for several years, fundamental studies aiming to understand the mechanisms both in the gas phase and at the interface have been conducted. The use of atmospheric plasmas is also expanding because they allow one to avoid the constraints of high vacuum and require low cost systems, despite more complex mechanisms involving a more difficult understanding of the underlying processes.

Low-pressure studies showed that plasmas could be used as a mean to alter the characteristics of treated-surfaces. It was shown that subjecting polymeric surfaces to oxygen or nitrogen plasmas could lead to the grafting of oxygenated or nitrogen-containing polar functions [1]. In the case of O2 plasmas, C=O, OH or COOH could be grafted depending on the plasma

conditions. Regarding nitrogen plasmas, it was shown that NH3 plasmas induced a higher

grafting of amine functions than N2 plasmas. Medard et al. showed that the use of CO2

plasmas could increase the percentage of carboxylic functions [2]. Most of the studies focus on the surfaces functionalization by low-pressure plasmas in view of improving the hydrophilicity.

The creation of hydrophobic surfaces proceeds from another approach. In most studies, plasmas containing a CxFy fluorinated gas (e.g. CF4, C2F4, C2F6, C4F8) are used for the

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Moreover, some studies were conducted to achieve (super)hydrophobic surfaces using fluorinated precursors. For instance, experiments using tetrafluoroethyelene (TFE) in a RF plasma discharge showed the formation of (super)hydrophobic coatings defined by a diffraction peak characteristic of PTFE (2θ = 18°) [8]. The formation of these coatings seems to indicate the presence of a mechanism able to assemble ordered (CF2-CF2)n chains at the

substrate surface. A magnetron sputtering plasma of CF4 showed that an excess of fluorine

tends to decrease the hydrophobic properties because of damages to the structure [9]. In general, the (super)hydrophobic property is related to the roughness of the film as described by Wenzel [10] and Cassie-Baxter [11].

In a series of studies, our group investigated the surface modifications of some polymers with different F/C ratios (PE, PVF, PVDF, PTFE) by O2 and N2 low-pressure plasmas [12], [13],

[14], [15]. Both the choice of the reactive gas and of the power has an effect on the surface modification as shown by XPS. A singular behaviour was observed when treating the PTFE by RF oxygen plasmas: while for all of the above polymer/gas couples a decrease in the WCA is observed, the WCA of the PTFE/O2 couple drastically increases. This singularity was

attributed to the increase in the roughness of the PTFE. The necessity of having charged species (supposed to be the electrons) and atomic oxygen to increase the roughness was highlighted at low pressure [12]. Our group proposed the existence of an etching mechanism based on the species detected in the gas phase (CO, CO2, and F) and on the significant

increase in the base pressure. AFM measurements confirmed the change in the morphology of the surface and so the etching process.

In another study, our group attempted to apply the same concept by means of the oxidant post-discharge of an atmospheric plasma [16]. An increase in the WCA was observed in the Ar-O2 post-discharge but not in a pure argon plasma which tends to confirm the observations

at low pressure. Along with the wettability properties, an increase in the roughness was also observed by AFM. However, the presence of electrons or other charged species in the post-discharge was not confirmed as well as the impact of the atomic oxygen density on the increased roughness.

In parallel, our laboratory has developed original synthesis methods of organics coatings by liquid-based precursors at atmospheric pressure [17], [18], [19] and patented methods to obtain fluorinated and chlorinated coatings by atmospheric plasma [20], [21].

3. Scientific strategy

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a. Plasma treatments of PTFE in the post-discharge of an atmospheric plasma torch In order to explain the processes occurring at the plasma/polymer interface, we first characterized the chemistry of the post-discharge used for the treatment of the polymer. The influence of the helium and oxygen flow rates as well as the impact of the gap (distance torch/surface) were evaluated by mass spectrometry and optical emission spectroscopy. The second step in this study was dedicated to the surface processing of PTFE by the post-discharge of an atmospheric plasma torch. Our initial goal was to study the texturization process leading to (super)hydrophobic surfaces using helium-oxygen plasmas. Indeed, according to the literature, oxygen was identified as being essential to increase the hydrophobicity of the PTFE. The influence of the kinematic parameters of the motion robot associated to the plasma torch and the plasma parameters were studied by mass loss, water contact angle measurements (WCA), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and X-ray diffraction (XRD).

In view of the results obtained using He-O2 plasma, we investigated the surface properties of

PTFE modified by a pure helium treatment. The use of different characterization techniques to analyse both the gas phase and the surface allowed us to propose a model describing the two processes.

This study led to the publication of four articles entitled: “Chemical mechanisms inducing a DC current measured in the flowing post-discharge of an RF He-O2 plasma torch” [22],

“PTFE surface etching in the post-discharge of a scanning RF plasma torch: evidence of ejected fluorinated species” [23], “Etching processes operating on a PTFE surface exposed to He and He-O2 atmospheric post-discharges” [24] and “Competitive and synergistic effects

between excimer VUV radiation and O radicals on the etching mechanisms of polyethylene and fluoropolymer surfaces treated by an atmospheric He–O2 post-discharge” [25].

b. Plasma treatments of PTFE in a dielectric barrier discharge

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c. Plasma polymerization of CxFy coatings in a dielectric barrier discharge

The last part of this work was dedicated to the deposition and the texturization of fluorinated coatings from liquid precursors in the dielectric barrier discharge. The effects of the nature of the precursor, the nature of the carrier gas, the plasma power, and the precursor flow rate were investigated in terms of chemical composition, wettability, topography and crystallinity by secondary ion mass spectrometry (SIMS), XPS, WCA, AFM and XRD. The impact of the deposition rate and the layer thickness on the hydrophobic properties as well as the polymerization processes through the gas phase characterization will also be discussed.

The study of plasma polymerization of liquid precursors was the subject of some publications and patents entitled: “One Step Polymerization of Sulfonated Polystyrene Films in a Dielectric Barrier Discharge” [18], “Plasma polymerization of C4Cl6 and C2H2Cl4 at

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4. References

[1] E.M. Liston, L. Martinu, and M.R. Wertheimer, J. Adhes. Sci. Technol., vol. 7, no. 10, p. 1091, 1993.

[2] N. Medard, J-C. Soutif, and F. Poncin-Epaillard, Surf. Coat. Technol., vol. 160, p. 197, 2002. [3] A. Milella, F. Palumbo, P. Favia, G. Cicala, and R. d'Agostino, Plasma Process. Polym., vol. 1, p.

164, 2004.

[4] G. Cicala, A. Milella, F. Palumbo, P. Rossini, P. Favia, and R. d'Agostino, Macromolecules, vol. 35, p. 8920, 2002.

[5] R. Prat, Y.J. Koh, Y. Babukutty, M. Kogoma, S. Okazaki, and M. Kodama, Polymer, vol. 41, p. 7355, 2000.

[6] F. Fanelli, R. d'Agostino, and F. Fracassi, Plasma Process. Polym., vol. 4, p. 797, 2007.

[7] I.P. Vinogradov, A. Dinkelmann, and A. Lunk, Surf. Coat. Technol., vol. 174-175, p. 509, 2003. [8] P. Favia, G. Cicala, A. Milella, F. Palumbo, P. Rossini, and R. d'Agostino, Surf. Coat. Technol.,

vol. 169-170, p. 609, 2003.

[9] Y. Zhou, X. Song, M. Yu, B. Wang, and H. Yan, Surf. Rev. Lett., vol. 13, no. 1, p. 117, 2006. [10] R.N. Wenzel, Ind. Eng. Chem., vol. 28, p. 988, 1936.

[11] A.B.D. Cassie and S. Baxter, Trans. Faraday Soc., vol. 40, p. 546, 1944.

[12] N. Vandencasteele, B. Broze, S. Collette, C. De Vos, P. Viville, R. Lazzaroni, and F. Reniers,

Langmuir, vol. 26, p. 16503, 2010.

[13] N. Vandencasteele, D. Merche, and F. Reniers, Surf. Interface Anal., vol. 38, p. 526, 2006. [14] N. Vandencasteele, B. Nisol, P. Viville, R. Lazzaroni, D.G. Castner, and F. Reniers, Plasma

Process. Polym., vol. 7, p. 661, 2008.

[15] N. Vandencasteele, H. Fairbrother, and F. Reniers, Plasma Process. Polym., vol. 2, p. 493, 2005. [16] E.A.D. Carbone, N. Boucher, M. Sferrazza, and F. Reniers, Surf. Interface Anal., vol. 42, p. 1014,

2010.

[17] B. Nisol, C. Poleunis, P. Bertrand, and F. Reniers, Plasma Process. Polym., vol. 7, no. 8, p. 715, 2010.

[18] D. Merche, C. Poleunis, S. Yunus, P. Bertrand, P. De Keyze, and F. Reniers, Plasma Process.

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[19] J. Hubert, C. Poleunis, A. Delcorte, P. Laha, J. Bossert, S. Lambeets, A. Ozkan, P. Bertrand, H. Terryn, and F. Reniers, Polymer, vol. 54, no. 16, p. 4085, 2013.

[20] N. Vandencasteele, O. Bury, and F. Reniers, "Method for depositing a fluorinated layer from a precursor monomer," WO/2009/030763, 2009.

[21] F. Reniers and J. Hubert, "Method for polymer plasma deposition," WO/2012/007466, 2010. [22] T. Dufour, J. Hubert, N. Vandencasteele, and F. Reniers, Plasma Source Sci. Technol., vol. 21, p.

045013 (10pp), 2012.

[23] T. Dufour, J. Hubert, P. Viville, C.Y. Duluard, S. Desbief, R. Lazzaroni, and F. Reniers, Plasma

Process. Polym., vol. 9, no. 8, p. 820, 2012.

[24] J. Hubert, T. Dufour, N. Vandencasteele, S. Desbief, R. Lazzaroni, and F. Reniers, Langmuir, vol. 28, no. 25, p. 9466, 2012.

[25] T. Dufour, J. Hubert, N. Vandencasteele, P. Viville, R. Lazzaroni, and F. Reniers, J. Phys. D:

Appl. Phys., vol. 46, p. 315203 (14pp), 2013.

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CHAPTER 2

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INTRODUCTION

The Nobel laureate physicist Wolfgang Pauli once said: “God made the bulk; surfaces were

invented by the devil”, but this witty observation hides a highly complex reality. Pauli

explained that these characteristics were due to the fact that the surfaces and interfaces define a boundary between a material and its surrounding environment, influencing the interactions with the latter. While inside the solid, each atom is surrounded by other similar atoms, the surfaces atoms can interact with three different neighbours: atoms from the bulk, other atoms from the same surface and atoms in the adjacent phase. As a result, the properties of the surface are different from the bulk of the material, and surface atoms exhibit a high chemical reactivity that makes them a favoured medium for chemical and biological processes in nature and technological applications [1].

Over millions of years of evolution, the surfaces in nature have been improved to enhance the interactions with the external world. For instance, plants leaves developed a (super)hydrophobic effect preventing them from wetting and giving them a self-cleaning property. Several animals on earth or in the aquatic world developed adhesive at their surfaces in order to easily climb substrates (e.g. gecko or ladybird foot pads) or attach to rocks to counteract ocean currents [2]. For examples of these particular adhesive surfaces, refer to Figure 1.

Figure 1 - (a)-(b) Gecko feet exhibiting reversible adhesion [3], (c)-(d) Adhesive tube feet of the sea star Asterias rubens [4]

From a technological perspective, nature appears to be a splendid source of ideas in order to improve the everyday life of our society. Mimicking biology or nature has then become a booming discipline called “biomimetics”. In 1957, Otto Schmitt proposed this term while he was developing a physical device that mimicked the electrical action of nerves [5]. Many more examples can be enumerated, such as the photovoltaic cells inspired from the photosynthesis process, or the micro-structured surfaces leading to drag reduction for fluid flow (texture of shark skin).

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1. (Super)hydrophobic surfaces: The Lotus effect

Hydrophobic and water-repellent abilities of several plants leaves have been known for a long time. Since the beginning of the 20th century, researchers have observed that surfaces with high water contact angles could be obtained by the deposition of several coatings [6], [7]. Based on this knowledge, a few empirical models were proposed to illustrate the surface wetting property such as the well-known Wenzel [8] and Cassie-Baxter [9] models. The scanning electron microscope (SEM) studies since the 1970s have revealed that the hydrophobicity of the leaf surface was related to its micro-structure. Indeed, the water repellent effect is mostly caused by the epicuticular wax crystalloids that cover the cuticular surface in a regular microrelief of about 1-5 µm in height [10]. The most famous example is the leaves of the Lotus (Nelumbo nucifera), with water contact angles of about 160° and a very low hysteresis (quantifying the rolling-drop angle) [11], [12]. Neinhuis and Barthlott, in 1997, obtained pictures of a lotus leaf as seen in Figure 2 [13], [14], where the structures consist of a combination of a two-scale roughness: 10 µm (rough structure) and 100 nm (fine structure).

Figure 2 – (Super)hydrophobic surface in nature. (a) Lotus leaves show self-cleaning properties, note that dust is accumulated in the water droplet at the centre of the leaves [15]. (b) SEM image of the

surface structures on the Lotus leaf (scale bar = 50 µm) [14].

The self-cleaning effect of the lotus leaf is evident and the underlying mechanism has been studied. Neinhuis and Barthlott illustrated the behaviour of a water droplet and the dust present at wax smooth and wax rough surfaces in Figure 3. If the leaf shows high enough hydrophobicity, the water drop is almost spherical and easily rolls across the surface as shown in Figure 3(b), increasing the amount of dirt particles picked up on its way [16]. In the case of waxy leaves which do not have the rough microstructures, the self-cleaning will be much less efficient. Because of the nonslip boundary condition, the water droplet falls across the dirt particles but the particles are mainly displaced to the sides of the droplet and re-deposited behind it, as illustrated in Figure 3(a).

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The wax has a water contact angle of about 103-110° [17], [18], but is not highly hydrophobic. The (super)hydrophobic property of the Lotus leaves is then presumed to be due to the combination of roughness and wax, as illustrated in the Figure 3(b).

Figure 3 – Diagram demonstrating the cleaning mechanism on (a) smooth and (b) rough super-hydrophobic surfaces [13]

In the animal kingdom, water repellent structures are also quite common and allow, for example, some water-walking insects to float and move on water [19]. It was indeed observed that the cuticle of these insects and spiders is covered by a waxy layer, with morphologies such as hairs or scales. Water strider legs are an example of extreme hydrophobicity as they present a high density of micro-sized hairs that are covered by grooves. Figure 4 represents the non-wetting leg of a water strider with a water contact angle of about 168°.

Figure 4 – SEM images of water strider leg. (a) WCA (b) spindle-like structure (scale bar 20µm) (c) nanoscale grooved structure (scale bar 200 nm) [19]

Inspired by nature, artificial (super)hydrophobic surfaces have been fabricated by combining rough surface morphology and low surface energy coatings. Several approaches detailed below have been developed to manufacture these specific surfaces.

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2. Methods for the fabrication of hydrophobic and (super)hydrophobic

surfaces

Techniques to create (super)hydrophobic surfaces can be divided into two main categories: top-down and bottom-up approaches. Top-down manufacturing involves a reduction of the materials dimensions to the nanoscale while the bottom-up approach is characterized by the synthesis of small units into the desired structure. Typical techniques of the top-down approach are lithography, template and plasma treatment. Examples of bottom-up methods include chemical deposition, layer-by layer (LBL) deposition, or sol-gel process. These few samples (non-exhaustive list) of practical synthesis of (super)hydrophobic surfaces are detailed below.

2.1. Top-down approaches

a. Lithography

Lithography methods are well-established techniques to create regular micro/nano-patterns, in which a design is transferred from a master onto a substrate surface, allowing copies to be made. In photolithography for example, a photoresist (e.g. photoactive polymer) is exposed to radiation through a photomask and then the exposed or unexposed parts are removed leaving a positive or negative image of the mask on the surface. The patterned surface is then either used like this or used as a mask on the substrate for deposition or further etching. Depending on the radiation source (UV, X-ray, e-beam, etc.), different categories can be defined. However, for the preparation of (super)hydrophobic surfaces, an extra surface treatment step of hydrophobization is needed.

Öner and McCarthy used photolithography to transfer patterns to silicon wafers before hydrophobization by silanization reagents. WCAs higher than 150° were reached [20]. Martines et al. fabricated nanopillars and nanopits by using e-beam lithography and an extra treatment by the octadecyltrichlorosilan. They obtained a (super)hydrophobicity characterized by a WCA of 164° and hysteresis of 1° [21]. (Super)hydrophobic ZnO nano-in-micro hierarchical structures were obtained from UV lithography followed by hydrothermal synthesis, and characterized by a WCA higher than 160° [22].

b. Template

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Figure 5 – Illustration of the steps in the Lotus leaf imprinting and SEM images of (a) a natural Lotus leaf and (b) its positive PDMS replica [12].

Sun et al. made an imprint from a Lotus leaf cast using PDMS as pre-polymer. After casting and lifting off the PDMS, a negative replication of the Lotus leaf structure was obtained (see scheme in Figure 5). The cast or negative template was then used as a master for the preparation of a positive replica of the lotus leaf. SEM images of the positive replica showed small structures with an average distance of 6 µm which was very similar to the original Lotus leaf structures and a WCA of 160° as shown in Figure 5 [12].

c. Plasma treatment

Plasma treatment can involve an etching of the surface by reactive species such as oxygen, chlorine or fluorine that are generated in the gas discharge. The technique easily leading to rough surfaces has been widely used for the preparation of hydrophobic surfaces.

For instance, low-density polyethylene (LDPE) was treated by O2 and CF4 plasmas to get

water contact angles of 170° with low hysteresis and roughness up to 400 nm [25], [26]. The hydrophobicity of the polytetrafluoroethylene (PTFE) was increased by a low-pressure O2

plasma to a value higher than 150° for a RRMS of about 500 nm [27]. In some cases, plasma

can also be combined with lithography or template-based methods [28]. b

a

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2.2. Bottom-up approaches

a. Chemical deposition

The term chemical deposition refers to chemical reactions where the products self-assemble and depose onto a substrate. Depending on the conditions, chemical deposition can denote several techniques: chemical bath deposition (CBD), chemical vapour deposition (CVD) and electrochemical deposition.

For instance, CBD was used for the deposition of highly textured zinc oxide cones onto glass substrates from a mixture of Zn(NO3)2 and ammonia, with a WCA of 150° obtained after heat

treatment [29]. The chemical vapour deposition is a process in which chemical precursors are carried to the vapour phase to be decomposed or react on a heated substrate. Huang et al. formed aligned carbon nanotubes on a Fe-N coated silicon substrate followed by the deposition of a ZnO layer giving WCAs of about 160° [30]. Compounds such as CF4 can also

be used in the CVD process to introduce fluorine on carbon nanotubes, increasing the hydrophobicity [31]. A one-step CVD of silica-based precursors (e.g. tetraethylorthosilicate + vinyltrimethoxylisane) showed WCAs higher than 150° [32],

Specific chemical vapour depositions have been developed and show promising results. Plasma-enhanced CVD (PECVD) uses plasma energy to activate the precursor. Hydrophobic surfaces were obtained by PECVD of compounds such as fluorocarbons [33], [34], or C2H2

leading to carbon nanotube forests [35]. The additional use of microwave energy was able to depose siloxane surfaces from methylsilane compounds [36]. The Aerosol assisted CVD (AACVD) is another method to create thin films where the precursor solution is vaporised by ultra-sonic vibration and atomiser devices [37]. (Super)hydrophobic films were produced from the silicone elastomer (Sylgard ® 184) on glass substrate giving a WCA of 165° with a low hysteresis [38].

Finally, electrochemical deposition of zinc oxide [39], nickel and copper [40] led to hydrophobic surfaces after modification with hydrophobic SAMs such as fluoroalkylsilane.

b. Layer-by-layer (LBL) deposition

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Figure 6 – SEM images of the honeycomb-like structure (a) without nanoparticles (b) fully treated film (c) WCA of the (super)hydrophobic surface [41]

For example, multilayers assembled from poly(allylamine hydrochloride) and poly(acrylic acid) in an appropriate combination of acidic treatments could induce pores of 10 µm and a honeycomb-like structure [41]. This porous structure was then used for SiO2 nanoparticles

deposition followed by a CVD of a semifluorinated silane. The authors recorded WCAs as high as 172° at the surface of the fully treated films. The important steps of this process are illustrated in Figure 6.

c. Sol-gel process

Sol-gel is a two-steps process [42]. The first reaction consists in the hydrolysis of precursors which are usually metal alkoxides because they react readily with water.

Si(OR)4 + H2O à HO-Si(OR)3 + ROH

Depending on the amount of water and catalyst present, hydrolysis may go to completion and all the OR groups are replaced by OH. The second step is the condensation of the hydrolyzed molecules liberating water or alcohol

(OR)3Si-OH + HO-Si(OR)3 à (OR)3Si-O-Si(OR)3 + H2O

During the network formation process, a large amount of solvent is impregnated in the network and a gel is formed.

As the low surface energy material is usually included in the sol-gel process, no post-hydrophobization step is required. For example, surface energy and roughness were controlled by the use of colloidal silica nanoparticles and fluoroalkylsilane coupling agent in a TEOS-based sol-gel. The spin-coated solution onto substrates showed WCAs of 150° [43].

We have enumerated a non-exhaustive list of methods leading to hydrophobic and/or (super)hydrophobic surfaces. Although we divided these methods into the top-down and bottom-up approaches, a combination of several techniques might be of great interest to get

a b

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the two-scale roughness characteristic of the Lotus effect. As previously mentioned, artificial (super)hydrophobic surfaces are made by combining rough surface morphology and low surface energy coatings. The two basic approaches can then be summarized in the two following categories: making a rough surface from a low surface energy material (mostly top-down methods) and modifying a rough surface with a material of low surface energy (bottom-up methods being related to the control of the chemistry).

Most of the methods listed above need several steps in order to induce a roughness besides controlling the chemistry. Among all these methods, the use of plasmas is a very promising synthetic route to produce (super)hydrophobic surfaces and has not been widely studied especially at atmospheric pressure. Moreover, this approach has the advantage of reducing the number of steps required to modify the surface of materials. Tailoring the surface of polymeric materials is of particular interest as polymers are omnipresent in our daily life because of their wide range of applications. The two main pathways involving the plasma processing of polymers are: the direct modification of polymer surfaces and the plasma deposition of thin polymeric films. The direct modification of polymer surfaces can also be sub-divided into two processes: the etching of a surface and the grafting of functions such as the incorporation of fluorine at the surface of the material called fluorination.

Surface terminal group WCA (°)

CF3 120

CF2 108

CH3 111

CH2 94

Table 1 – Water contact angle on smooth surfaces [44]

As stated before, both roughness and low surface energy are necessary for the hydrophobic character. Indeed, the highest known WCA on a smooth low-energy surface is about 110-120° (see Table 1) [44]. In this work, we then selected fluorocarbons compounds presenting the lower surface energy (e.g. about 20 mN/m for PTFE [45]).

2.3. Focus on plasma modifications of (fluoro)polymers

Plasma treatments of fluoropolymer surfaces have drawn a special interest and particularly in the case of PTFE because of its outstanding properties such as high thermal stability, low coefficient of friction, hydrophobicity, and chemical inertness which explain its use for biocompatibility and self-cleaning applications [46].

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for device performance and reliability [49], [50]. Several plasma treatments showed potential in decreasing the water contact angle with the use of gases such as ammonia [51], [52], hydrogen [52], N2/H2 [53], CH4/N2 [54], or air [55], [56].

a. Treatment of PTFE by inert gases

The use of inert argon or helium gases also seems to lead to a (small) decrease in the hydrophobicity of the PTFE. An atmospheric plasma jet supplied with argon was used to vary the water contact angle (85-105°) as a function of the treatment time. This decrease in the WCA was mostly related to the defluorination and the small incorporation of oxygen into the polymer [57]. Very high wettability and strong oxidation of the PTFE (up to 8% of oxygen) were observed in a low-pressure argon discharge [58]. The oxygen is assumed to come from the ambient residual atmosphere in the plasma reaction chamber. An earlier study in argon at low pressure recorded a WCA of 79° in which the authors assumed that this wettability was mainly due to the loss of a significant fraction of the fluorine atoms compared to the amount of oxygen uptake [59]. Moreover, argon was identified to be responsible for an etching or sputtering of the PTFE as fluorocarbons films were deposited onto a substrate during the RF magnetron plasma [60].

Similarly to argon plasmas, helium plasmas operating at atmospheric pressure usually lead to quite a strong decrease (50°) in WCA with a defluorination and the incorporation of oxygen observed by XPS [50], [61]. The group of Pappas, working with atmospheric He discharges, displayed an increase in the adhesion strength of PTFE to polyurethane. Preliminary results indicated that the plasma treatment improves the metal adhesion (thin Au/Pd film) to the surface [62]. Helium was also identified to be responsible for the sputtering of PTFE in a low-pressure plasma as PTFE-like films were deposited [63], [64]. Indeed, these authors measured fluorocarbon species in the gas phase with a UV emission spectrometer and analysed the fluorinated fragments deposited onto substrates. Another study highlighted the ejection of solid polymer and mass loss when a PTFE sample was submitted to a glow discharge supplied with helium [65].

b. Treatment of PTFE by O2-containing gases

Treatments of PTFE by oxygen-containing plasmas are more controversial, as both hydrophilic and hydrophobic modifications were obtained. For instance, some studies showed that oxygen radio-frequency plasmas could lead to an etching of the surface characterized by oxygen grafting and a decrease in WCA [56], [66], [67]. Liu et al. made a comparative study between a RF oxygen plasma at low pressure and an air-DBD plasma and showed that, in terms of energy amount, both types of plasmas could improve the PTFE surface wettability [56]. The chemistry of the PTFE varied a lot in the RF O2 plasma; authors assumed that the

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Wilson et al. showed that O2 treatments resulted in a slight defluorination whereas Ar, N2 or

NH3 treatments changed significantly the chemistry. Moreover, the wettability was correlated

with the surface chemical changes [67]. Salapare et al. studied the hydrophilic and (super)hydrophobic character of PTFE treated by O2 low-pressure plasmas [68]. They

observed distinct behaviours for low and high-energy plasmas. In a low-energy O2 plasma, the

WCA decreases to 87° while in the higher-energy plasma, it goes up to 151° with a reduction in the hysteresis. These variations were explained by the variation of roughness induced by the plasma as no chemical modification was observed in all conditions.

Many other studies also observed that an oxygen plasma treatment could induce a roughening of the samples with no change in the chemical composition [69], [70], [27]. Ryan et al. accomplished a quite complete study investigating the surface treatment of PTFE by several gases such as O2, H2, N2, He, Ar and CF4 [69]. The oxygen plasma did not induce any

variation in the chemical composition but its use led to the rougher surface. The hydrogen glow discharge caused the greatest loss of fluorine, probably because of the favourable formation of HF. Nitrogen and noble gases seemed to promote the reorganization of the structure leading to the formation of a fibrillar microtexture. CF4 being a source of fluorine,

they caused chain rupture followed by fluorination of CF2 leading to the detection of CF3

group in the XPS analysis. More recently, Vandencasteele et al. highlighted the synergetic role of charged species (electrons) and atomic oxygen in the etching of PTFE by low-pressure oxygen plasmas [27]. A mechanism was proposed based on the ejected species detected in the gas phase (CO, CO2 and F besides a significant increase in the pressure). The analysis by

AFM confirmed the change in the structure of the sample. (Super)hydrophobic PTFE surfaces were created by an Ar + O2 low-pressure plasma [71]. Although no change in the chemical

composition was recorded, leaf-like micro-protrusions were observed after a 4 hour-plasma treatment related to the WCA of 158° and characterized by a roughness of about 2 µm. Most of the studies were realized at low pressure, only few researches showed results focused on the (super)hydrophobicity of PTFE by O2 in atmospheric plasmas. The treatment of PTFE

by the post-discharge of an Ar-O2 plasma torch induced an increase in the WCA (130°) while

it was not observed in the pure argon plasma [72]. AFM measurements proved a similar roughness variation than the results at low pressure but no complete study of the post-discharge was made. Trigwell et al. showed that PTFE was chemically resistant to an atmospheric pressure glow discharge supplied with He-O2 with a WCA up to 125° [73].

c. Treatment of other (fluoro)polymers

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mostly with the hydrogen on PVDF [75]. In order to improve the hydrophobicity of these polymers, a fluorination is necessary and tetrafluoromethaneplasma treatments are a typical source of fluorine. For instance, additionally to the increase in fluorine content, the WCA of PVDF was increased up to 160° by CF4 plasmas and an etching property was highlighted by

the increase in roughness [76], [77].

Finally, hydrophobic surfaces can also be obtained by fluorination of hydrocarbon polymers. For instance, LDPE was treated by O2 and CF4 plasmas to get water contact angles of 170°

with low hysteresis and roughness up to 400 nm [25]. Due to its etching properties, CF4 is

assumed to be a good gas for fluorination, instead of lower F/C compounds (e.g. C4F8) which

will mostly lead to a thin film deposition [78].

2.4. Focus on plasma deposition of fluorocarbons

The second main pathway in the realization of hydrophobic surfaces assisted by plasmas is the plasma deposition of low surface energy coatings, such as fluorocarbon films. The first studies of fluorocarbon plasmas date back to the 70s when CF4 was used in microelectronics

for the etching process of SiO2 [79], and some fluorocarbon gases were employed for plasma

polymerization, additionally to the sputtering of PTFE surfaces [80]. Since then, many reviews have been published regarding the plasma polymerization of fluorocarbons along with the development of diagnostics of the gas phase and surface [81], [82], [83].

The decomposition of fluorocarbons gases in a discharge leads to the production of CFx,

fluorine atoms and ions which can either react in the gas phase or diffuse to the surface and take part in the film growth. The CFx being involved in the growth of the fluoropolymer film,

their concentration influences the polymerization rate. Fluorine atoms, on the contrary, promote the etching of the surface and the deposition rate is therefore a result of the competition between these two processes [84]. The F/CFx density ratio in the plasma has then

been proposed as one of the most relevant internal parameters to describe the deposition of the fluoropolymers. The choice of precursor based on the F/C ratio is therefore essential to select the desired process. The polymerizing capacity follows, for example, this sequence: C2F4 >

C3F8 > C2F6 > CF4 and explains why CF4 is more often used for fluorination of polymer

surfaces than plasma polymerization [85], [86].

a. Low pressure plasmas

The literature about the deposition of fluorocarbons by plasma is very rich, especially at low pressure. These deposition processes can be sub-divided into two categories: the sputtering of a fluorinated target such as a PTFE surface and the PECVD of some compounds, usually gaseous precursors (e.g. TFE, CF4, C2F6, C3F8, C4F8, etc.).

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termination of the chains. Yasuda proposed the competitive ablation and polymerization mechanism (CAP) depending on the etching character of the plasma. In order to consider the effect of species in contact with the surface while the film is growing at the surface, a complete model was developed by d’Agostino et al. which describes plasma polymerization of fluorocarbons in a continuous wave mode (CW) [81]. The ion-activated growth model (AGM) combines the contribution of low-energy ion bombardment to activate the substrate surface and of CFx radicals formed in the glow discharge growing the coating. A simplified

AGM scheme is expressed in the following reactions.

Fragmentation of the monomer in the plasma

Monomer n CFx (1 ≤ x ≤ 3) (a)

Ion-activation of the coating (or substrate)

I+ (low energy) + (coating)n (coating)n* + I+ (b)

Growth of the coating

CFx CFx (adsorbed) (c)

CFx (adsorbed) + (coating)n* (coating)n+1 (d)

The reaction (a) describes the production of the CFx precursors of the polymerization. If many

F atoms are produced in this step, an etching route can occur and compete with the deposition as mentioned before. Moreover, if an intense production of radicals occurs, radicals can also polymerize in the gas phase. The ion-energy is crucial in this model; at low ion-energy, surface defective sites (e.g. dangling bonds) are created and act as preferential adsorption sites for CFx radicals. In the case of high ion-energy, desorption of the precursor is induced and

sputtering of the coating can even occur. The two last steps describe the adsorption-desorption equilibrium of CFx radicals on the substrate or coating and the following reaction of

polymerization with the active sites coming from the ion bombardment. The intensity of the ion bombardment plays on the retention of the monomer structure; soft treatments can then be applied such as positioning the substrate in the afterglow or using a pulsed mode.

Regarding the characterization of plasma-polymerized coatings, many researchers highlighted different structures and properties (WCA, roughness, or deposition rates) depending on the plasma conditions. Many of these papers report the control of the chemistry but do not lead to WCAs higher than 100-110°. For instance, C2F6, C3F8 and C4F8 gaseous precursors were

studied in CW, pulsed and downstream (afterglow) RF plasmas [87]. The structure of the films deposited by CW plasma was highly cross-linked while longer pulse off times and downstream plasma led to a domination of the CF2 component [87]. The group of Fisher

observed similar effects, also for c-C4F8 and the hexafluoropropylene oxide, where a highly

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pulsed sources reduce trapped radicals in the film, lowers deposition surface temperatures, and decrease high-energy ion bombardment and UV flux to the surface. A DBD was also used at low pressure to study the effect of the frequency on several properties [90]. The authors showed that the increase in frequency led to higher deposition rates and an increase in roughness but not enough to induce change in the WCA (105-110°).

However, some low-pressure systems succeeded in getting high WCAs. A very interesting and well-known example is the ribbon surface structure deposited in modulated RF glow discharges at low duty cycle values (<10%), fed with TFE [91]. This surface, obtained by d’Agostino et al., was characterized by a surprising certain degree of crystallinity, and exhibited a WCA higher than 150°. They could not explain this growth mechanism but supposed it to be related to the specific time evolution of the CF2 emission intensity observed

by OES. In the deposition conditions of super(hydrophobic) and structured coatings, the CF2

emission intensity grows until a plateau is reached while for higher duty cycles, they observed a quasi-linear trend. A low duty cycle pulsed RF plasma also recently proved its efficiency in the deposition of (super)hydrophobic surfaces with WCAs of about 150° [92]. The XPS results of C2H2F4 plasma polymerization revealed an increase in the CF2 content for lower

duty cycles, along with a gradual formation of ribbon-like structures. A more recent study showed that by controlling the negative bias applied and the gaseous mixture of Ar and CF4,

the wettability of the surface could be tuned from a WCA of 120° up to 150° [93]. This phenomenon was attributed to the sputtering of the fluorinated groups on the surface of the growing films during the treatment; the F/C is indeed decreasing with the DC bias but the increase in the roughness is predominant.

The RF magnetron sputtering of PTFE was developed as an alternative route to create fluorocarbon coatings. This technique was extensively studied by the group of Biederman, who investigated, for example, the increase in argon pressure inducing a decrease in the deposition rate. The roughness of the deposited films was quite similar for pressures up to 50 Pa, before drastically rising up for 70 Pa, while the WCA increased almost linearly with the pressure and the chemical composition was changed from a highly disordered to a predominant –CF2– structure [60]. More recently, they showed that (super)hydrophobic

coatings (WCA 173°) could be obtained when the distance from the sputtered target was longer, and was a combined effect of the roughness and F/C ratio. Thanks to gas phase mass spectrometry analysis, they highlighted the presence of longer CxFy fluorocarbon molecules

reaching the surface and thus contributing to the formation of the coating besides the well-known CF2 [94].

b. Atmospheric pressure plasmas

The precursors used at atmospheric pressure are usually identical to those used in low-pressure processes. In the case of fluorocarbons depositions, gaseous precursors such as CF4,

C2F4, C2F6, C3F8, C3HF7 or c-C4F8 are most often utilized. However, the use of a plasma gas is

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irradiates the molecules precursor, generating an activated species, usually a radical. The processes of atmospheric plasmas are much more complicated due to the increase in the collision frequency. However, the plasma polymerization process is usually considered as almost similar to the radical mechanism proposed by Yasuda at low pressure [82]. A major difference between low and atmospheric pressure is the excitation step. At low pressure, the electrons are the most probable species colliding with the precursor, while in atmospheric plasma, the reactions involving the high-energy metastable species from the plasma gas need to be taken into account.

One of the first investigations on the deposition of fluorocarbons by atmospheric plasmas was reported by Yokoyama et al. working with a mixture of C2F4 and helium [95]. Coatings with a

F/C ratio about 1.4-1.7 were obtained with deposition rates as high as 2 µm/h.

Since many years, the group of d’Agostino has studied the deposition of fluorocarbons as well at low pressure [81], [84], [91] as at atmospheric pressure [96], [97]. They tried to demonstrate that atmospheric pressure DBDs could be useful in tailoring the coatings properties, with the example of He-C3F6 and He-C3F8-H2 [96]. First, they analysed the

homogeneity properties of the discharge and reported that the frequency had an influence on the transition from homogeneous to “filamentary” regime. They showed that higher deposition rates were obtained with the unsaturated precursor C3F6 (maximum 35 nm/min).

Moreover, varying the excitation frequency mainly affects the deposition rate, while the concentration of H2 in the feed allows the control of the chemical composition of the films,

additionally to the influence of the deposition rate (from 1 to 12 nm/min). Indeed, scavenging of fluorine atoms by hydrogen give rise to HF in the gas phase which is not an etching reagent and is pumped out of the discharge [84]. D’Agostino was one of the first researchers who did the most complete gas phase study by optical emission spectroscopy (OES), both at low and atmospheric pressure [96] and clearly identified the continuum of CF2+ component centred at

290 nm.

Vinogradov et al. have also been very active in this domain [98], [99]. They reported the influence of additive gases such as H2 and O2 in a fluorocarbon-containing filamentary DBD.

As in the case of the glow discharge in helium [96], the addition of H2 results in a consistent

increase in the deposition rate, while the addition of oxygen tends to consume the CF2 trough

the formation of COF, CO, CO2 and/or F, and then reduces the deposition rate [98]. The

intensity of the “amorphous” PTFE peak observed by FTIR-absorption spectroscopy at 740 cm-1 was also decreased. Moreover, the presence of oxygen is known to shift the process from deposition to etching [81]. The competition between deposition and etching was also investigated in atmospheric pressure DBDs with Ar-CF4-H2 and Ar-CF4-O2 [97]. They

showed that it was quite similar to that observed in low-pressure plasmas with the main difference being the negligible effect of ion bombardment at atmospheric pressure due to high collision probability limiting the ion energy. In order to detect the influence of the oxygen on the etch rate of a fluorocarbon coating, thickness measurements were performed [100]. Without oxygen, an etch rate of 7 nm/min was detected, indicating an etching character of CF4 due to fluorine atoms as showed at low pressure [84]. The addition of oxygen enhanced

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Vinogradov et al. also performed some spectroscopic measurements of the gas phase [99]. FTIR analysis confirmed the etching character of CF4 compared to precursors with lower F/C

ratios, as the CF and CF2 radicals density were two orders of magnitude higher in discharges

of c-C4F8. Although studies at low pressure reported a strong correlation between the CF2

radical concentration and the polymer deposition [88], [101], Vinogradov et al. showed that the CF2 concentration could not be the primary key parameter for the deposition process

[102]. Indeed, C3F8 and C3HF7 have very different deposition rates but almost same absolute

CF2 concentration. Based on the observation of larger fluorocarbon molecules, Vogelsang et

al. suggested that the high CxFy fragments produced by the incomplete dissociation of the

precursor and/or the formation of reactive molecules in the gas phase might then play a significant role in the deposition process [103].

Most of these studies looked at the chemical composition and if hydrophobic analysis were displayed, a water contact angle reaching a limit at about 100-110° was measured. Only few works reported in the literature succeeded in getting higher WCAs at atmospheric pressure. WCAs of about 120-130° maximum were obtained by plasma polymerization of C4F8 and

C3F8 in a DBD, and were strongly dependent on the substrate roughness [104]. The use of

in-line atmospheric RF plasma of He-CF4-H2 allowed to form superhydrophobic coating

consisting mostly of CFx nanoparticulates [105]. He/c-C4F8 microwave plasma were used to

synthesized spherical fluorocarbon particles in the gas phase before their deposition onto a positively charged substrate [106].

c. PECVD from fluorine-based liquid precursor

It is worth noticing that most of the precursors utilized in the plasma-enhanced CVD at low and atmospheric pressures are gaseous. Although it is not a new area of research at low pressure, only few studies have reported the use of fluorinated liquid solutions as precursors of the reaction. Hozumi et al. used fluoroalkyl silane liquid precursor (CF3(CF2)nCH2

-CH2Si(OCH3)3; n=0, 5 or 7) in a RF PECVD process to produce coatings with a WCA of

112° [107]. An increase in the deposition rate was obtained with an acrylate-based fluorinated precursor (1H,1H,2H,2H-perfluorodecyl acrylate-1) in a low-pressure RF plasma system but no wettability measurements were performed [108].

More recently, researchers focused on the use of liquid-based precursors in atmospheric plasmas [109], [110], [111], [112], [113]. In the fluorinated area, Borcia et al. focused on the deposition of chlorine and fluorine-based monomers such as C2Cl3F3 in a DBD system [112],

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reactions takes places with every new pulse. The wettability properties are highly dependent on the distance between the jet and the substrate and for small distances, WCA up to 115° were obtained with chemical compositions very different from a precursor to another.

Although some studies report the use of perfluorohexane, most of those do not report the use of pure CxFy compounds [114], [115], [116]. The deposition of perfluorohexane in a DBD

TEXTURIZATION OF FLUORINATED MATERIALS

HYDROPHOBIC AND SUPERHYDROPHOBIC SURFACES BY

MEANS OF ATMOSPHERIC PLASMAS: SYNTHESIS AND