Julie Hubert

Jérémy Mertens

Thierry Dufour

Nicolas Vandencasteele

François Reniers

Pascal Viville

Roberto Lazzaroni

M Raes

Herman Terryn

H Terryn C Affiliations

Synthesis and texturization processes of (super)-hydrophobic fluorinated surfaces by atmospheric plasma

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. Introduction

Super-hydrophobic thin films are attractive in applications where water-repellent, anti-fouling, and self-cleaning properties are desirable. A super-hydrophobic surface is one that repels water to such an extent that the droplet is almost spherical and easily rolls across the surface. They are generally characterized by a high water contact angle (WCA) (> 150°) and a low tilting angle or a low contact angle hysteresis (< 10°); the hysteresis being defined by the difference between the advancing contact angle and the receding contact angle [START_REF] Bhushan | Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction[END_REF] . The highest known WCA of a smooth low-energy surface is comprised between 110° and 120° depending on the chemical group present at the surface (CH3, CF2, CF3) [START_REF] Decker | Contact line structure and dynamics on surfaces with contact angle hysteresis[END_REF] . Super-hydrophobic surfaces are therefore obtained by combining rough surface morphology and low surface energy coatings [START_REF] Decker | Contact line structure and dynamics on surfaces with contact angle hysteresis[END_REF] . Such coatings have been deposited using wet chemistry with low surface energy materials but these methods usually require multiple processing steps and utilize solvents [START_REF] Öner | Ultrahydrophobic surfaces: Effects of topography length scales on wettability[END_REF][START_REF] Zhai | Stable superhydrophobic coatings from polyelectrolyte multilayers[END_REF][START_REF] Schondelmaier | Orientation and self-assembly of hydrophobic fluoroalkylsilanes[END_REF][START_REF] Brinker | Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing[END_REF] . The use of plasmas is then a very promising synthetic route to produce super-hydrophobic surfaces since this approach has the advantage of reducing the number of steps required to modify the surface of materials. Moreover, the use of atmospheric plasmas presents many benefits such as reducing the treatment time as well as the costs related to high-vacuum systems. Working under atmospheric pressure can therefore be advantageous in industrial applications since the process can be easily implemented in a continuous production line. The two main pathways involving the plasma processing of polymers are (i) the direct modification of polymer surfaces and (ii) the plasma deposition of thin polymeric films.

The plasma treatments of fluoropolymer surfaces have drawn a special interest and particularly in the case of the polytetrafluoroethylene (PTFE) because of its outstanding properties such as high thermal stability, low friction coefficient, hydrophobicity, and chemical inertness which explain its use for biocompatibility and self-cleaning applications [START_REF] Ebnesajjad | Introduction to Fluoropolymers: Materials, Technology, and Applications[END_REF] . The treatment of PTFE by plasma has been investigated in terms of both hydrophobic [START_REF] Morra | Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene)[END_REF][START_REF] Ryan | Surface texturing of PTFE film using nonequilibrium plasmas[END_REF][START_REF] Vandencasteele | Evidence of the synergetic role of charged species and atomic oxygen in the molecular etching of PTFE surfaces for hydrophobic surface synthesis[END_REF] and hydrophilic [START_REF] Badey | A comparative study of the effects of ammonia and hydrogen plasma downstream treatment on the surface modification of polytetrafluoroethylene[END_REF][START_REF] Pringle | Ammonia plasma treatment of PTFE under known plasma conditions[END_REF][START_REF] Liu | Comparative study on the effect of RF and DBD plasma treatment on PTFE surface modification[END_REF][START_REF] Wilson | Plasma modification of PTFE surfaces. Part I: Surfaces immediately following plasma treatment[END_REF] surfaces. Inert gases such as helium and argon improve the wettability of the PTFE by decreasing its WCA through either a defluorination of the polymer surface or an incorporation of residual oxygen [START_REF] Sarani | Surface modification of PTFE using an atmospheric pressure plasma jet in argon and argon 1 CO2[END_REF][START_REF] Kolska | PTFE surface modification by Ar plasma and its characterization[END_REF][START_REF] Youxian | Surface modification of poly(tetrafluoroethylene) by gas plasma treatment[END_REF][START_REF] Tanaka | Investigation of a new reactant for fluorinated polymer surface treatments with atmospheric pressure glow plasma to improve the adhesive strength[END_REF][START_REF] Zettsu | Plasma-chemical surface functionalization of flexible substrates at atmospheric pressure[END_REF] . Moreover, it has been shown that, at low pressure, those gases could induce a sputtering of the PTFE and produce a film onto a substrate made of fluorocarbon species ejected from the polymer [START_REF] Stelmashuk | Plasma polymer films rf sputtered from PTFE under various argon pressures[END_REF][START_REF] Golub | Similarity of plasma-polymerized tetrafluoroethylene and fluoropolymer films deposited by rf sputtering of poly(tetrafluoroethylene)[END_REF][START_REF] Mathias | The decompostion of polytetrafluoroethylene in a glow discharge[END_REF][START_REF] Ryan | Plasma polymerization of sputtered poly(tetrafluoroethylene)[END_REF] .

The treatment of PTFE by O2-containing gases is more controversial as both hydrophobic and hydrophilic modifications have been obtained. For instance, some studies showed that oxygen radio-frequency (RF) plasmas could lead to an etching of the surface characterized by oxygen grafting and as a result a decrease in WCA [START_REF] Liu | Comparative study on the effect of RF and DBD plasma treatment on PTFE surface modification[END_REF][START_REF] Wilson | Plasma modification of PTFE surfaces. Part I: Surfaces immediately following plasma treatment[END_REF][START_REF] Sarani | Surface modification of PTFE using an atmospheric pressure plasma jet in argon and argon 1 CO2[END_REF][START_REF] Kolska | PTFE surface modification by Ar plasma and its characterization[END_REF][START_REF] Youxian | Surface modification of poly(tetrafluoroethylene) by gas plasma treatment[END_REF][START_REF] Tanaka | Investigation of a new reactant for fluorinated polymer surface treatments with atmospheric pressure glow plasma to improve the adhesive strength[END_REF][START_REF] Zettsu | Plasma-chemical surface functionalization of flexible substrates at atmospheric pressure[END_REF][START_REF] Stelmashuk | Plasma polymer films rf sputtered from PTFE under various argon pressures[END_REF][START_REF] Golub | Similarity of plasma-polymerized tetrafluoroethylene and fluoropolymer films deposited by rf sputtering of poly(tetrafluoroethylene)[END_REF][START_REF] Mathias | The decompostion of polytetrafluoroethylene in a glow discharge[END_REF][START_REF] Ryan | Plasma polymerization of sputtered poly(tetrafluoroethylene)[END_REF][START_REF] Wilson | Surface chemistry and wettability of plasma-treated PTFE[END_REF] . Salapare et al. observed that in a O2 lowpressure plasma distinct behaviors (hydrophobic or hydrophilic) were obtained depending on the plasma energy [START_REF] Salapare | Stability of the hydrophilic and superhydrophobic properties of oxygen plasma-treated poly(tetrafluoroethylene) surfaces[END_REF] . Many other studies also demonstrated that an oxygen plasma treatment could induce a roughening of the samples responsible for the increase in WCA but with no change in the chemical composition [START_REF] Morra | Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene)[END_REF][START_REF] Ryan | Surface texturing of PTFE film using nonequilibrium plasmas[END_REF][START_REF] Vandencasteele | Evidence of the synergetic role of charged species and atomic oxygen in the molecular etching of PTFE surfaces for hydrophobic surface synthesis[END_REF] . Ryan and Badyal observed that the oxygen plasma did not induce any variation in the chemical composition but its use led to a surface texturization [START_REF] Ryan | Surface texturing of PTFE film using nonequilibrium plasmas[END_REF] . 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 [START_REF] Vandencasteele | Evidence of the synergetic role of charged species and atomic oxygen in the molecular etching of PTFE surfaces for hydrophobic surface synthesis[END_REF] . Super-hydrophobic PTFE surfaces were also created by an Ar+O2 low-pressure plasma [START_REF] Barshilia | Superhydrophobic polytetrafluoroethylene surfaces with leaf-like micro-protrusions through Ar 1 O2 plasma etching process[END_REF] . Although no change in the chemical composition was recorded, leaf-like micro-protrusions were observed after a 4 h-plasma treatment related to the WCA of 158° and characterized by a root mean square (RMS) roughness of about 2 µm. Most of the studies were realized at low pressure; only few research showed results focused on the superhydrophobicity of PTFE by O2 in atmospheric plasmas. For instance, the treatment of PTFE by the post-discharge of an Ar-O2 plasma torch induced an increase in the WCA (130°) and in the roughness while it was not observed in the pure argon plasma [START_REF] Carbone | How to increase the hydrophobicity of PTFE surfaces using an r.f. atmospheric-pressure plasma torch[END_REF] . 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° [START_REF] Trigwell | Electrostatic properties of PE and PTFE subjected to atmospheric pressure plasma treatment; correlation of experimental results with atomistic modelling[END_REF] .

Low-pressure plasma deposition of fluorocarbon films has been widely studied in the last decades [START_REF] Mackie | Characterization of pulsed-plasma-polymerized aromatic films[END_REF][START_REF] Agostino | Plasma Deposition, Treatment and Etching of Polymers[END_REF][START_REF] Henry | Synthesis of superhydrophobic PTFE like thin films by self-nanostructuration in a hybrid plasma process[END_REF][START_REF] Wang | Deposition of fluorocarbon film with 1,1,1,2-tetrafluoroethane pulsed plasma polymerization[END_REF][START_REF] Favia | Deposition of super-hydrophobic fluorocarbon coatings in modulated RF glow discharges[END_REF][START_REF] Mackie | Comparison of pulsed and continuous-wave deposition of thin films from saturated fluorocarbon/H2 inductively coupled rf plasmas[END_REF] . Gaseous monomers (e.g., CF4, C2F4, C2F6, C3F8, C3HF7 or c-C4F8) polymerized into the discharge led to the deposition of coatings with different properties depending on the plasma source or the feed gases. The F/CFx ratio was identified as one of the most relevant internal indicators to describe the deposition of the fluoropolymers because it defines the competition between the etching and the polymerization processes [START_REF] Agostino | Advanced Plasma Technology[END_REF][START_REF] Hopkins | Nonequilibrium glow discharge fluorination of polymer surfaces[END_REF][START_REF] Strobel | Plasma fluorination of polyolefins[END_REF] . The precursors used at atmospheric pressure are usually identical to those used in lowpressure processes but the use of a plasma gas (usually helium or argon) is most often required when working at atmospheric pressure. The groups of d'Agostino [START_REF] Agostino | Plasma Deposition, Treatment and Etching of Polymers[END_REF][START_REF] Favia | Deposition of super-hydrophobic fluorocarbon coatings in modulated RF glow discharges[END_REF][START_REF] Agostino | Advanced Plasma Technology[END_REF][START_REF] Fanelli | Atmospheric pressure PECVD of fluorocarbon coatings from glow dielectric barrier discharges[END_REF][START_REF] Fanelli | Deposition and etching of fluorocarbon thin films in atmospheric pressure DBDs fed with Ar-CF4-H2 and Ar-CF4-O2 mixtures[END_REF] and Vinogradov [START_REF] Vinogradov | Spectroscopic diagnostics of DBD in Ar/fluorocarbon mixtures-Correlation between plasma parameters and properties of deposited polymer films[END_REF][START_REF] Vinogradov | Deposition of fluorocarbon polymer films in a dielectric barrier discharge (DBD)[END_REF] have been very active in this domain at both low and atmospheric pressures. For instance, they widely studied the influence of reactive gases such as H2 and O2 on the chemistry of the films and the deposition rate. They showed that the addition of oxygen tends to consume the CF2 through the formation of COF, CO, CO2, and/or F, then reducing the deposition rate and shifting the process from deposition to etching. Regarding the wettability properties, only few works reported in the literature succeeded in getting WCAs higher than 100-110°, at atmospheric pressure [START_REF] Kim | Superhydrophobic CFx coating via in-line atmospheric RF plasma of He-CF4-H2[END_REF][START_REF] Liu | Surface modification of materials by dielectric barrier discharge deposition of fluorocarbon films[END_REF][START_REF] Nagai | Atmospheric pressure fluorocarbon-particle plasma chemical vapor deposition for hydrophobic film coating[END_REF] .

As previously mentioned, the synergy of the roughness and the low-energy surface is required to enhance the hydrophobicity. Many studies have investigated the combination of those two factors to create super-hydrophobic surfaces where authors combine several techniques; from creating roughness into a material followed by an hydrophobization step to obtain the desired properties. For instance, photolithography was used to transfer patterns to silicon wafers before hydrophobization by silanization reagents [START_REF] Öner | Ultrahydrophobic surfaces: Effects of topography length scales on wettability[END_REF] , electrochemical deposition of zinc oxide, nickel, and copper leads to hydrophobic surfaces after modification with hydrophobic self-assembly monolayers such as fluoroalkylsilane [START_REF] Hsieh | Electrochemical deposition and superhydrophobic behavior of ZnO nanorod arrays[END_REF][START_REF] Wang | Synthesis of biomimetic superhydrophobic surface through electrochemical deposition on porous alumina[END_REF] , an oxygen plasma etching of paper fibers followed by the deposition of a pentafluoroethane thin film.47 Additionally to those methods, the roughness might be obtained by the incorporation of nanoparticles which ones can be deposited on the substrate by solvent evaporation [START_REF] Shearer | Composite SiO2/TiO2 and amine polymer/TiO2 nanoparticles produced using plasma-enhanced chemical vapor deposition[END_REF] , sol-gel method [START_REF] Lakshmi | Fabrication of superhydrophobic and oleophobic sol-gel nanocomposite coating[END_REF][START_REF] Valipour Motlagh | Shahtahmassebi: Super-liquid-repellent coating on the carbon steel surface[END_REF] , electrophoretic path [START_REF] Charlot | Submicron coating of SiO2 nanoparticles from electrophoretic deposition[END_REF] , or spray-deposition methods [START_REF] Ogihara | Simple method for preparing superhydrophobic paper: Spray-deposited hydrophobic[END_REF][START_REF] Zhang | Spray-coating of superhydrophobic aluminum alloys with enhanced mechanical robustness[END_REF] . Once the nanoparticles are deposited, the hydrophobization step is performed through the synthesis of a low surface energy film such as fluorine-based compounds. Few studies have reported the interest of making hybrid coatings from nanoparticles; such as the use of a sol-gel process in the presence of tetraethoxysilane [START_REF] Fabbri | Surface properties of fluorinated hybrid coatings[END_REF] , the use of a vacuum-based method combining the deposition of nanoparticles by means of a gas aggregation source with conventional RF magnetron sputtering of PTFE [START_REF] Kylian | Hydrophobic and superhydrophobic coatings based on nanoparticles overcoated by fluorocarbon plasma polymer[END_REF] , or also the incorporation of hydrophobically modified silica nanoparticles in PTFE emulsion [START_REF] Basu | Fabrication of superhydrophobic nanocomposite coatings using polytetrafluoroethylene and silica nanoparticles[END_REF] .

In the present study, we show that cold atmospheric plasma is an efficient technology to easily obtain (super)-hydrophobic surfaces from fluorinated compounds. Three pathways are described: (i) the modification of an existing fluoropolymer surface, (ii) the deposition of a fluorocarbon film and (iii) the enhancement of the film hydrophobicity through the addition of nanoparticles. Based on the integrated study of both the plasma phase and the resulting material surface, polymerization and texturization processes are proposed. The influence of the gas phase on the etching of the PTFE and the deposition and texturization of CxFy compounds are described. Although helium and argon have both demonstrated their efficiency in the plasma deposition of fluorocarbons, studies usually deal with the effects of a single gas, either argon or helium. Moreover, these last results are compared with superhydrophobic hybrid coatings made of nanoparticles covered by a fluorinated layer deposited by a dielectric barrier discharge (DBD).

II. Experimental part II.1. Materials

One mm thick PTFE samples were supplied by Goodfellow. The samples were cleaned in pure methanol (AnalaR Normapur, VWR) and pure iso-octane (GR for analysis, Merck), before being exposed to the plasma postdischarge. The liquid precursors namely perfluoro-2-methyl-2-pentene (C6F12) and perfluorohexane (C6F14) were provided by Fluorochem and were used without any further purification (Fig. 1). 80 nm SiO2 nanoparticles from Alfa Aesar are dispersed in demineralized water without any surfactant. Silicon wafers (100) from Compart Technology Ltd. (Tamworth, UK) were used as substrates after being cleaned with methanol and isooctane. The glow discharge was sustained with argon (Air Liquide, ALPHAGAZ 1), helium (Air Liquide, ALPHAGAZ 1), or O2 (Air Liquide, ALPHAGAZ 1).

II.2. The plasma sources

The PTFE sample surfaces were exposed to the linear postdischarge of an RF atmospheric plasma torch, the Atomflo™ 400L-Series from SurfX Technologies (Redondo Beach, California), supplied with He and He-O2 and integrated to a robotic system detailed in a previous paper [START_REF] Dufour | PTFE surface etching in the postdischarge of a scanning RF plasma torch: Evidence of ejected fluorinated species[END_REF] . The kinematic parameters used in this study are the following ones: power 90-120 W, He = 15 L/min, O2 = 0-0.1 L/min, while the kinematic parameters are the scanning length (LS = 10 mm), the scanning velocity (vS = 25 mm/s), the number of scans (NS = 1000), and the torch-to-sample distance (gS = 0.5-1 mm). In all our experiments, the scanning plasma source moved back and forth along a single axis. According to the used kinematic parameters (LS = 10 mm, vS = 25 mm/s), the treatment time is the number of scans multiplied by the time required to scan one scanning length (0.4 s). The width of the plasma torch slit being 0.8 mm, the exposure time of a spot (0.8 mm) is estimated at 0.032 s. For a number of scans of 1000, the total treatment time is 400 s while the exposure time of each spot is about 32 s. The plasma-polymerized (pp)-fluorinated films were deposited with a home-built DBD described in previous studies [START_REF] Hubert | Plasma polymerization of C4Cl6 and C2H2Cl4 at atmospheric pressure[END_REF][START_REF] Hubert | Chemical and physical effects of the carrier gas on the atmospheric pressure PECVD of fluorinated precursors[END_REF] . The vapors of the precursor were introduced into the discharge by means of the carrier gas (second flow = 0.2 L/min) bubbling into the liquid, after being diluted with a primary flow of the carrier gas with a total carrier gas flow of 5 L/min. The injected amounts for 0.2 L/min are: Ar-C6F12 782 6 44 mg/min, He-C6F12 783 6 52 mg/min, Ar-C6F14 768 6 84 mg/min, and He-C6F14 767 6 101 mg/min. The operating frequency was set at 17.1 kHz with an output power set at 50 W for the results presented in this paper, supplied by an AFS-G10S power generator (AFS Entwicklungs-und Vertriebs GmbH, Horgau, Germany). The deposition time varied from 30 to 360 s.

II.3. Water contact angles (WCA)

A drop shape analyzer (Krüss DSA 100, Krüss GmbH, Hamburg, Germany) was used to measure dynamic WCAs onto the samples, according to the sessile drop method. The fitting method used is the "Tangent 1". Advancing and receding contact angles were both measured by growing and shrinking the size of a single drop on the surface sample, from 0 to 15 µL and back to 0 µL at a rate of 30 µL/min. The receding angles are not presented in this paper but information can be found in Refs [START_REF] Hubert | Chemical and physical effects of the carrier gas on the atmospheric pressure PECVD of fluorinated precursors[END_REF][START_REF] Hubert | Etching processes of polytetrafluoroethylene surfaces exposed to He and He-O2 atmospheric postdischarges[END_REF] .

II.4. X-ray photoelectron spectroscopy (XPS)

XPS analysis was performed on a Physical Electronics PHI-5600 photoelectron spectrometer (Eden Prairie, Minnesota). Survey spectra were used to determine the elemental chemical composition of the surface. Narrow region photoelectron spectra were used for the chemical study of the C 1s. The spectra were acquired using the Mg anode (1253.6 eV) operating at 300 W. Wide surveys were acquired at a pass-energy of 187.5 eV with a five-scans accumulation (time/step: 50 ms, eV/step: 0.8) and highresolution spectra of the C 1s peaks were recorded at a pass-energy of 23.5 eV with an accumulation of 5 scans (time/step: 200 ms, eV/step: 0.05). The elementary composition was calculated after the removal of a Shirley background and using the sensitivity coefficients from the manufacturer's handbook: SC = 0.205, SF = 1, and SO = 0.63.

II.5. Atomic force microscopy (AFM)

AFM was used to analyze the surface morphology of the deposited films. AFM images were recorded in air with a Nanoscope IIIa microscope (Brüker, Karlsruhe, Germany) operating in tapping mode. The probes were commercially available silicon tips with a spring constant of 24-52 N/m, a resonance frequency lying in the 264-339 kHz range, and a typical radius of curvature in the 5-10 nm range. The images presented here are height images recorded with a sampling resolution of 512  512 data points and a scan size of 5  5 µm 2 .

II.6. Profilometry

The thickness measurements were performed using a stylus profiler Brücker dektak XT (Brüker, Karlsruhe, Germany). The stylus with a 2 µm radius scans the surface with a force of 1 mg and the measurement was controlled and analyzed with the Vision 64 software. The thickness was estimated based on the height difference between the substrate surface and the surface of the coating and used to estimate the deposition rates. The roughness of the samples containing nanoparticles was estimated by a 3D simulation of the surface obtained by the sum of 150 scans covering a surface of 0.9 mm 2 . The RRMS was then calculated by the software. The force of the stylus for these measurements was set at 0.2 mg.

II.7. Scanning electron microscopy (SEM)

SEM measurements were performed on a JEOL JSM-7000F (JEOL, Tokyo, Japan) equipped by a Schottky field emission gun. The analyses were essentially topographic with a magnification of 5000. The electrons were accelerated by a 5 kV electrical field.

II.8. Mass loss measurements and XPS of aluminum trapping ejected species

The PTFE samples were weighted before and after the plasma treatments to evaluate mass variations. A Sartorius BA110S Basic series analytical balance characterized by a capacity of 110 g and a readability of 0.1 mg was used. Moreover, during the plasma treatment, an aluminum foil was placed close to the samples. As aluminum is known to be an efficient fluorine trap [START_REF] Lagow | Fluorinated functionalized polymers[END_REF] , we then analyzed the foil by XPS, looking for the presence of fluorinated species.

II.9. Optical emission and absorption spectroscopy (OES-OAS)

OES was performed with a SpectraPro-2500i spectrometer from ACTON Research Corporation (Acton, Massachusetts) (0.500 m focal length, triple grating imaging). The light emitted by the discharge was collected by an optical fiber and transmitted to the entrance slit (50 µm) of the monochromator. Each optical emission spectrum was acquired with the 1800 grooves/mm grating (blazed at 500 nm) and recorded over 10 accumulations with an exposure time of 50 ms for the DBD and 30 accumulations, 25 ms for the post-discharge. To tackle intensity variations as a function of the plasma conditions, the emissions of all the species were divided by the emission of the whole spectra (i.e., a continuum ranging from 250 to 850 nm). Line-absorption spectroscopy was applied to study He (2 3 S) metastable states, more precisely on the transition 2 3 S-3 3 P at 388.9 nm [START_REF] Dufour | Chemical mechanisms inducing a dc current measured in the flowing postdischarge of an RF He-O2 plasma torch[END_REF] . The absorption rate A is related to the emission intensities by the relation A = (IL + IP -IL+P)/IL, where IP is the light intensity emitted from the post-discharge, IL is the line intensity from the external lamp and IL+P is the line intensity from the two light sources [START_REF] Gavare | Determination of the number densities of argon metastables in argon-hydrogen plasma by absorption and self-absorption methods[END_REF][START_REF] Li | Density measurement of helium metastable states by absorption spectroscopy in an inductively coupled plasma[END_REF] .

II.10. Mass spectrometry (MS)

MS of the gas phase was performed with a Hiden analytical atmospheric gas analysis-quadrupole gas analyzer (QGA; Hiden Analytical, Warrington, UK). The gases were collected through a perfluoroalkoxy capillary located between the electrodes at the boundary of the plasma region. The secondary electron multiplier detector (SED) was used to detect fragments with low partial pressures (10 -6 -10 -13 Torr). To avoid excessive fragmentation of the precursor in the ionization chamber, the electron energy was set at 35 eV. The provided software MASsoft7 was used either to analyze the partial pressure as a function of the m/z ratio or to follow the partial pressure as a function of time for several specific m/z ratios, simultaneously.

III. Results & Discussion

As previously mentioned, three pathways are presented to show the efficiency of the cold atmospheric plasma to easily obtain (super)-hydrophobic surfaces from fluorinated compounds.

III.1. PTFE surface properties enhanced by atmospheric post-discharge III.1.1. Characterization of the surface modifications

The first evaluated method to enhance the hydrophobicity of a surface is the use of an atmospheric plasma torch to modify polymer properties. In this study, PTFE surfaces were submitted to the post-discharge of a RF plasma torch supplied in He or in He-O2 to study wettability variations. By comparing pure helium and He-O2 plasma treatments, it is shown that oxygen has a significant impact on the PTFE wettability and its presence is required to enhance the hydrophobicity of the polymer surface. As illustrated in Fig. 2, helium plasma treatments lead to a small decrease in the WCA while an increase in the hydrophobicity is observed by adding oxygen. Moreover, we previously showed that advancing WCA as high as 155° (with a contact angle hysteresis < 10°) could be obtained for higher number of scans and therefore a higher treatment time [START_REF] Dufour | PTFE surface etching in the postdischarge of a scanning RF plasma torch: Evidence of ejected fluorinated species[END_REF] .

Although modifications in wettability were observed, the relative surface chemical composition measured by XPS was kept unchanged as shown in Fig. 3. Indeed, only a strong CF2 component at 292.2 eV and a small CC component (284.6 eV) mixed up with the satellite peak of CF2 are detected at the surface with the same High-Resolution C 1s profile independently of the plasma conditions (native, He, He-O2). It is known that a high roughness and a low-energy surface are required to get (super)-hydrophobic properties [START_REF] Decker | Contact line structure and dynamics on surfaces with contact angle hysteresis[END_REF][START_REF] Sun | Bioinspired surfaces with special wettability[END_REF][START_REF] Chen | Is the lotus leaf superhydrophobic[END_REF][START_REF] Miwa | Effects of the surface roughness on sliding angles of water droplets on superhydrophobic surfaces[END_REF] . According to the above results in terms of surface composition (which does not change), the variation in the WCA is assumed to be only induced by morphology modifications. Indeed, the RMS roughness slightly decreases for a pure helium plasma (from 17 to 15 nm), while this roughness is much higher when oxygen is added (up to 58 nm). This significant variation between both plasma treatments is illustrated by the AFM images in Fig. 4 and justifies the wettability behaviors. Moreover, these results are correlated with those previously observed at low pressure [START_REF] Vandencasteele | Evidence of the synergetic role of charged species and atomic oxygen in the molecular etching of PTFE surfaces for hydrophobic surface synthesis[END_REF][START_REF] Vandencasteele | Plasma-modified PTFE for biological applications: Correlation between protein-resistant properties and surface characteristics[END_REF] . Although the surface modifications by a pure helium plasma are less noticeable in terms of wettability and morphology, this treatment however appears to induce the highest etching of the PTFE surface. Indeed, higher mass losses and higher concentrations of fluorinated compounds (% F values from survey spectra and % CF2 values from C 1s high resolution spectra) were detected onto an aluminum foil, indicating that more species are ejected from the PTFE sample using pure helium treatment as illustrated in Fig. 5 (pure helium plasma corresponds to 0 mL/min O2 flow rate). From the XPS study of the aluminum foil, the ejection of a higher amount of material in the pure He plasma can also be evidenced by the attenuation of the signal coming from the aluminum substrate indicating a thicker deposit of polymer (i.e., O2 100 mL/min: 8% F, 17% C, 39% O and 36% Al compared to no oxygen: 43% F, 27% C, 12% O and 18% Al).

III.1.2. Characterization of the post-discharge

OES-OAS of the post-discharge allowed detecting the emissive species existing in the post-discharge and analyzing their evolution as a function of the oxygen flow rate. Absorption spectroscopy showed that helium metastable atoms were present in the postdischarge and were consumed by the addition of oxygen leading to the formation of atomic oxygen according to the following reactions [START_REF] Léveillé | Atomic oxygen production and exploration of reaction mechanisms in a He-O2 atmospheric pressure glow discharge torch[END_REF] : consumption of the helium metastable through the Penning Ionization with oxygen (1) and Production of atomic oxygen (2):

𝐻𝑒 𝑚 + 𝑂 2 → 𝑂 2 + + 𝐻𝑒 + 𝑒 (1) 𝑂 2 + + 𝑒 → 2𝑂 (2)
The evolution of the most relevant species observed by spectroscopy is illustrated in Fig. 6. The intensity of O2 + ions (525 nm) permanently increases as a function of the oxygen flow rate.

Several mechanisms are usually considered to explain their production [dissociative transfer with He2 + ions, charge transfer reaction involving O2 molecules and O + ions or the electron impact ionization of O2 [START_REF] Dufour | Chemical mechanisms inducing a dc current measured in the flowing postdischarge of an RF He-O2 plasma torch[END_REF] . Considering the species observed or absent from the post-discharge and the weak electron densities measured in typical He-O2 RF post-discharge [START_REF] Gonzales | Surface activation of poly(methyl methacrylate) via remote atmospheric pressure plasma[END_REF] , the mechanism explaining the production of O2 + ions is assumed to be the Penning ionization of O2 molecules by He metastable atoms [Eq. ( 1)]. These He metastable species were indeed experimentally evidenced by OAS as seen in Fig. 6(a) and their consumption by oxygen are consistent with the production of O2 + . Moreover, their presence can also be emphasized by the observation of N2 + ions. Indeed, N2 molecules easily quench He m to produce N2 + through the Penning ionization of N2 [START_REF] Massines | The role of dielectric barrier discharge atmosphere and physics on polypropylene surface treatment[END_REF][START_REF] Nersisyan | Characterization of a dielectric barrier discharge operating in an open reactor with flowing helium[END_REF] . Their intensity decreases with the oxygen flow rate since helium metastable atoms are consumed by O2. Finally, the most probable channel to produce atomic oxygen is the electron impact dissociation of O2 + [Eq. ( 2)] since the rate constant of this reaction is important compared to other reactions [START_REF] Dufour | Chemical mechanisms inducing a dc current measured in the flowing postdischarge of an RF He-O2 plasma torch[END_REF] . The plateau observed for the highest oxygen flow rate can be interpreted as a result from equilibrium between the production and the consumption mechanisms. According to the results obtained from OES-OAS and surface characterization, etching processes explaining the two different behaviors are proposed. In a He-O2 plasma, no modification of the surface composition was observed but the roughness and therefore the WCA were increased inducing the (super)hydrophobic property. In this condition, we assume an anisotropic etching where the oxygen atoms etch preferentially the amorphous phase of the polymer. Oxygen atoms are indeed known to etch more easily the amorphous region of a polymer than the crystal phase [START_REF] Vinogradov | Spectroscopic diagnostics of DBD in Ar/fluorocarbon mixtures-Correlation between plasma parameters and properties of deposited polymer films[END_REF][START_REF] Youngblood | Ultrahydrophobic polymer surfaces prepared by simultaneous ablation of polypropylene and sputtering of poly(tetrafluoroethylene) using radio frequency plasma[END_REF] , creating alveolar structures at the surface playing a role on the increase in roughness. Since the etching of the PTFE at atmospheric pressure is higher in absence of oxygen but leads to a smoothening of the surface (slight decrease in WCA and RRMS), we assume a layer-by-layer random physical etching without any preferential orientation. Studies have shown that helium could be responsible for the sputtering of PTFE [START_REF] Golub | Similarity of plasma-polymerized tetrafluoroethylene and fluoropolymer films deposited by rf sputtering of poly(tetrafluoroethylene)[END_REF][START_REF] Mathias | The decompostion of polytetrafluoroethylene in a glow discharge[END_REF][START_REF] Ryan | Plasma polymerization of sputtered poly(tetrafluoroethylene)[END_REF] . For instance, the treatment of PTFE by a glow discharge supplied in helium induced the ejection of solid polymer and a mass loss of the polymer [START_REF] Mathias | The decompostion of polytetrafluoroethylene in a glow discharge[END_REF] . The main product identified in the helium plasma-induced decomposition of PTFE was the TFE monomer (CF2=CF2). Moreover, the RF plasma sputtering of PTFE is considered to involve scission of -(CF2)n-chains yielding smaller -(CF2)m-(m << n) segments, which are deposited at the substrate surfaces and form a structure dominated by -(CF2)-groups [START_REF] Horie | Plasma-structure dependence of the growth mechanism of plasma-polymerized fluorocarbon films with residual radicals[END_REF] . The scission of -(CF2)-chains is therefore supposed to be due to the highly energetic helium metastable atoms. Indeed, the helium metastable species, being consumed by the reaction with oxygen through the Penning ionization, are not available any more to etch the PTFE surface [START_REF] Horie | Plasma-structure dependence of the growth mechanism of plasma-polymerized fluorocarbon films with residual radicals[END_REF][START_REF] Strobel | Plasma Surface Modification of Polymers: Relevance to Adhesion[END_REF] . The increase in the oxygen flow rate leads therefore to a reduction of mass losses and of species detected onto the aluminum foil. To have a better understanding and visualization, the two processes are depicted in a schematic representation in Fig. 7. The second method using plasmas to create (super)-hydrophobic surfaces is the Plasma-Enhanced Chemical Vapor Deposition (PECVD) of precursors containing hydrophobic groups such as CFx groups which present the lower surface energy. Two liquid precursors, namely, perfluoro-2-methyl-2-pentene (C6F12) and perfluorohexane (C6F14) were used to deposit coatings in argon and helium. The films produced by both precursors have shown similar properties when comparing treatments in argon and helium.

The study of the plasma phase by MS and optical spectroscopy, completed by electrical measurements highlighted the influence of the gas nature and of the discharge regime (filamentary versus glow) on the fragmentation of the precursors [START_REF] Fanelli | Deposition and etching of fluorocarbon thin films in atmospheric pressure DBDs fed with Ar-CF4-H2 and Ar-CF4-O2 mixtures[END_REF] . Figure 8 presents the evolution of the intensity of some significant m/z fragments detected my MS, in a He (dashed curves) or Ar (full curves) plasma.

The energetic streamers present in the filamentary argon discharge induce a higher fragmentation evidenced by the decrease in the main fragments intensity in the mass spectrometer as seen in Fig. 8(a). In helium, the effect of the plasma is less pronounced, but all the intensities of the detected fragments increase when the discharge is turned on, as shown by the dashed curves in Fig. 8(a). C6F14 is held up as an example since all its main fragments are detected in the mass spectrometer range, but a similar behavior has been observed for C6F12.

fragments generated during the atmospheric pressure PECVD of C6F14 as a function of time, in argon (full curves) and helium (dashed curves). (0.2 L/min C6F14-carrier gas, 5 L/min carrier gas total, 50 W). Figure 9. Scheme of the possible fragmentation of C6F14 (left) in the mass spectrometer and (right) in the plasma, followed by the mass spectrometer (CxFy + and CwFz + assuming to be CF3 + , C3F7 + and C3F5 + according to MS results).

Despite the complexity of the reactions in the discharges, differences between the two systems ("MS" versus "plasma + MS") are briefly illustrated in the following scheme (Fig. 9). As the fragmentation of the precursor is carried out upstream of the mass spectrometer, some of the C3F7 + , CF3 + , and C3F5 + ions are already present in the discharge (especially with an Ar plasma) and would be even more fragmented by the mass spectrometer, explaining their decreasing intensity and the increasing intensity of smaller fragments such as CF or CF2.

Additionally to these observations, we highlighted a crucial role of the oxygen impurities in the plasma polymerization. MS results have shown that oxygenated fragments such as CO2 + , COF + , or COF2 + were highly detected in the helium plasma compared to the argon discharge [Fig. 8(b)]. The efficiency of generating atomic oxygen in helium plasma has also been evidenced by OES [START_REF] Hubert | Chemical and physical effects of the carrier gas on the atmospheric pressure PECVD of fluorinated precursors[END_REF] . The emission intensity of the atomic oxygen lines (e.g., 777 nm) is indeed strongly increased when O2 is added to the helium discharge while small variations are observed in argon. This behavior towards oxygen can be explained by the excited states of helium that are energetic enough to ionize most atoms and molecules and are therefore very efficient to excite impurities. Indeed, metastables atoms of helium have energies of 19.82 eV and 20.62 eV while the 11.55 eV and 11.72 eV argon metastable atoms are too low to induce the Penning ionization of O2 (ionization potential of 12.07 eV [START_REF] Massines | Physics and chemistry in a glow dielectric barrier discharge at atmospheric pressure: Diagnostics and modelling[END_REF][START_REF] Ricard | Plasmas réactifs[END_REF] ).

III.2.2. Characterization of the film properties

The surface properties reflect the results obtained from the plasma phase. The deposition rates are more than 5 times higher in argon than in helium (i.e., 240 and 25 nm/min for C6F12 in argon and helium, respectively; 25 and 5 nm/min for C6F14). The difference between the two precursors is assumed to be related to the higher reactivity of the C=C double bond present in C6F12, similarly to the case of hydrocarbon or acrylate molecules [START_REF] Kobayashi | Plasma polymerization of saturated and unsaturated hydrocarbons[END_REF][START_REF] Yasuda | Some aspects of plasma polymerization investigated by pulsed R.F. discharge[END_REF][START_REF] Batan | The impact of double bonds in the APPECVD of acrylate-like precursors[END_REF] . The surface composition of the films polymerized in both argon and helium estimated from XPS measurements are in good correlation with the behavior of the plasma phase. The C6F12 is held up as an example. The chemical structure of the pp-C6F12 film is better preserved in the case of helium since a large amount of CF3 is detected, while the argon plasma leads to a more disordered XPS spectrum containing a higher concentration of CF2 (Table I).

Ar-C6F12

Table I. Surface composition of pp-C6F12 films as a function of the carrier gas (0.2 L/min C6F12-carrier gas, 5 L/min carrier gas total, 50 W).

Given the structure of the precursor in Fig. 1, as well as the lower electron density, the lower energy and the lower micro-discharges density [START_REF] Massines | Physics and chemistry in a glow dielectric barrier discharge at atmospheric pressure: Diagnostics and modelling[END_REF][START_REF] Fridman | Plasma Chemistry[END_REF][START_REF] Eliasson | Modeling and applications of silent discharge plasmas[END_REF][START_REF] Kunhardt | Generation of large-volume, atmosphericpressure, nonequilibrium plasmas[END_REF] of the helium discharge, we suggest that the precursor structure is more preserved when using helium. Moreover, a small percentage of oxygen (about 3 %) is detected only in the films polymerized in helium. As previously detailed, helium is known to be reactive toward oxygen impurities [START_REF] Massines | Physics and chemistry in a glow dielectric barrier discharge at atmospheric pressure: Diagnostics and modelling[END_REF] . This property is important as it can explain why small amounts of oxygen are detected in the film and why the energetic helium species are less involved in the fragmentation/polymerization of the monomer. These species could indeed be consumed by the impurities.

The hydrophobic and morphological properties depend on the nature of the precursor and the gas phase as shown in Fig. 10. Although all the coatings are hydrophobic with a WCA of at least 115°, only the filamentary argon discharge induces a significant roughening of the surface, hence an increase in the hydrophobicity. Indeed, WCAs of 135° and 140° associated to RRMS higher than 40 nm and 60 nm were obtained for Ar-C6F12 and Ar-C6F14, respectively, while the homogeneous helium discharge leads to a smooth, low-energy surface characterized by a WCA of 115° and a RRMS of about 1 nm, for similar thicknesses.

Journal of Materials

Figure 11. 55 µm 2 AFM images of (a) He-C6F12, (b) Ar-C6F12 and (c) Ar-C6F14 pp-films. (0.2 L/min C6F14-carrier gas, 5 L/min carrier gas total, 50 W, thickness of about 300 nm).

The AFM images in Fig. 11 illustrate the three typical morphologies of films deposited from He-C6F12, Ar-C6F12 or Ar-C6F14. The first picture [Fig. 11(a)] shows the smooth surface obtained from a He-C6F12 plasma deposition. The second is characteristic of the Ar-C6F12 films and the last one of the Ar-C6F14 films. For a similar thickness, the Ar-C6F14 plasma seems to induce a higher roughness and hydrophobicity as shown in Fig. 10. Moreover, as illustrated in Fig. 11(c), this C6F14 precursor induces a roughness structure totally different from the C6F12 precursor. The alveolar structure of approximately 100 nm are quite regular and could be characteristic of the filamentary discharge inducing a localized etching of the surface in competition with the polymerization. The presence of the C=C double bond in C6F12 and its lower F/C ratio could promote the polymerization character instead of the etching, as it is the case for C6F14. The etching could be responsible this alveolar structure observed in Ar-C6F14 coatings. Moreover, the alveolar structure could induce the slightly higher WCAs recorded on these coatings. The results shown before confirm that a high roughness combined with a low-energy surface is required to produce (super)hydrophobic coatings. The low-energy surface was brought by the fluorinated groups of the coatings but the intrinsic roughness resulting from the filamentary argon discharge was not high enough to reach a super-hydrophobic state, i.e. a WCA higher than 150°. Indeed, the highest advancing WCA obtained was about 140° with a high contact angle hysteresis of 20-30°. Therefore, we developed a new process in which the addition of nanoparticles enhanced the surface texturization. Typical double-scale roughness is usually required to obtain super-hydrophobic surfaces with similar wettability properties to the lotus leaf present in nature [START_REF] Simsek | Dual scale roughness driven perfectly hydrophobic surfaces prepared by electrospraying a polymer in good solvent-poor solvent systems[END_REF][START_REF] Wu | Fabrication of superhydrophobic surfaces with double-scale roughness[END_REF] Neinhuis and Barthlott, in 1997, obtained pictures of a lotus leaf as seen where the structures consist of a combination of a two-scale roughness: 10 lm (rough structure) and 100 nm (fine structure) [START_REF] Barthlott | Purity of the sacred lotus, or escape from contamination in biological surfaces[END_REF][START_REF] Neinhuis | Characterization and distribution of water-repellent, self-cleaning plant surfaces[END_REF] . Additional studies showed that particles with a mean diameter of 60 and 70 nm were efficient to increase the hydrophobicity of the surface [START_REF] Kylian | Hydrophobic and superhydrophobic coatings based on nanoparticles overcoated by fluorocarbon plasma polymer[END_REF][START_REF] Ming | With: Superhydrophobic films from raspberry-like particles[END_REF] .

In this study, nanoparticles were deposited on a silicon substrate through the evaporation of a 0.05 g/L dispersion of 80 nm SiO2 nanoparticles (volume of the solution 1.5 mL for a substrate surface of about 20 cm 2 ). After evaporation of the solvent (water), the substrate was covered by a C6F12 film synthesized by atmospheric plasma using argon or helium as carrier gas. The C6F12 precursor has been chosen for this study because of its higher deposition rate and therefore the more accurate characterization of the deposited films. WCAs were measured to study the influence of the additional roughness on the wettability properties of the film.

As presented in Fig. 12, higher WCAs are observed when nanoparticles are located under the plasma synthesized film. An increase in the contact angle value of about 15° was measured with the nanoparticles when helium was used as carrier gas while only a rise of 8° was measured with argon, for an identical deposition time. Since the deposition rates are different for the two gases (i.e., 240 and 25 nm/min for argon and helium, respectively), we also report the WCA values for coatings with a similar thickness. From Fig. 12, we show that the argon plasma leads to higher WCAs as the presence of nanoparticles with helium plasma reaches a maximum WCA of about 130°. To discriminate between the two factors responsible for those wettability properties (surface roughness and the coating chemical composition), we carried out XPS measurements. As shown in the C 1s high resolution spectra represented in Fig. 13, the presence of nanoparticles has no significant influence on the chemical properties of the upper layer of the plasma synthesized film. This absence of chemical variations has been observed regardless of the carrier gas (argon or helium), and could indicate that the main part of the polymerization occurs in the plasma phase and does not depend on the substrate and/or that the SiO2 nanoparticles are not reactive towards the fluorinated precursor. According to those results, the increase in WCAs does not result from a change in chemical composition.

Figure 13. HR C 1s spectra of pp-C6F12 films in argon with SiO2 nanoparticles (red curve) and without SiO2 nanoparticles (black dashed curve). (0.2 L/min C6F12-carrier gas, 5 L/min carrier gas total, 50 W).

Roughness measurements of the films were performed using a stylus profilometer on a 900 µm 2 surface. The RRMS of the film synthesized in argon and helium plasma without any nanoparticles are 192 and 30 nm, respectively. The increase in the WCA with the addition of nanoparticles appears to be related to the rise in the RRMS value reaching up to 434 and 396 nm for argon and helium, respectively. The 5 and 12 % WCA increase in argon and helium in presence of SiO2 nanoparticles is explained by the thickness of the coatings. Indeed, as previously mentioned, the deposition rate of the fluorinated precursor in helium is much lower than in argon. This variation involves a thinner fluorinated covering layer for an equal deposition time. Since the nanoparticles are covered by a thinner layer in the case of helium, their effect on the roughness will be much more significant. Additional SEM images in Fig. 14 show that the film synthesized from helium is initially smooth and the nanoparticles are clearly visible, leading to a higher increase in the roughness and therefore in the wettability. Argon films are initially rough and the presence of nanoparticles has a weaker influence on the morphology and the resulting properties.

III.3.2. Effect of nanoparticles: Synthesis of super-hydrophobic surfaces

Super-hydrophobic films were synthesized by increasing the concentration of nanoparticles in the evaporated solutions. The influence of the concentration of nanoparticles was investigated by using 0.05, 0.5, 1, 2, and 5 g/L dispersions. Higher concentrations of nanoparticles lead to higher film roughness as shown in Fig. 15. The roughness RMS measured for a film of C6F12 synthesized by atmospheric plasma without any nanoparticles is of 192 nm while the presence of nanoparticles induces roughness According to the wettability measurements, the roughness obtained with nanoparticles dispersions of 0.5 g/L or higher concentrations leads to a super-hydrophobic character of the surface. Indeed, for those plasma depositions, the advancing and receding WCAs are higher than 160° with a very low contact angle hysteresis (< 5°). The surface is slippery since the water droplet does not adhere and roll across the surface of the sample as shown on Fig. 15. This result shows that the combination of rough surface morphology and low surface energy coatings required to obtain a super-hydrophobic surface is brought by the presence of nanoparticles and the low surface energy CxFy coating, respectively.

The process of overcoating nanoparticles by fluorocarbon films at atmospheric pressure appears to be efficient to create superhydrophobic surfaces. Kylian et al. reported the formation of slippery surfaces from a similar process but operated at low pressure through a magnetron sputtering of PTFE with lower deposition rates (50 nm/min versus 240 nm/min) than those presented in this article [START_REF] Kylian | Hydrophobic and superhydrophobic coatings based on nanoparticles overcoated by fluorocarbon plasma polymer[END_REF] . This work shows that a simple and rapid process operated at atmospheric pressure, without constraints from vacuum systems and using only water as a solvent, can lead to promising results. Further studies using hydrocarbon precursors instead of fluorocarbon should be performed since CH2 and CH3 chemical groups are known to be hydrophobic. Indeed, Fanelli et al. showed that super-hydrophobic thin films could be obtained from aerosol-assisted atmospheric cold plasma deposition [START_REF] Fanelli | Aerosol-assisted atmospheric cold plasma deposition and characterization of superhydrophobic organic-inorganic nanocomposite thin films[END_REF] .

IV. Conclusion

The synthesis and texturization processes of fluorinated surfaces by means of atmospheric plasma were investigated and presented through an integrated study of both the plasma phase and the resulting material surface. Three methods producing superhydrophobic surfaces and/or enhancing their hydrophobicity were evaluated:

-The modification of a fluorinated PTFE surface by the postdischarge of a RF plasma torch -The plasma deposition of fluorinated coatings by a DBD -The incorporation of nanoparticles into those fluorinated films. In the first approach, the addition of oxygen to the helium carrier gas showed a different behavior towards the surface properties. Oxygen is indeed required to enhance the hydrophobicity of the PTFE through its roughness increase but does not induce any change on the chemical composition. Atomic oxygen has been assumed to be responsible for this morphology modification since it etches preferably the amorphous phase; while the highly energetic helium metastables are supposed to be the main species responsible for the scission of -(CF2)-chains involving a higher etching without any preferential orientation.

The second approach focused on the deposition and intrinsic texturization of CxFy films by PECVD in a DBD. We showed that the nature of the carrier gas is a crucial parameter since argon and helium plasmas induce different chemical composition, morphology and therefore hydrophobicity. Indeed, only argon could induce WCA higher than 115° by the creation of alveolar structure in the films while in helium, the films remain smooth. The characterization of the discharges allowed us to correlate the gas phase behavior and the surface properties since we identified a higher fragmentation in argon and the presence of oxygen in helium.

Finally, the results obtained from the PECVD of fluorocarbon precursors were compared with the hybrid coatings made of nanoparticles covered by a fluorinated film deposited by the DBD. The addition of nanoparticles induces an enhancement of the hydrophobicity through the roughness increase but does not alter the chemical composition of the film. Moreover, superhydrophobic coatings characterized by the nonadherence of the water droplet at their surfaces were produced for high concentrations of nanoparticles.

V. Acknowledgments

This work was part of the I.A.P (Interuniversity Attraction Pole) programs "PSI _ Physical Chemistry of Plasma Surface Interactions-IAP-VII/12, P7/34-financially supported by the Belgian Federal Office for Science Policy (BELSPO). This work was also financially supported by the FNRS (Belgian National Fund for Scientific Research) for an "Aspirant Grant (FRFC Grant No. 2.4543.04)". Research in Mons is also supported by the Région Wallonne/European Commission FEDER program (SMARTFILM project) and FNRS-FRFC.

Figure 1 .

 1 Figure 1. Structures of the two liquid precursors C6F12 and C6F14 and the two C6F14 isomers.

Figure 2 . 2 CPSFigure 3 .

 223 Figure 2. Advancing WCAs of untreated PTFE and He and He-O2 plasmatreated PTFE, with the associated pictures of the static WCA. (P = 90 W, Ls = 10 mm, vs = 25 mm/s, gs = 500 µm, Ns = 1000, (He) = 15 L/min, (O2) = 0 and 100 mL/min).

Figure 4 .

 4 Figure 4. 55 µm 2 AFM 3D images of a PTFE surface treated by (a) pure He plasma (RRMS 15 nm) and (b) He-O2 plasma (RRMS 58 nm), with a Z scale of 511 nm. (Ls = 10 mm, vs = 25 mm/s, gs = 500 lm, Ns = 1000, (He) = 15 L/min,  (O2) = 0 and 100 mL/min, P = 90 W).

Figure 5 .

 5 Figure 5. (a) Relative mass losses of PTFE as a function of the oxygen flow rate and (b) relative surface composition of the fluorinated fragments ejected from the PTFE on the aluminum foil. (Ls = 10 mm, vs = 25 mm/s, gs = 1 mm, Ns = 1000, (He) = 15 L/min, P = 90 W).

Figure 6 .

 6 Figure 6. (a) Transmission rate of He 2 3 S at 388.9 nm and (b) optical emission intensity of O (844.6 nm), O2 + (525 nm) and N2 + (391 nm) in the He-O2 post-discharge (gs = 1 mm, (He) = 15 L/min, P = 120 W).

Figure 7 .

 7 Figure 7. Scheme of the two possible etching processes. In a pure helium plasma, the most important fragment produced is CF2.

Figure 8 .

 8 Figure 8. Intensity of the (a) CF3 + , C2F5 + , C3F5 + and C3F7 + and (b) CO2 + , COF + , COF2 + fragments generated during the atmospheric pressure PECVD of C6F14 as a function of time, in argon (full curves) and helium (dashed curves). (0.2 L/min C6F14-carrier gas, 5 L/min carrier gas total, 50 W).

Figure 10 .

 10 Figure 10. Advancing WCA and roughness RMS of the silicon wafer substrate and the pp films. Illustration of the drop behavior of (left) Ar-C6F14 and (right) He-C6F12 coatings. (0.2 L/min precursor-carrier gas, 5 L/min carrier gas total, 50 W).

III. 3 .

 3 Effect of nanoparticles on the hydrophobic fluorinated coatings properties III.3.1. Effect of nanoparticles: Enhancement of the hydrophobic property

Figure 12 .

 12 Figure 12. Advancing WCA of the pp films without nanoparticles (light gray) and with nanoparticles (red) for an identical deposition time of 2 min (left) and a similar thickness of about 200 nm (right). (0.2 L/min C6F12carrier gas, 5 L/min carrier gas total, 50 W).

Figure 14 .

 14 Figure 14. SEM images (magnification 5000) and summary of the effect of the nanoparticles on the properties of coatings synthesized from argon and helium plasmas.

Figure 15 .

 15 Figure 15. Advancing WCAs and roughness RMS as a function of the SiO2 nanoparticles concentration for C6F12 coatings (Ar-C6F12 50 W, 2 min).

4

  Journal

of Materials Research, 2015, Vol. 30, Issue 21, 3177-3191, http://dx.doi.org/10.1557/jmr.2015.279

Journal of MaterialsResearch, 2015, Vol. 30, Issue 21, 3177-3191, http://dx.doi.org/10.1557/jmr.2015.279

Journal of Materials

Research, 2015, Vol. 30, Issue 21, 3177-3191, http://dx.doi.org/10.1557/jmr.2015.279 VI. References


A Ozkan

T Dufour

T Silva

N Britun

R Snyders

A Bogaerts

F Reniers

The influence of power and frequency on the filamentary behavior of a flowing DBD -Application to the splitting of CO2 Authors

In this experimental study, a flowing dielectric barrier discharge operating at atmospheric pressure is used for the splitting of CO2 into O2 and CO. The influence of the applied frequency and plasma power on the microdischarge properties is investigated to understand their role on the CO2 conversion. Electrical measurements are carried out to explain the conversion trends and to characterize the microdischarges through their number, their lifetime, their intensity and the induced electrical charge. Their influence on the gas and electrode temperatures is also evidenced through optical emission spectroscopy and infrared imaging. It is shown that, in our configuration, the conversion depends mostly on the charge delivered in the plasma and not on the effective plasma voltage when the applied power is modified. Similarly, at constant total current, a better conversion is observed at low frequencies, where a less filamentary discharge regime with a higher effective plasma voltage than that at a higher frequency is obtained.

Introduction

Many industries still employ combustion processes from fossil fuels (natural gas, oil, coal, lignite, etc.), in particular the automotive and aerospace sectors, causing large emission of polluting and greenhouse gases [1]. As a result, carbon dioxide appears as one of the most problematic gases for the environment, mostly because it is responsible for global warming [2,3].

To prevent the release of large quantities of CO2 into the atmosphere [4], carbon capture and storage in geological reservoirs is one of the solutions which has been considered over the past decades. However, this approach presents several disadvantages including costs, environmental assessments and long-term risks. Moreover, it barely meets the principles of sustainable development since CO2 is still considered a waste for long term storage. Nowadays, a more eco-friendly approach has emerged: CO2 can be considered as a feedstock to convert into value-added products such as syngas, when mixing it with a Hsource gas. In this respect, electrochemical and photochemical CO2 reduction processes have already been successfully applied either to obtain CO, formic acid, formaldehyde, methanol and methane using electrical energy, or to obtain CO and formateusing light [START_REF] Aresta | Carbon Dioxide as Chemical Feedstock[END_REF][START_REF] Christophe | [END_REF][7]. Also, splitting pure CO2 into CO and O2 is of interest since CO is more reactive than CO2. It can be utilized in various catalytic processes such as the Monsanto process (combined with methanol to produce acetic acid) [8,9] and the Fischer-Tropsch process (combined with H2 to produce liquid hydrocarbons) [10].

In recent years, cold plasmas have been pointed out in the research literature as a new approach for the valorization of CO2 [11]. They operate far from thermodynamic equilibrium, i.e. the electrons are mostly heated by the applied electric power and can activate the gas by electron impact excitation, ionization and dissociation. New reactive species can, therefore, be created and converted more easily into new molecules, whilst the gas can remain at (or near) room temperature. Plasma can also allow the synthesis of specific organic molecules when combined with heterogeneous catalysis [12,13]. For instance, CO2-CH4 treatments using Ni catalysts have already been used for the production of syngas and small amounts of organic molecules [14,15]. Furthermore, plasma allows the possibility to store peak currents from sustainable energy into new fuels, since it can easily be switched on and off. Finally, the capture, transport and storage of CO2 might become no more necessary, as dedicated plasma sources could allow their direct and online implementation on industrial smokestack outputs.

Various types of plasma sources have already been employed for CO2 conversion, e.g. coronas [16,17], gliding arcs [18][19][20][21][22], microwave plasmas [11,23,24] and dielectric barrier discharges (DBDs) [25][26][27][28][29]. In this article, the latter plasma source is investigated in a tubular configuration. The main advantage is the treatment of the entire gas flow, since all the gas passes through the active discharge region, in contrast to coronas and gliding arcs. Moreover, a DBD reactor has a simple design that can easily be upscaled for industrial applications, as already demonstrated for the ozone synthesis [30][31][32]. We have already demonstrated that our flowing DBD setup presents a good compromise between a reasonable energy efficiency and a high CO2 conversion for low specific energy input (SEI) [33]. Even if a DBD does not yet provide an economically feasible solution for CO2 splitting [28,34], it remains a promising approach when combined with catalysts in packed bed configuration. In such a case, it could allow for the selective production of value-added chemicals and also improve the energy efficiency of CO2 splitting [27,[35][36][37]. Despite the economic limitations of a DBD for this type of application, it is important to better understand the electrical characteristics, and their effect on CO2 dissociation.

The present study is performed for a pure CO2 discharge in order to investigate the physical characteristics of the plasma. A complete study of the chemical mechanisms is beyond the scope of the present article but, as predicted by the simulations of Aerts et al, CO2 splitting in a DBD plasma is mainly dictated by electron impact dissociation to form CO and O atoms [34]. If CO2 is mixed with CH4 to produce syngas (CO and H2), the chemical pathways are more complicated [28,38]. In some studies, CO2 is also mixed with a rare gas such as argon or helium [33,[39][40][41]. In this case, no extra products are obtained, but the chemistry can still be affected, leading to a higher absolute CO2 conversion with a lower effective conversion and energy efficiency [41]. Moreover, adding helium also affects the physical characteristics, because the discharge becomes more homogeneous, while adding argon still results in a filamentary discharge [33,41].

In this experimental study, we investigate the influence of frequency and power on the filamentary behavior of a flowing DBD, operating at atmospheric pressure, to understand the role of the microdischarges in CO2 conversion. For this purpose, we performed a detailed electrical characterization using a numerical method to extract data that are usually underestimated or poorly described in the research literature, i.e. the number and lifetime of microdischarges, the conduction current and electrical charge per half period or for a given residence time. Moreover, the temperature of the gas and the electrode are determined to obtain a better understanding of CO2 splitting mechanisms.

Experimental methods

Reactor

A tubular DBD reactor dedicated to the treatment of CO2 was designed, as shown in figure 1. It consists of an alumina tube, with inner and outer diameter of 26 and 30 mm, respectively, a concentric inner electrode powered by an AC high voltage, and an outer electrode, surrounding the alumina tube, which is grounded. The inner electrode is a copper rod with a diameter of 22 mm and a length of 120 mm, while the outer electrode is a stainless steel mesh that can easily be rolled around the alumina tube, which acts as dielectric barrier. The gap, i.e. the distance separating the inner electrode from the dielectric barrier, is 2 mm, while the length of the outer electrode is 100 mm, which defines the length of the plasma zone. This length is sufficient to ensure a long enough residence time. CO2 was inserted in the reactor using a mass flow controller, keeping the flow rate fixed at 200 mLn.min -1 . The gas enters via several inlets of 0.75 mm diameter, arranged into a circular pattern, then travels through the tubular reactor and finally flows out of the reactor via several outlets (with the same configuration as the inlet). The power applied to the high-voltage electrode is provided by an AFS generator G10S-V with a maximum power of 1000 W. This generator can be coupled to a transformer with the ability to tune the frequency in a range between 1 and 30 kHz. Figure 1 represents the entire experimental setup with all the diagnostic devices described in the following paragraphs.

Mass spectrometry (MS)

The gas flowing out of the DBD is characterized by a Hiden analytical QGA mass spectrometer (Warrington, UK), using a secondary electron multiplier (SEM) detector for an electron energy in the ionization chamber set to 70 eV. MASsoft7 software is used to monitor the partial pressure variations with specific m/z rations as a function of time.

Electrical measurements

All of the electrical measurements are performed using a digital oscilloscope (Tektronix DPO 3032). The applied voltage (V1) was measured with a high voltage probe (Tektronix P6015A) while the DBD voltage is determined by subtracting V2 from V1, as represented in figure 1. An external capacitor (10 nF) was placed in series with the DBD to evaluate first its electrical charge QP(t) and subsequently the power absorbed by the plasma via the Lissajous method [42]. The total current of the discharge (IDBD) was recorded using a Pearson 2877 Rogowski coil placed in series with the DBD and presenting the ability to respond to fast changing currents due to its low inductance (rise time of 2 ns and drop rate of 0.2% μs -1 ). The total current is the sum of the plasma current (i.e. conduction current) and dielectric current (i.e. capacitive current):

𝐼 𝐷𝐵𝐷 (𝑡) = 𝐼 𝑝𝑙 (𝑡) + 𝐼 𝑑𝑖𝑒𝑙 (𝑡)

At atmospheric pressure DBD in CO2 usually operates in the filamentary mode [25,34,43,44]. The microdischarges are characterized through their current peaks observed in the oscillograms (see figure 2) and considering the dielectric current Idiel as the baseline for determining the microdischarge properties. Numerical methods from the Origin software analyzer toolbox were applied to determine the number of microdischarges (Nmd), their peak intensity, electrical charge (Qmd) and lifetime (Lmd). The intensity and lifetime of each microdischarge are evaluated considering the peak magnitude and FWHM, respectively. Its electric charge is determined by integrating the peak area from the baseline (see figure 2(c)). In the case of Nmd, the 'filtering by height' method (including the local maximum option) is used. The accuracy of this numerical method is verified by comparing the provided Nmd value with the one measured from a visual counting.

As the difference between the two approaches always remains lower than 5%, the numerical method is considered to be sufficiently accurate.

Figure 2. (a) Oscillogram of the total current and determination of the baseline; (b) counting of the current peaks; (c) signal processing for the electrical characterization of the microdischarges.

The main electrical parameters of the DBD were calculated and reported in table 1. Three different voltages were considered: the voltage over the entire DBD reactor (VDBD), the voltage over the dielectric barrier (Vdiel) calculated with  as a dummy variable, and the effective voltage of the plasma (Vpl,eff). The electric charges of the DBD (QDBD), the dielectric barrier (Qdiel), the plasma (Qpl) and the charge per micro-discharge (Qmd) are calculated from the current or voltage. Finally, the capacitances of the entire DBD (CDBD), the dielectric barrier (Cdiel) and the plasma (Cpl) are given, considering for Cdiel a dependence on the following parameters: the tubular barrier length (L), its external and internal radius (Rext and Rint, respectively) and the dielectric constant of alumina (εr). In the present work, the expression of the DBD capacitance is adopted from the homogeneous model of Godyak & Lieberman based on the equivalent circuit model of a capacitive discharge [START_REF] Lieberman | Principles of Plasma Discharges and Materials Processing[END_REF].

Optical emission spectroscopy (OES)

The OES measurements were performed with an Andor Shamrock-500i spectrometer including an Andor DU420A-OE CCD camera. Each spectrum was acquired for an exposure time of 5s and 7 accumulations. Silva et al have shown that in a CO2 microwave plasma at low pressure, the rotational temperature of the CO band is similar to the gas temperature [START_REF] Silva | [END_REF]. However, as the CO Angstrom band is not very suitable at atmospheric pressure for gas temperature measurements [47], we analyzed the first positive system (FPS) of N2 by adding 10% of nitrogen to CO2. Thereupon, the gas temperature has been measured via a line-ratio peak formula, specific to this system [48,49]. The peaks at 775.3 and at 773.9 nm were considered.

Infrared imaging

2D temperature profiles of the grounded outer electrode were achieved with an infrared camera (FLIR E40) with a resolution of 160 × 120 pixels and a thermal sensitivity lower than 0.07 °C at 30 °C. FLIR ResearchIR software is used to control, record and analyze the temperature profiles in a range from -20 °C to +650 °C. The emissivity coefficient of the stainless steel electrode is about 0.45, and this value is introduced in the software. The temperature was calibrated to room temperature.

CO2 conversion and energy efficiency

The effect of the frequency on the CO2 conversion (CO2) and on the CO2 energy efficiency (CO2) is investigated for an absorbed power fixed at 55 W (determined by the Lissajous method; applied power = 60 W). It was noticed that the absorbed power always remained constant for a given applied power of 60 W, regardless of the operating frequency used. CO2 is calculated according to the following equation, where I stands for the mass spectrometer intensity assigned to CO2:

𝜒 𝐶𝑂2 (%) = 100. 𝐼 𝐶𝑂2 𝑝𝑙𝑎𝑠𝑚𝑎 𝑂𝐹𝐹 -𝐼 𝐶𝑂2 𝑝𝑙𝑎𝑠𝑚𝑎 𝑂𝑁 𝐼 𝐶𝑂2 𝑝𝑙𝑎𝑠𝑚𝑎 𝑂𝐹𝐹 (1)
The energy efficiency of the CO2 conversion is calculated (in %) from CO2, the enthalpy of the splitting reaction (CO2 → CO + ½ O2), namely H 0 298K = 279.8 kJ.mol -1 = 2.9 eV.molecule -1 and the SEI, according to the following equation:

𝜂 𝐶𝑂2 (%) = 𝜒 𝐶𝑂2 (%). Δ𝐻 298𝐾 𝑜 (𝑒𝑉. 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 ) 𝑆𝐸𝐼 (𝑒𝑉. 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 ) (2)
The SEI corresponds to the energy density (Ed in J.cm -3 ), converted into eV•molecule -1 . The energy density depends on the power absorbed by the plasma and the gas flow rate.

𝐸 𝑑 (𝐽. 𝑐𝑚 -3 ) = 𝑃𝑙𝑎𝑠𝑚𝑎 𝑝𝑜𝑤𝑒𝑟 (𝐽. 𝑠 -1 ) 𝐺𝑎𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 (𝑐𝑚 3 . 𝑠 -1 )

(3) 𝑆𝐸𝐼 (𝑒𝑉.𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 ) = 𝐸 𝑑,(𝐽.𝑐𝑚 -3 ) × 6.24 10 (𝑒𝑉.𝐽 -1 )

18

× 24500 (𝑐𝑚 3 .𝑚𝑜𝑙 -1 )

6.022 10 (𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒.𝑚𝑜𝑙 -1 )

(4)

As shown in figure 3, an increase in the frequency from 16.2 to 28.6 kHz leads to a slight drop in the CO2 conversion and energy efficiency. This is probably attributed to a drop in the density of electrons involved in the CO2 splitting, although the total electron density in the whole discharge remains constant, as is shown below. The reason why there is a drop in conversion may be related to the fact that the shape of the electron energy distribution function (EEDF) is different when the plasma is in the filamentary regime (high frequency) or less filamentary regimeglow regime (low frequency) [33,50]. Depending on the shape of the tail of EEDF, the number of electrons which can contribute to the breaking of the C = O bonds present in CO2 might be higher for low frequency, i.e. electrons which are located on the right side of the EEDF over 5.52 eV (value of the C = O bond). Also, CO2 > CO2 in the whole frequency range because the SEI is in the order of ≈4.2 eV•molecule -1 , and thus always higher than ΔH298K.

Electrical characterization

A complementary, electrical characterization was carried out to determine whether this trend in conversion and energy efficiency can be correlated with electrical measurements. As observed in figure 4, the DBD voltage (VDBD) decreases as a function of the frequency (by almost 500 V in this frequency range).

For a DBD operating in its glow mode, the voltage is the sum of two components: plasma voltage (Vpl) and dielectric voltage (Vdiel) [51][52][53][54]. As in our case the DBD operates in the filamentary mode, the presence of micro-discharges and remnant surface charges from previous micro-discharges leads to a potentially inhomogeneous electric field distribution. Therefore, the measured voltage has to be considered as an effective plasma voltage (Vpl,eff), i.e. a spatial average of the plasma voltage with potential local magnitude variations.

As the total electrical charge is conserved through the entire DBD source (Qpl = Qdiel), the relation Cpl that Vpl,eff represents almost 82% of VDBD, as represented by the red curve in figure 4. Similar values have also been reported in previous studies; for instance, in [44] Vpl,eff was stated to represent even 98% of VDBD. It is clear that VDBD and Vpl,eff also decrease upon increasing frequency, and this can probably explain the drop in CO2 conversion and energy efficiency, shown in figure 3 above, as a lower voltage might give rise to lower electron energies, and thus less efficient CO2 conversion by electron impact excitationdissociation. The dielectric voltage is obtained by subtracting Vpl,eff from VDBD. It is almost constant in the entire frequency range with a value close to 450 V. To enhance the CO2 conversion, one could increase Vpl,eff (hence the electric field in the gap) by decreasing Vdiel, e.g. by reducing the dielectric thickness. As the plasma current corresponds to a large number of individual peaks, and not to a continuous periodic function, studying its variation with respect to the frequency requires a dedicated analytical study. The number of peaks can be analyzed (i) per period or half period and (ii) for the entire CO2 residence time or a given analysis time. In the present work, these variations are studied as follows: -Per half period rather than per period since the current peak distributions can differ depending on their location on the positive or negative half periods. -For an analysis time fixed at 120 μs which is a more reasonable time window for this study than the residence time of CO2. The latter one is estimated to 4.5 s considering the geometrical characteristics of the tubular discharge reactor and the CO2 flow rate fixed at 200 mLn•min -1 .

The oscillograms of the total current (IDBD = Ipl + Idiel) are plotted in figure 5 at 16.2 kHz, 22.9 kHz and 28.6 kHz. The comparison of these three oscillograms shows that increasing the frequency induces a decrease in the number of peaks per half period, which is not illogical, as the time of a half period drops upon higher frequency. Also, for a given frequency, the peak distribution in the positive half periods appears different from that in the negative half periods, which might be attributed to the asymmetry of the tubular discharge, where only one electrode is covered by a dielectric [55]. These oscillograms were analyzed using the aforementioned numerical method to determine the microdischarge characteristics with higher accuracy.

The effect of the frequency on the number of microdischarges (Nmd) is depicted in figure 6(a) per half period and for τanal = 120 μs. As mentioned above, a higher frequency results in a lower number of microdischarges per half period (whatever the sign of the half period) but it remains unchanged for τanal = 120 μs. Therefore, the total number of microdischarges cannot explain the decrease in the CO2 conversion. The same applies to the average lifetime of the microdischarges illustrated in figure 6(b) as it remains close to 15 ns, regardless the operating frequency. As shown in figure 6(c), a rise in the frequency induces a slight drop in the plasma charge per half period (with Qpl + = Qpl -). As the effect of the frequency on CO2 has to be explained based on the residence time, it is more relevant to consider QDBD 120 , as plotted in figure 6(d). In this figure, the electrical charge of the DBD increases with frequency due to its dielectric component (Qdiel) and not due to its plasma component (Qpl). In other words, the frequency does not seem to change the electrical properties of the plasma but the electric field distribution through the dielectric barrier. Plotting these currents in figure 6(e) indicates that their variations versus the frequency are not that very significant: only a slight increase of 20% (from 11 mA to 14 mA) is observed for a half period. The discharge current also increases slightly for a given residence time. This is logical, as the voltage slightly drops upon rising frequency, so the current should slightly increase to keep the same power. Hence, it is clear that the microdischarge characteristics are not correlated to CO2 conversion.

An alternative and convenient representation to evaluate how the frequency could change the microdischarge characteristics is to plot the distribution of their lifetimes versus their number (NS) for several frequencies, as presented in figure 7. Most of the microdischarges (NS ≈ 1200) exhibit a lifetime comprised between 7 and 25 ns. A small fraction of microdischarges exhibits a lifetime under 7 ns and above 25 ns. The drop in CO2 conversion upon rising frequency may not be induced by the microdischarges. Only the drop of the discharge voltage seems to be responsible for the drop in CO2 conversion.

Gas temperature and electrode temperature

A slight increase in the conduction current as well as in the microdischarge lifetime may have impact on the power dissipated by the discharge by the Joule effect since it is proportional to the square of IDBD. This assumption has been verified by investigating the influence of the frequency on the gas temperature (obtained from OES measurements) as well as on the electrode temperature (obtained from IR imaging). Figure 8(a) shows the 2D temperature profiles of the outer grounded electrode, obtained by placing the IR camera perpendicular to the gas flow. The electrode temperature appears rather uniform in the whole discharge region, regardless of the frequency. As illustrated in figure 8(b), increasing the frequency from 16.2 kHz to 28.6 kHz induces an increase in the gas temperature (from 523 to 560 K) and a slight increase in the average temperature of the ground electrode (from 375 K to 405 K). The difference Tgas-Telectrode can be considered as a representation of Joule losses. A rise in the frequency induces an increase in the gas temperature that can be linked to a decrease of the effective plasma voltage (see figure 4). Indeed, a low frequency plasma gives place to a higher effective plasma voltage, i.e. to a higher electric field where electrons can be more readily accelerated. The increase in gas temperature upon rising frequency may be correlated with the lower CO2 conversion at high frequencies since more energy may be spent for heating and less to decompose CO2. The gas temperature values which are found with the OES spectra are in good agreement, and in the same order of magnitude, with temperature values determined by other methods for a CO2 DBD in the same kind of conditions [56]. 3.2. Effect of the power

CO2 conversion and energy efficiency

Figure 9 shows that the CO2 conversion rises linearly with power, reaching a value as high as 28% at 100 W. In the same power range, a slight decrease in the energy efficiency is measured from 12.5% to 11%. These trends are logical, because more energy is put into the system, leading to a higher electron density [57,58], and to more CO2 conversion. At the same time, the rise in CO2 conversion is less pronounced than the rise in SEI, thus the energy efficiency drops upon increasing power, as predicted by equation ( 2). An electrical characterization was carried out to determine whether this conversion trend is related to specific properties of the filaments.

Electrical characterization

Firstly, the influence of the absorbed power on the DBD voltage was studied, as plotted in figure 10, which shows a clear increase from 3.8 kV to 4.2 kV, as expected. Vpl,eff represents 89% of the DBD voltage at 3 W and it drops to 80% at 97 W. The effective plasma voltage is thus not really correlated with the increase in CO2 but the influence of the filaments has to be investigated as well for a complete interpretation of the behavior.

As the oscillograms present a very symmetric signal for the positive and negative sign of T/2, we only focused our study on the filaments for a certain analysis time. Figure 11(a) depicts the number of microdischarges within a fixed τanal, as well as their mean lifetime, as a function of the absorbed power. Increasing the power leads to a rise in the number of peaks as well as their mean lifetime. The power effect is, therefore, very different from the frequency effect where no influence was observed both on Nmd and the microdischarge lifetime (see figures 6(a) and (b)). Also, figure 11(b) illustrates the electrical charge of the DBD versus the absorbed power: a clear increase was observed from 20 W to 100 W. This is attributable to the increase in the plasma charge (which corresponds therefore to an increase of the discharge current), even if the dielectric component remains always higher than the plasma component in the entire power range. The latter one, however, remained rather constant because the operating frequency is fixed. Thus, it is clear that the rise in number of microdischarges, in the microdischarge lifetime, and in the total electric charge of the DBD, are correlated with the rise in CO2 conversion with higher power. The distribution of the microdischarge lifetimes versus their number (Nmd) is represented in figure 12. It appears that decreasing the power leads mostly to the production of short lifetime microdischarges (cf. line a). Indeed, the percentage of microdischarges which has less than 7 ns of lifetime is 31, 23 and 8% for a plasma power of 23, 65 and 97 W, respectively. However, increasing the power leads to a greater extent to the production of long-lifetime microdischarges (cf. line b). Indeed, the percentage of microdischarges which has more than 25 ns of lifetime is 3, 5 and 12% for a plasma power of 23, 65 and 97 W, respectively. This effect is not observed when tuning the frequency.

3.2.3. Gas temperature and electrode temperature Furthermore, we also checked whether the rising CO2 conversion is correlated with the gas temperature and the temperature of the grounded electrode. In figure 13(a), the 2D profiles of the temperature at the outer grounded electrode are plotted for 20 W, 50 W, 80 W and 100 W. The electrode temperature appears not entirely uniform in the whole discharge region, and it is clearly correlated with power. As illustrated in figure 13(b), the temperature of the grounded electrode increases linearly with power and reaches a mean value as high as 430 K at 100 W after 5 min of process time. On the other hand, the gas temperature increases by almost 40 K as a function of the power as can be observed in figure 13(b). Increasing the plasma power generally enhances the electron impact processes and, thus, could lead to gas heating. When the plasma is ignited, the gas heating is responsible for an increase of the wall temperature. The Joule losses are quite significant since both the alumina dielectric and the mesh stainless steel electrode have quite high thermal conductivities.

Plasma

Conclusion

The splitting of CO2 into CO and O2 was studied at atmospheric pressure in a tubular DBD operating in pure CO2. The influence of the operating frequency and applied plasma power on the discharge behavior and on CO2 conversion and energy efficiency was discussed.

Table 2 summarizes the results of all the experiments to correlate the CO2 conversion with the process parameters by increasing the frequency and the power. We can observe how the frequency and power affect the CO2 conversion and energy efficiency, as well as other plasma parameters, for τanal = 120 μs. It is clear that these two parameters, which can easily be controllable on the high voltage AC generator, have an important impact on the CO2 splitting process. It was observed that both parameters (especially the power) strongly affect the charge delivered in the plasma, the number of microdischarge which occurs for a certain time, the gas temperature, the effective plasma voltage, the total number of electron and also the number of electron which has enough energy to split CO2, depending on the shape of the EEDF and, thus, defining the energy efficiency of the splitting process.

Increasing the frequency may induce an increase in Te, accounting for the measured increase in Tgas. Moreover, the gas heating also leads to a slight increase in the outer electrode temperature. As the microdischarge distributions do not depend on the frequency, and the same applies to the microdischarge average lifetime, the number and electrical charge of the microdischarges, we consider that the microdischarges do not affect the CO2 conversion in the frequency study. The most relevant parameter to explain the decrease in CO2 with rising frequency is the drop of the effective plasma voltage. The increase in the gas temperature with the power may itself result from a rise in the electron impact processes. This could explain the increase of the CO2 conversion in close correlation with the electrical characterizations.

Figure 1 .

 1 Figure 1. Schematic diagram of the experimental set-up.

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  Plasma Sources Science & Technology, 2016, Vol. 25, No 2, 025013 (11pp), http://dx.doi.org/10.1088/0963-0252/25/2/025013

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  Plasma Sources Science & Technology, 2016, Vol. 25, No 2, 025013 (11pp), http://dx.doi.org/10.1088/0963-0252/25/2/025013 3. Results and discussion 3.1. Effect of the frequency 3.1.1.

Figure 3 .

 3 Figure 3. CO2 conversion and energy efficiency as a function of the frequency, Pabs = 55 W, Φ(CO2) = 200 mLn•min -1 .

Figure 4 .

 4 Figure 4. RMS voltages as a function of frequency at Pabs = 55 W, Φ(CO2) = 200 mLn•min -1 .

Figure 5 .Figure 6 .

 56 Figure 5. Current profiles as a function of time for 3 different frequencies.

Figure 7 .

 7 Figure 7. Distribution of the microdischarges lifetime for τanal = 120 μs and Pabs = 55 W.

Figure 8 .

 8 Figure 8. (a) 2D temperature profiles of the grounded electrode; (b) Gas and grounded electrode temperature as a function of frequency. Pabs = 55 W, process time = 5 min.

Figure 9 . 1 .Figure 10 .

 9110 Figure 9. CO2 conversion and energy efficiency as a function of the absorbed power, for f = 28.6 kHz, and Φ(CO2) = 200 mLn•min -1 .

PlasmaFigure 11 .

 11 Figure 11. Characteristics of the microdischarges as a function of the absorbed power: (a) number of current peaks and mean lifetime (b) charge and current for τanal = 120 μs.

Figure 12 .

 12 Figure 12. Distribution of the microdischarges lifetime for f = 28.6 kHz, τanal = 120 μs.

Figure 13 .

 13 Figure 13. (a) 2D profiles of the grounded electrode temperature for various powers; (b) Gas temperature and grounded electrode temperature as a function of power (f = 28.6 kHz, process time = 5 min).

Table 1 . Main electrical parameters of the discharge.

 1

Table 2 . Summary of interpretation for the CO2 splitting.

 2

Acknowledgments

The authors acknowledge financial support from the IAPVII/12, P7/34 (Inter-university Attraction Pole) program 'PSI-Physical Chemistry of Plasma-Surface Interactions', financially supported by the Belgian Federal Office for Science Policy (BELSPO). A Ozkan would like to thank the financial support given by 'Fonds David et Alice Van Buuren'.