Alp Ozkan

Thierry Dufour

Gregory Arnoult

Philippe De Keyzer

Annemie Bogaerts

François Reniers

Affiliations

CO2-CH4 conversion and syngas formation at atmospheric pressure using a multi-electrode dielectric barrier discharge

teaching and research institutions in France or abroad, or from public or private research centers.

I. Introduction

Almost 72 % of the total greenhouse effect is attributed to water vapor and clouds, the remainder being mainly the result of CO2 [1]. Natural greenhouse gas emissions are responsible for bringing the average temperature of the Earth to +15 °C (instead of -18°C) by absorbing its infrared radiation. However, anthropogenic activities reinforce this situation, leading to an increase of greenhouse gas concentrations in the atmosphere [START_REF] Seinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF][START_REF] Canadell | [END_REF]. In that respect, carbon dioxide (but also methane) figures among the most important greenhouse gases produced by industries and taking part to the global warming. Its production has increased for many decades. Today, it represents 29 gigatons of emission per year and is expected to increase to 36 or 43 gigatons/year, depending upon the energy world policies, i.e. how we will use existing and new energy sources [4]. For this reason, the remediation of CO2 has received increasing attention in recent years.

Until now, four approaches have been considered to reduce the industrial CO2 footprint: using renewable energy, using non carbon energy resources, CO2 capture and CO2 reforming [5][6][START_REF] Aresta | Carbon Dioxide as Chemical Feedstock[END_REF]. The latter approach aims at using carbon dioxide as a feedstock and transforming it into value-added products such as carbon monoxide and oxygen, as shown in (1):

𝐶𝑂 2 → 1 2 𝑂 2 + 𝐶𝑂 Δ𝐺 298𝐾 0 = +257.2 𝑘𝐽. 𝑚𝑜𝑙 -1 (1)
This aforementioned reaction is thermodynamically limited and highly endothermic. According to Le Chatelier's principle, a high reaction temperature and a low CO2 partial pressure are required to achieve a high conversion [START_REF] Aresta | Carbon Dioxide as Chemical Feedstock[END_REF][START_REF] Nikoo | [END_REF][9]. Owing to the high thermodynamic stability of the CO2 molecule in standard conditions, its dissociation can only be achieved through endothermic reactions requiring an external energy source. In that respect, conventional chemistry processes have already been used, such as electroreduction of CO2 [6]. Besides, non-thermal atmospheric plasma processes can be employed such as corona discharges [10,11], dielectric barrier discharges (DBD) [12][13][14][15][16][17][18], gliding-arcs [19,20] and plasma jets [21,22]. Low pressure plasma sources can also be used such as microwave discharges [23,[START_REF] Cho | Carbon Dioxide Utilization for Global Sustainability[END_REF]. Among these sources, most of the energy required for the dissociation of CO2 depends on the electron energy distribution function (EEDF). Carbon dioxide can be mixed with methane to form carbon monoxide and molecular hydrogen in [START_REF] Seinfeld | Atmospheric Chemistry and Physics: From Air Pollution to Climate Change[END_REF], but also other products of interest can be formed, such as oxygenated organic molecules and hydrocarbons [START_REF] Luque | [END_REF]26].

𝐶𝑂 2 + 𝐶𝐻 4 → 2𝐻 2 + 2𝐶𝑂 Δ𝐺 298𝐾 0 = +170.8 𝑘𝐽. 𝑚𝑜𝑙 -1 (2)
The conversion of CO2 and CH4 by an atmospheric dielectric barrier discharge (DBD) is reported in this study, using Ar as a carrier gas to generate more metastable species and therefore stabilize the discharge. Using a tubular DBD offers a promising and innovative solution since the transformation of CO2 can be performed ''on line'', i.e. directly at the output of industrial chimneys instead of releasing the CO2 into the atmosphere and hence increase the greenhouse effect. Therefore, it does not require capture, transport or storage of CO2 and, for instance, could partially close the carbon loop if coupled to green electricity. By using gas chromatography (GC), we demonstrate that this process is efficient to obtain CO and value-added products. Three parameters are evaluated: the CO2 and CH4 flow rates, the power supplied to the DBD and the nature of the carrier gas (Ar or He).

The energy efficiency of the CO2 conversion is estimated and compared with those of similar plasma sources.

II. Experimental setup

II.1. DBD reactor

A cylindrical multi-electrode DBD reactor dedicated to the treatment of elevated gas flow rates has been designed as shown in Figure 1. It consists of a 2 mm thick tube made in quartz with an external diameter of 34 mm and a length of 100 mm (so as to ensure a long residence time). The gas enters via 16 inlets of 0.75 mm in diameter arranged into a circular pattern, then travels longitudinally through the tubular reactor and finally flows out of the reactor via 16 outlets (same configuration as the inlet). The discharge is generated between six AC high-voltage tubular electrodes set at equal distance from a central tubular electrode which is grounded. The power applied to the high-voltage electrodes is provided by an AFS Generator G10S-V with a maximum power of 1000 W and a variable frequency in the range between 1 and 30 kHz. The distance between the grounded electrode and each high-voltage electrode is the same as the distance between two high-voltage electrodes, namely 3 mm. The grounded electrode is a copper rod with a diameter of 5 mm and a length of 100 mm, while the high-voltage electrodes are copper wires approximately 1 mm in diameter and with the same length of 100 mm. The high-voltage electrodes are encompassed into alumina dielectric tubes with 0.75 mm thickness, as depicted in Figure 1.

II.2. Entire set-up

A schematic of the entire experimental setup is shown in Figure 2. Argon, carbon dioxide and methane are introduced into the reactor via Aalborg volumetric flow meters able to measure flow rates as high as 1800, 120 and 120 mL.min -1 , respectively. Argon (or helium) is used as the carrier gas to initiate and maintain the discharge. The total flow rate of the gas mixture supplying the DBD reactor remains fixed at 1920 mL.min -1 while the CO2 and CH4 flow rates are both varied from 0 to 120 mL.min -1 .

Figure 2. Schematic diagram of the entire experimental set up.

The products resulting from the plasma phase reactions are analyzed downstream of the reactor with an online gas chromatograph (Agilent 6890N) equipped with a 60/80 Carboxen 1000 column (Supelco 1-2390-U). The products are analyzed with two detectors: a thermal conductivity detector (TCD) and a flame ionization detector (FID). The conversion of CO2 and CH4 are calculated according to Eqs. ( 3) and ( 4), respectively, where A represents the peak area assigned to CO2 or CH4 in the chromatogram: The selectivities of H2, O2, CO, C2H6 and C2H4 have been calculated as reported in Table 1, listed as H, O or C based selectivities, depending on the plasma composition (CH4, CO2, CO2/CH4 respectively).

𝐶𝑂 2 𝑐𝑜𝑛𝑣 (%) = 𝜒 𝐶𝑂2 = 100. 𝐴 𝐶𝑂2 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑝𝑙𝑎𝑠𝑚𝑎 -𝐴 𝐶𝑂2 𝑤𝑖𝑡ℎ 𝑝𝑙𝑎𝑠𝑚𝑎 𝐴 𝐶𝑂2 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑝𝑙𝑎𝑠𝑚𝑎 (3)

III. Results

III.1. Effect of the CO2 and CH4 flow rates

The plasma is generated in a mixture of CO2, CH4 and Ar (or He) to investigate the effect of the reactive gas flow rates on their conversion. The Ar flow rate is set to 1800 mL.min -1 while the CO2 and CH4 flow rates can be tuned between 0 and 120 mL.min -1 , but the sum of both is always equal to 120 mL.min -1 . Figure 3 represents the CO2 and CH4 conversions as a function of the CO2 and CH4 flow rates. Both for CO2 and CH4, an increase in the flow rate is always correlated with a decrease in its conversion. Indeed, for CO2 flow rates increasing from 20 to 120 mL.min -1 , CO2 decreases from 8.3 % to 6.1 % while CH4 decreases from 21.5 % to 10.9 % when the CH4 flow rates rise from 20 to 120 mL.min -1 . This figure illustrates also that CH4 is always converted to a larger extent than CO2, whatever the individual gas flow rates. Chemical reactions in the plasma lead to the dissociation of these molecules, thus generating products that can also recombine to form new species such as H2, O2, CO, C2H4 and C2H6 whose volumetric fractions (fV) are plotted in Figure 4a as a function of the CO2 and CH4 flow rates. Each fV fraction is calculated as the ratio of the product flow rate to the CO2/CH4 mixture flow rate, multiplied by 100. The main products are molecular hydrogen (fV,max[H2] = 7.73 %), carbon monoxide (fV,max[CO] = 8.13 %) and molecular oxygen (fV,max[O2] = 3.98%), the latter being detected only if no CH4 is injected in the discharge. Other products such as ethylene and ethane are also formed but in smaller proportions (fV,max[C2H4] = 0.52 % and fV,max[C2H6] = 1.51 %). The production of CO is more important with an increase in the CO2 flow rate, reaching a plateau of approximately 8.10 % for CO2 flow rates higher than 80 mL.min - 1 . In the same way, the production of hydrogen, ethane and ethylene increases with the CH4 flow rate. The production of O2 is only present for pure CO2 plasma while it disappears after CH4 addition. That probably means that the CH4 reactive species interact with oxygen in the discharge. It is quite logical that the decomposition of CO2 favors the production of CO and O2 while the decomposition of CH4 leads to the production of H2, C2H4 and C2H6 but also of carbon black powder (not detected by gas chromatography). The selectivities of these products have also been calculated using the formulas from Table 1 hence the durability of the reactor. To prevent these problems, coke deposit can easily be removed by cleaning the inner walls of the reactor and polishing them with sandpaper. Another convenient way is to apply a pure CO2 or pure O2 plasma to remove the coke deposit.

Selec

Table 1 Formulas for the H, O or C based selectivities of H2, O2, CO, C2H6 and C2H4 (n is the number of moles).

III.2. Effect of the power

Figure 5 shows the CH4 and CO2 conversions versus the power applied to the DBD in the range between 30 W and 80 W for Ar = 1680 mL.min -1 , CO2 = CH4 = 120 mL.min -1 and an AC frequency of 19.5 kHz. The CO2 conversion increases from 2.0 % to 7.5 % upon rising power, while the CH4 conversion increases from 6.7 % to 14.8% in the same power range. The two conversions can be considered as linearly increasing with the power since their correlation coefficients are r 2 (CO2) = 0.976 and r 2 (CH4) = 0.899. The slopes of both curves are almost the same, consistently with the results of Zheng et al. performed in a two-electrode DBD reactor [27]. It is also clear that the methane conversion is always higher than CO2 (difference of at least 5 %) thanks to its lower bond energy. The volumetric fractions of H2, CO, C2H4, C2H6 plotted in Figure 6 versus the power indicate that the production of syngas (hydrogen and carbon monoxide) also increases linearly with the power, and both products are formed nearly equally, yielding a syngas ratio close to 1. A linear increase is also observed in the case of C2H4 and C2H6, although the slopes are less significant.

III.3. Effect of the carrier gas

The influence of the carrier gas (argon or helium) is investigated for the same flow rate set to 1800 mL.min -1 and the reactive gas flow rates set to CO2 = CH4 = 60 mL.min -1 . The nature of the carrier gas seems to have an important impact on the conversion of CO2 and CH4; see Figure 7a. The conversion of CH4 is indeed higher in the presence of helium than with argon (respectively 21.4 % and 16.4 %) while the opposite effect is observed for the conversion of CO2 since CO2 = 6.8% with helium and CH4 = 11.5 % with argon. It is also worth mentioning that for the same plasma power (60 W) and frequency (17.1 kHz), a filamentary discharge and a glow discharge are obtained with argon and helium, respectively, as shown in Figure 7b and7c. Plasmas are complex media where several hundred reactions of production and consumption can occur [28,29]. The most plausible mechanisms for the formation and consumption of intermediate and value-added products in the CO2/CH4 gas mixture are listed in Table 2. In the following sections, we explain how the most important reaction products are formed.

IV.1.2. Production of hydrogen

Several chemical reactions give rise to the production of molecular hydrogen through the dissociation of hydrocarbon species upon collision with an atom (R2), an electron (R5) or an H radical (R6, R7 and R8). The rate constants of these reactions are in the order of 10 -13 -10 -10 cm 3 .s -1 , except for R5 which is somewhat higher (7.88  10 -9 cm 3 .s -1 ) as the collision occurs between an energetic electron and an ion. Although the rate constant of R9 is a bit lower than the other ones (1.44  10 -14 cm 3 .s -1 ), the recombination of two H radicals may be considered as very important since v = k11.

[H] 2 and H is produced in many other reactions such as R3, R4, R11, R12, R16, R17, R18, R19 and R25. Electron impact reactions R16 and R17 are not described with a rate constant but with a cross section s which depends on the electron temperature.

IV.1.3. Production of CO

The formation of CO is directly correlated with the dissociation of CO2. The reactions responsible for the production of CO are given by R2, R10, R11, R12, R21, R22, and R23 [44]. (R21) is electron impact dissociation of CO2 into CO and O, which is the most important process in CO2 splitting. When CH4 is present, the O atoms will be further consumed by R2, R11, R20, R23 and R25 and this explains the higher CO2 conversion when more CH4 is present in the gas mixture. Indeed, as stated by the Le Chatelier's principle, the dissociation is more favorable as one (or both) of the reaction products is constantly consumed. This effect has been demonstrated in the literature: Tagawa et al. have observed an increasing CO2 conversion by placing an O2 trapper membrane into a CO2/CH4 discharge in order to separate O2 from the gas stream. As a consequence, the CO/CO2 equilibrium is more shifted to CO [45].

IV.1.4. Production of ethane

The recombination of two CH3 • radicals can lead to the production of ethane according to reaction (R13). R14 and R15 could also lead to the production of ethane but are less probable. Indeed, as computed by Snoeckx et al. in the case of a similar atmospheric DBD source supplied in CH4-CO2, the density of CH3 is always higher than the one of C2H5 [29].

IV.1.5. Production of ethylene

The formation of ethylene may result from a two-step collisional mechanism, where first an electron collision leads to the dissociation of C2H6 into C2H5 • and H radicals (R16), followed by a second electron collision with C2H5 • resulting in the abstraction of a H radical to produce ethylene (R17). This simple mechanism can explain why fV[C2H4] is always lower than fV[C2H6].

IV.1.6. Other reaction products

Formaldehyde traces have also been detected. Their formation can result from CH3 radicals (R25) or to a lower extent from CH2 radicals (R26). Other oxygenated products have not been detected at the conditions under study, probably because their amounts are under the limit of detection of the gas chromatography detectors.

According to the literature, the formation of other oxygenated organic molecules such as acetic acid or methanol may also occur in a plasma [17,46,47]. The higher volumetric fraction of H2, compared to ethane and ethylene, can be explained according to several chemical reactions (R2, R5, R6, R7, R8 and R9). Indeed, there are more reactions for H2 and H • formation compared to reactions for C2H6 and C2H4 formation. Moreover, there are more reactions consuming C2H6 or C2H4 than consuming H2. C2H6 or C2H4 is indeed very easily consumed once it is produced. That is why the H2 amount is always higher than the amounts of C2H6 and C2H4.

IV.2. Effect of the power

The linear increase of CO2 and CH4 conversions as a function of the power results from a linear increase in the electron density (Figure 5). Indeed, the dissociation of C-H and C=O bonds requires energies of a few eV that may be mostly transferred from the electrons. An increase in the plasma power can induce higher electron temperatures and higher electron densities. In our case, the increase in electron temperature may be assumed as negligible since in a classical DBD, it would induce a stronger filamentary regime that has not been observed here. Increasing the plasma power can also induce higher electron densities that can be assumed as linearly depending on the power if the electron permeability and the electric field profile are considered as weakly dependent on the applied power.

To produce C2H4 and C2H6, a linear increase upon increasing power is also observed, but the slopes are less pronounced than for CO and H2. This is probably because the production of these molecules is not simply based on one electron impact reaction, like the formation of H2 from CH4 and the splitting of CO2 into CO. Indeed, to obtain C2H6, two CH3 • radicals are necessary (R13) while to obtain C2H4, two electronic collisions with C2H6 are required (R16 and R17).

IV.3. Effect of the carrier gas

According to Figure 7, CH4 is always higher than CO2 whatever the nature of the carrier gas. Indeed, in a plasma, the dissociation of CH4 is easier than for CO2 since the bond dissociation energy of C-H (4.48 eV) is lower than the bond dissociation energy of C=O (5.52 eV) [START_REF] Stans | Bond Dissociation Energies in Simple Molecules[END_REF]. However, the fact that CH4 is more efficiently dissociated in He than in Ar, whereas CO2 is more efficiently dissociated in Ar than in He, is less straightforward. The reason is that the shape of the electron energy distribution function (EEDF) is different when the plasma is in the filamentary regime (Ar) or in the glow regime (He). The EEDF of these two regimes is sketched in Figure 8, assuming Maxwellian distributions (thermodynamic equilibrium) for the sake of clarity [START_REF] Godyak | [END_REF]. The bond dissociation energies of C-H and C=O are also reported in Figure 8. In the filamentary regime, the EEDF is characterized by (i) a number of warm electrons much lower than in a glow discharge but also by (ii) a tail extending toward higher energies, meaning that the hot electrons (even if not in a large number) can be involved into new collisional processes, which require a stronger activation energy [START_REF] Lieberman | Principles of Plasma Discharges and Materials Processing[END_REF]. In the case of the CH4 dissociation, all the electrons that contribute to breaking of the C-H bonds, must be located at the right side of BDE(C-H) and under the EEDF curves: this corresponds to the area A1 in the glow regime (He) and A3 in the filamentary regime (Ar) (see insert in Figure 8). As A1 is larger than A3, more electrons can participate to the dissociation of CH4 in the case of He, hence this explains why (CH4)He > (CH4)Ar.

On the other hand, a higher electron energy is needed for breaking the C=O bonds of CO2: all electrons that contribute to this bond breaking, must be located at the right side of BDE(C=O), and under the EEDF curves: this corresponds to area A2 in the glow regime (He) and to area A4 in the filamentary regime (Ar). As A4 is larger than A2, more electrons can participate to the dissociation of CO2 in Ar than in He, and this explains why (CO2)Ar > (CO2)He. Finally, if we consider the areas which correspond to the electrons that can contribute to the dissociation of C-H and C=O bonds for both the glow and filamentary regimes, it appears that A1 > A3 > A4 > A2.

Hence, this corresponds to (CH4)He > (CH4)Ar > (CO2)Ar > (CO2)He, which is indeed observed in Figure 7. Therefore, the CO2 conversion is the lowest in helium since A2 is the smallest among the four areas. In other terms, the number of electrons available in a He discharge for the conversion of CO2 is very small as the energy of these electrons has to be equal to or higher than the activation energy to break C=O (i.e. 5.52 eV). In summary, the nature of the carrier gas -and consequently the regime (glow or filamentary) of the DBD -directly impacts the shape of the EEDF and therefore the electron collision processes that may occur.

Figure 8. Schematic sketch of the EEDFs in the case of a glow discharge (He) and a filamentary discharge (Ar) at thermodynamic equilibrium. Also indicated are the bond dissociation energies (BDE) for C-H and C=O bonds. The insert shows the fractions of electrons that can contribute to dissociation of C-H and C=O bonds in both regimes (see text for more explanation).

IV.4. Conversion, specific energy input and energy efficiency: comparison with literature

The specific energy input (SEI) corresponds to the energy density (Ed) in J.cm -3 and can also be expressed in eV.molecule -1 as defined by equations ( 5) and (6). The energy efficiency of the CO2 conversion (CO2) has been calculated (in %) from the conversion

The same equation can be written for the energy efficiency of the CH4 conversion (CH4). Hence, the energy efficiency is separately defined for CO2 and CH4 in this article. Eq. [START_REF] Aresta | Carbon Dioxide as Chemical Feedstock[END_REF] indicates that an increase in the SEI systematically induces a decrease in , at least when the conversion stays constant, and this means that we should have a SEI value as low as possible to obtain a more energy efficient process. This is indeed clear from Figure 9, where the energy efficiencies of both CH4 and CO2 clearly drop upon higher SEI. For a SEI as low as 5.7 eV.molecule -1 , max(CO2) = 3.3 % while max(CH4) = 4.9 %. A comparison of our multiple electrode DBD reactor with other atmospheric plasma sources is presented in Table 3. This table reports various plasma sources: DBD, AC glow discharges, pulsed corona and gliding arcs with different geometries and specific operating parameters, namely: frequency, power given by authors, nature of the carrier gas or reactive gas, and CO2 flow rate. Note that some of these experiments apply to pure CO2 splitting, while others refer to dry reforming (i.e., conversion of both CO2 and CH4). However, we focus here only on the CO2 conversion. Also, it should be noted that some experiments were carried out for the pure greenhouse gases, while others made use of a carrier gas. The conversion and energy efficiency are in general higher in a carrier gas but it is obviously less interesting for applications. The optimal CO2 conversions for all these cases are plotted in Figure 10 as a function of the corresponding SEI while their energy efficiencies are plotted in Figure 11 as a function of the CO2 flow rates. In these figures, each squared number refers to one of the plasma sources listed in Table 3. A first remark is that no atmospheric plasma source can reach a CO2 higher than 25 %. Moreover, no general trend can be deduced: the cloud of points indicates that some plasma sources are very energy-consuming with high CO2 (#8, #14 and #15) while some others are much more dedicated to CO2 reforming at a lower energy cost (#1, #4, #7, #17, #18 and #19) since they are located close to the vertical line at 2.56 eV.molecule -1 , standing for the enthalpy of reaction (2). The plasma source #7 shows a good energy efficiency, but is not suitable as it can handle CO2 flow rates of only 0.8 mL.min -1 . The plasma sources #17 and #19 present interesting conversions for SEI as low as ours, but with the disadvantage of their geometry, which is a pulsed corona and a gliding arc, respectively. Indeed, the advantage of using a tubular DBD lies in the ability to place it at the nozzle exit of a combustion process to treat the entire gas flow since all the gas passes through the discharge zone. On the contrary, a pulsed corona and a gliding arc can exhibit ''dead volumes'' where the gas passes through the reactor without being treated in the plasma zone. Furthermore, the corona source is not adapted for high flow rates treatment as its discharge volume is not that important, which makes it a good candidate only to handle low flow rates. The plasma source #4 is an interesting alternative to our plasma process. In our case, the CO2 conversion is not so high but the SEI is quite low, so this yields a good energy efficiency, as shown in Figure 11 (#1). Our plasma source shows a good compromise between a high energy efficiency and the treatment of a significant CO2 flow rate, probably thanks to the multielectrode configuration.

V. Conclusion

The production of syngas (CO and H2), C2H4 and C2H6 has been achieved at atmospheric pressure in a dielectric barrier discharge operating in CO2 and CH4, with Ar or He as carrier gases. The main mechanisms responsible for the production of these compounds have been discussed. In this study, the effect of the concentration of CO2/CH4 in the mixture on the conversion has been demonstrated. Furthermore, the effect of power has also been reported, showing a linear increase in the CO2 and CH4 conversions but also in the production of syngas as a function of the supplied power. Finally, the energy efficiency of the CO2 conversion has been calculated and compared with those of other atmospheric plasma sources. Our DBD reactor offers very encouraging results as it offers one of the best compromises between a high energy efficiency and the treatment of a large CO2 flow rate.

Figure 1 .

 1 Figure 1. Schematic diagram of the DBD reactor.

Figure 3 .

 3 Figure 3. CO2 and CH4 conversions as a function of the CO2 and CH4 flow rates with Tot = 1920 mL.min -1 , Ar = 1800 mL.min -1 , CO2 = CH4 = 120 mL.min -1 , plasma power = 45 W, frequency = 19.5 kHz.

1 )Figure 4 .

 14 Figure 4. (a) Volumetric fractions of H2, CO, O2, C2H4 and C2H6 (using the TCD) and (b) C-based selectivity of the quantified gaseous products as a function of the CO2 and CH4 flow rates with Tot = 1920 mL.min -1 , Ar = 1800 mL.min -1 , CO2 = CH4 = 120 mL.min -1 , power = 45 W, f = 19.5 kHz.

Figure 5 . 6 Figure 6 .

 566 Figure 5. Conversions of CO2 and CH4 versus the power (Ar = 1680 mL.min -1 ; CO2 = CH4 = 120 mL.min -1 ; frequency = 19.5 kHz).

4 Figure 7 .

 47 Figure 7. (a) Conversions of CH4 and CO2 in Ar/CO2/CH4 and He/CO2/CH4 plasmas with Tot = 1920 mL.min -1 , Ar or He = 1800 mL.min -1 , CO2 =

Figure 9 .

 9 Figure 9. Energy efficiency as a function of specific energy input in our experimental set-up with Tot = 1920 mL.min -1 , Ar = 1800 mL.min -1 , CO2 = CH4 = 120 mL.min -1 , plasma power = 45 W, frequency = 19.5 kHz.

Figure 10 .Figure 11 .

 1011 Figure 10. CO2 conversions vs SEI for the various plasma sources listed in Table3.

Table 2 Reaction pathways for the formation and consumption of intermediate and value-added products of CH4 and CO2 conversion.

 2 𝐶𝐻 3 • + 𝐶𝐻 2 • → 𝐶 2 𝐻 4 + 𝐻 • 7.0110 -11 cm 3 .s -1[32] (R20) 𝐶 2 𝐻 5 • + 𝑂 • → 𝐶 2 𝐻 4 + 𝑂𝐻 • 4.4010 -11 cm 3 .s -1[31]

		Reaction	Rate constant	Ref.
	(R1) 𝐶𝐻 4 + 𝑒 → 𝐶𝐻 3 • + 𝐻 • + 𝑒 3* (cross section) [30]
	(R2)	𝐶𝐻 2 • + 𝑂 • → 𝐶𝑂 + 𝐻 2	5.5310 -11 cm 3 .s -1 [31]
	(R3) (R4) (R5)	𝐶𝐻 3 • → 𝐶𝐻 2 • + 𝐻 • 𝐶𝐻 3 + + 𝑒 → 𝐶𝐻 2 • + 𝐻 • 𝐶𝐻 3 + + 𝑒 → 𝐶𝐻 • + 𝐻 2	1.6910 -08 cm 3 .s -1 [32] 2.2510 -08 cm 3 .s -1 [30, 33] 7.8810 -09 cm 3 .s -1 [30, 33]
	(R6)	𝐶𝐻 3 • + 𝐻 • → 𝐶𝐻 2 • + 𝐻 2	1.0010 -10 cm 3 .s -1 [32]
	(R7)	𝐶𝐻 4 + 𝐻 • → 𝐶𝐻 3 • + 𝐻 2	5.8310 -13 cm 3 .s -1 [32]
	(R8)	𝐶𝐻 4 + + 𝐻 • → 𝐶𝐻 3 + + 𝐻 2	1.0010 -11 cm 3 .s -1 [34]
	(R9)	𝐻 • + 𝐻 • → 𝐻 2	1.4410 -14 cm 3 .s -1 [35]
	(R10) 𝐶𝑂 2 + 𝐻 • → 𝐶𝑂 + 𝑂𝐻 •	1.4010 -29 cm 3 .s -1 [36]
	(R11)	𝐶𝐻 • + 𝑂 • → 𝐶𝑂 + 𝐻 •	6.9010 -11 cm 3 .s -1 [32]
	(R12) 𝐶𝑂 2 + 𝐶𝐻 • → 2𝐶𝑂 + 𝐻 •	9.6810 -13 cm 3 .s -1 [31]
	(R13)	𝐶𝐻 3 • + 𝐶𝐻 3 • → 𝐶 2 𝐻 6	4.2010 -11 cm 3 .s -1 [37]
	(R14)	𝐶 2 𝐻 5 • + 𝐻 • → 𝐶 2 𝐻 6	2.2510 -10 cm 3 .s -1 [38]
	(R15) 𝐶 2 𝐻 5 • + 𝐶𝐻 4 → 𝐶 2 𝐻 6 + 𝐶𝐻 3 • 1.8310 -24 cm 3 .s -1 [36]
	(R16) 𝐶 2 𝐻 6 + 𝑒 → 𝐶 2 𝐻 5 • + 𝐻 • + 𝑒	18*	[39]
	(R17) 𝐶 2 𝐻 5 • + 𝑒 → 𝐶 2 𝐻 4 + 𝐻 • + 𝑒	19*	[39]
	(R18) 𝐶𝐻 4 + 𝐶𝐻 • → 𝐶 2 𝐻 4 + 𝐻 •	9.7410 -11 cm 3 .s -1 [32]
	(R19)

  CO2, the enthalpy of (2) namely H 0 298K = 247.3 kJ.mol -1 = 2.56 eV.molecule -1 and the SEI value, according to equation[START_REF] Aresta | Carbon Dioxide as Chemical Feedstock[END_REF].

	𝐸 𝑑[𝐽.𝑐𝑚 -3 ] =	𝑃𝑜𝑤𝑒𝑟 [𝐽.𝑠 -1 ] 𝐺𝑎𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 [𝑐𝑚 3 .𝑠 -1 ]	(5)
	𝑆𝐸𝐼 𝑒𝑉.𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 =	𝐸 𝑑[𝐽.𝑐𝑚 -3 ] × 6.24 × 10 [𝑒𝑉.𝐽 -1 ] 18 6.022 × 10 [𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒.𝑚𝑜𝑙 -1 ] × 24500 [𝑐𝑚 3 .𝑚𝑜𝑙 -1 ] 23	(6)
	𝜂 𝐶𝑂 2 (%) =	𝑜 𝜒 𝐶𝑂2 (%) × Δ𝐻 298𝐾 [𝑒𝑉.𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 ] 𝑆𝐸𝐼 [𝑒𝑉.𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒 -1 ]
	Journal of CO2 utilization, 2015, Vol. 9, 74-81, http://dx.doi.org/10.1016/j.jcou.2015.01.002

Table 3 Comparison of various plasma sources dedicated to the conversion of CO2 at atmospheric pressure. The CO2 conversions vs SEI and the energy efficiency as a function of CO2 flow rate for all these cases are reported in Figures 10 and 11, respectively.

 3

	#	Plasma source Type Geometry (kHz) (W) Freq. Pow. Carrier or reactive gas	CO2 (mL/min)	Ref.
	1	DBD	Tube multi-electrodes	19.5	45	Ar	120	Our reactor
	2	DBD	Tube	30	100	/	50	16
	3	DBD	Plane	30	500	CH4	500	47
	4	DBD	Tube	2.2	45	Ar	80	27
	5	DBD	Plane	25	15	He/CH4	5	26
	6	DBD	Tube	25	100	CH4	60	51
	7	DBD	Tube	8.1 0.11	/	0.8	44
	8	DBD	Tube	20	74	CH4	15	52
	9			8.1 2.78	He	2.5
	10			8.1 3.64	Ar	2.5
	11			8.1 5.25	N2	2.5
	12 13	AC glow discharge	Tube	8.1 2.78 8.1 3.64	He Ar	1.5 1.5	53
	14			8.1 5.25	N2	1.5
	15			8.1 3.64	Ar	0.75
	16			8.1 5.25	N2	0.75
	17	Pulsed corona	Electrode tip	20-200 9	/	47.5	54
	18	DBD	Tube	30	60	Ar/CH4	60	55
	19	Gliding arc	"V" shaped electrode	20	225	/	2000	20

Journal of CO2utilization, 2015, Vol. 9, 74-81, http://dx.doi.org/10.1016/j.jcou.2015.01.002

VI. Acknowledgements

The authors acknowledge financial support from the IAP/7 (Interuniversity Attraction Pole) program 'PSI-Physical Chemistry of Plasma-Surface Interactions', financially supported by the Belgian Federal Office for Science Policy (BELSPO).

VII. References


Julie Hubert

Nicolas Vandencasteele

Jérémy Mertens

Pascal Viville

Thierry Dufour

Cédric Barroo

Thierry Visart De Bocarmé

Roberto Lazzaroni

François Reniers

Chemical and Physical Effects of the Carrier Gas on the Atmospheric Pressure PECVD of Fluorinated Precursors

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

I. Introduction

Low-pressure plasma deposition of fluorocarbon films has been widely studied in the last decades [1][START_REF] Agostino | Plasma Deposition[END_REF][START_REF] Henry | [END_REF][4][5][6] . The main goal of those works was to evaluate the capability of tuning the chemistry of the coatings by controlling parameters such as the plasma source or the feed gases (monomers and additives). For instance, gaseous precursors such as C2F6, C3F8 and C4F8 were studied in continuous wave, pulsed, and downstream radio-frequency plasmas [7] . The structure of the films deposited by continuous/wave (CW) plasma is highly cross-linked while pulsed plasma with long pulse-off times and afterglow plasma leads to the prevalence of the CF2 component (i.e., less cross-linking). Favia et al. [5] . obtained spectacular ribbon surface structures characterized by a strong CF2 component, a small degree of crystallinity, and large water contact angles (WCA) higher than 150° when TFE was deposited in modulated RF glow discharges at a low duty cycle. Hydrogen addition to saturated fluorocarbon monomers allows the deposition of coatings with variable F/C ratios and crosslinking degrees [6] .

The precursors used at atmospheric pressure are usually identical to those used in low-pressure processes (e.g., CF4, C2F4, C2F6, C3F8, C3HF7, or c-C4F8), but the use of a carrier gas (usually helium or argon) is most often required when working at atmospheric pressure. 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. One of the first investigations on the deposition of fluorocarbons by atmospheric plasmas was reported by Yokoyama et al. [8] , working with a mixture of C2F4 and helium. The same mixture was used to modify the surface of poly(vinyl chloride) (PVC) and showed WCA of 100-110° but the modified surface was smoother that the pristine surface [9] . Fanelli et al. [10] demonstrated that atmospheric pressure dielectric barrier discharge (DBD) could also be useful in tailoring the coating properties while controlling the frequency or the addition of hydrogen. 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 increase in the deposition rate (from 1 to 12 nm.min -1 ). Vinogradov et al. [11] reported the influence of additive gases such as H2 and O2 in a fluorocarbon-containing filamentary DBD. The presence of oxygen is known to shift the process from deposition to etching and induces then a decrease in the deposition rate [START_REF] Agostino | Plasma Deposition[END_REF]12] .

It is worth noticing that most of the precursors utilized in the plasma-enhanced chemical vapor deposition (PECVD) 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 [13- 15] . Moreover, while some studies report the use of perfluorohexane, most of those do not report the use of pure CxFy compounds and the highest WCAs obtained are 115°[ [START_REF] Paulussen | Chemical and physical properties of hydrophobic coatings obtained in a dielectric barrier discharge[END_REF][START_REF] Hochart | [END_REF][18] . In the present study, the atmospheric pressure PECVD deposition and texturization of hydrophobic coatings using vapors from fluorinated liquid precursors at room temperature are investigated. We have studied the main effects of the nature of the carrier gas (argon and helium) through the characterizations of the plasma phase and the surface of the deposited layers. The filamentary/homogeneous character of the discharge is analyzed by a high-speed camera and electrical measurements. The influence of the carrier gas on the fragmentation of the precursor studied by mass spectrometry is discussed, along with the importance of the presence of impurities. The chemical composition, wettability, and morphology of the coatings are determined from X-ray photoelectron spectroscopy (XPS), WCA, and atomic force microscopy (AFM) measurements, respectively. Although a few studies have reported an influence of the carrier gas on the morphology of plasma-polymerized coatings, this work is, as far as we know, the first one aimed at studying such influence in the case of hydrophobic fluorinated films [19,20] . The effect of the carrier gas is indeed often neglected while it can be a key parameter for inducing an enhancement of the hydrophobic

II. Experimental section

II.1. Materials

The liquid precursorsnamelyperfluoro-2-methyl-2-pentene (C6F12) and perfluorohexane (C6F14) were provided by Fluorochem and were used without any further purification. Perfluorohexane is a mixture of two isomers, represented in Figure 1. Silicon Wafers (100) from Compart Technology Ltd. were used as substrates after being cleaned with methanol and isooctane. The glow discharge was sustained with argon (Air Liquide, ALPHAGAZ 1) or helium (Air Liquide, ALPHAGAZ 1).

II.2. Plasma polymerization

The plasma-polymerized (pp)-fluorinated films were deposited with a home-built dielectric barrier discharge (DBD) described in a previous study [21] . Both electrodes used were copper disks of 45 mm diameter. The top electrode was covered with a 1.5 mm thick Pyrex glass acting as a dielectric. The inter electrode gap was fixed at 3 mm to avoid arcing at atmospheric pressure. To improve the film homogeneity, the grounded electrode was connected to a rotary system powered by a DC generator, thus allowing the rotation of the substrate at 60-80 rpm.

In order to prevent contamination, the chamber was first pumped down to a pressure of 2 Torr. The atmospheric pressure was then reached by injecting the carrier gas (argon or helium). At that stage, the vapors of the precursor were introduced into the discharge by means of the carrier gas (second flow -0.2 L.min -1 ) 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 -1 . The precursor feed was estimated by measuring the mass variation of the bubbler for several flow rates. The injected amounts for 0.2 L.min -1 are as follows:

-Ar-C6F12: 782  44 mg.min -1 , -He-C6F12: 783  52 mg.min -1 , -Ar-C6F14: 768  84 mg.min -1 -He-C6F14: 767  101 mg.min -1 .

The operating frequency was set at 17.1 kHz with an output power set at 50 W for the presented results, supplied by an AFSG10S (AFS entwicklungs -und vertriebs gmbh, Horgau, Germany) power generator. The deposition time varied from 30 to 360 s but most of the results presented correspond to films with a deposition time of 180 s for argon and 360 s for helium.

II.3. Electrical measurements

The charge-voltage Lissajous method was used to determine the energy absorbed. A 10 nF capacitor (Cmeas) was connected in series with the bottom electrode of the DBD device and the ground [22] . The voltages at the top electrode and across the capacitor were measured using two Tektronix voltage probes (P6015A and P6139B resp.) connected to a digital oscilloscope (Tektronix DPO 3032). The actual plasma power can be calculated by measuring the charge accumulation Q at the capacitor Cmeas:

𝑄 = 𝐶 𝑚𝑒𝑎𝑠 × 𝑈 𝑐
with UC the voltage across the capacitor. Plotting Q as a function of the voltage U at the top electrode gives a so-called Lissajous figure. The area enclosed by this diagram is the electrical energy consumed by the plasma per voltage cycle, which, multiplied by the operating frequency gives the actual plasma power [22] . The estimation of the absorbed energy is reported in

II.4. High speed camera

The behavior of the DBD was followed via the brightness signal extracted from videos recorded with a Photron FastCam SA4 camera with an acquisition speed of 10 000 -100 000 frames per second (fps). The profile lines are established by probing the brightness along a selected line of the video. The histograms are established by probing the brightness distribution over regions of interest by the Photron FastCam Viewer software. The level of brightness is expressed as a grayscale corresponding, in our case, to 256 levels of brightness.

Ar-plasma videos were acquired at 100 000 fps with a spatial definition of 192  128 pixels. Since the brightness of the helium plasma is much less intense, the videos were acquired with an acquisition speed of 10 000 fps to collect enough light for the shutter to acquire the image. It has to be noted that experiments for argon at 10 000 fps also present the same phenomenology (streamers visible), but with a higher brightness. All other parameters of acquisition were kept constant.

II.5. Mass spectrometry (MS)

Mass spectrometry of the gas phase was performed with a Hiden analytical atmospheric gas analysis-QGA. The gases were collected through a PFA capillary located between the electrodes at the boundary of the plasma region. The secondary electron multiplier detector (SEM) was used to detect fragments with low partial pressures (10 -6 -10 -13 Torr). In order 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.

II.6. Optical emission spectroscopy (OES)

Optical emission spectroscopy was performed with the SpectraPro-2500i spectrometer from ACTON research Corporation (0.500m focal length, triple grating imaging). The light emitted by the discharge was collected by an optical fiber and transmitted to the entrance slit (50mm) of the monochromator. Each optical emission spectrum was acquired with the 1800 grooves.mm -1 grating (blazed at 500 nm) and recorded over 10 accumulations with an exposure time of 50 ms. 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).

II.7. Water contact angles (WCA)

A drop shape analyzer (Kruss DSA100) was used to measure static and dynamic water contact angles (WCA) 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 mL and back to 0 mL at a rate of 30 mL.min -1 . Advancing contact angles are the maximum angles observed during the droplet growth. The receding contact angles are measured by removing water from the droplet and are considered as the contact angles just before the contact surface reduction and distortion of the droplet [23] . All conditions lead to hydrophobic coatings characterized by an advancing WCA higher than 115°. The receding contact angles have been estimated from the dynamic measurements and the hysteresis seems to be constant whatever the advancing contact angle. A high hysteresis of about 20-30° would be indicative of a sticky surface characteristic of the Wenzel-type wetting mechanism and not the slippery Cassie-Baxter model [24,25] . For the sake of clarity, only the advancing WCA will be further discussed.

II.8. X-ray photoelectron spectroscopy

XPS analysis was performed on a Physical Electronics PHI-5600 photoelectron spectrometer. Survey scans 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 and F 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 high-resolution spectra of the C 1s peaks (and F 1s) 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 coming from the manufacturer's handbook: SC = 0.205, SF = 1, and SO = 0.63. The FWHM of the C 1s peak was around 2 with a constraint of 1.9-2.1 for the peak fitting.

II.9. Atomic force microscopy (AFM)

Atomic force microscopy (AFM) was used to analyze the surface morphology of the deposited films. AFM images were recorded in air with a Nanoscope IIIa (Veeco, Santa Barbara, CA) microscope operating in tapping mode. The probes were commercially available silicon tips with a spring constant of 24-52 N.m -1 , 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.10. Profilometry

The thickness measurements were performed with a stylus profiler Brücker dektak XT. The stylus with a 2mm radius scans the surface with a force of 1mg (0.01 mN) and the measurement was controlled and analyzed with the Vision 64 (Brüker, Billerica, MA) software. The thickness was estimated based on the height difference between the substrate surface and the surface of the coating. The height difference was obtained by placing an adhesive tape on the extremity of the substrate before the plasma deposition to maintain a non-coated area and by scratching the coating surface after deposition. The thickness value was evaluated by taking the average of each profile and used to estimate the deposition rates.

III. Results and Discussion

III.1. Characterization of the filamentary and glow-like discharges

Depending on the conditions (gas, voltage, etc.), an atmospheric pressure dielectric barrier discharge can operate in a filamentary or glow mode [26] . In most cases, DBDs are non-uniform and consist of numerous microdischarges, with a lifetime of 10-100 ns and an electron density of 10 14 -10 15 cm -3 [27][START_REF] Fridman | Plasma Chemistry[END_REF][START_REF] Eliasson | [END_REF][30] . These micro-discharges (or streamers) are randomly distributed in the gas gap due to the dielectric material covering the electrodes. Glow (homogeneous) discharges are characterized by a uniform distribution of species constituting the plasma and electron densities of 10 10 -10 11 cm - 3 [31,32] . Depending on the conditions, homogeneous and filamentary discharges can be obtained for a given carrier gas [33] . However, it is much more complex to form a homogeneous discharge with argon than helium as illustrated in Figure 2 for C6F12 polymerized in the DBD source. The presence of micro-discharges inducing the filamentary character of the argon discharge compared to the more homogeneous helium discharge is clearly observed in Figure 2. The brightness profile has been plotted along a line represented by the purple segment on the pictures. Along these lines, the streamers are made visible by peaks of higher brightness. In the case of Ar-C6F12, several peaks can be observed which are the steamers gathered on a single picture extracted from longer times of acquisition. This picture is representative of the behavior observed in several videos. However, the number of streamers remains approximately constant over the time of the whole video. In the case of He-C6F12, the one dimensional brightness line extracted from the picture is rather different: the brightness remains constant and stable without any peak, highlighting the homogeneity of the discharge. As in the previous case, the profile line is similar for longer acquisition times. The histograms of Figure 2 represent the brightness distribution over a larger region of interest which comprises the entire region where the plasma is developed. In the He-C6F12 plasma, all the events in the discharge are similar, which leads to a rather homogeneous distribution of the brightness centered at a low level of brightness (<50 luma).

Conversely, the histogram of brightness of the Ar-C6F12 plasma shows a broader distribution, which indicates the occurrence of events such as the presence of streamers involving events with higher brightness. It has to be noted that the mean intensity of brightness is much higher in the case of Ar-plasma than in the case of He-plasma.

As a supplement to the visual characterization of the discharge, the current curves presented in Figure 3 evidence the highly filamentary character of the argon discharge compared to helium. Moreover, a blue color discharge is observed only for argon plasmas. Although rarely discussed in the literature, this color seems to be characteristic of CF2 emission and would be the signature of a higher fragmentation, higher CF2 concentration in the films, and higher deposition rate recorded for argon plasmas [34] . It has to be noted that these observations are similar for the C6F14 precursor.

III.2. Characterization of the plasma phase

To evaluate the fragmentation in argon and helium plasmas, mass spectra as a function of the m/z ratio have been plotted to identify the interesting fragments and then, their intensity has been studied as a function of time. The mass spectrum of the C6F14 precursor is presented in Figure 4a, and is similar to the one referenced by the National Institute of Standards and Technology [35] . Both measured and reference spectra have been considered to define the most probable fragments released from the fragmentation in the mass spectrometer. The considered spectrum was measured in an argon atmosphere but the fragments are identical in the case of helium and differ only in intensity. For the sake of clarity and to focus on CxFy fragments, we only present m/z higher than 50. Considering the structure of the precursors in Figure 4b, the fragments released from the mass spectrometer are assumed to be direct products of the main fragmentation of the precursors. For the perfluorohexane, the C3F7 + fragment arises from the symmetric rupture of the n-C6F14. The formation of CF3 + occurs by fragmentation of the end of both configurations, so does the formation of C2F5 + . The presence of C3F5 + fragments can be explained, for example, by the release of the CF3 + groups in the isomer structure of the molecule.

To observe the direct effect of the plasma on the fragmentation, we monitored all the selected m/z ratios as a function of time and laid out the most relevant variations. The period before 800 s corresponds to the gas/precursor flowing in the reactor before plasma ignition, and the intensity of the corresponding monitored fragment is referred to as the baseline. The plasma was then turned on and the behavior of the partial pressure of the fragments was followed for about 5 min, before switching off the plasma again. At that point, the precursor was still injected in the reactor until the intensity returned to the baseline.

First, it should be mentioned that mass spectrometry is not a direct method to describe chemistry kinetics in the discharge because species produced in the plasma must travel throughout all the spectrometer capillary and the ionization chamber of the mass spectrometer induces a second fragmentation. However, interesting information can still be extracted. Indeed, in the argon plasma, the intensity of three of the main fragments (CF3 + , C3F5 + , and C3F7 + ) coming from the fragmentation of C6F14 in the mass spectrometer decreases, while the intensity of many other (small) fragments such as CF, CF2, C2F4, or C3F3 (not presented here) increases. In helium, the effect of the plasma is less pronounced, but all the intensities of the detected fragments are increased when the discharge is turned on, as shown by the dashed curves in Figure 5. We assume that these observations can be interpreted as a sign of higher fragmentation in argon. Despite the complexity of the reactions in the discharge, the differences between the two systems ("mass spectrometry" vs. "plasma + mass spectrometry") can be briefly summarized in the next scheme (Figure 6), where only the main detected fragments are discussed. 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 and would be even more fragmented by the mass spectrometer, explaining their decreasing intensity. We suggest that the reason why the C2F5 + fragment is not reduced is because it is a possible product of the fragmentation of C3F7 + . Indeed, C3F7 + can lead to CF3 + , C2F4 + , CF2 + , C2F5 + ; possible products from C3F5 + are CF + , C2F4 + , CF2 + , C2F3 + , and CF3 + can provide CF2 + or CF + with a release of fluorine atoms which would mostly be detected as negative charged species. Unfortunately, negative spectra cannot be obtained from our mass spectrometer.

According to Figure 5, the consumption of the most intense fragments did not occur in helium plasma. The interaction between the molecules of the precursor and the active species of the plasma is supposed to be quite distinct compared to argon plasma. As previously mentioned, the electrons and microdischarges densities in helium are much lower and could then explain the lower fragmentation.

Moreover, helium is known to possess excited states that are very efficient to excite all the impurities present between the electrodes [32] . The carrier gas can indeed be excited into electronic excited metastable states which induce Penning ionization of other species if the energetic level of the metastable is higher than the ionization potential of that species [START_REF] Ricard | Plasmas Reactifs. Societe FranScaise du Vide[END_REF] . In the case of helium, the metastable atoms are energetic enough (1s 2 s 3 S1 and 1s 2 s 1 S0 states with energies of 19.82 and 20.62 eV, respectively) to ionize most atoms and molecules, explaining this efficiency to excite impurities. This is in contrast with the argon metastable atoms which have energies (11.55 and 11.72 eV for 3p 5 4s 3 P2 and 3p 5 4s 3 P0 states, respectively) lower than the ionization potential of O2 (12.07 eV) or N2 (15.6 eV) [32,[START_REF] Ricard | Plasmas Reactifs. Societe FranScaise du Vide[END_REF] .

The efficiency of generating atomic oxygen in helium plasmas is illustrated in Figure 7 which represents the emission intensity of the line at 777 nm. Indeed, adding O2 to helium plasmas induces a strong increase in the atomic oxygen emission while small variations are observed in the argon discharges. The involvement of oxygen might be crucial for the plasma polymerization, and oxygen-containing species are an important part of the products detected in helium plasmas. The helium excited states seem indeed to activate oxygen impurities much more easily than argon since products such as CO2 + , COF + , or COF2 + were strongly detected by mass spectrometry (see Figure 8).

Figure 8. Intensity of the CO2 + , COF + , and COF2 + fragments generated during the APPECVD of C6F14 as a function of time, in helium (dashed curves) and argon (full curves) (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

Plasma Processes & Polymers, 2015, Vol. 12, Issue 10, 1174-1185, http://dx.doi.org/10.1002/ppap.201500025

We mostly discuss the case of C6F14 as, according to the literature, all its main fragments have their m/z ratios lower than 200 and can thus be detected [35] . The case of C6F12 is more complicated to interpret because all its main fragments are not observable with our mass spectrometer (C5F9 + and C6F11 + at m/z of 231 and 281, respectively) and because of the presence of high reactive C=C double bonds. The intensity of C4F7 + at 181 is decreasing while the intensity of small fragments is increasing. This could be explained either by the fragmentation of the precursor or by the oligomer formation in the gas phase as suggested by Nisol et al. [START_REF] Nisol | [END_REF] and the higher deposition rate detailed in the next section. However, a better interpretation of the C6F12 behavior requires information of higher m/z ratios in order to analyze a possible polymerization.

III.3. Surface properties of the deposited coatings III.3.1. Deposition rates

The comparison of the deposition rates shows that both the precursors and the carrier gases induce a different reactivity, as presented in Table 2. Much faster deposition is obtained with the C6F12 precursor, which is assumed to be related to the higher reactivity of the C=C double bond present in the molecule. This behavior was also highlighted in the case of hydrocarbon or acrylate molecules, among others [38][39][40] . Moreover, Yasuda et al. [39] stated that the plasma polymerization of unsaturated compounds proceeds by the opening of double or triple bonds. If this interpretation is correct, most of the energy would then be consumed to break the double bond for polymerization. A strong drop in the deposition rate is also observed when helium is used instead of argon, although the same amount of the precursor is injected into the discharge. This can be related to the observation by Jiang et al. [41] that a higher amount of activated precursor is produced along the streamers. The streamers hitting the surface cause a local heating and an activation of the surface which induces a higher growth rate at these locations [41] . The streamers being randomly distributed, they move and cover the entire substrate surface.

III.3.2. Chemical composition of the coatings

The chemical composition of the fluorinated coatings has been characterized by XPS. According to the literature, CxFy films deposited by plasma are usually characterized by a 5-component curve fitting of the XPS C 1s high-resolution peak [10,11,42,43] . The fitting of the C 1s peak of pp-C6F12 films was therefore achieved with CF3 (294.2  0.2 eV), CF2 (292.2  0.2 eV), CF (289.9  0.2 eV), CCF (287.8  0.2 eV) and CC (284.9  0.2 eV) components; with the CCF being a C-C bond shifted to higher binding energy because of the electronegative environment. The curves are represented in Figure 9. The F/C ratio of the C6F12 and C6F14 precursors is 2 and 2.3, respectively. In the plasma-polymerized films, the ratio decreases down to 1.17 and 1.32, respectively, in argon and down to 1.11 and 1.37, respectively, in helium. This partial defluorination of the precursor explains the presence of branched components such as CCF and CC, as evidenced in the C 1s high-resolution peak. Despite this observation, the much more intense percentage of CF2 in the pp-C6F14 (e.g., CF2 is 10 % higher in the pp-C6F14/Ar than in the pp-C6F12/Ar film as seen in Table 3) seems to indicate that the signature of the precursor is partly preserved in the coating. The detection of the silicon substrate for the He-C6F14 coatings indicates that the deposited layer is very thin and confirms the very low deposition rate previously reported. We will not detail this spectrum further as the contribution from the substrate might induce errors in the interpretation.

Additionally to the precursor, the choice of the carrier gas influences the surface chemical composition of the coating. The composition of the films polymerized in helium is indeed rather different from those polymerized in argon (Figure 9). In a helium plasma, the pp-C6F12 coatings have higher concentrations of CF3 and CCF groups, for identical plasma conditions. Given the structure of the precursor (Figure 1), as well as the lower electron density, the lower electron energy and the lower micro-discharges density of the more homogeneous (glow-like) helium discharge, we could therefore assume that the precursor structure is more preserved when using helium. Moreover, it has to be noted that a small incorporation of oxygen (< 3 %) was observed only with helium. Helium has been shown to be reactive toward oxygen impurities [31] . This property is important as it can explain why small amounts of oxygen are present in the film polymerized in helium, whatever the experimental conditions and why energetic helium species are less involved in the fragmentation/polymerization of the monomer (although the fragmentation is assumed to be performed mainly by electron impact). These species can indeed be consumed by the impurities.

III.3.3. Hydrophobicity and morphology of the coatings

According to Figure 10, both the carrier gas and the precursor have a significant impact on the morphology of the coating, thus inducing variations in the hydrophobicity. In helium plasmas, the water contact angle and the roughness are below 120° and 2 nm, respectively, which indicates the presence of a smooth, lowenergy surface. This is not surprising due to the more homogeneous discharge and stable WCAs recorded as a function of the treatment time in the case of tetrafluoromethane deposition in helium at atmospheric pressure [8] .

The argon-deposited coatings are very different since the roughness required to increase the hydrophobicity can be obtained by the filamentary character of those discharges. Advancing water contact angles as high as 140° are then reached.

In contrast with helium plasmas, the precursor plays a role in the morphology of the coatings. For a similar thickness of approximately 300 nm, Ar-C6F14 discharges induce not only a higher roughness and hydrophobicity but also a roughness structure totally different from the C6F12 precursor as shown in Figure 11. The alveolar structures of approximately 100 nm are quite regular and could be characteristic of the filamentary discharge inducing an oriented 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.

IV. Conclusion

The plasma polymerization deposition of fluorinated coatings was investigated as a function of the carrier gas (argon and helium), in the case of the precursors perfluoro-2-methyl-2-pentene (C6F12) and perfluorohexane (C6F14). We showed that helium and argon plasmas react differently toward the fluorocarbon precursors. The nature of the carrier gas induces a drastic effect on the chemical composition, morphology, and therefore hydrophobicity of the films and has been, as far as we know, never discussed in terms of hydrophobic fluorocarbons. These differences are evaluated and are assumed to come from the generation of both glow-like discharges (in helium) versus filamentary discharges (higher electron energies and densities) in argon. We showed that hydrophobic surfaces with water contact angles (WCA) higher than 115° were obtained only in the presence of argon. This is because of the filamentary argon discharge induces higher deposition rates and rougher coatings than the more homogeneous helium discharge. The formation of activated sites on the growing film and the activated monomeric species is much more important for the argon plasma. Moreover, the easier activation of impurities by helium metastable states seems to lead to the presence of a small percentage of oxygen in the coating, a lower deposition rate and a weaker fragmentation/polymerization of the monomer.

In addition to the surface characterization, mass spectrometry and optical emission spectroscopy measurements were performed in order to describe the gas phase. Both techniques allowed identifying the presence of oxygen and its involvement in the reactions taking place in helium plasmas. The analysis of fragments issued from the mass spectrometry while the plasma is ignited seems to confirm the higher fragmentation in argon. However, the limitation in the m/z ratio of the apparatus prevents us from analyzing fragments resulting from a possible polymerization in the gas phase. Supplementary information from IR in the gas phase would then be very useful, since by-products such as COF2 or COF could be detected in addition to products originating from the polymerization.

Figure 1 .

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

Figure 2 .

 2 Figure 2. Behavior of the discharge as a function of the carrier gas (a) Ar-C6F12 (b) He-C6F12 (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas), including pictures of the discharges, brightness profiles versus distance, and histograms reporting number of occurrences versus brightness (obtained from a high-speed camera-acquisition time: Ar:1/100 000 s, He: 1/10 000 s).

Figure 3 .

 3 Figure 3. Voltage and current curves of (a) Ar-C6F12 and (b) He-C6F12 plasma (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

Figure 4 .

 4 Figure 4. (a) Mass spectrum of Ar-C6F14 (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas), (b) List and structures of the most probable fragments according to the mass spectrum and the literature.

Figure 5 .Figure 6 .

 56 Figure 5. Intensity of the CF3 + , C2F5 + , C3F5 + , and C3F7 + fragments generated during the APPECVD of C6F14 as a function of time, in helium (dashed curves) and argon ( full curves) (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

Figure 7 .

 7 Figure 7. Emission intensity of atomic oxygen at 777 nm as a function of the carrier gas and the addition of molecular oxygen (50 W, 0.2 L.min -1monomer/carrier gas, 5 L.min -1 total carrier gas, 0.1 L.min -1 O2 L.min -1 ).

Figure 9 .

 9 Figure 9. XPS spectra of (a) Ar-C6F12, (b) Ar-C6F14, (c) He-C6F12, and (d) He-C6F14 plasma-polymerized films (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

Figure 10 .

 10 Figure 10. Advancing WCA and Roughness RMS of the plasma polymerized films. Illustration of the drop behavior of (left) Ar-C6F14 and (right) He-C6F12 coatings (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min - 1 total carrier gas).

Figure 11 . 5 

 115 Figure 11. 5  5 µm 2 AFM images of (a) Ar-C6F12, (b) Ar-C6F14, (c) He-C6F12, and (d) He-C6F14 plasma-polymerized films (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas). The vertical color scale corresponds to 300nm for images a and b, and to 30 nm for images c and d.

Table 1

 1

	Plasma condition	Absorbed energy (mJ)
	Ar-C6F12	2.21  0.05
	He-C6F12	2.36  0.08
	Ar-C6F14	2.31  0.06
	+Oxygen	2.37  0.05
	He-C6F14	2.42  0.09
	+Oxygen	2.49  0.08

for each condition discussed in this article. The voltage and the current curves were measured by a high voltage probe (Tektronix P6015A) and a Rogowski coil (Pearson current monitor model 2877) both connected to a digital oscilloscope (Tektronix DPO 3032).

Table 1 . Estimation of the absorbed energy according to the Lissajous method (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas, O2 0.1 L.min -1 ).

 1

Table 2 . Average deposition rates of C6F12 and C6F14 in argon and helium (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

 2

	Deposition rate (nm.min -1 )	C6F12	C6F14
	Argon	240  20	50  5
	Helium	25  3	< 5

Table 3 . Surface composition of pp-CxFy films as a function of the precursor and the carrier gas (50 W, 0.2 L.min -1 monomer/carrier gas, 5 L.min -1 total carrier gas).

 3

V. Acknowledgements

This work was part of the IAP (Interuniversity Attraction Pole) programs ''PSI-Physical Chemistry of Plasma Surface Interactions'' and ''Functional Supramolecular Systems'' 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)''. CB thanks the Fonds de la Recherche Scientifique (F.R.S.-FNRS) for financial support (PhD grant), TVdB and CB also thank the Wallonia-Brussels Federation (Action de Recherches Concertees n8AUWB 2010-2015/ ULB15) for the acquisition of equipment. Research in Mons is also supported by the Region Wallonne/European Commission FEDER program (SMARTFILM project) and FNRS-FRFC. The authors want to thank Dr. Jennifer Christophe for the colored pictures of the discharges.

VI. References