M Tulodziecki

J.-M Tarascon

P L Taberna

C Guéry

Electrodeposition Growth of Oriented ZnO Deposits in Ionic Liquid Media

ZnO is a material of great interest for microelectronics because it offers a panel of attractive physical properties. It has been grown for decades as thin films from a wide variety of physical-chemical deposition techniques, most of them leading to films oriented with the polar direction perpendicular to the substrate. Here we report the electrodeposition of ZnO in ionic liquids (EMImTFSI (1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide), BMPTFSI (1-Butyl-1-Methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) or BMImTf (1-butyl-3-methylimidazolium trifluoromethanesulfonate)), together with the feasibility of manipulating the growth orientation of the ZnO deposits by adding the proper co-solvent to ionic liquid. We establish solvent dielectric constant as a key parameter to tailor the deposit growth direction. Moreover, via the use of Electrochemical Quartz Crystal Microbalance (EQCM) measurements, we determine the importance of the competition between Zn 2+ and O 2 reduction to properly trigger the transport phenomena leading to the growth of ZnO. This work provides insight which can be implemented to the growth of other binary and ternary oxides deposits in ionic liquid media, a field still poorly explored.

Room temperature Ionic Liquids (RTIL) possess many attractive chemical/physical properties. They are non-flammable, display very high thermal stability together with negligible vapor pressure and are good solvents for numerous salts and polymers. 1,2 Such properties are at the origin of their wide use in many research fields with an ongoing increasing interest in the synthesis of inorganic materials; such a synthesis involving either ionothermal 3 or electrodeposition. [START_REF] Endres | Electrodeposition from Ionic Liquids[END_REF] Moreover by changing the nature of the cations and anions, ILs can be designed for specific applications. [START_REF] Earle | [END_REF] Owing to their relatively high ion conductivity, (0.2 -1 • cm -1 ) together with their high thermal and electrochemical stability (electrochemical window up to 4-5 V are reported), they stand as an alternative promising medium to water in the field of electrochemistry. [START_REF] Endres | Electrodeposition from Ionic Liquids[END_REF] Such attractive characteristics have so far been widely exploited for the electrodeposition of metals and semiconductors [START_REF] Endres | Electrodeposition from Ionic Liquids[END_REF][6][7][8][9] and very recently for binary compounds like CuS. 10 Additionally, control over the morphology and structure of the deposit can be achieved via tuning of the deposition conditions (temperature, wide potential window, changing cations and anions of IL, additives, mixtures of solvents, etc. [START_REF] Endres | Electrodeposition from Ionic Liquids[END_REF][6][7][8][9][10] ). Last, the specific interactions of IL with the electrode surface upon applied potential (which have been recently studied by a few groups) and the deposit itself may provide additional new possibilities to adjust/orient nucleation-growth mechanisms. [11][12][13][14][15] Reduction of oxygen in Ionic Liquids is being extensively studied by many groups. [16][17][18] A common belief is that oxygen undergoes a 2 steps reduction enlisting sequentially the formation of superoxide and later on peroxide anions, both of them are stable at room temperature in ionic liquid media. Such a medium was recently exploited to electrochemically synthesize metal oxides like NiO, 19 ZnO 20 or CeO 2 . 21 Zinc oxide is a non toxic direct wide-bandgap (3.3 eV) transparent semiconductor material widely used in the fields of chemical sensors, piezoelectric transducers, light emitting devices, photosensors and photovoltaic cells. 22 ZnO thin films can be deposited by several techniques such as sputtering, 23 pulsed laser deposition, 24,25 spray pyrolysis, 26 chemical bath deposition, 27 and electrodeposition, [28][29][30][31] the latter being the simplest. A variety of morphologies has been obtained depending on the used technique and conditions. 32 Physical synthesis techniques 24 as well as deposition from polar solvents like H 2 O 28,30 and DMSO 31 mostly lead to ZnO material oriented along the polar direction z [001] or lack of any orientation. Recently, 2 groups have shown that the preferential orientation along non-polar directions <hk0> can be obtained by either electrodeposition in RTIL 20 z E-mail: claude.guery@u-picardie.fr or by pulsed laser deposition; 25 also partial tilt from z direction was observed by electrodeposition from a water-isopropanol mixture. 33 Prasad et al. 33 suggested that a change in dielectric constant of the solution could be responsible for the different orientations of the ZnO crystals with favorable polar orientation for solutions with high dielectric constant. This non-polar orientation is especially interesting for photovoltaic cells applications, 34 light emitting devices 35 and surface acoustic wave devices, 36 hence our desire to revisit the growth of ZnO in ionic liquid media following a recent paper by Azaceta et al. 20 In this study we report the influence of temperature, substrate, nature of the ionic liquid, and electrochemical deposition conditions (potential and others) on the morphology and crystal structure of electrochemically grown ZnO films, and we show how the EQCM experiments can help to investigate the nucleation-growth mechanism of ZnO films. Finally the explanation of the growth of ZnO in nonpolar direction will be given.

Experimental

ZnO deposition.-The electrochemical bath was composed of Zn(TFSI) 2 (Solvionic) solution in IL either EMImTFSI (1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide), BMPTFSI (1-Butyl-1-Methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) or BMImTf (1-butyl-3-methylimidazolium trifluoromethanesulfonate) ordered from Solvionic 99,9% pure (H 2 O < 0,005%) with the final solution being saturated with oxygen (Alphagaz, water content < 1 ppm) that was purified by passing through the column filled with dry P 2 O 5 . Prior to being used, the ILs were purified under vacuum at 75 • C for 2 days; the final water content was ∼6 ppm as checked by Karl Fischer measurement. High purity DMSO and toluene (stored in the glove box, water <0.1 ppm) were used for the mixture baths (1:38 volumetric ratio solvent:IL). The Zn(TFSI) 2 salt was chosen as zinc cation precursor in order to avoid addition of highly corrosive chlorides (ZnCl 2 ). If not otherwise specified the concentration of zinc cations at the beginning of deposition was 0.12 M. A three-electrode set-up was used for electrochemical measurements (chronoamperometry and cyclic voltammetry) with the WE and CE electrodes oriented horizontally (only in the case of indium tin oxide substrate the configuration was vertical). Three different substrates were used as working electrodes (WE) -Platinum (Pt), Stainless steel (SS) and Indium Tin Oxide (ITO), with respective surfaces areas of S = 1.25 cm 2 for SS and Pt and 0.75 cm 2 for ITO. The counter electrode (CE) was a platinum grid with high surface area (S ≈ 2 cm 2 ). The Ag wire was used as the pseudo reference electrode due to its stability at high temperature. All the values of potential will be given versus the pseudo Ag reference electrode if not otherwise specified. Prior to being used, the WE and CE electrodes were cleaned in 2M nitric acid in ultrasonic bath for 1h, and then intensively rinsed with distilled water and ethanol. All the components of the cell were dried in the oven (50 • C) for 2 hours before the experiment. The cell was immersed in the oil bath while the temperature was controlled with a ±2 • C difference between heating and deposition bath.

The electrochemical measurements were carried out with Autolab PGSTAT 30 potentiostat (Eco Chemie BV). All the CVs measurements were performed starting from the OCV with a reduction scan followed by the oxidation one. The potential range for electrodeposition was chosen in the range of IL electrochemical stability window (-0.8 to -1.9 V versus Ag) and the electrodeposition time was selected according to other conditions (1h to 4h). The electrodeposition was performed at 2 different temperatures 20 and 100 • C. The bath was bubbled with oxygen before and during the measurement. The obtained deposits were thoroughly rinsed with acetone and soaked in dichloromethane for 4h in order to remove adsorbed IL from the surface.

The morphology and crystal structure of deposits were analyzed using Philips XL 30 field emission gun FEG microscope and Bruker D4 diffractometer with copper radiation, respectively. The Transmission electron microscopy measurements were performed on FEI TEC-NAI F20 S-TWIN microscope.

The EQCM measurements were carried out using commercial SEIKO microbalance with AT-cut 9 MHz quartz covered with platinum on both sides. The electroactive geometric surface area was equal to S = 0.196 cm 2 . A simultaneous measurement of the quartz frequency and resistance was done in order to subtract the influence of bath viscosity and density on the frequency change as it was observed in AgTFSI solution. 37 Prior to the measurement, the EQCM was calibrated, with 0.05M solution of AgTFSI in EMImTFSI at 100 • C with 2 mV • s -1 scanning rate, to determine proportionality constant ( m = C • f) 38 between the change of quartz frequency ( f) and change of deposited mass ( m). A constant C value of 1.065 ng/Hz close to the theoretical one of 1.068 ng/Hz was found suggesting that issues regarding viscosity of the bath or roughness of the substrate can be neglected. So, to analyze the EQCM data, the function M/z (molar mass over number of exchanged electrons) was simply calculated according to equation 1 and Faraday's laws of electrolysis, assuming that the reaction efficiency is near 100%.

M z = FC d f d Q [1]
Under such conditions, the expected value of M/z for the growth of ZnO, which relies on a 2 e -process, is 41 g • mol -1 . The CVs (scanning rate 2mV/s) measurement was performed in 0.12 M Zn(TFSI) 2 solution in EMImTFSI at 100 • C under O 2 atmosphere. During the experiment, the oxygen was only flushed over the solution not to disturb microbalance performances.

Results and Discussion

CV of bath components.-We initially surveyed the influence of temperature and concentration of the zinc cations on the reduction of oxygen and found, in agreement with Azaceta et al., that both cations content and temperature enhance the reduction of oxygen. Since our studies were performed with different substrates using different solutions, we judged useful to report the CV curves for the bath components separately and all together at 100 • C on Pt substrate.

Figure 1 presents the CVs curves of bath components. The reduction stability of EMImTFSI was limited to -2.0 V (Fig. 1a, black curve). For the pure IL saturated with O 2 (Fig. 1b, red curve) two reduction waves were observed, one (A) centered at E = -1 V (onset at E = -0.7 V, see inset on Figure 1a) and a second (B) at E = -1.4 V, consistent with the two-step oxygen reduction process reported in the literature. 16 For the 0.12 M Zn(TFSI) 2 solution under Ar atmosphere (Fig. 1c, green curve) we observed a single reduction peak (D) at E = -1.23 V due to the reduction of Zn 2+ to Zn 0 with, however, a small preceding wave (C) which can be due to the underpotential - deposition of Zn. Two peaks located at E = -0.41 and -0.13 V (E and F, respectively) are observed on oxidation. Although we don't have a clear explanation, these two peaks, whose amplitude ratio was found to vary with the number of scans (annex Fig. 1*), could be rooted in either the existence of two zinc morphologies with different oxidation energies (as suggested by Abbott et al. based on simultaneous in situ AFM and EQCM measurements) 39,40 or different kinds of complexation of Zn 2+ cations by TFSI -anions. Last, when O 2 and Zn 2+ are present in the solution (Fig. 1d, blue curve), we noted the appearance of two reduction waves centered at E = -1.20 and -1.70 V (A and B respectively) and no oxidation signals. The first reduction wave is shifted to negative values as compared to what was found for O 2 in pure EMImTFSI (Fig. 1b, red curve) and this independently of the substrate SS or ITO (annex Fig. 3a, 3b*) that we were using. It could be caused by a change of physical parameters of IL in the vicinity of WE (viscosity, change in double layer) when zinc cations are added.

To grasp further insight into the nature of those two reduction peaks we decided to carry EQCM measurements while performing cyclic voltammetry at a rate of 2mV/s (Fig. 2). Such a slow rate was purposely used to obtain better resolution (because of lower ohmic drop) as well as a good microbalance response. Figures 2a and2b represent CVs curve and change of M/z value versus potential, respectively. At the beginning of reduction (Fig. 2b red curve) a sharp increase in M/z value (M/z = 56) is observed, most probably due to the nucleation process (formation of deposits) as well as adsorption of the ions on to the electrode. 41 Once the thin compact layer is formed, the M/z stabilizes at a value of 41 g • mol -1 , as expected for zinc oxide deposition. This means also that the reaction is almost 100% efficient as in our previous assumption. Additionally, during the first reduction peak a drop of resistance of about 20 ohms is observed (annex Fig. 2b*), and it corresponds to a decrease in viscosity/density of the solution near the electrode surface caused by depletion of zinc cations, as mentioned in the experimental part. According to the final deposition product ZnO (M/z = 41 g • mol -1 ) we could assign the first reduction peak to the formation of superoxide ions as indicated on Figure 2a. Turning to the second reduction peak a "sinusoidal-type" variation of the M/z value is observed. It is concomitant with the appearance of a slightly gray deposit, indicating the deposit of metallic zinc (M/z = 33 for the 2 electron reduction reaction of Zinc cations). The variation of the M/z value between 33 and 41 further indicates an alternating process of reduction of zinc cations to metallic zinc and oxygen to peroxide ions as indicated on Figure 2a. On the reverse sweep a very small and broad oxidation wave was observed while the gray color of deposit diminished. The absence of well-defined oxidation peak for Zn means that the reduction/oxidation process of Zn/Zn 2+ couple in the presence of oxygen is poorly reversible, probably due to the increasing amount of a nearly insulating ZnO deposit.

The above experimental protocol was implemented to study the effect of changing the nature of the substrate, the nature of the ionic liquid, the temperature or the solvents. Whatever the modifications made, we shortly found that the overall shape of the CV curve collected on Pt substrate was preserved (annex Fig. 3,4*) with therefore minor changes on the onset positions and amplitudes of the redox peaks. Such shifts on the onset potentials of the reduction peaks are summarized, for instance, in annex (Tables I, II and III*) for different conditions.

Potentiostatic electrodeposition.-All the electrodeposition conditions are summarized in Table I. From now on if not otherwise specified, the deposition conditions: 0.12 M Zn(TFSI) 2 solution in EMImTFSI, T = 100 • C, E = -1.00 V, t = 2h will be considered as the reference conditions. Based on EQCM measurement we limited the potential deposition range (for deposition at 100 C) from -0.8 V to -1.2 V to avoid electrodeposition of metallic Zinc, at this potential range formation of super oxide occurs. In case of mixtures DMSO and toluene with IL the experiment had to be terminated before planned time due to a change in bath color.

Figure 3a (black curve) represents the current profile of chronoamperometric experiment at reference conditions. We initially observed a huge increase in current which then slightly decreased prior to stabilizing and reaching steady state. This is the typical characteristic of a nucleation-growth mechanism with the current downturn corresponding to the time at which the grains start to grow (e.g., decrease in the current is due to increase in surface area). 40 Once the thin layer is created there is a stabilization of the current most likely due to mass transport limitation (e.g., diffusion of O 2 toward the electrode).

The current profile for deposition at 100 • C at lower potential (-1.20 V) significantly differs (Fig. 3a, green curve) since the characteristic valley on the I = f(t) is no longer observed. In contrast, after 400 s (as indicated by black lines and arrows) there is a high decrease in current, probably due to deposition of ZnO with different morphologies, and then after 1500 s (see arrow on the curve) a second very sharp decrease in current most likely due to the deposition of metallic zinc. Surprisingly this behavior is no longer observed when the deposition proceeds at RT and lower potential (-1.85 V) since the I = f(t) curve shows a diffusion limited current process (Fig. 3a red curve) associated to an excess of oxygen (O 2 x-) which captures Zn 2+ prior to being reduced. Most probably at such low potential we have the formation of peroxide ions instead of superoxide that changes the mechanism of ZnO synthesis compared to the deposition at 100 C at potentials higher than -1.2 V.

Changes in the nature of the ionic liquids are found to affect the overall shape of the current profile (Fig. 3b). For instance, with a BMPTFSI and BMImTf solution, the process spreads over time as the initial valley in the current profile prior stabilization takes more than 2000s while for EMImTFSI solution it is a few hundreds seconds (200s). The time difference effect is most likely nested in the specific interaction of IL with the surface of newly deposited particles and substrate that probably differs according to the type of IL.

Turning to the nature of the substrate we found that it had a greater impact on the current profile (Fig. 3c) since, for instance, the characteristic valley on the I-t curve for nucleation and growth mechanism is not very well defined in the case of SS substrate (Fig. 3c green curve).

Last, we experienced that the addition of extra solvent as toluene or DMSO did not influence the shape of the current profile (Fig. 3d). This is in contrast with the addition of H 2 O (Fig. 3d red curve) which slightly changes the shape of the current profile with i) the absence of valley on I = f(t) curve, and ii) the appearance of a wavy current profile which we assigned to changes in the morphology of the deposits as it will be discussed later.

Deposit morphology.-Figures 4, 5 present the SEM images of

individual deposits with at first the reference deposit, obtained at 100 • C with a 0.12 M Zn(TFSI) 2 solution in EMImTFSI when clamping the potential at -1.00 V for 2 hours, and which is transparent. It is composed of a thin compact layer (∼500nm) with, in addition, 200-300 nm grains on top as can be seen in Figures 4a and4b. It is worth noting that cracks are not initially present in the film but are developing upon long beam exposure (annex Fig. 5*). To grasp more insight in the primary particles we carried TEM measurements which indicate that the deposit is made of very small crystallites (6 nm) as deduced by HRTEM measurement (annex Fig. 6*). From EDX we also spotted peaks associated to F and S suggesting the presence of residual ionic liquid that could not be fully eliminated even after several washings with methane dichloride. This does not come as a total surprise since we previously experienced a similar surface contamination for all the powders we have so far prepared by ionothermal synthesis even after copious washing either in dimethyl chlorine or ethyl acetate. 42 Lowering the electrodeposition potential to -1.2 V leads to an inhomogeneous deposit (Fig. 4c,4d) composed of 3 different layers as suggested by the current profile (Fig. 3a green curve). Departing from the Pt substrate we first found i) a compact thin layer of ZnO ∼ 500 nm thick (layer 1, Fig. 4c) which is alike the one grown at -1.0 V, ii) another compact layer of ZnO grains with hexagonal shape (layer 2, Fig. 4c and4d), and iii) last a non adhesive sponge-like layer composed of Zn metal (layer 3, Fig. 4d inset). Such an inhomogeneous deposition is believed to be due to the rapid decrease in oxygen concentration near the sample deposit surface. This is consistent with early reports which mentioned that the concentration of the ingredients within the bath has a huge influence on the deposit morphology. 30 Turning to the room temperature deposition at -1.85 V in 0.12 M Zn(TFSI) 2 in EMImTFSI, we obtained (Fig. 4e) a grayish layer deposit which shows cracks together with a copious amount of 1μm big grains. Interestingly, by decreasing the concentration of Zn cations by a factor of 10, we obtained a different morphology with namely the alignment of ZnO grains in a strip-like shape with equal spacing (5 μm) between strips (see arrows Fig. 4f). Such a pattern, which we found to be intrinsic to the deposit growth, can be nested in a specific interaction of IL with substrate (coupling of the platinum wave functions with the π orbitals of the imidazolium cations as early mentioned [11][12][13][14][15] favoring such an alignment. The local arrangement of IL (e.g. liquidcrystal type) could cause the orientation in deposit too. This is possible since we early demonstrated in our ionothermal synthesis work the feasibility of using ionic liquids as structural directing agent. 43 Such a specific IL-substrate interaction seems to disappear with temperature since we found that at 100 • C neither the type of the IL (Fig. 5a and5b) nor the substrate influence (Fig. 5c and5d) significantly affect the morphology of the deposits with the exception of the deposit density. In all the cases a thin transparent layer topped with a few ZnO grains was obtained as in the case of the reference deposit. Last, electrodeposition was also carried out with the additions of many solvents, such as toluene, DMSO, H 2 O to our ionic liquid media. In the case of DMSO:IL mixture the 250 nm deposit reveals many valleys and hills as well as some cracks (Fig. 5e). In contrast a lower surface roughness was observed for the 300 nm deposits grown in a toluene:IL mixture (Fig. 5f) with therefore the presence of hole-like spots, which could originate from a partial evaporation of trapped toluene molecules (Fig. 5f inset). The morphology of the deposits grown in H 2 O:IL mixture (annex Fig. 7*) totally changed, and it is composed of grains with very sharp edges which create a compact layer. Nevertheless, by tuning the deposition time we found out that transparent ZnO thin film alike in our reference can be obtained.

Crystal Structure.-Figure 6, 7 present the XRD patterns of all deposits as well as the projection of stack of ZnO unit cells along polar direction. The amplitude of the peaks is very low with respect to the substrate ones due to tiny amounts of deposit. Moreover, they are broad as compared to commercial ZnO powder confirming the presence of small crystallites as indicated by microscopy. For the reference deposit (Fig. 6b), many of the peaks expected from the ZnO hexagonal crystal structure are absent suggesting a preferential orientation of the deposit. The appearance of the (100) and (110) Bragg peaks together with the absence of (002) indicates a high orientation of the reference deposit in hk0 direction.

We found that the orientation of the deposit is lost (Fig. 6b) when the deposition is performed at lower potential (-1.2 V) and 100 • C, due to the non-homogeneity of the deposit (3 layers). The samples prepared at RT were featureless (Fig. 6b) due to poor crystallinity as confirmed by the SAED patterns which are quite diffused. Nevertheless, the ED patterns were found to improve upon long exposure to electron beam implying a high temperature-driven crystallization.

The non-polar orientation ( hk0 growth direction) remained unchanged whatever the nature of IL or substrate we have been using (Fig. 7a and7b). This suggests that this specific orientation is most likely nested in the nature of the solvent which could affect the dielectric constant (Imidazolium and pyrrolidinium based IL possess very low dielectric constant value in range of 10-15 44 ) or viscosity of the electrolytic bath. To test this hypothesis we have performed electrodeposition from mixtures of solvents starting with non-aqueous ones. The XRD powder pattern collected for a deposit grown in DMSO:IL mixture (increase in dielectric constant value while high viscosity) reveals the presence of the (002) Bragg peak which implies the loss of the non-polar growth direction (Fig. 7c). In contrast, films prepared from toluene:IL mixture (decrease in viscosity while maintaining low dielectric constant) are alike those obtained in IL alone (Fig. 7c). This indirectly suggests that the dielectric constant of the solvent plays a significant part in controlling deposit orientation. Now turning to H 2 O:IL media, the results are quite different (annex Fig. 8*). The XRD pattern shows 4 peaks out of which two only match the 2θ positions corresponding to the (100) and (110) Bragg peaks of hexagonal ZnO. The amplitude of the two remaining ones, which we identified as corresponding to zinc hydroxide Zn(OH) 2 from both IR and XRD measurements, were found to change with the electrolytic bath conditions. We found that pure Zn(OH) 2 thin films can be obtained at RT from H 2 O:IL mixture; such a phase was shown to transform upon heat-treatment at 200 • C into hexagonal ZnO keeping its non polar orientation, giving us a second route to obtain this specific orientation. Moreover for deposition at 100 • C the longer the bath was kept at 100 • C the lower the amount of Zn(OH) 2 was obtained, due to evaporation of water from the bath. After 2hrs preheating, only pure ZnO could be deposited.

Discussion

ZnO is mainly grown in the polar direction via physical deposition techniques. This contrast with our present work which indicates that ZnO thin film deposits can be grown in the non-polar direction via electrodeposition provided that we manipulate the dielectric constant of electrolytic bath composition. It now pertains to us to provide a rational explanation to account for such an observation and to return to the EQCM measurements.

The sinusoidal variation of M/z value during the second reduction wave (Fig. 2b) suggests that the reactions taking place at the electrode are ruled by transport phenomena, hence the importance of the huge difference in diffusion coefficients of the reacting species Zn 2+ (D RT = ∼10 -12 ), 45 17 ), Zn (C = 0.12 M), making the diffusion of molecular oxygen much faster than zinc cations. It results that, depending on the chosen deposition potential, which can be either before reduction of zinc cations (high potential) or after the onset of zinc reduction (low potential), two scenarios can be envisioned.

At high potential V) and 100 • C the growth of ZnO can be explained as follows. Firstly, the produced superoxide accumulates near the electrode due to its low diffusion coefficient as compared to molecular oxygen, and this is experimentally confirmed by the small lag between the frequency change and beginning of oxygen reduction observed during EQCM measurements (annex Fig. 2a*). Later superoxide ions O 2 -react with Zn 2+ causing its depletion near the electrode (as indicated by drop in resistance annex Fig. 2b*) with the end result being a transportation controlled deposition of compact ZnO layer. Occasionally, we have additional growth of ZnO grains over this thin layer which can be due to either changes in the oxygen concentration at the vicinity of the electrode or changes in the conductivity of the active surface, via non homogeneity of thicknesses, since ZnO is highly resistive. The reduction of Zn 2+ cannot occur since the potential is too high.

For the 100 • C deposition at low potential (-1.2 V where reduction of zinc cations is possible Fig. 1c), even though the concentration of Zinc cations is around 25 times higher than molecular oxygen (C O2 = 0.005 M 17 ), the first steps of electrodeposition are similar to the high potential deposition. At the beginning, only O 2 is reduced and the thin layer of ZnO is formed. After a certain time (400s) a thin layer is formed (Fig. 3a) and the growth of second ZnO morphology starts; further deposition of Zn occurs (after 1500 s) due to a decrease in concentration of oxygen in the vicinity of the electrode (the flux of oxygen is not sufficient to react with all Zn 2+ near the double layer). This is also confirmed by EQCM measurements (Fig. 2), where alternative reactions of zinc and oxygen reduction occur, because the M/z value varies between 41 and 33 at low potentials. In contrast, when the deposition is done at RT no zinc reduction occurred at very low potential (-1.85 V), even after a long time (4h), indicating that the Zn 2+ reduction is totally suppressed (Fig. 3a). Changes in the concentration of molecular oxygen in the bath at low and high temperatures, change of the reaction mechanism (formation of peroxides that may form a blocking diffusion layer) as well as changes in the coordination of Zinc cations upon heating could be at the origin of such a difference.

Turning to the deposit growth direction we show that it can be switched from polar (along 00l ) to non polar (along hk0 ) depending upon the composition of the electrodeposition bath. It was earlier reported that highly polar solvents like water, 30 DMSO 31 or mixture of DMSO with IL (as we reported) lead to deposits either oriented in the polar direction or with random orientation. Here we have pushed this observation further by showing that solvents with low dielectric constant EMImTFSI (ε = 12.0) 44 and more so toluene (ε = 2.38) lead to deposits oriented along the non-polar direction, clearly stressing the importance of solvent dielectric constant in ruling the ZnO growth direction. Moreover, we show that using ionic liquid conjointly with another solvent such as toluene enables to overcome the poor ionic liquid viscosity so as to improve transfer phenomena. Needless to say that dilution effect (IL + Solvent) can also influence the coordination of the zinc cations and hence drastically affect the electrodeposition process as it is observed with the addition of water.

Conclusions

We have reported the electrodeposition growth in ionic liquid media of ZnO deposits on Pt, SS, ITO substrates. From combined XRD and SEM/TEM measurements we have shown that neither the substrate, cation nor anion character of IL (possessing low dielectric constant value) does influence the morphology or crystal structure of the deposit in contrast to both deposition potential and temperature, with for the latter amorphous deposits for films grown at RT as opposed to well crystalline and hk0 oriented deposits for 100 • C grown films. We have provided direct evidence, via well selected solvent additives as DMSO and toluene, having different dielectric constants (ε = 46.7 and ε = 2.38, respectively), that the film growth orientation is mainly governed by solvent dielectric constant. So it results that mixtures of ILs-solvents can be manipulated to orient on the demand the growth direction of the ZnO deposits, provided the absence of water traces usually promoting the deposition of metal hydroxide.

More broadly we have shown how EQCM measurements can be used to follow the nucleation-growth mechanism process and to probe the type of reactions taking place at the electrolyte-substrate interface. When the electrodeposition is done at low potential, such measurements have shown competing processes associated to the reduction of zinc cations and molecular oxygen, with the reduction of zinc starting when the concentration of oxygen reaches a low critical value. Needless to say that the huge difference in the diffusion coefficients and concentration of molecular oxygen and zinc cations, is a key in determining the reacting schemes leading to the formation of ZnO deposits, with the feasibility of having different reacting pathways depending on the growth temperature and deposition potential. The use of further characterization techniques such as in situ STM, AFM and EQCM is immediately apparent to grasp further details on the ZnO growth mechanism depending on the type of ionic liquid or waterbased electrolytic baths. Overall we hope that this work will serve as an impetus to implement the electrodeposition technique to the growth of various binary and ternary oxides as we are presently doing with success since copper, cobalt and manganese oxides have already been grown as it will be reported in a coming paper.

Figure 1 .

 1 Figure 1. CVs curves of electrochemical bath components separately and all together. (a) black curve -pure ionic liquid under Argon atmosphere, (b) red curve -pure ionic liquid under oxygen atmosphere, (c) green curve -0,12 M Zn(TFSI) 2 solution under Ar atmosphere, (d) blue curve -0,12 M Zn(TFSI) 2 solution under O 2 atmosphere. T = 100 • C, WE -Pt, CE -Pt, RE-Ag, scan rate 50 mV • s -1 . The inset -magnification of the reduction step for all the curves.

Figure 2 .

 2 Figure 2. EQCM measurements: (a) I versus E curve, (b) M/z function versus potential, red (reduction sweep) and blue (oxidation sweep) curve. 0.12 M Zn(TFSI) 2 solution under O 2 atmosphere. α corresponds to ZnO deposition, β corresponds to Zn deposition. T = 100 • C, WE -Pt on quartz, CE -Pt, RE-Ag, scan rate 2 mV • s -1 .

Figure 3 .

 3 Figure 3. Current profiles of chronopotentiometric experiments. (a) different temperatures and potential, (b) different Ionic Liquids, (c) different substrates, (d) mixtures of IL with other solvents. Inset represent the beginning of the deposition.

Figure 4 .

 4 Figure 4. SEM images of particular deposits: (a,b) 100 • C, -1.00 V, 0.12 M Zn(TFSI) 2 in EMImTFSI on Pt (reference deposit); (c,d) 100 • C, -1.20 V, 0.12 M Zn(TFSI) 2 in EMImTFSI on Pt; (e) RT, -1.85 V, 0.12 M Zn(TFSI) 2 in EMImTFSI on Pt; (f) RT, -1.85 V, 0.012 M Zn(TFSI) 2 in EMImTFSI on Pt.

Figure 5 .

 5 Figure 5. SEM images of particular deposits: (a) 100 • C, -0.85 V, 0.12 M Zn(TFSI) 2 in BMPTFSI on Pt, (b) 100 • C, -0.90 V, 0.12 M Zn(TFSI) 2 in BMImTf on Pt (c) 100 • C, -1.10 V, 0.12 M Zn(TFSI) 2 in EMImTFSI on SS, (d) 100 • C, -1.12 V, 0.12 M Zn(TFSI) 2 in EMImTFSI on ITO, (e) 100 • C, -1.20 V, 0.12 M Zn(TFSI) 2 in DMSO:EMImTFSI on Pt, (f) 100 • C, -1.25 V, 0.12 M Zn(TFSI) 2 in toluen:EMImTFSI on Pt.

Figure 6 .

 6 Figure 6. (a) Projection of stack of ZnO unit cells along z direction. (b) XRD patterns of deposits and substrates, (from the bottom to top), Pt substrate; ZnO powder; RT deposit from EMImTFSI solution; 100 • C deposit from EMImTFSI on Pt at -1 V; 100 • C deposit from EMImTFSI on Pt at -1.2 V.

Figure 7 .

 7 Figure 7. XRD patterns of deposits and substrates: (a) (from the bottom to top) different IL deposits, EMImTFSI; BMPTFSI; BMImTf. (b) different substrates, 100 • C deposit from EMImTFSI on Pt; SS raw substrate; deposit on SS; ITO raw substrate; deposit on ITO. (c) mixtures of IL with other solvents, 100 • C deposit from EMImTFSI on Pt; IL:DMSO; IL:toluene.

  O 2 (D RT = ∼10 -10 ) 17 and intermediate species O 2 -(D RT = ∼10 -11 ) 17 as well as difference in concentrations O 2 (C RT = 0.005 M

Table I . Electrodeposition conditions for all the baths. The deposit obtained from the conditions marked with star * is a reference deposit.

 I

	Chronoamperometric deposition
	mixture	Substrate	Temp / • C	E/V	T/s
	EMImTFSI*	Pt	100	-1.00	7200
	EMImTFSI	Pt	RT	-1.85	14400
	EMImTFSI	Pt	100	-1.20	5100
	EMImTFSI	SS	100	-1.10	7200
	EMImTFSI	ITO	100	-1.12	7200
	BMPTFSI	Pt	100	-0.85	7200
	BMImTf	Pt	100	-0.90	7200
	H 2 O: EMImTFSI	Pt	100	-0.90	7200
	DMSO: EMImTFSI	Pt	100	-1.20	3000
	Toluene: EMImTFSI	Pt	100	-1.25

Acknowledgment

The authors thank Prof. L. Dupont (LRCS, Amiens) for the TEM measurements as well as P. Simon for fruitful discussions.


M Tulodziecki

.-M Tarascon

P L Taberna

C Guéry

Importance of the double layer structure in the electrochemical deposition of Co from soluble Co 2 + -based precursors in Ionie Liquid media

Keywords:

Electrodeposition in Ionie Liquid (IL) media is being widely used to obtain different metals, alloys and more recently binary compounds. However, the understanding of this process is still slim due to poor knowledge in the chemistry ofILs and also the complex structure of the double layer at the electrode sur face. lt is then difficult to predict deposition conditions when trying to synthesize a desired deposit. ln this work, we provide insight into the processes taking place at the electrode by detailed studies of Co 2 • reduc tion in ionic liquid media at elevated temperature, which have revealed an unusual Cyclic Voltammetry profile. A drastic drop in the reaction rate was observed, together with a well-shaped reduction peak dur ing the oxidation sweep. Using Electrochemical Quartz Crystal Microbalance, lmpedance Spectroscopy measurements and varying bath parameters, we ascribed those phenomena to the reconstruction of the double layer structure upon imposed polarization. Similar behaviors were observed for other systems (Mn 2 •, Ni 2 •, Zn 2 •). Finally, this work provides new insight into the electrochemical processes in Ils. We anticipate that it will also result in significant improvement in preparing metal and other deposits in the ionic liquid media.

t. Introduction

Ionie liquids (Ils) have received great attention in recent years as a media for electrodeposition thanks to their attractive chemi cal/physical properties. They are non-flammable, display very high thermal stability together with negligible vapor pressure, and are good sol vents for numerous salts and polymers. Their wide window of electrochemical stability and relatively high conductivity have allowed the electrodeposition of less noble metals, semiconductors and alloys that could not be accomplished in aqueous solution due to its limited potential window and thermal stability [START_REF] Aal | lnterfacial electrochemistry and electrodeposition from some ionic liquids: In situ scanning tunneling microscopy, plasma electrochem istry, selenium and macroporous materials[END_REF][2][3][START_REF] Freyland | Nanoscale electrodeposition of metals and semiconductors from ionic liquids[END_REF][START_REF] Hapiot | Electrochemical Reactivity in Room-Temperature Ionie Liquids[END_REF][START_REF] Simka | Electrodeposition of metals from non aqueous solutions[END_REF][START_REF] Su | The Electrode/Ionie Liquid Interface: Electric Double Layer and Meta! Electrodeposition[END_REF]. More recently binary compounds such as sulfides [START_REF] Chen | Growth ofsingle-crystal copper sulfide thin films via electrodeposition in ionic liquid media for lithium ion batteries[END_REF][START_REF] Chen | Exploring sulfur solubility in ionic liquids for the electrodeposition of sulfide films with their electrochemical reactivity toward lithium[END_REF] and metal oxides [START_REF] Azaceta | Electrochemical reduction of 02 inl butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid containing Zn 2 • cations: deposition of non-polar oriented ZnO nanocrystalline films[END_REF][START_REF] Azaceta | Niü cathodic electrochemical deposition from an aprotic ionic liquid: Building metal oxide n-p heterojunctions[END_REF][START_REF] Tulodziecki | Electrodeposition Growth ofOriented ZnO Deposits in Ionie Liquid Media[END_REF] have been electrodeposited from IL baths, opening new possibilities for this media.

An issue of primary importance regarding electrodeposition processes is the structure of the double layer and its behavior under applied potential which is different in Ils, aqueous solution and by adding strong organic complexing agents as TU [START_REF] Fukui | The effect of organic additives in electrode position of Co from an amide-type ionic Iiquid[END_REF][START_REF] Saito | Complexation of cobalt by natural organic ligands in the Sargasso Sea as determined by a new high-sensitivity electrochemical cobalt speciation method suitable for open ocean work[END_REF]. Despite ail these researches, issues dealing with double layer structur ing and cation complexation, which are of crucial importance for understanding and planning electrochemical processes in IL media, are still not well known, hence our motivation to address them.

In this study, we perform a detailed investigation of the Co 2+ /Co electrochemical reduction process in ionic liquid media by explor ing the influence of temperature, substrate, nature of the ionic liquid (with focus on EMlmTFSI), and complexation of Co 2+ . Special attention is devoted to the fundamental aspects of the processes taking place at the electrode surface via the complementary use of Electrochemical Quartz Crystal Microbalance (EQCM), Cyclic Voltammetry (CV) and Impedance Spectroscopy (IS) measure ments. Sorne interesting phenomena, never reported previously for the Co 2+ /Co system, have been unravelled. Finally we attempt to exp Iain them based on our results and the large amount of literature data on ionic liquids.

Experimental

The electrochemical bath was composed of Co(TFSih (Solvionic) solution in IL either EMlmTFSI, OMlmTFSI, BMPTFSI, BMlmTfO or tBMNTFSI ordered from Solvionic, 99.9% pure (H 2 O < 0.005%), Table 1. Prior to being used, the Ils were purified under vacuum at 75 °c for 2 days; the final water content was ~6 ppm as checked by Karl Fischer measurement. The complexing agent Thiourea (from Aldrich) was used as received. The Co(TFSih sait was chosen as cobalt cation precursor in order to avoid the addition of highly corrosive chlorides (CoCl 2 ). If not otherwise specified, the initial concentration of cobalt cations was 0.12 M. Ali the substances were kept in an argon-filled glove box (0 2 and H 2 O<0.1 ppm). ln ail TFSI-based Ils ( containing only Co 2 + cations) the col or of the bath was purple indicating octahedral coordination of cobalt cations [Co(TFSlhJ-. We have just noted a minor change of the bath color (0.12 M Co(TFSih in EMlmTFSI) from purple (RT) to dark purple upon heating (up to 200 °c), suggesting only a slight tempera ture influence on the cobalt coordination. This contrasts with the results presented by Katayama et al. where the solution of Co(TFSih in BMPTFSI changed to blue at high temperature [21 ]. However Raman/IR results have recently revealed the absence of change in coordination number as a function of temperature, although the conformational equilibrium in the coordination sphere (between two most stable states cis and trans) has been found to be strongly dependent [START_REF] I< | Solvation Struc tures of Sorne Transition Metal(ll) Ions in a Room-Temperature Ionie Liquid, 1-Ethyl-3-methylimidazol iu m Bis( trifl uoromethanesu lfonyl )amide[END_REF].

A three-electrode set-up was used for electrochemical mea surements ( chronoamperometry, cyclic voltammetry, impedance spectroscopy). Five different substrates were used as working elec trodes (WE): platinum dise (Ptd) 5=1.78 cm 2 , platinum planar electrode ( Ptp) S = 0.020 cm 2 , glassy carbon planar electrode ( GC) 5=0.071 cm-2 , platinum and stainless steel wires (Ptw and SSw respectively). The surface area of the last one was estimated accord ing to the length of the dipped part (surface ranging from 0,017 to 0,033 cm 2 ). The counter electrodes (CE) were platinum grids with high surface area (S ""2 cm 2 and 10 cm 2 depending on the chosen WE). An Ag wire covered with Ag 2 O was used as the pseudo ref erence electrode (RE). The stability of RE at high temperature has been checked by me ans of OCP stability and reproducibility. It was found stable enough for the purpose of our experiments. Ali the values of potential will be given versus the pseudo Ag/Ag 2 O refer ence electrode if not otherwise specified. Prior to being used, the platinum WEs were annealed in 0.5 M H 2 SO 4 in order to obtain a stress-free and clean surface (the CV profile patterns were compa rable with now standard ones representative of an impurity free Pt surface) [START_REF] Sugawara | Dissolution Mech anism of Platinum in Sulfuric Acid Solution[END_REF]. Next, the electrodes were rinsed with distilled water, ethanol, acetone and deionized water. The CE electrode was cleaned in 0.5 M ni tric acid in an ultrasonic bath for 1 h, and then inten sively rinsed with distilled water, ethanol, acetone and deionized water. Ali the components of the cell were dried in an oven at T= 50 °C for 2 hours before the experiment. Preparation of the solu tion and assembly of the cell were done in a glove box. The cell was immersed in an oil bath while the temperature was controlled with ± 2 °c difference between heating and deposition bath.

The electrochemical measurements were carried out with an Autolab PGSTAT 30 potentiostat (Eco Chemie BV). The impedance measurements were conducted with a Biologie VSP potentiostat; collection of impedance data started after the system reached steady state conditions at fixed potential. The chosen frequency ranged from 1 MHz to 50mHz while the amplitude of sinusoidal potential was set to 10 mV. Ali the CV measurements were per formed starting from the OCP with a reduction scan followed by the oxidation one if not otherwise specified.

The obtained deposits on Ptd were soaked in chloroform to remove adsorbed IL from the surface. Their morphology was further analyzed using a Philips XL 30 field emission gun FEG microscope.

EQCM measurements were carried out using a commercial SEIKO microbalance with AT-eut 9 MHz quartz covered with plat inum on both sides. The electro-active geometric surface area was equal to S = 0.196 cm 2 • Simultaneous measurements of the quartz frequency and motional resistance were done in order to follow bath the change in mass and the variation in Co 2 + concentration in the vicinity of the electrode (by following the change of viscos ity and density product of the solution) [START_REF] Tulodziecki | Electrodeposition Growth ofOriented ZnO Deposits in Ionie Liquid Media[END_REF][START_REF] Bruckenstein | Experimental Aspects of Use of the Quartz Crystal Microbalance in Solution[END_REF][START_REF] Liu | Quartz Crystal Microbal ance Technique for Analysis of Cooling Crystallization[END_REF]. The EQCM was calibrated with CV measurement in 0.05 M solution of AgTFSI in EMimTFSI, at 100 °(, with 2 mV•s-1 scanning rate. The proportion ality constant (�m=C.�f) was determined between the change of quartz frequency (�f) and change of deposited mass (�m) [START_REF] Sauerbrey | Verwendung von Schwingquarzen zurWagung Dunner Schichten und zur Mikrowagung[END_REF]. A constant value C= 1.065-10-9 g-Hz-1 close to the theoretical one Ct = 1.068-10-9 g-Hz-1 was found, suggesting that issues regarding viscosity of the bath or roughness of the substrate can be neglected. So, to analyze the EQCM data, the function M/z (molar mass over number of exchanged electrons) was simply calculated according to Sauerbrey equation (1) and Faraday's laws of electrolysis (2), assuming that the reaction efficiency is near 100% (3). The M/z value was calculated from the �f and �Q difference between each adjacent point.

.<1 m = -C ,1 f ,1m = ( ,1 FQ) ( �) M =-(FC) ,1f z ,1 Q (1) (2) (3)
The expected value of M/z for the growth of Co, which relies on a 2 e-process, is 29.5 g•moI-1 . The CV with scanning rate 2mV-s-1 and probe time 1s, was performed in 0.12 M Co(TFSih solution in EMimTFSI at 100 °c under Ar atmosphere.

Ali the experiments were done several times to check the repeatability. We would also like to mention that Co 2+ /Co + symbols are mostly written as cobalt species in the solu tion instead of complexated forms, which they are in reality ([Co(TFSihJ-/[Co(TFSI)x)Y-), although we corne back to the latter when it is needed for the sake of clarity.

Results

Cyclic Voltammetry in EMlmTFSI

We initially checked the electrochemical response of 0.12 M Co 2+ solution in EMimTFSI IL, at 100 °c, on 3 different sub strates: Ptp, GC and SSw (Fig. 1a,b,c respectively). Whatever the nature of the substrate, CVs revealed identical features during the Abbreviations for the IL components and their structures. The reduction starts at a high overpotential of ~0.8 V as compared to the coupled oxida tion process ( onset ~0 V). Subsequently, between -0.9 V and -1.1 V a diffusion limited process appears as the current density tends to stabilize. At lower potentials, the reduction current density sud denly drops and reaches a minimum at -1.20V {Il). Afterwards, the reduction current density increases again and reaches the maxi mum at -1.38V (III), which we again assigned to the reduction of Co 2+ . The value of the Iimiting current density below -1.38 Vis simi lar to the one of diffusion Iimited reaction taking place between-0.9 and-1.1 V. On the reverse sweep {Fig. la, red curve), the current density progressively decreases and reaches a minimum at slightly higher potentials (-1.18 V) in comparison to the reduction sweep (-1.20V). Above -1.18V, a very sharp reduction peak centered at -1.09 V is observed {IV), followed by a diffusion limited and then a diffusion/activation limited reduction process. The oxidation sig nal at more positive potentials was ascribed to Co stripping. The reversibility (the charge ratio between oxidation and reduction process) of the electrochemical reaction is above 75%; however, it varies according to the chosen reverse potential reaching almost 100% at higher potentials (Supplementary Materials (SM), Fig. 1 *). Such aforementioned features, namely the drop in current density around -1.2 V and the oxidation peak on the reverse sweep, were

EQCM measurement

Both the huge drop in current during the reduction sweep and the well-defined reduction peak during the oxidation sweep are rare phenomena in electrochemistry. To gain further insight into the origins of these unusual features, we have conducted an EQCM measurement which is an elegant technique to indirectly deter mine the mass of the deposit as well as the concentration of Co 2 + near the electrode via quartz frequency and motional resistance changes, respectively. Cyclic voltammetry at a rate of 2 mV -s-1 , at 100 °c, in 0.12 M Co{TFSih was performed. The reverse poten tial was chosen at the minimum of the current drop during the reduction sweep (-1.20V).

Figs 2a,b,c,d show current, quartz frequency, quartz resistance and M/z variations versus potential, respectively. The current pro file is almost identical to the one obtained on the Ptp electrode in the same conditions (Fig. 1 a). A shift in potential is visible most Iikely due to minor differences in the Pt surface, which in case of quartz is electrochemically mirror polished as opposed to mechan ically. Whatever the origin of the shift, a drop in current density with the minimum at -1.20 V and a very sharp reduction peak dur ing the oxidation sweep are still observed. The reduction of Co 2 + species starts at slightly Iower potentials of -0.87 V and is centered at-0.96 V. A time derivative of the current (SM, Fig. 2a*) reveals two weak anomalies at-0.88 and-0.92 V. Further, comparing CV (Fig. 2a) and variation in quartz frequency (Fig. 2b) between -0.87 and -0.90 V (within the blue dashed lines potential window, Fig. 2 ), a high reduction current density is observed while only small changes in frequency occurred ( clearly visible on the time derivative of the frequency which is proportional to the current according to Sauer bay and Faradays equations -SM, Fig. 2b*). Additionally, a small decrease in motional resistance (Fig. 2c) is observed, attributed to the change of viscosity/density properties of the solution in the vicinity of the electrode. ln our opinion, this part of the reduction process involves only soluble species with 1e-reduction process of Co 2+ ( equation 4 ), and the aforementioned variation in motional resistance is due to the change in the coordination sphere of Co 2+ as it is reduced.

Co 2+

+ e -� co + soluble � soluble

Co:ta luble + e-� COsoild Co:;;; uble + 2 e-� Cosoild

(4) (5) (6)
Below -0.9V, there is a significant decrease in frequency that is accompanied by the appearance of a dark deposit on the quartz, indicating the consumption of Co 2+ /Co + cations according to equations ( 5) and ( 6). This leads to a decrease of solution vis cosity/density, in agreement with the quartz motional resistance change (Fig. 2c) that suddenly starts to decrease and then stabilize at R = 915 Qin the region of the diffusion Iimiting step (-1.0/-1.1 V). Calculated M/z values (Fig. 2d) start to progressively increase reach ing a maximum of 42 g-moJ-1 at E=-0.97V. This maximum most likely originates from the simultaneous reduction of Co 2+ and co + species which should theoretically give M/z values of 29.5 g-moJ-1 and 59 g-moJ-1 , respectively.Afterwards, M/z stabilizes at a value of 29-30 g-moJ-1 indicating a direct 2 electron Co 2+ reduction process at lower potentials (equation 6). Once the current density starts to drop (below -1.1 V, blue dashed line Fig. 2), the frequency tends to stabilize due to a slowdown in the deposition rate; more impor tantly, the motional resistance increases dramatically (increase in viscosity and density product) reaching 935 Q. This is in relation with the diffusion of the Co 2+ in the vicinity of the electrode, equal izing its concentration with the bulk one.

When the potential is reversed, a huge and very sharp reduction peak is observed. It is linked to both a sudden increase in the depo sition rate and a decrease in motional resistance (drop in viscosity and density product). This means that Co 2+ reduction resumes rapidly with therefore the onset of a diffusion limiting process (as further suggested by the constancy of the motional resistance) at the plateau of the motional resistance value (R= 905 Q). Overall, from the current profile, quartz frequency and quartz motional resistance variation, it seems that the reduction of Co 2+ around -1.2 Vis inhibited by a reversible potential triggered phenomenon.

At this stage, it should be recalled that the quartz motional resistance does not solely depend on the bath viscosity and density product; other parameters such as viscoelasticity, rough ness or adhesion of the deposit also contribute to the motional resistance [START_REF] Liu | Quartz Crystal Microbal ance Technique for Analysis of Cooling Crystallization[END_REF]31 ]. However, in our case, the shape of the motional resistance curve follows the change in current density which is pro portional to the change of Co 2+ concentration. The change in Co 2+ content affects the structure of IL in the vicinity of the electrode (diffuse layer of Co 2+ ) that acts as a viscoelastic film on the quartz electrode. Any variation in IL structure leads to changes in the viscosity/density parameter consequently affecting the motional resistance. Additionally, note that during the diffusion limited reduction process (-1. 1 V to -0.8 V on oxidation sweep and -1.0 to -1.1 V on reduction sweep ), the motional resistance remains almost constant (R""' 905 Q). Moreover its value is close to the quartz resis tance in pure IL solution at 100 ° C (R= 895 Q), while there is still a progressive evolution of the deposit. This led us to conclude that the growth of the Co deposit has negligible impact on the resis tance which mostly depends on the properties of the diffuse layer (viscosity/density) that is related to Co 2+ concentration.

Influence of the bath parameters

ln order to further investigate the origin of the sudden drop in current density during reduction at -1.20 V (Fig. 1 ), the influence of temperature, IL formulation, and complexation of Co 2+ cations on the CV response were checked. Ali the experiments were conducted on the Ptw electrode if not otherwise specified. The CV curve col lected at 100 °C in 0.12 M Co(TFSl)z in EMlmTFSI (Fig. 3a) is a blank test that will be treated as a reference through the rest of this paper.

Temperature

Fig. 3 represents CV curves of 0.12 M Co(TFSI)z solutions at dif ferent temperatures. At RT (Fig. 3b, black curve), a single peak [START_REF] Aal | lnterfacial electrochemistry and electrodeposition from some ionic liquids: In situ scanning tunneling microscopy, plasma electrochem istry, selenium and macroporous materials[END_REF] with huge overpotential ( ~ 1.5 V) is observed as opposed to two at 100 °C. However during the next cycle (Fig. 3b, red curve) an additional reduction peak (Il) appears at slightly higher potentials (onset at -1 V). The intensity of the reduction peak (II) progres sively increases with the number of cycles while the intensity of the reduction peak (1) decreases (Fig. 3b, inset); this suggests that in both reactions the same species, namely the Co 2+ cations, are reduced. The reduction/oxidation process at RT is poorly reversible (33%) as on the reverse sweep, only a small oxidation peak (III) is visible during the first cycle (Fig. 3b, black curve). The reversibility improves upon cycling, due to the appearance of a new reduction peak (II) that is almost 100% reversible (SM, Fig. 3*). On the other hand, at high temperature (T = 150 °c, Fig. 3c ), the same phenomena (onset of 2 reduction peaks together with a sudden drop in the current density) are observed alike at 100 °c. The minimum of the current density drop is slightly shifted to higher potentials (-1.18 V).

IL nature

One of the advantages of ILs is the possibility of playing with their formulation by substituting either their cation or anion, and hence changing greatly their physical and chemical properties and their interactions with cobalt cations and the substrate.

First, substitution of TFSI-with Tm-was investigated. Once the Co(TFSI)z is added to the TfQ-based IL, its color changes to blue meaning that the coordination is no longer octahe dral ([Co(TFSihJ-) as in the case of EMimTFSI, but tetrahedral ([Co(Tf0) 4 J 2 -) as deduced from the collected UV-VIS spectra (SM, Fig. 4*). Fig. 4a shows the CV curve of 0.12 M Co(TFSI) 2 solution in EMimTfO IL at 100 °c. Although the shape of the peaks as well as their onsets change significantly (the reduction of Co 2+ starts at ---------------------� -2 -1

� 5 4+---'-'--------------------� - higher potentials), the two reduction peaks separated by a drop in current density are still visible, as well as the reduction peak on the oxidation sweep (SM, Fig. 5*).

The influence of imidazolium ring back bone was checked by substituting the EMlm + cation with OMlm + . The successive CV curves of 0.12 M Co(TFSlh on the Ptw at 100 °C in OMlmTFSI IL are reported on Fig. 4b. On the first scan (black curve), a small redue tian peak (I) starting at -0.89 V and centered at -1.12 V followed by a huge reduction peak (Il) with the onset at-1.30 V and centered at -1.62 V are observed; bath features were assigned to the reduction of Co 2+ . ln the next scans, the intensity of bath the reduction peak (1) and the oxidation peak (Ill) increase. That is very similar to the CVs obtained in 0.12 M Co(TFSlh in EMlmTFSI at RT (Fig. 3b ); never theless, in the case of OMlmTFSI at 100 °c the intensity of the peak at -1.60 V (Il) does not change upon cycling. At higher temperature ( 150 °C), the intensity of the first peak is greatly enhanced in com parison to the second one (Fig. 4c). Additionally, a very clear drop in current density is visible, reaching again the minimum at -1.20 V. Similarly, a reduction peak on the oxidation sweep is observed as well (more clear when the potential is reversed at -1.20 V, (SM, Fig. 6*). Lastly, the IL cation was replaced by BMP + and tBMW. ln bath cases, two reduction peaks are visible (Fig. Sa andb, respectively); however, the drop in current density is not so significant as for the imidazolium based Ils described above (the density current at the minimum of the drop is very high as compared to blank test, Fig. 3a 10+---�---�-�-��-�------ even at high temperatures (SM, Fig. 7*). Additionally, the minimum of the current density drop is shifted to lower potentials, -1.34 V for BMP + and -1.35 V for tBMW. So overall, it turns out that the nature of the cation solely influ ences both the current drop during reduction and the reduction peak on the reverse sweep.

Complex charge

As a last part of our survey, the influence of the charge of the cobalt complex was investigated, which is negatively charged in either TFSI-or nobased IL. To assess such a parameter, a neutral complexing agent thiourea (known to produce positive [Co{TU) 4 ] 2+ complexes) was added to a 0.12 M Co{TFSl) i solution in EMimTFSI IL [START_REF] Fukui | The effect of organic additives in electrode position of Co from an amide-type ionic Iiquid[END_REF]. Upon addition of TU, the solution rapidly turns blue, mean ing that the complexation changed from octahedral [Co{TFSlhJ to tetrahedral [Co{TU) 4 ] 2+ . Fig. 6 displays the CV curves on the Ptp electrode of EMimTFSI solution containing 0.48 M TU (black curve) and 0.48 TU +0.12 M Co{TFSl) i (red curve). The same Ptp electrode with constant surface area was used in both cases to directly com pare the intensities. The stability window for the TU, which ranges from -1.0 to 0.8 V, can be widened upon the addition of metal cations. Lastly, the onset of TU oxidation shifts to 1 V (SM, Fig. 8*) while its reduction is highly hindered (Fig. 6). Indeed since the cur rent profile indicates steady state diffusion (Co 2 + reduction) while the solution with only TU reveals a continuous increase in current density. Reduction of the Co 2+ starts at relatively high potential E = -0.5 V and is followed by a single reduction wave without any current drop and reduction peak on the reverse scan, unlike previously described systems. Following the oxidation sweep, an additional oxidation wave is observed starting at 0.41 V (apart from Co strip ping process at 0.3 V), however, for the sake of conciseness, it will not be discussed further. We can thus conclude from the afore mentioned results that the charge of the complex is also one of the crucial parameters responsible for the unusual behavior observed during the reduction/oxidation sweeps.

Impedance spectroscopy: Nyquist and Cole-Cole plots

The dynamic aspect of the system was tested by potentio electrochemical impedance spectroscopy at different potentials (-0.80; -1.00; -1.16; -1.20; -1.26; -1.50 V). For each experiment, a newly prepared electrode was used. Ali the impedance spectra were collected after the steady state was reached at each applied potential. Fig. 7a reports the Nyquist impedance curves of 0.12 M Co{TFSI) i solution at 100 °c on the Ptp electrode. At -0.80 V (full black squares), a semicircle ( 42 !J-cm 2 of diameter) is observed and can be assigned to parallel association of the charge trans fer resistance and the double layer capacitance. As the potential decreases (-1,00 V), a sharp increase of the imaginary part, proba bly related to diffusion limitation, is noticed. At -1.14 V (green full triangles) and -1.20 V (blue full triangles) for lower frequencies, a second semicircle appears with the opposite sign of curvature, crossing the negative part of the real axis at finite frequencies (neg ative impedance). This is in agreement with our CV curve, where in between -1.1 and -1.2 V a negative slope on the voltammo gram is observed (Fig. la) leading to negative value of polarization resistance. The negative impedance further disappears at Iower potentials (-1.26 and -1.50 V, brown empty circle and purple empty square, respectively) at the expense of a half semicircle (however a Warburg impedance cannot be excluded).

The above mentioned negative impedance is a part of wider con sideration on the stability nature of the system. If the intersection of the impedance spectra happened in the origin of the complex impedance plane, a Hopf-bifurcation (since the crossing happens at finite frequencies) would appear leading to an oscillatory current profile at fixed potential [START_REF] Koper | Non-linear phenomena in electrochemical systems[END_REF]. The shift of the intersection can be done by either applying a sufficient external resistance or changing the deposition parameters. To our surprise, a very clear oscilla tion current could be obtained at fixed potential of -1.16 V without any additional resistance (SM, Fig. 9*). In our opinion, the freshly deposited Co can act as the additional resistance, so that the oscil lation will be triggered by the thickness and the morphology of the deposit. Whatever the origin of the Hopf-bifurcation, the existence of an oscillation indicates an alternative transition between two antagonistic metastable states having highly different kinetics.

The representation of the same impedance data in a com plex capacitance type curve brings some additional information (Fig. 7b ). At -1.26 and -1.00 V, only a single semicircle assigned to the charging of a double layer is present. This contrasts with the -----�----... -----�----�----��---• data collected at -1.14 and -1.20 V which present two semi ci rel es, indicative of capacitive phenomena with fast and slow kinetics. Similar results have already been observed in the case of BMPFAP IL, in which the second semicircle was ascribed to charge flow caused by the surface reconstruction (herringbone structures) [START_REF] Atkin | An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(l 11 )interface: potential depend ent solvation layers and the herringbone reconstruction[END_REF][START_REF] Drüschler | New insights into the interface between a single-crystalline metal electrode and an extremely pure ionic liquid: slow interfacial processes and the influence of temperature on interfacial dynamics[END_REF]. At this juncture, it is worth mentioning that in pure EMimTFSI IL at 100 °C, the negative impedance and two capacitive semi ci rel es were not observed in the wide potential range of its stability.

Potentiostatic e/ectrodeposition

Having completed the fundamental studies on Co 2+ reduc tion, we would like now to check the influence of the reported phenomena on the synthesis of Co deposits. Potentiostatic depo sition at three different potentials (-0.95; -1.20 and -1.40V) for 10 min were performed on a Pt dise (Fig. 8a). The morphologies of obtained deposits were checked by SEM (Fig. 8b,c). The j-t curves obtained at -0.95 and -1.40V almost overlap from the beginning; divergence begins after 125 s, Ieading to different final current den sity values of j = 2.0 mAcm-2 andj = 2.6 mAcm-2 , respectively. The curve obtained at -1.20 V shows significantly smaller current den sity (0.95 mAcm-2 at the end of the deposition), in agreement with the CV curve, where at this potential a minimum of the current drop was reached. For the deposits taking place at -1.40 and -0.95 V, the j-t data reveal valleys during the first 25 s of deposition (Fig. 8 inset), which we believe to be associated to a 3D nucleation pro cess with diffusion controlled growth [START_REF] Smith | Time Resolved in Situ Liquid Atomic Force Microscopy and Simultaneous Acoustic Impedance Electrochem ical Quartz Crystal Microbalance Measurements: A Study of Zn Deposition[END_REF]. However in the case of the process at -1.20 V, the current decreases drastically from the first seconds. The Elovich equation, commonly used to describe the chemisorption-like phenomena, has been used to fit the first thirty seconds of current profile obtained at -1.20 V ( equation 7) [START_REF] Wu | Characteristics ofElovich equation used for the analysis of adsorption kinetics in dye-chitosan systems[END_REF].

-�; = a • exp (b • Q) [START_REF] Su | The Electrode/Ionie Liquid Interface: Electric Double Layer and Meta! Electrodeposition[END_REF] where a, b are fitting parameters and Qis the coverage ratio. Taking into account that Qo = 0 at t 0 = 0 and Q = Qn at t = tn (for n>0), the integrated form of equation 7 is:

Qn = a' + b' • ( c + tn) (8)
where a', b' and c are the new fitting parameters. We have assumed that the current is driven by the coverage ratio of an "adsorption like" phenomenon, so that the concentration of cobalt species in the vicinity of the electrode is constant (the current is not limited by the diffusion phenomenon, which is justified since the deposi tion current at -1.20 Vis much smaller than at -0.95 V). The current equation can be written as:

i = -d d Q dt (9)
where d is the proportionality constant. So the derived equation takes the form:

i = B:t (10)
where A and B are the new fitting parameters. The brown triangle curve on the inset (Fig. Sa) represents the fitting with equation 1 O. A considerable difference between fit and experimental data is observed; the fit curve tends toj = 0 as t ➔ ex, while the experimental data tends to lil = li oo I that is higher than O. The reason for such a difference most likely lies in the fact that the Elovich equation solely describes pure adsorption phenomena. Therefore, we found that a decent fit (R 2 = 0.991) could be achieved (Fig. Sa, inset -blue circles) by inserting an additional parameter i o: to equation 10, representing the non-zero current also called the leakage current. Thus using equation ( 11), we could nicely simulate for short deposition times, the experimental results with the parameters as follows: A= -1.90 mC.cm-2 , B=-2.46 s andj 00 =-0.71 mA-cm-2 .

. A . ( )

J = B + t + loo 11
Such a correlation suggests that the blocking of the reduction originates from drastic changes in the initial native double layer. For times greater than 30 s, the fitting is no longer valid since an increase in current density is observed, unlike the Elovich equation prediction. This is indicative of the perturbation of the "adsorption" phenomenon by the morphology of the deposited film.

The obtained deposits at -0.95 and -1.20 V were shiny and very adhesive, unlike the one at -1 .40 V which detached once the sub strate was removed from the solution. In bath cases, they consist of a thin layer made of parti cl es ranging from 20 to 100 nm on top of which we could visualize largely dispersed 500 nm to 1 µm size par ti cl es (Fig. Sb,c). Lastly, the cracks/scratches observed through our deposits are simply coming from the morphology of the substrate.

Discussion

We have reported that the reduction mechanism of Co 2+ in ionic Iiquid media is rather complex, dependent on many parameters and displaying unusual behavior such as: i) sudden drop in reduction current density at -1.2 V, ii) reduction peak on the oxidation sweep, iii) current profile following the Elovich law, and iv) existence of tricky charge motions as deduced from the negative impedance and the delicate balance between two capacitive semicircles.

First of ail, we should recall that the double layer of pristine Ionie liquids in contact with a metal surface is highly structured, especially under additional polarization (Graph la, b) where the oscillatory like concentration profiles of cations and anions were observed experimentally and theoretically [START_REF] Endres | The interface ionic liquid(s )/electrode(s ): ln situ SIM and AFM measurements[END_REF][START_REF] Merlet | On the molecular origin of supercapacitance in nanoporous carbon electrodes[END_REF]161. Now, a legit imate question deals with the localization of the cobalt species depending upon concentration, a pp lied potential and so on. We will consider two different scenarios enlisting 1) an uniform distribu tion of the [Co(TFSih 1-complex throughout the double layer, hence disturbing its structure (diffusion of [Co(TFSl)JJ-complex with out any obstacle) -Graph 1 c, 2) the exclusion of the [ Co(TFSih J complex within the compact layer of the double layer structure ( dif fusion of [Co(TFSihJ-occurs through the compact layer) -Graph 1 d. Next, we are trying to decide which of these proposed scenar ios is the most likely to be in agreement with our experiments and existing literature so as to better understand the electrochemical electrodeposition of Co in ionic liquid media, and further try to extend it to other systems.

From the EQCM results, we have a straightforward evidence for an "accumulation" of [Co(TFSlh J-complex during the current den sity drop and further its spontaneous consumption on the reverse sweep (leading to very sharp reduction peak). This is in agree ment with our IS spectra that showed the negative impedance in the potential range of sud den current density drop (-1.1 /-1.2 V), correlated with the negative slope on the polarization curve. Such behavior is related to the inhibition of the electrochemical process at the electrode as it was already investigated by other groups in aqueous solution[37I and more recently in IL[37,38I. The inhibition may be caused by many different phenomena such as: desorption of electrochemically active species, creation of passive layer, adsorp tion of surfactant or changes in double layer structuration. Further, the appearance of an oscillating current profile at fixed potential (-1.16 V) suggests an alternative transition between two antago nistic metastable states having highly different kinetics [32,37-39I. Additionally, as investigated by impedance spectroscopy during the current drop phenomenon, a second capacitive semicircle occurred (at low frequencies) that could be assigned to slow surface recon struction, as for example formation of herringbone structures, although those structures were found only at single crystal oriented electrodes[33,34I. Furthermore, chemisorption-like phenomena are consistent with the current profile of potentiostatic deposition at -1.20V, which could be well fitted with the modified Elovich equation. Ali these results suggest that strong interactions between the electrode and the electrolyte occur, which is believed to be linked to the creation of a reversible energetic barrier related to the reconstruction of the double layer upon a pp lied potential.

We also found that the nature of the Ionie Liquid cations as well as the charge of the cobalt complex strongly affect the exist ence and amplitude of the current density drop. More specifically, the current density drop is very clear when either the cation of IL is based on the ring structure with delocalized electrons (imid azolium based cation) or that the cobalt complex is negatively charged. To the extreme, this drop weakens or even disappears for IL based either on pyrrolidinium or ammonium cations and when positively charged cobalt complex is present. As those two parameters are the major ones influencing on the structure of the double layer, it is becoming doubtful that the scenario 1 (Graph le) does occur; indeed the changes in double layer would not affect the state of the [ Co(TFSih 1-complex. However it would have tremen dous impact in scenario 2. With an increase in negative polarization ( decrease in potential ), the concentration of anions in the first layer of the double layer decreases, because of the electrostatic repul sion. As a consequence, the created cations layer moves towards the electrode surface ( thinning of the layer) and finally the double layer gets more structured ( e.g. higher number of layers) and com pact (Graph 1 b). This leads to an increase in bath the high potential damping and the viscosity nearby the interface. Those changes can highly affect the electrochemical process taking place at the elec trode by disturbing bath [Co(TFSl)JJ-diffusion and charge transfer in scenario 2. Additionally, because of its negative charge, the cobalt complex will sit on the second layer (negative charge layer); thus a first cation layer stands as a barrier hindering cobalt from being reduced. Once the charge of the cobalt complex is changed to posi tive, thanks to the migration flux, it can be directly incorporated in the first layer resulting in the Jack of current drop. Overall, taking into account ail the above statements, we believe that the ener getic barrier at -1.20V concerns the restructuring of the double layer under applied potential. Now, it remains to explain why the aforementioned barrier is clearer in imidazolium based ionic liquids. Baldelli et al. [START_REF] Baldelli | Surface Structure at the Ionie Liquid-Electrified Meta! Interface[END_REF] found that the imidazolium ring changes the conformation from perpendicular to cofacial versus the substrate surface as the neg ative polarization increases, due to its delocalized positive charge. Consequently, it produces more compact layer (as illustrated on Graph 1 b ), highly inhibiting the reaction rate. However, this still does not explain the increase in current below -1.2 V since accord ing to this theory, the current should drop continuously and not exhibit a minimum as it is visible on the CVs curve. The actual charge transfer reaction plane (place where the electron exchange hap pens) is not localized at the surface of the electrode but at a distance from it (on the magnitude of few angstroms, Graph la, b) [START_REF] Aa Franco | A Dynamic Mechanistic Mode! of an Electrochemical Interface[END_REF]. As mentioned above with an increase in negative polarization, the first cation layer becomes thinner and closer to the substrate. Addition ally, in case of imidazolium cation based IL metal-pi interactions can occur so that finally the first cation layer becomes conductive and further reduction of [Co(TFSlhJ-complex positioned on sec ond layer, can take place. This phenomenon could explain the very Iow adhesion of the deposit obtained at -1.40 V, as a layer of IL cations remains between the substrate and deposit. It would also justify the decrease in oxidation efficiency when the reverse sweep is chosen below the barrier phenomenon (SI, Fig. 1 *). The mecha nism of the electron transfer at low and high potentials due to the change in double layer is probably very different; more sophisti cated experiments must be performed to probe the validity of this statement. Lastly, it should be noted that in both potential ranges -1.0/-1, 1 V and -1.3/-1.4 V, the reaction limiting parameters are the same since the current values are very similar. On the other hand, if the phenomena are purely caused by surface reconstruction, as in the formation of herringbone structures, the further increase in current would be attributed to the instability of such structures at Iower potentials [START_REF] Atkin | An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(l 11 )interface: potential depend ent solvation layers and the herringbone reconstruction[END_REF]. Another important aspect is the dynamics of this phenomenon. We found that the energetic barrier (minimum of the current drop) shifts to more negative values with an increase in the scanning rate (SM, Fig. 10*), suggesting that it is a rather slow phenomenon (in the range of few seconds). Furthermore, the negative loop observed in impedance spectra is visible at frequencies as low as 0.15 Hz. This is in agreement with previous statements on the origin of the bar rier since the changes in the structure of the double layer, which is governed by ion transfer, do not occur rapidly. This can also justify the difference in CV shapes between RT and 100 °c in EMimTFSI IL. At low temperatures, the barrier can actually be enhanced (strong interactions between ions and the substrate ), while the recons tructions slowed down (high viscosity/low conductivity); so that only one reduction peak appears at very low potentials during the first scan.

It must be recalled that this is not the only system within which the current suddenly drops leading to a split of the reduction waves or/and the reduction peak on oxidative sweeps. When Co 2+ cations are substituted with Mn 2 +, we experienced the same phenomena, though shifted to more negative potentials (Fig. 9b). With Ni 2 + cations, we solely observe a reduction peak on the reverse sweep (Fig. 9c). Finally, for Zn 2+ cations, neither a drop in current nor the reduction on the reverse sweep (although some small valleys are present) are visible (Fig. 9d). However, the barrier phenomenon for Zn 2+ system was clearly visible in Ionie Liquid containing stronger complexing anions EMlmDCA [START_REF] Deng | Dicyanamide anion based ionic liquids for electrodeposition of metals[END_REF], as studied by Forsytch et al. [START_REF] Simons | High current density, efficient cycling of Zn 2 • in 1-ethyl-3-methylimidazolium dicyanamide ionic liquid: The effect ofzn 2 • sait and water concentration[END_REF]. Baldelli et al. [START_REF] Baldelli | Surface Structure at the Ionie Liquid-Electrified Meta! Interface[END_REF] claimed thatthe BMlmdca IL creates a more struc tured double layer than EMlmTFSI (with higher amount of layers that were ascribed to strong coordinating ability of DCA-); addi tionally, the viscosity of this IL (19.8 cp) is much smaller than EMlmTFSI (34 cp), so that this phenomenon could be observed already at RT. ln ail cases, the imidazolium cation and negatively charged metal complex are present, therefore there are still other parameters to be considered. Among them, the complex strength may be an important parameter as the DCA-is a much stronger complexing agent than TFS1-( difference in reduction of Zn both IL). The charge density of the complex (the relative charge taking into account the size of the complex) may also be a signifi cant issue. Other systems such as Mn(TFSih in BMPDCA [START_REF] Deng | Dicyanamide anion based ionic liquids for electrodeposition of metals[END_REF], PdCl 2 in BMlmCl [START_REF] Jayakumar | Electro chemical behavior of ruthenium (Ill), rhodium (Ill) and palladium (II) in 1-butyl-3-methylimidazolium chloride ionic liquid[END_REF] and BMPDCA [START_REF] Huang | Voltammetric behavior of Pd(II) and Ni(II) ions and electrodeposition of PdNi bimetal in N-butyl-N-methylpyrrolidinium dicyanamide ionic liquid[END_REF], NiCl 2 in BMPDCA [START_REF] Huang | Voltammetric behavior of Pd(II) and Ni(II) ions and electrodeposition of PdNi bimetal in N-butyl-N-methylpyrrolidinium dicyanamide ionic liquid[END_REF], or Zn(TfO) in BMPTf0 [START_REF] Liu | Template-assisted electrode position of highly ordered macroporous zinc structures from an ionic liquid[END_REF] also exhibit uncommon CV curves. Sorne authors suggest that the two reduction peaks are due to the existence of 2 different metal complexes. This might be true in systems where the metal sait and IL have different anions; it becomes doubtful in our systems where both anions are the same, namely TFSJ-. Addition ally it is not limited to divalent metallic cations since in a solution of TaF 5 in BMPTSI, EMimTFSI and PMimTFSI ILs similar behavior was observed [START_REF] Ispas | On the electrodeposition of tantalum from three different ionic liquids with the bis(trifluoromethyl sulfonyl) amide anion[END_REF]. Although we suspect that the uncommon shape of the CV curve is due to double layer reconstruction, we still do not know what exactly its origin is or how the different metal cation complexes affect it. More detailed experimental work is needed on this tapie as well as theoretical modeling, both are ongoing.

Conclusions

We have reported detailed studies on the reduction of Co 2+ cations in Ionie Liquid media at elevated temperatures, based on the Electrochemical Quartz Crystal Microbalance, Cyclic Voltammetry, Chronoamperometric, lmpedance Spectroscopy, Scanning Electron Microscopy and UV-Vis Spectroscopy experiments. An uncommon shape of the CV curve during reduction has been spotted with the sudden current density drop reaching its minimum at -1.2 V and well defined reduction peak on the reverse sweep. We think this effect to be rooted in the creation of a reversible energetic bar rier that prevents from Co 2 + reduction. By a precise adjustment of different bath parameters (substrate, IL formulation, Co 2 + com plexation), we found that this phenomenon greatly depends on the nature of the ionic Iiquid cation and the charge of metal complex although other parameters such as the strength of the cation-anion interaction within the metal complex together with the electronic delocalization should be considered as well. Therefore, we believe that this barrier is nested in the reconstruction of the double layer under applied polarization. Such a phenomenon is not specific to the reduction of Co 2 + in EMimTFSI IL since we show herein that it occurs as well for Mn 2 +, Ni 2+ cations while it has already been reported for Zn 2 +, Pb 2 +, and Pd 2 +.

Finally, the paper shows the importance of tuning precisely the bath solution and specifically the ionic liquid formulation to grow deposits having the desired properties (such as good adhesion) which could find applications within the field of batteries, pho tovoltaic and microelectronics. We hope this better understanding of the double layer structuring, when working with ionic liquids, will help the community in harvesting ail the advantages that ionic liquid media present.

  and oxidation sweeps {Fig. 1 I, II, III, IV , V, insets). Follow ing the reduction sweep on the Ptp electrode (Fig. la, black curve), the first reduction peak with an onset at -0.70V and centered at -0.88 V (I) is ascribed to Co 2+ reduction.

  observed for the other substrates with slight shifts in onset and peak potentials ( Fig.1 b, c).

Fig

  Fig. t. CV's curves of 0.12 M Co(TFSl)2 solution in EMlmTFSI at 100 °C, on: a) Ptp, b) GC. c) SSw electrode. The black curve presents the reduction sweep while the red oxidation sweep. CE-Pt, RE-Ag/Ag2 0, scanning rate 5 mv.s-1 . lnset: magnification of negative potential part.

4Fig. 2 .

 2 Fig. 2. EQCM measurement a) current versus potential, b) quartz frequency change versus potential. c) quartz motional resistance versus potential, d) calculated M/z value for the reduction sweep-green curve and oxidation sweep-gray curve, the black dashed line indicates the theoretical M/z value of Co formation. Blue dashed lines divide the CV curve into regions where different phenomena occur. 0.12 M Co(TFSlh, T= 100 °C, WE-Pt mirror polished on quartz, CE-Pt, RE-Ag/Ag2 0, scan rate 2 mv.s-1 .

1 :Fig. 3 .

 13 Fig. 3. CV's curves of0.12 M Co(TFSI)i solution in EMimTFSI at different tempera tures a) blank test at 100 °( b) 2 successive scans at RT, inset represents first and ninth cycle, c) 150 °C. WE-Ptw, CE-Pt, RE-Ag/Ag20, scanning rate 5 mv.s-1 .

Fig. 4 .

 4 Fig.4. CV of0.12 M Co(TFSI)i solution: a) in EMimTfO at 100 °(, b) in OMimTFSI at 100 °( 3 successive scans, c) in OMlmTFSI at 150 °C. WE-Ptw, CE-Pt, RE-Ag/Ag2 O, scanning rates mv-s-1 .

Fig. 5 .

 5 Fig. 5. CV of0.12M Co(TFSI)i solution in a) BMPTFSI at 100 °C and b) tBMNTFSI at 100 °C. WE-Ptw, CE-Pt, RE-Ag/Ag2 O, scanning rate 5 mv-s-1 •

Fig. 6 .

 6 Fig. 6. CV curves ofEMimTFSI solution containing 0.48 M TU -black curve and 0.12 M Co(TFSI)i/0.48 M TU-red curve, at 100 'C on Ptp electrode. CE-Pt, RE-Ag/Ag2 O, scan ning rates mv-s-1 .

6 . 10 Real Capacitance / mF•cm- 2 Fig. 7 .

 61027 Fig. 7. lmpedance spectroscopy of0.12 M Co(TFSl)2 solution at 100 °C at different potentials a) Nyquist plots: -0.SV black filled squares, -1.00V red filled circles, -1.14 V green filled triangles, -1.20 V blue filled tilted triangles, -1.26 V brown open circles, -1.S0V purple open squares. b) Complex capacitance curves: -1.00V red fi lied ci rel es, -1.14 V green fi lied triangles, -1.20 V blue filled tilted triangles, -1.26 V brown open circles. The inset represents zoom of the curves at very high frequencies. WE-Ptp, CE-Pt, RE-Ag/ Ag20.

- 30 .

 30 .......

Fig. 8 .

 8 Fig. 8. a) Chronoamperograms of deposition processes obtained at 3 different potentials: -0.95 V-black curve, -1.20 V-red curve, -1.40 V-green curve, for 10 minutes. lnset represents first 30 seconds of Chronoamperograms, the brown triangles and blue circles represent fitting curves using Elovich and modified Elovich equations, respectively. 0.12 M Co(TFSI)i, T= 100 °C, WE-Pt dise, CE-Pt, RE-Ag/ Ag2 O. b,c-SEM images of deposits obtained at-0.95 V and -1.20V respectively.

Graph 1 .

 1 Draft of the double layer, top part in pure EMlmTFSI IL at a) OCP and b)under negative polarization, bottom part-2 possibilities of double layer in EMlmTFSI containing (Co(TFSI), J-complex under negative polarization c) totally changing the structure of double layer, d) being incorporated in the pristine structure of double layer.

Fig. 9 .

 9 Fig. 9. CV curves of EMlmTFSI solution at 100 °C containing: a) 0.12 M Co(TFSlh, b) 0.12 M Mn(TFSlh, c) 0.12 M Ni(TFSl)i , d) 0.12 M Zn(TFSl)i. WE-Ptw, CE-Pt, RE-Ag/Ag2 0, scanning rate 5 mV-s-1 .

Table 1

 1

Acknowledgment

The authors thank P. Simon for fruitful discussions as well as to ALISTORE European Research lnstitute for the financial support to conduct the research.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta. 2014.03.042.