Laura Thiebaud

Sophie Legeai
email: sophie.legeai@univ-lorraine.fr

Jaafar Ghanbaja

Nicolas Stein

Electrodeposition of high aspect ratio single crystalline tellurium nanowires from piperidinium-based ionic liquid

Keywords: Electrodeposition, Nanowires, Tellurium, Ionic liquid, Template-free High Resolution Transmission Electron Microscopy Tellurium, Nanowires, Ionic liquid, Electrodeposition

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

Introduction

Tellurium (Te) compounds can be used in thermoelectric devices, where they convert heat into electricity and conversely. When the diameter of a material is constrained to the nanometer scale and its length is several orders of magnitude larger, the material can be considered as a one-dimensional nanostructure and the thermoelectric conversion efficiency can be significantly improved [START_REF] Hicks | EFFECT OF QUANTUM-WELL STRUCTURES ON THE THERMOELECTRIC FIGURE OF MERIT[END_REF][START_REF] Boukai | Silicon nanowires as efficient thermoelectric materials[END_REF][START_REF] Hochbaum | Enhanced thermoelectric performance of rough silicon nanowires[END_REF][START_REF] Caballero-Calero | Thermoelectric nanowires: A brief prospective[END_REF]. Indeed, the decrease of one dimension to the characteristic length scales of the material like the exciton Bohr radius and the phonon mean free path allows increasing the Seebeck coefficient and reducing the lattice thermal conductivity by phonon surface scattering respectively.

Electrodeposition of nanowires usually needs the use of a porous template [START_REF] Xiao | Recent progress in electrodeposition of thermoelectric thin films and nanostructures[END_REF][START_REF] Mohanty | Electrodeposition: a versatile and inexpensive tool for the synthesis of nanoparticles, nanorods, nanowires, and nanoclusters of metals[END_REF]. The growth takes places in the template pores whose geometries allow to control the shape and the size of nanowires. However, template-assisted electrodeposition involves multiples fabrication steps, like the preparation of the template and its removal. A single step template-free electrodeposition would be more convenient and easier for a further use of the nanowires. It has been shown in literature that ionic liquids, in addition to have high electrochemical and thermal stabilities, can act as capping agents allowing the formation of nanostructures without using a template [START_REF] Yang | Single-step large-scale and template-free electrochemical growth of Ni-Zn alloy filament arrays from a zinc chloride based ionic liquid[END_REF][START_REF] Steichen | Synthesis of trigonal selenium nanorods by electrodeposition from an ionic liquid at high temperature[END_REF][START_REF] Hsieh | Speciation of cobalt-chloride-based ionic liquids and electrodeposition of Co wires[END_REF][START_REF] Absalan | Two-approach study for preparing stable colloidal gold nanoparticles in organic solvents by using 1-dodecy1-3methylimidazolium bromide as an efficient capping and phase transfer agent[END_REF]. Very few works deal with Te nanowires electrodeposition in roomtemperature ionic liquids (RTILs). Only Al-Salman [START_REF] Al-Salman | Template-Free Electrochemical Synthesis of High Aspect Ratio Sn Nanowires in Ionic Liquids: A General Route to Large-Area Metal and Semimetal Nanowire Arrays?[END_REF] and co-workers have recently synthesized long polycrystalline Te nanowires of several micrometers in length with low diameters ranging between 25 and 35 nm in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid containing SiCl4. The authors observed nanostructuring of Te deposits only in the presence of SiCl4 and concluded that this salt acts as a capping agent that promotes the growth of Te nanowires. However, the use of SiCl4 leads to the presence of an amorphous passivating layer on the nanowires surface which could affect the electronic properties of the material.

In a previous work [START_REF] Szymczak | Template-free electrodeposition of tellurium nanostructures in a room-temperature ionic liquid[END_REF], we showed that free standing single crystalline Te nanowires can be obtained in a piperidinium-based ionic liquids mixture: EOPipTFSI:EOPipBr 95:5 (mol:mol) (1-ethyl-1-octylpiperidinium cation, bis(trifluoromethylsulfonyl)imide and bromide anions).

More recently [START_REF] Thiebaud | Tuning the morphology of Te one-dimensional nanostructures by template-free electrochemical deposition in an ionic liquid[END_REF], we showed that the shape and the size of the deposited nanostructures can be controlled by varying some experimental parameters that influence the deposition kinetics, namely applied potential and Te IV concentration. Using low concentration conditions and under diffusional control, we obtained self-standing nanowires with a mean length of 1.72 ± 0.15 µm and a mean diameter of 132 ± 13 nm leading to a moderate aspect ratio of 13.2 ± 2.4, which turn into hollow nanostructures in high overpotential conditions due to an insufficient species supply at the electrode.

In this work, the synthesis of high aspect ratio Te nanowires was obtained by varying the halide content in the electrolyte. We first present a voltammetric study of the influence of EOPipBr content on the electrochemical system Te IV /Te 0 . It appears that in pure EOPipTFSI or for very low amount of EOPipBr, a complex voltammetric behavior is observed. Several reduction signals appear before bulk deposition, all corresponding to the reduction of Te IV in Te 0 . The influence of EOPipBr content on the morphology of nanostructures deposited at bulk deposition potential is then presented as well as the influence of applied potential for low EOPipBr content.

Experimental

TeCl4 (99mol%, Alfa Aesar) was used as Te IV precursor. EOPipTFSI and EOPipBr were synthesized according to [START_REF] Triolo | Nanoscale organization in piperidinium-based room temperature ionic liquids[END_REF][START_REF] Traore | New insight into indium electrochemistry in a Tf2N-based room-temperature ionic liquid[END_REF], characterized by nuclear magnetic resonance analysis (Bruker Spectrospin 300 MHz) and stored in an argon filled glove box (MBraunLabstar; O2/H2O levels < 1 ppm). As the viscosity is high at room temperature, ranging from 400 to 1400 mPa.s depending on EOPipBr content, but strongly decreases with the temperature to reach 15 to 20 mPa.s, the experiments were performed at 100°C [START_REF] Szymczak | Electrodeposition of stoichiometric bismuth telluride Bi2Te3 using a piperidinium ionic liquid binary mixture[END_REF]. Moreover, in our previous studies [START_REF] Szymczak | Template-free electrodeposition of tellurium nanostructures in a room-temperature ionic liquid[END_REF], we showed that a better crystallinity of the deposits is obtained at this temperature.

Electrochemical experiments were performed in a glove box using a Biologic potentiostat (VSP300). A platinum (Pt) disk (Metrohm) with a diameter of 3 mm and a Pt wire (Radiometer) were used respectively as working and counter electrodes along with an Ag I /Ag 0 reference electrode (RE) for voltammetric studies. This reference electrode consists in a 1 mm Ag wire (Alfa Aesar) immersed in a 10 mM Ag(CF3SO3) solution in BMICF3SO3

(1-butyl-1-methylimidazolium trifluoromethanesulfonate) ionic liquid. The reference electrode (RE) is placed in a glass capillary which is separated from the electrochemical cell by a glass frit. Voltammetric curves were obtained by scanning the potential from the open circuit potential towards the negative direction. Platinum-coated glass slides (25×10×1 mm, Pt coating thickness 300 nm), supplied by Applications Couches Minces (ACM), were used both as working and counter electrodes for electrodeposition experiments. A Pt wire quasi reference electrode (QRE) was used for potentiostatic deposition, instead of the Ag I /Ag 0 RE, to avoid Ag I species diffusion towards the bulk solution that could lead to Ag 0 co-deposition.

A voltammetric curve was systematically recorded with the Pt QRE before each deposition experiment. The volume of the bath was equal to 5 mL leading to an immersed slide surface of 0.5 cm 2 . Deposits were thoroughly rinsed three times with acetone and dried under air before analysis. oxidation, as already reported by Yamagata et al. in a 1-butyl-1-methylpyrrolidinium ionic liquid [START_REF] Yamagata | Electrochemical behavior of several iron complexes in hydrophobic room-temperature ionic liquids[END_REF]. The oxidation of Br -occurs in two steps, probably corresponding to the following reactions, as observed in other aprotic media [START_REF] Iwasita | Kinetics of the bromine-tribromide-bromide redox processes on platinum electrodes in acetonitrile solutions[END_REF][START_REF] Popov | Studies on the chemistry of halogen and of polyhalides. XVI. Voltammetry of bromine and interhalogen species in acetonitrilr[END_REF]:

The morphology of the deposits was analyzed by Scanning Electron

3 Br - Br3 -+ 2 e -(Eq 1) 2 Br3 - 3 Br2 + 2 e -(Eq 2)
and occurring respectively for E > 0.30 V and E > 0.71 V vs Ag I /Ag, respectively.

The influence of EOPipBr content (up to 80:20 (mol:mol)) on the electrochemical behavior of tellurium was studied at a Te IV concentration of 5 mM corresponding to the solubility of TeCl4 at 100°C in pure EOPipTFSI without EOPipBr. Moreover, we have shown in a previous study that the lower the Te IV concentration, the higher the aspect ratio [START_REF] Thiebaud | Tuning the morphology of Te one-dimensional nanostructures by template-free electrochemical deposition in an ionic liquid[END_REF]. The corresponding voltammograms are presented in Figure 2.

One can observe that the open circuit potential shift to more positive values as the EOPipBr content decreases. As the OCP lies between the reduction of Te(IV) species and the oxidation of Br -ions, this could be interpreted as a change in Te(IV) speciation, namely the formation Te(IV) halogenated-Te(IV) complexes of lower stability as EOPipBr content decreases. This hypothesis is reinforced by the differences between the voltammetric curves presented in Figure 2. For an EOPipBr content ≥ 0.5 mol%, the shape of the voltammograms is similar and presents two reduction peaks : C1 and C2 which correspond respectively to the reduction of Te IV into Te 0 and of Te 0 into Te -II according to the literature [START_REF] Jeng | Electrochemistry of tellurium(IV) in the basic aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt[END_REF][START_REF] Tsai | Voltammetric study and electrodeposition of tellurium, lead, and lead telluride in room-temperature ionic liquid 1-ethyl-3methylimidazolium tetrafluoroborate[END_REF]. This latter signal represents the limit for the deposition of Te 0 films. The C1 peak shifts to more negative values as the halide content increases (from -0.58 V to -0.67 V vs Ag I /Ag 0 ).

For EOPipBr ≤ 0.2 mol%, the reduction behavior of Te IV is clearly different. In the 0.2 mol% EOPipBr solution, a second reduction signal appears at -0.15 V vs Ag I /Ag 0 (C1'). In the solution free of EOPipBr, namely in pure EOPipTFSI ionic liquid, we can observe two additional reduction peaks at 0.18 V (C1'') and -0.18 V (C1') vs Ag I /Ag 0 . The presence of these peaks at more positive potentials values is responsible for the positive shift of the OCP. XRD analyses of deposits obtained at C1' and C1 '' peak potential values confirm that Te 0 was obtained in both cases proving that these signals correspond to the reduction of Te IV in Te 0 . These multiple signals could be related to the speciation of Te IV that can exist under several TeXy 4-y forms in halide containing solutions, the value of y depending on the halide content and TeX6 2-being the complex supposed to be predominant in halide-rich media [START_REF] Jeng | Electrochemistry of tellurium(IV) in the basic aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt[END_REF]. In order to support this hypothesis, the Table 1 presents the ratio values of X -to Te IV concentrations in the different tested ionic liquid mixtures.

Only one signal is observed in high EOPipBr content media (≥ 0.5 mol%), in which Te IV probably exists as TeX6 2-and we can assume that the following reaction occurs [START_REF] Jeng | Electrochemistry of tellurium(IV) in the basic aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt[END_REF]: TeX6 2-+ 4 e - Te 0 + 6 X - (Eq 3)

In pure EOPipTFSI or for low halide contents, Te IV probably exists under lower halidecoordinated complexes that are reduced at less negative potential values than TeX6 2-. The presence of several signals could be due to the co-existence of several species in the ionic liquid, as supposed by Saha et al. [START_REF] Saha | Electrodeposition of Selenium in a Hydrophobic Room-Temperature Ionic Liquid[END_REF] in the case of Se. Another hypothesis can be made, that was proposed by several authors that already observed this kind of behavior using halide salts in TFSI-based ionic liquids (In [START_REF] Boukai | Silicon nanowires as efficient thermoelectric materials[END_REF], Ta [START_REF] Borisenko | In situ STM and EQCM studies of tantalum electrodeposition from TaF5 in the air-and waterstable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide[END_REF] and Sb [START_REF] Hsieh | Sun, 1-Butyl-1-Methylpyrrolidinium Dicyanamide Room Temperature Ionic Liquid for Electrodeposition of Antimony[END_REF] electrochemical studies). The formation of the Te 0 deposit by halide-complexes reduction generates halide ions (Eq 3). In these poorhalide media, this leads to a significant increase of halide ions concentration in the vicinity of the electrode. Considering that ionic liquids are viscous media in which the mass transport is very slow, this enrichment could be responsible of the multiple signals observed, due to the formation of higher halide complexes that would be reduced at more negative potential values (C1' and C1).The C1 peak would correspond to the reduction of TeX6 2-regardless of halide content, according to its potential value. These hypotheses concerning Te(IV) speciation will be checked in further work by Raman analyses that allow to determine the speciation of Te(IV) in the electrolyte (ref Raman milieu aq 1967 TeBrCl)

One can also observe on Figure 1 a hysteresis that appears as the bromide content decreases, due to a higher capacitive current. This can be highlighted by varying the scan rate v (not shown here) : as the EOPipBr content decreases, the slope of log (v) = f(log i) deviates from 0.5 and increases, indicating a surface-controlled current. This is probably due to a difference in the double layer structure as the composition of the electrolyte changes. Endres and coworkers have shown by STM analysis that the anions of ionic liquids can be strongly adsorbed at the electrode, and so the nature of the anion can play a role in the double layer structure [START_REF] Carstens | Combined STM, AFM, and DFT Study of the Highly Ordered Pyrolytic Graphite/1-Octyl-3-methyl-imidazolium Bis(trifluoromethylsulfonyl)imide Interface[END_REF].

3.2.Influence of EOPipBr content on the morphology of Te deposits

Te electrodeposition was first studied by applying a potential value corresponding to the most negative peak (C1) in pure EOPipTFSI and for the different EOPipBr content (mol%)

corresponding to the voltammetric study presented in Figure 2. The surface of obtained deposits is entirely covered of nanowires whatever the electrolyte composition. All the deposits SEM images are then not shown here, and the Figure 3 only presents images of the deposits obtained in pure EOPipTFSI and for three EOPipBr contents.

One can observe in Figure 3 that the nanowires obtained in pure EOPipTFSI are thinner and not faceted compared to those obtained in the presence of EOPipBr. All obtained deposits with EOPipBr content ranging from 0.5 to 20 mol% present the same kind of morphology.

The nanowires present a hexagonal shape characteristic of the crystallographic structure of the elemental Te (space group P3121). All the nanowires are single crystalline and grow along the

[001] direction whatever the electrolyte composition. Figure 4 presents an example of selected TEM image and corresponding SAED diffractogram.

A statistic study of TEM images was carried out on a dozen of nanostructures per image to determine the mean dimensions of nanowires (length and diameter) as a function of EOPipBr content. Corresponding values and calculated aspect ratio are presented in Figure 5.

One can observe on Figure 5 that the diameter of the nanowires increases with EOPipBr content, the thinner nanowires being obtained in pure EOPipTFSI, around 40 ± 20 nm in diameter for a mean length of 2.5 ± 0.5 µm leading to the best aspect ratio of 92 ± 58. The evolution of the nanowire length is more complex. The nanowires obtained in pure EOPipTFSI and for a low EOPipBr content (0.2 mol%) are much longer that those obtained for higher bromide contents. For EOPipBr ≥ 0.5 mol%, the length is lower that for lower contents but increases then with EOPipBr concentration for higher contents. As nanowires grow then in diameter as much as in length, the aspect ratio remains constant around 14 ± 1.

In these overpotential conditions (C1 peak), we can thus distinguish two kind of nanowires:

the longest nanowires obtained in pure EOPipTFSI and at low bromide concentration (0.2 mol%) that reach high aspect ratios (92 and 36, respectively) and the ones obtained with higher EOPipBr content (≥ 0.5 mol%) which are faceted and have an mean aspect ratio of 13.5.

As observed in Figure 2, in the case of low bromide content (0. In pure EOPipTFSI (Figure 6), the deposit obtained at the less negative peak C1'' presents a morphology different from what was previously observed. The substrate is poorly covered of longer and larger nanowires laying on the surface, between 30 and 70 µm in length, and around 194 ± 20 nm in diameter, that tend to agglomerate.

As previously observed on Figure 6, both deposits obtained at C1' are composed of mixed long hair-like nanowires laying down on the surface, but denser and much thinner than those obtained at C1'' peak potential. Without bromide (Figure 7a), the interconnected nanowires present a diameter of 60 ± 30 nm and a mean length of 3.65 ± 0.35 µm. With the addition of a low amount of EOPipBr (Figure 7b), the deposit seems to better cover the surface. It is composed of 70 ± 10 µm long nanowires with a low diameter which reaches 50 ± 20 nm leading to a high aspect ratio of almost 2000. These morphological characteristics are promising for thermoelectric performances that are currently under evaluation.

We have realized TEM (Figures 8a and8b) and HRTEM (Figure 8c) studies of the deposit presented in Figure 7b.

The HRTEM analyses show that the hair-like structures are single crystalline with a smooth surface (Figure 8c). No halide-containing surface layer was observed unlike observed in other works [START_REF] Al-Salman | Template-Free Electrochemical Synthesis of High Aspect Ratio Sn Nanowires in Ionic Liquids: A General Route to Large-Area Metal and Semimetal Nanowire Arrays?[END_REF]. The Te crystallographic structure is highlighted with the image of inverse FFT (Figure 8d). The atomic arrangement is clearly displayed and a good agreement is observed with the (100) orthogonal projection of the hexagonal extended cell of Te. Thus the white spots correspond to the atoms having the same relative coordinates the [001] axis, which are colored in red in the schematic cell. On both sides are placed gray spots, corresponding to atoms in blue with other coordinates. Moreover the interplanar distance d001

was identified and measured equal to 0.59 nm. All these observations show unambiguously that the growth direction is along the c-axis and that these nanowires are single crystalline with no local defects.

Conclusion

We have shown in this study that single crystalline Te nanowires can be obtained in EOPipTFSI ionic liquid whatever the electrolyte composition, namely with or without the addition of halides by the mean of EOPipBr. Self-standing faceted nanowires are obtained for EOPipBr concentration higher than 0.5%. By decreasing the bromide concentration, the speciation of Te IV probably changes and depending on the involved Te IV species, hair-like nanowires can be deposited, that are exempt of surface contaminant. The hair-like nanowires obtained using a low EOPipBr content electrolyte (0.2 mol%) are the most promising for thermoelectric applications with their low diameter and their high aspect ratio.

  Microscopy (SEM, Philips XL30 S FEG) at 5 kV. Transmission Electron Microscopy (TEM) investigations were carried out using a JEM -ARM 200F Cold Field Emission Gun operating at 200 kV (point resolution 0.19 nm in TEM mode) fitted with a GIF Quantum ER. The growth direction of the nanostructures was determined by selected area electron diffraction (SAED). The diffraction spots were indexed according to the JCPDS card No. 36-1452 of the tellurium hexagonal phase. The crystalline structure of the deposits was investigated by X-ray powder diffraction measurements (XRD, Bruker D8Advance) with Cu Kα radiation (λ =1.5406Å) at 40 kV and 40 mA.

3 .Figure 1 .

 31 Figure 1. In the presence of EOPipBr, two anodic signals are observed corresponding to Br -

  2 mol%) and in pure EOPipTFSI, additional reduction signals are observed (C1'' and C1'). Deposits were then realized at each peak potential for each electrolyte composition. Corresponding SEM images are shown in Figure 6 (C1'') and Figure 7 (C1').

Figure 1 :

 1 Figure 1: Electrochemical stability of EOPipTFSI and EOPipTFSI:EOPipBr 95:5 electrolyte.

Figure 2 :

 2 Figure 2: Cathodic behavior of Te IV in EOPipTFSI for several EOPipBr contents (mol%).

Figure 3 :

 3 Figure 3: SEM images of Te nanostructures electrodeposited at C1 peak potential in pure

Figure 4 :

 4 Figure 4: TEM images and SAED pattern of Te nanowires obtained without EOPipBr.

Figure 5 :

 5 Figure 5: Evolution of a) length and diameter and b) aspect ratio of Te nanowires obtained at C1 peak potential according to the EOPipBr concentration (mol%), [Te IV ] = 5 mM.

Figure 6 :

 6 Figure 6 : SEM images of Te nanowires deposited at C1'' peak potential in pure EOPipTFSI. [Te IV ] = 5 mM. T = 100°C, Q = 2 C/cm 2 , Pt-coated glass slide.

Figure 7 :

 7 Figure 7 : SEM images of Te nanostructures deposited at C1' peak potential (a) in pure EOPipTFSI and (b) with a EOPipBr content of 0.2 mol%. [Te IV ] = 5 mM. T = 100°C, Q = 2 C/cm 2 , Pt-coated glass slide.

Figure 8 :

 8 Figure 8 : TEM characterization of Te hair-like nanowires obtained at C1' potential peak in

Table 1 :

 1 2 , Pt-coated glass slide. X -to Te IV concentration ratio according to the EOPipBr content (mol%). [X -] = [Cl -]

	EOPipBr content
		[X -]/[Te IV ]
	(mol%)
	0	4.00
	0.2	4.97
	0.5	6.43
	1	8.88
	2	13.78
	5	28.68
	20	107.00

+ [Br -]


Laura Thiebaud

Sophie Legeai
email: sophie.legeai@univ-lorraine.fr

Jaafar Ghanbaja

Nicolas Stein

Synthesis of

Keywords: Core-shell nanowires, Ionic liquid, Electrodeposition, Bismuth, Tellurium

come

Introduction

Tellurium exhibits outstanding thermoelectric properties and has a high Seebeck coefficient (500 μV.K -1 [START_REF] Bodiul | Thermoelectric properties of films and monocrystalline whiskers of tellurium[END_REF]). However its thermal conductivity is high (≈ 3 W.m -1 .K -1 [START_REF] Goncalves | New Approaches to Thermoelectric Materials[END_REF]), leading to low thermoelectric efficiency. Nanostructuring, especially the formation of nanowires, is an effective way for decreasing the lattice thermal conductivity by increasing the diffusive phonon scattering [START_REF] Caballero-Calero | Thermoelectric nanowires: A brief prospective[END_REF]. Te nanowires are easily synthesized due to the anisotropic crystalline structure of Te, which seems to favor one-dimensional growth. However, its electrical conductivity is quite low, between 2 and 3.8 S.cm -1 depending on crystal orientation [START_REF] Epstein | Electrical properties of tellurium at the melting point and in the liquid state[END_REF], which limits its potential as a candidate for thermoelectric systems. To overcome this problem, advanced nanostructures, such as core-shell nanowires with a Te core capped by an electrical conducting shell eg of bismuth (σBi = 6.25.10 3 S.cm -1 [START_REF] Cheng | Thermoelectric properties of a monolayer bismuth[END_REF]) are proposed as one possible way of improving the electronic properties of the 1D material. Indeed, compared to conventional nanowires, core-shell nanostructures permit combination of the electronic properties of the materials involved and also further decrease the lattice thermal conductivity by interface phonon scattering [START_REF] Zhang | Enhanced Thermoelectric properties of core/shell heterostructure nanowire composites[END_REF]. Thus, the synthesis of core-shell structures based on Te nanowires capped by a Bi shell should lead to an interesting thermoelectric material with a very low lattice thermal conductivity, a high Seebeck coefficient and high electrical conductivity.

Te-Bi nanowires have already been synthesized by Zhang et al. using a two-step phase solution method [START_REF] Zhang | Enhanced Thermoelectric properties of core/shell heterostructure nanowire composites[END_REF]. However, in this case the Bi shell does not completely cover the surface of the nanowires and presents a rough surface with saw-like edges. In contrast, electrodeposition is a coating process which produces a homogeneous covering layer. Bi electrodeposition is usually performed in aqueous media [START_REF] Osipovich | Bismuth underpotential deposition on tellurium[END_REF][START_REF] Zhu | The underpotential deposition of bismuth and tellurium on cold rolled silver substrate by ECALE[END_REF][START_REF] Schoenleber | Influence of tartaric acid on diffusion coefficients of Bi III ,Sb III ,Te IV in aqueous medium: Application of electrodeposition of thermoelectric films[END_REF][START_REF] Ananthi | Facile one-step direct electrodeposition of bismuth nanowires on glassy carbon electrode for selective determination of folic acid[END_REF]. However, Bi III precursor dissolution requires an acidic medium which could corrode the Te nanowires. An alternative is to use an ionic liquid electrolyte. As the viscosity of ionic liquids is very high (typically > 50 mPa.s at 25°C), the resulting deposition rate is also slower than in aqueous media and hence more favourable to the production of thin, homogeneous layers on complex structures such as nanowire arrays [START_REF]Electrodeposition from Ionic Liquids[END_REF].

Few works deal with Bi electrodeposition using ionic liquids. In an imidazolium-based ionic liquid, a Bi thin layer was obtained by underpotential deposition (UPD) on a gold substrate [START_REF] Fu | An in situ scanning tunneling microscopic study of electrodeposition of bismuth on Au(111) in a 1-butyl-3methylimidazolium tetrafluoroborate ionic liquid. Precursor adsorption and underpotential deposition[END_REF][START_REF] Pan | Electrocrystallization of Bi on Au(111) in an acidic chloroaluminate ionic liquid[END_REF] by electrochemical reduction of Bi III adsorbed species. On scanning towards more negative potentials, over-potential deposition (OPD) occurs, and the morphology of the Bi deposit varies from a needle-like structure to a monolayer/thin film [START_REF] Pan | Electrocrystallization of Bi on Au(111) in an acidic chloroaluminate ionic liquid[END_REF]. Deep eutectic solvents, consisting in a melt of choline chloride with oxalic acid [START_REF] Agapescu | Electrodeposition of bismuth, tellurium, and bismuth telluride thin films from choline chloride-oxalic acid ionic liquid[END_REF] or malonic acid [START_REF] Golgovici | Electrochemical deposition of BiTe films from choline chloridemalonic acid mixture as ionic liquid[END_REF], have also been used to deposit Bi. This article reports the fabrication of Te-Bi core-shell nanowires by two-step electrochemical deposition in piperidinium-based ionic liquids. The first step involves the template-free electrodeposition of single crystal Te nanowires in a binary mixture of 1-ethyl-1-octylpiperidinium bis(trifluoromethylsulfonyl)imide and 1-ethyl-1-octyl-piperidinium bromide (EOPipTFSI:EOPipBr, 95:5 mole%). The second step concerns the electrodeposition of a Bi thin film on the Te nanowires, leading to core-shell Te-Bi nanostructures. The electrochemical behaviour of the Bi III /Bi 0 system in EOPipTFSI ionic liquid appears quite complex and exhibits several Bi III to Bi 0 reduction signals. The influence of various experimental parameters on Bi shell deposition was studied: these included applied potential, Bi III concentration, density of Te nanowires and applied coulometric charge.

Material and methods

The electrochemical experiments were performed in a glove box (MBraun Labstar, O2/H2O levels < 1 ppm) using a Biologic potentiostat (VSP300). Platinum-coated glass slides (Applications Couches Minces ACM, 25×10×1 mm, Pt coating thickness 300 nm) were used as working and counter electrodes in the electrodeposition experiments, while a Pt wire was used as a quasi-reference electrode (QRE).

The Te-Bi core-shell nanowires were synthesized using a two-step procedure. First, single crystalline Te nanowires were obtained using TeCl4 (99 %, Alfa Aesar) at a concentration of either 5 or 20 mM in EOPipTFSI:EOPipBr 95:5 mole%, as reported in a previous work [START_REF] Thiebaud | Tuning the morphology of Te one-dimensional nanostructures by template-free electrochemical deposition in an ionic liquid[END_REF].

Te-Bi core-shell nanowires were then obtained by electrodepositing a thin Bi film on the Te nanowires. The EOPipTFSI ionic liquid was used as the electrolyte and BiBr3 (99 %, Alfa Aesar) as the Bi III precursor. All the ionic liquids used in this work were synthesized and purified according to a previously published procedure [START_REF] Thiebaud | Tuning the morphology of Te one-dimensional nanostructures by template-free electrochemical deposition in an ionic liquid[END_REF].

The Te-Bi nanowires were washed several times with acetone and dried before characterization by scanning electron microscopy (SEM, Philips XL30 S FEG, 5 kV) and transmission electron microscopy (TEM, JEM-ARM 200F Cold Field Emission Gun, 200 kV, point resolution 0.19 nm in TEM mode) fitted with a GIF Quantum ER. The direction of growth of the nanostructures was determined by selected area electron diffraction (SAED).

Results and Discussion

Electrodeposition of Te nanowires in an ionic liquid

Te nanowires were obtained in EOPipTFSI:EOPipBr 95:5 mole% by applying a constant potential corresponding to Te IV reduction. Figure 1 shows SEM images of the deposits obtained for two different Te IV concentrations. The nanowires are self-standing single crystals with a growth axis along the [001] direction. As shown previously [START_REF] Thiebaud | Electrodeposition of high aspect ratio single crystalline tellurium nanowires from piperidinium-based ionic liquid[END_REF], the density of the nanowires increases with [Te IV ]. Nanowires synthesized for [Te IV ] = 5 mM (Figure 1a) have a mean length of 1.70 ± 0.15 µm and most of them have a diameter between 50 and 100 nm with an average of 83 ± 37 nm. When the Te concentration is increased to 20 mM (Figure 1b), the deposit is denser and is composed of nanowires that are 1.0 ± 0.1 μm in length and have quite similar diameters, mostly around 140 ± 20 nm. Whatever the substrate, three reduction peaks are observed, at -0.49 V (C1), -0.70 V (C2) and -0.99 V (C3) vs Pt, which appear before the reduction of Te 0 to Te -II at -1.5 V [START_REF] Jeng | Electrochemistry of tellurium(IV) in the basic aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt[END_REF]. These signals all correspond to Bi 0 deposition, as confirmed by XRD analysis (not shown here). The electrochemical behaviour of Bi III is the same as that observed by Fu et al. in tetrafluoroborate medium [START_REF] Fu | An in situ scanning tunneling microscopic study of electrodeposition of bismuth on Au(111) in a 1-butyl-3methylimidazolium tetrafluoroborate ionic liquid. Precursor adsorption and underpotential deposition[END_REF] but is totally different to what was observed by Szymczak et al. in EOPipTFSI:EOPipBr 95:5 mole% [START_REF] Szymczak | Electrodeposition of stoichiometric bismuth telluride Bi2Te3 using a piperidinium ionic liquid binary mixture[END_REF] and in DESs [START_REF] Agapescu | Electrodeposition of bismuth, tellurium, and bismuth telluride thin films from choline chloride-oxalic acid ionic liquid[END_REF][START_REF] Golgovici | Electrochemical deposition of BiTe films from choline chloridemalonic acid mixture as ionic liquid[END_REF]. Indeed, only one reduction peak is observed in these halide-rich media. As in the case of Te [START_REF] Thiebaud | Electrodeposition of high aspect ratio single crystalline tellurium nanowires from piperidinium-based ionic liquid[END_REF], In [START_REF] Traore | New insight into indium electrochemistry in a Tf2N-based room-temperature ionic liquid[END_REF] and Ta [START_REF] Borisenko | In situ STM and EQCM studies of tantalum electrodeposition from TaF5 in the air-and waterstable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide[END_REF] electrochemical systems, the low amount of halides in the TFSI-based ionic liquid leads to complex cathodic behaviour. As previously suggested in the literature [START_REF] Thiebaud | Electrodeposition of high aspect ratio single crystalline tellurium nanowires from piperidinium-based ionic liquid[END_REF][START_REF] Traore | New insight into indium electrochemistry in a Tf2N-based room-temperature ionic liquid[END_REF], this could be due to the reduction of successive BiBrx 3-x complexes to Bi 0 during the potential scan. The low mass transport rate during the formation of the Bi 0 deposit, due to the high viscosity of the ionic liquid (η = 15 mPa.s at 100°C [START_REF] Szymczak | Electrodeposition of stoichiometric bismuth telluride Bi2Te3 using a piperidinium ionic liquid binary mixture[END_REF]), could favour an increase in the concentration of halide ions in the vicinity of the electrode and thus modify the [Br -]/[Bi 3+ ] concentration ratios at the electrode/electrolyte interface. Successive BiBrx 3-x complexes could then be formed, leading to multiple cathodic signals corresponding to their successive reduction. The C1 peak would thus correspond to the reduction of BiBr3 into Bi 0 regardless of halide content and the later signals to the higher halide complex reductions outlined below:

BiBr3 + 3e - Bi 0 + 3Br - BiBr4 -+ 3e - Bi 0 + 4Br - BiBr5 2-+ 3e - Bi 0 + 5Br -

Electrodeposition of Bi on Te nanowires

Bismuth was electrodeposited on Te nanowires under different experimental conditions, using a coulometric charge of 50 mC/cm 2 . The influence of the deposition potential was studied by applying different values, namely the C1, C2 and C3 peak potentials. The corresponding SEM images are given in Figures 3a, 3b and3c, respectively. The influence of Bi III concentration can be seen in Figures 3a ([Bi III ] = 5 mM) and 3e ([Bi III ] = 10 mM), which show SEM images of deposits obtained at C1 peak potential. On comparing the SEM images, it seems that the highest current densities, corresponding to high overpotential values (C2 -Figure 3b, and C3 -Figure 3c), and the highest Bi III concentration (10 mM -Figure 3e) lead to preferential deposition on top of the Te nanowires. By contrast, for the lowest overpotential, C1, and the lowest Bi III concentration (Figure 3a), the deposit is rough but homogeneous and is composed of sheets surrounding the Te nanowires, as confirmed by TEM. It thus appears that, at higher growth rates, bismuth preferentially deposits on top of the nanowires, probably because of the highly anisotropic crystalline structure of tellurium [START_REF] Li | Facile electrochemical synthesis of tellurium nanorods and their photoconductive properties[END_REF]. 1a) and 3d (high density of Te NWs as shown in Figure 1b). It should be noted that when the density of tellurium nanowires is high, the nanostructures are interconnected by the deposited bismuth. So in this case, no individual core-shell nanowires are obtained (Figure 3d). However, by applying low growth rate conditions and using substrates with a low density of Te nanowires, a smooth bismuth shell is obtained on decreasing the coulometric charge down to 16 mC/cm 2 (Figure 4). The Bi deposit is homogeneously deposited around the Te nanowires (Figure 4a). The statistical analysis presented in Figure 4b shows that most of the nanowires have a diameter of between 75 and 125 nm with an average of 125.5 ± 37.0 nm; this is an increase of 42 nm compared to the initial diameter of the Te nanowires (Figure 1a). This is in accordance with the HRTEM analysis of Figure 4c. Indeed, the cross-sectional view confirms that the Bi deposit is homogeneous along the Te nanowires with a smooth interface and a thickness of around 20 nm (Figure 4, c andd). The Te and Bi crystallographic structures are highlighted in the Fast Fourier Transform images (Figure 4c). It appears that the bismuth shell is present as a polycrystalline phase whereas the Te core is a single crystal with a growth direction along the c-axis.

Conclusion

In this work, we have shown that it is possible to synthesize Te-Bi core-shell nanowires by a two-step electrodeposition procedure in ionic liquids. By optimizing the synthesis parameters, a homogeneous polycrystalline Bi layer 20 nm thick was deposited around Te nanowires of diameter 83.2 ± 36.9 nm. These nanostructures are promising materials for thermoelectric applications.

Figure 1 :

 1 Figure 1: SEM images of Te nanowires obtained in EOPipTFSI:EOPipBr 95:5 mole% for [Te IV ] = (a) 5 mM and (b) 20 mM. The corresponding histograms show the distribution of diameters of the nanowires.

Figure 2 :

 2 Figure 2: Linear sweep voltammograms showing the electrochemical cathodic behaviour of Bi III in EOPipTFSI on Pt-coated glass slides with and without Te nanowires (synthesized using Te IV = 5mM). [Bi III ] = 5 mM, T = 100°C, 5 mV.s -1 .

Figure 3 :

 3 Figure 3: SEM images of Te nanowires after Bi deposition in EOPipTFSI. Te nanowires synthesized at [Te IV ] = 5 mM, Bi deposition at the peak potentials: (a) C1, (b) C2 and (c) C3. The inset of Figure 3(a) represents a TEM image of a single nanowire. [Bi III ] = 5 mM, Q = 50 mC/cm 2 , T = 100°C. Te nanowires synthesized at [Te IV ] = 20 mM, Bi deposition at C1 peak potential for [Bi III ] = (d) 5 mM and (e) 10 mM. Q = 50 mC/cm 2 , T = 100°C.

Figure 4 :

 4 Figure 4: Morphological and structural analyses of Te-Bi core-shell nanowires: (a) SEM image of Te-Bi core-shell nanowires obtained after Bi electrodeposition on Te nanowires (Figure 1a type) at C1 peak potential in EOPipTFSI (inset shows the corresponding TEM image of a single nanowire); (b) histogram showing the diameter distribution of the core-shell nanowires; (c) Te-Bi interface with the corresponding Fast Fourier Transform of the Te core and the Bi shell; (d) EDX-mapping of Bi (yellow) and Te (green). [Bi III ] = 5 mM, Q = 16 mC/cm 2 , T = 100°C.