Electrodeposition of tellurium from eutectic based ionic liquid

M. Rostom Ali1 and Md. Moynul Islam1,2*

1Department of Applied Chemistry and Chemical Engineering, University of Rajshahi, Rajshahi-

6205, Bangladesh; 2Department of Chemistry, Bangladesh Army University of Engineering and

Technology, Qadirabad Cantonment, Natore-6431, Bangladesh

Abstract The electrodeposition of tellurium from a solution containing tellurium(IV) chloride (TeCl4) in choline chloride (ChCl)-ethylene glycol (EG) based ionic liquid has been carried out onto a copper cathode by constant current and constant potential methods at room temperature in absence and in presence of additives such as 40 ml·L-1 acetonitrile or 0.05 mol·L-1 P2O5 or a mixture of 0.10 g·L-1 polyethylene glycol and 40 ml·L-1 formic acid. The influences of TeCl4 concentration and additives on electrodeposition have been investigated. The deposit is characterized by XRD and the morphology of the deposited layers has been investigated by scanning electron microscope (SEM). The diffraction patterns and SEM images indicate the electrodeposited tellurium shows very different morphology which depends primarily on the electrolytic composition and polarization potentials. Additives significantly change the quality of tellurium deposits. The current efficiency for the deposition of tellurium is about 98%.

Keywords: Ionic liquid; Electrodeposition; Tellurium; Cyclic voltammetry; Additives

*Corresponding author: E-mail: moynul.acct.ru.44@gmail.com (Md. Moynul Islam)Mobile: +880-1731 412402

IntroductionTellurium is an important element due to its many-fold applications. These include gas sensors for detections of nitrogen dioxide and ammonia (Tsiulyanu and Mocreac, 2013), piezoelectrics (Royer and Dieulesaint, 1979), photoconductors (Li et al. 2012), photonic crystals (Pan et al. 2004), submillimeter wave detectors (Görtz et al. 1982) and thermoelectric devices (Bodiul et al. 2006). Tellurium compounds have also been used in making catalyst, fungicide and organic dye stuffs (Mathur and Tandon, 1986, Schmitz et al. 1972). Tellurium-based semiconductor compounds such as CdTe can be used to produce solar cells (Wu et al. 2012), ZnTe for optoelectronics and non-linear optics (Fauzi et al. 2013); and PbTe (Ivanova et al. 2007, Heremans et al. 2005) and Bi2Te3 (Martín-González et al. 2002, Manzano et al. 2013, Martín-González et al. 2013) can be used as thermoelectric materials. Tellurium is widely used in photography for toning prints especially silver prints (Mathur and Tandon, 1986). More than

90% of tellurium is used in steel and alloys production (Kenawy et al. 1995). It improves mechanical properties of the alloys and also increases their resistance to corrosion.The electrodeposition of tellurium from different baths in aqueous and non-aqueous solutions of tellurium ions and TeCl4 on platinum-sheet and copper-sheet in both acidic and basic baths has been described by several investigators (Alekperov, 1974, Fouda et al. 1987, Mishra et al. 1990, Jayasekera et al. 1994). Recently, the studies have been focused on the electrodeposition of tellurium from Te(IV) acidic solutions (Rosamilia and Miller, 1986, Mori et al. 1988, Dennison and Webster, 1991) due to the increased interest in the electrochemical deposition of semiconductor thin layers (Martín-González et al. 2002, Gregory et al. 1990, Han et al. 2003). It is reported that the electrodeposition of Te from both acidic and basic liquids is quite complicated (Endres, 2002).Therefore, aprotic room-temperature ionic liquids (RTILs), especially air- and water-stable ones, can be used for electrodeposition of tellurium due to their advantageous properties, including wide electrochemical potential window, acceptable conductivity compared to non-aqueous electrolytes and negligible vapour pressure (up to 300 oC) (Ohno, 2005, Endres and El Abedin, 2006, Abbott and McKenzie, 2006). During the past two decades ionic liquids (ILs) have gained attention as an alternative electrolyte media for the electrodeposition of various semiconductors and reactive metals, such as Ge, Si, Ta, and Al, due to their large electrochemical potential windows and low volatility (Endres et al. 2008). Abbott has recently developed a range of ionic compounds, which are fluid at room temperature. These ionic liquids (ILs) are based on choline chloride (2-hydroxy-ethyl-trimethyl ammonium chloride) has recently been found to have interesting perspectives in electrodeposition of semiconductors. In general, a eutectic-containing IL is formed due to the formation of hydrogen bonding between choline chloride (ChCl) and an amide, alcohol or carboxylic acid (Abott et al. 2004). These binary mixtures are also called ‘deep eutectic solvents’ (DESs), being potentially recyclable, biodegradable and with no harm on human health. These ILs have been used in our laboratory for the deposition of Ag (Ali et al. 2015), Cu (Ali et al. 2014), Ni (Ali et al. 2014), Cr (Ali et al. 2017), Co (Ali et al. 2018) and Se (Ali et al. 2019) at high current efficiency.

The aim of the present work is to prepare the thin film of tellurium on copper substrate by electrodeposition from a eutectic mixture of choline chloride and ethylene glycol (ethaline) containing TeCl4 at room temperature.

Materials and Methods:The chemicals and preparation of IL 1:2 (mole ratio) ChCl:EG was detailed in the previous papers [Ali et al. 2017, Ali et al. 2018). Tellurium tetrachloride anhydrous (TeCl4) (SIGMA-ALDRICH 99%) was used as received. Solutions were made so that a TeCl4 concentration ranging from 0.025 to 0.10 mol·L-1 were obtained.

The additives polyethylene glycol (HO(CH2CH2O)nCH2CH2OH) (RDH 97%), formic acid (HCOOH) (Merck 98%), acetonitrile (CH3CN) (Merck >99.9%) and di-phosphorus pentaoxide (P2O5) (Merck >97%) were used as received. Generally, the additives were added to the plating

bath to a concentration of 0.10 g·L-1 polyethylene glycol and 40 mL·L-1 formic acid mixture or 40 mL·L-1 acetonitrile or 0.05 mol·L-1 P2O5. All other chemicals were used as received. Potential step chronoamperometry, chronopotentiometry and cyclic voltammetry experiments were similar to those in the previous studies (Ali et al. 2017, Ali et al. 2018).

The electrodeposition of tellurium was carried out onto copper cathode under constant current and constant potential methods from IL containing TeCl4 at room temperature. The applied current densities and potentials for depositing tellurium in different plating operations were -6.0 ~ -25.0 A·m-2 and -0.30 ~ -0.80 V, respectively. The depositing time in different plating operations was 2 ~ 3 hr. Following each deposition, the resulting deposit was soaked firstly in toluene, then in deionized water, and finally washed with acetone to remove the residual IL. The deposit was then dried with cold air.

Surface morphologies and crystal structure of the deposits were examined with scanning electron microscope (XL 30 SEM, PHILIPS) and X-ray diffraction (Philips PW 1716 diffractometer, CuK radiation, 40 kV, 25 mA).

Results and Discussion:Cyclic voltammetry of Te(IV) in ChCl:2EG (ethaline) IL

Cyclic voltammogram has been taken in order to study the different deposition mechanism and to find the optimal deposition potential of tellurium deposition from ethaline. Figure 1 shows a series of cyclic voltammograms as a function of the switching potential recorded on a platinum electrode in 1:2:0.05 (mole ratio) ChCl:EG:TeCl4 IL at 25 oC with a scan rate of 10 mV·s-1. The rest potential is +0.40 V. The scan towards negative direction consists of first (C1) and second (C2) (small) reduction waves with the current starting to increase at 0.38 V and -0.49 V, respectively. Additional reduction wave C3 is observed with the current again starting to increase at -0.87 V. The reverse scan consists of first (Pa1) stripping peak at 0.50 V. Additional oxidation wave is observed with the current again starting to increase at 0.97 V. The effect of cathodic switching potential study on the cyclic voltammogram shows that the first (C1) and second (C2) reduction waves correspond to the oxidation peak (Pa1) (dashed, dashed-dotted and dashed-double-dotted curves in Fig. 1)

Comparison of the voltammogram obtained in the absence of TeCl4 (dotted curve in Fig. 1), it is noticed that a reduction wave appeared at -0.87 V in ethaline corresponding to the reduction of cationic species (Cat+) into this IL, while the oxidation wave appeared at 1.05 V in ethaline corresponds to the oxidation of chloride ions (anions) to molecular/gaseous chlorine according to following reaction (Ali et al. 2015).

2Cl-(ad) → Cl2 + 2e- (1)

From XRD analysis, pure tellurium has been detected in the deposit obtained at a deposition potential of -0.28 V (C1) and -0.60 V (C2) by constant potential method. Therefore, the increase of the cathodic current in the first (C1) and second (C2) reduction waves are obviously associated with the reduction of tellurium ion to metallic state according to the following reaction:

Te4+(ad) + 4e- → Te(ad) (at (C1) and (C2)) (2)

This strongly suggests the presence of two energetically different phases/morphologies of tellurium deposit which has been described in the previous papers (Ali et al. 2015, Abbott et al. 2009).Figure 2 shows the effect of TeCl4 concentrations on the cyclic voltammograms recorded on a platinum electrode in ChCl:2EG IL at 25 oC with a scan rate of 10 mV·s-1. It is readily seen from these voltammograms that the magnitude of current density in the reduction waves (C1) and (C2), which are attributed to the reduction of Te(IV) to Te(0), increases with the increase of TeCl4

concentrations added into the IL. The same phenomenon is also observed with the stripping peak Pa1, which shows also the magnitude of peak current density increase with the increase of the TeCl4 concentrations added into the IL. The increase in the magnitude of Te(IV) ion reduction current density with the increase of TeCl4 concentrations added into the IL is due to increased mass transport, which could act to promote the diffusion of tellurium ion to the electrode surface, encouraging bulk growth. Similar result has been reported for the deposition of Cu (Ali et al. 2014) and Ag (Ali et al. 2015) in this IL.

The effects of additives (e.g. either acetonitrile, P2O5 or a mixture of polyethylene glycol and formic acid) on the cyclic voltammograms recorded on a platinum electrode in ChCl:2EG:0.05 TeCl4 IL at 25 oC with a scan rate of 10 mV·s-1 are presented in the Fig. 3. It is clear from Figure 3 that the additions of 40 mL·L-1 acetonitrile, 0.05 mol·L-1 P2O5 or a mixture of 0.1 g·L-1

polyethylene glycol and 40 mL·L-1 formic acid (continuous, dashed and dash-dotted curves) cause significant changes in the shape and position of the voltammograms. This indicates that the presence of additives alters the deposition process of Te. It is interesting to note that the magnitude of the reduction current densities is slightly increased on the addition of additives as compared with additive free IL. However, the magnitude of the stripping peak current densities is significantly increased on the addition of either 0.05 mol·L-1 P2O5 or a mixture of 0.1 g·L-1

polyethylene glycol & 40 mL·L-1 formic acid. This is likely to be simply an effect of increased mass transport due to the addition of additives. Here the onset of Te reduction potential is shifted cathodically by 70 mV but the anodic peak potential (Pa1) is unchanged. The magnitude of stripping peak current density is unchanged on addition of 40 mL·L -1 acetonitrile into the IL (continuous curve in Fig. 3).

Electrodeposition of tellurium from ethalineFigure 4 shows the camera images of the electrodeposited tellurium layers onto copper

cathode under constant potential and constant current methods from ethylene glycol based IL. The deposits A and B are obtained at applied deposition potentials of -0.50 V and C at applied deposition current density of -8.50 A·m-2. The depositing times are 3 hr. The deposit A in absence of additive, B in presence of a mixture of 0.1 g·L-1 polyethylene glycol and 40 mL·L-1

formic acid and C in presence of 40 mL·L-1 acetonitrile. Acetonitrile enhanced the crystallinite of tellurium deposit. However, the deposit (B) is black in colour and it is peeling off.

Figure 5 represent the SEM images of tellurium electrodeposits obtained from ChCl:2EG IL containing 0.05 mol·L-1 TeCl4 in absence (A) and in presence (B and C) of additives at 25 oC under constant potential and constant current methods. The depositing time in different plating

operations are 3 hr. The deposits show very different morphology which depends primarily on the electrolytic composition and polarization potentials. The electrodeposits show vermicular-like (Fig. 5A), leaf-like (Fig. 5B) and crystalline with vermicular-like (Fig. 5C) fine structures that are in general more than 2 µm in length. Additives significantly change the quality of the deposit. The thickness of the deposited layers is approximately 4~5 µm, which has been calculated theoretically and measured by the weight gain method.

The acquired diffraction pattern of the deposit obtained from 1:2:0.05 mole ratio of ChCl:EG:TeCl4 IL in presence of additive (40 ml·L-1 acetonitrile) at applied deposition current density of -8.50 A·m-2 and deposition temperature at 25 oC is shown in Fig. 6. The diffraction peaks at 2 = 38.6o, 46.3o, 63.2o and 82.35o are for Te(102), Te(003), Te(113) and Te(213), respectively. The diffraction peaks are very sharp, indicating the deposit has the crystalline structure. The current efficiency for the deposition of pure tellurium is about 98%. However, additional diffraction peaks at 2 = 43.31o, 50.45o, 74.126o and 89.94o corresponding to Cu(111), Cu(200), Cu(220) and Cu(311) (base metal) respectively, are also observed in Fig. 6.

ConclusionThis research work shows that ionic liquid based on eutectic mixtures of choline chloride

and hydrogen bond donors such as ethylene glycol can be used as electrochemical solvents. Tellurium has been electrodeposited onto copper cathode from EG based IL (ethaline) containing TeCl4 in absence and in presence of additives (40 ml·L-1acetonitrile, 0.10 g·L-1 polyethylene glycol and 40 ml·L-1 formic acid mixture) at temperature 25 °C. Additives significantly change the quality of tellurium deposits. The diffraction patterns and SEM images indicate the electrodeposited tellurium shows very different morphology. The current efficiency for the deposition of tellurium is about 98%. Further study is necessary to improve the surface of the electrodeposited layer of tellurium.

AcknowledgementThe authors gratefully acknowledge the financial support from the Ministry of Science and

Technology, People’s Republic of Bangladesh to carry out this work.

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-1

-0.5

0

0.5

1

-1.2 -0.6 0 0.6 1.2

Without Tellurium(IV)Rev. Pot. = -1.14 VRev. Pot. = -0.0024Rev. Pot. = -0.21 VRev. Pot. = -0.71 V

E/V (vs. AgAgCl)

i/(m

A·c

m-2

)

Cat+ + e Cat

2Cl-

Cl 2

+ 2

e

C1 C2

Pa1

C3

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-0.6

-0.4

-0.2

0

0.2

-1.4 -0.8 -0.2 0.4 1E/V (vs. AgAgCl)

i/(m

A·c

m-2

)

C1

C2

Pa1

C3

0.025 molL-1 Te4+

0.05 molL-1 Te4+

0.10 molL-1 Te4+

0.20 molL-1 Te4+

-0.3

-0.1

0.1

0.3

-1.4 -0.8 -0.2 0.4 1E/V (vs. AgAgCl)

i/(m

A·c

m-2

)

C1C2

Pa1

C3

Without additive0.05 molL-1 P2O5Polyeth. & formic acid40 mLL-1 acetonitrile

C1'

-1

-0.5

0

0.5

1

-1.2 -0.6 0 0.6 1.2

Without Tellurium(IV)Rev. Pot. = -1.14 VRev. Pot. = -0.0024Rev. Pot. = -0.21 VRev. Pot. = -0.71 V

E/V (vs. AgAgCl)

i/(m

A·c

m-2)

Cat+ + e Cat

2Cl-

Cl 2

+ 2

e

C1 C2

Pa1

C3

Fig. 1 A series of cyclic voltammograms recorded on a platinum electrode in ChCl:2EG IL containing 0.05 mol·L-1 TeCl4 at 25 oC as a function of switching potential with a scan rate of 10 mV·s-1.

Fig. 2 Effect TeCl4 concentrations on the cyclic voltammograms recorded on a platinum electrode in ChCl:2EG IL at 25 oC with a scan rate of 10 mV·s-1.

-0.3

-0.1

0.1

0.3

-1.4 -0.8 -0.2 0.4 1E/V (vs. AgAgCl)

i/(m

A·c

m-2

)

C1C2

Pa1

C3

Without additive0.05 molL-1 P2O5Polyeth. & formic acid40 mLL-1 acetonitrile

C1'

Fig. 4 Camera images of electrodeposited tellurium on copper from ethaline containing 0.05 mol·L-1 TeCl4 at 25 oC. Applied deposition potential/current density: A. -0.50 V; B. -0.50 V; C. -8.50 A·m-2. B in presence of 0.1 g·L-1 Polyethylene glycol & 40 mL·L-1

formic acid mixture and C in presence of 40 mL·L-1 acetonitrile.

Fig. 3 Effect additives on the cyclic voltammograms recorded on a platinum electrode in ChCl:2EG IL containing 0.05 mol·L-1 TeCl4 at 25 oC with a scan rate of 10 mV·s-1.

0

40

80

120

160

30 45 60 75 90

Cu

(111

)

Cu

(200

)

Cu

(220

)

Inte

nsity

/I

2/deg(CuK)

Te(1

02)

Te(0

03)

Te(1

13)

Te(2

13)

Cu

(311

)

Fig. 5 SEM images of tellurium electrodeposits on copper substrate obtained from ethaline containing 0.05 mol·L-1 TeCl4 in absence (A) and in presence (B, C) of additives at 25 oC. Deposition condition: (A) -0.50 V, (B) -0.05 V, (C) -8.50 A·m-2. B in presence of 0.1 g·L-1 Polyethylene glycol & 40 mL·L-1 formic acid mixture and C in presence of 40 mL·L-1 acetonitrile.

0

40

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30 45 60 75 90

Cu

(111

)

Cu

(200

)

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nsity

/I

2/deg(CuK)

Te(1

02)

Te(0

03)

Te(1

13)

Te(2

13)

Cu

(311

)

Fig. 6 XRD pattern of Te deposit obtained from 0.05 mol·L-1 Te(IV) in ethaline at deposition current density of -8.50 A·m-2 with 40 mL·L-1 acetonitrile.