Electrodeposition of photovoltaic thin films from ionic liquids in ambient atmosphere: Gallium from a chloroaluminate ionic liquid

Journal Pre-proof Electrodeposition of photovoltaic thin films from ionic liquids in ambient atmosphere: Gallium from a chloroaluminate ionic liquid

Ashraf Bakkar, Volkmar Neubert PII:

S1572-6657(19)30924-5

DOI:

https://doi.org/10.1016/j.jelechem.2019.113656

Reference:

JEAC 113656

To appear in:

Journal of Electroanalytical Chemistry

Received date:

12 May 2019

Revised date:

12 November 2019

Accepted date:

14 November 2019

Please cite this article as: A. Bakkar and V. Neubert, Electrodeposition of photovoltaic thin films from ionic liquids in ambient atmosphere: Gallium from a chloroaluminate ionic liquid, Journal of Electroanalytical Chemistry(2019), https://doi.org/10.1016/ j.jelechem.2019.113656

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© 2019 Published by Elsevier.

Journal Pre-proof

Electrodeposition of photovoltaic thin films from ionic liquids in ambient atmosphere: Gallium from a chloroaluminate ionic liquid

Ashraf Bakkar 1,2,3,*, Volkmar Neubert 2 Metallurgical & Materials Engineering Department, Suez University, P.O. Box 43721, Suez, Egypt.

2

Institut für Materialprüfung und Werkstofftechnik (Dr. Neubert GmbH), Freiberger Strasse 1, 38678 Clausthal-Zellerfeld, Germany ([email protected].)

3

Department of Environmental Engineering, College of Engineering at Al-Lith, Umm Al-Qura University, Corniche Road, Al-Lith City, Saudi Arabia

*

Corresponding author. Tel.: +20 1203072902; fax: +20 623 360252. E-mail address: [email protected]

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Abstract

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At present time, semiconductive thin films used in photovoltaic cells can be successfully electrodeposited using ionic liquids in lab scale. However, scaling-up of this technology is

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hazardous due to the complexity involved to conduct electrodeposition in inert gas atmosphere.

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This paper reports on a novel approach for electrodeposition of gallium from a chloroaluminate ionic liquid composed of anhydrous aluminum chloride (AlCl3) and 1-ethyl-3methylimidazolium chloride (EMIC) in ambient atmosphere, after protection with a non waterabsorbable hydrocarbon layer. Cyclic voltammetry (CV) measurements were undertaken to characterize the electrodissolution and electrodeposition behavior of Ga. Potentiostatic electrodeposition experiments were conducted to deposit functional Ga layers on Pt, nickel, and mild steel substrates. SEM/EDX investigations revealed that Ga deposits have inhomogeneous

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Journal Pre-proof microstructure developed through the progressive nucleation-growth mechanism that has been controlled by diffusion of Ga cations.

Keywords: Photovoltaic cells; Electrodeposition; Semiconductors; Gallium; Ionic liquids; Cyclic voltammetry.

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1. Introduction

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Electrochemical synthesis of semiconductive thin films used in photovoltaic (PV) cells is of

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great interest and is superior over a large range of highly complicated techniques that generally

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depend on sputtering and deposition of the aimed films in ultra-vacuum atmosphere based on PVD or CVD processes. Electrodeposition is a significantly lower cost technique that is easy to

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be scaled up. It also gives the possibility to deposit elements, alloys and compounds with

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versatile compositions, microstructures, and properties [1-5].

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Most of the semiconductive elements, e.g. Si and Ge, are highly reactive and are difficult to be electrodeposited from aqueous solutions. Ionic liquids (ILs) have been recently widely

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researched for electrodeposition of such semiconductors [6-8]. Gallium (Ga) can be electrodeposited from aqueous solutions but it is often associated with excessive hydrogen evolution that results in rough and non-compact deposited films with low current efficiency [9,10]. Appropriate alternative solutions for Ga electrodeposition were non-aqueous electrolytes, namely ionic liquids. Over the past two decades, some research articles have been published on Ga electrodeposition in ionic liquids at ambient temperatures [10-15]. Chloroaluminate ionic liquids, based on AlCl3 and different organic salts e.g. 1-ethylpyridinium chloride or 1-ethyl-3-methylimidazolium chloride, are the first generation of ionic liquids. 2

Journal Pre-proof Although these ILs are extremely hygroscopic, they are still being studied for electrodeposition of aluminum and its alloys [16-21]. Electrodeposition of Ga in a chloroaluminate IL composed of AlCl3/1-Methyl-3-ethylimidazolium chloride (ratio 60/40 mol%) was described by Sun and co-workers [10]; Pure Ga metal was forwardly electrodeposited on a glassy carbon electrode as a result of direct reduction of Ga(I) species dissolved in the IL. Seddon et al [11] reported successful electrodeposition of Ga from a chlorogallate IL of GaCl3/1-octyl-3-

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methylimidazolium chloride with improved morphology when the IL is buffered with sodium

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chloride. In a comparison between a chloroaluminate IL and a chlorogallate IL, Freyland et al

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[12] studied the nanodeposition of Ga onto Au.

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The electrodeposition processes of Ga, as well as of other semiconductors, from ionic liquids

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must be carried out inside inert gas-filled glovebox, in which the humidity and oxygen contents must be kept near zero [10-13]. Air- and water-stable ILs composed of choline chloride/urea

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eutectic mixtures, namely deep eutectic ILs, have been used for electrodeposition of Ga for the

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synthesis of Cu(In,Ga)Se2 (CIGS) photovoltaic films [22]. However, the electrodeposition experiments have performed in inert atmosphere, because choline chloride/urea IL is

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hydroscopic and adsorbs water in air [23,24]. More air/water-stable ILs, that are classified as hydrophobic ILs and have larger electrochemical window, have been investigated for Ga electrodeposition [13-15]. Gasparotto et al [13] studied in situ Ga electrodeposition from GaCl3 dissolved in an air- and water-stable IL, namely 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, in an argon-filled glovebox with water and oxygen contents below 2 ppm. Likewise, Zhang et al [14] investigated Ga deposition from GaCl3 in the air- and water-stable IL 1-butyl-3-methylimidazolium trifluoro-methanesulfonate that was vacuum-dried to have water content of 237 ppm, because Ga (III) chloride has a very 3

Journal Pre-proof strong affinity to absorb water. It is reported that reactive semiconductors must be electrodeposited in inert gas atmosphere albeit from air- and water- stable ILs; Cation-bearing salts, e.g. SiCl4 and GeCl4, are extremely hygroscopic and absorb water forming reactive halides, like HCl and oxochloro compounds, that seem to prevent electrodeposition [25]. Thus, the electrodeposition of semiconductors in ionic liquids must be conducted in dry inert gas

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atmosphere due to hygroscopic nature of the ionic liquid, of the semiconductor-bearing precursor, or of both. This limits the scalability of semiconductor electrodeposition in industry.

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Aiming to overcome upon this limitation, the present paper investigates the application of a

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pioneer method developed by the authors [26,27] for Ga electrodeposition in air outside

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glovebox from a chloroaluminate IL, as a highly hygroscopic IL. The electrochemical behavior

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of Ga (I) was demonstrated in the IL aluminum chloride/1ethyl-3-methylimidazolium chloride (AlCl3/EMIC, 60/40 mol%) in ambient atmosphere outside the glovebox after protection of the

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electrolyte from air with a hydrocarbon layer of decane.

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2. Experimental Details

Ionic liquid used as electrolyte for electrodeposition was prepared by mixing 60 mol% of anhydrous aluminum chloride “AlCl3” (Fluka ≥ 99% purity) with 40 mol% of 1-ethyl-3methylimidazolium chloride (EMIC), with chemical formula “C6H11ClN2” (Fluka, ≥ 95% purity) in argon-filled glovebox. The ionic liquid mixture was then stirred in a small beaker inside the glovebox until the mixture was altogether converted into liquid because of the exothermic reaction between AlCl3 and EMIC. Thereafter, some of n-decane (C10H22 - with Merck purity) was added to float onto the ionic liquid surface and make a barrier layer that prevents any

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Journal Pre-proof reaction with air [27]. This facilitated to handle the ionic liquid-contained beaker outside the glovebox for conducting experiments of electrodeposition and cyclic voltammetry (CV) measurements in ambient atmosphere open to air. The decane-covered electrolyte was kept outside the glovebox for times ranged from 1 h to 10 h before CV measurements. The CV measurements and electrodeposition experiments were performed at 22 oC under lab

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atmosphere in an ordinary three-electrode cell, which consisted of a 100 ml beaker containing 50 ml of ionic liquid that was overlaid with a decane layer of 10 mm in thickness. The beaker was

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covered by a PVC sheet with slots for holding the electrodes to have a distance of 3 cm between

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the working electrode and the counter electrode, with keeping the reference electrode (Al wire)

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at ̴ 2 cm far apart from the both other electrodes. The CV measurements and electrodeposition

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experiments were carried out using a laboratory potentiostat model “Wenking LB 94 L” controlled with “CPCDAU 41” software. The electrolytic cell was put in a water bath in which

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water was circulated by a pump-equipped heater that was programmed to keep the water

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temperature at 22 oC. After electrodeposition experiments, gallium-plated specimens were rinsed

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in ethyl alcohol, dried and stored in a desiccator for next microstructure investigations. Ga electrode was in a strip shape with dimensions of 80 x 10 x 3 mm. It was prepared by melting Ga (99.99% Sigma-Aldrich) in a glass tube immersed in a water bath with 80 oC and pouring in a rectangular-shaped PVC mold at 18 oC, the temperature of lab atmosphere. Then, the Ga strip was pickled in concentrated HCl at 10 oC for 3 min, rinsed in distilled water and in acetone, and dried to be used in CV and electrodeposition experiments. Pt sheet was prepared via pickling in 10 % HCl, rinsing in distilled water, ultrasonic cleaning in acetone, and finally drying. Pt sheet used as a working electrode was then painted with lacquer exposing a bare surface area of 10 x 10 mm2. 5

Journal Pre-proof An Al wire (ø1.5 mm, 99.99% Sigma-Aldrich) was connected as a reference electrode in all electrochemical experiments. In CV and electrodeposition experiments, Ga electrode was connected as a counter electrode, and the working electrode was Pt, nickel, or mild steel. In Ga dissolution experiments, the Ga electrode was used as a working electrode, and Pt sheet was connected as a counter electrode; the electrolyte was gently moved by a small magnetic stirrer with velocity of 20 rpm. The gallium dissolution experiments were conducted using the

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potentiostat Type M-Lab controlled by the software program MLabSci444.

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Mild steel used was in the shape of strips with dimensions of 100 x 20 x 2 mm of grade A 516,

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with nominal composition (in wt.%: 0.21 C, 0.55–0.98 Mn, 0.13–0.45 Si, 0.040 S and 0.035 P).

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The steel strips were ground by SiC papers, polished with diamond paste till having a mirror-like

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surface, pickled for 5 min in 10% HCl, washed by tap water, rinsed ultrasonically in acetone, and dried. Finally, the steel strip was painted with an appropriate lacquer with leaving a free exposed

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surface area of 10 x 10 mm2 used for CV and electrodeposition experiments. Pure electrolytic

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nickel strips were used. Ni strips were prepared for CV and electrodeposition experiments with

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the same procedure used for mild steel. For measuring the typical current efficiency of Ga dissolution, the Ga electrode was weighed before and after dissolution, and the mass, (M) in grams, of Ga dissolved was found as: M = weight of Ga electrode before dissolution – its weight after dissolution

(1),

The theoretical weight (m) of dissolved Ga obtained from Faraday’s Law (m) was calculated as:

𝑚 =

𝑄 𝐴𝑤

(2),

𝑛𝐹

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Journal Pre-proof where Q is the quantity of electrical charge in “A.s”, namely coulombs (C), Aw is the atomic weight of Ga in grams (g.mol-1), n is the oxidation state of Ga, and F is Faraday’s constant 𝑀

𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (%) =

=

nFM 𝐴𝑤 .𝑄

𝑚

× 100

× 100

(3)

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Microstructural investigations of Ga layers were performed using a scanning electron

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microscope “SEM” (model CamScan Series 4) coupled with an energy dispersive X-ray analyzer

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(EDX).

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3. Results and Discussion

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3.1. Dissolution of gallium

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The anodic behavior of gallium was firstly investigated via cyclic voltammetry (CV) measurements of a gallium electrode in AlCl3/EMIC ionic liquid. Fig. 1 shows the anodic CVs of

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Ga electrode reversed at 1000 mVvs Al and 1600 mVvs Al. The dissolution of gallium initiated with the onset of the anodic sweep at 325 ± 5 mVvs Al (the open circuit potential “OCP” of Ga in the IL). The anodic current density increased forwardly with increasing the potential up to 49.5 mA.cm-2 at 1120 mVvs Al and then the current density decreased steeply and leveled off at 1520 mVvs Al. It is speculated that the Ga cations generated in the anodic process interacted with proper anions existent in the IL. This formed a protective layer at the metal/IL interface that inhibited further dissolution of Ga at potentials higher than 1520 mVvs Al and during the reverse sweep too. Jiang et al [25] reported a similar behavior for anodic dissolution of uranium in AlCl3/EMIC IL; 7

Journal Pre-proof It has been suggested that the oxidized cations interact with anions AlCl4- or Al2Cl7- forming a viscous layer at the metal/liquid interface that blocks the uranium surface and suppresses its anodic dissolution. However, the CV of Ga electrode reversed at 1000 mVvs Al (Fig. 1) showed active dissolution of Ga in both forward and reverse sweeps. Consequently, potentiostatic dissolution experiments of Ga precursors in AlCl3/EMIC IL were carried out at a potential of 700

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mVvs Al. In order to determine the oxidation state of Ga ions released during the anodic dissolution, Ga

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electrode was undergone potentiostatic electrodissolution experiments at 700 mVvs Al. The typical

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weight loss of Ga was measured in each experiment and compared with the theoretical weight

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loss calculated by Faraday’s Law “equation (2)”. Table 1 shows the comparison of the

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theoretical weight loss with the typical weight loss of Ga electrode measured after passage of a definite charge. As Ga can be found in the +1 oxidation state [10,29] and in the +3 oxidation

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state in its electrolytes [11-14], the oxidation state (n) was assumed as n=1 and as n=3 in

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equation (3) and the relevant current efficiency () is calculated. It is found that the oxidation state (n = 1) is the reasonable value, i.e. the Ga (I) is the product of the dissolution process. This

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is in consistence with the findings by Sun and co-workers [10] on studying the dissolution behavior of Ga in AlCl3/MEIC IL; They also emphasized the dissolution of Ga (I) through constructing the Nernst plots for Ga(1)/Ga(0) redox couples, and found that the Nernst slope is equal to that of one-electrode redox couple [10]. However, the slight increase of the resulting current efficiency (Table 1) to be a little more than 100% can be attributed to a somewhat chemical dissolution of Ga during the application of anodic potential. Free immersion experiments of Ga electrode in AlCl3/EMIC IL showed that the weight loss rate of Ga, if dissolved chemically, equals 0.000085 g.cm-2.h-1. 8

Journal Pre-proof 3.2. Cyclic Voltammetry The Ga (I)-containing electrolyte was prepared by electrochemical dissolution of Ga in AlCl3/EMIC IL at 700 mV for 6 h to have Ga concentration of 5.38 g/L. The cyclic voltammogram (CV) of Pt in the IL was plotted with starting the potential scanning from the open circuit potential (OCP) of Pt at 600 ±5 mVvs Al towards the negative potential. Fig. 2 shows

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two typical CVs recorded on Pt in the IL electrolytes; one CV was conducted in the electrolyte exposed to argon gas inside the glove box, and the other CV was monitored outside the glove

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box in ambient atmosphere for the electrolyte protected from air by an insulating layer of decane.

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It is clear that the two CVs are identical, thereby indicating that the decane protective layer

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above the ionic liquid does provide protection from humidity and oxygen in air. The authors

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have reported the successful using of decane in protecting the AlCl3/EMIC IL from air for

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electrodeposition of Al in ambient atmosphere [27]. The change of CV profile with the scanning potential (Fig. 2) showed a sharp increase of

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cathodic current density at 225 mVvs Al, that can correspond to gallium deposition. The cathodic

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current reached its peak at 150 mVvs Al, and decreased with continuing the cathodic sweep up to 50 mVvs Al. Then, reversing the potential sweep depicted a decay in the cathodic current without showing any cathodic current loop, in contrast to Al deposition from AlCl3/EMIC IL [18,27]. The absence of cathodic current loop evidences that the electrodeposition of Ga is not a nucleation-controlled process. This means that the nucleation of Ga crystallites is faster than or simultaneous with their bulk growth, when Ga is electrodeposited from Ga1+ in AlCl3/EMIC IL. Similar CVs without current loop were reported for electrodeposition of Ga onto a tungsten electrode from Ga1+ in a chloroaluminate IL [10], and onto Au (111) from Ga3+ in [Py1,4]TFSA IL [13] and from Ga3+ in two Lewis acidic ILs, a chloroaluminate IL and a chlorogallate IL [12]. 9

Journal Pre-proof By contrast, the CV profile depicted a current loop for Ga deposition onto a glassy carbon electrode from Ga1+ in a chloroaluminate IL [10], and onto a glassy carbon electrode and molybdenum from Ga3+ in [BMIM][TfO] IL [14]. The electrodeposition kinetics depends on the electrode material, Ga oxidation state, ionic liquid used, and electrodeposition parameters [14,24,30].

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The CV reverse scan (Fig. 2) showed a steeply increase in the current at the onset of the anodic sweep. The anodic current reached to its maximum peak at 320 mVvs Al and depicted an

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oxidation wave due to Ga dissolution. However, a further anodic peak at 360 mVvs Al, that

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appeared as a shoulder to the Ga stripping peak, could be attributed to dissolution of Ga-Pt

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alloy. This anodic behavior with two striping peaks, the main peak owing to Ga dissolution and

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the side peak ascribable to dissolution of Ga-substrate metal alloy, is consistent with some CV studies on Ga deposition onto Au (111) [12,13], and onto copper [31]. The formation of Pt-Ga

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alloy is thermodynamically possible according to the Ga-Pt phase diagram [32]. Low melting

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point of Ga (̴ 29.8 oC) facilitates its diffusion into the substrate metal [13], that leads to

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formation of Ga-Pt intermatallics [32]. Fig. 3 shows the CVs recorded on nickel and mild steel electrodes in Ga(1)-contained AlCl3/EMIC IL protected from air by decane layer at ambient atmosphere. The potential was scanned from the OCP of electrodes, 530 mVvs Al for nickel and 570 mVvs Al for steel, towards to negative potentials up to 50 mVvs Al. The CV profiles of nickel and steel are generally similar to each other, and also similar to the CV diagram of Pt. However, a clear variation of the electrodeposition characteristics can be observed by comparing the onset deposition potential of Ga, the cathodic deposition wave, and the anodic dissolution wave in each CV. Such variation of electrodeposition behavior depending on the substrate electrode was reported for Ga 10

Journal Pre-proof deposition on tungsten and glassy carbon electrodes [10] and on molybdenum and glassy carbon electrodes [14], and for In-Ga deposition on copper and molybdenum electrodes [22]. It is stated that the substrate electrode material affects significantly on electrodeposition behavior [27,30,33]. Similar to Pt, nickel and steel electrodes showed additional side anodic peaks at 330 mVvs Al and 360 mVvs Al that act as shoulders overlapped with the main stripping peaks of Ni and steel, respectively. These side anodic peaks can be referred to dissolution of Ga-Ni alloy [34]

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and Ga-Fe alloy [35] formed during deposition [36].

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Fig. 4a shows a series of CV profiles of Pt electrode at various scan rates. With increasing the

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scan rate (v), the cathodic peak potential (Epc) became more cathodic and the cathodic peak

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current (Ipc) increased. The variation of Ipc with the square root of scan rate (v1/2) was linear (Fig.

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4b), indicating that the electrodeposition process occurs under mass transfer (diffusion) control [18,37-40]. However, the Ipc-v1/2 variation line did not intercept the origin of the plot as expected

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for a simple diffusion-controlled process. This implies that an additional process other than

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diffusion occurs. This additional process was assumed to be reduction of some electroactive impurities present in the AlCl3/EMIC IL when used without drying or purification [18,37,38], as

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the same case of the IL used in the current study. Also, the non-interception of the Ipc-v1/2 line with the origin has been ascribed to the nucleation process during electrodeposition of Ni-Se alloy [40]. The CV profile of Pt at the scan rate of 50 mV/s (Fig. 4a) depicted a slight peak hardly observed at ̴ 500 mVvs Al. This slight peak can be speculated as a result of dissociation of moisture or reduction of impurity species present in the ionic liquid. The ionic liquid used in the present study was not previously dried unlike the ionic liquids mostly used in literature that had been dried before CV measurements (e.g. Refs. [13-16]). Mahony et al [41] illustrated through CV 11

Journal Pre-proof measurements that non-dried and wet ILs revealed slight cathodic peak preceding the main cathodc peak. Consequently, it can be stated that the electrodeposition of Ga is a diffusion-controlled process. In addition, an electrochemical reaction, assumed to be reduction of some electroactive impurities present in the IL, may precede Ga electrodeposition. Further chronoamperometry and

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voltammeteric studies are recommended for deeper understanding of Ga electrodeposition

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kinetics

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Fig. 5a depicts a series of CVs of mild steel electrode with holding the scanning potential at the cathodic peak (150 mVvs Al) for 0.0 s, 5 s, 10 s, 20 s, and 50 s. Clearly, with increasing the

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holding time of static cathodic potential, the anodic peak current increased and the oxidation

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wave expanded. This illustrates that the electrodeposition of gallium continued with extending

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the time of applying the static cathodic potential. However, Fig. 5b shows that the cathodic current decreased gradually with time during application of static potential (e.g. the cathodic

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current decreased from -1.17 mA to -0.34 mA when the static potential of 150 mVvs Al was

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applied for 50 s). This gradual decrease in the cathodic current is suggested to be a result of depletion of Ga ions in the electrolyte adjacent to the cathode surface, and the current thereafter depends on diffusion of Ga+1 from the electrolyte to the cathode surface. This illustrates how the Ga deposition is affected by diffusion of Ga cations. 3.3. Morphology Fig. 6 shows the typical morphology and chemical composition of Ga layers electrodeposited potentiostatically at 150 mVvs Al onto substrates of platinum, steel, and nickel for 2h, 4h, and 6 h, respectively. Comparison observation of SEM micrographs (Fig. 6 a,b,c) shows that the Ga 12

Journal Pre-proof deposits displayed different morphologies with changing the substrate metal and electrodeposition time. The morphology of crystallites deposited, as well as nucleation/growth kinetics, has been reported to be significantly influenced by the type of substrate metal [24,30,42]. However, the morphology results obtained can not emphasize that the change of morphology is only due to the substrate metal as the micrograph undertaken were on Ga layers

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electrodeposited for different times. It is notable that peaks resulting from the substrate metals (Pt, Fe, and Ni in in the EDX spectra

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Fig 6 d, e, and f, respectively) diminished with increasing the electrodeposition time. Decreasing

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of the peaks intensity resulting from the substrate metals demonstrates the increasing thickness

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of Ga layer deposited with extending the time of deposition.

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The morphology of resultant deposits depicts Ga crystallites with inhomogeneous size,

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particularly on nickel substrate (Fig 6 c). This inhomogeneous morphology is attributed to a nucleation/growth procedure named as progressive mechanism [29,43-45]. The progressive

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mechanism is characteristic with a slow rate of nucleation of new crystallites that will keep on

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occurring simultaneously with growth of prior nuclei. Thus, the morphology contains some crystallites larger than others.

In addition, being a diffusion-controlled process, the electrodeposition of gallium can be accompanied with adsorption of foreign electroactive species at particular crystal faces [44] that may suppress its dimensional growth while other nuclei continue to grow. Consequently, it can be suggested that the morphology results supported the CV measurements. The mass transfer affected the nucleation/growth kinetics and led to progressive nucleation mechanism.

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Conclusions Gallium was successfully electrodeposited in a highly hygroscopic chloroaluminate ionic liquid “AlCl3/EMIC (60/40 mol%)” under ambient atmosphere, after covering the ionic liquid with a decane layer that facilitated handling it outside glovebox. Ga+1 species were obtained by anodic

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dissolution of Ga electrode that was used as a consumable anode in the electrodeposition

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experiments.

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Cyclic voltammetry (CV) measurements conducted in air on the decan layer-overlaid IL depicted a CV profile identical to that was revealed by the IL inside argon-filled glovebox. Moreover, the

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CV measurements showed that Ga electrodeposition is a diffusion-controlled process. The morphology investigation showed that the microstructure of Ga deposits is influenced by the

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substrate metal. The Ga crystallites were inhomogeneous in size as a result of progressive

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nucleation/growth mechanism.

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By insulating the ionic liquid from air with a decane layer, it can be claimed that the electrodeposition of semiconductive thin films under ambient atmosphere has the potential to be studied in a pilot scale. Moreover, further studies are recommended on investigations on the stability of decane and deacane/ionic liquid double layer over extended periods of time.

Acknowledgement The authors would like to express their thanks to Ms. Silke Lenk, Institute of Metallurgy in TU Clausthal, for SEM and EDX investigations. The revision of English by Dr. Mirza Imran, Umm Al-Qura University, is also greatly appreciated. 14

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[18] A. Bakkar, V. Neubert, Electrodeposition and corrosion characterisation of micro- and nanocrystalline aluminium from AlCl3/1-ethyl-3-methylimidazoliumchloride ionic liquid, Electrochim. Acta 103 (2013) 211– 218.

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Table captions: Table 1 Comparison between typical measured weight loss and theoretical weight loss of Ga dissolved due to potentiostatic dissolution at 700 mV.

The anodic cyclic voltammograms of Ga electrode in AlCl3/EMIC (60/40 mol%) IL

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Fig. 1

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Figure captions:

overlaid with a decane layer under ambient atmosphere, reversed at 1000 mVvs Al and at The cyclic voltammogram of Pt electrode in Ga(I)-contained AlCl3/EMIC (60/40

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Fig. 2

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1600 mVvs Al; scan rate = 10 mV.s-1.

mol%) IL protected from air by decane protective layer at ambient atmosphere, in

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comparison with that recorded in the same IL but without deacne layer inside an argonfilled glove box; scan rate = 10 mV.s-1.

The cyclic voltammogram of nickel and mild steel electrodes in Ga(I)-contained

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Fig. 3

AlCl3/EMIC (60/40 mol%) IL protected from air by decane protective layer at ambient

(a) A set of typical cyclic voltammograms of Pt electrode in Ga(I)-contained AlCl3/EMIC (60/40 mol%) at scan rates of 5, 10, 20, and 50 mV.s-1

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Fig. 4

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atmosphere; scan rate = 10 mV.s-1.

(b) A plot of cathodic (Ipc) peak currents deduced from the CVs in (a) versus the square root of scanning rate. Fig. 5

(a) A series of cyclic voltammograms of mild steel electrode in Ga(I)-contained AlCl3/EMIC (60/40 mol%) with holding the potential at 150 mVvs Al for times of 0.0 s, 5 s, 10 s, 20 s, and 50 s. Scan rate = 10 mV.s-1; (b) Variation of scanning potential and the corresponding current with time.

Fig. 6

SEM micrograph of Ga layers electrodeposited at 150 mVvs Al (a) onto Pt for 2 h, (b) onto steel for 4 h, and (c) onto Ni for 6 h; (d), (e), and (f) are EDX spectra of areas shown in (a), (b), and (c), respectively.

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CRediT author statement

Ashraf Bakkar:

Conceptualization, Methodology, Software, Validation, Formal analysis,

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Resources, Writing - Review & Editing, Supervision, Project administration.

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Volkmar Neubert:

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Investigation, Data Curation, Writing - Original Draft, Visualization.

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Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Table 1: Comparison between typical measured weight loss and theoretical weight loss of Ga dissolved due to potentiostatic dissolution at 700 mV.

Theoretical weight loss (m (g/cm2) 0.0511

Current efficiency “”, (%) 100.39

6 h in ambient atmosphere

0.0517

0.0514

100.58

8 h in ambient atmosphere

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0.0536

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6 h in glovebox

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Measured weight loss (M *, g/cm2) 0.0513

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Experiment

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100.37

Typical measured weight loss calculated by Eq. (1) Theoretical weight loss calculated from Faraday’s Law by Eq. (2), assuming the oxidation state (n) = 1.  Current efficiency calculated by Eq. (3)

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Journal Pre-proof Graphical abstract 2.0

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Current density (mA.cm )

1.5 1.0 0.5 0.0 -0.5 -1.0

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-1.5 -2.0

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0

22

100

200

300

400

500

Potential vs. Al (mV)

600

700

800

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Highlights

Ga was electrodissolved and electrodeposited in a chloroaluminate ionic liquid.

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Covering ionic liquid with decane layer facilitated electrodeposition of Ga in air.

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The approach may facilitate scaling-up electrodeposition of semiconductors.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6