Micelle Structure in a Deep Eutectic Solvent for the Electrochemical

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Micelle Structure in a Deep Eutectic Solvent for the Electrochemical Preparation of Nanomaterials Yi-Ting Hsieh, and Yan-Ru Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01896 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Langmuir

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Micelle Structure in a Deep Eutectic Solvent for the

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Electrochemical Preparation of Nanomaterials

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Yi-Ting Hsieh* and Yan-Ru Liu

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Department of Chemistry, Soochow University, Taipei City 11102, Taiwan

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KEYWORDS. Self-assemble, Copper, Choline chloride- urea, micelle.

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ABSTRACT. The self-aggregation of a surfactant in a deep eutectic solvent (DES) for

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electrodeposition is reported. The physical properties and electrochemical behavior of an anionic

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surfactant, sodium dodecyl sulfate (SDS), in a widely used DES, a choline chloride-urea mixture

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(ChCl-urea) were investigated. Based on surface tension and the conductivity measurements, the

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SDS micelles that were formed in the ChCl-urea system remained stable at higher temperatures,

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i.e., 90 ºC. Cyclic voltammetric and chronoamperometric data indicate that the addition of SDS

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to the DES may alter the nucleation and the growth processes that occur in the electrodeposition

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process. SEM images show that the SDS adsorption prevent dendrite formation during the

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electrodeposion process. A simple mechanism for the formation of the SDS micelles in the DES

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system for electrodeposition is proposed.

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Introduction

ACS Paragon Plus Environment

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A liquid-solid interface represents a unique environment for surfactant micellization to occur.

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Micellized surfactants can be used in numerous applications such as separation, chemical

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reactions, the preparation of nanomaterials, and drug delivery1. The aggregation properties of

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surfactants, which include critical micelle concentration (cmc), liquid-solid surface properties

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and micelle structures play crucial roles in the above-mentioned applications. Developing novel

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media for use in self-assembling micelles from a surfactant could lead to the production of more

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useful self-organized materials2-3. The micellization of surfactants in ionic liquids is a subject of

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current interest4-9. Ionic liquids (ILs) have expanded available options for using new types of

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solvents that exhibit amphiphile self-assembly behavior. However, the extreme pH values found

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in many ionic liquids can have negative effects on the long-term stability of such preparations,

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and the fact that they are often toxic and costly represents major deterrents to their use10. Deep

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eutectic solvents (DESs) are a class of ionic liquids that share some of the properties of ionic

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liquids such as negligible vapor pressure, nonflammability, good conductivity and a wide

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electrochemical window. Unlike other ionic liquids which are formed from discrete anions and

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cations, DESs are generally mixtures of halide salts and hydrogen bond donors11-12, can be easily

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prepared from low cost, nontoxic materials and can be formed from biodegradable and

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biocompatible neutral species11, 13. Because DESs have disordered and H-bonded structures,

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recent investigations have found that they display a sufficient thermodynamic driving force for

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achieving micelle formation14-15. Therefore, numerous studies have focused on the self-assembly

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behavior of the surfactants in various DESs1-2, 10, 14, 16-18. Pal et al.17 examined the properties of

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the anionic surfactant sodium dodecyl sulfate (SDS) in a hydrated choline chloride-urea DES and

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produced micelles in this system. They revealed that the cmc and the size of the micelles could

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be controlled by varying the water content in the DES. They also reported that only anionic


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surfactants such as SDS and sodium dodecylbenzenesulfonate (SDBS) could be dissolved in a

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ChCl-urea mixture and that various the non-ionic surfactants such as Tween 80 and Triton X-100

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were insoluble in this system, and cationic surfactants (CTAB, and DeTAB) were only sparingly

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soluble in ChCl-urea. Arnold and Sanchez-Fernandez

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concentration and the water content can have an effect on the shape and size of SDS micelles,

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when they are prepared in a ChCl-urea system.

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particles were formed but, as the surfactant or water content was increased, an unusual transition

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in shape occurred with globular micelles being formed2. As mentioned above, many studies

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have focused on the shapes, sizes, and properties of micelles that are prepared in DESs, but the

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2, 10

reported that both the surfactant

At low concentrations, cylindrical-shaped

use of a micelle structure for preparing nano-materials has received limited attention.

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Surfactants are commonly used in electrodeposition processes in order to control the

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morphology and size, in attempts to produce bright and smooth deposits3, 19-21. The addition of

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organic compounds to an electroplating bath results in positive quality changes in the deposits

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due to the fact that the interfacial surface between the liquid and solid are changed. Choi and co-

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workers discovered that the SDS surfactant formed a template on the an electrode surface that

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was used for ZnO electrodeposition in a bath comprised of a nitrite solution3. Gome et al.19

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reported that Zn deposits obtained from an aqueous solution of SDS have a greater crystallinity

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and a higher grain size than the deposits that are produced in the presence of CTAB. The results

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may be due to the specific adsorption of the surfactant on the electrode surface, which would

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influence the kinetics of the electron transfer rate in electrodeposion. Spray et al.22 proposed that

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the SDS acted as a soft-template in the electrochemical synthesis of mesoporous SnO2 films.

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However, the above-mentioned examples all involved the use of aqueous solutions.


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The results of this study show that SDS behaviors in ChCl-urea DES and can be used in the

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electrochemical preparation of metal nanomaterials. The physical properties, electrochemical

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behavior of SDS in ChCl-urea DES were investigated in detail and the results are reported

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herein.

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understanding of the role of SDS in this system. This is an important issue and has an impact on

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the electrodeposition of water-sensitive materials.

Using the electrodeposition of Cu as an example, we were able to develop and

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Experimental

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Choline chloride (ChCl, Alfa, 98%), urea (J.T Baker, ACS), sodium dodecyl sulfate (SDS,

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Acros, 99%) and anhydrous CuCl (Aldrich, 99.9%) were used as received. The ChCl-urea deep

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eutectic solvent was synthesized according to a previous report23, and was stored at room

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temperature. Since the water content may influence the physical properties of a DES; the water

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content of the ChCl-urea was determined by Karl Fisher titrations using a Coulometer

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(Metrohm, 831 KF). The ChCl-urea was heated at 90 ℃ for an hour before the measurement, and

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the water content was determined to be 28.2 ppm.

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Surface tension measurements were made by using the digital tensiometer SEO-DST30 with a

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Du Noüy ring. The ring was cleaned with ethanol, water and heated to a red hot state with a

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burner and then cooled for three minutes before being used for each test.

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measurements were carried out on a Clean CON500 conductivity meter. The viscosity of the

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ChCl-urea DES with and without SDS was measured using a BrookField DV2T cone/plate

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viscometer with a CPA40Z spindle attached. The viscometer cone was controlled by an external

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water bath circulator. Viscosity was studied as a function of temperature. In this experiment, 0.5

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mL of a sample was placed in the viscometer cone and the viscosity was measured at different

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rotating speeds in the temperature range from 30 to 90℃.

Conductivity


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Electrochemical studies were carried out using a Metrohm Autolab PGSTAT101 controlled

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with the NOVA11.1 software program. A three-electrode system consisting of a silver wire

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reference electrode, a copper wire counter electrode, and a glassy carbon (GC) working electrode

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was used in this study.

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deionized (DI) water, were used as substrates for the electrodeposition. The electrolyte solution

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containing 50 mM CuCl was prepared from ChCl-urea DES. The surfactant SDS, at

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concentrations of 0, 20 and 60 mM, respectively, was added to this solution. There is no

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precipitation after the addition of the SDS in to the electrolytes. Cu films were electrodeposited

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at the peak potential at 90℃ under stirring. The as-deposited materials were rinsed with DI water

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Nickel foils (0.25cm2), cleaned with acetone, 2M nitric acid and

and ethanol and then dried in vacuum for further characterization. The crystallinity of the as-deposited Cu films was examined by powder X-ray diffraction

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(XRD, Bruker D2 phaser) with Cu Kα radiation. The surface structures and morphologies of the

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as-deposited films were also characterized by scanning electron microscopy (SEM, JEOL JSM-

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IT100) and transmission electron microscopy (TEM, JEOL JEM-2100F).

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Results and discussion

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Various concentrations of the anionic surfactant, SDS, was introduced into a ChCl-urea DES.

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The surface tension was measured to evaluate the critical micelle concentration for SDS in the

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ChCl-urea DES. Figure 1 shows information on the surface tension and conductivity versus the

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concentration of SDS in the ChCl-urea at 30 and 90 ℃. The surface tension of pure ChCl-urea

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was found to be 68 ± 0.5 mN m-1 which is slightly larger than the value reported by Arnold10.

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This can be attributed to differences in temperature, atmospheric moisture and the presence of

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impurities. Fig. 1(a) clearly shows that the surface tension of the mixture gradually decreases

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with increasing surfactant concentration at both 30 and 90 ℃, indicating that the surfactant was


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adsorbed at the air/solution interface. The surface tension decreases and eventually reaches a

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nearly constant value with increasing surfactant concentration.

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surfactant concentration where the surface tension becomes almost constant is assigned as the

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cmc. The absence of a minima in these plots confirms the high purity of the DES that was used

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in the study24. The assigned cmc of the SDS in ChCl-urea is ca. 2 mM, which is close to the

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results reported by Arnold10. The self-assembly of SDS molecules within the ChCl-urea were

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further investigated by electrical conductivity measurements which are shown in Figure 1(b).

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The value of the conductivity at 30 and 90 ℃ are significantly different due to the fact that the

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viscosity decreases with increasing temperature. However, the behaviors are similar at both

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temperatures. Conductivity titrations were used to determine the cmc for ionic surfactants in

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various media25-26. The conductivity increased rapidly when the SDS was initially added and

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reached a plateau at a certain SDS concentration. The slower diffusion of the surfactant micelle

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above the cmc results in the appearance of the plateau in the conductivity plot.18 Based on the

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results of surface tension and conductivity measurements, the cmc for SDS micelle formation in

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the ChCl-urea DES was determined to be around 2 mM and, even at higher temperatures, the

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micelle structure remained stable due to the high viscosity of the DES compared to water.

This break point for the

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Figure 1. Surface tension (a) and conductivity (b) of SDS in a choline chloride-urea DES against

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surfactant concentration at 30 and 90 ℃.

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Since the electrochemical reaction is sensitive to the viscosity of the media, the experimental

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viscosity data for the ChCl-urea system with various concentrations of surfactant, are shown in


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Figure 2, as a function of temperature. It was noted that the viscosities of these solutions were

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highly temperature-sensitive. The viscosity decreased with increasing temperature because of

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the weakening of both van der Waals forces and hydrogen bond interactions. At a lower

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temperature (< 50℃), the viscosity increases significantly with increasing surfactant

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concentration. As the temperature is increased further, however, their viscosities approach the

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same value. In order to minimize the influence of the viscosity of the electrolyte, all of our

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electrodepositions were carried out at a temperature of 90℃.

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Figure 2. Viscosity of the SDS in choline chloride-urea against the temperature.

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To ensure the formation of the micelle to be used for electrodeposition, we always maintain ≧

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20 mM SDS concentration in the electrolytes for the Cu electrodeposition experiments.

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According to results obtained from Hartely27 et al., the speciation of CuCl dissolved into ChCl-

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urea DES is [CuCl2]- and [CuCl3]2-. Figure 3 compares the cyclic voltammograms (CVs)

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without and with the presence of SDS at concentrations of 20 and 60 mM, which were recorded


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on a GC disc electrode immersed in a solution of 50 mM CuCl in ChCl-urea DES. When the

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potential was first scanned to negative and reversed at -0.8V, a redox couple (c1/a1) was

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observed. During the anodic scan, the typical current loop associated with a nucleation and

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growth mechanism was observed, indicating that the c1 peak related to Cu electrodeposition, and

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the oxidation peak a1 are related to Cu stripping. At a more positive potential, another oxidation

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peak (a2) was detected at 0.6V associated with Cu(I)/Cu(II) oxidation, similar to results reported

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by Abbott et al.23. However, the addition of SDS to the solution had a remarkable effect on the

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CVs. The Cu electrodeposited potential shifts to more negative potentials whereas the peak

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current decreases with increasing SDS concentration.

These phenomena suggest that SDS

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inhibits the Cu nucleation process and also the Cu electrodeposition by blocking the active sites

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of the substrate surface, due to the specific adsorption of SDS on the electrode surface, leading to

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an alteration in the structure of the double layer.

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Figure 3. Cyclic voltammograms of 50 mM Cu(I) in Reline with various SDS concentration

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recorded on GC electrode at 90℃. Scan rate 50mV s-1.


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To further elucidate the mechanism responsible for Cu nucleation with and without SDS in the

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electrolytes, chronoamperometry was performed. Nucleation experiments are very sensitive to

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the condition of the GC electrode surface and to eliminate this problem, the electrode was

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polished and carefully cleaned prior to each experiment.

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chronoamperometric experiments with and without SDS all show an increase with time before

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reaching a maximum then falling with an additional increase in time. Therefore, we normalized

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the measured current ݅ to the corresponding maximal current ( ݅௠ ) at time ‫ݐ‬௠ for various

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potentials in Figure 4.

The data obtained for the

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Scharifker and Hills28 developed a two extreme three-dimensional (3D) nucleation model to

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explain dimensionless chronamperometric results which are classified as instantaneous and

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progressive, respectively. Two equations for a theoretical dimensionless current and time plots

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for instantaneous and progressive change are shown below: ௜

ଵ.ଽହସଶ ቄ1 ௧ൗ ௧೘

ଶ

௧

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( )ଶ = ௜

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( )ଶ = ௜

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The chronoamperometric data were plotted for these two equations and the results are shown

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in Figure 4. As shown in Figure 4(a), the chronoamperometric data for the absence of SDS all fit

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reasonably well to the model of a 3-D progressive process where the nucleation of copper on the

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GC electrode in the ChCl-urea corresponds to a 3-D progressive process in the absence of SDS.

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On the other hand, Fig. 4(b) shows that, in the presence of SDS, all of the experimental data at t

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< tm fall between the theoretical curves for instantaneous and progressive kinetics that display an

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intermediate behavior, indicating that progressive three-dimensional nucleation only occurs on a

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finite number of active sites28-29. It switches to instantaneous nucleation when the reaction

೘

௜

೘

− exp ቂ−1.2564 ቀ௧ ቁቃቅ ,

ଵ.ଶଶହସ ൜1 ௧ൗ ௧೘

(1)

೘

௧

ଶ

ଶ

− exp ൤−2.3367 ቀ௧ ቁ ൨ൠ . ೘

(2)


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continues after the reaction reaches a maximum. The phenomenon indicates that the SDS

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adsorbs specifically to the electrode surface which may influence the nucleation and growth

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processes. The difference in the voltammogram and amperomogram in the presence of SDS

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should account for the different morphologies which are discussed below.

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Figure 4. Nucleation loop for Cu(I) in Reline (a)without (b) with 20mM SDS recorded on GC

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electrode at 90℃.


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We then performed a Cu electrodeposition on a Ni substrate under moderate hydrodynamic

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conditions in the electroplating bath by using a magnetic stirrer and then determined the

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crystallinity of the electrodeposits by powder X-ray diffraction (XRD) measurements. The XRD

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patterns of all of the electrodeposits obtained from the plating bath with/without SDS are shown

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in Figure 5. All of the diffraction peaks are indexed according to the standard cubic structure of

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Cu (JCPDS no. 04-0836). The results reveal that the specific adsorption of SDS on the electrode

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surface does not change the crystallinity of the product.

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Figure 5. XRD of Cu deposits obtained from 50mM Cu(I) in ChCl-urea at 90℃ in the presence of SDS (a) Ni substrate, (b) 0 mM, (c) 20 mM and (d) 60mM .

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Nevertheless, typical SEM and TEM images shown in Figure 6 show that the surface

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morphologies of the Cu films are altered after the addition of SDS. As can been seen in Fig.

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6(a), the Cu film contains nanometer sized grains, which are more compact at the bottom and

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have a large amount of dendrite structures on the top. TEM images shown in Fig. 6(d) indicate

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that the dendrite structure is dense. In addition, the electrodeposition of Cu in the presence of


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SDS in the electroplating bath under the same conditions as in Fig. 6(b) and (c) show that the

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dendrite structures disappear. The nuclei density decreases with increasing SDS concentration.

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The finding of a TEM study in Fig. 6(e) and (f) also reveal that spherical structures are formed. It

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has been reported30-31 that certain types of surfactants may change the surface chemistry of metal

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particles and restrain the aggregation of metal nuclei. The long chains of SDS molecules can act

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as a stabilizer/capping agent to adsorb to the nuclei surfaces via van der Waals interactions32.

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Moreover, the SDS adsorption on the electrode surface in ChCl-urea DES plays an important

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role for the electrodeposion of nanoparticles.

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Figure 6. SEM and TEM image of Cu electrodeposits obtained from 50mM Cu(I) in ChCl-urea

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in the presence of SDS (a,d) 0, (b,e) 20, and (c,f) 60 mM at 90℃. The applied potential was

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chosen at the peak potential.


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This interaction results in the modification of micelles which act as growth units to prevent

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dendrite structures from being formed.

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layers on the electrode by the surface forces, the choline cations of DES serve as counter-ions to

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neutralize the polar sulfate headgroup of the SDS molecules, and the Cu complex anions then

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accumulate near the micelle group surface. When the potential was applied from an open circuit

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potential to a sufficiently negative potential, some of the choline cations and SDS micelles

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started to form compact and thin interfacial layers on the electrode surface, and thus, a further

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reduction in the accumulation of Cu complex anions could take place33. Although Cu nuclei,

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which are continuously produced, can be formed on the electrode surface, the SDS micelles act

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as the stabilizer to prevent the subsequent attachment the Cu adatoms to the aggregate. Thus, the

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Cu particles are evenly distributed thus avoiding the formation of dendrite structures on the

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substrate.

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Conclusions

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At the beginning, the SDS micelles form interfacial

This study reports on a facile route for the electrochemical preparation of well distribution Cu

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nanoparticles via a SDS micelle from a ChCl-urea DES.

The specific adsorption of the

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surfactant on the electrode surface, the unique electrostatic interactions between the choline

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cations of DES and the sulfonate headgroup of SDS, and the formation of a double layer make

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this process different from that which occurs in an aqueous solution. Surface tension and

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conductivity measurements revealed that the SDS micelles were formed in the ChCl-urea DES.

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The process was investigated by detailed systematic electrochemical experiments.

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reduction potential shifts to more negative values with decreasing current density in the presence

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of SDS, indicating that a larger overpotential is required to initiate the nucleation and growth

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process. Chronoamperograms indicate that the presence of the SDS appears to influence the

The Cu


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nucleation process, as well as, the morphologies of the Cu electrodeposits. SEM images show

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that Cu particles are produced in the electrolyte with the SDS surfactant. Because the presence of

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SDS not only changes the double layer structures on the electrode interface they can also

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function as an inhibitor to prevent particle aggregation. As stated above, the novel system, which

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permits SDS to self-assemble in deep eutectic solvents, results in the development of a micelle

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structure for electrodeposition, a process that is cheaper and more environmentally friendly and

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can be extended to other metals or semiconductors.

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AUTHOR INFORMATION

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Corresponding Author

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Corresponding author: * [email protected]

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ACKNOWLEDGMENT

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This work was supported by the Ministry of Science and Technology, Taiwan. Thanks to Ms.

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C.-Y. Chien of Ministry of Science and Technology (National Taiwan University) for the

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assistance in TEM experiments.

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Terry, A. E.; Jackson, A. J. Micelle structure in a deep eutectic solvent: a small-angle scattering

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The influence of SDS micelles in ChCl-urea DES during the electrodeposion process.

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Micelle Structure in a Deep Eutectic Solvent for the Electrochemical

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