Transition from amorphous to crystalline state for nickel electrodeposited from an ionic liquid

Abstract

The microstructure of metallic nickel electrodeposited from an ionic liquid onto a copper substrate may be tuned and transition from amorphous to crystalline state can be achieved by regulating water content in the bath.

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Due to its widespread applications in catalysis,^1a conductors,^1b supercapacitors^1c and electronics,² metallic nickel has received an upsurge of interest; in particular, Ni particles, films, alloys and composites with tuneable crystallinities and microstructures, encompassing both grain size and shape, have been studied. Electrodeposition has been a very efficient and superior preparation method compared to its chemical counterparts, owing to its scope and good control of morphology and microstructure of the deposit. There have been numerous reports on electrodeposited nickel from aqueous,^3,4 nonaqueous,⁵ and surfactant-based organized media, such as micelles, reverse micelles and microemulsions.^6–9 In practice, the widespread use of aqueous media is restricted by bulk deposition, limited potential window and hydrogen evolution reaction.^3,4 Microemulsions^6,7 and reverse micellar^8,9 systems have so far been reported for deposition of cobalt and, to a limited extent, nickel. These systems offer controllable size and morphology by exploiting a cage-like confinement effect of surfactant-stabilized microcavities;¹⁰ however, the systematic control of crystallinity still remains a dream. Alternative electrolytic media that can provide a large electrochemical potential window, low volatility, recyclability, and controlled directional growth of the electrodeposited film are, therefore, critically sought after.¹¹

Ionic liquids (ILs) have recently drawn significant attention for their characteristic wide electrochemical potential windows.^5,11,12 Since they comprise entirely ions, ILs are expected to play a crucial role in the morphology, microstructure and crystallinity of the deposited metal through the control of diffusion of the metal ions to be reduced at the electrode surface. The electrodeposition of nickel has already been studied from chloroaluminate, chlorozincate,^5a,b,12 and eutectic-based IL^11a baths. These systems suffered from the drawback of air and moisture instability.^11a Zhu et al.^5e and Deng et al.^5d reported on the electrochemical behaviour of nickel ions in pyrrolidinium- and dicyanamide-based ILs, respectively. Both studies inferred that electrodeposition proceeds via three-dimensional progressive nucleation with diffusion-controlled growth on the substrates,¹³ but a water-free environment has been a pre-requisite. Recently, we successfully deposited metallic cobalt on a copper substrate under ambient conditions from a hydrophilic IL bath containing 1-ethyl-3-methylimidazolium ethylsulfonate ([EMI][EtSO₄]).¹⁴ However, an efficient means to deposit nickel with systematic control of crystallinity from IL under ambient conditions is yet to be explored. In this study, we have, for the first time, been successful in tuning the microstructure of metallic nickel electrodeposited on a copper substrate from dry [EMI][EtSO₄]. Furthermore, we have established the principle of controlling the crystalline/amorphous nature of the metallic deposit by deliberate addition of water.

Nickel chloride (NiCl₂; Merck, Germany), potassium chloride (KCl; Merck, Germany) and [EMI][EtSO₄] (Sigma Aldrich, Germany) used in this study were of analytical grade and used as received. All aqueous solutions were prepared with deionized water (specific conductivity = 0.055 μS cm⁻¹, BOECO pure, Model: BOE-8082060, Germany). Electrochemical measurements were carried out with a computer-controlled electrochemical workstation (Model: 600D, CH Instruments Inc., USA). Trace water from the IL was removed using the strategy of water electrolysis.¹⁵ A homemade copper disk electrode (purity of >99.0%) with an area of 0.009 cm² was used as the substrate. A silver (Ag) wire was used as a quasi-reference electrode in the IL medium and a platinum wire served as the counter electrode. The solution was purged with dry N₂ gas to ensure an inert environment for electrochemical measurements. Electrodeposition was performed by applying constant potentials of −0.95 V vs. Ag/AgCl for 10 min in aqueous medium. The potential applied in [EMI][EtSO₄] was −1.20 V vs. Ag quasi-reference electrode for 120 min, both in the absence and presence of water. Scanning electron microscope (SEM; Model: JSM-6490LA, JEOL, USA), energy dispersive X-ray (EDX; Model: JSM-6490LA, JEOL, USA) and X-ray diffraction (XRD; Model: D8 Advance, Bruker, Germany) analyses were performed to characterize the electrodeposited metallic nickel.

Fig. 1 shows the cyclic voltammogram (CV) as recorded for the redox reaction of nickel(II) at the copper electrode in aqueous solution. A well-defined cathodic peak was observed at ca. −0.95 V, and the current of the peak increased as the concentration of nickel salt was increased. Thus, the cathodic peak was due to reduction of nickel(II) to metallic nickel at the copper electrode.⁸

Fig. 1 CV of 0.03 M NiCl₂ measured at a copper electrode in aqueous solution containing 0.40 M KCl at a potential scan rate of 0.05 V s⁻¹.

Fig. 2 presents the CV measured at a copper electrode in a [EMI][EtSO₄] bath in the absence and presence of the nickel salt. No characteristic peak was observed in the absence of nickel(II) within the measured potential range from −0.60 to −1.60 V; in the presence of the nickel salt, a well-defined cathodic peak appeared at ca. −1.2 V due to reduction of nickel(II) (Fig. 2a), similar to the peak observed in aqueous solution.⁸ The process was a diffusion-controlled one and the increase in the concentration of nickel salt in the IL resulted in a linear increase in the cathodic peak current (i_pc; data not shown). Upon addition of water in the IL bath, a significant change in the shape of the CV was observed: the cathodic peak potential (E_pc) shifted toward negative potential, a shift that was accompanied by an increase in the i_pc (Fig. 2b and c). Although the shift of the E_pc due to the addition of water needs to be clarified, the increase in the i_pc can be ascribed to faster diffusion of nickel(II), resulting from a decrease in the viscosity of the medium when water was added to the IL bath. Electrodeposition of metallic nickel could be successfully carried out from aqueous solution, dry IL and IL containing water (vide infra).

Fig. 2 CVs recorded for 0.03 M NiCl₂ in [EMI][EtSO₄] containing (a) 0, (b) 1 and (c) 4% water at a copper electrode. CVs that are labelled with prime sign were recorded without NiCl₂.

Fig. 3 compares the SEM images of the nickel deposited on the copper substrate from aqueous solution, dry [EMI][EtSO₄] and [EMI][EtSO₄] containing 4% water. This comparison allowed us to monitor the influence of added water on the morphology of the deposit. The electrodeposition of nickel from both aqueous solution and IL was confirmed by EDX measurement (Fig. 4), and showed layer-like film structures (Fig. 3a). On the other hand, nickel deposits from dry IL formed a thin film (Fig. 3b). The SEM image displayed in Fig. 3c for [EMI][EtSO₄] containing 4% water exhibited significant and noticeable change in deposit morphology as compared to those from aqueous solution or the IL. The regularity and periodic nature of the particles in the thin film were marked and the shapes of the deposits were nearly spherical. The differences, observed even from SEM images, were quite promising and indicative of possible control of crystallinity.

Fig. 3 SEM images of nickel electrodeposits on copper substrates from (a) aqueous, (b) dry [EMI][EtSO₄] and (c) [EMI][EtSO₄] containing 4% water.

Fig. 4 EDX spectra of nickel electrodeposits from (a) aqueous solution, (b) dry [EMI][EtSO₄] and (c) [EMI][EtSO₄] containing 4% water on copper substrates.

The XRD pattern of nickel deposited from aqueous and IL baths are shown in Fig. 5. The characteristic peaks for deposited nickel as revealed in Fig. 5A(a) were detected at 2θ values of 40.30°, 44.50°, 51.84° and 66.37° for the reflection planes of NiO(012), Ni(111), Ni(220) and NiO(110), respectively. The XRD patterns clearly showed the characteristic responses expected for nickel with a face centred cubic (fcc) structure (JCPDS card no. 04-0850).¹⁶ The peaks at 2θ values of 43.29°, 50.43°, 74.13°, 89.93° and 95.14° were assigned to the planes of (111), (200), (220), (311), and (222), respectively, of the copper substrate. Interestingly, the nickel deposited from IL bath does not show any characteristic peaks except for those of the copper substrate (Fig. 5b), although responses of both copper and nickel can be seen in the EDX spectra (Fig. 4b); thus, the deposit is amorphous in nature.

Fig. 5 XRD patterns of nickel electrodeposits from (A) aqueous solution (a) and [EMI][EtSO₄] (b) and (B) [EMI][EtSO₄] containing (a) 2%, (b) 4% and (c) 6% water on copper substrates.

Addition of water to the bath containing the IL resulted in an interesting variation in the deposit morphology. Fig. 5B(a–c) shows the development of a single crystalline facet of Ni(201) at 2θ value of 89.93° when water was added to the IL, although the facet disappears in the deposit made from 100% water. In pure water, different facets, namely, Ni(111), Ni(220) and Ni(110), indicate polycrystalline nature, whereas the facets for NiO(110) and NiO(012) indicated that the deposited metal was prone to oxidation when deposited from pure water. This was supported by the cracks observed in the SEM image shown in Fig. 3a and is in agreement with other literature reports.⁸ The crystallite sizes of the nickel electrodeposits on copper substrates from [EMI][EtSO₄] with different water contents (2–6%) have been calculated for Ni(201) plane from the XRD patterns. Fig. 6 shows the variation of crystallite size with respect to water content in the IL bath. It is clear that the crystallinity is tuneable through manipulation of the composition of the medium in the bath for electrodeposition.

Fig. 6 Crystallite size of nickel(201) electrodeposited from [EMI][EtSO₄] on copper substrates as a function of water content.

The analyses of electrochemical, SEM, EDX and XRD results clearly show that the nickel on copper substrate electrodeposited from IL is amorphous, whereas that from aqueous solution is crystalline. The microstructure changes from aqueous medium to the IL and the definite size and shape of the deposit may be achieved with an optimum addition of water in the IL, which is not surprising. The amorphous growth of nickel on copper substrate in pure IL is associated with a slower diffusion rate of nickel(II) ions and a lower wettability of solid nickel¹⁷ versus aqueous solution. As water is added, the crystallization of nickel nuclei becomes increasingly favoured due to changes in the microscopic environment of the IL.¹⁸ In the pure state, ILs in general are believed to exist in the form of larger aggregates or clusters through interactions on the microscopic level; these interactions break down upon the addition of water into smaller aggregates or ion pairs. At an optimum mole fraction of the IL, ions are isolated by water molecules and attain increased number of charged species and higher ionic mobility. In aqueous solution, the diffusion of nickel(II) species to the electrode surface under the influence of potential is presumably faster, which may contribute to the formation of the observed layer-like film structure (Fig. 3b), rather than the particles of definite shape that were observed from the IL system with added water (Fig. 3c).

Such control of the microstructure and crystallinity of the metal deposit is intriguing and contradicts the notion that water is a problem to be overcome when using IL media. The amount of water to be added, however, should vary depending on the structure of the ILs and their aggregation behaviour. Work is now in progress to systematically study the crystallinity, size, shape and morphology of nickel electrodeposits from ILs with different cations and anions in the presence of different amounts of water. It is very interesting to note that deliberate addition of water to hydrophilic ILs can be advantageous for electrodeposition of nickel, resulting in desirable and tuneable microstructure and morphology for task-specific applications.

Acknowledgements

The authors acknowledge the use of laboratory equipment for the present work procured under the subproject, CP 231 of the Higher Education Quality Enhancement Project of the Ministry of Education, Government of Bangladesh. The authors also express their gratitude to the Centre for Advanced Research in Sciences of Dhaka University, Bangladesh for permitting the use of its SEM and EDX facilities. The authors acknowledge Prof. Y. Katayama, Department of Applied Chemistry, Keio University, Japan, for useful discussion.

Notes and references

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