Tuning the electrodeposition parameters of silver to

Electrochimica Acta 81 (2012) 98–105

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Tuning the electrodeposition parameters of silver to yield micro/nano structures from room temperature protic ionic liquids Bryan H.R. Suryanto 1 , Christian A. Gunawan 1 , Xunyu Lu 1 , Chuan Zhao ∗,1 School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

a r t i c l e

i n f o

Article history: Received 18 May 2012 Received in revised form 17 July 2012 Accepted 17 July 2012 Available online 24 July 2012 Keywords: Electrodeposition Silver nanoparticles Protic ionic liquids Oxygen reduction reaction Electrocatalysis

a b s t r a c t Controlled electrodeposition of silver onto glassy carbon, gold and indium tin oxide-coated glass substrates has been achieved from three room temperature protic ionic liquids (PILs), ethylammonium nitrate, triethylammonium methylsulfonate, and bis(2-methoxyethyl)ammonium acetate. Cyclic voltammetric, chronoamperometric, together with microscopic and X-ray techniques reveal that micro/nanostructured Ag thin films of controlled morphology, size, density, and uniformity can be achieved by tuning the electrodeposition parameters such as potential, time, types of PILs, substrate materials, and ionic liquid viscosity by altering the water content. Chronoamperometric results provide direct evidence that electrodeposition of Ag in protic ionic liquids takes place through a progressive nucleation and diffusion-controlled 3D growth mechanism. The as prepared Ag micro/nanoparticles have been employed as electrocatalysts for oxygen reduction reaction and exhibit excellent catalytic activity. The study provides promise for using protic ionic liquids as alternative electrolytes to conventional aprotic ionic liquids for electrodeposition of metals and nanostructured electrocatalysts. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Electrodeposition is a fundamentally and technologically important process. It has been applied widely in industry that ranges from decorative and functional anti-corrosion to wearresistant coatings. Aqueous electrolytes are the most commonly used media in electrodeposition, and they have significant limitations such as narrow potential windows, gas evolution, necessity for complexing agents and environmental impact, since all of the water must eventually be returned to the water course [1–3]. Several alternative electrolytes have been found to substitute for aqueous solvent and overcome their limitations such as organic solvents. However, although organic solvents may have a wider electrochemical potential window than aqueous electrolytes but the volatility, toxicity and handling are the major issues for their applications at industrial scale [3]. Ionic liquids (ILs) are becoming increasingly important as electrolytes in electrodeposition of metals and can possibly circumvent these limitations. Generally speaking, ILs offer advantages such as large electrochemical potential window, high ionic conductivity and simplicity of handling due to their non-volatility and inflammability. In addition, many studies have demonstrated IL electrolytes are favourable for electrodeposition of nanocrystalline

∗ Corresponding author. Tel.: +61 2 9385 4645; fax: +61 2 9385 6141. E-mail address: [email protected] (C. Zhao). 1 ISE member. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.066

metals, while in aqueous media pulsed electrodeposition and addition of additives are required, which often complicates the reaction mechanisms significantly [1,4]. However, ionic liquids have not been widely used at industrial scale. One of the major barriers is the high cost of ILs. Additionally, the viscosity of most ILs is quite high compared to molecular solvents, which results in poor mass transport. The problem is complicated by the poor solubility of simple metal salts (e.g. chloride, nitrate, and sulphate) in most suitable ILs for electrodeposition, presumably due to the weak coordinating ability of the anions of ILs such as tetrafluoroborate [BF4 ]− , hexafluorophosphate [PF6 ]− or bistriflimide ([NTf2 ]− ). The lack of toxicity data also prevents the wide use of ILs [5]. Ionic liquids can be divided into two categories, aprotic ionic liquids (AILs) and protic ionic liquids (PILs). Most, if not all, ILs used in metal electrodeposition so far are aprotic ILs (AILs). PILs are formed by the variety of proton transfer and association equilibria from Brønsted acid to Brønsted bases [6–8]. Compared to AILs, importantly, PILs can be synthesised at much lower cost due to their straightforward synthetic procedure and ubiquity of starting materials such as amines and acids, which are well known in industry and fully characterised in terms of toxicity, biodegradability, cost, and environmental effects. It also has been found that the fluidities and attendant conductivities of PILs tend to be much higher than AILs, for reasons that are not completely clear [9]. PILs have attracted considerable attention and an increasingly large number of PILs have emerged over the past few years [10–14]. Recently we also have reported systematic accounts of the electrochemical properties of PILs for potential applications as electrolytes in

B.H.R. Suryanto et al. / Electrochimica Acta 81 (2012) 98–105

electrochemical systems [6]. Studies also have demonstrated that PILs are excellent candidates as electrolytes for fuel cell and for electrodeposition of conducting polymers [15]. However, the application of PILs as electrolytes for metal deposition has not been explored so far. Silver is a technologically very important metal for decorative, functional coatings and interconnect materials in computer chips because of its innate characteristics of having the lowest resistivity (1.6 ␮ cm−2 ) of all known common materials except superconductors [16]. Silver also has been recognised as a promising electrocatalysts alternative to platinum for the reduction of O2 , oxidation of methanol, deoxygenation of styrene and reduction of H2 O2 [17–20], with only 1% of the price of platinum [21]. Electrodeposition of silver is mostly carried out in aqueous electrolytes, which has significant environmental impact. For example, cyanide and thiosulphate based electrolytes have been widely used for silver electrodeposition, however, they are toxic and considered as environmental hazard [22,23]. Electrodeposition of silver also has been reported in aprotic ILs such as 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-butylmethylpyrollidinium bis(trifluoromethanesulfonyl)imide [24–27]. In AILs, Ag deposition usually follows progressive nucleation and growth processes, which are significantly affected by the water content in the AILs [24–27]. In this study, we report, for the first time, the electrodeposition of silver from room temperature protic ionic liquids (PILs). Three PILs, ethylammonium nitrate (EAN), triethylammonium methylsulfonate ([Et3 N][MeSO4 ], TAMS) and bis(2-methoxyethyl) ammonium acetate ([(MeOEt)2 NH][AcOH], MOEAA) have been prepared and their electrochemical potential windows have been investigated. The electrochemical behaviors of silver in the PILs have been examined by cyclic voltammetry and chronoamperometry, and the electrodeposited Ag was examined by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) elemental analysis. The electrodeposition mechanism of Ag in PILs media also has been compared with aqueous media and an aprotic IL, 1-butyl-3methylimidazolium-bistriflimide ([Bmim][NTf2 ]).

99

Fig. 1. Electrochemical potential windows obtained at a GC electrode under bench-top conditions for (a) ethylammonium nitrate (EAN), (b) triethylammonium methylsulfonate (TAMS) and (c) bis(2-methoxyethyl) ammonium acetate (MOEAA).

Electrochemical Workstation (CH Instrument, Texas, USA) using a three electrodes electrochemical cell. Glassy carbon (GC), platinum, and gold macro-disc electrodes were used as the working electrodes for voltammetric studies. Prior to experiments, the working electrodes were polished with an aqueous 0.05 ␮m alumina slurry (CH Instrument, Texas, USA) on microcloth polishing cloth (Buehler, Illinois, USA) then thoroughly rinsed with deionised water and acetone, then dried with low-lint tissue (Kimberly-Clark, Wisconsin, USA). An Ag wire sealed in a glass tube containing PIL of interest and 0.1 M Ag+ with a glass frit separating it from the test solution was used as the reference electrode, to which all potentials are referred. A platinum wire was used as the counter electrode. The electrodepositions of silver were conducted on 1 cm2 of GC, gold plates, and indium tin oxide (ITO)-coated glass (PrazisionsGlas and Optik GmbH, Iserlohn, Germany, 10 cm−2 sheet resistance), respectively. ITO was cleaned using sonication in acetone for 10 min followed by rinsing with absolute ethanol and drying under N2 stream.

2. Experimental

2.3. Physical characterization

2.1. Materials

All the ILs were used on the bench-top laboratory condition and their water contents were measured by Karl–Fischer (KF) titration using a 831 KF Coulometer (Metrohm, Switzerland). Scanning electron microscopy measurements were performed using FEI Nova SEM 230 with a Bruker energy dispersive X-ray (EDX) system operated on 10 kV accelerating voltage.

AgNO3 and AgBF4 were purchased from Prolabo and Sigma–Aldrich, respectively. KNO3 was purchased from Ajax APS. Ethylammonium nitrate (EAN, >97%) was purchased from Io-Li-Tec (Germany), high purity 1-butyl-3-methylimidazoliumbistriflimide ([Bmim][NTf2 ]) was purchased from Merck (Germany). Triethylammonium methylsulfonate (TAMS) and bis(2methoxyethyl) ammonium acetate (MOEAA) were synthesised according to the procedure described before [28]. Briefly, the starting reagents were purified, dried and handled under inert atmosphere. The equivalent value of neat Brønsted acids as well as Brønsted bases were then added into a round-bottom flask simultaneously while stirred vigorously to dissipate the exothermic reaction heat. The addition rate was determined by the reaction volume and was set between 0.2 and 10 ml min−1 . The as prepared PIL samples are highly pure, as confirmed by 1 H NMR and 13 C NMR and usually contain minimal water content in the range of ∼100 ppm [28]. 2.2. Electrochemical measurements All the electrochemical measurements were undertaken under normal bench-top laboratory conditions at 25 ± 0.2 ◦ C with a CH760

3. Results and discussion 3.1. Potential windows of the PILs Ionic liquids are hygroscopic materials and can take up significant amount of water from atmosphere when exposed to air. The protonic nature of PILs is also such that most PILs are highly hydrophilic. Therefore, the presence of water is almost inevitable in real applications using PILs. Fig. 1 shows the electrochemical potential windows obtained at a GC electrode in EAN, TAMS and MOEAA under bench-top laboratory conditions. KF titration measurements show that under bench top conditions EAN, TAMS and MOEAA contain 2.7 wt.%, 0.8 wt.% and 1.9 wt.% of water, respectively. The largest electrochemical potential window among the three “wet” PILs is obtained with TAMS (3.72 V). EAN shows a relatively large window of 3.30 V, while the smallest potential window of 2.14 V is obtained in MOEAA. These significant variations in

100


Fig. 2. Cyclic voltammogram obtained at the scan rate of 100 mV s−1 with a GC electrode in 0.1 M AgNO3 in EAN.

electrochemical potential window magnitude are due to variation of anion and cation species of the ionic liquids. Some minor peaks could also be observed within the potential window which can be attributed to the dissolved oxygen and impurities [29]. It has been established that the presence of water in ILs can substantially narrow the electrochemical potential window of ILs. In this study, the potential windows of the PILs were measured under both “benchtop” and “dried” conditions. It shows that the “bench-top” PILs have significantly smaller potential windows than the “dried” ones (EAN, 3.45 V, TAMS, 4.73 V and MOEAA, 2.37 V), respectively. Furthermore, it is observed that the electrochemical potential window also depends strongly on the materials of the working electrode. The largest potential window is normally obtained at GC electrodes. Pt and Au electrodes normally exhibit smaller cathodic potential limits than GC electrodes. These noble metal electrodes are more sensitive to the reduction of protons and impurities such as water, as is in conventional organic solvent (electrolyte) media [30]. Large electrochemical potential window is considered to be one of the major advantages of ionic liquids for electrodeposition of metals. Fig. 1 shows that even with the presence of proton and absorbed water from gas phase, quite decent potential window can still be achieved in PILs, which are comparable to some AILs, such as [Bmim][NTf2 ] (4.2 V, 0.10 wt.% water content). 3.2. Electrochemical behaviour of Ag+ in PILs Fig. 2 shows a typical cyclic voltammograms obtained on a GC electrode in EAN containing 0.1 M AgNO3 . Silver reduction process was initiated at the potential of −0.20 V on the initial cathodic scan. The reverse potential scan produces a current loop at ca. −0.31 V and −0.10 V, indicative of a nucleation and growth mechanism for Ag deposition [26], and a sharp stripping peak at 0.65 V. The pair of peaks is ascribed to the reductive deposition of Ag+ onto the electrode surface and subsequent oxidative stripping of the silver metal. A decrease of the reduction onset potential to −0.03 V is observed in the subsequent scans, which also implies the nucleation and growth processes for silver deposition [31]. The Ag stripping peak currents are always higher than deposition peak currents. Moreover, the absence of underpotential deposition indicates minimal electrodeposited Ag–GC interaction. Similar voltammetric behavior also has been observed on gold and ITO electrodes and with other two PILs, TAMS and MOEAA, with slight variation in peak potentials. Fig. 3a shows that the Ag reduction peak potential shifts by 0.193 V to more negative values when the scan rate is increased

Fig. 3. (a) Linear sweep voltammograms obtained at a GC electrode for the reduction of 0.1 M Ag+ in EAN at the scan rate of (i) 12.5 mV s−1 , (ii) 25 mV s−1 , (iii) 50 mV s−1 , (iv) 100 mV s−1 , and (v) 150 mV s−1 , respectively. (b) Dependence of peak potential, Ep on log v, and (c) dependence of peak current, Ip on v1/2 for the reduction of Ag+ in EAN.

from 12.5 mV s−1 to 150 mV s−1 , as a consequence of uncompensated iR drop. Electrodeposition of Ag onto GC electrode is an electrochemically irreversible process, which is also confirmed by a linear relationship between the peak potential (Ep ) and log v (Fig. 3b). The coefficient of charge transfer, ˛, is calculated to be 0.62 using Eq. (1) [30,32]: |Ep − Ep/2 | =

1.857RT ˛n˛ F

(1)

where R is the gas constant, F is the Faraday’s constant and T is an absolute temperature. Fig. 3c shows that a linear relationship between the peak current (Ip ) and v1/2 . The positive intercept on the current axis is indicative


101

Fig. 4. SEM images of silver electrodeposited onto a GC electrode from EAN containing 0.1 M Ag+ with different deposition parameters. (a, b) Edep = −0.2 V, t = 30 s; (c, d) Edep = −0.6 V, t = 30 s; (e, f) Edep = −0.2 V, t = 60 s; (g, h) Edep = −0.2 V, t = 300 s.

of the nucleation process of Ag deposition [33]. By assuming the process is fully irreversible and neglecting the electrode area change introduced by metallic silver deposition, the diffusion coefficient (D) of Ag+ in “bench-top” EAN was calculated using Eq. (2) [30,32]: Ip = 0.4958nF 3/2 CAD1/2 v1/2

˛n 1/2 ˛

RT

(2)

where A is the surface area of working electrodes, v is the value of scan rate, C is the concentration of silver ion inside ionic liquid, D is the diffusion coefficient, ˛ is the charge transfer coefficient and n˛ is the number of electrons. Thus the diffusion coefficient of 2.0 × 10−6 cm2 s−1 was obtained, which is significantly smaller than that in aqueous electrolyte solution (2.6 × 10−5 cm2 s−1 , 0.1 M KNO3 ), but more than one order of magnitude larger than that in viscous aprotic ionic liquids, e.g. [Bmim][BF4 ] (4.2 × 10−7 cm2 s−1 ) [25]. 3.3. Tuning the electrodeposition parameters to yield different morphologies of silver 3.3.1. Deposition potential Controlled potential electrolysis at Edep = −0.2 V for 30 s in EAN at a GC electrode leads to deposition of firmly attached, pale gray films on the electrode surface by visual inspection. SEM images reveal the formation of well-separated clusters of Ag (Fig. 4a). Inspection of images at higher magnification (Fig. 4b) shows that each cluster consists of silver microcrystals with rather smooth surface. The diameters of the Ag microcrystals formed generally lie in the range of 1–5 ␮m with some even smaller crystals being present. Distinctly different morphology is achieved when electrolysis is carried out at a more negative potential, Edep = −0.6 V. A smooth, grey film derived from a mixture of anisotropic short microneedles and dendritic crystal structures is now formed (Fig. 4c and d). The anisotropic crystal growth may be attributed to couloumbic attraction between discrete Ag crystal plane and Ag+ in PILs, Ag+ thus will experience stronger coulombic attraction at more negative potentials and restrain the surface diffusion of Ag ad-atom and obstructing the creation of new nuclei [24]. EDX analysis confirms that electrodeposited materials are pure Ag in all samples. 3.3.2. Deposition time Controlled potential electrolysis at Edep = −0.2 V for 60 s, a grey film composed of silver clusters are obtained (Fig. 4e and f).

In comparison to shorter deposition time of 30 s (Fig. 4a and e), longer deposition time leads to higher density of silver clusters. Significant changes in morphology are observed when the deposition time is extended to 300 s, where a white-grey thin film with densely packed silver microcrystals is obtained (Fig. 4g and h). The formed silver crystals are smaller in size and more uniform than that obtained at 30 s and 60 s (Fig. 4a and e). This meets the expectation that Ag+ are being deposited at more nuclei as the deposition time is increased. 3.3.3. Types of PILs Triethylammonium methylsulfonate (TAMS) and bis(2methoxyethyl) ammonium acetate (MOEAA) are two protic ionic liquids which exhibit relatively low viscosity (100 mPa s for TAMS and 10.1 mPa s for MOEAA), good conductivity, large potential windows [28]. Fig. 5 shows that distinctly different surface morphologies are obtained using different PILs, while all other parameters (deposition potential, time and substrate) remain unchanged. Using TAMS as electrolyte leads to formation of a thin film of nanocrystals of silver with a typical width of 200 nm. Somewhat unexpectedly, deposition in MOEAA results in formation of separated rose-like clusters with a typical diameter of 1 ␮m. Each rose cluster is formed by a number of nano-sheets having a thickness of ca. 60 nm. To the best of our knowledge, such rossete structure has not been reported before and might proven to be useful in electrocatalytic application as the surface area of these structures is larger than the regular deposits [34,35]. The detailed mechanisms for formation of different morphologies in the three PILs are not yet fully understood. Slight variations in Ag reduction potentials obtained in three PILs in principle could contribute to the difference in morphology. Furthermore, it has been postulated that different interfacial properties of the electrode/protic ionic liquids interfaces, such as surface tensions, adsorptions, electric double layer structures, etc., have played significant roles [1]. 3.3.4. Electrode substrate materials Substrate providing not only mechanical support to the electrodeposit, but affects significantly the structural and uniformity of a film. Electrodeposition of silver onto GC, gold and ITO glass has been carried out in EAN to demonstrate the effect of substrate (Fig. 5). Fig. 4e reveals that well-separated clusters of silver microcrystals are formed at GC electrode under conditions of Edep = −0.2 V, and t = 60 s. Electrodeposition under the same conditions onto gold electrode surfaces results in separated individual

102


Fig. 5. SEM images of silver electrodeposited onto a GC substrate at Edep = −0.2 V for 60 s from (a) TAMS and (b) MOEAA. Images (c) and (d) are silver deposited from EAN at Edep = −0.2 V for 60 s on gold and ITO substrate, respectively. The inset illustrates higher magnification image of the middle crystal.

single crystals with a wide range of size distribution from 200 nm to 3 ␮m (Fig. 5c). In contrast, electrodeposition on ITO glass results in more uniform distribution of single crystals of silver with a typical width ranging from 100 nm to 250 nm (Fig. 5d). The results show that the morphology of the electrodeposited silver is strongly substrate dependent. Similar phenomenon has often been reported in electrodeposition of metals in conventional media, and is generally attributed to different mechanical, magnetic catalytic and electric properties of the substrate materials [36].

3.3.5. Water content in PILs As mentioned earlier, most PILs are hydrophilic. Our recent results show that some “dried” PILs can absorb up to 15.8 wt.% of water when exposed to water-saturated nitrogen gas environment [37]. The presence of water in an ionic liquid is known to significantly influence physical properties such as viscosity, density, surface tension and hence increase the diffusion coefficients of metal ions [38,39]. These changes in physical properties may affect the morphology of electrodeposited materials. With respect to metal ions, aquated cations such as [Ag(H2 O)x ]+ may be formed. All of these modifications inevitably will alter the properties of the ionic liquid and the solutes contained therein and subsequently affect the electrodeposition processes. Recently we also demonstrated that water content can be used as a tool to tune the morphology of electrodeposited charge transfer complexes, AgTCNQ (TCNQ = tetracyanoquinodimethane) from an aprotic ionic liquid [Bmim][BF4 ] [40]. Smaller crystals of AgTCNQ were obtained in “dried” [Bmim][BF4 ] than that in “wet” [Bmim][BF4 ], as a result of smaller diffusion coefficients of TCNQ and Ag+ , which produces a slower growth process at the electrode surface and hence smaller structures [40].

The cyclic voltammetric studies of 0.01 M Ag+ in EAN reveal a significant increment on peak current of silver reduction process along with increasing water content. This could be ascribed to the decrease of viscosity in EAN and consequently the increase of diffusion coefficient of Ag+ . The changes of viscosity result into the transformation of surface morphology. Fig. 6 shows the SEM images of silver deposited onto GC substrate at Edep = −0.2 V in very “wet” EAN (15.8 wt.%) for 60 s. Dendritic structures are now obtained, in comparison to the granular structures obtained with normal “bench-top” EAN (2.7 wt.%, Fig. 4e and f). It is also suggested that the addition of water increases the conductivity of ILs as water–ion interactions are energetically smaller than ion–ion interactions allowing increase in conductivity [41]. Consequently, addition of water can be used as a tool for controlling silver deposits morphology via controllable changes in physical properties of ionic liquids. 3.4. Detection of progress nucleation and diffusion-controlled growth mechanism Chronoamperometries were employed to provide information related to the type of nucleation and growth involved in electrodeposition of silver in three PILs. Fig. 7a shows a series of chronoamperometric current–time (j–t) transients in EAN obtained at a GC electrode when the potential is stepped from 0.6 V, where no faradic process occurs, to progressively more negative values over the range of 0.05 V to −0.375 V, which corresponds to potentials encompassing the Ag deposition. All transients exhibit current maxima which shift to shorter time as the potential stepped into more negative values. Capacitance current predominated at a very short time and decayed at longer time for all j–t data. We observed that the faradic current become more prevailing at longer time


103

Fig. 6. SEM images of silver deposited onto a GC electrode for 60 s from (a) “bench-top” EAN (water content 2.7 wt.%). Image (b) is the central part of (a).

which associated to the growth of independent Ag crystals and/or the birth of new crystals. All the transients were decreasing at different rates after the maximum current density were reached and eventually merge into a common current–time profile (Fig. 7a). The observed behaviours can be explained by the development and overlap of the hemispherical nuclei and finally unite into planar electrode which responsible to the linear diffusion [40]. Using the j–t transients the diffusion coefficient (D) of Ag+ in “bench-top” EAN, TAMS and MOEAA were also calculated using the Cottrell equation (Eq. (3)) [30]: j=

nFD1/2 C 1/2 t 1/2

(3)

The D values in each PIL were obtained by analyzing the part after the current maximum on j–t curves obtained under 5 high electrodeposition potentials. The averaged values of D were

summarised in Table 1. Gunawardena et al. also have shown that the number density (No ) of nuclei formed can be calculated from the rising portion of the j–t curve when the diffusion field of the nuclei do not overlap using Eq. (4) [42]: j = 1.04nF(2DC)3/2 M 1/2 No t 1/2 r −1/2

(4)

where j is the current density, M is the atomic mass number of deposited metal, and is the density of the deposited metal. The No for Ag nucleation process in “bench-top” EAN, TAMS and MOEAA has been calculated using Eq. (4) and the results are summarised in Table 1. The No obtained are found to increase with the electrodeposition potential, which is consistent with previous studies and is due to the increase of driving force and the rate of nucleation [27,42]. To determine whether the nucleation type is instantaneous or progressive, the current transients obtained from Ag+

Fig. 7. (a) Chronoamperometric current transient for electrodeposition of silver onto a GC electrode in EAN containing 0.01 M AgNO3 at deposition potential of (i) −0.375 V, (ii) −0.275 V, (iii) −0.175 V, (iv) −0.100 V and (v) 0.05 V, respectively. (b–d) Comparison of normalised experimental data obtained in (b) EAN, (c) TAMS and (d) MOEAA with theoretical plots based on the 3D instantaneous and progressive nucleation and diffusion-controlled growth process. The potential steps were all initiated at potential at which no faradic current was observed.

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Table 1 Calculated nucleation number density (No ) and diffusion coefficient (D) of Ag+ in “bench-top” EAN, TAMS and MOEAA from chronoamperometric j–t transients. PILs

Electrodeposition potential vs. (Ag/Ag+ )/V

Nucleation number density (No )

Diffusion coefficient (D)/cm2 s−1

EAN

−0.175 −0.275 −0.375

1.93 × 10 3.92 × 106 1.60 × 107

1.10 × 10−6

TAMS

−0.175 −0.275 −0.375

5.17 × 104 4.62 × 105 5.42 × 106

2.30 × 10−6

MOEAA

−0.2 −0.3 −0.4

1.03 × 107 2.97 × 107 1.50 × 108

4.35 × 10−6

6

electrodeposition in three PILs are presented in a non-dimensional form and compared to values derived from a theoretical model proposed by Scharifker and Hills [43]. The theoretical model is based on a 3D nucleation and diffusion-controlled growth, and is commonly found in the electrocrystallization of metal and semiconductors in ILs. For instantaneous nucleation and growth, the theoretical model is described by Eq. (5) [43]:

I 2 Imax

= 1.9542

t tmax

1 − exp

1.2564t 2

(5)

tmax

For progressive nucleation and growth the model can be described as Eq. (6) [43]:

I 2 Imax

= 1.2254

t

max

t

1 − exp

−2.33674t 2

2

2 tmax

(6)

where Imax is the current obtained at tmax . Fig. 7b–d contains sets of non-dimensional experimental plots obtained in EAN, TAMS and MOEAA, versus those predicted for the limiting cases of instantaneous and progressive 3D nucleation with diffusion-controlled growth [43]. In the three PILs studied, experimental data match closer to the theoretical curve derived from a progressive nucleation. For comparison, electrodeposition of silver was also carried out in an aprotic ionic liquid [Bmim][NTf2 ] and an aqueous solution of 0.1 M KNO3 . The current-transients in [Bmim][NTf2 ] (not shown) displays an instantaneous nucleation with mixed kinetics and diffusion control of growth [44], which is in accordance with studies reported by other groups in [Bmim][NTf2 ] and [Bmim][BF4 ] [24,25]. In aqueous media (0.1 M KNO3 ), the instantaneous nucleation mechanism follows [25]. 3.5. Applications of electrodeposited silver as electrocatalysts for oxygen reduction reaction Platinum is the most commonly used electrocatalysts for many important processes such as oxygen reduction reaction (ORR). However, platinum is a very expensive material and the supply of Pt is also not sustainable. Recent investigations have shown acceptable electrocatalysis activity from Ag with significantly lower cost than Pt [21]. In the study, a GC electrode modified with silver microparticles were prepared by electrodeposition of Ag from EAN containing 0.1 M Ag+ with Edep = −0.2 V for 300 s (see morphology in Fig. 4g), and applied for ORR. Fig. 8 shows the cyclic voltammograms obtained in an oxygen saturated phosphate buffer solution (PBS, pH = 7.0). The onset potential for O2 reduction at a bare GC electrode is detected at ca. −0.145 V vs. Ag/AgCl with an irreversible peak at −0.6 V. The identity of peak is confirmed by purging the solution with O2 for 5 min, which leads to the increases of the peak current. The process is attributed to the reduction of oxygen [45]. Using the GC electrode with electrodeposited Ag particles, significant increase in oxygen reduction peak current is observed. Concomitantly, the onset potential and the peak potential shift to −0.070 V and −0.44 V,

Fig. 8. Cyclic voltammograms for oxygen reduction reactions obtained at a scan rate at 100 mV s−1 with (i) a bare GC electrode and (ii) GC electrode decorated with Ag nanoparticles in 0.1 M PBS solution saturated with O2 .

respectively, indicating significant electrocatalytic activity of the silver particles [17]. Further works are currently in progress to investigate the dependence of the catalytic effect on silver morphologies for ORR.

4. Conclusions Controlled electrodepositions of silver micro- and nanoparticles have been achieved from three room temperature protic ionic liquids. Voltammetric and microscopic investigations suggest that the electrodeposition processes are strongly dependent on deposition potential, time, type of PIL, electrode material, and water content. A range of nano- and microstructured Ag thin films can thus be obtained by tuning the electrodeposition parameters. Electrodeposition of Ag+ from the three PILs appears to follow a progressive nucleation and diffusion-controlled growth mechanism, as verified by using chronoamperometry. This study demonstrates that PILs are promising electrolytes for electrodeposition of technologically important metals, with major advantage of low cost and low toxicity in comparison to aprotic ionic liquids. Further work is also under the way to prepare novel PILs and to establish their use as “green” electrolytes for electrodeposition of metals, and the applications of the metal nanoparticles as electrocatalysts for electrochemical energy conversion.

Acknowledgements We are grateful to UNSW Mark Wainwright Analytical Center for providing access to their SEM facilities. The study was financed by Australian Research Council (DP110102569).


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