Electrodeposition of CuZn from Chlorozincate Ionic Liquid: From

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Electrodeposition of CuZn from Chlorozincate Ionic Liquid: From Hollow Tubes to Segmented Nanowires Yi-Ting Hsieh, Ren-Wei Tsai, Chung-Jui Su, and I-Wen Sun* Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan ABSTRACT: The growth of hollow tubes, nanowire array, and segmented porous nanowire arrays made of Cu−Zn alloys in a Lewis acidic ZnCl2-1-ethyl3-methylimidazolium chloride ionic liquid via direct electrodeposition without the need for a template is presented. The formation of each of type of structure is described. Hollow tubes result from the uneven overpotential gradient created at low Cu(I) concentration and low applied deposition overpotential. Nanowire arrays form under mass-transport-limited conditions, in which the ionic liquid speciation plays an important role. Segmented porous nanowire arrays are obtained by increasing the Cu(I) concentration to enhance the concentration profile vibration near the growth surface. The electrodeposited nanowire arrays show very good efficiency for the electrocatalytic reduction of nitrate ions in alkaline aqueous solution.

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INTRODUCTION The synthesis of micro-/nanostructured materials is of interest because of the wide applications of these materials, including catalysis, electronics, sensors, and biomedicine.1−3 Among the various techniques applied to synthesize micro-/nanostructures, electrodeposition stands out as a convenient and cost-effective technology. Control over of the morphology of the deposits can be achieved by varying parameters such as the concentration of precursors, deposition potential, deposition charge, and temperature. For example, when the deposition is performed with a fairly low overpotential (less negative potential), the deposition is at or near equilibrium conditions, under which the deposits grow in a manner that minimizes surface energy, leading to crystals with smooth facets. When the deposition is performed at a large overpotential that is far away from equilibrium, a depletion zone (diffusion layer) of the precursor forms, and the deposit growth becomes mass-transfer-limited. In this case, deposits with a branched structure, such as dendrites, are often obtained.4 The morphology can further be modified by additives that preferentially adsorb on certain positions of the growing deposits and thus affect the relative growth rate of the surface. Although many interesting structures, including polyhedral crystals and dendrites, have been obtained by direct electrodeposition,5−7 structures such as hollow tubes and wires are almost exclusively produced using template-assisted methods,8 in which the nanowires or tubes are electrodeposited into the channels of a premade template followed by the removal of the template to expose the deposited wires or tubes. Ionic liquids (ILs) are a class of salts that are liquid at temperatures below 100 °C, differentiating them from hightemperature molten salts that contain primarily inorganic ions. Most ILs contain large organic cations and inorganic anions. ILs have attracted a lot of attention in recent years for their © 2014 American Chemical Society

potential in applications such as chemical synthesis, catalysis, separation science, mass spectroscopy, energy devices, electrochemistry, and more. General advantages of ILs over water and conventional molecular organic solvents include a wide electrochemical window, high thermal stability, negligible vapor pressure, low flammability, and intrinsic ionic conductivity.9,10 Furthermore, aprotic ILs can eliminate the problems associated with hydrogen evolution that occurs during the electroreduction carried in protic electrolytes. The higher available operating temperature of ILs favors the formation of deposits with good crystallinity.11 These advantages make ILs suitable electrolytes for the electrodeposition of many materials that is not viable using molecular solvents. Various metals, their alloys, and polymers have been electrodeposited from various ILs.12−14 These studies have shown that a specific IL may be important in controlling the morphology of a metallic deposit. For example, bulk aromatic cations usually lead to small architectures. Regarding the electrodeposition of micro-/nanostructures from ILs, templatebased methods have been adopted for the deposition of nanoporous Cu15 and Si16 using polystyrene spheres, Al,17 Ag,18 and Si19 nanorods and/or tubes using polycarbonate membranes, and CuZnSn nanorods20 using anodized aluminum oxide membrane. Template-free methods have also been used for the direct deposition of monodispersed metal nanocrystals,21,22 (polyhedral) iron pseudorods and rings,23 and Te tubes.24 We have previously reported the direct deposition of hollow CuSn tubes from 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA) IL,25 and NiZn,26 FeCoZn,27 AuZn,28and Co29 nanowire arrays from the corresponding metal chlorideReceived: July 9, 2014 Revised: August 26, 2014 Published: September 2, 2014 22347

dx.doi.org/10.1021/jp506833s | J. Phys. Chem. C 2014, 118, 22347−22355

The Journal of Physical Chemistry C

Article

was analyzed by X-ray diffraction (XRD, Shimadzu, XRD7000). Electroreduction of nitrate ions was performed at room temperature with a three-electrode cell controlled by a CHI 6142C potentiostat (CH Instruments, U.S.). A platinum spiral wire and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The electrolyte contained 0.016 M NaNO3 (Riedel-de Haen), 0.1 M NaHCO3 (J.T. Baker), 0.006 M NaCl (SHOWA), and 0.0028 M Na2SO4 (Shimakyu) prepared using ultrapure water (18 MΩ cm). Before each experiment, the solution was purged with Ar for 10 min to remove dissolved oxygen.

EMIC ILs without the need for a template. Metal-chloridebased ILs are obtained when metal chlorides such as AlCl3, ZnCl2, SnCl2, and CoCl2 are mixed with a certain organic halide (eg., choline chloride or EMIC). Metal-chloride-based ILs are unique because their Lewis acidity can be adjusted by varying the molar ratio of the metal chloride and the organic chloride. Depending on the Lewis acidity of the IL, metal anions may be present in various forms. Take chlorozincate as an example.30 The IL contains >33 mol % ZnCl2, and the Zn(II) species may exist as various reducible Lewis acidic species, such as ZnCl3− (or Zn2Cl62−), Zn2Cl5−, and Zn3Cl7− that are reducible within the cathodic electrochemical limit of the melt. Fully coordinated ZnCl42− is not reducible within the cathodic limit of the IL. It was found that the direct electrodeposition of Zn alloy nanowire arrays can be correlated to the speciation conversion near the electrode/solution interface during the deposition process.26 The present study demonstrates that, without the need for template, the morphology of CuZn electrodeposited from chlorozincate can be tuned from hollow tubes to straight nanowire arrays, and segmented porous nanowire arrays can obtained by carefully controlling the deposition potential and the precursor concentrations.

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RESULTS AND DISCUSSION To determine the electrochemistry of CuCl in 40−60 mol % ZnCl2-EMIC IL with various concentrations, cyclic voltammetry (CV) was performed. Figure 1a−c shows the typical cyclic

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EXPERIMENTAL SECTION The EMIC was prepared by reacting methyl imidazolium (99.9%, Aldrich) and ethyl chloride according to the method described in the literature31 and purified by recrystallization from acetonitrile. The prepared EMIC was vacuum-dried at 100 °C for 24 h, whereas ZnCl2 (98%, Sigma-Aldrich) was vacuumdried at 120 °C for 24 h, and both chemicals were stored in a nitrogen-filled glovebox (Vacuum Atmospheres Co.) in which the moisture and oxygen level in the box was kept lower than 1 ppm. In this way, the water residual in the resulting ZnCl2EMIC should be negligible. The colorless 40−60 mol % ZnCl2EMIC melt was prepared in the glovebox by mixing 40 mol % of ZnCl2 (99.99%, Aldrich) and 60 mol % of EMIC in a beaker followed by heating at 90 °C for 2 days. CuZn films were electrodeposited under quiescent conditions from 40−60 mol % ZnCl2-EMIC IL with various concentrations of CuCl (99.999%, Strem), in a standard three-electrode cell using an Autolab 302 N potentiostat/gavanostat controlled with GPES and Nova 1.9 software. Because the IL is very viscous, to reduce the viscosity, all of the deposition experiments were carried out at 90 °C under a purified nitrogen atmosphere in a glovebox. It was visually seen that even at this temperature the viscosity of the IL is still high as compared to that of water. A Zn wire (99.95%) placed in a separate fritted glass tube containing pure 40−60 mol % ZnCl2-EMIC IL was used as the counter electrode. The reference electrode was also a Zn wire placed in a fritted tube containing pure 50−50 mol % ZnCl2-EMIC IL. A tungsten electrode (area = 0.0785 cm2; purity = 99.95%, diameter = 0.5 mm, Alfa Aesar) was used as the working electrode. All of the substrates were cleaned with acetone, 2 M HNO3, and deionized water, and then dried in a vacuum before use. The morphologies of the CuZn deposits were examined using high-resolution scanning electron microscopy (HITACHI, SU8000), environmental scanning electron microscopy (ESEM, FEI Quanta 400 F, Phillips), and high-resolution transmission electron microscopy (HR-TEM, JEOL JEM2100F CS STEM,). The crystal structure of the CuZn deposits

Figure 1. Cyclic voltammograms of 40−60 mol % ZnCl2-EMIC ionic liquid containing (a) 1, (b) 3, and (c) 5 mol % of CuCl, respectively, recorded on a tungsten electrode at 90 °C. The potential scan rate was 50 mV s−1.

voltammograms of the IL containing 1, 3, and 5 mol % of CuCl, respectively, recorded on the W electrode at 90 °C. All of the cyclic voltammograms were scanned cathodically from the open-circuit potential (OCP) (+0.84 V) and reversed at −0.3 V with a scan rate 50 mV s−1. When the concentration of Cu(I) is low (curves a and b), the redox peak current of Cu(I)/Cu (c1) is weak and difficult to observe as compared to the reduction peak (c2) of bulk Zn deposited at −0.2 V. When the concentration of Cu(I) is increased to 5 mol % (curve (c)), two distinct reduction peaks can be clearly observed at 0.3 and −0.2 V (c1 and c2), respectively. Peak c1 is related to the reduction of Cu(I) to Cu metal. On the reverse scan, two oxidation peaks, corresponding to Zn stripping (a2) and the CuZn alloy and nonalloyed Cu (a1), can be observed. When the Cu(I) concentration is increased, the a2 oxidation wave diminishes and the a1 wave increases, indicating that more of the deposit formed the CuZn alloy, which is consistent with the results reported by Chen et al. for Cu(I) in Lewis acidic ZnCl2EMIC IL.32 The morphologies and compositions of the CuZn electrodeposits varied with the concentration of Cu(I) and electrodeposition potential. Three CuZn deposits with distinct 22348



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layer by layer, with each successive layer slightly wider than the previous layer. This observation agrees with the SEM image shown in the inset of Figure 3a. The selected area electron diffraction (SAED) pattern recorded for the sample of Figure 3b is shown in Figure 3c. This pattern reveals the good crystalline structure of the CuZn tube, and can be indexed to the (110), (220), (211), and (222) planes of Cu4.21Zn8.79. The crystal structure of the CuZn deposited on a W substrate was also examined with powder XRD. As shown in Figure 3d, in addition to the peaks due to the W substrate, the diffraction peaks can be indexed to CuZn5 and Cu4.21Zn 8.79 phases (JCPDS card nos. 35-1151 and 01-076-3507, respectively). The presence of CuZn5 in the XRD pattern but its absence in the SAED pattern suggests that CuZn5 probably formed during the initial deposition stage at the W substrate but not in the tubes. As mentioned above, hollow CuZn tubes were obtained at −0.15 V. The current−time curve shown in Figure 2a indicates that the deposition process was performed within the chargetransfer-controlled overpotential range. Because the crystal growth is limited by reduction charge transfer without forming a significant depletion layer, crystals with smooth facets with minimized surface energy are expected.4 Thus, the appearance of a hollow tube structure is an unusual feature. Choi33,34 electrodeposited crystals with a void volume at low applied deposition overpotentials. She pointed out that deviation from faceting polyhedral crystals may result from the uneven deposition overpotential experienced by different parts of a crystal during electrochemical growth. By referring to Choi’s work, the growth of hollow CuZn tubes here is proposed as follows. Initially, CuZn crystal is electrodeposited. As the crystal grows, a concentration gradient of the precursors forms, and according to the Nernst equation, a reduction potential gradient forms, which means that a gradient of the applied overpotential for the deposition is established across the crystal with the central part experiencing a lower overpotential than that experienced by the periphery part. Therefore, when the CuZn crystal grows to a certain size, the overpotential gradient becomes so large that the applied deposition potential used in this case cannot provide an overpotential large enough to the central part to support its growth; therefore, only the periphery part of the crystal grows, resulting in the hollow tube structure. Because the overpotential gradient expands as the deposition proceeds, the tube grows layer by layer, with each successive layer slightly wider than the previous one, leading to a pyramidal shape. This process is depicted in Scheme 1. In this case, the low diffusion rate of the metal ions resulting from the high viscosity of the IL may contribute to the development of the concentration gradient and thus the overpotential gradient. Formation of CuZn Nanowires. As shown in Figure 2b, the current−time curve recorded for the electrodeposition performed in a 40−60 mol % ZnCl2-EMIC IL containing 3 mol % CuCl at a more negative applied potential (−0.2 V) differs from that shown in Figure 2a. Because of the higher Cu(I) concentration and larger applied overpotential, the reduction charge transfer rate is significantly faster, and the current is much higher than that in Figure 2a, and the deposition is considered to be mass-transfer-limited. Under this situation, smooth crystals are not expected. Figure 2b shows that after the decay of the charging current, the current increases due to the formation of nuclei and the fast growth of the deposit into the solution until a plateau at which a steady diffusion layer thickness is achieved, and the diffusion-limited geometric surface as well as the current increase only slightly with time.

morphologies, hollow tubes, nanowires, and segmented porous nanowires, were obtained via potentiostatic electrolysis with various solution compositions and deposition potentials. Typical current−time curves recorded for the three types of structure are displayed in Figure 2. It is clear from this figure

Figure 2. Current−time curve during potentiostatic electrolysis experiments of quiescent 40−60 mol % ZnCl2-EMIC ionic liquid containing: (a) 1, (b), 3, and (c) 5 mol % Cu(I) on a tungsten wire working electrode at 90 °C. Deposition potential was (a) −0.15 V, (b) −0.2 V, and (c) −0.2 V.

that the current−time behaviors of the individual experiments are very different, in accordance with the different morphologies. The formation of each of these CuZn structures is described below. Formation of Hexagonal CuZn Tubes. Hexagonal hollow CuZn tubes were obtained by electrodeposition on various metal substrates, such as W and Pt, from a 40−60 mol % ZnCl2-EMIC IL containing 1 mol % Cu(I) at an applied potential of −0.15 V at 90 °C. The current−time curve recorded during this deposition is shown in Figure 2a, which shows that at this low overpotential, the deposition current was fairly low, reaching a stable plateau during the electrolysis, indicating that the deposition process was performed within the charge-transfer-controlled overpotential range. Figure 3a shows the SEM image of a typical W substrate electrodeposited with CuZn tubes. This figure reveals that the W surface is covered by numerous CuZn tubes of different sizes, indicating that the deposition process might involve progressive nucleation; the larger tubes result from the earlier nucleation, and the smaller tubes result from the latter nucleation. The high-magnification SEM image of a typical CuZn tube show that is displaced in the inset of Figure 3a shows the well-defined hexagonal facets and in-plane, c-axis orientation of the CuZn tube with an openended hollow structure. The thickness of the wall is around 150 nm, and the width of the tube mouth is around 7 μm. Furthermore, this image shows that the tube might grow layer by layer. The side-view TEM image of a CuZn tube with a length of 30 μm is depicted in Figure 3b. This figure shows that the tube initiated from a narrow base and gradually widened in diameter as it grew, forming a pyramidal shape. The enlarged TEM image of this tube, shown in the inset of Figure 3b, shows a slightly lower contract in the interior than at the periphery, evidencing the hollow nature of the CuZn pyramid. Moreover, as indicated by arrows in this image, the tube seemed to grow 22349



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Figure 3. (a) SEM images; inset, the enlarge image of a single tube; (b) TEM images; inset, top portion of the tube; (c) SAED pattern; and (d) XRD pattern of the deposits obtained at −0.15 V from the 40−60 mol % ZnCl2-EMIC ionic liquid containing 1 mol % of CuCl.

When Cu(I) and Zn(II) species are reduced, chloride ions are released from the corresponding chloride complexes to the electrode−solution interface and undergo further complex reactions with other species, For example, for Zn(II), the reducible ZnCl3− is reduced as

This current−time behavior is somewhat similar to the growth of nanowires within the nanochannels of the template used in template-based methods.35,36 The SEM image in Figure 4a shows that deposits with a morphology dramatically different from that in Figure 3 were obtained; numerous parallel nanowires with length longer than 10 μm were deposited. The nanowires were removed from the substrate and examined with TEM. The TEM image shown in Figure 4b reveals that the CuZn nanowires were fairly uniform in diameter along their growth direction. Further, the corresponding SAED pattern (Figure 4c) of the samples reveals spotty rings, indicating the polycrystalline nature of the nanowires. Figure 4d shows the XRD pattern of the nanowire. In addition to the peaks of the W substrate, the three diffraction peaks at 37.8°, 43.1°, and 68.3° correspond to the (321), (330), and (541) planes, respectively, and can be indexed as a cubic structure phase of space group I4̅3m Cu4.21Zn8.79 (no. 01-0763507). No additional peaks for other phases appeared. The formation of CuZn nanowires is related to the application of a higher overpotential and the speciation of the chlorozincate IL. The 40−60 mol % ZnCl2-EMIC IL contains reducible ZnCl3− and nonreducible ZnCl42−. When the deposition was performed at a high overpotential where the deposition is mass-transport-limited, Cu and Zn nuclei formed rapidly, providing growth sites for further deposition of CuZn.

ZnCl3− + 2e− → Zn + 3Cl−

(1)

The three chloride ions released in eq 1 react with nearby ZnCl3− to form the nonreducible ZnCl42− as follows: 3Cl− + 3ZnCl3− → 3ZnCl4 2 −

(2)

This results in a total reduction of: 4ZnCl3− + 2e− → Zn + 3ZnCl4 2 −

(3)

Equation 3 indicates that 4 ZnCl3− complex ions are consumed for each Zn atom that is deposited. Therefore, the ZnCl3− concentration at the electrode surface depletes much faster than it would if reaction 2 was absent. A depletion zone of ZnCl3− thus forms quickly at the electrode/solution interface, in which the concentration of reducible Zn(II) species is unusually low. As illustrated in Scheme 2, as the deposition continues, the depletion zones expand and overlap between adjacent growth sites, prohibiting further deposition within this region. Similar reactions may occur for Cu(I). As a 22350



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Formation of Segmented Porous CuZn Nanowire. The copper content in the aforementioned CuZn deposits is lower than that of zinc. To increase the copper content in the deposit, the concentration of CuCl was increased to 5 mol % in the 40− 60 mol % ZnCl2-EMIC IL for electrodeposition. As depicted in Figure 2c, the i−t curve for this solution performed at −0.2 V displays a distinct characteristic, current oscillation (the current density for the segmented porous nanowire oscillates between 2 and 3 mA cm−2) that does not appear in curves a and b for hollow tubes and nanowires, respectively, indicating that the deposits morphology obtained in this case must differ from that shown in Figures 3 and 4. Figure 5a shows the SEM image of the CuZn nanowires electrodeposited from this solution at −0.2 V. A distinguishing feature of this SEM is that the nanowires display a segmented morphology; that is, the nanowires periodically change their diameter along the growth direction. Typically, the nanowires exhibit the same morphological characteristics over lengths of >30 μm. As more clearly illustrated in the TEM image shown in Figure 5b, the segments are linked to each other only by a short connection with a reduced diameter in the center, symmetric to the longitudinal axis. The high-angle annular dark-field, HAADF, image shown in the inset of Figure 5b confirms that the CuZn nanowire exhibits a porous structure. The energydispersive X-ray spectroscopy (EDS) result reveals a Cu/Zn atomic ratio of 94.65/5.35. The EDS line scan (data not shown) performed in the middle along the longitudinal axis of a single nanowire also reveals that the distribution of Cu and Zn atoms is uniform. Figure 5c shows that the XRD pattern of the as-deposited CuZn nanowires displays two growth planes, which can be indexed to (111) and (200) for Cu (JCPDS card no. 85-1326) and Cu0.7Zn0.3 (JCPDS card no. 03-065-9062) phases, respectively. This result indicates that the segmented porous nanowire is a mixture of the two component crystals, which is consistent with the high Cu/Zn atomic ratio observed by EDS. The HR-TEM image (Figure 5d) shows that the growth orientations of the porous nanowire are [111] and [200], and the SAED pattern shown in the inset of Figure 5d indicates that the porous nanowire is polycrystalline, in agreement with the XRD data. The crystalline structures of Cu0.7Zn0.3 and Cu are both face-centered cubic (aCu0.7Zn0.3 = 0.3684 nm and aCu = 0.3615 nm), and the lattice fringes are similar for the same crystal face; therefore, the diffraction patterns are almost overlapping, displaying one set of diffraction points. The deposition of segmented nanowires can be explained using current oscillation seen in the current−time curve depicted in Figure 2c. The current oscillation means that the deposition rate varies with time due to the oscillation of the local electrolyte (Zn(II) and Cu(I)) concentration profiles in the vicinity of the electrode surface. The current is high when the electrolyte concentration is high. As the deposition proceeds, the concentration decreases, resulting in a decline in the reduction current, until more electrolyte diffuses into the reduction zone to make the reduction current rise again. The periodic change in the concentration profile leads to the deposition of nanowires with periodically changed diameters, that is, a wide diameter during high current and a reduced diameter during low current. Electrodeposition of segmented Cu wires through concentration oscillation has been obtained by Zhang et al.39 in a specially designed thin layer cell in aqueous solution in which the diffusion rate was reduced by carrying the deposition at a temperature near 0 °C. However, in

Scheme 1. Illustration of the Evolution of Cu−Zn Tube via Overpotential-Gradient-Limited Growth at −0.15 V from the 40−60 mol % ZnCl2-EMIC IL Containing 1 mol % CuCla

a

The gray portions represent the growing deposits, and the dashed lines represent the overpotential gradient. Note the overpotential in the central part is lower than that in the periphery region. The deposits grew layer by layer.

result, the concentration is higher in the growth front, and the deposits grow from their tips preferentially along the vertical direction, forming CuZn nanowires. Thus, the unique speciation conversion during the deposition process plays an important role in the direct deposition of nanowires. The leveling rather than decaying of the deposition current shown in Figure 2b is indicative of the sluggish diffusion layer thickening during the deposition, like an expanding dropping mercury electrode used in a polarography experiment, in which the continuously expanding mercury toward the diffusion front creates an “effective” convection and thus compensates partially the thickening of the diffusion layer, and thus the Faradaic current does not decay during the lifetime of the mercury droop.37 In the present study, the continuous growth of the nanowire tips toward the solution front compensates for the diffusion layer thickening. The growth of nanowires being affected by the overlapping of the depletion zones between adjacent growing nanowires is similar to the growth of nanowires within nanochannels in the anodized aluminum oxide (AAO) used in template-based methods.35,36,38 In the case of template-assisted methods, the lateral growth of the deposits was restricted by the nanochannel wall, whereas in the current study, the lateral growth of the deposit was restricted by the overlapped depletion zones. The size and morphology of the nanowires obtained with the template-assisted methods are determined by the physical size and distribution of nanochannels of the template, and are more uniform. On the other hand, the size and morphology of the nanowires obtained in this study are largely dependent on the metal speciation and the high viscosity of the IL, and are less uniformly distributed. The high viscosity reduces the diffusion rate of the solutes and favors the formation of depletion zones, which may be difficult to form in electrolytes with low viscosity. 22351



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Figure 4. (a) SEM image, (b) TEM images, (c) SAED pattern, and (d) XRD pattern of the deposits obtained at −0.20 V from 40−60 mol % ZnCl2EMIC ionic liquid containing 3 mol % of CuCl.

Scheme 2. Illustration of the Growth of Cu−Zn Nanowire Arrays at −0.20 V from the 40−60 mol % ZnCl2-EMIC IL Containing 3 mol % CuCla

a

The gray regions are the growing deposits, and the dashed line regions represent the depletion zones. Deposition time: t3 > t2 > t1.

is chemically oxidized by the cations of the nobler metal, may occur in which the electrochemically deposited Zn is galvanically dissolved when in contact with the Cu(I) cation:

our case, simple diffusion may not be the only factor that accounts for the periodic changing of the concentration profile. Note that while current oscillation occurred in the IL containing 5 mol % Cu(I), it did not occur when the IL contained a lower Cu(I) concentration (3 mol %). Therefore, the high Cu(I) concentration may also play a role in the current oscillation and deposition of segmented nanowires. Recall that the reduction potential for Cu(I)/Cu is more positive than that for Zn(II)/Zn. It is, then, possible that a galvanic metal displacement (GMD) reaction,40 in which the less noble metal

2Cu(I) + Zn → Zn(II) + 2Cu

(4)

This reaction affects the actual deposition rate and concentration profiles of these two species near the double layer of the electrode. It is possible that with 5 mol % Cu(I), the GMD reaction rate is significantly faster, leading to faster 22352



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Figure 5. (a) SEM image; (b) TEM image; inset, HAADF image; (c) XRD pattern; and (d) HR-TEM; inset, SAED pattern, of the deposits obtained at −0.20 V from 40−60 mol % ZnCl2-EMIC ionic liquid containing 5 mol % of CuCl.

electrode, reduction waves at ca. −0.9 V (C1) and −1.2 V (C2) can be observed. According to the literature,43 the C1 wave is related to the reduction of nitrate to nitrite anions through the reaction:

variation in the concentration profiles. Along with the natural diffusion, the current shows oscillation results. With lower Cu(I) concentration (3 mol %), the GMD reaction was not significant enough to cause current oscillation, and thus no segmented nanowires were produced. Note that the overall Cu content in the as-deposited segmented nanowires is much higher than that of Zn, even though the Zn(II) concentration is much higher than the Cu(I) concentration in the IL solution. The GMD reaction may be one of the reasons for this result. Moreover, the high viscosity of the IL may also contribute to the current oscillation similar to the case of the deposition of segmented Cu wire in aqueous electrolyte at temperature near 0 °C.39 Nitrate is a well-known water pollutant. The electrochemical reduction of nitrate has been widely studied on Cu-based electrodes, and alloying Cu with Zn can improve the activity for nitrate electroreduction.41,42 Here, the electroreduction of nitrate on pure Cu and the CuZn nanowires was investigated. Figure 6a shows the cyclic voltammograms recorded in a blank electrolyte solution (pH 8.32) without NO3−. No significant reduction peaks for the three electrodes are observed except for the hydrogen evolution reaction at −1.2, −1.1, and −1.05 V for pure Cu, nanowire, and segmented porous nanowire electrodes, respectively. Figure 6b shows the voltammograms of nitrate solution after blank background subtraction. On the pure Cu

NO3− + H 2O + 2e− → NO2− + 2OH−

(5)

Furthermore, the C2 wave is derived from the reduction of nitrite to various products:44 NO2− + 4H 2O + 4e− → NH 2OH + 5OH−

(6)

NO2− + 5H 2O + 6e− → NH3 + 7OH−

(7)

2NO2− + 4H 2O + 6e− → N2 + 8OH−

(8)

The C1 wave slightly shifted to a more positive potential at ca. −0.85 V, and the C2 waves are at −1.0 and −1.15 V for the segmented porous nanowire and nanowire electrodes, respectively, indicating that these two electrodes required less overpotential for the electroreduction of nitrate. The higher peak currents for nitrate reduction on as-prepared segmented porous nanowire and nanowire electrodes could be attributed to a higher effective surface area. Nanowire and segmented porous nanowire electrodes provide a larger surface area and higher electrocatalytic activity for the electroreduction of nitrate. 22353



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in alkaline aqueous solution. This study demonstrated the utility of using metal-chloride-based ILs for the direct electrodeposition of materials with interesting morphologies, which cannot easily be achieved using other solvents. Thus, this approach facilitates the electrodeposition of interesting micro-/ nanostructured materials.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-6-2757575 ext 65355. Fax: +886-6-2740552. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology Republic of China, Taiwan, and the Ministry of Education, Taiwan, R.O.C., The Aim for the Top University Project to the National Cheng Kung University (NCKU).

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Figure 6. Cyclic voltammograms of Cu and CuZn electrodes in (a) 0.1 M NaHCO3 and (b) 0.0158 M NaNO3 and 0.1 M NaHCO3 alkaline solution (after deducing the blank current). The scan rate was 10 mV/ s.

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CONCLUSION CuZn alloys with controllable morphologies, hexagonal tubes, nanowires, and segmented nanowires, were fabricated electrochemically from 40−60 mol % ZnCl2-EMIC IL without using a template or additive. The composition and morphologies of the CuZn deposits can be controlled simply by tailoring the deposition potential and the Cu/Zn molar ratio in the electrolyte. At low overpotential (that is, the applied potential is less negative than −0.15 V), hexagonal tubes are obtained. This is attributed to the lower overpotential experienced at the growing central part, promoting the growth of the periphery part. When the overpotential is increased (more negative than −0.2 V) to make the deposition mass-transport-limited, nanowires form because of the fast overlapping of the reducible metal species depletion zones between growing deposits, leading to preferential vertical growth. Increasing the concentration of Cu(I) to 5 mol % enhances the concentration oscillation, and, as a result, the deposition rate oscillates, leading to the segmented porous nanowires. The high viscosity of the IL may contribute to the formation of these structures. As compared to the pure Cu electrode, the nanowires and segmented porous nanowire electordes showed superior performance in terms of the electroreduction of nitrate ions 22354



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Electrodeposition of CuZn from Chlorozincate Ionic Liquid: From

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