Template-Free Electrochemical Synthesis of High Aspect Ratio Sn

Mar 10, 2015 - Perhaps more importantly, this approach is not restricted to the synthesis of Sn nanowires and can be applied to other metals and semim...
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Template-Free Electrochemical Synthesis of High Aspect Ratio Sn Nanowires in Ionic Liquids: A General Route to Large-Area Metal and Semimetal Nanowire Arrays? Rihab Al-Salman,*,† Heino Sommer,†,‡ Torsten Brezesinski,*,† and Jürgen Janek†,§ †

Battery and Electrochemistry Laboratory, Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ BASF SE, 67056 Ludwigshafen, Germany § Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany S Supporting Information *

ABSTRACT: A facile low-temperature and template-free synthesis route to high aspect ratio Sn nanowires is described. The nanowires are prepared by electrochemical deposition from SnCl4/ionic liquid solutions containing SiCl4 onto a variety of substrates, including glassy carbon as well as Cu, Al, and Sn foils. SiCl4 is found to strongly promote the growth of nanowires under specific conditions, while only bulk structures are achieved in the absence of SiCl4. The nanowires exhibit a unique hair-like morphology with very high areal density (≥5 × 109 cm−2), and they have diameters in the sub-30 nm size range over lengths of up to 90 μm, leading to aspect ratios as high as 5 × 103. Diffraction, spectroscopy, and microscopy studies establish that the nanowires are of high quality and consist of single phase, anisometric βSn crystallites. The morphology of the deposits is very uniform over centimeter-sized areas and can be tailored by changing key parameters such as the concentration of SiCl4 and/or SnCl4 and the electrode potential. Perhaps more importantly, this approach is not restricted to the synthesis of Sn nanowires and can be applied to other metals and semimetals with an anisotropic crystal structure (e.g., Zn and Te).

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tin since β-Sn, with space group I41/amd (D19 4h), is the thermodynamically favored phase at ambient conditions. We are aware of only a few literature reports on the templatefree electrochemical synthesis of Sn nanowires and nanostructures. For example, Deng et al. described the preparation of curled 200 nm diameter nanowires by electrodeposition from 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA) at 40 °C;10 they were interwoven and formed a macroporous structure. By increasing the temperature to 70 °C, longer nanowires with diameters ranging from 70 to 200 nm were achieved. Naor-Pomerantz et al. reported on the electrodeposition of rhenium−tin nanowires from aqueous solution.11 They obtained materials having a Sn-rich core and Re-rich shell. However, the Sn:Re ratio range was limited to 0.67−2.0, which thus implies high rhenium content in the nanowires. Recently, Nam et al. described the synthesis of Sn nanofibers by template-free electrodeposition from aqueous solution containing the nonionic surfactant Triton X-100.12 According to the authors, the Sn nanofibers had a thickness of ∼370 nm and length of 37.1 μm (i.e., aspect ratio of 100).

INTRODUCTION

Tin (Sn) nanowires/nanostructures possess a high surface-tovolume ratio which makes them attractive for use in catalysis, in electronics, and as sensors.1,2 In addition, Sn is considered to be a next generation anode material for Li-ion batteries because it has a high theoretical specific capacity of 993 mAh g−1 nanoscale Sn is expected to solve for the structure degradation associated with the large volume changes upon electrochemical reaction with Li.3,4 High quality Sn nanostructures can be prepared by vacuum deposition techniques such as physical vapor deposition5 and template-assisted methods,1,6,7 both of which are disadvantageous for a technological process, though. The former are elaborate and expensive, while the latter typically involve multiple steps, including removal of the template. In the case of electrochemical synthesis, the template must be sputtered with a layer of a conductive material prior to deposition, which adds a further step. Consequently, template-free electrochemical synthesis would be an attractive route for facile and “low cost” fabrication of nanoscale Sn. In recent years, it has been shown that the intrinsic anisotropic structure of some materials is responsible for their tendency to exhibit one-dimensional growthunder specific conditionswithout using any template.8,9 This also applies for © XXXX American Chemical Society

Received: January 16, 2015 Revised: March 2, 2015

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properties such as high thermal stability, good electrical conductivity, negligible vapor pressure, and large operating potential range.16 In addition, and compared to molecular solvents, ILs exhibit interesting interfacial behavior at metal surfaces (thanks to their ionic nature), especially under the applied potentials, which seems to strongly affect deposition processes.17,18 Sn was first deposited from a mixture of SnCl4 and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI). Figure 1a shows

The preparation of needle-like structures by template-free electrodeposition from Sn(II)-containing methanesulfonic acid electrolyte at 25 °C has also been reported.13 The needle morphology was obtained by controlling the electrode potential and concentration of two additives, namely, Triton X-100 and a Pb(II) salt. Electrodeposition of Sn from SnCl2/1-ethyl-3methylimidazolium tetrafluoroborate (EMIM-BF4) solution also gave micrometer-thick needles at room temperature.14 Mackay et al. succeeded in synthesizing nanoneedles (from basic solution of sodium stannate at temperatures above 40 °C) with diameters of 200 nm at the bottom and 20 nm at the tip and lengths of up to 5 μm.15 Herein, we report on a novel hair-like morphology of nanocrystalline Sn obtained via low-temperature and templatefree electrochemical deposition from SnCl4/ionic liquid solutions containing SiCl4. The Si−Sn system was chosen because of its immiscibility (two-phase system with very low mutual solubility) at room temperature and the fact that the deposition rates of Si and Sn are different, which thus gives rise to structuring at the nanometer level. We demonstrate that the nanowires, with an average diameter of 25 nm and lengths of up to 90 μm, have a very high areal density, and the deposits are of uniform morphology over large areas. In practice, the latter is rarely achieved for electrochemically generated nanostructures. Moreover, the one-pot synthesis described in this work not only is straightforward and scalable but can also be extended to other materials with anisotropic crystal structure. Tellurium (Te) and zinc (Zn) nanowires are two successful examples introduced herein. A possible growth mechanism is discussed.

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Figure 1. (a) Cyclic voltammogram of a Cu foil with an area of 0.5 cm2 at a scan rate of 10 mV s−1 in 0.1 M SnCl4/BMP-TFSI solution (solid line) and in pure BMP-TFSI (dashed line). (b) SEM images at different magnifications of the Sn deposit at −2.1 V.

the cyclic voltammetric (CV) curve of a Cu foil in 0.1 M SnCl4/ BMP-TFSI solution. Three distinct cathodic peaks are observed. The first two at −1.1 and −1.4 V vs Pt quasireference might be attributed to the reduction of SnCl4 to SnCl2 and/or underpotential deposition (UPD) of Sn. The rising current at about −1.6 V is assigned to bulk deposition of Sn where a gray deposit starts to cover the surface of the Cu electrode. The peaks in the back scan are mainly due to Sn oxidation since both the IL and Cu are electrochemically stable in the potential range investigated, as can be seen from the CV curve in pure BMP-TFSI (dashed line in Figure 1a). The Sn deposition also occurs at a potential where the IL is stable. The reduction peak at −2.4 V, observed before the bulk decomposition of BMP+ at about −2.6 V, is most likely due to some inorganic impurities, like Li+ or K+, associated with the metathesis synthesis of ILs.19 Figure 1b shows low- and high-magnification SEM images of the deposit obtained by applying a constant potential of −2.1 V for 1.5 h. As is seen, the material has an ill-defined morphology and consists of particles several hundreds of nanometers in diameter, which in some spots grow anisotropically to form large leave-like architecturesappearing like typical dendrite nuclei in an early stage of growth. Silicon (Si) can also be deposited from this IL.20 The CV curve of a Cu foil in 0.5 M SiCl4/BMP-TFSI solution is given in Figure 2 and shows only one main reduction peak at −2.4 V. For the cathodic codeposition of Sn and Si from BMP-TFSI, we started with a 5:1 molar ratio of SiCl4:SnCl4. In the following, this is considered as a reference experiment to which all others will be compared. The CV curve of a Cu foil in 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution (see Figure 2) shows, similar to that in 0.1 M SnCl4/BMP-TFSI solution, two distinct reduction peaks before the bulk deposition of Sn (and probably Si) occurs. At −2.0 V, a grayish black deposit starts forming which turns dark black with deposition time. Chemical analysis by EDS (see

EXPERIMENTAL SECTION

1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI, Io-Li-Tec), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI, Io-Li-Tec), and 1butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP-FAP, Merck) were purchased in the highest available quality and used after drying under vacuum at 100 °C for several hours (water content ≤3 ppm). SnCl 4 (99.995%), SiCl4 (99.998%), SiBr 4 (99.995%), Si(OCOCH3)4 (98%), GeCl4 (99.999%), Sn(OCOCH3)4, TeCl4 (99%), and ZnCl2 (≥99.995%) were purchased from SigmaAldrich and Alfa Aesar and used as received. Cu foil (>99.9%, GOULD Electronics), Al foil (99.5%, Nippon Foil), glassy carbon (Alfa Aesar), and Sn foil (99.95%, Goodfellow) were used as working electrodes. A Pt wire (99.997%, Alfa Aesar) and Pt mesh (99.9%, Alfa Aesar) were used as quasi-reference and counter electrodes, respectively. The electrodepositions were performed under nonconvection conditions (i.e., without stirring) at a temperature of 25 °C inside an argon-filled glovebox from MBraun, with [O2] < 1 ppm and [H2O] < 1 ppm, using a BioLogic potentiostat. After deposition, the material was quickly removed from the solution and carefully rinsed with dry acetone to avoid side reactions with SnCl4 and other species. The deposits were characterized by scanning electron microscopy (SEM, LEO 1530 instrument operated at 10 kV), transmission electron microscopy (TEM, FEI Tecnai G2 F20 Super-Twin operated at 200 kV), energydispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD, STOE Stadi P with Cu Kα radiation and Dectris Mythen strip detector). X-ray photoelectron spectroscopy (XPS) data were acquired on a VersaProbe PHI 5000 Scanning ESCA Microprobe from Physical Electronics with monochromatic Al Kα X-ray source. The C 1s signal at 284.8 eV was used as energy reference to correct for charging.

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RESULTS AND DISCUSSION In the present work, we used ionic liquids (ILs) as electrolyte solvents because they exhibit a unique combination of B

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consist of particles with elongated shape which appear stacked together, and some of the wires are interconnected. However, the latter is not necessarily a result of the deposition process removal of the IL solution by washing with acetone could make the nanowires stick together. Both the areal density and maximum length of the nanowires are estimated (from the SEM images) to be ≥5 × 109 cm−2 and 90 μm, respectively. To the best of our knowledge, Sn wires with such a high aspect ratio have not yet been reported. Also, it is noteworthy to mention that the deposits are uniform in terms of morphology over centimeter-sized areas (see also low-magnification SEM images in Figure S2 of the Supporting Information). This is an interesting and highly useful result which suggests that the overall process is scalable. While template-free electrodeposition from ILs can produce high quality metal, semimetal, and alloy nanomaterials,21−25 it is challenging to obtain uniform nanowires with a high aspect ratio. Normally the ratio is smaller than 500. Figure 3 further presents a high-resolution TEM image and EDS maps of the nanowires. The HRTEM image demonstrates the crystalline nature, with pure β-Sn phase, and indicates the presence of a 1−3 nm thick amorphous surface layer (see also HRTEM images at different magnifications in Figures S3a−c of the Supporting Information). The crystallinity and phase composition was also confirmed by selected area electron diffraction (see Figure S3d of the Supporting Information). According to EDS, the amorphous surface layer consists of Sn, Si, O, and C (and some N and Cl). Both the carbon and oxygen might originate from unavoidable contamination/oxidation during sample processing and analysis. In this regard, it is important to note that Si electrodeposited from ILs is typically fully amorphous and, thus, highly sensitive toward oxidation. Whether the presence of Si is due to some deposition onto the nanowires or strongly adsorbing Si-containing species that cannot be removed by simply washing the nanowires is unclear at present. The microstructure of the nanowires was also investigated via XRD. A representative pattern is shown in Figure 4a. From these data, it can be seen that the material is indeed single phase. In the range of 2θ from 25° to 55°, only three distinct peaks corresponding to tetragonal Sn (beta-phase), with space group I41/amd, can be observed, namely, the (200), (101), and (211) reflections. These peaks are symmetric and there are no apparent shifts from their equilibrium positions. Consequently, strain effects are negligible. In addition, it can be seen that the (200) and (211) peaks are significantly broader than the (101) peak, thereby suggesting that the crystallites are anisometric, as already indicated by electron microscopy. The crystallite size was determined by applying the Scherrer equation to the line broadening of the peaks. This analysis provides values in the range of 22−24 nm and 35−37 nm. Taken together, the XRD data are consistent with the electron microscopy results, and collectively they demonstrate that the nanowires are of very high quality. Both the elemental composition of the nanowires (and especially the amorphous surface layer) and the oxidation state of Sn and Si were examined by XPS. A survey spectrum, with peaks only for Sn, Si, O, C, Cl, N, and F evident, is given in Figure 4b. All these elements are part of the deposition bath solution. Detail spectra of the Sn 3d and Si 2p as well as the O 1s, C 1s, and Cl 2p core excitations are shown in Figures 4c,d and S4 (Supporting Information), respectively. The 2p (except for Si 2p) and 3d level spectra were fitted using relative area

Figure 2. Cyclic voltammogram of a Cu foil with an area of 0.5 cm2 at a scan rate of 10 mV s−1 in 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution. The current overlap between the forward and back scans at a potential of approximately −2.0 V is likely a result of nucleation phenomena. The cyclic voltammograms obtained in 0.1 M SnCl4/ BMP-TFSI and 0.5 M SiCl4/BMP-TFSI solutions are shown as green and orange curves for comparison.

Figure S1 of the Supporting Information) shows the material obtained after 1.5 h deposition time to be primarily composed of Sn; only trace amounts of Si are found (∼1:12 atomic ratio). However, electron microscopy demonstrates that adding SiCl4 to the reaction solution strongly promotes the growth of a unique hair-like nanowire structure. Representative SEM and bright-field TEM images at different magnifications are shown in panels (a−c) of Figure 3. From these data, it can be seen that the nanowires have fairly uniform diameters ranging from 20 to 30 nm, and they are closely packed. It is also evident, especially from the higher magnification images, that the nanowires

Figure 3. Electron microscopy of the Sn nanowires deposited at −2.0 V from 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution. SEM images at different magnifications in (a, b) and TEM image in (c) demonstrating the nanowires are uniform in size and shape. (d) HRTEM image showing the (101) lattice planes of β-Sn with a d-spacing of ∼2.78 Å. (e−h) EDS maps showing the distribution of Sn, Si, O, and C along two nanowires. The side length of the images is 80 nm. C

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images of the corresponding deposits are shown in Figure 5. The Cu electrode is completely covered by nanowires after 20

Figure 4. (a) XRD data of the Sn nanowires deposited at −2.0 V from 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution. The line pattern shows Joint Committee on Powder Diffraction Standards reference card no. 4-0673 for β-Sn. Peaks denoted with an asterisk mark can be attributed to the Cu foil. (b) XPS survey spectrum. The F 2s, Sn 4d, and O 2s core level regions at low binding energy are indicated by an asterisk mark. (c, d) XPS detail spectra of the Sn 3d and Si 2p core excitations. Solid curves in black, blue, and green are fits to the data, and solid orange curves represent the sums of the fits.

ratios of 1:2 and 2:3 for the component peaks, respectively; the full width at half-maximum was constrained to be the same for all peaks of a given core level. The Sn 3d spectrum can be deconvoluted into six peaks, with binding energies of 483.91 eV (Sn0), 485.61 eV (Sn2+), and 486.57 eV (Sn4+) due to emissions from the 3d5/2 levels and binding energies of 492.34 eV (Sn0), 494.06 eV (Sn2+), and 495.02 eV (Sn4+) due to emissions from the 3d3/2 levels. The broad peak at 498.2 eV is assigned to a plasmon loss peak. The Si 2p spectrum can be fitted to two peaks, with binding energies of 100.79 and 102.96 eV (Si4+). The peak at higher binding energy is due to SiO2, while the minor one can be attributed to Si+ (or Si2+). As mentioned above, the presence of both oxidized Sn and Si−O species at the surface of the nanowires cannot be prevented, even though the oxygen exposure was kept at a minimumthe sample was transferred to the XPS chamber for surface examination without exposing it to air. It should also be noted that SnCl4 can react with metallic Sn when the applied potential is off. The presence of SiO2 and some tin oxide(s) is also evident from the O 1s spectrum (see Supporting Information Figure S4), showing two characteristic peaks at 530.18 and 532.35 eV with a relative area ratio of approximately 1:7. The atomic Si:Sn:C:O:Cl ratio from quantitative analysis is 17.2:11.3:29.8:38.1:3.6, which corroborates the results from EDS and further demonstrates that the amorphous surface layer is rich in Si, O, and C. On the basis of all these data, we conclude that Si-containing species function as a kind of capping agent in the synthesis. To see the nanowire growth in early stages, shorter deposition times of 20, 10, and 5 min were used. The SEM

Figure 5. SEM images of the Sn deposits at −2.0 V from 0.5 M SiCl4/ 0.1 M SnCl4/BMP-TFSI solution showing the nanowire growth as a function of time: (a) 5 min, (b) 10 min, and (c) 20 min. The wires grow from predeposited micrometer-sized crystals.

min, which thus implies that the overall growth rate is rather fast. Reducing the deposition time to 10 min reveals that the Sn wires do not grow directly from the substrate but instead from micrometer-sized crystals. This can be more clearly seen from the SEM data for the deposit after 5 minthe seed crystals have a platelet-like shape and appear quasi-transparent (i.e., they must be very thin). To gain more insight into the anisotropic growth, the influence of various experimental parameters on the morphology of the Sn deposit was studied in detail. In the following sections we summarize our primary findings. Effect of the Concentrations of SiCl4 and SnCl4. The concentration of SiCl4 was varied from 0.01 to 0.7 M. As can be seen in Figure 6, even a very low concentration in the SnCl4/ BMP-TFSI solution strongly affects the morphology of the Sn deposit (see also Figure 1b for comparison)a worm-like structure with ∼50 nm features is observed. The structure changes quite a bit as the concentration is increased to 0.1 M. The resulting deposit consists of 10−20 nm diameter D

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solvent instead of BMP-TFSI. The CV curve of a Cu foil in 0.5 M SiCl4/0.1 M SnCl4/EMIM-TFSI solution (not shown) looks very similar to that in BMP-TFSI but with higher current values. This is likely a result of the lower room temperature viscosity of EMIM-TFSI (∼27 mPa s), as compared to BMPTFSI (∼60 mPa s). Figure S6a of the Supporting Information shows the deposit from EMIM-TFSI after applying a potential of −1.9 V for 1.5 hnanowires similar to those from BMPTFSI solution, with diameters ranging from 20 to 30 nm, are obtained. However, we noticed that some of the wires are branching out (Y-shaped branching), the reason for which is unclear. Such kind of branching was not observed in the case of BMP-TFSI. To test for the anion effect, we used 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMPFAP). This particular IL was chosen because FAP is even a weaker coordinating anion than TFSI. Dissolving 0.5 M of SiCl4 and 0.1 M of SnCl4 produced an opaque solution, which might be related to lower solubility of the chloride species, especially that of SnCl4. However, we tried to deposit Sn from this solution and found that applying a potential of −2.5 V (where bulk deposition of Sn occurs) for 1.5 h results in formation of a material with ill-defined morphology, consisting of both Sn nanowires and probably bulk Si (see Figure S6b of the Supporting Information). Chemical analysis of the deposit by EDS revealed a relatively high Si:Sn ratio of ∼1:3 (compare to ∼1:12 for the deposit from BMP-TFSI). One has to bear in mind that exchanging the IL causes several changes to the physicochemical properties of the system: the viscosity and, thus, ion mobility are altered, and the interfacial behavior and interactions of solutes with the IL are affected as well, all of which will influence the deposition process. Therefore, the effect of the IL on the morphology can hardly be predicted. Nevertheless, our research data clearly show that the Sn deposits from the TFSI-based ILs employed in this work have quite similar morphology, whereas the material obtained from BMP-FAP solution is of much lower quality overall. Effect of the Substrate. To find out whether the substrate (working electrode) has any influence on the formation of the Sn nanowires, glassy carbon as well as aluminum and tin foils was also used. All of the experiments were carried out under identical conditions. The results show that nanowires are formed in a similar manner, i.e., growth from predeposited micrometer-sized crystals, irrespective of substrate. However, in the case of Sn foil, we observed that free-standing nanowires can be obtained when the surface is mechanically polished prior to deposition to remove the native oxide layer (see Figure S7 of the Supporting Information). Effect of the Deposition Potential. Sn was also deposited at lower and higher negative potentials (namely, −1.75 and −2.3 V) relative to that applied in our reference experiment. At −1.75 V, Sn is predominantly obtained, as confirmed by EDS; Si has a much more negative deposition potential. The morphology of the deposit is not uniform, though (see Figure S8a of the Supporting Information). Both micrometer-sized crystals and relatively thick and curled nanowires are observed. In contrast, a porous network consisting of short sub-20 nm diameter Sn nanowires is achieved at −2.3 V (see Figure S8b of the Supporting Information). This difference can be explained by the fact that the potential has a profound effect on the deposition rate and is further known to affect atomistic processes. For example, at high overpotentials, more nucleation

Figure 6. SEM images at different magnifications of the Sn deposits at −2.0 V from 0.01 M SiCl4/0.1 M SnCl4/BMP-TFSI (a), 0.1 M SiCl4/ 0.1 M SnCl4/BMP-TFSI (b), and 0.7 M SiCl4/0.1 M SnCl4/BMPTFSI (c) solutions showing the effect of the concentration of SiCl4 on the morphology.

nanowires and appears rather porous, with cavity sizes ranging from 20 to 100 nm. Higher concentrations than 0.5 M also produce micrometer-long nanowires. However, the Sn deposit from 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution is more uniform in size and shape across the whole sample, even after several hours deposition time. The concentration of SnCl4 was both decreased to 0.05 M and increased to 0.5 M. In the latter case, only micrometersized dendritic structures are obtained (see Figure S5a of the Supporting Information). A concentration of 0.05 M, by contrast, gives Sn nanowires, but the deposit is not homogeneous (see Figure S5b of the Supporting Information). Overall, these data establish that the concentrations of SiCl4 and SnCl4 have a strong impact on the structure, morphology, and uniformity of the deposit. This is certainly due, in part, to different diffusion rates at different concentrations of the reactive species in solution and, thus, different deposition rates. Effect of the Ionic Liquid. The effect of the IL on the morphology was first tested by changing the cation. For this purpose, we used 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) as the electrolyte E

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(a) Deposition of Sn onto Cu, (b) formation of “large” crystallites with platelet-like shape and depletion of SnCl4 from the surface, (c) adsorption of SiClx/IL species on the deposit and diffusion of SnCl4 molecules, (d) selective deposition of Sn due to surface passivation and formation of anisometric β-Sn crystallites, and (e) “tip effect” enhanced nanowire growth.

a

low concentration results in the formation of porous threedimensional networks of much shorter nanowires. In the latter case, it is likely that no effective passivation of the Sn crystal planes is achieved. A similar structure was obtained from 0.5 M SiCl4/0.1 M SnCl4/BMP-TFSI solution at more negative potential. The key question is whether the inhibiting species are simply SiCl4 molecules or complexes resulting from the interaction between SiCl4 and TFSI−. IR spectroscopy data of mixtures of BMP-based ILs (including BMP-TFSI) with SiCl4 from Pulletikurthi et al. suggest the formation of complexes.28 It should be noted, though, that adsorption of bare IL cannot be ruled out.29 Both BMP-TFSI and EMIM-TFSI have been shown to have a strong affinity to metal surfaces such as Au. AFM showed that multiple solvation layers are present at the Au surface, and STM measurements even revealed surface restructuring due to adsorption of BMP+.17 In this work, we also utilized other silicon compounds, namely, SiBr4 and Si(OCOCH3)4, and we found that nanowires of similar quality can be obtained with SiBr4 (see Figure S9a of the Supporting Information). In contrast, Si(OCOCH3)4 produced inhomogeneous deposits consisting of both nanowires and nanosheets (see Figure S9b of the Supporting Information). This might be due to different interactions of Si(OCOCH3)4 with the IL species which would lead to different interfacial behavior. In addition, SiCl4 was replaced by GeCl4both are relatively close from a chemical point of viewand the results show that no Sn deposits are formed for GeCl4 concentrations of ≥0.1 M but only Ge with high Cl content. This can be explained by the facts that (1) Ge has a higher deposition rate in BMP-TFSI than Si and (2) GeCl4 can dissolve Sn to form GeCl2. Nevertheless, dendritic Sn structures made of nanowires, with diameters of less than 15 nm and lengths of up to 100 nm, are obtained at low GeCl 4 concentration of 0.01 M (see Figure S9c of the Supporting Information). Also, the deposit from BMP-TFSI solution containing Sn(OCOCH3)4 exhibits predominantly nanowire structure (see Figure S9d of the Supporting Information). From these results, we conclude that the Sn precursor only plays a minor role in the nanowire growth; it simply acts as the source of the tin ions. In contrast, the silicon compound plays a

processes take place, and thus, rapid depletion of SnCl4 in the vicinity of the crystallites occurs. This inhibits further crystal growththe deposition process is mainly controlled by the diffusion of SnCl4 to the surface because the experiments were performed under nonconvection conditions at room temperature. If we suppose that there is a competition between Sn deposition and adsorption of SiCl4 and/or SiClx/IL complexes, one can expect that the probability for adsorption is significantly higher when SnCl4 is depleted from the surface (see proposed mechanism in Scheme 1). We note that the deposition potential of −2.3 V did not give nanostructured Sn in the absence of SiCl4, which confirms our hypothesis that SiCl4 and/or SiClx/IL species are somehow acting as capping agents. On the basis of all of these findings, we conclude that, apart from the anisotropic crystal structure of Sn, interfacial effects are key factors influencing the morphology of the deposit. As already briefly discussed above, additives are particularly effective at altering the structure and morphology of electrodeposited metalsthey may affect the propensity for crystal growth and formation (competition between nucleation and growth). For example, the presence of sodium gluconate in the sulfate bath for Sn plating was found to enhance the nucleation and to decrease the growth rate.26 The nanowires studied here consist of anisometric crystallites, as shown by electron microscopy and XRD, which thus indicates that progressive nucleation takes place during deposition and further growth of the crystallites is inhibited. Furthermore, selective adsorption of Si-containing species appears to strongly affect the overall growth. This might also explain why nanowires grow directly from the substrate when mechanically polished Sn foil is used as substrate. However, it is important to note that some metals show a growth rate dependence on orientation even in the absence of additives. This phenomenon was proven by Xu et al. in their study on template-assisted electrodeposition of Ni/Cu superlattice nanowires.27 The effect of both additive concentration and applied potential on the morphology of the Sn deposit is evident when looking at Figure 6 as well as Figures S5 and S8 of the Supporting Information. Micrometer-long nanowires are obtained at relatively high concentrations of SiCl4, while a F

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Figure 8a,c,d. The low- and high-magnification SEM images show 20−30 nm diameter wires of different lengths and

major role in the growth mechanism because it acts as a capping agent. The fact that Si(IV) halides work more efficiently than Si(OCOCH3)4 is probably due to stronger interactions with the IL (Cl− and Br− are better leaving groups). In addition to the aforementioned factors, the so-called autocatalytic crystal growth or “tip effect”, as described by Nakanishi et al.30 and Djenizian et al.6 for Sn dendrite/ nanowire deposition, is likely to have an influence as well. The authors of both works considered that under diffusion controlled conditions the tip of the deposit has a hemispherical diffusion layer, as opposed to a lateral diffusion layer at the sides, which gives rise to higher current densities and, thus, enhances one-dimensional growth (see Scheme 1). Overall, we believe that the nanowire growth is a result of synergistic effects of several factors, amplifying the intrinsic tendency of Sn for directed growth. Synthesis of Te- and Zn-Based Nanowires. The impact of the SiCl4/IL system on the morphology of the deposited Sn is quite interesting in the sense that it reproducibly yields uniform high aspect ratio Sn nanowires over centimeter-sized areas. This raises the question of whether our method can be applied to other systems. As mentioned previously, materials with an anisotropic crystal structure typically show a stronger tendency toward one-dimensional growth. For example, hexagonal Te, with space group P3121 (D43), has been shown to grow as nanowires (without using any template) under specific conditions.31,32 So, we decided to try synthesizing Te nanowires in a similar manner by simply substituting SnCl4 with TeCl4 in the SiCl4/BMP-TFSI solution. For comparison, we used the same solution but without addition of SiCl4. The electrodeposition was performed at a constant potential of −1.2 V vs Pt quasi-reference for 1 h. The SEM images in Figures 7

Figure 8. Electron microscopy of the Zn deposits at −2.0 V from 0.05 M ZnCl2/EMIM-TFSI solution with (a) and without (b) addition of SiCl4 (0.5 M). (a, b) Low- and high-magnification SEM images. (c) HAADF-STEM image of a single nanowire and (d) corresponding EDS map showing the distribution of Si, Zn, and O.

HAADF-STEM confirms the presence of a ∼5 nm thick surface layer. This layer again contains Si and O (see also EDS maps in Figure S12 of the Supporting Information). Lastly, we note that Zn deposited in the absence of SiCl4 but under otherwise identical conditions has a completely different morphology. It consists of agglomerates of 100−200 nm hexagonal platelets (see Figure 8b). These results are quite promising at this early stage. On the one hand, they prove the validity of our system (i.e., IL + SiX4, with X = halide) for the synthesis of nanowires of materials other than Sn. On the other hand, they establish that SnCl4, in fact, plays a minor role (or even no role) in the directional growth of Sn. Given that the synthesis of both Zn and Te was not optimized, we strongly believe that this approach might pave the way for the preparation of a wide range of metals and semimetals in the form of nanowires.

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CONCLUSION Sn deposits with leave-like architecturereminiscent of dendrite nucleican be obtained from a mixture of SnCl4 and BMP-TFSI. Interestingly, the addition of SiCl4 to the electrolyte solution strongly promotes the growth of high quality nanowires 20−30 nm in diameter over lengths of up to 90 μm under specific conditions. The resulting material is single phase β-Sn and large-area arrays of both entangled and partially aligned nanowires (with very high areal density) can be achieved on a variety of substrates including, among others, Cu, Al, and glassy carbon. The use of Si(IV) halides in general is beneficial for the nanowire growth. Moreover, we have successfully synthesized Te and Zn nanowires by using the same system, which thus proves that our approach is of a more general applicability. Although the exact growth mechanism is still under investigation, we strongly believe that a synergistic

Figure 7. Morphology of the nanocrystalline Te-based deposit at −1.2 V from 0.5 M SiCl4/0.2 M TeCl4/BMP-TFSI solution.

and S10 of the Supporting Information show very clearly that SiCl4 strongly promotes the growth of Te nanowires, as in the case of Sn. They have diameters of 25−35 nm and lengths of several micrometers. XRD analysis (see Figure S11 of the Supporting Information) indicates that the nanocrystalline material consists of both hexagonal Te and hexagonal Cu2−xTe (Cu foil served as working electrode). Furthermore, we have also successfully prepared Zn nanowires by electrodeposition from 0.5 M SiCl4/0.05 M ZnCl2/EMIM-TFSI solution at room temperature. SEM and high-angle annular dark-field scanning TEM images as well as an EDS map of the material obtained at −2.0 V are given in G

DOI: 10.1021/acs.chemmater.5b00200 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

(19) Endres, F.; Abbott, A. P.; MacFarlane, D. R. Electrodeposition from Ionic Liquids; Wiley-VCH: Weinheim, 2008. (20) Al-Salman, R.; El Abedin, S. Z.; Endres, F. Phys. Chem. Chem. Phys. 2008, 10, 4650. (21) Hsieh, Y.-T.; Lai, M.-C.; Huang, H.-L.; Sun, I. W. Electrochim. Acta 2014, 117, 217. (22) Hsieh, Y.-T.; Sun, I. W. Chem. Commun. 2014, 50, 246. (23) Hsieh, Y.-T.; Tsai, R.-W.; Su, C.-J.; Sun, I. W. J. Phys. Chem. C 2014, 118, 22347. (24) Yang, J.-M.; Gou, S.-P.; Sun, I. W. Chem. Commun. 2010, 46, 2686. (25) Yang, J.-M.; Hsieh, Y.-T.; Zhuang, D.-X.; Sun, I. W. Electrochem. Commun. 2011, 13, 1178. (26) Torrent-Burgués, J.; Guaus, E.; Sanz, F. J. Appl. Electrochem. 2002, 32, 225. (27) Xu, S. H.; Fei, G. T.; Zhu, X. G.; Zhang, L. D. CrystEngComm 2013, 15, 4070. (28) Pulletikurthi, G.; Lahiri, A.; Carstens, T.; Borisenko, N.; Zein El Abedin, S.; Endres, F. J. Solid State Electrochem. 2013, 17, 2823. (29) Peppler, K.; Pölleth, M.; Meiss, S.; Rohnke, M.; Janek, J. Z. Phys. Chem. 2006, 220, 1507. (30) Nakanishi, S.; Fukami, K.; Tada, T.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 9556. (31) Mayers, B.; Xia, Y. J. Mater. Chem. 2002, 12, 1875. (32) Szymczak, J.; Legeai, S.; Diliberto, S.; Migot, S.; Stein, N.; Boulanger, C.; Chatel, G.; Draye, M. Electrochem. Commun. 2012, 24, 57.

effect of different factors is responsible for the one-dimensional growth, especially the anisotropic crystal structure and interfacial effects.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data from SEM, HRTEM, SAED, XPS, EDS, and XRD. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(R.A.-S.) E-mail: [email protected]. *(T.B.) E-mail: [email protected]. Author Contributions

R.A.-S. performed all electrochemical experiments, and the manuscript was written through contributions from R.A.-S. and T.B. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Andreas Eichhoefer for assistance with XRD, Hauke Metelmann for XPS, and eye of science for providing high quality SEM images. This study is part of projects being funded within the BASF International Network for Batteries and Electrochemistry.

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DOI: 10.1021/acs.chemmater.5b00200 Chem. Mater. XXXX, XXX, XXX−XXX