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Modification of the Electrolyte/Electrode Interface for the TemplateFree Electrochemical Synthesis of Metal Nanowires from Ionic Liquids Abhishek Lahiri, Maryam Shapouri Ghazvini, Giridhar Pulletikurthi, Tong Cui, Volker Klemm, David Rafaja, and Frank Endres J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00166 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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The Journal of Physical Chemistry Letters

Modification of the Electrolyte/Electrode Interface for the Template-Free Electrochemical Synthesis of Metal Nanowires from Ionic Liquids Abhishek Lahiri1, Maryam Shapouri Ghazvini1, Giridhar Pulletikurthi1, Tong Cui1, Volker Klemm2, David Rafaja2, Frank Endres1 1

Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str 6, 38678, Clausthal-Zellerfeld, Germany

2

Institute of Materials Science, Freiberg University of Technology, Gustav-Zeuner-Str. 5, 09599, Freiberg, Germany

Abstract In electrochemistry, the electrode/electrolyte interface (EEI) governs the charge/mass transfer processes and controls the nucleation/growth phenomena. The EEI in ionic liquids (IL) can be controlled by changing the cation/anion of the IL, salt concentration, electrode potential and temperature. In this paper, we show that adding a dopant salt leads to the deposition of nanowires. To illustrate, zinc nanowires were electrodeposited from ZnCl2/1-butyl-1methylpyrrolidinium trifluoromethylsulfonate in presence of GaCl3 as a dopant salt. The choice of Zn salt and its ratio to GaCl3 were found to be crucial for Zn nanowires formation. AFM studies revealed that the solvation structure of Au(111)/IL changes significantly in the presence of GaCl3 and ZnCl2. Chronoamperometry showed changes in the nucleation/growth process, consequently leading to the formation of nanowires. A similar approach was adopted to synthesise Sn nanowires. Thus, modification of the EEI by adding a dopant to ILs can be a viable method to obtain nanowires.

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One-dimensional nanostructures (nanowires, nanotubes, nanobelts etc.) have shown some unique properties compared to their bulk structures such as size dependent excitation/emission, quantised conductance, metal-insulator transition etc. 1-4 One-dimensional structures are usually synthesised by vacuum techniques such as chemical vapour deposition (CVD), physical vapour deposition (PVD), vapour-liquid-solid (VLS) etc. or by thermal evaporation

5-8

. In vacuum techniques, the

growth of the nanowires takes place by a catalytic growth mechanism wherein a metal seed acts as a catalyst. Using this technique, semiconductor and metal nanowires have been synthesised 11

9-

. In thermal evaporation, the metal or metal salt is evaporated and condensed to form self-

catalysed nanowires 8. Among various techniques, electrodeposition of nanowires is a versatile method

12, 13

. Template-

assisted electrodeposition is a common technique for developing nanowires wherein nanoporous polycarbonate or anodised aluminium oxide (AAO) is used as a sacrificial template, and the pore diameter governs the size of the deposited nanowires

13, 14

. Alternatively, a template-free

electrodeposition method for obtaining nanostructures would be very attractive. Ionic liquids are useful media for the electrodeposition of various metals, alloys and semiconductor nanostructures. It was shown that with a change in the cation/anion of the ionic liquids as well as addition of other solvents such as water changes the solvation structure at the EEI which influences the deposit growth/morphology

15-18

. However, studies on the solvation

layer in presence of two salts and its effect on deposit morphology have not yet been reported. Recently a unique approach for the template-free electrodepositing SnSi, Sn and Sb nanowires was presented

19-21

. Elbasiony et al19 showed that SnSi nanowires could be electrodeposited

without the use of a template on Au from 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) containing SnCl4 and SnCl2 salts wherein the electrode potential was shown to be important for the formation of nanowires. In another approach, Al-Salman et al 20 showed that Sn nanowires could be electrodeposited in [Py1,4]TFSA in the presence of both SiCl4 and SnCl4 wherein Sn nanowires were formed despite the high concentration of SiCl4 which should have led to deposition of Si as well

20, 21

. They proposed that the presence of SiCl4 or species of SiCl4 in

the ionic liquid acted as a capping agent which led to the growth of Sn nanowires 20. Yang et al 22 showed that Zn-Ni alloy nanowires could be electrodeposited from their chloride salts in Lewis acidic ZnCl2-EMImCl. They postulated that the growth mechanism of nanowires could be due to 2 ACS Paragon Plus Environment

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the formation of Ni-Zn alloy nuclei at the initial stages, followed by a release of chloride ions which can change the originally Lewis acidic electrolyte to a basic one near the electrode and restrict further nucleation. This leads to a deposition that takes place in a unidirectional manner thus forming nanowires. In the present paper we have focused on the influence of an added salt (GaCl3) in addition to the precursor salt (ZnCl2/Zn(TfO)2) on the nanostructure of the electrode/electrolyte interface for the electrodeposition of Zn nanowires. The growth of nanowires could be achieved in [Py1,4]TfO by using 0.4 M ZnCl2 and 0.05 M GaCl3. However, on changing from ZnCl2 to Zn(TfO)2, nanowires could not be obtained. The influence of GaCl3 on the formation of nanowires and interfacial processes during electrodeposition was investigated using in situ atomic force microscope (AFM). Furthermore, scanning electron microscope (SEM) and Transmission Electron Microscopy (TEM) in combination with energy dispersive X-ray (EDX) and selected area electron diffraction (SAED) were used to evaluate the morphology, growth direction and composition of the nanowires. Finally, we also showed that a similar method can be successfully used to electrodeposit Sn nanowires. Figure 1a shows the cyclic voltammogram of 0.4 M ZnCl2 in [Py1,4]TfO on Au. The CV shows quite a broad reduction peak in the cathodic regime (C1) which is due to the deposition of Zn. A current-crossover appears at -2.8 V, which could be related to a nucleation process. Such phenomenon could be related to the arrangement of IL ion-pairs adjacent to the electrode in the compact layer

23

. The anodic peak A1 is related to the oxidation of electrodeposited Zn. On

addition of 50 mM GaCl3 to the ZnCl2 containing electrolyte, marginal changes are observed in the CV (figure 1b). Reduction peaks C1, and C2 are observed. The first reduction process can be related to alloying of Zn and/or Ga with Au and the reduction process C2 can be related to the deposition of Zn on the working electrode. In the anodic regime, the peak A1 corresponds to the oxidation of electrodeposited Zn. Figure 1c shows the morphology of the Zn deposit from 0.4 M ZnCl2+0.05 M GaCl3 in [Py1,4]TfO which shows the formation of short Zn nanowires. Repeated experiments showed that the nanowires length varied between 200 nm and 1µm (fig S1). However, very fine nanoparticles of 20 and 30 nm in size were formed from 0.4 M ZnCl2 in [Py1,4]TfO (fig S2) which indicates that GaCl3 influences the growth of the nanowires. The CV of 0.4 M Zn(TfO)2 in [Py1,4]TfO is shown in fig 1d wherein three reduction peaks C1, C2 and C3 are observed. The peak C1 is related to the formation of Zn-Au alloy and the peak C2 is associated 3 ACS Paragon Plus Environment


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with the bulk deposition of Zn 24. Peak C3 can be related to the decomposition of [Py1,4]+ cation which occurs below -3.0 V.

16, 19

The oxidation peaks A1 and A2 correspond to the oxidation of

electrodeposited Zn and the decomposed IL, respectively. On addition of GaCl3 to Zn(TfO)2 in [Py1,4]TfO (fig 1e), again three reduction peaks C1, C2 and C3 are observed. The reduction peaks C1 and C2 can be related to the alloying of Ga with Au

25

and Zn with the electrode23,

respectively. Peak C3 can be related to the bulk deposition of Zn. In the oxidation regime, peak A1 can be related to the reduction process C3. The peak A2 can be associated with the reduction process C2 and the oxidation peak A3 can be associated with the reduction process C1. On deposition at -2.6 V from 0.4 M Zn(TfO)2+0.05 M GaCl3 in [Py1,4]TfO, agglomerated spherical particles are seen (fig 1f).

Fig 1: CVs of (a) 0.4 M ZnCl2 in [Py1,4]TfO on polycrystalline gold; (b) 0.4 M ZnCl2 +0.05M GaCl3 in [Py1,4]TfO; (c) SEM of Zn deposited from 0.4 M ZnCl2 +0.05M GaCl3 in [Py1,4]TfO; (d) CV of 0.4 M Zn(TfO)2 in [Py1,4]TfO; (e) CV of 0.4 M Zn(TfO)2+0.05 M GaCl3 in [Py1,4]TfO; (f) Microstructure of Zn deposited from 0.4 M Zn(TfO)2 + 0.05 M GaCl3 in [Py1,4]TfO The striking difference here is that when the same experiments were carried out with Zinc trifluoromethylsulfonate (Zn(TfO)2), nanowires were not observed in the presence of GaCl3. The growth of such spherical structures suggests that deposition of Zn occurs from Zn(TfO)2 complexes with TfO-. It has been shown using Raman spectroscopy that [Zn(TfO)x]n- is formed upon dissolving Zn(TfO)2 in [Py1,4]TfO

26

. The Zn complex is expected to be different on 4

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dissolving ZnCl2 in [Py1,4]TfO which appears to have changed the nucleation/growth process. Thus, it appears that nucleation/growth is one of the influencing factors for obtaining templatefree nanowires, which differs when changing the Zn(II) precursors. Figure 2a shows a TEM image of a Zn nanowire with a diameter of about 30 nm and a length of about 100 nm. The SAED pattern of the nanowire (figure 2b) reveals that the nanowire grows along perpendicular to the lattice planes Zn(110). The high-resolution TEM image in figure 2c shows the lattice fringes of the Zn nanowire with an average interplanar distance of 0.25 nm, consistent with the separation between the (0001) crystal planes of hexagonal Zn

27

. The cross-

sectional EDX spectra of the Zn nanowire (figure 2d) is shown in figure 2e which demonstrates that higher concentration of Ga is present at the edges of Zn nanowire.

Fig 2: (a) TEM image of a Zn nanowire; (b) Selected area electron diffraction pattern (SAED) of the nanowire shown in figure 2a; (c) Lattice fringes of the Zn nanowire show the Zn (0001) plane (d, e) EDX profile of Zn and Ga along the cross-section of the nanowire shown in figure 2d In order to understand the influence of GaCl3 on the growth of Zn nanowires, in situ AFM force spectroscopy and imaging were performed. Figure 3 compares the force-distance curves of [Py1,4]TfO, 0.4M ZnCl2 in [Py1,4]TfO and 0.05 M GaCl3 along with 0.4M ZnCl2 in [Py1,4]TfO on Au(111). At open circuit potential (OCP), as the AFM cantilever approaches the Au(111) surface in [Py1,4]TfO (fig 3a) , two steps are observed at 0.5 nm and 1.0 nm which are related to rupturing 5 ACS Paragon Plus Environment


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of the solvation layers. Taking a cubic symmetry of an ion-pair of [Py1,4]TfO into account, the size of [Py1,4]TfO is found to be ~ 0.72 nm. As the innermost layer in figure 3a shows a separation of 0.5 nm, this layer can be related to IL ion-pair wherein the pyrrolidinium cations adsorb strongly on to the electrode surface. This is substantiated by the fact that in the case of 1butyl-1-methylpyrrolidinium bis(trifluoromethlysulfonyl)amide ([Py1,4]TFSI), both neutron reflectrometry and in situ STM have shown the presence of cations on Au substrate

28, 29

.

Furthermore, on comparing the force-distance results of [Py1,4]TfO with [Py1,4]TFSI, it was shown that the innermost layer width of [Py1,4]TFSI was 0.6 nm whereas the ion-pair dimension was calculated to be 0.75 nm assuming a cubic symmetry 15.

Figure 3: Force-distance curves of [Py1,4]TfO at different electrode potentials (a) OCP; (b) -1.0 V and; (c) -2.0 V; (d) force-distance curves of 0.4M ZnCl2 in [Py1,4]TfO at OCP; (e) -1.0 V and; (f) -1.5 V; (g) Force-distance curves of 0.4 M ZnCl2 + 0.05 M GaCl3 in [Py1,4]TfO at OCP; (h)-1.0 V and; (i) -1.5 V


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As TfO- anion is smaller than TFSI- anion, a smaller separation is observed in figure 3a. The second layer also occurs at a further distance of 0.5 nm from the innermost layer and is more or less consistent with the presence of an ion-pair. On changing the potential to -1.0 V (fig 3b), three solvation layers are seen. The innermost layer decreases to 0.4 nm which is due to stronger attraction forces between electrode surface and cations of the IL ion-pairs

15, 30

. The second and

third layers occur at a further distance of ~0.7 nm which relates well with the presence of an ionpair that are unaffected by the surface charge. On changing the electrode potential to -2.0 V (fig 3c), the innermost layer shows a separation of 0.3 nm which can be related to an even higher attraction force between the electrode surface and cations compared to the one observed at -1.0 V. It appears that the second solvation layer is also affected due to the higher applied negative bias, which shows a separation of ~0.5 nm and can be related to the IL ion-pair. The third layer occurs at a further distance of 0.6 nm which also related to the presence of an ion-pair On addition of 0.4 M ZnCl2 to [Py1,4]TfO, at OCP (fig 3d), only a double layer is observed at ~0.4 nm. Compared to [Py1,4]TfO in figure 3a, no multilayered structure is observed and therefore the observed double layer could be related to the presence of metal complexes at the interface and is consistent with the previously observed results of Zn(II) salts dissolved in ILs 31. On changing the potential to -1.0 V (fig 3e), still a double layer structure is seen. However, on changing the potential to -1.5 V (fig 3f), three solvation layers are observed. The increase in number of solvation layers indicate that the Au(111) surface changed due to the formation of a Zn-Au alloy as seen from the AFM image of Au(111) with 0.4 M ZnCl2 which shows a rough surface (fig S3). Furthermore, the same experiments performed with 0.2 M ZnCl2 (fig S4) showed the formation of an alloy on the terraces of Au (111). Zinc-gold alloy formation has been observed for Zn(TfO)2 in [EMIm]TfO using in situ STM 24. Thus, it can be said that additional solvation layers observed in figure 3f is due to the formation of an alloy. Further evidence with in situ scanning tunneling microscopy is required to support the formation of alloy in this system. On addition of 0.05 M GaCl3 along with 0.4 M ZnCl2 to [Py1,4]TfO at OCP (fig 3g), three solvation layers are observed. The innermost layer has a width of 0.3 nm, which can be due to the presence of GaCl3-TfO- complexes along with ZnCl2-TfO- complexes. The second layer occurs at a further separation of 0.4 nm, which can also be related to complexes of ZnCl2/GaCl3 with TfO-. The third layer occurs at a separation of 0.8 nm, which can be attributed to the presence of an IL ion-pair. On changing the potential to -1.0 V (fig 3h) and -1.5 V (fig 3i), only a double layer is 7 ACS Paragon Plus Environment


observed at 0.3 nm. This is in contrast to that observed in figure 3f. Furthermore, from in situ AFM images, nuclei’s in the size range of 100-200 nm can be observed (fig S5). The change in the interfacial structures and the formation of nuclei’s might have led to changes in the growth mechanism. On changing the Zn salt from ZnCl2 to Zn(TfO)2, Zn nanowires could not be obtained and therefore, we looked in to the EEI structure of 0.4 M Zn(TfO)2/[Py1,4]TfO and compared it with the results shown in figure 3. A comparison of the force-distance profiles of 0.4 M Zn(TfO)2/[Py1,4]TfO at various applied electrode potentials is shown in Fig. S6. A single layer with an approximate width of 0.5 nm can be identified at OCP (fig S6a), which could be related to the presence of Zn-TfO- complexes

26

. Two distinct layers were observed upon changing the

electrode potential to -0.5 V and -1.0 V (fig S6b, S6c) with a separation distance of 0.5 and 0.6 nm from the electrode surface. These two layers can be associated with the presence of Zn-TfOcomplexes and to the presence of an IL ion-pair, respectively. Furthermore, the force required to rupture the innermost layer was found to be larger (~ 8 nN at -1 V), which is nearly twice the value observed for rupturing the innermost layer in the case of -0.5 V (~4 nN). Such an increase in force could be related to a stronger adsorption of the species due to a more negative electrode potential. On changing the potential to -1.5 V, again a double layer formation with a separation distance of 0.5 nm is seen (fig S6d) and can be associated with Zn-TfO complexes. In comparison, the solvation structures at the EEI (fig 3 and fig S6) are different at all applied potentials. It appears that the Zn(II) complexes differ in the two electrolytes thus influencing the layer widths of the solvation structure. For example, GaCl3 and ZnCl2 can form complexes like [GaClx(TfO)y]n- and [ZnClx(TfO)y]n- for (GaCl3+ZnCl2)/[Py1,4]TfO whereas the complexes can be in the form of [GaClx(TfO)y]n- and [Zn(TfO)x]n- for (GaCl3+Zn(TfO)2)/[Py1,4]TfO. Furthermore, the force required to rupture the innermost layers at -1.5 V for 0.4 M ZnCl2/[Py1,4]TfO (Fig.3f) is more than two times compared to that for 0.4 M Zn(TfO)2/[Py1,4]TfO (fig S6d) and a change in adsorption strength might have influenced the growth of the deposit. To support the above results, we have performed chronoamperometry for 0.4 M ZnCl2/[Py1,4]TfO and for 0.4 M Zn(TfO)2/[Py1,4]TfO with and without GaCl3 under the deposition conditions of Zn. A comparison of chronoamperograms of 0.4M ZnCl2/[Py1,4]TfO and of 0.4M ZnCl2+ 0.05 M GaCl3/[Py1,4]TfO recorded on gold at -2.6 V is shown in Fig.4a. The current-time transients are 8 ACS Paragon Plus Environment

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characterised by an increase in current due to formation and growth of nuclei on the substrate until a current maximum (im) is attained at a time maximum (t1) followed by Cottrell behaviour. For ZnCl2 alone in [Py1,4]TfO, the current-time profile shows a single step with a rapid increase in current reaching a current maximum (-1.1 mA) in 0.8 s. For ZnCl2 and GaCl3 in [Py1,4]TfO, the chronoamperogram shows two discrete steps (inset fig 4a), a first step at ~ 0.3 s (-0.15 mA) and another one at ~ 2 s (-0.8 mA). Furthermore, the current maximum (im) is attained at longer time t2 (inset of Fig.4a). This suggests that the nucleation and growth of zinc is distinctly different in the presence of GaCl3 in ZnCl2/[Py1,4]TfO . The current-time transient exhibits quite a different form involving two growth processes for Zn(TfO)2/[Py1,4]TfO in the absence and in the presence of GaCl3 at −2.6 V as shown in Figure 4b. The first process (t1) for Zn(TfO)2+0.05M GaCl3/[Py1,4]TfO occurs at a longer time interval than that in the absence of GaCl3 (t1) which can be related to the co-deposition of a Zn-Ga alloy and/or the formation of an adsorption layer on the electrode surface (see inset of Fig.4b). The second process (t2), which occurs at a longer time and with a lower current than that in the absence of GaCl3 (t2), probably is a result of the deposition of zinc on the adsorbed layers. From the potentiostatic step experiments we can conclude at a minimum that the growth of the Zn deposition from [Py1,4]TfO differs not only upon changing the Zn(II) source but also on the addition of GaCl3. Thus, based on the above results, we propose a possible mechanism for the growth of the nanowires as shown in scheme 1. For ZnCl2 in [Py1,4]TfO, the influence of solvation layers leads to the deposition of Zn nanoparticles (Scheme 1a). On addition of GaCl3 to this liquid, the formation of ZnGa alloy nuclei’s take place and a change in the solvation layers occur (Scheme 1b), which was supported by AFM experiments (see Fig. S5). As depletion of GaCl3 in the ionic liquid takes place, Zn deposits primarily on the nucleation sites which lead to a growth in a unidirectional manner as seen in Scheme 1c. The rate of deposition is also an influencing factor for the growth as a high rate of deposition may lead to the formation of dendrites/microcrystalline deposits.



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Fig. 4: Comparison of current-time transients recorded on gold at RT and at -2.6 V. (a) 0.4 M ZnCl2/[Py1,4]TfO and

ZnCl2+GaCl3/[Py1,4]TfO; (b) 0.4 M Zn(TfO)2/[Py1,4]TfO

and

ZnCl2+GaCl3/[Py1,4]TfO In our case the growth of the deposits seems to be influenced by interfacial layers as we observe a lower rate of deposition with another step in the chronoamperogram for ZnCl2+GaCl3/[Py1,4]TfO (fig 4a). However, a single step with a higher current has been observed for ZnCl2/[Py1,4]TfO. Moreover, two distinct steps have been noticed for both Zn(TfO)2/[Py1,4]TfO and for Zn(TfO)2+GaCl3/[Py1,4]TfO (fig 4b) wherein the maximum current for the Zn deposition occurred at a longer time for the latter case. Here, the slower deposition rate controls the growth of particles and leads to the formation of agglomerated nanostructures. However, only a single step with a diffusion limited growth can be seen (

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