Template free synthesis of beaded aluminium sub-microwires via pulse potential electrodeposition

Abstract

Beaded aluminum wires consisting of rope-like wires with evenly-spaced beading are formed by using a square-wave pulse potential electrodeposition method from a chloroaluminate ionic liquid without the use of a template. The physical dimensions of the beaded aluminum wires such as the diameter and spacing between the nodules can be tuned by changing the pulse potentials and durations, which control the growth mechanism.

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The fabrication of micro-/nanostructured materials, in particular one-dimensional (1-D) micro-/nanowires is of interest because of the wide applications of these materials, including catalysis, electronics, sensors, magnetic recording, and biomedicine.^1,2 Electrodeposition stands out as a convenient, low-cost, energy-efficient technique among the various popular techniques for the fabrication of such micro-/nanostructures. Currently, both template-assisted methods and template-free methods have been explored for the fabrication of the 1-D metals, alloys, semiconductors, and composite materials. In the former, the products are electrodeposited into the nanochannels of a pre-made porous template such as anodic aluminum oxide or track-etched polymer membrane^2,3 followed by removal of the template after electrodeposition. In contrast, direct template-free electrodeposition^4,5 which does not require the preparation and removal of template before and after the electrodeposition process are less explored for the fabrication of 1-D materials because it is difficult to guide the crystal growth to follow 1-D direction without the template channels.

It has been shown that the properties of the 1-D materials can be manipulated by changing their microstructures.^6,7 In this regard, pulse potential electrodeposition appears to be a versatile technique. With this technique, segmented multicomponent nanowires with alternatively layered elements^1–3 and segmented single component nanowires with modulated density⁸ along the wire axis have been prepared. However, almost all of the 1-D materials produced so far have uniform diameter along the wire axis. Electrodeposition of wires with varied diameters along the wire axis is scarce. Only recently has it been demonstrated that segmented Pt wires consisting of linked Pt segments are formed through periodically pulse the deposition potential between cathodic and anodic values during the electrodeposition into ion track-etched polycarbonate (PC) membranes.⁸ In each channel of the PC template, the Pt wire was deposited at a cathodic potential and partially dissolved at an anodic potential to constrict the diameter, producing Pt segments of which the distance between the segments decreases with decreasing anodic duration. To our knowledge, fabrication of beaded (segmented) wires using template-free method has not been reported. In view of the fact the template-free methods would offer the advantage of time and material saving over the template-assisted methods, herein, we use aluminum as an example to explore, for the first time, the template-free electrodeposition of beaded metal wires, and tuning the beaded wire dimension by pulse potential deposition between a more negative cathodic potential and a less negative cathodic potential from a chloroaluminate ionic liquid (IL).

Ionic liquids (ILs) have proven to be suitable electrolytes for electrochemistry studies.⁹ Compared with aqueous solutions and organic solvents, ILs have distinct advantages, such as high chemical and thermal stability, negligible vapor pressure, and a wide electrochemical window. Nanowires composed of elements that are hard to be electrodeposited from aqueous electrolytes have been prepared from IL with both template-assisted methods^10–13 and template-free methods.^14–18 Among the various type of ILs, Lewis acidic chloroaluminate ILs is especially useful for electrodeposition of Al¹⁹ and for the Al ion battery.²⁰ In addition to the many examples on electrodeposition of Al films and coatings from ILs,^21–23 electrodeposition of Al wires has also been reported. For example, Endres et al. used a PC template to deposit Al wires for the use as electrode in Li ion batteries.^24,25 On the other hand, direct template-free electrodeposition of Al wires has been recently achieved from the Lewis acidic AlCl₃/trimethylamine hydrochloride (TMHC) chloroaluminate ILs by potentiostatic²⁶ and galvanostatic²⁷ methods, respectively. The template-free formation of Al wires is interesting and is related to the unique speciation conversion during the deposition process. In the Lewis acidic chloroaluminate, the reducible species is Al₂Cl₇⁻ anion. Upon reduction, three chloride ions and one non-reducible AlCl₄⁻ anion are released to the electrode–solution interface as indicated by the reaction: Al₂Cl₇⁻ + 3e⁻ → Al + AlCl₄⁻ + 3Cl⁻. The released chloride ions then react with nearby Al₂Cl₇⁻ anions to form another three AlCl₄⁻ anions; 3Cl⁻ + 3Al₂Cl₇⁻ → 3AlCl₄⁻. Therefore, the total reduction is 4Al₂Cl₇⁻ + 3e⁻ → Al + 3AlCl₄⁻, which indicates that the Al₂Cl₇⁻ concentration at the electrode surface depletes rapidly because 4Al₂Cl₇⁻ complex ions are consumed for each Al atom that is deposited. A depletion zone (diffusion layer) of Al₂Cl₇⁻ thus establishes quickly at the electrode/solution interface at a large applied overpotential (more negative potential), renders dramatic changes in the viscosity and double-layer structure, and hinders further deposition within this region. As a result, the deposits grow from their tips preferentially along the vertical direction towards the growth front rather than lateral direction, leading to the Al nanowires. It was observed that the diameter of the Al wires decreases with increasing deposition overpotential.²⁶ Therefore, electrodeposition of wires with variable diameters along their growing axis could be expected by periodically changing the applied overpotential. The following reports the results on the template-free electrodeposition of beaded Al wires, and tuning of the beaded wire dimension by pulse potential deposition between a more negative cathodic potential and a less negative cathodic potential with different pulse durations. The beaded Al wires were electrodeposited directly on a tungsten electrode in the 60/40 mol% AlCl₃/TMHC melt at 30 °C in a N₂-filled glove box using a programmed square-wave potential electrodeposition procedure as illustrated in Fig. 1A. The W electrode was first subjected to a potential step from open circuit potential (OCP) to a more negative potential (E₁) (−0.20 V or −0.25 V vs. Al/Al³⁺) for a duration of t₁, then the potential was stepped to a less negative potential (E₂) (−0.05 V or −0.10 V) for a duration of t₂. The formation of the beaded Al wires is likely to proceed following the scheme proposed in Fig. 1A. Duplicated micrographs of a single beaded Al wire in Fig. 1A is used to illustrate the growth front (indicated by a black line) of the wire at various growth stages corresponding to E₁ and E₂. When the deposition potential is first applied at E₁, the deposition is initiated by the formation of nuclei which grow quickly during duration t₁. A thick depletion zone of the precursor is rapidly established at the electrode surface because the reduction occurs at a large cathodic overpotential that is far away from equilibrium, rendering the deposit growth becomes mass-transfer limited. Therefore, the deposited crystal starts to develop tip which is closest to the bulk solution so that the diffusive transport and thus, the growth rate becomes faster at the tip along the vertical direction than other directions, leading to the formation of a wire which extends in its length with time (Fig. 1A(a) and (b)). When the deposition potential is stepped to and remained at E₂ with a smaller cathodic overpotential for the duration t₂, the diffusion layer thickening is relieved due to the reduced charge-transfer rate, favoring the lateral growth of the growing Al wire, and hence increases the wire diameter to produce a nodule (Fig. 1A(c) and (d)). By repeating the deposition between E₁ and E₂, the diameter of the deposited wires is periodically varied and beaded Al wires are formed. The low magnification overview scanning electron microscopy (SEM) image of a representative W electrode deposited with Al by the square-wave pulse potential electrodeposition process is presented in Fig. 1B. This micrograph depicts that the W surface is covered by rope-like filaments (wires) with evenly-spaced beads.

Fig. 1 (A) Schematics of the mechanism proposed for the formation of the beaded Al sub-microwires by pulse potential deposition; (B) SEM image and (C) representative TEM image of beaded Al wires obtained by pulse-deposition.

A TEM image with a higher magnification of the typical beaded Al wires is presented in Fig. 1C. This image reveals more clearly that the beads are linked to each other. The detailed descriptions of the morphology are given below.

Typically, the wires exhibit the same morphological characteristics over lengths longer than 20 μm, proving that the programmed square-wave potential deposition procedure is indeed applicable for producing the beaded Al wires. Illustrated in Fig. 2 are the current–time transients recorded during the pulse deposition of aluminum with two different E₁ potentials (−0.2 V and −0.25 V, respectively) while t₁ and t₂ were fixed at 3 s and E₂ was fixed at −0.1 V. The enlarged i–t transients of three consecutive pulse circles are displayed in the inset of this figure. This figure shows that, during each pulse circle, the cathodic current jumps up immediately upon the applying of E₁, and continuously ascends until the potential was stepped to E₂. The continuous ascending of the cathodic current during t₁ can be attributed to the enlarging surface area of the growing wires due to the fast growth of the wires along the axial direction. The cathodic current drops, however, when the potential was stepped to E₂ and descends during t₂ in agreement with the reduced applied overpotential which reduces the deposition rate. Moreover, the figure reveals that a more negative E₁ results in higher cathodic current, and therefore a faster wire growth is expected.

Fig. 2 Typical curves of time dependence of the pulse deposition potential and observed current during the growth of Al sub-microwires.

As pictured in Fig. 3A, the beaded Al can be characterized in terms of the space between the centers of two adjacent beads, S_N, the diameter of the wire, d_W, the diameter of the beads, d_N, and the interval defined as (S_N − d_N). Values of these measurements assessed from the SEM images of the beaded wires obtained with various t₁, t₂, E₁, and E₂ combinations are collected in Table 1. As demonstrated by the SEM images in Fig. 3C, the space between the beads, S_N, of the beaded Al wire obtained with E₁ = −0.25 V is 740 nm, which is longer than the space, 440 nm, seen in Fig. 3B for that obtained with E₁ = −0.20 V, showing that the electrodeposited wire length is proportional to the diffusion current in accordance with the i–t transients in Fig. 2, i.e. more negative E₁ gives rise to faster axial growth. Furthermore, the SEM micrograph shown in Fig. 3D reveals that the space between the beads was increased to 1107 nm when t₁ is increased to 5 s. Similar behaviour is observed under other potential pulse combinations as given in Table 1. Table 1 also shows that (S_N − d_N) exhibits the same relationship as S_N with E₁. This further supports that the axial growth of the wires is dominated at E₁ during t₁. It is, however, interesting to note that a negative interval, −40 nm, is observed for the case of E₁ = −0.2 V, t₁ = 1 s, E₂ = −0.1 V, t₂ = 3 s. This negative interval value indicates that with such a short t₁, the transition time between wire and bead is so short that the adjacent beads overlap with each other. As shown in Table 1, increase t₂ also increases the (S_N − d_N) but not as effective as t₁ does. Due to the narrower d_W, values of the d_N/d_W ratio of the beaded Al wires obtained with E₁ = −0.25 V are approximately kept around 2.0. On the other hand, the ratio of d_N/d_W of the beaded Al wires obtained with E₁ = −0.20 V increases from approximately 1.4 to 2.6 as t₂ increases from 3 s to 5 s. Overall, Table 1 indicates that the physical morphology of the beaded Al wires can be tuned by changing the pulse parameters.

Fig. 3 SEM images of the beaded Al sub-microwires prepared by pulse electrodeposition at different E₁, and E₂: (A) −0.2 V, 5 s//−0.1 V, 3 s; (B) −0.2 V, 3 s//−0.1 V, 3 s, (C) −0.25 V, 3 s//−0.1 V, 3 s; and (D) −0.25 V, 5 s//−0.1 V, 3 s.

Table 1 Physical dimensions of beaded Al wires prepared from the Lewis acidic chloroaluminate ionic liquid at 30 °C under various potential pulse combinations

Pulse parameters, E1, t1//E2, t2	Space, SN (nm)	Wire diameter, dW (nm)	Nodule diameter, dN (nm)	Interval, SN − dN	Diameter ratio, dN/dW
−0.2 V, 3 s//−0.05 V, 3 s	332	205	296	36	1.44
−0.2 V, 5 s//−0.05 V, 3 s	552	231	338	214	1.46
−0.2 V, 3 s//−0.05 V, 5 s	376	123	319	57	2.59
−0.2 V, 1 s//−0.1 V, 3 s	230	210	270	−40	1.28
−0.2 V, 3 s//−0.1 V, 3 s	440	200	400	40	2.00
−0.2 V, 5 s//−0.1 V, 3 s	580	180	310	270	1.72
−0.2 V, 3 s//−0.1 V, 5 s	566	166	395	171	2.38
−0.25 V, 3 s//−0.1 V, 3 s	740	173	350	390	2.02
−0.25 V, 5 s//−0.1 V, 3 s	1107	141	278	829	1.97
−0.25 V, 3 s//−0.1 V, 5 s	801	156	311	490	1.99

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In summary, beaded Al wires consist of evenly spaced beads were successively prepared from the Lewis acidic AlCl₃/TMHC ionic liquid by direct electrochemical pulse deposition without the use of a template. The unique growth mechanism of the beaded wires in this study is different from and has not been observed in the template-assisted methods.

The physical dimensions of the wires could be tuned by varying the deposition parameters including pulse potentials and pulse durations. For instance, the growth of the space between the beads is dominated by the large overpotential pulse E₁ and duration t₁. Moreover, this work suggests that other chlorometallic ionic liquids could be ideal electrolytes for the direct pulse potential deposition of beaded wires including single metal and multicomponents materials. Therefore, it provides a new option, in addition to the template-assisted methods, for the fabrication of beaded wires. Preliminary experiments confirmed that magnetic AlCo beaded wires can also be prepared using this template-free pulse-potential deposit strategy. Given that there are a large number of pulse deposition parameter combinations, preparation of beaded wire materials with various dimensions including constant spacing and variable spacing should be feasible. Such beaded wires may offer some advantages over the wires with uniform diameter. More detailed mechanism of this template-free pulse deposition process deserves further investigation.

Acknowledgements

This work was supported by the Ministry of Science and Technology, Taiwan.

Notes and references

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