Fabrication of Various Nickel Nanostructures by Manipulating the One

Journal of The Electrochemical Society, 154 共6兲 E77-E83 共2007兲

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0013-4651/2007/154共6兲/E77/7/$20.00 © The Electrochemical Society

Fabrication of Various Nickel Nanostructures by Manipulating the One-Step Electrodeposition Process Yi-Wen Chung,a Ing-Chi Leu,b,z Jian-Hang Lee,a Jung-Hsien Yen,a and Min-Hsiung Hona a

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Department of Materials Science and Engineering, National United University, Miao-Li 360, Taiwan

b

By employing colloidal crystal templates with different colloid diameters, nickel well-ordered macroporous structures 共inverse opal兲 with different void diameters can be fabricated via a template-mediated electrodeposition process. It was found that the different degrees of filling colloidal crystal template produces different nickel nanostructures during the electrodeposition process. Moreover, the filling behavior for electrodeposition of nickel into the template can be characterized in terms of the current transients recorded. Because of preferential nickel-filling into the interstitial spaces among the colloids, fabrication of well-ordered nickel nanorod and monolayer porous arrays are manipulated by controlling electrodeposition times accurately at 5 and 10 s, respectively. However, the macroporous structure covered with hollow sphere arrays on the surface can be formed in 120 s due to preferential nickel-coating around the colloidal surface. Therefore, various kinds of nickel-based structures derived from colloidal crystals can be constructed via a one-step, template-mediated electrodeposition process. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2717501兴 All rights reserved. Manuscript submitted September 22, 2006; revised manuscript received January 4, 2007. Available electronically April 3, 2007.

Recently, there has been extensive interest in the creation of porous materials with three-dimensional periodicity due to their potential applications, such as catalysts,1 chemical2 and gas3 sensors, photonic4 and optoelectronic devices,5 thermal insulation materials,6 and membranes.7 In general, the atomic clusters,8 nanoparticles,9 and nanostructures10 of materials exhibit size-dependent properties that are profoundly different from their corresponding bulk material counterparts. Hence, it can be anticipated that porous material in the nano or submicrometer scale with a larger surface area will exhibit improved performance in applications, such as those requiring transport through the pores or interaction with the surface. Moreover, it has been suggested that metals with well-ordered porous networks possess interesting photonic properties.11 For porous metals prepared using conventional methods, such as powder sintering, slip casting and fiber metallurgy, it is difficult to achieve materials with high surface areas.12 Although some microporous13 or mesoporous14 metals exhibit high surface areas, a significant fraction of the pores is not easily accessible to large fillers due to the relatively small pore diameters. Thus, well-ordered macroporous metals fabricated via a template-mediated method have attracted a great deal of attention recently due to their efficient process and wider variety of applications. Template-mediated technology is an important process for forming special nanostructures, including one-dimensional 共1D兲 nanofiber15 and three-dimensional 共3D兲 well-ordered macroporous structures.16 By utilizing a nanochannel-filling technology, a replica of the host template can be obtained. The entire process has two key factors. The first is the choice of filling technology. Many methods15,17-19 for nanochannel filling have been developed in which the filling for nanochannels is performed by chemical vapor deposition,17 a dipping process,18 and electrophoresis,15 but the top of the template usually encounters the issue of jamming. However, the mechanism of filling nanochannels by using an electrodeposition20 is from bottom to top, and the jamming of template surfaces can be avoided.21 Hence, electrodeposition is a powerful tool for the fabrication of nanostructures with high structural quality and aspect ratio.22 The fabrication of high-quality and appropriate templates is the second factor for creating high-quality replicas via a template-mediated process.23-25 For example, the colloidal crystal, in which monodispersed colloids self-assemble into an ordered 3D artificial crystal structure, often acts as the template for forming macroporous structures.26-31 Hence, by combining a high-

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quality template and efficient nanochannel-filling technology, the template-mediated process can be easily manipulated. In this report we prepared a well-ordered nickel macroporous structure via a template-mediated electrodeposition process. Because the colloidal crystal template via the capillary-enhanced process exhibits the face-centered cubic 共fcc兲 structure, the nickel replica, which was constructed by using a template-mediated electrodeposition process, has the same structure. In addition, according to the relationship between current values and electrodeposition time, the behavior of nickel electrodeposition in the interstitial spaces of colloidal crystal template can be determined. Hence, wellordered nickel nanorod, nanoporous arrays, and macroporous structure can be fabricated via a one-step process. Further, the nickel macroporous structures covered with hollow sphere arrays on the surface are also constructed due to different interstitial-spaces-filling mechanisms.

Experimental Preparation of monodispersed colloidal particles.— The monodispersed P共St-co-MAA兲 microspheres were prepared by using an emulsifier-free-emulsion copolymerization of styrene 共St兲 and methacrylic acid 共MAA兲 based on the method proposed by Wang and Pan.32 By controlling the concentration of MAA and reaction time, two batches with different diameters of colloidal particles 共180 and 300 nm with a relative standard deviation smaller than 5%兲 were formed. Fabrication of colloidal crystal template.— An efficient process for preparing high-quality colloidal crystals, called capillaryenhanced method, was used in this study.31 First, the substrate coated with indium tin oxide 共ITO兲 thin film was placed horizontally at the bottom of a container which was filled with an aqueous dispersion of the P共St-co-MAA兲 microspheres obtained by ultrasonication. The formation of colloidal crystals can be obtained by the self-assembly of microspheres at 45°C and under a high-humidity 共90%兲 condition. After evaporation was terminated in 24 h, the substrate was picked up with a colloidal crystal film thereon. Synthesis of nickel macroporous structure.— Cathodic dc electrodeposition was conducted in a conventional three-electrode cell. The aqueous solution used for Ni deposition33 was composed of NiSO4 共2.13 M兲, NiCl2 共0.35 M兲, and H3BO3 共0.43 M兲. The working electrode was a conductive indium-tin oxide 共ITO兲-glass substrate with a deposited colloidal crystal film. A platinum plate and silver/silver chloride were used as the counter and reference electrodes, respectively. After finishing nickel electrodeposition, the col-

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Figure 3. SEM surface images of colloidal crystal filled with nickel via an electrodeposition process. The inset figures indicate that colloidal crystal templates are either unfilled or filled with nickel.

Results and Discussion

Figure 1. SEM images of well-ordered colloidal crystals prepared by a capillary-enhanced method: 共a兲 top view and 共b兲 side view.

loidal crystal template can be removed by calcinations at 450°C for 4 h. Finally, nickel well-ordered macroporous film was formed on the substrate. Instrumentation and characterization.— Surface morphology was observed using a Hitachi S4100 field emission scanning electron microscope, and the quality of colloidal crystal template was gauged by using a UV-2001 UV-visible spectrometer from Hitachi. Electrochemical synthesis of nickel was performed by a potentiostat 共EG&G, model 263兲.

The colloidal crystal templates for the synthetic macroporous structure studied in this work were prepared using an efficient technology described in Ref. 31. SEM images of the 共111兲 surface of the synthetic reveal a close-packed arrangement, as shown in Fig. 1a, where some vacancies are found in this well-ordered array. A typical result is shown in Fig. 1b, where the different facets of square and hexagonal arrangements can be clearly observed at the facets 共100兲 and 共111兲, respectively. In fact, a square arrangement can only correspond to a 兵100其-type surface of a fcc structure, instead of in the hexagonal close-packed 共hcp兲. Hence, this colloidal crystal has a fcc structure. According to discussion of modified Bragg’s law,34 a highquality colloidal crystal in a large area can diffract light and exhibits a stop band in its transmission spectrum. Figure 2 shows that the colloidal crystals consisted of 180 and 300 nm colloidal particles and have absorptive peaks at 451 and 654 nm, respectively. The result means that the colloidal crystals are of good crystallinity. Figure 3 shows the nickel electrochemically deposited into the colloidal crystal template from a nickel sulfate solution at −1.0 V 共Ag/AgCl兲. Comparing the two insets, the interstitial space of colloidal crystal template had been filled. Because the filling behavior of the electrodeposition process is from bottom to top, surface jam-

Figure 2. The absorptive spectra of colloidal crystals with the different colloid diameters of 共a兲 300 and 共b兲 180 nm.



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Figure 4. Chronoamperometric response of nickel electrodeposition in the interstitial spaces of colloidal crystal template with 180 and 300 nm colloid diameter.

ming is avoided. After the nickel electrodeposition process, the wellordered colloidal crystal was not destroyed due to the presintering process performed at 135°C for 2 h. Figure 4 shows a typical current-time transient for nickel deposition into the colloidal crystal template with 180 and 300 nm colloidal diameter, respectively, which is divided into three stages according to the filling behaviors. In the initial stage, from 0 to 15 s, nickel ions in the electroplating solution are induced to migrate by the applied potential and accumulate onto the working electrode, resulting in the increase of current values. Subsequently, nickel is formed at the bottom of the template not masked by the colloids by a reduction reaction, as depicted in Fig. 5a. In the second stage, from 15 to 100 s, nickel metal forms in the interstitial spaces of the template on the working electrode, as depicted in Fig. 5b. The current values are at a stable state. This phenomenon of smooth current curve with small oscillation has also been reported for nickel-filling the interstitial spaces of colloidal crystal template by Sumida et al.35 In the third stage, 100–200 s, the level of nickel-filling into the interstitial spaces almost reaches the surface of the colloidal crystal template, as depicted in Fig. 5c. The time-dependence variation of current differs from that in the second stage. In general, the behavior of electrodeposition can be described explicitly by employing Faraday’s law.36 Hence, it can be used to explain the behavior of nickelfilling the colloidal crystal template, which is expressed as W=

ItM ZF

关1兴

where W is the weight of electroplated material, I and t are electroplating current and time, respectively, M is the molecular weight of electroplated material, Z is the electric charge, and F is the Faraday constant. Then Eq. 1 can be modified, as shown in Eq. 2

冉冊

1 tM I A= ␳h ZF

关2兴

where ␳ is the density of electroplated material. The values of A and h indicate the area and height of electroplated material, respectively. According to the modified Faraday’s law 共Eq. 2兲, the current value 共I兲 increases with increasing area 共A兲. When the height of nickelfilling is above the surface of the colloidal crystal template, the electrochemical reaction area changes from projected area of the interstitial spaces into planes. The growth of nickel metal extends from the spaces among the colloids toward the colloidal surface, until colloidal particles on the template surface are covered completely. Hence, current value gradually elevates due to the gradual increase of electrochemical reaction area. The result differs from

Figure 5. Schematic diagrams of nickel infiltration into colloidal crystal templates at different electrodeposition stages of 共a兲 0–15, 共b兲 15–100, 共c兲 100–200 s.

that in the electrodeposition of polypyrrole into the voids of colloidal crystal,37 where a rapid increase in current is suggested to be caused mainly by a rapid increase of the electrochemical reaction area when the growing surface reaches the membrane/bulk solution interface. After removing the colloidal crystal template by calcinations at 450°C for 4 h, nickel 共with the presence of NiO on the surface兲 macroporous structures are obtained. Moreover, porous structure with different pore sizes can be formed by employing a colloidal crystal template having different colloid diameters. Figure 6a shows the SEM image of the surface of a 3D ordered nickel replica formed by depositing into a colloidal crystal template with 180 nm colloid particles. It is apparent that a close-packed arrangement on the topmost plane of the replica is exhibited. The inset of Fig. 6a shows a symmetry indicating that the spherical holes are located above three



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Figure 6. SEM surface images of 3D well-ordered nickel macroporous structure prepared by using colloidal crystal templates with the colloid diameters of 180 nm at the 共a兲 top view and 共b兲 side view.

Figure 7. SEM surface images of 3D well-ordered nickel macroporous structure prepared using colloidal crystal templates with the 300 nm colloid diameters at the 共a兲 top view and 共b兲 side view.

neighboring hollow sites in the crystal. Each cavity formed by the polystyrene particles has three dark spots corresponding to the contact points with the three particles in the layer below. In the cross section of Fig. 6b, the well-ordered macroporous structures are demonstrated to be fabricated from bottom to top. These nickel nanostructures are also called inverse opals. As shown in Fig. 7a, the nickel porous arrays in a large area, fabricated by depositing into colloidal crystal template with 300 nm colloid particles, appear well-ordered in the free surface of the 共111兲 texture. The inset of Fig. 7a also shows that each air sphere rests on three neighboring spheres below along the 关111兴 direction. Moreover, the same 3D macroporous structure consisting of many spherical void layers from the side view can be also observed, as shown in Fig. 7b. Hence, nickel inverse opals can be obtained via a templatemediated electrodeposition process. In general, the height of nickel-filling into the colloidal crystal template increases with increasing electrodeposition time. Further, by using the colloidal crystal templates consisting of the different colloid diameters, different kinetics relationships for nickel-filling occurs with different electrodeposition current values. Hence, the height of nickel-filling in the colloidal crystal with 180 and 300 nm colloid diameter, respectively, can be discussed according to a modified Faraday’s law 共Eq. 3兲, as depicted by the following

almost equal to 3.47 at the same time. From the observation of Fig. 6b and 7b, the ratio between the height 共4.1 ␮m兲 for nickel-filling into colloidal crystal with 300 nm colloid diameter and 180 nm 共1.2 ␮m兲 is equal to 3.42. It can be demonstrated that essentially both areas not occupied by the colloids are the same for the case using 180 and 300 nm colloid particles. These areas indicate the nickel-filling sites, shown as shaded areas of Fig. 8. First, both black trigonal regions for colloids with 180 and 300 nm diameter can be calculated to be 1317 and 3657 nm2, respectively. The numbers of the trigonal region for colloids with 180 and 300 nm diameter in the

Ah =

冉冊

1 tM I ␳ ZF

关3兴

The nickel-filling volume 共Ah兲 increases with increasing the current values. As depicted in Fig. 4, the ratio between current values 共−2.6 mA兲 upon filling a colloidal crystal with 300 nm colloid diameter and 180 nm 共−0.75 mA兲 is equal to 3.47 during the electrodeposition time of 100 s. This also results in a ratio of nickelfilling volume between 300 and 180 nm colloidal diameter being

Figure 8. The illustration of projected area to be filled by Ni in the interstitial space among colloid particles.


Journal of The Electrochemical Society, 154 共6兲 E77-E83 共2007兲 colloidal crystal 共of substrate area 0.503 cm2兲 are equal to 3.58 and 1.3 ⫻ 109, respectively. Finally, the product of the area and the number per unit area, i.e., the entire projected area for nickel-filling 共A兲, is equal to 4715 ⫻ 109 and 4754 ⫻ 109 nm2 for the colloid with 180 and 300 nm diameter, respectively. The area 共A兲 between the 180 and 300 nm colloidal system is almost the same. Because both cases possess the same area for nickel-filling, the ratio of their heights for nickel filling can be estimated according to the ratio of their electrodeposition currents in the semiquantitative analysis. Nickel ions in the solution migrate toward the working electrode with the colloidal crystal template in the electrodeposition process. Subsequently, nickel metal reduces and deposits into the interstitial spaces of the colloidal crystal template. Because the numbers of 180 nm colloid particles filled in the constant volume is larger than the ones of the 300 nm colloid particles filled in the same volume, the nanochannels composed of interstitial spaces of 180 nm colloidal stacks include more turns. These turns cause a larger resistance for ionic fluid near the electrical double layers in the fluid system.38 Therefore, the migration speed of nickel ions in the interstitial spaces of colloidal crystal with 180 nm diameter is lower than for the case of 300 nm colloid particles. According to the semiquantitative analysis, the current values in the 300 nm colloidal system are larger than ones in the 180 nm colloidal system. In the initial stage of the nickel electrodeposition process, from 0 to 15 s, the different morphologies of the porous array can be obtained by manipulating electrodeposition time. Figure 9a shows a monolayer of nickel porous array fabricated via an electrodeposition process for 10 s. Each air hole is surrounded by six spherical voids, which is a hcp network of circular holes. The pattern of pore array indicates a replica of colloidal crystal in 2D. The inset of Fig. 9a shows a cross-sectional image of a nickel macroporous monolayer in which only one layer of nickel macropore exists. When electrodeposition time is decreased to 5 s, nanodot arrays can be obtained as shown in Fig. 9b. This structural arrangement is similar to the porous arrays in a close-packed network. The inset figure with a high magnification shows that one hole is surrounded by six nickel nanodots. These sites occupied by nickel nanodots are the interstitial spaces among three neighboring colloidal particles. Because these sites covered by ITO thin film are exposed to the electrodeposition solution, they possess a large free surface area for the nickel reduction and deposition. Hence, the nickel ions in the electrodeposition solution migrate toward these sites and more easily produce nanodots of nickel metal via an electrochemical reduction reaction. Further, the phenomenon of nickel-filling interstitial spaces at the template bottom can be described according to the chronoamperometric response at short time, as shown in Fig. 9c. From 0 to 3 s, because of the supply of a negative potential, nickel ions with positive charge near the electrode in the solution migrate fast and accumulate onto the ITO-glass substrate not covered by colloids. Subsequently, nickel metal is reduced at these sites, resulting in a rapid increase of the electrodeposition current. Because of the presence of time-dependent diffusion regime,39 the increasing trend of electrodeposition current became smaller from 3 to 8 s. Nevertheless, these nickel ions are reduced to metal gradually at the same sites. Meanwhile, the nickel nanorod is fabricated in 5 s. Due to the existence of a colloidal crystal template, the nickel metals cannot grow freely. The actual electrode area is the cross section of the interstitial spaces of the template, which leads to a stable current during the period of Ni filling into the template. Hence, current is about the same from 8 to 15 s. The nickel macroporous monolayer also forms in 10 s via the template-mediated process. Therefore, three types of current variation during electrodeposition respectively lead to the fabrication of nanorods and a macroporous monolayer. When the electrodeposition time proceeds for 120 s, a novel nickel nanostructure consisting of macroporous structure covered with a hollow sphere array can be formed after removing colloidal crystal template, as shown in Fig. 10a. From the figure inset, the nickel hollow spherical nanostructures can be observed clearly. The

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Figure 9. SEM images of a 2D nickel nanostructure based on 共a兲 a macroporous monolayer and 共b兲 nanodot arrays prepared by a templatemediated electrodeposition process for 10 and 5 s, respectively. 共c兲 Chronoamperometric response of nickel electrodeposition in the interstitial spaces of colloidal crystal template with 300 nm colloid diameter for 15 s.

behavior of nickel-filling interstitial spaces on the top surface of colloidal crystal template is different from the nanostructure at the bottom and middle. At the template bottom and middle, nickel metal is reduced and grows at the interstitial spaces among the colloidal


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Journal of The Electrochemical Society, 154 共6兲 E77-E83 共2007兲 macroporous structure covered with hollow sphere arrays is fabricated, resulting from preferential adsorption coating of nickel around the colloids. Because this nanostructure possesses both a large surface area and a 3D well-ordered arrangement, it has potential for many applications, including magnetic devices,41 sensors,42 and surface-enhanced Raman spectroscopy.43 Conclusions A high-quality, colloidal crystal template can be fabricated via the capillary-enhanced method. The mechanism for nickel-filling into the interstitial spaces of colloidal crystal templates can be divided into four stages according to the current values during the electroplating process. Further, the ratio of height of nickel-filling colloidal crystal with 300 and 180 nm diameter can be estimated using the ratio of their current values in the second stage of the electrodeposition process. In addition, well-ordered nanorods and nanoporous arrays can be formed in 5 and 10 s, respectively, and the nickel hollow spherical nanostructures can be obtained on the surface for an electrodeposition time of 120 s. Therefore, various kinds of nickel nanostructure can be produced via the one-step electrodeposition process. Moreover, their forming mechanism can be described according to the relation of current-time responses. Acknowledgments Financial support from the National Science Council, Taiwan, through contract no. NSC93-2216-E-006-039 is greatly appreciated. National Cheng Kung University assisted in meeting the publication costs of this article.

References

Figure 10. 共a兲 SEM surface image of a macroporous structure covered with hollow sphere arrays prepared for 120 s by a template-mediated electrodeposition process. 共b兲 The illustration for forming a nickel macroporous structure 共left figure兲 and a hollow-sphere-array-covered macroporous structure 共right figure兲.

particles according to previous results, as depicted in the left of Fig. 10b. This mechanism results in a well-ordered, macroporous array on the top of the nanostructure, as shown in Fig. 7b. However, they are produced and coated around the colloidal surface, instead of interstitial spaces, on the template top. The mechanism of forming a hollow sphere array on the top is depicted in the right of Fig. 10b. This is attributed to an electrostatics-induced effect between colloidal particles and the nickel ions.40 The zeta-potential of P共St-co-MAA兲 colloidal particles is measured to be negative in our preliminary study 共not shown here兲. A great number of positive nickel ions in the solution are attracted toward the working electrode by the applied constant potential 关−1 V to 共Ag/AgCl兲兴 during the electrodeposition process. This phenomenon results in many nickel ions being adsorbed onto the colloidal surface at the template top, until the colloidal crystal templates are filled with nickel metal and reach the top of the template. Finally, nickel metal is reduced preferentially at these sites, adsorbing nickel ions. Subsequently, nickel metal grows and completely coats the colloidal surface. After removing the template, the nickel hollow sphere nanostructures on the surface can be obtained if the process is terminated before the complete filling of the interstitial spaces. Therefore, at the bottom of colloidal crystal templates, well-ordered nanodot and nanoporous arrays form more easily due to the preferential filling of nickel into the interstitial spaces among the colloids. On the contrary, a

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