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Template-assisted electrodeposition is an important technique for synthesizing metallic nanomaterials with controlled shape and size. Arrays of nanostructured materials with specific arrangements can be prepared by this method, employing either an active or restrictive template as a cathode in an electrochemical cell. An overview of the template-assisted electrodeposition process to synthesize metallic nanostructures is presented in this article.

INTRODUCTION

Metals at the nanometer scale exhibit unusual behavior compared to their bulk counterparts. The manifestation of quantum phenomena in the electron transport of metallic nanowires is an example, as are the interesting magnetic properties of nanoparticles (transition metals) due to spin-polarized transport.^1,2 Such unusual properties have generated interest in nanostructured materials for potential applications in biomedical sciences, electronics, optics, magnetism, and energy storage.^3,4 These nanostructured materials can be synthesized by a number of techniques, such as inert gas condensation, plasma processing, physical and chemical vapor deposition, electrodeposition, mechanical alloying, rapid solidification, sol-gel, micro-emulsion, spark erosion, and severe plastic deformation.⁵ Although the use of many such techniques is limited to laboratory scale, inert gas condensation, mechanical alloying, and electrodeposition are among the methods that are available for commercial use. In this article, an overview of nanomaterials synthesis using electrodeposition techniques is presented.

ELECTRODEPOSITION TECHNIQUES

The formation of nuclei on the electrode substrate was extensively studied on the basis of macroscopic thermodynamic considerations. The nucleation of nanostructures on the electrode substrate during electrodeposition is influenced by the crystal structure of the substrate, specific free surface energy, adhesion energy, lattice orientation of the electrode surface, and crystallographic lattice mismatch at the nucleus-substrate interface boundary.^6,7 The final size distribution of the electrodeposits, however, strongly depends on the kinetics of the nucleation and growth. The electrodeposition process involves the formation of either an instantaneous or a progressive nucleation.⁸ In the case of instantaneous nucleation, all the nuclei form instantaneously on the electrode substrate, and subsequently grow with the time of electrodeposition. In contrast, the number of nuclei that are formed is a function of time of electrodeposition in the progressive nucleation. These nuclei gradually grow and overlap, and therefore, the progressive nucleation process exhibits zones of reduced nucleation rate around the growing stable nuclei. The electrodeposition method consists of an electrochemical cell and accessories for applying controlled current at a certain voltage. The cell usually contains a reference electrode, a specially designed cathode, and an anode or counter electrode. The cathode substrate on which electrodeposition of the nanostructure takes place can be made of either nonmetallic or metallic materials. Using the surface of the cathode as a template, various desired nanostructures or morphologies can be synthesized for specific applications. Such a template-assisted electrodeposition process can be broadly divided into two groups: active template-assisted and restrictive template-based electrodeposition.

Active Template-Assisted Electrodeposition

The formation of nanostructures in active template-based synthesis results from growth of the nuclei that invariably nucleate at the holes and defects of the electrode substrate. Subsequent growth of these nuclei at the template yields the desired surface morphology of the nanostructures, which can therefore be synthesized by choosing the appropriate surface of the electrode. Highly oriented pyrolitic graphite (HOPG), for example, is used extensively as an electrode substrate for the electrodeposition of silver,^9–11 gold,¹² molybdenum,¹³ palladium,^14–16 and platinum¹⁷ nanostructures. Electrodeposition initiates at the step edges, dislocations, and defect sites of the electrode surface. Vázquez et al.⁹ studied the early stages of overpotential deposition of silver on HOPG from silver-ion-containing acidic solution. It was shown that the initial deposition occurred at the step edges of the electrode surface, and silver nuclei were not found on the flat HOPG surface domains. The electrodeposition study¹⁰ of silver on the HOPG substrate using in-situ scanning tunneling microscopy revealed that at low over-potentials, the deposition process on the atomically flat terraces of HOPG was inhibited because of weak substrate-deposit interaction leading to preferential silver deposition at step edges and other surface defects. Such step-edge deposition involves the preferential nucleation and subsequent growth of nanoparticles on a certain crystal surface. Figure 1 shows a schematic diagram of step-edge synthesis. Favier et al.¹⁶ attempted to fabricate hydrogen sensors and switches with the help of palladium nanoparticles electrodeposited onto the graphite step edges. Stable, two dimensionally branched palladium islands, 100–160 nm in size, were electrodeposited¹⁴ on HOPG from an aqueous acidic palladium chloride solution with an excess of sodium perchlorate at a potential of a few millivolts above the threshold potential of the hydrogen evolution reaction. Platinum nanocrystals were deposited¹⁷ on the basal plane of HOPG from dilute (1.0 mM) PtCl₆ ^2–- containing electrolytes using a pulsed potentiostatic method. The deposition of platinum nanocrystals occurred via an instantaneous nucleation and diffusion-limited growth mechanism which resulted in a narrow particle size distribution for mean crystallite diameters smaller than 40 Å.

The active template-assisted deposition includes numerous other techniques. Nickel nanoparticles with a narrow size distribution have been electrodeposited on a graphite surface using the hydrogen co-evolution method.¹⁸ The electrode position of metal at large overpotential invariably results in instantaneous nucleation, which is non-selective, mostly due to the formation of a large number of nuclei on defect-free terraces as well as at defects such as step edges on the surfaces. The nuclei density is expected to be of the order of 10⁹cm^–2. The dependence of the number of nuclei on time was investigated⁶ in a wide range of voltage intervals using different glassy carbon electrodes. The data for the steady-state nucleation rate were interpreted on the basis of both classical and atomistic concepts of nucleation phenomena.

Since electrodeposition occurs at the defect sites, arrays of palladium nanoparticles have been synthesized¹⁹ by electrodepositing them on the surface of an aluminum layer obtained from a digital video disc (DVD) that usually has a 50 µm thick aluminum layer between two polycarbonate layers. The aluminum layer with arrays of laser holes was exposed by peeling off one polycarbonate layer, and palladium nanoparticles from a palladium chloride solution were electrodeposited on the holes. Pits in the aluminum layer were created during data recording, and palladium nanoparticles preferentially deposited into the pits, making arrays of metallic nanoparticles. The blank aluminum template and the aluminum template with arrays of electrodeposited palladium nanoparticles are depicted in Figures 2a and 2b, respectively.

a	b
Figure 2. Scanning-electron micrographs of an aluminum template (a) blank and (b) with electrodeposited arrays of palladium nanoparticles.		Figure 3. Scanning electron micrographs of palladium particles on a freshly cleaved graphite surface.

In order to study the role of electrode surface on electrodeposition, palladium nanoparticles were deposited on a freshly cleaved graphite surface.²⁰ A microscopic investigation found that the palladium nanoparticles were faceted and their shape was mostly spherical, as shown in Figure 3. The faceted characteristic of these particles is more clearly observed in the inset of Figure 3. Nuclei formed at almost all parts of the electrode surface; however, the presence of a large number of nanoparticles near the edges and boundaries (Figure 3) indicates a preferential nucleation process.

In another study,²¹ the growth of the palladium nuclei was found to be at the grain interior during electrodeposition on an AISI 316 grade stainless-steel electrode. It is well known that nucleation occurs preferentially at the holes or disorder state of the substrate on which the metal is to be deposited. Scanning electron micrographs of the surface of the stainless steel after electrodeposition of palladium nanoparticles at low and high magnifications are shown in Figures 4a and 4b, respectively. Although nucleation occurred preferentially at the grain boundaries, the extensive growth took place only for nuclei within the grain interior, as shown in Figure 4b. The probable reason for the discrepancy in deposition of palladium particles on the grain rather than on the grain boundary is the presence of oxide ridges at the grain boundaries where the formation of chromium oxide is easy at room temperature. Stainless steel consists of 18 wt.% chromium, which forms thin film oxides at room temperature. The oxidation process is more favorable at the grain boundaries because the formation of oxide nuclei at room temperature is energetically favorable due to the presence of defects. Although the formation of nuclei on such stainless-steel templates during electrodeposition of palladium was easy, subsequent growth of nuclei on top of the oxide ridge at the grain boundaries of the steel substrate was slow compared to that of the nuclei at the grain interior. The slow growth rate was attributed to the difficulty in the charge transfer due to the presence of highly resistive oxide film underneath the nuclei at the grain boundaries of the stainless-steel template.

a	b
Figure 4. Scanning electron micrographs (a) bare and (b) palladium-deposited stainless-steel surfaces after thermal etching.		Figure 5. A scanning electron micrograph of palladium particles on carbon tape.

Figure 5 presents the role of amorphous carbon substrate as a template for the electrodeposition of palladium nanoparticles.²⁰ Although the template consists of carbon, the shape of the electrodeposited metallic particle differs from that of the cleaved graphite surface. In the case of the amorphous carbon substrate, the nuclei formed at random locations and fractal growth behavior was observed. In addition, the attachment between the deposits and carbon substrate was weak. All these results strongly indicate that the surface of the active template plays an important role in shape and size distribution of metallic nanoparticles during electrodeposition. Table I presents the various active template substrates used for template-assisted electrodeposition of metallic nanostructures of mostly precious metals such as silver, gold, platinum, and palladium.

Table I. Various Active-Template Substrates Used for Electrodeposition of Metallic Nanostructures
Template Substrate		Elements Deposited	Size and Shape of Nanostructures	Ref.
Highly oriented		Ag	Flat island of diameter 0.29 nm	9
	pyrolytic		Clusters of 10 nm diameter	10
	graphite (HOPG)		Disc (diameter 20–60 nm and height 1.5–5 nm)	11
		Au	Aggregate (size 500 nm and height 70 nm)	12
		Mo	Nanowire of 15 nm diameter	13
		Pd	Island of 200–300 nm grain size range	15
			Two-dimensional branched island (100–160 nm)	14
			Mesowires of grain size 50–300 nm	16
		Pt		17
Graphite surface		Pd		20
		Ni	Spherical particles of 20–600 nm size	18
Al foil with laser hole		Pd	Arrays of nanoparticles with 5 nm diameter	19
Stainless-steel grain interior		Pd	Agglomerated particles (average size 70 nm)	21
Carbon tape		Pd	Meso-sized particles	20

Restrictive Template-Based Electrodeposition

Another template-assisted synthesis of mostly metal nanowires involves the deposition of metal into the cylindrical pores or channels of an inert, nonconductive nanoporous electrode material. Track-etch membranes,²² porous alumina,²³ and other nanoporous structures including conductive polymers, metals, semiconductors, carbons, and other solid materials have been used as templates to prepare nanometer-sized particles, fibrils, rods, and tubules. Various electrode substrates used for restrictive template-assisted electrodeposition of metallic nanostructures are presented in Table II.

Table II. Various Restrictive Template Substrates Used for Electrodeposition of Metallic Nanostructures
Template Substrate		Deposited	Size and Shape of Nanostructures	Ref.
Nanoindented holes		Ni-Fe island	Diameter of 5 nm	26
Polyethylene glycol		Nanostructured Pd	Surface areas 50 m2g–1	24
	and polyvinyl
	pyrrolidone
Semiconductor InAs		Magnetic nanowires	Magnetic nanowire of width 20 nm	25
	quantum well
Mesoporous silica		Co nanowires	Dimension is not reported	29
		Ni and Co	Nanowires with 30 nm diameter	27
		Pd nanowire	Diameter of 7 nm	28

Palladium deposits with high specific surface areas, up to 50 m²g^–1, have been synthesized²⁴ from palladium chloride solutions with additions of polyethylene glycol and polyvinyl pyrrolidone. Details of the globular structure of the deposits depend on the polymer additives. A comparison with electrochemically determined true surface areas demonstrates the rare coalescence of nanoparticles.

Figure 6. A schematic electrode arrangement for the synthesis of nanowires though restrictive template-assisted electrodeposition.

Fasol and Runge²⁵ deposited magnetic permalloy (Ni-Fe) nanowires on the edge of an InAs quantum well. Carrey et al.²⁶ used a nanoindented Al₂O₃-Al foil to preferentially electrodeposit Ni-Fe nanoparticulate arrays from an electrodeposition bath containing a solution of nickel and iron sulfate. Fabrications of arrays of nickel and cobalt nanowires have been reported through electrodeposition at constant potential by Whitney et al.²⁷ In order to fabricate arrays of nanowires on a polycarbonate porous membrane, a copper film was first sputter-deposited to make the working electrode. The electrodeposition solution was confined to the bare side of the membrane so that deposition was initiated onto the copper film through the pores. It was observed²⁷ that the electrodeposition current was directly proportional to the area of electrodeposits at any given potential under a pseudo-steady-state condition. Thin films composed of ordered arrays of palladium nanowires have been electrochemically synthesized²⁸ using silica mesoporous channels. The experimental set-up showing the electrochemical cell along with a restrictive template for electrodeposition is schematically presented in Figure 6. Mesoporous channels have been deposited on a conductive glass substrate. In this method, nanowires with the face-centered-cubic crystal structure (Fm3m) are continuously deposited from the conductive substrate upward until the mesoporous channels are filled.

MORPHOLOGY AND CHARACTERIZATION OF DEPOSITS

The surface morphology of the electrodeposits depends on the structure and chemical composition of the electrode surface as well as other electrochemical parameters. The morphologies of the nanostructures prepared using electrodeposition techniques are usually characterized using transmission electron microscopy and non-contact atomic force microscopy. The electrodeposition of gold¹² on HOPG from acidic aqueous solutions was studied by using electrochemical techniques complemented with ex-situ scanning tunneling microscopy. The kinetics of gold electrodeposition are consistent with a nucleation and three-dimensional growth process. A model including a potential-dependent energy barrier at step edges accounts for the morphology transition for gold electrodeposition on HOPG. It was shown¹² that dendritic aggregates are produced when the gold surface is negatively charged at potential less than the potential of zero charge. Such growth behavior results from the anisotropic corner surface diffusion caused by the varying energy barriers for the diffusion of surface atoms at step edges, as has been observed for metal deposition from vapor. Rounded kinked gold crystals are formed when the gold surface is positively charged at potential greater than the potential of zero charge. This was attributed to the leveling and lowering of barrier heights at step edges induced by Cl– ion adsorption on gold aggregates. A potentiostatic-pulse method was used¹¹ to electrochemically deposit silver nanocrystallites on the atomically smooth graphite basal-plane surface. The particles were disc-shaped and well separated, with a height of 1.5–5 nm and an apparent diameter of 20–60 nm.

It has been often found that the result of electrodeposition using a carbon substrate is weaker than that with a metallic substrate. The surface morphologies of the nanostructures that are formed on carbon substrates have been found to be faceted.

CONCLUSION

Template-assisted electrodeposition is a versatile technique to synthesize various kinds of arrays, nanoparticles, and nanowires with a desired structure. Its flexibility gives the opportunity to have nanomaterials of desired surface morphologies.

The synthesis of nanostructures using template-assisted electrodeposition is an emerging area in nanoscience and technology with the possibility of numerous applications in micro and nanodevices. Although a variety of templates and electrodeposition processes are reported in the literature, the universal application of such templates for depositing metallic nanostructures with controlled and well-defined morphologies is missing. The major problem with using electrodeposition to synthesize nanostructure is the preparation of proper templates. Electrodeposition is highly influenced by the surface characteristics of the electrode substrate, and the shape and size of the deposit depend on the substrate. Therefore, further study of electrode surfaces and subsequent surface engineering are essential to develop active templates for the successful synthesis of nanostructures using electrodeposition. In order to understand the preferential deposition on a particular site of the electrode substrate, further studies on the fundamentals of the nucleation and growth of nanoparticles are needed.

ACKNOWLEDGEMENTS

The authors acknowledge NASA Glenn, Florida Solar Energy Center— University of Central Florida, Florida Space Grant Consortium for funding the arrayed nanostructure research. S. Seal further acknowledges the Office of Naval Research—Young Investigator Award (ONR YIP-2002), the National Science Foundation, and Department of Defense, Department of Energy, and NASA Small Business Innovation Research Phase I and II through Plasma Process for funding nanotechnology research.

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For more information, contact S. Seal, University of Central Florida, AMPAC, Eng1, Room 381, Orlando, Florida 32816; (407) 823-5277; fax (407) 882-1462; e-mail sseal@pegasus.cc.ucf.edu and sseal@mail.ucf.edu.