Electrochemical Deposition of Co− Sb Thin Films and Nanowires

Oct 1, 2010 - (916) 604-6707. ... Properties of Cobalt Antimony Thin Films Deposited on Flexible Substrates by Radio Frequency Magnetron Sputtering...
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Ind. Eng. Chem. Res. 2010, 49, 11385–11392

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Electrochemical Deposition of Co-Sb Thin Films and Nanowires Dat V. Quach, Ruxandra Vidu,* Joanna R. Groza, and Pieter Stroeve Department of Chemical Engineering & Materials Science, UniVersity of California, DaVis, California 95616

Co-Sb thin films and nanowires were grown on gold-coated polymer templates by electrochemical deposition under different potentiostatic conditions from -0.8 to -1.0 V vs Ag/AgCl. The growth of nanowires was 3D and went through several growth stages starting from the gold-coated wall. Overgrowth of nanowires out of the porous template did not produce a fully dense film; instead, Sb-rich columnar structures (mushroom caps) were developed and connected by Co-rich needle-like structures. Compared to nanowires, the mushroom caps had a lower content of Co. With a more negative deposition potential, the content of Co in the nanowires and the mushroom caps increased. A composition of Co:Sb ) 1:3 in nanowires was obtained at a constant deposition potential of -0.965 V. 1. Introduction Thermoelectrics are renewable energy materials that benefit from low dimensionality. Cobalt triantimonide and its derivatives are considered to be the most suitable thermoelectric materials for applications in the range around 600 K. CoSb3 exhibits excellent electrical transport properties, one of the highest values for hole mobility in a semiconductor due to a high degree of covalent bonding. Cobalt and antimony form three intermediate compounds (CoSb, CoSb2, and CoSb3).1 CoSb3 has a very narrow range of solid solubility, forms a eutectic with Sb at about 621 °C, and melts incongruently at 873 °C.1 It belongs to a group that has the skutterudite (CoAs3) structure indicated by MX3 where M represents a metal atom and X a pnictide atom. The compound crystallizes in a body-centered cubic structure with the space group Im3. The unit cells consists of eight corner-shared MX6 octahedra, which produce a large void at the center of (MX6)8 clusters occupying the body center position in the unit cell. This open site may be further filled with large, “rattling”, rare earth ions (La or Ce) to decreases thermal conductivity. CoSb3 is a semiconductor with a band gap of 0.5 eV as determined experimentally2,3 and 0.57 eV by calculations.4 Thermoelectric and electrical transport properties of CoSb3 are sensitive to dopant concentrations. Important dopants include Fe and Ni (substituted for Co) and Sn and Te (substituted for Sb).3,5-16 Thin films of CoSb3 have been prepared by different techniques including pulsed laser deposition12 and dc magnetron sputtering.17 Although most studies on skutterudites have been focused on the effects of rattler and dopant concentrations on thermoelectric properties, only recent work concentrated on lowdimension skutterudites such as thin films and nanowires. Unlike other thermoelectric materials such as Bi2Te3, CoSb3 has a cage structure that offers a unique opportunity for nanowires to combine the rattling effect and quantum confinement in obtaining higher figure of merit ZT and more efficient thermoelectric devices. Since theoretical calculations first predicted a drastic increase of ZT for thermoelectric nanowires, well beyond the stagnant bulk value of 1, significant progress has been made in synthesizing these structures by way of electrodeposition. Most of the electroplating baths were developed first for thin films and then * To whom correspondence should be addressed. Tel.: (916) 6046707. E-mail: [email protected], [email protected].

demonstrated for nanowires using nanoporous templates. However, the literature on electrochemical deposition of cobalt antimonide is scarce. Cheng et al.18 studied the codeposition of cobalt and antimony in citric-based solutions and the growth behavior of Co-Sb alloy thin films under various deposition conditions. Behnke et al.19 reported on nanowires with Co:Sb ≈ 1:3 using an electrochemical deposition process that required an additional postdeposition annealing to react Co and Sb on adjacent layers and form CoSb3. After annealing, EDS pointed to the presence of cobalt-rich phases in small amounts or noncrystalline due to the antimony loss. Therefore, postdeposition annealing is a challenging method to form crystalline CoSb3. Recently, Chen et al.20,21 reported a study on the synthesis of crystalline CoSb3 nanowires using electrochemical deposition without any additional treatment. CoSb3 nanowire arrays were deposited on an alumina template from a solution of SbO+, Co2+, and tartaric acid. It was found that the stoichiometry of the product was very sensitive to the pH value, and the 1:3 ratio of Co:Sb was obtained at a pH ) 2.5. During the codeposition process, nucleation and growth of CoSb3 occurred quickly to form nanowires that had a preferred (420) orientation. The above studies show the feasibility of synthesis of the Co-Sb thin films and nanowires; however, there is a lack of systematic studies on deposition and growth to produce controlled nanoscale cobalt antimonide thermoelectric materials and gain a better understanding of the electrochemical deposition process. We present in this work a comparative electrochemical and compositional study between thin film and nanowire depositions, both grown on a gold-coated polycarbonate tracketched (PCTE) membrane. Thin films of cobalt antimonide were grown on the nanostructured Au surface. Nanowires were grown inside the pores of the PCTE membrane. This study addresses the differences in morphology and composition of cobalt antimonide Co-Sb materials. 2. Experimental Section Thin films and nanowires were both grown on a gold-coated polycarbonate track-etched (PCTE) membrane. First, a thin layer of Au was deposited on one side of the PCTE membrane. The Au-coated template was then placed on a copper tape and mounted in between two plastic tapes, exposing either the Au layer for thin film deposition or the uncoated side of the PCTE membrane for nanowire growth. Figure 1 shows the two setup configurations for nanowires (Figure 1a) and thin film deposition

10.1021/ie101173u  2010 American Chemical Society Published on Web 10/01/2010

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Figure 1. Setup configurations for (a) nanowires and (b) thin film growth; (c) SEM micrograph of a 400 nm PCTE membrane coated with Au (micrometer bar ) 1 µm).

(Figure 1b). Thin films of cobalt antimonide were grown on the nanostructured Au surface. Nanowires were grown inside the pores of the PCTE membrane. The thin film deposition was performed on a nanostructured gold surface, and nanowires were grown electrochemically by template synthesis. Polycarbonate track-etched membranes with a thickness of 10 µm, pore density of 1 × 108 pores/cm2, and pore diameter of 400 nm, obtained from GE Osmonics, were used as templates for electrochemical deposition of CoSb3 nanowires. The area occupied by the pores for the 400 nm membranes was 12.57%. Au was coated on the rough side of the PCTE membrane using an SEM sputtering device. Electrolytes were prepared by dissolving 0.003 M Sb2O3 and 0.172 M CoSO4 · 7H2O in aqueous solution containing 0.125 M potassium citrate monobasic and 0.196 M citric acid. Sb2O3, CoSO4 · 7H2O, C6H7KO7 (potassium citrate monobasic), and C6H8O7 (citric acid) were used as received from Sigma-Aldrich. Deionized water (Milli Q l8-MΩ) was used for preparing solutions and rinsing. Cathodic electrodeposition was performed under potentiostatic conditions with a conventional three-electrode setup consisting of a computer-controlled bipotentiostat (model AFCBP1). The reference electrode was Ag/AgCl electrode (3 M NaCl). All potentials are given here relative to the Ag/AgCl (0.194 V vs SHE). The counter electrode was an Au wire. Electrolyte was purged with N2 for at least 15 min before each experiment. Samples were mounted in special holder with a circular area of 0.3846 cm2 exposing to the electrolyte either the gold surface or the open-pore membrane. Electrochemical characterization of the CoSb3 nanowires including cyclic voltammetry (CV) and deposition were performed using a bipotentiostat (model AFCBP1, Pine Instrument Co.). Structural characterization was performed using XL30-SFEG, a high-resolution scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS) capability. 3. Results and Discussion Co-Sb thin film deposition was performed on the nanostructured gold surface, and CoSb3 nanowires were grown electrochemically by template synthesis. The Au coating was characterized by EDS and XRD. Results have shown that the gold film is polycrystalline and has (111) preferred orientation. Figure 1c shows the Au-coated side of the membrane. The Au layer is thin and does not cover the pores. Instead, a conductive ring is

Figure 2. Cyclic voltammetry of Au layer on 400 nm PCTE membrane in 50 mM H2SO4, V ) 50 mV/s: initial CV, before any treatment is applied to the surface, CV recorded after 10 cycles between 0 and 1.5 V, and CV recorded after holding the potential at 0.9 V for 15 min.

formed at the base of the pore (Figure 1a), which provides the electrical contact for deposition. 3.1. CoSb3 Film on Au. 3.1.1. Electrochemical Treatment of Au Surface. Cyclic voltammetry (CV) was used to monitor the electrochemical reactions during Co-Sb deposition and to observe the conditions (i.e., potential/current) for deposition of CoSb3 nanowires. Before each CV experiment, the Au surface was subjected to an electrochemical treatment to improve the surface quality and crystallinity of the Au film and the reproducibility of the deposition experiments.22-24 The electrochemical treatment consists of cleaning the surface by cycling the potential between 0 and 1.5 V for 10-15 times followed by an electrochemical annealing carried out at a constant potential of 0.9 V for 15 min. On the basis of an enhanced surface diffusion, this treatment is very effective in reorganizing Au atoms in the surface and smoothening the surface.22 Figure 2 shows cyclic voltammetry of the Au layer in 50 mM H2SO4 at a sweep rate of 50 mV/s in all three preparation states: (i) initial surface, before any treatment is applied to the surface, (ii) surface after 10 cycles between 0 and 1.5 V, and (iii) surface obtained after holding the potential at 0.9 V for 15 min. Although the position of Au oxidation peaks for the initial surface indicates a polycrystalline Au surface, the peak at 1.3 V becomes predominant, shifts to about 1.2 V, and becomes sharper after the EC treatment. The configuration of the Au oxidation peaks points to a rearrangement of the Au atoms in the surface during the electrochemical annealing. The rearrangement of the atoms is due to high surface diffusion of Au atoms under applied potential.22,25-29 3.1.2. Co-Sb Film on Au. After electrochemical annealing of the Au surface, Co-Sb was deposited on Au film from a solution containing 0.003 M Sb2O3 + 0.172 M CoSO4 · 7H2O + 0.125 M potassium citrate + 0.196 M citric acid. A small amount of Sb2O3 can be dissolved in water according to the following equation: Sb2O3 + H2O ) 2HSbO2

log[HSbO2] ) -3.92

(1)

In acidic solution, antimony can also exist in the form of SbO+: Sb2O3 + 2H+ ) 2SbO+ + H2O

log[SbO+] ) -3.05 - pH

(2)

Combining eqs 1 and 2, equilibrium between HSbO2 and SbO+ can be established at a given pH:

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+

SbO + H2O ) HSbO2 + H

log([HSbO2]/[SbO ]) ) -0.87 + pH (3)

While the concentration of HSbO2 in a saturated aqueous solution is always 1.20 × 10-4 M, the concentration of SbO+ varies with pH. For pH < 0.87, the amount of SbO+ in the solution exceeds that of HSbO2. Since the pH of the as-prepared solution is 2.29, the concentration of HSbO2 in the solution is 26 times greater than that of free SbO+. Noteworthy is that the equilibrium amounts of both HSbO2 and free SbO+ in the solution are small compared to the starting 0.003 M Sb2O3, and the rest of the initial Sb2O3 input stays in complexes between SbO+ and citrates. As the deposition proceeds, depletion of HSbO2 and SbO+ is counterbalanced by dissociation of citrate complexes, so that new equilibrium is established. Electrodeposition of CoSb3 involves first the reduction of the absorbed Co2+ and HSbO2 on the electrode to elemental Co and Sb: Co2+ + 2e- ) Co(s)

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+

E0 ) -0.277 + 0.0295 log[Co2+] (4)

Figure 3. Cyclic voltammetry of Au in (a) 0.003 M Sb2O3 + 0.125 M potassium citrate + 0.196 M citric acid, (b) 0.172 M CoSO4 · 7H2O + 0.125 M potassium citrate + 0.196 M citric acid, and (c) 0.003 M Sb2O3 + 0.172 M CoSO4 · 7H2O + 0.125 M potassium citrate + 0.196 M citric acid, V ) 5 mV/s.

EH/H+ ) 0.0592 log[H+] ) -0.136 V vs SHE or -0.330 V vs Ag/AgCl HSbO2 + 3H+ + 3e- ) Sb + 2H2O E0 ) 0.230 - 0.0591 pH + 0.0197 log[HSbO2]

SbO+ + 2H+ + 3e- ) Sb + H2O E0 ) 0.212 - 0.0394 pH + 0.0197 log[SbO+]

(5)

ESb/SbO+ ) 0.212 - 0.0394 pH + 0.0197 log[SbO+] ) 0.016 V vs SHE or -0.178 V vs Ag/AgCl

(6)

Then, the reduced Co and Sb atoms react with each other to form CoSb3. The overall reaction can be expressed as Co2+ + 3HSbO2 + 9H+ + 11e- ) CoSb3 + 6H2O

ESb/HSbO2 ) 0.230 - 0.0591 pH + 0.0197 log[HSbO2] ) 0.017 V vs SHE or -0.177 V vs Ag/AgCl

(7)

In aqueous solutions, there is a large separation between the reduction potential of Co(II) and Sb(III). Under acidic conditions, the standard reduction potentials of the redox couples Co2+/Co0 and HSbO2/Sb0 (or SbO+/Sb0) are -0.277 and 0.230 V (or 0.212 V) vs SHE, respectively, which is 0.507 V (or 0.489 V) apart.30 In these conditions, it is difficult to achieve controlled deposition rates for Co and Sb. However, the difference between the reduction potentials of Co2+ and HSbO2/SbO+ can be reduced by controlling the composition of the electrolyte. Theoretically, the concentrations of HSbO2, SbO+, and Co2+ ions in the solution affect the potential at which Sb and Co deposit on the cathode. The concentrations of HSbO2 and SbO+ are 1.20 × 10-4 and 4.57 × 10-6 M, respectively, as determined from eqs 1 and 2. The equilibrium concentration of free Co2+ is calculated from the MINEQL+ software for the as-prepared solution and equals 5.56 × 10-2 M. Of the initial 0.172 M CoSO4, about 0.116 M (or 67.4%) of Co2+ exists in the form of citrate complexes. The concentration of H+ is 5.13 × 10-3 M. The standard potentials ESb/HSbO2, ESb/SbO+, ECo/Co2+, and EH/H+ are calculated as follows: ECo/Co2+ ) -0.277 + 0.0295 log(Co2+) ) -0.314V vs SHE or -0.508 V vs Ag/AgCl

The gap between ESb/HSbO2 (or ESb/SbO+) and ECo/Co2+ is shortened from 0.507 V (or 0.489 V) under standard conditions to 0.331 V (or 0.330 V) in this solution. Although the gap between the standard potentials are still relatively large and hydrogen evolution is unavoidable, the main purpose of using citrate and citric acid is to form complexes with SbO+ and to increase the solubility of Sb2O3. Without the formation of complexes, at pH ) 2.29 the maximum amount of Sb2O3 that can be dissolved in an aqueous solution is about 6.00 × 10-5 M (i.e., 1.20 × 10-4 M HSbO2). In this example, when citrate/citric acid is used and complexes are formed at the same pH, the total amount of Sb2O3 dissolved in the solution is increased to 3.0 × 10-3 M or 50 times greater than the normal condition. Complexes also affect codepositions and formation of compound. Figure 3 shows the cyclic voltammetry of Au in solutions containing only Co2+, SbO+, or both ions. All voltammetry curves were scanned first in the negative direction from 0.4 V. For Sb deposition (Figure 3a), the reduction point is seen at a potential of -0.6 V in a forward scan. The corresponding oxidation potential is around -0.05 V. Sb-Au is among the systems that show underpotential deposition (UPD). The cyclic voltammogram for antimony UPD on the Au(111) in acidic solution contains two redox couples corresponding to the deposition and stripping of Sb atomic layers.31 One cathodic peak as a composite of two peaks of two different Sb UPD structures on Au(111) and Au (100) was reported by Yan et al.,32,33 suggesting that the UPD process is most sensitive to the surface orientation. In the case of polycrystalline Au with nanostructured morphology, the absence of UPD peaks might be explained by the fact that surface roughness hinders the UPD process. The bulk Sb deposition in the acidic solution (pH ) 1.5) does not occur until -200 mV.31

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A simple electrochemical reducing process, for example, SbO+ f Sb0, is accepted widely and described as SbO+ + 3e+ 2H+ f Sb0 + H2O. The process is more complicated since three electrons cannot be obtained in one electrochemical reducing step. It is well known that complex reagents are always needed in the solution in order to increase the solubility of Sb(III) and also its stability. Previous work showed that the electrochemical reduction of SbO+ on a Au electrode surface is comprised of three parts: the irreversible reduction of adsorbed SbO+, the reversible UPD of dissociative SbO+, and the irreversible over potential deposition (OPD) of dissociative SbO+.34 For Co deposition (Figure 3b), the reduction point is seen around -0.3 V in a forward scan. The corresponding oxidation potential is around -0.2 V. Like Sb, Co UPD on Au nanostructured electrode is characterized by a distinct hysteresis in the potential of deposition and stripping peaks. This is an indication of a kinetic hindrance in the Co adsorption/desorption process and may be a consequence of a first-order phase transition. Such phase transitions are most often associated with nucleation and growth processes. The formation of stable growth centers requires an overpotential that results in a shift between the anodic and the cathodic UPD waves.35,36 Co-Sb deposition from a solution containing both Co and Sb ions (Figure 3c) starts around -0.4 V. The oxidation peak at -0.2 V is consistent with the cathodic waves resulting from the reduction of Co(II) to Co metal. In Figure 3, the negative potential limit was set to -1.2 V, negative to the potential where H2 evolution commences on Au surface (see Figure 2). It can be seen that on the first potential scan toward more negative potential there are two reduction waves. The waves overlap to some extent. The back scan toward more positive potentials shows also two overlapping reduction waves, and substantial cathodic current is observed at all potentials negative to -0.3 V. When a second scan is recorded immediately after the first scan, without any additional treatment, the current is similar to the first scan but not identical. Surface modification due to the interaction between Sb and Au33 may be responsible for the differences in consecutive CVs. The crossover in the voltammograms of Figure 3 are most commonly associated with systems that involve the nucleation and growth of a new phase on the electrode surface. Hence, the voltammograms on the reverse and second scans appear to be for the reduction of Co and Co-Sb at a newly formed Sb-Au surface. All the voltammograms imply that the nucleation of the Co-Sb phase is a complicated process requiring a large overpotential. The most negative cathodic waves show an increase in height and may result from a larger contribution from hydrogen adsorption on the growing Co-Sb surface. Figure 4 shows a typical image of the Co-Sb film on Au. The film is relatively uniform and deposits on the entire membrane surface, including the holes. Local variations in morphology may be associated with the particular nature of the nanostructured Au surface and possible hydrogen evolution. 3.2. Deposition of Co-Sb Nanowires. Nanowires were grown in the same solution as the deposition of Co-Sb film, i.e., 0.003 M Sb2O3 + 0.172 M CoSO4 · 7H2O + 0.125 M potassium citrate + 0.196 M citric acid. Deposition was investigated in the potential range from -0.9 to -1.2 V. Deposition data and composition results of Co-Sb nanowires were compared to that of the film obtained in the same conditions. Figure 5 shows the current recorded during Co-Sb

Figure 4. SEM of Co-Sb film on Au substrate. Deposition conditions: solution, 0.003 M Sb2O3 + 0.172 M CoSO4 · 7H2O + 0.125 M potassium citrate + 0.196 M citric acid; time, 1 h; applied potential, -1.1 V vs Ag/ AgCl.

Figure 5. Current variation during the deposition of nanowires and films at -0.9 V vs Ag/AgCl as a function of time, along with the four growth stages during deposition of nanowires. Current spikes at the end of nanowire growth are due to hydrogen evolution. (Note: same legend as in Figure 1.)

electrodeposition of Co-Sb film and nanowires as a function time. Both current variations bear the fingerprint of nanoscale processes. For the Co-Sb film, the current increases in the beginning of the process as more nuclei form on the Au nanostructured surface. After about 10 min, the current stabilizes at a constant value and the film grows at a steady rate. Unlike thin films, the formation of Co-Sb nanowires involves three main processes that take into consideration the template characteristics.20 First, Co2+ and SbO+ ions are driven by both electric field and concentration gradient into the nanopores of the template, while the diffusion of HSbO2 is only influenced by its concentration gradient. The charge applied to the electrode surface provides the electrons to produce elemental Co and Sb (reactions 4 and 5). Then, the reaction between Co and Sb results in the formation of CoSb3 and the growth of nanowires. The growth of nanowires inside these template membranes can be divided into four stages as seen in Figure 5. The first

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Figure 6. (a) SEM images of the overgrown nanowire surface (electrodeposition at -0.9 V for 3000 s). (b) Schematics of the isotropic (left) and anisotropic (right) growth of the mushroom caps. (Note: same legend as in Figure 1.)

stage (I) corresponds to an initial nucleation and growth of the nuclei inside the pores. The nucleation process starts at the bottom of the pores where the Au ring acts as an electrode. At this very initial stage of deposition, in the first 2 min of deposition, the variation of the current is similar to that recorded for thin films, suggesting a similar mechanism. Also, the fluctuation of the responding current strongly implies that there is a progressive nucleation. These nuclei grow freely in both radial and axial directions inside the template to a point where the nuclei start to overlap in the radial direction causing a local maximum in the absolute value of the current. This marks the end of the first stage. Stage II is a continuous growth of nuclei, which overlap/impinge inside the template. The growth is not uniform. Figure 5 suggests that there are nanowires in stage II that start to mushroom while other nanowires are still growing inside the template. Stage III is associated with mushrooming, where nanowires continue to grow over the template. The signature of the third stage is a rapid increase in the absolute value of the current which marks a sharp increase in surface area. At this point, the growth of nanowires inside the template is complete. The last stage, stage IV, is characterized by a rather constant current which is parallel to that of a film. The difference in current between the film and the last stage of nanowire growth is due to the roughness of each surface. Figure 6 shows the morphology of the Co-Sb surface (stage IV) formed by the mushroom cups of overgrown nanowires, i.e., that nanowires grew over the template surface. The uniqueness of this surface is given by the pillar-like structure of the mushroom cups. This morphology is completely different from the Co-Sb film (Figure 4).

In the process of nanowires electrodeposition, the deposit fills the pores of a template from the bottom. For extended deposition time, the deposit grows isotropically over the template, resulting in semispherical mushroom caps. This type of isotropic growth has been generally observed and is expected in the template synthesis process. However, the SEM micrographs in Figure 6 clearly show free-standing pillars, completely different from the thin film surface. A preferential growth in the vertical direction occurred, resulting in free-standing pillars. The pillars started to grow from one or more pores, where the conducting surface is exposed. After the pores were filled, the deposit grew anisotropically in the vertical direction and maintained the shape but not the size of the pores. Therefore, micrometer-size pillars were obtained with a diameter of 5-10 µm, defined by the membrane pattern. Mushroom caps are much bigger than nanowires, suggesting that the lateral growth is significant in the beginning of the nanowire overgrow. However, the individual surface pillars show preferential growth in the vertical direction, independently of each other. This peculiar behavior explains why the current during overgrowth of nanowires is not stable but slightly increases in time (Figure 5, the last stage of film growth). The Co-Sb pillars stand straight up, suggesting they are rigid enough to withstand the hydrodynamic force during the plating. When a much higher stirring rate was used, damages to the membrane were often observed. The resulting surface of the overgrown nanowires is not fully dense (Figure 6a) and cannot be directly compared to the film (Figure 4). The SEM micrographs suggest that on the PCTE template, vertical growth is much easier than lateral growth.

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Figure 7. (a) SEM micrograph of mushrooms caps formed at -1.0 V; (b and c) X-ray maps for Co and Sb, respectively.

Figure 8. (a) SEM micrograph of nanorods grown inside a 400 nm PCTE template at -0.965 V; (b) SEM image of a nanowire tip showing multiple terraces of various orientations suggesting the impingement of growing nuclei; (c) EDS on nanowires.

At this growth rate it may take a significant amount of time to cover completely the template surface. A closer look at beginning of the mushrooming process (Figure 7) unveils important aspects of the deposition of Co and Sb on a nanostructured surface. As shown in Figure 7, mushroom caps contain both Co and Sb; however, bridges between the mushroom heads are predominantly made of Co. It has been suggested that Sb grows according to the Staranski-Krastanov mode (layer + island), while the growth of Co follows the Frank-van der Merwe mode (layer by layer).37 Figure 7 suggests that Co does not incorporate uniformly in the overgrown nanowires; only the narrow channels in between the largest mushroom heads have Co-rich phase with a needle-like structure. The Co enrichment in between the cobalt antimonide pillars is probably induced by a local depletion in HSbO2/SbO+ that promotes a faster growth of Co structures. A similar needle-like structure has been previously reported for electrochemically deposited Co metal.38 It is worth mentioning that the Co-enriched bridges were formed only in between the largest overgrown nanowire heads, i.e., after the mushrooms reached a critical size. These new results indicate a more complex mechanism that was first suggested by Chen et al.20 and needs further investigation.

Figure 8 shows the SEM image of nanowires after the template was partially dissolved in dichloromethane. Nanowires have a slightly larger diameter than the initial template size and a rough surface. This appearance is probably induced by the template itself. Although hydrogen evolution is unavoidable, the nanowires do not show signs of porosity. A better look at the tip of a nanowire (Figure 8b) shows a solid structure with what seems to be multiple terraces. For a 400 nm membrane, a constant potential deposition at -0.965 V results in CoSb3 nanowires with a ratio of Co to Sb of 1:3 (Figure 8c). EDS results show that the elemental composition of nanowires, nanowire heads, and cauliflower-like films obtained in one sample is different. Deposits inside the template have a higher content of Co. For example, while EDS of nanowires grown inside a 400 nm PCTE template gives an average ratio of Co:Sb of 0.175, the composition of the overgrown nanowire caps obtained under the same deposition condition shows virtually no Co. Figure 9 shows the ratio of Co to Sb for nanowires and overgrown nanowire heads. Results are compared with data obtained by Cheng et al.37 Cheng’s data were obtained for cobalt antimonide films deposited on stainless steel. Compared to nanowires, the deposition of a film of a composition similar to

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Figure 9. Atomic ratio of Co to Sb as a function of deposition potential for 400 nm nanorods and their resultant mushrooms/“films”. (At -0.8 V, Co:Sb ) 0 for films in Cheng et al.37 and for both nanowires and mushrooms in this study.)

nanowire takes place at more negative potential. Mushroom caps formed by overgrown nanowire show a lower Co:Sb ratio when compared to film or nanowires. It is possible that Sb deposition inside nanopores is a diffusion-controlled process, and the small pore size of the template hinders its deposition. 4. Conclusions Cobalt antimonide thin films and nanowires were electrochemically deposited at constant potential at room temperature in aqueous citric electrolyte solutions. The Co-Sb thin film deposited on an Au nanostructured surface has a granular, cauliflower-like morphology and a composition that is different from the mushroom-like shape film formed by overgrown nanowires. The composition of Co-Sb obtained at a given deposition potential differs from thin films to nanowires and mushroom caps. At a given deposition potential in the potential range from -0.8 to 1.0 V, the ratio of Co to Sb is the highest for nanowires. At an applied potential of -965 mV, we obtained stoichiometric CoSb3 nanowires. Overgrown nanowires do not result in a compact film. Anisotropic overgrown mushroom caps result in pillar-like structures that eventually connect to each other through a Co-rich net morphology. Acknowledgment This work was supported by the National Science Foundation (NSF) under Grant 0930554. Literature Cited (1) Feschotte, P.; Lorin, D. The Binary-Systems Fe-Sb, Co-Sb and NiSb. J. Less-Common Met. 1989, 155 (2), 255–269. (2) Dudkin, L. D.; Abrikosov, N. K. A Physicochemical Investigation of Cobalt Antimonides. J. Inorg. Chem.-USSR 1956, 1 (9), 169–180. (3) Dudkin, L. D.; Abrikosov, N. K. On the Doping of the Semiconductor Compound Cosb3. SoViet Phys.-Solid State 1959, 1 (1), 126–133. (4) Singh, D. J.; Pickett, W. E. Skutterudite Antimonides - Quasi-Linear Bands and Unusual Transport. Phys. ReV. B 1994, 50 (15), 11235–11238. (5) Chitroub, M.; Besse, F.; Scherrer, H. Thermoelectric properties of semi-conducting compound CoSb3 doped with Pd and Te. J. Alloys Compd. 2009, 467 (1-2), 31–34. (6) Itoh, T.; Hattori, E.; Kitagawa, K. Thermoelectric properties of ironand lanthanum-doped CoSb3 compounds by pulse discharge sintering. J. Propulsion Power 2008, 24 (2), 359–364.

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ReceiVed for reView May 27, 2010 ReVised manuscript receiVed July 27, 2010 Accepted August 28, 2010 IE101173U