Segmented All-Platinum Nanowires with Controlled Morphology

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Segmented All-Platinum Nanowires with Controlled Morphology through Manipulation of the Local Electrolyte Distribution in Fluidic Nanochannels during Electrodeposition Markus Rauber,*,†,‡ Joachim Bro¨tz,† Jinglai Duan,§ Jie Liu,§ Sven Mu¨ller,‡ Reinhard Neumann,‡ Oliver Picht,‡ Maria Eugenia Toimil-Molares,‡ and Wolfgang Ensinger† Department of Material- and Geo-Sciences, Technische UniVersita¨t Darmstadt, Darmstadt, Germany, Materials Research Department, GSI Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Darmstadt, Germany, and Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China ReceiVed: September 17, 2010

Synthesis of segmented all-Pt nanowires is achieved by a template-assisted method. The combination of a suitably chosen electrolyte/template system with pulse-reverse electrodeposition allows the formation of welldefined segments linked to nanowires. Manipulation of the morphology is obtained by controlling the electrokinetic effects on the local electrolyte distribution inside the nanochannels during the nanowire growth process, allowing a deviation from the continuously cylindrical geometry given by the nanoporous template. The length of the segments can be adjusted as a function of the cathodic pulse duration. Applying constant pulses leads to segments with homogeneous shape and dimensions along most of the total wire length. X-ray diffraction demonstrates that the preferred crystallite orientation of the polycrystalline wires varies with the average segment length. The results are explained considering transitions in texture formation with increasing thickness of the electrodeposit. A mechanism of segment formation is proposed based on structural characterizations. Nanowires with controlled segmented morphology are of great technological importance, because of the possibility to precisely control their substructure as a means of tuning their electrical, thermal, and optical properties. The concept we present in this work for electrodeposited platinum and track-etched polycarbonate membranes can be applied to other selected materials as well as templates and constitutes a general method to controlled nanostructuring and synthesis of shape controlled nanostructures. Introduction One-dimensional (1-D) nanostructures have become a focus of intensive research. Compared to their bulk counterparts, they exhibit new physical and chemical characteristics that predestine them as promising candidates for advanced technological applications.1 It is generally accepted that the peculiar properties of nanowires are a direct consequence of their shape anisotropy and reduced dimensionality.2,3 A common fabrication technique is based on electrodeposition into nanochannels of a porous template. In this straightforward approach, nanowires are generated by reduction of ions from an electrolyte solution adopting the morphology of the host material.4 Since the dimensions of the nanostructure are predetermined by the template, changing the internal structure of the nanowire by the deposition process can be identified as a key issue, which must be addressed to further control and tune the properties of nanowires of a given material. In particular, the electrodeposition parameters have a strong influence on the preferred growth orientation and crystal structure.5 The template-directed electrochemical method has been widely used to produce a large variety of metals, semiconductors, and composite materials. Hence, the influence of deposition parameters on the nanowire structure has been investigated thoroughly, and much research work concerning the growth mechanisms inside high-aspectratio templates is reported.6-11 It was demonstrated that the * To whom correspondence should be addressed. Phone: +49 (0)615971-2715. E-mail: [email protected]. † Technische Universita¨t Darmstadt. ‡ GSI Helmholtzzentrum für Schwerionenforschung GmbH. § Institute of Modern Physics.

structures of nanowires consisting of various metals can be tuned from typical polycrystalline to single crystalline.12,13 However, single-crystal growth via electrodeposition is very difficult for high melting point metals such as Rh and Pt.2 Since these metals are relatively insensitive to the electrodeposition parameters, additional methods for tuning their structures are desirable. Manipulating their morphologies is important for providing control over properties required for applications in the field of catalysts, sensors, and electronic devices. The template-based electrochemical method also proved to be an effective approach for obtaining length and sequence controlled multicomponent layered structures (e.g., Bi/BiSb, Co/ Pt, Fe/Au).9,14-16 While extensive research work has been dedicated to segmented multicomponent nanowires, whose readily distinguishable structure elements are made of different compositions, only scarce knowledge is available about segmented 1-D nanostructures consisting of constant composition or only one chemical element.17,18 Very recently, Shpaisman et al. reported the deposition of CdSe nanowires with constant diameter and uniform chemical composition, but exhibiting modulated “density” along the wire axis.19 However, segmented single-element nanowires cannot be directly synthesized by the template electrodeposition methods reported so far. Studies on nanowires have shown that their properties (e.g., thermal conductivity, electrical conductivity, catalytic and optical properties) depend not only on size, and crystallinity, but also on the nanowire morphology.20-22 In particular, surface roughness has been demonstrated to play a crucial role in the decrease of thermal conductivity in Si nanowires, and interesting plasmonic effects have been predicted also for nanowires showing constrictions along their length.23,24 Successful manipulation and

10.1021/jp108889c  2010 American Chemical Society Published on Web 12/08/2010

Segmented All-Platinum Nanowires additional control of the morphology of nanowires, electrodeposited inside templates, would open up new opportunities to tune the surface characteristics of nanowires for specific thermal, catalytic, and optical applications. In the case of Pt, cylindrical nanowires have been synthesized by various techniques including template electrodeposition, chemical reduction, and electrospinning.25-27 Other shape controlled Pt nanostructures have been generated by a number of methods.28-31 Here, we report the synthesis of segmented all-Pt nanowires by template electrodeposition into ion track etched polycarbonate (PC) membranes. Pulse-reverse (PR) deposition, in combination with a selected electrolyte-template system, is used to influence the local distribution of Pt ions inside the nanochannels and generate nanowires, consisting of linked segments, by a kinetically controlled growth process. This novel growth mechanism enables the design of nanowires with a preset number of segments, and thus of both interfaces and diameter constrictions. On the basis of structural characterizations, we propose a mechanism responsible for nucleation and growth of segments. Experimental Section Templates for nanowire deposition were fabricated by heavyion irradiation and subsequent chemical etching. First, 30-µmthick PC foils (Makrofol N, Bayer Leverkusen) were irradiated at the UNILAC heavy-ion linear accelerator of GSI with Au25+ ions with a kinetic energy E ) 11.1 MeV/nucleon, applying fluences of 1 × 108-4 × 109 ions/cm2. Second, chemical etching in aqueous 6 M NaOH solution at 50 °C led to the formation of track-etched membranes with cylindrical nanochannels. A thin gold layer was subsequently sputter-deposited on one side of the membrane and reinforced electrochemically with copper serving as cathode for nanowire array growth. PR electrodeposition into the nanochannels was performed at 65 °C from an alkaline platinum bath (Platinum-OH, Metakem) in a twoelectrode cell. The cathodic pulse was applied at a potential Uc ) -1.3 V for different pulse durations tc ranging from 1 to 20 s. The anodic pulse was invariably timed to ta ) 1 s at Ua ) +0.4 V. After electrodeposition, the polymer template was dissolved in dichloromethane for nanowire characterization. Field-emission scanning electron microscopy (FESEM) was employed to investigate the morphology of randomly distributed nanowires on the cathode layer and of isolated nanowires on Si substrate. A more detailed morphological and structural characterization was obtained by transmission electron microscopy (TEM). The texture of nanowire arrays was studied by X-ray diffraction (XRD), while the wires were still embedded in the membrane, by using a STOE four-circle diffractometer with Co KR radiation. Results and Discussion Figure 1 shows a representative example of a pulse-reverse deposition with cathodic pulse duration tc ) 5 s and anodic pulse duration ta ) 1 s, applied for the growth of modulated Pt nanowires. The curve represents the charge Qp employed during each pulse as a function of the respective pulse number N, and it shows the same three characteristic zones (1-3) as the current-time curves, recorded for metallic nanowires grown under direct current (dc) deposition conditions.6,8 After a charge decrease due to the growing linear diffusion layer in zone 1, an almost constant regime is reached in zone 2, while the nanochannels are filled with Pt. Zone 3 represents the rapid increase of charge, as caps are formed on top of the membrane surface enlarging the electrode area. Qp was calculated by

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Figure 1. Graph illustrating the development of the deposited charge of cathodic pulses Qp and the average segment length LS with the pulse number N. Three regimes describing the main events are indicated (1-3). The inset shows the applied potential U and the resulting current I as function of time for three consecutive pulses.

integrating the correspondent current vs time response, shown in the inset for three consecutive pulses. It is noticeable that coalescence features following the initial nucleation can be identified in the inset curve. The Faraday law relates the charge needed for each segment growth with the segment’s geometric characteristics. Using Qp ) zFFLSπr2w/M, with z denoting the ion charge state, F the Faraday constant, F the density of platinum, r the pore/wire radius, w the total number of nanowires, M the molar mass, and LS the average length of segments, we calculated LS for each pulse number N corresponding to the segment number. The results demonstrate that the length of the segments can be controlled by the duration of the cathodic pulse, and that the efficiency of the deposition reaction is about 100%. Figure 2 presents FESEM images of Pt nanowires exhibiting their characteristic segmented morphology obtained by PR electrodeposition. Figure 2a depicts several nanowires consisting of segments that maintain their homogeneous dimensions (diameter 45 nm, length 50 nm) almost along the entire nanowire. Typically, the nanowires exhibit the same morphological characteristics over lengths >20 µm. In addition, highmagnification images (Figure 2b,c) reveal both sharp interfaces and pronounced diameter constrictions over the full wire length. Parts d and e of Figure 2 display nanowires produced by PR deposition using pulse durations tc ) 5 and 20 s, respectively, resulting in current-time transients shown in the insets. The images displaying the sections of the nanowires with lengths 55 and 200 nm, respectively, prove that the segment length can be well controlled as a function of the cathodic pulse duration. By modulating the potential, sequence controlled nanowires can also be produced for potential applications as information storage or bar-coded nanowires (see Supporting Information, Figure S1). The segment length is very uniform along the wire axis. A slight and continuous length increase along the axis proceeds due to the rise in diffusion limited current with growing amount of Pt deposited in the template. In other words, electrodeposition in the membrane nanochannels can be described as occurring on a recessed nanoelectrode array. During nanowire growth, the cathode surface translates toward the upper membrane surface, and thus the effective length of the recessed nanoelectrode decreases continuously. For pulses of the same duration, the

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Figure 2. FESEM images of segmented Pt nanowires: (a-c) nanowires deposited under PR with tc ) 5 s at different magnifications (diameter 45 nm); (d, e) nanowires produced with different cathodic pulse durations (tc ) 5 and 20 s, respectively, diameter 50 nm). The insets show corresponding current-time curves recorded during PR electrodeposition.

electrodeposited segment length will be proportional to the diffusion current. Electrochemical growth of segmented nanowires constitutes therefore a powerful method to investigate transport processes in nanochannels. Representative TEM images of several segmented nanowires are depicted in Figure 3. The selected area electron diffraction (SAED) pattern shown in the inset indicates the polycrystalline structure of the corresponding nanowires. Interestingly, the micrographs reveal that the segments are linked to each other only by a short connection with reduced diameter in the center, rotationally symmetric to the wire axis (Figure 3a). The nanowires thus do not fully adopt the morphology of the nanochannels, in contrast to the common behavior when applying dc voltage. Void space observed between the segments is essential to create clear separations. It is evident that these wires exist in a thermodynamic nonequilibrium state, and that the deviation from the cylindrical geometry predetermined by the template is a consequence of a growth process predominantly controlled by kinetics. To study the influence of the cathodic pulse characteristics on the resulting nanowire morphology, we electrodeposited Pt in different membranes with identical characteristics. We applied cathodic pulses, all with Uc ) -1.3 V but different durations tc ) 5, 2.5, and 1 s, while keeping the anodic pulse voltage and duration constant at Uc ) +0.4 V and ta ) 1 s. TEM images in Figure 3b-d demonstrate (as previous images in Figure 2) that the segment length decreases with shorter pulse duration tc, until individual segments can hardly be recognized (Figure 3d).

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Figure 3. TEM images of Pt nanowires produced by PR electrodeposition with different cathodic pulse durations tc at -1.3 V: (a, b) segmented nanowires (tc ) 5 s); (c) segmented nanowire (tc ) 2.5 s); (d) continuous nanowire (tc ) 1 s). The anodic pulse was timed to ta ) 1 s at +0.4 V for all experiments. The inset represents the corresponding SAED pattern.

However, we could also observe a complete disappearance of the segmented morphology when the cathodic pulse duration tc was much shorter than 1 s. The growth mechanism of segmented nanowires is assumed to be based on a slow ion diffusion in the pores, together with a kinetically controlled reduction at the electrode during the cathodic pulse. During the anodic pulse, the ions have additional time to diffuse to the electrode, thus decreasing the concentration gradient in the nanochannel. Consequently, the electrolyte concentration is higher on the electrode surface at the beginning of a deposition pulse and decreases during the cathodic pulse. For very short cathodic pulses, the average ion concentration in the vicinity of the electrode remains high. Appropriate differences in the local electrolyte distribution, essential for segment formation, cannot be established. In addition, as the average current density is decreased for short cathodic pulses, the growth process becomes less controlled by kinetics. Hence, the wires appear continuously cylindrical without clear signs of segmentation, but rather displaying irregular grain boundaries, indicating a typical three-dimensional (3-D) nucleationcoalescence mechanism. The crystal structure of nanowire arrays, produced by dc and PR deposition with different cathodic pulse durations, was investigated systematically by XRD, while keeping the wires embedded in the template. These arrays were deposited in templates fabricated in exactly the same manner, leading to arrangements of similar wire diameter, length, and areal density. The related X-ray diffractograms shown in Figure 4 demonstrate that the crystallographic orientation is sensitive to variation of

Segmented All-Platinum Nanowires

Figure 4. (a) X-ray diffractograms of segmented nanowire arrays produced by PR electrodeposition (1-5) using different cathodic pulse durations (1, 2.5, 5, 10, and 20 s). Peaks not labeled originate from Cu and CuO in the cathode layer. Vertical lines represent the intensity distribution of a Pt powder sample. (b) TC100 versus segment length. The inset shows the development of the FWHM of the Pt 200 diffraction peaks.

the deposition parameters. We observe a correlation of segment length with texture. In particular, significant changes are found with regard to 〈200〉 reflections. In the case of dc deposition and PR deposition using long cathodic pulses of 20 s, the wires are continuously cylindrical or composed of long segments, and appear rather polycrystalline without preferential orientation. With decreasing pulse duration tc (from 20 to 5 s) and segment length (from 139 to 50 nm), the intensity of the 〈200〉 reflection becomes stronger as a distinct preferred orientation in the 〈100〉 direction is developed. Further decrease in cathodic pulse duration leads again to a polycrystalline structure with random orientation. This observation is supported by texture coefficients TChkl calculated for the different hkl reflections considering the first four reflections (111, 200, 220, and 311).

I(hikili)/I0(hikili)

TC(hikili) )

n

(1/n)

∑ (I(hjkjlj)/I0(hjkjlj)) j)1

where I0(hikili) are the intensities of the (hikili) lattice planes of a standard Pt powder sample and I(hikili) are the intensities of the (hikili) lattice planes under analysis; n is the total number of

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22505 diffraction planes, taken into account for calculations, and represents the maximum value of the texture coefficient. For TC(hikili) > 1 there exists a preferred crystallographic orientation perpendicular to the (hikili) planes. The theoretical maximum value of TC is 4, representing nanowires with all crystallites oriented in the same direction along the wire axis. The texture coefficient TC100 of the Pt wires is plotted versus the average segment length in Figure 4b. It reaches its highest value of 2.35 for a cathodic pulse duration of 5 s. The average segment length was calculated using Faraday’s law and experimentally determined by FESEM, as explained above. The development of a preferred growth orientation is assumed to be caused mainly by nucleation and growth of crystallites during the cathodic pulse and not during postdeposition processing. Because different crystal faces show different rates of growth, crystallites with different orientations compete in growth rate. The generation of a preferred orientation of the crystallites either can be an effect of preferential nucleation or can originate from the competitive growth process following the coalescence stage.32,33 Assuming the second case, texture formation is a process that develops with deposition time and thickness of the deposit. Transitions in phase formation have been observed, and it was shown that, after a critical thickness is reached, phase formation changes from a thermodynamically to a kinetically controlled process.34,35 Since preferred orientation, created during the initial step of electrodeposition, often deviates from the orientation of the crystallites formed during following growth, the deposit texture and its degree of perfection will finally be controlled by the thickness of the electrodeposit.36 In the case of segmented nanowires, the segment length is identical with the deposition thickness. We assume that nearly all segments of a sample are almost identical, regarding the development of texture along their axis, because the electrodeposition conditions before each deposition pulse and mass transport during growth are very similar. By investigating samples with segments of different lengths, information about the development of preferred orientation is obtained. Our experimental results reported in Figure 4 show that segments differing in length have different textures. Therefore, a transition in phase formation must occur after a critical thickness is reached. The shortest observed deposits, measuring an average thickness of 13.2 nm, exhibit no clearly preferred orientation, with all considered TChkl values close to 1. By increasing the average segment length to 29.5 nm, a preferred orientation in the 〈100〉 direction appears. We may suggest that in the initial stage of electrocrystallization the orientation of the Pt nuclei is random, leading to a newly coalesced deposit with random orientation. The texture of thicker deposits is a result of competitive growth mechanisms of crystallites, taking place in a stage of growth subsequent to the coalescence stage. Grains of low surface energy grow faster than those of high surface energy. After formation of the first layers, the {100} surface, representing a low-energy surface, is favored. Consequently, a 〈100〉 texture is developed as the fraction of crystallites oriented in this direction prevails. The degree of perfection of the 〈100〉 texture increases with the average segment length, until it reaches a maximum for segments with an average length of 50.3 nm. As the cathodic pulse duration is further increased to grow longer segments, the period of time of each pulse, in which the reduction reaction is limited by the diffusion-driven transport, is expanded. The gain of the diffusion-limited regime shifts the process toward a mechanism being mainly controlled by kinetics, allowing energetically higher surfaces such as the {311} and {220} surfaces to grow increasingly, and hence the 〈100〉

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texture vanishes. The structure becomes rather randomly oriented, again for segments with an average length of 138.8 nm and cylindrical nanowires produced under dc. The average size of crystallites oriented in the 〈100〉 direction was derived from the full width at half-maximum (FWHM) of the Pt 200 reflections in the diffractograms presented in Figure 4a. The development as a function of tc is plotted in the inset of Figure 4b. The average size of the crystallites with 〈100〉 orientation increases with the segment length. This finding may be a result of the preferred growth of existing crystallites with 〈100〉 orientation and hints at a granular texture. Finally, as the segments grow longer, the average size of grains with 〈100〉 orientation appears smaller again, since the deposition conditions favor increasingly surfaces other than {100}. Thus, the fraction of smaller crystallites oriented in the 〈100〉 direction increases with the average segment length. Estimations using the Scherrer equation result in grains with 〈100〉 orientation of an average size of 5-7 nm. These results are in good agreement with observations made by TEM. It must be carefully considered that increasing tc without changing ta leads to a decrease of the average electrolyte concentration near the growing nanowire. As a consequence, limitations due to mass transport gain influence even at the beginning of a deposition pulse. Phase formation becomes more and more controlled by kinetics, and the transition to the development of a polycrystalline deposit with random orientation should start earlier along the segment. Moreover, it should be taken into account that structural changes in the nanowire-electrolyte interface induced by anodic pulses may also contribute to the development of texture of electrodeposited nanowires.37 However, since only the electrode surface is affected, the influenced fraction of volume is relatively small under the applied conditions and may only be important for very short cathodic pulses. Different growth processes have been proposed for nanowires.9,12 The polycrystalline Pt nanowires produced by dc plating seem to follow a typical 3-D nucleation-coalescence mechanism.38 Several nuclei are formed at the same time, independently of each other, on the layer growth front and grow to 3-D clusters. As deposition is continued, isolated clusters coalesce, and the free space between clusters is subsequently filled until a complete hole-free film is created. In contrast to the typical 3-D nucleation-coalescence growth mode, the formation of a continuous deposit is not observed for Pt nanowire deposition in the case of PR plating. The SEM and TEM images indicate that nucleation and growth of 3-D clusters occur only on a circular area at the center of each nanochannel, when a cathodic pulse is applied at the initial state of segment growth. We believe that the localized formation of nuclei, and thus also the parabolic shape of the segment front faces (Figure 3b,c), is directed by the Pt ion concentration and is a direct consequence of the influence of electrokinetic effects on the local electrolyte distribution in small fluidic nanochannels. Due to the combination of ion track etched PC membranes and the strong alkaline Pt electrolyte solution, the nanochannel walls acquire a negative charge, and an electrical double layer (EDL) of significant thickness is established.39 While the negative charges on the channel walls remain stationary, the mobile charges in the EDL result in a flow toward the cathode, if an electrical field is applied. This flow depends on the thickness of the EDL, which in turn depends among other things on the pH value and the ion concentration and increases with rising pH and a decrease in the electrolyte concentration. For fluidic nanochannels the flow profile is parabolic in shape

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Figure 5. (a) Scheme of concentration-directed 3-D nucleation mechanism. (b) Schematic illustration of a segmented nanowire with parabolic front faces of the segments.

and becomes more pronounced for increasing double layer thicknesses.40 This means that the ion velocity is higher in the center of the nanochannel than near the channel wall. Although the overall mass transport in the nanochannels closed from one side is driven by diffusion, electrokinetic processes can exert a strong influence. In particular, when the external field is reversed, the interactions are expected to be strong in the vicinity of the growing nanoelectrode where the electrolyte concentration is low. This exactly happens, whenever deposition is stopped or when a new segment is deposited. During the anodic pulse, the positive Pt ions are rejected from the front face of the segment formed immediately before. By applying a cathodic pulse, the ions follow the reversed field and move toward the nanowire. Because of the transport profile the ions move fastest in the very center of the nanochannel. The redistribution of ions leads to local charge differences. Figure 5a shows a schematic illustration of the proposed concentration-directed nucleation-coalescence growth process. At the center of the growth front, the concentration of the Pt species is higher than near the channel wall and hence is also the nucleation rate. We assume that after the stage of coalescence, a stable growth process occurs at which a parabolic shape of the growing front is maintained in the majority of cases. The whole mechanism is not understood entirely, and further investigations are underway. Our concept may be applied to other suitably chosen systems, to create segmented nanowires consisting of other materials, and represents a general route to controlled nanostructuring. Conclusions In summary, we developed a simple and highly effective route to produce segmented single-metal nanowires by using a selected electrolyte/template system in combination with PR plating. To our knowledge, this is the first time that segmented all-Pt nanowires have been fabricated via template electrodeposition. It was shown that the dimensions of the segments can be controlled accurately, and that the texture is a function of the segment length. This work not only makes a contribution to the understanding of the growth process of electrodeposited nanowires predominantly controlled by kinetics, but also demonstrates that these wires are a powerful tool to investigate transport processes in fluidic nanochannels. The segmented single-metal nanowires may have a great potential for applications because of the possibility to tune their properties by precisely controlling the structure. Acknowledgment. This work was in part supported by BMBF (International Bureau, WTZ Grant CHN 08/039). J.L.

Segmented All-Platinum Nanowires and J.D. acknowledge support from NSFC grants (Nos. 10775161 and 10805062). Supporting Information Available: Structural characterization including TEM and FESEM images of segmented Pt nanowires, and details of the synthesis of length and sequence controlled nanowires used to store binary information. TChkl values of segmented nanowire arrays. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (2) Kline, T. R.; Tian, M.; Wang, J.; Sen, A.; Chan, M. W. H.; Mallouk, T. E. Inorg. Chem. 2006, 45, 7555. (3) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (4) Martin, C. R. Science 1994, 266, 1961. (5) Karim, S.; Toimil-Molares, M. E.; Maurer, F.; Miehe, G.; Ensinger, W.; Liu, J.; Cornelius, T. W.; Neumann, R. Appl. Phys. A: Mater. Sci. Process. 2006, 84, 403. (6) Schuchert, I. U.; Toimil-Molares, M. E.; Dobrev, D.; Vetter, J.; Neumann, R.; Martin, M. J. Electrochem. Soc. 2003, 150, C189. (7) Philippe, L.; Kacem, N.; Michler, J. J. Phys. Chem. C 2007, 111, 5229. (8) Valizadeh, S.; George, J. M.; Leisner, P.; Hultman, L. Electrochim. Acta 2001, 47, 865. (9) Dou, X.; Li, G.; Lei, H. Nano Lett. 2008, 8, 1286. (10) Dou, X.; Zhu, Y.; Huang, X.; Li, L.; Li, G. J. Phys. Chem. B 2006, 110, 21572. (11) Pan, H.; Liu, B.; Yi, J.; Poh, C.; Lim, S.; Ding, J.; Feng, Y.; Huan, C. H. A.; Lin, J. J. Phys. Chem. B 2005, 109, 3094. (12) Tian, M.; Wang, J.; Kurtz, J.; Mallouk, T. E.; Chan, M. H. W. Nano Lett. 2003, 3, 919. (13) Toimil-Molares, M. E.; Buschmann, V.; Dobrev, D.; Neumann, R.; Scholz, R.; Schuchert, I. U.; Vetter, J. AdV. Mater. 2001, 13, 62. (14) Wang, L.; Yu-Zhang, K.; Metrot, A.; Bonhomme, P.; Troyon, M. Thin Solid Films 1996, 288, 86. (15) Choi, J. R.; Oh, S. J.; Ju, H.; Cheon, J. Nano Lett. 2005, 5, 2179. (16) Lee, J. H.; Wu, J. H.; Liu, H. L.; Cho, J. U.; Cho, M. K.; An, B. H.; Min, J. H.; Noh, S. J.; Kim, Y. K. Angew. Chem., Int. Ed. 2007, 46, 3663. (17) Huang, X. P.; Han, W.; Shi, Z. L.; Wu, D.; Wang, M.; Peng, R. W.; Ming, N. B. J. Phys. Chem. C 2009, 113, 1694.

J. Phys. Chem. C, Vol. 114, No. 51, 2010 22507 (18) Laocharoensuk, R.; Sattayasamitsathit, S.; Burdick, J.; Kanatharana, P.; Thavarungkul, P.; Wang, J. ACS Nano 2007, 1, 403. (19) Shpaisman, N.; Givan, U.; Patolsky, F. ACS Nano 2010, 4, 1901. (20) Toimil-Molares, M. E.; Ho¨hberger, E. M.; Schaeflein, C.; Blick, R. H.; Neumann, R.; Trautmann, C. Appl. Phys. Lett. 2003, 82, 2139. (21) Karim, S.; Ensinger, W.; Cornelius, T. W.; Neumann, R. Physica E 2008, 40, 3173. (22) Vo¨lklein, F.; Reith, H.; Cornelius, T. W.; Rauber, M.; Neumann, R. Nanotechnology 2009, 20, 325706. (23) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Nature 2008, 451, 163. (24) Large, N.; Saviot, L.; Margueritat, J. r. m.; Gonzalo, J.; Afonso, C. N.; Arbouet, A.; Langot, P.; Mlayah, A.; Aizpurua, J. Nano Lett. 2009, 9, 3732. (25) Choi, S. M.; Kim, J. H.; Jung, J. Y.; Yoon, E. Y.; Kim, W. B. Electrochim. Acta 2008, 53, 5804. (26) Shui, J.; Li, J. C. M. Nano Lett. 2009, 9, 1307. (27) Teng, X.; Han, W. Q.; Ku, W.; Hu¨cker, M. Angew. Chem. 2008, 120, 2085. (28) Lim, B.; Lu, X.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Lee, E. P.; Xia, Y. Nano Lett. 2008, 8, 4043. (29) Mayers, B.; Jiang, X.; Sunderland, D.; Cattle, B.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 13364. (30) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 126, 635. (31) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (32) Amblard, J.; Froment, M.; Maurin, G.; Spyrellis, N.; TrevisanSouteyrand, E. Electrochim. Acta 1983, 28, 909. (33) Amblard, J.; Froment, M.; Maurin, G.; Mercier, D.; Trevisan-Pikacz, E. J. Electroanal. Chem. 1982, 134, 345. (34) Switzer, J. A.; Kothari, H. M.; Bohannan, E. W. J. Phys. Chem. B 2002, 106, 4027. (35) Fukumoto, Y.; Kawashima, Y.; Hayashi, T. Surf. Coat. Technol. 1986, 27, 145. (36) Kozlov, V. M.; Bicelli, L. P. Mater. Chem. Phys. 2002, 77, 289. (37) Maurer, F.; Bro¨tz, J.; Karim, S.; Toimil-Molares, M. E.; Trautmann, C.; Fuess, H. Nanotechnology 2007, 18, 135709. (38) Paunovic, M. Schlesinger, M. Fundamentals of Electrochemical Deposition, 2nd ed.; Wiley-Interscience: Hoboken, NJ, 2006. (39) Kemery, P. J.; Steehler, J. K.; Bohn, P. W. Langmuir 1998, 14, 2884. (40) Yuan, Z.; Garcia, A. L.; Lopez, G. P.; Petsev, D. N. Electrophoresis 2007, 28, 595.

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