Planar Arrays of Nanoporous Gold Nanowires - ACS Publications

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Planar arrays of nanoporous gold nanowires: when electrochemical dealloying meets nanopatterning Adrien Chauvin, Cyril Delacote, Leopoldo Molina-Luna, Michael Duerrschnabel, Mohammed Boujtita, Damien Thiry, Ke Du, Junjun Ding, Chang-Hwan Choi, Pierre-Yves Tessier, and Abdel-Aziz El Mel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11244 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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ACS Applied Materials & Interfaces

Planar arrays of nanoporous gold nanowires: when electrochemical dealloying meets nanopatterning Adrien Chauvin,† Cyril Delacôte,‡ Leopoldo Molina-Luna,║ Michael Duerrschnabel,║ Mohammed Boujtita,‡ Damien Thiry,† Ke Du,§,1 Junjun Ding,§ Chang-Hwan Choi,§ Pierre-Yves Tessier,† and Abdel-Aziz El Mel†* †

Institut des Matériaux Jean Rouxel, IMN, Université de Nantes, CNRS, 2 rue de la Houssinière B.P. 32229, 44322 Nantes cedex 3, France

‡

CEISAM, Université de Nantes, CNRS, 2 rue de la Houssinière, 44322 Nantes Cedex 3, France

║

Technische Universität Darmstadt, Department of Material- and Geosciences, Alarich-WeissStrasse 2, 64287 Darmstadt, Germany

§

Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA

1

Current address: Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States

ACS Paragon Plus Environment

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ABSTRACT

Nanoporous materials are of great interest for various technological applications including sensors based on surface enhanced Raman scattering, catalysis, and biotechnology. Currently, tremendous efforts are dedicated to the development of porous one-dimensional materials to improve the properties of such class of materials. The main drawback of the synthesis approaches reported so far includes (i) the short length of the porous nanowires which cannot reach the macroscopic scale and (ii) the poor organization of the nanostructures obtained by the end of the synthesis process. In this work, we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with a length up to a few centimeters. This two-step approach consists of the growth of gold/copper alloy nanowires by magnetron co-sputtering on a nanograted silicon substrate, serving as a physical template, followed by a selective dissolution of copper by an electrochemical anodic process in diluted sulfuric acid. We demonstrate that the pore size of the nanowires can be tailored between 6 and 21 nm by tuning the dealloying voltage between 0.2 V and 0.4 V and the dealloying time within the range 150-600 s. We further show that the initial gold content (11 to 26 at. %) and the diameter of the gold/copper alloy nanowires (135 to 250 nm) are two important parameters that must carefully be selected to precisely control the porosity of the material.

KEYWORDS Nanoporous, nanowires, dealloying, gold, copper


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INTRODUCTION With a three-dimensional open porosity and an extremely high specific surface area, sponge-like nanoporous metals constituted of interconnected nanoligaments rank among the top multifunctional materials exhibiting exceptional properties such as enhanced catalytic activity,1,2 superhydrophobicity,3 and high electric field enhancements thanks to surface plasmons.4 They have demonstrated impressive capabilities in various modern technological areas relevant for optics,5–7 mechanics,8–10 biotechnology,11,12 and nanofluidics.13 Currently, nanoporous metals are fabricated using various methods including dealloying,14,15 galvanic replacement,16–19 and dual-templating.20,21 Among these techniques, dealloying is the most commonly used approach to prepare nanoporous metals since it can be applied to a broad range of metals including gold,22–24 copper,25 platinum,26 silver,27 and palladium.28 In case of binary alloys, the dealloying effect describes the leaching of the less noble element of the alloy leaving behind a nanoporous skeleton of the most noble one exhibiting the highest oxidation potential.14 Free-corrosion, which consists in the direct exposure of a metal alloy to a highly corrosive electrolyte such as nitric acid, is the simplest way to apply the dealloying approach.29,30 Dealloying can be also carried out using an electrochemical process in a less corrosive electrolyte.31 In comparison to free-corrosion, electrochemical dealloying provides better results since it allows an accurate real-time control of the dealloying process via a direct monitoring of the electrochemical parameters.32,33 Most of the studies in literature dealing with the dealloying approach report on the growth of nanoporous metals in a sheet-like structure; the sheet thickness may vary between hundred of nanometers and a few microns.14,15 Synthesizing such nanoporous material in a one-


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dimensional (1D) structure, such as nanowires and nanotubes, is among the best ways allowing enhancing its active specific surface area.34,35 Compared to the sheet-like two-dimensional (2D) structure, the improvement in case of the 1D structure originates from the anisotropic shape and the high aspect ratio of the material.36 In contrast to spherical nanoparticles,31,37,38 in general, 1D nanostructures display additional advantages in terms of manipulation and electrical contacting which facilitate their integration in nanodevices.39 In most of cases, 1D porous metal nanostructures are fabricated by applying the freecorrosion approach to a binary metal alloy nanowires electroplated into cylindrical nanopores of a host template membrane such as porous anodic aluminum oxide.40,41 Such strategy requires, however, an additional intermediate stage before carrying out the dealloying process; this stage consists in the removal of the host template after the growth of the nanowire alloys.39,42,43 Another drawback to such a template approach is the short length of the porous nanowires which stays limited by the thickness of the template membrane making it impossible to reach the macroscopic scale. This in turns makes the integration of porous nanowires in practical devices more challenging. The advantages of using ultra long nanowire has been reported in literature.44,45 Here we report on a new strategy to prepare highly ordered ultra-long porous gold nanowires with a tunable porosity and a length up to few centimeters. Our two-step approach involves the growth of Au-Cu alloy nanowires by magnetron co-sputtering of gold and copper targets over a nanograted substrate serving as a physical template46 followed by electrochemical dealloying (Figure 1a) allowing to transform the nanowires from solid (Figure 1b) to nanoporous (Figure 1c). In particular, we aim to control in an accurate manner the nanoporosity created within the nanowires by i) adjusting the Au-Cu precursor nanowires characteristics (i.e.,


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composition and dimensions) and ii) by carefully selecting the appropriate electrochemical dealloying parameters (i.e., dealloying time and potential). RESULTS AND DISCUSSION Choice of the electrochemical conditions. When carefully examining the cyclic voltammogram recorded on the as-grown Au-Cu nanowires, one can identify a peak at 0.1 V that can be attributed to the oxidation of Cu (Figure 1d). This means that by selecting a dealloying potential higher than 0.1 V, one can dissolve copper from the alloy. For this reason, the lowest dealloying potential employed in our study was 0.2 V and the highest dealloying potential was 0.5 V.47 The shortest dealloying duration was fixed to 150 s based on a several chronoamperometry curves recorded at different potential values; more precisely, from the typical chronoamperometry curve shown in Figure 1e, one can remark that for a dealloying time exceeding 150 s, the current density is very stable and it exhibits its lowest value.

Influence of the dealloying conditions. In this section we explore the impact of the dealloying potential and time on the formation of nanoporosity in Au-Cu nanowires. For this purpose, Au-Cu alloy nanowires with 11 at. % of Au and 200 nm in diameter were used. As shown in the plan-view SEM micrographs, the pore size is found to increases with an increase of the dealloying time and potential (Figure 2). The nanowires dealloyed at 0.2 V for 150 s (Figure 2a) and 300 s (Figure 2b) exhibit a morphological modification with the formation of superficial pores. Nanoporosity appears for samples dealloyed for 600 s (Figure 2c); the pore size, however, remains small (11 nm). Increasing the dealloying potential to 0.3 V allows generating a more significant porosity within the nanowires (Figure 2d-f). The pore size becomes larger as the


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dealloying time increases. Simultaneously to the observed increase in pore size, the diameter of the nanowires is found to decrease when increasing the dealloying time. For the samples dealloyed at 0.4 V, the formation of a porous structure was the most dramatic (Figure 2g-i). In addition, the nanowires are found to be constituted of nanoligaments. Furthermore, it can be seen that the nanoligaments width increases from 13 to 20 nm as increasing the dealloying time from 150 and 300 s. To gather additional information about the three-dimensional distribution of the nanopores, cross-sectional SEM micrographs were also recorded on all the samples (Figure 3). At 0.2 V, for less than 300 s (Figure 3a and 3b), no pores can be noticed but rather periodic nanolayers can be clearly seen. The formation of such multilayered structures was explained in details in our recent work.48 Briefly, the formation of the nanolayers is related to the sample rotation applied during the co-sputter deposition of the gold-copper alloy nanowires.48,49 Such rotation results in the formation of periodic Au-poor/Au-rich nanolayers. When dealloying such a nanolayered structure, the Au-poor layers are quickly dealloyed while the Au-rich layers resist to the dissolution process for the treatment durations not exceeding 300 s. When reaching 600 s (Figure 3c), the nanolayered structure disappears and a granular/porous morphology is observed. Similar granular and porous morphology was also identified at 0.3 V, for dealloying durations between 150 and 300 s (Figure 3d and 3e). However, when reaching 600 s, the nanoporous structure consisting of interconnected ligaments appears (Figure 3f). Contrary to the previous conditions, for any selected dealloying time, the nanowires treated at 0.4 V (Figure 3g-i) exhibit porous and interconnected ligament morphology. To analyze the evolution of the pore size (Figure 4a) and the decrease in diameter (Figure 4b) of the nanowires as modifying the dealloying parameters (i.e., potential and time), statistical


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study was carried out based on the SEM micrographs recorded on all the samples. The decrease in diameter, δ, of the nanowires is calculated according to equation 1:

δ=

di − d f di

× 100

Where di and df represent the initial (i.e., before dealloying) and the final diameter (i.e., after dealloying) of the nanowires, respectively. According to Figure 4a, for all the used potentials, the pore size increases as increasing the dealloying time. By selecting an appropriate combination between the dealloying time and potential, one can control the pore size within the range of 6-18 nm. The same evolution is observed for the decrease in diameter, which was more pronounced with the increase of the dealloying time (Figure 4b). This reflects the shrinkage in wires diameter as the dealloying process proceeds in time. Such reduction of wires diameter becomes more significant as the dealloying potential increases. For example, when fixing the dealloying time to 300 s and varying the dealloying potential from 0.2 to 0.4 V, the decrease ratio in diameter increases from 20% up to 40%. This diameter reduction is associated with the dissolution of copper occurring during the dealloying process. To validate this assumption, the copper residue remaining in the nanowires after dealloying for different durations was measured using EDS (Figure 4c). As expected, for the three selected potentials, the Cu content within the nanowires decreases as the dealloying process proceeds in time. Such a decrease in Cu content is more significant as the dealloying potential becomes more important in the process. For instance, for 150 s of dealloying, the Cu content at 0.2 V is six times higher than the one at 0.4 V (~55 at. % at 0.2 V and ~8 at. % at 0.4 V). It should be noted here that contrary to the dealloying at 0.2 V where the


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significant variation in Cu content is observed between 150 and 600 s, at 0.4 V there is almost no significant variation in Cu content within this range of dealloying time; this means that the most important fraction of Cu is dissolved in the electrolyte within the first 150 seconds of the process. To electrochemically monitor the transformation of the nanowires from solid to porous, cyclic voltammograms were carried out on the nanowires before and after dealloying. At 0.2 V (Figure 5a), the peak located at 0.1 V is assigned the electrochemical oxidation of Cu0 into Cu2+. A decrease in the global signal intensity becomes more significant as increasing the dealloying time; it reflects the decrease in Cu content within the nanowires which is in good agreement with the EDS results presented and discussed previously. To explore the impact of the dealloying potential, the current density was recorded as a function of the dealloying time at different potentials (Figure 5b). For the three potentials, the curves consist of four parts with different slopes (defined within four different time ranges: 0-0.2 s, 0.2-1 s, 1-50 s and above 50 s); this reflects the presence of different regimes taking place during the dealloying process. One contribution to the presence of these regimes can be the rapid change in surface composition of the nanowires. More precisely, during the early stage of dealloying, the amount of Cu at the surface rapidly decreases resulting in the enrichment of the nanowires surface with gold which hinder the dissolution of copper. The breakdown of this passivation gold layer in a further stage induces a sudden change in current density due to the enhancement in copper dissolution. The curve’s slope varies from a potential to another since the kinetics of the formation and breakdown of this passivation layer is dependent on the leaching potential.50 The evolution of the electrolyte resistance inside the pores during the leaching process, which must impact the


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outward diffusion of the extracted ions from the nanopores, may also contribute to the existence of these different regimes.51 The cyclic voltammograms carried out on nanowires dealloyed for 300 s at different potentials are plotted in Figure 5c. For a dealloying potential of 0.2 V, only one oxidation peak is expected at a potential higher than 0.2 V. For 0.3 V, the voltammogram exhibits two peaks located at 0.1 and 0.3 V. The same peaks can be also identified for 0.4 V. Most probably, the observed peaks at different dealloying potentials represent the oxidation peak of copper; the position of the oxidation peak is expected to be different according to the environment from which the Cu is being extracted. This behavior is also observed for the sample dealloyed at 0.2, 0.3 and 0.4 V for 150 s or 600 s (Figure S1). To validate it, cyclic voltammograms were recorded on two different electrodes deposited by the same sputter deposition process used to grow the nanowires: pure gold thin film and gold thin film covered with a 5 nm thick copper layer (Figure 5d). For the pure gold electrode, no peaks were detected. In case of the pure gold film, no oxidation peaks were detected. For the gold electrode covered with a 5 nm thick Cu layer, in addition to the oxidation peak at 0.1 V reflecting the oxidation peak of Cu from Cu0 to Cu2+,50 another peak located at 0.3 V is also detected. One can conclude that the additional peak observed at 0.3 VSCE originates from the oxidation of Cu atoms surrounded by gold. In such a case gold can stabilize Cu in the alloy causing the increase of the copper’s leaching potential. This result is in good agreement with most of reports in literature.50,52,53 This means that the position of the oxidation peak is highly dependent on the composition of the environment from which the Cu atoms are being dissolved. In case of our nanowires, the environment varies as a function of the dealloying time and potential. After dealloying at 0.2 V, the peak at 0.1 V was not observed even with high amount of Cu residue (~30 at. %) remaining within the nanowires; it is


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expected that in such a case the copper residue remaining in the nanowire is alloyed with gold. For this reason, the oxidation and removal of such copper residue requires higher dealloying voltage (e.g. 0.3 and 0.4 V). When increasing the dealloying voltage up to 0.3 or 0.4 V, the oxidation peak of Cu at 0.1 V becomes visible probably due to the redeposition of the dissolved copper on the surface of the pores occurring when recording the cyclic voltammograms. It is important to note that the peak at 0.1 V is less intense at 0.4 V than at 0.3 V since the amount of Cu present is less important at 0.4 than at 0.3 V (Figure 4c).In addition, the peak at 0.3 V can be also detected indicating the dissolution of copper from a gold-rich alloy. In respect to a leaching voltage of 0.2 V, at a leaching voltage of 0.3 or 0.4 V the copper residue remaining within the gold-rich alloy is expected to become more accessible explaining the reason why the peak at 0.3 V is detected. Moreover, referring to our previous study, our nanowires are expected to be mainly formed of a mixing of various gold-copper alloy phases (i.e. AuCu3, AuCu and Au2Cu3).48 The spatial distribution of the copper residue within the nanowires was investigated using STEM-EDS (Figure 6). The nanolayered structure of the nanowires dealloyed at 0.2 V for 300 s is further confirmed by STEM (Figure S2). The spatial distribution of gold and copper overlaps indicating no obvious phase separation between gold and copper (Figure 6a). The EDS mapping recorded on the nanowires dealloyed at 0.3 V for 300 s shows that, in addition to the presence of Cu in their volume, a small fraction of Cu is also present on the surface of the nanoligaments (Figure 6b). In addition, a clear decrease in the Cu signal can be clearly seen in respect to the wires dealloyed at 0.2 V which confirms the reduction in the amount of the Cu residue when increasing the dealloying potential. A similar case is remarked for the nanowires dealloyed at 0.4


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V for 300 s (Figure 6c), where the copper residue was found to be less important compared to the two previous conditions.

Influence of the nanowires diameter. Although the dealloying process demonstrated in the previous section allows controlling the porosity generated within the nanowires, the mechanical stability of the nanostructures was found to deteriorate drastically when increasing the dealloying time (Figure 7). Although the nanowires dealloyed for 300 s at 0.2 V stay laid on the substrate surface, they become highly fractured by the end of the dealloying process (Figure 7a). More details concerning the formation of the observed cracks are presented in Figure S3. Increasing the dealloying potential to 0.3 or 0.4 V results in the formation of more significant cracks and blisters leading to the delamination of the nanowires from the substrate (Figure 7b and 7c). Similarly to the case of thin films, one may overcome this drawback by modifying the characteristics of the as-grown Au-Cu alloy nanowires.32,54 For this purpose, we investigated the influence of the initial nanowires diameter on the final organization of the nanoobjects over the surface. For this study, the diameter of the as-grown nanowires was tuned between 135 and 250 nm by adjusting the co-sputter deposition time between 60 s and 170 s. For all the experiments, the dealloying time and potential were fixed to 300 s and 0.3 V, respectively. As it can be seen on the plan-view (Figure 8a-c) and the cross-section (Figure 8d-f) SEM micrographs, varying the initial diameter of the as-grown nanowires while fixing the dealloying parameters does not impact the final porosity generated within the nanoobjects. The pore size is around 14 nm, the decrease in diameter is about 30%, and the copper residue determined by EDS after dealloying is ~14 at. %. The thinnest nanowires with an initial diameter of 135 nm show a high fidelity to the grated structures and no delamination can be seen (Figure 8g). For the wires with a diameter of


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200 nm (Figure 8h), a significant delamination can be remarked. For the largest nanowires, with an initial diameter around 250 nm (Figure 8i), the delamination becomes extremely high; when disengaged from the substrate, the porous nanowires tend to form bundles. The cracking and delamination phenomena originate from the volume shrinkage of the material for about 30% due to the dealloying process.55,56 If we suppose that there is a good adhesion of the nanowires on the substrate surface, when their volume shrinks (Figure 4b) they cannot accommodate such volume change which in turns results in a huge in-plane internal tensile stress.57 There are competition between the increase in stress due to the removal of the less noble metal from the material constrained to the substrate and the decrease in stress due to diffusion of the noble ions resulting in the Coble creep deformation of the nanowires.32 Increasing the diameter of the nanowires leads to the increase in the internal residual stress making the delamination of thick nanowires easier than the thinner ones.58,59 Moreover, due to the low diameter of the nanowires compared to their extremely high length, the generated stress is more significant along the wire axis than laterally (i.e., perpendicular to the wire axis); as a consequence, fractures form in the nanowires perpendicularly to the wire axis. Increasing the dealloying potential is expected to increase the internal stress within the nanowires due to the coarsening of the material observed at high potentials;57 in addition to the fracturing process, such an increase in internal stress results in the complete delamination of the nanowires from the surface.

Influence of the gold content. Another parameter that can impact the mechanical strength of the porous nanowires is the initial gold content in the as-grown Au-Cu nanowires. Gold-based alloys with high gold content show an improved mechanical stability after dealloying compared to the ones containing low gold content.32 Thus, another way to overcome the cracking


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and delamination issues discussed in the previous section is by increasing the gold content within the as-grown Au-Cu nanowires. To explore this point, we have synthesized thick Au-Cu nanowires (i.e., 250 nm) with two additional Au contents: 23 and 26 at. %. All the samples were dealloyed for 300 s by applying a dealloying potential of 0.3 V. Compared to the nanowires with 11 at. % (Figure 8c, f and i), the nanowires show an enhanced mechanical stability since no cracks or delamination were observed (Figure 9). When examining the morphology of the nanowires, however, almost no pores were created (Figure 9c-d and e-f). This effect can be related to the passivation of the surface by a gold-rich layer when working at high gold content.60 To generate nanoporosity within these Au-rich nanowires, a higher dealloying potential is necessary.61 For this reason, we used a dealloying potential of 0.5 V to dealloy these two types of nanowires (Figure 10). For 5 min of dealloying at 0.5 V, the nanowires with an initial Au content of 23 at. % become nanoporous (pore size ~24 nm) with a skeleton constituted of interconnected nanoligaments (Figure 10a and 10b); the nanowires show an excellent organization after dealloying and a very limited amount of cracks can be identified (Figure 10c). In case of nanowires with an initial Au content of 26 at. %, a low porosity can be observed with a pore size of about 20 nm (Figure 10d-f). The low porosity observed in this case is related to the surface passivation by Au which is the limiting factor for the development of a three-dimensional open porosity within the material. The evolution of the Cu residue within the wires after dealloying at 0.5 V for 5 min is plotted in Figure 10g as a function of the initial Au content within the nanowires. An increase in the amount of the Cu residue is observed when increasing the initial Au content within the wires. This is related to the surface passivation by Au which becomes more significant as the Au content within the as-grown nanowires increases. This result is consistent with most of reports in literature dealing with dealloying of binary alloys.61,62


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CONCLUSION In this report, we have demonstrated a two-step approach for the fabrication of highly ordered and ultra-long porous gold nanowires with a macroscopic length and a tunable pore size and diameter. This approach consists in the electrochemical dealloying of Au-Cu alloy nanowires grown by magnetron co-sputtering over a nanopatterned silicon template substrate. We have shown that by tuning the dealloying potential between 0.2 and 0.4 V as well as the dealloying time between 150 s and 600 s, the pore size can be adjusted within the range of 6-21 nm. The evolution in pore size was accompanied by a diameter reduction ranging between 22% and 43%. Meanwhile, the dealloying process was found to deteriorate the organization of the nanowires over the substrate. We have proposed and demonstrated two solutions allowing overcoming this drawback and synthesizing porous nanowires with an excellent organization and very low structural damage, including: (i) reducing the initial diameter of the Au-Cu alloy nanowires and (ii) increasing the gold content within the as-grown nanowires. METHODS Nanopatterned substrates. The nanopatterned silicon substrates used to grow the Au-Cu nanowire arrays were prepared by laser interference lithography followed by deep reactive ion etching.63 Such substrates consist of silicon nanograted structures (120 nm in width and 1000 nm in height) of 220 nm in pitch and an aspect ratio of ~6. The formation of nanowires on the top of the nanograted structures is related to the shadowing effect taking place during the deposition of the material. However, an extremely low amount of material, that cannot be detected with SEM, is expected to deposit on the walls of the nanogratings and at the bottom of the nanotrenches.46


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Growth of Au-Cu alloy nanowires. The Au-Cu nanowires (Figure 1b), used to prepare the porous Au nanowires, were synthesized by DC co-sputtering in pure argon plasma of an Au (diameter: 76.2 mm; purity: 99.99%) and a Cu (diameter: 76.2 mm; purity: 99.99%) targets in a co-focal geometry. The distance between the targets and the substrate was 130 mm. To ensure a homogenous deposition over a large surface area, the samples were rotated at a speed of 5 turns/min. The deposition of the Au-Cu nanowire was then carried out at a pressure of 0.5 Pa without applying any intentional heating to the substrate. For all the depositions, the base pressure was less than 4·10-5 Pa. In order to control the composition of the nanowires, the electrical power applied to the Cu target was fixed to 300 W whereas the one applied to the Au target was tuned. Three electrical powers applied to the Au target were selected (30, 50 and 100 W) allowing growing nanowires with 11, 23 and 26 at. % of Au, respectively. Moreover, three depositions times were selected to tune the initial diameter of the as-grown nanowires (60 s, 140 s and 170 s) allowing growing wires with 135, 200 and 250 nm +/- 3 nm, respectively (Figure S4).The diameter of the nanowires in this case corresponds to the diagonal dimension of the wire’s section. The pore size and diameters of nanowires were determined from the SEM images using an image processing software (visiometer).

Electrochemical dealloying process. The porous gold nanowires (Figure 1c) were prepared by electrochemical dealloying of Au-Cu alloy nanowires. These experiments consist in the selective dissolution of Cu by an anodic process. All experiments were performed with a Biologic SP-300 potentiostat. Platinum wire and saturated calomel electrode (SCE) served as the


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counter and the reference electrode, respectively. All working electrode potentials are provided with respect to the SCE reference electrode. Electrochemical treatments were performed in diluted H2SO4 at 0.1 M (pH less than 1) as supporting electrolyte. This condition is selected according to Pourbaix diagram of copper.64 The contact to the working electrode (i.e., Au-Cu nanowire arrays) was made using a crocodile clips at the tip of a sample allowing injecting the current along the nanowires axis. The treated geometrical surface was typically around 0.5 cm2 (the samples were 1 cm in length and 0.5 cm in width). An analysis by cyclic voltammetry (between -0.2 V and the dealloying potential at 50 mV.s-1) was carried out before and after the dealloying process (Figure 1d) to check the current injection in the nanowires as well as the electrochemical properties of the samples. It should be noted that the potentiodynamic polarization curves recorded on Au-Cu nanowires with different Au content were found to be very similar (data not presented here). The dealloying was proceed by applying an anodic potential (typically from +0.2 V to 0.5 V) during a determined time (from 150 s to 600 s) (Figure 1e). During the dealloying process, a magnetic stirring (650 rpm) was applied to ensure a better diffusion of the Cu2+ leached during the process. By the end of each treatment, the samples were removed from the electrolytic solution and dipped in distilled water and then rinsed with methanol.

Characterization. The SEM micrographs were recorded using a JEOL JSM 7600 F microscope operating at 5 kV. The chemical compositions of the films were determined by energy dispersive X-ray spectroscopy (EDS) integrated in a JEOL 5600 microscope operating at 5 kV. Scanning transmission electron microscopy (STEM) analyses, i.e. high angle annular dark-


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field (HAADF) imaging, and energy-dispersive X-ray spectroscopy (EDS) were carried out using a Jeol JEM 2100F field emission transmission electron microscope operated at 200 kV and equipped with an Oxford X-max80 EDS detector. The EDS maps were acquired in such a way that the number of counts was maximized while keeping the detector dead time below 30%. The map size was 256×256 pixels. The acquisition time was 1000-2000 s for the EDS maps.


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ASSOCIATED CONTENT Supporting Information. Cyclic voltammograms recorded at different voltages and time; STEM micrograph of dealloyed sample at different voltages; formation of cracks in the nanowires dealloyed during 300 s at 0.4 V; morphology of the as-grown Au/Cu nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected], Tel: +33 240376325, Fax: +33 240373995 Notes The authors declare no competing financial interest.

Present Addresses 1 Current address: Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States

ACKNOWLEDGEMENTS The authors gratefully acknowledge F. Petitgas (IMN, Nantes) and N. Stephant (Nantes University) for their technical assistance on the co-sputtering system and on the SEM, respectively. The JEOL JEM-2100F transmission electron microscope employed for this work was partially funded by the German Research Foundation (DFG/INST163/2951). M. D.


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acknowledges financial support from the LOEWE research cluster RESPONSE (Hessen, Germany). D. Thiry would like to thank “la Région des Pays de la Loire-France” for financially assisting this research project through the “Post-Doctorats internationaux” program. ABBREVIATIONS SEM, Scanning Electron Microscopy; EDS, Energy dispersive X-ray microscopy; SCE, Standard Calomel Electrode; STEM, scanning transmission electron microscopy; HAADF, high-angle annular dark-field imaging. REFERENCES (1)

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Figure 1. (a) Scheme of the two-step approach developed in this work to prepare highly ordered porous gold nanowire arrays consisting in the deposition of Au-Cu alloy nanowires on a nanograted template substrate followed by an electrochemical dealloying in diluted sulfuric acid (0.1 M); RE, WE and CE represent the reference, the working and the counter electrode, respectively. Typical plan-view SEM micrograph showing the morphology of Au-Cu nanowires with an initial gold content of 23 at. % (b) before (as-grown) and (c) after dealloying for 300 s at 0.5 V; scale bar: 100 nm. Typical (d) cyclic voltammogram recorded at 0.5 V and (e) chronoamperometry curve recorded at 0.2 V.


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Figure 2. Plan-view SEM micrographs showing the morphological evolution of an Au-Cu alloy nanowire as a function of the dealloying time and potential: (a-c) 0.2 V, (d-f) 0.3 V, and (g-i) 0.4 V. The initial gold content within the as-grown nanowires was fixed to 11 at. % and the diameter to 200 nm. Scale bar: 100 nm.


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Figure 3. Cross-sectional SEM micrographs showing the morphological evolution of Au-Cu nanowires with an initial Au content of 11 at. % and a diameter of 200 nm as a function of the dealloying time and potential. Different dealloying potentials were explored: (a-c) 0.2 V, (d-f) 0.3 V, and (g-i) 0.4 V. Scale bar: 100 nm.


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Figure 4. Evolution of (a) the pore size, (b) the decrease in nanowire’s diameter δ, and (c) the copper content plotted for different potentials as a function of the dealloying time: (open red squares) 0.2 V, (open blue triangles) 0.3 V and (open green diamonds) 0.4 V. The pore size and the decrease in nanowire’s diameter were determined from the SEM micrographs recorded on the samples, whereas the Cu content was evaluated using EDS analysis.


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Figure 5. (a) Cyclic voltammograms recorded on nanowire arrays before (in green) and after dealloying in diluted H2SO4 (0.1 M) at 0.2 V for different durations (black: 150 s, red: 300 s and blue: 600 s); due to the very low signal, the cyclic voltammograms recorded on the dealloyed samples where multiplied by five to be visible on the graph. (b) Chronoamperometry curves recorded at different potentials during 300 s. (c) Cyclic voltammograms recorded at different potentials after dealloying for 300 s Au-Cu nanowires with an initial Au content of 11 at. % and a diameter of 200 nm. (d) Voltammograms recorded on two thin film electrodes: 5 nm thick layer of copper stacked to a 200 nm of gold thin film (red) and a pure gold film of 200 nm in diameter (blue) deposited on silicon substrates.


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Figure 6. HAADF-STEM micrograph and the corresponding EDS mapping of Au/Cu nanowire with 11 at. % of gold after dealloying for 300 s at: (a) 0.2 V, (b) 0.3 V, and (c) 0.4 V. The arrangement of images is from the left to the right: HAADF, Au-L EDS map corresponding to 9.616-9.811 keV (∆E=0,196 keV), Cu-K EDS map corresponding to 7.958-8.138 keV (∆E =0,180 keV), and RGB composite EDS map. For the RGB composite EDS map, Au-L was used for the red and Cu-K for the green channel, respectively.


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Figure 7. Plan-view SEM images showing the organization of the nanowires over the substrate surface after dealloying for 300 s at: (a) 0.2 V, (b) 0.3 V and (c) 0.4 V. The pristine Au-Cu alloy nanowires contain 11 at. % of Au with an initial diameter of 200 nm. Scale bar: 1 µm.


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Figure 8. SEM micrographs showing the impact of the initial diameter of the nanowires on their final organization after dealloying at 0.3 V for 300 s. Nanowires with three different initial diameters were considered: (a,d,g) 135 nm, (b,e,h) 200 nm, and (c,f,i) 250 nm. The nanowires composition was fixed to 11 at. % of gold. All the samples were dealloyed at 0.3 V for 300 s. White scale bar: 100 nm; black scale bar: 1 µm.


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Figure 9. (a-d) Plan-view and (e and f) cross-sectional SEM micrographs showing the impact of the gold content on the organization and morphology of nanowires after dealloying at 0.3 V for 300 s. Two Au contents were explored: (a, c and e) 23 at. % and (b, d and f) 26 at. %. Black scale bar: 1 µm; white scale bar: 100 nm.


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Figure 10. (a,c,d,f) Plan-view and (b and e) cross-sectional SEM micrographs showing the morphology of nanowires dealloyed at 0.5 V for 300 s; nanowires with two different initial Au contents were studied: (a-c) 23 at. % and (c-f) 26 at. %. (g) Evolution of the copper residue within the porous nanowires as function of the initial gold content at different potentials: 0.3 V (open blue triangles) and 0.5 V (open black squares). White scale bar: 100nm; black scale bar: 1 µm.


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