Galvanic Replacement Reaction: A Route to Highly Ordered Bimetallic

Article pubs.acs.org/JPCC

Galvanic Replacement Reaction: A Route to Highly Ordered Bimetallic Nanotubes Abdel-Aziz El Mel,*,† Meriem Chettab,† Eric Gautron,† Adrien Chauvin,† Bernard Humbert,† Jean-Yves Mevellec,† Cyril Delacote,‡ Damien Thiry,† Nicolas Stephant,† Junjun Ding,§ Ke Du,§ Chang-Hwan Choi,§ and Pierre-Yves Tessier† †

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 § Department of Mechanical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States S Supporting Information *

ABSTRACT: Hollow bimetallic nanostructures are of great importance for various applications. Understanding the fundamental mechanisms occurring during the synthesis of such nanomaterials by wet chemistry remains very challenging. This Article reports a mechanistic study on the galvanic replacement reaction between planar arrays of silver nanowires grown site-specifically on tall silicon nanogratings and HAuCl4 in lack of any stabilizing or capping agent, which might complicate and alter the conversion process of silver nanowires into silver−gold nanotubes. The direct contact of the silver nanowires with the substrate is found to modify the reaction as compared to nanowires in suspension. We show that when using diluted HAuCl4, AgCl nanoclusters precipitate on the surface during the process resulting in an increased surface roughness of the nanotubes. Overcoming this drawback requires saturating the HAuCl4 solution with NaCl; this allows distributing the anodic and cathodic sites over the surface of the nano-objects in a homogeneous manner, allowing in turn obtaining nanotubes with a smooth surface. For both protocols (i.e., pure HAuCl4 or HAuCl4 saturated with NaCl), the conversion rate was found to increase with the concentration of HAuCl4 in the solution. We further show that the kinetic of the reaction and the surface roughness of the nanotubes become more important when raising the temperature from 0 to 100 °C. Furthermore, we show that by using the proposed approach, one can synthesize double-walled bimetallic nanotubes.

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INTRODUCTION Nanoengineering of metals is an emerging field receiving impressive attention from many research communities.1−6 It covers the growth and the assembly of nanostructured metals and their integration in a broad range of applications including SERS-based sensing,7 catalysis,8 and biotechnology.9 Most of the approaches developed so far to synthesize metal nanotubes involve one or more of the following processes: template approaches,10,11 galvanic replacement,9,12−15 Oswald ripening,16 anion exchange,14 Kirkendall effect,14,17,18 and dealloying.19 Among these processes, galvanic replacement is an appealing and versatile route to fabricate hollow and porous metal nanostructures.12,20−22 Briefly, this process involves (i) the oxidation of sacrificial template metal nanostructures in an electrolyte containing ions of another metal exhibiting a higher reduction potential and (ii) the plating of this last on the surface of the template metal. According to the concentration of the metal ions in the solution, the treatment time, and the temperature, the nanostructures can be transformed from solid into hollow or nanoporous. © XXXX American Chemical Society

Galvanic replacement has been applied to template nanostructures of different materials (Ag,9,15,23 Cu,24 and metal oxides25) with various shapes (e.g., nanowires,2,9,26−28 nanospheres,2,21,29,30 nanoprisms,23 and nanocubes28,31−38). Although a broad range of hollow and porous nanostructures were successfully grown by galvanic replacements, the synthesis approaches were in most of the cases limited by the chemical conversion of nanostructures dispersed in a liquid phase (i.e., nanostructures in suspension) containing stabilizing agents.12,13 However, in addition to the fact that the performance and the properties of the material were reported to deteriorate,39 the presence of additives in the solution such as poly(vinylpyrrolidone) (PVP), used in general as a capping agent to avoid the agglomeration of the nanostructure templates in suspension, may complicate and alter the transformation process, giving rise to nanostructures with undesired Received: June 24, 2016 Revised: July 14, 2016

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DOI: 10.1021/acs.jpcc.6b06393 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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morphologies.32 Using metal templates attached to a substrate surface may allow overcoming the above-mentioned drawbacks because no additives are required to ensure the dispersion of the nanostructures. From a fundamental point of view, this would be extremely interesting because the mechanisms of the chemical reaction can be studied without any disruption originating from the presence of additives and binders in the solution. In terms of application, creating organized hollow nanostructures attached to a solid substrate is of great interest because it simplifies their manipulation so they can be directly integrated and used in practical devices such as SERS-based sensors. Moreover, the organization of such nanostructures over the substrate surface in a periodic manner may give rise to some unexpected enhancement in the behavior of the material that does not exist when using a bundle or an isolated metal nanostructure. Very recently, much attention was shown for the conversion chemistry of template nanostructures organized on a solid substrate.40−44 However, the reported studies were limited to the comprehension of the conversion process of zerodimensional nanostructures. Here, we apply for the first time galvanic replacement in gold(III) chloride (HAuCl4) to an array of highly ordered silver nanowires grown over a nanopatterned silicon substrate (Figure 1a). Two protocols

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MATERIALS AND METHODS Fabrication of Nanopatterned Template Substrate of Silicon. The nanopatterned silicon substrates were prepared by a two-step approach consisting of the creation of a photoresist mask pattern over a silicon wafer using laser interference lithography followed by deep reactive ion etching. This allows creating periodic nanogratings of silicon (width of about 120 nm, a height of 1 μm, and a pitch of 240 nm) on a 4-in. wafer scale. Growth Conditions of Silver Nanowires. The Ag nanowires were grown on a nondoped nanograted silicon substrate by DC magnetron sputtering of a silver target (diameter, 76.2 mm; purity, 99.99%) using a pure argon plasma. Because no HF cleaning was applied to the silicon substrate prior to the nanowires deposition, a native oxide layer was present on the top surface of the substrate. The target-tosubstrate distance was 130 mm. The electrical power applied to the target was 200 W; the deposition time was 120 s. The substrates were rotated during the growth at a speed of 20 rpm. The working pressure was fixed at 0.5 Pa. For all depositions, the base pressure was less than 4 × 10−5 Pa. Galvanic Replacement Process. The galvanic replacement was applied using HAuCl4 ordered from Sigma-Aldrich. The sample was first placed at the bottom of a beaker filled with the HAuCl4 solution. The size of the sample is 1 × 1 cm2. Considering (i) nanowires with a cylindrical shape with a diameter of 150 nm and a length of 1 cm, (ii) a substrate with a size of 1 × 1 cm2, and (iii) a patterns pitch of 240 nm, the number and the global volume of the Ag nanowires deposited over the surface can be determined. Using the density of bulk silver (10.5 g cm−3), the global mass of the deposited silver involved in the reaction can be evaluated to ∼0.074 mg. The nanowires were located on the top-side of the substrate facing the solution. According to the used protocol, the HAuCl4 solution can be either saturated or not with NaCl. Contrary to the reports in the literature, we have intentionally selected an extremely high volume of HAuCl4 solution (15 mL) so the conversion process of the nanowires will not be limited by the amount of gold in the used volume of solution. After the desired treatment time was reached, to stop the galvanic replacement reaction, the sample is dipped for 60 min in 100 mL of distilled water saturated with NaCl. The NaCl is used to remove the AgCl precipitates forming over the surface during the galvanic replacement reaction. After the reaction was stopped and any undesirable AgCl precipitates were removed from the surface, the sample is then immersed for 10 min in 300 mL of distilled water to remove the residues of NaCl from the surface of the sample. This last step is subsequently repeated in three different beakers filled with distilled water. Material Characterization. The SEM micrographs of the samples were recorded on a JEOL JSM 7600F microscope operating at 5 kV. For the cross-sectional observations, the samples were cleaved using a diamond tip to open the nanotubes. The chemical composition of the nanoobjects was evaluated using energy dispersive X-ray spectroscopy (EDS) performed with a SAMx SDD system mounted on a JEOL JSM 5800LV scanning microscope operating at 5 kV. Transmission electron microscopy (TEM) imaging and selected-area electron diffraction were carried out on a Hitachi H9000NAR microscope (LaB6 filament, 300 kV, point-to-point resolution: 0.18 nm).

Figure 1. (a) Scheme of the nanofabrication approach developed in this work to prepare an ordered array of Au−Ag nanotubes in a planar configuration. (1) Growth of Ag nanowires over nanograted silicon structures and (2) transformation of the solid Ag nanowires into Au− Ag nanotubes by means of galvanic replacement using HAuCl4. (b) Cross-section and (c) plane-view scanning electron microscope images (SEM) showing the Ag nanowire arrays as-grown over the nanopatterned silicon substrate. (d) TEM micrograph of an Ag nanowire and (e) the associated selected-area electron diffraction pattern indexed according to the crystallographic database of facecentered cubic silver (JCPDS no. 004-0783).

are explored allowing synthesizing a broad panel of gold−silver hollow nanostructures. In the first protocol, the galvanic replacement is applied in a diluted HAuCl4 solution, whereas in the second one it is carried out in a diluted HAuCl4 solution saturated with NaCl. For both protocols, we investigate the impact of the concentration of HAuCl4 and the treatment time on the final morphology and structure of the nanotubes. Moreover, we also investigate the influence of the temperature on the conversion rate of the reaction. On the basis of the collected data, a scenario is proposed to explain the fundamental mechanisms occurring at the atomic scale during the conversion process of the nanoobjects from solid to hollow. B


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RESULTS AND DISCUSSION Growth of the Ag Nanowires Template. Figure 1b and c shows SEM micrographs of the Ag nanowire arrays used as template for the growth of Au−Ag nanotubes. The nanowires are ∼150 nm in diameter and periodically separated by a gap of about 70 nm; the length of the nanowires is defined by the length of the nanograting structures, which may reach several centimeters depending on the size of the substrate.45 In this study, the length of the nanowires was fixed to 1 cm. The TEM micrograph (Figure 1d) and the associated selected area electron diffraction (SAED) pattern (Figure 1e) reveal the granular morphology and the polycrystalline structure of the nanowires. In addition, the presence of continuous rings in the SAED pattern indicates that the Ag crystallites forming the nanowire do not exhibit a preferential crystallographic orientation. Conversion in HAuCl4. In the first protocol, the galvanic replacement process was applied using a diluted 0.1 mM HAuCl4 solution at room temperature. This concentration was selected after a careful examination of the studies reporting on galvanic replacement in the literature.12 After 5 min of conversion, one can clearly remark a change in the morphology of the nanowires, and large colloids are formed on the surface of the sample (Figure 2a). In addition, the surface of the

found to be very similar to the ones observed in case of 5 min of treatment, but the pinholes are no longer visible. As demonstrated by X-ray photoelectron spectroscopy, the formed cubes are the products of AgCl precipitation (Figure S1). The AgCl precipitation can be easily removed by rinsing the samples in distilled water saturated with NaCl (Figure S2). Although 15 min of treatment allows reaching a more significant hollowing level, the nanowires are still not fully empty (Figure 2f). When reaching 60 min, the cubes observed previously on the surface of the sample grow larger (Figure 2g), while the morphology and roughness of the nanowires remain similar (Figure 2h). However, contrary to the two previous conditions, when the nanowires are treated for 60 min, they transform into fully hollow nanotubes (Figure 2i). It should be pointed out that the shell of the formed nanotubes is discontinuous at the region in direct contact with the nanograting silicon structures (Figure S3). This leads to the conclusion that all of the silver at the nanowire/substrate interface was consumed during the reaction and a very low amount of gold was plated at this region. The mechanisms behind the formation of such “hemi-shells” will be discussed more in detail later. The formation of such hollow nanostructures is further confirmed by TEM (Figure S4). The nanoobjects were found to become larger in diameter when increasing the treatment time, resulting in the decrease in the gap separating the nanoobjects (Figure S5). Increasing the concentration of HAuCl4 to 1 mM results in an increase in both the amount of AgCl precipitation as well as the surface roughness of the nanoobjects (Figure S6). In addition, the transformation kinetic of the conversion process becomes faster because the nanoobjects were found to agglomerate during the process. Furthermore, the nanowires treated in 1 mM HAuCl4 showed poor adhesion to the substrate, and in some cases they delaminated completely when rinsed with distilled water. Because the delamination issue is a real hurdle for the preparation of the specimens for cross-section SEM observation, only top-view SEM micrographs were recorded on the partially delaminated samples (Figure S6). Impact of NaCl on the Conversion Process. In the second protocol, we used HAuCl4 diluted in distilled water saturated with NaCl. The presence of NaCl has the advantage to avoid the precipitation of AgCl on the surface.46 Indeed, adding NaCl to the solution allows solubilizing the AgCl byproduct by the formation of AgCl2− soluble in aqueous solution. First, we explored the conversion in 0.1 mM concentrated HAuCl4 saturated with NaCl. When examining the surface of the nanostructures after conversion, it can be clearly seen that no AgCl precipitates on the surface of the sample, whereas it did for the case of the samples prepared without adding NaCl to the solution (Figure 3a−c). In addition, the surface of the nanoobjects was found to be less rough as compared to the protocol described previously (i.e., treatment without NaCl) (Figure 3d−f). For 5 min of treatment, the nanoobjects were not found to be fully empty (Figure 3g). Increasing the conversion time to 15 min triggers the carving process of the nanoobjects (Figure 3h). When reaching 60 min, the nanoobjects become fully hollow (Figure 3i). Similar to the previous protocol, increasing the conversion time was found to result in an increase in the global diameter of the nanoobjects (Figure S5). However, such increase stays less remarkable in case of the presence of NaCl in the solution. It should be noted that for all of the conversion times, pinholes were found to form within the shell of the nanotubes. Such pinholes serve as extraction sites of Ag+ during the carving

Figure 2. Plane-view (a,b,d,e,g,h) and cross-section (c,f,i) SEM micrographs showing the impact of the treatment time in 0.1 mM HAuCl4 on the morphology of the Ag nanowires. Different exposure durations were investigated: (a−c) 5 min, (d−f) 15 min, and (g−i) 60 min. Black scale bar, 500 nm; white scale bar, 100 nm.

nanowires becomes extremely rough (Figure 2b) as compared to the one before treatment (Figure 1c), and they are constituted of spherical nanodomes; in addition, some pinholes can be seen at the surface of the nanoobjects. From the crosssection view of these nanostructures, one can conclude that they were partially carved and they did not fully transform into nanotubes (Figure 2c). In addition, one can remark the presence of a very small amount of residues on the top region of the side-walls of the nanogratings. This is probably the product of the reaction of the extremely low amount of silver deposited on the nanogratings side-walls and HAuCl4 taking place during the conversion process. Increasing the treatment duration to 15 min was found to result in the formation of large cubes over the surface (Figure 2d). On the other hand, the morphology and roughness of the nanowires (Figure 2e) are C


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kinetic of the galvanic replacement process with the increase in the concentration of the solution. For longer conversion duration of 15 min, no precipitation forms on the surface either (Figure 4d), and the objects surface stays smooth (Figure 4e). In such a condition, the nanoobjects become fully hollow (Figure 4f). Further increasing the conversion time to 30 min results in the formation of nanoporous nanowires instead of nanotubes (Figures 4g−i). The generation of porosity in the nanotube shells is a consequence of the dealloying process occurring during the treatment in the solution.47 During this stage, the Au−Ag shell undergoes a selective dissolution process of silver in the solution; simultaneously, a surface diffusion of Au takes place at the alloy/electrolyte interface. As a consequence of these two events, by the end of the process, the Au−Ag alloy transforms into a porous skeleton of Au constituted of interconnected nanoligaments.48,49 Impact of Conversion Temperature. The conversion temperature is an important parameter that can impact the galvanic replacement process. For this purpose, we explored the transformation of the nanoobjects in 0.1 mM of HAuCl4 at different temperatures: 0, 25, 60, and 100 °C (Figure 5). All of

Figure 3. Plane-view (a−f) and cross-section (g−i) SEM micrographs showing the impact of the exposure time in 0.1 mM HAuCl4 saturated with NaCl on the morphology of the Ag nanowires. Different exposure durations were investigated: (a,d,g) 5 min, (b,e,h) 15 min, and (c,f,i) 60 min. Black scale bar, 500 nm; white scale bar, 100 nm.

process. It is difficult to evaluate the length-scale over which these portals form as they do not exhibit the same size and are randomly distributed over the external surface of the nanotubes. To explore the impact of the concentration, the same experiments were repeated in 1 mM HAuCl4 saturated with NaCl (Figure 4). For 5 min of conversion, no AgCl

Figure 5. SEM micrographs showing the morphological evolution of the nanowires treated in 0.1 mM of HAuCl4 at several temperatures for different durations. Scale bar: 200 nm.

the samples were rinsed after treatment in distilled water saturated with NaCl to remove the AgCl precipitates at the surface of the nanoobjects. The results at 25 °C, as discussed earlier, show that the nanotubes have a rough surface and are constituted of nanodomes. Increasing the treatment time was found to induce an increase in the diameter of the tubes and a decrease in the gap separating the nanoobjects. When reducing the conversion temperature to 0 °C, one can remark that the surface of the nanoobjects is less rough; in addition, the increase in diameter (and the decrease in gap between the nanoobjects) is less significant as compared to the sample treated at 25 °C. This indicates a reduction in the kinetic of the conversion process. Raising the temperature to 60 °C was found to accelerate the conversion process. One can clearly see that even for a short treatment time (e.g., 5 min), the roughness of the nanoobjects becomes more pronounced as compared to the conversion at lower temperatures (i.e., 25 and 0 °C). Moreover, similar to the two previous conditions, the roughness becomes more phenomenal when extending the conversion time. Large nanospheres were found to form on the surface of the objects after 60 min of conversion at 60 °C; in addition, the walls of the nanoobjects start transforming from hollow to nanoporous due to the dealloying process taking over after 60 min of conversion at 60 °C. This effect was not observed for the two other temperatures, which supports the

Figure 4. Plane-view (a,b,d,e,g,h) and cross-section (c,f,i) SEM micrographs showing the impact of the exposure time in 1 mM HAuCl4 saturated with NaCl on the morphology of the Ag nanowires. Different exposure durations were investigated: (a−c) 5 min, (d−f) 15 min, and (g−i) 30 min. Black scale bar, 500 nm; white scale bar, 100 nm.

precipitation can be seen on the surface of the sample (Figure 4a), and the nanowires surface is smooth (Figure 4b). As it can be seen on the cross-section SEM micrograph, the nanowires become almost hollow after 5 min of conversion (Figure 4c); in addition, the nanoobjects diameter reaches higher values as compared to the case of using 0.1 mM of HAuCl4 saturated with NaCl (Figure S5). This reflects an acceleration of the D


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the solution. It is important to mention here that the amount of gold detected for the samples treated for 60 min, 10 at. %, is relatively low as compared to the hollowing degree observed for such nanoobjects (Figures 2i and 3i). There are several factors that can probably lead to such a result: (i) The dissolution of silver and the plating of gold take place according to the following reaction:

supposition that the conversion process becomes faster with raising the conversion temperature. When reaching 100 °C, for 5 min of conversion, the silver nanowires transform into chains of highly faceted hollow nanospheres with large pinholes (Figure S7). Increasing the conversion time to 15 min induces an increase in size of the formed nanospheres. To check if these nanospheres are hollow or solid, we scratched the surface of the sample with a diamond tip to delaminate and flip some of these nanochains on their top side. Figure S8 shows clearly that the formed nanospheres are fully hollow. At such a temperature, for conversion durations exceeding 15 min, the nanoobjects were found to completely delaminate in the solution and thus could not be imaged by SEM. Reaction Rate and Conversion Mechanisms. To explore the impact of the used solution on the reaction rate, EDS was employed to evaluate the amount of Au deposited by the end of the reaction (Figure 6a). It can be seen that for the

3Ag + AuCl−4 → 3Ag + + 4Cl− + Au

This means that the plating of one atom of gold requires dissolving 3 atoms of silver. For this reason, there will be 3 times less plated gold than the dissolved silver. (ii) A fraction of gold plates on the AgCl precipitates during the conversion process. When the samples were washed with the saturated solution of NaCl, these AgCl precipitates together with the gold plated on them are removed from the sample, resulting in a non negligible drop in the amount of gold that it is expected to plate on the nanoobjects. For 1 mM of HAuCl4, the Au content rises from ∼20 to ∼100 at. % when increasing the conversion time from 5 to 15 min. Adding the NaCl to the 1 mM HAuCl4 solution leads to a reduction in the kinetic of the conversion process; in such a condition, a duration of 30 min is required to reach 95 at. % of Au within the nanoobjects. The gold concentration was not only dependent on the conversion time, but also on the conversion temperature (Figure 6b). When plotting the gold content measured for different conversion durations as a function of the conversion temperature, one can conclude that for all of the selected conversion durations, the gold content is found to increase with the conversion temperature. Moreover, such evolution becomes more significant for long conversion durations as compared to short ones. For example, for 5 min of conversion, even at 100 °C, one cannot reach the saturation with gold; on the other hand, for 15 and 60 min of conversion, the saturation regime can be reached at 100 and 60 °C, respectively. According to our experimental data, there exist two different pathways that can be followed during the conversion process. The stages of these conversion pathways are summarized in Figure 6c. The first pathway (Figure 6c, top) describes the transformation in a solution of HAuCl4 without NaCl, while the second one (Figure 6c, bottom) represents the transformation in a solution of HAuCl4 saturated with NaCl. During both cases, the fundamental mechanisms of the galvanic replacement reaction describing the dissolution and plating are the same as that detailed in the literature with the exception that in our case one must take into account the presence of the substrate and the polycrystalline structure of the nanowires.12 In the case of the first mechanism (i.e., pure HAuCl4), the carving process of the nanoobjects occurs mainly through the base of the nanowires (i.e., nanowire/substrate interface); this results in the formation of asymmetric shells, that is, a very thick wall at the upper side of the tube, and it becomes thinner while approaching to the contact region with the silicon substrate. One can conclude that the Ag dissolution sites are mainly located at the regions close to the wire/silicon interface, while the Au reduction sites are located on the upper surface of the nanoobjects. In addition to the asymmetric shape, the nanoobjects in such conditions exhibit a very high roughness. There are several factors for the observed roughness. The first factor can be the solid-state diffusion of gold within the Au−Ag alloy forming during the reaction. As solid-state diffusion is a temperature-dependent process, it is expected that the diffusion

Figure 6. (a) Evolution of the Au concentration within the nanoobjects as a function of the conversion time in four different solutions: (■) 0.1 mM HAuCl4 saturated with NaCl, (green ▲) 0.1 mM HAuCl4, (red ●) 1 mM HAuCl4 saturated with NaCl, and (purple ◆) 1 mM HAuCl4. (b) Evolution of the gold content in the nanoobjects as a function of the conversion temperature; the treatment was carried out in a 0.1 mM HAuCl4 solution for three different durations: 5 min (□), 15 min (red ○), and 60 min (blue △). (c) Scheme showing the different stages of the conversion process from solid Ag nanowires into Ag−Au nanotubes in case of using HAuCl4 (top) and NaCl saturated HAuCl4 solution (bottom), respectively.

solution with 0.1 mM of HAuCl4, the Au content increases very slowly from 1 to 10 at. %. Similar evolution is observed in case of the solution with 0.1 mM HAuCl4 saturated with NaCl; in such a case, however, the Au concentrations are slightly lower in comparison to the solution without NaCl. This indicates a slight decrease in the conversion kinetic when adding NaCl to E


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The Journal of Physical Chemistry C of Au in the alloy is quite low at low temperature as compared to the case of conversion at high temperatures. As a consequence of such significant solid-state diffusion at high temperatures, the gold atoms are driven to form separated clusters during the conversion process instead of a continuous layer; these clusters form nanodomes by the end of the conversion process. Such “clustering” process occurring as a consequence of solid-state diffusion becomes more significant with increasing temperature during the reaction. For this reason, most probably, when reaching 100 °C (see previous section), we obtain by the end of the conversion process separated nanospheres instead of a continuous nanotube. Another factor for the roughening effect can be the precipitation of AgCl on the surface of the object during the reaction.28,50 In fact, during the reaction, AgCl precipitates on the surface in a form of tiny clusters. The formed AgCl nanoclusters may evolve in two different ways. In the first case, they continue to grow until forming large AgCl microparticles that can be removed by rinsing the samples in a saturated solution of NaCl (Figure S2). In the other case, some of the tiny clusters of AgCl can be covered by a thin Au layer and their growth stops. It is the coverage of the small AgCl nanoclusters by the plated gold that leads to the high surface roughness of the nanoobjects (Figure S9). In such a case, the AgCl grains play the role of a template on which the gold is plated. A similar effect was observed by Zhang et al. while studying the galvanic replacement reaction between silver nanocubes and K2PtCl4.32 When rinsed in NaCl, however, the AgCl dissolves and results in the formation of fully hollow nanostructures (Figure S8). The presence of the AgCl grains may also disrupt the distribution of the cathodic and anodic sites on the surface, which promotes the deposition of Au in a nonhomogeneous way. Some of the anodic dissolution sites of Ag (the holes formed at the early stage of the reaction) may also be obstructed due to their filling with the AgCl precipitation. As a consequence, the dissolution of Ag is promoted at the anodic sites present close to the interfacial region with silicon where the adhesion of the AgCl precipitation is expected to be less important. In the second mechanism, the roughness is reduced due to the saturation of the solution with NaCl that prevents the precipitation of the AgCl crystallites on the surface (Figure 6c, bottom). In the presence of a large amount of NaCl, the oxidized Ag produced during the reaction are rapidly complexed with the Cl− resulting in the formation of AgCl2− instead of AgCl nanocrystallites. In such a case, the anodic dissolution sites of Ag and the cathodic plating sites of Au are distributed homogeneously over the surface of the nanowires; thanks to this uniform sites distribution, the cavities form homogeneously within the nanoobjects, resulting in the formation of nanotubes with isotropic shells and low roughness. Double-Walled Nanotubes. With the knowledge acquired in the fabrication of single-walled nanotubes presented above, we have developed an approach, described in Figure 7a, to synthesize in a controlled way an array of Au−Ag double-walled nanotubes (Figure 7b). As compared to single-walled nanotubes, the fabrication of double-walled nanotubes requires additional deposition and treatment steps. For the synthesis of the first shell, Ag nanowires are deposited by magnetron sputtering for 40 s (Figure 7a, step 1). After their growth, the Ag nanowires are treated for 5 min in a solution of HAuCl4 with 1 mM of concentration saturated with NaCl (Figure 7a, step 2). After their fabrication, the single-walled nanotubes are then covered with a silver layer deposited by magnetron

Figure 7. (a) Scheme of the different steps required to fabricate double-walled Au−Ag nanotubes: (1) growth of Ag nanowires, (2) conversion of the Ag nanowires into Au−Ag nanowires by galvanic replacement for 5 min in a solution of HAuCl4 with 1 mM saturated with NaCl, (3) coating of the Au−Ag nanotubes with an Ag layer using magnetron sputtering, and (4) carving of the silver layer and formation of the double-walled nanotube. (b) SEM micrograph of double-walled Au−Ag nanotubes synthesized following the steps described in panel a. Scale bar: 100 nm.

sputtering for 40 s (Figure 7a, step 3). The sample is then treated once again in 1 mM concentrated HAuCl4 saturated with NaCl for 4 min (Figure 7a, step 4). This allows obtaining the double-walled nanotubes presented in Figure 7b.

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CONCLUSION We have investigated the conversion process of planar arrays of silver nanowires into bimetallic silver−gold nanotubes by means of galvanic replacement in HAuCl4 in the absence of any stabilizer or capping agent such as PVP. We have demonstrated that the fundamental mechanisms occurring during the reaction between the silver nanowires attached to the silicon template substrate and the HAuCl4 are different as compared to the wellknown ones reported in the literature for silver nanowires in suspension. We have further shown that the conversion rate of the reaction becomes more significant with increasing the concentration of the HAuCl4 solution. As a consequence, when the silver nanowires transform into fully hollow nanotubes, their surface becomes extremely rough. To lower the surface roughness of the nanotubes, we have employed a protocol in which the silver nanowires are transformed in the HAuCl4 solution saturated with NaCl. The presence of NaCl in the solution has allowed: (i) distributing the anodic and cathodic sites homogeneously over the surface of the nanowires; and (ii) avoiding the precipitation of AgCl cyrstallites over the surface of the nanoobjects during the reaction. We have further shown that the surface roughness can be exacerbated by increasing the conversion temperature. In light of these results, we have proposed a mechanistic model to explain the galvanic replacement reaction between the silver nanowires attached to a substrate and the HAuCl4 solutions. Laying on the knowledge collected on the synthesis of single-walled bimetallic nanotubes by galvanic replacement, we have further shown that our approach can be employed to synthesize an array of highly ordered double-walled bimetallic nanotubes. In addition to the fundamental comprehension provided in this study, creating organized hollow nanostructures arranged directly on a solid substrate will enable a simple development of practical nanomaterials-based devices such as SERS-based sensors.

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Chemical state and electron microscopy data recorded on specimens obtained by galvanic replacement in different conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +33 240376325. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

D.T. would like to thank the “la Région des Pays de la LoireFrance” for financially assisting this project research through the “Post-Doctorats internationaux” program.

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ABBREVIATIONS TEM, transmission electron microscopy; SEM, scanning electron microscopy

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Galvanic Replacement Reaction: A Route to Highly Ordered Bimetallic

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